Amplified Gliosis and Interferon-Associated Inflammation in the Aging Brain following Diffuse Traumatic Brain Injury

A global view of altered ligand-receptor interactions in bone marrow aging based on single-cell sequencing

Altered cell-cell communication is a hallmark of aging, but its impact on bone marrow aging remains poorly understood. Based on a common and effective pipeline and single-cell transcriptome sequencing, we detected 384,124 interactions including 2575 ligand-receptor pairs and 16 non-adherent bone marrow cell types in old and young mouse and identified a total of 5560 significantly different interactions, which were then verified by flow cytometry and quantitative real-time PCR. These differential ligand-receptor interactions exhibited enrichment for the senescence-associated secretory phenotypes. Further validation demonstrated supplementing specific extracellular ligands could modify the senescent signs of hematopoietic stem cells derived from old mouse. Our work provides an effective procedure to detect the ligand-receptor interactions based on single-cell sequencing, which contributes to understand mechanisms and provides a potential strategy for intervention of bone marrow aging.

Pharmacological inhibition of STING reduces neuroinflammation-mediated damage post-traumatic brain injury

BACKGROUND AND PURPOSE

Traumatic brain injury (TBI) remains a major public health concern worldwide with unmet effective treatment. Stimulator of interferon genes (STING) and its downstream type-I interferon (IFN) signalling are now appreciated to be involved in TBI pathogenesis. Compelling evidence have shown that STING and type-I IFNs are key in mediating the detrimental neuroinflammatory response after TBI. Therefore, pharmacological inhibition of STING presents a viable therapeutic opportunity in combating the detrimental neuroinflammatory response after TBI.

EXPERIMENTAL APPROACH

This study investigated the neuroprotective effects of the small-molecule STING inhibitor n-(4-iodophenyl)-5-nitrofuran-2-carboxamide (C-176) in the controlled cortical impact mouse model of TBI in 10- to 12-week-old male mice. Thirty minutes post-controlled cortical impact surgery, a single 750-nmol dose of C-176 or saline (vehicle) was administered intravenously. Analysis was conducted 2 h and 24 h post-TBI.

KEY RESULTS

Mice administered C-176 had significantly smaller cortical lesion area when compared to vehicle-treated mice 24 h post-TBI. Quantitative temporal gait analysis conducted using DigiGait™ showed C-176 administration attenuated TBI-induced impairments in gait symmetry, stride frequency and forelimb stance width. C-176-treated mice displayed a significant reduction in striatal gene expression of pro-inflammatory cytokines Tnf-α, Il-1β and Cxcl10 compared to their vehicle-treated counterparts 2 h post-TBI.

CONCLUSION AND IMPLICATIONS

This study demonstrates the neuroprotective activity of C-176 in ameliorating acute neuroinflammation and preventing white matter neurodegeneration post-TBI. This study highlights the therapeutic potential of small-molecule inhibitors targeting STING for the treatment of trauma-induced inflammation and neuroprotective potential.

Traumatic brain injury alters the effects of class II invariant peptide (CLIP) antagonism on chronic meningeal CLIP + B cells, neuropathology, and neurobehavioral impairment in 5xFAD mice

BACKGROUND

Traumatic brain injury (TBI) is a significant risk factor for Alzheimer's disease (AD), and accumulating evidence supports a role for adaptive immune B and T cells in both TBI and AD pathogenesis. We previously identified B cell and major histocompatibility complex class II (MHCII)-associated invariant chain peptide (CLIP)-positive B cell expansion after TBI. We also showed that antagonizing CLIP binding to the antigen presenting groove of MHCII after TBI acutely reduced CLIP + splenic B cells and was neuroprotective. The current study investigated the chronic effects of antagonizing CLIP in the 5xFAD Alzheimer's mouse model, with and without TBI.

METHODS

12-week-old male wild type (WT) and 5xFAD mice were administered either CLIP antagonist peptide (CAP) or vehicle, once at 30 min after either sham or a lateral fluid percussion injury (FPI). Analyses included flow cytometric analysis of immune cells in dural meninges and spleen, histopathological analysis of the brain, magnetic resonance diffusion tensor imaging, cerebrovascular analysis, and assessment of motor and neurobehavioral function over the ensuing 6 months.

RESULTS

9-month-old 5xFAD mice had significantly more CLIP + B cells in the meninges compared to age-matched WT mice. A one-time treatment with CAP significantly reduced this population in 5xFAD mice. Importantly, CAP also improved some of the immune, histopathological, and neurobehavioral impairments in 5xFAD mice over the ensuing six months. Although FPI did not further elevate meningeal CLIP + B cells, it did negate the ability of CAP to reduce meningeal CLIP + B cells in the 5xFAD mice. FPI at 3 months of age exacerbated some aspects of AD pathology in 5xFAD mice, including further reducing hippocampal neurogenesis, increasing plaque deposition in CA3, altering microgliosis, and disrupting the cerebrovascular structure. CAP treatment after injury ameliorated some but not all of these FPI effects.

Amyloid precursor protein induces reactive astrogliosis

AIM

Astrocytes respond to stressors by acquiring a reactive state characterized by changes in their morphology and function. Molecules underlying reactive astrogliosis, however, remain largely unknown. Given that several studies observed increase in the Amyloid Precursor Protein (APP) in reactive astrocytes, we here test whether APP plays a role in reactive astrogliosis.

METHODS

We investigated whether APP instigates reactive astroglios by examining in vitro and in vivo the morphology and function of naive and APP-deficient astrocytes in response to APP and well-established stressors.

RESULTS

Overexpression of APP in cultured astrocytes led to remodeling of the intermediate filament network, enhancement of cytokine production, and activation of cellular programs centered around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion abrogated remodeling of the intermediate filament network and blunted expression of IFN-stimulated gene products in response to lipopolysaccharide. Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein observed canonically in astrocytes in response to TBI.

CONCLUSIONS

The APP thus represents a candidate molecular inducer and regulator of reactive astrogliosis. This finding has implications for understanding pathophysiology of neurodegenerative and other diseases of the nervous system characterized by reactive astrogliosis and opens potential new therapeutic avenues targeting APP and its pathways to modulate reactive astrogliosis.

STING inhibition suppresses microglia-mediated synapses engulfment and alleviates motor functional deficits after stroke

Ischemic stroke is the leading cause of adult disability. Ischemia leads to progressive neuronal death and synapse loss. The engulfment of stressed synapses by microglia further contributes to the disruption of the surviving neuronal network and related brain function. Unfortunately, there is currently no effective target for suppressing the microglia-mediated synapse engulfment. Stimulator of interferon genes (STING) is an important participant in innate immune response. In the brain, microglia are the primary cell type that mediate immune response after brain insult. The intimate relationship between STING and microglia-mediated neuroinflammation has been gradually established. However, whether STING affects other functions of microglia remains elusive. In this study, we found that STING regulated microglial phagocytosis of synapses after photothrombotic stroke. The treatment of STING inhibitor H151 significantly improved the behavioral performance of injured mice in grid-walking test, cylinder test, and adhesive removal test after stroke. Moreover, the puncta number of engulfed SYP or PSD95 in microglia was reduced after consecutive H151 administration. Further analysis showed that the mRNA levels of several complement components and phagocytotic receptors were decreased after STING inhibition. Transcriptional factor STAT1 is known for regulating most of the decreased molecules. After STING inhibition, the nucleus translocation of phosphorylated STAT1 was also suppressed in microglia. Our data uncovered the novel regulatory effects of STING in microglial phagocytosis after stroke, and further emphasized STING as a potential drug-able target for post-stroke functional recovery.

STING-Dependent Signaling in Microglia or Peripheral Immune Cells Orchestrates the Early Inflammatory Response and Influences Brain Injury Outcome

While originally identified as an antiviral pathway, recent work has implicated that cyclic GMP-AMP-synthase-Stimulator of Interferon Genes (cGAS-STING) signaling is playing a critical role in the neuroinflammatory response to traumatic brain injury (TBI). STING activation results in a robust inflammatory response characterized by the production of inflammatory cytokines called interferons, as well as hundreds of interferon stimulated genes (ISGs). Global knock-out (KO) mice inhibiting this pathway display neuroprotection with evidence that this pathway is active days after injury; yet, the early neuroinflammatory events stimulated by STING signaling remain understudied. Furthermore, the source of STING signaling during brain injury is unknown. Using a murine controlled cortical impact (CCI) model of TBI, we investigated the peripheral immune and microglial response to injury utilizing male chimeric and conditional STING KO animals, respectively. We demonstrate that peripheral and microglial STING signaling contribute to negative outcomes in cortical lesion volume, cell death, and functional outcomes postinjury. A reduction in overall peripheral immune cell and neutrophil infiltration at the injury site is STING dependent in these models at 24 h. Transcriptomic analysis at 2 h, when STING is active, reveals that microglia drive an early, distinct transcriptional program to elicit proinflammatory genes including interleukin 1-β (IL-1β), which is lost in conditional knock-out mice. The upregulation of alternative innate immune pathways also occurs after injury in these animals, which supports a complex relationship between brain-resident and peripheral immune cells to coordinate the proinflammatory response and immune cell influx to damaged tissue after injury.

Unveiling the role of astrocytes in postoperative cognitive dysfunction

Alzheimer's disease (AD) is the most common neurodegenerative disorder, characterized by progressive cognitive decline and the accumulation of amyloid-beta plaques, tau tangles, and neuroinflammation in the brain. Postoperative cognitive dysfunction (POCD) is a prevalent and debilitating condition characterized by cognitive decline following neuroinflammation and oxidative stress induced by procedures. POCD and AD are two conditions that share similarities in the underlying mechanisms and pathophysiology. Compared to normal aging individuals, individuals with POCD are at a higher risk for developing AD. Emerging evidence suggests that astrocytes, the most abundant glial cells in the central nervous system, play a critical role in the pathogenesis of these conditions. Comprehensive functions of astrocyte in AD has been extensively explored, but very little is known about POCD may experience late-onset AD pathogenesis. Herein, in this context, we mainly explore the multifaceted roles of astrocytes in the context of POCD, highlighting their involvement in neuroinflammation, neurotransmitter regulation, synaptic plasticity and neurotrophic support, and discuss how POCD may augment the onset of AD. Additionally, we discuss potential therapeutic strategies targeting astrocytes to mitigate or prevent POCD, which hold promise for improving the quality of life for patients undergoing surgeries and against AD in the future.

Impaired cortical neuronal homeostasis and cognition after diffuse traumatic brain injury are dependent on microglia and type I interferon responses

Neuropsychiatric complications including depression and cognitive decline develop in the years after traumatic brain injury (TBI), negatively affecting quality of life. Microglial and type 1 interferon (IFN-I) responses are associated with the transition from acute to chronic neuroinflammation after diffuse TBI in mice. Thus, the purpose of this study was to determine if impaired neuronal homeostasis and increased IFN-I responses intersected after TBI to cause cognitive impairment. Here, the RNA profile of neurons and microglia after TBI (single nucleus RNA-sequencing) with or without microglia depletion (CSF1R antagonist) was assessed 7 dpi. There was a TBI-dependent suppression of cortical neuronal homeostasis with reductions in CREB signaling, synaptogenesis, and synaptic migration and increases in RhoGDI and PTEN signaling (Ingenuity Pathway Analysis). Microglial depletion reversed 50% of TBI-induced gene changes in cortical neurons depending on subtype. Moreover, the microglial RNA signature 7 dpi was associated with increased stimulator of interferon genes (STING) activation and IFN-I responses. Therefore, we sought to reduce IFN-I signaling after TBI using STING knockout mice and a STING antagonist, chloroquine (CQ). TBI-associated cognitive deficits in novel object location and recognition (NOL/NOR) tasks at 7 and 30 dpi were STING dependent. In addition, TBI-induced STING expression, microglial morphological restructuring, inflammatory (Tnf, Cd68, Ccl2) and IFN-related (Irf3, Irf7, Ifi27) gene expression in the cortex were attenuated in STINGKO mice. CQ also reversed TBI-induced cognitive deficits and reduced TBI-induced inflammatory (Tnf, Cd68, Ccl2) and IFN (Irf7, Sting) cortical gene expression. Collectively, reducing IFN-I signaling after TBI with STING-dependent interventions attenuated the prolonged microglial activation and cognitive impairment.

Candidate Genetic and Molecular Drivers of Dysregulated Adaptive Immune Responses After Traumatic Brain Injury

Abstract Neuroinflammation is a significant and modifiable cause of secondary injury after traumatic brain injury (TBI), driven by both central and peripheral immune responses. A substantial proportion of outcome after TBI is genetically mediated, with an estimated heritability effect of around 26%, but because of the comparatively small datasets currently available, the individual drivers of this genetic effect have not been well delineated. A hypothesis-driven approach to analyzing genome-wide association study (GWAS) datasets reduces the burden of multiplicity testing and allows variants with a high prior biological probability of effect to be identified where sample size is insufficient to withstand data-driven approaches. Adaptive immune responses show substantial genetically mediated heterogeneity and are well established as a genetic source of risk for numerous disease states; importantly, HLA class II has been specifically identified as a locus of interest in the largest TBI GWAS study to date, highlighting the importance of genetic variance in adaptive immune responses after TBI. In this review article we identify and discuss adaptive immune system genes that are known to confer strong risk effects for human disease, with the dual intentions of drawing attention to this area of immunobiology, which, despite its importance to the field, remains under-investigated in TBI and presenting high-yield testable hypotheses for application to TBI GWAS datasets.

Antagonism of the brain P2X7 ion channel attenuates repeated social defeat induced microglia reactivity, monocyte recruitment and anxiety-like behavior in male mice

Chronic stress is linked to increased anxiety. Repeated social defeat (RSD) in mice causes anxiety that is dependent on activated neurons, reactive microglia, and accumulation of monocytes in the brain. This response requires interactions between the immune system and central nervous system (CNS). Neuronal activation within threat appraisal regions is a key response to RSD, however, it is unclear how microglia become activated. One potential explanation is that microglia express a purinergic non-selective ligand gated adenosine-triphosphate (ATP) receptor 7 (P2X7). Activation of P2X7 promotes the release of chemokines and cytokines, and recruitment of monocytes to the brain. Thus, the purpose of this study was to determine if a novel P2X7 antagonist blocked neuronal and microglia interactions and the corresponding anxiety following RSD. Male mice were administered (i.p.) a P2X7 antagonist, JNJ-54471300, prior to each cycle of RSD. Fourteen hours after RSD, behavioral deficits including social avoidance and anxiety-like were determined. Moreover, several immune parameters were assessed. RSD caused neuronal activation in stress-responsive regions, monocyte production and release, splenomegaly, and social avoidance. These parameters were unaffected by P2X7 antagonism. RSD-associated proportional area of Iba-1+ microglia, monocyte accumulation in the brain, IL-1β mRNA expression in enriched myeloid cells, plasma IL-6, and anxiety-like behavior were ameliorated by P2X7 antagonism. Gene expression analysis in the hippocampus and amygdala showed regional specific responses to RSD and some were reversed with P2X7 antagonism. Overall, blocking P2X7 activation attenuated RSD-induced microglia reactivity with corresponding reduction in neuroinflammation, monocyte accumulation, and anxiety-like behavior in male mice.

Amyloid precursor protein induces reactive astrogliosis

We present in vitro and in vivo evidence demonstrating that Amyloid Precursor Protein (APP) acts as an essential instigator of reactive astrogliosis. Cell-specific overexpression of APP in cultured astrocytes led to remodelling of the intermediate filament network, enhancement of cytokine production and activation of cellular programs centred around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion in cultured astrocytes abrogated remodelling of the intermediate filament network and blunted expression of IFN stimulated gene (ISG) products in response to lipopolysaccharide (LPS). Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein (GFAP) observed canonically in astrocytes in response to TBI. Thus, APP represents a molecular inducer and regulator of reactive astrogliosis.

Microglia moonlighting after traumatic brain injury: aging and interferons influence chronic microglia reactivity

Most of the individuals who experience traumatic brain injury (TBI) develop neuropsychiatric and cognitive complications that negatively affect recovery and health span. Activation of multiple inflammatory pathways persists after TBI, but it is unclear how inflammation contributes to long-term behavioral and cognitive deficits. One outcome of TBI is microglial priming and subsequent hyper-reactivity to secondary stressors, injuries, or immune challenges that further augment complications. Additionally, microglia priming with aging contributes to exaggerated glial responses to TBI. One prominent inflammatory pathway, interferon (IFN) signaling, is increased after TBI and may contribute to microglial priming and subsequent reactivity. This review discusses the contributions of microglia to inflammatory processes after TBI, as well as the influence of aging and IFNs on microglia reactivity and chronic inflammation after TBI.

Delayed CO2 postconditioning promotes neurological recovery after cryogenic traumatic brain injury by downregulating IRF7 expression

AIMS

Few treatments are available in the subacute phase of traumatic brain injury (TBI) except rehabilitation training. We previously reported that transient CO2 inhalation applied within minutes after reperfusion has neuroprotective effects against cerebral ischemia/reperfusion injury. In this study, it was hypothesized that delayed CO2 postconditioning (DCPC) starting at the subacute phase may promote neurological recovery of TBI.

METHODS

Using a cryogenic TBI (cTBI) model, mice received DCPC daily by inhaling 5%/10%/20% CO2 for various time-courses (one/two/three cycles of 10-min inhalation/10-min break) at Days 3-7, 3-14 or 7-18 after cTBI. Beam walking and gait tests were used to assess the effect of DCPC. Lesion size, expression of GAP-43 and synaptophysin, amoeboid microglia number and glia scar area were detected. Transcriptome and recombinant interferon regulatory factor 7 (Irf7) adeno-associated virus were applied to investigate the molecular mechanisms.

RESULTS

DCPC significantly promoted recovery of motor function in a concentration and time-course dependent manner with a wide therapeutic time window of at least 7 days after cTBI. The beneficial effects of DCPC were blocked by intracerebroventricular injection of NaHCO3 . DCPC also increased puncta density of GAP-43 and synaptophysin, and reduced amoeboid microglia number and glial scar formation in the cortex surrounding the lesion. Transcriptome analysis showed many inflammation-related genes and pathways were altered by DCPC, and Irf7 was a hub gene, while overexpression of IRF7 blocked the motor function improvement of DCPC.

CONCLUSIONS

We first showed that DCPC promoted functional recovery and brain tissue repair, which opens a new therapeutic time window of postconditioning for TBI. Inhibition of IRF7 is a key molecular mechanism for the beneficial effects of DCPC, and IRF7 may be a potential therapeutic target for rehabilitation after TBI.

Divergent Spatial Learning, Enhanced Neuronal Transcription, and Blood-Brain Barrier Disruption Develop During Recovery from Post-Injury Sleep Fragmentation

Traumatic brain injury (TBI) causes pathophysiology that may significantly decrease quality of life over time. A major propagator of this response is chronic, maladaptive neuroinflammation, which can be exacerbated by stressors such as sleep fragmentation (SF). This study determined whether post-TBI SF had lasting behavioral and inflammatory effects even with a period of recovery. To test this, male and female mice received a moderate lateral fluid percussion TBI or sham surgery. Half the mice were left undisturbed, and half were exposed to daily SF for 30 days. All mice were then undisturbed between 30 and 60 days post-injury (DPI), allowing mice to recover from SF (SF-R). SF-R did not impair global Barnes maze performance. Nonetheless, TBI SF-R mice displayed retrogression in latency to reach the goal box within testing days. These nuanced behavioral changes in TBI SF-R mice were associated with enhanced expression of neuronal processing/signaling genes and indicators of blood-brain barrier (BBB) dysfunction. Aquaporin-4 (AQP4) expression, a marker of BBB integrity, was differentially altered by TBI and TBI SF-R. For example, TBI enhanced cortical AQP4 whereas TBI SF-R mice had the lowest cortical expression of perivascular AQP4, dysregulated AQP4 polarization, and the highest number of CD45+ cells in the ipsilateral cortex. Altogether, post-TBI SF caused lasting, divergent behavioral responses associated with enhanced expression of neuronal transcription and BBB disruption even after a period of recovery from SF. Understanding lasting impacts from post-TBI stressors can better inform both acute and chronic post-injury care to improve long-term outcome post-TBI.

Traumatic brain injury induces an adaptive immune response in the meningeal transcriptome that is amplified by aging

Traumatic Brain Injury (TBI) is a major cause of disability and mortality, particularly among the elderly, yet our mechanistic understanding of how age renders the post-traumatic brain vulnerable to poor clinical outcomes and susceptible to neurological disease remains poorly understood. It is well established that dysregulated and sustained immune responses contribute to negative outcomes after TBI, however our understanding of the interactions between central and peripheral immune reservoirs is still unclear. The meninges serve as the interface between the brain and the immune system, facilitating important bi-directional roles in healthy and disease settings. It has been previously shown that disruption of this system exacerbates inflammation in age related neurodegenerative disorders such as Alzheimer's disease, however we have an incomplete understanding of how the meningeal compartment influences immune responses after TBI. Here, we examine the meningeal tissue and its response to brain injury in young (3-months) and aged (18-months) mice. Utilizing a bioinformatic approach, high-throughput RNA sequencing demonstrates alterations in the meningeal transcriptome at sub-acute (7-days) and chronic (1 month) timepoints after injury. We find that age alone chronically exacerbates immunoglobulin production and B cell responses. After TBI, adaptive immune response genes are up-regulated in a temporal manner, with genes involved in T cell responses elevated sub-acutely, followed by increases in B cell related genes at chronic time points after injury. Pro-inflammatory cytokines are also implicated as contributing to the immune response in the meninges, with ingenuity pathway analysis identifying interferons as master regulators in aged mice compared to young mice following TBI. Collectively these data demonstrate the temporal series of meningeal specific signatures, providing insights into how age leads to worse neuroinflammatory outcomes in TBI.

The role of mtDAMPs in the trauma-induced systemic inflammatory response syndrome

Systemic inflammatory response syndrome (SIRS) is a non-specific exaggerated defense response caused by infectious or non-infectious stressors such as trauma, burn, surgery, ischemia and reperfusion, and malignancy, which can eventually lead to an uncontrolled inflammatory response. In addition to the early mortality due to the "first hits" after trauma, the trauma-induced SIRS and multiple organ dysfunction syndrome (MODS) are the main reasons for the poor prognosis of trauma patients as "second hits". Unlike infection-induced SIRS caused by pathogen-associated molecular patterns (PAMPs), trauma-induced SIRS is mainly mediated by damage-associated molecular patterns (DAMPs) including mitochondrial DAMPs (mtDAMPs). MtDAMPs released after trauma-induced mitochondrial injury, including mitochondrial DNA (mtDNA) and mitochondrial formyl peptides (mtFPs), can activate inflammatory response through multiple inflammatory signaling pathways. This review summarizes the role and mechanism of mtDAMPs in the occurrence and development of trauma-induced SIRS.

Short-term high-fat diet consumption impairs synaptic plasticity in the aged hippocampus via IL-1 signaling

More Americans are consuming diets higher in saturated fats and refined sugars than ever before. These trends could have serious consequences for the older population because high-fat diet (HFD) consumption, known to induce neuroinflammation, has been shown to accelerate and aggravate memory declines. We have previously demonstrated that short-term HFD consumption, which does not evoke obesity-related comorbidities, produced profound impairments to hippocampal-dependent memory in aged rats. These impairments were precipitated by increases in proinflammatory cytokines, primarily interleukin-1 beta (IL-1β). Here, we explored the extent to which short-term HFD consumption disrupts hippocampal synaptic plasticity, as measured by long-term potentiation (LTP), in young adult and aged rats. We demonstrated that (1) HFD disrupted late-phase LTP in the hippocampus of aged, but not young adult rats, (2) HFD did not disrupt early-phase LTP, and (3) blockade of the IL-1 receptor rescued L-LTP in aged HFD-fed rats. These findings suggest that hippocampal memory impairments in aged rats following HFD consumption occur through the deterioration of synaptic plasticity and that IL-1β is a critical driver of that deterioration.

Models of traumatic brain injury-highlights and drawbacks

Traumatic brain injury (TBI) is the leading cause for high morbidity and mortality rates in young adults, survivors may suffer from long-term physical, cognitive, and/or psychological disorders. Establishing better models of TBI would further our understanding of the pathophysiology of TBI and develop new potential treatments. A multitude of animal TBI models have been used to replicate the various aspects of human TBI. Although numerous experimental neuroprotective strategies were identified to be effective in animal models, a majority of strategies have failed in phase II or phase III clinical trials. This failure in clinical translation highlights the necessity of revisiting the current status of animal models of TBI and therapeutic strategies. In this review, we elucidate approaches for the generation of animal models and cell models of TBI and summarize their strengths and limitations with the aim of exploring clinically meaningful neuroprotective strategies.

Aging with TBI vs. Aging: 6-month temporal profiles for neuropathology and astrocyte activation converge in behaviorally relevant thalamocortical circuitry of male and female rats

Traumatic brain injury (TBI) manifests late-onset and persisting clinical symptoms with implications for sex differences and increased risk for the development of age-related neurodegenerative diseases. Few studies have evaluated chronic temporal profiles of neuronal and glial pathology that include sex as a biological variable. After experimental diffuse TBI, late-onset and persisting somatosensory hypersensitivity to whisker stimulation develops at one-month post-injury and persists to at least two months post-injury in male rats, providing an in vivo model to evaluate the temporal profile of pathology responsible for morbidity. Whisker somatosensation is dependent on signaling through the thalamocortical relays of the whisker barrel circuit made up of glutamatergic projections between the ventral posteromedial nucleus of the thalamus (VPM) and primary somatosensory barrel cortex (S1BF) with inhibitory (GABA) innervation from the thalamic reticular nucleus (TRN) to the VPM. To evaluate the temporal profiles of pathology, male and female Sprague Dawley rats ( n = 5-6/group) were subjected to sham surgery or midline fluid percussion injury (FPI). At 7-, 56-, and 168-days post-injury (DPI), brains were processed for amino-cupric silver stain and glial fibrillary acidic protein (GFAP) immunoreactivity, where pixel density of staining was quantified to determine the temporal profile of neuropathology and astrocyte activation in the VPM, S1BF, and TRN. FPI induced significant neuropathology in all brain regions at 7 DPI. At 168 DPI, neuropathology remained significantly elevated in the VPM and TRN, but returned to sham levels in the S1BF. GFAP immunoreactivity was increased as a function of FPI and DPI, with an FPI × DPI interaction in all regions and an FPI × Sex interaction in the S1BF. The interactions were driven by increased GFAP immunoreactivity in shams over time in the VPM and TRN. In the S1BF, GFAP immunoreactivity increased at 7 DPI and declined to age-matched sham levels by 168 DPI, while GFAP immunoreactivity in shams significantly increased between 7 and 168 days. The FPI × Sex interaction was driven by an overall greater level of GFAP immunoreactivity in FPI males compared to FPI females. Increased GFAP immunoreactivity was associated with an increased number of GFAP-positive soma, predominantly at 7 DPI. Overall, these findings indicate that FPI, time post-injury, sex, region, and aging with injury differentially contribute to chronic changes in neuronal pathology and astrocyte activation after diffuse brain injury. Thus, our results highlight distinct patterns of pathological alterations associated with the development and persistence of morbidity that supports chronic neuropathology, especially within the thalamus. Further, data indicate a convergence between TBI-induced and age-related pathology where further investigation may reveal a role for divergent astrocytic phenotypes associated with increased risk for neurodegenerative diseases.