Background
Magnesium is the second most abundant cation within the cell and plays an important role in many intracellular biochemical functions [1]. Despite the abundance and importance of magnesium, little is known about how eukaryotic cells regulate their magnesium content.
Intracellular free Mg2+ concentration is in the order of 0.5 mM which is 1–2% of the total cellular magnesium [2]. Accordingly, intracellular Mg2+ is maintained below the concentration predicted from the transmembrane electrochemical potential. Intracellular Mg2+ concentration is finely regulated likely by precise controls of Mg2+ entry, Mg2+ efflux, and intracellular storage compartments [3]. The transporters comprising these pathways have only begun to be identified.
Few magnesium transporters have been identified at the molecular level. Schweyen and colleagues have demonstrated that the mitochondrial RNA splicing2 (Mrs2) gene encodes a protein that is present in yeast and mammalian inner mitochondrial membranes [4,5]. Mrs2 mediates high capacity Mg2+ influx in isolated yeast mitochondria driven by the inner membrane potential but also transports a range of divalent cations such as Ni2+, Co2+, and Cu2+ [6]. Overexpression of Mrs2 increases influx while deletion of the gene abolishes uptake suggesting that it is the major mitochondrial system. This data suggests that Mrs2 protein may mediate Mg2+ transport in mammalian mitochondria. Nadler et al first identified TRPM7, a widely expressed member of the transient receptor potential melastatin (TRPM) ion channel family, that produces a Mg2+ current in a wide variety of cells [7]. TRPM7 is regulated by intracellular Mg·ATP levels and is similarly permeable to both major divalent cations, Ca2+ and Mg2+, but also many of the trace elements, such as Zn2+, Mn2+, and Co2+ [8]. Using a positional cloning approach, Schlingmann et al [9] and Walder et al [10] found that hypomagnesemia with secondary hypocalcemia (HSH) was caused by mutations in TRPM6, a new member of the TRPM family. HSH is an inherited disease affecting both intestinal and renal Mg2+ absorption [3]. The functional characteristics of the TRPM6 transporter have not been fully elucidated [11,12]. Other magnesium transporters have been functionally described but they have not been characterized at the molecular level [13-18]. It is disparaging that, despite the significance of cellular Mg2+, only three specific magnesium transporters have been described in mammalian cells to date.
Mammalian magnesium homeostasis is a balance of epithelial intestinal magnesium absorption and renal magnesium excretion. The kidney plays a major role in control of vertebrate magnesium balance, in part, by active magnesium transport within the distal tubule of the nephron [2]. Using the Madin-Darby canine kidney (MDCK) cell line obtained from canine distal tubules and immortalized mouse distal convoluted tubule cells (MDCT), we have shown that Mg2+ entry is through specific and regulated magnesium pathways that are controlled by a variety of hormonal influences [19]. However these hormones do not provide selective control as they also affect calcium and in some cases sodium and potassium transport [19]. Selective and sensitive control of cellular Mg2+ transport is regulated by intrinsic mechanisms so that culture in media containing low magnesium results in upregulation of Mg2+ uptake in these cells. This adaptive increase in Mg2+ entry was shown to be dependent on de novo transcription since prior treatment of the epithelial cells with actinomycin D prevented the adaptation to low extracellular magnesium [20]. The data suggest that epithelial cells can somehow sense the environmental magnesium and through transcription- and translation-dependent processes alter Mg2+ transport and maintain magnesium balance. These conclusions using isolated epithelial cells are consonant with our views of magnesium conservation in the intact kidney [2].
In an attempt to identify genes underlying cellular changes resulting from adaptation to low extracellular magnesium, we used oligonucleotide microarray analysis to screen for magnesium-regulated transcripts in epithelial cells. This approach revealed one transcript whose relative level was dramatically altered by extracellular magnesium. Thus, this transcript potentially represented a species of mRNA whose synthesis was regulated by changes in cell magnesium. In this study, we describe the identification and characterization of this novel transcript referred to as MagT1. Our data indicate MagT1 may mediate Mg2+ transport in a wide variety of cells and may play a role in control of cellular magnesium homeostasis.