Do phosphatidylinositides modulate vertebrate phototransduction?

Mammalian rod cyclic nucleotide gated (CNG) channels (i.e., α plus β subunits) are strongly inhibited by phosphatidylinositol 4,5-bisphosphate (PIP₂) when they are expressed in Xenopus oocytes and studied in giant membrane patches. Cytoplasmic Mg-ATP inhibits CNG currents similarly, and monoclonal antibodies to PIP₂ reverse the effect and hyperactivate currents. When α subunits are expressed alone, PIP₂ inhibition is less strong; olfactory CNG channels are not inhibited. In giant patches from rod outer segments, inhibition by PIP₂ 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 PIP₂. First, the cGMP concentration producing half-maximal CNG channel activity (K(1/2)) is decreased by PIP₂ antibody in the presence of PDE inhibitors. Second, the activation of PDE activity by several nucleotides, monitored electrophysiologically and biochemically, is reversed by PIP₂ antibody. Third, exogenous PIP₂ can enhance PDE activation by nucleotides.