The synaptic vesicle cycle: is kissing overrated?

Synaptic Vesicle Recycling and the Endolysosomal System: A Reappraisal of Form and Function

The endolysosomal system is present in all cell types. Within these cells, it performs a series of essential roles, such as trafficking and sorting of membrane cargo, intracellular signaling, control of metabolism and degradation. A specific compartment within central neurons, called the presynapse, mediates inter-neuronal communication via the fusion of neurotransmitter-containing synaptic vesicles (SVs). The localized recycling of SVs and their organization into functional pools is widely assumed to be a discrete mechanism, that only intersects with the endolysosomal system at specific points. However, evidence is emerging that molecules essential for endolysosomal function also have key roles within the SV life cycle, suggesting that they form a continuum rather than being isolated processes. In this review, we summarize the evidence for key endolysosomal molecules in SV recycling and propose an alternative model for membrane trafficking at the presynapse. This includes the hypotheses that endolysosomal intermediates represent specific functional SV pools, that sorting of cargo to SVs is mediated via the endolysosomal system and that manipulation of this process can result in both plastic changes to neurotransmitter release and pathophysiology via neurodegeneration.

Dynamin photoinactivation blocks Clathrin and α-adaptin recruitment and induces bulk membrane retrieval

Drosophila Dynamin prevents bulk membrane endocytosis through effects on AP2- and Clathrin-mediated stabilization of endocytic pits.

Dynamin is a well-known regulator of synaptic endocytosis. Temperature-sensitive dynamin (shi^ts1) mutations in Drosophila melanogaster or deletion of some of the mammalian Dynamins causes the accumulation of invaginated endocytic pits at synapses, sometimes also on bulk endosomes, indicating impaired membrane scission. However, complete loss of dynamin function has not been studied in neurons in vivo, and whether Dynamin acts in different aspects of synaptic vesicle formation remains enigmatic. We used acute photoinactivation and found that loss of Dynamin function blocked membrane recycling and caused the buildup of huge membrane-connected cisternae, in contrast to the invaginated pits that accumulate in shi^ts1 mutants. Moreover, photoinactivation of Dynamin in shi^ts1 animals converted these pits into bulk cisternae. Bulk membrane retrieval has also been seen upon Clathrin photoinactivation, and superresolution imaging indicated that acute Dynamin photoinactivation blocked Clathrin and α-adaptin relocalization to synaptic membranes upon nerve stimulation. Hence, our data indicate that Dynamin is critically involved in the stabilization of Clathrin- and AP2-dependent endocytic pits.

Fusion pore regulation in peptidergic vesicles

Regulated exocytosis, which involves fusion of secretory vesicles with the plasma membrane, is an important mode of communication between cells. In this process, signalling molecules that are stored in secretory vesicles are released into the extracellular space. During the initial stage of fusion, the interior of the vesicle is connected to the exterior of the cell with a narrow, channel-like structure: the fusion pore. It was long believed that the fusion pore is a short-lived intermediate state leading irreversibly to fusion pore dilation. However, recent results show that the diameter of the fusion pore can fluctuate, suggesting that the fusion pore is a subject of stabilization. A possible mechanism is addressed in this article, involving the local anisotropicity of membrane constituents that can stabilize the fusion pore. The molecular nature of such a stable fusion pore to predict how interacting molecules (proteins and/or lipids) mediate changes that affect the stability of the fusion pore and exocytosis is also considered. The fusion pore likely attains stability via multiple mechanisms, which include the shape of the lipid and protein membrane constituents and the interactions between them.

The fusion pore and vesicle cargo discharge modulation

Exocytosis, the merger of the vesicle membrane with the plasma membrane, is thought to mediate the release of hormones and neurotransmitters from secretory vesicles. The work of Bernard Katz and colleagues decades ago considered that vesicle cargo discharge initially requires the delivery of secretory vesicles to the plasma membrane where vesicles dock and are primed for fusion with the plasma membrane. Then, upon stimulation, the vesicle and the plasma membranes fuse to form a transient fusion pore through which cargo molecules diffuse out of the vesicle lumen into the extracellular space. Katz and colleagues considered this process to occur in an all-or-none fashion. However, recent studies show that this may not be so simple. The aim of this overview is to highlight the novel findings that indicate that fusion pores are subject to regulations, which affect the release competence of a single vesicle. Here we discuss the elementary properties of spontaneous and stimulated peptidergic vesicle discharge, which appears to be modulated, at least in pituitary lactotrophs, by fusion pore conductance (pore diameter) and fusion pore gating (kinetics).

Phosphatidylinositol metabolism and membrane fusion

Membrane fusion underlies many cellular events, including secretion, exocytosis, endocytosis, organelle reconstitution, transport from endoplasmic reticulum to Golgi and nuclear envelope formation. A large number of investigations into membrane fusion indicate various roles for individual members of the phosphoinositide class of membrane lipids. We first review the phosphoinositides as membrane recognition sites and their regulatory functions in membrane fusion. We then consider how modulation of phosphoinositides and their products may affect the structure and dynamics of natural membranes facilitating fusion. These diverse roles underscore the importance of these phospholipids in the fusion of biological membranes.

Regulated exocytosis: merging ideas on fusing membranes

Cellular trafficking pathways end with fusion reactions at the target. These reactions have been studied extensively for many decades, but recent studies have been particularly productive in providing new solutions to old problems, especially in some of the most complex fusion reactions, like synaptic vesicle secretion in neurons. Here, we discuss new studies that begin to merge ideas on three central questions: (A) are all releasable vesicles equally likely to undergo fusion, (B) do different fusion modes contribute to synaptic transmission, and (C) which molecular events are 'upstream' and which ones 'downstream' of SNARE complex assembly.

Elementary properties of spontaneous fusion of peptidergic vesicles: fusion pore gating

The release of hormones and neurotransmitters by regulated exocytosis requires the delivery of secretory vesicles to the plasma membrane, where they dock and become primed for fusion with the plasma membrane. Upon stimulation a fusion pore is formed through which cargo molecules diffuse out of the vesicle lumen into the extracellular space. After the cargo release the fusion pore either closes (kiss-and-run, transient exocytosis), fluctuates between an open and a closed state (for short times, fusion pore flickering, or for rather longer periods, 'pulsing pore') or expands irreversibly (full fusion exocytosis). In almost all secretory cells spontaneous secretion of vesicle cargo can be detected in the absence of stimulation. Spontaneous and stimulated exocytosis were thought to exhibit similar properties at elementary level, differing only in the probability of occurrence. However, recent studies indicate that spontaneous exocytosis differs from the stimulated one in many respects, therefore opening questions about the physiological role of spontaneous exocytosis. In this report we address the elementary properties of spontaneous and stimulated peptidergic vesicle discharge which appears to be modulated by fusion pore conductance (diameter) and fusion pore gating.

Macroendocytosis and endosome processing in snake motor boutons

We have examined the processing of endosomes formed by macroendocytosis (ME), or bulk membrane retrieval, in active motor terminal boutons at the snake nerve-muscle synapse. Endocytic probes were imaged at light (FM1-43) and electron (horseradish peroxidase (HRP)) levels over stimulus frequencies representing low, intermediate and high levels of use. Endosomes formed rapidly (1-2 s) at all frequencies, concomitant with clathrin-mediated vesicular endocytosis (CME). Endosomes dissipated rapidly into vesicles (approximately 10 s). The dissipation rate was not influenced by activity. Many endosomes split into clusters of 2-20 smaller endosomes of varying size. Vesicles budded from these smaller endosomes, from large endosomes that had not undergone fission into smaller ones, and from precursor membrane infoldings that had not yet internalized. In snake, exocytosed vesicular membrane is not competent for reuse until after a delay (> 3 min). We found that time required for endosome processing is not responsible for this delay. Endosome processing might, however, limit availability of some vesicles for release at very high levels of use. Generally, endosome processing paralleled that of vesicles internalized directly from the plasma membrane via CME, regardless of stimulus frequency. There was no evidence for differential recruitment of ME versus CME depending upon level of use.