Chloride is a critical component of all living cells. It is also the single most dominant diffusible anion inside of most cells - the others are mostly impermeable organic anions. Since cytoplasmic electroneutrality is maintained under normal physiological environment, changes in cellular chloride level is accompanied by total cell solute content. Because of high water permeability of cell membranes, changes in cell solute content are accompanied by changes in cellular volume. Voltage-gated chloride channels regulate cellular traffic of chloride ion.
The chloride channels (CIC or CLC) perform several functions including the regulation of cell volume, membrane potential stabilization, signal transudation, and transepithelial transport. Chloride ions are present in the extra cellular fluid. They are important for the maintenance of Homeostasis and in the signal transduction in the human body.
The transport of chloride ions takes place through the chloride channels. Any change or modulation in activity of chloride channels can cause different types of undesirable effects, as chloride channels display a variety of important physiological and cellular roles that include regulation of pH, volume homeostasis, organic solute transport, cell migration, cell proliferation and differentiation. A number of different gene products have been shown to function as chloride channels.
Classification of Chloride Channels
Based on sequence homology, the chloride channels can be subdivided into a number of groups,
Group I: Termed as GABA receptor family. It is present mainly in the central nervous system, where main function is to hyperpolarize neuronal membrane and hence inhibits neurotransmission. GABA receptor activation is due to the ligand binding. The various drugs like benzodiazepine and barbiturates acts through this receptor. The drugs are helpful in the treatment of Insomnia, anxiety and epilepsy disorders of the Central Nervous System.
Group II: The second group of chloride channels is CLC family. The importance of such group can be seen from the diseases that develop when the channel does not function normally. The main function of these channels is in maintaining membrane potential, cell volume regulation & chloride transport across cell membrane. These channels are voltage or volume gated. Since the cloning of first member of CLC from Torpedo electric organ (CLC-0), many mammalian homologs have been identified. In mammals, CLC proteins form a super family of at least 9 different genes (CLC1-7 also known as CLCN1-7 & CLCK1-2 or CLC Ka & CLCKb).
Additional forms of these proteins are obtained by alternate splicing. A detailed sequence comparison of CLC has revealed three closely related groups: One group contains CLC-1, CLC-2 and the renal CLCK-1 & CLCK-2, a second group comprises of CLC-3, CLC-4 and CLC-5; The third group has CLC-6 & CLC-7. All CLC proteins (~700-1000 da) are predicted to contain 10 (possibly 12) transmembrane domains with intracellular N and C- terminals. Except CLC-1 and CLC-K1/K2 that are specific for kidney, most other CLC are widely distributed in various tissues.
Diseases related with CLC channels:
Myotonia Congenita (Thomsen's disease):
In skeletal muscle the major ion channel that stabilizes the membrane potential in the CLC-1 chloride channel. Inherited diseases that affect this channel is myotonia congenital is associated with muscle stiffness. In these patients, the non-functioning CLC-1 chloride channel is unable to repolarize the membrane, giving rise to repeated action potential.
Barter's Syndrome:
Rare disorder, which is associated with renal salt wasting and hypokalemic alkalosis, is due to the defective transport of chloride ions and associated ions in the thick ascending loop of Henle in renal tubules. Mutations in a member of transporters or the chloride channel CLC-Kb have been implicated as the inherited defect that results in the disease.
Dent's Diseases :
This disease affects the kidney organ, characterized by low molecular weight proteinuria and hypercalciuria. Again the basis of the problem has been traced to mutated forms of a chloride channel, CLC-5, that results in either total loss or a greatly reduced chloride current.
Goup III: The third group of chloride channels is CFTR i.e. cystic fibrosis transmembrane conductance regulator. Cystic fibrosis is an autosomal recessive genetic disorder occurs 1 in every 2500 births. The main symptoms associated with it are abnormal electrolyte transport in a variety of epithelium. CFTR is mainly present in epithelial cells, lung, pancreas, and testis. It activation occurs due to phosphorylation and ATP hydrolysis. Cystic fibrosis occurs due to deletion mutation in CFTR gene at 508 position. CFTR gene spans about 188 kb of genomic DNA and contains about 27 axons. The CFTR protein is 1480 amino acid transmembrane protein 70% of cystic fibrosis patients have defective processing, the other type of mutations that cause cystic fibrosis are:
i. Defective protein production
ii. Defective regulation
iii. Defective conduction
iv. Defective RNA synthesis.
Role of Chloride Channel in abnormal mucus production in respiratory diseases
It has been discovered that novel calcium activated chloride channel (hCLCA1) regulates abnormal mucus production. Mucus overproduction and small airways plugging is one of the hallmarks of asthma, COPD (Chronic obstructive pulmonary disease) and chronic bronchitis. So in these diseases mucoregulatory therapy may be of benefit. This hCLCA-1 has potential as an important therapeutic target for asthma and other respiratory and sinus diseases.
This discovery of gene in lungs comes from asthmatic mice and the identification of equivalent human gene. Further research has demonstrated that hCLCA-1 appears to regulate inducible lung and sinus mucus production and to participate in regulating antigen stimulated epithelial cell function, in a number of pulmonary and sinus disorders. Genaera company has developed Lomucin (tm) which is intended to block the hCCLA-1 dependent mucus overproduction present in respiratory and sinus disorders and therapy provides a new strategy for opening the airways and easing breathing in patients with there diseases. The drug is presently in clinical trials.
Lomucin may provide a unique advantage over conventional treatments because it appears to specifically inhibit the over production of abnormal gel forming mucin that that clog the small airways in chronic lung diseases whereas current therapies usually attempt the deal with mucus clearance once it is present in the airways, not in preventing excess mucus production. So Lomucin will become the future drug for the treatment of respiratory and sinus disorders.
Though, less discussed in scientific forum, chloride channel plays an important role in the maintenance of the normal physiology of the body and homeostasis. The advances in understanding the molecular basis of the various diseases and involvement of the chloride channel, newer targets are to be found and the lead compounds are to be discovered for their therapeutic/clinical uses.
--The authors are with Pharmacy Group, Birla Institute of Technology and Science, Pilani