Pancreatic endocrine beta cells secrete insulin to maintain blood glucose homeostasis. Reduced beta cell mass due to perturbations in beta cell development or loss during adulthood result in the disease diabetes mellitus, yet the mechanisms regulating beta cell differentiation and maintenance are poorly understood. Researchers have identified two stages of endocrine cell development during embryogenesis. During the first stage, known as the primary transition, multipotent pancreatic progenitors expand and become specified to the acinar, endocrine, and ductal lineages. Following the primary transition, progenitors undergo rapid differentiation to produce mature acinar, endocrine, and ductal cells, during a stage called the secondary transition. The mechanisms that regulate both progenitor cell specification and differentiation are subject of intense investigation, as a better understanding of these processes is necessary to effectively develop a cell-based therapy for diabetes.

Using conditional loss- and gain-of-function mouse models for the transcription factor Nkx6.1, we discovered Nkx6.1 is a master-regulator of endocrine cell specification differentiation. Specifically, we found that Nkx6.1, together with its paralog Nkx6.2, repress the pre-acinar transcription factor Ptf1a, thereby allocating pancreatic progenitors to the endocrine lineage. Given this function for Nkx6.1 in cell specification, we subsequently investigated whether Nkx6.1 expression was necessary and/or sufficient to specify beta cell fate in endocrine progenitors. We found that Nkx6.1 specifies endocrine progenitors to the beta cell lineage, while repressing alternative pancreatic endocrine lineage choices. Moreover, we provide evidence that endocrine cell type specification occurs through the concerted activities of multiple transcription factors, rather than through a linear hierarchical differentiation program. In addition to its role in beta cell specification, we also identified a role for Nkx6.1 in the delamination and migration of endocrine progenitors, a process that is critical for forming functional clusters of endocrine cells, called islets. Finally, we examined the effects of elevated Nkx6.1 expression in beta cells and contrary to conclusions from previous in vitro studies, show that it does not improve beta cell function or expand beta cell mass in vivo. Together, our data demonstrate that Nkx6.1 is both necessary and sufficient at every stage of pancreas morphogenesis to generate functional beta cells.