Microglia processes associate with diffusely injured axons following mild traumatic brain injury in the micro pig

A strike to the head: Parallels between the pediatric and adult human and the rodent in traumatic brain injury

Traumatic brain injury (TBI) is a condition that occurs commonly in children from infancy through adolescence and is a global health concern. Pediatric TBI presents with a bimodal age distribution, with very young children (0-4 years) and adolescents (15-19 years) more commonly injured. Because children's brains are still developing, there is increased vulnerability to the effects of head trauma, which results in entirely different patterns of injury than in adults. Pediatric TBI has a profound and lasting impact on a child's development and quality of life, resulting in long-lasting consequences to physical, cognitive, and emotional development. Chronic issues like learning disabilities, behavioral problems, and emotional disturbances can develop. Early intervention and ongoing support are critical for minimizing these long-term deficits. Many animal models of TBI exist, and each varies significantly, displaying different characteristics of clinical TBI. The neurodevelopment differs in the rodent from the human in timing and effect, so TBI outcomes in the juvenile rodent can thus vary from the human child. The current review compares findings from preclinical TBI work in juvenile and adult rodents to clinical TBI research in pediatric and adult humans. We focus on the four brain regions most affected by TBI: the prefrontal cortex, corpus callosum, hippocampus, and hypothalamus. Each has its unique developmental projections and thus is impacted by TBI differently. This review aims to compare the healthy neurodevelopment of these four brain regions in humans to the developmental processes in rodents.

Preliminary study of the consciousness-promotion mechanism of electroacupuncture in comatose patients with diffuse axonal injuries

BACKGROUND

Diffuse axonal injury (DAI) accounts for 30-40% of total neurotrauma cases, and the majority among them manifest with consciousness disturbance. At present, the understanding of the treatment of coma and awakening in patients with DAIs is still limited. This study is characterized by the use of electroacupuncture along with conventional Western medicine to promote consciousness more effectively in comatose patients with DAIs, shorten their time spent in a coma, and gain time for more favorable treatments during follow-up rehabilitation in order to improve the cure rate, reduce the morbidity rate, and achieve better therapeutic effects.

METHODS

In this randomized controlled study, 145 comatose patients with DAIs (type III) were divided into the treatment group (N.=71) and control group (N.=74). The patients in the control group were treated with conventional Western medicine, while those in the treatment group were treated with both electroacupuncture and conventional treatment. The Glasgow Coma Scale (GCS) scores and consciousness-promotion rates of both groups were observed before treatment as well as 10, 20, and 30 days after treatment. Meanwhile, serum acetylcholinesterase E (AchE) concentrations in both groups were measured with ELISA, while AchE activity was determined with the rate method. Correlations between GCS score, AchE concentration, and AchE activity in the treatment group were analyzed by using the stepwise multiple regression method.

RESULTS

The GCS scores in the treatment group showed significant increases after the first, second, and third courses of treatment when compared to the pre-treatment scores (P<0.05). After 1 course of treatment, the GCS scores in the control group were not statistically significantly different compared to the pre-treatment scores (P>0.05), whereas after 2 and 3 courses of treatment, the differences were of greater statistical significance (P<0.05). Statistically significant differences between the two groups were found in GCS scores in the same course of treatment (P<0.05). The consciousness-promotion rates between the two groups after the same treatment course were statistically significantly different (P<0.05). Both the standardized regression coefficients and partial correlation coefficients showed that AchE concentration had a certain influence on GCS score (|Beta|=0.3601; r Y2.1=0.726).

CONCLUSIONS

Conventional Western medicine combined with electroacupuncture treatment may promote the consciousness of patients with DAIs and shorten the amount of time they spend comatose. Furthermore, the neurotransmitter AchE may play a role in the pathophysiological mechanism of consciousness promotion.

Histological Characterisation of a Sheep Model of Mild Traumatic Brain Injury: A Pilot Study

Large animal models of mild traumatic brain injury (mTBI) are needed to elucidate the pathophysiology of mechanical insult to a gyrencephalic brain. Sheep (ovis aries) are an attractive model for mTBI because of their neuroanatomical similarity to humans; however, few histological studies of sheep mTBI models have been conducted. We previously developed a sheep mTBI model to pilot methods for investigating the mechanical properties of brain tissue after injury. Here, we sought to histologically characterize the cortex under the impact site in this model. Three animals received a closed skull mTBI with unconstrained head motion, delivered with an impact stunner, and 3 sham animals were anesthetized but did not receive an impact. Magnetic resonance imaging (MRI) of the brain was performed before and after the impact and revealed variable degrees of damage to the skull and brain. Fluorescent immunohistochemistry revealed regions of hemorrhage in the cortex underlying the impact site in 2 of 3 mTBI sheep, the amount of which correlated with the degree of damage observed on the post-impact MRI scans. Labeling for microtubule-associated protein 2 and neuronal nuclear protein revealed changes in cellular anatomy, but, unexpectedly, glial fibrillary acidic protein and ionized calcium-binding adaptor molecule 1 labeling were relatively unchanged compared to sham animals. Our findings provide preliminary evidence of vascular and neuronal damage with limited glial reactivity and highlight the need for further in-depth histological assessment of large animal mTBI models.

Characterization of Traumatic Brain Injury in a Gyrencephalic Ferret Model Using the Novel Closed Head Injury Model of Engineered Rotational Acceleration (CHIMERA)

Traumatic brain injury (TBI) results from mechanical force to the brain and leads to a series of biochemical responses that further damage neurons and supporting cells. Clinically, most TBIs result from an impact to the intact skull, making closed head TBI pre-clinical models highly relevant. However, most of these closed head TBI models use lissencephalic rodents, which may not transduce biomechanical load in the same manner as gyrencephalic humans. To address this translational gap, this study aimed to characterize acute axonal injury and microglial responses in ferrets-the smallest gyrencephalic mammal. Injury was induced in male ferrets (Mustela furo; 1.20-1.51 kg; 6-9 months old) with the novel Closed Head Injury Model of Engineered Rotational Acceleration (CHIMERA) model. Animals were randomly allocated to either sham (n = 4), a 22J (joules) impact (n = 4), or a 27J impact (n = 4). Axonal injury was examined histologically with amyloid precursor protein (APP), neurofilament M (RMO 14.9) (RMO-14), and phosphorylated tau (AT180) and the microglial response with ionized calcium-binding adaptor molecule 1 at 24 h post-injury in gray and white matter regions. Graded axonal injury was observed with modest increases in APP and RMO-14 immunoreactivity in the 22J TBI group, mostly within the corpus callosum and fornix and more extensive diffuse axonal injury encompassing gray matter structures like the thalamus and hypothalamus in the 27J group. Accompanying microglial activation was only observed in the 27J group, most prominently within the white matter tracts in response to the larger amounts of axonal injury. The 27J, but not the 22J, group showed an increase in AT180 within the base of the sulci post-injury. This could suggest that the strain may be highest in this region, demonstrating the different responses in gyrencephalic compared to lissencephalic brains. The CHIMERA model in ferrets mimic many of the histopathological features of human closed head TBI acutely and provides a promising model to investigate the pathophysiology of TBI.

Validation and characterization of a novel blood-brain barrier platform for investigating traumatic brain injury

The Blood-Brain Barrier (BBB) is a highly-selective physiologic barrier responsible for maintaining cerebral homeostasis. Innovative in vitro models of the BBB are needed to provide useful insights into BBB function with CNS disorders like traumatic brain injury (TBI). TBI is a multidimensional and highly complex pathophysiological condition that requires intrinsic models to elucidate its mechanisms. Current models either lack fluidic shear stress, or neglect hemodynamic parameters important in recapitulating the human in vivo BBB phenotype. To address these limitations in the field, we developed a fluid dynamic novel platform which closely mimics these parameters. To validate our platform, Matrigel-coated Transwells were seeded with brain microvascular endothelial cells, both with and without co-cultured primary human astrocytes and bone-marrow mesenchymal stem cells. In this article we characterized BBB functional properties such as TEER and paracellular permeability. Our platform demonstrated physiologic relevant decreases in TEER in response to an ischemic environment, while directly measuring barrier fluid fluctuation. These recordings were followed with recovery, implying stability of the model. We also demonstrate that our dynamic platform is responsive to inflammatory and metabolic cues with resultant permeability coefficients. These results indicate that this novel dynamic platform will be a valuable tool for evaluating the recapitulating BBB function in vitro, screening potential novel therapeutics, and establishing a relevant paradigm to evaluate the pathophysiology of TBI.

Concussion: Beyond the Cascade

Sport concussion affects millions of athletes each year at all levels of sport. Increasing evidence demonstrates clinical and physiological recovery are becoming more divergent definitions, as evidenced by several studies examining blood-based biomarkers of inflammation and imaging studies of the central nervous system (CNS). Recent studies have shown elevated microglial activation in the CNS in active and retired American football players, as well as in active collegiate athletes who were diagnosed with a concussion and returned to sport. These data are supportive of discordance in clinical symptomology and the inflammatory response in the CNS upon symptom resolution. In this review, we will summarize recent advances in the understanding of the inflammatory response associated with sport concussion and broader mild traumatic brain injury, as well as provide an outlook for important research questions to better align clinical and physiological recovery.

The Central Fluid Percussion Brain Injury in a Gyrencephalic Pig Brain: Scalable Diffuse Injury and Tissue Viability for Glial Cell Immunolabeling following Long-Term Refrigerated Storage

Traumatic brain injury (TBI) affects millions of people annually; however, our knowledge of the diffuse pathologies associated with TBI is limited. As diffuse pathologies, including axonal injury and neuroinflammatory changes, are difficult to visualize in the clinical population, animal models are used. In the current study, we used the central fluid percussion injury (CFPI) model in a micro pig to study the potential scalability of these diffuse pathologies in a gyrencephalic brain of a species with inflammatory systems very similar to humans. We found that both axonal injury and microglia activation within the thalamus and corpus callosum are positively correlated with the weight-normalized pressure pulse, while subtle changes in blood gas and mean arterial blood pressure are not. We also found that the majority of tissue generated up to 10 years previously is viable for immunofluorescent labeling after long-term refrigeration storage. This study indicates that a micro pig CFPI model could allow for specific investigations of various degrees of diffuse pathological burdens following TBI.

Temporal changes in the microglial proteome of male and female mice after a diffuse brain injury using label-free quantitative proteomics

Traumatic brain injury (TBI) triggers neuroinflammatory cascades mediated by microglia, which promotes tissue repair in the short-term. These cascades may exacerbate TBI-induced tissue damage and symptoms in the months to years post-injury. However, the progression of the microglial function across time post-injury and whether this differs between biological sexes is not well understood. In this study, we examined the microglial proteome at 3-, 7-, or 28-days after a midline fluid percussion injury (mFPI) in male and female mice using label-free quantitative proteomics. Data are available via ProteomeXchange with identifier PXD033628. We identified a reduction in microglial proteins involved with clearance of neuronal debris via phagocytosis at 3- and 7-days post-injury. At 28 days post-injury, pro-inflammatory proteins were decreased and anti-inflammatory proteins were increased in microglia. These results indicate a reduction in microglial clearance of neuronal debris in the days post-injury with a shift to anti-inflammatory function by 28 days following TBI. The changes in the microglial proteome that occurred across time post-injury did not differ between biological sexes. However, we did identify an increase in microglial proteins related to pro-inflammation and phagocytosis as well as insulin and estrogen signaling in males compared with female mice that occurred with or without a brain injury. Although the microglial response was similar between males and females up to 28 days following TBI, biological sex differences in the microglial proteome, regardless of TBI, has implications for the efficacy of treatment strategies targeting the microglial response post-injury.

Microglial activation persists beyond clinical recovery following sport concussion in collegiate athletes

Introduction

In concussion, clinical and physiological recovery are increasingly recognized as diverging definitions. This study investigated whether central microglial activation persisted in participants with concussion after receiving an unrestricted return-to-play (uRTP) designation using [18F]DPA-714 PET, an in vivo marker of microglia activation.

Methods

Eight (5 M, 3 F) current athletes with concussion (Group 1) and 10 (5 M, 5 F) healthy collegiate students (Group 2) were enrolled. Group 1 completed a pre-injury (Visit1) screen, follow-up Visit2 within 24 h of a concussion diagnosis, and Visit3 at the time of uRTP. Healthy participants only completed assessments at Visit2 and Visit3. At Visit2, all participants completed a multidimensional battery of tests followed by a blood draw to determine genotype and study inclusion. At Visit3, participants completed a clinical battery of tests, brain MRI, and brain PET; no imaging tests were performed outside of Visit3.

Results

For Group 1, significant differences were observed between Visits 1 and 2 (p < 0.05) in ImPACT, SCAT5 and SOT performance, but not between Visit1 and Visit3 for standard clinical measures (all p > 0.05), reflecting clinical recovery. Despite achieving clinical recovery, PET imaging at Visit3 revealed consistently higher [18F]DPA-714 tracer distribution volume (VT) of Group 1 compared to Group 2 in 10 brain regions (p < 0.001) analyzed from 164 regions of the whole brain, most notably within the limbic system, dorsal striatum, and medial temporal lobe. No notable differences were observed between clinical measures and VT between Group 1 and Group 2 at Visit3.

Discussion

Our study is the first to demonstrate persisting microglial activation in active collegiate athletes who were diagnosed with a sport concussion and cleared for uRTP based on a clinical recovery.

Acute Blood Levels of Neurofilament Light Indicate One-Year White Matter Pathology and Functional Impairment in Repetitive Mild Traumatic Brain Injured Mice

Mild traumatic brain injury (mTBI) mostly causes transient symptoms, but repeated (r)mTBI can lead to neurodegenerative processes. Diagnostic tools to evaluate the presence of ongoing occult neuropathology are lacking. In a mouse model of rmTBI, we investigated MRI and plasma biomarkers of brain damage before chronic functional impairment arose. Anesthetized adult male and female C57BL/6J mice were subjected to rmTBI or a sham procedure. Sensorimotor deficits were evaluated up to 12 months post-injury in SNAP and Neuroscore tests. Cognitive function was assessed in the novel object recognition test at six and 12 months. Diffusion tensor imaging (DTI) and structural magnetic resonance imaging (MRI) were performed at six and 12 months to examine white matter and structural damage. Plasma levels of neurofilament light (NfL) were assessed longitudinally up to 12 months. Brain histopathology was performed at 12 months. Independent groups of mice were used to examine the effects of 2-, 7- and 14-days inter-injury intervals on acute plasma NfL levels and on hyperactivity. Twelve months after an acute transient impairment, sensorimotor functions declined again in rmTBI mice (p < 0.001 vs sham), but not earlier. Similarly, rmTBI mice showed memory impairment at 12 (p < 0.01 vs sham) but not at 6 months. White matter damage examined by DTI was evident in rmTBI mice at both six and 12 months (p < 0.001 vs sham). This was associated with callosal atrophy (p < 0.001 vs sham) evaluated by structural MRI. Plasma NfL at one week was elevated in rmTBI (p < 0.001 vs sham), and its level correlated with callosal atrophy at 12 months (Pearson r = 0.72, p < 0.01). Histopathology showed thinning of the corpus callosum and marked astrogliosis in rmTBI mice. The NfL levels were higher in mice subjected to short (2 days) compared with longer (7 and 14 days) inter-injury intervals (p < 0.05), and this correlated with hyperactivity in mice (Pearson r = 0.50; p < 0.05). These findings show that rmTBI causes white matter pathology detectable by MRI before chronic functional impairment. Early quantification of plasma NfL correlates with the degree of white matter atrophy one year after rmTBI and can serve to monitor the brain's susceptibility to a second mTBI, supporting its potential clinical application to guide the return to practice in sport-related TBI.

Understanding microglial responses in large animal models of traumatic brain injury: an underutilized resource for preclinical and translational research

Traumatic brain injury (TBI) often results in prolonged or permanent brain dysfunction with over 2.8 million affected annually in the U.S., including over 56,000 deaths, with over 5 million total survivors exhibiting chronic deficits. Mild TBI (also known as concussion) accounts for over 75% of all TBIs every year. Mild TBI is a heterogeneous disorder, and long-term outcomes are dependent on the type and severity of the initial physical event and compounded by secondary pathophysiological consequences, such as reactive astrocytosis, edema, hypoxia, excitotoxicity, and neuroinflammation. Neuroinflammation has gained increasing attention for its role in secondary injury as inflammatory pathways can have both detrimental and beneficial roles. For example, microglia-resident immune cells of the central nervous system (CNS)-influence cell death pathways and may contribute to progressive neurodegeneration but also aid in debris clearance and neuroplasticity. In this review, we will discuss the acute and chronic role of microglia after mild TBI, including critical protective responses, deleterious effects, and how these processes vary over time. These descriptions are contextualized based on interspecies variation, sex differences, and prospects for therapy. We also highlight recent work from our lab that was the first to describe microglial responses out to chronic timepoints after diffuse mild TBI in a clinically relevant large animal model. The scaled head rotational acceleration of our large animal model, paired with the gyrencephalic architecture and appropriate white:gray matter ratio, allows us to produce pathology with the same anatomical patterns and distribution of human TBI, and serves as an exemplary model to examine complex neuroimmune response post-TBI. An improved understanding of microglial influences in TBI could aid in the development of targeted therapeutics to accentuate positive effects while attenuating detrimental post-injury responses over time.

The Integrin Pathway Partially Mediates Stretch-Induced Deficits in Primary Rat Microglia

Stretch-injured microglia display significantly altered morphology, function and inflammatory-associated gene expression when cultured on a synthetic fibronectin substrate. However, the mechanism by which stretch induces these changes is unknown. Integrins, such as α5β1, mediate microglial attachment to fibronectin via the RGD binding peptide; following integrin ligation the integrin-associated signaling enzyme, focal adhesion kinase (FAK), autophosphorylates tyrosine residue 397 and mediates multiple downstream cellular processes. We therefore hypothesize that blocking the RGD binding/integrin pathway with a commercially available RGD peptide will mimic the stretch-induced morphological alterations and functional deficits in microglia. Further, we hypothesize that upregulation of stretch-inhibited downstream integrin signaling will reverse these effects. Using primary rat microglia, we tested the effects of RGD binding peptide and a FAK activator on cellular function and structure and response to stretch-injury. Similar to injured cells, RGD peptide administration significantly decreases media nitric oxide (NO) levels and iNOS expression and induced morphological alterations and migratory deficits. While stretch-injury and RGD peptide administration decreased phosphorylation of the tyrosine 397 residue on FAK, 20 nM of ZINC 40099027, an activator specific to the tyrosine 397 residue, rescued the stretch-induced decrease in FAK phosphorylation and ameliorated the injury-induced decrease in media NO levels, iNOS expression and inflammatory associated gene expression. Additionally, treatment alleviated morphological changes observed after stretch-injury and restored normal migratory behavior to control levels. Taken together, these data suggest that the integrin/FAK pathway partially mediates the stretch-injured phenotype in microglia, and may serve as a pathway to modulate microglial responses.

Regional variances depict a unique glial-specific inflammatory response following closed-head injury

Mild traumatic brain injuries (mTBI) constitute a significant health concern with clinical symptoms ranging from headaches to cognitive deficits. Despite the myriad of symptoms commonly reported following this injury, there is still a lack of knowledge on the various pathophysiological changes that occur. Preclinical studies are at the forefront of discovery delineating the changes that occur within this heterogeneous injury, with the emergence of translational models such as closed-head impact models allowing for further exploration of this injury mechanism. In the current study, male rats were subjected to a closed-head controlled cortical impact (cCCI), producing a concussion (mTBI). The pathological effects of this injury were then evaluated using immunoflourescence seven days following. The results exhibited a unique glial-specific inflammatory response, with both the ipsilateral and contralateral sides of the cortex and hippocampus showing pathological changes following impact. Overall these findings are consistent with glial changes reported following concussions and may contribute to subsequent symptoms.

Assessment of behavioral, neuroinflammatory, and histological responses in a model of rat repetitive mild fluid percussion injury at 2 weeks post-injury

Repetitive mild traumatic brain injury (rmTBI) is a prominent public health concern, with linkage to debilitating chronic sequelae. Developing reliable and well-characterized preclinical models of rmTBI is imperative in the investigation of the underlying pathophysiological mechanisms, as models can have varying parameters, affecting the overall pathology of the resulting injury. The lateral fluid percussion injury (FPI) model is a reliable and frequently used method of TBI replication in rodent subjects, though it is currently relatively underutilized in rmTBI research. In this study, we have performed a novel description of a variation of the lateral repetitive mild FPI (rmFPI) model, showing the graded acute behavioral impairment and histopathology occurring in response to one, two or four mild FPI (1.25 atm) or sham surgeries, implemented 24h apart. Beam walking performance revealed significant motor impairment in injured animals, with dysfunction increasing with additional injury. Based upon behavioral responses and histological observations, we further investigated the subacute pathophysiological outcomes of the dual FPI (dFPI). Immunoreactivity assessments showed that dFPI led to regionally-specific reductions in the post-synaptic protein neurogranin and increased subcortical white matter staining of the presynaptic protein synaptophysin at 2 weeks following dFPI. Immunohistochemical assessments of the microglial marker Iba-1 showed a striking increase in in several brain regions, and assessment of the astrocytic marker GFAP showed significantly increased immunoreactivity in the subcortical white matter and thalamus. With this study, we have provided a novel account of the subacute post injury outcomes occurring in response to a rmFPI utilizing these injury and frequency parameters, and thereby also demonstrating the reliability of the lateral FPI model in rmTBI replication.

Post-Injury Buprenorphine Administration Is Associated with Long-Term Region-Specific Glial Alterations in Rats

Traumatic brain injury (TBI) is a major leading cause of death and disability. While previous studies regarding focal pathologies following TBI have been done, there is a lack of information concerning the role of analgesics and their influences on injury pathology. Buprenorphine (Bup), an opioid analgesic, is a commonly used analgesic in experimental TBI models. Our previous studies investigated the acute effects of Buprenorphine-sustained release-Lab (Bup-SR-Lab) on diffuse neuronal/glial pathology, neuroinflammation, cell damage, and systemic physiology. The current study investigated the longer-term chronic outcomes of Bup-SR-Lab treatment at 4 weeks following TBI utilizing a central fluid percussion injury (cFPI) model in adult male rats. Histological assessments of physiological changes, neuronal damage, cortical and thalamic cytokine expression, microglial and astrocyte morphological changes, and myelin alterations were done, as we had done in our acute study. In the current study the Whisker Nuisance Task (WNT) was also performed pre- and 4w post-injury to assess changes in somatosensory sensitivity following saline or Bup-SR-Lab treatment. Bup-SR-Lab treatment had no impact on overall physiology or neuronal damage at 4w post-injury regardless of region or injury, nor did it have any significant effects on somatosensory sensitivity. However, greater IL-4 cytokine expression with Bup-SR-Lab treatment was observed compared to saline treated animals. Microglia and astrocytes also demonstrated region-specific morphological alterations associated with Bup-SR-Lab treatment, in which cortical microglia and thalamic astrocytes were particularly vulnerable to Bup-mediated changes. There were discernable injury-specific and region-specific differences regarding myelin integrity and changes in specific myelin basic protein (MBP) isoform expression following Bup-SR-Lab treatment. This study indicates that use of Bup-SR-Lab could impact TBI-induced glial alterations in a region-specific manner 4w following diffuse brain injury.

Long-term in vivo imaging of mouse spinal cord through an optically cleared intervertebral window

The spinal cord accounts for the main communication pathway between the brain and the peripheral nervous system. Spinal cord injury is a devastating and largely irreversible neurological trauma, and can result in lifelong disability and paralysis with no available cure. In vivo spinal cord imaging in mouse models without introducing immunological artifacts is critical to understand spinal cord pathology and discover effective treatments. We developed a minimally invasive intervertebral window by retaining the ligamentum flavum to protect the underlying spinal cord. By introducing an optical clearing method, we achieve repeated two-photon fluorescence and stimulated Raman scattering imaging at subcellular resolution with up to 15 imaging sessions over 6-167 days and observe no inflammatory response. Using this optically cleared intervertebral window, we study neuron-glia dynamics following laser axotomy and observe strengthened contact of microglia with the nodes of Ranvier during axonal degeneration. By enabling long-term, repetitive, stable, high-resolution and inflammation-free imaging of mouse spinal cord, our method provides a reliable platform in the research aiming at interpretation of spinal cord physiology and pathology.

Crosstalk between Microglia and Neurons in Neurotrauma: An Overview of the Underlying Mechanisms

Microglia are the resident immune cells of the brain and play a crucial role in housekeeping and maintaining homeostasis of the brain microenvironment. Upon injury or disease, microglial cells become activated, at least partly, via signals initiated by injured neurons. Activated microglia, thereby, contribute to both neuroprotection and neuroinflammation. However, sustained microglial activation initiates a chronic neuroinflammatory response which can disturb neuronal health and disrupt communications between neurons and microglia. Thus, microglia-neuron crosstalk is critical in a healthy brain as well as during states of injury or disease. As most studies focus on how neurons and microglia act in isolation during neurotrauma, there is a need to understand the interplay between these cells in brain pathophysiology. This review highlights how neurons and microglia reciprocally communicate under physiological conditions and during brain injury and disease. Furthermore, the modes of microglia-neuron communication are exposed, focusing on cell-contact dependent signaling and communication by the secretion of soluble factors like cytokines and growth factors. In addition, it has been discussed that how microglia-neuron interactions could exert either beneficial neurotrophic effects or pathologic proinflammatory responses. We further explore how aberrations in microglia-neuron crosstalk may be involved in central nervous system (CNS) anomalies, namely traumatic brain injury (TBI), neurodegeneration, and ischemic stroke. A clear understanding of how the microglia-neuron crosstalk contributes to the pathogenesis of brain pathologies may offer novel therapeutic avenues of brain trauma treatment.

Age-at-Injury Determines the Extent of Long-Term Neuropathology and Microgliosis After a Diffuse Brain Injury in Male Rats

Traumatic brain injury (TBI) can occur at any age, from youth to the elderly, and its contribution to age-related neuropathology remains unknown. Few studies have investigated the relationship between age-at-injury and pathophysiology at a discrete biological age. In this study, we report the immunohistochemical analysis of naïve rat brains compared to those subjected to diffuse TBI by midline fluid percussion injury (mFPI) at post-natal day (PND) 17, PND35, 2-, 4-, or 6-months of age. All brains were collected when rats were 10-months of age (n = 6–7/group). Generalized linear mixed models were fitted to analyze binomial proportion and count data with R Studio. Amyloid precursor protein (APP) and neurofilament (SMI34, SMI32) neuronal pathology were counted in the corpus callosum (CC) and primary sensory barrel field (S1BF). Phosphorylated TAR DNA-binding protein 43 (pTDP-43) neuropathology was counted in the S1BF and hippocampus. There was a significantly greater extent of APP and SMI34 axonal pathology and pTDP-43 neuropathology following a TBI compared with naïves regardless of brain region or age-at-injury. However, age-at-injury did determine the extent of dendritic neurofilament (SMI32) pathology in the CC and S1BF where all brain-injured rats exhibited a greater extent of pathology compared with naïve. No significant differences were detected in the extent of astrocyte activation between brain-injured and naïve rats. Microglia counts were conducted in the S1BF, hippocampus, ventral posteromedial (VPM) nucleus, zona incerta, and posterior hypothalamic nucleus. There was a significantly greater proportion of deramified microglia, regardless of whether the TBI was recent or remote, but this only occurred in the S1BF and hippocampus. The proportion of microglia with colocalized CD68 and TREM2 in the S1BF was greater in all brain-injured rats compared with naïve, regardless of whether the TBI was recent or remote. Only rats with recent TBI exhibited a greater proportion of CD68-positive microglia compared with naive in the hippocampus and posterior hypothalamic nucleus. Whilst, only rats with a remote brain-injury displayed a greater proportion of microglia colocalized with TREM2 in the hippocampus. Thus, chronic alterations in neuronal and microglial characteristics are evident in the injured brain despite the recency of a diffuse brain injury.

Diverse changes in microglia morphology and axonal pathology during the course of 1 year after mild traumatic brain injury in pigs

Over 2.8 million people experience mild traumatic brain injury (TBI) in the United States each year, which may lead to long-term neurological dysfunction. The mechanical forces that are caused by TBI propagate through the brain to produce diffuse axonal injury (DAI) and trigger secondary neuroinflammatory cascades. The cascades may persist from acute to chronic time points after injury, altering the homeostasis of the brain. However, the relationship between the hallmark axonal pathology of diffuse TBI and potential changes in glial cell activation or morphology have not been established in a clinically relevant large animal model at chronic time points. In this study, we assessed the tissue from pigs subjected to rapid head rotation in the coronal plane to generate mild TBI. Neuropathological assessments for axonal pathology, microglial morphological changes, and astrocyte reactivity were conducted in specimens out to 1-year post-injury. We detected an increase in overall amyloid precursor protein pathology, as well as periventricular white matter and fimbria/fornix pathology after a single mild TBI. We did not detect the changes in corpus callosum integrity or astrocyte reactivity. However, detailed microglial skeletal analysis revealed changes in morphology, most notably increases in the number of microglial branches, junctions, and endpoints. These subtle changes were most evident in periventricular white matter and certain hippocampal subfields, and were observed out to 1-year post-injury in some cases. These ongoing morphological alterations suggest persistent change in neuroimmune homeostasis. Additional studies are needed to characterize the underlying molecular and neurophysiological alterations, as well as potential contributions to neurological deficits.

Design and Evaluation of an In Vitro Mild Traumatic Brain Injury Modeling System Using 3D Printed Mini Impact Device on the 3D Cultured Human iPSC Derived Neural Progenitor Cells

Despite significant progress in understanding the disease mechanism of traumatic brain injury (TBI), promising preclinical therapeutics have seldom been translated into successful clinical outcomes, partially because the model animals have physiological and functional differences in the central nervous system (CNS) compared to humans. Human relevant models are thus urgently required. Here, an in vitro mild TBI (mTBI) modeling system is reported based on 3D cultured human induced pluripotent stem cells (iPSC) derived neural progenitor cells (iPSC-NPCs) to evaluate consequences of single and repetitive mTBI using a 3D printed mini weight-drop impact device. Computational simulation is performed to understand the single/cumulative effects of weight-drop impact on the NPC differentiated neurospheres. Experimental results reveal that neurospheres show reactive astrogliosis and glial scar formation after repetitive (10 hits) mild impacts, while no astrocyte activation is found after one or two mild impacts. A 3D co-culture model of human microglia cells with neurospheres is further developed. It is found that astrocyte response is promoted even after two mild impacts, possibly caused by the chronic neuroinflammation after microglia activation. The in vitro mTBI modeling system recapitulates several hallmarks of the brain impact injury and might serve as a good platform for future drug screening.

Microglia: A Potential Drug Target for Traumatic Axonal Injury

Traumatic axonal injury (TAI) is a major cause of death and disability among patients with severe traumatic brain injury (TBI); however, no effective therapies have been developed to treat this disorder. Neuroinflammation accompanying microglial activation after TBI is likely to be an important factor in TAI. In this review, we summarize the current research in this field, and recent studies suggest that microglial activation plays an important role in TAI development. We discuss several drugs and therapies that may aid TAI recovery by modulating the microglial phenotype following TBI. Based on the findings of recent studies, we conclude that the promotion of active microglia to the M2 phenotype is a potential drug target for the treatment of TAI.

Microglial Displacement of GABAergic Synapses Is a Protective Event during Complex Febrile Seizures

Complex febrile seizures (FSs) lead to a high risk of intractable temporal lobe epilepsy during adulthood, yet the pathological process of complex FSs is largely unknown. Here, we demonstrate that activated microglia extensively associated with glutamatergic neuronal soma displace surrounding GABAergic presynapses in complex FSs. Patch-clamp electrophysiology establishes that the microglial displacement of GABAergic presynapses abrogates a complex-FS-induced increase in GABAergic neurotransmission and neuronal excitability, whereas GABA exerts an excitatory action in this immature stage. Pharmacological inhibition of microglial displacement of GABAergic presynapses or selective ablation of microglia in CD11b^DTR mice promotes the generation of complex FSs. Blocking or deleting the P2Y₁₂ receptor (P2Y₁₂R) reduces microglial displacement of GABAergic presynapses and shortens the latency of complex FSs. Together, microglial displacement of GABAergic presynapses, regulated by P2Y₁₂R, reduces neuronal excitability to mitigate the generation of complex FSs. Microglial displacement is a protective event during the pathological process of complex FSs.

Focal white matter lesions induce long-lasting axonal degeneration, neuroinflammation and behavioral deficits

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) with episodes of inflammatory demyelination and remyelination. While remyelination has been linked with functional recovery in MS patients, there is evidence of ongoing tissue damage despite complete myelin repair. In this study, we investigated the long-term consequences of an acute demyelinating white matter CNS lesion. For this purpose, acute demyelination was induced by 5-week-cuprizone intoxication in male C57BL/6 J mice, and the tissues were examined after a 7-month recovery period. While myelination and oligodendrocyte densities appeared normal, ongoing axonal degeneration and glia cell activation were found in the remyelinated corpus callosum. Neuropathologies were paralleled by subtle gait abnormalities evaluated using DigiGait™ high speed ventral plane videography. Gene array analyses revealed increased expression levels of various inflammation related genes, among protein kinase c delta (PRKCD). Immunofluorescence stains revealed predominant microglia/macrophages PRKCD expression in both, cuprizone tissues and post-mortem MS lesions. These results support the hypothesis that chronic microglia/macrophages driven tissue injury represents a key aspect of progressive neurodegeneration and functional decline in MS.

Buprenorphine alters microglia and astrocytes acutely following diffuse traumatic brain injury

Traumatic brain injury (TBI) is a common phenomenon, accounting for significant cost and adverse health effects. While there is information about focal pathologies following TBI, knowledge of more diffuse processes is lacking, particularly regarding how analgesics affect this pathology. As buprenorphine is the most commonly used analgesic in experimental TBI models, this study investigated the acute effects of the opioid analgesic buprenorphine (Bup-SR-Lab) on diffuse neuronal/glial pathology, neuroinflammation, cell damage, and systemic physiology. We utilized a model of central fluid percussion injury (CFPI) in adult male rats treated with a single subcutaneous bolus of Bup-SR-Lab or saline 15 min post-injury. Microscopic assessments were performed at 1 day post-injury. Cell impermeable dextran was infused intraventricularly prior to sacrifice to assess neuronal membrane disruption. Axonal injury was assessed by investigating labeling of the anterogradely transported amyloid precursor protein. Neuroinflammation was assessed by analyzing Iba-1 + microglial and GFAP + astrocyte histological/morphological features as well as cytokine levels in both regions of interest (ROIs). Myelin pathology was assessed by evaluating the expression of myelin basic protein (MBP) and the propensity of MBP + myelin debris. Acute physiologic data showed no difference between groups except for reduction in weight loss following cFPI in Bup treated animals compared to saline. There were no discernable differences in axonal injury or membrane disruption between treatment groups. Cytokine levels were consistent between Bup and saline treated animals, however, microglia and astrocytes revealed region specific histological changes at 1d following Bup treatment. Myelin integrity and overall MBP expression showed no differences between Bup and saline treated animals, but there were significant regional differences in MBP expression between the cortex and thalamus. These data suggest effects of Bup treatment on weight following CFPI and potential regional specificity of Bup-associated microglial and astrocyte alterations, but very little change in other acute pathology at 1-day post-injury. Overall, this preliminary study indicates that use of Bup-SR-Lab in preclinical work does have effects on acute glial pathology, however, longer term studies will be needed to assess potential effects of Bup treatment on more chronic pathological progressions.

From biomechanics to pathology: predicting axonal injury from patterns of strain after traumatic brain injury

The relationship between biomechanical forces and neuropathology is key to understanding traumatic brain injury. White matter tracts are damaged by high shear forces during impact, resulting in axonal injury, a key determinant of long-term clinical outcomes. However, the relationship between biomechanical forces and patterns of white matter injuries, associated with persistent diffusion MRI abnormalities, is poorly understood. This limits the ability to predict the severity of head injuries and the design of appropriate protection. Our previously developed human finite element model of head injury predicted the location of post-traumatic neurodegeneration. A similar rat model now allows us to experimentally test whether strain patterns calculated by the model predicts in vivo MRI and histology changes. Using a controlled cortical impact, mild and moderate injuries (1 and 2 mm) were performed. Focal and axonal injuries were quantified with volumetric and diffusion 9.4 T MRI at 2 weeks post injury. Detailed analysis of the corpus callosum was conducted using multi-shell diffusion MRI and histopathology. Microglia and astrocyte density, including process parameters, along with white matter structural integrity and neurofilament expression were determined by quantitative immunohistochemistry. Linear mixed effects regression analyses for strain and strain rate with the employed outcome measures were used to ascertain how well immediate biomechanics could explain MRI and histology changes. The spatial pattern of mechanical strain and strain rate in the injured cortex shows good agreement with the probability maps of focal lesions derived from volumetric MRI. Diffusion metrics showed abnormalities in the corpus callosum, indicating white matter changes in the segments subjected to high strain, as predicted by the model. The same segments also exhibited a severity-dependent increase in glia cell density, white matter thinning and reduced neurofilament expression. Linear mixed effects regression analyses showed that mechanical strain and strain rate were significant predictors of in vivo MRI and histology changes. Specifically, strain and strain rate respectively explained 33% and 28% of the reduction in fractional anisotropy, 51% and 29% of the change in neurofilament expression and 51% and 30% of microglia density changes. The work provides evidence that strain and strain rate in the first milliseconds after injury are important factors in determining patterns of glial and axonal injury and serve as experimental validators of our computational model of traumatic brain injury. Our results provide support for the use of this model in understanding the relationship of biomechanics and neuropathology and can guide the development of head protection systems, such as airbags and helmets.

Player position in American football influences the magnitude of mechanical strains produced in the location of chronic traumatic encephalopathy pathology: A computational modelling study

American football players are frequently exposed to head impacts, which can cause concussions and may lead to neurodegenerative diseases such as chronic traumatic encephalopathy (CTE). Player position appears to influence the risk of concussion but there is limited work on its effect on the risk of CTE. Computational modelling has shown that large brain deformations during head impacts co-localise with CTE pathology in sulci. Here we test whether player position has an effect on brain deformation within the sulci, a possible biomechanical trigger for CTE. We physically reconstructed 148 head impact events from video footage of American Football games. Players were separated into 3 different position profiles based on the magnitude and frequency of impacts. A detailed finite element model of TBI was then used to predict Green-Lagrange strain and strain rate across the brain and in sulci. Using a one-way ANOVA, we found that in positions where players were exposed to large magnitude and low frequency impacts (e.g. defensive back and wide receiver), strain and strain rate across the brain and in sulci were highest. We also found that rotational head motion is a key determinant in producing large strains and strain rates in the sulci. Our results suggest that player position has a significant effect on impact kinematics, influencing the magnitude of deformations within sulci, which spatially corresponds to where CTE pathology is observed. This work can inform future studies investigating different player-position risks for concussion and CTE and guide design of prevention systems.

Traumatic Brain Injury Causes Chronic Cortical Inflammation and Neuronal Dysfunction Mediated by Microglia

Traumatic brain injury (TBI) can lead to significant neuropsychiatric problems and neurodegenerative pathologies, which develop and persist years after injury. Neuroinflammatory processes evolve over this same period. Therefore, we aimed to determine the contribution of microglia to neuropathology at acute [1 d postinjury (dpi)], subacute (7 dpi), and chronic (30 dpi) time points. Microglia were depleted with PLX5622, a CSF1R antagonist, before midline fluid percussion injury (FPI) in male mice and cortical neuropathology/inflammation was assessed using a neuropathology mRNA panel. Gene expression associated with inflammation and neuropathology were robustly increased acutely after injury (1 dpi) and the majority of this expression was microglia independent. At 7 and 30 dpi, however, microglial depletion reversed TBI-related expression of genes associated with inflammation, interferon signaling, and neuropathology. Myriad suppressed genes at subacute and chronic endpoints were attributed to neurons. To understand the relationship between microglia, neurons, and other glia, single-cell RNA sequencing was completed 7 dpi, a critical time point in the evolution from acute to chronic pathogenesis. Cortical microglia exhibited distinct TBI-associated clustering with increased type-1 interferon and neurodegenerative/damage-related genes. In cortical neurons, genes associated with dopamine signaling, long-term potentiation, calcium signaling, and synaptogenesis were suppressed. Microglial depletion reversed the majority of these neuronal alterations. Furthermore, there was reduced cortical dendritic complexity 7 dpi, reduced neuronal connectively 30 dpi, and cognitive impairment 30 dpi. All of these TBI-associated functional and behavioral impairments were prevented by microglial depletion. Collectively, these studies indicate that microglia promote persistent neuropathology and long-term functional impairments in neuronal homeostasis after TBI.SIGNIFICANCE STATEMENT Millions of traumatic brain injuries (TBIs) occur in the United States alone each year. Survivors face elevated rates of cognitive and psychiatric complications long after the inciting injury. Recent studies of human brain injury link chronic neuroinflammation to adverse neurologic outcomes, suggesting that evolving inflammatory processes may be an opportunity for intervention. Here, we eliminate microglia to compare the effects of diffuse TBI on neurons in the presence and absence of microglia and microglia-mediated inflammation. In the absence of microglia, neurons do not undergo TBI-induced changes in gene transcription or structure. Microglial elimination prevented TBI-induced cognitive changes 30 d postinjury (dpi). Therefore, microglia have a critical role in disrupting neuronal homeostasis after TBI, particularly at subacute and chronic timepoints.

Operation Brain Trauma Therapy: An Exploratory Study of Levetiracetam Treatment Following Mild Traumatic Brain Injury in the Micro Pig

Operation brain trauma therapy (OBTT) is a drug- and biomarker-screening consortium intended to improve the quality of preclinical studies and provide a rigorous framework to increase the translational potential of experimental traumatic brain injury (TBI) treatments. Levetiracetam (LEV) is an antiepileptic agent that was the fifth drug tested by OBTT in three independent rodent models of moderate to severe TBI. To date, LEV has been the most promising drug tested by OBTT and was therefore advanced to testing in the pig. Adult male micro pigs were subjected to a mild central fluid percussion brain injury followed by a post-injury intravenous infusion of either 170 mg/kg LEV or vehicle. Systemic physiology was assessed throughout the post-injury period. Serial serum samples were obtained pre-injury as well as at 1 min, 30 min, 1 h, 3 h, and 6 h post-injury for a detailed analysis of the astroglial biomarker glial fibrillary acidic protein (GFAP) and ubiquitin carboxy-terminal hydrolase L1. Tissue was collected 6 h following injury for histological assessment of diffuse axonal injury using antibodies against the amyloid precursor protein (APP). The animals showed significant increases in circulating GFAP levels from baseline to 6 h post-injury; however, LEV treatment was associated with greater GFAP increases compared to the vehicle. There were no differences in the numbers of APP+ axonal swellings within the pig thalamus with LEV treatment; however, significant alterations in the morphological properties of the APP+ axonal swellings, including reduced swelling area and increased swelling roundness, were observed. Additionally, expression of the neurite outgrowth marker, growth-associated protein 43, was reduced in axonal swellings following LEV treatment, suggesting potential effects on axonal outgrowth that warrant further investigation.

Shaping Neuronal Fate: Functional Heterogeneity of Direct Microglia-Neuron Interactions

The functional contribution of microglia to normal brain development, healthy brain function, and neurological disorders is increasingly recognized. However, until recently, the nature of intercellular interactions mediating these effects remained largely unclear. Recent findings show microglia establishing direct contact with different compartments of neurons. Although communication between microglia and neurons involves intermediate cells and soluble factors, direct membrane contacts enable a more precisely regulated, dynamic, and highly effective form of interaction for fine-tuning neuronal responses and fate. Here, we summarize the known ultrastructural, molecular, and functional features of direct microglia-neuron interactions and their roles in brain disease.

Mechanosensation in traumatic brain injury

Traumatic brain injury (TBI) is distinct from other neurological disorders because it is induced by a discrete event that applies extreme mechanical forces to the brain. This review describes how the brain senses, integrates, and responds to forces under both normal conditions and during injury. The response to forces is influenced by the unique mechanical properties of brain tissue, which differ by region, cell type, and sub-cellular structure. Elements such as the extracellular matrix, plasma membrane, transmembrane receptors, and cytoskeleton influence its properties. These same components also act as force-sensors, allowing neurons and glia to respond to their physical environment and maintain homeostasis. However, when applied forces become too large, as in TBI, these components may respond in an aberrant manner or structurally fail, resulting in unique pathological sequelae. This so-called "pathological mechanosensation" represents a spectrum of cellular responses, which vary depending on the overall biomechanical parameters of the injury and may be compounded by repetitive injuries. Such aberrant physical responses and/or damage to cells along with the resulting secondary injury cascades can ultimately lead to long-term cellular dysfunction and degeneration, often resulting in persistent deficits. Indeed, pathological mechanosensation not only directly initiates secondary injury cascades, but this post-physical damage environment provides the context in which these cascades unfold. Collectively, these points underscore the need to use experimental models that accurately replicate the biomechanics of TBI in humans. Understanding cellular responses in context with injury biomechanics may uncover therapeutic targets addressing various facets of trauma-specific sequelae.

Applying a novel 3D hydrogel cell culture to investigate activation of microglia due to rotational kinematics associated with mild traumatic brain injury

Many investigations on mild traumatic brain injury (mTBI) aim to further understand how cells in the brain react to the mechanical forces associated with the injury. While it is known that rapid head rotation is a mechanism contributing to mTBI, establishing definitive thresholds for head rotation has proved challenging. One way to advance determining mechanisms and thresholds for injury is through in vitro models. Here, an apparatus has been designed that is capable of delivering rotational forces to three-dimensional (3D) hydrogel cell cultures. Using an in vitro model, we test the hypothesis that rotational kinematics can activate microglia suspended in a 3-dimensional mixed glia environment (absent neurons). The impact apparatus was able to deliver peak angular velocities of approximately 45 rad/s, a magnitude for angular velocity that in select literature is associated with diffuse brain injury. However, no measurable glial cell reactivity was observed in response to the rotational kinematics through any of the chosen metrics (nitric oxide, pro-inflammatory cytokine release and proportion of amoeboid activated microglia). The results generated from this study suggest that rotation of the glia alone did not cause activation - in future work we will investigate the effect of neuronal contributions in activating glia.

Morphological analyzes of microglia heterogeneity and dynamics during photoreceptor degeneration in vitro: Presumptive dark microglia in porcine retina

In the adult retina, ramifying microglia interact with the outer plexiform layer (OPL) monitoring the synaptic integrity between photoreceptors and post-synaptic target cells. Microglia are reactive during photoreceptor diseases, but their disease-related function(s) are not fully understood. Retinal explant cultures are model systems used to study degenerative events including photoreceptor degeneration and gliosis. Our culture paradigm, with adult porcine retinas subjected to coculture with human A-retinal pigment epithelia-19 (ARPE) cells, is an experimental approach resulting in improved photoreceptor survival and reduced gliosis. Under the in vitro pathological conditions with photoreceptor degeneration, reactive Iba1-and CD11b-immunoreactive microglia and their processes positioned in proximity with the OPL and among photoreceptor outer segments. Coculture for 3 days with ARPE-cells resulted in a significantly increased density of microglia at the OPL. After 5 days of culture, the density of microglia at the OPL was similar between coculture and control specimens. Electron microscopy revealed the presence of two subtypes of microglia: one exhibiting a dark nucleus and cytosol with dilated endoplasmic reticulum, vacuoles, endosomes and mitochondrial variations. This subtype localized close to synaptic structures in the OPL. The other subtype appeared as pale phagocytic microglia localized among degenerating outer segments. The Iba1-and CD11b-immunoreactive microglia in degenerating retina may be of two separate subtypes, which differ in localization, subcellular morphology and perhaps function.

Circulating GFAP and Iba-1 levels are associated with pathophysiological sequelae in the thalamus in a pig model of mild TBI

Serum biomarkers are promising tools for evaluating patients following traumatic brain injury (TBI). However, their relationship with diffuse histopathology remains unclear. Additionally, translatability is a focus of neurotrauma research, however, studies using translational animal models are limited. Here, we evaluated associations between circulating biomarkers and acute thalamic histopathology in a translational micro pig model of mTBI. Serum samples were collected pre-injury, and 1 min-6 h following mTBI. Markers of neuronal injury (Ubiquitin Carboxy-terminal Hydrolase L1 [UCH-L1]), microglial/macrophage activation (Ionized calcium binding adaptor molecule-1 [Iba-1]) and interleukin-6 [IL-6]) and astrogliosis/astrocyte damage (glial fibrillary acidic protein [GFAP]) were measured. Axonal injury and histological features of neurons and glia were also investigated using immunofluorescent labeling and correlated to serum levels of the associated biomarkers. Consistent with prior experimental and human studies, GFAP, was highest at 6 h post-injury, while no substantial changes were observed in UCH-L1, Iba-1 or IL-6 over 6 h. This study also found promising associations between thalamic glial histological signatures and ensuing release of Iba-1 and GFAP into the circulation. Our findings suggest that in diffuse injury, monitoring serum Iba-1 and GFAP levels can provide clinically relevant insight into the underlying acute pathophysiology and biomarker release kinetics following mTBI, providing previously underappreciated diagnostic capability.

Cerebral Immunohistochemical Characterization of the H2S and the Oxytocin Systems in a Porcine Model of Acute Subdural Hematoma

The hydrogen sulfide (H₂S) and the oxytocin/oxytocin receptor (OT/OTR) systems interact in trauma and are implicated in vascular protection and regulation of fluid homeostasis. Acute brain injury is associated with pressure-induced edema formation, blood brain barrier disruption, and neuro-inflammation. The similarities in brain anatomy: size, gyrencephalic organization, skull structure, may render the pig a highly relevant model for translational medicine. Cerebral biomarkers for pigs for pathophysiological changes and neuro-inflammation are limited. The current study aims to characterize the localization of OT/OTR and the endogenous H₂S producing enzymes together with relevant neuro-inflammatory markers on available porcine brain tissue from an acute subdural hematoma (ASDH) model. In a recent pilot study, anesthetized pigs underwent ASDH by injection of 20 mL of autologous blood above the left parietal cortex and were resuscitated with neuro-intensive care measures. After 54 h of intensive care, the animals were sacrificed, the brain was removed and analyzed via immunohistochemistry. The endogenous H₂S producing enzymes cystathionine-ɤ-lyase (CSE) and cystathionine-β-synthase (CBS), the OTR, and OT were localized in neurons, vasculature and parenchyma at the base of sulci, where pressure-induced injury leads to maximal stress in the gyrencephalic brain. The pathophysiological changes in response to brain injury in humans and pigs, we show here, are comparable. We additionally identified modulators of brain injury to further characterize the pathophysiology of ASDH and which may indicate future therapeutic approaches.

Microglial process convergence on axonal segments in health and disease

Microglia dynamically interact with neurons influencing the development, structure, and function of neuronal networks. Recent studies suggest microglia may also influence neuronal activity by physically interacting with axonal domains responsible for action potential initiation and propagation. However, the nature of these microglial process interactions is not well understood. Microglial-axonal contacts are present early in development and persist through adulthood, implicating microglial interactions in the regulation of axonal integrity in both the developing and mature central nervous system. Moreover, changes in microglial-axonal contact have been described in disease states such as multiple sclerosis (MS) and traumatic brain injury (TBI). Depending on the disease state, there are increased associations with specific axonal segments. In MS, there is enhanced contact with the axon initial segment and node of Ranvier, while, in TBI, microglia alter interactions with axons at the site of injury, as well as at the axon initial segment. In this article, we review the interactions of microglial processes with axonal segments, analyzing their associations with various axonal domains and how these interactions may differ between MS and TBI. Furthermore, we discuss potential functional consequences and molecular mechanisms of these interactions and how these may differ among various types of microglial-axonal interactions.

Mossy cell hypertrophy and synaptic changes in the hilus following mild diffuse traumatic brain injury in pigs

BACKGROUND

Each year in the USA, over 2.4 million people experience mild traumatic brain injury (TBI), which can induce long-term neurological deficits. The dentate gyrus of the hippocampus is notably susceptible to damage following TBI, as hilar mossy cell changes in particular may contribute to post-TBI dysfunction. Moreover, microglial activation after TBI may play a role in hippocampal circuit and/or synaptic remodeling; however, the potential effects of chronic microglial changes are currently unknown. The objective of the current study was to assess neuropathological and neuroinflammatory changes in subregions of the dentate gyrus at acute to chronic time points following mild TBI using an established model of closed-head rotational acceleration induced TBI in pigs.

METHODS

This study utilized archival tissue of pigs which were subjected to sham conditions or rapid head rotation in the coronal plane to generate mild TBI. A quantitative assessment of neuropathological changes in the hippocampus was performed via immunohistochemical labeling of whole coronal tissue sections at 3 days post-injury (DPI), 7 DPI, 30 DPI, and 1 year post-injury (YPI), with a focus on mossy cell atrophy and synaptic reorganization, in context with microglial alterations (e.g., density, proximity to mossy cells) in the dentate gyrus.

RESULTS

There were no changes in mossy cell density between sham and injured animals, indicating no frank loss of mossy cells at the mild injury level evaluated. However, we found significant mossy cell hypertrophy at 7 DPI and 30 DPI in anterior (> 16% increase in mean cell area at each time; p = <  0.001 each) and 30 DPI in posterior (8.3% increase; p = <  0.0001) hippocampus. We also found dramatic increases in synapsin staining around mossy cells at 7 DPI in both anterior (74.7% increase in synapsin labeling; p = <  0.0001) and posterior (82.7% increase; p = <  0.0001) hippocampus. Interestingly, these morphological and synaptic alterations correlated with a significant change in microglia in proximity to mossy cells at 7 DPI in anterior and at 30 DPI in the posterior hippocampus. For broader context, while we found that there were significant increases in microglia density in the granule cell layer at 30 DPI (anterior and posterior) and 1 YPI (posterior only) and in the molecular layer at 1 YPI (anterior only), we found no significant changes in overall microglial density in the hilus at any of the time points evaluated post-injury.

CONCLUSIONS

The alterations of mossy cell size and synaptic inputs paired with changes in microglia density around the cells demonstrate the susceptibility of hilar mossy cells after even mild TBI. This subtle hilar mossy cell pathology may play a role in aberrant hippocampal function post-TBI, although additional studies are needed to characterize potential physiological and cognitive alterations.

Acute Kahweol Treatment Attenuates Traumatic Brain Injury Neuroinflammation and Functional Deficits

Traumatic brain injury (TBI) affects millions worldwide with devastating long-term effects on health and cognition. Emerging data suggest that targeting the immune response may offer promising strategies to alleviate TBI outcomes; kahweol, an anti-inflammatory diterpene that remains in unfiltered coffee, has been shown to be beneficial in neuronal recovery. Here, we examined whether kahweol could alleviate brain trauma-induced injury in a mouse model of TBI and its underlying mechanisms. TBI was induced by controlled cortical impact (CCI) and various doses of kahweol were intraperitoneally administered following injury. Contusion volume, brain edema, neurobehavioral deficits, and protein expression and activity were evaluated in both short-term and long-term recovery. We found that kahweol treatments significantly reduced secondary brain injury and improved neurobehavioral outcomes in TBI mice. These changes were accompanied by the attenuation of proinflammatory cytokine secretion, decreased microglia/macrophage activation, and reduction of neutrophil and leukocyte infiltration. In addition, continuous kahweol treatment further improved short-term TBI outcomes compared to single-dosage. Collectively, our data showed that kahweol protects against TBI by reducing immune responses and may serve as a potential therapeutic intervention for TBI patients.

Depletion of Microglia Attenuates Dendritic Spine Loss and Neuronal Apoptosis in the Acute Stage of Moderate Traumatic Brain Injury in Mice

Microglia are the primary immune cells in the central nervous system and undergo significant morphological and transcriptional changes after traumatic brain injury (TBI). However, their exact contribution to the pathogenesis of TBI is still debated and remains to be elucidated. In the present study, thy-1 GFP mice received a colony-stimulating factor 1 receptor inhibitor (PLX3397) for 21 consecutive days, then were subjected to moderate fluid percussion injury (FPI). Brain samples were collected at 1 day and 3 days after FPI for flow cytometry analysis, immunofluorescence, dendrite spine quantification, terminal deoxynucleotidyl transferase dUTP nick end labeling assay, and Western blot. We found that PLX3397 treatment significantly attenuated the percentages of resident microglia and infiltrated immune cells. Depletion of microglia promoted neurite outgrowth, preserved dendritic spines and reduced total brain cell and neuronal apoptosis after FPI, which was accompanied by decreased the protein levels of endoplasmic reticulum stress marker proteins, C/EBP-homologous protein and inositol-requiring kinase 1α. Taken together, these findings suggest that microglial depletion may exert beneficial effects in the acute stage of FPI.

Toward development of clinically translatable diagnostic and prognostic metrics of traumatic brain injury using animal models: A review and a look forward

Traumatic brain injury is a leading cause of cognitive and behavioral deficits in children in the US each year. There is an increasing interest in both clinical and pre-clinical studies to discover biomarkers to accurately diagnose traumatic brain injury (TBI), predict its outcomes, and monitor its progression especially in the developing brain. In humans, the heterogeneity of TBI in terms of clinical presentation, injury causation, and mechanism has contributed to the many challenges associated with finding unifying diagnosis, treatment, and management practices. In addition, findings from adult human research may have little application to pediatric TBI, as age and maturation levels affect the injury biomechanics and neurophysiological consequences of injury. Animal models of TBI are vital to address the variability and heterogeneity of TBI seen in human by isolating the causation and mechanism of injury in reproducible manner. However, a gap between the pre-clinical findings and clinical applications remains in TBI research today. To take a step toward bridging this gap, we reviewed several potential TBI tools such as biofluid biomarkers, electroencephalography (EEG), actigraphy, eye responses, and balance that have been explored in both clinical and pre-clinical studies and have shown potential diagnostic, prognostic, or monitoring utility for TBI. Each of these tools measures specific deficits following TBI, is easily accessible, non/minimally invasive, and is potentially highly translatable between animals and human outcomes because they involve effort-independent and non-verbal tasks. Especially conspicuous is the fact that these biomarkers and techniques can be tailored for infants and toddlers. However, translation of preclinical outcomes to clinical applications of these tools necessitates addressing several challenges. Among the challenges are the heterogeneity of clinical TBI, age dependency of some of the biomarkers, different brain structure, life span, and possible variation between temporal profiles of biomarkers in human and animals. Conducting parallel clinical and pre-clinical research, in addition to the integration of findings across species from several pre-clinical models to generate a spectrum of TBI mechanisms and severities is a path toward overcoming some of these challenges. This effort is possible through large scale collaborative research and data sharing across multiple centers. In addition, TBI causes dynamic deficits in multiple domains, and thus, a panel of biomarkers combining these measures to consider different deficits is more promising than a single biomarker for TBI. In this review, each of these tools are presented along with the clinical and pre-clinical findings, advantages, challenges and prospects of translating the pre-clinical knowledge into the human clinical setting.

The complexity of neuroinflammation consequent to traumatic brain injury: from research evidence to potential treatments

This review recounts the definitions and research evidence supporting the multifaceted roles of neuroinflammation in the injured brain following trauma. We summarise the literature fluctuating from the protective and detrimental properties that cytokines, leukocytes and glial cells play in the acute and chronic stages of TBI, including the intrinsic factors that influence cytokine responses and microglial functions relative to genetics, sex, and age. We elaborate on the pros and cons that cytokines, chemokines, and microglia play in brain repair, specifically neurogenesis, and how such conflicting roles may be harnessed therapeutically to sustain the survival of new neurons. With a brief review of the clinical and experimental findings demonstrating early and chronic inflammation impacts on outcomes, we focus on the clinical conditions that may be amplified by neuroinflammation, ranging from acute seizures to chronic epilepsy, neuroendocrine dysfunction, dementia, depression, post-traumatic stress disorder and chronic traumatic encephalopathy. Finally, we provide an overview of the therapeutic agents that have been tested to reduce inflammation-driven secondary pathological cascades and speculate the future promise of alternative drugs.

Carnosic Acid Improves Outcome after Repetitive Mild Traumatic Brain Injury

In the majority of cases, the cognitive and behavioral impairments resulting from a mild traumatic brain injury (TBI) (also referred to as concussion) wane within days to weeks. In contrast, these impairments can persist for months to years after repetitive mild TBI (rmTBI). The cellular and molecular mechanisms underlying these impairments are not well understood. In the present study, we examined the consequences of rmTBI (three weight drops each separated by 72 h) on brain tissue respiration, pathology, and cognitive performance in mice. The transcription factor nuclear factor-erythroid 2-realted factor 2 (Nrf2) has been demonstrated to enhance the expression of numerous cytoprotective genes. Carnosic acid (CA) has been shown to activate Nrf2 and suppress the proinflammatory transcription factor nuclear factor kappa B (NF-κB). Because contemporaneous activation of cytoprotective genes and inhibition of proinflammatory genes can be beneficial, we questioned whether CA can be used to mitigate the pathobiology of rmTBI. The rmTBI increased hippocampal adenosine triphosphate-linked tissue respiration and proton leak that were unaffected by CA treatment. The rmTBI also caused significant motor and cognitive dysfunction, as tested using the foot fault, Barnes maze, and novel object recognition tasks. These impairments occurred in the absence of visible neuronal or dendritic loss. Post-rmTBI administration of CA significantly improved motor and cognitive function, and decreased Gfap and Iba1 immunoreactivities within white matter tracks. Taken together, these results show that rmTBI can cause cognitive impairments in the absence of overt neuronal pathologies, and post-injury treatment with CA can lessen some of these impairments.

Enduring Neuroprotective Effect of Subacute Neural Stem Cell Transplantation After Penetrating TBI

Traumatic brain injury (TBI) is the largest cause of death and disability of persons under 45 years old, worldwide. Independent of the distribution, outcomes such as disability are associated with huge societal costs. The heterogeneity of TBI and its complicated biological response have helped clarify the limitations of current pharmacological approaches to TBI management. Five decades of effort have made some strides in reducing TBI mortality but little progress has been made to mitigate TBI-induced disability. Lessons learned from the failure of numerous randomized clinical trials and the inability to scale up results from single center clinical trials with neuroprotective agents led to the formation of organizations such as the Neurological Emergencies Treatment Trials (NETT) Network, and international collaborative comparative effectiveness research (CER) to re-orient TBI clinical research. With initiatives such as TRACK-TBI, generating rich and comprehensive human datasets with demographic, clinical, genomic, proteomic, imaging, and detailed outcome data across multiple time points has become the focus of the field in the United States (US). In addition, government institutions such as the US Department of Defense are investing in groups such as Operation Brain Trauma Therapy (OBTT), a multicenter, pre-clinical drug-screening consortium to address the barriers in translation. The consensus from such efforts including “The Lancet Neurology Commission” and current literature is that unmitigated cell death processes, incomplete debris clearance, aberrant neurotoxic immune, and glia cell response induce progressive tissue loss and spatiotemporal magnification of primary TBI. Our analysis suggests that the focus of neuroprotection research needs to shift from protecting dying and injured neurons at acute time points to modulating the aberrant glial response in sub-acute and chronic time points. One unexpected agent with neuroprotective properties that shows promise is transplantation of neural stem cells. In this review we present (i) a short survey of TBI epidemiology and summary of current care, (ii) findings of past neuroprotective clinical trials and possible reasons for failure based upon insights from human and preclinical TBI pathophysiology studies, including our group's inflammation-centered approach, (iii) the unmet need of TBI and unproven treatments and lastly, (iv) present evidence to support the rationale for sub-acute neural stem cell therapy to mediate enduring neuroprotection.

Acute drivers of neuroinflammation in traumatic brain injury

Neuroinflammation is initiated as a result of traumatic brain injury and can exacerbate evolving tissue pathology. Immune cells respond to acute signals from damaged cells, initiate neuroinflammation, and drive the pathological consequences over time. Importantly, the mechanism(s) of injury, the location of the immune cells within the brain, and the animal species all contribute to immune cell behavior following traumatic brain injury. Understanding the signals that initiate neuroinflammation and the context in which they appear may be critical for understanding immune cell contributions to pathology and regeneration. Within this paper, we review a number of factors that could affect immune cell behavior acutely following traumatic brain injury.

The pig as a preclinical traumatic brain injury model: current models, functional outcome measures, and translational detection strategies

Traumatic brain injury (TBI) is a major contributor of long-term disability and a leading cause of death worldwide. A series of secondary injury cascades can contribute to cell death, tissue loss, and ultimately to the development of functional impairments. However, there are currently no effective therapeutic interventions that improve brain outcomes following TBI. As a result, a number of experimental TBI models have been developed to recapitulate TBI injury mechanisms and to test the efficacy of potential therapeutics. The pig model has recently come to the forefront as the pig brain is closer in size, structure, and composition to the human brain compared to traditional rodent models, making it an ideal large animal model to study TBI pathophysiology and functional outcomes. This review will focus on the shared characteristics between humans and pigs that make them ideal for modeling TBI and will review the three most common pig TBI models-the diffuse axonal injury, the controlled cortical impact, and the fluid percussion models. It will also review current advances in functional outcome assessment measures and other non-invasive, translational TBI detection and measurement tools like biomarker analysis and magnetic resonance imaging. The use of pigs as TBI models and the continued development and improvement of translational assessment modalities have made significant contributions to unraveling the complex cascade of TBI sequela and provide an important means to study potential clinically relevant therapeutic interventions.

Experimental Designs for Repeated Mild Traumatic Brain Injury: Challenges and Considerations

Mild traumatic brain injury (mild TBI) is a growing public concern, as evidence mounts that even brain injuries classified as "mild" can result in persistent neurological dysfunction. Multiple brain injuries heighten the likelihood of worsened or more prolonged symptomatology and may trigger long-term neurodegeneration. Animal models provide a logical platform to identify key parameters, such as loading forces, duration between injuries, and number of injuries, which contribute to additive or synergistic damage after repeated mild TBI. Despite the tremendous increase in research productivity in the field of repeated mild TBI, relatively few studies have been designed in such a way as to provide experimental-based insights into the dependence of cellular and functional outcomes on the prescribed parameters of mild TBI. In this review, we summarize how standard models of TBI have been adapted to produce mild TBI and highlight commonly observed aspects of neuropathology replicated in rodent models of mild TBI. The complexity of designing studies of repeated TBI is discussed, including challenges of incorporating appropriate control groups, informative experimental design, and relevant outcome measures. We then feature studies that provide a well-controlled, within-study design varying either the number of injuries or the interinjury interval. Harnessing the power of experimental models of TBI to elucidate which injury parameters are critical contributors to acute and chronic damage after repeated injury can further efforts at prevention and provide improved models for testing mechanisms and therapeutic interventions.

Traumatic brain injury-induced neuronal damage in the somatosensory cortex causes formation of rod-shaped microglia that promote astrogliosis and persistent neuroinflammation

Microglia undergo dynamic structural and transcriptional changes during the immune response to traumatic brain injury (TBI). For example, TBI causes microglia to form rod-shaped trains in the cerebral cortex, but their contribution to inflammation and pathophysiology is unclear. The purpose of this study was to determine the origin and alignment of rod microglia and to determine the role of microglia in propagating persistent cortical inflammation. Here, diffuse TBI in mice was modeled by midline fluid percussion injury (FPI). Bone marrow chimerism and BrdU pulse-chase experiments revealed that rod microglia derived from resident microglia with limited proliferation. Novel data also show that TBI-induced rod microglia were proximal to axotomized neurons, spatially overlapped with dense astrogliosis, and aligned with apical pyramidal dendrites. Furthermore, rod microglia formed adjacent to hypertrophied microglia, which clustered among layer V pyramidal neurons. To better understand the contribution of microglia to cortical inflammation and injury, microglia were eliminated prior to TBI by CSF1R antagonism (PLX5622). Microglial elimination did not affect cortical neuron axotomy induced by TBI, but attenuated rod microglial formation and astrogliosis. Analysis of 262 immune genes revealed that TBI caused profound cortical inflammation acutely (8 hr) that progressed in nature and complexity by 7 dpi. For instance, gene expression related to complement, phagocytosis, toll-like receptor signaling, and interferon response were increased 7 dpi. Critically, these acute and chronic inflammatory responses were prevented by microglial elimination. Taken together, TBI-induced neuronal injury causes microglia to structurally associate with neurons, augment astrogliosis, and propagate diverse and persistent inflammatory/immune signaling pathways.