Phosphatidyl inositol-phospholipase C in squid photoreceptor membrane is activated by stable metarhodopsin via GTP-binding protein, Gq

Signaling roles of phosphoinositides in the retina

The field of phosphoinositide signaling has expanded significantly in recent years. Phosphoinositides (also known as phosphatidylinositol phosphates or PIPs) are universal signaling molecules that directly interact with membrane proteins or with cytosolic proteins containing domains that directly bind phosphoinositides and are recruited to cell membranes. Through the activities of phosphoinositide kinases and phosphoinositide phosphatases, seven distinct phosphoinositide lipid molecules are formed from the parent molecule, phosphatidylinositol. PIP signals regulate a wide range of cellular functions, including cytoskeletal assembly, membrane budding and fusion, ciliogenesis, vesicular transport, and signal transduction. Given the many excellent reviews on phosphoinositide kinases, phosphoinositide phosphatases, and PIPs in general, in this review, we discuss recent studies and advances in PIP lipid signaling in the retina. We specifically focus on PIP lipids from vertebrate (e.g., bovine, rat, mouse, toad, and zebrafish) and invertebrate (e.g., Drosophila, horseshoe crab, and squid) retinas. We also discuss the importance of PIPs revealed from animal models and human diseases, and methods to study PIP levels both in vitro and in vivo. We propose that future studies should investigate the function and mechanism of activation of PIP-modifying enzymes/phosphatases and further unravel PIP regulation and function in the different cell types of the retina.

Identification and characterization of a protostome homologue of peropsin from a jumping spider

Peropsin, a member of the opsin family, has characteristics of two functionally distinct opsin-groups, that is, amino acid residues conserved among opsins for light-sensing and a retinal-photoisomerase-like molecular property. Although such a bilateral feature of peropsin seems to be important for understanding the diversity of the opsin family, previous studies have been limited to higher deuterostome, vertebrate and amphioxus peropsins. Here, we report a protostome peropsin homologue from a jumping spider. We found a spider opsin that shares amino acid homology and conserved amino acid residues with known peropsins. The spider opsin-based pigment heterologously expressed in cultured cells exhibited photoisomerase-like isomerization characteristics and a bistable nature. Based on the characteristics of both the amino acid homology and its photochemical properties, we concluded that the spider opsin is the first protostome peropsin homologue. These results show that peropsin existed before the deuterostome-protostome split like other members of the opsin family. In addition, the spider peropsin was localized to non-visual cells in the retina, and fluorescence from reduced retinal chromophore was also observed in the region where peropsin was localized. These findings provide the first demonstration that the peropsin can form a photosensitive pigment in vivo and underlie non-visual function.

Jellyfish vision starts with cAMP signaling mediated by opsin-G(s) cascade

Light sensing starts with phototransduction in photoreceptor cells. The phototransduction cascade has diverged in different species, such as those mediated by transducin in vertebrate rods and cones, by G(q)-type G protein in insect and molluscan rhabdomeric-type visual cells and vertebrate photosensitive retinal ganglion cells, and by G(o)-type G protein in scallop ciliary-type visual cells. Here, we investigated the phototransduction cascade of a prebilaterian box jellyfish, the most basal animal having eyes containing lens and ciliary-type visual cells similar to vertebrate eyes, to examine the similarity at the molecular level and to obtain an implication of the origin of the vertebrate phototransduction cascade. We showed that the opsin-based pigment functions as a green-sensitive visual pigment and triggers the G(s)-type G protein-mediated phototransduction cascade in the ciliary-type visual cells of the box jellyfish lens eyes. We also demonstrated the light-dependent cAMP increase in the jellyfish visual cells and HEK293S cells expressing the jellyfish opsin. The first identified prebilaterian cascade was distinct from known phototransduction cascades but exhibited significant partial similarity with those in vertebrate and molluscan ciliary-type visual cells, because all involved cyclic nucleotide signaling. These similarities imply a monophyletic origin of ciliary phototransduction cascades distributed from prebilaterian to vertebrate.

Cephalochordate melanopsin: evolutionary linkage between invertebrate visual cells and vertebrate photosensitive retinal ganglion cells

Animal photoreceptor cells can be classified into two distinct types, depending on whether the photopigment is borne on the membrane of a modified cilium (ciliary type) or apical microvilli (rhabdomeric type) [1]. Ciliary photoreceptors are well known as vertebrate rods and cones and are also found in several invertebrates. The rhabdomeric photoreceptor, in contrast, is a predominant type of invertebrate visual cell, but morphologically identifiable rhabdomeric photoreceptors have never been found in vertebrates. It is hypothesized that the rhabdomeric photoreceptor cell had evolved to be the photosensitive retinal ganglion cell for the vertebrate circadian photoentrainment [2, 3 and 4] owing to the fact that some molecules involved in cell differentiation are common among them [5]. We focused on the cephalochordate amphioxus because it is the closest living invertebrate to the vertebrates, and interestingly, it has rhabdomeric photoreceptor cells for putative nonvisual functions [6]. Here, we show that the amphioxus homolog of melanopsin [7, 8 and 9], the circadian photopigment in the photosensitive retinal ganglion cells of vertebrates, is expressed in the rhabdomeric photoreceptor cells of the amphioxus and that its biochemical and photochemical properties, not just its primary structure, are considerably similar to those of the visual rhodopsins in the rhabdomeric photoreceptor cells of higher invertebrates. The cephalochordate rhabdomeric photoreceptor represents an evolutionary link between the invertebrate visual photoreceptor and the vertebrate circadian photoreceptor.

Ultrastructure and localization of a visual Gq protein in hypertrophied epitoke ocelli of Perinereis brevicirris (Polychaeta, Annelida)

Functional ultrastructural changes in the rhabdomeric photoreceptors of the cerebral ocelli are described for normal and sexually mature (epitoke) Perinereis brevicirris (Polychaeta, Annelida). With sexual maturation, the cerebral ocelli hypertrophied, increasing in volume to 5.5 times that of ocelli in the normal state, and the thickness of the retinal layer increased up to 10 times. Perinereis ocelli have a pigmented retinal layer consisting of at least two cell types: photoreceptor cell (PR) and pigmented supporting cells (PS). In epitoke ocelli, PR bear well-developed rhabdomeric microvilli, multilamellar bodies, and numerous cytoplasmic membranous structures, including vesicles, smooth endoplasmic reticulum, and secondary lysosomes. Localization of a visual Gq protein in the ocelli was studied with anti-GqC antibody. The antibody strongly labeled not only microvilli and multilamellar bodies throughout the retinal layer, but also secondary lysosomes and vesicles in the cytoplasm of the PR in the epitoke ocelli, although labeling was observed only in the microvilli and multilamellar bodies in normal ocelli. Reverse transcription/polymerase chain reaction analysis revealed that the amount of G protein alpha subunit mRNA in the epitoke head increased by roughly twice that of the normal head. Since Gq protein is essential for phototransduction in Perinereis ocelli, these results suggest that the sites are involved in photoreceptive membrane turnover, which occurs much more extensively in epitoke ocelli. Thus, epitoke ocelli may represent a model system for studying rhabdomeric photoreceptive membrane turnover.

Diversity of visual pigments from the viewpoint of G protein activation--comparison with other G protein-coupled receptors

The visual pigment present in the photoreceptor cells of the retina is a member of the family of G protein-coupled receptors and contains an 11-cis-retinal as a light-absorbing chromophore. Light induces conformational changes in the protein moiety of the visual pigment through cis-trans isomerization of the chromophore, which leads to the activation of a G protein-mediated signal transduction cascade that eventually generates an electrical response of the photoreceptor cells. So far, various types of visual pigments have been identified from a variety of photoreceptor cells and the structure-function relationship of visual pigments has been widely investigated by means of biophysical, biochemical and molecular biological techniques. Recent identifications of visual pigment-like proteins in the extra-ocular cells emphasize the importance of the visual pigment family as the photoreceptive molecules in not only visual but also non-visual photoreception. This article reviews the functional diversity of visual pigments from the viewpoint of the molecular mechanisms of photoreception and G protein activation. In addition, the similarity and difference of G protein activation mechanism between visual pigment and other G protein-coupled receptors are discussed for furthering our understanding of the common mechanism of G protein activation by G protein-coupled receptors.

Retinal phospholipase C from squid is a regulator of Gq alpha GTPase activity

The phospholipase C (PLC) pathway is the major signaling mechanism of photoactivation in invertebrate photoreceptors. Here we report the cloning of a cDNA encoding a 140-kDa retinal PLC that is uniquely expressed in squid photoreceptors. This cDNA encodes a protein with multiple distinct modular domains: PH, X and Y catalytic, and C2 domains, as well as G- and P-box motifs and two GTP/ATP binding motifs. The PLC was stimulated by activated squid Gq alpha but not by squid Gq beta gamma or mammalian beta gamma subunits. The PLC was inhibited by monophosphate, diphosphate and triphosphate nucleotides but not cyclic nucleosides. We also tested the ability of PLC-140 to regulate the GTPase activity of Gq alpha in the rhabdomeric membranes. Depletion of PLC-140 from the rhabdomeric membranes decreased the GTP hydrolysis but not GTP gamma S binding to the membranes. Reconstitution of purified PLC-140 with membranes accelerated Gq alpha GTPase activity by fivefold at a concentration of 2.5 microM. Our data suggest that PLC-140 plays an important role in both the activation and inactivation pathways of invertebrate visual transduction.

InsP(3)-induced Ca(2+) release in permeabilized invertebrate photoreceptors: a link between phototransduction and Ca(2+) stores

Using the low-affinity fluorescent Ca(2+) indicators, Mag-Fura-2 and Mag-Fura Red, we studied light- and InsP(3)-induced Ca(2+) release in permeabilized microvillar photoreceptors of the medicinal leech, Hirudo medicinalis. Two major components of the phosphoinositide signaling pathway, phospholipase-C and the InsP(3) receptor, were characterized immunologically and appropriately localized in photoreceptors. Whereas phospholipase-C was abudantly expressed in photoreceptive microvilli, InsP(3) receptors were found mostly in submicrovillar endoplasmic reticulum (SER). Permeabilization of the peripheral plasma membrane with saponin allowed direct measurements of luminal free Ca(2+) concentration (Ca(L)) changes. Confocal Ca(2+) imaging using Mag-Fura Red demonstrated that Ins(1,4,5)P(3) mobilizes Ca(2+) from SER. As detected with Mag-Fura-2, a brief 50ms light flash activated rapid Ca(2+) depletion of SER, followed by an effective refilling within 1min of dark adaptation after the light flash. Sensitivity to Ins(1,4,5)P(3) of the Ca(2+) release from SER in leech photoreceptors was accompanied by irreversible uncoupling of phototransduction from Ca(2+) release. Depletion of Ca(2+) stores was induced by Ins(1,4,5)P(3)(EC(50)= 4.75 microM) and the hyper-potent agonist adenophostin A (EC(50)/40nM) while the stereoisomer L-myo Ins(1,4,5)P(3) was totally inactive. Ins(1,4,5)P(3)- or adenophostin A-induced Ca(2+) release was inhibited by 0.1-1mg/ml heparin. The Ca(2+) pump inhibitors, cyclopiazonic acid and thapsigargin, in the presence of Ins(1,4,5)P(3), completely depleted Ca(2+) stores in leech photoreceptors.

Inositol-1,4,5-trisphosphate-binding proteins controlling the phototransduction cascade of invertebrate visual cells

The main phototransduction cascade in invertebrate visual cells involves the turnover of phosphatidylinositol, an important biochemical mechanism common to many signal-transduction systems. Light-activated rhodopsin stimulates guanine nucleotide exchange on the Gq class of G-protein, which activates phospholipase C to hydrolyze phosphatidylinositol 4,5-bisphosphate to inositol-1,4,5-trisphosphate and diacylglycerol. Subsequently, inositol-1,4,5-trisphosphate-binding proteins continue the signal cascade. Here, we report on the first inositol-1,4,5-trisphosphate-binding proteins demonstrated in an invertebrate visual system with our investigation of the photosensitive rhabdoms of squid. We screened the ability of proteins to interact with inositol-1,4,5-trisphosphate by affinity column chromatography with an inositol-1,4,5-trisphosphate analogue. We detected an inositol-1,4,5-trisphosphate-binding affinity in phospholipase C, receptor kinase and five other proteins in the cytosolic fraction and, surprisingly, rhodopsin in the membrane fraction. A binding assay with (3)H-labelled inositol-1,4,5-trisphosphate demonstrated the inositol-1,4,5-trisphosphate affinity of each of the purified proteins. Since rhodopsin, receptor kinase and phospholipase C are involved upstream of phosphatidylinositol turnover in the signal cascade, our result suggests that phosphatidylinositol turnover is important in feedback pathways in the signalling system.

Light transduction in invertebrate hyperpolarizing photoreceptors: possible involvement of a Go-regulated guanylate cyclase

The hyperpolarizing receptor potential of scallop ciliary photoreceptors is attributable to light-induced opening of K(+)-selective channels. Having previously demonstrated the activation of this K(+) current by cGMP, we examined upstream events in the transduction cascade. GTP-gamma-S produced persistent excitation after a flash, accompanied by decreased sensitivity and acceleration of the photocurrent, whereas GDP-beta-S only inhibited responsiveness, consistent with the involvement of a G-protein. Because G(o) (but not G(t) nor G(q)) recently has been detected in the ciliary retinal layer of a related species, we tested the effects of activators of G(o); mastoparan peptides induced an outward current suppressible by blockers of the light-sensitive conductance such as l-cis-diltiazem. In addition, intracellular dialysis with the A-protomer of pertussis toxin (PTX) depressed the photocurrent. The mechanisms that couple G-protein stimulation to changes in cGMP were investigated. Intracellular IBMX enhanced the photoresponse with little effect on the baseline current, a result that argues against regulation by light of phosphodiesterase activity. LY83583, an inhibitor of guanylate cyclase (GC), exerted a reversible, dose-dependent suppression of the photocurrent. By contrast, ODQ, an antagonist of NO-sensitive GC, and YC-1, an activator of NO-sensitive GC, failed to alter the light response or the holding current; furthermore, the NO synthase inhibitor N-methyl- l-arginine was inert, indicating that the NO signaling pathway is not implicated. Taken together, these results suggest a novel type of phototransduction cascade in which stimulation of a PTX-sensitive G(o) may activate a membrane GC to induce an increase in cGMP and the consequent opening of light-dependent channels.

Structural and functional differences of two forms of GTP-binding protein, Gq, in the cephalopod retina

The major GTP-binding protein (G-protein) in the rhabdomeric photoreceptor membranes of the squid (Watasenia scintillans) has been identified as a Gq-class G-protein. Anti-Gq alpha antibodies recognized a protein not only in the photoreceptor membranes but also in soluble fractions of the retina. The 42 kD protein in the soluble fractions (soluble Gq alpha) had the same molecular mass and the same reactivities to anti-Gq antibodies as those of membrane-bound Gq alpha. The G beta subunit was scarcely detected in the soluble fractions, being found mostly in the membrane fraction, indicating soluble Gq alpha exists in monomeric form. Soluble Gq alpha had no effect on the GTPase activity of the photoreceptor membranes, suggesting that it does not interact with photoactivated rhodopsin or G beta gamma. Soluble Gq alpha would be an inactive form of Gq alpha. In the retina of Octopus fangsiao, soluble Gq alpha was scarcely detected after dark adaptation, but increased during subsequent light exposure and decreased on returning to dark adaptation. These results with Octopus suggest that functional membrane-bound Gq alpha is converted to soluble Gq alpha on exposure to light. Transformation of membrane-bound Gq alpha into the soluble form by hydroxylamine suggests that the difference between membrane-bound and soluble Gq alpha is associated with the attachment of fatty acid(s).

Regulation of squid visual phospholipase C by activated G-protein alpha

Phospholipase C (PLC) is the key enzyme in the phototransduction cascade of invertebrate rhabdomeric photoreceptors. In addition to 130 kDa PLC, a 95 kDa protein recognized by antibody against the catalytic site of PLC was found in the squid retina. The PLC-like 95 kDa protein (95 kDa PLC) was produced from 130 kDa PLC by an intrinsic protease in the presence of calcium. The 130 kDa PLC was stimulated by the active form of Gq-class G-protein alpha (Gq alpha), but the 95 kDa PLC was not, although their PLC activity was similar. A 35 kDa fragment, the counterpart of 95 kDa PLC, was not recognized by antibodies against catalytic site or N-terminal site of the 130 kDa PLC, indicating that the cleavage site is on the C-terminal side beyond the catalytic site. In the presence of a large excess of the 35 kDa fragment, 95 kDa PLC was stimulated by Gq alpha to a similar extent as intact 130 kDa PLC. These results indicate that the C-terminal polypeptide of PLC is necessary for regulation of its enzyme activity by Gq alpha. The uncoupling of PLC from Gq alpha, caused by limited proteolysis, is therefore a candidate regulatory mechanism of the phototransduction cascade in rhabdomeric photoreceptors.

A novel photointermediate of octopus rhodopsin activates its G-protein

The photointermediate of octopus rhodopsin responsible for G-protein activation was examined by a GTPgammaS-binding assay in a reconstituted system with purified rhodopsin and photoreceptor G-protein. When octopus rhodopsin alone was incubated in the dark after illumination, its ability to stimulate GTPgammaS-binding by the G-protein decreased in a time-dependent manner. We associate this decay with the decay of a novel photointermediate, transient acid metarhodopsin, which lies between mesorhodopsin and acid metarhodopsin. Spectroscopic evidence for its existence was suggested by its effects on the turbidity of the vesicles. These results suggest that the transient acid metarhodopsin, not the stable final photoproduct, acid metarhodopsin, activates a G-protein in octopus photoreceptors.

Molecular and immunological characterization of a Gq protein from ventral and lateral eye of the horseshoe crab Limulus polyphemus

GTP binding proteins of the Gq family have been implicated in phototransduction in rhabdomeral photoreceptors. In this study we used molecular and immunochemical techniques to characterize a GTP-binding protein alpha subunit of the Gq family in ventral and lateral photoreceptors of the horseshoe crab, Limulus polyphemus. Both ventral photoreceptors and lateral eye retinular cells became strongly labeled with an antibody directed against the common carboxyl tail of two Gq family proteins, G alpha q and G alpha 11. This antibody also labeled a 42 kDa band on Western blots of proteins from ventral photoreceptor cell bodies, ventral photoreceptor axons, lateral eyes and lateral optic nerves. The reverse transcription-polymerase chain reaction (RT-PCR), along with degenerate oligonucleotide primers designed against conserved regions of known G alpha q and G alpha 11 proteins, was used to isolate a cDNA from ventral eye RNA which encodes a protein with high identity to known Gq proteins. Ribonuclease protection assays showed that the corresponding message was expressed in ventral eye, but these assays, as well as Northern blots, failed to detect expression in lateral eye. Therefore, while photoreceptors of both ventral and lateral eyes contain a Gq-like protein, the mRNA encoding the Gq protein in the ventral eye may differ in nucleotide sequence from its lateral eye counterpart.

Rhodopsin, Gq and phospholipase C activation in cephalopod photoreceptors

We present characterization of the rhodopsin, Gq and phosphatidylinositol-specific phospholipase C (PLC) from the signal transduction pathway of cephalopod photoreceptors. Cephalopod rhodopsins are unique in possessing a C-terminal extension of proline-rich repeats, and they have a strong tendency to form ordered arrays. Two-dimensional arrays of a full-length and C-terminally-truncated cephalopod rhodopsin have been obtained. The C termini appear to cluster the rhodopsins into small groups. An AlF4(-)-activated Gq alpha subunit has been isolated and shown to activate a partially purified PLC beta. This 130 kDa PLC, isolated by absorption on heparin agarose, showed a specific activity of 195 nmol of phosphatidylinositol 4,5-bisphosphate hydrolysed per milligram of protein per minute in the presence of 1.6 microM free calcium.

Light-modulated subcellular localization of the alpha-subunit of GTP-binding protein Gq in crayfish photoreceptors

Gq-type GTP-binding protein (Gq) plays an important role in invertebrate visual phototransduction. The subcellular localization of the alpha subunit of visual Gq in crayfish photoreceptor was investigated immunocytochemically and biochemically to demonstrate the details of the rhodopsin-Gq interaction. The localization of Gq(alpha) changed depending on the light condition. In the dark, Gq(alpha) was localized in the whole rhabdoms as the membrane-bound form. In the light, half of the Gq(alpha) was localized in the cytoplasm as the soluble form. The translocation of Gq(alpha) was reversible. The light-modulated translocation possibly controls the amount of Gq that can be activated by rhodopsin. In vitro hydroxylamine treatment of rhabdomeric membranes suggested that the translocation was regulated by the fatty-acid modification of Gq(alpha).

Squid photoreceptor phospholipase C is stimulated by membrane Gq alpha but not by soluble Gq alpha

Phospholipase C (PLC) was purified from squid retina. Soluble Gq alpha, membrane Gq alpha and G beta gamma were isolated from GTP gamma S-treated and light-illuminated photoreceptor membranes. The membrane Gq alpha stimulated phosphatidyl inositol-phospholipase C (PI-PLC) activity in a dose-dependent manner. Soluble Gq alpha and membrane G beta gamma showed no stimulating effects on PLC. GTP gamma S-binding was found exclusively in membrane fraction, with very little present in the KCl-soluble fraction which contained soluble Gq alpha. These results indicate that light-activated rhodopsin activates PLC through membrane-bound Gq alpha and suggest that the rhodopsin/Gq/PLC cascade might be the pathway of phototransduction in squid photoreceptors.