Revisiting CFTR inhibition: a comparative study of CFTRinh -172 and GlyH-101 inhibitors.

BACKGROUND AND PURPOSE: For decades, inhibitors of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel have been used as tools to investigate the role and function of CFTR conductance in cystic fibrosis research. In the early 2000s, two new and potent inhibitors of CFTR, CFTRinh -172 and GlyH-101, were described and are now widely used to inhibit specifically CFTR. However, despite some evidence, the effects of both drugs on other types of Cl(-) -conductance have been overlooked. In this context, we explore the specificity and the cellular toxicity of both inhibitors in CFTR-expressing and non-CFTR-expressing cells. EXPERIMENTAL APPROACH: Using patch-clamp technique, we tested the effects of CFTRinh -172 and GlyH-101 inhibitors on three distinct types of Cl(-) currents: the CFTR-like conductance, the volume-sensitive outwardly rectifying Cl(-) conductance (VSORC) and finally the Ca(2+) -dependent Cl(-) conductance (CaCC). We also explored the effect of both inhibitors on cell viability using live/dead and cell proliferation assays in two different cell lines. KEY RESULTS: We confirmed that these two compounds were potent inhibitors of the CFTR-mediated Cl(-) conductance. However,GlyH-101 also inhibited the VSORC conductance and the CaCC at concentrations used to inhibit CFTR. The CFTRinh -172 did not affect the CaCC but did inhibit the VSORC, at concentrations higher than 5 µM. Neither inhibitor (20 µM; 24 h exposure) affected cell viability, but both were cytotoxic at higher concentrations. CONCLUSIONS AND IMPLICATIONS: Both inhibitors affected Cl(-) conductances apart from CFTR. Our results provided insights into their use in mouse models.

Mechanisms underlying Andersen's syndrome pathology in skeletal muscle are revealed in human myotubes.

Andersen's syndrome is a rare disorder that has been defined with a triad: periodic paralysis, cardiac arrhythmia, and development anomalies. Muscle weakness has been reported in two-thirds of the patients. KCNJ2 remains the only gene linked to Andersen's syndrome; this gene encodes for the alpha-subunit of the strong inward-rectifier K+ channel Kir2.1. Several studies have shown that Andersen's syndrome mutations lead to a loss of function of the K+ channel activity in vitro. However, ex vivo studies on isolated patient muscle tissue have not been reported. We have performed muscle biopsies of controls and patients presenting with clinically and genetically defined Andersen's syndrome disorder. Myoblasts were cultured and characterized morphologically and functionally using the whole cell patch-clamp technique. No morphological difference was observed between Andersen's syndrome and control myoblasts at each passage of the cell culture. Cellular proliferation and viability were quantified in parallel with direct cell counts and showed no difference between control and Andersen's syndrome patients. Moreover, our data show no significant difference in myoblast fusion index among Andersen's syndrome and control patients. Current recordings carried out on myotubes revealed the absence of an inwardly rectifying Ba2+-sensitive current in affected patient cells. One consequence of the Ik1 current loss in Andersen's syndrome myotubes is a shift of the resting membrane potential toward depolarizing potentials. Our data describe for the first time the functional consequences of Andersen's syndrome mutations ex vivo and provide clues to the K+ channel pathophysiology in skeletal muscle.

PIP2 binding residues of Kir2.1 are common targets of mutations causing Andersen syndrome.

BACKGROUND: Mutations in KCNJ2, the gene encoding the inward-rectifying K+ channel Kir2.1, cause the cardiac, skeletal muscle, and developmental phenotypes of Andersen-Tawil syndrome (ATS; also known as Andersen syndrome). Although pathogenic mechanisms have been proposed for select mutations, a common mechanism has not been identified. METHODS: Seventeen probands presenting with symptoms characteristic of ATS were evaluated clinically and screened for mutations in KCNJ2. The results of mutation analysis were combined with those from previously studied subjects to assess the frequency with which KCNJ2 mutations cause ATS. RESULTS: Mutations in KCNJ2 were discovered in nine probands. These included six novel mutations (D71N, T75R, G146D, R189I, G300D, and R312C) as well as previously reported mutations R67W and R218W. Six probands possessed mutations of residues implicated in binding membrane-associated phosphatidylinositol 4,5-bisphosphate (PIP2). In total, mutations in PIP(2)-related residues accounted for disease in 18 of 29 (62%) reported KCNJ2 -based probands with ATS. Also reported is that mutation R67W causes the full clinical triad in two unrelated males. CONCLUSIONS: The novel mutations corresponding to residues involved in Kir2.1 channel-PIP2 interactions presented here as well as the overall frequency of mutations occurring in these residues indicate that defects in PIP2 binding constitute a major pathogenic mechanism of ATS. Furthermore, screening KCNJ2 in patients with the complex phenotypes of ATS was found to be invaluable in establishing or confirming a disease diagnosis as mutations in this gene can be identified in the majority of patients.

Impairment of slow inactivation as a common mechanism for periodic paralysis in DIIS4-S5.

BACKGROUND: Mutations in the human skeletal muscle sodium channels are associated with hyperKPP, hypoKPP, paramyotonia congenita, and potassium-aggravated myotonia. This article describes the clinical manifestations of a patient with hyperKPP carrying a mutation (L689I) occurring in the linker DIIS4-S5 and its functional expression in a mammalian system. OBJECTIVE: To correlate the clinical manifestations of hyperkalemic periodic paralysis (hyperKPP) with the functional expression of a sodium channel mutation. METHODS: The mutation was introduced into a mammalian expression vector and expressed in the human embryonic kidney 293 cells. The functional expression of the L689I and that of the wild-type channels was monitored using the whole cell voltage-clamp technique. RESULTS: There was no change in the kinetics of fast inactivation, and inactivation curves were indistinguishable from that of wild-type channels. However, the L689I mutation caused a hyperpolarizing shift in the voltage dependence of activation and the mutant channels showed an impaired slow inactivation process. In addition, the mutant channels have a larger persistent current at -40 mV where window current may occur. CONCLUSIONS: The L689I mutation has similar effects to the T704M mutation and causes hyperKPP in this family. Because both of these hyperKPP mutations cause episodic muscle weakness, and because patients harboring another mutation (I693T) also can have episodic weakness, it is hypothesized that mutations occurring in this region of the sodium channel may cause episodic weakness through an impaired slow inactivation process coupled with enhanced activation.

Sodium channel inactivation defects are associated with acetazolamide-exacerbated hypokalemic periodic paralysis.

A novel mutation in a family with hypokalemic periodic paralysis is described. The mutation R672S is located in the voltage sensor segment S4 of domain II in the SCN4A gene encoding the human skeletal muscle voltage-gated sodium channel. Functional expression of the R672S channels in human embryonic kidney 293 cells revealed a small but significant hyperpolarizing shift in the steady-state fast inactivation, and a dramatic enhancement in channel slow inactivation. These two defects are mainly due to a slow recovery of the mutant channels from fast and/or slow inactivation. Our data may help explain the mechanism underlying hypokalemic periodic paralysis and the patient's worsening from acetazolamide.

Mutations in Kir2.1 cause the developmental and episodic electrical phenotypes of Andersen's syndrome.

Andersen's syndrome is characterized by periodic paralysis, cardiac arrhythmias, and dysmorphic features. We have mapped an Andersen's locus to chromosome 17q23 near the inward rectifying potassium channel gene KCNJ2. A missense mutation in KCNJ2 (encoding D71V) was identified in the linked family. Eight additional mutations were identified in unrelated patients. Expression of two of these mutations in Xenopus oocytes revealed loss of function and a dominant negative effect in Kir2.1 current as assayed by voltage-clamp. We conclude that mutations in Kir2.1 cause Andersen's syndrome. These findings suggest that Kir2.1 plays an important role in developmental signaling in addition to its previously recognized function in controlling cell excitability in skeletal muscle and heart.

MiRP2 forms potassium channels in skeletal muscle with Kv3.4 and is associated with periodic paralysis.

The subthreshold, voltage-gated potassium channel of skeletal muscle is shown to contain MinK-related peptide 2 (MiRP2) and the pore-forming subunit Kv3.4. MiRP2-Kv3.4 channels differ from Kv3.4 channels in unitary conductance, voltage-dependent activation, recovery from inactivation, steady-state open probability, and block by a peptide toxin. Thus, MiRP2-Kv3.4 channels set resting membrane potential (RMP) and do not produce afterhyperpolarization or cumulative inactivation to limit action potential frequency. A missense mutation is identified in the gene for MiRP2 (KCNE3) in two families with periodic paralysis and found to segregate with the disease. Mutant MiRP2-Kv3.4 complexes exhibit reduced current density and diminished capacity to set RMP. Thus, MiRP2 operates with a classical potassium channel subunit to govern skeletal muscle function and pathophysiology.

A double mutation in families with periodic paralysis defines new aspects of sodium channel slow inactivation.

Hyperkalemic periodic paralysis (HyperKPP) is an autosomal dominant skeletal muscle disorder caused by single mutations in the SCN4A gene, encoding the human skeletal muscle voltage-gated Na(+) channel. We have now identified one allele with two novel mutations occurring simultaneously in the SCN4A gene. These mutations are found in two distinct families that had symptoms of periodic paralysis and malignant hyperthermia susceptibility. The two nucleotide transitions predict phenylalanine 1490-->leucine and methionine 1493-->isoleucine changes located in the transmembrane segment S5 in the fourth repeat of the alpha-subunit Na(+) channel. Surprisingly, this mutation did not affect fast inactivation parameters. The only defect produced by the double mutant (F1490L-M1493I, expressed in human embryonic kidney 293 cells) is an enhancement of slow inactivation, a unique behavior not seen in the 24 other disease-causing mutations. The behavior observed in these mutant channels demonstrates that manifestation of HyperKPP does not necessarily require disruption of slow inactivation. Our findings may also shed light on the molecular determinants and mechanism of Na(+) channel slow inactivation and help clarify the relationship between Na(+) channel defects and the long-term paralytic attacks experienced by patients with HyperKPP.

Functional consequences of chloride channel gene (CLCN1) mutations causing myotonia congenita.

OBJECTIVE: To determine the functional consequences of missense mutations within the skeletal muscle chloride channel gene CLCN1 that cause myotonia congenita. BACKGROUND: Myotonia congenita is a genetic muscle disease associated with abnormalities in the skeletal muscle voltage-gated chloride (ClC-1) channel. In order to understand the molecular basis of this inherited disease, it is important to determine the physiologic consequences of mutations found in patients affected by it. METHODS: The authors used a mammalian cell (human embryonic kidney 293) expression system and the whole-cell voltage-clamp technique to functionally express and physiologically characterize five CLCN1 mutations. RESULTS: The I329T mutation shifted the voltage dependence of open probability of ClC-1 channels to the right by 192 mV, and the R338Q mutation shifted it to the right by 38 mV. In addition, the I329T ClC-1 channels deactivated to a lesser extent than normal at negative potentials. The V165G, F167L, and F413C ClC-1 channels also shifted the voltage dependence of open probability, but only by +14 to +20 mV. CONCLUSIONS: The functional consequences of these mutations form the physiologic argument that these are disease-causing mutations and could lead to myotonia congenita by impairing the ability of the skeletal muscle voltage-gated chloride channels to maintain normal muscle excitability. Understanding of genetic and physiologic defects may ultimately lead to better diagnosis and treatment of patients with myotonia congenita.

Characterization of a new sodium channel mutation at arginine 1448 associated with moderate Paramyotonia congenita in humans.

1. Paramyotonia congenita is a temperature-sensitive skeletal muscle disorder caused by missense mutations that occur in the adult skeletal muscle voltage-gated sodium channel. We report here the identification of a new genetic mutation in a family with the paramyotonia congenita phenotype. 2. Single-strand conformation polymorphism analysis and DNA sequencing showed that the defect was linked to a single nucleotide substitution causing an amino acid change from an arginine to a serine at position 1448 in the human sodium channel alpha-subunit. 3. Expression of the altered protein in human embryonic kidney (HEK) 293 cells revealed several defects in channel function: (i) the rate of fast inactivation was slower in the mutant channel compared with wild-type, (ii) steady-state fast inactivation was shifted towards hyperpolarizing potentials, (iii) the R1448S channels deactivated much more slowly, and (iv) the mutant channels recovered from the fast inactivated state more rapidly. 4. By contrast, the activation curve, steady-state slow inactivation and the rate of onset and recovery from slow inactivation were not altered by the R1448S mutation. 5. These data show that the defects observed in the sodium channel function could well explain the onset of the paramyotonia congenita in this family and emphasize the role of segment S4 of domain IV in sodium channel inactivation.

Activation and inactivation of the voltage-gated sodium channel: role of segment S5 revealed by a novel hyperkalaemic periodic paralysis mutation.

Hyperkalaemic periodic paralysis, paramyotonia congenita, and potassium-aggravated myotonia are three autosomal dominant skeletal muscle disorders linked to the SCN4A gene encoding the alpha-subunit of the human voltage-sensitive sodium channel. To date, approximately 20 point mutations causing these disorders have been described. We have identified a new point mutation, in the SCN4A gene, in a family with a hyperkalaemic periodic paralysis phenotype. This mutation predicts an isoleucine-to-phenylalanine substitution at position 1495 located in the transmembrane segment S5 in the fourth homologous domain of the human alpha-subunit sodium channel. Introduction of the I1495F mutation into the wild-type channels disrupted the macroscopic current inactivation decay and shifted both steady-state activation and inactivation to the hyperpolarizing direction. The recovery from fast inactivation was slowed, and there was no effect on channel deactivation. Additionally, a significant enhancement of slow inactivation was observed in the I1495F mutation. In contrast, the T704M mutation, a hyperkalaemic periodic paralysis mutation located in the cytoplasmic interface of the S5 segment of the second domain, also shifted activation in the hyperpolarizing direction but had little effect on fast inactivation and dramatically impaired slow inactivation. These results, showing that the I1495F and T704M hyperkalaemic periodic paralysis mutations both have profound effects on channel activation and fast-slow inactivation, suggest that the S5 segment maybe in a location where fast and slow inactivation converge.

Voltage sensitivity and conformational change of isolated S4L45 fragments from sodium channels are tuned to proline.

Peptide fragments reproducing the sequences of S4 segments extended with L45 linkers from the four homologous domains of the electric eel sodium channel were chemically synthesized and purified to allow circular dichroism studies in various solvents and conductance assays in planar lipid bilayers. Repeats III (with proline) and IV (lacking proline) present the lowest and highest helicities, respectively. The conformational transition (from helix to beta-strand) shown to occur on an increase of solvent dielectric constant is broader with repeat III. Analytical ultracentrifugation (interference fringe pattern) is consistent with a monodispersion of the peptide. In macroscopic conductance experiments, the proline containing peptides (repeats I, II and especially III) display higher voltage-sensitivities than repeat IV. The apparent and averaged number of monomers per intramembrane conducting aggregate is 4-5. The influence of proline is confirmed in similar experiments carried out on homologous S4 segments of repeat IV of the human skeletal muscle sodium channel comparing the wild type and an analogue where the fourth arginine was substituted with a proline. Thus, both conformational switching and voltage-sensitivity appear correlated to the presence and position of a single proline residue. Since voltage sensors are likely to experience different polarity environments in the channel open and closed states, our results suggest an alternative gating mechanism, i.e. a voltage-driven conformational change of S4L45s. The data also implies a plausible functional asymmetry, namely a "three- or four-stroke" activation sequentially involving the four domains of the sodium channel.

Sodium channel fragments: contributions to voltage sensitivity and ion selectivity.

The peptide strategy was employed to resolve structure-function relationships in the voltage-dependent sodium channel Two families of motifs were studied: the four voltage sensors S4 extended with the short cytoplasmic linkers L45 and the four P-regions, between S5 and S6, each from the homologous domains of the electric eel sodium channel. Macroscopic conductance experiments conducted with synthetic S4L45s in neutral lipid planar bilayers pointed to a moderate voltage-sensitivity for repeat IV which has no proline, whereas S4L45 of repeats I and II (Pro 19) and especially of repeat III (Pro 14) were much more voltage-sensitive. The influence both of Pro and its position within the sequence was confirmed by comparing the human skeletal muscle channel isoform D4/S4 wild-type and the R4P analogue. Circular dichroism spectroscopy shows highest and lowest helicities for repeats IV and III. The conformational transition (from helix to extended, mainly beta forms), which occurs when the solvent dielectric constant increases, was broader with repeat III. These structural and functional correlates suggest alternative gating mechanisms. The different contributions of each repeat also have effects at the level of the main selectivity filter, which suggests self-recognition between the four P-regions is a key component of intact sodium channel selectivity. In addition, the P-region from domain III is significantly voltage-sensitive and molecular dynamics simulations show that the C-terminal part of P-regions is mainly helical whilst the N-terminus tends to unfold. Such specializations of the four domains both in gating and selectivity are independently confirmed in recent electrophysiological studies.

Mechanism of modulation of the voltage-gated skeletal and cardiac muscle sodium channels by fatty acids.

Voltage-gated rat skeletal muscle and cardiac Na+ channels are modulated by exogenous unsaturated fatty acids. Application of 1-10 microM arachidonic or oleic acids reversibly depressed Na+ channel conductance and shifted the inactivation curve to hyperpolarizing potentials. These effects were not prevented by inhibitors of lipoxygenase, cyclooxygenase, cytochrome P-450 epoxygenase, or protein kinase C. Neither palmitic acid nor methyl ester oleate had an effect on the inward Na+ current, suggesting that trivial variations in membrane fluidity are not responsible for the Na+ current depression or kinetic changes. Arachidonic acid altered fast Na+ inactivation without changing the slow inactivation kinetics. Moreover, skeletal muscle Na+ channel gating currents were markedly decreased by 2 microM arachidonic acid. Finally, nonstationary noise analysis indicated that both the number of channels and the open probability were slightly decreased without change in the single-channel conductance. These data suggest that unsaturated fatty acids such as arachidonic and oleic acids 1) specifically regulate voltage-gated Na+ channels and 2) interact directly with Na+ channels, perhaps at a fatty acid binding domain, by decreasing the total gating charge and altering fast-inactivation kinetics.

Enhancement of the Shaker B delta6-46 current by fatty acids depends on the activation of the lipoxygenase metabolic pathway.

Voltage-gated K+ channels are modulated by extracellular free unsaturated fatty acids. Both increases and attenuations of K+ channels activities have been observed. We studied the effect of cis-unsaturated fatty acids on Shaker B delta6-46 and the endogenous outward rectifier K+ channels expressed in COS cell line, using the whole cell recording technique. For both K+ channels, exogenously applied unsaturated fatty acids dramatically increased the outward K+ currents. This enhancement was not mediated by cylooxygenase or epoxygenase (P450) enzymes. However, nordihydroguaiaretic acid, a lipoxygenase metabolic pathway blocker, did prevent the arachidonic acid-induced Shaker current enhancement.

Serine-1321-independent regulation of the mu 1 adult skeletal muscle Na+ channel by protein kinase C.

The adult skeletal muscle Na+ channel mu1 possesses a highly conserved segment between subunit domains III and IV containing a consensus protein kinase C (PKC) phosphorylation site that, in the neuronal isoform, acts as a master control for "convergent" regulation by PKC and cAMP-dependent protein kinase. It lacks an approximately 200-aa segment between domains I and II though to modulate channel gating. We here demonstrate that mu1 is regulated by PKC (but not cAMP-dependent protein kinase) in a manner distinct from that observed for the neuronal isoforms, suggesting that under the same conditions muscle excitation could be uncoupled from motor neuron input. Maximal phosphorylation by PKC, in the presence of phosphatase inhibitors, reduced peak Na+ currents by approximately 90% by decreasing the maximal conductance, caused a -15 mV shift in the midpoint of steady-state inactivation, and caused a slight speeding of inactivation. Surprisingly, these effects were not affected by mutation of the conserved serine (serine-1321) in the interdomain III-IV loop. the pattern of current suppression and gating modification by PKC resembles the response of muscle Na+ channels to inhibitory factors present in the serum and cerebrospinal fluid of patients with Guillain-Barré syndrome, multiple sclerosis, and idiopathic demyelinating polyradiculoneuritis.

[Purification of sodium channel from squid mantle and reincorporation in planar lipid bilayers]. / Purification du canal sodique du manteau de calmar et réincorporation dans des bicouches lipidiques planes.

The purification of voltage-dependent sodium channels from mantle muscles of the squid is monitored through increase in the specific binding of tritiated saxitoxin (3H-STX) of membrane extracts solubilized with Lubrol-PX, chromatographed over an ion-exchange resin then through an affinity column coupling lectins (WGA) and Sepharose. The achieved purification factor is 300 and the electrophoretic pattern of purified fractions shows a single band at 260 kD. Reincorporation of these fractions into planar lipid bilayers yields two conductance levels (in 0.5 M NaCl). The most frequently observed level, at 15 pS, is in agreement with in situ recordings. The other level, at 150 pS, equally blocked by tetrodotoxin, could be explained by some flexibility of the open state or alternatively by cooperative interactions between channels.