The nonselective cation channel TRPM7 is unusual in that the C-terminal region contains a functional serine-threonine kinase domain. Knockout of TRPM7 is embryonic lethal in mice, and T cell–specific knockout results in a developmental block in most of the cells. However, a few of these TRPM7-deficient T cells escape the block. Desai et al. found that these TRPM7-deficient T cells were resistant to apoptosis triggered by T cell receptor restimulation, a process called activation-induced cell death, or by cross-linking of the cell death receptor Fas. Knockdown of TRPM7 in a T cell line also resulted in resistance to Fas-mediated cell death, suggesting that the difference in Fas-mediated apoptosis in the knockout T cells was not due to a developmental defect. Transfection of a kinase-inactivated mutant of TRPM7, but not of a channel-inactivated form, in a glioblastoma cell line (LN18) augmented Fas-mediated activation of caspase-3, a marker of apoptosis. Indeed, activation of Fas in LN18 cells resulted in the appearance of a C-terminal cleavage product of TRPM7, which contains the kinase domain, and in vitro cleavage assays revealed that TRPM7 was a substrate for caspase-8 and caspase-3. Mutation of the caspase consensus cleavage site Asp1510 abolished cleavage in vitro or in cells. LN18 cells transfected with various mutants of TRPM7 showed that the presence of the caspase-mediated cleavage site (and thus cleavage) and channel activity were necessary for TRPM7 to enhance Fas-mediated apoptosis. Electrophysiological analysis showed that the cleaved channel exhibited greater activity than the uncleaved channel. Experiments performed with the B cell line SKW6.4 suggested that TRPM7 contributed to the internalization of Fas, a process that has been proposed to contribute to apoptotic signaling: Knockdown of TRPM7 inhibited Fas internalization but did not prevent formation of the caspase-activation complex. Thus, release of the kinase domain of TRPM7 in response to caspase cleavage appears to enhance channel activity, which promotes the internalization of activated Fas and its apoptotic signaling.
B. N. Desai, G. Krapivinsky, B. Navarro, L. Krapivinsky, B. C. Carter, S. Febvay, M. Delling, A. Penumaka, I. S. Ramsey, Y. Manasian, D. E. Clapham, Cleavage of TRPM7 releases the kinase domain from the ion channel and regulates its participation in Fas-induced apoptosis. Dev. Cell 22, 1149–1162 (2012). [Online Journal]