Chronic Histological Outcomes of Indirect Traumatic Optic Neuropathy in Adolescent Mice: Persistent Degeneration and Temporally Regulated Glial Responses

Abstract

Injury to the optic nerve, termed, traumatic optic neuropathy (TON) is a known comorbidity of traumatic brain injury (TBI) and is now known to cause chronic and progressive retinal thinning up to 35 years after injury. Although animal models of TBI have described the presence of optic nerve degeneration and research exploring acute mechanisms is underway, few studies in humans or animals have examined chronic TON pathophysiology outside the retina. We used a closed-head weight-drop model of TBI/TON in 6-week-old male C57BL/6 mice. Mice were euthanized 7-, 14-, 30-, 90-, and 150-days post-injury (DPI) to assess histological changes in the visual system of the brain spanning a total of 12 regions. We show chronic elevation of FluoroJade-C, indicative of neurodegeneration, throughout the time course. Intriguingly, FJ-C staining revealed a bimodal distribution of mice indicating the possibility of subpopulations that may be more or less susceptible to injury outcomes. Additionally, we show that microglia and astrocytes react to optic nerve damage in both temporally and regionally different ways. Despite these differences, astrogliosis and microglial changes were alleviated between 14–30 DPI in all regions examined, perhaps indicating a potentially critical period for intervention/recovery that may determine chronic outcomes.

Model matters: Differential outcomes in traumatic optic neuropathy pathophysiology between blunt and blast-wave mediated head injuries

“…We next examined brains for evidence of neurodegeneration using FluoroJade-B. Doing this, we observed a pattern of axonal degeneration consistent with previous studies in blunt injured mice with axonal degeneration seen bilaterally in subcortical visual regions (LGN, DNT, SC, OT, chiasm) (Evanson et al, 2018;Hetzer et al, 2021b). Surprisingly, however, blast tissue only exhibited unilateral FJ-B staining in the right hemisphere, which is contralateral to the blast exposure.…”

“…In addition to the optic regions discussed in the main text, we also looked at the dorsal terminal nucleus, a branch of the optic nerve in the accessory optic system, which plays a role in the brain's coordination of the optokinetic response. As with the SC, (A) we found significantly increased bilateral FJB (degeneration) in blunt injured mice (previously reported in this model (Hetzer et al, 2021b)) with only unilateral increases in rblast mice in the right hemisphere. (B) There was no positive staining in any sham mice, but positive FJB is clearly seen in (C) all injured groups including single blast, but it was not enough to reach significance.…”

Complement propagates visual system pathology following traumatic brain injury

“…In a recent systematic review of murine blast-TBI, 35 identi ed studies investigated changes in the retina and optic nerve, and none examined changes in the LGN (40). A study using a milder closed head weight drop model and following mice up to 5 months after injury identi ed increased uoro-Jade staining identifying degenerating neurons and astrogliosis in the dLGN and vLGN at multiple timepoints, although they did not detect differences in the soma area of microglia, and microglia morphological changes or counts were not assessed in detail (41). However, this is a different type of injury and is also signi cantly milder than the TBI model used in the current study.…”

Oxidative stress in the brain and retina after traumatic injury

“…Glial cell activation remains a common factor in secondary neurodegeneration following the initial injury in both TBI and TON (Rodgers et al, 2012;Bond and Rex, 2014;Choi et al, 2014;Tao et al, 2017;Evanson et al, 2018;DeJulius et al, 2021;Hetzer et al, 2021;Zhou et al, 2021;Priester et al, 2022). As previously stated, after injury the BBB/BRB can break down, allowing for systemic macrophages to invade the CNS and take on an active microglial morphology (Bond and Rex, 2014;Brooks et al, 2014;McMenamin et al, 2019;DeJulius et al, 2021).…”