Molecular cloning and functional characterization of a novel receptor-activated TRP Ca2+ channel from mouse brain.
Characterization of mammalian homologues of Drosophila TRP proteins, which induce light-activated Ca2+ conductance in photoreceptors, has been an important clue to understand molecular mechanisms underlying receptor-activated Ca2+ influx in vertebrate cells. We have here isolated cDNA that encodes a novel TRP homologue, TRP5, predominantly expressed in the brain. Recombinant expression of the TRP5 cDNA in human embryonic kidney cells dramatically potentiated extracellular Ca2+-dependent rises of intracellular Ca2+ concentration ([Ca2+]i) evoked by ATP. These [Ca2+]i transients were inhibited by SK&F96365, a blocker of receptor-activated Ca2+ entry, and by La3+. Expression of the TRP5 cDNA, however, did not significantly affect [Ca2+]i transients induced by thapsigargin, an inhibitor of endoplasmic reticulum Ca2+-ATPases. ATP stimulation of TRP5-transfected cells pretreated with thapsigargin to deplete internal Ca2+ stores caused intact extracellular Ca2+-dependent [Ca2+]i transients, whereas ATP suppressed [Ca2+]i in thapsigargin-pretreated control cells. Furthermore, in ATP-stimulated, TRP5-expressing cells, there was no significant correlation between Ca2+ release from the internal Ca2+ store and influx of extracellular Ca2+. Whole-cell mode of patch-clamp recording from TRP5-expressing cells demonstrated that ATP application induced a large inward current in the presence of extracellular Ca2+. Omission of Ca2+ from intrapipette solution abolished the current in TRP5-expressing cells, whereas 10 nM intrapipette Ca2+ was sufficient to support TRP5 activity triggered by ATP receptor stimulation. Permeability ratios estimated from the zero-current potentials of this current were PCa:PNa:PCs = 14.3:1. 5:1. Our findings suggest that TRP5 directs the formation of a Ca2+-selective ion channel activated by receptor stimulation through a pathway that involves Ca2+ but not depletion of Ca2+ store in mammalian cells.
Okada
T
T
Department of Information Physiology, National Institute for Physiological Sciences, Okazaki 444, Japan.
Shimizu
S
S
Wakamori
M
M
Maeda
A
A
Kurosaki
T
T
Takada
N
N
Imoto
K
K
Mori
Y
Y
D000595:Amino Acid Sequence; D000818:Animals; D001921:Brain / Q000166:cytology / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000235:genetics* / Q000378:metabolism; D027682:Cation Transport Proteins; D003001:Cloning, Molecular; D003600:Cytosol / Q000378:metabolism; D018076:DNA, Complementary; D006801:Humans; D051379:Mice; D008807:Mice, Inbred BALB C; D008969:Molecular Sequence Data; D009474:Neurons / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D050052:TRPC Cation Channels
D015220:Calcium Channels; D027682:Cation Transport Proteins; D018076:DNA, Complementary; D012333:RNA, Messenger; D011994:Recombinant Proteins; D050052:TRPC Cation Channels; C491780:TRPC5 protein, human; C112021:Trpc5 protein, mouse; D002118:Calcium