The N terminus of amphiphysin II mediates dimerization and plasma membrane targeting

The phospho-regulated amphiphysin/endophilin interaction is required for synaptic vesicle endocytosis

The multidomain adaptor protein amphiphysin-1 (Amph1) is an important coordinator of clathrin-mediated endocytosis in non-neuronal cells and synaptic vesicle (SV) endocytosis at central nerve terminals. Amph1 contains a lipid-binding N-BAR (Bin/Amphiphysin/Rvs) domain, central proline-rich (PRD) and clathrin/AP2 (CLAP) domains, and a C-terminal SH3 domain. Amph1 interacts with both lipids and proteins, with all of these interactions required for SV endocytosis, with the exception of the Amph1 PRD. The Amph1 PRD associates with the endocytosis protein endophilin A1, however, the role of this interaction in SV endocytosis has not been investigated. In this study, we set out to determine whether the Amph1 PRD and its interaction with endophilin A1 was essential for efficient SV endocytosis at typical small central synapses. To achieve this, domain-specific interactions of Amph1 were validated using in vitro GST pull-down assays, with the role of these interactions in SV endocytosis determined in molecular replacement experiments in primary neuronal culture. Using this approach, we confirmed important roles for CLAP and SH3 domain interactions of Amph1 in the control of SV endocytosis. Importantly, we identified the interaction site for endophilin A1 within the Amph1 PRD and exploited specific binding mutants to reveal a key role for this interaction in SV endocytosis. Finally, we determined that the formation of the Amph1-endophilin A1 complex is dependent on the phosphorylation status of Amph1-S293 within the PRD and that the phosphorylation status of this residue is essential for efficient SV regeneration. This work, therefore, reveals a key role for the dephosphorylation-dependent Amph1-endophilin A1 interaction in efficient SV endocytosis.

HNF1β-associated cyst development and electrolyte disturbances are not explained by BAIAP2L2 expression

Mutations or deletions in transcription factor hepatocyte nuclear factor 1 homeobox β (HNF1β) cause renal cysts and/or malformation, maturity-onset diabetes of the young and electrolyte disturbances. Here, we applied a comprehensive bioinformatic approach on ChIP-seq, RNA-seq, and gene expression array studies to identify novel transcriptional targets of HNF1β explaining the kidney phenotype of HNF1β patients. We identified BAR/IMD Domain Containing Adaptor Protein 2 Like 2 (BAIAP2L2), as a novel transcriptional target of HNF1β and validated direct transcriptional activation of the BAIAP2L2 promoter by a reporter luciferase assay. Using mass spectrometry analysis, we show that BAIAP2L2 binds to other members of the I-BAR domain-containing family: BAIAP2 and BAIAP2L1. Subsequently, the role of BAIAP2L2 in maintaining epithelial cell integrity in the kidney was assessed using Baiap2l2 knockout cell and mouse models. Kidney epithelial cells lacking functional BAIAP2L2 displayed normal F-actin distribution at cell-cell contacts and formed polarized three-dimensional spheroids with a lumen. In vivo, Baiap2l2 knockout mice displayed normal kidney and colon tissue morphology and serum and urine electrolyte concentrations were not affected. Altogether, our study is the first to characterize the function of BAIAP2L2 in the kidney in vivo and we report that mice lacking BAIAP2L2 exhibit normal electrolyte homeostasis and tissue morphology under physiological conditions.

The Alzheimer susceptibility gene BIN1 induces isoform-dependent neurotoxicity through early endosome defects

The Bridging Integrator 1 (BIN1) gene is a major susceptibility gene for Alzheimer’s disease (AD). Deciphering its pathophysiological role is challenging due to its numerous isoforms. Here we observed in Drosophila that human BIN1 isoform1 (BIN1iso1) overexpression, contrary to human BIN1 isoform8 (BIN1iso8) and human BIN1 isoform9 (BIN1iso9), induced an accumulation of endosomal vesicles and neurodegeneration. Systematic search for endosome regulators able to prevent BIN1iso1-induced neurodegeneration indicated that a defect at the early endosome level is responsible for the neurodegeneration. In human induced neurons (hiNs) and cerebral organoids, BIN1 knock-out resulted in the narrowing of early endosomes. This phenotype was rescued by BIN1iso1 but not BIN1iso9 expression. Finally, BIN1iso1 overexpression also led to an increase in the size of early endosomes and neurodegeneration in hiNs. Altogether, our data demonstrate that the AD susceptibility gene BIN1, and especially BIN1iso1, contributes to early-endosome size deregulation, which is an early pathophysiological hallmark of AD pathology.

BAR domain proteins-a linkage between cellular membranes, signaling pathways, and the actin cytoskeleton

Actin filament assembly typically occurs in association with cellular membranes. A large number of proteins sit at the interface between actin networks and membranes, playing diverse roles such as initiation of actin polymerization, modulation of membrane curvature, and signaling. Bin/Amphiphysin/Rvs (BAR) domain proteins have been implicated in all of these functions. The BAR domain family of proteins comprises a diverse group of multi-functional effectors, characterized by their modular architecture. In addition to the membrane-curvature sensing/inducing BAR domain module, which also mediates antiparallel dimerization, most contain auxiliary domains implicated in protein-protein and/or protein-membrane interactions, including SH3, PX, PH, RhoGEF, and RhoGAP domains. The shape of the BAR domain itself varies, resulting in three major subfamilies: the classical crescent-shaped BAR, the more extended and less curved F-BAR, and the inverse curvature I-BAR subfamilies. Most members of this family have been implicated in cellular functions that require dynamic remodeling of the actin cytoskeleton, such as endocytosis, organelle trafficking, cell motility, and T-tubule biogenesis in muscle cells. Here, we review the structure and function of mammalian BAR domain proteins and the many ways in which they are interconnected with the actin cytoskeleton.

Gas7b (growth arrest specific protein 7b) regulates neuronal cell morphology by enhancing microtubule and actin filament assembly

Neurons undergo several morphological changes as a part of normal neuron maturation process. Alzheimer disease is associated with increased neuroproliferation and impaired neuronal maturation. In this study, we demonstrated that Gas7b (growth arrest specific protein 7b) expression in a neuronal cell line, Neuro 2A, induces cell maturation by facilitating formation of dendrite-like processes and/or filopodia projections and that Gas7b co-localizes with neurite microtubules. Molecular analysis was performed to evaluate whether Gas7b associates with actin filaments and microtubules, and the data revealed two novel roles of Gas7b in neurite outgrowth: we showed that Gas7b enhances bundling of several microtubule filaments and connects microtubules with actin filaments. These results suggest that Gas7b governs neural cell morphogenesis by enhancing the coordination between actin filaments and microtubules. We conclude that lower neuronal Gas7b levels may impact Alzheimer disease progression.

Specific amino acids in the BAR domain allow homodimerization and prevent heterodimerization of sorting nexin 33

SNX33 (sorting nexin 33) is a homologue of the endocytic protein SNX9 and has been implicated in actin polymerization and the endocytosis of the amyloid precursor protein. SNX33 belongs to the large family of BAR (Bin/amphiphysin/Rvs) domain-containing proteins, which alter cellular protein trafficking by modulating cellular membranes and the cytoskeleton. Some BAR domains engage in homodimerization, whereas other BAR domains also mediate heterodimerization between different BAR domain-containing proteins. The molecular basis for this difference is not yet understood. Using co-immunoprecipitations we report that SNX33 forms homodimers, but not heterodimers, with other BAR domain-containing proteins, such as SNX9. Domain deletion analysis revealed that the BAR domain, but not the SH3 (Src homology 3) domain, was required for homodimerization of SNX33. Additionally, the BAR domain prevented the heterodimerization between SNX9 and SNX33, as determined by domain swap experiments. Molecular modelling of the SNX33 BAR domain structure revealed that key amino acids located at the BAR domain dimer interface of the SNX9 homodimer are not conserved in SNX33. Replacing these amino acids in SNX9 with the corresponding amino acids of SNX33 allowed the mutant SNX9 to heterodimerize with SNX33. Taken together, the present study identifies critical amino acids within the BAR domains of SNX9 and SNX33 as determinants for the specificity of BAR domain-mediated interactions and suggests that SNX9 and SNX33 have distinct molecular functions.

The NECAP PHear domain increases clathrin accessory protein binding potential

AP-2 is a key regulator of the endocytic protein machinery driving clathrin-coated vesicle (CCV) formation. One critical function, mediated primarily by the AP-2 alpha-ear, is the recruitment of accessory proteins. NECAPs are alpha-ear-binding proteins that enrich on CCVs. Here, we have solved the structure of the conserved N-terminal region of NECAP 1, revealing a unique module in the pleckstrin homology (PH) domain superfamily, which we named the PHear domain. The PHear domain binds accessory proteins bearing FxDxF motifs, which were previously thought to bind exclusively to the AP-2 alpha-ear. Structural analysis of the PHear domain reveals the molecular surface for FxDxF motif binding, which was confirmed by site-directed mutagenesis. The reciprocal analysis of the FxDxF motif in amphiphysin I identified distinct binding requirements for binding to the alpha-ear and PHear domain. We show that NECAP knockdown compromises transferrin uptake and establish a functional role for NECAPs in clathrin-mediated endocytosis. Our data uncover a striking convergence of two evolutionarily and structurally distinct modules to recognize a common peptide motif and promote efficient endocytosis.

Major Cdk5-dependent phosphorylation sites of amphiphysin 1 are implicated in the regulation of the membrane binding and endocytosis

Amphiphysin 1 (amph 1) is an endocytic protein enriched in the nerve terminals that functions in the clathrin-mediated endocytosis. It acts as membrane curvature sensor, a linker of clathrin coat proteins, and an enhancer of dynamin Guanosine Triphosphatase (GTPase) activity. Amph 1 undergoes phosphorylation by cyclin-dependent kinase 5 (Cdk5), at five phosphorylation sites, serine 262, 272, 276, 285, and threonine 310, as determined by mass spectrometry (MS). We show here that Cdk5-dependent phosphorylation of amph 1 is enhanced in the presence of lipid membranes. Analysis by tandem liquid chromatograph MS revealed that the phosphorylation occurs at two phosphorylation sites. The phosphorylation was markedly decreased by mutation either Ser276 or Ser285 of amph 1 to alanine (S276A and S285A). Furthermore, mutation of both sites (S276, 285A) completely eliminated the phosphorylation. Functional studies indicated that binding of amph 1 to lipid membrane was attenuated by Cdk5-dependent phosphorylation of wild type amph 1, but not of the S276, 285A form. Interestingly, endocytosis was increased in rat pheochromocytoma cells expressing amph 1 S276, 285A in comparison with wild type. These results suggest that Ser276 and Ser285 are regulatory Cdk5 phosphorylation sites of amph 1 in the lipid-bound state. Phosphorylation at these sites alters binding of amph 1 to lipid membranes, and may be an important regulatory aspect in the regulation of synaptic vesicle 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.

The BAR domain proteins: molding membranes in fission, fusion, and phagy

The Bin1/amphiphysin/Rvs167 (BAR) domain proteins are a ubiquitous protein family. Genes encoding members of this family have not yet been found in the genomes of prokaryotes, but within eukaryotes, BAR domain proteins are found universally from unicellular eukaryotes such as yeast through to plants, insects, and vertebrates. BAR domain proteins share an N-terminal BAR domain with a high propensity to adopt alpha-helical structure and engage in coiled-coil interactions with other proteins. BAR domain proteins are implicated in processes as fundamental and diverse as fission of synaptic vesicles, cell polarity, endocytosis, regulation of the actin cytoskeleton, transcriptional repression, cell-cell fusion, signal transduction, apoptosis, secretory vesicle fusion, excitation-contraction coupling, learning and memory, tissue differentiation, ion flux across membranes, and tumor suppression. What has been lacking is a molecular understanding of the role of the BAR domain protein in each process. The three-dimensional structure of the BAR domain has now been determined and valuable insight has been gained in understanding the interactions of BAR domains with membranes. The cellular roles of BAR domain proteins, characterized over the past decade in cells as distinct as yeasts, neurons, and myocytes, can now be understood in terms of a fundamental molecular function of all BAR domain proteins: to sense membrane curvature, to bind GTPases, and to mold a diversity of cellular membranes.

Cell-type-specific pathways of neurotensin endocytosis

The neurotensin receptor subtype 1 (NTS1) is a G-protein-coupled receptor (GPCR) mediating a large number of central and peripheral effects of neurotensin. Upon stimulation, NTS1 is rapidly internalized and targeted to lysosomes. This process depends on the interaction of the phosphorylated receptor with beta-arrestin. Little is known about other accessory endocytic proteins potentially involved. Here, we investigated the involvement of dynamin, amphiphysin, and intersectin in the internalization of NTS1 receptor-ligand complexes in transfected COS-7 and HEK 293 cells, by using the transferrin receptor as an internal control for the constitutive endocytic pathway. We found that NTS1 endocytosis was not only arrestin-dependent, but also dynamin-dependent in both COS-7 and HEK 293 cells, whereas internalization of the transferrin receptor was independent of arrestin but required dynamin. Overexpression of the SH3 domain of amphiphysin II had no effect on receptor internalization in either cell type. By contrast, overexpression of full-length intersectin or of its SH3 domain (but not of its EH domain) inhibited NTS1 internalization in COS-7 but not in HEK 293 cells. This difference between COS-7 and HEK 293 cells was not attributable to differences in endogenous intersectin levels between the two cell lines. Indeed, the same constructs inhibited transferrin endocytosis equally well in COS-7 and HEK 293 cells. However, immunogold electron microscopy revealed that internalized NTS1 receptors were associated with clathrin-coated pits in COS-7 cells but with smooth vesicles in HEK 293 cells, suggesting that NTS1 internalization proceeds via different endocytic pathways in these two cell types.

Adaptors for clathrin coats: structure and function

Clathrin-coated vesicles (CCVs) are responsible for the transport of proteins between various compartments of the secretory and endocytic systems. Clathrin forms a scaffold around these vesicles that is linked to membranes by clathrin adaptors. The adaptors simultaneously bind to clathrin and to transmembrane proteins and/or phospholipids and can also interact with each other and with other components of the CCV formation machinery. The result is a collection of proteins that can make multiple, moderate strength (microM Kd) interactions and thereby establish the dynamic regulatable networks to drive vesicle genesis at the correct time and place in the cell. This review focuses on the structure of clathrin adaptors and how these structures provide functional information on the mechanism of CCV formation.

Expression of the endocytic proteins dynamin and amphiphysin in rat gastric enterochromaffin-like cells

Dynamin and amphiphysin play crucial roles in a variety of endocytic processes. Previous investigations of expression and functions of these proteins were performed mostly on neurons. The aim of this study was to investigate the presence and interaction of dyn and amph in gastric enterochromaffin-like cells. These endocrine cells of the gastric mucosa play a pivotal role in the regulation of acid secretion. Exocytosis of histamine-containing secretory vesicles has been described in detail. However, the mechanisms of endocytosis are unknown in this neuroendocrine cell type. Using RT-PCR and western blotting, we detected dynamin-1, -2 and -3 in highly enriched isolated enterochromaffin-like cells. Dynamin-1 and -2 were expressed at similar high levels, whereas dynamin-3 was of low abundance. Immunofluorescence microscopy located dynamin-1 and -2 to the cytoplasm and cell surface, whereas dynamin-3 was distributed differently in the perinuclear area. The presence of amphiphysin-1 and -2 RNAs was revealed by RT-PCR and a new splice variant of amphiphysin-2 was detected. Amphiphysin-1 and -2 were also detected in enterochromaffin-like cells by immunohistochemistry in the same locations as dynamin-1 and -2. Amphiphysin-1 and dynamin-1 co-immunoprecipitated with amphiphysin-2. In addition, dynamin-1 and amphiphysin-2 partially colocalized at the plasma membrane. Our results confirm the interaction of dynamin and amphiphysin and imply a role in endocytosis in enterochromaffin-like cells. To our knowledge, this is the first demonstration of the co-expression of all three dynamin isoforms in a non-tumor cell.

Stimulation of Dynamin GTPase Activity by Amphiphysin

Dynamin functions in the fission of endocytic pits in the process of clathrin-mediated endocytosis. Dynamin GTPase activity is essential for its fission activity, and it is stimulated by self-assembly as well as by interacting with its binding partners, such as microtubules, SH3 domain containing proteins, or inositol phospholipids. Amphiphysin 1, SH3 domain-containing binding partner of dynamin 1, is proposed to cooperatively function in endocytosis. Amphiphysin 1 is essential for dynamin-dependent synaptic vesicle recycling in the synapse, and it enhances dynamin-dependent vesicle formation in vitro. In order to elucidate the molecular mechanism underlying the amphiphysin's effect, we measured dynamin GTPase activity in the presence of both amphiphysin 1 and lipid membranes. We describe here in detail the procedure of the dynamin GTPase assay and the results demonstrating stimulatory effect of amphiphysin on dynamin GTPase activity, which is highly dependent on the liposome size.

The stimulatory action of amphiphysin on dynamin function is dependent on lipid bilayer curvature

Amphiphysin is a major dynamin-binding partner at the synapse; however, its function in fission is unclear. Incubation of large unilamellar liposomes with mice brain cytosol led to massive formation of small vesicles, whereas cytosol of amphiphysin 1 knockout mice was much less efficient in this reaction. Vesicle formation from large liposomes by purified dynamin was also strongly enhanced by amphiphysin. In the presence of liposomes, amphiphysin strongly affected dynamin GTPase activity and the recruitment of dynamin to the liposomes, but this activity was highly dependent on liposome size. Deletion from amphiphysin of its central proline-rich stretch dramatically potentiated its effect on dynamin, possibly by relieving an inhibitory intramolecular interaction. These results suggest a model in which maturation of endocytic pits correlates with the oligomerization of dynamin with either amphiphysin or other proteins with similar domain structure. Formation of these complexes is coupled to the activation of dynamin GTPase activity, thus explaining how deep invagination of the pit leads to fission.

A novel dynamin-associating molecule, formin-binding protein 17, induces tubular membrane invaginations and participates in endocytosis

Dynamin associates with a variety of SH3 domain-containing molecules via a C-terminal proline-rich motif and takes part, with them, in endocytic processes. Here, we have investigated a new dynamin-associating molecule, formin-binding protein 17 (FBP17), involved in deforming the plasma membrane and in endocytosis. FBP17 formed tubular invaginations originating from the plasma membrane. Its N-terminal Fer/CIP4 homology domain, a coiled-coil domain, and a proline-rich motif were required for tubular invagination and self-assembly, by which tubular invagination might be induced. Using anti-FBP17 antibody, we detected positive immunoreactions in the testis that were restricted to the germ cells. We also detected FBP17 in the brain by immunoblotting and in situ hybridization. When COS cells expressing enhanced green fluorescent protein-tagged FBP17 were incubated with fluorescently labeled transferrin, epidermal growth factor, and cholera toxin, these molecules co-localized with FBP17-induced tubular invaginations, suggesting that FBP17 is involved in dynamin-mediated endocytosis in both a clathrin-dependent and -independent manner. These observations therefore indicate that FBP17 interacts with dynamin and regulates endocytosis by forming vesicotubular structures.

Two distinct interaction motifs in amphiphysin bind two independent sites on the clathrin terminal domain β-propeller

During the assembly of clathrin-coated vesicles, many peripheral membrane proteins, including the amphiphysins, use LLDLD-type clathrin-box motifs to interact with the N-terminal beta-propeller domain (TD) of clathrin. The 2.3 A-resolution structure of the clathrin TD in complex with a TLPWDLWTT peptide from amphiphysin 1 delineates a second clathrin-binding motif, PWXXW (the W box), that binds at a site on the TD remote from the clathrin box-binding site. The presence of both sequence motifs within the unstructured region of the amphiphysins allows them to bind more tightly to free TDs than do other endocytic proteins that contain only clathrin-box motifs. This property, along with the propensity of the N-terminal BAR domain to bind curved membranes, will preferentially localize amphiphysin and its partner, dynamin, to the periphery of invaginated clathrin lattices.

Role of Amphiphysin II in Somatostatin Receptor Trafficking in Neuroendocrine Cells

Amphiphysins are SH3 domain-containing proteins thought to function in clathrin-mediated endocytosis. To investigate the potential role of amphiphysin II in cellular trafficking of G protein-coupled somatostatin (SRIF) receptors, we generated an AtT-20 cell line stably overexpressing amphiphysin IIb, a splice variant that does not bind clathrin. Endocytosis of (125)I-[d-Trp(8)]SRIF was not affected by amphiphysin IIb overexpression. However, the maximal binding capacity (B(max)) of the ligand on intact cells was significantly lower in amphiphysin IIb overexpressing than in non-transfected cells. This difference was no longer apparent when the experiments were performed on crude cell homogenates, suggesting that amphiphysin IIb overexpression interferes with SRIF receptor targeting to the cell surface and not with receptor synthesis. Accordingly, immunofluorescence experiments demonstrated that, in amphiphysin overexpressing cells, sst(2A) and sst(5) receptors were segregated in a juxtanuclear compartment identified as the trans-Golgi network. Amphiphysin IIb overexpression had no effect on corticotrophin-releasing factor 41-stimulated adrenocorticotropic hormone secretion, suggesting that it is not involved in the regulated secretory pathway. Taken together, these results suggest that amphiphysin II is not necessary for SRIF receptor endocytosis but is critical for its constitutive targeting to the plasma membrane. Therefore, amphiphysin IIb may be an important component of the constitutive secretory pathway.

A putative role for intramolecular regulatory mechanisms in the adaptor function of amphiphysin in endocytosis

Amphiphysin 1 is a brain-specific protein enriched at the synapse and a major binding partner of several components of the clathrin-mediated endocytic machinery (Proc Natl Acad Sci USA 93 (1996) 331). It interacts with clathrin-coat proteins, dynamin, and membranes (Nat Cell Biol 1 (1999) 33; JBC). A role of amphiphysin in synaptic vesicle recycling is supported by both acute and chronic perturbation studies (Science 276 (1997) 259; Neuron 33 (2002) 789). Here we show that amphiphysin directly stimulates clathrin recruitment onto liposomes in an in vitro assay. Amphiphysin-dependent clathrin-coat recruitment is enhanced by the interaction of amphiphysin with dynamin. We also show that the amphiphysin SH3 domain binds full-length amphiphysin, likely via an internal poly-proline region, and that clathrin recruitment onto liposomes by amphiphysin is enhanced in the presence of the isolated amphiphysin SH3 domain. Expression of a mutant amphiphysin harboring two amino acid substitutions in the SH3 domain, and therefore unable to bind proline-containing motifs, induces an accumulation of large intracellular aggregates including amphiphysin, clathrin, AP-2, and other endocytic proteins, as well as a concomitant block of transferrin endocytosis. Thus, putative intramolecular interactions between the amphiphysin COOH-terminal SH3 domain and its internal poly-proline region may regulate clathrin recruitment onto membranes.

Targeted disruption of the murine Bin1/Amphiphysin II gene does not disable endocytosis but results in embryonic cardiomyopathy with aberrant myofibril formation

The mammalian Bin1/Amphiphysin II gene encodes an assortment of alternatively spliced adapter proteins that exhibit markedly divergent expression and subcellular localization profiles. Bin1 proteins have been implicated in a variety of different cellular processes, including endocytosis, actin cytoskeletal organization, transcription, and stress responses. To gain insight into the physiological functions of the Bin1 gene, we have disrupted it by homologous recombination in the mouse. Bin1 loss had no discernible impact on either endocytosis or phagocytosis in mouse embryo-derived fibroblasts and macrophages, respectively. Similarly, actin cytoskeletal organization, proliferation, and apoptosis in embryo fibroblasts were all unaffected by Bin1 loss. In vivo, however, Bin1 loss resulted in perinatal lethality. Bin1 has been reported to affect muscle cell differentiation and T-tubule formation. No striking histological abnormalities were evident in skeletal muscle of Bin1 null embryos, but severe ventricular cardiomyopathy was observed in these embryos. Ultrastructurally, myofibrils in ventricular cardiomyocytes of Bin1 null embryos were severely disorganized. These results define a developmentally critical role for the Bin1 gene in cardiac muscle development.

Sorting nexin 4 and amphiphysin 2, a new partnership between endocytosis and intracellular trafficking

Endocytosis is a regulated physiological process by which membrane receptors and their extracellular ligands are internalized. After internalization, they enter the endosomal trafficking pathway for sorting and processing. Amphiphysins consist of a family of proteins conserved throughout evolution that are crucial elements of the endocytosis machinery in mammalian cells. They act as adaptors for a series of proteins important for the endocytic process, such as dynamin. In order to improve our knowledge of amphiphysin function, we performed a two-hybrid screen with the N-terminal part of murine amphiphysin 2 (residues 1-304). One of the interacting clones corresponded to sorting nexin 4 (SNX4), a member of the SNX family of proteins which are suspected to regulate vesicular trafficking. This interaction was confirmed in vivo by co-immunoprecipitation. Immunofluorescence analysis revealed that amphiphysin 2 might bind reticulo-vesicular structures present throughout the cell body and be associated with SNX4 on these structures. In an endocytosis assay, overexpressed C-terminal or full-length SNX4 was able to inhibit transferrin receptor endocytosis as efficiently as the SH3 domain of amphiphysin 2. At lower levels of expression, SNX4 colocalized with transferrin-containing vesicles, some of which were also positive for amphiphysin 2. These results indicate that SNX4 may be part of the endocytic machinery or, alternatively, that SNX4 may associate with key elements of endocytosis such as amphiphysin 2 and sequester them when overexpressed. The presence of amphiphysin 2 on intracellular vesicles and its interplay with SNX4, which is likely to take part in intracellular trafficking, suggest that amphiphysin 2 is not only a regulator of the early steps of endocytosis. It could also play a role at the surface of the endocytic vesicle that has just been formed and of the future endosomes, in order to regulate intracellular trafficking.

Endocytosis and the cytoskeleton

In this review we describe the potential roles of the actin cytoskeleton in receptor-mediated endocytosis in mammalian cells and summarize the efforts of recent years in establishing a relationship between these two cellular functions. With molecules such as dynamin, syndapin, HIP1R, Abp1, synaptojanin, N-WASP, intersectin, and cortactin a set of molecular links is now available and it is likely that their further characterization will reveal the basic principles of a functional interconnection between the membrane cytoskeleton and the vesicle-budding machinery. We will therefore discuss proteins involved in endocytic clathrin coat formation and accessory factors to control and regulate coated vesicle formation but we will also focus on actin cytoskeletal components such as the Arp2/3 complex, spectrin, profilin, and motor proteins involved in actin dynamics and organization. Additionally, we will discuss how phosphoinositides, such as PI(4,5)P2, small GTPases thought to control the actin cytoskeleton, such as Rho, Rac, and Cdc42, or membrane trafficking, such as Rab GTPases and ARF proteins, and different kinases may participate in the functional connection of actin and endocytosis. We will compare the concepts and different molecular mechanisms involved in mammalian cells with yeast as well as with specialized cells, such as epithelial cells and neurons, because different model organisms often offer complementary advantages for further studies in this thriving field of current cell biological research.

Electroconvulsive shock increases the phosphorylation of amphiphysin II in the rat cerebellum

Amphiphysin II (Amph2) is known to undergo rapid dephosphorylation and phosphorylation at nerve terminals. After in vivo electroconvulsive shock (ECS) in the rat cerebellum, we found an electrophoretic mobility retardation of Amph2, which suggested an increased degree of phosphorylation above the non-stimulated level. This shifted signal was observed from 1 min, reached the maximum level at 5 min and extended beyond 2 h after ECS. The shifted band was markedly decreased by the phosphatase treatment. Pretreatment with cyclosporin A augmented the mobility retardation of Amph2 after ECS. Our results indicate that ECS induces the phosphorylation of Amph2 in the rat cerebellum.

Amphiphysins: raising the BAR for synaptic vesicle recycling and membrane dynamics. Bin-Amphiphysin-Rvsp

Amphiphysins, members of the BAR (Bin-Amphiphysin-Rvsp) protein super family, have been postulated to play a key role in clathrin-mediated endocytosis of synaptic vesicles (SVs). This review focuses on recent genetic studies of the role of amphiphysins in SV recycling and membrane morphogenesis. In the mouse, brain-specific amphiphysin I and II regulate, but are not essential for, SV recycling. The role of this regulation appears important, as mice deficient in these proteins have seizures and are deficient in learning and memory. In the fruit fly Drosophila melanogaster, amphiphysin is found in muscles and is enriched at postsynaptic membranes of neuromuscular junctions (NMJs); however, it does not play a role in SV recycling. Rather, amphiphysin in fly muscles appears to regulate the organization and structure of the muscle T-tubule system and possibly the subsynaptic reticulum. Amphiphysin is also involved in membrane organization in both neurons and non-neuronal cells in Drosophila. These studies reveal pleiotropic functions for amphiphysins in clathrin-mediated endocytosis and the regulation of membrane dynamics, perhaps through the actin cytoskeleton.

Synthesis and function of 3-phosphorylated inositol lipids

The 3-phosphorylated inositol lipids fulfill roles as second messengers by interacting with the lipid binding domains of a variety of cellular proteins. Such interactions can affect the subcellular localization and aggregation of target proteins, and through allosteric effects, their activity. Generation of 3-phosphoinositides has been documented to influence diverse cellular pathways and hence alter a spectrum of fundamental cellular activities. This review is focused on the 3-phosphoinositide lipids, the synthesis of which is acutely triggered by extracellular stimuli, the enzymes responsible for their synthesis and metabolism, and their cell biological roles. Much knowledge has recently been gained through structural insights into the lipid kinases, their interaction with inhibitors, and the way their 3-phosphoinositide products interact with protein targets. This field is now moving toward a genetic dissection of 3-phosphoinositide action in a variety of model organisms. Such approaches will reveal the true role of the 3-phosphoinositides at the organismal level in health and disease.

Decreased synaptic vesicle recycling efficiency and cognitive deficits in amphiphysin 1 knockout mice

The function of the clathrin coat in synaptic vesicle endocytosis is assisted by a variety of accessory factors, among which amphiphysin (amphiphysin 1 and 2) is one of the best characterized. A putative endocytic function of amphiphysin was supported by dominant-negative interference studies. We have now generated amphiphysin 1 knockout mice and found that lack of amphiphysin 1 causes a parallel dramatic reduction of amphiphysin 2 selectively in brain. Cell-free assembly of endocytic protein scaffolds is defective in mutant brain extracts. Knockout mice exhibit defects in synaptic vesicle recycling that are unmasked by stimulation and suggest impairments at multiple stages of the cycle. These defects correlate with increased mortality due to rare irreversible seizures and with major learning deficits, suggesting a critical role of amphiphysin for higher brain functions.

β1PIX, the PAK-interacting exchange factor, requires localization via a coiled-coil region to promote microvillus-like structures and membrane ruffles

PIX is a Rho-family guanine nucleotide exchange factor that binds PAK. We previously described two isoforms of PIX that differ in their N termini. Here, we report the identification of a new splice variant of βPIX, designated β2PIX, that is the dominant species in brain and that lacks the region of ∼120 residues with predicted coiled-coil structure at the C terminus of β1PIX. Instead, β2PIX contains a serine-rich C terminus. To determine whether these splice variants differ in their cellular function, we studied the effect of expressing these proteins in HeLa cells. We found that the coiled-coil region plays a key role in the localization of β1PIX to the cell periphery and is also responsible for PIX dimerization. Overexpression of β1, but not β2PIX, drives formation of membrane ruffles and microvillus-like structures (via activation of Rac1 and Cdc42, respectively), indicating that its function requires localized activation of these GTPases. Thus, β1PIX, like other RhoGEFs, exerts specific morphological functions that are dependent on its intracellular location and are mediated by its C-terminal dimerization domain.

Amphiphysin is necessary for organization of the excitation-contraction coupling machinery of muscles, but not for synaptic vesicle endocytosis in Drosophila

Amphiphysins 1 and 2 are enriched in the mammalian brain and are proposed to recruit dynamin to sites of endocytosis. Shorter amphiphysin 2 splice variants are also found ubiquitously, with an enrichment in skeletal muscle. At the Drosophila larval neuromuscular junction, amphiphysin is localized postsynaptically and amphiphysin mutants have no major defects in neurotransmission; they are also viable, but flightless. Like mammalian amphiphysin 2 in muscles, Drosophila amphiphysin does not bind clathrin, but can tubulate lipids and is localized on T-tubules. Amphiphysin mutants have a novel phenotype, a severely disorganized T-tubule/sarcoplasmic reticulum system. We therefore propose that muscle amphiphysin is not involved in clathrin-mediated endocytosis, but in the structural organization of the membrane-bound compartments of the excitation-contraction coupling machinery of muscles.

Drosophila Amphiphysin is a post-synaptic protein required for normal locomotion but not endocytosis

Clathrin-mediated endocytosis is required to recycle synaptic vesicles for fast and efficient neurotransmission. Amphiphysins are thought to be multiprotein adaptors that may contribute to this process by bringing together many of the proteins required for endocytosis. Their in vivo function, however, has yet to be determined. Here, we show that the Drosophila genome encodes a single amphiphysin gene that is broadly expressed during development. We also show that, unlike its vertebrate counterparts, Drosophila Amphiphysin is enriched postsynaptically at the larval neuromuscular junction. To determine the role of Drosophila Amphiphysin, we also generated null mutants which are viable but give rise to larvae and adults with pronounced locomotory defects. Surprisingly, the locomotory defects cannot be accounted for by alterations in the morphology or physiology of the neuromuscular junction. Moreover, using stimulus protocols designed to test endocytosis under moderate and extreme vesicle cycling, we could not detect any defect in the neuromuscular junction of the amphiphysin mutant. Taken together, our findings suggest that Amphiphysin is not required for viability, nor is it absolutely required for clathrin-mediated endocytosis. However, Drosophila Amphiphysin function is required in both larvae and adults for normal locomotion.

The dephosphins: dephosphorylation by calcineurin triggers synaptic vesicle endocytosis

When nerve terminals in the brain are stimulated, a group of phosphoproteins called the dephosphins are coordinately dephosphorylated by calcineurin, the Ca(2+)-dependent protein phosphatase. Amazingly, the seven presently known dephosphins are not structurally related, yet each has been independently shown to be essential for synaptic vesicle endocytosis (SVE). Nowhere else in biology is there a similar example of the coordinated dephosphorylation of such a large group of proteins each sharing roles in the same biological response. This suggests that dephosphorylation and phosphorylation of the dephosphins is essential for SVE. Recent studies in synaptosomes have confirmed this view, with calcineurin-mediated dephosphorylation of the dephosphins essential for triggering SVE. The phosphorylation cycle of the dephosphins might regulate SVE by targeting the proteins to sites of action and by stimulating the assembly of several large essential endocytic protein complexes.

Transforming growth factor beta receptor signaling and endocytosis are linked through a COOH terminal activation motif in the type I receptor

Transforming growth factor beta (TGF-beta) coordinates a number of biological events important in normal and pathophysiological growth. In this study, deletion and substitution mutations were used to identify receptor motifs modulating TGF-beta receptor activity. Initial experiments indicated that a COOH-terminal sequence between amino acids 482-491 in the kinase domain of the type I receptor was required for ligand-induced receptor signaling and down-regulation. These 10 amino acids are highly conserved in mammalian, Xenopus, and Drosophila type I receptors. Although mutation or deletion of the region (referred to as the NANDOR BOX, for nonactivating non-down-regulating) abolishes TGF-beta-dependent mitogenesis, transcriptional activity, type I receptor phosphorylation, and down-regulation in mesenchymal cultures, adjacent mutations also within the kinase domain are without effect. Moreover, a kinase-defective type I receptor can functionally complement a mutant BOX expressing type I receptor, documenting that when the BOX mutant is activated, it has kinase activity. These results indicate that the sequence between 482 and 491 in the type I receptor provides a critical function regulating activation of the TGF-beta receptor complex.

Interaction of two structurally distinct sequence types with the clathrin terminal domain beta-propeller

The amino-terminal domain of the clathrin heavy chain, which folds into a seven-bladed beta-propeller, binds directly to several endocytic proteins via short sequences based on the consensus residues LLDLD. In addition to a single LLDLD-based, type I clathrin-binding sequence, both amphiphysin and epsin contain a second, distinct sequence that is also capable of binding to clathrin directly. Here, we analyzed these sequences, which we term type II sequences, and show that the (257)LMDLA sequence in rat epsin 1 appears to be a weak clathrin-binding variant of the sequence (417)PWDLW originally found in human amphiphysin II. The structural features of the type II sequence required for association with clathrin are distinct from the LLDLD-based sequence. In the central segment of amphiphysin, the type I and type II sequences cooperate to effect optimal clathrin binding and the formation of sedimentable assemblies. Together, the data provide evidence for two interaction surfaces upon certain endocytic accessory proteins that could cooperate with other coat components to enhance clathrin bud formation at the cell surface.

Accessory factors in clathrin-dependent synaptic vesicle endocytosis

Clathrin-mediated endocytosis is a special form of vesicle budding important for the internalization of receptors and extracellular ligands, for the recycling of plasma membrane components, and for the retrieval of surface proteins destined for degradation. In nerve terminals, clathrin-mediated endocytosis is crucial for synaptic vesicle recycling. Recent structural studies have provided molecular details of coat assembly. In addition, biochemical and genetic studies have identified numerous accessory proteins that assist the clathrin coat in its function at synapses and in other systems. This review summarizes these advances with a special focus on accessory factors and highlights new aspects of clathrin-mediated endocytosis revealed by the study of these factors.

Tandem arrangement of the clathrin and AP-2 binding domains in amphiphysin 1 and disruption of clathrin coat function by amphiphysin fragments comprising these sites

Amphiphysin 1 and 2 are proteins implicated in the recycling of synaptic vesicles in nerve terminals. They interact with dynamin and synaptojanin via their COOH-terminal SH3 domain, whereas their central regions contain binding sites for clathrin and for the clathrin adaptor AP-2. We have defined here amino acids of amphiphysin 1 crucial for binding to AP-2 and clathrin. Overexpression in Chinese hamster ovary cells of an amphiphysin 1 fragment that binds both AP-2 and clathrin resulted in a segregation of clathrin, which acquired a diffuse distribution, from AP-2, which accumulated at patches also positive for Eps15. These effects correlated with a block in clathrin-mediated endocytosis. A fragment selectively interacting with clathrin produced a similar effect. These results can be explained by the binding of amphiphysin to the NH(2)-terminal domain of clathrin and by a competition with the binding of this domain to the beta-subunit of AP-2 and AP180. The interaction of amphiphysin 1 with either clathrin or AP-2 did not prevent its interaction with dynamin, supporting the existence of tertiary complexes between these proteins. Together with previous evidence indicating a direct interaction between amphiphysin and membrane lipids, these findings support a model in which amphiphysin acts as a multifunctional adaptor linking the membrane to coat proteins and coat proteins to dynamin and synaptojanin.

Sequential steps in clathrin-mediated synaptic vesicle endocytosis

Synaptic vesicles are recycled with remarkable speed and precision in nerve terminals. A major recycling pathway involves clathrin-mediated endocytosis at endocytic zones located around sites of release. Different 'accessory' proteins linked to this pathway have been shown to alter the shape and composition of lipid membranes, to modify membrane-coat protein interactions, and to influence actin polymerization. These include the GTPase dynamin, the lysophosphatidic acid acyl transferase endophilin, and the phosphoinositide phosphatase synaptojanin. Protein perturbation studies in living nerve terminals are now beginning to link the actions of these proteins with morphologically defined steps of endocytosis.

Amphiphysin IIm, a novel amphiphysin II isoform, is required for macrophage phagocytosis

Phagocytosis of pathogens by macrophages initiates the innate immune response, which in turn orchestrates the adaptive immune response. Amphiphysin II participates in receptor-mediated endocytosis, in part, by recruiting the GTPase dynamin to the nascent endosome. We demonstrate here that a novel isoform of amphiphysin II associates with early phagosomes in macrophages. We have ablated the dynamin-binding site of this protein and shown that this mutant form of amphiphysin II inhibits phagocytosis at the stage of membrane extension around the bound particles. We define a signaling cascade in which PI3K is required to recruit amphiphysin II to the phagosome, and amphiphysin II in turn recruits dynamin. Thus, amphiphysin II facilitates a critical initial step in host response to infection.