VPS41 Facilitates Formation of a Subpopulation of Insulin Granules

Motivation: Diabetes mellitus is a severe, chronic medical condition that affects roughly 10% of Americans. It is a result of insulin hormone impairment or resistance. One of the many ways in which insulin can be impaired is due to defects in the secretion by pancreatic beta cells.

Problem: Beta cells contain two distinct subpopulations of insulin granules, characterized by the presence of either synaptotagmin-5 or synaptotagmin-7. These granule subpopulations display unique release properties and are differentially impaired under conditions mimicking type 1 and type 2 diabetes respectively. Additionally, the absence of the protein VPS41 in mice leads to type 2 diabetes due to a partial defect in insulin secretion. Our work tests the hypothesis that VPS41 contributes to the formation of synaptotagmin-5 granules.

Approach: Insulinoma cells (INS-1) were used as the wild type (WT) condition and for making the VPS41 knockout (KO) and subsequent rescue conditions. The cells were then immunostained for insulin, synaptotagmin-5, and synaptotagmin-7. Mander’s colocalization coefficient of insulin with synaptotagmin-5 and synaptotagmin-7 was determined using a custom MATLAB script.

Results: VPS41 KO cells showed significantly less colocalization of insulin with synaptotagmin-5 as compared to the WT and rescue. In contrast, we found no change in Syt7 colocalization with insulin in absence of VPS41.Implications: The data suggest that VPS41 plays a critical role in the biogenesis of synaptotagmin-5 positive insulin granules. Further characterization of VPS41 in the insulin secretory pathway will afford a greater understanding of diabetes and could likely lead to novel treatments for the disease