Quantification of rat retinal growth and vascular population changes after single and split doses of proton irradiation: translational study using stereology methods

Radiation-induced ophthalmic risks of long duration spaceflight: Current investigations and interventions

PURPOSE

As the average duration of space missions increases, astronauts will experience longer periods of exposure to risks of long duration space flight including microgravity and radiation. The risks from long-term exposure to space radiation remains ill-defined. We review the current literature on the possible and known risks of radiation on the eye (including radiation retinopathy) after long duration spaceflight.

METHODS

A PubMed and Google Scholar search of the English language ophthalmic literature was performed from inception to July 11, 2022. The following search terms were utilized independently or in conjunction to build this manuscript: "Radiation Retinopathy", "Spaceflight", "Space Radiation", "Spaceflight Associated Neuro-Ocular Syndrome", "Microgravity", "Hypercapnia", "Radiation Shield", "Cataract", and "SANS". A concise and selective approach of references was conducted in including relevant original studies and reviews.

RESULTS

A total of 65 papers were reviewed and 47 papers were included in our review.

CONCLUSION

We discuss the potential and developing countermeasures to mitigate these radiation risks in preparation for future space exploration. Given the complex nature of space radiation, no single approach will fully reduce the risks of developing radiation maculopathy in long-duration spaceflight. Understanding and appropriately overcoming the risks of space radiation is key to becoming a multi-planetary species.

Short-Term and Long-Term Effects after Exposure to Ionizing Radiation and Visible Light on Retina and Retinal Pigment Epithelium of Mouse Eye

A comparative in vivo study of the effects of ionizing radiation (accelerated protons) and visible light (400-700 nm) on the retina and retinal pigment epithelium (RPE) of the mouse eye was carried out. Using the methods of fluorescence spectroscopy and high-performance liquid chromatography (HPLC), we analyzed the relative composition of retinoids in chloroform extracts obtained from the retinas and RPEs immediately after exposure of animals to various types of radiation and 4.5 months after they were exposed and maintained under standard conditions throughout the period. The fluorescent properties of chloroform extracts were shown to change upon exposure to various types of radiation. This fact indicates the accumulation of retinoid oxidation and degradation products in the retina and RPE. The data from fluorescence and HPLC analyses of retinoids indicate that when exposed to ionizing radiation, retinoid oxidation processes similar to photooxidation occur. Both ionizing radiation and high-intensity visible light have been shown to be characterized by long-term effects. The action of any type of radiation is assumed to activate the mechanism of enhanced reactive oxygen species production, resulting in a long-term damaging effect.

Evidence of Spaceflight-Induced Adverse Effects on Photoreceptors and Retinal Function in the Mouse Eye

The goal of the present study was to characterize acute oxidative damage in ocular structure and retinal function after exposure to spaceflight, and to evaluate the efficacy of an antioxidant in reducing spaceflight-induced changes in the retina. Ten-week-old adult C57BL/6 male mice were flown aboard the ISS on Space-X 24 over 35 days, and returned to Earth alive. The mice received a weekly injection of a superoxide dismutase mimic, MnTnBuOE-2-PyP 5+ (BuOE), before launch and during their stay onboard the ISS. Ground control mice were maintained on Earth under identical environmental conditions. Before the launch, intraocular pressure (IOP) was measured using a handheld tonometer and retinal function was evaluated using electroretinogram (ERG). ERG signals were recorded when the mouse eye was under dark-adapted conditions in response to ultraviolet monochromatic light flashes. Within 20 h after splashdown, IOP and ERG assessments were repeated before euthanasia. There were significant increases in body weight for habitat control groups post-flight compared to pre-flight measurements. However, the body weights were similar among flight groups before launch and after splashdown. The IOP measurements were similar between pre- and post-flight groups with no significant differences between BuOE-treated and saline controls. Immunofluorescence evaluation showed increases in retinal oxidative stress and apoptotic cell death after spaceflight. BuOE treatment significantly decreased the level of the oxidative stress biomarker. ERG data showed that the average amplitudes of the a- and b-wave were significantly decreased (39% and 32% by spaceflight, respectively) compared to that of habitat ground controls. These data indicate that spaceflight conditions induce oxidative stress in the retina, which may lead to photoreceptor cell damage and retinal function impairment.

The Protective Role of Neurogenetic Components in Reducing Stress-Related Effects during Spaceflights: Evidence from the Age-Related Positive Memory Approach

Fighting stress-related effects during spaceflight is crucial for a successful mission. Emotional, motivational, and cognitive mechanisms have already been shown to be involved in the decrease of negative emotions. However, emerging evidence is pointing to a neurogenetic profile that may render some individuals more prone than others to focusing on positive information in memory and increasing affective health. The relevance for adaptation to the space environment and the interaction with other stressors such as ionizing radiations is discussed. In particular, to clarify this approach better, we will draw from the psychology and aging literature data. Subsequently, we report on studies on candidate genes for sensitivity to positive memories. We review work on the following candidate genes that may be crucial in adaptation mechanisms: ADRA2B, COMT, 5HTTLPR, CB1, and TOMM40. The final aim is to show how the study of genetics and cell biology of positive memory can help us to reveal the underlying bottom-up pathways to also increasing positive effects during a space mission.

The individual and combined effects of spaceflight radiation and microgravity on biologic systems and functional outcomes

Both microgravity and radiation exposure in the spaceflight environment have been identified as hazards to astronaut health and performance. Substantial study has been focused on understanding the biology and risks associated with prolonged exposure to microgravity, and the hazards presented by radiation from galactic cosmic rays (GCR) and solar particle events (SPEs) outside of low earth orbit (LEO). To date, the majority of the ground-based analogues (e.g., rodent or cell culture studies) that investigate the biology of and risks associated with spaceflight hazards will focus on an individual hazard in isolation. However, astronauts will face these challenges simultaneously Combined hazard studies are necessary for understanding the risks astronauts face as they travel outside of LEO, and are also critical for countermeasure development. The focus of this review is to describe biologic and functional outcomes from ground-based analogue models for microgravity and radiation, specifically highlighting the combined effects of radiation and reduced weight-bearing from rodent ground-based tail suspension via hind limb unloading (HLU) and partial weight-bearing (PWB) models, although in vitro and spaceflight results are discussed as appropriate. The review focuses on the skeletal, ocular, central nervous system (CNS), cardiovascular, and stem cells responses.

Acute Effect of Low-Dose Space Radiation on Mouse Retina and Retinal Endothelial Cells

There is concern that degradation of vision as a result of space flight may compromise both mission goals and long-term quality of life after space travel. The visual disturbances may be due to a combination of intracerebral pressure changes and exposure to ionizing radiation. The retina and the retinal vasculature play important roles in vision, yet have not been studied extensively in relationship to space travel and space radiation. The goal of the current study was to characterize oxidative damage and apoptosis in retinal endothelial cells after whole-body gamma-ray, proton and oxygen (¹⁶O) ion radiation exposure at 0.1 to 1 Gy. Six-month-old male C57Bl/6J mice were whole-body irradiated with 600 MeV/n ¹⁶O ions (0, 0.1, 0.25, 1 Gy), solar particle event (SPE)-like protons (0, 0.1, 0.25, 0.5 Gy) or ⁶⁰Co gamma rays (0, 0.1, 0.25, 0.5 Gy). Eyes were isolated for examining endothelial nitric oxide synthase (eNOS) expression and characterization of apoptosis in retina and retinal endothelial cells at two weeks postirradiation. The expression of eNOS was significantly increased in the retina after proton and ¹⁶O ion exposure. ¹⁶O ions induced over twofold increase in eNOS expression compared to proton exposure at two weeks postirradiation ( P < 0.05). TUNEL assays showed dose-dependent increases in apoptosis in the retina after irradiation. Low doses of ¹⁶O ions elicited apoptosis in the mouse retinal endothelial cells with the most robust changes observed after 0.1 Gy irradiation ( P < 0.05) compared to controls. Data also showed that ¹⁶O ions induced a higher frequency of apoptosis in retinal endothelial cells compared to protons ( P < 0.05). In summary, our study revealed that exposure to low-dose ionizing radiation induced oxidative damage and apoptosis in the retina. Significant changes in retinal endothelial cells occur at doses as low as 0.1 Gy. There were significant differences in the responses of endothelial cells among the radiation types examined here.

Review: cerebral microvascular pathology in ageing and neurodegeneration

This review of age-related brain microvascular pathologies focuses on topics studied by this laboratory, including anatomy of the blood supply, tortuous vessels, venous collagenosis, capillary remnants, vascular density and microembolic brain injury. Our studies feature thick sections, large blocks embedded in celloidin, and vascular staining by alkaline phosphatase. This permits study of the vascular network in three dimensions, and the differentiation of afferent from efferent vessels. Current evidence suggests that there is decreased vascular density in ageing, Alzheimer's disease and leukoaraiosis, and cerebrovascular dysfunction precedes and accompanies cognitive dysfunction and neurodegeneration. A decline in cerebrovascular angiogenesis may inhibit recovery from hypoxia-induced capillary loss. Cerebral blood flow is inhibited by tortuous arterioles and deposition of excessive collagen in veins and venules. Misery perfusion due to capillary loss appears to occur before cell loss in leukoaraiosis, and cerebral blood flow is also reduced in the normal-appearing white matter. Hypoperfusion occurs early in Alzheimer's disease, inducing white matter lesions and correlating with dementia. In vascular dementia, cholinergic reductions are correlated with cognitive impairment, and cholinesterase inhibitors have some benefit. Most lipid microemboli from cardiac surgery pass through the brain in a few days, but some remain for weeks. They can cause what appears to be a type of vascular dementia years after surgery. Donepezil has shown some benefit. Emboli, such as clots, cholesterol crystals and microspheres can be extruded through the walls of cerebral vessels, but there is no evidence yet that lipid emboli undergo such extravasation.

A review of string vessels or collapsed, empty basement membrane tubes

String vessels are thin connective tissue strands, remnants of capillaries, with no endothelial cells; they do not carry blood flow. They occur in numerous species, particularly in the central nervous system, but can occur in any tissue where capillaries have died. String vessels are often associated with pathologies such as Alzheimer's disease, ischemia, and irradiation, but are also found in normal human brains from preterm babies to the aged. They provide a record of the original blood vessel location, but gradually disappear after months or years. There have been numerous studies of string vessels (acellular capillaries) in the retina, because retinal vessels can be seen in great detail in whole mounts after trypsin digestion. Capillary regression occurs by apoptosis, synchronously along capillary segments, with macrophages engulfing apoptotic endothelial cells. Macrophages may cause the apoptosis, or the regression may be triggered by loss of the endothelial cell survival factor VEGF. VEGF expression is induced by hypoxia and promotes capillary growth. Cessation of blood flow eliminates the shear stress that helps maintain endothelial cell survival. Capillaries can re-grow by proliferation and migration of endothelial cells into empty basement membrane tubes, which provide a structural scaffold, replete with signaling molecules. This is a problem in tumor control, but useful for recovery from capillary loss. There is an age-related waning of VEGF expression in response to hypoxia. This causes an age-related decline in cerebral angiogenesis and results in neuronal loss. It may also contribute to the proposed age-related loss of brain reserve.

High-LET radiation-induced response of microvessels in the Hippocampus

The hippocampus is critical for learning and memory, and injury to this structure is associated with cognitive deficits. The response of the hippocampal microvessels after a relatively low dose of high-LET radiation remains unclear. In this study, endothelial population changes in hippocampal microvessels exposed to (56)Fe ions at doses of 0, 0.5, 2 and 4 Gy were quantified using unbiased stereological techniques. Twelve months after exposure, mice that received 0.5 Gy or 2 Gy of iron ions showed a 34% or 29% loss of endothelial cells, respectively, in the hippocampal cornu ammonis region 1 (CA1) compared to age-matched controls or mice that received 4 Gy (P < 0.05). We suggest that this "U-shaped" dose response indicates a repopulation from a sensitive subset of endothelial cells that occurred after 4 Gy that was stimulated by an initial rapid loss of endothelial cells. In contrast to the CA1, in the dentate gyrus (DG), there was no significant difference in microvessel cell and length density between irradiated groups and age-matched controls. Vascular topology differences between CA1 and DG may account for the variation in dose response. The correlation between radiation-induced alterations in the hippocampal microvessels and their functional consequences must be investigated in further studies.

Microvascular changes in the white mater in dementia

Our studies of the brain microvascular system have focused on some aspects not commonly studied by other research groups because we use some techniques not often used by others. Our observations tend to add new details to the pathological picture rather than contradict the mainstream findings. We use large, thick celloidin sections which provide a three dimensional view of vascular networks, and alkaline phosphatase (AP) staining which allows one to differentiate between afferent and efferent vessels. We found millions of lipid microemboli in the brains of patients after cardiac surgery, and concluded that they caused vascular dementia in many patients. We previously proposed an animal model of vascular dementia using brain irradiation, which induces capillary loss. Lipid emboli might also be used to create an animal model of vascular dementia. The deep white matter is vulnerable to chronic hypoperfusion because the blood vessels supplying this region arise from the border-zone and have the longest course of all vessels penetrating the cerebrum. In cases with leukoaraiosis (LA), we found periventricular venous collagenosis (PVC), resulting in stenosis. Thirteen of 20 subjects older than 60 years had PVC, and 10 of 13 subjects with severe PVC had LA. Vascular stenosis might induce chronic ischemia and/or edema in the deep white matter, leading to LA. We suggest three mechanisms for a possible genetic predisposition to PVC: i) a predisposition to excessive venous collagenosis; ii) an indirect effect that causes chronic periventricular ischemia with a reactive over-production of collagen; and iii) mechanical damage to small vessels due to increased pulsatile motion. We found tortuous arterioles supplying the deep white matter beginning at about age 50. We also found a trend toward an increase in tortuosity in LA. If tortuosity is a factor in LA, it is probably significant in only a subset of cases. String vessels, remnants of capillaries, occur commonly in the brain, and are increased in ischemia, AD, and irradiation. Capillary injury or shutdown of blood flow can lead to capillary loss and string vessel formation. We found string vessels in brains from preterm babies to the very old. They seem to disappear after some months or years. We found an early loss of capillaries in LA, followed in a few years by the disappearance of string vessels. LA lesions do not progress to cortical cavitating lesions. Our findings raise three questions. 1. Why is the capillary loss arrested before infarction? 2. Why is there a floor below which the vascular density will not fall? 3. Why does the process which initiates string vessels shut down? We explain the vascular changes in LA as follows. LA induces apoptosis with loss of oligodendrocytes. Capillaries and neuropil are lost. Increased oxygen extraction from the blood in the deep white matter in LA implies that there are too many cells for the remaining capillaries. Thus, the capillaries appear to die first. But why do they stop dying? Perhaps a minimum number of capillaries are needed to transport the arterial blood to the venous system. Once the capillaries stop dying, no more string vessels are formed, and the string vessels gradually disappear.

Radioprotective effect of a metalloporphyrin compound in rat eye model

PURPOSE

The purpose of this study was to evaluate the efficacy of the antioxidant Mn (III) tetrakis (N-ethylpyridinium-2-yl) porphyrin (MnTE-2-PyP) in protecting ocular tissue and retinal microvasculature from radiation damage.

MATERIALS AND METHODS

75 rats were treated with Mn TE-2-PyP at 2.5 micro g/injection into one eye an hour before proton irradiation. The radiation was delivered in a single fraction to total doses of 8 Gray (Gy) or 28 Gy; Rats were sacrificed 3 days and 3, 6, 9, and 12 months thereafter for histology and quantification of photoreceptor cell populations and retinal capillary changes.

RESULTS

By 6 months following radiation, there was significant loss of retinal outer and inner nuclear layers in eyes receiving radiation only (8 and 28 Gy) (p < 0.05) compared to their controls and to the eyes of rats treated with radiation plus metalloporphyrin. Retinal microvessel length density decreased significantly 6 months following 28 Gy (p < 0.05) compared to their controls and to MnTE-2-PyP treated rats. By 12 months following irradiation, irradiated eyes showed extensive damage to the photoreceptor layer, whereas the eyes of animals receiving radiation plus MnTE-2-PyP showed almost no morphological damage. MnTE-2-PyP treatment also suppressed radiation-induced apoptosis in our study.

CONCLUSIONS

These results demonstrated that MnTE-2-PyP protected both photoreceptors and retinal capillaries from radiation damage, suggesting that this metalloporphyrin antioxidant is effective in regulating the damage induced by proton radiation.

Capillary loss precedes the cognitive impairment induced by fractionated whole-brain irradiation: a potential rat model of vascular dementia

Brain tumor patients who are long-term survivors after whole-brain irradiation (WBI) often suffer cognitive impairment, including dementia. Although the pathogenic mechanisms remain poorly understood, our studies suggest that radiation-induced cognitive impairment may be a form of vascular dementia. We used a fractionated dose of gamma-rays that is biologically similar to that given to brain tumor patients. The brains of adult rats were irradiated with 40 Gy, in eight 5 Gy fractions over 4 weeks. Cognitive function was assessed prior to WBI and up to 9 months post-irradiation using a partially-baited radial arm maze. A significant increase in working memory errors was found in the irradiated rats by two-way ANOVA (p=0.0042). The increased errors occurred primarily at 6 and 9 months (p < 0.05, student's t-test). Vessel density was quantified using a stereology method with computerized image processing and analysis. Vessel density was unchanged 24 h after the last dose, but significantly decreased (p=0.002), by approximately 30%, from 10 weeks to 52 weeks. Thus, cognitive impairment arose after brain capillary loss in irradiated rats that show no other gross brain pathology. Capillary loss may play an important role in radiation-induced dementia and this may be a model of vascular dementia.

A quantitative study of the effects of ionizing radiation on endothelial cells and capillary-like network formation

The initial events of angiogenesis comprise endothelial cell activation, migration, and proliferation. The characteristics of retinal endothelial cells and capillaries are significantly altered in a number of diseases including cancer. Since radiation has been shown as a useful tool in radiotherapy by altering the proliferative changes, it is important to evaluate the responses of the endothelial cells and the capillary network to radiation. We quantified functional and kinetic responses of endothelial cells and capillaries to radiation in an in vitro model. An in vitro angiogenesis model was introduced in our study with endothelial cells cultured on an extracellular matrix gel in which hollow tube-like structures could be rapidly formed. Vessel formation was quantified using stereological techniques. The cell cycle kinetics of endothelial cells and accumulation of DNA damage after radiation were measured using laser scanning cytometry. To study the response of proliferative capillary-like structures to radiation, the vessel network was irradiated with 2 gray (Gy). To evaluate functional and kinetic responses and differentiation of endothelial cells to radiation, cells were irradiated with 2 and 6 Gy. Progressive time- and dose-dependent loss of endothelial cells occurred starting 24 hours after radiation. Vessel growth was significantly retarded at the higher dose. A significant percentage of DNA breaks were detected dose-dependently. A large percentage of G1 cells were measured in the irradiated endothelial cell population when compared to the respective sham-treated control population. These results indicate that radiation-induced endothelial cell injuries destroy the integrity of vascular structure. We postulated that apoptosis may represent a biologically relevant mechanism of radiation-induced endothelial cell damage.

Vascular Damage after Fractionated Whole-Brain Irradiation in Rats

Whole-brain irradiation of animals and humans has been reported to lead to late delayed structural (vascular damage, demyelination, white matter necrosis) and functional (cognitive impairment) alterations. However, most of the experimental data on late delayed radiation-induced brain injury have been generated with large single doses or short fractionation schemes that may provide a less accurate indication of the events that occur after clinical whole-brain radiotherapy. The pilot study reported here investigates cerebral vascular pathology in male Fischer 344 rats after whole-brain irradiation with a fractionated total dose of 137Cs gamma rays that is expected to be biologically similar to that given to brain tumor patients. The brains of young adult rats (4 months old) were irradiated with a total dose of 40 Gy, given as eight 5-Gy fractions twice per week for 4 weeks. Brain capillary and arteriole pathology was studied using an alkaline phosphatase enzyme histochemistry method; vessel density and length were quantified using a stereology method with computerized image processing and analysis. Vessel density and length were unchanged 24 h after the last dose, but at 10 weeks postirradiation, both were substantially decreased. After 20 weeks, the rate of decline in the vessel density and length in irradiated rats was similar to that in unirradiated age-matched controls. No gross gliosis or demyelination was observed 12 months postirradiation using conventional histopathology techniques. We suggest that the early (10-week) and persistent vascular damage that occurs after a prolonged whole-brain irradiation fractionation scheme may play an important role in the development of late delayed radiation-induced brain injury.

Stereology techniques have--or should have--a role in preclinical radiation therapy

A paper by Kubinova et al. (Radiat. Res. 000, 000-000, 2003) introduced to radiation biology the techniques of stereology required to quantify the dose response of irradiated brain populations. A paper by Mao et al. (Radiat. Res. 000, 000-000, 2003) and earlier papers by Archambeau et al. applied these techniques to quantify the population changes in the vasculature of the retina. This presentation reviews in broad terms the evolution of the need to quantify population and kinetic techniques and how the need has been met. The in vitro and in vivo descriptive and clonogenic techniques used regularly in radiation biology and in clinical therapy will not be replaced by stereology. While stereology is applicable to all tissue, it proves to be an important technique that allows the investigator to quantify cell population parameters in late-responding and non-proliferative populations.

Stereology techniques in radiation biology

Clinicians involved in conventional radiation therapy are very concerned about the dose-response relationships of normal tissues. Before proceeding to new clinical protocols, radiation biologists involved with conformal proton therapy believe it is necessary to quantify the dose response and tolerance of the organs and tissues that will be irradiated. An important focus is on the vasculature. This presentation reviews the methodology and format of using confocal microscopy and stereological methods to quantify tissue parameters, cell number, tissue volume and surface area, and vessel length using the microvasculature as a model tissue. Stereological methods and their concepts are illustrated using an ongoing study of the dose response of the microvessels in proton-irradiated hemibrain. Methods for estimating the volume of the brain and the brain cortex, the total number of endothelial cells in cortical microvessels, the length of cortical microvessels, and the total surface area of cortical microvessel walls are presented step by step in a way understandable for readers with little mathematical background. It is shown that stereological techniques, based on a sound theoretical basis, are powerful and reliable and have been used successfully.