Identification of Paraoxonase‐2 as a Novel Modulator of the Epithelial Na+ Channel

Abstract

Paraoxonase‐2 (PON‐2) is a membrane‐bound lactonase with unique anti‐oxidative and anti‐atherosclerotic properties, and thus has been associated with cardiovascular diseases. PON‐2 shares key structural elements with MEC‐6, an essential component of the C. elegans mechanosensitive ion channel complex required for the worm's touch response. At the core of this channel complex, MEC‐6 interacts with two pore‐forming subunits, MEC‐4 and MEC‐10 and enhances channel activity. MEC‐4 and MEC‐10 belong to the same family as the epithelial Na+ channel (ENaC), a key mediator of Na+ uptake in the aldosterone sensitive distal nephron (ASDN). We therefore examined PON‐2 expression in the ASDN, and immunofluorescence microscopy indicated tubular‐specific expression of PON‐2 that included aquaporin 2‐expressing cells. When PON‐2 was co‐expressed with individual ENaC subunits in HEK293 cells, PON‐2 and each of the three ENaC subunits could be co‐immunoprecipitated. In contrast, PON‐2 co‐immunoprecipitated only the α subunit of ENaC when all three subunits were expressed. When PON‐2 was co‐expressed with ENaC in Xenopus oocytes, both whole cell Na+ currents and ENaC surface expression were reduced. This inhibitory effect was ENaC‐specific, as PON‐2 had no effect on ROMK activity. However, the PON‐2 inhibitory effect was lost with a PON‐2 mutant (D268A) that abrogates the lactonase activity of PON‐2, likely as a result of interfering with calcium binding. Together, our results indicate that PON‐2 interacts with ENaC and modulates channel expression.