Changes in Photoreceptor Outer Segment Structures in Horses with Equine Recurrent Uveitis (ERU)

Abstract

Abstract: : Purpose: ERU is a spontaneous disease with high prevalence in the equine population. The disease shares similarities with recurrent uveitis in man. Glycoproteins on the cell surface and extracellular matrix are involved in a variety of cellular functions and intercellular interactions. There are various kinds of N-linked glycoproteins in the retina as rhodopsin and IRBP which play a role in retinal diseases. We examined the sugar residues on the retinal tissue in various stages of ERU using plant lectins to determine possible changes in glycoproteins. Methods: Fourteen eyes of horses showing no signs of ophthalmic disease by direct ophthalmoscopy were used as the eye healthy control group. Twenty-three eyes of horses with ERU were included in the second group. All horses were slaughtered due to reasons unrelated to this study. Lectin histochemistry was performed by the ABC method. After rehydration, sections were incubated with plant lectins (5µg/ml) for one hour at RT. Binding was visualized by DAB or by incubation in naphtol-AS-biphosphate as the phosphatase substrate and hexazotized New Fuchsin. Control stainings were performed by the preabsorption of lectins with an excess amount (100mM concentration) of respective specific sugar residues. A second control was performed by use of TBS to replace the biotinylated lectins. Results: The ß-N-acetylgalactosamine-specific lectins, PNA, RCA-I and Jacalin (JAC) labeled cones and rods in the outer segments, whereas the α-N-acetylgalactosamine-specific lectin BSL-I labeled specifically cones. This is in contrast to findings in a wide variety of species, where PNA is used as a cone specific marker. PNA labels the cone sheats in horses but also the outer segments of rods. This labeling pattern was observed in all healthy eyes. In 12 ERU horses, the PNA labeling was absent in rods and cones despite clear JAC staining of these structures. This was not observed in the other 11 ERU eyes. The loss of PNA labeling could not be correlated with the structural destruction of the eye. Western Blots using whole retinal extracts of pigs and horses revealed the staining of two additional proteins in the horse retina by PNA compared to the pig. Conclusions: We could observe changes in the sugar residues of photoreceptor outer segments in 50% of horses with recurrent uveitis.

Keywords: photoreceptors • uveitis-clinical/animal model • cell membrane/membrane specializations