Detection of inositol 1,4,5-trisphosphate kinase in retina. A direct demonstration of phosphorylation of inositol 1,4,5-trisphosphate by ATP

Inositol-1,4,5-trisphosphate receptors in the vertebrate retina

Evidence has shown an activation of phosphatidylinositol 4,5-bisphosphate (PIP2) specific phospholipase C (PtdIns-PLC) by light in the vertebrate retina and rod outer segments (ROS), suggesting important roles for its two metabolites, 1,2-diacylglycerol (DG) and inositol-1,4,5-trisphosphate [Ins(1,4,5)P3]. DG activates protein kinase C (PKC) and Ins(1,4,5)P3 releases bound intracellular calcium. Since Ca2+ plays an important role in light adaptation, the presence of Ins(1,4,5)P3 receptors in ROS may indicate a regulatory role of Ins(1,4,5)P3 to the free Ca2+ content. In the present study, we investigated the Ins(1,4,5)P3 receptors in whole retinal membranes and several subcellular fractions prepared from bovine retinas. Scatchard analyses of binding data for retinal membrane preparations showed a single, high-affinity binding site with equilibrium dissociation constant (Kd) of 24 +/- 2 nM and maximal binding capacity (Bmax) of 353 +/- 15 fmol/mg protein at pH 7.4. Specific binding was found in both small and large synaptosomal preparations representing inner and outer plexiform layers, respectively. A detectable, but low abundance of Ins(1,4,5)P3-specific binding in ROS was observed at both pH 7.4 and 8.3, but no specific binding of Ins(1,4,5)P3 was found in isolated outer segment discs. The binding of Ins(1,4,5)P3 in ROS was reduced by addition of ATP, suggesting a regulatory role for this nucleotide. Addition of calcium, sodium, and potassium ions also reduced specific binding of Ins(1,4,5)P3. Immunocytochemical studies indicate intense staining in the inner segment and extending to the ROS. Inner and outer plexiform layers were also stained. These findings show that the Ins(1,4,5)P3 receptor is present in photoreceptor cells and inner and outer plexiform layers in the vertebrate retina.

Li(+)-induced structural changes of synaptic ribbons are related to the phosphoinositide metabolism in photoreceptor synapses

Synaptic ribbons are specialized cytoskeletal components of the presynaptic exocytotic machinery in photoreceptors. In cone photoreceptors, these structures are highly dynamic disappearing during darkness and reforming in the light phase. In this study we wanted to characterize the second messengers involved in the regulation of these cyclic changes. We show that synaptic ribbons in cone photoreceptors are very sensitive to both Li+ and inositol 1,4,5-trisphosphate suggesting that inositol polyphosphates might play a physiological role in the disassembly of synaptic ribbons. The presence of the phosphoinositide pathway was demonstrated in isolated photoreceptor synapses. The phosphoinositide metabolism in photoreceptor synapses was dark-activated and correlated with the disappearance of synaptic ribbons during dark-adaptation. Since Li+ and inositol 1,4,5-trisphosphate only influenced synaptic ribbons in cones but not in rods the dark-activated phosphoinositide metabolism should be largely ascribed to cone synapses.

Ca2+/calmodulin independent inositol 1,4,5-trisphosphate 3-kinase activity in guinea pig peritoneal macrophages

1. The Ca2+/calmodulin (CaM) independent activity of inositol 1,4,5-trisphosphate (InsP3) 3-kinase in macrophages could be separated from the dependent activity by serial column chromatography, gel filtration, Orange A and DEAE-5PW. 2. An InsP3 analog which has an aminobenzoyl group on the 2nd carbon of the inositol ring inhibited the conversion of [3H]InsP3 to [3H]InsP4 (inositol 1,3,4,5-tetrakisphosphate) in a dose-dependent manner. The concentration required for half-maximal inhibition (IC50) with the Ca2+/CaM independent enzyme activity was also dependent on the free Ca2+ concentration, as with the dependent activity. 3. These results suggest that a conformational change in the enzyme occurs in response to a change in free Ca2+ concentration, and thus the potency to recognize the InsP3 analog would change, even when the Ca2+/CaM independent enzyme activity was used.