Abstract
Abstract: :
Purpose: Osteopontin (OPN) is a glycosylated phosphoprotein that has been implicated in a number of physiological and pathological processes. A recent study has shown OPN to be localised to the ganglion cell layer of the retina. One aim of this study was to investigate whether OPN levels in the retina are reduced in paradigms of ganglion cell death, i.e. whether the molecule can be used as a marker for ganglion cell survival. However, since expression of OPN is enhanced by a variety of toxicants in the brain, a second aim was to ascertain whether levels of OPN are elevated, rather than decreased, after injury to the retina, i.e. whether OPN is a marker for retinal injury. Methods: Wistar rats received one of the following insults in one eye while the fellow eye served as a control: (1) optic nerve transection followed by 21 days of recovery; (2) intravitreal injection of NMDA followed by 7 days of recovery; (3) intravitreal injection of kainate followed by 3 days of recovery; (4) high intraocular pressure–induced ischemia for 45 min followed by 1–7 days of recovery; (5) exposure to bright light (approximately 2000 lux) for 48h followed by 3 days of recovery. Rats were then killed, total retinal mRNA was extracted and levels of OPN and other mRNAs were assessed using semi–quantitative RT–PCR. In some animals, retinas were analysed for OPN protein content by immunoblotting and immunohistochemistry using standard methodologies. Results: Optic nerve transection, high intraocular pressure–induced ischemia and intravitreal injection of NMDA and kainate all caused a loss of ganglion cells, which was evidenced by a reduction in the total mRNA levels of the ganglion cell markers NF–L and Thy–1 in the retina. In contrast, OPN mRNA levels were not reduced in any of these injury paradigms. In fact, levels of OPN mRNA were found to be significantly elevated after kainate and ischemia–reperfusion. Increases in OPN mRNA levels were also observed in retinas that had been injured by exposure to intense light. These findings were supported by the results of the immunoblotting and immunohistochemistry experiments. All five injury paradigms also caused upregulations in the levels of the glial cell marker GFAP and the trophic factors FGF–2 and CNTF. Conclusions: The present studies show that OPN cannot be used as a marker for ganglion cell survival in paradigms of ganglion cell injury. Instead, the data suggest that enhanced levels of OPN play a role in the response of the retina to toxicant injury. Studies are in progress to investigate these observations further.
Keywords: retinal degenerations: cell biology • retina • inflammation