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Mg2+ plays a vital role in platelet function, but despite implications for life-threatening conditions such as stroke or myocardial infarction, the mechanisms controlling [Mg2+]i in megakaryocytes (MKs) and platelets are largely unknown. Transient receptor potential melastatin-like 7 channel (TRPM7) is a ubiquitous, constitutively active cation channel with a cytosolic α-kinase domain that is critical for embryonic development and cell survival. Here we report that impaired channel function of TRPM7 in MKs causes macrothrombocytopenia in mice (Trpm7fl/fl-Pf4Cre) and likely in several members of a human pedigree that, in addition, suffer from atrial fibrillation. The defect in platelet biogenesis is mainly caused by cytoskeletal alterations resulting in impaired proplatelet formation by Trpm7fl/fl-Pf4Cre MKs, which is rescued by Mg2+ supplementation or chemical inhibition of non-muscle myosin IIA heavy chain activity. Collectively, our findings reveal that TRPM7 dysfunction may cause macrothrombocytopenia in humans and mice., Although Mg2+ is vital for platelet activation and aggregation, its regulation in these cells is still largely unknown. Here, the authors show that TRPM7, a cation channel and a protein kinase, regulates thrombopoiesis and platelet size by affecting the cytoskeleton of these cells in mice and humans.

Mg2+ is the second-most abundant cation in animal cells and is an essential cofactor in numerous enzymatic reactions. The molecular mechanisms controlling Mg2+ balance in the organism are not well understood. In this study, we report identification of TRPM7, a bifunctional protein containing a protein kinase fused to an ion channel, as a key regulator of whole body Mg2+ homeostasis in mammals. We generated TRPM7-deficient mice with the deletion of the kinase domain. Homozygous TRPM7Δkinase mice demonstrated early embryonic lethality, whereas heterozygous mice were viable, but developed signs of hypomagnesaemia and revealed a defect in intestinal Mg2+ absorption. Cells derived from heterozygous TRPM7Δkinase mice demonstrated reduced TRPM7 currents that had increased sensitivity to the inhibition by Mg2+. Embryonic stem cells lacking TRPM7 kinase domain displayed a proliferation arrest phenotype that can be rescued by Mg2+ supplementation. Our results demonstrate that TRPM7 is essential for the control of cellular and whole body Mg2+ homeostasis., Magnesium is an essential element of the diet and is a cofactor for many enzymes. In this study, the channel kinase TRPM7 is shown to be essential for magnesium homeostasis, and heterozygous mice lacking the kinase domain show a defect in absorption of magnesium from the diet.