Purpose: The temporal series of molecular events that occur in dying retinal ganglion cells is poorly understood. We have examined the change in expression of a normally-expressed ganglion cell marker gene, Thy1, relative to the kinetics of cell loss caused by acute and chronic damaging stimuli.
Methods: For acute experiments, mice were subjected to optic nerve crush or intravitreal injections of N-methyl-D-aspartate (NMDA) to induce ganglion cell death. RNase protection analysis was used to quantify Thy1 mRNA levels from total retina RNA and in situ hybridization was used to monitor the pattern of Thy1 positive cells. Changes in Thy1 expression were compared to the time course of cell loss induced by each treatment. To induce elevated intraocular pressure (IOP), the episcleral veins of rats were injected with hypertonic saline, which scleroses Schlemm's Canal and the trabecular meshwork. Elevated IOP was monitored every day for 35 days after which the animals were sacrificed and the retinas harvested for quantitative RT-PCR or fixed for in situ hybridization studies. Evaluation of glaucomatous damage caused by elevated IOP was determined from histological sections of the optic nerves of all rat eyes.
Results: After optic nerve crush, Thy1 mRNA levels decreased within 24 h, although the number of expressing cells did not decline until 7 days. Both measures showed a loss of Thy1 well in advance of cell loss, which was detected by 2 weeks after surgery. This change in expression was not dependent on execution of the cell death program since a similar decrease was detected in Bax-/- ganglion cells, which are resistant to cell death induced by optic nerve crush. Thy1 mRNA levels and the number of expressing cells also decreased within 6 h after NMDA injection, in advance of cell loss, which was detected by 24 h. Similarly, elevated intraocular pressure was associated with a decrease in mRNA and expressing cells in a pressure-dependent manner. In moderately hypertensive rat eyes, the number of cells expressing Thy1 decreased before significant cell loss in the retina. Virtually no Thy1-expressing cells were detected in eyes with severe disease.
Conclusions: Thy1 mRNA abundance and expressing cells, decreased in advance of detectable ganglion cell loss caused by three different modalities of damage. This change is independent of the committed step of cell death.