Overnutrition induces β-cell differentiation through prolonged activation of β-cells in zebrafish larvae

Insulin from islet beta-cells maintains glucose homeostasis by stimulating peripheral tissues to remove glucose from circulation. Persistent elevation of insulin demand increases beta-cell number through self-replication or differentiation (neogenesis) as part of a compensatory response. However, it is not well understood how a persistent increase in insulin demand is detected. We have previously demonstrated that a persistent increase in insulin demand by overnutrition induces compensatory beta-cell differentiation in zebrafish. Here, we use a series of pharmacological and genetic analyses to show that prolonged stimulation of existing beta-cells is necessary and sufficient for this compensatory response. In the absence of feeding, tonic, but not intermittent, pharmacological activation of beta-cell secretion was sufficient to induce beta-cell differentiation. Conversely, drugs that block-cell secretion, including an ATP- sensitive potassium K-ATP) channel agonist and an L- type Ca2+ channel blocker, suppressed overnutrition- induced beta-cell differentiation. Genetic experiments specifically targeting beta-cells confirm existing beta-cells as the overnutrition sensor. First, inducible expression of a constitutively active KATP channel in beta-cells suppressed the overnutrition effect. Second, inducible expression of a dominant- negative KATP mutant induced beta-cell differentiation independent of nutrients. Third, sensitizing beta-cell metabolism by transgenic expression of a hyperactive glucokinase potentiated differentiation. Finally, ablation of the existing beta-cells abolished the differentiation response. Taken together, these data establish that overnutrition induces beta-cell differentiation in larval zebrafish through prolonged activation of beta-cells. These findings demonstrate an essential role for existing beta-cells in sensing overnutrition and compensating for their own insufficiency by recruiting additional beta-cells.