PROJECT SUMMARY/ABSTRACT Urea cycle disorders (UCDs) are common inborn errors of hepatic metabolism. With improved therapies such as nitrogen-scavenging agents to prevent elevated ammonia levels, patients with UCDs have increased survival. However, even in the absence of hyperammonemia, patients with UCDs may have chronic liver disease. Liver disease in UCDs can manifest as abnormal serum transaminases, hepatomegaly, hepatic fibrosis, or hepatocellular carcinoma. Among the UCDs, the highest prevalence of chronic liver disease occurs in argininosuccinate lyase deficiency (ASLD). Importantly, the cause for liver disease in UCDs such as ASLD is unknown, and liver disease has not been prevented by standard therapies. Moreover, there are no therapeutic strategies specifically targeting liver disease in ASLD or other UCDs. One common histopathologic finding in ASLD and other UCDs is excess hepatic glycogen deposition. However, the mechanism underlying hepatic glycogen accumulation and its consequences on hepatic function in UCDs are unknown. Hepatic glycogen deposition is associated with liver disease in glycogen storage disorders and diabetic glycogenic hepatopathy. Thus, our central hypothesis is that urea cycle dysfunction and accumulation of ammonia and other toxic metabolites disrupt hepatic energy metabolism, including glycogen metabolism, and cause liver disease in UCDs. Studies using current mouse models of ASLD and other distal UCDs have been complicated by the small size and shortened lifespan. To overcome this challenge and facilitate our proposed studies, we have manipulated mouse models of ASLD and citrullinemia to extend the lifespan and improve growth. For the proposed studies, we will use biochemical studies, genetic manipulation and stable isotope studies in these mouse modes to address the following questions: 1) What is the biochemical basis of hepatic glycogen accumulation in ASLD? 2) Does normalization of hepatic glycogen levels prevent liver disease in ASLD? Insights from these studies have the potential to have significant impact on our understanding of the relationship between urea cycle dysfunction and hepatic glycogen metabolism. In addition, the results may inform chronic management strategies for patients with UCDs and may lend insights into new treatment approaches for this group of disorders. On broader terms, our studies may elucidate mechanisms that contribute to the regulation of hepatic glucose flux in more common disorders of glucose metabolism.