Exocytosis in Type 2 Diabetes - Functional and Genetic Studies of Hormone Secretion
Type 2 Diabetes (T2D) is characterized by dysregulated beta-and alpha-cell hormone secretion leading to elevated blood glucose levels. Several proteins are crucial in maintaining exocytosis of the hormone-containing granules such as Syntaxin1A (Stx1A), SNAP25, Munc-18 (Stxbp1) and the family of Synaptotagmins (Syts). The aim of this thesis has been to investigate functional and genetic events that participate in the exocytotic process.
First we show that SNAP25 is essential for cAMP-dependent rapid exocytosis in insulin-secreting cells, and that the effect may be mediated by binding to cAMP-GEFII (paper I). In mouse alpha-cells, SNAP25 and Stx1A display a glucose-dependent localization where immunoneutralization of SNAP25 and Stx1A strongly reduce the exocytotic response (paper II). In paper III, the mRNA expression levels of 23 genes with known function in exocytosis was compared in human beta-cells from non-diabetic (ND) and T2D donors which revealed that STX1A, SYT4, SYT7, SYT11 and SYT13 is nominally down-regulated in T2D beta-cells and concomitantly correlate positively with glucose stimulated insulin secretion (GSIS) and negatively with HbA1c levels. Furthermore, silencing of Syt4, Syt7 and Syt13 in INS1-832/13 cells correlate with reduced levels of GSIS. In paper IV exocytosis in human beta-cells was studied to generate a risk score consisting of 4 SNPs in or near TCF7L2, KCNJ11, ADRA2A and KCNQ1 that successfully predict reduced exocytosis, calcium-sensitivity, granular docking and GSIS. Finally, in paper V, knock-down of rno-microRNA-335 in insulin-secreting cells was found to increase Stxbp1 expression which correlates with increased exocytosis.
Within the scope of this thesis we establish that exocytosis is an important part of glucose homeostasis and that a broad range of factors can influence the exocytotic outcome. In this context, regulation of exocytosis span from the transcription of DNA, translation, transport and function of exocytotic proteins as well as their intricate interactions with one another. All while maintaining the correct cues in response to increased levels of intracellular calcium either amplified by second messengers or triggered by glucose-stimulated coupling. Taken together, extensive research on the molecular mechanisms participating in hormone exocytosis may well contribute to future treatment of the reduced insulin secretion apparent in T2D.