White Matter is the Predilection Site of Late-Delayed Radiation-Induced Brain Injury in Non-Human Primates

Total-Body Irradiation Alters White Matter Volume and Microstructural Integrity in Rhesus Macaques

PURPOSE

Long-term survivors of brain irradiation can experience irreversible injury and cognitive impairment. T1-weighted and diffusion tensor magnetic resonance imaging (MRI) are used to evaluate brain volume and white matter (WM) microstructure in neurodevelopmental and neurodegenerative conditions. The goal of this study was to evaluate the long-term effects of single-dose total-body irradiation (TBI) or TBI with 5% partial-body sparing on brain volumetrics and WM integrity in macaques.

METHODS AND MATERIALS

We used MRI scans from a cohort of male rhesus macaques (age range, 3.6-22.8 years) to compare global and regional brain volumes and WM diffusion in survivors of TBI (T1-weighted, n = 137; diffusion tensor imaging, n = 121; dose range, 3.5-10 Gy) with unirradiated controls (T1-weighted, n = 48; diffusion tensor imaging, n = 38).

RESULTS

In all regions of interest, radiation affected age-related changes in fractional anisotropy, which tended to increase across age in both groups but to a lesser extent in the irradiated group (interaction P < .01). Depending on the region of interest, mean diffusivity decreased or remained the same across age in unirradiated animals, whereas it increased or did not change in irradiated animals. The increases in mean diffusivity were driven by changes in radial diffusivity, which followed similar trends across age. Axial diffusivity did not differ by irradiation status. Age-related changes in relative volumes in controls reflected normal trends in humans, with increasing WM and decreasing gray matter until middle age. Cerebrospinal fluid (CSF) volume did not differ across age in controls. WM volume was lower and CSF volume was higher in young irradiated macaques. WM volume was similar between groups, and CSF volume lower in older irradiated macaques. Gray matter volume was unaffected by radiation.

CONCLUSIONS

TBI results in delayed WM expansion and long-term disruption of WM integrity. Diffusion changes suggest that myelin injury in WM is a hallmark of late-delayed radiation-induced brain injury.

Resilience, aging, and response to radiation exposure (RARRE) in nonhuman primates: a resource review

The Wake Forest nonhuman primate (NHP) Radiation Late Effects Cohort (RLEC) is a unique and irreplaceable population of aging NHP radiation survivors which serves the nation's need to understand the late effects of radiation exposure. Over the past 16 years, Wake Forest has evaluated > 250 previously irradiated rhesus macaques (Macaca mulatta) that were exposed to single total body irradiation (IR) doses of 1.14-8.5 Gy or to partial body exposures of up to 10 Gy (5% bone marrow sparing) or 10.75 Gy (whole thorax). Though primarily used to examine IR effects on disease-specific processes or to develop radiation countermeasures, this resource provides insights on resilience across physiologic systems and its relationship with biological aging. Exposure to IR has well documented deleterious effects on health, but the late effects of IR are highly variable. Some animals exhibit multimorbidity and accumulated health deficits, whereas others remain relatively resilient years after exposure to total body IR. This provides an opportunity to evaluate biological aging at the nexus of resilient/vulnerable responses to a stressor. Consideration of inter-individual differences in response to this stressor can inform individualized strategies to manage late effects of radiation exposure, and provide insight into mechanisms underlying systemic resilience and aging. The utility of this cohort for age-related research questions was summarized at the 2022 Trans-NIH Geroscience Interest Group's Workshop on Animal Models for Geroscience. We present a brief review of radiation injury and its relationship to aging and resilience in NHPs with a focus on the RLEC.

Measurements of cerebral microvascular blood flow, oxygenation, and morphology in a mouse model of whole-brain irradiation-induced cognitive impairment by two-photon microscopy and optical coherence tomography: evidence for microvascular injury in the cerebral white matter

Whole-brain irradiation (WBI, also known as whole-brain radiation therapy) is a mainstay treatment modality for patients with multiple brain metastases. It is also used as a prophylactic treatment for microscopic tumors that cannot be detected by magnetic resonance imaging. WBI induces a progressive cognitive decline in ~ 50% of the patients surviving over 6 months, significantly compromising the quality of life. There is increasing preclinical evidence that radiation-induced injury to the cerebral microvasculature and accelerated neurovascular senescence plays a central role in this side effect of WBI. To better understand this side effect, male C57BL/6 mice were first subjected to a clinically relevant protocol of fractionated WBI (5 Gy, two doses per week, for 4 weeks). Nine months post the WBI treatment, we applied two-photon microscopy and Doppler optical coherence tomography to measure capillary red-blood-cell (RBC) flux, capillary morphology, and microvascular oxygen partial pressure (PO2) in the cerebral somatosensory cortex in the awake, head-restrained, WPI-treated mice and their age-matched controls, through a cover-glass-sealed chronic cranial window. Thanks to the extended penetration depth with the fluorophore - Alexa680, measurements of capillary blood flow properties (e.g., RBC flux, speed, and linear density) in the cerebral subcortical white matter were enabled. We found that the WBI-treated mice exhibited a significantly decreased capillary RBC flux in the white matter. WBI also caused a significant reduction in capillary diameter, as well as a large (although insignificant) reduction in segment density at the deeper cortical layers (e.g., 600-700 μm), while the other morphological properties (e.g., segment length and tortuosity) were not obviously affected. In addition, we found that PO2 measured in the arterioles and venules, as well as the calculated oxygen saturation and oxygen extraction fraction, were not obviously affected by WBI. Lastly, WBI was associated with a significant increase in the erythrocyte-associated transients of PO2, while the changes of other cerebral capillary PO2 properties (e.g., capillary mean-PO2, RBC-PO2, and InterRBC-PO2) were not significant. Collectively, our findings support the notion that WBI results in persistent cerebral white matter microvascular impairment, which likely contributes to the WBI-induced brain injury and cognitive decline. Further studies are warranted to assess the WBI-induced changes in brain tissue oxygenation and malfunction of the white matter microvasculature as well.

Neurocognitive and radiological changes after cranial radiation therapy in humans and rodents: a systematic review

BACKGROUND

Radiation-induced brain injury is a common long-term side effect for brain cancer survivors, leading to a reduced quality of life. Although there is growing research pertaining to this topic, the relationship between cognitive and radiologically detected lesions of radiation-induced brain injury in humans remains unclear. Furthermore, clinically translatable similarities between rodent models and human findings are also undefined. The objective of this review is to then identify the current evidence of radiation-induced brain injury in humans and to compare these findings to current rodent models of radiation-induced brain injury.

METHODS

This review includes an examination of the current literature on cognitive and radiological characteristics of radiation-induced brain injury in humans and rodents. A thorough search was conducted on PubMed, Web of Science, and Scopus to identify studies that performed cognitive assessments and magnetic resonance imaging techniques on either humans or rodents after cranial radiation therapy. A qualitative synthesis of the data is herein reported.

RESULTS

A total of 153 studies pertaining to cognitively or radiologically detected radiation injury of the brain are included in this systematic review; 106 studies provided data on humans while 47 studies provided data on rodents. Cognitive deficits in humans manifest across multiple domains after brain irradiation. Radiological evidence in humans highlight various neuroimaging-detectable changes post-irradiation. It is unclear, however, whether these findings reflect ground truth or research interests. Additionally, rodent models do not comprehensively reproduce characteristics of cognitive and radiological injury currently identified in humans.

CONCLUSION

This systematic review demonstrates that associations between and within cognitive and radiological radiation-induced brain injuries often rely on the type of assessment. Well-designed studies that evaluate the spectrum of potential injury are required for a precise understanding of not only the clinical significance of radiation-induced brain injury in humans, but also how to replicate injury development in pre-clinical models.

Lung transcriptome of nonhuman primates exposed to total- and partial-body irradiation

The focus of radiation biodosimetry has changed recently, and a paradigm shift for using molecular technologies of omic platforms in addition to cytogenetic techniques has been observed. In our study, we have used a nonhuman primate model to investigate the impact of a supralethal dose of 12 Gy radiation on alterations in the lung transcriptome. We used 6 healthy and 32 irradiated animal samples to delineate radiation-induced changes. We also used a medical countermeasure, γ-tocotrienol (GT3), to observe any changes. We demonstrate significant radiation-induced changes in the lung transcriptome for total-body irradiation (TBI) and partial-body irradiation (PBI). However, no major influence of GT3 on radiation was noted in either comparison. Several common signaling pathways, including PI3K/AKT, GADD45, and p53, were upregulated in both exposures. TBI activated DNA-damage-related pathways in the lungs, whereas PTEN signaling was activated after PBI. Our study highlights the various transcriptional profiles associated with γ- and X-ray exposures, and the associated pathways include LXR/RXR activation in TBI, whereas pulmonary wound-healing and pulmonary fibrosis signaling was repressed in PBI. Our study provides important insights into the molecular pathways associated with irradiation that can be further investigated for biomarker discovery.

Microglia as Therapeutic Target for Radiation-Induced Brain Injury

Radiation-induced brain injury (RIBI) after radiotherapy has become an increasingly important factor affecting the prognosis of patients with head and neck tumor. With the delivery of high doses of radiation to brain tissue, microglia rapidly transit to a pro-inflammatory phenotype, upregulate phagocytic machinery, and reduce the release of neurotrophic factors. Persistently activated microglia mediate the progression of chronic neuroinflammation, which may inhibit brain neurogenesis leading to the occurrence of neurocognitive disorders at the advanced stage of RIBI. Fully understanding the microglial pathophysiology and cellular and molecular mechanisms after irradiation may facilitate the development of novel therapy by targeting microglia to prevent RIBI and subsequent neurological and neuropsychiatric disorders.

Cross-translational models of late-onset cognitive sequelae and their treatment in pediatric brain tumor survivors

Pediatric brain tumor treatments have a high success rate, but survivors are at risk of cognitive sequelae that impact long-term quality of life. We summarize recent clinical and animal model research addressing pathogenesis or evaluating candidate interventions for treatment-induced cognitive sequelae. Assayed interventions encompass a broad range of approaches, including modifications to radiotherapy, modulation of immune response, prevention of treatment-induced cell loss or promotion of cell renewal, manipulation of neuronal signaling, and lifestyle/environmental adjustments. We further emphasize the potential of neuroimaging as a key component of cross-translation to contextualize laboratory research within broader clinical findings. This cross-translational approach has the potential to accelerate discovery to improve pediatric cancer survivors' long-term quality of life.

Modulation of the Complement System by Neoplastic Disease of the Central Nervous System

The complement system is a highly conserved component of innate immunity that is involved in recognizing and responding to pathogens. The system serves as a bridge between innate and adaptive immunity, and modulation of the complement system can affect the entire host immune response to a foreign insult. Neoplastic diseases have been shown to engage the complement system in order to evade the immune system, gain a selective growth advantage, and co-opt the surrounding environment for tumor proliferation. Historically, the central nervous system has been considered to be an immune-privileged environment, but it is now clear that there are active roles for both innate and adaptive immunity within the central nervous system. Much of the research on the role of immunological modulation of neoplastic disease within the central nervous system has focused on adaptive immunity, even though innate immunity still plays a critical role in the natural history of central nervous system neoplasms. Here, we review the modulation of the complement system by a variety of neoplastic diseases of the central nervous system. We also discuss gaps in the current body of knowledge and comment on future directions for investigation.

Meeting Report: A Poly-Pharmacy Approach to Mitigate Acute Radiation Syndrome

The National Institute of Allergy and Infectious Diseases, Radiation and Nuclear Countermeasures Program, was tasked by the United States Congress and the U.S. Department of Health and Human Services to identify and fund early-to-mid-stage development of medical countermeasures (MCMs) to treat radiation-induced injuries. In developing MCMs to treat various sub-syndromes (e.g., hematopoietic, gastrointestinal, lung), it is important to investigate whether a poly-pharmacy approach (i.e., drug cocktails) can provide additive benefits to mitigate injuries arising from the acute radiation syndrome (ARS). In addition, potential drug-drug interactions must be examined. For this reason, a workshop was held, which centered on understanding the current state of research investigating poly-pharmacy approaches to treat radiation injuries. The first session set the stage with an introduction to the concept of operations or support available for the response to a nuclear incident, as this is the key to any emergency response, including MCM availability and distribution. The second session followed the natural history of ARS in both humans and animal models to underscore the complexity of ARS and why a poly-pharmacy approach may be necessary. The third session featured talks from investigators conducting current MCM poly-pharmacy research. The meeting closed with a focus on regulatory considerations for the development of poly-pharmacy approaches or combination treatments for ARS.

All for one, though not one for all: team players in normal tissue radiobiology

PURPOSE

As part of the special issue on 'Women in Science', this review offers a perspective on past and ongoing work in the field of normal (non-cancer) tissue radiation biology, highlighting the work of many of the leading contributors to this field of research. We discuss some of the hypotheses that have guided investigations, with a focus on some of the critical organs considered dose-limiting with respect to radiation therapy, and speculate on where the field needs to go in the future.

CONCLUSIONS

The scope of work that makes up normal tissue radiation biology has and continues to play a pivotal role in the radiation sciences, ensuring the most effective application of radiation in imaging and therapy, as well as contributing to radiation protection efforts. However, despite the proven historical value of preclinical findings, recent decades have seen clinical practice move ahead with altered fractionation scheduling based on empirical observations, with little to no (or even negative) supporting scientific data. Given our current appreciation of the complexity of normal tissue radiation responses and their temporal variability, with tissue- and/or organ-specific mechanisms that include intra-, inter- and extracellular messaging, as well as contributions from systemic compartments, such as the immune system, the need to maintain a positive therapeutic ratio has never been more urgent. Importantly, mitigation and treatment strategies, whether for the clinic, emergency use following accidental or deliberate releases, or reducing occupational risk, will likely require multi-targeted approaches that involve both local and systemic intervention. From our personal perspective as five 'Women in Science', we would like to acknowledge and applaud the role that many female scientists have played in this field. We stand on the shoulders of those who have gone before, some of whom are fellow contributors to this special issue.

Stem-Cell Therapy as a Potential Strategy for Radiation-Induced Brain Injury

Radiation therapy is a standard and effective non-surgical treatment for primary brain tumors and metastases. However, this strategy inevitably results in damage of normal brain tissue, causing severe complications, especially the late-delayed cognitive impairment. Due to the multifactorial and complex pathological effects of radiation, there is a lack of effective preventative and restorative treatments for the irradiated brain. Stem-cell therapy has held considerable promise for decades in the treatment of central nervous system (CNS) disorders because of its unique capacity for tissue repair and functional integrity. Currently, there is growing interest in using stem cells as a novel option to attenuate the adverse effects of irradiation. In the present review, we discuss recent studies evaluating stem-cell therapies for the irradiated brain and their therapeutic effects on ameliorating radiation-related brain injury as well as their potential challenges in clinical applications. We discuss these works in context of the pathogenesis of radiation-induced injury to CNS tissue in an attempt to elucidate the potential mechanisms of engrafted stem cells to reverse radiation-induced degenerative processes.

Microglia in Cancer Therapy-Related Cognitive Impairment

Millions of cancer survivors experience a persistent neurological syndrome that includes deficits in memory, attention, information processing, and mental health. Cancer therapy-related cognitive impairment can cause mild to severe disruptions to quality of life for these cancer survivors. Understanding the cellular and molecular underpinnings of this disorder will facilitate new therapeutic strategies aimed at ameliorating these long-lasting impairments. Accumulating evidence suggests that a range of cancer therapies induce persistent activation of the brain's resident immune cells, microglia. Cancer therapy-induced microglial activation disrupts numerous mechanisms of neuroplasticity, and emerging findings suggest that this impairment in plasticity is central to cancer therapy-related cognitive impairment. This review explores reactive microglial dysregulation of neural circuit structure and function following cancer therapy.

Reduction of pTau and APP levels in mammalian brain after low-dose radiation

Brain radiation can occur from treatment of brain tumors or accidental exposures. Brain radiation has been rarely considered, though, as a possible tool to alter protein levels involved in neurodegenerative disorders. We analyzed possible molecular and neuropathology changes of phosphorylated-Tau (pTau), all-Tau forms, β-tubulin, amyloid precursor protein (APP), glial fibrillary acidic protein (GFAP), ionized calcium binding adaptor molecule 1 (IBA-1), myelin basic protein (MBP), and GAP43 in Frontal Cortex (FC), Hippocampus (H) and Cerebellum (CRB) of swine brains following total-body low-dose radiation (1.79 Gy). Our data show that radiated-animals had lower levels of pTau in FC and H, APP in H and CRB, GAP43 in CRB, and higher level of GFAP in H versus sham-animals. These molecular changes were not accompanied by obvious neurohistological changes, except for astrogliosis in the H. These findings are novel, and might open new perspectives on brain radiation as a potential tool to interfere with the accumulation of specific proteins linked to the pathogenesis of various neurodegenerative disorders.

Emerging mechanistic underpinnings and therapeutic targets for chemotherapy-related cognitive impairment

PURPOSE OF REVIEW

Modern innovations in cancer therapy have dramatically increased the number of cancer survivors. An unfortunately frequent side-effect of cancer treatment is enduring neurological impairment. Persistent deficits in attention, concentration, memory, and speed of information processing afflict a substantial fraction of cancer survivors following completion of these life-saving therapies. Here, we highlight chemotherapy-related cognitive impairment (CRCI) and discuss the current understanding of mechanisms underlying CRCI.

RECENT FINDINGS

New studies emphasize the deleterious impact of chemotherapeutic agents on glial-glial and neuron-glial interactions that shape the form, function and plasticity of the central nervous system. An emerging theme in cancer therapy-related cognitive impairment is therapy-induced microglial activation and consequent dysfunction of both neural precursor cells and mature neural cell types. Recent work has highlighted the complexity of dysregulated intercellular interactions involving oligodendrocyte lineage cells, microglia, astrocytes, and neurons following exposure to traditional cancer therapies such as methotrexate. This new understanding of the mechanistic underpinnings of CRCI has elucidated potential therapeutic interventions, including colony-stimulating factor 1 receptor inhibition, TrkB agonism, and aerobic exercise.

SUMMARY

Traditional cancer therapies induce lasting alterations to multiple neural cell types. Therapy-induced microglial activation is a critical component of the cause of CRCI, contributing to dysregulation of numerous processes of neural plasticity. Therapeutic targeting of microglial activation or the consequent dysregulation of neural plasticity mechanisms are emerging.

Effects of Focal Radiation on [18 F]-Fluoro-D-Glucose Positron Emission Tomography in the Brains of Miniature Pigs: Preliminary Findings on Local Metabolism

OBJECTIVES

It would be a medically important advance if durable and focal neuromodulation of the brain could be delivered noninvasively and without ablation. This ongoing study seeks to elucidate the effects of precisely delivered ionizing radiation upon focal brain metabolism and the corresponding cellular integrity at that target. We hypothesize that focally delivered ionizing radiation to the brain can yield focal metabolic changes without lesioning the brain in the process.

MATERIALS AND METHODS

We used stereotactic radiosurgery to deliver doses from 10 Gy to 120 Gy to the left primary motor cortex (M1) of Lee Sung miniature pigs (n = 8). One additional animal served as a nonirradiated control. We used positron emission tomography-computed tomography (PET-CT) to quantify radiation dose-dependent effects by calculating the ratio of standard uptake values (SUV) of 2-deoxy-2-[¹⁸ F]-fluoro-D-glucose (¹⁸ F-FDG) between the radiated (left) and irradiated (right) hemispheres across nine months.

RESULTS

We found that the FDG-PET SUV ratio at the targeted M1 was significantly lowered from the pre-radiation baseline measurements for animals receiving 60 Gy or higher, with the effect persisting at nine months after radiosurgery. Only at 120 Gy was a lesion suggesting ablation visible at the M1 target. Animals treated at 60-100 Gy showed a reduced signal in the absence of an identifiable lesion, a result consistent with the occurrence of neuromodulation.

CONCLUSION

Focal, noninvasive, and durable changes in brain activity can be induced without a magnetic resonance imaging (MRI)-visible lesion, a result that may be consistent with the occurrence of neuromodulation. This approach may provide new venues for the investigation of neuromodulatory treatments for disorders involving dysfunctional brain circuits. Postmortem pathological analysis is needed to elucidate whether there have been morphological changes not detected by MRI.

Proceedings of the 2019 National Toxicology Program Satellite Symposium

The 2019 annual National Toxicology Program Satellite Symposium, entitled "Pathology Potpourri," was held in Raleigh, North Carolina, at the Society of Toxicologic Pathology's 38th annual meeting. The goal of this symposium was to present and discuss challenging diagnostic pathology and/or nomenclature issues. This article presents summaries of the speakers' talks along with select images that were used by the audience for voting and discussion. Various lesions and topics covered during the symposium included aging mouse lesions from various strains, as well as the following lesions from various rat strains: rete testis sperm granuloma/fibrosis, ovarian cystadenocarcinoma, retro-orbital schwannoma, periductal cholangiofibrosis of the liver and pancreas, pars distalis hypertrophy, chronic progressive nephropathy, and renal tubule regeneration. Other cases included polyovular follicles in young beagle dogs and a fungal blood smear contaminant. One series of cases challenged the audience to consider how immunohistochemistry may improve the diagnosis of some tumors. Interesting retinal lesions from a rhesus macaque emphasized the difficulty in determining the etiology of any particular retinal lesion due to the retina's similar response to vascular injury. Finally, a series of lesions from the International Harmonization of Nomenclature and Diagnostic Criteria Non-Rodent Fish Working Group were presented.