The CpG island shore of the GLT-1 gene acts as a methylation-sensitive enhancer

Astrocytic lineage commitment and brain region-dependent specialization of glia are partly ascribed to epigenetic processes. Clearance of glutamate is an essential task, which astrocytes assume in a temporal-spatial fashion by distinct glutamate transporter expression. Glutamate transporter subtype 1 (GLT-1) is predominant in cortex (CTX), while it plays an inferior role in cerebellum (CER). Here, we set out to identify regulatory elements that could account for the differences in brain region-specific activity as well as response to dexamethasone (DEX) or epigenetic factors. We found a distal promoter element at the shore of the CpG island exhibiting enhancer function in response to DEX in reporter gene assays. This shore region showed slight enrichment in repressive trimethyl-histone H3 (Lys27) and under-representation of acetyl-histone H4 (H4ac) marks in DEX nonresponsive CER astrocytes as determined by chromatin immunoprecipitation. In addition, CpG sites of the shore region displayed higher methylation in CER than in CTX cells. Targeted in vitro methylation of CpG sites within the shore abrogated the stimulatory effects of DEX. Interestingly, the shore was characterized by a pronounced epigenetic plasticity in CTX cells since DEX exposure elicited an increase of H4ac in CTX in comparison to DEX nonresponsive CER. The transcriptional activity of this region was also affected by histone deacetylase inhibitors in a methylation- and brain region-dependent manner. Together, our study highlights the impact of an epigenetically adaptive DNA element of the GLT-1 promoter being decisive for brain region-specific activity and reactivity.