Christina Bark

An Ancient Duplication of Exon 5 in the Snap25 Gene Is Required for Complex Neuronal Development/Function

Alternative splicing is an evolutionary innovation to create functionally diverse proteins from a limited number of genes. SNAP-25 plays a central role in neuroexocytosis by bridging synaptic vesicles to the plasma membrane during regulated exocytosis. The SNAP-25 polypeptide is encoded by a single copy gene, but in higher vertebrates a duplication of exon 5 has resulted in two mutually exclusive splice variants, SNAP-25a and SNAP-25b. To address a potential physiological difference between the two SNAP-25 proteins, we generated gene targeted SNAP-25b deficient mouse mutants by replacing the SNAP-25b specific exon with a second SNAP-25a equivalent. Elimination of SNAP-25b expression resulted in developmental defects, spontaneous seizures, and impaired short-term synaptic plasticity. In adult mutants, morphological changes in hippocampus and drastically altered neuropeptide expression were accompanied by severe impairment of spatial learning. We conclude that the ancient exon duplication in the Snap25 gene provides additional SNAP-25-function required for complex neuronal processes in higher eukaryotes.

Overexpression of rat neuronal calcium sensor-1 in rodent NG108-15 cells enhances synapse formation and transmission

1 The role of rat neuronal calcium sensor‐1 (NCS‐1), a Ca2+‐binding protein, in synapse formation and transmitter release was examined in mouse neuroblastoma × rat glioma hybrid NG108‐15 cells in culture. 2 Wild‐type NG108‐15 cells expressed rodent NCS‐1. Endogenous NCS‐1 was partially co‐localized with the synaptic protein SNAP‐25 at the plasma membrane in both cell bodies and processes, but not with the Golgi marker β‐COP, an individual coat subunit of the coatomer complex present on Golgi‐derived vesicles. 3 In NG108‐15 cells co‐cultured with rat myotubes, partial co‐localization of SNAP‐25 and NCS‐1 was observed at the plasma membrane of neurites and growth cones, some of which had synaptic contacts to muscle cells. 4 Transient co‐transfection of the rat NCS‐1 cDNA and green fluorescent protein (GFP) resulted in NCS‐1 overexpression in about 30 % of the cells as determined by fluorescence microscopy. 5 The rate of functional synapse formation with co‐cultured rat myotubes increased 2‐fold as determined by the presence of miniature endplate potentials (MEPPs) in NCS‐1‐overexpressing NG108‐15 cells compared to non‐ and mock‐transfected cells. 6 The number of neurites per cell, branches per neurite and length of neurites was slightly less in cells that were either transiently transfected (GFP‐NCS‐1‐fluorescence positive) or stably transformed with NCS‐1 compared to GFP‐NCS‐1‐negative, non‐transfected or mock‐transfected NG108‐15 cells. 7 The number of action potentials that elicited endplate potentials increased in NG108‐15 cells stably transformed with rat NCS‐1. The mean number of quanta per impulse (m) increased 5‐fold. 8 These results show that NCS‐1 functions to facilitate synapse formation, probably because of the increased quantal content of evoked acetylcholine release.

Nephrin Is Expressed on the Surface of Insulin Vesicles and Facilitates Glucose-Stimulated Insulin Release

OBJECTIVE Nephrin, an immunoglobulin-like protein essential for the function of the glomerular podocyte and regulated in diabetic nephropathy, is also expressed in pancreatic β-cells, where its function remains unknown. The aim of this study was to investigate whether diabetes modulates nephrin expression in human pancreatic islets and to explore the role of nephrin in β-cell function. RESEARCH DESIGN AND METHODS Nephrin expression in human pancreas and in MIN6 insulinoma cells was studied by Western blot, PCR, confocal microscopy, subcellular fractionation, and immunogold labeling. Islets from diabetic (n = 5) and nondiabetic (n = 7) patients were compared. Stable transfection and siRNA knockdown in MIN-6 cells/human islets were used to study nephrin function in vitro and in vivo after transplantation in diabetic immunodeficient mice. Live imaging of green fluorescent protein (GFP)-nephrin–transfected cells was used to study nephrin endocytosis. RESULTS Nephrin was found at the plasma membrane and on insulin vesicles. Nephrin expression was decreased in islets from diabetic patients when compared with nondiabetic control subjects. Nephrin transfection in MIN-6 cells/pseudoislets resulted in higher glucose-stimulated insulin release in vitro and in vivo after transplantation into immunodeficient diabetic mice. Nephrin gene silencing abolished stimulated insulin release. Confocal imaging of GFP-nephrin–transfected cells revealed nephrin endocytosis upon glucose stimulation. Actin stabilization prevented nephrin trafficking as well as nephrin-positive effect on insulin release. CONCLUSIONS Our data suggest that nephrin is an active component of insulin vesicle machinery that may affect vesicle-actin interaction and mobilization to the plasma membrane. Development of drugs targeting nephrin may represent a novel approach to treat diabetes.

Munc18-1 and Munc18-2 Proteins Modulate β-Cell Ca2+ Sensitivity and Kinetics of Insulin Exocytosis Differently

Fast neurotransmission and slower hormone release share the same core fusion machinery consisting of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. In evoked neurotransmission, interactions between SNAREs and the Munc18-1 protein, a member of the Sec1/Munc18 (SM) protein family, are essential for exocytosis, whereas other SM proteins are dispensable. To address if the exclusivity of Munc18-1 demonstrated in neuroexocytosis also applied to fast insulin secretion, we characterized the presence and function of Munc18-1 and its closest homologue Munc18-2 in β-cell stimulus-secretion coupling. We show that pancreatic β-cells express both Munc18-1 and Munc18-2. The two Munc18 homologues exhibit different subcellular localization, and only Munc18-1 redistributes in response to glucose stimulation. However, both Munc18-1 and Munc18-2 augment glucose-stimulated hormone release. Ramp-like photorelease of caged Ca2+ and high resolution whole-cell patch clamp recordings show that Munc18-1 and Munc18-2 overexpression shift the Ca2+ sensitivity of the fastest phase of insulin exocytosis differently. In addition, we reveal that Ca2+ sensitivity of exocytosis in β-cells depends on the phosphorylation status of the Munc18 proteins. Even though Munc18-1 emerges as the key SM-protein determining the Ca2+ threshold for triggering secretory activity in a stimulated β-cell, Munc18-2 has the ability to increase Ca2+ sensitivity and thus mediates the release of fusion-competent granules requiring a lower cytoplasmic-free Ca2+ concentration, [Ca2+]i. Hence, Munc18-1 and Munc18-2 display distinct subcellular compartmentalization and can coordinate the insulin exocytotic process differently as a consequence of the actual [Ca2+]i.

Differential sorting of SNAP-25a and SNAP-25b proteins in neuroblastoma cells.

Exocytosis is mediated by high-affinity interactions between different SNARE proteins. The existence of several variants of each SNARE protein suggests that the specificity of fusion may be directed by unique combination of SNARE family members. We examined if two alternatively spliced variants of synaptosomal-associated protein of 25 kD, SNAP-25a and SNAP-25b, possessed distinct cellular distribution if coexpressed within the same neuroblastoma cell. Double-labelling immunofluorescence histochemistry in combination with confocal laser microscopy of individual cell clones revealed a different subcellular localisation pattern for the two SNAP-25 variants. Sucrose density gradient centrifugation of cell homogenates followed by Western blotting showed that the SNAP-25 protein variants associated with intracellular organelles of different density. Taken together, this study shows that two alternatively spliced variants of SNAP-25, differing in only nine amino acids, possess distinct properties at the level of intracellular trafficking, suggesting that the cellular localisation of SNAP-25 protein is regulated at the level of mRNA splicing.

Heterogeneous expression of SNARE proteins SNAP-23, SNAP-25, Syntaxin1 and VAMP in human parathyroid tissue

In regulated exocytosis synaptosomal-associated protein of 25kDa (SNAP-25) is one of the key-players in the formation of SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptor) complex and membrane fusion. SNARE proteins are essentially expressed in neurons, neuroendocrine and endocrine cells. Whether parathyroid cells express these proteins is not known. In this study, we have examined the expression of the SNARE protein SNAP-25 and its cellular homologue SNAP-23, as well as syntaxin1 and VAMP (vesicle-associated membrane protein) in samples of normal parathyroid tissue, chief cell adenoma, and parathyroid carcinoma, using immunohistochemistry and Western blot analysis. SNAP-23 and VAMP were evenly expressed in all studied parathyroid tissues using immunohistochemistry and/or Western blot analysis. SNAP-25 (and Syntaxin1) was not expressed in normal parathyroid tissue, but in approximately 20% of chief cell adenomas, and in approximately 45% of parathyroid carcinoma samples. It is likely that the SNARE proteins SNAP-23 and VAMP play a role in the stimulus-secretion coupling and exocytosis of parathyroid hormone as these proteins were expressed in all of the parathyroid samples we studied. In particular, preferential expression of SNAP-23 rather than SNAP-25 provides an explanation of the high level of PTH secretion that occurs under conditions of low cytoplasmic free Ca(2+) concentration (around 0.1micromol/l). SNAP-25 (and Syntaxin1) appears to be a tumour-specific protein(s) in parathyroid tissues since its expression was restricted to pathological tissues.

Replacing SNAP-25b with SNAP-25a expression results in metabolic disease

Significance Our changed lifestyle, including decreased physical activity and increased food consumption, is leading to a pandemic of obesity and type 2 diabetes, the metabolic syndrome. Impaired release of hormones, such as insulin, from excitable cells usually is considered a symptom, not a cause, of this syndrome. Here, however, we show that a small genetic modification, replacing synaptosomal-associated protein of 25 kDa (SNAP-25), SNAP-25b with SNAP-25a in the machinery mediating stimulus-dependent release of hormones and neurotransmitters is sufficient to provoke hypothalamic dysfunction, obesity, and type 2 diabetes. When combined with a Western diet, this genetic condition triggers severe diabesity. Our work expands previous knowledge supporting the notion that many individuals have an increased susceptibility to developing metabolic disease because of a genetic predisposition. Synaptosomal-associated protein of 25 kDa (SNAP-25) is a key molecule in the soluble N-ethylmaleimide–sensitive factor attachment protein (SNARE) complex mediating fast Ca2+-triggered release of hormones and neurotransmitters, and both splice variants, SNAP-25a and SNAP-25b, can participate in this process. Here we explore the hypothesis that minor alterations in the machinery mediating regulated membrane fusion can increase the susceptibility for metabolic disease and precede obesity and type 2 diabetes. Thus, we used a mouse mutant engineered to express normal levels of SNAP-25 but only SNAP-25a. These SNAP-25b–deficient mice were exposed to either a control or a high-fat/high-sucrose diet. Monitoring of food intake, body weight, hypothalamic function, and lipid and glucose homeostases showed that SNAP-25b–deficient mice fed with control diet developed hyperglycemia, liver steatosis, and adipocyte hypertrophy, conditions dramatically exacerbated when combined with the high-fat/high-sucrose diet. Thus, modified SNARE function regulating stimulus-dependent exocytosis can increase the vulnerability to and even provoke metabolic disease. When combined with a high-fat/high-sucrose diet, this vulnerability resulted in diabesity. Our SNAP-25b–deficient mouse may represent a diabesity model.

SNAP‐25 and Gene‐targeted Mouse Mutants

The evolutionary conserved soluble N‐ethylmaleimide‐sensitive factor attachment protein receptor (SNARE) fusion machinery is the operational unit in the release of neurotransmitters and hormones from excitable cells. The SNARE core complex consists of three proteins named SNAP‐25 (synaptosomal‐associated protein of 25 kD), syntaxin 1, and VAMP (vesicle‐associated membrane protein)/synaptobrevin. Syntaxin 1 is, together with SNAP‐25, localized to the plasma membrane, whereas VAMP/synaptobrevin is a component of secretory vesicles. In concert with the SNAREs, accessory factors govern the docking and priming of secretory vesicles prior to trans‐SNARE complex formation and ultimately Ca2+‐triggered fusion pore opening at the plasma membrane. The synaptic SNAP‐25 protein exists as two closely related protein variants, named SNAP‐25a and SNAP‐25b. SNAP‐25a and SNAP‐25b are both encoded from a single copy gene and generated by obligate alternative splicing between two similar exon 5 sequences. Exon 5 spans a region of SNAP‐25 that is subject to posttranslational palmitoylation and implicated in membrane anchoring of this cytosolic protein. The alternative splicing is strictly developmentally and neuroanatomically regulated, but the biological relevance of the distinct expression of these two similar protein variants is still a question of debate. However, recent findings in gene‐targeted mouse mutants have started to unravel the importance that physiological levels of total SNAP‐25 protein are present and, importantly, that this is accompanied by a balanced expression of SNAP‐25a and SNAP‐25b.

SNAP-25b-deficiency increases insulin secretion and changes spatiotemporal profile of Ca 2+ oscillations in β cell networks

SNAP-25 is a protein of the core SNARE complex mediating stimulus-dependent release of insulin from pancreatic β cells. The protein exists as two alternatively spliced isoforms, SNAP-25a and SNAP-25b, differing in 9 out of 206 amino acids, yet their specific roles in pancreatic β cells remain unclear. We explored the effect of SNAP-25b-deficiency on glucose-stimulated insulin release in islets and found increased secretion both in vivo and in vitro. However, slow photo-release of caged Ca2+ in β cells within pancreatic slices showed no significant differences in Ca2+-sensitivity, amplitude or rate of exocytosis between SNAP-25b-deficient and wild-type littermates. Therefore, we next investigated if Ca2+ handling was affected in glucose-stimulated β cells using intracellular Ca2+-imaging and found premature activation and delayed termination of [Ca2+]i elevations. These findings were accompanied by less synchronized Ca2+-oscillations and hence more segregated functional β cell networks in SNAP-25b-deficient mice. Islet gross morphology and architecture were maintained in mutant mice, although sex specific compensatory changes were observed. Thus, our study proposes that SNAP-25b in pancreatic β cells, except for participating in the core SNARE complex, is necessary for accurate regulation of Ca2+-dynamics.

SNAP-25a and SNAP-25b differently mediate interactions with Munc18-1 and Gβγ subunits

SNAP-25 is a protein involved in regulated membrane fusion and part of the SNARE complex. It exists as two splicing variants, SNAP-25a and SNAP-25b, which differ in 9 out of 206 amino acids. SNAP-25 together with Syntaxin 1 and VAMP-2 forms the ternary SNARE complex essential for mediating activity-dependent release of hormones and neurotransmitters. The functional difference between SNAP-25a and SNAP-25b is poorly understood as both can participate in SNARE complexes and mediate membrane fusion. However, we recently demonstrated that SNAP-25b-deficiency results in metabolic disease and increased insulin secretion. Here we investigated if SNAP-25a and SNAP-25b differently affect interactions with other SNAREs and SNARE-interacting proteins in mouse hippocampus. Adult mice almost exclusively express the SNAP-25b protein in hippocampus whereas SNAP-25b-deficient mice only express SNAP-25a. Immunoprecipitation studies showed no significant differences in amount of Syntaxin 1 and VAMP-2 co-precipitated with the different SNAP-25 isoforms. In contrast, Munc18-1, that preferentially interacts with SNAP-25 via Syntaxin 1 and/or the trimeric SNARE complex, demonstrated an increased ability to bind protein-complexes containing SNAP-25b. Moreover, we found that both SNAP-25 isoforms co-precipitated the Gβγ subunits of the heterotrimeric G proteins, an interaction known to play a role in presynaptic inhibition. We have identified Gβ1 and Gβ2 as the interacting partners of both SNAP-25 isoforms in mouse hippocampus, but Gβ2 was less efficiently captured by SNAP-25a. These results implicate that the two SNAP-25 isoforms could differently mediate protein interactions outside the ternary SNARE core complex and thereby contribute to modulate neurotransmission.