Date

4-29-2009

UMMS Affiliation

Graduate School of Biomedical Sciences, MD/PhD Program

Document Type

Dissertation, Doctoral

Subjects

Diabetes Mellitus, Type 1; Leptin; Haptoglobins; Biological Markers; Viruses; Academic Dissertations; Dissertations, UMMS

Disciplines

Life Sciences | Medicine and Health Sciences

Abstract

Several viral infections have been associated with human type 1 diabetes (T1D), although it has proven difficult to unequivocally establish them as causative agents. In rodent models, however, viruses have definitely been established to cause T1D. The treatment of weanling BBDR rats with the combination of a TLR3 ligand, pIC, and an ssDNA parvovirus, KRV, precipitates T1D in nearly 100% of rats within a short, predictable timeframe. In this dissertation, we utilized the BBDR rat model to (1) identify early serum biomarkers that could predict T1D precipitated by viral induction and (2) test the efficacy of leptin, a therapeutic agent, which may have the ability to prevent diabetes onset, reverse new onset diabetes and prevent autoimmune recurrence of diabetes in rats transplanted with syngeneic islet grafts.

Identification of biomarkers has long served as an invaluable tool for disease prediction. In BBDR rats, we identified an acute phase response protein, haptoglobin, as a potential biomarker for pIC + KRV induced T1D using the global proteomic profiling techniques, 2D gel analysis and iTRAQ. Upon validating this biomarker, we determined that haptoglobin was sensitive in predicting T1D in the pIC + KRV model, in which nearly 100% of the rats become diabetic, but not in models where diabetes expression was variable (KRV only or RCMV only models). However, analysis of the serum kinetics of haptoglobin and its functional capacity in the blood has given us insights into the potential role of early phase reactants in modulating virally mediated T1D.

An alternative means of regulating T1D pathogenesis is through leptin. Leptin is a hormone with pleotropic roles in the body, particularly affecting energy metabolism and immune regulation. These characteristics make leptin an intriguing candidate for therapeutic testing in T1D models. Our studies have determined that high doses of leptin delivered via an adenovirus (AdLeptin) or alzet pump delivery system can prevent diabetes in > 90% of rats treated with pIC + KRV. We further showed that serum hyperleptinemia was associated with decreased body weight, decreased non-fasting serum insulin levels and lack of islet insulitis in pIC + KRV treated rats pretreated with AdLeptin compared with those pretreated with PBS. We discovered that hyperleptinemia induced a profound decrease in splenic weight and splenic cellularity, including reductions in CD4+ and CD8+ T cells, DC/MACs and B cells. These findings indicate a potential mechanism whereby hyperleptinemia protects rats from virally induced T1D through the promotion of peripheral immunosuppression.

Among pIC + KRV treated rats, we have also found that leptin therapy can reverse hyperglycemia in a subset of new onset diabetics for up to 20 days. In the absence of exogenous insulin, leptin treatment of new onset diabetics prevented the rapid weight loss associated with osmotic diuresis, as well as the ketosis observed in vehicle treated diabetic rats. Overall, these findings point to the therapeutic value of leptin in maintaining glycemic control and preventing ketosis in an insulin deficient state, in the absence of exogenous insulin therapy.

Additionally, we have also determined that AdLeptin treatment can prolong the survival of syngeneic islets transplanted into diabetic BBDR rats for up to 50 days post transplant. Although hyperleptinemia generated by AdLeptin was unable to prevent insulitis into islet grafts, this insulitis did not appear to be destructive as islet grafts continued to stain positively for insulin when compared with control rats whose grafts succumbed to recurrent autoimmunity. In the various therapeutic settings in which we have tested leptin treatment, we have found this hormone to have significant beneficial effects. These findings merit further evaluation of leptin as a therapeutic agent in human T1D.

 
 

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