When is a control not a control? Reactive microglia occur throughout the control contralateral visual pathway in experimental glaucoma

Abstract

Background: Glaucoma remains a leading cause of irreversible blindness worldwide. Multiple animal models show retinal ganglion cell injuries that replicate features of glaucoma. In these experiments the contralateral eye is commonly used as an internal control. As in humans, in rodents and non-human primates there is significant cross-over of retinal ganglion cell axons from the ipsilateral to the contralateral side at the level of the optic chiasm which may confound findings when damage is restricted to one eye. While neuroinflammation may be a critical event that damages retinal ganglion cells and their axons during glaucoma progression the effect of unilateral damage on the contralateral visual pathway has largely been unexplored.Methods: Ocular hypertensive glaucoma was induced unilaterally or bilaterally by intracameral injection of paramagnetic beads in adult Brown Norway rats. Retinal ganglion cell neurodegeneration and dysfunction were assessed. Neuroinflammation was quantified in the retina, optic nerve head, optic nerve, lateral geniculate nucleus, and superior colliculus by high resolution imaging, and in the retina by flow cytometry and protein arrays.Results: Following ocular hypertensive stress the retinal vasculature remodels, peripheral monocytes enter the retina, and microglia become highly reactive. This effect is more marked in animals with bilateral induction of ocular hypertensive glaucoma. In rats where glaucoma was induced unilaterally there was significant microglia activation in the contralateral (control) eye. Microglial activation extended into the optic nerve and terminal visual thalami, where it was similar across hemispheres irrespective of whether ocular hypertension was unilateral or bilateral. Conclusions:Ocular hypertensive glaucoma in the rat recapitulates many of the features of human glaucoma. In this model microglia become reactive throughout the visual pathway during glaucoma pathogenesis. These effects are not isolated to the experimental eye and its pathway and significant neuroinflammation occurs in the contralateral 'control' eye and pathway. These data suggest that caution is warranted when using the contralateral eye as control in unilateral models of glaucomatous damage.

GLP-1 Receptor Agonist NLY01 Reduces Retinal Inflammation and Neuron Death Secondary to Ocular Hypertension

“…Transcriptomic data from the DBA/2J mouse model of glaucoma suggest that early inflammation is driven by CD11b + CD11c + cells, while CD11b + CD11c À cells initially adopt an anti-inflammatory pattern of gene expression (Tribble et al, 2020a;Williams et al, 2019). Our results support this finding by demonstrating that CD11b + CD11c + cells upregulated IL-1a, TNF-a, and C1q expression prior to contribution from CD11b + CD11c À cells.…”

“…Several observations support this multi-hit hypothesis. First, in animal models of unilateral glaucoma, where one eye has eIOP and the other eye with normal IOP is used as an internal control, microglial activation and inflammation can be observed sans RGC loss in the control optic nerve ( Tribble et al, 2020b ). Second, neuronal injury, via either optic nerve crush or eIOP, is a necessary precursor for astrocyte-mediated neuroinflammatory cell death ( Guttenplan et al, 2020 ).…”

“…This paired approach reduced the impact of biological variability through the use of an internal control. However, recent work has shown that unilateral induction of eIOP in rats resulted in microglial reactivity throughout the visual pathway, including in the contralateral, normotensive optic nerve ( Tribble et al, 2020b ). While most of our experiments normalized the microbead eye against the normotensive control eye, one exception to this can be found in Figures 3A – 3C , where absolute protein levels were measured using ELISA.…”

Nicotinamide provides neuroprotection in glaucoma by protecting against mitochondrial and metabolic dysfunction

“…The retinal metabolome is significantly altered following 3 days of OHT, a time point at which IOP is high but no detectable RGC loss has occurred (15). HC revealed heterogeneity across and within conditions, but with a division between samples such that NAM treated eyes were largely grouped and distinguished ( Figure 4A).…”

“…Rat ocular hypertension was induced using a paramagnetic bead model as previously described (15). Rats were anesthetized with an intraperitoneal injection of ketamine (37.5 mg/kg) and medetomidine hydrochloride (1.25 mg/kg).…”