Maintenance-Type DNA Methyltransferase Is Highly Expressed in Post-Mitotic Neurons and Localized in the Cytoplasmic Compartment

Maintenance-type DNA methyltransferase (Dnmt1) is usually down-regulated in non-proliferating cells. In the present study, we detected significant expression of Dnmtl protein in adult mouse brain where the majority of the cells are in a post-mitotic state. A significant amount of Dnmtl protein was fractionated into the post-nuclear fraction for both cerebrum and cerebellum. The Dnmtl in this fraction was enzymatically active. An immunofluorescence study revealed that Dnmtl protein was mainly expressed in neurons and seemed to be localized in the cytoplasmic compartment. Primary culturing of neurons confirmed the expression and localization of Dnmtl in the cytoplasmic compartment. The findings that the Dnmtl transcript in the brain utilized the somatic-type exon and that the apparent size of the Dnmtl protein in the cytoplasm was identical to that in proliferating culture cells indicate that the cytoplasmic Dnmtl in neurons was of the somatic-type.