Distinct endocytic pathways control the rate and extent of synaptic vesicle protein recycling

What is the function of neuronal AP-3?

Neurotransmission requires the proper organization and rapid recycling of synaptic vesicles. Rapid retrieval has been suggested to occur either by kiss-and-stay or kiss-and-run mechanisms, whereas classical recycling is mediated by clathrin-dependent endocytosis. Molecular coats are key components in the selection of cargos, AP-2 (adaptor protein 2) playing a prominent role in synaptic vesicle endocytosis. Another coat protein, AP-3, has been implicated in synaptic vesicle biogenesis and in the generation of secretory and lysosomal-related organelles. In the present review, we will particularly focus on the recent data concerning the recycling of synaptic vesicles and the function of AP-3 and the v-SNARE (vesicular soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) TI-VAMP (tetanus neurotoxin-insensitive vesicle-associated membrane protein) in these processes. We propose that AP-3 plays an important regulatory role in neurons which contributes to the basal and stimulated exocytosis of synaptic vesicles.

Identification and characterization of SV31, a novel synaptic vesicle membrane protein and potential transporter

Synaptic vesicle proteins govern all relevant functions of the synaptic vesicle life cycle, including vesicle biogenesis, vesicle transport, uptake and storage of neurotransmitters, and regulated endocytosis and exocytosis. In spite of impressive progress made in the past years, not all known vesicular functions can be assigned to defined protein components, suggesting that the repertoire of synaptic vesicle proteins is still incomplete. We have identified and characterized a novel synaptic vesicle membrane protein of 31 kDa with six putative transmembrane helices that, according to its membrane topology and phylogenetic relation, may function as a vesicular transporter. The vesicular allocation is demonstrated by subcellular fractionation, heterologous expression, immunocytochemical analysis of brain sections and immunoelectron microscopy. The protein is expressed in select brain regions and contained in subpopulations of nerve terminals that immunostain for the vesicular glutamate transporter 1 and the vesicular GABA transporter VGaT (vesicular amino acid transporter) and may attribute specific and as yet undiscovered functions to subsets of glutamatergic and GABAergic synapses.

UNC-46 is required for trafficking of the vesicular GABA transporter

Mutations in unc-46 in Caenorhabditis elegans cause defects in all behaviors that are mediated by GABA. Here we show that UNC-46 is a sorting factor that localizes the vesicular GABA transporter to synaptic vesicles. The UNC-46 protein is related to the LAMP (lysosomal associated membrane protein) family of proteins and is localized at synapses. In unc-46 mutants, the vesicular transporter is not found specifically in synaptic vesicles but rather is diffusely spread along the axon. Mislocalization of the transporter severely reduces the frequency of miniature currents, but the remaining currents are normal in amplitude. Because the number of synaptic vesicles is not depleted, it is likely that only a fraction of vesicles harbor the transporter in unc-46 mutants. Our data indicate that the transporter and UNC-46 have mutual roles in sorting. The vesicular GABA transporter recruits UNC-46 to synaptic vesicle precursors in the cell body, and UNC-46 sorts the transporter at the cell body and during endocytosis at the synapse.

Kiss-and-run, Collapse and ‘Readily Retrievable’ Vesicles

Two models of synaptic vesicle recycling have been intensely debated for decades: kiss-and-run, in which the vesicle opens and closes transiently, presumably through a small fusion pore, and full fusion, in which the vesicle collapses into the plasma membrane and is retrieved by clathrin-coat-dependent processes. Conceptually, it seems that kiss-and-run would be faster and would retrieve vesicles with greater fidelity. Is this the case? This review discusses recent evidence for both models. We conclude that both mechanisms allow for high fidelity of vesicle recycling. Also, the presence in the plasma membrane of a depot of previously fused vesicles that are already interacting with the endocytotic machinery (the 'readily retrievable' vesicles) allows full fusion to trigger quite fast endocytosis, further blurring the efficiency differences between the two models.

Clathrin-mediated endocytosis at synapses

Neurons are communication specialists that convert electrical into chemical signals at specialized cell-cell junctions termed synapses. Arrival of an action potential triggers calcium-regulated exocytosis of neurotransmitter (NT) from small synaptic vesicles (SVs), which then diffuses across the synaptic cleft and binds to postsynaptic receptors to elicit specific changes within the postsynaptic cell. Endocytosis of pre- and postsynaptic membrane proteins including SV components and postsynaptic NT receptors is essential for the proper functioning of the synapse. During the past several years, we have witnessed enormous progress in our understanding of the mechanics of clathrin-mediated endocytosis (CME) and its role in regulating exo-endocytic vesicle cycling at synapses. Here we summarize the molecular machinery used for recognition of synaptic membrane protein cargo and its clathrin-dependent internalization, and describe the inventory of tools that can be used to monitor vesicle cycling at synapses or to inhibit CME in a stage-specific manner.

Monitoring synaptic function at the neuromuscular junction of a mouse expressing synaptopHluorin

We monitored presynaptic exocytosis and vesicle recycling at neuromuscular junctions of transgenic mice expressing synaptopHluorin (spH), using simultaneous optical and electrophysiological recordings. Synaptic transmission was indistinguishable from that in wild-type controls. Fluorescence rose during and decayed monotonically after stimulus trains to the nerve, with amplitudes and decay times increasing with the amount of stimulation. The relatively large size of synaptic terminals allowed us to examine the spatial profile of fluorescence changes. We identified hot spots of exocytosis, which were stable with repeated trains. Photobleach experiments showed that spH freshly exposed by nerve stimulation was not preferentially retrieved by compensatory endocytosis; instead, most retrieved spH preexisted in the surface membrane. Finally, we compared fluorescence and electrical [summed end-plate potentials (EPPs)] estimates of exocytosis, which diverged during repeated trains, as fluorescence exceeded summed EPPs, although the average amplitude of miniature EPPs was unchanged. This might reflect exocytosis of spH-containing, acetylcholine-free ("empty") vesicles or other organelles during intense stimulation.

Neuronal and non-neuronal functions of the AP-3 sorting machinery

Vesicles selectively exchange lipids, membrane proteins and luminal contents between organelles along the exocytic and endocytic routes. The repertoire of membrane proteins present in these vesicles is crucial for their targeting and function. Vesicle composition is determined at the time of their biogenesis by cytosolic coats. The heterotetrameric protein adaptor protein complex 3 (AP-3), a coat component, participates in the generation of a diverse group of secretory organelles and lysosome-related organelles. Recent work has shed light on the mechanisms that regulate AP-3 and the trafficking pathways controlled by this adaptor. Phenotypic analysis of organisms carrying genetic deficiencies in the AP-3 pathway highlight its role regulating the targeting of lysosomal, melanosomal and synaptic vesicle-specific membrane proteins. Synaptic vesicles from AP-3-deficient mice possess altered levels of neurotransmitter and ion transporters, molecules that ultimately define the type and amount of neurotransmitter stored in these vesicles. These findings reveal a complex picture of how AP-3 functions in multiple tissues, including neuronal tissue, and expose potential links between endocytic sorting mechanisms and the pathogenesis of psychiatric disorders such as schizophrenia.

Do different endocytic pathways make different synaptic vesicles?

At a wide range of synapses, synaptic vesicles reside in distinct pools that respond to different stimuli. The recycling pool supplies the vesicles required for release in response to modest stimulation, whereas the reserve pool is mobilized only by strong stimulation. Multiple pathways have been proposed for the recycling of synaptic vesicles after exocytosis, but the relationship of these pathways to the different synaptic vesicle pools has remained unclear. Synaptic vesicle proteins have also been assumed to undergo recycling as a unit. However, emerging data indicate that differences in the association with distinct endocytic adaptors such as the heterotetrameric adaptor AP3 influence the trafficking of individual synaptic vesicle proteins, affecting the composition of synaptic vesicles and hence their functional characteristics. These observations might begin to account for differences in the properties of different vesicle pools.

Membrane topology of the Drosophila vesicular glutamate transporter

The vesicular glutamate transporters (VGLUTs) are responsible for packaging glutamate into synaptic vesicles, and are part of a family of structurally related proteins that mediate organic anion transport. Standard computer-based predictions of transmembrane domains have led to divergent topological models, indicating the need for experimentally derived predictions. Here we present data on the topology of the VGLUT ortholog from Drosophila melanogaster (DVGLUT). Using immunofluorescence assays of DVGLUT transiently localized to the plasma membrane of heterologously transfected cells, we have determined the accessibility of epitope tags inserted into the lumenal/extracellular face of the protein. Using immunoisolation, we have identified complementary tagged sites that face the cytoplasm. Our data show that DVGLUT contains 10 hydrophobic regions that completely span the membrane (TMs 1-10) and that the amino and carboxyl termini are cytosolic. Importantly, between TMs 4 and 5 is an unforeseen cytosolic loop of some 50 residues. Other domains exposed to the cytosol include loops between TMs 6-7 and 8-9, and regions C-terminal to TM2 and N-terminal to TM3. Between TM2 and 3 is a potentially hydrophobic, but topologically ambiguous region. Lumenal domains include sequences between TMs 1-2, 3-4, 5-6, 7-8 and 9-10. These data provide a basis for determining structure-function relationships for DVGLUT and other related proteins.

Two synaptojanin 1 isoforms are recruited to clathrin-coated pits at different stages

Phosphoinositides are thought to play an important role in clathrin-coated pit (CCP) dynamics. Biochemical and structural studies have shown a direct interaction of phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P2] with endocytic clathrin adaptors, whereas functional studies using cell-free systems or intact cells have demonstrated the importance of PI(4,5)P2 synthesis and dephosphorylation in clathrin coating and uncoating, respectively. Furthermore, genetic manipulations of kinases and phosphatases involved in PI(4,5)P2 metabolism result in major defects in synaptic vesicle recycling and other forms of clathrin-dependent endocytosis. However, live imaging studies of these enzymes at CCPs have not been conducted. We have used multicolor total internal reflection fluorescence microscopy (TIRFM) to visualize the spatial-temporal recruitment of synaptojanin 1 (SJ1), a polyphosphoinositide phosphatase, and its binding partner endophilin to CCPs. Strikingly, we observed differential temporal recruitment of the two major SJ1 splice variants to CCPs. The 145-kDa isoform, the predominant isoform expressed in the brain, was rapidly recruited as a "burst," together with endophilin, at a late stage of CCP formation. In contrast, the nonneuronal ubiquitously expressed 170-kDa isoform of SJ1 was present at all stages of CCP formation. These results raise the possibility that dynamic phosphoinositide metabolism may occur throughout the lifetime of a CCP.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

Rustling synaptic vesicle cargo after exocytosis

In this issue of Neuron, Voglmaier et al. provide new evidence that the retrieval of synaptic vesicle transporters after exocytosis proceeds along at least two different endocytic pathways. This work provides new insight into the mechanisms of sorting synaptic vesicle cargo at the cell surface.