ROLE OF ACTIN IN REGULATION OF EPITHELIAL SODIUM-CHANNELS BY CFTR

Cystic fibrosis (CF) airway epithelia exhibit enhanced Na+ reabsorptio n in parallel with diminished Cl- secretion. We tested the hypothesis that actin plays a role in the regulation of a cloned epithelial Na+ c hannel (ENaC) by the cystic fibrosis transmembrane conductance regulat or (CFTR). We found that immunopurified bovine tracheal CFTR coreconst ituted into a planar lipid bilayer with alpha,beta,gamma-rat ENaC (rEN aC) decreased single-channel open probability (P-o) of rENaC in the pr esence of actin by over 60%, a significantly greater effect than was o bserved in the absence of actin (similar to 20%). In the presence of a ctin, protein kinase A plus ATP activated both CFTR and rENaC, but CFT R was activated in a sustained manner, whereas the activation of rENaC was transitory. ATP alone could also activate ENaC transiently in the presence of actin but had no effect on CFTR. Stabilizing short actin filaments at a fixed length with gelsolin (at a ratio to actin of 2:1) produced a sustained activation of alpha,beta,gamma-rENaC in both the presence or absence of CFTR. Gelsolin alone (i.e., in the absence of actin) had no effect on the conductance or P-o of either CFTR or rENaC . We have also found that short actin filaments produced their modulat ory action on alpha-rENaC independent of the presence of the beta- or gamma-rENaC subunits. In contrast, CFTR did not affect any properties of the channel formed by alpha-rENaC alone, i.e., in the absence of be ta- or gamma-rENaC. These results indicate that CFTR can directly down regulate single Na+ channel activity, which may account for the observ ed differences between Na+ transport in normal and CF-affected airway epithelia. Moreover, the presence of actin confers an enhanced modulat ory ability of CFTR on Na+ channels.