Abstract
Increased glucagon secretion from the pancreatic alpha cell is the first and most important defense against hypoglycemia. In type 1 diabetes this defensive mechanism is lost, increasing the mortality risk. In addition to the reduced glucagon response to hypoglycemia in T1D, basal glucagon secretion is elevated further stimulating hyperglycemia and desensitizing autocrine signaling pathways. Furthermore, alpha cells lose their ability to respond appropriately to rising glucose levels. In healthy individuals, rising glucose levels lead to increased insulin secretion, resulting in inhibition of glucagon secretion. In T1D however, the loss of beta cells results in the absence of these paracrine signals that mediate glucagon suppression under hyperglycemic conditions. We tested the hypothesis that reactivating paracrine and autocrine signaling pathways pharmacologically reverses alpha cell glucose blindness. We used living pancreas slices, which allow functional assessments of damaged and infiltrated islets within their native environment. We studied tissues from non-diabetic donors and donors with type 1 diabetes to determine alpha cell responses to (a) changes in glycemia, (b) agonists, antagonists, and positive allosteric modulators of autocrine and paracrine signaling pathways, and (c) reference stimuli such as adrenaline and KCl depolarization using functional recordings. We further performed in vivo studies using mouse models with defective glucose counterregulation to determine whether alpha cell responses to hypoglycemia could be restored. We found that human alpha cells in slices from type 1 diabetic donors had normal glucagon content and responded to KCl depolarization but failed to respond to decreases in glucose concentration. Furthermore, we found severely diminished Ca2+ responses to both lowering in glucose concentration and glutamate receptor stimulation. Our in vitro results not only reproduced important features seen in vivo but also revealed mechanistic defects related to autocrine and paracrine inputs. Importantly, reestablishing these inputs pharmacologically rescued glucagon secretion in response to a lowering in glucose concentration in human tissue slices/islets from donors with T1D as well as to hypoglycemia in the nonobese diabetic (NOD) mouse model. Several of these signaling molecules are used in other medical indications and could be repurposed to develop therapies aimed at preventing hypoglycemia in T1D.
Presentation:
Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.