Principles of exocytosis and membrane fusion

Fine tuning of the net charge alternation of polyzwitterion surfaced lipid nanoparticles to enhance cellular uptake and membrane fusion potential

Lipid nanoparticles (LNPs) coated with functional and biocompatible polymers have been widely used as carriers to deliver oligonucleotide and messenger RNA therapeutics to treat diseases. Poly(ethylene glycol) (PEG) is a representative material used for the surface coating, but the PEG surface-coated LNPs often have reduced cellular uptake efficiency and pharmacological activity. Here, we demonstrate the effect of pH-responsive ethylenediamine-based polycarboxybetaines with different molecular weights as an alternative structural component to PEG for the coating of LNPs. We found that appropriate tuning of the molecular weight around polycarboxybetaine-modified LNP, which incorporated small interfering RNA, could enhance the cellular uptake and membrane fusion potential in cancerous pH condition, thereby facilitating the gene silencing effect. This study demonstrates the importance of the design and molecular length of polymers on the LNP surface to provide effective drug delivery to cancer cells.

Membrane progesterone receptor induces meiosis in Xenopus oocytes through endocytosis into signaling endosomes and interaction with APPL1 and Akt2

The steroid hormone progesterone (P4) mediates many physiological processes through either nuclear receptors that modulate gene expression or membrane P4 receptors (mPRs) that mediate nongenomic signaling. mPR signaling remains poorly understood. Here we show that the topology of mPRβ is similar to adiponectin receptors and opposite to that of G-protein-coupled receptors (GPCRs). Using Xenopus oocyte meiosis as a well-established physiological readout of nongenomic P4 signaling, we demonstrate that mPRβ signaling requires the adaptor protein APPL1 and the kinase Akt2. We further show that P4 induces clathrin-dependent endocytosis of mPRβ into signaling endosome, where mPR interacts transiently with APPL1 and Akt2 to induce meiosis. Our findings outline the early steps involved in mPR signaling and expand the spectrum of mPR signaling through the multitude of pathways involving APPL1.

The steroid hormone progesterone mediates many physiological processes through either nuclear receptors that modulate gene expression, or membrane progesterone receptors (mPRs) that mediate non-genomic signaling. This study shows that non-genomic mPRβ signaling progresses through clathrin-dependent endocytosis into signaling endosomes where it interacts with and activates APPL1 and Akt2 to induce oocyte meiosis.

Targeted Quantitative Proteomic Approach for High-Throughput Quantitative Profiling of Small GTPases in Brain Tissues of Alzheimer's Disease Patients

Neurodegenerative disorders, including Alzheimer's disease (AD), are prevalent among the elderly. Small GTPases of the Ras superfamily are essential regulators of intracellular trafficking and signal transduction. In this study, we develop a targeted quantification method for small GTPase proteins, where the method involves scheduled multiple-reaction monitoring analysis and the use of synthetic stable isotope-labeled peptides as internal standards or surrogate standards. We further applied this method to examine the altered expression of small GTPase proteins in post-mortem frontal cortex tissues from AD patients with different degrees of disease severity. We were able to achieve sensitive and reproducible quantifications of 80 small GTPases in brain tissue samples from 15 patients. Our results revealed substantial up-regulations of several synaptic GTPases, i.e., RAB3A/C, RAB4A/B, and RAB27B, in tissues from patients with higher degrees of AD pathology, suggesting that aberrant synaptic trafficking may modulate the progression of AD. The method should be generally applicable for high-throughput targeted quantification of small GTPase proteins in other tissue and cellular samples.

Emerging evidence for the modulation of exocytosis by signalling lipids

Membrane fusion is a key event in exocytosis of neurotransmitters and hormones stored in intracellular vesicles. In this process, soluble N‐ethylmaleimide sensitive factor attachment protein receptor (SNARE) proteins are essential components of the exocytotic molecular machinery, while lipids have been seen traditionally as structural elements. However, the so‐called signalling lipids, such as sphingosine and arachidonic acid, interact with SNAREs and directly modulate the frequency and mode of fusion events. Interestingly, recent work has proved that the sphingosine analogue FTY‐720, used in the treatment of multiple sclerosis, mimics the effects of signalling lipids. In the present Review, we discuss recent investigations suggesting that endogenous signalling lipids and synthetic analogues can modulate important physiological aspects of secretion, such as quantal release, vesicle recruitment into active sites, vesicle transport and even organelle fusion in the cytosol. Therefore, these compounds are far from being merely structural components of cellular membranes.

Enteric glia regulate gut motility in health and disease

The enteric nervous system, often referred to as the second brain, is the largest assembly of neurons and glia outside the central nervous system. The enteric nervous system resides within the wall of the digestive tract and regulates local gut reflexes involved in gastrointestinal motility and fluid transport; these functions can be accomplished in the absence of the extrinsic innervation from the central nervous system. It is neurons and their circuitry within the enteric nervous system that govern the gut reflexes. However, it is becoming clear that enteric glial cells are also actively involved in this process through the bidirectional signaling with neurons and other cells in the gut wall. We synthesize the recently discovered modulatory roles of enteric gliotransmission in gut motility and provide our perspective for future lines of research.

Munc13 proteins control regulated exocytosis in mast cells

Mast cells (MCs) are involved in host defenses against pathogens and inflammation. Stimulated MCs release substances stored in their granules via regulated exocytosis. In other cell types, Munc13 (mammalian homolog of Caenorhabditis elegans uncoordinated gene 13) proteins play essential roles in regulated exocytosis. Here, we found that MCs express Munc13-2 and -4, and we studied their roles using global and conditional knock-out (KO) mice. In a model of systemic anaphylaxis, we found no difference between WT and Munc13-2 KO mice, but global and MC-specific Munc13-4 KO mice developed less hypothermia. This protection correlated with lower plasma histamine levels and with histological evidence of defective MC degranulation but not with changes in MC development, distribution, numbers, or morphology. In vitro assays revealed that the defective response in Munc13-4-deficient MCs was limited to regulated exocytosis, leaving other MC secretory effector responses intact. Single cell capacitance measurements in MCs from mouse mutants differing in Munc13-4 expression levels in their MCs revealed that as levels of Munc13-4 decrease, the rate of exocytosis declines first, and then the total amount of exocytosis decreases. A requirement for Munc13-2 in MC exocytosis was revealed only in the absence of Munc13-4. Electrophysiology and EM studies uncovered that the number of multigranular compound events (i.e. granule-to-granule homotypic fusion) was severely reduced in the absence of Munc13-4. We conclude that although Munc13-2 plays a minor role, Munc13-4 is essential for regulated exocytosis in MCs, and that this MC effector response is required for a full anaphylactic response.

Role of phosphatidylinositol-4,5-bisphosphate 3-kinase signaling in vesicular trafficking

Phosphatidylinositol-4,5-bisphosphate 3-kinases (PI3Ks) are regulatory enzymes involved in the generation of lipid species that modulate cellular signaling pathways through downstream effectors to influence a variety of cellular functions. Years of intensive study of PI3Ks have produced a significant body of literature in many areas, including that PI3K can mediate intracellular vesicular trafficking and through these actions contribute to a number of important physiological functions. This review focuses on the crucial roles that PI3K and AKT, a major downstream partner of PI3K, play in the regulation of vesicle trafficking during various forms of vesicular endocytosis and exocytosis.

Proteome Analysis of the Hemolymph, Mushroom Body, and Antenna Provides Novel Insight into Honeybee Resistance against Varroa Infestation

Varroa destructor has been identified as a major culprit responsible for the losses of millions of honeybee colonies. Varroa sensitive hygiene (VSH) is a suite of behaviors from adult bees to suppress mite reproduction by uncapping and/or removing mite infested pupae from a sealed brood. Despite the efforts to elucidate the molecular underpinnings of VSH, they remain largely unknown. We investigated the proteome of mushroom bodies (MBs) and antennae of adult bees with and without VSH from a stock selected for VSH based on their response to artificially Varroa-infected brood cells by near-infrared camera observation. The pupal hemolymph proteome was also compared between the VSH-line and the line that was not selected for VSH. The identified 8609 proteins in the hemolymph, MBs, and antennae represent the most depth coverage of the honeybee proteome (>55%) to date. In the hemolymph, the VSH-line adapts a unique strategy to boost the social immunity and drive pupal organogenesis by enhancing energy metabolism and protein biosynthesis. In MBs, the up-regulated proteins implicated in neuronal sensitivity suggest their roles to promote the execution of VSH by activation of synaptic vesicles and calcium channel activities. In antennae, the highly expressed proteins associated with sensitivity of olfactory senses and signal transmissions signify their roles by inputting a strong signal to the MBs for initiating VSH. These observations illustrate that the enhanced social immunities and olfactory and neuronal sensitivity play key roles in the combat against Varroa infestation. The identified candidate markers may be useful for accelerating marker-associated selection for VSH to aid in resistance to a parasite responsible for decline in honeybee health.

Role of Intermediate Filaments in Vesicular Traffic

Intermediate filaments are an important component of the cellular cytoskeleton. The first established role attributed to intermediate filaments was the mechanical support to cells. However, it is now clear that intermediate filaments have many different roles affecting a variety of other biological functions, such as the organization of microtubules and microfilaments, the regulation of nuclear structure and activity, the control of cell cycle and the regulation of signal transduction pathways. Furthermore, a number of intermediate filament proteins have been involved in the acquisition of tumorigenic properties. Over the last years, a strong involvement of intermediate filament proteins in the regulation of several aspects of intracellular trafficking has strongly emerged. Here, we review the functions of intermediate filaments proteins focusing mainly on the recent knowledge gained from the discovery that intermediate filaments associate with key proteins of the vesicular membrane transport machinery. In particular, we analyze the current understanding of the contribution of intermediate filaments to the endocytic pathway.

Conformational states of syntaxin-1 govern the necessity of N-peptide binding in exocytosis of PC12 cells and Caenorhabditis elegans

Syntaxin-1 is the central SNARE protein for neuronal exocytosis. It interacts with Munc18-1 through its cytoplasmic domains, including the N-terminal peptide (N-peptide). Here we examine the role of the N-peptide binding in two conformational states ("closed" vs. "open") of syntaxin-1 using PC12 cells and Caenorhabditis elegans. We show that expression of "closed" syntaxin-1A carrying N-terminal single point mutations (D3R, L8A) that perturb interaction with the hydrophobic pocket of Munc18-1 rescues impaired secretion in syntaxin-1-depleted PC12 cells and the lethality and lethargy of unc-64 (C. elegans orthologue of syntaxin-1)-null mutants. Conversely, expression of the "open" syntaxin-1A harboring the same mutations fails to rescue the impairments. Biochemically, the L8A mutation alone slightly weakens the binding between "closed" syntaxin-1A and Munc18-1, whereas the same mutation in the "open" syntaxin-1A disrupts it. Our results reveal a striking interplay between the syntaxin-1 N-peptide and the conformational state of the protein. We propose that the N-peptide plays a critical role in intracellular trafficking of syntaxin-1, which is dependent on the conformational state of this protein. Surprisingly, however, the N-peptide binding mode seems dispensable for SNARE-mediated exocytosis per se, as long as the protein is trafficked to the plasma membrane.

Analysis of shape and spatial interaction of synaptic vesicles using data from focused ion beam scanning electron microscopy (FIB-SEM)

The spatial interactions of synaptic vesicles in synapses were assessed after a detailed characterization of size, shape, and orientation of the synaptic vesicles. We hypothesized that shape and orientation of the synaptic vesicles are influenced by their movement toward the active zone causing deviations from spherical shape and systematic trends in their orientation. We studied three-dimensional representations of synapses obtained by manual annotation of focused ion beam scanning electron microscopy (FIB-SEM) images of male mouse brain. The configurations of synaptic vesicles were regarded as marked point patterns, where the points are the centers of the vesicles, and the mark of a vesicle is given by its size, shape, and orientation characteristics. Statistics for marked point processes were employed to study spatial interactions between vesicles. We found that the synaptic vesicles in excitatory synapses appeared to be of oblate ellipsoid shape and in inhibitory synapses appeared to be of cigar ellipsoid shape, and followed a systematic pattern regarding their orientation toward the active zone. Moreover, there was strong evidence of spatial alignment in the orientations of pairs of synaptic vesicles, and of repulsion between them only in excitatory synapses, beyond that caused by their physical extent.

Distinct yet overlapping roles of Rab GTPases on synaptic vesicles

Exo-endocytotic cycling of synaptic vesicles (SVs) is one of the most intensely studied membrane trafficking pathways. It is governed by sets of conserved proteins including Rab GTPases. Long considered to define the identity and composition of a subcellular organelle, it has become increasingly evident that multiple Rabs co-exist on intracellular compartments, each contributing to its membrane organization and specialised function. Indeed, we have recently demonstrated that at least 11 distinct Rab proteins co-exist on highly purified SVs. These include Rabs involved in exocytosis (Rab3a/b/c and Rab27b) and intermediates of SV recycling such as early endosomes (Rab4, Rab5, Rab10, Rab11b and Rab14). Interestingly, we found that while two of these proteins, namely Rab3a and Rab27b, exhibited differential cycling dynamics on SV membranes; they played complementary roles during Ca(2+)-triggered neurotransmitter release. The implications of these findings in the SV trafficking cycle are discussed.

Quantal Basis of Secretory Granule Biogenesis and Inventory Maintenance: the Surreptitious Nano-machine Behind It

Proteins are molecular machines with the capacity to perform diverse physical work as response to signals from the environment. Proteins may be found as monomers or polymers, two states that represent an important subset of protein interactions and generate considerable functional diversity, leading to regulatory mechanisms closely akin to decision-making in service systems. Polymerization is not unique to proteins. Other cell compartments (e.g. secretory granules) or tissue states (e.g. miniature end plate potential) are associated with polymerization of some sort, leading to information transport. This data-processing mechanism has similarities with (and led us to the investigation of) granule homotypic polymerization kinetics. Using information theory, we demonstrate the role played by the heterogeneity induced by polymerization: granule size distribution and the stealthy machine behind granule life cycle increase system entropy, which modulates the source/receiver potential that affects communication between the cell and its environment. The granule inventory management by the same nano-machine is discussed.

Apocrine secretion in Drosophila salivary glands: subcellular origin, dynamics, and identification of secretory proteins

In contrast to the well defined mechanism of merocrine exocytosis, the mechanism of apocrine secretion, which was first described over 180 years ago, remains relatively uncharacterized. We identified apocrine secretory activity in the late prepupal salivary glands of Drosophila melanogaster just prior to the execution of programmed cell death (PCD). The excellent genetic tools available in Drosophila provide an opportunity to dissect for the first time the molecular and mechanistic aspects of this process. A prerequisite for such an analysis is to have pivotal immunohistochemical, ultrastructural, biochemical and proteomic data that fully characterize the process. Here we present data showing that the Drosophila salivary glands release all kinds of cellular proteins by an apocrine mechanism including cytoskeletal, cytosolic, mitochondrial, nuclear and nucleolar components. Surprisingly, the apocrine release of these proteins displays a temporal pattern with the sequential release of some proteins (e.g. transcription factor BR-C, tumor suppressor p127, cytoskeletal β-tubulin, non-muscle myosin) earlier than others (e.g. filamentous actin, nuclear lamin, mitochondrial pyruvate dehydrogenase). Although the apocrine release of proteins takes place just prior to the execution of an apoptotic program, the nuclear DNA is never released. Western blotting indicates that the secreted proteins remain undegraded in the lumen. Following apocrine secretion, the salivary gland cells remain quite vital, as they retain highly active transcriptional and protein synthetic activity.

G protein-dependent basal and evoked endothelial cell vWF secretion

von Willebrand factor (vWF) secretion by endothelial cells (ECs) is essential for hemostasis and thrombosis; however, the molecular mechanisms are poorly understood. Interestingly, we observed increased bleeding in EC-Gα13(-/-);Gα12(-/-) mice that could be normalized by infusion of human vWF. Blood from Gα12(-/-) mice exhibited significantly reduced vWF levels but normal vWF multimers and impaired laser-induced thrombus formation, indicating that Gα12 plays a prominent role in EC vWF secretion required for hemostasis and thrombosis. In isolated buffer-perfused mouse lungs, basal vWF levels were significantly reduced in Gα12(-/-), whereas thrombin-induced vWF secretion was defective in both EC-Gαq(-/-);Gα11(-/-) and Gα12(-/-) mice. Using siRNA in cultured human umbilical vein ECs and human pulmonary artery ECs, depletion of Gα12 and soluble N-ethylmaleimide-sensitive-fusion factor attachment protein α (α-SNAP), but not Gα13, inhibited both basal and thrombin-induced vWF secretion, whereas overexpression of activated Gα12 promoted vWF secretion. In Gαq, p115 RhoGEF, and RhoA-depleted human umbilical vein ECs, thrombin-induced vWF secretion was reduced by 40%, whereas basal secretion was unchanged. Finally, in vitro binding assays revealed that Gα12 N-terminal residues 10-15 mediated the binding of Gα12 to α-SNAP, and an engineered α-SNAP binding-domain minigene peptide blocked basal and evoked vWF secretion. Discovery of obligatory and complementary roles of Gα12 and Gαq/11 in basal vs evoked EC vWF secretion may provide promising new therapeutic strategies for treatment of thrombotic disease.

Function suggests nano-structure: towards a unified theory for secretion rate, a statistical mechanics approach

The inventory of secretory granules along the plasma membrane can be viewed as maintained in two restricted compartments. The release-ready pool represents docked granules available for an initial stage of fast, immediate secretion, followed by a second stage of granule set-aside secretion pool, with significantly slower rate. Transmission electron microscopy ultra-structural investigations correlated with electrophysiological techniques and mathematical modelling have allowed the categorization of these secretory vesicle compartments, in which vesicles can be in various states of secretory competence. Using the above-mentioned approaches, the kinetics of single vesicle exocytosis can be worked out. The ultra-fast kinetics, explored in this study, represents the immediately available release-ready pool, in which granules bound to the plasma membrane are exocytosed upon Ca(2+) influx at the SNARE rosette at the base of porosomes. Formalizing Dodge and Rahamimoff findings on the effect of calcium concentration and incorporating the effect of SNARE transient rosette size, we postulate that secretion rate (rate), the number (X) of intracellular calcium ions available for fusion, calcium capacity (0 ≤ M ≤ 5) and the fusion nano-machine size (as measured by the SNARE rosette size K) satisfy the parsimonious M-K relation rate ≈ C × [Ca(2+)](min(X,M))e(-K/2).

Comparative studies of Munc18c and Munc18-1 reveal conserved and divergent mechanisms of Sec1/Munc18 proteins

Sec1/Munc18 (SM) family proteins are essential for every vesicle fusion pathway. The best-characterized SM protein is the synaptic factor Munc18-1, but it remains unclear whether its functions represent conserved mechanisms of SM proteins or specialized activities in neurotransmitter release. To address this question, we dissected Munc18c, a functionally distinct SM protein involved in nonsynaptic exocytic pathways. We discovered that Munc18c binds to the trans-SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex and strongly accelerates the fusion rate. Further analysis suggests that Munc18c recognizes both vesicle-rooted SNARE and target membrane-associated SNAREs, and promotes trans-SNARE zippering at the postdocking stage of the fusion reaction. The stimulation of fusion by Munc18c is specific to its cognate SNARE isoforms. Because Munc18-1 regulates fusion in a similar manner, we conclude that one conserved function of SM proteins is to bind their cognate trans-SNARE complexes and accelerate fusion kinetics. Munc18c also binds syntaxin-4 monomer but does not block target membrane-associated SNARE assembly, in agreement with our observation that six- to eightfold increases in Munc18c expression do not inhibit insulin-stimulated glucose uptake in adipocytes. Thus, the inhibitory "closed" syntaxin binding mode demonstrated for Munc18-1 is not conserved in Munc18c. Unexpectedly, we found that Munc18c recognizes the N-terminal region of the vesicle-rooted SNARE, whereas Munc18-1 requires the C-terminal sequences, suggesting that the architecture of the SNARE/SM complex likely differs across fusion pathways. Together, these comparative studies of two distinct SM proteins reveal conserved as well as divergent mechanisms of SM family proteins in intracellular vesicle fusion.

Doc2b promotes GLUT4 exocytosis by activating the SNARE-mediated fusion reaction in a calcium- and membrane bending-dependent manner

Reconstitution of GLUT4 vesicle fusion in a defined fusion system shows that the C2-domain factor Doc2b activates the SNARE-dependent fusion reaction by a calcium- and membrane bending–dependent mechanism. Of importance, certain features of Doc2b function appear to be distinct from how synaptotagmin-1 promotes synaptic release.

The glucose transporter GLUT4 plays a central role in maintaining body glucose homeostasis. On insulin stimulation, GLUT4-containing vesicles fuse with the plasma membrane, relocating GLUT4 from intracellular reservoirs to the cell surface to uptake excess blood glucose. The GLUT4 vesicle fusion reaction requires soluble N-ethylmaleimide–sensitive factor attachment protein receptors (SNAREs) as the core fusion engine and a group of regulatory proteins. In particular, the soluble C2-domain factor Doc2b plays a key role in GLUT4 vesicle fusion, but its molecular mechanism has been unclear. Here we reconstituted the SNARE-dependent GLUT4 vesicle fusion in a defined proteoliposome fusion system. We observed that Doc2b binds to GLUT4 exocytic SNAREs and potently accelerates the fusion kinetics in the presence of Ca²⁺. The stimulatory activity of Doc2b requires intact Ca²⁺-binding sites on both the C2A and C2B domains. Using electron microscopy, we observed that Doc2b strongly bends the membrane bilayer, and this membrane-bending activity is essential to the stimulatory function of Doc2b in fusion. These results demonstrate that Doc2b promotes GLUT4 exocytosis by accelerating the SNARE-dependent fusion reaction by a Ca²⁺- and membrane bending–dependent mechanism. Of importance, certain features of Doc2b function appear to be distinct from how synaptotagmin-1 promotes synaptic neurotransmitter release, suggesting that exocytic Ca²⁺ sensors may possess divergent mechanisms in regulating vesicle fusion.

SNARE complex-mediated degranulation in mast cells

Mast cell function and dysregulation is important in the development and progression of allergic and autoimmune disease. Identifying novel proteins involved in mast cell function and disease progression is the first step in the design of new therapeutic strategies. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are a family of proteins demonstrated to mediate the transport and fusion of secretory vesicles to the membrane in mast cells, leading to the subsequent release of the vesicle cargo through an exocytotic mechanism. The functional role[s] of specific SNARE family member complexes in mast cell degranulation has not been fully elucidated. Here, we review recent and historical data on the expression, formation and localization of various SNARE proteins and their complexes in murine and human mast cells. We summarize the functional data identifying the key SNARE family members that appear to participate in mast cell degranulation. Furthermore, we discuss the utilization of RNA interference (RNAi) methods to validate SNARE function and the use of siRNA as a therapeutic approach to the treatment of inflammatory disease. These studies provide an overview of the specific SNARE proteins and complexes that serve as novel targets for the development of new therapies to treat allergic and autoimmune disease.

A novel function of Noc2 in agonist-induced intracellular Ca2+ increase during zymogen-granule exocytosis in pancreatic acinar cells

Noc2, a putative Rab effector, contributes to secretory-granule exocytosis in neuroendocrine and exocrine cells. Here, using two-photon excitation live-cell imaging, we investigated its role in Ca²⁺-dependent zymogen granule (ZG) exocytosis in pancreatic acinar cells from wild-type (WT) and Noc2-knockout (KO) mice. Imaging of a KO acinar cell revealed an expanded granular area, indicating ZG accumulation. In our spatiotemporal analysis of the ZG exocytosis induced by agonist (cholecystokinin or acetylcholine) stimulation, the location and rate of progress of ZG exocytosis did not differ significantly between the two strains. ZG exocytosis from KO acinar cells was seldom observed at physiological concentrations of agonists, but was normal (vs. WT) at high concentrations. Flash photolysis of a caged calcium compound confirmed the integrity of the fusion step of ZG exocytosis in KO acinar cells. The decreased ZG exocytosis present at physiological concentrations of agonists raised the possibility of impaired elicitation of calcium spikes. When calcium spikes were evoked in KO acinar cells by a high agonist concentration: (a) they always started at the apical portion and traveled to the basal portion, and (b) calcium oscillations over the 10 µM level were observed, as in WT acinar cells. At physiological concentrations of agonists, however, sufficient calcium spikes were not observed, suggesting an impaired [Ca²⁺]_i-increase mechanism in KO acinar cells. We propose that in pancreatic acinar cells, Noc2 is not indispensable for the membrane fusion of ZG per se, but instead performs a novel function favoring agonist-induced physiological [Ca²⁺]_i increases.

Clavulanic acid increases dopamine release in neuronal cells through a mechanism involving enhanced vesicle trafficking

Clavulanic acid is a CNS-modulating compound with exceptional blood-brain barrier permeability and safety profile. Clavulanic acid has been proposed to have anti-depressant activity and is currently entering Phase IIb clinical trials for the treatment of Major Depressive Disorder (MDD). Studies have also shown that clavulanic acid suppresses anxiety and enhances sexual functions in rodent and primate models by a mechanism involving central nervous system (CNS) modulation, although its detailed mechanism of action has yet to be elucidated. To further examine its potential as a CNS modulating agent as well as its mechanism of action, we investigated the effects of clavulanic acid in neuronal cells. Our results indicate that clavulanic acid enhances dopamine release in PC12 and SH-SY5Y cells without affecting dopamine synthesis. Furthermore, using affinity chromatography we were able to identify two proteins, Munc18-1 and Rab4 that potentially bind to clavulanic acid and play a critical role in neurosecretion and the vesicle trafficking process. Consistent with this result, an increase in the translocation of Munc18-1 and Rab4 from the cytoplasm to the plasma membrane was observed in clavulanic acid treated cells. Overall, these data suggest that clavulanic acid enhances dopamine release in a mechanism involving Munc18-1 and Rab4 modulation and warrants further investigation of its therapeutic use in CNS disorders, such as depression.

Vacuolar H(+)-ATPase subunits Voa1 and Voa2 cooperatively regulate secretory vesicle acidification, transmitter uptake, and storage

Voa1 and Voa2 cooperatively regulate the acidification and transmitter uptake/storage of dense-core vesicles, although they might not be as critical for exocytosis as recently proposed.

The Vo sector of the vacuolar H⁺-ATPase is a multisubunit complex that forms a proteolipid pore. Among the four isoforms (a1–a4) of subunit Voa, the isoform(s) critical for secretory vesicle acidification have yet to be identified. An independent function of Voa1 in exocytosis has been suggested. Here we investigate the function of Voa isoforms in secretory vesicle acidification and exocytosis by using neurosecretory PC12 cells. Fluorescence-tagged and endogenous Voa1 are primarily localized on secretory vesicles, whereas fluorescence-tagged Voa2 and Voa3 are enriched on the Golgi and early endosomes, respectively. To elucidate the functional roles of Voa1 and Voa2, we engineered PC12 cells in which Voa1, Voa2, or both are stably down-regulated. Our results reveal significant reductions in the acidification and transmitter uptake/storage of dense-core vesicles by knockdown of Voa1 and more dramatically of Voa1/Voa2 but not of Voa2. Overexpressing knockdown-resistant Voa1 suppresses the acidification defect caused by the Voa1/Voa2 knockdown. Unexpectedly, Ca²⁺-dependent peptide secretion is largely unaffected in Voa1 or Voa1/Voa2 knockdown cells. Our data demonstrate that Voa1 and Voa2 cooperatively regulate the acidification and transmitter uptake/storage of dense-core vesicles, whereas they might not be as critical for exocytosis as recently proposed.

Resolving the function of distinct Munc18-1/SNARE protein interaction modes in a reconstituted membrane fusion assay

Sec1p/Munc18 proteins and SNAP receptors (SNAREs) are key components of the intracellular membrane fusion machinery. Compartment-specific v-SNAREs on a transport vesicle pair with their cognate t-SNAREs on the target membrane and drive lipid bilayer fusion. In a reconstituted assay that dissects the sequential assembly of t-SNARE (syntaxin 1·SNAP-25) and v-/t-SNARE (VAMP2·syntaxin 1·SNAP-25) complexes, and finally measures lipid bilayer merger, we resolved the inhibitory and stimulatory functions of the Sec1p/Munc18 protein Munc18-1 at the molecular level. Inhibition of membrane fusion by Munc18-1 requires a closed conformation of syntaxin 1. Remarkably, the concurrent preincubation of Munc18-1-inhibited syntaxin 1 liposomes with both VAMP2 liposomes and SNAP-25 at low temperature releases the inhibition and effectively stimulates membrane fusion. VAMP8 liposomes can neither release the inhibition nor exert the stimulatory effect, demonstrating the need for a specific Munc18-1/VAMP2 interaction. In addition, Munc18-1 binds to the N-terminal peptide of syntaxin 1, which is obligatory for a robust stimulation of membrane fusion. In contrast, this interaction is neither required for the inhibitory function of Munc18-1 nor for the release of this block. These results indicate that Munc18-1 and the neuronal SNAREs already have the inherent capability to function as a basic stage-specific off/on switch to control membrane fusion.

Basolateral sorting of syntaxin 4 is dependent on its N-terminal domain and the AP1B clathrin adaptor, and required for the epithelial cell polarity

Generation of epithelial cell polarity requires mechanisms to sort plasma membrane proteins to the apical and basolateral domains. Sorting involves incorporation into specific vesicular carriers and subsequent fusion to the correct target membranes mediated by specific SNARE proteins. In polarized epithelial cells, the SNARE protein syntaxin 4 localizes exclusively to the basolateral plasma membrane and plays an important role in basolateral trafficking pathways. However, the mechanism of basolateral targeting of syntaxin 4 itself has remained poorly understood. Here we show that newly synthesized syntaxin 4 is directly targeted to the basolateral plasma membrane in polarized Madin-Darby canine kidney (MDCK) cells. Basolateral targeting depends on a signal that is centered around residues 24–29 in the N-terminal domain of syntaxin 4. Furthermore, basolateral targeting of syntaxin 4 is dependent on the epithelial cell-specific clathrin adaptor AP1B. Disruption of the basolateral targeting signal of syntaxin 4 leads to non-polarized delivery to both the apical and basolateral surface, as well as partial intercellular retention in the trans-Golgi network. Importantly, disruption of the basolateral targeting signal of syntaxin 4 leads to the inability of MDCK cells to establish a polarized morphology which suggests that restriction of syntaxin 4 to the basolateral domain is required for epithelial cell polarity.

Palaeontological evidence of membrane relationship in step-by-step membrane fusion

Studies on membrane fusion in living cells indicate that initiation of membrane fusion is a transient and hard to capture process. Despite previous research, membrane behaviour at this point is still poorly understood. Recent palaeobotanical research has revealed snapshots of membrane fusion in a 15-million-year-old fossil pinaceous cone. To reveal the membrane behaviour during the fusion, we conducted more observations on the same fossil material. Several discernible steps of membrane fusion have been fixed naturally and observed in the fossil material. This observation provides transmission electron microscope (TEM) images of the transient intermediate stage and clearly shows the relationship between membranes. Observing such a transient phenomenon in fossil material implies that the fixing was most likely accomplished quickly by a natural process. The mechanism behind this phenomenon is clearly worthy of further enquiry.

Small-interfering RNA-mediated identification and regulation of the ternary SNARE complex mediating RBL-2H3 mast cell degranulation

Dysregulation of mast cell function contributes to allergic and autoimmune disease that affects more than 70 million persons in the United States alone. Identifying novel mast cell targets that mediate disease or disease progression is required for the development of innovative therapeutics for the treatment of allergy/asthma and autoimmune disease. RNA interference technologies offer hope both as basic research tools for target identification and as potential, novel, specific therapeutics. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are a family of evolutionarily conserved proteins that have been postulated to mediate the transport and fusion of inflammatory mediator-laden vesicles to the membrane in mast cells leading to their subsequent exocytosis. The functional role(s) of specific SNARE family member complexes in mast cell degranulation has not been fully elucidated. Here, we characterize the functional importance of SNARE complexes in FcεRI receptor-mediated degranulation of RBL-2H3 cells utilizing RNA interference. We demonstrate that ternary SNARE complexes of synaptosomal-associated protein-23, Syntaxin 4 and vesicle-associated membrane protein-7 (VAMP-7) or VAMP-8 are directly involved in mast cell degranulation. Additionally, we evaluate the siRNAs directed against these molecules as potential therapeutic agents for disease intervention. These studies have identified specific SNARE proteins and complexes that serve as novel targets for the development of siRNA therapies to treat allergic and autoimmune disease.

The "acrosomal synapse": Subcellular organization by lipid rafts and scaffolding proteins exhibits high similarities in neurons and mammalian spermatozoa

Mammalian spermatozoa are highly polarized cells composed of two morphological and functional units, each optimized for a special task. Although the apparent division into head and tail may as such represent the anatomical basis to avoid random diffusion of their special sets of signaling proteins and lipids, recent findings demonstrate the presence of lipid raft-derived membrane platforms and specific scaffolding proteins, thus indicating that smaller sub-domains exist in the two functional units of male germ cells. The aim of this review is to summarize new insights into the principles of subcellular organization in mammalian spermatozoa. Special emphasis is placed on recent observations indicating that an "acrosomal synapse" is formed by lipid raft-derived membrane micro-environments and multidomain scaffolding proteins. Both mechanisms appear to be responsible for ensuring the attachment of the huge acrosomal vesicle to the overlaying plasma membrane, as well as for preventing an accidental spontaneous loss of the single acrosome.

SNARE bundle and syntaxin N-peptide constitute a minimal complement for Munc18-1 activation of membrane fusion

Whittling away SNARE complex components reveals essential domains for Munc18-1–mediated membrane fusion.

Sec1/Munc18 (SM) proteins activate intracellular membrane fusion through binding to cognate SNAP receptor (SNARE) complexes. The synaptic target membrane SNARE syntaxin 1 contains a highly conserved H_abc domain, which connects an N-peptide motif to the SNARE core domain and is thought to participate in the binding of Munc18-1 (the neuronal SM protein) to the SNARE complex. Unexpectedly, we found that mutation or complete removal of the H_abc domain had no effect on Munc18-1 stimulation of fusion. The central cavity region of Munc18-1 is required to stimulate fusion but not through its binding to the syntaxin H_abc domain. SNAP-25, another synaptic SNARE subunit, contains a flexible linker and exhibits an atypical conjoined Q_bc configuration. We found that neither the linker nor the Q_bc configuration is necessary for Munc18-1 promotion of fusion. As a result, Munc18-1 activates a SNARE complex with the typical configuration, in which each of the SNARE core domains is individually rooted in the membrane bilayer. Thus, the SNARE four-helix bundle and syntaxin N-peptide constitute a minimal complement for Munc18-1 activation of fusion.

CaMKIIalpha interacts with multi-PDZ domain protein MUPP1 in spermatozoa and prevents spontaneous acrosomal exocytosis

The success of acrosomal exocytosis, a complex process with a variety of inter-related steps, relies on the coordinated interaction of participating signaling molecules. Since the acrosome reaction resembles Ca(2+)-regulated exocytosis in neurons, we investigated whether cognate neuronal binding partners of the multi-PDZ domain protein MUPP1, which recruits molecules that control the initial tethering and/or docking between the acrosomal vesicle and the plasma membrane, are also expressed in spermatozoa, and whether they contribute to the regulation of acrosomal secretion. We observed that CaMKIIalpha colocalizes with MUPP1 in the acrosomal region of epididymal spermatozoa where the kinase selectively binds to a region encompassing PDZ domains 10-11 of MUPP1. Furthermore, we found that pre-treating mouse spermatozoa with a CaMKII inhibitor that directly blocks the catalytic region of the kinase, as well as a competitive displacement of CaMKIIalpha from PDZ domains 10-11, led to a significant increase in spontaneous acrosomal exocytosis. Since Ca(2+)-calmodulin releases CaMKIIalpha from the PDZ scaffolding protein, MUPP1 represents a central signaling platform to dynamically regulate the assembly and disassembly of binding partners pertinent to acrosomal secretion, thereby precisely adjusting an increase in Ca(2+) to synchronized fusion pore formation.

Lipid mixing and content release in single-vesicle, SNARE-driven fusion assay with 1-5 ms resolution

A single-vesicle, fluorescence-based, SNARE-driven fusion assay enables simultaneous measurement of lipid mixing and content release with 5 ms/frame, or even 1 ms/frame, time resolution. The v-SNARE vesicles, labeled with lipid and content markers of different color, dock and fuse with a planar t-SNARE bilayer supported on glass. A narrow (<5 ms duration), intense spike of calcein fluorescence due to content release and dequenching coincides with inner-leaflet lipid mixing within 10 ms. The spike provides more sensitive detection of productive hemifusion events than do lipid labels alone. Consequently, many fast events previously thought to be prompt, full fusion events are now reclassified as productive hemifusion. Both full fusion and hemifusion occur with a time constant of 5-10 ms. At 60% phosphatidylethanolamine lipid composition, productive and dead-end hemifusion account for 65% of all fusion events. However, quantitative analysis shows that calcein is released into the space above the bilayer (vesicle bursting), rather than the thin aqueous space between the bilayer and glass. Evidently, at the instant of inner-leaflet mixing, flattening of the vesicle increases the internal pressure beyond the bursting point. This may be related to in vivo observations suggesting that membrane lysis often competes with membrane fusion.

Proteomic characterization of lipid raft proteins in amyotrophic lateral sclerosis mouse spinal cord

Familial amyotrophic lateral sclerosis (ALS) has been linked to mutations in the copper/zinc superoxide dismutase (SOD1) gene. The mutant SOD1 protein exhibits a toxic gain-of-function that adversely affects the function of neurons. However, the mechanism by which mutant SOD1 initiates ALS is unclear. Lipid rafts are specialized microdomains of the plasma membrane that act as platforms for the organization and interaction of proteins involved in multiple functions, including vesicular trafficking, neurotransmitter signaling, and cytoskeletal rearrangements. In this article, we report a proteomic analysis using a widely used ALS mouse model to identify differences in spinal cord lipid raft proteomes between mice overexpressing wild-type (WT) and G93A mutant SOD1. In total, 413 and 421 proteins were identified in the lipid rafts isolated from WT and G93A mice, respectively. Further quantitative analysis revealed a consortium of proteins with altered levels between the WT and G93A samples. Functional classification of the 67 altered proteins revealed that the three most affected subsets of proteins were involved in: vesicular transport, and neurotransmitter synthesis and release; cytoskeletal organization and linkage to the plasma membrane; and metabolism. Other protein changes were correlated with alterations in: microglia activation and inflammation; astrocyte and oligodendrocyte function; cell signaling; cellular stress response and apoptosis; and neuronal ion channels and neurotransmitter receptor functions. Changes of selected proteins were independently validated by immunoblotting and immunohistochemistry. The significance of the lipid raft protein changes in motor neuron function and degeneration in ALS is discussed, particularly for proteins involved in vesicular trafficking and neurotransmitter signaling, and the dynamics and regulation of the plasma membrane-anchored cytoskeleton.

Friends and foes in synaptic transmission: the role of tomosyn in vesicle priming

Priming is the process by which vesicles become available for fusion at nerve terminals and is modulated by numerous proteins and second messengers. One of the prominent members of this diverse family is tomosyn. Tomosyn has been identified as a syntaxin-binding protein; it inhibits vesicle priming, but its mode of action is not fully understood. The inhibitory activity of tomosyn depends on its N-terminal WD40-repeat domain and is regulated by the binding of its SNARE motif to syntaxin. Here, we describe new physiological information on the function of tomosyn and address possible interpretations of these results in the framework of the recently described crystal structure of the yeast tomosyn homolog Sro7. We also present possible molecular scenarios for vesicle priming and the involvement of tomosyn in these processes.

The JIP3 scaffold protein UNC-16 regulates RAB-5 dependent membrane trafficking at C. elegans synapses

How endosomes contribute to the maintenance of vesicular structures at presynaptic terminals remains controversial and poorly understood. Here, we have investigated synaptic endosomal compartments in the presynaptic terminals of C. elegans GABAergic motor neurons. Using RAB reporters, we find that several subsynaptic compartments reside in, or near, presynaptic regions. Loss of function in the C. elegans JIP3 protein, UNC-16, causes a RAB-5-containing compartment to accumulate abnormally at presynaptic terminals. Ultrastructural analysis shows that synapses in unc-16 mutants contain reduced number of synaptic vesicles, accompanied by an increase in the size and number of cisternae. FRAP analysis revealed a slow recovery of RAB-5 in unc-16 mutants, suggestive of an impairment of RAB-5 activity state and local vesicular trafficking. Overexpression of RAB-5:GDP partially suppresses, whereas overexpression of RAB-5:GTP enhances, the synaptic defects of unc-16 mutants. Our data demonstrate a novel function of UNC-16 in the regulation of synaptic membrane trafficking and suggest that the synaptic RAB-5 compartment contributes to synaptic vesicle biogenesis or maintenance.

The proteome of the presynaptic active zone: from docked synaptic vesicles to adhesion molecules and maxi-channels

The presynaptic proteome controls neurotransmitter release and the short and long term structural and functional dynamics of the nerve terminal. Using a monoclonal antibody against synaptic vesicle protein 2 we immunopurified a presynaptic compartment containing the active zone with synaptic vesicles docked to the presynaptic plasma membrane as well as elements of the presynaptic cytomatrix. Individual protein bands separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were subjected to nanoscale-liquid chromatography electrospray ionization-tandem mass spectrometry. Combining this method with 2-dimensional benzyldimethyl-n-hexadecylammonium chloride/sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization time of flight and immunodetection we identified 240 proteins comprising synaptic vesicle proteins, components of the presynaptic fusion and retrieval machinery, proteins involved in intracellular signal transduction, a large variety of adhesion molecules and proteins potentially involved in regulating the functional and structural dynamics of the pre-synapse. Four maxi-channels, three isoforms of voltage-dependent anion channels and the tweety homolog 1 were co-isolated with the docked synaptic vesicles. As revealed by in situ hybridization, tweety homolog 1 reveals a distinct expression pattern in the rodent brain. Our results add novel information to the proteome of the presynaptic active zone and suggest that in particular proteins potentially involved in the short and long term structural modulation of the mature presynaptic compartment deserve further detailed analysis.

Biogenesis of Dense-Core Secretory Granules

Dense core granules (DCGs) are vesicular organelles derived from outbound traffic through the eukaryotic secretory pathway. As DCGs are formed, the secretory pathway can also give rise to other types of vesicles, such as those bound for endosomes, lysosomes, and the cell surface. DCGs differ from these other vesicular carriers in both content and function, storing highly concentrated cores’ of condensed cargo in vesicles that are stably maintained within the cell until a specific extracellular stimulus causes their fusion with the plasma membrane. These unique features are imparted by the activities of membrane and lumenal proteins that are specifically delivered to the vesicles during synthesis. This chapter will describe the DCG biogenesis pathway, beginning with the sorting of DCG proteins from proteins that are destined for other types of vesicle carriers. In the trans-Golgi network (TGN), sorting occurs as DCG proteins aggregate, causing physical separation from non-DCG proteins. Recent work addresses the nature of interactions that produce these aggregates, as well as potentially important interactions with membranes and membrane proteins. DCG proteins are released from the TGN in vesicles called immature secretory granules (ISGs). The mechanism of ISG formation is largely unclear but is not believed to rely on the assembly of vesicle coats like those observed in other secretory pathways. The required cytosolic factors are now beginning to be identified using in vitro systems with purified cellular components. ISG transformation into a mature fusion-competent, stimulus-dependent DCG occurs as endoproteolytic processing of many DCG proteins causes continued condensation of the lumenal contents. At the same time, proteins that fail to be incorporated into the condensing core are removed by a coat-mediated budding mechanism, which also serves to remove excess membrane and membrane proteins from the maturing vesicle. This chapter will summarize the work leading to our current view of granule synthesis, and will discuss questions that need to be addressed in order to gain a more complete understanding of the pathway.

Prolactin secretion sites contain syntaxin-1 and differ from ganglioside monosialic acid rafts in rat lactotrophs

In neuroendocrine cells, discharge of hormones follows the fusion of exocytotic vesicles with the plasma membrane at confined sites; however, the molecular nature of these distinct sites remains poorly understood. We studied intact pituitary lactotrophs and plasma membrane lawns by confocal microscopy in conjunction with antibodies against rat prolactin (rPRL), soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) proteins (syntaxin-1 and synaptobrevin-2,) and fluorescent cholera toxin subunit B (CT-B), a marker of ganglioside monosialic acid (GM1) lipid rafts, to examine 1) whether rPRL vesicles discharge cargo at GM1 rafts, 2) whether discharging rPRL vesicles interact with SNAREs, and 3) to examine the overlap of GM1 rafts, rPRL, and syntaxin-1 sites in plasma membrane lawns. In intact cells, immunofluorescently labeled rPRL poorly colocalized (<6%) with CT-B. In conditions favoring endocytotic trafficking, vesicle SNARE synaptobrevin-2 modestly colocalized (35%) with CT-B, whereas it highly colocalized (58%) with retrieved rPRL. Although partial mixing between rPRL and CT-B intracellular trafficking pathways is likely, our results indicated that rPRL discharge involves interactions with plasma membrane SNAREs, but not with GM1 rafts. In support of this, the plasma membrane SNARE syntaxin-1 poorly colocalized with CT-B (<5%), whereas it highly colocalized (75%) with rPRL in inside-out plasma membrane lawns. Spontaneous and stimulated rPRL discharge in live lactotrophs is thus associated with plasma membrane sites enriched with SNARE proteins, however, spatially confined to plasma membrane areas other than GM1 rafts.

Identifying the targets of the amplifying pathway for insulin secretion in pancreatic beta-cells by kinetic modeling of granule exocytosis

A kinetic model for insulin secretion in pancreatic beta-cells is adapted from a model for fast exocytosis in chromaffin cells. The fusion of primed granules with the plasma membrane is assumed to occur only in the "microdomain" near voltage-sensitive L-type Ca(2+)-channels, where [Ca(2+)] can reach micromolar levels. In contrast, resupply and priming of granules are assumed to depend on the cytosolic [Ca(2+)]. Adding a two-compartment model to handle the temporal distribution of Ca(2+) between the microdomain and the cytosol, we obtain a unified model that can generate both the fast granule fusion and the slow insulin secretion found experimentally in response to a step of membrane potential. The model can simulate the potentiation induced in islets by preincubation with glucose and the reduction in second-phase insulin secretion induced by blocking R-type Ca(2+)-channels (Ca(V)2.3). The model indicates that increased second-phase insulin secretion induced by the amplifying signal is controlled by the "resupply" step of the exocytosis cascade. In contrast, enhancement of priming is a good candidate for amplification of first-phase secretion by glucose, cyclic adenosine 3':5'-cyclic monophosphate, and protein kinase C. Finally, insulin secretion is enhanced when the amplifying signal oscillates in phase with the triggering Ca(2+)-signal.

Regulation of the expression of components of the exocytotic machinery of insulin-secreting cells by microRNAs

Fine-tuning of insulin secretion from pancreatic β-cells participates in blood glucose homeostasis. Defects in this process can lead to chronic hyperglycemia and diabetes mellitus. Several proteins controlling insulin exocytosis have been identified, but the mechanisms regulating their expression remain poorly understood. Here, we show that two non-coding microRNAs, miR124a and miR96, modulate the expression of proteins involved in insulin exocytosis and affect secretion of the β-cell line MIN6B1. miR124a increases the levels of SNAP25, Rab3A and synapsin-1A and decreases those of Rab27A and Noc2. Inhibition of Rab27A expression is mediated by direct binding to the 3′-untranslated region of Rab27A mRNA. The effect on the other genes is indirect and linked to changes in mRNA levels. Over-expression of miR124a leads to exaggerated hormone release under basal conditions and a reduction in glucose-induced secretion. miR96 increases mRNA and protein levels of granuphilin, a negative modulator of insulin exocytosis, and decreases the expression of Noc2, resulting in lower capacity of MIN6B1 cells to respond to secretagogues. Our data identify miR124a and miR96 as novel regulators of the expression of proteins playing a critical role in insulin exocytosis and in the release of other hormones and neurotransmitters.

Evolution of insect proteomes: insights into synapse organization and synaptic vesicle life cycle

BACKGROUND

The molecular components in synapses that are essential to the life cycle of synaptic vesicles are well characterized. Nonetheless, many aspects of synaptic processes, in particular how they relate to complex behaviour, remain elusive. The genomes of flies, mosquitoes, the honeybee and the beetle are now fully sequenced and span an evolutionary breadth of about 350 million years; this provides a unique opportunity to conduct a comparative genomics study of the synapse.

RESULTS

We compiled a list of 120 gene prototypes that comprise the core of presynaptic structures in insects. Insects lack several scaffolding proteins in the active zone, such as bassoon and piccollo, and the most abundant protein in the mammalian synaptic vesicle, namely synaptophysin. The pattern of evolution of synaptic protein complexes is analyzed. According to this analysis, the components of presynaptic complexes as well as proteins that take part in organelle biogenesis are tightly coordinated. Most synaptic proteins are involved in rich protein interaction networks. Overall, the number of interacting proteins and the degrees of sequence conservation between human and insects are closely correlated. Such a correlation holds for exocytotic but not for endocytotic proteins.

CONCLUSION

This comparative study of human with insects sheds light on the composition and assembly of protein complexes in the synapse. Specifically, the nature of the protein interaction graphs differentiate exocytotic from endocytotic proteins and suggest unique evolutionary constraints for each set. General principles in the design of proteins of the presynaptic site can be inferred from a comparative study of human and insect genomes.

Compartmentalized cAMP signalling in regulated exocytic processes in non-neuronal cells

Cyclic adenosine monophosphate (cAMP) is a central second messenger controlling a plethora of vital functions. Studies of cAMP dynamics in living cells have revealed markedly inhomogeneous concentrations of the second messenger in different compartments. Moreover, cAMP effectors such as cAMP-dependent protein kinase (PKA) and cAMP-activated GTP-exchange factors (Epacs) are tethered to specific cellular sites. Both the tailoring of cAMP concentrations, and the activities of cAMP-dependent signalling systems at specific cellular locations are prerequisites for most, if not all, cAMP-dependent processes. This review focuses on the role of compartmentalized cAMP signalling in exocytic processes in non-neuronal cells. Particularly, the insertion of aquaporin-2 into the plasma membrane of renal principal cells as an example for a cAMP-dependent exocytic process in a non-secretory cell type, renin secretion from juxtaglomerular cells as a cAMP-triggered exocytosis from an endocrine cell, insulin release from pancreatic beta-cells as a Ca2+-mediated and cAMP-potentiated exocytic processes in an endocrine cell, and cAMP- or Ca2+ -triggered H+ secretion from gastric parietal cells as an exocytic process in an exocrine cell are discussed. The selected examples of cAMP-regulated exocytic pathways are reviewed with regard to key proteins involved: adenylyl cyclases, phosphodiesterases, PKA, A kinase anchoring proteins (AKAPs) and Epacs.

Fluctuations of hippocampal neuronal protein levels over the estrous cycle in the rat

Hippocampal function is known to be estrous-cycle-dependent but information on estrous-cycle-dependent protein expression is limited. It was therefore the aim to study protein levels of the neuronal network over the estrous cycle in the hippocampus of female rats and in males showing protein chemical neuroanatomy in this area. Female and male OFA Sprague-Dawley rats were used and females were grouped to proestrous, estrous, metestrous and diestrous by using vaginal smears. Hippocampal tissue was taken, proteins extracted, run on two-dimensional gel electrophoresis and proteins were identified by mass spectrometry methods (MALDI-TOF-TOF and nano-LC-ESI-MS/MS). Spot volumes were quantified with specific software. A Synapsin-1 expression form was differentially regulated between proestrous and diestrous, a Synapsin IIa expression form was differentially regulated between proestrous and metestrous, the sum of ERC-2 proteins organizing the cytomatrix at the nerve terminals active zone was showing sex-dependent levels in the proestrous phase and Neurofilament triplet L protein was differentially expressed between the estrous phase and males. The findings may represent estrous-cycle-dependent hippocampal synaptic function that has been shown already in terms of electrophysiology and neuroanatomy. Neurofilament changes over the estrous cycle may reflect endoskeleton changes over the estrous cycle. We learn from this study, although increasing complexity of protein knowledge, that the estrous cycle and not only the sex per se has to be taken into account for design of future studies and interpretation of previous work at the protein level.

Productive hemifusion intermediates in fast vesicle fusion driven by neuronal SNAREs

An in vitro fusion assay uses fluorescence microscopy of labeled lipids to monitor single v-SNARE vesicle docking and fusion events on a planar lipid bilayer containing t-SNAREs. For vesicles and bilayer comprising phosphatidylcholine (POPC, 84-85% by mol) and phosphatidylserine (DOPS, 15% by mol), previous work demonstrated prompt, full fusion (tau(fus) = 25 ms). Substitution of 20-60% phosphatidylethanolamine (DOPE) for phosphatidylcholine in the v-SNARE vesicle with either 0 or 20% DOPE included in the t-SNARE bilayer gives rise to hemifusion events. Labeled lipids diffuse into the planar bilayer as two temporally distinct waves, presumably hemifusion of the outer leaflet followed by inner leaflet (core) fusion. The fusion kinetics with DOPE is markedly heterogeneous. Some vesicle/docking site pairs exhibit prompt, full fusion while others exhibit hemifusion. Hemifusion events are roughly half productive (leading to subsequent core fusion within 20 s) and half dead-end. In qualitative accord with expectations from studies of protein-free vesicle-vesicle fusion, the hemifusion rate k(hemi) is 15-20 times faster than the core fusion rate k(core), and the fraction of hemifusion events increases with increasing percentage of DOPE. This suggests similar underlying molecular pathways for protein-free and neuronal SNARE-driven fusion. Removal of phosphatidylserine from the v-SNARE vesicle has no effect on docking or fusion.

Inhibition of N-ethylmaleimide-sensitive factor protects against myocardial ischemia/reperfusion injury

Exocytosis of endothelial granules promotes thrombosis and inflammation and may contribute to the pathophysiology of early reperfusion injury following myocardial ischemia. TAT-NSF700 is a novel peptide that reduces endothelial exocytosis by inhibiting the ATPase activity and disassembly activity of N-ethylmaleimide-sensitive factor (NSF), a critical component of the exocytic machinery. We hypothesized that TAT-NSF700 would limit myocardial injury in an in vivo murine model of myocardial ischemia/reperfusion injury. Mice were subjected to 30 minutes of ischemia followed by 24 hours of reperfusion. TAT-NSF700 or the scrambled control peptide TAT-NSF700scr was administered intravenously 20 minutes before the onset of ischemia. Myocardial ischemia/reperfusion caused endothelial exocytosis, myocardial infarction, and left ventricular dysfunction. However, TAT-NSF700 decreased von Willebrand factor levels after myocardial ischemia/reperfusion, attenuated myocardial infarct size by 47%, and preserved left ventricular structure and function. These data suggest that drugs targeting endothelial exocytosis may be useful in the treatment of myocardial injury following ischemia/reperfusion.

Vesicular traffic at the cell membrane regulates oocyte meiotic arrest

Vertebrate oocytes are maintained in meiotic arrest for prolonged periods of time before undergoing oocyte maturation in preparation for fertilization. Cyclic AMP (cAMP) signaling plays a crucial role in maintaining meiotic arrest, which is released by a species-specific hormonal signal. Evidence in both frog and mouse argues that meiotic arrest is maintained by a constitutively active G-protein coupled receptor (GPCR) leading to high cAMP levels. Because activated GPCRs are typically targeted for endocytosis as part of the signal desensitization pathway, we were interested in determining the role of trafficking at the cell membrane in maintaining meiotic arrest. Here we show that blocking exocytosis, using a dominant-negative SNAP25 mutant in Xenopus oocytes, releases meiotic arrest independently of progesterone. Oocyte maturation in response to the exocytic block induces the MAPK and Cdc25C signaling cascades, leading to MPF activation, germinal vesicle breakdown and arrest at metaphase of meiosis II with a normal bipolar spindle. It thus replicates all tested aspects of physiological maturation. Furthermore, inhibiting clathrin-mediated endocytosis hinders the effectiveness of progesterone in releasing meiotic arrest. These data show that vesicular traffic at the cell membrane is crucial in maintaining meiotic arrest in vertebrates, and support the argument for active recycling of a constitutively active GPCR at the cell membrane.

Identification and characterization of SV31, a novel synaptic vesicle membrane protein and potential transporter

Synaptic vesicle proteins govern all relevant functions of the synaptic vesicle life cycle, including vesicle biogenesis, vesicle transport, uptake and storage of neurotransmitters, and regulated endocytosis and exocytosis. In spite of impressive progress made in the past years, not all known vesicular functions can be assigned to defined protein components, suggesting that the repertoire of synaptic vesicle proteins is still incomplete. We have identified and characterized a novel synaptic vesicle membrane protein of 31 kDa with six putative transmembrane helices that, according to its membrane topology and phylogenetic relation, may function as a vesicular transporter. The vesicular allocation is demonstrated by subcellular fractionation, heterologous expression, immunocytochemical analysis of brain sections and immunoelectron microscopy. The protein is expressed in select brain regions and contained in subpopulations of nerve terminals that immunostain for the vesicular glutamate transporter 1 and the vesicular GABA transporter VGaT (vesicular amino acid transporter) and may attribute specific and as yet undiscovered functions to subsets of glutamatergic and GABAergic synapses.

Role of Habc domain in membrane trafficking and targeting of syntaxin 1A

Membrane syntaxin plays essential roles in exocytosis in eukaryotic cells. The conservative H(abc) domain in plasma membrane syntaxins implies important roles for syntaxin targeting and function. Our previous study showed H(abc) domain was necessary for the trafficking and cluster distribution of syntaxin 1A on the plasma membrane. Here we identified which of the three domains (H(a), H(b) and H(c)) was essential for Stx1A trafficking and clustering. We found that, in INS-1 cells, the mutant truncated with either H(a), H(b) or H(c) domain could be sorted to the cell surface by a different mechanism compared to that of whole H(abc) truncated mutant. In contrast to wild type Stx1A, none of the mutants showed cluster distribution at the functional sites, suggesting that the physiological localization of Stx1A relies on intact H(abc) domain. Furthermore Munc18-1 is found not to be essential for Stx1A cluster distribution, despite important role in stabilizing membrane delivery of Stx1A.

The Multi-PDZ domain protein MUPP1 as a lipid raft-associated scaffolding protein controlling the acrosome reaction in mammalian spermatozoa

The success of acrosomal exocytosis, a complex process with a variety of interrelated steps, relies on the coordinated interaction of participating signaling molecules. Since scaffolding proteins are known to spatially organize sequential signaling pathways, we examined whether the Multi-PDZ domain protein MUPP1, recently identified in mammalian spermatozoa, is functionally active in controlling acrosomal secretion in mammalian sperm cells. To address this question, permeabilized mouse sperm were loaded with inhibitory antibodies against MUPP1 as well as with a photosensitive Ca(2+) chelator which allows a controlled release of acrosomal Ca(2+). The results revealed that MUPP1 controls initial tethering and docking of the acrosomal vesicle, whereas syntaxin 2, a t-SNARE protein also expressed in the acrosomal cap of mammalian spermatozoa, appears to take part in the final process of acrosomal fusion. Interestingly, using immunogold electron microscopy, it was found that MUPP1 is detectable in the region of the periacrosomal membrane. Furthermore, in isolated detergent-insoluble glycolipid-enriched membrane domains from epididymal spermatozoa, MUPP1 was found to show a striking association with the Triton X-100 insoluble membrane fraction, which did not change significantly upon sperm capacitation or partial chemical extraction of cholesterol. This evidence points to a role of MUPP1 as a membrane raft-associated molecular organizer, and suggests that mammalian spermatozoa may use a scaffolding protein and distinct membrane subdomains to spatially organize components involved in the process of acrosomal exocytosis.

Analysis of the synaptic vesicle proteome using three gel-based protein separation techniques

Synaptic vesicles are key organelles in neurotransmission. Their functions are governed by a unique set of integral and peripherally associated proteins. To obtain a complete protein inventory, we immunoisolated synaptic vesicles from rat brain to high purity and performed a gel-based analysis of the synaptic vesicle proteome. Since the high hydrophobicity of integral membrane proteins hampers their resolution by gel electrophoretic techniques, we applied in parallel three different gel electrophoretic methods for protein separation prior to MS. Synaptic vesicle proteins were subjected to either 1-D SDS-PAGE along with nano-LC ESI-MS/MS or to the 2-D gel electrophoretic techniques benzyldimethyl-n-hexadecylammonium chloride (BAC)/SDS-PAGE, and double SDS (dSDS)-PAGE in combination with MALDI-TOF-MS. We demonstrate that the combination of all three methods provides a comprehensive survey of the proteinaceous inventory of the synaptic vesicle membrane compartment. The identified synaptic vesicle proteins include transporters, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), synapsins, rab and rab-interacting proteins, additional guanine nucleotide triphosphate (GTP) binding proteins, cytoskeletal proteins, and proteins modulating synaptic vesicle exo- and endocytosis. In addition, we identified novel proteins of unknown function. Our results demonstrate that the parallel application of three different gel-based approaches in combination with mass spectrometry permits a comprehensive analysis of the synaptic vesicle proteome that is considerably more complex than previously anticipated.

Mechanisms controlling the expression of the components of the exocytotic apparatus under physiological and pathological conditions

The last decade has witnessed spectacular progress in the identification of the protein apparatus required for exocytosis of neurotransmitters, peptide hormones and other bioactive products. In striking contrast, our knowledge of the mechanisms determining the expression of the components of the secretory machinery has remained rudimentary. Since modifications in secretory functions are associated with several physiological processes and contribute to the development of human pathologies, a better knowledge of the control of the expression of the genes involved in exocytosis is urgently needed. Recent studies have led to the identification of transcription factors and other regulatory molecules such as microRNAs that modulate the cellular level of key controllers of the exocytotic process. These findings furnish a new perspective for understanding how secretory functions can adapt to normal physiological conditions and shed light on the mechanisms involved in the development of important human diseases such as diabetes mellitus characterized by defective release of bioactive compounds.