Mutations in the Human VPS41 Protein and Their Effect on Insulin Secretion
The protein VPS41 is an established subunit of the homotypic fusion and protein sorting (HOPS) complex, which acts as a tethering complex and facilitates endolysosomal fusion. However, VPS41 may function independently of this complex in the formation of insulin granules contributing to secretion of insulin, a process that is still poorly understood. The absence of VPS41 disrupts insulin regulated secretion, alters secretory granule morphology, and leads to a reduction in the number of secretory granules in pancreatic beta cells. Here we investigate how point mutations in the human VPS41 gene, expressed in a KO background, impact insulin secretion, HOPS complex function, and exocytosis in rat insulinoma (INS-1) cells. These mutations have been associated with type 2 diabetes in humans but have not be functionally tested. Notably, we found that 3 mutations (T52R, R416C, and E432K) led to a decrease in stimulated insulin secretion, and 2 of those (T52R and R416) had no impact on HOPS function, indicating that the role of VPS41 in insulin secretion is independent of the HOPS complex. Additionally, we verified that loss of endogenous rat VPS41 disrupts regulated exocytosis using TIRF imaging. These KO INS-1 cells, on average, had significantly fewer exocytotic events over a 1-minute period than the hWT cells did in response to glucose stimulation. Overall, these results not only add support for previous findings, but they also identify regions of interest within the VPS41 gene that should be investigated further. T52R, R416C, and E432K all reside within the GTPase-binding domain of the protein, suggesting that VPS41’s role in insulin secretion may be mediated through interactions with a small GTPase.