title: G-protein-mediated inhibition of the Trp channel TRPM1 requires the Gβγ dimer.
abstract: ON bipolar cells are critical for the function of the ON pathway in the visual system. They express a metabotropic glutamate receptor (mGluR6) that, when activated, couples to the G(o) class of G protein. The channel that is primarily responsible for the synaptic response has been recently identified as the transient receptor potential cation channel subfamily M member 1 (TRPM1); TRPM1 is negatively coupled to the mGluR6/Go cascade such that activation of the cascade results in closure of the channel. Light indirectly opens TRPM1 by reducing transmitter release from presynaptic photoreceptors, resulting in a decrease in mGluR6 activation. Conversely, in the dark, binding of synaptic glutamate to mGluR6 inhibits TRPM1 current. Closure of TRPM1 by G-protein activation in the dark is a critical step in the process of ON bipolar cell signal transduction, but the precise pathway linking these two events is not understood. To address this question, we measured TRPM1 activity in retinal bipolar cells, in human ependymal melanocytes (HEMs) that endogenously express TRPM1, and in HEK293 cells transfected with TRPM1. Dialysis of the Gβγ subunit dimer, but not Gα(o), closed TRPM1 channels in every cell type that we tested. In addition, activation of an endogenous G-protein-coupled receptor pathway in HEK293 cells that releases Gβγ without activating Go protein also closed TRPM1 channels. These results suggest a model in which the Gβγ dimer that is released as a result of the dissociation from Gα(o) upon activation of mGluR6 closes the TRPM1 channel, perhaps via a direct interaction.
authors.affiliation: Eye Center, Renmin Hospital of Wuhan University, Eye Institute of Wuhan University, Wuhan, China 430060. yinshen@whu.edu.cn
authors.forename: Yin
authors.initials: Y
authors.lastname: Shen
authors.forename: Melissa Ann F
authors.initials: MA
authors.lastname: Rampino
authors.forename: Reed C
authors.initials: RC
authors.lastname: Carroll
authors.forename: Scott
authors.initials: S
authors.lastname: Nawy
mesh_terms: D000596:Amino Acids / Q000494:pharmacology; D000818:Animals; D002211:Capsaicin / Q000031:analogs & derivatives / Q000494:pharmacology; D002478:Cells, Cultured; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000378:metabolism*; D044387:GTP-Binding Protein beta Subunits / Q000737:chemistry / Q000378:metabolism*; D044388:GTP-Binding Protein gamma Subunits / Q000737:chemistry / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D008027:Light; D008564:Membrane Potentials / Q000187:drug effects / Q000528:radiation effects; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D011485:Protein Binding / Q000187:drug effects / Q000528:radiation effects; D055503:Protein Multimerization; D018094:Receptors, Metabotropic Glutamate / Q000037:antagonists & inhibitors / Q000378:metabolism; D051245:Retinal Bipolar Cells / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D050053:TRPM Cation Channels / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D014966:Xanthenes / Q000494:pharmacology
chemical_list: D000596:Amino Acids; D018691:Excitatory Amino Acid Antagonists; D044387:GTP-Binding Protein beta Subunits; D044388:GTP-Binding Protein gamma Subunits; C114624:LY 341495; D018094:Receptors, Metabotropic Glutamate; D050053:TRPM Cation Channels; C116297:Trpm1 protein, mouse; D014966:Xanthenes; C113897:metabotropic glutamate receptor 6; D018698:Glutamic Acid; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; C071423:capsazepine; D002211:Capsaicin