Skeletal muscle munc18c and syntaxin 4 in human obesity

Assembly-promoting protein Munc18c stimulates SNARE-dependent membrane fusion through its SNARE-like peptide

Intracellular vesicle fusion requires the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and their cognate Sec1/Munc18 (SM) proteins. How SM proteins act in concert with trans-SNARE complexes to promote membrane fusion remains incompletely understood. Munc18c, a broadly distributed SM protein, selectively regulates multiple exocytotic pathways, including GLUT4 exocytosis. Here, using an in vitro reconstituted system, we discovered a SNARE-like peptide (SLP), conserved in Munc18-1 of synaptic exocytosis, is crucial to the stimulatory activity of Munc18c in vesicle fusion. The direct stimulation of the SNARE-mediated fusion reaction by SLP further supported the essential role of this fragment. Interestingly, we found SLP strongly accelerates the membrane fusion rate when anchored to the target membrane but not the vesicle membrane, suggesting it primarily interacts with t-SNAREs in cis to drive fusion. Furthermore, we determined the SLP fragment is competitive with the full-length Munc18c protein and specific to the cognate v-SNARE isoforms, supporting how it could resemble Munc18c’s activity in membrane fusion. Together, our findings demonstrate that Munc18c facilitates SNARE-dependent membrane fusion through SLP, revealing that the t-SNARE-SLP binding mode might be a conserved mechanism for the stimulatory function of SM proteins in vesicle fusion.

Targeted stabilization of Munc18-1 function via pharmacological chaperones

Heterozygous de novo mutations in the neuronal protein Munc18-1 cause syndromic neurological symptoms, including severe epilepsy, intellectual disability, developmental delay, ataxia, and tremor. No disease-modifying therapy exists to treat these disorders, and while chemical chaperones have been shown to alleviate neuronal dysfunction caused by missense mutations in Munc18-1, their required high concentrations and potential toxicity necessitate a Munc18-1-targeted therapy. Munc18-1 is essential for neurotransmitter release, and mutations in Munc18-1 have been shown to cause neuronal dysfunction via aggregation and co-aggregation of the wild-type protein, reducing functional Munc18-1 levels well below hemizygous levels. Here, we identify two pharmacological chaperones via structure-based drug design, that bind to wild-type and mutant Munc18-1, and revert Munc18-1 aggregation and neuronal dysfunction in vitro and in vivo, providing the first targeted treatment strategy for these severe pediatric encephalopathies.

Exocytosis proteins as novel targets for diabetes prevention and/or remediation?

Diabetes remains one of the leading causes of morbidity and mortality worldwide, affecting an estimated 422 million adults. In the US, it is predicted that one in every three children born as of 2000 will suffer from diabetes in their lifetime. Type 2 diabetes results from combinatorial defects in pancreatic β-cell glucose-stimulated insulin secretion and in peripheral glucose uptake. Both processes, insulin secretion and glucose uptake, are mediated by exocytosis proteins, SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes, Sec1/Munc18 (SM), and double C2-domain protein B (DOC2B). Increasing evidence links deficiencies in these exocytosis proteins to diabetes in rodents and humans. Given this, emerging studies aimed at restoring and/or enhancing cellular levels of certain exocytosis proteins point to promising outcomes in maintaining functional β-cell mass and enhancing insulin sensitivity. In doing so, new evidence also shows that enhancing exocytosis protein levels may promote health span and longevity and may also harbor anti-cancer and anti-Alzheimer's disease capabilities. Herein, we present a comprehensive review of the described capabilities of certain exocytosis proteins and how these might be targeted for improving metabolic dysregulation.

Doc2b serves as a scaffolding platform for concurrent binding of multiple Munc18 isoforms in pancreatic islet β-cells

Biphasic glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells involves soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor (SNARE) protein-regulated exocytosis. SNARE complex assembly further requires the regulatory proteins Munc18c, Munc18-1 and Doc2b. Munc18-1 and Munc18c are required for first- and second-phase GSIS respectively. These distinct Munc18-1 and Munc18c roles are related to their transient high-affinity binding with their cognate target (t-)SNAREs, Syntaxin 1A and Syntaxin 4 respectively. Doc2b is essential for both phases of GSIS, yet the molecular basis for this remains unresolved. Because Doc2b binds to Munc18-1 and Munc18c via its distinct C2A and C2B domains respectively, we hypothesized that Doc2b may provide a plasma membrane-localized scaffold/platform for transient docking of these Munc18 isoforms during GSIS. Towards this, macromolecular complexes composed of Munc18c, Doc2b and Munc18-1 were detected in β-cells. In vitro interaction assays indicated that Doc2b is required to bridge the interaction between Munc18c and Munc18-1 in the macromolecular complex; Munc18c and Munc18-1 failed to associate in the absence of Doc2b. Competition-based GST-Doc2b interaction assays revealed that Doc2b could simultaneously bind both Munc18-1 and Munc18c. Hence these data support a working model wherein Doc2b functions as a docking platform/scaffold for transient interactions with the multiple Munc18 isoforms operative in insulin release, promoting SNARE assembly.

Munc18c mediates exocytosis of pre-docked and newcomer insulin granules underlying biphasic glucose stimulated insulin secretion in human pancreatic beta-cells

Objective

Pancreatic beta-cells express three Munc18 isoforms. Much is known about the roles of Munc18a (pre-docked secretory granules-SGs) and Munc18b (newcomer SGs and SG–SG fusion) in insulin exocytosis. Although shown to influence glucose-stimulated insulin secretion (GSIS) in rodents the precise role of Munc18c in insulin SG exocytosis has not been elucidated. We here examined the role of Munc18c in human pancreatic beta-cells.

Methods

Munc18c-shRNA/RFP lenti-virus (versus control virus) was used to knock down the expression level of Munc18c in human islets or single beta-cells. Insulin secretion and granule exocytosis were measured by performing islets perifusion, single-cell patch clamp capacitance measurements and total internal reflection fluorescence microscopy (TIRFM).

Results

Munc18c is most abundant in the cytosol of human beta-cells. Endogenous function of Munc18c was assessed by knocking down (KD) its islet expression by 70% employing lenti-shRNA virus. Munc18c-KD caused reduction in cognate syntaxin-4 islet expression but not in other exocytotic proteins, resulting in the reduction in GSIS in first- (by 42%) and second phases (by 35%). Single cell analyses of RFP-tagged Munc18c-KD beta-cells by patch clamp capacitance measurements showed inhibition in both readily-releasable pool (by 52%) and mobilization from the reserve pool (by 57%). TIRFM to assess single SG behavior showed that Munc18c-KD inhibition of first phase GSIS was attributed to reduction in exocytosis of pre-docked and newcomer SGs, and second phase inhibition attributed entirely to reduction in newcomer SG fusion (SGs that undergo minimal residence or docking time at the plasma membrane before fusion).

Conclusion

Munc18c is involved in the distinct molecular machineries that affect exocytosis of both predocked and newcomer SG pools that underlie Munc18c's role in first and second phases of GSIS, respectively.

Regulation of the SNARE-interacting protein Munc18c tyrosine phosphorylation in adipocytes by protein-tyrosine phosphatase 1B

BACKGROUND

Protein-tyrosine phosphatase 1B (PTP1B) is a physiological regulator of insulin signaling and adiposity and is a drug target for the treatment of obesity and diabetes. The molecular mechanisms underlying PTP1B metabolic actions require additional investigation.

RESULTS

Herein, we identify Munc18c as a novel PTP1B substrate in adipocytes and in vivo. We demonstrate nutritional regulation of Munc18c in adipose tissue revealing decreased expression upon high fat feeding. In addition, PTP1B deficiency leads to elevated Munc18c tyrosine phosphorylation and dissociation from syntaxin4. At the molecular level, we identify Munc18c Tyr218/219 and Tyr521 as key residues that mediate Munc18c interaction with PTP1B. Further, we uncover an essential role of Munc18c total tyrosine phosphorylation in general, and Tyr218/219 and Tyr521 in particular, in regulating its interactions and glucose uptake in adipocytes.

CONCLUSION

In conclusion, our findings identify PTP1B as the first known tyrosine phosphatase for Munc18c and a regulator of its phosphorylation and function in adipocytes.

Munc18c in adipose tissue is downregulated in obesity and is associated with insulin

Objective

Munc18c is associated with glucose metabolism and could play a relevant role in obesity. However, little is known about the regulation of Munc18c expression. We analyzed Munc18c gene expression in human visceral (VAT) and subcutaneous (SAT) adipose tissue and its relationship with obesity and insulin.

Materials and Methods

We evaluated 70 subjects distributed in 12 non-obese lean subjects, 23 overweight subjects, 12 obese subjects and 23 nondiabetic morbidly obese patients (11 with low insulin resistance and 12 with high insulin resistance).

Results

The lean, overweight and obese persons had a greater Munc18c gene expression in adipose tissue than the morbidly obese patients (p<0.001). VAT Munc18c gene expression was predicted by the body mass index (B = −0.001, p = 0.009). In SAT, no associations were found by different multiple regression analysis models. SAT Munc18c gene expression was the main determinant of the improvement in the HOMA-IR index 15 days after bariatric surgery (B = −2148.4, p = 0.038). SAT explant cultures showed that insulin produced a significant down-regulation of Munc18c gene expression (p = 0.048). This decrease was also obtained when explants were incubated with liver X receptor alpha (LXRα) agonist, either without (p = 0.038) or with insulin (p = 0.050). However, Munc18c gene expression was not affected when explants were incubated with insulin plus a sterol regulatory element-binding protein-1c (SREBP-1c) inhibitor (p = 0.504).

Conclusions

Munc18c gene expression in human adipose tissue is down-regulated in morbid obesity. Insulin may have an effect on the Munc18c expression, probably through LXRα and SREBP-1c.

Doc2b is a key effector of insulin secretion and skeletal muscle insulin sensitivity

Exocytosis of intracellular vesicles, such as insulin granules, is carried out by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) and Sec1/Munc18 (SM) proteins. An additional regulatory protein, Doc2b (double C2 domain), has recently been implicated in exocytosis from clonal β-cells and 3T3-L1 adipocytes. Here, we investigated the role of Doc2b in insulin secretion, insulin sensitivity, and the maintenance of whole-body glucose homeostasis. Doc2b heterozygous (Doc2b(+/-)) and homozygous (Doc2b(-/-)) knockout mice exhibited significant whole-body glucose intolerance and peripheral insulin resistance, compared with wild-type littermates. Correspondingly, Doc2b(+/-) and Doc2b(-/-) mice exhibited decreased responsiveness of pancreatic islets to glucose in vivo, with significant attenuation of both phases of insulin secretion ex vivo. Peripheral insulin resistance correlated with ablated insulin-stimulated glucose uptake and GLUT4 vesicle translocation in skeletal muscle from Doc2b-deficient mice, which was coupled to impairments in Munc18c-syntaxin 4 dissociation and in SNARE complex assembly. Hence, Doc2b is a key positive regulator of Munc18c-syntaxin 4-mediated insulin secretion as well as of insulin responsiveness in skeletal muscle, and thus a key effector for glucose homeostasis in vivo. Doc2b's actions in glucose homeostasis may be related to its ability to bind Munc18c and/or directly promote fusion of insulin granules and GLUT4 vesicles in a stimulus-dependent manner.

Regulation of glucose transporter translocation in health and diabetes

To enhance glucose uptake into muscle and fat cells, insulin stimulates the translocation of GLUT4 glucose transporters from intracellular membranes to the cell surface. This response requires the intersection of insulin signaling and vesicle trafficking pathways, and it is compromised in the setting of overnutrition to cause insulin resistance. Insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases and through the Rho GTPase TC10α to act on other targets. In unstimulated cells, GLUT4 is incorporated into specialized storage vesicles containing IRAP, LRP1, sortilin, and VAMP2, which are sequestered by TUG, Ubc9, and other proteins. Insulin mobilizes these vesicles directly to the plasma membrane, and it modulates the trafficking itinerary so that cargo recycles from endosomes during ongoing insulin exposure. Knowledge of how signaling and trafficking pathways are coordinated will be essential to understanding the pathogenesis of diabetes and the metabolic syndrome and may also inform a wide range of other physiologies.

Munc18c phosphorylation by the insulin receptor links cell signaling directly to SNARE exocytosis

SNARE complex assembly and mobilization of GLUT4 vesicles is coordinated through direct targeting of Munc18c by the insulin receptor tyrosine kinase.

How the Sec1/Munc18–syntaxin complex might transition to form the SNARE core complex remains unclear. Toward this, Munc18c tyrosine phosphorylation has been correlated with its dissociation from syntaxin 4. Using 3T3-L1 adipocytes subjected to small interfering ribonucleic acid reduction of Munc18c as a model of impaired insulin-stimulated GLUT4 vesicle exocytosis, we found that coordinate expression of Munc18c–wild type or select phosphomimetic Munc18c mutants, but not phosphodefective mutants, restored GLUT4 vesicle exocytosis, suggesting a requirement for Munc18c tyrosine phosphorylation at Tyr219 and Tyr521. Surprisingly, the insulin receptor (IR) tyrosine kinase was found to target Munc18c at Tyr521 in vitro, rapidly binding and phosphorylating endogenous Munc18c within adipocytes and skeletal muscle. IR, but not phosphatidylinositol 3-kinase, activation was required. Altogether, we identify IR as the first known tyrosine kinase for Munc18c as part of a new insulin-signaling step in GLUT4 vesicle exocytosis, exemplifying a new model for the coordination of SNARE assembly and vesicle mobilization events in response to a single extracellular stimulus.

The SNARE protein SNAP23 and the SNARE-interacting protein Munc18c in human skeletal muscle are implicated in insulin resistance/type 2 diabetes

OBJECTIVE

Our previous studies suggest that the SNARE protein synaptosomal-associated protein of 23 kDa (SNAP23) is involved in the link between increased lipid levels and insulin resistance in cardiomyocytes. The objective was to determine whether SNAP23 may also be involved in the known association between lipid accumulation in skeletal muscle and insulin resistance/type 2 diabetes in humans, as well as to identify a potential regulator of SNAP23.

RESEARCH DESIGN AND METHODS

We analyzed skeletal muscle biopsies from patients with type 2 diabetes and healthy, insulin-sensitive control subjects for expression (mRNA and protein) and intracellular localization (subcellular fractionation and immunohistochemistry) of SNAP23, and for expression of proteins known to interact with SNARE proteins. Insulin resistance was determined by a euglycemic hyperinsulinemic clamp. Potential mechanisms for regulation of SNAP23 were also investigated in the skeletal muscle cell line L6.

RESULTS

We showed increased SNAP23 levels in skeletal muscle from patients with type 2 diabetes compared with that from lean control subjects. Moreover, SNAP23 was redistributed from the plasma membrane to the microsomal/cytosolic compartment in the patients with the type 2 diabetes. Expression of the SNARE-interacting protein Munc18c was higher in skeletal muscle from patients with type 2 diabetes. Studies in L6 cells showed that Munc18c promoted the expression of SNAP23.

CONCLUSIONS

We have translated our previous in vitro results into humans by showing that there is a change in the distribution of SNAP23 to the interior of the cell in skeletal muscle from patients with type 2 diabetes. We also showed that Munc18c is a potential regulator of SNAP23.

Exocytosis mechanisms underlying insulin release and glucose uptake: conserved roles for Munc18c and syntaxin 4

Type 2 diabetes has been coined "a two-hit disease," as it involves specific defects of glucose-stimulated insulin secretion from the pancreatic beta cells in addition to defects in peripheral tissue insulin action required for glucose uptake. Both of these processes, insulin secretion and glucose uptake, are mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein core complexes composed of syntaxin, SNAP-23/25, and VAMP proteins. The SNARE core complex is regulated by the Sec1/Munc18 (SM) family of proteins, which selectively bind to their cognate syntaxin isoforms with high affinity. The process of insulin secretion uses multiple Munc18-syntaxin isoform pairs, whereas insulin action in the peripheral tissues appears to use only the Munc18c-syntaxin 4 pair. Importantly, recent reports have linked obesity and Type 2 diabetes in humans with changes in protein levels and single nucleotide polymorphisms (SNPs) of Munc18 and syntaxin isoforms relevant to these exocytotic processes, although the molecular mechanisms underlying the observed phenotypes remain incomplete (5, 104, 144). Given the conservation of these proteins in two seemingly disparate processes and the need to design and implement novel and more effective clinical interventions, it will be vitally important to delineate the mechanisms governing these conserved SNARE-mediated exocytosis events. Thus, we provide here an up-to-date historical review of advancements in defining the roles and molecular mechanisms of Munc18-syntaxin complexes in the pathophysiology of Type 2 diabetes.

Munc18c depletion selectively impairs the sustained phase of insulin release

OBJECTIVE

The Sec1/Munc18 protein Munc18c has been implicated in Syntaxin 4-mediated exocytosis events, although its purpose in exocytosis has remained elusive. Given that Syntaxin 4 functions in the second phase of glucose-stimulated insulin secretion (GSIS), we hypothesized that Munc18c would also be required and sought insight into the possible mechanism(s) using the islet beta-cell as a model system.

RESEARCH DESIGN AND METHODS

Perifusion analyses of isolated Munc18c- (-/+) or Munc18c-depleted (RNAi) mouse islets were used to assess biphasic secretion. Protein interaction studies used subcellular fractions and detergent lysates prepared from MIN6 beta-cells to determine the mechanistic role of Munc18c in Syntaxin 4 activation and docking/fusion of vesicle-associated membrane protein (VAMP)2-containing insulin granules. Electron microscopy was used to gauge changes in granule localization.

RESULTS

Munc18c (-/+) islets secreted approximately 60% less insulin selectively during second-phase GSIS; RNAi-mediated Munc18c depletion functionally recapitulated this in wild-type and Munc18c (-/+) islets in a gene dosage-dependent manner. Munc18c depletion ablated the glucose-stimulated VAMP2-Syntaxin 4 association as well as Syntaxin 4 activation, correlating with the deficit in insulin release. Remarkably, Munc18c depletion resulted in aberrant granule localization to the plasma membrane in response to glucose stimulation, consistent with its selective effect on the second phase of secretion.

CONCLUSIONS

Collectively, these studies demonstrate an essential positive role for Munc18c in second-phase GSIS and suggest novel roles for Munc18c in granule localization to the plasma membrane as well as in triggering Syntaxin 4 accessibility to VAMP2 at a step preceding vesicle docking/fusion.

Involvement of TNF-alpha in abnormal adipocyte and muscle sortilin expression in obese mice and humans

AIMS/HYPOTHESIS

Insulin resistance is caused by numerous factors including inflammation. It is characterised by defective insulin stimulation of adipocyte and muscle glucose transport, which requires the glucose transporter GLUT4 translocation towards the plasma membrane. Defects in insulin signalling can cause insulin resistance, but alterations in GLUT4 trafficking could also play a role. Our goal was to determine whether proteins controlling GLUT4 trafficking are altered in insulin resistance linked to obesity.

METHODS

Using real-time RT-PCR, we searched for selected transcripts that were differentially expressed in adipose tissue and muscle in obese mice and humans. Using various adipocyte culture models and in vivo mice treatment, we searched for the involvement of TNF-alpha in these alterations in obesity.

RESULTS

Sortilin mRNA and protein were downregulated in adipose tissue from obese db/db and ob/ob mice, and also in muscle. Importantly, sortilin mRNA was also decreased in morbidly obese human diabetic patients. Sortilin and TNF-alpha (also known as TNF) mRNA levels were inversely correlated in mice and human adipose tissues. TNF-alpha decreased sortilin mRNA and protein levels in cultured mouse and human adipocytes, an effect partly prevented by the peroxisome proliferator-activated receptor gamma activator rosiglitazone. TNF-alpha also inhibited adipocyte and muscle sortilin mRNA when injected to mice.

CONCLUSIONS/INTERPRETATION

Sortilin, an essential player in adipocyte and muscle glucose metabolism through the control of GLUT4 localisation, is downregulated in obesity and TNF-alpha is likely to be involved in this defect. Chronic low-grade inflammation in obesity could thus contribute to insulin resistance by modulating proteins that control GLUT4 trafficking.