Endocytosis of synaptotagmin 1 is mediated by a novel, tryptophan-containing motif

Synaptic vesicle proteins are selectively delivered to axons in mammalian neurons

Neurotransmitter-filled synaptic vesicles (SVs) mediate synaptic transmission and are a hallmark specialization in neuronal axons. Yet, how SV proteins are sorted to presynaptic nerve terminals remains the subject of debate. The leading model posits that these proteins are randomly trafficked throughout neurons and are selectively retained in presynaptic boutons. Here, we used the RUSH (retention using selective hooks) system, in conjunction with HaloTag labeling approaches, to study the egress of two distinct transmembrane SV proteins, synaptotagmin 1 and synaptobrevin 2, from the soma of mature cultured rat and mouse neurons. For these studies, the SV reporter constructs were expressed at carefully controlled, very low levels. In sharp contrast to the selective retention model, both proteins selectively and specifically entered axons with minimal entry into dendrites. However, even moderate overexpression resulted in the spillover of SV proteins into dendrites, potentially explaining the origin of previous non-polarized transport models, revealing the limited, saturable nature of the direct axonal trafficking pathway. Moreover, we observed that SV constituents were first delivered to the presynaptic plasma membrane before incorporation into SVs. These experiments reveal a new-found membrane trafficking pathway, for SV proteins, in classically polarized mammalian neurons and provide a glimpse at the first steps of SV biogenesis.

The recycling regulation of sodium-hydrogen exchanger isoform 3(NHE3) in epithelial cells

As the main exchanger of electroneutral NaCl absorption, sodium-hydrogen exchanger isoform 3 (NHE3) circulates in the epithelial brush border (BB) and intracellular compartments in a multi-protein complex. The size of the NHE3 complex changes during rapid regulation events. Recycling regulation of NHE3 in epithelial cells can be roughly divided into three stages. First, when stimulated by Ca²⁺, cGMP, and cAMP-dependent signaling pathways, NHE3 is converted from an immobile complex found at the apical microvilli (MV) into an easily internalized and mobile form that relocates to a compartment near the base of the MV. Second, NHE3 is internalized by clathrin and albumin-dependent pathways into cytoplasmic endosomal compartments, where the complex is reprocessed and reassembled. Finally, NHE3 is translocated from the recycling endosomes (REs) to the apex of epithelial cells, a process that can be stimulated by an increase in sodium-glucose cotransporter 1 (SGLT1) activity, epidermal growth factor receptor (EGFR) signaling, Ca²⁺ signaling, and binding to βPix and SH3 and multiple ankyrin repeat domains 2 (Shank2) proteins. This review describes the molecular steps and protein interactions involved in the recycling movement of NHE3 from the apex of epithelial cells, into vesicles, where it is reprocessed and reassembled, and returned to its original location on the plasma membrane, where it exerts its physiological function.

Function of Drosophila Synaptotagmins in membrane trafficking at synapses

The Synaptotagmin (SYT) family of proteins play key roles in regulating membrane trafficking at neuronal synapses. Using both Ca²⁺-dependent and Ca²⁺-independent interactions, several SYT isoforms participate in synchronous and asynchronous fusion of synaptic vesicles (SVs) while preventing spontaneous release that occurs in the absence of stimulation. Changes in the function or abundance of the SYT1 and SYT7 isoforms alter the number and route by which SVs fuse at nerve terminals. Several SYT family members also regulate trafficking of other subcellular organelles at synapses, including dense core vesicles (DCV), exosomes, and postsynaptic vesicles. Although SYTs are linked to trafficking of multiple classes of synaptic membrane compartments, how and when they interact with lipids, the SNARE machinery and other release effectors are still being elucidated. Given mutations in the SYT family cause disorders in both the central and peripheral nervous system in humans, ongoing efforts are defining how these proteins regulate vesicle trafficking within distinct neuronal compartments. Here, we review the Drosophila SYT family and examine their role in synaptic communication. Studies in this invertebrate model have revealed key similarities and several differences with the predicted activity of their mammalian counterparts. In addition, we highlight the remaining areas of uncertainty in the field and describe outstanding questions on how the SYT family regulates membrane trafficking at nerve terminals.

Disorders of synaptic vesicle fusion machinery

The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative disorders, revealing that the presynaptic machinery governing synaptic vesicle fusion is compromised in many of these neurological disorders. This builds upon decades of research showing that disturbance to neurotransmitter release via toxins can cause acute neurological dysfunction. In this review, we focus on disorders of synaptic vesicle fusion caused either by toxic insult to the presynapse or alterations to genes encoding the key proteins that control and regulate fusion: the SNARE proteins (synaptobrevin, syntaxin-1 and SNAP-25), Munc18, Munc13, synaptotagmin, complexin, CSPα, α-synuclein, PRRT2 and tomosyn. We discuss the roles of these proteins and the cellular and molecular mechanisms underpinning neurological deficits in these disorders.

Cellular endocytic compartment localization of expressed canine CD1 molecules

CD1 molecules are glycoproteins present primarily on dendritic cells (DCs), which recognize and present a variety of foreign- and self-lipid antigens to T-cells. Humans have five different CD1 isoforms that survey distinct cellular compartments allowing for recognition of a large repertoire of lipids. The canine CD1 family consists of seven functional CD1 molecules (canine CD1a2, CD1a6, CD1a8, CD1a9, CD1b, CD1c and CD1e) and one presumed non-functional isoform (canine CD1d) due to a disrupted gene structure. The aim of this study was to describe in vitro steady-state localization ptterns of canine CD1 isoforms and their correlation with endocytic organelles. GFP-fused canine CD1 293T cell transfectants were stained with markers for early endocytic compartments (EEA-1) and late endocytic/lysosomal compartments (LAMP-1), respectively, and analyzed by confocal microscopy. Canine CD1a molecules localized to the plasma membrane and partially to the early endocytic compartment, but not to late endosomes or lysosomes. In contrast, canine CD1b was highly associated with late endosomal/lysosomal compartments and showed a predominant intracellular expression pattern. Canine CD1c protein expression localized more promiscuously to both the early endosomal compartments and the late endosomal/lysosomal compartments. The canine CD1e molecule showed a strictly intracellular expression with a partial overlap with late endosomal/lysosomal compartments. Lastly, canine CD1d was expressed abnormally showing only a diminished GFP expression. In conclusion, canine CD1 transfectants show distinct localization patterns that are similar to human CD1 proteins with the exception of the canine CD1d isoform, which most likely is non-functional. These findings imply that canine CD1 localization overall resembles human CD1 trafficking patterns. This knowledge is important for the understanding of lipid antigen-receptor immunity in the dog.

Ca2+ regulates the Drosophila Stoned-A and Stoned-B proteins interaction with the C2B domain of Synaptotagmin-1

The dicistronic Drosophila stoned gene is involved in exocytosis and/or endocytosis of synaptic vesicles. Mutations in either stonedA or stonedB cause a severe disruption of neurotransmission in fruit flies. Previous studies have shown that the coiled-coil domain of the Stoned-A and the µ-homology domain of the Stoned-B protein can interact with the C2B domain of Synaptotagmin-1. However, very little is known about the mechanism of interaction between the Stoned proteins and the C2B domain of Synaptotagmin-1. Here we report that these interactions are increased in the presence of Ca(2+). The Ca(2+)-dependent interaction between the µ-homology domain of Stoned-B and C2B domain of Synaptotagmin-1 is affected by phospholipids. The C-terminal region of the C2B domain, including the tryptophan-containing motif, and the Ca(2+) binding loop region that modulate the Ca(2+)-dependent oligomerization, regulates the binding of the Stoned-A and Stoned-B proteins to the C2B domain. Stoned-B, but not Stoned-A, interacts with the Ca(2+)-binding loop region of C2B domain. The results indicate that Ca(2+)-induced self-association of the C2B domain regulates the binding of both Stoned-A and Stoned-B proteins to Synaptotagmin-1. The Stoned proteins may regulate sustainable neurotransmission in vivo by binding to Ca(2+)-bound Synaptotagmin-1 associated synaptic vesicles.

Stoned

The stoned proteins, stoned A (STNA) and stoned B (STNB), are essential for normal vesicle trafficking in Drosophila melanogaster neurons, and deletion of the stoned locus is lethal. Although there is a growing body of research aimed at defining the roles of these proteins, particularly for STNB where homologues have now been identified in all multicellular species, their functions and mechanisms of action are not yet established. The two proteins are structurally unrelated, consistent with two distinct cellular functions. The evidence suggests a critical requirement for stoned proteins in recycling/regulation or specification of a competent synaptic vesicle pool. As stoned proteins may be specific to a particular pathway of endocytosis, studies of their function are likely to be valuable in distinguishing between the different mechanisms of membrane retrieval and their respective contributions to synaptic vesicle recycling, a subject of considerable scientific debate. In this review, we examine the published literature on stoned and comment on the available data, conclusions from these analyses and how they may relate to alternative models of vesicle cycling.

Molecular mechanism of the synaptotagmin-SNARE interaction in Ca2+-triggered vesicle fusion

In neurons, SNAREs, synaptotagmin, and other factors catalyze Ca²⁺-triggered fusion of vesicles with the plasma membrane. The molecular mechanism of this process remains an enigma, especially regarding the interaction between synaptotagmin and SNAREs. Here we characterized this interaction by single-molecule fluorescence microscopy and crystallography. The two rigid Ca²⁺-binding domains of synaptotagmin 3 undergo large relative motions in solution. Interaction with SNARE complex amplifies a particular state of the two domains that is further enhanced by Ca²⁺. This state is represented by the first SNARE-induced Ca²⁺-bound crystal structure of a synaptotagmin fragment containing both domains. The arrangement of the Ca²⁺-binding loops of this structure of synaptotagmin 3 matches that of SNARE-bound synaptotagmin 1, suggesting a conserved feature of synaptotagmins. The loops resemble the membrane-interacting loops of certain viral fusion proteins in the postfusion state, suggesting unexpected similarities between both fusion systems.

Cyclic AMP-mediated endocytosis of intestinal epithelial NHE3 requires binding to synaptotagmin 1

The apical membrane Na(+)-H(+) exchanger (NHE)3 is regulated by cAMP-dependent phosphorylation, which inhibits its activity through membrane endocytosis. The clathrin complex adaptor protein synaptotagmin 1 (Syt 1) appears to be essential to this process, but little is known about its expression in intestinal epithelial cells or interaction with NHE3. The intestinal epithelial expression and apical location of Syt 1 were determined by Syt 1 mRNA profiling and immunolocalization. Tandem mass spectrometry was used for protein identification. Bis(sulfosuccinimidyl) suberate (BS(3)) cross linking suggested that NHE3 and Syt 1 were in a membrane complex following cAMP stimulation of Caco2BBE (Brush Border Expressions) cells. To investigate the regulation of NHE3 appearance in a Syt 1-containing membrane compartment, doxycycline-inducible hemaglutinin (HA)-tagged NHE3 was expressed in Caco2BBE cells. HA-NHE3 correctly targeted to the apical membrane, where, upon cAMP stimulation, it was internalized with a Syt 1-containing compartment. Site-directed mutagenesis of NHE3 showed that serine 605 (S605) was pivotal to NHE3 and Syt 1 association and internalization. Direct Syt 1 interaction with NHE3 was suggested by fluorescence resonance energy transfer (FRET) analysis. The physiological role of S552 was less clear. By FRET, this serine residue appeared to be involved in cAMP-induced Syt 1 binding of NHE3. However, when HA-tagged NHE3 S552A was expressed in Caco2 cells, the mutated construct was not inserted into the apical membrane. We conclude that intestinal epithelial Syt 1 plays an important role in cAMP-stimulated endocytosis of apical NHE3 through cAMP-dependent phosphorylation of S605 that is required for NHE3 and Syt 1 association.

Arabidopsis synaptotagmin SYTA regulates endocytosis and virus movement protein cell-to-cell transport

Synaptotagmins are calcium sensors that regulate synaptic vesicle exo/endocytosis. Thought to be exclusive to animals, they have recently been characterized in plants. We show that Arabidopsis synaptotagmin SYTA regulates endosome recycling and movement protein (MP)-mediated trafficking of plant virus genomes through plasmodesmata. SYTA localizes to endosomes in plant cells and directly binds the distinct Cabbage leaf curl virus (CaLCuV) and Tobacco mosaic virus (TMV) cell-to-cell movement proteins. In a SYTA knockdown line, CaLCuV systemic infection is delayed, and cell-to-cell spread of TMV and CaLCuV movement proteins is inhibited. A dominant-negative SYTA mutant causes depletion of plasma membrane-derived endosomes, produces large intracellular vesicles attached to plasma membrane, and inhibits cell-to-cell trafficking of TMV and CaLCuV movement proteins, when tested in an Agrobacterium-based leaf expression assay. Our studies show that SYTA regulates endocytosis, and suggest that distinct virus movement proteins transport their cargos to plasmodesmata for cell-to-cell spread via an endocytic recycling pathway.

Plasma membrane repair in plants

Resealing is the membrane-repair process that enables cells to survive disruption, preventing the loss of irreplaceable cell types and eliminating the cost of replacing injured cells. Given that failure in the resealing process in animal cells causes diverse types of muscular dystrophy, plasma membrane repair has been extensively studied in these systems. Animal proteins with Ca(2+)-binding domains such as synaptotagmins and dysferlin mediate Ca(2+)-dependent exocytosis to repair plasma membranes after mechanical damage. Until recently, no components or proof for membrane repair mechanisms have been discovered in plants. However, Arabidopsis SYT1 is now the first plant synaptotagmin demonstrated to participate in Ca(2+)-dependent repair of membranes. This suggests a conservation of membrane repair mechanisms between animal and plant cells.

Two-way traffic on the road to plasma membrane repair

Ca(2+) influx through plasma membrane wounds triggers a rapid-repair response that is essential for cell survival. Earlier studies showed that repair requires the exocytosis of intracellular vesicles. Exocytosis was thought to promote resealing by 'patching' the plasma membrane lesion or by facilitating bilayer restoration through reduction in membrane tension. However, cells also rapidly repair lesions created by pore-forming proteins, a form of injury that cannot be resealed solely by exocytosis. Recent studies indicate that, in cells injured by pores or mechanical abrasions, exocytosis is followed by lesion removal through endocytosis. Describing the relationship between wound-induced exocytosis and endocytosis has implications for the understanding of muscular degenerative diseases that are associated with defects in plasma membrane repair.

Regulation of insulin secretion and GLUT4 trafficking by the calcium sensor synaptotagmin VII

Insulin regulates blood glucose by promoting uptake by fat and muscle, and inhibiting production by liver. Insulin-stimulated glucose uptake is mediated by GLUT4, which translocates from an intracellular compartment to the plasma membrane. GLUT4 traffic and insulin secretion both rely on calcium-dependent, regulated exocytosis. Deletion of the voltage-gated potassium channel Kv1.3 results in constitutive expression of GLUT4 at the plasma membrane. Inhibition of channel activity stimulated GLUT4 translocation through a calcium dependent mechanism. The synaptotagmins (Syt) are calcium sensors for vesicular traffic, and Syt VII mediates lysosomal and secretory granule exocytosis. We asked if Syt VII regulates insulin secretion by pancreatic beta cells, and GLUT4 translocation in insulin-sensitive tissues mouse model. Syt VII deletion (Syt VII -/-) results in glucose intolerance and a marked decrease in glucose-stimulated insulin secretion in vivo. Pancreatic islet cells isolated from Syt VII -/- cells secreted significantly less insulin than islets of littermate controls. Syt VII deletion disrupted GLUT4 traffic as evidenced by constitutive expression of GLUT4 present at the plasma membrane of fat and skeletal muscle cells and unresponsiveness to insulin. These data document a key role for Syt VII in peripheral glucose homeostasis through its action on both insulin secretion and GLUT4 traffic.

Eps15 and Dap160 control synaptic vesicle membrane retrieval and synapse development

Epidermal growth factor receptor pathway substrate clone 15 (Eps15) is a protein implicated in endocytosis, endosomal protein sorting, and cytoskeletal organization. Its role is, however, still unclear, because of reasons including limitations of dominant-negative experiments and apparent redundancy with other endocytic proteins. We generated Drosophila eps15-null mutants and show that Eps15 is required for proper synaptic bouton development and normal levels of synaptic vesicle (SV) endocytosis. Consistent with a role in SV endocytosis, Eps15 moves from the center of synaptic boutons to the periphery in response to synaptic activity. The endocytic protein, Dap160/intersectin, is a major binding partner of Eps15, and eps15 mutants phenotypically resemble dap160 mutants. Analyses of eps15 dap160 double mutants suggest that Eps15 functions in concert with Dap160 during SV endocytosis. Based on these data, we hypothesize that Eps15 and Dap160 promote the efficiency of endocytosis from the plasma membrane by maintaining high concentrations of multiple endocytic proteins, including dynamin, at synapses.

Synaptotagmin I binds intestinal epithelial NHE3 and mediates cAMP- and Ca2+-induced endocytosis by recruitment of AP2 and clathrin

Apical membrane sodium hydrogen exchanger 3 (NHE3), a major pathway for non-nutrient-dependent intestinal Na(+) absorption, is tightly regulated by second messenger systems that affect its functional activity and membrane trafficking. However, the events and components involved in NHE3 regulation are only partially understood. We report that the adaptor protein synaptotagmin I (Syt I) plays a pivotal role in cAMP- and Ca(2+)-induced cargo recognition of NHE3 and initiation of its endocytosis. Both mouse small intestine (jejunum) and Caco-2BBe Syt I coimmunoprecipitated with NHE3, particularly following increases in cellular cAMP or Ca(2+). Following short interfering RNA (siRNA) suppression of Syt I expression, cAMP- and Ca(2+)-induced inhibition of NHE3 activity were still observed but NHE3 endocytosis was blocked, as assessed by (22)Na influx and apical membrane biotin labeling, respectively. Similar effects on NHE3 inhibition and endocytosis were observed by siRNA suppression of either the mu-subunit of the adaptor protein 2 (AP2) complex or the heavy chain of clathrin. Coimmunoprecipitation analyses of NHE3 with these adaptor proteins revealed that cAMP- and Ca(2+)-induced NHE3-Syt I interaction preceded and was required for recruitment of AP2 and the clathrin complex. Confocal microscopy confirmed both the time sequence and protein associations of these events. We conclude that Syt I plays a pivotal role in mediating cAMP- and Ca(2+)-induced endocytosis of NHE3 (but not in inhibition of activity) through cargo recognition of NHE3 and subsequent recruitment of AP2-clathrin assembly required for membrane endocytosis.

C2B polylysine motif of synaptotagmin facilitates a Ca2+-independent stage of synaptic vesicle priming in vivo

Synaptotagmin I, a synaptic vesicle protein required for efficient synaptic transmission, contains a highly conserved polylysine motif necessary for function. Using Drosophila, we examined in which step of the synaptic vesicle cycle this motif functions. Polylysine motif mutants exhibited an apparent decreased Ca2+ affinity of release, and, at low Ca2+, an increased failure rate, increased facilitation, and increased augmentation, indicative of a decreased release probability. Disruption of Ca2+ binding, however, cannot account for all of the deficits in the mutants; rather, the decreased release probability is probably due to a disruption in the coupling of synaptotagmin to the release machinery. Mutants exhibited a major slowing of recovery from synaptic depression, which suggests that membrane trafficking before fusion is disrupted. The disrupted process is not endocytosis because the rate of FM 1-43 uptake was unchanged in the mutants, and the polylysine motif mutant synaptotagmin was able to rescue the synaptic vesicle depletion normally found in syt(null) mutants. Thus, the polylysine motif functions after endocytosis and before fusion. Finally, mutation of the polylysine motif inhibits the Ca2+-independent ability of synaptotagmin to accelerate SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)-mediated fusion. Together, our results demonstrate that the polylysine motif is required for efficient Ca2+-independent docking and/or priming of synaptic vesicles in vivo.

Intersectin-1L nucleotide exchange factor regulates secretory granule exocytosis by activating Cdc42

Rho GTPases are key regulators of the actin cytoskeleton in membrane trafficking events. We previously reported that Cdc42 facilitates exocytosis in neuroendocrine cells by stimulating actin assembly at docking sites for secretory granules. These findings raise the question of the mechanism activating Cdc42 in exocytosis. The neuronal guanine nucleotide exchange factor, intersectin-1L, which specifically activates Cdc42 and is at an interface between membrane trafficking and actin dynamics, appears as an ideal candidate to fulfill this function. Using PC12 and chromaffin cells, we now show the presence of intersectin-1 at exocytotic sites. Moreover, through an RNA interference strategy coupled with expression of various constructs encoding the guanine nucleotide exchange domain, we demonstrate that intersectin-1L is an essential component of the exocytotic machinery. Silencing of intersectin-1 prevents secretagogue-induced activation of Cdc42 revealing intersectin-1L as the factor integrating Cdc42 activation to the exocytotic pathway. Our results extend the current role of intersectin-1L in endocytosis to a function in exocytosis and support the idea that intersectin-1L is an adaptor that coordinates exo-endocytotic membrane trafficking in secretory cells.

A pre-synaptic to-do list for coupling exocytosis to endocytosis

Synaptic vesicles are made locally in the nerve terminal during recycling of membrane. Synaptic vesicle proteins must be sorted and concentrated on the plasma membrane, packaged into a budding vesicle of precise size and cut away from the synaptic surface. Adaptors, scaffolds, BAR-domain and ENTH-domain proteins all must be coordinated to carry out this sequence of events prior to the action of dynamin. Details of how this is orchestrated at nerve terminals are just beginning to emerge.

Drosophila synaptotagmin Inull mutants show severe alterations in vesicle populations but calcium-binding motif mutants do not

Synaptotagmin I is a synaptic vesicle protein postulated to mediate vesicle docking, vesicle recycling, and the Ca(2+) sensing required to trigger vesicle fusion. Analysis of synaptotagmin I knockouts (sytI(NULL) mutants) in both Drosophila and mice led to these hypotheses. Although much research on the mechanisms of synaptic transmission in Drosophila is performed at the third instar neuromuscular junction, the ultrastructure of this synapse has never been analyzed in sytI(NULL) mutants. Here we report severe synaptic vesicle depletion, an accumulation of large vesicles, and decreased vesicle docking at sytI(NULL) third instar neuromuscular junctions. Mutations in synaptotagmin I's C(2)B Ca(2+)-binding motif nearly abolish synaptic transmission and decrease the apparent Ca(2+) affinity of neurotransmitter release. Although this result is consistent with disruption of the Ca(2+) sensor, synaptic vesicle depletion and/or redistribution away from the site of Ca(2+) influx could produce a similar phenotype. To address this question, we examined vesicle distributions at neuromuscular junctions from third instar C(2)B Ca(2+)-binding motif mutants and transgenic wild-type controls. The number of docked vesicles and the overall number of synaptic vesicles in the vicinity of active zones was unchanged in the mutants. We conclude that the near elimination of synaptic transmission and the decrease in the Ca(2+) affinity of release observed in C(2)B Ca(2+)-binding motif mutants is not due to altered synaptic vesicle distribution but rather is a direct result of disrupting synaptotagmin I's ability to bind Ca(2+). Thus, Ca(2+) binding by the C(2)B domain mediates a post-docking step in fusion.

AP180 maintains the distribution of synaptic and vesicle proteins in the nerve terminal and indirectly regulates the efficacy of Ca2+-triggered exocytosis

AP180 plays an important role in clathrin-mediated endocytosis of synaptic vesicles (SVs) and has also been implicated in retrieving SV proteins. In Drosophila, deletion of its homologue, Like-AP180 (LAP), has been shown to increase the size of SVs but decrease the number of SVs and transmitter release. However, it remains elusive whether a reduction in the total vesicle pool directly affects transmitter release. Further, it is unknown whether the lap mutation also affects vesicle protein retrieval and synaptic protein localization and, if so, how it might affect exocytosis. Using a combination of electrophysiology, optical imaging, electron microscopy, and immunocytochemistry, we have further characterized the lap mutant and hereby show that LAP plays additional roles in maintaining both normal synaptic transmission and protein distribution at synapses. While increasing the rate of spontaneous vesicle fusion, the lap mutation dramatically reduces impulse-evoked transmitter release at steps downstream of calcium entry and vesicle docking. Notably, lap mutations disrupt calcium coupling to exocytosis and reduce calcium cooperativity. These results suggest a primary defect in calcium sensors on the vesicles or on the release machinery. Consistent with this hypothesis, three vesicle proteins critical for calcium-mediated exocytosis, synaptotagmin I, cysteine-string protein, and neuronal synaptobrevin, are all mislocalized to the extrasynaptic axonal regions along with Dap160, an active zone marker (nc82), and glutamate receptors in the mutant. These results suggest that AP180 is required for either recycling vesicle proteins and/or maintaining the distribution of both vesicle and synaptic proteins in the nerve terminal.

Kinetic efficiency of endocytosis at mammalian CNS synapses requires synaptotagmin I

At nerve terminals, synaptic vesicle components are retrieved from the cell surface and recycled for local reuse soon after exocytosis. The kinetics of this coupling is critical for the proper functioning of synapses during repetitive action potential firing, because deficiencies in this process lead to abnormal depletion of the releasable vesicle pool. Although the molecular basis of this coupling is poorly understood, numerous biochemical data point to a role for synaptotagmin I (SytI), an essential synaptic vesicle protein required for fast calcium-dependent exocytosis. Here, using synapto-pHluorin in an approach that allows the dissection of endocytosis and exocytosis into separate components during periods of stimulation, we examined exocytic-endocytic coupling in synapses from SytI knockout mice and their WT littermates. We show that endocytosis is significantly impaired in the absence of SytI with the relative rates of endocytosis compared with exocytosis reduced approximately 3-fold with respect to WT. Thus, in addition to regulating exocytosis, SytI also controls the kinetic efficiency of endocytosis at nerve terminals.

Recognition of a basic AP-2 binding motif within the C2B domain of synaptotagmin is dependent on multimerization

Synaptotagmin is a multifunctional membrane protein that may regulate exo-endocytic cycling of synaptic vesicles at the presynaptic plasmalemma. Its C2B domain has been postulated to interact with a variety of effector molecules including acidic phospholipids, phosphoinositides, SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), calcium channels, and the clathrin adaptor complex AP-2. Here we report that a basic motif within the C2B domain is required and sufficient for binding to AP-2 via its mu2 subunit and that this interaction is dependent on multimerization of the AP-2 binding site. Moreover, we show that upon fusion to a plasma membrane reporter protein this sequence is sufficient to target the chimeric molecule for internalization. We hypothesize that basic motifs within multimeric membrane proteins may represent a novel type of clathrin/AP-2-dependent endocytosis signal.