Differentiation of pancreatic endocrine progenitors reversibly blocked by premature induction of MafA

Specification and maturation of insulin+ cells accompanies a transition in expression of Maf family of transcription factors. In development, MafA is expressed after specification of insulin+ cells that are expressing another Maf factor, MafB; after birth, these insulin+ MafA+ cells stop MafB expression and gain glucose responsiveness. Current differentiation protocols for deriving insulin-producing β-cells from stem cells result in β-cells lacking both MafA expression and glucose-stimulated insulin secretion. So driving expression of MafA, a β-cell maturation factor in endocrine precursors could potentially generate glucose-responsive MafA+ β cells. Using inducible transgenic mice, we characterized the final stages of β-cell differentiation and maturation with MafA pause/release experiments. We found that forcing MafA transgene expression, out of its normal developmental context, in Ngn3+ endocrine progenitors blocked endocrine differentiation and prevented the formation of hormone+ cells. However, this arrest was reversible such that with stopping the transgene expression, the cells resumed their differentiation to hormone+ cells, including α-cells, indicating that the block likely occurred after progenitors had committed to a specific hormonal fate. Interestingly, this delayed resumption of endocrine differentiation resulted in a greater proportion of immature insulin+MafB+ cells at P5, demonstrating that during maturation the inhibition of MafB in β-cell transitioning from insulin+MafB+ to insulin+MafB− stage is regulated by cell-autonomous mechanisms. These results demonstrate the importance of proper context of initiating MafA expression on the endocrine differentiation and suggest that generating mature Insulin+MafA+ β-cells will require the induction of MafA in a narrow temporal window to achieve normal endocrine differentiation.