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Abstract: SA-PO1036

Dietary Mg2+ Restriction Downregulates NCC Through a SPAK-Independent Pathway

Session Information

Category: Fluid and Electrolytes

  • 901 Fluid and Electrolytes: Basic

Authors

  • Ferdaus, Mohammed Zubaerul, Oregon Health & Science University, Portland, Oregon, United States
  • Miller, Lauren N., Oregon Health & Science University, Portland, Oregon, United States
  • McCormick, James A., Oregon Health & Science University, Portland, Oregon, United States
Background

Hypomagnesemia is often observed in critically ill patients and is associated with lower kidney function and life-threatening complications. The distal convoluted tubule (DCT) is the major site of active Mg2+ reabsorption, determining the final urinary Mg2+ excretion. The Na+-Cl- cotransporter (NCC), expressed along the DCT, plays a key role in ion homeostasis. Dietary interventions have a strong effect on NCC.

Methods

We studied the effect of dietary Mg2+ manipulation on NCC in C57BL/6J mice by assessing levels of total NCC (tNCC), phospho-NCC (pNCC) and other relevant proteins with blood electrolyte analysis.

Results

We observed that abundances of tNCC and pNCC, a surrogate for NCC activity, were lower following acute (3 days) and chronic dietary Mg2+ (14 days) restriction, compared with normal and high Mg2+ diets. However, pNCC:tNCC was unchanged, suggesting the primary effect is on tNCC abundance. It is well-established that low blood [K+] induces NCC phosphorylation. We observed that while tNCC was still downregulated by combined K+ and Mg2+ restriction, pNCC abundance was strongly increased compared with normal and Mg2+ deficient diets. These data suggest that distinct signaling pathways mediate the effects of dietary Mg2+ and K+ restriction on NCC. STE20 (Sterile 20)/SPS-1 related proline/alanine-rich kinase (SPAK) directly phosphorylates NCC, and SPAK knockout (KO) mice have ~90% lower tNCC abundance compared with wild type mice. Dietary Mg2+ restriction of SPAK KO mice further decreased tNCC, suggesting a strong effect of dietary Mg2+ restriction on tNCC abundance, and showing that SPAK was not involved in mediating the effect of dietary Mg2+ restriction. The E3 ubiquitin-protein ligase neural precursor cell expressed developmentally downregulated gene 4-like (Nedd4-2) induces tNCC degradation on a high Na+ diet, but targets the epithelial sodium channel (ENaC) during K+ restriction. As activated Nedd4-2 promotes self-degradation, its abundance reflects its activity. We found that Nedd4-2 abundance was lower on Mg2+ deficient diet compared with normal diet, suggesting activation of Nedd4-2 by Mg2+ restriction. However, abundances of ENaC subunits were unaffected.

Conclusion

Overall, our data suggest that dietary Mg2+ deficiency may alter NCC function by downregulating total abundance of NCC by activating Nedd4-2 selectively along the DCT.

Funding

  • NIDDK Support