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Lee Ling Yang, Orson L. Mortiz; Characterization of the Localization and Cleavage of Protocadherin-21 in Rod and Cone Photoreceptors of Several Species. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1146.
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© ARVO (1962-2015); The Authors (2016-present)
Previous studies in mice suggest that protocadherin-21 (pcdh-21) is involved in photoreceptor disk synthesis. Using X. laevis as a model organism, we aim to investigate the role of pcdh-21 in regulating rod and cone photoreceptor disk morphogenesis.
We generated antibodies targeting the N- and C-terminus of pcdh-21 and examined its localization in X.laevis photoreceptors using confocal microscopy. We compared this localization with that of other species, including X.tropicalis, zebrafish, mouse and pig. Immunoblotting was used to study the proteolytic processing of pcdh-21 in these species.
In X. laevis retinas, immunofluorescence staining showed that pcdh-21 was differentially localized in rods and cones. In rods, pcdh-21 labeling was most intense in the inner and outer segment (OS) plasma membrane, and in most cases, also appeared as a thin band at the base of the rod OS. High levels of pcdh-21 were observed in the disk lamellae of a small minority of rods. Pcdh-21 expression was higher in cones than rods. Longitudinal sections showed that pcdh-21 was localized to the cone inner segment plasma membrane. Cross sections double labeled with anti-acetylated tubulin and pcdh-21 antibodies showed that pcdh-21 was localized to the rims of cone disks, except at the side closest to the connecting cilium. In zebrafsh retinas, pcdh-21 was localized to the rims of cone lamellae but it was not detected in rods. In mouse and pig retinas, pcdh-21 was concentrated at the base of the OS. Immunoblot showed that a significant proportion of pcdh-21 was cleaved in mouse, but not X. laevis, photoreceptors.
Our results indicate that pcdh-21 is differentially localized in rods and cones, and that localization is not conserved across species. In mammals with rod-dominant retinas, pcdh-21 localization was most intense at the base of the rod OS. In lower vertebrate retinas with higher cone to rod ratio, pcdh-21 expression is significantly greater in cone outer segments, where it is localized to the disk rim region, whereas it is localized to the plasma membrane and base of the OS in the rods. Proteolytic processing may be unique to mammalian pcdh-21.
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