Preparation of chicken iodopsin

Cone outer segment extracellular matrix as binding domain for interphotoreceptor retinoid-binding protein

Cones are critically dependent on interphotoreceptor retinoid-binding protein (IRBP) for retinoid delivery in the visual cycle. Cone-dominant vertebrates offer an opportunity to uncover the molecular basis of IRBP's role in this process. Here, we explore the association of IRBP with the interphotoreceptor matrix (IPM) of cones vs. rods in cone dominant retinas from chicken (Gallus domesticus), turkey (Meleagris gallopavo), and pig (Sus scrofa). Retinas were detached and fixed directly or washed in saline prior to fixation. Disassociated photoreceptors with adherent matrix were also prepared. Under 2 mM CaCl(2) , insoluble matrix was delaminated from saline washed retinas. The distribution of IRBP, as well as glycans binding peanut agglutinin (cone matrix) and wheat germ agglutinin (rod/cone matrix), was defined by confocal microscopy. Retina flat mounts showed IRBP diffusely distributed in an interconnecting, lattice-like pattern throughout the entire matrix. Saline wash replaced this pattern with fluorescent annuli surrounding individual cone outer segments. In isolated cones and matrix sheets, IRBP colocalized with the peanut agglutinin binding matrix glycans. Our results reveal a wash-resistant association of IRBP with a matrix domain immediately surrounding cone outer segments. The cone matrix sheath may be responsible for IRBP-mediated cone targeting of 11-cis retinoids.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Immunocytochemical identification of outer segment proteins in the rd chicken

Immunoreactivities of two monoclonal antibodies (MAbs) that recognize cone photopigments were tested in the retinas of congenitally blind retinal degenerate (rd) chicks and compared to normally sighted carrier chicks, heterozygous for the mutation. MAb OS-2 had been previously determined to label rod and most cone outer segment membranes in normal chick retinas and is believed to bind to an epitope that is common to several photopigments in chickens. MAb COS-1 labels specifically middle-to-long-wavelength-sensitive cone photopigments in a number of vertebrate species. In rd chicks MAb OS-2 labeled the same number of rod outer segments at the same densities as carrier chicks. However, cone outer segments were less frequently and significantly less heavily labeled with this MAb at all ages tested (1 day, 1 week and 2 weeks post hatching). MAb COS-1 labeled the same number of cone outer segments in both rd and carrier retinas at 1 day of age, however, those outer segments that were labeled in rd specimens had significantly fewer gold particles on them. At both 1 week and 2 weeks of age, rd chick retinas had a significant reduction in numbers of cone outer segments labeled by COS-1. These findings support the hypothesis that the cone photopigment protein is abnormal in the rd chick model of hereditary blindness and retinal degeneration.

Monoclonal antibodies to chicken iodopsin

The protein moiety of chicken iodopsin, R-photopsin, was purified from the chicken retina using a sucrose flotation method followed by two steps of column chromatography. Apparent molecular weights of R-photopsin and scotopsin (the protein moiety of chicken rhodopsin), which was partly purified in the process of purification of R-photopsin, were estimated to be 34,000 and 36,000, respectively, by sodium docecylsulfate-polyacrylamide gel electrophoresis. Using the purified R-photopsin as an antigen, four kinds of hybridoma cells which secreted monoclonal antibodies specific for R-photopsin and iodopsin were prepared. The antibodies thus obtained reacted with neither other chicken cone visual pigments nor rhodopsin as analyzed by immunoblots and immunoprecipitation methods. All the monoclonal antibodies stained the majority of the cone outer segments in chicken retina, while an antiserum raised against cattle rhodopsin stained the rod outer segments as well as some cone outer segments in the retina.

Activation of phosphodiesterase by chicken iodopsin

Activation of guanosine 3',5'-cyclic monophosphate phosphodiesterase in outer-segment membrane of chicken retina was investigated. Irradiation of dark-adapted chicken outer segment membrane for bleaching of iodopsin increased the enzyme activity twice as much as that in the dark in the presence of GTP. Further irradiation of the sample for bleaching of rhodopsin in the membrane induced some additional activation of the enzyme. However, chicken iodopsin activated the enzyme in frog rod outer segment membrane without irradiation, while chicken rhodopsin did not. Irradiation of chicken iodopsin increased the enzyme activity twice as much as that in the dark.