Identification of a splice variant of the rat and human mineralocorticoid receptor genes

Lysine-Cysteine-Serine-Tryptophan inserted into the DNA-binding domain of human mineralocorticoid receptor increases transcriptional activation by aldosterone

Due to alternative splicing in an ancestral DNA-binding domain (DBD) of the mineralocorticoid receptor (MR), humans contain two almost identical MR transcripts with either 984 amino acids (MR-984) or 988 amino acids (MR-988), in which their DBDs differ by only four amino acids, Lys,Cys,Ser,Trp (KCSW). Human MRs also contain mutations at two sites, codons 180 and 241, in the amino terminal domain (NTD). Together, there are five distinct full-length human MR genes in GenBank. Human MR-984, which was cloned in 1987, has been extensively studied. Human MR-988, cloned in 1995, contains KCSW in its DBD. Neither this human MR-988 nor the other human MR-988 genes have been studied for their response to aldosterone and other corticosteroids. Here, we report that transcriptional activation of human MR-988 by aldosterone is increased by about 50 % compared to activation of human MR-984 in HEK293 cells transfected with the TAT3 promoter, while the half-maximal response (EC50) is similar for aldosterone activation of MR-984 and MR-988. Transcriptional activation of human MR also depends on the amino acids at codons 180 and 241. Interestingly, in HEK293 cells transfected with the MMTV promoter, transcriptional activation by aldosterone of human MR-988 is similar to activation of human MR-984, indicating that the promoter has a role in the regulation of the response of human MR-988 to aldosterone. The physiological responses to aldosterone and other corticosteroids in humans with MR genes containing KCSW and with differences at codons 180 and 241 in the NTD warrant investigation.

Novel Evolution of Mineralocorticoid Receptor in Humans Compared to Chimpanzees, Gorillas, and Orangutans

We identified five distinct full-length human mineralocorticoid receptor (MR) genes containing either 984 amino acids (MR-984) or 988 amino acids (MR-988), which can be distinguished by the presence or absence of Lys, Cys, Ser, and Trp (KCSW) in their DNA-binding domain (DBD) and mutations at codons 180 and 241 in their amino-terminal domain (NTD). Two human MR-KCSW genes contain either (Val-180, Val-241) or (Ile-180, Val-241) in their NTD, and three human MR-984 genes contain either (Ile-180, Ala-241), (Val-180, Val-241), or (Ile-180, Val-241). Human MR-KCSW with (Ile-180, Ala-241) has not been cloned. In contrast, chimpanzees contain four MRs: two MR-988s with KCSW in their DBD, or two MR-984s without KCSW in their DBD. Chimpanzee MRs only contain (Ile180, Val-241) in their NTD. A chimpanzee MR with either (Val-180, Val-241) or (Ile-180, Ala-241) in the NTD has not been cloned. Gorillas and orangutans each contain one MR-988 with KCSW in the DBD and one MR-984 without KCSW, and these MRs only contain (Ile-180, Val-241) in their NTD. A gorilla MR or orangutan MR with either (Val-180, Val-241) or (Ile-180, Ala-241) in the NTD has not been cloned. Together, these data suggest that human MRs with (Val-180, Val-241) or (Ile-180, Ala-241) in the NTD evolved after humans and chimpanzees diverged from their common ancestor. Considering the multiple functions in human development of the MR in kidney, brain, heart, skin, and lungs, as well as MR activity in interaction with the glucocorticoid receptor, we suggest that the evolution of human MRs that are absent in chimpanzees may have been important in the evolution of humans from chimpanzees. Investigation of the physiological responses to corticosteroids mediated by the MR in humans, chimpanzees, gorillas, and orangutans may provide insights into the evolution of humans and their closest relatives.

Reduced steroid activation of elephant shark GR and MR after inserting four amino acids from the DNA-binding domain of lamprey corticoid receptor-1

Atlantic sea lamprey contains two corticoid receptors (CRs), CR1 and CR2, that have identical amino acid sequences, except for a four amino acid insert (Thr-Arg-Gln-Gly) in the CR1 DNA-binding domain (DBD). Steroids are stronger transcriptional activators of CR2 than of CR1 suggesting that the insert reduces the transcriptional response of lamprey CR1 to steroids. The DBD in elephant shark mineralocorticoid receptor (MR) and glucocorticoid receptor (GR), which are descended from a CR, lack these four amino acids, suggesting that a CR2 is their common ancestor. To determine if, similar to lamprey CR1, the presence of this insert in elephant shark MR and GR decreases transcriptional activation by corticosteroids, we inserted these four CR1-specific residues into the DBD of elephant shark MR and GR. Compared to steroid activation of wild-type elephant shark MR and GR, cortisol, corticosterone, aldosterone, 11-deoxycorticosterone and 11-deoxycortisol had lower transcriptional activation of these mutant MR and GR receptors, indicating that the absence of this four-residue segment in the DBD in wild-type elephant shark MR and GR increases transcriptional activation by corticosteroids.

Aldosterone: Renal Action and Physiological Effects

Aldosterone exerts profound effects on renal and cardiovascular physiology. In the kidney, aldosterone acts to preserve electrolyte and acid-base balance in response to changes in dietary sodium (Na+ ) or potassium (K+ ) intake. These physiological actions, principally through activation of mineralocorticoid receptors (MRs), have important effects particularly in patients with renal and cardiovascular disease as demonstrated by multiple clinical trials. Multiple factors, be they genetic, humoral, dietary, or otherwise, can play a role in influencing the rate of aldosterone synthesis and secretion from the adrenal cortex. Normally, aldosterone secretion and action respond to dietary Na+ intake. In the kidney, the distal nephron and collecting duct are the main targets of aldosterone and MR action, which stimulates Na+ absorption in part via the epithelial Na+ channel (ENaC), the principal channel responsible for the fine-tuning of Na+ balance. Our understanding of the regulatory factors that allow aldosterone, via multiple signaling pathways, to function properly clearly implicates this hormone as central to many pathophysiological effects that become dysfunctional in disease states. Numerous pathologies that affect blood pressure (BP), electrolyte balance, and overall cardiovascular health are due to abnormal secretion of aldosterone, mutations in MR, ENaC, or effectors and modulators of their action. Study of the mechanisms of these pathologies has allowed researchers and clinicians to create novel dietary and pharmacological targets to improve human health. This article covers the regulation of aldosterone synthesis and secretion, receptors, effector molecules, and signaling pathways that modulate its action in the kidney. We also consider the role of aldosterone in disease and the benefit of mineralocorticoid antagonists. © 2023 American Physiological Society. Compr Physiol 13:4409-4491, 2023.

Mineralocorticoid receptor in non-alcoholic fatty liver disease

Liver diseases are the fourth common death in Europe responsible for about 2 million death per year worldwide. Among the known detrimental causes for liver dysfunction are virus infections, intoxications and obesity. The mineralocorticoid receptor (MR) is a ligand-dependent transcription factor activated by aldosterone or glucocorticoids but also by pathological milieu factors. Canonical actions of the MR take place in epithelial cells of kidney, colon and sweat glands and contribute to sodium reabsorption, potassium secretion and extracellular volume homeostasis. The non-canonical functions can be initiated by inflammation or an altered micro milieu leading to fibrosis, hypertrophy and remodeling in various tissues. This narrative review summarizes the evidence regarding the role of MR in portal hypertension, non-alcoholic fatty liver disease, liver fibrosis and cirrhosis, demonstrating that inhibition of the MR in vivo seems to be beneficial for liver function and not just for volume regulation. Unfortunately, the underlying molecular mechanisms are still not completely understood.

Glucocorticoids: Dr. Jekyll and Mr. Hyde of Hippocampal Neuroinflammation

Glucocorticoids (GCs) are an important component of adaptive response of an organism to stressogenic stimuli, a typical stress response being accompanied by elevation of GC levels in blood. Anti-inflammatory effects of GCs are widely used in clinical practice, while pro-inflammatory effects of GCs are believed to underlie neurodegeneration. This is particularly critical for the hippocampus, brain region controlling both cognitive function and emotions/affective behavior, and selectively vulnerable to neuroinflammation and neurodegeneration. The hippocampus is believed to be the main target of GCs since it has the highest density of GC receptors potentially underlying high sensitivity of hippocampal cells to severe stress. In this review, we analyzed the results of studies on pro- and anti-inflammatory effects of GCs in the hippocampus in different models of stress and stress-related pathologies. The available data form a sophisticated, though often quite phenomenological, picture of a modulatory role of GCs in hippocampal neuroinflammation. Understanding the dual nature of GC-mediated effects as well as causes and mechanisms of switching can provide us with effective approaches and tools to avert hippocampal neuroinflammatory events and as a result to prevent and treat brain diseases, both neurological and psychiatric. In the framework of a mechanistic view, we propose a new hypothesis describing how the anti-inflammatory effects of GCs may transform into the pro-inflammatory ones. According to it, long-term elevation of GC level or preliminary treatment with GC triggers accumulation of FKBP51 protein that suppresses activity of GC receptors and activates pro-inflammatory cascades, which, finally, leads to enhanced neuroinflammation.

An alternative splicing variant of mineralocorticoid receptor discovered in preeclampsia tissues and its effect on endothelial dysfunction

The pathophysiology of preeclampsia (PE) remains unclear. PE spiral artery remodeling dysfunction and PE offspring cardiovascular future development has been a worldwide concern. We collected placental and umbilical artery samples from nor-motensive and PE pregnancies. Mineralocorticoid receptor (MR) and its alternative splicing variant (ASV) expression and their biological effects on PE were examined. An MR ASV was found to be highly expressed in all PE samples and slightly expressed in about half of the normotensive samples (umbilical artery, ~57.58%; placenta, ~36.84%). The MR ASV expression was positively associated with blood pressure in both groups. The MR ASV protein changed the aldosterone-induced expression pattern of MR target genes related to ion exchanges and cell signaling pathways. The MR ASV can also impair the proliferation, migration, and tube formation ability of endothelial cells. These findings indicate that MR ASV in PE placenta plays a pathogenic role in PE pathophysiology, especially in endothelial dysfunction, and the existence of the MR ASV in PE umbilical artery provides a new direction in the study of PE offspring with increased risk of cardiovascular diseases.

HuR-Dependent Editing of a New Mineralocorticoid Receptor Splice Variant Reveals an Osmoregulatory Loop for Sodium Homeostasis

Aldosterone and the Mineralocorticoid Receptor (MR) control hydroelectrolytic homeostasis and alterations of mineralocorticoid signaling pathway are involved in the pathogenesis of numerous human diseases, justifying the need to decipher molecular events controlling MR expression level. Here, we show in renal cells that the RNA-Binding Protein, Human antigen R (HuR), plays a central role in the editing of MR transcript as revealed by a RNA interference strategy. We identify a novel Δ6 MR splice variant, which lacks the entire exon 6, following a HuR-dependent exon skipping event. Using isoform-specific TaqMan probes, we show that Δ6 MR variant is expressed in all MR-expressing tissues and cells and demonstrate that extracelullar tonicity regulates its renal expression. More importantly, this splice variant exerts dominant-negative effects on transcriptional activity of the full-length MR protein. Collectively, our data highlight a crucial role of HuR as a master posttranscriptional regulator of MR expression in response to osmotic stress. We demonstrate that hypotonicity, not only enhances MR mRNA stability, but also decreases expression of the Δ6 MR variant, thus potentiating renal MR signaling. These findings provide compelling evidence for an autoregulatory feedback loop for the control of sodium homeostasis through posttranscriptional events, likely relevant in renal pathophysiological situations.

Aldosterone resistance: structural and functional considerations and new perspectives

Aldosterone plays an essential role in the maintenance of fluid and electrolyte homeostasis in the distal nephron. Loss-of-function mutations in two key components of the aldosterone response, the mineralocorticoid receptor and the epithelial sodium channel ENaC, lead to type 1 pseudohypoaldosteronism (PHA1), a rare genetic disease of aldosterone resistance characterized by salt wasting, dehydration, failure to thrive, hyperkalemia and metabolic acidosis. This review describes the clinical, biological and genetic characteristics of the different forms of PHA1 and highlights recent advances in the understanding of the pathogenesis of the disease. We will also discuss genotype-phenotype correlations and new clinical and genetic entities that may prove relevant for patient's care in neonates with renal salt losing syndromes and/or failure to thrive.

Corticosteroid receptor signalling modes and stress adaptation in the brain

Adrenal glucocorticoid hormones modulate neuronal activity to support an adaptive response to stress. They modulate brain circuitry mediating physiological responses, emotion and cognitive processing. Chronically elevated glucocorticoid exposure is however linked to the development of mental disease. Glucocorticoid effects depend on mineralo- and glucocorticoid receptors, which are powerful transcription factors, but also can act via a diversity of non-genomic mechanisms. Here, I review generic factors that determine neuronal glucocorticoid sensitivity, in relation to brain function. First, pre-receptor mechanisms determine ligand availability. Second, there may be considerable variation in the receptor splice- and translation variants. Third, other transcription factors and many transcriptional coregulators interact with steroid receptors, determining nature and magnitude of steroid responses, in part through epigenetic regulation of DNA accessibility. Which factors underlie adaptive and pathogenic effects of stress hormones is largely unknown. Genome-wide identification of the receptor-DNA interactions in specific behavioural and physiological contexts provides a way of assessing the complete genomic range of glucocorticoid modes of action. Novel ligands that induce selective activation of particular receptor signalling modes will aid our understanding of receptor signalling and may allow selective targeting of glucocorticoid effects in emotional or cognitive domains, in research and, hopefully, in clinical settings.

Functional characterization of chicken glucocorticoid and mineralocorticoid receptors

Glucocorticoid (GR) and mineralocorticoid (MR) receptors are ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily. Little is known about the function of GR and MR in avian species. Recently, the chicken homologue of the GR (cGR) gene was cloned, and its tissue-specific expression was characterized, whereas the full-length sequence of the chicken MR (cMR) gene remains unknown. Therefore, the aims of this project were to clone the full-length cMR and to functionally characterize both chicken receptors. Cos-7 cells were transiently transfected with cGR or cMR expression vectors along with a glucocorticoid response element-luciferase (GRE-Luc) reporter construct. Transfected cells were then treated with increasing doses of corticosterone (CORT) or aldosterone (ALDO) alone and with GR or MR antagonists (ZK98299 and spironolactone, respectively). Transactivation of cGR or cMR was evaluated by luciferase assay. CORT and ALDO induced cGR- and cMR-driven transcriptional activity in a dose-dependent manner. Each receptor responded to both steroids, but cMR transcriptional activity was induced by lower levels of CORT and ALDO than cGR. Coexpression of both chicken corticosteroid receptors in Cos-7 cells had no synergistic or additive effect on CORT- or ALDO-induced transcriptional activity. Corticosteroid-dependent transactivation of cGR and cMR was partially blocked by antagonists. ZK98299 showed high specificity to cGR, while spironolactone had agonist properties toward both receptors. Immunocytochemistry was used to assess the cellular localization of both receptors. Corticosteroids induced translocation of both receptors into the nucleus. The functional properties of cGR and cMR determined in this study will be helpful in defining the physiological roles of GR and MR in avian species.

Characterization of conserved tandem donor sites and intronic motifs required for alternative splicing in corticosteroid receptor genes

Alternative splicing events from tandem donor sites result in mRNA variants coding for additional amino acids in the DNA binding domain of both the glucocorticoid (GR) and mineralocorticoid (MR) receptors. We now show that expression of both splice variants is extensively conserved in mammalian species, providing strong evidence for their functional significance. An exception to the conservation of the MR tandem splice site (an A at position +5 of the MR+12 donor site in the mouse) was predicted to decrease U1 small nuclear RNA binding. In accord with this prediction, we were unable to detect the MR+12 variant in this species. The one exception to the conservation of the GR tandem splice site, an A at position +3 of the platypus GRgamma donor site that was predicted to enhance binding of U1 snRNA, was unexpectedly associated with decreased expression of the variant from the endogenous gene as well as a minigene. An intronic pyrimidine motif present in both GR and MR genes was found to be critical for usage of the downstream donor site, and overexpression of TIA1/TIAL1 RNA binding proteins, which are known to bind such motifs, led to a marked increase in the proportion of GRgamma and MR+12. These results provide striking evidence for conservation of a complex splicing mechanism that involves processes other than stochastic spliceosome binding and identify a mechanism that would allow regulation of variant expression.

Stress and innate immunity in carp: corticosteroid receptors and pro-inflammatory cytokines

The stress hormone cortisol is deeply involved in immune regulation in all vertebrates. Common carp (Cyprinus carpio L.) express four corticoid receptors that may modulate immune responses: three glucocorticoid receptors (GR); GR1, with two splice variants (GR1a and GR1b), GR2 and a single mineralocorticoid receptor (MR). All receptors are expressed as of 4 days post-fertilization and may thus play a critical role in development and functioning of the adult immune system. Immune tissues and cells predominantly express mRNA for GRs compared to mRNA for the MR. Three-dimensional protein structure modeling predicts, and transfection assays confirm that alternative splicing of GR1 does not influence the capacity to induce transcription of effector genes. When tested for cortisol activation, GR2 is the most sensitive corticoid receptor in carp, followed by the MR and GR1a and GR1b. Lipopolysacharide (LPS) treatment of head kidney phagocytes quickly induces GR1 expression and inhibits GR2 expression. Cortisol treatment in vivo enhances GR1a and MR mRNA expression, but only mildly, and cortisol treatment in vitro does not affect receptor expression of phagocytes. Cortisol has no direct effect on the LPS-induced receptor profile. Therefore, an immune rather than a stress stimulus regulates GR expression. Cortisol administered at stress levels to phagocytes in vitro significantly inhibits LPS-induced expression of the pro-inflammatory cytokines tumor necrosis factor alpha (TNF-alpha) and interleukin-12 (IL-12) (subunit p35) and of inducible nitric oxide synthase (iNOS) expression. A physiologically differential function for GR1 and GR2 in the immune response of fish to infection is indicated.

Postnatal ontogeny of expression of the corticosteroid receptor genes in mammalian brains: inter-species and intra-species differences

Corticosteroids are important mediators of homeostasis and stress, and exert their effects via two transcription-factor receptors, mineralocorticoid receptor (MR) and glucocorticoid receptor (GR). Both receptors are expressed in the brain in a region-specific manner, and regulate neuroendocrine and behavioral functions. Stress during early development has been demonstrated to lead to long-term alterations in MR and GR levels and in the phenotypes that they mediate. To date, however, nearly all of this evidence has been obtained in rats, and there is actually no clear basis for extrapolation to other species. The current comparative review presents data, as available, on the following aspects of GR and MR gene expression in mouse and rat (Rodentia), tree shrew (Scandentia), common marmoset, squirrel monkey, rhesus macaque and human (Primates): (1) species-typical adult expression of MR mRNA and GR mRNA in hypothalamus, amygdala, hippocampus and neocortex; (2) species-typical neonate, infant, juvenile/adolescent and adult expression of MR mRNA and GR mRNA in hippocampus. (1) and (2) allow for identification of inter-species consistencies and differences in the relative levels of MR and GR expression across brain regions and ontogenetic stages. In addition, data are presented on (3) within-species inter-individual variation in MR and GR expression and causes thereof, including polymorphism and early life stress. Integrating the evidence in (1)-(3), it is noted that, should the expression levels of MR and GR at the time of early-life stress determine the latter's effects on the formers' long-term expression levels and functioning, then the long-term effects of early life stress on corticosteroid receptor expression and function will be species-, brain-region- and receptor-type-specific.

Molecular pathogenesis of renal pseudohypoaldosteronism type 1

Pseudohypoaldosteronism is a rare heterogeneous syndrome of mineralocorticoid resistance resulting in insufficient potassium and hydrogen secretion. Pseudohypoaldosteronism type 1 is characterized by mineralocorticoid resistance leading to neonatal salt loss, dehydration and failure to thrive. At least two different forms of pseudohypoaldosteronism type 1 can be distinguished, showing either a systemic or renal form of mineralocorticoid resistance. This review offers an overview on transepithelial sodium reabsorption and pseudohypoaldosteronism in general, and focuses on the underlying molecular pathology of the renal-restricted pseudohypoaldosteronism type 1 form caused by heterozygous mutations in the mineralocorticoid receptor-coding gene NR3C2. The investigation of several NR3C2 mutants in vitro has resulted in important progress in the understanding of the physiology of the mineralocorticoid receptor. However, there are still some families or individuals suffering from renal pseudohypoaldosteronism type 1 in whom no genetic defect was found in the NR3C2 or other genes such as SCNN1A, SCNN1B, SCNN1G, NEDD4 or SGK1 that are involved in the epithelial salt transport machinery. Further research in these cases may enable the identification of other pathologies leading to renal pseudohypoaldosteronism type 1 and permit deeper insights into the epithelial sodium reabsorption process.

Postnatal ontogeny of hippocampal expression of the mineralocorticoid and glucocorticoid receptors in the common marmoset monkey

The mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) are nuclear transcription factors that mediate many of the basal and stress functions and effects of the corticosteroid hormones, including those related to brain development. Despite this, relatively little is known about the postnatal ontogeny of MR and GR gene and protein expression in the central nervous system, and this is particularly true of the primates, including humans. Here we describe the postnatal ontogeny of central MR and GR gene and protein expression in the common marmoset monkey. By developing marmoset-specific riboprobes and using in situ hybridization, it was demonstrated that MR mRNA expression in the dentate gyrus and Ammon's horn was significantly greater in marmoset infants (aged 4-6 weeks) than in neonates (1-2 days), juveniles (4-5 months) and adults (3-6 years), with expression in the latter three ontogenetic stages being broadly similar. In the same subjects and ontogenetic stages, GR mRNA expression was developmentally consistent in the marmoset dentate gyrus and Ammon's horn, as well as in the paraventricular nucleus of the hypothalamus. Qualitative immunohistochemical comparison of infants and adults demonstrated that MR protein expression in the hippocampus was, as for mRNA, also greater in infants than adults, and that hippocampal GR protein was, as for mRNA, also similar in infants and adults. The increase in MR mRNA expression between the stages of neonate and infant co-occurred with a reduction in basal plasma ACTH and cortisol titres. The ontogenetic profiles of MR and GR gene expression in the marmoset monkey are therefore fundamentally different from those described for the rat and the mouse. This evidence for the postnatal ontogeny of central corticosteroid nuclear receptor expression in a primate is important for understanding both the developmental stage-specific significance of stress exposure and its potential long-term effects on health and disease.

11-deoxycorticosterone is a potent agonist of the rainbow trout (Oncorhynchus mykiss) mineralocorticoid receptor

The teleost fish are thought to lack the mineralocorticoid hormone aldosterone but possess mineralocorticoid receptor (MR) homologs. Here we describe the characterization of two rainbow trout (Oncorhynchus mykiss) MRs, called rtMRa and rtMRb. The open reading frame of rtMRa cDNA encoded a protein of 1041 amino acids. The rtMRb predicted protein sequence is similar, differing in only 10 amino acids in the nonconserved A/B domain and lacking a three-amino acid insertion between the two zinc fingers of the C domain. Expression of rtMR mRNA (sum of both forms), measured in juvenile trout by real-time RT-PCR, shows that the transcripts are ubiquitous. Expression was significantly higher in brain than the other tissues studied (eye, trunk kidney, head kidney, gut, gills, liver, spleen, ovary, heart, white muscle, skin). Hormonal stimulation of receptor transactivation activity was studied in COS-7 cells transiently cotransfected with receptor cDNA and a mouse mammary tumor virus-luciferase reporter. The mineralocorticoids 11-deoxycorticosterone and aldosterone were more potent enhancers of rtMRa transcriptional activity (EC50 = 1.6 +/- 0.5 x 10(-10) and 1.1 +/- 0.4 x 10(-10) M, respectively) than the glucocorticoids cortisol and 11-deoxycortisol (EC50 = 1.1 +/- 0.3 x 10(-9) and 3.7 +/- 1.9 x 10(-9) M, respectively). A similar response was observed in transactivation assays with rtMRb. These results are discussed in the view of reported circulating levels of corticosteroids in trout.

New Naturally Occurring Missense Mutations of the Human Mineralocorticoid Receptor Disclose Important Residues Involved in Dynamic Interactions with Deoxyribonucleic Acid, Intracellular Trafficking, and Ligand Binding

We have investigated the functional consequences of three naturally occurring amino acid substitutions of the human mineralocorticoid receptor (hMR). These mutations are located in the DNA-binding domain and the ligand-binding domain (LBD) and are associated with autosomal dominant or sporadic type I pseudohypoaldosteronism. All mutant receptors bound specifically to glucocorticoid-responsive elements but presented modified transcriptional properties. The DNA-binding domain mutant G633R, which possesses a normal affinity for a glucocorticoid-responsive element, displayed altered interaction with, and a reduced dissociation rate from, DNA. Its intracellular localization in the absence of hormone was predominantly nuclear in comparison with predominant cytoplasmic location of hMR. Hormone-dependent nuclear cluster formation was comparable to wild-type hMR. These results and the three-dimensional modeling of the interaction of R633 with DNA suggest that altered interaction dynamics with DNA as well as modified intracellular localization may be responsible for submaximal transcriptional potency of hMR. Two LBD mutations, Q776R and L979P, were also investigated. Our data confirm the fundamental role of amino acid Q776 for anchoring the C3 ketone group of steroids in the ligand-binding pocket. Analysis of LBD conformation of mutant P979 demonstrates the relevance of hydrophobic interactions in the extreme C-terminal tail of the hMR for the correct ligand-binding competent state of the receptor. Our data underline the importance of studying naturally occurring mutants to identify crucial residues involved in hMR function.

The origin and functions of multiple human glucocorticoid receptor isoforms

Glucocorticoid hormones are necessary for life and are essential in all aspects of human health and disease. The actions of glucocorticoids are mediated by the glucocorticoid receptor (GR), which binds glucocorticoid hormones and regulates gene expression, cell signaling, and homeostasis. Decades of research have focused on the mechanisms of action of one isoform of GR, GRa. However, in recent years, increasing numbers of human GR (hGR) isoforms have been reported. Evidence obtained from this and other laboratories indicates that multiple hGR isoforms are generated from one single hGR gene via mutations and/or polymorphisms, transcript alternative splicing, and alternative translation initiation. Each hGR protein, in turn, is subject to a variety of posttranslational modifications, and the nature and degree of posttranslational modification affect receptor function. We summarize here the processes that generate and modify various hGR isoforms with a focus on those that impact the ability of hGR to regulate target genes. We speculate that unique receptor compositions and relative receptor proportions within a cell determine the specific response to glucocorticoids. Unchecked expression of some isoforms, for example hGRbeta, has been implicated in various diseases.

Inactivating mutations of the mineralocorticoid receptor in Type I pseudohypoaldosteronism

Type I pseudohypoaldosteronism (PHA1) is a rare form of mineralocorticoid resistance characterized by neonatal renal salt wasting and failure to thrive. Typical biochemical features include high levels of plasma aldosterone and renin, hyponatremia and hyperkalemia. Different mutations of the human mineralocorticoid receptor (hMR) gene have been identified in subjects affected by the autosomal dominant or sporadic form of the disease. Our laboratory has investigated a large number of subjects with familial and sporadic PHA1. Several different mutations have been detected, which are localized in different coding exons of the hMR gene. These mutations either create truncated proteins, either affect specific amino acids involved in receptor function. In this paper, we review hMR mutations described to date in PHA1 and their functional characterization. We discuss the absence of mutations in some kindreds and the role of precise phenotypic and biological examination of patients to allow for identification of other genes potentially involved in the disease.

Corticosteroid receptors, 11 beta-hydroxysteroid dehydrogenase, and the heart

Mineralocorticoid and glucocorticoid hormones are known as corticosteroid hormones and are synthesized mainly in the adrenal cortex; however, more recently the enzymes involved in their synthesis have been found in a variety of cells and tissues, including the heart. The effects of these hormones are mediated via both cytoplasmic mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs), which act as ligand-inducible transcription factors. In addition, rapid, nongenomically mediated effects of these steroids can occur that may be via novel corticosteroid receptors. The lipophilic nature of these hormones allows them to pass freely through the cell membrane, although the intracellular concentration of mineralocorticoids and glucocorticoids is dependent on several cellular factors. The main regulators of intracellular glucocorticoid levels are 11 beta-hydroxysteroid dehydrogenase (11 beta HSD) isoforms. 11 beta HSD1 acts predominantly as a reductase in vivo, facilitating glucocorticoid action by converting circulating receptor-inactive 11-ketoglucocorticoids to active glucocorticoids. In contrast, 11 beta HSD 2 acts exclusively as an 11 beta-dehydrogenase and decreases intracellular glucocorticoids by converting them to their receptor-inactive 11-ketometabolites. Furthermore, P-glycoproteins, by actively pumping steroids out of cells, can selectively decrease steroids and local steroid synthesis can increase steroid concentrations. Receptor concentration, receptor modification, and receptor-protein interactions can also significantly impact on the corticosteroid response. This review details the receptors and possible mechanisms involved in both mediating and modulating corticosteroid responses. In addition, direct effects of corticosteroids on the heart are described including a discussion of the corticosteroid receptors and the mechanisms involved in mediating their effects.

Different inactivating mutations of the mineralocorticoid receptor in fourteen families affected by type I pseudohypoaldosteronism

We have analyzed the human mineralocorticoid receptor (hMR) gene in 14 families with autosomal dominant or sporadic pseudohypoaldosteronism (PHA1), a rare form of mineralocorticoid resistance characterized by neonatal renal salt wasting and failure to thrive. Six heterozygous mutations were detected. Two frameshift mutations in exon 2 (insT1354, del8bp537) and one nonsense mutation in exon 4 (C2157A, Cys645stop) generate truncated proteins due to premature stop codons. Three missense mutations (G633R, Q776R, L979P) differently affect hMR function. The DNA binding domain mutant R633 exhibits reduced maximal transactivation, although its binding characteristics and ED(50) of transactivation are comparable with wild-type hMR. Ligand binding domain mutants R776 and P979 present reduced or absent aldosterone binding, respectively, which is associated with reduced or absent ligand-dependent transactivation capacity. Finally, P979 possesses a transdominant negative effect on wild-type hMR activity, whereas mutations G633R and Q776R probably result in haploinsufficiency in PHA1 patients. We conclude that hMR mutations are a common feature of autosomal dominant PHA1, being found in 70% of our familial cases. Their absence in some families underscores the importance of an extensive investigation of the hMR gene and the role of precise diagnostic procedures to allow for identification of other genes potentially involved in the disease.

Peptide insertion in the DNA-binding domain of fish glucocorticoid receptor is encoded by an additional exon and confers particular functional properties

The trout glucocorticoid receptor (rtGR) contains an additional sequence of nine amino acids located between the two zinc fingers of the DNA-binding domain (DBD) (Endocrinology 136 (1995) 3774). Polymerase chain reaction on trout genomic DNA and sequencing were performed in the DBD region, demonstrating that this peptide is encoded by an additional exon of 27 nucleotides between the two exons encoding the two zinc fingers of other nuclear receptors. This additional sequence in the rtGR confers a better binding affinity of the receptor to a single GRE, as shown by gel shift experiments with GST-DBDrtGR fusion proteins, deleted or not of the nine amino acids (Delta9). This higher affinity is correlated with a higher constitutive transcriptional activity of the receptor on a reporter gene driven by a single GRE, but not with the ligand-induced transcriptional activity. Nevertheless, on a double GRE, the wild type and rtGR-Delta9 are equally active on both constitutive or dexamethasone-induced transcriptional activity. This original DBD structure could have emerged during evolution such as to allow better regulation of glucocorticoid dependent genes in relation to the large spectrum of cortisol physiological functions in fish.

New ideas about aldosterone signaling in epithelia

The systemic actions of aldosterone are well documented; however, in comparison, our understanding of the cellular and molecular mechanisms by which aldosterone orchestrates these actions is rudimentary. Aldosterone exerts most of its physiological actions by modifying gene expression. It is now apparent that aldosterone represses almost as many genes as it induces. Several aldosterone-sensitive genes, including serum and glucocorticoid-inducible kinase (sgk) and small, monomeric Kirsten Ras GTP-binding protein (Ki-ras) have recently been identified. The molecular mechanisms and elements bestowing corticosteroid sensitivity on these and many other genes are becoming clear. Induction of Ki-Ras and Sgk is necessary and sufficient for some portion of aldosterone action in epithelia. These two signaling factors are components of a converging pathway with phosphatidylinositol 3-kinase positioned between them that enables both stabilizing the epithelial Na(+) channel (ENaC) in the open state as well as increasing the number of ENaC in the apical membrane. This aldosterone-induced signaling pathway contains many potential sites for feedback regulation and cross talk from other cascades and potentially impinges directly on the activity of transport proteins and/or cellular differentiation to modify electrolyte transport.

A new human MR splice variant is a ligand-independent transactivator modulating corticosteroid action

Aldosterone effects are mediated by the MR, which possesses the same affinity for mineralocorticoids and glucocorticoids. In addition to the existence of mechanisms regulating intracellular hormone availability, we searched for human MR splice variants involved in tissue-specific corticosteroid function. We have identified a new human MR isoform, hMRDelta5,6, resulting from an alternative splicing event skipping exons 5 and 6 of the human MR gene. hMRDelta5,6 mRNAs are expressed in several human tissues at different levels compared with wild-type human MR, as shown by real time PCR. Introduction of a premature stop codon results in a 75-kDa protein lacking the entire hinge region and ligand binding domain. Interestingly, hMRDelta5,6 is still capable of binding to DNA and acts as a ligand-independent transactivator, with maximal transcriptional induction corresponding to approximately 30-40% of aldosterone-activated wild-type human MR. Coexpression of hMRDelta5,6 with human MR or human GR increases their transactivation potential at high doses of hormone. Finally, hMRDelta5,6 is able to recruit the coactivators, steroid receptor coactivator 1, receptor interacting protein 140, and transcription intermediary factor 1alpha, which enhance its transcriptional activity. Ligand-independent transactivation and enhancement of both wild-type MR and GR activities by hMRDelta5,6 suggests that this new variant might play a role in modulating corticosteroid effects in target tissues.

Role of central mineralocorticoid receptors in cardiovascular disease

Mineralocorticoids act directly through their receptors in specific centers in the central nervous system, kidneys, heart, and vascular smooth muscle to mediate hemodynamic homeostasis. These steroids also modulate renal and cardiovascular function indirectly through the autonomic nervous system. Complex homeostatic mechanisms under normal hormonal control become pathogenic when there is an excess of regulatory hormone. Experiments in which mineralocorticoid receptor antagonists or antisense oligodeoxynucleotides were administered centrally have clearly shown that centrally mediated effects on salt appetite, baroreceptor function, and autonomic drive to the renal and cardiovascular systems are crucial to the pathogenesis of hypertension and cardiovascular disease of hyperaldosteronism, and certain forms of genetic hypertension.

An alternatively spliced rat mineralocorticoid receptor mRNA causing truncation of the steroid binding domain

We attempted to clone the putative 11-dehydrocorticosterone receptor by RT-PCR with two degenerate primers from highly homologous regions of the DNA and steroid binding domains of the receptor subfamily. In doing so, we have identified an alternatively spliced variant mRNA of the rat mineralocorticoid (MR) with a ten bp deletion in the C-terminal steroid binding domain. This deletion results in a truncated MR receptor of 807 amino acids in comparison to the wild type of 981 amino acids. The deletion variant was expressed in colon, kidney, heart, liver, aorta and brain tissues. The relative abundance of the deletion variant compared to the wild type MR was estimated to be 6% in rat kidney and 4% in hippocampus. This deletion was also detected in human kidney by RT-PCR. Site-directed mutagenesis was used to create the eukaryotic expression plasmid pCR3-rMRdel10 from the wild type for a transactivation assay using the luciferase reporter system in CV-1 cells. The deletion variant had the same baseline transactivation activity as the wild type MR, but did not respond to aldosterone or corticosterone stimulation. Co-transfection of MR with the deletion variant had no significant effect on transactivation activity of the MR, indicating that the deletion variant is unlikely to serve as a negative regulator of MR function.

In vivo tissue specific modulation of rat insulin receptor gene expression in an experimental model of mineralocorticoid excess

Insulin receptor (IR) gene expression at the mRNA level was investigated in hindlimb skeletal muscle, epididymal adipose tissue and in the liver of rats exposed to prolonged in vivo administration of deoxycorticosterone acetate (DOCA). Following treatment, plasma insulin levels were reduced while glucose levels increased compared to values in control rats. DOCA-treated animals showed an increase in blood pressure and a reduction in body weight. This treatment also induced hypokalemia and decreased plasma protein levels. Sodium levels were unaffected. Moreover, no differences in DNA and protein content or in the indicator of cell size (protein/DNA) were observed in the skeletal muscle or adipose tissue of animals. In contrast, there was a clear increase in the protein and DNA contents of the liver with no change in the indicator of cell size. Northern blot assays revealed 2 major IR mRNA species of approximately 9.5 and 7.5 Kb in the 3 tissues from control animals. DOCA treatment induced no change in the levels of either RNA species in skeletal muscle. However, a decrease of approximately 22% was detected in the levels of both species in adipose tissue whereas the liver showed an increase of 64%. These results provide the first evidence for an in vivo tissue-specific modulation of IR mRNA levels under experimental conditions of mineralocorticoid excess.

Brain corticosteroid receptor balance in health and disease

In this review, we have described the function of MR and GR in hippocampal neurons. The balance in actions mediated by the two corticosteroid receptor types in these neurons appears critical for neuronal excitability, stress responsiveness, and behavioral adaptation. Dysregulation of this MR/GR balance brings neurons in a vulnerable state with consequences for regulation of the stress response and enhanced vulnerability to disease in genetically predisposed individuals. The following specific inferences can be made on the basis of the currently available facts. 1. Corticosterone binds with high affinity to MRs predominantly localized in limbic brain (hippocampus) and with a 10-fold lower affinity to GRs that are widely distributed in brain. MRs are close to saturated with low basal concentrations of corticosterone, while high corticosterone concentrations during stress occupy both MRs and GRs. 2. The neuronal effects of corticosterone, mediated by MRs and GRs, are long-lasting, site-specific, and conditional. The action depends on cellular context, which is in part determined by other signals that can activate their own transcription factors interacting with MR and GR. These interactions provide an impressive diversity and complexity to corticosteroid modulation of gene expression. 3. Conditions of predominant MR activation, i.e., at the circadian trough at rest, are associated with the maintenance of excitability so that steady excitatory inputs to the hippocampal CA1 area result in considerable excitatory hippocampal output. By contrast, additional GR activation, e.g., after acute stress, generally depresses the CA1 hippocampal output. A similar effect is seen after adrenalectomy, indicating a U-shaped dose-response dependency of these cellular responses after the exposure to corticosterone. 4. Corticosterone through GR blocks the stress-induced HPA activation in hypothalamic CRH neurons and modulates the activity of the excitatory and inhibitory neural inputs to these neurons. Limbic (e.g., hippocampal) MRs mediate the effect of corticosterone on the maintenance of basal HPA activity and are of relevance for the sensitivity or threshold of the central stress response system. How this control occurs is not known, but it probably involves a steady excitatory hippocampal output, which regulates a GABA-ergic inhibitory tone on PVN neurons. Colocalized hippocampal GRs mediate a counteracting (i.e., disinhibitory) influence. Through GRs in ascending aminergic pathways, corticosterone potentiates the effect of stressors and arousal on HPA activation. The functional interaction between these corticosteroid-responsive inputs at the level of the PVN is probably the key to understanding HPA dysregulation associated with stress-related brain disorders. 5. Fine-tuning of HPA regulation occurs through MR- and GR-mediated effects on the processing of information in higher brain structures. Under healthy conditions, hippocampal MRs are involved in processes underlying integration of sensory information, interpretation of environmental information, and execution of appropriate behavioral reactions. Activation of hippocampal GRs facilitates storage of information and promotes elimination of inadequate behavioral responses. These behavioral effects mediated by MR and GR are linked, but how they influence endocrine regulation is not well understood. 6. Dexamethasone preferentially targets the pituitary in the blockade of stress-induced HPA activation. The brain penetration of this synthetic glucocorticoid is hampered by the mdr1a P-glycoprotein in the blood-brain barrier. Administration of moderate amounts of dexamethasone partially depletes the brain of corticosterone, and this has destabilizing consequences for excitability and information processing. 7. The set points of HPA regulation and MR/GR balance are genetically programmed, but can be reset by early life experiences involving mother-infant interaction. 8. (ABSTRACT TRUNCATED)

Corticosteroids in the brain. Cellular and molecular actions

The rat adrenal hormone corticosterone reaches the brain and binds to intracellular receptors. These receptors comprise high-affinity mineralocorticoid and lower-affinity glucocorticoid receptors that, upon activation, affect the transcription rate of specific genes. The two receptor types are discretely localized in the brain, with particularly high expression levels in the hippocampus. Here we review recent studies showing that electrical properties and structural aspects of hippocampal principal neurons are specifically regulated by mineralocorticoid- or glucocorticoid-receptor activation. The molecular mechanisms by which these cellular effects could be accomplished are discussed.

Identification of a splice variant of the rat estrogen receptor beta gene

Recently a second estrogen receptor termed estrogen receptor beta (ERbeta) has been cloned and characterized, and shown to be expressed at the highest levels in ovarian granulosa cells and prostatic epithelium. In the course of amplifying a region of the ligand-binding domain of the rat ERbeta cDNA we identified a second, larger transcript which appears to arise through differential splicing. The second isoform has 54 nucleotides inserted after position 1372 encoding 18 additional amino acids. Both isoforms are expressed at similar relative abundance in a range of tissues.