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Orson Moritz, Lee Ling Yang; Characterization of protocadherin-21 localization in rod and cone photoreceptors of several species. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3735.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the localization of the photoreceptor-specific protein protocadherin-21 across species in order to understand its role in rod and cone photoreceptor disk morphogenesis.
We generated antibodies targeting the N- and C-terminus of X.laevis protocadherin-21. The anti-N-terminal antibody cross-reacts with protocadherin-21 of multiple species, including X.tropicalis, zebrafish, mouse, pig and human. We therefore compared its localization across species using immunofluorescence microscopy. Immunoblotting was used to study the proteolytic processing of pcdh-21 in retinas of multiple species.
In X.laevis retina, protocadherin-21 expression was highest in cones where it was found in the inner segment plasma membrane, as well as open disk rims of the cone outer segment. In X.laevis rods, protocadherin-21 labeling was most intense in the inner and outer segment (OS) plasma membrane, and in most rods, also appeared as a thin band at the base of the OS. High levels of protocadherin-21 were observed in the disk lamellae of a small minority of rods. In zebrafsh retinas, protocadherin-21 was localized to the rims of cone lamellae but was not detected in rods. In mouse, pig and human rods, protocadherin-21 was concentrated at the base of the OS and, occasionally at one side of the basal OS, exhibiting a L-shaped labeling pattern. In human cones, protocadherin-21 was found exclusively at the base of the OS. The identity of the specific structures involved is currently under investigation using electron microscopy techniques. Immunoblots showed that, unlike mouse protocadherin-21, X. laevis protocadherin-21 was not proteolytically cleaved in-vivo.
In rods and cones of all species, protocadherin-21 was localized to nascent disks at the base of the OS, suggesting a conserved role in disk assembly. However, depending on the cell type and species, we found protocadherin-21 localized to other cellular regions. In mammals with rod-dominant retinas, protocadherin-21 localization was most intense at the base of the rod and cone OS. In lower vertebrate retinas with higher cone to rod ratios, protocadherin-21 expression was significantly greater in cones, and was prominently localized throughout cone disk rims and rod plasma membrane. Our results suggest that protocadherin-21 has cell type- and species- specific roles or mechanisms.
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