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Missense and truncating variants in the X-chromosome-linked CLCN4 gene, resulting in reduced or complete loss-of-function (LOF) of the encoded chloride/proton exchanger ClC-4, were recently demonstrated to cause a neurocognitive phenotype in both males and females. Through international clinical matchmaking and interrogation of public variant databases we assembled a database of 90 rare CLCN4 missense variants in 90 families: 41 unique and 18 recurrent variants in 49 families. For 43 families, including 22 males and 33 females, we collated detailed clinical and segregation data. To confirm causality of variants and to obtain insight into disease mechanisms, we investigated the effect on electrophysiological properties of 59 of the variants in Xenopus oocytes using extended voltage and pH ranges. Detailed analyses revealed new pathophysiological mechanisms: 25% (15/59) of variants demonstrated LOF, characterized by a shift of the voltage-dependent activation to more positive voltages, and nine variants resulted in a toxic gain-of-function, associated with a disrupted gate allowing inward transport at negative voltages. Functional results were not always in line with in silico pathogenicity scores, highlighting the complexity of pathogenicity assessment for accurate genetic counselling. The complex neurocognitive and psychiatric manifestations of this condition, and hitherto under-recognized impacts on growth, gastrointestinal function, and motor control are discussed. Including published cases, we summarize features in 122 individuals from 67 families with CLCN4-related neurodevelopmental condition and suggest future research directions with the aim of improving the integrated care for individuals with this diagnosis.

The genetic causes of global developmental delay (GDD) and intellectual disability (ID) are diverse and include variants in numerous ion channels and transporters. Loss-of-function variants in all five endosomal/lysosomal members of the CLC family of Cl- channels and Cl-/H+ exchangers lead to pathology in mice, humans, or both. We have identified nine variants in CLCN3, the gene encoding CIC-3, in 11 individuals with GDD/ID and neurodevelopmental disorders of varying severity. In addition to a homozygous frameshift variant in two siblings, we identified eight different heterozygous de novo missense variants. All have GDD/ID, mood or behavioral disorders, and dysmorphic features; 9/11 have structural brain abnormalities; and 6/11 have seizures. The homozygous variants are predicted to cause loss of ClC-3 function, resulting in severe neurological disease similar to the phenotype observed in Clcn3-/- mice. Their MRIs show possible neurodegeneration with thin corpora callosa and decreased white matter volumes. Individuals with heterozygous variants had a range of neurodevelopmental anomalies including agenesis of the corpus callosum, pons hypoplasia, and increased gyral folding. To characterize the altered function of the exchanger, electrophysiological analyses were performed in Xenopus oocytes and mammalian cells. Two variants, p.Ile607Thr and p.Thr570Ile, had increased currents at negative cytoplasmic voltages and loss of inhibition by luminal acidic pH. In contrast, two other variants showed no significant difference in the current properties. Overall, our work establishes a role for CLCN3 in human neurodevelopment and shows that both homozygous loss of ClC-3 and heterozygous variants can lead to GDD/ID and neuroanatomical abnormalities.

ClC-1 protein channels facilitate rapid passage of chloride ions across cellular membranes, thereby orchestrating skeletal muscle excitability. Malfunction of ClC-1 is associated with myotonia congenita, a disease impairing muscle relaxation. Here, we present the cryo-electron microscopy (cryo-EM) structure of human ClC-1, uncovering an architecture reminiscent of that of bovine ClC-K and CLC transporters. The chloride conducting pathway exhibits distinct features, including a central glutamate residue ("fast gate") known to confer voltage-dependence (a mechanistic feature not present in ClC-K), linked to a somewhat rearranged central tyrosine and a narrower aperture of the pore toward the extracellular vestibule. These characteristics agree with the lower chloride flux of ClC-1 compared with ClC-K and enable us to propose a model for chloride passage in voltage-dependent CLC channels. Comparison of structures derived from protein studied in different experimental conditions supports the notion that pH and adenine nucleotides regulate ClC-1 through interactions between the so-called cystathionine-β-synthase (CBS) domains and the intracellular vestibule ("slow gating"). The structure also provides a framework for analysis of mutations causing myotonia congenita and reveals a striking correlation between mutated residues and the phenotypic effect on voltage gating, opening avenues for rational design of therapies against ClC-1-related diseases.

Tuberous sclerosis complex (TSC) is a rare multisystem disorder caused by heterozygous loss-of-function pathogenic variants in the tumour suppressor genes TSC1 and TSC2 encoding the tuberin and hamartin proteins, respectively. Both TSC1 and TSC2 inhibit the mammalian target of rapamycin (mTOR) complexes pathway, which is crucial for cell proliferation, growth, and differentiation, and is stimulated by various energy sources and hormonal signaling pathways. Pathogenic variants in TSC1 and TSC2 lead to mTORC1 hyperactivation, producing benign tumours in multiple organs, including the brain and kidneys, and drug-resistant epilepsy, a typical sign of TSC. Brain tumours, sudden unexpected death from epilepsy, and respiratory conditions are the three leading causes of morbidity and mortality. Even though several therapeutic options are available for the treatment of TSC, there is further need for a better understanding of the pathophysiological basis of the neurologic and other manifestations seen in TSC, and for novel therapeutic approaches. This review provides an overview of the main current therapies for TSC and discusses recent studies highlighting the repurposing of approved drugs and the emerging role of novel targets for future drug design. [ABSTRACT FROM AUTHOR]

The nine-member CLC gene family of Cl− chloride-transporting membrane proteins is divided into plasma membrane-localized Cl− channels and endo-/lysosomal Cl−/H+ antiporters. Accessory proteins have been identified for ClC-K and ClC-2 channels and for the lysosomal ClC-7, but not the other CLCs. Here, we identified TMEM9 Domain Family Member B (TMEM9B), a single-span type I transmembrane protein of unknown function, to strongly interact with the neuronal endosomal ClC-3 and ClC-4 transporters. Co-expression of TMEM9B with ClC-3 or ClC-4 dramatically reduced transporter activity in Xenopus oocytes and transfected HEK cells. For ClC-3, TMEM9B also induced a slow component in the kinetics of the activation time course, suggesting direct interaction. Currents mediated by ClC-7 were hardly affected by TMEM9B, and ClC-1 currents were only slightly reduced, demonstrating specific interaction with ClC-3 and ClC-4. We obtained strong evidence for direct interaction by detecting significant Förster Resonance Energy Transfer (FRET), exploiting fluorescence lifetime microscopy-based (FLIM-FRET) techniques between TMEM9B and ClC-3 and ClC-4, but hardly any FRET with ClC-1 or ClC-7. The discovery of TMEM9B as a novel interaction partner of ClC-3 and ClC-4 might have important implications for the physiological role of these transporters in neuronal endosomal homeostasis and for a better understanding of the pathological mechanisms in CLCN3- and CLCN4-related pathological conditions. [ABSTRACT FROM AUTHOR]

Melatonin is a pleiotropic biofactor and an effective antioxidant and free radical scavenger and, as such, can be protective in oxidative stress‐related brain conditions including epilepsy and aging. To test the potential protective effect of melatonin on brain homeostasis and identify the corresponding molecular targets, we established a new model of oxidative stress‐related aging neuroglia represented by U‐87 MG cells exposed to D‐galactose (D‐Gal). This model was characterized by a substantial elevation of markers of oxidative stress, lipid peroxidation, and protein oxidation. The function of the inward rectifying K+ channel Kir2.1, which was identified as the main Kir channel endogenously expressed in these cells, was dramatically impaired. Kir2.1 was unlikely a direct target of oxidative stress, but the loss of function resulted from a reduction of protein abundance, with no alterations in transcript levels and trafficking to the cell surface. Importantly, melatonin reverted these changes. All findings, including the melatonin antioxidant effect, were reproduced in heterologous expression systems. We conclude that the glial Kir2.1 can be a target of oxidative stress and further suggest that inhibition of its function might alter the extracellular K+ buffering in the brain, therefore contributing to neuronal hyperexcitability and epileptogenesis during aging. Melatonin can play a protective role in this context. [ABSTRACT FROM AUTHOR]

Cortical spreading depression (CSD), a wave of depolarization followed by depression of cortical activity, is a pathophysiological process implicated in migraine with aura and various other brain pathologies, such as ischemic stroke and traumatic brain injury. To gain insight into the pathophysiology of CSD, we generated a mouse model for a severe monogenic subtype of migraine with aura, familial hemiplegic migraine type 3 (FHM3). FHM3 is caused by mutations in SCN1A, encoding the voltage-gated [Na.sup.+] channel [Na.sub.V]1.1 predominantly expressed in inhibitory interneurons. Homozygous [Scn1a.sup.L1649Q] knock-in mice died prematurely, whereas heterozygous mice had a normal lifespan. Heterozygous [Scn1a.sup.L1649Q] knock-in mice compared with WT mice displayed a significantly enhanced susceptibility to CSD. We found L1649Q to cause a gain-of-function effect with an impaired [Na.sup.+]*-channel inactivation and increased ramp [Na.sup.+] currents leading to hyperactivity of fast-spiking inhibitory interneurons. Brain slice recordings using [K.sup.+]-sensitive electrodes revealed an increase in extracellular [K.sup.+] in the early phase of CSD in heterozygous mice, likely representing the mechanistic link between interneuron hyperactivity and CSD initiation. The neuronal phenotype and premature death of homozygous [Scn1a.sup.L1649Q] knock-in mice was partially rescued by GS967, a blocker of persistent [Na.sup.+] currents. Collectively, our findings identify interneuron hyperactivity as a mechanism to trigger CSD., Introduction With a prevalence of 10% to 15% in the general population, migraine is one of the most common neurological diseases (1, 2) and is rated as one of the [...]

ClC‐6 and ClC‐7 are closely related, intracellular Cl−/H+ antiporters belonging to the CLC family of channels and transporters. They localize to acidic late endosomes and lysosomes and probably function in ionic homeostasis of these contiguous compartments. ClC‐7 transport function requires association with the accessory protein Ostm1, whereas ClC‐6 transport does not. To elucidate their roles in endo‐lysosomes, we measured Cl−‐ and pH‐dependences of over‐expressed wild‐type ClC‐6 and ClC‐7, as well as disease‐associated mutants, using high‐resolution recording protocols. Lowering extracellular Cl− (corresponding to luminal Cl− in endo‐lysosomes) reduced ClC‐6 currents, whereas it increased transport activity of ClC‐7/Ostm1. Low extracellular Cl− activated ClC‐7/Ostm 1 under acidic extracellular conditions, as well as under conditions of low intracellular chloride. Activation is conserved in ClC‐7Y713C, a variant displaying disrupted PI(3,5)P2 inhibition. Detailed biophysical analysis of disease‐associated ClC‐6 and ClC‐7 gain‐of‐function (GoF) variants, ClC‐6Y553C and ClC‐7Y713C, and the ClC‐7Y577C and ClC‐6Y781C correlates, identified additional functional nuances distinguishing ClC‐6 and ClC‐7. ClC‐7Y577C recapitulated GoF produced by ClC‐6Y553C. ClC‐6Y781C displayed transport activation qualitatively similar to ClC‐7Y713C, although current density did not differ from that of wild‐type ClC‐6. Finally, rClC‐7R760Q, homologous to hClC‐7R762Q, an osteopetrosis variant with fast gating kinetics, appeared indifferent to extracellular Cl−, identifying altered Cl− sensitivity as a plausible mechanism underlying disease. Collectively, the present studies underscore the distinct roles of ClC‐6 and ClC‐7 within the context of their respective localization to late endosomes and lysosomes. In particular, we suggest the atypical inhibition of ClC‐7 by luminal Cl− serves to limit excessive intraluminal Cl− accumulation. Key points: ClC‐6 and ClC‐7 are late endosomal and lysosomal 2 Cl−/1 H+ exchangers, respectively. When targeted to the plasma membrane, both activate slowly at positive voltages.ClC‐6 activity is decreased in low extracellular (i.e. luminal) chloride, whereas ClC‐7 is activated by low luminal chloride, even at acidic pH.The functional gain‐of‐function phenotypes of the ClC‐6 and ClC‐7 disease mutations ClC‐6Y553C and ClC‐7Y715C are maintained when introduced in their respective homologues, ClC‐7Y577C and ClC‐6Y781C, with all mutations retaining chloride dependence of the respective wild type (WT).An osteopetrosis mutation of ClC‐7 displaying fast gating kinetics (R762Q) was less sensitive to extracellular chloride compared to WT.The opposing substrate dependences of ClC‐6 and ClC‐7 Cl− / H+ exchangers point to non‐overlapping physiological functions, leading us to propose that inhibition of ClC‐7 by luminal chloride and protons serves to prevent osmotic stress imposed by hyper‐accumulation of chloride. [ABSTRACT FROM AUTHOR]

Endosomes and lysosomes are intracellular vesicular organelles with important roles in cell functions such as protein homeostasis, clearance of extracellular material, and autophagy. Endolysosomes are characterized by an acidic luminal pH that is critical for proper function. Five members of the gene family of voltage-gated ChLoride Channels (CLC proteins) are localized to endolysosomal membranes, carrying out anion/proton exchange activity and thereby regulating pH and chloride concentration. Mutations in these vesicular CLCs cause global developmental delay, intellectual disability, various psychiatric conditions, lysosomal storage diseases, and neurodegeneration, resulting in severe pathologies or even death. Currently, there is no cure for any of these diseases. Here, we review the various diseases in which these proteins are involved and discuss the peculiar biophysical properties of the WT transporter and how these properties are altered in specific neurodegenerative and neurodevelopmental disorders. [ABSTRACT FROM AUTHOR]

This document is an editorial published in the journal Frontiers in Pharmacology titled "Intracellular ion channels in health and disease." The authors discuss the importance of studying ion channels and transporters in intracellular compartments for understanding cellular physiology and disease. They highlight recent advances in the field, including research on vesicular chloride/proton exchangers, calcium regulation in cardiac hypertrophy and Parkinson's disease, and calcium-sensing receptors and channels in skin function and dermatological diseases. The authors believe that further research in this area will provide valuable insights and potential therapeutic approaches. [Extracted from the article]