Type 2 diabetes susceptibility gene GRK5 regulates physiological pancreatic β-cell proliferation via phosphorylation of HDAC5 in mice

Objective Diabetes onset is accompanied with β-cell deficiency, and thus restoring functional β-cell mass is a promising therapy for those with diabetes. However, the regulatory mechanisms controlling β-cell mass are not fully understood. Previously, we demonstrated that the transcription factor SOX4 is required for β-cell proliferation in the prediabetic state. To elucidate potential mechanisms by which SOX4 regulates β-cell mass, we performed RNA sequencing (RNA-seq) using pancreatic β-cell specific SOX4 knockout mice (βSOX4 KO). The RNA-seq revealed decreased expression of GRK5, a known type 2 diabetes susceptibility gene whose association with diabetes has not been fully elucidated. Therefore, we aimed to clarify the function of GRK5 in pancreatic β cells. Methods We generated a novel pancreatic β cell-specific GRK5 knockout mass (βGRK5 KO) and evaluated glucose tolerance and metabolic changes by body weight measurement, oral glucose tolerance test, and insulin tolerance test. The number of pancreatic β cells was quantified by immunohistochemistry. Glucose loading and Ca2+ imaging was performed on isolated islets to evaluate insulin secretory capacity. To elucidate the mechanism of βGRK5 on β cell mass regulation, we performed RNA-seq of isolated islets and identified the signaling pathways that could be regulated by GRK5. Furthermore, in vitro experiments were conducted using human islets and mouse βGRK5 KO islets to clarify the direct effects of GRK5 on these pathways. Results βGRK5 KO islets showed impaired glucose tolerance and insulin secretion, but no change in body weight or insulin resistance, suggesting that the main cause of impaired glucose tolerance is impaired insulin secretion. Isolated islets showed no abnormalities in insulin secretory capacity or changes in calcium influx, but histologically showed a decrease in β cell mass. Consistent with the decreased β cell mass in βGRK5 KO, the cell cycle inhibitor gene Cdkn1a was upregulated in βGRK5 KO islets; this phenocopies the βSOX4 KO. Furthermore, we found that Grk5 positively regulates facultative increases in β cell mass through activity-dependent phosphorylation of HDAC5 and subsequent transcription of immediate early genes (IEGs) such as Nr4a1, Fosb, Junb, Arc, Egr1 and Srf. Conclusions Our results suggest that GRK5 is associated with type 2 diabetes through regulation of β cell mass. GRK5 could become a potential target of cell therapy to preserve functional β cells during the progression towards frank diabetes.