Beta‐cell‐specific Loss of the Inhibitory G protein, Gαz, has Sex‐dependent Effects on Development and Pathophysiology of Type 1 Diabetes

Abstract

The mechanisms that underlie the beta‐cell pathophysiology of Type 1 Diabetes (T1D) are not fully understood. Our lab has discovered a role for the unique inhibitory G‐protein, Gαz, in beta‐cell function, survival, and replication in both normal conditions and those of pathological stress. Gαz, when acting as a canonical inhibitory G‐protein, reduces production of cAMP through inhibition of adenylate cyclase. Cyclic AMP acts as essential player in the amplification of glucose‐stimulated insulin secretion, while also mediating pathways that regulate beta‐cell death and replication. Previous work in our lab has shown that non‐obese diabetic (NOD) mice with global loss of Gαz are protected from developing T1D‐like hyperglycemia through increased beta‐cell function, increased beta‐cell replication, and decreased beta‐cell death. In order to determine if Gαz in the beta‐cell was solely responsible for the protected phenotype, we generated a beta‐cell‐specific Gαzknockout mouse by breeding GzFlox/Flox mice with Rat Insulin Promoter (RIP) driven CreHerrmice, on the NOD background for more than 10 generations. Experimental mice were sNP genotyped to ensure preservation of NOD loci. Weekly random‐fed blood glucose measurements were taken beginning at weaning. Loss of beta‐cell Gαz(GαzβKO), only provided a modest protection against diabetes development; 69% of Gαz βKO females remained euglycemic over the first 30 weeks of life compared to 59% in Cre+ NOD controls. Consistent with this phenotype, neither pancreases from Gαzβ‐cell‐KO males nor females had increased beta‐cell fractional area as compared to Cre+ controls. Overall, both euglycemic Gαzβ‐cell‐KO males and females had improved glucose tolerance at 12 weeks of age, with this effect persisting at 28 weeks of age only in females. Yet, the mean insulitis score at 28 weeks‐of‐age was dramatically reduced in GαzβKO males only, and this score was strongly correlated with 28‐week glucose tolerance test AUC. The effects of Gαzβ‐cell‐KO on plasma insulin and ex vivo islet insulin secretion are also differentially regulated by sex: effects that appear to correlate with baseline expression of the constitutively‐active splice variant of the Gαz coupled EP3 receptor. Finally, changes in intra‐islet hormone production and secretion in full‐body vs. beta‐cell‐specific Gαz‐KO mice provide a potential mechanism for the incomplete protection from hyperglycemia in Gαzβ‐cell‐KO mice. Overall, our results show a divergent, sex‐dependent role for beta‐cell Gαz in T1D pathophysiology and implicate Gαz as a hub of intra‐islet cell‐cell signaling.