The GIT family of proteins forms multimers and associates with the presynaptic cytomatrix protein Piccolo

The cytoskeletal matrix assembled at active zones (CAZ) is implicated in defining neurotransmitter release sites. However, little is known about the molecular mechanisms by which the CAZ is organized. Here we report a novel interaction between Piccolo, a core component of the CAZ, and GIT proteins, multidomain signaling integrators with GTPase-activating protein activity for ADP-ribosylation factor small GTPases. A small region (approximately 150 amino acid residues) in Piccolo, which is not conserved in the closely related CAZ protein Bassoon, mediates a direct interaction with the Spa2 homology domain (SHD) domain of GIT1. Piccolo and GIT1 colocalize at synaptic sites in cultured neurons. In brain, Piccolo forms a complex with GIT1 and various GIT-associated proteins, including betaPIX, focal adhesion kinase, liprin-alpha, and paxillin. Point mutations in the SHD of GIT1 differentially interfere with the association of GIT1 with Piccolo, betaPIX, and focal adhesion kinase, suggesting that these proteins bind to the SHD by different mechanisms. Intriguingly, GIT proteins form homo- and heteromultimers through their C-terminal G-protein-coupled receptor kinase-binding domain in a tail-to-tail fashion. This multimerization enables GIT1 to simultaneously interact with multiple SHD-binding proteins including Piccolo and betaPIX. These results suggest that, through their multimerization and interaction with Piccolo, the GIT family proteins are involved in the organization of the CAZ.     

Physical and functional interaction of the active zone proteins, CAST, RIM1, and Bassoon, in neurotransmitter release

We have recently isolated a novel cytomatrix at the active zone (CAZ)-associated protein, CAST, and found it directly binds another CAZ protein RIM1 and indirectly binds Munc13-1 through RIM1; RIM1 and Munc13-1 directly bind to each other and are implicated in priming of synaptic vesicles. Here, we show that all the CAZ proteins thus far known form a large molecular complex in the brain, including CAST, RIM1, Munc13-1, Bassoon, and Piccolo. RIM1 and Bassoon directly bind to the COOH terminus and central region of CAST, respectively, forming a ternary complex. Piccolo, which is structurally related to Bassoon, also binds to the Bassoon-binding region of CAST. Moreover, the microinjected RIM1- or Bassoon-binding region of CAST impairs synaptic transmission in cultured superior cervical ganglion neurons. Furthermore, the CAST-binding domain of RIM1 or Bassoon also impairs synaptic transmission in the cultured neurons. These results indicate that CAST serves as a key component of the CAZ structure and is involved in neurotransmitter release by binding these CAZ proteins.     

Structural and functional dissection of the Abp1 ADFH actin-binding domain reveals versatile in vivo adapter functions

Abp1 is a multidomain protein that regulates the Arp2/3 complex and links proteins involved in endocytosis to the actin cytoskeleton. All of the proposed cellular functions of Abp1 involve actin filament binding, yet the actin binding site(s) on Abp1 have not been identified, nor has the importance of actin binding for Abp1 localization and function in vivo been tested. Here, we report the crystal structure of the Saccharomyces cerevisiae Abp1 actin-binding actin depolymerizing factor homology (ADFH) domain and dissect its activities by mutagenesis. Abp1-ADFH domain and ADF/cofilin structures are similar, and they use conserved surfaces to bind actin; however, there are also key differences that help explain their differential effects on actin dynamics. Using point mutations, we demonstrate that actin binding is required for localization of Abp1 in vivo, the lethality caused by Abp1 overexpression, and the ability of Abp1 to activate Arp2/3 complex. Furthermore, we genetically uncouple ABP1 functions that overlap with SAC6, SLA1, and SLA2, showing they require distinct combinations of activities and interactions. Together, our data provide the first structural and functional view of the Abp1-actin interaction and show that Abp1 has distinct cellular roles as an adapter, linking different sets of ligands for each function.     

Mammalian Abp1, a signal-responsive F-actin-binding protein, links the actin cytoskeleton to endocytosis via the GTPase dynamin

The actin cytoskeleton has been implicated in endocytosis, yet few molecular links to the endocytic machinery have been established. Here we show that the mammalian F-actin-binding protein Abp1 (SH3P7/HIP-55) can functionally link the actin cytoskeleton to dynamin, a GTPase that functions in endocytosis. Abp1 binds directly to dynamin in vitro through its SH3 domain. Coimmunoprecipitation and colocalization studies demonstrated the in vivo relevance of this interaction. In neurons, mammalian Abp1 and dynamin colocalized at actin-rich sites proximal to the cell body during synaptogenesis. In fibroblasts, mAbp1 appeared at dynamin-rich sites of endocytosis upon growth factor stimulation. To test whether Abp1 functions in endocytosis, we overexpressed several Abp1 constructs in Cos-7 cells and assayed receptor-mediated endocytosis. While overexpression of Abp1's actin-binding modules did not interfere with endocytosis, overexpression of the SH3 domain led to a potent block of transferrin uptake. This implicates the Abp1/dynamin interaction in endocytic function. The endocytosis block was rescued by cooverexpression of dynamin. Since the addition of the actin-binding modules of Abp1 to the SH3 domain construct also fully restored endocytosis, Abp1 may support endocytosis by combining its SH3 domain interactions with cytoskeletal functions in response to signaling cascades converging on this linker protein.     

Stonin 2 is an AP-2-dependent endocytic sorting adaptor for synaptotagmin internalization and recycling

Clathrin-mediated endocytosis is involved in the internalization, recycling, and degradation of cycling membrane receptors as well as in the biogenesis of synaptic vesicle proteins. While many constitutively internalized cargo proteins are recognized directly by the clathrin adaptor complex AP-2, stimulation-dependent endocytosis of membrane proteins is often facilitated by specialized sorting adaptors. Although clathrin-mediated endocytosis appears to be a major pathway for presynaptic vesicle cycling, no sorting adaptor dedicated to synaptic vesicle membrane protein endocytosis has been indentified in mammals. Here, we show that stonin 2, a mammalian ortholog of Drosophila stoned B, facilitates clathrin/AP-2-dependent internalization of synaptotagmin and targets it to a recycling vesicle pool in living neurons. The ability of stonin 2 to facilitate endocytosis of synaptotagmin is dependent on its association with AP-2, an intact mu-homology domain, and functional AP-2 heterotetramers. Our data identify stonin 2 as an AP-2-dependent endocytic sorting adaptor for synaptotagmin internalization and recycling.     

Sorting nexin 9 interacts with dynamin 1 and N-WASP and coordinates synaptic vesicle endocytosis

Sorting nexin 9 (SNX9) is a member of the sorting nexin family of proteins, each of which contains a characteristic Phox homology domain. SNX9 is widely expressed and plays a role in clathrin-mediated endocytosis, but it is not known if it is present in neuronal cells. We report that SNX9 is expressed in the presynaptic compartment of cultured hippocampal neurons, where it binds to dynamin-1 and N-WASP. Overexpression of full-length SNX9 or a C-terminal truncated version caused severe defects in synaptic vesicle endocytosis during, as well as after, stimulation. Knockdown of SNX9 with short interfering RNA also reduced synaptic vesicle endocytosis, and the W39A mutation of SNX9 abolished the inhibitory effect of SNX9 on endocytosis. Rescue experiments showed that most of the effect of SNX9 on endocytosis results from its interaction with dynamin 1, although its interaction with N-WASP contributes in some degree. We further showed that SNX9 dimerizes through its C-terminal domain, suggesting that it may interact simultaneously with dynamin 1 and N-WASP. We propose that SNX9 interacts with dynamin-1 and N-WASP in presynaptic terminals, where it links actin dynamics and synaptic vesicle endocytosis.     

The phosphoinositide phosphatase Sjl2 is recruited to cortical actin patches in the control of vesicle formation and fission during endocytosis

The Saccharomyces cerevisiae synaptojanin-like proteins (Sjl1, Sjl2, and Sjl3) are phosphoinositide (PI) phosphatases that regulate PI metabolism in the control of actin organization and membrane trafficking. However, the primary sites of action for each of the yeast synaptojanin-like proteins remain unclear. In this study, we show that Sjl2 is localized to cortical actin patches, sites of endocytosis. Cortical recruitment of Sjl2 requires the actin patch component Abp1. Consistent with this, the SH3 domain-containing protein Abp1 physically associates with Sjl2 through its proline-rich domain. Furthermore, abp1Delta mutations confer defects resembling loss of SJL2; sjl1Delta abp1Delta double-mutant cells exhibit invaginated plasma membranes and impaired endocytosis, findings similar to those for sjl1Delta sjl2Delta mutant cells. Thus, Abp1 acts as an adaptor protein in the localization or concentration of Sjl2 during late stages of endocytic vesicle formation. Overexpression of the Hip1-related protein Sla2 delayed the formation of extended plasma membrane invaginations in sjl2ts cells, indicating that Sla2 may become limiting or misregulated in cells with impaired PI phosphatase activity. Consistent with this, the cortical actin patch protein Sla2 is mislocalized in sjl1Delta sjl2Delta mutant cells. Together, our studies suggest that PI metabolism by the synaptojanin-like proteins coordinately directs actin dynamics and membrane invagination, in part by regulation of Sla2.     

Human stoned B interacts with AP-2 and synaptotagmin and facilitates clathrin-coated vesicle uncoating

Synaptic vesicle biogenesis involves the recycling of synaptic vesicle components by clathrin-mediated endocytosis from the presynaptic membrane. stoned B, a protein encoded by the stoned locus in Drosophila melanogaster has been shown to regulate vesicle recycling by interacting with synaptotagmin. We report here the identification and characterization of a human homolog of stoned B (hStnB). Human stoned B is a brain-specific protein which co-enriches with other endocytic proteins such as AP-2 in a crude synaptic vesicle fraction and at nerve terminals. A domain with homology to the medium chain of adaptor complexes binds directly to both AP-2 and synaptotagmin and competes with AP-2 for the same binding site within synaptotagmin. Finally we show that the µ2 homology domain of hStnB stimulates the uncoating of both clathrin and AP-2 adaptors from clathrin-coated vesicles. We hypothesize that hStnB regulates synaptic vesicle recycling by facilitating vesicle uncoating.     

Activation of the Arp2/3 complex by the actin filament binding protein Abp1p

The actin-related protein (Arp) 2/3 complex plays a central role in assembly of actin networks. Because distinct actin-based structures mediate diverse processes, many proteins are likely to make spatially and temporally regulated interactions with the Arp2/3 complex. We have isolated a new activator, Abp1p, which associates tightly with the yeast Arp2/3 complex. Abp1p contains two acidic sequences (DDW) similar to those found in SCAR/WASp proteins. We demonstrate that mutation of these sequences abolishes Arp2/3 complex activation in vitro. Genetic studies indicate that this activity is important for Abp1p functions in vivo. In contrast to SCAR/WASp proteins, Abp1p binds specifically to actin filaments, not monomers. Actin filament binding is mediated by the ADF/cofilin homology (ADF-H) domain of Abp1p and is required for Arp2/3 complex activation in vitro. We demonstrate that Abp1p recruits Arp2/3 complex to the sides of filaments, suggesting a novel mechanism of activation. Studies in yeast and mammalian cells indicate that Abp1p is involved functionally in endocytosis. Based on these results, we speculate that Abp1p may link Arp2/3-mediated actin assembly to a specific step in endocytosis.     

Nerve growth factor-induced phosphorylation of amphiphysin-1 by casein kinase 2 regulates clathrin-amphiphysin interactions

Amphiphysins interact directly with clathrin and have a function in clathrin-mediated synaptic vesicle recycling and clathrin-mediated endocytosis. The neuronal isoform amphiphysin-1 is a serine/threonine phosphoprotein that is dephosphorylated upon stimulation of synaptic vesicle endocytosis. Rephosphorylation was stimulated by nerve growth factor. We analysed the regulation of amphiphysin-clathrin interactions by phosphorylation. The N-terminal domain of clathrin bound to unphosphorylated amphiphysin-1, but not to the phosphorylated protein. A search for possible phosphorylation sites revealed two casein kinase 2 consensus motifs in close proximity to the clathrin binding sites in amphiphysin-1 and -2. We mutagenized these residues (T350 and T387) to glutamate, mimicking a constitutive phosphorylation. The double mutant showed a strong reduction in clathrin binding. The assumption that casein kinase 2 phosphorylates amphiphysin-1 at T350 and T387 was corroborated by experiments showing that: (i) casein kinase 2 phosphorylated these residues directly in vitro, (ii) when expressed in HeLa cells, the glutamate mutant showed reduced phosphorylation, and (iii) casein kinase 2 inhibitors blocked nerve growth factor-induced phosphorylation of endogenous amphiphysin-1 in PC12 cells. These observations are consistent with the hypothesis that, upon activation by nerve growth factor, casein kinase 2 phosphorylates amphiphysin-1 and thereby regulates the endocytosis of clathrin-coated vesicles via the interaction between clathrin and amphiphysin.     

Mammalian actin binding protein 1 is essential for endocytosis but not lamellipodia formation: functional analysis by RNA interference

Mammalian actin binding protein 1 (mAbp1, also called SH3P7/Hip55) is structurally and functionally related to yeast Abp1 and to cortactin, both of which have been implicated in endocytotic processes. mAbp1 associates through its SH3 domain with dynamin, a large GTPase essential for vesicle fission. To clarify the function of mAbp1, we specifically knocked down its expression in human embryonic kidney 293T cells, using RNA interference (RNAi). Co-transfection of a short interfering RNA (siRNA) together with a plasmid coding for a surface marker, followed by purification of transfected cells, enabled us to obtain a cell population having up to 90% inhibition of mAbp1 expression. In mAbp1-knocked down cells, transferrin (Tf) receptor endocytosis was significantly inhibited and intracellular distribution of the early endosomal compartment was modified. In contrast, in these cells actin and microtubule filaments appeared normal, and formation of lamellipodia induced by active Rac was not inhibited. This study provides definitive evidence that mAbp1 is indispensable for receptor-mediated endocytosis.     

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 Igamma, the major phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P(2)]-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)P(2) synthesis, actin dynamics, and endocytosis, and further supports a functional link between actin and clathrin-mediated endocytosis.     

Evidence for physical and functional interactions among two Saccharomyces cerevisiae SH3 domain proteins, an adenylyl cyclase-associated protein and the actin cytoskeleton.

In a variety of organisms, a number of proteins associated with the cortical actin cytoskeleton contain SH3 domains, suggesting that these domains may provide the physical basis for functional interactions among structural and regulatory proteins in the actin cytoskeleton. We present evidence that SH3 domains mediate at least two independent functions of the Saccharomyces cerevisiae actin-binding protein Abp1p in vivo. Abp1p contains a single SH3 domain that has recently been shown to bind in vitro to the adenylyl cyclase-associated protein Srv2p. Immunofluorescence analysis of Srv2p subcellular localization in strains carrying mutations in either ABP1 or SRV2 reveals that the Abp1p SH3 domain mediates the normal association of Srv2p with the cortical actin cytoskeleton. We also show that a site in Abp1p itself is specifically bound by the SH3 domain of the actin-associated protein Rvs167p. Genetic analysis provides evidence that Abp1p and Rvs167p have functions that are closely interrelated. Abp1 null mutations, like rvs167 mutations, result in defects in sporulation and reduced viability under certain suboptimal growth conditions. In addition, mutations in ABP1 and RVS167 yield similar profiles of genetic "synthetic lethal" interactions when combined with mutations in genes encoding other cytoskeletal components. Mutations which specifically disrupt the SH3 domain-mediated interaction between Abp1p and Srv2p, however, show none of the shared phenotypes of abp1 and rvs167 mutations. We conclude that the Abp1p SH3 domain mediates the association of Srv2p with the cortical actin cytoskeleton, and that Abp1p performs a distinct function that is likely to involve binding by the Rvs167p SH3 domain. Overall, work presented here illustrates how SH3 domains can integrate the activities of multiple actin cytoskeleton proteins in response to varying environmental conditions.     

Endocytosis at ribbon synapses

Unlike conventional synaptic terminals that release neurotransmitter episodically in response to action potentials, neurons of the visual, auditory and vestibular systems encode sensory information in graded signals that are transmitted at their synapses by modulating the rate of continuous release. The synaptic ribbon, a specialized structure found at the active zones of these neurons, is necessary to sustain the high rates of exocytosis required for continuous release. To maintain the fidelity of synaptic transmission, exocytosis must be balanced by high-capacity endocytosis, to retrieve excess membrane inserted during vesicle fusion. Capacitance measurements following vesicle release in ribbon-type neurons indicate two kinetically distinct phases of compensatory endocytosis, whose relative contributions vary with stimulus intensity. The two phases can be independently regulated and may reflect different underlying mechanisms operating on separate pools of recycling vesicles. Electron microscopy shows diversity among ribbon-type synapses in the relative importance of clathrin-mediated endocytosis versus bulk membrane retrieval as mechanisms of compensatory endocytosis. Ribbon synapses, like conventional synapses, make use of multiple endocytosis pathways to replenish synaptic vesicle pools, depending on the physiological needs of the particular cell type.     

SH3P7/mAbp1 deficiency leads to tissue and behavioral abnormalities and impaired vesicle transport

The intracellular adaptor protein SH3P7 is the mammalian ortholog of yeast actin-binding protein 1 and thus alternatively named as mAbp1 (or HIP55). Structural properties, biochemical analysis of its interaction partners and siRNA studies implicated mAbp1 as an accessory protein in clathrin-mediated endocytosis (CME). Here, we describe the generation and characterization of mice deficient for SH3P7/mAbp1 owing to targeted gene disruption in embryonic stem cells. Mutant animals are viable and fertile without obvious deficits during the first weeks of life. Abnormal structure and function of organs including the spleen, heart, and lung is observed at about 3 months of age in both heterozygous and homozygous mouse mutants. A moderate reduction of both receptor-mediated and synaptic endocytosis is observed in embryonic fibroblasts and in synapses of hippocampal neurons, respectively. Recycling of synaptic vesicles in hippocampal boutons is severely impaired and delayed four-fold. The presynaptic defect of SH3P7/mAbp1 mouse mutants is associated with their constricted physical capabilities and disturbed neuromotoric behaviour. Our data reveal a nonredundant role of SH3P7/mAbp1 in CME and places its function downstream of vesicle fission.     

Cellular and structural insight into dynamin function during endocytic vesicle formation: a tale of 50 years of investigation

Dynamin is one of the major proteins involved in endocytosis. First identified 50 years ago in a genetic screen in Drosophila melanogaster, it has become a central player in many forms of endocytosis, such as clathrin-mediated endocytosis or synaptic vesicle endocytosis, as well as other important cellular processes such as actin remodelling. Decades of work using biochemical and structural studies, cell-free assays, live cell imaging, acute inhibition and genetic studies have led to important insights on its mode of action. Dynamin is a remarkable mechano-GTPase, which can do a lot to membranes on its own but which is, in cells, at the centre of a vast protein and lipid network and cannot work in isolation. This review summarizes the main features of dynamin structure and function and its central role in membrane remodelling events, and give an update on the latest results.     

Formation of an endophilin-Ca2+ channel complex is critical for clathrin-mediated synaptic vesicle endocytosis

A tight balance between synaptic vesicle exocytosis and endocytosis is fundamental to maintaining synaptic structure and function. Calcium influx through voltage-gated Ca2+ channels is crucial in regulating synaptic vesicle exocytosis. However, much less is known about how Ca2+ regulates vesicle endocytosis or how the endocytic machinery becomes enriched at the nerve terminal. We report here a direct interaction between voltage-gated Ca2+ channels and endophilin, a key regulator of clathrin-mediated synaptic vesicle endocytosis. Formation of the endophlin-Ca2+ channel complex is Ca2+ dependent. The primary Ca2+ binding domain resides within endophilin and regulates both endophilin-Ca2+ channel and endophilin-dynamin complexes. Introduction into hippocampal neurons of a dominant-negative endophilin construct, which constitutively binds to Ca2+ channels, significantly reduces endocytosis-mediated uptake of FM 4-64 dye without abolishing exocytosis. These results suggest an important role for Ca2+ channels in coordinating synaptic vesicle recycling by directly coupling to both exocytotic and endocytic machineries.     

Identification and Immunocytochemical Characterization of Piccolino,a Novel Piccolo Splice Variant Selectively Expressed at Sensory Ribbon Synapses of the Eye and Ear

Piccolo is one of the largest cytomatrix proteins present at active zones of chemical synapses, where it is suggested to play a role in recruiting and integrating molecules relevant for both synaptic vesicle exo- and endocytosis. Here we examined the retina of a Piccolo-mutant mouse with a targeted deletion of exon 14 in the Pclo gene. Piccolo deficiency resulted in its profound loss at conventional chemical amacrine cell synapses but retinal ribbon synapses were structurally and functionally unaffected. This led to the identification of a shorter, ribbon-specific Piccolo variant, Piccolino, present in retinal photoreceptor cells, bipolar cells, as well as in inner hair cells of the inner ear. By RT-PCR analysis and the generation of a Piccolino-specific antibody we show that non-splicing of intron 5/6 leads to premature translation termination and generation of the C-terminally truncated protein specifically expressed at active zones of ribbon synapse containing cell types. With in situ proximity ligation assays we provide evidence that this truncation leads to the absence of interaction sites for Bassoon, Munc13, and presumably also ELKS/CAST, RIM2, and the L-type Ca^{2 +} channel which exist in the full-length Piccolo at active zones of conventional chemical synapses. The putative lack of interactions with proteins of the active zone suggests a function of Piccolino at ribbon synapses of sensory neurons different from Piccolo’s function at conventional chemical synapses.     

Inhibitory role of endophilin 3 in receptor-mediated endocytosis

Endophilin 1 (Endo1) participates in synaptic vesicle biogenesis through interactions of its Src homology 3 domain with the polyphosphoinositide phosphatase Synaptojanin and the GTPase Dynamin. Endo1 has also been reported to affect endocytosis by converting membrane curvature via its lysophosphatidic acid acyltransferase activity. Here we report that a closely related isoform of Endo1, Endo3, inhibits clathrin-mediated endocytosis. Mutational analyses showed that the variable region of Endo3 is important in regulating transferrin endocytosis. In the brain, Endo3 is co-localized with dopamine D2 receptor in olfactory nerve terminals and inhibits its clathrin-mediated endocytosis in COS-7 cells. Furthermore, overexpression of Endo3 in an olfactory epithelium-derived cell line suppressed dopamine D2 receptor-mediated endocytosis and therefore accelerated its dopamine-induced differentiation. These results indicate that Endo3 may act as a negative regulator of clathrin-mediated endocytosis in brain neurons.     

Giant reticulospinal synapse in lamprey: molecular links between active and periactive zones

Deciphering the function of synaptic release sites is central to understanding neuronal communication. Here, we review studies of the lamprey giant reticulospinal synapse, a model that can be used to dissect synaptic vesicle trafficking at single release sites. The presynaptic axon is large and contains active zones that are spatially separated from each other. During activity, synaptic vesicle membrane is shuttled between the active zone and the periactive zone at which endocytosis occurs. Recent studies have shown that the periactive zone contains an actin-rich cytomatrix that expands during synaptic activity. This cytomatrix has been implicated in multiple functions that include (1) activity-dependent trafficking of proteins between the synaptic vesicle cluster and the periactive zone, (2) synaptic vesicle endocytosis, and (3) the movement of newly formed synaptic vesicles to the vesicle cluster. The actin cytomatrix thus provides a link between the active zone and the periactive zone; this link appears to be critical for sustained cycling of synaptic vesicles.This work was supported by Swedish Research Council grants (K2004-33X-11287-10A, LB; K2005-32X-13473-06A, OS).