Low Phosphate Stimulates Na‐H Exchanger Regulatory Factor I (NHERF1) Trafficking

NHERF1 anchors the type IIa sodium phosphate cotransporter (Npt2a) in the brush border membrane (BBM). We have shown that Npt2a associates with NHERF1 in the ER/Golgi and that low phosphate, a stimulus for Npt2a exocytosis, causes a transient decrease in BBM NHERF1 expression in OK cells, a model of proximal tubule. We hypothesized that low phosphate stimulates trafficking of NHERF1 between BBM and Golgi. To address this hypothesis, OK cells placed in low phosphate (0.1 mmol) medium for 4 or 8h were fractionated and blotted for NHERF1 and Npt2a. NHERF1 BBM expression progressively decreased while ER/Golgi expression increased at 4 and 8 hours. Npt2a expression increased at all times in the BBM. Using total internal reflection microscopy, plasma membrane (PM) expression of GFP‐tagged Npt2a and mCherry‐tagged NHERF1 showed progressive increase in Npt2a but cyclic variations in NHERF1 PM expression over 4h of low phosphate treatment. Particle tracking averages within the PM showed 0.02 NHERF1 trajectories and 0.034 Npt2a trajectories/min/μm 2 , corresponding to 0.84 NHERF1 and 1.28 Npt2a particles/min (filtered for 50 frame persistence of particle signal). We conclude that low phosphate exerts a differential effect on NHERF1 and Npt2a trafficking. NHERF1 traffics dynamically in and out of the PM and to the Golgi in response to low phosphate while Npt2a exhibits greater movement within PM and progressively accumulates in the PM. These findings are consistent with a chaperone function for NHERF1 in Npt2a exocytosis. Support: VA, University of Louisville