Induction of heat shock proteins 27 and 72 in retinal ganglion cells after acute pressure-induced ischaemia

Background:  We wanted to investigate whether heat shock protein (HSP) 27 and HSP 72 are induced in retinal ganglion cells (RGCs) after acute intraocular pressure (IOP)-induced ischaemia.
Methods:  Retinal ischaemia was induced by acutely increasing IOP to 100–110 mmHg for 30 or 90 min unilaterally in Sprague Dawley rats. A fluorescent tracer (fluorogold, FG) was applied to the superior colliculi to label RGCs. Twenty-four hours, 1 week or 2 weeks after of IOP elevation, rats were killed, RGCs counted, and immunohistochemical labelling of the retina was performed. HSP-positive RGCs were counted and normalized HSP RGC counts determined.
Results:  The ratio of FG-positive labelled RGCs in the experimental to the contralateral eye as a marker of RGC survival remained unchanged after 30 min of ischaemia: 1.09 ± 0.11 at 1 week, and 0.94 ± 0.28 at 2 weeks. After 90 min of ischaemia RGC survival decreased to 0.19 ± 0.14 at 1 week, and 0.20 ± 0.14 at 2 weeks. After 30 min of ischaemia, the normalized HSP 27- and HSP 72-positive RGC count was detected at highest levels (HSP 27: 5.42 ± 1.18; HSP 72: 12.23 ± 1.24) at 2 weeks compared with controls, whereas after 90 min ischaemia it was detected at higher levels at 1 week (HSP 27: 52.63 ± 3.65; HSP 72: 206.84 ± 60.38), as well as at 2 weeks (HSP 27: 89.00 ± 17.21; HSP 72: 191.00 ± 50.05).
Conclusion:  These results demonstrate an enhanced induction of HSP 27 and HSP 72 after 90 min acute IOP-induced ischaemia. In contrast to 30 min ischaemia, we showed time-dependent loss of RGCs after 90 min of ischaemia after 1 week or 2 weeks.