|Analysis of CFTR Function and Expression|
The CFTR protein was identified as a cAMP regulated Cl¯ and bicarbonate channel expressed in the apical membrane of epithelial cells. The secretory properties transmitted by CFTR are regulated by the b2 adrenergic pathway in exocrine tissues. The molecular pathology, connecting the basic defect to the symptoms of the disease, is described comprehensively for the sweat gland: the impermeability of the apical membrane of sweat duct cells results in a decreased reabsorption of Cl¯ and, parallel to this, a decreased reabsorption of the counterion Na+. Reduction of NaCl absorption thus leads to elevated NaCl concentrations in sweat of CF patients.
The basic defect in CF, i.e. an impaired chloride permeability of the apical membrane of epithelial cells, manifests in all CFTR expressing tissues and thus is seen not only in the patient's sweat gland, but also in the epithelia of the lower nasal turbinate and the rectum. Hence, diagnosis of CF can be done by assessing the basic defect in the sweat gland (pilocarpine iontophoresis sweat test), the nasal epithelium (nasal potential difference measurement) and intestinal tissue (intestinal current measurement, ICM), both of which are done here at the MHH.
Besides these electrophysiological measurements, the maturation state of CFTR can be assessed by immunocytochemistry and protein biochemistry. The CFTR protein biochemistry from patient samples is hard to tackle, as the protein suffers from most manipulations of the tissue. Analysis by immunoprecipitation and by direct Western blot of lysates is feasible from small human tissue samples.
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