Patent Grant
7144983
This application relates to proteins which are involved in the growth, regulation or maintenance of nervous tissue, particularly neurons. In particular, it relates to pantropic neurotrophic factors which have multiple neurotrophic specificities (MNTS variants).
The survival and maintenance of differentiated function of vertebrate neurons is influenced by the availability of specific proteins referred to as neurotrophins. Developing neurons depend for survival on the supply of these factors from their target fields and the limited production of neurotrophins results in death of superfluous neurons (for reviews, see (1); (2)). The various neurotrophins differ functionally in their ability to support survival of distinct neuronal populations in the central and the peripheral nerve system (3), (4); (5), (80).
The neurotrophin family is a highly homologous family which includes NT3 (6), (7); (5); (8); (9); (10), nerve growth factor (NGF) (11); (12), brain-derived neurotrophic factor (BDNF) (13); (14)) and neurotrophin 4/5 (NT4/5) ((15), (16), (17).
Studies suggest that neurotrophins transduce intracellular signalling at least in part through the ligand-dependent activation of a class of tyrosine kinase-containing receptors of Mr=140–145,000 known as the trks (18); (19) (21); (20) (22); (23); (24); (25); (26). Thus, the signal transduction pathway of neurotrophins is initiated by this high-affinity binding to and activation of specific tyrosine kinase receptors and subsequent receptor autophosphorylation (19); (27). Although there is some degree of cross-receptor interaction between the neurotrophins and the different trks, the predominant specificity appears to be NGF/trkA, BDNF/trkB, and NT3/trkC while NT4/5 appears to interact primarily with trkB as efficiently as BDNF (27); (19) (21); (25); (22); (28); (18); (28a). While trkC responds exclusively to NT3 (25); (26), trkA and trkB can respond in vitro under certain circumstances to multiple neurotrophins (6); (23). However, the neuronal environment does restrict trkA and trkB in their ability to respond to non-preferred neurotrophic ligands (29). In addition to the trk family of receptors, the neurotrophins can also bind to a different class of receptor termed the p75 low affinity NGF receptor (p75; (30); (31)) which has an unknown mechanism of transmembrane signalling but is structurally related to a receptor gene family which includes the tumor necrosis factor receptor (TNFR), CD40, 0×40, and CD27 (32); (33); (34), (35); (36); (37)). The role of the gp75 in the formation of high-affinity binding sites and in the signal transduction pathway of neurotrophins is as yet unclear (for reviews see (38); (39)).
An examination of the primary amino acid sequence of the neurotrophins reveals seven regions of 7–10 residues each which account for 85% of the sequence divergence among the family members.
Nerve growth factor (NGF) is a 120 amino acid polypeptide homodimeric protein that has prominent effects on developing sensory and sympathetic neurons of the peripheral nervous system. NGF acts via specific cell surface receptors on responsive neurons to support neuronal survival, promote neurite outgrowth, and enhance neurochemical differentiation. NGF actions are accompanied by alterations in neuronal membranes (40), (41), in the state of phosphorylation of neuronal proteins (42), (43), and in the abundance of certain mRNAs and proteins likely to play a role in neuronal differentiation and function (see, for example (44)).
Forebrain cholinergic neurons also respond to NGF and may require NGF for trophic support. (45). Indeed, the distribution and ontogenesis of NGF and its receptor in the central nervous system (CNS) suggest that NGF acts as target-derived neurotrophic factor for basal forebrain cholinergic neurons (46), (81).
Little is known about the NGF amino acid residues necessary for the interaction with the trkA-tyrosine kinase receptor. Significant losses of biological activity and receptor binding were observed with purified homodimers of human and mouse NGF, representing homogenous truncated forms modified at the amino and carboxy termini. (47); (48); (49). The 109 amino acid species (10–118)hNGF, resulting from the loss of the first 9 residues of the N-terminus and the last two residues from the C-terminus of purified recombinant human NGF, is 300-fold less efficient in displacing mouse [125]NGF from the human trkA receptor compared to (1–118)HNGF (49). It is 50- to 100-fold less active in dorsal root ganglion and sympathetic ganglion survival compared to (1–118)hNGF (48). The (1–118)HNGF has considerably lower trkA tyrosine kinase autophosphorylation activity (49).
NT3 transcription has been detected in a wide array of peripheral tissues (e.g. kidney, liver, skin) as well as in the central nerve system (e.g. cerebellum, hippocampus) (5); (7), (82). During development, NT3 mRNA transcription is most prominent in regions of the central nervous system in which proliferation, migration and differentiation of neurons are ongoing (50). Supporting evidence for a role in neuronal development includes the promoting effect of NT3 on neural crest cells (51) and the stimulation of the proliferation of oligodendrocyte precursor cells in vivo (79). NT3 also supports in vitro the survival of sensory neurons from the nodose ganglion (NG) (7); (5), (83) and a population of muscle sensory neurons from dorsal root ganglion (DRG) (52). In addition to these in vitro studies, a recent report showed that NT3 prevents in vivo the degeneration of adult central noradrenergic neurons of the locus coerulus in a model that resembles the pattern of cell loss found in Alzheimer's disease. Currently, there are no published reports concerning the amino acid residues necessary for trkC binding.
There has been some limited attempts to create chimeric or pan-neurotrophic factors. (See (53); (56); (54), (55)).
It is an object of the invention to provide pantropic neurotrophins and to produce useful quantities of these pantropic neurotrophins using recombinant DNA techniques.
It is a further object of the invention to provide recombinant nucleic acids encoding pantropic neurotrophins, and expression vectors and host cells containing the nucleic acid encoding the pantropic neurotrophins.
An additional object of the invention is to provide methods for producing the pantropic neurotrophins, and for treating neuronal disorders of a patient.
In accordance with the foregoing objects, the present invention provides recombinant pantropic neurotrophins, and isolated or recombinant nucleic acids which encode the neurotrophins of the present invention. Also provided are expression vectors which comprise DNA encoding a pantropic neurotrophin operably linked to transcriptional and translational regulatory DNA, and host cells which contain the nucleic acids.
An additional aspect of the present invention provides methods for producing pantropic neurotrophins which comprises culturing a host cell transformed with an expression vector and causing expression of the nucleic acid encoding the pantropic neurotrophin to produce a recombinant neurotrophin.
Additionally provided are methods of treating a neural disorder comprising administering the pantropic neurotrophins of the present invention to a patient.
Additional objects and features of the invention will be apparent to those skilled in the art from the following detailed description and appended claims when taken in conjunction with the figures.
The response of medium from mock transfected cells was subtracted from each data point and was 23%, 23% and 29% for the 200 pg/ml, 1000 pg/ml and 5000 pg/ml experiment, respectively. (D) Response of PC12/trkC cells induced by conditioned medium containing either NT-3 or R68A mutant. Percentage of cells with neurites induced by different doses of neurotrophins. The sum of cells with and without neurites was constant for NT3 and R68A for all doses. (E) Survival of neurons from DRG. Response induced by NT-3, R68A or R114A/K115A expressed as number of surviving cells. Results are the mean value of triplicate determinations ±SD. The response induced by mock-transfected conditioned medium was subtracted from data points and was 20±4 surviving cells.
Single letter codes for the amino acids are used herein, as is known in the art, according to the following table:
Thus, the identification of an amino acid residue is the single letter amino acid code followed by the position number of the residue. It is to be understood that the position number corresponds to the particular neurotrophin backbone; thus, D15A NT3 means that the aspartic acid at position 15 of NT3 is changed to an alanine. This aspartic acid, found within a “constant region” as defined below, corresponds to position 16 of NGF, since NGF has an additional amino acid at its N-terminus, as shown in
The present invention provides pantropic neurotrophins. Generally, a neurotrophin is a protein involved in the development, regulation and maintenance of the nervous system, and in particular of neurons. Currently, there are at least five known important neurotrophic factors: nerve growth factor (NGF), neurotrophin-3 (NT3), neurotrophin-4 (NT4, also sometimes called neurotrophin-5 (NT5) or NT4/5), brain-derived neurotrophic factor (BDNF), and ciliary neurotrophic factor (CNTF).
By the term “pantropic neurotrophins” or “pantropic neurotrophic factors”, or grammatical equivalents, herein is meant a neurotrophin which, unlike naturally occurring neurotrophins, has multiple neurotrophin specificities. That is, it contains domains which confer different neurotrophin specificities. In one embodiment, this means that the pantropic neurotrophins of the present invention will bind to a variety of neurotrophic receptors. Thus, for example, naturally occurring NGF, which is the natural or native ligand for the trkA receptor, does not bind appreciably to either the trkB or trkC receptor with high affinity; for example, NGF binds to these receptors with a 500-1000 fold lower KD than BDNF or NT3, respectively. However, a pantropic NGF, i.e. a pantropic neurotrophin whose amino acid backbone is based on NGF, may bind to at least the trkA, trkB and p75 receptor. Alternatively, a pantropic NGF will bind to the trkA, trkC and p75 receptor. A preferred embodiment allows the binding of the trkA, trkB, trkC and p75 receptor. Similarly, naturally occurring BDNF and NT4/5, which are the natural ligands for the trkB receptor, do not bind appreciably to either the trkA or trkC receptor as above. Thus pantropic BDNF or NT4/5 will bind to trkB and any combination of trkA, trkC and p75, as shown above for pantropic NGF.
In alternative embodiments, the naturally occurring neurotrophin will bind with poor affinity to several neurotrophin receptors. In this embodiment, the pantropic neurotrophin binds to these receptors with affinities higher than normally found, similar to the affinities seen for the natural ligand. For example, NT3 binds strongly to trkC, and weakly to trkA and trkB. Thus, a pantropic NT3 binds to trkC with its normal binding affinity, and will bind to either trkA with an affinity similar to the trkA natural ligand, NGF, or to trkB with an affinity similar to the trkB natural ligands BDNF or NT4/5, or both.
In the preferred embodiment, the binding affinity of the pantropic neurotrophin for neurotrophin receptors is at least about 50–60%, preferably about 75–80%, and most preferably about 90% of the binding affinity of the natural ligand. Thus, a pantropic NGF will bind to the trkB or trkC receptor with at least 50% of the binding of BDNF or NT4/5, or NT3, respectively. This affinity is measured by a variety of ways, as will appreciated by those skilled in the art. The preferred method is the use of competition assays, as shown in (84) and in Example 2. Generally, bindino affinities are reported as IC50, that is, the concentration of unlabeled competitor which inhibits 50% of the binding of labeled ligand to the receptor.
In alternative embodiments, the pantropicity of the neurotrophin is measured not by binding affinity to neurotrophin receptors, but rather by the neuronal survival or neurite outgrowth assays. Thus, all neurotrophins support the survival of embryonic neural crest-derived sensory neurons (77), (78), (7), (17). Survival of embryonic sympathetic neurons is only supported by NGF, while survival of placode-derived sensory neurons is supported by NT3 and BDNF (85). Survival of sensory neurons of the dorsal root ganglion is supported by both NGF and BDNF (13). NT3 elicits neurite outgrowth of sensory neurons from dorsal root ganglion, sympathetic chain ganglia, and nodose ganglion, as well as supports survival of nodose ganglia neurons and dorsal root ganglion neurons. Thus, neuronal survival assays or neurite outgrowth assays can be run to determine the pantropicity of the pantropic neurotrophins.
Thus, neurotrophin specificity is determined by the neurotrophin receptor binding, and the neuronal survival assays and/or neurite outgrowth assays. Thus, a pantropic neurotrophin with NGF specificity means a neurotrophin which exhibits at least the binding characteristics, neuronal survival assay specificity, or the neurite outgrowth assay specificity of NGF. Similarly, a pantropic neurotrophin with BDNF, NT3 or NT4/5 specificity exhibits at least the binding characteristics, neuron survival assay specificity, or neurite outgrowth assay specificity of BDNF, NT3 or NT4/5, respectively.
In an additional embodiment, pantropic neurotrophins are made by constructing covalent heterodimers. Normally, neurotrophins are homodimers, comprising two identical monomers which are non-covalently associated. In this embodiment, as outlined below, pantropicity is conferred by each monomer containing domains which confer different neurotrophic specificity. Alternatively, pantropicity may be created by covalently attaching two different neurotrophins with different specificities to create a covalent heterodimer. Thus, for example, a NGF monomer may be covalently attached to a NT3 monomer, resulting in a pantropic neurotrophin with both NGF and NT3 specificity. Similarly, covalent heterodimers may be made with any combination of NGF, NT3, NT4/5, BDNF or CNTF to create pantropic neurotrophins with at least two specificities. In addition, this procedure may be done with monomers which are themselves pantropic, resulting in covalent dimers of any combination of pantropic and single specificity monomers. Thus, a pantropic covalent dimer may be a homodimer of two pantropic monomers. However, to be included within the definition of the present invention, the pantropic covalent dimer must have at least two, and preferably three, neurotrophin specificities.
The covalent attachment is preferably done as a direct fusion of the nucleic acid, such that when the protein is expressed, the C-terminus of the first monomer is attached directly to the N-terminus of the second monomer, creating a single nucleic acid encoding the dimer. In alternative embodiments, a linker may be used, such as short repeats of glycine, or glycine and serine; for example, a linker such as gly-gly or gly-gly-ser-gly-gly (SEQ ID NO: 8) may be used. This is done using techniques well known in the art. Other techniques for the covalent attachment of proteins are well known in the art.
Pantropic neurotrophins accomplish pantropic binding, or, as discussed above, pantropic neuronal survival, by containing domains which confer neurotrophin receptor specificity or binding. A domain may be defined in one of two ways. In the first embodiment, a domain is a portion of the neurotrophin which confers some neurotrophic specificity. In this embodiment, a single monomer of the pantropic neurotrophin contains one or several domains which confer different specificities. The domains can range in size from a single amino acid to about 10–15 amino acids. The domain may be comprised of a combination of amino acids from a different neurotrophin than the host neurotrophin, i.e. a domain from one neurotrophin may be substituted into a second neurotrophin, conferring pantropicity to the second neurotrophin. Alternatively, the domain may result from amino acid substitutions which are not based on homology to existing neurotrophins, as outlined below. In the preferred embodiment, the domain comprises a continuous sequence of amino acids; that is, a single stretch of amino acids is replaced. In other embodiments, the domain may be comprised of discontinuous amino acids; for example, several regions within the neurotrophin may confer specificity, and thus replacements at several positions within the neurotrophin are necessary for pantropicity.
In some embodiments, there is more than one domain within a neurotrophin which can confer neurotrophic specificity, which will depend on the particular neurotrophin. BDNF, gor example, has a number of domains which appear to confer BDNF specificity. The present invention shows that a single amino acid change in NT3, from aspartic acid at position 15 to an alanine, confers BDNF specificity to NT3. This domain can also be imported into the NGF and NT4/5 sequences at the positions that correspond to position 15 in NT3; i.e. position 16 in NGF or position 18 in NT4/5. It should be understood that the corresponding amino acids are determined by an examination of the alignment of the sequences, as shown in
Similarly, NT3 has a number of domains which may confer NT3 specificity when substituted into a different neurotrophin. A number of residues of NT3 have been shown to be important in NT3 trkC receptor binding as well as bioactivity assays. Specifically, mutations at positions R103, D105, K80, Q83, E54, R56, T22, Y51, V97, Y11, E7, R8, E10 and R68 all contribute to NT3 specificity, since mutations at these positions in NT3 cause decreases in NT3 activity. Of these, K80, Q83, T22, and V97 are within variable regions as shown in
NGF has a number of domains which may confer NGF specificity when substituted into a different neurotophin. The N-terminal amino acids of NGF confer NGF specificity when substituted for the N-terminal residues of NT3. Specifically, the 7 N-terminal amino acids (SSSHPIF) (SEQ ID NO: 11) of NGF may be substituted for the 6 N-terminal amino acids of NT3 (YAEHKS) (SEQ ID NO: 12), resulting in a pantropic NT3 with NGF specificity. The exact number of NGF N-terminal residues is not crucial; as shown in the Examples, and particularly in Example 3, the histidine at amino acid position 4 appears to be quite important for NGF specificity; thus from about 4 to about 10 N-terminal residues may be exchanged although in some embodiments, a single amino acid change will be sufficient. Similarly, a number of other residues of NGF have been shown to be important in NGF trkA receptor binding as well as bioactivity assays. For example, there are a number of residues which, when mutated, lose NGF activity. This shows the importance of the residue for NGF specificity. These residues include, but are not limited to, H4, P5, V18, V20, G23, D30, Y52, R59, R69, H75, Y79, T81, and R103. Of these, D30, R59, Y79, and T81 are in “variable regions”, i.e. regions which vary between the different neurotrophins, as shown in
Once identified, the residues important in neurotrophin specificity can be replaced by any of the other amino acid residues using techniques described in the examples and well-known techniques for site-directed mutagenesis. Generally, the amino acids to be substituted are chosen on the basis of characteristics understood by those skilled in the art. For example, when small alterations in the characteristics are desired, substitutions are generally made in accordance with the following table:
Substantial changes in function or immunological identity are made by selecting substitutions that are less conservative than those shown in Table 1. For example, substitutions may be made which more significantly affect: the structure of the polypeptide backbone in the area of the alteration, for example the alpha-helical or beta-sheet structure; the charge or hydrophobicity of the molecule at the target site; or the bulk of the side chain. The substitutions which in general are expected to produce the greatest chanoes in the polypeptide's properties are those in which (a) a hydrophilic residue, e.g., seryl or threonyl, is substituted for (or by) a hydrophobic residue, e.g., leucyl, isoleucyl, phenylalanyl, valyl or alanyl; (b) a cysteine or proline is substituted for (or by) any other residue; (c) a residue having an electropositive side chain, e.g., lysyl, arginyl, or histidyl, is substituted for (or by) an electronegative residue, e.g., glutamyl or aspartyl; or (d) a residue having a bulky side chain, e.g., phenylalanine, is substituted for (or by) one not having a side chain, e.g., glycine. In a preferred embodiment, the residues are changed to alanine residues.
Other domains within each neurotrophin may be found using the techniques disclosed herein. Specifically, the modelling techniques of Example 1 allow the identification of putative specificity sites. In addition, homologue-scanning mutagenesis, random mutagenesis, cassette mutagenesis, may all be used to generate putative pantropic neurotrophins which may then be screened for receptor binding using the techniques described in the Examples and well-known in the art.
In the context of a covalent heterodinier, a domain may also refer to the entire neurotrophin monomer. Thus, a pantropic covalent heterodimer can be comprised of a domain which confers NT3 specificity, i.e. the NT3 monomer, covalently attached to a domain that confers BDNF specificity, i.e. the BDNF monomer. Similarly, an NT3 monomer may be paired with an NGF monomer, or an NGF monomer may be paired with a BDNF monomer. In addition, covalent heterodimers may be made with NT4/5 and CNTF monomers as well. In these embodiments, the domain is large, and generally comprises most or all of the wild-type neurotrophin amino acid sequence.
In the broadest embodiment, a pantropic neurotrophin binds to at least three different neurotrophin receptors. In the preferred embodiment, the pantropic neurotrophin binds to at least four different neurotrophin receptors.
By the term “neurotrophin receptor” or grammatical equivalents herein is meant a receptor which binds a neurotrophin ligand. In some embodiments, the neurotrophin receptor is a member of the tyrosine kinase family of receptors, generally referred to as the “trk” receptors, which are expressed on the surface of distinct neuronal populations. The trk family includes, but is not limited to, trkA (also known as p140); trkB (also known as p145trkB); and trkC (also known as p145trkC). In other embodiments, the neurotrophin receptor is p75NGFR, also called p75 or low-affinity nerve growth factor receptor (LNGFR). It is to be understood that other as yet undiscovered neurotrophin receptors may also bind the pantropic neurotrophins of the present invention, as will be easily ascertainable by those skilled in the art.
In a preferred embodiment, the pantropic neurotophin is a pantropic NT3. In this context, a pantropic NT3 is a pantropic neurotrophin which has an amino acid sequence homologous to the amino acid sequence to NT3, with domains which confer other neurotrophin specificities. In the preferred embodiment, the domains are substituted for NT3 residues: that is, some number of amino acids are deleted from the NT3 sequence, and an identical or similar number of amino acids are substituted, conferring an additional specificity. For example, the MNTS-1 (multiple neurotrophic specificities-1) pantropic NT3 comprises the first 7 amino acids of NGF replacing the 6 N-terminal residues of NT3, plus the D15A substitution. The MNTS-1 pantropic NT3 has NT3, NGF, and BDNF specificities, and also binds to the p75 receptor. Other pantropic NT3s are made using minimal changes within the N-terminus. For example, since H4 and P5 are conserved among NGFs, and 2 hydrophobic residues in positions 6 and 7 are conserved, the following variants are made: 1) YASHPIF (SEQ ID NO: 13)-hNT3; 2) YAHPIF (SEQ ID NO: 14)-hNT3; 3) YASHPIS (SEQ ID NO: 15)-hNT3; 4) YAEHPIF (SEQ ID NO: 16)-hNT3; and 5) YAQHPIF (SEQ ID NO: 17)-hNT3.
In a preferred embodiment, the pantropic neurotrophin is pantropic NGF. In this context, a pantropic NGF is a pantropic neurotrophin which has an amino acid sequence homologous to the amino acid sequence of NGF, with domains which confer other neurotrophin specificities. In the preferred embodiment, the domains are substituted for NGF residues; that is, some number of amino acids are deleted from the NGF sequence, and an identical or similar number of amino acids are substituted, conferring an additional specificity. For example, a pantropic NGF is made with a D16A substitution, which confers BDNF specificity, plus substitutions in the pre-variable region 1 (V18E+V20L+G23T) and in variable region 4 (Y79Q+T81K+H84Q+F86Y+K88R). Alternatively, the substitutions in the pre-variable region 1 can be made with only single amino acid substitutions in variable region 4; for example, V18E+V20L+G23T and one of Y79Q, T81K, H84Q, F86Y, or K88R may be made.
In one embodiment, the pantropic neurotrophin is a pantropic NT4/5. For example, NGF specificity may be conferred on NT4/5 by replacing the N-terminal 9 amino acids of NT4/5 with the N-terminal 7 amino acids of NGF.
In one embodiment, binding to the p75 receptor by the pantropic neurotrophin has been substantiallydiminished oreliminated. For example, as shown in
In addition to the amino acid changes outlined above, those skilled in the art understand that some variability of the amino acid sequence is tolerated without altering the specificity and characteristics of the neurotrophin. Thus, pantropic neurotrophins can have amino acid substitutions, insertions or deletions compared to the wild-type sequences which do not affect pantropicity but are merely variations of the sequence. In some embodiments, these mutations will be found within the same positions identified as important to specificity; i.e. in some cases, neutral mutations may be made without changing neurotrophin specificity.
The pantropic neurotrophins of the present invention can be made in a variety of ways, using recombinant technology. By the term “recombinant nucleic acid” herein is meant nucleic acid in a form not normally found in nature. That is, a recombinant nucleic acid is flanked by a nucleotide sequence not naturally flanking the nucleic acid or has a sequence not normally found in nature. Recombinant nucleic acids can be originally formed in vitro by the manipulation of nucleic acid by restriction endonucleases, or alternatively using such techniques as polymerase chain reaction. It is understood that once a recombinant nucleic acid is made and reintroduced into a host cell or organism, it will replicate non-recombinantly, i.e., using the in vivo cellular machinery of the host cell rather than in vitro manipulations; however, such nucleic acids, once produced recombinantly, although subsequently replicated non-recombinantly, are still considered recombinant for the purposes of the invention.
Similarly, a “recombinant protein” is a protein made using recombinant techniques, i.e., through the expression of a recombinant nucleic acid as depicted above. A recombinant protein is distinguished from naturally occurring protein by at least one or more characteristics. For example, the protein may be isolated away from some or all of the proteins and compounds with which it is normally associated in its wild type host. The definition includes the production pantropic neurotrophins from one organism in the same or different organism or host cell. For example, the protein may be made in the same organism from which it is derived but at a significantly higher concentration than is normally seen, e.g., through the use of a inducible or high expression promoter, such that increased levels of the protein is made. Alternatively, the protein may be in a form not normally found in nature, as in the addition of an epitope tag or amino acid substitutions, insertions and deletions.
Using the nucleic acids of the invention which encode pantropic neurotrophins, a variety of expression vectors are made. The expression vectors may be either self-replicating extrachromosomal vectors or vectors which integrate into a host genome. Generally, expression vectors include transcriptional and translational regulatory nucleic acid operably linked to the nucleic acid encoding the pantropic neurotrophin. “Operably linked” in this context means that the transcriptional and translational regulatory DNA is positioned relative to the coding sequence of the pantropic neurotrophin in such a manner that transcription is initiated. Generally, this will mean that the promoter and transcriptional initiation or start sequences are positioned 5′ to the pantropic neurotrophin coding region. The transcriptional and translational regulatory nucleic acid will generally be appropriate to the host cell used to express the pantropic neurotrophin; for example, transcriptional and translational regulatory nucleic acid sequences from mammalian cells will be used to express the pantropic neurotrophin in mammalian cells. Numerous types of appropriate expression vectors, and suitable regulatory sequences are known in the art for a variety of host cells.
In general, the transcriptional and translational regulatory sequences may include, but are not limited to, promoter sequences, signal sequences, ribosomal binding sites, transcriptional start and stop sequences, translational start and stop sequences, termination and poly A signal sequences, and enhancer or activator sequences. In a preferred embodiment, the regulatory sequences include a promoter and transcriptional start and stop sequences.
Promoter sequences encode either constitutive or inducible promoters. Hybrid promoters, which combine elements of more than one promoter, are also known in the art, and are useful in the invention.
In addition, the expression vector may comprise additional elements. For example, the expression vector may have two replication systems, thus allowing it to be maintained in two organisms, for example in mammalian cells for expression and in a procaryotic host for cloning and amplification. Furthermore, for integrating expression vectors, the expression vector contains at least one sequence homologous to the host cell genome, and preferably two homologous sequences which flank the expression construct. The integrating vector may be directed to a specific locus in the host cell by selecting the appropriate homologous sequence for inclusion in the vector. Constructs for integrating vectors are well known in the art.
In addition, in a preferred embodiment, the expression vector contains a selectable marker gene to allow the selection of transformed host cells. Selection genes are well known in the art and will vary with the host cell used.
The pantropic neurotrophins of the invention are produced by culturing a host cell transformed with an expression vector containing nucleic acid encoding a pantropic neurotrophin, under the appropriate conditions to induce or cause expression of the pantropic neurotrophin. The conditions appropriate for pantropic neurotrophin expression will vary with the choice of the expression vector and the host cell, and will be easily ascertained by one skilled in the art. For example, the use of constitutive promoters in the expression vector will require optimizing the growth and proliferation of the host cell, while the use of an inducible or repressible promoter requires the appropriate growth conditions for induction or derepression.
In a preferred embodiment, the pantropic neurotrophin is purified or isolated after expression. The pantropic neurotrophins may be isolated or purified in a variety of ways known to those skilled in the art depending on what other components are in the sample. Standard purification methods include electrophoretic, molecular, immunological and chromatographic techniques, including ion exchange, hydrophobic, affinity, and reverse-phase HPLC chromatography, and chromatofocusing. Ultrafiltration and diafiltration techniques, in conjunction with protein concentration, are also useful. For general guidance in suitable purification techniques, see (57). The degree of purification necessary will vary depending on the use of the pantropic neurotrophin. In some instances no purification will be necessary.
Appropriate host cells include yeast, bacteria, archebacteria, fungi such as filamentous fungi, and plant and animal cells, including mammalian cells. Of particular interest are Saccharomyces cerevisiae and other yeasts, E. coli, Bacillus subtilis, Pichia pastoris, SF9 cells, C129 cells, 293 cells, Neurospora, and CHO, COS, HeLa cells, immortalized mammalian myeloid and lymphoid cell lines. A preferred host cell is a mammalian cell, and the most preferred host cells include CHO cells, COS-7 cells, and human fetal kidney cell line 293.
In a preferred embodiment, the pantropic neurotrophins of the invention are expressed in mammalian cells. Mammalian expression systems are also known in the art.
Some genes may be expressed more efficiently when introns are present. Several cDNAs, however, have been efficiently expressed from vectors that lack splicing signals. Thus, in some embodiments, the nucleic acid encoding the pantropic neurotrophin includes introns.
The methods of introducing exogenous nucleic acid into mammalian hosts, as well as other hosts, is well known in the art, and will vary with the host cell used, and include dextran-mediated transfection, calcium phosphate precipitation, polybrene mediated transfection, protoplast fusion, electroporation, encapsulation of the polynucleotide(s) in liposomes, and direct microinjection of the DNA into nuclei.
In one embodiment, pantropic neurotrophins are produced in yeast cells. Yeast expression systems are well known in the art, and include expression vectors for Saccharomyces cerevisiae, Candida albicans and C. maltosa, Hansenula polymorpha, Kluyveromyces fragilis and K. lactis, Pichia guillerimondii and P. pastoris, Schizosaccharomyces pombe, and Yarrowia lipolytica. The methods of introducing exogenous nucleic acid into yeast hosts, as well as other hosts, is well known in the art, and will vary with the host cell used.
In a preferred embodiment, pantropic neurotrophins are expressed in bacterial systems. Expression vectors for bacteria are well known in the art, and include vectors for Bacillus subtilis, E. coli, Streptococcus cremoris, and Streptococcus lividans, among others. The bacterial expression vectors are transformed into bacterial host cells using techniques well known in the art, such as calcium chloride treatment, electroporation, and others.
In one embodiment, pantropic neurotrophins are produced in insect cells. Expression vectors for the transformation of insect cells, and in particular, baculovirus-based expression vectors, are well known in the art. Materials and methods for baculovirus/insect cell expression systems are commercially available in kit form; for example the “MaxBac” kit from Invitrogen in San Diego.
Recombinant baculovirus expression vectors have been developed for infection into several insect cells. For example, recombinant baculoviruses have been developed for Aedes aegypti, Autrographa californica, Bombyx mori, Drosophila melangaster, Spodoprera frugiperda, and Trichoplusia ni.
Once expressed, pantropic neurotrophins are used as neurotrophic factors. These pantropic neurotrophins may be utilized in various diagnostic and therapeutic applications.
The pantropic neurotrophins of the present invention are useful in diagnostic methods of detecting neurotrophin receptors. For example, the pantropic neurotrophins of the present invention may be labelled. By a “labelled pantropic neurotrophin” herein is meant a pantropic neurotrophin that has at least one element, isotope or chemical compound attached to enable the detection of the pantropic neurotrophin or the pantropic neurotrophin bound to a neurotrophin receptor. In general, labels fall into three classes: a) isotopic labels, which may be radioactive or heavy isotopes; b) immune labels, which may be antibodies or antigens; and c) colored or fluorescent dyes. The labels may be incorporated into the pantropic neurotrophin at any position. Once labelled, the pantropic neurotrophins are used to detect neurotrophin receptors, either in vitro or in vivo. For example, the presence of neurotrophin receptors can be an indication of the presence of certain cell types, useful in diagnosis. That is, a subpopulation of certain cell types may be shown by the bindino of the labelled pantropic neurotrophin to the cells via the receptors.
Additionally, the pantropic neurotrophins of the present invention are useful as standards in neurotrophin assays. For example, the activity of a pantropic neurotrophin in any particular assay may be determined using known neurotrophin standards, and then the pantropic neurotrophin may be used in the diagnosis and quantification of neurotrophins.
Furthermore, the pantropic neurotrophins of the present invention are useful as components of culture media for use in culturing nerve cells in vivo, since many nerve cell cultures require growth factors. As will be understood by those skilled in the art, the pantropic neurotrophins of the present invention can replace other neurotrophic factors which are frequently used as media components. The amount of the pantropic neurotrophins to be added can be easily determined using standard assays.
The pantropic neurotrophins of the present invention are also useful to generate antibodies, which can be used in the diagnosis, identification, and localization of neurotrophins or neurotrophin antibodies within an organism or patient. For example, the pantropic neurotrophins can be used to make polyclonal or monoclonal antibodies as is well known by those skilled in the art. The antibodies can then be labelled and used to detect the presence, or absence, of the neurotrophins. Thus, diagnosis of neural disorders associated with neurotrophins may be detected. Alternatively, the antibodies are detected indirectly, by using a second antibody. For example, primary antibodies may be made in mice or rabbits, and then labelled anti-mouse or anti-rabbit antibodies are used to detect the primary antibodies. Either of these methods, as well as similar methods well known in the art, allow the detection of neurotrophins in a variety of tissues.
In addition, the antibodies generated to the pantropic neurotrophins of the present invention are also useful for the purification of neurotrophins and pantropic neurotrophins. Since generally the amino acid substitutions of the pantropic neurotrophins are small, many immune epitopes are shared by the neurotrophins and pantropic neurotrophins. Thus, antibodies generated to the pantropic neurotrophins will bind naturally occurring neurotrophins, and thus are useful in purification. For example, purification schemes based on affinity chromatography techniques can be used, as are well known in the art.
In the preferred embodiment, the pantropic neurotrophins of the present invention are administered to a patient to treat neural disorders. By “neural disorders” herein is meant disorders of the central and/or peripheral nervous system that are associated with neuron degeneration or damage. Specific examples of neural disorders include, but are not limited to, Alzheimer's disease, Parkinson's disease, Huntington's chorea, stroke, ALS, peripheral neuropathies, and other conditions characterized by necrosis or loss of neurons, whether central, peripheral, or motorneurons, in addition to treating damaged nerves due to trauma, burns, kidney disfunction or injury. For example, peripheral neuropathies associated with certain conditions, such as neuropathies associated with diabetes, AIDS, or chemotherapy may be treated using the pantropic neurotrophins of the present invention. Additionally, the administration of NT3 prevents the in vivo degeneration of adult central noradreneroic neurons of the locus coerulus in a model that resembles the pattern of cell loss found in Alzheimer's disease (86) In addition, the addition of NT3 has been shown to enhance sprouting of corticospinal tract during development, as well as after adult spinal cord lesions (58). In fact, when NT3 was administered with antibodies which inhibit myelin-associated growth inhibitory proteins, long-distance regeneration was seen. Thus, the pantropic neurotrophins of the present invention can be used in place of NT3 in this application.
In this embodiment, a therapeutically effective dose of a pantropic neurotrophin is administered to a patient. By “therapeutically effective dose” herein is meant a dose that produces the effects for which it is administered. The exact dose will depend on the disorder to be treated, and will be ascertainable by one skilled in the art using known techniques. In general, the pantropic neurotrophins of the present invention are administered at about 1 μg/kg to about 100 mg/kg per day. In addition, as is known in the art, adjustments for age as well as the body weight, general health, sex, diet, time of administration, drug interaction and the severity of the disease may be necessary, and will be ascertainable with routine experimentation by those skilled in the art.
A “patient” for the purposes of the present invention includes both humans and other animals and organisms. Thus the methods are applicable to both human therapy and veterinary applications.
The administration of the pantropic neurotrophins of the present invention can be done in a variety of ways, including, but not limited to, orally, subcutaneously, intravenously, intracerebrally, intranasally, transdermally, intraperitoneally, intramuscularly, intrapulmonary, vaginally, rectally, or intraocularly. The pantropic neurotrophins may be administered continuously by infusion into the fluid reservoirs of the CNS, although bolus injection is acceptable, using techniques well known in the art, such as pumps or implantation. In some instances, for example, in the treatment of wounds, the pantropic neurotrophins may be directly applied as a solution or spray.
The pharmaceutical compositions of the present invention comprise a pantropic neurotrophin in a form suitable for administration to a patient. In the preferred embodiment, the pharmaceutical compositions are in a water soluble form, and may include such things as carriers, excipients, stabilizers, buffers, salts, antioxidants, hydrophilic polymers, amino acids, carbohydrates, ionic or nonionic surfactants, and polyethylene or propylene glycol. The pantropic neurotrophins may be in a time-release form for implantation, or may be entrapped in microcapsules using techniques well known in the art.
The following examples serve to more fully describe the manner of using the above-described invention, as well as to set forth the best modes contemplated for carrying out various aspects of the invention. It is understood that these examples in no way serve to limit the true scope of this invention, but rather are presented for illustrative purposes.
The coordinates for the three-dimensional structure of mouse NGF were obtained from N. Q. McDonald and T. L. Blundell. The molecular modeling for human NT-3 was performed on a Silicon Graphics Iris Workstation using the interactive program InsightII. The representations of NT-3 structures were produced using the program MidasPlus. (University of California at San Francisco).
When the three-dimensional structure of mouse NGF (mNGF) became available (59) a rational approach to the structural basis of neurotrophic function using protein engineering techniques became possible. The structure of mNGF consists of a tightly associated dimer of two identical amino acid polypeptide chains. The fold of each monomer is formed by extended segments of twisted anti-parallel β-sheets linked by turns. The molecule has an elongated shape and provides a flat hydrophobic surface that forms the interface of the associated monomers (59). A striking feature of the structure is the arrangement of the disulfide bonds, now known as the cysteine-knot motif (60). This motif is also found in the otherwise unrelated TGF-β (61); (60) and PDGF-BB (87). Several regions of the mNGF structure, including the amino and carboxy termini and the loop between residues 43 and 48 were not well defined, indicating higghly flexible structural elements.
The sequence of human NT-3 (hNT-3) is 56% identical and 70% similar to mNGF (
The first set of mutations probed both conserved and non-conserved residues, located mainly in β-strands, that are surface exposed and therefore potentially involved in binding to the trkC and gp75 receptors. The current hypothesis proposed for NGF function (55) is that divergent residues located in loops connecting β-strands and the termini are major determinants for receptor binding and specificity. A second set of hNT-3 mutants evaluated the importance of these residues to interaction of hNT-3 and its receptors. The total set of mutants covered essentially the entire surface of the NT-3 molecule.
Human NT-3 was previously cloned, sequenced and subcloned into a pRK-type vector which allows for production of double and single stranded DNA in E. coli, as well as expression of mature NT-3 in a mammalian system under control of the cytomegalo virus promoter (65). Mutagenesis on this vector was performed according to the method of Kunkel (66) (67). After transformation into the E. coli strain XL1-Blue, colonies were screened for the presence of the desired mutation by sequencing single-stranded DNA using the Sequenase version 2.0 kit (U.S. Biochemical Corp.). The entire sequence coding for the mature NT-3 was verified for all positive clones. Double-stranded DNA was isolated from XL-1 Blue with the QIAGEN DNA purification kit (Qiagen Inc., Chatsworth Calif.). This DNA was subsequently used for transfection of the fetal human kidney cell line 293 (68). All other recombinant DNA manipulations were performed as described (69). Well known techniques are used to generate the primers for all the mutations. The primer for the D15A mutation was 5′-GGTCACCCACAAGCTTTCACTGGCACATACCGAG-3′(SEQ ID NO. 7; and the primer for the S1 mutant (the N-terminal swap of the 6 N-terminal amino acids of NT3 for the 7 N-terminal amino acids of NGF) was 5′-GTACTCCCCTCGGTGGAAGATGGGATGGCTCGAGGACCGTTTCCGC CGTG-3′. (SEQ. ID NO.9)
Expression of Wild-Type and Mutant Neurotrophins
Plasmid DNA containing either the hNT-3 or mutant hNT-3 coding sequences was introduced into the human fetal kidney cell line 293 by calcium phosphate precipitation (70). The 75% confluent cells were transfected with 10 μg of plasmid DNA per 15 mm cell culture dish and incubated for 15 h in serum containing medium. Then the medium was removed and exchanged by serum-free medium (PSO4)supplemented with 10 mg/l recombinant bovine insulin, 1 mg/l transferrin and trace elements. The supernatant was collected after 48 and 96 hours concentrated approximately 20-fold with centriprep-10 filtration units (Amicon, Beverly Mass.) and sterile filtered.
Quantification of Neurotrophin Mutants
The specific hNT-3 ELISA was based on a Protein A purified polyclonal antiserum from guinea pig (Genentech). Each well of a 96-well plate (MaxiSorp; Nunc, Kamstrup, Denmark) was coated overnight at 4° C. with 100 μl of 4 μg/ml antiserum in 0.05M sodium carbonate buffer (pH 9.6). After a 1 h blocking step with blocking buffer (PBS+0.5% BSA+0.01% Thimerosal, pH 7.4), the wells were washed six times with ELISA buffer (PBS+0.5% BSA+0.05% Tween-20+0.01% Thimerosal, pH 7.4). Purified recombinant hNT-3 or samples of hNT-3 mutants of unknown concentrations were diluted in ELISA buffer to a volume of 100 μl and added to the wells. The plates were incubated for 2 h at room temperature with continous shaking. After a wash with ELISA buffer, the wells were incubated with 100 μl biotinylated anti-hNT-3 antibody (Genentech) for 2h and again washed with ELISA buffer. 100 μl of a 1:50000 dilution of streptavidin/horse radish peroxidase (Zymed, 43–4323) was added to the wells and incubated for 30 min., followed by a wash step with ELISA buffer. Finally, the color was developed for 15–20 min. using 100 μl of a PBS solution containing 0.012% H2O2 and 0.04% o-phenylenediamine. The reaction was stopped by addition of 50 μl of 4.5 N H2SO4. The absorption was read at 490 nm and at 405 nm on a Vmax kinetic microplate reader (Molecular Devices, Palo Alto Calif.). The standard curve was determined using purified recombinant hNT-3 (Genentech) at concentrations of 50, 25, 12.5, 6.25, 3.13, 1.56 and 0.78 ng/ml. The samples with unknown NT-3 concentration were serially diluted 1:10, 1:30, 1:90, 1:270, 1:810, 1:2430, 1:7290 and 1:21870 in order to obtain multiple data-points per sample. The standard curve was determined using a four-parameter fit of the data points obtained from the assay of the standard protein.
The amounts of NT-3 mutants after concentration varied between 120 ng/ml and 36 μg/ml. The ELISA assay did not detect any NT-3 in supernatants from mock transfected cells nor did it crossreact with recombinant human NGF from supernatants of NGF transfected cells (data not shown). For each set of expressions of NT-3 mutants a native hNT-3 expression was performed and quantified by ELISA in parallel in order to obtain a comparative wt concentration for receptor binding studies. All mutants were expressed, quantified and assayed at least twice.
Iodination
Purified recombinant hNT-3, hBDNF and hNGF (Genentech) were labeled by lactoperoxidase treatment using a modification of the Enzymobead radioiodination reagent (Bio-Rad) procedure (71). Usually, 2 μg of the neurotrophins were iodinated to specific activities ranging from 3000–3500 cpm/fmol. The labeled material was stored at 4° C. and used within 2 weeks of preparation.
Binding Assays
Cell based binding assays made use of preparations of membranes from stable cell lines expressing rat trkC (NIH3T3/trkC, (26)). Competitive displacement assays were performed as described previously (26). Mutants were assayed for binding affinity to the trkC receptor twice for each of the multiple expressions with a duplicate set of data points. This procedure allowed estimation of the error of affinity determination for each of the mutants. Unpurified recombinant NT-3 from transiently expressing cells was compared with purified NT-3 for its ability to displace 125-I labeled NT-3 from trkC receptors expressed on NIH13T3 cells. Both displaced labeled NT-3 with similar IC-50:7 pM and 9 pM for unpurified NT-3 and pure NT-3. This indicated that unpurified NT-3 from supernatants of expressing 293 cells could be quantified precisely and subsequently used for receptor binding studies. The specificity of the binding assays was demonstrated by the inability of NGF, BDNF and supernatant of mock transfected cells to displace bound labeled NT-3 from trkC (data not shown). Receptor immunoadhesin proteins were constructed using human trkA, trkB, trkC and gp75 extracellular domains fused to immunoglobulin constant domains (Genentech, unpublished results). A 96-well plate (Corning, ELISA wells strips) was coated with 100 μl of 5 μg/ml goat F(ab′)2 anti-human Fc IgG (Organon Technika, West Chester, Pa.) in coating buffer for 15h at 4–8° C. The wells were aspirated, washed 3 times with PBS and incubated for 2h with 100 μl of a 40 ng/ml solution of the receptor immunoadhesin protein in binding buffer (Leibovitz's L-15 medium supplemented with 5 mg/ml BSA (Intergen, Purchase, PA), 0.1 mg/ml horse heart cytochrome C (Sigma) and 20 mM HEPES, pH7.2). After a wash step with PBS, 50 μl of binding buffer was immediately added to the wells in order to prevent drying. Each of the native and mutant protein stock solutions was serially diluted, using binding buffer, to give a concentration range of 4096-2 pM. 25 μl of serial dilution was added per well, followed by 25 μl of labeled neurotrophins. The final concentration of labeled neurotrophins in each well was approximately 50 pM for trkA, trkB and trkC assays and 100 pM for gp75 binding assays. After 3h of incubation at room temperature, the wells were washed with PBS+0.5% Tween-20 and the bound radioactivity was counted. All displacement experiments were analyzed by applying a four-parameter fit procedure on the data set with the Kaleidagraph software package. All binding results in bar graphs are expressed as IC-50 mut/IC-50 wt.
Stimulation of Autophosphorylation of trk Receptors on PC12 Cell Lines by Neurotrophic Factors
Approximately 1×107 cells were treated at 37° C. for 5 min with 25 ng/ml neurotrophin. NP-40 plate lysis and immunoprecipitation with antiserum 443 (pan-trk) or 656 (trkC specific) was done as previously described (26). The phosphotyrosine content was analyzed by Western transfer using monoclonal antibody 4G10 as previously described (23). 4G10 was detected as previously described (26).
Differentiation Assays on PC12 and PC12 Cells Expressing trkB and trkC.
Approximately 103 PC12 cells expressing the different trk family members (trkC; (26) trkB; Soppet, unpublished observations), were plated into 35 mm collagen-coated tissue culture dishes containing a total of 2 ml of medium. PC12 cells expressing trkC were assayed at three different concentrations (10 ng/ml, 1 ng/ml, 100 pg/ml) and the parental PC12 cells expressing only trkA or PC12 cells expressing trkB were treated with 10 ng/ml of NT-3 mutant supernatants. For each treatment, at least 200 cells were counted. The proportion of neurite-bearing cells was determined by counting the number of cells containing processes at least twice the length of the cell body after 3–4 days.
Dissection of Embryonic Tissues and Neuronal Cultures.
Chick embryos at different stages of development were obtained by incubating white Leghorn chick egos (SPAFAS, Reinholds, Pa.) at 38° C. in an egg incubator for the required time. Dorsal root ganglia, nodose ganglia from embryonic day 8 (E8), and sympathetic ganglia from embryonic day 11 (E11), were dissected in Leibowitz-15 (L-15) media containing 1× penicillin/streptomycin using watch-maker's forceps and electrolytically sharpened tungsten needles. Embryonic chicken ganglia were trypsinized at 37° C. for 20 min and then washed in culture medium (F14 with 10% heat-inactivated horse serum and 5% heat-inactivated fetal calf serum) and were gently triturated with a fire-polished pipette to give a single-cell suspension. Chick embryo cells were plated onto 35-mm dishes that had been coated with polyornithine (0.5 mg/ml in 0.15 M borate buffer at pH 8.6, overnight) and laminin (20 m/ml for 4–6 hr at 37_C) in 2 ml of culture medium in presence of 2 ng/ml of neurotrophin, or at the concentrations noted in the text. All cells with a neuronal morphology within a 5×5-mm grid in the center of each dish were counted 72 hr later.
The results are shown in Tables 3 and 4.
Several charged and uncharged residues are conserved among NGF proteins from other species. In particular, His4, Pro5, and His8 are conserved in 7 of 8 known NGF sequences; Arg9 exists only in human and chicken NGF while Met predominates at this position of NGF of other species. Ten mutants were generated by oligonucleotide-directed mutagenesis that either: 1) replaced some of the charged residues of the N-terminus of hNGF with alanine individually or together, 2) replaced His4 with negatively charged aspartic acid which resides in position 3 of the N-terminal hBDNF sequence (65), or 3) generated chimeric hNGF molecules which contained the first 5 or 6 residues of hBDNF or hNT3, respectively, or other variable regions of hNT3. The resulting mutant constructs were generated in vectors containing a human CMV promoter (70) and were expressed transiently in human 293 cells as described below.
Purified recombinant (1–118), (6–118), and (10–118) were purified from transfected CHO cell line conditioned media utilizing reversed-phase HPLC and high performance ion-exchange chromatography as described by Burton et al. (1992) (48) and Kahle et al. (1992) (49), and characterized by N-terminal sequence analysis, SDS-PAGE and amino acid analysis (data not shown). These processed variants result from in siti proteolysis during conditioning of the CH0 cell media by as of yet uncharacterized proteolytic enzymes or processing pathways. The purity of each form was 99% based on SDS-PAGE and the concentration was determined by quantitative amino acid analysis. Purification and analysis of the H4D mutant 2(20 μg from 300 ml media) and the N-terminal hNT3/hNGF mutant 6 (5 μg from 300 ml media) was performed from serum-free media conditioned by transfected 293 cells (see below) as just described for the N-terminal truncated variants.
Mutagenesis was performed by the oligonucleotide-directed method (72) with modifications as indicated in the BioRad Muta-Gene kit (66); BioRad, Richmond, Calif.). The mutations were verified by DNA sequencing of single stranded phagemid clones by the chain termination method (73). The hNGF mutants were expressed in conditioned media following transient transfection of human 293 cells (68) (70). The media used for collection was 50:50 F12/DMEM serum-free media containing the N2 supplement and was collected following 48 hours in the serum-free media. The conditioned media was concentrated 10-fold using Amicon concentrators. The concentration of hNGF mutants was determined by an enzyme-linked immunoassay (ELISA) utilizing purified rabbit anti-hNGF polyclonal antibodies. The concentration of each mutant varied from 3–8 μg/ml. Each mutant was expressed at least three times and the concentration determined by ELISA 2–3 independent times.
Mutants were also analyzed by metabolic labelling of transfected 293 cells (60 mm plates, 1.2 ml media) by the addition of 200 μCi each of 35S-methionine and cysteine (Amersham). After 18 hrs, media is collected and reacted with either rabbit anti-hNGF polyclonal antibody or mouse monoclonal antibody for 3–4 hrs at 4° C., collected by precipitation with Protein-A beads (Pharmacia), and applied to 15% acrylamide SDS-PAGE gels (Novex). Following electrophoresis the gels were dried and placed next to X-ray film. Non-radiolabelled mutants were produced as described above and 0.1 μg aliquots were lyophilized, redissolved in SDS-PAGE sample buffer, electrophoresed on same gels, and transported onto nitrocellulose according to standard protocols (BioRad). The blot was treated with rabbit anti-hNGF polyclonal or mouse anti-hNGF monoclonal antibody overnight at 4° C., washed, and mutants detected with alkaline phosphatase-coupled goat anti-rabbit or anti-mouse IgG antibodies.
Receptor Binding, trkA Autophosphorylation, and PC12 Neurite Outgrowth Assays
[125]hNGF was produced using the Enzymobead method (BioRad), according to the method of Escandon (71). The specific radioactivity, determined by TCA precipitation of aliquots of the starting reaction mixture and gel filtration-chromatographed [125I]hNGF, averaged 60–90 μCi/μg Receptor binding assays were performed overnight at 4° C. on NIH3T3 cells recombinantly expressing rat trkA cells (Kindly supplied by Dr. Luis Parada), p75-expressing A875 human melanoma cells (ATCC), and rat PC12 cells (Kindly supplied by Dr. Louis Reichardt) as described for trkB-expressing NIH3T3 cells (23). The concentration of NIH3T3-trkA and A875-p75 cells used was 1×106 cells per ml; 5×105 cells per ml for PC12 cells. The final concentration of [125I]hNGF was 50 pM in a volume of 0.2 ml. The non-specific binding, defined as the [125I]hNGF bound in the presence of 1×10−6 M unlabelled hNGF, varied between 15–25% in most cases for the NIH3T3 trk cells, 20–35% for the p75-A875 melanoma cells, and 20–30% for the trkA+p75 PC12 cells using the filter binding assay. The data was fitted to a displacement isotherm and an IC50 was calculated utilizing a 4-parameter equation within the Kaleidagraph program. In some instances receptor binding was performed with cells at 25° C. for 90 min, and bound [125I]hNGF was separated from free by sucrose cushion-centrifugation.
Autophosphorylation of trkA was performed at 37° C. for 5 min and the extent of phosphorylation was determined by a variation of the method described by Kaplan (19). Triton X-100 lysed trk A cells were immunoprecipitated with agarose bead-immobilized antiphosphotyrosine monoclonal antibody 4G10 (UBI), electrophoresed on SDS-PAGE (8% acrylamide-Novex), immunoblotted, and probed with rabbit anti-trkA polyclonal antibody (Kindly provided by Dr. David Kaplan). Detection of trkA was by alkaline phosphatase (AP)-coupled goat anti-rabbit IgG antibody (TAGO). PC12 cells were grown to 20–30% confluency on Primaria polycationic 24-well plates, the media changed to serum-free DMEM high glucose supplemented with N2 containing wild type or mutant variants of hNGF. After 48 hours, the number of cells projecting neurites longer than two cell bodies were counted in a representative visual field and expressed as a percentage of the total cells within the field, usually 100–140 cells. The activity of each mutant and NGF control was determined at least twice in separate experiments. The percent of responsive cells at maximal concentrations of hNGF varied from 55–75% between experiments with the mean of 63% calculated from 13 determinations. To account for the variation in the maximal response between experiments, this mean value was used to normalize all the data.
Inhibition of [125I]hNGF Binding to trkA and p75 Cells by a Monoclonal Antibody to hNGF
Under the same conditions as the filter binding assay described above, increasing concentrations of an anti-hNGF monoclonal antibody were added to 25 pM [125I]hNGF and incubated for 30 min at 25° C. Then 1×106 cells per ml of either NIH3T3-trkA or A875-p75 cells were added (0.2 ml final volume) and incubated for 4° C. overnight with vigorous mixing. The samples were then diluted and filtered on Whatman GF/C filters and counted.
Results
The results are shown in Tables 5 and 6.
aTrkA autophosphorylation was performed at 1 × 10−10, 1 × 10−9, and 1 × 10−8 M as described in the Experimental Procedures and the legends for FIGS. 2 and 5. The values represent the ratio of densitometric area of the immunoblotted autophosphorylated p140trkA band following stimulation of NIH3T3-trkA cells by hNGF structural variants versus (1–118) hNGF (truncated hNGF) or wild type hNGF (mutants).
bPC12 cell differentiation was determined by neurite outgrowth described in the Experimental Procedures and the legend to FIG. 6. The EC50 values and ratios are taken from the data in FIG. 6 and represent the average from two separate experiments for each mutant.
To initiate characterization of the N-terminal amino acid residues necessary for full hNGF activity, the (6–118) truncated form of hNGF was isolated from conditioned media of CHO cells recombinantly-expressing hNGF. The nine amino acid truncated form (10–118)hNGF was generated by limited proteolysis as described (48). The (6–118) and (10–118)hNGF were purified by high-performance ion-exchange chromatography (HPIEC) and characterized by reverse-phase HPLC, N-terminal sequence analysis, SDS-PAGE, and amino acid analysis (Data not shown).
The relative potency of purified (6–118)hNGF to displace [125I]hNGF from cell lines expressing trkA, p75, and trkA+p75 were then compared to those of (10–118)hNGF, (1–118) or (1–120)hNGF, and (1–118)mNGF(
Deletion of the first five amino acids results in a 9-fold loss of binding to NIH3T3 cells recombinanLly expressing rat trkA while little difference in binding occurs with p75-expressing A875 human melanoma cells (no change) or with PC12 cells expressing trkA+p75 (3-fold). In contrast, a 265- and 82-fold loss of binding to trkA and PC12 cells, respectively, were observed for (10–118)hNGF compared to (1–118)hNGF while a 10-fold loss in binding to p75 occurs. The intermediate potency of displacement by (10–118)hNGF observed with BC12 cells, relative to the trkA and p75 cells, suggests contributions by both receptors to the profile of the displacement isotherm (
The mutant forms of hNGF can be detected by metabolic labelling followed by immunoprecipitation, or immunoblot analysis of non-labelled conditioned media, and are represented as fully-processed polypeptides of 14 kD (
The replacement of all three charged amino acids to alanine (Mut 4:H4A+H8A+R9A) resulted in the loss of detectable competitive displacement of [125I]hNGF from trkA at 4° C. over a concentration range of wild-type hNGF that completely displace the tracer (IC50=1×10−10 M; maximum displacement=1×10−9 M,
His4 and Arg9 variants were then analyzed individually. The N-terminal region of both hNT3 and hBDNF contains a hislidine which sugoests the possibility of a conserved functional role. Replacement of His4 of hNGF by either alanine (mutant 1) or aspartic acid (mutant 2) result in dramatic loss of trkA binding, autophosphorylation, and PC12 cell differentiation (
To test whether the specific N-terminal sequence of hNGF is required for neurotrophin interaction with trkA, chimeric mutants (mutants 5 and 6) were generated by replacing the N-terminus of hNGF (SSSHPIF) (SEQ ID NO: 11) with that of hBDNF (HSDPA) (SEQ ID NO: 18) or hNT3 (YAEHKS) (SEQ ID NO: 12). These mutants would therefore retain the dibasic His8, Arg9 residues of HNGF. Even at 10-fold higher concentrations of (1–118)hNGF which result in complete receptor displacement at 4° C., the resulting chimeric neurotrophins were unable to displace [125I]HNGF from trkA (
Chimeric mutants were also generated to initially compare the role of two other variable regions of hNGF as possible determinants of trkA receptor specificity. Six residues within beta-turn variable region 3 (Arg59-Ser66) and seven residues within beta turn variable region 5 (Met92-Ala98) were exchanged with the corresponding hNT3 residues in mutants 7 and 8, respectively. Mutant 7 was slightly more potent in displacing [125I]NGF than hNGF from trkA while mutant 8 bound less well to p75 (3-5 fold). Otherwise these mutants displayed little difference from hNGF in their trkA and p75 binding profiles, ability to support trkA autophosphorylation or PC12 neurite outgrowth (
To determine the relative levels of expression of the Mr=14,000 fully processed form of the structural variants of hNGF, monoclonal and polyclonal antibodies to hNGF were tested for their ability to recognize mutants by immunoblotting (
Identification of Target Residues for Mutational Analysis
NGF and its neurotrophin family members NT3, BDNF and NT4/5 share approximately 56% sequence identity. The receptor binding specificity may be determined in part by the amino sequence differences among the neurotrophin family members. These residues may bind directly to the trk receptor, or function as inhibitory constraints on the trk interactions of other variable, or conserved residues. Domain-swap mutants of hNGF were generated between hNGF and hNT3 to test the role of the divergent residues in determining trk receptor specificity. A comparison of neurotrophin primary sequences reveals that there are 7 regions of 7–10 amino acids each which contain most (80%) of the sequence differences (
The selection of individual residues of hNGF for mutagenesis was determined primarily by their position within the x-ray crystal structure of murine NGF. It was assumed that minimal structural change results from the sequence differences between human and mouse NGF since the replaced residues are mostly functionally conserved (10/12). Computer-generated modelling of murine NGF, based on the x-ray crystal structure coordinates and described in example 1, reveals amino acids which project side chains into the solvent and could interact with trkA and gp75 receptors. Some of these include variable residues which were significantly modified by the domain-swap mutations, however, many represent residues conserved between hNGF and hNT3 within variable and conserved regions. Residues predicted to have minimal side-chain exposure, such as those implicated in formino the dimeric interface (F12, V14, W21, F49, Y52, F54, W76, T85, F86, W99, F10, T106, A107, V109, V111), hydrophobic interior (V36, V38, F53, 171, A89, 1102, and 1104), and structurally-dependent and buried hydrogen bonding (Q51, S78, T91, R100), were minimally modified. Of the disulfide bond-forming cysteine residues, and glycines and alanines, only A97 was modified. Exceptions were made in the following cases: residues I30 and Y52 which have surface side chains although they appear at the dimer interface, residues L39, L90, M92 and A97 which also form a hydrophobic surface patch, and D16, K25, D30. E55, K57, R59, R69, D72, H75 which exhibit some side-chain solvent exposure although hydrogen bonded. Most residues were changed to alanine (64); in some instances other replacements were made to maintain structure while testing the role of a specific functionality in receptor interactions.
Production and Receptor Binding Characterization of hNGF Variants
Mutagenesis, expression, and protein characterization of hNGF variants was performed as described in example 3. Following oligonucleotide-directed mutagenesis, all mutants were verified by dideoxynucleotide sequencing. The hNGF mutants were expressed in human 293 cells (
TrkA Autophosphorylation Activity and PC12 Cell Differentiation Bioassay
Biochemical activation of trkA kinase by hNGF variants was determined by assessing trkA autophosphorylation as described in example 3. A quantitative assay was developed (89) which permits dose-dependent determinations of the EC50 for trkA autophosphorylation. A trkA receptor variant, containing a peptide epitope tag derived from a Herpes simplex surface protein, was stably expressed in CHO cells in 96 well plates (88). The affinity of the epitope-tagged trkA for hNGF is identical to that of the normal receptor (88). The cells (duplicate wells for each concentration) are stimulated with 8 increasing concentrations of hNGF variant (10 pM-10 nM) for 10 minutes at 37° C. The cells are lysed with Triton X-100 lysis buffer as described in example 3, and transferred to a plate coated with a monoclonal antibody directed to the epitope tag. After binding, the captured trkA is then reacted with a HRP-conjugated antiphosphotyrosine monoclonal antibody, and the color reaction developed. The absorbance is then read and plotted versus concentration. The EC50 for hNGF is 100–120 pM. Differentiation of PC12 cells was performed as described in example 3, however, cells were first grown or primed in NGF for 7–10 days. Cells bearing neurites were then harvested and plated in 24 well dishes in normal growth media, and either in the presence or absence of hNGF variant. The percentage of cells bearing neurites after 72 hours were quantifyied as described in example 2. hNGF/hNT3 pan-neurotrophic variants were evaluated for hNT3-like trkC bioactivity in trkC-transfected PC12 cells which did not respond to hNGF (Kindly provided by Drs. Pantelis Tsolfous and Luis Parada, NCI).
Mutagenic Analysis of Variable Residues by hNGF/hNT3 Chimera
Thirteen chimeric mutants were generated by replacing several residues, or all, of each of the 7 variable regions of hNGF, with the corresponding region of hNT3 (See
The loss of trkA binding and receptor activation suggests that the variable residues within the N-terminus and pre-variable region 1 contribute to trk receptor specificity. This possibility was tested by determining receptor binding to trkC-IgG immunoadhesion and neurite outgrowth in trkC-transfected PC12 cells which do not respond to NGF. Surprisingly, trkC interactions were not conferred by the N-terminus of hNT3 (Mutant 6, FIGS. 22A,B), however, the four amino acid swap in variable region 4 of hNGF (T81K, H84Q, F86Y, K88R) resulted in a significant trkC interaction (FIGS. 22A,B). The lower trkC affinity and potency of neurite outgrowth of this variant indicates that other regions likely contribute to efficient trkC interactions. Pre-variable region 1 mutant is now being evaluated for trkC interactions, as are the contributions of the individual residues within variable region 4. However, overlapping mutations within variable region 4 suggest that multiple residues of V4 may be necessary for the trk specificity For example, the beta-turn 3/4 variant, exchanging three variable residues which overlaps at T81K (S73, Y79Q, T81K), activates neurite outgrowth in trkC-PCI2 cells only 5–10% as well as the variable region 4 mutant (
Although the N-terminal domain of the neurotrophins appears not to be a general trk specificity domain, this region of hNGF appears to be a major determinant of trkA interaction. The replacement of the first six residues of NT3 with the first seven residues of hNGF results in a pantropic variant which binds and activates both trkA and trkC with high affinity and potency (
Mutagenic Analysis of Individual Variable and Conserved hNGF Residues:
Structural nwdel of hNGF residues which interact with trkA and gp75. Using the crystal structure of murine NGF as described above, we evaluated by point mutagenesis 45 residues of hNGF, many of which have side chain functionalilies exposed to the solvent and are capable of trkA or gp75 interactions. Competition binding analysis reveals that H4, P5A, S13, D30, I31, Y52. R59, R69, Y79, T81, and R103 mutations affect trkA binding 1.8-10 fold, while mutations of residues E41, K57, D72, N77 increase binding 1.5-2 fold (
Autophosphorylation analysis using epitope-tagged trkA indicated decreases in the potency of activation by 1.5-6 fold by mutations in residues H4, P5, D30, Y52, R69, Y79+T81, and R103 (
Residues interacting with the trkA and gp75 receptors were modelled by computer-generated on the structure of murine NGF. Two major trkA interacting regions were found by this analysis: 1) The N-terminus (H4, P5), with unknown crystal structure, and 2) A surface formed by Y79, T81, H84 and R103 of beta sheets C and D. Residues V18, V20, G23, Y52, R59 and R69 of beta sheets A and B make some contributions to an extended surface which would wrap around the beta sheet strands. Near the area of Y52 and the beta sheet A residues are D30 and I31 of the second protomer. These two residues project relatively little surface area into the solvent, however, it is possible that they contribute to a continuous binding surface formed with the beta-sheet residues.
Two major p75 interacting regions were found: 1) Variable region 1 of one protomer and beta-sheet B and C of the other protomer, 2) Conserved residues within the C-terminus and beta-turn 3, also from different protomers. In contrast to the trkA-interesting residues within a cleft formed by the pairs of beta sheets, the p75 interacting residues appear to be well exposed. As shown by (54) K32 and K34 project from the variable region of beta-hairpin turn 1. We find the adjacent residues K50 and Y52 from the other protomer contribute to p75 binding. K88, which contributes significantly to the p75 binding, is in this region but is not highly exposed. The other binding surface is composed of K74 (beta-turn 3), R114 and K115 (C-terminus) of one terminus, and F12, R69 from the other protomer.
Other potential pantropic molecules are now being constructed and evaluated based on the mutagenesis analysis presented above. A pan trkA/trkC molecule can be generated by the following changes in hNGF: 1) pre-variable region 1 (V18E+V20L+G23T) plus variable region 4 (Y79Q+T81K+ H84Q+F86Y+K88R); 2) pre-variable region 1 plus minimal residues replacements of variable region 4. A pan trkA/trkC molecule can be generated by replacing minimal changes within the first seven residues of the N-terminus of hNGF and replacing the first 6 residues of hNT3. Since H4 and P5 are conserved among NGFs, and 2 hydrophobic residues in positions 6 and 7 are conserved, the following variants have been made: 1) YASHPIF(SEQ ID NO: 13)-hNT3; 2) YAHPIF (SEQ ID NO: 14)-hNT3; 3) YASHPIS (SEQ ID NO: 15)-hNT3; 4) YAEHPIF (SEQ ID NO: 16)-hNT3; and 5) YAQHPIF (SEQ ID NO: 17)-hNT3.
This is a divisional application of, and claims priority under 35 USC §120 to, U.S. application Ser. No. 08/794,028 filed on Feb. 3, 1997 (now U.S. Pat. No. 6,503,728), which is a continuation of, and claims priority under 35 USC §120 to, U.S. application Ser. No. 08/253,937 filed on Jun. 3, 1994, now abandoned.
| Number | Date | Country |
|---|---|---|
| WO 9202620 | Feb 1992 | WO |
| WO 9318066 | Sep 1993 | WO |
| WO 9325684 | Dec 1993 | WO |
| WO 9412539 | Jun 1994 | WO |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 08794028 | Feb 1997 | US |
| Child | 10230865 | US |
