Basolateral chloride transporters in autosomal dominant polycystic kidney disease

Transepithelial Cl- secretion mediated by apical cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels plays a key role in cyst fluid accumulation in autosomal dominant polycystic kidney disease (ADPKD). The molecular identity of the basolateral transporter(s) responsible for Cl- entry in ADPKD cells is unknown, although pharmacological studies suggest that a bumetanide-sensitive Na+-K+-2Cl(-) cotransporter (NKCC/BSC) is involved. We investigated the expression of NKCC1, CFTR and anion exchanger type I (AE1) in ADPKD kidneys and cultured ADPKD cells. Immunoblotting and immunoprecipitation detected NKCC1 at similar to170 kDa in ADPKD cells and kidney extracts. Immunostaining located NKCC1 in one-third of ADPKD cysts, with a pattern of basolateral reactivity. Staining of serial sections showed that cysts positive for NKCC1 also stained for CFTR. Additional studies demonstrated that AE1 is expressed in ADPKD kidneys, and is located at the basolateral pole of CFTR-positive ADPKD cysts that do not express NKCC1. RT-PCR and sequence analyses confirmed the selective expression of NKCC1 or AEI in cultured ADPKD cells that also express CFTR. The fact that most CFTR-positive ADPKD cysts also express NKCC1 suggests that transepithelial Cl- secretion in ADPKD involves molecular mechanisms similar to secretory epithelia. AE1 might be an alternative basolateral pathway for Cl- in a minority of cysts.