Do phosphatidylinositides modulate vertebrate phototransduction?

Abstract

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.