Converting human pluripotent stem cells into beta-cells: recent advances and future challenges

Abstract

Purpose of review: The transplantation of insulin-producing beta-cells derived from human embryonic stem cells and induced pluripotent stem cells (collectively termed pluripotent stem cells or PSCs) holds great promise for therapy of diabetes mellitus. The purpose of this review is to summarize recent advances in this area, emphasizing the importance of studies of endocrine pancreas development in efforts to direct PSC differentiation into endocrine cells, as well as to outline the major challenges remaining before transplantation of PSC-derived beta-cells can become a reality.

Recent findings: Although several protocols to generate glucose-responsive pancreatic beta-cells in vitro have been described, the most successful approaches are those that most closely mimic embryonic development of the endocrine pancreas. Until recently, cells generated by these methods have exhibited immature pancreatic endocrine phenotypes. However, protocols that generate more functional beta-like cells have now been described. In addition, small molecules are being used to improve protocols to direct differentiation of PSCs into endoderm and pancreatic lineages.

Summary: Advances over the last decade suggest that generating functional beta-cells from human PSCs is achievable. However, there are aspects of beta-cell development that are not well understood and are hampering generation of PSC-derived beta-cells. In particular, the signaling pathways that instruct endocrine progenitor cells to differentiate into mature and functional beta-cells are poorly understood. Other significant obstacles remain, including the need for safe and cost-effective differentiation methods for large-scale generation of transplantation quality beta-cells, methods to prevent immune rejection of grafted tissues, and amelioration of the risks of tumorigenesis.

Figure 1. Comparison of mouse pancreas organogenesis and the directed differentiation of human PSCs to β-cells

Depicted are the different stages of mouse embryonic pancreas development in vivo (left), with darker shaded areas representing developmental intermediates that have been recapitulated in the directed differentiation of pluripotent stem cells (PSCs) to pancreatic beta cells in vitro (center). Critical signaling pathways that were identified in embryonic studies (center column, left) are activated or inhibited in these protocols using growth factors and small molecules (Pathway activators: Wnt, Wnt3a; Nodal, Activin A; FGF, FGF7 or FGF10; BMP, BMP4; GLP1, Exendin 4. Pathway inhibitors: PI3K, wortmannin; BMP, noggin; HH, cyclopamine; Notch, DAPT). Markers used to identify cells at the indicated developmental stages are shown to the right of the center column. Efficient differentiation is indicated by co-expression of these markers at the protein level in a high proportion of cells. Evolving iPSC technologies are currently being used for reprogramming patient-derived cells into autologous PSCs. When efficient, therapeutic-quality differentiation protocols are established, iPSCs converted to mature beta cells could be used for cell replacement therapy for people with diabetes (right column). PI3K, Phosphoinositide 3-kinase; RA, retinoic acid; HH, hedgehog; GLP1, glucagon-like peptide 1; d panc, dorsal pancreas; v panc, ventral pancreas. Schematics in the left panel have been adapted from previous reviews [7, 8] with permission.