Sunday, October 6, 2024
10/6/2024
Modified Project Summary/Abstract Section Type 2 diabetes (T2D) affects ~10% of the US adult population, and another 84.1 million have prediabetes. The β-cell dysfunction is an early and essential event in an impaired glucose tolerant state (IGT) that leads to diabetes, including obesity-linked T2D. The formation of SNARE complexes is rate-limiting for the fusion of insulin granules to the plasma membrane (PM) for insulin secretion. A PM protein syntaxin is a central component for forming SNARE complexes that modulate biphasic insulin secretion. Our understanding of factors that inhibit the formation of SNARE complexes to reduce insulin secretion in an IGT state is lacking. In this regard, we have identified Tomosyn-1, an endogenous inhibitor of insulin secretion that functions by binding to syntaxin. Our long- term goal is to understand how Tomosyn-1 function in β-cells can be manipulated to improve insulin secretion in IGT to treat and prevent prediabetes and T2D. Increasing protein abundance and/or functional activity of Tomosyn-1 reduces insulin secretion. We show that β-cell-specific loss of Tomosyn-1 increased insulin secretion, improving glucose clearance in mice. Increased Tomosyn-1 protein abundance in obese mouse and T2D human islets is associated with reduced insulin secretion in IGT. As such, islets of diet-induced obese mice hemizygous for Tomosyn-1 exhibited increases in insulin secretion. We have identified phosphorylation sites in Tomosyn-1 in response to β-cell signaling pathways, which increase insulin secretion, and an E3-ubiquitin ligase, Hrd1, which increases Tomosyn-1 turnover. Excitingly, we discovered that Syt9, which marks a subpopulation of insulin granules, binds Tomosyn-1 and inhibits the formation of the SNARE complexes. Improved glucose tolerance in Syt9-null mice directly results from enhanced insulin secretion from β-cells. Interactions of Tomosyn- 1 with syntaxin and Syt9 provide strong evidence for Tomosyn-1 as a key modulator of the fusion of insulin granules for the insulin secretion process. We aim to determine how Tomosyn-1 inhibits insulin secretion in pathophysiology and physiology at the molecular level and how its inhibitory function is regulated. We hypothesize that Tomosyn-1 is a critical protein in the exocytosis machinery that regulates SNARE complex- mediated insulin secretion in response to nutritional cues, and specific post-translational modifications of Tomosyn-1 increase insulin secretion. For one year of bridge funding, we plan to a) conduct metabolic phenotyping using Tomosyn-1βKO and control mice in young (6 weeks) and older (>30 weeks) mice, b) determine Tomosyn-1 protein abundance in human T2D islet β-cells, and c) assess the binding of Syt9 with Stx1A and Stx4 and Tomosyn-1 in T2D islets. These experiments are aimed to address select reviewers’ comments to provide a stronger rationale for investigating the role of Tomosyn-1 in regulating insulin secretion and that increases in Tomosyn-1 binding with Syt9 are implicated in reducing insulin secretion in T2D human islets. This will improve our understanding of how β-cells fine-tune insulin secretion and there is a loss in fusion competency of insulin granules during IGT.
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