Optic nerve crush does not induce retinal ganglion cell loss in the contralateral eye.

Abstract

PurposeOptic nerve crush (ONC) is a model for studying optic nerve trauma. Unilateral ONC induces massive retinal ganglion cell (RGC) degeneration in the affected eye, leading to vision loss within a month. A common assumption has been that the non-injured contralateral eye is unaffected due to the minimal anatomical decussation of the RGC projections at the chiasm. Yet, recently, microglia, the brain-resident macrophages, have shown a responsive phenotype in the contralateral eye after ONC. Whether RGC loss accompanies this phenotype is still controversial. MethodsUsing the available RGCode algorithm and developing our own RGC-Quant deep-learning-based tool, we quantify RGCs total number and density across the entire retina after ONC. ResultsWe confirm a short-term microglia response in the contralateral eye after ONC, but this did not affect microglia number. Furthermore, we cannot confirm the previously reported RGC loss between naive and contralateral retinas five weeks after ONC induction across the commonly used Cx3cr1creERT2 and C57BL6/J mouse models. Neither sex nor the direct comparison of the RGC markers Brn3a and RBPMS, with Brn3a co-labeling, on average, 89% of the RBPMS+-cells, explained this discrepancy, suggesting that the early microglia-responsive phenotype does not have immediate consequences on the RGC number. ConclusionsOur results corroborate that unilateral optic nerve injury elicits a microglial response in the uninjured contralateral eye but without RGC loss. Therefore, the contralateral eye should be treated separately and not as an ONC control.