Differential mRNA expression of the two mineralocorticoid receptor splice variants within the human brain: Structure analysis of their different DNA binding domains

In human brain tissue, cortisol action, at basal concentrations, is mediate d by the mineralocorticoid receptor (MR). An in-frame insertion of 12 bp in the MR-DNA-binding domain due to alternative splice site usage between exo ns 3 and 4 results in an MR mRNA splice variant (MR+4) encoding a receptor protein with four additional amino acids compared to the wild-type MR prote in. To elucidate the questions of sex, age, and/or tissue dependent differe nces of the relative amount of the two mRNA subtypes, we examined 131 fresh human brain tissue samples from temporal and frontal lobe or hippocampus. One hundred and twenty samples were obtained from patients with epilepsy an d 11 samples from patients with brain tumours. A small but significant diff erence of the MR+4 mRNA splice variant proportions in cortex (9.5 +/- 0.8%) and subcortical white matter (6.6 +/- 0.7%) of the temporal lobe could be detected, indicating differential MR splice variant expression within these brain areas. Moreover, the splice variant ratios in samples of the tempora l lobe cortex collected from patients with epilepsy differed from samples o f patients with brain tumours. These data point to an altered expression of the MR splice variants in epilepsy, and strengthen the supposition of a ti ssue specific alternative splicing of the MR mRNA. The frequent occurence o f the MR+4 transcript raises the question of its functional significance. F or this reason, an MR+4 DNA-binding-domain structure model was generated by computer-based homology modelling based on the known glucocorticoid recept or structure. The data obtained revealed no distorting effect of the insert ed four amino acids on the adjacent secondary structures, thereby suggestin g that both zinc fingers retain their function. The resulting structure of the MR+4 model leads to the supposition that the receptor retains its funct ion. Moreover, databank analysis with respect to this kind of steroid recep tor variation and our own sequence data of the closely related progesterone receptor sustained the hypothesis that only corticosteroid receptors were affected by this alternative splicing event.