Investigating the role of microRNA-7 in pancreatic islet development

Abstract

Diabetes represents a progressively worsening global health crisis. The primary proximal cause of symptoms in both type 1 and type 2 diabetes is a relative or absolute deficiency of functional β-cell mass. Replenishing β-cell mass through islet transplantation can cure diabetes, however cadaveric islets are scarce and re- quire patient immunosuppression. Step-wise in vitro differentiation of pluripotent stem cells into β-cells could represent a viable alternative source for transplanta- tion. However, current limitations in our understanding of in vivo islet endocrine cell differentiation mean that ‘β-like’ cells generated by existing differentiation protocols are produced at low efficiency and lack numerous markers of functional maturity. Mice with a genetic inactivation of Dicer1, a key component of the microRNA (miRNA) biogenesis pathway, show aberrant islet endocrine cell de- velopment, revealing a key role for miRNAs in this process. However, precisely which miRNAs influence β-cell differentiation from stem cells and how they are integrated into endocrine specification networks is unknown. Using a combination of reporter mouse models and RT-qPCR, we show that the miRNA-7 (miR-7) gene family is highly expressed in embryonic pancreatic endocrine progenitors and plays a pivotal role in driving β-cell differentiation. Genetic deletion of the miR-7 gene family in endocrine progenitors leads to reduced α-, β-, and δ-cell mass and sub- sequent adult-onset hyperglycaemia. We find that reduced endocrine cell mass is due to a failure of endocrine progenitors to delaminate from the epithelial plexus, associated with a reduction in neurogenin-3 (Neurog3) expression and increased Sry-box transcription factor 9 (Sox9). Lineage tracing experiments reveal that a significant number of endocrine progenitors lacking miR-7 differentiate to ductal cells and are retained in the adult pancreatic duct. Our findings suggest that increasing miR-7 expression in in vitro differentiation protocols may be used to improve differentiation efficiency and the terminal maturity of stem-cell derived β-cells.