Aldosterone binding in isolated tubules II. An autoradiographic study of concentration dependency in the rabbit nephron

Cnksr3 is a direct mineralocorticoid receptor target gene and plays a key role in the regulation of the epithelial sodium channel

Aldosterone is the principal hormonal regulator of sodium homeostasis in vertebrates. It exerts its actions through the mineralocorticoid receptor (MR) that regulates the transcription of specific target genes. In recent years, a number of MR target genes have been identified that are involved in the regulation of the epithelial sodium channel (ENaC), a key modulator of renal sodium absorption. Here we report the identification of cnksr3 as a direct MR target gene that is up-regulated in response to physiological concentrations of aldosterone. The cnksr3 promoter exhibits two functional aldosterone-responsive regions, which were bound by the MR as assessed by chromatin immunoprecipitation (ChIP). In vivo, CNKSR3 was highly expressed in the renal cortical collecting duct (CCD), the prime target segment of aldosterone-regulated sodium retention in the kidney. CCD cell lines stably overexpressing or silencing CNKSR3 were electrophysiologically analyzed and show that CNKSR3 expression correlated with and is required for ENaC-mediated transepithelial sodium transport. In parallel, CNKSR3 expression led to decreased MEK phosphorylation. We conclude that CNKSR3, a homologue of scaffold proteins involved in MAPK pathway regulation, is a direct target of MR and is required for the maintenance of transepithelial sodium transport in the kidney.

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.

Sodium and calcium transport pathways along the mammalian distal nephron: from rabbit to human

The final adjustment of renal sodium and calcium excretion is achieved by the distal nephron, in which transepithelial ion transport is under control of various hormones, tubular fluid composition, and flow rate. Acquired or inherited diseases leading to deranged renal sodium and calcium balance have been linked to dysfunction of the distal nephron. Diuretic drugs elicit their effects on sodium balance by specifically inhibiting sodium transport proteins in the apical plasma membrane of distal nephron segments. The identification of the major apical sodium transport proteins allows study of their precise distribution pattern along the distal nephron and helps address their cellular and molecular regulation under various physiological and pathophysiological settings. This review focuses on the topological arrangement of sodium and calcium transport proteins along the cortical distal nephron and on some aspects of their functional regulation. The availability of data on the distribution of transporters in various species points to the strengths, as well as to the limitations, of animal models for the extrapolation to humans.

Multiple aspects of mineralocorticoid selectivity

Aldosterone regulates renal sodium reabsorption through binding to the mineralocorticoid receptor (MR). Because the glucocorticoid receptor (GR) is expressed together with the MR in aldosterone target cells, glucocorticoid hormones bound to GR may also intervene to modulate physiological functions in these cells. In addition, each steroid can bind both receptors, and the MR has equal affinity for aldosterone and glucocorticoid hormones. Several cellular and molecular mechanisms intervene to allow specific aldosterone regulatory effects, despite the large prevalence of glucocorticoid hormones in the plasma. They include the local metabolism of the glucocorticoid hormones into inactive derivatives by the enzyme 11beta-hydroxysteroid dehydrogenase; the intrinsic properties of the MR that discriminate between ligands through differential contacts; the possibility of forming homo- or heterodimers between MR and GR, leading to differential transactivation properties; and the interactions of MR and GR with other regulatory transcription factors. The relative contribution of each of these successive mechanisms may vary among aldosterone target cells (epithelial vs. nonepithelial) and according to the hormonal context. All these phenomena allow fine tuning of cellular functions depending on the degree of cooperation between corticosteroid hormones and other factors (hormonal or tissue specific). Such interactions may be altered in pathophysiological situations.

Distribution of 11 beta-hydroxysteroid dehydrogenase along the rabbit nephron

It has been recently proposed that 11 beta-hydroxysteroid dehydrogenase (11 beta-OHSD) is responsible for aldosterone tissue specificity. A 11 beta-OHSD deficiency has been invoked as a cause of the syndrome of apparent mineralocorticoid excess, and 11 beta-OHSD inhibition by liquorice has been invoked to explain the hypertension induced by this drug. Since the renal tubule is composed of aldosterone-sensitive and insensitive segments, we determined the distribution of 11 beta-OHSD along the rabbit tubule. Pools of tubular segments isolated by microdissection were incubated for 2 h at 37 degrees C in the presence of [3H]corticosterone (3H-B, 8.10(-9) M). Afterwards, the amounts of 3H-B and of the metabolite 11-dehydrocorticosterone (3H-A) were determined using HPLC analysis. In the proximal tubule, in either its convoluted or straight portion, and in the medullary thick ascending limb, the amount of 3H-A was 19.6 +/- 3.8% (n = 12), 17.9 +/- 3.4 (n = 8), and 15.0 +/- 2.2 (n = 4), respectively, of the sum of 3H-A + 3H-B. In the cortical ascending limb and the collecting tubule in its cortical and medullary parts, it was 74.7 +/- 6.8% (n = 4), 74.1 +/- 4.9 (n = 9) and 64.6 +/- 14.1 (n = 3), respectively. In both proximal and cortical collecting tubule, addition of carbenoxolone 8.10(-4) M, an inhibitor of 11 beta-OHSD, almost completely inhibited the conversion of 3H-B to 3H-A. Thus, 11 beta-OHSD activity was high in the aldosterone-sensitive segments, and low in the aldosterone-insensitive segments. These results strongly favor the hypothesis that 11 beta-OHSD is a key enzyme in mineralocorticoid tissue specificity along the rabbit nephron. They reinforce the notion that a defect in 11 beta-OHSD plays a major role in the syndrome of apparent mineralocorticoid excess and liquorice-induced hypertension.

Immunohistochemical localization of renal mineralocorticoid receptor by using an anti-idiotypic antibody that is an internal image of aldosterone

A monoclonal antibody (H10E), generated by an auto-anti-idiotypic procedure and directed at the aldosterone-binding site of mineralocorticoid receptor (MR), was used in immunohistochemical studies to localize MR in rabbit kidney preparations. In agreement with earlier physiological and biochemical observations, MR was detected in connecting and cortical collecting tubules. Additionally, MR was detected in the distal tubules, the medullary and papillary collecting ducts, and in the epithelial cells lining the papilla. The internal image properties of the antibody were exploited to assess the specificity of MR detection by means of competition studies with hormones and antihormones. Immunostaining was completely abolished by preincubation with aldosterone but not with RU 486, a steroid antagonist that does not bind MR. On the cellular level, immunostaining occurred in the cytoplasm and, in the majority of cells, in the nucleus as well. The nucleocytoplasmic distribution of MR was unaffected by adrenalectomy or by administration of aldosterone. The availability of this specific monoclonal antibody makes it feasible to study MR expression in other target tissues and in pathological disorders.

Mineralocorticoid effect on K+ permeability of the rabbit cortical collecting tubule

Mineralocorticoid hormones stimulate Na+ absorption and K+ secretion by the cortical collecting tubule. There is good evidence that this stimulation involves increasing luminal membrane Na+ permeability and the turnover rate (or number) of the Na+-K+ pumps. These experiments were designed to examine whether mineralocorticoid hormones also increase cell K+ permeability. Using 42K tracer measurements in tubules treated with amiloride to inhibit active Na+ and K+ transport, passive K+ permeation increased with increasing mineralocorticoid effect. Net Na+ absorption and the (passive) K+ efflux rate coefficient (KK) showed a linear relationship. The stimulatory effect was evident in vitro since 0.2 microM aldosterone added to the bath of tubules harvested from NaCl-loaded rabbits increased KK at 3 hrs while time controls showed no change. Since these tubules were also treated with amiloride, this increase in KK was not dependent on increasing Na+ absorption. The results indicate that in addition to the well-described effects of aldosterone on Na+ permeability and cell metabolism, the mineralcorticoid effect includes an increase in cellular K+ permeability.