TRPM7 Kinase Inactivation Results in Macrophage Alkalinization and Increased Phagocytic Activity

Transient receptor potential melastatin 7 (TRPM7) is a unique protein that is both an ion channel and a cytosolic protein kinase. Normally, TRPM7 channels are tonically inhibited by cytoplasmic Mg ²⁺ , polyamines and acidic pH. Channel basal activity can be increased by alkalinization or Mg ²⁺ depletion. We have used the TRPM7 kinase-dead (KD K1646R) mouse to investigate the consequences of kinase inactivation in immune cell function. It also serves as a convenient system to elucidate kinase-channel interactions at a functional level. CD4 and CD8 T lymphocytes isolated from the spleens of TRPM7 KD mice have defects in blastogenesis, proliferation and reduced store-operated calcium entry (SOCE) without a change in TRPM7 current magnitudes. Here we have investigated the effect of kinase inactivation in peritoneal and splenic macrophages. We examined phagocytosis using multiple approaches, finding that KD splenic macrophages phagocytize latex beads, zymosan particles and opsonized red blood cells more efficiently compared to WT. We observed somewhat higher basal cytosolic Ca ²⁺ levels in KD macrophages compared to WT, but no differences in SOCE were apparent. Addition of calcium ionophore ionomycin overnight did not affect phagocytosis rates. By contrast, we found that cytosolic pH was alkalinized in KD mouse cells. In order to test if the KD effects on phagocytosis and pH were linked, we incubated the macrophages in presence of sodium-hydrogen exchanger 1 (NHE1) blockers and found that both pH and phagocytosis returned to normal, WT, levels. Moreover, the basal TRPM7 channel activity (current magnitude at break-in) was also reduced. WT and KD channel sensitivity to Mg ²⁺ was similar. In summary, we have identified a new role for TRPM7 kinase as a regulator of cellular pH and phagocytosis.