Investigation of novel therapies for hyperammonemia

Ammonia is a toxic compound daily produced and consumed during cell metabolism in the body. It is converted to the less toxic substance urea by the urea cycle in the liver. Moreover, the urea cycle operates anatomically and functionally in sequence with the hepatic glutamine synthetase (GS) to detoxify ammonia and both systems are required for ammonia homeostasis. Urea cycle or GS deficiencies and liver diseases lead to defective ammonia removal causing hyperammonemia. Hyperammonemia is a life- threatening condition which may results in hepatic encephalopathy and death, if left untreated. Available therapies for hyperammonemia are partially effective and novel and improved therapies are required. Considering the cooperative role of GS in ammonia detoxification, gene therapy based on hepatic GS augmentation was investigated as a potential treatment against hyperammonemia. Thus, upregulation of hepatic GS protects wild-type mice against acute hyperammonemia. Notably, liver specific GS overexpression improved ammonia detoxification also in Cps1-deficient mice. As a result, hepatic GS augmentation therapy has potential for treatment of both primary and secondary forms of hyperammonemia. Furthermore ammonia can induce modification of O-GlcNAcylation levels. O- GlcNAcylation is a dynamic post-translational modification catalyzed by the enzyme O-GlcNAc transferase (OGT) whereas the O-GlcNAcase (OGA) removes O-GlcNAc. O-GlcNAcylation, once modulated, may be a new therapeutic target to treat hyperammonemia. Hence, the role of hepatic O-GlcNAcylation in ammonia detoxification was investigated. In vivo modulation of O-GlcNAcylation through both pharmacological (small molecules) and OGA knockdown approaches improved ammonia detoxification. Neither expression of genes encoding urea cycle enzymes nor protein levels of these enzymes were affected by OGA inhibition, suggesting that hepatic O-GlcNAcylation regulates ammonia clearance capacity by enzymatic activity of the urea cycle. Finally, hepatic O-GlcNAcylation plays an important role in ammonia detoxification. Therefore OGA is a novel therapeutic target for treatment of hyperammonemia of both urea cycle disorders and acquired hyperammonemia.