Rabbit distal convoluted tubule coexpresses NaCl cotransporter and 11 beta-hydroxysteroid dehydrogenase II mRNA

Structure-function relationships in the sodium chloride cotransporter

The thiazide sensitive Na+:Cl− cotransporter (NCC) is the principal via for salt reabsorption in the apical membrane of the distal convoluted tubule (DCT) in mammals and plays a fundamental role in managing blood pressure. The cotransporter is targeted by thiazide diuretics, a highly prescribed medication that is effective in treating arterial hypertension and edema. NCC was the first member of the electroneutral cation-coupled chloride cotransporter family to be identified at a molecular level. It was cloned from the urinary bladder of the Pseudopleuronectes americanus (winter flounder) 30 years ago. The structural topology, kinetic and pharmacology properties of NCC have been extensively studied, determining that the transmembrane domain (TM) coordinates ion and thiazide binding. Functional and mutational studies have discovered residues involved in the phosphorylation and glycosylation of NCC, particularly on the N-terminal domain, as well as the extracellular loop connected to TM7-8 (EL7-8). In the last decade, single-particle cryogenic electron microscopy (cryo-EM) has permitted the visualization of structures at high atomic resolution for six members of the SLC12 family (NCC, NKCC1, KCC1-KCC4). Cryo-EM insights of NCC confirm an inverted conformation of the TM1-5 and TM6-10 regions, a characteristic also found in the amino acid-polyamine-organocation (APC) superfamily, in which TM1 and TM6 clearly coordinate ion binding. The high-resolution structure also displays two glycosylation sites (N-406 and N-426) in EL7-8 that are essential for NCC expression and function. In this review, we briefly describe the studies related to the structure-function relationship of NCC, beginning with the first biochemical/functional studies up to the recent cryo-EM structure obtained, to acquire an overall view enriched with the structural and functional aspects of the cotransporter.

Molecular Basis, Diagnostic Challenges and Therapeutic Approaches of Bartter and Gitelman Syndromes: A Primer for Clinicians

Gitelman and Bartter syndromes are rare inherited diseases that belong to the category of renal tubulopathies. The genes associated with these pathologies encode electrolyte transport proteins located in the nephron, particularly in the Distal Convoluted Tubule and Ascending Loop of Henle. Therefore, both syndromes are characterized by alterations in the secretion and reabsorption processes that occur in these regions. Patients suffer from deficiencies in the concentration of electrolytes in the blood and urine, which leads to different systemic consequences related to these salt-wasting processes. The main clinical features of both syndromes are hypokalemia, hypochloremia, metabolic alkalosis, hyperreninemia and hyperaldosteronism. Despite having a different molecular etiology, Gitelman and Bartter syndromes share a relevant number of clinical symptoms, and they have similar therapeutic approaches. The main basis of their treatment consists of electrolytes supplements accompanied by dietary changes. Specifically for Bartter syndrome, the use of non-steroidal anti-inflammatory drugs is also strongly supported. This review aims to address the latest diagnostic challenges and therapeutic approaches, as well as relevant recent research on the biology of the proteins involved in disease. Finally, we highlight several objectives to continue advancing in the characterization of both etiologies.

The Mineralocorticoid Receptor in Salt-Sensitive Hypertension and Renal Injury

Hypertension and its comorbidities pose a major public health problem associated with disease-associated factors related to a modern lifestyle, such high salt intake or obesity. Accumulating evidence has demonstrated that aldosterone and its receptor, the mineralocorticoid receptor (MR), have crucial roles in the development of salt-sensitive hypertension and coexisting cardiovascular and renal injuries. Accordingly, clinical trials have repetitively shown the promising effects of MR blockers in these diseases. We and other researchers have identified novel mechanisms of MR activation involved in salt-sensitive hypertension and renal injury, including the obesity-derived overproduction of aldosterone and ligand-independent signaling. Moreover, recent advances in the analysis of cell-specific and context-dependent mechanisms of MR activation in various tissues-including a classic target of aldosterone, aldosterone-sensitive distal nephrons-are now providing new insights. In this review, we summarize recent updates to our understanding of aldosterone-MR signaling, focusing on its role in salt-sensitive hypertension and renal injury.

WNK4 kinase: from structure to physiology

With no lysine kinase-4 (WNK4) belongs to a serine-threonine kinase family characterized by the atypical positioning of its catalytic lysine. Despite the fact that WNK4 has been found in many tissues, the majority of its study has revolved around its function in the kidney, specifically as a positive regulator of the thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule of the nephron. This is explained by the description of gain-of-function mutations in the gene encoding WNK4 that causes familial hyperkalemic hypertension. This disease is mainly driven by increased downstream activation of the Ste20/SPS1-related proline-alanine-rich kinase/oxidative stress responsive kinase-1-NCC pathway, which increases salt reabsorption in the distal convoluted tubule and indirectly impairs renal K⁺ secretion. Here, we review the large volume of information that has accumulated about different aspects of WNK4 function. We first review the knowledge on WNK4 structure and enumerate the functional domains and motifs that have been characterized. Then, we discuss WNK4 physiological functions based on the information obtained from in vitro studies and from a diverse set of genetically modified mouse models with altered WNK4 function. We then review in vitro and in vivo evidence on the different levels of regulation of WNK4. Finally, we go through the evidence that has suggested how different physiological conditions act through WNK4 to modulate NCC activity.

Structure-function relationships in the renal NaCl cotransporter (NCC)

The thiazide-sensitive Na⁺-Cl^- cotransporter (NCC) is the major pathway for salt reabsorption in the distal convoluted tubule, serves as a receptor for thiazide-type diuretics, and is involved in inherited diseases associated with abnormal blood pressure. The functional and structural characterization of NCC from different species has led us to gain insights into the structure-function relationships of the cotransporter. Here we present an overview of different studies that had described these properties. Additionally, we report the cloning and characterization of the NCC from the spiny dogfish (Squalus acanthias) kidney (sNCC). The purpose of the present study was to determine the main functional, pharmacological and regulatory properties of sNCC to make a direct comparison with other NCC orthologous. The sNCC cRNA encodes a 1033 amino acid membrane protein, when expressed in Xenopus oocytes, functions as a thiazide-sensitive Na-Cl cotransporter with NCC regulation and thiazide-inhibition properties similar to mammals, rather than to teleosts. However, the K_m values for ion transport kinetics are significantly higher than those observed in the mammal species. In summary, we present a review on NCC structure-function relationships with the addition of the sNCC information in order to enrich the NCC cotransporter knowledge.

Thiazide-sensitive Na+-Cl- cotransporter: genetic polymorphisms and human diseases

The thiazide-sensitive Na(+)-Cl(-) cotransporter (TSC) is responsible for the major sodium chloride reabsorption pathway, which is located in the apical membrane of the epithelial cells of the distal convoluted tubule. TSC is involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl(-) concentration below or above its electrochemical potential equilibrium. In addition, TSC serves as the target of thiazide-type diuretics that are the first line of therapy for the treatment of hypertension in the clinic, and its mutants are also reported to be associated with the hereditary disease, Gitelman's syndrome. This review aims to summarize the publications with regard to the TSC by focusing on the association between TSC mutants and human hypertension as well as Gitelman's syndrome.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Hypertrophy in the Distal Convoluted Tubule of an 11β-Hydroxysteroid Dehydrogenase Type 2 Knockout Model

Na(+) transport in the renal distal convoluted tubule (DCT) by the thiazide-sensitive NaCl cotransporter (NCC) is a major determinant of total body Na(+) and BP. NCC-mediated transport is stimulated by aldosterone, the dominant regulator of chronic Na(+) homeostasis, but the mechanism is controversial. Transport may also be affected by epithelial remodeling, which occurs in the DCT in response to chronic perturbations in electrolyte homeostasis. Hsd11b2(-/-) mice, which lack the enzyme 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2) and thus exhibit the syndrome of apparent mineralocorticoid excess, provided an ideal model in which to investigate the potential for DCT hypertrophy to contribute to Na(+) retention in a hypertensive condition. The DCTs of Hsd11b2(-/-) mice exhibited hypertrophy and hyperplasia and the kidneys expressed higher levels of total and phosphorylated NCC compared with those of wild-type mice. However, the striking structural and molecular phenotypes were not associated with an increase in the natriuretic effect of thiazide. In wild-type mice, Hsd11b2 mRNA was detected in some tubule segments expressing Slc12a3, but 11βHSD2 and NCC did not colocalize at the protein level. Thus, the phosphorylation status of NCC may not necessarily equate to its activity in vivo, and the structural remodeling of the DCT in the knockout mouse may not be a direct consequence of aberrant corticosteroid signaling in DCT cells. These observations suggest that the conventional concept of mineralocorticoid signaling in the DCT should be revised to recognize the complexity of NCC regulation by corticosteroids.

Chloride transport

Chloride transport along the nephron is one of the key actions of the kidney that regulates extracellular volume and blood pressure. To maintain steady state, the kidney needs to reabsorb the vast majority of the filtered load of chloride. This is accomplished by the integrated function of sequential chloride transport activities along the nephron. The detailed mechanisms of transport in each segment generate unique patterns of interactions between chloride and numerous other individual components that are transported by the kidney. Consequently, chloride transport is inextricably intertwined with that of sodium, potassium, protons, calcium, and water. These interactions not only allow for exquisitely precise regulation but also determine the particular patterns in which the system can fail in disease states.

Molecular physiology of SPAK and OSR1: two Ste20-related protein kinases regulating ion transport

SPAK (Ste20-related proline alanine rich kinase) and OSR1 (oxidative stress responsive kinase) are members of the germinal center kinase VI subfamily of the mammalian Ste20 (Sterile20)-related protein kinase family. Although there are 30 enzymes in this protein kinase family, their conservation across the fungi, plant, and animal kingdom confirms their evolutionary importance. Already, a large volume of work has accumulated on the tissue distribution, binding partners, signaling cascades, and physiological roles of mammalian SPAK and OSR1 in multiple organ systems. After reviewing this basic information, we will examine newer studies that demonstrate the pathophysiological consequences to SPAK and/or OSR1 disruption, discuss the development and analysis of genetically engineered mouse models, and address the possible role these serine/threonine kinases might have in cancer proliferation and migration.

The WNKs: atypical protein kinases with pleiotropic actions

WNKs are serine/threonine kinases that comprise a unique branch of the kinome. They are so-named owing to the unusual placement of an essential catalytic lysine. WNKs have now been identified in diverse organisms. In humans and other mammals, four genes encode WNKs. WNKs are widely expressed at the message level, although data on protein expression is more limited. Soon after the WNKs were identified, mutations in genes encoding WNK1 and -4 were determined to cause the human disease familial hyperkalemic hypertension (also known as pseudohypoaldosteronism II, or Gordon's Syndrome). For this reason, a major focus of investigation has been to dissect the role of WNK kinases in renal regulation of ion transport. More recently, a different mutation in WNK1 was identified as the cause of hereditary sensory and autonomic neuropathy type II, an early-onset autosomal disease of peripheral sensory nerves. Thus the WNKs represent an important family of potential targets for the treatment of human disease, and further elucidation of their physiological actions outside of the kidney and brain is necessary. In this review, we describe the gene structure and mechanisms regulating expression and activity of the WNKs. Subsequently, we outline substrates and targets of WNKs as well as effects of WNKs on cellular physiology, both in the kidney and elsewhere. Next, consequences of these effects on integrated physiological function are outlined. Finally, we discuss the known and putative pathophysiological relevance of the WNKs.

The thiazide-sensitive Na+-Cl- cotransporter: molecular biology, functional properties, and regulation by WNKs

The thiazide-sensitive Na+-Cl(-) cotransporter is the major salt reabsorption pathway in the distal convoluted tubule, which is located just after the macula densa at the beginning of the aldosterone-sensitive nephron. This cotransporter was identified at the molecular level in the early 1990s by the pioneering work of Steven C. Hebert and coworkers, opening the molecular area, not only for the Na+-Cl(-) cotransporter but also for the family of electroneutral cation-coupled chloride cotransporters that includes the loop diuretic-sensitive Na+-K+-2Cl(-) cotransporter of the thick ascending limb of Henle's loop. This work honoring the memory of Steve Hebert presents a brief review of our current knowledge about salt and water homeostasis generated as a consequence of cloning the cotransporter, with particular emphasis on the molecular biology, physiological properties, human disease due to decreased or increased activity of the cotransporter, and regulation of the cotransporter by a family of serine/threonine kinases known as WNK. Thus one of the legacies of Steve Hebert is a better understanding of salt and water homeostasis.

Novel mouse strain with Cre recombinase in 11beta-hydroxysteroid dehydrogenase-2-expressing cells

Here we describe the generation and characterization of a mouse strain that expresses an improved Cre (iCre) recombinase (48) under the control of the endogenous 11beta-hydroxysteroid dehydrogenase type 2 (11HSD2) promoter. Progeny of 11HSD2/iCre and ROSA26 reporter mice were used to determine the pattern of iCre expression by measuring the activity of the LacZ gene product beta-galactosidase in a panel of tissues. On Cre recombinase activity, intense beta-galactosidase activity (X-gal staining) was observed in the classic mineralocorticoid target segments of the kidney, as well as in the colon, and both female and male reproductive organs. Weaker iCre expression was detected in the lung and heart. In the brain, strong iCre activity was present in cardiovascular centers that are known to express 11HSD2 and mineralocorticoid receptors [nucleus tractus solitarius (NTS) and amygdala] as well as in the granular layer of the cerebellum. iCre expression was weaker in neonatal kidney and colon than in the adult but was present in the hair follicles and cartilage. These results indicate that in the 11HSD2/iCre strain iCre expression faithfully represents the expression pattern of endogenous 11HSD2. Thus this mouse model represents the first Cre deleter strain that can be used to eliminate desired genes in every mineralocorticoid target tissue. This mouse model should serve as a useful resource for investigators who want to study the function of genes involved in aldosterone action and genes in other pathways that are selectively expressed in these cells.

Beyond the signal sequence: protein routing in health and disease

Receptors, hormones, enzymes, ion channels, and structural components of the cell are created by the act of protein synthesis. Synthesis alone is insufficient for proper function, of course; for a cell to operate effectively, its components must be correctly compartmentalized. The mechanism by which proteins maintain the fidelity of localization warrants attention in light of the large number of different molecules that must be routed to distinct subcellular loci, the potential for error, and resultant disease. This review summarizes diseases known to have etiologies based on defective protein folding or failure of the cell's quality control apparatus and presents approaches for therapeutic intervention.

Expression of the potassium channel ROMK in adult and fetal human kidney

The renal potassium channel ROMK is a crucial element of K+ recycling and secretion in the distal tubule and the collecting duct system. Mutations in the ROMK gene (KCNJ1) lead to hyperprostaglandin E syndrome/antenatal Bartter syndrome, a life-threatening hypokalemic disorder of the newborn. The localization of ROMK channel protein, however, remains unknown in humans. We generated an affinity-purified specific polyclonal anti-ROMK antibody raised against a C-terminal peptide of human ROMK. Immunoblotting revealed a 45 kDa protein band in both rat and human kidney tissue. In human kidney sections, the antibody showed intense staining of epithelial cells in the cortical and medullary thick ascending limb (TAL), the connecting tubule, and the collecting duct. Moreover, a strong expression of ROMK protein was detected in cells of the macula densa. In epithelial cells of the TAL expression of ROMK protein was mainly restricted to the apical membrane. In human fetal kidney expression of ROMK protein was detected mainly in distal tubules of mature nephrons but not or only marginally in the collecting system. No expression was found in early developmental stages such as comma or S shapes, indicating a differentiation-dependent expression of ROMK protein. In summary, these findings support the proposed role of ROMK channels in potassium recycling and in the regulation of K+ secretion and present a rationale for the phenotype observed in patients with ROMK deficiency.

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

Electroneutral cation-Cl−cotransporters compose a family of solute carriers in which cation (Na+or K+) movement through the plasma membrane is always accompanied by Cl−in a 1:1 stoichiometry. Seven well-characterized members include one gene encoding the thiazide-sensitive Na+−Cl−cotransporter, two genes encoding loop diuretic-sensitive Na+−K+−2Cl−cotransporters, and four genes encoding K+−Cl−cotransporters. These membrane proteins are involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl−concentration below or above its electrochemical potential equilibrium. In addition, members of this family play an important role in cardiovascular and neuronal pharmacology and pathophysiology. Some of these cotransporters serve as targets for loop diuretics and thiazide-type diuretics, which are among the most commonly prescribed drugs in the world, and inactivating mutations of three members of the family cause inherited diseases such as Bartter's, Gitelman's, and Anderman's diseases. Major advances have been made in the past decade as consequences of molecular identification of all members in this family. This work is a comprehensive review of the knowledge that has evolved in this area and includes molecular biology of each gene, functional properties of identified cotransporters, structure-function relationships, and physiological and pathophysiological roles of each cotransporter.

Interactions between 11beta-hydroxysteroid dehydrogenase and COX-2 in kidney

The syndrome of apparent mineralocorticoid excess (SAME) is an autosomal recessive form of salt-sensitive hypertension caused by deficiency of the kidney type 2 11beta-hydroxysteroid dehydrogenase (11betaHSD2). In this disorder, cortisol is not inactivated by 11betaHSD2, occupies mineralocorticoid receptors (MRs), and causes excessive sodium retention and hypertension. In renal medulla, prostaglandins derived from cyclooxygenase-2 (COX-2) stimulate sodium and water excretion, and renal medullary COX-2 expression increases after mineralocorticoid administration. We investigated whether medullary COX-2 also increases in rats with 11betaHSD2 inhibition and examined its possible role in the development of hypertension. 11betaHSD2 inhibition increased medullary and decreased cortical COX-2 expression in adult rats and induced high blood pressure in high-salt-treated rats. COX-2 inhibition had no effect on blood pressure in control animals but further increased blood pressure in high-salt-treated rats with 11betaHSD2 inhibition. COX-1 inhibition had no effect on blood pressure in either control or experimental animals. 11betaHSD2 inhibition also led to medullary COX-2 increase and cortical COX-2 decrease in weaning rats, primarily through activation of MRs. In the suckling rats, medullary COX-2 expression was very low, consistent with a urinary concentrating defect. 11betaHSD2 inhibition had no effect on either cortical or medullary COX-2 expression in the suckling rats, consistent with low levels of circulating corticosterone in these animals. These data indicate that COX-2 plays a modulating role in the development of hypertension due to 11betaHSD2 deficiency and that 11betaHSD2 regulates renal COX-2 expression by preventing glucocorticoid access to MRs during postnatal development.

Long-term adaptation of renal ion transporters to chronic diuretic treatment

Loop and thiazide diuretics are clinically useful to induce negative sodium balance. However, with chronic treatment, their effects tend to be blunted since the kidney adapts to diuretics. Molecular identification of the renal ion transporters has provided us with a new understanding of the mechanisms of intrarenal adaptation to diuretics at molecular levels. In the kidney, loop and thiazide diuretics are secreted from the proximal tubule via the organic anion transporter-1 (OAT1) and exert their diuretic action by binding to the Na-K-2Cl cotransporter type 2 (NKCC2) in the thick ascending limb and the Na-Cl cotransporter (NCC) in the distal convoluted tubule, respectively. Recent studies in animal models suggest that abundance of these ion transporters is affected by long-term diuretic administration. Downstream from the primary site of diuretic action, an increase in epithelial Na+ channel (ENaC) abundance is induced by chronic furosemide or hydrochlorothiazide treatment. This adaptation is consistent with previous reports showing cellular hypertrophy and increased Na+ absorption in distal tubular segments. The abundance of NKCC2 and NCC is increased by furosemide and hydrochlorothiazide, respectively. This compensatory upregulation suggests that either diuretic may activate the ion transporter within the primary site of action. In the proximal tubule, the abundance of OAT1 is increased by chronic treatment with furosemide or hydrochlorothiazide. This upregulation of OAT1 seems to be induced by substrate stimulation, lessening diuretic tolerance associated with long-term diuretic use.

Segment-specific expression of 2P domain potassium channel genes in human nephron

BACKGROUND

The 2P domain potassium (K2P) channels are a recently discovered ion channel superfamily. Structurally, K2P channels are distinguished by the presence of two pore forming loops within one channel subunit. Functionally, they are characterized by their ability to pass potassium across the physiologic voltage range. Thus, K2P channels are also called open rectifier, background, or leak potassium channels. Patch clamp studies of renal tubules have described several open rectifier potassium channels that have as yet eluded molecular identification. We sought to determine the segment-specific expression of transcripts for the 14 known K2P channel genes in human nephron to identify potential correlates of native leak channels.

METHODS

Human kidney samples were obtained from surgical cases and specific nephron segments were dissected. RNA was extracted and used as template for the generation of cDNA libraries. Real-time polymerase chain reaction (PCR) (TaqMan) was used to analyze gene expression.

RESULTS

We found significant (P < 0.05) expression of K2P10 in glomerulus, K2P5 in proximal tubule and K2P1 in cortical thick ascending limb of Henle's loop (cTAL) and in distal nephron segments. In addition, we repeatedly detected message for several other K2P channels with less abundance, including K2P3 and K2P6 in glomerulus, K2P10 in proximal tubule, K2P5 in thick ascending limb of Henle's loop, and K2P3, K2P5, and K2P13 in distal nephron segments.

CONCLUSION

K2P channels are expressed in specific segments of human kidney. These results provide a step toward assigning K2P channels to previously described native renal leaks.

A single nucleotide polymorphism alters the activity of the renal Na+:Cl- cotransporter and reveals a role for transmembrane segment 4 in chloride and thiazide affinity

The thiazide-sensitive Na+:Cl- cotransporter is the major salt transport pathway in the distal convoluted tubule of the kidney, and a role of this cotransporter in blood pressure homeostasis has been defined by physiological studies on pressure natriuresis and by its involvement in monogenic diseases that feature arterial hypotension or hypertension. Data base analysis revealed that 135 single nucleotide polymorphisms along the human SLC12A3 gene that encodes the Na+:Cl- cotransporter have been reported. Eight are located within the coding region, and one results in a single amino acid change; the residue glycine at the position 264 is changed to alanine (G264A). This residue is located within the fourth transmembrane domain of the predicted structure. Because Gly-264 is a highly conserved residue, we studied the functional properties of this polymorphism by using in vitro mutagenesis and the heterologous expression system in Xenopus laevis oocytes. G264A resulted in a significant and reproducible reduction ( approximately 50%) in (22)Na+ uptake when compared with the wild type cotransporter. The affinity for extracellular Cl- and for thiazide diuretics was increased in G264A. Western blot analysis showed similar immunoreactive bands between the wild type and the G264A cotransporters, and confocal images of oocytes injected with enhanced green fluorescent protein-tagged wild type and G264A cotransporter showed no differences in the protein surface expression level. These observations suggest that the G264A polymorphism is associated with reduction in the substrate translocation rate of the cotransporter, due to a decrease in the intrinsic activity. Our study also reveals a role of the transmembrane segment 4 in defining the affinity for extracellular Cl- and thiazide diuretics.

Molecular physiology of cation-coupled Cl- cotransport: the SLC12 family

The electroneutral cation-chloride-coupled cotransporter gene family ( SLC12) was identified initially at the molecular level in fish and then in mammals. This nine-member gene family encompasses two major branches, one including two bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporters and the thiazide-sensitive Na(+):Cl(-) cotransporter. Two of the genes in this branch ( SLC12A1 and SLC12A3), exhibit kidney-specific expression and function in renal salt reabsorption, whereas the third gene ( SLC12A2) is expressed ubiquitously and plays a key role in epithelial salt secretion and cell volume regulation. The functional characterization of both alternatively-spliced mammalian Na(+)-K(+)-2Cl(-) cotransporter isoforms and orthologs from distantly related species has generated important structure-function data. The second branch includes four genes ( SLC12A4- 7) encoding electroneutral K(+)-Cl(-) cotransporters. The relative expression level of the neuron-specific SLC12A5 and the Na(+)-K(+)-2Cl(-) cotransporter SLC12A2 appears to determine whether neurons respond to GABA with a depolarizing, excitatory response or with a hyperpolarizing, inhibitory response. The four K(+)-Cl(-) cotransporter genes are co-expressed to varying degrees in most tissues, with further roles in cell volume regulation, transepithelial salt transport, hearing, and function of the peripheral nervous system. The transported substrates of the remaining two SLC12 family members, SLC12A8 and SLC12A9, are as yet unknown. Inactivating mutations in three members of the SLC12 gene family result in Mendelian disease; Bartter syndrome type I in the case of SLC12A1, Gitelman syndrome for SLC12A3, and peripheral neuropathy in the case of SLC12A6. In addition, knockout mice for many members of this family have generated important new information regarding their respective physiological roles.

Cortisol and the renal handling of electrolytes: role in glucocorticoid-induced hypertension and bone disease

Hypertension and osteoporosis are characteristic clinical features in patients with Cushing's syndrome or in those on glucocorticoid (GC) treatment. These two distinct complications of GC excess share one common denominator: an abnormal handling of cations, sodium (Na(+)) and calcium (Ca(2+)), either primarily or in part by the kidney tubule. The principal mechanism of GC-induced hypertension is overstimulation of the non-selective mineralocorticoid receptor (MR), resulting in renal Na(+) retention, volume expansion and finally to an increase in blood pressure. In mineralocorticoid target organs, such as the kidney, the MR is protected from GC occupation by the enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2), a gate-keeping enzyme, which converts cortisol to receptor-inactive cortisone. This enzyme allows aldosterone to be the physiological agonist of the MR despite significantly higher circulating levels of cortisol. Kinetic properties of 11betaHSD2 suggest that saturability of this enzyme can already be achieved at high-normal physiological plasma cortisol levels, thereby leading to ovestimualtion of the MR by cortisol in states of GC excess. The mechanisms of GC action on bone turnover are more complex. GCs increase bone resorption, inhibit bone formation and have an indirect action on bone by decreasing intestinal Ca(2+) absorption, but also inducing a sustained renal Ca(2+) excretion. The latter appears to be mediated through stimulation of the MR by GC. The prevention and treatment of GC-induced hypertension and osteoporosis include the use of the minimal effective dose of GC, some general measures, and the use of some specific drugs. Modulation of renal Na(+) and Ca(2+) excretion with some, but not all, diuretics represents an important specific (for hypertension) or supportive (for bone disease) therapeutic intervention.

Cloning and localization of KCC4 in rabbit kidney: expression in distal convoluted tubule

Cl-dependent K secretion is a feature of renal distal tubules and collecting ducts. Recent cloning and identification of K-Cl cotransporter proteins led us to search for additional novel KCC isoforms expressed in the renal distal nephron. A human expressed sequence tag (EST) with high homology to KCC1 was identified. The rabbit isoform was cloned by homology using degenerate primers and rapid amplification of cDNA ends (RACE). Our isoform is the rabbit homologue of mouse and human KCC4 published previously. The 4.35-kb rabbit KCC4 cDNA encodes a protein of 1,106 amino acids. Antibodies were generated to both NH2-terminal and COOH-terminal fusion proteins. Northern and Western blot analyses showed widespread mRNA and protein expression in many rabbit organs, in renal cortex, outer medulla, and inner medulla but not in skeletal muscle. Immunohistochemical localization of KCC4 showed expression exclusively along the basolateral membrane in many nephron segments. The distal convoluted tubule and connecting tubule exhibited the highest level of KCC4 immunoreactivity, followed by the medullary thick ascending limb. A low level of immunoreactivity was detected in the proximal tubule and collecting ducts. We postulate that KCC4 mediates potassium and chloride exit from the cell and may play an important role in salt absorption by the distal convoluted tubule.

Modulation of renal calcium handling by 11 beta-hydroxysteroid dehydrogenase type 2

Reduced concentration of serum ionized calcium and increased urinary calcium excretion have been reported in primary aldosteronism and glucocorticoid-treated patients. A reduced activity of the 11 beta-hydroxysteroid dehydrogenase type 2 (11 beta HSD2) results in overstimulation of the mineralocorticoid receptor by cortisol. Whether inhibition of the 11 beta HSD2 by glycyrrhetinic acid (GA) may increase renal calcium excretion is unknown. Serum and urinary electrolyte and creatinine, serum ionized calcium, urinary calcium excretion, and the steroid metabolites (THF+5 alpha THF)/THE as a parameter of 11 beta HSD2 activity were repeatedly measured in 20 healthy subjects during baseline conditions and during 1 wk of 500 mg/d GA. One week of GA induced a maximal increment of 93% in (THF+5 alpha THF)/THE. Ambulatory BP was significantly higher at day 7 of GA than at baseline (126/77 +/- 10/7 versus 115/73 +/- 8/6 mmHg; P < 0.001 for systolic; P < 0.05 for diastolic). During GA administration, serum ionized calcium decreased from 1.26 +/- 0.05 to 1.18 +/- 0.04 mmol/L (P < 0.0001), and absolute urinary calcium excretion was enhanced from 29.2 +/- 3.6 to 31.9 +/- 3.1 micromol/L GFR (P < 0.01). Fractional calcium excretion increased from 2.4 +/- 0.3 to 2.7 +/- 0.3% (P < 0.01) and was negatively correlated to the fractional sodium excretion during GA (R = -0.35; P < 0.001). Moreover, serum potassium correlated positively with serum ionized calcium (R = 0.66; P < 0.0001). Inhibition of 11 beta HSD2 activity is sufficient to significantly increase the fractional excretion of calcium and decrease serum ionized calcium, suggesting decreased tubular reabsorption of this divalent cation under conditions of renal glucocorticoid/mineralocorticoid excess. The likely site of steroid-regulated renal calcium handling appears to be the distal tubule.

Functional expression of mutations in the human NaCl cotransporter: evidence for impaired routing mechanisms in Gitelman's syndrome

Gitelman's syndrome is an autosomal recessive renal tubular disorder characterized by hypokalemic metabolic alkalosis, hypomagnesemia, and hypocalciuria. This disorder results from mutations in the thiazide-sensitive NaCl cotransporter (NCC). To elucidate the functional implications of mutations associated with this disorder, metolazone-sensitive (22)Na(+) uptake, subcellular localization, and glycosidase-sensitive glycosylation of human NCC (hNCC) were determined in Xenopus laevis oocytes expressing FLAG-tagged wild-type or mutant hNCC. Injection of 10 ng of FLAG-tagged hNCC cRNA resulted in metolazone-sensitive (22)Na(+) uptake of 3.4 +/- 0.2 nmol Na(+)/oocyte per 2 h. Immunocytochemical analysis revealed sharp immunopositive staining at the plasma membrane. In agreement with this finding, a broad endoglycosidase H-insensitive band of 130 to 140 kD was present in Western blots of total membranes. The plasma membrane localization of this complex-glycosylated protein was confirmed by immunoblotting of purified plasma membranes. The mutants could be divided into two distinct classes. Class I mutants (G439S, T649R, and G741R) exhibited no significant metolazone-sensitive (22)Na(+) uptake. Immunopositive staining was present in a diffuse band just below the plasma membrane. This endoplasmic reticulum and/or pre-Golgi complex localization was further suggested by the complete absence of the endoglycosidase H-insensitive band. Class II mutants (L215P, F536L, R955Q, G980R, and C985Y) demonstrated significant metolazone-sensitive (22)Na(+) uptake, although uptake was significantly lower than that obtained with wild-type hNCC. The latter mutants could be detected at and below the oocyte plasma membrane, and immunoblotting revealed the characteristic complex-glycosylated bands. In conclusion, this study substantiates NCC processing defects as the underlying pathogenic mechanism in Gitelman's syndrome.

Interaction with grp58 increases activity of the thiazide-sensitive Na-Cl cotransporter

The thiazide-sensitive sodium-chloride cotransporter (NCC) is expressed by distal convoluted tubule cells of the mammalian kidney. We used yeast two-hybrid screening to identify that glucose-regulated protein 58 (grp58), a protein induced by glucose deprivation, binds to the COOH terminus of the NCC. Immunoprecipitation of rat kidney cortex homogenates using a guinea pig anti-NCC antibody confirmed that grp58 associates with the NCC in vivo. Northern blots indicated that grp58 is highly expressed in rat kidney cortex. Immunofluorescence showed that grp58 protein abundance in kidney is highest in epithelial cells of the distal nephron, where it colocalizes with NCC near the apical membrane. To determine whether this interaction has a functional significance, NCC and grp58 cRNA were coexpressed in Xenopus laevis oocytes. In oocytes overexpressing grp58, sodium uptake was increased compared with control. Because oocytes express endogenous grp58, antisense experiments were performed to evaluate whether endogenous grp58 affected NCC activity in oocytes. Sodium uptake was lower in oocytes injected with both antisense grp58 cRNA and sense NCC compared with sense NCC oocytes. Western blot analysis did not show any effect of grp58 expression on processing of the NCC. These data indicate a novel, functionally important interaction between grp58 and the NCC in rat kidney cortex.

Localization of thiazide-sensitive Na(+)-Cl(-) cotransport and associated gene products in mouse DCT

The mammalian distal nephron develops a complex assembly of specialized cell types to accomplish the fine adjustment of urinary electrolyte composition. The epithelia of the distal convoluted tubule (DCT), the connecting tubule (CNT), and the cortical collecting duct (CCD) show an axial structural heterogeneity that has been functionally elucidated by the localization of proteins involved in transepithelial ion transport. We compared the distribution of the thiazide-sensitive Na(+)-Cl(-) cotransporter (TSC), basolateral Na(+)/Ca(2+) exchanger (Na/Ca), cytosolic calcium-binding proteins calbindin D(28K) and parvalbumin, and the key enzyme for selective aldosterone actions, 11 beta-hydroxysteroid-dehydrogenase 2 (11HSD2), in the distal convolutions of the mouse. In the mouse, as opposed to the rat, we found no clear subsegmentation of the DCT into a proximal (DCT1) and a distal (DCT2) portion. The TSC was expressed along the entire DCT. Na/Ca and calbindin D(28K) were similarly expressed along most of the DCT, with minor exceptions in the initial portion of the DCT. Both were also present in the CNT. Parvalbumin was found in the entire DCT, with an occasional absence from short end portions of the DCT, and was not present in CNT. 11HSD2 was predominantly located in the CNT and CCD. Short end portions of DCT only occasionally showed the 11HSD2 signal. We also observed an overlap of 11HSD2 immunoreactivity and mRNA staining. Our observations will have implications in understanding the physiological effects of gene disruption and targeting experiments in the mouse.

Renal tubule-specific transcription and chromosomal localization of rat thiazide-sensitive Na-Cl cotransporter gene

The molecular mechanism underlying the renal expression localization of the thiazide-sensitive Na-Cl cotransporter (TSC) gene was studied. The TSC gene was localized to chromosome 19p12-14. In cultured cells, tissue-specific transcription activity of the 5'-flanking region of the rat rTSC gene (5'FL/rTSC) was demonstrated, and the major promoter region was located between position -580 and -141. To further examine the tissue-specific transcription, transgenic rats harboring the 5'FL/rTSC fused upstream of the LacZ gene were generated. Immunohistochemical analysis clearly showed that LacZ gene expression was co-localized to distal convoluted tubules (DCT) with TSC, indicating that the 5'FL/rTSC regulates the renal tubule-specific TSC expression. Because a transcription factor, HFH-3 (hepatocyte nuclear factor-3/folk head homologue-3), had also been localized to DCT, a possible role of the putative cis-acting element (HFH-3/rTSC, -400/-387 position) for HFH-3 binding in the tissue-specific transcription was examined. Deletion and mutation analyses suggested that transcription of the HFH-3/rTSC was actually responsive to HFH-3, and electrophoretic mobility shift assay showed a direct binding of in vitro synthesized HFH-3 to the HFH-3/rTSC. In conclusion, the rTSC gene is localized to rat chromosome 19p12--24. The transcription regulatory region of the TSC gene confers DCT-specific gene expression. DCT-specific transcription factor HFH-3 may be involved in the renal tubule-specific transcription of TSC gene.

The distal convoluted tubule of rabbit kidney does not express a functional sodium channel

We sought to assess whether the distal convoluted tubule (DCT) segment of the rabbit nephron expresses a functional epithelial sodium channel. First, the transepithelial voltage (V(te), lumen vs. bath) was measured in isolated perfused DCT segments (assessed separately in the upstream half and the downstream half of the DCT). V(te) was zero and not affected by amiloride or barium in the upstream DCT. V(te) was sometimes negative in the downstream DCT and depolarized by amiloride and hyperpolarized by barium, suggesting inclusion of connecting tubule (CNT) cells. To determine expression of epithelial sodium channel (ENaC) mRNA subunits by the upstream DCT, rabbit alpha-, beta-, and gamma-ENaC cDNA fragments were cloned and primers were selected for single-nephron RT-PCR analysis. Although alpha-ENaC was expressed by the DCT, beta- and gamma-ENaC were not detected in the DCT. In contrast, the CNT, CCD, and outer medullary collecting duct (OMCD) expressed all three subunits. Nedd4 was also not detected in the DCT but was expressed by the CNT, CCD, and OMCD. When upstream DCT fragments were grown to confluent monolayers in primary culture, the epithelia exhibited negative voltages and high transepithelial resistances and expressed mRNA for all three ENaC subunits as well as for Nedd4. The absence of a negative voltage and failure to detect transcript for beta- and gamma-ENaC and Nedd4 in the native rabbit DCT suggest that the sodium channel is not a significant pathway for sodium absorption by this segment. The phenotype conversion observed when DCT cells are grown in culture does not rule out the possibility that there may be conditions in which the DCT in the intact kidney expresses sodium channel activity. The results are consistent with the notion that DCT sodium transport is predominantly, if not exclusively, electroneutral.

Magnesium transport in the renal distal convoluted tubule

The distal tubule reabsorbs approximately 10% of the filtered Mg(2+), but this is 70-80% of that delivered from the loop of Henle. Because there is little Mg(2+) reabsorption beyond the distal tubule, this segment plays an important role in determining the final urinary excretion. The distal convoluted segment (DCT) is characterized by a negative luminal voltage and high intercellular resistance so that Mg(2+) reabsorption is transcellular and active. This review discusses recent evidence for selective and sensitive control of Mg(2+) transport in the DCT and emphasizes the importance of this control in normal and abnormal renal Mg(2+) conservation. Normally, Mg(2+) absorption is load dependent in the distal tubule, whether delivery is altered by increasing luminal Mg(2+) concentration or increasing the flow rate into the DCT. With the use of microfluorescent studies with an established mouse distal convoluted tubule (MDCT) cell line, it was shown that Mg(2+) uptake was concentration and voltage dependent. Peptide hormones such as parathyroid hormone, calcitonin, glucagon, and arginine vasopressin enhance Mg(2+) absorption in the distal tubule and stimulate Mg(2+) uptake into MDCT cells. Prostaglandin E(2) and isoproterenol increase Mg(2+) entry into MDCT cells. The current evidence indicates that cAMP-dependent protein kinase A, phospholipase C, and protein kinase C signaling pathways are involved in these responses. Steroid hormones have significant effects on distal Mg(2+) transport. Aldosterone does not alter basal Mg(2+) uptake but potentiates hormone-stimulated Mg(2+) entry in MDCT cells by increasing hormone-mediated cAMP formation. 1,25-Dihydroxyvitamin D(3), on the other hand, stimulates basal Mg(2+) uptake. Elevation of plasma Mg(2+) or Ca(2+) inhibits hormone-stimulated cAMP accumulation and Mg(2+) uptake in MDCT cells through activation of extracellular Ca(2+)/Mg(2+)-sensing mechanisms. Mg(2+) restriction selectively increases Mg(2+) uptake with no effect on Ca(2+) absorption. This intrinsic cellular adaptation provides the sensitive and selective control of distal Mg(2+) transport. The distally acting diuretics amiloride and chlorothiazide stimulate Mg(2+) uptake in MDCT cells acting through changes in membrane voltage. A number of familial and acquired disorders have been described that emphasize the diversity of cellular controls affecting renal Mg(2+) balance. Although it is clear that many influences affect Mg(2+) transport within the DCT, the transport processes have not been identified.

Distal tubular electrolyte transport during inhibition of renal 11beta-hydroxysteroid dehydrogenase

To test the proposal that the enzyme 11beta-hydroxysteroid dehydrogenase (11beta-HSD) confers aldosterone specificity on mineralocorticoid receptors in the distal nephron by inactivating glucocorticoids, we performed a free-flow micropuncture study of distal tubular function in adrenalectomized rats infused with high-dose corticosterone. One-half of the rats were additionally given intravenous carbenoxolone (CBX; 6 mg/h) to inhibit renal 11beta-HSD activity. Although this maneuver lowered fractional Na(+) excretion (1.1 +/- 0.2 vs. 1.9 +/- 0.2%, P < 0.01), Na(+) reabsorption within the accessible distal tubule was found to be similar in the two groups of animals. In contrast, distal tubular K(+) secretion was enhanced in CBX-treated rats: fractional K(+) deliveries to the early and late distal collection sites in the corticosterone-alone group were 13 +/- 1 and 20 +/- 3%, respectively (not significant), whereas corresponding data in the CBX-treated group were 9 +/- 1 and 24 +/- 2% (P < 0.01). This stimulation of distal K(+) secretion provides the first direct in vivo evidence that 11beta-HSD normally prevents corticosterone from exerting a mineralocorticoid-like effect in the distal tubule. The reduction in fractional Na(+) excretion during inhibition of 11beta-HSD, in the absence of a change in end-distal Na(+) delivery, suggests enhanced Na(+) reabsorption in the collecting ducts.

Mammalian distal tubule: physiology, pathophysiology, and molecular anatomy

The distal tubule of the mammalian kidney, defined as the region between the macula densa and the collecting duct, is morphologically and functionally heterogeneous. This heterogeneity has stymied attempts to define functional properties of individual cell types and has led to controversy concerning mechanisms and regulation of ion transport. Recently, molecular techniques have been used to identify and localize ion transport pathways along the distal tubule and to identify human diseases that result from abnormal distal tubule function. Results of these studies have clarified the roles of individual distal cell types. They suggest that the basic molecular architecture of the distal nephron is surprisingly similar in mammalian species investigated to date. The results have also reemphasized the role played by the distal tubule in regulating urinary potassium excretion. They have clarified how both peptide and steroid hormones, including aldosterone and estrogen, regulate ion transport by distal convoluted tubule cells. Furthermore, they highlight the central role that the distal tubule plays in systemic calcium homeostasis. Disorders of distal nephron function, such as Gitelman's syndrome, nephrolithiasis, and adaptation to diuretic drug administration, emphasize the importance of this relatively short nephron segment to human physiology. This review integrates molecular and functional results to provide a contemporary picture of distal tubule function in mammals.

Defective processing and expression of thiazide-sensitive Na-Cl cotransporter as a cause of Gitelman's syndrome

Gitelman's syndrome is an autosomal recessive disorder of salt wasting and hypokalemia caused by mutations in the thiazide-sensitive Na-Cl cotransporter. To investigate the pathogenesis of Gitelman's syndrome, eight disease mutations were introduced into the mouse thiazide-sensitive Na-Cl cotransporter and studied by functional expression in Xenopus oocytes. Sodium uptake into oocytes that expressed the wild-type clone was more than sevenfold greater than uptake into control oocytes. Uptake into oocytes that expressed the mutated transporters was not different from control. Hydrochlorothiazide reduced Na uptake by oocytes expressing the wild-type gene to control values but had no effect on oocytes expressing the mutant clones. Western blots of oocytes injected with the wild-type clone showed bands representing glycosylated (125 kDa) and unglycosylated (110 kDa) forms of the transport protein. Immunoblot of oocytes expressing the mutated clones showed only the unglycosylated protein, indicating that protein processing was disrupted. Immunocytochemistry with an antibody against the transport protein showed intense membrane staining of oocytes expressing the wild-type protein. Membrane staining was completely absent from oocytes expressing mNCC(R948X); instead, diffuse cytoplasmic staining was evident. In summary, the results show that several mutations that cause Gitelman's syndrome are nonfunctional because the mutant thiazide-sensitive Na-Cl cotransporter is not processed normally, probably activating the "quality control" mechanism of the endoplasmic reticulum.

Intrarenal distribution of the colonic H,K-ATPase mRNA in rabbit

BACKGROUND

Evidence suggests that the colonic H,K-ATPase isoform is expressed in the kidney and that a mRNA species highly homologous to the rat and guinea pig HKalpha2 is expressed in the cortical collecting duct (CCD) of the rabbit. The goals of this study were to determine if this mRNA is the rabbit homologue of HKalpha2 or a novel isoform and to determine intrarenal distribution of the HKalpha2 mRNA in rabbit.

METHODS

5'-RACE and Dye Deoxy Terminator chemistry were used to determine the full-length sequence of the rabbit HKalpha2 mRNA. The intrarenal distribution of HKalpha2 mRNA was determined in microdissected nephron segments, connecting tubule (CNT), and CCD cells isolated by immunodissection, as well as in the three cell types of the CCD. Principal cells and alpha- and beta-intercalated cells were isolated by fluorescence-activated cell sorting. HKalpha2 mRNA levels were determined by quantitative reverse transcription-polymerase chain reaction (RT-PCR) or single-nephron RT-PCR (SN-RTPCR).

RESULTS

The full-length sequence of the rabbit kidney HKalpha2 mRNA was determined. This transcript is identical to the one expressed in rabbit distal colon. In microdissected nephron segments, strong HKalpha2 amplicons were present in the CNT, CCD, and outer medullary collecting duct (OMCD), whereas no signal was detected in the proximal tubule, distal convoluted tubule, think ascending limb, and inner medullary collecting duct. Roughly comparable levels of HKalpha2 mRNA were present in all three CCD cell types, and the highest levels were observed in a subpopulation most likely corresponding to CNT cells.

CONCLUSIONS

These results suggest that the HKalpha2 mRNA is expressed in rabbit collecting duct is identical in size and sequence to the one expressed in rabbit distal colon. HKalpha2 mRNA in the rabbit kidney is selectively expressed in the CNT, CCD, and OMCD, and all three collecting duct subtypes express its mRNA.

Paracellin-1, a renal tight junction protein required for paracellular Mg2+ resorption

Epithelia permit selective and regulated flux from apical to basolateral surfaces by transcellular passage through cells or paracellular flux between cells. Tight junctions constitute the barrier to paracellular conductance; however, little is known about the specific molecules that mediate paracellular permeabilities. Renal magnesium ion (Mg2+) resorption occurs predominantly through a paracellular conductance in the thick ascending limb of Henle (TAL). Here, positional cloning has identified a human gene, paracellin-1 (PCLN-1), mutations in which cause renal Mg2+ wasting. PCLN-1 is located in tight junctions of the TAL and is related to the claudin family of tight junction proteins. These findings provide insight into Mg2+ homeostasis, demonstrate the role of a tight junction protein in human disease, and identify an essential component of a selective paracellular conductance.

Cell localization and ontogeny of sodium transport pathways in the distal nephron: perspectives in function and failure

The expression and function of ion co-transporters/exchangers/channels in the distal nephron have recently been defined. The role of cation-chloride co-transporters and proteins implicated in aldosterone target cell function are reported in the adult and during ontogeny. Volume disorders can currently be related to identified gene products acting in defined nephron sites.