OR24-6 Liver-Specific Inhibition of Sialic Acid Biosynthesis Improves Systemic Glucose Homeostasis in Mice
Authors: Holland William , Kohler Jennifer , Mineo Chieko , Oh Dayoung , Shaul Philip , Yu Liming , Peng Jun 
Published: November 2022

Abstract
Glycosylation is an important posttranslational modification that modulates protein expression and function. Sialic acid is the most abundant terminal monosaccharide of N-linked glycosylation in mammalian cells, and its biosynthesis is controlled by the rate-limited enzyme UDP-GlcNAc 2-epimerase/ManNac kinase (GNE). There is a positive association between the abundance of terminally sialylated plasma glycoproteins and the risk of type 2 diabetes (T2DM). The majority of glycosylated plasma proteins are liver-derived, and key hepatic receptors involved in glucose metabolism, such as the insulin receptor (IR) and glucagon receptor (GCGR), are terminally sialylated. Whether and how protein sialylation in the liver influences glucose homeostasis is unknown. To address this question we generated floxed GNE mice and at 4 weeks of age injected males with either AAV8 encoding a liver-specific targeted Cre recombinase or GFP, to yield mice lacking or normally expressing GNE in liver (GNELKO and GNELWT, respectively). Two weeks post AAV8 injection, GNELKO showed a &gt;98% loss of GNE protein selectively in liver and drastic reductions in the sialylation of liver and plasma proteins detected by glycan-specific lectin-based analyses. Whereas body weights/body composition and plasma lipids were unaltered, GNELKO had lower fasting serum glucose and greater glucose tolerance and insulin sensitivity compared to GNELWT; serum insulin or glucose-stimulated insulin secretion were unchanged. In GNELKO mice hyperinsulinemic-euglycemic clamps displayed a loss of insulin suppression of hepatic glucose production, hepatic IRa and IRb subunit expression and Akt and GSK phosphorylation were decreased, and liver glycogen content was lowered &gt;85%. However, there was also evidence of markedly impaired GCGR function in GNELKO mice, with serum glucagon levels raised 18-fold and attenuation of hepatic cAMP production, CREB phosphorylation and gluconeogenesis in response to glucagon. Regarding extrahepatic mechanisms, GNELKO mice displayed enhanced peripheral glucose disposal, and this was associated with increased IRb, Akt and GSK phosphorylation in skeletal muscle, and with serum FGF21 levels elevated by 406%. In additional studies, when invoked following 12 weeks of high fat diet feeding, liver GNE silencing caused dramatic improvements in glucose tolerance and insulin sensitivity. To evaluate the relevance of these findings to humans, liver GNE transcript levels were queried in a publicly available cohort, and they were higher in T2DM patients compared to non-diabetic controls in two independent cohorts (n= 54 and n=9 for Cohort I and n=4 and n=4 for Cohort II, respectively). Collectively these observations reveal that protein sialylation in the liver has dramatic impact on both insulin and glucagon action in the liver, and on peripheral glucose disposal. Further interrogation of the modulation of glucose control mechanisms by protein sialylation may lead to novel approaches to combatting glucose intolerance in T2DM and other conditions.
Presentation: Monday, June 13, 2022 12:15 p.m. - 12:30 p.m.