Insulin is a peptide hormone made in islet beta-cells that is packaged in storage vesicles, termed dense core granules. Insulin is initially synthesized as the prohormone precursor, proinsulin, in the endoplasmic reticulum. Proinsulin is subsequently trafficked to the Golgi, where it is sorted along with other cargo for packaging into regulated secretory vesicles. Insulin secretory vesicles emerge from the terminal Golgi, or trans-Golgi network, where they enter the storage pool to be used for glucose-stimulated insulin secretion. For more information on the beta-cell, check out our recent review.
To study proinsulin trafficking, our lab has developed fluorescent pulse-chase reporters. In the movie and images below, we use the RUSH system to visualize proinsulin trafficking in primary beta-cells. See our recent publication for detailed information on using this system.
Early in response to insulin resistance and hyperglycemia, islet beta-cells increase insulin production, in part, through remodeling the secretory system. Our lab is currently exploring cellular mechanisms regulating this process. Following prolonged exposure to hyperglycemia, beta-cell function deteriorates, resulting in loss of glucose-regulation insulin secretion, which contributes to further loss of glucose control. We hypothesize that critical defects in proinsulin trafficking and insulin granule formation underlie the development of beta-cell dysfunction. Using a fluorescent reporter, termed proCpepSNAP, we have recently shown that proinsulin trafficking is delayed in a dietary model of diabetes (Western diet, WD) leading to a decrease in the formation of insulin secretory granules (below). For more information, see our recent publication.
In support of these observations, images below demonstrate the loss of insulin granules in Type 2 diabetic beta-cells.
We are currently exploring alterations to the ER and Golgi as possible mechanisms leading to defects in proinsulin trafficking. For example, Golgi cisternae from diabetic animals are distended and highly vesiculated, which may contribute to defects in proinsulin packaging and export. Please see our recent publication for more information.
In addition, we have shown that the ER becomes hyperoxidized and may struggle to properly fold proinsulin described in this publication.
Using information learned from our ex vivo islet studies, we explore human islet function in vivo using transplantation models.