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Post-synaptic Density Enriched for Culprits in Human Brain Disorders

21 December 2010. Researchers have identified 1,461 proteins in the human cortical post-synaptic density (PSD), a multi-protein complex found on the receiving end of synapses in the brain. As reported in Nature Neuroscience on December 16, mutations in the genes encoding some of these proteins result in 133 human nervous system diseases, highlighting the importance of the PSD and its potential involvement in other brain disorders, including schizophrenia.

Named for the darkly staining region on the post-synaptic side of synapses visible by electron microscopy, the PSD comprises an interconnected network of proteins that includes neurotransmitter receptors, cell adhesion molecules, and scaffolding and signaling proteins. Once thought of as a static structure, the PSD is now recognized as a dynamic machine that can shuffle around some of its component parts according to a cell's needs. The new study throws some light on this complicated machine by essentially coming up with its parts list. It also found a connection between mutations in these parts and human disease and phenotypes, arguing that the PSD is a place of particular interest when looking for the origins of brain disorders.

"We've developed a kind of template or roadmap for investigating human synapse function," Seth Grant, senior author of the study and professor at the Wellcome Trust Sanger Institute in the United Kingdom, told SRF. "This could help drug discovery for treatment of various human diseases."

Though no one doubts that synapses are vital for the brain to work properly, no one had ever tallied up the number of diseases stemming from malfunctions in synaptic components. "Now we've put a number down on the table for PSD diseases, though we expect that number to rise with time," said Grant.

Parts list
First authors Álex Bayés and Louie van de Lagemaat and colleagues took a different approach from typical genetic studies of disease, which often deliver a list of seemingly unrelated genes. Instead, the team started with proteins that are already known to function together and asked which of them, when mutated, resulted in disease. They began by extracting the PSD proteins from neocortical tissue taken from nine adults undergoing brain surgery. The nine samples were pooled and divided into three groups. Using mass spectrometry, the researchers identified 1,461 proteins total, each encoded by a different gene. Of these, 748 proteins were found in all three groups, representing a "consensus" protein set. These numbers are similar in magnitude to those found for proteomic characterizations of the mouse PSD (Collins et al., 2006).

Though the vast majority of the proteins on this list (see the Genes2Cognition website) had never been connected to the PSD before, many are familiar. Some, like GABA receptors, are embedded in the PSD, whereas others, like MAP kinase, have additional jobs beyond the PSD in other parts of the neuron and in other cell types. Some of these PSD genes already have a reputation in human disease, including HTT for Huntington's and ApoE in Alzheimer's. NRXN1, a gene whose deletion is associated with schizophrenia and autism, also turned up.

Cross-reference to disease
This list allowed the researchers to systematically track the effects of mutation in each of these PSD genes on human health. The researchers limited themselves to diseases in the Online Mendelian Inheritance in Man (OMIM) database, which catalogs monogenic diseases. They found that 269 diseases in the OMIM stemmed from mutations to 199 of these PSD genes, and 133 of these diseases were nervous system disorders. Similar proportions resulted when looking only at mutations in the consensus PSD. These included neurodegenerative diseases like Alzheimer's, as well as mental retardation, motor disorders, and epilepsy. Though the number of diseases linked to mutations in the human PSD was higher than expected, this number is likely to increase once complex psychiatric diseases like schizophrenia are included, Grant told SRF.

The team also tried to get beyond disease classifications by describing in phenotypic terms what these PSD proteins might be doing. For this, they turned to the Human Phenotype Ontology (HPO) database, which categorizes the constellation of signs and symptoms associated with different diseases into discrete phenotypes. Mutations to the human PSD were associated more often with cognitive and motor phenotypes than were mutations to other neuronal genes, arguing that the PSD is particularly enriched for susceptibility to these kinds of symptoms. This analysis also revealed that multiple genes contributed to particular phenotypes; for example, 40 PSD genes were related to mental retardation and 20 to muscle spasticity.

Future work will have to fill in the picture of exactly how these many proteins work together. From a therapeutic standpoint, it will be particularly interesting to see which ones act through a common mechanism within the PSD. These subsets of proteins may bridge the gap between the encouraging sign that many diseases stem from malfunctions in the same synaptic structure and the distress at seeing its very long parts list. Multiple proteins that converge on the same machinery within the PSD could reveal new targets for drug design, potentially providing a way of normalizing PSD function and of treating multiple diseases.—Michele Solis.

Reference:
Bayés Á, van de Lagemaat LN, Collins MO, Croning MDR, Whittle IR, Choudhary JS, Grant SGN. Characterization of the proteome, diseases and evolution of the human post-synaptic density. Nat Neurosci. 2010 Dec 16. Abstract

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Related News: DISC1 and SNAP23 Emerge In NMDA Receptor Signaling

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

Related News: WCPG 2011—A Capital Day for CNVs in Schizophrenia

Comment by:  John McGrath, SRF Advisor
Submitted 17 September 2011
Posted 20 September 2011

De novo CNVs are associated with advanced paternal age in a mouse model
While the association between advanced paternal age and an increased risk of various neuropsychiatric disorders such as schizophrenia and autism is now well established, the mechanism underpinning this finding remains unclear. Putative mechanisms include de-novo mutations and/or epigenetic mechanisms. In light of the growing body of evidence linking copy number variants (CNVs) with these same disorders, we used a mouse model to explore the hypothesis that the offspring of older males have an increased risk of de-novo CNVs. C57BL/6J sires that were three- and 12-16 months old were mated with three-month-old dams to create control offspring and offspring of old sires, respectively. Applying genomewide microarray screening technology, seven distinct CNVs were identified in a set of 12 offspring and their parents.

Competitive quantitative PCR confirmed these CNVs in the original set and also established their frequency in an independent set of 77 offspring and their parents. On the basis of the combined samples, six de-novo CNVs were detected in the offspring of older sires, whereas none were detected in the control group. Two of the CNVs were associated with behavioral and/or neuroanatomical phenotypic features. One of the de-novo CNVs involved Auts2 (autism susceptibility candidate 2), and other CNVs included genes linked to schizophrenia, autism, and brain development.

Our results support the hypothesis that the offspring of older fathers have an increased risk of neurodevelopmental disorders such as schizophrenia and autism by generation of de-novo CNVs in the male germline.

References:

T Flatscher-Bader, CJ Foldi, S Chong, E Whitelaw, RJ Moser, THJ Burne, DW Eyles, JJ McGrath. (2011) Increased de novo copy number variants in the offspring of older males. Translational Psychiatry. View the free full-text article.

View all comments by John McGrath

Related News: Dissecting Cognition at the Synapse

Comment by:  Jennifer Barnett (Disclosure)
Submitted 13 December 2012
Posted 13 December 2012

Cognitive function is highly heritable (Devlin et al., 1997), yet we have relatively little understanding of which genes regulate either general intelligence or specific cognitive functions. A long list of mutations can cause the large cognitive impairments that we class as learning disability—including many of the same CNVs associated with cognitive disorders such as autism and schizophrenia (Guilmatre et al., 2009). Prior to the GWAS era, it was generally assumed that normal variation—outside of the range of learning disability—would be regulated by common variants of small effect. Yet GWAS, a technology well suited to detecting common variants of small effect, has not massively increased our understanding of the genetic basis of cognition.

One explanation for this relative lack of success is that, compared with quantitative phenotypes such as height or BMI, the measurement of cognition can be time consuming and therefore costly, so really large-scale studies are rare. Moreover, any two studies are very unlikely to use identical cognitive tests, introducing error to the phenotypic measurement and making it difficult to combine datasets. In this context, the Nithianantharajah paper is a beautiful example of how translational research using relatively simple but neuroscience-led assays can increase our understanding of the genetics of cognition, while avoiding the need for the ever-increasing sample sizes.

In particular, I was impressed by the building up of related but increasingly complex forms of cognition across the rodent tasks, and the use of the closest possible analogues between mouse and human assays. (Disclosure: I am employed by Cambridge Cognition, the suppliers of the CANTAB tests used in the human phenotyping.) Adding to these very careful phenotyping methodologies, the parallel experiment across both mouse and human "knockouts" is a really elegant piece of translational neuroscience.

Like many traits underlying brain function, it is inherently easy to believe that variants that have large effects on cognition would create strong evolutionary advantages or disadvantages. The authors here demonstrate not only that a related family of genes affects multiple aspects of cognitive function, but also that variation in these genes produces different cognitive tendencies in the mouse, including reciprocal effects of variants of Dlg2 and Dlg3, which seem, at least at first pass, to be conserved in human behavior.

There is a lot to digest in this and the companion paper by Ryan et al., but the methodology appears to have been very useful here in understanding cognition, and may provide useful insights for researchers trying to decipher other aspects of the schizophrenia phenotype.

References:

Devlin B, Daniels M, Roeder K (1997). The heritability of IQ. Nature.388(6641):468-71. Abstract

Guilmatre A, Dubourg C, Mosca AL, Legallic S, Goldenberg A, Drouin-Garraud V, Layet V, Rosier A, Briault S, Bonnet-Brilhault F, Laumonnier F, Odent S, Le Vacon G, Joly-Helas G, David V, Bendavid C, Pinoit JM, Henry C, Impallomeni C, Germano E, Tortorella G, Di Rosa G, Barthelemy C, Andres C, Faivre L, Frébourg T, Saugier Veber P, Campion D. (2009). Recurrent rearrangements in synaptic and neurodevelopmental genes and shared biologic pathways in schizophrenia, autism, and mental retardation. Arch Gen Psychiatry;66(9):947-56. Abstract

View all comments by Jennifer Barnett