Stem cells to pancreatic beta-cells: new sources for diabetes cell therapy

Abstract

The number of patients worldwide suffering from the chronic disease diabetes mellitus is growing at an alarming rate. Insulin-secreting beta-cells in the islet of Langerhans are damaged to different extents in diabetic patients, either through an autoimmune reaction present in type 1 diabetic patients or through inherent changes within beta-cells that affect their function in patients suffering from type 2 diabetes. Cell replacement strategies via islet transplantation offer potential therapeutic options for diabetic patients. However, the discrepancy between the limited number of donor islets and the high number of patients who could benefit from such a treatment reflects the dire need for renewable sources of high-quality beta-cells. Human embryonic stem cells (hESCs) are capable of self-renewal and can differentiate into components of all three germ layers, including all pancreatic lineages. The ability to differentiate hESCs into beta-cells highlights a promising strategy to meet the shortage of beta-cells. Here, we review the different approaches that have been used to direct differentiation of hESCs into pancreatic and beta-cells. We will focus on recent progress in the understanding of signaling pathways and transcription factors during embryonic pancreas development and how this knowledge has helped to improve the methodology for high-efficiency beta-cell differentiation in vitro.

Figure 1

Comparison of pancreatic organogenesis in mice and step-wise hESCs to β-cell differentiation. During embryonic pancreas development, the mammalian pancreatic primordium is derived from the DE that subsequently gives rise to the primitive gut and posterior foregut. After the induction of pancreatic epithelium, endocrine cells proliferate and aggregate to form the islet of Langerhans, aggregates of endocrine cells that also harbor the insulin-secreting β-cells. For simplification, only the endoderm-derived pancreatic epithelium is shown in the figure, whereas the surrounding mesenchymal tissues have been omitted. Multiple signaling pathways have been shown to play critical roles during distinct steps of pancreas and β-cell development. Based on this knowledge, D'Amour et al. and Kroon et al. (122,123) developed a step-wise differentiation protocol for generating β-cells from hESCs. By adding growth factors and small molecule inhibitors in culture medium, hESCs can be induced to form DE, primitive gut (PG), posterior foregut (PF), pancreatic endoderm and endocrine precursors (PE). Fully matured β-cells can be obtained after transplantation of endocrine precursors into immunocompromised mice. Differentiation efficiencies are indicated by the percentage of target cells at the end of each stage. Activating pathways and molecules are shown in green and inhibitory pathways and molecules are shown in red. RA, Retinoic acid; Cyc, cyclopamine; Nog, Noggin; Hh, Hedgehog; VEGF, vascular endothelial growth factor; d. Panc, dorsal pancreas; v. Panc, ventral pancreas; KGF, keratinocyte growth factor.

Figure 2

Signals from tissues surrounding pancreatic endoderm promote islet development and endocrine cell maturation. Around E13, developing endocrine cells delaminate from the duct-like pancreatic epithelium and aggregate in endocrine cell clusters. Signaling molecules secreted from pancreatic mesenchyme, blood vessels, and migrating neural crest cells are required for the formation of the islet of Langerhans. Vascular endothelial cells and nerve fibers are also important regulators of hormone synthesis and secretion in the mature islets.