Higher Expression of TRPM7 Channels in Murine Mature B Lymphocytes than Immature Cells

Higher Expression of TRPM7 Channels in Murine Mature B Lymphocytes than Immature Cells

TRPM7, a cation channel protein permeable to various metal ions such as Mg<SUP>2&#8290;</SUP>, is ubiquitously expressed in variety of cells including lymphocytes. The activity of TRPM7 is tightly regulated by intracellular Mg<SUP>2&#8290;</SUP>, thus named Mg<SUP>2&#8290;</SUP>-inhibited cation (MIC) current, and its expression is known to be critical for the viability and proliferation of B lymphocytes. In this study, the level of MIC current was compared between immature (WEHI-231) and mature (Bal-17) B lymphocytes. In both cell types, an intracellular dialysis with Mg<SUP>2&#8290;</SUP>-free solution (140 mM CsCl) induced an outwardly-rectifying MIC current. The peak amplitude of MIC current and the permeability to divalent cation (Mn<SUP>2&#8290;</SUP>) were several fold higher in Bal-17 than WEHI-231. Also, the level of mRNAs for TRPM7, a molecular correspondence of the MIC channel, was significantly higher in Bal-17 cells. The amplitude of MIC was further increased, and the relation between current and voltage became linear under divalent cation-free conditions, demonstrating typical properties of the TRPM7. The stimulation of B cell receptors (BCR) by ligation with antibodies did not change the amplitude of MIC current. Also, increase of extracellular [Mg<SUP>2&#8290;</SUP>]<SUB>c</SUB> to enhance the Mg<SUP>2&#8290;</SUP> influx did not affect the BCR ligation-induced death of WEHI-231 cells. Although the level of TRPM7 was not directly related with the cell death of immature B cells, the remarkable difference of TRPM7 might indicate a fundamental change in the permeability to divalent cations during the development of B cells.