Propuesta de indicadores para el nuevo ANE

15. Mishra NK, Peleg Y, Cirri E, Belogus T, Lifshitz Y, Voelker DR, Apell H-J, Garty H, Karlish SJD. FXYD proteins stabilize Na,K-ATPase: amplification of specific phosphatidylserine-protein interactions. J. Biol. Chem. 286: 9699–9712, 2011.

16. Richette P, Ayoub G, Lahalle S, Vicaut E, Badran A-M, Joly F, Messing B, Bardin T. Hypomagnesemia associated with chondrocalcinosis: a cross-sectional study. Arthritis Rheum. 57: 1496–1501, 2007.

17. Yang L, Frindt G, Palmer LG. Magnesium modulates ROMK channel-mediated potassium secretion. J. Am. Soc. Nephrol. 21: 2109–2116, 2010.

Abstract

Hypomagnesemia is a common clinical cause of muscle cramps, epilepsy and cardiac arrhythmias. Regulation of the body magnesium (Mg2+_{) balance is of critical importance} and is largely regulated by the kidney and the intestine. In the distal convoluted tubule (DCT) of the kidney, a transcellular reabsorption process determines the final urinary Mg2+ excretion. The basolateral Mg2+ _{extrusion mechanism in DCT is still unknown, but recent} findings suggest that proteins of the SLC41 family may contribute to cellular Mg2+ extrusion. The aim of this study was, therefore, to characterize the role of SLC41A3 in Mg2+ homeostasis using the Slc41a3 knockout mouse.

To this end, a tissue expression screening was performed by RT-PCR, showing that

Slc41a3 is the only SLC41 isoform with enriched expression in DCT compared to other

segments in the kidney. Interestingly, serum and urinary electrolyte determinations demonstrated that Slc41a3 knockout mice suffer from hypomagnesemia with renal Mg2+ wasting. Serum and urinary sodium, potassium and calcium levels were not affected. To determine the effect of SLC41A3 on intestinal Mg2+ _{absorption, the Mg}2+ _absorption capacity was measured using the stable 25_Mg2+ _isotope. 25_Mg2+ _{uptake was similar in wild} type and knockout mouse, although Slc41a3 knockout animals exhibited increased intestinal expression of Mg2+ _{transporters Trpm6 and Slc41a1. Remarkably, 10% of the}

Slc41a3 knockout mice developed severe unilateral hydronephrosis, as demonstrated by

the presence of transitional epithelium lining the fluid cavity. Feeding the Slc41a3 knockout mice a low Mg2+ _{diet may have instigated the formation of hydronephrosis.}

In conclusion, SLC41A3 was established as a new important factor for renal Mg2+ handling. In the future, SLC41A3 mutations should be considered in patients with unilateral hydronephrosis and/or hypomagnesemia.

Introduction

The distal convoluted tubule (DCT) is essential for magnesium (Mg2+_{) homeostasis (4).} Although only 10 % of the filtered Mg2+ _{is reabsorbed in this segment of the kidney, it} determines the final urinary Mg2+ _{excretion since no reabsorption takes place beyond this} segment. In the DCT, Mg2+ _{enters the cell via the Transient Receptor Potential Melastatin} type 6 (TRPM6) divalent cation channel (31), while the basolateral Mg2+ _extrusion mechanism remains to be identified. Disturbances in this latter process result in hypomagnesemia, which is characterized by a variety of clinical symptoms including cardiac arrhythmias, muscle cramps and fatigue (4). Additionally, drugs affecting the DCT such as epidermal growth factor receptor inhibitors, calcineurin inhibitors and diuretics may reduce blood Mg2+ _{levels (15). Therefore, understanding the molecular mechanism} underlying Mg2+ _{reabsorption in the DCT is of major importance for the treatment of} patients with hypomagnesemia.

In a recent transcriptome screening study, Solute carrier family 41 member 3 (Slc41a3) was identified as highly regulated by dietary Mg2+ _{intake (3). In this study, mice were fed} with various Mg2+_{-containing diets after which DCT segments were isolated. Using a} quantitative gene expression microarray, the Mg2+_{-sensitive genes in the DCT were} identified. Slc41a3 was among the genes with increased expression in response to a Mg2+_{-deficient diet.}

SLC41A3 was originally described by Quamme and colleagues as part of the solute carrier family 41 of putative Mg2+ _{transporters (22). Although SLC41A3 has never been} studied in detail, few studies have addressed the structure, regulation and function of its close homologue SLC41A1 (52 % identity at amino acid level)(12, 13, 17). There is controversy about the structure of SLC41A1; some groups support a 10 transmembrane domain structure, while others suggest 11 transmembrane domains with an extracellular C-terminus (17, 28). On the functional level, initial experiments in Xenopus laevis oocytes demonstrated Mg2+ _{currents for SLC41A1 and SLC41A3 using physiological Mg}2+ concentrations, proposing a channel-like function (5, 22). Indeed, SLC41 proteins posses conserved pore regions with the MgtE bacterial Mg2+ _{channel (9, 17, 32). In contrast, recent} experiments using Mg2+_{-sensitive fluorescent probes showed Na}+_{-dependent Mg}2+ transport, suggesting that SLC41A1 may be the long-sought Na+_/Mg2+_{-exchanger (13, 14).} Interestingly, an exon-skipping mutation in SLC41A1 resulted in a nephronophthisis-like phenotype in a patient eventually requiring renal transplantation (10). Whether SLC41A3 may have a similar function as SLC41A1 has never been examined. However, in contrast to

Slc41a3, Mg2+_{-sensitive regulation of Slc41a1 expression was not observed in the DCT} transcriptome study (3).

The aim of the present study was to characterize the role of SLC41A3 in renal and intestinal Mg2+ _{(re)absorption. Slc41a3 knockout mice (Slc41a3}-/-_{) were analyzed for ion} homeostasis and kidney development. Furthermore, by challenging the mice with low

Mg2+ _{diets the compensatory mechanisms in intestine, kidney and brain were examined} in detail. Using the 25_{Mg isotope, intestinal Mg}2+ _{uptake studies were performed to} determine the role of SLC41A3 in the intestine.

Results