Microglia and infiltrating T-cells adopt long-term, age-specific, transcriptional changes after traumatic brain injury

Introduction: Traumatic brain injury (TBI) afflicts over 3 million Americans every year. Patients over 65 years of age suffer increased mortality as well as greater long-term neurocognitive and neuropsychiatric morbidity compared to younger adults. Microglia, the resident macrophages of the brain, are complicit in both. Our published and preliminary data have demonstrated a significant age-effect in which aged microglia are more prone to adopt a constitutively activated state associated with worse neurocognitive and neuropsychiatric outcomes. Therefore, we hypothesized that aged microglia would fail to return to a homeostatic state after TBI but instead adopt a long-term injury-associated state within the brain of aged mice as compared to young-adult mice after TBI. Methods: Young-adult (14-weeks) and aged (80-weeks) C57BL/6 mice underwent TBI via controlled cortical impact vs. sham injury. We utilized single-cell RNA sequencing to examine age-associated cellular responses after TBI. Four months post-TBI or sham injury, brains were harvested, and CD45+ cells (N=4,000 cells) were isolated via florescence-activated cell sorting. cDNA libraries were prepared via the 10x Genomics Chromium Single Cell 3' Reagent Kit, followed by sequencing on a HiSeq 4000 instrument. The raw data were processed using the Cell Ranger pipeline mapped to the mm10 mouse reference genome and Seurat following standard workflow. Seurat and GOrilla were used for downstream clustering, differential gene expression, and pathway analysis. All cell types were annotated using canonical markers and top expressed genes. ProjecTILs was additionally used to interpret T cell states. Results: Microglia from young-adult and aged mice have distinct transcriptional profiles pre-injury and markedly different transcriptional responses post-injury compared to young-adult mice. Pre-injury, aged mice demonstrated a disproportionate immune cell infiltration, including T cells, as compared to young-adult mice (aged versus young: 45.5% vs. 14.5%). Post-injury, the disparity was amplified with a proportional decrease in homeostatic microglia and greater increased infiltrating T cells compared to young-adult mice (Microglia: 27.5% vs. 71%; T cell: 45.5% vs. 4.5%). Of note, aged mice post-injury had a subpopulation of unique, age-specific, immune-inflammatory microglia resembling gene profiles of neurodegenerative disease-associated microglia (DAM) with enriched pathways involved in leukocyte recruitment and Alzheimer’s disease pathogenesis (FDR < 0.05). Contrastingly, post-injury, aged mice demonstrate a heterogenous T-cell infiltration with gene profiles corresponding to CD8 effector memory, CD8 native-like, CD4, and double-negative T cells (75.9%, 2.5%, 12.9%, and 8.6%, respectively) and enriched pathways including tau protein binding, macromolecule synthesis, and cytokine-mediated signaling pathways (FDR < 0.05). Conclusion: We hypothesized that aged microglia would fail to return to a homeostatic state after TBI and adopt a long-term, injury-associated state within the brain of aged mice as compared to young-adult mice after TBI. In particular, our data suggest an age-dependent reduction of homeostatic microglia post-injury yet an upregulation in a unique microglial subpopulation with a distinct immuno-inflammatory profile. Furthermore, aged subjects demonstrated a markedly disproportionate inflammatory infiltrate after TBI predominated by the presence of CD8+ T cells. In addition, post-injury, brain trauma reorganized the T cell milieu, especially CD8 effector memory T cells, via upregulating genes associated with macromolecule biosynthesis process and negative regulation of neuronal death, possibly linking TBI with its long-term sequelae and complications. Taken together, our data showed that age-specific gene signature changes in the T-cell infiltrates and the microglial subpopulation contributes to increased vulnerability of the aged brain to TBI. Age should be an a priori consideration in future TBI clinical trials.

A Brief Review of In Vitro Models for Injury and Regeneration in the Peripheral Nervous System

Nerve axonal injury and associated cellular mechanisms leading to peripheral nerve damage are important topics of research necessary for reducing disability and enhancing quality of life. Model systems that mimic the biological changes that occur during human nerve injury are crucial for the identification of cellular responses, screening of novel therapeutic molecules, and design of neural regeneration strategies. In addition to in vivo and mathematical models, in vitro axonal injury models provide a simple, robust, and reductionist platform to partially understand nerve injury pathogenesis and regeneration. In recent years, there have been several advances related to in vitro techniques that focus on the utilization of custom-fabricated cell culture chambers, microfluidic chamber systems, and injury techniques such as laser ablation and axonal stretching. These developments seem to reflect a gradual and natural progression towards understanding molecular and signaling events at an individual axon and neuronal-soma level. In this review, we attempt to categorize and discuss various in vitro models of injury relevant to the peripheral nervous system and highlight their strengths, weaknesses, and opportunities. Such models will help to recreate the post-injury microenvironment and aid in the development of therapeutic strategies that can accelerate nerve repair.

A Levee to the Flood: Pre-injury Neuroinflammation and Immune Stress Influence Traumatic Brain Injury Outcome

Increasing evidence demonstrates that aging influences the brain's response to traumatic brain injury (TBI), setting the stage for neurodegenerative pathology like Alzheimer's disease (AD). This topic is often dominated by discussions of post-injury aging and inflammation, which can diminish the consideration of those same factors before TBI. In fact, pre-TBI aging and inflammation may be just as critical in mediating outcomes. For example, elderly individuals suffer from the highest rates of TBI of all severities. Additionally, pre-injury immune challenges or stressors may alter pathology and outcome independent of age. The inflammatory response to TBI is malleable and influenced by previous, coincident, and subsequent immune insults. Therefore, pre-existing conditions that elicit or include an inflammatory response could substantially influence the brain's ability to respond to traumatic injury and ultimately affect chronic outcome. The purpose of this review is to detail how age-related cellular and molecular changes, as well as genetic risk variants for AD affect the neuroinflammatory response to TBI. First, we will review the sources and pathology of neuroinflammation following TBI. Then, we will highlight the significance of age-related, endogenous sources of inflammation, including changes in cytokine expression, reactive oxygen species processing, and mitochondrial function. Heightened focus is placed on the mitochondria as an integral link between inflammation and various genetic risk factors for AD. Together, this review will compile current clinical and experimental research to highlight how pre-existing inflammatory changes associated with infection and stress, aging, and genetic risk factors can alter response to TBI.

Refined Analysis of Chronic White Matter Changes after Traumatic Brain Injury and Repeated Sports-Related Concussions: Of Use in Targeted Rehabilitative Approaches?

Traumatic brain injury (TBI) or repeated sport-related concussions (rSRC) may lead to long-term memory impairment. Diffusion tensor imaging (DTI) is helpful to reveal global white matter damage but may underestimate focal abnormalities. We investigated the distribution of post-injury regional white matter changes after TBI and rSRC. Six patients with moderate/severe TBI, and 12 athletes with rSRC were included ≥6 months post-injury, and 10 (age-matched) healthy controls (HC) were analyzed. The Repeatable Battery for the Assessment of Neuropsychological Status was performed at the time of DTI. Major white matter pathways were tracked using q-space diffeomorphic reconstruction and analyzed for global and regional changes with a controlled false discovery rate. TBI patients displayed multiple classic white matter injuries compared with HC (p < 0.01). At the regional white matter analysis, the left frontal aslant tract, anterior thalamic radiation, and the genu of the corpus callosum displayed focal changes in both groups compared with HC but with different trends. Both TBI and rSRC displayed worse memory performance compared with HC (p < 0.05). While global analysis of DTI-based parameters did not reveal common abnormalities in TBI and rSRC, abnormalities to the fronto-thalamic network were observed in both groups using regional analysis of the white matter pathways. These results may be valuable to tailor individualized rehabilitative approaches for post-injury cognitive impairment in both TBI and rSRC patients.

New group memberships formed after an acquired brain injury predicts post-traumatic growth: A prospective study.

Predicting positive psychosocial outcomes following an Acquired Brain Injury (ABI) remains a challenge. Considerable research demonstrates that social group memberships can have positive effects on psychological well-being, particularly during life transitions. Social group memberships are argued to help people derive a sense of self. This prospective study examined if social group memberships (number of groups and connectedness with groups) could predict posttraumatic growth (PTG) in those affected by ABI. Thirty-six participants (10 females, Mage = 46.56, SD = 11.46) engaged in community rehabilitation services completed measures at two time-points. Mediation analyses demonstrated that the number of new group memberships (groups formed post-injury) predicted greater PTG at time 2, via stronger connectedness with these new group memberships (controlling for initial PTG). The observed results suggest that a focus on developing and strengthening connections with new group memberships may promote positive adjustment after brain injury.

Thalamo-Prefrontal Connectivity Correlates with Early Command-Following After Severe Traumatic Brain injury

Recovery of consciousness after traumatic brain injury (TBI) is heterogeneous and difficult to predict. Structures such as the thalamus and prefrontal cortex are thought to be important in facilitating consciousness. We sought to investigate whether the integrity of thalamo-prefrontal circuits, assessed via diffusion tensor imaging (DTI), was associated with the return of goal-directed behavior after severe TBI. We classified a cohort of severe TBI patients (N = 25, 20 males) into Early and Late/Never outcome groups based on their ability to follow commands within 30 days post-injury. We assessed connectivity between whole thalamus, and mediodorsal thalamus (MD), to prefrontal cortex (PFC) subregions including dorsolateral PFC (dlPFC), medial PFC (mPFC), anterior cingulate (ACC), and orbitofrontal (OFC) cortices. We found that the integrity of thalamic projections to PFC subregions (L OFC, L and R ACC, and R mPFC) was significantly associated with Early command-following. This association persisted when the analysis was restricted to prefrontal-mediodorsal (MD) thalamus connectivity. In contrast, dlPFC connectivity to thalamus was not significantly associated with command-following. Using the integrity of thalamo-prefrontal connections, we created a linear regression model that demonstrated 72% accuracy in predicting command-following after a leave-one-out analysis. Together, these data support a role for thalamo-prefrontal connectivity in the return of goal-directed behavior following TBI.

Hippocampal Expression of Cytochrome P450 1B1 in Penetrating Traumatic Brain Injury

Hippocampal dysfunction contributes to multiple traumatic brain injury sequala. Female rodents’ outcome is superior to male which has been ascribed the neuroprotective sex hormones 17β-estradiol and progesterone. Cytochrome P450 1B1 (CYP1B1) is an oxidative enzyme influencing the neuroinflammatory response by creating inflammatory mediators and metabolizing neuroprotective 17β-estradiol and progesterone. In this study, we aimed to describe hippocampal CYP1B1 mRNA expression, protein presence of CYP1B1 and its key redox partner Cytochrome P450 reductase (CPR) in both sexes, as well as the effect of penetrating traumatic brain injury (pTBI). A total 64 adult Sprague Dawley rats divided by sex received pTBI or sham-surgery and were assigned survival times of 1-, 3-, 5- or 7 days. CYP1B1 mRNA was quantified using in-situ hybridization and immunohistochemistry performed to verify protein colocalization. CYP1B1 mRNA expression was present in all subregions but greatest in CA2 irrespective of sex, survival time or intervention. At 3-, 5- and 7 days post-injury, expression in CA2 was reduced in male rats subjected to pTBI compared to sham-surgery. Females subjected to pTBI instead exhibited increased expression in all CA subregions 3 days post-injury, the only time point expression in CA2 was greater in females than in males. Immunohistochemical analysis confirmed neuronal CYP1B1 protein in all hippocampal subregions, while CPR was limited to CA1 and CA2. CYP1B1 mRNA is constitutively expressed in both sexes. In response to pTBI, females displayed a more urgent but brief regulatory response than males. This indicates there may be sex-dependent differences in CYP1B1 activity, possibly influencing inflammation and neuroprotection in pTBI.

Fibro-adipogenesis in Injured Rotator Cuff Muscle

Purpose of Review Fibro-adipogenic progenitors were first characterized in 2010 and later found to contribute significantly to muscle regeneration and mediate degenerative changes in muscle following injury. These progenitors were also found to have an influence on the rotator cuff muscle’s response to chronic injury which is defined by fibrosis accompanied by massive fatty degeneration. The purpose of this review is to highlight progenitor cells, their contribution to fibro-adipogenesis in rotator cuff tissue, and the factors influencing fibro-adipogenesis in this tissue. Recent Findings Fibro-adipogenic progenitors are a key mediator of the fatty infiltration notably prevalent in rotator cuff injury. Relative to other muscle groups, the rotator cuff has relatively high rates of fibro-adipogenesis following massive chronic rotator cuff tears. This may be linked to the pre-injury density of fibro-adipogenic progenitors in muscle tissue affecting post-injury levels of fibro-adipogenesis. In addition, suprascapular nerve injury in rat models of rotator cuff tears has demonstrated worse, histologic, and biomechanical properties and lower healing rates of rotator cuff repairs. However, fatty infiltration in the rotator cuff following suprascapular nerve compression has been shown to be reversible following release of the nerve compression. Summary The fibro-adipogenic response to acute and chronic injury in rotator cuff tissue is determined by a complex array of factors including progenitor cell influence, transcriptional pathways, chronicity of the injury, anatomic location of injury, microenvironmental influences, and the severity of nerve involvement. Elucidating the complex interactions of these factors will provide potential targets for therapeutic intervention in vivo.

Factors associated with injury and re-injury occurrence in female pole dancers

The aim of the study was to recognise what participant-, training- and post-injury-related factors are associated with an injury and re-injury occurrence in female pole dancers (PDs). 320 female PDs fulfilled a custom survey. 1050 injuries were reported by 276 PDs, 59% of injuries were related to lower extremity, 39% to upper extremity and 10% to spine and trunk. 156 PDs reported sustaining a re-injury, and overall, 628 re-injuries were reported. The median weekly pole-specific training session volume was 90 min and 240 min in the low and high qualified group, respectively. The total training volume was 180 min in the low qualified PDs and 240 min in the high qualified group. PDs with higher height and spending more time on pole-specific training in studio and on other forms of training have higher odds of sustaining an injury. PDs with lower level of experience in training, who sustained an injury, and who had a shorter pause between the moment of injury and the return to performance, and thus who did not fully recover, have higher odds of sustaining a re-injury. Sport-specific injury prevention strategies should be developed and implemented in this cohort, since over 85% of pole dancers reported sustaining some kind of injury.

Kinetics of LYVE-1-Positive M2-Like Macrophages in Developing and Repairing Dental Pulp in Vivo and Their Pro-Angiogenic Activity in Vitro

Tissue-resident macrophages expressing lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1) are found in multiple tissues and organs. We aimed to evaluate the dynamics and biological function of LYVE-1+ macrophages in dental pulp during post-injury tissue remodeling. Immunofluorescence staining of mouse embryos revealed that LYVE-1+ macrophages colonized dental pulp before birth. In mature rat molar dental pulp, LYVE-1+ macrophages were the main subset of macrophages expressing CD163, an M2 marker, and were distributed throughout the tissue. In response to dental pulp injury induced by cavity preparation, LYVE-1+ macrophages quickly disappeared from the affected area of the pulp and gradually repopulated during the wound healing process. RAW264.7 mouse macrophages cultured with a mixture of macrophage colony-stimulating factor, interleukin-4, and dexamethasone increased LYVE-1 expression, whereas lipopolysaccharide-stimulation decreased LYVE-1 expression. Enforced expression of Lyve1 in RAW264.7 cells resulted in increased mRNA expression of matrix metalloproteinase 2 (Mmp2), Mmp9, and vascular endothelial growth factor A (Vegfa). Lyve1-expressing macrophages promoted the migration and tube formation of human umbilical vein endothelial cells. In conclusion, LYVE-1+ tissue-resident M2-like macrophages in dental pulp showed dynamism in response to pulp injury, and possibly play an important role in angiogenesis during wound healing and tissue remodeling.