Pancreatic beta cells secrete insulin responding to rises in blood glucose levels. Patients lacking sufficient insulin production due to the autoimmune destruction of beta cells in type 1 diabetes (T1D)1, or due to beta cell dysfunction in type 2 diabetes (T2D)2, are typically treated with the administration of exogenous insulin. Despite this life-saving therapy, it cannot precisely match the exquisite control of blood glucose as achieved by dynamic insulin secretion from bona fide beta cells. As such, patients often suffer the consequences of life-threatening hypoglycemic episodes and other complications resulting from chronic hyperglycemic excursions. Transplantation of human cadaveric islets successfully restores tight glycemic control in T1D patients but is limited by the availability of islet donors and difficulties in purifying healthy islets for transplantation3,4. This challenge can, in principle, be solved by using hPSCs as an alternative starting material.
Current strategies for generating insulin-secreting islets from hPSCs in vitro often aim to mimic the process of embryonic pancreas development in vivo5,6. This requires knowledge of the responsible signaling pathways and timed addition of corresponding soluble factors to mimic critical stages of the developing embryonic pancreas. The pancreatic program initiates with the commitment into definitive endoderm, which is marked by transcription factors forkhead box A2 (FOXA2) and sex-determining region Y-box 17 (SOX17)7. Successive differentiation of definitive endoderm involves the formation of a primitive gut tube, patterning into a posterior foregut that expresses the pancreatic and duodenal homeobox 1 (PDX1)7,8,9, and epithelial expansion into pancreatic progenitors that co-express PDX1 and NK6 homeobox 1 (NKX6.1)10,11.
Further commitment to endocrine islet cells is accompanied by the transient expression of pro-endocrine master regulator neurogenin-3 (NGN3)12 and stable induction of key transcription factors neuronal differentiation 1 (NEUROD1) and NK2 homeobox 2 (NKX2.2)13. The major hormone-expressing cells, such as insulin-producing beta cells, glucagon-producing alpha cells, somatostatin-producing delta cells, and pancreatic polypeptide-producing PPY cells, are subsequently programmed. With this knowledge, as well as discoveries from extensive, high-throughput drug screening studies, recent advancements have enabled the generation of hPSC-islets with cells resembling beta cells capable of insulin secretion14,15,16,17,18,19.
Step-wise protocols have been reported for generating glucose-responsive beta cells6,14,18,19. Built upon these studies, the present protocol involves the use of a pancreatic progenitor kit for generating PDX1+/NKX6.1+ pancreatic progenitor cells in a planar culture, followed by microwell plate aggregation into uniform-sized clusters and further differentiation toward insulin-secreting hPSC-islets with the R-protocol in a static 3D suspension culture. Quality control analyses, including flow cytometry, immunostaining, and functional assessment, are performed for rigorous characterization of the differentiating cells. This paper provides a detailed description of each step of the directed differentiation and outlines the in vitro characterization approaches.