Magnesium is an essential cofactor in many cellular processes, and aberrations in magnesium homeostasis can have life-threatening consequences. The kidney plays a central role in maintaining serum magnesium within a narrow range (0.70 to 1.10 mmol/L). Along the proximal tubule and thick ascending limbs, magnesium reabsorption occurs via paracellular pathways. Members of the claudin family form the magnesium pores in these segments, and also regulate magnesium reabsorption by adjusting the transepithelial voltage that drives it. Along the distal convoluted tubule transcellular reabsorption via heteromeric TRPM6/7 channels predominates, though paracellular reabsorption may also occur. In this segment, the NaCl cotransporter plays a critical role in determining transcellular magnesium reabsorption. While the general machinery involved in renal magnesium reabsorption has been identified by studying genetic forms of magnesium imbalance, the mechanisms regulating it are poorly understood. This review discusses pathways of renal magnesium reabsorption by different segments of the nephron, emphasizing newer findings that provide insight into regulatory process, and outlining critical unanswered questions.

Background: Mutations in the ubiquitin ligase scaffold protein Cullin 3 (CUL3) cause the disease Familial Hyperkalemic Hypertension (FHHt). In the kidney, mutant CUL3 (CUL3-Δ9) increases abundance of With-No-Lysine [K] Kinase 4 (WNK4), inappropriately activating Sterile 20/SPS-1-related proline/alanine-rich kinase (SPAK), which then phosphorylates and hyperactivates the Na+-Cl- cotransporter (NCC). The precise mechanism by which CUL3-Δ9 causes FHHt has been unclear. We tested the hypothesis that reduced abundances of CUL3 and of Kelch-like 3 (KLHL3), the CUL3 substrate adaptor for WNK4, are mechanistically important. Since JAB1, an enzyme that inhibits CUL3 activity by removing the ubiquitin-like protein NEDD8, cannot interact with CUL3-Δ9, we also determined whether Jab1 disruption mimicked the effects of CUL3-Δ9 expression. Methods: We used an inducible renal tubule-specific system to generate several mouse models expressing CUL3-Δ9, mice heterozygous for both CUL3 and KLHL3 (Cul3+/−/Klhl3+/−), and mice with short-term Jab1 disruption (to avoid renal injury associated with long-term disruption). Results: Renal KLHL3 was higher in Cul3−/− mice, but lower in Cul3−/−/Δ9 mice and in the Cul3+/−/Δ9 FHHt model, suggesting KLHL3 is a target for both WT and mutant CUL3. Cul3+/−/Klhl3+/− mice displayed increased WNK4-SPAK activation and phospho-NCC abundance, and an FHHt-like phenotype with increased plasma [K+] and salt-sensitive blood pressure. Short-term Jab1 disruption in mice lowered abundances of CUL3 and KLHL3, and increased abundances of WNK4 and phospho-NCC. Conclusions:Jab1-/- mice and Cul3+/−/Klhl3+/− mice recapitulated the effects of CUL3-Δ9 expression on WNK4-SPAK-NCC. Our data suggest that degradation of both KLHL3 and CUL3 plays a central mechanistic role in CUL3-Δ9-mediated FHHt.

Background: MR (mineralocorticoid receptor) antagonists are recommended for patients with resistant hypertension even when circulating aldosterone levels are not high. Although aldosterone activates MR to increase epithelial sodium channel (ENaC) activity, glucocorticoids also activate MR but are metabolized by 11βHSD2 (11β-hydroxysteroid dehydrogenase type 2). 11βHSD2 is expressed at increasing levels from distal convoluted tubule (DCT) through collecting duct. Here, we hypothesized that MR maintains ENaC activity in the DCT2 and early connecting tubule in the absence of aldosterone. Methods: We studied AS (aldosterone synthase)-deficient (AS −/− ) mice, which were backcrossed onto the same C57BL6/J strain as kidney-specific MR knockout (KS-MR −/− ) mice. KS-MR −/− mice were used to compare MR expression and ENaC localization and cleavage with AS −/− mice. Results: MR was highly expressed along DCT2 through the cortical collecting duct (CCD), whereas no 11βHSD2 expression was observed along DCT2. MR signal and apical ENaC localization were clearly reduced along both DCT2 and CCD in KS-MR −/− mice but were fully preserved along DCT2 and were partially reduced along CCD in AS −/− mice. Apical ENaC localization and ENaC currents were fully preserved along DCT2 in AS −/− mice and were not increased along CCD after low salt. AS −/− mice exhibited transient Na + wasting under low-salt diet, but administration of the MR antagonist eplerenone to AS −/− mice led to hyperkalemia and decreased body weight with higher Na + excretion, mimicking the phenotype of MR −/− mice. Conclusions: Our results provide evidence that MR is activated in the absence of aldosterone along DCT2 and partially CCD, suggesting glucocorticoid binding to MR preserves sodium homeostasis along DCT2 in AS −/− mice.

The genetic disease Gitelman syndrome, knockout mice, and pharmacological blockade with thiazide diuretics have revealed that reduced activity of the NaCl cotransporter (NCC) promotes renal Mg2+ wasting. NCC is expressed along the distal convoluted tubule (DCT), and its activity determines Mg2+ entry into DCT cells through transient receptor potential channel subfamily M, member 6 (TRPM6). Several other genetic forms of hypomagnesemia lower the drive for Mg2+ entry by inhibiting activity of the basolateral Na+-K+-ATPase, and reduced NCC activity may do the same. Lower intracellular Mg2+ may promote further Mg2+ loss by directly decreasing activity of the Na+-K+-ATPase. Lower intracellular Mg2+ may also lower Na+-K+-ATPase indirectly by downregulating NCC. Lower NCC activity also induces atrophy of DCT cells, decreasing the available number of TRPM6 channels. Conversely, a mouse model with increased NCC activity was recently shown to display normal Mg2+ handling. Moreover, recent studies have identified calcineurin and uromodulin (UMOD) as regulators of both NCC and Mg2+ handling by the DCT. Calcineurin inhibitors paradoxically cause hypomagnesemia in a state of NCC activation, but this may be related to direct effects on TRPM6 gene expression. In Umod−/− mice, the cause of hypomagnesemia may be partly due to both decreased NCC expression and lower TRPM6 expression on the cell surface. This mini-review discusses these new findings, and the possible role of altered Na+ flux through NCC and ultimately the Na+-K+-ATPase in Mg2+ reabsorption by the DCT.

Edited by Joel M. Gottesfeld This study was supported by Grants-in-Aid for Scientific Research (KAKENHI) 17H06416, 15K21706, and 26460099 from the Ministry of Education, Culture, Sports, Science, and Technology, Japan and also supported by the Takeda Science Foundation. The authors declare that they have no conflicts of interest with the contents of this article. This article contains Table S1 and Figs. S1-S6.