Cystic fibrosis transmembrane conductance regulator (CFTR) and renal function

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is expressed in all nephron segments. Although mutations in CFTR are not associated with major changes in renal function, drug excretion by the kidneys is altered in cystic fibrosis (CF) as is the ability of the kidneys to concentrate and dilute the urine and excrete a salt load. It is not clear if these changes in renal function are secondary to decreased extracellular fluid volume caused by excessive losses of NaCl in sweat and feces or if they are related to primary defects in renal function caused by mutations in CFTR. Considerable evidence supports a role for CFTR in mediating Cl- secretion by the distal tubule, principal cells in the cortical collecting duct (CCD) and the inner medullary collecting duct (IMCD). In addition, CFTR is responsible for Cl- secretion into the lumen of cysts in polycystic kidneys and, therefore, contributes to cyst enlargement. Under some conditions--when Na+ absorption across the apical membrane of principal cells in the CCD is stimulated and the apical membrane potential is depolarized--the electrochemical gradient for Cl- will support Cl- absorption via CFTR Cl- channels. In addition to its function as a 3',5'-cAMP-activated Cl- channel, CFTR may play a role in intracellular vesicle acidification, protein processing, protein trafficking, secretion of ATP and the regulation of the epithelial Na channel (ENaC) and the secretory K+ channel (ROMK2) which mediate Na+ and K+ transport, respectively, across the CCD. Thus, CFTR may regulate Na+ and K+ excretion by the kidneys. The most common mutation in CFTR is delta F508, a mutation which causes improper folding of CFTR such that it is retained within the endoplasmic reticulum where it is degraded. Thus, in the majority of cases, CF is a trafficking disease. However, nothing is known about the intracellular trafficking of CFTR in the kidney. In preliminary studies we have developed a living cell model to study the intracellular trafficking of CFTR and delta F508-CFTR in renal epithelial cells in real time. Our ultimate goal is to elucidate the intracellular trafficking of CFTR and to identify therapeutic approaches to restore normal function to renal cells in CF and to block CFTR-mediated Cl- secretion in cysts in polycystic kidneys.