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Wild type but not deltaF508 CFTR inhibits Na+ conductance when coexpressed in Xenopus oocytes
1Physiologisches Institut der Albert-Ludwigs-Universität Freiburg, Germany.
Abstract:
Airway epithelial cells bearing mutations of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) possess an increased Na+ conductance along with their well described defect of cAMP dependent Cl- conductance. Currently it is not clear, how this occurs, and whether it is due to a CFTR control of epithelial Na+ conductances which might be defective in CF patients. In the present study, we have tried to identify possible interactions between both CFTR and the epithelial Na+ conductance by overexpressing respective cRNAs in Xenopus oocytes. The expression of all three (alpha, beta, gamma) subunits of the rat epithelial Na+ channel (rENaC) and wild type (wt) CFTR resulted in the expected amiloride sensitive Na+ and IBMX (1 mmol/l) activated Cl- currents, respectively. The amiloride sensitive Na+ conductance was, however, inhibited when the wt-CFTR Cl- conductance was activated by phosphodiesterase inhibition (IBMX). In contrast, IBMX had no such effect in deltaF508 and Na+ channels coexpressing oocytes. These results suggest that wt-CFTR, but not deltaF508-CFTR, is a cAMP dependent downregulator of epithelial Na+ channels. This may explain the higher Na+ conductance observed in airway epithelial cells of CF patients.
Insights
Cystic fibrosis (CF) transmembrane conductance regulator (CFTR) normally downregulates epithelial sodium (Na+) channels. The deltaF508 mutation prevents this, explaining increased Na+ conductance in CF airway cells.
Area of Science:
- Cellular physiology
- Ion channel function
- Genetic disease mechanisms
Background:
- Cystic fibrosis (CF) airway epithelial cells exhibit elevated Na+ conductance alongside defective Cl- transport.
- The precise mechanism linking CFTR mutations to altered Na+ channel activity remains unclear.
Purpose of the Study:
- To investigate the interaction between the cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial Na+ channels (ENaC).
- To determine if CFTR modulates ENaC activity in a cAMP-dependent manner.
Main Methods:
- Overexpression of rat epithelial Na+ channel (rENaC) subunits (alpha, beta, gamma) and wild-type (wt) or deltaF508 mutant CFTR cRNAs in Xenopus oocytes.
- Measurement of amiloride-sensitive Na+ currents and IBMX-activated Cl- currents.
- Assessment of the effect of cAMP-dependent CFTR activation on ENaC activity.
Main Results:
- Coexpression of wt-CFTR and rENaC subunits resulted in amiloride-sensitive Na+ currents.
- Activation of wt-CFTR by IBMX led to inhibition of the amiloride-sensitive Na+ conductance.
- IBMX did not affect Na+ conductance in oocytes expressing deltaF508-CFTR and rENaC.
Conclusions:
- Wild-type CFTR acts as a cAMP-dependent downregulator of epithelial Na+ channels.
- The deltaF508-CFTR mutation impairs this downregulatory function.
- This CFTR-mediated regulation of ENaC may explain the increased Na+ conductance observed in cystic fibrosis airway epithelia.