Transient Receptor Potential Melastatin 7 Ion Channels Regulate Magnesium Homeostasis in Vascular Smooth Muscle Cells

Abstract

Magnesium modulates vascular smooth muscle cell (VSMC) function. However, molecular mechanisms regulating VSMC Mg2+ remain unknown. Using biochemical, pharmacological, and genetic tools, the role of transient receptor potential membrane melastatin 7 (TRPM7) cation channel in VSMC Mg2+ homeostasis was evaluated. Rat, mouse, and human VSMCs were studied. Reverse transcriptase polymerase chain reaction and immunoblotting demonstrated TRPM7 presence in VSMCs (membrane and cytosol). Angiotensin II (Ang II) and aldosterone increased TRPM7 expression. Gene silencing using small interfering RNA (siRNA) against TRPM7, downregulated TRPM7 (mRNA and protein). Basal [Mg2+]i, measured by mag fura-2AM, was reduced in siRNA-transfected cells (0.39+/-0.01 mmol/L) versus controls (0.54+/-0.01 mmol/L; P<0.01). Extracellular Mg2+ dose-dependently increased [Mg2+]i in control cells (Emax 0.70+/-0.02 mmol/L) and nonsilencing siRNA-transfected cells (Emax 0.71+/-0.04 mmol/L), but not in siRNA-transfected cells (Emax 0.5+/-0.01 mmol/L). The functional significance of TRPM7 was evaluated by assessing [Mg2+]i and growth responses to Ang II in TRPM7 knockdown cells. Acute Ang II stimulation decreased [Mg2+]i in control and TRPM7-deficient cells in a Na+-dependent manner. Chronic stimulation increased [Mg2+]i in control, but not in siRNA-transfected VSMCs. Ang II-induced DNA and protein synthesis, measured by 3[H]-thymidine and 3[H]-leucine incorporation, respectively, were increased in control and nonsilencing cells, but not in TRPM7 knockdown VSMCs. Our data indicate that VSMCs possess membrane-associated, Ang II-, and aldosterone-regulated TRPM7 channels, which play a role in regulating basal [Mg2+]i, transmembrane Mg2+ transport and DNA and protein synthesis. These novel findings identify TRPM7 as a functionally important regulator of Mg2+ homeostasis and growth in VSMCs.