Role of with-no-lysine [K] kinases in the pathogenesis of Gordon's syndrome

Deletion of Kir5.1 Impairs Renal Ability to Excrete Potassium during Increased Dietary Potassium Intake

BACKGROUND

The basolateral potassium channel in the distal convoluted tubule (DCT), comprising the inwardly rectifying potassium channel Kir4.1/Kir5.1 heterotetramer, plays a key role in mediating the effect of dietary potassium intake on the thiazide-sensitive NaCl cotransporter (NCC). The role of Kir5.1 (encoded by Kcnj16) in mediating effects of dietary potassium intake on the NCC and renal potassium excretion is unknown.

METHODS

We used electrophysiology, renal clearance, and immunoblotting to study Kir4.1 in the DCT and NCC in Kir5.1 knockout (Kcnj16-/- ) and wild-type (Kcnj16+/+ ) mice fed with normal, high, or low potassium diets.

RESULTS

We detected a 40-pS and 20-pS potassium channel in the basolateral membrane of the DCT in wild-type and knockout mice, respectively. Compared with wild-type, Kcnj16-/- mice fed a normal potassium diet had higher basolateral potassium conductance, a more negative DCT membrane potential, higher expression of phosphorylated NCC (pNCC) and total NCC (tNCC), and augmented thiazide-induced natriuresis. Neither high- nor low-potassium diets affected the basolateral DCT's potassium conductance and membrane potential in Kcnj16-/- mice. Although high potassium reduced and low potassium increased the expression of pNCC and tNCC in wild-type mice, these effects were absent in Kcnj16-/- mice. High potassium intake inhibited and low intake augmented thiazide-induced natriuresis in wild-type but not in Kcnj16-/- mice. Compared with wild-type, Kcnj16-/- mice with normal potassium intake had slightly lower plasma potassium but were more hyperkalemic with prolonged high potassium intake and more hypokalemic during potassium restriction.

CONCLUSIONS

Kir5.1 is essential for dietary potassium's effect on NCC and for maintaining potassium homeostasis.

Hyperkalemia in young children: blood pressure checked?

Hyperkalemia in young children is a rare phenomenon and in many cases caused by hemolysis in the specimen due to difficulties in obtaining a sample. However, hyperkalemia can also be a sign of a rare Mendelian syndrome known as familial hyperkalemic hypertension or pseudohypoaldosteronism type II. This disease is characterized by hyperkalemia, hypertension, and mild hyperchloremic metabolic acidosis (with normal anion gap) despite normal glomerular filtration. Full recovery of these abnormalities with thiazide diuretics is essential not to miss the diagnosis of this syndrome. We describe two young patients with hyperkalemia as an incidental finding who were subsequently diagnosed with this rare endocrine disorder. Genetic testing revealed mutations in two recently discovered genes, the study of which has helped to unravel the pathophysiologic pathways.

CONCLUSION

In patients with hyperkalemia and a normal glomerular filtration rate, the clinician should actively search for abnormalities in blood pressure since recognizing this condition can lead to simple, cheap, and effective treatment. What is Known: • True Hyperkalemia is rare in pediatrics and can be a sign of FHHt. What is New: • KLHL3 & CUL3 are recently discovered genes helping unravel the pathophysiologic pathway of FHHt.

(WNK)ing at death: With-no-lysine (Wnk) kinases in neuropathies and neuronal survival

Members of With-no-lysine (WNK) family of serine-threonine kinase are key regulators of chloride ion transport in diverse cell types, controlling the activity and the surface expression of cation-chloride (Na(+)/K(+)-Cl(-)) co-transporters. Mutations in WNK1 and WNK4 are linked to a hereditary form of hypertension, and WNKs have been extensively investigated pertaining to their roles in renal epithelial ion homeostasis. However, some members of the WNK family and their splice isoforms are also expressed in the mammalian brain, and have been implicated in aspects of hereditary neuropathy as well as neuronal and glial survival. WNK2, which is exclusively enriched in neurons, is well known as an anti-proliferative tumor suppressor. WNK3, on the other hand, appears to promote cell survival as its inhibition enhances neuronal apoptosis. However, loss of WNK3 has been recently shown to reduce ischemia-associated brain damage. In this review, I surveyed the potentially context-dependent roles of WNKs in neurological disorders and neuronal survival.

Developmental changes in renal tubular transport-an overview

The adult kidney maintains a constant volume and composition of extracellular fluid despite changes in water and salt intake. The neonate is born with a kidney that has a small fraction of the glomerular filtration rate of the adult and immature tubules that function at a lower capacity than that of the mature animal. Nonetheless, the neonate is also able to maintain a constant extracellular fluid volume and composition. Postnatal renal tubular development was once thought to be due to an increase in the transporter abundance to meet the developmental increase in glomerular filtration rate. However, postnatal renal development of each nephron segment is quite complex. There are isoform changes of several transporters as well as developmental changes in signal transduction that affect the capacity of renal tubules to reabsorb solutes and water. This review will discuss neonatal tubular function with an emphasis on the differences that have been found between the neonate and adult. We will also discuss some of the factors that are responsible for the maturational changes in tubular transport that occur during postnatal renal development.

Domain-Swapping Switch Point in Ste20 Protein Kinase SPAK

The related protein kinases SPAK and OSR1 regulate ion homeostasis in part by phosphorylating cation cotransporter family members. The structure of the kinase domain of OSR1 was determined in the unphosphorylated inactive form and, like some other Ste20 kinases, exhibited a domain-swapped activation loop. To further probe the role of domain swapping in SPAK and OSR1, we have determined the crystal structures of SPAK 63-403 at 3.1 Å and SPAK 63-390 T243D at 2.5 Å resolution. These structures encompass the kinase domain and different portions of the C-terminal tail, the longer without and the shorter with an activating T243D point mutation. The structure of the T243D protein reveals significant conformational differences relative to unphosphorylated SPAK and OSR1 but also has some features of an inactive kinase. Both structures are domain-swapped dimers. Sequences involved in domain swapping were identified and mutated to create a SPAK monomeric mutant with kinase activity, indicating that monomeric forms are active. The monomeric mutant is activated by WNK1 but has reduced activity toward its substrate NKCC2, suggesting regulatory roles for domain swapping. The structure of partially active SPAK T243D is consistent with a multistage activation process in which phosphorylation induces a SPAK conformation that requires further remodeling to build the active structure.

[Electrolyte disorders as a hallmark of monogenetic diseases]

In daily clinical practice, the term electrolyte generally refers to sodium, potassium, chloride, calcium, and magnesium ions. In addition to their many functions, such as neuronal and muscular transmission, some electrolytes also contribute to osmolality and maintenance of electrochemical gradients, which, in turn enable many transport processes. The absorption and reabsorption of electrolytes occurs via polarized cell assemblies, i.e., epithelia. Besides the intestine (absorption), the most important organ is the kidney. Here, following glomerular filtration, electrolytes are reabsorbed via trans- and paracellular mechanisms along the renal tubular system. In the past, the identification and elucidation of transport-associated monogenetic disorders has contributed tremendously to our understanding of the physiology and pathophysiology of such transport mechanisms. Sodium reabsorption mechanisms along the tubular system have been characterized by means of pharmacological compounds for a long time. However, only with the development of novel molecular genetic tools and approaches has it been possible to clarify the genetic basis of distinct diseases. As examples, we discuss here Bartter and Gitelman syndrome, and other sodium disorders such as pseudohypoaldosteronism and Liddle Syndrome. Diagnosis, clinical presentation, and therapy are briefly described. Furthermore, examples of magnesium homeostasis disorders are also presented, the molecular mechanisms and pathophysiology of which could also be characterized by the identification of different human mutations.

Regulation of OSR1 and the sodium, potassium, two chloride cotransporter by convergent signals

The Ste20 family protein kinases oxidative stress-responsive 1 (OSR1) and the STE20/SPS1-related proline-, alanine-rich kinase directly regulate the solute carrier 12 family of cation-chloride cotransporters and thereby modulate a range of processes including cell volume homeostasis, blood pressure, hearing, and kidney function. OSR1 and STE20/SPS1-related proline-, alanine-rich kinase are activated by with no lysine [K] protein kinases that phosphorylate the essential activation loop regulatory site on these kinases. We found that inhibition of phosphoinositide 3-kinase (PI3K) reduced OSR1 activation by osmotic stress. Inhibition of the PI3K target pathway, the mammalian target of rapamycin complex 2 (mTORC2), by depletion of Sin1, one of its components, decreased activation of OSR1 by sorbitol and reduced activity of the OSR1 substrate, the sodium, potassium, two chloride cotransporter, in HeLa cells. OSR1 activity was also reduced with a pharmacological inhibitor of mTOR. mTORC2 phosphorylated OSR1 on S339 in vitro, and mutation of this residue eliminated OSR1 phosphorylation by mTORC2. Thus, we identify a previously unrecognized connection of the PI3K pathway through mTORC2 to a Ste20 protein kinase and ion homeostasis.

A molecular update on pseudohypoaldosteronism type II

The DCT (distal convoluted tubule) is the site of microregulation of water reabsorption and ion handling in the kidneys, which is mainly under the control of aldosterone. Aldosterone binds to and activates mineralocorticoid receptors, which ultimately lead to increased sodium reabsorption in the distal part of the nephron. Impairment of mineralocorticoid signal transduction results in resistance to aldosterone and mineralocorticoids, and, therefore, causes disturbances in electrolyte balance. Pseudohypoaldosteronism type II (PHAII) or familial hyperkalemic hypertension (FHHt) is a rare, autosomal dominant syndrome characterized by hypertension, hyperkalemia, metabolic acidosis, elevated or low aldosterone levels, and decreased plasma renin activity. PHAII is caused by mutations in the WNK isoforms (with no lysine kinase), which regulate the Na-Cl and Na-K-Cl cotransporters (NCC and NKCC2, respectively) and the renal outer medullary potassium (ROMK) channel in the DCT. This review focuses on new candidate genes such as KLHL3 and Cullin3, which are instrumental to unraveling novel signal transductions pathways involving NCC, to better understand the cause of PHAII along with the molecular mechanisms governing the pathophysiology of PHAII and its clinical manifestations.

Interactions with WNK (with no lysine) family members regulate oxidative stress response 1 and ion co-transporter activity

Two of the four WNK (with no lysine (K)) protein kinases are associated with a heritable form of ion imbalance culminating in hypertension. WNK1 affects ion transport in part through activation of the closely related Ste20 family protein kinases oxidative stress-responsive 1 (OSR1) and STE20/SPS1-related proline-, alanine-rich kinase (SPAK). Once activated by WNK1, OSR1 and SPAK phosphorylate and stimulate the sodium, potassium, two chloride co-transporters, NKCC1 and NKCC2, and also affect other related ion co-transporters. We find that WNK1 and OSR1 co-localize on cytoplasmic puncta in HeLa and other cell types. We show that the C-terminal region of WNK1 including a coiled coil is sufficient to localize the fragment in a manner similar to the full-length protein, but some other fragments lacking this region are mislocalized. Photobleaching experiments indicate that both hypertonic and hypotonic conditions reduce the mobility of GFP-WNK1 in cells. The four WNK family members can phosphorylate the activation loop of OSR1 to increase its activity with similar kinetic constants. C-terminal fragments of WNK1 that contain three RFXV interaction motifs can bind OSR1, block activation of OSR1 by sorbitol, and prevent the OSR1-induced enhancement of ion co-transporter activity in cells, further supporting the conclusion that association with WNK1 is required for OSR1 activation and function at least in some contexts. C-terminal WNK1 fragments can be phosphorylated by OSR1, suggesting that OSR1 catalyzes feedback phosphorylation of WNK1.

Syndromes of impaired ion handling in the distal nephron: pseudohypoaldosteronism and familial hyperkalemic hypertension

The distal nephron, which is the site of the micro-regulation of water absorption and ion handling in the kidneys, is under the control of aldosterone. Impairment of the mineralocorticoid signal transduction pathway results in resistance to the action of aldosterone and of mineralocorticoids in general. Herein, we review two syndromes in which ion handling in the distal nephron is impaired: pseudohypoaldosteronism (PHA) and familial hyperkalemic hypertension (FHH). PHA is a rare inherited syndrome characterized by mineralocorticoid resistance, which leads to salt loss, hypotension, hyperkalemia and metabolic acidosis. There are two types of this syndrome: a renal (autosomal dominant) type due to mutations of the mineralocorticoid receptor (MR), and a systemic (autosomal recessive) type due to mutations of the epithelial sodium channel (ENaC). There is also a transient form of PHA, which may be due to urinary tract infections, obstructive uropathy or several medications. FHH is a rare autosomal dominant syndrome, characterized by salt retention, hypertension, hyperkalemia and metabolic acidosis. In FHH, mutations of WNK (with-no-lysine kinase) 4 and 1 alter the activity of several ion transportation systems in the distal nephron. The study of the pathophysiology of PHA and FHH greatly elucidated our understanding of the renin-angiotensin-aldosterone system function and ion handling in the distal nephron. The physiological role of the distal nephron and the pathophysiology of diseases in which the renal tubule is implicated may hence be better understood and, based on this understanding, new drugs can be developed.

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.

WNK1 is required for mitosis and abscission

WNK [with no lysine (K)] protein kinases are found in all sequenced multicellular and many unicellular organisms. WNKs influence ion balance. Two WNK family members are associated with a single gene form of hypertension. RNA interference screens have implicated WNKs in survival and growth, and WNK1 is essential for viability of mice. We found that the majority of WNK1 is localized on cytoplasmic puncta in resting cells. During cell division, WNK1 localizes to mitotic spindles. Therefore, we analyzed mitotic phenotypes in WNK1 knockdown cells. A large percentage of WNK1 knockdown cells fail to complete cell division, displaying defects in mitotic spindles and also in abscission and cell survival. One of the best-characterized WNK1 targets is the protein kinase OSR1 (oxidative stress responsive 1). OSR1 regulates ion cotransporters, is activated in response to osmotic stress by WNK family members, and is largely associated with WNK1. In resting cells, the majority of OSR1, like WNK1, is on cytoplasmic puncta. OSR1 is also in nuclei. In contrast to WNK1, however, OSR1 does not concentrate around spindles during mitosis and does not show a WNK1-like localization pattern in mitotic cells. Knockdown of OSR1 has only a modest effect on cell survival and does not lead to spindle defects. We conclude that decreased cell survival associated with loss of WNK1 is attributable to defects in chromosome segregation and abscission and is independent of the effector kinase OSR1.

Protein kinase WNK1 promotes cell surface expression of glucose transporter GLUT1 by regulating a Tre-2/USP6-BUB2-Cdc16 domain family member 4 (TBC1D4)-Rab8A complex

One mechanism by which mammalian cells regulate the uptake of glucose is the number of glucose transporter proteins (GLUT) present at the plasma membrane. In insulin-responsive cells types, GLUT4 is released from intracellular stores through inactivation of the Rab GTPase activating protein Tre-2/USP6-BUB2-Cdc16 domain family member 4 (TBC1D4) (also known as AS160). Here we describe that TBC1D4 forms a protein complex with protein kinase WNK1 in human embryonic kidney (HEK293) cells. We show that WNK1 phosphorylates TBC1D4 in vitro and that the expression levels of WNK1 in these cells regulate surface expression of the constitutive glucose transporter GLUT1. WNK1 was found to increase the binding of TBC1D4 to regulatory 14-3-3 proteins while reducing its interaction with the exocytic small GTPase Rab8A. These effects were dependent on the catalytic activity because expression of a kinase-dead WNK1 mutant had no effect on binding of 14-3-3 and Rab8A, or on surface GLUT1 levels. Together, the data describe a pathway regulating constitutive glucose uptake via GLUT1, the expression level of which is related to several human diseases.

Serum and glucocorticoid-induced kinase (SGK) 1 and the epithelial sodium channel are regulated by multiple with no lysine (WNK) family members

The four WNK (with no lysine (K)) protein kinases affect ion balance and contain an unusual protein kinase domain due to the unique placement of the active site lysine. Mutations in two WNKs cause a heritable form of ion imbalance culminating in hypertension. WNK1 activates the serum- and glucocorticoid-induced protein kinase SGK1; the mechanism is noncatalytic. SGK1 increases membrane expression of the epithelial sodium channel (ENaC) and sodium reabsorption via phosphorylation and sequestering of the E3 ubiquitin ligase neural precursor cell expressed, developmentally down-regulated 4-2 (Nedd4-2), which otherwise promotes ENaC endocytosis. Questions remain about the intrinsic abilities of WNK family members to regulate this pathway. We find that expression of the N termini of all four WNKs results in modest to strong activation of SGK1. In reconstitution experiments in the same cell line all four WNKs also increase sodium current blocked by the ENaC inhibitor amiloride. The N termini of the WNKs also have the capacity to interact with SGK1. More detailed analysis of activation by WNK4 suggests mechanisms in common with WNK1. Further evidence for the importance of WNK1 in this process comes from the ability of Nedd4-2 to bind to WNK1 and the finding that endogenous SGK1 has reduced activity if WNK1 is knocked down by small interfering RNA.

WNK kinases and blood pressure control

The WNK (With No K-Lysine) family of proteins is widely expressed and has been shown to promote blood pressure homeostasis through a variety of mechanisms. Members of this family have been reported to affect sodium/chloride cotransporters, sodium/potassium/chloride cotransporters, potassium/chloride cotransporters, the renal outer medullary potassium channel, and the epithelial sodium channel, directly and indirectly. Mutations in WNK1 and WNK4 were shown to cause pseudohypoaldosteronism type II, a Mendelian disorder characterized by hypertension, hyperkalemia, and acidosis. Because of the complexity of the renal system, it has been difficult to completely define the role of these kinases in kidney function. This article reviews current knowledge of the role of these proteins in ion homeostasis and volume control.

A novel locus for arterial hypertension on chromosome 1p36 maps to a metabolic syndrome trait cluster in the Sorbs, a Slavic population isolate in Germany

OBJECTIVE

Genome-wide linkage studies and genome-wide association studies have not as yet identified major genes contributing to primary hypertension in the general population. This state-of-affairs suggests considerable heterogeneity with small contributing effects for primary hypertension, or other complex genetic traits, in outbred populations. Isolated populations, as recent data from Iceland and French Canada suggest, could offer a solution to this problem.

METHODS

We studied a Slavic isolate in Germany, the Sorbs, and genotyped 1040 polymorphic microsatellite markers in 87 multigeneration families.

RESULTS

Our genome-wide linkage scan revealed a locus on chromosome 1p36.13 at D1S3669-D1S2826 (40.95 cM Marshfield coordinates; logarithm of the odds = 3.45, nominal P = 0.00003) that reached genome-wide significance (P = 0.004), indicating the increased power in isolated populations. The chromosome 1 locus maps to a region in which traits such as diabetes, hyperlipidemia, obesity and BMI cluster.

CONCLUSION

Our results suggest that this locus contributes to the metabolic syndrome, and that further attention in this and other populations is warranted.

LINGO-1 interacts with WNK1 to regulate nogo-induced inhibition of neurite extension

LINGO-1 is a component of the tripartite receptor complexes, which act as a convergent mediator of the intracellular signaling in response to myelin-associated inhibitors and lead to collapse of growth cone and inhibition of neurite extension. Although the function of LINGO-1 has been intensively studied, its downstream signaling remains elusive. In the present study, a novel interaction between LINGO-1 and a serine-threonine kinase WNK1 was identified by yeast two-hybrid screen. The interaction was further validated by fluorescence resonance energy transfer and co-immunoprecipitation, and this interaction was intensified by Nogo66 treatment. Morphological evidences showed that WNK1 and LINGO-1 were co-localized in cortical neurons. Furthermore, either suppressing WNK1 expression by RNA interference or overexpression of WNK1-(123-510) attenuated Nogo66-induced inhibition of neurite extension and inhibited the activation of RhoA. Moreover, WNK1 was identified to interact with Rho-GDI1, and this interaction was attenuated by Nogo66 treatment, further indicating its regulatory effect on RhoA activation. Taken together, our results suggest that WNK1 is a novel signaling molecule involved in regulation of LINGO-1 mediated inhibition of neurite extension.

Mutations in the nervous system--specific HSN2 exon of WNK1 cause hereditary sensory neuropathy type II

Hereditary sensory and autonomic neuropathy type II (HSANII) is an early-onset autosomal recessive disorder characterized by loss of perception to pain, touch, and heat due to a loss of peripheral sensory nerves. Mutations in hereditary sensory neuropathy type II (HSN2), a single-exon ORF originally identified in affected families in Quebec and Newfoundland, Canada, were found to cause HSANII. We report here that HSN2 is a nervous system-specific exon of the with-no-lysine(K)-1 (WNK1) gene. WNK1 mutations have previously been reported to cause pseudohypoaldosteronism type II but have not been studied in the nervous system. Given the high degree of conservation of WNK1 between mice and humans, we characterized the structure and expression patterns of this isoform in mice. Immunodetections indicated that this Wnk1/Hsn2 isoform was expressed in sensory components of the peripheral nervous system and CNS associated with relaying sensory and nociceptive signals, including satellite cells, Schwann cells, and sensory neurons. We also demonstrate that the novel protein product of Wnk1/Hsn2 was more abundant in sensory neurons than motor neurons. The characteristics of WNK1/HSN2 point to a possible role for this gene in the peripheral sensory perception deficits characterizing HSANII.

The thiazide-sensitive Na-Cl cotransporter is regulated by a WNK kinase signaling complex

The pathogenesis of essential hypertension remains unknown, but thiazide diuretics are frequently recommended as first-line treatment. Recently, familial hyperkalemic hypertension (FHHt) was shown to result from activation of the thiazide-sensitive Na-Cl cotransporter (NCC) by mutations in WNK4, although the mechanism for this effect remains unknown. WNK kinases are unique members of the human kinome, intimately involved in maintaining electrolyte balance across cell membranes and epithelia. Previous work showed that WNK1, WNK4, and a kidney-specific isoform of WNK1 interact to regulate NCC activity, suggesting that WNK kinases form a signaling complex. Here, we report that WNK3, another member of the WNK kinase family expressed by distal tubule cells, interacts with WNK4 and WNK1 to regulate NCC in both human kidney cells and Xenopus oocytes, further supporting the WNK signaling complex hypothesis. We demonstrate that physiological regulation of NCC in oocytes results from antagonism between WNK3 and WNK4 and that FHHt-causing WNK4 mutations exert a dominant-negative effect on wild-type (WT) WNK4 to mimic a state of WNK3 excess. The results provide a mechanistic explanation for the divergent effects of WT and FHHt-mutant WNK4 on NCC activity, and for the dominant nature of FHHt in humans and genetically modified mice.

SLC26A9 is a Cl(-) channel regulated by the WNK kinases

SLC26A9 is a member of the SLC26 family of anion transporters, which is expressed at high levels in airway and gastric surface epithelial cells. The transport properties and regulation of SLC26A9, and thus its physiological function, are not known. Here we report that SLC26A9 is a highly selective Cl(-) channel with minimal OH(-)/HCO(3)(-) permeability that is regulated by the WNK kinases. Expression in Xenopus oocytes and simultaneous measurement of membrane potential or current, intracellular pH (pH(i)) and intracellular Cl(-) (Cl(-)(i)) revealed that expression of SLC26A9 resulted in a large Cl(-) current. SLC26A9 displays a selectivity sequence of I(-) > Br(-) > NO(3)(-) > Cl(-) > Glu(-), but it conducts Br(-) > Cl(-) > I(-) > NO(3)(-) > Glu(-), with NO(3)(-) and I(-) inhibiting the Cl(-) conductance. Similarly, expression of SLC26A9 in HEK cells resulted in a large Cl(-) current. Although detectable, OH(-) and HCO(3)(-) fluxes in oocytes expressing SLC26A9 were very small. Moreover, HCO(3)(-) had no discernable effect on the Cl(-) current, the reversal potential in the presence or absence of Cl(-)(o) and, importantly, HCO(3)(-) had no effect on Cl(-) fluxes. These findings indicate that SLC26A9 is a Cl(-) channel with minimal OH(-)/HCO(3)(-) permeability. Co-expression of SLC26A9 with the WNK kinases WNK1, WNK3 or WNK4 inhibited SLC26A9 activity, and the inhibition was independent of WNK kinase activity. Immunolocalization in oocytes and cell surface biotinylation in HEK cells indicated that the WNK-mediated inhibition of SLC26A9 activity is caused by reduced SLC26A9 surface expression. Expression of SLC26A9 in the airway and the response of the WNKs to homeostatic stress raise the possibility that SLC26A9 serves to mediate the response of the airway to stress.

A review of the genetics of essential hypertension

PURPOSE OF REVIEW

Essential hypertension affects more than 20% of the adult population, and has a multifactorial origin arising from an interaction between susceptibility genes and environmental factors. Several strategies have been used to identify hypertension susceptibility genes. This review highlights recent efforts in genetic dissection of essential hypertension.

RECENT FINDINGS

Recently, further chromosomal regions harboring blood pressure loci have emerged in genome-wide linkage studies. Findings from a new systematic two-dimensional genome scan are presented, as well as sex-specific loci linked to hypertension in inbred rodent models. Many case-control association studies have been carried out, but results so far have been equivocal. This review discusses some interesting studies combining linkage and association strategies using gene-gene interactions, and studies the use of haplotypes instead of SNPs. Two novel hypertension susceptibility genes are presented, and a short summary on new insights into genes of the renin-angiotensin and adrenergic systems is given.

SUMMARY

To date, linkage and association studies have not been convincing. Genome-wide association studies may prove to be an effective approach to the problems posed by complex traits. Combined with candidate gene approaches, it is hoped this strategy will yield convincing evidence for genes associated with essential hypertension.

WNK4-mediated regulation of renal ion transport proteins

Point mutations in WNK4 [for With No K (lysine)], a serine-threonine kinase that is expressed in the distal nephron of the kidney, are linked to familial hyperkalemic hypertension (FHH). The imbalanced electrolyte homeostasis in FHH has led to studies toward an understanding of WNK4-mediated regulation of ion transport proteins in the kidney. A growing number of ion transport proteins for Na(+), K(+), Ca(2+), and Cl(-), including ion channels and transporters in the transcellular pathway and claudins in the paracellular pathway, are shown to be regulated by WNK4 from studies using models ranging from Xenopus laevis oocytes to transgenic and knockin mice. WNK4 regulates these transport proteins in different directions and by different cellular mechanisms. The common theme of WNK4-mediated regulation is to alter the abundance of ion transport proteins at the plasma membrane, with the exception of claudins, which are phosphorylated in the presence of WNK4. The regulation of WNK4 can be blocked by the full-length WNK1, whose action is in turn antagonized by a kidney-specific WNK1 variant lacking the kinase domain. In addition, WNK4 also activates stress-related serine-threonine kinases to regulate members of the SLC12 family members of cation-chloride cotransporters. In many cases, the FHH-causing mutants of WNK4 exhibit differences from wild-type WNK4 in regulating ion transport proteins. These regulations well explain the clinical features of FHH and provide insights into the multilayered regulation of ion transport processes in the distal nephron.

Potassium transport in the maturing kidney

The distal nephron and colon are the primary sites of regulation of potassium (K(+)) homeostasis, responsible for maintaining a zero balance in adults and net positive balance in growing infants and children. Distal nephron segments can either secrete or reabsorb K(+) depending on the metabolic needs of the organism. In the healthy adult kidney, K(+) secretion predominates over K(+) absorption. Baseline K(+) secretion occurs via the apical low-conductance secretory K(+) (SK) channel, whereas the maxi-K channel mediates flow-stimulated net urinary K(+) secretion. The K(+) retention characteristic of the neonatal kidney appears to be due not only to the absence of apical secretory K(+) channels in the distal nephron but also to a predominance of apical H-K-adenosine triphosphatase (ATPase), which presumably mediates K(+) absorption. Both luminal and peritubular factors regulate the balance between K(+) secretion and absorption. Perturbation in any of these factors can lead to K(+) imbalance. In turn, these factors may serve as effective targets for the treatment of both hyper-and hypokalemia. The purpose of this review is to present an overview of recent advances in our understanding of mechanisms of K(+) transport in the maturing kidney.

Biological Cross-talk between WNK1 and the Transforming Growth Factor β-Smad Signaling Pathway

WNKs (with no lysine (K)), unique serine/threonine protein kinases, have been best studied in the context of cell volume regulation and ion homeostasis. Here we describe a biological link between WNKs and transforming growth factor (TGF) beta-Smad signaling. Both WNK1 and WNK4 directly bind to and phosphorylate Smad2. Knockdown of WNK1 in HeLa cells using small interfering RNA reduces Smad2 protein expression; this decrease is at least partially due to down-regulation of Smad2 transcription. In contrast, phosphorylated Smad2 significantly accumulated in the nucleus as a consequence of depletion of WNK1, resulting in Smad-mediated transcriptional responses. In addition, TGFbeta-induced target gene transcripts were increased in WNK1 small interfering RNA cells. These findings suggest WNK1 as a dual modulator of TGFbeta-Smad signaling pathways.

Molecular pathogenesis of pseudohypoaldosteronism type II: generation and analysis of a Wnk4(D561A/+) knockin mouse model

WNK1 and WNK4 mutations have been reported to cause pseudohypoaldosteronism type II (PHAII), an autosomal-dominant disorder characterized by hyperkalemia and hypertension. To elucidate the molecular pathophysiology of PHAII, we generated Wnk4(D561A/+) knockin mice presenting the phenotypes of PHAII. The knockin mice showed increased apical expression of phosphorylated Na-Cl cotransporter (NCC) in the distal convoluted tubules. Increased phosphorylation of the kinases OSR1 and SPAK was also observed in the knockin mice. Apical localization of the ROMK potassium channel and transepithelial Cl(-) permeability in the cortical collecting ducts were not affected in the knockin mice, whereas activity of epithelial Na(+) channels (ENaC) was increased. This increase, however, was not evident after hydrochlorothiazide treatment, suggesting that the regulation of ENaC was not a genetic but a secondary effect. Thus, the pathogenesis of PHAII caused by a missense mutation of WNK4 was identified to be increased function of NCC through activation of the OSR1/SPAK-NCC phosphorylation cascade.

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.

Advances in genetic hypertension

PURPOSE OF REVIEW

Mendelian forms of hypertension are rare genetic disorders that cause severe hypertension. This review will explore the recently identified molecular mechanisms and pathogenesis of genetic disorders that cause hypertension in children.

RECENT FINDINGS

Hypertension is now believed to be a polygenic disorder resulting from the interaction of multiple genes and the environment. A few forms of severe hypertension have been linked to single genes. The genes responsible for these disorders have all been cloned and all participate in pathways involved in heightened renal sodium reabsorption. The increased sodium reabsorption arises in the distal nephron and leads to volume expansion and hypertension.

SUMMARY

Investigating forms of monogenic hypertension has advanced the understanding of sodium transport and volume control by the kidney. Future studies will identify novel genes, pathways and treatment targets important in the fight against primary hypertension.