Abstract
Secondary neurodegeneration is thought to play an important role in the pathology of neurodegenerative disease, which potential therapies may target. However, the quantitative assessment of the degree of secondary neurodegeneration is difficult. The present study describes a novel algorithm from which estimates of primary and secondary degeneration are computed using well-established rodent models of partial optic nerve transection (pONT) and ocular hypertension (OHT). Brn3-labelled retinal ganglion cells (RGCs) were identified in whole-retinal mounts from which RGC density, nearest neighbour distances and regularity indices were determined. The spatial distribution and rate of RGC loss were assessed and the percentage of primary and secondary degeneration in each non-overlapping segment was calculated. Mean RGC number (82 592±681) and RGC density (1695±23.3 RGC/mm2) in naïve eyes were comparable with previous studies, with an average decline in RGC density of 71±17 and 23±5% over the time course of pONT and OHT models, respectively. Spatial analysis revealed greatest RGC loss in the superior and central retina in pONT, but significant RGC loss in the inferior retina from 3 days post model induction. In comparison, there was no significant difference between superior and inferior retina after OHT induction, and RGC loss occurred mainly along the superior/inferior axis (~30%) versus the nasal–temporal axis (~15%). Intriguingly, a significant loss of RGCs was also observed in contralateral eyes in experimental OHT. In conclusion, a novel algorithm to automatically segment Brn3a-labelled retinal whole-mounts into non-overlapping segments is described, which enables automated spatial and temporal segmentation of RGCs, revealing heterogeneity in the spatial distribution of primary and secondary degenerative processes. This method provides an attractive means to rapidly determine the efficacy of neuroprotective therapies with implications for any neurodegenerative disorder affecting the retina.
Highlights
This study describes a method for segmentation of Brn3a-labelled retinal whole-mounts to extract greater information regarding the spatial distribution and rate of retinal ganglion cells (RGCs) loss in the partial optic nerve transection (pONT) and ocular hypertension (OHT) models of retinal neurodegeneration than has previously been achieved (Figure 1)
Using spatial and temporal analysis of RGC loss in these models, we reveal distinctive patterns of primary and secondary degeneration that correlate with previous observations and retinal nerve fibre layer (RNFL) changes in clinical glaucoma
Significant RGC loss was reported in the contralateral eyes of the OHT model, lending support to the hypothesis that systemic effects can exacerbate secondary RGC degeneration in clinical glaucoma.[49]
Summary
Secondary neurodegeneration is reported to play a significant role in many diseases of growing socioeconomic and health-economic concern, including neurodegenerative diseases, spinal cord injuries and optic neuropathies.[1,2,3,4,5,6,7] It is thought to occur through a combination of excitotoxicity, inflammation and oxidative stress pathways, which provide attractive therapeutic targets as secondary neurodegeneration is clinically more modifiable than primary.[8,9,10] substantial progress has been made in our understanding of these disorders in recent years, including recognition of similarities between these conditions,[11,12] there are presently few effective disease-modifying therapies to slow or reverse the course of neurodegeneration. (but not exclusively), this manifests as an initial permanent loss of peripheral vision, leading to central vision defects as the disease progresses.[14,15] Presently, there is no curative treatment for glaucoma, with existing therapies targeting raised intraocular pressure (IOP), the major and modifiable risk factor.[16,17] Glaucoma is a complex and multifactorial disease, with variation in disease progression, and vision loss continuing to occur in some patients, despite well-controlled IOPs,[18] suggesting that secondary RGC degeneration plays an important role in glaucoma pathology.[10,19]
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