The Synaptic VesicleCycle Revisited

What are the molecular mechanisms that guide the synaptic vesicle pathway? The components of synaptic vesicles have been largely characterized, several molecules of active zones are known, and a large number of proteins with a function in endocytosis has been identified. Nevertheless, except for membrane fusion, little is known about the molecular correlates of the various stages of the different vesicle cycles. Descriptions of general functions, of vesicles moving here or there, have been much easier to come by than an understanding of the mechanisms involved. For the most part, we only have reasonable hypotheses coupled with negative data. The actin or microtubule cytoskeleton is unlikely to be centrally involved since it is primarily, if not exclusively, localized outside of the vesicle cluster (Dunaevsky and Connor 2000xDunaevsky, A and Connor, E.A. J. Neurosci. 2000; 20: 6007–6012PubMedSee all ReferencesDunaevsky and Connor 2000). Intrinsic proteins of the synaptic vesicles must be the final endpoints of all molecules acting on the vesicles during their different pathways; a systematic analysis of these proteins along the lines that have already started may provide a definitive test of this hypothesis (reviewed in Fernandez-Chacon and Sudhof 1999xFernandez-Chacon, R and Sudhof, T.C. Annu. Rev. Physiol. 1999; 61: 753–776Crossref | PubMed | Scopus (144)See all ReferencesFernandez-Chacon and Sudhof 1999). However, up to now genetic deletions of various synaptic vesicle proteins mostly cause changes in release rates and synaptic plasticity. In particular, no synaptic vesicle protein that is selectively essential for the biogenesis or recycling of synaptic vesicles has been identified. There is a general lack of insight into what keeps a vesicle together and how it is routed in the presynaptic nerve terminals. Although endocytosis has been exceedingly well studied in nonneuronal cells and the components of the endocytic machinery are highly enriched in nerve terminals (e.g., see Maycox et al. 1992xMaycox, P.R, Link, E, Reetz, A, Morris, S.A, and Jahn, R. J. Cell Biol. 1992; 118: 1379–1388Crossref | PubMedSee all ReferencesMaycox et al. 1992), it is unclear, for example, if clathrin coats are only involved in slow endocytosis or also in the fast “kiss-and-stay” reuse pathway. The molecular understanding of the nerve terminal is far behind the exquisite description of vesicles moving in and out of the active zone. Synaptic vesicle cycling is functionally relatively simple compared to the mechanistic complexity of molecular interactions. Even with three nested pathways, the vesicle cycle is still less complex than the interaction cycle of a protein such as Rab3A which interacts with at least five other proteins during the vesicle cycle in an ordered sequence (Rabphilin, RIM, Rab3-GAP, Rab3-GDP/GTP exchange protein, GDI; Sudhof 1995xSudhof, T.C. Nature. 1995; 375: 488–493Crossref | PubMedSee all ReferencesSudhof 1995). The need for such a complexity is given by the plasticity of the terminals, both structurally and functionally, that allows synapses to change release dramatically over short time periods.In view of this complexity, what then are the primary questions that we can hope to address molecularly in the near future? The fusion process, and its relation to the RRP, probably holds the greatest promise for understanding. It seems likely that vesicles in the RRP are prefused when they are stimulated for exocytosis by Ca2+ in order to allow the extraordinary speed of Ca2+ action. Recent results suggest that this prefused state may correspond to assembled SNARE complexes (Lonart and Sudhof 2000xLonart, G and Sudhof, T.C. J. Biol. Chem. 2000; 275: 27703–27707PubMedSee all ReferencesLonart and Sudhof 2000). Although it is still unclear if the SNAREs are executive or merely catalytic in fusion, elucidation of how and where they act in synaptic vesicle fusion in conjunction with the essential fusion protein munc18-1 will provide decisive insight into the mechanism of release. A second promising approach is the focus on proteins that reversibly associate with synaptic vesicles during the pathway. For example, Rab3A dissociates from synaptic vesicles after extensive stimulation while Rab5 does not (Fischer von Mollard et al. 1994xFischer von Mollard, G, Stahl, B, Walch-Solimena, C, Takei, K, Daniels, L, Khoklatchev, A, De Camilli, P, Sudhof, T.C, and Jahn, R. Eur. J. Cell Biol. 1994; 65: 319–326PubMedSee all ReferencesFischer von Mollard et al. 1994). It is possible that this dissociation occurs only in the direct recycling and the endosomal recycling pathways because docked vesicles (which presumably correspond to the RRP) contain Rab3A (reviewed in Sudhof 1995xSudhof, T.C. Nature. 1995; 375: 488–493Crossref | PubMedSee all ReferencesSudhof 1995). Is there a cycle of GTP hydrolysis/GDP to GTP exchange on docked vesicles, or does GTP hydrolysis only occur in preparation to true endocytosis? As mentioned above, synaptic vesicles quantitatively contain Rab5, a marker for endosomes. However, since most of the vesicles belong to the resting pool of vesicles that are not actively participating, it is impossible to tell if locally recycling vesicles (i.e., the recycling pool) contain or lack Rab5. Biochemically, the lack of Rab5 on 10% of the vesicles would easily have been missed. One idea thus is that the transition of vesicles from one pool to the next could be guided by the acquisition or loss of Rab5 from these vesicles and that different complements of Rab proteins identify different vesicle pools. The elegant physiological description of presynaptic terminals that has now been achieved by the work of 15xPyle, J.L, Kavalali, E.T, Piedras-Renteria, E.S, and Tsien, R.W. Neuron. 2000; 28: 221–231Abstract | Full Text | Full Text PDF | PubMedSee all References, 19xStevens, C.F and Williams, J.H. Proc. Natl. Acad. USA. 2000; in pressSee all References, and others will provide the opportunity to elucidate the molecular mechanisms that underlie these pathways.*E-mail: tsudho@mednet.swmed.edu