␣-Synuclein is a key protein in Parkinson's disease (PD) because it accumulates as fibrillar aggregates in pathologic hallmark features in affected brain regions, most notably in nigral dopaminergic neurons. Intraneuronal levels of this protein appear critical in mediating its toxicity, because multiplication of its gene locus leads to autosomal dominant PD, and transgenic animal models overexpressing human ␣-synuclein manifest impaired function or decreased survival of dopaminergic neurons. Here, we show that microRNA-7 (miR-7), which is expressed mainly in neurons, represses ␣-synuclein protein levels through the 3 -untranslated region (UTR) of ␣-synuclein mRNA. Importantly, miR-7-induced down-regulation of ␣-synuclein protects cells against oxidative stress. Further, in the MPTP-induced neurotoxin model of PD in cultured cells and in mice, miR-7 expression decreases, possibly contributing to increased ␣-synuclein expression. These findings provide a mechanism by which ␣-synuclein levels are regulated in neurons, have implications for the pathogenesis of PD, and suggest miR-7 as a therapeutic target for PD and other ␣-synucleinopathies.Parkinson's disease ͉ neuroprotection ͉ MPTP model ͉ microRNA P arkinson's disease (PD) is a common neurodegenerative disorder that affects 1% of the population over 65. It is characterized by disabling motor abnormalities including tremor, slow movements, rigidity, and poor balance. These impairments stem from the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. Eventually, large percentages of patients develop dementia and hallucinations when the pathology involves other brain regions as well. Although the majority of Parkinson cases appear to be sporadic, the disorder runs in families in Ϸ15-20% of the cases. To date, 5 distinct genes have been identified to cause PD including ␣-synuclein, parkin, dj-1, pink1, and lrrk2 (1). Understanding how mutations in these genes cause neurodegeneration is crucial in the development of treatments that might slow or stop the disease progression.

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Summary In Escherichia coli, phase variation of the outer membrane protein Ag43 encoded by the agn43 gene is mediated by DNA methylation and the global regu‐lator OxyR. Transcription of agn43 occurs (ON phase) when three Dam target sequences in the agn43 regulatory region are methylated, which prevents the repressor OxyR from binding. Conversely, transcription is repressed (OFF) when these Dam target sequences are unmethylated and OxyR binds. A change in expression phase requires a concomitant change in the DNA methylation state of these Dam target sequences. To gain insight into the process of inheritance of the expression phase and the DNA methylation state, protein–DNA interactions at agn43 were examined. Binding of OxyR at agn43 was sufficient to protect the three GATC sequences contained within its binding site from Dam‐dependent methylation in vitro, suggesting that no other factors are required to maintain the unmethylated state and OFF phase. To maintain the methylated state of the ON phase, however, Dam must access the hemimethylated agn43 region after DNA replication, and OxyR binding must not occur. OxyR bound hemimethylated agn43 DNA, but the affinity was severalfold lower than for unmethylated DNA. This presumably contributes to the maintenance of the methylated state but, at the same time, may allow for infrequent OxyR binding and a switch to the OFF phase. Hemimethylated agn43 DNA was also a binding substrate for the sequestration protein SeqA. Thus, SeqA, OxyR and Dam may compete for the same hemimethylated agn43 DNA that is formed after DNA replication in an ON phase cell. In isolates with a mutant seqA allele, agn43 phase variation rates were altered and resulted in a bias to the OFF phase. In part, this can be attributed to the observed decrease in the level of DNA methylation in the seqA mutant.

Apoptosis signal-regulating kinase 1 (ASK1), a member of the mitogen-activated protein kinase 3 family, is activated by oxidative stress. The death-signaling pathway mediated by ASK1 is inhibited by DJ-1, which is linked to recessively inherited Parkinson's disease (PD). Considering that DJ-1 deficiency exacerbates the toxicity of the mitochondrial complex I inhibitor 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), we sought to investigate the direct role and mechanism of ASK1 in MPTP-induced dopamine neuron toxicity. In the present study, we found that MPTP administration to wild-type mice activates ASK1 in the midbrain. In ASK1 null mice, MPTP-induced motor impairment was less profound, and striatal dopamine content and nigral dopamine neuron counts were relatively preserved compared to wild-type littermates. Further, microglia and astrocyte activation seen in wild-type mice challenged with MPTP was markedly attenuated in ASK1−/− mice. These data suggest that ASK1 is a key player in MPTP-induced glial activation linking oxidative stress with neuroinflammation, two well recognized pathogenetic factors in PD. These findings demonstrate that ASK1 is an important effector of MPTP-induced toxicity and suggest that inhibiting this kinase is a plausible therapeutic strategy for protecting dopamine neurons in PD.