Traumatic brain injury (TBI) patients often exhibit slowed information processing speed that can underlie diverse symptoms. Processing speed depends on neural circuit function at synapses, in the soma, and along axons. Long axons in white matter (WM) tracts are particularly vulnerable to TBI. We hypothesized that disrupted axon-myelin interactions that slow or block action potential conduction in WM tracts may contribute to slowed processing speed after TBI. Concussive TBI in male/female mice was used to produce traumatic axonal injury in the corpus callosum (CC), similar to WM pathology in human TBI cases. Compound action potential velocity was slowed along myelinated axons at 3 d after TBI with partial recovery by 2 weeks, suggesting early demyelination followed by remyelination. Ultrastructurally, dispersed demyelinated axons and disorganized myelin attachment to axons at paranodes were apparent within CC regions exhibiting traumatic axonal injury. Action potential conduction is exquisitely sensitive to paranode abnormalities. Molecular identification of paranodes and nodes of Ranvier detected asymmetrical paranode pairs and abnormal heminodes after TBI. Fluorescent labeling of oligodendrocyte progenitors in NG2CreER;mTmG mice showed increased synthesis of new membranes extended along axons to paranodes, indicating remyelination after TBI. At later times after TBI, an overall loss of conducting axons was observed at 6 weeks followed by CC atrophy at 8 weeks. These studies identify a progression of both myelinated axon conduction deficits and axon-myelin pathology in the CC, implicating WM injury in impaired information processing at early and late phases after TBI. Furthermore, the intervening recovery reveals a potential therapeutic window.SIGNIFICANCE STATEMENT Traumatic brain injury (TBI) is a major global health concern. Across the spectrum of TBI severities, impaired information processing can contribute to diverse functional deficits that underlie persistent symptoms. We used experimental TBI to exploit technical advantages in mice while modeling traumatic axonal injury in white matter tracts, which is a key pathological feature of human TBI. A combination of approaches revealed slowed and failed signal conduction along with damage to the structure and molecular composition of myelinated axons in the white matter after TBI. An early regenerative response was not sustained yet reveals a potential time window for intervention. These insights into white matter abnormalities underlying axon conduction deficits can inform strategies to improve treatment options for TBI patients.

Experimental brain trauma activates quiescent neural stem cells (NSCs) to increase neuronal progenitor cell proliferation in the adult rodent brain. Previous studies have shown focal brain contusion in the form of a unilateral controlled cortical impact (CCI) stimulates NSCs to bilaterally increase neurogenesis in the adult hippocampus.

The role of epigenetic regulators in the control of adult neurogenesis is largely undefined. We show that the histone demethylase enzyme Kdm5b (Jarid1b) negatively regulates neurogenesis from adult subventricular zone (SVZ) neural stem cells (NSCs) in culture. shRNA-mediated depletion of Kdm5b in proliferating adult NSCs decreased proliferation rates and reduced neurosphere formation in culture. When transferred to differentiation culture conditions, Kdm5b-depleted adult NSCs migrated from neurospheres with increased velocity. Whole-genome expression screening revealed widespread transcriptional changes with Kdm5b depletion, notably the up-regulation of reelin (Reln), the inhibition of steroid biosynthetic pathway component genes and the activation of genes with intracellular transport functions in cultured adult NSCs. Kdm5b depletion increased extracellular reelin concentration in the culture medium and increased phosphorylation of the downstream reelin signaling target Disabled-1 (Dab1). Sequestration of extracellular reelin with CR-50 reelin-blocking antibodies suppressed the increase in migratory velocity of Kdm5b-depleted adult NSCs. Chromatin immunoprecipitation revealed that Kdm5b is present at the proximal promoter of Reln, and H3K4me3 methylation was increased at this locus with Kdm5b depletion in differentiating adult NSCs. Combined the data suggest Kdm5b negatively regulates neurogenesis and represses Reln in neural stem cells from the adult SVZ.

Leukemia/lymphoma-related factor (LRF), a zinc-finger transcription factor encoded by Zbtb7a, is a protooncogene that regulates differentiation in diverse cell lineages, and in the CNS, its function is relatively unexplored. This study is the first to examine the role of LRF in CNS pathology. We first examined LRF expression in a murine viral model of spinal cord demyelination with clinically relevant lesion characteristics. LRF was rarely expressed in oligodendrocyte progenitors (OP) yet, was detected in nuclei of the majority of oligodendrocytes in healthy adult CNS and during remyelination. Plp/CreERT :Zbtb7afl/fl mice were then used with cuprizone demyelination to determine the effect of LRF knockdown on oligodendrocyte repopulation and remyelination. Cuprizone was given for 6 weeks to demyelinate the corpus callosum. Tamoxifen was administered at 4, 5, or 6 weeks after the start of cuprizone. Tamoxifen-induced knockdown of LRF impaired remyelination during 3 or 6-week recovery periods after cuprizone. LRF knockdown earlier within the oligodendrocyte lineage using NG2CreERT :Zbtb7afl/fl mice reduced myelination after 6 weeks of cuprizone. LRF knockdown from either the Plp/CreERT line or the NG2CreERT line did not significantly change OP or oligodendrocyte populations. In vitro promoter assays demonstrated the potential for LRF to regulate transcription of myelin-related genes and the notch target Hes5, which has been implicated in control of myelin formation and repair. In summary, in the oligodendrocyte lineage, LRF is expressed mainly in oligodendrocytes but is not required for oligodendrocyte repopulation of demyelinated lesions. Furthermore, LRF can modulate the extent of remyelination, potentially by contributing to interactions regulating transcription.

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.

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.

Noninvasive detection of mild traumatic brain injury (mTBI) is important for evaluating acute through chronic effects of head injuries, particularly after repetitive impacts. To better detect abnormalities from mTBI, we performed longitudinal studies (baseline, 3, 6, and 42 days) using magnetic resonance diffusion tensor imaging (DTI) and diffusion kurtosis imaging (DKI) in adult mice after repetitive mTBI (r-mTBI; daily × 5) or sham procedure. This r-mTBI produced righting reflex delay and was first characterized in the corpus callosum to demonstrate low levels of axon damage, astrogliosis, and microglial activation, without microhemorrhages. High-resolution DTI-DKI was then combined with post-imaging pathological validation along with behavioral assessments targeted for the impact regions. In the corpus callosum, only DTI fractional anisotropy at 42 days showed significant change post-injury. Conversely, cortical regions under the impact site (M1-M2, anterior cingulate) had reduced axial diffusivity (AD) at all time points with a corresponding increase in axial kurtosis (Ka) at 6 days. Post-imaging neuropathology showed microglial activation in both the corpus callosum and cortex at 42 days after r-mTBI. Increased cortical microglial activation correlated with decreased cortical AD after r-mTBI (r = -0.853; n = 5). Using Thy1-YFP-16 mice to fluorescently label neuronal cell bodies and processes revealed low levels of axon damage in the cortex after r-mTBI. Finally, r-mTBI produced social deficits consistent with the function of this anterior cingulate region of cortex. Overall, vulnerability of cortical regions is demonstrated after mild repetitive injury, with underlying differences of DTI and DKI, microglial activation, and behavioral deficits.

Mild traumatic brain injury (mTBI) is highly prevalent but lacks both research tools with adequate sensitivity to detect cellular alterations that accompany mild injury and pre-clinical models that are able to robustly mimic hallmark features of human TBI. To address these related challenges, high-resolution diffusion tensor MRI (DTI) analysis was performed in a model of mild TBI in the ferret - a species that, unlike rodents, share with humans a gyrencephalic cortex and high white matter (WM) volume. A set of DTI image analysis tools were optimized and implemented to explore key features of DTI alterations in ex vivo adult male ferret brains (n = 26), evaluated 1 day to 16 weeks after mild controlled cortical impact (CCI). Using template-based ROI analysis, lesion overlay mapping and DTI-driven tensor-based morphometry (D-TBM) significant differences in DTI and morphometric values were found and their dependence on time after injury evaluated. These observations were also qualitatively compared with immunohistochemistry staining of neurons, astrocytes, and microglia in the same tissue. Focal DTI abnormalities including reduced cortical diffusivity were apparent in 12/13 injured brains with greatest lesion extent found acutely following CCI by ROI overlay maps and reduced WM FA in the chronic period was observed near to the CCI site (ANOVA for FA in focal WM: time after CCI p = 0.046, brain hemisphere p = 0.0012) often in regions without other prominent MRI abnormalities. Global abnormalities were also detected, especially for WM regions, which demonstrated reduced diffusivity (ANOVA for Trace: time after CCI p = 0.007) and atrophy that appeared to become more extensive and bilateral with longer time after injury (ANOVA for D-TBM Log of the Jacobian values: time after CCI p = 0.007). The findings of this study extend earlier work in rodent models especially by evaluation of focal WM abnormalities that are not influenced by partial volume effects in the ferret. There is also substantial overlap between DTI and morphometric findings in this model and those from human studies of mTBI implying that the combination of DTI tools with a human-similar model system can provide an advantageous and informative approach for mTBI research.

Although rodent TBI studies provide valuable information regarding the effects of injury and recovery, an animal model with neuroanatomical characteristics closer to humans may provide a more meaningful basis for clinical translation. The ferret has a high white/gray matter ratio, gyrencephalic neocortex, and ventral hippocampal location. Furthermore, ferrets are amenable to behavioral training, have a body size compatible with pre-clinical MRI, and are cost-effective.

White matter damage is an important consequence of traumatic brain injury (TBI) in humans. Unlike rodents, ferrets have a substantial amount of white matter and a gyrencephalic brain; therefore, they may represent an ideal small mammal model to study human-pertinent consequences of TBI. Here we report immunohistochemical and behavioral results after a controlled cortical impact (CCI) injury to the sensorimotor cortex of adult male ferrets. We assessed inflammation in the neocortex and white matter, and behavior at 1 day post injury and 1, 4, and 16 weeks post injury (WPI). CCI in the ferret produced inflammation that originated in the neocortex near the site of the injury and progressed deep into the white matter with time. The density of microglia and astrocytes increased in the neocortex near the injury, peaking at 4WPI and remaining elevated at 16WPI. Microglial morphology in the neocortex was significantly altered in the first 4 weeks, but showed a return toward normal at 16 weeks. Clusters of microglial cells in the white matter persisted until 16WPI. We assessed motor and cognitive behavior using the open field, novel object recognition, T-maze, and gait tests. A transient deficit in memory occurred at 4WPI, with a reduction of rearing and motor ability at 12 and 16WPI. Behavioral impairments coincide with features of the inflammatory changes in the neocortex revealed by immunohistochemistry. The ferret represents an important animal model to explore ongoing damage in the white matter and cerebral cortex after TBI.

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.

Traumatic brain injury (TBI) is often more complicated than a single head injury. An extreme example of this point may be military service members who experience a spectrum of exposures over a prolonged period under stressful conditions. Understanding the effects of complex exposures can inform evaluation and care to prevent persistent symptoms. We designed a longitudinal series of non-invasive procedures in adult mice to evaluate the effects of prolonged mild stress and head injury exposures. We assessed anxiety, depression, and sleep-wake dysfunction as symptoms that impact long-term outcomes after mild TBI. Unpredictable chronic mild stress (UCMS) was generated from a varied sequence of environmental stressors distributed within each of 21 days. Subsequently, mice received a mild blast combined with closed-head mild TBI on 5 days at 24-h intervals. In males and females, UCMS induced anxiety without depressive behavior. A major finding was reproducible sleep-wake dysfunction through 6- to 12-month time points in male mice that received UCMS with repetitive blast plus TBI events, or surprisingly after just UCMS alone. Specifically, male mice exhibited hypersomnia with increased sleep during the active/dark phase and fragmentation of longer wake bouts. Sleep-wake dysfunction was not found with TBI events alone, and hypersomnia was not found in females under any conditions. These results identify prolonged stress and sex differences as important considerations for sleep-wake dysfunction. Furthermore, this reproducible hypersomnia with impaired wakefulness is similar to the excessive daytime sleepiness reported in patients, including patients with TBI, which warrants further clinical screening, care, and treatment development.

During the acute time period following traumatic brain injury (TBI), noninvasive brain imaging tools such as magnetic resonance imaging (MRI) can provide important information about the clinical and pathological features of the injury and may help predict long-term outcomes. In addition to standard imaging approaches, several quantitative MRI techniques including relaxometry and diffusion MRI have been identified as promising reporters of cellular alterations after TBI and may provide greater sensitivity and specificity for identifying brain abnormalities especially in mild TBI. However, for these imaging tools to be useful, it is crucial to define their relationship with the neurophysiological response to brain injury. Recently, a model of controlled cortical impact (CCI) has been developed in the ferret which has many advantages compared with rodent models (e.g., gyrencephalic cortex and high white matter volume). The objective of this study was to evaluate quantitative MRI metrics in the ferret CCI model, including T2 values and diffusion tensor imaging (DTI) metrics, during the acute time period. Longitudinal quantitative comparisons of in vivo MRI and DTI metrics were evaluated to identify abnormalities and characterize their spatial patterns in the ferret brain. Ex vivo MRI and DTI maps were then compared with histological staining for glial and neuronal abnormalities. The main findings of this article describe T2, diffusivity, and anisotropy markers of tissue change during the acute time period following mild TBI, and ex vivo analyses suggest that MRI and DTI markers are sensitive to subtle cellular alterations in this model. This was confirmed by comparison with immunohistochemistry, also showing altered markers in regions of MRI and DTI change.