Apollo-NADP ⁺ reveals in vivo adaptation of NADPH/NADP ⁺ metabolism in electrically activated pancreatic β cells

Several genetically encoded sensors have been developed to study live cell NADPH/NADP + dynamics, but their use has been predominantly in vitro. Here, we developed an in vivo assay using the Apollo-NADP + sensor and microfluidic devices to measure endogenous NADPH/NADP + dynamics in the pancreatic β cells of live zebrafish embryos. Flux through the pentose phosphate pathway, the main source of NADPH in many cell types, has been reported to be low in β cells. Thus, it is unclear how these cells compensate to meet NADPH demands. Using our assay, we show that pyruvate cycling is the main source of NADP + reduction in β cells, with contributions from folate cycling after acute electrical activation. INS1E β cells also showed a stress-induced increase in folate cycling and further suggested that this cycling requires both increased glycolytic intermediates and cytosolic NAD + . Overall, we show in vivo application of the Apollo-NADP + sensor and reveal that β cells are capable of adapting NADPH/NADP + redox during stress.