N‐acetylcysteine prevents loss of dopaminergic neurons in the EAAC1−/− mouse

Parkinson’s disease (PD) leads to cell death in several neuronal populations, particularly the dopaminergic neurons of the substantia nigra pars compacta (SNc). Between 5% and 10% of PD can now be attributed to heritable genetic mutations, but the cause of the more common, sporadic PD remains elusive. Several lines of evidence suggest that oxidative stress and depletion of glutathione, a major endogenous antioxidant, contributes to neuronal death in both hereditary and sporadic PD. Oxidative stress and dopaminergic neuronal death are produced by chemical agents epidemiologically associated with PD, such as paraquat and rotenone.1,2 Postmortem studies of PD patients show increased lipid and protein oxidation products in the SNc and markedly reduced levels of glutathione.3,4 Moreover, the glutathione depletion precedes dopaminergic neuronal death in PD5 and does not occur in other neurodegenerative disorders affecting the SNc,6 thus suggesting a specific and causal role for glutathione depletion in PD pathogenesis. Development of interventions to slow or prevent neuronal death in PD has been hindered by a paucity of animal models of the chronic, sustained neuronal oxidative stress observed in human PD.7,8 6-Hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and other toxins used to generate animal models of PD produce massive, acute oxidative stress in the targeted neuronal populations, leading to rapid cell death and motor abnormalities. These models generate a phenotype resembling human PD, but they do not recapitulate the chronic oxidative stress and slow neurodegeneration that occurs over decades in human PD. The EAAC1−/− mouse may be a more useful model in this respect because this mouse strain exhibits chronic, neuron-specific oxidative stress9. EAAC1 (also termed EAAT3 and SLC1A1) is an excitatory amino acid transporter expressed selectively by eurons in the central nervous system (CNS).10,11 It is the primary route for neuronal uptake of cysteine,9,12,13 the rate-limiting substrate in glutathione synthesis.14 Mice lacking EAAC1 have decreased neuronal glutathione content, increased markers of neuronal oxidative stress, and a slow, age-dependent reduction in overall brain size9 ; however, the effect of EAAC1 deficiency on SNc dopaminergic neurons has not previously been reported. The striking depletion of glutathione in dopaminergic neurons in PD, coupled with the importance of glutathione for neuronal survival, has led several authors to suggest that preventing this glutathione depletion may be neuroprotective in PD.15,16 Glutathione is involved in the elimination of peroxides and nitrosylated proteins in both the cytosol and mitochondria. N-acetylcysteine (NAC) is a cell membrane–permeable form of cysteine that can cross the blood-brain barrier and replete neuroal glutathione content.9,17,18 NAC is well-tolerated and currently in clinical use for other indications. Here, using the EAAC1−/− mouse as a model of chronic neuronal oxidative stress, we show an age-dependent loss of SNc dopaminergic neurons in the EAAC1−/− mouse, and further show that this neuronal loss is reduced by oral administration of NAC. These results provide a rationale for evaluating glutathione repletion as a disease-modifying therapy for PD.