Neuroprotective functions of DJ-1 and MsrA: Implications for Parkinson\u27s disease

Parkinson\u27s disease (PD) is a neurodegenerative disorder that involves a loss of dopaminergic neurons in the substantia nigra. Mitochondrial dysfunction, proteasome inhibition, and protein aggregation play roles in the pathogenesis of PD. Rare cases of early-onset PD have been linked to loss-of-function mutations in the gene encoding DJ-1, a protein with antioxidant and chaperone activities. We found that DJ-1 protects dopaminergic neurons against various PD-related insults including inhibition of mitochondrial complex I, proteasome dysfunction, and over-expression of a mutant form of α-synuclein linked to autosomal dominant PD. The neuroprotective mechanism of DJ-1 is insult-specific. DJ-1 acts as an antioxidant in cells exposed to rotenone treatment, and this antioxidant activity correlates with increased levels of reduced GSH. In contrast, DJ-1 induces an increase in levels of heat shock protein 70 (Hsp70) in response to proteasome inhibition or α-synuclein over-expression. DJ-1 readily undergoes oxidation at cysteine 106, and modification of this site to the cysteine sulfinic acid (yielding the \u272O\u27 form of DJ-1) has been proposed to serve as an oxidative stress sensor function in neurons. To address whether cysteine 106 oxidation is necessary for DJ-1-mediated neuroprotection against different PD-related insults, we characterized the neuroprotective activities of C106A, a variant that cannot convert to the 2O form. We found that C106A DJ-1 is less protective against rotenone neurotoxicity compared to the wild-type protein, but it is equally protective against toxicity elicited by the proteasome inhibitor MG132 and mutant α-synuclein. These results suggest that the oxidation of C106 is critical for its antioxidant function. Another unique feature of DJ-1 is that it is localized to various subcellular compartments, including the cytosol, the nucleus, and mitochondria. To test the function of DJ-1 in different subcellular compartments, we fused a mitochondrial localization sequence (MLS) and a nuclear localization sequence (NLS) to wild-type DJ-1. We found that MLS DJ-1 was protective against dopaminergic cell death induced by rotenone and mutant α-synuclein, whereas NLS DJ-1 was only protective against neurotoxicity elicited by MG132. These results suggest that both the oxidation and subcellular localization of DJ-1 regulate its neuroprotective functions. In other studies, we characterized neuroprotective activities of MsrA, an antioxidant repair enzyme. MsrA plays a critical role in the antioxidant response by repairing oxidized methionine residues on proteins and by participating in cycles of methionine oxidation and reduction that have the net effect of consuming reactive oxygen species (ROS). MsrA levels decrease during aging, and a deficiency in the enzyme results in impaired dopamine signaling in animal models. We found that MsrA is protective against toxicity elicited by rotenone and mutant α-synuclein (but not MG132) in primary midbrain cultures. The protective function of MsrA in this model involves the repair of oxidized proteins (including α-synuclein) rather than a ROS scavenging activity. In summary, DJ-1 and MsrA alleviate dopaminergic cell death induced by various PD-related insults through complementary neuroprotective activities. Our findings suggest that targeted upregulation of these activities may provide therapeutic benefits to PD patients