Human CNNM2 is not a Mg(2+) transporter per se.

CNNM2 is associated with the regulation of serum Mg concentration, and when mutated, with severe familial hypomagnesemia. The function and cellular localization of CNNM2 and its isomorphs (Iso) remain controversial. The objective of this work was to examine the following: (1) the transcription-responsiveness of CNNM2 to Mg starvation, (2) the cellular localization of Iso1 and Iso2, (3) the ability of Iso1 and Iso2 to transport Mg(2+), and (4) the complex-forming ability and spectra of potential interactors of Iso1 and Iso2. The five main findings are as follows. (1) Mg-starvation induces CNNM2 overexpression that is markedly higher in JVM-13 cells (lymphoblasts) compared with Jurkat cells (T-lymphocytes). (2) Iso1 and Iso2 localize throughout various subcellular compartments in transgenic HEK293 cells overexpressing Iso1 or Iso2. (3) Iso1 and Iso2 do not transport Mg(2+) in an electrogenic or electroneutral mode in transgenic HEK293 cells overexpressing Iso1 or Iso2. (4) Both Iso1 and Iso2 form complexes of a higher molecular order. (5) The spectrum of potential interactors of Iso1 is ten times smaller than that of Iso2. We conclude that sensitivity of CNNM2 expression to extracellular Mg(2+) depletion depends on cell type. Iso1 and Iso2 exhibit a dispersed pattern of cellular distribution; thus, they are not exclusively integral to the cytoplasmic membrane. Iso1 and Iso2 are not Mg(2+) transporters per se. Both isomorphs form protein complexes, and divergent spectra of potential interactors of Iso1 and Iso2 indicate that each isomorph has a distinctive function. CNNM2 is therefore the first ever identified Mg(2+) homeostatic factor without being a Mg(2+) transporter per se.

Overexpression of Na+/Mg2+ exchanger SLC41A1 attenuates pro-survival signaling.

The Na+/Mg2+ exchanger SLC41A1 (A1), a key component of intracellular Mg homeostasis (IMH), is the major cellular Mg2+ efflux system, and its overexpression decreases [Mg2+]intracellular. IMH plays an important role in the regulation of many cellular processes, including cellular signaling. However, whether the overexpression of A1 and the consequent drop of [Mg2+]i impact on intracellular signaling is unknown. To examine the latter, we utilized dynamic mass redistribution (DMR) assay, PathScan® RTK signaling antibody (PRSA) array, confirmatory Western blot (WB) analyses of phosphorylation of kinases selected by PRSA, and mag-fura 2-assisted fast filter spectrometry (FFS). We demonstrate here that the overexpression of A1 quantitatively and qualitatively changes the DMR signal evoked by the application of PAR-1-selective activating peptide and/or by changing [Mg2+]extracellular in HEK293 cells. PRSA profiling of the phosphorylation of important signaling nodes followed by confirmatory WB has revealed that, in HEK293 cells, A1 overexpression significantly attenuates the phosphorylation of Akt/PKB on Thr308 and/or Ser473 and of Erk1/2 on Thr202/Tyr204 in the presence of 0 or 1 mM (physiological) Mg2+ in the bath solution. The latter is also true for SH-SY5Y and HeLa cells. Overexpression of A1 in HEK293 cells significantly lowers [Mg2+]i in the presence of [Mg2+]e = 0 or 1 mM. This correlates with the observed attenuation of prosurvival Akt/PKB - Erk1/2 signaling in these cells. Thus, A1 expression status and [Mg2+]e (and consequently also [Mg2+]i) modulate the complex physiological fingerprint of the cell and influence the activity of kinases involved in anti-apoptotic and, hence, pro-survival events in cells.