Behavioral, Neurochemical, and Histological Characterization of Mice Deficient for Parkin, DJ-1, and Antioxidant Proteins
Seamans, Katherine Webster
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Parkinson’s disease is a progressive neurodegenerative disease characterized by a loss of dopaminergic neurons in the substantia nigra. The cause of Parkinson’s disease remains uncertain, however, evidence implicates mitochondrial dysfunction and oxidative stress with selective vulnerability of dopaminergic neurons. Although most cases of Parkinson’s disease are sporadic, 5-10% of cases are caused by mutations in a single gene. Loss-of-function mutations in parkin and DJ-1 were the first to be linked to recessively inherited parkinsonism. Surprisingly, mice bearing similar loss-of-function mutations in parkin and DJ-1 do not show age-dependent loss of nigral dopaminergic neurons or depletion of dopamine in the striatum. Although the normal cellular functions of Parkin and DJ-1 remain unclear, we hypothesized that Parkin and DJ-1 protect cells from oxidative stress and that loss-of-function mutations in these genes cause neurodegeneration in Parkinson’s disease by rendering cells more sensitive to mitochondrial dysfunction and oxidative stress. We crossed mice deficient for Parkin and DJ-1 with mice deficient for the major mitochondrial antioxidant protein Mn-superoxide dismutase or Cu/Zn-superoxide dismutase. Previous studies have shown that mice with reduced levels of Cu/Zn-superoxide dismutase or Mn-superoxide dismutase are more sensitive to dopaminergic neurotoxins whereas mice with increased levels of superoxide dismutase are more resistant to dopaminergic neurotoxins. We predicted that reducing levels of antioxidant proteins in parkin-/-DJ-1-/- mice would result in age-dependent nigral cell loss, striatal dopamine depletion or behavioral abnormalities. Characterization of these mice for behavioral abnormalities, neurotransmitter defects and neuropathology, revealed significant behavioral abnormalities in the mutant mice even in the absence of significant changes to dopamine levels in the striatum, dopamine receptor density, or dopaminergic neuron numbers. Aged parkin-/-DJ-1-/- and Mn-superoxide dismutase triple deficient mice have a surprising enhanced rotorod performance without the presence of an anxiety phenotype or hyperactivity. Cu/Zn-superoxide dismutase and Mn-superoxide dismutase triple deficient mice have elevated levels of dopamine in the striatum, however none of the mice present with nigral cell loss. Levels of D1-like and D2-like dopamine receptors in the striatum were unchanged. It is evident from our studies that on a parkin/DJ-1 null background, additional loss of major antioxidant proteins does not lead to a progressive loss of dopaminergic neurons in mice.