TRPM7 channels conduct metal cations such as Na+, K+, Ca2+ and Mg2+. In the presence of external Ca2+/Mg2+ TRPM7 has a steeply outwardly rectifying current-voltage (I-V) relation. In the absence of Ca2+/Mg2+ the IV becomes semi-linear. This has been explained by the removal of pore blockade by divalent cations (e.g. Ca2+o/Mg2+o). TRPM7 channels are inhibited by cytoplasmic Mg2+ in a voltage-independent manner, primarily by a reversible reduction in the overall number of conducting channels. Here, we have examined the consequences of external Ca2+ removal and reintroduction on TRPM7 current kinetics. In whole-cell patch clamp with low internal Mg2+, we rapidly and repeatedly exchanged 2 mM Ca2+ with divalent cation free (DVF) Na+ or Cs+ containing solutions to compare the time dependent changes in current. This maneuver resulted in a declining current in DVF following Ca2+ exposure, which we call inactivation. This was followed by a slowly rising current when Ca2+o was reintroduced. The decay of monovalent current occurred over 1-4 minutes and was monoexponential in most cases. Internal Mg2+ and spermine, partially inhibiting TRPM7, revealed more pronounced decay/potentiation cycles than seen with Mg2+-free internal solutions. Inactivation was observed with both Na+ and Cs+ as charge carriers and extracellular Mg2+ could substitute for the potentiating Ca2+ effect. Current decay in DVF was distinct from TRPM7 rundown, which we previously documented in patch-clamp recordings. Inactivation persisted in perforated-patch recordings, which allow the maintenance of physiological Mg2+ concentrations and prevent rundown. We have also characterized TRPM7 point mutations that abolish Ca2+ potentiation/inactivation. TRPM7 current behavior in response to switches between divalent-containing and divalent-free solutions resembled the Ca2+ potentiation/depotentiation described for calcium release activated calcium (CRAC) channels. We conclude that TRPM7 response to extracellular ions depends on intracellular Mg2+/spermine concentrations, presenting a novel case of inside-out signaling.
& Kozak, J. A.
(2018). TRPM7 Current Inactivation: Evidence for Inside-Out Signaling. Biophysical Journal, 114 (3), 642A, Supplement 1.