Kidney Week

Abstract: TH-PO1023

Novel Point Mutation in NBCe1/SLC4A4 Displays Dominant-Negative Inhibition of Wild-Type NBCe1A Activity in Mammalian Cells and Xenopus Oocytes

Session Information

• Acid Base: Basic
  November 02, 2017 | Location: Hall H, Morial Convention Center
  Abstract Time: 10:00 AM - 10:00 AM

Category: Fluid, Electrolytes, and Acid-Base

• 701 Acid-Base: Basic

Authors

• Rossano, Adam Joseph, Mayo Clinic College of Medicine, Rochester, Minnesota, United States

Adam Joseph Rossano,

• Holmes, Heather L., Mayo Clinic College of Medicine, Rochester, Minnesota, United States

Heather L. Holmes,

• Romero, Michael F., Mayo Clinic College of Medicine, Rochester, Minnesota, United States

Michael F. Romero,

Background

NBCe1 (electrogenic Na⁺/HCO₃^- cotransporter 1; SLC4A4) is present at the proximal tubule basolateral membrane where it controls systemic pH by reclaiming HCO₃^-. Human recessive NBCe1 mutations cause proximal renal tubular acidosis, cataracts, and glaucoma. A recently discovered novel point mutation (NBCe1-Mut) produces similar clinical findings, but its biophysical characterization is lacking.

Methods

NBCe1A activity was assessed by voltage clamp in Xenopus oocytes. Membrane trafficking was followed using HA-tagged constructs. NBCe1A activity was also assayed in human trabecular meshwork cells (NTM-5) and retinal pigment epithelia (RPE) by co-transfecting GFP-tagged NBCe1 constructs with pHire (RFP-based pH indicator) to monitor intracellular pH.

Results

In oocytes, CO₂/HCO₃^- (CB) addition elicited strong inward currents with NBCe1A-WT (WT). Currents in E91R (low-function) and Mut injected oocytes were indistinguishable from water controls. WT/E91R coinjections produced currents similar to WT, but co-injection of WT/Mut reduced maximum current to ~35% of WT-alone. HA tracking revealed that Mut trafficking to the plasma membrane was impaired compared to WT/E91R, and co-injection of WT/Mut constructs decreased successful trafficking to ~50% of WT-alone. Exposure of human cells expressing pHire to CB solutions produced a characteristic fall in pH followed by a gradual Na⁺-sensitive pH increase (HCO₃^- entry) via endogenous NBCe1. Cells transfected with WT displayed more rapid HCO₃^- entry than neighboring untransfected cells as well as dramatic acidification upon Na⁺ removal. pH transients in E91R transfected cells were indistinguishable from those in untransfected neighbors while expression of Mut slowed HCO₃^- entry and diminished the rate of acidification upon Na⁺ removal in both cell types studied.

Conclusion

NBCe1A-Mut displays poor transport and trafficking to the plasma membrane. Surprisingly, Mut can impair WT function and localization when co-expressed in amphibian and human cells. These results suggest a unique dominant-negative phenotype which requires further mechanistic analysis.

Funding

• NIDDK Support