The A-kinase anchoring protein 15 regulates feedback inhibition of the epithelial Na⁺ channel

Protein kinase A anchoring proteins or AKAPs regulate the activity of many ion channels. Protein kinase A (PKA) is a well-recognized target of AKAPs, with other kinases now emerging as additional targets. We examined the roles of epithelial-expressed AKAPs in regulating the epithelial Na⁺ channel (ENaC). Experiments used heterologous expression with AKAP15, AKAP-KL, and AKAP79 in Xenopus oocytes. Experiments were carried out under high and low Na⁺ conditions, as Na⁺ loading is known to affect the baseline activity of ENaC in a PKC-dependent mechanism. ENaC activity was unaffected by AKAP79 and AKAP-KL expression. However, oocytes coexpressing AKAP15 exhibited an 80% and 91% reduction in the amiloride-sensitive, whole-cell conductance in high and low Na⁺ conditions, respectively. The reduced channel activity was unaffected by PKA activation or inhibition, indicating a PKA-independent mechanism. Expression with a membrane-targeting domain, mutant form of AKAP15 (AKAP15m) prevented the decrease of ENaC activity, but only under low Na⁺ conditions. In high sodium conditions, coexpression with AKAP15m led to an increase of ENaC activity to levels similar to those observed under low Na⁺. These results indicate that membrane-associated AKAP15 reduces ENaC activity whereas the cytoplasmically associated one may participate in the channel's feedback inhibition by intracellular Na⁺, a process known to involve PKC. This hypothesis was further confirmed in coexpression experiments, which demonstrated functional and physical interaction between AKAP15 and PKCα. We propose that AKAP15 regulates ENaC via a novel PKA-independent pathway.