Rescue of radiation-induced cognitive impairment through cranial transplantation of human embryonic stem cells

A bibliometric analysis of radiation-induced brain injury: a research of the literature from 1998 to 2023

BACKGROUND

Radiation-induced brain injury (RIBI) is a debilitating sequela after cranial radiotherapy. Research on the topic of RIBI has gradually entered the public eye, with more innovations and applications of evidence-based research and biological mechanism research in the field of that. This was the first bibliometric analysis on RIBI, assessing brain injury related to radiation articles that were published during 1998-2023, to provide an emerging theoretical basis for the future development of RIBI.

METHODS

Literature were obtained from the Web of Science Core Collection (WOSCC) from its inception to December 31, 2023. The column of publications, author details, affiliated institutions and countries, publication year, and keywords were also recorded.

RESULTS

A total of 2543 journal articles were selected. The annual publications on RIBI fluctuated within a certain range. Journal of Neuro-oncology was the most published journal and Radiation Oncology was the most impactful one. LIMOLI CL was the most prolific author with 37 articles and shared the highest h-index with BARNETT GH. The top one country and institutions were the USA and the University of California System, respectively. Clusters analysis of co-keywords demonstrated that the temporal research trends in this field primarily focused on imaging examination and therapy for RIBI.

CONCLUSION

This study collects, visualizes, and analyzes the literature within the field of RIBI over the last 25 years to map the development process, research frontiers and hotspots, and cutting-edge directions in clinical practice and mechanisms related to RIBI.

Wharton's jelly mesenchymal stem cells: Future regenerative medicine for clinical applications in mitigation of radiation injury

Wharton's jelly mesenchymal stem cells (WJ-MSCs) are gaining significant attention in regenerative medicine for their potential to treat degenerative diseases and mitigate radiation injuries. WJ-MSCs are more naïve and have a better safety profile, making them suitable for both autologous and allogeneic transplantations. This review highlights the regenerative potential of WJ-MSCs and their clinical applications in mitigating various types of radiation injuries. In this review, we will also describe why WJ-MSCs will become one of the most probable stem cells for future regenerative medicine along with a balanced view on their strengths and weaknesses. Finally, the most updated literature related to both preclinical and clinical usage of WJ-MSCs for their potential application in the regeneration of tissues and organs will also be compiled.

Extracellular vesicles from GABAergic but not glutamatergic neurons protect against neurological dysfunction following cranial irradiation

Cranial irradiation used to control brain malignancies invariably leads to progressive and debilitating declines in cognition. Clinical efforts implementing hippocampal avoidance and NMDAR antagonism, have sought to minimize dose to radiosensitive neurogenic regions while normalizing excitatory/inhibitory (E/I) tone. Results of these trials have yielded only marginal benefits to cognition, prompting current studies to evaluate the potential of systemic extracellular vesicle (EV) therapy to restore neurocognitive functionality in the irradiated brain. Here we tested the hypothesis that EVs derived from inhibitory but not excitatory neuronal cultures would prove beneficial to cognition and associated pathology. Rats subjected to a clinically relevant, fractionated cranial irradiation paradigm were given multiple injections of either GABAergic- or glutamatergic-derived EV and subjected to behavioral testing. Rats treated with GABAergic but not glutamatergic EVs showed significant improvements on hippocampal- and cortical-dependent behavioral tasks. While each treatment enhanced levels of the neurotrophic factors BDNF and GDNF, only GABAergic EVs preserved granule cell neuron dendritic spine density. Additional studies conducted with GABAergic EVs, confirmed significant benefits on amygdala-dependent behavior and modest changes in synaptic plasticity as measured by long-term potentiation. These data point to a potentially more efficacious approach for resolving radiation-induced neurological deficits, possibly through a mechanism able to restore homeostatic E/I balance.

New Radiobiological Principles for the CNS Arising from Space Radiation Research

Traditionally, the brain has been regarded as a relatively insensitive late-reacting tissue, with radiologically detectable damage not being reported at doses < 60 Gy. When NASA proposed interplanetary exploration missions, it was required to conduct an intensive health and safety evaluation of cancer, cardiovascular, and cognitive risks associated with exposure to deep space radiation (SR). The SR dose that astronauts on a mission to Mars are predicted to receive is ~300 mGy. Even after correcting for the higher RBE of the SR particles, the biologically effective SR dose (<1 Gy) would still be 60-fold lower than the threshold dose for clinically detectable neurological damage. Unexpectedly, the NASA-funded research program has consistently reported that low (<250 mGy) doses of SR induce deficits in multiple cognitive functions. This review will discuss these findings and the radical paradigm shifts in radiobiological principles for the brain that were required in light of these findings. These included a shift from cell killing to loss of function models, an expansion of the critical brain regions for radiation-induced cognitive impediments, and the concept that the neuron may not be the sole critical target for neurocognitive impairment. The accrued information on how SR exposure impacts neurocognitive performance may provide new opportunities to reduce neurocognitive impairment in brain cancer patients.

A rigorous behavioral testing platform for the assessment of radiation-induced neurological outcomes

Behavioral testing is a popular and reliable method of neurocognitive assessment of rodents but the lack of standard operating procedures has led to a high variation of protocols in use. Therefore, there exists a strong need to standardize protocols for a combined behavioral platform in order to maintain consistency across institutions and assist newcomers in the field. This paper provides details on the methodology of several behavioral tasks which have been validated in identifying radiation induced cognitive impairment as well as provide guidance on timescales and best practices. The cognitive assessments outlined here are optimized for rodent studies and either target learning and memory (open field task, object in updated location, novel object recognition, object in place, and temporal order) or mood and cognition (social interaction, elevated plus maze, light dark box, forced swim test, and fear extinction). We have utilized this platform successfully in evaluating cognitive injury induced by various radiation types, doses, fractionation schedules and also with ultra-high dose rate FLASH radiotherapy. Recommended materials and software are provided as well as advice on methods of data analysis. In this way a comprehensive behavioral platform is described with broad applicability to assess cognitive endpoints critical to therapeutic outcome.

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.

Nanoparticles-based anti-aging treatment of Alzheimer's disease

Age is the strongest risk factor for Alzheimer's disease (AD). In recent years, the relationship between aging and AD has been widely studied, with anti-aging therapeutics as the treatment for AD being one of the mainstream research directions. Therapeutics targeting senescent cells have shown improvement in AD symptoms and cerebral pathological changes, suggesting that anti-aging strategies may be a promising alternative for AD treatment. Nanoparticles represent an excellent approach for efficiently crossing the blood-brain barrier (BBB) to achieve better curative function and fewer side effects. Thereby, nanoparticles-based anti-aging treatment may exert potent anti-AD therapeutic efficacy. This review discusses the relationship between aging and AD and the application and prospect of anti-aging strategies and nanoparticle-based therapeutics in treating AD.

Recent Neurotherapeutic Strategies to Promote Healthy Brain Aging: Are we there yet?

Owing to the global exponential increase in population ageing, there is an urgent unmet need to develop reliable strategies to slow down and delay the ageing process. Age-related neurodegenerative diseases are among the main causes of morbidity and mortality in our contemporary society and represent a major socio-economic burden. There are several controversial factors that are thought to play a causal role in brain ageing which are continuously being examined in experimental models. Among them are oxidative stress and brain inflammation which are empirical to brain ageing. Although some candidate drugs have been developed which reduce the ageing phenotype, their clinical translation is limited. There are several strategies currently in development to improve brain ageing. These include strategies such as caloric restriction, ketogenic diet, promotion of cellular nicotinamide adenine dinucleotide (NAD+) levels, removal of senescent cells, 'young blood' transfusions, enhancement of adult neurogenesis, stem cell therapy, vascular risk reduction, and non-pharmacological lifestyle strategies. Several studies have shown that these strategies can not only improve brain ageing by attenuating age-related neurodegenerative disease mechanisms, but also maintain cognitive function in a variety of pre-clinical experimental murine models. However, clinical evidence is limited and many of these strategies are awaiting findings from large-scale clinical trials which are nascent in the current literature. Further studies are needed to determine their long-term efficacy and lack of adverse effects in various tissues and organs to gain a greater understanding of their potential beneficial effects on brain ageing and health span in humans.

Stem cell therapy for Alzheimer's disease: An overview of experimental models and reality

Alzheimer's disease (AD) is a neurodegenerative disorder. The pathology of AD is characterized by extracellular amyloid beta (Aβ) plaques, neurofibrillary tangles composed of hyperphosphorylated tau, neuronal death, synapse loss, and brain atrophy. Many therapies have been tested to improve or at least effectively modify the course of AD. Meaningful data indicate that the transplantation of stem cells can alleviate neuropathology and significantly ameliorate cognitive deficits in animal models with Alzheimer's disease. Transplanted stem cells have shown their inherent advantages in improving cognitive impairment and memory dysfunction, although certain weaknesses or limitations need to be overcome. This review recapitulates rodent models for AD, the therapeutic efficacy of stem cells, influencing factors, and the underlying mechanisms behind these changes. Stem cell therapy provides perspective and challenges for its clinical application in the future.

Neural stem cell transplantation alleviates functional cognitive deficits in a mouse model of tauopathy

The mechanisms of the transplantation of neural stem cells (NSCs) in the treatment of Alzheimer’s disease remain poorly understood. In this study, NSCs were transplanted into the hippocampal CA1 region of the rTg (tau P301L) 4510 mouse model, a tauopathy model that is thought to reflect the tau pathology associated with Alzheimer’s disease. The results revealed that NSC transplantation reduced the abnormal aggregation of tau, resulting in significant improvements in the short-term memory of the tauopathy model mice. Compared with wild-type and phosphate-buffered saline (PBS)-treated mice, mice that received NSC transplantations were characterized by changes in the expression of multiple proteins in brain tissue, particularly those related to the regulation of tau aggregation or misfolding. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis and Gene Ontology (GO) function analysis revealed that these proteins were primarily enriched in pathways associated with long-term potentiation, neurogenesis, and other neurobiological processes. Changes in the expression levels of key proteins were verified by western blot assays. These data provided clues to improve the understanding of the functional capacity associated with NSC transplantation in Alzheimer’s disease treatment. This study was approved by the Beijing Animal Ethics Association and Ethics Committee of Beijing Institute of Technology (approval No. SYXK-BIT-school of life science-2017-M03) in 2017.

Therapeutic effects of stem cells in different body systems, a novel method that is yet to gain trust: A comprehensive review

Stem cell therapy has been used to treat several types of diseases, and it is expected that its therapeutic uses shall increase as novel lines of evidence begin to appear. Furthermore, stem cells have the potential to make new tissues and organs. Thus, some scientists propose that organ transplantation will significantly rely on stem cell technology and organogenesis in the future. Stem cells and its robust potential to differentiate into specific types of cells and regenerate tissues and body organs, have been investigated by numerous clinician scientists and researchers for their therapeutic effects. Degenerative diseases in different organs have been the main target of stem cell therapy. Neurodegenerative diseases such as Alzheimer's, musculoskeletal diseases such as osteoarthritis, congenital cardiovascular diseases, and blood cell diseases such as leukemia are among the health conditions that have benefited from stem cell therapy advancements. One of the most challenging parts of the process of incorporating stem cells into clinical practice is controlling their division and differentiation potentials. Sometimes, their potential for  uncontrolled growth will make these cells tumorigenic. Another caveat in this process is the ability to control the differentiation process. While stem cells can easily differentiate into a wide variety of cells,  a paracrine effect controlled activity, being in an appropriate medium will cause abnormal differentiation leading to treatment failure. In this review, we aim to provide an overview of the therapeutic effects of stem cells in diseases of various organ systems. In order to advance this new treatment to its full potential, researchers should focus on establishing methods to control the differentiation process, while policymakers should take an active role in providing adequate facilities and equipment for these projects. Large population clinical trials are a necessary tool that will help build trust in this method. Moreover, improving social awareness about the advantages and adverse effects of stem cell therapy is required to develop a rational demand in the society, and consequently, healthcare systems should consider established stem cell-based therapeutic methods in their treatment algorithms.

Stem Cell Therapy for Post-Traumatic Stress Disorder: A Novel Therapeutic Approach

Stem cell therapy is a rapidly evolving field of regenerative medicine being employed for the management of various central nervous system disorders. The ability to self-renew, differentiate into specialized cells, and integrate into neuronal networks has positioned stem cells as an ideal mechanism for the treatment of epilepsy. Epilepsy is characterized by repetitive seizures caused by imbalance in the GABA and glutamate neurotransmission following neuronal damage. Stem cells provide benefit by reducing the glutamate excitotoxicity and strengthening the GABAergic inter-neuron connections. Similar to the abnormal neuroanatomic location in epilepsy, post-traumatic stress disorder (PTSD) is caused by hyperarousal in the amygdala and decreased activity of the hippocampus and medial prefrontal cortex. Thus, stem cells could be used to modulate neuronal interconnectivity. In this review, we provide a rationale for the use of stem cell therapy in the treatment of PTSD.

Stem cells as a potential therapeutic option for treating neurodegenerative diseases

In future, neurodegenerative diseases will take over cancer's place and become the major cause of death in the world, especially in developed countries. Advancements in the medical field and its facilities have led to an increase in the old age population, and thus contributing to the increase in number of people suffering from neurodegenerative diseases. Economically it is of a great burden to society and the affected family. No current treatment aims to replace, protect, and regenerate lost neurons; instead, it alleviates the symptoms, extends the life span by a few months and creates severe side effects. Moreover, people who are affected are physically dependent for performing their basic activities, which makes their life miserable. There is an urgent need for therapy that could be able to overcome the deficits of conventional therapy for neurodegenerative diseases. Stem cells, the unspecialized cells with the properties of self-renewing and potency to differentiate into various cells types can become a potent therapeutic option for neurodegenerative diseases. Stem cells have been widely used in clinical trials to evaluate their potential in curing different types of ailments. In this review, we discuss the various types of stem cells and their potential use in the treatment of neurodegenerative disease based on published preclinical and clinical studies.

The Cannabinoid Receptor 1 Reverse Agonist AM251 Ameliorates Radiation-Induced Cognitive Decrements

Despite advancements in the radiotherapeutic management of brain malignancies, resultant sequelae include persistent cognitive dysfunction in the majority of survivors. Defining the precise causes of normal tissue toxicity has proven challenging, but the use of preclinical rodent models has suggested that reductions in neurogenesis and microvascular integrity, impaired synaptic plasticity, increased inflammation, and alterations in neuronal structure are contributory if not causal. As such, strategies to reverse these persistent radiotherapy-induced neurological disorders represent an unmet medical need. AM251, a cannabinoid receptor 1 reverse agonist known to facilitate adult neurogenesis and synaptic plasticity, may help to ameliorate radiation-induced CNS impairments. To test this hypothesis, three treatment paradigms were used to evaluate the efficacy of AM251 to ameliorate radiation-induced learning and memory deficits along with disruptions in mood at 4 and 12 weeks postirradiation. Results demonstrated that acute (four weekly injections) and chronic (16 weekly injections) AM251 treatments (1 mg/kg) effectively alleviated cognitive and mood dysfunction in cranially irradiated mice. The beneficial effects of AM251 were exemplified by improved hippocampal- and cortical-dependent memory function on the novel object recognition and object in place tasks, while similar benefits on mood were shown by reductions in depressive- and anxiety-like behaviors on the forced swim test and elevated plus maze. The foregoing neurocognitive benefits were associated with significant increases in newly born (doublecortin+) neurons (1.7-fold), hippocampal neurogenesis (BrdU+/NeuN+mature neurons, 2.5-fold), and reduced expression of the inflammatory mediator HMGB (1.2-fold) in the hippocampus of irradiated mice. Collectively, these findings indicate that AM251 ameliorates the effects of clinically relevant cranial irradiation where overall neurological benefits in memory and mood coincided with increased hippocampal cell proliferation, neurogenesis, and reduced expression of proinflammatory markers.

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.

Human neural stem cell-derived extracellular vesicles mitigate hallmarks of Alzheimer's disease

Background

Regenerative therapies to mitigate Alzheimer’s disease (AD) neuropathology have shown very limited success. In the recent era, extracellular vesicles (EVs) derived from multipotent and pluripotent stem cells have shown considerable promise for the treatment of dementia and many neurodegenerative conditions.

Methods

Using the 5xFAD accelerated transgenic mouse model of AD, we now show the regenerative potential of human neural stem cell (hNSC)-derived EVs on the neurocognitive and neuropathologic hallmarks in the AD brain. Two- or 6-month-old 5xFAD mice received single or two intra-venous (retro-orbital vein, RO) injections of hNSC-derived EVs, respectively.

Results

RO treatment using hNSC-derived EVs restored fear extinction memory consolidation and reduced anxiety-related behaviors 4–6 weeks post-injection. EV treatment also significantly reduced dense core amyloid-beta plaque accumulation and microglial activation in both age groups. These results correlated with partial restoration of homeostatic levels of circulating pro-inflammatory cytokines in the AD mice. Importantly, EV treatment protected against synaptic loss in the AD brain that paralleled improved cognition. MiRNA analysis of the EV cargo revealed promising candidates targeting neuroinflammation and synaptic function.

Conclusions

Collectively, these data demonstrate the neuroprotective effects of systemic administration of stem cell-derived EVs for remediation of behavioral and molecular AD neuropathologies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-021-00791-x.

Alzheimer's disease and its treatment by different approaches: A review

Alzheimer's disease (AD) is a neurodegenerative disorder that impairs mental ability development and interrupts neurocognitive function. This neuropathological condition is depicted by neurodegeneration, neural loss, and development of neurofibrillary tangles and Aβ plaques. There is also a greater risk of developing AD at a later age for people with cardiovascular diseases, hypertension and diabetes. In the biomedical sciences, effective treatment for Alzheimer's disease is a severe obstacle. There is no such treatment to cure Alzheimer's disease. The drug present in the market show only symptomatic relief. The cause of Alzheimer's disease is not fully understood and the blood-brain barrier restricts drug efficacy are two main factors that hamper research. Stem cell-based therapy has been seen as an effective, secure, and creative therapeutic solution to overcoming AD because of AD's multifactorial nature and inadequate care. Current developments in nanotechnology often offer possibilities for the delivery of active drug candidates to address certain limitations. The key nanoformulations being tested against AD include polymeric nanoparticles (NP), inorganic NPs and lipid-based NPs. Nano drug delivery systems are promising vehicles for targeting several therapeutic moieties by easing drug molecules' penetration across the CNS and improving their bioavailability. In this review, we focus on the causes of the AD and their treatment by different approaches.

Therapeutic Potential of Mesenchymal Stromal Cells and Extracellular Vesicles in the Treatment of Radiation Lesions-A Review

Ionising radiation-induced normal tissue damage is a major concern in clinic and public health. It is the most limiting factor in radiotherapy treatment of malignant diseases. It can also cause a serious harm to populations exposed to accidental radiation exposure or nuclear warfare. With regard to the clinical use of radiation, there has been a number of modalities used in the field of radiotherapy. These includes physical modalities such modified collimators or fractionation schedules in radiotherapy. In addition, there are a number of pharmacological agents such as essential fatty acids, vasoactive drugs, enzyme inhibitors, antioxidants, and growth factors for the prevention or treatment of radiation lesions in general. However, at present, there is no standard procedure for the treatment of radiation-induced normal tissue lesions. Stem cells and their role in tissue regeneration have been known to biologists, in particular to radiobiologists, for many years. It was only recently that the potential of stem cells was studied in the treatment of radiation lesions. Stem cells, immediately after their successful isolation from a variety of animal and human tissues, demonstrated their likely application in the treatment of various diseases. This paper describes the types and origin of stem cells, their characteristics, current research, and reviews their potential in the treatment and regeneration of radiation induced normal tissue lesions. Adult stem cells, among those mesenchymal stem cells (MSCs), are the most extensively studied of stem cells. This review focuses on the effects of MSCs in the treatment of radiation lesions.

Can a comparison of clinical and deep space irradiation scenarios shed light on the radiation response of the brain?

Not surprisingly, our knowledge of the impact of radiation on the brain has evolved considerably. Decades of work have struggled with identifying the critical cellular targets in the brain, the latency of functional change and understanding how irradiation alters the balance between excitatory and inhibitory circuits. Radiation-induced cell kill following clinical fractionation paradigms pointed to both stromal and parenchymal targets but also defined an exquisite sensitivity of neurogenic populations of newly born cells in the brain. It became more and more apparent too, that acute (days) events transpiring after exposure were poorly prognostic of the late (months-years) waves of radiation injury believed to underlie neurocognitive deficits. Much of these gaps in knowledge persisted as NASA became interested in how exposure to much different radiation types, doses and dose rates that characterize the space radiation environment might impair central nervous system functionality, with possibly negative implications for deep space travel. Now emerging evidence from researchers engaged in clinical, translational and environmental radiation sciences have begun to fill these gaps and have uncovered some surprising similarities in the response of the brain to seemingly disparate exposure scenarios. This article highlights many of the commonalities between the vastly different irradiation paradigms that distinguish clinical treatments from occupational exposures in deep space.

Extracellular Vesicle-Derived miR-124 Resolves Radiation-Induced Brain Injury

Radiation-induced cognitive dysfunction (RICD) is a progressive and debilitating health issue facing patients following cranial radiotherapy to control central nervous system cancers. There has been some success treating RICD in rodents using human neural stem cell (hNSC) transplantation, but the procedure is invasive, requires immunosuppression, and could cause other complications such as teratoma formation. Extracellular vesicles (EV) are nanoscale membrane-bound structures that contain biological contents including mRNA, miRNA, proteins, and lipids that can be readily isolated from conditioned culture media. It has been previously shown that hNSC-derived EV resolves RICD following cranial irradiation using an immunocompromised rodent model. Here, we use immunocompetent wild-type mice to show that hNSC-derived EV treatment administered either intravenously via retro-orbital vein injection or via intracranial transplantation can ameliorate cognitive deficits following 9 Gy head-only irradiation. Cognitive function assessed on the novel place recognition, novel object recognition, and temporal order tasks was not only improved at early (5 weeks) but also at delayed (6 months) postirradiation times with just a single EV treatment. Improved behavioral outcomes were also associated with reduced neuroinflammation as measured by a reduction in activated microglia. To identify the mechanism of action, analysis of EV cargo implicated miRNA (miR-124) as a potential candidate in the mitigation of RICD. Furthermore, viral vector-mediated overexpression of miR-124 in the irradiated brain ameliorated RICD and reduced microglial activation. Our findings demonstrate for the first time that systemic administration of hNSC-derived EV abrogates RICD and neuroinflammation in cranially irradiated wild-type rodents through a mechanism involving miR-124. SIGNIFICANCE: Radiation-induced neurocognitive decrements in immunocompetent mice can be resolved by systemic delivery of hNSC-derived EVs involving a mechanism dependent on expression of miR-124.

Prevention and treatment of radiotherapy-induced side effects

Radiotherapy remains a mainstay of cancer treatment, being used in roughly 50% of patients. The precision with which the radiation dose can be delivered is rapidly improving. This precision allows the more accurate targeting of radiation dose to the tumor and reduces the amount of surrounding normal tissue exposed. Although this often reduces the unwanted side effects of radiotherapy, we still need to further improve patients' quality of life and to escalate radiation doses to tumors when necessary. High-precision radiotherapy forces one to choose which organ or functional organ substructures should be spared. To be able to make such choices, we urgently need to better understand the molecular and physiological mechanisms of normal tissue responses to radiotherapy. Currently, oversimplified approaches using constraints on mean doses, and irradiated volumes of normal tissues are used to plan treatments with minimized risk of radiation side effects. In this review, we discuss the responses of three different normal tissues to radiotherapy: the salivary glands, cardiopulmonary system, and brain. We show that although they may share very similar local cellular processes, they respond very differently through organ-specific, nonlocal mechanisms. We also discuss how a better knowledge of these mechanisms can be used to treat or to prevent the effects of radiotherapy on normal tissue and to optimize radiotherapy delivery.

The Acute and Early Effects of Whole-Brain Irradiation on Glial Activation, Brain Metabolism, and Behavior: a Positron Emission Tomography Study

Purpose

Radiotherapy is a frequently applied treatment modality for brain tumors. Concomitant irradiation of normal brain tissue can induce various physiological responses. The aim of this study was to investigate whether acute and early-delayed effects of brain irradiation on glial activation and brain metabolism can be detected with positron emission tomography (PET) and whether these effects are correlated with behavioral changes.

Procedures

Rats underwent 0-, 10-, or 25-Gy whole-brain irradiation. At 3 and 31 days post irradiation, 1-(2-chlorophenyl)-N-[¹¹C]methyl-(1-methylpropyl)-3-isoquinoline carboxamide ([¹¹C]PK11195) and 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) PET scans were acquired to detect changes in glial activation (neuroinflammation) and glucose metabolism, respectively. The open-field test (OFT) was performed on days 6 and 27 to assess behavioral changes.

Results

Twenty-five-gray-irradiated rats showed higher [¹¹C]PK11195 uptake in most brain regions than controls on day 3 (striatum, hypothalamus, accumbens, septum p < 0.05), although some brain regions had lower uptake (cerebellum, parietal association/retrosplenial visual cortex, frontal association/motor cortex, somatosensory cortex, p < 0.05). On day 31, several brain regions in 25-Gy-irradiated rats still showed significantly higher [¹¹C]PK11195 uptake than controls and 10-Gy-irradiated group (p < 0.05). Within-group analysis showed that [¹¹C]PK11195 uptake in individual brain regions of 25-Gy treated rats remained stable or slightly increased between days 3 and 31. In contrast, a significant reduction (p < 0.05) in tracer uptake between days 3 and 31 was found in all brain areas of controls and 10-Gy-irradiated animals. Moreover, 10-Gy treatment led to a significantly higher [¹⁸F]FDG uptake on day 3 (p < 0.05). [¹⁸F]FDG uptake decreased between days 3 and 31 in all groups; no significant differences between groups were observed anymore on day 31, except for increased uptake in the hypothalamus in the 10-Gy group. The OFT did not show any significant differences between groups.

Conclusions

Non-invasive PET imaging indicated that brain irradiation induces neuroinflammation and a metabolic flare, without causing acute or early-delayed behavioral changes.

Electronic supplementary material

The online version of this article (10.1007/s11307-020-01483-y) contains supplementary material, which is available to authorized users.

Functional equivalence of stem cell and stem cell-derived extracellular vesicle transplantation to repair the irradiated brain

Cranial radiotherapy, although beneficial for the treatment of brain tumors, inevitably leads to normal tissue damage that can induce unintended neurocognitive complications that are progressive and debilitating. Ionizing radiation exposure has also been shown to compromise the structural integrity of mature neurons throughout the brain, an effect believed to be at least in part responsible for the deterioration of cognitive health. Past work has shown that cranially transplanted human neural stem cells (hNSCs) or their extracellular vesicles (EVs) afforded long-term beneficial effects on many of these cognitive decrements. To provide additional insight into the potential neuroprotective mechanisms of cell-based regenerative strategies, we have analyzed hippocampal neurons for changes in structural integrity and synaptic remodeling after unilateral and bilateral transplantation of hNSCs or EVs derived from those same cells. Interestingly, hNSCs and EVs similarly afforded protection to host neurons, ameliorating the impact of irradiation on dendritic complexity and spine density for neurons present in both the ipsilateral and contralateral hippocampi 1 month following irradiation and transplantation. These morphometric improvements were accompanied by increased levels of glial cell-derived growth factor and significant attenuation of radiation-induced increases in postsynaptic density protein 95 and activated microglia were found ipsi- and contra-lateral to the transplantation sites of the irradiated hippocampus treated with hNSCs or hNSC-derived EVs. These findings document potent far-reaching neuroprotective effects mediated by grafted stem cells or EVs adjacent and distal to the site of transplantation and support their potential as therapeutic agents to counteract the adverse effects of cranial irradiation.

Neural stem cell therapy for neurovascular injury in Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia, is characterized by progressive neurodegeneration leading to severe cognitive decline and eventual death. AD pathophysiology is complex, but neurotoxic accumulation of amyloid-β (Aβ) and hyperphosphorylation of Tau are believed to be main drivers of neurodegeneration in AD. The formation and deposition of Aβ plaques occurs in the brain parenchyma as well as in the cerebral vasculature. Thus, proper blood-brain barrier (BBB) and cerebrovascular functioning are crucial for clearance of Aβ from the brain, and neurovascular dysfunction may be a critical component of AD development. Further, neuroinflammation and dysfunction of angiogenesis, neurogenesis, and neurorestorative capabilities play a role in AD pathophysiology. Currently, there is no effective treatment to prevent or restore loss of brain tissue and cognitive decline in patients with AD. Based on multifactorial and complex pathophysiological cascades in multiple Alzheimer's disease stages, effective AD therapies need to focus on targeting early AD pathology and preserving cerebrovascular function. Neural stem cells (NSCs) participate extensively in mammalian brain homeostasis and repair and exhibit pleiotropic intrinsic properties that likely make them attractive candidates for the treatment of AD. In the review, we summarize the current advances in knowledge regarding neurovascular aspects of AD-related neurodegeneration and discuss multiple actions of NSCs from preclinical studies of AD to evaluate their potential for future clinical treatment of AD.

Distinct modes of death in human neural stem and glioblastoma cells irradiated with carbon-ion radiation and gamma-rays

Purpose: Accumulated damage in neural stem cells (NSCs) during brain tumor radiotherapy causes cognitive dysfunction to the patients. Carbon-ion radiotherapy can reduce undesired irradiation of normal tissues more efficiently than conventional photon radiotherapy. This study elucidates the responses of NSCs to carbon-ion radiation.Methods: Human NSCs and glioblastoma A-172 cells were irradiated with carbon-ion radiation and γ-rays, which have different linear-energy-transfer (LET) values of 108 and 0.2 keV/μm, respectively. After irradiation, growth rates were measured, apoptotic cells were detected by flow cytometry, and DNA synthesizing cells were immunocytochemically visualized.Results: Growth rates of NSCs and A-172 cells were decreased after irradiation. The percentages of apoptotic cells were remarkably increased in NSCs but not in A-172 cells. In contrast, the fractions of DNA synthesizing A-172 cells were decreased in a dose-dependent manner. These results indicate that apoptosis induction and DNA synthesis inhibition contribute to the growth inhibition of NSCs and glioblastoma cells, respectively. In addition, high-LET carbon ions induced more profound effects than low-LET γ-rays.Conclusions: Apoptosis is an important clinical target to protect NSCs during brain tumor radiotherapy using carbon-ion radiation as well as conventional X-rays.

A computational model of radiolytic oxygen depletion during FLASH irradiation and its effect on the oxygen enhancement ratio

Recent results from animal irradiation studies have demonstrated the potential of ultra-high dose rate irradiation (also known as FLASH) for reducing radiation toxicity in normal tissues. However, despite mounting evidence of a 'FLASH effect', a mechanism has yet to be elucidated. This article hypothesizes that the radioprotecting effect of FLASH irradiation could be due to the specific sparing of hypoxic stem cell niches, which have been identified in several organs including the bone marrow and the brain. To explore this hypothesis, a new computational model is presented that frames transient radiolytic oxygen depletion (ROD) during FLASH irradiation in terms of its effect on the oxygen enhancement ratio (OER). The model takes into consideration oxygen diffusion through the tissue, its consumption by metabolic cells, and its radiolytic depletion to estimate the relative decrease in radiosensitivity of cells receiving FLASH irradiation. Based on this model and the following parameters (oxygen diffusion constant [Formula: see text]  =  2 · 10^-5 cm² s^-1, oxygen metabolic rate m  =  3 mmHg s^-1, ROD rate L _ROD  =  [Formula: see text] mmHg Gy^-1, prescribed dose D _p  =  10 Gy, and capillary oxygen tension p ₀  =  40 mmHg), several predictions are made that could be tested in future experiments: (1) the FLASH effect should gradually disappear as the radiation pulse duration is increased from  <1 s to 10 s; (2) dose should be deposited using the smallest number of radiation pulses to achieve the greatest FLASH effect; (3) a FLASH effect should only be observed in cells that are already hypoxic at the time of irradiation; and (4) changes in capillary oxygen tension (increase or decrease) should diminish the FLASH effect.

Neural regeneration therapies for Alzheimer's and Parkinson's disease-related disorders

Neurodegenerative diseases are devastating mental illnesses without a cure. Alzheimer's disease (AD) characterized by memory loss, multiple cognitive impairments, and changes in personality and behavior. Although tremendous progress has made in understanding the basic biology in disease processes in AD and PD, we still do not have early detectable biomarkers for these diseases. Just in the United States alone, federal and nonfederal funding agencies have spent billions of dollars on clinical trials aimed at finding drugs, but we still do not have a drug or an agent that can slow the AD or PD disease process. One primary reason for this disappointing result may be that the clinical trials enroll patients with AD or PD at advances stages. Although many drugs and agents are tested preclinical and are promising, in human clinical trials, they are mostly ineffective in slowing disease progression. One therapy that has been promising is 'stem cell therapy' based on cell culture and pre-clinical studies. In the few clinical studies that have investigated therapies in clinical trials with AD and PD patients at stage I. The therapies, such as stem cell transplantation - appear to delay the symptoms in AD and PD. The purpose of this article is to describe clinical trials using 1) stem cell transplantation methods in AD and PD mouse models and 2) regenerative medicine in AD and PD mouse models, and 3) the current status of investigating preclinical stem cell transplantation in patients with AD and PD.

Long-term neurocognitive benefits of FLASH radiotherapy driven by reduced reactive oxygen species

Here, we highlight the potential translational benefits of delivering FLASH radiotherapy using ultra-high dose rates (>100 Gy⋅s-1). Compared with conventional dose-rate (CONV; 0.07-0.1 Gy⋅s-1) modalities, we showed that FLASH did not cause radiation-induced deficits in learning and memory in mice. Moreover, 6 months after exposure, CONV caused permanent alterations in neurocognitive end points, whereas FLASH did not induce behaviors characteristic of anxiety and depression and did not impair extinction memory. Mechanistic investigations showed that increasing the oxygen tension in the brain through carbogen breathing reversed the neuroprotective effects of FLASH, while radiochemical studies confirmed that FLASH produced lower levels of the toxic reactive oxygen species hydrogen peroxide. In addition, FLASH did not induce neuroinflammation, a process described as oxidative stress-dependent, and was also associated with a marked preservation of neuronal morphology and dendritic spine density. The remarkable normal tissue sparing afforded by FLASH may someday provide heretofore unrealized opportunities for dose escalation to the tumor bed, capabilities that promise to hasten the translation of this groundbreaking irradiation modality into clinical practice.

Human Mesenchymal Stem Cells Prevent Neurological Complications of Radiotherapy

Radiotherapy is a highly effective tool for the treatment of brain cancer. However, radiation also causes detrimental effects in the healthy tissue, leading to neurocognitive sequelae that compromise the quality of life of brain cancer patients. Despite the recognition of this serious complication, no satisfactory solutions exist at present. Here we investigated the effects of intranasal administration of human mesenchymal stem cells (hMSCs) as a neuroprotective strategy for cranial radiation in mice. Our results demonstrated that intranasally delivered hMSCs promote radiation-induced brain injury repair, improving neurological function. This intervention confers protection against inflammation, oxidative stress, and neuronal loss. hMSC administration reduces persistent activation of damage-induced c-AMP response element-binding signaling in irradiated brains. Furthermore, hMSC treatment did not compromise the survival of glioma-bearing mice. Our findings encourage the therapeutic use of hMSCs as a non-invasive approach to prevent neurological complications of radiotherapy, improving the quality of life of brain tumor patients.

Long-term survival and late complications of intensity-modulated radiotherapy for recurrent nasopharyngeal carcinoma

BACKGROUND

To evaluate the effectiveness and toxicities of intensity-modulated radiotherapy (IMRT) for locally recurrent nasopharyngeal carcinoma (NPC).

METHODS

One hundred and eighty-four previously irradiated NPC patients with recurrent disease and re-irradiated by IMRT between February 2005 to May 2013 had been reviewed. The disease was re-staged I in 33, II in 27, III in 70 and IV in 54 patients. Seventy-five percent of the patients received cisplatin-based chemotherapy.

RESULTS

The median survival time was 33 months. The 3-year actuarial rates of local recurrence-free survival (LRFS), distant metastases-free survival (DMFS), and overall survival (OS) rates were 85.1, 91.1, and 46.0%, respectively. About 53% of the patients experienced Grade 3-4 late toxicities. Forty-four patients died of massive hemorrhage of the nasopharynx caused by radiation induced mucosal necrosis. Multivariate analysis indicated that chemotherapy and time interval between initial radiotherapy and re-irradiation were independent predictors for DMFS.

CONCLUSION

IMRT is an effective method for patients with locally recurrent NPC. Massive hemorrhage of the nasopharynx is the major sever late complication and also the leading cause of death. Early recurrence is negative factor for DMFS. Combination of chemotherapy can improve DMFS, but not for OS. Optimal salvage treatment strategies focusing on improvement of survival and minimization of late toxicities are warranted.

Central Nervous System Responses to Simulated Galactic Cosmic Rays

In preparation for lunar and Mars missions it is essential to consider the challenges to human health that are posed by long-duration deep space habitation via multiple stressors, including ionizing radiation, gravitational changes during flight and in orbit, other aspects of the space environment such as high level of carbon dioxide, and psychological stress from confined environment and social isolation. It remains unclear how these stressors individually or in combination impact the central nervous system (CNS), presenting potential obstacles for astronauts engaged in deep space travel. Although human spaceflight research only within the last decade has started to include the effects of radiation transmitted by galactic cosmic rays to the CNS, radiation is currently considered to be one of the main stressors for prolonged spaceflight and deep space exploration. Here we will review the current knowledge of CNS damage caused by simulated space radiation with an emphasis on neuronal and glial responses along with cognitive functions. Furthermore, we will present novel experimental approaches to integrate the knowledge into more comprehensive studies, including multiple stressors at once and potential translation to human functions. Finally, we will discuss the need for developing biomarkers as predictors for cognitive decline and therapeutic countermeasures to prevent CNS damage and the loss of cognitive abilities.

Grafting Neural Stem and Progenitor Cells Into the Hippocampus of Juvenile, Irradiated Mice Normalizes Behavior Deficits

The pool of neural stem and progenitor cells (NSPCs) in the dentate gyrus of the hippocampus is reduced by ionizing radiation. This explains, at least partly, the learning deficits observed in patients after radiotherapy, particularly in pediatric cases. An 8 Gy single irradiation dose was delivered to the whole brains of postnatal day 9 (P9) C57BL/6 mice, and BrdU-labeled, syngeneic NSPCs (1.0 × 10⁵ cells/injection) were grafted into each hippocampus on P21. Three months later, behavior tests were performed. Irradiation impaired novelty-induced exploration, place learning, reversal learning, and sugar preference, and it altered the movement pattern. Grafting of NSPCs ameliorated or even normalized the observed deficits. Less than 4% of grafted cells survived and were found in the dentate gyrus 5 months later. The irradiation-induced loss of endogenous, undifferentiated NSPCs in the dentate gyrus was completely restored by grafted NSPCs in the dorsal, but not the ventral, blade. The grafted NSPCs did not exert appreciable effects on the endogenous NSPCs; however, more than half of the grafted NSPCs differentiated. These results point to novel strategies aimed at ameliorating the debilitating late effects of cranial radiotherapy, particularly in children.

A Novel Murine Model of Radiation Keratopathy

Purpose

Radiation therapy results in severe chronic keratopathy and dry eye disease. We developed a novel mouse model for radiation keratopathy to allow future mechanistic studies.

Methods

Six to 8-week-old BALB/c mice underwent sublethal irradiation to the head only from a Cesium-137 irradiator, 2 × 550 rad, 3-hours apart. Irradiated mice were clinically evaluated by corneal fluorescein staining (CFS) at 1, 2, and 3 months, after which corneas were excised and immunofluorescence histochemistry performed with anti-CD45, anti-MHC class II, and anti-β-tubulin antibodies.

Results

The survival rate after irradiation was 100%. Mice demonstrated significant CFS and hair loss around the eyes. Corneal nerve density decreased in the central and peripheral corneas (P < 0.01) at 2 and 3 months, respectively. CD45+ immune cell densities increased in the central and peripheral corneas (P < 0.005, P < 0.001) at 2 and 3 months, respectively. MHC class II, a sign of antigen presenting cell activation, significantly increased after irradiation in the central and peripheral corneas at 2 and 3 months (P = 0.02). A strong inverse correlation was noted between decreased corneal nerves and increase in CD45+ cells in the central cornea at 2 (P = 0.04, r = -0.89) and 3 months (P = 0.03, r = -0.91) after irradiation.

Conclusions

We present a model of radiation keratopathy and demonstrate significant nerve loss and increase in immune cell influx and activation within months. This model will enable future investigations to understand the effects of radiation therapy on the eye, and to study mechanisms of neuro-immune crosstalk in the cornea.

Mesenchymal Stem Cells Attenuate Radiation-Induced Brain Injury by Inhibiting Microglia Pyroptosis

Radiation-induced brain injury (RI) commonly occurs in patients who received head and neck radiotherapy. However, the mechanism of RI remains unclear. We aimed to evaluate whether pyroptosis was involved in RI and the impact of mesenchymal stem cells (MSCs) on it. BALB/c male mice (6–8 weeks) were cranially irradiated (15 Gy), and MSCs were transplanted into the bilateral cortex 2 days later; then mice were sacrificed 1 month later. Meanwhile, irradiated BV-2 microglia cells (10 Gy) were cocultured with MSCs for 24 hours. We observed that irradiated mice brains presented NLRP3 and caspase-1 activation. RT-PCR then indicated that it mainly occurred in microglia cells but not in neurons. Further, irradiated BV-2 cells showed pyroptosis and increased production of IL-18 and IL-1β. RT-PCR also demonstrated an increased expression of several inflammasome genes in irradiated BV-2 cells, including NLRP3 and AIM2. Particularly, NLRP3 was activated. Knockdown of NLRP3 resulted in decreased LDH release. Noteworthily, in vivo, MSCs transplantation alleviated radiation-induced NLRP3 and caspase-1 activation. Moreover, in vitro, MSCs could decrease caspase-1 dependent pyroptosis, NLRP3 inflammasome activation, and ROS production induced by radiation. Thus, our findings proved that microglia pyroptosis occurred in RI. MSCs may act as a potent therapeutic tool in attenuating pyroptosis.

Stem Cell Therapies for the Resolution of Radiation Injury to the Brain

Purpose of review

To encapsulate past and current research efforts focused on stem cell transplantation strategies to resolve radiation-induced cognitive dysfunction.

Recent Findings

Transplantation of human stem cells in the irradiated brain was first shown to resolve radiation-induced cognitive dysfunction in a landmark paper by Acharya et al., appearing in PNAS in 2009. Since that time, work from the same laboratory as well as other groups have reported on the beneficial (as well as detrimental) effects of stem cell grafting after cranial radiation exposure. Improved learning and memory found many months after engraftment has since been associated with a preservation of host neuronal morphology, a suppression of neuroinflammation, improved myelination and increased cerebral blood flow. Interestingly, many (if not all) of these beneficial effects can be demonstrated by substituting stem cells with microvesicles derived from human stem cells during transplantation, thereby eliminating many of the more long-standing concerns related to immunorejection and teratoma formation.

Summary

Stem cell and microvesicle transplantation into the irradiated brain of rodents has uncovered some unexpected benefits that hold promise for ameliorating many of adverse neurocognitive complications associated with major cancer treatments. Properly developed, such approaches may provide much needed clinical recourse to millions of cancer survivors suffering from the unintended side effects of their cancer therapies.

[Cognitive deficits following brain tumor radiation therapy]

Brain radiation is an important treatment option for malignant and benign brain diseases. The possible acute or chronic impact of radiation therapy on cognitive performance is important for daily functioning and quality of life. A detailed evaluation of cognitive impairment is important in the context of how to control disease progression. The susceptibility of the hippocampus to radiation-induced neuronal damage and its important role in memory highlight that therapeutic strategies require precision medicine.

Alzheimer's disease, dementia, and stem cell therapy

Alzheimer’s disease (AD) represents arguably the most significant social, economic, and medical crisis of our time. Characterized by progressive neurodegenerative pathology, AD is first and foremost a condition of neuronal and synaptic loss. Repopulation and regeneration of depleted neuronal circuitry by exogenous stem cells is therefore a rational therapeutic strategy. This review will focus on recent advances in stem cell therapies utilizing animal models of AD, as well as detailing the human clinical trials of stem cell therapies for AD that are currently undergoing development.

The Promise of Targeting Macrophages in Cancer Therapy

Cancer therapy has developed around the concept of killing, or stopping the growth of, the cancer cells. Molecularly targeted therapy is the modern expression of this paradigm. Increasingly, however, the realization that the cancer has co-opted the normal cells of the stroma for its own survival has led to the concept that the tumor microenvironment (TME) could be targeted for effective therapy. In this review, we outline the importance of tumor-associated macrophages (TAM), a major component of the TME, in the response of tumors to cancer therapy. We discuss the normal role of macrophages in wound healing, the major phenotypes of TAMs, and their role in blunting the efficacy of cancer treatment by radiation and anticancer drugs, both by promoting tumor angiogenesis and by suppressing antitumor immunity. Finally, we review the many preclinical studies that have shown that the response of tumors to irradiation and anticancer drugs can be improved, sometimes markedly so, by depleting TAMs from tumors or by suppressing their polarization from an M1 to an M2 phenotype. The data clearly support the validity of clinical testing of combining targeting TAMs with conventional therapy. Clin Cancer Res; 23(13); 3241-50. ©2017 AACR.

Radiation-induced brain injury: low-hanging fruit for neuroregeneration

Brain radiation is a fundamental tool in neurooncology to improve local tumor control, but it leads to profound and progressive impairments in cognitive function. Increased attention to quality of life in neurooncology has accelerated efforts to understand and ameliorate radiation-induced cognitive sequelae. Such progress has coincided with a new understanding of the role of CNS progenitor cell populations in normal cognition and in their potential utility for the treatment of neurological diseases. The irradiated brain exhibits a host of biochemical and cellular derangements, including loss of endogenous neurogenesis, demyelination, and ablation of endogenous oligodendrocyte progenitor cells. These changes, in combination with a state of chronic neuroinflammation, underlie impairments in memory, attention, executive function, and acquisition of motor and language skills. Animal models of radiation-induced brain injury have demonstrated a robust capacity of both neural stem cells and oligodendrocyte progenitor cells to restore cognitive function after brain irradiation, likely through a combination of cell replacement and trophic effects. Oligodendrocyte progenitor cells exhibit a remarkable capacity to migrate, integrate, and functionally remyelinate damaged white matter tracts in a variety of preclinical models. The authors here critically address the opportunities and challenges in translating regenerative cell therapies from rodents to humans. Although valiant attempts to translate neuroprotective therapies in recent decades have almost uniformly failed, the authors make the case that harnessing human radiation-induced brain injury as a scientific tool represents a unique opportunity to both successfully translate a neuroregenerative therapy and to acquire tools to facilitate future restorative therapies for human traumatic and degenerative diseases of the central nervous system.

Changes in Imaging and Cognition in Juvenile Rats After Whole-Brain Irradiation

PURPOSE

In pediatric cancer survivors treated with whole-brain irradiation (WBI), long-term cognitive deficits and morbidity develop that are poorly understood and for which there is no treatment. We describe similar cognitive defects in juvenile WBI rats and correlate them with alterations in diffusion tensor imaging and magnetic resonance spectroscopy (MRS) during brain development.

METHODS AND MATERIALS

Juvenile Fischer rats received clinically relevant fractionated doses of WBI or a high-dose exposure. Diffusion tensor imaging and MRS were performed at the time of WBI and during the subacute (3-month) and late (6-month) phases, before behavioral testing.

RESULTS

Fractional anisotropy in the splenium of the corpus callosum increased steadily over the study period, reflecting brain development. WBI did not alter the subacute response, but thereafter there was no further increase in fractional anisotropy, especially in the high-dose group. Similarly, the ratios of various MRS metabolites to creatine increased over the study period, and in general, the most significant changes after WBI were during the late phase and with the higher dose. The most dramatic changes observed were in glutamine-creatine ratios that failed to increase normally between 3 and 6 months after either radiation dose. WBI did not affect the ambulatory response to novel open field testing in the subacute phase, but locomotor habituation was impaired and anxiety-like behaviors increased. As for cognitive measures, the most dramatic impairments were in novel object recognition late after either dose of WBI.

CONCLUSIONS

The developing brains of juvenile rats given clinically relevant fractionated doses of WBI show few abnormalities in the subacute phase but marked late cognitive alterations that may be linked with perturbed MRS signals measured in the corpus callosum. This pathomimetic phenotype of clinically relevant cranial irradiation effects may be useful for modeling, mechanistic evaluations, and testing of mitigation approaches.

Elimination of microglia improves cognitive function following cranial irradiation

Cranial irradiation for the treatment of brain cancer elicits progressive and severe cognitive dysfunction that is associated with significant neuropathology. Radiation injury in the CNS has been linked to persistent microglial activation, and we find upregulation of pro-inflammatory genes even 6 weeks after irradiation. We hypothesize that depletion of microglia in the irradiated brain would have a neuroprotective effect. Adult mice received acute head only irradiation (9 Gy) and were administered a dietary inhibitor (PLX5622) of colony stimulating factor-1 receptor (CSF1R) to deplete microglia post-irradiation. Cohorts of mice maintained on a normal and PLX5662 diet were analyzed for cognitive changes using a battery of behavioral tasks 4–6 weeks later. PLX5622 treatment caused a rapid and near complete elimination of microglia in the brain within 3 days of treatment. Irradiation of animals given a normal diet caused characteristic behavioral deficits designed to test medial pre-frontal cortex (mPFC) and hippocampal learning and memory and caused increased microglial activation. Animals receiving the PLX5622 diet exhibited no radiation-induced cognitive deficits, and exhibited near complete loss of IBA-1 and CD68 positive microglia in the mPFC and hippocampus. Our data demonstrate that elimination of microglia through CSF1R inhibition can ameliorate radiation-induced cognitive deficits in mice.

Adenosine Kinase Inhibition Protects against Cranial Radiation-Induced Cognitive Dysfunction

Clinical radiation therapy for the treatment of CNS cancers leads to unintended and debilitating impairments in cognition. Radiation-induced cognitive dysfunction is long lasting; however, the underlying molecular and cellular mechanisms are still not well established. Since ionizing radiation causes microglial and astroglial activation, we hypothesized that maladaptive changes in astrocyte function might be implicated in radiation-induced cognitive dysfunction. Among other gliotransmitters, astrocytes control the availability of adenosine, an endogenous neuroprotectant and modulator of cognition, via metabolic clearance through adenosine kinase (ADK). Adult rats exposed to cranial irradiation (10 Gy) showed significant declines in performance of hippocampal-dependent cognitive function tasks [novel place recognition, novel object recognition (NOR), and contextual fear conditioning (FC)] 1 month after exposure to ionizing radiation using a clinically relevant regimen. Irradiated rats spent less time exploring a novel place or object. Cranial irradiation also led to reduction in freezing behavior compared to controls in the FC task. Importantly, immunohistochemical analyses of irradiated brains showed significant elevation of ADK immunoreactivity in the hippocampus that was related to astrogliosis and increased expression of glial fibrillary acidic protein (GFAP). Conversely, rats treated with the ADK inhibitor 5-iodotubercidin (5-ITU, 3.1 mg/kg, i.p., for 6 days) prior to cranial irradiation showed significantly improved behavioral performance in all cognitive tasks 1 month post exposure. Treatment with 5-ITU attenuated radiation-induced astrogliosis and elevated ADK immunoreactivity in the hippocampus. These results confirm an astrocyte-mediated mechanism where preservation of extracellular adenosine can exert neuroprotection against radiation-induced pathology. These innovative findings link radiation-induced changes in cognition and CNS functionality to altered purine metabolism and astrogliosis, thereby linking the importance of adenosine homeostasis in the brain to radiation injury.

Transplantation of human neural stem cells restores cognition in an immunodeficient rodent model of traumatic brain injury

Traumatic brain injury (TBI) in humans can result in permanent tissue damage and has been linked to cognitive impairment that lasts years beyond the initial insult. Clinically effective treatment strategies have yet to be developed. Transplantation of human neural stem cells (hNSCs) has the potential to restore cognition lost due to injury, however, the vast majority of rodent TBI/hNSC studies to date have evaluated cognition only at early time points, typically <1month post-injury and cell transplantation. Additionally, human cell engraftment and long-term survival in rodent models of TBI has been difficult to achieve due to host immunorejection of the transplanted human cells, which confounds conclusions pertaining to transplant-mediated behavioral improvement. To overcome these shortfalls, we have developed a novel TBI xenotransplantation model that utilizes immunodeficient athymic nude (ATN) rats as the host recipient for the post-TBI transplantation of human embryonic stem cell (hESC) derived NSCs and have evaluated cognition in these animals at long-term (≥2months) time points post-injury. We report that immunodeficient ATN rats demonstrate hippocampal-dependent spatial memory deficits (Novel Place, Morris Water Maze), but not non-spatial (Novel Object) or emotional/anxiety-related (Elevated Plus Maze, Conditioned Taste Aversion) deficits, at 2-3months post-TBI, confirming that ATN rats recapitulate some of the cognitive deficits found in immunosufficient animal strains. Approximately 9-25% of transplanted hNSCs survived for at least 5months post-transplantation and differentiated into mature neurons (NeuN, 18-38%), astrocytes (GFAP, 13-16%), and oligodendrocytes (Olig2, 11-13%). Furthermore, while this model of TBI (cortical impact) targets primarily cortex and the underlying hippocampus and generates a large lesion cavity, hNSC transplantation facilitated cognitive recovery without affecting either lesion volume or total spared cortical or hippocampal tissue volume. Instead, we have found an overall increase in host hippocampal neuron survival in hNSC transplanted animals and demonstrate that a correlation exists between hippocampal neuron survival and cognitive performance. Together, these findings support the use of immunodeficient rodents in models of TBI that involve the transplantation of human cells, and suggest that hNSC transplantation may be a viable, long-term therapy to restore cognition after brain injury.

Cranial grafting of stem cell-derived microvesicles improves cognition and reduces neuropathology in the irradiated brain

Cancer survivors face a variety of challenges as they cope with disease recurrence and a myriad of normal tissue complications brought on by radio- and chemotherapeutic treatment regimens. For patients subjected to cranial irradiation for the control of CNS malignancy, progressive and debilitating cognitive dysfunction remains a pressing unmet medical need. Although this problem has been recognized for decades, few if any satisfactory long-term solutions exist to resolve this serious unintended side effect of radiotherapy. Past work from our laboratory has demonstrated the neurocognitive benefits of human neural stem cell (hNSC) grafting in the irradiated brain, where intrahippocampal transplantation of hNSC ameliorated radiation-induced cognitive deficits. Using a similar strategy, we now provide, to our knowledge, the first evidence that cranial grafting of microvesicles secreted from hNSC affords similar neuroprotective phenotypes after head-only irradiation. Cortical- and hippocampal-based deficits found 1 mo after irradiation were completely resolved in animals cranially grafted with microvesicles. Microvesicle treatment was found to attenuate neuroinflammation and preserve host neuronal morphology in distinct regions of the brain. These data suggest that the neuroprotective properties of microvesicles act through a trophic support mechanism that reduces inflammation and preserves the structural integrity of the irradiated microenvironment.