Intermediate filaments in bluegill retinal pigment epithelial cells
The retinal pigment epithelium (RPE) is a layer of cuboidal cells located between the neural retina and choroid layers of the eye. In teleosts, long and short apical processes are characteristic of these cells (Zinn and Marmor, 1979). Furthermore, unlike higher vertebrates, teleosts lack the ability to adjust their pupil diameter in accordance to different light intensities. However, they have evolved a process called retinomotor movements which involve changes in the relative positions of the rod and cone photoreceptors and melanin pigment granules within the RPE. In lit environments, the light sensitive rods elongate, burying their outer segments between the processes of the RPE, while cones contract away from the RPE. Pigment granules migrate into the apical processes where they function as a protective shield by absorbing light that would otherwise impinge on the rods. In dark environments, rods contract and cones elongate while pigment granules migrate into the cell body of the RPE (Burnside and Nagle, 1983). In eukaryotes, cell shape, architecture, and motility depend on the cytoskeleton which is composed of microtubules (MTs; 25 nm diameter), microfilaments (MFs; 6-7 nm diameter) and intermediate filaments (IFs; 8-12 nm diameter) (Alberts et al., 1994). Studies conducted on the microfilament and microtubule networks in teleost RPE cells have suggested a role for these cytoskeletal elements in the pigment granule translocation associated with light and dark adaptation (Klyne and Ali, 1981; Burnside et al., 1983; Bruenner and Burnside, 1986; Troutt and Burnside 1989; Dearry et al., 1990; King-Smith et al., 1997). However, little is known of the role of intermediate filaments in teleost RPE cells or in eukaryotic cells in general. The objective of this study was to determine the intermediate filament type or types expressed in bluegill RPE cells and to investigate whether the intermediate filament network is reorganized as pigment granules translocate. Previous immunolabeling experiments revealed the presence of intermediate filaments of unknown type extending from the level of the circumferential microfilament bundles (CMBs) toward the apical processes in bluegill RPE cells. In this investigation, Western blot analysis has revealed the filamentous network to be composed of the protein vimentin with a relative molecular mass of 50 kDa. Corroborative immunolabeling studies have verified the intermediate filament type as vimentin and ruled out the presence of cytokeratin intermediate filaments in isolated RPE cells. Furthermore, when pigment granules were induced to aggregate into the cell body by the application of cAMP, the density of vimentin intermediate filaments increased in the apical processes of isolated RPE cells compared to the density of intermediate filaments in the processes of cells with pigment granules dispersed.
Osteichthyes, Epithelial cells, Cytoplasmic filaments, Retinal pigment epithelium
Pérez, E. (1999). <i>Intermediate filaments in bluegill retinal pigment epithelial cells</i> (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.