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

A Novel Role for DOC2B in Ameliorating Palmitate-Induced Glucose Uptake Dysfunction in Skeletal Muscle Cells via a Mechanism Involving β-AR Agonism and Cofilin

Diet-related lipotoxic stress is a significant driver of skeletal muscle insulin resistance (IR) and type 2 diabetes (T2D) onset. β2-adrenergic receptor (β-AR) agonism promotes insulin sensitivity in vivo under lipotoxic stress conditions. Here, we established an in vitro paradigm of lipotoxic stress using palmitate (Palm) in rat skeletal muscle cells to determine if β-AR agonism could cooperate with double C-2-like domain beta (DOC2B) enrichment to promote skeletal muscle insulin sensitivity under Palm-stress conditions. Previously, human T2D skeletal muscles were shown to be deficient for DOC2B, and DOC2B enrichment resisted IR in vivo. Our Palm-stress paradigm induced IR and β-AR resistance, reduced DOC2B protein levels, triggered cytoskeletal cofilin phosphorylation, and reduced GLUT4 translocation to the plasma membrane (PM). By enhancing DOC2B levels in rat skeletal muscle, we showed that the deleterious effects of palmitate exposure upon cofilin, insulin, and β-AR-stimulated GLUT4 trafficking to the PM and glucose uptake were preventable. In conclusion, we revealed a useful in vitro paradigm of Palm-induced stress to test for factors that can prevent/reverse skeletal muscle dysfunctions related to obesity/pre-T2D. Discerning strategies to enrich DOC2B and promote β-AR agonism can resist skeletal muscle IR and halt progression to T2D.

Epigallocatechin gallate inhibits SNARE-dependent membrane fusion by blocking trans-SNARE assembly

Insulin secretion is a signal-triggered process that requires membrane fusion between the secretory granules and plasma membrane in pancreatic β cells. The exocytosis of insulin is mediated by target-soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs) on the plasma membrane and vesicle-SNAREs on the vesicles, which assemble into a quaternary trans-SNARE complex to initiate the fusion. Expression of fusion proteins is reduced in the islets of patients with type II diabetes, indicating that SNARE-mediated fusion defect is closely related to insulin-based metabolic diseases. Previous studies have suggested that epigallocatechin gallate (EGCG) has an inhibitory effect on membrane fusion. In the present study, we performed in vitro reconstitution assays to unravel the molecular mechanisms of EGCG in SNARE-mediated insulin secretory vesicle fusion. Our data show that EGCG efficiently inhibits insulin secretory SNARE-mediated membrane fusion. Mechanistic studies indicated that EGCG blocks the formation of the trans-SNARE complex. Furthermore, calcium/synaptotagmin-7-stimulated fusion kinetics were largely reduced by EGCG, confirming that it is a potential regulator of SNARE-dependent insulin secretion. Our findings suggest that the trans-SNARE complex might be a promising target for controlling SNARE-dependent vesicle fusion.

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.

Exocytosis Proteins: Typical and Atypical Mechanisms of Action in Skeletal Muscle

Insulin-stimulated glucose uptake in skeletal muscle is of fundamental importance to prevent postprandial hyperglycemia, and long-term deficits in insulin-stimulated glucose uptake underlie insulin resistance and type 2 diabetes. Skeletal muscle is responsible for ~80% of the peripheral glucose uptake from circulation via the insulin-responsive glucose transporter GLUT4. GLUT4 is mainly sequestered in intracellular GLUT4 storage vesicles in the basal state. In response to insulin, the GLUT4 storage vesicles rapidly translocate to the plasma membrane, where they undergo vesicle docking, priming, and fusion via the high-affinity interactions among the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) exocytosis proteins and their regulators. Numerous studies have elucidated that GLUT4 translocation is defective in insulin resistance and type 2 diabetes. Emerging evidence also links defects in several SNAREs and SNARE regulatory proteins to insulin resistance and type 2 diabetes in rodents and humans. Therefore, we highlight the latest research on the role of SNAREs and their regulatory proteins in insulin-stimulated GLUT4 translocation in skeletal muscle. Subsequently, we discuss the novel emerging role of SNARE proteins as interaction partners in pathways not typically thought to involve SNAREs and how these atypical functions reveal novel therapeutic targets for combating peripheral insulin resistance and diabetes.

Calcium promotes α-synuclein liquid-liquid phase separation to accelerate amyloid aggregation

α-Synuclein (α-Syn) is an aggregation-prone protein whose accumulation in Lewy bodies leads to neurodegenerative diseases like Parkinson's disease (PD). Calcium plays a critical role in neurons, and calcium dysregulation is one of the risk factors of PD. It is known that Ca²⁺ interacts with α-Syn and affects its assembly. However, how Ca²⁺ regulates α-Syn aggregation remains unclear. Here, we reported that Ca²⁺ accelerates α-Syn amyloid aggregation through the modulation of protein phase separation. We observed that Ca²⁺ promotes the formation of α-Syn liquid droplets but does not change the protein fluidity inside the droplets. Further studies showed Ca²⁺-involved α-Syn droplets are still able to fuse. A metal chelator eliminated Ca²⁺-induced enlargement of α-Syn droplets, suggesting the influence of Ca²⁺ on α-Syn assembly could be reversed at the stage of liquid-liquid phase separation (LLPS). Interestingly, our data showed Ca²⁺ still promoted α-Syn phase separation in the presence of the lipid membranes. In addition, Ca²⁺/α-syn droplets could efficiently recruit lipid vesicles to the surface of these condensates. Our findings demonstrate that Ca²⁺ facilitates α-Syn phase separation to accelerate amyloid aggregation and pave the path for understanding the implications of Ca²⁺ in α-Syn accumulation and PD.

In Vitro Reconstitution Studies of SNAREs and Their Regulators Mediating GLUT4 Vesicle Fusion

The GLUT4 vesicle fusion is mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and a variety of regulatory proteins. For example, synip and tomosyn negatively regulate GLUT4 SNARE-mediated membrane fusion. Here we describe in vitro reconstituted assays to determine the molecular mechanisms of SNAREs, synip, and tomosyn. These methods can also be extended to the studies of other types of membrane fusion events.

Manganese promotes α-synuclein amyloid aggregation through the induction of protein phase transition

α-Synuclein (α-Syn) is the major protein component of Lewy bodies, a key pathological feature of Parkinson’s disease (PD). The manganese ion Mn²⁺ has been identified as an environmental risk factor of PD. However, it remains unclear how Mn²⁺ regulates α-Syn aggregation. Here, we discovered that Mn²⁺accelerates α-Syn amyloid aggregation through the regulation of protein phase separation. We found that Mn²⁺ not only promotes α-Syn liquid-to-solid phase transition but also directly induces soluble α-Syn monomers to form solid-like condensates. Interestingly, the lipid membrane is integrated into condensates during Mn²⁺-induced α-Syn phase transition; however, the preformed Mn²⁺/α-syn condensates can only recruit lipids to the surface of condensates. In addition, this phase transition can largely facilitate α-Syn amyloid aggregation. Although the Mn²⁺-induced condensates do not fuse, our results demonstrated that they could recruit soluble α-Syn monomers into the existing condensates. Furthermore, we observed that a manganese chelator reverses Mn²⁺-induced α-Syn aggregation during the phase transition stage. However, after maturation, α-Syn aggregation becomes irreversible. These findings demonstrate that Mn²⁺ facilitates α-Syn phase transition to accelerate the formation of α-Syn aggregates and provide new insights for targeting α-Syn phase separation in PD treatment.

Omega-3 polyunsaturated fatty acids promote SNAREs mediated GLUT4 vesicle docking and fusion

Glucose homeostasis imbalance and insulin resistance (IR) are major contributors to the incidence of type 2 diabetes. Omega-3 polyunsaturated fatty acids (PUFAs) are key ingredients for maintaining cellular functions and improving insulin sensitivity. However, how omega-3 PUFAs modulate the dynamic process of glucose transport at the cellular level remains unclear. Here we unraveled eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) may regulate the glucose transporter 4 (GLUT4) vesicle trafficking in both normal and IR adipocytes. Both omega-3 PUFAs significantly increase glucose consumption within a range of 10-32% in the basal state. Furthermore, both EPA (200 μM) and DHA (100 μM) may significantly promote the serine/threonine protein kinase (Akt) phosphorylation by 70% and 40% in the physiological state of adipocytes, respectively. Both omega-3 PUFAs significantly advanced the Akt phosphorylation in a dose-dependent way and showed a ∼2-fold increase at the dose of 200 μM in the IR pathological state. However, they could not up-regulate the expression of GLUT4 and insulin-regulated aminopeptidase protein. We further revealed that both omega-3 PUFAs dynamically promote insulin-stimulated GLUT4 vesicle translocation and soluble N-ethylmaleimide-sensitive factor attachment protein receptor mediated vesicle docking and fusion to the plasma membrane via specifically modulating the expression of vesicle-associated membrane protein 2. Understanding the mechanisms by which omega-3 PUFAs modulate cellular metabolism and IR in peripheral tissues may provide novel insights into the potential impact of omega-3 PUFAs on the metabolic function and the management of IR.

Mechanism of copper nanoparticle toxicity in rainbow trout olfactory mucosa

Chemosensory perception is crucial for fish reproduction and survival. Direct contact of olfactory neuroepithelium to the surrounding environment makes it vulnerable to contaminants in aquatic ecosystems. Copper nanoparticles (CuNPs), which are increasingly used in commercial and domestic applications due their exceptional properties, can impair fish olfactory function. However, the molecular events underlying olfactory toxicity of CuNPs are largely unexplored. Our results suggested that CuNPs were bioavailable to olfactory mucosal cells. Using RNA-seq, we compared the effect of CuNPs and copper ions (Cu²⁺) on gene transcript profiles of rainbow trout (Oncorhynchus mykiss) olfactory mucosa. The narrow overlap in differential gene expression between the CuNP- and Cu²⁺-exposed fish revealed that these two contaminants exert their effects through distinct mechanisms. We propose a transcript-based conceptual model that shows that olfactory signal transduction, calcium homeostasis, and synaptic vesicular signaling were affected by CuNPs in the olfactory sensory neurons (OSNs). Neuroregenerative pathways were also impaired by CuNPs. In contrast, Cu²⁺ did not induce toxicity pathways and rather upregulated regeneration pathways. Both Cu treatments reduced immune system pathway transcripts. However, suppression of transcripts that were associated with inflammatory signaling was only observed with CuNPs. Neither oxidative stress nor apoptosis were triggered by Cu²⁺ or CuNPs in mucosal cells. Dysregulation of transcripts that regulate function, maintenance, and reestablishment of damaged olfactory mucosa represents critical mechanisms of toxicity of CuNPs. The loss of olfaction by CuNPs may impact survival of rainbow trout and impose an ecological risk to fish populations in contaminated environments.

Genetic evidence for an inhibitory role of tomosyn in insulin-stimulated GLUT4 exocytosis

Exocytosis is a vesicle fusion process driven by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). A classic exocytic pathway is insulin-stimulated translocation of the glucose transporter type 4 (GLUT4) from intracellular vesicles to the plasma membrane in adipocytes and skeletal muscles. The GLUT4 exocytic pathway plays a central role in maintaining blood glucose homeostasis and is compromised in insulin resistance and type 2 diabetes. A candidate regulator of GLUT4 exocytosis is tomosyn, a soluble protein expressed in adipocytes. Tomosyn directly binds to GLUT4 exocytic SNAREs in vitro but its role in GLUT4 exocytosis was unknown. In this work, we used CRISPR-Cas9 genome editing to delete the two tomosyn-encoding genes in adipocytes. We observed that both basal and insulin-stimulated GLUT4 exocytosis was markedly elevated in the double knockout (DKO) cells. By contrast, adipocyte differentiation and insulin signaling remained intact in the DKO adipocytes. In a reconstituted liposome fusion assay, tomosyn inhibited all the SNARE complexes underlying GLUT4 exocytosis. The inhibitory activity of tomosyn was relieved by NSF and α-SNAP, which act in concert to remove tomosyn from GLUT4 exocytic SNAREs. Together, these studies revealed an inhibitory role for tomosyn in insulin-stimulated GLUT4 exocytosis in adipocytes. We suggest that tomosyn-arrested SNAREs represent a reservoir of fusion capacity that could be harnessed to treat patients with insulin resistance and type 2 diabetes.

Complexin-2 redistributes to the membrane of muscle cells in response to insulin and contributes to GLUT4 translocation

Insulin stimulates glucose uptake in muscle cells by rapidly redistributing vesicles containing GLUT4 glucose transporters from intracellular compartments to the plasma membrane (PM). GLUT4 vesicle fusion requires the formation of SNARE complexes between vesicular VAMP and PM syntaxin4 and SNAP23. SNARE accessory proteins usually regulate vesicle fusion processes. Complexins aide in neuro-secretory vesicle-membrane fusion by stabilizing trans-SNARE complexes but their participation in GLUT4 vesicle fusion is unknown. We report that complexin-2 is expressed and homogeneously distributed in L6 rat skeletal muscle cells. Upon insulin stimulation, a cohort of complexin-2 redistributes to the PM. Complexin-2 knockdown markedly inhibited GLUT4 translocation without affecting proximal insulin signalling of Akt/PKB phosphorylation and actin fiber remodelling. Similarly, complexin-2 overexpression decreased maximal GLUT4 translocation suggesting that the concentration of complexin-2 is finely tuned to vesicle fusion. These findings reveal an insulin-dependent regulation of GLUT4 insertion into the PM involving complexin-2.

β-Glucan-Induced Trained Immunity Protects against Leishmania braziliensis Infection: a Crucial Role for IL-32

American tegumentary leishmaniasis is a vector-borne parasitic disease caused by Leishmania protozoans. Innate immune cells undergo long-term functional reprogramming in response to infection or Bacillus Calmette-Guérin (BCG) vaccination via a process called trained immunity, conferring non-specific protection from secondary infections. Here, we demonstrate that monocytes trained with the fungal cell wall component β-glucan confer enhanced protection against infections caused by Leishmania braziliensis through the enhanced production of proinflammatory cytokines. Mechanistically, this augmented immunological response is dependent on increased expression of interleukin 32 (IL-32). Studies performed using a humanized IL-32 transgenic mouse highlight the clinical implications of these findings in vivo. This study represents a definitive characterization of the role of IL-32γ in the trained phenotype induced by β-glucan or BCG, the results of which improve our understanding of the molecular mechanisms governing trained immunity and Leishmania infection control.

The Cytosolic Phospholipase A2α N-terminal C2 Domain Binds and Oligomerizes on Membranes with Positive Curvature

Group IV phospholipase A₂α (cPLA₂α) regulates the production of prostaglandins and leukotrienes via the formation of arachidonic acid from membrane phospholipids. The targeting and membrane binding of cPLA₂α to the Golgi involves the N-terminal C2 domain, whereas the catalytic domain produces arachidonic acid. Although most studies of cPLA₂α concern its catalytic activity, it is also linked to homeostatic processes involving the generation of vesicles that traffic material from the Golgi to the plasma membrane. Here we investigated how membrane curvature influences the homeostatic role of cPLA₂α in vesicular trafficking. The cPLA₂α C2 domain is known to induce changes in positive membrane curvature, a process which is dependent on cPLA₂α membrane penetration. We showed that cPLA₂α undergoes C2 domain-dependent oligomerization on membranes in vitro and in cells. We found that the association of the cPLA₂α C2 domain with membranes is limited to membranes with positive curvature, and enhanced C2 domain oligomerization was observed on vesicles ~50 nm in diameter. We demonstrated that the cPLA₂α C2 domain localizes to cholesterol enriched Golgi-derived vesicles independently of cPLA₂α catalytic activity. Moreover, we demonstrate the C2 domain selectively localizes to lipid droplets whereas the full-length enzyme to a much lesser extent. Our results therefore provide novel insight into the molecular forces that mediate C2 domain-dependent membrane localization in vitro and in cells.

BAG6 contributes to glucose uptake by supporting the cell surface translocation of the glucose transporter GLUT4

Defective translocation of glucose transporter 4 (GLUT4) to the cell surface is a key feature of insulin resistance in type 2 diabetes. Therefore, elucidating the mechanism of GLUT4 translocation is of primary importance. The mammalian Bag6/Bat3 gene has been suggested to be linked with potential obesity- and diabetes-associated loci, while its function in the control of glucose incorporation into the cytoplasm has not been investigated. In this study, we established a series of cell lines that stably expressed GLUT4 with three tandem repeats of the antigenic peptide inserted into its 1st extracellular loop. With these cell lines, we found that the depletion of endogenous BAG6 downregulated the cell surface expression of GLUT4, concomitant with the reduced incorporation of a glucose analog into the cells. Defective intracellular translocation of GLUT4 in BAG6-depleted cells is similar to the case observed for the depletion of Rab8a, an essential regulator of insulin-stimulated GLUT4 translocation. In addition, we observed that the assembly of syntaxin 6 into the endoplasmic reticulum membrane was slightly disturbed under BAG6 depletion. Given that Rab8a and syntaxin 6 are critical for GLUT4 translocation, we suggest that BAG6 may play multiple roles in the trafficking of glucose transporters to the cell surface.

This article has an associated First Person interview with the first author of the paper.

Summary: BAG6 is critical for the insulin-stimulated translocation of GLUT4 from its peri-nuclear storage compartments to the cell surface.

Synaptotagmin-1 and Doc2b Exhibit Distinct Membrane-Remodeling Mechanisms

Synaptotagmin-1 (Syt1) is a calcium sensor protein that is critical for neurotransmission and is therefore extensively studied. Here, we use pairs of optically trapped beads coated with SNARE-free synthetic membranes to investigate Syt1-induced membrane remodeling. This activity is compared with that of Doc2b, which contains a conserved C₂AB domain and induces membrane tethering and hemifusion in this cell-free model. We find that the soluble C₂AB domain of Syt1 strongly affects the probability and strength of membrane-membrane interactions in a strictly Ca²⁺- and protein-dependent manner. Single-membrane loading of Syt1 yielded the highest probability and force of membrane interactions, whereas in contrast, Doc2b was more effective after loading both membranes. A lipid-mixing assay with confocal imaging reveals that both Syt1 and Doc2b are able to induce hemifusion; however, significantly higher Syt1 concentrations are required. Consistently, both C₂AB fragments cause a reduction in the membrane-bending modulus, as measured by a method based on atomic force microscopy. This lowering of the energy required for membrane deformation may contribute to Ca²⁺-induced fusion.

DOC2B promotes insulin sensitivity in mice via a novel KLC1-dependent mechanism in skeletal muscle

AIMS/HYPOTHESIS

Skeletal muscle accounts for >80% of insulin-stimulated glucose uptake; dysfunction of this process underlies insulin resistance and type 2 diabetes. Insulin sensitivity is impaired in mice deficient in the double C2 domain β (DOC2B) protein, while whole-body overexpression of DOC2B enhances insulin sensitivity. Whether insulin sensitivity in the skeletal muscle is affected directly by DOC2B or is secondary to an effect on other tissues is unknown; the underlying molecular mechanisms also remain unclear.

METHODS

Human skeletal muscle samples from non-diabetic or type 2 diabetic donors were evaluated for loss of DOC2B during diabetes development. For in vivo analysis, new doxycycline-inducible skeletal-muscle-specific Doc2b-overexpressing mice fed standard or high-fat diets were evaluated for insulin and glucose tolerance, and insulin-stimulated GLUT4 accumulation at the plasma membrane (PM). For in vitro analyses, a DOC2B-overexpressing L6-GLUT4-myc myoblast/myotube culture system was coupled with an insulin resistance paradigm. Biochemical and molecular biology methods such as site-directed mutagenesis, co-immunoprecipitation and mass spectrometry were used to identify the molecular mechanisms linking insulin stimulation to DOC2B.

RESULTS

We identified loss of DOC2B (55% reduction in RNA and 40% reduction in protein) in the skeletal muscle of human donors with type 2 diabetes. Furthermore, inducible enrichment of DOC2B in skeletal muscle of transgenic mice enhanced whole-body glucose tolerance (AUC decreased by 25% for female mice) and peripheral insulin sensitivity (area over the curve increased by 20% and 26% for female and male mice, respectively) in vivo, underpinned by enhanced insulin-stimulated GLUT4 accumulation at the PM. Moreover, DOC2B enrichment in skeletal muscle protected mice from high-fat-diet-induced peripheral insulin resistance, despite the persistence of obesity. In L6-GLUT4-myc myoblasts, DOC2B enrichment was sufficient to preserve normal insulin-stimulated GLUT4 accumulation at the PM in cells exposed to diabetogenic stimuli. We further identified that DOC2B is phosphorylated on insulin stimulation, enhancing its interaction with a microtubule motor protein, kinesin light chain 1 (KLC1). Mutation of Y301 in DOC2B blocked the insulin-stimulated phosphorylation of DOC2B and interaction with KLC1, and it blunted the ability of DOC2B to enhance insulin-stimulated GLUT4 accumulation at the PM.

CONCLUSIONS/INTERPRETATION

These results suggest that DOC2B collaborates with KLC1 to regulate insulin-stimulated GLUT4 accumulation at the PM and regulates insulin sensitivity. Our observation provides a basis for pursuing DOC2B as a novel drug target in the muscle to prevent/treat type 2 diabetes.

Ethanolic Extract of Folium Sennae Mediates the Glucose Uptake of L6 Cells by GLUT4 and Ca2

In today’s world, diabetes mellitus (DM) is on the rise, especially type 2 diabetes mellitus (T2DM), which is characterized by insulin resistance. T2DM has high morbidity, and therapies with natural products have attracted much attention in the recent past. In this paper, we aimed to study the hypoglycemic effect and the mechanism of an ethanolic extract of Folium Sennae (FSE) on L6 cells. The glucose uptake of L6 cells was investigated using a glucose assay kit. We studied glucose transporter 4 (GLUT4) expression and AMP-activated protein kinase (AMPK), protein kinase B (PKB/Akt), and protein kinase C (PKC) phosphorylation levels using western blot analysis. GLUT4 trafficking and intracellular Ca²⁺ levels were monitored by laser confocal microscopy in L6 cells stably expressing IRAP-mOrange. GLUT4 fusion with plasma membrane (PM) was observed by myc-GLUT4-mOrange. FSE stimulated glucose uptake; GLUT4 expression and translocation; PM fusion; intracellular Ca²⁺ elevation; and the phosphorylation of AMPK, Akt, and PKC in L6 cells. GLUT4 translocation was weakened by the AMPK inhibitor compound C, PI3K inhibitor Wortmannin, PKC inhibitor Gö6983, G protein inhibitor PTX/Gallein, and PLC inhibitor U73122. Similarly, in addition to PTX/Gallein and U73122, the IP3R inhibitor 2-APB and a 0 mM Ca²⁺-EGTA solution partially inhibited the elevation of intracellular Ca²⁺ levels. BAPTA-AM had a significant inhibitory effect on FSE-mediated GLUT4 activities. In summary, FSE regulates GLUT4 expression and translocation by activating the AMPK, PI3K/Akt, and G protein–PLC–PKC pathways. FSE causes increasing Ca²⁺ concentration to complete the fusion of GLUT4 vesicles with PM, allowing glucose uptake. Therefore, FSE may be a potential drug for improving T2DM.

VAMP8, a vesicle-SNARE required for RAB37-mediated exocytosis, possesses a tumor metastasis suppressor function

We previously identified a metastasis suppressor RAB37 small GTPase that regulated exocytosis of tissue inhibitor of metalloproteinases 1 (TIMP1) to suppress lung cancer metastasis. Here, we show that vesicle-associated membrane protein 8 (VAMP8), a v-SNARE (vesicle soluble N-ethylmaleimide-sensitive factor activating protein receptor), interacts with RAB37 and drives the secretion of TIMP1 to inhibit tumor metastases. Confocal and total internal reflection fluorescence microscopic images demonstrated that VAMP8 co-localized with RAB37 and facilitated trafficking of RAB37-TIMP1 vesicles. Reconstitution experiments using tail-vein injection and lung-to-lung metastasis in mice showed that VAMP8 was essential for RAB37-regulated vesicle trafficking of TIMP1 to suppress cancer metastasis. Lung cancer patients with low VAMP8 showed distant metastasis, poor overall survival and progression-free survival. Importantly, multivariate Cox regression analysis indicated that patients with low VAMP8/low RAB37 expression profile showed significantly high risk of death (hazard ratio = 3.42, P < 0.001) even after adjusting for tumor metastasis parameter. Our findings reveal that VAMP8 is a novel v-SNARE crucial for RAB37-mediated exocytic transport of TIMP1 to suppress lung tumor metastasis. VAMP8 possesses a tumor metastasis suppressor function with a prognostic value in lung cancer.

Doc2b Protects β-Cells Against Inflammatory Damage and Enhances Function

Loss of functional β-cell mass is an early feature of type 1 diabetes. To release insulin, β-cells require soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes, as well as SNARE complex regulatory proteins like double C2 domain-containing protein β (Doc2b). We hypothesized that Doc2b deficiency or overabundance may confer susceptibility or protection, respectively, to the functional β-cell mass. Indeed, Doc2b^+/- knockout mice show an unusually severe response to multiple-low-dose streptozotocin (MLD-STZ), resulting in more apoptotic β-cells and a smaller β-cell mass. In addition, inducible β-cell-specific Doc2b-overexpressing transgenic (βDoc2b-dTg) mice show improved glucose tolerance and resist MLD-STZ-induced disruption of glucose tolerance, fasting hyperglycemia, β-cell apoptosis, and loss of β-cell mass. Mechanistically, Doc2b enrichment enhances glucose-stimulated insulin secretion (GSIS) and SNARE activation and prevents the appearance of apoptotic markers in response to cytokine stress and thapsigargin. Furthermore, expression of a peptide containing the Doc2b tandem C2A and C2B domains is sufficient to confer the beneficial effects of Doc2b enrichment on GSIS, SNARE activation, and apoptosis. These studies demonstrate that Doc2b enrichment in the β-cell protects against diabetogenic and proapoptotic stress. Furthermore, they identify a Doc2b peptide that confers the beneficial effects of Doc2b and may be a therapeutic candidate for protecting functional β-cell mass.

Doc2B acts as a calcium sensor for vesicle priming requiring synaptotagmin-1, Munc13-2 and SNAREs

Doc2B is a cytosolic protein with binding sites for Munc13 and Tctex-1 (dynein light chain), and two C2-domains that bind to phospholipids, Ca²⁺ and SNAREs. Whether Doc2B functions as a calcium sensor akin to synaptotagmins, or in other calcium-independent or calcium-dependent capacities is debated. We here show by mutation and overexpression that Doc2B plays distinct roles in two sequential priming steps in mouse adrenal chromaffin cells. Mutating Ca²⁺-coordinating aspartates in the C2A-domain localizes Doc2B permanently at the plasma membrane, and renders an upstream priming step Ca²⁺-independent, whereas a separate function in downstream priming depends on SNARE-binding, Ca²⁺-binding to the C2B-domain of Doc2B, interaction with ubMunc13-2 and the presence of synaptotagmin-1. Another function of Doc2B – inhibition of release during sustained calcium elevations – depends on an overlapping protein domain (the MID-domain), but is separate from its Ca²⁺-dependent priming function. We conclude that Doc2B acts as a vesicle priming protein.

RABIF/MSS4 is a Rab-stabilizing holdase chaperone required for GLUT4 exocytosis

Rab GTPases are switched from their GDP-bound inactive conformation to a GTP-bound active state by guanine nucleotide exchange factors (GEFs). The first putative GEFs isolated for Rabs are RABIF (Rab-interacting factor)/MSS4 (mammalian suppressor of Sec4) and its yeast homolog DSS4 (dominant suppressor of Sec4). However, the biological function and molecular mechanism of these molecules remained unclear. In a genome-wide CRISPR genetic screen, we isolated RABIF as a positive regulator of exocytosis. Knockout of RABIF severely impaired insulin-stimulated GLUT4 exocytosis in adipocytes. Unexpectedly, we discovered that RABIF does not function as a GEF, as previously assumed. Instead, RABIF promotes the stability of Rab10, a key Rab in GLUT4 exocytosis. In the absence of RABIF, Rab10 can be efficiently synthesized but is rapidly degraded by the proteasome, leading to exocytosis defects. Strikingly, restoration of Rab10 expression rescues exocytosis defects, bypassing the requirement for RABIF. These findings reveal a crucial role of RABIF in vesicle transport and establish RABIF as a Rab-stabilizing holdase chaperone, a previously unrecognized mode of Rab regulation independent of its GDP-releasing activity. Besides Rab10, RABIF also regulates the stability of two other Rab GTPases, Rab8 and Rab13, suggesting that the requirement of holdase chaperones is likely a general feature of Rab GTPases.

Update on GLUT4 Vesicle Traffic: A Cornerstone of Insulin Action

Glucose transport is rate limiting for dietary glucose utilization by muscle and fat. The glucose transporter GLUT4 is dynamically sorted and retained intracellularly and redistributes to the plasma membrane (PM) by insulin-regulated vesicular traffic, or 'GLUT4 translocation'. Here we emphasize recent findings in GLUT4 translocation research. The application of total internal reflection fluorescence microscopy (TIRFM) has increased our understanding of insulin-regulated events beneath the PM, such as vesicle tethering and membrane fusion. We describe recent findings on Akt-targeted Rab GTPase-activating proteins (GAPs) (TBC1D1, TBC1D4, TBC1D13) and downstream Rab GTPases (Rab8a, Rab10, Rab13, Rab14, and their effectors) along with the input of Rac1 and actin filaments, molecular motors [myosinVa (MyoVa), myosin1c (Myo1c), myosinIIA (MyoIIA)], and membrane fusion regulators (syntaxin4, munc18c, Doc2b). Collectively these findings reveal novel events in insulin-regulated GLUT4 traffic.

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.

Promoting Glucose Transporter-4 Vesicle Trafficking along Cytoskeletal Tracks: PAK-Ing Them Out

Glucose is the principal cellular energy source in humans and maintenance of glucose homeostasis is critical for survival. Glucose uptake into peripheral skeletal muscle and adipose tissues requires the trafficking of vesicles containing glucose transporter-4 (GLUT4) from the intracellular storage compartments to the cell surface. Trafficking of GLUT4 storage vesicles is initiated via the canonical insulin signaling cascade in skeletal muscle and fat cells, as well as via exercise-induced contraction in muscle cells. Recent studies have elucidated steps in the signaling cascades that involve remodeling of the cytoskeleton, a process that underpins the mechanical movement of GLUT4 vesicles. This review is focused upon an alternate phosphoinositide-3 kinase-dependent pathway involving Ras-related C3 botulinum toxin substrate 1 signaling through the p21-activated kinase p21-activated kinase 1 and showcases related signaling events that co-regulate both the depolymerization and re-polymerization of filamentous actin. These new insights provide an enriched understanding into the process of glucose transport and yield potential new targets for interventions aimed to improve insulin sensitivity and remediate insulin resistance, pre-diabetes, and the progression to type 2 diabetes.

Angiotensin II activates different calcium signaling pathways in adipocytes

Angiotensin II (Ang II) is an important mammalian neurohormone involved in reninangiotensin system. Ang II is produced both constitutively and locally by RAS systems, including white fat adipocytes. The influence of Ang II on adipocytes is complex, affecting different systems of signal transduction from early Са(2+) responses to cell proliferation and differentiation, triglyceride accumulation, expression of adipokine-encoding genes and adipokine secretion. It is known that white fat adipocytes express all RAS components and Ang II receptors (АТ1 and АТ2). The current work was carried out with the primary white adipocytes culture, and Са(2+) signaling pathways activated by Ang II were investigated using fluorescent microscopy. Са(2+)-oscillations and transient responses of differentiated adipocytes to Ang II were registered in cells with both small and multiple lipid inclusions. Using inhibitory analysis and selective antagonists, we now show that Ang II initiates periodic Са(2+)-oscillations and transient responses by activating АТ1 and АТ2 receptors and involving branched signaling cascades: 1) Ang II → Gq → PLC → IP3 → IP3Rs → Ca(2+) 2) Gβγ → PI3Kγ → PKB 3) PKB → eNOS → NO → PKG 4) CD38 → cADPR → RyRs → Ca(2+) In these cascades, AT1 receptors play the leading role. The results of the present work open a perspective of using Ang II for correction of signal resistance of adipocytes often observed during obesity and type 2 diabetes.

The trans-SNARE-regulating function of Munc18-1 is essential to synaptic exocytosis

The fusion of neurotransmitter-filled synaptic vesicles with the plasma membrane requires two classes of molecules-SNAP receptor (SNARE) and Sec1/Munc18 (SM) protein. Reconstitution studies suggest that the SM protein Munc18-1 promotes the zippering of trans-SNARE complexes and accelerates the kinetics of SNARE-dependent membrane fusion. However, the physiological role of this trans-SNARE-regulating function in synaptic exocytosis remains to be established. Here we first demonstrate that two mutations in the vesicle-anchored v-SNARE selectively impair the ability of Munc18-1 to promote trans-SNARE zippering, whereas other known Munc18-1/SNARE-binding modes are unaffected. In cultured neurons, these v-SNARE mutations strongly inhibit spontaneous as well as evoked neurotransmitter release, providing genetic evidence for the trans-SNARE-regulating function of Munc18-1 in synaptic exocytosis. Finally, we show that the trans-SNARE-regulating function of Munc18-1 is compromised by a mutation associated with Ohtahara Syndrome, a severe form of epilepsy.

Reconstituting Intracellular Vesicle Fusion Reactions: The Essential Role of Macromolecular Crowding

Intracellular vesicle fusion is mediated by SNAREs and Sec1/Munc18 (SM) proteins. Despite intensive efforts, the SNARE-SM mediated vesicle fusion reaction has not been faithfully reconstituted in biochemical assays. Here, we present an unexpected discovery that macromolecular crowding is required for reconstituting the vesicle fusion reaction in vitro. Macromolecular crowding is known to profoundly influence the kinetic and thermodynamic behaviors of macromolecules, but its role in membrane transport processes such as vesicle fusion remains unexplored. We introduced macromolecular crowding agents into reconstituted fusion reactions to mimic the crowded cellular environment. In this crowded assay, SNAREs and SM proteins acted in concert to drive efficient membrane fusion. In uncrowded assays, by contrast, SM proteins failed to associate with the SNAREs and the fusion rate decreased more than 30-fold, close to undetectable levels. The activities of SM proteins were strictly specific to their cognate SNARE isoforms and sensitive to biologically relevant mutations, further supporting that the crowded fusion assay accurately recapitulates the vesicle fusion reaction. Using this crowded fusion assay, we also showed that the SNARE-SM mediated fusion reaction can be modulated by two additional factors: NSF and α-SNAP. These findings suggest that the vesicle fusion machinery likely has been evolutionarily selected to function optimally in the crowded milieu of the cell. Accordingly, macromolecular crowding should constitute an integral element of any reconstituted fusion assay.

Synaptotagmin's role in neurotransmitter release likely involves Ca(2+)-induced conformational transition

Neuronal exocytosis is mediated by a Ca(2+)-triggered membrane fusion event that joins synaptic vesicles and presynaptic membrane. In this event, synaptotagmin I plays a key role as a Ca(2+) sensor protein that binds to and bends the presynaptic membrane with its C2B domain, and thereby initiates membrane fusion. We report free energy calculations according to which C2B-induced membrane bending is preceded by a Ca(2+)- and membrane-dependent conformational transition. In this transition C2B attaches to the membrane, moves its C-terminal helix from the orientation seen in the available (but membrane-free) crystal/NMR structures as pointing away from the membrane (helix-up), to an orientation pointing toward the membrane (helix-down). In the C2B helix-down state, lipid tails in the proximal membrane bilayer leaflet interact with the moved helix and become disordered, whereas tails in the distal leaflet, to keep in contact with the proximal leaflet, become stretched and ordered. The difference in lipid tail packing between the two leaflets results in an imbalance of pressure across the membrane, and thereby causes membrane bending. The lipid-disordering monitored in the simulations is well suited to facilitate Ca(2+)-triggered membrane fusion.

Structural elements that underlie Doc2β function during asynchronous synaptic transmission

Double C2-like domain-containing proteins alpha and beta (Doc2α and Doc2β) are tandem C2-domain proteins proposed to function as Ca(2+) sensors for asynchronous neurotransmitter release. Here, we systematically analyze each of the negatively charged residues that mediate binding of Ca(2+) to the β isoform. The Ca(2+) ligands in the C2A domain were dispensable for Ca(2+)-dependent translocation to the plasma membrane, with one exception: neutralization of D220 resulted in constitutive translocation. In contrast, three of the five Ca(2+) ligands in the C2B domain are required for translocation. Importantly, translocation was correlated with the ability of the mutants to enhance asynchronous release when overexpressed in neurons. Finally, replacement of specific Ca(2+)/lipid-binding loops of synaptotagmin 1, a Ca(2+) sensor for synchronous release, with corresponding loops from Doc2β, resulted in chimeras that yielded slower kinetics in vitro and slower excitatory postsynaptic current decays in neurons. Together, these data reveal the key determinants of Doc2β that underlie its function during the slow phase of synaptic transmission.

The cholesterol-lowering agent methyl-β-cyclodextrin promotes glucose uptake via GLUT4 in adult muscle fibers and reduces insulin resistance in obese mice

Insulin stimulates glucose uptake in adult skeletal muscle by promoting the translocation of GLUT4 glucose transporters to the transverse tubule (T-tubule) membranes, which have particularly high cholesterol levels. We investigated whether T-tubule cholesterol content affects insulin-induced glucose transport. Feeding mice a high-fat diet (HFD) for 8 wk increased by 30% the T-tubule cholesterol content of triad-enriched vesicular fractions from muscle tissue compared with triads from control mice. Additionally, isolated muscle fibers (flexor digitorum brevis) from HFD-fed mice showed a 40% decrease in insulin-stimulated glucose uptake rates compared with fibers from control mice. In HFD-fed mice, four subcutaneous injections of MβCD, an agent reported to extract membrane cholesterol, improved their defective glucose tolerance test and normalized their high fasting glucose levels. The preincubation of isolated muscle fibers with relatively low concentrations of MβCD increased both basal and insulin-induced glucose uptake in fibers from controls or HFD-fed mice and decreased Akt phosphorylation without altering AMPK-mediated signaling. In fibers from HFD-fed mice, MβCD improved insulin sensitivity even after Akt or CaMK II inhibition and increased membrane GLUT4 content. Indinavir, a GLUT4 antagonist, prevented the stimulatory effects of MβCD on glucose uptake. Addition of MβCD elicited ryanodine receptor-mediated calcium signals in isolated fibers, which were essential for glucose uptake. Our findings suggest that T-tubule cholesterol content exerts a critical regulatory role on insulin-stimulated GLUT4 translocation and glucose transport and that partial cholesterol removal from muscle fibers may represent a useful strategy to counteract insulin resistance.

The p10 FAST protein fusion peptide functions as a cystine noose to induce cholesterol-dependent liposome fusion without liposome tubulation

The reovirus p10 fusion-associated small transmembrane (FAST) proteins are the smallest known membrane fusion proteins, and evolved specifically to mediate cell-cell, rather than virus-cell, membrane fusion. The 36-40-residue ectodomains of avian reovirus (ARV) and Nelson Bay reovirus (NBV) p10 contain an essential intramolecular disulfide bond required for both cell-cell fusion and lipid mixing between liposomes. To more clearly define the functional, biochemical and biophysical features of this novel fusion peptide, synthetic peptides representing the p10 ectodomains of ARV and NBV were analyzed by solution-state NMR spectroscopy, circular dichroism spectroscopy, fluorescence spectroscopy-based hydrophobicity analysis, and liposome binding and fusion assays. Results indicate that disulfide bond formation promotes exposure of hydrophobic residues, as indicated by bis-ANS binding and time-dependent peptide aggregation under aqueous conditions, implying the disulfide bond creates a small, geometrically constrained, cystine noose. Noose formation is required for peptide partitioning into liposome membranes and liposome lipid mixing, and electron microscopy revealed that liposome-liposome fusion occurs in the absence of liposome tubulation. In addition, p10 fusion peptide activity, but not membrane partitioning, is dependent on membrane cholesterol.

Dynamic shaping of cellular membranes by phospholipids and membrane-deforming proteins

All cellular compartments are separated from the external environment by a membrane, which consists of a lipid bilayer. Subcellular structures, including clathrin-coated pits, caveolae, filopodia, lamellipodia, podosomes, and other intracellular membrane systems, are molded into their specific submicron-scale shapes through various mechanisms. Cells construct their micro-structures on plasma membrane and execute vital functions for life, such as cell migration, cell division, endocytosis, exocytosis, and cytoskeletal regulation. The plasma membrane, rich in anionic phospholipids, utilizes the electrostatic nature of the lipids, specifically the phosphoinositides, to form interactions with cytosolic proteins. These cytosolic proteins have three modes of interaction: 1) electrostatic interaction through unstructured polycationic regions, 2) through structured phosphoinositide-specific binding domains, and 3) through structured domains that bind the membrane without specificity for particular phospholipid. Among the structured domains, there are several that have membrane-deforming activity, which is essential for the formation of concave or convex membrane curvature. These domains include the amphipathic helix, which deforms the membrane by hemi-insertion of the helix with both hydrophobic and electrostatic interactions, and/or the BAR domain superfamily, known to use their positively charged, curved structural surface to deform membranes. Below the membrane, actin filaments support the micro-structures through interactions with several BAR proteins as well as other scaffold proteins, resulting in outward and inward membrane micro-structure formation. Here, we describe the characteristics of phospholipids, and the mechanisms utilized by phosphoinositides to regulate cellular events. We then summarize the precise mechanisms underlying the construction of membrane micro-structures and their involvements in physiological and pathological processes.

Munc18c: a controversial regulator of peripheral insulin action

Insulin resistance, a hallmark of impaired glucose tolerance and type 2 diabetes (T2D), arises from dysfunction of insulin action and subsequent glucose uptake by peripheral tissues, predominantly skeletal muscle and fat. Exocytosis of glucose transporter (GLUT4)-containing vesicles facilitated by soluble NSF (N-ethylmaleimide-sensitive factor) attachment receptor (SNARE) protein isoforms, and Munc18c (mammalian homolog of Unc-18c) mediates this glucose uptake. Emerging evidences, including recent human clinical studies, point to pivotal roles for Munc18c in peripheral insulin action in adipose and skeletal muscle. Intriguing new advances are also initiating debates regarding the molecular mechanism(s) controlling Munc18c action. The objective of this review is therefore to present a balanced perspective of new continuities and controversies surrounding the regulation and requirement for Munc18c in the regulation of peripheral insulin action.

DOC2 isoforms play dual roles in insulin secretion and insulin-stimulated glucose uptake

AIMS/HYPOTHESIS

Glucose-stimulated insulin secretion (GSIS) and insulin-stimulated glucose uptake are processes that rely on regulated intracellular vesicle transport and vesicle fusion with the plasma membrane. DOC2A and DOC2B are calcium-sensitive proteins that were identified as key components of vesicle exocytosis in neurons. Our aim was to investigate the role of DOC2 isoforms in glucose homeostasis, insulin secretion and insulin action.

METHODS

DOC2 expression was measured by RT-PCR and western blotting. Body weight, glucose tolerance, insulin action and GSIS were assessed in wild-type (WT), Doc2a (-/-) (Doc2aKO), Doc2b (-/-) (Doc2bKO) and Doc2a (-/-)/Doc2b (-/-) (Doc2a/Doc2bKO) mice in vivo. In vitro GSIS and glucose uptake were assessed in isolated tissues, and exocytotic proteins measured by western blotting. GLUT4 translocation was assessed by epifluorescence microscopy.

RESULTS

Doc2b mRNA was detected in all tissues tested, whereas Doc2a was only detected in islets and the brain. Doc2aKO and Doc2bKO mice had minor glucose intolerance, while Doc2a/Doc2bKO mice showed pronounced glucose intolerance. GSIS was markedly impaired in Doc2a/Doc2bKO mice in vivo, and in isolated Doc2a/Doc2bKO islets in vitro. In contrast, Doc2bKO mice had only subtle defects in insulin secretion in vivo. Insulin action was impaired to a similar degree in both Doc2bKO and Doc2a/Doc2bKO mice. In vitro insulin-stimulated glucose transport and GLUT4 vesicle fusion were defective in adipocytes derived from Doc2bKO mice. Surprisingly, insulin action was not altered in muscle isolated from DOC2-null mice.

CONCLUSIONS/INTERPRETATION

Our study identifies a critical role for DOC2B in insulin-stimulated glucose uptake in adipocytes, and for the synergistic regulation of GSIS by DOC2A and DOC2B in beta cells.

The N- and C-terminal domains of tomosyn play distinct roles in soluble N-ethylmaleimide-sensitive factor attachment protein receptor binding and fusion regulation

Tomosyn negatively regulates SNARE-dependent exocytic pathways including insulin secretion, GLUT4 exocytosis, and neurotransmitter release. The molecular mechanism of tomosyn, however, has not been fully elucidated. Here, we reconstituted SNARE-dependent fusion reactions in vitro to recapitulate the tomosyn-regulated exocytic pathways. We then expressed and purified active full-length tomosyn and examined how it regulates the reconstituted SNARE-dependent fusion reactions. Using these defined fusion assays, we demonstrated that tomosyn negatively regulates SNARE-mediated membrane fusion by inhibiting the assembly of the ternary SNARE complex. Tomosyn recognizes the t-SNARE complex and prevents its pairing with the v-SNARE, therefore arresting the fusion reaction at a pre-docking stage. The inhibitory function of tomosyn is mediated by its C-terminal domain (CTD) that contains an R-SNARE-like motif, confirming previous studies carried out using truncated tomosyn fragments. Interestingly, the N-terminal domain (NTD) of tomosyn is critical (but not sufficient) to the binding of tomosyn to the syntaxin monomer, indicating that full-length tomosyn possesses unique features not found in the widely studied CTD fragment. Finally, we showed that the inhibitory function of tomosyn is dominant over the stimulatory activity of the Sec1/Munc18 protein in fusion. We suggest that tomosyn uses its CTD to arrest SNARE-dependent fusion reactions, whereas its NTD is required for the recruitment of tomosyn to vesicle fusion sites through syntaxin interaction.

Doc2b enrichment enhances glucose homeostasis in mice via potentiation of insulin secretion and peripheral insulin sensitivity

AIMS/HYPOTHESIS

Insulin secretion from pancreatic beta cells and insulin-stimulated glucose uptake into skeletal muscle are processes regulated by similar isoforms of the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) and mammalian homologue of unc-18 (Munc18) protein families. Double C2 domain β (Doc2b), a SNARE- and Munc18-interacting protein, is implicated as a crucial effector of glycaemic control. However, whether Doc2b is naturally limiting for these processes, and whether Doc2b enrichment might exert a beneficial effect upon glycaemia in vivo, remains undetermined.

METHODS

Tetracycline-repressible transgenic (Tg) mice engineered to overexpress Doc2b simultaneously in the pancreas, skeletal muscle and adipose tissues were compared with wild-type (Wt) littermate mice regarding glucose and insulin tolerance, islet function in vivo and ex vivo, and skeletal muscle GLUT4 accumulation in transverse tubule/sarcolemmal surface membranes. SNARE complex formation was further assessed using Doc2b overexpressing L6-GLUT4-myc myoblasts to derive mechanisms relatable to physiological in vivo analyses.

RESULTS

Doc2b Tg mice cleared glucose substantially faster than Wt mice, correlated with enhancements in both phases of insulin secretion and peripheral insulin sensitivity. Heightened peripheral insulin sensitivity correlated with elevated insulin-stimulated GLUT4 vesicle accumulation in cell surface membranes of Doc2b Tg mouse skeletal muscle. Mechanistic studies demonstrated Doc2b enrichment to enhance syntaxin-4-SNARE complex formation in skeletal muscle cells.

CONCLUSIONS/INTERPRETATION

Doc2b is a limiting factor in SNARE exocytosis events pertinent to glycaemic regulation in vivo. Doc2b enrichment may provide a novel means to simultaneously boost islet and skeletal muscle function in vivo in the treatment and/or prevention of diabetes.

Cellular and molecular interactions of phosphoinositides and peripheral proteins

Anionic lipids act as signals for the recruitment of proteins containing cationic clusters to biological membranes. A family of anionic lipids known as the phosphoinositides (PIPs) are low in abundance, yet play a critical role in recruitment of peripheral proteins to the membrane interface. PIPs are mono-, bis-, or trisphosphorylated derivatives of phosphatidylinositol (PI) yielding seven species with different structure and anionic charge. The differential spatial distribution and temporal appearance of PIPs is key to their role in communicating information to target proteins. Selective recognition of PIPs came into play with the discovery that the substrate of protein kinase C termed pleckstrin possessed the first PIP binding region termed the pleckstrin homology (PH) domain. Since the discovery of the PH domain, more than ten PIP binding domains have been identified including PH, ENTH, FYVE, PX, and C2 domains. Representative examples of each of these domains have been thoroughly characterized to understand how they coordinate PIP headgroups in membranes, translocate to specific membrane docking sites in the cell, and function to regulate the activity of their full-length proteins. In addition, a number of novel mechanisms of PIP-mediated membrane association have emerged, such as coincidence detection-specificity for two distinct lipid headgroups. Other PIP-binding domains may also harbor selectivity for a membrane physical property such as charge or membrane curvature. This review summarizes the current understanding of the cellular distribution of PIPs and their molecular interaction with peripheral proteins.

Cellular regulation of glucose uptake by glucose transporter GLUT4

GLUT4 is regulated by its intracellular localization. In the absence of insulin, GLUT4 is efficiently retained intracellularly within storage compartments in muscle and fat cells. Upon insulin stimulation (and contraction in muscle), GLUT4 translocates from these compartments to the cell surface where it transports glucose from the extracellular milieu into the cell. Its implication in insulin-regulated glucose uptake makes GLUT4 not only a key player in normal glucose homeostasis but also an important element in insulin resistance and type 2 diabetes. Nevertheless, how GLUT4 is retained intracellularly and how insulin acts on this retention mechanism is largely unclear. In this review, the current knowledge regarding the various molecular processes that govern GLUT4 physiology is discussed as well as the questions that remain.

Mutations that disrupt Ca²⁺-binding activity endow Doc2β with novel functional properties during synaptic transmission

Double C2-domain protein (Doc2) is a Ca(2+)-binding protein implicated in asynchronous and spontaneous neurotransmitter release. Here we demonstrate that each of its C2 domains senses Ca(2+); moreover, the tethered tandem C2 domains display properties distinct from the isolated domains. We confirm that overexpression of a mutant form of Doc2β, in which two acidic Ca(2+) ligands in the C2A domain and two in the C2B domain have been neutralized, results in markedly enhanced asynchronous release in synaptotagmin 1-knockout neurons. Unlike wild-type (wt) Doc2β, which translocates to the plasma membrane in response to increases in [Ca(2+)](i), the quadruple Ca(2+)-ligand mutant does not bind Ca(2+) but is constitutively associated with the plasma membrane; this effect is due to substitution of Ca(2+) ligands in the C2A domain. When overexpressed in wt neurons, Doc2β affects only asynchronous release; in contrast, Doc2β Ca(2+)-ligand mutants that constitutively localize to the plasma membrane enhance both the fast and slow components of synaptic transmission by increasing the readily releasable vesicle pool size; these mutants also increase the frequency of spontaneous release events. Thus, mutations in the C2A domain of Doc2β that were intended to disrupt Ca(2+) binding result in an anomalous enhancement of constitutive membrane-binding activity and endow Doc2β with novel functional properties.

Regulation of myosin light chain kinase during insulin-stimulated glucose uptake in 3T3-L1 adipocytes

Myosin II (MyoII) is required for insulin-responsive glucose transporter 4 (GLUT4)-mediated glucose uptake in 3T3-L1 adipocytes. Our previous studies have shown that insulin signaling stimulates phosphorylation of the regulatory light chain (RLC) of MyoIIA via myosin light chain kinase (MLCK). The experiments described here delineate upstream regulators of MLCK during insulin-stimulated glucose uptake. Since 3T3-L1 adipocytes express two MyoII isoforms, we wanted to determine which isoform was required for insulin-stimulated glucose uptake. Using a siRNA approach, we demonstrate that a 60% decrease in MyoIIA protein expression resulted in a 40% inhibition of insulin-stimulated glucose uptake. We also show that insulin signaling stimulates the phosphorylation of MLCK. We further show that MLCK can be activated by calcium as well as signaling pathways. We demonstrate that adipocytes treated with the calcium chelating agent, 1,2-b (iso-aminophenoxy) ethane-N,N,N',N'-tetra acetic acid, (BAPTA) (in the presence of insulin) impaired the insulin-induced phosphorylation of MLCK by 52% and the RLC of MyoIIA by 45% as well as impairing the recruitment of MyoIIA to the plasma membrane when compared to cells treated with insulin alone. We further show that the calcium ionophore, A23187 alone stimulated the phosphorylation of MLCK and the RLC associated with MyoIIA to the same extent as insulin. To identify signaling pathways that might regulate MLCK, we examined ERK and CaMKII. Inhibition of ERK2 impaired phosphorylation of MLCK and insulin-stimulated glucose uptake. In contrast, while inhibition of CaMKII did inhibit phosphorylation of the RLC associated with MyoIIA, inhibition of CAMKIIδ did not impair MLCK phosphorylation or translocation to the plasma membrane or glucose uptake. Collectively, our results are the first to delineate a role for calcium and ERK in the activation of MLCK and thus MyoIIA during insulin-stimulated glucose uptake in 3T3-L1 adipocytes.

The C2B domain is the primary Ca2+ sensor in DOC2B: a structural and functional analysis

DOC2B (double-C2 domain) protein is thought to be a high-affinity Ca(2+) sensor for spontaneous and asynchronous neurotransmitter release. To elucidate the molecular features underlying its physiological role, we determined the crystal structures of its isolated C2A and C2B domains and examined their Ca(2+)-binding properties. We further characterized the solution structure of the tandem domains (C2AB) using small-angle X-ray scattering. In parallel, we tested structure-function correlates with live cell imaging tools. We found that, despite striking structural similarity, C2B binds Ca(2+) with considerably higher affinity than C2A. The C2AB solution structure is best modeled as two domains with a highly flexible orientation and no difference in the presence or absence of Ca(2+). In addition, kinetic studies of C2AB demonstrate that, in the presence of unilamellar vesicles, Ca(2+) binding is stabilized, as reflected by the ~10-fold slower rate of Ca(2+) dissociation than in the absence of vesicles. In cells, isolated C2B translocates to the plasma membrane (PM) with an EC50 of 400 nM while the C2A does not translocate at submicromolar Ca(2+) concentrations, supporting the biochemical observations. Nevertheless, C2AB translocates to the PM with an ~2-fold lower EC50 and to a greater extent than C2B. Our results, together with previous studies, reveal that the C2B is the primary Ca(2+) sensing unit in DOC2B, whereas C2A enhances the interaction of C2AB with the PM.

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

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

Synip arrests soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent membrane fusion as a selective target membrane SNARE-binding inhibitor

The vesicle fusion reaction in regulated exocytosis requires the concerted action of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) core fusion engine and a group of SNARE-binding regulatory factors. The regulatory mechanisms of vesicle fusion remain poorly understood in most exocytic pathways. Here, we reconstituted the SNARE-dependent vesicle fusion reaction of GLUT4 exocytosis in vitro using purified components. Using this defined fusion system, we discovered that the regulatory factor synip binds to GLUT4 exocytic SNAREs and inhibits the docking, lipid mixing, and content mixing of the fusion reaction. Synip arrests fusion by binding the target membrane SNARE (t-SNARE) complex and preventing the initiation of ternary SNARE complex assembly. Although synip also interacts with the syntaxin-4 monomer, it does not inhibit the pairing of syntaxin-4 with SNAP-23. Interestingly, synip selectively arrests the fusion reactions reconstituted with its cognate SNAREs, suggesting that the defined system recapitulates the biological functions of the vesicle fusion proteins. We further showed that the inhibitory function of synip is dominant over the stimulatory activity of Sec1/Munc18 proteins. Importantly, the inhibitory function of synip is distinct from how other fusion inhibitors arrest SNARE-dependent membrane fusion and therefore likely represents a novel regulatory mechanism of vesicle fusion.