Chloride channels are a functionally and structurally diverse group of anion-selective channels involved in processes including the regulation of the excitability of neurones, skeletal, cardiac and smooth muscle, cell volume regulation, transepithelial salt transport, the acidification of internal and extracellular compartments, the cell cycle and apoptosis. Excluding the transmitter-gated GABA and glycine receptors, well-characterized chloride channels can be classified as certain members of the voltage-sensitive ClC subfamily, calcium-activated channels, high- (maxi) conductance channels, the cystic fibrosis transmembrane conductance regulator (CFTR) and volume-regulated channels.
Chloride channels are a class of channel proteins that can transport chloride ions and other anions on the plasma membrane of cell membranes or organelles. Excluding the transmitter-gated γ-aminobutyric acid (GABA) and glycine receptors, well-characterized chloride channels can be classified as voltage-gated chloride channel (ClC), cystic fibrosis transmembrane conductance regulator (CFTR), and calcium-activated chloride channel (CaCC), volume-regulated chloride channels (VRAC), ligand activated chloride channels. The main function of chloride channels is to regulate transepithelial transport, membrane potential and cellular immune responses. Abnormal expression of chloride channel protein is closely related to the occurrence and development of tumors, which are relevant to the proliferation, apoptosis, migration and invasion of gastric cancer.
In 2002, the MacKinnon group presented the X-ray structures of two prokaryotic ClC Cl- channels from Salmonella enterica serovar typhimurium and Escherichia coli at 3.0 and 3.5 Å, respectively. The crystal reveals that the bacterial ClC protein is composed of 18 α-helices (from A to R), most of which do not cross the membrane entirely. It has a complex topology structure with an N-terminal polypeptide (from B to I) and a C-terminal polypeptide (from J to Q). Eighteen α-helices are similar in structure and opposite in direction, forming an anti-parallel pseudo-double symmetric structure. ClC-1 can be inhibited by micromolar concentrations of anthracene-9-carboxylic acid (9AC) and 2-(p- chlorophenyl) propionic acid (CPP) analogues. ClC-2 is a widely expressed and self-regulating chloride channel that is activated by increased cell volume. ClC-Ka and ClC-Kb are expressed in the ascending and descending segments of the medullary canal, respectively, providing a cross-cell pathway for chloride reabsorption. Benzofuran carboxylic acid can block ClC-Ka and ClC-Kb, and its half inhibition concentration is less than 10 μmol·L-1.
CFTR is a chloride-conducting transmembrane protein. Functional failure of CFTR results in mucus retention and chronic infection and subsequently in local airway inflammation that is harmful to the lungs. There are mainly two types of CFTR inhibitors: thiazolidinone and pyrimidine (thio) ketone. CFTRinh-172 is a human specific inhibitor, which can inhibit the Cl- transport in humans, mice, rats, and pigs. VX-809 (Lumacaftor) as an another inhibitor corrected CFTR mutations in CF by promoting maturation of the mutant CFTR (F508del-CFTR). IOWH032 is a synthetic CFTR inhibitor, which shows inhibitory activity against CFTR (T84-CFTR).
CaCCs play a crucial role in various physiological processes such as epithelial fluid transport, intestinal motility regulation, smooth muscle contraction, neuronal excitation and nociception and is closely related to various diseases such as chronic constipation, hypertension, congenital megacolon, tumor and diarrhea. At present, bestrophins are most likely to be CaCC modulators. Studies of heterologous expression, mutagenesis, and RNA interference (RNAi) knockouts have shown that bestrophins trigger calcium activation, which leads to chloride influx.
The cell swells during hypotonic shock and VRAC is activated. When current and voltage are instantaneously generated, the VRACs are generally rectified outwards, and when the membrane potential reaches the excitatory potential, a larger ion current is generated. VRACs are permeable to certain organic compounds, such as 2-aminoethanesulfonic acid. However, there is also data indicating that chloride and 2-aminoethanesulfonic acid channels are separated.
Ligand activated chloride channels are mainly activated by γ-aminobutyric acid (GABA) and glycine. Inhibitory synaptic transmission is associated with GABA and glycine receptors, both of which play an important role in the brain and spinal cord, respectively. The binding of GABAA, GABAC and glycine receptors to ligands promotes the opening of chloride channels. GABAB receptor is an unrelated protein belonging to the G protein-coupled receptor subfamily. GABA receptors have multiple subunits, such as α1–6, β1–3, γ1–3, δ, ε, π, θ, and ρ1–3; various combinations of these subunits produce GABAA and GABAB receptors. Glycine receptors have four α subunits and one β subunit, and two subunits are usually combined in 2α1:3β. GABAA receptors are targets for sedative and hypnotic drugs, such as barbiturates and benzodiazepines and its modulators have been used clinically.
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Raimund, D., Ernest, B. C., Martine, C., Brian T. C., Roderick M. (2002) X-ray structure of a ClC chloride channel at 3.0 Å reveals the molecular basis of anion selectivity. Nature, 415, 287–294.