Pharmacologic Rescue of Hyperammonemia-induced Neurotoxicity by Inhibition of Ornithine Aminotransferase in a Zebrafish Model of Acute Hyperammonemic Encephalopathy

Background: Acute hyperammonemic encephalopathy is a life-threatening manifestation of individuals with urea cycle disorders (UCD), which is associated with high mortality rates and severe neurological sequelae in survivors. The outcome of UCD individuals with neonatal onset is still poor and has not relevantly improved over the last three decades. Mortality rates of UCD individuals with neonatal onset are still as high as 30 to 50%, highlighting the need for a better comprehension of the pathophysiological basis of hyperammonemia-induced brain damage and the development of novel therapeutic strategies.

Methods: To characterize the mechanisms of hyperammonemia-induced neurotoxicity and to investigate the efficacy of therapeutic interventions, we established a zebrafish model of acute hyperammonemic encephalopathy by exposure of zebrafish larvae to 10 mM ammonium acetate at developmental age 4 days post fertilization. Exposure to 10 mM sodium acetate was used as negative control.

Results: Acute hyperammonemia strongly enhances transamination-dependent formation of osmolytic glutamine and excitatory glutamate, thereby inducing neurotoxicity and death in zebrafish larvae via synergistically acting overactivation of NMDA receptors and bioenergetic impairment induced by transamination-dependent withdrawal of 2-oxoglutarate from the tricarboxylic acid (TCA) cycle. Depletion of 2-oxoglutarate leads to a broad TCA cycle dysfunction ultimately causing impaired oxidative phosphorylation with ATP shortage, decreased ATP/ADP-ratio and elevated lactate concentrations. Anaplerotic supply of carbon backbones to the TCA cycle by propionate oxidation and increased glycolysis are not sufficient to maintain proper bioenergetic function under hyperammonemic conditions. Intriguingly, specific inhibition of ornithine aminotransferase (OAT) by 5-fluoromethylornithine rescues zebrafish from lethal concentrations of ammonium acetate and corrects hyperammonemia-induced biochemical alterations.

Conclusion: OAT inhibition is a promising and effective therapeutic approach for the prevention of neurotoxicity and mortality in acute hyperammonemic encephalopathy.

References

1. Zielonka M, Breuer M, Okun JG, Carl M, Hoffmann GF, Kölker S. Pharmacologic rescue of hyperammonemia-induced toxicity in zebrafish by inhibition of ornithine aminotransferase. PLoS One 2018;13(9):e0203707

2. Zielonka M, Probst J, Carl M, Hoffmann GF, Kölker S, Okun JG. Bioenergetic dysfunction in a zebrafish model of acute hyperammonemic decompensation. Exp Neurol 2019;314:91–99

No conflict of interest has been declared by the author(s).