Identification of sites responsible for the potentiating effect of niflumic acid on ClC-Ka kidney chloride channels

BACKGROUND AND PURPOSE

ClC-K kidney Cl(-) channels are important for renal and inner ear transepithelial Cl(-) transport, and are potentially interesting pharmacological targets. They are modulated by niflumic acid (NFA), a non-steroidal anti-inflammatory drug, in a biphasic way: NFA activates ClC-Ka at low concentrations, but blocks the channel above approximately 1 mM. We attempted to identify the amino acids involved in the activation of ClC-Ka by NFA.

EXPERIMENTAL APPROACH

We used site-directed mutagenesis and two-electrode voltage clamp analysis of wild-type and mutant channels expressed in Xenopus oocytes. Guided by the crystal structure of a bacterial CLC homolog, we screened 97 ClC-Ka mutations for alterations of NFA effects.

KEY RESULTS

Mutations of five residues significantly reduced the potentiating effect of NFA. Two of these (G167A and F213A) drastically altered general gating properties and are unlikely to be involved in NFA binding. The three remaining mutants (L155A, G345S and A349E) severely impaired or abolished NFA potentiation.

CONCLUSIONS AND IMPLICATIONS

The three key residues identified (L155, G345, A349) are localized in two different protein regions that, based on the crystal structure of bacterial CLC homologs, are expected to be exposed to the extracellular side of the channel, relatively close to each other, and are thus good candidates for being part of the potentiating NFA binding site. Alternatively, the protein region identified mediates conformational changes following NFA binding. Our results are an important step towards the development of ClC-Ka activators for treating Bartter syndrome types III and IV with residual channel activity.

Statins and fenofibrate affect skeletal muscle chloride conductance in rats by differently impairing ClC-1 channel regulation and expression

BACKGROUND AND PURPOSE

Statins and fibrates can produce mild to life-threatening skeletal muscle damage. Resting chloride channel conductance (gCl), carried by the ClC-1 channel, is reduced in muscles of rats chronically treated with fluvastatin, atorvastatin or fenofibrate, along with increased resting cytosolic calcium in statin-treated rats. A high gCl, controlled by the Ca(2+)-dependent protein kinase C (PKC), maintains sarcolemma electrical stability and its reduction alters muscle function. Here, we investigated how statins and fenofibrate impaired gCl.

EXPERIMENTAL APPROACH

In rats treated with fluvastatin, atorvastatin or fenofibrate, we examined the involvement of PKC in gCl reduction by the two intracellular microelectrodes technique and ClC-1 mRNA level by quantitative real time-polymerase chain reaction. Direct drug effects were tested by patch clamp analysis on human ClC-1 channels expressed in human embryonic kidney (HEK) 293 cells.

KEY RESULTS

Chelerythrine, a PKC inhibitor, applied in vitro on muscle dissected from atorvastatin-treated rats fully restored gCl, suggesting the involvement of this enzyme in statin action. Chelerythrine partially restored gCl in muscles from fluvastatin-treated rats but not in those from fenofibrate-treated rats, implying additional mechanisms for gCl impairment. Accordingly, a decrease of ClC-1 channel mRNA was found in both fluvastatin- and fenofibrate-treated rat muscles. Fenofibric acid, the in vivo metabolite of fenofibrate, but not fluvastatin, rapidly reduced chloride currents in HEK 293 cells.

CONCLUSIONS AND IMPLICATIONS

Our data suggest multiple mechanisms underlie the effect of statins and fenofibrate on ClC-1 channel conductance. While statins promote Ca(2+)-mediated PKC activation, fenofibrate directly inhibits ClC-1 channels and both fluvastatin and fenofibrate impair expression of mRNA for ClC-1.

Role of tumour necrosis factor alpha, but not of cyclo-oxygenase-2-derived eicosanoids, on functional and morphological indices of dystrophic progression in mdx mice: a pharmacological approach

The role of tumour necrosis factor (TNF)-alpha or cyclo-oxygenase-2 (COX-2) eicosanoids in dystrophinopathies has been evaluated by chronically treating (4-8 weeks) adult dystrophic mdx mice with the anti-TNF-alpha etanercept (0.5 mg/kg) or the COX-2 inhibitor meloxicam (0.2 mg/kg). Throughout the treatment period the mdx mice underwent a protocol of exercise on treadmill in order to worsen the pathology progression; gastrocnemious muscles from exercised mdx mice showed an intense staining for TNF-alpha by immunohistochemistry. In vivo, etanercept, but not meloxicam, contrasted the exercise-induced forelimb force drop. Electrophysiological recordings ex vivo, showed that etanercept counteracted the decrease in chloride channel function (gCl), a functional index of myofibre damage, in both diaphragm and extensor digitorum longus (EDL) muscle, meloxicam being effective only in EDL muscle. None of the drugs ameliorated calcium homeostasis detected by electrophysiology and/or spectrofluorimetry. Etanercept, more than meloxicam, effectively reduced plasma creatine kinase (CK). Etanercept-treated muscles showed a reduction of connective tissue area and of pro-fibrotic cytokine TGF-beta1 vs. untreated ones; however, the histological profile was weakly ameliorated. In order to better evaluate the impact of etanercept treatment on histology, a 4-week treatment was performed on 2-week-old mdx mice, so to match the first spontaneous degeneration cycle. The histology profile of gastrocnemious was significantly improved with a reduction of degenerating area; however, CK levels were only slightly lower. The present results support a key role of TNF-alpha, but not of COX-2 products, in different phases of dystrophic progression. Anti-TNF-alpha drugs may be useful in combined therapies for Duchenne patients.

Niflumic acid inhibits chloride conductance of rat skeletal muscle by directly inhibiting the CLC-1 channel and by increasing intracellular calcium

BACKGROUND AND PURPOSE

Given the crucial role of the skeletal muscle chloride conductance (gCl), supported by the voltage-gated chloride channel CLC-1, in controlling muscle excitability, the availability of ligands modulating CLC-1 are of potential medical as well as toxicological importance. Here, we focused our attention on niflumic acid (NFA), a molecule belonging to the fenamates group of non-steroidal anti-inflammatory drugs (NSAID).

EXPERIMENTAL APPROACH

Rat muscle Cl(-) conductance (gCl) and heterologously expressed CLC-1 currents were evaluated by means of current-clamp (using two-microelectrodes) and patch-clamp techniques, respectively. Fura-2 fluorescence was used to determine intracellular calcium concentration, [Ca(2+)](i), in native muscle fibres.

KEY RESULTS

NFA inhibited native gCl with an IC(50) of 42 muM and blocked CLC-1 by interacting with an intracellular binding site. Additionally, NFA increased basal [Ca(2+)](i) in myofibres by promoting a mitochondrial calcium efflux that was not dependent on cyclooxygenase or CLC-1. A structure-activity study revealed that the molecular conditions that mediate the two effects are different. Pretreatment with the Ca-dependent protein kinase C (PKC) inhibitor chelerythrine partially inhibited the NFA effect. Therefore, in addition to direct channel block, NFA also inhibits gCl indirectly by promoting PKC activation.

CONCLUSIONS AND IMPLICATIONS

These cellular effects of NFA on skeletal muscle demonstrate that it is possible to modify CLC-1 and consequently gCl directly by interacting with channel proteins and indirectly by interfering with the calcium-dependent regulation of the channel. The effect of NFA on mitochondrial calcium stores suggests that NSAIDs, widely used drugs, could have potentially dangerous side-effects.

Carboxylic acids and skeletal muscle chloride channel conductance: effects on the biological activity induced by the introduction of an aryloxyalkyl group alpha to the carboxylic function of 4-chloro-phenoxyacetic acid

2-(4-Chloro-phenoxy)propanoic and 2-(4-chloro-phenoxy)butanoic acids are compounds known to block chloride membrane conductance in rat striated muscle by interaction with a specific receptor. In the present study, a series of chiral analogues has been prepared and tested to evaluate the influence of a second aryloxy moiety introduced in the side-chain at a variable distance from the stereogenic centre. The results show that this chemical modification is detrimental for biological activity which, however, is increased by lengthening the alkyl chain up to three methylenic groups, then decreases to remain constant in the next analogues of the series. A possible explanation for this is proposed on the basis of steric effects and/or different approach of the molecules to the receptor.

Mechanism of block of single protopores of the Torpedo chloride channel ClC-0 by 2-(p-chlorophenoxy)butyric acid (CPB)

We investigated in detail the mechanism of inhibition by the S(-) enantiomer of 2-(p-chlorophenoxy)butyric acid (CPB) of the Torpedo Cl(-)channel, ClC-0. The substance has been previously shown to inhibit the homologous skeletal muscle channel, CLC-1. ClC-0 is a homodimer with probably two independently gated protopores that are conductive only if an additional common gate is open. As a simplification, we used a mutant of ClC-0 (C212S) that has the common gate "locked open" (Lin, Y.W., C.W. Lin, and T.Y. Chen. 1999. J. Gen. Physiol. 114:1-12). CPB inhibits C212S currents only when applied to the cytoplasmic side, and single-channel recordings at voltages (V) between -120 and -80 mV demonstrate that it acts independently on individual protopores by introducing a long-lived nonconductive state with no effect on the conductance and little effect on the lifetime of the open state. Steady-state macroscopic currents at -140 mV are half-inhibited by approximately 0.5 mM CPB, but the inhibition decreases with V and vanishes for V > or = 40 mV. Relaxations of CPB inhibition after voltage steps are seen in the current responses as an additional exponential component that is much slower than the gating of drug-free protopores. For V = 60 mV) with an IC50 of approximately 30-40 mM. Altogether, these findings support a model for the mechanism of CPB inhibition in which the drug competes with Cl(-) for binding to a site of the pore where it blocks permeation. CPB binds preferentially to closed channels, and thereby also strongly alters the gating of the single protopore. Since the affinity of CPB for open WT pores is extremely low, we cannot decide in this case if it acts also as an open pore blocker. However, the experiments with the mutant K519E strongly support this interpretation. CPB block may become a useful tool to study the pore of ClC channels. As a first application, our results provide additional evidence for a double-barreled structure of ClC-0 and ClC-1.

Alteration of excitation-contraction coupling mechanism in extensor digitorum longus muscle fibres of dystrophic mdx mouse and potential efficacy of taurine

No clear data is available about functional alterations in the calcium-dependent excitation-contraction (e-c) coupling mechanism of dystrophin-deficient muscle of mdx mice. By means of the intracellular microelectrode "point" voltage clamp method, we measured the voltage threshold for contraction (mechanical threshold; MT) in intact extensor digitorum longus (EDL) muscle fibres of dystrophic mdx mouse of two different ages: 8 - 12 weeks, during the active regeneration of hind limb muscles, and 6 - 8 months, when regeneration is complete. The EDL muscle fibres of 8 - 12-week-old wildtype animals had a more negative rheobase voltage (potential of equilibrium for contraction- and relaxation-related calcium movements) with respect to control mice of 6 - 8 months. However, at both ages, the EDL muscle fibres of mdx mice contracted at more negative potentials with respect to age-matched controls and had markedly slower time constants to reach the rheobase. The in vitro application of 60 mM taurine, whose normally high intracellular muscle levels play a role in e-c coupling, was without effect on 6 - 8-month-old wildtype EDL muscle, while it significantly ameliorated the MT of mdx mouse. HPLC determination of taurine content at 6 - 8 months showed a significant 140% rise of plasma taurine levels and a clear trend toward a decrease in amino acid levels in hind limb muscles, brain and heart, suggesting a tissue difficulty in retaining appropriate levels of the amino acid. The data is consistent with a permanent alteration of e-c coupling in mdx EDL muscle fibres. The alteration could be related to the proposed increase in intracellular calcium, and can be ameliorated by taurine, suggesting a potential therapeutic role of the amino acid.

Pharmacological characterization of chloride channels belonging to the ClC family by the use of chiral clofibric acid derivatives

The enantiomers of 2-(p-chlorophenoxy)propionic acid (CPP) and of its analogs with substitutions on the asymmetric carbon atom were tested on human ClC-1 channel, the skeletal muscle chloride channel, after heterologous expression in Xenopus laevis oocytes, to gain insight in the mechanism of action of these stereoselective modulators of macroscopic chloride conductance (gCl) of rat striated fibers. By means of two microelectrode voltage clamp recordings, we found that S(-)-CPP shifted the activation curve of the ClC-1 currents toward more positive potentials and decreased the residual conductance at negative membrane potential; both effects probably account for the decrease of gCl at resting potential of native muscle fibers. Experiments on expressed Torpedo marmorata ClC-0 channels and a mutant lacking the slow gate suggest that S(-)-CPP could act on the fast gate of the single protochannels constituting the double-barreled structure of ClC-0 and ClC-1. The effect of S(-)-CPP on ClC-1 was markedly increased at low external pH (pH = 6), possibly for enhanced diffusion through the membrane (i.e., because the compound was effective only when applied to the cytoplasmic side during patch clamp recordings). The R(+)-isomer had little effect at concentrations as high as 1 mM. The CPP analogs with an ethyl, a phenyl, or an n-propyl group in place of the methyl group on the asymmetric center showed a scale of potency and a stereoselective behavior on ClC-1 similar to that observed for blocking gCl in native muscle fibers. The tested compounds were selective toward the ClC-1 channel. In fact, they were almost ineffective on an N-terminal deletion mutant of ClC-2 that is volume- and pH-independent while they blocked wild-type ClC-2 currents only at high concentrations and independently of pH and drug configuration, suggesting a different mechanism of action compared with ClC-1. No effects were observed on ClC-5 that shows less than 30% homology with ClC-1. Thus, CPP-like compounds may be useful both to gain insight into biophysical properties of ClC-1 and for searching tissue-specific therapeutic agents.

Effects of HMG-CoA reductase inhibitors on excitation-contraction coupling of rat skeletal muscle

3-Hydroxy-3-methyl glutaryl coenzyme A (HMG-CoA) reductase inhibitors currently used as cholesterol-lowering drugs produce side effects in patients, one of which is myopathy. In the present study we compared the effect of a 3-month chronic treatment with two different compounds, simvastatin and pravastatin, on the excitation-contraction coupling of rat skeletal muscle fibers, the mechanism which links membrane depolarization to the movements of cytosolic Ca2+ from intracellular stores. The voltage threshold for mechanical activation of extensor digitorum longus muscle fibers in response to depolarizing pulses of various durations was studied in vitro by the two intracellular microelectrode method in 'point' voltage clamp mode. Simvastatin (5-50 mg/kg) modified the mechanical threshold of striated fibers in a dose-dependent manner. The muscle fibers of rats treated with 10 mg/kg and 50 mg/kg of simvastatin needed significantly less depolarization to contract than did untreated fibers at each pulse duration, suggesting that levels of cytosolic Ca2+ were higher. Consequently, the rheobase voltage for fiber contraction was significantly shifted toward more negative potentials with respect to controls by 2.4 mV and 7.1 mV in the 10 mg/kg and 50 mg/kg simvastatin-treated animals, respectively. Pravastatin treatment at 100 mg/kg did not produce any alteration of excitation-contraction coupling since the rheobase voltage was similar to that of controls. The different physicochemical properties of the two drugs may underlie the different effect observed because lipophilic agents, such as simvastatin, have been shown to affect sterol biosynthesis in many tissues, whereas the hydrophilic pravastatin is hepato-selective.

Phosphorylation and IGF-1-mediated dephosphorylation pathways control the activity and the pharmacological properties of skeletal muscle chloride channels

1. In the present study we tested the hypothesis that insulin-like growth factor-1 (IGF-1) modulates resting chloride conductance (G(Cl)) of rat skeletal muscle by activating a phosphatase and that the chloride channel, based on the activity of phosphorylating-dephosphorylating pathways, has different sensitivity to specific ligands, such as the enantiomers of 2-(p-chlorophenoxy) propionic acid (CPP). 2. For this purpose G(Cl) in EDL muscle isolated from adult rat was first lowered by treatment with 5 nM 4-beta-phorbol 12,13 dibutyrate (4-beta-PDB), presumably activating protein kinase C (PKC). The effects of IGF-1 and of the enantiomers of CPP on G(Cl) were then tested. 3. IGF-1 (3.3 nM) had no effect of G(Cl) on EDL muscle fibres in normal physiological solution, whereas it completely counteracted the 30% decrease of G(Cl) induced by 4-beta-PDB. No effects of IGF-1 were observed on G(Cl) lowered by the phosphatase inhibitor okadaic acid (0.25 microM). 4. Ceramide, reported to activate on okadaic acid-sensitive phosphatase, mimicked the effects of IGF-1. In fact, N-acetyl-sphingosine (2.5-5 microM), not very effective in control conditions, increased the G(Cl) lowered by the phorbol ester, but not the G(Cl) lowered by okadaic acid. 5. In the presence of 4-beta-PDB, G(Cl) was differently affected by the enantiomers of CPP. The S(-)-CPP was remarkably less potent in producing the concentration-dependent reduction of G(Cl), whereas the R(+)-CPP caused an increase of G(Cl) at all the concentrations tested. 6. In conclusion, the PKC-induced lowering of G(Cl) is counteracted by IGF-1 through an okadaic acid sensitive phosphatase, and this effect can have therapeutic relevance in situations characterized by excessive channel phosphorylation. In turn the phosphorylation state of the channel can modulate the effects and the therapeutic potential of direct channel ligands.

Comparison of excitability parameters and sodium channel behavior of fast- and slow-twitch rat skeletal muscles for the study of the effects of hindlimb suspension, a model of hypogravity

When mammals are constrained to hypogravity, their neuromuscular apparatus undergoes modifications which rend difficult postural maintenance and muscular activity upon the return to normal gravitational conditions. Muscle atrophy and differetial gene expression are particularly evident in slow-twitch antigravity muscles such as the soleus. During hypogravity, most of the metabolic and contractile properties characteristic of slow-twitch muscles shift toward to those of fast-twitch muscles. For example, the expression of the fast isoforms of both the myosin heavy-chain and the sarcoplasmic reticulum calcium pump increases in slow-twitch muscle during hypogravity. Thus, modifications of the contractile machinery and calcium handling are likely to be involved in the hypogravity-induced slow-twitch muscle impariment. Fast- and slow-twitch muscles differ also in their electrical properties. Resting membrane potential (RMP) is more negative by about 10 mV in fast muscles compared to slow ones. Differences in action potential (AP) shape as well as in the number of elicitable APs have been also observed between both muscle types, which may reslut from the reported differences in chloride conductance and sodium current. Little is known about the potential modification fo muscle electrical properties during hypogravity, apart a negative shift of the RMP in soleus muscle. Thus this study was performed at the aim to compare the excitability parameters and sodium channel behavior of rat fast-twitch and slow-twitch muscle fibers. The characterization of these properties specific for each muscle-type will give us the basis for the study of the effect of hypogravity.

Potential targets for skeletal muscle impairment by hypogravity: basic characterization of resting ionic conductances and mechanical threshold of rat fast- and slow-twitch muscle fibers

Prolonged hypogravity such as during space flights affects skeletal muscle function by inducing postural changes as well as reduced muscle strength and locomotion capacity. Also in rats, space flight as well as useful models of groundbased hypogravity induce marked atrophy in the slow-twitch soleus (SOL) muscle as opposed to slight or none in the fast-twitch ones such as extensor digitorum longus (EDL). Biochemical and histological studies on hindlimb suspended animals, showed a hypogravity-induced impairment of muscle function involving the transition of slow-twitch muscle type, responsible for postural control, toward the fast-twitch phenotype by modification of excitation-contraction pattern. In slow muscles of rats, hindlimb suspension induced upregulation of the fast isoform of myosin heavy-chain and increased expression of fast Ca2+ pump mRNA and protein, which is consistent with the increased Ca(2+)-dependent ATPase activity and the speeding of muscle relaxation, typical of fast muscles. Little is known about the modifications induced by hypogravity in the sarcolemmal ion channels function, which controls the pattern of muscle excitability and contractility. The normally high resting chloride conductance, which is required for the electrical stabilization of mammalian muscle fibers, may be a target of hypogravity modifications since a pharmacological block of this parameter determines, though an increase of excitability, the transition of the fast-twitch muscle phenotype toward the slow one either in adult or in developing rats. Hypogravity also induced increased expression of dihydropyridine receptors in soleus muscle, that are normally lower than that found in the fast ones. In this study, we characterized the electrical and contractile properties of rat extensor digitorum longus (EDL) and slow-twitch soleus SOL muscles fibers at the aim to better understand the molecular mechanisms leading to fiber transition.