A functional promoter directing expression of a novel type of rat mineralocorticoid receptor mRNA in brain

N-terminal truncations in sex steroid receptors and rapid steroid actions

Sex steroid receptors act as ligand activated nuclear transcription factors throughout the body, including the brain. However, post-translational modification of these receptors can direct them to extranuclear sites, including the plasma membrane, where they are able to initiate rapid signaling. Because of the conserved domain structure of these receptors, alternative exon splicing can result in proteins with altered nuclear and extranuclear actions. Although much attention has focused on internal and C-terminal splice variants, both estrogen and androgen receptors undergo N-terminal truncations, as well. These truncated proteins not only influence the transcriptional activity of the full-length receptors, but also associate with caveolin and initiate signaling at the plasma membrane. Such actions may have important physiological consequences in neuronal, endothelial, and cancer signaling and cell survival.

Temporal control of glucocorticoid neurodynamics and its relevance for brain homeostasis, neuropathology and glucocorticoid-based therapeutics

Glucocorticoids mediate plethora of actions throughout the human body. Within the brain, they modulate aspects of immune system and neuroinflammatory processes, interfere with cellular metabolism and viability, interact with systems of neurotransmission and regulate neural rhythms. The influence of glucocorticoids on memory and emotional behaviour is well known and there is increasing evidence for their involvement in many neuropsychiatric pathologies. These effects, which at times can be in opposing directions, depend not only on the concentration of glucocorticoids but also the duration of their presence, the temporal relationship between their fluctuations, the co-influence of other stimuli, and the overall state of brain activity. Moreover, they are region- and cell type-specific. The molecular basis of such diversity of effects lies on the orchestration of the spatiotemporal interplay between glucocorticoid- and mineralocorticoid receptors, and is achieved through complex dynamics, mainly mediated via the circadian and ultradian pattern of glucocorticoid secretion. More sophisticated methodologies are therefore required to better approach the study of these hormones and improve the effectiveness of glucocorticoid-based therapeutics.

The mineralocorticoid signaling pathway throughout development: expression, regulation and pathophysiological implications

The mineralocorticoid signaling pathway has gained interest over the past few years, considering not only its implication in numerous pathologies but also its emerging role in physiological processes during kidney, brain, heart and lung development. This review aims at describing the setting and regulation of aldosterone biosynthesis and the expression of the mineralocorticoid receptor (MR), a nuclear receptor mediating aldosterone action in target tissues, during the perinatal period. Specificities concerning MR expression and regulation during the development of several major organs are highlighted. We provide evidence that MR expression is tightly controlled in a tissue-specific manner during development, which could have major pathophysiological implications in the neonatal period.

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.

Defense of adrenocorticosteroid receptor expression in rat hippocampus: effects of stress and strain

Neuronal mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) proteins are glucocorticoid-activated transcription factors that bind identical DNA response elements yet transduce distinct physiological/transcriptional actions. The present study assessed regulation of adrenocorticosteroid receptor RNA and protein following intermittent stress exposure, using Sprague Dawley (S-D) and stress-hyperresponsive Fischer 344 (F344) rat strains. The F344 (but not S-D) strain showed enhanced acute stress responsivity and enhanced corticosterone secretion following prolonged stress. F344 rats also showed reduced responsiveness to a novel stressor after prolonged stress exposure, suggestive of enhanced glucocorticoid negative feed-back. Upon prolonged stress, F344 rats down-regulated MR hnRNA in CA1, CA3, and dentate gyrus. Transcriptional changes were accompanied by decreased expression of the alpha 5' messenger RNA (mRNA) form, consistent with altered promoter utilization. In contrast, alpha 5' splice variant, full-length mRNA, and MR protein expression were not affected by stress in either strain, implying that transcriptional changes do not affect overall mRNA or protein expression. GR protein was increased in pyramidal and granule cell somata/nuclei of F344 rats despite lack of a change in mRNA expression. These data suggest that prolonged stress elicits restricted changes in MR and GR expression in the F344 strain only. Overall, stable expression of adrenocorticosteroid receptors is rigorously defended in hippocampal neurons, apparently through transcriptional and posttranscriptional mechanisms.

Multiple promoters direct the tissue-specific expression of novel N-terminal variant human vitamin D receptor gene transcripts

The effects of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] are mediated by the vitamin D receptor (VDR), a member of the nuclear receptor superfamily of transcriptional regulators. We have identified upstream exons of the human (h) VDR gene that are incorporated into variant transcripts, two of which encode N-terminal variant receptor proteins. Expression of the hVDR gene, which spans more than 60 kb and consists of at least 14 exons, is directed by two distinct promoters. A tissue-specific distal promoter generates unique transcripts in tissues involved in calcium regulation by 1, 25-(OH)2D3 and can direct the expression of a luciferase reporter gene in a cell line-specific manner. These major N-terminal differences in hVDR transcripts, potentially resulting in structural differences in the expressed receptor, may contribute to cellular responsiveness to 1,25-(OH)2D3 through tissue differences in the regulation of VDR expression.

Alpha, beta, and gamma mineralocorticoid receptor messenger ribonucleic acid splice variants: differential expression and rapid regulation in the developing hippocampus

Two different types of corticoid receptor molecules bind circulating corticosterone in brain: mineralocorticoid receptors (MR) and glucocorticoid receptors. MR exhibit the highest affinity for the endogenous glucocorticoid in the rat, corticosterone. During development, low corticosterone levels influence neurogenesis, and these effects are probably MR mediated. Three MR complementary DNA clones, alpha, beta, and gamma, have been identified in the rodent. All of these MR complementary DNA clones have identical coding regions, but differ significantly at the 5'-untranslated end. Although the functional significance of these three messenger RNA (mRNA) species remains unknown, one hypothesis is that they reflect the ability of the brain to regulate the expression of MR, allowing multiple factors to differentially control transcription in a tissue- and time-specific manner. To investigate this possibility, we examined the presence of these distinct mRNA forms in the developing rat hippocampus (HC). In situ hybridization with specific alpha, beta, and gamma complementary RNA probes was performed in the HC of 3-, 5-, 7-, 12-, 14-, 28-, 35-, and 65-day-old animals. We found that there is differential expression of these forms in each of the HC subfields from infancy to adulthood. y expression appears to be associated with periods of cell birth and increased axonal sprouting. beta expression, on the other hand, may be best linked to periods of synaptogenesis, growth of commissural and associative terminal fields, and possibly active pruning. To explore the possibility that the differential gene expression may be related to corticosterone environment, adrenalectomy was performed. A rapid modulation of the MR mRNA variants (14 h) in an age- and site-specific fashion was seen. These findings suggest that the variation in expression and regulation during development of the multiple MR transcripts could reflect a complex pattern of developmental regulation that may involve a multitude of factors unique to each postnatal age and to the different neuronal populations within the hippocampal formation.

Influence of gonadal steroids on brain corticosteroid receptors: a minireview

Sex differences exist in the functioning of the two brain corticosteroid receptor systems. Ovarian steroid replacement alters receptor mRNA expression, receptor binding capacities, and receptor affinity. The abundance of both mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) message can be reduced by estrogen. Progesterone is able to partially antagonize the action of estrogen and to induce MR transcription. The effect of estrogen on receptor binding capacity is more modest than its transcriptional actions. Estrogen decreases MR binding more reliably than it does GR. Progesterone has high affinity for the MR and can substantially reduce MR affinity for corticoids. Androgen apparently regulates corticoid receptor transcription but may not affect binding capacity. Estrogen and androgen are both more potent in regulating pituitary-adrenal function than would be suggested by their actions on receptor binding parameters.

Distribution of adrenocorticoid receptors in the rat CNS measured by competitive PCR and cytosolic binding

Combined quantitative polymerase chain reaction (PCR) and cytosolic binding assay techniques are used to measure mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) mRNA, Kd, and Bmax in various rat central nervous system (CNS) regions, namely amygdala, hypothalamus, hippocampus, cortex, pituitary, and cervical, thoracic, and lumbar spinal cord. Two internal standards (i.s.) cDNA were cloned for quantitative PCR purposes. The i.s. templates differed from the respective wild-type (wt) templates for a single base-pair mutation introduced by PCR that generated a unique restriction site, thus allowing amplification products arising from coamplification of wt and i.s. to be distinguished. Results show that cerebellum, which displayed average Bmax values for both receptors, contained the highest level of MR and GR mRNA. Hippocampus also had a high level of MR mRNA. Low mRNA content was found in the hypothalamus for MR and GR as well as in the cortex for GR. High Bmax values for both MR and GR were found in the lumbar spinal cord, despite a modest mRNA content. The lowest Bmax values were found in the cortex for both receptors. It is, therefore, concluded that mRNA content and Bmax are not closely correlated in the rat CNS. These data suggest a differential regulation of various adrenocorticoid receptor isoforms. Moreover, this quantitative PCR method is very sensitive and can be used to assay small amounts of material in order to obtain absolute measurements of mRNA expression.

Ketanserin selectively blocks acute stress-induced changes in NGFI-A and mineralocorticoid receptor gene expression in hippocampal neurons

Serotonin and glucocorticoids interact at the hippocampus to alter neuronal function. Serotonin and antidepressant drugs increase glucocorticoid receptor and mineralocorticoid receptor gene expression in hippocampal neurons over a few days. The effects of serotonin are mediated via ketanserin-sensitive "serotonin-2 type" receptors and induction of cyclic AMP, although the subsequent molecular mechanisms are unclear. Recently, we have shown that chronic environmental manipulations which induce glucocorticoid receptor gene expression in specific hippocampal subfields of the rat are associated with congruent induction of the transcription factor NGFI-A (zif268, krox24, egr-1) and repression of AP-2; both factors may bind to the glucocorticoid receptor gene promoter. However, any relationship between serotonin and these transcription factors is unknown. Here, we show that acute restraint stress, which causes serotonin release at the hippocampus, induces hipppocampal NGFI-A, but represses activator protein-2 and mineralocorticoid receptor gene expression within 90 min. These changes are sustained for 4 h, but not 12 h. Ketanserin attenuates the stress-induced rise in NGFI-A and fall in mineralocorticoid receptor gene expression, and partly also the fall in AP-2 messenger RNA expression. These data suggest that restraint stress, acting via serotonin release and ketanserin-sensitive serotonin receptors, produces rapid, transient and specific changes in transcription factor gene expression in hippocampal neurons. Any link between these effects and the control of glucocorticoid and mineralocorticoid receptor expression with chronic serotonin or antidepressant treatment remains to be elucidated.

Dopaminergic regulation of AP-1 transcription factor DNA binding activity in rat striatum

Dopaminergic modulation of the DNA binding activity of AP-1, Sp1, CREB and AP-2 transcription factors was examined in rat striatal nuclear extracts by gel shift assay. AP-1 binding was selectively increased in the striatum following depletion of dopamine by 6-hydroxydopamine-induced lesion of the nigrostriatal pathway or after reserpine treatment. The D1 agonist SKF 38393 dose-dependently increased AP-1 binding; this effect was significantly increased in reserpine-treated rats and even more markedly enhanced in denervated striatum. The D2/D3 agonist quinpirole, administered alone, did not affect striatal activator protein-1 binding; in combination, quinpirole and SKF 38393 acted synergistically in normal and reserpine-treated rats but not in 6-hydroxydopamine-lesioned rats, suggesting that mechanisms underlying D1-D2/D3 interactions are altered after dopamine denervation. Most, but not all, of the changes in AP-1 binding activity observed in this study are consistent with changes in levels of Fos/Jun family proteins observed after similar treatments. These results support the hypothesis that D1 receptor stimulation activates striatonigral neurons and modulates expression of AP-1-related genes in these neurons, while D2 receptor stimulation mediates tonic inhibition of AP-1 expression and activity in the striatopallidal neurons. Moreover, the findings provide evidence that the loss of dopaminergic input to the striatum, as occurs in Parkinson's disease, induces long-lasting alterations in the regulation of striatal gene expression which may contribute to the disease's progress.

The human mineralocorticoid receptor gene promoter: its structure and expression

The structure and expression of a clone containing the promoter region, all of exon 1, and part of the first intron of the human mineralocorticoid receptor (hMR) gene is presented. The clone has three sets of CAAT and TATA elements, one located at the very 5'-end of the clone, one located just 5'- to the start of transcription, and one set located in intron A, approximately 300 bp into the intron. The major start of transcription site by primer extension analysis and ribonuclease protection assays is located 26 bp downstream of a TATA-like box (TTTAA) and 90 and 143 bp downstream, respectively, of two CCAAT boxes. Putative cis-transcription factor binding sites are as follows: two potential AP1 sites, one potential AP2 site, two ATF/CREB sites, six potential GC boxes or SP1 sites, one potential perfect half-palindromic estrogen response element, and three potential PEA3 sites. Therefore, the hMR promoter region contains elements characteristic of both regulated genes and "housekeeping" genes. CAT assays of overlapping deletions of the promoter region demonstrated tissue-specific regulation in human neuroepithelioma (SK-N-MC-IXC) and non-neuronal, peripheral choriocarcinoma cell lines (JEG-3).

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

The sequence of a splice variant of the rat mineralocorticoid receptor (MR) gene is presented. A cDNA clone corresponding to rat MR was isolated from a rat brain cDNA library. Sequence analysis of the region corresponding to the DNA binding domain revealed the presence of a 12 base pair (bp) insertion. Analysis of mRNA from several rat tissues suggests that the variant is less abundant than the wild type in most tissues. The insertion variant is also a product of the human MR gene, the identical splice variant was also observed in human white blood cell mRNA. Unlike other splice variants reported for the MR, this variant alters the encoded protein by the addition of four amino acid residues in the DNA binding domain. The altered protein may influence the affinity of the MR for mineralocorticoid or glucocorticoid response elements.

Human mineralocorticoid receptor genomic structure and identification of expressed isoforms

Most of the known effects of aldosterone are mediated by the mineralocorticoid receptor, an intracellular receptor belonging to the steroid/thyroid hormone/retinoic acid receptor superfamily. We determined the genomic structure of the human MR (hMR) and identified 10 exons in the gene, including two exons (1 alpha and 1 beta) that encode different 5'-untranslated sequence. Expression of the two different hMR variants is under the control of two different promoters that contain no obvious TATA element, but multiple GC boxes. Our results indicate that hMR expression is regulated by alternative promoters perhaps in a tissue- or developmental-specific manner.

Stress regulation of mineralocorticoid receptor heteronuclear RNA in rat hippocampus

Localization and regulation of mineralocorticoid receptor (MR) heteronuclear RNA (hnRNA) was assessed in rat hippocampus using an intron-directed in situ hybridization approach. The presence of hnRNA in the cell nucleus reflects recent gene transcriptional events and can be used as an index of neuronal transcriptional activation or inhibition. In the present study, sections incubated with an MR intron probe labeled cells in all regions known to contain MR mRNA. Signal generated by the intron probe was localized specifically to the nuclear compartment, consistent with recognition of hnRNA. Analysis of distribution across hippocampus indicated that MR hnRNA was particularly abundant in dentate gyrus (DG). In contrast, MR mRNA shows similar levels of expression across all hippocampal subfields. Exposure of animals to acute restraint markedly reduced MR hnRNA levels in CA1 and DG 60 min and 120 min following stress exposure, consistent with reduced gene transcription. These data support the hypothesis that stress-induced glucocorticoid secretion can rapidly affect transcription of steroid receptor genes. MR mRNA levels did not decrease over comparable time periods, suggesting either a significant lag-time between transcriptional changes and changes in cytoplasmic mRNA pools or stimulus-driven alterations in post-transcriptional RNA processing.

Corticosteroid receptors and the central nervous system

In mammalian systems, the physiological mineralocorticoid is aldosterone (aldo), and the physiological glucocorticoid cortisol (F), or corticosterone (B) in rats and mice. Receptors (MR) with high affinity for aldo, B and F are found in both epithelia and the central nervous system (CNS); receptors (GR) with lower affinity for F and B, and still lower for aldo, are found in essentially all cells. Both MR and GR bind to and activate canonical pentadecamer response elements in transfected cells and in epithelia, wherein MR aldo, B and F all act as agonists. In vivo, in epithelial cells a low Km, NAD-dependent, 11 beta hydroxysteroid dehydrogenase (11 beta OHSD) converts B and F, but not aldo, to receptor-inactive 11-keto congeners, thus allowing aldo to occupy epithelial MR and produce sodium retention. The CNS differs markedly in terms of MR/GR in a number of ways: (i) most but not all MR in the CNS are functionally unprotected, despite the presence of a low Km, NADP-preferring 11 beta OHSD, so that they operate as high-affinity GR; (ii) in such CNS 'MR', aldo antagonizes the effects of B, and vice versa, in contrast with epithelia; (iii) also in contrast with epithelia, activated GR in the CNS do not mimic activated MR, suggesting considerable if not total specificity at the response element level. These differences suggest that glucocorticoids have two distinct domains of action in the CNS, mediated by 'MR' at low B/F concentrations, and GR at higher concentrations; secondly, they suggest that the nuclear recognition and response elements mediating these effects are other than canonical pentadecamer sequences.

Mineralocorticoid and glucocorticoid receptors in the brain. Implications for ion permeability and transmitter systems

In this review we have argued that corticosteroid hormones represent an endocrine signal that can influence neuronal communication. The steroids bind to intracellular receptors in the brain, resulting in slow effects that involve gene transcription, but they may also evoke rapid effects via membrane receptors. The signal carried by the corticosteroids is therefore divergent with respect to the dimension of space and time. Within the rat brain, at least two intracellular receptor subtypes, i.e. MRs and GRs, bind corticosterone. The affinity, density and localization of the MRs is different from the GRs, although the actual properties may vary somewhat depending on the condition of the animal. In general, due to the difference in affinity, low corticosteroid levels result in a predominant MR occupation, while higher steroid levels additionally occupy GRs. Recent studies indicate that predominant MR occupation is important for the maintenance of ongoing transmission in certain brain regions and for neuroprotection. By contrast, additional GR occupation (for a limited period of time) results in an attenuation of local excitability; yet, prolonged exposure to high steroid levels may become an endangering condition for neurons. Since predominant MR occupation on the one hand and additional GR occupation on the other hand induce different cellular actions, the ratio of MR/GR occupation is an important factor determining the net effect of corticosteroid hormones in the brain. How coordinated MR- and GR-mediated effects control neuronal communication under various physiological and pathological conditions will be a challenge for future research.