TRPM7 channels are activated by cell swelling and involved in the cell volume regulation

Abstract

In animal cells, it is known that under hypoosmotic conditions, an intracellular Ca²⁺ increase initially occurs followed by a regulatory volume decrease (RVD) which is attained by effluxes of K⁺, Cl⁻ and organic osmolytes and the resulting extraction of osmotically obliged water. The present study was performed to identify the volume-regulatory Ca²⁺ influx pathway under hypoosmotic conditions in HeLa cells. Whole-cell recordings showed that osmotic swelling activates TRPM7-like cation currents. The currents showed outward rectification and sharp sensitivity to Mg²⁺, Gd³⁺, SKF96365 and ruthenium red (RR). These TRPM7 channel blockers also inhibited the intracellular Ca²⁺ increase in response to osmotic swelling and the following RVD. RT-PCR studies demonstrated expression of TRPM7 mRNA in HeLa cells. The siRNA silencing of TRPM7 significantly suppressed not only expression of TRPM7 mRNA but also whole-cell TRPM7-like channel currents, Ca²⁺ influx and RVD in HeLa cells upon a hypotonic challenge. Thus, we conclude that the endogenous activity of swelling-activated cation channel exhibits the properties identical to the hallmark biophysical and pharmacological features of TRPM7, and that the TRPM7 channel plays an important role in the RVD process by serving as the volume-regulatory Ca²⁺ influx pathway in the human epithelial cells. [Jpn J Physiol 55 Suppl:S76 (2005)]