Dendritic Spine Loss and Chronic White Matter Inflammation in a Mouse Model of Highly Repetitive Head Trauma

Water-Soluble Lynx1 Upregulates Dendritic Spine Density and Stimulates Astrocytic Network and Signaling

Secreted and membrane-tethered mammalian neuromodulators from the Ly6/uPAR family are involved in regulation of many physiological processes. Some of them are expressed in the CNS in the neurons of different brain regions and target neuronal membrane receptors. Thus, Lynx1 potentiates nicotinic acetylcholine receptors (nAChRs) in the brain, while others like Lypd6 and Lypd6b suppress it. However, the mechanisms underlying the regulation of cognitive processes by these neuromodulators remain unclear. Here, we showed that water-soluble analogue of Lynx1 (ws-Lynx-1) targets α7-nAChRs both in the hippocampal neurons and astrocytes. Incubation of astrocytes with ws-Lynx1 increased expression of connexins 30 and 43; α4, α5, and β4 integrins; and E- and P-cadherins. Ws-Lynx1 reduced secretion of pro-inflammatory adhesion factors ICAM-1, PSGL-1, and VCAM-1 and downregulated secretion of CD44 and NCAM, which inhibit synaptic plasticity. Moreover, increased astrocytic secretion of the dendritic growth activator ALCAM and neurogenesis regulator E-selectin was observed. Incubation of neurons with ws-Lynx1 potentiated α7-nAChRs and upregulated dendritic spine density. Thus, the pro-cognitive activity of ws-Lynx1 observed previously can be explained by stimulation of astrocytic network and signaling together with up-regulation of spinogenesis, potentiation of the α7-nAChRs, and neuronal and synaptic plasticity. For comparison, influence of water-soluble analogues of a set of Ly6/uPAR proteins (SLURP-1, SLURP-2, Lypd6, Lypd6b, and PSCA) on dendritic spine density and diameter was studied. Data obtained give new insights on the role of Ly6/uPAR proteins in the brain and open new prospects for the development of drugs to improve cognitive function.

Extracranial Effects of Traumatic Brain Injury: A Narrative Review

BACKGROUND

Traumatic brain injury (TBI) is often associated with other injuries and comorbidities. However, even isolated TBI directly leads to dysfunction in multiple body systems outside the central nervous system. These extracranial effects of TBI target systems including the autonomic nervous, cardiovascular, renal, pulmonary, immune, gastrointestinal, and hemostasis systems, as well as causing significant alteration to systemic metabolism.

AIM

This review is intended to outline the effects of TBI on other body systems, and place these in context with treatment considerations for these patients.

SIGNIFICANCE

Systemic effects of TBI have implications for acute and critical care management of patients with TBI, including pharmacologic treatment. They also affect treatment decisions in chronic TBI care, as well as TBI-unrelated routine medical care for patients with chronic TBI. In addition, extracranial effects of TBI should be considered in research settings.

CONCLUSIONS

It is important for clinicians and researchers to be aware of these extracranial effects, and consider their effects on pathology, treatment decisions, and interpretation of research findings.

HISTOLOGICAL COMPARISON OF REPEATED MILD WEIGHT DROP AND LATERAL FLUID PERCUSSION INJURY MODELS OF TRAUMATIC BRAIN INJURY IN FEMALE AND MALE RATS

ABSTRACT

In preclinical traumatic brain injury (TBI) research, the animal model should be selected based on the research question and outcome measures of interest. Direct side-by-side comparisons of different injury models are essential for informing such decisions. Here, we used immunohistochemistry to compare the outcomes from two common models of TBI, lateral fluid percussion (LFP) and repeated mild weight drop (rmWD) in adult female and male Wistar rats. Specifically, we measured the effects of LFP and rmWD on markers of cerebrovascular and tight junction disruption, neuroinflammation, mature neurons, and perineuronal nets in the cortical site of injury, cortex adjacent to injury, dentate gyrus, and the CA 2/3 area of the hippocampus. Animals were randomized into the LFP or rmWD group. On day 1, the LFP group received a craniotomy, and on day 4, injury (or sham procedure; randomly assigned). The rmWD animals underwent either injury or isoflurane only (randomly assigned) on each of those 4 days. Seven days after injury, brains were harvested for analysis. Overall, our observations revealed that the most significant disruptions were evident in response to LFP, followed by craniotomy only, whereas rmWD animals showed the least residual changes compared with isoflurane-only controls, supporting consideration of rmWD as a mild injury. LFP led to longer-lasting disruptions, perhaps more representative of moderate TBI. We also report that craniotomy and LFP produced greater disruptions in females relative to males. These findings will assist the field in the selection of animal models based on target severity of postinjury outcomes and support the inclusion of both sexes and appropriate control groups.

Temporal expression of brainstem neurotrophic proteins following mild traumatic brain injury

BDNF, a neurotrophic factor, and its receptors have been implicated in the pathophysiology of mild traumatic brain injury (mTBI). The brainstem houses many vital functions, that are also associated with signs and symptoms of mTBI, but has been understudied in mTBI animal models. We determined the extent to which neurotrophic protein and associated receptor expression is affected within the brainstem of adult rats following mTBI. Their behavioral function was assessed and temporal expression of the 'negative' regulators of neuronal function (p75, t-TrkB, and pro-BDNF) and 'positive' neuroprotective (FL-TrkB and m-BDNF) protein isoforms were determined via western blot and immunohistochemistry at 1, 3, 7, and 14 post-injury days (PID) following mTBI or sham (control) procedure. Within the brainstem, p75 expression increased at PID 1 vs. sham animals. t-TrkB and pro-BDNF expression increased at PID 7 and 14. The 'positive' protein isoforms of FL-TrkB and m-BDNF expression were increased only at PID 7. The ratio of t-TrkB:FL-TrkB (negative:positive) was substantial across groups and time points, suggesting a negative impact of neurotrophic signaling on neuronal function. Additional NeuN experiments revealed cell death occurring within a subset of neurons within the medulla. While behavioral measures improved by PID 7-14, negative neurotrophic biochemical responses persisted. Despite the assertion that mTBI produces "mild" injury, evidence of cell death was observed in the medulla. Ratios of TrkB and BDNF isoforms with conflicting functions suggest that future work should specifically measure each subtype since they induce opposing downstream effects on neuronal function.

Profiling the neuroimmune cascade in 3xTg-AD mice exposed to successive mild traumatic brain injuries

Repetitive mild traumatic brain injuries (rmTBI) sustained within a window of vulnerability can result in long term cognitive deficits, depression, and eventual neurodegeneration associated with tau pathology, amyloid beta (Aβ) plaques, gliosis, and neuronal and functional loss. However, a comprehensive study relating acute changes in immune signaling and glial reactivity to neuronal changes and pathological markers after single and repetitive mTBIs is currently lacking. In the current study, we addressed the question of how repeated injuries affect the brain neuroimmune response in the acute phase of injury (< 24 h) by exposing the 3xTg-AD mouse model of tau and Aβ pathology to successive (1x-5x) once-daily weight drop closed-head injuries and quantifying immune markers, pathological markers, and transcriptional profiles at 30 min, 4 h, and 24 h after each injury. We used young adult 2-4 month old 3xTg-AD mice to model the effects of rmTBI in the absence of significant tau and Aβ pathology. We identified pronounced sexual dimorphism in this model, with females eliciting more diverse changes after injury compared to males. Specifically, females showed: (1) a single injury caused a decrease in neuron-enriched genes inversely correlated with inflammatory protein expression and an increase in AD-related genes within 24 h, (2) each injury significantly increased a group of cortical cytokines (IL-1α, IL-1β, IL-2, IL-9, IL-13, IL-17, KC) and MAPK phospho-proteins (phospho-Atf2, phospho-Mek1), several of which co-labeled with neurons and correlated with phospho-tau, and (3) repetitive injury caused increased expression of genes associated with astrocyte reactivity and macrophage-associated immune function. Collectively our data suggest that neurons respond to a single injury within 24 h, while other cell types, including astrocytes, transition to inflammatory phenotypes within days of repetitive injury.

Purinergic Astrocyte Signaling Driven by TNF-α After Cannabidiol Administration Restores Normal Synaptic Remodeling Following Traumatic Brain Injury

Traumatic brain injury (TBI) is a prevalent form of cranial trauma that results in neural conduction disruptions and damage to synaptic structures and functions. Cannabidiol (CBD), a primary derivative from plant-based cannabinoids, exhibits a range of beneficial effects, including analgesic, sedative, anti-inflammatory, anticonvulsant, anti-anxiety, anti-apoptotic, and neuroprotective properties. Nevertheless, the effects of synaptic reconstruction and the mechanisms underlying these effects remain poorly understood. TBI is characterized by increased levels of tumor necrosis factor-alpha (TNF-α), a cytokine integral for the modulation of glutamate release by astrocytes. In the present study, the potential of CBD in regulating aberrant glutamate signal transmission in astrocytes following brain injury, as well as the underlying mechanisms involved, were investigated using immunofluorescence double staining, enzyme-linked immunosorbent assay (ELISA), western blot analysis, hematoxylin and eosin (H&E) staining, Nissl staining, transmission electron microscopy, and RT-qPCR. In this study, we examined the impact of CBD on neuronal synapses, focusing on the TNF-α-driven purinergic signaling pathway. Specifically, our research revealed that CBD pretreatment effectively reduced the secretion of TNF-α induced by astrocyte activation following TBI. This reduction inhibited the interaction between TNF-α and P2Y1 receptors, leading to a decrease in the release of neurotransmitters, including Ca2+ and glutamate, thereby initiating synaptic remodeling. Our study showed that CBD exhibits significant therapeutic potential for TBI-related synaptic dysfunction, offering valuable insights for future research and more effective TBI treatments. Further exploration of the potential applications of CBD in neuroprotection is required to develop innovative clinical strategies.

Decoupling the mutual promotion of inflammation and oxidative stress mitigates cognitive decline and depression-like behavior in rmTBI mice by promoting myelin renewal and neuronal survival

BACKGROUND

Repetitive mild traumatic brain injury (rmTBI) can lead to somatic, emotional, and cognitive symptoms that persist for years after the initial injury. Although the ability of various treatments to promote recovery after rmTBI has been explored, the optimal time window for early intervention after rmTBI is unclear. Previous research has shown that hydrogen-rich water (HRW) can diffuse through the blood-brain - barrier, attenuate local oxidative stress, and reduce neuronal apoptosis in patients with severe traumatic brain injury. However, research on the effect of HRW on rmTBI is scarce.

AIMS

The objectives of this study were to explore the following changes after rmTBI and HRW treatment: (i) temporal changes in inflammasome activation and oxidative stress-related protein expression through immunoblotting, (ii) temporal changes in neuron/myelin-related metabolite concentrations in vivo through magnetic resonance spectroscopy, (iii) myelin structural changes in late-stage rmTBI via immunofluorescence, and (iv) postinjury anxiety/depression-like behaviors and spatial learning and memory impairment.

RESULTS

NLRP-3 expression in the rmTBI group was elevated at 7 and 14 DPI, and inflammasome marker levels returned to normal at 30 DPI. Oxidative stress persisted throughout the first month postinjury. HRW replacement significantly decreased Nrf2 expression in the prefrontal cortex and hippocampal CA2 region at 14 and 30 DPI, respectively. Edema and local gliosis in the hippocampus and restricted diffusion in the thalamus were observed on MR-ADC images. The tCho/tCr ratio in the rmTBI group was elevated, and the tNAA/tCr ratio was decreased at 30 DPI. Compared with the mice in the other groups, the mice in the rmTBI group spent more time exploring the open arms in the elevated plus maze (P < 0.05) and were more active in the maze (longer total distance traveled). In the sucrose preference test, the rmTBI group exhibited anhedonia. In the Morris water maze test, the latency to find the hidden platform in the rmTBI group was longer than that in the sham and HRW groups (P < 0.05).

CONCLUSION

Early intervention with HRW can attenuate inflammasome assembly and reduce oxidative stress after rmTBI. These changes may restore local oligodendrocyte function, promote myelin repair, prevent axonal damage and neuronal apoptosis, and alleviate depression-like behavior and cognitive impairment.

Histological comparison of repeated mild weight drop and lateral fluid percussion injury models of traumatic brain injury (TBI) in female and male rats

Traumatic brain injury (TBI) heterogeneity has led to the development of several preclinical models, each modeling a distinct subset of outcomes. Selection of an injury model should be guided by the research question and the specific outcome measures of interest. Consequently, there is a need for conducting direct comparisons of different TBI models. Here, we used immunohistochemistry to directly compare the outcomes from two common models, lateral fluid percussion (LFP) and repeat mild weight drop (rmWD), on neuropathology in adult female and male Wistar rats. Specifically, we used immunohistochemistry to measure the effects of LFP and rmWD on cerebrovascular and tight junction disruption, inflammatory markers, mature neurons and perineuronal nets in the cortical site of injury, cortex adjacent to injury, dentate gyrus, and the CA2/3 area of the hippocampus. Animals were randomized into either LFP or rmWD groups. The LFP group received a craniotomy prior to LFP (or corresponding sham procedure) three days later, while rmWD animals underwent either weight drop or sham (isoflurane only) on each of those four days. After a recovery period of 7 days, animals were euthanized, and brains were harvested for analysis of RECA-1, claudin-5, GFAP, Iba-1, CD-68, NeuN, and wisteria floribunda lectin. Overall, our observations revealed that the most significant disruptions were evident in response to LFP, followed by craniotomy-only, while rmWD animals showed the least residual changes compared to isoflurane-only controls. These findings support consideration of rmWD as a mild, transient injury. LFP leads to longer-lasting disruptions that are more closely associated with a moderate TBI. We further show that both craniotomy and LFP produced greater disruptions in females relative to males at 7 days post-injury. These findings support the inclusion of a time-matched experimentally-naïve or anesthesia-only control group in preclinical TBI research to enhance the validity of data interpretation and conclusions.

Associations of prior concussion severity with brain microstructure using mean apparent propagator magnetic resonance imaging

Magnetic resonance imaging (MRI) diffusion studies have shown chronic microstructural tissue abnormalities in athletes with history of concussion, but with inconsistent findings. Concussions with post-traumatic amnesia (PTA) and/or loss of consciousness (LOC) have been connected to greater physiological injury. The novel mean apparent propagator (MAP) MRI is expected to be more sensitive to such tissue injury than the conventional diffusion tensor imaging. This study examined effects of prior concussion severity on microstructure with MAP-MRI. Collegiate-aged athletes (N = 111, 38 females; ≥6 months since most recent concussion, if present) completed semistructured interviews to determine the presence of prior concussion and associated injury characteristics, including PTA and LOC. MAP-MRI metrics (mean non-Gaussian diffusion [NG Mean], return-to-origin probability [RTOP], and mean square displacement [MSD]) were calculated from multi-shell diffusion data, then evaluated for associations with concussion severity through group comparisons in a primary model (athletes with/without prior concussion) and two secondary models (athletes with/without prior concussion with PTA and/or LOC, and athletes with/without prior concussion with LOC only). Bayesian multilevel modeling estimated models in regions of interest (ROI) in white matter and subcortical gray matter, separately. In gray matter, the primary model showed decreased NG Mean and RTOP in the bilateral pallidum and decreased NG Mean in the left putamen with prior concussion. In white matter, lower NG Mean with prior concussion was present in all ROI across all models and was further decreased with LOC. However, only prior concussion with LOC was associated with decreased RTOP and increased MSD across ROI. Exploratory analyses conducted separately in male and female athletes indicate associations in the primary model may differ by sex. Results suggest microstructural measures in gray matter are associated with a general history of concussion, while a severity-dependent association of prior concussion may exist in white matter.

Evolving brain and behaviour changes in rats following repetitive subconcussive head impacts

There is growing concern that repetitive subconcussive head impacts, independent of concussion, alter brain structure and function, and may disproportionately affect the developing brain. Animal studies of repetitive subconcussive head impacts are needed to begin to characterize the pathological basis and mechanisms underlying imaging and functional effects of repetitive subconcussive head impacts seen in humans. Since repetitive subconcussive head impacts have been largely unexplored in animals, we aimed to characterize the evolution of imaging, behavioural and pathological effects of repetitive subconcussive head impacts in awake adolescent rodents. Awake male and female Sprague Dawley rats (postnatal Day 35) received 140 closed-head impacts over the course of a week. Impacted and sham-impacted animals were restrained in a plastic cone, and unrestrained control animals were included to account for effects of restraint and normal development. Animals (n = 43) underwent repeated diffusion tensor imaging prior to and over 1 month following the final impact. A separate cohort (n = 53) was assessed behaviourally for fine motor control, emotional-affective behaviour and memory at acute and chronic time points. Histological and immunohistochemical analyses, which were exploratory in nature due to smaller sample sizes, were completed at 1 month following the final impact. All animals tolerated the protocol with no overt changes in behaviour or stigmata of traumatic brain injury, such as alteration of consciousness, intracranial haemorrhage or skull fracture. We detected longitudinal, sex-dependent diffusion tensor imaging changes (fractional anisotropy and axial diffusivity decline) in corpus callosum and external capsule of repetitive subconcussive head impact animals, which diverged from both sham and control. Compared to sham animals, repetitive subconcussive head impact animals exhibited acute but transient mild motor deficits. Repetitive subconcussive head impact animals also exhibited chronic anxiety and spatial memory impairment that differed from the control animals, but these effects were not different from those seen in the sham condition. We observed trends in the data for thinning of the corpus callosum as well as regions with elevated Iba-1 in the corpus callosum and cerebral white matter among repetitive subconcussive head impact animals. While replication with larger study samples is needed, our findings suggest that subconcussive head impacts cause microstructural tissue changes in the developing rat brain, which are detectable with diffusion tensor imaging, with suggestion of correlates in tissue pathology and behaviour. The results point to potential mechanisms underpinning consequences of subconcussive head impacts that have been described in humans. The congruence of our imaging findings with human subconcussive head impacts suggests that neuroimaging could serve as a translational bridge to advance study of injury mechanisms and development of interventions.

Repeated Mild Traumatic Brain Injury and JZL184 Produce Sex-Specific Increases in Anxiety-Like Behavior and Alcohol Consumption in Wistar Rats

Alcohol use disorder (AUD) is highly comorbid with traumatic brain injury (TBI). Previously, using a lateral fluid percussion model (LFP) (an open-head injury model) to generate a single mild to moderate traumatic brain injury (TBI) we showed that TBI produces escalation in alcohol drinking, that alcohol exposure negatively impacts TBI outcomes, and that the endocannabinoid degradation inhibitor (JZL184) confers significant protection from behavioral and neuropathological outcomes in male rodents. In the present study, we used a weight drop model (a closed-head injury model) to produce repeated mild TBI (rmTBI; three TBIs separated by 24 hours) in male and female rats to examine the sex-specific effects on anxiety-like behavior and alcohol consumption, and whether systemic treatment with JZL184 would reverse TBI effects on those behaviors. In two separate studies, adult male and female Wistar rats were subjected to rmTBI or sham procedure using the weight drop model. Physiological measures of injury severity were collected from all animals. Animals in both studies were allowed to consume alcohol using an intermittent 2-bottle choice procedure (12 pre-TBI sessions and 12 post-TBI sessions). Neurological severity and neurobehavioral scores (NSS and NBS, respectively) were tested 24 hours after the final injury. Anxiety-like behavior was tested at 37-38 days post-injury in Study 1-, and 6-8-days post-injury in Study 2. Our results show that females exhibited reduced respiratory rates relative to males with no significant differences between Sham and rmTBI, no effect of rmTBI or sex on righting reflex, and increased neurological deficits in rmTBI groups in both studies. In Study 1, rmTBI increased alcohol consumption in female but not male rats. Male rats consistently exhibited higher levels of anxiety-like behavior than females. The rmTBI did not affect anxiety-like behavior 37-38 days post-injury. In Study 2, rmTBI once again increased alcohol consumption in female but not male rats, and repeated systemic treatment with JZL184 did not affect alcohol consumption. Also in Study 2, rmTBI increased anxiety-like behavior in males but not females and repeated systemic treatment with JZL184 produced an unexpected increase in anxiety-like behavior 6-8 days post-injury. In summary, rmTBI increased alcohol consumption in female rats, systemic JZL184 treatment did not alter alcohol consumption, and both rmTBI and systemic JZL184 treatment increased anxiety-like behavior 6-8 days post-injury in males but not females, highlighting robust sex differences in rmTBI effects.

Single Neuron Modeling Identifies Potassium Channel Modulation as Potential Target for Repetitive Head Impacts

Traumatic brain injury (TBI) and repetitive head impacts can result in a wide range of neurological symptoms. Despite being the most common neurological disorder in the world, repeat head impacts and TBI do not have any FDA-approved treatments. Single neuron modeling allows researchers to extrapolate cellular changes in individual neurons based on experimental data. We recently characterized a model of high frequency head impact (HFHI) with a phenotype of cognitive deficits associated with decreases in neuronal excitability of CA1 neurons and synaptic changes. While the synaptic changes have been interrogated in vivo, the cause and potential therapeutic targets of hypoexcitability following repetitive head impacts are unknown. Here, we generated in silico models of CA1 pyramidal neurons from current clamp data of control mice and mice that sustained HFHI. We use a directed evolution algorithm with a crowding penalty to generate a large and unbiased population of plausible models for each group that approximated the experimental features. The HFHI neuron model population showed decreased voltage gated sodium conductance and a general increase in potassium channel conductance. We used partial least squares regression analysis to identify combinations of channels that may account for CA1 hypoexcitability after HFHI. The hypoexcitability phenotype in models was linked to A- and M-type potassium channels in combination, but not by any single channel correlations. We provide an open access set of CA1 pyramidal neuron models for both control and HFHI conditions that can be used to predict the effects of pharmacological interventions in TBI models.

Profiling the neuroimmune cascade in 3xTg mice exposed to successive mild traumatic brain injuries

Repetitive mild traumatic brain injuries (rmTBI) sustained within a window of vulnerability can result in long term cognitive deficits, depression, and eventual neurodegeneration associated with tau pathology, amyloid beta (Aβ) plaques, gliosis, and neuronal and functional loss. However, we have limited understanding of how successive injuries acutely affect the brain to result in these devastating long-term consequences. In the current study, we addressed the question of how repeated injuries affect the brain in the acute phase of injury (<24hr) by exposing the 3xTg-AD mouse model of tau and Aβ pathology to successive (1x, 3x, 5x) once-daily weight drop closed-head injuries and quantifying immune markers, pathological markers, and transcriptional profiles at 30min, 4hr, and 24hr after each injury. We used young adult mice (2-4 months old) to model the effects of rmTBI relevant to young adult athletes, and in the absence of significant tau and Aβ pathology. Importantly, we identified pronounced sexual dimorphism, with females eliciting more differentially expressed proteins after injury compared to males. Specifically, females showed: 1) a single injury caused a decrease in neuron-enriched genes inversely correlated with inflammatory protein expression as well as an increase in AD-related genes within 24hr, 2) each injury significantly increased expression of a group of cortical cytokines (IL-1α, IL-1β, IL-2, IL-9, IL-13, IL-17, KC) and MAPK phospho-proteins (phospho-Atf2, phospho-Mek1), several of which were co-labeled with neurons and correlated with phospho-tau, and 3) repetitive injury caused increased expression of genes associated with astrocyte reactivity and immune function. Collectively our data suggest that neurons respond to a single injury within 24h, while other cell types including astrocytes transition to inflammatory phenotypes within days of repetitive injury.

Human amnionic progenitor cell secretome mitigates the consequence of traumatic optic neuropathy in a mouse model

Traumatic optic neuropathy (TON) is a condition in which acute injury to the optic nerve from direct or indirect trauma results in vision loss. The most common cause of TON is indirect injury to the optic nerve caused by concussive forces that are transmitted to the optic nerve. TON occurs in up to 5% of closed-head trauma patients and there is currently no known effective treatment. One potential treatment option for TON is ST266, a cell-free biological solution containing the secretome of amnion-derived multipotent progenitor (AMP) cells. We investigated the efficacy of intranasal ST266 in a mouse model of TON induced by blunt head trauma. Injured mice treated with a 10-day regimen of ST266 showed an improvement in spatial memory and learning, a significant preservation of retinal ganglion cells, and a decrease in neuropathological markers in the optic nerve, optic tract, and dorsal lateral geniculate nucleus. ST266 treatment effectively downregulated the NLRP3 inflammasome-mediated neuroinflammation pathway after blunt trauma. Overall, treatment with ST266 was shown to improve functional and pathological outcomes in a mouse model of TON, warranting future exploration of ST266 as a cell-free therapeutic candidate for testing in all optic neuropathies.

JZL184 increases anxiety-like behavior and does not reduce alcohol consumption in female rats after repeated mild traumatic brain injury

Alcohol use disorder (AUD) is highly comorbid with traumatic brain injury (TBI). Previously, using a lateral fluid percussion model (LFP) (an open model of head injury) to generate a single mild to moderate traumatic brain injury (TBI), we showed that TBI produces escalation in alcohol drinking, that alcohol exposure negatively impacts TBI outcomes, and that the endocannabinoid degradation inhibitor (JZL184) confers significant protection from behavioral and neuropathological outcomes in male rodents. In the present study, we used a weight drop model (a closed model of head injury) to produce a repeated mild TBI (rmTBI, 3 TBIs, spaced by 24 hours) to examine the sex-specific effects on alcohol consumption and anxiety-like behavior in rats, and whether systemic treatment with JZL184 would reverse TBI effects on those behaviors in both sexes. In two separate studies, adult male and female Wistar rats were subjected to rmTBI or sham using the weight drop model. Physiological measures of injury severity were collected from all animals. Animals in both studies were allowed to consume alcohol using an intermittent 2-bottle choice procedure (12 pre-TBI sessions and 12 post-TBI sessions). Neurological severity and neurobehavioral scores (NSS and NBS, respectively) were tested 24 hours after the final injury. Anxiety-like behavior was tested at 37-38 days post-injury in Study 1, and 6-8 days post-injury in Study 2. Our results show that females exhibited reduced respiratory rates relative to males with no significant differences between Sham and rmTBI, no effect of rmTBI or sex on righting reflex, and increased neurological deficits in rmTBI groups in both studies. In Study 1, rmTBI increased alcohol consumption in female but not male rats. Male rats consistently exhibited higher levels of anxiety-like behavior than females. rmTBI did not affect anxiety-like behavior 37-38 days post-injury. In Study 2, rmTBI once again increased alcohol consumption in female but not male rats, and repeated systemic treatment with JZL184 did not affect alcohol consumption. Also in Study 2, rmTBI increased anxiety-like behavior in males but not females and repeated systemic treatment with JZL184 produced an unexpected increase in anxiety-like behavior 6-8 days post-injury. In summary, rmTBI increased alcohol consumption in female rats, systemic JZL184 treatment did not alter alcohol consumption, and both rmTBI and sub-chronic systemic JZL184 treatment increased anxiety-like behavior 6-8 days post-injury in males but not females, highlighting robust sex differences in rmTBI effects.

Dimethyl itaconate ameliorates the deficits of goal-directed behavior in Toxoplasma gondii infected mice

BACKGROUND

The neurotrophic parasite Toxoplasma gondii (T. gondii) has been implicated as a risk factor for neurodegenerative diseases. However, there is only limited information concerning its underlying mechanism and therapeutic strategy. Here, we investigated the effects of T. gondii chronic infection on the goal-directed cognitive behavior in mice. Moreover, we evaluated the preventive and therapeutic effect of dimethyl itaconate on the behavior deficits induced by the parasite.

METHODS

The infection model was established by orally infecting the cysts of T. gondii. Dimethyl itaconate was intraperitoneally administered before or after the infection. Y-maze and temporal order memory (TOM) tests were used to evaluate the prefrontal cortex-dependent behavior performance. Golgi staining, transmission electron microscopy, indirect immunofluorescence, western blot, and RNA sequencing were utilized to determine the pathological changes in the prefrontal cortex of mice.

RESULTS

We showed that T. gondii infection impaired the prefrontal cortex-dependent goal-directed behavior. The infection significantly downregulated the expression of the genes associated with synaptic transmission, plasticity, and cognitive behavior in the prefrontal cortex of mice. On the contrary, the infection robustly upregulated the expression of activation makers of microglia and astrocytes. In addition, the metabolic phenotype of the prefrontal cortex post infection was characterized by the enhancement of glycolysis and fatty acid oxidation, the blockage of the Krebs cycle, and the disorder of aconitate decarboxylase 1 (ACOD1)-itaconate axis. Notably, the administration of dimethyl itaconate significantly prevented and treated the cognitive impairment induced by T. gondii, which was evidenced by the improvement of behavioral deficits, synaptic ultrastructure lesion and neuroinflammation.

CONCLUSION

The present study demonstrates that T. gondii infection induces the deficits of the goal-directed behavior, which is associated with neuroinflammation, the impairment of synaptic ultrastructure, and the metabolic shifts in the prefrontal cortex of mice. Moreover, we report that dimethyl itaconate has the potential to prevent and treat the behavior deficits.

Ketogenic Diet Modulates Neuroinflammation via Metabolites from Lactobacillus reuteri After Repetitive Mild Traumatic Brain Injury in Adolescent Mice

Repetitive mild traumatic brain injury (rmTBI) is associated with a range of neural changes which is characterized by axonal injury and neuroinflammation. Ketogenic diet (KD) is regarded as a potential therapy for facilitating recovery after moderate-severe traumatic brain injury (TBI). However, its effect on rmTBI has not been fully studied. In this study, we evaluated the anti-neuroinflammation effects of KD after rmTBI in adolescent mice and explored the potential mechanisms. Experimentally, specific pathogen-free (SPF) adolescent male C57BL/6 mice received a sham surgery or repetitive mild controlled cortical impacts consecutively for 7 days. The uninjured mice received the standard diet, and the mice with rmTBI were fed either the standard diet or KD for 7 days. One week later, all mice were subjected to behavioral tests and experimental analysis. Results suggest that KD significantly increased blood beta-hydroxybutyrate (β-HB) levels and improved neurological function. KD also reduced white matter damage, microgliosis, and astrogliosis induced by rmTBI. Aryl hydrocarbon receptor (AHR) signaling pathway, which was mediated by indole-3-acetic acid (3-IAA) from Lactobacillus reuteri (L. reuteri) in gut and activated in microglia and astrocytes after rmTBI, was inhibited by KD. The expression level of the toll-like receptor 4 (TLR4)/myeloid differentiation primary response 88 (MyD88) in inflammatory cells, which mediates the NF-κB pathway, was also attenuated by KD. Taken together, our results indicated that KD can promote recovery following rmTBI in adolescent mice. KD may modulate neuroinflammation by altering L. reuteri in gut and its metabolites. The inhibition of indole/AHR pathway and the downregulation of TLR4/MyD88 may play a role in the beneficial effect of KD against neuroinflammation in rmTBI mice.

The meningeal transcriptional response to traumatic brain injury and aging

Emerging evidence suggests that the meningeal compartment plays instrumental roles in various neurological disorders, however, we still lack fundamental knowledge about meningeal biology. Here, we utilized high-throughput RNA sequencing (RNA-seq) techniques to investigate the transcriptional response of the meninges to traumatic brain injury (TBI) and aging in the sub-acute and chronic time frames. Using single-cell RNA sequencing (scRNA-seq), we first explored how mild TBI affects the cellular and transcriptional landscape in the meninges in young mice at one-week post-injury. Then, using bulk RNA-seq, we assessed the differential long-term outcomes between young and aged mice following TBI. In our scRNA-seq studies, we highlight injury-related changes in differential gene expression seen in major meningeal cell populations including macrophages, fibroblasts, and adaptive immune cells. We found that TBI leads to an upregulation of type I interferon (IFN) signature genes in macrophages and a controlled upregulation of inflammatory-related genes in the fibroblast and adaptive immune cell populations. For reasons that remain poorly understood, even mild injuries in the elderly can lead to cognitive decline and devastating neuropathology. To better understand the differential outcomes between the young and the elderly following brain injury, we performed bulk RNA-seq on young and aged meninges 1.5 months after TBI. Notably, we found that aging alone induced upregulation of meningeal genes involved in antibody production by B cells and type I IFN signaling. Following injury, the meningeal transcriptome had largely returned to its pre-injury signature in young mice. In stark contrast, aged TBI mice still exhibited upregulation of immune-related genes and downregulation of genes involved in extracellular matrix remodeling. Overall, these findings illustrate the dynamic transcriptional response of the meninges to mild head trauma in youth and aging.

Altered amyloid precursor protein, tau-regulatory proteins, neuronal numbers and behaviour, but no tau pathology, synaptic and inflammatory changes or memory deficits, at 1 month following repetitive mild traumatic brain injury

Repetitive mild traumatic brain injury, commonly experienced following sports injuries, results in various secondary injury processes and is increasingly recognised as a risk factor for the development of neurodegenerative conditions such as chronic traumatic encephalopathy, which is characterised by tau pathology. We aimed to characterise the underlying pathological mechanisms that might contribute to the onset of neurodegeneration and behavioural changes in the less-explored subacute (1-month) period following single or repetitive controlled cortical impact injury (five impacts, 48 h apart) in 12-week-old male and female C57Bl6 mice. We conducted motor and cognitive testing, extensively characterised the status of tau and its regulatory proteins via western blot and quantified neuronal populations using stereology. We report that r-mTBI resulted in neurobehavioural deficits, gait impairments and anxiety-like behaviour at 1 month post-injury, effects not seen following a single injury. R-mTBI caused a significant increase in amyloid precursor protein, an increased trend towards tau phosphorylation and significant changes in kinase/phosphatase proteins that may promote a downstream increase in tau phosphorylation, but no changes in synaptic or neuroinflammatory markers. Lastly, we report neuronal loss in various brain regions following both single and repeat injuries. We demonstrate herein that repeated impacts are required to promote the initiation of a cascade of biochemical events that are consistent with the onset of neurodegeneration subacutely post-injury. Identifying the timeframe in which these changes occur and the pathological mechanisms involved will be crucial for the development of future therapeutics to prevent the onset or mitigate the progression of neurodegeneration following r-mTBI.

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.

Aberrant cortical spine dynamics after concussive injury are reversed by integrated stress response inhibition

Traumatic brain injury (TBI) is a leading cause of long-term neurological disability in the world and the strongest environmental risk factor for the development of dementia. Even mild TBI (resulting from concussive injuries) is associated with a greater than twofold increase in the risk of dementia onset. Little is known about the cellular mechanisms responsible for the progression of long-lasting cognitive deficits. The integrated stress response (ISR), a phylogenetically conserved pathway involved in the cellular response to stress, is activated after TBI, and inhibition of the ISR-even weeks after injury-can reverse behavioral and cognitive deficits. However, the cellular mechanisms by which ISR inhibition restores cognition are unknown. Here, we used longitudinal two-photon imaging in vivo after concussive injury in mice to study dendritic spine dynamics in the parietal cortex, a brain region involved in working memory. Concussive injury profoundly altered spine dynamics measured up to a month after injury. Strikingly, brief pharmacological treatment with the drug-like small-molecule ISR inhibitor ISRIB entirely reversed structural changes measured in the parietal cortex and the associated working memory deficits. Thus, both neural and cognitive consequences of concussive injury are mediated in part by activation of the ISR and can be corrected by its inhibition. These findings suggest that targeting ISR activation could serve as a promising approach to the clinical treatment of chronic cognitive deficits after TBI.

Tau pathology, metal dyshomeostasis and repetitive mild traumatic brain injury: an unexplored link paving the way for neurodegeneration

Repetitive mild traumatic brain injury (r-mTBI), commonly experienced by athletes and military personnel, causes changes in multiple intracellular pathways, one of which involves the tau protein. Tau phosphorylation plays a role in several neurodegenerative conditions including chronic traumatic encephalopathy (CTE), a progressive neurodegenerative disorder linked to repeated head trauma. There is now mounting evidence suggesting that tau phosphorylation may be regulated by metal ions (such as iron, zinc and copper), which themselves are implicated in ageing and neurodegenerative disorders such as Alzheimer's disease (AD). Recent work has also shown that a single TBI can result in age-dependent and region-specific modulation of metal ions. As such, this review explores the link between TBI, CTE, ageing and neurodegeneration with a specific focus on the involvement of (and interaction between) tau pathology and metal dyshomeostasis. The authors highlight that metal dyshomeostasis has yet to be investigated in the context of repeat head trauma or CTE. Given the evidence that metal dyshomeostasis contributes to the onset and/or progression of neurodegeneration, and that CTE itself is a neurodegenerative condition, this brings to light an uncharted link that should be explored. The development of adequate models of r-mTBI and/or CTE will be crucial in deepening our understanding of the pathological mechanisms that drive the clinical manifestations in these conditions and also in the development of effective therapeutics targeted towards slowing progressive neurodegenerative disorders.

Ayurvedic formulations amalaki rasayana and rasa sindoor improve age-associated memory deficits in mice by modulating dendritic spine densities

BACKGROUND

Emerging reports indicate that age-associated cognitive decline begins with the transition from young to middle-aged, and this neurological condition manifests mainly due to the progressive impairment in the adaptive homeostasis process. Moreover, cognitive decline is associated with neurodegenerative changes in older adults.

OBJECTIVE

Previous studies have shown that the administration of Ayurvedic formulations restores the homeostatic pathways and ameliorates neurodegeneration in animal models of neurodegenerative diseases. Therefore, we wanted to check whether Ayurvedic formulations can rescue or delay the age-associated cognitive decline in middle-aged mice.

MATERIAL AND METHODS

We fed two-month-old mice with amalaki aasayana (AR, 1025 mg/kg per day) or rasa sindoor (RS, 41 mg/kg per day) mixed in a gelatin-based jelly for six months. Mice eating regular chow or blank jelly served as control. Subsequently, we looked at the improvements in the cognitive and behavioural traits of the treated animals. We have also analysed the effect of these formulations on the dendritic processes of neurons, glial activation, and the formation of corpora amylacea.

RESULTS

We found a significant improvement in episodic, working- and reference-spatiotemporal memory in animals fed on AR or RS. Microscopic analyses revealed a significant increase in the dendritic spine density in the apical dendrites of the hippocampal pyramidal neurons. The treatment, however, did not significantly affect gliosis and corpora amylacea in the brains.

CONCLUSIONS

Both AR and RS showed beneficial effects on memory functions of the middle-aged mice, possibly due to their effect on the dendritic spine densities. Our findings provide strong evidence to conclude that formulations AR and RS can prevent or delay the onset of age-associated cognitive decline.

Temporal patterns of microglial activation in white matter following experimental mild traumatic brain injury: a systematic literature review

Mild traumatic brain injuries (mTBIs) are a prevalent form of injury that can result in persistent neurological impairments. Microglial activation has become increasingly recognized as a key process regulating the pathology of white matter in a wide range of brain injury and disease contexts. As white matter damage is known to be a major contributor to the impairments that follow mTBI, microglia have rightfully become a common target of investigation for the development of mTBI therapies and biomarkers. Recent work has demonstrated that the efficacy of microglial manipulation as a therapeutic intervention following injury or disease is highly time-sensitive, emphasizing the importance of advancing our understanding of the dynamics of post-mTBI microglial activation from onset to resolution. Current reporting of microglial activation in experimental studies of mTBI is non-standardized, which has limited our ability to identify concrete patterns of post-mTBI microglial activation over time. In this review, we examine preclinical studies of mTBI that report on microglial activation in white matter regions to summarize our current understanding of these patterns. Specifically, we summarize timecourses of post-mTBI microglial activation in white matter regions of the brain, identify factors that influence this activation, examine the temporal relationship between microglial activation and other post-mTBI assessments, and compare the relative sensitivities of various methods for detecting microglial activation. While the lack of replicated experimental conditions has limited the extent of conclusions that can confidently be drawn, we find that microglia are activated over a wide range of timecourses following mTBI and that microglial activation is a long-lasting outcome of mTBI that may resolve after most typical post-mTBI assessments, with the exception of those measuring oligodendrocyte lineage cell integrity. We identify several understudied parameters of post-mTBI microglial activation in white matter, such as the inclusion of female subjects. This review summarizes our current understanding of the progression of microglial activation in white matter structures following experimental mTBI and offers suggestions for important future research directions.

Axonal injury following mild traumatic brain injury is exacerbated by repetitive insult and is linked to the delayed attenuation of NeuN expression without concomitant neuronal death in the mouse

Mild traumatic brain injury (mTBI) affects brain structure and function and can lead to persistent abnormalities. Repetitive mTBI exacerbates the acute phase response to injury. Nonetheless, its long‐term implications remain poorly understood, particularly in the context of traumatic axonal injury (TAI), a player in TBI morbidity via axonal disconnection, synaptic loss and retrograde neuronal perturbation. In contrast to the examination of these processes in the acute phase of injury, the chronic‐phase burden of TAI and/or its implications for retrograde neuronal perturbation or death have received little consideration. To critically assess this issue, murine neocortical tissue was investigated at acute (24‐h postinjury, 24hpi) and chronic time points (28‐days postinjury, 28dpi) after singular or repetitive mTBI induced by central fluid percussion injury (cFPI). Neurons were immunofluorescently labeled for NeuroTrace and NeuN (all neurons), p‐c‐Jun (axotomized neurons) and DRAQ5 (cell nuclei), imaged in 3D and quantified in automated manner. Single mTBI produced axotomy in 10% of neurons at 24hpi and the percentage increased after repetitive injury. The fraction of p‐c‐Jun+ neurons decreased at 28dpi but without neuronal loss (NeuroTrace), suggesting their reorganization and/or repair following TAI. In contrast, NeuN+ neurons decreased with repetitive injury at 24hpi while the corresponding fraction of NeuroTrace+ neurons decreased over 28dpi. Attenuated NeuN expression was linked exclusively to non‐axotomized neurons at 24hpi which extended to the axotomized at 28dpi, revealing a delayed response of the axotomized neurons. Collectively, we demonstrate an increased burden of TAI after repetitive mTBI, which is most striking in the acute phase response to the injury. Our finding of widespread axotomy in large fields of intact neurons contradicts the notion that repetitive mTBI elicits progressive neuronal death, rather, emphasizing the importance of axotomy‐mediated change.

Delayed dosing of minocycline plus N-acetylcysteine reduces neurodegeneration in distal brain regions and restores spatial memory after experimental traumatic brain injury

Multiple drugs to treat traumatic brain injury (TBI) have failed clinical trials. Most drugs lose efficacy as the time interval increases between injury and treatment onset. Insufficient therapeutic time window is a major reason underlying failure in clinical trials. Few drugs have been developed with therapeutic time windows sufficiently long enough to treat TBI because little is known about which brain functions can be targeted if therapy is delayed hours to days after injury. We identified multiple injury parameters that are improved by first initiating treatment with the drug combination minocycline (MINO) plus N-acetylcysteine (NAC) at 72 h after injury (MN72) in a mouse closed head injury (CHI) experimental TBI model. CHI produces spatial memory deficits resulting in impaired performance on Barnes maze, hippocampal neuronal loss, and bilateral damage to hippocampal neurons, dendrites, spines and synapses. MN72 treatment restores Barnes maze acquisition and retention, protects against hippocampal neuronal loss, limits damage to dendrites, spines and synapses, and accelerates recovery of microtubule associated protein 2 (MAP2) expression, a key protein in maintaining proper dendritic architecture and synapse density. These data show that in addition to the structural integrity of the dendritic arbor, spine and synapse density can be successfully targeted with drugs first dosed days after injury. Retention of substantial drug efficacy even when first dosed 72 h after injury makes MINO plus NAC a promising candidate to treat clinical TBI.

Repetitive Traumatic Brain Injury Is Associated With TDP-43 Alterations, Neurodegeneration, and Glial Activation in Mice

Increasing evidence points to a relationship between repetitive mild traumatic brain injury (mTBI), the Tar DNA binding protein 43 (TDP-43) pathology and some neurodegenerative diseases, but the underlying pathophysiological mechanisms are still unknown. We examined TDP-43 regulation, neurodegeneration, and glial responses following repetitive mTBI in nontransgenic mice and in animals with overexpression of human mutant TDP-43 protein (TDP-43G348C). In the frontal cortices of the injured nontransgenic animals, early TDP-43 cytoplasmatic translocation and overexpression of the protein and its pathological forms were detected. In the injured animals of both genotypes, neurodegeneration and pronounced glial activity were detected in the optic tract. In TDP-43G348C mice, these changes were significantly higher at day 7 after the last mTBI compared with the values in the nontransgenic animals. Results of this study suggest that the changes in the TDP-43 regulation in the frontal cortices of the nontransgenic animals were a transient stress response to the brain injury. Repetitive mTBI did not produce additional TDP-43 dysregulation or neurodegeneration or pronounced gliosis in the frontal cortex of TDP-43G348C mice. Our research also suggests that overexpression of mutated human TDP-43 possibly predisposes the brain to more intense neurodegeneration and glial activation in the optic tract after repetitive mTBI.

Long-Term Effects of Repetitive Mild Traumatic Injury on the Visual System in Wild-Type and TDP-43 Transgenic Mice

Little is known about the impairments and pathological changes in the visual system in mild brain trauma, especially repetitive mild traumatic brain injury (mTBI). The goal of this study was to examine and compare the effects of repeated head impacts on the neurodegeneration, axonal integrity, and glial activity in the optic tract (OT), as well as on neuronal preservation, glial responses, and synaptic organization in the lateral geniculate nucleus (LGN) and superior colliculus (SC), in wild-type mice and transgenic animals with overexpression of human TDP-43 mutant protein (TDP-43^G348C) at 6 months after repeated closed head traumas. Animals were also assessed in the Barnes maze (BM) task. Neurodegeneration, axonal injury, and gliosis were detected in the OT of the injured animals of both genotypes. In the traumatized mice, myelination of surviving axons was mostly preserved, and the expression of neurofilament light chain was unaffected. Repetitive mTBI did not induce changes in the LGN and the SC, nor did it affect the performance of the BM task in the traumatized wild-type and TDP-43 transgenic mice. Differences in neuropathological and behavioral assessments between the injured wild-type and TDP-43^G348C mice were not revealed. Results of the current study suggest that repetitive mTBI was associated with chronic damage and inflammation in the OT in wild-type and TDP-43^G348C mice, which were not accompanied with behavioral problems and were not affected by the TDP-43 genotype, while the LGN and the SC remained preserved in the used experimental conditions.

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.

Inflammation in Traumatic Brain Injury

There is an increasing evidence that inflammation contributes to clinical and functional outcomes in traumatic brain injury (TBI). Many successful target-engaging, lesion-reducing, symptom-alleviating, and function-improving interventions in animal models of TBI have failed to show efficacy in clinical trials. Timing and immunological context are paramount for the direction, quality, and intensity of immune responses to TBI and the resulting neuroanatomical, clinical, and functional course. We present components of the immune system implicated in TBI, potential immune targets, and target-engaging interventions. The main objective of our article is to point toward modifiable molecular and cellular mechanisms that may modify the outcomes in TBI, and contribute to increasing the translational value of interventions that have been identified in animal models of TBI.

High-frequency head impact causes chronic synaptic adaptation and long-term cognitive impairment in mice

Repeated head impact exposure can cause memory and behavioral impairments. Here, we report that exposure to non-damaging, but high frequency, head impacts can alter brain function in mice through synaptic adaptation. High frequency head impact mice develop chronic cognitive impairments in the absence of traditional brain trauma pathology, and transcriptomic profiling of mouse and human chronic traumatic encephalopathy brain reveal that synapses are strongly affected by head impact. Electrophysiological analysis shows that high frequency head impacts cause chronic modification of the AMPA/NMDA ratio in neurons that underlie the changes to cognition. To demonstrate that synaptic adaptation is caused by head impact-induced glutamate release, we pretreated mice with memantine prior to head impact. Memantine prevents the development of the key transcriptomic and electrophysiological signatures of high frequency head impact, and averts cognitive dysfunction. These data reveal synapses as a target of high frequency head impact in human and mouse brain, and that this physiological adaptation in response to head impact is sufficient to induce chronic cognitive impairment in mice.

Neuroimmune cleanup crews in brain injury

Traumatic brain injury (TBI) is a leading cause of death and disability. Mounting evidence indicates that the immune system is critically involved in TBI pathogenesis, where it is deployed to dispose of neurotoxic material generated from head trauma and to instruct the wound healing process. However, the immune response to brain damage must be carefully held in check as aberrant regulation of immune signaling can lead to deleterious neuroinflammation, brain pathology, and neurological dysfunction. Efficient clearance of neurotoxic material by microglia (the brain's resident phagocytes) and the glymphatic-meningeal lymphatic drainage system are paramount to keeping the immune system in balance following head trauma. In this review, we highlight emerging evidence that defines pivotal roles for microglia and the recently discovered glymphatic-meningeal lymphatic system in TBI pathogenesis.

The Delayed Neuropathological Consequences of Traumatic Brain Injury in a Community-Based Sample

The late neuropathological effects of traumatic brain injury have yet to be fully elucidated, particularly with respect to community-based cohorts. To contribute to this critical gap in knowledge, we designed a multimodal neuropathological study, integrating traditional and quantitative approaches to detect pathologic changes in 532 consecutive brain autopsies from participants in the Adult Changes in Thought (ACT) study. Diagnostic evaluation including assessment for chronic traumatic encephalopathy (CTE) and quantitative immunoassay-based methods were deployed to examine levels of pathological (hyperphosphorylated) tau (pTau) and amyloid (A) β in brains from ACT participants with (n = 107) and without (n = 425) history of remote TBI with loss of consciousness (w/LOC). Further neuropathological assessments included immunohistochemistry for α-synuclein and phospho-TDP-43 pathology and astro- (GFAP) and micro- (Iba1) gliosis, mass spectrometry analysis of free radical injury, and gene expression evaluation (RNA sequencing) in a smaller sub-cohort of matched samples (49 cases with TBI and 49 non-exposed matched controls). Out of 532 cases, only 3 (0.6%-none with TBI w/LOC history) showed evidence of the neuropathologic signature of chronic traumatic encephalopathy (CTE). Across the entire cohort, the levels of pTau and Aβ showed expected differences for brain region (higher levels in temporal cortex), neuropathological diagnosis (higher in participants with Alzheimer's disease), and APOE genotype (higher in participants with one or more APOE ε4 allele). However, no differences in PHF-tau or Aβ1-42 were identified by Histelide with respect to the history of TBI w/LOC. In a subset of TBI cases with more carefully matched control samples and more extensive analysis, those with TBI w/LOC history had higher levels of hippocampal pTau but no significant differences in Aβ, α-synuclein, pTDP-43, GFAP, Iba1, or free radical injury. RNA-sequencing also did not reveal significant gene expression associated with any measure of TBI exposure. Combined, these findings suggest long term neuropathological changes associated with TBI w/LOC may be subtle, involve non-traditional pathways of neurotoxicity and neurodegeneration, and/or differ from those in autopsy cohorts specifically selected for neurotrauma exposure.

Early Life Stress Exacerbates Outcome after Traumatic Brain Injury

The neurocognitive impairments associated with mild traumatic brain injury (TBI) often resolve within 1-2 weeks; however, a subset of people exhibit persistent cognitive dysfunction for weeks to months after injury. The factors that contribute to these persistent deficits are unknown. One potential risk factor for worsened outcome after TBI is a history of stress experienced by a person early in life. Early life stress (ELS) includes maltreatment such as neglect, and interferes with the normal construction of cortical and hippocampal circuits. We hypothesized that a history of ELS contributes to persistent learning and memory dysfunction following a TBI. To explore this interaction, we modeled ELS by separating Sprague Dawley pups from their nursing mothers from post-natal days 2-14 for 3 h daily. At 2 months of age, male rats received sham surgery or mild to moderate parasagittal fluid-percussion brain injury. We found that the combination of ELS with TBI in adulthood impaired hippocampal-dependent learning, as assessed with contextual fear conditioning, the water maze task, and spatial working memory. Cortical atrophy was significantly exacerbated in TBI animals exposed to ELS compared with normal-reared TBI animals. Changes in corticosterone in response to restraint stress were prolonged in TBI animals that received ELS compared with TBI animals that were normally reared or sham animals that received ELS. Our findings indicate that ELS is a risk factor for worsened outcome after TBI, and results in persistent learning and memory deficits, worsened cortical pathology, and an exacerbation of the hormonal stress response.

The Immune System's Role in the Consequences of Mild Traumatic Brain Injury (Concussion)

Mild traumatic brain injury (mild TBI), often referred to as concussion, is the most common form of TBI and affects millions of people each year. A history of mild TBI increases the risk of developing emotional and neurocognitive disorders later in life that can impact on day to day living. These include anxiety and depression, as well as neurodegenerative conditions such as chronic traumatic encephalopathy (CTE) and Alzheimer's disease (AD). Actions of brain resident or peripherally recruited immune cells are proposed to be key regulators across these diseases and mood disorders. Here, we will assess the impact of mild TBI on brain and patient health, and evaluate the recent evidence for immune cell involvement in its pathogenesis.

Repetitive Traumatic Brain Injury Causes Neuroinflammation before Tau Pathology in Adolescent P301S Mice

Repetitive closed head injury (rCHI) is commonly encountered in young athletes engaged in contact and collision sports. Traumatic brain injury (TBI) including rCHI has been reported to be an important risk factor for several tauopathies in studies of adult humans and animals. However, the link between rCHI and the progression of tau pathology in adolescents remains to be elucidated. We evaluated whether rCHI can trigger the initial acceleration of pathological tau in adolescent mice and impact the long-term outcomes post-injury. To this end, we subjected adolescent transgenic mice expressing the P301S tau mutation to mild rCHI and assessed tau hyperphosphorylation, tangle formation, markers of neuroinflammation, and behavioral deficits at 40 days post rCHI. We report that rCHI did not accelerate tau pathology and did not worsen behavioral outcomes compared to control mice. However, rCHI induced cortical and hippocampal microgliosis and corpus callosum astrocytosis in P301S mice by 40 days post-injury. In contrast, we did not find significant microgliosis or astrocytosis after rCHI in age-matched WT mice or sham-injured P301S mice. Our data suggest that neuroinflammation precedes the development of Tau pathology in this rCHI model of adolescent repetitive mild TBI.

An update on the association between traumatic brain injury and Alzheimer's disease: Focus on Tau pathology and synaptic dysfunction

L.P. Li, J.W. Liang and H.J. Fu. An update on the association between traumatic brain injury and Alzheimer's disease: Focus on Tau pathology and synaptic dysfunction. NEUROSCI BIOBEHAV REVXXX-XXX,2020.-Traumatic brain injury (TBI) and Alzheimer's disease (AD) are devastating conditions that have long-term consequences on individual's cognitive functions. Although TBI has been considered a risk factor for the development of AD, the link between TBI and AD is still in debate. Aggregation of hyperphosphorylated tau and intercorrelated synaptic dysfunction, two key pathological elements in both TBI and AD, play a pivotal role in mediating neurodegeneration and cognitive deficits, providing a mechanistic link between these two diseases. In the first part of this review, we analyze the experimental literatures on tau pathology in various TBI models and review the distribution, biological features and mechanisms of tau pathology following TBI with implications in AD pathogenesis. In the second part, we review evidences of TBI-mediated structural and functional impairments in synapses, with a focus on the overlapped mechanisms underlying synaptic abnormalities in both TBI and AD. Finally, future perspectives are proposed for uncovering the complex relationship between TBI and neurodegeneration, and developing potential therapeutic avenues for alleviating cognitive deficits after TBI.

Microglia dynamics in adolescent traumatic brain injury

Repetitive, mild traumatic brain injuries (RmTBIs) are increasingly common in adolescents and encompass one of the largest neurological health concerns in the world. Adolescence is a critical period for brain development where RmTBIs can substantially impact neurodevelopmental trajectories and life-long neurological health. Our current understanding of RmTBI pathophysiology suggests key roles for neuroinflammation in negatively regulating neural health and function. Microglia, the brain’s resident immune population, play important roles in brain development by regulating neuronal number, and synapse formation and elimination. In response to injury, microglia activate to inflammatory phenotypes that may detract from these normal homeostatic, physiological, and developmental roles. To date, however, little is known regarding the impact of RmTBIs on microglia function during adolescent brain development. This review details key concepts surrounding RmTBI pathophysiology, adolescent brain development, and microglia dynamics in the developing brain and in response to injury, in an effort to formulate a hypothesis on how the intersection of these processes may modify long-term trajectories.

Meningeal lymphatic dysfunction exacerbates traumatic brain injury pathogenesis

Traumatic brain injury (TBI) is a leading global cause of death and disability. Here we demonstrate in an experimental mouse model of TBI that mild forms of brain trauma cause severe deficits in meningeal lymphatic drainage that begin within hours and last out to at least one month post-injury. To investigate a mechanism underlying impaired lymphatic function in TBI, we examined how increased intracranial pressure (ICP) influences the meningeal lymphatics. We demonstrate that increased ICP can contribute to meningeal lymphatic dysfunction. Moreover, we show that pre-existing lymphatic dysfunction before TBI leads to increased neuroinflammation and negative cognitive outcomes. Finally, we report that rejuvenation of meningeal lymphatic drainage function in aged mice can ameliorate TBI-induced gliosis. These findings provide insights into both the causes and consequences of meningeal lymphatic dysfunction in TBI and suggest that therapeutics targeting the meningeal lymphatic system may offer strategies to treat TBI.

Assessment of Long-Term Effects of Sports-Related Concussions: Biological Mechanisms and Exosomal Biomarkers

Concussion or mild traumatic brain injury (mTBI) in athletes can cause persistent symptoms, known as post-concussion syndrome (PCS), and repeated injuries may increase the long-term risk for an athlete to develop neurodegenerative diseases such as chronic traumatic encephalopathy (CTE), and Alzheimer's disease (AD). The Center for Disease Control estimates that up to 3.8 million sport-related mTBI are reported each year in the United States. Despite the magnitude of the phenomenon, there is a current lack of comprehensive prognostic indicators and research has shown that available monitoring tools are moderately sensitive to short-term concussion effects but less sensitive to long-term consequences. The overall aim of this review is to discuss novel, quantitative, and objective measurements that can predict long-term outcomes following repeated sports-related mTBIs. The specific objectives were (1) to provide an overview of the current clinical and biomechanical tools available to health practitioners to ensure recovery after mTBIs, (2) to synthesize potential biological mechanisms in animal models underlying the long-term adverse consequences of mTBIs, (3) to discuss the possible link between repeated mTBI and neurodegenerative diseases, and (4) to discuss the current knowledge about fluid biomarkers for mTBIs with a focus on novel exosomal biomarkers. The conclusions from this review are that current post-concussion clinical tests are not sufficiently sensitive to injury and do not accurately quantify post-concussion alterations associated with repeated mTBIs. In the current review, it is proposed that current practices should be amended to include a repeated symptom inventory, a cognitive assessment of executive function and impulse control, an instrumented assessment of balance, vestibulo-ocular assessments, and an improved panel of blood or exosome biomarkers.

The overexpression of RBM3 alleviates TBI-induced behaviour impairment and AD-like tauopathy in mice

The therapeutic hypothermia is an effective tool for TBI‐associated brain impairment, but its side effects limit in clinical routine use. Hypothermia up‐regulates RNA‐binding motif protein 3 (RBM3), which is verified to protect synaptic plasticity. Here, we found that cognitive and LTP deficits, loss of spines, AD‐like tau pathologies are displayed one month after TBI in mice. In contrast, the deficits of LTP and cognitive, loss of spines and tau abnormal phosphorylation at several sites are obviously reversed in TBI mice combined with hypothermia pre‐treatment (HT). But, the neuroprotective role of HT disappears in TBI mouse models under condition of blocking RBM3 expression with RBM3 shRNA. In other hand, overexpressing RBM3 by AAV‐RBM3 plasmid can mimic HT‐like neuroprotection against TBI‐induced chronic brain injuries, such as improving LTP and cognitive, loss of spines and tau hyperphosphorylation in TBI mouse models. Taken together, hypothermia pre‐treatment reverses TBI‐induced chronic AD‐like pathology and behaviour deficits in RBM3 expression dependent manner, RBM3 may be a potential target for neurodegeneration diseases including Alzheimer disease.

Alcohol consumption during adolescence alters the hippocampal response to traumatic brain injury

Binge drinking is the consumption of large volumes of alcohol in short periods and exerts its effects on the central nervous system, including the hippocampus. We have previously shown that binge drinking alters mitochondrial dynamics and induces neuroinflammation in the hippocampus of adolescent rats. Mild traumatic brain injury (mTBI), is regularly linked to alcohol consumption and share mechanisms of brain damage. In this context, we hypothesized that adolescent binge drinking could prime the development of brain damage generated by mTBI. We found that alcohol binge drinking induced by the "drinking in the dark" (DID) paradigm increases oxidative damage and astrocyte activation in the hippocampus of adolescent mice. Interestingly, adolescent animals submitted to DID showed decreased levels of mitofusin 2 that controls mitochondrial dynamics. When mTBI was evaluated as a second challenge, hippocampi from animals previously submitted to DID showed a reduction in dendritic spine number and a different spine profile. Mitochondrial performance could be compromised by alterations in mitochondrial fission in DID-mTBI animals. These data suggest that adolescent alcohol consumption can modify the progression of mTBI pathophysiology. We propose that mitochondrial impairment and oxidative damage could act as priming factors, modifying predisposition against mTBI effects.

The effects of mild closed head injuries on tauopathy and cognitive deficits in rodents: Primary results in wild type and rTg4510 mice, and a systematic review

In humans, the majority of sustained traumatic brain injuries (TBIs) are classified as 'mild' and most often a result of a closed head injury (CHI). The effects of a non-penetrating CHI are not benign and may lead to chronic pathology and behavioral dysfunction, which could be worsened by repeated head injury. Clinical-neuropathological correlation studies provide evidence that conversion of tau into abnormally phosphorylated proteotoxic intermediates (p-tau) could be part of the pathophysiology triggered by a single TBI and enhanced by repeated TBIs. However, the link between p-tau and CHI in rodents remains controversial. To address this question experimentally, we induced a single CHI or two CHIs to WT or rTg4510 mice. We found that 2× CHI increased tau phosphorylation in WT mice and rTg4510 mice. Behavioral characterization in WT mice found chronic deficits in the radial arm water maze in 2× CHI mice that had partially resolved in the 1× CHI mice. Moreover, using Manganese-Enhanced Magnetic Resonance Imaging with R1 mapping - a novel functional neuroimaging technique - we found greater deficits in the rTg4510 mice following 2× CHI compared to 1× CHI. To integrate our findings with prior work in the field, we conducted a systematic review of rodent mild repetitive CHI studies. Following Prisma guidelines, we identified 25 original peer-reviewed papers. Results from our experiments, as well as our systematic review, provide compelling evidence that tau phosphorylation is modified by experimental mild TBI studies; however, changes in p-tau levels are not universally reported. Together, our results provide evidence that repetitive TBIs can result in worse and more persistent neurological deficits compared to a single TBI, but the direct link between the worsened outcome and elevated p-tau could not be established.

Progressive long-term spatial memory loss following repeat concussive and subconcussive brain injury in mice, associated with dorsal hippocampal neuron loss, microglial phenotype shift, and vascular abnormalities

There is considerable concern about the long‐term deleterious effects of repeat head trauma on cognition, but little is known about underlying mechanisms and pathology. To examine this, we delivered four air blasts to the left side of the mouse cranium, a week apart, with an intensity that causes deficits when delivered singly and considered “concussive,” or an intensity that does not yield significant deficits when delivered singly and considered “subconcussive.” Neither repeat concussive nor subconcussive blast produced spatial memory deficits at 4 months, but both yielded deficits at 14 months, and dorsal hippocampal neuron loss. Hierarchical cluster analysis of dorsal hippocampal microglia across the three groups based on morphology and expression of MHCII, CX3CR1, CD68 and IBA1 revealed five distinct phenotypes. Types 1A and 1B microglia were more common in sham mice, linked to better neuron survival and memory, and appeared mildly activated. By contrast, 2B and 2C microglia were more common in repeat concussive and subconcussive mice, linked to poorer neuron survival and memory, and characterized by low expression levels and attenuated processes, suggesting they were de‐activated and dysfunctional. In addition, endothelial cells in repeat concussive mice exhibited reduced CD31 and eNOS expression, which was correlated with the prevalence of type 2B and 2C microglia. Our findings suggest that both repeat concussive and subconcussive head injury engender progressive pathogenic processes, possibly through sustained effects on microglia that over time lead to increased prevalence of dysfunctional microglia, adversely affecting neurons and blood vessels, and thereby driving neurodegeneration and memory decline.

Retinal nerve fibre changes in sports-related repetitive traumatic brain injury

IMPORTANCE

There is limited literature on the use of optical coherence tomography in the assessment of retinal nerve fibre layer (RNFL) thickness in sports-related repetitive mild traumatic brain injury.

BACKGROUND

To evaluate RNFL thickness in professional rugby league players. RNFL thinning may serve as a proxy for wider white matter degeneration.

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Thirteen retired Australian professional rugby league players were recruited.

METHODS

Participants underwent binocular optical coherence tomography to measure RNFL thickness. Each participant underwent a complete ophthalmic assessment to exclude concurrent disease.

MAIN OUTCOME MEASURES

RNFL thickness of each eye were compared with a normative database.

RESULTS

Participants had played professional Rugby League for 18 years on average and reported sustaining 15 sports-related concussions throughout their career. The RNFL in participants was four micrometres thinner than that of matched normative data. Cohort average RNFL thickness was reduced in 12 out of 14 optical coherence testing parameters. These findings were statistically significant in the left inferonasal [P = .013] and left nasal [P = .006] sectors. There was no statistically significant relationship between RNFL thickness and other visual measures.

CONCLUSIONS AND RELEVANCE

This study is the first to demonstrate RNFL thinning in a cohort of retired Australian professional Rugby League players. RNFL changes have been shown to correlate with cerebral white matter loss and neurodegeneration. Optical coherence tomography may serve as a safe and economical means of screening for repetitive traumatic brain injury related neurodegeneration in contact sport athletes.

CRMP2 improves memory deficits by enhancing the maturation of neuronal dendritic spines after traumatic brain injury

Our recent study investigated the role of collapsin response mediator protein-2 (CRMP2) on dendritic spine morphology and memory function after traumatic brain injury (TBI). First, we examined the density and morphology of dendritic spines in Thy1-GFP mice on the 1 st day (P1D) and 7th day (P7D) after controlled cortical impact injury (CCI). The dendritic spine density in the hippocampus was decreased on P1D, in which mainly mushroom-type and thin-type spines were lost. The density of dendritic spines was increased on P7D, most of which were of the thin type. Next, we explored the expression of CRMP2 on P1D and P7D. CRMP2 expression was decreased on P1D, but the levels of the CRMP2 breakdown product were increased. On P7D, the expression pattern was the opposite. Then, we constructed CRMP2 overexpression and knockdown plasmids and transfected them into cultured neurons in vitro. CRMP2 increased the dendritic spine density of cultured neurons and the proportion of mushroom-type spines, while CRMP2-shRNA reduced the dendritic spine density and the proportion of mushroom-type spines. To determine the role of CRMP2 in dendritic spines after TBI, we stereotactically injected the CRMP2 overexpression and knockdown viruses into the hippocampus and found that CRMP2 increased the dendritic spine density and the proportion of mushroom-type spines after TBI. Meanwhile, as suggested by the morphological changes, fear conditioning behavioral experiments confirmed that CRMP2 improved memory deficits after TBI.

Tau overexpression exacerbates neuropathology after repeated mild head impacts in male mice

Repeated mild traumatic brain injury (rmTBI) can lead to development of chronic traumatic encephalopathy (CTE), which is characterized by progressive neurodegeneration with presence of white matter damage, gliosis and hyper-phosphorylated tau. While animal models of rmTBI have been documented, few characterize the molecular pathogenesis and expression profiles of relevant injured brain regions. Additionally, while the usage of transgenic tau mice in rmTBI is prevalent, the effects of tau on pathological outcomes has not been well studied. Here we characterized a 42-impact closed-head rmTBI paradigm on 3-4 month old male C57BL/6 (WT) and Tau-overexpressing mice (Tau58.4). This injury paradigm resulted in chronic gliosis, T-cell infiltration, and demyelination of the optic nerve and associated white matter tracts at 1-month post-injury. At 3-months post-injury, Tau58.4 mice showed progressive neuroinflammation and neurodegeneration in multiple brain regions compared to WT mice. Corresponding to histopathology, RNAseq of the optic nerve tract at 1-month post-injury showed significant upregulation of inflammatory pathways and downregulation of myelin synthetic pathways in both genotypes. However, Tau58.4 mice showed additional changes in neurite development, protein processing, and cell stress. Comparisons with published transcriptomes of human Alzheimer's Disease and CTE revealed common signatures including neuroinflammation and downregulation of protein phosphatases. We next investigated the demyelination and T-cell infiltration phenotypes to determine whether these offer potential avenues for therapeutic intervention. Tau58.4 mice were treated with the histamine H3 receptor antagonist GSK239512 for 1-month post-injury to promote remyelination of white matter lesions. This restored myelin gene expression to sham levels but failed to repair the histopathologic lesions. Likewise, injured T-cell-deficient Rag2/Il2rg (R2G2) mice also showed evidence for inflammation and loss of myelin. However, unlike immune-competent mice, R2G2 mice had altered myeloid cell gene expression and fewer demyelinated lesions. Together this data shows that rmTBI leads to chronic white matter inflammatory demyelination and axonal loss exacerbated by human tau overexpression but suggests that immune-suppression and remyelination alone are insufficient to reverse damage.

Subchronic Pathobiological Response Following Chronic Repetitive Mild Traumatic Brain Injury in an Aged Preclinical Model of Amyloid Pathogenesis

Repetitive mild traumatic brain injury (r-mTBI) is a risk factor for Alzheimer disease (AD). The precise nature of how r-mTBI leads to, or precipitates, AD pathogenesis remains unclear. In this study, we explore subchronic effects of chronic r-mTBI (12-impacts) administered over 1-month in aged-PS1/APP mice and littermate controls. We investigate specific mechanisms that may elucidate the molecular link between AD and r-mTBI, focusing primarily on amyloid and tau pathology, amyloid processing, glial activation states, and associated clearance mechanisms. Herein, we demonstrate r-mTBI in aged PS1/APP mice does not augment, glial activation, amyloid burden, or tau pathology (with exception of pS202-positive Tau) 1 month after exposure to the last-injury. However, we observed a decrease in brain soluble Aβ42 levels without any appreciable change in peripheral soluble Aβ42 levels. This was accompanied by an increase in brain insoluble to soluble Aβ42 ratio in injured PS1/APP mice compared with sham injury. A parallel reduction in phagocytic receptor, triggering receptor expressed on myeloid cells 2, was also observed. This study demonstrates very subtle subchronic effects of r-mTBI on a preexisting amyloid pathology background, which may be on a continuum toward a slow and worsening neurodegenerative outcome compared with sham injury, and therefore, have many implications, especially in the elderly population exposed to TBI.

Comparison of post-traumatic changes in circulating and bone marrow leukocytes between BALB/c and CD-1 mouse strains

This manuscript emerged from a larger third-party funded project investigating a new poly-trauma model and its influence upon secondary sepsis. The present sub-study compared selected leukocyte subpopulations in the circulation and bone marrow after polytrauma in BALB/c versus CD-1 mice. Animals underwent unilateral femur fracture, splenectomy and hemorrhagic shock. We collected blood and bone marrow for flow cytometry analysis at 24h and 48h post-trauma. Circulating granulocytes (Ly6G+CD11+) increased in both strains after trauma. Only in BALB/c mice circulating CD8+ T-lymphocytes decreased within 48h by 30%. Regulatory T-cells (Tregs, CD4+CD25+CD127low) increased in both strains by approx. 32%. Circulating Tregs and lymphocytes (CD11b-Ly6G-MHC-2+) were always at least 1.5-fold higher in BALB/c, while the bone marrow MHC-2 expression decreased in CD-1 mice (p<0.05). Overall, immune responses to polytrauma were similar in both strains. Additionally, BALB/c expressed higher level of circulating regulatory T-cells and MHC-2-positive lymphocytes compared to CD-1 mice.

Animal models of closed-skull, repetitive mild traumatic brain injury

The underlying mechanisms that result in neurophysiological changes and cognitive sequelae in the context of repetitive mild traumatic brain injury (rmTBI) remain poorly understood. Animal models provide a unique opportunity to examine cellular and molecular responses using histological assessment, which can give important insights on the neurophysiological changes associated with the evolution of brain injury. To better understand the potential cumulative effects of multiple concussions, the focus of animal models is shifting from single to repetitive head impacts. With a growing body of literature on this subject, a review and discussion of current findings is valuable to better understand the neuropathology associated with rmTBI, to evaluate the current state of the field, and to guide future research efforts. Despite variability in experimental settings, existing animal models of rmTBI have contributed to our understanding of the underlying mechanisms following repeat concussion. However, how to reconcile the various impact methods remains one of the major challenges in the field today.