An endophilin-dynamin complex promotes budding of clathrin-coated vesicles during synaptic vesicle recycling

Association of Endophilin B1 with Cytoplasmic Vesicles

Endophilins are SH3- and BAR domain-containing proteins implicated in membrane remodeling and vesicle formation. Endophilins A1 and A2 promote the budding of endocytic vesicles from the plasma membrane, whereas endophilin B1 has been implicated in vesicle budding from intracellular organelles, including the trans-Golgi network and late endosomes. We previously reported that endophilins A1 and A2 exist almost exclusively as soluble dimers in the cytosol. Here, we present results of fluorescence fluctuation spectroscopy analyses indicating that, in contrast, the majority of endophilin B1 is present in multiple copies on small, highly mobile cytoplasmic vesicles. Formation of these vesicles was enhanced by overexpression of wild-type dynamin 2, but suppressed by expression of a catalytically inactive dynamin 2 mutant. Using dual-color heterospecies partition analysis, we identified the epidermal growth factor receptor on endophilin B1 vesicles. Moreover, a proportion of endophilin B1 vesicles also contained caveolin, whereas clathrin was almost undetectable on those vesicles. These results raise the possibility that endophilin B1 participates in dynamin 2-dependent formation of a population of transport vesicles distinct from those generated by A-type endophilins.

SH3 Domain-Containing Protein 2 Plays a Crucial Role at the Step of Membrane Tubulation during Cell Plate Formation

During cytokinesis in plants, trans-Golgi network-derived vesicles accumulate at the center of dividing cells and undergo various structural changes to give rise to the planar cell plate. However, how this conversion occurs at the molecular level remains elusive. In this study, we report that SH3 Domain-Containing Protein 2 (SH3P2) in Arabidopsis thaliana plays a crucial role in converting vesicles to the planar cell plate. SH3P2 RNAi plants showed cytokinesis-defective phenotypes and produced aggregations of vesicles at the leading edge of the cell plate. SH3P2 localized to the leading edge of the cell plate, particularly the constricted or curved regions of the cell plate. The BAR domain of SH3P2 induced tubulation of vesicles. SH3P2 formed a complex with dynamin-related protein 1A (DRP1A) and affected DRP1A accumulation to the cell plate. Based on these results, we propose that SH3P2 functions together with DRP1A to convert the fused vesicles to tubular structures during cytokinesis.

Transmembrane Pathways and Mechanisms of Rod-like Paclitaxel Nanocrystals through MDCK Polarized Monolayer

Drug nanocrystals (NCs) appear to be favorable to improving oral bioavailability of poorly water-soluble drugs as evidenced by the great success they have had in the market. However, the pathway and mechanism of drug NCs through epithelial membrane are still unclear. In an attempt to clarify their transport features, paclitaxel nanocrystals (PTX-NCs), and paclitaxel hybrid NCs with lipophilic carbocyanine dyes, were prepared and characterized as the models. The endocytosis, intracellular trafficking, paracellular transport, and transcytosis of PTX-NCs were carefully investigated with Förster resonance energy transfer (FRET) analysis, as well as a colocalization assay, flow cytometry, gene silencing, Western-blot, transepithelial electrical resistance (TEER) study and other approaches on MDCK cells. It was found that rod-like PTX-NCs could transport through the monolayer intact, and the process of endocytosis proved to be time and energy dependent. Endoplasmic reticulum (ER) and Golgi complexes were colocalized with PTX-NCs in cells, so the ER-Golgi complexes/Golgi complexes-basolateral membrane pathway may be involved in the intracellular trafficking and transcytosis of PTX-NCs. It was demonstrated here that cav-1, dynamin, and actin filament modulated the endocytosis process, and Cdc 42 regulated the transcytosis process. In addition, no paracellular transport of PTX-NCs was observed. These findings demonstrated that the rod-like nanocrystals not only enhanced the transcytosis of PTX compared with microparticles of raw drug materials but also changed the pathways of drug delivery. This study certainly provides insight for the oral absorption mechanism of nanocrystals of poorly soluble drugs.

Dynamin Functions and Ligands: Classical Mechanisms Behind

Dynamin is a GTPase that plays a vital role in clathrin-dependent endocytosis and other vesicular trafficking processes by acting as a pair of molecular scissors for newly formed vesicles originating from the plasma membrane. Dynamins and related proteins are important components for the cleavage of clathrin-coated vesicles, phagosomes, and mitochondria. These proteins help in organelle division, viral resistance, and mitochondrial fusion/fission. Dysfunction and mutations in dynamin have been implicated in the pathophysiology of various disorders, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, Charcot-Marie-Tooth disease, heart failure, schizophrenia, epilepsy, cancer, dominant optic atrophy, osteoporosis, and Down's syndrome. This review is an attempt to illustrate the dynamin-related mechanisms involved in the above-mentioned disorders and to help medicinal chemists to design novel dynamin ligands, which could be useful in the treatment of dynamin-related disorders.

Membrane fission by dynamin: what we know and what we need to know

The large GTPase dynamin is the first protein shown to catalyze membrane fission. Dynamin and its related proteins are essential to many cell functions, from endocytosis to organelle division and fusion, and it plays a critical role in many physiological functions such as synaptic transmission and muscle contraction. Research of the past three decades has focused on understanding how dynamin works. In this review, we present the basis for an emerging consensus on how dynamin functions. Three properties of dynamin are strongly supported by experimental data: first, dynamin oligomerizes into a helical polymer; second, dynamin oligomer constricts in the presence of GTP; and third, dynamin catalyzes membrane fission upon GTP hydrolysis. We present the two current models for fission, essentially diverging in how GTP energy is spent. We further discuss how future research might solve the remaining open questions presently under discussion.

How curvature-generating proteins build scaffolds on membrane nanotubes

Bin/Amphiphysin/Rvs (BAR) domain proteins control the curvature of lipid membranes in endocytosis, trafficking, cell motility, the formation of complex subcellular structures, and many other cellular phenomena. They form 3D assemblies that act as molecular scaffolds to reshape the membrane and alter its mechanical properties. It is unknown, however, how a protein scaffold forms and how BAR domains interact in these assemblies at protein densities relevant for a cell. In this work, we use various experimental, theoretical, and simulation approaches to explore how BAR proteins organize to form a scaffold on a membrane nanotube. By combining quantitative microscopy with analytical modeling, we demonstrate that a highly curving BAR protein endophilin nucleates its scaffolds at the ends of a membrane tube, contrary to a weaker curving protein centaurin, which binds evenly along the tube's length. Our work implies that the nature of local protein-membrane interactions can affect the specific localization of proteins on membrane-remodeling sites. Furthermore, we show that amphipathic helices are dispensable in forming protein scaffolds. Finally, we explore a possible molecular structure of a BAR-domain scaffold using coarse-grained molecular dynamics simulations. Together with fluorescence microscopy, the simulations show that proteins need only to cover 30-40% of a tube's surface to form a rigid assembly. Our work provides mechanical and structural insights into the way BAR proteins may sculpt the membrane as a high-order cooperative assembly in important biological processes.

Endocytic membrane trafficking and neurodegenerative disease

Neurodegenerative diseases are amongst the most devastating of human disorders. New technologies have led to a rapid increase in the identification of disease-related genes with an enhanced appreciation of the key roles played by genetics in the etiology of these disorders. Importantly, pinpointing the normal function of disease gene proteins leads to new understanding of the cellular machineries and pathways that are altered in the disease process. One such emerging pathway is membrane trafficking in the endosomal system. This key cellular process controls the localization and levels of a myriad of proteins and is thus critical for normal cell function. In this review we will focus on three neurodegenerative diseases; Parkinson disease, amyotrophic lateral sclerosis, and hereditary spastic paraplegias, for which a large number of newly discovered disease genes encode proteins that function in endosomal membrane trafficking. We will describe how alterations in these proteins affect endosomal function and speculate on the contributions of these disruptions to disease pathophysiology.

CPG2 Recruits Endophilin B2 to the Cytoskeleton for Activity-Dependent Endocytosis of Synaptic Glutamate Receptors

Internalization of glutamate receptors at the postsynaptic membrane via clathrin-mediated endocytosis (CME) is a key mechanism for regulating synaptic strength. A role for the F-actin cytoskeleton in CME is well established, and recently, PKA-dependent association of candidate plasticity gene 2 (CPG2) with the spine-cytoskeleton has been shown to mediate synaptic glutamate receptor internalization. Yet, how the endocytic machinery is physically coupled to the actin cytoskeleton to facilitate glutamate receptor internalization has not been demonstrated. Moreover, there has been no distinction of endocytic-machinery components that are specific to activity-dependent versus constitutive glutamate receptor internalization. Here, we show that CPG2, through a direct physical interaction, recruits endophilin B2 (EndoB2) to F-actin, thus anchoring the endocytic machinery to the spine cytoskeleton and facilitating glutamate receptor internalization. Regulation of CPG2 binding to the actin cytoskeleton by protein kinase A directly impacts recruitment of EndoB2 and clathrin. Specific disruption of EndoB2 or the CPG2-EndoB2 interaction impairs activity-dependent, but not constitutive, internalization of both NMDA- and AMPA-type glutamate receptors. These results demonstrate that, through direct interactions with F-actin and EndoB2, CPG2 physically bridges the spine cytoskeleton and the endocytic machinery, and this tripartite association is critical specifically for activity-dependent CME of synaptic glutamate receptors.

Binocular pattern deprivation interferes with the expression of proteins involved in primary visual cortex maturation in the cat

Background

Binocular pattern deprivation from eye opening (early BD) delays the maturation of the primary visual cortex. This delay is more pronounced for the peripheral than the central visual field representation within area 17, particularly between the age of 2 and 4 months [Laskowska-Macios, Cereb Cortex, 2014].

Results

In this study, we probed for related dynamic changes in the cortical proteome. We introduced age, cortical region and BD as principal variables in a 2-D DIGE screen of area 17. In this way we explored the potential of BD-related protein expression changes between central and peripheral area 17 of 2- and 4-month-old BD (2BD, 4BD) kittens as a valid parameter towards the identification of brain maturation-related molecular processes. Consistent with the maturation delay, distinct developmental protein expression changes observed for normal kittens were postponed by BD, especially in the peripheral region. These BD-induced proteomic changes suggest a negative regulation of neurite outgrowth, synaptic transmission and clathrin-mediated endocytosis, thereby implicating these processes in normal experience-induced visual cortex maturation. Verification of the expression of proteins from each of the biological processes via Western analysis disclosed that some of the transient proteomic changes correlate to the distinct behavioral outcome in adult life, depending on timing and duration of the BD period [Neuroscience 2013;255:99-109].

Conclusions

Taken together, the plasticity potential to recover from BD, in relation to ensuing restoration of normal visual input, appears to rely on specific protein expression changes and cellular processes induced by the loss of pattern vision in early life.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0137-7) contains supplementary material, which is available to authorized users.

Synaptic vesicle exocytosis and increased cytosolic calcium are both necessary but not sufficient for activity-dependent bulk endocytosis

Activity‐dependent bulk endocytosis (ADBE) is the dominant synaptic vesicle (SV) endocytosis mode in central nerve terminals during intense neuronal activity. By definition this mode is triggered by neuronal activity; however, key questions regarding its mechanism of activation remain unaddressed. To determine the basic requirements for ADBE triggering in central nerve terminals, we decoupled SV fusion events from activity‐dependent calcium influx using either clostridial neurotoxins or buffering of intracellular calcium. ADBE was monitored both optically and morphologically by observing uptake of the fluid phase markers tetramethylrhodamine‐dextran and horse radish peroxidase respectively. Ablation of SV fusion with tetanus toxin resulted in the arrest of ADBE, but had no effect on other calcium‐dependent events such as activity‐dependent dynamin I dephosphorylation, indicating that SV exocytosis is necessary for triggering. Furthermore, the calcium chelator EGTA abolished ADBE while leaving SV exocytosis intact, demonstrating that ADBE is triggered by intracellular free calcium increases outside the active zone. Activity‐dependent dynamin I dephosphorylation was also arrested in EGTA‐treated neurons, consistent with its proposed role in triggering ADBE. Thus, SV fusion and increased cytoplasmic free calcium are both necessary but not sufficient individually to trigger ADBE.

Activity‐dependent bulk endocytosis (ADBE) is the dominant synaptic vesicle (SV) endocytosis mode in central nerve terminals during intense neuronal activity. To determine the minimal requirements for ADBE triggering, we decoupled SV fusion events from activity‐dependent calcium influx using either clostridial neurotoxins or buffering of intracellular calcium. We found that SV fusion and increased cytoplasmic free calcium are both necessary but not sufficient to trigger ADBE.

Endophilin-A2 functions in membrane scission in clathrin-independent endocytosis

During endocytosis, energy is invested to narrow the necks of cargo-containing plasma membrane invaginations to radii at which the opposing segments spontaneously coalesce, thereby leading to the detachment by scission of endocytic uptake carriers. In the clathrin pathway, dynamin uses mechanical energy from GTP hydrolysis to this effect, assisted by the BIN/amphiphysin/Rvs (BAR) domain-containing protein endophilin. Clathrin-independent endocytic events are often less reliant on dynamin, and whether in these cases BAR domain proteins such as endophilin contribute to scission has remained unexplored. Here we show, in human and other mammalian cell lines, that endophilin-A2 (endoA2) specifically and functionally associates with very early uptake structures that are induced by the bacterial Shiga and cholera toxins, which are both clathrin-independent endocytic cargoes. In controlled in vitro systems, endoA2 reshapes membranes before scission. Furthermore, we demonstrate that endoA2, dynamin and actin contribute in parallel to the scission of Shiga-toxin-induced tubules. Our results establish a novel function of endoA2 in clathrin-independent endocytosis. They document that distinct scission factors operate in an additive manner, and predict that specificity within a given uptake process arises from defined combinations of universal modules. Our findings highlight a previously unnoticed link between membrane scaffolding by endoA2 and pulling-force-driven dynamic scission.

Dynamins and BAR Proteins-Safeguards against Cancer

Dynamins and BAR proteins are crucial in a wide variety of cellular processes for their ability to mediate membrane remodeling, such as membrane curvature and membrane fission and fusion. In this review, we highlight dynamins and BAR proteins and the cellular mechanisms that are involved in the initiation and progression of cancer. We specifically discuss the roles of the seproteinsin endocytosis, endo-lysosomal trafficking, autophagy, and apoptosis as these processes are all tightly linked to membrane remodeling and cancer.

Vesicle uncoating regulated by SH3-SH3 domain-mediated complex formation between endophilin and intersectin at synapses

Neurotransmission involves the exo-endocytic cycling of synaptic vesicle (SV) membranes. Endocytic membrane retrieval and clathrin-mediated SV reformation require curvature-sensing and membrane-bending BAR domain proteins such as endophilin A. While their ability to sense and stabilize curved membranes facilitates membrane recruitment of BAR domain proteins, the precise mechanisms by which they are targeted to specific sites of SV recycling has remained unclear. Here, we demonstrate that the multi-domain scaffold intersectin 1 directly associates with endophilin A to facilitate vesicle uncoating at synapses. Knockout mice deficient in intersectin 1 accumulate clathrin-coated vesicles at synapses, a phenotype akin to loss of endophilin function. Intersectin 1/endophilin A1 complex formation is mediated by direct binding of the SH3B domain of intersectin to a non-canonical site on the SH3 domain of endophilin A1. Consistent with this, intersectin-binding defective mutant endophilin A1 fails to rescue clathrin accumulation at neuronal synapses derived from endophilin A1-3 triple knockout (TKO) mice. Our data support a model in which intersectin aids endophilin A recruitment to sites of clathrin-mediated SV recycling, thereby facilitating vesicle uncoating.

Endophilin marks and controls a clathrin-independent endocytic pathway

Endocytosis is required for internalization of micronutrients and turnover of membrane components. Endophilin has been assigned as a component of clathrin-mediated endocytosis. Here we show in mammalian cells that endophilin marks and controls a fast-acting tubulovesicular endocytic pathway that is independent of AP2 and clathrin, activated upon ligand binding to cargo receptors, inhibited by inhibitors of dynamin, Rac, phosphatidylinositol-3-OH kinase, PAK1 and actin polymerization, and activated upon Cdc42 inhibition. This pathway is prominent at the leading edges of cells where phosphatidylinositol-3,4-bisphosphate-produced by the dephosphorylation of phosphatidylinositol-3,4,5-triphosphate by SHIP1 and SHIP2-recruits lamellipodin, which in turn engages endophilin. This pathway mediates the ligand-triggered uptake of several G-protein-coupled receptors such as α2a- and β1-adrenergic, dopaminergic D3 and D4 receptors and muscarinic acetylcholine receptor 4, the receptor tyrosine kinases EGFR, HGFR, VEGFR, PDGFR, NGFR and IGF1R, as well as interleukin-2 receptor. We call this new endocytic route fast endophilin-mediated endocytosis (FEME).

BIN1 membrane curvature sensing and generation show autoinhibition regulated by downstream ligands and PI(4,5)P2

In striated muscles, invaginations from the plasma membrane, termed transverse tubules (T-tubule), function in the excitation–contraction coupling machinery. BIN1 (isoform8) plays a critical role in the biogenesis of T-tubules. BIN1 contains an N-terminal BAR domain to sense and induce membrane curvature, an isoform8-specific polybasic motif (exon10) as the phosphoinositide binding module and a C-terminal Src homology 3 (SH3) domain for the recruitment of downstream proteins such as dynamin 2. Previous studies of N-BAR domains focused on elucidating mechanisms of membrane curvature sensing and generation (MC-S&G). Less is known about how MC-S&G is regulated. We found that the SH3 domain binds to the exon10 motif more strongly compared to the proline-rich domain (PRD) of dynamin 2. Furthermore, we found that the MC-S&G ability of full-length BIN1 is inhibited on membranes lacking PI(4,5)P₂. Addition of PI(4,5)P₂ in the membrane activates BIN1 to sense and induce membrane curvature. Co-presence of the SH3 domain and exon10 motif leads to the strongest phosphoinositide-mediated control of BIN1 function. Addition of SH3 domain ligand (such as PRD peptides), as well as addition of the water-soluble PI(4,5)P₂ analogue, can both enhance the MC-S&G ability of BIN1 on membranes without PI(4,5)P₂, indicating that the key to activate BIN1 is to disrupt the exon10–SH3 interaction. The nonsense mutation K436X, found in centronuclear myopathy (CNM) patients, abolishes SH3 domain binding with either exon10 or the PRD motif, resulting in increased membrane deformation capacity. Our results suggest an autoinhibition model for BIN1 that involves a synergistic regulation by membrane composition and protein–protein interactions.

Regulating dynamin dynamics during endocytosis

Dynamin is a large GTPase that mediates plasma membrane fission during clathrin-mediated endocytosis. Dynamin assembles into polymers on the necks of budding membranes in cells and has been shown to undergo GTP-dependent conformational changes that lead to membrane fission in vitro. Recent efforts have shed new light on the mechanisms of dynamin-mediated fission, yet exactly how dynamin performs this function in vivo is still not fully understood. Dynamin interacts with a number of proteins during the endocytic process. These interactions are mediated by the C-terminal proline-rich domain (PRD) of dynamin binding to SH3 domain-containing proteins. Three of these dynamin-binding partners (intersectin, amphiphysin and endophilin) have been shown to play important roles in the clathrin-mediated endocytosis process. They promote dynamin-mediated plasma membrane fission by regulating three important sequential steps in the process: recruitment of dynamin to sites of endocytosis; assembly of dynamin into a functional fission complex at the necks of clathrin-coated pits (CCPs); and regulation of dynamin-stimulated GTPase activity, a key requirement for fission.

Lipid cell biology. Polyunsaturated phospholipids facilitate membrane deformation and fission by endocytic proteins

Phospholipids (PLs) with polyunsaturated acyl chains are extremely abundant in a few specialized cellular organelles such as synaptic vesicles and photoreceptor discs, but their effect on membrane properties is poorly understood. Here, we found that polyunsaturated PLs increased the ability of dynamin and endophilin to deform and vesiculate synthetic membranes. When cells incorporated polyunsaturated fatty acids into PLs, the plasma membrane became more amenable to deformation by a pulling force and the rate of endocytosis was accelerated, in particular, under conditions in which cholesterol was limiting. Molecular dynamics simulations and biochemical measurements indicated that polyunsaturated PLs adapted their conformation to membrane curvature. Thus, by reducing the energetic cost of membrane bending and fission, polyunsaturated PLs may help to support rapid endocytosis.

A dynamin mutant defines a superconstricted prefission state

Dynamin is a 100 kDa GTPase that organizes into helical assemblies at the base of nascent clathrin-coated vesicles. Formation of these oligomers stimulates the intrinsic GTPase activity of dynamin, which is necessary for efficient membrane fission during endocytosis. Recent evidence suggests that the transition state of dynamin's GTP hydrolysis reaction serves as a key determinant of productive fission. Here, we present the structure of a transition-state-defective dynamin mutant K44A trapped in a prefission state at 12.5 Å resolution. This structure constricts to 3.7 nm, reaching the theoretical limit required for spontaneous membrane fission. Computational docking indicates that the ground-state conformation of the dynamin polymer is sufficient to achieve this superconstricted prefission state and reveals how a two-start helical symmetry promotes the most efficient packing of dynamin tetramers around the membrane neck. These data suggest a model for the assembly and regulation of the minimal dynamin fission machine.

Activity-dependent facilitation of Synaptojanin and synaptic vesicle recycling by the Minibrain kinase

Phosphorylation has emerged as a crucial regulatory mechanism in the nervous system to integrate the dynamic signalling required for proper synaptic development, function and plasticity, particularly during changes in neuronal activity. Here we present evidence that Minibrain (Mnb; also known as Dyrk1A), a serine/threonine kinase implicated in autism spectrum disorder and Down syndrome, is required presynaptically for normal synaptic growth and rapid synaptic vesicle endocytosis at the Drosophila neuromuscular junction (NMJ). We find that Mnb-dependent phosphorylation of Synaptojanin (Synj) is required, in vivo, for complex endocytic protein interactions and to enhance Synj activity. Neuronal stimulation drives Mnb mobilization to endocytic zones and triggers Mnb-dependent phosphorylation of Synj. Our data identify Mnb as a synaptic kinase that promotes efficient synaptic vesicle recycling by dynamically calibrating Synj function at the Drosophila NMJ, and in turn endocytic capacity, to adapt to conditions of high synaptic activity.

Arf6 exchange factor EFA6 and endophilin directly interact at the plasma membrane to control clathrin-mediated endocytosis

Members of the Arf family of small G proteins are involved in membrane traffic and organelle structure. They control the recruitment of coat proteins, and modulate the structure of actin filaments and the lipid composition of membranes. The ADP-ribosylation factor 6 (Arf6) isoform and the exchange factor for Arf6 (EFA6) are known to regulate the endocytic pathway of many different receptors. To determine the molecular mechanism of the EFA6/Arf6 function in vesicular transport, we searched for new EFA6 partners. In a two-hybrid screening using the catalytic Sec7 domain as a bait, we identified endophilin as a new partner of EFA6. Endophilin contains a Bin/Amphiphysin/Rvs (BAR) domain responsible for membrane bending, and an SH3 domain responsible for the recruitment of dynamin and synaptojanin, two proteins involved, respectively, in the fission and uncoating of clathrin-coated vesicles. By using purified proteins, we confirmed the direct interaction, and identified the N-BAR domain as the binding motif to EFA6A. We showed that endophilin stimulates the catalytic activity of EFA6A on Arf6. In addition, we observed that the Sec7 domain competes with flat but not with highly curved lipid membranes to bind the N-BAR. In cells, expression of EFA6A recruits endophilin to EFA6A-positive plasma membrane ruffles, whereas expression of endophilin rescues the EFA6A-mediated inhibition of transferrin internalization. Overall, our results support a model whereby EFA6 recruits endophilin on flat areas of the plasma membrane to control Arf6 activation and clathrin-mediated endocytosis.

Synthesis of the Pitstop family of clathrin inhibitors

This protocol describes the synthesis of two classes of clathrin inhibitors, Pitstop 1 and Pitstop 2, along with two inactive analogs that can be used as negative controls (Pitstop inactive controls, Pitnot-2 and Pitnot-2-100). Pitstop-induced inhibition of clathrin TD function acutely interferes with clathrin-mediated endocytosis (CME), synaptic vesicle recycling and cellular entry of HIV, whereas clathrin-independent internalization pathways and secretory traffic proceed unperturbed; these reagents can, therefore, be used to investigate clathrin function, and they have potential pharmacological applications. Pitstop 1 is synthesized in two steps: sulfonation of 1,8-naphthalic anhydride and subsequent reaction with 4-amino(methyl)aniline. Pitnot-1 results from the reaction of 4-amino(methyl)aniline with commercially available 4-sulfo-1,8-naphthalic anhydride potassium salt. Reaction of 1-naphthalene sulfonyl chloride with pseudothiohydantoin followed by condensation with 4-bromobenzaldehyde yields Pitstop 2. The synthesis of the inactive control commences with the condensation of 4-bromobenzaldehyde with the rhodanine core. Thioketone methylation and displacement with 1-napthylamine affords the target compound. Although Pitstop 1-series compounds are not cell permeable, they can be used in biochemical assays or be introduced into cells via microinjection. The Pitstop 2-series compounds are cell permeable. The synthesis of these compounds does not require specialist equipment and can be completed in 3-4 d. Microwave irradiation can be used to reduce the synthesis time. The synthesis of the Pitstop 2 family is easily adaptable to enable the synthesis of related compounds such as Pitstop 2-100 and Pitnot-2-100. The procedures are also simple, efficient and amenable to scale-up, enabling cost-effective in-house synthesis for users of these inhibitor classes.

BAR domain scaffolds in dynamin-mediated membrane fission

Biological membranes undergo constant remodeling by membrane fission and fusion to change their shape and to exchange material between subcellular compartments. During clathrin-mediated endocytosis, the dynamic assembly and disassembly of protein scaffolds comprising members of the bin-amphiphysin-rvs (BAR) domain protein superfamily constrain the membrane into distinct shapes as the pathway progresses toward fission by the GTPase dynamin. In this Review, we discuss how BAR domain protein assembly and disassembly are controlled in space and time and which structural and biochemical features allow the tight regulation of their shape and function to enable dynamin-mediated membrane fission.

Endophilin A1 induces different membrane shapes using a conformational switch that is regulated by phosphorylation

Membrane remodeling is controlled by proteins that can promote the formation of highly curved spherical or cylindrical membranes. How a protein induces these different types of membrane curvature and how cells regulate this process is still unclear. Endophilin A1 is a protein involved in generating endocytotic necks and vesicles during synaptic endocytosis and can transform large vesicles into lipid tubes or small and highly curved vesicles in vitro. By using EM and electron paramagnetic resonance of endophilin A1, we find that tubes are formed by a close interaction with endophilin A1's BIN/amphiphysin/Rvs (BAR) domain and deep insertion of its amphipathic helices. In contrast, vesicles are predominantly stabilized by the shallow insertion of the amphipathic helical wedges with the BAR domain removed from the membrane. By showing that the mechanism of membrane curvature induction is different for vesiculation and tubulation, these data also explain why previous studies arrived at different conclusions with respect to the importance of scaffolding and wedging in the membrane curvature generation of BAR proteins. The Parkinson disease-associated kinase LRRK2 phosphorylates S75 of endophilin A1, a position located in the acyl chain region on tubes and the aqueous environment on vesicles. We find that the phosphomimetic mutation S75D favors vesicle formation by inhibiting this conformational switch, acting to regulate endophilin A1-mediated curvature. As endophilin A1 is part of a protein superfamily, we expect these mechanisms and their regulation by posttranslational modifications to be a general means for controlling different types of membrane curvature in a wide range of processes in vivo.

Dynamin photoinactivation blocks Clathrin and α-adaptin recruitment and induces bulk membrane retrieval

Drosophila Dynamin prevents bulk membrane endocytosis through effects on AP2- and Clathrin-mediated stabilization of endocytic pits.

Dynamin is a well-known regulator of synaptic endocytosis. Temperature-sensitive dynamin (shi^ts1) mutations in Drosophila melanogaster or deletion of some of the mammalian Dynamins causes the accumulation of invaginated endocytic pits at synapses, sometimes also on bulk endosomes, indicating impaired membrane scission. However, complete loss of dynamin function has not been studied in neurons in vivo, and whether Dynamin acts in different aspects of synaptic vesicle formation remains enigmatic. We used acute photoinactivation and found that loss of Dynamin function blocked membrane recycling and caused the buildup of huge membrane-connected cisternae, in contrast to the invaginated pits that accumulate in shi^ts1 mutants. Moreover, photoinactivation of Dynamin in shi^ts1 animals converted these pits into bulk cisternae. Bulk membrane retrieval has also been seen upon Clathrin photoinactivation, and superresolution imaging indicated that acute Dynamin photoinactivation blocked Clathrin and α-adaptin relocalization to synaptic membranes upon nerve stimulation. Hence, our data indicate that Dynamin is critically involved in the stabilization of Clathrin- and AP2-dependent endocytic pits.

Intradimer/Intermolecular interactions suggest autoinhibition mechanism in endophilin A1

Endophilin A1 is a homodimeric membrane-binding endocytic accessory protein with a high dimerization affinity. Its function has been hypothesized to involve autoinhibition. However, the autoinhibition mechanism, as well as the physicochemical basis for the high dimerization affinity of endophilin in solution, have remained unclear. In this contribution, we use a Förster resonance energy transfer (FRET) method to investigate the homodimerization mechanism and intradimer molecular interactions in endophilin. For the endophilin N-BAR domain (which lacks the SH3 domain including a linker region of the full length protein), we observe a large temperature dependence of the dimerization affinity and dimer dissociation kinetics, implying large dimerization enthalpy and dissociation activation enthalpy, respectively. Our evaluation of the protein concentration dependence of dimer dissociation kinetics implies that endophilin reversibly forms monomers via a dissociation/reassociation mechanism. Furthermore, we use a kinetic method that allows us to compare the dissociation kinetics of full-length endophilin to that of truncated mutants. We find that mutants that lack either H0 helix or SH3 domain show significantly faster dissociation kinetics relative to full-length endophilin. This observation supports the presence of an intradimer, intermonomer cross-interaction between H0 helix and SH3 domain from different subunits within a homodimer. Because the H0 helix is known to play a significant role in endophilin’s membrane interactions, our measurements support a syngergistic model where these interactions are inhibited in the absence of SH3 domain binding ligands such as dynamin’s prolin rich domains, and where the binding of these ligands may be suppressed for non-membrane-bound endophilin.

Endophilin B is required for the Drosophila oocyte to endocytose yolk downstream of Oskar

The nutritional environment is crucial for Drosophila oogenesis in terms of controlling hormonal conditions that regulate yolk production and the progress of vitellogenesis. Here, we discovered that Drosophila Endophilin B (D-EndoB), a member of the endophilin family, is required for yolk endocytosis as it regulates membrane dynamics in developing egg chambers. Loss of D-EndoB leads to yolk content reduction, similar to that seen in yolkless mutants, and also causes poor fecundity. In addition, mutant egg chambers exhibit an arrest at the previtellogenic stage. D-EndoB displayed a crescent localization at the oocyte posterior pole in an Oskar-dependent manner; however, it did not contribute to pole plasm assembly. D-EndoB was found to partially colocalize with Long Oskar and Yolkless at the endocytic membranes in ultrastructure analysis. Using an FM4-64 dye incorporation assay, D-EndoB was also found to promote endocytosis in the oocyte. When expressing the full-length D-endoB(FL) or D-endoB(ΔSH3) mutant transgenes in oocytes, the blockage of vitellogenesis and the defect in fecundity in D-endoB mutants was restored. By contrast, a truncated N-BAR domain of the D-EndoB only partially rescued these defects. Taken together, these results allow us to conclude that D-EndoB contributes to the endocytic activity downstream of Oskar by facilitating membrane dynamics through its N-BAR domain in the yolk uptake process, thereby leading to normal progression of vitellogenesis.

Quantifying the dynamic interactions between a clathrin-coated pit and cargo molecules

Clathrin-mediated endocytosis takes place through the recruitment of cargo molecules into a growing clathrin-coated pit (CCP). Despite the importance of this process to all mammalian cells, little is yet known about the interaction dynamics between cargo and CCPs. These interactions are difficult to study because CCPs display a large degree of lifetime heterogeneity and the interactions with cargo molecules are time dependent. We use single-molecule total internal reflection fluorescence microscopy, in combination with automatic detection and tracking algorithms, to directly visualize the recruitment of individual voltage-gated potassium channels into forming CCPs in living cells. We observe association and dissociation of individual channels with a CCP and, occasionally, their internalization. Contrary to widespread ideas, cargo often escapes from a pit before abortive CCP termination or endocytic vesicle production. Thus, the binding times of cargo molecules associating to CCPs are much shorter than the overall endocytic process. By measuring tens of thousands of capturing events, we build the distribution of capture times and the times that cargo remains confined to a CCP. An analytical stochastic model is developed and compared with the measured distributions. Due to the dynamic nature of the pit, the model is non-Markovian and it displays long-tail power law statistics. The measured distributions and model predictions are in excellent agreement over more than five orders of magnitude. Our findings identify one source of the large heterogeneities in CCP dynamics and provide a mechanism for the anomalous diffusion of proteins in the plasma membrane.

Screening and identification of dynamin-1 interacting proteins in rat brain synaptosomes

Dynamin-1 is a multi-domain GTPase that is crucial for the fission stage of synaptic vesicle recycling and vesicle trafficking. In this study, we constructed prokaryotic expression plasmids for the four functional domains of dynamin-1, which are pGEX-4T-2-PH, pGEX-4T-2-PRD, pGEX-4T-2-GED and pGEX-4T-2-GTPase. Glutathione S-transferase pull-down, co-immunoprecipitation (co-IP), and liquid chromatography/mass spectrometry were used to screen and identify dynamin-1 interacting proteins in rat brain synaptosomes. We identified a set of 63 candidate protein interactions, including 36 proteins interacting with dynamin-1 C-terminal proline-rich domain (PRD), 14 with pleckstrin-homology domain (PH), 7 with GTPase effector domain (GED) and 6 with GTPase domain, consisting of synaptic vesicle-associated proteins, cytoskeletal proteins, metabolic enzymes and other proteins. We selected three previously unreported dynamin-1 interacting proteins to verify their interaction with dynamin-1 under native conditions. Using co-IP, we found that Rab GDP-dissociation inhibitor (Rab GDI) and chloride channel 3 (ClC-3) do interact with dynamin-1, but not with TUC-4b (the TOAD-64/Ulip/CRMP (TUC) family member). Those novel interactions detected in our study offer valuable insight into the protein-protein interacting network that could enhance our understanding of dynamin-1 mediated synaptic vesicle recycling.

The architectural relationship of components controlling mast cell endocytosis

Eukaryotic cells use multiple routes for receptor internalization. Here, we examine the topographical relationships of clathrin-dependent and clathrin-independent endocytic structures on the plasma membranes of leukemia-derived mast cells. The high affinity IgE receptor (FcεRI) utilizes both pathways, whereas transferrin receptor serves as a marker for the classical clathrin-mediated endocytosis pathway. Both receptors were tracked by live-cell imaging in the presence or absence of inhibitors that established their differential dependence on specific endocytic adaptor proteins. The topology of antigen-bound FcεRI, clathrin, dynamin, Arf6 and Eps15-positive structures were analyzed by 2D and 3D immunoelectron microscopy techniques, revealing their remarkable spatial relationships and unique geometry. We conclude that the mast cell plasma membrane has multiple specialized domains for endocytosis. Their close proximity might reflect shared components, such as lipids and adaptor proteins, that facilitate inward membrane curvature. Intersections between these specialized domains might represent sorting stations that direct cargo to specific endocytic pathways.

Clathrin-coated vesicles from brain have small payloads: a cryo-electron tomographic study

Clathrin coats, which stabilize membrane curvature during endocytosis and vesicular trafficking, form highly polymorphic fullerene lattices. We used cryo-electron tomography to visualize coated particles in isolates from bovine brain. The particles range from ∼66 to ∼134nm in diameter, and only 20% of them (all ⩾80nm) contain vesicles. The remaining 80% are clathrin "baskets", presumably artifactual assembly products. Polyhedral models were built for 54 distinct coat geometries. In true coated vesicles (CVs), most vesicles are offset to one side, leaving a crescent of interstitial space between the coat and the membrane for adaptor proteins and other components. The latter densities are fewer on the membrane-proximal side, which may represent the last part of the vesicle to bud off. A small number of densities - presumably cargo proteins - are associated with the interior surface of the vesicles. The clathrin coat, adaptor proteins, and vesicle membrane contribute almost all of the mass of a CV, with most cargoes accounting for only a few percent. The assembly of a CV therefore represents a massive biosynthetic effort to internalize a relatively diminutive payload. Such a high investment may be needed to overcome the resistance of membranes to high curvature.

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.

Interchangeable adaptors regulate mitochondrial dynamin assembly for membrane scission

Mitochondrial fission is mediated by the dynamin-related GTPases Dnm1/Drp1 (yeast/mammals), which form spirals around constricted sites on mitochondria. Additional membrane-associated adaptor proteins (Fis1, Mdv1, Mff, and MiDs) are required to recruit these GTPases from the cytoplasm to the mitochondrial surface. Whether these adaptors participate in both GTPase recruitment and membrane scission is not known. Here we use a yeast strain lacking all fission proteins to identify the minimal combinations of GTPases and adaptors sufficient for mitochondrial fission. Although Fis1 is dispensable for fission, membrane-anchored Mdv1, Mff, or MiDs paired individually with their respective GTPases are sufficient to divide mitochondria. In addition to their role in Drp1 membrane recruitment, MiDs coassemble with Drp1 in vitro. The resulting heteropolymer adopts a dramatically different structure with a narrower diameter than Drp1 homopolymers assembled in isolation. This result demonstrates that an adaptor protein alters the architecture of a mitochondrial dynamin GTPase polymer in a manner that could facilitate membrane constriction and severing activity.

Kinetics of endophilin N-BAR domain dimerization and membrane interactions

The recruitment to plasma membrane invaginations of the protein endophilin is a temporally regulated step in clathrin-mediated endocytosis. Endophilin is believed to sense or stabilize membrane curvature, which in turn likely depends on the dimeric structure of the protein. The dynamic nature of the membrane association and dimerization of endophilin is thus functionally important and is illuminated herein. Using subunit exchange Förster resonance energy transfer (FRET), we determine dimer dissociation kinetics and find a dimerization equilibrium constant orders of magnitude lower than previously published values. We characterize N-BAR domain membrane association kinetics under conditions where the dimeric species predominates, by stopped flow, observing prominent electrostatic sensitivity of membrane interaction kinetics. Relative to membrane binding, we find that protein monomer/dimer species equilibrate with far slower kinetics. Complementary optical microscopy studies reveal strikingly slow membrane dissociation and an increase of dissociation rate constant for a construct lacking the amphipathic segment helix 0 (H0). We attribute the slow dissociation kinetics to higher-order protein oligomerization on the membrane. We incorporate our findings into a kinetic scheme for endophilin N-BAR membrane binding and find a significant separation of time scales for endophilin membrane binding and subsequent oligomerization. This separation may facilitate the regulation of membrane trafficking phenomena.

Cooperative recruitment of dynamin and BIN/amphiphysin/Rvs (BAR) domain-containing proteins leads to GTP-dependent membrane scission

Dynamin mediates various membrane fission events, including the scission of clathrin-coated vesicles. Here, we provide direct evidence for cooperative membrane recruitment of dynamin with the BIN/amphiphysin/Rvs (BAR) proteins, endophilin and amphiphysin. Surprisingly, endophilin and amphiphysin recruitment to membranes was also dependent on binding to dynamin due to auto-inhibition of BAR-membrane interactions. Consistent with reciprocal recruitment in vitro, dynamin recruitment to the plasma membrane in cells was strongly reduced by concomitant depletion of endophilin and amphiphysin, and conversely, depletion of dynamin dramatically reduced the recruitment of endophilin. In addition, amphiphysin depletion was observed to severely inhibit clathrin-mediated endocytosis. Furthermore, GTP-dependent membrane scission by dynamin was dramatically elevated by BAR domain proteins. Thus, BAR domain proteins and dynamin act in synergy in membrane recruitment and GTP-dependent vesicle scission.

Dynamin: expanding its scope to the cytoskeleton

The large GTPase dynamin is well known for its actions on budded cellular membranes to generate vesicles, most often, clathrin-coated endocytic vesicles. The scope of cellular processes in which dynamin-mediated vesicle formation occurs, has expanded to include secretory vesicle formation at the Golgi, from other endosomes and nonclathrin structures, such as caveolae, as well as membrane remodeling during exocytosis and vesicle fusion. An intriguing new facet of dynamin's sphere of influence is the cytoskeleton. Cytoskeletal filament networks maintain cell shape, provide cell movement, execute cell division and orchestrate vesicle trafficking. Recent evidence supports the hypothesis that dynamin influences actin filaments and microtubules via mechanisms that are independent of its membrane-remodeling activities. This chapter discusses this emerging evidence and considers possible mechanisms of action.

Implications of receptor-mediated endocytosis and intracellular trafficking dynamics in the development of antibody drug conjugates

The use of antibody-drug conjugates (ADCs) as a therapeutic platform to treat cancer has recently gained substantial momentum. This therapeutic modality has the potential to increase the efficacy and reduce the systemic toxicity associated with current therapeutic regimens. The efficacy of ADCs, however, relies on the proper exploitation of intracellular sorting dynamics of the antigen as well as the specificity, selectivity and pharmacokinetic properties of the antibody itself. Our understanding of endocytosis and endosomal trafficking of receptors has appreciably increased in recent years, as improvements in the assays used to study these events have resolved many of the molecular mechanisms regulating these processes. As a result, we now have the knowledge necessary to exploit these pathways efficiently to improve the efficacy of antibody-based therapy. This review discusses some recent studies that have explored how endo/lysosomal dynamics can affect the efficacy of engineered therapeutic antibodies, including ADCs.

Nonlinear sorting, curvature generation, and crowding of endophilin N-BAR on tubular membranes

The curvature of biological membranes is controlled by membrane-bound proteins. For example, during endocytosis, the sorting of membrane components, vesicle budding, and fission from the plasma membrane are mediated by adaptor and accessory proteins. Endophilin is a peripherally binding membrane protein that functions as an endocytic accessory protein. Endophilin's membrane tubulation capacity is well known. However, to understand the thermodynamic and mechanical aspects of endophilin function, experimental measurements need to be compared to quantitative theoretical models. We present measurements of curvature sorting and curvature generation of the endophilin A1 N-BAR domain on tubular membranes pulled from giant unilamellar vesicles. At low concentration, endophilin functions primarily as a membrane curvature sensor; at high concentrations, it also generates curvature. We determine the spontaneous curvature induced by endophilin and observe sigmoidal curvature/composition coupling isotherms that saturate at high membrane tensions and protein solution concentrations. The observation of saturation is supported by a strong dependence of lateral diffusion coefficients on protein density on the tether membrane. We develop a nonlinear curvature/composition coupling model that captures our experimental observations. Our model predicts a curvature-induced phase transition among two states with varying protein density and membrane curvature. This transition could act as a switch during endocytosis.

Endophilin isoforms have distinct characteristics in interactions with N-type Ca2+ channels and dynamin I

OBJECTIVE

Formation of the endophilin II-Ca(2+) channel complex is Ca(2+)-dependent in clathrin-mediated endocytosis. However, little is known about whether the other two endophilin isoforms have the same features. The present study aimed to investigate the characteristics of the interactions of all three isoforms with Ca(2+) channels and dynamin I.

METHODS

N-type Ca(2+) channel C-terminal fragments (NCFs) synthesized with a (3)H-leucine-labeled kit, were incubated with endophilin-GST fusion proteins, followed by pull-down assay. Results were counted on a scintillation counter. In addition, the different endophilin isoforms were each co-transfected with dynamin I into 293T cells, followed by flow cytometry and co-immunoprecipitation assay. Immunostaining was performed and an image analysis program was used to evaluate the overlap coefficient of cells expressing endophilin and dynamin I.

RESULTS

All three isoforms interacted with NCF. Endophilins I and II demonstrated clear Ca(2+)-dependent interactions with NCF, whereas endophilin III did not. Co-immunoprecipitation showed that, compared to endophilin I/II, the interaction between endophilin III and dynamin I was significantly increased. Similar results were obtained from flow cytometry. Furthermore, endophilin III had a higher overlap coefficient with dynamin I in co-transfected 293T cells.

CONCLUSION

Endophilin isoforms have distinct characteristics in interactions with NCF and dynamin I. Endophilin III binding to NCF is Ca(2+)-independent, implying that it plays a different role in clathrin-mediated endocytosis.

Structural basis of membrane bending by the N-BAR protein endophilin

Functioning as key players in cellular regulation of membrane curvature, BAR domain proteins bend bilayers and recruit interaction partners through poorly understood mechanisms. Using electron cryomicroscopy, we present reconstructions of full-length endophilin and its N-terminal N-BAR domain in their membrane-bound state. Endophilin lattices expose large areas of membrane surface and are held together by promiscuous interactions between endophilin's amphipathic N-terminal helices. Coarse-grained molecular dynamics simulations reveal that endophilin lattices are highly dynamic and that the N-terminal helices are required for formation of a stable and regular scaffold. Furthermore, endophilin accommodates different curvatures through a quantized addition or removal of endophilin dimers, which in some cases causes dimerization of endophilin's SH3 domains, suggesting that the spatial presentation of SH3 domains, rather than affinity, governs the recruitment of downstream interaction partners.

A feedback loop between dynamin and actin recruitment during clathrin-mediated endocytosis

Clathrin-mediated endocytosis proceeds by a sequential series of reactions catalyzed by discrete sets of protein machinery. The final reaction in clathrin-mediated endocytosis is membrane scission, which is mediated by the large guanosine triophosphate hydrolase (GTPase) dynamin and which may involve the actin-dependent recruitment of N-terminal containing BIN/Amphiphysin/RVS domain containing (N-BAR) proteins. Optical microscopy has revealed a detailed picture of when and where particular protein types are recruited in the ∼20-30 s preceding scission. Nevertheless, the regulatory mechanisms and functions that underpin protein recruitment are not well understood. Here we used an optical assay to investigate the coordination and interdependencies between the recruitment of dynamin, the actin cytoskeleton, and N-BAR proteins to individual clathrin-mediated endocytic scission events. These measurements revealed that a feedback loop exists between dynamin and actin at sites of membrane scission. The kinetics of dynamin, actin, and N-BAR protein recruitment were modulated by dynamin GTPase activity. Conversely, acute ablation of actin dynamics using latrunculin-B led to a ∼50% decrease in the incidence of scission, an ∼50% decrease in the amplitude of dynamin recruitment, and abolished actin and N-BAR recruitment to scission events. Collectively these data suggest that dynamin, actin, and N-BAR proteins work cooperatively to efficiently catalyze membrane scission. Dynamin controls its own recruitment to scission events by modulating the kinetics of actin and N-BAR recruitment to sites of scission. Conversely actin serves as a dynamic scaffold that concentrates dynamin and N-BAR proteins at sites of scission.

Recruitment of endophilin to clathrin-coated pit necks is required for efficient vesicle uncoating after fission

Endophilin is a membrane-binding protein with curvature-generating and -sensing properties that participates in clathrin-dependent endocytosis of synaptic vesicle membranes. Endophilin also binds the GTPase dynamin and the phosphoinositide phosphatase synaptojanin and is thought to coordinate constriction of coated pits with membrane fission (via dynamin) and subsequent uncoating (via synaptojanin). We show that although synaptojanin is recruited by endophilin at bud necks before fission, the knockout of all three mouse endophilins results in the accumulation of clathrin-coated vesicles, but not of clathrin-coated pits, at synapses. The absence of endophilin impairs but does not abolish synaptic transmission and results in perinatal lethality, whereas partial endophilin absence causes severe neurological defects, including epilepsy and neurodegeneration. Our data support a model in which endophilin recruitment to coated pit necks, because of its curvature-sensing properties, primes vesicle buds for subsequent uncoating after membrane fission, without being critically required for the fission reaction itself.

Dynamin, a membrane-remodelling GTPase

Dynamin, the founding member of a family of dynamin-like proteins (DLPs) implicated in membrane remodelling, has a critical role in endocytic membrane fission events. The use of complementary approaches, including live-cell imaging, cell-free studies, X-ray crystallography and genetic studies in mice, has greatly advanced our understanding of the mechanisms by which dynamin acts, its essential roles in cell physiology and the specific function of different dynamin isoforms. In addition, several connections between dynamin and human disease have also emerged, highlighting specific contributions of this GTPase to the physiology of different tissues.

Role of phosphoinositides at the neuronal synapse

Synaptic transmission is amongst the most sophisticated and tightly controlled biological phenomena in higher eukaryotes. In the past few decades, tremendous progress has been made in our understanding of the molecular mechanisms underlying multiple facets of neurotransmission, both pre- and postsynaptically. Brought under the spotlight by pioneer studies in the areas of secretion and signal transduction, phosphoinositides and their metabolizing enzymes have been increasingly recognized as key protagonists in fundamental aspects of neurotransmission. Not surprisingly, dysregulation of phosphoinositide metabolism has also been implicated in synaptic malfunction associated with a variety of brain disorders. In the present chapter, we summarize current knowledge on the role of phosphoinositides at the neuronal synapse and highlight some of the outstanding questions in this research field.

Yeast dynamin Vps1 and amphiphysin Rvs167 function together during endocytosis

Dynamins are a conserved family of proteins involved in many membrane fusion and fission events. Previously, the dynamin-related protein Vps1 was shown to localize to endocytic sites, and yeast carrying deletions for genes encoding both the BAR domain protein Rvs167 and Vps1 had a more severe endocytic scission defect than either deletion alone. Vps1 and Rvs167 localize to endocytic sites at the onset of invagination and disassemble concomitant with inward vesicle movement. Rvs167-GFP localization is reduced in cells lacking vps1 suggesting that Vps1 influences Rvs167 association with the endocytic complex. Unlike classical dynamins, Vps1 does not have a proline-arginine domain that could interact with SH3 domain-containing proteins. Thus, while Rvs167 has an SH3 domain, it is not clear how an interaction would be mediated. Here, we demonstrate an interaction between Rvs167 SH3 domain and the single type I SH3-binding motif in Vps1. Mutant Vps1 that cannot bind Rvs167 rescues all membrane fusion/fission functions associated with Vps1 except for endocytic function, demonstrating the specificity and mechanistic importance of the interaction. In vitro, an Rvs161/Rvs167 heterodimer can disassemble Vps1 oligomers. Overall, the data support the idea that Vps1 and the amphiphysins function together to mediate scission during endocytosis in yeast.

Regulation of synaptic vesicle budding and dynamin function by an EHD ATPase

Eps15 homology domain-containing proteins (EHDs) are conserved ATPases implicated in membrane remodeling. Recently, EHD1 was found to be enriched at synaptic release sites, suggesting a possible involvement in the trafficking of synaptic vesicles. We have investigated the role of an EHD1/3 ortholog (l-EHD) in the lamprey giant reticulospinal synapse. l-EHD was detected by immunogold at endocytic structures adjacent to release sites. In antibody microinjection experiments, perturbation of l-EHD inhibited synaptic vesicle endocytosis and caused accumulation of clathrin-coated pits with atypical, elongated necks. The necks were covered with helix-like material containing dynamin. To test whether l-EHD directly interferes with dynamin function, we used fluid-supported bilayers as in vitro assay. We found that l-EHD strongly inhibited vesicle budding induced by dynamin in the constant presence of GTP. l-EHD also inhibited dynamin-induced membrane tubulation in the presence of GTPγS, a phenomenon linked with dynamin helix assembly. Our in vivo results demonstrate the involvement of l-EHD in clathrin/dynamin-dependent synaptic vesicle budding. Based on our in vitro observations, we suggest that l-EHD acts to limit the formation of long, unproductive dynamin helices, thereby promoting vesicle budding.

Overlapping role of dynamin isoforms in synaptic vesicle endocytosis

The existence of neuron-specific endocytic protein isoforms raises questions about their importance for specialized neuronal functions. Dynamin, a GTPase implicated in the fission reaction of endocytosis, is encoded by three genes, two of which, dynamin 1 and 3, are highly expressed in neurons. We show that dynamin 3, thought to play a predominantly postsynaptic role, has a major presynaptic function. Although lack of dynamin 3 does not produce an overt phenotype in mice, it worsens the dynamin 1 KO phenotype, leading to perinatal lethality and a more severe defect in activity-dependent synaptic vesicle endocytosis. Thus, dynamin 1 and 3, which together account for the overwhelming majority of brain dynamin, cooperate in supporting optimal rates of synaptic vesicle endocytosis. Persistence of synaptic transmission in their absence indicates that if dynamin plays essential functions in neurons, such functions can be achieved by the very low levels of dynamin 2.