Evaluating Ocular Response in the Retina and Optic Nerve Head after Single and Fractionated High-Energy Protons

Abstract

There are serious concerns about possible late radiation damage to ocular tissue from prolonged space radiation exposure, and occupational and medical procedures. This study aimed to investigate the effects of whole-body high-energy proton exposure at a single dose on apoptosis, oxidative stress, and blood-retina barrier (BRB) integrity in the retina and optic nerve head (ONH) region and to compare these radiation-induced effects with those produced by fractionated dose. Six-month-old C57BL/6 male mice were either sham irradiated or received whole-body high energy proton irradiation at an acute single dose of 0.5 Gy or 12 equal dose fractions for a total dose of 0.5 Gy over twenty-five days. At four months following irradiation, mice were euthanized and ocular tissues were collected for histochemical analysis. Significant increases in the number of apoptotic cells were documented in the mouse retinas and ONHs that received proton radiation with a single or fractionated dose (p < 0.05). Immunochemical analysis revealed enhanced immunoreactivity for oxidative biomarker, 4-hydroxynonenal (4-HNE) in the retina and ONH following single or fractionated protons with more pronounced changes observed with a single dose of 0.5 Gy. BRB integrity was also evaluated with biomarkers of aquaporin-4 (AQP-4), a water channel protein, a tight junction (TJ) protein, Zonula occludens-1 (ZO-1), and an adhesion molecule, the platelet endothelial cell adhesion molecule-1 (PECAM-1). A significantly increased expression of AQP-4 was observed in the retina following a single dose exposure compared to controls. There was also a significant increase in the expression of PECAM-1 and a decrease in the expression of ZO-1 in the retina. These changes give a strong indication of disturbance to BRB integrity in the retina. Interestingly, there was very limited immunoreactivity of AQP-4 and ZO-1 seen in the ONH region, pointing to possible lack of BRB properties as previously reported. Our data demonstrated that exposure to proton radiation of 0.5 Gy induced oxidative stress-associated apoptosis in the retina and ONH, and changes in BRB integrity in the retina. Our study also revealed the differences in BRB biomarker distribution between these two regions. In response to radiation insults, the cellular response in the retina and ONH may be differentially regulated in acute or hyperfractionated dose schedules.

Multi-Domain Touchscreen-Based Cognitive Assessment of C57BL/6J Female Mice Shows Whole-Body Exposure to 56Fe Particle Space Radiation in Maturity Improves Discrimination Learning Yet Impairs Stimulus-Response Rule-Based Habit Learning

“…In principle, in vivo whole-body fractionated exposure to single (or mixed) particles of space radiation has translational relevance even beyond NASA ( Held et al, 2016 ; Simonsen et al, 2020 ). In the brain, there is a limited literature on the effect of fractionated vs. non-fractionated in vivo whole-body exposure to a space radiation-relevant single particle; more studies have been done in other systems, such as the cardiac system ( Leith et al, 1982 ; Chang et al, 2007 ; Rivera et al, 2013 ; Whoolery et al, 2017 , 2020 ; Davis et al, 2021 ; Mao et al, 2021 ). An increasing number of in vivo studies on space radiation and cognition deliver fractions of mixed—rather than single—beams ( Kiffer et al, 2018 ; Krukowski et al, 2018 ; Raber et al, 2019 ; Holden et al, 2021 ).…”

Ionizing radiation, cerebrovascular disease, and consequent dementia: A review and proposed framework relevant to space radiation exposure

“…For example, in mice irradiated with low-dose gamma radiation using a cobalt-57 plate (0.01 cGy/h for a total dose of 0.04 Gy) combined with hindlimb unloading to simulate microgravity, there was increased expression of aquaporin-4 after 9 months, suggesting reduced blood brain barrier integrity ( Bellone et al, 2016 ). Increased aquaporin-4 and reduced tight junction protein expressions were also observed in the brain retinal barrier of mice after whole-body proton irradiation of 0.5 Gy of either single or fractionated doses indicating a radiation-induced reduction in blood retinal barrier integrity ( Mao et al, 2021 ). However, the impact of ionizing radiation on the blood brain barrier may be time dependent.…”