imaging of β-cell function reveals glucose-mediated heterogeneity of β-cell functional development.

How pancreatic beta-cells acquire function in vivo is a long-standing mystery due to the lack of technology to visualize beta-cell function in living animals. Here, we applied a high-resolution two-photon light-sheet microscope for the first in vivo imaging of Ca2+ activity of every beta-cell in Tg (ins:Rcamp1.07) zebrafish. We reveal that the heterogeneity of beta-cell functional development in vivo occurred as two waves propagating from the islet mantle to the core, coordinated by islet vascularization. Increasing amounts of glucose induced functional acquisition and enhancement of beta-cells via activating calcineurin/nuclear factor of activated T-cells (NFAT) signaling. Conserved in mammalians, calcineurin/NFAT prompted high-glucose-stimulated insulin secretion of neonatal mouse islets cultured in vitro. However, the reduction in low-glucose-stimulated insulin secretion was dependent on optimal glucose but independent of calcineurin/NFAT. Thus, combination of optimal glucose and calcineurin activation represents a previously unexplored strategy for promoting functional maturation of stem cell-derived beta-like cells in vitro.