Synchronized proinsulin trafficking reveals delayed Golgi export accompanies β-cell secretory dysfunction in a rodent model of hyperglycemia

Abstract

The pancreatic islet beta-cell′s preference for release of newly synthesized insulin requires careful coordination of insulin exocytosis with sufficient insulin granule production to ensure that insulin stores exceed peripheral demands for glucose homeostasis. Thus, the cellular mechanisms regulating insulin granule production are critical to maintaining beta-cell function. In this report, we utilized the synchronous protein trafficking system, RUSH, in primary beta-cells to evaluate proinsulin transit through the secretory pathway leading to insulin granule formation. We demonstrate that the trafficking, processing, and secretion of the proinsulin RUSH reporter, proCpepRUSH, are consistent with current models of insulin maturation and release. Using a rodent dietary model of hyperglycemia and beta-cell dysfunction, we show that proinsulin trafficking is impeded at the Golgi and coincides with the decreased appearance of nascent insulin granules at the plasma membrane. Ultrastructural analysis of beta-cells from diabetic leptin receptor deficient mice revealed gross morphological changes in Golgi structure, including shortened and swollen cisternae, and partial Golgi vesiculation, which are consistent with defects in secretory protein export. Collectively, this work highlights the utility of the proCpepRUSH reporter in studying proinsulin trafficking dynamics and suggests that altered Golgi export function contributes to beta-cell secretory defects in the pathogenesis of Type 2 diabetes.

Long-term ketogenic diet causes hyperlipidemia, liver dysfunction, and glucose intolerance from impaired insulin trafficking and secretion in mice

“…Then the infusion amounts were changed based on blood glucose levels that were taken every 3-5 minutes with a target glucose level of 250mg/ dl. Tail bleeds were also performed at baseline 0, 5,10,15,20,25,30,60,90, 120 so that insulin levels could be measured.…”

“…60 Misfolded proinsulin released from the ER is put forth as cause of Golgi dilation and altered morphology. 60 Incidentally, misfolded proinsulin has been suggested to occur in early stages of T2D, 61 so it is possible that increases in misfolded proinsulin cause Golgi swelling and further impair insulin trafficking and secretion.…”

“…A recent study characterized the disrupted protein trafficking observed in islets of diabetic LepR KO db/db mice and uncovered distended and vesiculated Golgi very similar to what we see in our KD mice. 60 Misfolded proinsulin released from the ER is put forth as cause of Golgi dilation and altered morphology. 60 Incidentally, misfolded proinsulin has been suggested to occur in early stages of T2D, 61 so it is possible that increases in misfolded proinsulin cause Golgi swelling and further impair insulin trafficking and secretion.…”