Juxtanodin in retinal pigment epithelial cells: Expression and biological activities in regulating cell morphology and actin cytoskeleton organization

Juxtanodin (JN, also known as ermin) was initially identified as an actin cytoskeleton-related oligodendroglial protein in the rat central nervous system. It was subsequently also found in the rat olfactory neuroepithelium, especially at the apical junctional belt of the sustentacular cells. We further examined JN expression and functional roles in the retina using fluorescence histochemistry, confocal microscopy, immuno-electron microscopy, molecular biology, and cell culture. Prominent JN expression was found in the photoreceptor-supporting retinal pigment epithelium (RPE), especially in a zone corresponding to the apices of RPE cells, at the roots of the RPE microvilli, and at the base of RPE cells next to the Bruch's membrane. Partial co-localization of JN immunoreactivity with F-actin (labeled with phalloidin) was observed at the apices and bases of RPE cells. No JN was detected in other cell types of the retina. In cultured human RPE cell line ARPE-19, expression of extrinsic JN up-regulated formation of actin cytoskeleton stress fibers, caused redistribution of more F-actin fibers to the cell periphery, and promoted spreading/enlargement of transfected cells. These findings suggest possible roles of JN in RPE molecular transport, phagocytosis and formation of outer blood-retinal barrier, or possible involvement of JN expression perturbations in pathogenesis of such retinal disorders as proliferative vitreoretinopathy and age-related macular degeneration.