Protein glycosylation is an abundant and highly regulated post-translational modification. This enzymatic process forms a repertoire of diverse glycan linkages that comprise the glycome and which is essential for cell function. The mammalian genome encodes more than two hundred enzymes, including glycosyltransferases and glycosidases, many of which operate in the Golgi apparatus of the secretory pathway and are responsible for glycan synthesis and catabolism. Investigating the biological roles of secretory protein glycosylation is facilitated by studies of intact organisms bearing genetic and enzymatic defects that restrict glycan linkage formation. This research proposal continues our program to establish the biological functions of mammalian glycans by using the mouse as a model system, and with a focus among the glycan linkages that promote hybrid and complex N-glycan branching. Recently, we have discovered that a deficit of the Mgat4a-encoded GnT-4a glycosyltransferase results in the pathogenesis of type 2 diabetes. The disease mechanism appears to involve the loss of pancreatic beta cell glucose transport and thereby glucose-stimulated insulin secretion, arising from a defect in glucose transporter-2 (Glut-2) glycosylation and retention at the cell surface. Notably, significantly reduced GnT-4a glycosylation results among wild-type mice that acquire type 2 diabetes associated with a similar failure of pancreatic beta cell function following administration of a high-fat diet. Preliminary data indicate that enforced GnT-4a glycosylation, and perhaps Glut-2 over-expression may suppress dietary-induced type 2 diabetes. The research proposed herein will thus investigate whether these molecular and cellular alterations represent a key pathogenic trigger that provokes beta cell failure and hyperglycemia in pathogenesis of high-fat diet-induced type 2 diabetes, and whether this defect promotes further metabolic dysfunction including the development of hepatic steatosis and insulin resistance. This research program will also determine whether the highly related GnT-4b isozyme is involved in similar mechanisms that govern glucose and insulin metabolism. Recent data further indicate that the HNF-11 homeobox transcription factor, the known diabetogenic defect in human MODY3, normally promotes Mgat4a and Glut2 RNA expression in animals fed the standard diet. The relevance of these findings to disease pathogenesis will be determined among MODY3 mouse models of type 2 diabetes that bear enforced GnT-4a glycosylation and Glut-2 glycoprotein expression. The potential that these findings can be translated to the human population will be addressed as preliminary data reveal a similar mechanism of glycan-mediated regulation of Glut- and Glut-2 cell surface expression exists in human pancreatic beta cells.
The proposed studies are focused upon identifying and inhibiting the pathogenic mechanism by which a high- fat diet causes type 2 diabetes, using the mouse model of this widespread mammalian response. Preliminary work completed supports the hypothesis that pancreatic beta cell failure to transport glucose is a fundamental pathogenic trigger that can be further resolved to a coordinate defect in protein glycosylation by the GnT4a glycosyltransferase.
This research aims to provide a molecular explanation of how the high-fat, or western- style, diet contributes to the ongoing global epidemic of Human type 2 diabetes, and an experimental approach by which disease pathogenesis may be averted.
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