The newly published paper by Katherine Roche and Paul...
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.
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
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