GFAP gene methylation in different neural cell types from rat brain.

It is generally believed that specific demethylation processes take place in the promoter of tissue-specific genes during development. It has been suggested that hypomethylation of the -1500/-1100 domain of the 5' flanking regulatory region of the rat glial fibrillary acidic protein gene may be specific for neuroectodermal derivatives such as neurons and astrocytes. In the present work the methylation status of one of those seven CG sites (the -1176) of the 'neuroectoderm-specific domain' was analyzed. In agreement with the neuroectoderm hypothesis, the -1176 site is highly demethylated in astroglial, oligodendroglial and neuronal cells, but heavily methylated in microglial and fibroblast cells. The three different glial population are derived from the same tissue (cerebral hemispheres of newborn rats) but have a different embryological origin: oligodendrocytes and astrocytes originate from neuroectoderm, while microglia is of mesodermal origin. It is not clear if GFAP-negative neuronal cells maintain such demethylation in the advanced stage of maturation or if they undergo a second phase of de novo methylation. In order to clarify this point we used a subcellular fractionation method which allowed us to separate two different nuclear populations from adult rat cerebral hemispheres: one enriched in neuronal nuclei (called N1) and the other enriched in glial nuclei (N2). A higher methylation level of the -1176 site was detected in the N1 fraction, suggesting the GFAP gene undergo a de novo methylation process during neuronal maturation. This observation is in agreement with recent results showing a de novo methylation of the -1176 site during postnatal brain development. We hypothesize that a DNA demethylation process takes place in neuroectodermal precursor cells and that the -1176 site persists demethylated at the earlier stages of neuronal differentiation (immature neurons) and becomes fully methylated at more advanced stages of differentiation.