Patent Application
20210108239
The present invention relates to processes to make neosaxitoxin and analogues thereof, and intermediates in the production of neosaxitoxin in recombinant host cells. Neosaxitoxin may be used in the production of pharmaceutical compositions.
Voltage-gated sodium channels (VGSCs) are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's plasma membrane. They play an important role in the initiation of action potentials.
It has long been known that blocking such channels may be useful in preventing the transmission of pain impulses; VGSCs are now well-validated targets for the treatment of pain. In particular, a number of VGSC antagonists are currently being investigated in the production of analgesics and anaesthetics.
Saxitoxin and neosaxitoxin both act as specific blockers of VGSCs. These compounds are therefore potentially useful in the treatment of pain. Saxitoxin (SXT), also known as paralytic shellfish toxin (PST), is a neurotoxin produced by many cyanobacteria (e.g. Anabaena, Aphanizomenon, Cylindrospermopsis, Lyngbya, Planktothrix, Raphidiopsis, Scytonema) and dinoflagellates (Alexandrium, Gymnodinium and Pyrodinium). Saxitoxin is one of the most lethal non-proteinaceous neurotoxins known to date and reversibly binds VGSCs, causing paralysis. It is responsible for many cases of human food poisoning, due to the ingestion of contaminated filter-feeding aquatic animals (e.g. crustaceans, molluscs, shellfish) who bioaccumulate the toxin.
However, there are a number of analogues of SXT, including neosaxitoxin (which is an N1-hydroxylated analogue of SXT), which, although they have high specific toxicities, they have no systemic toxicity when administered in low dosages. They are therefore potentially useful as human therapeutics.
In June 2015, the German drugmaker Grunenthal entered into a collaboration with the Chilean company Proteus S.A. and the US-based Boston Children's Hospital to develop the use of neosaxitoxin as a novel anaesthetic for local anaesthesia and post-operative pain management.
There is also a market for other saxitoxin analogues, such as gonyautoxin which has been clinically tested as a muscle relaxant.
Proteus S.A. has filed patent applications (e.g. US 2015/0099879) relating to the production and harvesting of neosaxitoxin and saxitoxin from cells which naturally-produce neosaxitoxin and saxitoxin (e.g. cyanobacterial cells).
A biosynthetic pathway for the production of saxitoxin was first proposed by Shimizu et al. (J. Am. Chem. Soc. (1984), 106, 6433-6434). It was not until 2008, however, that the saxitoxin (sxt) gene cluster was first identified; this was in the cyanobacterium Cylindrospermopsis raciborskii T3 (Kellmann et al., Appl. Environ. Microbiol. 2008, 74, 4044-4053). The sxt gene cluster consists of 34 genes or open reading frames (ORFs). This lead Kellmann et al. to propose a revised (10-step) biosynthetic pathway for the production of saxitoxin (see
Four other sxt gene clusters have since been identified in different genera: Anabaena circinalis 131C and Aphanizomenon sp. NH-5 (Mihali et al., BMC Biochem. 10, 8 (2009)); Lyngbya wollei (Mihali et al., PloS One 6, e14657 (2011)); and Raphidiopsis brookii D9 (Stucken, K. et al., PLoS ONE 5, e9235 (2010)). It is notable, however, that there are several differences in these gene clusters, including the absence of some genes and duplications of others. Furthermore, the architecture of the sxt cluster is rearranged in some genomes, resulting in the biosynthesis of saxitoxin analogues. This has made the elucidation of the biosynthetic pathways for the production of saxitoxin and neosaxitoxin particularly difficult, thus preventing the production of saxitoxin and neosaxitoxin by recombinant routes.
In the pathway proposed by Kellmann (2008), it was suggested that the first steps involved the sxtA gene product. The sxtA gene codes for a multi-domain protein related to polyketide synthases (PKS). SxtA was proposed by Kellmann to catalyse the condensation of arginine and one methylated acetate unit to produce a 4-amino-3-oxo-guanidinoheptane (AOGH) intermediate. This process was proposed to occur stepwise, and was catalysed by the four domains of SxtA. The acetyltransferase domain selectively was said to tether acetyl-CoA onto the pantetheinyl arm of the holo-acyl carrier protein. SAM-dependent methylation of the acetyl moiety, catalysed by the first SxA domain, was said to result in the formation of propionate. The final step in AOGH biosynthesis was said to be the condensation of propionate to arginine, catalysed by the class II aminotransferase domain. AOGH was used as a substrate for the biosynthesis of saxitoxin by downstream enzymes encoded by other members of the sxt gene cluster. However, the structure of AOGH was not confirmed due to lack of a chemical standard.
Although Tsuchiya and co-workers have recently elucidated a synthetic route for AOGH (Tsuchiya, S. et al. Org. Biomol. Chem. 12, 3016-3020 (2014); Tsuchiya, S. et al., Chem. Eur. J. 21, 7835-7840 (2015)), this study did not investigate the involvement of SxtA or any other Sxt proteins in the production of AOGH. This step is a key one in the production of saxitoxin and neosaxitoxin.
In summary, therefore, the only commercially-available high-yield route which is currently useable to make saxitoxin and neosaxitoxin is to isolate it from cells (such as cyanobacteria) which naturally produce it.
The current methods for the production of neosaxitoxin are not capable of producing neosaxitoxin in the quantities which are needed to manufacture the desired pharmaceutical compositions. Therefore, there exists a need for improved methods to produce neosaxitoxin.
The biosynthetic pathway for the production of neosaxitoxin has now been elucidated in sufficient detail to enable the production of neosaxitoxin by a recombinant route. In particular, out of the 34 sxt genes or ORFs which were identified by Kellmann (2008), those that are necessary for the production of neosaxitoxin have identified.
It has now been found that the biosynthetic pathway which was proposed by Kellmann (2008) is incorrect and that the Kellmann pathway refers to some genes which are not necessary for the recombinant production of neosaxitoxin; the Kellmann (2008) pathway also fails to refer to some sxt genes which are necessary for the production of neosaxitoxin.
Thus the invention provides for the first time a recombinant route for the production of neosaxitoxin and analogues thereof, as well as a recombinant route for the production of various intermediates in the production of neosaxitoxin and analogues thereof.
The invention also facilitates the production of saxitoxin and other analogues thereof, such as gonyautoxin.
In one embodiment, the invention provides a process for producing neosaxitoxin or an analogue thereof, the process comprising the steps:
(A) contacting the substrates:
Preferably, the process is carried out in a host cell which comprises nucleic acid molecules encoding said Sxt polypeptides.
The invention also provides a process for producing neosaxitoxin or an analogue thereof in a host cell, the process comprising the step:
(A) culturing a host cell which comprises nucleic acid molecules encoding the Sxt polypeptides A, B, D, G, H, I, S, T, U, V, W and X in a culture medium in the presence of the substrates:
(i) S-adenosylmethionine,
(ii) arginine,
(iii) acetyl-CoA, malony-CoA or propionyl-CoA, and
(iv) carbamoyl phosphate
preferably under conditions which are suitable for the production of neosaxitoxin or an analogue thereof. Preferably, the host cell additionally comprises a nucleic acid molecule encoding a PPTase.
Preferably, the process additionally comprises the step of isolating and/or purifying neosaxitoxin or an analogue thereof from the host cells or from the culture medium.
As used herein, the term “neosaxitoxin” refers to a compound having the following structure:
or a stereoisomer thereof.
The host cells may be any cells which are capable of expressing the nucleic acid molecules encoding all of the specified Sxt polypeptides. The host cell is preferably a recombinant host cell.
As used herein, the term “recombinant” refers to the fact that the host cells are not wild-type host cells, e.g. they have been modified by the introduction of one of more nucleic acid molecules encoding one or more of the specified Sxt polypeptides.
The host cells may be prokaryotic or eukaryotic cells. For example, the host cells may be bacterial cells. The bacteria may be a Gram positive or Gram negative bacteria. The Gram positive bacteria may be selected from the group consisting of Actinobacteria, Firmicutes and Tenericutes. The Gram negative bacteria may be selected from the group consisting of Aquificae, Bacteroidetes/Fibrobacteres-Chlorobi (FCB group), Deinococcus-Thermus, Fusobacteria, Gemmatimonadetes, Nitrospirae, Planctomycetes-Verrucomicrobia/Chlamydiae (PVC group), Proteobacteria, Spirochaetes and Synergistetes.
Preferably, the host cell is of the Phylum Proteobacteria; more preferably of the Class Gammaproteobacteria; more preferably of the Family Enterobacteriaceae; and even more preferably of the Genus Escherichia. Most preferably, the host cell is of the species E. coli. In some embodiments, the host cells are of the genus Pseudomonas.
Eukaryotic cells may also be used, e.g. yeast cells and mammalian cells. Since neosaxitoxin is toxic to many eukaryotic cells, the eukaryotic host cells are preferably ones which are not susceptible to neosaxitoxin toxicity. Such cells may be naturally non-susceptible (e.g. yeast cells) or they may be engineered to be non-susceptible (e.g. mammalian cells which co-express a neosaxitoxin antagonist).
Alternatively, the host cells may be ones which do not secrete neosaxitoxin, i.e. any neosaxitoxin which is produced is retained within the cells (thus preventing it from exerting its toxic effect). The host cells may also be plant cells.
In some embodiments, the host cells are heterotrophs. The host cell may be a photoheterotroph or a chemoheterotroph. A heterotroph is an organism that cannot fix carbon and uses organic carbon for growth. Heterotrophs can be further divided based on how they obtain energy: if the heterotroph uses light for energy, then it is a photoheterotroph; if the heterotroph uses chemical energy, it is a chemoheterotroph. In some embodiments, the host cells are not autotrophs. Autotrophs can be photoautotrophs or chemoautotrophs.
Neosaxitoxin is produced naturally by several species of marine dinoflagellates and freshwater cyanobacteria. The invention does not relate to the natural production of neosaxitoxin by such wild-type host cell species. Consequently, in some embodiments of the invention, the host cells are not dinoflagellates or cyanobacteria.
In particular, the host cells are preferably not selected from the group consisting of Cylindrospermopsis raciborskii, Anphanizomenon flos-aquae, Aphanizomenon (APh) issatschenkoi (usaceb) Proskina-Lavrenco, Aphanizomenon gracile (Lemm) Lemm, Anabaena circinalis, Lyngbya wollei and Alexandrium tamarens.
In particular, the host cells are preferably not selected from the group consisting of Anabaena, Aphanizomenon, Cylindrospermopsis, Lyngbya, Planktothrix, Raphidiopsis, Scytonema and dinoflagellates (Alexandrium, Gymnodinium and Pyrodinium).
The host cells may, however, be recombinant dinoflagellates or recombinant cyanobacteria, e.g. dinoflagellates or cyanobacteria which have been modified compared to the wild-type dinoflagellates or cyanobacteria (for example, by addition of one or more genes, preferably by the addition of one or more sxt genes). The host cells comprise and/or express nucleic acid molecules encoding the specified Sxt proteins.
Preferably, the Sxt polypeptide and sxt genes are obtained from a cyanobacteria or a dinoflagellate. More preferably, the Sxt polypeptide and sxt genes are obtained from Anabaena, Aphanizomenon, Cylindrospermopsis, Lyngbya, Planktothrix, Raphidiopsis, Scytonema, Alexandrium, Gymnodinium or Pyrodinium. Even more preferably, the Sxt polypeptide and sxt genes are obtained from Cylindrospermopsis raciborskii, Anphanizomenon flos-aquae, Aphanizomenon (APh) issatschenkoi (usaceb) Proskina-Lavrenco, Aphanizomenon gracile (Lemm) Lemm, Anabaena circinalis, Lyngbya wollei or Alexandrium tamarens. Most preferably, the Sxt polypeptide and sxt genes are obtained from C. raciborskii T3 strain. In some embodiments, the Sxt polypeptide and sxt genes may be obtained from Dolichosporum circinale 134C or A. minutum.
The nucleic acid molecules encoding the specified Sxt proteins may be heterologous molecules, i.e. ones that do not occur naturally in the wild-type host cell.
As used herein, the term SxtA preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 10 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a polyketide synthase related protein with four catalytic domains: sxtA1: methyltransferase; sxtA2: GNAT acetyl transferase; sxtA3: acyl carrier protein; and sxtA4: class II aminotransferase.
As used herein, the term sxtA preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 11 or 12, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a polyketide synthase related protein with four catalytic domains: sxtA1: methyltransferase; sxtA2: GNAT acetyl transferase; sxtA3: acyl carrier protein; and sxtA4: class II aminotransferase.
As used herein, the term SxtB preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 7 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a cytidine deaminase.
As used herein, the term sxtB preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 8 or 9, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a cytidine deaminase. The SxtB polypeptide may also be capable of cyclisation.
As used herein, the term SxtC preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 4 or a sequence having at least 50% amino acid sequence identity thereto and encoding a regulatory subunit. As used herein, the term SxtC preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 5 or 6, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a regulatory subunit. The SxtC polypeptide may also be capable of decarbamoylation.
As used herein, the term SxtD preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 1 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a desaturase. As used herein, the term sxtD preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 2 or 3, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a desaturase.
As used herein, the term SxtE preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 13 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a chaperone-like protein. As used herein, the term sxtE preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 14 or 15, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a chaperone-like protein.
As used herein, the term SxtF preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 16 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a sodium-driven multidrug and toxic compound extrusion protein. As used herein, the term sxtF preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 17 or 18, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a sodium-driven multidrug and toxic compound extrusion protein.
As used herein, the term SxtG preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 19 or a sequence having at least 50% amino acid sequence identity thereto and having the function of an amidino transferase. As used herein, the term sxtG preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 20 or 21, or a sequence having at least 50% nucleotide sequence identity thereto and encoding an amidino transferase.
As used herein, the term SxtH preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 22 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a dioxygenase. As used herein, the term sxtH preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 23 or 24, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a dioxygenase. The SxtH polypeptide may also be capable of C12 hydroxylation.
As used herein, the term SxtI preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 25 or a sequence having at least 50% amino acid sequence identity thereto and having the function of an O-carbamoyl transferase. As used herein, the term sxtI preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 26 or 27, or a sequence having at least 50% nucleotide sequence identity thereto and encoding an O-carbamoyl transferase.
As used herein, the term SxtJ preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 28 or a sequence having at least 50% amino acid sequence identity thereto. As used herein, the term sxtJ preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 29 or 30, or a sequence having at least 50% nucleotide sequence identity thereto.
As used herein, the term SxtK preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 31 or a sequence having at least 50% amino acid sequence identity thereto. As used herein, the term sxtK preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 32 or 33, or a sequence having at least 50% nucleotide sequence identity thereto. sxtJ and sxtK are often associated with 0-carbamoyltransferases. They may be regulatory subunits, and/or mediate binding of SxtI to other proteins or to the membrane.
As used herein, the term SxtL preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 34 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a GDSL-lipase. As used herein, the term sxtL preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 35 or 36, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a GDSL-lipase. The SxtL polypeptide may also be capable of decarbamoylation.
As used herein, the term SxtM preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 37 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a sodium-driven multidrug and toxic compound extrusion protein. As used herein, the term sxtM preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 38 or 39, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a sodium-driven multidrug and toxic compound extrusion protein.
As used herein, the term SxtN preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 40 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a sulfotransferase. As used herein, the term sxtN preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 41 or a sequence having at least 50% nucleotide sequence identity thereto and encoding a sulfotransferase.
As used herein, the term SxtO preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 72 or a sequence having at least 50% amino acid sequence identity thereto and having the function of an adenylylsulfate kinase. As used herein, the term sxtO preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 73 or a sequence having at least 50% nucleotide sequence identity thereto and encoding an adenylylsulfate kinase. The SxtO polypeptide may also be capable of PAPS biosynthesis.
As used herein, the term SxtP preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 69 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a putative saxitoxin-binding protein. As used herein, the term sxtP preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 70 or 71, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a putative saxitoxin-binding protein.
As used herein, the term SxtQ preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 66 or a sequence having at least 50% amino acid sequence identity thereto. As used herein, the term sxtQ preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 67 or 68, or a sequence having at least 50% nucleotide sequence identity thereto.
As used herein, the term SxtR preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 63 or a sequence having at least 50% amino acid sequence identity thereto and having the function of an acyl-CoA N-acyltransferase. As used herein, the term sxtR preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 64 or 65, or a sequence having at least 50% nucleotide sequence identity thereto and encoding an acyl-CoA N-acyltransferase.
As used herein, the term SxtS preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 57 or a sequence having at least 50% amino acid sequence identity thereto and capable of epoxidation and ring formation of a neosaxitoxin precursor. As used herein, the term sxtS preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 58 or 59, or a sequence having at least 50% nucleotide sequence identity thereto and capable of epoxidation and ring formation of a neosaxitoxin precursor.
As used herein, the term SxtT preferably refers to a polypeptide having the amino acid sequence given in SEQ ID N: 54 or a sequence having at least 50% amino acid sequence identity thereto and capable of C-11 hydroxylation of a neosaxitoxin precursor. As used herein, the term sxtT preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 55 or 56, or a sequence having at least 50% nucleotide sequence identity thereto and capable of C-11 hydroxylation of a neosaxitoxin precursor.
As used herein, the term SxtU preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 51 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a short-chain alcohol dehydrogenase. As used herein, the term sxtU preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 52 or 53, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a short-chain alcohol dehydrogenase. The SxtU polypeptide may also be capable of C1 reduction.
As used herein, the term SxtV preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 48 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a FAD-dependent succinate dehydrogenase/fumarate reductase. As used herein, the term sxtV preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 49 or 50, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a FAD-dependent succinate dehydrogenase/fumarate reductase. The SxtV polypeptide may also encode a dioxygenase reductase.
As used herein, the term SxtW preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 45 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a ferredoxin. As used herein, the term sxtW preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 46 or 47, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a ferredoxin. The SxtW polypeptide may also encode an electron carrier.
As used herein, the term SxtX preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 42 or a sequence having at least 50% amino acid sequence identity thereto and capable of the N-1 hydroxylation of neosaxitoxin. As used herein, the term sxtX preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 43 or 44, or a sequence having at least 50% nucleotide sequence identity thereto and capable of the N-1 hydroxylation of neosaxitoxin.
As used herein, the term SxtY preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 74 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a phosphate-dependent transcriptional regulator. As used herein, the term sxtY preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 75 or a sequence having at least 50% nucleotide sequence identity thereto and encoding a phosphate-dependent transcriptional regulator. The SxtY polypeptide may also be capable of signal transduction.
As used herein, the term SxtZ preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 76 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a histidine kinase. As used herein, the term sxtZ preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 77 or a sequence having at least 50% nucleotide sequence identity thereto and encoding a histidine kinase.
The functions and capabilities of some of the sxt genes are further illustrated in
As used herein, the term ORF5 preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 60 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a cyanophage S-PM2 protein CAF34141-like protein. ORF 5 is also known in the art as ORF24. As used herein, the term orf5 preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 61 or 62, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a cyanophage S-PM2 protein CAF34141-like protein.
Preferably, the host cell additionally comprises and/or expresses a nucleic acid molecule encoding a 4′-phosphopantetheinyl transferase (PPTase). As used herein, the term PPTase preferably refers to a polypeptide having the amino acid sequence given in SEQ ID NO: 78 or 90 or a sequence having at least 50% amino acid sequence identity thereto and having the function of a phosphopantetheinyl transferase. As used herein, the term PPTase gene preferably refers to a nucleic acid molecule having the nucleotide sequence given in SEQ ID NO: 79, 80 or 89, or a sequence having at least 50% nucleotide sequence identity thereto and encoding a phosphopantetheinyl transferase. Preferably, the PPTase is encoded by the Bacillus subtilis sfp gene (SEQ ID NO: 89). Most preferably, the PPTase is from C. raciborskii T3 strain. In some embodiments, 1-20 (e.g. 20) of the first twenty amino acids of the PPTase may be removed in order to increase the solubility of the PPTase. Additionally, the first V in the amino acid sequence may be changed to M.
Preferably, the nucleotide sequences of the nucleic acid molecules are codon-optimised for the host cell.
The Sxt polypeptides, ORF and PPTases are preferably defined herein as having at least 50% amino acid sequence identity to a reference amino acid sequence. Preferably, the Sxt, ORF and PPTase polypeptides have at least 60%, 70%, 80%, 90%, 95%, 98% or 99% amino acid sequence identity to the specified reference polypeptides. The sxt nucleic acid molecules, orf and PPTase nucleic acid molecules are preferably defined herein as having at least 50% nucleotide sequence identity to a reference nucleotide sequence. Preferably, the sxt, orf and PPTase nucleic acid molecules have at least 60%, 70%, 80%, 90%, 95%, 98% or 99% nucleotide sequence identity to the specified reference nucleic acid molecules.
The nucleic acid molecules may be DNA or RNA. Percentage amino acid sequence identities and nucleotide sequence identities may be obtained using the BLAST methods of alignment (Altschul et al. (1997), “Gapped BLAST and PSI-BLAST: a new generation of protein database search programs”, Nucleic Acids Res. 25:3389-3402; and http://www.ncbi.nlm.nih.gov/BLAST). Preferably the standard or default alignment parameters are used. Standard protein-protein BLAST (blastp) may be used for finding similar sequences in protein databases. Like other BLAST programs, blastp is designed to find local regions of similarity. When sequence similarity spans the whole sequence, blastp will also report a global alignment, which is the preferred result for protein identification purposes. Preferably the standard or default alignment parameters are used. In some instances, the “low complexity filter” may be taken off. BLAST protein searches may also be performed with the BLASTX program, score=50, wordlength=3. To obtain gapped alignments for comparison purposes, Gapped BLAST (in BLAST 2.0) can be utilized as described in Altschul et al. (1997) Nucleic Acids Res. 25: 3389. Alternatively, PSI-BLAST (in BLAST 2.0) can be used to perform an iterated search that detects distant relationships between molecules. (See Altschul et al. (1997) supra). When utilizing BLAST, Gapped BLAST, PSI-BLAST, the default parameters of the respective programs may be used. With regard to nucleotide sequence comparisons, MEGABLAST, discontiguous-megablast, and blastn may be used to accomplish this goal. Preferably the standard or default alignment parameters are used. MEGABLAST is specifically designed to efficiently find long alignments between very similar sequences. Discontiguous
MEGABLAST may be used to find nucleotide sequences which are similar, but not identical, to the nucleic acids of the invention. The BLAST nucleotide algorithm finds similar sequences by breaking the query into short subsequences called words. The program identifies the exact matches to the query words first (word hits). The BLAST program then extends these word hits in multiple steps to generate the final gapped alignments. In some embodiments, the BLAST nucleotide searches can be performed with the BLASTN program, score=100, wordlength=12. One of the important parameters governing the sensitivity of BLAST searches is the word size. The most important reason that blastn is more sensitive than MEGABLAST is that it uses a shorter default word size (11). Because of this, blastn is better than MEGABLAST at finding alignments to related nucleotide sequences from other organisms. The word size is adjustable in blastn and can be reduced from the default value to a minimum of 7 to increase search sensitivity. A more sensitive search can be achieved by using the newly-introduced discontiguous megablast page (www.ncbi.nlm.nih.gov/Web/Newsltr/FallWinter02/blastlab.html). This page uses an algorithm which is similar to that reported by Ma et al. (Bioinformatics. 2002 March; 18(3): 440-5). Rather than requiring exact word matches as seeds for alignment extension, discontiguous megablast uses non-contiguous word within a longer window of template. In coding mode, the third base wobbling is taken into consideration by focusing on finding matches at the first and second codon positions while ignoring the mismatches in the third position. Searching in discontiguous MEGABLAST using the same word size is more sensitive and efficient than standard blastn using the same word size. Parameters unique for discontiguous megablast are: word size: 11 or 12; template: 16, 18, or 21; template type: coding (0), non-coding (1), or both (2).
The Sxt polypeptide sequences of the Cylindrospermopsis raciborskii T3 sxt genes are available from GenBank under accession no. DQ787200 (http://www.ncbi.nlm.nih.qov/nuccore/DQ787200). The amino acid sequences of the Cylindrospermopsis raciborskii T3 Sxt polypeptides are given herein in the attached “SEQUENCES” section. The invention is not, however, limited to the sequences of those polypeptides; the invention encompasses polypeptides from other species which have the same function as the recited Cylindrospermopsis raciborskii T3 polypeptides.
In the processes of the invention, the substrates are contacted with the Sxt polypeptides A, B, D, G, H, I, S, T, U, V, W and X, or the host cells comprise one or more nucleic acid molecules coding for such polypeptides. In some embodiments of the invention, the substrates are not contacted with one or more of the Sxt polypeptides A, B, D, G, H, I, S, T, U, V, W and X, or the host cells does not comprise one or more nucleic acid molecules coding for such polypeptides. In some embodiments of the processes of the invention, other Sxt polypeptides or nucleic acid molecules coding for such polypeptides may or may not be used.
If a process of the invention does use a particular Sxt polypeptide, the substrates will be contacted with that Sxt polypeptide and/or the host cells will comprise a nucleic acid molecule coding for such a polypeptide. If a process of the invention does not use a particular Sxt polypeptide, the substrates will not be contacted with that Sxt polypeptide and/or the host cells will not comprise a nucleic acid molecule coding for such a polypeptide.
In some embodiments, the processes of the invention may or may not additionally use one or more Sxt polypeptides independently selected from the group consisting of Sxt C, E, J, K, L, and R, or nucleic acid molecules coding for such polypeptides. Preferably, the processes of the invention do additionally use one or more Sxt polypeptides selected from the group consisting of Sxt C, E, J, K, L, and R (preferably C and/or E), or nucleic acid molecules coding for such polypeptides.
In some embodiments, the process of the invention additionally uses Sxt E or a nucleic acid molecule coding for such polypeptide. In other embodiments, the process of the invention additionally does not use Sxt E or a nucleic acid molecule coding for such polypeptide.
In some embodiments, the processes of the invention may or may not utilise the Sxt Q polypeptide or a nucleic acid molecule coding for such a polypeptide.
In some embodiments, the processes of the invention may or may not utilise the Sxt R polypeptide or a nucleic acid molecule coding for such a polypeptide.
In some embodiments, the processes of the invention may or may not utilise the ORF24 polypeptide or a nucleic acid molecule coding for such a polypeptide.
In other embodiments, the processes of the invention may or may not use one or more Sxt polypeptides independently selected from the group consisting of Sxt F, M, N, O, P, Y, Z, ORF3, ORF4, ORF29, ORF34, OMPR or HISA, or nucleic acid molecules coding for such polypeptides. Preferably, the processes of the invention do not use one or more Sxt polypeptides selected from the group consisting of Sxt F, M, N, O, P, Y, Z, ORF3, ORF4, ORF29, ORF34, OMPR or HISA, or nucleic acid molecules coding for such polypeptides.
In some embodiments of the invention, the processes of the invention may or may not use one or more Sxt polypeptides (or a nucleic acid molecule coding for such a polypeptide) independently selected from the group consisting of:
Sxt ACT may be required to make the Lyngbya wollei-specific “weird” analogues of saxitoxin (LWTX-1 to -6). These carry a methyl-acetylester side chain instead of a carbamoyl side chain.
Sxt DIOX and Sxt SUL may be required to make C-11 sulfated toxin analogues, such as gonyautoxins 1 to 4 & C-1 to C-4 toxins.
SxtN1 and SxtN2 may be required to make various N-sulfocarbamoyl analogues (C-toxins). SxtN from C. raciborskii T3 might be inactive due to a mutation in the catalytic site.
In other embodiments, the processes of the invention may or may not use one or more Sxt polypeptides independently selected from the group consisting of ORF24, P and Q, or nucleic acid molecules coding for such polypeptides. Preferably, the processes of the invention do not use one or more Sxt polypeptides selected from the group consisting of ORF24, P and Q, or nucleic acid molecules coding for such polypeptides.
The host cells of the invention may be produced using standard molecular biology techniques (e.g. Green & Sambrook, “Molecular Cloning: A Laboratory Manual”, Fourth Edition, 2012) and with reference to the Examples disclosed herein.
The coding sequences of each of the specified polypeptides will be operably associated with suitable regulatory elements which facilitate the production of the specified polypeptides in the host cells. For example, each coding sequence will be operably associated with a suitable promoter and terminator element. Preferably, these regulatory elements are optimised for use in the host cells. For example, if the host cells are E. coli, then E. coli regulatory elements (e.g. promoters, terminators, ribosome binding sequences) are preferably used.
One or more of the nucleic acid molecules may be integrated into the genome of the host cells. One or more of the nucleic acid molecules may be present in the host cells in the form of a plasmid or vector. Preferably, all of the specified nucleic acids are integrated into the genomes of the host cells. One or more of the specified nucleic acids may be inserted within a sugar operon within the host cell genome.
The nucleic acid molecules may be present in the form of operons or fragments of gene clusters, i.e. coding for more than one of the desired polypeptides. These may be independently transformed into the host cells. For example, a first nucleic acid molecule may comprise open reading frames encoding sxtA, sxtB and sxtC; a second nucleic acid molecule may comprise open reading frames encoding sxtD, sxtE, sxtG, sxtH, sxtI, sxtJ, sxtK and sxtL; a third nucleic acid molecule may comprise open reading frames encoding sxtQ, sxtR, orf24, sxtS, sxtT, sxtU, sxtV, sxtW and sxtX; and/or a fourth nucleic acid molecule may comprise open reading frames encoding sxtF, sxtP and sxtM. In some embodiments, the third nucleic acid molecule may comprise open reading frames encoding sxtQ, sxtR, sxtS, sxtT, sxtU, sxtV, sxtW and sxtX; or sxtS, sxtT, sxtU, sxtV, sxtW and sxtX.
The nucleic acid molecules may also comprise appropriate selection markers (e.g. genes coding for antibiotic resistance). The nucleic acid molecules may also comprise further control elements such that the expression of one more of the polypeptides is inducible.
In some preferred embodiments, the expression of SxtA is inducible.
In some embodiments of the invention, one or more of the nucleic acid molecules or a nucleic acid molecule which encodes a functionally-equivalent polypeptide may already be endogenously present in the host cell (in the host cell genome or in an endogenous plasmid).
The invention also extends to all host cells as defined herein. In particular, the invention provides a host cell which comprises nucleic acid molecules coding for one or more Sxt polypeptides independently selected from the group consisting of A, B, D, G, H, I, S, T, U, V, W and X. The host cell may or may not additionally comprise nucleic acid molecules coding for one or more Sxt polypeptides independently selected from the group consisting of Sxt C, E, J, K, L, and/or R. The host cell may or may not additionally comprise a nucleic acid molecule coding for Sxt Q.
The host cell may or may not additionally comprise one or more nucleic acid molecules coding for one or more or all of the Sxt polypeptides independently selected from the group consisting of Sxt F, M, N, O, P, Y, Z, ORF3, ORF4, ORF29, ORF34, OMPR or HISA. Preferably, the host cell does not comprise nucleic acid molecules coding for one or more or all of the Sxt polypeptides selected from the group consisting of Sxt F, M, N, O, P, Y, Z, ORF3, ORF4, ORF29, ORF34, OMPR or HISA.
The reaction medium and culture medium provide appropriate for the production of neosaxitoxin or an analogue thereof. The conditions will also include appropriate pH and temperature, as can readily be determined by the skilled person. The host cells are cultured in a culture medium under conditions which are suitable for the production of neosaxitoxin or an analogue thereof. Suitable culture media are well known in the art. These will be selected according to the host cells which are being used. Preferably, the culture medium will be an aqueous medium.
The starting substrates for the production of neosaxitoxin or the analogue or variant thereof are:
(i) S-adenosylmethionine,
(ii) arginine,
(iii) acetyl-CoA, malony-CoA or propionyl-CoA, and
(iv) carbamoyl phosphate.
Hence appropriate concentrations of the above substrates need to be available in the reaction medium and culture medium at the start of the process. Appropriate concentrations of the above substrates may readily be determined by the person of skill in the art. Arginine may readily be taken up into the host cells as a substrate from the surrounding culture medium. The other substrates (i.e. S-adenosylmethionine, acetyl-CoA, malony-CoA, propionyl-CoA and carbamoyl phosphate) are unstable. Preferably, these substrates are produced in sufficient amounts within the host cells (they are present in all living cells). The host cells may readily be modified to increase production of these substrates in ways which are routine in the art, if necessary.
In some embodiments of the invention, the process is carried out at a temperature of 14-24° C.; preferably at 16-22° C.; even preferably at about 17, 18, 19, 20 or 21° C.; and most preferably at about 19° C.
Preferably, neosaxitoxin is isolated and/or purified from the reaction or culture medium. Such isolation/purification may be by any suitable means.
In embodiments of the invention wherein the process is carried out in host cells, neosaxitoxin will be produced in the host cells. The host cells may therefore be separated from the culture medium (e.g. by filtration or centrifugation); the host cells may then be lysed; and neosaxitoxin harvested.
Neosaxitoxin may be isolated from the reaction or culture medium by solid phase extraction over a C-18 reverse-phase resin to remove hydrophobic compounds; neosaxitoxin would be present in the flow-through. Solid phase extraction using cation-exchange resin on activated charcoal may also be used. For further purification techniques, reference may be made to US 2015/0099879.
The invention also provides processes to produce various intermediates in the production of neosaxitoxin, as defined below. These processes may be carried out in cell-free media or in host cells which comprise nucleic acid molecules encoding the appropriate Sxt polypeptide.
In other embodiments, the invention provides a process for producing a compound of Formula I [intermediate 4]:
wherein R is OH, the process comprising the steps:
(A) contacting a compound of Formula II [intermediate 3]
wherein R is OH, with Sxt S, and optionally alpha-ketoglutarate, and optionally molecular oxygen; and (B) isolating or purifying a compound of Formula I from the reaction medium.
The invention also provides a process for producing a compound of Formula I [intermediate 4]:
wherein R is OH, the process comprising the steps:
(A) culturing a host cell which comprises a nucleic acid molecule encoding Sxt S in a culture medium in the presence of a compound of Formula II (intermediate 3):
wherein R is OH, under conditions such that Sxt S is produced and Sxt S converts compounds of Formula II to compounds of Formula I.
In other embodiments, the invention provides a process for producing a compound of Formula I [intermediate 4′]:
wherein R is OH, the process comprising the steps:
(A) contacting a compound of Formula II [intermediate 3]
wherein R is OH, with Sxt S, and optionally alpha-ketoglutarate and optionally molecular oxygen; and (B) isolating or purifying a compound of Formula I from the reaction medium.
The invention also provides a process for producing a compound of Formula I [intermediate 4′]:
wherein R is OH, the process comprising the steps:
(A) culturing a host cell which comprises a nucleic acid molecule encoding Sxt S in a culture medium in the presence of a compound of Formula II (intermediate 3):
wherein R is OH, under conditions such that Sxt S is produced and Sxt S converts compounds of Formula II to compounds of Formula I.
In other embodiments, the invention provides a process for producing a compound of Formula I [intermediate 5]:
wherein R is OH, the process comprising the steps:
(A) contacting a compound of Formula II [intermediate 4]:
wherein R is OH, with Sxt D, and optionally NADH, and optionally NADPH; and
(B) isolating or purifying a compound of Formula I from the reaction medium.
The invention also provides a process for producing a compound of Formula I [intermediate 5]:
wherein R is OH, the process comprising the steps:
(A) culturing a host cell which comprises a nucleic acid molecule encoding Sxt D in a culture medium in the presence of a compound of Formula II [intermediate 4]:
wherein R is OH, under conditions such that Sxt D is produced and Sxt D converts compounds of Formula II to compounds of Formula I.
In other embodiments, the invention provides a process for producing a compound of Formula I [intermediate 5′]:
wherein R is OH, the process comprising the steps:
(A) contacting a compound of Formula II [intermediate 4′]:
wherein R is OH, with Sxt D, and optionally NADH, and optionally NADPH; and
(B) isolating or purifying a compound of Formula I from the reaction medium.
The invention also provides a process for producing a compound of Formula I [intermediate 5′]:
wherein R is OH, the process comprising the steps:
(A) culturing a host cell which comprises a nucleic acid molecule encoding Sxt D in a culture medium in the presence of a compound of Formula II [intermediate 4′]:
wherein R is OH, under conditions such that Sxt D is produced and Sxt D converts compounds of Formula II to compounds of Formula I.
In other embodiments, the invention provides a process for producing a compound of Formula I [intermediate 6]
wherein R is OH, the process comprising the steps:
(A) contacting a compound of Formula II [intermediate 5]:
wherein R is OH, with Sxt S, and optionally alpha-ketoglutarate and optionally molecular oxygen; and (B) isolating or purifying a compound of Formula I from the reaction medium.
The invention also provides a process for producing a compound of Formula I [intermediate 6]:
wherein R is OH, the process comprising the steps:
(A) culturing a host cell which comprises a nucleic acid molecule encoding Sxt S in a culture medium in the presence of a compound of Formula II [intermediate 5]:
wherein R is OH, under conditions such that Sxt S is produced and Sxt S converts compounds of Formula II to compounds of Formula I.
In other embodiments, the invention provides a process for producing a compound of Formula I [intermediate 6′]:
wherein R is OH, the process comprising the steps:
(A) contacting a compound of Formula II [intermediate 5′]:
wherein R is OH, with Sxt S, and optionally alpha-ketoglutarate and optionally molecular oxygen; and (B) isolating or purifying a compound of Formula I from the reaction medium.
The invention also provides a process for producing a compound of Formula I [intermediate 6′]:
wherein R is OH, the process comprising the steps:
(A) culturing a host cell which comprises a nucleic acid molecule encoding Sxt S in a culture medium in the presence of a compound of Formula II [intermediate 5′]:
wherein R is OH, under conditions such that Sxt S is produced and Sxt S converts compounds of Formula II to compounds of Formula I.
In other embodiments, the invention provides a process for producing a compound of Formula I [intermediate 7]:
wherein R is OH, the process comprising the steps:
(A) contacting a compound of Formula II [intermediate 6]:
wherein R is OH, with Sxt S, and optionally alpha-ketoglutarate and optionally molecular oxygen; and (B) isolating or purifying a compound of Formula I from the reaction medium.
The invention also provides a process for producing a compound of Formula I [intermediate 7]:
wherein R is OH, the process comprising the steps:
(A) culturing a host cell which comprises a nucleic acid molecule encoding Sxt S in a culture medium in the presence of a compound of Formula II [intermediate 6]:
wherein R is OH, under conditions such that Sxt S is produced and Sxt S converts compounds of Formula II to compounds of Formula I.
In other embodiments, the invention provides a process for producing a compound of Formula I [intermediate 7′]:
wherein R is OH, the process comprising the steps:
(A) contacting a compound of Formula II [intermediate 6′]:
wherein R is OH, with Sxt S, and optionally alpha-ketoglutarate and optionally molecular oxygen; and (B) isolating or purifying a compound of Formula I from the reaction medium.
The invention also provides a process for producing a compound of Formula I [intermediate 7′]:
wherein R is OH, the process comprising the steps:
(A) culturing a host cell which comprises a nucleic acid molecule encoding Sxt S in a culture medium in the presence of a compound of Formula II [intermediate 6′]:
wherein R is OH, under conditions such that Sxt S is produced and Sxt S converts compounds of Formula II to compounds of Formula I.
The invention also encompasses the above processes wherein R is H.
The term “neosaxitoxin analogue” as used herein encompasses analogues and variants of neosaxitoxin, such as those compounds referred to below (preferably saxitoxin).
The elucidation of the saxitoxin and neosaxitoxin biosynthetic pathways as disclosed herein also enables the production of various saxitoxin, neosaxitoxin and gonyautoxin variants, such as the variants shown below (with reference to the structure of saxitoxin):
The elucidation of the saxitoxin and neosaxitoxin biosynthetic pathways as disclosed herein also enables the production of the following neosaxitoxin analogues and variants (shown below with reference to the structure of saxitoxin):
Natural Derivatives of Paralytic Shellfish Toxins (Oshima 1995). Abbreviations used are, STX: saxitoxin; GTX: gonyautoxin; C: C-toxin; dc: decarbamoyl; do: deoxy.
(Abbreviations used are, LTX: Lyngbya wollei toxin; GC: Gymnodinium catenatum toxin)
Molecular Structure of zeteki toxin AB from the golden frog Atelopus zeteki (Yotsu-Yamashita et al. 2004).
Hence, in a further aspect, the invention provides a process for the production of a saxitoxin variant as defined in the above tables, or an intermediate in the production thereof, which comprises a process as disclosed herein for the production of neosaxitoxin or an intermediate in the production of neosaxitoxin, wherein that process has been modified to produce the saxitoxin variant, or an intermediate in the production thereof.
For example, in the production of saxitoxin, the use of a Sxt X polypeptide or sxt X gene may be unnecessary because the Sxt X polypeptide is responsible of the N1-hydroxylation step in the production of neosaxitoxin (and saxitoxin is not N1-hydroxylated).
Prior to sulfonation at C-11 to produce C-11 sulfated toxins, such as GTX-1 to -4, carbon C-11 needs to be hydroxylated. This is putatively carried out by SxtDIOX.
The sulfotransferase, SxtN, putatively catalyses an N-sulfonation to produce N-sulfocarbamoyl toxins, such as GTX5/6 and C1-4 toxins. It is uncertain where in the pathway these reactions occur. However, it is likely that they occur prior to the formation of STX, i.e. on intermediates rather than on the end-product of the pathway.
In a further aspect, there is provided a process of the invention for producing neosaxitoxin or an analogue or variant thereof, wherein the process additionally comprises the step of contacting the neosaxitoxin or the analogue or variant thereof with an SxtN or SxtDIOX polypeptide. For example, the host cell may be one which additionally comprises genes encoding sxtN and/or sxtDIOX.
In such a way, saxitoxin may be converted to GTX-5 by SxtN (by sulphation of the carbamoyl side chain). Similarly, saxitoxin may be converted to 11-hydroxy saxitoxin by SxtDIOX. This step would precede the C-11 sulphation to convert saxitoxin to GTX-2/3 (or neosaxitoxin to GTX-4/1).
There is a strong possibility that the array of toxins produced in a given strain is the result of a combination of each enzyme having different kinetics (reaction speed), and varying relaxed substrate specificities towards intermediate metabolites with various modifications.
Intermediate 8 is converted by 0-carbamoyltransferase SxtI to intermediate 9. Intermediate 8 as well as intermediate 9 may both be the substrate for dioxygenases SxtH and SxtT converting intermediate 8 to dcSTX and intermediate 9 to STX. An analogous pathway is likely for the production of neoSTX and dcneoSTX.
Conversions of intermediate 8 to intermediate 9 and decarbamoylsaxitoxin, and intermediate 9 to saxitoxin.
Conversion of hydroxylated intermediate 8 to hydroxylated intermediate 9 and decarbamoylneosaxitoxin, and hydroxylated intermediate 9 to neosaxitoxin.
In yet a further embodiment, the invention provides neosaxitoxin or an analogue thereof which is produced by a process of the invention. The neosaxitoxin or analogue thereof which is produced by a process of the invention may further be converted into a salt, particularly into a pharmaceutically-acceptable salt thereof with an inorganic or organic acid or base.
Acids which may be used for this purpose include hydrochloric acid, hydrobromic acid, sulphuric acid, sulphonic acid, methanesulphonic acid, phosphoric acid, fumaric acid, succinic acid, lactic acid, citric acid, tartaric acid, maleic acid, acetic acid, trifluoroacetic acid and ascorbic acid. Bases which may be suitable for this purpose include alkali and alkaline earth metal hydroxides, e.g. sodium hydroxide, potassium hydroxide or caesium hydroxide, ammonia and organic amines such as diethylamine, triethylamine, ethanolamine, diethanolamine, cyclohexylamine and dicyclohexylamine. Procedures for salt formation are conventional in the art.
The neosaxitoxin or analogue thereof which is produced by a process of the invention or a salt thereof may further be formulated for use in a pharmaceutical composition. Hence, in a further aspect, there is provided a process of the invention for producing neosaxitoxin or analogue thereof or a salt thereof which additionally comprises the step of formulating the isolated or purified neosaxitoxin, or a salt thereof, in a pharmaceutical composition. Preferably, the step comprises combining isolated or purified neosaxitoxin or analogue thereof or a salt thereof with one or more pharmaceutically acceptable carriers, adjuvants and/or excipients.
In particular, neosaxitoxin or an analogue thereof or a salt thereof may be formulated with one or more conventional carriers, diluents and/or excipients according to techniques well known in the art. The pharmaceutically acceptable carriers, adjuvants and/or excipients may be a preservative.
The compositions may be adapted for oral administration or for parenteral administration, for example by intradermal, subcutaneous, intraperitoneal, intravenous, or intramuscular injection. Suitable pharmaceutical forms thus include plain or coated tablets, capsules, suspensions and solutions containing the active component optionally together with one or more conventional inert carriers and/or diluents, such as corn starch, lactose, sucrose, microcrystalline cellulose, magnesium stearate, polyvinylpyrrolidone, citric acid, tartaric acid, water, water/ethanol, water/glycerol, water/sorbitol, water/polyethyleneglycol, propylene glycol, stearylalcohol, carboxymethylcellulose or fatty substances such as hard fat or suitable mixtures of any of the above.
Alternatively, neosaxitoxin or a salt thereof may be formulated for topical administration, e.g. in the form of a gel, cream, emulsion, paste, etc., e.g. comprising neosaxitoxin or a salt thereof together with a conventional diluent, carrier or excipient.
In other embodiments, neosaxitoxin or a salt thereof may be formulated for transdermal administration.
In
In
The present invention is further illustrated by the following Examples, in which parts and percentages are by weight and degrees are Celsius, unless otherwise stated. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, various modifications of the invention in addition to those shown and described herein will be apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims.
The disclosure of each reference set forth herein is incorporated herein by reference in its entirety.
Subcloning of sxtA and sfp in pET28b
PCR amplifications of sxtA from Cylindrospermopsis raciborskii T3 and sfp from Bacillus subtilis were performed from genomic DNA using Velocity polymerase (Bioline). The manufacturer's protocols were followed, with an annealing temperature of 55° C. and extension times of 4 min and 1 min for sxtA and sfp, respectively (sxtA primers: forward, 5′-GGGCTTTCATATGTTACAAAAGATTAA-3′ (SEQ ID NO: 81) and reverse, 5′-AAAGTATGCGGCCGCATGCTTGAGTAT-3′ (SEQ ID NO: 82); sfp primers: forward 5′-GGCATCCATGGGCAAGATTTACGGAA-3′ (SEQ ID NO: 83) and reverse, 5′-GGCATCTCGAGTTATAAAAGCGCTTCG-3′ (SEQ ID NO: 84)). PCR amplicons were purified (DNA clean and concentrator 5× kit, ZymoResearch) and digested with NdeI/NotI (sxtA) or NcoI/XhoI (sfp; New England Biolabs). Digested products were purified and ligated into pET-28b (Novagen), cut with the same enzymes, and purified via agarose gel electrophoresis (DNA recovery kit, ZymoResearch). After transformation in electrocompetent Escherichia coli GB2005 cells, positive clones were screened by PCR of the purified plasmid (PureLink Miniprep, Invitrogen) with the universal primers T7 promoter and T7 terminator. The inserts were sequenced (Ramaciotti Center at UNSW, Australia) for verification.
For co-expression with sxtA, the sfp gene was amplified from pET28b::sfp using Velocity polymerase (Bioline) with primers for the T7 promoter and T7 terminator (5′-GGTTAAGATCTGAAATTAATACGACTC-3′ (SEQ ID NO: 85), 5′-TTTTAAGATCTTTTCAGCAAAAAACCC-3′ (SEQ ID NO: 86)). The manufacturer's protocols were followed with an annealing temperature of 55° C. and an extension time of 1 min. Amplicons were purified (DNA clean and concentrator 5× kit, ZymoResearch) and digested with BglII (New England Biolabs). Digested products were purified and ligated into a pET28b::sxtA cut with the same enzyme, and purified as described previously. After transformation in electrocompetent Escherichia coli GB2005 cells, the positive clones were screened by PCR of the purified plasmid (PureLink Miniprep, Invitrogen). The insert was sequenced (Ramaciotti Center at UNSW, Australia) for verification.
Expression of sxtA with Sfp and Purification of Holo-SxtA
For expression of sxtA, 0.5 mL of overnight E. coli BL21 (DE3) transformants containing pET28b::sxtA,sfp and pRARE plasmids (Invitrogen) was subcultured in 50 mL Lysogeny Broth (LB) medium supplemented with 50 μg·mL−1 kanamycin and 30 μg·mL−1 chloramphenicol and incubated at 30° C. under agitation (200 rpm) until an optical density of 0.8-1.0 at 600 nm. Cultures were then induced with 200 μM isopropyl β-D-thiogalactoside (IPTG) and incubated overnight at 18° C. with agitation and pelleted by centrifugation (4000 rpm, at 4° C. for 20 min, Hitachi CR22GIII centrifuge, R10A5 rotor). The cell pellet was frozen until further purification.
Pelleted cells were resuspended in 10 mL of lysis buffer (20 mM sodium phosphate buffer, pH 7.4, 500 mM NaCl, 20 mM imidazole), and lysed by sonication (Branson Digital Sonifier M450, 3 mm probe, 30% of amplitude, 3 min at 4° C. with cycles of 15 s power on and 59 s off). The resulting suspension was centrifuged (20 000 rpm, at 4° C. for 60 min, Hitachi CR22GIII centrifuge, R20A2 rotor), and the supernatant was loaded on a Ni-affinity column (1 mL HiTrap column, fitted on an AKTApurifier, GE Healthcare), equilibrated with 20 mM sodium phosphate buffer, pH 7.4, 500 mM NaCl, 20 mM imidazole (butter A). After injection, the column was washed (35 mL of buffer A, 1 mL·min−1) and the proteins were eluted using 20 mM sodium phosphate buffer, pH 7.4, 500 mM NaCl, 500 mM imidazole (buffer B) and a stepwise gradient of 0% to 20% buffer B in 20 min, followed by 10 min at 20%, then a linear gradient from 20% to 100% in 20 min, and a final wash at 100% for 10 min. The collected fractions (1 mL, detection at 280 nm) were analyzed by 10% polyacrylamide gel electrophoresis under denaturing conditions (SDS-PAGE), and the fractions containing the pure protein were pooled. The protein solution was desalted and concentrated via centrifugal filter (Amicon Ultra-4 centrifugal filter unit 100 k) with 50 mM HEPES-150 mM NaCl pH7.4 and then glycerol was added to a final concentration of 10% (w/v). Protein concentration was determined by using protein assay kit (Bio-Rad) and stored at −80° C.
Cloning of the Acyl Carrier Protein Domain of sxtA for Expression Alone and with Sfp
The acyl carrier protein of sxtA (sxtA-ACP) coding gene was amplified using Velocity polymerase (Bioline) (primers: forward 5′-ATATCCATGGGACCTGGTGATCGCAAAGGA-3′ (SEQ ID NO: 87) and reverse 5′-TATCTCGAGAGTGTTGATTTCGTTGGCTG-3′ (SEQ ID NO: 88)). Manufacturer protocols were followed with an annealing temperature of 54° C. and an extension time of 1.5 min. PCR amplicons were purified, digested, and ligated into pET-28b using the same method as described for sfp. After transformation in electrocompetent Escherichia coli GB2005 cells, the positive clones were screened by with universal primers T7 promoter and T7 terminator and sequenced as described previously. For co-expression with sfp, the gene was cloned in pET28b::sxtA-ACP plasmid, as described previously.
For expression of sxtA-ACP, 10 mL of overnight E. coli BL21 (DE3) transformants containing pET28b::sxtA-ACP and pET28b::sxtA-ACP,sfp plasmids were grown in 1 L Lysogeny broth (LB) supplemented with 50 μg·mL−1 kanamycin and 30 μg·mL−1 chloramphenicol and incubated at 37° C. under agitation (200 rpm) until the induction with 100 μM IPTG, as described above. Cells were collected by centrifugation (4000 rpm, at 4° C. for 20 min, Hitachi CR22GIII centrifuge, R10A5), resuspended in 15 mL of Lysis buffer, and disrupted by sonication (Branson Digital Sonifier M450, 3 mm probe, 30% of amplitude, 3 min at 4° C. with cycles of 1 s power on followed by 4 s off). The resulting suspension was centrifuged (20 000 g, at 4° C. for 60 min, Hitachi CR22GIII centrifuge, R20A2 rotor), and the supernatant was loaded on a Ni-affinity column (1 mL HiTrap column, fitted on an AKTApurifier, GE Healthcare) previously equilibrated with 20 mM sodium phosphate buffer, pH 7.4, 500 mM NaCl, 20 mM imidazole (buffer A). After injection, the column was washed (35 mL of buffer A, 1 mL·min−1) and the proteins were eluted using 20 mM sodium phosphate buffer, pH 7.4, 500 mM NaCl, 500 mM imidazole (buffer B) and a linear gradient from 0% to 20% B in 20 min, then 10 min at 20%, then a linear gradient from 20% to 100% in 20 min, and a final wash at 100% for 20 min. The collected fractions (1 mL, detection at 280 nm) were analyzed by 15% polyacrylamide gel electrophoresis under denaturing conditions (SDS-PAGE), and fractions containing the pure protein were pooled together. The protein solution was desalted and concentrated using centrifucation filtration (Amicon Ultra-15 centrifugal filter unit 3 k) with 50 mM HEPES-150 mM NaCl, pH 7.4, freezed in liquid nitrogen and stored at −80° C. Protein concentration was determined by using protein assay kit (Bio-Rad).
SxtA was successfully detected after purification on IMAC and SDS-PAGE (
Protein mass was detected by MALDI-TOF-TOF mass spectrometry. For matrix preparation, 10 mg of 3 5-dimethoxy-4-hydroxyl cinnamic acid was added into 1 mL 80% acetonitrile with 0.1% TFA, and 1 mL of matrix was mixed with 1 μl of protein sample on the surface of a MALDI target plate, followed by analysis by Bruker ultrafleXtreme MALDI-TOF/TOF with a YAG laser. Data acquisition was performed in the positive ion mode and the instrument calibrated immediately prior to each analysis. Analysis was performed in the linear delayed extraction mode acquiring 100 averaged spectra. The results are shown in
BL21 (DE3) transformants containing pRARE and pET28b::sxtA,sfp were resuspended in methanol acidified with 0.1% of acid acetic. The cells were disrupted by sonication (Branson Digital Sonifier M450, 3 mm probe, 30% of amplitude, 2 min at 4° C. with cycles of 5 s power on and 15 s off). The resulting suspension was centrifuged (20,000 rpm, at 4° C. for 30 min, Hitachi CR22GIII centrifuge, R20A2 rotor). The supernatant was dried under rotary evaporation and store at −20° C. until further analysis.
To determine the biosynthetic products of SxtA, intracellular metabolites of E. coli cells containing pET28b::sxtA,sfp and pRARE were chemically extracted and analysed by LC-MS.
Samples were resuspended in 95:5 acetonitrile:water (typically 1 mL) and transferred to HPLC vials for mass spectrometric analysis. LC-MS analysis was performed on 10 μL of sample using a Dionex U3000 UHPLC interfaced to a Q-Exactive Plus (ThermoFisher Scientific) via a heated electrospray interface. Ionisation was performed in positive mode under default source conditions (as suggested by the manufacturer's Tune software). Samples were injected onto a Waters BEH HILIC column (2.1×100 mm, 1.9 μm). Chromatographic conditions were as in Turner et al. with no modifications.
The mass spectrometer was run in data dependent analysis mode, with six MS/MS spectra being collected after every full scan mass spectrum. Inclusion lists of toxins and metabolites from the literature where used to prioritise their detections.
Comparison of the mass spectra between induced cells and non-induced controls showed the presence of a molecular ion at 1.1 min of m/z 187.06 [M+H]+, corresponding to the protonated mass of the AOGH(C8H18N4O). Secondary ion fragmentation of this molecular ion resulted in a fragmentation pattern closely resembling that of AOGH. The experiment was realised in eight replicates and mass data were analysed by Progenesis 01 to quantify the difference of expression between induced and non-induced extraction. The putative AOGH compound was observed to be overexpressed in the induced cells, compared to non-induced controls (Figure), with a maximum fold change equal to infinity (entirely absent in negative controls) and an ANOVA p-value of 1.1×10−16.
The compound at 187.06 Da mass was purified and analysed NMR confirmed the structure of the AOGH.
The following three open reading frames (ORFs) from C. raciborskii T3 were placed in a nucleotide fragment labelled “sxt1”: sxtA, sxtB and sxtC. Each ORF was codon optimised for expression in E. coli by Life Technology using GeneArt software.
EcoRI restriction sites were added at both ends of the fragment. In addition, homologous regions of 50 bp to the lacZ (H1) and lacA (H2) genes were added to the 5′- and 3′-end, respectively, for integration by RED ET mediated recombination into the E. coli genome. Downstream of H1, a terminator was added to prevent readthrough, followed by the transcription factor gene xylS and its Ps2 promoter. (XylS is the transcription factor that is required for transcription from the Pm promoter, after binding to its inducer, toluic acid.) This was followed by a terminator and then Pm promoter for the transcription of the downstream sxt genes. Upstream of each ORF, a ribosomal binding site (RBS) was added. Finally, a kanamycin resistance marker with two flanking FRT sites for excision by flippase-recombinase (GeneBridges), and homology arm H2 were added, including a terminal EcoRI site.
The following sxt genes were included in the “sxt2” fragment: sxtD, sxtE, sxtG, sxtH, sxtI, sxtJ, sxtK, sxtL. Each ORF was codon optimised for expression in E. coli by Life Technology using GeneArt software.
The sxt2 fragment was designed to have terminal EcoRI sites, homology arms for recombination with the malE (H1) and malG (H2) genes of E. coli, and a Pm promoter upstream of the sxt genes. This fragment also contained the kanR resistance cassette with FRT sites like fragment sxt1.
Three variants of sxt3 fragment were designed. The following sxt genes were included in the “sxt3” fragment, variant 1: sxtQ, sxtR, orf24, sxtS, sxtT, sxtU, sxtV, sxtW, sxtX. The following sxt genes were included in the “sxt3” fragment, variant 2: sxtQ, sxtR, sxtS, sxtT, sxtU, sxtV, sxtW, sxtX. The following sxt genes were included in the “sxt3” fragment, variant 3: sxtS, sxtT, sxtU, sxtV, sxtW, sxtX. Each ORF was codon optimised for expression in E. coli by Life Technology using GeneArt software.
Each variant of the sxt3 fragment was designed to have terminal EcoRI sites, homology arms for recombination with the xylF (H1) and xylB (H2) genes of E. coli, and a Pm promoter upstream of the sxt genes. Each variant of this fragment also contained the kanR resistance cassette with FRT sites like fragment sxt1.
The following sxt genes were included in the “sxt4” fragment: sxtF, sxtP, sxtM. Each ORF was codon optimised for expression in E. coli by Life Technology using GeneArt software.
The sxt4 fragment was designed to have terminal EcoRI sites, homology arms for recombination with the melR (H1) and melB (H2) genes of E. coli, and a Pm promoter upstream of sxt genes. This fragment also contained the kanR resistance cassette with FRT sites like fragment sxt1.
It is desirable to use a phosphopantetheinyl transferase (PPTase) to convert the apo-form of the polyketide synthase, SxtA, into its holo-form.
The entire genomes of Raphidiopsis brooki D9 and Cylindrospermopsis racoborskii CS-505 have been sequenced and the sequences deposited in GenBank. Each of these strains only contain a single PPTase gene. Specific PCR primers were designed based on the D9 and CS-505 PPTase genes to PCR amplify and sequence the PPTase gene from C. raciborskii T3.
A synthetic gene construct was designed for integration of the C. raciborskii T3 PPTase gene into the maltose operon of E. coli BL21(DE3). The PPTase gene was codon optimised for expression in E. coli and the construct was fitted with a T7 promoter and a beta-lactamase gene for selection for ampicillin resistance. The start codon was also changed to ATG.
The gene construct was then integrated into the maltose operon of E. coli BL21(DE3) (Life Technology) using the Quick & Easy E. coli Gene Deletion Kit following the manufacturer's instructions (Gene Bridges, catalog No. K006), and as depicted in
The sxtI fragment was produced by Life Technology inside a pMA-RQ (ampR) plasmid backbone. This plasmid will be called pSxt1 herein. The pSxt1 plasmid was transformed into E. coli BL21(DE3) PPTase, which carries the phosphopantetheinyl transferase from Cylindrospermopsis raciborskii T3 in its genome. The PPTase gene is under T7 promoter control (IPTG-inducible), and the sxtA, B, and C genes on pSxt1 are under Pm promoter control (m-toluoic acid inducible).
The goal of this experiment was to measure the expression and activity of the sxtA gene in E. coli at three different temperatures, 5° C., 19° C., and 30° C., at constant inducer concentrations (IPTG (0.5 mM) and m-toluic acid (1 mM)). The experimental strain was E. coli BL21(DE3) PPTase pSxt1, and the negative control was E. coli BL21 (DE3) (parent strain). Enzymatic product of only SxtA was expected, as the substrates for SxtB and SxtC are the enzymatic products of other enzymes, such as SxtG, which are not encoded on pSxt1.
Seed-cultures of BL21(DE3) (parent strain) and BL21(DE3) PPTase pSxt1 were grown overnight at 37° C. in, respectively, LB and LB 50 μg/ml ampicillin and kanamycin. The following day, triplicates volumes of 200 ml LB medium with kanamycin 50 μg/ml were inoculated each with 2 ml seed culture of E. coli BL21(DE3) PPTase pSxt1, and incubated at 30° C. until the culture reached an OD600 of ˜0.6.
For the negative control, triplicate volumes of 200-ml LB without antibiotics were inoculate each with 2 ml seed culture from E. coli BL21(DE3) and incubated at 30° C. to an OD600 of ˜0.6. All cultures were then transferred to the corresponding temperature, and acclimatised for 30 minutes. Subsequently, IPTG (0.5 mM final concentration) and m-toluic acid (1 mM final concentration) was added to each culture, which were then incubated for 48 hours at 5° C., and for 18 hours at 19° C., and at 30° C.
Samples for SDS-PAGE as well as for RT-PCR were taken before induction, and at the end of the experiment. After 18 hours induction, the cultures were harvested by centrifugation 7.500 rpm for 12 min. The cell pellets were frozen until extraction for LC-MS analysis
The cell pellets were resuspended in 1 ml water and sonicated on ice (output control 4, duty cycle 50%, 7 minutes in total in cycles of 2-3 minutes). After sonication, 4 ml acetonitrile was added, the samples were vortexed, and centrifuged to remove cell debris and other particulates.
LC-MS analysis was carried out at the Hormone laboratory, Haukeland University Hospital on a Waters Xevo TQ-S coupled to an iclass Acquity UPLC that was fitted with a 50×2.1 mm Acquity BEH amide column. Mobile phase A consisted of 10 mM ammonium formate pH 3.0 and mobile phase B consisted of 95% acetonitrile with 10 mM ammonium formate pH 3.0. The flow-rate was 0.4 ml per min, and the column temperature was held at 40° C. A gradient over 3 min was applied from 85% B to 70% B. The injection volume was always 1 μl.
Analytes were ionised in positive mode by electrospray ionisation. The following mrm transitions were measured. Arginine 175.05->60.00, 175->70.02, 4-amino-3-oxo-guanidinoheptane: 187.1->170.1, 187.1->128.1, 187.1->110.1, 187.1->72.08, 187.1->60.05, according to Tsuchiya et al. (2014) and Tsuchiya et al. (2015).
A large arginine peak was detected in all culture treatments, as well as for pure arginine solution (data not shown), whereas a 4-amino-3-oxo-guanidinoheptane signal was absent from the arginine solution and all controls. Furthermore, a 4-amino-3-oxo-guanidinoheptane signal was absent from all E. coli BL21(DE3) PPTase pSxt1 cultures, apart from the culture induced at 19° C.
This experiment demonstrated that SxtA could be expressed in a catalytically-active form in E. coli BL21(DE3) using a synthetic DNA construct. In this experiment, expression occurred only at a growth temperature of 19° C., and after induction with 0.5 mM IPTG and 1 mM toluic acid.
The sxtI construct was integrated into the lac operon of E. coli BL21(DE3) PPTase as follows:
The synthetic DNA construct, sxt1, was PCR amplified using 2 μl template DNA, 0.4 μM primers, Phusion DNA polymerase in a 50 μl reaction volume.
The PCR cycles were as follows: 98 C for 1 min, 30 cycles: 98 C for 5 sec, 72 C for 2 min, 72 C for 5 min, then hold at 4° C.
The PCR product (8019 bp) was then used for RED ET mediated recombination, after its purification by gel electrophoresis and gel extraction. Prior to recombination, E. coli BL21(DE3) PPTase was transformed with the plasmid pRED (GeneBridges) by electroporation, and transcription of pRED encoded recombinases was induced according to the manufacturer's instructions. Subsequently, the transformed strain was electroporated with the PCR product described above (2.5 kV, 200 Ohm, 25 μF, 150 ng PCR product in 5 μl), followed by recombination into the lac operon.
The electroporated cells were plated onto LB agar with 15 μg/ml kanamycin, and grown overnight at 37° C. Resulting clones were picked and plated onto MacConkey (with lactose) agar with 15 μg/ml kanamycin, and grown for at least 24 h at 37° C. Resulting white colonies were isolated into pure culture, and tested by PCR and sequencing for correct and mutation-free integration of fragment sxtI into the lac operon.
Upon confirmation, a clone was chosen to remove the kanamycin resistance cassette by flippase-recombination using plasmid p707, according to the manufacturer's protocol (GeneBridges). Resulting clones were screened for kanamycin resistance, and the successful removal of the kanamycin resistance cassette was verified by PCR. In addition, PCR was used to verify that the remainder of the sxtI fragment was intact.
The resulting strain was then used for the integration of fragment sxt2 into the maltose operon, using the same approach. The resulting strain, where fragments sxtI and sxt2 were integrated, and after removal of the kanamycin resistance cassette, was used by the same approach to integrate each of three variants of sxt3 fragment into the xylose operon.
Sxt3 Variant E. coli BL21(DE3) Strains
The 4′-phosphopantetheinyl transferase gene sfp from Bacillus subtilis (Pfeifer B A, et al. (2001) Science 291:1790-2) was then used to replace the T3 PPTase in each of the above strains to produced additional strains.
The resulting strains were then used for the integration of fragment sxt4 into the melobiose operon of E. coli BL21(DE3) T3PPTase and E. coli BL21(DE3) sfp. The kanamycin cassette was not removed. Diagrams of the sxtI-4 fragments are given in
Strain E. coli BL21(DE3) sfp NSX3v3 was used as the parent strain to create the following strains where individual or sets of genes were deleted by homologous recombination methods.
LC-MS was used to detect neosaxitoxin and intermediates in the production of neosaxitoxin. LC-MS analysis was performed on a ThermoFisher Scientific Q Exative Hybrid Quadrupole mass spectrometer that was fitted with an ESI II ion source, and coupled online to a Dionex 3000 Ultimate RSLC Ultrapressure liquid chromatography instrument.
Mass/Charge Ratios (m/z) of Possible Saxitoxin Analogues and Intermediate Metabolites that Might by Produced by E. coli BL21(DE3) NSX3 Strains
E. coli BL21(DE3) T3PPtase NSX3v1 was grown in 20 ml LB broth in a 100 ml flask at 19° C. with shaking (200 rpm). The culture was induced with 0.2 mM IPTG, 1 mM toluic acid, when it reached an OD600 of 0.9, and grown for 18 hours to an OD600=5.
The cells were harvested by centrifugation, and extracted in 800 μl of 0.1 M acetic acid in methanol. The lysate was centrifuged, and the supernatant diluted 1:5 with acetonitrile prior to LC-MS analysis.
Intermediates 1 and 8 were detected in the extract. The spectra from Intermediate 8 are shown in
A seed culture of E. coli sfp NSX3v3 was prepared in 50 ml Hi YE medium, and incubated overnight at 30° C. (225 rpm). A New Brunswick BioFlo 110 fermentor (2 L volume) equipped a heater sleeve, water cooling, a pH and dissolved oxygen probe was set up for a fermentation experiment according to the manufacturer's instructions. The fermentor was filled with 2 L Hi-YE medium base, and autoclaved. Stock solutions of yeast extract, magnesium chloride, glucose and glycerol were aseptically added to the fermenter after autoclavation. In addition, 2 ml sterile solution of 5% antifoam 204 (Sigma Aldrich) was added to the fermenter. The medium was inoculated with 20 ml overnight seed-culture, which had an OD600 of 5.71. The OD600 of the starting culture was measured at 0.15. The culture was incubated at 19° C. with an initial agitation at 50 rpm, which was increased to 600 rpm after 26 hours incubation. The culture was purged with pressurised air at a flow-rate of 2 l/hour. The pH and dissolved oxygen levels were continuously monitored, and the growth was measured off-line after taking samples. The pH (pH 7.1) was stable during the entire experiment, as well as the dissolved oxygen level (100%). The culture was induced with 0.5 mM m-toluic acid and 0.2 mM IPTG after 22 hours incubation (OD600=10.6), and incubated for a further 22 hours. Sterile glucose solution (400 g/I) was fed to the culture at a flow-rate of 2.9 ml/min between 24 and 26.3 hours of the experiment (total 11.4 g), and 1.8 ml of a sterile 1 M magnesium sulfate solution was added at 26.3 hours of the experiment in an attempt to stimulate growth and production of neoSTX. The experiment was terminated after 45 hours, when the culture was harvested by centrifugation (5000×5, 30 minutes at 4° C.). The cell pellet was washed twice with 50 ml ice-cold MQ water (5.000×g, 10 min, 4° C.), weighed, and stored at −20° C. until analysis. The total biomass obtained was 23.77 g.
The cells from the fermentation experiments were lysed by boiling in 0.1% formic acid, and the extract was diluted 1:5 with acetonitrile with 0.1% formic acid prior to LC-MS analysis (see
The native genes sxtA, sxtB, sxtC were cloned into pVB vector (Vectrons Biosolutions), and the resulting vector pVB-sxtABC was transformed into E. coli BAP1. BAP1 pVB-sxtABC was grown in 10 ml TB medium at 19° C. with shaking at 200 rpm for 24 hours. The cells were harvested by centrifugation, and washed twice with ice-cold MQ water, the pellets were weighed, and stored at −20° C. until analysis.
The pellets were resuspended (1:3 wet weight:volume) in various lysis solutions:
1. 50% methanol with 25 mM sodium hydroxide
2. 50% methanol with 0.5% ammonium hydroxide
3. 50% methanol with 0.1% formic acid
4. 0.1 mM hydrochloric acid
5. 0.1% formic acid
6. 0.5% ammonium hydroxide
Treatments 1, 2 and 3 were sonicated for 2 minutes using a Branson Sonifier 250, equipped with a microprobe (output 2, duty cycle 50%). Treatments 3, to 5 were boiled for 5 minutes in a boiling water batch. The extracts were centrifuged for 10 minutes at 18000×g (4° C). 250 μl supernatant were mixed with 750 μl acetonitrile with 0.4% formic acid, and centrifuged (10 minutes at 18000×g, 4° C.). The supernatant was transferred to autosampler vials for LC-MS analysis of Intermediate 1 (see
5 minutes boiling in 0.1% formic acid was regarded as the most suitable method, as it did not cause interference with HILIC LC-MS, and produced a minimal amount of precipitation during extraction.
Seed-cultures of the E. coli strains were prepared in 10 ml LB broth and grown overnight at 30° C. with shaking at 200 rpm. The following strains were compared for neoSTX production levels:
E. coli BL21(DE3) sfp
E. coli BL21(DE3) T3PPTase sxt1 sxt2
E. coli BL21(DE3) T3PPTase NSX3v1
E. coli BL21(DE3) sfp NSX3v1
E. coli BL21(DE3) sfp NSX3v2
E. coli BL21(DE3) sfp NSX3v3
E. coli BL21(DE3) sfp NSX3v4
E. coli BL21(DE3) sfp NSX3v5
E. coli BL21(DE3) sfp NSX3v6
E. coli BL21(DE3) sfp NSX3v7
30 ml Terrific Broth (TB) with 0.4% glycerol and buffered to pH 6.8 with 89 mM phosphate buffer were inoculated with 300 μl overnight seed-culture to an approximate OD600 of 0.05. Cultures were incubated at 30° C. with shaking at 225 rpm until they reached an OD600 of 0.4. The cultures were then transferred to 19° C. (with shaking 225 rpm), and grown to an OD600 of 0.5. For induction of sfp and sxt genes, 0.05 mM IPTG (0.05 mM final concentration) and toluic acid (0.5 mM final concentration) were added to the cultures, which incubated at 19° C. with shaking (225 rpm) for a further 48 hours. Each strain was grown in triplicate sub-cultures. 25 ml of each culture was harvested by centrifugation at 3000×g for 10 min at 4° C. Aliquots of 1 nil supernatant was stored at −20° C., and the remaining supernatant discarded. The cell pellets were washed with twice with respectively 25 and 5 ml ice-cold MQ water (3000×g for 10 min at 4° C.). Cell pellets were weighed and stored at −20° C.
Toxin Extraction from Cell Pellets
Bacterial cell pellet was resuspended in 0.1% formic acid at a ratio of 1:4 (wwt:vol) and boiled for 5 minutes. The extract was briefly cooled on ice, centrifuged at 16.000×g for 10 minutes, and the supernatant collected. For LC-MS analysis, the supernatant was diluted with 90% acetonitrile:10% methanol:0.1% formic acid containing 25 nM saxitoxin-15N4 internal standard at a ratio of 1:5. The sample was centrifuged (16.000×g for 10 minutes, 4° C.) and the supernatant transferred to autosampler vials for LC-MS analysis.
Toxin Extraction from Growth Media
Growth media (200 μl) was acidified with 2 μl 10% formic acid, boiled for 5 minutes, cooled on ice, and centrifuged at 16000×g for 10 minutes at 4° C. The supernatant was transferred to a new tube, and 40 μl internal standard was added (25 nM saxitoxin-15N4 in 10% methanol 90% acetonitrile and 0.1% formic acid). The tube was centrifuged at 16000×g 4° C. for 10 minutes, and the supernatant transferred to an HPLC auto-sampler vial for LC-MS analysis. The results are shown in
E. coli Cultures for MNBA
Seed-cultures of the E. coli strains BL21(DE3) sfp and BL21(DE3) sfp NSX3v3 were prepared in 10 ml LB broth and grown overnight at 30° C. with shaking at 200 rpm. 50 ml Terrific Broth (TB) with 0.4% glycerol and buffered to pH 6.8 with 89 mM phosphate buffer were inoculated with 500 μl overnight seed-culture to an approximate OD600 of 0.05. Cultures were incubated at 30° C. with shaking at 225 rpm until they reached an OD600 of 0.4. The cultures were then transferred to 19° C. (with shaking 225 rpm), and grown to an OD600 of 0.5. For induction of sfp and sxt genes, 0.05 mM IPTG (0.05 mM final concentration) and toluic acid (0.5 mM final concentration) were added to the cultures, which incubated at 19° C. with shaking (225 rpm) for a further 48 hours. Each strain was grown in triplicate sub-cultures. Cultures were harvested by centrifugation at 2500×g for 10 min at 4° C. The cell pellets were washed with twice with respectively 25 ice-cold MQ water (2500×g for 10 min at 4° C.). Cell pellets were weighed and stored at −20° C.
Bacterial cells were extracted by weak cation exchange solid phase extraction (WCX-SPE) for the MNBA, using Accell Plus CM cartridges (360 mg, 1.1 ml, WAT010910, Waters) according to the following protocol.
1.2 g cell pellet was resuspended in 3.6 ml 0.15% formic acid and lysed by boiling for 5 minutes. The cell extract was cleared by centrifugation at 16.000×g for 10 minutes.
Columns were conditioned with 3.6 ml methanol, followed by 3.6 ml 0.15% formic acid. 3 ml sample was loaded, and the column was washed with 1.8 ml MQ water, followed by 1.8 ml acetonitrile. The column was then dried, and the sample eluted in 2×3.6 ml methanol with 5% formic acid. The eluate was dried by vacuum centrifugation, and the sample reconstituted in 200 μl 0.01% formic acid.
A mouse neuroblastoma assay was used (as described by Humpage et al. (2007). Environ. Toxicol. Chem. 26:1512-9), using the mouse neuro-2a cell line (CCL131).
A calibrator curve for the MNBA assay was prepared with and without biological matrix using a certified reference material for neoSTX (CRM-NEO-c, lot 2009-02-18, 65.6 μM in 3 mM HCl). The results are shown in
A saxitoxin ELISA Kit (Abraxis PN 52255B, Microtiter Plate 96T) was used to detect neoSTX produced in E. coli. The kit employs polyclonal saxitoxin antibodies, which have 1.3% cross-reactivity to neoSTX. Cultures of E. coli T3PPTase NSX3v3 were prepared and cell lysates obtained. Samples were extracted by solid phase extraction on SampliQ silica columns (Agilent PN 5982-2211, 1 ml, 100 mg) Jansson D and Astot C (2015)m J Chromatogr A 1417:41-8). Extracted and evaporated samples were dissolved in 100 μl sample buffer provided by the STX ELISA kit. A 1:1000 dilution of E. coli T3PPTase NSX3v3 extract in sample buffer was also prepared. The assay was calibrated by a 2 point standard curve using Std 0 as the blank, Std 1 (0.0668 nM STX) and STd 5 (1.3365 nM STX) provided by the kit. The reference sample was used to estimate the accuracy of the STX ELISA kit for neoSTX, whereas the recovery of neoSTX during SPE was estimated based on the ratio of the extracted reference sample versus the reference sample. Results of the assay are shown in the Table below. The estimated accuracy was 119%, whereas the recovery during SPE was 92%. The assay detected an equivalent of 1.16 nM STX in the cell extract of E. coli T3PPTase NSX3v3. Converted to neoSTX, this amounts to 89.1 nM, and a yield of approximately 223 pmol neoSTX per liter of E. coli culture.
STX ELISA. Std 0, Std 1, and Std 5 were provided by the STX ELISA kit. The ELISA assay was calibrated by 2 points (y=−0.4627+0.7597). The concentration of neoSTX was calculated on the basis of 1.3% cross reactivity of the STX antibody according to the manufacturer. ND: none detected. The recovery of neoSTX by SPE was 92%.
A number of pVB constructs were made using the following elements:
(i) the Pm promoter;
(ii) the sxtA synthetic (codon-optimised for E. coli) or the sxt native gene; and
(iii) a 8 or 10 bp spacer between the ribosome binding site (RBS) and start codon.
The sequence of the Pm promoter with the 8 bp spacer between RBS and start codon of SxtA is shown below. The RBS and start codon are shown with capital letters, the spacer is underlined:
The sequence of the Pm promoter with the 10 bp spacer between RBS and start codon of SxtA is shown below. The RBS and start codon are shown with capital letters, the spacer is underlined:
The four pVB-sxtA plasmids were independently transformed into E. coli BL21(DE3) sfp.
Cultures of each of the four E. coli BL21(DE3) sfp with pVB-sxtA variants were prepared and cells were harvested. Cell pellets were extracted and the presence of Intermediate 1 was analysed by LC-MS. The results are shown in
In this experiment, the following PPTases were compared:
1. T3PPT: E. coli BL21(DE3) T3PPTase NSX3v1
2. T3PPT: E. coli BL21(DE3) T3PPTase NSX3v1 pET28B-NsPPT
(pET vector with phosphopantetheinyl transferase from Nodularia spumigena)
3. Ala18T3PPT: E. coli BL21(DE3) T3PPTase NSX3v1 pET30b-Ala18T3PPTase
(T3PPTase where there first 18 amino acids were removed to increase solubility of expressed protein.)
The strains were cultured for 18 hours in 20 ml LB broth at 19° C. with shaking 200 rpm. Strains with pET vector were grown in the presence of 50 μg/ml kanamycin. Cultures were either grown without inducer, or induced with 0.2 mM IPTG and 1 mM toluic acid at OD600 ca. 0.8. There was clear background expression of sxt and PPTase genes in the absence of inducer.
Intermediate 8 and neoSTX were measured, and used as an indicator for the effectiveness of the PPTase. The results are shown in
Saxitoxin may be converted to GTX-5 by SxtN (i.e. by sulfonation of the carbamoyl side-chain). Similarly, saxitoxin may be converted 11-hydroxy STX by SxtDIOX. This is a step preceding C-11 sulfonation to convert STX to GTX-2/3 (or neosaxitoxin to GTX-4/1).
sxtN from Scytonema cf. crispum UCFS15 was cloned into an appropriate expression vector, and placed under the control of the IPTG-inducible T7 promoter, and provided with a hexa-histidine tag on its N- and C-terminus.
sxtO from S. cf. crispum UCFS15 was cloned into an appropriate expression vector, and placed under the control of the IPTG-inducible T7 promoter, and provided with a hexa-histidine tag on its N- and C-terminus.
The expression vectors were expressed in E. coli and the Sxt proteins were purified using No-NTA resin (Novagen). Purity of the proteins was assessed on SDS gels.
The sulfotransferase activity of SxtN was tested using saxitoxin or GTX2. The results were determined using HPLC-MS/MS. Using saxitoxin as substrate, a peak at 2.13 min was identified that was not present in the control. This contained a major peak of m/z 380.10, which fragmented (LC-MS/MS) to m/z of 300.14 and 282.13, proposed to be GTX5 (shown in
The dioxygenase activity of SxtDIOX was tested using saxitoxin. The results were determined using HPLC-MS/MS. The substrate was identified in both the assay and the control. Further, a peak of m/z 316.14>296.13 at 1.66 min that was not present in the control was identified in the assay, suggesting the presence of hydroxylated saxitoxin (
The accompanying Sequence Listing is fully incorporated herein as part of the description.
E. coli lactose operon.
coli lactose operon.
coli melobiose operon.
coli maltose operon.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1602576.9 | Feb 2016 | GB | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16077281 | Aug 2018 | US |
| Child | 17074607 | US |
