Experimental Traumatic Brain Injury Identifies Distinct Early and Late Phase Axonal Conduction Deficits of White Matter Pathophysiology, and Reveals Intervening Recovery

Enhancing axonal myelination: Clemastine attenuates cognitive impairment in a rat model of diffuse traumatic brain injury

Traumatic brain injury (TBI) has a significant impact on cognitive function, affecting millions of people worldwide. Myelin loss is a prominent pathological feature of TBI, while well-functioning myelin is crucial for memory and cognition. Utilizing drug repurposing to identify effective drug candidates for TBI treatment has gained attention. Notably, recent research has highlighted the potential of clemastine, an FDA-approved allergy medication, as a promising pro-myelinating drug. Therefore, in this study, we aim to investigate whether clemastine can enhance myelination and alleviate cognitive impairment following mild TBI using a clinically relevant rat model of TBI. Mild diffuse TBI was induced using the Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA). Animals were treated with either clemastine or an equivalent volume of the vehicle from day 1 to day 14 post-injury. Following treatment, memory-related behavioral tests were conducted, and myelin pathology in the cortex and hippocampus was assessed through immunofluorescence staining and ProteinSimple® capillary-based immunoassay. Our results showed that TBI leads to significant myelin loss, axonal damage, glial activation, and a decrease in mature oligodendrocytes in both the cortex and hippocampus. The TBI animals also exhibited notable deficits in memory-related tests. In contrast, animals treated with clemastine showed an increase in mature oligodendrocytes, enhanced myelination, and improved performance in the behavioral tests. These preliminary findings support the therapeutic value of clemastine in alleviating TBI-induced cognitive impairment, with substantial clinical translational potential. Our findings also underscore the potential of remyelinating therapies for TBI.

Disrupted Maturation of White Matter Microstructure After Concussion Is Associated With Internalizing Behavior Scores in Female Children

BACKGROUND

Some children who experience concussions, particularly females, develop long-lasting emotional and behavioral problems. Establishing the potential contribution of preexisting behavioral problems and disrupted white matter maturation has been challenging due to a lack of preinjury data.

METHODS

From the Adolescent Brain Cognitive Development cohort, 239 (90 female) children age 12.1 ± 0.6 years who experienced a concussion after study entry at 10.0 ± 0.6 years were compared to 6438 (3245 female) children without head injuries who were age 9.9 ± 0.6 years at baseline and 12.0 ± 0.6 years at follow-up. The Child Behavior Checklist was used to assess internalizing and externalizing behavior at study entry and follow-up. In the children with magnetic resonance imaging data available (concussion n = 134, comparison n = 3520), deep and superficial white matter was characterized by neurite density from restriction spectrum image modeling of diffusion magnetic resonance imaging. Longitudinal ComBat harmonization removed scanner effects. Linear regressions modeled 1) behavior problems at follow-up controlling for baseline behavior, 2) impact of concussion on white matter maturation, and 3) contribution of deviations in white matter maturation to postconcussion behavior problems.

RESULTS

Only female children with concussion had higher internalizing behavior problem scores. The youngest children with concussion showed less change in superficial white matter neurite density over 2 years than children with no concussion. In females with concussion, less change in superficial white matter neurite density was correlated with increased internalizing behavior problem scores.

CONCLUSIONS

Concussions in female children are associated with emotional problems beyond preinjury levels. Injury to superficial white matter may contribute to persistent internalizing behavior problems in females.

Identifying the Phenotypes of Diffuse Axonal Injury Following Traumatic Brain Injury

Diffuse axonal injury (DAI) is a significant feature of traumatic brain injury (TBI) across all injury severities and is driven by the primary mechanical insult and secondary biochemical injury phases. Axons comprise an outer cell membrane, the axolemma which is anchored to the cytoskeletal network with spectrin tetramers and actin rings. Neurofilaments act as space-filling structural polymers that surround the central core of microtubules, which facilitate axonal transport. TBI has differential effects on these cytoskeletal components, with axons in the same white matter tract showing a range of different cytoskeletal and axolemma alterations with different patterns of temporal evolution. These require different antibodies for detection in post-mortem tissue. Here, a comprehensive discussion of the evolution of axonal injury within different cytoskeletal elements is provided, alongside the most appropriate methods of detection and their temporal profiles. Accumulation of amyloid precursor protein (APP) as a result of disruption of axonal transport due to microtubule failure remains the most sensitive marker of axonal injury, both acutely and chronically. However, a subset of injured axons demonstrate different pathology, which cannot be detected via APP immunoreactivity, including degradation of spectrin and alterations in neurofilaments. Furthermore, recent work has highlighted the node of Ranvier and the axon initial segment as particularly vulnerable sites to axonal injury, with loss of sodium channels persisting beyond the acute phase post-injury in axons without APP pathology. Given the heterogenous response of axons to TBI, further characterization is required in the chronic phase to understand how axonal injury evolves temporally, which may help inform pharmacological interventions.

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.

Premature white matter microstructure in female children with a history of concussion

Childhood concussion may interfere with neurodevelopment and influence cognition. Females are more likely to experience persistent symptoms after concussion, yet the sex-specific impact of concussion on brain microstructure in children is understudied. This study examined white matter and cortical microstructure, based on neurite density (ND) from diffusion-weighted MRI, in 9-to-10-year-old children in the Adolescent Brain Cognitive Development Study with (n = 336) and without (n = 7368) a history of concussion, and its relationship with cognitive performance. Multivariate regression was used to investigate relationships between ND and group, sex, and age in deep and superficial white matter, subcortical structures, and cortex. Partial least square correlation was performed to identify associations between ND and performance on NIH Toolbox tasks in children with concussion. All tissue types demonstrated higher ND with age, reflecting brain maturation. Group comparisons revealed higher ND in deep and superficial white matter in females with concussion. In female but not male children with concussion, there were significant associations between ND and performance on cognitive tests. These results demonstrate a greater long-term impact of childhood concussion on white matter microstructure in females compared to males that is associated with cognitive function. The increase in ND in females may reflect premature white matter maturation.

Traumatic Axonal Injury in the Optic Nerve: The Selective Role of SARM1 in the Evolution of Distal Axonopathy

Traumatic axonal injury (TAI), thought to be caused by rotational acceleration of the head, is a prevalent neuropathology in traumatic brain injury (TBI). TAI in the optic nerve is a common finding in multiple blunt-force TBI models and hence a great model to study mechanisms and treatments for TAI, especially in view of the compartmentalized anatomy of the visual system. We have previously shown that the somata and the proximal, but not distal, axons of retinal ganglion cells (RGC) respond to DLK/LZK blockade after impact acceleration of the head (IA-TBI). Here, we explored the role of the sterile alpha and TIR-motif containing 1 (SARM1), the key driver of Wallerian degeneration (WD), in the progressive breakdown of distal and proximal segments of the optic nerve following IA-TBI with high-resolution morphological and classical neuropathological approaches. Wild type and Sarm1 knockout (KO) mice received IA-TBI or sham injury and were allowed to survive for 3, 7, 14, and 21 days. Ultrastructural and microscopic analyses revealed that TAI in the optic nerve is characterized by variable involvement of individual axons, ranging from apparent early disconnection of a subpopulation of axons to a range of ongoing axonal and myelin perturbations. Traumatic axonal injury resulted in the degeneration of a population of axons distal and proximal to the injury, along with retrograde death of a subpopulation of RGCs. Quantitative analyses on proximal and distal axons and RGC somata revealed that different neuronal domains exhibit differential vulnerability, with distal axon segments showing more severe degeneration compared with proximal segments and RGC somata. Importantly, we found that Sarm1 KO had a profound effect in the distal optic nerve by suppressing axonal degeneration by up to 50% in the first 2 weeks after IA-TBI, with a continued but lower effect at 3 weeks, while also suppressing microglial activation. Sarm1 KO had no evident effect on the initial traumatic disconnection and did not ameliorate the proximal optic axonopathy or the subsequent attrition of RGCs, indicating that the fate of different axonal segments in the course of TAI may depend on distinct molecular programs within axons.

Mature and Myelinating Oligodendrocytes Are Specifically Vulnerable to Mild Fluid Percussion Injury in Mice

Myelin loss and oligodendrocyte death are well documented in patients with traumatic brain injury (TBI), as well as in experimental animal models after moderate-to-severe TBI. In comparison, mild TBI (mTBI) does not necessarily result in myelin loss or oligodendrocyte death, but causes structural alterations in the myelin. To gain more insight into the impact of mTBI on oligodendrocyte lineage in the adult brain, we subjected mice to mild lateral fluid percussion injury (mFPI) and characterized the early impact (1 and 3 days post-injury) on oligodendrocytes in the corpus callosum using multiple oligodendrocyte lineage markers (platelet-derived growth factor receptor [PDGFR]-α, glutathione S-transferase [GST]-π, CC1, breast carcinoma-amplified sequence 1 [BCAS1], myelin basic protein [MBP], myelin-associated glycoprotein [MAG], proteolipid protein [PLP], and FluoroMyelin™). Two regions of the corpus callosum in relation to the impact site were analyzed: areas near (focal) and anterior (distal) to the impact site. mFPI did not result in oligodendrocyte death in either the focal or distal corpus callosum, nor impact on oligodendrocyte precursors (PDGFR-α+) and GST-π+ oligodendrocyte numbers. In the focal but not distal corpus callosum, mFPI caused a decrease in CC1+ as well as BCAS1+ actively myelinating oligodendrocytes and reduced FluoroMyelin intensity without altering myelin protein expression (MBP, PLP, and MAG). Disruption in node-paranode organization and loss of Nav1.6+ nodes were observed in both the focal and distal regions, even in areas without obvious axonal damage. Altogether, our study shows regional differences in mature and myelinating oligodendrocyte in response to mFPI. Further, mFPI elicits a widespread impact on node-paranode organization that affects regions both close to and remotely located from the site of injury.

Brain Trauma and the Secondary Cascade in Humans: Review of the Potential Role of Vitamins in Reparative Processes and Functional Outcome

An estimated sixty-nine million people sustain a traumatic brain injury each year. Trauma to the brain causes the primary insult and initiates a secondary biochemical cascade as part of the immune and reparative response to injury. The secondary cascade, although a normal physiological response, may also contribute to ongoing neuroinflammation, oxidative stress and axonal injury, continuing in some cases years after the initial insult. In this review, we explain some of the biochemical mechanisms of the secondary cascade and their potential deleterious effects on healthy neurons including secondary cell death. The second part of the review focuses on the role of micronutrients to neural mechanisms and their potential reparative effects with regards to the secondary cascade after brain injury. The biochemical response to injury, hypermetabolism and excessive renal clearance of nutrients after injury increases the demand for most vitamins. Currently, most research in the area has shown positive outcomes of vitamin supplementation after brain injury, although predominantly in animal (murine) models. There is a pressing need for more research in this area with human participants because vitamin supplementation post-trauma is a potential cost-effective adjunct to other clinical and therapeutic treatments. Importantly, traumatic brain injury should be considered a lifelong process and better evaluated across the lifespan of individuals who experience brain injury.

The cortical NG2-glia response to traumatic brain injury

Traumatic brain injury (TBI) is a significant worldwide cause of morbidity and mortality. A chronic neurologic disease bearing the moniker of "the silent epidemic," TBI currently has no targeted therapies to ameliorate cellular loss or enhance functional recovery. Compared with those of astrocytes, microglia, and peripheral immune cells, the functions and mechanisms of NG2-glia following TBI are far less understood, despite NG2-glia comprising the largest population of regenerative cells in the mature cortex. Here, we synthesize the results from multiple rodent models of TBI, with a focus on cortical NG2-glia proliferation and lineage potential, and propose future avenues for glia researchers to address this unique cell type in TBI. As the molecular mechanisms that regulate NG2-glia regenerative potential are uncovered, we posit that future therapeutic strategies may exploit cortical NG2-glia to augment local cellular recovery following TBI.

Inflammation and immunomodulation in central nervous system injury - B cells as a novel therapeutic opportunity

Acute injury to the central nervous system (CNS) remains a complex and challenging clinical need. CNS injury initiates a dynamic neuroinflammatory response, mediated by both resident and infiltrating immune cells. Following the primary injury, dysregulated inflammatory cascades have been implicated in sustaining a pro-inflammatory microenvironment, driving secondary neurodegeneration and the development of lasting neurological dysfunction. Due to the multifaceted nature of CNS injury, clinically effective therapies for conditions such as traumatic brain injury (TBI), spinal cord injury (SCI), and stroke have proven challenging to develop. No therapeutics that adequately address the chronic inflammatory component of secondary CNS injury are currently available. Recently, B lymphocytes have gained increasing appreciation for their role in maintaining immune homeostasis and regulating inflammatory responses in the context of tissue injury. Here we review the neuroinflammatory response to CNS injury with particular focus on the underexplored role of B cells and summarize recent results on the use of purified B lymphocytes as a novel immunomodulatory therapeutic for tissue injury, particularly in the CNS.

Microfluidic Platforms Promote Polarization of Human-Derived Retinal Ganglion Cells That Model Axonopathy

Purpose

Axons depend on long-range transport of proteins and organelles which increases susceptibility to metabolic stress in disease. The axon initial segment (AIS) is particularly vulnerable due to the high bioenergetic demand of action potential generation. Here, we prepared retinal ganglion cells derived from human embryonic stem cells (hRGCs) to probe how axonal stress alters AIS morphology.

Methods

hRGCs were cultured on coverslips or microfluidic platforms. We assayed AIS specification and morphology by immunolabeling against ankyrin G (ankG), an axon-specific protein, and postsynaptic density 95 (PSD-95), a dendrite-specific protein. Using microfluidic platforms that enable fluidic isolation, we added colchicine to the axon compartment to lesion axons. We verified axonopathy by measuring the anterograde axon transport of cholera toxin subunit B and immunolabeling against cleaved caspase 3 (CC3) and phosphorylated neurofilament H (SMI-34). We determined the influence of axon injury on AIS morphology by immunolabeling samples against ankG and measuring AIS distance from soma and length.

Results

Based on measurements of ankG and PSD-95 immunolabeling, microfluidic platforms promote the formation and separation of distinct somatic-dendritic versus axonal compartments in hRGCs compared to coverslip cultures. Chemical lesioning of axons by colchicine reduced hRGC anterograde axon transport, increased varicosity density, and enhanced expression of CC3 and SMI-34. Interestingly, we found that colchicine selectively affected hRGCs with axon-carrying dendrites by reducing AIS distance from somas and increasing length, thus suggesting reduced capacity to maintain excitability.

Conclusions

Thus, microfluidic platforms promote polarized hRGCs that enable modeling of axonopathy.

Translational Relevance

Microfluidic platforms may be used to assay compartmentalized degeneration that occurs during glaucoma.

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

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

Intravenous immunoglobulin (IVIG) promotes brain repair and improves cognitive outcomes after traumatic brain injury in a FcγRIIB receptor-dependent manner

Intravenous immunoglobulin (IVIG) is a promising immune-modulatory therapy for limiting harmful inflammation and associated secondary tissue loss in neurotrauma. Here, we show that IVIG therapy attenuates spatial learning and memory deficits following a controlled cortical impact mouse model of traumatic brain injury (TBI). These improvements in cognitive outcomes were associated with increased neuronal survival, an overall reduction in brain tissue loss, and a greater preservation of neural connectivity. Furthermore, we demonstrate that the presence of the main inhibitory FcγRIIB receptor is required for the beneficial effects of IVIG treatment in TBI, with our results simultaneously highlighting the role of this receptor in reducing secondary damage arising from brain injury.

Stem Cell Factor and Granulocyte Colony-Stimulating Factor Promote Remyelination in the Chronic Phase of Severe Traumatic Brain Injury

Severe traumatic brain injury (TBI) causes long-term disability and death in young adults. White matter is vulnerable to TBI damage. Demyelination is a major pathological change of white matter injury after TBI. Demyelination, which is characterized by myelin sheath disruption and oligodendrocyte cell death, leads to long-term neurological function deficits. Stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF) treatments have shown neuroprotective and neurorestorative effects in the subacute and chronic phases of experimental TBI. Our previous study has revealed that combined SCF and G-CSF treatment (SCF + G-CSF) enhances myelin repair in the chronic phase of TBI. However, the long-term effect and mechanism of SCF + G-CSF-enhanced myelin repair remain unclear. In this study, we uncovered persistent and progressive myelin loss in the chronic phase of severe TBI. SCF + G-CSF treatment in the chronic phase of severe TBI enhanced remyelination in the ipsilateral external capsule and striatum. The SCF + G-CSF-enhanced myelin repair is positively correlated with the proliferation of oligodendrocyte progenitor cells in the subventricular zone. These findings reveal the therapeutic potential of SCF + G-CSF in myelin repair in the chronic phase of severe TBI and shed light on the mechanism underlying SCF + G-CSF-enhanced remyelination in chronic TBI.

DCC/netrin-1 regulates cell death in oligodendrocytes after brain injury

Hallmark pathological features of brain trauma are axonal degeneration and demyelination because myelin-producing oligodendrocytes (OLs) are particularly vulnerable to injury-induced death signals. To reveal mechanisms responsible for this OL loss, we examined a novel class of "death receptors" called dependence receptors (DepRs). DepRs initiate pro-death signals in the absence of their respective ligand(s), yet little is known about their role after injury. Here, we investigated whether the deleted in colorectal cancer (DCC) DepR contributes to OL loss after brain injury. We found that administration of its netrin-1 ligand is sufficient to block OL cell death. We also show that upon acute injury, DCC is upregulated while netrin-1 is downregulated in perilesional tissues. Moreover, after genetically silencing pro-death activity using DCCD1290N mutant mice, we observed greater OL survival, greater myelin integrity, and improved motor function. Our findings uncover a novel role for the netrin-1/DCC pathway in regulating OL loss in the traumatically injured brain.

Stem cell factor and granulocyte colony-stimulating factor promote remyelination in the chronic phase of severe traumatic brain injury

Severe traumatic brain injury (TBI) causes long-term disability and death in young adults. White matter is vulnerable to TBI damage. Demyelination is a major pathological change of white matter injury after TBI. Demyelination which is characterized by myelin sheath disruption and oligodendrocyte cell death leads to long-term neurological function deficits. Stem cell factor (SCF) and granulocyte colonyâ€"stimulating factor (G-CSF) treatments have shown neuroprotective and neurorestorative effects in the subacute and chronic phases of experimental TBI. Our previous study has revealed that combined SCF and G-CSF treatment (SCF+G-CSF) enhances myelin repair in the chronic phase of TBI. However, the long-term effect and mechanism of SCF+G-CSF-enhanced myelin repair remain unclear. In this study, we uncovered persistent and progressive myelin loss in the chronic phase of severe TBI. SCF+G-CSF treatment in the chronic phase of severe TBI enhanced remyelination in the ipsilateral external capsule and striatum. The SCF+G-CSF-enhanced myelin repair is positively correlated with the proliferation of oligodendrocyte progenitor cells in the subventricular zone. These findings reveal the therapeutic potential of SCF+G-CSF in myelin repair in the chronic phase of severe TBI and shed light on the mechanism underlying SCF+G-CSF-enhanced remyelination in chronic TBI.

Traumatic axonopathy in spinal tracts after impact acceleration head injury: Ultrastructural observations and evidence of SARM1-dependent axonal degeneration

Traumatic axonal injury (TAI) and the associated axonopathy are common consequences of traumatic brain injury (TBI) and contribute to significant neurological morbidity. It has been previously suggested that TAI activates a highly conserved program of axonal self-destruction known as Wallerian degeneration (WD). In the present study, we utilize our well-established impact acceleration model of TBI (IA-TBI) to characterize the pathology of injured myelinated axons in the white matter tracks traversing the ventral, lateral, and dorsal spinal columns in the mouse and assess the effect of Sterile Alpha and TIR Motif Containing 1 (Sarm1) gene knockout on acute and subacute axonal degeneration and myelin pathology. In silver-stained preparations, we found that IA-TBI results in white matter pathology as well as terminal field degeneration across the rostrocaudal axis of the spinal cord. At the ultrastructural level, we found that traumatic axonopathy is associated with diverse types of axonal and myelin pathology, ranging from focal axoskeletal perturbations and focal disruption of the myelin sheath to axonal fragmentation. Several morphological features such as neurofilament compaction, accumulation of organelles and inclusions, axoskeletal flocculation, myelin degeneration and formation of ovoids are similar to profiles encountered in classical examples of WD. Other profiles such as excess myelin figures and inner tongue evaginations are more typical of chronic neuropathies. Stereological analysis of pathological axonal and myelin profiles in the ventral, lateral, and dorsal columns of the lower cervical cord (C6) segments from wild type and Sarm1 KO mice at 3 and 7 days post IA-TBI (n = 32) revealed an up to 90% reduction in the density of pathological profiles in Sarm1 KO mice after IA-TBI. Protection was evident across all white matter tracts assessed, but showed some variability. Finally, Sarm1 deletion ameliorated the activation of microglia associated with TAI. Our findings demonstrate the presence of severe traumatic axonopathy in multiple ascending and descending long tracts after IA-TBI with features consistent with some chronic axonopathies and models of WD and the across-tract protective effect of Sarm1 deletion.

Genetic deletion of Krüppel-like factor 11 aggravates traumatic brain injury

Background

The long-term functional recovery of traumatic brain injury (TBI) is hampered by pathological events, such as parenchymal neuroinflammation, neuronal death, and white matter injury. Krüppel-like transcription factor 11 (KLF 11) belongs to the zinc finger family of transcription factors and actively participates in various pathophysiological processes in neurological disorders. Up to now, the role and molecular mechanisms of KLF11 in regulating the pathogenesis of brain trauma is poorly understood.

Methods

KLF11 knockout (KO) and wild-type (WT) mice were subjected to experimental TBI, and sensorimotor and cognitive functions were evaluated by rotarod, adhesive tape removal, foot fault, water maze, and passive avoidance tests. Brain tissue loss/neuronal death was examined by MAP2 and NeuN immunostaining, and Cresyl violet staining. White matter injury was assessed by Luxol fast blue staining, and also MBP/SMI32 and Caspr/Nav1.6 immunostaining. Activation of cerebral glial cells and infiltration of blood-borne immune cells were detected by GFAP, Iba-1/CD16/32, Iba-1/CD206, Ly-6B, and F4/80 immunostaining. Brian parenchymal inflammatory cytokines were measured with inflammatory array kits.

Results

Genetic deletion of KLF11 worsened brain trauma-induced sensorimotor and cognitive deficits, brain tissue loss and neuronal death, and white matter injury in mice. KLF11 genetic deficiency in mice also accelerated post-trauma astrocytic activation, promoted microglial polarization to a pro-inflammatory phenotype, and increased the infiltration of peripheral neutrophils and macrophages into the brain parenchyma. Mechanistically, loss-of-KLF11 function was found to directly increase the expression of pro-inflammatory cytokines in the brains of TBI mice.

Conclusion

KLF11 acts as a novel protective factor in TBI. KLF11 genetic deficiency in mice aggravated the neuroinflammatory responses, grey and white matter injury, and impaired long-term sensorimotor and cognitive recovery. Elucidating the functional importance of KLF11 in TBI may lead us to discover novel pharmacological targets for the development of effective therapies against brain trauma.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02638-0.

Concussion leads to widespread axonal sodium channel loss and disruption of the node of Ranvier

Despite being a major health concern, little is known about the pathophysiological changes that underly concussion. Nonetheless, emerging evidence suggests that selective damage to white matter axons, or diffuse axonal injury (DAI), disrupts brain network connectivity and function. While voltage-gated sodium channels (NaChs) and their anchoring proteins at the nodes of Ranvier (NOR) on axons are key elements of the brain's network signaling machinery, changes in their integrity have not been studied in context with DAI. Here, we utilized a clinically relevant swine model of concussion that induces evolving axonal pathology, demonstrated by accumulation of amyloid precursor protein (APP) across the white matter. Over a two-week follow-up post-concussion with this model, we found widespread loss of NaCh isoform 1.6 (Nav1.6), progressive increases in NOR length, the appearance of void and heminodes and loss of βIV-spectrin, ankyrin G, and neurofascin 186 or their collective diffusion into the paranode. Notably, these changes were in close proximity, yet distinct from APP-immunoreactive swollen axonal profiles, potentially representing a unique, newfound phenotype of axonal pathology in DAI. Since concussion in humans is non-fatal, the clinical relevance of these findings was determined through examination of post-mortem brain tissue from humans with higher levels of acute traumatic brain injury. Here, a similar loss of Nav1.6 and changes in NOR structures in brain white matter were observed as found in the swine model of concussion. Collectively, this widespread and progressive disruption of NaChs and NOR appears to be a form of sodium channelopathy, which may represent an important substrate underlying brain network dysfunction after concussion.

STAT1 Contributes to Microglial/Macrophage Inflammation and Neurological Dysfunction in a Mouse Model of Traumatic Brain Injury

Traumatic brain injury (TBI) triggers a plethora of inflammatory events in the brain that aggravate secondary injury and impede tissue repair. Resident microglia (Mi) and blood-borne infiltrating macrophages (MΦ) are major players of inflammatory responses in the post-TBI brain and possess high functional heterogeneity. However, the plasticity of these cells has yet to be exploited to develop therapies that can mitigate brain inflammation and improve the outcome after TBI. This study investigated the transcription factor STAT1 as a key determinant of proinflammatory Mi/MΦ responses and aimed to develop STAT1 as a novel therapeutic target for TBI using a controlled cortical impact model of TBI on adult male mice. TBI induced robust upregulation of STAT1 in the brain at the subacute injury stage, which occurred primarily in Mi/MΦ. Intraperitoneal administration of fludarabine, a selective STAT1 inhibitor, markedly alleviated proinflammatory Mi/MΦ responses and brain inflammation burden after TBI. Such phenotype-modulating effects of fludarabine on post-TBI Mi/MΦ were reproduced by tamoxifen-induced, selective KO of STAT1 in Mi/MΦ (STAT1 mKO). By propelling Mi/MΦ away from a detrimental proinflammatory phenotype, STAT1 mKO was sufficient to reduce long-term neurologic deficits and brain lesion size after TBI. Importantly, short-term fludarabine treatment after TBI elicited long-lasting improvement of TBI outcomes, but this effect was lost on STAT1 mKO mice. Together, our study provided the first line of evidence that STAT1 causatively determines the proinflammatory phenotype of brain Mi/MΦ after TBI. We also showed promising preclinical data supporting the use of fludarabine as a novel immunomodulating therapy to TBI.SIGNIFICANCE STATEMENT The functional phenotype of microglia and macrophages (Mi/MΦ) critically influences brain inflammation and the outcome after traumatic brain injury (TBI); however, no therapies have been developed to modulate Mi/MΦ functions to treat TBI. Here we report, for the first time, that the transcription factor STAT1 is a key mediator of proinflammatory Mi/MΦ responses in the post-TBI brain, the specific deletion of which ameliorates neuroinflammation and improves long-term functional recovery after TBI. We also show excellent efficacy of a selective STAT1 inhibitor fludarabine against TBI-induced functional deficits and brain injury using a mouse model, presenting STAT1 as a promising therapeutic target for TBI.

Unraveling axonal mechanisms of traumatic brain injury

Axonal swellings (AS) are one of the neuropathological hallmark of axonal injury in several disorders from trauma to neurodegeneration. Current evidence proposes a role of perturbed Ca²⁺ homeostasis in AS formation, involving impaired axonal transport and focal distension of the axons. Mechanisms of AS formation, in particular moments following injury, however, remain unknown. Here we show that AS form independently from intra-axonal Ca²⁺ changes, which are required primarily for the persistence of AS in time. We further show that the majority of axonal proteins undergoing de/phosphorylation immediately following injury belong to the cytoskeleton. This correlates with an increase in the distance of the actin/spectrin periodic rings and with microtubule tracks remodeling within AS. Observed cytoskeletal rearrangements support axonal transport without major interruptions. Our results demonstrate that the earliest axonal response to injury consists in physiological adaptations of axonal structure to preserve function rather than in immediate pathological events signaling axonal destruction.

Purkinje cell vulnerability induced by diffuse traumatic brain injury is linked to disruption of long-range neuronal circuits

Cerebellar dysfunction is commonly observed following traumatic brain injury (TBI). While direct impact to the cerebellum by TBI is rare, cerebellar pathology may be caused by indirect injury via cortico-cerebellar pathways. To address the hypothesis that degeneration of Purkinje cells (PCs), which constitute the sole output from the cerebellum, is linked to long-range axonal injury and demyelination, we used the central fluid percussion injury (cFPI) model of widespread traumatic axonal injury in mice. Compared to controls, TBI resulted in early PC loss accompanied by alterations in the size of pinceau synapses and levels of non-phosphorylated neurofilament in PCs. A combination of vDISCO tissue clearing technique and immunohistochemistry for vesicular glutamate transporter type 2 show that diffuse TBI decreased mossy and climbing fiber synapses on PCs. At 2 days post-injury, numerous axonal varicosities were found in the cerebellum supported by fractional anisotropy measurements using 9.4 T MRI. The disruption and demyelination of the cortico-cerebellar circuits was associated with poor performance of brain-injured mice in the beam-walk test. Despite a lack of direct input from the injury site to the cerebellum, these findings argue for novel long-range mechanisms causing Purkinje cell injury that likely contribute to cerebellar dysfunction after TBI.

Chronic Cortical Inflammation, Cognitive Impairment, and Immune Reactivity Associated with Diffuse Brain Injury Are Ameliorated by Forced Turnover of Microglia

Traumatic brain injury (TBI) is associated with an increased risk of cognitive, psychiatric, and neurodegenerative complications that may develop after injury. Increased microglial reactivity following TBI may underlie chronic neuroinflammation, neuropathology, and exaggerated responses to immune challenges. Therefore, the goal of this study was to force turnover of trauma-associated microglia that develop after diffuse TBI and determine whether this alleviated chronic inflammation, improved functional recovery and attenuated reduced immune reactivity to lipopolysaccharide (LPS) challenge. Male mice received a midline fluid percussion injury (mFPI) and 7 d later were subjected to a forced microglia turnover paradigm using CSF1R antagonism (PLX5622). At 30 d postinjury (dpi), cortical gene expression, dendritic complexity, myelin content, neuronal connectivity, cognition, and immune reactivity were assessed. Myriad neuropathology-related genes were increased 30 dpi in the cortex, and 90% of these gene changes were reversed by microglial turnover. Reduced neuronal connectivity was evident 30 dpi and these deficits were attenuated by microglial turnover. TBI-associated dendritic remodeling and myelin alterations, however, remained 30 dpi independent of microglial turnover. In assessments of functional recovery, increased depressive-like behavior, and cognitive impairment 30 dpi were ameliorated by microglia turnover. To investigate microglial priming and reactivity 30 dpi, mice were injected intraperitoneally with LPS. This immune challenge caused prolonged lethargy, sickness behavior, and microglial reactivity in the TBI mice. These extended complications with LPS in TBI mice were prevented by microglia turnover. Collectively, microglial turnover 7 dpi alleviated behavioral and cognitive impairments associated with microglial priming and immune reactivity 30 dpi.SIGNIFICANCE STATEMENT A striking feature of traumatic brain injury (TBI), even mild injuries, is that over 70% of individuals have long-term neuropsychiatric complications. Chronic inflammatory processes are implicated in the pathology of these complications and these issues can be exaggerated by immune challenge. Therefore, our goal was to force the turnover of microglia 7 d after TBI. This subacute 7 d postinjury (dpi) time point is a critical transitional period in the shift toward chronic inflammatory processes and microglia priming. This forced microglia turnover intervention in mice attenuated the deficits in behavior and cognition 30 dpi. Moreover, microglia priming and immune reactivity after TBI were also reduced with microglia turnover. Therefore, microglia represent therapeutic targets after TBI to reduce persistent neuroinflammation and improve recovery.

Acute axon damage and demyelination are mitigated by 4-aminopyridine (4-AP) therapy after experimental traumatic brain injury

Damage to long axons in white matter tracts is a major pathology in closed head traumatic brain injury (TBI). Acute TBI treatments are needed that protect against axon damage and promote recovery of axon function to prevent long term symptoms and neurodegeneration. Our prior characterization of axon damage and demyelination after TBI led us to examine repurposing of 4-aminopyridine (4-AP), an FDA-approved inhibitor of voltage-gated potassium (Kv) channels. 4-AP is currently indicated to provide symptomatic relief for patients with chronic stage multiple sclerosis, which involves axon damage and demyelination. We tested clinically relevant dosage of 4-AP as an acute treatment for experimental TBI and found multiple benefits in corpus callosum axons. This randomized, controlled pre-clinical study focused on the first week after TBI, when axons are particularly vulnerable. 4-AP treatment initiated one day post-injury dramatically reduced axon damage detected by intra-axonal fluorescence accumulations in Thy1-YFP mice of both sexes. Detailed electron microscopy in C57BL/6 mice showed that 4-AP reduced pathological features of mitochondrial swelling, cytoskeletal disruption, and demyelination at 7 days post-injury. Furthermore, 4-AP improved the molecular organization of axon nodal regions by restoring disrupted paranode domains and reducing Kv1.2 channel dispersion. 4-AP treatment did not resolve deficits in action potential conduction across the corpus callosum, based on ex vivo electrophysiological recordings at 7 days post-TBI. Thus, this first study of 4-AP effects on axon damage in the acute period demonstrates a significant decrease in multiple pathological hallmarks of axon damage after experimental TBI.

Immediate induction of varicosities by transverse compression but not uniaxial stretch in axon mechanosensation

Uniaxial stretch is believed to drive diffuse axonal injury (DAI) in mild traumatic brain injury (mTBI). Axonal varicosities are enlarged structures along axonal shafts and represent a hallmark feature of DAI. Here we report that axonal varicosities initiate in vivo immediately after head impact and are mainly induced by transverse compression but not uniaxial stretch. Vertical and lateral impacts to the mouse head induced axonal varicosities in distinct brain regions before any changes of microglial markers. Varicosities preferentially formed along axons perpendicular to impact direction. In cultured neurons, whereas 50% uniaxial strain was needed to rapidly induce axonal varicosities in a nanowrinkled stretch assay, physiologically-relevant transverse compression effectively induced axonal varicosities in a fluid puffing assay and can generate large but nonuniform deformation simulated by finite element analysis. Therefore, impact strength and direction may determine the threshold and spatial pattern of axonal varicosity initiation, respectively, partially resulting from intrinsic properties of axon mechanosensation.

Periostin aggravates the early phase of traumatic brain injury via the MAPK/ERK pathway

OBJECTIVES

Periostin is found associated with trauma severity and mortality following head injury. In this study, the role and mechanism of periostin in the traumatic brain injury were investigated.

METHODS

Male Sprague-Dawley adult rats underwent sham or TBI modeling. Vehicle or recombinant periostin was administered intracerebroventricularly at 30 minutes post-TBI, and U0126, a specific MEK1/2 inhibitor, was administered intravenously at 30 minutes pre-TBI. Garcia neuroscore, limb function and brain water content assessments, as well as TUNEL and Western blotting assays were performed to evaluate the status of the above rats at 24 hours post-TBI. Finally, the motor test and Morris water maze test were performed to measure the effects of periostin and U0126 in the late phase of TBI.

RESULTS

Periostin expression significantly increased 24 hours post-TBI. Treatment with R-periostin aggravated post-TBI neurobehavioral impairment, brain edema, induced apoptosis and raised the quantity of phospho-p38, phospho-JNK, phospho-ERK and MMP-9, and lowered the expression of ZO-1. However, U0126, a kind of inhibitor of MEK, lowered the quantities of phospho-ERK and MMP-9, raised the expression of ZO-1, and suppressed apoptosis. U0126 also ameliorated the neurobehavioral impairments and brain edema induced by R-periostin. Additionally, U0126 didn't inhibit the expression of periostin in the early phase of TBI model. IU0126 was also able to ameliorate the pathological conditions in the late phase of TBI.

DISCUSSION

Periostin could aggravate neurobehavioral impairments and brain edema following TBI, and was involved in the early phase of TBI via the MAPK/ERK pathway.

Chronic Histological Outcomes of Indirect Traumatic Optic Neuropathy in Adolescent Mice: Persistent Degeneration and Temporally Regulated Glial Responses

Injury to the optic nerve, termed, traumatic optic neuropathy (TON) is a known comorbidity of traumatic brain injury (TBI) and is now known to cause chronic and progressive retinal thinning up to 35 years after injury. Although animal models of TBI have described the presence of optic nerve degeneration and research exploring acute mechanisms is underway, few studies in humans or animals have examined chronic TON pathophysiology outside the retina. We used a closed-head weight-drop model of TBI/TON in 6-week-old male C57BL/6 mice. Mice were euthanized 7-, 14-, 30-, 90-, and 150-days post-injury (DPI) to assess histological changes in the visual system of the brain spanning a total of 12 regions. We show chronic elevation of FluoroJade-C, indicative of neurodegeneration, throughout the time course. Intriguingly, FJ-C staining revealed a bimodal distribution of mice indicating the possibility of subpopulations that may be more or less susceptible to injury outcomes. Additionally, we show that microglia and astrocytes react to optic nerve damage in both temporally and regionally different ways. Despite these differences, astrogliosis and microglial changes were alleviated between 14-30 DPI in all regions examined, perhaps indicating a potentially critical period for intervention/recovery that may determine chronic outcomes.

Oligodendrocyte progenitor proliferation is disinhibited following traumatic brain injury in leukemia inhibitory factor heterozygous mice

Traumatic brain injury (TBI) is a significant problem that affects over 800,000 children each year. As cell proliferation is disturbed by injury and required for normal brain development, we investigated how a pediatric closed head injury (CHI) would affect the progenitors of the subventricular zone (SVZ). Additionally, we evaluated the contribution of leukemia inhibitory factor (LIF) using germline LIF heterozygous mice (LIF Het), as LIF is an injury-induced cytokine, known to influence neurogenesis and gliogenesis. CHIs were performed on P20 LIF Het and wild-type (WT) mice. Ki-67 immunostaining and stereology revealed that cell proliferation increased ~250% in injured LIF Het mice compared to the 30% increase observed in injured WT mice at 48-hr post-CHI. OLIG2+ cell proliferation increased in the SVZ and white matter of LIF Het injured mice at 48-hr recovery. Using an 8-color flow cytometry panel, the proliferation of three distinct multipotential progenitors and early oligodendrocyte progenitor cell proliferation was significantly increased in LIF Het injured mice compared to WT injured mice. Supporting its cytostatic function, LIF decreased neurosphere progenitor and oligodendrocyte progenitor cell proliferation compared to controls. In highly enriched mouse oligodendrocyte progenitor cell cultures, LIF increased phospho-protein kinase B after 20 min and increased phospho-S6 ribosomal protein at 20 and 40 min of exposure, which are downstream targets of the mammalian target of rapamycin pathway. Altogether, our data provide new insights into the regulatory role of LIF in suppressing neural progenitor cell proliferation and, in particular, oligodendrocyte progenitor cell proliferation after a mild TBI.

Traumatic Brain Injury: Mechanisms of Glial Response

Traumatic brain injury (TBI) is a heterogeneous disorder that involves brain damage due to external forces. TBI is the main factor of death and morbidity in young males with a high incidence worldwide. TBI causes central nervous system (CNS) damage under a variety of mechanisms, including synaptic dysfunction, protein aggregation, mitochondrial dysfunction, oxidative stress, and neuroinflammation. Glial cells comprise most cells in CNS, which are mediators in the brain’s response to TBI. In the CNS are present astrocytes, microglia, oligodendrocytes, and polydendrocytes (NG2 cells). Astrocytes play critical roles in brain’s ion and water homeostasis, energy metabolism, blood-brain barrier, and immune response. In response to TBI, astrocytes change their morphology and protein expression. Microglia are the primary immune cells in the CNS with phagocytic activity. After TBI, microglia also change their morphology and release both pro and anti-inflammatory mediators. Oligodendrocytes are the myelin producers of the CNS, promoting axonal support. TBI causes oligodendrocyte apoptosis, demyelination, and axonal transport disruption. There are also various interactions between these glial cells and neurons in response to TBI that contribute to the pathophysiology of TBI. In this review, we summarize several glial hallmarks relevant for understanding the brain injury and neuronal damage under TBI conditions.

Utility of Pupillary Light Reflex Metrics as a Physiologic Biomarker for Adolescent Sport-Related Concussion

Importance

Concussion diagnosis remains clinical, without objective diagnostic tests available for adolescents. Known deficits in visual accommodation and autonomic function after concussion make the pupillary light reflex (PLR) a promising target as an objective physiological biomarker for concussion.

Objective

To determine the potential utility of PLR metrics as physiological biomarkers for concussion.

Design, Setting, and Participants

Prospective cohort of adolescent athletes between ages 12 and 18 years recruited between August 1, 2017, and December 31, 2018. The study took place at a specialty concussion program and private suburban high school and included healthy control individuals (n = 134) and athletes with a diagnosis of sport-related concussion (SRC) (n = 98). Analysis was completed June 30, 2020.

Exposures

Sports-related concussion and pupillometry assessments.

Main Outcomes and Measures

Pupillary light reflex metrics (maximum and minimum pupillary diameter, peak and average constriction/dilation velocity, percentage constriction, and time to 75% pupillary redilation [T75]).

Results

Pupillary light reflex metrics of 134 healthy control individuals and 98 athletes with concussion were obtained a median of 12.0 days following injury (interquartile range [IQR], 5.0-18.0 days). Eight of 9 metrics were significantly greater among athletes with concussion after Bonferroni correction (maximum pupil diameter: 4.83 mm vs 4.01 mm; difference, 0.82; 99.44% CI, 0.53-1.11; minimum pupil diameter: 2.96 mm vs 2.63 mm; difference, 0.33; 99.4% CI, 0.18-0.48; percentage constriction: 38.23% vs 33.66%; difference, 4.57; 99.4% CI, 2.60-6.55; average constriction velocity: 3.08 mm/s vs 2.50 mm/s; difference, 0.58; 99.4% CI, 0.36-0.81; peak constriction velocity: 4.88 mm/s vs 3.91 mm/s; difference, 0.97; 99.4% CI, 0.63-1.31; average dilation velocity, 1.32 mm/s vs 1.22 mm/s; difference, 0.10; 99.4% CI, 0.00-0.20; peak dilation velocity: 1.83 mm/s vs 1.64 mm/s; difference, 0.19; 99.4% CI, 0.07-0.32; and T75: 1.81 seconds vs 1.51 seconds; difference, 0.30; 0.10-0.51). In exploratory analyses, sex-based differences were observed, with girls with concussion exhibiting longer T75 (1.96 seconds vs 1.63 seconds; difference, 0.33; 99.4% CI, 0.02-0.65). Among healthy control individuals, diminished PLR metrics (eg, smaller maximum pupil size 3.81 mm vs 4.22 mm; difference, -0.41; 99.4% CI, -0.77 to 0.05) were observed after exercise.

Conclusions and Relevance

These findings suggest that enhancement of PLR metrics characterize acute adolescent concussion, while exercise produced smaller pupil sizes and overall slowing of PLR metrics, presumably associated with fatigue. Quantifiable measures of the PLR may serve in the future as objective physiologic biomarkers for concussion in the adolescent athlete.

Genetic inactivation of SARM1 axon degeneration pathway improves outcome trajectory after experimental traumatic brain injury based on pathological, radiological, and functional measures

Traumatic brain injury (TBI) causes chronic symptoms and increased risk of neurodegeneration. Axons in white matter tracts, such as the corpus callosum (CC), are critical components of neural circuits and particularly vulnerable to TBI. Treatments are needed to protect axons from traumatic injury and mitigate post-traumatic neurodegeneration. SARM1 protein is a central driver of axon degeneration through a conserved molecular pathway. Sarm1−/− mice with knockout (KO) of the Sarm1 gene enable genetic proof-of-concept testing of the SARM1 pathway as a therapeutic target. We evaluated Sarm1 deletion effects after TBI using a concussive model that causes traumatic axonal injury and progresses to CC atrophy at 10 weeks, indicating post-traumatic neurodegeneration. Sarm1 wild-type (WT) mice developed significant CC atrophy that was reduced in Sarm1 KO mice. Ultrastructural classification of pathology of individual axons, using electron microscopy, demonstrated that Sarm1 KO preserved more intact axons and reduced damaged or demyelinated axons. Longitudinal MRI studies in live mice identified significantly reduced CC volume after TBI in Sarm1 WT mice that was attenuated in Sarm1 KO mice. MR diffusion tensor imaging detected reduced fractional anisotropy in both genotypes while axial diffusivity remained higher in Sarm1 KO mice. Immunohistochemistry revealed significant attenuation of CC atrophy, myelin loss, and neuroinflammation in Sarm1 KO mice after TBI. Functionally, Sarm1 KO mice exhibited beneficial effects in motor learning and sleep behavior. Based on these findings, Sarm1 inactivation can protect axons and white matter tracts to improve translational outcomes associated with CC atrophy and post-traumatic neurodegeneration.

Hippocampal-Dependent Cognitive Dysfunction following Repeated Diffuse Rotational Brain Injury in Male and Female Mice

Cognitive dysfunction is a common, often long-term complaint following acquired traumatic brain injury (TBI). Cognitive deficits suggest dysfunction in hippocampal circuits. The goal of the studies described here is to phenotype in both male and female mice the hippocampal-dependent learning and memory deficits resulting from TBI sustained by the Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA) device—a model that delivers both a contact–concussion injury as well as unrestrained rotational head movement. Mice sustained either sham procedures or four injuries (0.7 J, 24-h intervals). Spatial learning and memory skills assessed in the Morris water maze (MWM) approximately 3 weeks following injuries were significantly impaired by brain injuries; however, slower swimming speeds and poor performance on visible platform trials suggest that measurement of cognitive impairment with this test is confounded by injury-induced motor and/or visual impairments. A separate experiment confirmed hippocampal-dependent cognitive deficits with trace fear conditioning (TFC), a behavioral test less dependent on motor and visual function. Male mice had greater injury-induced deficits on both the MWM and TFC tests than female mice. Pathologically, the injury was characterized by white matter damage as observed by silver staining and glial fibrillary acidic protein (astrogliosis) in the optic tracts, with milder damage seen in the corpus callosum, and fimbria and brainstem (cerebral peduncles) of some animals. No changes in the density of GABAergic parvalbumin-expressing cells in the hippocampus, amygdala, or parietal cortex were found. This experiment confirmed significant sexually dimorphic cognitive impairments following a repeated, diffuse brain injury.

Behavioral and Myelin-Related Abnormalities after Blast-Induced Mild Traumatic Brain Injury in Mice

In civilian and military settings, mild traumatic brain injury (mTBI) is a common consequence of impacts to the head, sudden blows to the body, and exposure to high-energy atmospheric shockwaves from blast. In some cases, mTBI from blast exposure results in long-term emotional and cognitive deficits and an elevated risk for certain neuropsychiatric diseases. Here, we tested the effects of mTBI on various forms of auditory-cued fear learning and other measures of cognition in male C57BL/6J mice after single or repeated blast exposure (blast TBI; bTBI). bTBI produced an abnormality in the temporal organization of cue-induced freezing behavior in a conditioned trace fear test. Spatial working memory, evaluated by the Y-maze task performance, was also deleteriously affected by bTBI. Reverse-transcription quantitative real-time polymerase chain reaction (RT-qPCR) analysis for glial markers indicated an alteration in the expression of myelin-related genes in the hippocampus and corpus callosum 1-8 weeks after bTBI. Immunohistochemical and ultrastructural analyses detected bTBI-related myelin and axonal damage in the hippocampus and corpus callosum. Together, these data suggest a possible link between blast-induced mTBI, myelin/axonal injury, and cognitive dysfunction.

Role of innate inflammation in traumatic brain injury

Traumatic brain injury is one of the leading causes of morbidity and mortality throughout the world. Its increasing incidence, in addition to its fundamental role in the development of neurodegenerative disease, proves especially concerning. Despite extensive preclinical and clinical studies, researchers have yet to identify a safe and effective neuroprotective strategy. Following brain trauma, secondary injury from molecular, metabolic, and cellular changes causes progressive cerebral tissue damage. Chronic neuroinflammation following traumatic brain injuries is a key player in the development of secondary injury. Targeting this phenomenon for development of effective neuroprotective therapies holds promise. This strategy warrants a concrete understanding of complex neuroinflammatory mechanisms. In this review, we discuss pathophysiological mechanisms such as the innate immune response, glial activation, blood-brain barrier disruption, activation of immune mediators, as well as biological markers of traumatic brain injury. We then review existing and emerging pharmacological therapies that target neuroinflammation to improve functional outcome.

Repetitive Blast Exposure Produces White Matter Axon Damage without Subsequent Myelin Remodeling: In Vivo Analysis of Brain Injury Using Fluorescent Reporter Mice

The potential effects of blast exposure on the brain health of military personnel have raised concerns and led to increased surveillance of blast exposures. Neuroimaging studies have reported white matter abnormalities in brains of service members with a history of blast exposure. However, blast effects on white matter microstructure remain poorly understood. As a novel approach to screen for white matter effects, transgenic mice that express fluorescent reporters to sensitively detect axon damage and myelin remodeling were exposed to simulated repetitive blasts (once/day on 5 consecutive days). Axons were visualized using Thy1-YFP-16 reporter mice that express yellow fluorescent protein (YFP) in a broad spectrum of neurons. Swelling along damaged axons forms varicosities that fill with YFP. The frequency and size of axonal varicosities were significantly increased in the corpus callosum (CC) and cingulum at 3 days after the final blast exposure, versus in sham procedures. CC immunolabeling for reactive astrocyte and microglial markers was also significantly increased. NG2CreER;mTmG mice were given tamoxifen (TMX) on days 2 and 3 after the final blast to induce fluorescent labeling of newly synthesized myelin membranes, indicating plasticity and/or repair. Myelin synthesis was not altered in the CC over the intervening 4 or 8 weeks after repetitive blast exposure. These experiments show the advantages of transgenic reporter mice for analysis of white matter injury that detects subtle, diffuse axon damage and the dynamic nature of myelin sheaths. These results show that repetitive low-level blast exposures produce infrequent but significant axon damage along with neuroinflammation in white matter.

Neuron-Oligodendrocyte Interactions in the Structure and Integrity of Axons

The myelination of axons by oligodendrocytes is a highly complex cell-to-cell interaction. Oligodendrocytes and axons have a reciprocal signaling relationship in which oligodendrocytes receive cues from axons that direct their myelination, and oligodendrocytes subsequently shape axonal structure and conduction. Oligodendrocytes are necessary for the maturation of excitatory domains on the axon including nodes of Ranvier, help buffer potassium, and support neuronal energy metabolism. Disruption of the oligodendrocyte-axon unit in traumatic injuries, Alzheimer's disease and demyelinating diseases such as multiple sclerosis results in axonal dysfunction and can culminate in neurodegeneration. In this review, we discuss the mechanisms by which demyelination and loss of oligodendrocytes compromise axons. We highlight the intra-axonal cascades initiated by demyelination that can result in irreversible axonal damage. Both the restoration of oligodendrocyte myelination or neuroprotective therapies targeting these intra-axonal cascades are likely to have therapeutic potential in disorders in which oligodendrocyte support of axons is disrupted.

Interleukin-4 improves white matter integrity and functional recovery after murine traumatic brain injury via oligodendroglial PPARγ

Long-term neurological recovery after severe traumatic brain injury (TBI) is strongly linked to the repair and functional restoration of injured white matter. Emerging evidence suggests that the anti-inflammatory cytokine interleukin-4 (IL-4) plays an important role in promoting white matter integrity after cerebral ischemic injury. Here, we report that delayed intranasal delivery of nanoparticle-packed IL-4 boosted sensorimotor neurological recovery in a murine model of controlled cortical impact, as assessed by a battery of neurobehavioral tests for up to five weeks. Post-injury IL-4 treatment failed to reduce macroscopic brain lesions after TBI, but preserved the structural and functional integrity of white matter, at least in part through oligodendrogenesis. IL-4 directly facilitated the differentiation of oligodendrocyte progenitor cells (OPCs) into mature myelin-producing oligodendrocytes in primary cultures, an effect that was attenuated by selective PPARγ inhibition. IL-4 treatment after TBI in vivo also failed to stimulate oligodendrogenesis or improve white matter integrity in OPC-specific PPARγ conditional knockout (cKO) mice. Accordingly, IL-4-afforded improvements in sensorimotor neurological recovery after TBI were markedly impaired in the PPARγ cKO mice compared to wildtype controls. These results support IL-4 as a potential novel neurorestorative therapy to improve white matter functionality and mitigate the long-term neurological consequences of TBI.

Acute treatment with TrkB agonist LM22A-4 confers neuroprotection and preserves myelin integrity in a mouse model of pediatric traumatic brain injury

Young children have a high risk of sustaining a traumatic brain injury (TBI), which can have debilitating life-long consequences. Importantly, the young brain shows particular vulnerability to injury, likely attributed to ongoing maturation of the myelinating nervous system at the time of insult. Here, we examined the effect of acute treatment with the partial tropomyosin receptor kinase B (TrkB) agonist, LM22A-4, on pathological and neurobehavioral outcomes after pediatric TBI, with the hypothesis that targeting TrkB would minimize tissue damage and support functional recovery. We focused on myelinated tracts-the corpus callosum and external capsules-based on recent evidence that TrkB activation potentiates oligodendrocyte remyelination. Male mice at postnatal day 21 received an experimental TBI or sham surgery. Acutely post-injury, extensive cell death, a robust glial response and disruption of compact myelin were evident in the injured brain. TBI or sham mice then received intranasal saline vehicle or LM22A-4 for 14 days. Behavior testing was performed from 4 weeks post-injury, and brains were collected at 5 weeks for histology. TBI mice showed hyperactivity, reduced anxiety-like behavior, and social memory impairments. LM22A-4 ameliorated the abnormal anxiolytic phenotype but had no effect on social memory deficits. Use of spectral confocal reflectance microscopy detected persistent myelin fragmentation in the external capsule of TBI mice at 5 weeks post-injury, which was accompanied by regionally distinct deficits in oligodendrocyte progenitor cells and post-mitotic oligodendrocytes, as well as chronic reactive gliosis and atrophy of the corpus callosum and injured external capsule. LM22A-4 treatment ameliorated myelin deficits in the perilesional external capsule, as well as tissue volume loss and the extent of reactive gliosis. However, there was no effect of this TrkB agonist on oligodendroglial populations detected at 5 weeks post-injury. Collectively, our results demonstrate that targeting TrkB immediately after TBI during early life confers neuroprotection and preserves myelin integrity, and this was associated with some improved neurobehavioral outcomes as the pediatric injured brain matures.

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

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

Rapid and Reversible Development of Axonal Varicosities: A New Form of Neural Plasticity

Axonal varicosities are enlarged, heterogeneous structures along axonal shafts, profoundly affecting axonal conduction and synaptic transmission. They represent a key pathological feature believed to develop via slow accumulation of axonal damage that occurs during irreversible degeneration, for example in mild traumatic brain injury (mTBI), Alzheimer's and Parkinson's diseases, and multiple sclerosis. Here this review first discusses recent in vitro results showing that axonal varicosities can be rapidly and reversibly induced by mechanical stress in cultured primary neurons from the central nervous system (CNS). This notion is further supported by in vivo studies revealing the induction of axonal varicosities across various brain regions in different mTBI mouse models, as a prominent feature of axonal pathology. Limited progress in understanding intrinsic and extrinsic regulatory mechanisms of axonal varicosity induction and development is further highlighted. Rapid and reversible formation of axonal varicosities likely plays a key role in CNS neuron mechanosensation and is a new form of neural plasticity. Future investigation in this emerging research field may reveal how to reverse axonal injury, contributing to the development of new strategies for treating brain injuries and related neurodegenerative diseases.

The genetic basis of inter-individual variation in recovery from traumatic brain injury

Traumatic brain injury (TBI) is one of the leading causes of death among young people, and is increasingly prevalent in the aging population. Survivors of TBI face a spectrum of outcomes from short-term non-incapacitating injuries to long-lasting serious and deteriorating sequelae. TBI is a highly complex condition to treat; many variables can account for the observed heterogeneity in patient outcome. The limited success of neuroprotection strategies in the clinic has led to a new emphasis on neurorestorative approaches. In TBI, it is well recognized clinically that patients with similar lesions, age, and health status often display differences in recovery of function after injury. Despite this heterogeneity of outcomes in TBI, restorative treatment has remained generic. There is now a new emphasis on developing a personalized medicine approach in TBI, and this will require an improved understanding of how genetics impacts on long-term outcomes. Studies in animal model systems indicate clearly that the genetic background plays a role in determining the extent of recovery following an insult. A candidate gene approach in human studies has led to the identification of factors that can influence recovery. Here we review studies of the genetic basis for individual differences in functional recovery in the CNS in animals and man. The application of in vitro modeling with human cells and organoid cultures, along with whole-organism studies, will help to identify genes and networks that account for individual variation in recovery from brain injury, and will point the way towards the development of new therapeutic approaches.

Mechanosensation in traumatic brain injury

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

MEK inhibitor trametinib attenuates neuroinflammation and cognitive deficits following traumatic brain injury in mice

Microglia-mediated neuroinflammation is one of the hallmark pathological features following traumatic brain injury (TBI) that contributes to aggravated brain damage and cognitive deficits. These pathologies require novel effective treatments to improve prognosis. Trametinib, a mitogen-activated protein kinase inhibitor approved by the Food and Drug Administration in treating various malignant tumors, has been shown to exert anti-inflammatory effects. The present study demonstrated that TBI mice treated with trametinib exhibited improved cognitive function. Trametinib treatment rescued oligodendrocytes and decreased infiltrating microglial density in the TBI area. Furthermore, this study revealed that ameliorated lipopolysaccharides (LPS) induced inflammatory reaction in microglial cells. Besides, trametinib attenuated inflammation factors expression during the early stages of TBI. In addition, trametinib inhibited LPS-induced microglial chemotactic activity. In conclusion, the results indicate that trametinib efficiently suppresses microglia-induced neuroinflammation and improves cognitive function of TBI mice, providing a potential therapy strategy for TBI patients.

Experimentally Induced Sepsis Causes Extensive Hypomyelination in the Prefrontal Cortex and Hippocampus in Neonatal Rats

Neonatal sepsis is associated with cognitive deficit in the later life. Axonal myelination plays a pivotal role in neurotransmission and formation of learning and memory. This study aimed to explore if systemic lipopolysaccharide (LPS) injection would induce hypomyelination in the prefrontal cortex and hippocampus in developing septic neonatal rats. Sprague-Dawley rats (1-day old) were injected with LPS (1 mg/kg) intraperitoneally. By electron microscopy, axonal hypomyelination was evident in the subcortical white matter and hippocampus. The expression of myelin proteins including CNPase, MBP, PLP and MAG was downregulated in both areas of the brain at 7, 14 and 28 days after LPS injection. The frequency of MBP and PLP-positive oligodendrocyte was significantly reduced using in situ hybridization in the cerebral cortex and hippocampus at the corresponding time points after LPS injection, whereas the expression of NG2 and PDGFRα was noticeably increased. In tandem with this was reduction of Olig1 and Olig2 expressions which are involved in differentiation/maturation of OPCs. Expression of NFL, NFM, and NFH was significantly downregulated, indicating that axon development was disrupted after LPS injection. Morris Water Maze behavioral test, Open field test, Rotarod test, and Pole test were used to evaluate neurological behaviors of 28 days rats. The rats in the LPS group showed the impairment of motor coordination, balance, memory, and learning ability and represented bradykinesia and anxiety-like behavior. The present results suggest that following systemic LPS injection, differentiation/maturation of OPCs was affected which may be attributed to the inhibition of transcription factors Olig1 and Olig2 expression resulting in impairment to axonal development. It is suggested that this would ultimately lead to axonal hypomyelination in the prefrontal cortex and hippocampus, which may be associated with neurological deficits in later life.

Bone morphogenetic proteins: New insights into their roles and mechanisms in CNS development, pathology and repair

Bone morphogenetic proteins (BMPs) are a highly conserved and diverse family of proteins that play essential roles in various stages of development including the formation and patterning of the central nervous system (CNS). Bioavailability and function of BMPs are regulated by input from a plethora of transcription factors and signaling pathways. Intriguingly, recent literature has uncovered novel roles for BMPs in regulating homeostatic and pathological responses in the adult CNS. Basal levels of BMP ligands and receptors are widely expressed in the adult brain and spinal cord with differential expression patterns across CNS regions, cell types and subcellular locations. Recent evidence indicates that several BMP isoforms are transiently or chronically upregulated in the aged or pathological CNS. Genetic knockout and pharmacological studies have elucidated that BMPs regulate several aspects of CNS injury and repair including cell survival and differentiation, reactive astrogliosis and glial scar formation, axon regeneration, and myelin preservation and repair. Several BMP isoforms can be upregulated in the injured or diseased CNS simultaneously yet exert complementary or opposing effects on the endogenous cell responses after injury. Emerging studies also show that dysregulation of BMPs is associated with various CNS pathologies. Interestingly, modulation of BMPs can lead to beneficial or detrimental effects on CNS injury and repair mechanisms in a ligand, temporally or spatially specific manner, which reflect the complexity of BMP signaling. Given the significance of BMPs in neurodevelopment, a better understanding of their role in the context of injury may provide new therapeutic targets for the pathologic CNS. This review will provide a timely overview on the foundation and recent advancements in knowledge regarding the role and mechanisms of BMP signaling in the developing and adult CNS, and their implications in pathological responses and repair processes after injury or diseases.

Repetitive Head Trauma Induces Chronic Traumatic Encephalopathy by Multiple Mechanisms

Exposure to repetitive neurotrauma increases lifetime risk for developing progressive cognitive deficits, neurobehavioral abnormalities, and chronic traumatic encephalopathy (CTE). CTE is a tau protein neurodegenerative disease first identified in boxers and recently described in athletes participating in other contact sports (notably American football, ice hockey, rugby, and wrestling) and in military veterans with blast exposure. Currently, CTE can only be diagnosed by neuropathological examination of the brain after death. The defining diagnostic lesion of CTE consists of patchy perivascular accumulations of hyperphosphorylated tau protein that localize in the sulcal depths of the cerebral cortex. Neuronal abnormalities, axonopathy, neurovascular dysfunction, and neuroinflammation are triggered by repetitive head impacts (RHIs) and likely act as catalysts for CTE pathogenesis and progression. However, the specific mechanisms that link RHI to CTE are unknown. This review will explore two important areas of CTE pathobiology. First, we will review what is known about the biomechanical properties of RHI that initiate CTE-related pathologies. Second, we will provide an overview of key features of CTE neuropathology and how these contribute to abnormal tau hyperphosphorylation, accumulation, and spread.

Time course and diagnostic utility of NfL, tau, GFAP, and UCH-L1 in subacute and chronic TBI

OBJECTIVE

To determine whether neurofilament light (NfL), glial fibrillary acidic protein (GFAP), tau, and ubiquitin C-terminal hydrolase-L1 (UCH-L1) measured in serum relate to traumatic brain injury (TBI) diagnosis, injury severity, brain volume, and diffusion tensor imaging (DTI) measures of traumatic axonal injury (TAI) in patients with TBI.

METHODS

Patients with TBI (n = 162) and controls (n = 68) were prospectively enrolled between 2011 and 2019. Patients with TBI also underwent serum, functional outcome, and imaging assessments at 30 (n = 30), 90 (n = 48), and 180 (n = 59) days, and 1 (n = 84), 2 (n = 57), 3 (n = 46), 4 (n = 38), and 5 (n = 29) years after injury.

RESULTS

At enrollment, patients with TBI had increased serum NfL compared to controls (p < 0.0001). Serum NfL decreased over the course of 5 years but remained significantly elevated compared to controls. Serum NfL at 30 days distinguished patients with mild, moderate, and severe TBI from controls with an area under the receiver-operating characteristic curve (AUROC) of 0.84, 0.92, and 0.92, respectively. At enrollment, serum GFAP was elevated in patients with TBI compared to controls (p < 0.001). GFAP showed a biphasic release in serum, with levels decreasing during the first 6 months of injury but increasing over the subsequent study visits. The highest AUROC for GFAP was measured at 30 days, distinguishing patients with moderate and severe TBI from controls (both 0.89). Serum tau and UCH-L1 showed weak associations with TBI severity and neuroimaging measures. Longitudinally, serum NfL was the only biomarker that was associated with the likely rate of MRI brain atrophy and DTI measures of progression of TAI.

CONCLUSIONS

Serum NfL shows greater diagnostic and prognostic utility than GFAP, tau, and UCH-L1 for subacute and chronic TBI.

CLASSIFICATION OF EVIDENCE

This study provides Class III evidence that serum NfL distinguishes patients with mild TBI from healthy controls.