Markers and Mechanisms of β-cell Dedifferentiation

About this book

Human and murine diabetes is characterized by pancreatic β-cell dedifferentiation, a process in which β-cells lose expression of markers of maturity and gain those of endocrine progenitors. Failing β-cells inappropriately metabolize lipids over carbohydrates and exhibit impaired mitochondrial oxidative phosphorylation. Therefore, pathways involved in mitochondrial fuel selection and catabolism may represent potential targets for the prevention or reversal of dedifferentiation. In chapter I of this dissertation, we isolated and functionally characterized failing β-cells from various experimental models of diabetes. We found a striking enrichment in the expression of aldehyde dehydrogenase 1 isoform A3 (Aldh1a3) as β-cells become dedifferentiated. Flow-sorted Aldh1a3-expressing (ALDH+) islet cells demonstrate impaired glucose-induced insulin secretion, are depleted of Foxo1 and MafA, and include a Neurogenin3-positive subset. RNA sequencing analysis demonstrated that ALDH+ cells are characterized by: (i) impaired oxidative phosphorylation and mitochondrial complex I, IV, and V; (ii) activated RICTOR; and (iii) progenitor cell markers. We propose that impaired mitochondrial function marks the progression from metabolic inflexibility to dedifferentiation in the natural history of β-cell failure. In chapter II of this dissertation, we report that cytochrome b5 reductase 3 (Cyb5r3) is a FoxO1-regulated mitochondrial oxidoreductase critical to β cell function. Expression of Cyb5r3 is greatly decreased in multiple murine models of diabetes, and in vitro Cyb5r3 knockdown leads to increased ROS generation and impairment of respiration, mitochondrial function, glucose-stimulated insulin secretion, and calcium mobilization. In vivo, mice with β-cell-specific ablation of Cyb5r3 (B-Cyb5r3) display impaired glucose tolerance with decreased insulin secretion, and their islets have significantly lower basal respiration and glucose-stimulated insulin secretion. B-Cyb5r3 β-cells lose expression of Glut2, MafA, and Pdx1 expression despite a compensatory increase in FoxO1 expression. Our data suggest that Cyb5r3 is a critical mediator of FoxO1’s protective response in β-cells, and that loss of Cyb5r3 expression is an early event in β-cell failure.