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DISC1 and SNAP23 Emerge In NMDA Receptor Signaling

10 February 2010. The hypothesis that schizophrenia results from underachieving N-methyl D-aspartate (NMDA) receptors or, at the very least, disturbed glutamate neurotransmission, has spurred researchers to seek ways to tweak NMDA-R function. Two new studies in Nature Neuroscience may clarify some of the mechanisms involved. Akira Sawa and colleagues find that DISC1, a gene associated with schizophrenia and other major mental disorders in a large Scottish family, helps regulate the plasticity of dendritic spines in response to NMDA-receptor activation. Stimulating NMDA receptors causes DISC1 to limit kalirin-7’s access to the GTPase Rac1, thereby affecting the formation of dendritic spines. In other work, Paul Roche, Katherine Roche, and others report that SNAP23, a relative of the schizophrenia-associated synaptic trafficking protein SNAP25, regulates NMDA receptors in the post-synaptic membrane. Together, the studies offer new information on the details of NMDA receptor regulation and potential insights into how that function might be altered, or treated, in schizophrenia.

The disrupted-in-schizophrenia 1, or DISC1, gene sits at the crossroads of various pathways, where it binds proteins involved in neural development and signaling (see SRF related news story; see SRF related news story). Support for its importance in schizophrenia comes from multiple lines of genetic and biological research (Porteous et al., 2006; also see SRF live discussion).

In the brain, most excitatory synaptic neurotransmission takes place on dendritic spines, but it has been reported that patients with schizophrenia have too few of these structures (see SRF related news story), perhaps due to DISC1 abnormalities (see SRF related news story). Sawa, first author Akiko Hayashi-Takagi, both of Johns Hopkins University in Baltimore, Maryland, and their colleagues sought to understand how DISC1 governs the building of these spines. Their paper appeared on February 7 in an advance online publication of Nature Neuroscience.

To study the effects of DISC1, the researchers injected lentivirus carrying two short-hairpin RNAs (shRNAs) into the prefrontal cortex of adult rats to knock down expression of DISC1 and its main isoforms. Two days later, dendritic spines had multiplied and grown. Furthermore, expression of the AMPA-type glutamate receptor subunit GluR1 on the cell surface had increased, along with the frequency of miniature excitatory post-synaptic currents (mEPSCs).

Overseeing construction
Hayashi-Takagi and colleagues asked if these effects could result from DISC1 interacting with two of its known partner proteins, kalirin-7 and post-synaptic density-95 (PSD-95). Prior evidence suggests that kalirin-7, a guanine nucleotide exchange factor, is necessary for neurons to remodel dendritic spines in response to activity (see SRF related news story), and kalirin-7 knockout mice display changes in glutamatergic signaling, cognition, and behavior reminiscent of schizophrenia (see SRF related news story). PSD-95 binds with NMDA receptors and kalirin-7 (see SRF related news story).

Hayashi-Takagi and colleagues used immunoprecipitation to identify DISC1’s binding partners in cortical neurons and rat brains. As they predicted, DISC1 interacted with kalirin-7 and PSD-95. It did not interact with guanine nucleotide exchange factors other than kalirin-7.

The researchers did not stop there. To learn which domain of DISC1 binds with kalirin-7, they studied the effects of deleting various DISC1 segments. They noted that binding did not occur if DISC1 was missing a certain sequence of 44 amino acids. Overexpressing DISC1 reduced spine size in neurons, but only if it contained that key segment. These results support the idea that DISC1 and kalirin-7 binding regulates spines.

Since NMDA receptor activation launches kalirin-7 signaling, the researchers applied electroconvulsive treatment to activate neurons in homogenized brain tissue. This lessened interactions of kalirin-7 with PSD-95, kalirin-7 with DISC1, and DISC1 with PSD-95. To confirm that the decreased binding resulted from specific activation of NMDA receptors, the investigators also targeted the receptors by withdrawing the inhibitory influence of amino-5-phosphonovaleric acid. This tactic destroyed the complex that DISC1 had formed with PSD-95 and kalirin-7. With kalirin-7 now freed from DISC1’s grip, it triggered Rac1 to control spine formation. In the longer term, the investigators showed that disruption of DISC1 expression led to spine shrinkage in rat primary cortical neurons, which could be rescued by full-length DISC1 but not the kalirin binding domain mutant.

Controlling traffic
Changes in SNARE (soluble N-ethylmaleimide-sensitive-factor attachment protein receptor) proteins, which regulate vesicle trafficking and neurotransmitter release, might also help explain the synaptic abnormalities seen in schizophrenia (for a review, see Johnson et al., 2008). Studies show altered levels of synaptosomal-associated protein 25 (SNAP25), in various brain regions in the disorder (see Corradini et al., 2009; also see SRF related comments). Mice induced to express mutant human DISC1 show decreased SNAP25 levels and a schizophrenic phenotype (Pletnikov et al., 2008). Furthermore, polymorphisms in the gene encoding SNAP25 have been associated with schizophrenia in some studies (see SZGene entry for SNAP25). Prior research finds SNAP25 (see SRF related news story) expression mainly in presynaptic membranes in brain neurons. There it forms part of the assembly that moves vesicles holding neurotransmitter to the cell membrane. By binding with other SNARE proteins, it joins the vesicle membrane to the cell membrane, releasing neurotransmitter into the synapse.

Unlike SNAP25, the structurally similar SNAP23 appears throughout the body (Ravichandran et al., 1996), including the brain. Paul Roche, Katherine Roche, and their colleagues at the National Institutes of Health in Bethesda, Maryland, raise an interesting question: If SNAP25 abounds in the brain and binds relatively well with other SNARE proteins, what is the similar SNAP23 doing there? Their paper in Nature Neuroscience, published online on January 31, finds SNAP25 only at presynaptic sites in the neuron, while its cousin works the other side of the synapse. The researchers, including first author Young Ho Suh, show that SNAP23 governs the transport and function of NMDA receptors.

Different neighborhoods
Suh and colleagues started by seeing where the two proteins appeared in rat hippocampal neurons in culture. After labeling the cells with antibodies for SNAP23 and SNAP25, they found SNAP25 in the axons and SNAP23 in the cell body and dendrites. Indeed, the two proteins seemed to have staked out their own separate turfs. As Suh and colleagues write, “The exclusive localization of SNAP23 on dendrites suggests that SNAP23 is involved in post-synaptic membrane trafficking events.”

To follow up on this finding, Suh and colleagues differentially centrifuged brain homogenate from rats to see which cell parts associated with the two proteins. SNAP25 fell out with the fraction that was enriched with synaptic vesicles. In contrast, SNAP23’s distribution echoed that of elements of the post-synaptic density. It showed up at the same sites as post-synaptic density-95, as well as NMDA receptor subunits NR2A and NR2B.

The researchers used light and electron microscopy to learn more about SNAP23’s presence within neurons. It revealed SNAP23 enrichment near NMDA and AMPA glutamate receptors, but not at inhibitory synapses.

Different jobs
Next, the researchers produced genetically altered mice that had half the normal levels of SNAP23. On the surface of their cortical neurons, the altered mice made less of the NMDA receptor subunits NR1 and NR2B than wild-type mice did. In contrast, their surface expression of AMPA receptor subunits, a GABA receptor, and a presynaptic metabotropic glutamate receptor seemed normal. Suh and colleagues write, “These data indicate that reduced expression of endogenous SNAP23 regulates the surface expression of NMDA receptors in neurons.”

Yet, the possibility remained that SNAP25 might do the same thing. To compare the two proteins’ effects, the researchers used lentivirus to deliver shRNA that would curb production of SNAP23 or SNAP25. Targeting SNAP23 in this way curbed expression of NMDA receptor subunits NR2A, NR2B, and NR1 at the neuronal surface. It also modestly reduced surface expression of two AMPA receptor subunits. In contrast, the SNAP25 knockdown did not affect NMDA or AMPA receptor subunits.

A final test of the proteins’ roles involved taking voltage-clamp recordings of whole pyramidal neurons from the hippocampal CA1 region. Cells that expressed SNAP23, but not SNAP25 shRNA showed weakened NMDA-evoked currents. Not only did SNAP23 regulate these currents, it also affected NMDA excitatory post-synaptic currents, apparently by controlling the number of NMDA receptors at synapses.

The study by Suh and colleagues suggests that SNAP23 controls the trafficking and behavior of post-synaptic NMDA receptors. Whether the protein causes their dysfunction in schizophrenia (see SRF Current Hypotheses by Bita Moghaddam and by Daniel Javitt) remains to be seen. While some studies have looked at SNAP25 in regard to schizophrenia and related phenotypes, a PubMed search using the terms “SNAP23” and “schizophrenia” came up empty. That may change. Researchers have been seeking promising targets to treat NMDA dysfunction in the disease (see SRF related news story; SRF related news story). Knowing more about the mechanisms involved may help.—Victoria L. Wilcox.

References:
Hayashi-Takagi A, Takaki M, Graziane N, Seshadri S, Murdoch H, Dunlop AJ, Makino Y, Seshadri AJ, Ishizuka K, Srivastava DP, Xie Z, Baraban JM, Houslay MD, Tomoda T, Brandon NJ, Kamiya A, Yan Z, Penzes P, Sawa A. Disrupted-in-schizophrenia 1 (DISC1) regulates spines of the glutamate synapse via Rac1. Nat Neurosci. 2010 Feb 7. Abstract

Suh YH, Terashima A, Petralia RS, Wenthold RJ, Isaac JTR, Roche KW, Roche PA. A neuronal role for SNAP-23 in post-synaptic glutamate receptor trafficking. Nat Neurosci. 2010 Jan 31. Abstract

Comments on News and Primary Papers
Comment by:  Jacqueline Rose
Submitted 2 March 2010
Posted 2 March 2010
  I recommend the Primary Papers

The newly published paper by Katherine Roche and Paul Roche reports SNAP-23 expression in neuron dendrites and examines the possible role of this neuronal SNAP-23 protein. To this point, SNAP-23 has traditionally been discussed in reference to vesicle trafficking in epithelial cells (see Rodriguez-Boulan et al., 2005 for review), so it is of interest to determine the function of SNAP-23 in neurons. Suh et al. report that surface NMDA receptor expression and NMDA-mediated currents are inhibited following SNAP-23 knockdown. Further, SNAP-23 knockdown results in a specific decrease in NR2B subunit insertion; previously, the NR2B subunit has been reported to preferentially localize to recycling endosomes compared to NR2A (Lavezzari et al., 2004). Given these findings, it is reasonable to conclude that SNAP-23 may be involved in maintaining NMDA receptor surface expression possibly by binding to NMDA-specific recycling endosomes.

Interestingly, there is recent evidence that PKC-induced NMDA receptor insertion is mediated by another neuronal SNARE protein; postsynaptic SNAP-25 (Lau et al., 2010). It is possible that activity-induced NMDA receptor trafficking is mediated by SNAP-25, while baseline maintenance of NMDA receptor levels relies on SNAP-23. Other evidence to suggest a strictly regulatory role for SNAP-23 in neuronal NMDA insertion is the finding that activity-dependent receptor insertion from early endosomes has previously been reported to be restricted to AMPA-type glutamate receptors (Park et al., 2004). However, it is possible that activity-induced insertion of AMPA receptors occurs via a distinct endosome pool than NMDA receptors; AMPA and NMDA receptor trafficking has been reported to proceed by distinct vesicle trafficking pathways (Jeyifous et al., 2009).

Although SNAP-23 may not be involved in activity-dependent early endosome receptor trafficking, it is possible that SNAP-23 operates in other pathways linked to activity-induced NMDA receptor trafficking. For instance, SNAP-23 may be the SNARE protein by which lipid raft shuttling of NMDA receptors occurs. SNAP-23 has been found to preferentially associate with lipid rafts over SNAP-25 in PC12 cells (Salaün et al., 2005). As well, NMDA receptors have been found to associate with lipid raft associated proteins flotilin-1 and -2 in neurons (Swanwick et al., 2009). Lipid raft trafficking of NMDA receptors to post-synaptic densities has been reported to follow global ischemia (Besshoh et al., 2005), and the possibility remains that under certain circumstances, NMDA trafficking occurs by lipid raft association to SNAP-23.

Taken together, the discovery of post-synaptic SNARE proteins offers several avenues of research to determine their roles and functions in glutamatergic synapse organization. Further, investigating disruption of synaptic receptor organization presents several possibilities for potential etiologies of disorders linked to compromised glutamate signaling like schizophrenia.

References:

Besshoh, S., Bawa, D., Teves, L., Wallace, M.C. and Gurd, J.W. (2005). Increased phosphorylation and redistribution of NMDA receptors between synaptic lipid rafts and post-synaptic densities following transient global ischemia in the rat brain. Journal of Neurochemistry, 93: 186-194. Abstract

Jeyifous, O., Waites, C.L., Specht, C.G., Fujisawa, S., Schubert, M., Lin, E.I., Marshall, J., Aoki, C., de Silva, T., Montgomery, J.M., Garner, C.C. and Green, W.N. (2009). SAP97 and CASK mediate sorting of NMDA receptors through a previously unknown secretory pathway. Nature Neuroscience, 12: 1011-1019. Abstract

Lau, C.G., Takayasu, Y., Rodenas-Ruano, A., Paternain, A.V., Lerma, J., Bennet, M.V.L. and Zukin, R.S. (2010). SNAP-25 is a target of protein kinase C phosphorylation critical to NMDA receptor trafficking. Journal of Neuroscience, 30: 242-254. Abstract

Lavezzari, G., McCallum, J., Dewey, C.M. and Roche, K.W. (2004). Subunit-specific regulation of NMDA receptor endocytosis. Journal of Neuroscience, 24: 6383-6391. Abstract

Park, M., Penick, E.C., Edward, J.G., Kauer, J.A. and Ehlers, M.D. (2004). Recycling endosomes supply AMPA receptors for LTP. Science, 305: 1972-1975. Abstract

Rodriguez-Boulan, E., Kreitzer, G. and Müsch, A. (2005) Organization of vesicular trafficking in epithelia. Nature Reviews: Molecular Cell Biology, 6: 233-247. Abstract

Salaün, C., Gould, G.W. and Chamberlain, L.H. (2005). The SNARE proteins SNAP-25 and SNAP-23 display different affinities for lipid rafts in PC12 cells. Journal of Biological Chemistry, 280: 1236-1240. Abstract

Suh, Y.H., Terashima, A., Petralia, R.S., Wenthold, R.J., Isaac, J.T.R., Roche, K.W. and Roche, P.A. (2010). A neuronal role for SNAP-23 in postsynaptic glutamate receptor trafficking. Nat Neurosci. 2010 Mar;13(3):338-43. Abstract

Swanwick, C.C., Shapiro, M.E., Chang, Y.Z. and Wenthold, R.J. (2009). NMDA receptors interact with flotillin-1 and -2, lipid raft-associated proteins. FEBS Letters, 583: 1226-1230. Abstract

View all comments by Jacqueline Rose

Comments on Related News


Related News: Messing with DISC1 Protein Disturbs Development, and More

Comment by:  Anil Malhotra, SRF Advisor
Submitted 21 November 2005
Posted 21 November 2005

The relationship between DISC1 and neuropsychiatric disorders, including schizophrenia, schizoaffective disorder, and bipolar disorder, has now been observed in several studies. Moreover, a number of studies have demonstrated that DISC1 appears to impact neurocognitive function. Nevertheless, the molecular mechanisms by which DISC1 could contribute to impaired CNS function are unclear, and these two papers shed light on this critical issue.

Millar et al. (2005) have followed the same strategy that they so successfully utilized in their initial DISC1 studies, identifying a translocation that associated with a psychotic illness. In contrast to DISC1, in which a pedigree was identified with a number of translocation carriers, this manuscript is based upon the identification of a single translocation carrier, who appears to manifest classic signs of schizophrenia, without evidence of mood dysregulation. Two genes are disrupted by this translocation: cadherin 8 and phosphodiesterase 4B (PDE4B). The researchers' elegant set of experiments provides compelling biological evidence that PDE4B interacts with DISC1 and suggests a mechanism mediated by cAMP for DISC1/PDE4B effects on basic molecular processes underlying learning, memory, and perhaps psychosis. It remains possible that PDE4B (and DISC1) are proteins fundamentally involved in cognitive processes, and that the observed relationship to psychotic illnesses represents a final common pathway of neurocognitive impairment. This would be consistent with data from our group (Lencz et al., in press) demonstrating that verbal memory impairment specifically predicts onset of psychosis in at-risk subjects. Similarly, Burdick et al. (2005) found that our DISC1 risk genotypes (Hodgkinson et al., 2004) were associated with impaired verbal working memory. Finally, Callicott et al. (2005) found that a DISC1 risk SNP, Ser704Cys, predicted hippocampal dysfunction, an SNP which we (DeRosse et al., unpublished data) have also found to link with the primary psychotic symptoms (persecutory delusions) manifested by the patient in the Millar et al. study. This body of evidence supports the notion that these proteins play fundamental roles in the key clinical manifestations of schizophrenia.

Kamiya et al. (2005) provide another potential mechanism for these effects, suggesting that a DISC1 mutation may disrupt cerebral cortical development, hinting that studies examining the role of DISC1 genotypes on brain structure and function in the at-risk schizophrenia pediatric patients may be fruitful.

Taken together, these papers add considerable new data suggesting that DISC1 plays a key role in the etiology of schizophrenia, and places DISC1 at the forefront of the rapidly growing body of schizophrenia candidate genes.

References:
Burdick KE, Hodgkinson CA, Szeszko PR, Lencz T, Ekholm JM, Kane JM, Goldman D, Malhotra AK. DISC1 and neurocognitive function in schizophrenia. Neuroreport 2005; 16(12):1399-1402. Abstract

Callicott JH, Straub RE, Pezawas L, Egan MF, Mattay VS, Hariri AR, Verchinski BA, Meyer-Lindenberg A, Balkissoon R, Kolachana B, Goldberg TE, Weinberger DR. Variation in DISC1 affects hippocampal structure and function and increases risk for schizophrenia. Proc Natl Acad Sci USA 2005; 102(24): 8627-8632. Abstract

Hodgkinson CA, Goldman D, Jaeger J, Persaud S, Kane JM, Lipsky RH, Malhotra AK. Disrupted in Schizophrenia (DISC1): Association with schizophrenia, schizoaffective disorder, and bipolar disorder. Am J Hum Genet 2004; 75:862-872. Abstract

Lencz T, Smith CW, McLaughlin D, Auther A, Nakayama E, Hovey L, Cornblatt BA. Generalized and specific neurocognitive deficits in prodromal schizophrenia. Biological Psychiatry (in press).

View all comments by Anil Malhotra

Related News: Messing with DISC1 Protein Disturbs Development, and More

Comment by:  Angus Nairn
Submitted 29 December 2005
Posted 31 December 2005
  I recommend the Primary Papers

This study describes an interesting genetic link between PDE4B (phosphodiesterase 4B) and schizophrenia that may be related to a physical interaction with DISC1 (disrupted in schizophrenia 1), another gene associated with the psychiatric disorder. The study is highly suggestive of a role for the PDE4B/DISC1 complex in schizophrenia. However, the mechanistic model suggested by the authors whereby DISC1 sequesters PDE4B in an inactive state seems overly speculative, given the results presented in this paper and in prior studies that have examined the regulation of PDE4B by phosphorylation in the absence of DISC1.

View all comments by Angus Nairn

Related News: Messing with DISC1 Protein Disturbs Development, and More

Comment by:  Patricia Estani
Submitted 2 January 2006
Posted 2 January 2006
  I recommend the Primary Papers

Related News: Dendritic Spine Research—Putting Meat on the Bones

Comment by:  Amanda Jayne Law, SRF Advisor
Submitted 13 February 2006
Posted 13 February 2006

The formation of dendritic spines during development and their structural plasticity in the adult brain are critical aspects of synaptogenesis and synaptic plasticity. Actin is the major cytoskeletal source of dendritic spines, and polymerization/depolymerization of actin is the primary determinant of spine motility and morphogenesis. Some, but not all, postmortem studies in schizophrenia have identified reduced dendritic spine density in neurons of the hippocampal formation and dorsolateral prefrontal cortex (for review, see Honer et al., 2000); however, little is known about the underlying pathogenic mechanisms affecting synaptic function in the disease.

Many different factors and proteins are known to control dendritic spine development and remodeling (see Ethell and Pasquale, 2005). Comprehensive investigation of the effectors and signaling pathways involved in regulating actin dynamics may provide insight into the molecular mechanisms mediating altered cortical microcircuitry in the disease.

David Lewis and colleagues have previously reported reduced spine density in the basilar dendrites of pyramidal neurons in laminar III of the DLPFC (though this is not clearly a laminar-specific finding). In their current study, Hill et al. extended these investigations to examine gene expression levels for members of the RhoGTPase family of intracellular signaling molecules (e.g., Cdc42, Rac1, RhoA, Duo), and Debrin, an F-actin binding protein, all of which are critical signal transduction molecules involved in spine formation and maintenance. Their aim was to determine whether alterations in the expression of one of more molecules may underlie the reduced spine density seen in the disorder. Hill et al. report that reductions in Cdc42 and Duo mRNA are observed in the DLPFC in schizophrenia and correlate with spine density on deep layer III pyramidal neurons. This paper provides preliminary evidence that "gene expression levels of certain mRNAs encoding proteins known to be key regulators of dendritic spines are reduced in the DLPFC in schizophrenia." However, the paper also reports that these two mRNAs are reduced in lamina where significant reductions in spine density are not observed in schizophrenia. These results may suggest, as the authors discuss, that reduced expression of Cdc42 and Duo might contribute to, but is not sufficient to cause reduced, spine density.

Synaptic dysfunction has received increasing attention as a key feature of schizophrenia’s neuropathology and possibly its genetic etiology (Law et al., 2004). Neuregulin 1 (NRG1), a lead schizophrenia susceptibility gene, is known to be a critical upstream regulator of signal transduction pathways modulating cytoskeletal dynamics, playing pivotal roles in synapse formation and function. We have previously reported that isoform-specific alterations of the NRG1 gene and its primary receptor, ErbB4, are apparent in the brain in schizophrenia and related to genetic risk for the disease (Law et al, 2005a, Law et al, 2005b). Altered NRG1/ErbB4 signaling in schizophrenia may be a pathway to aberrant cortical neurodevelopment and synaptic function via dysregulation of specific intracellular signaling pathways linked to actin. The lack of significant alterations in gene expression levels for proteins such as Rac1 and RhoA in the DLPFC (gray matter, as reported by Hill and colleagues) in schizophrenia might be because the primary defect may not lie with the expression of these molecules but with the upstream modulation of their function and activity. Therefore, investigation of the proteins themselves, their phosphorylation status and activity, will be useful in understanding how genes effect molecular pathways that mediate biological risk for schizophrenia. The study of intracellular signaling cascades may be a route to a closer understanding of the biological mechanisms underpinning the association of genes such as NRG1 and ErbB4 with schizophrenia and their relationship to its neuropathology.

References:

Ethell IM, Pasquale EB. Molecular mechanisms of dendritic spine development and remodeling. Prog Neurobiol. 2005 Feb;75(3):161-205. Epub 2005 Apr 2. Review. Abstract

Honer G, Young C, and Falkai P, 2000. Synaptic Pathology in the Neuropathology of Schizophrenia, Progress and interpretation. Oxford University Press, edited by Paul J Harrison and Gareth W. Roberts, pp105-136.

Law AJ, Weickert CS, Hyde TM, Kleinman JE, Harrison PJ. Reduced spinophilin but not microtubule-associated protein 2 expression in the hippocampal formation in schizophrenia and mood disorders: molecular evidence for a pathology of dendritic spines. Am J Psychiatry. 2004 Oct;161(10):1848-55. Abstract

Law, 2005a. Soc Neurosci Abstract, SFN Annual Meeting, Washington DSC, 2005. Neuregulin1 and schizophrenia: A pathway to altered cortical circuits. Also See SfN 2005 research news: Cortical Deficits in Schizophrenia: Have Genes, Will Hypothesize.

Law 2005b ACNP Abstract, Neuropsychopharmacology, vol. 30, Supplement 1. SNPing away at NRG1 and ErbB4 gene expression in schizophrenia.

View all comments by Amanda Jayne Law

Related News: Messing with DISC1 Protein Disturbs Development, and More

Comment by:  Ali Mohammad Foroughmand
Submitted 16 December 2006
Posted 16 December 2006
  I recommend the Primary Papers

Related News: Studies Explore Glutamate Receptors as Target for Schizophrenia Monotherapy

Comment by:  Dan Javitt, SRF Advisor
Submitted 3 September 2007
Posted 3 September 2007

A toast to success, or new wine in an old skin?
Patil et al. present a landmark study. It is the kind of study that represents the best of how science should work. It pulls together the numerous strands of schizophrenia research from the last 50 years, from the development of PCP psychosis as a model for schizophrenia in the late 1950s, through the links to glutamate, the discovery of metabotropic receptors, and the seminal discovery in 1998 by Moghaddam and Adams that metabotropic glutamate 2/3 receptor (mGluR2/3) agonists reverse the neurochemical and behavioral effects of PCP in rodents (Moghaddam and Adams, 1998. The story would not be possible without the elegant medicinal chemistry of Eli Lilly, which provided the compounds needed to test the theories; the research support of NIMH and NIDA, who have been consistent supporters of the “PCP theory”; or the hard work of academic investigators, who provided the theories and the platforms for testing. The study is large and the effects robust. Assuming they replicate (and there is no reason to suspect that they will not), this compound, and others like it, will represent the first rationally developed drugs for schizophrenia. Patients will benefit, drug companies will benefit, and academic investigators and NIH can feel that they have played their role in new treatment development.

Nevertheless, it is always the prerogative of the academic investigator to ask for more. In this case, we do not yet know if this will be a revolution in the treatment of schizophrenia, or merely a platform shift. What is striking about the study, aside from the effectiveness of LY2140023, is the extremely close parallel in both cross-sectional and temporal pattern of response between it and olanzapine. Both drugs change positive and negative symptoms in roughly equal proportions, despite their different pharmacological targets. Both drugs show approximately equal slopes over a 4-week period. There is no intrinsic reason why symptoms should require 4 or more weeks to resolve, or why negative and positive symptoms should change in roughly the same proportion with two medications from two such different categories, except that evidently they do.

There are many things about mGluR2/3 agonists that we do not yet know. The medication used here was administered at a single, fixed dose. It is possible that a higher dose might have been better, and that optimal results have not yet been achieved. The medications were used in parallel. It is possible that combined medication might be more effective than treatment with either class alone. The study was stopped at 4 weeks, with the trend lines still going down. It is possible that longer treatment duration in future studies might lead to even more marked improvement and that the LY and olanzapine lines might separate. No cognitive data are reported. It is possible that marked cognitive improvement will be observed with these compounds when cognition is finally tested, in which case a breakthrough in pharmacotherapy will clearly have been achieved.

If one were to look at the glass as half empty, then the question is why the metabotropic agonist did not beat olanzapine, and why the profiles of response were so similar. If these compounds work, as suggested in the article by modulating mesolimbic dopamine, then it is possible that metabotropic agonists will share the same therapeutic limitations as current antipsychotics—good drugs certainly and without the metabolic side effects of olanzapine, but not “cures.” The recent study with the glycine transport inhibitor sarcosine by Lane and colleagues showed roughly similar overall change in PANSS total (-17.1 pts) to that reported in this study, but larger change in negative symptoms (-5.5 pts), and less change in positive symptoms (-2.3 pts) in a similar type of patient population. Onset of effect in the sarcosine study also appeared somewhat faster. The sarcosine study was smaller (n = 20) and did not include a true placebo group. As with the Lilly study, it was only 4 weeks in duration, and did not include cognitive measures. It also included only two, possibly non-optimized doses. As medications become increasingly available to test a variety of mechanisms, side-by-side comparisons will become increasingly important.

There are also causes for concern and effects to be watched. For example, a side effect signal was observed for affect lability in the LY group, at about the same prevalence rate as weight increase in the olanzapine group. What this means for the mechanism and how this will effect treatment remains to be determined. Since these medications are agonists, there is concern that metabotropic receptors may downregulate over time. Thus, whether treatment effects increase, decrease, or remain constant over the course of long-term treatment will need to be determined. Nevertheless, 50 years since the near-contemporaneous discovery of both PCP and chlorpromazine, it appears that glutamatergic drugs for schizophrenia may finally be on the horizon.

References:

Moghaddam B, Adams BW. Reversal of phencyclidine effects by a group II metabotropic glutamate receptor agonist in rats. Science. 1998 Aug 28;281(5381):1349-52. Abstract

View all comments by Dan Javitt

Related News: Studies Explore Glutamate Receptors as Target for Schizophrenia Monotherapy

Comment by:  Gulraj Grewal
Submitted 4 September 2007
Posted 4 September 2007
  I recommend the Primary Papers

Related News: Architect of Synaptic Plasticity Links Spine Form and Function

Comment by:  Akira Sawa, SRF Advisor
Submitted 29 December 2007
Posted 29 December 2007

Synaptic disturbance in the pathology of schizophrenia is a well-established idea. Lewis’s lab has reported decreased synaptic spine density in brains from patients with schizophrenia (Glantz and Lewis, 2000). Although it is unclear whether this is primary or secondary, expression of kalirin-7-associated molecules is decreased (Hill et al., 2006). Thus, kalirin-7-associated cellular signaling in synaptic spines may have implication for the pathology of schizophrenia. In this sense, I regard the recent publication from Penzes’s lab as very interesting in schizophrenia research.

It is still unclear whether kalirin-7 may interact with genetic susceptibility factors for schizophrenia, such as ErbB4 and DISC1. Until the protein interactions are tested by co-immunoprecipitation at endogenous protein levels, as well as validated by cell staining, we cannot tell whether or not such factors are really associated with the kalirin-7 pathway. This putative protein interaction of kalirin-7 with DISC1 or ErbB4 will be an important issue to address in the future.

In Penzes’s neuronal cultures, he has focused on spine formation in pyramidal neurons, but not in interneurons. Thus, the mechanism proposed in his study will be useful to consider possible pathology in pyramidal neurons in brains of patients with schizophrenia.

View all comments by Akira Sawa

Related News: Studies Explore Glutamate Receptors as Target for Schizophrenia Monotherapy

Comment by:  Shoreh Ershadi
Submitted 8 June 2008
Posted 9 June 2008
  I recommend the Primary Papers

Related News: DISC1 Players Gird For Adult Neurodevelopment

Comment by:  Kevin J. Mitchell
Submitted 8 October 2009
Posted 8 October 2009

The seminal identification of mutations in DISC1 associated with schizophrenia and other psychiatric disorders raises several obvious questions: what does the DISC1 protein normally do? What are its biochemical and cellular functions, and what processes are affected by its mutation? How do defects in these cellular processes ultimately lead to altered brain function and psychopathology? Which brain systems are affected and how? Similar questions could be asked for the growing number of other genes that have been implicated by the identification of putatively causal mutations, including NRG1, ERBB4, NRXN1, CNTNAP2, and many copy number variants. Finding the points of biochemical or phenotypic convergence for these proteins or mutations may be key to understanding how mutations in so many different genes can lead to a similar clinical phenotype and to suggesting points of common therapeutic intervention.

The papers by Kim et al. and Enomoto et al. add more detail to the complex picture of the biochemical interactions of DISC1 and its diverse cellular functions. The links with Akt and PTEN signaling are especially interesting, given the previous implication of these proteins in schizophrenia and autism. Akt, in particular, may provide a link between Nrg1/ErbB4 signaling and DISC1 intracellular functions.

These studies also reinforce the importance of DISC1 and its interacting partners in neurodevelopment, specifically in cell migration and axonal extension. In particular, they highlight the roles of these proteins in postnatal hippocampal development and adult hippocampal neurogenesis. They also raise the question of which extracellular signals and receptors regulate these processes through these signalling pathways. The Nrg1/ErbB4 pathway has already been implicated, but there are a multitude of other cell migration and axon guidance cues known to regulate hippocampal development, some of which, for example, semaphorins, signal through the PTEN pathway.

Whether or how disruptions in these developmental processes contribute to psychopathology also remains unclear. It seems likely that the effects of mutations in any of these genes will be highly pleiotropic and have effects in many brain systems. The reported pathology in schizophrenia is not restricted to hippocampus but extends to cortex, thalamus, cerebellum, and many other regions. Similarly, while the cognitive deficits receive a justifiably large amount of attention, given that they may have the most clinical impact, motor and sensory deficits are also a stable and consistent part of the syndrome that must be explained. Pleiotropic effects on prenatal and postnatal development, as well as on adult processes, may actually be the one common thread characterizing the genes so far implicated. These new papers represent the first steps in the kinds of detailed biological studies that will be required to make explanatory links from mutations, through biochemical and cellular functions, to effects on neuronal networks and ultimately psychopathology.

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Related News: DISC1 Players Gird For Adult Neurodevelopment

Comment by:  Peter PenzesMichael Cahill
Submitted 8 October 2009
Posted 8 October 2009

DISC1 disruption by chromosomal translocation cosegregates with several neuropsychiatric disorders, including schizophrenia (Blackwood et al., 2001; Millar et al., 2000). Recent attention has focused on the effects of DISC1 on the structure and function of the dentate gyrus, one of the few brain regions that exhibit neurogenesis throughout life. The downregulation of DISC1 has several deleterious effects on the dentate gyrus, including aberrant neuronal migration (Duan et al., 2007). However, the mechanisms through which DISC1 regulates the structure and function of the dentate gyrus remain unknown. The dentate gyrus and its output to the CA3 area, the mossy fiber, show several abnormalities in schizophrenia and other neuropsychiatric diseases (Kobayashi, 2009). Thus, understanding how a gene associated with neuropsychiatric disease, DISC1, mechanistically impacts the dentate gyrus is an important question with much clinical relevance.

The recent papers by Kim et al. and Enomoto et al. characterize an interaction between DISC1 and girdin (also known as KIAA1212), and reveal how girdin, and the interaction between DISC1 and girdin, impact axon development, dendritic development, and the proper positioning of newborn neurons in the dentate gyrus. Girdin normally stimulates the function of AKT (Anai et al., 2005), and Kim et al. show that DISC1 binds to girdin and inhibits its function. Thus, the loss of DISC1 leaves girdin unopposed, resulting in excessive AKT signaling. Indeed, the developmental defects in neurons lacking DISC1 can be rescued by pharmacologically blocking the activation of an AKT downstream target. However, as shown by Enomoto et al., the loss of girdin produces deleterious effects on neuronal morphology, suggesting that a proper balance of girdin function is crucial.

Collectively, these studies thoroughly characterize the interaction between DISC1 and girdin, and shed much light on the consequences of this interaction on neuronal morphology as well as on the positioning of neurons in the dentate gyrus. The role of girdin in the pathology of neuropsychiatric diseases is unknown, and remains an interesting question for the future. Characterizing the molecules that act up- or downstream of DISC1 remains an important area of investigation and could aid the development of pharmacological interventions in the future. It’s intriguing that DISC1 acting through girdin regulates the activity of AKT as AKT1 was previously identified as a schizophrenia risk gene (Emamian et al., 2004). This suggests a convergence of multiple schizophrenia-associated genes in a shared pathway, and thus it will be important to determine if the DISC1-girdin-AKT1 pathway is particularly vulnerable in neuropsychiatric disorders.

References:

Blackwood DH, Fordyce A, Walker MT, St Clair DM, Porteous DJ, Muir WJ. Schizophrenia and affective disorders--cosegregation with a translocation at chromosome 1q42 that directly disrupts brain-expressed genes: clinical and P300 findings in a family. Am J Hum Genet . 2001 Aug 1 ; 69(2):428-33. Abstract

Millar JK, Christie S, Semple CA, Porteous DJ. Chromosomal location and genomic structure of the human translin-associated factor X gene (TRAX; TSNAX) revealed by intergenic splicing to DISC1, a gene disrupted by a translocation segregating with schizophrenia. Genomics . 2000 Jul 1 ; 67(1):69-77. Abstract

Duan X, Chang JH, Ge S, Faulkner RL, Kim JY, Kitabatake Y, Liu XB, Yang CH, Jordan JD, Ma DK, Liu CY, Ganesan S, Cheng HJ, Ming GL, Lu B, Song H. Disrupted-In-Schizophrenia 1 regulates integration of newly generated neurons in the adult brain. Cell . 2007 Sep 21 ; 130(6):1146-58. Abstract

Kobayashi K. Targeting the hippocampal mossy fiber synapse for the treatment of psychiatric disorders. Mol Neurobiol . 2009 Feb 1 ; 39(1):24-36. Abstract

Anai M, Shojima N, Katagiri H, Ogihara T, Sakoda H, Onishi Y, Ono H, Fujishiro M, Fukushima Y, Horike N, Viana A, Kikuchi M, Noguchi N, Takahashi S, Takata K, Oka Y, Uchijima Y, Kurihara H, Asano T. A novel protein kinase B (PKB)/AKT-binding protein enhances PKB kinase activity and regulates DNA synthesis. J Biol Chem . 2005 May 6 ; 280(18):18525-35. Abstract

Emamian ES, Hall D, Birnbaum MJ, Karayiorgou M, Gogos JA. Convergent evidence for impaired AKT1-GSK3beta signaling in schizophrenia. Nat Genet . 2004 Feb 1 ; 36(2):131-7. Abstract

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