Synaptic vesicle endocytosis: the races, places, and molecular faces

Dysfunction of Synaptic Vesicle Endocytosis in Parkinson's Disease

Parkinson’s disease (PD) is the second most common neurodegenerative disorder after Alzheimer’s disease. It is a chronic and progressive disorder estimated to affect at least 4 million people worldwide. Although the etiology of PD remains unclear, it has been found that the dysfunction of synaptic vesicle endocytosis (SVE) in neural terminal happens before the loss of dopaminergic neurons. Recently, accumulating evidence reveals that the PD-linked synaptic genes, including DNAJC6, SYNJ1, and SH3GL2, significantly contribute to the disruptions of SVE, which is vital for the pathogenesis of PD. In addition, the proteins encoded by other PD-associated genes such as SNCA, LRRK2, PRKN, and DJ-1 also play key roles in the regulation of SVE. Here we present the facts about SVE-related genes and discussed their potential relevance to the pathogenesis of PD.

α-Synuclein Dimers Impair Vesicle Fission during Clathrin-Mediated Synaptic Vesicle Recycling

α-Synuclein is a presynaptic protein that regulates synaptic vesicle (SV) trafficking. In Parkinson’s disease (PD) and several other neurodegenerative disorders, aberrant oligomerization and aggregation of α-synuclein lead to synaptic dysfunction and neurotoxicity. Despite evidence that α-synuclein oligomers are generated within neurons under physiological conditions, and that altering the balance of monomers and oligomers contributes to disease pathogenesis, how each molecular species of α-synuclein impacts SV trafficking is currently unknown. To address this, we have taken advantage of lamprey giant reticulospinal (RS) synapses, which are accessible to acute perturbations via axonal microinjection of recombinant proteins. We previously reported that acute introduction of monomeric α-synuclein inhibited SV recycling, including effects on the clathrin pathway. Here, we report the effects of α-synuclein dimers at synapses. Similar to monomeric α-synuclein, both recombinant α-synuclein dimers that were evaluated bound to small liposomes containing anionic lipids in vitro, but with reduced efficacy. When introduced to synapses, the α-synuclein dimers also induced SV recycling defects, which included a build up of clathrin-coated pits (CCPs) with constricted necks that were still attached to the plasma membrane, a phenotype indicative of a vesicle fission defect. Interestingly, both α-synuclein dimers induced longer necks on CCPs as well as complex, branching membrane tubules, which were distinct from the CCPs induced by a dynamin inhibitor, Dynasore. In contrast, monomeric α-synuclein induced a buildup of free clathrin-coated vesicles (CCVs), indicating an inhibition of clathrin-mediated endocytosis at a later stage during the clathrin uncoating process. Taken together, these data further support the conclusion that excess α-synuclein impairs SV recycling. The data additionally reveal that monomeric and dimeric α-synuclein produce distinct effects on clathrin-mediated endocytosis, predicting different molecular mechanisms. Understanding what these mechanisms are could help to further elucidate the normal functions of this protein, as well as the mechanisms underlying PD pathologies.

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

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.

A Bcl-xL-Drp1 complex regulates synaptic vesicle membrane dynamics during endocytosis

Following exocytosis, the rate of recovery of neurotransmitter release is determined by vesicle retrieval from the plasma membrane and by recruitment of vesicles from reserve pools within the synapse, the latter of which is dependent on mitochondrial ATP. The Bcl-2 family protein Bcl-x_L, in addition to its role in cell death, regulates neurotransmitter release and recovery in part by increasing ATP availability from mitochondria. We now find, however, that, Bcl-x_L directly regulates endocytotic vesicle retrieval in hippocampal neurons through protein/protein interaction with components of the clathrin complex. Our evidence suggests that, during synaptic stimulation, Bcl-x_L translocates to clathrin-coated pits in a calmodulin-dependent manner and forms a complex of proteins with the GTPase Drp1, Mff and clathrin. Depletion of Drp1 produces misformed endocytotic vesicles. Mutagenesis studies suggest that formation of the Bcl-x_L-Drp1 complex is necessary for the enhanced rate of vesicle endocytosis produced by Bcl-x_L, thus providing a mechanism for presynaptic plasticity.

A role for an Hsp70 nucleotide exchange factor in the regulation of synaptic vesicle endocytosis

Neurotransmission requires a continuously available pool of synaptic vesicles (SVs) that can fuse with the plasma membrane and release their neurotransmitter contents upon stimulation. After fusion, SV membranes and membrane proteins are retrieved from the presynaptic plasma membrane by clathrin-mediated endocytosis. After the internalization of a clathrin-coated vesicle, the vesicle must uncoat to replenish the pool of SVs. Clathrin-coated vesicle uncoating requires ATP and is mediated by the ubiquitous molecular chaperone Hsc70. In vitro, depolymerized clathrin forms a stable complex with Hsc70*ADP. This complex can be dissociated by nucleotide exchange factors (NEFs) that release ADP from Hsc70, allowing ATP to bind and induce disruption of the clathrin:Hsc70 association. Whether NEFs generally play similar roles in vesicle trafficking in vivo and whether they play such roles in SV endocytosis in particular is unknown. To address this question, we used information from recent structural and mechanistic studies of Hsp70:NEF and Hsp70:co-chaperone interactions to design a NEF inhibitor. Using acute perturbations at giant reticulospinal synapses of the sea lamprey (Petromyzon marinus), we found that this NEF inhibitor inhibited SV endocytosis. When this inhibitor was mutated so that it could no longer bind and inhibit Hsp110 (a NEF that we find to be highly abundant in brain cytosol), its ability to inhibit SV endocytosis was eliminated. These observations indicate that the action of a NEF, most likely Hsp110, is normally required during SV trafficking to release clathrin from Hsc70 and make it available for additional rounds of endocytosis.

Reduction of AP180 and CALM produces defects in synaptic vesicle size and density

Clathrin assembly proteins AP180 and CALM regulate the assembly of clathrin-coated vesicles (CCVs), which mediate diverse intracellular trafficking processes, including synaptic vesicle (SV) recycling at the synapse. Although studies using several invertebrate model systems have indicated a role for AP180 in SV recycling, less is known about AP180's or CALM's function in the synapse of mammalian neurons. In this study, we examined synapses of rat hippocampal neurons in which the level of AP180 or CALM had been reduced by RNA interference (RNAi). Using light microscopy, we visualized synaptic puncta in these AP180- or CALM-reduced neurons by co-expressing Synaptophysin::EGFP (Syp::EGFP). We found that neurons with reduced AP180 or reduced CALM had smaller Syp::EGFP-illuminated puncta. Using electron microscopy, we further examined the ultrastructure of the AP180- or CALM-reduced presynaptic terminals. We found that SVs became variably enlarged in both the AP180-reduced and CALM-reduced presynaptic terminals. Lower AP180 and CALM also reduced the density of SVs and the size of SV clusters. Our findings demonstrate that in the presynaptic terminals of hippocampal neurons, AP180 and CALM have a similar role in regulating synaptic vesicles. This overlapping activity may be necessary for high-precision and high-efficacy SV formation during endocytosis.

Endocytosis at the nanoscale

Endocytosis is a fundamental process in which eukaryotic cells internalise molecules and macromolecules via deformation of the membrane and generation of membrane-bound carriers. Functional aspects are not only limited to uptake of nutrients, but also play a primary role in evolutionary conserved processes such as the regulation of plasma membrane protein activity (i.e. signal-transducing receptors, small-molecule transporters and ion channels), cell motility and mitosis. The macromolecular nature of the material transported by endocytosis makes this route one of the most important targets for nanomedicine. Indeed, many nanoparticle formulations have been customised to enter cells through endocytosis and deliver the cargo within the cell. In this critical review, we present an overview of the biology of endocytosis and discuss its implications in cell internalisation of nanoparticles. We discuss how nanoparticle size, shape and surface chemistry can control this process effectively. Finally, we discuss different drug delivery strategies on how to evade lysosomal degradation to promote effective release of the cargo (376 references).

Membrane trafficking events underlying axon repair, growth, and regeneration

Two central challenges for the field of neurobiology are to understand how axons grow and make proper synaptic connections under normal conditions and how they repair their membranes and mount regenerative responses after injury. At the most reductionist level, the first step toward addressing these challenges is to delineate the cellular and molecular processes by which an axon extends from its cell body. Underlying axon extension are questions of appropriate timing and mechanisms that establish or maintain the axon's polarity, initiate growth cone formation, and promote axon outgrowth and synapse formation. After injury, the problem is even more complicated because the neuron must also repair its damaged membrane, redistribute or manufacture what it needs in order to survive, and grow and form new synapses within a more mature, complex environment. While other reviews have focused extensively on the signaling events and cytoskeletal rearrangements that support axon outgrowth and regeneration, we focus this review instead on the underlying membrane trafficking events underlying these processes. Though the mechanisms are still under active investigation, the key roles played by membrane trafficking events during axon repair, growth, and regeneration have been elucidated through elegant comparative studies in both invertebrate and vertebrate organisms. Taken together, a model emerges indicating that the critical requirements for ensuring proper membrane sealing and axon extension include iterative bouts of SNARE mediated exocytosis, endocytosis, and functional links between vesicles and the actin cytoskeleton, similar to the mechanisms utilized during synaptic transmission. This article is part of a Special Issue entitled 'Neuronal Function'.

Clathrin assembly proteins AP180 and CALM in the embryonic rat brain

Clathrin-coated vesicles are known to play diverse and pivotal roles in cells. The proper formation of clathrin-coated vesicles is dependent on, and highly regulated by, a large number of clathrin assembly proteins. These assembly proteins likely determine the functional specificity of clathrin-coated vesicles, and together they control a multitude of intracellular trafficking pathways, including those involved in embryonic development. In this study, we focus on two closely related clathrin assembly proteins, AP180 and CALM (clathrin assembly lymphoid myeloid leukemia protein), in the developing embryonic rat brain. We find that AP180 begins to be expressed at embryonic day 14 (E14), but only in postmitotic cells that have acquired a neuronal fate. CALM, on the other hand, is expressed as early as E12, by both neural stem cells and postmitotic neurons. In vitro loss-of-function studies using RNA interference (RNAi) indicate that AP180 and CALM are dispensable for some aspects of embryonic neurogenesis but are required for the growth of postmitotic neurons. These results identify the developmental stage of AP180 and CALM expression and suggest that each protein has distinct functions in neural development.

Role of HRB in clathrin-dependent endocytosis

Human immunodeficiency virus Rev-binding protein (HRB), also called human Rev-interacting protein (hRIP) or Rev/Rex activation domain binding (RAB) is a partner of the tyrosine kinase substrate EPS15, and it has been recovered in the AP-2 interactome. EPS15 and AP-2 are involved in endocytosis, but the function of HRB in this process is still unknown. Here we identified HRB as a partner of the vesicular SNARE tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP, also called VAMP7) in yeast two-hybrid screens and using biochemical assays. In HeLa cells, HRB localized both in the nucleus and in the cytoplasm. In the cytoplasm, HRB colocalized with clathrin-, AP-2-, EPS15-, and transferrin receptor-containing vesicles. We did not see significant colocalization between HRB and TI-VAMP in HeLa cells, and we saw partial colocalization with green fluorescent protein-TI-VAMP in stably expressing Madin-Darby canine kidney cells. Nevertheless using a pHLuorin-tagged TI-VAMP construct, we found that HRB and TI-VAMP colocalize close to the plasma membrane after 5 min of anti-green fluorescent protein antibody uptake. These results suggest that TI-VAMP and HRB may interact only during the early stages of endocytosis. Furthermore uptake experiments followed by fluorescence-activated cell sorting showed that the endocytosis of fluorescent transferrin and pHLuorin-TI-VAMP is strongly reduced in HRB knockdown cells. Altogether these results suggest that HRB is involved in clathrin-dependent endocytosis and recruits TI-VAMP in this process.

AP180 and CALM in the developing hippocampus: expression at the nascent synapse and localization to trafficking organelles

Genetic and biochemical evidence has established that clathrin assembly protein AP180 is required for the proper assembly of synaptic vesicles via clathrin-mediated endocytosis. The assembly protein CALM, the ubiquitously expressed homolog of AP180, also regulates the formation of clathrin-coated vesicles. In this study we found high expression levels of AP180 and CALM in hippocampal tissues as early as embryonic day 18, before the expression of synaptophysin. We also used immunoelectron microscopy to establish the distribution of AP180 and CALM in the developing hippocampal synapses. We found AP180 and CALM in synapses at all developmental stages and in nonsynaptic growing processes. In addition to localization on the plasma membrane and clathrin-coated vesicles that originated from the plasma membrane, we also report the presence of AP180 and CALM on other types of membrane structures. Our observations link AP180 and CALM to multiple vesicular organelles and raise the possibility that these proteins may play additional roles in developing neurons.

The transmitter release-site CaV2.2 channel cluster is linked to an endocytosis coat protein complex

Synaptic vesicles (SVs) are triggered to fuse with the surface membrane at the presynaptic transmitter release site (TRSs) core by Ca2+ influx through nearby attached CaV2.2 channels [see accompanying paper: Khanna et al. (2007)Eur. J. Neurosci., 26, 547-559] and are then recovered by endocytosis. In this study we test the hypothesis that the TRS core is linked to an endocytosis-related protein complex. This was tested by immunostaining analysis of the chick ciliary ganglion calyx presynaptic terminal and biochemical analysis of synaptosome lysate, using CaV2.2 as a marker for the TRS. We noted that CaV2.2 clusters abut heavy-chain (H)-clathrin patches at the transmitter release face. Quantitative coimmunostaining analysis (ICA/ICQ method) demonstrated a strong covariance of release-face CaV2.2 staining with that for the AP180 and intersectin endocytosis adaptor proteins, and a moderate covariance with H- or light-chain (L)-clathrin and dynamin coat proteins, consistent with a multimolecular complex. This was supported by coprecipitation of these proteins with CaV2.2 from brain synaptosome lysate. Interestingly, the channel neither colocalized nor coprecipitated with the endocytosis cargo-capturing adaptor AP2, even though this protein both colocalized and coprecipitated with H-clathrin. Fractional recovery analysis of the immunoprecipitated CaV2.2 complex by exposure to high NaCl (approximately 1 m) indicated that AP180 and S-intersectin adaptors are tightly bound to CaV2.2 while L-intersectin, H- and L-clathrin and dynamin form a less tightly linked subcomplex. Our results are consistent with two distinct clathrin endocytosis complexes: an AP2-containing, remote, non-TRS complex and a specialised, AP2-lacking, TRS-associated subcomplex linked via a molecular bridge. The most probable role of this subcomplex is to facilitate SV recovery after transmitter release.

UNC-13 and UNC-10/rim localize synaptic vesicles to specific membrane domains

Synaptic vesicles undergo a maturation step, termed priming, in which they become competent to fuse with the plasma membrane. To morphologically define the site of vesicle priming and identify fusion-competent synaptic vesicles, we combined a rapid physical-fixation technique with immunogold staining and high-resolution morphometric analysis at Caenorhabditis elegans neuromuscular junctions. In these presynaptic terminals, a subset of synaptic vesicles contact the plasma membrane within approximately 100 nm of a presynaptic dense projection. UNC-13, a protein required for vesicle priming, localizes to this same region of the plasma membrane. In an unc-13 null mutant, few synaptic vesicles contact the plasma membrane, suggesting that membrane-contacting synaptic vesicles represent the morphological correlates of primed vesicles. Interestingly, a subpopulation of membrane-contacting vesicles, located within 30 nm of a dense projection, are unperturbed in unc-13 mutants. We show that UNC-10/Rim, a protein implicated in presynaptic plasticity, localizes to dense projections and that loss of UNC-10/Rim causes an UNC-13-independent reduction in membrane-contacting synaptic vesicles within 30 nm of the dense projections. Our data together identify a discrete domain for vesicle priming within 100 nm of dense projections and further suggest that UNC-10/Rim and UNC-13 separately contribute to the membrane localization of synaptic vesicles within this domain.

Kiss-and-run and full-collapse fusion as modes of exo-endocytosis in neurosecretion

Neurotransmitters and hormones are released from neurosecretory cells by exocytosis (fusion) of synaptic vesicles, large dense-core vesicles and other types of vesicles or granules. The exocytosis is terminated and followed by endocytosis (retrieval). More than fifty years of research have established full-collapse fusion and clathrin-mediated endocytosis as essential modes of exo-endocytosis. Kiss-and-run and vesicle reuse represent alternative modes, but their prevalence and importance have yet to be elucidated, especially in neurons of the mammalian CNS. Here we examine various modes of exo-endocytosis across a wide range of neurosecretory systems. Full-collapse fusion and kiss-and-run coexist in many systems and play active roles in exocytotic events. In small nerve terminals of CNS, kiss-and-run has an additional role of enabling nerve terminals to conserve scarce vesicular resources and respond to high-frequency inputs. Full-collapse fusion and kiss-and-run will each contribute to maintaining cellular communication over a wide range of frequencies.

Clathrin and synaptic vesicle endocytosis: studies at the squid giant synapse

The role of clathrin-mediated endocytosis in SV (synaptic vesicle) recycling has been studied by combining molecular biology, physiology and electron microscopy at the squid giant synapse. Procedures that prevent clathrin from assembling into membrane coats, such as impairment of binding of the AP180 and AP-2 adaptor proteins, completely prevent membrane budding during endocytosis. These procedures also reduce exocytosis, presumably an indirect effect of a reduction in the number of SVs following block of endocytosis. Disrupting the binding of auxilin to Hsc70 (heat-shock cognate 70) prevents clathrin-coated vesicles from uncoating and also disrupts SV recycling. Taken together, these results indicate that a clathrin-dependent pathway is the primary means of SV recycling at this synapse under physiological conditions.

Partially overlapping distribution of epsin1 and HIP1 at the synapse: analysis by immunoelectron microscopy

Synapses of neurons use clathrin-mediated endocytic pathways for recycling of synaptic vesicles and trafficking of neurotransmitter receptors. Epsin 1 and huntingtin-interacting protein 1 (HIP1) are endocytic accessory proteins. Both proteins interact with clathrin and the AP2 adaptor complex and also bind to the phosphoinositide-containing plasma membrane via an epsin/AP180 N-terminal homology (ENTH/ANTH) domain. Epsin1 and HIP1 are found in neurons; however, their precise roles in synapses remain largely unknown. Using immunogold electron microscopy, we examine and compare the synaptic distribution of epsin1 and HIP1 in rat CA1 hippocampal synapse. We find that epsin1 is located across both sides of the synapse, whereas HIP1 displays a preference for the postsynaptic compartment. Within the synaptic compartments, espin1 is distributed similarly throughout, whereas postsynaptic HIP1 is concentrated near the plasma membrane. Our results suggest a dual role for epsin1 and HIP1 in the synapse: as broadly required factors for promoting clathrin assembly and as adaptors for specific endocytic pathways.

Dynamics of Endocytic Vesicle Creation

Clathrin-mediated endocytosis is the main path for receptor internalization in metazoans and is essential for controlling cell integrity and signaling. It is driven by a large array of protein and lipid interactions that have been deciphered mainly by biochemical and genetic means. To place these interactions into context, and ultimately build a fully operative model of endocytosis at the molecular level, it is necessary to know the kinetic details of the role of each protein in this process. In this review, we describe the recent efforts made, by using live cell imaging, to define clear steps in the formation of endocytic vesicles and to observe the recruitment of key proteins during membrane invagination, the scission of a newly formed vesicle, and its movement away from the plasma membrane.

Clathrin-dependent and clathrin-independent retrieval of synaptic vesicles in retinal bipolar cells

Synaptic vesicles can be retrieved rapidly or slowly, but the molecular basis of these kinetic differences has not been defined. We now show that substantially different sets of molecules mediate fast and slow endocytosis in the synaptic terminal of retinal bipolar cells. Capacitance measurements of membrane retrieval were made in terminals in which peptides and protein domains were introduced to disrupt known interactions of clathrin, the AP2 adaptor complex, and amphiphysin. All these manipulations caused a selective inhibition of the slow phase of membrane retrieval (time constant approximately 10 s), leaving the fast phase (approximately 1 s) intact. Slow endocytosis after strong stimulation was therefore dependent on the formation of clathrin-coated membrane. Fast endocytosis occurring after weaker stimuli retrieves vesicle membrane in a clathrin-independent manner. All compensatory endocytosis required GTP hydrolysis, but only a subset of released vesicles were primed for fast, clathrin-independent endocytosis.

Synaptic distribution of the endocytic accessory proteins AP180 and CALM

Clathrin-coated vesicles mediate a variety of endocytosis pathways in cells, including endocytic events at synapses. AP180 and clathrin assembly lymphoid myeloid leukemia protein (CALM) are clathrin accessory proteins that promote the formation of clathrin-coated vesicles. Both proteins bind to membrane lipids through their epsin N-terminal homology domains and interact with clathrin and related protein components through their carboxyl-terminal peptide motifs. We examine their neuronal expression and synaptic distribution. We show that both proteins are detected in synapses but demonstrate different distribution patterns. AP180 is located predominantly in presynaptic profiles, whereas CALM is found nonselectively in pre- and postsynaptic profiles and also in perisynaptic processes. These observations reveal an unexpected relationship between AP180 and the presumed non-neuronal homologue CALM. We propose that both AP180 and CALM function as endocytic accessory proteins at synapses, but each may regulate distinct clathrin pathways.

Regulation of exocytosis in neurons and neuroendocrine cells

Neurons communicate with one another through the release of molecules from synaptic vesicles and large dense core granules through the process of exocytosis. During exocytosis, molecules are released to the extracellular space through a fusion pore, which can either dilate, resulting in full fusion, or close, resulting in incomplete exocytosis, often referred to as 'kiss and run' exocytosis. Recently, there has been much interest in the regulation of this process in both neurons and neuroendocrine cells. There has been much recent work that addresses the existence of incomplete exocytosis in neurons and neuroendocrine cells, as well as recent work probing the molecular components and modulation of the fusion pore.

Fast endocytosis is inhibited by GABA-mediated chloride influx at a presynaptic terminal

Although multiple kinetic components of synaptic vesicle endocytosis have been identified, it has remained unclear whether neurons can differentially modulate these components. Using membrane capacitance measurements from isolated goldfish bipolar cell terminals, we found that the kinetics of endocytosis in retinal slices (single exponential decay; tau > 10 s) were significantly slower than those in acutely dissociated terminals (double exponential decay; tau(fast) approximately 1-2 s; tau(slow) > 10 s). Surprisingly, GABA(A) and/or GABA(C) receptor antagonists restored the fast component of endocytosis to terminals in retinal slices. Blocking GABAergic feedback from reciprocal synapses or removing external Cl(-) ions also allowed for fast endocytosis. Elevating internal Cl(-) via the patch pipette invariably slowed endocytosis, even in terminals dialyzed with increased Ca(2+) buffer. These results suggest a new role for GABA and Cl(-) ions in blocking the trigger for fast endocytosis at this ribbon-type synapse.

A role for talin in presynaptic function

Talin, an adaptor between integrin and the actin cytoskeleton at sites of cell adhesion, was recently found to be present at neuronal synapses, where its function remains unknown. Talin interacts with phosphatidylinositol-(4)-phosphate 5-kinase type Iγ, the major phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P₂]–synthesizing enzyme in brain. To gain insight into the synaptic role of talin, we microinjected into the large lamprey axons reagents that compete the talin–PIP kinase interaction and then examined their effects on synaptic structure. A dramatic decrease of synaptic actin and an impairment of clathrin-mediated synaptic vesicle endocytosis were observed. The endocytic defect included an accumulation of clathrin-coated pits with wide necks, as previously observed after perturbing actin at these synapses. Thus, the interaction of PIP kinase with talin in presynaptic compartments provides a mechanism to coordinate PI(4,5)P₂ synthesis, actin dynamics, and endocytosis, and further supports a functional link between actin and clathrin-mediated endocytosis.

Ligand-dependent and -independent transforming growth factor-beta receptor recycling regulated by clathrin-mediated endocytosis and Rab11

Proteins in the transforming growth factor-beta (TGF-beta) family recognize transmembrane serine/threonine kinases known as type I and type II receptors. Binding of TGF-beta to receptors results in receptor down-regulation and signaling. Whereas previous work has focused on activities controlling TGF-beta signaling, more recent studies have begun to address the trafficking properties of TGF-beta receptors. In this report, it is shown that receptors undergo recycling both in the presence and absence of ligand activation, with the rates of internalization and recycling being unaffected by ligand binding. Recycling occurs as receptors are most likely internalized through clathrin-coated pits, and then returned to the plasma membrane via a rab11-dependent, rab4-independent mechanism. Together, the results suggest a mechanism wherein activated TGF-beta receptors are directed to a distinct endocytic pathway for down-regulation and clathrin-dependent degradation after one or more rounds of recycling.

Heterogeneity of endocytic proteins: distribution of clathrin adaptor proteins in neurons and glia

Clathrin adaptor protein (AP)180 is a synaptic protein that regulates the assembly of clathrin-coated vesicles. Several endocytic proteins including AP2, CALM, and epsin 1 have functions or molecular structures similar to AP180. We determined if AP180 associates with functional synapses in cultured hippocampal neurons. We also compared the expression pattern of AP180 with the other endocytic proteins. The distribution of AP180 corresponded with the synaptic vesicle-associated protein synapsin I, and with functional presynaptic terminals labeled with the styryl dye FM1-43. Synaptic AP2 colocalized with AP180, but the distribution of AP2 was not limited to synapses of neurons and it was also expressed in glia. CLAM and epsin 1 immunoreactivities were also detected in both neurons and glia. Unlike AP180, the neuronal immunoreactivity of CALM was not intense in the synaptic puncta. Epsin 1 immunoreactivity was found in both synaptic and extrasynaptic sites, and its synaptic distribution only partially overlapped with that of AP180. These results support roles for AP180 in synaptic function in neurons. The findings also provide information on the distribution of AP2, CALM, and epsin 1 in cells of the nervous system that suggest different roles for these endocytic proteins in the biology of these cells.

Neurotransmitter Release at Central Synapses

Our understanding of synaptic transmission has grown dramatically during the 15 years since the first issue of Neuron was published, a growth rate expected from the rapid progress in modern biology. As in all of biology, new techniques have led to major advances in the cell and molecular biology of synapses, and the subject has evolved in ways (like the production of genetically engineered mice) that could not even be imagined 15 years ago. My plan for this review is to summarize what we knew about neurotransmitter release when Neuron first appeared and what we recognized we did not know, and then to describe how our views have changed in the intervening decade and a half. Some things we knew about synapses--"knew" in the sense that the field had reached a consensus--are no longer accepted, but for the most part, impressive advances have led to a new consensus on many issues. What I find fascinating is that in certain ways nothing has changed--many of the old arguments persist or recur in a different guise--but in other ways the field would be unrecognizable to a neurobiologist time-transported from 1988 to 2003.

[Neuromuscular synapse: molecular mechanisms of acetylcholine vesicular exocytosis]

The transmission of excitation from motor neurones to muscle fibers at the neuromuscular junction is made through liberation of acetylcholine. This is concentrated into vesicles according to an exchange with H(+) ions. Increase of intracellular calcium concentration leads some vesicles to mobilise and target with the neuron plasma membrane, where they are docked and preconditioned via the formation of a SNARE complex between vesicular proteins and plasma membrane proteins. Energy and control of these transports and reactions are provided by several kinds of enzymes and chaperone molecules. Arrival of an action potential causes depolarisation of the motoneurone axon terminal and opening of large conductance calcium channels. The last step, directly linked to calcium input, leads the membranes to fuse and acetylcholine to immediately leave the neuron towards the synaptic space.