Anionic Phospholipids Regulate Native and Expressed Epithelial Sodium Channel (ENaC)

Abstract

Using patch clamp techniques, we found that the epithelial sodium channel (ENaC) activity in the apical membrane of A6 distal nephron cells showed a sudden rundown beginning at 4 min after forming the inside-out configuration. This sudden rundown was prevented by addition of anionic phospholipids such as phosphatidylinositol 4,5-bisphosphate (PIP(2)), phosphatidylinositol 3,4,5-trisphosphate (PIP(3)), and phosphatidylserine (PS) to the "cytoplasmic" bath. Conversely, chelation of endogenous PIP(2) with anti-PIP(2) antibody, hydrolysis of PIP(2) with either exogenous phospholipase C (PLC) or activation of endogenous PLC by extracellular ATP, or application of the positively charged molecule, poly-L-lysine, accelerated channel rundown. However, neutral phosphatidylcholine had no effect on ENaC activity. By two-electrode voltage clamp recordings, we demonstrated that PIP(2) and PIP(3) significantly increased amiloride-sensitive current in Xenopus oocytes injected with cRNAs of rat alpha-, beta-, and gamma-ENaC. However, PIP(2) and PIP(3) did not affect surface expression of ENaC, indicating that PIP(2) and PIP(3) regulate ENaC at the level of the inner plasma membrane through a mechanism that is independent of ENaC trafficking. These data suggest that anionic phospholipids may mediate the regulation of ENaC by PLC- or phosphoinositide 3-kinase-coupled receptors.