A single point mutation reveals gating of the human ClC-5 Cl-/H+ antiporter

Structural basis of pH-dependent activation in a CLC transporter

CLCs are dimeric chloride channels and anion/proton exchangers that regulate processes such as muscle contraction and endo-lysosome acidification. Common gating controls their activity; its closure simultaneously silences both protomers, and its opening allows them to independently transport ions. Mutations affecting common gating in human CLCs cause dominant genetic disorders. The structural rearrangements underlying common gating are unknown. Here, using single-particle cryo-electron microscopy, we show that the prototypical Escherichia coli CLC-ec1 undergoes large-scale rearrangements in activating conditions. The slow, pH-dependent remodeling of the dimer interface leads to the concerted opening of the intracellular H+ pathways and is required for transport. The more frequent formation of short water wires in the open H+ pathway enables Cl- pore openings. Mutations at disease-causing sites favor CLC-ec1 activation and accelerate common gate opening in the human CLC-7 exchanger. We suggest that the pH activation mechanism of CLC-ec1 is related to the common gating of CLC-7.

Biophysical Aspects of Neurodegenerative and Neurodevelopmental Disorders Involving Endo-/Lysosomal CLC Cl-/H+ Antiporters

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.

Functional and clinical studies reveal pathophysiological complexity of CLCN4-related neurodevelopmental condition

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.

Functional Characterization of CLCN4 Variants Associated With X-Linked Intellectual Disability and Epilepsy

Early/late endosomes, recycling endosomes, and lysosomes together form the endo-lysosomal recycling pathway. This system plays a crucial role in cell differentiation and survival, and dysregulation of the endo-lysosomal system appears to be important in the pathogenesis of neurodevelopmental and neurodegenerative diseases. Each endo-lysosomal compartment fulfils a specific function, which is supported by ion transporters and channels that modify ion concentrations and electrical gradients across endo-lysosomal membranes. CLC-type Cl–/H+ exchangers are a group of endo-lysosomal transporters that are assumed to regulate luminal acidification and chloride concentration in multiple endosomal compartments. Heterodimers of ClC-3 and ClC-4 localize to various internal membranes, from the endoplasmic reticulum and Golgi to recycling endosomes and late endosomes/lysosomes. The importance of ClC-4-mediated ion transport is illustrated by the association of naturally occurring CLCN4 mutations with epileptic encephalopathy, intellectual disability, and behavioral disorders in human patients. However, how these mutations affect the expression, subcellular localization, and function of ClC-4 is insufficiently understood. We here studied 12 CLCN4 variants that were identified in patients with X-linked intellectual disability and epilepsy and were already characterized to some extent in earlier work. We analyzed the consequences of these mutations on ClC-4 ion transport, subcellular trafficking, and heterodimerization with ClC-3 using heterologous expression in mammalian cells, biochemistry, confocal imaging, and whole-cell patch-clamp recordings. The mutations led to a variety of changes in ClC-4 function, ranging from gain/loss of function and impaired heterodimerization with ClC-3 to subtle impairments in transport functions. Our results suggest that even slight functional changes to the endosomal Cl–/H+ exchangers can cause serious neurological symptoms.

The Role of the Lysosomal Cl−/H+ Antiporter ClC-7 in Osteopetrosis and Neurodegeneration

CLC proteins comprise Cl⁻ channels and anion/H⁺ antiporters involved in several fundamental physiological processes. ClC-7 is a lysosomal Cl⁻/H⁺ antiporter that together with its beta subunit Ostm1 has a critical role in the ionic homeostasis of lysosomes and of the osteoclasts’ resorption lacuna, although the specific underlying mechanism has so far remained elusive. Mutations in ClC-7 cause osteopetrosis, but also a form of lysosomal storage disease and neurodegeneration. Interestingly, both loss-of- and gain-of-function mutations of ClC-7 can be pathogenic, but the mechanistic implications of this finding are still unclear. This review will focus on the recent advances in our understanding of the biophysical properties of ClC-7 and of its role in human diseases with a focus on osteopetrosis and neurodegeneration.

Unique variants in CLCN3, encoding an endosomal anion/proton exchanger, underlie a spectrum of neurodevelopmental disorders

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.

Large transient capacitive currents in wild-type lysosomal Cl-/H+ antiporter ClC-7 and residual transport activity in the proton glutamate mutant E312A

ClC-7 is a lysosomal 2 Cl⁻/1 H⁺ antiporter of the CLC protein family, which comprises Cl⁻ channels and other Cl⁻/H⁺ antiporters. Mutations in ClC-7 and its associated β subunit Ostm1 lead to osteopetrosis and lysosomal storage disease in humans and mice. Previous studies on other mammalian CLC transporters showed that mutations of a conserved, intracellularly located glutamate residue, the so-called proton glutamate, abolish steady-state transport activity but increase transient capacitive currents associated with partial reactions of the transport cycle. In contrast, we observed large, transient capacitive currents for the wild-type ClC-7, which depend on external pH and internal, but not external, Cl⁻. Very similar transient currents were observed for the E312A mutant of the proton glutamate. Interestingly, and unlike in other mammalian CLC transporters investigated so far, the E312A mutation strongly reduces, but does not abolish, stationary transport currents, potentially explaining the intermediate phenotype observed in the E312A mouse line.

Divergent Cl- and H+ pathways underlie transport coupling and gating in CLC exchangers and channels

The CLC family comprises H+-coupled exchangers and Cl- channels, and mutations causing their dysfunction lead to genetic disorders. The CLC exchangers, unlike canonical 'ping-pong' antiporters, simultaneously bind and translocate substrates through partially congruent pathways. How ions of opposite charge bypass each other while moving through a shared pathway remains unknown. Here, we use MD simulations, biochemical and electrophysiological measurements to identify two conserved phenylalanine residues that form an aromatic pathway whose dynamic rearrangements enable H+ movement outside the Cl- pore. These residues are important for H+ transport and voltage-dependent gating in the CLC exchangers. The aromatic pathway residues are evolutionarily conserved in CLC channels where their electrostatic properties and conformational flexibility determine gating. We propose that Cl- and H+ move through physically distinct and evolutionarily conserved routes through the CLC channels and transporters and suggest a unifying mechanism that describes the gating mechanism of both CLC subtypes.

Extracellular pH and intracellular phosphatidylinositol 4,5-bisphosphate control Cl- currents in guinea pig detrusor smooth muscle cells

Cl^- channels serve as key regulators of excitability and contractility in vascular, intestinal, and airway smooth muscle cells. We recently reported a Cl^- conductance in detrusor smooth muscle (DSM) cells. Here, we used the whole cell patch-clamp technique to further characterize biophysical properties and physiological regulators of the Cl^- current in freshly isolated guinea pig DSM cells. The Cl^- current demonstrated outward rectification arising from voltage-dependent gating of Cl^- channels rather than the Cl^- transmembrane gradient. An exposure of DSM cells to hypotonic extracellular solution (Δ 165 mOsm challenge) did not increase the Cl^- current providing strong evidence that volume-regulated anion channels do not contribute to the Cl^- current in DSM cells. The Cl^- current was monotonically dependent on extracellular pH, larger and lower in magnitude at acidic (5.0) and basic pH (8.5) values, respectively. Additionally, intracellularly applied phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] analog [PI(4,5)P₂-diC8] increased the average Cl^- current density by approximately threefold in a voltage-independent manner. The magnitude of the DSM whole cell Cl^- current did not depend on the cell surface area (cell capacitance) regardless of the presence or absence of PI(4,5)P₂-diC8, an intriguing finding that underscores the complex nature of Cl^- channel expression and function in DSM cells. Removal of both extracellular Ca²⁺ and Mg²⁺ did not affect the DSM whole cell Cl^- current, whereas Gd³⁺ (1 mM) potentiated the current. Collectively, our recent and present findings strongly suggest that Cl^- channels are critical regulators of DSM excitability and are regulated by extracellular pH, Gd³⁺, and PI(4,5)P₂.

Properties of single-channel and whole cell Cl- currents in guinea pig detrusor smooth muscle cells

Multiple types of Cl^- channels regulate smooth muscle excitability and contractility in vascular, gastrointestinal, and airway smooth muscle cells. However, little is known about Cl^- channels in detrusor smooth muscle (DSM) cells. Here, we used inside-out single channel and whole cell patch-clamp recordings for detailed biophysical and pharmacological characterizations of Cl^- channels in freshly isolated guinea pig DSM cells. The recorded single Cl^- channels displayed unique gating with multiple subconductive states, a fully opened single-channel conductance of 164 pS, and a reversal potential of -41.5 mV, which is close to the E_Cl of -65 mV, confirming preferential permeability to Cl^-. The Cl^- channel demonstrated strong voltage dependence of activation (half-maximum of mean open probability, V_0.5, ~-20 mV) and robust prolonged openings at depolarizing voltages. The channel displayed similar gating when exposed intracellularly to solutions containing Ca²⁺-free or 1 mM Ca²⁺. In whole cell patch-clamp recordings, macroscopic current demonstrated outward rectification, inhibitions by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) and niflumic acid, and insensitivity to chlorotoxin. The outward current was reversibly reduced by 94% replacement of extracellular Cl^- with I^-, Br^-, or methanesulfonate (MsO^-), resulting in anionic permeability sequence: Cl^->Br^->I^->MsO^-. While intracellular Ca²⁺ levels (0, 300 nM, and 1 mM) did not affect the amplitude of Cl^- current and outward rectification, high Ca²⁺ slowed voltage-step current activation at depolarizing voltages. In conclusion, our data reveal for the first time the presence of a Ca²⁺-independent DIDS and niflumic acid-sensitive, voltage-dependent Cl^- channel in the plasma membrane of DSM cells. This channel may be a key regulator of DSM excitability.

Modulation of ClC-3 gating and proton/anion exchange by internal and external protons and the anion selectivity filter

KEY POINTS

The ClC-3 2Cl- /1H+ exchanger modulates endosome pH and Cl- concentration. We investigated the relationships between ClC-3-mediated ion transport (steady-state transport current, ISS ), gating charge (Q) and cytoplasmic alkalization. ClC-3 transport is functionally unidirectional. ClC-5 and ClC-3 display indistinguishable exchange ratios, but ClC-3 cycling is less "efficient", as reflected by a large Q/ISS . An M531A mutation predicted to increase water-wire stability and cytoplasmic proton supply improves efficiency. Protonation (pH 5.0) of the outer glutamate gate (Gluext ; E224) reduces Q, inhibits transport, and weakens coupling. Removal of the central tyrosine anion gate (Y572S) greatly increases uncoupled anion current. Tyrosine -OH removal (Y572F) alters anion selectivity and impairs coupling. E224 and Y572 act as anion barriers, and contribute to gating. The Y572 side chain and -OH regulate Q movement kinetics and voltage dependence. E224 and Y572 interact to create a "closed" inner gate conformation that maintains coupling during cycling.

ABSTRACT

We utilized plasma membrane-localized ClC-3 to investigate relationships between steady-state transport current (ISS ), gating charge (Q) movement, and cytoplasmic alkalization rate. ClC-3 exhibited lower transport efficiency than ClC-5, as reflected by a larger Q/ISS ratio, but an indistinguishable Cl- /H+ coupling ratio. External SCN- reduced H+ transport rate and uncoupled anion/H+ exchange by 80-90%. Removal of the external gating glutamate ("Gluext ") (E224A mutation) reduced Q and abolished H+ transport. We hypothesized that Methionine 531 (M531) impedes "water wire" H+ transfer from the cytoplasm to E224. Accordingly, an M531A mutation decreased the Q/ISS ratio by 50% and enhanced H+ transport. External protons (pH 5.0) inhibited ISS and markedly reduced Q while shifting the Q-voltage (V) relationship positively. The Cl- /H+ coupling ratio at pH 5.0 was significantly increased, consistent with externally protonated Gluext adopting an outward/open position. Internal "anion gate" removal (Y572S) dramatically increased ISS and impaired coupling, without slowing H+ transport rate. Loss of both gates (Y572S/E224A) resulted in a large "open pore" conductance. Y572F (removing only the phenolic hydroxide) and Y572S shortened Q duration similarly, resulting in faster Q kinetics at all voltages. These data reveal a complex relationship between Q and ion transport. Q/ISS must be assessed together with coupling ratio to properly interpret efficiency. Coupling and transport rate are influenced by the anion, internal proton supply and external protons. Y572 regulates H+ coupling as well as anion selectivity, and interacts directly with E224. Disruption of this "closed gate" conformation by internal protons may represent a critical step in the ClC-3 transport cycle.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Helix O modulates voltage dependency of CLC-1

The chloride channel (CLC) family of proteins consists of channels and transporters that share similarities in architecture and play essential roles in physiological functions. Among the CLC family, CLC-1 channels have the representative homodimeric double-barreled structure carrying two gating processes. One is protopore gating that acts on each pore independently by glutamate residue (E_ext). The other is common gating that closes both pores simultaneously in association with large conformational changes across each subunit. In skeletal muscle, CLC-1 is associated with maintaining normal sarcolemmal excitability, and a number of myotonic mutants were reported to modify the channel gating of CLC-1. In this study, we characterized highly conserved helix O as a key determinant of structural stability in CLC-1. Supporting this hypothesis, myotonic mutant (G523D) at N-terminal of helix O showed the activation at hyperpolarizing membrane potentials with a reversed voltage dependency. However, introducing glutamate at serine residue (S537) at the C-terminal of the helix O on G523D restored WT-like voltage dependency of the common gate and showed proton insensitive voltage dependency. To further validate this significant site, site-specific mutagenesis experiments was performed on V292 that is highly conserved as glutamate in antiporter and closely located to S537 and showed that this area is essential for channel function. Taken together, the results of our study suggest the importance of helix O as the main contributor for stable structure of evolutionary conserved CLC proteins and its key role in voltage dependency of the CLC-1. Furthermore, the C-terminal of the helix O can offer a clue for possible proton involvement in CLC-1 channel.

A tale of two CLCs: biophysical insights toward understanding ClC-5 and ClC-7 function in endosomes and lysosomes

The CLC protein family comprises both Cl(-) channels and H(+) -coupled anion transporters. The understanding of the critical role of CLC proteins in a number of physiological functions has greatly contributed to a revision of the classical paradigm that attributed to Cl(-) ions only a marginal role in human physiology. The endosomal ClC-5 and the lysosomal ClC-7 are the best characterized human CLC transporters. Their dysfunction causes Dent's disease and osteopetrosis, respectively. It had been originally proposed that they would provide a Cl(-) shunt conductance allowing efficient acidification of intracellular compartments. However, this model seems to conflict with the transport properties of these proteins and with recent physiological evidence. Currently, there is no consensus on their specific physiological role. CLC proteins present also a number of peculiar biophysical properties, such as the dimeric architecture, the co-existence of intrinsically different thermodynamic modes of transport based on similar structural principles, and the gating mechanism recently emerging for the transporters, just to name a few. This review focuses on the biophysical properties and physiological roles of ClC-5 and ClC-7.

F-/Cl- selectivity in CLCF-type F-/H+ antiporters

Many bacterial species protect themselves against environmental F(-) toxicity by exporting this anion from the cytoplasm via CLC(F) F(-)/H(+) antiporters, a subclass of CLC superfamily anion transporters. Strong F(-) over Cl(-) selectivity is biologically essential for these membrane proteins because Cl(-) is orders of magnitude more abundant in the biosphere than F(-). Sequence comparisons reveal differences between CLC(F)s and canonical Cl(-)-transporting CLCs within regions that, in the canonical CLCs, coordinate Cl(-) ion and govern anion transport. A phylogenetic split within the CLC(F) clade, manifested in sequence divergence in the vicinity of this ion-binding center, raises the possibility that these two CLC(F) subclades might exhibit differences in anion selectivity. Several CLC(F) homologues from each subclade were examined for F(-)/Cl(-) selectivity of anion transport and equilibrium binding. Differences in both of these anion-selectivity metrics correlate with sequence divergence among CLC(F)s. Chimeric constructs identify two residues in this region that largely account for the subclade differences in selectivity. In addition, these experiments serendipitously uncovered an unusually steep, Cl(-)-specific voltage dependence of transport that greatly enhances F(-) selectivity at low voltage.

Salt, chloride, bleach, and innate host defense

Salt provides 2 life-essential elements: sodium and chlorine. Chloride, the ionic form of chlorine, derived exclusively from dietary absorption and constituting the most abundant anion in the human body, plays critical roles in many vital physiologic functions, from fluid retention and secretion to osmotic maintenance and pH balance. However, an often overlooked role of chloride is its function in innate host defense against infection. Chloride serves as a substrate for the generation of the potent microbicide chlorine bleach by stimulated neutrophils and also contributes to regulation of ionic homeostasis for optimal antimicrobial activity within phagosomes. An inadequate supply of chloride to phagocytes and their phagosomes, such as in CF disease and other chloride channel disorders, severely compromises host defense against infection. We provide an overview of the roles that chloride plays in normal innate immunity, highlighting specific links between defective chloride channel function and failures in host defense.

Mutations associated with Dent's disease affect gating and voltage dependence of the human anion/proton exchanger ClC-5

Dent's disease is associated with impaired renal endocytosis and endosomal acidification. It is linked to mutations in the membrane chloride/proton exchanger ClC-5; however, a direct link between localization in the protein and functional phenotype of the mutants has not been established until now. Here, two Dent's disease mutations, G212A and E267A, were investigated using heterologous expression in HEK293T cells, patch-clamp measurements and confocal imaging. WT and mutant ClC-5 exhibited mixed cell membrane and vesicular distribution. Reduced ion currents were measured for both mutants and both exhibited reduced capability to support endosomal acidification. Functionally, mutation G212A was capable of mediating anion/proton antiport but dramatically shifted the activation of ClC-5 toward more depolarized potentials. The shift can be explained by impeded movements of the neighboring gating glutamate Glu_ext, a residue that confers major part of the voltage dependence of ClC-5 and serves as a gate at the extracellular entrance of the anion transport pathway. Cell surface abundance of E267A was reduced by ~50% but also dramatically increased gating currents were detected for this mutant and accordingly reduced probability to undergoing cycles associated with electrogenic ion transport. Structurally, the gating alternations correlate to the proximity of E267A to the proton glutamate Glu_in that serves as intracellular gate in the proton transport pathway and regulates the open probability of ClC-5. Remarkably, two other mammalian isoforms, ClC-3 and ClC-4, also differ from ClC-5 in gating characteristics affected by the here investigated disease-causing mutations. This evolutionary specialization, together with the functional defects arising from mutations G212A and E267A, demonstrate that the complex gating behavior exhibited by most of the mammalian CLC transporters is an important determinant of their cellular function.

ClC-5: Physiological role and biophysical mechanisms

Cl(-) transport in animal cells has fundamental physiological roles and it is mediated by a variety of protein families, one of them being the CLC family of ion channels and transporters. Besides their physiological relevance, CLC proteins show peculiar biophysical properties. This review will focus on a member of the CLC protein family, the endosomal Cl(-)/H(+) antiporter ClC-5. ClC-5 mutations cause Dent's disease, a renal syndrome due to defective protein reabsorption in the proximal tubule. This established the critical function of ClC-5 for endocytosis. However, our understanding of ClC-5's molecular role in endosomes and of its biophysical properties has proved elusive in spite of important progress achieved in the last two decades. Early models in which ClC-5 would provide a shunt conductance to enable efficient endosomal acidification conflicted with the antiport activity of ClC-5 that has more recently emerged. Currently, the physiological role of ClC-5 is hotly debated and its biophysical properties are still not fully understood.

Thermal sensitivity of CLC and TMEM16 chloride channels and transporters

Cl(-) transport is of fundamental importance in the most diverse physiological contexts and it is mediated by a variety of ion channels and transporters belonging to different protein families. In particular, the recently identified TMEM16 protein family comprises the long sought Ca(2+)-activated Cl(-) channel (CaCC) and the activity of one of its members, TMEM16A, is highly dependent on temperature and is involved in thermal nociception. Among the other protein families mediating Cl(-) transport, CLC proteins are also regulated by temperature although so far the physiological implications of this dependence are unknown.