367-OR: Elucidating Mechanisms of α-Cell Dysfunction in T1D

We previously showed that α cell function and gene expression in islets from donors with type 1 diabetes (T1D) was significantly compromised. The reason (s) for these intrinsic α cell changes are unknown but may include loss of α-to-β cell contact, chronic hyperglycemia, and/or repeated hypoglycemic events. To test these concurrent hypotheses, we modeled human T1D islets using α cell-enriched (“T1D-like") human pseudoislets. The synchronous acquisition of intracellular Ca²⁺ and hormone response showed impaired intracellular Ca²⁺ signaling and glucagon secretion in T1D-like pseudoislets compared to controls containing both α and β cells. For modeling of chronic hyperglycemia, we transplanted pseudoislets into Nod-SCID-IL2Rγ^null; RIP-Diphtheria Toxin Receptor mice made diabetic by diphtheria toxin (DT) -induced β cell depletion. In DT-treated mice, α cells in T1D-like pseudoislets maintained expression of MAFB and ARX but began to express the β cell-enriched transcription factor NKX6.1 (control vs. T1D-like, 3.43±2.49 vs. 54.8±20.8% NKX6.1+ α cells; P=0.046, N=3-5 mice, 2 donors/group) . To mimic repeated hypoglycemic events, we intermittently exposed T1D-like pseudoislets to low glucose (1.7 mM) (3 events x 3hr each) in vitro, using those remaining in basal glucose (5 mM) as control. Following multiple low glucose (MLG) exposures, T1D-like pseudoislets had reduced glucagon secretion in the presence of both 1.7 mM (control vs. MLG, 1.51±0.18 vs. 0.79±0.13 % glucagon content; P=0.011, N=4 donors) and 16.7 mM glucose (control vs. MLG, 0.79±0.13 vs. 0.39±0.% glucagon content; P=0.023, N=4) . Native human islets exposed to MLG had similar changes in glucagon response (control vs. MLG, 1.7mM: 0.63±0.vs. 0.22±0.% glucagon content, P=0.007; 16.7mM: 0.18±0.vs. 0.05±0.% glucagon content, P=0.029; N=4 donors) . These data suggest that loss of α-to-β cell contact and repeated exposure to low glucose impair α cell function. Further, exposure to chronic hyperglycemia may lead to changes in α cell identity state after β cell loss.