Highlights d Human b cell dysfunction induced by metabolic stress may be persistent or transient d Specific transcriptomic changes associate with durable or reversible damage d Type 2 diabetes (T2D) b cells have several functional and molecular alterations d T2D and irreversibly or temporarily impaired b cells share key transcriptome traits

The mechanisms that control proliferation, or lack thereof, in adult human β cells are poorly understood. Controlled induction of proliferation could dramatically expand the clinical application of islet cell transplantation and represents an important component of regenerative approaches to a functional cure of diabetes. Adult human β cells are particularly resistant to common proliferative targets and often dedifferentiate during proliferation. Here we show that expression of the transcription factor E2F3 has a role in regulating β-cell quiescence and proliferation. We found human islets have virtually no expression of the pro-proliferative G 1/S transcription factors E2F1-3, but an abundance of inhibitory E2Fs 4-6. In proliferative human insulinomas, inhibitory E2Fs were absent, while E2F3 is expressed. Using this pattern as a "roadmap" for proliferation, we demonstrated that ectopic expression of nuclear E2F3 induced significant expansion of insulin-positive cells in both rat and human islets. These cells did not undergo apoptosis and retained their glucose-responsive insulin secretion, showing the ability to reverse diabetes in mice. Our results suggest that E2F4-6 may help maintain quiescence in human β cells and identify E2F3 as a novel target to induce proliferation of functional β cells. Refinement of this approach may increase the islets available for cell-based therapies and research and could provide important cues for understanding in vivo proliferation of β cells.

GPR40 is a clinically validated molecular target for the treatment of diabetes. Many GPR40 agonists have been identified to date, with the partial agonist fasiglifam (TAK-875) reaching phase III clinical trials before its development was terminated due to off-target liver toxicity. Since then, attention has shifted toward the development of full agonists that exhibit superior efficacy in preclinical models. Full agonists bind to a distinct binding site, suggesting conformational plasticity and a potential for biased agonism. Indeed, it has been suggested that alternative pharmacology may be required for meaningful efficacy. In this study, we described the discovery and characterization of Compound A, a newly identified GPR40 allosteric full agonist highly efficacious in human islets at potentiating glucose-stimulated insulin secretion. We compared Compound A-induced GPR40 activity to that induced by both fasiglifam and AM-1638, another allosteric full agonist previously reported to be highly efficacious in preclinical models, at a panel of G proteins. Compound A was a full agonist at both the Gq and Gi2 pathways, and in contrast to fasiglifam Compound A also induced G12 coupling. Compound A and AM-1638 displayed similar activity at all pathways tested. The G/G-mediated signaling pathway has been linked to protein kinase D activation as well as actin remodeling, well known to contribute to the release of insulin vesicles. Our data suggest that the pharmacology of GPR40 is complex and that G12/G13-mediated signaling, which may contribute to GPR40 agonists therapeutic efficacy, is a specific property of GPR40 allosteric full agonists.