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Tyrosinase is a key enzyme for the biosynthesis of melanin pigments in peripheral tissues such as skin and retina. Although tyrosinase activity is specifically detected in melanocytes, several studies have shown the expression and enzymatic activity of tyrosinase in the central nervous system, especially in the midbrain substantia nigra. In the present study, we investigated the antioxidative effects of tyrosinase on protein damage in the substantia nigra of mice. C57BL/10JMsHir (B10) and tyrosinase-deficient albino B10.C-Tyrc/Hir (B10-c) mice were intraperitoneally administered retinol palmitate to induce oxidative stress, and the protein carbonyl content, a hallmark of protein oxidative damage, was examined in the substantia nigra. Retinol palmitate administration was found to decrease catalase activity in the substantia nigra of both B10 and B10-c mice, suggesting the induction of oxidative stress due to imbalanced antioxidant systems. In this model, we found that tyrosinase deficiency markedly increases the protein carbonyl content in the substantia nigra. Thus, we concluded that tyrosinase activity prevents protein damage in the substantia nigra of mice that were challenged with oxidative stress. These findings provide novel insight into the physiological role of tyrosinase in the central nervous system.

Although valproate (VPA) is used widely in the treatment of bipolar mood disorder and epilepsy, the precise mechanism of action in the brain remains elusive. In this study, we investigated the effects of subchronic VPA administrations on the expression of the cyclin-dependent kinase inhibitor (Cdkn) family in the hippocampus of adult mice. The administration of VPA specifically increased hippocampal p21 expression involving both mRNA and protein levels, but other members of the Cdkn family were not affected. We identified two CpG islands in the p21 gene regulatory region, located distal and proximal to the transcription start site. VPA altered genomic DNA methylation patterns in the distal region, but not in the proximal promoter region. However, no change was found in DNA methyltransferase (Dnmt) 1 or Dnmt3a protein levels, suggesting an involvement in active demethylation mechanisms. These findings suggest that VPA alters the gene expression of cell cycle regulators by modulating promoter DNA methylation, and this resulted in altered hippocampal cell proliferation. These findings promote understanding of the actions of VPA in the brain.

The DNA-demethylating agent 5-azacytidine (5-azaC) causes extensive genomic demethylation of 5-methyl-cytosine residues and reduces DNA methyltransferase activity in cells. This study evaluated the effect of 5-azaC on neuronal differentiation in proliferating NG108-15 neuronal cells, which exhibit cholinergic traits. The expression of choline acetyltransferase, an enzyme responsible for acetylcholine synthesis, was increased at both the mRNA and protein level, and neurite outgrowth was markedly induced with an increase of neurofilament-heavy chain protein, in the 5-azaC-treated cells. These findings show that global DNA demethylation markedly induces the expression of the neurotransmitter phenotype and morphological differentiation in NG108-15 neuronal cells as a model for cholinergic neuron.