The expression of heat shock protein 27 in retinal ganglion and glial cells in a rat glaucoma model.

The heat shock protein 27 kDa (HSP27) is a member of proteins that are highly inducible under various forms of cellular stress. This study describes constitutive HSP27 expression in rat retina and stress-associated expression of HSP27 in an experimental rat glaucoma model. Glaucoma was induced unilaterally using laser photocoagulation of the episcleral and limbal veins. Three and seven days after the elevation of intraocular pressure (IOP), groups of rats were killed. The second laser treatment was performed for those rats killed 14 and 21 days after the first laser treatment. The RGCs were labeled with a retrograde tracer 7 days before kill. The expression of HSP27 was analyzed by Western blotting in retinas of rats killed on day 14 after the first laser treatment. Retinal astrocytes, Müller cells and HSP27-positive cells were visualized using immunohistochemical methods both from retinal whole-mounts and paraffin sections. The total number of retrogradely labeled RGCs decreased by 23.2% after 7 days, 28% after 14 days, and 29.3% after 21 days of elevated IOP when compared with controls. A significant decrease of glial fibrillary acidic protein (GFAP)-immunoreactive retinal astrocytes in laser-treated eyes was observed compared with the controls (accounted for 44.9%, 38.2% and 35% of the control values in the 7-day, 14-day and 21-day groups, respectively). The expression of HSP27 in RGCs and retinal astrocytes was also increased in laser-treated eyes when compared with controls in all groups. However, glycinergic and cholinergic cells in the inner nuclear layer and the highest number of RGCs and astrocytes that expressed HSP27 were found in the 14-day group of rats. The constitutive expression of HSP27 was observed only in retinal astrocytes and Müller cells. This study suggests that constitutive HSP27 expression is a cell-type specific phenomenon in the rat retina. However, at the same time, HSP27 might be considered as a marker for neuronal injury in the rat glaucoma model.