The Role of Gm4951 in Nonalcoholic Fatty Liver Disease

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PROJECT SUMMARY Nonalcoholic fatty liver disease (NAFLD) is becoming a global human health problem. Our long-term goal is to understand molecular mechanisms of NAFLD, and to translate this knowledge into novel therapeutic strategies. Due to the physiologic similarities between humans and mice, and the propensity of mice to develop a disease closely mimicking NAFLD when fed a high fat diet (HFD), mice have provided us with fundamental insights into NAFLD pathogenesis. In humans and mice, genetic variation influences the rate and severity of hepatosteatosis under a given set of environmental conditions. To identify genes that influence the process, we utilized unbiased forward genetic screening and highly automated meiotic mapping to identify mutations that cause NAFLD in mice sensitized by a HFD. Two semi-dominant missense alleles of predicted gene 4951 (Gm4951), named Oily and Carboniferous, were detected in this screen. As distinct from most NAFLD mutants that are associated with obesity, our preliminary data showed that Gm4951 deficient mice had dramatically increased hepatic lipid accumulation without a concomitant increase of body weight on a HFD. Gm4951 was highly expressed in hepatocytes. Knockout of Gm4951 increased lipid content and overexpression of Gm4951 decreased lipid content of primary hepatocytes in vitro, suggesting hepatocyte-intrinsic regulation of lipid content. Gm4951 knockout livers showed decreased expression of lipid oxidation genes. Mass spectrometry analysis of endogenous GM4951 interacting proteins revealed interaction with lipid droplet protein Hydroxysteroid 17β- dehydrogenase 13 (HSD17B13) and lipid oxidation enzymes. Moreover, the transcription of Gm4951 in hepatocytes was activated by interferon gamma (IFN-γ), which effectively decreased lipid content, much as when GM4951 was overexpressed. These results led to our central hypothesis that GM4951 is critical for promoting lipid oxidation, and limits hepatic lipid accumulation. To test this hypothesis, we propose to pursue three Specific Aims. Aim 1 will further investigate the development of NAFLD in GM4951 deficient mice. Aim 2 will determine the precise mechanistic role of GM4951 in regulating lipid metabolism. Aim 3 will study the transcriptional regulation of Gm4951, including the liver-specific expression pattern and the inducible expression by IFN-γ. Understanding how to activate GM4951 and what’s the human homolog of GM4951 would offer approaches to preventing or treating NAFLD. These will be studied in Aim 3 as well. The NAFLD phenotype caused by GM4951 deficiency is fundamentally distinct from the classic obesity-associated NAFLD mouse models. GM4951 is specifically expressed in the liver and operates there to limit lipid accumulation. Thus, finding ways to activate GM4951 will provide a new means of reducing hepatic lipid content. Completion of the proposed work will suggest new therapeutic targets to combat NAFLD.