Depletion of insulin receptors leads to β-cell hyperplasia in zebrafish

Hyperglycemia in type 2 diabetes results from an inability of insulin to regulate gluconeogenesis. To characterize the role of the insulin/insulin receptor pathway in glycometabolism and type 2 diabetes, we created a zebrafish model in which insulin receptors a and b (insra and insrb) have been ablated. We first observed that insra and insrb were both expressed abundantly during embryonic development and in various adult tissues. Increased expression of insulin and number of β-cells were observed in insra-/-/insrb-/- fish together with higher glucose in insra-/-, insrb-/-, or insra-/-/insrb-/- fish, indicating that insra and insrb were knocked out effectively. However, compared to the wild-type fish, insra-/-/insrb-/- fish died between 5 and 16days post-fertilization (dpf) with severe pericardial edema and increased level of cell apoptosis, which was not induced by increased total body glucose content. Increased gluconeogenesis and decreased glycolysis were also observed in both single and double knockout fish, but no mortality or malformation was observed in single knockout fish. Given the importance of insulin receptors in glucose homeostasis and embryonic development, transcriptome analysis was used to provide an important model of defective insulin signaling and to study its developmental consequences in zebrafish. The results indicated that both insra and insrb played a pivotal role in glucose metabolism and embryonic development, and insra was more critical than insrb in the insulin signaling pathway.