Highlights d Adult human pancreatic beta cells can be induced to proliferate at high rates d Driven by synergy between DYRK1A inhibitors and TGFb superfamily inhibitors d Reflects activation of cyclins and CDKs accompanied by CDK inhibitor suppression d Proliferation occurs in type 2 diabetic beta cells, with enhanced differentiation SUMMARYSmall-molecule inhibitors of dual-specificity tyrosine-regulated kinase 1A (DYRK1A) induce human beta cells to proliferate, generating a labeling index of 1.5%-3%. Here, we demonstrate that combined pharmacologic inhibition of DYRK1A and transforming growth factor beta superfamily (TGFbSF)/SMAD signaling generates remarkable further synergistic increases in human beta cell proliferation (average labeling index, 5%-8%, and as high as 15%-18%), and increases in both mouse and human beta cell numbers. This synergy reflects activation of cyclins and cdks by DYRK1A inhibition, accompanied by simultaneous reductions in key cell-cycle inhibitors (CDKN1C and CDKN1A). The latter results from interference with the basal Trithorax-and SMAD-mediated transactivation of CDKN1C and CDKN1A.Notably, combined DYRK1A and TGFb inhibition allows preservation of beta cell differentiated function. These beneficial effects extend from normal human beta cells and stem cell-derived human beta cells to those from people with type 2 diabetes, and occur both in vitro and in vivo.

The scarcity of donors and need for immunosuppression limit pancreatic islet transplantation to a few patients with labile type 1 diabetes. Transplantation of encapsulated stem cell-derived islets (SC islets) might extend the applicability of islet transplantation to a larger cohort of patients. Transplantation of conformal-coated islets into a confined well-vascularized site allows long-term diabetes reversal in fully MHC-mismatched diabetic mice without immunosuppression. Here, we demonstrated that human SC islets reaggregated from cryopreserved cells display glucose-stimulated insulin secretion in vitro. Importantly, we showed that conformally coated SC islets displayed comparable in vitro function with unencapsulated SC islets, with conformal coating permitting physiological insulin secretion. Transplantation of SC islets into the gonadal fat pad of diabetic NOD-scid mice revealed that both unencapsulated and conformal-coated SC islets could reverse diabetes and maintain human-level euglycemia for more than 80 days. Overall, these results provide support for further evaluation of safety and efficacy of conformal-coated SC islets in larger species.

Single cell genomics is a powerful tool to study cellular heterogeneity and discover novel cell types. Recent studies used single cell RNA sequencing (scRNA-seq) to analyze the transcriptomes of individual pancreatic islet cells. Islets are a complex mixture of endocrine cells and therefore represent an ideal tissue type for single cell transcriptomic analysis. Adult human islets consist of five known endocrine cell types (α, β, δ, γ, ε) and multiple less welldefined non-endocrine cells. In this review, we discuss the scRNA-seq studies performed on human fetal, adult, diseased and pluripotent stem cell-derived islets in recent years. Since 2015, ~30,000 adult human islet cells have been analyzed using several scRNA-seq technologies. Studies provide a complete catalogue of all islet cell types and subtypes found throughout human development from fetus to adulthood. Islets from patients with Type 2 diabetes have also been analyzed with scRNA-seq unraveling multiple mechanisms of islet dysfunction. Advances in stem cell differentiation protocols and cell therapy manufacturing are bringing stem cell-derived islets (SC-Islets) closer to clinical trials. In 2018, more than 60,000 SC-Islet cells were analyzed using scRNA-seq technologies. Lessons learned include SC-Islet cell populations, lineage trajectories and comparative analyses to adult human islet cell transcriptomes. Studies have also identified and characterized the non-islet, off-target cell populations revealing potential strategies for their elimination.