Axotomy-induced retinal ganglion cell death in adult mice: quantitative and topographic time course analyses

The fate of retinal ganglion cells after optic nerve injury has been thoroughly described in rat, but not in mice, despite the fact that this species is amply used as a model to study different experimental paradigms that affect retinal ganglion cell population. Here we have analyzed, quantitatively and topographically, the course of mice retinal ganglion cells loss induced by intraorbital nerve transection. To do this, we have doubly identified retinal ganglion cells in all retinas by tracing them from their main retinorecipient area, the superior colliculi, and by their expression of BRN3A (product of Pou4f1 gene). In rat, this transcription factor is expressed by a majority of retinal ganglion cells; however in mice it is not known how many out of the whole population of these neurons express it. Thus, in this work we have assessed, as well, the total population of BRN3A positive retinal ganglion cells. These were automatically quantified in all whole-mounted retinas using a newly developed routine. In control retinas, traced-retinal ganglion cells were automatically quantified, using the previously reported method (Salinas-Navarro et al., 2009b). After optic nerve injury, though, traced-retinal ganglion cells had to be manually quantified by retinal sampling and their total population was afterwards inferred. In naïve whole-mounts, the mean (±standard deviation) total number of traced-retinal ganglion cells was 40,437(±3196) and of BRN3A positive ones was 34,697(±1821). Retinal ganglion cell loss was first significant for both markers 5 days post-axotomy and by day 21, the last time point analyzed, only 15% or 12% of traced or BRN3A positive retinal ganglion cells respectively, survived. Isodensity maps showed that, in control retinas, BRN3A and traced-retinal ganglion cells were distributed similarly, being densest in the dorsal retina along the naso-temporal axis. After axotomy the progressive loss of BRN3A positive retinal ganglion cells was diffuse and affected the entire retina. In conclusion, this is the first study assessing the values, in terms of total number and density, of the retinal ganglion cells surviving axotomy from 2 till 21 days post-lesion. Besides, we have demonstrated that BRN3A is expressed by 85.6% of the total retinal ganglion cell population, and because BRN3A positive retinal ganglion cells show the same spatial distribution and temporal course of degeneration than traced ones, BRN3A is a reliable marker to identify, quantify and assess, ex-vivo, retinal ganglion cell loss in this species.