SUMMARY The pancreatic islets of Langerhans regulate glucose homeostasis. The loss of insulin-producing β cells within islets results in diabetes, and islet transplantation from cadaveric donors can cure the disease. In vitro production of whole islets, not just β cells, will benefit from a better understanding of endocrine differentiation and islet morphogenesis. We used single-cell mRNA sequencing to obtain a detailed description of pancreatic islet development. Contrary to the prevailing dogma, we find islet morphology and endocrine differentiation to be directly related. As endocrine progenitors differentiate, they migrate in cohesion and form bud-like islet precursors, or “peninsulas” (literally “almost islands”). α cells, the first to develop, constitute the peninsular outer layer, and β cells form later, beneath them. This spatiotemporal collinearity leads to the typical core-mantle architecture of the mature, spherical islet. Finally, we induce peninsula-like structures in differentiating human embryonic stem cells, laying the ground for the generation of entire islets in vitro.

Highlights d scRNA-seq of hESC differentiation into b cells in vitro d Progenitor and endocrine compartments express different WNT pathway components d APC deletion in mouse endocrine precursors prevents endocrine differentiation d Small molecule inhibition of the WNT pathway in vitro increases endocrine yield

Features of the fission yeast Schizosaccharomyces pombe cAMP/PKA pathway make S. pombe particularly amenable for heterologous expression of cAMP pathway proteins such as GαS subunits and their cognate adenylyl cyclases, PKA catalytic and regulatory subunits, and cyclic nucleotide phosphodiesterases. We have constructed two PKA-repressed reporters for use in high-throughput screens to detect compounds that elevate or reduce PKA activity, thus facilitating the discovery of both inhibitors and activators of these target proteins. Here, we describe steps to construct screening strains and to optimize and conduct these screens.