The pathogenesis of tissue-specific autoimmune disease reflects innate or acquired defects in immunological tolerance but remains poorly understood. In Type 1 diabetes (T1D), a cell type-specific defect in immunological tolerance results in the destruction of pancreatic beta cells. Although dysregulation of the immune system represents a pathogenic component of this disease, the reason why pancreatic beta cells are targeted is not understood. Discovering the bases of tissue-specific autoimmunity will enhance our understanding of the mechanisms of immunological tolerance and will aid in the development of preventative and curative approaches to autoimmune diseases such as T1D. This research proposal is focused on a hypothesis supported by significant preliminary data that the susceptibility of pancreatic beta cells to immunological attack in autoimmune diabetes reflects low levels of toleragenic sialic acids. In the past decade, novel studies have linked the post-translational modification of proteins by sialyltransferases with immunological regulation. Sialyltransferases are conserved and highly regulated enzymes that generate sialic acid linkages on the distal ends of glycan chains. Sialic acid linkages can form ligands of various receptors including the Siglec receptors of immune cells. Siglecs bind sialic acids in cis and trans at cell surfaces and thereby control the co-localization and signaling of immune cell receptors. For example, Siglec binding to cell surface sialic acid linkages induces immunological tolerance and inhibits the onset of autoimmunity in various mouse models at least in part by blocking autoantibody production coincident with induction of B lymphocyte anergy and apoptosis. Most mammalian cell surfaces are covered with toleragenic sialic acid linkages. However we have unexpectedly observed low levels of sialic acid linkages present on the surface of normal pancreatic beta cells. This intrinsic low level of sialic acids appears advantageous in normal physiological contexts requiring glucose uptake and sensing, but appears disadvantageous in the presence of a dysfunctional immune system. To identify the mechanistic features of immunological tolerance to pancreatic beta cells, we have initially studied the Non- Obese Diabetic ShiLT/J (NOD) mouse because of its well-defined disease signs that include spontaneous insulitis progressing to beta cell destruction, and because of the large body of immunological knowledge that has been achieved using this model of human T1D. We have generated and analyzed transgenic and syngeneic NOD mice bearing increased expression of sialic acids on pancreatic beta cell glycoproteins. Remarkably, our findings reveal that augmentation of sialic acid linkages protects NOD mice from insulitis and the immunological destruction of pancreatic beta cells. The research proposed herein will investigate sialic acid linkage expression in normal human and mouse tissues, compare results of augmented expression of ?2-3 and ?2-6 sialic acid linkages in beta cells, and undertake immunological studies to acquire mechanistic insights that will ultimately be needed to understand how sialic acids participate in immunological tolerance.
This research proposal will investigate recent discoveries demonstrating that protein glycosylation has important and unexpected functions in pancreatic beta cells, including the regulation of blood glucose homeostasis and the maintenance of immunological tolerance. Data recently published and acquired indicates that among normal tissues surveyed, pancreatic beta cells are unexpectedly deficient in sialic acid linkages, while augmentation of sialyltransferase expression alters blood glucose homeostasis and diminishes the frequency of onset of autoimmune diabetes. This proposal will investigate the roles of sialic acids in glycoprotein expression at the beta cell surface and immune tolerance in the onset of insulitis and autoimmune disease.
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