Modes and mechanisms of synaptic vesicle recycling
Introduction
Neuronal communication depends on the regulated release of neurotransmitters from synaptic vesicles (SVs) by calcium-triggered exocytic fusion at specialized presynaptic release sites within so-called active zones (AZs) [1•, 2•]. To prevent expansion of the presynaptic plasma membrane and a corresponding loss of lateral membrane tension (i.e. tension within the plane of the membrane) SV exocytosis is followed by compensatory endocytic membrane retrieval. Moreover, SVs of correct size and composition have to be reformed in order to sustain neurotransmission. Finally, recent data suggest that endocytosis is also required to clear release sites [3] from previously exocytosed material (i.e. SV proteins, SNARE complexes, etc.) [4, 5, 6•, 7•], which may be rate-limiting for exocytosis at fast-firing synapses [3]. As synapses differ vastly with respect to their SV pool sizes and firing pattern, it is conceivable that synapses have evolved specific separate mechanisms to maintain membrane homeostasis and to replenish the readily releasable SV pool. The exact nature and molecular components of the various modes of SV recycling and the underlying mechanisms remain a matter of debate.
Section snippets
Proposed modes of SV recycling
Pioneering studies based on electron microscopy (EM) analysis of stimulated nerve-muscle preparations suggested that SV membranes are recycled by clathrin-mediated endocytosis (CME) from cisternal structures located away from AZs [8]. Using the same preparation, but a different stimulation paradigm Ceccarelli et al. [9] proposed an alternative clathrin-independent rapid mode of SV recycling by kiss-and-run exocytosis/endocytosis (K&R) of SVs at AZs involving a transient fusion pore. While
Mechanisms and components of presynaptic membrane retrieval and SV reformation
Key distinctive features of K&R, UFE, ADBE, and CME are their differential dependence on clathrin and its associated factors (i.e. AP-2, AP180) as well as the speed and fidelity at which SV membrane and protein retrieval and SV reformation occur (Figure 1, Table 1). In K&R, UFE, and ADBE membrane internalization post-fusion is independent of clathrin and its main recruitment factor AP-2, while membrane retrieval via CME strictly depends on clathrin/AP-2 and dynamin in essentially all cell types
Sorting of SV proteins and other presynaptic cargo
Sustained rapid neurotransmission not only depends on release site clearance and the restoration of lateral membrane tension post-exocytic fusion, but also requires that SVs of correct size and composition are reformed. Imaging analysis indeed suggests that SV protein copy numbers are tightly controlled [47], presumably at multiple levels. High-resolution time-lapse imaging has revealed that newly exocytosed SV proteins rapidly disperse post-fusion until confined within the presynaptic bouton
Modulation of SV recycling
A number of endocytic proteins, the so-called dephosphins (i.e. dynamin 1 [57], synaptojanin 1, AP180, amphiphysin 1), have long been known to be functionally regulated via activity-dependent dephosphorylation by calcineurin, a serine/threonine protein phosphatase with a key role in SV recycling [58]. Recent studies have uncovered additional mechanisms, which regulate SV recycling at different types of synapses. For example, LRRK2, a kinase mutated in Parkinson's disease, modulates presynaptic
Conclusion and perspectives
In spite of decades of studies on the modes and mechanisms of SV recycling and the rich molecular information gathered in recent years, many key questions remain controversial. Cumulating evidence suggests that the mechanisms of presynaptic membrane retrieval and reformation of functional SVs are at least partially separable with distinct phenotypes of genetic manipulations of endocytic proteins on the kinetics of SV protein retrieval and the fidelity of SV protein sorting and SV reformation [1•
Conflict of interest statement
None of the authors of this work declare any conflict of interest.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
Our own research was supported by grants from the Deutsche Forschungsgemeinschaft (DFG) [SFB958/A01; SFB958/A07; HA 2686/8-1 and MA4735/1-1] and the Neurocure Cluster of Excellence (DFG-Exc-257).
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