Binding of kappa-conotoxin PVIIA to Shaker K+ channels reveals different K+ and Rb+ occupancies within the ion channel pore

The x-ray structure of the KcsA channel at different [K(+)] and [Rb(+)] provided insight into how K(+) channels might achieve high selectivity and high K(+) transit rates and showed marked differences between the occupancies of the two ions within the ion channel pore. In this study, the binding of kappa-conotoxin PVIIA (kappa-PVIIA) to Shaker K(+) channel in the presence of K(+) and Rb(+) was investigated. It is demonstrated that the complex results obtained were largely rationalized by differences in selectivity filter occupancy of this 6TM channels as predicted from the structural work on KcsA. kappa-PVIIA inhibition of the Shaker K(+) channel differs in the closed and open state. When K(+) is the only permeant ion, increasing extracellular [K(+)] decreases kappa-PVIIA affinity for closed channels by decreasing the "on" binding rate, but has no effect on the block of open channels, which is influenced only by the intracellular [K(+)]. In contrast, extracellular [Rb(+)] affects both closed- and open-channel binding. As extracellular [Rb(+)] increases, (a) binding to the closed channel is slightly destabilized and acquires faster kinetics, and (b) open channel block is also destabilized and the lowest block seems to occur when the pore is likely filled only by Rb(+). These results suggest that the nature of the permeant ions determines both the occupancy and the location of the pore site from which they interact with kappa-PVIIA binding. Thus, our results suggest that the permeant ion(s) within a channel pore can determine its functional and pharmacological properties.     

A variable residue in the pore of Kv1 channels is critical for the high affinity of blockers from sea anemones and scorpions

Animal toxins are associated with well defined selectivity profiles; however the molecular basis for this property is not understood. To address this issue we refined our previous three-dimensional models of the complex between the sea anemone toxin BgK and the S5-S6 region of Kv1.1 (Gilquin, B., Racape, J., Wrisch, A., Visan, V., Lecoq, A., Grissmer, S., Ménez, A., and Gasparini, S. (2002) J. Biol. Chem. 277, 37406-37413) using a docking procedure that scores and ranks the structures by comparing experimental and back-calculated values of coupling free energies DeltaDeltaGint obtained from double-mutant cycles. These models further highlight the interaction between residue 379 of Kv1.1 and the conserved dyad tyrosine residue of BgK. Because the nature of the residue at position 379 varies from one channel subtype to another, we explored how these natural mutations influence the sensitivity of Kv1 channel subtypes to BgK using binding and electrophysiology experiments. We demonstrated that mutations at this single position indeed suffice to abolish or enhance the sensitivity of Kv1 channels for BgK and other sea anemone and scorpion toxins. Altogether, our data suggest that the residue at position 379 of Kv1 channels controls the affinity of a number of blocking toxins.     

A marine snail neurotoxin shares with scorpion toxins a convergent mechanism of blockade on the pore of voltage-gated K channels.

kappa-Conotoxin-PVIIA (kappa-PVIIA) belongs to a family of peptides derived from a hunting marine snail that targets to a wide variety of ion channels and receptors. kappa-PVIIA is a small, structurally constrained, 27-residue peptide that inhibits voltage-gated K channels. Three disulfide bonds shape a characteristic four-loop folding. The spatial localization of positively charged residues in kappa-PVIIA exhibits strong structural mimicry to that of charybdotoxin, a scorpion toxin that occludes the pore of K channels. We studied the mechanism by which this peptide inhibits Shaker K channels expressed in Xenopus oocytes with the N-type inactivation removed. Chronically applied to whole oocytes or outside-out patches, kappa-PVIIA inhibition appears as a voltage-dependent relaxation in response to the depolarizing pulse used to activate the channels. At any applied voltage, the relaxation rate depended linearly on the toxin concentration, indicating a bimolecular stoichiometry. Time constants and voltage dependence of the current relaxation produced by chronic applications agreed with that of rapid applications to open channels. Effective valence of the voltage dependence, zdelta, is approximately 0.55 and resides primarily in the rate of dissociation from the channel, while the association rate is voltage independent with a magnitude of 10(7)-10(8) M-1 s-1, consistent with diffusion-limited binding. Compatible with a purely competitive interaction for a site in the external vestibule, tetraethylammonium, a well-known K-pore blocker, reduced kappa-PVIIA's association rate only. Removal of internal K+ reduced, but did not eliminate, the effective valence of the toxin dissociation rate to a value <0.3. This trans-pore effect suggests that: (a) as in the alpha-KTx, a positively charged side chain, possibly a Lys, interacts electrostatically with ions residing inside the Shaker pore, and (b) a part of the toxin occupies an externally accessible K+ binding site, decreasing the degree of pore occupancy by permeant ions. We conclude that, although evolutionarily distant to scorpion toxins, kappa-PVIIA shares with them a remarkably similar mechanism of inhibition of K channels.     

Inhibition of single Shaker K channels by kappa-conotoxin-PVIIA

kappa-Conotoxin-PVIIA (kappa-PVIIA) is a 27-residue basic (+4) peptide from the venom of the predator snail Conus purpurascens. A single kappa-PVIIA molecule interrupts ion conduction by binding to the external mouth of Shaker K channels. The blockade of Shaker by kappa-PVIIA was studied at the single channel level in membrane patches from Xenopus oocytes. The amplitudes of blocked and closed events were undistinguishable, suggesting that the toxin interrupts ion conduction completely. Between -20 and 40 mV kappa-PVIIA increased the latency to the first opening by one order of magnitude in a concentration-independent fashion. Because kappa-PVIIA has higher affinity for the closed channels at high enough concentration to block >90% of the resting channels, the dissociation rate could be estimated from the analysis of the first latency. At 0 mV, the dissociation rate was 20 s(-1) and had an effective valence of 0.64. The apparent closing rate increased linearly with [kappa-PVIIA] indicating an association rate of 56 microM(-1) s(-1). The toxin did not modify the fraction of null traces. This result suggests that the structural rearrangements in the external mouth contributing to the slow inactivation preserve the main geometrical features of the toxin-receptor interaction.     

Depolarization-induced release of glycine and beta-alanine from plasma membrane vesicles derived from rat brain synaptosomes

Glycine and beta-alanine actively loaded into brain synaptic plasma membrane vesicles were released into the external medium by using the classical depolarization agents high K+ and veratridine. This release occurs via a Ca2+-independent process. Measurements of membrane depolarization using tetraphenylphosphonium uptake show a close correlation between changes in the membrane potential and stimulation of the efflux process. Results shown herein and previously reported by our group (Aragón, M.C. and Giménez, C. (1986) Biochim. Biophys. Acta 855, 257-264; Agulló, L., Jiménez, B., Aragón, M.C. and Giménez, C. (1986) Eur. J. Biochem. 159, 611-617), suggest that the glycine and beta-alanine transport systems in synaptic plasma membranes are susceptible of modulation by changes in ionic fluxes and hence in the membrane potential, similar to those occurring during depolarization and repolarization.     

Na(+) interaction with the pore of Shaker B K(+) channels: zero and low K(+) conditions.

The Shaker B K(+) conductance (G(K)) collapses (in a reversible manner) if the membrane is depolarized and then repolarized in, 0 K(+), Na(+)-containing solutions (Gómez-Lagunas, F. 1997. J. Physiol. 499:3-15; Gómez-Lagunas, F. 1999. Biophys. J. 77:2988-2998). In this work, the role of Na(+) ions in the collapse of G(K) in 0-K(+) solutions, and in the behavior of the channels in low K(+) was studied. The main findings are as follows. First, in 0-K(+) solutions, the presence of Na(+) ions is an important factor that speeds the collapse of G(K). Second, external Na(+) fosters the drop of G(K) by binding to a site with a K(d) = 3.3 mM. External K(+) competes, in a mutually exclusive manner, with Na(o)(+) for binding to this site, with an estimated K(d) = 80 microM. Third, NMG and choline are relatively inert regarding the stability of G(K); fourth, with [K(o)(+)] = 0, the energy required to relieve Na(i)(+) block of Shaker (French, R.J., and J.B. Wells. 1977. J. Gen. Physiol. 70:707-724; Starkus, J.G., L. Kuschel, M. Rayner, and S. Heinemann. 2000. J. Gen. Physiol. 110:539-550) decreases with the molar fraction of Na(i)(+) (X(Na,i)), in an extent not accounted for by the change in Delta(mu)(Na). Finally, when X(Na,i) = 1, G(K) collapses by the binding of Na(i)(+) to two sites, with apparent K(d)s of 2 and 14.3 mM.     

Energetic and structural interactions between delta-dendrotoxin and a voltage-gated potassium channel

Dendrotoxin proteins isolated from Mamba snake venom block potassium channels with a high degree of specificity and selectivity. Using site-directed mutagenesis we have identified residues that constitute the functional interaction surfaces of delta-dendrotoxin and its voltage-gated potassium channel receptor. delta-Dendrotoxin uses a triangular patch formed by seven side-chains (Lys3, Tyr4, Lys6, Leu7, Pro8, Arg10, Lys26) to block K(+) currents carried by a Shaker potassium channel variant. The inhibitory surface of the toxin interacts with channel residues at Shaker positions 423, 425, 427, 431, and 449 near the pore. Amino acid mutations that interact across the toxin-channel interface were identified by mutant cycle analysis. These results constrain the possible orientation of dendrotoxin with respect to the K(+) channel structure. We propose that dendrotoxin binds near the pore entryway but does not act as a physical plug.     

Mutational analysis of the antagonist-binding site of the histamine H(1) receptor.

We combined in a previously derived three-dimensional model of the histamine H(1) receptor (Ter Laak, A. M., Timmerman, H., Leurs, H., Nederkoorn, P. H. J., Smit, M. J., and Donne-Op den Kelder, G. M. (1995) J. Comp. Aid. Mol. Design. 9, 319-330) a pharmacophore for the H(1) antagonist binding site (Ter Laak, A. M., Venhorst, J., Timmerman, H., and Donné-Op de Kelder, G. M. (1994) J. Med. Chem. 38, 3351-3360) with the known interacting amino acid residue Asp(116) (in transmembrane domain III) of the H(1) receptor and verified the predicted receptor-ligand interactions by site-directed mutagenesis. This resulted in the identification of the aromatic amino acids Trp(167), Phe(433), and Phe(436) in transmembrane domains IV and VI of the H(1) receptor as probable interaction points for the trans-aromatic ring of the H(1) antagonists. Subsequently, a specific interaction of carboxylate moieties of two therapeutically important, zwitterionic H(1) antagonists with Lys(200) in transmembrane domain V was predicted. A Lys(200) --> Ala mutation results in a 50- (acrivastine) to 8-fold (d-cetirizine) loss of affinity of these zwitterionic antagonists. In contrast, the affinities of structural analogs of acrivastine and cetirizine lacking the carboxylate group, triprolidine and meclozine, respectively, are unaffected by the Lys(200) --> Ala mutation. These data strongly suggest that Lys(200), unique for the H(1) receptor, acts as a specific anchor point for these "second generation" H(1) antagonists.     

Generating a high affinity scorpion toxin receptor in KcsA-Kv1.3 chimeric potassium channels

The crystal structure of the bacterial K(+) channel, KcsA (Doyle, D. A., Morais, C. J., Pfuetzner, R. A., Kuo, A., Gulbis, J. M., Cohen, S. L., Chait, B. T., and MacKinnon, R. (1998) Science 280, 69-77), and subsequent mutagenesis have revealed a high structural conservation from bacteria to human (MacKinnon, R., Cohen, S. L., Kuo, A., Lee, A., and Chait, B. T. (1998) Science 280, 106-109). We have explored this conservation by swapping subregions of the M1-M2 linker of KcsA with those of the S5-S6 linker of the human Kv-channel Kv1.3. The chimeric K(+) channel constructs were expressed in Escherichia coli, and their multimeric state was analyzed after purification. We used two scorpion toxins, kaliotoxin and hongotoxin 1, which bind specifically to Kv1.3, to analyze the pharmacological properties of the KcsA-Kv1.3 chimeras. The results demonstrate that the high affinity scorpion toxin receptor of Kv1.3 could be transferred to KcsA. Our biochemical studies with purified KcsA-Kv1.3 chimeras provide direct chemical evidence that a tetrameric channel structure is necessary for forming a functional scorpion toxin receptor. We have obtained KcsA-Kv1.3 chimeras with kaliotoxin affinities (IC(50) values of approximately 4 pm) like native Kv1.3 channels. Furthermore, we show that a subregion of the S5-S6 linker may be an important determinant of the pharmacological profile of K(+) channels. Using available structural information on KcsA and kaliotoxin, we have developed a structural model for the complex between KcsA-Kv1.3 chimeras and kaliotoxin to aid future pharmacological studies of K(+) channels.     

A novel conus peptide ligand for K+ channels

Voltage-gated ion channels determine the membrane excitability of cells. Although many Conus peptides that interact with voltage-gated Na(+) and Ca(2+) channels have been characterized, relatively few have been identified that interact with K(+) channels. We describe a novel Conus peptide that interacts with the Shaker K(+) channel, kappaM-conotoxin RIIIK from Conus radiatus. The peptide was chemically synthesized. Although kappaM-conotoxin RIIIK is structurally similar to the mu-conotoxins that are sodium channel blockers, it does not affect any of the sodium channels tested, but blocks Shaker K(+) channels. Studies using Shaker K(+) channel mutants with single residue substitutions reveal that the peptide interacts with the pore region of the channel. Introduction of a negative charge at residue 427 (K427D) greatly increases the affinity of the toxin, whereas the substitutions at two other residues, Phe(425) and Thr(449), drastically reduced toxin affinity. Based on the Shaker results, a teleost homolog of the Shaker K(+) channel, TSha1 was identified as a kappaM-conotoxin RIIIK target. Binding of kappaM-conotoxin RIIIK is state-dependent, with an IC(50) of 20 nm for the closed state and 60 nm at 0 mV for the open state of TSha1 channels.     

Asn792 participates in the hydrogen bond network around the K+-binding pocket of gastric H,K-ATPase

Asn792 present in M5 of gastric H,K-ATPase is highly conserved within the P-type ATPase family. A direct role in K+ binding was postulated for Na,K-ATPase but was not found in a recent model for gastric H,K-ATPase (Koenderink, J. B., Swarts, H. G. P., Willems, P. H. G. M., Krieger, E., and De Pont, J. J. H. H. M. (2004) J. Biol. Chem. 279, 16417-16424). Therefore, its role in K+ binding and E1/E2 conformational equilibrium in gastric H,K-ATPase was studied by site-directed mutagenesis and expression in Sf9 cells. N792Q and N792A, but not N792D and N792E, had a markedly reduced K+ affinity in both the ATPase and dephosphorylation reactions. In addition, N792A shifted the conformational equilibrium to the E1 form. In double mutants, the effect of N792A on K+ sensitivity was overruled by either E820Q (K(+)-independent activity) or E343D (no dephosphorylation activity). Models were made for the mutants based on the E2 structure of Ca(2+)-ATPase. In the wild-type model the acid amide group of Asn792 has hydrogen bridges to Lys791, Ala339, and Val341. Comparison of the effects of the various mutants suggests that the hydrogen bridge between the carbonyl oxygen of Asn792 and the amino group of Lys791 is essential for the K+ sensitivity and the E2 preference of wild-type enzyme. Moreover, there was a high positive correlation (r = 0.98) between the in silico calculated energy difference of the E2 form (mutants versus wild type) and the experimentally measured IC50 values for vanadate, which reflects the direction of the E2<-->E1 conformational equilibrium. These data strongly support the validity of the model in which Asn792 participates in the hydrogen bond network around the K(+)-binding pocket.     

Reviews

Book reviewed in this article: Møller , A. P. 1994: Sexual selection and the barn swallow. Wehner, R., Lehrer, M. & Harvey , W. R. (eds) 1996: Navigation, migration, and homing. Maclean , G. L. 1996: Ecophysiology of desert birds. Keulen, C., Loneux, M., Poncin, P. & Ruwet , J.-Cl. 1994: La Biodiversité: une étude de cas — ‘le site de Lanaye’ en Meuse belgo-néerlandaise. Gansloßer, U., Hodges, J. K. & Kaumanns , W. (eds) 1995: Research and captive propagation. Henry , J. D. 1996: Red fox. The catlike canine.     

U-type inactivation of Kv3.1 and Shaker potassium channels.

We previously concluded that the Kv2.1 K(+) channel inactivates preferentially from partially activated closed states. We report here that the Kv3.1 channel also exhibits two key features of this inactivation mechanism: a U-shaped voltage dependence measured at 10 s and stronger inactivation with repetitive pulses than with a single long depolarization. More surprisingly, slow inactivation of the Kv1 Shaker K(+) channel (Shaker B Delta 6--46) also has a U-shaped voltage dependence for 10-s depolarizations. The time and voltage dependence of recovery from inactivation reveals two distinct components for Shaker. Strong depolarizations favor inactivation that is reduced by K(o)(+) or by partial block by TEA(o), as previously reported for slow inactivation of Shaker. However, depolarizations near 0 mV favor inactivation that recovers rapidly, with strong voltage dependence (as for Kv2.1 and 3.1). The fraction of channels that recover rapidly is increased in TEA(o) or high K(o)(+). We introduce the term U-type inactivation for the mechanism that is dominant in Kv2.1 and Kv3.1. U-type inactivation also makes a major but previously unrecognized contribution to slow inactivation of Shaker.     

Voltage-dependent structural interactions in the Shaker K(+) channel

Using a strategy related to intragenic suppression, we previously obtained evidence for structural interactions in the voltage sensor of Shaker K(+) channels between residues E283 in S2 and R368 and R371 in S4 (Tiwari-Woodruff, S.K., C.T. Schulteis, A.F. Mock, and D. M. Papazian. 1997. Biophys. J. 72:1489-1500). Because R368 and R371 are involved in the conformational changes that accompany voltage-dependent activation, we tested the hypothesis that these S4 residues interact with E283 in S2 in a subset of the conformational states that make up the activation pathway in Shaker channels. First, the location of residue 283 at hyperpolarized and depolarized potentials was inferred by substituting a cysteine at that position and determining its reactivity with hydrophilic, sulfhydryl-specific probes. The results indicate that position 283 reacts with extracellularly applied sulfhydryl reagents with similar rates at both hyperpolarized and depolarized potentials. We conclude that E283 is located near the extracellular surface of the protein in both resting and activated conformations. Second, we studied the functional phenotypes of double charge reversal mutations between positions 283 and 368 and between 283 and 371 to gain insight into the conformations in which these positions approach each other most closely. We found that combining charge reversal mutations at positions 283 and 371 stabilized an activated conformation of the channel, and dramatically slowed transitions into and out of this state. In contrast, charge reversal mutations at positions 283 and 368 stabilized a closed conformation, which by virtue of the inferred position of 368 corresponds to a partially activated (intermediate) closed conformation. From these results, we propose a preliminary model for the rearrangement of structural interactions of the voltage sensor during activation of Shaker K(+) channels.     

NMR solution structures of delta-conotoxin EVIA from Conus ermineus that selectively acts on vertebrate neuronal Na+ channels

Delta-conotoxin EVIA, from Conus ermineus, is a 32-residue polypeptide cross-linked by three disulfide bonds forming a four-loop framework. delta-Conotoxin EVIA is the first conotoxin known to inhibit sodium channel inactivation in neuronal membranes from amphibians and mammals (subtypes rNa(v)1.2a, rNa(v)1.3, and rNa(v)1.6), without affecting rat skeletal muscle (subtype rNa(v)1.4) and human cardiac muscle (subtype hNa(v)1.5) sodium channel (Barbier, J., Lamthanh, H., Le Gall, F., Favreau, P., Benoit, E., Chen, H., Gilles, N., Ilan, N., Heinemann, S. F., Gordon, D., Ménez, A., and Molgó, J. (2004) J. Biol. Chem. 279, 4680-4685). Its structure was solved by NMR and is characterized by a 1:1 cis/trans isomerism of the Leu(12)-Pro(13) peptide bond in slow exchange on the NMR time scale. The structure of both cis and trans isomers could be calculated separately. The isomerism occurs within a specific long disordered loop 2, including residues 11-19. These contribute to an important hydrophobic patch on the surface of the toxin. The rest of the structure matches the "inhibitor cystine-knot motif" of conotoxins from the "O superfamily" with a high structural order. To probe a possible functional role of the Leu(12)-Pro(13) cis/trans isomerism, a Pro(13) --> Ala delta-conotoxin EVIA was synthesized and shown to exist only as a trans isomer. P13A delta-conotoxin EVIA was estimated only two times less active than the wild-type EVIA in binding competition to rat brain synaptosomes and when injected intracerebroventricularly into mice.     

Characterization of a novel radiolabeled peptide selective for a subpopulation of voltage-gated potassium channels in mammalian brain.

BgK, a 37-amino acid voltage-gated potassium (Kv) 1 channel blocker isolated from the sea anemone Bunodosoma granulifera, can be modified at certain positions to alter its pharmacological profile (Alessandri-Haber, N., Lecoq, A., Gasparini, S., Grangier-Macmath, G., Jacquet, G., Harvey, A. L., de Medeiros, C., Rowan, E. G., Gola, M., Ménez, A., and Crest, M. (1999) J. Biol. Chem. 274, 35653-35661). In the present study, we report the design of two BgK analogs that have been radiolabeled with (125)INa. Whereas BgK(W5Y/Y26F) and its radiolabeled derivative, (125)I-BgK(W5Y/Y26F), bind to Kv1.1, Kv1.2, and Kv1.6 channels with potencies similar to those for the parent peptide, BgK, BgK(W5Y/F6A/Y26F) and its monoiodo-tyrosine derivative, (125)I-BgK(W5Y/F6A/Y26F), display a distinctive and unique pharmacological profile; they bind with high affinity to homomultimeric Kv1.1 and Kv1.6 channels, but not to Kv1.2 channels. Interaction of BgK(W5Y/F6A/Y26F) with potassium channels depends on the nature of a residue in the mouth of the channel, at a position that determines channel sensitivity to external tetraethylammonium. In native brain tissue, (125)I-BgK(W5Y/F6A/Y26F) binds to a population of Kv1 channels that appear to consist of at least two sensitive (Kv1.1 and/or Kv1.6) subunits, in adjacent position. Given its unique pharmacological properties, (125)I-BgK(W5Y/F6A/Y26F) represents a new tool for studying subpopulations of Kv1 channels in native tissues.     

Lysophospholipids open the two-pore domain mechano-gated K(+) channels TREK-1 and TRAAK

The two-pore (2P) domain K(+) channels TREK-1 and TRAAK are opened by membrane stretch as well as arachidonic acid (AA) (Patel, A. J., Honoré, E., Maingret, F., Lesage, F., Fink, M., Duprat, F., and Lazdunski, M. (1998) EMBO J. 17, 4283-4290; Maingret, F., Patel, A. J., Lesage, F., Lazdunski, M., and Honoré, E. (1999) J. Biol. Chem. 274, 26691-26696; Maingret, F., Fosset, M., Lesage, F., Lazdunski, M. , and Honoré, E. (1999) J. Biol. Chem. 274, 1381-1387. We demonstrate that lysophospholipids (LPs) and platelet-activating factor also produce large specific and reversible activations of TREK-1 and TRAAK. LPs activation is a function of the size of the polar head and length of the acyl chain but is independent of the charge of the molecule. Bath application of lysophosphatidylcholine (LPC) immediately opens TREK-1 and TRAAK in the cell-attached patch configuration. In excised patches, LPC activation is lost, whereas AA still produces maximal opening. The carboxyl-terminal region of TREK-1, but not the amino terminus and the extracellular loop M1P1, is critically required for LPC activation. LPC activation is indirect and may possibly involve a cytosolic factor, whereas AA directly interacts with either the channel proteins or the bilayer and mimics stretch. Opening of TREK-1 and TRAAK by fatty acids and LPs may be an important switch in the regulation of synaptic function and may also play a protective role during ischemia and inflammation.     

Shaker and ether-à-go-go K+ channel subunits fail to coassemble in Xenopus oocytes.

Members of different voltage-gated K+ channel subfamilies usually do not form heteromultimers. However, coassembly between Shaker and ether-à-go-go (eag) subunits, members of two distinct K+ channel subfamilies, was suggested by genetic and functional studies (Zhong and Wu. 1991. Science. 252: 1562-1564; Chen, M.-L., T. Hoshi, and C.-F. Wu. 1996. Neuron. 17:535-542). We investigated whether Shaker and eag form heteromultimers in Xenopus laevis oocytes using electrophysiological and biochemical approaches. Coexpression of Shaker and eag subunits produced K+ currents that were virtually identical to the sum of separate Shaker and eag currents, with no change in the kinetics of Shaker inactivation. According to the results of dominant negative and reciprocal coimmunoprecipitation experiments, the Shaker and eag proteins do not interact. We conclude that Shaker and eag do not coassemble to form heteromultimers in Xenopus oocytes.     

Evidence against functional interaction between aquaporin-4 water channels and Kir4.1 potassium channels in retinal Müller cells

Indirect evidence suggests that the Müller/glial cell water channel aquaporin-4 (AQP4) modulates K(+) channel function of the closely associated Kir4.1 protein. We used patch clamp to compare Kir4.1 K(+) channel function in freshly isolated Müller cells from retinas of wild-type (+/+) and AQP4 knock-out (-/-) mice. Immunocytochemistry showed a comparable Kir4.1 protein expression pattern in Müller cells from +/+ and -/- retinas, with greatest expression at their end feet. Osmotic water permeability was >4-fold reduced in -/- than in +/+ Müller cells. Resting membrane potential did not differ significantly in +/+ versus -/- Müller cells (-64 +/- 1 versus -64 +/- 1 mV, S.E., n = 24). Whole-cell K(+) currents recorded with a micropipette inserted into the cell soma were Ba(2+)-sensitive and showed no significant differences in magnitude in +/+ versus -/- Müller cells (1.3 +/- 0.1 versus 1.2 +/- 0.1 nA at -160 mV) or in inwardly rectifying current-voltage relationships. Spatially resolved K(+) currents generated by pulsed K(+) injections along Müller cell bodies were also comparable in +/+ versus -/- Müller cells. Single-channel cell-attached patch clamp showed comparable unitary conductance, current-voltage data, and open probability in +/+ versus -/- Müller cells. Thus, contrary to the generally accepted view, our results provide direct evidence against functionally significant AQP4 modulation of Müller cell Kir4.1 K(+) channel function.