Tuesday 4 May 2004
Regulation of CFTR protein by the surface receptor beta adrenergic receptor is mediated through the ezrin/radixin/moesin binding phosphoprotein 50 (EBP50), which binds both the C-termini CFTR and b2AR through their PDZ binding motifs.
In the resting state, CFTR, b2AR, and EBP50 exist as a macromolecular complex on the apical surface of epithelial cells. Upon agonist activation of the b2AR, the adenulate cyclase is stimulated through the G protein pathway, leading to an increase in cAMP. The elevated concentration of cAMP activates PKA, which is anchored near CFTR via interaction with Ezrin. The phosphorylation of CFTR by PKA disrupts the complex and leads to compartmentalized and specific signaling of the channel.
mutations in cAMP-regulated chloride channel CTFR in cystic fibrosis
CFTR mutations classification
Since the CFTR gene was cloned in 1989, more than 800 disease-causing muta-tions have been identified. Various mutations can be grouped into six "classes" based on their effect on the CFTR protein:
- Class I: Defective protein synthesis.
These mutations are associated with complete lack of CFTR protein at the apical surface of epithelial cells.
- Class II: Abnormal protein folding, processing, and trafficking.
These mutations result in defective processing of the protein from the endoplasmic reticulum to the Golgi apparatus; the protein does not become fully folded and glycosylated and is instead degraded before it reaches the cell surface. The most common cystic fibrosis gene abnormality in patients with this disease is a Class II mutation that leads to a deletion of three nucleotides coding for phenylalanine at amino acid position 508 ([utri ]F508). Worldwide, this mutation can be found in approximately 70% of cystic fibrosis patients. Class II mutations are also associated with complete lack of CFTR protein at the apical surface of epithelial cells.
- Class III: Defective regulation.
Mutations in this class prevent activation of CFTR by preventing ATP binding and hydrolysis, an essential prerequisite for ionic passage (see above). Thus, there is a normal amount of CFTR on the apical surface, but it is nonfunctional.
- Class IV: Decreased conductance.
These mutations typically occur in the transmembrane domain of CFTR, which forms the ionic pore for chloride transport. There is a normal amount of CFTR at the apical membrane, but with reduced function. This class is usually associated with a milder phenotype of cystic fibrosis.
- Class V: Reduced abundance.
These mutations typically affect intronic splice sites or the CFTR promoter, such that there is a reduced amount of normal protein. As discussed subsequently, Class V mutations are also associated with a milder phenotype of cystic fibrosis.
- Class VI: Altered regulation of separate ion channels.
As previously described, CFTR is involved in the regulation of multiple distinct cellular ion channels. Mutations in this class affect the regulatory role of CFTR. In some cases, a given mutation affects the conductance by CFTR as well as regulation of other ion channels. For example, the deltaF508 mutation is both a Class II and Class VI mutation.