Genetic and functional linkage of Kir5.1 and Kir2.1 channel subunits

Role of calcineurin in regulating renal potassium (K+) excretion: Mechanisms of calcineurin inhibitor-induced hyperkalemia

Calcineurin, protein phosphatase 2B (PP2B) or protein phosphatase 3 (PP3), is a calcium-dependent serine/threonine protein phosphatase. Calcineurin is widely expressed in the kidney and regulates renal Na+ and K+ transport. In the thick ascending limb, calcineurin plays a role in inhibiting NKCC2 function by promoting the dephosphorylation of the cotransporter and an intracellular sorting receptor, called sorting-related-receptor-with-A-type repeats (SORLA), is involved in modulating the effect of calcineurin on NKCC2. Calcineurin also participates in regulating thiazide-sensitive NaCl-cotransporter (NCC) in the distal convoluted tubule. The mechanisms by which calcineurin regulates NCC include directly dephosphorylation of NCC, regulating Kelch-like-3/CUL3 E3 ubiquitin-ligase complex, which is responsible for WNK (with-no-lysin-kinases) ubiquitination, and inhibiting Kir4.1/Kir5.1, which determines NCC expression/activity. Finally, calcineurin is also involved in regulating ROMK (Kir1.1) channels in the cortical collecting duct and Cyp11 2 expression in adrenal zona glomerulosa. In summary, calcineurin is involved in the regulation of NKCC2, NCC, and inwardly rectifying K+ channels in the kidney, and it also plays a role in modulating aldosterone synthesis in adrenal gland, which regulates epithelial-Na+-channel expression/activity. Thus, application of calcineurin inhibitors (CNIs) is expected to abrupt calcineurin-mediated regulation of transepithelial Na+ and K+ transport in the kidney. Consequently, CNIs cause hypertension, compromise renal K+ excretion, and induce hyperkalemia.

Novel KCNJ16 variants identified in a Chinese patient with hypokalemic metabolic acidosis

BACKGROUND

Biallelic pathogenic variants in the KCNJ16 gene result in hypokalemic tubulopathy and deafness (HKTD) (MIM #619406), which is a rare autosomal recessive disease characterized by hypokalemic tubulopathy with renal salt wasting, disturbed acid-base homeostasis, and sensorineural deafness. Currently, nine individuals with HKTD have been reported, and seven pathogenic variants in KCNJ16 have been revealed.

METHODS

A 5-year-6-month-old Chinese female patient displayed hypokalemic metabolic acidosis, salt wasting, renin-angiotensin-aldosterone system (RAAS) activation, arrhythmia, myocardial damage, cardiogenic shock and secondary diffuse brain oedema. Trio-based whole-exome sequencing (WES) was applied to detect the genetic cause.

RESULTS

Novel compound heterozygous variants, c.190A>C (p.Thr64Pro) and c.628C>G (p.His210Asp), in KCNJ16 were detected in the patient, and these variants were inherited from the patient's mother and father, respectively. Then, we systematically reviewed the available clinical manifestations of individuals with HKTD. We found that HKTD patients are at risk of cardiogenic shock and secondary diffuse brain oedema, which urges clinicians to make early diagnoses with prompt treatments.

CONCLUSION

These findings expand the variant spectrum of KCNJ16, enrich the clinical characteristics of HKTD, and provide a solid base for the genetic counseling, diagnosis and treatment of this condition.

Nutrigenomics of inward rectifier potassium channels

Inwardly rectifying potassium (Kir) channels play a key role in maintaining the resting membrane potential and supporting potassium homeostasis. There are many variants of Kir channels, which are usually tetramers in which the main subunit has two trans-membrane helices attached to two N- and C-terminal cytoplasmic tails with a pore-forming loop in between that contains the selectivity filter. These channels have domains that are strongly modulated by molecules present in nutrients found in different diets, such as phosphoinositols, polyamines and Mg²⁺. These molecules can impact these channels directly or indirectly, either allosterically by modulation of enzymes or via the regulation of channel expression. A particular type of these channels is coupled to cell metabolism and inhibited by ATP (KATP channels, essential for insulin release and for the pathogenesis of metabolic diseases like diabetes mellitus). Genomic changes in Kir channels have a significant impact on metabolism, such as conditioning the nutrients and electrolytes that an individual can take. Thus, the nutrigenomics of ion channels is an important emerging field in which we are attempting to understand how nutrients and diets can affect the activity and expression of ion channels and how genomic changes in such channels may be the basis for pathological conditions that limit nutrition and electrolyte intake. In this contribution we briefly review Kir channels, discuss their nutrigenomics, characterize how different components in the diet affect their function and expression, and suggest how their genomic changes lead to pathological phenotypes that affect diet and electrolyte intake.

Diagnostics in skeletal muscle channelopathies

INTRODUCTION

Skeletal muscle channelopathies (SMCs) are a heterogenous group of disorders, caused by mutations in skeletal ion channels leading to abnormal muscle excitability, resulting in either delayed muscle relaxation (myotonia) which characterizes non-dystrophic myotonias (NDMs), or membrane transient inactivation, causing episodic weakness, typical of periodic paralyses (PPs).

AREAS COVERED

SMCs include myotonia congenita, paramyotonia congenita, and sodium-channel myotonia among NDMs, and hyper-normokalemic, hypokalemic, or late-onset periodic paralyses among PPs. When suspecting an SMC, a structured diagnostic approach is required. Detailed personal and family history and clinical examination are essential, while neurophysiological tests should confirm myotonia and rule out alternative diagnosis. Moreover, specific electrodiagnostic studies are important to further define the phenotype of de novo cases and drive molecular analyses together with clinical data. Definite diagnosis is achieved through genetic testing, either with Sanger sequencing or multigene next-generation sequencing panel. In still unsolved patients, more advanced techniques, as exome-variant sequencing or whole-genome sequencing, may be considered in expert centers.

EXPERT OPINION

The diagnostic approach to SMC is still mainly based on clinical data; moreover, definite diagnosis is sometimes complicated by the difficulty to establish a proper genotype-phenotype correlation. Lastly, further studies are needed to allow the genetic characterization of unsolved patients.

The effect of high-dietary K+ (HK) on Kir4.1/Kir5.1 and ROMK in the distal convoluted tubule (DCT) is not affected by gender and Cl- content of the diet

Basolateral potassium channels in the distal convoluted tubule (DCT) are composed of inwardly-rectifying potassium channel 4.1 (Kir4.1) and Kir5.1. Kir4.1 interacts with Kir5.1 to form a 40 pS K⁺ channel which is the only type K⁺ channel expressed in the basolateral membrane of the DCT. Moreover, Kir4.1/Kir5.1 heterotetramer plays a key role in determining the expression and activity of thiazide-sensitive Na-Cl cotransport (NCC). In addition to Kir4.1/Kir5.1, Kir1.1 (ROMK) is expressed in the apical membrane of the late DCT (DCT2) and plays a key role in mediating epithelial Na⁺ channel (ENaC)-dependent K⁺ excretion. High dietary-K⁺-intake (HK) stimulates ROMK and inhibits Kir4.1/Kir5.1 in the DCT. Inhibition of Kir4.1/Kir5.1 is essential for HK-induced suppression of NCC whereas the stimulation of ROMK is important for increasing ENaC-dependent K⁺ excretion during HK. We have now used the patch-clamp-technique to examine whether gender and Cl^- content of K⁺-diet affect HK-induced inhibition of basolateral Kir4.1/Kir5.1 and HK-induced stimulation of ROMK. Single-channel-recording shows that basolateral 40 pS K⁺ channel (Kir4.1/Kir5.1) activity of the DCT defined by NP_o was 1.34 (1% KCl, normal K, NK), 0.95 (5% KCl) and 1.03 (5% K⁺-citrate) in male mice while it was 1.47, 1.02 and 1.05 in female mice. The whole-cell recording shows that Kir4.1/Kir5.1-mediated-K⁺ current of the early-DCT (DCT1) was 1,170 pA (NK), 725 pA (5% KCl) and 700 pA (5% K⁺-citrate) in male mice whereas it was 1,125 pA, 674 pA and 700 pA in female mice. Moreover, K⁺-currents (I_K) reversal potential of DCT (an index of membrane potential) was -63 mV (NK), -49 mV (5% KCl) and -49 mV (5% K-citrate) in the male mice whereas it was -63 mV, -50 mV and -50 mV in female mice. Finally, TPNQ-sensitive whole-cell ROMK-currents in the DCT2 /initial-connecting tubule (CNT) were 910 pA (NK), 1,520 pA (5% KCl) and 1,540 pA (5% K⁺-citrate) in male mice whereas the ROMK-mediated K⁺ currents were 1,005 pA, 1,590 pA and 1,570 pA in female mice. We conclude that the effect of HK intake on Kir4.1/Kir5.1 of the DCT and ROMK of DCT2/CNT is similar between male and female mice. Also, Cl^- content in HK diets has no effect on HK-induced inhibition of Kir4.1/Kir5.1 of the DCT and HK-induced stimulation of ROMK in DCT2/CNT.

Kir5.1 channels: potential role in epilepsy and seizure disorders

Inwardly rectifying potassium (Kir) channels are broadly expressed in many mammalian organ systems, where they contribute to critical physiological functions. However, the importance and function of the Kir5.1 channel (encoded by the KCNJ16 gene) have not been fully recognized. This review focuses on the recent advances in understanding the expression patterns and functional roles of Kir5.1 channels in fundamental physiological systems vital to potassium homeostasis and neurological disorders. Recent studies have described the role of Kir5.1-forming Kir channels in mouse and rat lines with mutations in the Kcnj16 gene. The animal research reveals distinct renal and neurological phenotypes, including pH and electrolyte imbalances, blunted ventilatory responses to hypercapnia/hypoxia, and seizure disorders. Furthermore, it was confirmed that these phenotypes are reminiscent of those in patient cohorts in which mutations in the KCNJ16 gene have also been identified, further suggesting a critical role for Kir5.1 channels in homeostatic/neural systems health and disease. Future studies that focus on the many functional roles of these channels, expanded genetic screening in human patients, and the development of selective small-molecule inhibitors for Kir5.1 channels, will continue to increase our understanding of this unique Kir channel family member.

Inwardly rectifying K+ channels 4.1 and 5.1 (Kir4.1/Kir5.1) in the renal distal nephron

The inwardly rectifying potassium channel (Kir) 4.1 (encoded by KCNJ10) interacts with Kir5.1 (encoded by KCNJ16) to form a major basolateral K⁺ channel in the renal distal convoluted tubule (DCT), connecting tubule (CNT), and the cortical collecting duct (CCD). Kir4.1/Kir5.1 heterotetramer plays an important role in regulating Na⁺ and K⁺ transport in the DCT, CNT, and CCD. A recent development in the field has firmly established the role of Kir4.1/Kir5.1 heterotetramer of the DCT in the regulation of thiazide-sensitive Na-Cl cotransporter (NCC). Changes in Kir4.1/Kir5.1 activity of the DCT are an essential step for the regulation of NCC expression/activity induced by dietary K⁺ and Na⁺ intakes and play a role in modulating NCC by type 2 angiotensin II receptor (AT2R), bradykinin type II receptor (BK2R), and β-adrenergic receptor. Since NCC activity determines the Na⁺ delivery rate to the aldosterone-sensitive distal nephron (ASDN), a distal nephron segment from late DCT to CCD, Kir4.1/Kir5.1 activity plays a critical role not only in the regulation of renal Na⁺ absorption but also in modulating renal K⁺ excretion and maintaining K⁺ homeostasis. Thus, Kir4.1/Kir5.1 activity serves as an important component of renal K⁺ sensing mechanism. The main focus of this review is to provide an overview regarding the role of Kir4.1 and Kir5.1 of the DCT and CCD in the regulation of renal K⁺ excretion and Na⁺ absorption.

Altered Expression of Ion Channels in White Matter Lesions of Progressive Multiple Sclerosis: What Do We Know About Their Function?

Despite significant advances in our understanding of the pathophysiology of multiple sclerosis (MS), knowledge about contribution of individual ion channels to axonal impairment and remyelination failure in progressive MS remains incomplete. Ion channel families play a fundamental role in maintaining white matter (WM) integrity and in regulating WM activities in axons, interstitial neurons, glia, and vascular cells. Recently, transcriptomic studies have considerably increased insight into the gene expression changes that occur in diverse WM lesions and the gene expression fingerprint of specific WM cells associated with secondary progressive MS. Here, we review the ion channel genes encoding K⁺, Ca²⁺, Na⁺, and Cl^- channels; ryanodine receptors; TRP channels; and others that are significantly and uniquely dysregulated in active, chronic active, inactive, remyelinating WM lesions, and normal-appearing WM of secondary progressive MS brain, based on recently published bulk and single-nuclei RNA-sequencing datasets. We discuss the current state of knowledge about the corresponding ion channels and their implication in the MS brain or in experimental models of MS. This comprehensive review suggests that the intense upregulation of voltage-gated Na⁺ channel genes in WM lesions with ongoing tissue damage may reflect the imbalance of Na⁺ homeostasis that is observed in progressive MS brain, while the upregulation of a large number of voltage-gated K⁺ channel genes may be linked to a protective response to limit neuronal excitability. In addition, the altered chloride homeostasis, revealed by the significant downregulation of voltage-gated Cl^- channels in MS lesions, may contribute to an altered inhibitory neurotransmission and increased excitability.

Defects in KCNJ16 Cause a Novel Tubulopathy with Hypokalemia, Salt Wasting, Disturbed Acid-Base Homeostasis, and Sensorineural Deafness

BACKGROUND

The transepithelial transport of electrolytes, solutes, and water in the kidney is a well-orchestrated process involving numerous membrane transport systems. Basolateral potassium channels in tubular cells not only mediate potassium recycling for proper Na⁺,K⁺-ATPase function but are also involved in potassium and pH sensing. Genetic defects in KCNJ10 cause EAST/SeSAME syndrome, characterized by renal salt wasting with hypokalemic alkalosis associated with epilepsy, ataxia, and sensorineural deafness.

METHODS

A candidate gene approach and whole-exome sequencing determined the underlying genetic defect in eight patients with a novel disease phenotype comprising a hypokalemic tubulopathy with renal salt wasting, disturbed acid-base homeostasis, and sensorineural deafness. Electrophysiologic studies and surface expression experiments investigated the functional consequences of newly identified gene variants.

RESULTS

We identified mutations in the KCNJ16 gene encoding KCNJ16, which along with KCNJ15 and KCNJ10, constitutes the major basolateral potassium channel of the proximal and distal tubules, respectively. Coexpression of mutant KCNJ16 together with KCNJ15 or KCNJ10 in Xenopus oocytes significantly reduced currents.

CONCLUSIONS

Biallelic variants in KCNJ16 were identified in patients with a novel disease phenotype comprising a variable proximal and distal tubulopathy associated with deafness. Variants affect the function of heteromeric potassium channels, disturbing proximal tubular bicarbonate handling as well as distal tubular salt reabsorption.

A Comprehensive Map of mRNAs and Their Isoforms across All 14 Renal Tubule Segments of Mouse

BACKGROUND

The repertoire of protein expression along the renal tubule depends both on regulation of transcription and regulation of alternative splicing that can generate multiple proteins from a single gene.

METHODS

A full-length, small-sample RNA-seq protocol profiled transcriptomes for all 14 renal tubule segments microdissected from mouse kidneys.

RESULTS

This study identified >34,000 transcripts, including 3709 that were expressed in a segment-specific manner. All data are provided as an online resource (https://esbl.nhlbi.nih.gov/MRECA/Nephron/). Many of the genes expressed in unique patterns along the renal tubule were solute carriers, transcription factors, or G protein-coupled receptors that account for segment-specific function. Mapping the distribution of transcripts associated with Wnk-SPAK-PKA signaling, renin-angiotensin-aldosterone signaling, and cystic diseases of the kidney illustrated the applications of the online resource. The method allowed full-length mapping of RNA-seq reads, which facilitated comprehensive, unbiased characterization of alternative exon usage along the renal tubule, including known isoforms of Cldn10, Kcnj1 (ROMK), Slc12a1 (NKCC2), Wnk1, Stk39 (SPAK), and Slc14a2 (UT-A urea transporter). It also identified many novel isoforms with segment-specific distribution. These included variants associated with altered protein structure (Slc9a8, Khk, Tsc22d1, and Scoc), and variants that may affect untranslated, regulatory regions of transcripts (Pth1r, Pkar1a, and Dab2).

CONCLUSIONS

Full-length, unbiased sequencing of transcripts identified gene-expression patterns along the mouse renal tubule. The data, provided as an online resource, include both quantitative and qualitative differences in transcripts. Identification of alternative splicing along the renal tubule may prove critical to understanding renal physiology and pathophysiology.

Inwardly rectifying potassium channel 5.1: Structure, function, and possible roles in diseases

Inwardly rectifying potassium (Kir) channels make it easier for K⁺ to enter into a cell and subsequently regulate cellular biological functions. Kir5.1 (encoded by KCNJ16) alone can form a homotetramer and can form heterotetramers with Kir4.1 (encoded by KCNJ10) or Kir4.2 (encoded by KCNJ15). In most cases, homomeric Kir5.1 is non-functional, while heteromeric Kir5.1 on the cell membrane contributes to the inward flow of K⁺ ions, which can be regulated by intracellular pH and a variety of signaling mechanisms. In the form of a heterotetramer, Kir5.1 regulates Kir4.1/4.2 activity and is involved in the maintenance of nephron function. Actually, homomeric Kir5.1 may also play a very important role in diseases, including in the ventilatory response to hypoxia and hypercapnia, hearing impairment, cardiovascular disease and cancer. With an increase in the number of studies into the roles of Kir channels, researchers are paying more attention to the pathophysiological functions of Kir5.1. This minireview provides an overview regarding these Kir5.1 roles.

Expression, localization, and functional properties of inwardly rectifying K+ channels in the kidney

Inwardly rectifying K+ (Kir) channels are expressed in multiple organs and cell types and play critical roles in cellular function. Most notably, Kir channels are major determinants of the resting membrane potential and K+ homeostasis. The renal outer medullary K+ channel (Kir1.1) was the first renal Kir channel identified and cloned in the kidney over two decades ago. Since then, several additional members, including classical and ATP-regulated Kir family classes, have been identified to be expressed in the kidney and to contribute to renal ion transport. Although the ATP-regulated Kir channel class remains the most well known due to severe pathological phenotypes associated with their mutations, progress is being made in defining the properties, localization, and physiological functions of other renal Kir channels, including those localized to the basolateral epithelium. This review is primarily focused on the current knowledge of the expression and localization of renal Kir channels but will also briefly describe their proposed functions in the kidney.

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.

AMP-Activated Protein Kinase (AMPK)-Dependent Regulation of Renal Transport

AMP-activated kinase (AMPK) is a serine/threonine kinase that is expressed in most cells and activated by a high cellular AMP/ATP ratio (indicating energy deficiency) or by Ca²⁺. In general, AMPK turns on energy-generating pathways (e.g., glucose uptake, glycolysis, fatty acid oxidation) and stops energy-consuming processes (e.g., lipogenesis, glycogenesis), thereby helping cells survive low energy states. The functional element of the kidney, the nephron, consists of the glomerulus, where the primary urine is filtered, and the proximal tubule, Henle's loop, the distal tubule, and the collecting duct. In the tubular system of the kidney, the composition of primary urine is modified by the reabsorption and secretion of ions and molecules to yield final excreted urine. The underlying membrane transport processes are mainly energy-consuming (active transport) and in some cases passive. Since active transport accounts for a large part of the cell's ATP demands, it is an important target for AMPK. Here, we review the AMPK-dependent regulation of membrane transport along nephron segments and discuss physiological and pathophysiological implications.

Expression of Kir2.1 Inward Rectifying Potassium Channels in Optic Nerve Glia: Evidence for Heteromeric Association with Kir4.1 and Kir5.1

Inward rectifying potassium (Kir) channels comprise a large family with diverse biophysical properties. A predominant feature of central nervous system (CNS) glia is their expression of Kir4.1, which as homomers are weakly rectifying channels, but form strongly rectifying channels as heteromers with Kir2.1. However, the extent of Kir2.1 expression and their association with Kir4.1 in glia throughout the CNS is unclear. We have examined this in astrocytes and oligodendrocytes of the mouse optic nerve, a typical CNS white matter tract. Western blot and immunocytochemistry demonstrates that optic nerve astrocytes and oligodendrocytes express Kir2.1 and that it co-localises with Kir4.1. Co-immunoprecipitation analysis provided further evidence that Kir2.1 associate with Kir4.1 and, moreover, Kir2.1 expression was significantly reduced in optic nerves and brains from Kir4.1 knock-out mice. In addition, optic nerve glia express Kir5.1, which may associate with Kir2.1 to form silent channels. Immunocytochemical and co-immunoprecipitation analyses indicate that Kir2.1 associate with Kir5.1 in optic nerve glia, but not in the brain. The results provide evidence that astrocytes and oligodendrocytes may express heteromeric Kir2.1/Kir4.1 and Kir2.1/Kir5.1 channels, together with homomeric Kir2.1 and Kir4.1 channels. In astrocytes, expression of multiple Kir channels is the biophysical substrate for the uptake and redistribution of K+ released during neuronal electrical activity known as ‘potassium spatial buffering’. Our findings suggest a similar potential role for the diverse Kir channels expressed by oligodendrocytes, which by way of their myelin sheaths are intimately associated with the sites of action potential propagation and axonal K+ release.

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.

Downregulation of inwardly rectifying potassium channel 5.1 expression in C57BL/6J cochlear lateral wall

Age-related hearing loss (AHL) is one of the most common sensory disorders among elderly persons. The inwardly rectifying potassium channel 5.1 (Kir5.1) plays a vital role in regulating cochlear K(+) circulation which is necessary for normal hearing. The distribution of Kir5.1 in C57BL/6J mice cochleae, and the relationship between the expression of Kir5.1 and the etiology of AHL were investigated. Forty C57BL/6J mice were randomly divided into four groups at 4, 12, 24 and 52 weeks of age respectively. The location of Kir5.1 was detected by immunofluorescence technique. The mRNA and protein expression of Kir5.1 was evaluated in mice cochleae using real-time polymerase-chain reactions (RT-PCR) and Western blotting respectively. Kir5.1 was detected in the type II and IV fibrocytes of the spiral ligament in the cochlear lateral wall of C57BL/6J mice. The expression levels of Kir5.1 mRNA and protein in the cochleae of aging C57BL/6J mice were down-regulated. It was suggested that the age-related decreased expression of Kir5.1 in the lateral wall of C57BL/6J mice was associated with hearing loss. Our results indicated that Kir5.1 may play an important role in the pathogenesis of AHL.

Recent advances in therapeutic strategies that focus on the regulation of ion channel expression

A number of different ion channel types are involved in cell signaling networks, and homeostatic regulatory mechanisms contribute to the control of ion channel expression. Profiling of global gene expression using microarray technology has recently provided novel insights into the molecular mechanisms underlying the homeostatic and pathological control of ion channel expression. It has demonstrated that the dysregulation of ion channel expression is associated with the pathogenesis of neural, cardiovascular, and immune diseases as well as cancers. In addition to the transcriptional, translational, and post-translational regulation of ion channels, potentially important evidence on the mechanisms controlling ion channel expression has recently been accumulated. The regulation of alternative pre-mRNA splicing is therefore a novel therapeutic strategy for the treatment of dominant-negative splicing disorders. Epigenetic modification plays a key role in various pathological conditions through the regulation of pluripotency genes. Inhibitors of pre-mRNA splicing and histone deacetyalase/methyltransferase have potential as potent therapeutic drugs for cancers and autoimmune and inflammatory diseases. Moreover, membrane-anchoring proteins, lysosomal and proteasomal degradation-related molecules, auxiliary subunits, and pharmacological agents alter the protein folding, membrane trafficking, and post-translational modifications of ion channels, and are linked to expression-defect channelopathies. In this review, we focused on recent insights into the transcriptional, spliceosomal, epigenetic, and proteasomal regulation of ion channel expression: Ca(2+) channels (TRPC/TRPV/TRPM/TRPA/Orai), K(+) channels (voltage-gated, KV/Ca(2+)-activated, KCa/two-pore domain, K2P/inward-rectifier, Kir), and Ca(2+)-activated Cl(-) channels (TMEM16A/TMEM16B). Furthermore, this review highlights expression of these ion channels in expression-defect channelopathies.

A digital atlas of ion channel expression patterns in the two-week-old rat brain

The approximately 350 ion channels encoded by the mammalian genome are a main pillar of the nervous system. We have determined the expression pattern of 320 channels in the two-week-old (P14) rat brain by means of non-radioactive robotic in situ hybridization. Optimized methods were developed and implemented to generate stringently coronal brain sections. The use of standardized methods permits a direct comparison of expression patterns across the entire ion channel expression pattern data set and facilitates recognizing ion channel co-expression. All expression data are made publically available at the Genepaint.org database. Inwardly rectifying potassium channels (Kir, encoded by the Kcnj genes) regulate a broad spectrum of physiological processes. Kcnj channel expression patterns generated in the present study were fitted with a deformable subdivision mesh atlas produced for the P14 rat brain. This co-registration, when combined with numerical quantification of expression strengths, allowed for semi-quantitative automated annotation of expression patterns as well as comparisons among and between Kcnj subfamilies. The expression patterns of Kcnj channel were also cross validated against previously published expression patterns of Kcnj channel genes.

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.

Disease-targeted sequencing of ion channel genes identifies de novo mutations in patients with non-familial Brugada syndrome

Brugada syndrome (BrS) is one of the ion channelopathies associated with sudden cardiac death (SCD). The most common BrS-associated gene (SCN5A) only accounts for approximately 20–25% of BrS patients. This study aims to identify novel mutations across human ion channels in non-familial BrS patients without SCN5A variants through disease-targeted sequencing. We performed disease-targeted multi-gene sequencing across 133 human ion channel genes and 12 reported BrS-associated genes in 15 unrelated, non-familial BrS patients without SCN5A variants. Candidate variants were validated by mass spectrometry and Sanger sequencing. Five de novo mutations were identified in four genes (SCNN1A, KCNJ16, KCNB2, and KCNT1) in three BrS patients (20%). Two of the three patients presented SCD and one had syncope. Interestingly, the two patients presented with SCD had compound mutations (SCNN1A:Arg350Gln and KCNB2:Glu522Lys; SCNN1A:Arg597* and KCNJ16:Ser261Gly). Importantly, two SCNN1A mutations were identified from different families. The KCNT1:Arg1106Gln mutation was identified in a patient with syncope. Bioinformatics algorithms predicted severe functional interruptions in these four mutation loci, suggesting their pivotal roles in BrS. This study identified four novel BrS-associated genes and indicated the effectiveness of this disease-targeted sequencing across ion channel genes for non-familial BrS patients without SCN5A variants.

Mislocalization of K+ channels causes the renal salt wasting in EAST/SeSAME syndrome

The Kir4.1/Kir5.1 channel mediates basolateral K(+) recycling in renal distal tubules; this process is critical for Na(+) reabsorption at the tubules. Mutations in Kir4.1 are associated with EAST/SeSAME syndrome, a genetic disorder characterized by renal salt wasting. In this study, we found that MAGI-1 anchors Kir4.1 channels (Kir4.1 homomer and Kir4.1/Kir5.1 heteromer) and contributes to basolateral K(+) recycling. The Kir4.1 A167V mutation associated with EAST/SeSAME syndrome caused mistrafficking of the mutant channels and inhibited their expression on the basolateral surface of tubular cells. These findings suggest mislocalization of the Kir4.1 channels contributes to renal salt wasting.

A genome-wide association study identifies novel susceptibility genetic variation for thyrotoxic hypokalemic periodic paralysis

Several lines of evidence have pointed out that genetic components have roles in thyrotoxic hypokalemic periodic paralysis (TTPP). In this study, for the first time we performed genome-wide association study (GWAS) in male hyperthyroid subjects in order to identify genetic loci conferring susceptibility to TTPP. We genotyped 78 Thai male TTPP cases and 74 Thai male hyperthyroid patients without hypokalemia as controls with Illumina Human-Hap610 Genotyping BeadChip. Among the SNPs analyzed in the GWAS, rs312729 at chromosome 17q revealed the lowest P-value for association (P=2.09 × 10(-7)). After fine mapping for linkage disequilibrium blocks surrounding the landmark SNP, we found a significant association of rs623011; located at 75 kb downstream of KCNJ2 on chromosome 17q, reached the GWAS significance after Bonferroni's adjustment (P=3.23 × 10(-8), odds ratio (OR)=6.72; 95% confidence interval (CI)=3.11-14.5). The result was confirmed in an independent cohort of samples consisting of 28 TTPP patients and 48 controls using the same clinical criteria diagnosis (replication analysis P=3.44 × 10(-5), OR=5.13; 95% CI=1.87-14.1; combined-analysis P=3.71 × 10(-12), OR=5.47; 95% CI=3.04-9.83).

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.

Cytoskeletal basis of ion channel function in cardiac muscle

The heart is a force-generating organ that responds to self-generated electrical stimuli from specialized cardiomyocytes. This function is modulated by sympathetic and parasympathetic activity. In order to contract and accommodate the repetitive morphological changes induced by the cardiac cycle, cardiomyocytes depend on their highly evolved and specialized cytoskeletal apparatus. Defects in components of the cytoskeleton affect the ability of the cell to compensate at both functional and structural levels in the long term. In addition to structural remodeling, the myocardium becomes increasingly susceptible to altered electrical activity, leading to arrhythmogenesis. The development of arrhythmias secondary to structural remodeling defects has been noted, although the detailed molecular mechanisms are still elusive. Here, the author reviews the current knowledge of the molecular and functional relationships between the cytoskeleton and ion channels, and discusses the future impact of new data on molecular cardiology research and clinical practice.

Novel de novo mutation in the KCNJ2 gene in a patient with Andersen-Tawil syndrome

Andersen-Tawil syndrome is a rare autosomal-dominant disease characterized by episodic muscle weakness, cardiac arrhythmias, and dysmorphic features. Mutations in the KCNJ2 gene (which encodes an inward-rectifying potassium channel protein, Kir2.1) have been reported to be responsible for this disorder. Reported here is a novel de novo mutation in the KCNJ2 gene in a patient with Andersen-Tawil syndrome. This mutation predicts the substitution of alanine for glycine at position 146 (Gly146Ala, c.437G > C) of Kir2.1 and is located at the extracellular pore loop region that serves as a principal ion-selective filter. The patient did not respond to acetazolamide, but experienced an improvement of the paralytic symptoms on treatment with a combination of spironolactone, amiloride, and potassium supplements.

A 2.3Mb deletion of 17q24.2-q24.3 associated with 'Carney Complex plus'

We present a 12-year-old with a de novo interstitial deletion of approximately 2.3Mb in chromosome band 17q24.2-q24.3, which was identified by array CGH. The most characteristic features in this case are posterior laryngeal cleft and the presence of numerous freckles and lentigines in childhood. Growth restriction, microcephaly and moderate mental retardation are also prominent features but are frequently seen with other chromosomal anomalies. The microdeletion causes haploinsufficiency of PRKAR1A (protein kinase, cAMP-dependent, regulatory 1alpha), which is known to cause Carney Complex but this diagnosis alone does not account for all of her problems and she therefore has 'Carney Complex plus'. This report illustrates the practical benefits associated with a clear cytogenetic diagnosis, as regular endocrinological and cardiac screening is required.

Immunohistochemical distribution of inwardly rectifying K+ channels in the medulla oblongata of the rat

The inwardly rectifying K+ channels, Kir1.1, Kir2.3 and Kir4.1-Kir5.1, are the candidate chemosensory molecules for CO2/H+. We determined the mRNA expression and immunohistochemical localization of these channels in the medulla oblongata of the rat. RT-PCR analysis revealed mRNAs of Kir1.1, Kir2.3, Kir4.1 and Kir5.1 were detected in the medulla. The immunoreactivities for Kir1.1, Kir2.3, Kir4.1, and Kir5.1 were observed in the medulla, and immunolabeling pattern was varied by the subunit. Immunoreactivities for Kir1.1 and Kir2.3 were observed in the nerve cell bodies and glial cells both in the chemosensory areas [nucleus tractus solitarius (NTS), nucleus raphe obscurus (RO), pre-Bötzinger complex (PreBötC)] and non-chemosensory area [hypoglossal nucleus (XII), inferior olive nucleus (IO)]. Kir4.1 immunoreactivity was observed in the glial cells and neuropil, especially in XII and IO. Kir5.1 immunoreactivity was observed in the nerve cell bodies in the XII, RO, and PreBötC, but not in the NTS or IO. In the NTS, a dense network of varicose nerve fibers showed immunoreactivity for Kir5.1. Our findings suggest that Kir channels may not act specific to the central chemoreception, but regulate the ionic properties of cellular membranes in various neurons and glial cells.

Ancient intron insertion sites and palindromic genomic duplication evolutionally shapes an elementally functioning membrane protein family

Background

In spite of the recent accumulation of genomic data, the evolutionary pathway in the individual genes of present-day living taxa is still elusive for most genes. Among ion channels, inward K⁺ rectifier (IRK) channels are the fundamental and well-defined protein group. We analyzed the genomic structures of this group and compared them among a phylogenetically wide range with our sequenced Halocynthia roretzi, a tunicate, IRK genomic genes.

Results

A total of 131 IRK genomic genes were analyzed. The phylogenic trees of amino acid sequences revealed a clear diversification of deuterostomic IRKs from protostomic IRKs and suggested that the tunicate IRKs are possibly representatives of the descendants of ancestor forms of three major groups of IRKs in the vertebrate. However, the exon-intron structures of the tunicate IRK genomes showed considerable similarities to those of Caenorhabditis. In the vertebrate clade, the members in each major group increased at least four times those in the tunicate by various types of global gene duplication. The generation of some major groups was inferred to be due to anti-tandem (palindromic) duplication in early history. The intron insertion points greatly decreased during the evolution of the vertebrates, remaining as a unique conservation of an intron insertion site in the portion of protein-protein interaction within the coding regions of all vertebrate G-protein-activated IRK genes.

Conclusion

From the genomic survey of a family of IRK genes, it was suggested that the ancient intron insertion sites and the unique palindromic genomic duplication evolutionally shaped this membrane protein family.

Non-equivalent role of TM2 gating hinges in heteromeric Kir4.1/Kir5.1 potassium channels

Comparison of the crystal structures of the KcsA and MthK potassium channels suggests that the process of opening a K⁺ channel involves pivoted bending of the inner pore-lining helices at a highly conserved glycine residue. This bending motion is proposed to splay the transmembrane domains outwards to widen the gate at the “helix-bundle crossing”. However, in the inwardly rectifying (Kir) potassium channel family, the role of this “hinge” residue in the second transmembrane domain (TM2) and that of another putative glycine gating hinge at the base of TM2 remain controversial. We investigated the role of these two positions in heteromeric Kir4.1/Kir5.1 channels, which are unique amongst Kir channels in that both subunits lack a conserved glycine at the upper hinge position. Contrary to the effect seen in other channels, increasing the potential flexibility of TM2 by glycine substitutions at the upper hinge position decreases channel opening. Furthermore, the contribution of the Kir4.1 subunit to this process is dominant compared to Kir5.1, demonstrating a non-equivalent contribution of these two subunits to the gating process. A homology model of heteromeric Kir4.1/Kir5.1 shows that these upper “hinge” residues are in close contact with the base of the pore α-helix that supports the selectivity filter. Our results also indicate that the highly conserved glycine at the “lower” gating hinge position is required for tight packing of the TM2 helices at the helix-bundle crossing, rather than acting as a hinge residue.

Inwardly rectifying potassium channels (Kir) in central nervous system glia: a special role for Kir4.1 in glial functions

Glia in the central nervous system (CNS) express diverse inward rectifying potassium channels (Kir). The major function of Kir is in establishing the high potassium (K+) selectivity of the glial cell membrane and strongly negative resting membrane potential (RMP), which are characteristic physiological properties of glia. The classical property of Kir is that K+ flows inwards when the RMP is negative to the equilibrium potential for K+ (E(K)), but at more positive potentials outward currents are inhibited. This provides the driving force for glial uptake of K+ released during neuronal activity, by the processes of "K+ spatial buffering" and "K+ siphoning", considered a key function of astrocytes, the main glial cell type in the CNS. Glia express multiple Kir channel subtypes, which are likely to have distinct functional roles related to their differences in conductance, and sensitivity to intracellular and extracellular factors, including pH, ATP, G-proteins, neurotransmitters and hormones. A feature of CNS glia is their specific expression of the Kir4.1 subtype, which is a major K+ conductance in glial cell membranes and has a key role in setting the glial RMP. It is proposed that Kir4.1 have a primary function in K+ regulation, both as homomeric channels and as heteromeric channels by co-assembly with Kir5.1 and probably Kir2.0 subtypes. Significantly, Kir4.1 are also expressed by oligodendrocytes, the myelin-forming cells of the CNS, and the genetic ablation of Kir4.1 results in severe hypomyelination. Hence, Kir, and in particular Kir4.1, are key regulators of glial functions, which in turn determine neuronal excitability and axonal conduction.

PDZ binding motif-dependent localization of K+ channel on the basolateral side in distal tubules

Kir5.1, a nonfunctional inwardly rectifying K(+) channel by itself, can form functional channels by assembling with other proteins. We previously showed that Kir5.1 assembled with Kir4.1 and functioned as an acid-base regulator in the kidney. In this study, we examined the intrarenal distribution of Kir5.1 by RT-PCR analysis on dissected nephron segments and immunohistochemical analysis with the specific anti-Kir5.1 antibody. Strong expression of Kir5.1 was detected in distal convoluted tubules, and weak expression was also detected in thick ascending limb of Henle's loop. Colocalization of Kir5.1 with Kir4.1 indicated expression of Kir5.1/Kir4.1 heteromer in these nephron segments. In a renal epithelial cell line, Madin-Darby canine kidney cells, heteromer formation with Kir4.1 changed the localization of Kir5.1 from intracellular components to the cell surface. The COOH-terminal cytoplasmic portion that includes the PDZ binding motif of Kir4.1 was responsible for this intracellular localization. These data suggest the signals on the COOH terminus of Kir4.1, including PDZ binding motif, determine the intracellular localization of Kir5.1/Kir4.1 heteromer in distal tubules.

Differential Assembly of Inwardly Rectifying K+ Channel Subunits, Kir4.1 and Kir5.1, in Brain Astrocytes

The inwardly rectifying K+ channel subunit Kir5.1 is expressed abundantly in the brain, but its precise distribution and function are still largely unknown. Because Kir5.1 is co-expressed with Kir4.1 in retinal glial Muller cells, we have compared the biochemical and immunological properties of Kir5.1 and Kir4.1 in the mouse brain. Immunoprecipitation experiments suggested that brain expressed at least two subsets of Kir channels, heteromeric Kir4.1/5.1 and homomeric Kir4.1. Immunolabeling using specific antibodies showed that channels comprising Kir4.1 and Kir5.1 subunits were assembled in a region-specific fashion. Heteromeric Kir4.1/5.1 was identified in the neocortex and in the glomeruli of the olfactory bulb. Homomeric Kir4.1 was confined to the hippocampus and the thalamus. Homomeric Kir5.1 was not identified. Kir4.1/5.1 and Kir4.1 expression appeared to occur only in astrocytes, specifically in the membrane domains facing the pia mater and blood vessels or in the processes surrounding synapses. Both Kir4.1/5.1 and Kir4.1 could be associated with PDZ domain-containing syntrophins, which might be involved in the subcellular targeting of these astrocyte Kir channels. Because heteromeric Kir4.1/5.1 and homomeric Kir4.1 have distinct ion channel properties (Tanemoto, M., Kittaka, N., Inanobe, A., and Kurachi, Y. (2000) J. Physiol. (Lond.) 525, 587-592 and Tucker, S. J., Imbrici, P., Salvatore, L., D'Adamo, M. C., and Pessia, M. (2000) J. Biol. Chem. 275, 16404-16407), it is plausible that these channels play differential physiological roles in the K+ -buffering action of brain astrocytes in a region-specific manner.

Proximal renal tubular acidosis in TASK2 K+ channel-deficient mice reveals a mechanism for stabilizing bicarbonate transport

The acid- and volume-sensitive TASK2 K+ channel is strongly expressed in renal proximal tubules and papillary collecting ducts. This study was aimed at investigating the role of TASK2 in renal bicarbonate reabsorption by using the task2 -/- mouse as a model. After backcross to C57BL6, task2 -/- mice showed an increased perinatal mortality and, in adulthood, a reduced body weight and arterial blood pressure. Patch-clamp experiments on proximal tubular cells indicated that TASK2 was activated during HCO3- transport. In control inulin clearance measurements, task2 -/- mice showed normal NaCl and water excretion. During i.v. NaHCO3 perfusion, however, renal Na+ and water reabsorption capacity was reduced in -/- animals. In conscious task2 -/- mice, blood pH, HCO3- concentration, and systemic base excess were reduced but urinary pH and HCO3- were increased. These data suggest that task2 -/- mice exhibit metabolic acidosis caused by renal loss of HCO3-. Both in vitro and in vivo results demonstrate the specific coupling of TASK2 activity to HCO3- transport through external alkalinization. The consequences of the task2 gene inactivation in mice are reminiscent of the clinical manifestations seen in human proximal renal tubular acidosis syndrome.

Gastric parietal cell secretory membrane contains PKA- and acid-activated Kir2.1 K+ channels

Our objective was to identify and localize a K+ channel involved in gastric HCl secretion at the parietal cell secretory membrane and to characterize and compare the functional properties of native and recombinant gastric K+ channels. RT-PCR showed that mRNA for Kir2.1 was abundant in rabbit gastric mucosa with lesser amounts of Kir4.1 and Kir7.1, relative to beta-actin. Kir2.1 mRNA was localized to parietal cells of rabbit gastric glands by in situ RT-PCR. Resting and stimulated gastric vesicles contained Kir2.1 by Western blot analysis at approximately 50 kDa as observed with in vitro translation. Immunoconfocal microscopy showed that Kir2.1 was present in parietal cells, where it colocalized with H+ -K+ -ATPase and ClC-2 Cl- channels. Function of native K+ channels in rabbit resting and stimulated gastric mucosal vesicles was studied by reconstitution into planar lipid bilayers. Native gastric K+ channels exhibited a linear current-voltage relationship and a single-channel slope conductance of approximately 11 pS in 400 mM K2SO4. Channel open probability (Po) in stimulated vesicles was high, and that of resting vesicles was low. Reduction of extracellular pH plus PKA treatment increased resting channel Po to approximately 0.5 as measured in stimulated vesicles. Full-length rabbit Kir2.1 was cloned. When stably expressed in Chinese hamster ovary (CHO) cells, it was activated by reduced extracellular pH and forskolin/IBMX with no effects observed in nontransfected CHO cells. Cation selectivity was K+ = Rb+ >> Na+ = Cs+ = Li+ = NMDG+. These findings strongly suggest that the Kir2.1 K+ channel may be involved in regulated gastric acid secretion at the parietal cell secretory membrane.

Potassium channels in epithelial transport

Epithelial cells in the kidney, gastrointestinal tract and exocrine glands are engaged in vectorial transport of salt and nutrients. In these tissues, K(+) channels play an important role for the stabilization of membrane voltage and maintenance of the driving force for electrogenic transport. Luminal K(+) channels represent an exit pathway for the excretion of K(+) in secreted fluid, urine and faeces, thereby effecting body K(+) homeostasis. Indeed, the expression and function of several luminal K(+) channels is modulated by hormones regulating water, Na(+), and K(+) metabolism. In addition to net transport of K(+) in the serosal (or apical) direction, K(+) channels can be coupled functionally to K(+)-transporting ATPases such as the basolateral Na(+)/K(+) ATPase or the luminal H(+)/K(+) ATPase. These ATPases export Na(+) or H(+) and take up K(+), which is then recycled via K(+) channels. This review gives a short overview on the molecular identity of epithelial K(+) channels and summarizes the different mechanisms of K(+) channel function during transport in epithelial cells.

Identification of domains that control the heteromeric assembly of Kir5.1/Kir4.0 potassium channels

Heteromultimerization between different inwardly rectifying (Kir) potassium channel subunits is an important mechanism for the generation of functional diversity. However, little is known about the mechanisms that control this process and that prevent promiscuous interactions in cells that express many different Kir subunits. In this study, we have examined the heteromeric assembly of Kir5.1 with other Kir subunits and have shown that this subunit exhibits a highly selective interaction with members of the Kir4.0 subfamily and does not physically associate with other Kir subunits such as Kir1.1, Kir2.1, and Kir6.2. Furthermore, we have identified regions within the Kir4.1 subunit that appear to govern the specificity of this interaction. These results help us to understand the mechanisms that control Kir subunit recognition and assembly and how cells can express many different Kir channels while maintaining distinct subpopulations of homo- and heteromeric channels within the cell.

An inwardly rectifying K+ channel in bovine parotid acinar cells: possible involvement of Kir2.1

Using electrophysiological and molecular techniques, we investigated the molecular nature of an inwardly rectifying K+ channel in bovine parotid acinar (BPA) cells and examined its role in setting resting membrane potential. In whole-cell recordings from freshly isolated BPA cells, a predominant current was a K+ current rectified strongly in the inward direction. An inward conductance of the inwardly rectifying K+ (Kir) current was proportional to [K+]o(0.57). The selectivity sequence based on permeability ratios was K+ (1.00) > Rb+ (0.63) >> Li+ (0.04) = Na+ (0.02) and the sequence based on conductance ratios was K+ (1.00) >> Rb+ (0.03) = Li+ (0.03) = Na+ (0.02). The current was blocked by extracellular Ba2+ and Cs+ in a voltage- and a concentration-dependent manner, with a Kd at 0 mV of 11.6 microM and 121 mM, respectively. Cell-attached patch measurements identified 27 pS K+ channels as being the most likely to mediate whole-cell Kir currents. Addition of Ba2+ (100 microM) to the bathing solution reversibly depolarized the resting membrane potential in intact unstimulated cells. RT-PCR of RNA from bovine parotid cells revealed transcripts of bovine Kir2.1 (bKir2.1). HEK293 cells stably expressing bKir2.1 cloned from bovine parotid exhibited whole-cell and single channel Kir currents, of which electrophysiological characteristics were quantitatively similar to those of native Kir currents. Immunohistochemical studies showed a bKir2.1 immunoreactivity in BPA cells. Collectively, these results suggest that Kir2.1 may mediate native Kir currents responsible for setting resting membrane potential in BPA cells and might be, at least in part, involved in spontaneous secretion in ruminant parotid glands.

Identification of human Kir2.2 (KCNJ12) gene encoding functional inward rectifier potassium channel in both mammalian cells and Xenopus oocytes

Arginine residue at position 285 (R285) in the intracellular C-terminal domain of inward rectifier potassium channel Kir2.2 is conserved in many species, but missing in previously reported human Kir2.2 sequences. We here identified the human Kir2.2 gene in normal individuals, which contained R285 in the deduced amino-acid sequence (hKir2.2/R285). All 30 individuals we examined were homozygous for Kir2.2/R285 gene. The hKir2.2/R285 was electrophysiologically functional in both mammalian cells and Xenopus oocytes. However, the hKir2.2 missing R285 was functional only in Xenopus oocytes, but not in mammalian cells. Thus, R285 in Kir2.2 is important for its functional expression in mammalian cells.

Kir2.4 and Kir2.1 K(+) channel subunits co-assemble: a potential new contributor to inward rectifier current heterogeneity

Heteromeric channel assembly is a potential source of physiological variability. The potential significance of Kir2 subunit heterotetramerization has been controversial, but recent findings suggest that heteromultimerization of Kir2.1-3 may be significant. This study was designed to investigate whether the recently described Kir2.4 subunit can form heterotetramers with the important subunit Kir2.1, and if so, to investigate whether the resulting heterotetrameric channels are functional. Co-expression of either dominant negative Kir2.1 or Kir2.4 subunits in Xenopus oocytes with either wild-type Kir2.1 or 2.4 strongly decreased resulting current amplitude. To examine physical association between Kir2.1 and Kir2.4, Cos-7 cells were co-transfected with a His(6)-tagged Kir2.1 subunit (Kir2.1-His(6)) and a FLAG-tagged Kir2.4 subunit (Kir2.4-FLAG). After pulldown with a His(6)-binding resin, Kir2.4-FLAG could be detected in the eluted cell lysate by Western blotting, indicating co-assembly of Kir2.1-His(6) and Kir2.4-FLAG. Expression of a tandem construct containing covalently linked Kir2.1 and 2.4 subunits led to robust current expression. Kir2.1-Kir2.4 tandem subunit expression, as well as co-injection of Kir2.1 and Kir2.4 cRNA into Xenopus oocytes, produced currents with barium sensitivity greater than that of Kir2.1 or Kir2.4 subunit expression alone. These results show that Kir2.4 subunits can co-assemble with Kir2.1 subunits, and that co-assembled channels are functional, with properties different from those of Kir2.4 or Kir2.1 alone. Since Kir2.1 and Kir2.4 mRNAs have been shown to co-localize in the CNS, Kir2.1 and Kir2.4 heteromultimers might play a role in the heterogeneity of native inward rectifier currents.

AL, Benson DW. KCNJ2 mutation results in Andersen syndrome with sex-specific cardiac and skeletal muscle phenotypes

Evaluation of candidate loci culminated in the identification of a heterozygous missense mutation (R67W) in KCNJ2, the gene encoding the inward-rectifying potassium current, Kir2.1, in 41 members of a kindred in which ventricular arrhythmias (13 of 16 female members [81%]) and periodic paralysis (10 of 25 male members [40%]) segregated as autosomal dominant traits with sex-specific variable expressivity. Some mutation carriers exhibited dysmorphic features, including hypertelorism, small mandible, syndactyly, clinodactyly, cleft palate, and scoliosis, which, together with cardiodysrhythmic periodic paralysis, have been termed "Andersen syndrome." However, no individual exhibited all manifestations of Andersen syndrome, and this diagnosis was not considered in the proband until other family members were examined. Other features seen in this kindred included unilateral dysplastic kidney and cardiovascular malformation (i.e., bicuspid aortic valve, bicuspid aortic valve with coarctation of the aorta, or valvular pulmonary stenosis), which have not been previously associated. Nonspecific electrocardiographic abnormalities were identified in some individuals, but none had a prolonged QT interval. Biophysical characterization of R67W demonstrated loss of function and a dominant-negative effect on Kir2.1 current. These findings support the suggestion that, in addition to its recognized role in function of cardiac and skeletal muscle, KCNJ2 plays an important role in developmental signaling.

Expression pattern and functional characteristics of two novel splice variants of the two-pore-domain potassium channel TREK-2

Two novel alternatively spliced isoforms of the human two-pore-domain potassium channel TREK-2 were isolated from cDNA libraries of human kidney and fetal brain. The cDNAs of 2438 base pairs (bp) (TREK-2b) and 2559 bp (TREK-2c) encode proteins of 508 amino acids each. RT-PCR showed that TREK-2b is strongly expressed in kidney (primarily in the proximal tubule) and pancreas, whereas TREK-2c is abundantly expressed in brain. In situ hybridization revealed a very distinct expression pattern of TREK-2c in rat brain which partially overlapped with that of TREK-1. Expression of TREK-2b and TREK-2c in human embryonic kidney (HEK) 293 cells showed that their single-channel characteristics were similar. The slope conductance at negative potentials was 163 +/- 5 pS for TREK-2b and 179 +/- 17 pS for TREK-2c. The mean open and closed times of TREK-2b at -84 mV were 133 +/- 16 and 109 +/- 11 micros, respectively. Application of forskolin decreased the whole-cell current carried by TREK-2b and TREK-2c. The sensitivity to forskolin was abolished by mutating a protein kinase A phosphorylation site at position 364 of TREK-2c (construct S364A). Activation of protein kinase C (PKC) by application of phorbol-12-myristate-13-acetate (PMA) also reduced whole-cell current. However, removal of the putative TREK-2b-specific PKC phosphorylation site (construct T7A) did not affect inhibition by PMA. Our results suggest that alternative splicing of TREK-2 contributes to the diversity of two-pore-domain K+ channels.

An inward rectifier K(+) channel at the basolateral membrane of the mouse distal convoluted tubule: similarities with Kir4-Kir5.1 heteromeric channels

In this study, K(+) channels present in the basolateral membrane of the distal convoluted tubule (DCT) were investigated using patch-clamp methods. In addition, Kir4.1, Kir4.2 and Kir5.1 inward rectifier channels were investigated using RT-PCR and immunohistochemistry (Kir4.1). DCTs were microdissected from collagenase-treated mouse kidneys. One type of K(+) channel was detected in about 50 % of cell-attached patches from the DCT basolateral membrane; this channel was inwardly rectifying and had an inward conductance (g(in)) of approximately 40 pS at an external [K(+)] of 145 mM. The current-voltage relationship was linear when inside-out patches were exposed to a Mg(2+)-free medium. Mg(2+) at a concentration of 1.2 mM considerably reduced the outward conductance (g(out)), yielding a g(in)/g(out) ratio of approximately 4.7. The polycation spermine (5 x 10(-7) M) reduced the open probability (P(o)) by 50 %. Channel activity was dependent upon the intracellular pH, with acid pH decreasing, and basic pH increasing, P(o). Internal ATP (2 mM) and Ca(2+) (up to 10(-3) M) had no effect. Channel activity declined irreversibly when the inner side of the patch was exposed to Mg(2+). Kir4.1, Kir4.2 and Kir5.1 mRNAs were all detected in the DCT. The Kir4.1 protein co-localised with the Na(+)-Cl(-) cotransporter, which is specific to the DCT, and was located on basolateral membranes. The DCT K(+) channel differs from other functionally identified renal K(+) channels with regard to its inhibition by spermine and insensitivity to internal ATP and Ca(2+). At the current state of knowledge, the channel is similar to Kir4.1-Kir5.1 and Kir4.2-Kir5.1 heteromeric channels, but not to Kir4.1 or Kir4.2 homomeric channels.

Properties and regulation of organic cation transport in freshly isolated human proximal tubules

The kidney, and more specifically the proximal tubule, is the main site of elimination of cationic endogenous metabolites and xenobiotics. Although numerous studies exist on renal organic cation transport of rat and rabbit, no information is available from humans. Therefore, we examined organic cation transport and its regulation across the basolateral membrane of isolated human proximal tubules. mRNA for the cation transporters hOCT1 and hOCT2 as well as hOCTN1 and hOCTN2 was detected in these tubules. Organic cation transport across the basolateral membrane of isolated collapsed proximal tubules was recorded with the fluorescent dye 4-(4-dimethylamino)styryl-N-methylpyridinium (ASP(+)). Depolarization of the cells by rising extracellular K(+) concentration to 145 mm reduced ASP(+) uptake by 20 +/- 5% (n = 15), indicating its electrogeneity. The substrates of organic cation transport tetraethylammonium (K(i) = 63 microm) and cimetidine (K(i) = 11 microm) as well as the inhibitor quinine (K(i) = 2.9 microm) reduced ASP(+) uptake concentration dependently. Maximal inhibition reached with these substances was approximately 60%. Stimulation of protein kinase C with 1,2-dioctanoyl-sn-glycerol (DOG, 1 microm) or ATP (100 microm) inhibited ASP(+) uptake by 30 +/- 3 (n = 16) and 38 +/- 13% (n = 6), respectively. The effect of DOG could be reduced with calphostin C (0.1 microm, n = 7). Activation of adenylate cyclase by forskolin (1 microm) decreased ASP(+) uptake by 29 +/- 3% (n = 10). hANP (10 nm) or 8-bromo-cGMP (100 microm) also decreased ASP(+) uptake by 17 +/- 3 (n = 9) or 32 +/- 5% (n = 10), respectively. We show for the first time that organic cation transport across the basolateral membrane of isolated human proximal tubules, most likely mediated via hOCT2, is electrogenic and regulated by protein kinase C, the cAMP- and the cGMP-dependent protein kinases.