Regulation of ENaC and CFTR biogenesis by ERp29 (869.9)

The Epithelial Na+ Channel (ENaC) plays an important role in blood pressure homeostasis and, in conjunction with the Cystic Fibrosis Transmembrane Conductance regulator (CFTR), regulates airway surface liquid volume. ERp29 is a 29 kD thioredoxin‐homologous protein which primarily resides in the lumen of the endoplasmic reticulum (ER). But unlike the thioredoxins, ERp29 has only a single Cysteine instead of the usual C‐X‐X‐C motif. Our group has previously demonstrated that ERp29 promotes biogenesis of CFTR. As the biogenesis of CFTR and ENaC have a number of similar features, the present studies tested the hypothesis that ERp29 would also regulate ENaC biogenesis and functional expression, as well as further probed the mechanism by which ERp29 promotes CFTR biogenesis. In MDCK epithelial cells expressing epitope tagged αβγ‐ENaC, overexpression of wt ERp29 increased the abundance of the cleaved form of γ‐ENaC, as well as ENaC functional expression (as defined by amiloride‐sensitive Isc in Ussing chamber experiments.) In contrast, siRNA‐mediated depletion of ERp29 or overexpression of a mutant ERp29 lacking its single Cysteine (C157S ERp29) decreased ENaC functional expression. Interestingly, these observations were not associated with altered expression of β‐ENaC at the apical surface, suggesting that ERp29 may modulate the cleavage state, and therefore open probability (Po) of ENaC at the apical surface. Finally, ERp29 overexpression promotes the interaction of both ENaC and CFTR with the Sec24D cargo recognition component of coat complex II ER exit machinery. Together these findings suggest a key role for ERp29 in the biogenesis of ENaC and CFTR; for ENaC this may include directing channel for cleavage by furin in the Golgi and delivery to the apical membrane in a higher Po form. Grant Funding Source : Supported by R01 DK58046