Molecular and functional characterization of a novel mouse transient receptor potential protein homologue TRP7. Ca²⁺-permeable cation channel that is constitutively activated and enhanced by stimulation of g protein- coupled receptor

Characterization of mammalian homologues of Drosophila transient receptor potential protein (TRP) is an important clue to understand molecular mechanisms underlying Ca²⁺ influx activated in response to stimulation of G(q) protein-coupled receptors in vertebrate cells. Here we have isolated cDNA encoding a novel seventh mammalian TRP homologue, TRP7, from mouse brain. TRP7 showed abundant RNA expression in the heart, lung, and eye and moderate expression in the brain, spleen, and testis. TRP7 recombinantly expressed in human embryonic kidney cells exhibited distinctive functional features, compared with other TRP homologues. Basal influx activity accompanied by reduction in Ca²⁺ release from internal stores was characteristic of TRP7-expressing cells but was by far less significant in cells expressing TRP3, which is structurally the closest to TRP7 in the TRP family. TRP7 induced Ca²⁺ influx in response to ATP receptor stimulation at ATP concentrations lower than those necessary for activation of TRP3 and for Ca²⁺ release from the intracellular store, which suggests that the TRP7 channel is activated independently of Ca²⁺ release. In fact, TRP7 expression did not affect capacitative Ca²⁺ entry induced by thapsigargin, whereas TRP7 greatly potentiated Mn²⁺ influx induced by diacylglycerols without involvement of protein kinase C. Nystatin-perforated and conventional whole-cell patch clamp recordings from TRP7-expressing cells demonstrated the constitutively activated and ATP-enhanced inward cation currents, both of which were initially blocked and then subsequently facilitated by extracellular Ca²⁺ at a physiological concentration. Impairment of TRP7 currents by internal perfusion of the Ca²⁺ chelator 1,2-bis(2- aminophenoxy)ethane-N,N,N',V'-tetraacetic acid revealed an essential role of intracellular Ca²⁺ in activation of TRP7, and their potent activation by the diacylglycerol analogue suggests that the TRP7 channel is a new member of diacylglycerol-activated cation channels. Relative permeabilities indicate that TRP7 is slightly selective to divalent cations. Thus, our findings reveal an interesting correspondence of TRP7 to the background and receptor stimulation-induced cation currents in various native systems.