Mouse genomic associations with in vitro sensitivity to simulated space radiation

Exposure to ionizing radiation is considered by NASA to be a major health hazard for deep space exploration missions. Ionizing radiation sensitivity is modulated by both genomic and environmental factors. Understanding their contributions is crucial for designing experiments in model organisms, evaluating the risk of deep space (i.e. high-linear energy transfer, or LET, particle) radiation exposure in astronauts, and also selecting therapeutic irradiation regimes for cancer patients. We identified single nucleotide polymorphisms in 15 strains of mice, including 10 collaborative cross model strains and 5 founder strains, associated with spontaneous and ionizing radiation-induced in vitro DNA damage quantified based on immunofluorescent tumor protein p53 binding protein (53BP1) positive nuclear foci. Statistical analysis suggested an association with pathways primarily related to cellular signaling, metabolism, tumorigenesis and nervous system damage. We observed different genomic associations in early (4 and 8 h) responses to different LET radiation, while later (24 hour) DNA damage responses showed a stronger overlap across all LETs. Furthermore, a subset of pathways was associated with spontaneous DNA damage, suggesting 53BP1 positive foci as a potential biomarker for DNA integrity in mouse models. Our results suggest several mouse strains as new models to further study the impact of ionizing radiation and validate the identified genetic loci. We also highlight the importance of future human in vitro studies to refine the association of genes and pathways with the DNA damage response to ionizing radiation and identify targets for space travel countermeasures.

Astrocytes regulate vascular endothelial responses to simulated deep space radiation in a human organ-on-a-chip model

Central nervous system (CNS) damage by galactic cosmic ray radiation is a major health risk for human deep space exploration. Simulated galactic cosmic rays or their components, especially high Z-high energy particles such as ⁵⁶Fe ions, cause neurodegeneration and neuroinflammation in rodent models. CNS damage can be partially mediated by the blood-brain barrier, which regulates systemic interactions between CNS and the rest of the body. Astrocytes are major cellular regulators of blood-brain barrier permeability that also modulate neuroinflammation and neuronal health. However, astrocyte roles in regulating CNS and blood-brain barrier responses to space radiation remain little understood, especially in human tissue analogs. In this work, we used a novel high-throughput human organ-on-a-chip system to evaluate blood-brain barrier impairments and astrocyte functions 1-7 days after exposure to 600 MeV/n ⁵⁶Fe particles and simplified simulated galactic cosmic rays. We show that simulated deep space radiation causes vascular permeability, oxidative stress, inflammation and delayed astrocyte activation in a pattern resembling CNS responses to brain injury. Furthermore, our results indicate that astrocytes have a dual role in regulating radiation responses: they exacerbate blood-brain barrier permeability acutely after irradiation, followed by switching to a more protective phenotype by reducing oxidative stress and pro-inflammatory cytokine and chemokine secretion during the subacute stage.

Quantification of radiation-induced DNA double strand break repair foci to evaluate and predict biological responses to ionizing radiation

Radiation-induced foci (RIF) are nuclear puncta visualized by immunostaining of proteins that regulate DNA double-strand break (DSB) repair after exposure to ionizing radiation. RIF are a standard metric for measuring DSB formation and repair in clinical, environmental and space radiobiology. The time course and dose dependence of their formation has great potential to predict in vivo responses to ionizing radiation, predisposition to cancer and probability of adverse reactions to radiotherapy. However, increasing complexity of experimentally and therapeutically setups (charged particle, FLASH …) is associated with several confounding factors that must be taken into account when interpreting RIF values. In this review, we discuss the spatiotemporal characteristics of RIF development after irradiation, addressing the common confounding factors, including cell proliferation and foci merging. We also describe the relevant endpoints and mathematical models that enable accurate biological interpretation of RIF formation and resolution. Finally, we discuss the use of RIF as a biomarker for quantification and prediction of in vivo radiation responses, including important caveats relating to the choice of the biological endpoint and the detection method. This review intends to help scientific community design radiobiology experiments using RIF as a key metric and to provide suggestions for their biological interpretation.

Fundamental Biological Features of Spaceflight: Advancing the Field to Enable Deep-Space Exploration

Research on astronaut health and model organisms have revealed six features of spaceflight biology that guide our current understanding of fundamental molecular changes that occur during space travel. The features include oxidative stress, DNA damage, mitochondrial dysregulation, epigenetic changes (including gene regulation), telomere length alterations, and microbiome shifts. Here we review the known hazards of human spaceflight, how spaceflight affects living systems through these six fundamental features, and the associated health risks of space exploration. We also discuss the essential issues related to the health and safety of astronauts involved in future missions, especially planned long-duration and Martian missions.

NASA GeneLab RNA-seq consensus pipeline: standardized processing of short-read RNA-seq data

With the development of transcriptomic technologies, we are able to quantify precise changes in gene expression profiles from astronauts and other organisms exposed to spaceflight. Members of NASA GeneLab and GeneLab-associated analysis working groups (AWGs) have developed a consensus pipeline for analyzing short-read RNA-sequencing data from spaceflight-associated experiments. The pipeline includes quality control, read trimming, mapping, and gene quantification steps, culminating in the detection of differentially expressed genes. This data analysis pipeline and the results of its execution using data submitted to GeneLab are now all publicly available through the GeneLab database. We present here the full details and rationale for the construction of this pipeline in order to promote transparency, reproducibility, and reusability of pipeline data; to provide a template for data processing of future spaceflight-relevant datasets; and to encourage cross-analysis of data from other databases with the data available in GeneLab.

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Analysis of omics data from different spaceflight studies presents unique challenges

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A standardized pipeline for RNA-seq analysis eliminates data processing variation

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The GeneLab RNA-seq pipeline includes QC, trimming, mapping, quantification, and DGE

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Space-relevant data processed with this pipeline are available at genelab.nasa.gov

Knowledge Network Embedding of Transcriptomic Data from Spaceflown Mice Uncovers Signs and Symptoms Associated with Terrestrial Diseases

There has long been an interest in understanding how the hazards from spaceflight may trigger or exacerbate human diseases. With the goal of advancing our knowledge on physiological changes during space travel, NASA GeneLab provides an open-source repository of multi-omics data from real and simulated spaceflight studies. Alone, this data enables identification of biological changes during spaceflight, but cannot infer how that may impact an astronaut at the phenotypic level. To bridge this gap, Scalable Precision Medicine Oriented Knowledge Engine (SPOKE), a heterogeneous knowledge graph connecting biological and clinical data from over 30 databases, was used in combination with GeneLab transcriptomic data from six studies. This integration identified critical symptoms and physiological changes incurred during spaceflight.

53BP1 Repair Kinetics for Prediction of In Vivo Radiation Susceptibility in 15 Mouse Strains

We present a novel mathematical formalism to predict the kinetics of DNA damage repair after exposure to both low- and high-LET radiation (X rays; 350 MeV/n 40Ar; 600 MeV/n 56Fe). Our method is based on monitoring DNA damage repair protein 53BP1 that forms radiation-induced foci (RIF) at locations of DNA double-strand breaks (DSB) in the nucleus and comparing its expression in primary skin fibroblasts isolated from 15 mice strains. We previously reported strong evidence for clustering of nearby DSB into single repair units as opposed to the classic "contact-first" model where DSB are considered immobile. Here we apply this clustering model to evaluate the number of remaining RIF over time. We also show that the newly introduced kinetic metrics can be used as surrogate biomarkers for in vivo radiation toxicity, with potential applications in radiotherapy and human space exploration. In particular, we observed an association between the characteristic time constant of RIF repair measured in vitro and survival levels of immune cells collected from irradiated mice. Moreover, the speed of DNA damage repair correlated not only with radiation-induced cellular survival in vivo, but also with spontaneous cancer incidence data collected from the Mouse Tumor Biology database, suggesting a relationship between the efficiency of DSB repair after irradiation and cancer risk.

Circulating miRNA Spaceflight Signature Reveals Targets for Countermeasure Development

We have identified and validated a spaceflight-associated microRNA (miRNA) signature that is shared by rodents and humans in response to simulated, short-duration and long-duration spaceflight. Previous studies have identified miRNAs that regulate rodent responses to spaceflight in low-Earth orbit, and we have confirmed the expression of these proposed spaceflight-associated miRNAs in rodents reacting to simulated spaceflight conditions. Moreover, astronaut samples from the NASA Twins Study confirmed these expression signatures in miRNA sequencing, single-cell RNA sequencing (scRNA-seq), and single-cell assay for transposase accessible chromatin (scATAC-seq) data. Additionally, a subset of these miRNAs (miR-125, miR-16, and let-7a) was found to regulate vascular damage caused by simulated deep space radiation. To demonstrate the physiological relevance of key spaceflight-associated miRNAs, we utilized antagomirs to inhibit their expression and successfully rescue simulated deep-space-radiation-mediated damage in human 3D vascular constructs.

Neutrophil-to-Lymphocyte Ratio: A Biomarker to Monitor the Immune Status of Astronauts

A comprehensive understanding of spaceflight factors involved in immune dysfunction and the evaluation of biomarkers to assess in-flight astronaut health are essential goals for NASA. An elevated neutrophil-to-lymphocyte ratio (NLR) is a potential biomarker candidate, as leukocyte differentials are altered during spaceflight. In the reduced gravity environment of space, rodents and astronauts displayed elevated NLR and granulocyte-to-lymphocyte ratios (GLR), respectively. To simulate microgravity using two well-established ground-based models, we cultured human whole blood-leukocytes in high-aspect rotating wall vessels (HARV-RWV) and used hindlimb unloaded (HU) mice. Both HARV-RWV simulation of leukocytes and HU-exposed mice showed elevated NLR profiles comparable to spaceflight exposed samples. To assess mechanisms involved, we found the simulated microgravity HARV-RWV model resulted in an imbalance of redox processes and activation of myeloperoxidase-producing inflammatory neutrophils, while antioxidant treatment reversed these effects. In the simulated microgravity HU model, mitochondrial catalase-transgenic mice that have reduced oxidative stress responses showed reduced neutrophil counts, NLR, and a dampened release of selective inflammatory cytokines compared to wildtype HU mice, suggesting simulated microgravity induced oxidative stress responses that triggered inflammation. In brief, both spaceflight and simulated microgravity models caused elevated NLR, indicating this as a potential biomarker for future in-flight immune health monitoring.

Ionizing radiation-induced risks to the central nervous system and countermeasures in cellular and rodent models

PURPOSE

Harmful effects of ionizing radiation on the Central Nervous System (CNS) are a concerning outcome in the field of cancer radiotherapy and form a major risk for deep space exploration. Both acute and chronic CNS irradiation induce a complex network of molecular and cellular alterations including DNA damage, oxidative stress, cell death and systemic inflammation, leading to changes in neuronal structure and synaptic plasticity with behavioral and cognitive consequences in animal models. Due to this complexity, countermeasure or therapeutic approaches to reduce the harmful effects of ionizing radiation include a wide range of protective and mitigative strategies, which merit a thorough comparative analysis.

MATERIALS AND METHODS

We reviewed current approaches for developing countermeasures to both targeted and non-targeted effects of ionizing radiation on the CNS from the molecular and cellular to the behavioral level.

RESULTS

We focus on countermeasures that aim to mitigate the four main detrimental actions of radiation on CNS: DNA damage, free radical formation and oxidative stress, cell death, and harmful systemic responses including tissue death and neuroinflammation. We propose a comprehensive review of CNS radiation countermeasures reported for the full range of irradiation types (photons and particles, low and high linear energy transfer) and doses (from a fraction of gray to several tens of gray, fractionated and unfractionated), with a particular interest for exposure conditions relevant to deep-space environment and radiotherapy. Our review reveals the importance of combined strategies that increase DNA protection and repair, reduce free radical formation and increase their elimination, limit inflammation and improve cell viability, limit tissue damage and increase repair and plasticity.

CONCLUSIONS

The majority of therapeutic approaches to protect the CNS from ionizing radiation have been limited to acute high dose and high dose rate gamma irradiation, and few are translatable from animal models to potential human application due to harmful side effects and lack of blood-brain barrier permeability that precludes peripheral administration. Therefore, a promising research direction would be to focus on practical applicability and effectiveness in a wider range of irradiation paradigms, from fractionated therapeutic to deep space radiation. In addition to discovering novel therapeutics, it would be worth maximizing the benefits and reducing side effects of those that already exist. Finally, we suggest that novel cellular and tissue models for developing and testing countermeasures in the context of other impairments might also be applied to the field of CNS responses to ionizing radiation.

Dose, LET and Strain Dependence of Radiation-Induced 53BP1 Foci in 15 Mouse Strains Ex Vivo Introducing Novel DNA Damage Metrics

We present a comprehensive comparative analysis on the repair of radiation-induced DNA damage ex vivo in 15 strains of mice, including 5 inbred reference strains and 10 collaborative-cross strains, of both sexes, totaling 5 million skin fibroblast cells imaged by three-dimensional highthroughput conventional microscopy. Non-immortalized primary skin fibroblasts derived from 76 mice were subjected to increasing doses of both low- and high-LET radiation (X rays; 350 MeV/n 40Ar; 600 MeV/n 56Fe), which are relevant to carcinogenesis and human space exploration. Automated image quantification of 53BP1 radiation-induced foci (RIF) formation and repair during the first 4-48 h postirradiation was performed as a function of dose and LET. Since multiple DNA double-strand breaks (DSBs) are induced in a dose- and LET-dependent manner, our data suggest that when DSBs are formed within the same discrete nuclear region, referred to as the "repair domain", novel mathematical formalisms used to report RIF allowed us to conclude that multiple DSBs can be present in single RIF. Specifically, we observed that the number of RIF per Gy was lower for higher X-ray doses or higher LET particles (i.e., 600 MeV/n ⁵⁶Fe), suggesting there are more DSBs per RIF when the local absorbed dose increases in the nucleus. The data also clearly show that with more DSBs per RIF, it becomes more difficult for cells to fully resolve RIF. All 15 strains showed the same dose and LET dependence, but strain differences were preserved under various experimental conditions, indicating that the number and sizes of repair domains are modulated by the genetic background of each strain.

Reduced gravity contributes to Neutrophil to Lymphocyte Ratio shifting and promotion of the Oxidative Stress Response

Spaceflight can cause immune system dysfunction, such as elevated white blood cells (WBC) and polymorphonuclear neutrophils (PMN), along with unchanged or reduced lymphocyte counts. A high PMN to lymphocyte ratio (NLR) can acts as a poor prognosis in cancer and a biomarker for subclinical inflammation however, the NLR has not been identified as a predictor of astronaut health during spaceflight. CBC data collected on board the International Space Station (ISS) was repurposed to determine the granulocyte to lymphocyte ratio (GLR) in humans and the NLR in rodents. The results displayed a progressive increase in GLR and NLR during spaceflight and at landing. The mechanism for increased NLR was assessed in vitro using the microgravity-analog, rotating wall vessel (RWV), with human WBCs. The results indicated that simulated microgravity led to increased GLR and NLR profiles, and production of reactive oxygen species (ROS) and myeloperoxidase (MPO). Interestingly, simulated microgravity increased the number of matured PMNs that showed impaired phagocytic function, while treatment with tert-Butyl hydroperoxide (TBHP), also reduced PMN phagocytosis. In addition, 30-days of simulated microgravity (hindlimb unloading) in mice, indicated an increased NLR and MPO gene expression, which were mitigated in mitochondrial catalase overexpressing transgenic mice, suggesting ROS scavenging is essential for maintaining homeostatic immunity. Collectively, we propose that the health status of astronauts during future short- and long-term space missions can be monitored by their NLR profile, in addition to utilizing this measurement as a tool for oxidative stress response countermeasure development to restore homeostatic immunity.

Disease-modifying therapies alter gut microbial composition in MS

Objective

To determine the effects of the disease-modifying therapies, glatiramer acetate (GA) and dimethyl fumarate (DMF), on the gut microbiota in patients with MS.

Methods

Participants with relapsing MS who were either treatment-naive or treated with GA or DMF were recruited. Peripheral blood mononuclear cells were immunophenotyped. Bacterial DNA was extracted from stool, and amplicons targeting the V4 region of the bacterial/archaeal 16S rRNA gene were sequenced (Illumina MiSeq). Raw reads were clustered into Operational Taxonomic Units using the GreenGenes database. Differential abundance analysis was performed using linear discriminant analysis effect size. Phylogenetic investigation of communities by reconstruction of unobserved states was used to investigate changes to functional pathways resulting from differential taxon abundance.

Results

One hundred sixty-eight participants were included (treatment-naive n = 75, DMF n = 33, and GA n = 60). Disease-modifying therapies were associated with changes in the fecal microbiota composition. Both therapies were associated with decreased relative abundance of the Lachnospiraceae and Veillonellaceae families. In addition, DMF was associated with decreased relative abundance of the phyla Firmicutes and Fusobacteria and the order Clostridiales and an increase in the phylum Bacteroidetes. Despite the different changes in bacterial taxa, there was an overlap between functional pathways affected by both therapies.

Interpretation

Administration of GA or DMF is associated with differences in gut microbial composition in patients with MS. Because those changes affect critical metabolic pathways, we hypothesize that our findings may highlight mechanisms of pathophysiology and potential therapeutic intervention requiring further investigation.

Comparing Photon and Charged Particle Therapy Using DNA Damage Biomarkers

Treatment modalities for cancer radiation therapy have become increasingly diversified given the growing number of facilities providing proton and carbon-ion therapy in addition to the more historically accepted photon therapy. An understanding of high-LET radiobiology is critical for optimization of charged particle radiation therapy and potential DNA damage response. In this review, we present a comprehensive summary and comparison of these types of therapy monitored primarily by using DNA damage biomarkers. We focus on their relative profiles of dose distribution and mechanisms of action from the level of nucleic acid to tumor cell death.

Astrocytes: Integrative Regulators of Neuroinflammation in Stroke and Other Neurological Diseases

Astrocytes regulate neuroinflammatory responses after stroke and in other neurological diseases. Although not all astrocytic responses reduce inflammation, their predominant function is to protect the brain by driving the system back to homeostasis after injury. They receive multidimensional signals within the central nervous system and between the brain and the systemic circulation. Processing this information allows astrocytes to regulate synapse formation and maintenance, cerebral blood flow, and blood-brain barrier integrity. Similarly, in response to stroke and other central nervous system disorders, astrocytes detect and integrate signals of neuronal damage and inflammation to regulate the neuroinflammatory response. Two direct regulatory mechanisms in the astrocyte arsenal are the ability to form both physical and molecular barriers that seal the injury site and localize the neuroinflammatory response. Astrocytes also indirectly regulate the inflammatory response by affecting neuronal health during the acute injury and axonal regrowth. This ability to regulate the location and degree of neuroinflammation after injury, combined with the long time course of neuroinflammation, makes astrocytic signaling pathways promising targets for therapies.

Immune cell-specific transcriptional profiling highlights distinct molecular pathways controlled by Tob1 upon experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system characterized by focal lymphocytic infiltration, demyelination and neurodegeneration. Despite the recent advances in understanding MS molecular basis, no reliable biomarkers have been identified yet to monitor disease progression. Our group has previously reported that low levels of TOB1 in CD4(+) T cells are strongly associated with a higher risk of MS conversion in individuals experiencing an initial demyelinating event. Consistently, Tob1 ablation in mice exacerbates the clinical phenotype of the MS model experimental autoimmune encephalomyelitis (EAE). To shed light on Tob1 molecular functions in the immune system, we have conducted the first cell-based transcriptomic analysis in Tob1(-/-) and wildtype mice upon EAE. Next-generation sequencing was employed to characterize the changes in gene expression in T and B cells at pre- and post-symptomatic EAE stages. Remarkably, we found only modest overlap among the different genetic signatures, suggesting that Tob1 may control distinct genetic programs in the different cytotypes. This hypothesis was corroborated by gene ontology and global interactome analyses, which highlighted specific cellular pathways in each cellular subset before and after EAE induction. In summary, our work pinpoints a multifaceted activity of Tob1 in both homeostasis and disease progression.

Astrocytic transforming growth factor-beta signaling reduces subacute neuroinflammation after stroke in mice

Astrocytes limit inflammation after CNS injury, at least partially by physically containing it within an astrocytic scar at the injury border. We report here that astrocytic transforming growth factor-beta (TGFβ) signaling is a second, distinct mechanism that astrocytes utilize to limit neuroinflammation. TGFβs are anti-inflammatory and neuroprotective cytokines that are upregulated subacutely after stroke, during a clinically accessible time window. We have previously demonstrated that TGFβs signal to astrocytes, neurons and microglia in the stroke border days after stroke. To investigate whether TGFβ affects astrocyte immunoregulatory functions, we engineered "Ast-Tbr2DN" mice where TGFβ signaling is inhibited specifically in astrocytes. Despite having a similar infarct size to wildtype controls, Ast-Tbr2DN mice exhibited significantly more neuroinflammation during the subacute period after distal middle cerebral occlusion (dMCAO) stroke. The peri-infarct cortex of Ast-Tbr2DN mice contained over 60% more activated CD11b(+) monocytic cells and twice as much immunostaining for the activated microglia and macrophage marker CD68 than controls. Astrocytic scarring was not altered in Ast-Tbr2DN mice. However, Ast-Tbr2DN mice were unable to upregulate TGF-β1 and its activator thrombospondin-1 2 days after dMCAO. As a result, the normal upregulation of peri-infarct TGFβ signaling was blunted in Ast-Tbr2DN mice. In this setting of lower TGFβ signaling and excessive neuroinflammation, we observed worse motor outcomes and late infarct expansion after photothrombotic motor cortex stroke. Taken together, these data demonstrate that TGFβ signaling is a molecular mechanism by which astrocytes limit neuroinflammation, activate TGFβ in the peri-infarct cortex and preserve brain function during the subacute period after stroke.

Astrocytic TGF-β signaling limits inflammation and reduces neuronal damage during central nervous system Toxoplasma infection

The balance between controlling infection and limiting inflammation is particularly precarious in the brain because of its unique vulnerability to the toxic effects of inflammation. Astrocytes have been implicated as key regulators of neuroinflammation in CNS infections, including infection with Toxoplasma gondii, a protozoan parasite that naturally establishes a chronic CNS infection in mice and humans. In CNS toxoplasmosis, astrocytes are critical to controlling parasite growth. They secrete proinflammatory cytokines and physically encircle parasites. However, the molecular mechanisms used by astrocytes to limit neuroinflammation during toxoplasmic encephalitis have not yet been identified. TGF-β signaling in astrocytes is of particular interest because TGF-β is universally upregulated during CNS infection and serves master regulatory and primarily anti-inflammatory functions. We report in this study that TGF-β signaling is activated in astrocytes during toxoplasmic encephalitis and that inhibition of astrocytic TGF-β signaling increases immune cell infiltration, uncouples proinflammatory cytokine and chemokine production from CNS parasite burden, and increases neuronal injury. Remarkably, we show that the effects of inhibiting astrocytic TGF-β signaling are independent of parasite burden and the ability of GFAP(+) astrocytes to physically encircle parasites.

Delayed administration of a small molecule tropomyosin-related kinase B ligand promotes recovery after hypoxic-ischemic stroke

BACKGROUND AND PURPOSE

Stroke is the leading cause of long-term disability in the United States, yet no drugs are available that are proven to improve recovery. Brain-derived neurotrophic factor stimulates neurogenesis and plasticity, processes that are implicated in stroke recovery. It binds to both the tropomyosin-related kinase B and p75 neurotrophin receptors. However, brain-derived neurotrophic factor is not a feasible therapeutic agent, and no small molecule exists that can reproduce its binding to both receptors. We tested the hypothesis that a small molecule (LM22A-4) that selectively targets tropomyosin-related kinase B would promote neurogenesis and functional recovery after stroke.

METHODS

Four-month-old mice were trained on motor tasks before stroke. After stroke, functional test results were used to randomize mice into 2 equally, and severely, impaired groups. Beginning 3 days after stroke, mice received LM22A-4 or saline vehicle daily for 10 weeks.

RESULTS

LM22A-4 treatment significantly improved limb swing speed and accelerated the return to normal gait accuracy after stroke. LM22A-4 treatment also doubled both the number of new mature neurons and immature neurons adjacent to the stroke. Drug-induced differences were not observed in angiogenesis, dendritic arborization, axonal sprouting, glial scar formation, or neuroinflammation.

CONCLUSIONS

A small molecule agonist of tropomyosin-related kinase B improves functional recovery from stroke and increases neurogenesis when administered beginning 3 days after stroke. These findings provide proof-of-concept that targeting of tropomyosin-related kinase B alone is capable of promoting one or more mechanisms relevant to stroke recovery. LM22A-4 or its derivatives might therefore serve as "pro-recovery" therapeutic agents for stroke.

TGFβ signaling in the brain increases with aging and signals to astrocytes and innate immune cells in the weeks after stroke

Background

TGFβ is both neuroprotective and a key immune system modulator and is likely to be an important target for future stroke therapy. The precise function of increased TGF-β1 after stroke is unknown and its pleiotropic nature means that it may convey a neuroprotective signal, orchestrate glial scarring or function as an important immune system regulator. We therefore investigated the time course and cell-specificity of TGFβ signaling after stroke, and whether its signaling pattern is altered by gender and aging.

Methods

We performed distal middle cerebral artery occlusion strokes on 5 and 18 month old TGFβ reporter mice to get a readout of TGFβ responses after stroke in real time. To determine which cell type is the source of increased TGFβ production after stroke, brain sections were stained with an anti-TGFβ antibody, colocalized with markers for reactive astrocytes, neurons, and activated microglia. To determine which cells are responding to TGFβ after stroke, brain sections were double-labelled with anti-pSmad2, a marker of TGFβ signaling, and markers of neurons, oligodendrocytes, endothelial cells, astrocytes and microglia.

Results

TGFβ signaling increased 2 fold after stroke, beginning on day 1 and peaking on day 7. This pattern of increase was preserved in old animals and absolute TGFβ signaling in the brain increased with age. Activated microglia and macrophages were the predominant source of increased TGFβ after stroke and astrocytes and activated microglia and macrophages demonstrated dramatic upregulation of TGFβ signaling after stroke. TGFβ signaling in neurons and oligodendrocytes did not undergo marked changes.

Conclusions

We found that TGFβ signaling increases with age and that astrocytes and activated microglia and macrophages are the main cell types that undergo increased TGFβ signaling in response to post-stroke increases in TGFβ. Therefore increased TGFβ after stroke likely regulates glial scar formation and the immune response to stroke.