Kcnj16 (Kir5.1) Gene Ablation Causes Subfertility and Increases the Prevalence of Morphologically Abnormal Spermatozoa

The ability of spermatozoa to swim towards an oocyte and fertilize it depends on precise K+ permeability changes. Kir5.1 is an inwardly-rectifying potassium (Kir) channel with high sensitivity to intracellular H+ (pHi) and extracellular K+ concentration [K+]o, and hence provides a link between pHi and [K+]o changes and membrane potential. The intrinsic pHi sensitivity of Kir5.1 suggests a possible role for this channel in the pHi-dependent processes that take place during fertilization. However, despite the localization of Kir5.1 in murine spermatozoa, and its increased expression with age and sexual maturity, the role of the channel in sperm morphology, maturity, motility, and fertility is unknown. Here, we confirmed the presence of Kir5.1 in spermatozoa and showed strong expression of Kir4.1 channels in smooth muscle and epithelial cells lining the epididymal ducts. In contrast, Kir4.2 expression was not detected in testes. To examine the possible role of Kir5.1 in sperm physiology, we bred mice with a deletion of the Kcnj16 (Kir5.1) gene and observed that 20% of Kir5.1 knock-out male mice were infertile. Furthermore, 50% of knock-out mice older than 3 months were unable to breed. By contrast, 100% of wild-type (WT) mice were fertile. The genetic inactivation of Kcnj16 also resulted in smaller testes and a greater percentage of sperm with folded flagellum compared to WT littermates. Nevertheless, the abnormal sperm from mutant animals displayed increased progressive motility. Thus, ablation of the Kcnj16 gene identifies Kir5.1 channel as an important element contributing to testis development, sperm flagellar morphology, motility, and fertility. These findings are potentially relevant to the understanding of the complex pHi- and [K+]o-dependent interplay between different sperm ion channels, and provide insight into their role in fertilization and infertility.

Kir 5.1-dependent CO2 /H+ -sensitive currents contribute to astrocyte heterogeneity across brain regions

Astrocyte heterogeneity is an emerging concept in which astrocytes within or between brain regions show variable morphological and/or gene expression profiles that presumably reflect different functional roles. Recent evidence indicates that retrotrapezoid nucleus (RTN) astrocytes sense changes in tissue CO2/ H+ to regulate respiratory activity; however, mechanism(s) by which they do so remain unclear. Alterations in inward K+ currents represent a potential mechanism by which CO2 /H+ signals may be conveyed to neurons. Here, we use slice electrophysiology in rats of either sex to show that RTN astrocytes intrinsically respond to CO2 /H+ by inhibition of an inward rectifying potassium (Kir ) conductance and depolarization of the membrane, while cortical astrocytes do not exhibit such CO2 /H+ -sensitive properties. Application of Ba2+ mimics the effect of CO2 /H+ on RTN astrocytes as measured by reductions in astrocyte Kir -like currents and increased RTN neuronal firing. These CO2 /H+ -sensitive currents increase developmentally, in parallel to an increased expression in Kir 4.1 and Kir 5.1 in the brainstem. Finally, the involvement of Kir 5.1 in the CO2 /H+ -sensitive current was verified using a Kir5.1 KO rat. These data suggest that Kir inhibition by CO2 /H+ may govern the degree to which astrocytes mediate downstream chemoreceptive signaling events through cell-autonomous mechanisms. These results identify Kir channels as potentially important regional CO2 /H+ sensors early in development, thus expanding our understanding of how astrocyte heterogeneity may uniquely support specific neural circuits and behaviors.

Kcnj16 knockout produces audiogenic seizures in the Dahl salt-sensitive rat

Kir5.1 is an inwardly rectifying potassium (Kir) channel subunit abundantly expressed in the kidney and brain. We previously established the physiologic consequences of a Kcnj16 (gene encoding Kir5.1) knockout in the Dahl salt-sensitive rat (SSKcnj16-/-), which caused electrolyte/pH dysregulation and high-salt diet-induced mortality. Since Kir channel gene mutations may alter neuronal excitability and are linked to human seizure disorders, we hypothesized that SSKcnj16-/- rats would exhibit neurological phenotypes, including increased susceptibility to seizures. SSKcnj16-/- rats exhibited increased light sensitivity (fMRI) and reproducible sound-induced tonic-clonic audiogenic seizures confirmed by electroencephalography. Repeated seizure induction altered behavior, exacerbated hypokalemia, and led to approximately 38% mortality in male SSKcnj16-/- rats. Dietary potassium supplementation did not prevent audiogenic seizures but mitigated hypokalemia and prevented mortality induced by repeated seizures. These results reveal a distinct, nonredundant role for Kir5.1 channels in the brain, introduce a rat model of audiogenic seizures, and suggest that yet-to-be identified mutations in Kcnj16 may cause or contribute to seizure disorders.

Isoflurane inhibits a Kir4.1/5.1-like conductance in neonatal rat brainstem astrocytes and recombinant Kir4.1/5.1 channels in a heterologous expression system

All inhalation anesthetics used clinically including isoflurane can suppress breathing; since this unwanted side effect can persist during the postoperative period and complicate patient recovery, there is a need to better understand how isoflurane affects cellular and molecular elements of respiratory control. Considering that astrocytes in a brainstem region known as the retrotrapezoid nucleus (RTN) contribute to the regulation of breathing in response to changes in CO2/H+ (i.e., function as respiratory chemoreceptors), and astrocytes in other brain regions are highly sensitive to isoflurane, we wanted to determine whether and how RTN astrocytes respond to isoflurane. We found that RTN astrocytes in slices from neonatal rat pups (7-12 days postnatal) respond to clinically relevant levels of isoflurane by inhibition of a CO2/H+-sensitive Kir4.1/5.1-like conductance [50% effective concentration (EC50) = 0.8 mM or ~1.7%]. We went on to confirm that similar levels of isoflurane (EC50 = 0.53 mM or 1.1%) inhibit recombinant Kir4.1/5.1 channels but not homomeric Kir4.1 channels expressed in HEK293 cells. We also found that exposure to CO2/H+ occluded subsequent effects of isoflurane on both native and recombinant Kir4.1/5.1 currents. These results identify Kir4.1/5.1 channels in astrocytes as novel targets of isoflurane. These results suggest astrocyte Kir4.1/5.1 channels contribute to certain aspects of general anesthesia including altered respiratory control.NEW & NOTEWORTHY An unwanted side effect of isoflurane anesthesia is suppression of breathing. Despite this clinical significance, effects of isoflurane on cellular and molecular elements of respiratory control are not well understood. Here, we show that isoflurane inhibits heteromeric Kir4.1/5.1 channels in a mammalian expression system and a Kir4.1/5.1-like conductance in astrocytes in a brainstem respiratory center. These results identify astrocyte Kir4.1/5.1 channels as novel targets of isoflurane and potential substrates for altered respiratory control during isoflurane anesthesia.

Relationship between the renin-angiotensin-aldosterone system and renal Kir5.1 channels

Kir5.1 (encoded by the Kcnj16 gene) is an inwardly rectifying K+ (Kir) channel highly expressed in the aldosterone-sensitive distal nephron of the kidney, where it forms a functional channel with Kir4.1. Kir4.1/Kir5.1 channels are responsible for setting the transepithelial voltage in the distal nephron and collecting ducts and are thereby major determinants of fluid and electrolyte distribution. These channels contribute to renal blood pressure control and have been implicated in salt-sensitive hypertension. However, mechanisms pertaining to the impact of K ir4.1/Kir5.1-mediated K+ transport on the renin-angiotensin-aldosterone system (RAAS) remain unclear. Herein, we utilized a knockout of Kcnj16 in the Dahl salt-sensitive rat (SSKcnj16-/-) to investigate the relationship between Kir5.1 and RAAS balance and function in the sensitivity of blood pressure to the dietary Na+/K+ ratio. The knockout of Kcnj16 caused substantial elevations in plasma RAAS hormones (aldosterone and angiotensin peptides) and altered the RAAS response to changing the dietary Na+/K+ ratio. Blocking aldosterone with spironolactone caused rapid mortality in SSKcnj16-/- rats. Supplementation of the diet with high K+ was protective against mortality resulting from aldosterone-mediated mechanisms. Captopril and losartan treatment had no effect on the survival of SSKcnj16-/- rats. However, neither of these drugs prevented mortality of SSKcnj16-/- rats when switched to high Na+ diet. These studies revealed that the knockout of Kcnj16 markedly altered RAAS regulation and function, suggesting Kir5.1 as a key regulator of the RAAS, particularly when exposed to changes in dietary sodium and potassium content.

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.

Genetic mutation of Kcnj16 identifies Kir5.1-containing channels as key regulators of acute and chronic pH homeostasis

Acute and chronic homeostatic pH regulation is critical for the maintenance of optimal cellular function. Renal mechanisms dominate global pH regulation over longer time frames, and rapid adjustments in ventilation compensate for acute pH and CO2 changes. Ventilatory CO2 and pH chemoreflexes are primarily determined by brain chemoreceptors with intrinsic pH sensitivity likely driven by K+ channels. Here, we studied acute and chronic pH regulation in Kcnj16 mutant Dahl salt-sensitive (SS Kcnj16-/-) rats; Kcnj16 encodes the pH-sensitive inwardly rectifying K+ 5.1 (Kir5.1) channel. SS Kcnj16-/- rats hyperventilated at rest, likely compensating for a chronic metabolic acidosis. Despite their resting hyperventilation, SS Kcnj16-/- rats showed up to 45% reduction in the ventilatory response to graded hypercapnic acidosis vs. controls. SS Kcnj16-/- rats chronically treated with bicarbonate or the carbonic anhydrase inhibitor hydrochlorothiazide had partial restoration of arterial pH, but there was a further reduction in the ventilatory response to hypercapnic acidosis. SS Kcnj16-/- rats also had a nearly absent hypoxic ventilatory response, suggesting major contributions of Kir5.1 to O2- and CO2-dependent chemoreflexes. Although previous studies demonstrated beneficial effects of a high-K+ diet (HKD) on cardiorenal phenotypes in SS Kcnj16-/- rats, HKD failed to restore the observed ventilatory phenotypes. We conclude that Kir5.1 is a key regulator of renal H+ handling and essential for acute and chronic regulation of arterial pH as determinants of the ventilatory CO2 chemoreflex.-Puissant, M. M., Muere, C., Levchenko, V., Manis, A. D., Martino, P., Forster, H. V., Palygin, O., Staruschenko, A., Hodges, M. R. Genetic mutation of Kcnj16 identifies Kir5.1-containing channels as key regulators of acute and chronic pH homeostasis.

Kir4.1/Kir5.1 in the DCT plays a role in the regulation of renal K+ excretion

The aim of this mini review is to provide an overview regarding the role of inwardly rectifying potassium channel 4.1 (Kir4.1)/Kir5.1 in regulating renal K+ excretion. Deletion of Kir4.1 in the kidney inhibited thiazide-sensitive NaCl cotransporter (NCC) activity in the distal convoluted tubule (DCT) and slightly suppressed Na-K-2Cl cotransporter (NKCC2) function in the thick ascending limb (TAL). Moreover, increased dietary K+ intake inhibited, whereas decreased dietary K+ intake stimulated, the basolateral potassium channel (a Kir4.1/Kir5.1 heterotetramer) in the DCT. The alteration of basolateral potassium conductance is essential for the effect of dietary K+ intake on NCC because deletion of Kir4.1 in the DCT abolished the effect of dietary K+ intake on NCC. Since potassium intake-mediated regulation of NCC plays a key role in regulating renal K+ excretion and potassium homeostasis, the deletion of Kir4.1 caused severe hypokalemia and metabolic alkalosis under control conditions and even during increased dietary K+ intake. Finally, recent studies have suggested that the angiotensin II type 2 receptor (AT2R) and bradykinin-B2 receptor (BK2R) are involved in mediating the effect of high dietary K+ intake on Kir4.1/Kir5.1 in the DCT.

Norepinephrine-Induced Stimulation of Kir4.1/Kir5.1 Is Required for the Activation of NaCl Transporter in Distal Convoluted Tubule

The stimulation of β-adrenergic receptor increases thiazide-sensitive NaCl cotransporter (NCC), an effect contributing to salt-sensitive hypertension by sympathetic stimulation. We now test whether the stimulation of β-adrenergic receptor-induced activation of NCC is achieved through activating basolateral Kir4.1 in the distal convoluted tubule (DCT). Application of norepinephrine increased the basolateral 40 pS K+ channel (Kir4.1/Kir5.1 heterotetramer) in the DCT. The stimulatory effect of norepinephrine on the K+ channel was mimicked by cAMP analogue but abolished by inhibiting PKA (protein kinase A). Also, the effect of norepinephrine on the K+ channel in the DCT was recapitulated by isoproterenol but not by α-adrenergic agonist and blocked by propranolol, suggesting that norepinephrine effect on the K+ channel was mediated by β-adrenergic receptor. The whole-cell recording shows that norepinephrine and isoproterenol increased DCT K+ currents and shifted the K+ current ( IK) reversal potential to negative range (hyperpolarization). Continuous norepinephrine perfusion (7 days) increased DCT K+ currents, hyperpolarized IK reversal potential, and increased the expression of total NCC/phosphorylated NCC, but it had no significant effect on the expression of NKCC2 (type 2 Na-Cl-K cotransporter) and ENaC-α (epithelial Na channel-α subunit). Renal clearance study demonstrated that norepinephrine perfusion augmented thiazide-induced urinary Na+ excretion only in wild-type but not in kidney-specific Kir4.1 knockout mice, suggesting that Kir4.1 is required for mediating the effect of norepinephrine on NCC. However, norepinephrine perfusion did not affect urinary K+ excretion. We conclude that the stimulation of β-adrenergic receptor activates the basolateral Kir4.1 in the DCT and that the activation of Kir4.1 is required for norepinephrine-induced stimulation of NCC.

[The function and regulation of basolateral Kir4.1 and Kir4.1/Kir5.1 in renal tubules]

Basolateral inwardly-rectifying K+ channels (Kir) 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. Kir4.1 and Kir4.1/Kir5.1 heterotetramer are abundantly expressed in the basolateral membrane of late thick ascending limb (TAL), distal convoluted tubule (DCT), connecting tubule (CNT) and cortical collecting duct (CCD). Loss-of-function mutations in KCNJ10 cause EAST/SeSAME syndrome in humans associated with epilepsy, ataxia, sensorineural deafness and water-electrolyte metabolism imbalance, which is characterized by salt wasting, hypomagnesaemia, hypokalaemia and metabolic alkalosis. In contrast, mice lacking Kir5.1 have severe renal phenotype apart from hypokalaemia such as high chlorine metabolic acidosis and hypercalcinuria. The genetic knockout or functional inhibition of Kir4.1 suppresses Na-Cl cotransporter (NCC) expression and activity in the DCT. However, the downregulation of Kir4.1 increases epithelial Na+ channel (ENaC) expression in the collecting duct. Recently, factors regulating expression and activity of Kir4.1 and Kir4.1/Kir5.1 were identified, such as cell acidification, dopamine, insulin and insulin-like growth factor-1. The involved mechanisms include PKC, PI3K, Src family protein tyrosine kinases and WNK-SPAK signal transduction pathways. Here we review the progress of renal tubule basolateral Kir, and mainly discuss the function and regulation of Kir4.1 and Kir4.1/Kir5.1.

Kir5.1 regulates Nedd4-2-mediated ubiquitination of Kir4.1 in distal nephron

Kir4.1/5.1 heterotetramer participates in generating the negative cell membrane potential in distal convoluted tubule (DCT) and plays a critical role in determining the activity of Na-Cl cotransporter (NCC). Kir5.1 contains a phosphothreonine motif at its COOH terminus (AA249-252). Coimmunoprecipitation showed that Nedd4-2 was associated with Kir5.1 in HEK293 cells cotransfected with Kir5.1 or Kir4.1/Kir5.1. GST pull-down further confirmed the association between Nedd4-2 and Kir5.1. Ubiquitination assay showed that Nedd4-2 increased the ubiquitination of Kir4.1/Kir5.1 heterotetramer in the cells cotransfected with Kir4.1/Kir5.1, but it has no effect on Kir4.1 or Kir5.1 alone. Patch-clamp and Western blot also demonstrated that coexpression of Nedd4-2 but not Nedd4-1 decreased K currents and Kir4.1 expression in the cells cotransfected with Kir4.1 and Kir5.1. In contrast, Nedd4-2 fails to inhibit Kir4.1 in the absence of Kir5.1 or in the cells transfected with the inactivated form of Nedd4-2 (Nedd4-2C821A). Moreover, the mutation of TPVT motif in the COOH terminus of Kir5.1 largely abolished the association of Nedd4-2 with Kir5.1 and abolished the inhibitory effect of Nedd4-2 on K currents in HEK293 cells transfected with Kir4.1 and Kir5.1 mutant (Kir5.1T249A). Finally, the basolateral K conductance in the DCT and Kir4.1 expression is significantly increased in the kidney-specific Nedd4-2 knockout or in Kir5.1 knockout mice in comparison to their corresponding wild-type littermates. We conclude that Nedd4-2 binds to Kir5.1 at the phosphothreonine motif of the COOH terminus, and the association of Nedd4-2 with Kir5.1 facilitates the ubiquitination of Kir4.1, thereby regulating its plasma expression in the DCT.

Distal tubule basolateral potassium channels: cellular and molecular mechanisms of regulation

PURPOSE OF REVIEW

Multiple clinical and translational evidence support benefits of high potassium diet; however, there many uncertainties underlying the molecular and cellular mechanisms determining effects of dietary potassium. Kir4.1 and Kir5.1 proteins form a functional heteromer (Kir4.1/Kir5.1), which is the primary inwardly rectifying potassium channel on the basolateral membrane of both distal convoluted tubule (DCT) and the collecting duct principal cells. The purpose of this mini-review is to summarize latest advances in our understanding of the evolution, physiological relevance and mechanisms controlling these channels.

RECENT FINDINGS

Kir4.1 and Kir5.1 channels play a critical role in determining electrolyte homeostasis in the kidney and blood pressure, respectively. It was reported that Kir4.1/Kir5.1 serves as potassium sensors in the distal nephron responding to variations in dietary intake and hormonal stimuli. Global and kidney specific knockouts of either channel resulted in hypokalemia and severe cardiorenal phenotypes. Furthermore, knock out of Kir5.1 in Dahl salt-sensitive rat background revealed the crucial role of the Kir4.1/Kir5.1 channel in salt-induced hypertension.

SUMMARY

Here, we focus on reviewing novel experimental evidence of the physiological function, expression and hormonal regulation of renal basolateral inwardly rectifying potassium channels. Further investigation of molecular and cellular mechanisms controlling Kir4.1 and Kir4.1/Kir5.1-mediating pathways and development of specific compounds targeting these channels function is essential for proper control of electrolyte homeostasis and blood pressure.

Potassium intake modulates the thiazide-sensitive sodium-chloride cotransporter (NCC) activity via the Kir4.1 potassium channel

Kir4.1 in the distal convoluted tubule plays a key role in sensing plasma potassium and in modulating the thiazide-sensitive sodium-chloride cotransporter (NCC). Here we tested whether dietary potassium intake modulates Kir4.1 and whether this is essential for mediating the effect of potassium diet on NCC. High potassium intake inhibited the basolateral 40 pS potassium channel (a Kir4.1/5.1 heterotetramer) in the distal convoluted tubule, decreased basolateral potassium conductance, and depolarized the distal convoluted tubule membrane in Kcnj10flox/flox mice, herein referred to as control mice. In contrast, low potassium intake activated Kir4.1, increased potassium currents, and hyperpolarized the distal convoluted tubule membrane. These effects of dietary potassium intake on the basolateral potassium conductance and membrane potential in the distal convoluted tubule were completely absent in inducible kidney-specific Kir4.1 knockout mice. Furthermore, high potassium intake decreased, whereas low potassium intake increased the abundance of NCC expression only in the control but not in kidney-specific Kir4.1 knockout mice. Renal clearance studies demonstrated that low potassium augmented, while high potassium diminished, hydrochlorothiazide-induced natriuresis in control mice. Disruption of Kir4.1 significantly increased basal urinary sodium excretion but it abolished the natriuretic effect of hydrochlorothiazide. Finally, hypokalemia and metabolic alkalosis in kidney-specific Kir4.1 knockout mice were exacerbated by potassium restriction and only partially corrected by a high-potassium diet. Thus, Kir4.1 plays an essential role in mediating the effect of dietary potassium intake on NCC activity and potassium homeostasis.

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.

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.

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.

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.

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.

Insulin and IGF-1 activate Kir4.1/5.1 channels in cortical collecting duct principal cells to control basolateral membrane voltage

Potassium Kir4.1/5.1 channels are abundantly expressed at the basolateral membrane of principal cells in the cortical collecting duct (CCD), where they are thought to modulate transport rates by controlling transepithelial voltage. Insulin and insulin-like growth factor-1 (IGF-1) stimulate apically localized epithelial sodium channels (ENaC) to augment sodium reabsorption in the CCD. However, little is known about their actions on potassium channels localized at the basolateral membrane. In this study, we implemented patch-clamp analysis in freshly isolated murine CCD to assess the effect of these hormones on Kir4.1/5.1 at both single channel and cellular levels. We demonstrated that K(+)-selective conductance via Kir4.1/5.1 is the major contributor to the macroscopic current recorded from the basolateral side in principal cells. Acute treatment with 10 μM amiloride (ENaC blocker), 100 nM tertiapin-Q (TPNQ; ROMK inhibitor), and 100 μM ouabain (Na(+)-K(+)-ATPase blocker) failed to produce a measurable effect on the macroscopic current. In contrast, Kir4.1 inhibitor nortriptyline (100 μM), but not fluoxetine (100 μM), virtually abolished whole cell K(+)-selective conductance. Insulin (100 nM) markedly increased the open probability of Kir4.1/5.1 and nortriptyline-sensitive whole cell current, leading to significant hyperpolarization of the basolateral membrane. Inhibition of the phosphatidylinositol 3-kinase cascade with LY294002 (20 μM) abolished action of insulin on Kir4.1/5.1. IGF-1 had similar stimulatory actions on Kir4.1/5.1-mediated conductance only when applied at a higher (500 nM) concentration and was ineffective at 100 nM. We concluded that both insulin and, to a lesser extent, IGF-1 activate Kir4.1/5.1 channel activity and open probability to hyperpolarize the basolateral membrane, thereby facilitating Na(+) reabsorption in the CCD.

Molecular insights into the possible role of Kir4.1 and Kir5.1 in thyroid hormone biosynthesis

INTRODUCTION

Thyroid morphogenesis is a complex process. Inwardly rectifying potassium (Kir) genes play a role in hormone release, cell excitability, pH and K(+) homeostasis in many tissues.

OBJECTIVES

To investigate the thyroid developmental expression of three members, Kir4.1, Kir4.2 and Kir5.1, in mice. To postulate the K(+) channel role in thyroid hormone secretion.

MATERIAL AND METHODS

Quantitative RT-PCR analysis of Kir4.1, Kir4.2 and Kir5.1 in mice of different stages (E13.5-E18.5).

RESULTS

mRNA for Kir4.1, Kir4.2 and Kir5.1 were identified and increased with age in mice. Both Kir4.1 and Kir4.2 genes are better expressed after E16.5. Kir4.2 greatly increases from E13.5 to E16.5 (p ≤ 0.05).

CONCLUSION

Quantitative PCR shows that the mouse thyroid presents increased expression for Kir channels during development. The role of Kir in thyroid morphogenesis and differentiation might be understood in future studies. We speculate that thyroglobulin trafficking might be modulated by Kir4.1/5.1.

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.

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.

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.

The disruption of central CO2 chemosensitivity in a mouse model of Rett syndrome

People with Rett syndrome (RTT) have breathing instability in addition to other neuropathological manifestations. The breathing disturbances contribute to the high incidence of unexplained death and abnormal brain development. However, the cellular mechanisms underlying the breathing abnormalities remain unclear. To test the hypothesis that the central CO(2) chemoreception in these people is disrupted, we studied the CO(2) chemosensitivity in a mouse model of RTT. The Mecp2-null mice showed a selective loss of their respiratory response to 1-3% CO(2) (mild hypercapnia), whereas they displayed more regular breathing in response to 6-9% CO(2) (severe hypercapnia). The defect was alleviated with the NE uptake blocker desipramine (10 mg·kg(-1)·day(-1) ip, for 5-7 days). Consistent with the in vivo observations, in vitro studies in brain slices indicated that CO(2) chemosensitivity of locus coeruleus (LC) neurons was impaired in Mecp2-null mice. Two major neuronal pH-sensitive Kir currents that resembled homomeric Kir4.1 and heteromeric Ki4.1/Kir5.1 channels were identified in the LC neurons. The screening of Kir channels with real-time PCR indicated the overexpression of Kir4.1 in the LC region of Mecp2-null mice. In a heterologous expression system, an overexpression of Kir4.1 resulted in a reduction in the pH sensitivity of the heteromeric Kir4.1-Kir5.1 channels. Given that Kir4.1 and Kir5.1 subunits are also expressed in brain stem respiration-related areas, the Kir4.1 overexpression may not allow CO(2) to be detected until hypercapnia becomes severe, leading to periodical hyper- and hypoventilation in Mecp2-null mice and, perhaps, in people with RTT as well.

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

The heteromeric inwardly rectifying Kir4.1/Kir5.1 K(+) channel underlies the basolateral K(+) conductance in the distal nephron and is extremely sensitive to inhibition by intracellular pH. The functional importance of Kir4.1/Kir5.1 in renal ion transport has recently been highlighted by mutations in the human Kir4.1 gene (KCNJ10) that result in seizures, sensorineural deafness, ataxia, mental retardation, and electrolyte imbalance (SeSAME)/epilepsy, ataxia, sensorineural deafness, and renal tubulopathy (EAST) syndrome, a complex disorder that includes salt wasting and hypokalemic alkalosis. Here, we investigated the role of the Kir5.1 subunit in mice with a targeted disruption of the Kir5.1 gene (Kcnj16). The Kir5.1(-/-) mice displayed hypokalemic, hyperchloremic metabolic acidosis with hypercalciuria. The short-term responses to hydrochlorothiazide, an inhibitor of ion transport in the distal convoluted tubule (DCT), were also exaggerated, indicating excessive renal Na(+) absorption in this segment. Furthermore, chronic treatment with hydrochlorothiazide normalized urinary excretion of Na(+) and Ca(2+), and abolished acidosis in Kir5.1(-/-) mice. Finally, in contrast to WT mice, electrophysiological recording of K(+) channels in the DCT basolateral membrane of Kir5.1(-/-) mice revealed that, even though Kir5.1 is absent, there is an increased K(+) conductance caused by the decreased pH sensitivity of the remaining homomeric Kir4.1 channels. In conclusion, disruption of Kcnj16 induces a severe renal phenotype that, apart from hypokalemia, is the opposite of the phenotype seen in SeSAME/EAST syndrome. These results highlight the important role that Kir5.1 plays as a pH-sensitive regulator of salt transport in the DCT, and the implication of these results for the correct genetic diagnosis of renal tubulopathies is discussed.

Respiratory responses to hypercapnia and hypoxia in mice with genetic ablation of Kir5.1 (Kcnj16)

Inward rectifier (Kir) potassium channels contribute to the control of electrical activity in excitable tissues and their activity is modulated by many biochemical factors, including protons. Heteromeric Kir4.1-Kir5.1 channels are highly pH sensitive within the physiological range of pH changes and are strongly expressed by the peripheral chemosensors as well as in the brainstem pH-sensitive areas which mediate respiratory responses to changes in blood and brain levels of P(CO(2))/[H(+)]. In the present study, Kir5.1 knockout mice (Kir5.1(-/-)) were used to determine the role of these channels in the chemosensory control of breathing. We found that Kir5.1(-/-) mice presented with persistent metabolic acidosis and a clear respiratory phenotype. Despite metabolic acidosis, ventilation at rest and in hyperoxic hypercapnia were similar in wild-type and Kir5.1(-/-) mice. Ventilatory responses to hypoxia and normoxic hypercapnia were significantly reduced in Kir5.1(-/-) mice; however, carotid body chemoafferent responses to hypoxia and CO(2) were not affected. In the in situ brainstem-spinal cord preparations with denervated peripheral chemoreceptors, resting phrenic nerve activity and phrenic nerve responses to respiratory acidosis or isohydric hypercapnia were also similar in Kir5.1(-/-) and wild-type mice. In in situ preparations of Kir5.1(-/-) mice with intact peripheral chemoreceptors, application of CN(-) resulted in a significantly reduced phrenic nerve response, suggesting that the relay of peripheral chemosensory information to the CNS is compromised. We suggest that this compensatory modulation of the peripheral chemosensory inputs develops in Kir5.1(-/-) mice in order to counteract the effect of continuing metabolic acidosis on the activity of the peripheral chemoreceptors. These results therefore suggest that despite their intrinsic pH sensitivity, Kir4.1-Kir5.1 channels are dispensable for functional central and peripheral respiratory chemosensitivity.

Genetic inactivation of Kcnj16 identifies Kir5.1 as an important determinant of neuronal PCO2/pH sensitivity

The molecular identity of ion channels which confer PCO(2)/pH sensitivity in the brain is unclear. Heteromeric Kir4.1/Kir5.1 channels are highly sensitive to inhibition by intracellular pH and are widely expressed in several brainstem nuclei involved in cardiorespiratory control, including the locus coeruleus. This has therefore led to a proposed role for these channels in neuronal CO(2) chemosensitivity. To examine this, we generated mutant mice lacking the Kir5.1 (Kcnj16) gene. We show that although locus coeruleus neurons from Kcnj16((+/+)) mice rapidly respond to cytoplasmic alkalinization and acidification, those from Kcnj16((-/-)) mice display a dramatically reduced and delayed response. These results identify Kir5.1 as an important determinant of PCO(2)/pH sensitivity in locus coeruleus neurons and suggest that Kir5.1 may be involved in the response to hypercapnic acidosis.

Astrocytes in the retrotrapezoid nucleus sense H+ by inhibition of a Kir4.1-Kir5.1-like current and may contribute to chemoreception by a purinergic mechanism

Central chemoreception is the mechanism by which CO(2)/pH sensors regulate breathing in response to tissue pH changes. There is compelling evidence that pH-sensitive neurons in the retrotrapezoid nucleus (RTN) are important chemoreceptors. Evidence also indicates that CO(2)/H(+)-evoked adenosine 5'-triphosphate (ATP) release in the RTN, from pH-sensitive astrocytes, contributes to chemoreception. However, mechanism(s) by which RTN astrocytes sense pH is unknown and their contribution to chemoreception remains controversial. Here, we use the brain slice preparation and a combination of patch-clamp electrophysiology and immunohistochemistry to confirm that RTN astrocytes are pH sensitive and to determine mechanisms by which they sense pH. We show that pH-sensitive RTN glia are immunoreactive for aldehyde dehydrogenase 1L1, a marker of astrocytes. In HEPES buffer the pH-sensitive current expressed by RTN astrocytes reversed near E(K(+)) (the equilibrium potential for K(+)) and was inhibited by Ba(2+) and desipramine (blocker of Kir4.1-containing channels), characteristics most consistent with heteromeric Kir4.1-Kir5.1 channels. In bicarbonate buffer, the sodium/bicarbonate cotransporter also contributed to the CO(2)/H(+)-sensitive current in RTN astrocytes. To test the hypothesis that RTN astrocytes contribute to chemoreception by a purinergic mechanism, we used fluorocitrate to selectively depolarize astrocytes while measuring neuronal activity. We found that fluorocitrate increased baseline activity and pH sensitivity of RTN neurons by a P2-receptor-dependent mechanism, suggesting that astrocytes may release ATP to activate RTN chemoreceptors. We also found in bicarbonate but not HEPES buffer that P2-receptor antagonists decreased CO(2) sensitivity of RTN neurons. We conclude that RTN astrocytes sense CO(2)/H(+) in part by inhibition of a Kir4.1-Kir5.1-like current and may provide an excitatory purinergic drive to pH-sensitive neurons.

Molecular mechanisms of EAST/SeSAME syndrome mutations in Kir4.1 (KCNJ10)

Inwardly rectifying potassium channel Kir4.1 is critical for glial function, control of neuronal excitability, and systemic K(+) homeostasis. Novel mutations in Kir4.1 have been associated with EAST/SeSAME syndrome, characterized by mental retardation, ataxia, seizures, hearing loss, and renal salt waste. Patients are homozygous for R65P, G77R, C140R or T164I; or compound heterozygous for A167V/R297C or R65P/R199Stop, a deletion of the C-terminal half of the protein. We investigated the functional significance of these mutations by radiotracer efflux and inside-out membrane patch clamping in COSm6 cells expressing homomeric Kir4.1 or heteromeric Kir4.1/Kir5.1 channels. All of the mutations compromised channel function, but the underlying mechanisms were different. R65P, T164I, and R297C caused an alkaline shift in pH sensitivity, indicating that these positions are crucial for pH sensing and pore gating. In R297C, this was due to disruption of intersubunit salt bridge Glu(288)-Arg(297). C140R breaks the Cys(108)-Cys(140) disulfide bond essential for protein folding and function. A167V did not affect channel properties but may contribute to decreased surface expression in A167V/R297C. In G77R, introduction of a positive charge within the bilayer may affect channel structure or gating. R199Stop led to a dramatic decrease in surface expression, but channel activity was restored by co-expression with intact subunits, suggesting remarkable tolerance for truncation of the cytoplasmic domain. These results provide an explanation for the molecular defects that underlie the EAST/SeSAME syndrome.

Kir5.1 underlies long-lived subconductance levels in heteromeric Kir4.1/Kir5.1 channels from Xenopus tropicalis

The inwardly-rectifying potassium channel subunit Kir5.1 selectively co-assembles with members of the Kir4.0 subfamily to form novel pH-sensitive heteromeric channels with unique single channel properties. In this study, we have cloned orthologs of Kir4.1 and Kir5.1 from the genome of the amphibian, Xenopus tropicalis (Xt). Heteromeric XtKir4.1/XtKir5.1 channels exhibit similar macroscopic current properties to rat Kir4.1/Kir5.1 with a faster time-dependent rate of activation. However, single channel analysis of heteromeric XtKir4.1/XtKir5.1 channels reveals that they have markedly different long-lived, multi-level subconductance states. Furthermore, we demonstrate that the XtKir5.1 subunit is responsible for these prominent subconductance levels. These results are consistent with a model in which the slow transitions between sublevel states represent the movement of individual subunits. These novel channels now provide an excellent model system to determine the structural basis of subconductance levels and contribution of heteromeric pore architecture to this process.

Modulation of Kir4.1 and Kir4.1-Kir5.1 channels by extracellular cations

This work demonstrates that extracellular Na(+) modulates the cloned inwardly rectifying K(+) channels Kir4.1 and Kir4.1-Kir5.1. Whole-cell patch clamp studies on astrocytes have previously indicated that inward potassium currents are regulated by external Na(+). We expressed Kir4.1 and Kir4.1-Kir5.1 in Xenopus oocytes to disclose if Kir4.1 and/or Kir4.1-Kir5.1 at the molecular level are responsible for the observed effect of [Na(+)](o) and to investigate the regulatory mechanism of external cations further. Our results showed that Na(+) has a biphasic modulatory effect on both Kir4.1 and Kir4.1-Kir5.1 currents. Depending on the Na(+)-concentration and applied voltage, the inward Kir4.1/Kir4.1-Kir5.1 currents are either enhanced or reduced by extracellular Na(+). The Na(+) activation was voltage-independent, whereas the Na(+)-induced reduction of the Kir4.1 and Kir4.1-Kir5.1 currents was both concentration-, time- and voltage-dependent. Our data indicate that the biphasic effect of extracellular Na(+)on the Kir4.1 and Kir4.1-Kir5.1 channels is caused by two separate mechanisms.

KCNQ1 is the luminal K+ recycling channel during stimulation of gastric acid secretion

Parietal cell (PC) proton secretion via H(+)/K(+)-ATPase requires apical K(+) recycling. A variety of K(+) channels and transporters are expressed in the PC and the molecular nature of the apical K(+) recycling channel is under debate. This study was undertaken to delineate the exact function of KCNQ1 channels in gastric acid secretion. Acid secretory rates and electrophysiological parameters were determined in gastric mucosae of 7- to 8-day-old KCNQ1(+/+), (+/-) and (-/-) mice. Parietal cell ultrastructure, abundance and gene expression levels were quantified. Glandular structure and PC abundance, and housekeeping gene expression did not differ between the KCNQ1(-/-) and (+/+) mucosae. Microvillar secretory membranes were intact, but basal acid secretion was absent and forskolin-stimulated acid output reduced by approximately 90% in KCNQ1(-/-) gastric mucosa. Application of a high K(+) concentration to the luminal membrane restored normal acid secretory rates in the KCNQ1(-/-) mucosa. The study demonstrates that the KCNQ1 channel provides K(+) to the extracellular K(+) binding site of the H(+)/K(+)-ATPase during acid secretion, and no other gastric K(+) channel can substitute for this function.

Modulation of Kir4.1 and Kir4.1-Kir5.1 channels by small changes in cell volume

The K+ channels Kir4.1 and Kir4.1-Kir5.1 are expressed in the glial cells of the CNS and are involved in regulation of the K+ homeostasis. Several studies have shown that Kir4.1 channels are co-localized with aquaporins (AQP4) in the glial endfeet, and a putative functional coupling between the Kir channels and aquaporins is therefore debated. To test a possible volume-sensitivity of the Kir channels, the Kir4.1 or Kir4.1-Kir5.1 channels were expressed in Xenopus oocytes with or without co-expression of aquaporins and subsequently exposed to cell volume alterations. Our results show an increase in Kir4.1 and Kir4.1-Kir5.1 currents upon swelling of the oocytes and a reduction in the current when the oocytes were shrunk. The volume-dependent changes in channel activity were not due to changes in the kinetics of the channels. These findings implicate a putative functional interaction between the Kir channels and aquaporins via small, fast cell volume changes in the glial cells.

Kir4.1/Kir5.1 channel forms the major K+ channel in the basolateral membrane of mouse renal collecting duct principal cells

K(+) channels in the basolateral membrane of mouse cortical collecting duct (CCD) principal cells were identified with patch-clamp technique, real-time PCR, and immunohistochemistry. In cell-attached membrane patches, three K(+) channels with conductances of approximately 75, 40, and 20 pS were observed, but the K(+) channel with the intermediate conductance (40 pS) predominated. In inside-out membrane patches exposed to an Mg(2+)-free medium, the current-voltage relationship of the intermediate-conductance channel was linear with a conductance of 38 pS. Addition of 1.3 mM internal Mg(2+) had no influence on the inward conductance (G(in) = 35 pS) but reduced outward conductance (G(out)) to 13 pS, yielding a G(in)/G(out) of 3.2. The polycation spermine (6 x 10(-7) M) reduced its activity on inside-out membrane patches by 50% at a clamp potential of 60 mV. Channel activity was also dependent on intracellular pH (pH(i)): a sigmoid relationship between pH(i) and channel normalized current (NP(o)) was observed with a pK of 7.24 and a Hill coefficient of 1.7. By real-time PCR on CCD extracts, inwardly rectifying K(+) (Kir)4.1 and Kir5.1, but not Kir4.2, mRNAs were detected. Kir4.1 and Kir5.1 proteins cellularly colocalized with aquaporin 2 (AQP2), a specific marker of CCD principal cells, while AQP2-negative cells (i.e., intercalated cells) showed no staining. Dietary K(+) had no influence on the properties of the intermediate-conductance channel, but a Na(+)-depleted diet increased its open probability by approximately 25%. We conclude that the Kir4.1/Kir5.1 channel is a major component of the K(+) conductance in the basolateral membrane of mouse CCD principal cells.

K(ir) and K(v) channels regulate electrical properties and proliferation of adult neural precursor cells

The functional significance of the electrophysiological properties of neural precursor cells (NPCs) was investigated using dissociated neurosphere-derived NPCs from the forebrain subventricular zone (SVZ) of adult mice. NPCs exhibited hyperpolarized resting membrane potentials, which were depolarized by the K(+) channel inhibitor, Ba(2+). Pharmacological analysis revealed two distinct K(+) channel families: Ba(2+)-sensitive K(ir) channels and tetraethylammonium (TEA)-sensitive K(v) (primarily K(DR)) channels. Ba(2+) promoted mitogen-stimulated NPC proliferation, which was mimicked by high extracellular K(+), whereas TEA inhibited proliferation. Based on gene and protein levels in vitro, we identified K(ir)4.1, K(ir)5.1 and K(v)3.1 channels as the functional K(+) channel candidates. Expression of these K(+) channels was immunohistochemically found in NPCs of the adult mouse SVZ, but was negligible in neuroblasts. It therefore appears that expression of K(ir) and K(v) (K(DR)) channels in NPCs and related changes in the resting membrane potential could contribute to NPC proliferation and neuronal lineage commitment in the neurogenic microenvironment.

Protein kinase C dependent inhibition of the heteromeric Kir4.1-Kir5.1 channel

Heteromultimerization of Kir4.1 and Kir5.1 leads to a channel with distinct functional properties. The heteromeric Kir4.1-Kir5.1 channel is expressed in the eye, kidney and brainstem and has CO(2)/pH sensitivity in the physiological range, suggesting a candidate molecule for the regulation of K(+) homeostasis and central CO(2) chemoreception. It is known that K(+) transport in renal epithelium and brainstem CO(2) chemosensitivity are subject to modulation by hormones and neurotransmitters that activate distinct intracellular signaling pathways. If the Kir4.1-Kir5.1 channel is involved in pH-dependent regulation of cellular functions, it may also be regulated by some of the intracellular signaling systems. Therefore, we undertook studies to determine whether PKC modulates the heteromeric Kir4.1-Kir5.1 channel. The channel expressed using a Kir4.1-Kir5.1 tandem dimer construct was inhibited by the PKC activator PMA in a dose-dependent manner. The channel inhibition was produced via reduction of the P(open). The effect of PMA was abolished by specific PKC inhibitors. In contrast, exposure of oocytes to forskolin (a PKA activator) had no significant effect on Kir4.1-Kir5.1 currents. The channel inhibition appeared to be independent of PIP(2) depletion and PKC-dependent internalization. Several consensus sequences of potential PKC phosphorylation sites were identified in the Kir4.1 and Kir5.1 subunits by sequence scan. Although the C-terminal peptides of both Kir4.1 and Kir5.1 were phosphorylated in vitro, site-directed mutagenesis of individual residues failed to reveal the PKC phosphorylation sites suggesting that the channel may have multiple phosphorylation sites. Taken together, these results suggest that the Kir4.1-Kir5.1 but not the homomeric Kir4.1 channel is strongly inhibited by PKC activation.

Control of pH and PIP2 gating in heteromeric Kir4.1/Kir5.1 channels by H-Bonding at the helix-bundle crossing

Inhibition by intracellular H(+) (pH gating) and activation by phosphoinositides such as PIP(2) (PIP(2)-gating) are key regulatory mechanisms in the physiology of inwardly-rectifying potassium (Kir) channels. Our recent findings suggest that PIP(2) gating and pH gating are controlled by an intra-subunit H-bond at the helix-bundle crossing between a lysine in TM1 and a backbone carbonyl group in TM2. This interaction only occurs in the closed state and channel opening requires this H-bond to be broken, thereby influencing the kinetics of PIP(2) and pH gating in Kir channels. In this addendum, we explore the role of H-bonding in heteromeric Kir4.1/Kir5.1 channels. Kir5.1 subunits do not possess a TM1 lysine. However, homology modelling and molecular dynamics simulations demonstrate that the TM1 lysine in Kir4.1 is capable of H-bonding at the helix-bundle crossing. Consistent with this, the rates of pH and PIP2 gating in Kir4.1/Kir5.1 channels (two H-bonds) were intermediate between those of wild-type homomeric Kir4.1 (four H-bonds) and Kir4.1(K67M) channels (no H-bonds) suggesting that the number of H-bonds in the tetrameric channel complex determines the gating kinetics. Furthermore, in heteromeric Kir4.1(K67M)/Kir5.1 channels, where the two remaining H-bonds are disrupted, we found that the gating kinetics were similar to Kir4.1(K67M) homomeric channels despite the fact that these two channels differ considerably in their PIP(2) affinities. This indicates that Kir channel PIP(2) affinity has little impact on either the PIP(2) or pH gating kinetics.

Interaction of the Ca2+-sensing receptor with the inwardly rectifying potassium channels Kir4.1 and Kir4.2 results in inhibition of channel function

The Ca(2+)-sensing receptor (CaR), a G protein-coupled receptor, is expressed in many epithelial tissues including the parathyroid glands, kidney, and GI tract. Although its role in regulating PTH levels and Ca(2+) metabolism are best characterized, it may also regulate salt and water transport in the kidney as demonstrated by recent reports showing association of potent gain-of-function mutations in the CaR with a Bartter-like, salt-wasting phenotype. To determine whether this receptor interacts with novel proteins that control ion transport, we screened a human adult kidney cDNA library with the COOH-terminal 219 amino acid cytoplasmic tail of the CaR as bait using the yeast two-hybrid system. We identified two independent clones coding for approximately 125 aa from the COOH terminus of the inwardly rectifying K(+) channel, Kir4.2. The CaR and Kir4.2 as well as Kir4.1 (another member of Kir4 subfamily) were reciprocally coimmunoprecipitated from HEK-293 cells in which they were expressed, but the receptor did not coimmunoprecipitate with Kir5.1 or Kir1.1. Both Kir4.1 and Kir4.2 were immunoprecipitated from rat kidney extracts with the CaR. In Xenopus laevis oocytes, expression of the CaR with either Kir4.1 or Kir4.2 channels resulted in inactivation of whole cell current as measured by two-electrode voltage clamp, but the nonfunctional CaR mutant CaR(R796W), and that does not coimmunoprecipitate with the channels, had no effect. Kir4.1 and the CaR were colocalized in the basolateral membrane of the distal nephron. The CaR interacts directly with Kir4.1 and Kir4.2 and can decrease their currents, which in turn could reduce recycling of K(+) for the basolateral Na(+)-K(+)-ATPase and thereby contribute to inhibition of Na(+) reabsorption.

Modulation of Kir4.2 rectification properties and pHi-sensitive run-down by association with Kir5.1

Inwardly rectifying K+ channels (Kir) comprise seven subfamilies that can be subdivided further on the basis of cytosolic pH (pHi) sensitivity, rectification strength and kinetics, and resistance to run-down. Although distinct residues within each channel subunit define these properties, heteromeric association with other Kir subunits can modulate them. We identified such an effect in the wild-type forms of Kir4.2 and Kir5.1 and used this to further understand how the functional properties of Kir channels relate to their structures. Kir4.2 and a Kir4.2-Kir5.1 fusion protein were expressed in HEK293 cells. Inward currents from Kir4.2 were stable over 10 min and pHi-insensitive (pH 6 to 8). Conversely, currents from Kir4.2-Kir5.1 exhibited a pHi-sensitive run-down at slightly acidic pHi. At pHi 7.2, currents in response to voltage steps positive to EK were essentially time independent for Kir4.2 indicating rapid block by Mg2+. Coexpression with Kir5.1 significantly increased the blocking time constant, and increased steady-state outward current characteristic of weak rectifiers. Recovery from blockade at negative potentials was voltage dependent and 2 to 10 times slower in the homomeric channel. These results show that Kir5.1 converts Kir4.2 from a strong to a weak rectifier, rendering it sensitive to pHi, and suggesting that Kir5.1 plays a role in fine-tuning Kir4.2 activity.

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.

An inwardly rectifying potassium channel in apical membrane of Calu-3 cells

Patch clamp methods and reverse transcription-polymerase chain reaction (RT-PCR) were used to characterize an apical K+ channel in Calu-3 cells, a widely used model of human airway gland serous cells. In cell-attached and excised apical membrane patches, we found an inwardly rectifying K+ channel (Kir). The permeability ratio was PNa/PK = 0.058. In 30 patches with both cystic fibrosis transmembrane conductance regulator and Kir present, we observed 79 cystic fibrosis transmembrane conductance regulator and 58 Kir channels. The average chord conductance was 24.4 +/- 0.5 pS (n = 11), between 0 and -200 mV, and was 9.6 +/- 0.7 pS (n = 8), between 0 and 50 mV; these magnitudes and their ratio of approximately 2.5 are most similar to values for rectifying K+ channels of the Kir4.x subfamilies. We attempted to amplify transcripts for Kir4.1, Kir4.2, and Kir5.1; of these only Kir4.2 was present in Calu-3 lysates. The channel was only weakly activated by ATP and was relatively insensitive to internal pH. External Cs+ and Ba2+ blocked the channel with Kd values in the millimolar range. Quantitative modeling of Cl- secreting epithelia suggests that secretion rates will be highest and luminal K+ will rise to 16-28 mm if 11-25% of the total cellular K+ conductance is placed in the apical membrane (Cook, D. I., and Young, J. A. (1989) J. Membr. Biol. 110, 139-146). Thus, we hypothesize that the K+ channel described here optimizes the rate of secretion and is involved in K+ recycling for the recently proposed apical H+ -K+ -ATPase in Calu-3 cells.