81-OR: The Rnf20 Ubiquitin Ligase Is Required for Beta-Cell Identity and Function

Abstract

Type 1 and Type 2 Diabetes are characterized by a loss of functional β-cell mass. With the incidence of diabetes rising globally, a better understanding of the gene regulatory mechanisms maintaining β-cell functional identity is imperative to improve diabetes therapies. Islet 1 (Isl1) is a LIM-homeodomain class transcription factor that is essential for β-cell maturation and function. Our previous Isl1 studies revealed that it interacts with the histone ubiquitin ligases Ring Finger 20 (Rnf20) and Rnf40 in mouse β-cells, mouse islets, and human islets. Rnf20 and Rnf40 act as homo- or heterodimeric complexes wherein they monoubiquitinate histone 2B to regulate transcription. In mouse β-cell lines, we demonstrated that Rnf20 and Rnf40 are important for β-cell gene expression and insulin secretion, perhaps mediated by their occupancy of Slc2a2 and MafA. Additionally, using proximity ligation assay we showed a robust increase in Isl1 and Rnf20 interactions under high glucose conditions. Based on these findings, we hypothesized that at least Rnf20 is required for β-cell function in adult islets. To evaluate the contribution of Rnf complexes to β-cell maintenance, we developed an adult-inducible Rnf20 β-cell-specific knockout mouse model, termed Rnf20Δβ-cell. By 8 weeks of age (2 weeks after tamoxifen induction) male and female Rnf20Δβ-cell mice became hyperglycemic, severely glucose intolerant, and had reduced plasma insulin levels following a glucose challenge. Additionally, Rnf20Δβ-cell mice had reduced β-cell mass, in parallel with dysregulated expression of glucose-stimulated insulin secretion and β-cell identity mRNAs including Ins I, Slc2a2, Ucn3, Ngn3, and Aldh1a3. Our in vivo findings resemble those observed in adult Isl1 knockout mice. Paired with similar changes in gene expression in the Rnf20 and Isl1 models, and the glucose-responsive interaction between Isl1 and Rnf20 in vitro, our data support the hypothesis that Isl1-Rnf complexes are critical regulators of β-cell function. Disclosure T.Pierre: None. Y.Liu: None. C.S.Hunter: None. Funding National Institutes of Health (DK111483, DK128132)