Acute and Persistent Alterations of Cerebellar Inflammatory Networks and Glial Activation in a Rat Model of Pediatric Mild Traumatic Brain Injury

Mild traumatic brain injury increases vulnerability to post-traumatic stress disorder in rats and the possible role of hippocampal DNA methylation

Introduction

Clinical studies have established that patients with mild traumatic brain injury (mTBI) are at an increased risk for developing post-traumatic stress disorder (PTSD), suggesting that mTBI increases vulnerability to subsequent PTSD onset. However, preclinical animal studies investigating this link remain scarce, and the specific biological mechanism through which mTBI increases vulnerability to PTSD is largely unknown.

Methods

In this study, we modeled mTBI in rats using a mild, closed-head, weight-drop injury, followed 72 h later by exposure to single prolonged stress (SPS) to simulate PTSD. Then, we investigated the impact of mTBI on subsequent PTSD development by observing the behaviors of rats in a series of validated behavioral tests and further explored the possible role of hippocampal DNA methylation.

Results

We found that, compared with rats in the PTSD-only group, those in the mTBI + PTSD group exhibited higher anxiety levels, higher depression levels, and impaired spatial learning and memory as determined in the open field test, the forced swimming test, and the Morris water maze test, respectively. Rats in the mTBI + PTSD group also exhibited higher hippocampal DNMT3b protein expression compared with those in the PTSD group.

Conclusion

In conclusion, our results demonstrated that mTBI increases vulnerability to PTSD in rats, possibly through alterations in hippocampal DNA methylation patterns.

Minimally invasive serial collection of cerebrospinal fluid reveals sex-dependent differences in neuroinflammation in a rat model of mild traumatic brain injury

Traumatic brain injuries (TBI) are the seventh leading cause of disability globally with 48.99 million prevalent cases and 7.08 million years lived with diability. Approximately 80 % of TBI patients are diagnosed with mild TBI (mTBI), or concussion, caused by nonpenetrating mechanical trauma to the head or body along with sudden rotational motion of the head. Studies investigating the temporal dynamics of neuroinflammation after mTBI are greatly needed. Without longitudinal studies, translating preclinical studies to clinical studies remains challenging as the difference in timing remains poorly understood. In this study, we describe a method of minimally invasive serial cerebrospinal fluid (CSF) collection that enables longitudinal investigation of CSF inflammation. The method described in this study can easily be adapted by any laboratory prepared for animal studies. Multiplex immunoassay of serially collected and singly collected CSF samples show collection frequency does not alter protein expression in the CSF. Further, sex-dependent differences in TBI have been reported, but remain poorly understood. This study establishes a framework for assessing sex difference in neuroinflammation after a concussion. We showed that results vary based on the framing of the statistical test. However, it is evident that males experience a more robust inflammatory response to a single concussion than females.

The Neuroimmune System and the Cerebellum

The recognition that there is an innate immune system of the brain, referred to as the neuroimmune system, that preforms many functions comparable to that of the peripheral immune system is a relatively new concept and much is yet to be learned. The main cellular components of the neuroimmune system are the glial cells of the brain, primarily microglia and astrocytes. These cell types preform many functions through secretion of signaling factors initially known as immune factors but referred to as neuroimmune factors when produced by cells of the brain. The immune functions of glial cells play critical roles in the healthy brain to maintain homeostasis that is essential for normal brain function, to establish cytoarchitecture of the brain during development, and, in pathological conditions, to minimize the detrimental effects of disease and injury and promote repair of brain structure and function. However, dysregulation of this system can occur resulting in actions that exacerbate or perpetuate the detrimental effects of disease or injury. The neuroimmune system extends throughout all brain regions, but attention to the cerebellar system has lagged that of other brain regions and information is limited on this topic. This article is meant to provide a brief introduction to the cellular and molecular components of the brain immune system, its functions, and what is known about its role in the cerebellum. The majority of this information comes from studies of animal models and pathological conditions, where upregulation of the system facilitates investigation of its actions.

Short-term behavioral and histological findings following a single concussive and repeated subconcussive brain injury in a rodent model

PRIMARY OBJECTIVE

It is unclear of the correlation between a mild traumatic brain injury (mTBI) and repeated subconcussive (RSC) impacts with respect to injury biomechanics. Thus, the present study was designed to determine the behavioral and histological differences between a single mTBI impact and RSC impacts with subdivided cumulative kinetic energies of the single mTBI impact.

RESEARCH DESIGN

Adult male Sprague-Dawley rats were randomly assigned to a single mTBI impact, RSC impact, sham, or repeated sham groups.

METHODS AND PROCEDURES

Following a weight drop injury, anxiety-like behavior and general locomotive activity and were assessed using the open field test, while motor coordination was evaluated using a rotarod unit. Neuronal loss, astrogliosis, and microgliosis were assessed using NeuN, GFAP and Iba-1 immunohistochemistry. All assessments were undertaken at 3- and 7-days post impact.

MAIN OUTCOMES AND RESULTS

No behavioral disturbances were observed in injury groups, however, both injury groups did lead to microgliosis following 3-days post-impact.

CONCLUSIONS

No pathophysiological differences were observed between a single mTBI impact and RSC impacts of the same energy input. Even though a cumulative injury threshold for RSC impacts was not determined, a threshold still may exist where no pathodynamic shift occurs.

The Emerging Landscape of the Cerebellum after a Pediatric Traumatic Brain Injury: From Diaschisis to Sociality

There is an expanding interest in the cerebellum in the context of focal and diffuse traumatic injuries to the cerebral cortex. In the adult brain, preclinical studies have revealed acute as well as progressive loss of Purkinje cells in the cerebellum coincident with microglial activation. This pathogenesis, remote to the site of the primary injury, is termed "diaschisis." Here we consider traumatic injuries to the developing brain, where the cerebellum likewise undergoes neurodegeneration. As injury is superimposed on a young brain, long-term adverse consequences may reflect diaschisis that is compounded by disruption of brain development.

Pediatric Traumatic Brain Injury: Models, Therapeutics, and Outcomes

Pediatric traumatic brain injury (TBI) is a significant healthcare issue, but potential treatments are absent despite robust investigation in several clinical trials. Factors attributed to clinical TBI, such as heterogeneity of injury and single-dose pharmacological treatments as well as timing of administration, may be reasons for the negative studies. Preclinical models of TBI can reduce some of the impediments by highlighting differences in injury depending on injury severity and location and by conducting dose response studies, thus providing better therapeutic targets and pharmacological profiles for clinical use. In this chapter, there were sufficient reports to make comparisons between the models in terms of pathophysiology, behavioral dysfunction, and the efficacy of therapeutic interventions. The models used to date include controlled cortical impact (CCI), weight drop, fluid percussion, and abusive head trauma. Several therapeutics were identified after CCI injury but none in the other models, which underscores the need for studies evaluating the therapies reported after CCI injury as well as novel potential approaches.

Repeated mild traumatic brain injury causes sex-specific increases in cell proliferation and inflammation in juvenile rats

Childhood represents a period of significant growth and maturation for the brain, and is also associated with a heightened risk for mild traumatic brain injuries (mTBI). There is also concern that repeated-mTBI (r-mTBI) may have a long-term impact on developmental trajectories. Using an awake closed head injury (ACHI) model, that uses rapid head acceleration to induce a mTBI, we investigated the acute effects of repeated-mTBI (r-mTBI) on neurological function and cellular proliferation in juvenile male and female Long-Evans rats. We found that r-mTBI did not lead to cumulative neurological deficits with the model. R-mTBI animals exhibited an increase in BrdU + (bromodeoxyuridine positive) cells in the dentate gyrus (DG), and that this increase was more robust in male animals. This increase was not sustained, and cell proliferation returning to normal by PID3. A greater increase in BrdU + cells was observed in the dorsal DG in both male and female r-mTBI animals at PID1. Using Ki-67 expression as an endogenous marker of cellular proliferation, a robust proliferative response following r-mTBI was observed in male animals at PID1 that persisted until PID3, and was not constrained to the DG alone. Triple labeling experiments (Iba1+, GFAP+, Brdu+) revealed that a high proportion of these proliferating cells were microglia/macrophages, indicating there was a heightened inflammatory response. Overall, these findings suggest that rapid head acceleration with the ACHI model produces an mTBI, but that the acute neurological deficits do not increase in severity with repeated administration. R-mTBI transiently increases cellular proliferation in the hippocampus, particularly in male animals, and the pattern of cell proliferation suggests that this represents a neuroinflammatory response that is focused around the mid-brain rather than peripheral cortical regions. These results add to growing literature indicating sex differences in proliferative and inflammatory responses between females and males. Targeting proliferation as a therapeutic avenue may help reduce the short term impact of r-mTBI, but there may be sex-specific considerations.

Multimodal magnetic resonance imaging of youth sport-related concussion reveals acute changes in the cerebellum, basal ganglia, and corpus callosum that resolve with recovery

Magnetic resonance imaging (MRI) can provide a number of measurements relevant to sport-related concussion (SRC) symptoms; however, most studies to date have used a single MRI modality and whole-brain exploratory analyses in attempts to localize concussion injury. This has resulted in highly variable findings across studies due to wide ranging symptomology, severity and nature of injury within studies. A multimodal MRI, symptom-guided region-of-interest (ROI) approach is likely to yield more consistent results. The functions of the cerebellum and basal ganglia transcend many common concussion symptoms, and thus these regions, plus the white matter tracts that connect or project from them, constitute plausible ROIs for MRI analysis. We performed diffusion tensor imaging (DTI), resting-state functional MRI, quantitative susceptibility mapping (QSM), and cerebral blood flow (CBF) imaging using arterial spin labeling (ASL), in youth aged 12-18 years following SRC, with a focus on the cerebellum, basal ganglia and white matter tracts. Compared to controls similar in age, sex and sport (N = 20), recent SRC youth (N = 29; MRI at 8 ± 3 days post injury) exhibited increased susceptibility in the cerebellum (p = 0.032), decreased functional connectivity between the caudate and each of the pallidum (p = 0.035) and thalamus (p = 0.021), and decreased diffusivity in the mid-posterior corpus callosum (p < 0.038); no changes were observed in recovered asymptomatic youth (N = 16; 41 ± 16 days post injury). For recent symptomatic-only SRC youth (N = 24), symptom severity was associated with increased susceptibility in the superior cerebellar peduncles (p = 0.011) and reduced activity in the cerebellum (p = 0.013). Fewer days between injury and MRI were associated with reduced cerebellar-parietal functional connectivity (p < 0.014), reduced activity of the pallidum (p = 0.002), increased CBF in the caudate (p = 0.005), and reduced diffusivity in the central corpus callosum (p < 0.05). Youth SRC is associated with acute cerebellar inflammation accompanied by reduced cerebellar activity and cerebellar-parietal connectivity, as well as structural changes of the middle regions of the corpus callosum accompanied by functional changes of the caudate, all of which resolve with recovery. Early MRI post-injury is important to establish objective MRI-based indicators for concussion diagnosis, recovery assessment and prediction of outcome.

The neuroprotection of hyperbaric oxygen therapy against traumatic brain injury via NF-κB/MAPKs-CXCL1 signaling pathways

It is well known that hyperbaric oxygen (HBO) therapy achieves neuroprotective effects by modulating neuroinflammatory responses. However, its underlying therapeutic mechanisms are not yet fully elucidated. Based on our previous studies, we further investigated whether HBO therapy exerts neuroprotective effects in vivo by regulating the nuclear factor-kappa B (NF-κB)/ mitogen-activated protein kinases (MAPKs) chemokine (C-X-C motif) ligand (CXCL)1 inflammatory pathway. In our study, a rat model of traumatic brain injury (TBI) was established by controlled cortical impact (CCI) to verify that the expression of CXCL1 and chemokine (C-X-C motif) receptor (CXCR)2 increased after TBI, and CXCL1 was mainly expressed in astrocytes, while CXCR2 was mainly expressed in neurons. Increased apoptosis of cortical nerve cells in the injured cortex was also found after TBI. Reduced nerve cell apoptosis with improved neurological function was observed after application of a CXCR2 antagonist. The expression of phospho-extracellular signal-regulated kinase (p-ERK), phospho-c-Jun N-terminal kinase (p-JNK) and p-NF-κB increased after TBI, and application of ERK, JNK and NF-κB inhibitors decreased expression of CXCL1 and CXCR2 in rats. We further found that HBO therapy down-regulated the expression of p-ERK, p-JNK, p-NF-κB, CXCL1, and CXCR2, and reduced nerve cell apoptosis, improved the neurological function of TBI rats, and ultimately alleviated the secondary injury. In conclusion, HBO therapy may exert neuroprotective effect by regulating the NF-κB/MAPKs (JNK and ERK)-CXCL1 inflammatory pathways following TBI, which probably provide the theoretical and experimental basis for the clinical application of HBO therapy in the treatment of TBI.

Traumatic Brain Injury: An Age-Dependent View of Post-Traumatic Neuroinflammation and Its Treatment

Traumatic brain injury (TBI) is a leading cause of death and disability all over the world. TBI leads to (1) an inflammatory response, (2) white matter injuries and (3) neurodegenerative pathologies in the long term. In humans, TBI occurs most often in children and adolescents or in the elderly, and it is well known that immune responses and the neuroregenerative capacities of the brain, among other factors, vary over a lifetime. Thus, age-at-injury can influence the consequences of TBI. Furthermore, age-at-injury also influences the pharmacological effects of drugs. However, the post-TBI inflammatory, neuronal and functional consequences have been mostly studied in experimental young adult animal models. The specificity and the mechanisms underlying the consequences of TBI and pharmacological responses are poorly understood in extreme ages. In this review, we detail the variations of these age-dependent inflammatory responses and consequences after TBI, from an experimental point of view. We investigate the evolution of microglial, astrocyte and other immune cells responses, and the consequences in terms of neuronal death and functional deficits in neonates, juvenile, adolescent and aged male animals, following a single TBI. We also describe the pharmacological responses to anti-inflammatory or neuroprotective agents, highlighting the need for an age-specific approach to the development of therapies of TBI.

Paternal exposure to exercise and/or caffeine and alcohol modify offspring behavioral and pathophysiological recovery from repetitive mild traumatic brain injury in adolescence

Only recently has the scope of parental research expanded to include the paternal sphere with epidemiological studies implicating stress, nutrition and alcohol consumption in the neurobiological and behavioral characteristics of offspring. This study was designed to determine if paternal exposure to caffeine, alcohol and exercise prior to conception would improve or exacerbate offspring recovery from adolescent repetitive mild traumatic brain injury (RmTBI). Sires received 7 weeks of standard drinking water, or caffeine and ethanol and were housed in regular cages or cages with running wheels, prior to being mated to control females. At postnatal day 40, offspring were administered RmTBI or sham injuries and were assessed for post concussive symptomology. Post-mortem quantitative real-time polymerase chain reaction (qRT-PCR) was used to assess gene expression in the prefrontal cortex (PFC), nucleus accumbens (NAc) and changes in telomere length. Additionally, enzyme-linked immunosorbent assay (ELISA's) were run on serum to detect levels of cytokines, chemokines and sex hormones. Paternal experience did not improve or exacerbate RmTBI behavioral outcomes. However, female and male offspring displayed unique responses to RmTBI and paternal experience, resulting in changes in physical, behavioral and molecular outcomes. Injury and paternal exercise modified changes in female offspring, whereas male offspring were affected by paternal exercise, caffeine and alcohol treatment. Additionally, paternal experience and RmTBI modified expression of many genes in the PFC, NAc, telomere length and levels of sex hormones. Although further exploration is required to understand the heterogeneity that exists in disease risk and resiliency, this study provides corroborating evidence that paternal experiences prior to conception influences offspring development.

The expression of repulsive guidance molecule a after traumatic brain injury: Time-course changes in gene expression in a murine model of controlled cortical impact

INTRODUCTION

Repulsive guidance molecule a (RGMa) is a key protein that negatively regulates neuronal regeneration as its inhibition enhances axonal growth and promotes functional recovery in animal models of spinal cord injury. However, the role of RGMa in traumatic brain injury (TBI) remains elusive. This study aimed to clarify TBI-responsive RGMa expression in a murine model.

METHODS

Adult male C57BL/6J mice were subjected to controlled cortical impact. Brains were extracted 6 hours and 1, 3, 7, 14 and 21 days after injury (n = 6 in each group). Changes in the messenger RNA (mRNA) expression of RGMa and its receptor, neogenin, were evaluated by quantitative polymerase chain reaction in the damaged area of the cortex and contralateral cortex, along with expression measurement of inflammation-related molecules. Neurological deficit was also assessed by the cylinder test.

RESULTS

Neurological score was consistently lower in the TBI group compared to the sham group throughout the experimental period. The mRNA expressions of representative inflammatory cytokine TNF-α and chemokine receptor CCR2 were remarkably increased in the injured cortex on day 1 and gradually decreased over time, although remaining at higher values at least until day 14. The mRNA expressions of RGMa and neogenin were significantly suppressed in the damaged cortex until day 3. Interestingly, RGMa expression was suppressed most on day 1 and recovered over time.

CONCLUSION

In the acute phase of TBI, gene expression of inflammatory cytokines significantly increased, and gene expressions of RGMa and neogenin significantly decreased in the inflammatory milieu of the damaged area. Despite the subsequent remission of inflammation, RGMa gene expression recovered to the normal level 1 week after TBI. Intrinsic regenerative response to acute brain injury might be hampered by the following recovery of RGMa expression, hinting at the possibility of functional RGMa inhibition as a new, effective maneuver against TBI.

Neuroinflammation and Precision Medicine in Pediatric Neurocritical Care: Multi-Modal Monitoring of Immunometabolic Dysfunction

The understanding of molecular biology in neurocritical care (NCC) is expanding rapidly and recognizing the important contribution of neuroinflammation, specifically changes in immunometabolism, towards pathological disease processes encountered across all illnesses in the NCC. Additionally, the importance of individualized inflammatory responses has been emphasized, acknowledging that not all individuals have the same mechanisms contributing towards their presentation. By understanding cellular processes that drive disease, we can make better personalized therapy decisions to improve patient outcomes. While the understanding of these cellular processes is evolving, the ability to measure such cellular responses at bedside to make acute care decisions is lacking. In this overview, we review cellular mechanisms involved in pathological neuroinflammation with a focus on immunometabolic dysfunction and review non-invasive bedside tools that have the potential to measure indirect and direct markers of shifts in cellular metabolism related to neuroinflammation. These tools include near-infrared spectroscopy, transcranial doppler, elastography, electroencephalography, magnetic resonance imaging and spectroscopy, and cytokine analysis. Additionally, we review the importance of genetic testing in providing information about unique metabolic profiles to guide individualized interpretation of bedside data. Together in tandem, these modalities have the potential to provide real time information and guide more informed treatment decisions.

Repeated mild traumatic brain injuries impair visual discrimination learning in adolescent mice

Cognitive deficits following a mild traumatic brain injury (mTBI) are common and are associated with learning deficits in school-age children. Some of these deficits include problems with long-term memory, working memory, processing speeds, attention, mental fatigue, and executive function. Processing speed deficits have been associated with alterations in white matter, but the underlying mechanisms of many of the other deficits are unclear. Without a clear understanding of the underlying mechanisms we cannot effectively treat these injuries. The goal of these studies is to validate a translatable touchscreen discrimination/reversal task to identify deficits in executive function following a single or repeated mTBIs. Using a mild closed skull injury model in adolescent mice we were able to identify clear deficits in discrimination learning following repeated injuries that were not present from a single mTBI. The repeated injuries were not associated with any deficits in motor-based behavior but did induce a robust increase in astrocyte activation. These studies provide an essential platform to interrogate the underlying neurological dysfunction associated with these injuries.