Actin regulates the turnover of insulin secretory granules

Introduction: Insulin secretion is required for regulation of glucose homeostasis and its impairment leads to diabetes. Insulin is produced by the pancreatic b-cells, it is stored within insulin secretory granules (SGs) and released upon glucose stimulation. Newly synthesized insulin is preferentially released and mobile insulin SGs are more prone to undergo exocytosis. This suggests that SG age and mobility are key factors affecting insulin secretion.

Methods: We used live-cell imaging combined with pharmacological treatment and automated particle tracking to analyze the dynamics and interaction with cytoskeleton of 3- to 5- and 28- to 30-hour-old (hence younger and older) cortical SGs labeled with insulin-SNAP reporter construct (Ivanova et al., 2013).

Results: The majority of both younger and older SGs was immobile or had restricted mobility, with highly dynamic and processively moving SGs being a minority. Highly dynamic and processive SGs moved on microtubules regardless of their age, but their mean speed and the diffusion coefficient were greater in the case of older SGs. In contrast, F-actin restricted the mobility of younger SGs, but it supported that of the older SGs. Moreover, we identified multigranular bodies (MGBs) consisting of several SGs coated with F-actin and moving on microtubules. These MGBs were more frequent among older SG population. Positivity of MGBs for the lysosomal and autophagosomal markers suggest that they represent cellular degradatory compartment. Finally, removal of these highly dynamic MGBs from the analyses resulted in the reversal of our original assumption – younger bona-fide SGs displayed greater mobility then their older counterparts.

Conclusions: Our data provide novel insights into the relationship between SG age and dynamics and point on F-actin as a potential marker of SG aging, conceivably associated with the disposal of aged SGs via an autophagosomal-lysosomal pathway.