ClC-1 Chloride Channel: Inputs on the Structure-Function Relationship of Myotonia Congenita-Causing Mutations

Identification of a crosstalk between ClC-1 C-terminal CBS domains and the transmembrane region

CLC channels and transporters have large C-terminal regions which contain two cystathionine β-synthetase (CBS) domains. It has been hypothesized that conformational changes in these domains upon nucleotide binding modulate the gating of the CLC dimer. It is not clear how rearrangements that occur in the CBS domains are transmitted to the ion pathway, as CBS domains interact with the rest of the channel at multiple locations and some of these sites are not visible in recent solved cryogenic electron microscopy structures or are difficult to model using the AlphaFold server. Using ClC-1 as a model, we started working with a described ClC-1 mutation (H835R) located in the first alpha helix of the CBS2 domain which changes the voltage dependence of gating. We then identified several residues located in the disorganized loop after helix R (R-linker) that revert the phenotype of this mutation. We additionally proved that R-linker's function is connected to the CBS2 domain as current intensity, plasma membrane levels and gating defects of several R-linker variants were corrected by adding the mutation H835R. Furthermore, cross-linking studies using newly developed split-cysless ClC-1 channels containing specific cysteine mutants in the R-linker and the CBS2 domain indicate that these two regions are in close contact. Considering these new results, we propose that conformational changes occurring in the CBS domains could be transmitted to the CLC intracellular chloride binding site by means of its interaction with the R-linker. KEY POINTS: CBS domains, which are present as pairs in CLC proteins, are involved in the regulation by nucleotides of CLC gating. It is not clear how CBS domains interact with different regions of the transmembrane (TM) region to regulate gating. Using ClC-1 as a model, we investigated how a mutation in the second CBS domain dramatically changes the voltage dependence of gating taking advantage of recently solved structures and AlphaFold models of CLC proteins. Thus, we identified in the linker after helix R (R-linker) several revertant mutations of the gating defects caused by a mutation in the second CBS2 domain and vice versa, indicating that these two regions functionally interact. These interactions were biochemically proven by employing cysteine cross-linkings using a newly developed split-cysless ClC-1 channel. Based on these findings we experimentally prove that the R-linker is a key element for the transmission of the CBS conformational rearrangements to the TM region.

Pharmacological therapy of non-dystrophic myotonias

Objectives

Non-dystrophic myotonias (NDM) are rare diseases due to mutations in the voltage-gated sodium (Nav1.4) and chloride (ClC-1) channels expressed in skeletal muscle fibers. We provide an up-to-date review of pharmacological treatments available for NDM patients and experimental studies aimed at identifying alternative treatments and at better understanding the mechanisms of actions.

Methods

Literature research was performed using PubMed and ClinicalTrial.gov.

Results

Today, the sodium channel blocker mexiletine is the drug of choice for treatment of NDM. Alternative drugs include other sodium channel blockers and the carbonic anhydrase inhibitor acetazolamide. Preclinical studies suggest that activators of ClC-1 channels or voltage-gated potassium channels may have antimyotonic potential.

Conclusions

An increasing number of antimyotonic drugs would help to design a precision therapy to address personalized treatment of myotonic individuals.

Case report: A CLCN1 complex variant mutation in exon 15 in a mixed-breed dog with hereditary myotonia

At 4 months of age, a male dog was presented with a complaint of a stiff gait following a startle response. Neurological examination revealed no deficits, but clinical myotonia was easily induced upon requesting the patient to jump. Additionally, myotonia of the upper lip muscles was observed upon manipulation. Hereditary myotonia was suspected, and electromyography confirmed the presence of myotonic potentials. Genetic testing of the myotonic patient identified a complex of mutations, including c.[1636_1639 delins AACGGG] and c.[1644 A>T], both located in exon 15 of the CLCN1 gene leading to the formation of a premature stop codon. Genetic investigations of the mother and four littermates revealed that, except for one littermate who was wild type, all others carried a copy of the mutated gene. To the best of the authors' knowledge, these mutations have not been previously reported.

CryoEM structures of the human CLC-2 voltage-gated chloride channel reveal a ball-and-chain gating mechanism

CLC-2 is a voltage-gated chloride channel that contributes to electrical excitability and ion homeostasis in many different tissues. Among the nine mammalian CLC homologs, CLC-2 is uniquely activated by hyperpolarization, rather than depolarization, of the plasma membrane. The molecular basis for the divergence in polarity of voltage gating among closely related homologs has been a long-standing mystery, in part because few CLC channel structures are available. Here, we report cryoEM structures of human CLC-2 at 2.46 - 2.76 Å, in the presence and absence of the selective inhibitor AK-42. AK-42 binds within the extracellular entryway of the Cl--permeation pathway, occupying a pocket previously proposed through computational docking studies. In the apo structure, we observed two distinct conformations involving rotation of one of the cytoplasmic C-terminal domains (CTDs). In the absence of CTD rotation, an intracellular N-terminal 15-residue hairpin peptide nestles against the TM domain to physically occlude the Cl--permeation pathway. This peptide is highly conserved among species variants of CLC-2 but is not present in other CLC homologs. Previous studies suggested that the N-terminal domain of CLC-2 influences channel properties via a "ball-and-chain" gating mechanism, but conflicting data cast doubt on such a mechanism, and thus the structure of the N-terminal domain and its interaction with the channel has been uncertain. Through electrophysiological studies of an N-terminal deletion mutant lacking the 15-residue hairpin peptide, we support a model in which the N-terminal hairpin of CLC-2 stabilizes a closed state of the channel by blocking the cytoplasmic Cl--permeation pathway.