GFAP gene methylation in different neural cell types from rat brain

It is generally believed that specific demethylation processes take place i n the promoter of tis sue-specific genes during development. II has been su ggested that hypomethylation of the -1500/-1100 domain of the 5' flanking r egulatory region of the rat glial fibrillary acidic protein gene may be spe cific for neuroectodermal derivatives such as neurons and astrocytes. In th e present work the methylation status of one of those seven CG sites (the - 1176) of the 'neuroectoderm-specific domain' was analyzed. In agreement wit h the neuroectoderm hypothesis, the -1176 site is highly demethylated in as troglial, oligodendroglial and neuronal cells, but heavily methylated in mi croglial and fibroblast cells. The three different glial population are der ived 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 d e novo methylation. In order to clarify this point we used a subcellular fr actionation method which allowed us to separate two different nuclear popul ations from adult rat cerebral hemispheres: one enriched in neuronal nuclei (called N1) and the other enriched in glial nuclei (N2). A higher methylat ion 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 me thylation of the -1176 site during postnatal brain development. We hypothes ize that a DNA demethylation process takes place in neuroectodermal precurs or cells and that the -1176 site persists demethylated at the earlier stage s of neuronal differentiation (immature neurons) and becomes fully methylat ed at more advanced stages of differentiation. (C) 1999 ISDN. Published by Elsevier Science Ltd. All rights reserved.