In this study, we demonstrate that cells lacking the regulator of Mg2+ homeostasis, TRPM7, increase gene expression of the Mg2+ transporter MagT1, indicating that MagT1 function might be beneficial in situations of TRPM7-deficiency. This notion is further supported by our results showing that overexpression of MagT1 enables TRPM7−/− cells to regain viability and some growth under normal levels of Mg2+. This is in contrast to TRPM7−/− cells that require extracellular Mg2+ levels an order of magnitude higher than physiologic concentrations.
Our findings about MagT1
gene expression regulation are reminiscent of work done in HEK and MDCT cells where MagT1
expression was also upregulated in response to hypomagnesia [15
]. Together with our data, these studies suggest that when cells are sensing suboptimal extracellular Mg2+
, they respond by transcriptional upregulation of MagT1,
probably in an effort to avoid intracellular Mg2+
deficiency by increasing Mg2+
uptake. Importantly, TRPM7 itself is not required to mediate this hypomagnesia-induced MagT1
upregulation given that this takes place in DT40 cells deprived of TRPM7. Since TRPM7 is described as a master regulator of Mg2+
regulation, it could be required to sense environmental Mg2+
availability and elicit upregulation of cellular Mg2+
transport capacity, but this appears not to be the case for MagT1
. From our review of the literature, MagT1
gene expression does not appear to be uniformly regulated in all tissues. Other studies have shown that in low concentrations of Mg2+
, mammary epithelial MagT1
expression remains unchanged [20
], while rumen epithelial cells actually downregulate MagT1
]. These results imply that there may be differential regulation of Mg2+
and its transport machinery in various cell types.
We observed that overexpression of MagT1 in TRPM7−/−
cells is capable of augmenting growth and increasing cytosolic Mg2+
recovery in the absence of supplemental Mg2+
. This suggests that MagT1 enhances Mg2+
accumulation in TRPM7−/−
cells, and supports the role of MagT1 as a Mg2+
influx system. These results further establish TRPM7−/−
DT40 cells as a valid model system to test functionality of Mg2+
transport pathways. This experimental approach has been used previously to demonstrate that SLC41A2, another putative Mg2+
transporter, can indeed function as such. Similarly to MagT1, SLC41A2 restores partial growth of the TRPM7−/−
cells under normagnesic conditions [21
]. In the past, we have also used this system to demonstrate that TRPM7’s closest homologue and only other known channel-kinase fusion, TRPM6, cannot functionally compensate for TRPM7 in DT40s [19
]. Together, the only partial TRPM7-complementations reported with either SLC41A2 or MagT1 suggest that TRPM7 remains essential for DT40s to proliferate and accrue Mg2+
when grown in physiological concentrations of Mg2+
. Future studies will need to determine whether overexpression of multiple Mg2+
transporters in the absence of TRPM7 can lead to a complete rescue of the growth phenotype without supplemental Mg2+
. In summary, our data suggests that MagT1 is an integral component of Mg2+
uptake in TRPM7−/−
- We analyzed the role of the Mg2+ transporter MagT1 in DT40 B cells lacking the Mg2+ regulator TRPM7.
- Gene expression of MagT1 is upregulated in TRPM7−/− cells.
- Overexpressing MagT1 in TRPM7−/− cells partially rescues their growth in the absence of excess Mg2+.
- Overexpressing MagT1 in TRPM7−/− cells partially rescues their ability to uptake Mg2+.