Phosphoinositide Binding Differentially Regulates NHE1 Na+/H+ Exchanger-dependent Proximal Tubule Cell Survival

Tubular atrophy predicts chronic kidney disease progression, and is caused by proximal tubular epithelial cell (PTC) apoptosis. The normally quiescent Na+/H+ exchanger-1 (NHE1) defends against PTC apoptosis, and is regulated by PI(4,5)P-2 binding. Because of the vast array of plasma membrane lipids, we hypothesized that NHE1-mediated cell survival is dynamically regulated by multiple anionic inner leaflet phospholipids. In membrane overlay and surface plasmon resonance assays, the NHE1 C terminus bound phospholipids with low affinity and according to valence (PIP3 > PIP2> PIP = PA > PS). NHE1-phosphoinositide binding was enhanced by acidic pH, and abolished by NHE1 Arg/Lys to Ala mutations within two juxtamembrane domains, consistent with electrostatic interactions. PI(4,5)P-2-incorporated vesicles were distributed to apical and lateral PTC domains, increased NHE1-regulated Na+/H+ exchange, and blunted apoptosis, whereas NHE1 activity was decreased in cells enriched with PI(3,4,5)P-3, which localized to basolateral membranes. Divergent PI(4,5)P-2 and PI(3,4,5)P-3 effects on NHE1-dependent Na+/H+ exchange and apoptosis were confirmed by selective phosphoinositide sequestration with pleckstrin homology domain-containing phospholipase C delta and Akt peptides, PI 3-kinase, and Akt inhibition in wild-type and NHE1-null PTCs. The results reveal an on-off switch model, whereby NHE1 toggles between weak interactions with PI(4,5)P-2 and PI(3,4,5)P-3. In response to apoptotic stress, NHE1 is stimulated by PI(4,5)P-2, which leads to PI 3-kinase activation, and PI(4,5)P-2 phosphorylation. The resulting PI(3,4,5)P-3 dually stimulates sustained, downstream Akt survival signaling, and dampens NHE1 activity through competitive inhibition and depletion of PI(4,5)P-2.