Chapter 8 Biochemistry of the cyclic nucleotide-related enzymes in rod photoreceptors

Effect of light on phosphatidate phosphohydrolase activity of retina rod outer segments: the role of transducin

The aim of the present paper is to evaluate the modulation of phosphatidate phosphohydrolase (PAPase) and diacylglyceride lipase (DGL) activities in bovine rod outer segment (ROS) under dark and light conditions and to evaluate the role of transducin (T) in this phenomenon. In dark-adapted ROS membranes exposed to light, PAPase activity is inhibited by 20% with respect to the activity found under dark conditions. To determine whether the retinal G protein, T, participates in the regulation of PAPase activity in these membranes, the effects of GTPgammaS and GDPbetaS on enzyme activity were examined. Under dark conditions in the presence of GTPgammaS, which stabilizes T in its active form (Talpha + Tbetagamma), enzyme activity was inhibited and approached control values under light conditions. GDPbetaS, on the other hand, which stabilizes the inactive state of T (Talphabetagamma), stimulated PAPase activity by 36% with respect to control light conditions. ADP-ribosylation by cholera and pertussis toxin was also studied. In ADP-rybosilated ROS membranes with pertussis toxin under dark conditions, PAPase activity was 36% higher than the activity found under control light conditions. ADP-ribosylation by CTx, on the other hand, inhibited PAPase activity by 22%, with respect to dark control conditions, mimicking light effect. The effects of GTPgammaS and GDPbetaS and conditions of ADP-ribosylation by PTx and CTx on DGL activity were similar to those of PAPase activities. Based on NEM sensitivity we have also demonstrated that the PAPase present in ROS is the PAP 2 isoform. Our findings therefore suggest that light inhibition of PAP 2 in ROS is a transducin-mediated mechanism.

Concentration effects on reactions in membranes: rhodopsin and transducin

The reaction rate of two laterally-diffusing species in a biological membrane shows a maximum at some concentration of reactants, because an increase in the concentration of reactants tends to increase the reaction rate by the law of mass action but decreases the diffusion rate of the reactants. The activation of transducin by rhodopsin in the disk membrane of the rod outer segment is described in terms of a steady-state diffusion model with concentration-dependent diffusion coefficients. The optimum concentrations of reactants are obtained from contour plots of the reaction rate as a function of rhodopsin and transducin concentrations, and the sensitivity of the results to the assumed values of the variables is examined. To determine whether the observed concentrations are in fact those yielding the maximum reaction rate, several variables must be known more accurately.

Effects of platelet-activating factor on electrophysiology of isolated retinas and their inhibition by BN 52021, a specific PAF-acether receptor antagonist

The effects of (R)PAF-acether have been tested on the isolated rat retina model. Results indicate that (R)PAF-acether inhibits the electrophysiological response (electroretinogram) elicited on isolated retina by a brief light flash. Immediately after the administration of (R)PAF-acether, an irreversible decrease of the electroretinogram b-wave amplitude is observed. This effect is dose-dependent (2 X 10(-11) M, 2 X 10(-9) M, 2 X 10(-7) M) and partially inhibited by simultaneous administration of Ginkgolide B (BN 52021; 2 X 10(-5) M). These results suggest the existence of (R)PAF-acether-specific receptors inside the retina.

Regulation of neuronal adenylate cyclase

It appears that several components function in a spirit of integrated cooperation toward the intracellular regulation of neurotransmitter responsiveness. We have demonstrated that cytoskeletal proteins might interact with GNs and that GNs and GNi might interact with one another. At this juncture, it appears that both of these phenomena might occur only in cells of neural origin. Calmodulin and antidepressants may also affect adenylate cyclase in nervous tissue alone. The effects of AAGTP are different in nervous tissue from other tissues, and experiments with that nucleotide have led to the discovery of a new, 32 kDa GTP-binding protein which appears only in neural crest cells. Appreciation of the intricacies of signal transduction through the adenylate cyclase system are developing along with our understanding of that system. When combined with the complexity of neurotransmitter responsiveness, comprehension of the combined systems remains in its infancy, destined to grow as well as to surprise and delight all who are interested.

Receptor-coupled activation of phosphoinositide-specific phospholipase C by an N protein

Cleavage of phosphatidylinositol 4,5-bisphosphate by phospholipase C results in the production of two important second messengers: inositol-1,4,5-trisphosphate and 1,2-diacylglycerol. Although several receptors promote this cleavage, the molecular details of phospholipase C activation have remained unresolved. In this study, occupancy of a Ca2+-mobilizing receptor, the oligopeptide chemoattractant receptor on human polymorphonuclear leukocyte plasma membranes, was found to lead to the activation of a guanine nucleotide regulatory (N) protein by guanosine 5'-triphosphate. The activated N protein then stimulated a polyphosphoinositide-specific phospholipase C by reducing the Ca2+ requirement for expression of this activity from superphysiological to normal intracellular concentrations. Therefore, the N protein-mediated activation of phospholipase C may be a key step in the pathway of cellular activation by chemoattractants and certain other hormones.

Immunochemical study of the cyclic nucleotide system of retinal photoreceptor membranes: antibodies raised to phosphodiesterase, its protein inhibitor and GTP-binding proteins

Monospecific precipitating antibodies raised to phosphodiesterase, its protein inhibitor and GTP-binding proteins of bovine retina photoreceptor membranes were obtained. The characterization of the antibodies was carried out by immunochemical methods and according to their functional properties as determined by their effect on enzyme activities. The antibodies were used to study the distribution of immunolike proteins in different animal retinas and to purify the inhibitor protein by the method of immunoaffinity chromatography.

Electrophysiological consequences of leukotrienes applied on isolated rat retina

Scotopic vision is the result of a cascade of light-dependent biochemical events in rod outer segments (ROS) involving mainly a cGMP-modulation of sodium current. This modification of ionic currents induces changes of membrane potential which generates electroretinographic (ERG) waves. As (i) ERG disturbances are commonly recorded in hypoxic and inflammatory retinal diseases (ii) leukotrienes (LTs), a very potent mediators of inflammation, disturb ionic exchanges in several artificial or natural membrane systems, we undertook the investigation of the effects of LTs on ERG record in mammalian isolated retina. LTB4, LTC4 and LTD4, all induced a dose-dependent marked reduction of the b wave amplitude of ERG. This effect is correlated with a significant decrease in the survival time of the retina. The analysis of the modification of ERG indicates that LTs exhibit a real toxic effect since b wave is mainly affected while P III wave is unchanged. Comparatively with other nervous cells, this phenomenon may be attributed to an increase in Na+ permeability of ROS. It is suggested that LTs may be involved in the development of inflammatory or ischemic retinal diseases.

Light-induced conformational changes in the extradiscal regions of bovine rhodopsin

Light-induced conformational changes occurring at the cytosolic surface of rhodopsin were investigated by performing limited digestions of native and illuminated visual pigment with thermolysin, Arg-C endoproteinase, papain and proteinase K. A higher susceptibility of the extradiscal regions of the bleached pigment to the proteases were observed together with altered capacities of the digested bleached rhodopsins to activate the cGMP phosphodiesterase. The overall results strongly suggest that light induces conformational changes not only in the C-terminal end but also in the second and the third extradiscal loop of rhodopsin.

Photoaffinity identification of colchicine-solubilized regulatory subunit from rat brain adenylate cyclase

Five GTP binding proteins in rat cerebral cortex synaptic membranes were identified by photoaffinity labelling with [3H] or [32P](P3-azido-anilido)-P1-5' GTP (AAGTP). When AAGTP-treated membranes were incubated with colchicine or vinblastine and subsequently washed, a single AAGTP-labelled protein of 42 kD was released into the supernatant. About 30% of the total labelled 42-kD protein was released into supernatants from membranes pretreated with colchicine or vinblastine compared with 15% released from control membranes. The amount of adenylate cyclase regulatory subunit (G unit) remaining in these membranes was assessed with reconstitution studies after inactivating the adenylate cyclase catalytic moiety with N-ethylmaleimide (NEM). Forty to fifty percent of functional G units were lost from membranes treated with colchicine prior to washing. This 40-50% loss of functional G unit after colchicine treatment corresponds to the previously observed 42% loss of NaF and guanylyl-5'-imidodiphosphate [Gpp(NH)p]-activated adenylate cyclase. Release of the AAGTP-labelled 42-kD protein from colchicine-treated synaptic membranes is double that from lumicolchicine-treated membranes. This colchicine-mediated release of 42-kD protein correlates with a doubling of functional G unit released from synaptic membranes after colchicine treatment. These findings suggest multiple populations of the G unit within the synaptic plasma membrane, some of which may interact with cytoskeletal components.