Inducible Alterations of Glutathione Levels in Adult Dopaminergic Midbrain Neurons Result in Nigrostriatal Degeneration

Abstract

Parkinson's disease is a neurodegenerative disorder characterized by the preferential loss of midbrain dopaminergic neurons in the substantia nigra (SN). One of the earliest detectable biochemical alterations that occurs in the Parkinsonian brain is a marked reduction in SN levels of total glutathione (glutathione plus glutathione disulfide), occurring before losses in mitochondrial complex I (CI) activity, striatal dopamine levels, or midbrain dopaminergic neurodegeneration associated with the disease. Previous in vitro data from our laboratory has suggested that prolonged depletion of dopaminergic glutathione results in selective impairment of mitochondrial complex I activity through a reversible thiol oxidation event. To address the effects of depletion in dopaminergic glutathione levels in vivo on the nigrostriatal system, we created genetically engineered transgenic mouse lines in which expression of ␥-glutamyl cysteine ligase, the rate-limiting enzyme in de novo glutathione synthesis, can be inducibly downregulated in catecholaminergic neurons, including those of the SN. A novel method for isolation of purified dopaminergic striatal synaptosomes was used to study the impact of dopaminergic glutathione depletion on mitochondrial events demonstrated previously to occur in vitro as a consequence of this alteration. Dopaminergic glutathione depletion was found to result in a selective reversible thiol-oxidation-dependent mitochondrial complex I inhibition, followed by an age-related nigrostriatal neurodegeneration. This suggests that depletion in glutathione within dopaminergic SN neurons has a direct impact on mitochondrial complex I activity via increased nitric oxide-related thiol oxidation and age-related dopaminergic SN cell loss.

Oxidative and nitrative protein modifications in Parkinson's disease

“…Indeed, we also observe the presence of 3NT modifications in complex I subunits following glutathione depletion but as in our case enzymatic inhibition is reversible, this would argue against them playing a functional role in glutathione-mediated effects [91]. In further in vivo studies, we were able to demonstrate that inducible dopaminergic glutathione depletion in the adult animal results in S-nitrosylation of complex I subunits which coincides with a selective decrease in complex I activity and an agedependent neurodegeneration of dopaminergic SN neurons [92]. Coupled with our in vitro data, this strongly implies that a possible mechanism of the hallmark complex I inhibition in these cells could be a consequence of early glutathione depletion.…”

Neuronal Glutathione Content and Antioxidant Capacity can be Normalized In Situ by N-acetyl Cysteine Concentrations Attained in Human Cerebrospinal Fluid

“…A causal role for oxidative stress and GSH depletion in PD is supported by both clinical and animal studies [1,[3][4][5]. In particular, factors that cause global impairments in neuronal GSH metabolism cause cytotoxicity preferentially in the neuronal populations most affected in PD [6][7][8].…”

Substantia nigra dopaminergic neurons and striatal interneurons are engaged in three parallel but interdependent postnatal neurotrophic circuits

“…Posterior studies with this and other models of DA SNpc postnatal depletion will be required to validate our results and provide mechanistic insights. However, no tamoxifen‐regulated Cre line has so far proven efficiency in recombining postnatal DA SNpc, something that could be overcome with bitransgenic systems containing Th or Pitx3‐tet, and TRE‐Cre lines (Chinta et al, 2007; Lin et al, 2012; Tillack, Aboutalebi, & Kramer, 2015). …”