Chloride channel openers refer to a specific category of drugs designed to modulate chloride channels in the human body. Chloride channels are anion-selective channels which are involved in a wide variety of physiological functions and processes such as the regulation of neuroexcitation, transepithelial salt transport, and smooth muscle contraction. Due to their distribution throughout the body, diversity, functionality, and associated pathology, chloride channels represent an ideal target for the development of channel modulating drugs such as chloride channel openers.
Chloride channel modulators include chloride channel openers, and chloride channel blockers, both of which modulate the transport of chloride ions through chloride channels. Chloride channel openers specifically work by either preventing the closure of chloride channels, or promoting their opening, thus helping to facilitate movement of chloride ions into a cell.
In general, mutations in various chloride channels throughout the human body can result in many pathologies, such as macular degeneration, myotonia, cystic fibrosis, and hyperekplexia. Chloride channel openers have been proposed to treat a variety of such pathologies, most notably including cystic fibrosis, a genetic disorder in which a defect in the cystic fibrosis transmembrane conductance regulator protein impacts chloride ion transport across epithelial cells.
Chloride channels remain a somewhat under-explored target for drug development despite their biological significance, due to various issues associated with the development of successful chloride channel drugs. As a result, the development of chloride channel modulators such as chloride channel opening drugs has been limited. The relative structural complexity of chloride channels, their varied roles in biological processes, and the problems associated with the development of drugs with high specificity emphasize the necessity for further research in this area.
Chloride Channels Ion Channels Ion channels are pore-forming proteins which help facilitate the transport of ions across membranes, typically plasma membranes or the membranes of organelles within cells . They are considered to be the second largest drug target for existing drugs, after G protein-coupled receptors. There is a wide variety of ion channels, and they are typically characterized by their gating mechanism, such as ligand-gated channels or voltage-gated channels, and the ion being transported, such as sodium (Na), potassium (K), or chloride (Cl) ions.
Voltage-gated ion channels Several different intracellular transport channels, with varying functions, exist throughout the human body to facilitate routine upkeep and maintenance. Such transport channels, including ligand gated and voltage gated channels, regulate uptake of chemical stimulants that trigger neuronal function. Chloride (Cl-), the most abundant anion in the human body, as well as Sodium (Na+) and Potassium (K+), determine the electrochemical potential across a cell. The difference in concentration of chloride determines if a cell will depolarize or hyperpolarize the plasma membrane, resulting in a neural response. Chloride channels are a type of voltage-gated ion channel, and are responsible for regulating chloride ion transport and therefore neural excitation or inhibition.
Physiological significance of chloride ion channels Chloride channels are present throughout the body and have important roles in many physiological processes. Some functions of chloride channels include ion homeostasis, cell volume regulation, and modulation of electrical excitability. In the central nervous system (CNS), chloride channels are responsible for both direct modulation of neuronal activity and indirect control of neuronal functions through release of gliotransmitters by astrocytes through gating of organic anions such as GABA. Chloride channels also partake in cell apoptosis caused by endoplasmic reticulum (ER) stress, in addition to their roles in modulation of membrane potential and gating anions. ER stress notably plays a role in neurological disorders such as Alzheimer's (AD).
Chloride ion transport Chloride ions stimulate chloride channels to open, and so the active and efficient transport of chloride ions is crucial to neural function. Chloride is transported across necessary cell membranes via chloride transporting proteins. The function of these proteins is crucial for appropriate neural pathology and dysfunction of these proteins is characteristic of diseases and disorders such as epilepsy and autism.
Gene families of chloride channels Although opinions vary among experts regarding classification of the different chloride channels, they can generally be classified into 4 families of chloride channels that have been identified, with the notable exceptions of ligand gated GABA and glycine receptors: ClC channel proteins, CFTR (cystic fibrosis transmembrane conductance regulator) channels, calcium-activated chloride channels, and voltage-dependent anion selective channels. Groups are assigned by molecular properties and variance in activation stimuli. CLC channel proteins, which are expressed on cell membranes, organelles, and vesicles, are of particular interest for the development of chloride channel openers due to their regulation of chloride ion transport and gradients for many cellular functions.
ClC Channel Proteins ClC channel proteins, which were discovered in the 1990s, are found within the plasma and intracellular membranes of cells. Expressed as either anion channels or anion/protein exchangers, these proteins can join to form homomeric or heteromeric dimers. These proteins have both individual pathways as well as advanced pathway options when incorporated into a dimer. The specific channel and exchanger biological function roles vary based on organism type. The ClC-1 channel protein, the first identified member of this protein family, elicited extraordinary scientific interest when it was discovered to possess two gates or channels which functioned independently of one another This so-called "double-barreled" structure poses both a potential for new understanding and very complex drug design utilizing the surprising structure of the ClC-1 protein.
CFTR Channels The cystic fibrosis transmembrane conductance regulator (CFTR) is a phosphorylation-dependent epithelial chloride channel located mostly within the apical membrane of epithelial cells to control the rate of chloride ion flow. The CFTR channels are therefore critical to determining transepithelial salt transport, fluid flow, and concentrations of ions. CFTR chloride channels have important roles in various aspects of the human body, such as in fluid and electrolyte secretion within the intestines, pancreas, and sweat glands. The structure of the CFTR consists of five domains: two nucleotide-binding domains, two membrane-spanning domains, and a single regulatory domain. Each of the domains has a different contribution to the overall function of the CFTR chloride channel. The membrane-spanning domains create the channel pore, while the phosphorylation of the regulatory domain controls the activity of the channels, and ATP hydrolysis from the nucleotide-binding domains modulates gating of the channels. When CFTR channels malfunction, the transport of chloride ions across epithelial cells is disturbed, impacting the function of various organs lined by these cells, and leading to the genetic disorder cystic fibrosis. As a result, the CFTR channels also represent a key target for the development of chloride channel agonists.
Calcium-activated Chloride Channels The calcium-activated chloride channel family (CaCCs) are present throughout a wide range of tissues, participating in several physiological processes including epithelial secretion, sensory transduction, and smooth muscle contraction. CaCCs are activated by cytosolic calcium ions, and moderate transmembrane anion transport in response to an increase in the intracellular concentration of calcium ions. The most notable CaCC is formed by TMEM16A, which is present in several tissues of the body. TMEM16A has a variety of different functions within the various tissues it is present in, such as in chloride ion secretion with the airway epithelia, making it an important target for the development of chloride modulating drugs which treat cystic fibrosis.
Voltage-dependent Anion Selective Channels Voltage-dependent anion selective channels (VDAC), which are also known as mitochondrial porins, are channels permeable to a variety of anions, cations, and other metabolites including ATP. At lower transmembrane potentials, VDAC channels are more selective for anions such as chloride ions, as opposed to cations, but at higher transmembrane potentials, they favor cations over anions. Therefore, these large-pore channels likely play a key role in regulating the transport of metabolites, including chloride ions, in and out of the mitochondria, and could potentially present as a target for chloride channel modulating drugs. However, other types of chloride channels would likely present a more viable option for the development of channel modulator drugs such as chloride channel agonists.
GABA receptors GABAergic receptors, which receive GABA, the most prevalent inhibitory neurotransmitter found within the mammalian CNS, can be further divided into three subclasses: GABA-A, GABA-B, and GABA-C. Both GABA-A and GABA–C receptors are ionotropic ligand-gated chloride channels, while the GABA-B receptor is a G-protein-linked metabotropic receptor. The ionotropic GABA-A and GABA-C receptors can be activated by GABA to open and allow entry of negatively chloride ions into a cell, playing a significant role in the control of neuronal excitability. As a result, both GABA-A and GABA-C receptors, particularly GABA-A receptors, represent prominent targets for the development of chloride channel agonist drugs.
GABA-A receptor inhibition GABA-A receptors are GABA-gated anion channels which are involved in the function of rapid inhibitory synaptic transmission through the vertebrate CNS . These receptors are coupled with intrinsic chloride channels that are triggered to open through the binding of GABA, which is an inhibitory neurotransmitter. When activated, GABAergic inhibition of two types, phasic and tonic, will occur. Phasic GABA-A receptor-mediated inhibition is a result of a brief exposure of postsynaptic GABA-A receptors to high concentrations of GABA. Alternatively, tonic GABA-A receptor-mediated inhibition results from an activation of extrasynaptic receptors by low concentrations of ambient GABA. Somewhere between 75% and 90% of GABA-A inhibition in the CNS is tonic.
GABA-A receptor structure and disease GABA-A receptors are part of the cys-loop pentameric ligand-gated ion channel family, which includes multiple neurotransmitter-gated channels. GABAA receptors are assembled from five subunits. Such subunits and splice variants can be distinguished as α1-α6, β1-β3, γ1-γ3, δ, ε, π and θ. GABA-A receptor subunit mutations are believed to be a potential cause for many neurological and CNS disorders. For example, epilepsy related sleep disturbances are believed to be partially caused by improper activation of the β3 GABA-A receptor subunit. Various other pathological mood disorders including anxiety and schizophrenia are major therapeutic targets for GABA-A. GABA-A therapeutics are a related therapy that could potentially be treated or targeted by chloride ion channel opener drugs.
Perception of chloride ion gated channels as drug targets