Minocycline Transiently Reduces Microglia/Macrophage Activation but Exacerbates Cognitive Deficits Following Repetitive Traumatic Brain Injury in the Neonatal Rat

Developmental Functions of Microglia: Impact of Psychosocial and Physiological Early Life Stress

Microglia play numerous important roles in brain development. From early embryonic stages through adolescence, these immune cells influence neuronal genesis and maturation, guide connectivity, and shape brain circuits. They also interact with other glial cells and structures, influencing the brain's supportive microenvironment. While this central role makes microglia essential, it means that early life perturbations to microglia can have widespread effects on brain development, potentially resulting in long-lasting behavioral impairments. Here, we will focus on the effects of early life psychosocial versus physiological stressors in rodent models. Psychosocial stress refers to perceived threats that lead to stress axes activation, including prenatal stress, or chronic postnatal stress, including maternal separation and resource scarcity. Physiological stress refers to with physical threats, including maternal immune activation, postnatal infection, and traumatic brain injury. Differing sources of early life stress have varied impacts on microglia, and these effects are moderated by factors such as developmental age, brain region, and sex. Overall, these stressors appear to either 1) upregulate basal microglia numbers and activity throughout the lifespan, while possibly blunting their responsivity to subsequent stressors, or 2) shift the developmental curve of microglia, resulting in differential timing and function, impacting the critical periods they govern. Either could contribute to behavioral dysfunctions that occur after the resolution of early life stress. Exploring how different stressors impact microglia, as well as how the experience of multiple stressors interacts to alter microglia's developmental functions, could deepen our understanding of how early life stress changes the brain's developmental trajectory.

Noninvasive microvascular imaging in newborn rats using high-frequency ultrafast ultrasound

Ultrasound imaging stands as the predominant modality for neonatal health assessment, with recent advancements in ultrafast Doppler (μDoppler) technology offering significant promise in fields such as neonatal brain imaging. Combining μDoppler with high-frequency ultrasound (HF-μDoppler) presents a potential efficient avenue to enhance in vivo microvascular imaging in small animals, notably newborn rats, a crucial preclinical animal model for neonatal disease and development research. It is necessary to verify the imaging performance of HF-μDoppler in preclinical trials. This study investigates the microvascular imaging capabilities of HF-μDoppler using a 30 MHz high-frequency linear array probe in newborn rats. Results demonstrate the clarity of cerebral microvascular imaging in rats aged 1 to 7 postnatal days, extending to whole-body microvascular imaging, encompassing the central nervous system, including the brain and spinal cord. In conclusion, HF-μDoppler technology emerges as a reliable imaging tool, offering a new perspective for preclinical investigations into neonatal diseases and development.

Beyond nimodipine: advanced neuroprotection strategies for aneurysmal subarachnoid hemorrhage vasospasm and delayed cerebral ischemia

The clinical management of aneurysmal subarachnoid hemorrhage (SAH)-associated vasospasm remains a challenge in neurosurgical practice, with its prevention and treatment having a major impact on neurological outcome. While considered a mainstay, nimodipine is burdened by some non-negligible limitations that make it still a suboptimal candidate of pharmacotherapy for SAH. This narrative review aims to provide an update on the pharmacodynamics, pharmacokinetics, overall evidence, and strength of recommendation of nimodipine alternative drugs for aneurysmal SAH-associated vasospasm and delayed cerebral ischemia. A PRISMA literature search was performed in the PubMed/Medline, Web of Science, ClinicalTrials.gov, and PubChem databases using a combination of the MeSH terms "medical therapy," "management," "cerebral vasospasm," "subarachnoid hemorrhage," and "delayed cerebral ischemia." Collected articles were reviewed for typology and relevance prior to final inclusion. A total of 346 articles were initially collected. The identification, screening, eligibility, and inclusion process resulted in the selection of 59 studies. Nicardipine and cilostazol, which have longer half-lives than nimodipine, had robust evidence of efficacy and safety. Eicosapentaenoic acid, dapsone and clazosentan showed a good balance between effectiveness and favorable pharmacokinetics. Combinations between different drug classes have been studied to a very limited extent. Nicardipine, cilostazol, Rho-kinase inhibitors, and clazosentan proved their better pharmacokinetic profiles compared with nimodipine without prejudice with effective and safe neuroprotective role. However, the number of trials conducted is significantly lower than for nimodipine. Aneurysmal SAH-associated vasospasm remains an area of ongoing preclinical and clinical research where the search for new drugs or associations is critical.

Current state of neuroprotective therapy using antibiotics in human traumatic brain injury and animal models

TBI is a leading cause of death and disability in young people and older adults worldwide. There is no gold standard treatment for TBI besides surgical interventions and symptomatic relief. Post-injury infections, such as lower respiratory tract and surgical site infections or meningitis are frequent complications following TBI. Whether the use of preventive and/or symptomatic antibiotic therapy improves patient mortality and outcome is an ongoing matter of debate. In contrast, results from animal models of TBI suggest translational perspectives and support the hypothesis that antibiotics, independent of their anti-microbial activity, alleviate secondary injury and improve neurological outcomes. These beneficial effects were largely attributed to the inhibition of neuroinflammation and neuronal cell death. In this review, we briefly outline current treatment options, including antibiotic therapy, for patients with TBI. We then summarize the therapeutic effects of the most commonly tested antibiotics in TBI animal models, highlight studies identifying molecular targets of antibiotics, and discuss similarities and differences in their mechanistic modes of action.

Treating Traumatic Brain Injury with Minocycline

Traumatic brain injury (TBI) results in both rapid and delayed brain damage. The speed, complexity, and persistence of TBI present large obstacles to drug development. Preclinical studies from multiple laboratories have tested the FDA-approved anti-microbial drug minocycline (MINO) to treat traumatic brain injury. At concentrations greater than needed for anti-microbial action, MINO readily inhibits microglial activation. MINO has additional pleotropic effects including anti-inflammatory, anti-oxidant, and anti-apoptotic activities. MINO inhibits multiple proteins that promote brain injury including metalloproteases, caspases, calpain, and polyADP-ribose-polymerase-1. At these elevated doses, MINO is well tolerated and enters the brain even when the blood-brain barrier is intact. Most preclinical studies with a first dose of MINO at less than 1 h after injury have shown improved multiple outcomes after TBI. Fewer studies with more delayed dosing have yielded similar results. A small number of clinical trials for TBI have established the safety of MINO and suggested some drug efficacy. Studies are also ongoing that either improve MINO pharmacology or combine MINO with other drugs to increase its therapeutic efficacy against TBI. This review builds upon a previous, recent review by some of the authors (Lawless and Bergold, Neural Regen Res 17:2589-92, 2022). The present review includes the additional preclinical studies examining the efficacy of minocycline in preclinical TBI models. This review also includes recommendations for a clinical trial to test MINO to treat TBI.

Repeated mild closed head injury in neonatal rats results in sustained cognitive deficits associated with chronic microglial activation and neurodegeneration

Abusive head trauma in infants is a consequence of multiple episodes of abuse and results in axonal injury, brain atrophy, and chronic cognitive deficits. Anesthetized 11-day-old rats, neurologically equivalent to infants, were subjected to 1 impact/day to the intact skull for 3 successive days. Repeated, but not single impact(s) resulted in spatial learning deficits (p < 0.05 compared to sham-injured animals) up to 5 weeks postinjury. In the first week following single or repetitive brain injury, axonal and neuronal degeneration, and microglial activation were observed in the cortex, white matter, thalamus, and subiculum; the extent of the histopathologic damage was significantly greater in the repetitive-injured animals compared to single-injured animals. At 40 days postinjury, loss of cortical, white matter and hippocampal tissue was evident only in the repetitive-injured animals, along with evidence of microglial activation in the white matter tracts and thalamus. Axonal injury and neurodegeneration were evident in the thalamus up to 40 days postinjury in the repetitive-injured rats. These data demonstrate that while single closed head injury in the neonate rat is associated with pathologic alterations in the acute post-traumatic period, repetitive closed head injury results in sustained behavioral and pathologic deficits reminiscent of infants with abusive head trauma.

Abusive Head Trauma Animal Models: Focus on Biomarkers

Abusive head trauma (AHT) is a serious traumatic brain injury and the leading cause of death in children younger than 2 years. The development of experimental animal models to simulate clinical AHT cases is challenging. Several animal models have been designed to mimic the pathophysiological and behavioral changes in pediatric AHT, ranging from lissencephalic rodents to gyrencephalic piglets, lambs, and non-human primates. These models can provide helpful information for AHT, but many studies utilizing them lack consistent and rigorous characterization of brain changes and have low reproducibility of the inflicted trauma. Clinical translatability of animal models is also limited due to significant structural differences between developing infant human brains and the brains of animals, and an insufficient ability to mimic the effects of long-term degenerative diseases and to model how secondary injuries impact the development of the brain in children. Nevertheless, animal models can provide clues on biochemical effectors that mediate secondary brain injury after AHT including neuroinflammation, excitotoxicity, reactive oxygen toxicity, axonal damage, and neuronal death. They also allow for investigation of the interdependency of injured neurons and analysis of the cell types involved in neuronal degeneration and malfunction. This review first focuses on the clinical challenges in diagnosing AHT and describes various biomarkers in clinical AHT cases. Then typical preclinical biomarkers such as microglia and astrocytes, reactive oxygen species, and activated N-methyl-D-aspartate receptors in AHT are described, and the value and limitations of animal models in preclinical drug discovery for AHT are discussed.

Neuro-Inflammation Modulation and Post-Traumatic Brain Injury Lesions: From Bench to Bed-Side

Head trauma is the most common cause of disability in young adults. Known as a silent epidemic, it can cause a mosaic of symptoms, whether neurological (sensory-motor deficits), psychiatric (depressive and anxiety symptoms), or somatic (vertigo, tinnitus, phosphenes). Furthermore, cranial trauma (CT) in children presents several particularities in terms of epidemiology, mechanism, and physiopathology-notably linked to the attack of an immature organ. As in adults, head trauma in children can have lifelong repercussions and can cause social and family isolation, difficulties at school, and, later, socio-professional adversity. Improving management of the pre-hospital and rehabilitation course of these patients reduces secondary morbidity and mortality, but often not without long-term disability. One hypothesized contributor to this process is chronic neuroinflammation, which could accompany primary lesions and facilitate their development into tertiary lesions. Neuroinflammation is a complex process involving different actors such as glial cells (astrocytes, microglia, oligodendrocytes), the permeability of the blood-brain barrier, excitotoxicity, production of oxygen derivatives, cytokine release, tissue damage, and neuronal death. Several studies have investigated the effect of various treatments on the neuroinflammatory response in traumatic brain injury in vitro and in animal and human models. The aim of this review is to examine the various anti-inflammatory therapies that have been implemented.

The effect of minocycline on beta-amyloid-induced memory and learning deficit in male rats: A behavioral, biochemical, and histological study

OBJECTIVES

Minocycline hydrochloride is a semi-synthetic, second-generation tetracycline with neuroprotective, neurorestorative, anti-amyloidogenic, anti-inflammatory, antioxidant, and anti-apoptotic properties. The present study was designed to investigate the potential protective effects of minocycline against beta-amyloid (Aβ)-induced Alzheimer's disease (AD), recognition memory decline, and the possible involved anti-apoptotic mechanisms.

METHODS

The rats were treated with minocycline (50 and 100 mg/kg/day; P.O.) after AD induction for 30 days. Behavioral functions were assessed by employing standard behavioral tests, including novel object recognition (NOR) and passive avoidance learning (PAL) tasks. Then, total antioxidant capacity (TAC) and total oxidant status (TOS) were measured in blood serum using ELISA kits. Apoptosis and the number of Aβ plaques were examined by the TUNEL and Congo red staining, respectively.

RESULTS

Treatment of Aβ rats with minocycline improved memory deficit in the PAL task and a decline in recognition memory in the NOR test. Minocycline at 50 and 100 mg/kg significantly reduced the TOS levels and increased the TAC levels (P < 0.0001). Also, minocycline at 50 and 100 mg/kg reduced the apoptotic index in the hippocampus of Aβ rats. After Congo red staining, the minocycline group showed improved cell morphology and markedly fewer Aβ plaques.

CONCLUSIONS

Minocycline reduced memory and learning deficit in behavioral experiments after Aβ injection, which may be due to its anti-inflammatory and anti-apoptotic effects.

Defining Experimental Variability in Actuator-Driven Closed Head Impact in Rats

Traumatic brain injury (TBI) is a world-wide health challenge that lacks tools for diagnosis and treatment. There is a need for translational preclinical models to effectively design clinical tools, however, the diversity of models is a barrier to reproducible studies. Actuator-driven closed head impact (AD-CHI) models have translational advantages in replicating the pathophysiological and behavioral outcomes resulting from impact TBI. The main advantages of AD-CHI protocols include versatility of impact parameters such as impact angle, velocity, depth, and dwell time with the ability to interchange tip types, leading to consistent outcomes without the need for craniectomy. Sources of experimental variability within AD-CHI rat models are identified within this review with the aim of supporting further characterization to improve translational value. Primary areas of variability may be attributed to lack of standardization of head stabilization methods, reporting of tip properties, and performance of acute neurological assessments. AD-CHI models were also found to be more prevalently used among pediatric and repeated TBI paradigms. As this model continues to grow in use, establishing the relationships between impact parameters and associated injury outcomes will reduce experimental variability between research groups and encourage meaningful discussions as the community moves towards common data elements.

Microglial polarization in TBI: Signaling pathways and influencing pharmaceuticals

Traumatic brain injury (TBI) is a serious disease that threatens life and health of people. It poses a great economic burden on the healthcare system. Thus, seeking effective therapy to cure a patient with TBI is a matter of great urgency. Microglia are macrophages in the central nervous system (CNS) and play an important role in neuroinflammation. When TBI occurs, the human body environment changes dramatically and microglia polarize to one of two different phenotypes: M1 and M2. M1 microglia play a role in promoting the development of inflammation, while M2 microglia play a role in inhibiting inflammation. How to regulate the polarization direction of microglia is of great significance for the treatment of patients with TBI. The polarization of microglia involves many cellular signal transduction pathways, such as the TLR-4/NF-κB, JAK/STAT, HMGB1, MAPK, and PPAR-γ pathways. These provide a theoretical basis for us to seek therapeutic drugs for the patient with TBI. There are several drugs that target these pathways, including fingolimod, minocycline, Tak-242 and erythropoietin (EPO), and CSF-1. In this study, we will review signaling pathways involved in microglial polarization and medications that influence this process.

A deep learning-based toolbox for Automated Limb Motion Analysis (ALMA) in murine models of neurological disorders

In neuroscience research, the refined analysis of rodent locomotion is complex and cumbersome, and access to the technique is limited because of the necessity for expensive equipment. In this study, we implemented a new deep learning-based open-source toolbox for Automated Limb Motion Analysis (ALMA) that requires only basic behavioral equipment and an inexpensive camera. The ALMA toolbox enables the consistent and comprehensive analyses of locomotor kinematics and paw placement and can be applied to neurological conditions affecting the brain and spinal cord. We demonstrated that the ALMA toolbox can (1) robustly track the evolution of locomotor deficits after spinal cord injury, (2) sensitively detect locomotor abnormalities after traumatic brain injury, and (3) correctly predict disease onset in a multiple sclerosis model. We, therefore, established a broadly applicable automated and standardized approach that requires minimal financial and time commitments to facilitate the comprehensive analysis of locomotion in rodent disease models.

Presenting ALMA toolbox, an open source Python repository for automatic analysis of mouse locomotion using bodypart coordinates from markerless pose estimation tools. ALMA is capable of analyzing both healthy and CNS-injured mice. ALMA is also capable of predicting onset of disease in a multiple sclerosis model.

Acute minocycline administration reduces brain injury and improves long-term functional outcomes after delayed hypoxemia following traumatic brain injury

Clinical trials of therapeutics for traumatic brain injury (TBI) demonstrating preclinical efficacy for TBI have failed to replicate these results in humans, in part due to the absence of clinically feasible therapeutic windows for administration. Minocycline, an inhibitor of microglial activation, has been shown to be neuroprotective when administered early after experimental TBI but detrimental when administered chronically to human TBI survivors. Rather than focusing on the rescue of primary injury with early administration of therapeutics which may not be clinically feasible, we hypothesized that minocycline administered at a clinically feasible time point (24 h after injury) would be neuroprotective in a model of TBI plus delayed hypoxemia. We first explored several different regimens of minocycline dosing with the initial dose 24 h after injury and 2 h prior to hypoxemia, utilizing short-term neuropathology to select the most promising candidate. We found that a short course of minocycline reduced acute microglial activation, monocyte infiltration and hippocampal neuronal loss at 1 week post injury. We then conducted a preclinical trial to assess the long-term efficacy of a short course of minocycline finding reductions in hippocampal neurodegeneration and synapse loss, preservation of white matter myelination, and improvements in fear memory performance at 6 months after injury. Timing in relation to injury and duration of minocycline treatment and its impact on neuroinflammatory response may be responsible for extensive neuroprotection observed in our studies.

Neuroinflammation as an etiological trigger for depression comorbid with inflammatory bowel disease

Patients with inflammatory bowel disease (IBD) suffer from depression at higher rates than the general population. An etiological trigger of depressive symptoms is theorised to be inflammation within the central nervous system. It is believed that heightened intestinal inflammation and dysfunction of the enteric nervous system (ENS) contribute to impaired intestinal permeability, which facilitates the translocation of intestinal enterotoxins into the blood circulation. Consequently, these may compromise the immunological and physiological functioning of distant non-intestinal tissues such as the brain. In vivo models of colitis provide evidence of increased blood–brain barrier permeability and enhanced central nervous system (CNS) immune activity triggered by intestinal enterotoxins and blood-borne inflammatory mediators. Understanding the immunological, physiological, and structural changes associated with IBD and neuroinflammation may aid in the development of more tailored and suitable pharmaceutical treatment for IBD-associated depression.

Microglia: A Potential Drug Target for Traumatic Axonal Injury

Traumatic axonal injury (TAI) is a major cause of death and disability among patients with severe traumatic brain injury (TBI); however, no effective therapies have been developed to treat this disorder. Neuroinflammation accompanying microglial activation after TBI is likely to be an important factor in TAI. In this review, we summarize the current research in this field, and recent studies suggest that microglial activation plays an important role in TAI development. We discuss several drugs and therapies that may aid TAI recovery by modulating the microglial phenotype following TBI. Based on the findings of recent studies, we conclude that the promotion of active microglia to the M2 phenotype is a potential drug target for the treatment of TAI.

Unravelling neuroinflammation in abusive head trauma with radiotracer imaging

Abusive head trauma (AHT) is a leading cause of mortality and morbidity in child abuse, with a mortality rate of approximately 25%. In survivors, the prognosis remains dismal, with high prevalence of cerebral palsy, epilepsy and neuropsychiatric disorders. Early and accurate diagnosis of AHT is challenging, both clinically and radiologically, with up to one-third of cases missed on initial examination. Moreover, most of the management in AHT is supportive, reflective of the lack of clear understanding of specific pathogenic mechanisms underlying secondary insult, with approaches targeted toward decreasing intracranial hypertension and reducing cerebral metabolism, cell death and excitotoxicity. Multiple studies have elucidated the role of pro- and anti-inflammatory cytokines and chemokines with upregulation/recruitment of microglia/macrophages, oligodendrocytes and astrocytes in severe traumatic brain injury (TBI). In addition, recent studies in animal models of AHT have demonstrated significant upregulation of microglia, with a potential role of inflammatory cascade contributing to secondary insult. Despite the histological and biochemical evidence, there is a significant dearth of specific imaging approaches to identify this neuroinflammation in AHT. The primary motivation for development of such imaging approaches stems from the need to therapeutically target neuroinflammation and establish its utility in monitoring and prognostication. In the present paper, we discuss the available data suggesting the potential role of neuroinflammation in AHT and role of radiotracer imaging in aiding diagnosis and patient management.

Microglia-Mediated Neurodegeneration in Perinatal Brain Injuries

Perinatal brain injuries, including encephalopathy related to fetal growth restriction, encephalopathy of prematurity, neonatal encephalopathy of the term neonate, and neonatal stroke, are a major cause of neurodevelopmental disorders. They trigger cellular and molecular cascades that lead in many cases to permanent motor, cognitive, and/or behavioral deficits. Damage includes neuronal degeneration, selective loss of subclasses of interneurons, blocked maturation of oligodendrocyte progenitor cells leading to dysmyelination, axonopathy and very likely synaptopathy, leading to impaired connectivity. The nature and severity of changes vary according to the type and severity of insult and maturation stage of the brain. Microglial activation has been demonstrated almost ubiquitously in perinatal brain injuries and these responses are key cell orchestrators of brain pathology but also attempts at repair. These divergent roles are facilitated by a diverse suite of transcriptional profiles and through a complex dialogue with other brain cell types. Adding to the complexity of understanding microglia and how to modulate them to protect the brain is that these cells have their own developmental stages, enabling them to be key participants in brain building. Of note, not only do microglia help build the brain and respond to brain injury, but they are a key cell in the transduction of systemic inflammation into neuroinflammation. Systemic inflammatory exposure is a key risk factor for poor neurodevelopmental outcomes in preterm born infants. Based on these observations, microglia appear as a key cell target for neuroprotection in perinatal brain injuries. Numerous strategies have been developed experimentally to modulate microglia and attenuate brain injury based on these strong supporting data and we will summarize these.

Niche Cells Crosstalk In Neuroinflammation After Traumatic Brain Injury

Traumatic brain injury (TBI) is recognized as the disease with high morbidity and disability around world in spite of the work ongoing in neural protection. Due to heterogeneity among the patients, it's still hard to acquire satisfying achievements in clinic. Neuroinflammation, which exists since primary injury occurs, with elusive duality, appear to be of significance from recovery of injury to neurogenesis. In recent years, studied have revealed that communication in neurogenic niche is more than “cell to cell” communication, and study on NSCs represent it as central role in the progress of neural regeneration. Hence, the neuroinflammation-affecting crosstalk after TBI, and clarifying definitive role of NSCs in the course of regeneration is a promising subject for researchers, for its great potential in overcoming the frustrating status quo in clinic, promoting welfare of TBI patient.

Stem Cell Therapy for Pediatric Traumatic Brain Injury

There has been a growing interest in the potential of stem cell transplantation as therapy for pediatric brain injuries. Studies in pre-clinical models of pediatric brain injury such as Traumatic Brain Injury (TBI) and neonatal hypoxia-ischemia (HI) have contributed to our understanding of the roles of endogenous stem cells in repair processes and functional recovery following brain injury, and the effects of exogenous stem cell transplantation on recovery from brain injury. Although only a handful of studies have evaluated these effects in models of pediatric TBI, many studies have evaluated stem cell transplantation therapy in models of neonatal HI which has a considerable overlap of injury pathology with pediatric TBI. In this review, we have summarized data on the effects of stem cell treatments on histopathological and functional outcomes in models of pediatric brain injury. Importantly, we have outlined evidence supporting the potential for stem cell transplantation to mitigate pathology of pediatric TBI including neuroinflammation and white matter injury, and challenges that will need to be addressed to incorporate these therapies to improve functional outcomes following pediatric TBI.

The Role of Microglia in Cerebral Traumatic Injury and its Therapeutic Implications

Microglia have a variety of functions in the brain such as synaptic remodeling, damage repair of the central nervous system (CNS), and CNS’ inflammatory response to peripheral infections. The response depends on the type of insult and infection and includes a range of variety of activation states, the duration of which will decide the outcome. In response to traumatic brain injury (TBI), early activation can lead to early restoration of function, while prolonged and continuous activation can cause neurodegeneration states. Current evidence, however, states that this may not be the case. In this article, we discuss this seldom understood topic of microglia response to TBI, and analyze their distribution, function and possible sites of manipulation. Animal studies have allowed genetic and pharmacological manipulations of microglia activation, in order to define their role. Microglia activation can be remote to the site of injury, and thus their manipulation may play a significant role in the response to any trauma.

Longitudinal optical imaging technique to visualize progressive axonal damage after brain injury in mice reveals responses to different minocycline treatments

A high-resolution, three-dimensional, optical imaging technique for the murine brain was developed to identify the effects of different therapeutic windows for preclinical brain research. This technique tracks the same cells over several weeks. We conducted a pilot study of a promising drug to treat diffuse axonal injury (DAI) caused by traumatic brain injury, using two different therapeutic windows, as a means to demonstrate the utility of this novel longitudinal imaging technique. DAI causes immediate, sporadic axon damage followed by progressive secondary axon damage. We administered minocycline for three days commencing one hour after injury in one treatment group and beginning 72 hours after injury in another group to demonstrate the method’s ability to show how and when the therapeutic drug exerts protective and/or healing effects. Fewer varicosities developed in acutely treated mice while more varicosities resolved in mice with delayed treatment. For both treatments, the drug arrested development of new axonal damage by 30 days. In addition to evaluation of therapeutics for traumatic brain injury, this hybrid microlens imaging method should be useful to study other types of brain injury and neurodegeneration and cellular responses to treatment.

Prx2 (Peroxiredoxin 2) as a Cause of Hydrocephalus After Intraventricular Hemorrhage

Background and Purpose- Our recent study demonstrated that release of Prx2 (peroxiredoxin 2) from red blood cells (RBCs) is involved in the inflammatory response and brain injury after intracerebral hemorrhage. The current study investigated the role of extracellular Prx2 in hydrocephalus development after experimental intraventricular hemorrhage. Methods- There were 4 parts in this study. First, Sprague-Dawley rats received an intraventricular injection of lysed RBC or saline and were euthanized at 1 hour for Prx2 measurements. Second, rats received an intraventricular injection of Prx2, deactivated Prx2, or saline. Third, lysed RBC was coinjected with conoidin A, a Prx2 inhibitor, or vehicle. Fourth, rats received Prx2 injection and were treated with minocycline or saline (i.p.). The effects of Prx2 and the inhibitors were examined using magnetic resonance imaging assessing ventriculomegaly, histology assessing ventricular wall damage, and immunohistochemistry to assess inflammation, particularly at the choroid plexus. Results- Intraventricular injection of lysed RBC resulted in increased brain Prx2 and hydrocephalus. Intraventricular injection of Prx2 alone caused hydrocephalus, ventricular wall damage, activation of choroid plexus epiplexus cells (macrophages), and an accumulation of neutrophils. Conoidin A attenuated lysed RBC-induced injury. Systemic minocycline treatment reduced the epiplexus cell activation and hydrocephalus induced by Prx2. Conclusions- Prx2 contributed to the intraventricular hemorrhage-induced hydrocephalus, probably by inducing inflammatory responses in choroid plexus and ventricular wall damage.

Minocycline-induced microbiome alterations predict cafeteria diet-induced spatial recognition memory impairments in rats

Diets rich in sugar and saturated fat are associated with cognitive impairments in both humans and rodents with several potential mechanisms proposed. To test the involvement of diet-induced pro-inflammatory signaling, we exposed rats to a high-fat, high-sugar cafeteria diet, and administered the anti-inflammatory antibiotic minocycline. In the first experiment minocycline was coadministered across the diet, then in a second, independent cohort it was introduced following 4 weeks of cafeteria diet. Cafeteria diet impaired novel place recognition memory throughout the study. Minocycline not only prevented impairment in spatial recognition memory but also reversed impairment established in rats following 4 weeks cafeteria diet. Further, minocycline normalized diet-induced increases in hippocampal pro-inflammatory gene expression. No effects of minocycline were seen on adiposity or dietary intake across the experiments. Cafeteria diet and minocycline treatment significantly altered microbiome composition. The relative abundance of Desulfovibrio_OTU31, uniquely enriched in vehicle-treated cafeteria-fed rats, negatively and significantly correlated with spatial recognition memory. We developed a statistical model that accurately predicts spatial recognition memory based on Desulfovibrio_OTU31 relative abundance and fat mass. Thus, our results show that minocycline prevents and reverses a dietary-induced diet impairment in spatial recognition memory, and that spatial recognition performance is best predicted by changes in body composition and Desulfovibrio_OTU31, rather than changes in pro-inflammatory gene expression.

Post-Ischaemic Immunological Response in the Brain: Targeting Microglia in Ischaemic Stroke Therapy

Microglia, the major endogenous immune cells of the central nervous system, mediate critical degenerative and regenerative responses in ischaemic stroke. Microglia become "activated", proliferating, and undergoing changes in morphology, gene and protein expression over days and weeks post-ischaemia, with deleterious and beneficial effects. Pro-inflammatory microglia (commonly referred to as M1) exacerbate secondary neuronal injury through the release of reactive oxygen species, cytokines and proteases. In contrast, microglia may facilitate neuronal recovery via tissue and vascular remodelling, through the secretion of anti-inflammatory cytokines and growth factors (a profile often termed M2). This M1/M2 nomenclature does not fully account for the microglial heterogeneity in the ischaemic brain, with some simultaneous expression of both M1 and M2 markers at the single-cell level. Understanding and regulating microglial activation status, reducing detrimental and promoting repair behaviours, present the potential for therapeutic intervention, and open a longer window of opportunity than offered by acute neuroprotective strategies. Pharmacological modulation of microglial activation status to promote anti-inflammatory gene expression can increase neurogenesis and improve functional recovery post-stroke, based on promising preclinical data. Cell-based therapies, using preconditioned microglia, are of interest as a method of therapeutic modulation of the post-ischaemic inflammatory response. Currently, there are no clinically-approved pharmacological options targeting post-ischaemic inflammation. A major developmental challenge for clinical translation will be the selective suppression of the deleterious effects of microglial activity after stroke whilst retaining (or enhancing) the neurovascular repair and remodelling responses of microglia.

Docosahexaenoic acid decreased neuroinflammation in rat pups after controlled cortical impact

Traumatic brain injury (TBI) is the leading cause of acquired neurologic disability in children, yet specific therapies to treat TBI are lacking. Therapies that decrease the inflammatory response and enhance a reparative immune action may decrease oxidative damage and improve outcomes after TBI. Docosahexaenoic acid (DHA) modulates the immune response to injury in many organs. DHA given in the diet before injury decreased rat pup cognitive impairment, oxidative stress and white matter injury in our developmental TBI model using controlled cortical impact (CCI). Little is known about DHA effects on neuroinflammation in the developing brain. Further, it is not known if DHA given after developmental TBI exerts neuroprotective effects. We hypothesized that acute DHA treatment would decrease oxidative stress and improve cognitive outcome, associated with decreased pro-inflammatory activation of microglia, the brain's resident macrophages.

METHODS

17-day-old rat pups received intraperitoneal DHA or vehicle after CCI or SHAM surgery followed by DHA diet or continuation of REG diet to create DHACCI, REGCCI, SHAMDHA and SHAMREG groups. We measured brain nitrates/nitrites (NOx) at post injury day (PID) 1 to assess oxidative stress. We tested memory using Novel Object Recognition (NOR) at PID14. At PID 3 and 7, we measured reactivity of microglial activation markers Iba1, CD68 and CD206 and astrocyte marker GFAP in the injured cortex. At PID3, 7 and 30 we measured mRNA levels of inflammation-related genes and transcription factors in flow-sorted brain cells.

RESULTS

DHA decreased oxidative stress at PID1 and pro-inflammatory microglial activation at PID3. CCI increased mRNA levels of two interferon regulatory family transcription factors, blunted by DHA, particularly in microglia-enriched cell populations at PID7. CCI increased mRNA levels of genes associated with "pro- " and "anti-" inflammatory activity at PID3, 7 and 30. Most notably within the microglia-enriched population, DHA blunted increased mRNA levels of pro-inflammatory genes at PID 3 and 7 and of anti-inflammatory genes at PID 30. Particularly in microglia, we observed parallel activation of pro-inflammatory and anti-inflammatory genes. DHA improved performance on NOR at PID14 after CCI.

CONCLUSIONS

DHA decreased oxidative stress and histologic and mRNA markers of microglial pro-inflammatory activation in rat pup brain acutely after CCI associated with improved short term cognitive function. DHA administration after CCI has neuroprotective effects, which may result in part from modulation of microglial activation toward a less inflammatory profile in the first week after CCI. Future and ongoing studies will focus on phagocytic function and reactive oxygen species production in microglia and macrophages to test functional effects of DHA on neuroinflammation in our model. Given its favorable safety profile in children, DHA is a promising candidate therapy for pediatric TBI.

Neuroimmune responses in the developing brain following traumatic brain injury

Traumatic brain injury (TBI) is one of the leading causes of both acute and long-term morbidity in the pediatric population, leading to a substantial, long-term socioeconomic burden. Despite the increase in the amount of pre-clinical and clinical research, treatment options for TBI rely heavily on supportive care with very limited targeted interventions that improve the acute and chronic sequelae of TBI. Other than injury prevention, not much can be done to limit the primary injury, which consists of tissue damage and cellular destruction. Secondary injury is the result of the ongoing complex inflammatory pathways that further exacerbate tissue damage, resulting in the devastating chronic outcomes of TBI. On the other hand, some level of inflammation is essential for neuronal regeneration and tissue repair. In this review article we discuss the various stages of the neuroimmune response in the immature, pediatric brain in the context of normal maturation and development of the immune system. The developing brain has unique features that distinguish it from the adult brain, and the immune system plays an integral role in CNS development. Those features could potentially make the developing brain more susceptible to worse outcomes, both acutely and in the long-term. The neuroinflammatory reaction which is triggered by TBI can be described as a highly intricate interaction between the cells of the innate and the adaptive immune systems. The innate immune system is triggered by non-specific danger signals that are released from damaged cells and tissues, which in turn leads to neutrophil infiltration, activation of microglia and astrocytes, complement release, as well as histamine release by mast cells. The adaptive immune response is subsequently activated leading to the more chronic effects of neuroinflammation. We will also discuss current attempts at modulating the TBI-induced neuroinflammatory response. A better understanding of the role of the immune system in normal brain development and how immune function changes with age is crucial for designing therapies to appropriately target the immune responses following TBI in order to enhance repair and plasticity.

Depletion of microglia immediately following traumatic brain injury in the pediatric rat: Implications for cellular and behavioral pathology

The inflammatory response is a significant component of the pathophysiology of pediatric traumatic brain injury. High levels of inflammatory mediators have been found in the cerebrospinal fluid of brain-injured children which have been linked to poor prognosis. Targeting aspects of the inflammatory response in the hopes of finding a viable post-injury therapeutic option has gained attention. Microglia are largely responsible for perpetuating the injury-induced inflammatory response but in the developing brain they play beneficial roles in both normal and disease states. Following closed head injury in the neonate rat, depletion of microglia with intracerebral injections of liposomes containing clodronate was associated with an increase in neurodegeneration in the early post-injury period (3 days) relative to those injected with empty liposomes suggestive of a decrease in clearance of dying cells. In sham-injured animals, microglia repopulated the clodrosome-mediated depleted brain regions over a period of 2-4 weeks and exhibited morphology typical of a resting phenotype. In brain-injured animals, the repopulated microglia in clodrosome-injected animals exhibited rod-like and amoeboid morphologies. However, fluoro-Jade B reactivity in these brain regions was more extensive than in empty liposome-injected animals suggesting that the active microglia may be unable to clear dying neurons. This was accompanied by an induction of hyperexcitability in the local cortical circuitry. Depletion of microglia within the white matter tracts and the thalamus did not affect the extent of injury-induced traumatic axonal injury. Increased neurodegeneration in the dorsal subiculum was not accompanied by any changes to injury-induced deficits in spatial learning and memory. These data suggest that activation of microglia may be important for removal of dying neurons in the traumatically-injured immature brain.

Minocycline prevents the development of depression-like behavior and hippocampal inflammation in a rat model of Alzheimer's disease

RATIONALE

Considerable clinical and experimental studies have shown that depression-related disorders are the most common neuropsychiatric symptoms in Alzheimer's disease (AD), affecting as many as 20-40% of patients. An increasing amount of evidence shows that monoamine-based antidepressant treatments are not completely effective for depression treatment in patients with dementia. Minocycline, a second-generation tetracycline antibiotic, has been gaining research and clinical attention for the treatment of different neuropsychiatric disorders, and more recently depression symptom in humans.

METHODS

In the present study, we investigated the effects of Aβ1-42 administration alone or in combination with minocycline treatment on depression-like behaviors and anti/pro-inflammatory cytokines such as interleukin(IL)-10, IL-β, and tumor necrosis factor (TNF)-α in the hippocampus of rats.

RESULTS

Our results showed that Aβ1-42 administration increased depression-related behaviors in sucrose preference test, tail suspension test, novelty-suppressed feeding test, and forced swim test. We also found significant increases in IL-1β and TNF-α levels in the hippocampus of Aβ1-42-treated rats. Interestingly, minocycline treatment significantly reversed depression-related behaviors and the levels of hippocampal cytokines in Aβ1-42-treated rats.

CONCLUSION

These findings support the idea that there is a significant relationship among AD, depression-related symptoms, and pro-inflammatory cytokines in the brain, and suggest that antidepressant-like impacts of minocycline could be due to its anti-inflammatory properties. This drug could be of potential interest for the treatment of depression in patients with Alzheimer's disease.

Plasticity of microglia in remote regions after focal brain injury

The CNS is endowed with an intrinsic ability to recover from and adapt secondary compensatory mechanisms to injury. The basis of recovery stems from brain plasticity, defined as the brain's ability to make adaptive changes on structural and functional levels, ranging from molecular, synaptic, and cellular changes in response to alterations in their environment. In this multitude of responses, microglia have an active role and contribute to brain plasticity through their dynamic responses. This review will provide an overview of microglial responses in the context of acute CNS injury and their function in post-traumatic repair and assess the changes that are induced by damage in remote areas from, but functionally connected to, the primary site of injury. In the second section, we highlight the effects of several therapeutic approaches, with particular interest paid to specialized pro-resolving lipid mediators, in modulating microglial responses in remote regions and enhancing long-term functional recovery via suppression of neurodegenerative cascades that are induced by damage, which may contribute to a translational bridge from bench to bedside.

Enduring Neuroprotective Effect of Subacute Neural Stem Cell Transplantation After Penetrating TBI

Traumatic brain injury (TBI) is the largest cause of death and disability of persons under 45 years old, worldwide. Independent of the distribution, outcomes such as disability are associated with huge societal costs. The heterogeneity of TBI and its complicated biological response have helped clarify the limitations of current pharmacological approaches to TBI management. Five decades of effort have made some strides in reducing TBI mortality but little progress has been made to mitigate TBI-induced disability. Lessons learned from the failure of numerous randomized clinical trials and the inability to scale up results from single center clinical trials with neuroprotective agents led to the formation of organizations such as the Neurological Emergencies Treatment Trials (NETT) Network, and international collaborative comparative effectiveness research (CER) to re-orient TBI clinical research. With initiatives such as TRACK-TBI, generating rich and comprehensive human datasets with demographic, clinical, genomic, proteomic, imaging, and detailed outcome data across multiple time points has become the focus of the field in the United States (US). In addition, government institutions such as the US Department of Defense are investing in groups such as Operation Brain Trauma Therapy (OBTT), a multicenter, pre-clinical drug-screening consortium to address the barriers in translation. The consensus from such efforts including “The Lancet Neurology Commission” and current literature is that unmitigated cell death processes, incomplete debris clearance, aberrant neurotoxic immune, and glia cell response induce progressive tissue loss and spatiotemporal magnification of primary TBI. Our analysis suggests that the focus of neuroprotection research needs to shift from protecting dying and injured neurons at acute time points to modulating the aberrant glial response in sub-acute and chronic time points. One unexpected agent with neuroprotective properties that shows promise is transplantation of neural stem cells. In this review we present (i) a short survey of TBI epidemiology and summary of current care, (ii) findings of past neuroprotective clinical trials and possible reasons for failure based upon insights from human and preclinical TBI pathophysiology studies, including our group's inflammation-centered approach, (iii) the unmet need of TBI and unproven treatments and lastly, (iv) present evidence to support the rationale for sub-acute neural stem cell therapy to mediate enduring neuroprotection.

The influence of head injury on olfactory and gustatory function

Head injury, particularly that resulting in brain injury, is a significant public health concern. For example, annual incidence rates of traumatic brain injury, a common consequence of head injury, range from 54 to 60 million people worldwide, including 2.2-3.6 million people whose trauma is moderate to severe. Trauma to the face and brain, including blast injuries common in modern warfare, can result in alterations in the ability to both smell and taste. In the case of smell, these include total loss of function (anosmia), decreased sensitivity (hyposmia), alterations in odor quality (dysosmia), and hallucination (phantosmia). Although taste dysfunction, i.e., altered perception of such basic taste-bud-mediated sensations as sweet, sour, bitter, salty, and savory (umami), can be similarly influenced by head trauma, the effects are typically more subtle and less studied. The present review provides an up-to-date assessment of what is known about the impact of head injury on quantitative measures of taste and smell function, including the influences of severity, type of injury, location of insults, prognosis, and approaches to therapy.

Knowledge Gaps and Emerging Research Areas in Intrauterine Growth Restriction-Associated Brain Injury

Intrauterine growth restriction (IUGR) is a complex global healthcare issue. Concerted research and clinical efforts have improved our knowledge of the neurodevelopmental sequelae of IUGR which has raised the profile of this complex problem. Nevertheless, there is still a lack of therapies to prevent the substantial rates of fetal demise or the constellation of permanent neurological deficits that arise from IUGR. The purpose of this article is to highlight the clinical and translational gaps in our knowledge that hamper our collective efforts to improve the neurological sequelae of IUGR. Also, we draw attention to cutting-edge tools and techniques that can provide novel insights into this disorder, and technologies that offer the potential for better drug design and delivery. We cover topics including: how we can improve our use of crib-side monitoring options, what we still need to know about inflammation in IUGR, the necessity for more human post-mortem studies, lessons from improved integrated histology-imaging analyses regarding the cell-specific nature of magnetic resonance imaging (MRI) signals, options to improve risk stratification with genomic analysis, and treatments mediated by nanoparticle delivery which are designed to modify specific cell functions.

The histone demethylase KDM6B in the medial prefrontal cortex epigenetically regulates cocaine reward memory

Epigenetic remodeling contributes to synaptic plasticity via modification of gene expression, which underlies cocaine-induced long-term memory. A prevailing hypothesis in drug addiction is that drugs of abuse rejuvenate developmental machinery to render reward circuitry highly plastic and thus engender drug memories to be highly stable. Identification and reversal of these pathological pathways are therefore critical for cocaine abuse treatment. Previous studies revealed an interesting finding in which the mRNA of histone lysine demethylase, KDM6B, is upregulated in the medial prefrontal cortex (mPFC) during early cocaine withdrawal. However, whether and how it contributes to drug-seeking behavior remain unknown. Here we used a conditioned place preference paradigm to investigate the potential role of KDM6B in drug-associated memory. We found that KDM6B protein levels selectively increased in the mPFC during cocaine withdrawal. Notably, systemic injection of KDM6B inhibitor, GSK-J4, disrupted both reconsolidation of cocaine-conditioned memory and cocaine-primed reinstatement, suggesting dual effects of KDM6B in cocaine reward memory. In addition, we found that NMDAR expression and function were both enhanced during early cocaine withdrawal in mPFC. Injection of GSK-J4 selectively reversed this cocaine-induced increase of NR2A expression and synaptic function, suggesting that mal-adaptation of cocaine-induced synaptic plasticity in mPFC largely underlies KDM6B-mediated cocaine-associated memory. Altogether, these data suggest that KDM6B plays an essential role in cocaine-associated memory, which mainly acts through enhancing cocaine-induced synaptic plasticity in the mPFC. Our findings revealed a novel role of KDM6B in cocaine-associated memory and inhibition of KDM6B is a potential strategy to alleviate drug-seeking behavior.

Bidirectional Microglia-Neuron Communication in Health and Disease

Microglia are ramified cells that exhibit highly motile processes, which continuously survey the brain parenchyma and react to any insult to the CNS homeostasis. Although microglia have long been recognized as a crucial player in generating and maintaining inflammatory responses in the CNS, now it has become clear, that their function are much more diverse, particularly in the healthy brain. The innate immune response and phagocytosis represent only a little segment of microglia functional repertoire that also includes maintenance of biochemical homeostasis, neuronal circuit maturation during development and experience-dependent remodeling of neuronal circuits in the adult brain. Being equipped by numerous receptors and cell surface molecules microglia can perform bidirectional interactions with other cell types in the CNS. There is accumulating evidence showing that neurons inform microglia about their status and thus are capable of controlling microglial activation and motility while microglia also modulate neuronal activities. This review addresses the topic: how microglia communicate with other cell types in the brain, including fractalkine signaling, secreted soluble factors and extracellular vesicles. We summarize the current state of knowledge of physiological role and function of microglia during brain development and in the mature brain and further highlight microglial contribution to brain pathologies such as Alzheimer’s and Parkinson’s disease, brain ischemia, traumatic brain injury, brain tumor as well as neuropsychiatric diseases (depression, bipolar disorder, and schizophrenia).

Therapeutic strategies to target acute and long-term sequelae of pediatric traumatic brain injury

Pediatric traumatic brain injury (TBI) remains one of the leading causes of morbidity and mortality in children. Experimental and clinical studies demonstrate that the developmental age, the type of injury (diffuse vs. focal) and sex may play important roles in the response of the developing brain to a traumatic injury. Advancements in acute neurosurgical interventions and neurocritical care have improved and led to a decrease in mortality rates over the past decades. However, survivors are left with life-long behavioral deficits underscoring the need to better define the cellular mechanisms underlying these functional changes. A better understanding of these mechanisms some of which begin in the acute post-traumatic period may likely lead to targeted treatment strategies. Key considerations in designing pre-clinical experiments to test therapeutic strategies in pediatric TBI include the use of age-appropriate and pathologically-relevant models, functional outcomes that are tested as animals age into adolescence and beyond, sex as a biological variable and the recognition that doses and dosing strategies that have been demonstrated to be effective in animal models of adult TBI may not be effective in the developing brain. This article is part of the Special Issue entitled "Novel Treatments for Traumatic Brain Injury".

Minocycline promotes posthemorrhagic neurogenesis via M2 microglia polarization via upregulation of the TrkB/BDNF pathway in rats

Intracerebral hemorrhage (ICH) is a devastating disease worldwide with increasing mortality. The present study investigated whether minocycline was neuroprotective and induced M2 microglial polarization via upregulation of the TrkB/BDNF pathway after ICH. ICH was induced via injection of autologous blood into 150 Sprague-Dawley rats. A selective TrkB antagonist [N2-2-2-oxoazepan-3-yl amino] carbonyl phenyl benzo (b) thiophene-2-carboxamide (ANA 12)] and agonist [ N-[2-(5-hydroxy-1H-indol-3-yl) ethyl]-2-oxopiperidine-3-carboxamide (HIOC)] were used to investigate the mechanism of minocycline-induced neuroprotection. Minocycline improved ICH-induced neurological deficits and reduced M1 microglia marker protein (CD68, CD16) expression as well as M2 microglial polarization (CD206 and arginase 1 protein). Minocycline administration enhanced microglia-neuron cross talk and promoted the proliferation of neuronal progenitor cells, such as DCX- and Tuj-1-positive cells, 24 h after ICH. Minocycline also increased M2 microglia-derived brain-derived neurotrophic factors (BDNF) and the upstream TrkB pathway. ANA 12 reversed the neuroprotective effects of minocycline. HIOC exhibited the same effects as minocycline and accelerated neurogenesis after ICH. This study demonstrated for the first time that minocycline promoted M2 microglia polarization via upregulation of the TrkB/BDNF pathway and promoted neurogenesis after ICH. This study contributes to our understanding of the therapeutic potential of minocycline in ICH. NEW & NOTEWORTHY The present study gives several novel points: 1) Minocycline promotes neurogenesis after intracerebral hemorrhage in rats. 2) Minocycline induces activated M1 microglia into M2 neurotrophic phenotype. 3) M2 microglia secreting BDNF remodel the damaged neurocircuit.

Antineuroinflammation of Minocycline in Stroke

Accumulating research substantiates the statement that inflammation plays an important role in the development of stroke. Both proinflammatory and anti-inflammatory mediators are involved in the pathogenesis of stroke, an imbalance of which leads to inflammation. Anti-inflammation is a kind of hopeful strategy for the prevention and treatment of stroke. Substantial studies have demonstrated that minocycline, a second-generation semisynthetic antibiotic belonging to the tetracycline family, can inhibit neuroinflammation, inflammatory mediators and microglia activation, and improve neurological outcome. Experimental and clinical data have found the preclinical and clinical potential of minocycline in the treatment of stroke due to its anti-inflammation properties and anti-inflammation-induced pathogeneses, including antioxidative stress, antiapoptosis, inhibiting leukocyte migration and microglial activation, and decreasing matrix metalloproteinases activity. Hence, it suggests a great future for minocycline in the therapeutics of stroke that diminish the inflammatory progress of stroke.

Pre-clinical models in pediatric traumatic brain injury-challenges and lessons learned

PURPOSE

Despite the enormity of the problem and the lack of new therapies, research in the pre-clinical arena specifically using pediatric traumatic brain injury (TBI) models is limited. In this review, some of the key models addressing both the age spectrum of pediatric TBI and its unique injury mechanisms will be highlighted. Four topics will be addressed, namely, (1) unique facets of the developing brain important to TBI model development, (2) a description of some of the most commonly used pre-clinical models of severe pediatric TBI including work in both rodents and large animals, (3) a description of the pediatric models of mild TBI and repetitive mild TBI that are relatively new, and finally (4) a discussion of challenges, gaps, and potential future directions to further advance work in pediatric TBI models.

METHODS

This narrative review on the topic of pediatric TBI models was based on review of PUBMED/Medline along with a synthesis of information on key factors in pre-clinical and clinical developmental brain injury that influence TBI modeling.

RESULTS

In the contemporary literature, six types of models have been used in rats including weight drop, fluid percussion injury (FPI), impact acceleration, controlled cortical impact (CCI), mechanical shaking, and closed head modifications of CCI. In mice, studies are largely restricted to CCI. In large animals, FPI and rotational injury have been used in piglets and shake injury has also been used in lambs. Most of the studies have been in severe injury models, although more recently, studies have begun to explore mild and repetitive mild injuries to study concussion.

CONCLUSIONS

Given the emerging importance of TBI in infants and children, the morbidity and mortality that is produced, along with its purported link to the development of chronic neurodegenerative diseases, studies in these models merit greater systematic investigations along with consortium-type approaches and long-term follow-up to translate new therapies to the bedside.

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.

Role of microglia in a mouse model of paediatric traumatic brain injury

The cognitive and behavioural deficits caused by traumatic brain injury (TBI) to the immature brain are more severe and persistent than TBI in the mature brain. Understanding this developmental sensitivity is critical as children under four years of age sustain TBI more frequently than any other age group. Microglia (MG), resident immune cells of the brain that mediate neuroinflammation, are activated following TBI in the immature brain. However, the type and temporal profile of this activation and the consequences of altering it are still largely unknown. In a mouse model of closed head weight drop paediatric brain trauma, we characterized i) the temporal course of total cortical neuroinflammation and the phenotype of ex vivo isolated CD11B-positive microglia/macrophage (MG/MΦ) using a battery of 32 markers, and ii) neuropathological outcome 1 and 5days post-injury. We also assessed the effects of targeting MG/MΦ activation directly, using minocycline a prototypical microglial activation antagonist, on these processes and outcome. TBI induced a moderate increase in both pro- and anti-inflammatory cytokines/chemokines in the ipsilateral hemisphere. Isolated cortical MG/MΦ expressed increased levels of markers of endogenous reparatory/regenerative and immunomodulatory phenotypes compared with shams. Blocking MG/MΦ activation with minocycline at the time of injury and 1 and 2days post-injury had only transient protective effects, reducing ventricular dilatation and cell death 1day post-injury but having no effect on injury severity at 5days. This study demonstrates that, unlike in adults, the role of MG/MΦ in injury mechanisms following TBI in the immature brain may not be negative. An improved understanding of MG/MΦ function in paediatric TBI could support translational efforts to design therapeutic interventions.

Microglial Activation in Traumatic Brain Injury

Microglia have a variety of functions in the brain, including synaptic pruning, CNS repair and mediating the immune response against peripheral infection. Microglia rapidly become activated in response to CNS damage. Depending on the nature of the stimulus, microglia can take a number of activation states, which correspond to altered microglia morphology, gene expression and function. It has been reported that early microglia activation following traumatic brain injury (TBI) may contribute to the restoration of homeostasis in the brain. On the other hand, if they remain chronically activated, such cells display a classically activated phenotype, releasing pro-inflammatory molecules, resulting in further tissue damage and contributing potentially to neurodegeneration. However, new evidence suggests that this classification is over-simplistic and the balance of activation states can vary at different points. In this article, we review the role of microglia in TBI, analyzing their distribution, morphology and functional phenotype over time in animal models and in humans. Animal studies have allowed genetic and pharmacological manipulations of microglia activation, in order to define their role. In addition, we describe investigations on the in vivo imaging of microglia using translocator protein (TSPO) PET and autoradiography, showing that microglial activation can occur in regions far remote from sites of focal injuries, in humans and animal models of TBI. Finally, we outline some novel potential therapeutic approaches that prime microglia/macrophages toward the beneficial restorative microglial phenotype after TBI.

White matter damage after traumatic brain injury: A role for damage associated molecular patterns

Traumatic brain injury (TBI) is a leading cause of mortality and long-term morbidity worldwide. Despite decades of pre-clinical investigation, therapeutic strategies focused on acute neuroprotection failed to improve TBI outcomes. This lack of translational success has necessitated a reassessment of the optimal targets for intervention, including a heightened focus on secondary injury mechanisms. Chronic immune activation correlates with progressive neurodegeneration for decades after TBI; however, significant challenges remain in functionally and mechanistically defining immune activation after TBI. In this review, we explore the burgeoning evidence implicating the acute release of damage associated molecular patterns (DAMPs), such as adenosine 5'-triphosphate (ATP), high mobility group box protein 1 (HMGB1), S100 proteins, and hyaluronic acid in the initiation of progressive neurological injury, including white matter loss after TBI. The role that pattern recognition receptors, including toll-like receptor and purinergic receptors, play in progressive neurological injury after TBI is detailed. Finally, we provide support for the notion that resident and infiltrating macrophages are critical cellular targets linking acute DAMP release with adaptive immune responses and chronic injury after TBI. The therapeutic potential of targeting DAMPs and barriers to clinical translational, in the context of TBI patient management, are discussed.

Vascular impairment as a pathological mechanism underlying long-lasting cognitive dysfunction after pediatric traumatic brain injury

Traumatic brain injury (TBI) is the leading cause of death and disability in children. Indeed, the acute mechanical injury often evolves to a chronic brain disorder with long-term cognitive, emotional and social dysfunction even in the case of mild TBI. Contrary to the commonly held idea that children show better recovery from injuries than adults, pediatric TBI patients actually have worse outcome than adults for the same injury severity. Acute trauma to the young brain likely interferes with the fine-tuned developmental processes and may give rise to long-lasting consequences on brain's function. This review will focus on cerebrovascular dysfunction as an important early event that may lead to long-term phenotypic changes in the brain after pediatric TBI. These, in turn may be associated with accelerated brain aging and cognitive dysfunction. Finally, since no effective treatments are currently available, understanding the unique pathophysiological mechanisms of pediatric TBI is crucial for the development of new therapeutic options.

The far-reaching scope of neuroinflammation after traumatic brain injury

The 'silent epidemic' of traumatic brain injury (TBI) has been placed in the spotlight as a result of clinical investigations and popular press coverage of athletes and veterans with single or repetitive head injuries. Neuroinflammation can cause acute secondary injury after TBI, and has been linked to chronic neurodegenerative diseases; however, anti-inflammatory agents have failed to improve TBI outcomes in clinical trials. In this Review, we therefore propose a new framework of targeted immunomodulation after TBI for future exploration. Our framework incorporates factors such as the time from injury, mechanism of injury, and secondary insults in considering potential treatment options. Structuring our discussion around the dynamics of the immune response to TBI - from initial triggers to chronic neuroinflammation - we consider the ability of soluble and cellular inflammatory mediators to promote repair and regeneration versus secondary injury and neurodegeneration. We summarize both animal model and human studies, with clinical data explicitly defined throughout this Review. Recent advances in neuroimmunology and TBI-responsive neuroinflammation are incorporated, including concepts of inflammasomes, mechanisms of microglial polarization, and glymphatic clearance. Moreover, we highlight findings that could offer novel therapeutic targets for translational and clinical research, assimilate evidence from other brain injury models, and identify outstanding questions in the field.

Differential effects of minocycline on microglial activation and neurodegeneration following closed head injury in the neonate rat

The role of microglia in the pathophysiology of injury to the developing brain has been extensively studied. In children under the age of 4 who have sustained a traumatic brain injury (TBI), markers of microglial/macrophage activation were increased in the cerebrospinal fluid and were associated with worse neurologic outcome. Minocycline is an antibiotic that decreases microglial/macrophage activation following hypoxic-ischemia in neonatal rodents and TBI in adult rodents thereby reducing neurodegeneration and behavioral deficits. In study 1, 11-day-old rats received an impact to the intact skull and were treated for 3days with minocycline. Immediately following termination of minocycline administration, microglial reactivity was reduced in the cortex and hippocampus (p<0.001) and was accompanied by an increase in the number of fluoro-Jade B profiles (p<0.001) suggestive of a reduced clearance of degenerating cells; however, this effect was not sustained at 7days post-injury. Although microglial reactivity was reduced in the white matter tracts (p<0.001), minocycline treatment did not reduce axonal injury or degeneration. In the thalamus, minocycline treatment did not affect microglial reactivity, axonal injury and degeneration, and neurodegeneration. Injury-induced spatial learning and memory deficits were also not affected by minocycline. In study 2, to test whether extended dosing of minocycline may be necessary to reduce the ongoing pathologic alterations, a separate group of animals received minocycline for 9days. Immediately following termination of treatment, microglial reactivity and neurodegeneration in all regions examined were exacerbated in minocycline-treated brain-injured animals compared to brain-injured animals that received vehicle (p<0.001), an effect that was only sustained in the cortex and hippocampus up to 15days post-injury (p<0.001). Whereas injury-induced spatial learning deficits remained unaffected by minocycline treatment, memory deficits appeared to be significantly worse (p<0.05). Sex had minimal effects on either injury-induced alterations or the efficacy of minocycline treatment. Collectively, these data demonstrate the differential effects of minocycline in the immature brain following impact trauma and suggest that minocycline may not be an effective therapeutic strategy for TBI in the immature brain.