5 '-heterogeneity of glucocorticoid receptor messenger RNA is tissue specific: Differential regulation of variant transcripts by early-life events

Glucocorticoid receptor (GR) gene expression is regulated in a complex tiss ue-specific manner, notably by early-life environmental events that program tissue GR levels. We have identified and characterized several new rat GR mRNAs. All encode a common protein, but differ in their 5'-leader sequences as a consequence of alternate splicing of, potentially, 11 different exon 1 sequences. Most are located in a 3-kb CpG island, upstream of exon 2, tha t exhibits substantial promoter activity in transfected cells. Ribonuclease (RNase) protection analysis demonstrated significant levels of six alterna te exons 1 in vivo in rat, with differences between liver, hippocampus, and thymus reflecting tissue-specific differences in promoter activity. Two of the alternate exons 1 (exons 1(6) and 1(10)) were expressed in all tissues examined, together present in 77-87% of total GR mRNA. The remaining GR tr anscripts contained tissue-specific alternate first exons. Importantly, tis sue-specific first exon usage was altered by perinatal environmental manipu lations. Postnatal handling, which permanently increases GR in the hippocam pus, causing attenuation of stress responses, selectively elevated GR mRNA containing the hippocampus-specific exon 1(7). Prenatal glucocorticoid expo sure, which increases hepatic GR expression and produces adult hyperglycemi a, decreased the proportion of hepatic GR mRNA containing the predominant e xon 1(10), suggesting an increase in a minor exon 1 variant. Such tissue sp ecificity of promoter usage allows differential GR regulation and programmi ng.