Insulin secretion from pancreatic β-cells occurs in a biphasic manner. The loss of the early phase of insulin secretion, which occurs due to the inability of insulin granules to fuse with the plasma membrane (PM) via the formation of the SNARE complex, acts as a key determinant in impaired glucose tolerance and development of type 2 diabetes (T2D). Thus, it is critical to elucidate molecular mechanisms underlying early-phase insulin secretion. Tomosyn-2 binds to Stx1A, a key component of the SNARE complex, and functions as an endogenous inhibitor of insulin secretion. The mechanism by which Tomosyn-2 regulates the fusion of insulin granules is not completely understood. Our preliminary data show that Tomosyn-2 binds, colocalizes, and co-fractionates with synaptotagmin-9 (Syt9) at high glucose; Syt9 protein is present in the insulin granule membrane, and its role in insulin secretion remains poorly defined. Our data show that mice lacking Syt9 (Syt9-/-) have improved glucose clearance, increased in vivo early-phase insulin secretion at 5 min and 15 min post-glucose challenge with no change in insulin tolerance vs. the wild type control mice, suggesting that Syt9 regulates the early phase of insulin secretion from β-cells. Strikingly, Syt9-/- mouse islets have reduced Tomosyn-2 levels (∼50%) without altering the levels of other key SNARE proteins. The knockdown of Tomosyn in INS1 β-cells treated with KCL prevents the localization of Syt9 with Stx1A. These results show that Syt9 binding stabilizes Tomosyn-2 protein in a non-fusogenic complex by potentially blocking insulin granules’ access to the PM-Stx1A for membrane fusion and insulin secretion.
In summary, our data reveal a novel molecular complex in Stx1A-Tomosyn-2-Syt-9 to regulate the early-phase insulin secretion.
A. Pathak: None. H. Alsharif: None. J. E. Trombley: None. M. Rahman: None. S. Bhatnagar: None.
National Institutes of Health (R01DK120684-01)