ASL metabolically regulates tyrosine hydroxylase activity in the locus coeruleus

Argininosuccinic-lyase (ASL) is the only enzyme in mammalians that endogenously generates arginine, the substrate of nitric oxide synthase for generating nitric oxide (NO). Argininosuccinic-aciduria (ASA) is a urea cycle disorder (UCD) caused by germ-line mutations in the ASL gene. We have previously shown that even in the absence of documented hyperammonemia, ASA patients have a yet unexplained higher incidence of neurocognitive abnormalities compare to other UCD patients. To dissect the role for ASL in the brain, we first assayed its expression in wild-type mouse brains. Surprisingly, we found that ASL is prominently expressed in the locus coeruleus (LC), a brain stem region that is the main source of norepinephrine synthesis. To understand the role of ASL in the LC we generated a conditional mouse model and a neuronal cell-line in which we silenced the expression of ASL. We find that ASL deficiency in the LC decreases NO levels resulting in decreased tyrosine hydroxylase (TH) levels and activity. Consequently, we demonstrate reduced norepinephrine availability and accumulation of tyrosine aggregates in neurons, as well as high tyrosine levels in the CSF of ASA patients. Phenotypically, loss of ASL alters the stress response as manifested by a persistent locomotor activity, high blood pressure and increases seizure sensitivity. Supplementation with NO donors rescues TH level, thus normalizing the abnormal stress response. Jointly, our results shed light on the neurological delays in ASA, and more broadly identify ASL as a new player in the metabolic regulation of the LC.