Reversible inhibition of endocytosis in cultured neurons from the Drosophila temperature-sensitive mutant shibirets1

Zebrafish dnm1a gene plays a role in the formation of axons and synapses in the nervous tissue

Classical dynamins (DNMs) are GTPase proteins engaged in endocytosis, a fundamental process for cargo internalization from the plasma membrane. In mammals, three DNM genes are present with different expression patterns. DNM1 is expressed at high levels in neurons, where it takes place in the recycling of synaptic vesicles; DNM2 is ubiquitously expressed, while DNM3 is found in the brain and in the testis. Due to the conservation of genes in comparison to mammals, we took advantage of a zebrafish model for functional characterization of dnm1a, ortholog of mammalian DNM1. Our data strongly demonstrated that dnm1a has a nervous tissue-specific expression pattern and plays a role in the formation of both axon and synapse. This is the first in vivo study that collects evidence about the effects of dnm1a loss of function in zebrafish, thus providing a new excellent model to be used in different scientific fields.

Skin disorders and interstitial lung disease: Part II-The spectrum of cutaneous diseases with lung disease association

Part 2 of this 2-part CME introduces dermatologists to noninfectious inflammatory skin diseases associated with pulmonary involvement. In many cases, dermatologists may be the first physicians recognizing respiratory complications associated with these diagnoses. Because pulmonary involvement is often the leading cause of morbidity and mortality, dermatologists should be comfortable screening and monitoring for lung disease in high-risk patients, recognizing cutaneous stigmata of lung disease in these patients and referring to pulmonary specialists, when appropriate, for prompt treatment initiation. Some treatments used for skin disease may not be appropriate in the context of lung disease and hence, choosing a holistic approach is important. Interstitial lung disease and pulmonary hypertension are the most common pulmonary complications and a significant cause of mortality in autoimmune connective tissue diseases, especially systemic sclerosis, dermatomyositis, and mixed connective tissue disease. Pulmonary complications, notably interstitial lung disease, are also common and life-threatening in sarcoidosis and vasculitis, while they are variable in neutrophilic and autoimmune blistering diseases.

Disruption of Endocytosis with the Dynamin Mutant shibirets1 Suppresses Seizures in Drosophila

One challenge in modern medicine is to control epilepsies that do not respond to currently available medications. Since seizures consist of coordinated and high-frequency neural activity, our goal was to disrupt neurotransmission with a synaptic transmission mutant and evaluate its ability to suppress seizures. We found that the mutant shibire, encoding dynamin, suppresses seizure-like activity in multiple seizure-sensitive Drosophila genotypes, one of which resembles human intractable epilepsy in several aspects. Because of the requirement of dynamin in endocytosis, increased temperature in the shi(ts1) mutant causes impairment of synaptic vesicle recycling and is associated with suppression of the seizure-like activity. Additionally, we identified the giant fiber neuron as critical in the seizure circuit and sufficient to suppress seizures. Overall, our results implicate mutant dynamin as an effective seizure suppressor, suggesting that targeting or limiting the availability of synaptic vesicles could be an effective and general method of controlling epilepsy disorders.

Remembering Obaid Siddiqi, a pioneer in the study of temperature-sensitive paralytic mutants in Drosophila

Although Obaid Siddiqi's major research focus in neurogenetics was on chemosensation and olfaction in Drosophila, he made seminal contributions to the study of temperature-sensitive paralytic mutants that paved the way for research that we and many other investigators have continued to pursue. Here we recount Siddiqi's investigation and the impact it had on our own studies especially at a formative stage of our careers. We acknowledge our debt to Obaid Siddiqi and remember him fondly as an inspired and inspiring scientist, mentor, role model and human being.

Increased dynamin-1 and -2 protein expression in the aged gerbil hippocampus

Dynamin (DNM) plays roles in membrane dynamics, vesicle formation, and transport. In the present study, we compared DNM-1 and DNM-2 protein expressions between the adult (postnatal month 6) and aged (postnatal month 24) gerbil hippocampus using immunohistochemistry and western blot analysis. DNM-1 and DNM-2 immunoreactivities were primarily observed in hippocampal principal neurons: pyramidal cells in the hippocampus proper (CA1-CA3) and granule cells in the dentate gyrus. DNM-1 and DNM-2 immunoreactivities in principal neurons were significantly increased in the aged group compared with the adult group. In addition, DNM-1 and DNM-2 protein levels as well as phospho-DNM-1 level were significantly increased in the aged group. These results indicate that the increases of DNM-1 and DNM-2 protein expressions may reflect the age-related changes in hippocampal function.

Exocyst-mediated membrane trafficking is required for branch outgrowth in Drosophila tracheal terminal cells

Branching morphogenesis, the process by which cells or tissues generate tree-like networks that function to increase surface area or in contacting multiple targets, is a common developmental motif in multicellular organisms. We use Drosophila tracheal terminal cells, a component of the insect respiratory system, to investigate branching morphogenesis that occurs at the single cell level. Here, we show that the exocyst, a conserved protein complex that facilitates docking and tethering of vesicles at the plasma membrane, is required for terminal cell branch outgrowth. We find that exocyst-deficient terminal cells have highly truncated branches and show an accumulation of vesicles within their cytoplasm and are also defective in subcellular lumen formation. We also show that vesicle trafficking pathways mediated by the Rab GTPases Rab10 and Rab11 are redundantly required for branch outgrowth. In terminal cells, the PAR-polarity complex is required for branching, and we find that the PAR complex is required for proper membrane localization of the exocyst, thus identifying a molecular link between the branching and outgrowth programs. Together, our results suggest a model where exocyst mediated vesicle trafficking facilitates branch outgrowth, while de novo branching requires cooperation between the PAR and exocyst complexes.

Adherens junction distribution mechanisms during cell-cell contact elongation in Drosophila

During Drosophila gastrulation, amnioserosa (AS) cells flatten and spread as an epithelial sheet. We used AS morphogenesis as a model to investigate how adherens junctions (AJs) distribute along elongating cell-cell contacts in vivo. As the contacts elongated, total AJ protein levels increased along their length. However, genetically blocking this AJ addition indicated that it was not essential for maintaining AJ continuity. Implicating other remodeling mechanisms, AJ photobleaching revealed non-directional lateral mobility of AJs along the elongating contacts, as well as local AJ removal from the membranes. Actin stabilization with jasplakinolide reduced AJ redistribution, and live imaging of myosin II along elongating contacts revealed fragmented, expanding and contracting actomyosin networks, suggesting a mechanism for lateral AJ mobility. Actin stabilization also increased total AJ levels, suggesting an inhibition of AJ removal. Implicating AJ removal by endocytosis, clathrin endocytic machinery accumulated at AJs. However, dynamin disruption had no apparent effect on AJs, suggesting the involvement of redundant or dynamin-independent mechanisms. Overall, we propose that new synthesis, lateral diffusion, and endocytosis play overlapping roles to populate elongating cell-cell contacts with evenly distributed AJs in this in vivo system.

Mutations in palmitoyl-protein thioesterase 1 alter exocytosis and endocytosis at synapses in Drosophila larvae

Infantile-onset neuronal ceroid lipofuscinosis (INCL) is a severe pediatric neurodegenerative disorder produced by mutations in the gene encoding palmitoyl-protein thioesterase 1 (Ppt1). This enzyme is responsible for the removal of a palmitate group from its substrate proteins, which may include presynaptic proteins like SNAP-25, cysteine string protein (CSP), dynamin, and synaptotagmin. The fruit fly, Drosophila melanogaster, has been a powerful model system for studying the functions of these proteins and the molecular basis of neurological disorders like the NCLs. Genetic modifier screens and tracer uptake studies in Ppt1 mutant larval garland cells have suggested that Ppt1 plays a role in endocytic trafficking. We have extended this analysis to examine the involvement of Ppt1 in synaptic function at the Drosophila larval neuromuscular junction (NMJ). Mutations in Ppt1 genetically interact with temperature sensitive mutations in the Drosophila dynamin gene shibire, accelerating the paralytic behavior of shibire mutants at 27 °C. Electrophysiological work in NMJs of Ppt1-deficient larvae has revealed an increase in miniature excitatory junctional potentials (EJPs) and a significant depression of evoked EJPs in response to repetitive (10 hz) stimulation. Endocytosis was further examined in Ppt1-mutant larvae using FM1-43 uptake assays, demonstrating a significant decrease in FM1-43 uptake at the mutant NMJs. Finally, Ppt1-deficient and Ppt1 point mutant larvae display defects in locomotion that are consistent with alterations in synaptic function. Taken together, our genetic, cellular, and electrophysiological analyses suggest a direct role for Ppt1 in synaptic vesicle exo- and endocytosis at motor nerve terminals of the Drosophila NMJ.

Single vesicle imaging indicates distinct modes of rapid membrane retrieval during nerve growth

Background

During nerve growth, cytoplasmic vesicles add new membrane preferentially to the growth cone located at the distal tip of extending axons. Growth cone membrane is also retrieved locally, and asymmetric retrieval facilitates membrane remodeling during growth cone repulsion by a chemorepellent gradient. Moreover, growth inhibitory factors can stimulate bulk membrane retrieval and induce growth cone collapse. Despite these functional insights, the processes mediating local membrane remodeling during axon extension remain poorly defined.

Results

To investigate the spatial and temporal dynamics of membrane retrieval in actively extending growth cones, we have used a transient labeling and optical recording method that can resolve single vesicle events. Live-cell confocal imaging revealed rapid membrane retrieval by distinct endocytic modes based on spatial distribution in Xenopus spinal neuron growth cones. These modes include endocytic "hot-spots" triggered at the base of filopodia, at the lateral margins of lamellipodia, and along dorsal ridges of the growth cone. Additionally, waves of endocytosis were induced when individual filopodia detached from the substrate and fused with the growth cone dorsal surface or with other filopodia. Vesicle formation at sites of membrane remodeling by self-contact required F-actin polymerization. Moreover, bulk membrane retrieval by macroendocytosis correlated positively with the substrate-dependent rate of axon extension and required the function of Rho-family GTPases.

Conclusions

This study provides insight into the dynamic membrane remodeling processes essential for nerve growth by identifying several distinct modes of rapid membrane retrieval in the growth cone during axon extension. We found that endocytic membrane retrieval is intensified at specific subdomains and may drive the dynamic membrane ruffling and re-absorption of filopodia and lamellipodia in actively extending growth cones. The findings offer a platform for determining the molecular mechanisms of distinct endocytic processes that may remodel the surface distribution of receptors, ion channels and other membrane-associated proteins locally to drive growth cone extension and chemotactic guidance.

Drosophila basement membrane collagen col4a1 mutations cause severe myopathy

Recent data from clinical and mammalian genetic studies indicate that COL4A1 mutations manifest with basement membrane defects that result in muscle weakness, cramps, contractures, dystrophy and atrophy. In-depth studies of mutant COL4A1-associated muscle phenotype, however, are lacking and significant details of the muscle-specific pathomechanisms remain unknown. In this study, we have used a comprehensive set of Drosophila col4a1 and col4a2 mutants and a series of genetic and mutational analyses, gene, protein expression, and immunohistochemistry experiments in order to establish a Drosophila model and address some of these questions. The Drosophila genome contains two type IV collagen genes, col4a1 and col4a2. Mutant heterozygotes of either gene are viable and fertile, whereas homozygotes are lethal. In complementation analysis of all known mutants of the locus and a complementation matrix derived from these data we have identified the dominant lesions within the col4a1, but not within the col4a2 gene. Expression of a col4a1 transgene partially rescued the dominant and recessive mutant col4a1 alleles but not the col4a2 mutations that were all recessive. Partial complementation suggested that col4a1 gene mutations have strong antimorph effect likely due to the incorporation of the mutant protein into the triple helix. In col4a1 mutants, morphological changes of the oviduct muscle included severe myopathy with centronuclear myofibers leading to gradual development of female sterility. In larval body wall muscles ultrastructural changes included disturbance of A and I bands between persisting Z bands. In the most severely affected DTS-L3 mutant, we have identified four missense mutations within the coding region of the col4a1 gene two of which affected the Y within the Gly-X-Y unit and a 3' UTR point mutation. In conclusion, our Drosophila mutant series may serve as an effective model to uncover the mechanisms by which COL4A1 mutations result in compromised myofiber-basement membrane interactions and aberrant muscle function.

Growth cone morphology and spreading are regulated by a dynamin-cortactin complex at point contacts in hippocampal neurons

Neuronal growth cone (GC) migration and targeting are essential processes for the formation of a neural network during embryonic development. Currently, the mechanisms that support directed motility of GCs are not fully defined. The large GTPase dynamin and an interacting actin-binding protein, cortactin, have been localized to GCs, although the function performed by this complex is unclear. We have found that cortactin and the ubiquitous form of dynamin (Dyn) 2 exhibit a striking co-localization at the base of the transition zone of advancing GCs of embryonic hippocampal neurons. Confocal and total internal reflection fluorescence microscopies demonstrate that this basal localization represents point contacts. Exogenous expression of wild-type Dyn2 and cortactin leads to large, exceptionally flat, and static GCs, whereas disrupting this complex has no such effect. We find that excessive GC spreading is induced by Dyn2 and cortactin over-expression and substantial recruitment of the point contact-associated, actin-binding protein α-actinin1 to the ventral GC membrane. The distributions of other point contact proteins such as vinculin or paxillin appear unchanged. Immunoprecipitation experiments show that both Dyn2 and cortactin reside in a complex with α-actinin1. These findings provide new insights into the role of Dyn2 and the actin cytoskeleton in GC adhesion and motility.

Dynamin binding protein gene expression and memory performance in aged rats

Previous studies have shown that messenger RNA (mRNA) of the dynamin-binding protein (DNMBP), a scaffold protein regulating actin cytoskeleton and synaptic vesicle pools, is lower in neuropathologically-confirmed Alzheimer's brains. Here we investigated whether a deficit in long term memory formation during physiological aging is also associated with lower DNMBP expression. Hippocampal DNMBP mRNA was quantified by quantitative real time reverse transcriptase polymerase chain reaction (qRT-PCR) following inhibitory avoidance task in aged (26- to 27-month-old) rats that, according to memory performance, were ranked as good responders (GR) and bad responders (BR), in adult (3-month-old), late-adult (19-month-old), and aged (26-27-month-old) naive animals. We found that DNMBP mRNA levels were significantly higher in naive adults versus late adult and aged naive rats, in GR versus BR, and in pooled GR and BR versus aged-matched controls. Our data provide the first evidence that hippocampal DNMBP mRNA expression is reduced during physiological aging, and suggest that the capability to increase the expression of this mRNA may be a requirement for preserving long term memory formation during aging.

Drosophila models of cardiac disease

The fruit fly Drosophila melanogaster has emerged as a useful model for cardiac diseases, both developmental abnormalities and adult functional impairment. Using the tools of both classical and molecular genetics, the study of the developing fly heart has been instrumental in identifying the major signaling events of cardiac field formation, cardiomyocyte specification, and the formation of the functioning heart tube. The larval stage of fly cardiac development has become an important model system for testing isolated preparations of living hearts for the effects of biological and pharmacological compounds on cardiac activity. Meanwhile, the recent development of effective techniques to study adult cardiac performance in the fly has opened new uses for the Drosophila model system. The fly system is now being used to study long-term alterations in adult performance caused by factors such as diet, exercise, and normal aging. The fly is a unique and valuable system for the study of such complex, long-term interactions, as it is the only invertebrate genetic model system with a working heart developmentally homologous to the vertebrate heart. Thus, the fly model combines the advantages of invertebrate genetics (such as large populations, facile molecular genetic techniques, and short lifespan) with physiological measurement techniques that allow meaningful comparisons with data from vertebrate model systems. As such, the fly model is well situated to make important contributions to the understanding of complicated interactions between environmental factors and genetics in the long-term regulation of cardiac performance.

Perturbing dynamin reveals potent effects on the Drosophila circadian clock

Background

Transcriptional feedback loops are central to circadian clock function. However, the role of neural activity and membrane events in molecular rhythms in the fruit fly Drosophila is unclear. To address this question, we expressed a temperature-sensitive, dominant negative allele of the fly homolog of dynamin called shibire^ts1 (shi^ts1), an active component in membrane vesicle scission.

Principal Findings

Broad expression in clock cells resulted in unexpectedly long, robust periods (>28 hours) comparable to perturbation of core clock components, suggesting an unappreciated role of membrane dynamics in setting period. Expression in the pacemaker lateral ventral neurons (LNv) was necessary and sufficient for this effect. Manipulation of other endocytic components exacerbated shi^ts1's behavioral effects, suggesting its mechanism is specific to endocytic regulation. PKA overexpression rescued period effects suggesting shi^ts1 may downregulate PKA pathways. Levels of the clock component PERIOD were reduced in the shi^ts1-expressing pacemaker small LNv of flies held at a fully restrictive temperature (29°C). Less restrictive conditions (25°C) delayed cycling proportional to observed behavioral changes. Levels of the neuropeptide PIGMENT-DISPERSING FACTOR (PDF), the only known LNv neurotransmitter, were also reduced, but PERIOD cycling was still delayed in flies lacking PDF, implicating a PDF-independent process. Further, shi^ts1 expression in the eye also results in reduced PER protein and per and vri transcript levels, suggesting that shibire-dependent signaling extends to peripheral clocks. The level of nuclear CLK, transcriptional activator of many core clock genes, is also reduced in shi^ts1 flies, and Clk overexpression suppresses the period-altering effects of shi^ts1.

Conclusions

We propose that membrane protein turnover through endocytic regulation of PKA pathways modulates the core clock by altering CLK levels and/or activity. These results suggest an important role for membrane scission in setting circadian period.

Drosophila as a genetic and cellular model for studies on axonal growth

One of the most fascinating processes during nervous system development is the establishment of stereotypic neuronal networks. An essential step in this process is the outgrowth and precise navigation (pathfinding) of axons and dendrites towards their synaptic partner cells. This phenomenon was first described more than a century ago and, over the past decades, increasing insights have been gained into the cellular and molecular mechanisms regulating neuronal growth and navigation. Progress in this area has been greatly assisted by the use of simple and genetically tractable invertebrate model systems, such as the fruit fly Drosophila melanogaster. This review is dedicated to Drosophila as a genetic and cellular model to study axonal growth and demonstrates how it can and has been used for this research. We describe the various cellular systems of Drosophila used for such studies, insights into axonal growth cones and their cytoskeletal dynamics, and summarise identified molecular signalling pathways required for growth cone navigation, with particular focus on pathfinding decisions in the ventral nerve cord of Drosophila embryos. These Drosophila-specific aspects are viewed in the general context of our current knowledge about neuronal growth.

The timing of endocytosis after activation of a G-protein-coupled receptor in a sensory neuron

Endocytosis is a fundamental cellular event in membrane retrieval after exocytosis and in the regulation of receptor-mediated signal transduction. In contrast to the well-studied depolarization-induced membrane recycling, little is known about the kinetics of ligand-induced endocytosis of G-protein-coupled receptors in neurons. Here we investigated the kinetics of ligand-receptor binding-induced endocytosis in rat sensory neurons using a membrane capacitance assay. The time constant of ADP-induced endocytosis of P2Y-receptors was determined as 1.7 s. The ADP-induced endocytosis was blocked by antagonists against P2Y, phosphorylation, and clathrin. However, block of dynamin was without effect. The ADP-induced endocytosis was confirmed independently by a single vesicle image technique using a styryl FM2-10. Finally, the receptors were internalized in response to ADP, as determined by GFP-labeled P2Y. We conclude that ligand-receptor binding leads to rapid endocytosis in the cytoplasm of rat dorsal root ganglion neurons.

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.

Novel Caenorhabditis elegans unc-119 axon outgrowth defects correlate with behavioral phenotypes that are partially rescued by nonneural unc-119

UNC-119 function is necessary for the correct development of the Caenorhabditis elegans nervous system. Worms mutant for unc-119 exhibit nervous system structural defects, including supernumerary axon branches, defasciculated nerve fibers, and choice point errors. Axons of both mechanosensory (ALM) and chemo- sensory (ASI) neurons have elongation defects within the nerve ring. Expressing unc-119 cDNA in mechanosensory neurons rescues the elongation defect of ALM axons, but expression in ASI neurons does not rescue ASI axon elongation defects. Neither gross movement nor dauer larva formation defects are rescued in either case. However, expressing a construct including introns under the control of the same promoters results in substantial rescue of phenotypic defects. In these cases reporter expression expands to tissues outside those specified by the promoter, notably into head muscles. Surprisingly, expressing an unc-119 cDNA construct under the control of a muscle-specific promoter fully rescues the dauer formation defect and substantially rescues movement. Thus, although UNC-119 normally acts in a cell-autonomous fashion, the cell-nonautonomous rescue of neural function suggests that it either acts at the cell surface or that it can be transported into the cell from the extracellular environment and play its normal role.

Fate of constitutive endocytic vesicles formed in the growth cone: transport of vesicles from one growth cone to another in the same neuron

A developing neuron must have multiple paths of communication coordinating events among all its parts. One of these is the transport to the cell body of endocytic vesicles formed in growth cones. In order to observe this at single cell resolution, we developed a technique in which the fluorescent dye FM1-43 was applied to a single growth cone and newly formed constitutive endocytic vesicles were labeled. Using low light, time-lapse microscopy we were able to follow the movement of these vesicles throughout the neuron. The vast majority of the transported vesicles went to the cell body. However, many were observed to enter secondary neurites and to be transported to other growth cones. These new, more direct paths of transport that link the multiple growth cones of a neuron may play a role in several important developmental events involving interactions between the multiple neurites of a single neuron.

Calcium- and dynamin-independent endocytosis in dorsal root ganglion neurons

Synaptic vesicle endocytosis is believed to require calcium and the GTPase dynamin. We now report a form of rapid endocytosis (RE) in dorsal root ganglion (DRG) neurons that, unlike previously described forms of endocytosis, is independent of calcium and dynamin. The RE is tightly coupled to calcium-independent but voltage-dependent secretion (CIVDS). Using FM dye and capacitance measurements, we show that membrane depolarization induces RE in the absence of calcium. Inhibition of dynamin function does not affect RE. The magnitude of RE is proportional to that of preceding CIVDS and stimulation frequency. Inhibitors of protein kinase A (PKA) suppress RE induced by high-frequency depolarization, while PKA activators enhance RE induced by low-frequency depolarization. Biochemical experiments demonstrate that depolarization directly upregulates PKA activity in calcium-free medium. These results reveal a calcium- and dynamin-independent form of endocytosis, which is controlled by neuronal activity and PKA-dependent phosphorylation, in DRG neurons.

Synaptic vesicles: test for a role in presynaptic calcium regulation

Membrane-bound organelles such as mitochondria and the endoplasmic reticulum play an important role in neuronal Ca(2+) homeostasis. Synaptic vesicles (SVs), the organelles responsible for exocytosis of neurotransmitters, occupy more of the volume of presynaptic nerve terminals than any other organelle and, under some conditions, can accumulate Ca(2+). They are also closely associated with voltage-gated Ca(2+) channels (VGCCs) that trigger transmitter release by admitting Ca(2+) into the nerve terminal in response to action potentials (APs). We tested the hypothesis that SVs can modulate Ca(2+) signals in the presynaptic terminal. This has been a difficult question to address because neither pharmacological nor genetic approaches to block Ca(2+) permeation of the SV membrane have been available. To investigate the possible role of SVs in Ca(2+) regulation, we used imaging techniques to compare Ca(2+) dynamics in motor nerve terminals before and after depletion of SVs. We used the temperature-sensitive Drosophila dynamin mutant shibire, in which SVs can be eliminated by stimulation. There was no difference in the amplitude or time course of Ca(2+) responses during high-frequency trains of APs, or single APs, in individual presynaptic boutons before and after depletion of SVs. SVs have a limited role, if any, in the rapid sequestration of Ca(2+) within the neuronal cytosol or the synaptic microdomain. We also conclude that SVs are not important for regulation of synaptic VGCCs.

Tuba, a novel protein containing bin/amphiphysin/Rvs and Dbl homology domains, links dynamin to regulation of the actin cytoskeleton

Tuba is a novel scaffold protein that functions to bring together dynamin with actin regulatory proteins. It is concentrated at synapses in brain and binds dynamin selectively through four N-terminal Src homology-3 (SH3) domains. Tuba binds a variety of actin regulatory proteins, including N-WASP, CR16, WAVE1, WIRE, PIR121, NAP1, and Ena/VASP proteins, via a C-terminal SH3 domain. Direct binding partners include N-WASP and Ena/VASP proteins. Forced targeting of the C-terminal SH3 domain to the mitochondrial surface can promote accumulation of F-actin around mitochondria. A Dbl homology domain present in the middle of Tuba upstream of a Bin/amphiphysin/Rvs (BAR) domain activates Cdc42, but not Rac and Rho, and may thus cooperate with the C terminus of the protein in regulating actin assembly. The BAR domain, a lipid-binding module, may functionally replace the pleckstrin homology domain that typically follows a Dbl homology domain. The properties of Tuba provide new evidence for a close functional link between dynamin, Rho GTPase signaling, and the actin cytoskeleton.

Unique biochemical and behavioral alterations in Drosophila shibire(ts1) mutants imply a conformational state affecting dynamin subcellular distribution and synaptic vesicle cycling

Dynamin is a GTPase protein that is essential for clathrin-mediated endocytosis of synaptic vesicle membranes. The Drosophila dynamin mutation shi(ts1) changes a single residue (G273D) at the boundary of the GTPase domain. In cell fractionation of homogenized fly heads without monovalent cations, all dynamin was in pellet fractions and was minimally susceptible to Triton-X extraction. Addition of Na(+) or K(+) can extract dynamin to the cytosolic (supernatant) fraction. The shi(ts1) mutation reduced the sensitivity of dynamin to salt extraction compared with other temperature-sensitive alleles or wild type. Sensitivity to salt extraction in shi(ts1) was enhanced by GTP and nonhydrolyzable GTP-gammaS. The shi(ts1) mutation may therefore induce a conformational change, involving the GTP binding site, that affects dynamin aggregation. Temperature-sensitive shibire mutations are known to arrest endocytosis at restrictive temperatures, with concomitant accumulation of presynaptic collared pits. Consistent with an effect upon dynamin aggregation, intact shi(ts1) flies recovered much more slowly from heat-induced paralysis than did other temperature-sensitive shibire mutants. Moreover, a genetic mutation that lowers GTP abundance (awd(msf15)), which reduces the paralytic temperature threshold of other temperature-sensitive shibire mutations that lie closer to consensus GTPase motifs, did not reduce the paralytic threshold of shi(ts1). Taken together, the results may link the GTPase domain to conformational shifts that influence aggregation in vitro and endocytosis in vivo, and provide an unexpected point of entry to link the biophysical properties of dynamin to physiological processes at synapses.

Trafficking of the vesicular acetylcholine transporter in SN56 cells: a dynamin-sensitive step and interaction with the AP-2 adaptor complex

The pathways by which synaptic vesicle proteins reach their destination are not completely defined. Here we investigated the traffic of a green fluorescent protein (GFP)-tagged version of the vesicular acetylcholine transporter (VAChT) in cholinergic SN56 cells, a model system for neuronal processing of this cargo. GFP-VAChT accumulates in small vesicular compartments in varicosities, but perturbation of endocytosis with a dominant negative mutant of dynamin I-K44A impaired GFP-VAChT trafficking to these processes. The protein in this condition accumulated in the cell body plasma membrane and in large vesicular patches therein. A VAChT endocytic mutant (L485A/L486A) was also located at the plasma membrane, however, the protein was not sorted to dynamin I-K44A generated vesicles. A fusion protein containing the VAChT C-terminal tail precipitated the AP-2 adaptor protein complex from rat brain, suggesting that VAChT directly interacts with the endocytic complex. In addition, yeast two hybrid experiments indicated that the C-terminal tail of VAChT interacts with the micro subunit of AP-2 in a di-leucine (L485A/L486A) dependent fashion. These observations suggest that the di-leucine motif regulates sorting of VAChT from the soma plasma membrane through a clathrin dependent mechanism prior to the targeting of the transporter to varicosities.

Endocytosis in Drosophila: progress, possibilities, prognostications

By many outside the field, endocytosis is often perceived as a "house-keeping" function performed via identical mechanisms in yeast and man. Recent discoveries have done much to reduce this misperception. (1) Endocytosis occurs via different mechanisms and different pathways in different cellular contexts. (2) Molecular mechanisms that regulate homologous pathways in unicellular and multicellular organisms show considerable variance. (3) Temporally controlled endocytosis of specific regulatory molecules underlies several important and intricate biological processes including synapse formation, synaptic plasticity, cell fate determination, and morphogen gradient formation. Interactions between endocytosis and cytoskeletal and signaling pathways have been particularly revealing. In this intellectual context, Drosophila has become invaluable as a metazoan genetic model in which to understand the many faces of endocytosis. This review discusses two aspects of work in Drosophila: (a) its contributions toward understanding fundamental mechanisms that underlie the operation of endocytic pathways; (b) how analyses in Drosophila provide insights into varied biological processes regulated by endocytosis. In addition, while offering our commentary on merits and limitations of Drosophila work, we speculate on likely areas for contributions and future research on endocytosis in Drosophila.

Genetic interaction between shibire and comatose mutations in Drosophila suggest a role for snap-receptor complex assembly and disassembly for maintenance of synaptic vesicle cycling

NSF is an ATPase required for the fusion of secretory vesicles with plasma membrane. Conditional comatose (Drosophila homolog of N-ethylmaleimide sensitive fusion factor (NSF)) mutations in Drosophila block synaptic transmission at restrictive temperature. Current models hold that NSF-mediated dissociation of SNARE (SNAp REceptor) complexes on mature synaptic vesicles primes them for exocytic release. Paralysis in comt mutants thus reflects defective exocytosis due to buildup of unresolved SNARE complexes. Here, we analyze effects of blocking synaptic vesicle recycling on behavioral, physiological and biochemical phenotypes of comt. Behavioral recovery of comt animals and recovery of comt synapses, as assayed by electroretinograms, after exposure to high temperature is faster if synaptic vesicle recycling is simultaneously blocked using shi(ts) mutants. Concurrently, 7S complex buildup in comt shi double mutants is substantially lower than in comt mutants alone. In addition, we find that 7S complexes can form on presynaptic plasma membrane if NSF is inhibited after synaptic-vesicle depletion. Thus, our experiments demonstrate a need for continuous NSF activity required not only for dissociating cis-SNARE complexes on plasma membrane after exocytosis, but also for maintaining these cis-SNARE complexes in a dissociated state.

Trafficking of green fluorescent protein tagged-vesicular acetylcholine transporter to varicosities in a cholinergic cell line

Synaptic vesicle proteins are suggested to travel from the trans-Golgi network to active zones via tubulovesicular organelles, but the participation of different populations of endosomes in trafficking remains a matter of debate. Therefore, we generated a green fluorescent protein (GFP)-tagged version of the vesicular acetylcholine transporter (VAChT) and studied the localization of VAChT in organelles in the cell body and varicosities of living cholinergic cells. GFP-VAChT is distributed to both early and recycling endosomes in the cell body and is also observed to accumulate in endocytic organelles within varicosities of SN56 cells. GFP-VAChT positive organelles in varicosities are localized close to plasma membrane and are labeled with FM4-64 and GFP-Rab5, markers of endocytic vesicles and early endosomes, respectively. A GFP-VAChT mutant lacking a dileucine endocytosis motif (leucine residues 485 and 486 changed to alanine residues) accumulated at the plasma membrane in SN56 cells. This endocytosis-defective GFP-VAChT mutant is localized primarily at the somal plasma membrane and exhibits reduced neuritic targeting. Furthermore, the VAChT mutant did not accumulate in varicosities, as did VAChT. Our data suggest that clathrin-mediated internalization of VAChT to endosomes at the cell body might be involved in proper sorting and trafficking of VAChT to varicosities. We conclude that genesis of competent cholinergic secretory vesicles depends on multiple interactions of VAChT with endocytic proteins.

The products of the Drosophila stoned locus interact with synaptic vesicles via synaptotagmin

The stoned locus of Drosophila melanogaster encodes two novel proteins, stonedA (STNA) and stonedB (STNB), both of which are expressed in the nervous system. Flies with defects at the stoned locus have abnormal behavior and altered synaptic transmission. Genetic interactions, in particular with the shibire (dynamin) mutation, indicated a presynaptic function for stoned and suggested an involvement in vesicle cycling. Immunological studies revealed colocalization of the stoned proteins at the neuromuscular junction with the integral synaptic vesicle protein synaptotagmin (SYT). We show here that stoned interacts genetically with synaptotagmin to produce a lethal phenotype. The STNB protein is found by co-immunoprecipitation to be associated with synaptic vesicles, and glutathione S-transferase pull-downs demonstrate an in vitro interaction between the micro2-homology domain of STNB and the C2B domain of the SYTI isoform. The STNA protein is also found in association with vesicles, and it too exhibits an in vitro association with SYTI. However, we find that the bulk of STNA is in a nonmembranous fraction. By using the shibire mutant to block endocytosis, STNB is shown to be present on some synaptic vesicles before exocytosis. However, STNB is not associated with all synaptic vesicles. We hypothesize that STNB specifies a subset of synaptic vesicles with a role in the synaptic vesicle cycle that is yet to be determined.

Dynamin, encoded by shibire, is central to cardiac function

Employing the Drosophila heart, a model system for genetic and molecular investigation of cardiac physiology, we demonstrate here an essential role for the protein dynamin, encoded by the Drosophila gene shibire(ts) (shi(ts)), in maintaining normal heart function. In flies bearing two temperature-sensitive alleles of shi, shi(ts1) and shi(ts2), heartbeat is both slower and less rhythmic than in wild-type animals. Serotonin and norepinephrine, normally cardioacceleratory in wild type, are without effect in flies bearing the shi mutation. Electrocardiogram (EKG) analysis reveals a bigeminal beat in mutant hearts, unlike the single electrical pulse in wild-type. The gene no action potential (temperature sensitive), with previously-described cardiac aberrations similar to those of shi, interacts with shi: shi/shi;nap/nap mutants have almost wild-type heart function. J. Exp. Zool. 289:81-89, 2001.

Gradient formation of the TGF-beta homolog Dpp

Secreted morphogens such as the Drosophila TGF-beta homolog Decapentaplegic (Dpp) are thought to spread through target tissues and form long-range concentration gradients providing positional information. Using a GFP-Dpp fusion, we monitored a TGF-beta family member trafficking in situ throughout the target tissue and forming a long-range concentration gradient. Evidence is presented that long-range Dpp movement involves Dpp receptor and Dynamin functions. We also show that the rates of endocytic trafficking and degradation determine Dpp signaling range. We propose a model where the gradient is formed via intracellular trafficking initiated by receptor-mediated endocytosis of the ligand in receiving cells with the gradient slope controlled by endocytic sorting of Dpp toward recycling versus degradation.

Membrane recycling in the neuronal growth cone revealed by FM1-43 labeling

Membrane dynamics within the chick ciliary neuronal growth cone were investigated by using the membrane-impermeant dye FM1-43. A depolarization-evoked endocytosis was observed that shared many properties with the synaptic vesicle recycling previously described at the presynaptic terminal. In addition, in the absence of depolarization a basal level of constitutive endocytotic activity was observed at approximately 30% of the rate of evoked endocytosis. This constitutive endocytosis accounted for large amounts of membrane retrieval: the equivalent of the entire growth cone surface area could be internalized within a 30 min period. Endosomes generated via constitutive and evoked processes were highly mobile and could move considerable distances both within the growth cone and out to the neurite. In addition to their different requirements for formation, evoked and constitutive endosomes displayed a significant difference in release properties. After a subsequent depolarization of labeled growth cones, evoked endosomes were released although constitutive endosomes were not released. Furthermore, treatment with latrotoxin released evoked endosomes, but not constitutive endosomes. Although the properties of evoked endosomes are highly reminiscent of synaptic vesicles, constitutive endosomes appear to be a separate pool resulting from a distinct and highly active process within the neuronal growth cone.

Morphology of the pupal heart, adult heart, and associated tissues in the fruit fly, Drosophila melanogaster

The early pupal heart of the fruit fly Drosophila melanogaster has recently been the subject of intense physiological and molecular work, yet it has not been well described, nor has it been compared with the heart of the adult fly. In the work reported here, the hearts of adults and early pupae of D. melanogaster were studied by scanning and transmission electron microscopy and by light microscopy. The hearts of adults and early pupae both consist of a tube of circular striated muscle one cell in thickness. The alary muscles, which suspend the heart, are more delicate in the adult compared to the early pupa. The pericardial cells in both early pupae and adults are connected to the heart by connective tissue radiating from the alary muscles or dorsal diaphragm. We confirm that four major changes occur in the heart during metamorphosis: 1) a conical chamber is formed de novo in the first and second abdominal segments; 2) the adult heart curves to conform to the contour of the abdomen; 3) a layer of longitudinal striated muscle appears on the ventral surface of the heart; 4) a fourth pair of ostia is added to the three already present in the early pupa; and note additionally that 5) the ostia appear as simple openings in the heart of the early pupa but are valve-like in the adult.

Differential distribution of dynamin isoforms in mammalian cells

Dynamins are 100-kDa GTPases that are essential for clathrin-coated vesicle formation during receptor-mediated endocytosis. To date, three different dynamin genes have been identified, with each gene expressing at least four different alternatively spliced forms. Currently, it is unclear whether these different dynamin gene products perform distinct or redundant cellular functions. Therefore, the focus of this study was to identify additional spliced variants of dynamin from rat tissues and to define the distribution of the dynamin family members in a cultured rat epithelial cell model (Clone 9 cells). After long-distance reverse transcription (RT)-PCR of mRNA from different rat tissues, the full-length cDNAs encoding the different dynamin isoforms were sequenced and revealed four additional spliced variants for dynamin I and nine for dynamin III. Thus, in rat tissues there are a total of at least 25 different mRNAs produced from the three dynamin genes. Subsequently, we generated stably transfected Clone 9 cells expressing full-length cDNAs of six different spliced forms tagged with green fluorescent protein. Confocal or fluorescence microscopy of these transfected cells revealed that many of the dynamin proteins associate with distinct membrane compartments, which include clathrin-coated pits at the plasma membrane and the Golgi apparatus, and several undefined vesicle populations. These results indicate that the dynamin family is more extensive than was originally predicted and suggest that the different dynamin proteins are localized to distinct cytoplasmic or membrane compartments.

Probable mechanisms underlying interallelic complementation and temperature-sensitivity of mutations at the shibire locus of Drosophila melanogaster

The shibire locus of Drosophila melanogaster encodes dynamin, a GTPase required for the fission of endocytic vesicles from plasma membrane. Biochemical studies indicate that mammalian dynamin is part of a complex containing multiple dynamin subunits and other polypeptides. To gain insight into sequences of dynamin critical for its function, we have characterized in detail a collection of conditional and lethal shi alleles. We describe a probable null allele of shi and show that its properties are distinct from those of two classes of lethal alleles (termed I and II) that show intergroup, interallelic complementation. Sequenced class I alleles, which display dominant properties, carry missense mutations in conserved residues in the GTPase domain of dynamin. In contrast, the sequenced class II alleles, which appear completely recessive, carry missense mutations in conserved residues of a previously uncharacterized "middle domain" that lies adjacent to the GTPase region. These data suggest that critical interactions mediated by this middle domain are severely affected by the class II lethal mutations; thus, the mutant sequences should be very useful for confirming the in vivo relevance of interactions observed in vitro. Viable heteroallelic combinations of shi lethals show rapid and reversible temperature-sensitive paralytic phenotypes hitherto only described for the ts alleles of shi. When taken together with the molecular analysis of shi mutations, these observations suggest that the GTPase domain of dynamin carries an intrinsically temperature-sensitive activity: hypomorphic mutations that reduce this activity at low temperatures result in conditional temperature-sensitive phenotype. These observations explain why screens for conditional paralytic mutants in Drosophila inevitably recover ts alleles of shi at high frequencies.

Dynamin at the neck of caveolae mediates their budding to form transport vesicles by GTP-driven fission from the plasma membrane of endothelium

The molecular mechanisms mediating cell surface trafficking of caveolae are unknown. Caveolae bud from plasma membranes to form free carrier vesicles through a "pinching off" or fission process requiring cytosol and driven by GTP hydrolysis (Schnitzer, J.E., P. Oh, and D.P. McIntosh. 1996. Science. 274:239-242). Here, we use several independent techniques and functional assays ranging from cell-free to intact cell systems to establish a function for dynamin in the formation of transport vesicles from the endothelial cell plasma membrane by mediating fission at the neck of caveolae. This caveolar fission requires interaction with cytosolic dynamin as well as its hydrolysis of GTP. Expression of dynamin in cytosol as well as purified recombinant dynamin alone supports GTP-induced caveolar fission in a cell-free assay whereas its removal from cytosol or the addition to the cytosol of specific antibodies for dynamin inhibits this fission. Overexpression of mutant dynamin lacking normal GTPase activity not only inhibits GTP-induced fission and budding of caveolae but also prevents caveolae-mediated internalization of cholera toxin B chain in intact and permeabilized endothelial cells. Analysis of endothelium in vivo by subcellular fractionation and immunomicroscopy shows that dynamin is concentrated on caveolae, primarily at the expected site of action, their necks. Thus, through its ability to oligomerize, dynamin appears to form a structural collar around the neck of caveolae that hydrolyzes GTP to mediate internalization via the fission of caveolae from the plasma membrane to form free transport vesicles.

Amphiphysin I antisense oligonucleotides inhibit neurite outgrowth in cultured hippocampal neurons

Amphiphysin I is an SH3 domain-containing neuronal protein, enriched in axon terminals, which was reported to act as a physiological binding partner for dynamin I in synaptic vesicle endocytosis. Rvs167 and Rvs161, the yeast homologs of amphiphysin I, have been implicated in endocytosis, actin function, and cell polarity. Now we have explored the possibility that amphiphysin I also may have a role in actin dynamics and cell polarity by testing the effect of amphiphysin I suppression on neurite outgrowth. Freshly plated hippocampal neurons were exposed to antisense oligonucleotides via a new delivery system based on a polycationic amphipathic polymer, PS980. Western blot analysis revealed that amphiphysin I levels steadily increased with neuronal differentiation, whereas in antisense-treated cultures amphiphysin I levels were reduced to approximately 10% of control levels at 48 hr. Concomitantly, a collapse of growth cones and a severe inhibition of neurite outgrowth and axon formation were observed. A similar effect was observed previously after dynamin I suppression in the same culture system (). We also have found that amphiphysin I and dynamin I colocalize in developing neurons at all developmental stages and that a pool of both proteins is colocalized with actin patches at the leading edge of growth cones. Our findings suggest a conserved role of the amphiphysin protein family in the dynamics of the cortical cell cytoskeleton and provide new evidence for a close functional link between amphiphysin I and dynamin I.

Two isoforms of Drosophila dynamin in wild-type and shibire(ts) neural tissue: different subcellular localization and association mechanisms

The temperature-sensitive mutations of the shibire (shi) gene in Drosophila cause endocytic arrest, resulting in neurotransmission block and paralysis at high temperatures. However, underlying mechanism for the defects is not yet known. We examined the subcellular distribution of dynamin, a product of the shi gene, by immunoblotting and immunocytochemical assays. Two isoforms of dynamin with apparent M(r) of 92 kD and 94 kD have been detected in wild-type and shi(n) adult neural tissue. The two isoforms were reproducibly associated with different subcellular fractions of head homogenates. The 94kD isoform is fractionated in the low speed (2.000 x g) pellet containing plasma membrane fragments, and the 92kD isoform in the high speed (130,000 x g) pellet. In this procedure, very little dynamin remained in the high speed supernatant fraction. The 94 kD isoform represents the majority (65-75%) of total dynamin and appears to be a peripheral membrane protein. It can be extracted from the low speed membrane pellet by high salt, Na2CO3 (pH 11) or Triton X-100 treatments. Extracted 94kD dynamin from both wild-type and mutant homogenates is able to reassociate with artificial phospholipid vesicles at both permissive and restrictive temperatures. Binding of the 94 kD dynamin to liposomes appears to be pH-dependent, varying most significantly within the physiological pH range, which may be functionally important. The 92 kD isoform cannot be released by high salt or Na2CO3 treatments and only a small fraction is released by Triton X-100, suggesting a different mechanism of association with cell structures. The distribution of the two isoforms is not altered by the presence of stabilized microtubules in homogenates. No apparent degradation or subcellular redistribution of mutant dynamin was detected in two shi(n) alleles after heat shock or block of the dynamin GTPase activity, suggesting that intracellular redistribution or degradation of mutant dynamin are not involved in the endocytosis arrest in these mutants. These observations resemble the effect of endocytosis arrest by GTP-gamma-S in rat brain synaptosomes (Takei et al., 1995), in which dynamin is trapped at the neck of invaginated pits but is absent in the clathrin-coated distal end undergoing internalization. Our finding that endocytosis arrest by shi(n) mutations and GTP-gamma-S do not lead to cumulation of dynamin in the low speed pellet fraction further suggests that the 94 kD isoform remains associated with the plasma membrane during coated vesicle pinch-off and that the two isoforms do not appear to correspond to different functional states of dynamin but are likely to be involved in separate cellular compartments within the membrane cycling pathway (e.g., the plasma membrane, endosomes, and endoplasmic reticulum).

DNM1, a dynamin-related gene, participates in endosomal trafficking in yeast

We identified DNM1, a novel dynamin-related gene in Saccharomyces cerevisiae. Molecular and genetic mapping showed that DNM1 is the most proximal gene to the right of centromere 12, and is predicted to encode a protein of 85 kD, designated Dnm1p. The protein exhibits 41% overall identity with full-length dynamin I and 55% identity with the most highly conserved 400-amino acid GTPase region. Our findings show that like mammalian dynamin, Dnm1p participates in endocytosis; however, it is unlikely to be a cognate homologue. Cells with a disruption in the DNM1 gene showed mating response defects consistent with a delay in receptor-mediated endocytosis. The half-life of the Ste3p pheromone receptor was increased two- to threefold in the dnm1 mutant, demonstrating that Dnm1p participates in the constitutive turnover of the receptor. To define the step in the endocytic pathway at which Dnm1p acts, we analyzed mutant strains at both early and late steps of the process. Initial internalization of epitope-tagged pheromone receptor or of labeled pheromone proceeded with wild-type kinetics. However, delivery of the internalized receptor to the vacuole was greatly impeded during ligand-induced endocytosis. These data suggest that during receptor-mediated endocytosis, Dnm1p acts after internalization, but before fusion with the vacuole. The dnm1 mutant was not defective for sorting of vacuolar proteins, indicating that Dnm1p is not required for transport from the late endosome to the vacuole. Therefore, we suggest that Dnm1p participates at a novel step before fusion with the late endosome.

Golgi and vacuolar membrane proteins reach the vacuole in vps1 mutant yeast cells via the plasma membrane

The Vps1 protein of Saccharomyces cerevisiae is an 80-kD GTPase associated with the Golgi apparatus. Vps1p appears to play a direct role in the retention of late Golgi membrane proteins, which are mislocalized to the vacuolar membrane in its absence. The pathway by which late Golgi and vacuolar membrane proteins reach the vacuole in vps1 delta mutants was investigated by analyzing transport of these proteins in vps1 delta cells that also contained temperature sensitive mutations in either the SEC4 or END4 genes, which are required for a late step in secretion and the internalization step of endocytosis, respectively. Not only was vacuolar transport of a Golgi membrane protein blocked in the vps1 delta sec4-ts and vps1 delta end4-ts double mutant cells at the non-permissive temperature but vacuolar delivery of the vacuolar membrane protein, alkaline phosphatase was also blocked in these cells. Moreover, both proteins expressed in the vps1 delta end4-ts cells at the elevated temperature could be detected on the plasma membrane by a protease digestion assay indicating that these proteins are transported to the vacuole via the plasma membrane in vps1 mutant cells. These data strongly suggest that a loss of Vps1p function causes all membrane traffic departing from the late Golgi normally destined for the prevacuolar compartment to instead be diverted to the plasma membrane. We propose a model in which Vps1p is required for formation of vesicles from the late Golgi apparatus that carry vacuolar and Golgi membrane proteins bound for the prevacuolar compartment.

The Saccharomyces cerevisiae MVP1 gene interacts with VPS1 and is required for vacuolar protein sorting

The VPS1 gene of Saccharomyces cerevisiae encodes an 80-kDa GTPase that associates with Golgi membranes and is required for the sorting of proteins to the yeast vacuole. Vps1p is a member of a growing family of high-molecular-weight GTPases that are found in a number of organisms and are involved in a variety of cellular processes. Vps1p is most similar to mammalian dynamin and the Drosophila Shibire protein, both of which have been shown to play a role in an early step of endocytosis. To identify proteins that interact with Vps1p, a genetic screen was designed to isolate multicopy suppressors of dominant-negative vps1 mutations. One such suppressor, MVP1, that exhibits genetic interaction with VPS1 and is itself required for vacuolar protein sorting has been isolated. Overproduction of Mvp1p will suppress several dominant alleles of VPS1, and suppression is dependent on the presence of wild-type Vps1p. MVP1 encodes a 59-kDa hydrophilic protein, Mvp1p, which appears to colocalize with Vps1p in vps1d and vps27 delta yeast cells. We therefore propose that Mvp1p and Vps1p act in concert to promote membrane traffic to the vacuole.

Intermediates in synaptic vesicle recycling revealed by optical imaging of Drosophila neuromuscular junctions

We show that uptake and release of the styryl dye FM1-43 may be used to monitor synaptic vesicle exocytosis and recycling at Drosophila larval neuromuscular junctions. At Drosophila nerve terminals, FM1-43 specifically labels subsynaptic domains enriched in synaptotagmin, in a manner that requires Ca2+, membrane depolarization, and shibire (shi) function. Endocytosis rates, very low in unstimulated synapses, are induced severalfold by the exocytosis of synaptic vesicles. Using shi(ts)1 mutant synapses to separate synaptic vesicle fusion and recycling temporally, we show that recycling events subsequent to the shi block do not require extracellular Ca2+. We suggest that two distinct intermediate stages in vesicle recycling may be trapped and analyzed at Drosophila neuromuscular junctions.

Effects of mutant rat dynamin on endocytosis

Dynamin is a 100-kD microtubule-activated GTPase. Recent evidence has revealed a high degree of sequence homology with the product of the Drosophila gene shibire, mutations in which block the recycling of synaptic vesicles and, more generally, the formation of coated and non-coated vesicles at the plasma membrane. We have now transfected cultured mammalian COS-7 cells with both wild-type and mutant dynamin cDNAs. Point mutations in the GTP-binding consensus sequence elements of dynamin equivalent to dominant negative mutations in ras, and an NH2-terminal deletion of the entire GTP-binding domain of dynamin, block transferrin uptake and alter the distribution of clathrin heavy chain and alpha-, but not gamma-, adaptin. COOH-terminal deletions reverse these effects, identifying this portion of dynamin as a site of interaction with other components of the endocytic pathway. Over-expression of neither wild-type nor mutant forms of dynamin affected the distribution of microtubules. These results demonstrate a specific role for dynamin and for GTP in the initial stages of receptor-mediated endocytosis.

Development and maintenance of a simple reflex circuit in small-patch mosaics of Drosophila: effects of altered neuronal function and developmental arrest

A combined genetic, anatomical, and behavioral approach has been undertaken to study the developmental and functional plasticity of identified bristle mechanosensory neurons in Drosophila. A stereotyped grooming reflex in decapitated flies enabled simple but reliable assessments of the functional output of individual bristle sensory cells to correlate with their axonal projections and terminal arbors revealed by the cobalt backfill technique. Construction of small-patch mosaics that contain only a single mutant bristle allowed functional perturbation of individual neurons within an otherwise normal environment. Mutations that affect nerve excitability and membrane recycling have been used to examine their effects on neuronal pathfinding, arborization, and the initiation and maintenance of functional connections. Previous studies (Burg and Wu, 1986, J. Neurosci. 6:2968-2976; 1989, Dev. Biol. 131:505-514) have demonstrated that para(ts)nap(ts) double-mutant sensory neurons, in which action potentials are unconditionally blocked by defects in sodium currents, and eag Sh double-mutant sensory cells, in which membrane excitability is increased by alterations in potassium currents, can establish and maintain central projections that are indistinguishable from their functionally normal counterparts. Mutations of the shi(ts) gene cause a temperature-sensitive, reversible block of the membrane recycling process, resulting in arrest of neuronal growth in culture (Kim and Wu, 1987, J. Neurosci. 7:3245-3255) and depletion of synaptic vesicles that leads to transmission blockade at established synapses (Ikeda, Ozawa, and Hagiwara, 1976, Nature 259:489-491; Koenig and Ikeda, 1983, J. Neurobiol. 14:411-419; 1989, J. Neurosci. 9:3844-3860). Prolonged heat treatments (up to 16% of total development time) of small-patch shi(ts) mosaics at different pupal stages did not prevent the establishment of central projections characteristic of the various sensory cell types. However, none of the shi(ts) sensory neurons heat-pulsed during the initial or the final 16% of pupal development were able to initiate the reflex behavior, although a proportion of those treated in other periods apparently established functional contacts with appropriate targets to support the characteristic cleaning reflex. The possibility exists that the membrane recycling process blocked in shi(ts) cells provides a crucial mechanism for cell-cell interactions taking place during initial differentiation and final synaptic stabilization, and possibly competition, in the developing sensory neuron. Heat treatments of adult shi(ts) mosaics blocked the reflex initiated by the mutant (but not the surrounding normal) bristles, as expected from the effect of synaptic block.(ABSTRACT TRUNCATED AT 400 WORDS)