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1 Cell Physiology, National Institute for Physiological Sciences, Okazaki , Aichi, Japan
* To whom correspondence should be addressed. E-mail: okada{at}nips.ac.jp.
Stretch- and swelling-activated cation (SSAC) channels play essential roles not only in sensing and transducing external mechanical stresses but also in regulating cell volume in living cells. However, the molecular nature of SSAC channel has not been clarified. In human epithelial HeLa cells, single-channel recordings in cell-attached and inside-out patches revealed expression of a Mg2+- and Gd3+-sensitive non-selective cation channel which is exquisitely sensitive to membrane stretch. Whole-cell recordings revealed that the macroscopic cationic currents exhibit TRPM7-like properties such as outward rectification and sensitivity to Mg2+ and Gd3+. The whole-cell cation current was augmented by osmotic cell swelling. RT-PCR and Western blotting demonstrated molecular expression of TRPM7 in HeLa cells. Treatment with small interfering RNA (siRNA) targeted against TRPM7 led to abolition of single stretch-activated cation channel currents and of swelling-activated, whole-cell cation currents in HeLa cells. The silencing of TRPM7 by siRNA reduced the rate of cell volume recovery after osmotic swelling. A similar inhibition of the regulatory volume decrease (RVD) was also observed when extracellular Ca2+ was removed or Gd3+ was applied. It is thus concluded that TRPM7 represents the SSAC channel endogenously expressed in HeLa cells and that, by serving as a swelling-induced Ca2+ influx pathway, it plays an important role in cell volume regulation.
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