Do phosphatidylinositides modulate vertebrate phototransduction?
Mammalian rod cyclic nucleotide gated (CNG) channels (i.e., alpha plus beta subunits) are strongly inhibited by phosphatidylinositol 4, 5-bisphosphate (PIP(2)) when they are expressed in Xenopus oocytes and studied in giant membrane patches. Cytoplasmic Mg-ATP inhibits CNG currents similarly, and monoclonal antibodies to PIP(2) reverse the effect and hyperactivate currents. When alpha subunits are expressed alone, PIP(2) inhibition is less strong; olfactory CNG channels are not inhibited. In giant patches from rod outer segments, inhibition by PIP(2) is intermediate. Other anionic lipids (e.g., phosphatidyl serine and phosphatidic acid), a phosphatidylinositol-specific phospholipase C, and full-length diacylglycerol have stimulatory effects. Although ATP also potently inhibits cGMP-activated currents in rod patches, the following findings indicate that ATP is used to transphosphorylate GMP, generated from cGMP, to GTP. First, a phosphodiesterase (PDE) inhibitor, Zaprinast, blocks inhibition by ATP. Second, inhibition can be rapidly reversed by exogenous regulator of G-protein signaling 9, suggesting G-protein activation by ATP. Third, the reversal of ATP effects is greatly slowed when cyclic inosine 5'-monophosphate is used to activate currents, as expected for slow inosine 5' triphosphate hydrolysis by G-proteins. Still, other results remain suggestive of regulatory roles for PIP(2). First, the cGMP concentration producing half-maximal CNG channel activity (K(1/2)) is decreased by PIP(2) antibody in the presence of PDE inhibitors. Second, the activation of PDE activity by several nucleotides, monitored electrophysiologically and biochemically, is reversed by PIP(2) antibody. Third, exogenous PIP(2) can enhance PDE activation by nucleotides.
Womack
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Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75235-9040, USA.
Gordon
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Wensel
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Hilgemann
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D000255:Adenosine Triphosphate / Q000494:pharmacology* / Q000502:physiology; D000818:Animals; D002417:Cattle; D006152:Cyclic GMP / Q000378:metabolism; D007289:Cyclic IMP / Q000378:metabolism; D054815:Cyclic Nucleotide-Gated Cation Channels; D019852:Diacylglycerol Kinase / Q000494:pharmacology / Q000502:physiology; D006160:Guanosine Triphosphate / Q000494:pharmacology* / Q000502:physiology; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000494:pharmacology* / Q000502:physiology; D010770:Phosphotransferases / Q000494:pharmacology / Q000502:physiology; D020710:RGS Proteins / Q000494:pharmacology* / Q000502:physiology; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects* / Q000502:physiology; D014785:Vision, Ocular / Q000187:drug effects* / Q000502:physiology; D014981:Xenopus
D054815:Cyclic Nucleotide-Gated Cation Channels; D007473:Ion Channels; D019269:Phosphatidylinositol 4,5-Diphosphate; D020710:RGS Proteins; C116925:regulator of g-protein signaling 9; D007289:Cyclic IMP; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D010770:Phosphotransferases; D019852:Diacylglycerol Kinase; D006152:Cyclic GMP