Ultrastructure of Diaschisis Lesions after Traumatic Brain Injury

Metabolic Diaschisis in Mild Traumatic Brain Injury

Neurophysiological diaschisis presents in traumatic brain injury (TBI) as functional impairment distant to the lesion site caused by axonal neuroexcitation and deafferentation. Diaschisis studies in TBI models have evaluated acute phase functional and microstructural changes. Here, in vivo biochemical changes and cerebral blood flow (CBF) dynamics following TBI are studied with magnetic resonance. Behavioral assessments, magnetic resonance spectroscopy (MRS), and CBF measurements on rats followed cortical impact TBI. Data were acquired pre-TBI and 1-3 h, 2-days, 7-days, and 14-days post-TBI. MRS was performed on the ipsilateral and contralateral sides in the cortex, striatum, and thalamus. Metabolites measured by MRS included N-acetyl aspartate (NAA), aspartate (Asp), lactate (Lac), glutathione (GSH), and glutamate (Glu). Lesion volume expanded for 2 days post-TBI and then decreased. Ipsilateral CBF dropped acutely versus baseline on both sides (-62% ipsilateral, -48% contralateral, p < 0.05) but then recovered in cortex, with similar changes in ipsilateral striatum. Metabolic changes versus baseline included increased Asp (+640% by Day 7 post-TBI, p < 0.05) and Lac (+140% on Day 2 post-TBI, p < 0.05) in ipsilateral cortex, while GSH (-67% acutely, p < 0.05) and NAA decreased (-50% on Day 2, p < 0.05). In contralateral cortex Lac decreased (-73% acutely, p < 0.05). Analysis of variance showed significance for Side (p < 0.05), Time after TBI (p < 0.05), and interactions (p < 0.005) for Asp, GSH, Lac, and NAA. Transient decreases of GSH (-30%, p < 0.05, acutely) and NAA (-23% on Day 2, p < 0.05) occurred in ipsilateral striatum with reduced GSH (-42%, p < 0.005, acutely) in the contralateral striatum. GSH was decreased in ipsilateral thalamus (-59% ipsilateral on Day 2, p < 0.05). Delayed increases of total choline were seen in the contralateral thalamus were noted as well (+21% on Day 7 post-TBI, p < 0.05). Both CBF and neurometabolite concentration changes occurred remotely from the TBI site, both ipsilaterally and contralaterally. Decreased Lac levels on the contralateral cortex following TBI may be indicative of reduced anaerobic metabolism during the acute phase. The timing and locations of the changes suggest excitatory and inhibitory signaling processes are affecting post-TBI metabolic fluctuations.

Exploring Potential Mechanisms Accounting for Iron Accumulation in the Central Nervous System of Patients with Alzheimer's Disease

Elevated levels of iron occur in both cortical and subcortical regions of the CNS in patients with Alzheimer's disease. This accumulation is present early in the disease process as well as in more advanced stages. The factors potentially accounting for this increase are numerous, including: (1) Cells increase their uptake of iron and reduce their export of iron, as iron becomes sequestered (trapped within the lysosome, bound to amyloid β or tau, etc.); (2) metabolic disturbances, such as insulin resistance and mitochondrial dysfunction, disrupt cellular iron homeostasis; (3) inflammation, glutamate excitotoxicity, or other pathological disturbances (loss of neuronal interconnections, soluble amyloid β, etc.) trigger cells to acquire iron; and (4) following neurodegeneration, iron becomes trapped within microglia. Some of these mechanisms are also present in other neurological disorders and can also begin early in the disease course, indicating that iron accumulation is a relatively common event in neurological conditions. In response to pathogenic processes, the directed cellular efforts that contribute to iron buildup reflect the importance of correcting a functional iron deficiency to support essential biochemical processes. In other words, cells prioritize correcting an insufficiency of available iron while tolerating deposited iron. An analysis of the mechanisms accounting for iron accumulation in Alzheimer's disease, and in other relevant neurological conditions, is put forward.

Spautin-1 Protects Against Mild TBI-Induced Anxiety-Like Behavior in Mice via Immunologically Silent Apoptosis

Anxiety is reportedly one of the most common mental changes after traumatic brain injury (TBI). Perineuronal nets (PNNs) produced by astrocytes in the lateral hypothalamus (LHA) that surround gamma-aminobutyric acid-ergic (GABAergic) neurons have been associated with anxiety. The potent anti-tumor effects of Spautin-1, a novel autophagy inhibitor, have been documented in malignant melanoma; moreover, the inhibition of autophagy is reported to mitigate anxiety disorders. However, little is known about the ability of spautin-1 to alleviate anxiety. In this study, we sought to investigate whether spautin-1 could alleviate anxiety-like behaviors post-TBI by reducing the loss of PNNs in the LHA. A mild TBI was established in mice through Feeney's weight-drop model. Then, Spautin-1 (20 mmol/2 μl) was immediately administered into the left lateral ventricle. Behavioral and pathological changes were assessed at 24 h, 7 days, 30 days, 31 days and 32 days after TBI by the neurological severity scores (NSS), open field test (OFT), elevated plus-maze (EPM) test, western blot, immunofluorescence assays and electron microscopy. Spautin-1 significantly reversed TBI-induced decreased time in the central zone during OFT and in the open-arm during the EPM test. Spautin-1 also increased PNNs around GABAergic neurons indicated by WFA- plus GAD2- positive A2-type astrocytes and attenuated M1-type microglia in the LHA 32 days after TBI compared to TBI alone. Moreover, compared to mice that only underwent TBI, spautin-1 downregulated autophagic vacuoles, abnormal organelles, the expression of Beclin 1, USP13, phospho-TBK1, and phospho-IRF3 and upregulated the levels of cleaved caspase-3, -7 and -9, but failed to increase TUNEL-positive cells in the LHA at 24 h. Spautin-1 alleviated anxiety-like behavior in mice exposed to mild TBI; this protective mechanism may be associated with decreased PNNs loss around GABAergic neurons via immunologically silent apoptosis induced by the caspase cascade.

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

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

An AGS-associated mutation in ADAR1 catalytic domain results in early-onset and MDA5-dependent encephalopathy with IFN pathway activation in the brain

BACKGROUND

Aicardi-Goutières syndrome (AGS) is a severe neurodegenerative disease with clinical features of early-onset encephalopathy and progressive loss of intellectual abilities and motor control. Gene mutations in seven protein-coding genes have been found to be associated with AGS. However, the causative role of these mutations in the early-onset neuropathogenesis has not been demonstrated in animal models, and the mechanism of neurodegeneration of AGS remains ambiguous.

METHODS

Via CRISPR/Cas-9 technology, we established a mutant mouse model in which a genetic mutation found in AGS patients at the ADAR1 coding gene (Adar) loci was introduced into the mouse genome. A mouse model carrying double gene mutations encoding ADAR1 and MDA-5 was prepared using a breeding strategy. Phenotype, gene expression, RNA sequencing, innate immune pathway activation, and pathologic studies including RNA in situ hybridization (ISH) and immunohistochemistry were used for characterization of the mouse models to determine potential disease mechanisms.

RESULTS

We established a mouse model bearing a mutation in the catalytic domain of ADAR1, the D1113H mutation found in AGS patients. With this mouse model, we demonstrated a causative role of this mutation for the early-onset brain injuries in AGS and determined the signaling pathway underlying the neuropathogenesis. First, this mutation altered the RNA editing profile in neural transcripts and led to robust IFN-stimulated gene (ISG) expression in the brain. By ISH, the brains of mutant mice showed an unusual, multifocal increased expression of ISGs that was cell-type dependent. Early-onset astrocytosis and microgliosis and later stage calcification in the deep white matter areas were observed in the mutant mice. Brain ISG activation and neuroglial reaction were completely prevented in the Adar D1113H mutant mice by blocking RNA sensing through deletion of the cytosolic RNA receptor MDA-5.

CONCLUSIONS

The Adar D1113H mutation in the ADAR1 catalytic domain results in early-onset and MDA5-dependent encephalopathy with IFN pathway activation in the mouse brain.

Somatostatin interneurons exhibit enhanced functional output and resilience to axotomy after mild traumatic brain injury

Mild traumatic brain injury (mTBI) gives rise to a remarkable breadth of pathobiological consequences, principal among which are traumatic axonal injury and perturbation of the functional integrity of neuronal networks that may arise secondary to the elimination of the presynaptic contribution of axotomized neurons. Because there exists a vast diversity of neocortical neuron subtypes, it is imperative to elucidate the relative vulnerability to axotomy among different subtypes. Toward this end, we exploited SOM-IRES-Cre mice to investigate the consequences of the central fluid percussion model of mTBI on the microanatomical integrity and the functional efficacy of the somatostatin (SOM) interneuron population, one of the principal subtypes of neocortical interneuron. We found that the SOM population is resilient to axotomy, representing only 10% of the global burden of inhibitory interneuron axotomy, a result congruous with past work demonstrating that parvalbumin (PV) interneurons bear most of the burden of interneuron axotomy. However, the intact structure of SOM interneurons after injury did not translate to normal cellular function. One day after mTBI, the SOM population is more intrinsically excitable and demonstrates enhanced synaptic efficacy upon post-synaptic layer 5 pyramidal neurons as measured by optogenetics, yet the global evoked inhibitory tone within layer 5 is stable. Simultaneously, there exists a significant increase in the frequency of miniature inhibitory post-synaptic currents within layer 5 pyramidal neurons. These results are consistent with a scheme in which 1 day after mTBI, SOM interneurons are stimulated to compensate for the release from inhibition of layer 5 pyramidal neurons secondary to the disproportionate axotomy of PV interneurons. The enhancement of SOM interneuron intrinsic excitability and synaptic efficacy may represent the initial phase of a dynamic process of attempted autoregulation of neocortical network homeostasis secondary to mTBI.

Programming axonal mitochondrial maintenance and bioenergetics in neurodegeneration and regeneration

Mitochondria generate ATP essential for neuronal growth, function, and regeneration. Due to their polarized structures, neurons face exceptional challenges to deliver mitochondria to and maintain energy homeostasis throughout long axons and terminal branches where energy is in high demand. Chronic mitochondrial dysfunction accompanied by bioenergetic failure is a pathological hallmark of major neurodegenerative diseases. Brain injury triggers acute mitochondrial damage and a local energy crisis that accelerates neuron death. Thus, mitochondrial maintenance defects and axonal energy deficits emerge as central problems in neurodegenerative disorders and brain injury. Recent studies have started to uncover the intrinsic mechanisms that neurons adopt to maintain (or reprogram) axonal mitochondrial density and integrity, and their bioenergetic capacity, upon sensing energy stress. In this review, we discuss recent advances in how neurons maintain a healthy pool of axonal mitochondria, as well as potential therapeutic strategies that target bioenergetic restoration to power neuronal survival, function, and regeneration.

Time Course of Remote Neuropathology Following Diffuse Traumatic Brain Injury in the Male Rat

Traumatic brain injury (TBI) can affect different regions throughout the brain. Regions near the site of impact are the most vulnerable to injury. However, damage to distal regions occurs. We investigated progressive neuropathology in the dorsal hippocampus (near the impact) and cerebellum (distal to the impact) after diffuse TBI. Adult male rats were subjected to midline fluid percussion injury or sham injury. Brain tissue was stained by the amino cupric silver stain. Neuropathology was quantified in sub-regions of the dorsal hippocampus at 1, 7, and 28 days post-injury (DPI) and coronal cerebellar sections at 1, 2, and 7 DPI. The highest observed neuropathology in the dentate gyrus occurred at 7 DPI which attenuated by 28 DPI, whereas the highest observed neuropathology was at 1 DPI in the CA3 region. There was no significant neuropathology in the CA1 region at any time point. Neuropathology was increased at 7 DPI in the cerebellum compared to shams and stripes of pathology were observed in the molecular layer perpendicular to the cerebellar cortical surface. Together these data show that diffuse TBI can result in neuropathology across the brain. By describing the time course of pathology in response to TBI, it is possible to build the temporal profile of disease progression.

Single-Nucleus RNA Sequencing Reveals that Decorin Expression in the Amygdala Regulates Perineuronal Nets Expression and Fear Conditioning Response after Traumatic Brain Injury

Traumatic brain injury (TBI) is a risk factor for posttraumatic stress disorder (PTSD). Augmented fear is a defining characteristic of PTSD, and the amygdala is considered the main brain region to process fear. The mechanism by which the amygdala is involved in fear conditioning after TBI is still unclear. Using single-nucleus RNA sequencing (snRNA-seq), transcriptional changes in cells in the amygdala after TBI are investigated. In total, 72 328 nuclei are obtained from the sham and TBI groups. 7 cell types, and analysis of differentially expressed genes (DEGs) reveals widespread transcriptional changes in each cell type after TBI are identified. In in vivo experiments, it is demonstrated that Decorin (Dcn) expression in the excitatory neurons of the amygdala significantly increased after TBI, and Dcn knockout in the amygdala mitigates TBI-associated fear conditioning. Of note, this effect is caused by a Dcn-mediated decrease in the expression of perineuronal nets (PNNs), which affect the glutamate-γ-aminobutyric acid balance in the amygdala. Finally, the results suggest that Dcn functions by interacting with collagen VI α3 (Col6a3). Consequently, the findings reveal transcriptional changes in different cell types of the amygdala after TBI and provide direct evidence that Dcn relieves fear conditioning by regulating PNNs.

Pretreatment Normal WM Magnetization Transfer Ratio Predicts Risk of Radiation Necrosis in Patients with Medulloblastoma

BACKGROUND AND PURPOSE

Radiation necrosis, for which abnormal WM enhancement is a hallmark, is an uncommon complication of craniospinal irradiation in children with medulloblastoma. The magnetization transfer ratio measures macromolecular content, dominated by myelin in the WM. We investigated whether the pretreatment supratentorial (nonsurgical) WM magnetization transfer ratio could predict patients at risk for radiation necrosis after radiation therapy for medulloblastoma.

MATERIALS AND METHODS

Ninety-five eligible patients with medulloblastoma (41% female; mean age, 11.0 [SD, 5.4]  years) had baseline balanced steady-state free precession MR imaging before proton or photon radiation therapy. Associations among baseline supratentorial magnetization transfer ratio, radiation necrosis (spontaneously resolving/improving parenchymal enhancement within the radiation field)³, age, and the presence of visible brain metastases were explored by logistic regression and parametric/nonparametric techniques as appropriate.

RESULTS

Twenty-three of 95 (24.2%) children (44% female; mean age, 10.7 [SD, 6.7] years) developed radiation necrosis after radiation therapy (19 infratentorial, 1 supratentorial, 3 both). The mean pretreatment supratentorial WM magnetization transfer ratio was significantly lower in these children (43.18 versus 43.50, P = .03). There was no association between the supratentorial WM magnetization transfer ratio and age, sex, risk/treatment stratum, or the presence of visible brain metastases.

CONCLUSIONS

A lower baseline supratentorial WM magnetization transfer ratio may indicate underlying structural WM susceptibility to radiation necrosis and may identify children at risk for developing radiation necrosis after craniospinal irradiation for medulloblastoma.

Changes in Brain Matrix Glycan Sulfation Associate With Reactive Gliosis and Motor Coordination in Mice With Head Trauma

Perineuronal nets (PNNs) are extracellular matrix (ECM) structures that enmesh and regulate neurocircuits involved in motor and sensory function. Maladaptive changes to the composition and/or abundance of PNNs have been implicated in preclinical models of neuroinflammation and neurocircuit destabilization. The central nervous system (CNS) is limited in its capacity to repair and reorganize neural networks following traumatic brain injury (TBI) and little is known about mechanisms of ECM repair in the adult brain after TBI. In this study, adult male C57BL/6 mice were subjected to a TBI via a controlled cortical impact (CCI) to the right motor and somatosensory cortices. At 7 days following CCI, histological analysis revealed a loss of Wisteria floribunda agglutinin (WFA) positive PNN matrices in the ipsilateral cortex. PNNs are comprised of chondroitin sulfate (CS) and dermatan sulfate (DS)-glycosaminoglycans (GAGs), the composition of which are known to influence neuronal integrity and repair. Using an innovative liquid chromatography tandem mass spectrometry (LC-MS/MS) method, we analyzed the relative abundance of six specific CS/DS-GAG isomers (Δ4S-, Δ6S-, Δ4S6S-, Δ2S6S-, Δ0S-CS, and Δ2S4S-DS) from fixed-brain sections after CCI injury. We report a significant shift in CS/DS-GAG sulfation patterns within the rostro-caudal extent of the injury site from mice exposed to CCI at 7 days, but not at 1 day, post-CCI. In the ipsilateral thalamus, the appearance of WFA⁺ puncta occurred in tandem with gliosis at 7 days post-CCI, but weakly colocalized with markers of gliosis. Thalamic WFA⁺ puncta showed moderate colocalization with neuronal ubiquitin C-terminal hydrolase L1 (UCHL1), a clinical biomarker for TBI injury. A shift in CS/DS-GAG sulfation was also present in the thalamus including an increase of 6S-CS, which is a specific isomer that associates with the presence of glial scarring. Upregulation of the 6S-CS-specific sulfotransferase (CHST3) gene expression was accompanied by reactive gliosis in both the ipsilateral cortex and thalamus. Moreover, changes in 6S-CS extracted from the thalamus positively correlated with deficits in motor coordination after CCI. Collectively, these data argue that CCI alters CS/DS-GAG sulfation in association with the spatiotemporal progression of neurorepair. Therapeutic interventions targeting restoration of CS/DS-GAG sulfation patterns may improve outcomes from TBI.

Microglia as hackers of the matrix: sculpting synapses and the extracellular space

Microglia shape the synaptic environment in health and disease, but synapses do not exist in a vacuum. Instead, pre- and postsynaptic terminals are surrounded by extracellular matrix (ECM), which together with glia comprise the four elements of the contemporary tetrapartite synapse model. While research in this area is still just beginning, accumulating evidence points toward a novel role for microglia in regulating the ECM during normal brain homeostasis, and such processes may, in turn, become dysfunctional in disease. As it relates to synapses, microglia are reported to modify the perisynaptic matrix, which is the diffuse matrix that surrounds dendritic and axonal terminals, as well as perineuronal nets (PNNs), specialized reticular formations of compact ECM that enwrap neuronal subsets and stabilize proximal synapses. The interconnected relationship between synapses and the ECM in which they are embedded suggests that alterations in one structure necessarily affect the dynamics of the other, and microglia may need to sculpt the matrix to modify the synapses within. Here, we provide an overview of the microglial regulation of synapses, perisynaptic matrix, and PNNs, propose candidate mechanisms by which these structures may be modified, and present the implications of such modifications in normal brain homeostasis and in disease.

Abcc8 (Sulfonylurea Receptor-1) Impact on Brain Atrophy after Traumatic Brain Injury Varies by Sex

Females have been understudied in pre-clinical and clinical traumatic brain injury (TBI), despite distinct biology and worse clinical outcomes versus males. Sulfonylurea receptor 1 (SUR1) inhibition has shown promising results in predominantly male TBI. A phase II trial is ongoing. We investigated whether SUR1 inhibition effects on contusional TBI differ by sex given that this may inform clinical trial design and/or interpretation. We studied the moderating effects of sex on post-injury brain tissue loss in 142 male and female ATP-binding cassette transporter subfamily C member 8 (Abcc8) wild-type, heterozygote, and knockout mice (12-15 weeks). Monkey fibroblast-like cells and mouse brain endothelium-derived cells were used for in vitro studies. Mice were injured with controlled cortical impact and euthanized 21 days post-injury to assess contusion, brain, and hemisphere volumes (vs. genotype- and sex-matched naïves). Abcc8 knockout mice had smaller contusion volumes (p = 0.012) and larger normalized contralateral (right) hemisphere volumes (nRHV; p = 0.03) after injury versus wild type. This was moderated by sex: Contusions were smaller (p = 0.020), nRHV was higher (p = 0.001), and there was less global atrophy (p = 0.003) in male, but not female, knockout versus wild-type mice after TBI. Less atrophy occurred in males for each copy of Abcc8 lost (p = 0.023-0.002, all outcomes). In vitro, sex-determining region Y (SRY) stimulated Abcc8 promoter activity and increased Abcc8 expression. Loss of Abcc8 strongly protected against post-traumatic cerebral atrophy in male, but not female, mice. This may partly be mediated by SRY on the Y-chromosome. Sex differences may have important implications for ongoing and future trials of SUR1 blockade.

Aicardi-Goutières syndrome-associated mutation at ADAR1 gene locus activates innate immune response in mouse brain

Background

Aicardi-Goutières syndrome (AGS) is a severe infant or juvenile-onset autoimmune disease characterized by inflammatory encephalopathy with an elevated type 1 interferon-stimulated gene (ISG) expression signature in the brain. Mutations in seven different protein-coding genes, all linked to DNA/RNA metabolism or sensing, have been identified in AGS patients, but none of them has been demonstrated to activate the IFN pathway in the brain of an animal. The molecular mechanism of inflammatory encephalopathy in AGS has not been well defined. Adenosine Deaminase Acting on RNA 1 (ADAR1) is one of the AGS-associated genes. It carries out A-to-I RNA editing that converts adenosine to inosine at double-stranded RNA regions. Whether an AGS-associated mutation in ADAR1 activates the IFN pathway and causes autoimmune pathogenesis in the brain is yet to be determined.

Methods

Mutations in the ADAR1 gene found in AGS patients were introduced into the mouse genome via CRISPR/Cas9 technology. Molecular activities of the specific p.K999N mutation were investigated by measuring the RNA editing levels in brain mRNA substrates of ADAR1 through RNA sequencing analysis. IFN pathway activation in the brain was assessed by measuring ISG expression at the mRNA and protein level through real-time RT-PCR and Luminex assays, respectively. The locations in the brain and neural cell types that express ISGs were determined by RNA in situ hybridization (ISH). Potential AGS-related brain morphologic changes were assessed with immunohistological analysis. Von Kossa and Luxol Fast Blue staining was performed on brain tissue to assess calcification and myelin, respectively.

Results

Mice bearing the ADAR1 p.K999N were viable though smaller than wild type sibs. RNA sequencing analysis of neuron-specific RNA substrates revealed altered RNA editing activities of the mutant ADAR1 protein. Mutant mice exhibited dramatically elevated levels of multiple ISGs within the brain. RNA ISH of brain sections showed selective activation of ISG expression in neurons and microglia in a patchy pattern. ISG-15 mRNA was upregulated in ADAR1 mutant brain neurons whereas CXCL10 mRNA was elevated in adjacent astroglia. No calcification or gliosis was detected in the mutant brain.

Conclusions

We demonstrated that an AGS-associated mutation in ADAR1, specifically the p.K999N mutation, activates the IFN pathway in the mouse brain. The ADAR1 p.K999N mutant mouse replicates aspects of the brain interferonopathy of AGS. Neurons and microglia express different ISGs. Basal ganglia calcification and leukodystrophy seen in AGS patients were not observed in K999N mutant mice, indicating that development of the full clinical phenotype may need an additional stimulus besides AGS mutations. This mutant mouse presents a robust tool for the investigation of AGS and neuroinflammatory diseases including the modeling of potential “second hits” that enable severe phenotypes of clinically variable diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02217-9.

Role of the Dopaminergic System in the Striatum and Its Association With Functional Recovery or Rehabilitation After Brain Injury

Disabilities are estimated to occur in approximately 2% of survivors of traumatic brain injury (TBI) worldwide, and disability may persist even decades after brain injury. Facilitation or modulation of functional recovery is an important goal of rehabilitation in all patients who survive severe TBI. However, this recovery tends to vary among patients because it is affected by the biological and physical characteristics of the patients; the types, doses, and application regimens of the drugs used; and clinical indications. In clinical practice, diverse dopaminergic drugs with various dosing and application procedures are used for TBI. Previous studies have shown that dopamine (DA) neurotransmission is disrupted following moderate to severe TBI and have reported beneficial effects of drugs that affect the dopaminergic system. However, the mechanisms of action of dopaminergic drugs have not been completely clarified, partly because dopaminergic receptor activation can lead to restoration of the pathway of the corticobasal ganglia after injury in brain structures with high densities of these receptors. This review aims to provide an overview of the functionality of the dopaminergic system in the striatum and its roles in functional recovery or rehabilitation after TBI.

Progressive Neurodegeneration Across Chronic Stages of Severe Traumatic Brain Injury

OBJECTIVE

To examine the trajectory of structural gray matter changes across 2 chronic periods of recovery in individuals who have sustained severe traumatic brain injury (TBI), adding to the growing literature indicating that neurodegenerative processes occur in the months to years postinjury.

PARTICIPANTS

Patients who experienced posttraumatic amnesia of 1 hour or more, and/or scored 12 or less on the Glasgow Coma Scale at the emergency department or the scene of the accident, and/or had positive brain imaging findings were recruited while receiving inpatient care, resulting in 51 patients with severe TBI.

METHODS

Secondary analyses of gray matter changes across approximately 5 months, 1 year, and 2.5 years postinjury were undertaken, using an automated segmentation protocol with improved accuracy in populations with morphological anomalies. We compared patients and matched controls on regions implicated in poorer long-term clinical outcome (accumbens, amygdala, brainstem, hippocampus, thalamus). To model brain-wide patterns of change, we then conducted an exploratory principal component analysis (PCA) on the linear slopes of all regional volumes across the 3 time points. Finally, we assessed nonlinear trends across earlier (5 months-1 year) versus later (1-2.5 years) time-windows with PCA to compare degeneration rates across time. Chronic degeneration was predicted cortically and subcortically brain-wide, and within specific regions of interest.

RESULTS

(1) From 5 months to 1 year, patients showed significant degeneration in the accumbens, and marginal degeneration in the amygdala, brainstem, thalamus, and the left hippocampus when examined unilaterally, compared with controls. (2) PCA components representing subcortical and temporal regions, and regions from the basal ganglia, significantly differed from controls in the first time-window. (3) Progression occurred at the same rate across both time-windows, suggesting neither escalation nor attenuation of degeneration across time.

CONCLUSION

Localized yet progressive decline emphasizes the necessity of developing interventions to offset degeneration and improve long-term functioning.

Traumatic Brain Injury: Ultrastructural Features in Neuronal Ferroptosis, Glial Cell Activation and Polarization, and Blood-Brain Barrier Breakdown

The secondary injury process after traumatic brain injury (TBI) results in motor dysfunction, cognitive and emotional impairment, and poor outcomes. These injury cascades include excitotoxic injury, mitochondrial dysfunction, oxidative stress, ion imbalance, inflammation, and increased vascular permeability. Electron microscopy is an irreplaceable tool to understand the complex pathogenesis of TBI as the secondary injury is usually accompanied by a series of pathologic changes at the ultra-micro level of the brain cells. These changes include the ultrastructural changes in different parts of the neurons (cell body, axon, and synapses), glial cells, and blood–brain barrier, etc. In view of the current difficulties in the treatment of TBI, identifying the changes in subcellular structures can help us better understand the complex pathologic cascade reactions after TBI and improve clinical diagnosis and treatment. The purpose of this review is to summarize and discuss the ultrastructural changes related to neurons (e.g., condensed mitochondrial membrane in ferroptosis), glial cells, and blood–brain barrier in the existing reports of TBI, to deepen the in-depth study of TBI pathomechanism, hoping to provide a future research direction of pathogenesis and treatment, with the ultimate aim of improving the prognosis of patients with TBI.

Drebrin controls scar formation and astrocyte reactivity upon traumatic brain injury by regulating membrane trafficking

The brain of mammals lacks a significant ability to regenerate neurons and is thus particularly vulnerable. To protect the brain from injury and disease, damage control by astrocytes through astrogliosis and scar formation is vital. Here, we show that brain injury in mice triggers an immediate upregulation of the actin-binding protein Drebrin (DBN) in astrocytes, which is essential for scar formation and maintenance of astrocyte reactivity. In turn, DBN loss leads to defective astrocyte scar formation and excessive neurodegeneration following brain injuries. At the cellular level, we show that DBN switches actin homeostasis from ARP2/3-dependent arrays to microtubule-compatible scaffolds, facilitating the formation of RAB8-positive membrane tubules. This injury-specific RAB8 membrane compartment serves as hub for the trafficking of surface proteins involved in astrogliosis and adhesion mediators, such as β1-integrin. Our work shows that DBN-mediated membrane trafficking in astrocytes is an important neuroprotective mechanism following traumatic brain injury in mice.

Ultrastructural Dendritic Changes Underlying Diaschisis After Capsular Infarct

Diaschisis has been described as functional depression distant to the lesion. A variety of neuroscientific approaches have been used to investigate the mechanisms underlying diaschisis. However, few studies have examined the pathological changes in diaschisis at ultrastructural level. Here, we used a rat model of capsular infarct that consistently produces diaschisis in ipsilesional and contralesional motor and sensory cortices. To verify the occurrence of diaschisis and monitor time-dependent changes in diaschisis, we performed longitudinal 2-deoxy-2-[18F]-fluoro-d-glucose microPET (FDG-microPET) study. We also used light and electron microscopy to identify the microscopic and ultrastructural changes at the diaschisis site at 7, 14, and 21 days after capsular infarct modeling (CIM). FDG-microPET showed the occurrence of diaschisis after CIM. Light microscopic examinations revealed no significant histopathological changes at the diaschisis site except a mild degree of reactive astrogliosis. However, electron microscopy revealed swollen, hydropic degeneration of axial dendrites and axodendritic synapses, although the neuronal soma (including nuclear chromatin and cytoplasmic organelles) and myelinated axons were relatively well preserved up to 21 days after injury. Furthermore, number of axodendritic synapses was significantly decreased after CIM. These data indicate that a circumscribed subcortical white-matter lesion produces ultrastructural pathological changes related to the pathogenesis of diaschisis.

Deficiency of urokinase-type plasminogen activator and its receptor affects social behavior and increases seizure susceptibility

Extracellular proteolysis initiated by the binding of urokinase-type plasminogen activator (uPA) to its receptor (uPAR) regulates the development of inhibitory neuronal circuits in the cerebral cortex and tissue remodeling after epileptogenic brain injury. To study the function of different components of the uPA-uPAR system on behavior and epileptogenesis, and to complement our previous studies on naïve and injured mice deficient in the uPA-encoding gene Plau or the uPAR-encoding gene Plaur, we analyzed the behavioral phenotype, seizure susceptibility, and perineuronal nets surrounding parvalbumin-positive inhibitory interneurons in Plau and Plaur (double knockout dKO) mice. In a climbing test, dKO mice showed reduced interest towards the environment as compared with Wt mice (p < 0.01). In a social approach test, however, dKO mice spent more time than Wt mice exploring the compartment containing a stranger mouse than the empty compartment (p < 0.05). Moreover, in a social interaction test, dKO mice exhibited increased contact time (p < 0.01). Compared with Wt mice, the dKO mice also had a longer single contact duration (p < 0.001) with the stranger mouse. In the elevated plus-maze, grooming, and marble burying tests, the anxiety level of dKO mice did not differ from that of Wt mice. Rearing time in an exploratory activity test, and spatial learning and memory in the Morris swim navigation task were also comparable between dKO and Wt mice. In the pentylenetetrazol (PTZ) seizure-susceptibility test, dKO mice had a shorter latency to the first epileptiform spike (p = 0.0001) and a greater total number of spikes (p < 0.001) than Wt mice. The dKO genotype did not affect the number of cortical perineuronal nets. Our findings indicate that Plau/Plaur-deficiency leads to a more social phenotype toward other mice with diminished interest in the surrounding environment, and increased seizure susceptibility.

Lithium Improves Dopamine Neurotransmission and Increases Dopaminergic Protein Abundance in the Striatum after Traumatic Brain Injury

Experimental models of traumatic brain injury (TBI) recapitulate secondary injury sequela and cognitive dysfunction reported in patients afflicted with a TBI. Impairments in neurotransmission are reported in multiple brain regions in the weeks following experimental TBI and may contribute to behavioral dysfunction. Formation of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex is an important mechanism for neurotransmitter exocytosis. We previously showed that lithium treatment attenuated hippocampal decreases in α-synuclein and VAMP2, enhanced SNARE complex formation, and improved cognitive performance after TBI. However, the effect of TBI on striatal SNARE complex formation is not known. We hypothesized lithium treatment would attenuate TBI-induced impairments in evoked dopamine release and increase the abundance of synaptic proteins associated with dopamine neurotransmission. The current study evaluated the effect of lithium (1 mmol/kg/day) administration on striatal evoked dopamine neurotransmission, SNARE complex formation, and proposed actions of lithium, including inhibition of GSK3β, assessment of synaptic marker protein abundance, and synaptic proteins important for dopamine synthesis and transport following controlled cortical impact (CCI). Sprague-Dawley rats were subjected to CCI or sham injury and treated daily with lithium chloride or vehicle for 7 days post-injury. We provide novel evidence that CCI reduces SNARE protein and SNARE complex abundance in the striatum at 1 week post-injury. Lithium administration improved evoked dopamine release and increased the abundance of α-synuclein, D2 receptor, and phosphorylated tyrosine hydroxylase in striatal synaptosomes post-injury. These findings show that lithium treatment attenuated dopamine neurotransmission deficits and increased the abundance of synaptic proteins important for dopamine signaling after TBI.

Longitudinal Microstructural Changes in Traumatic Brain Injury in Rats: A Diffusional Kurtosis Imaging, Histology, and Behavior Study

BACKGROUND AND PURPOSE

Traumatic brain injury is a major public health problem worldwide. Accurately evaluating the brain microstructural changes in traumatic brain injury is crucial for the treatment and prognosis assessment. This study aimed to assess the longitudinal brain microstructural changes in traumatic brain injury in the rat using diffusional kurtosis imaging.

MATERIALS AND METHODS

Diffusional kurtosis imaging was performed in a group of 5 rats at preinjury and 3, 14, and 28 days after traumatic brain injury. The diffusional kurtosis imaging parameters were measured in the bilateral cortex, hippocampus, and corpus callosum. Another 4 groups of 5 rats were used in brain immunohistochemistry analysis of neuron (neuron-specific nuclear protein [NeuN]), astroglia (glial fibrillary acidic protein [GFAP]), microglia (ionized calcium binding adaptor molecule 1 [Iba-1]), and myelin (myelin basic protein [MBP]) in the same area as the diffusional kurtosis imaging parameter measurements. Furthermore, 2 groups of 6 rats underwent a Morris water maze test at 28 days after traumatic brain injury. The diffusional kurtosis imaging parameters, immunohistochemistry results, and Morris water maze test results were compared longitudinally or between traumatic brain injury and control groups.

RESULTS

Compared with baseline, traumatic brain injury in the rat showed higher mean kurtosis and mean diffusivity values in the ipsilateral perilesional cortex and hippocampus and lower fractional anisotropy values in the corpus callosum (P < .05). The traumatic brain injury group showed higher staining of GFAP and Iba-1 and lower immunohistochemistry staining of NeuN and MBP in all ipsilateral ROIs (P < .05). There was no significant difference in the contralateral ROIs in diffusional kurtosis imaging parameters or immunohistochemistry results. The Morris water maze test revealed lower platform crossing times in the probe test (P < .05).

CONCLUSIONS

Our study indicated that there were longitudinal changes in diffusional kurtosis imaging parameters, accompanied by multiple pathologic changes at different time points following traumatic brain injury, and that mean kurtosis is more sensitive to detect microstructural changes, especially in gray matter, than mean diffusivity and fractional anisotropy.

Minocycline Attenuates High Mobility Group Box 1 Translocation, Microglial Activation, and Thalamic Neurodegeneration after Traumatic Brain Injury in Post-Natal Day 17 Rats

In response to cell injury, the danger signal high mobility group box-1 (HMGB) is released, activating macrophages by binding pattern recognition receptors. We investigated the role of the anti-inflammatory drug minocycline in attenuating HMGB1 translocation, microglial activation, and neuronal injury in a rat model of pediatric traumatic brain injury (TBI). Post-natal day 17 Sprague-Dawley rats underwent moderate-severe controlled cortical impact (CCI). Animals were randomized to treatment with minocycline (90 mg/kg, intraperitoneally) or vehicle (saline) at 10 min and 20 h after injury. Shams received anesthesia and craniotomy. We analyzed HMGB1 translocation (protein fractionation and Western blotting), microglial activation (Iba-1 immunohistochemistry), neuronal death (Fluoro-Jade-B [FJB] immunofluorescence), and neuronal cell counts (unbiased stereology). Behavioral assessments included motor and Morris-water maze testing. Nuclear to cytosolic translocation of HMGB1 in the injured brain was attenuated in minocycline versus vehicle-treated rats at 24 h (p < 0.001). Treatment with minocycline reduced microglial activation in the ipsilateral cortex, hippocampus, and thalamus (p < 0.05 vs. vehicle, all regions); attenuated neurodegeneration (FJB-positive neurons) at seven days (p < 0.05 vs. vehicle); and increased thalamic neuronal survival at 14 days (naïve 22773 ± 1012 cells/mm³, CCI + vehicle 11753 ± 464, CCI + minocycline 17047 ± 524; p < 0.001). Minocycline-treated rats demonstrated delayed motor recovery early after injury but had no injury effect on Morris-water maze whereas vehicle-treated rats performed worse than sham on the final two days of testing (both p < 0.05 vs. vehicle). Minocycline globally attenuated HMGB1 translocation and microglial activation in injured brain in a pediatric TBI model and afforded selective thalamic neuroprotection. The HMGB1 translocation and thalamic injury may represent novel mechanistic and regional therapeutic targets in pediatric TBI.