Syndapin is dispensable for synaptic vesicle endocytosis at the Drosophila larval neuromuscular junction

Distinct Bin/Amphiphysin/Rvs (BAR) family proteins may assemble on the same tubule to regulate membrane organization in vivo

Intracellular membrane tubules play a crucial role in diverse cellular processes, and their regulation is facilitated by Bin-Amphiphysin-Rvs (BAR) domain-containing proteins. This study investigates the roles of Drosophila ICA69 (dICA69) (an N-BAR protein) and Drosophila CIP4 (dCIP4) (an F-BAR protein), focusing on their impact on in vivo membrane tubule organization. In contrast to the prevailing models of BAR-domain protein function, we observed colocalization of endogenous dICA69 with dCIP4-induced tubules, indicating their potential recruitment for tubule formation and maintenance. Moreover, actin-regulatory proteins such as Wasp, SCAR, and Arp2/3 were recruited at the site of CIP4-induced tubule formation. An earlier study indicated that F-BAR proteins spontaneously segregate from the N-BAR domain proteins during membrane tubule formation. In contrast, our observation supports a model in which different BAR-domain family members can associate with the same tubule and cooperate to fine-tune the tubule width, possibly by recruiting actin modulators during the generation of tubules. Our data suggests that cooperative activities of distinct BAR-domain family proteins may determine the length and width of the membrane tubule in vivo.

PACSIN proteins in vivo: Roles in development and physiology

Protein kinase C and casein kinase substrate in neurons (PACSINs), or syndapins (synaptic dynamin‐associated proteins), are a family of proteins involved in the regulation of cell cytoskeleton, intracellular trafficking and signalling. Over the last twenty years, PACSINs have been mostly studied in the in vitro and ex vivo settings, and only in the last decade reports on their function in vivo have emerged. We first summarize the identification, structure and cellular functions of PACSINs, and then focus on the relevance of PACSINs in vivo. During development in various model organisms, PACSINs participate in diverse processes, such as neural crest cell development, gastrulation, laterality development and neuromuscular junction formation. In mouse, PACSIN2 regulates angiogenesis during retinal development and in human, PACSIN2 associates with monosomy and embryonic implantation. In adulthood, PACSIN1 has been extensively studied in the brain and shown to regulate neuromorphogenesis, receptor trafficking and synaptic plasticity. Several genetic studies suggest a role for PACSIN1 in the development of schizophrenia, which is also supported by the phenotype of mice depleted of PACSIN1. PACSIN2 plays an essential role in the maintenance of intestinal homeostasis and participates in kidney repair processes after injury. PACSIN3 is abundant in muscle tissue and necessary for caveolar biogenesis to create membrane reservoirs, thus controlling muscle function, and has been linked to certain genetic muscular disorders. The above examples illustrate the importance of PACSINs in diverse physiological or tissue repair processes in various organs, and associations to diseases when their functions are disturbed.

F-BAR domain protein Syndapin regulates actomyosin dynamics during apical cap remodeling in syncytial Drosophila embryos

Branched actin networks driven by Arp2/3 interact with actomyosin filaments in processes such as cell migration. Similar interactions occur in the syncytial Drosophila blastoderm embryo where expansion of apical caps by Arp2/3-driven actin polymerization occurs in interphase, and cap buckling at contact edges by Myosin II to form furrows takes place in metaphase. Here, we study the role of Syndapin (Synd), an F-BAR domain-containing protein, in apical cap remodeling prior to furrow extension. We found that depletion of synd resulted in larger apical caps. Super-resolution and TIRF microscopy showed that control embryos had long apical actin protrusions in caps during interphase and short protrusions during metaphase, whereas synd depletion led to formation of sustained long protrusions, even during metaphase. Loss of Arp2/3 function in synd mutants partly reverted defects in apical cap expansion and protrusion remodeling. Myosin II levels were decreased in synd mutants, an observation consistent with the expanded cap phenotype previously reported for Myosin II mutant embryos. We propose that Synd function limits branching activity during cap expansion and affects Myosin II distribution in order to bring about a transition in actin remodeling activity from apical cap expansion to lateral furrow extension.

Syndapin constricts microvillar necks to form a united rhabdomere in Drosophila photoreceptors

Drosophila photoreceptors develop from polarized epithelial cells that have apical and basolateral membranes. During morphogenesis, the apical membranes subdivide into a united bundle of photosensory microvilli (rhabdomeres) and a surrounding supporting membrane (stalk). By EMS-induced mutagenesis screening, we found that the F-Bin/Amphiphysin/Rvs (F-BAR) protein syndapin is essential for apical membrane segregation. The analysis of the super-resolution microscopy, STORM and the electron microscopy suggest that syndapin localizes to the neck of the microvilli at the base of the rhabdomere. Syndapin and moesin are required to constrict the neck of the microvilli to organize the membrane architecture at the base of the rhabdomere, to exclude the stalk membrane. Simultaneous loss of syndapin along with the microvilli adhesion molecule chaoptin significantly enhanced the disruption of stalk-rhabdomere segregation. However, loss of the factors involving endocytosis do not interfere. These results indicated syndapin is most likely functioning through its membrane curvature properties, and not through endocytic processes for stalk-rhabdomere segregation. Elucidation of the mechanism of this unconventional domain formation will provide novel insights into the field of cell biology.

RNAi Screen in Tribolium Reveals Involvement of F-BAR Proteins in Myoblast Fusion and Visceral Muscle Morphogenesis in Insects

In a large-scale RNAi screen in Tribolium castaneum for genes with knock-down phenotypes in the larval somatic musculature, one recurring phenotype was the appearance of larval muscle fibers that were significantly thinner than those in control animals. Several of the genes producing this knock-down phenotype corresponded to orthologs of Drosophila genes that are known to participate in myoblast fusion, particularly via their effects on actin polymerization. A new gene previously not implicated in myoblast fusion but displaying a similar thin-muscle knock-down phenotype was the Tribolium ortholog of Nostrin, which encodes an F-BAR and SH3 domain protein. Our genetic studies of Nostrin and Cip4, a gene encoding a structurally related protein, in Drosophila show that the encoded F-BAR proteins jointly contribute to efficient myoblast fusion during larval muscle development. Together with the F-Bar protein Syndapin they are also required for normal embryonic midgut morphogenesis. In addition, Cip4 is required together with Nostrin during the profound remodeling of the midgut visceral musculature during metamorphosis. We propose that these F-Bar proteins help govern proper morphogenesis particularly of the longitudinal midgut muscles during metamorphosis.

Synaptic Vesicle Endocytosis in Different Model Systems

Neurotransmission in complex animals depends on a choir of functionally distinct synapses releasing neurotransmitters in a highly coordinated manner. During synaptic signaling, vesicles fuse with the plasma membrane to release their contents. The rate of vesicle fusion is high and can exceed the rate at which synaptic vesicles can be re-supplied by distant sources. Thus, local compensatory endocytosis is needed to replenish the synaptic vesicle pools. Over the last four decades, various experimental methods and model systems have been used to study the cellular and molecular mechanisms underlying synaptic vesicle cycle. Clathrin-mediated endocytosis is thought to be the predominant mechanism for synaptic vesicle recycling. However, recent studies suggest significant contribution from other modes of endocytosis, including fast compensatory endocytosis, activity-dependent bulk endocytosis, ultrafast endocytosis, as well as kiss-and-run. Currently, it is not clear whether a universal model of vesicle recycling exist for all types of synapses. It is possible that each synapse type employs a particular mode of endocytosis. Alternatively, multiple modes of endocytosis operate at the same synapse, and the synapse toggles between different modes depending on its activity level. Here we compile review and research articles based on well-characterized model systems: frog neuromuscular junctions, C. elegans neuromuscular junctions, Drosophila neuromuscular junctions, lamprey reticulospinal giant axons, goldfish retinal ribbon synapses, the calyx of Held, and rodent hippocampal synapses. We will compare these systems in terms of their known modes and kinetics of synaptic vesicle endocytosis, as well as the underlying molecular machineries. We will also provide the future development of this field.

Filamin, a synaptic organizer in Drosophila, determines glutamate receptor composition and membrane growth

Filamin is a scaffolding protein that functions in many cells as an actin-crosslinker. FLN90, an isoform of the Drosophila ortholog Filamin/cheerio that lacks the actin-binding domain, is here shown to govern the growth of postsynaptic membrane folds and the composition of glutamate receptor clusters at the larval neuromuscular junction. Genetic and biochemical analyses revealed that FLN90 is present surrounding synaptic boutons. FLN90 is required in the muscle for localization of the kinase dPak and, downstream of dPak, for localization of the GTPase Ral and the exocyst complex to this region. Consequently, Filamin is needed for growth of the subsynaptic reticulum. In addition, in the absence of filamin, type-A glutamate receptor subunits are lacking at the postsynapse, while type-B subunits cluster correctly. Receptor composition is dependent on dPak, but independent of the Ral pathway. Thus two major aspects of synapse formation, morphological plasticity and subtype-specific receptor clustering, require postsynaptic Filamin.

DOI:http://dx.doi.org/10.7554/eLife.19991.001

The influence of postsynaptic structure on missing quanta at the Drosophila neuromuscular junction

Background

Synaptic transmission requires both pre- and post-synaptic elements for neural communication. The postsynaptic structure contributes to the ability of synaptic currents to induce voltage changes in postsynaptic cells. At the Drosophila neuromuscular junction (NMJ), the postsynaptic structure, known as the subsynaptic reticulum (SSR), consists of elaborate membrane folds that link the synaptic contacts to the muscle, but its role in synaptic physiology is poorly understood.

Results

In this study, we investigate the role of the SSR with simultaneous intra- and extra-cellular recordings that allow us to identify the origin of spontaneously occurring synaptic events. We compare data from Type 1b and 1s synaptic boutons, which have naturally occurring variations of the SSR, as well as from genetic mutants that up or down-regulate SSR complexity. We observed that some synaptic currents do not result in postsynaptic voltage changes, events we called ‘missing quanta’. The frequency of missing quanta is positively correlated with SSR complexity in both natural and genetically-induced variants. Rise-time and amplitude data suggest that passive membrane properties contribute to the observed differences in synaptic effectiveness.

Conclusion

We conclude that electrotonic decay within the postsynaptic structure contributes to the phenomenon of missing quanta. Further studies directed at understanding the role of the SSR in synaptic transmission and the potential for regulating ‘missing quanta’ will yield important information about synaptic transmission at the Drosophila NMJ.

Syndapin promotes pseudocleavage furrow formation by actin organization in the syncytial Drosophila embryo

F-BAR domain–containing proteins link the actin cytoskeleton to the membrane during membrane remodeling. Syndapin associates with the pseudocleavage furrow membrane and is essential for furrow morphology, actin organization, and extension downstream of initiation factor RhoGEF2.

Coordinated membrane and cytoskeletal remodeling activities are required for membrane extension in processes such as cytokinesis and syncytial nuclear division cycles in Drosophila. Pseudocleavage furrow membranes in the syncytial Drosophila blastoderm embryo show rapid extension and retraction regulated by actin-remodeling proteins. The F-BAR domain protein Syndapin (Synd) is involved in membrane tubulation, endocytosis, and, uniquely, in F-actin stability. Here we report a role for Synd in actin-regulated pseudocleavage furrow formation. Synd localized to these furrows, and its loss resulted in short, disorganized furrows. Synd presence was important for the recruitment of the septin Peanut and distribution of Diaphanous and F-actin at furrows. Synd and Peanut were both absent in furrow-initiation mutants of RhoGEF2 and Diaphanous and in furrow-progression mutants of Anillin. Synd overexpression in rhogef2 mutants reversed its furrow-extension phenotypes, Peanut and Diaphanous recruitment, and F-actin organization. We conclude that Synd plays an important role in pseudocleavage furrow extension, and this role is also likely to be crucial in cleavage furrow formation during cell division.

σ2-Adaptin Facilitates Basal Synaptic Transmission and Is Required for Regenerating Endo-Exo Cycling Pool Under High-Frequency Nerve Stimulation in Drosophila

The functional requirement of adapter protein 2 (AP2) complex in synaptic membrane retrieval by clathrin-mediated endocytosis is not fully understood. Here we isolated and functionally characterized a mutation that dramatically altered synaptic development. Based on the aberrant neuromuscular junction (NMJ) synapse, we named this mutation angur (a Hindi word meaning "grapes"). Loss-of-function alleles of angur show more than twofold overgrowth in bouton numbers and a dramatic decrease in bouton size. We mapped the angur mutation to σ2-adaptin, the smallest subunit of the AP2 complex. Reducing the neuronal level of any of the subunits of the AP2 complex or disrupting AP2 complex assembly in neurons phenocopied the σ2-adaptin mutation. Genetic perturbation of σ2-adaptin in neurons leads to a reversible temperature-sensitive paralysis at 38°. Electrophysiological analysis of the mutants revealed reduced evoked junction potentials and quantal content. Interestingly, high-frequency nerve stimulation caused prolonged synaptic fatigue at the NMJs. The synaptic levels of subunits of the AP2 complex and clathrin, but not other endocytic proteins, were reduced in the mutants. Moreover, bone morphogenetic protein (BMP)/transforming growth factor β (TGFβ) signaling was altered in these mutants and was restored by normalizing σ2-adaptin in neurons. Thus, our data suggest that (1) while σ2-adaptin facilitates synaptic vesicle (SV) recycling for basal synaptic transmission, its activity is also required for regenerating SVs during high-frequency nerve stimulation, and (2) σ2-adaptin regulates NMJ morphology by attenuating TGFβ signaling.

An Endocytic Scaffolding Protein together with Synapsin Regulates Synaptic Vesicle Clustering in the Drosophila Neuromuscular Junction

Many endocytic proteins accumulate in the reserve pool of synaptic vesicles (SVs) in synapses and relocalize to the endocytic periactive zone during neurotransmitter release. Currently little is known about their functions outside the periactive zone. Here we show that in the Drosophila neuromuscular junction (NMJ), the endocytic scaffolding protein Dap160 colocalizes during the SV cycle and forms a functional complex with the SV-associated phosphoprotein synapsin, previously implicated in SV clustering. This direct interaction is strongly enhanced under phosphorylation-promoting conditions and is essential for proper localization of synapsin at NMJs. In a dap160 rescue mutant lacking the interaction between Dap160 and synapsin, perturbed reclustering of SVs during synaptic activity is observed. Our data indicate that in addition to the function in endocytosis, Dap160 is a component of a network of protein-protein interactions that serves for clustering of SVs in conjunction with synapsin. During the SV cycle, Dap160 interacts with synapsin dispersed from SVs and helps direct synapsin back to vesicles. The proteins function in synergy to achieve efficient clustering of SVs in the reserve pool.

SIGNIFICANCE STATEMENT

We provide the first evidence for the function of the SH3 domain interaction in synaptic vesicle (SV) organization at the synaptic active zone. Using Drosophila neuromuscular junction as a model synapse, we describe the molecular mechanism that enables the protein implicated in SV clustering, synapsin, to return to the pool of vesicles during neurotransmitter release. We also identify the endocytic scaffolding complex that includes Dap160 as a regulator of the events linking exocytosis and endocytosis in synapses.

The EHD protein Past1 controls postsynaptic membrane elaboration and synaptic function

The conserved C-terminal EHD protein Past1 is required for postsynaptic membrane remodeling and synaptic transmission at the Drosophila neuromuscular junction. Past1 activity defines distinct synaptic microdomains of the BAR-domain proteins Syndapin and Amphiphysin, suggesting a new mechanism for elaboration of the postsynaptic membrane reticulum.

Membranes form elaborate structures that are highly tailored to their specialized cellular functions, yet the mechanisms by which these structures are shaped remain poorly understood. Here, we show that the conserved membrane-remodeling C-terminal Eps15 Homology Domain (EHD) protein Past1 is required for the normal assembly of the subsynaptic muscle membrane reticulum (SSR) at the Drosophila melanogaster larval neuromuscular junction (NMJ). past1 mutants exhibit altered NMJ morphology, decreased synaptic transmission, reduced glutamate receptor levels, and a deficit in synaptic homeostasis. The membrane-remodeling proteins Amphiphysin and Syndapin colocalize with Past1 in distinct SSR subdomains and collapse into Amphiphysin-dependent membrane nodules in the SSR of past1 mutants. Our results suggest a mechanism by which the coordinated actions of multiple lipid-binding proteins lead to the elaboration of increasing layers of the SSR and uncover new roles for an EHD protein at synapses.

Remembering K. S. Krishnan (1946-2014)

Dr. K. S. Krishnan was on the faculty of the Division of Biological Sciences at the Tata Institute of Fundamental Research (TIFR) in Mumbai, India, and later emeritus professor at the National Center for Biological Sciences (NCBS) in Bangalore, India. His research using fruit flies has contributed richly to our understanding of synaptic function and mechanisms of anesthetic action. Dr. Krishnan passed away suddenly of a heart attack on the 24th of May, 2014. Below a few of his students fondly recall how it was to work in his group.

Syndapin--a membrane remodelling and endocytic F-BAR protein

Syndapin [also called PACSIN (protein kinase C and casein kinase II interacting protein)] is an Fes-CIP4 homology Bin-amphiphysin-Rvs161/167 (F-BAR) and Src-homology 3 domain-containing protein. Three genes give rise to three main isoforms in mammalian cells. They each function in different endocytic and vesicle trafficking pathways and provide critical links between the cytoskeletal network in different cellular processes, such as neuronal morphogenesis and cell migration. The membrane remodelling activity of syndapin via its F-BAR domain and its interaction partners, such as dynamin and neural Wiskott-Aldrich syndrome protein binding to its Src-homology 3 domain, are important with respect to its function. Its various partner proteins provide insights into its mechanism of action, as well as its differential roles in these cellular processes. Signalling pathways leading to the regulation of syndapin function by phosphorylation are now contributing to our understanding of the broader functions of this family of proteins.

Comparison of parallel high-throughput RNA sequencing between knockout of TDP-43 and its overexpression reveals primarily nonreciprocal and nonoverlapping gene expression changes in the central nervous system of Drosophila

The human Tar-DNA binding protein, TDP-43, is associated with amyotrophic lateral sclerosis (ALS) and other neurodegenerative disorders. TDP-43 contains two conserved RNA-binding motifs and has documented roles in RNA metabolism, including pre-mRNA splicing and repression of transcription. Here, using Drosophila melanogaster as a model, we generated loss-of-function and overexpression genotypes of Tar-DNA binding protein homolog (TBPH) to study their effect on the transcriptome of the central nervous system (CNS). By using massively parallel sequencing methods (RNA-seq) to profile the CNS, we find that loss of TBPH results in widespread gene activation and altered splicing, much of which are reversed by rescue of TBPH expression. Conversely, TBPH overexpression results in decreased gene expression. Although previous studies implicated both absence and mis-expression of TDP-43 in ALS, our data exhibit little overlap in the gene expression between them, suggesting that the bulk of genes affected by TBPH loss-of-function and overexpression are different. In combination with computational approaches to identify likely TBPH targets and orthologs of previously identified vertebrate TDP-43 targets, we provide a comprehensive analysis of enriched gene ontologies. Our data suggest that TDP-43 plays a role in synaptic transmission, synaptic release, and endocytosis. We also uncovered a potential novel regulation of the Wnt and BMP pathways, many of whose targets appear to be conserved.

Barfly: sculpting membranes at the Drosophila neuromuscular junction

The ability of a cell to change the shape of its membranes is intrinsic to many cellular functions. Proteins that can alter or recognize curved membrane structures and those that can act to recruit other proteins which stabilize the membrane curvature are likely to be essential in cell functions. The BAR (Bin, amphiphysin, RVS167 homology) domain is a protein domain that can either induce lipidic membranes to curve or can sense curved membranes. BAR domains are found in several proteins at neuronal synapses. We will review BAR domain structure and the role that BAR domain containing proteins play in regulating the morphology and function of the Drosophila neuromuscular junction. In flies the BAR domain containing proteins, endophilin and syndapin affect synaptic vesicle endocytosis, whereas CIP4, dRich, nervous wreck and syndapin affect synaptic morphology. We will review the growing evidence implicating mutations in BAR domain containing proteins being the cause of human pathologies.

Sub-diffraction imaging on standard microscopes through Photobleaching Microscopy with non-linear Processing

Discerning organelles and molecules at nanometer resolution is revolutionizing biological sciences. However, such technology is still limitedly available for many cell biologists. We present here a novel approach using Photobleaching Microscopy with non-linear Processing (PiMP) for sub-diffraction imaging. Bleaching fluorophores both within the single molecule regime and beyond allows visualizing stochastic representations of sub-populations of fluorophores by imaging the same region over time. Our method is based on enhancing probable positions of the fluorophores underlying the images. The random nature of the bleached fluorophores is assessed by calculating the deviation of the local actual bleached fluorescence intensity to the average bleach expectation as given by the overall decay of intensity. Subtracting measured from estimated decay images yields differential images. Non-linear enhancement of maxima in these diffraction limited differential images approximates the positions of the underlying structure. Summing many such processed differential images yields a super-resolution PiMP image. PiMP allows multi-color, three-dimensional sub-diffraction imaging of cells and tissues using common fluorophores and can be implemented on standard widefield or confocal systems.

Proper synaptic vesicle formation and neuronal network activity critically rely on syndapin I

Synaptic transmission relies on effective and accurate compensatory endocytosis. F-BAR proteins may serve as membrane curvature sensors and/or inducers and thereby support membrane remodelling processes; yet, their in vivo functions urgently await disclosure. We demonstrate that the F-BAR protein syndapin I is crucial for proper brain function. Syndapin I knockout (KO) mice suffer from seizures, a phenotype consistent with excessive hippocampal network activity. Loss of syndapin I causes defects in presynaptic membrane trafficking processes, which are especially evident under high-capacity retrieval conditions, accumulation of endocytic intermediates, loss of synaptic vesicle (SV) size control, impaired activity-dependent SV retrieval and defective synaptic activity. Detailed molecular analyses demonstrate that syndapin I plays an important role in the recruitment of all dynamin isoforms, central players in vesicle fission reactions, to the membrane. Consistently, syndapin I KO mice share phenotypes with dynamin I KO mice, whereas their seizure phenotype is very reminiscent of fitful mice expressing a mutant dynamin. Thus, syndapin I acts as pivotal membrane anchoring factor for dynamins during regeneration of SVs.

Let's go bananas: revisiting the endocytic BAR code

Against the odds of membrane resistance, members of the BIN/Amphiphysin/Rvs (BAR) domain superfamily shape membranes and their activity is indispensable for a plethora of life functions. While crystal structures of different BAR dimers advanced our understanding of membrane shaping by scaffolding and hydrophobic insertion mechanisms considerably, especially life-imaging techniques and loss-of-function studies of clathrin-mediated endocytosis with its gradually increasing curvature show that the initial idea that solely BAR domain curvatures determine their functions is oversimplified. Diagonal placing, lateral lipid-binding modes, additional lipid-binding modules, tilde shapes and formation of macromolecular lattices with different modes of organisation and arrangement increase versatility. A picture emerges, in which BAR domain proteins create macromolecular platforms, that recruit and connect different binding partners and ensure the connection and coordination of the different events during the endocytic process, such as membrane invagination, coat formation, actin nucleation, vesicle size control, fission, detachment and uncoating, in time and space, and may thereby offer mechanistic explanations for how coordination, directionality and effectiveness of a complex process with several steps and key players can be achieved.

The F-BAR protein family Actin' on the membrane

A tight spatio-temporal coordination of the machineries controlling actin dynamics and membrane remodelling is crucial for a huge variety of cellular processes that shape cells into a multicellular organism. Dynamic membrane remodelling is achieved by a functional relationship between proteins that control plasma membrane curvature, membrane fission and nucleation of new actin filaments. The BAR/F-BAR-domain-containing proteins are prime candidates to couple plasma membrane curvature and actin dynamics in different morphogenetic processes. Here, we discuss recent findings on the membrane-shaping proteins of the F-BAR domain subfamily and how they regulate morphogenetic processes in vivo.

Structural requirements for PACSIN/Syndapin operation during zebrafish embryonic notochord development

PACSIN/Syndapin proteins are membrane-active scaffolds that participate in endocytosis. The structure of the Drosophila Syndapin N-terminal EFC domain reveals a crescent shaped antiparallel dimer with a high affinity for phosphoinositides and a unique membrane-inserting prong upon the concave surface. Combined structural, biochemical and reverse genetic approaches in zebrafish define an important role for Syndapin orthologue, Pacsin3, in the early formation of the notochord during embryonic development. In pacsin3-morphant embryos, midline convergence of notochord precursors is defective as axial mesodermal cells fail to polarize, migrate and differentiate properly. The pacsin3 morphant phenotype of a stunted body axis and contorted trunk is rescued by ectopic expression of Drosophila Syndapin, and depends critically on both the prong that protrudes from the surface of the bowed Syndapin EFC domain and the ability of the antiparallel dimer to bind tightly to phosphoinositides. Our data confirm linkage between directional migration, endocytosis and cell specification during embryonic morphogenesis and highlight a key role for Pacsin3 in this coupling in the notochord.

F-BAR proteins of the syndapin family shape the plasma membrane and are crucial for neuromorphogenesis

Coordinated functions of the actin cytoskeleton and microtubules, which require careful control in time and space, are indispensable for the drastic alterations of neuronal morphology during neuromorphogenesis and neuronal network formation. Actin filament formation driven by the Arp2/3 complex and its activator neural Wiskott-Aldrich syndrome protein (N-WASP) is important for proper axon development. The underlying molecular mechanisms for targeting to and specific activation of N-WASP at the neuronal plasma membrane, however, have thus far remained elusive. We show that syndapin I is critical for proper neuromorphogenesis and hereby uses N-WASP as a cytoskeletal effector. Upon N-WASP binding, syndapins release N-WASP autoinhibition. Syndapins hereby cooperate with Cdc42 and phosphatidyl-inositol-(4,5)-bisphosphate. Syndapins furthermore specifically bind to phosphatidylserine-containing membranes via their extended F-BAR domain. Dissecting the syndapin functions actin nucleation and direct membrane binding in vivo, we demonstrate that both functions are physiologically relevant and required. Constitutive plasma membrane-targeting experiments in vivo indicate that specifically actin nucleation at the cell cortex is triggered by syndapins. Consistent with syndapins steering N-WASP as downstream effector for cortical actin nucleation, syndapin-induced neuronal arborization is N-WASP and Cdc42 dependent. The functions of syndapin-N-WASP complexes in neuromorphogenesis were revealed by loss-of-function studies. Knockdown of syndapin I leads to impaired axon development and especially phenocopies the aberrant axon branching observed upon N-WASP and Arp2/3 complex deficiency. In contrast, proper length control involves another N-WASP-binding protein, Abp1. Our data thus reveal that syndapin I is crucial for neuromorphogenesis and that different N-WASP activators ensure fine control of N-WASP activity and have distinct functions during neuronal network formation.

Syndapin promotes formation of a postsynaptic membrane system in Drosophila

Syndapins belong to the F-BAR domain protein family whose predicted functions in membrane tubulation remain poorly studied in vivo. At Drosophila neuromuscular junctions, syndapin is associated predominantly with a tubulolamellar postsynaptic membrane system known as the subsynaptic reticulum (SSR). We show that syndapin overexpression greatly expands this postsynaptic membrane system. Syndapin can expand the SSR in the absence of dPAK and Dlg, two known regulators of SSR development. Syndapin's N-terminal F-BAR domain, required for membrane tubulation in cultured cells, is required for SSR expansion. Consistent with a model in which syndapin acts directly on postsynaptic membrane, SSR expansion requires conserved residues essential for membrane binding in vitro. However, syndapin's Src homology (SH) 3 domain, which negatively regulates membrane tubulation in cultured cells, is required for synaptic targeting and strong SSR induction. Our observations advance knowledge of syndapin protein function by 1) demonstrating the in vivo relevance of membrane remodeling mechanisms suggested by previous in vitro and structural analyses, 2) showing that SH3 domains are necessary for membrane expansion observed in vivo, and 3) confirming that F-BAR proteins control complex membrane structures.