Presynaptic membrane retrieval and endosome biology: defining molecularly heterogeneous synaptic vesicles

Abstract

The release and uptake of neurotransmitters by synaptic vesicles is a tightly controlled process that occurs in response to diverse stimuli at morphologically disparate synapses. To meet these architectural and functional synaptic demands, it follows that there should be diversity in the mechanisms that control their secretion and retrieval and possibly in the composition of synaptic vesicles within the same terminal. Here we pay particular attention to areas where such diversity is generated, such as the variance in exocytosis/endocytosis coupling, SNAREs defining functionally diverse synaptic vesicle populations and the adaptor-dependent sorting machineries capable of generating vesicle diversity. We argue that there are various synaptic vesicle recycling pathways at any given synapse and discuss several lines of evidence that support the role of the endosome in synaptic vesicle recycling.

Figure 1.

Synaptic vesicle recycling mechanisms and the origins of synaptic vesicle species. (A,B) Mechanisms of vesicle recycling at the presynaptic terminal. Note that vesicles have different sizes only as a way to add depth to the diagram. Kiss-and-run retrieval of vesicle membrane has been omitted for simplicity. The insert in A denotes the region of the neuron magnified in both diagrams. A and B differ in the level of activity of the synapse. A represents a resting synapse in which spontaneous fusion of vesicles occurs at the active zone. Membrane is retrieved by clathrin-mediated endocytosis (steps 1–2) to an early endosome (step 3). This endosome is capable of sorting synaptic membrane proteins into different vesicles that repopulate the synaptic vesicle pool located at the active zone. B represents a synapse undergoing evoked activity. Pathway 1 operates at low frequency and route 2–3 is recruited under high-frequency stimulation. Synaptic vesicle membrane is retrieved by clathrin-dynamin-mediated endocytosis directly from the plasma membrane adjacent to active zones in one step (step 1), or from deep plasma membrane invaginations or cisternae (step 2). These cisternae are also capable of sorting vesicle proteins into clathrin-dynamin-mediated budding profiles (step 2). Plasma-membrane-connected invaginations (step 2) give rise to endosomes (step 3) where membrane proteins get packed into different synaptic vesicle pools located at the active zone. Vesicle colors indicate different vesicular composition. Multivesicular body (MVB) accommodates targeting of presynaptic proteins to cell body lysosomes/degradative organelles as suggested by recent studies (Uytterhoeven et al. 2011; Maday et al. 2012).

Figure 2.

Endosome-sorting mechanisms at the nerve terminal. The image depicts a presynaptic early endosome as those presented in step 3 in Figure 1A or B. The proposed rab composition of the endosome is highlighted in one endosome yet different rabs could define distinct endosome populations. The endosome depicts three budding profiles with their proposed clathrin-adaptor-sorting mechanism, AP-1 and AP-3-BLOC-1, and a cholesterol-rich microdomain-dependent-sorting mechanism. The v-SNAREs VAMP4 and VAMP7 have been paired with adaptors known to bind these SNAREs. The degree to which these endosome-sorting mechanisms operate in diverse functional statuses of a synapse, namely, resting synapses, synchronous versus asynchronous evoked released, and low- versus high-frequency stimulation release, is discussed in the text.