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

Age dictates brain functional connectivity and axonal integrity following repetitive mild traumatic brain injuries in mice

Traumatic brain injuries (TBI) present a major public health challenge, demanding an in-depth understanding of age-specific symptoms and risk factors. Aging not only significantly influences brain function and plasticity but also elevates the risk of hospitalizations and death following TBIs. Repetitive mild TBIs (rmTBI) compound these issues, resulting in cumulative and long-term brain damage in the brain. In this study, we investigate the impact of age on brain network changes and white matter properties following rmTBI by employing a multi-modal approach that integrates resting-state functional magnetic resonance imaging (rsfMRI), graph theory analysis, diffusion tensor imaging (DTI), and neurite orientation dispersion and density imaging (NODDI). Our hypothesis is that the effects of rmTBI are worsened in aged animals, with this group showing more pronounced alterations in brain connectivity and white matter structure. Utilizing the closed-head impact model of engineered rotational acceleration (CHIMERA) model, we conducted rmTBIs or sham (control) procedures on young (2.5-3-months-old) and aged (22-months-old) male and female mice to model high-risk groups. Functional and structural imaging unveiled age-related reductions in communication efficiency between brain regions, while injuries induced opposhigh-risking effects on the small-world index across age groups, influencing network segregation. Functional connectivity analysis also identified alterations in 79 out of 148 brain regions by age, treatment (sham vs. rmTBI), or their interaction. Injuries exerted pronounced effects on sensory integration areas, including insular and motor cortices. Age-related disruptions in white matter integrity were observed, indicating alterations in various diffusion directions (mean diffusivity, radial diffusivity, axial diffusivity, and fractional anisotropy) and density neurite properties (dispersion index, intracellular and isotropic volume fraction). Neuroinflammation, assessed through Iba-1 and GFAP markers, correlated with higher dispersion in the optic tract, suggesting a neuroinflammatory response in injured aged animals compared to sham aged. These findings offer insight into the interplay between age, injuries, and brain connectivity, shedding light on the long-term consequences of rmTBI.

Chronic traumatic encephalopathy pathognomonic lesions occurring in isolation adjacent to infiltrative and non-infiltrative white matter lesions

Chronic traumatic encephalopathy (CTE) is defined by perivascular neuronal phosphorylated-tau accumulation at cortical sulcal depths. CTE has been mainly described in the context of repetitive, impact-type traumatic brain injury (rTBI), principally from contact sports. Rarely, CTE has been associated with single TBIs, including in relationship to healed leucotomy sites in brains from formerly institutionalized psychiatric patients without documented rTBI. Given that leucotomy principally involves severing of white matter, this could suggest involvement of axonal injury in CTE pathophysiology. We present three cases wherein isolated CTE pathology was identified adjacent to distinct white matter lesions. Case 1 is a 41-year-old man with history of hereditary hemorrhagic telangiectasia and resection of a cerebral arteriovenous malformation (AVM). Case 2 is a 46-year-old man with glioblastoma. Case 3 is a 52-year-old man with a remote cerebral infarct. Isolated CTE lesions were found adjacent to the aforementioned pathologies in each case. Additional CTE lesions were not identified despite extensive sampling. Multiple age-related tau astrogliopathy (ARTAG)-like lesions were also identified at other sulcal depths near the AVM resection site in Case 1. These cases may provide insights regarding the pathophysiology of the CTE pathognomonic lesion and the development of ARTAG-like pathology adjacent to long-standing mass lesions.

Age dictates brain functional connectivity and axonal integrity following repetitive mild traumatic brain injuries

Traumatic brain injuries (TBI) present a major public health challenge, demanding an in-depth understanding of age-specific signs and vulnerabilities. Aging not only significantly influences brain function and plasticity but also elevates the risk of hospitalizations and death following repetitive mild traumatic brain injuries (rmTBIs). In this study, we investigate the impact of age on brain network changes and white matter properties following rmTBI employing a multi-modal approach that integrates resting-state functional magnetic resonance imaging (rsfMRI), graph theory analysis, diffusion tensor imaging (DTI), and Neurite Orientation Dispersion and Density Imaging (NODDI). Utilizing the CHIMERA model, we conducted rmTBIs or sham (control) procedures on young (2.5-3 months old) and aged (22-month-old) male and female mice to model high risk groups. Functional and structural imaging unveiled age-related reductions in communication efficiency between brain regions, while injuries induced opposing effects on the small-world index across age groups, influencing network segregation. Functional connectivity analysis also identified alterations in 79 out of 148 brain regions by age, treatment (sham vs. rmTBI), or their interaction. Injuries exerted pronounced effects on sensory integration areas, including insular and motor cortices. Age-related disruptions in white matter integrity were observed, indicating alterations in various diffusion directions (mean, radial, axial diffusivity, fractional anisotropy) and density neurite properties (dispersion index, intracellular and isotropic volume fraction). Inflammation, assessed through Iba-1 and GFAP markers, correlated with higher dispersion in the optic tract, suggesting a neuroinflammatory response in aged animals. These findings provide a comprehensive understanding of the intricate interplay between age, injuries, and brain connectivity, shedding light on the long-term consequences of rmTBIs.

Identification of brain areas in mice with peak neural activity across the acute and persistent phases of post-traumatic headache

BACKGROUND

Post-traumatic headache is very common after a mild traumatic brain injury. Post-traumatic headache may persist for months to years after an injury in a substantial proportion of people. The pathophysiology underlying post-traumatic headache remains unknown but is likely distinct from other headache disorders. Identification of brain areas activated in acute and persistent phases of post-traumatic headache can provide insights into the underlying circuits mediating headache pain. We used an animal model of mild traumatic brain injury-induced post-traumatic headache and c-fos immunohistochemistry to identify brain regions with peak activity levels across the acute and persistent phases of post-traumatic headache.

METHODS

Male and female C57BL/6 J mice were briefly anesthetized and subjected to a sham procedure or a weight drop closed-head mild traumatic brain injury . Cutaneous allodynia was assessed in the periorbital and hindpaw regions using von Frey filaments. Immunohistochemical c-fos based neural activity mapping was then performed on sections from whole brain across the development of post-traumatic headache (i.e. peak of the acute phase at 2 days post- mild traumatic brain injury), start of the persistent phase (i.e. >14 days post-mild traumatic brain injury) or after provocation with stress (bright light). Brain areas with consistent and peak levels of c-fos expression across mild traumatic brain injury induced post-traumatic headache were identified and included for further analysis.

RESULTS

Following mild traumatic brain injury, periorbital and hindpaw allodynia was observed in both male and female mice. This allodynia was transient and subsided within the first 14 days post-mild traumatic brain injury and is representative of acute post-traumatic headache. After this acute post-traumatic headache phase, exposure of mild traumatic brain injury mice to a bright light stress reinstated periorbital and hindpaw allodynia for several hours - indicative of the development of persistent post-traumatic headache. Acute post-traumatic headache was coincident with an increase in neuronal c-fos labeling in the spinal nucleus of the trigeminal caudalis, primary somatosensory cortex, and the nucleus accumbens. Neuronal activation returned to baseline levels by the persistent post-traumatic headache phase in the spinal nucleus of the trigeminal caudalis and primary somatosensory cortex but remained elevated in the nucleus accumbens. In the persistent post-traumatic headache phase, coincident with allodynia observed following bright light stress, we observed bright light stress-induced c-fos neural activation in the spinal nucleus of the trigeminal caudalis, primary somatosensory cortex, and nucleus accumbens.

CONCLUSION

Examination of mild traumatic brain injury-induced changes in peak c-fos expression revealed brain regions with significantly increased neural activity across the acute and persistent phases of post-traumatic headache. Our findings suggest mild traumatic brain injury-induced post-traumatic headache produces neural activation along pain relevant pathways at time-points matching post-traumatic headache-like pain behaviors. These observations suggest that the spinal nucleus of the trigeminal caudalis, primary somatosensory cortex, and nucleus accumbens may contribute to both the induction and maintenance of post-traumatic headache.

Neuronal tau pathology worsens late-phase white matter degeneration after traumatic brain injury in transgenic mice

Traumatic brain injury (TBI) causes diffuse axonal injury which can produce chronic white matter pathology and subsequent post-traumatic neurodegeneration with poor patient outcomes. Tau modulates axon cytoskeletal functions and undergoes phosphorylation and mis-localization in neurodegenerative disorders. The effects of tau pathology on neurodegeneration after TBI are unclear. We used mice with neuronal expression of human mutant tau to examine effects of pathological tau on white matter pathology after TBI. Adult male and female hTau.P301S (Tg2541) transgenic and wild-type (Wt) mice received either moderate single TBI (s-TBI) or repetitive mild TBI (r-mTBI; once daily × 5), or sham procedures. Acutely, s-TBI produced more extensive axon damage in the corpus callosum (CC) as compared to r-mTBI. After s-TBI, significant CC thinning was present at 6 weeks and 4 months post-injury in Wt and transgenic mice, with homozygous tau expression producing additional pathology of late demyelination. In contrast, r-mTBI did not produce significant CC thinning except at the chronic time point of 4 months in homozygous mice, which exhibited significant CC atrophy (- 29.7%) with increased microgliosis. Serum neurofilament light quantification detected traumatic axonal injury at 1 day post-TBI in Wt and homozygous mice. At 4 months, high tau and neurofilament in homozygous mice implicated tau in chronic axon pathology. These findings did not have sex differences detected. Conclusions: Neuronal tau pathology differentially exacerbated CC pathology based on injury severity and chronicity. Ongoing CC atrophy from s-TBI became accompanied by late demyelination. Pathological tau significantly worsened CC atrophy during the chronic phase after r-mTBI.

Fixed Time-Point Analysis Reveals Repetitive Mild Traumatic Brain Injury Effects on Resting State Functional Magnetic Resonance Imaging Connectivity and Neuro-Spatial Protein Profiles

Repetitive mild traumatic brain injuries (rmTBIs) are serious trauma events responsible for the development of numerous neurodegenerative disorders. A major challenge in developing diagnostics and treatments for the consequences of rmTBI is the fundamental knowledge gaps of the molecular mechanisms responsible for neurodegeneration. It is both critical and urgent to understand the neuropathological and functional consequences of rmTBI to develop effective therapeutic strategies. Using the Closed-Head Impact Model of Engineered Rotational Acceleration, or CHIMERA, we measured neural changes following injury, including brain volume, diffusion tensor imaging, and resting-state functional magnetic resonance imaging coupled with graph theory and functional connectivity analyses. We determined the effect of rmTBI on markers of gliosis and used NanoString-GeoMx to add a digital-spatial protein profiling analysis of neurodegenerative disease-associated proteins in gray and white matter regions. Our analyses revealed aberrant connectivity changes in the thalamus, independent of microstructural damage or neuroinflammation. We also identified distinct changes in the levels of proteins linked to various neurodegenerative processes including total and phospho-tau species and cell proliferation markers. Together, our data show that rmTBI significantly alters brain functional connectivity and causes distinct protein changes in morphologically intact brain areas.

Altered Tau Kinase Activity in rTg4510 Mice after a Single Interfaced CHIMERA Traumatic Brain Injury

Traumatic brain injury (TBI) is an established risk factor for neurodegenerative diseases. In this study, we used the Closed Head Injury Model of Engineered Rotational Acceleration (CHIMERA) to investigate the effects of a single high-energy TBI in rTg4510 mice, a mouse model of tauopathy. Fifteen male rTg4510 mice (4 mo) were impacted at 4.0 J using interfaced CHIMERA and were compared to sham controls. Immediately after injury, the TBI mice showed significant mortality (7/15; 47%) and a prolonged duration of loss of the righting reflex. At 2 mo post-injury, surviving mice displayed significant microgliosis (Iba1) and axonal injury (Neurosilver). Western blotting indicated a reduced p-GSK-3β (S9):GSK-3β ratio in TBI mice, suggesting chronic activation of tau kinase. Although longitudinal analysis of plasma total tau suggested that TBI accelerates the appearance of tau in the circulation, there were no significant differences in brain total or p-tau levels, nor did we observe evidence of enhanced neurodegeneration in TBI mice compared to sham mice. In summary, we showed that a single high-energy head impact induces chronic white matter injury and altered GSK-3β activity without an apparent change in post-injury tauopathy in rTg4510 mice.

Can mild traumatic brain injury alter cognition chronically? A LIMBIC-CENC multicenter study

OBJECTIVE

While outcome from mild traumatic brain injury (mTBI) is generally favorable, concern remains over potential negative long-term effects, including impaired cognition. This study examined the link between cognitive performance and remote mTBIs within the Long-term Impact of Military-relevant Brain Injury Consortium-Chronic Effects of Neurotrauma Consortium (LIMBIC-CENC) multicenter, observational study of Veterans and service members (SMs) with combat exposure.

METHOD

Baseline data of the participants passing all cognitive performance validity tests (n = 1,310) were used to conduct a cross-sectional analysis. Using multivariable regression models that adjusted for covariates, including age and estimated preexposure intellectual function, positive mTBI history groups, 1-2 lifetime mTBIs (nonrepetitive, n = 614), and 3 + lifetime mTBIs (repetitive; n = 440) were compared to TBI negative controls (n = 256) on each of the seven cognitive domains computed by averaging Z scores of prespecified component tests. Significance levels were adjusted for multiple comparisons.

RESULTS

Neither of the mTBI positive groups differed from the mTBI negative control group on any of the cognitive domains in multivariable analyses. Findings were also consistently negative across sensitivity analyses (e.g., mTBIs as a continuous variable, number of blast-related mTBIs, or years since the first and last mTBI).

CONCLUSIONS

Our findings demonstrate that the average veteran or SM who experienced one or more mTBIs does not have postacute objective cognitive deficits due to mTBIs alone. A holistic health care approach including comorbidity assessment is indicated for patients reporting chronic cognitive difficulties after mTBI(s), and strategies for addressing misattribution may be beneficial. Future study is recommended with longitudinal designs to assess within-subjects decline from potential neurodegeneration. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Electromagnetic Controlled Closed-Head Model of Mild Traumatic Brain Injury in Mice

Highly reproducible animal models of traumatic brain injury (TBI), with well-defined pathologies, are needed for testing therapeutic interventions and understanding the mechanisms of how a TBI alters brain function. The availability of multiple animal models of TBI is necessary to model the different aspects and severities of TBI seen in people. This manuscript describes the use of a midline closed head injury (CHI) to develop a mouse model of mild TBI. The model is considered mild because it does not produce structural brain lesions based on neuroimaging or gross neuronal loss. However, a single impact creates enough pathology that cognitive impairment is measurable at least 1 month after injury. A step-by-step protocol to induce a CHI in mice using a stereotaxically guided electromagnetic impactor is defined in the paper. The benefits of the mild midline CHI model include the reproducibility of the injury-induced changes with low mortality. The model has been temporally characterized up to 1 year after the injury for neuroimaging, neurochemical, neuropathological, and behavioral changes. The model is complementary to open skull models of controlled cortical impact using the same impactor device. Thus, labs can model both mild diffuse TBI and focal moderate-to-severe TBI with the same impactor.

Recent Treatment Strategies in Alzheimer's Disease and Chronic Traumatic Encephalopathy

Neurotrauma has been well linked to the progression of neurodegenerative disease. Much work has been done characterizing chronic traumatic encephalopathy, but less has been done regarding the contribution to Alzheimer's Disease. This review focuses on AD and its association with neurotrauma. Emerging clinical trials are discussed as well as novel mechanisms. We then address how some of these mechanisms are shared with CTE and emerging pre-clinical studies. This paper is a user-friendly resource that summarizes the emerging findings and proposes further investigation into key areas of interest. It is intended to serve as a catalyst for both research teams and clinicians in the quest to improve effective treatment and diagnostic options.

Lateral Fluid Percussion Injury Causes Sex-Specific Deficits in Anterograde but Not Retrograde Memory

Cognitive impairment is a common symptom after traumatic brain injury (TBI). Memory, in particular, is often disrupted during chronic post-injury recovery. To understand the sex-specific effects of brain injury on retrograde and anterograde memory, we examined paired associate learning (PAL), spatial learning and memory, and fear memory after lateral fluid percussion TBI. We hypothesized that male and female mice would display unique memory deficits after TBI. PAL task acquisition was initiated via touchscreen operant conditioning 22 weeks before sham injury or TBI. Post-injury PAL testing occurred 7 weeks post-injury. Barnes maze and fear conditioning were completed at 14- and 15-weeks post-injury, respectively. Contrary to our expectations, behavioral outcomes were not primarily influenced by TBI. Instead, sex-specific differences were observed in all tasks which exposed task-specific trends in male TBI mice. Male mice took longer to complete the PAL task, but this was not affected by TBI and did not compromise the ability to make a correct choice. Latency to reach the goal box decreased across testing days in Barnes maze, but male TBI mice lagged in improvement compared to all other groups. Use of two learning indices revealed that male TBI mice were deficient in transferring information from 1 day to the next. Finally, acquisition and contextual retention of fear memory were similar between all groups. Cued retention of the tone-shock pairing was influenced by both injury and sex. Male sham mice displayed the strongest cued retention of fear memory, evidenced by increased freezing behavior across the test trial. In contrast, male TBI mice displayed reduced freezing behavior with repetitive tone exposure. An inverse relationship in freezing behavior to tone exposure was detected between female sham and TBI mice, although the difference was not as striking. Together, these studies show that retrograde memory is intact after lateral TBI. However, male mice are more vulnerable to post-injury anterograde memory deficits. These behaviors were not associated with gross pathological change near the site injury or in subcortical brain regions associated with memory formation. Future studies that incorporate pre- and post-injury behavioral analysis will be integral in defining sex-specific memory impairment after TBI.

Magnetic Resonance Imaging in Tauopathy Animal Models

The microtubule-associated protein tau plays an important role in tauopathic diseases such as Alzheimer's disease and primary tauopathies such as progressive supranuclear palsy and corticobasal degeneration. Tauopathy animal models, such as transgenic, knock-in mouse and rat models, recapitulating tauopathy have facilitated the understanding of disease mechanisms. Aberrant accumulation of hyperphosphorylated tau contributes to synaptic deficits, neuroinflammation, and neurodegeneration, leading to cognitive impairment in animal models. Recent advances in molecular imaging using positron emission tomography (PET) and magnetic resonance imaging (MRI) have provided valuable insights into the time course of disease pathophysiology in tauopathy animal models. High-field MRI has been applied for in vivo imaging in animal models of tauopathy, including diffusion tensor imaging for white matter integrity, arterial spin labeling for cerebral blood flow, resting-state functional MRI for functional connectivity, volumetric MRI for neurodegeneration, and MR spectroscopy. In addition, MR contrast agents for non-invasive imaging of tau have been developed recently. Many preclinical MRI indicators offer excellent translational value and provide a blueprint for clinical MRI in the brains of patients with tauopathies. In this review, we summarized the recent advances in using MRI to visualize the pathophysiology of tauopathy in small animals. We discussed the outstanding challenges in brain imaging using MRI in small animals and propose a future outlook for visualizing tau-related alterations in the brains of animal models.

Intranasal delivery of exosomes from human adipose derived stem cells at forty-eight hours post injury reduces motor and cognitive impairments following traumatic brain injury

The neuroprotective role of human adipose-derived stems cells (hASCs) has raised great interest in regenerative medicine due to their ability to modulate their surrounding environment. Our group has demonstrated that exosomes derived from hASC (hASCexo) are a cell-free regenerative approach to long term recovery following traumatic brain injury (TBI). Previously, we demonstrated the efficacy of exosome treatment with intravenous delivery at 3 h post TBI in rats. Here, we show efficacy of exosomes through intranasal delivery at 48 h post TBI in mice lengthening the therapeutic window of treatment and therefore increasing possible translation to clinical studies. Our findings demonstrate significant recovery of motor impairment assessed by an elevated body swing test in mice treated with exosomes containing MALAT1 compared to both TBI mice without exosomes and exosomes depleted of MALAT1. Significant cognitive improvement was seen in the reversal trial of 8 arm radial arm water maze in mice treated with exosomes containing MALAT1. Furthermore, cortical damage was significantly reduced in mice treated with exosomes containing MALAT1 as well as decreased MHCII+ staining of microglial cells. Mice without exosomes or treated with exosomes depleted of MALAT1 did not show similar recovery. Results demonstrate both inflammation related genes and NRTK3 (TrkC) are target genes modulated by hASC exosomes and further that MALAT1 in hASC exosomes regulates expression of full length TrkC thereby activating the MAPK pathway and promoting recovery. Exosomes are a promising therapeutic approach following TBI with a therapeutic window of at least 48 h and contain long noncoding RNA's, specifically MALAT1 that play a vital role in the mechanism of action.

Mouse closed head traumatic brain injury replicates the histological tau pathology pattern of human disease: characterization of a novel model and systematic review of the literature

Traumatic brain injury (TBI) constitutes one of the strongest environmental risk factors for several progressive neurodegenerative disorders of cognitive impairment and dementia that are characterized by the pathological accumulation of hyperphosphorylated tau (p-Tau). It has been questioned whether mouse closed-head TBI models can replicate human TBI-associated tauopathy. We conducted longitudinal histopathological characterization of a mouse closed head TBI model, with a focus on pathological features reported in human TBI-associated tauopathy. Male C57BL/6 J mice were subjected to once daily TBI for 5 consecutive days using a weight drop paradigm. Histological analyses (AT8, TDP-43, pTDP-43, NeuN, GFAP, Iba-1, MBP, SMI-312, Prussian blue, IgG, βAPP, alpha-synuclein) were conducted at 1 week, 4 weeks, and 24 weeks after rTBI and compared to sham operated controls. We conducted a systematic review of the literature for mouse models of closed-head injury focusing on studies referencing tau protein assessment. At 1-week post rTBI, p-Tau accumulation was restricted to the corpus callosum and perivascular spaces adjacent to the superior longitudinal fissure. Progressive p-Tau accumulation was observed in the superficial layers of the cerebral cortex, as well as in mammillary bodies and cortical perivascular, subpial, and periventricular locations at 4 to 24 weeks after rTBI. Associated cortical histopathologies included microvascular injury, neuroaxonal rarefaction, astroglial and microglial activation, and cytoplasmatic localization of TDP-43 and pTDP-43. In our systematic review, less than 1% of mouse studies (25/3756) reported p-Tau using immunostaining, of which only 3 (0.08%) reported perivascular p-Tau, which is considered a defining feature of chronic traumatic encephalopathy. Commonly reported associated pathologies included neuronal loss (23%), axonal loss (43%), microglial activation and astrogliosis (50%, each), and beta amyloid deposition (29%). Our novel model, supported by systematic review of the literature, indicates progressive tau pathology after closed head murine TBI, highlighting the suitability of mouse models to replicate pertinent human histopathology.

Space-occupying brain lesions, trauma-related tau astrogliopathy, and ARTAG: a report of two cases and a literature review

Astrocytes with intracellular accumulations of misfolded phosphorylated tau protein have been observed in advanced-stage chronic traumatic encephalopathy (CTE) and in other neurodegenerative conditions. There is a growing awareness that astrocytic tau inclusions are also relatively common in the brains of persons over 70 years of age-affecting approximately one-third of autopsied individuals. The pathologic hallmarks of aging-related tau astrogliopathy (ARTAG) include phosphorylated tau protein within thorn-shaped astrocytes (TSA) in subpial, subependymal, perivascular, and white matter regions, whereas granular-fuzzy astrocytes are often seen in gray matter. CTE and ARTAG share molecular and histopathologic characteristics, suggesting that trauma-related mechanism(s) may predispose to the development of tau astrogliopathy. There are presently few experimental systems to study the pathobiology of astrocytic-tau aggregation, but human studies have made recent progress. For example, leucotomy (also referred to as lobotomy) is associated with a localized ARTAG-like neuropathology decades after the surgical brain injury, suggesting that chronic brain injury of any type may predispose to later life ARTAG. To examine this idea in a different context, we report clinical and pathologic features of two middle-aged men who came to autopsy with large (> 6 cm in greatest dimension) arachnoid cysts that had physically displaced and injured the subjects' left temporal lobes through chronic mechanical stress. Despite the similarity of the size and location of the arachnoid cysts, these individuals had dissimilar neurologic outcomes and neuropathologic findings. We review the evidence for ARTAG in response to brain injury, and discuss how the location and molecular properties of astroglial tau inclusions might alter the physiology of resident astrocytes. These cases and literature review point toward possible mechanism(s) of tau aggregation in astrocytes in response to chronic brain trauma.

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.

Optimization and validation of a modified radial-arm water maze protocol using a murine model of mild closed head traumatic brain injury

Cognitive impairments can be a significant problem after a traumatic brain injury (TBI), which affects millions worldwide each year. There is a need for establish reproducible cognitive assays in rodents to better understand disease mechanisms and to develop therapeutic interventions towards treating TBI-induced impairments. Our goal was to validate and standardize the radial arm water maze (RAWM) test as an assay to screen for cognitive impairments caused by TBI. RAWM is a visuo-spatial learning test, originally designed for use with rats, and later adapted for mice. The present study investigates whether test procedures, such us the presence of extra-maze cues influences learning and memory performance. C57BL/6 mice were tested in an 8-arm RAWM using a four-day protocol. We demonstrated that two days of training, exposing the mice to extra-maze cues and a visible platform, influenced learning and memory performance. Mice that did not receive training performed poorer compared to mice trained. To further validate our RAWM protocol, we used scopolamine. We, also, demonstrated that a single mild closed head injury (CHI) caused deficits in this task at two weeks post-CHI. Our data supported the use of 7 trials per day and a spaced training protocol as key factor to unmask memory impairment following CHI. Here, we provide a detailed standard operating procedure for RAWM test, which can be applied to a variety of mouse models including neurodegenerative diseases and pathology, as well as when pharmacological approaches are used.

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.