Disease relevance of Stx4a

• These data demonstrate that the lack of syntaxin 4 expression results in abnormal movement of GLUT8 in response to insulin, decreased insulin-stimulated glucose uptake, and increased apoptosis [1].
• Introduction of the cytoplasmic domain of syntaxin 4, using either recombinant vaccinia virus or single-cell microinjection, resulted in an inhibition of insulin-stimulated GLUT4 but not GLUT1 translocation to the plasma membrane [2].

High impact information on Stx4a

• However, the Syn4(+/-) mice manifested impaired glucose tolerance with a 50% reduction in whole-body glucose uptake [3].
• Together, these data demonstrate that syntaxin 4 plays a critical physiological role in insulin-stimulated glucose uptake in skeletal muscle [3].
• In contrast, Syn4(+/-) mice displayed normal insulin-stimulated glucose uptake and metabolism in adipose tissue and liver [3].
• Syntaxin 4 heterozygous knockout mice develop muscle insulin resistance [3].
• Furthermore, treatment of the permeabilized adipocytes with glutathione S-transferase fusion proteins encoding soluble forms of VAMP-2 or syntaxin-4 also effectively blocked insulin-regulated GLUT4 translocation [4].

Biological context of Stx4a

• The t-SNARE syntaxin 4 is regulated during macrophage activation to function in membrane traffic and cytokine secretion [5].
• Syntaxin 4 is the t-SNARE critical for insulin-stimulated glucose transporter 4 (GLUT4)-plasma membrane fusion in adipocytes [1].
• Syntaxin 4 binding to Munc18c decreased as Munc18c phosphorylation levels increased in pervanadate-treated cells, suggesting that phosphorylation dissociates the Munc18c-Syntaxin 4 complex [6].
• Mutagenesis of one residue in this region, Y219F, significantly increased the affinity of Munc18c for Syntaxin 4, whereas mutation of three other candidate sites was without effect [6].
• Insulin relieves this inhibition by inducing the dissociation of Munc18c from syntaxin 4 and by sequestering Munc18c to an alternative plasma membrane binding site [7].

Anatomical context of Stx4a

• 5. In parallel, 22% of the blastocysts from Stx4a+/- matings had no significant insulin-stimulated translocation of GLUT8 whereas 77% displayed either partial or complete translocation to the apical plasma membrane [1].
• This difference in GLUT8 translocation directly correlated with one-third of blastocysts from Stx4a+/- mating having reduced rates of insulin-stimulated glucose uptake and 67% with wild-type rates [1].
• Syntaxin 4 formed a complex with Munc18c at the cell surface of macrophages [5].
• Interestingly, acute insulin treatment decreased the content of syntaxin 4 in low-density microsomes and caused a corresponding gain in the plasma membrane fraction, reminiscent of the recruitment of GLUT4 glucose transporters [8].
• Taken together, these data support the notion that Syntaxin 4-based SNARE complexes are essential for biphasic insulin granule fusion in pancreatic beta-cells [9].

Associations of Stx4a with chemical compounds

• An important finding was that female Syn4 transgenic mice exhibited an increased rate of glucose clearance during glucose tolerance tests that was repressible by the administration of tetracycline [10].
• Here, we use combined membrane fractionation, detergent solubility, and sucrose gradient flotation to demonstrate that the large majority (>70%) of SNAP-23 and a significant proportion of syntaxin 4 ( approximately 35%) are associated with plasma membrane lipid rafts in 3T3-L1 adipocytes [11].
• A measurable fraction of SNAP23 was sedimented with cytoskeletal elements when extracted with Triton X-100, unlike VAMP-2 and syntaxin-4, which were exclusively soluble in detergent [12].
• Finally, PIM displayed in vivo effects in macrophages by increasing accumulation of plasma membrane-endosomal syntaxin 4 and transferrin receptor on PIM-coated latex bead phagosomes [13].
• Disruption of F-actin-Syntaxin 4 binding correlated with enhanced glucose-stimulated insulin secretion, mediated by increased granule accumulation at the plasma membrane and increased Syntaxin 4 accessibility under basal conditions [14].

Physical interactions of Stx4a

• Membrane-bound Mlgl from MDCK cell lysates was coimmunoprecipitated with syntaxin 4, a component of the exocytic machinery at the basolateral membrane, but not with other plasma membrane soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins that are either absent from or not restricted to the basolateral membrane domain [15].
• This is the first localization of SNAP-23 within a polarized tissue and suggests that this t-SNAREs may interact with syntaxin-4 to mediate basolateral secretion [16].

Enzymatic interactions of Stx4a

• Syntaxin-4 was efficiently phosphorylated by casein kinase II (CKII) and cAMP-dependent protein kinase (PKA) (incorporating 0.8 and 3.9 mol of phosphate/mol of syntaxin-4, respectively), while syntaxin-1 was only strongly phosphorylated by CKII [17].

Other interactions of Stx4a

• Thus, syntaxin 4 functions as the necessary t-SNARE protein responsible for correct fusion of the GLUT8-containing vesicle with the plasma membrane in the mouse blastocyst [1].
• Functional studies involving the introduction of Syntaxin 4 cDNA or peptides into macrophages implicate this t-SNARE in a rate-limiting step of TNFalpha secretion and in membrane ruffling during macrophage activation [5].
• These data taken together strongly suggest a role for VAMP 2 in GLUT4 trafficking and also for syntaxin 4 [18].
• The plasma membranes of insulin-responsive cells have also been shown to contain syntaxin 4 and the 25 kDa synaptosome-associated protein (SNAP-25), two proteins that form a complex with VAMP 2 [18].
• Levels of the v-SNARE protein cellubrevin and of the t-SNARE protein syntaxin 4 were increased in this process in parallel to GLUT4 [19].

Analytical, diagnostic and therapeutic context of Stx4a

• Furthermore, reduction in syntaxin 4 protein levels in this tissue can account for the impairment in whole-body insulin-stimulated glucose metabolism in this animal model [3].
• In addition, immunoprecipitation of the expressed syntaxin 4 cytoplasmic domain resulted in an insulin-stimulated increase in the coimmunoprecipitation of GLUT4-containing vesicles [2].
• Moreover, immunofluorescent confocal microscopy revealed that Syntaxin 4 was principally localized to the beta-cells and not the alpha-cells of the mouse islet [9].
• Full-length munc18c(1-592), munc18c(1-139) and munc18c(1-225), but not munc18c(226-592), munc18c(1-100), munc18c(43-139) or munc18c(66-139), interacted with the cytoplasmic portion of syntaxin 4, Stx4(2-273), as assessed by yeast two-hybrid assay of growth on nutritionally deficient media and by beta-galactosidase reporter induction [20].

References