The mineralocorticoid receptor (MR) is a ligand-activated transcription factor, belonging to the nuclear receptor family, a diverse group of transcription factors that arose in multicellular animals [1–5]. The traditional physiological function of the MR is to maintain electrolyte balance by regulating sodium and potassium transport in epithelial cells in the kidney and colon [6–10]. In addition, the MR has important physiological functions in many other tissues, including brain, heart, skin and lungs [10–16].
The MR and its paralog, the glucocorticoid receptor (GR), descended from an ancestral corticoid receptor (CR) in a cyclostome (jawless fish) that evolved about 550 million years ago at the base of the vertebrate line [17–24]. A descendent of this ancestral steroid receptor, the CR in lamprey (Petromyzon marinus), is activated by aldosterone [25, 26] and other corticosteroids [25, 26]. Lampreys contain two CR isoforms, which differ only in the presence of a four amino acid insert Thr, Arg, Gln, Gly (TRQG) in their DNA-binding domain (DBD) [27] (Fig. 1). We found that several corticosteroids had a similar half-maximal response (EC50) for lamprey CR1 and CR2 [26]. However, these corticosteroids had a lower fold-activation of transcription for CR1, which contains the four amino acid insert, than for CR2 suggesting that the deletion of the four amino acid sequence in CR2 selected for increased transcriptional activation by corticosteroids of CR2 [26, 27].
A distinct MR and GR first appear in sharks and other cartilaginous fishes (Chondrichthyes) [19, 21, 28–31]. The DBD in elephant shark MR and GR lacks the four amino acid sequence found in lamprey CR1 [26] (Fig. 1). We inserted this four-residue sequence from lamprey CR1 into the DBD in elephant shark MR and GR and found that in HEK293 cells co-transfected with the TAT3 promoter, the mutant elephant shark MR and GR had lower transcriptional activation by corticosteroids than did their wild-type elephant shark MR and GR counterparts, indicating that the insertion of the four amino acid sequence into the DBD of wild-type elephant shark MR and GR had a similar effect on transcriptional activation as the KCSW insert had in the DBD of lamprey CR1 [27].
We then analyzed the DBD sequence of human MR, which had been cloned, sequenced and characterized by Arriza et al. [32], and found that like elephant shark MR, this human MR (MR1) lacks a four-residue segment in its DBD. This human MR has been widely studied [8, 10, 12, 13, 33–35]. Unexpectedly, our BLAST [36] search with the DBD from this human MR found a second, previously described, human MR splice variant with a KCSW insert (MR-KCSW) in its DBD [27, 37–39] (Fig. 1). As described later, further BLAST searches found two full-length human MRs with this insert (MR-KCSW) and three full-length human MRs without this insert. The three human MRs without the KCSW insert contain either (Ile-180, Ala-241) or (Val-180, Val-241) or (Ile-180, Val-241) in their amino terminal domain (NTD). The two human MR-KCSW splice variants contain either (Val-180, Val-241) or (Ile-180, Val-241) in their NTD. A human MR-KCSW with (Ile-180, Ala-241) has not been cloned.
Although the level of expression of MR-KCSW in human and rat tissues has been reported [37–39], transcriptional activation of either human or rat MR-KCSW by corticosteroids has not been reported. To remedy this deficiency, we have studies underway to characterize transcriptional activation of the five full-length human MRs by a panel of corticosteroids. Here, in this brief report, we describe our evolutionary analysis of human MR from a comparison of five full-length human MRs with four full-length MRs in chimpanzees and two full-length MRsin gorillas, and orangutans. Due to the multiple functions in human development of the MR alone [11, 15, 16, 40], as well as due to the MRs interaction with the GR [41–45], we suggest that the sequence divergence of human MR from chimpanzee MR may have been important in the evolution of humans from chimpanzees.