Differential Regulation of β‐cell Function by Tonically‐active and Receptor‐activated Gα _z : Implications for β‐cell Compensation and Failure

Adenylate cyclase(AC) synthesizes the second messenger molecule, cyclic adenosine monophosphate (cAMP). In the insulin‐secreting β‐cell of the pancreatic islet, cAMP has critical roles in glucose‐stimulated insulin secretion and β‐cell replication. The G protein α‐subunit, Gα z , has partial tonic inhibitory activity towards AC. Our previous work has confirmed Gα z ‐null mice secrete more insulin and are resistant to glucose intolerance via a significant increase in b‐cell mass. Gα z an also be activated by E‐prostanoid receptor 3 (EP3), a GPCR for the arachidonic acid metabolite, PGE2. Islets from diabetic mice and humans produce more PGE2 and express more EP3 than those from WT controls, while a EP3 antagonist restores insulin secretion. To determine the mechanisms behind the negative regulation of b‐cell function and replication by Ga z , we developed a β‐cell‐specific, cAMP biosensor expressed in intact islets, confirming, for the first time, an inverse relationship between β‐cell cAMP and Ca 2+ oscillations. Loss of Gα z has the expected effects on basal β‐cell cAMP levels, and in response to an agonist of the cAMP‐stimulatory glucagon‐like peptide 1 (GLP‐1) receptor. Consistent with a lack of EP3 expression in islets from lean mice, neither agonists nor antagonists of the EP3 receptor have any effects on β‐cell cAMP, Ca 2+ , or the coordination between the two, regardless of Gα z expression. C57BL/6J Lep ob (B6‐Ob) mice are a model of β‐cell compensation. We have shown the EP3:Gα z pathway is up‐regulated in B6‐Ob islets; raising the question as to whether up‐regulation of the this pathway is dysfunctional or beneficial. Here, we confirm B6‐Ob cAMP production and Ca 2+ oscillations are up‐regulated as compared to WT mice. In addition, the anti‐phase relationship between cAMP and Ca 2+ is lost as a result of reduce phosphodiesterase 1A expression. These results are also consistent with saturation of Ca 2+ ‐regulatable AC isozymes. The EP3 agonist, sulprostone, had no effect on cAMP levels, but blocked Ca 2+ influx, suggesting divergent signaling mechanisms between tonically‐active Gα z and receptor‐activated Gα z . The ability of activated Gα z to bind to Rap1GAP, sequestering it from AC, yields a hypothesis for this signaling divergence. Our current model is that cAMP signaling up‐regulated in the compensating β‐cell, ensures Rap1GAP is inactive towards Rap1, allowing Rap1 to promote cAMP‐mediated effects on exocytosis and β‐cell replication while serving as a sink for activated Gα z , inhibiting its action on AC isoforms critical for β‐cell function and mass. Support or Funding Information I01 BX003700‐01A1, VA BLR&D; R01 DK102598, NIH/NIDDK; and ADA Innovative Basic Science Award 1‐14‐BS‐115 This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

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