Kidney Week

Abstract: SA-PO1025

Urinary Uromodulin Is Increased in Magnesium Deficiency and Stimulates Tubular Magnesium Absorption via TRPM6

Session Information

• Fluid and Electrolytes: Basic - II
  October 27, 2018 | Location: Exhibit Hall, San Diego Convention Center
  Abstract Time: 10:00 AM - 12:00 PM

Category: Fluid and Electrolytes

• 901 Fluid and Electrolytes: Basic

Authors

• Wolf, Matthias, University of Texas Southwestern Medical Center, Dallas, Texas, United States

Matthias Wolf,

• Bal, Manjot S., University of Texas Southwestern Medical Center, Dallas, Texas, United States

Manjot S. Bal,

• Rivera, Carolina, University of Texas Southwestern Medical Center, Dallas, Texas, United States

Carolina Rivera,

• Bindels, René J., Radboud University Medical Center, Nijmegen, Netherlands

René J. Bindels,

• Hoenderop, Joost, Radboud University Medical Center, Nijmegen, Netherlands

Joost Hoenderop,

• Marciano, Denise K., University of Texas Southwestern Medical Center, Dallas, Texas, United States

Denise K. Marciano,

• Nie, Mingzhu, University of Texas Southwestern Medical Center, Dallas, Texas, United States

Mingzhu Nie,

Background

Uromodulin (UMOD) is the most abundant urinary protein in humans. We describe a novel role for Uromodulin in renal magnesium homeostasis. Mild to moderate chronic hypomagnesemia occurs in up to 15% of the population and is associated with type 2 diabetes mellitus, hypertension, and chronic kidney disease.

Methods

Whole-animal experiments for urinary excretion of magnesium and UMOD, and response to furosemide were performed in nine months-old wild-type (WT) and Uromodulin knockout (Umod^-/-) mice. Kidneys from both strains were studied for qRT-PCR studies and immunofluorescent TRPM6 expression. Whole-cell patch-clamp recording and biotinylation assays were performed in HEK293 cells to determine TRPM6 whole-cell current density and cell surface abundance.

Results

Umod^-/- mice, compared to WT, excreted more urinary magnesium (WT 18 ± 3 μmol/24h vs. Umod^-/- 34 ± 5 μmol/24h; p<0.05) and displayed upregulation of genes promoting renal magnesium absorption, confirming magnesium deficiency in Umod^-/- mice. The thick ascending limb in both strains responded the same to furosemide, indicating appropriate function. Fine-tuning of magnesium absorption occurs in the distal convoluted tubule (DCT) via the apical magnesium channel TRPM6. We found decreased apical TRPM6 staining in the DCT of Umod^-/- mice. Applying in vitro biotinylation assays and whole-cell patch-clamp recording we found that UMOD enhanced TRPM6 cell surface abundance and current density from the extracellular space (42 ± 6 vs 196 ± 10 pA/pF for control vs UMOD; p<0.0001). Co-immunoprecipitation studies showed that UMOD physically interacted with TRPM6, and thereby impaired dynamin-dependent TRPM6 endocytosis. UMOD depended on the urinary lectin galectin-1 to form a lattice together with a novel extracellular TRPM6 N-glycan to increase TRPM6 current density. To examine if a low magnesium state could modify urinary UMOD secretion we fed WT mice with a low magnesium diet and detected almost two-fold increased urinary UMOD excretion compared to the same mice on a regular diet.

Conclusion

Our data suggest that increased urinary UMOD secretion in low magnesium states defends against further urinary magnesium losses through upregulation of TRPM6 cell surface abundance by impairing TRPM6 endocytosis.

Funding

• NIDDK Support