ß-Cell keratin 8 maintains islet mechanical integrity, mitochondrial ultrastructure, and ß-cell glucose transporter 2 plasma membrane targeting.

Islet ß-cell dysfunction is an underlying factor for type I diabetes (T1D) development. Insulin sensing and secretion are tightly regulated in ß-cells at multiple subcellular levels. The epithelial intermediate filament (IF) 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 (GSIS) response in vivo, despite undisturbed systemic blood glucose regulation. K8flox/flox, Ins-Cre mice have a decreased sensitivity to STZ compared with 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.NEW & NOTEWORTHY Keratin 8 is the main cytoskeletal protein in the cytoplasmic intermediate filament network in ß-cells. Here for the first time, we assessed the ß-cell autonomous mechanical and nonmechanical roles of keratin 8 in ß-cell function. We demonstrated the importance of keratin 8 in islet and ß-cell structural integrity, maintaining mitochondrial morphology and GLUT2 plasma membrane targeting.

Keratin 7 Is a Constituent of the Keratin Network in Mouse Pancreatic Islets and Is Upregulated in Experimental Diabetes.

Keratin (K) 7 is an intermediate filament protein expressed in ducts and glands of simple epithelial organs and in urothelial tissues. In the pancreas, K7 is expressed in exocrine ducts, and apico-laterally in acinar cells. Here, we report K7 expression with K8 and K18 in the endocrine islets of Langerhans in mice. K7 filament formation in islet and MIN6 ß-cells is dependent on the presence and levels of K18. K18-knockout (K18‒/‒) mice have undetectable islet K7 and K8 proteins, while K7 and K18 are downregulated in K8‒/‒ islets. K7, akin to F-actin, is concentrated at the apical vertex of ß-cells in wild-type mice and along the lateral membrane, in addition to forming a fine cytoplasmic network. In K8‒/‒ ß-cells, apical K7 remains, but lateral keratin bundles are displaced and cytoplasmic filaments are scarce. Islet K7, rather than K8, is increased in K18 over-expressing mice and the K18-R90C mutation disrupts K7 filaments in mouse ß-cells and in MIN6 cells. Notably, islet K7 filament networks significantly increase and expand in the perinuclear regions when examined in the streptozotocin diabetes model. Hence, K7 represents a significant component of the murine islet keratin network and becomes markedly upregulated during experimental diabetes.

Keratin intermediate filaments in the colon: guardians of epithelial homeostasis.

Keratin intermediate filament proteins are major cytoskeletal components of the mammalian simple layered columnar epithelium in the gastrointestinal tract. Human colon crypt epithelial cells express keratins 18, 19 and 20 as the major type I keratins, and keratin 8 as the type II keratin. Keratin expression patterns vary between species, and mouse colonocytes express keratin 7 as a second type II keratin. Colonic keratin patterns change during cell differentiation, such that K20 increases in the more differentiated crypt cells closer to the central lumen. Keratins provide a structural and mechanical scaffold to support cellular stability, integrity and stress protection in this rapidly regenerating tissue. They participate in central colonocyte processes including barrier function, ion transport, differentiation, proliferation and inflammatory signaling. The cell-specific keratin compositions in different epithelial tissues has allowed for the utilization of keratin-based diagnostic methods. Since the keratin expression pattern in tumors often resembles that in the primary tissue, it can be used to recognize metastases of colonic origin. This review focuses on recent findings on the biological functions of mammalian colon epithelial keratins obtained from pivotal in vivo models. We also discuss the diagnostic value of keratins in chronic colonic disease and known keratin alterations in colon pathologies. This review describes the biochemical properties of keratins and their molecular actions in colonic epithelial cells and highlights diagnostic data in colorectal cancer and inflammatory bowel disease patients, which may facilitate the recognition of disease subtypes and the establishment of personal therapies in the future.