Interplay between mitochondria, primary cilium, diabetes and its complications

Diabetes is one of the major health problems of the 21 century. Dysfunction of the insulin secreting pancreatic β-cell together with insulin resistance is central to the pathogenesis of type 2 diabetes mellitus (T2DM). The cause of the disease and the underlying mechanisms linking hyperglycemia to diabetes complications are still unclear. This thesis is focused on two cellular organelles, the mitochondrion and the primary cilium, and their role in the pathophysiological mechanisms of diabetes and its complications. In the first paper, we studied the effect of hyperglycemia on cell biology and energy metabolism in human primary fibroblasts and endothelial cells. Acute hyperglycemia triggered a metabolic switch from mitochondrial respiration to aerobic glycolysis, which was persistent after prolonged exposure together with reduced ATP/ADP ratio without increase in reactive oxygen species (ROS). An acute decrease in mitochondrial transmembrane potential and cellular proliferation with changes in cytoskeletal reorganization was linked to the increased osmotic pressure induced by hyperglycemia. In the second and third papers we investigated the effect of hypoxia, a common feature in diabetes, and hyperoxia in pancreatic islets. Here, we found deleterious effects on mitochondrial content, respiration and glucose-stimulated insulin secretion. Preconditioning with the KATP channel opener diazoxide enhanced insulin release, HIF-1α and AMPK activation and improved β-cell survival in response to hypoxia. In the fourth paper, a role for the β-cell primary cilium in diabetes was reported. We found reduced first phase insulin secretion in ciliary defective cells and islets, and impaired glucose tolerance in a ciliopathy mouse model. These results were linked to impaired recruitment of insulin receptor A to the cilium, necessary for proper insulin signaling. Mitochondrial respiration and glucose uptake was unaffected by cilia impairment. Additionally, in vivo evidence of ciliary morphology alteration in the GK rat, a model of T2DM, supported a relationship between ciliary defect and T2DM. Preliminary results show that decreasing intracellular ATP and increasing mitochondrial ROS production impaired cilia morphology and/or number in two different cell types. Further, cilia were decreased in number with altered morphology in the kidneys of a mouse model of T2DM with diabetic nephropathy, characterized by increased ROS and altered mitochondrial metabolism. Finally, a reduction of 60-80% in mtDNA content (reported in diabetes) did not affect mitochondrial metabolism, respiration and energy production in two different cell types. In summary, mitochondrial dysfunctions during diabetes and its complications are most probably due to a combination of hyperglycemia and other factors such as hypoxia, depending on the cells and tissues involved. A proper ciliary/basal body function is necessary for insulin release and signaling in β-cell. Cilia morphology and number can be affected by mitochondrial dysfunction/ROS and thus related to diabetic complications such as diabetic nephropathy. LIST OF SCIENTIFIC PAPERS I. I. Moruzzi N, Del Sole M, Fato R, Gerdes JM, Berggren PO, Bergamini C, Brismar K. Short and prolonged exposure to hyperglycaemia in human fibroblasts and endothelial cells: metabolic and osmotic effects. Int J Biochem Cell Biol. 2014;53:66-76. II. II. Ma Z, Moruzzi N, Catrina SB, Hals I, Oberholzer J, Grill V, Björklund A. Preconditioning with associated blocking of Ca2+ inflow alleviates hypoxiainduced damage to pancreatic beta-cells. PLoS One. 2013;8(7):e67498. III. I . Ma Z, Moruzzi N, Catrina SB, Grill V, Björklund A. Hyperoxia inhibits glucose-induced insulin secretion and mitochondrial metabolism in rat pancreatic islets. Biochem Biophys Res Commun. 2014;443(1):223-8. IV. IV. Gerdes JM, Christou-Savina S, Xiong Y, Moede T, Moruzzi N, KarlssonEdlund P, Leibiger B. , Leibiger IB. , Ostenson CG, Beales PL. , Berggren PO. Ciliary dysfunction impairs beta-cell insulin secretion and promotes development of type 2 diabetes in rodents. Nat Commun. 2014;5:5308. PUBLICATIONS NOT INCLUDED IN THE THESIS I. Bergamini C. , Moruzzi N. , Volta F. , Faccioli L. , et al. Role of mitochondrial complex I and protective effect of CoQ10 supplementation in propofol induced cytotoxicity. J Bioenerg Biomembr. 2016 Aug;48(4):413-23. II. Bader E. , Migliorini A. , Gegg M. , Moruzzi N. , et al. Identification of proliferative and mature β-cells in the islets of Langerhans. Nature. 2016 Jul 21;535(7612):430-4. CONTENTS