Stable cation coordination at a single outer pore residue defines permeation properties in Kir channels

Normal vision and development in mice with low functional expression of Kir7.1 in heterozygosis for a blindness-producing mutation inactivating the channel

K+ channel Kir7.1 expressed at the apical membrane of the retinal pigment epithelium (RPE) plays an essential role in retinal function. An isoleucine-to-threonine mutation at position 120 of the protein is responsible for blindness-causing vitreo-retinal dystrophy. We have studied the molecular mechanism of action of Kir7.1-I120T in vitro by heterologous expression and in vivo in CRISPR-generated knockin mice. Full-size Kir7.1-I120T reaches the plasma membrane but lacks any activity. Analysis of Kir7.1 and the I120T mutant in mixed transfection experiments, and that of tandem tetrameric constructs made by combining wild type (WT) and mutant protomers, leads us to conclude that they do not form heterotetramers in vitro. Homozygous I120T/I120T mice show cleft palate and tracheomalacia and do not survive beyond P0, whereas heterozygous WT/I120T develop normally. Membrane conductance of RPE cells isolated from WT/WT and heterozygous WT/I120T mice is dominated by Kir7.1 current. Using Rb+ as a charge carrier, we demonstrate that the Kir7.1 current of WT/I120T RPE cells corresponds to approximately 50% of that in cells from WT/WT animals, in direct proportion to WT gene dosage. This suggests a lack of compensatory effects or interference from the mutated allele product, an interpretation consistent with results obtained using WT/- hemizygous mouse. Electroretinography and behavioral tests also show normal vision in WT/I120T animals. The hypomorphic ion channel phenotype of heterozygous Kir7.1-I120T mutants is therefore compatible with normal development and retinal function. The lack of detrimental effect of this degree of functional deficit might explain the recessive nature of Kir7.1 mutations causing human eye disease.NEW & NOTEWORTHY Human retinal pigment epithelium K+ channel Kir7.1 is affected by generally recessive mutations leading to blindness. We investigate one such mutation, isoleucine-to-threonine at position 120, both in vitro and in vivo in knockin mice. The mutated channel is inactive and in heterozygosis gives a hypomorphic phenotype with normal retinal function. Mutant channels do not interfere with wild-type Kir7.1 channels which are expressed concomitantly without hindrance, providing an explanation for the recessive nature of the disease.

The unique structural characteristics of the Kir 7.1 inward rectifier potassium channel: a novel player in energy homeostasis control

The inward rectifier potassium channel Kir7.1, encoded by the KCNJ13 gene, is a tetramer composed of two-transmembrane domain-spanning monomers, closer in homology to Kir channels associated with potassium transport such as Kir1.1, 1.2, and 1.3. Compared with other channels, Kir7.1 exhibits small unitary conductance and low dependence on external potassium. Kir7.1 channels also show a phosphatidylinositol 4,5-bisphosphate (PIP2) dependence for opening. Accordingly, retinopathy-associated Kir7.1 mutations mapped at the binding site for PIP2 resulted in channel gating defects leading to channelopathies such as snowflake vitreoretinal degeneration and Leber congenital amaurosis in blind patients. Lately, this channel's role in energy homeostasis was reported due to the direct interaction with the melanocortin type 4 receptor (MC4R) in the hypothalamus. As this channel seems to play a multipronged role in potassium homeostasis and neuronal excitability, we will discuss what is predicted from a structural viewpoint and its possible implications for hunger control.

Comparison of K+ Channel Families

K⁺ channels enable potassium to flow across the membrane with great selectivity. There are four K⁺ channel families: voltage-gated K (K_v), calcium-activated (K_Ca), inwardly rectifying K (K_ir), and two-pore domain potassium (K_2P) channels. All four K⁺ channels are formed by subunits assembling into a classic tetrameric (4x1P = 4P for the K_v, K_Ca, and K_ir channels) or tetramer-like (2x2P = 4P for the K_2P channels) architecture. These subunits can either be the same (homomers) or different (heteromers), conferring great diversity to these channels. They share a highly conserved selectivity filter within the pore but show different gating mechanisms adapted for their function. K⁺ channels play essential roles in controlling neuronal excitability by shaping action potentials, influencing the resting membrane potential, and responding to diverse physicochemical stimuli, such as a voltage change (K_v), intracellular calcium oscillations (K_Ca), cellular mediators (K_ir), or temperature (K_2P).

Altered phosphatidylinositol regulation of mutant inwardly rectifying K+ Kir7.1 channels associated with inherited retinal degeneration disease

KEY POINTS

Kir7.1 K⁺ channel expressed in retinal pigment epithelium is mutated in inherited retinal degeneration diseases. We study Kir7.1 in heterologous expression to test the hypothesis that pathological R162 mutation to neutral amino acids results in loss of a crucial site that binds PI(4,5)P₂ . Although R162W mutation inactivates Kir7.1, changes to smaller volume (e.g. Gln) amino acids are tolerated or even enhance function (Ala or Cys). Chemical modification of Kir7.1-R162C confirms that large residues of the size of Trp are incompatible with normal channel function even if positively charged. In addition to R162, K164 (and possibly K159) forms a binding site for the phosphoinositide and is essential for channel activity. R162 substitution with a large, neutral side chain like Trp exerts a dominant negative effect on Kir7.1 activity such that less than one fifth of the full activity is expected in a cell expressing the same amount of mutant and wild-type channels.

ABSTRACT

Mutations in the Kir7.1 K⁺ channel, highly expressed in retinal pigment epithelium, have been linked to inherited retinal degeneration diseases. Examples are mutations changing Arg 162 to Trp in snowflake vitreoretinal degeneration (SVD) and Gln in retinitis pigmentosa. R162 is believed to be part of a site that binds PI(4,5)P₂ and stabilises the open state. We have tested the hypothesis that R162 mutation to neutral amino acids will result in the loss of this crucial interaction to the detriment of channel function. Our findings indicate that although R612W mutation inactivates Kir7.1, changes to smaller volume (e.g. Gln) amino acids are tolerated or even enhance function (Ala or Cys). Cys chemical modification of Kir7.1-R162C confirms that large residues of the size of Trp are incompatible with normal channel function even if positively charged. Experiments titrating the levels of plasma membrane PI(4,5)P₂ with voltage-dependent phosphatase DrVSP reveal that, in addition to R162, K164 (and possibly K159) forms a binding site for the phosphoinositide and ensures channel activity. Finally, the use of a concatemeric approach shows that substitution of R162 with a large, neutral side chain mimicking a Trp residue exerts a dominant negative effect on Kir7.1 activity such that less than one fifth of the full activity is expected in heterozygous cells carrying the SVD mutation. Our results suggest that if mutations in the human KCNJ13 gene resulting in the neutralisation of R162 and Kir7.1 malfunction led to retinal degeneration diseases, their severity might depend on the nature of the side chain of the replacing amino acid.

A novel Kir7.1 splice variant expressed in various mouse tissues shares organisational and functional properties with human Leber amaurosis-causing mutations of this K+ channel

Kir7.1 is an inwardly rectifying K⁺ channel present in epithelia where it shares membrane localization with the Na⁺/K⁺-pump. In the present communication we report the presence of a novel splice variant of Kir7.1 in mouse tissues including kidney, lung, choroid plexus and retinal pigment epithelium (RPE). The variant named mKir7.1-SV2 lacks most of the C-terminus domain but is predicted to have the two transmembrane domains and permeation pathway unaffected. Similarly truncated predicted proteins, Kir7.1-R166X and Kir7.1-Q219X, would arise from mutations associated with Leber Congenital Amaurosis, a rare recessive hereditary retinal disease that results in vision loss at early age. We found that mKir7.1-SV2 and the pathological variants do not produce any channel activity when expressed alone in HEK-293 cells due to their scarce presence in the plasma membrane. Simultaneous expression with the full length Kir7.1 however leads to a reduction in activity of the wild-type channel that might be due to partial proteasome degradation of WT-mutant channel heteromers.

Tissue Distribution of Kir7.1 Inwardly Rectifying K+ Channel Probed in a Knock-in Mouse Expressing a Haemagglutinin-Tagged Protein

Kir7.1 encoded by the Kcnj13 gene in the mouse is an inwardly rectifying K⁺ channel present in epithelia where it shares membrane localization with the Na⁺/K⁺-pump. Further investigations of the localisation and function of Kir7.1 would benefit from the availability of a knockout mouse, but perinatal mortality attributed to cleft palate in the neonate has thwarted this research. To facilitate localisation studies we now use CRISPR/Cas9 technology to generate a knock-in mouse, the Kir7.1-HA that expresses the channel tagged with a haemagglutinin (HA) epitope. The availability of antibodies for the HA epitope allows for application of western blot and immunolocalisation methods using widely available anti-HA antibodies with WT tissues providing unambiguous negative control. We demonstrate that Kir7.1-HA cloned from the choroid plexus of the knock-in mouse has the electrophysiological properties of the native channel, including characteristically large Rb⁺ currents. These large Kir7.1-mediated currents are accompanied by abundant apical membrane Kir7.1-HA immunoreactivity. WT-controlled western blots demonstrate the presence of Kir7.1-HA in the eye and the choroid plexus, trachea and lung, and intestinal epithelium but exclusively in the ileum. In the kidney, and at variance with previous reports in the rat and guinea-pig, Kir7.1-HA is expressed in the inner medulla but not in the cortex or outer medulla. In isolated tubules immunoreactivity was associated with inner medulla collecting ducts but not thin limbs of the loop of Henle. Kir7.1-HA shows basolateral expression in the respiratory tract epithelium from trachea to bronchioli. The channel also appears basolateral in the epithelium of the nasal cavity and nasopharynx in newborn animals. We show that HA-tagged Kir7.1 channel introduced in the mouse by a knock-in procedure has functional properties similar to the native protein and the animal thus generated has clear advantages in localisation studies. It might therefore become a useful tool to unravel Kir7.1 function in the different organs where it is expressed.

Differential binding of monovalent cations to KcsA: Deciphering the mechanisms of potassium channel selectivity

This work explores whether the ion selectivity and permeation properties of a model potassium channel, KcsA, could be explained based on ion binding features. Non-permeant Na⁺ or Li⁺ bind with low affinity (millimolar K_D's) to a single set of sites contributed by the S1 and S4 sites seen at the selectivity filter in the KcsA crystal structure. Conversely, permeant K⁺, Rb⁺, Tl⁺ and even Cs⁺ bind to two different sets of sites as their concentration increases, consistent with crystallographic evidence on the ability of permeant species to induce concentration-dependent transitions between conformational states (non-conductive and conductive) of the channel's selectivity filter. The first set of such sites, assigned also to the crystallographic S1 and S4 sites, shows similarly high affinities for all permeant species (micromolar K_D's), thus, securing displacement of potentially competing non-permeant cations. The second set of sites, available only to permeant cations upon the transition to the conductive filter conformation, shows low affinity (millimolar K_D's), thus, favoring cation dissociation and permeation and results from the contribution of all S1 through S4 crystallographic sites. The differences in affinities between permeant and non-permeant cations and the similarities in binding behavior within each of these two groups, correlate fully with their permeabilities relative to K⁺, suggesting that binding is an important determinant of the channel's ion selectivity. Conversely, the complexity observed in permeation features cannot be explained just in terms of binding and likely relates to reported differences in the occupancy of the S2 and S3 sites by the permeant cations.

Focus on Kir7.1: physiology and channelopathy

Genetic studies have linked alterations in Kir7.1 channel to diverse pathologies. We summarize functional relevance of Kir7.1 channel in retinal pigment epithelium (RPE), regulation of channel function by various cytoplasmic metabolites, and mutations that cause channelopathies. At the apical membrane of RPE, K(+) channels contribute to subretinal K(+) homeostasis and support Na(+)/K(+) pump and Na(+)-K(+)-2Cl(-) cotransporter function by providing a pathway for K(+) secretion. Electrophysiological studies have established that barium- and cesium-sensitive inwardly rectifying K(+) (Kir) channels make up a major component of the RPE apical membrane K(+) conductance. Native human RPE expresses transcripts for Kir1.1, Kir2.1, Kir2.2, Kir3.1, Kir3.4, Kir4.2, and Kir6.1, albeit at levels at least 50-fold lower than Kir7.1. Kir7.1 is structurally similar to other Kir channels, consisting of 2 trans-membrane domains, a pore-forming loop that contains the selectivity filter, and 2 cytoplasmic polar tails. Within the cytoplasmic structure, clusters of amino acid sequences form regulatory domains that interact with cellular metabolites and control the opening and closing of the channel. Recent evidence indicated that intrinsic sequence motifs present in Kir7.1 control surface expression. Mutant Kir7.1 channels are associated with inherited eye pathologies such as Snowflake Vitreoretinal Degeneration (SVD) and Lebers Congenital Amaurosis (LCA16). Based on the current evidence, mutations implicated in channelopathies have the potential to be used for genetic testing to diagnose blindness due to Kir7.1.

Characterization of the R162W Kir7.1 mutation associated with snowflake vitreoretinopathy

KCNJ13 encodes Kir7.1, an inwardly rectifying K(+) channel that is expressed in multiple ion-transporting epithelia. A mutation in KCNJ13 resulting in an arginine-to-tryptophan change at residue 162 (R162W) of Kir7.1 was associated with snowflake vitreoretinal degeneration, an inherited autosomal-dominant disease characterized by vitreous degeneration and mild retinal degeneration. We used the Xenopus laevis oocyte expression system to assess the functional properties of the R162W (mutant) Kir7.1 channel and determine how wild-type (WT) Kir7.1 is affected by the presence of the mutant subunit. Recordings obtained via the two-electrode voltage-clamp technique revealed that injection of oocytes with mutant Kir7.1 cRNA resulted in currents and cation selectivity that were indistinguishable from those in water-injected oocytes, suggesting that the mutant protein does not form functional channels in the plasma membrane. Coinjection of oocytes with equal amounts of mutant and WT Kir7.1 cRNAs resulted in inward K(+) and Rb(+) currents with amplitudes that were ∼17% of those in oocytes injected with WT Kir7.1 cRNA alone, demonstrating a dominant-negative effect of the mutant subunit. Similar to oocytes injected with WT Kir7.1 cRNA alone, coinjected oocytes exhibited inwardly rectifying Rb(+) currents that were more than seven times larger than K(+) currents, indicating that mutant subunits did not alter Kir7.1 channel selectivity. Immunostaining of Xenopus oocytes or Madin-Darby canine kidney cells expressing mutant or WT Kir7.1 demonstrated distribution of both proteins primarily in the plasma membrane. Our data suggest that the R162W mutation suppresses Kir7.1 channel activity, possibly by negatively impacting gating by membrane phosphadidylinositol 4,5-bisphosphate.

Activator-induced dynamic disorder and molecular memory in human two-pore domain hTREK1 K channel

Ion channels are fundamental molecules in the nervous system that catalyze the flux of ions across the cell membrane. Ion channel flux activity is comparable to the catalytic activity of enzyme molecules. Saturating concentrations of substrate induce "dynamic disorder" in the kinetic rate processes of single-enzyme molecules and consequently, develop correlative "memory" of the previous history of activities. Similarly, binding of ions as substrate alone or in presence of agonists affects the catalytic turnover of single-ion channels. Here, we investigated the possible existence of dynamic disorder and molecular memory in the single human-TREK1-channel due to binding of substrate/agonist using the excised inside-out patch-clamp technique. Our results suggest that the single-hTREK1-channel behaves as a typical Michaelis-Menten enzyme molecule with a high-affinity binding site for K(+) ion as substrate. But, in contrast to enzyme, dynamic disorder in single-hTREK1-channel was not induced by substrate K(+) binding, but required allosteric modification of the channel molecule by the agonist, trichloroethanol. In addition, interaction of trichloroethanol with hTREK1 induced strong correlation in the waiting time and flux intensity, exemplified by distinct mode-switching between high and low flux activities. This suggested the induction of molecular memory in the channel molecule by the agonist, which persisted for several decades in time. Our mathematical modeling studies identified the kinetic rate processes associated with dynamic disorder. It further revealed the presence of multiple populations of distinct conformations that contributed to the "heterogeneity" and consequently, to the molecular memory phenomenon that we observed.

ELECTRONIC SUPPLEMENTARY MATERIAL

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Functional equivalency inferred from "authoritative sources" in networks of homologous proteins

A one-on-one mapping of protein functionality across different species is a critical component of comparative analysis. This paper presents a heuristic algorithm for discovering the Most Likely Functional Counterparts (MoLFunCs) of a protein, based on simple concepts from network theory. A key feature of our algorithm is utilization of the user's knowledge to assign high confidence to selected functional identification. We show use of the algorithm to retrieve functional equivalents for 7 membrane proteins, from an exploration of almost 40 genomes form multiple online resources. We verify the functional equivalency of our dataset through a series of tests that include sequence, structure and function comparisons. Comparison is made to the OMA methodology, which also identifies one-on-one mapping between proteins from different species. Based on that comparison, we believe that incorporation of user's knowledge as a key aspect of the technique adds value to purely statistical formal methods.

Modulation of the Kir7.1 potassium channel by extracellular and intracellular pH

Inwardly rectifying K(+) (K(ir)) channels in the apical membrane of the retinal pigment epithelium (RPE) contribute to extracellular K(+) homeostasis in the distal retina by mediating K(+) secretion. Multiple lines of evidence suggest that these channels are composed of Kir7.1. Previously, we showed that native K(ir) channels in bovine RPE are modulated by changes in intracellular pH in the physiological range. In the present study, we used the Xenopus laevis oocyte expression system to investigate the pH dependence of cloned human Kir7.1 channels and several point mutants involving histidine residues in the NH(2) and COOH termini. Kir7.1 channels were inhibited by strong extracellular acidification and modulated by intracellular pH in a biphasic manner, with maximal activity at about intracellular pH (pH(i)) 7.0 and inhibition by acidification or alkalinization. Replacement of histidine 26 (H26) in the NH(2) terminus with alanine eliminated the requirement of protons for channel activity and increased sensitivity to proton-induced inhibition, resulting in maximal channel activity at alkaline pH(i) and smaller whole cell currents at resting pH(i) compared with wild-type Kir7.1. When H26 was replaced with arginine, the pH(i) sensitivity profile was similar to that of the H26A mutant but with the pK(a) shifted to a more acidic value, giving rise to whole cell current amplitude at resting pH(i) that was comparable to that of wild-type Kir7.1. These results indicate that Kir7.1 channels are modulated by intracellular protons by diverse mechanisms and suggest that H26 is important for channel activation at physiological pH(i) and that it influences an unidentified proton-induced inhibitory mechanism.

Expression of Kir7.1 and a novel Kir7.1 splice variant in native human retinal pigment epithelium

Previous studies on bovine retinal pigment epithelium (RPE) established that Kir7.1 channels compose this epithelium's large apical membrane K+ conductance. The purpose of this study was to determine whether Kir7.1 and potential Kir7.1 splice variants are expressed in native adult human RPE and, if so, to determine their function and how they are generated. RT-PCR analysis indicated that human RPE expresses full-length Kir7.1 and a novel Kir7.1 splice variant, designated Kir7.1S. Analysis of the human Kir7.1 gene (KCNJ13) organization revealed that it contains three exons, two introns, and a novel alternative 5' splice site in exon 2. In human RPE, the alternative usage of two competing 5' splice sites in exon 2 gives rise to transcripts encoding full-length Kir7.1 and Kir7.1S, which is predicted to encode a truncated protein. Real-time PCR indicated that Kir7.1 transcript is nearly as abundant as GAPDH mRNA in human RPE whereas Kir7.1S transcript expression is 4-fold lower. Western blot analysis showed that the splice variant is translated in Xenopus oocytes injected with Kir7.1S cRNA and revealed the expression of full-length Kir7.1 but not Kir7.1S in adult human RPE. Co-expression of Kir7.1 with Kir7.1S in Xenopus oocytes had no effect on either the kinetics or amplitude of Kir7.1 currents. This study confirms the expression of Kir7.1 in human RPE, identifies a Kir7.1 splice variant resulting in predicted changes in protein sequence, and indicates that there is no functional interaction between this splice variant and full-length Kir7.1.

K+ activation of kir3.1/kir3.4 and kv1.4 K+ channels is regulated by extracellular charges

K+ activates many inward rectifier and voltage-gated K+ channels. In each case, an increase in K+ current through the channel can occur despite a reduced driving force. We have investigated the molecular mechanism of K+ activation of the inward rectifier K+ channel, Kir3.1/Kir3.4, and the voltage-gated K+ channel, Kv1.4. In the Kir3.1/Kir3.4 channel, mutation of an extracellular arginine residue, R155, in the Kir3.4 subunit markedly reduced K+ activation of the channel. The same mutation also abolished Mg2+ block of the channel. Mutation of the equivalent residue in Kv1.4 (K532) abolished K+ activation as well as C-type inactivation of the Kv1.4 channel. Thus, whereas C-type inactivation is a collapse of the selectivity filter, K+ activation could be an opening of the selectivity filter. K+ activation of the Kv1.4 channel was enhanced by acidic pH. Mutation of an extracellular histidine residue, H508, that mediates the inhibitory effect of protons on Kv1.4 current, abolished both K+ activation and the enhancement of K+ activation at acidic pH. These results suggest that the extracellular positive charges in both the Kir3.1/Kir3.4 and the Kv1.4 channels act as "guards" and regulate access of K+ to the selectivity filter and, thus, the open probability of the selectivity filter. Furthermore, these data suggest that, at acidic pH, protonation of H508 inhibits current through the Kv1.4 channel by decreasing K+ access to the selectivity filter, thus favoring the collapse of the selectivity filter.

Ionic permeation and conduction properties of neuronal KCNQ2/KCNQ3 potassium channels

Heteromeric KCNQ2/3 potassium channels are thought to underlie the M-current, a subthreshold potassium current involved in the regulation of neuronal excitability. KCNQ channel subunits are structurally unique, but it is unknown whether these structural differences result in unique conduction properties. Heterologously expressed KCNQ2/3 channels showed a permeation sequence of while showing a conduction sequence of A differential contribution of component subunits to the properties of heteromeric KCNQ2/3 channels was demonstrated by studying homomeric KCNQ2 and KCNQ3 channels, which displayed contrasting ionic selectivities. KCNQ2/3 channels did not exhibit an anomalous mole-fraction effect in mixtures of K(+) and Rb(+). However, extreme voltage-dependence of block by external Cs(+) was indicative of multi-ion pore behavior. Block of KCNQ2/3 channels by external Ba(2+) ions was voltage-independent, demonstrating unusual ionic occupation of the outer pore. Selectivity properties and block of KCNQ2 were altered by mutation of outer pore residues in a manner consistent with the presence of multiple ion-binding sites. KCNQ2/3 channel deactivation kinetics were slowed exclusively by Rb(+), whereas activation of KCNQ2/3 channels was altered by a variety of external permeant ions. These data indicate that KCNQ2/3 channels are multi-ion pores which exhibit distinctive mechanisms of ion conduction and gating.

Molecular basis of ion selectivity, block, and rectification of the inward rectifier Kir3.1/Kir3.4 K(+) channel

The glycine-tyrosine-glycine (GYG) sequence in the p-loop of K+ channel subunits lines a narrow pore through which K+ ions pass in single file intercalated by water molecules. Mutation of the motif can give rise to non-selective channels, but it is clear that other structural features are also required for selectivity because, for instance, a recently identified class of cyclic nucleotide-gated pacemaker channels has the GYG motif but are poorly K+ selective. We show that mutation of charged glutamate and arginine residues behind the selectivity filter in the Kir3.1/Kir3.4 K+ channel reduces or abolishes K+ selectivity, comparable with previously reported effects in the Kir2.1 K+ channel. It has been suggested that a salt bridge exists between the glutamate-arginine residue pair. Molecular modeling indicates that the salt bridge does exist, and that it acts as a "bowstring" to maintain the rigid bow-like structure of the selectivity filter and restrict selectivity to K+. The modeling shows that relaxation of the bowstring by mutation of the residue pair leads to enhanced flexibility of the p-loop, allowing permeation of other cations, including polyamines. In experiments, mutation of the residue pair can also abolish polyamine-induced inward rectification. The latter effect occurs because polyamines now permeate rather than block the channel, to the remarkable extent that large polyamine currents can be measured.

Regulation of inwardly rectifying K+ channels in retinal pigment epithelial cells by intracellular pH

Inwardly rectifying K+ (Kir) channels in the apical membrane of the retinal pigment epithelium (RPE) play a key role in the transport of K+ into and out of the subretinal space (SRS), a small extracellular compartment surrounding photoreceptor outer segments. Recent molecular and functional evidence indicates that these channels comprise Kir7.1 channel subunits. The purpose of this study was to determine whether Kir channels in the RPE are modulated by extracellular (pHo) or intracellular pH (pHi), both of which change upon illumination of the dark-adapted retina. The Kir current (IKir) in acutely dissociated bovine RPE cells was recorded in the whole-cell configuration while altering pHo or pHi. In cells dialysed with pipette solution buffered to pH 7.2, step changes in pHo from 7.4 to 8.0, 7.0 or 6.5 had little effect on IKir. Acidification to pHo 6.0, however, caused a transient activation of IKir followed by a slower inhibition. To determine the dependence of IKir on pHi, we altered pHi within individual RPE cells at constant pHo by imposing transmembrane acetate concentration gradients. These experiments revealed a biphasic relationship between IKir and pHi: IKir was maximal at about pHi 7.1, but decreased sharply at more acidic or alkaline levels. To evaluate the role of Kir7.1 channels in the pHi-dependent changes in IKir, we tested the effect of transmembrane acetate concentration gradients on Rb+ currents, which are 10-fold larger than K+ currents for this channel subtype. Inwardly rectifying Rb+ currents were maximal at about pHi 7.0 and were inhibited by intracellular alkalinization or acidification. We conclude that the Kir conductance in the RPE is modulated by intracellular pH in the physiological range and that this reflects the behaviour of Kir7.1 channels. This sensitivity to pHi may provide an important mechanism linking photoreceptor activity and RPE function.

Site-directed glycosylation tagging of functional Kir2.1 reveals that the putative pore-forming segment is extracellular

Inwardly rectifying K+ channels or Kirs are a large gene family and have been predicted to have two transmembrane segments, M1 and M2, intracellular N and C termini, and two extracellular loops, E1 and E2, separated by an intramembranous pore-forming segment, H5. H5 contains a stretch of eight residues that are similar in voltage-dependent K+ channels, Kvs, and this stretch is called the signature sequence of K+ channels. Because mutations in this sequence altered selectivity in Kvs, it has been designated as the selectivity filter. Previously, we used N-glycosylation substitution mutants to map the extracellular topology of a weak inwardly rectifying K+ channel, Kir1.1 or ROMK1, and found that the entire H5 segment was extracellular. We now report utilization of introduced N-glycosylation sites, NX(S/T), at positions Ser(128) in E1, and Gln(140), Ileu(143), and Phe(147) in the H5 sequence of a strong inwardly rectifying K+ channel, Kir2.1. Furthermore, we show that biotinylated channel proteins with N-linked oligosaccharides attached at positions 140 and 143 in the signature sequence are located at the cell surface. Mutant channels were functional as detected by whole-cell and single-channel recordings. Unlike Kir1.1, position Lys(117) was not occupied. We conclude that, for yet another K+ channel, the invariant G(Y/F)G sequence is extracellular rather than intramembranous.

Characterization of heteromultimeric G protein-coupled inwardly rectifying potassium channels of the tunicate tadpole with a unique pore property

Two cDNAs that encode the G protein-coupled inwardly rectifying K(+) channel (GIRK, Kir3) of tunicate tadpoles (tunicate G protein-coupled inwardly rectifying K(+) channel-A and -B; TuGIRK-A and -B) have been isolated. The deduced amino acid sequences showed approximately 60% identity with the mammalian Kir3 family. Detected by whole mount in situ hybridization, both TuGIRK-A and -B were expressed similarly in the neural cells of the head and neck region from the tail bud stage to the young tadpole stage. By co-injecting cRNAs of TuGIRK-A and G protein beta(1)/gamma(2) subunits (Gbetagamma) in Xenopus oocytes, an inwardly rectifying K(+) current was expressed. In contrast, coinjection of TuGIRK-B with Gbetagamma did not express any current. When both TuGIRK-A and -B were coexpressed together with Gbetagamma, an inwardly rectifying K(+) current was also detected. The properties of this current clearly differed from those of TuGIRK-A current, since it displayed a characteristic decline of the macroscopic conductance at strongly hyperpolarized potentials. TuGIRK-A/B current also differed from TuGIRK-A current in terms of the lower sensitivity to the Ba(2+) block, the higher sensitivity to the Cs(+) block, and the smaller single channel conductance. Taken together, we concluded that TuGIRK-A and -B form functional heteromultimeric G protein-coupled inwardly rectifying K(+) channels in the neural cells of the tunicate tadpole. By introducing a mutation of Lys(161) to Thr in TuGIRK-B, TuGIRK-A/B channels acquired a higher sensitivity to the Ba(2+) block and a slightly lower sensitivity to the Cs(+) block, and the decrease in the macroscopic conductance at hyperpolarized potentials was no longer observed. Thus, the differences in the electrophysiological properties between TuGIRK-A and TuGIRK-A/B channels were shown to be, at least partly, due to the presence of Lys(161) at the external mouth of the pore of the TuGIRK-B subunit.

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

We have identified several cDNAs for the human Kir5.1 subunit of inwardly rectifying K(+) channels. Alternative splicing of exon 3 and the usage of two alternative polyadenylation sites contribute to cDNA diversity. The hKir5.1 gene KCNJ16 is assigned to chromosomal region 17q23.1-24.2, and is separated by only 34 kb from the hKir2.1 gene (KCNJ2). In the brain, Kir5.1 mRNA is restricted to the evolutionary older parts of the hindbrain, midbrain and diencephalon and overlaps with Kir2.1 in the superior/inferior colliculus and the pontine region. In the kidney Kir5.1 and Kir2.1 are colocalized in the proximal tubule. When expressed in Xenopus oocytes, Kir5.1 is efficiently targeted to the cell surface and forms electrically silent channels together with Kir2.1, thus negatively controlling Kir2.1 channel activity in native cells.

Expression and permeation properties of the K(+) channel Kir7.1 in the retinal pigment epithelium

Bovine Kir7.1 clones were obtained from a retinal pigment epithelium (RPE)-subtracted cDNA library. Human RPE cDNA library screening resulted in clones encoding full-length human Kir7.1. Northern blot analysis indicated that bovine Kir7.1 is highly expressed in the RPE. Human Kir7.1 channels were expressed in Xenopus oocytes and studied using the two-electrode voltage-clamp technique. The macroscopic Kir7.1 conductance exhibited mild inward rectification and an inverse dependence on extracellular K+ concentration ([K+]o). The selectivity sequence based on permeability ratios was K+ (1.0) approximately Rb+ (0.89) > Cs+ (0.013) > Na+ (0.003) approximately Li+ (0.001) and the sequence based on conductance ratios was Rb+ (9.5) >> K+ (1.0) > Na+ (0.458) > Cs+ (0.331) > Li+ (0.139). Non-stationary noise analysis of Rb+ currents in cell-attached patches yielded a unitary conductance for Kir7.1 of approximately 2 pS. In whole-cell recordings from freshly isolated bovine RPE cells, the predominant current was a mild inwardly rectifying K+ current that exhibited an inverse dependence of conductance on [K+]o. The selectivity sequence based on permeability ratios was K+ (1.0) approximately Rb+ (0.89) > Cs+ (0.021) > Na+ (0.003) approximately Li+ (0.002) and the sequence based on conductance ratios was Rb+ (8.9) >> K+ (1.0) > Na+ (0.59) > Cs+ (0.23) > Li+ (0.08). In cell-attached recordings with Rb+ in the pipette, inwardly rectifying currents were observed in nine of 12 patches of RPE apical membrane but in only one of 13 basolateral membrane patches. Non-stationary noise analysis of Rb+ currents in cell-attached apical membrane patches yielded a unitary conductance for RPE Kir of approximately 2 pS. On the basis of this molecular and electrophysiological evidence, we conclude that Kir7.1 channel subunits comprise the K+ conductance of the RPE apical membrane.

Genomic structure and promoter analysis of the rat kir7.1 potassium channel gene (Kcnj13)

In the brain inwardly rectifying potassium channel Kir7.1 subunits are predominantly expressed in the choroid plexus and meninges. To investigate this tissue-specific expression pattern, we characterized the genomic organization and the 5' proximal promoter of the rat Kir7.1 gene (Kcnj13). Starting from the major transcriptional initiation site, three exons in Kcnj13 give rise to the dominant approximately 1.45 kb transcript in brain. Adjacent to the transcriptional start the minimal promoter which, uncommon for ion channels, contains a TATA- and CAAT-box is controlled by AP-1 factors and accounts for high gene expression levels. Luciferase reporter gene responses driven by the first 2.1 kb of the 5' flanking region were similarly high in epithelial FRTL-5 and neuronal N2A cells, suggesting that neuron-specific repressor elements are located remote from the non-selective minimal promoter.

Complex structure and regulation of expression of the rat gene for inward rectifier potassium channel Kir7.1

Genomic organization of the rat inward rectifier K(+) channel Kir7.1 was determined in an attempt to clarify how multiple species of its mRNA are generated in a tissue-specific manner and how its expression is regulated. The rat Kir7.1 gene spans >40 kilobases (kb) and consists of eight exons; the first four exons encode the 5'-untranslated region that is unusually long ( approximately 3 kb). The coding region is located in exons 5 and 6. In the testis, exon 4 is processed as four exons (4a-4d), whereas it is recognized as a single exon in the small intestine. The three major species of rat Kir7.1 mRNA (1.4, 2.2, and 3.2 kb) were found to arise from alternative usage of the two promoters and polyadenylation signals and by alternative splicing of the 5'-noncoding exons. The splicing pattern of the 5'-noncoding exons is quite complex and highly tissue-specific, suggesting that complex mechanisms may operate to regulate the Kir7.1 expression. Deletion and mutational analysis of the promoter activity indicated that the rat Kir7.1 gene is regulated by cAMP through a CCAAT element. The cAMP induction was also demonstrated using the rat follicular cell line FRTL-5 endogenously expressing Kir7.1.