Patent Application
20120321609
The present invention relates to a method of treating psychological disorders, and more particularly, to a method of modulating the interaction between a corticotropin releasing factor receptor and a serotonin receptor.
Anxiety and major depressive disorder often present as co-morbid disorders and the expression and severity of these disorders is commonly associated with stressful experiences. In response to stress, corticotropin releasing factor (CRF) regulates the activity of hypothalamic-pituitary-adrenal (HPA) axis and triggers changes in other neurotransmitters systems, such as serotonin (5-HT). CRF is also known to influence anxiety responses and CRF receptor 1 (CRFR1) may be particularly important in this regard. 5-HT also has diverse functional effects in the central nervous system, as well as in the periphery and plays an important role in modulating depression and anxiety-related behaviours in humans and rodents. In particular, pharmacological studies and knockout mice have demonstrated that 5-HT2A and 5-HT2C receptors contribute to anxiety and are pharmacological targets for the treatment of anxiety. The targeted deletion of either the 5-HT2AR, 5-HT2CR or CRFR1 in mice is also associated with a reduction in anxiety-related behaviour. However, little is known about the molecular mechanisms underlying the cross talk between these two important neurotransmitter systems at the cellular level.
CRF is a 41 amino acid peptide that activates the HPA axis to regulate adrenocorticotropin secretion by the pituitary gland in response to acute and chronic stress. CRF peptide acts through two subtypes of Gs-coupled G protein-coupled receptors (GPCRs) resulting in increased intracellular cAMP formation. Besides its endocrine function in the pituitary, CRF is also involved in a wide variety of effects not related to its pituitary activity indicating it also functions as either a neurotransmitter or neuromodulator in the brain. Consistent with its role as a neurotransmitter, CRF immunoreactive terminals, CRF binding sites and CRF receptor mRNA are widely distributed in areas of the brain that are unrelated to endocrine function. There are also fifteen genes encoding functional serotonin receptors (5-HTR) in the mammalian brain that are classified into 7 families (5-HT1 to 5-HT7), all of which are GPCRs except for 5-HT3Rs which are ionotropic receptors.
The 5-HT2 and CRF receptors each contribute to the regulation of anxiety behaviors and stress responses and CRF treatment is demonstrated to prolong 5-HT regulation of GABAergic inhibitory transmission. The molecular and cellular basis for the action of CRF on 5-HT signaling remains unknown, as agents that increase cAMP accumulation do not mimic the effect of CRFR activation.
It would be desirable, thus, to develop methods of regulating 5-HT2 or CRF receptors that would be useful in the treatment of psychological disorders such as anxiety and related behaviors.
The present invention relates to methods of desensitizing 5-HT2R signaling which are useful to treat psychological disorders.
In one aspect, a method of desensitizing 5-HT2R signaling is provided and comprises inhibiting CRFR1 activation of 5-HT2AR signaling by preventing trafficking of intracellular vesicles or endocytosis.
In another aspect, a method of desensitizing 5-HT2R signaling is provided and comprises inhibiting CRFR1 activation of 5-HT2AR signaling by blocking recycling of 5-HT2AR to the cell surface.
In another aspect, a method of desensitizing 5-HT2R signaling is provided and comprises blocking PDZ binding motifs in at least one of CRFR1, 5-HT2AR and 5-HT2CR.
In a further aspect, a method of desensitizing 5-HT2R signaling is provided and comprises blocking the interaction of a 5-HT2R or CRFR1 with a PDZ-domain-containing protein.
These and other aspects are described in the detailed description that follows by reference to the following figures.
The present invention relates to methods of desensitizing 5-HT2R signaling in a mammal. Such methods have been determined to be useful to treat psychological disorders in mammals.
The term “psychological disorder” includes anxiety-related disorders, depression and related disorders and stress-related disorders. Such disorders may include depression, schizophrenia, anxiety disorder, and bipolar disorder.
The term “mammal” is used herein to refer to either human or non-human mammals.
In a first aspect, 5-HT2R signaling may be desensitized to treat a psychological disorder by inhibiting CRFR1 activation of 5-HT2AR signaling. This may be achieved by administration of an agent that prevents trafficking of intracellular vesicles or endocytosis. In one embodiment, monensin was used to prevent such trafficking.
Desensitization of 5-HT2R signaling may also be achieved by blocking rapid recycling of GPCRs to the cell surface. In one embodiment, overexpression of a dominant-negative Rab GTPase that selectively prevents receptor recycling, such as Rab4-S28N mutant protein, was useful to block recycling of 5-HT2AR.
In another aspect, PDZ binding motifs in the carboxyl-terminal tail domains of at least one of CRFR1, 5-HT2AR or 5-HT2CR may be blocked to desensitize 5-HT2R signaling in a mammal. The amino acid sequences of CRFR1, 5-HT2AR or 5-HT2CR, including the PDZ binding motifs of each, are set out in
In a further aspect, a method of desensitizing 5-HT2R signaling is provided in which a PDZ-domain-containing protein is blocked from binding to the PDZ binding motif on a target 5-HT2AR or CRFR1 in the mammal. PDZ-domain-containing proteins that may be blocked include, but are not limited to, MAGI-1 PDZ1, MAGI-2 PDZ1, MAGI-3 PDZ1, PSD95 PDZ 1&2, PSD95 PDZ3, CAL PDZ, SAP97 PDZ 1&2, PTPN13 PDZ 4&5, PDZK2 PDZ1, MPP3 PDZ, ERBIN PDZ and MUPP1 PDZ 12. Techniques known in the art may be used to block PDZ-domain-containing proteins, and thereby desensitize 5-HT2R signaling.
Inhibitors that may be useful to block PDZ-domain-containing proteins include immunological inhibitors such as antibodies, e.g. polyclonal or monoclonal antibodies. Antibodies may be prepared using known methods. Monoclonal antibodies are prepared using the well-established hybridoma technology developed by Kohler and Milstein (Nature 256, 495-497 (1975)). Hybridoma cells can be screened immunochemically for production of antibodies specifically reactive with a selected SOX9 region and the monoclonal antibodies can be isolated. The term “antibody” as used herein is intended to include fragments thereof which also specifically react with a SOX9 protein according to the invention, as well as chimeric antibody derivatives, i.e., antibody molecules resulting from the combination of a variable non-human animal peptide region and a constant human peptide region.
Methods which prevent expression of the PDZ-domain proteins may also be employed including the use of antisense oligonucleotides, and RNA interfering nucleotides, e.g. siRNA, shRNA and miRNA. The term “antisense oligonucleotide” as used herein means a nucleotide sequence that is complementary to at least a portion of a target PDZ-domain protein-encoding nucleic acid sequence. The term “oligonucleotide” refers to an oligomer or polymer of nucleotide or nucleoside monomers consisting of naturally occurring bases, sugars, and intersugar (backbone) linkages. The term includes modified or substituted oligomers comprising non-naturally occurring monomers or portions thereof, which function similarly. The term also includes chimeric oligonucleotides which contain two or more chemically distinct regions. The antisense oligonucleotides of the present invention may be ribonucleic or deoxyribonucleic acids and may contain naturally occurring or modified bases. SiRNA technology may also be employed in which nucleic acid fragments such as siRNA fragments that correspond with regions of a gene encoding a PDZ-domain protein are used to block expression.
Antisense, siRNA, and other inhibitory nucleic acid molecules may be readily prepared using well-established methods of nucleic acid syntheses, given that the structure of target PDZ-domain proteins is known in the art. It will be appreciated by one of skill in the art that such inhibitory nucleic acids may be derived from specific regions of the target gene to provide more effective inhibition of gene expression, for example, the 5′ end of the gene. In addition, as one of skill in the art will appreciate, useful nucleic acid fragments may not correspond exactly with the target gene, but may incorporate sequence modifications, for example, addition, deletion or substitution of one or more of the nucleotide bases therein, provided that the modified siRNA retains the ability to bind to the target gene and block expression. Once prepared, oligonucleotides determined to be useful to inhibit gene expression, may be used to desensitize 5-HT2R signalling. A suitable oligonucleotide may be introduced into tissues or cells of a mammal using techniques in the art including vectors (retroviral vectors, adenoviral vectors and DNA virus vectors) or by using physical techniques such as microinjection.
In one embodiment, the PDZ-domain-containing protein is the SAP97, or synapse-associated protein 97, which is also known as Disks large homolog 1 (DLG1). The amino acid sequence of this protein is set out in
An inhibitory agent determined to be useful to desensitize 5-HT2R signaling may be administered to a mammal in need of treatment using any suitable mode of administration including, but not limited to, oral, subcutaneous, intravenous, intraperitoneal, intranasal, enteral, topical, sublingual, intramuscular, intra-arterial, intramedullary, intrathecal, inhalation, ocular, transdermal, vaginal or rectal means.
An inhibitory agent determined to be useful to desensitize 5-HT2R signaling may be administered to a mammal in need of treatment alone or in combination with a suitable pharmaceutically acceptable carrier. The expression “pharmaceutically acceptable” means acceptable for use in the pharmaceutical and veterinary arts, i.e. not being unacceptably toxic or otherwise unsuitable. Examples of pharmaceutically acceptable carriers include diluents, excipients and the like. Reference may be made to “Remington's: The Science and Practice of Pharmacy”, 21st Ed., Lippincott Williams & Wilkins, 2005, for guidance on drug formulations generally. The selection of adjuvant depends on the type of inhibitor and the intended mode of administration of the composition. In one embodiment of the invention, the compounds are formulated for administration by infusion, or by injection either subcutaneously, intravenously, intrathecally, intraspinally or as part of an artificial matrix, and are accordingly utilized as aqueous solutions in sterile and pyrogen-free form and optionally buffered or made isotonic. Thus, the compounds may be administered in distilled water or, more desirably, in saline, phosphate-buffered saline or 5% dextrose solution. Compositions for oral administration via tablet, capsule or suspension are prepared using adjuvants including sugars, such as lactose, glucose and sucrose; starches such as corn starch and potato starch; cellulose and derivatives thereof, including sodium carboxymethylcellulose, ethylcellulose and cellulose acetates; powdered tragancanth; malt; gelatin; talc; stearic acids; magnesium stearate; calcium sulfate; vegetable oils, such as peanut oils, cotton seed oil, sesame oil, olive oil and corn oil; polyols such as propylene glycol, glycerine, sorbital, mannitol and polyethylene glycol; agar; alginic acids; water; isotonic saline and phosphate buffer solutions. Wetting agents, lubricants such as sodium lauryl sulfate, stabilizers, tableting agents, anti-oxidants, preservatives, colouring agents and flavouring agents may also be present. Aerosol formulations, for example, for nasal delivery, may also be prepared in which suitable propellant adjuvants are used. Other adjuvants may also be added to the composition regardless of how it is to be administered, for example, anti-microbial agents may be added to the composition to prevent microbial growth over prolonged storage periods.
In another aspect of the invention, SAP97 may be used as a tool to screen for therapeutic agents. Candidate inhibitors may be screened to identify compounds that alter the interaction between SAP97 and CRFR1 or 5HT2R, and which may then modulate 5HT2R signaling. A cell-based system may be used for such screening utilizing a CRFR1 or 5HT2R-expressing cell line. The candidate compound is incubated with a CRFR1 or 5HT2R-expressing cell line in the presence of SAP97, either co-expressed or otherwise present in the cells. The effect of the candidate compound on the interaction of SAP97 with CRFR1 or 5HT2R is determined and compared to a control in which a CRFR1 or 5HT2R-expressing cell line is incubated in the presence of SAP97 but in the absence of the candidate compound. For example, the effect on activation of ERK1/2 signaling by the CRFR1, binding of SAP97 to the PDZ binding motif of CRFR1 or 5HT2R or receptor endocytosis may be determined using techniques well-established in the art such as those described in the specific examples that follow. A determination of an effect of the candidate compound on the SAP97 interaction with CRFR1/5HT2R that differs from the control (e.g. normal) indicates that the candidate compound may be a potential therapeutic for use to desensitize 5-HT2R signaling.
Embodiments of the invention are described in the following specific example which is not to be construed as limiting.
Plasmid Constructs.
The FLAG-tagged human (h) 5-HT2CR plasmid construct was generated by PCR and subcloned into pcDNA3.1 and the FLAG-tagged human (h) 5-HT2AR plasmid construct was previously described, (Abbas et al. J. Neurosci. 29, 7124-7136 (2009)). The FLAG-5-HT2CR-ASSV and HA-CRFR1-ΔTAV mutant receptors were constructed using the using the QuikChange™ site-directed mutagenesis kit (Stratagene). The HA-CRFR1 and GFP-Rab constructs were described previously (Holmes et al. J. Neurochem. 96, 934-949 (2006)). The CRFR2 cDNA clone was the kind gift of Dr. Wylie Vale.
Cell Cuture and Transfection.
HEK 293 cells were maintained in Eagle's minimal essential medium supplemented with 10% fetal bovine serum and gentamicin (100 μg/ml). Cells were seeded on 100 mm dishes at 80-90% density one day before transfection. Transfection was carried out using a modified calcium phosphate method as described previously (Conn et al. J. Pharmacol. Exp. Ther. 234, 195-203 (1985)). After transfection (approximately 17 hours), cells were washed with phosphate buffered saline (PBS), pooled and reseeded on appropriate dishes. Primary prefrontal cortical neurons were prepared from E18 CD1 mouse embryos as described previously (Holmes et al. 2006). Rat cortical neurons (R-cx-500, QBM cell science, Ottawa, Canada) were thawed and cultured for 6 days as suggested by manufacturers, then transfected with 4 μg of plasmid DNA encoding each receptor using lipofectamine. The University of Western Ontario Animal Care Committee approved all animal protocols.
Inositol Phosphate Formation.
Inositol phosphate formation in HEK 293 cells and mouse cortical neurons was determined by labeling cellular inositol lipids with 1 μCi/ml [3H] myo-inositol as previously described (Dhami et al. J. Biol. Chem. 279, 16614-16620 (2004)). Cells were then preincubated in either the presence or absence of CRF peptide for 30 min at 37° C. and then stimulated with increasing concentrations (0-10 μM) of 5-HT for 30 min at 37° C. Total [3H] inositol phosphate was purified from cell extracts by anion exchange chromatography [3H] inositol phosphate formation was determined by liquid scintillation counting as previously described (Dhamie et al. ibid). For inositol phosphate formation assay in brain slices, the protocol described by Conn and Sanders-Bush, (ibid) was utilized with minor modifications. Briefly, prefrontal cortex was isolated and cross-chopped (350×350 μm). Slices were suspended in Krebs Ringer Buffer (KRB) (108 mM NaCl 4.7 mM KCl, 1.2 mM MgSO4, 1.2 mM KH2PO4, 2.5 mM CaCl2, 25 mM NaHCO3, 10 mM Glucose) and incubated for 30 min at 37° C. in a shaking bath under an atmosphere of O2/CO2 (95:5). Slices were then washed 3 times with 15 ml warm KRB and incubated with 5 μCi/ml [3H] myo-Inositol for 90 min (200 μl of gravity packed slices per ml of KRB). To remove excess radioactive inositol, slices were washed with 40 volumes of warm KRB containing 10 mM unlabeled myo-Inositol and allowed to settle under gravity. Buffer was aspirated off and 30 μl of gravity packed slices were aliquoted into tubes containing 240 μl of KRB containing 10 mM LiCl, 10 μM pargyline and ascorbic acid (100 μM). Slices were incubated for 15 min at 37° C. Following LiCl incubation, slices were preincubated in the presence or absence of 500 nM CRF peptide for 45 min at 37° C. (final volume=270 μl). Slices were then stimulated with 5-HT for 45 min (final volume=300 μl). The reaction was terminated by the addition of 3 volumes of chloroform/methanol (2:1, v/v) for 15 min at room temperature. One volume each of chloroform and HCl 0.15N was then added and the tubes were vortexed for 1 min. The phases were separated either by centrifugation at 1600 rpm for 5 min. Total inositol phosphate was purified from slice extracts by anion exchange chromatography as described above. Raw data was normalized for protein content which was measured in triplicate samples of prelabeled slices using the Bio-Rad Dc Protein Assay Kit following the manufacturer's instructions.
cAMP Assay.
Protocol was carried out as suggested by manufacturer. Briefly, HEK 293 cells transiently expressing FLAG-5-HT2AR and HA-CRFR1 were seeded into 96-well plate (10,000 cells/well). Two days after transfection, cells were incubated in the absence or presence of 10 μM 5-HT in induction buffer [HBSS, 500 μM isobutyl-1-methylxanthine (IBMX)] for 30 minutes at 37° C. Cells were then incubated with increasing concentrations of CRF peptide for 30 minutes. Following stimulation, cells were solubilized with cAMP-Glo lysis buffer for 15 minutes with gentle shaking at room temperature. Lysates were carefully transferred to a white opaque 96-well plate and cAMP-Glo Detection solution containing protein kinase A was added for 20 minutes at room temperature followed by addition of Kinase-Glo Reagent for 10 min. Luminescence was measured using a Victor Reader (Perkin-Elmer, Walthan, Mass.).
Immunofluorescence Microscopy.
Immunofluoroescence was done as previously detailed on wt or 5-HT2A KO mice (Abbas et al. ibid). In brief, mice were transcardialy perfused with 4% paraformaldehyde in 1×PBS. Brains were then harvested and placed 12 h in 4% paraformaldehyde in 1×PBS at 4° C. and then placed in 30% sucrose in 1×PBS until they sank, then frozen on dry ice and stored at −80° C. Sections (30 μm) were free-floating in 1×PBS (one per well in a 24-well plate) and then permeabilized with 0.4% Triton X100 in 1×PBS for 1 hour. PBS 1×/0.4% Triton X100 containing 0.1% glycine, 0.1% lysine, 1% BSA and 1% normal donkey serum. Primary antibodies (anti 5-HT2A, rabbit polyclonal, Neuromics cat #RA24288 and anti CRFR1, goat polyclonal, Abcam cat #ab59023) were incubated in blocking buffer for 72 hours at 4° C. Sections were then washed five times in 1×PBS/0.4% triton (10 min each). Hoechst (2-5 μg/ml) and secondary antibodies: donkey Alexa Fluor 555 conjugated anti-goat (1:500) and donkey Alexa Fluor 488 conjugated anti-rabbit antibodies (1:500) (Invitrogen) were diluted in blocking buffer and slices were incubated for 1 hour at RT. Sections were washed five times in 1×PBS/0.4% triton (10 min each). Sections were mounted on slides and visualized by Zeiss LSM-510 META multophoton laser scanning microscope with a Zeiss 25×NA 1.2 oil immersion lens and appropriate filters.
Biotinylation of Cell Surface Receptor.
HEK 293 cells transiently expressing wild-type and truncated FLAG-5-HT2AR and HA-CRFR1 were seeded into 100 mm dishes and pre-incubated for 30 minutes in HBSS. Cells were then treated for 30 min with or without 500 nM CRF, washed twice with ice-cold HBSS and placed on ice for biotin labeling. Cell surface receptors were labeled on ice with biotin (1 mg/ml) for 1 hour. Following labeling, cells were washed 3 times with 10 mM glycine and then 2 times with HBSS, lysed and equal amounts of total protein were incubated with neutravidin beads for 2 hours with rotation at 4° C. Beads were then washed 3 times with lysis buffer and one time with PBS. Proteins were eluted from beads by addition of 50 ul of SDS loading buffer. Samples were resolved by SDS-PAGE, transferred to nitrocellulose membrane and subsequently immunoblotted as described above with rabbit polyclonal FLAG antibody.
Surgical Procedure.
Male CD-1 mice were obtained from Charles River Canada (St. Constant, Quebec) at 50-60 days of age, and were acclimatized to the laboratory for approximately 30 days before serving as experimental subjects. Mice were housed four per cage, until the time of surgery, after which they were housed individually. The vivarium was maintained on a 12-h light/dark cycle in a temperature (21° C.) controlled room with food and water freely available. Mice were anesthetized using isoflurane and stereotaxic surgery (David Kopf Instruments Model 940) was performed to install a cannulae into the medial prefrontal cortex. A Guide cannulae (Plastics One In), situated according to the mouse atlas of Franklin and Paxinos at Lateral=0.32 mm, D/V=2.25 mm, A/P=+2.68 mm. A dummy cannula, was inserted flush with guide. Approximately 1 week after behavioral testing mice were perfused with 4% paraformaldehyde. Brains were subsequently sectioned at 14 microns and stained with Cresyl violet for probe placement verification. Only the data from mice with correct probe placements were used in the analysis of the behavioral tests.
Drug Treatments.
One week after surgical recovery animals were infused with 1.0 μl of CRF (1.5 μg) or vehicle (Phoenix Pharmaceuticals) over a 5 min period through an internal cannulae situated 0.3 mm below the guide cannulae. Drug diffusion was permitted for 5 min, and then after a further 5 min period mice were injected intraperotineally with DOI (Sigma) at a dose of 0.15 mg/kg or saline. Behavioral testing was conducted 15 min after the DOI treatment. In a second experiment the procedure was identical to that of the preceding study, except that mice were pretreated i.p. with either vehicle or 0.25 mg/kg of M100907 in a volume of 0.3 ml immediately prior to the DOI treatment. As in the preceding study mice were then tested in the elevated plus maze test (n=6-8)/group. Once again, data were obtained from videotapes and the researcher was blind as to the treatments mice had received.
Behavioral Testing.
In an initial test, mice were placed in a 45×45 cm open field, with an inner square of 21×24×24 cm., for a 5 min period, during which the time to enter the center area, and the total time spent in the center portion of the arena was recorded. The plus maze test was then conducted 1 min after the open field assessment. Mice were individually placed in one of the enclosed arms of a plus-maze and the behavior of the animals was recorded over a 5 min period by a ceiling-mounted video camera. The amount of time spent in each of the arms, the number of arm entries (an arm entry was defined as all four of the paws being placed in an arm of the plus-maze). The elevated plus-maze had two arms enclosed by 21 cm high walls; whereas the remaining two arms were open (arms were 24.8 cm long×7.7 cm wide). The maze was situated in a dimly lit room, such that the closed arms were darkened, whereas open arms were somewhat illuminated. All behavioural experiments were blinded. All experiments complied with the guidelines set by the Canadian Council on Animal Care and were approved by the Carleton University Animal Care Committee.
Data Analysis.
The mean and the standard error of the mean were expressed for values obtained from the number of separate experiments indicated. Dose response data were analyzed using GraphPad Prism (GraphPad Software). Statistical significance was determined by analysis of variance and corrected for multinositol phosphatele comparisons. For behavioural testing data were analyzed by either a two factor (Drug infusion and DOI treatment) or three factor (Drug indusion, DOI treatment and M100907) analysis of variance (ANOVA), as appropriate, independently for each of the outcome measures. Follow-up tests were conducted by Bonferonni t tests corrected to maintain the a at 0.05.
CRFR1 Activation Enhances 5-HT2R Signaling.
The mechanism by which CRFR1, a receptor coupled Gαs-stimulated cAMP accumulation, might alter the signaling of Gαq/11-coupled receptors (5-HT2AR and 5-HT2CR) that stimulate increases in inositol phosphate formation was studied. Human embryonic kidney (HEK 293) cells that do not express endogenous CRFR1 or 5-HT2Rs were used to examine whether CRFR1 activation altered 5-HT2R signaling. In HEK 293 cells transfected to express either 5-HT2AR or 5-HT2CR in the absence of CRFR1, the treatment of cells with increasing concentrations of 5-HT resulted in a dose-dependent increase in inositol phosphate formation and pretreatment with CRF had no effect on the dose-response curves for inositol phosphate formation for either receptor (
It was essential to establish whether the augmented 5-HT2R signaling in response to CRF was observed in prefrontal cortical neurons. Therefore, it was determined whether or not both receptors were expressed in neurons from the prefrontal cortex of mice. Mouse prefrontal cortical slices were stained with polyclonal antibodies that recognized either endogenous 5-HT2AR or CRFR1 and Hoechst to mark cell nuclei. It was found that a subpopulation of neurons in the prefrontal cortex stained positive for both 5-HT2AR and CRFR1 protein. The specificity of the 5-HT2AR antibody was confirmed in parallel Western blot and immunofluorescent studies of prefrontal cortex from 5-HT2AR knockout mice. CRFR1 antibody specificity was confirmed in HEK293 cells expressing HA-CRFR1. CRF (500 nM) pretreatment of mouse neuronal cultures for 30 min was found to significantly increase 5-HT (50 μM)-stimulated [3H]-myo-inositol conversion to inositol phosphate. Importantly, in slices prepared from prefrontal cortex CRF pretreatment increased 5-HT-stimulated inositol phosphate formation by 2.3±0.2 fold and when the 5-HT2A/CR selective agonist 2,5-dimethoxy-4-iodoamphetamine (DOI; 10 μM) was used, CRF pretreatment increased inositol phosphate formation by 1.5±0.2 fold. Thus, consistent with what was observed in an overexpression system the pretreatment of endogenous CRF receptor increased 5-HT/DOI-stimulated inositol phosphate formation in prefrontal neuronal cultures and tissue.
The sensitization in 5-HT2R signaling was unique to CRFR1 and was independent of the activity of second messenger-dependent protein kinases activated by either receptor. Therefore, it was examined whether or not agonist-stimulated CRFR1 internalization contributed to the sensitization of 5-HT2R signaling. It was first determined whether the expression of a dominant-negative inhibitor of clathrin-mediated endocytosis (dynamin I-K44A) altered CRFR1-mediated increases in 5-HT2AR signaling in HEK 293 cells. Dynamin I-K44A expression completely eliminated CRFR1-dependent increases in 5-HT2AR-stimulated inositol phosphate formation following CRF pretreatment (
HA-epitope tagged CRFR1 and FLAG-epitope tagged 5-HT2R that were immunofluorescently labeled at the cell surface at 4° C. were localized and then allowed to warm to 37° C. in both HEK 293 cells and rat cortical neurons. Both FLAG-5-HT2AR and FLAG-5-HT2CR were internalized from the cell surface in the absence of agonist, whereas no constitutive endocytosis was observed for the HA-CRFR1. Similarly, in transfected neurons FLAG-5-HT2AR, but not CRFR1 was observed to internalize from the cell surface in the absence of agonist treatment. In contrast, when rat cortical neurons were warmed to 37° C. and treated with 100 nM CRF both HA-CRFR1 and FLAG-5-HT2AR (untreated) were endocytosed and were colocalized within the same intracellular vesicles. Similar to what was observed for the HA-CRFR1, agonist-stimulated HA-β2AR also colocalized with FLAG-5-HT2AR in vesicles after isoproterenol treatment, but this does not translate into an alteration in 5-HT2AR signaling (
To further assess the role of the intracellular trafficking of both the 5-HT2AR and CRFR1 in the CRF-dependent regulation of 5-HT2AR signaling, inhibition of receptor recycling with monensin was tested to determined if it would block CRF-mediated increases in 5-HT2AR signaling. Treatment of cells with 100 μM monensin did not affect 5-HT2AR signaling in the absence of CRF pretreatment (
All three receptors encoded class I PDZ domain interacting motifs at the end of their carboxyl-terminal tails and both the 5-HT2AR and 5-HT2CR. Therefore, it was examined whether the deletion of three amino acids from the 5-HT2AR (ΔSCV) and CRFR1 (ΔTAV) carboxyl-terminal tails would affect cell surface recruitment of the 5-HT2AR following CRF treatment. When tested, the deletion of either the 5-HT2AR or CRFR1 PDZ domain binding motifs attenuated the CRF-dependent increases in 5-HT2AR at the cell surface (
To assess the role of CRF in the regulation of 5-HT2R-mediated anxiety behaviour, two anxiety-related behaviours were examined in mice: (1) the latency for mice to enter the center of an open field and (2) the latency for mice to enter the open arm of an elevated plus maze. Having established the molecular mechanism by which CRFR1 activation sensitized 5-HT2R responses in vitro, it was determined whether or not the infusion of CRF peptide (1.5 μg) into the medial prefrontal cortex followed by the intraperitoneal administration of the 5-HT2R selective agonist DOI (0.15 mg/kg) would sensitize 5-HT-mediated anxiety-related behavioral responses. The latency of mice to enter the center of an open field varied as a function of the intracerebral infusion (CRF vs vehicle) x systemic challenge (DOI vs vehicle) interaction, F(1,35)=7.01, p<0.01. Follow-up analysis of the mean latencies for mice to enter the center square in a 5 min open field test revealed that neither the CRF nor the DOI treatments alone influenced performance relative to the vehicle-vehicle condition (
In a follow up series of experiments it was determined whether or not the synergistic effects of DOI and CRF treatment could be antagonized by the pretreatment of mice with the 5-HT2AR selective antagonist M100907. The latencies to enter the open arms of the plus maze varied as a function of the DOI×CRF×M100907 interaction, F(1,41)=6.00, p=0.018 (
The analysis of both the number of open-arm entries and the time spent in the open arms revealed responses which paralleled that of the response latencies (
The analysis of the entries to the closed arms indicated that behavior was significantly influenced by the DOI×CRF×M100907 interaction, F(1,41)=9.29, p<0.01 (
CRF was shown to act through CRFR1 to sensitize 5-HT2R-mediated signaling and anxiety behaviours thereby linking CRF-mediated stress responses to anxiety and depression. The present findings indicate that enhanced 5-HT2R sensitivity following CRF pretreatment in vivo was evidenced by increased anxiety-related behaviour in mice. This observation showed that CRF could potentiate 5HT2R mediated behaviours and has implications regarding the mechanisms by which stressors may exacerbate the anxiogenic effects of 5HT2R activation. Importantly, the behavioural data, which showed a functional interaction between CRF and 5-HT, were supported at the cellular level. Thus, it was demonstrated both that CRFR1 activation positively modulated 5-HT2R signaling in cortical neurons and that these two receptors were co-expressed in the same neuronal populations. The molecular mechanism underlying the sensitization of 5-HT2R signaling by CRFR1 required agonist-stimulated CRFR1 endocytosis and recycling which resulted in increased cell surface expression of 5-HT2Rs and increased second messenger responses to 5-HT treatment. These findings provide an additional mechanism by which receptor endocytosis and recycling contribute to the regulation of GPCR responsiveness in general and specifically show how CRFR1 activation can positively modulate 5-HT2R signaling thereby leading to pathophysiological behavioural responses.
Anxiety responses in both an open field emergence and in a plus-maze test were shown to be sensitized in mice that were pretreated with CRF administered to the prefrontal cortex, followed by systemic administration of a low dose of DOI. When administered alone, neither of these treatments affected performance in these tests, demonstrating that the CRF and DOI treatments acted synergistically to provoke the anxiety responses. The behavioral change could not be attributed to diminished motoric activity, as entries into the closed arms of the plus-maze were unaffected by the treatments. It should be said that when significantly higher doses of DOI were employed (0.625 and 1.25; data not shown) elevated arm entries were evident (as opposed to reduced open-arm entries), likely reflecting an overall arousal.
Thus, both the CRF and 5-HT systems when sufficiently activated will independently lead to anxiety responses. The 5-HT2AR selective antagonist M100907 itself also provoked marked reductions of open arm entries suggesting that M100907 could independently induce an anxiety-like response. As entries into the closed arm were observed, it was clear that the absolute failure to enter the open arm was not due to motor impairments, and instead it was likely that the reduced activity reflected an overall increase of anxiety. However, of particular significance, was the observation that the anxiety-provoking effects of CRF and DOI cotreatment were antagonized by M100907 pretreatment. Thus, the observations indicated that cross-talk between CRF- and 5-HT-mediated signaling processes occurred in the prefrontal cortex and that CRF sensitized 5-HT2-processes to promote stressor-like effects, such as anxiety.
Based on the present data, a multistep mechanism is proposed whereby CRF peptide activation of CRFR1 enhances 5-HT2R signaling by increasing the availability of 5-HT2R at the surface of cells to be activated by agonist and to couple to the activation of phospholinositol phosphatase Cβ-mediated inositol phosphate formation. Agonist-activation of CRFR1 promoted the dynamin-dependent internalization of CRFR1 into the intracellular endosomal compartment of the cell and 5-HT2AR and 5-HT2CR were internalized to endosomes in a constitutive manner. Thus, following agonist treatment internalized CRFR1 facilitated the cell surface recycling of 5-HT2R from endosomes resulting in increased 5-HT2R protein at the cell surface. The CRFR-dependent enhancement of 5-HT2R signaling also required the interaction of PDZ domain containing proteins with both receptors, since the deletion of PDZ binding motifs in the carboxyl-terminal tail domains of either CRFR1, 5-HT2AR or 5-HT2CR prevented CRF-mediated sensitization of 5-HT2R signaling. Interestingly, the activation of CRFR2, another CRFR expressed in the brain, did not sensitize 5-HT2AR signaling and consistent with this observation examination of the CRFR2 carboxyl-terminal tail revealed that the canonical PDZ binding motif was disrupted.
Sensitization of 5-HT2R signaling was dependent on receptor endocytosis as dynamin I-K44A expression could block this effect. This suggested that the internalization of either the CRFR1 or the 5-HT2Rs was essential for sensitizing 5-HT2R signaling. Several lines of evidence suggest that it is the internalization of CRFR1 that is essential for this effect. First, both 5-HT2AR and 5-HT2CR are found to be predominantly intracellular in neurons of the rat prefrontal cortex. Second, in the present study, it was found that both 5-HT2AR and 5-HT2CR were constitutively internalized in both HEK 293 cells and neurons, although cell surface expression of 5-HT2AR has been reported. However, the mechanism underlying the observed constitutive endocytosis was unclear and may be a consequence of the fact that the serum used to culture cells may contain 5-HT. Independent of the mechanism by which 5-HT2R were internalized, it is proposed that it was the internalization and recycling of the CRFR1 that dynamically regulated the subcellular equilibrium of 5-HT2R resulting in the redistribution of 5-HT2R to the cell surface resulting in the sensitization of 5-HT2R signaling.
The CRFR1-mediated increases in 5-HT2AR signaling were also blocked by either the treatment of cells with monensin, which prevents the trafficking of intracellular vesicles or the overexpression of a dominant-negative Rab4-S28N mutant protein that blocked rapid recycling of GPCRs to the cell surface. Thus, CRFR1 sensitization of 5-HT2R signaling required increased 5-HT2R recycling and cell surface expression. The intracellular localization of 5-HT2R may prevent over-stimulation of serotonergic synapses. The regulated recruitment of this intracellular pool of 5-HT2R may function to promote altered post-synaptic signal adaptation to physiological stimuli, such as CRF peptide release in response to stress leading to the activation of CRFR1 in 5-HT2R expressing neurons of the prefrontal cortex.
CRFR1-dependent alterations in 5-HT2R signaling required intact PDZ binding motifs at the carboxyl-terminal tails of both CRFR1 and 5-HT2Rs. Examples of PDZ domain containing proteins that interact with both 5-HT2AR and 5-HT2CR include MAGI-2, MPP3, MUPP1, PSD-95 and SAP97. Each of these PDZ domain containing proteins are comprised of multiple PDZ domains that would allow them to form complexes with more than one GPCR.
Materials:
Goat anti-glutathione-S-transferase (GST) antibodies as well as ECL Western blotting detection reagents were purchased from GE Healthcare (Oakville, ON, Canada). Rabbit anti-phospho-p44/42 MAP kinase (Thr202/Tyr402), and rabbit anti-p44/42 MAP kinase antibodies were obtained from Cell Signalling Technology (Pickering, ON, Canada). Rabbit anti-GFP antibody was obtained from Invitrogen/Life Technologies (Burlington, ON, Canada). Mouse anti-SAP97 antibody was obtained from Assay Designs/Enzo Life Sciences (Farmingdale, N.Y., USA). Alexa Fluor 647 anti-mouse IgG and Alexa Fluor 633 goat anti-mouse IgG Zenon antibodies were purchased from Invitrogen/Molecular Probes (Burlington, ON, Canada). cAMP GLO Assay was obtained from Promega (Madison, Wis., USA). Mouse anti-HA antibody and all other biochemical reagents were purchased from Sigma-Aldrich (Oakville, ON, Canada).
Plasmid Constructs:
The HA-CRFR1 and HA-CRFR1ΔTAV receptors were described previously (Holmes et al. (2006) J. Neurochem. 96, 934-949). The YFP-SAP97 and SAP97 single hairpin RNA (shRNA) constructs were graciously provided by Dr. Suleiman W. Bahouth (Neuroscience Institute, University of Tennessee Health Sciences Center) (Gardner et al. (2007) J. Biol. Chem. 282, 5085-5099.). The YFP-SAP97 was subcloned into the pEGFP1 vector. The EPAC cAMP biosensor was the gift of Drs. Ali Salahpour (University of Toronto) and Marc Caron (Duke University) (Barak et al. (2008) Mol. Pharmacol. 74, 585-594). The CRFR1 carboxyl-terminal tail was cloned into pGEX-4 with ECOR1/NotI.
Cell Culture and Transfection:
Human embryonic kidney (HEK 293) cells were maintained in Eagle's minimal essential medium supplemented with 10% fetal bovine serum. Cells were seeded on 10 cm dishes at 70-80% density 24 h prior to transfection. Transfection was performed using a modified calcium phosphate method, as described previously (Ferguson et al. (2004) Methods Mol. Biol. 237, 121-126). 1 μg of each construct was used for each transfection, with the exception that 3 μg of plasmid cDNA was used for all shRNA constructs. Empty pcDNA3.1 vector was used to equalize the total amount of plasmid cDNA used to transfect cells. 18 h post-transfection, cells were washed with phosphate buffered saline (PBS) and re-suspended with Trypsin 0.25% EDTA. Cells were then reseeded for experimentation. All experiments were conducted approximately 48 h after the initial transfection, with exception of transfections involving SAP97 shRNA, which were conducted 72 h after initial transfection to optimize the knockdown of endogenous SAP97, as confirmed by Western blotting.
PDZ Blot Overlay Assay:
GST and GST-CRFR1 peptides were generated by growing recombinant BL21 bacteria at 21° C. to an A600 of 0.6-1.0. Cultures were induced for 3 hrs with 1 mM IPTG, pelleted, resuspended in PBS containing protease inhibitors (1 mM AEBSF, 10 g/ml leupeptin, and 5 μg/ml aprotinin) and lysed by mild sonication. The bacterial lysates were cleared of cellular debris by centrifugation and then applied to Glutathione Sepharose 4B overnight at 4° C. GST and GST-CRFR1 peptides bound to the matrix were washed extensively in PBS-containing 0.3% Triton X-100. 100 nM of GST and GST-CRFR1 peptide in blot buffer (2% nonfat dry milk, 0.1% Tween 20, 50 mM NaCl, 10 mM Hepes, pH 7.4) were incubated with gridded nylon membranes that were spotted with His/S-tagged PDZ domain fusion proteins (1 μg/bin) for 1 h at room temperature. The arrays were. The arrays were then washed three times with blot buffer, and incubated with a horseradish peroxidase-conjugated anti-GST antibody (1:3000). Interactions of the GST fusion proteins with the various PDZ domains were then visualized via chemiluminescence using the enhanced chemiluminescence kit from GE Healthcare.
Co-Immunoprecipitation:
Transfected HEK 293 cells were seed on 10 cm dishes the day before the experiment. Cells were serum-starved for 1 hour in HEPES-buffered saline solution (HBSS), and dishes were treated with either HBSS alone or with 100 nM CRF agonist in HBSS for 30 min at 37° C. Cells were subsequently lysed in lysis buffer (50 mM Tris, pH 8.0, 150 mM NaCl, and 0.1% Triton X-100) containing protease inhibitors (1 mM AEBSF, 10 μg/ml leupeptin, and 5 aprotinin) for 20 min on a rocking platform at 4° C. Samples were collected into 1.5 mL Eppendorf tubes and centrifuged at 15,000 g for 15 min at 4° C. to pellet insoluble material. A Bronsted-Lowry protein assay was performed and 400 μg of protein was incubated for 1-2 h at 4° C. with Protein G Sepharose and mouse anti-HA antibody (1:50). After incubation, beads were washed 3 times with cold lysis buffer and incubated overnight at room temperature in 3×SDS Loading Buffer containing 2-mercaptoethanol. Samples were separated by SDS-PAGE, transferred to a nitrocellulose membrane, and immunoblotted to identify co-immunoprecipitated GFP-SAP97 (rabbit anti-GFP, 1:1000). An additional Western blot was performed to examine HA-CRFR1, HA-CRFR1ΔTAV (mouse anti-HA, 1:1000) and GFP-SAP97 (rabbit anti-GFP, 1:1000) protein expression.
Live HEK293 Cell Immunofluorescent Confocal Microscopy:
Following transfection, HEK 293 cells were re-seeded on 35-mm glass-bottom confocal dishes. Cells were serum-starved for 1 hour at 37° C. in HBSS, then labeled with mouse anti-HA antibody (1:200) and Zenon Alexa Fluor 647 mouse IgG1 Labeling Kit (Invitrogen) at 4° C. for 30 minutes. The cells were washed with HBSS and warmed to 37° C. for live imaging using a heated stage. Confocal microscopy was performed on a Zeiss LSM-510 META laser scanning confocal microscope using a Zeiss 63×, 1.3 NA, oil immersion lens. Co-localization studies were performed using dual excitation (488 nm, 633 nm) and emission (band pass 505-550 nm and long pass 650 nm for YFP/GFP and Alexa Fluor 647, respectively) filter sets. The specificity of labeling and absence of signal crossover were established by examination of single-labeled samples. Receptor endocytosis experiments were additionally stimulated with 500 nM CRF agonist (Tocris) and specified cells were re-imaged at regular intervals for up to 60 minutes.
Receptor Endocytosis:
Following transfection, HEK 293 cells were re-seeded into 12-well plates. Cells were serum-starved for 1 h at 37° C. in HBSS and then stimulated for certain periods of time with or without 500 nM CRF in HBSS at 37° C. Cells were washed with cold HBSS and treated with mouse anti-HA antibody (1:500) for 45 min on ice. Cells were washed with cold HBSS and additionally treated with Alexa Fluor 633 goat anti-mouse IgG (Invitrogen) (1:500) for 45 min on ice. Cells were washed with cold PBS and treated with 5 mM EDTA in PBS for 5 min on ice. Newly suspended HEK 293 cells were then transferred to flow cytometry tubes containing 4% formaldehyde in PBS. Samples were run on a FACSCalibur cytometer using BD CellQuest Pro software until 10,000 cells were counted. The geometric mean of fluorescence was determined using FlowJo analysis software, with less fluorescence corresponding to less CRFR1 on the membrane.
cAMP Assay:
The cAMP GLO Assay protocol was carried out as suggested by the manufacturer (Promega). Transfected HEK 293 cells were seeded into 96-well plate (˜10,000 cells per well). Cells were incubated in induction buffer (HBSS with 500 μM isobutyl-1-methylxanthine (IBMX) and increasing concentrations of CRF agonist for 30 min at 37° C. Following stimulation, cells were solubilized with cAMP-GLO Lysis Buffer for 15 min with gentle shaking at 20-23° C. cAMP-GLO detection solution containing protein kinase A was added for 20 min at 20-23° C., followed by the addition of Kinase-Glo Reagent for 10 min. Each solution was carefully transferred to a white, opaque, 96-well plate and Luminescence was measured using a Victor Plate Reader (Perkin-Elmer). SAP97 knockdown experiments were additionally performed using a BRET-based biosensor (EPAC) for cAMP and the protocol was adapted from Barak et al. (2008) Mol. Pharmacol. 74, 585-594.
ERK Phosphorylation:
Following transfection, HEK 293 cells were re-seeded into 6-well plates. Cells were serum-starved for 1 hour at 37° C. in HBSS and then stimulated with 500 nM CRF agonist for the duration of the described time-points. Cells were lysed with lysis buffer (50 mM Tris, pH 8.0, 150 mM NaCl, and 0.1% Triton X-100) containing protease inhibitors (1 mM AEBSF, 10 μg/ml leupeptin, and 5 μg/ml aprotinin) for 20 min on a rocking platform at 4° C. Samples were collected into 1.5 mL Eppendorf tubes and centrifuged at 15,000 g for 15 min at 4° C. to pellet insoluble material. A Bronsted-Lowry protein assay was performed and 50 μg of protein was incubated overnight at room temperature in 3×SDS Loading Buffer containing 2-mercaptoethanol. Samples were separated by SDS-PAGE, transferred to a nitrocellulose membrane, and immunoblotted for ERK1/2 (rabbit anti-p44/42 mitogen-activated protein kinase (MAPK), 1:1000), phospho-ERK1/2 (rabbit anti-phospho-p44/42 MAPK, 1:1000), SAP97 (mouse anti-SAP97, 1:1000), and HA-CRFR1 expression (mouse anti-HA, 1:1000), followed by a horseradish peroxidase-conjugated secondary anti-rabbit antibody (1:10,000) or anti-mouse antibody (1:10,000) where appropriate. Proteins were detected using chemiluminescence with the enhanced chemiluminescence kit from GE Healthcare.
Statistical Analysis:
Densitometric data were normalized first for protein expression and the maximum value was set to 100, with all other values displayed as a percentage thereof. One-way analysis of variance test (ANOVA) was performed to determine significance, followed by a post-hoc Tukey multiple comparison test or Bonferroni's multiple comparisons test to determine which means were significantly different (p<0.05) from one another.
Proteomic Analysis of CRFR1-Interacting PDZ Proteins:
Array of 96 class I PDZ domains spotted on a gridded nylon membrane, as described in Fam et al. (2005). Proc. Natl. Acad. Sci. U.S.A. 102, 8042-8047; and He et al. (2006) J. Biol. Chem. 281, 2820-2827, were used to identify potential CRFR1 interacting PDZ domain-containing proteins. The PDZ array was overlaid with either purified glutathione-S-transferase (GST)-CRFR1-carboxyl-terminal tail or GST (as a control) and as can be observed in
SAP97 is Co-Immunoprecipitated with CRFR1 in a PDZ-Binding Motif-Dependent Manner:
SAP97 was one of the candidate CRFR1 binding proteins identified in the proteomic PDZ domain screen (
SAP97 Recruitment to the Plasma Membrane is CRFR1 PDZ Binding Motif-Dependent:
When expressed alone in HEK 293 cells GFP-SAP97 was diffusely localized throughout the cytoplasm and did not exhibit localization to the plasma membrane (data not shown). However, when GFP-SAP97 was co-expressed with HA-CRFR1, the GFP-SAP97 was predominantly localized with the receptor at the plasma membrane. When the CRFR1 PDZ motif was deleted from the carboxyl-terminus of the receptor (ΔTAV) the resulting HA-CRFR1-ΔTAV mutant did not promote the plasma membrane localization of GFP-SAP97. Thus, this data in combination with the co-immunoprecipitation data indicated that SAP97 was a CRFR1 interacting protein and that this interaction was dependent upon the CRFR1 PDZ binding motif SAP97 antagonizes CRFR1 endocytosis in a PDZ motif-dependent manner: PDZ interactions have been reported to regulate the endocytosis and trafficking of several GPCRs. Therefore, the effect of overexpressing GFP-SAP97 on the endocytosis of the wild-type CRFR1 and the CRFR1 mutant lacking a PDZ binding motif (ΔTAV) were examined. In cells expressing only wild-type CRFR1, agonist treatment for 30 min with 500 nM CRF at 37° C. resulted in a 24±4% loss of cell surface HA-CRFR1 as measured by flow cytometry (
SAP97 Co-Localizes with CRFR1 During Receptor Endocytosis:
The overexpression of GFP-SAP97 antagonized HA-CRFR1 endocytosis, but did not completely block the internalization of the receptor. Therefore, it was examined whether or not internalized HA-CRFR1 was either internalized as a complex with GFP-SAP97 or was internalized independently of GFP-SAP97. To do this, HEK 293 cells were transfected with both HA-CRFR1 and GFP-SAP97 and the HA-CRFR1 was labeled with Alexa Fluor 633-conjugated monoclonal HA mouse antibody (1:1000 dilution) for 45 min on ice. Labeled cells were then live cell imaged by laser scanning confocal microscopy. Each cell was allowed to warm to 37° C. and was imaged prior to the addition of 500 nM CRF and then was consecutively imaged every 30 s for 20 min. It was found that prior to agonist treatment, Alexa Fluor 633-conjugated mouse monoclonal HA antibody-labeled CRFR1 was colocalized with GFP-SAP97 at the cell surface. However, upon CRF treatment limited internalization of HA-CRFR1 was observed 30 min after agonist stimulation and internalized HA-CRFR1 that was internalized was co-localized with GFP-SAP97 in endocytic vesicles. This indicated that the pool of CRFR1 that could be endocytosed redistributed GFP-SAP97 into the endosomal compartment, despite the role for SAP97 in antagonizing CRFR1 endocytosis.
SAP97 does not Regulate CRFR1-Mediated cAMP Signaling:
Because SAP97overexpression antagonized CRFR1 internalization and SAP97 down-regulation enhanced CRFR1 endocytosis, it was determined whether or not either SAP97 or the CRFR1 PDZ binding motif contributed to the regulation of CRFR1-mediated cAMP formation. In cells transfected with HA-CRFR1 with and without GFP-SAP97 and treated with increasing doses of CRF, there was no significant change in the maximum efficacy for CRF-stimulated cAMP formation (
CRFR1-Mediated ERK1/2 Phosphorylation is Dependent on Endogenous SAP97 Expression:
It was then examined whether endogenous SAP97 expression was required for CRFR1-mediated ERK1/2 phosphorylation. HEK293 cells were transiently transfected with and without HA-CRFR1 along with either scrambled shRNA or SAP97 shRNA to knockdown SAP97 expression. ERK1/2 phosphorylation in response to 500 nM CRF for 0, 2, 5, 15 and 30 min was determined by densitometric analysis of immunoblots. The treatment of non-transfected HEK 293 cells with 500 nM CRF led to an increase in detectable ERK1/2 phosphorylation at 5 min, which was likely due to endogenous CRFR2 that is expressed in these cells. However, in cells transfected with HA-CRFR1 and scrambled shRNA, 500 nM CRF treatment resulted in a more robust and sustained activation of ERK1/2 phosphorylation. shRNA knockdown of SAP97 protein expression led to attenuated CRFR1-mediated ERK1/2 phosphorylation following 500 nM CRF treatment, to levels that were comparable to those observed in non-transfected cells. The over-expression of GFP-SAP97 had no significant effect on ERK1/2 phosphorylation (data not shown). Taken together, these data suggested that SAP97 plays a direct role in regulating the activation of ERK1/2 signaling by the CRFR1, without modulating G protein coupling.
A subset of Class I PDZ proteins, including SAP97, have been found to interact with the CRFR1 C-tail on a proteomic PDZ domain array. SAP97 interacts with the carboxyl-terminal CRFR1 PDZ binding motif resulting in the recruitment of SAP97 to the cell surface. SAP97 functions to antagonize CRFR1 endocytosis, couples the receptor to the activation of the ERK1/2 signaling pathway, but does not alter CRFR1-mediated G protein coupling.
This is the first evidence for SAP97 as a regulator of CRFR1 trafficking and signaling. Specifically, SAP97 interacts with the CRFR1 PDZ binding motif to antagonize CRFR1 endocytosis and couples the receptor to the activation of ERK1/2 signaling without affecting G protein coupling.
The relevant portions of all references referred to herein are incorporated herein by reference.
This application claims the benefit of U.S. Provisional Patent Application No. 61/473,855, filed on Apr. 11, 2011, and incorporates such provisional patent application in its entirety by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61473855 | Apr 2011 | US |
