ABSTRACT
Islet ß-cell dysfunction is an underlying factor for type I diabetes (T1D) development. Insulin sensing and secretion is tightly regulated in ß-cells at multiple subcellular levels. The epithelial intermediate filament protein keratin (K) 8 is the main ß-cell keratin, constituting the filament network with K18. To identify the cell-autonomous functions of K8 in ß-cells, mice with targeted deletion of ß-cell K8 (K8flox/flox; Ins-Cre) were analyzed for islet morphology, ultrastructure and integrity, as well as blood glucose regulation and streptozotocin (STZ)-induced diabetes development. Glucose transporter 2 (GLUT2) localization was studied in ß-cells in vivo and in MIN6 cells with intact or disrupted K8/K18 filaments. Loss of ß-cell K8 leads to a major reduction in K18. Islets without ß-cell K8 are more fragile and these ß-cells display disjointed plasma membrane organization with less membranous E-cadherin and smaller mitochondria, with diffuse cristae. Lack of ß-cell K8 also leads to a reduced glucose stimulated insulin secretion response in vivo, despite undisturbed systemic blood glucose regulation. K8flox/flox; Ins-Cre mice have a decreased sensitivity to STZ compared to K8 wild-type mice, which is in line with decreased membranous GLUT2 expression observed in vivo, as GLUT2 is required for STZ uptake in ß-cells. In vitro, MIN6 cell plasma membrane GLUT2 is rescued in cells overexpressing K8/K18 filaments, but mistargeted in cells with disrupted K8/K18 filaments. ß-cell K8 is required for islet and ß-cell structural integrity, normal mitochondrial morphology and GLUT2 plasma membrane targeting, and has implications on STZ sensitivity as well as systemic insulin responses.
