The molecular mechanisms of the mammalian exocyst complex in exocytosis

Dense granule biogenesis, secretion, and function in Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite and the causative agent of Toxoplasmosis. A key to understanding and treating the disease lies with determining how the parasite can survive and replicate within cells of its host. Proteins released from specialized secretory vesicles, named the dense granules (DGs), have diverse functions that are critical for adapting the intracellular environment, and are thus key to survival and pathogenicity. In this review, we describe the current understanding and outstanding questions regarding dense granule biogenesis, trafficking, and regulation of secretion. In addition, we provide an overview of dense granule protein ("GRA") function upon secretion, with a focus on proteins that have recently been identified.

Kinase suppressor of Ras 1 and Exo70 promote fatty acid-stimulated neurotensin secretion through ERK1/2 signaling

Neurotensin is a peptide hormone released from enteroendocrine cells in the small intestine in response to fat ingestion. Although the mechanisms regulating neurotensin secretion are still incompletely understood, our recent findings implicate a role for extracellular signal-regulated kinase 1 and 2 as positive regulators of free fatty acid-stimulated neurotensin secretion. Previous studies have shown that kinase suppressor of Ras 1 acts as a molecular scaffold of the Raf/MEK/extracellular signal-regulated kinase 1 and 2 kinase cascade and regulates intensity and duration of extracellular signal-regulated kinase 1 and 2 signaling. Here, we demonstrate that inhibition of kinase suppressor of Ras 1 attenuates neurotensin secretion and extracellular signal-regulated kinase 1 and 2 signaling in human endocrine cells. Conversely, we show that overexpression of kinase suppressor of Ras 1 enhances neurotensin secretion and extracellular signal-regulated kinase 1 and 2 signaling. We also show that inhibition of extracellular signal-regulated kinase 2 and exocyst complex component 70, a substrate of extracellular signal-regulated kinase 2 and mediator of secretory vesicle exocytosis, potently inhibits basal and docosahexaenoic acid-stimulated neurotensin secretion, whereas overexpression of exocyst complex component 70 enhances basal and docosahexaenoic acid-stimulated neurotensin secretion. Together, our findings demonstrate a role for kinase suppressor of Ras 1 as a positive regulator of neurotensin secretion from human endocrine cells and indicate that this effect is mediated by the extracellular signal-regulated kinase 1 and 2 signaling pathway. Moreover, we reveal a novel role for exocyst complex component 70 in regulation of neurotensin vesicle exocytosis through its interaction with the extracellular signal-regulated kinase 1 and 2 signaling pathway.

Off the wall: The rhyme and reason of Neurospora crassa hyphal morphogenesis

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Chitin and β-1,3-glucan synthases are transported separately in chitosomes and macrovesicles.

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Chitin synthases occupy the core of the SPK; β-1,3-glucan synthases the outer layer.

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CHS-4 arrival to the SPK and septa is CSE-7 dependent.

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Rabs YPT-1 and YPT-31 localization at the SPK mimics that of chitosomes and macrovesicles.

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The exocyst acts as a tether between the SPK outer layer vesicles and the apical PM.

The fungal cell wall building processes are the ultimate determinants of hyphal shape. In Neurospora crassa the main cell wall components, β-1,3-glucan and chitin, are synthesized by enzymes conveyed by specialized vesicles to the hyphal tip. These vesicles follow different secretory routes, which are delicately coordinated by cargo-specific Rab GTPases until their accumulation at the Spitzenkörper. From there, the exocyst mediates the docking of secretory vesicles to the plasma membrane, where they ultimately get fused. Although significant progress has been done on the cellular mechanisms that carry cell wall synthesizing enzymes from the endoplasmic reticulum to hyphal tips, a lot of information is still missing. Here, the current knowledge on N. crassa cell wall composition and biosynthesis is presented with an emphasis on the underlying molecular and cellular secretory processes.

In vitro study of putative genomic biomarkers of nephrotoxicity through differential gene expression using gentamicin

Drug-induced nephrotoxicity is one of the most frequently observed effects in long-term pharmacotherapy. The effects of nephrotoxicity are commonly discovered later due to a lack of sensitivity in in vivo methods. Therefore, researchers have tried to develop in vitro alternative methods for early identification of toxicity. In this study, LLC-PK1 cells were exposed to gentamicin through MTT and trypan blue assay. Concentrations of 4 (low), 8 (medium) and 12 (high) mM, were used to evaluate differential gene expression. A panel of genes was selected based on gene expression changes. The search for sequences of mRNA encoding proteins previously associated with kidney damage was conducted in the databases of the National Center for Biotechnology Information (USA). RNA was extracted from the cells, and RT-qPCR was performed to evaluate differential expression profiles of the selected genes. Among the 11 analyzed genes, four proved to be differentially up-regulated in cells exposed to gentamicin: HAVcr1, caspase 3, ICAM-1 and EXOC6. According to this study's results, we suggest that these genes play an important role in the mechanism of in vitro nephrotoxicity caused by gentamicin and can be used as early in vitro biomarkers to identify nephrotoxicity when developing safer drugs.

The Par3 polarity protein is an exocyst receptor essential for mammary cell survival

The exocyst is an essential component of the secretory pathway required for delivery of basolateral proteins to the plasma membranes of epithelial cells. Delivery occurs adjacent to tight junctions (TJ), suggesting that it recognizes a receptor at this location. However, no such receptor has been identified. The Par3 polarity protein associates with TJs but has no known function in membrane traffic. We now show that, unexpectedly, Par3 is essential for mammary cell survival. Par3 silencing causes apoptosis, triggered by phosphoinositide trisphosphate depletion and decreased Akt phosphorylation, resulting from failure of the exocyst to deliver basolateral proteins to the cortex. A small region of PAR3 binds directly to Exo70 and is sufficient for exocyst docking, membrane-protein delivery and cell survival. PAR3 lacking this domain can associate with the cortex but cannot support exocyst function. We conclude that Par3 is the long-sought exocyst receptor required for targeted membrane-protein delivery.

The exocyst delivers basolateral proteins from the secretory pathway to the plasma membrane of epithelial cells close to tight junctions. Here the authors show that Par3 acts as a docking site for the exocyst to regulate polarized delivery of basolateral proteins and this is essential to prevent apoptosis and promote mammary cell survival.

The role of Exo70 in vascular smooth muscle cell migration

Background

As a key subunit of the exocyst complex, Exo70 has highly conserved sequence and is widely found in yeast, mammals, and plants. In yeast, Exo70 mediates the process of exocytosis and promotes anchoring and integration of vesicles with the plasma membrane. In mammalian cells, Exo70 is involved in maintaining cell morphology, cell migration, cell connection, mRNA splicing, and other physiological processes, as well as participating in exocytosis. However, Exo70’s function in mammalian cells has yet to be fully recognized. In this paper, the expression of Exo70 and its role in cell migration were studied in a rat vascular smooth muscle cell line A7r5.

Methods

Immunofluorescent analysis the expression of Exo70, α-actin, and tubulin in A7r5 cells showed a co-localization of Exo70 and α-actin, we treated the cells with cytochalasin B to depolymerize α-actin, in order to further confirm the co-localization of Exo70 and α-actin. We analyzed Exo70 co-localization with actin at the edge of migrating cells by wound-healing assay to establish whether Exo70 might play a role in cell migration. Next, we analyzed the migration and invasion ability of A7r5 cells before and after RNAi silencing through the wound healing assay and transwell assay.

Results

The mechanism of interaction between Exo70 and cytoskeleton can be clarified by the immunoprecipitation techniques and wound-healing assay. The results showed that Exo70 and α-actin were co-localized at the leading edge of migrating cells. The ability of A7r5 to undergo cell migration was decreased when Exo70 expression was silenced by RNAi. Reducing Exo70 expression in RNAi treated A7r5 cells significantly lowered the invasion and migration ability of these cells compared to the normal cells. These results indicate that Exo70 participates in the process of A7r5 cell migration.

Conclusions

This research is importance for the study on the pathological process of vascular intimal hyperplasia, since it provides a new research direction for the treatment of cardiovascular diseases such as atherosclerosis and restenosis after balloon angioplasty.

Regulation of L-type Ca2+ Channel Activity and Insulin Secretion by Huntingtin-associated Protein 1

Huntingtin-associated protein 1 (Hap1) was originally identified as a protein that binds to the Huntington disease protein, huntingtin. Growing evidence has shown that Hap1 participates in intracellular trafficking via its association with various microtubule-dependent transporters and organelles. Recent studies also revealed that Hap1 is involved in exocytosis such as insulin release from pancreatic β-cells. However, the mechanism underlying the action of Hap1 on insulin release remains to be investigated. We found that Hap1 knock-out mice had a lower plasma basal insulin level than control mice. Using cultured pancreatic β-cell lines, INS-1 cells, we confirmed that decreasing Hap1 reduces the number of secreted vesicles and inhibits vesicle exocytosis. Electrophysiology and imaging of intracellular Ca²⁺ measurements demonstrated that Hap1 depletion significantly reduces the influx of Ca²⁺ mediated by L-type Ca²⁺ channels (Cav). This decrease is not due to reduced expression of Cav1.2 channel mRNA but results from the decreased distribution of Cav1.2 on the plasma membrane of INS-1 cells. Fluorescence recovery after photobleaching showed a defective movement of Cav1.2 in Hap1 silencing INS-1 cells. Our findings suggest that Hap1 is important for insulin secretion of pancreatic β-cells via regulating the intracellular trafficking and plasma membrane localization of Cav1.2, providing new insight into the mechanisms that regulate insulin release from pancreatic β-cells.

Modeling neutralization of Shiga 2 toxin by A-and B-subunit-specific human monoclonal antibodies

A mathematical model for Shiga 2 toxin neutralization by A-and B-subunit-specific human monoclonal antibodies initially delivered in the extracellular domain is presented, taking into account toxin and antibodies interaction in the extracellular domain, diffusion of toxin, antibodies, and their reaction products toward the cell, the receptor-mediated toxin and complex composed of toxin and antibody to A-subunit internalization from the extracellular into the intracellular medium and excretion of this complex back to the extracellular environment via recycling endosomal carriers. The retrograde transport of the intact toxin to the endoplasmic reticulum and its anterograde movement back to the vicinity of the plasma membrane with its subsequent exocytotic removal to the extracellular space via the secretory vesicle pathway is also taken into account. The model is composed of a set of coupled PDEs. A mathematical model based on a system of ODEs for Shiga 2 toxin neutralization by antibodies in the absence of cell is also studied. Both PDE and ODE systems are solved numerically. Numerical results are illustrated by figures and discussed.

Disrupted-in-schizophrenia-1 (DISC1) Regulates Endoplasmic Reticulum Calcium Dynamics

Disrupted-in-schizophrenia-1 (DISC1) has emerged as a convincing susceptibility gene for multiple mental disorders, but its mechanistic link to the pathogenesis of schizophrenia related psychiatric conditions is yet to be further understood. Here, we showed that DISC1 localizes to the outer surface of the endoplasmic reticulum (ER). EXOC1, a subunit of the exocyst complex, interacted with DISC1 and affected its recruitment to inositol-1,4,5-trisphosphate receptor 1 (IP₃R1). Notably, knockdown of DISC1 and EXOC1 elicited an exaggerated ER calcium response upon stimulation of IP₃R agonists. Similar abnormal ER calcium responses were observed in hippocampal neurons from DISC1-deficient mutant mice. Moreover, perturbation of ER calcium dynamics upon DISC1 knockdown was effectively reversed by treatment with antipsychotic drugs, such as clozapine and haloperidol. These results collectively indicate that DISC1 is a regulatory factor in ER calcium dynamics, linking a perturbed intracellular calcium signaling and schizophrenia pathogenesis.

RhoGDI3 and RhoG: Vesicular trafficking and interactions with the Sec3 Exocyst subunit

RhoGDIs are negative regulators of small GTP-binding proteins of the Rho family, which have essential cellular functions in most aspects of actin-based morphology and motility processes. They extract Rho proteins from membranes, keep them in inactive rhoGDI/Rho complexes and eventually deliver them again to specific membranes in response to cellular signals. RhoGDI3, the most divergent member of the rhoGDI family, is well suited to document the underlying molecular mechanisms, since the active and inactive forms of its cellular target, RhoG, have well-separated subcellular localizations. In this study, we investigate trafficking structures and molecular interactions involved in rhoGDI3-mediated shuttling of RhoG between the Golgi and the plasma membrane.Bimolecular fluorescence complementation and acceptor-photobleaching FRET experiments suggest that rhoGDI3 and RhoG form complexes on Golgi and vesicular structures in mammalian cells. 4D-videomicroscopy confirms this localization, and show that RhoG/rhoGDI3-labelled structures are less dynamic than RhoG and rhoGDI3-labeled vesicles, consistent with the inhibitory function of rhoGDI3. Next, we identify the Exocyst subunit Sec3 as a candidate rhoGDI3 partner in cells. RhoGDI3 relocates a subcomplex of the Exocyst (Sec3 and Sec8) from the cytoplasm to the Golgi, while Sec6 is unaffected. Remarkably, Sec3 increases the level of GTP-bound endogenous RhoG, the RhoG-dependent induction of membrane ruffles, and the formation of intercellular tunneling nanotube-like protrusions.Altogether, our study identifies a novel link between vesicular traffic and the regulation of Rho proteins by rhoGDIs. It also suggests that components of the Exocyst machinery may be involved in RhoG functions, possibly regulated by rhoGDI3.

SEC-10 and RAB-10 coordinate basolateral recycling of clathrin-independent cargo through endosomal tubules in Caenorhabditis elegans

Despite the increasing number of regulatory proteins identified in clathrin-independent endocytic (CIE) pathways, our understanding of the exact functions of these proteins and the sequential manner in which they function remains limited. In this study, using the Caenorhabditis elegans intestine as a model, we observed a unique structure of interconnected endosomal tubules, which is required for the basolateral recycling of several CIE cargoes including hTAC, GLUT1, and DAF-4. SEC-10 is a subunit of the octameric protein complex exocyst. Depleting SEC-10 and several other exocyst components disrupted the endosomal tubules into various ring-like structures. An epistasis analysis further suggested that SEC-10 operates at the intermediate step between early endosomes and recycling endosomes. The endosomal tubules were also sensitive to inactivation of the Rab GTPase RAB-10 and disruption of microtubules. Taken together, our data suggest that SEC-10 coordinates with RAB-10 and microtubules to form the endosomal tubular network for efficient recycling of particular CIE cargoes.

The trafficking of the cellulose synthase complex in higher plants

BACKGROUND

Cellulose is an important constituent of plant cell walls in a biological context, and is also a material commonly utilized by mankind in the pulp and paper, timber, textile and biofuel industries. The biosynthesis of cellulose in higher plants is a function of the cellulose synthase complex (CSC). The CSC, a large transmembrane complex containing multiple cellulose synthase proteins, is believed to be assembled in the Golgi apparatus, but is thought only to synthesize cellulose when it is localized at the plasma membrane, where CSCs synthesize and extrude cellulose directly into the plant cell wall. Therefore, the delivery and endocytosis of CSCs to and from the plasma membrane are important aspects for the regulation of cellulose biosynthesis.

SCOPE

Recent progress in the visualization of CSC dynamics in living plant cells has begun to reveal some of the routes and factors involved in CSC trafficking. This review highlights the most recent major findings related to CSC trafficking, provides novel perspectives on how CSC trafficking can influence the cell wall, and proposes potential avenues for future exploration.

Exocyst complex protein expression in the human placenta

INTRODUCTION

Protein production and secretion are essential to syncytiotrophoblast function and are associated with cytotrophoblast cell fusion and differentiation. Syncytiotrophoblast hormone secretion is a crucial determinant of maternal-fetal health, and can be misregulated in pathological pregnancies. Although, polarized secretion is a key component of placental function, the mechanisms underlying this process are poorly understood.

OBJECTIVE

While the octameric exocyst complex is classically regarded as a master regulator of secretion in various mammalian systems, its expression in the placenta remained unexplored. We hypothesized that the syncytiotrophoblast would express all exocyst complex components and effector proteins requisite for vesicle-mediated secretion more abundantly than cytotrophoblasts in tissue specimens.

METHODS

A two-tiered immunobiological approach was utilized to characterize exocyst and ancillary proteins in normal, term human placentas. Exocyst protein expression and localization was documented in tissue homogenates via immunoblotting and immunofluorescence labeling of placental sections.

RESULTS

The eight exocyst proteins, EXOC1, 2, 3, 4, 5, 6, 7, and 8, were found in the human placenta. In addition, RAB11, an important exocyst complex modulator, was also expressed. Exocyst and Rab protein expression appeared to be regulated during trophoblast differentiation, as the syncytiotrophoblast expressed these proteins with little, if any, expression in cytotrophoblast cells. Additionally, exocyst proteins were localized at or near the syncytiotrophoblast apical membrane, the major site of placental secretion.

DISCUSSION/CONCLUSION

Our findings highlight exocyst protein expression as novel indicators of trophoblast differentiation. The exocyst's regulated localization within the syncytiotrophoblast in conjunction with its well known functions suggests a possible role in placental polarized secretion.

The endocytic activity of the flagellar pocket in Trypanosoma brucei is regulated by an adjacent phosphatidylinositol phosphate kinase

Phosphoinositides are spatially restricted membrane signaling molecules. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]--a phosphoinositide that is highly enriched in, and present throughout, the plasma membrane--has been implicated in endocytosis. Trypanosoma brucei has one of the highest known rates of endocytosis, a process it uses to evade the immune system. To determine whether phosphoinositides play a role in endocytosis in this organism, we have identified and characterized one of the enzymes that is responsible for generating PI(4,5)P2. Surprisingly, this phosphoinositide was found to be highly concentrated in the flagellar pocket, the only site of endocytosis and exocytosis in this organism. The enzyme (designated TbPIPKA, annotated as Tb927.10.1620) was present at the neck of the pocket, towards the anterior-end of the parasite. Depletion of TbPIPKA led to depletion of PI(4,5)P2 and enlargement of the pocket, the result of impaired endocytosis. Taken together, these data suggest that TbPIPKA and its product PI(4,5)P2 are important for endocytosis and, consequently, for homeostasis of the flagellar pocket.

The Neurospora crassa exocyst complex tethers Spitzenkörper vesicles to the apical plasma membrane during polarized growth

Fungal hyphae are among the most highly polarized cells. Hyphal polarized growth is supported by tip-directed transport of secretory vesicles, which accumulate temporarily in a stratified manner in an apical vesicle cluster, the Spitzenkörper. The exocyst complex is required for tethering of secretory vesicles to the apical plasma membrane. We determined that the presence of an octameric exocyst complex is required for the formation of a functional Spitzenkörper and maintenance of regular hyphal growth in Neurospora crassa. Two distinct localization patterns of exocyst subunits at the hyphal tip suggest the dynamic formation of two assemblies. The EXO-70/EXO-84 subunits are found at the peripheral part of the Spitzenkörper, which partially coincides with the outer macrovesicular layer, whereas exocyst components SEC-5, -6, -8, and -15 form a delimited crescent at the apical plasma membrane. Localization of SEC-6 and EXO-70 to the plasma membrane and the Spitzenkörper, respectively, depends on actin and microtubule cytoskeletons. The apical region of exocyst-mediated vesicle fusion, elucidated by the plasma membrane-associated exocyst subunits, indicates the presence of an exocytotic gradient with a tip-high maximum that dissipates gradually toward the subapex, confirming the earlier predictions of the vesicle supply center model for hyphal morphogenesis.

Exocyst subunits are involved in isoproterenol-induced amylase release from rat parotid acinar cells

Exocytosis of secretory granules in parotid acinar cells requires multiple events: tethering, docking, priming, and fusion with a luminal plasma membrane. The exocyst complex, which is composed of eight subunits (Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70, and Exo84) that are conserved in yeast and mammalian cells, is thought to participate in the exocytotic pathway. However, to date, no exocyst subunit has been identified in salivary glands. In the present study, we investigated the expression and function of exocyst subunits in rat parotid acinar cells. The expression of mRNA for all eight exocyst subunits was detected in parotid acinar cells by RT-PCR, and Sec6 and Sec8 proteins were localized on the luminal plasma membrane. Sec6 interacted with Sec8 after 5 min of stimulation with isoproterenol. In addition, antibodies to-Sec6 and Sec8 inhibited isoproterenol-induced amylase release from streptolysin O-permeabilized parotid acinar cells. These results suggest that an exocyst complex of eight subunits is required for amylase release from parotid acinar cells.

ERK1/2 regulate exocytosis through direct phosphorylation of the exocyst component Exo70

The exocyst is a multiprotein complex essential for exocytosis and plasma membrane remodeling. The assembly of the exocyst complex mediates the tethering of post-Golgi secretory vesicles to the plasma membrane prior to fusion. Elucidating the mechanisms regulating exocyst assembly is important for the understanding of exocytosis. Here we show that the exocyst component Exo70 is a direct substrate of the extracellular signal-regulated kinases 1/2 (ERK1/2). ERK1/2 phosphorylation enhances the binding of Exo70 to other exocyst components and promotes the assembly of the exocyst complex in response to epidermal growth factor (EGF) signaling. We further demonstrate that ERK1/2 regulates exocytosis, because blocking ERK1/2 signaling by a chemical inhibitor or the expression of an Exo70 mutant defective in ERK1/2 phosphorylation inhibited exocytosis. In tumor cells, blocking Exo70 phosphorylation inhibits matrix metalloproteinase secretion and invadopodia formation. ERK1/2 phosphorylation of Exo70 may thus coordinate exocytosis with other cellular events in response to growth factor signaling.

Neurotransmitter segregation: functional and plastic implications

Synaptic cotransmission is the ability of neurons to use more than one transmitter to convey synaptic signals. Cotransmission was originally described as the presence of a classic transmitter, which conveys main signal, along one or more cotransmitters that modulate transmission, later on, it was found cotransmission of classic transmitters. It has been generally accepted that neurons store and release the same set of transmitters in all their synaptic processes. However, some findings that show axon endings of individual neurons storing and releasing different sets of transmitters, are not in accordance with this assumption, and give support to the hypothesis that neurons can segregate transmitters to different synapses. Here, we review the studies showing segregation of transmitters in invertebrate and mammalian central nervous system neurons, and correlate them with our results obtained in sympathetic neurons. Our data show that these neurons segregate even classic transmitters to separated axons. Based on our data we suggest that segregation is a plastic phenomenon and responds to functional synaptic requirements, and to 'environmental' cues such as neurotrophins. We propose that neurons have the machinery to guide the different molecules required in synaptic transmission through axons and sort them to different axon endings. We believe that transmitter segregation improves neuron interactions during cotransmission and gives them selective and better control of synaptic plasticity.

Exorcising the exocyst complex

The exocyst complex is an evolutionarily conserved multisubunit protein complex implicated in tethering secretory vesicles to the plasma membrane. Originally identified two decades ago in budding yeast, investigations using several different eukaryotic systems have since made great progress toward determination of the overall structure and organization of the eight exocyst subunits. Studies point to a critical role for the complex as a spatiotemporal regulator through the numerous protein and lipid interactions of its subunits, although a molecular understanding of exocyst function has been challenging to elucidate. Recent progress demonstrates that the exocyst is also important for additional trafficking steps and cellular processes beyond exocytosis, with links to development and disease. In this review, we discuss current knowledge of exocyst architecture, assembly, regulation and its roles in a variety of cellular trafficking pathways.

Characterization of Mug33 reveals complementary roles for actin cable-dependent transport and exocyst regulators in fission yeast exocytosis

Although endocytosis and exocytosis have been extensively studied in budding yeast, there have been relatively few investigations of these complex processes in the fission yeast Schizosaccharomyces pombe. Here we identify and characterize fission yeast Mug33, a novel Tea1-interacting protein, and show that Mug33 is involved in exocytosis. Mug33 is a Sur7/PalI-family transmembrane protein that localizes to the plasma membrane at the cell tips and to cytoplasmic tubulovesicular elements (TVEs). A subset of Mug33 TVEs make long-range movements along actin cables, co-translocating with subunits of the exocyst complex. TVE movement depends on the type V myosin Myo52. Although mug33Δ mutants are viable, with only a mild cell-polarity phenotype, mug33Δ myo52Δ double mutants are synthetically lethal. Combining mug33 Δ with deletion of the formin For3 (for3Δ) leads to synthetic temperature-sensitive growth and strongly reduced levels of exocytosis. Interestingly, mutants in non-essential genes involved in exocyst function behave in a manner similar to mug33Δ when combined with myo52Δ and for3Δ. By contrast, combining mug33Δ with mutants in non-essential exocyst genes has only minor effects on growth. We propose that Mug33 contributes to exocyst function and that actin cable-dependent vesicle transport and exocyst function have complementary roles in promoting efficient exocytosis in fission yeast.

Distinct cytoskeleton populations and extensive crosstalk control Ciona notochord tubulogenesis

Cell elongation is a fundamental process that allows cells and tissues to adopt new shapes and functions. During notochord tubulogenesis in the ascidian Ciona intestinalis, a dramatic elongation of individual cells takes place that lengthens the notochord and, consequently, the entire embryo. We find a novel dynamic actin- and non-muscle myosin II-containing constriction midway along the anteroposterior aspect of each notochord cell during this process. Both actin polymerization and myosin II activity are required for the constriction and cell elongation. Discontinuous localization of myosin II in the constriction indicates that the actomyosin network produces local contractions along the circumference. This reveals basal constriction by the actomyosin network as a novel mechanism for cell elongation. Following elongation, the notochord cells undergo a mesenchymal-epithelial transition and form two apical domains at opposite ends. Extracellular lumens then form at the apical surfaces. We show that cortical actin and Ciona ezrin/radixin/moesin (ERM) are essential for lumen formation and that a polarized network of microtubules, which contributes to lumen development, forms in an actin-dependent manner at the apical cortex. Later in notochord tubulogenesis, when notochord cells initiate a bi-directional crawling movement on the notochordal sheath, the microtubule network rotates 90° and becomes organized as parallel bundles extending towards the leading edges of tractive lamellipodia. This process is required for the correct organization of actin-based protrusions and subsequent lumen coalescence. In summary, we establish the contribution of the actomyosin and microtubule networks to notochord tubulogenesis and reveal extensive crosstalk and regulation between these two cytoskeleton components.

Loss of huntingtin-associated protein 1 impairs insulin secretion from pancreatic β-cells

Hap1 was originally identified as a neuronal protein that interacts with huntingtin, the Huntington's disease (HD) protein. Later studies revealed that Hap1 participates in intracellular trafficking in neuronal cells and that this trafficking function can be adversely affected by mutant huntingtin. Hap1 is also present in pancreatic β-cells and other endocrine cells; however, the role of Hap1 in these endocrine cells remains unknown. Using the Cre-loxP system, we generated conditional Hap1 knockout mice to selectively deplete the expression of Hap1 in mouse pancreatic β-cells. Mutant mice with Hap1 deficiency in pancreatic β-cells had impaired glucose tolerance and decreased insulin release in response to intraperitoneally injected glucose. Using cultured pancreatic β-cell lines and isolated mouse pancreatic islets, we confirmed that decreasing Hap1 could reduce glucose-mediated insulin release. Electron microscopy suggested that there was a reduced number of insulin-containing vesicles docked at the plasma membrane of pancreatic islets in Hap1 mutant mice following intraperitoneal glucose injection. Glucose treatment decreased the phosphorylation of Hap1A in cultured β-cells and in mouse pancreatic tissues. Moreover, this glucose treatment increased Hap1's association with kinesin light chain and dynactin p150, both of which are involved in microtubule-dependent trafficking. These studies suggest that Hap1 is important for insulin release from β-cells via dephosphorylation that can regulate its intracellular trafficking function.

Deciphering subcellular processes in live imaging datasets via dynamic probabilistic networks

MOTIVATION

Designing mathematical tools that can formally describe the dynamics of complex intracellular processes remains a challenge. Live cell imaging reveals changes in the cellular states, but current simple approaches extract only minimal information of a static snapshot.

RESULTS

We implemented a novel approach for analyzing organelle behavior in live cell imaging data based on hidden Markov models (HMMs) and showed that it can determine the number and evolution of distinct cellular states involved in a biological process. We analyzed insulin-mediated exocytosis of single Glut4-vesicles, a process critical for blood glucose homeostasis and impaired in type II diabetes, by using total internal reflection fluorescence microscopy (TIRFM). HMM analyses of movie sequences of living cells reveal that insulin controls spatial and temporal dynamics of exocytosis via the exocyst, a putative tethering protein complex. Our studies have validated the proof-of-principle of HMM for cellular imaging and provided direct evidence for the existence of complex spatial-temporal regulation of exocytosis in non-polarized cells. We independently confirmed insulin-dependent spatial regulation by using static spatial statistics methods.

CONCLUSION

We propose that HMM-based approach can be exploited in a wide avenue of cellular processes, especially those where the changes of cellular states in space and time may be highly complex and non-obvious, such as in cell polarization, signaling and developmental processes.

Characterization of the Arabidopsis thaliana exocyst complex gene families by phylogenetic, expression profiling, and subcellular localization studies

*The exocyst is a complex of eight proteins (Sec3p, Sec5p, Sec6p, Sec8p, Sec10p, Sec15p, Exo70p and Exo84p) involved in tethering vesicles to the plasma membrane during regulated or polarized secretion. Here, the plant exocyst complex was explored in phylogenetic, expression, and subcellular localization studies. *Evolutionary relationships of predicted exocyst subunits were examined in the complete genomes of Arabidopsis thaliana, Oryza sativa, Populus trichocarpa and Physcomitrella patens. Furthermore, detailed expression profiling of the A. thaliana microarray databases was performed and subcellular localization patterns were studied. *Several plant exocyst subunit genes appear to have undergone gene expansion in a common ancestor and subsequent duplication events in independent plant lineages. Expression profiling revealed that the A. thaliana Exo70 gene family exhibits dynamic expression patterns, while the remaining exocyst subunit genes displayed more static profiles. Subcellular localization patterns for A. thaliana exocyst subunits ranged from cytosolic to endosomal compartments (with enrichment in the early endosomes and the trans-Golgi network). Interestingly, two endosomal-localized AtExo70 proteins also recruited other exocyst subunits to these compartments. *Overall subcellular localization patterns were observed that were also found in yeast and animal cells, and this, coupled with the evolutionary relationships, suggests that the exocyst may perform similar conserved functions in plants.

Perturbation of vacuolar maturation promotes listeriolysin O-independent vacuolar escape during Listeria monocytogenes infection of human cells

Listeria monocytogenes is a bacterial pathogen that replicates within the cytosol of infected host cells. The ability to rapidly escape the phagocytic vacuole is essential for efficient intracellular replication. In the murine model of infection, the pore-forming cytolysin listeriolysin O (LLO) is absolutely required for vacuolar dissolution, as LLO-deficient (DeltaLLO) mutants remain trapped within vacuoles. In contrast, in many human cell types DeltaLLO L. monocytogenes are capable of vacuolar escape at moderate to high frequencies. To better characterize the mechanism of LLO-independent vacuolar escape in human cells, we conducted an RNA interference screen to identify vesicular trafficking factors that play a role in altering vacuolar escape efficiency of DeltaLLO L. monocytogenes. RNA interference knockdown of 18 vesicular trafficking factors resulted in increased LLO-independent vacuolar escape. Our results suggest that knockdown of one factor, RABEP1 (rabaptin-5), decreased the maturation of vacuoles containing DeltaLLO L. monocytogenes. Thus, we provide evidence that increased vacuolar escape of DeltaLLO L. monocytogenes in human cells correlates with slower vacuolar maturation. We also determined that increased LLO-independent dissolution of vacuoles during RABEP1 knockdown required the bacterial broad-range phospholipase C (PC-PLC). We hypothesize that slowing the kinetics of vacuolar maturation generates an environment conducive for vacuolar escape mediated by the bacterial phospholipases.

Sec61beta, a subunit of the Sec61 protein translocation channel at the endoplasmic reticulum, is involved in the transport of Gurken to the plasma membrane

Background

Protein translocation across the membrane of the Endoplasmic Reticulum (ER) is the first step in the biogenesis of secretory and membrane proteins. Proteins enter the ER by the Sec61 translocon, a proteinaceous channel composed of three subunits, α, β and γ. While it is known that Sec61α forms the actual channel, the function of the other two subunits remains to be characterized.

Results

In the present study we have investigated the function of Sec61β in Drosophila melanogaster. We describe its role in the plasma membrane traffic of Gurken, the ligand for the Epidermal Growth Factor (EGF) receptor in the oocyte. Germline clones of the mutant allele of Sec61β show normal translocation of Gurken into the ER and transport to the Golgi complex, but further traffic to the plasma membrane is impeded. The defect in plasma membrane traffic due to absence of Sec61β is specific for Gurken and is not due to a general trafficking defect.

Conclusion

Based on our study we conclude that Sec61β, which is part of the ER protein translocation channel affects a post-ER step during Gurken trafficking to the plasma membrane. We propose an additional role of Sec61β beyond protein translocation into the ER.

The organelle proteome of the DT40 lymphocyte cell line

A major challenge in eukaryotic cell biology is to understand the roles of individual proteins and the subcellular compartments in which they reside. Here, we use the localization of organelle proteins by isotope tagging technique to complete the first proteomic analysis of the major organelles of the DT40 lymphocyte cell line. This cell line is emerging as an important research tool because of the ease with which gene knockouts can be generated. We identify 1090 proteins through the analysis of preparations enriched for integral membrane or soluble and peripherally associated proteins and localize 223 proteins to the endoplasmic reticulum, Golgi, lysosome, mitochondrion, or plasma membrane by matching their density gradient distributions to those of known organelle residents. A striking finding is that within the secretory and endocytic pathway a high proportion of proteins are not uniquely localized to a single organelle, emphasizing the dynamic steady-state nature of intracellular compartments in eukaryotic cells.

Ral-regulated interaction between Sec5 and paxillin targets Exocyst to focal complexes during cell migration

Changes in cellular behavior that cause epithelial cells to lose adhesiveness, acquire a motile invasive phenotype and metastasize to secondary sites are complex and poorly understood. Molecules that normally function to integrate adhesive spatial information with cytoskeleton dynamics and membrane trafficking probably serve important functions in cellular transformation. One such complex is the Exocyst, which is essential for targeted delivery of membrane and secretory proteins to specific plasma membrane sites to maintain epithelial cell polarity. Upon loss of cadherin-mediated adhesion in Dunning R3327-5'A prostate tumor cells, Exocyst localization shifts from lateral membranes to tips of protrusive membrane extensions. Here, it colocalizes and co-purifies with focal complex proteins that regulate membrane trafficking and cytoskeleton dynamics. These sites are the preferred destination of post-Golgi transport vesicles ferrying biosynthetic cargo, such as alpha(5)-integrin, which mediates adhesion of cells to the substratum, a process essential to cell motility. Interference with Exocyst activity impairs integrin delivery to plasma membrane and inhibits tumor cell motility and matrix invasiveness. Localization of Exocyst and, by extension, targeting of Exocyst-dependent cargo, is dependent on Ral GTPases, which control association between Sec5 and paxillin. Overexpression of Ral-uncoupled Sec5 mutants inhibited Exocyst interaction with paxillin in 5'A cells, as did RNAi-mediated reduction of either RalA or RalB. Reduction of neither GTPase significantly altered steady-state levels of assembled Exocyst in these cells, but did change the observed localization of Exocyst proteins.

Synaptic vesicle protein trafficking at the glutamate synapse

Expression of the integral and associated proteins of synaptic vesicles is subject to regulation over time, by region, and in response to activity. The process by which changes in protein levels and isoforms result in different properties of neurotransmitter release involves protein trafficking to the synaptic vesicle. How newly synthesized proteins are incorporated into synaptic vesicles at the presynaptic bouton is poorly understood. During synaptogenesis, synaptic vesicle proteins sort through the secretory pathway and are transported down the axon in precursor vesicles that undergo maturation to form synaptic vesicles. Changes in protein content of synaptic vesicles could involve the formation of new vesicles that either mix with the previous complement of vesicles or replace them, presumably by their degradation or inactivation. Alternatively, new proteins could individually incorporate into existing synaptic vesicles, changing their functional properties. Glutamatergic vesicles likely express many of the same integral membrane proteins and share certain common mechanisms of biogenesis, recycling, and degradation with other synaptic vesicles. However, glutamatergic vesicles are defined by their ability to package glutamate for release, a property conferred by the expression of a vesicular glutamate transporter (VGLUT). VGLUTs are subject to regional, developmental, and activity-dependent changes in expression. In addition, VGLUT isoforms differ in their trafficking, which may target them to different pathways during biogenesis or after recycling, which may in turn sort them to different vesicle pools. Emerging data indicate that differences in the association of VGLUTs and other synaptic vesicle proteins with endocytic adaptors may influence their trafficking. These observations indicate that independent regulation of synaptic vesicle protein trafficking has the potential to influence synaptic vesicle protein composition, the maintenance of synaptic vesicle pools, and the release of glutamate in response to changing physiological requirements.

A 20-nm step toward the cell membrane preceding exocytosis may correspond to docking of tethered granules

In endocrine cells, plasma membrane (PM)-bound secretory granules must undergo a number of maturation stages (i.e., priming) to become fusion-competent. Despite identification of several molecules involved in binding granules to the PM and priming them, the exact nature of events occurring at the PM still largely remains a mystery. In stimulated BON cells, we used evanescent wave microscopy to study trajectories of granules shortly before their exocytoses, which provided a physical description of vesicle-PM interactions at an unprecedented level of detail, and directly lead to an original mechanistic model. In these cells, tethered (T), nonfusogenic, vesicles are prevented from converting to fusogenic, docked (D) ones in resting conditions. Upon elevation of calcium, T-vesicles perform a 21-nm step toward the PM to become D, and fuse approximately 3 s thereafter. Our ability to directly visualize different modes of PM-attachment paves the way for clarifying the exact role of various molecules implicated in attachment and priming of granules in future studies.

Exocyst requirement for endocytic traffic directed toward the apical and basolateral poles of polarized MDCK cells

The octameric exocyst complex is associated with the junctional complex and recycling endosomes and is proposed to selectively tether cargo vesicles directed toward the basolateral surface of polarized Madin-Darby canine kidney (MDCK) cells. We observed that the exocyst subunits Sec6, Sec8, and Exo70 were localized to early endosomes, transferrin-positive common recycling endosomes, and Rab11a-positive apical recycling endosomes of polarized MDCK cells. Consistent with its localization to multiple populations of endosomes, addition of function-blocking Sec8 antibodies to streptolysin-O-permeabilized cells revealed exocyst requirements for several endocytic pathways including basolateral recycling, apical recycling, and basolateral-to-apical transcytosis. The latter was selectively dependent on interactions between the small GTPase Rab11a and Sec15A and was inhibited by expression of the C-terminus of Sec15A or down-regulation of Sec15A expression using shRNA. These results indicate that the exocyst complex may be a multipurpose regulator of endocytic traffic directed toward both poles of polarized epithelial cells and that transcytotic traffic is likely to require Rab11a-dependent recruitment and modulation of exocyst function, likely through interactions with Sec15A.

The Drosophila Dead end Arf-like3 GTPase controls vesicle trafficking during tracheal fusion cell morphogenesis

The Drosophila larval tracheal system consists of a highly branched tubular organ that becomes interconnected by migration-fusion events during embryonic development. Fusion cells at the tip of each branch guide migration, adhere, and then undergo extensive remodeling as the tracheal lumen extends between the two branches. The Drosophila dead end gene is expressed in fusion cells, and encodes an Arf-like3 GTPase. Analyses of dead end RNAi and mutant embryos reveal that the lumen fails to connect between the two branches. Expression of a constitutively active form of Dead end in S2 cells reveals that it influences the state of actin polymerization, and is present on particles that traffic along actin/microtubule-containing processes. Imaging experiments in vivo reveal that Dead end-containing vesicles are associated with recycling endosomes and the exocyst, and control exocyst localization in fusion cells. These results indicate that the Dead end GTPase plays an important role in trafficking membrane components involved in tracheal fusion cell morphogenesis and lumenal development.

Phosphatidylinositol 4,5-bisphosphate mediates the targeting of the exocyst to the plasma membrane for exocytosis in mammalian cells

The exocyst is an evolutionarily conserved octameric protein complex that tethers post-Golgi secretory vesicles at the plasma membrane for exocytosis. To elucidate the mechanism of vesicle tethering, it is important to understand how the exocyst physically associates with the plasma membrane (PM). In this study, we report that the mammalian exocyst subunit Exo70 associates with the PM through its direct interaction with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)). Furthermore, we have identified key conserved residues at the C-terminus of Exo70 that are crucial for the interaction of Exo70 with PI(4,5)P(2). Disrupting Exo70-PI(4,5)P(2) interaction abolished the membrane association of Exo70. We have also found that wild-type Exo70 but not the PI(4,5)P(2)-binding-deficient Exo70 mutant is capable of recruiting other exocyst components to the PM. Using the ts045 vesicular stomatitis virus glycoprotein trafficking assay, we demonstrate that Exo70-PI(4,5)P(2) interaction is critical for the docking and fusion of post-Golgi secretory vesicles, but not for their transport to the PM.