Mechanism of retinal ganglion cells death in secondary degeneration of the optic nerve

In central nervous system injury, the secondary degeneration process is known to play a major role in determining the final extent of impairment. Here, we investigated the mechanism of retinal ganglion cell (RGC) death in secondary degeneration of the optic nerve using a unique model that allows morphological separation between primary and secondary degeneration. A partial transection model was applied unilaterally in 110 Wistar rat eyes. The rate of apoptosis was evaluated in primary and secondary degeneration over a period of 6 months using the Hoechst staining technique. The involvement of caspase 3 and members of the Bcl-2 family (Bax, Bad, Bcl-2 and Bcl-xl) was evaluated at multiple time points for 6 months after the injury by immunohistochemistry and RT-PCR. We found that in secondary degeneration of the optic nerve, RGCs died by apoptosis from day 3-6 months following the injury, peaking at 3 months (16.3% ± 2.5% apoptotic cells, p < 0.01). Both primary and secondary degeneration of the optic nerve resulted in caspase 3 activation, which was longer and more intense in the former. Similarly, both primary and secondary degeneration led to significant (p < 0.05) downregulation of the pro-survival genes Bcl-2 and Bcl-x-L and up-regulation of the pro-apoptotic genes Bax and Bad (p < 0.05), with a suggested delay in secondary degeneration. Thus, secondary degeneration of the optic nerve leads to RGC apoptosis over long periods in a similar mechanism as in primary degeneration.