Survival of melanopsin expressing retinal ganglion cells long term after optic nerve trauma in mice

In this study we have compared the response to optic nerve crush (ONC) and to optic nerve transection (ONT) of the general population of retinal ganglion cells in charge of the image-forming visual functions that express Brn3a (Brn3a⁺RGCs) with that of the sub-population of non-image forming RGCs that express melanopsin (m⁺RGCs). Intact animals were used as control. ONT and ONC were performed at 0.5 mm from the optic disk, and retinas dissected 3, 5, 7, 14, 30, 45 or 90 days later (n = 5/injury/time point). In all the retinas, Brn3a⁺RGCs and m⁺RGCs were identified and their survival analyzed quantitatively and topographically. There were no differences in the course of RGC loss between lesions. The decrease of RGCs was significant at short time points (3 or 5 days for Brn3a⁺ or m⁺ RGCs, respectively) and, up to 14 days, the course of loss of both RGC populations was similar, surviving at this time point between 20 and 22% of their original population. However, while the loss of Brn3a⁺RGCs continues steadily up to 90 days when only 5-6% of them still remain, the loss of m⁺RGCs stops at 14 days, and the proportion of surviving m⁺RGCs remains constant up to 90 days (26-30%). In conclusion, m⁺RGC do not respond to axotomy in the same way than the rest of RGCs, and so whilst image-forming RGCs die in two exponential phases a quick one and a slow protracted one, non-image forming RGCs die only during the first quick phase.

Nerve fibre layer degeneration and retinal ganglion cell loss long term after optic nerve crush or transection in adult mice

We have investigated the long term effects of two different models of unilateral optic nerve (ON) lesion on retinal ganglion cells (RGCs) and their axons, in the injured and contralateral retinas of adult albino mice. Intact animals were used as controls. The left ON was intraorbitally crushed or transected at 0.5 mm from the optic disk and both retinas were analyzed at 2, 3, 5, 7, 14, 30, 45 or 90 days after injury. RGCs were immunoidentified with anti-Brn3a, and their axons with anti-highly phosphorylated axonal neurofilament subunit H (pNFH). After both lesions, RGC death in the injured retinas is first significant at day 3, and progresses quickly up to 7 days slowing down till 90 days. In the same retinas, the anatomical loss of RGC axons is not evident until day 30. However, by two days after both lesions there are changes in the expression pattern of pNFH: axonal beads, axonal club- or bulb-like formations, and pNFH⁺RGC somas. The number of pNFH⁺RGC somata peak at day 5 after either lesion and is significantly higher than in intact retinas at all time points. pNFH⁺RGC somata are distributed across the retina, in accordance with the pattern of RGC death which is diffuse and homogenous. In the contralateral retinas there is no RGC loss, but there are few pNFH⁺RGCs from day 2 to day 90. In conclusion, in albino mice, axotomy-induced RGC death precedes the loss of their intraretinal axons and occurs in two phases, a rapid and a slower, but steady, one. Injured retinas show similar changes in the pattern of pNFH expression and a comparable course of RGC loss.

Brain derived neurotrophic factor maintains Brn3a expression in axotomized rat retinal ganglion cells

The transcription factor Brn3a has been reported to be a good marker for adult rat retinal ganglion cells in control and injured retinas. However, it is still unclear if Brn3a expression declines progressively by the injury itself or otherwise its expression is maintained in retinal ganglion cells that, though being injured, are still alive, as might occur when assessing neuroprotective therapies. Therefore, we have automatically quantified the whole population of surviving Brn3a positive retinal ganglion cells in retinas subjected to intraorbital optic nerve transection and treated with either brain derived neurotrophic factor or vehicle. Brain derived neurotrophic factor is known to delay retinal ganglion cell death after axotomy. Thus, comparison of both groups would inform of the suitability of Brn3a as a retinal ganglion cell marker when testing neuroprotective molecules. As internal control, retinal ganglion cells were, as well, identified in all retinas by retrogradely tracing them with fluorogold. Our data show that at all the analyzed times post-lesion, the numbers of Brn3a positive retinal ganglion cells and of fluorogold positive retinal ganglion cells are significantly higher in the brain derived neurotrophic factor-treated retinas compared to the vehicle-treated ones. Moreover, detailed isodensity maps of the surviving Brn3a positive retinal ganglion cells show that a single injection of brain derived neurotrophic factor protects retinal ganglion cells throughout the entire retina. In conclusion, Brn3a is a reliable retinal ganglion cell marker that can be used to accurately measure the potential effect of a given neuroprotective therapy.