Distinct cellular locations of the syntaxin family of proteins in rat pancreatic acinar cells

Two roads diverged in a cell: insights from differential exosome regulation in polarized cells

Exosomes are extracellular vesicles involved in intercellular signaling, carrying various cargo from microRNAs to metabolites and proteins. They are released by practically all cells and are highly heterogenous due to their origin and content. Several groups of exosomes are known to be involved in various pathological conditions including autoimmune, neurodegenerative, and infectious diseases as well as cancer, and therefore a substantial understanding of their biogenesis and release is crucial. Polarized cells display an array of specific functions originated from differentiated membrane trafficking systems and could lead to hints in untangling the complex process of exosomes. Indeed, recent advances have successfully revealed specific regulation pathways for releasing different subsets of exosomes from different sides of polarized epithelial cells, underscoring the importance of polarized cells in the field. Here we review current evidence on exosome biogenesis and release, especially in polarized cells, highlight the challenges that need to be combatted, and discuss potential applications related to exosomes of polarized-cell origin.

Mechanisms linking hypertriglyceridemia to acute pancreatitis

Hypertriglyceridemia (HTG) is a metabolic disorder, defined when serum or plasma triglyceride concentration (seTG) is >1.7 mM. HTG can be categorized as mild to very severe groups based on the seTG value. The risk of acute pancreatitis (AP), a serious disease with high mortality and without specific therapy, increases with the degree of HTG. Furthermore, even mild or moderate HTG aggravates AP initiated by other important etiological factors, including alcohol or bile stone. This review briefly summarizes the pathophysiology of HTG, the epidemiology of HTG-induced AP and the clinically observed effects of HTG on the outcomes of AP. Our main focus is to discuss the pathophysiological mechanisms linking HTG to AP. HTG is accompanied by an increased serum fatty acid (FA) concentration, and experimental results have demonstrated that these FAs have the most prominent role in causing the consequences of HTG during AP. FAs inhibit mitochondrial complexes in pancreatic acinar cells, induce pathological elevation of intracellular Ca²⁺ concentration, cytokine release and tissue injury, and reduce the function of pancreatic ducts. Furthermore, high FA concentrations can induce respiratory, kidney, and cardiovascular failure in AP. All these effects may contribute to the observed increased AP severity and frequent organ failure in patients. Importantly, experimental results suggest that the reduction of FA production by lipase inhibitors can open up new therapeutic options of AP. Overall, investigating the pathophysiology of HTG-induced AP or AP in the presence of HTG and determining possible treatments are needed.

A CRISPR screen in intestinal epithelial cells identifies novel factors for polarity and apical transport

Epithelial polarization and polarized cargo transport are highly coordinated and interdependent processes. In our search for novel regulators of epithelial polarization and protein secretion, we used a genome-wide CRISPR/Cas9 screen and combined it with an assay based on fluorescence-activated cell sorting (FACS) to measure the secretion of the apical brush-border hydrolase dipeptidyl peptidase 4 (DPP4). In this way, we performed the first CRISPR screen to date in human polarized epithelial cells. Using high-resolution microscopy, we detected polarization defects and mislocalization of DPP4 to late endosomes/lysosomes after knockout of TM9SF4, anoctamin 8, and ARHGAP33, confirming the identification of novel factors for epithelial polarization and apical cargo secretion. Thus, we provide a powerful tool suitable for studying polarization and cargo secretion in epithelial cells. In addition, we provide a dataset that serves as a resource for the study of novel mechanisms for epithelial polarization and polarized transport and facilitates the investigation of novel congenital diseases associated with these processes.

Role of SNARE Proteins in the Insertion of KCa3.1 in the Plasma Membrane of a Polarized Epithelium

Targeting proteins to a specific membrane is crucial for proper epithelial cell function. KCa3.1, a calcium-activated, intermediate-conductance potassium channel, is targeted to the basolateral membrane (BLM) in epithelial cells. Surprisingly, the mechanism of KCa3.1 membrane targeting is poorly understood. We previously reported that targeting of KCa3.1 to the BLM of epithelial cells is Myosin-Vc-, Rab1-and Rab8-dependent. Here, we examine the role of the SNARE proteins VAMP3, SNAP-23 and syntaxin 4 (STX-4) in the targeting of KCa3.1 to the BLM of Fischer rat thyroid (FRT) epithelial cells. We carried out immunoblot, siRNA and Ussing chamber experiments on FRT cells, stably expressing KCa3.1-BLAP/Bir-A-KDEL, grown as high-resistance monolayers. siRNA-mediated knockdown of VAMP3 reduced BLM expression of KCa3.1 by 57 ± 5% (p ≤ 0.05, n = 5). Measurements of BLM-localized KCa3.1 currents, in Ussing chambers, demonstrated knockdown of VAMP3 reduced KCa3.1 current by 70 ± 4% (p ≤ 0.05, n = 5). Similarly, siRNA knockdown of SNAP-23 reduced the expression of KCa3.1 at the BLM by 56 ± 7% (p ≤ 0.01, n = 6) and reduced KCa3.1 current by 80 ± 11% (p ≤ 0.05, n = 6). Also, knockdown of STX-4 lowered the BLM expression of KCa3.1 by 54 ± 6% (p ≤ 0.05, n = 5) and reduced KCa3.1 current by 78 ± 11% (p ≤ 0.05, n = 5). Finally, co-immunoprecipitation experiments demonstrated associations between KCa3.1, VAMP3, SNAP-23 and STX-4. These data indicate that VAMP3, SNAP-23 and STX-4 are critical for the targeting KCa3.1 to BLM of polarized epithelial cells.

Susceptibility Factors and Cellular Mechanisms Underlying Alcoholic Pancreatitis

Alcohol is a major cause of acute and chronic pancreatitis. There have been some recent advances in the understanding of the mechanisms underlying alcoholic pancreatitis, which include perturbation in mitochondrial function and autophagy and ectopic exocytosis, with some of these cellular events involving membrane fusion soluble N-ethylmaleimide-sensitive factor receptor protein receptor proteins. Although new insights have been unraveled recently, the precise mechanisms remain complex, and their finer details have yet to be established. The overall pathophysiology of pancreatitis involves not only the pancreatic acinar cells but also the stellate cells and duct cells. Why only some are more susceptible to pancreatitis and with increased severity, while others are not, would suggest that there may be undefined protective factors or mechanisms that enhance recovery and regeneration after injury. Furthermore, there are confounding influences of lifestyle factors such as smoking and diet, and genetic background. Whereas alcohol and smoking cessation and a generally healthy lifestyle are intuitively the advice given to these patients afflicted with alcoholic pancreatitis in order to reduce disease recurrence and progression, there is as yet no specific treatment. A more complete understanding of the pathogenesis of pancreatitis from which novel therapeutic targets could be identified will have a great impact, particularly with the stubbornly high fatality (>30%) of severe pancreatitis. This review focuses on the susceptibility factors and underlying cellular mechanisms of alcohol injury on the exocrine pancreas.

Pancreatic duct-like cell line derived from pig embryonic stem cells: expression of uroplakin genes in pig pancreatic tissue

The isolation of a cell line, PICM-31D, with phenotypic characteristics like pancreatic duct cells is described. The PICM-31D cell line was derived from the previously described pig embryonic stem cell-derived exocrine pancreatic cell line, PICM-31. The PICM-31D cell line was morphologically distinct from the parental cells in growing as a monolayer rather than self-assembling into multicellular acinar-like structures. The PICM-31D cells were propagated for over a year at split ratios of 1:3 to 1:10 at each passage without change in phenotype or growth rate. Electron microscopy showed the cells to be a polarized epithelium of cuboidal cells joined by tight junction-like adhesions at their apical/lateral aspect. The cells contained numerous mucus-like secretory vesicles under their apical cell membrane. Proteomic analysis of the PICM-31D's cellular proteins detected MUC1 and MUC4, consistent with mucus vesicle morphology. Gene expression analysis showed the cells expressed pancreatic ductal cell-related transcription factors such as GATA4, GATA6, HES1, HNF1A, HNF1B, ONECUT1 (HNF6), PDX1, and SOX9, but little or no pancreas progenitor cell markers such as PTF1A, NKX6-1, SOX2, or NGN3. Pancreas ductal cell-associated genes including CA2, CFTR, MUC1, MUC5B, MUC13, SHH, TFF1, KRT8, and KRT19 were expressed by the PICM-31D cells, but the exocrine pancreas marker genes, CPA1 and PLA2G1B, were not expressed by the cells. However, the exocrine marker, AMY2A, was still expressed by the cells. Surprisingly, uroplakin proteins were prominent in the PICM-31D cell proteome, particularly UPK1A. Annexin A1 and A2 proteins were also relatively abundant in the cells. The expression of the uroplakin and annexin genes was detected in the cells, although only UPK1B, UPK3B, ANXA2, and ANXA4 were detected in fetal pig pancreatic duct tissue. In conclusion, the PICM-31D cell line models the mucus-secreting ductal cells of the fetal pig pancreas.

Postsynaptic SNARE Proteins: Role in Synaptic Transmission and Plasticity

Soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) proteins mediate membrane fusion events in eukaryotic cells. Traditionally recognized as major players in regulating presynaptic neurotransmitter release, accumulative evidence over recent years has identified several SNARE proteins implicated in important postsynaptic processes such as neurotransmitter receptor trafficking and synaptic plasticity. Here we analyze the emerging data revealing this novel functional dimension for SNAREs with a focus on the molecular specialization of vesicular recycling and fusion in dendrites compared to those at axon terminals and its impact in synaptic transmission and plasticity.

Molecular Regulatory Mechanism of Exocytosis in the Salivary Glands

Every day, salivary glands produce about 0.5 to 1.5 L of saliva, which contains salivary proteins that are essential for oral health. The contents of saliva, 0.3% proteins (1.5 to 4.5 g) in fluid, help prevent oral infections, provide lubrication, aid digestion, and maintain oral health. Acinar cells in the lobular salivary glands secrete prepackaged secretory granules that contain salivary components such as amylase, mucins, and immunoglobulins. Despite the important physiological functions of salivary proteins, we know very little about the regulatory mechanisms of their secretion via exocytosis, which is a process essential for the secretion of functional proteins, not only in salivary glands, but also in other secretory organs, including lacrimal and mammary glands, the pancreas, and prostate. In this review, we discuss recent findings that elucidate exocytosis by exocrine glands, especially focusing on the salivary glands, in physiological and pathological conditions.

Weibel-Palade Body Localized Syntaxin-3 Modulates Von Willebrand Factor Secretion From Endothelial Cells

Supplemental Digital Content is available in the text.

Objective—

Endothelial cells store VWF (von Willebrand factor) in rod-shaped secretory organelles, called Weibel-Palade bodies (WPBs). WPB exocytosis is coordinated by a complex network of Rab GTPases, Rab effectors, and SNARE (soluble NSF attachment protein receptor) proteins. We have previously identified STXBP1 as the link between the Rab27A-Slp4-a complex on WPBs and the SNARE proteins syntaxin-2 and -3. In this study, we investigate the function of syntaxin-3 in VWF secretion.

Approach and Results—

In human umbilical vein endothelial cells and in blood outgrowth endothelial cells (BOECs) from healthy controls, endogenous syntaxin-3 immunolocalized to WPBs. A detailed analysis of BOECs isolated from a patient with variant microvillus inclusion disease, carrying a homozygous mutation in STX3(STX3^−/−), showed a loss of syntaxin-3 protein and absence of WPB-associated syntaxin-3 immunoreactivity. Ultrastructural analysis revealed no detectable differences in morphology or prevalence of immature or mature WPBs in control versus STX3^−/− BOECs. VWF multimer analysis showed normal patterns in plasma of the microvillus inclusion disease patient, and media from STX3^−/− BOECs, together indicating WPB formation and maturation are unaffected by absence of syntaxin-3. However, a defect in basal as well as Ca²⁺- and cAMP-mediated VWF secretion was found in the STX3^−/− BOECs. We also show that syntaxin-3 interacts with the WPB-associated SNARE protein VAMP8 (vesicle-associated membrane protein-8).

Conclusions—

Our data reveal syntaxin-3 as a novel WPB-associated SNARE protein that controls WPB exocytosis.

Pancreatitis-Induced Depletion of Syntaxin 2 Promotes Autophagy and Increases Basolateral Exocytosis

BACKGROUND & AIMS

Pancreatic acinar cells are polarized epithelial cells that store enzymes required for digestion as inactive zymogens, tightly packed at the cell apex. Stimulation of acinar cells causes the zymogen granules to fuse with the apical membrane, and the cells undergo exocytosis to release proteases into the intestinal lumen. Autophagy maintains homeostasis of pancreatic acini. Syntaxin 2 (STX2), an abundant soluble N-ethyl maleimide sensitive factor attachment protein receptor in pancreatic acini, has been reported to mediate apical exocytosis. Using human pancreatic tissues and STX2-knockout (KO) mice, we investigated the functions of STX2 in zymogen granule-mediated exocytosis and autophagy.

METHODS

We obtained pancreatic tissues from 5 patients undergoing surgery for pancreatic cancer and prepared 80-μm slices; tissues were exposed to supramaximal cholecystokinin octapeptide (CCK-8) or ethanol and a low concentration of CCK-8 and analyzed by immunoblot and immunofluorescence analyses. STX2-KO mice and syntaxin 2+/+ C57BL6 mice (controls) were given intraperitoneal injections of supramaximal caerulein (a CCK-8 analogue) or fed ethanol and then given a low dose of caerulein to induce acute pancreatitis, or saline (controls); pancreata were isolated and analyzed by histology and immunohistochemistry. Acini were isolated from mice, incubated with CCK-8, and analyzed by immunofluorescence microscopy or used in immunoprecipitation experiments. Exocytosis was quantified using live-cell exocytosis and Ca2+ imaging analyses and based on formation of exocytotic soluble N-ethyl maleimide sensitive factor attachment protein receptor complexes. Dysregulations in autophagy were identified using markers, electron and immunofluorescence microscopy, and protease activation assays.

RESULTS

Human pancreatic tissues and dispersed pancreatic acini from control mice exposed to CCK-8 or ethanol plus CCK-8 were depleted of STX2. STX2-KO developed more severe pancreatitis after administration of supramaximal caerulein or a 6-week ethanol diet compared with control. Acini from STX2-KO mice had increased apical exocytosis after exposure to CCK-8, as well as increased basolateral exocytosis, which led to ectopic release of proteases. These increases in apical and basolateral exocytosis required increased formation of fusogenic soluble N-ethyl maleimide sensitive factor attachment protein receptor complexes, mediated by STX3 and STX4. STX2 bound ATG16L1 and prevented it from binding clathrin. Deletion of STX2 from acini increased binding of AT16L1 to clathrin, increasing formation of pre-autophagosomes and inducing autophagy. Induction of autophagy promoted the CCK-8-induced increase in autolysosome formation and the activation of trypsinogen.

CONCLUSIONS

In studies of human pancreatic tissues and pancreata from STX2-KO and control mice, we found STX2 to block STX3- and STX4-mediated fusion of zymogen granules with the plasma membrane and exocytosis and prevent binding of ATG16L1 to clathrin, which contributes to induction of autophagy. Exposure of pancreatic tissues to CCK-8 or ethanol depletes acinar cells of STX2, increasing basolateral exocytosis and promoting autophagy induction, leading to activation of trypsinogen.

Monoubiquitination of syntaxin 3 leads to retrieval from the basolateral plasma membrane and facilitates cargo recruitment to exosomes

Monoubiquitination of Stx3 leads to efficient endocytosis from the basolateral plasma membrane and trafficking into the multivesicular body/exosomal pathway. Stx3 plays a role in cargo recruitment into exosomes. This pathway is exploited by HCMV for virion excretion.

Syntaxin 3 (Stx3), a SNARE protein located and functioning at the apical plasma membrane of epithelial cells, is required for epithelial polarity. A fraction of Stx3 is localized to late endosomes/lysosomes, although how it traffics there and its function in these organelles is unknown. Here we report that Stx3 undergoes monoubiquitination in a conserved polybasic domain. Stx3 present at the basolateral—but not the apical—plasma membrane is rapidly endocytosed, targeted to endosomes, internalized into intraluminal vesicles (ILVs), and excreted in exosomes. A nonubiquitinatable mutant of Stx3 (Stx3-5R) fails to enter this pathway and leads to the inability of the apical exosomal cargo protein GPRC5B to enter the ILV/exosomal pathway. This suggests that ubiquitination of Stx3 leads to removal from the basolateral membrane to achieve apical polarity, that Stx3 plays a role in the recruitment of cargo to exosomes, and that the Stx3-5R mutant acts as a dominant-negative inhibitor. Human cytomegalovirus (HCMV) acquires its membrane in an intracellular compartment and we show that Stx3-5R strongly reduces the number of excreted infectious viral particles. Altogether these results suggest that Stx3 functions in the transport of specific proteins to apical exosomes and that HCMV exploits this pathway for virion excretion.

Syntaxin 4 mediates endosome recycling for lytic granule exocytosis in cytotoxic T-lymphocytes

Adaptive and innate immunity utilize the perforin-killing pathway to eliminate virus-infected or cancer cells. Cytotoxic T-lymphocytes (CTLs) and natural killer cells mediate this process by releasing toxic proteins at the contact area with target cells known as immunological synapse (IS). Formation of a stable IS and exocytosis of toxic proteins requires persistent fusion of Rab11a recycling endosomes with the plasma membrane (PM) that may assure the delivery of key effector proteins. Despite the importance of the recycling endosomal compartment, the membrane fusion proteins that control this process at the IS remain elusive. Here, by performing knockdown experiments we found that syntaxin 4 (STX4) is necessary for cytotoxic activity and CD107a degranulation against target cells in a similar fashion to syntaxin 11, which is involved in lytic granule (LG) exocytosis and immunodeficiency when it is mutated. Using total internal reflection fluorescent microscopy we identified that STX4 mediates fusion of EGFP-Rab11a vesicles at the IS. Immunoprecipitation experiments in lysates of activated CTLs indicate that endogenous STX4 may drive this fusion step by interacting with cognate proteins: Munc18-3/SNAP23/VAMP7 and/or VAMP8. These results reveal the role of STX4 in mediating fusion of Rab11a endosomes upstream of lytic granules (LGs) exocytosis and further demonstrate the importance of this pathway in controlling CTL-mediated cytotoxicity.

Cell polarity defines three distinct domains in pancreatic β-cells

The structural organisation of pancreatic β-cells in the islets of Langerhans is relatively unknown. Here, using three-dimensional (3D) two-photon, 3D confocal and 3D block-face serial electron microscopy, we demonstrate a consistent in situ polarisation of β-cells and define three distinct cell surface domains. An apical domain located at the vascular apogee of β-cells, defined by the location of PAR-3 (also known as PARD3) and ZO-1 (also known as TJP1), delineates an extracellular space into which adjacent β-cells project their primary cilia. A separate lateral domain, is enriched in scribble and Dlg, and colocalises with E-cadherin and GLUT2 (also known as SLC2A2). Finally, a distinct basal domain, where the β-cells contact the islet vasculature, is enriched in synaptic scaffold proteins such as liprin. This 3D analysis of β-cells within intact islets, and the definition of distinct domains, provides new insights into understanding β-cell structure and function.

Summary: 3D imaging methods identify three structural and functional domains within β-cells in islets: apical, lateral and basal.

Cargo-selective apical exocytosis in epithelial cells is conducted by Myo5B, Slp4a, Vamp7, and Syntaxin 3

The motor protein Myo5B and t-SNARE Stx3 drive cargo-selective apical exocytosis in polarized epithelial cells in a pathway dependent on v-SNARE–like Slp4a, v-SNARE Vamp7, Sec1/Munc18-like protein Munc18-2, and the Rab11/8 cascade.

Mutations in the motor protein Myosin Vb (Myo5B) or the soluble NSF attachment protein receptor Syntaxin 3 (Stx3) disturb epithelial polarity and cause microvillus inclusion disease (MVID), a lethal hereditary enteropathy affecting neonates. To understand the molecular mechanism of Myo5B and Stx3 interplay, we used genome editing to introduce a defined Myo5B patient mutation in a human epithelial cell line. Our results demonstrate a selective role of Myo5B and Stx3 for apical cargo exocytosis in polarized epithelial cells. Apical exocytosis of NHE3, CFTR (cystic fibrosis transmembrane conductance regulator), and GLUT5 required an interaction cascade of Rab11, Myo5B, Slp4a, Munc18-2, and Vamp7 with Stx3, which cooperate in the final steps of this selective apical traffic pathway. The brush border enzymes DPPIV and sucrase-isomaltase still correctly localize at the apical plasma membrane independent of this pathway. Hence, our work demonstrates how Myo5B, Stx3, Slp4a, Vamp7, Munc18-2, and Rab8/11 cooperate during selective apical cargo trafficking and exocytosis in epithelial cells and thereby provides further insight into MVID pathophysiology.

SNARE Protein Syntaxin-1 Colocalizes Closely with NMDA Receptor Subunit NR2B in Postsynaptic Spines in the Hippocampus

Syntaxins are a family of membrane-integrated proteins that are instrumental in exocytosis of vesicles. Syntaxin-1 is an essential component of the presynaptic exocytotic fusion machinery in the brain and interacts with several other proteins. Syntaxin-1 forms a four-helical bundle complex with proteins SNAP-25 and VAMP2 that drives fusion of vesicles with the plasma membrane in the active zone (AZ). Little is known, however, about the ultrastructural localization of syntaxin-1 at the synapse. We have analyzed the intrasynaptic expression of syntaxin-1 in glutamatergic hippocampal synapses in detail by using quantitative postembedding immunogold labeling. Syntaxin-1 was present in highest concentrations at the presynaptic AZ, supporting its role in transmitter release. Presynaptic plasma membrane lateral to the AZ, as well as presynaptic cytoplasmic (PreCy) vesicles were also labeled. However, syntaxin-1 was also significantly expressed in postsynaptic spines, where it was localized at the postsynaptic density (PSD), at postsynaptic lateral membranes and in postsynaptic cytoplasm. Postsynaptically, syntaxin-1 colocalized in the nanometer range with the N-methyl-D-aspartate (NMDA) receptor subunit NR2B, but only weakly with the AMPA receptor subunits GluA2/3. This observation points to the possibility that syntaxin-1 may be involved with NR2B vesicular trafficking from cytoplasmic stores to the postsynaptic plasma membrane, thus facilitating synaptic plasticity. Confocal immunofluorescence double labeling with PSD-95 and ultrastructural fractionation of synaptosomes also confirm localization of syntaxin-1 at the PSD.

Insulin secretion from beta cells in intact mouse islets is targeted towards the vasculature

AIMS/HYPOTHESIS

We set out to test the hypothesis that insulin secretion from beta cells is targeted towards the vasculature.

METHODS

The spatial location of granule fusion was identified by live-cell two-photon imaging of mouse pancreatic beta cells within intact islets, using sulforhodamine B labelling. Three-dimensional (3D) immunofluorescence of pancreatic slices was used to identify the location of proteins associated with neuronal synapses.

RESULTS

We demonstrated an asymmetric, non-random, distribution of sites of insulin granule fusion in response to glucose and focal targeting of insulin granule secretion to the beta cell membrane facing the vasculature. 3D immunofluorescence of islets showed that structural proteins, such as liprin, piccolo and Rab2-interacting molecule, normally associated with neuronal presynaptic targeting, were present in beta cells and enriched at the vascular face. In contrast, we found that syntaxin 1A and synaptosomal-associated protein 25 kDa (SNAP25) were relatively evenly distributed across the beta cells.

CONCLUSIONS/INTERPRETATION

Our results show that beta cells in situ, within intact islets, are polarised and target insulin secretion. This evidence for an 'endocrine synapse' has wide implications for our understanding of stimulus-secretion coupling in healthy islets and in disease.

Ca²⁺-regulated secretory granule exocytosis in pancreatic and parotid acinar cells

Protein secretion from acinar cells of the pancreas and parotid glands is controlled by G-protein coupled receptor activation and generation of the cellular messengers Ca(2+), diacylglycerol and cAMP. Secretory granule (SG) exocytosis shares some common characteristics with nerve, neuroendocrine and endocrine cells which are regulated mainly by elevated cell Ca(2+). However, in addition to diverse signaling pathways, acinar cells have large ∼1 μm diameter SGs (∼30 fold larger diameter than synaptic vesicles), respond to stimulation at slower rates (seconds versus milliseconds), demonstrate significant constitutive secretion, and in isolated acini, undergo sequential compound SG-SG exocytosis at the apical membrane. Exocytosis proceeds as an initial rapid phase that peaks and declines over 3 min followed by a prolonged phase that decays to near basal levels over 20-30 min. Studies indicate the early phase is triggered by Ca(2+) and involves the SG proteins VAMP2 (vesicle associated membrane protein2), Ca(2+)-sensing protein synatotagmin 1 (syt1) and the accessory protein complexin 2. The molecular details for regulation of VAMP8-mediated SG exocytosis and the prolonged phase of secretion are still emerging. Here we review the known regulatory molecules that impact the sequential exocytic process of SG tethering, docking, priming and fusion in acinar cells.

Tumor protein D52 controls trafficking of an apical endolysosomal secretory pathway in pancreatic acinar cells

Zymogen granule (ZG) formation in acinar cells involves zymogen cargo sorting from trans-Golgi into immature secretory granules (ISGs). ISG maturation progresses by removal of lysosomal membrane and select content proteins, which enter endosomal intermediates prior to their apical exocytosis. Constitutive and stimulated secretion through this mechanism is termed the constitutive-like and minor-regulated pathways, respectively. However, the molecular components that control membrane trafficking within these endosomal compartments are largely unknown. We show that tumor protein D52 is highly expressed in endosomal compartments following pancreatic acinar cell stimulation and regulates apical exocytosis of an apically directed endolysosomal compartment. Secretion from the endolysosomal compartment was detected by cell-surface antigen labeling of lysosome-associated membrane protein LAMP1, which is absent from ZGs, and had incomplete overlap with surface labeling of synaptotagmin 1, a marker of ZG exocytosis. Although culturing (16-18 h) of isolated acinar cells is accompanied by a loss of secretory responsiveness, the levels of SNARE proteins necessary for ZG exocytosis were preserved. However, levels of endolysosomal proteins D52, EEA1, Rab5, and LAMP1 markedly decreased with culture. When D52 levels were restored by adenoviral delivery, the levels of these regulatory proteins and secretion of both LAMP1 (endolysosomal) and amylase was strongly enhanced. These secretory effects were absent in alanine and aspartate substitutions of serine 136, the major D52 phosphorylation site, and were inhibited by brefeldin A, which does not directly affect the ZG compartment. Our results indicate that D52 directly regulates apical endolysosomal secretion and are consistent with previous studies, suggesting that this pathway indirectly regulates ZG secretion of digestive enzymes.

Syntaxin-3 regulates newcomer insulin granule exocytosis and compound fusion in pancreatic beta cells

AIMS/HYPOTHESIS

The molecular basis of the exocytosis of secretory insulin-containing granules (SGs) during biphasic glucose-stimulated insulin secretion (GSIS) from pancreatic beta cells remains unclear. Syntaxin (SYN)-1A and SYN-4 have been shown to mediate insulin exocytosis. The insulin-secretory function of SYN-3, which is particularly abundant in SGs, is unclear.

METHODS

Mouse pancreatic islets and INS-1 cells were treated with adenovirus carrying Syn-3 (also known as Stx3) or small interfering RNA targeting Syn-3 in order to examine insulin secretion by radioimmunoassay. The localisation and distribution of insulin granules were examined by confocal and electron microscopy. Dynamic single-granule fusion events were assessed using total internal reflection fluorescence microscopy (TIRFM).

RESULTS

Depletion of endogenous SYN-3 inhibited insulin release. TIRFM showed no change in the number or fusion competence of previously docked SGs but, instead, a marked reduction in the recruitment of newcomer SGs and their subsequent exocytotic fusion during biphasic GSIS. Conversely, overexpression of Syn-3 enhanced both phases of GSIS, owing to the increase in newcomer SGs and, remarkably, to increased SG-SG fusion, which was confirmed by electron microscopy.

CONCLUSIONS/INTERPRETATION

In insulin secretion, SYN-3 plays a role in the mediation of newcomer SG exocytosis and SG-SG fusion that contributes to biphasic GSIS.

Molecular control of compound Exocytosis: A key role for VAMP8

Exocytosis is the process of fusion of a membrane-bound vesicle with the cell membrane and subsequent release of the vesicle content to the outside. It is now widely accepted that SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) proteins are key components in the molecular machinery of exocytosis. SNARE proteins on the vesicle membrane selectively form complexes with specific SNAREs on the cell membrane. In a variant of exocytosis, called compound exocytosis, secretory vesicles still fuse with the cell membrane but vesicle-to-vesicle fusion enhances secretory output. Two types of compound exocytosis occur, either vesicles fuse with each other and then fuse with the cell membrane, or a vesicle fuses with the cell membrane and then becomes a target for further vesicles to fuse with it. It is expected that SNAREs are important for vesicle-to-vesicle fusion but the mechanism(s) that control these processes is unknown. In our recent paper (Behrendorff et al. 2011) we provide evidence that VAMP8 (a Q-SNARE) is essential in regulating compound exocytosis. Here we discuss the implications of our findings with reference to a new model for the control of vesicle-to-vesicle fusion.

Measuring calcium signals and exocytosis in tissues

BACKGROUND

Since the 1960s it has been clear that calcium is a key regulator of exocytosis. Early experiments directly showed that the secretory output was calcium dependent. But it has taken improvements in technology and clever experimentation to determine the relationships between the calcium signal and exocytosis. Today controversies still remain because of limitations in our ability to record both the calcium responses within the local domains that control secretion and in the methods used to record exocytosis.

SCOPE OF REVIEW

Here the techniques used to measure calcium and exocytosis are reviewed with a distinction being drawn between measurements in excitable cells versus measurements in non-excitable cells. The review has a focus on techniques that are relevant to in vitro studies of native tissues and recent in vivo recordings.

MAJOR CONCLUSIONS

There are a range of methods used to study the stimulus-secretion pathway. Each presents their own advantages and drawbacks. These are discussed with reference to the latest work determining the factors controlling exocytosis in tissues.

GENERAL SIGNIFICANCE

Stimulus-secretion coupling is the fundamental step in the control of neurotransmitter release, hormone secretion and protein secretion. Understanding secretory control is therefore important in understanding the physiological regulation of processes ranging from learning and memory to pancreatic secretion. Recent technological advances are now enabling us to study stimulus-secretion coupling within native tissues. This is helping us to understand the physiological complexities of secretory control. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Vesicle-associated membrane protein 8 (VAMP8) is a SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) selectively required for sequential granule-to-granule fusion

Compound exocytosis is found in many cell types and is the major form of regulated secretion in acinar and mast cells. Its key characteristic is the homotypic fusion of secretory granules. These then secrete their combined output through a single fusion pore to the outside. The control of compound exocytosis remains poorly understood. Although soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) such as syntaxin 2, SNAP23 (synaptosome-associated protein of 23 kDa), and SNAP25 have been suggested to play a role, none has been proven. Vesicle-associated membrane protein 8 (VAMP8) is a SNARE first associated with endocytic processes but more recently has been suggested as an R-SNARE in regulated exocytosis. Secretion in acinar cells is reduced when VAMP8 function is inhibited and is less in VAMP8 knock-out mice. Based on electron microscopy experiments, it was suggested that VAMP8 may be involved in compound exocytosis. Here we have tested the hypothesis that VAMP8 controls homotypic granule-to-granule fusion during sequential compound exocytosis. We use a new assay to distinguish primary fusion events (fusion with the cell membrane) from secondary fusion events (granule-granule fusion). Our data show the pancreatic acinar cells from VAMP8 knock-out animals have a specific reduction in secondary granule fusion but that primary granule fusion is unaffected. Furthermore, immunoprecipitation experiments show syntaxin 2 association with VAMP2, whereas syntaxin 3 associates with VAMP8. Taken together our data indicate that granule-to-granule fusion is regulated by VAMP8 containing SNARE complexes distinct from those that regulate primary granule fusion.

The syntaxin 4 N terminus regulates its basolateral targeting by munc18c-dependent and -independent mechanisms

To generate and maintain epithelial cell polarity, specific sorting of proteins into vesicles destined for the apical and basolateral domain is required. Syntaxin 3 and 4 are apical and basolateral SNARE proteins important for the specificity of vesicle fusion at the apical and basolateral plasma membrane domains, respectively, but how these proteins are specifically targeted to these domains themselves is unclear. Munc18/SM proteins are potential regulators of this process. Like syntaxins, they are crucial for exocytosis and vesicle fusion. However, how munc18c and syntaxin 4 regulate the function of each other is unclear. Here, we investigated the requirement of syntaxin 4 in the delivery of basolateral membrane and secretory proteins, the basolateral targeting of syntaxin 4, and the role of munc18c in this targeting. Depletion of syntaxin 4 resulted in significant reduction of basolateral targeting, suggesting no compensation by other syntaxin forms. Mutational analysis identified amino acids Leu-25 and to a lesser extent Val-26 as essential for correct localization of syntaxin 4. Recently, it was shown that the N-terminal peptide of syntaxin 4 is involved in binding to munc18c. A mutation in this region that affects munc18c binding shows that munc18c binding is required for stabilization of syntaxin 4 at the plasma membrane but not for its correct targeting. We conclude that the N terminus serves two functions in membrane targeting. First, it harbors the sorting motif, which targets syntaxin 4 basolaterally in a munc18c-independent manner and second, it allows for munc18c binding, which stabilizes the protein in a munc18c-dependent manner.

Complexin 2 modulates vesicle-associated membrane protein (VAMP) 2-regulated zymogen granule exocytosis in pancreatic acini

Complexins are soluble proteins that regulate the activity of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes necessary for vesicle fusion. Neuronal specific complexin 1 has inhibitory and stimulatory effects on exocytosis by clamping trans-SNARE complexes in a prefusion state and promoting conformational changes to facilitate membrane fusion following cell stimulation. Complexins are unable to bind to monomeric SNARE proteins but bind with high affinity to ternary SNARE complexes and with lower affinity to target SNARE complexes. Far less is understood about complexin function outside the nervous system. Pancreatic acini express the complexin 2 isoform by RT-PCR and immunoblotting. Immunofluorescence microscopy revealed complexin 2 localized along the apical plasma membrane consistent with a role in secretion. Accordingly, complexin 2 was found to interact with vesicle-associated membrane protein (VAMP) 2, syntaxins 3 and 4, but not with VAMP 8 or syntaxin 2. Introduction of recombinant complexin 2 into permeabilized acini inhibited Ca(2+)-stimulated secretion in a concentration-dependent manner with a maximal inhibition of nearly 50%. Mutations of the central α-helical domain reduced complexin 2 SNARE binding and concurrently abolished its inhibitory activity. Surprisingly, mutation of arginine 59 to histidine within the central α-helical domain did not alter SNARE binding and moreover, augmented Ca(2+)-stimulated secretion by 130% of control. Consistent with biochemical studies, complexin 2 colocalized with VAMP 2 along the apical plasma membrane following cholecystokinin-8 stimulation. These data demonstrate a functional role for complexin 2 outside the nervous system and indicate that it participates in the Ca(2+)-sensitive regulatory pathway for zymogen granule exocytosis.

Exocytosis, dependent on Ca2+ release from Ca2+ stores, is regulated by Ca2+ microdomains

The relationship between the cellular Ca2+ signal and secretory vesicle fusion (exocytosis) is a key determinant of the regulation of the kinetics and magnitude of the secretory response. Here, we have investigated secretion in cells where the exocytic response is controlled by Ca2+ release from intracellular Ca2+ stores. Using live-cell two-photon microscopy that simultaneously records Ca2+signals and exocytic responses, we provide evidence that secretion is controlled by changes in Ca2+ concentration [Ca2+] in relatively large-volume microdomains. Our evidence includes: (1) long latencies (>2 seconds) between the rise in [Ca2+] and exocytosis, (2) observation of exocytosis all along the lumen and not clustered around Ca2+ release hot-spots, (3) high affinity (Kd=1.75 microM) Ca2+dependence of exocytosis, (4) significant reduction in exocytosis in the presence of cytosolic EGTA, (5) spatial exclusion of secretory granules from the cell membrane by the endoplasmic reticulum, and (6) inability of local Ca2+ responses to trigger exocytosis. These results strongly indicate that the control of exocytosis, triggered by Ca2+ release from stores, is through the regulation of cytosolic[Ca2+] within a microdomain.

Trans-Golgi network syntaxin 10 functions distinctly from syntaxins 6 and 16

Syntaxin 10 is a soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) protein localized to the trans-Golgi network (TGN), where two other members of the syntaxin family, syntaxins 6 and 16, also reside. The role of syntaxin 10 in regulating TGN protein traffic is not yet defined. Syntaxin 10 co-localizes well with syntaxins 6 and 16 at the TGN in interphase cells, and appears to interact with both syntaxin 6 and 16 as evidenced by co-immunoprecipitation analyses. However, unlike syntaxin 6 and 16, neither syntaxin 10 antibodies nor its cytosolic domain inhibits endosome-TGN transport of shiga toxin. Syntaxin 16 knockdown with small interfering RNA (siRNA) affects the TGN localization of syntaxin 6 but not syntaxin 10, and clearly inhibits endosome-TGN transport. On the other hand, knockdown of syntaxin 10 expressions had no significant effect on the TGN localization of syntaxin 6 and 16, and did not inhibit endosome-TGN transport. Unlike syntaxin 16, syntaxin 10 does not interact specifically with Vps45, the Sec1/Munc18 (SM) family member at the TGN. On the other hand, syntaxin 10 reciprocally co-immunoprecipitated endosomal syntaxin 12/13, and knockdown of syntaxin 10 expressions affects the surface expression of transferrin receptor (TfR) and seems to induce the formation of an immobile TfR pool. Therefore, in spite of its co-localization and possible interaction with syntaxin 16, syntaxin 10 is not part of the syntaxin 16-based SNARE complex involved in endosome-TGN transport, and may have a hitherto unrecognized function in the TGN-endosome boundary.

Functional role of J domain of cysteine string protein in Ca2+-dependent secretion from acinar cells

The heat shock protein 70 family members Hsc70 and Hsp70 are known to play a protective role against the onset of experimental pancreatitis, yet their molecular function in acini is unclear. Cysteine string protein (CSP-alpha) is a zymogen granule (ZG) membrane protein characterized by an NH(2)-terminal "J domain" and a central palmitoylated string of cysteine residues. The J domain functions as a cochaperone by modulating the activity of Hsc70/Hsp70 family members. A role for CSP-alpha in regulating digestive enzyme exocytosis from pancreas was investigated by introducing CSP-alpha truncations into isolated acini following their permeabilization with Perfringolysin O. Incubation of acini with CSP-alpha(1-82), containing the J domain, significantly augmented Ca(2+)-stimulated amylase secretion. Effects of CSP-alpha(1-82) were concentration dependent, with a maximum 80% increase occurring at 200 microg/ml of protein. Although CSP-alpha(1-82) had no effects on basal secretion measured in the presence of < or =10 nM free Ca(2+), it did significantly augment GTP-gammaS-induced secretion under basal Ca(2+) conditions by approximately 25%. Mutation of the J domain to abolish its cochaperone activity failed to augment Ca(2+)-stimulated secretion, implicating the CSP-alpha/Hsc70 cochaperone system as a regulatory component of the secretory pathway. CSP-alpha physically associates with vesicle-associated membrane protein 8 (VAMP 8) on ZGs, and the CSP-alpha-VAMP 8 interaction was dependent on amino acids 83-112 of CSP-alpha. Immunofluorescence analysis of acinar lobules or purified ZGs confirmed the CSP-alpha colocalization with VAMP 8. These data establish a role for CSP-alpha in regulating digestive enzyme secretion and suggest that CSP-alpha and Hsc70 modulate specific soluble N-ethylmaleimide-sensitive attachment receptor interactions necessary for exocytosis.

VAMP8 is the v-SNARE that mediates basolateral exocytosis in a mouse model of alcoholic pancreatitis

In rodents and humans, alcohol exposure has been shown to predispose the pancreas to cholinergic or viral induction of pancreatitis. We previously developed a rodent model in which exposure to an ethanol (EtOH) diet, followed by carbachol (Cch) stimulation, redirects exocytosis from the apical to the basolateral plasma membrane of acinar cells, resulting in ectopic zymogen enzyme activation and pancreatitis. This redirection of exocytosis involves a soluble NSF attachment receptor (SNARE) complex consisting of syntaxin-4 and synapse-associated protein of 23 kDa (SNAP-23). Here, we investigated the role of the zymogen granule (ZG) SNARE vesicle-associated membrane protein 8 (VAMP8) in mediating basolateral exocytosis. In WT mice, in vitro EtOH exposure or EtOH diet reduced Cch-stimulated amylase release by redirecting apical exocytosis to the basolateral membrane, leading to alcoholic pancreatitis. Further reduction of zymogen secretion, caused by blockade of both apical and basolateral exocytosis and resulting in a more mild induction of alcoholic pancreatitis, was observed in Vamp8(-/-) mice in response to these treatments. In addition, although ZGs accumulated in Vamp8(-/-) acinar cells, ZG-ZG fusions were reduced compared with those in WT acinar cells, as visualized by electron microscopy. This reduction in ZG fusion may account for reduced efficiency of apical exocytosis in Vamp8(-/-) acini. These findings indicate that VAMP8 is the ZG-SNARE that mediates basolateral exocytosis in alcoholic pancreatitis and that VAMP8 is critical for ZG-ZG homotypic fusion.

AQP2 exocytosis in the renal collecting duct -- involvement of SNARE isoforms and the regulatory role of Munc18b

Vasopressin regulates the fusion of the water channel aquaporin 2 (AQP2) to the apical membrane of the renal collecting-duct principal cells and several lines of evidence indicate that SNARE proteins mediate this process. In this work MCD4 renal cells were used to investigate the functional role of a set of Q- and R-SNAREs, together with that of Munc18b as a negative regulator of the formation of the SNARE complex. Both VAMP2 and VAMP3 were associated with immunoisolated AQP2 vesicles, whereas syntaxin 3 (Stx3), SNAP23 and Munc18 were associated with the apical plasma membrane. Co-immunoprecipitation experiments indicated that Stx3 forms complexes with VAMP2, VAMP3, SNAP23 and Munc18b. Protein knockdown coupled to apical surface biotinylation demonstrated that reduced levels of the R-SNAREs VAMP2 and VAMP3, and the Q-SNAREs Stx3 and SNAP23 strongly inhibited AQP2 fusion at the apical membrane. In addition, knockdown of Munc18b promoted a sevenfold increase of AQP2 fused at the plasma membrane without forskolin stimulation. Taken together these findings propose VAMP2, VAMP3, Stx3 and SNAP23 as the complementary set of SNAREs responsible for AQP2-vesicle fusion into the apical membrane, and Munc18b as a negative regulator of SNARE-complex formation in renal collecting-duct principal cells.

Munc18/SNARE proteins’ regulation of exocytosis in guinea pig duodenal Brunner’s gland acini

AIM: To examine the molecular mechanism of exocytosis in the Brunner’s gland acinar cell.

METHODS: We used a submucosal preparation of guinea pig duodenal Brunner’s gland acini to visualize the dilation of the ductal lumen in response to cholinergic stimulus. We correlated this to electron microscopy to determine the extent of exocytosis of the mucin-filled vesicles. We then examined the behavior of SNARE and interacting Munc18 proteins by confocal microscopy.

RESULTS: One and 6 &mgr;mol/L carbachol evoked a dose-dependent dilation of Brunner’s gland acini lumen, which correlated to the massive exocytosis of mucin. Munc18c and its cognate SNARE proteins Syntaxin-4 and SNAP-23 were localized to the apical plasma membrane, and upon cholinergic stimulation, Munc18c was displaced into the cytosol leaving Syntaxin-4 and SNAP-23 intact.

CONCLUSION: Physiologic cholinergic stimulation induces Munc18c displacement from the Brunner’s gland acinar apical plasma membrane, which enables apical membrane Syntaxin-4 and SNAP-23 to form a SNARE complex with mucin-filled vesicle SNARE proteins to affect exocytosis.

Sdmg1 is a conserved transmembrane protein associated with germ cell sex determination and germline-soma interactions in mice

In mammals, the supporting cell lineage in an embryonic gonad communicates the sex-determining decision to various sexually dimorphic cell types in the developing embryo, including the germ cells. However, the molecular nature of the sex-determining signals that pass from the supporting cells to the germ cells is not well understood. We have identified a conserved transmembrane protein, Sdmg1, owing to its male-specific expression in mouse embryonic gonads. Sdmg1 is expressed in the Sertoli cells of embryonic testes from 12.5 dpc, and in granulosa cells of growing follicles in adult ovaries. In Sertoli cells, Sdmg1 is localised to endosomes, and knock-down of Sdmg1 in Sertoli cell lines causes mis-localisation of the secretory SNARE Stx2 and defects in membrane trafficking. Upregulation of Sdmg1 appears to be part of a larger programme of changes to membrane trafficking pathways in embryonic Sertoli cells, and perturbing secretion in male embryonic gonads in organ culture causes male-to-female germ cell sex reversal. These data suggest that changes that occur in the cell biology of embryonic Sertoli cells may facilitate the communication of male sex-determining decisions to the germ cells during embryonic development.

The use of syntaxin chimeras to study polarized protein trafficking in epithelial cells

The plasma membrane of epithelial cells has two physically separated membrane domains. This membrane polarization is essential for the function of epithelial cells. It has been well established that different plasma membrane syntaxin forms are expressed in epithelial cells. In addition, these syntaxin forms can have a polarized localization, suggesting that they may play a direct role in the specificity of polarized membrane delivery. To determine the mechanism of the polarized syntaxin localization, we have made several chimeras of syntaxin 3 and 4. This allowed us to identify the protein sequences involved in this polarized localization. Using this technique, we showed that targeting information of syntaxin 3 and 4 is located in the first 30 amino acids.

Alcohol redirects CCK-mediated apical exocytosis to the acinar basolateral membrane in alcoholic pancreatitis

The molecular mechanism of clinical alcohol-induced pancreatitis remains vague. We had reported that experimental high-dose cholecystokinin (CCK)-induced pancreatitis is in part because of excessive aberrant basolateral exocytosis. High-dose CCK caused Munc18c on basolateral plasma membrane (BPM) to dissociate from syntaxin (Syn)-4, activating Syn-4 to complex with plasma membrane (PM)-SNAP-23 and granule-VAMP to mediate basolateral exocytosis. We now hypothesize that alcohol could render the acinar cell BPM conducive to exocytosis by a similar mechanism. Weakly stimulating postprandial doses of alcohol (20-50 mM) inhibited postprandial low-dose CCK-stimulated secretion by blocking physiologic apical exocytosis and redirecting exocytosis to less-efficient basal PM (visualized by FM1-43 fluorescence imaging) and lateral PM sites (electron microscopy). Alcohol or low-dose CCK had no effect on PM-Munc18c, but alcohol preincubation enabled low-dose CCK to displace Munc18c from BPM, leading to SNARE complex assembly in the BPM. Similarly, alcohol diet-fed rats did not exhibit morphologic defects in the pancreas nor affected PM-Munc18c behavior, but subsequent intraperitoneal injections of low-dose CCK analog cerulein caused Munc18c displacement from BPM and cytosolic degradation, which contributed to pancreatitis. We conclude that alcohol induces BPM-Munc18c to become receptive to postprandial CCK-induced displacement into the cytosol, a process which facilitates SNARE complex assembly that in turn activates restricted BPM sites to become available for aberrant exocytosis into the interstitial space, where zymogen activation would take place and cause pancreatitis.

Purification and proteomics analysis of pancreatic zymogen granule membranes

Pancreatic zymogen granules (ZGs) are specialized for digestive enzyme storage and regulated secretion in exocrine pancreas and are a classical model for studying secretory granule function. To understand the function of this organelle, we have conducted a proteomic study to identify the ZG membrane (ZGM) proteins from ZGs purified by Percoll gradient centrifugation. By combining multiple separation strategies including two-dimensional gel electrophoresis and two-dimensional HPLC with tandem mass spectrometry, we identified 101 proteins from purified ZGMs including a large number of proteins previously unknown on ZGMs. To distinguish intrinsic membrane proteins from soluble and peripheral membrane proteins, a quantitative proteomics strategy was used to measure the enrichment of intrinsic membrane proteins through the purification steps by labeling crude, KBr-, and Na(2)CO(3)-washed ZGMs with multiplexed isobaric tags (iTRAQtrade mark), 114, 116, and 117, respectively. The proteins with 117:114 ratios greater than one correlated well with intrinsic membrane proteins that contain either known or predicted transmembrane domains.

SARA-regulated vesicular targeting underlies formation of the light-sensing organelle in mammalian rods

The light-sensing organelle of the vertebrate rod photoreceptor, the outer segment (OS), is a modified cilium containing approximately 1,000 stacked disc membranes that are densely packed with visual pigment rhodopsin. The mammalian OS is renewed every ten days; new discs are assembled at the base of the OS by a poorly understood mechanism. Our results suggest that discs are formed and matured in a process that involves specific phospholipid-directed vesicular membrane targeting. Rhodopsin-laden vesicles in the OS axonemal cytoplasm fuse with nascent discs that are highly specialized with abundant phosphatidylinositol 3-phosphate (PI3P). This membrane coupling is regulated by the FYVE domain-containing protein, SARA, through its direct interaction with PI3P, rhodopsin, and SNARE protein syntaxin 3. Our model, in contrast to the previously proposed evagination model, suggests that the vesicular delivery of rhodopsin in the OS concentrates rhodopsin into discs, and this process directly participates in disc biogenesis.

Identification of SNAREs that mediate zymogen granule exocytosis

A secretagogue-stimulated pancreatic acinar cell releases digestive enzymes from its apical pole. We attempted to identify the SNAREs involved in zymogen granule exocytosis. Antibodies against syntaxins 2 and 3, SNAP-23 and VAMP 8, and the corresponding recombinant SNAREs, inhibited amylase secretion from streptolysin O-permeabilised acini; other anti-SNARE antibodies and SNAREs had no effect. Botulinum neurotoxin C, which cleaved syntaxin 2 and (to a lesser extent) syntaxin 3, but not syntaxins 4, 7 or 8, also inhibited exocytosis. We propose that syntaxin 2, SNAP-23 and VAMP 8 mediate primary granule-plasma membrane fusion. Syntaxin 3 may be involved in secondary granule-granule fusion.

Recent insights into the cellular mechanisms of acute pancreatitis

In acute pancreatitis, initiating cellular events causing acinar cell injury includes co-localization of zymogens with lysosomal hydrolases, leading to premature enzyme activation and pathological exocytosis of zymogens into the interstitial space. This is followed by processes that accentuate cell injury; triggering acute inflammatory mediators, intensifying oxidative stress, compromising the microcirculation and activating a neurogenic feedback. Such localized events then progress to a systemic inflammatory response leading to multiorgan dysfunction syndrome with resulting high morbidity and mortality. The present review discusses some of the most recent insights into each of these cellular processes postulated to cause or propagate the process of acute pancreatitis, and also the role of alcohol and genetics.

Alcohol-induced protein kinase Calpha phosphorylation of Munc18c in carbachol-stimulated acini causes basolateral exocytosis

BACKGROUND & AIMS

Acute or chronic alcohol treatment does little to the exocrine pancreas but predisposes the pancreas to postprandial cholinergic stimulation that triggers cellular events leading to pancreatitis. This alcohol-induced susceptibility mechanism of pancreatitis is unknown.

METHODS

We employed alcohol-treated dispersed rat pancreatic acini and alcohol diet-fed rats to examine the effects of submaximal carbachol-induced changes in exocytosis (FM1-43 epifluorescence imaging and electron microscopy), Munc18c cellular translocation (confocal microscopy and subcellular fractionation), and protein kinase C (PKC) alpha-induced phosphorylation in relation to pancreatitis.

RESULTS

Acute low-dose alcohol (20 mmol/L) in vitro exposure or chronic alcohol diet reduces postprandial cholinergic-stimulated amylase secretion from rat pancreatic acinar cells by blocking apical exocytosis and redirecting exocytosis to less efficient basolateral plasma membrane sites. This ectopic exocytosis is mediated by PKCalpha-induced phosphorylation of Munc18c, causing Munc18c displacement from the basolateral plasma membrane into the cytosol in which it undergoes proteolytic degradation; these processes can be blocked by PKCalpha inhibition.

CONCLUSIONS

We conclude that sequential low-dose alcohol and postprandial cholinergic stimulation can induce PKCalpha-mediated Munc18c plasma membrane displacement. This relieves cognate SNARE proteins on zymogen granules and basolateral membrane to complex and consummate pathologic ectopic exocytosis at the basolateral surface. This change in vesicle trafficking may be related to the pathogenesis of pancreatitis.

Alcohol/cholecystokinin-evoked pancreatic acinar basolateral exocytosis is mediated by protein kinase C alpha phosphorylation of Munc18c

The pancreatic acinus is the functional unit of the exocrine pancreas whose role is to secrete zymogens into the gut lumen for food digestion via apical exocytosis. We previously reported that supramaximal CCK induced apical blockade and redirected exocytosis to ectopic sites on the basolateral plasma membrane (BPM) of this polarized cell, leading to pancreatitis. Basolateral exocytosis was mediated by protein kinase C phosphorylation of BPM Munc18c, causing its displacement into the cytosol and activation of BPM-bound Syntaxin-4 to form a SNARE complex. To mimic the conditions of alcoholic pancreatitis, we now examined whether 20 mm alcohol followed by submaximal CCK might mimic supramaximal CCK in inducing these pathologic exocytotic events. We show that a non-secretory but clinically relevant alcohol concentration (20 mm) inhibited submaximal CCK (50 pM)-stimulated amylase secretion by blocking apical exocytosis and redirecting exocytosis to less efficient BPM, indeed mimicking supramaximal CCK (10 nM) stimulation. We further demonstrate that basolateral exocytosis caused by both stimulation protocols is mediated by PKC alpha-induced phosphorylation of Munc18c: 1) PKC alpha is activated, which binds and induces phosphorylation of PM-Munc18c at a Thr site, and these events can be inhibited by PKC alpha blockade; 2) PKC alpha inhibition blocks Munc18c displacement from the BPM; 3) PKC alpha inhibition prevents basolateral exocytosis but does not rescue apical exocytosis. We conclude that 20 mm alcohol/submaximal CCK as well supramaximal CCK stimulation can trigger pathologic basolateral exocytosis in pancreatic acinar cells via PKC alpha-mediated activation of Munc18c, which enables Syntaxin-4 to become receptive in forming a SNARE complex in the BPM; and we further postulate this to be an underlying mechanism contributing to alcoholic pancreatitis.

Pancreatic acinar cells express vesicle-associated membrane protein 2- and 8-specific populations of zymogen granules with distinct and overlapping roles in secretion

Previous studies have demonstrated roles for vesicle-associated membrane protein 2 (VAMP 2) and VAMP 8 in Ca(2+)-regulated pancreatic acinar cell secretion, however, their coordinated function in the secretory pathway has not been addressed. Here we provide evidence using immunofluorescence microscopy, cell fractionation, and SNARE protein interaction studies that acinar cells contain two distinct populations of zymogen granules (ZGs) expressing either VAMP 2 or VAMP 8. Further, VAMP 8-positive granules also contain the synaptosome-associated protein 29, whereas VAMP 2-expressing granules do not. Analysis of acinar secretion by Texas red-dextran labeling indicated that VAMP 2-positive ZGs mediate the majority of exocytotic events during constitutive secretion and also participate in Ca(2+)-regulated exocytosis, whereas VAMP 8-positive ZGs are more largely involved in Ca(2+)-stimulated secretion. Previously undefined functional roles for VAMP and syntaxin isoforms in acinar secretion were established by introducing truncated constructs of these proteins into permeabilized acini. VAMP 2 and VAMP 8 constructs each attenuated Ca(2+)-stimulated exocytosis by 50%, whereas the neuronal VAMP 1 had no effects. In comparison, the plasma membrane SNAREs syntaxin 2 and syntaxin 4 each inhibited basal exocytosis, but only syntaxin 4 significantly inhibited Ca(2+)-stimulated secretion. Syntaxin 3, which is expressed on ZGs, had no effects. Collectively, these data demonstrate that individual acinar cells express VAMP 2- and VAMP 8-specific populations of ZGs that orchestrate the constitutive and Ca(2+)-regulated secretory pathways.

Apical targeting of syntaxin 3 is essential for epithelial cell polarity

In polarized epithelial cells, syntaxin 3 localizes to the apical plasma membrane and is involved in membrane fusion of apical trafficking pathways. We show that syntaxin 3 contains a necessary and sufficient apical targeting signal centered around a conserved FMDE motif. Mutation of any of three critical residues within this motif leads to loss of specific apical targeting. Modeling based on the known structure of syntaxin 1 revealed that these residues are exposed on the surface of a three-helix bundle. Syntaxin 3 targeting does not require binding to Munc18b. Instead, syntaxin 3 recruits Munc18b to the plasma membrane. Expression of mislocalized mutant syntaxin 3 in Madin-Darby canine kidney cells leads to basolateral mistargeting of apical membrane proteins, disturbance of tight junction formation, and loss of ability to form an organized polarized epithelium. These results indicate that SNARE proteins contribute to the overall specificity of membrane trafficking in vivo, and that the polarity of syntaxin 3 is essential for epithelial cell polarization.

Distribution of plasma membrane-associated syntaxins 1 through 4 indicates distinct trafficking functions in the synaptic layers of the mouse retina

Background

Syntaxins 1 through 4 are SNAP receptor (SNARE) proteins that mediate vesicular trafficking to the plasma membrane. In retina, syntaxins 1 and 3 are expressed at conventional and ribbon synapses, respectively, suggesting that synaptic trafficking functions differ among syntaxin isoforms. To better understand syntaxins in synaptic signaling and trafficking, we further examined the cell- and synapse-specific expression of syntaxins 1 through 4 in the mouse retina by immunolabeling and confocal microscopy.

Results

Each isoform was expressed in the retina and showed a unique distribution in the synaptic layers of the retina, with little or no colocalization of isoforms. Syntaxin 1 was present in amacrine cell bodies and processes and conventional presynaptic terminals in the inner plexiform layer (IPL). Syntaxin 2 was present in amacrine cells and their processes in the IPL, but showed little colocalization with syntaxin 1 or other presynaptic markers. Syntaxin 3 was found in glutamatergic photoreceptor and bipolar cell ribbon synapses, but was absent from putative conventional glutamatergic amacrine cell synapses. Syntaxin 4 was localized to horizontal cell processes in the ribbon synaptic complexes of photoreceptor terminals and in puncta in the IPL that contacted dopaminergic and CD15-positive amacrine cells. Syntaxins 2 and 4 often were apposed to synaptic active zones labeled for bassoon.

Conclusion

These results indicate that each syntaxin isoform has unique, non-redundant functions in synaptic signaling and trafficking. Syntaxins 1 and 3 mediate presynaptic transmitter release from conventional and ribbon synapses, respectively. Syntaxins 2 and 4 are not presynaptic and likely mediate post-synaptic trafficking.

Syntaxins 3 and 4 are concentrated in separate clusters on the plasma membrane before the establishment of cell polarity

Syntaxins 3 and 4 localize to the apical and basolateral plasma membrane, respectively, of epithelial cells where they mediate vesicle fusion. Here, we report that before establishment of cell polarity, syntaxins 3 and 4 are confined to mutually exclusive, submicron-sized clusters. Syntaxin clusters are remarkably uniform in size, independent of expression levels, and are distinct from caveolae and clathrin-coated pits. SNAP-23 partially colocalizes with both syntaxin 3 and 4 clusters. Deletion of the apical targeting signal of syntaxin 3 does not prevent sorting into clusters away from syntaxin 4. Syntaxin 3 and 4 cluster formation depends on different mechanisms because the integrity of syntaxin 3 clusters depends on intact microtubules, whereas syntaxin 4 clusters depend on intact actin filaments. Cholesterol depletion causes dispersion of syntaxin 3 but not syntaxin 4 clusters. In migrating cells, syntaxin clusters polarize to the leading edge, suggesting a role in polarized exocytosis. These results suggest that exocytosis occurs at small fusion sites exhibiting high local concentrations of SNARE proteins that may be required for efficient membrane fusion. The establishment of separate clusters for each syntaxin suggests that the plasma membrane is inherently polarized on an ultrastructural level even before the establishment of true cell polarity.

Compound Exocytosis: Mechanisms and Functional Significance

Compound exocytosis occurs in many cell types. It represents a specialized form of secretion in which vesicles undergo fusion with each other as well as with the plasma membrane. In most cases, compound exocytosis occurs sequentially, with deeper-lying vesicles fusing, after a delay, with vesicles that have already fused with the plasma membrane. However, in some cells, vesicles can also apparently fuse with each other intracellularly before any interaction with the plasma membrane. In this review, we discuss the general features of compound exocytosis, and the features that are specific to particular cells. We consider mechanisms that might impose the requirement for vesicles to fuse with the plasma membrane before they become able to fuse with each other, the possibility that there are biochemical differences between vesicle-plasma membrane fusion events and subsequent secondary homotypic vesicle fusion events, and the role that cytoskeletal elements might play in the stabilization of fused vesicles, in order to permit secondary fusion events. Finally, we discuss the likely physiological significance of compound exocytosis in the various cell types in which it exists.