Renal phenotype in mice lacking the Kir5.1 (Kcnj16) K+ channel subunit contrasts with that observed in SeSAME/EAST syndrome

HNF1B controls epithelial organization and cell polarity during ureteric bud branching and collecting duct morphogenesis

Kidney development depends crucially on proper ureteric bud branching giving rise to the entire collecting duct system. The transcription factor HNF1B is required for the early steps of ureteric bud branching, yet the molecular and cellular events regulated by HNF1B are poorly understood. We report that specific removal of Hnf1b from the ureteric bud leads to defective cell-cell contacts and apicobasal polarity during the early branching events. High-resolution ex vivo imaging combined with a membranous fluorescent reporter strategy show decreased mutant cell rearrangements during mitosis-associated cell dispersal and severe epithelial disorganization. Molecular analysis reveals downregulation of Gdnf-Ret pathway components and suggests that HNF1B acts both upstream and downstream of Ret signaling by directly regulating Gfra1 and Etv5 Subsequently, Hnf1b deletion leads to massively mispatterned ureteric tree network, defective collecting duct differentiation and disrupted tissue architecture, which leads to cystogenesis. Consistently, mRNA-seq analysis shows that the most impacted genes encode intrinsic cell-membrane components with transporter activity. Our study uncovers a fundamental and recurring role of HNF1B in epithelial organization during early ureteric bud branching and in further patterning and differentiation of the collecting duct system in mouse.

Essential role of Kir5.1 channels in renal salt handling and blood pressure control

Supplementing diets with high potassium helps reduce hypertension in humans. Inwardly rectifying K+ channels Kir4.1 (Kcnj10) and Kir5.1 (Kcnj16) are highly expressed in the basolateral membrane of distal renal tubules and contribute to Na+ reabsorption and K+ secretion through the direct control of transepithelial voltage. To define the importance of Kir5.1 in blood pressure control under conditions of salt-induced hypertension, we generated a Kcnj16 knockout in Dahl salt-sensitive (SS) rats (SSKcnj16-/-). SSKcnj16-/- rats exhibited hypokalemia and reduced blood pressure, and when fed a high-salt diet (4% NaCl), experienced 100% mortality within a few days triggered by salt wasting and severe hypokalemia. Electrophysiological recordings of basolateral K+ channels in the collecting ducts isolated from SSKcnj16-/- rats revealed activity of only homomeric Kir4.1 channels. Kir4.1 expression was upregulated in SSKcnj16-/- rats, but the protein was predominantly localized in the cytosol in SSKcnj16-/- rats. Benzamil, but not hydrochlorothiazide or furosemide, rescued this phenotype from mortality on a high-salt diet. Supplementation of high-salt diet with increased potassium (2% KCl) prevented mortality in SSKcnj16-/- rats and prevented or mitigated hypertension in SSKcnj16-/- or control SS rats, respectively. Our results demonstrate that Kir5.1 channels are key regulators of renal salt handling in SS hypertension.

Loss of transcriptional activation of the potassium channel Kir5.1 by HNF1β drives autosomal dominant tubulointerstitial kidney disease

Hepatocyte nuclear factor 1 homeobox B (HNF1β) is an essential transcription factor for the development and functioning of the kidney. Mutations in HNF1β cause autosomal dominant tubulointerstitial kidney disease characterized by renal cysts and maturity-onset diabetes of the young (MODY). Moreover, these patients suffer from a severe electrolyte phenotype consisting of hypomagnesemia and hypokalemia. Until now, genes that are regulated by HNF1β are only partially known and do not fully explain the phenotype of the patients. Therefore, we performed chIP-seq in the immortalized mouse kidney cell line mpkDCT to identify HNF1β binding sites on a genome-wide scale. In total 7,421 HNF1β-binding sites were identified, including several genes involved in electrolyte transport and diabetes. A highly specific and conserved HNF1β site was identified in the promoter of Kcnj16 that encodes the potassium channel Kir5.1. Luciferase-promoter assays showed a 2.2-fold increase in Kcnj16 expression when HNF1β was present. Expression of the Hnf1β p.Lys156Glu mutant, previously identified in a patient with autosomal dominant tubulointerstitial kidney disease, did not activate Kcnj16 expression. Knockdown of Hnf1β in mpkDCT cells significantly reduced the appearance of Kcnj16 (Kir5.1) and Kcnj10 (Kir4.1) by 38% and 37%, respectively. These results were confirmed in a HNF1β renal knockout mouse which exhibited downregulation of Kcnj16, Kcnj10 and Slc12a3 transcripts in the kidney by 78%, 83% and 76%, respectively, compared to HNF1β wild-type mice. Thus, HNF1β is a transcriptional activator of Kcnj16. Hence, patients with HNF1β mutations may have reduced Kir5.1 activity in the kidney, resulting in hypokalemia and hypomagnesemia.

Removable Backbone Modification Method for the Chemical Synthesis of Membrane Proteins

Chemical synthesis can produce water-soluble globular proteins bearing specifically designed modifications. These synthetic molecules have been used to study the biological functions of proteins and to improve the pharmacological properties of protein drugs. However, the above advances notwithstanding, membrane proteins (MPs), which comprise 20-30% of all proteins in the proteomes of most eukaryotic cells, remain elusive with regard to chemical synthesis. This difficulty stems from the strong hydrophobic character of MPs, which can cause considerable handling issues during ligation, purification, and characterization steps. Considerable efforts have been made to improve the solubility of transmembrane peptides for chemical ligation. These methods can be classified into two main categories: the manipulation of external factors and chemical modification of the peptide. This Account summarizes our research advances in the development of chemical modification especially the two generations of removable backbone modification (RBM) strategy for the chemical synthesis of MPs. In the first RBM generation, we install a removable modification group at the backbone amide of Gly within the transmembrane peptides. In the second RBM generation, the RBM group can be installed into all primary amino acid residues. The second RBM strategy combines the activated intramolecular O-to-N acyl transfer reaction, in which a phenyl group remains unprotected during the coupling process, which can play a catalytic role to generate the activated phenyl ester to assist in the formation of amide. The key feature of the RBM group is its switchable stability in trifluoroacetic acid. The stability of these backbone amide N-modifications toward TFA can be modified by regulating the electronic effects of phenol groups. The free phenol group is acylated to survive the TFA deprotection step, while the acyl phenyl ester will be quantitatively hydrolyzed in a neutral aqueous solution, and the free phenol group increases the electron density of the benzene ring to make the RBM labile to TFA. The transmembrane peptide segment bearing RBM groups behaves like a water-soluble peptide during fluorenylmethyloxycarbonyl based solid-phase peptide synthesis (Fmoc SPPS), ligation, purification, and characterization. The quantitative removal of the RBM group can be performed to obtain full-length MPs. The RBM strategy was used to prepare the core transmembrane domain Kir5.1[64-179] not readily accessible by recombinant protein expression, the influenza A virus M2 proton channel with phosphorylation, the cation-specific ion channel p7 from the hepatitis C virus with site-specific NMR isotope labels, and so on. The RBM method enables the practical engineering of small- to medium-sized MPs or membrane protein domains to address fundamental questions in the biochemical, biophysical, and pharmaceutical sciences.

Potassium Sensing by Renal Distal Tubules Requires Kir4.1

The mammalian distal convoluted tubule (DCT) makes an important contribution to potassium homeostasis by modulating NaCl transport. The thiazide-sensitive Na⁺/Cl^- cotransporter (NCC) is activated by low potassium intake and by hypokalemia. Coupled with suppression of aldosterone secretion, activation of NCC helps to retain potassium by increasing electroneutral NaCl reabsorption, therefore reducing Na⁺/K⁺ exchange. Yet the mechanisms by which DCT cells sense plasma potassium concentration and transmit the information to the apical membrane are not clear. Here, we tested the hypothesis that the potassium channel Kir4.1 is the potassium sensor of DCT cells. We generated mice in which Kir4.1 could be deleted in the kidney after the mice are fully developed. Deletion of Kir4.1 in these mice led to moderate salt wasting, low BP, and profound potassium wasting. Basolateral membranes of DCT cells were depolarized, nearly devoid of conductive potassium transport, and unresponsive to plasma potassium concentration. Although renal WNK4 abundance increased after Kir4.1 deletion, NCC abundance and function decreased, suggesting that membrane depolarization uncouples WNK kinases from NCC. Together, these results indicate that Kir4.1 mediates potassium sensing by DCT cells and couples this signal to apical transport processes.

Expression of Kir4.1 and Kir5.1 inwardly rectifying potassium channels in oligodendrocytes, the myelinating cells of the CNS

The inwardly rectifying K+ channel subtype Kir5.1 is only functional as a heteromeric channel with Kir4.1. In the CNS, Kir4.1 is localised to astrocytes and is the molecular basis of their strongly negative membrane potential. Oligodendrocytes are the specialised myelinating glia of the CNS and their resting membrane potential provides the driving force for ion and water transport that is essential for myelination. However, little is known about the ion channel profile of mature myelinating oligodendrocytes. Here, we identify for the first time colocalization of Kir5.1 with Kir4.1 in oligodendrocytes in white matter. Immunolocalization with membrane-bound Na+/K+-ATPase and western blot of the plasma membrane fraction of the optic nerve, a typical CNS white matter tract containing axons and the oligodendrocytes that myelinate them, demonstrates that Kir4.1 and Kir5.1 are colocalized on oligodendrocyte cell membranes. Co-immunoprecipitation provides evidence that oligodendrocytes and astrocytes express a combination of homomeric Kir4.1 and heteromeric Kir4.1/Kir5.1 channels. Genetic knock-out and shRNA to ablate Kir4.1 indicates plasmalemmal expression of Kir5.1 in glia is largely dependent on Kir4.1 and the plasmalemmal anchoring protein PSD-95. The results demonstrate that, in addition to astrocytes, oligodendrocytes express both homomeric Kir4.1 and heteromeric Kir4.1/Kir5.1 channels. In astrocytes, these channels are essential to their key functions of K+ uptake and CO2/H+ chemosensation. We propose Kir4.1/Kir5.1 channels have equivalent functions in oligodendrocytes, maintaining myelin integrity in the face of large ionic shifts associated with action potential propagation along myelinated axons.

Role and mechanisms of regulation of the basolateral Kir 4.1/Kir 5.1K+ channels in the distal tubules

Epithelial K+ channels are essential for maintaining electrolyte and fluid homeostasis in the kidney. It is recognized that basolateral inward-rectifying K+ (Kir ) channels play an important role in the control of resting membrane potential and transepithelial voltage, thereby modulating water and electrolyte transport in the distal part of nephron and collecting duct. Monomeric Kir 4.1 (encoded by Kcnj10 gene) and heteromeric Kir 4.1/Kir 5.1 (Kir 4.1 together with Kir 5.1 (Kcnj16)) channels are abundantly expressed at the basolateral membranes of the distal convoluted tubule and the cortical collecting duct cells. Loss-of-function mutations in KCNJ10 cause EAST/SeSAME tubulopathy in humans associated with salt wasting, hypomagnesaemia, metabolic alkalosis and hypokalaemia. In contrast, mice lacking Kir 5.1 have severe renal phenotype that, apart from hypokalaemia, is the opposite of the phenotype seen in EAST/SeSAME syndrome. Experimental advances using genetic animal models provided critical insights into the physiological role of these channels in electrolyte homeostasis and the control of kidney function. Here, we discuss current knowledge about K+ channels at the basolateral membrane of the distal tubules with specific focus on the homomeric Kir 4.1 and heteromeric Kir 4.1/Kir 5.1 channels. Recently identified molecular mechanisms regulating expression and activity of these channels, such as cell acidification, dopamine, insulin and insulin-like growth factor-1, Src family protein tyrosine kinases, as well as the role of these channels in NCC-mediated transport in the distal convoluted tubules, are also described.

Molecular bases of K+ secretory cells in the inner ear: shared and distinct features between birds and mammals

In the cochlea, mammals maintain a uniquely high endolymphatic potential (EP), which is not observed in other vertebrate groups. However, a high [K⁺] is always present in the inner ear endolymph. Here, we show that Kir4.1, which is required in the mammalian stria vascularis to generate the highly positive EP, is absent in the functionally equivalent avian tegmentum vasculosum. In contrast, the molecular repertoire required for K⁺ secretion, specifically NKCC1, KCNQ1, KCNE1, BSND and CLC-K, is shared between the tegmentum vasculosum, the vestibular dark cells and the marginal cells of the stria vascularis. We further show that in barn owls, the tegmentum vasculosum is enlarged and a higher EP (~+34 mV) maintained, compared to other birds. Our data suggest that both the tegmentum vasculosum and the stratified stria vascularis evolved from an ancestral vestibular epithelium that already featured the major cell types of the auditory epithelia. Genetic recruitment of Kir4.1 specifically to strial melanocytes was then a crucial step in mammalian evolution enabling an increase in the cochlear EP. An increased EP may be related to high-frequency hearing, as this is a hallmark of barn owls among birds and mammals among amniotes.

Extracellular K+ rapidly controls NaCl cotransporter phosphorylation in the native distal convoluted tubule by Cl- -dependent and independent mechanisms

KEY POINTS

High dietary potassium (K⁺ ) intake dephosphorylates and inactivates the NaCl cotransporter (NCC) in the renal distal convoluted tubule (DCT). Using several ex vivo models, we show that physiological changes in extracellular K⁺ , similar to those occurring after a K⁺ rich diet, are sufficient to promote a very rapid dephosphorylation of NCC in native DCT cells. Although the increase of NCC phosphorylation upon decreased extracellular K⁺ appears to depend on cellular Cl^- fluxes, the rapid NCC dephosphorylation in response to increased extracellular K⁺ is not Cl^- -dependent. The Cl^- -dependent pathway involves the SPAK/OSR1 kinases, whereas the Cl^- independent pathway may include additional signalling cascades.

ABSTRACT

A high dietary potassium (K⁺ ) intake causes a rapid dephosphorylation, and hence inactivation, of the thiazide-sensitive NaCl cotransporter (NCC) in the renal distal convoluted tubule (DCT). Based on experiments in heterologous expression systems, it was proposed that changes in extracellular K⁺ concentration ([K⁺ ]_ex ) modulate NCC phosphorylation via a Cl^- -dependent modulation of the with no lysine (K) kinases (WNK)-STE20/SPS-1-44 related proline-alanine-rich protein kinase (SPAK)/oxidative stress-related kinase (OSR1) kinase pathway. We used the isolated perfused mouse kidney technique and ex vivo preparations of mouse kidney slices to test the physiological relevance of this model on native DCT. We demonstrate that NCC phosphorylation inversely correlates with [K⁺ ]_ex , with the most prominent effects occurring around physiological plasma [K⁺ ]. Cellular Cl^- conductances and the kinases SPAK/OSR1 are involved in the phosphorylation of NCC under low [K⁺ ]_ex . However, NCC dephosphorylation triggered by high [K⁺ ]_ex is neither blocked by removing extracellular Cl^- , nor by the Cl^- channel blocker 4,4'-diisothiocyano-2,2'-stilbenedisulphonic acid. The response to [K⁺ ]_ex on a low extracellular chloride concentration is also independent of significant changes in SPAK/OSR1 phosphorylation. Thus, in the native DCT, [K⁺ ]_ex directly and rapidly controls NCC phosphorylation by Cl^- -dependent and independent pathways that involve the kinases SPAK/OSR1 and a yet unidentified additional signalling mechanism.

Basolateral Kir4.1 activity in the distal convoluted tubule regulates K secretion by determining NaCl cotransporter activity

PURPOSE OF REVIEW

Renal potassium (K) secretion plays a key role in maintaining K homeostasis. The classic mechanism of renal K secretion is focused on the connecting tubule and cortical collecting duct, in which K is uptaken by basolateral Na-K-ATPase and is secreted into the lumen by apical ROMK (Kir1.1) and Ca-activated big conductance K channel. Recently, genetic studies and animal models have indicated that inwardly rectifying K channel 4.1 (Kir4.1 or Kcnj10) in the distal convoluted tubule (DCT) may play a role in the regulation of K secretion in the aldosterone-sensitive distal nephron by targeting the NaCl cotransporter (NCC). This review summarizes recent progresses regarding the role of Kir4.1 in the regulation of NCC and K secretion.

RECENT FINDINGS

Kir4.1 is expressed in the basolateral membrane of the DCT, and plays a predominant role in contributing to the basolateral K conductance and in participating in the generation of negative membrane potential. Kir4.1 is also the substrate of src-family tyrosine kinase and the stimulation of src-family tyrosine kinase activates Kir4.1 activity in the DCT. The genetic deletion or functional inhibition of Kir4.1 depolarizes the membrane of the DCT, inhibits ste20-proline-alanine rich kinase, and suppresses NCC activity. Moreover, the downregulation of Kir4.1 increases epithelial Na channel expression in the collecting duct and urinary K excretion. Finally, mice with low Kir4.1 activity in the DCT are hypomagnesemia and hypokalemia.

SUMMARY

Recent progress in exploring the regulation and the function of Kir4.1 in the DCT strongly indicates that Kir4.1plays an important role in initiating the regulation of renal K secretion by targeting NCC and it may serves as a K sensor in the kidney.

Disruption of KCNJ10 (Kir4.1) stimulates the expression of ENaC in the collecting duct

Kcnj10 encodes the inwardly rectifying K(+) channel 4.1 (Kir4.1) and is expressed in the basolateral membrane of late thick ascending limb, distal convoluted tubule (DCT), connecting tubule (CNT), and cortical collecting duct (CCD). In the present study, we perform experiments in postneonatal day 9 Kcnj10(-/-) or wild-type mice to examine the role of Kir.4.1 in contributing to the basolateral K(+) conductance in the CNT and CCD, and to investigate whether the disruption of Kir4.1 upregulates the expression of the epithelial Na(+) channel (ENaC). Immunostaining shows that Kir4.1 is expressed in the basolateral membrane of CNT and CCD. Patch-clamp studies detect three types of K(+) channels (23, 40, and 60 pS) in the basolateral membrane of late CNT and initial CCD in wild-type (WT) mice. However, only 23- and 60-pS K(+) channels but not the 40-pS K(+) channel were detected in Kcnj10(-/-) mice, suggesting that Kir.4.1 is a key component of the 40-pS K(+) channel in the CNT/CCD. Moreover, the depletion of Kir.4.1 did not increase the probability of finding the 23- and 60-pS K(+) channel in the CNT/CCD. We next used the perforated whole cell recording to measure the K(+) reversal voltage in the CNT/CCD as an index of cell membrane potential. Under control conditions, the K(+) reversal potential was -69 mV in WT mice and -61 mV in Kcnj10(-/-) mice, suggesting that Kir4.1 partially participates in generating membrane potential in the CNT/CCD. Western blotting and immunostaining showed that the expression of ENaCβ and ENaCγ subunits from a renal medulla section of Kcnj10(-/-) mice was significantly increased compared with that of WT mice. Also, the disruption of Kir4.1 increased aquaporin 2 expression. We conclude that Kir4.1 is expressed in the CNT and CCD and partially participates in generating the cell membrane potential. Also, increased ENaC expression in medullary CD of Kcnj10(-/-) mice is a compensatory action in response to the impaired Na(+) transport in the DCT.

The expression, regulation, and function of Kir4.1 (Kcnj10) in the mammalian kidney

Kir4.1 is an inwardly rectifying potassium (K(+)) channel and is expressed in the brain, inner ear, and kidney. In the kidney, Kir4.1 is expressed in the basolateral membrane of the late thick ascending limb (TAL), the distal convoluted tubule (DCT), and the connecting tubule (CNT)/cortical collecting duct (CCD). It plays a role in K(+) recycling across the basolateral membrane in corresponding nephron segments and in generating negative membrane potential. The renal phenotypes of the loss-function mutations of Kir4.1 include mild salt wasting, hypomagnesemia, hypokalemia, and metabolic alkalosis, suggesting that the disruption of Kir4.1 mainly impairs the transport in the DCT. Patch-clamp experiments and immunostaining demonstrate that Kir4.1 plays a predominant role in determining the basolateral K(+) conductance in the DCT. However, the function of Kir4.1 in the TAL and CNT/CCD is not essential, because K(+) channels other than Kir4.1 are also expressed. The downregulation of Kir4.1 in the DCT reduced basolateral chloride (Cl(-)) conductance, suppressed the expression of ste20 proline-alanine-rich kinase (SPAK), and decreased Na-Cl cotransporter (NCC) expression and activity. This suggests that Kir4.1 regulates NCC expression by the modulation of the Cl(-)-sensitive with-no-lysine kinase-SPAK pathway.

Novel KCNJ10 Gene Variations Compromise Function of Inwardly Rectifying Potassium Channel 4.1

TheKCNJ10gene encoding Kir4.1 contains numerous SNPs whose molecular effects remain unknown. We investigated the functional consequences of uncharacterized SNPs (Q212R, L166Q, and G83V) on homomeric (Kir4.1) and heteromeric (Kir4.1-Kir5.1) channel function. We compared these with previously characterized EAST/SeSAME mutants (G77R and A167V) in kidney-derived tsA201 cells and in glial cell-derived C6 glioma cells. The membrane potentials of tsA201 cells expressing G77R and G83V were significantly depolarized as compared with WTKir4.1, whereas cells expressing Q212R, L166Q, and A167V were less affected. Furthermore, macroscopic currents from cells expressing WTKir4.1 and Q212R channels did not differ, whereas currents from cells expressing L166Q, G83V, G77R, and A167V were reduced. Unexpectedly, L166Q current responses were rescued when co-expressed with Kir5.1. In addition, we observed notable differences in channel activity between C6 glioma cells and tsA201 cells expressing L166Q and A167V, suggesting that there are underlying differences between cell lines in terms of Kir4.1 protein synthesis, stability, or expression at the surface. Finally, we determined spermine (SPM) sensitivity of these uncharacterized SNPs and found that Q212R-containing channels displayed reduced block by 1 μmSPM. At 100 μmSPM, the block was equal to or greater than WT, suggesting that the greater driving force of SPM allowed achievement of steady state. In contrast, L166Q-Kir5.1 channels achieved a higher block than WT, suggesting a more stable interaction of SPM in the deep pore cavity. Overall, our data suggest that G83V, L166Q, and Q212R residues play a pivotal role in controlling Kir4.1 channel function.

Roles and Regulation of Renal K Channels

More than two dozen types of potassium channels, with different biophysical and regulatory properties, are expressed in the kidney, influencing renal function in many important ways. Recently, a confluence of discoveries in areas from human genetics to physiology, cell biology, and biophysics has cast light on the special function of five different potassium channels in the distal nephron, encoded by the genes KCNJ1, KCNJ10, KCNJ16, KCNMA1, and KCNN3. Research aimed at understanding how these channels work in health and go awry in disease has transformed our understanding of potassium balance and provided new insights into mechanisms of renal sodium handling and the maintenance of blood pressure. This review focuses on recent advances in this rapidly evolving field.

Caveolin-1 Deficiency Inhibits the Basolateral K+ Channels in the Distal Convoluted Tubule and Impairs Renal K+ and Mg2+ Transport

Kcnj10 encodes the inwardly rectifying K(+) channel Kir4.1 in the basolateral membrane of the distal convoluted tubule (DCT) and is activated by c-Src. However, the regulation and function of this K(+) channel are incompletely characterized. Here, patch-clamp experiments in Kcnj10-transfected HEK293 cells demonstrated that c-Src-induced stimulation of Kcnj10 requires coexpression of caveolin-1 (cav-1), and immunostaining showed expression of cav-1 in the basolateral membrane of parvalbumin-positive DCT. Patch-clamp experiments detected a 40-pS inwardly rectifying K(+) channel, a heterotetramer of Kir4.1/Kir5.1, in the basolateral membrane of the early DCT (DCT1) in both wild-type (WT) and cav-1-knockout (KO) mice. However, the activity of this basolateral 40-pS K(+) channel was lower in KO mice than in WT mice. Moreover, the K(+) reversal potential (an indication of membrane potential) was less negative in the DCT1 of KO mice than in the DCT1 of WT mice. Western blot analysis demonstrated that cav-1 deficiency decreased the expression of the Na(+)/Cl(-) cotransporter and Ste20-proline-alanine-rich kinase (SPAK) but increased the expression of epithelial Na(+) channel-α. Furthermore, the urinary excretion of Mg(2+) and K(+) was significantly higher in KO mice than in WT mice, and KO mice developed hypomagnesemia, hypocalcemia, and hypokalemia. We conclude that disruption of cav-1 decreases basolateral K(+) channel activity and depolarizes the cell membrane potential in the DCT1 at least in part by suppressing the stimulatory effect of c-Src on Kcnj10. Furthermore, the decrease in Kcnj10 and Na(+)/Cl(-) cotransporter expression induced by cav-1 deficiency may underlie the compromised renal transport of Mg(2+), Ca(2+), and K(+).

KCNJ10 (Kir4.1) is expressed in the basolateral membrane of the cortical thick ascending limb

The aim of the present study is to examine the role of Kcnj10 (Kir.4.1) in contributing to the basolateral K conductance in the cortical thick ascending limb (cTAL) using Kcnj10(+/+) wild-type (WT) and Kcnj10(-/-) knockout (KO) mice. The patch-clamp experiments detected a 40- and an 80-pS K channel in the basolateral membrane of the cTAL. Moreover, the probability of finding the 40-pS K was significantly higher in the late part of the cTAL close to the distal convoluted tubule than those in the initial part. Immunostaining showed that Kcnj10 staining was detected in the basolateral membrane of the cTAL but the expression was not uniformly distributed. The disruption of Kcnj10 completely eliminated the 40-pS K channel but not the 80-pS K channel, suggesting the role of Kcnj10 in forming the 40-pS K channel of the cTAL. Also, the disruption of Kcnj10 increased the probability of finding the 80-pS K channel in the cTAL, especially in the late part of the cTAL. Because the channel open probability of the 80-pS K channel in KO was similar to those of WT mice, the increase in the 80-pS K channel may be achieved by increasing K channel number. The whole cell recording further showed that K reversal potential measured with 5 mM K in the bath and 140 mM K in the pipette was the same in the WT and KO mice. Moreover, Western blot and immunostaining showed that the disruption of Kcnj10 did not affect the expression of Na-K-Cl cotransporter 2 (NKCC2). We conclude that Kir.4.1 is expressed in the basolateral membrane of cTAL and that the disruption of Kir.4.1 has no significant effect on the membrane potential of the cTAL and NKCC2 expression.

Inward-rectifying potassium channelopathies: new insights into disorders of sodium and potassium homeostasis

Inward-rectifying potassium (Kir) channels allow more inward than outward potassium flux when channels are open in mammalian cells. At physiological resting membrane potentials, however, they predominantly mediate outward potassium flux and play important roles in regulating the resting membrane potential in diverse cell types and potassium secretion in the kidneys. Mutations of Kir channels cause human hereditary diseases collectively called Kir channelopathies, many of which are characterized by disorders of sodium and potassium homeostasis. Studies on these genetic Kir channelopathies have shed light on novel pathophysiological mechanisms, including renal sodium and potassium handling, potassium shifting in skeletal muscles, and aldosterone production in the adrenal glands. Here, we review several recent advances in Kir channels and their clinical implications in sodium and potassium homeostasis.

Molecular aspects of structure, gating, and physiology of pH-sensitive background K2P and Kir K+-transport channels

K(+) channels fulfill roles spanning from the control of excitability to the regulation of transepithelial transport. Here we review two groups of K(+) channels, pH-regulated K2P channels and the transport group of Kir channels. After considering advances in the molecular aspects of their gating based on structural and functional studies, we examine their participation in certain chosen physiological and pathophysiological scenarios. Crystal structures of K2P and Kir channels reveal rather unique features with important consequences for the gating mechanisms. Important tasks of these channels are discussed in kidney physiology and disease, K(+) homeostasis in the brain by Kir channel-equipped glia, and central functions in the hearing mechanism in the inner ear and in acid secretion by parietal cells in the stomach. K2P channels fulfill a crucial part in central chemoreception probably by virtue of their pH sensitivity and are central to adrenal secretion of aldosterone. Finally, some unorthodox behaviors of the selectivity filters of K2P channels might explain their normal and pathological functions. Although a great deal has been learned about structure, molecular details of gating, and physiological functions of K2P and Kir K(+)-transport channels, this has been only scratching at the surface. More molecular and animal studies are clearly needed to deepen our knowledge.

Mechanisms and regulation of renal magnesium transport

Magnesium's most important role is in the release of chemical energy. Although most magnesium is stored outside of the extracellular fluid compartment, the regulated value is blood magnesium concentration. Cellular magnesium and bone magnesium do not play a major role in the defense of blood magnesium concentration; the major role is played by the kidney, where the renal tubule matches the urinary magnesium excretion and the net entry of magnesium into the extracellular fluid. In the kidney, magnesium is reabsorbed in the proximal tubule, the thick ascending limb of the loop of Henle, and the distal convoluted tubule. Magnesium absorption is mainly paracellular in the proximal tubule and in the thick ascending limb of the loop of Henle, whereas it is transcellular in the distal convoluted tubule. Several hormones and extracellular magnesium itself alter the distal tubular handling of magnesium, but the hormone(s) regulating extracellular magnesium concentration remains unknown.

KCNJ10 determines the expression of the apical Na-Cl cotransporter (NCC) in the early distal convoluted tubule (DCT1)

The renal phenotype induced by loss-of-function mutations of inwardly rectifying potassium channel (Kir), Kcnj10 (Kir4.1), includes salt wasting, hypomagnesemia, metabolic alkalosis and hypokalemia. However, the mechanism by which Kir.4.1 mutations cause the tubulopathy is not completely understood. Here we demonstrate that Kcnj10 is a main contributor to the basolateral K conductance in the early distal convoluted tubule (DCT1) and determines the expression of the apical Na-Cl cotransporter (NCC) in the DCT. Immunostaining demonstrated Kcnj10 and Kcnj16 were expressed in the basolateral membrane of DCT, and patch-clamp studies detected a 40-pS K channel in the basolateral membrane of the DCT1 of p8/p10 wild-type Kcnj10(+/+) mice (WT). This 40-pS K channel is absent in homozygous Kcnj10(-/-) (knockout) mice. The disruption of Kcnj10 almost completely eliminated the basolateral K conductance and decreased the negativity of the cell membrane potential in DCT1. Moreover, the lack of Kcnj10 decreased the basolateral Cl conductance, inhibited the expression of Ste20-related proline-alanine-rich kinase and diminished the apical NCC expression in DCT. We conclude that Kcnj10 plays a dominant role in determining the basolateral K conductance and membrane potential of DCT1 and that the basolateral K channel activity in the DCT determines the apical NCC expression possibly through a Ste20-related proline-alanine-rich kinase-dependent mechanism.

Hypomagnesemia: a clinical perspective

Although magnesium is involved in a wide spectrum of vital functions in normal human physiology, the significance of hypomagnesemia and necessity for its treatment are under-recognized and underappreciated in clinical practice. In the current review, we first present an overview of the clinical significance of hypomagnesemia and normal magnesium metabolism, with a focus on renal magnesium handling. Subsequently, we review the literature for both congenital and acquired hypomagnesemic conditions that affect the various steps in normal magnesium metabolism. Finally, we present an approach to the routine evaluation and suggested management of hypomagnesemia.

Expedient total synthesis of small to medium-sized membrane proteins via Fmoc chemistry

Total chemical synthesis provides a unique approach for the access to uncontaminated, monodisperse, and more importantly, post-translationally modified membrane proteins. In the present study we report a practical procedure for expedient and cost-effective synthesis of small to medium-sized membrane proteins in multimilligram scale through the use of automated Fmoc chemistry. The key finding of our study is that after the attachment of a removable arginine-tagged backbone modification group, the membrane protein segments behave almost the same as ordinary water-soluble peptides in terms of Fmoc solid-phase synthesis, ligation, purification, and mass spectrometry characterization. The efficiency and practicality of the new method is demonstrated by the successful preparation of Ser64-phosphorylated M2 proton channel from influenza A virus and the membrane-embedded domain of an inward rectifier K(+) channel protein Kir5.1. Functional characterizations of these chemically synthesized membrane proteins indicate that they provide useful and otherwise-difficult-to-access materials for biochemistry and biophysics studies.

Development and validation of fluorescence-based and automated patch clamp-based functional assays for the inward rectifier potassium channel Kir4.1

The inward rectifier potassium (Kir) channel Kir4.1 plays essential roles in modulation of neurotransmission and renal sodium transport and may represent a novel drug target for temporal lobe epilepsy and hypertension. The molecular pharmacology of Kir4.1 is limited to neurological drugs, such as fluoxetine (Prozac(©)), exhibiting weak and nonspecific activity toward the channel. The development of potent and selective small-molecule probes would provide critically needed tools for exploring the integrative physiology and therapeutic potential of Kir4.1. A fluorescence-based thallium (Tl(+)) flux assay that utilizes a tetracycline-inducible T-Rex-HEK293-Kir4.1 cell line to enable high-throughput screening (HTS) of small-molecule libraries was developed. The assay is dimethyl sulfoxide tolerant and exhibits robust screening statistics (Z'=0.75±0.06). A pilot screen of 3,655 small molecules and lipids revealed 16 Kir4.1 inhibitors (0.4% hit rate). 3,3-Diphenyl-N-(1-phenylethyl)propan-1-amine, termed VU717, inhibits Kir4.1-mediated thallium flux with an IC50 of ∼6 μM. An automated patch clamp assay using the IonFlux HT workbench was developed to facilitate compound characterization. Leak-subtracted ensemble "loose patch" recordings revealed robust tetracycline-inducible and Kir4.1 currents that were inhibited by fluoxetine (IC50=10 μM), VU717 (IC50=6 μM), and structurally related calcium channel blocker prenylamine (IC50=6 μM). Finally, we demonstrate that VU717 inhibits Kir4.1 channel activity in cultured rat astrocytes, providing proof-of-concept that the Tl(+) flux and IonFlux HT assays can enable the discovery of antagonists that are active against native Kir4.1 channels.

KCNJ10 mutations display differential sensitivity to heteromerisation with KCNJ16

BACKGROUND/AIMS

Mutations in the inwardly-rectifying K(+)-channel KCNJ10/Kir4.1 cause autosomal recessive EAST syndrome (epilepsy, ataxia, sensorineural deafness and tubulopathy). KCNJ10 is expressed in the distal convoluted tubule of the kidney, stria vascularis of the inner ear and brain glial cells. Patients diagnosed clinically with EAST syndrome were genotyped and mutations in KCNJ10 were studied functionally.

METHODS

Patient DNA was amplified and sequenced, and new mutations were identified. Mutant and wild-type KCNJ10 constructs were cloned and heterologously expressed in Xenopus oocytes. Whole-cell K(+) currents were measured by 2-electrode voltage clamping and channel expression was analysed by Western blotting.

RESULTS

We identified 3 homozygous mutations in KCNJ10 (p.F75C, p.A167V and p.V91fs197X), with mutation p.A167V previously reported in a compound heterozygous state. Oocytes expressing wild-type human KCNJ10 showed inwardly rectified currents, which were significantly reduced in all of the mutants (p < 0.001). Specific inhibition of KCNJ10 currents by Ba(2+) demonstrated a large residual function in p.A167V only, which was not compatible with causing disease. However, co-expression with KCNJ16 abolished function in these heteromeric channels almost completely.

CONCLUSION

This study provides an explanation for the pathophysiology of the p.A167V KCNJ10 mutation, which had previously not been considered pathogenic on its own. These findings provide evidence for the functional cooperation of KCNJ10 and KCNJ16. Thus, in vitro ascertainment of KCNJ10 function may necessitate co-expression with KCNJ16.

Direct inhibition of basolateral Kir4.1/5.1 and Kir4.1 channels in the cortical collecting duct by dopamine

It is recognized that dopamine promotes natriuresis by inhibiting multiple transporting systems in the proximal tubule. In contrast, less is known about the molecular targets of dopamine actions on water-electrolyte transport in the cortical collecting duct (CCD). Epithelial cells in the CCD are exposed to dopamine, which is synthesized locally or secreted from sympathetic nerve endings. Basolateral K(+) channels in the distal renal tubule are critical for K(+) recycling and controlling basolateral membrane potential to establish the driving force for Na(+) reabsorption. Here, we demonstrate that Kir4.1 and Kir5.1 are highly expressed in the mouse kidney cortex and are localized to the basolateral membrane of the CCD. Using patch-clamp electrophysiology in freshly isolated CCDs, we detected highly abundant 40-pS and scarce 20-pS single channel conductances, most likely representing Kir4.1/5.1 and Kir4.1 channels, respectively. Dopamine reversibly decreased the open probability of both channels, with a relatively greater action on the Kir4.1/5.1 heterodimer. This effect was mediated by D2-like but not D1-like dopamine receptors. PKC blockade abolished the inhibition of basolateral K(+) channels by dopamine. Importantly, dopamine significantly decreased the amplitude of Kir4.1/5.1 and Kir4.1 unitary currents. Consistently, dopamine induced an acute depolarization of basolateral membrane potential, as directly monitored using current-clamp mode in isolated CCDs. Therefore, we demonstrate that dopamine inhibits basolateral Kir4.1/5.1 and Kir4.1 channels in CCD cells via stimulation of D2-like receptors and subsequently PKC. This leads to depolarization of the basolateral membrane and a decreased driving force for Na(+) reabsorption in the distal renal tubule.

Congenital heart defects in patients with deletions upstream of SOX9

Heterozygous loss-of-function coding-sequence mutations of the transcription factor SOX9 cause campomelic dysplasia, a rare skeletal dysplasia with congenital bowing of long bones (campomelia), hypoplastic scapulae, a missing pair of ribs, pelvic, and vertebral malformations, clubbed feet, Pierre Robin sequence (PRS), facial dysmorphia, and disorders of sex development. We report here two unrelated families that include patients with isolated PRS, isolated congenital heart defect (CHD), or both anomalies. Patients from both families carried a very similar ∼1 Mb deletion upstream of SOX9. Analysis of ChIP-Seq from mouse cardiac tissue for H3K27ac, a marker of active regulatory elements, led us to identify several putative cardiac enhancers within the deleted region. One of these elements is known to interact with Nkx2.5 and Gata4, two transcription factors responsible for CHDs. Altogether, these data suggest that disruption of cardiac enhancers located upstream of SOX9 may be responsible for CHDs in humans.

Novel diuretic targets

As the molecular revolution continues to inform a deeper understanding of disease mechanisms and pathways, there exist unprecedented opportunities for translating discoveries at the bench into novel therapies for improving human health. Despite the availability of several different classes of antihypertensive medications, only about half of the 67 million Americans with hypertension manage their blood pressure appropriately. A broader selection of structurally diverse antihypertensive drugs acting through different mechanisms would provide clinicians with greater flexibility in developing effective treatment regimens for an increasingly diverse and aging patient population. An emerging body of physiological, genetic, and pharmacological evidence has implicated several renal ion-transport proteins, or regulators thereof, as novel, yet clinically unexploited, diuretic targets. These include the renal outer medullary potassium channel, ROMK (Kir1.1), Kir4.1/5.1 potassium channels, ClC-Ka/b chloride channels, UTA/B urea transporters, the chloride/bicarbonate exchanger pendrin, and the STE20/SPS1-related proline/alanine-rich kinase (SPAK). The molecular pharmacology of these putative targets is poorly developed or lacking altogether; however, recent efforts by a few academic and pharmaceutical laboratories have begun to lessen this critical barrier. Here, we review the evidence in support of the aforementioned proteins as novel diuretic targets and highlight examples where progress toward developing small-molecule pharmacology has been made.

Regulation of potassium channel trafficking in the distal nephron

PURPOSE OF REVIEW

Potassium channels in the distal nephron are precisely controlled to regulate potassium secretion in accord with physiological demands. In recent years, it has become evident that membrane trafficking processes play a fundamental role. This short review highlights recent developments in elucidating the underlying mechanisms.

RECENT FINDINGS

Novel sorting signals in the renal potassium channels, and the elusive intracellular trafficking machinery that read and act on these signals have recently been identified. These new discoveries reveal that independent signals sequentially interact with different intracellular sorting, retention and internalization machineries to appropriately ferry the channels to and from the apical and basolateral membrane domains in sufficient numbers to regulate potassium balance.

SUMMARY

A new understanding of the basic mechanisms that control potassium channel density at polarized membrane domains has emerged, providing new insights into how potassium balance is achieved and how it goes awry in disease.

Src family protein tyrosine kinase regulates the basolateral K channel in the distal convoluted tubule (DCT) by phosphorylation of KCNJ10 protein

The loss of function of the basolateral K channels in the distal nephron causes electrolyte imbalance. The aim of this study is to examine the role of Src family protein tyrosine kinase (SFK) in regulating K channels in the basolateral membrane of the mouse initial distal convoluted tubule (DCT1). Single-channel recordings confirmed that the 40-picosiemen (pS) K channel was the only type of K channel in the basolateral membrane of DCT1. The suppression of SFK reversibly inhibited the basolateral 40-pS K channel activity in cell-attached patches and decreased the Ba(2+)-sensitive whole-cell K currents in DCT1. Inhibition of SFK also shifted the K reversal potential from -65 to -43 mV, suggesting a role of SFK in determining the membrane potential in DCT1. Western blot analysis showed that KCNJ10 (Kir4.1), a key component of the basolateral 40-pS K channel in DCT1, was a tyrosine-phosphorylated protein. LC/MS analysis further confirmed that SFK phosphorylated KCNJ10 at Tyr(8) and Tyr(9). The single-channel recording detected the activity of a 19-pS K channel in KCNJ10-transfected HEK293T cells and a 40-pS K channel in the cells transfected with KCNJ10+KCNJ16 (Kir.5.1) that form a heterotetramer in the basolateral membrane of the DCT. Mutation of Tyr(9) did not alter the channel conductance of the homotetramer and heterotetramer. However, it decreased the whole-cell K currents, the probability of finding K channels, and surface expression of KCNJ10 in comparison to WT KCNJ10. We conclude that SFK stimulates the basolateral K channel activity in DCT1, at least partially, by phosphorylating Tyr(9) on KCNJ10. We speculate that the modulation of tyrosine phosphorylation of KCNJ10 should play a role in regulating membrane transport function in DCT1.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Basolateral membrane K+ channels in renal epithelial cells

The major function of epithelial tissues is to maintain proper ion, solute, and water homeostasis. The tubule of the renal nephron has an amazingly simple structure, lined by epithelial cells, yet the segments (i.e., proximal tubule vs. collecting duct) of the nephron have unique transport functions. The functional differences are because epithelial cells are polarized and thus possess different patterns (distributions) of membrane transport proteins in the apical and basolateral membranes of the cell. K(+) channels play critical roles in normal physiology. Over 90 different genes for K(+) channels have been identified in the human genome. Epithelial K(+) channels can be located within either or both the apical and basolateral membranes of the cell. One of the primary functions of basolateral K(+) channels is to recycle K(+) across the basolateral membrane for proper function of the Na(+)-K(+)-ATPase, among other functions. Mutations of these channels can cause significant disease. The focus of this review is to provide an overview of the basolateral K(+) channels of the nephron, providing potential physiological functions and pathophysiology of these channels, where appropriate. We have taken a "K(+) channel gene family" approach in presenting the representative basolateral K(+) channels of the nephron. The basolateral K(+) channels of the renal epithelia are represented by members of the KCNK, KCNJ, KCNQ, KCNE, and SLO gene families.

Genetic defects in the hotspot of inwardly rectifying K(+) (Kir) channels and their metabolic consequences: a review

Inwardly rectifying potassium (Kir) channels are essential for maintaining normal potassium homeostasis and the resting membrane potential. As a consequence, mutations in Kir channels cause debilitating diseases ranging from cardiac failure to renal, ocular, pancreatic, and neurological abnormalities. Structurally, Kir channels consist of two trans-membrane domains, a pore-forming loop that contains the selectivity filter and two cytoplasmic polar tails. Within the cytoplasmic structure, clusters of amino acid sequences form regulatory domains that interact with cellular metabolites to control the opening and closing of the channel. In this review, we present an overview of Kir channel function and recent progress in the characterization of selected Kir channel mutations that lie in and near a C-terminal cytoplasmic 'hotspot' domain. The resultant molecular mechanisms by which the loss or gain of channel function leads to organ failure provide potential opportunities for targeted therapeutic interventions for this important group of channelopathies.