Different roles of insulin receptor a and b in maintaining blood glucose homeostasis in zebrafish

•Either insulin receptor a- or b-deficiency would results in insulin resistance in zebrafish.•Both insulin receptor a and b are essential in maintaining glycolysis.•Insulin receptor a is involved in inhibiting gluconeogenesis via down-regulating the transcriptional expression of foxo1a. An inability of insulin to signal glycolysis and gluconeogenesis would largely result in type 2 diabetes. In this study, the physiological roles of zebrafish insulin receptor a and b in maintaining blood glucose homeostasis were characterized. We observed that, though blood glucose in insra−/− fish and insrb−/− fish were comparable with the control siblings at 0 h postprandium (hpp), the most evident hyperglycemia have been observed in insra−/− fish from 1 hpp to 3 hpp. A mild increase of blood glucose in insrb−/− fish has been seen only at 1.5 hpp. The down-regulated expressions of glycolytic enzymes were observed in insra−/− fish and insrb−/− fish liver and muscle, together with the significantly decreased activities or concentrations of glycolytic enzymes. These results suggest that both Insra and Insrb were critical in glycolysis. Intriguingly, the up-regulated expressions of gluconeogenic enzymes, pck1 and g6pca.1, along with the elevated enzyme activities, were observed in insra−/− fish liver at 1 hpp and 1.5 hpp. Compared with the control fish, the elevated plasma insulin and lowered phosphorylated AKT were observed in insra−/− fish and insrb−/− fish, suggesting that there is an insulin resistance in insra−/− fish and insrb−/− fish. The increased levels of both transcriptions of foxo1a and Foxo1a protein abundance in the insra−/− fish liver have been found. When insra−/− fish treated with the Foxo1 inhibitor, the postprandial blood glucose levels could be normalized, accompanied with the normalized expression levels and enzyme activities of both pck1 and g6pca.1. Therefore, Insra and Insrb demonstrate a similar role in promoting glycolysis, but Insra is involved in inhibiting gluconeogenesis via down-regulating the expression of foxo1a. Our results indicate that Insra and Insrb exhibit diversified functions in maintaining glucose homeostasis in zebrafish.