Mg<sup>2+</sup> Restriction Downregulates NCC Through NEDD4-2 and Prevents Its Activation by Hypokalemia
November 07, 2019 | 05:18 PM - 05:30 PM
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Mg2+ Restriction Downregulates NCC Through NEDD4-2 and Prevents Its Activation by Hypokalemia
Advances in Fluid and Electrolyte Handling: Basic Physiology
November 07, 2019 | Location: 146 C, Walter E. Washington Convention Center
Abstract Time: 05:18 PM - 05:30 PM
Category: Fluid and Electrolytes
- 901 Fluid and Electrolytes: Basic
- McCormick, James A., Oregon Health and Science University, Portland, Oregon, United States
- Ferdaus, Mohammed Zubaerul, Oregon Health and Science University, Portland, Oregon, United States
- Blatt, Philip James, Oregon Health and Science University, Portland, Oregon, United States
- Nelson, Jonathan W., Oregon Health and Science University, Portland, Oregon, United States
- Terker, Andrew, Vanderbilt University, Nashvile, Tennessee, United States
- Staub, Olivier, University of Lausanne, Lausanne, Switzerland
- Lin, Daohong, New York Medical College, Valhalla, New York, United States
James A. McCormick, PhD
Mohammed Zubaerul Ferdaus,
Philip James Blatt,
Jonathan W. Nelson,
Hypomagnesemia is associated with lower kidney function and life-threatening complications, and sustains hypokalemia. The distal convoluted tubule (DCT) determines final urinary Mg2+ excretion, and via activity of the Na+-Cl- cotransporter (NCC) plays a key role in K+ homeostasis by metering Na+ delivery to distal segments. We previously showed that short-term (3 days) or long-term (14 days) Mg2+ restriction lowered abundances of total NCC (tNCC) and the active phosphorylated NCC (pNCC), but this did not involve the NCC-activating WNK-SPAK pathway.
We set out to further explore the mechanisms involved and determine interactions with K+ restriction, a strong activator of NCC, by performing dietary manipulations in mice then Western blotting.
We confirmed a previous report that long-term Mg2+ restriction does not alter NCC mRNA abundance, and found the same with short-term Mg2+ restriction. The E3 ubiquitin-protein ligase neural precursor cell expressed developmentally downregulated gene 4-2 (NEDD4-2) is known to target NCC for proteasomal degradation. We found that short-term Mg2+ restriction did not lower tNCC abundance in inducible nephron-specific NEDD4-2 knockout mice. We next examined interactions with K+ restriction. tNCC and pNCC abundances were similar after short- or long-term Mg2+ or combined Mg2+-K+ restriction, but were dramatically lower compared with low K+ diet, suggesting that Mg2+ restriction overrides the effects of K+ restriction on NCC. After combined Mg2+-K+ restriction, adding back K+ alone to the diet had no effect on tNCC abundance, but adding back Mg2+ either at the same time or after K+ replenishment increased tNCC abundance. NEDD4-2 mediates degradation of the epithelial sodium channel (ENaC) during dietary K+ restriction so we next examined the effect of Mg2+ restriction on ENaC by performing amiloride response tests. Compared with normal diet the natriuretic effect of amiloride was strongly blunted after K+ restriction but not after Mg2+ restriction.
Together, these data suggest that NEDD4-2 mediates proteasomal degradation of NCC during Mg2+ restriction, Mg2+ restriction exerts differential effects on NCC and ENaC, and sustained NCC downregulation may enhance distal Na+ delivery during states of hypomagnesemia, maintaining hypokalemia.
- NIDDK Support