Insulin Stimulates Syntaxin4 SNARE Complex Assembly via a Novel Regulatory Mechanism

Abstract

Insulin stimulates glucose transport into fat and muscle cells by increasing the exocytic trafficking rate of the GLUT4 facilitative glucose transporter from intracellular stores to the plasma membrane. Delivery of GLUT4 to the plasma membrane is mediated by formation of functional SNARE complexes containing syntaxin4, SNAP23, and VAMP2. Here we have used an in situ proximity ligation assay to integrate these two observations by demonstrating for the first time that insulin stimulation causes an increase in syntaxin4-containing SNARE complex formation in adipocytes. Furthermore, we demonstrate that insulin brings about this increase in SNARE complex formation by mobilizing a pool of syntaxin4 held in an inactive state under basal conditions. Finally, we have identified phosphorylation of the regulatory protein Munc18c, a direct target of the insulin receptor, as a molecular switch to coordinate this process. Hence, this report provides molecular detail of how the cell alters membrane traffic in response to an external stimulus, in this case, insulin.A major consequence of insulin binding its receptor on fat and muscle cells is a change in localization of the GLUT4 facilitative glucose transporter. In the absence of insulin, ϳ95% of the transporter is retained intracellularly. Activation of the insulin receptor tyrosine kinase culminates in a 10-to 20-fold increase in the amount of GLUT4 present at the cell surface, accounting for the increased rate of glucose transport into these cells (1). GLUT4 is sequestered away from the cell surface in the absence of insulin by continually cycling through two interrelated endosomal cycles (1). The first operates between the plasma membrane and early and recycling endosomes. This is a fast-trafficking loop that keeps steady-state levels of transporter at the cell surface low under basal conditions. GLUT4 is further sorted from this cycle into a more slowly operating loop between recycling endosomes, the transGolgi network, and a population of vesicles termed GSVs (GLUT4 storage vesicles). It is from GSVs that GLUT4 is mobilized to the cell surface in response to insulin (1).Like all eukaryotic membrane trafficking events, insulin-regulated trafficking of GLUT4 is mediated by formation of specific SNARE complexes. Cell surface delivery of GLUT4 is mediated by a SNARE complex containing the plasma membrane-localized syntaxin4 (Sx4) and SNAP23 t-SNAREs and the VAMP2 v-SNARE (2). SNARE complex formation contributes to specificity of membrane traffic and also provides energy for bilayer fusion (3). Thus, regulating SNARE complex assembly allows the cell to regulate membrane traffic, but whether insulin stimulates SNARE complex formation remains unknown. Sec1/Munc18 (SM) proteins are key regulators of membrane traffic exerting their effect through their cognate SNARE proteins, but their precise mode of action is not understood (3). The Munc18c SM protein binds to Sx4 and is required for insulin-regulated delivery of GLUT4 to the surface of fat and muscle cells (2). Intriguingly, M...

Studies of the regulated assembly of SNARE complexes in adipocytes

“…Our experiments have revealed no increase in the association of Sx4 with SNAP23, VAMP2 or Munc18c, consistent with Sx4 being associated with all of these proteins before insulin-stimulation as well as after [20]. They are not all in complex together because we have seen an increase in associations between VAMP2 and SNAP23 in response to insulin.…”

“…between SNAP23 and VAMP2 and between SNAP23 and Munc18c; both of which increased in response to insulin. (Note that SNAP23 does not bind directly to either VAMP2 or Munc18c, but Munc18c binds the assembled Sx4-SNAP23-VAMP2 complex containing both SNAP23 and VAMP2 in sufficient proximity to generate a PLA signal [20].) These data support a model in which insulin increases the number of assembled Sx4-SNAP23-VAMP2 SNARE complexes, and that these have Munc18c associated with them.…”

“…The addition of tags, such as those required for FRET or bipartite fluorescent protein approaches, has been shown to alter the function of SNAREs, making interpretation of microscopy studies problematic [19]. We have used recently a novel approach to address this question, using a microscopybased proximity ligation assay (PLA) [20]. PLA uses antibodies against two proteins of interest raised in separate species; these are detected subsequently using speciesspecific secondary antibodies to which short oligonucleotides of different defined sequence are attached covalently.…”

“…We have used this approach to examine insulin-stimulated changes in associations between Sx4, SNAP23, VAMP2 and the SM protein Munc18c in 3T3-L1 adipocytes [20]. We have analysed all six possible pairwise combinations between Sx4, SNAP23, VAMP2 and Munc18c under basal and insulin-stimulated conditions.…”

“…It is possible that not all changes in associations are detected using such an assay due to the possibility of certain epitopes being masked or revealed following insulin treatment. To address this, we have performed all of our analyses with antibodies against different regions of each of the proteins [20].…”