Preliminary study on the effect of trauma-induced secondary cellular hypoxia in brain injury

Rebuilding Microbiome for Mitigating Traumatic Brain Injury: Importance of Restructuring the Gut-Microbiome-Brain Axis

Traumatic brain injury (TBI) is a damage to the brain from an external force that results in temporary or permanent impairment in brain functions. Unfortunately, not many treatment options are available to TBI patients. Therefore, knowledge of the complex interplay between gut microbiome (GM) and brain health may shed novel insights as it is a rapidly expanding field of research around the world. Recent studies show that GM plays important roles in shaping neurogenerative processes such as blood-brain-barrier (BBB), myelination, neurogenesis, and microglial maturation. In addition, GM is also known to modulate many aspects of neurological behavior and cognition; however, not much is known about the role of GM in brain injuries. Since GM has been shown to improve cellular and molecular functions via mitigating TBI-induced pathologies such as BBB permeability, neuroinflammation, astroglia activation, and mitochondrial dysfunction, herein we discuss how a dysbiotic gut environment, which in fact, contributes to central nervous system (CNS) disorders during brain injury and how to potentially ward off these harmful effects. We further opine that a better understanding of GM-brain (GMB) axis could help assist in designing better treatment and management strategies in future for the patients who are faced with limited options.

Glucose transporters in brain in health and disease

Energy demand of neurons in brain that is covered by glucose supply from the blood is ensured by glucose transporters in capillaries and brain cells. In brain, the facilitative diffusion glucose transporters GLUT1-6 and GLUT8, and the Na⁺-d-glucose cotransporters SGLT1 are expressed. The glucose transporters mediate uptake of d-glucose across the blood-brain barrier and delivery of d-glucose to astrocytes and neurons. They are critically involved in regulatory adaptations to varying energy demands in response to differing neuronal activities and glucose supply. In this review, a comprehensive overview about verified and proposed roles of cerebral glucose transporters during health and diseases is presented. Our current knowledge is mainly based on experiments performed in rodents. First, the functional properties of human glucose transporters expressed in brain and their cerebral locations are described. Thereafter, proposed physiological functions of GLUT1, GLUT2, GLUT3, GLUT4, and SGLT1 for energy supply to neurons, glucose sensing, central regulation of glucohomeostasis, and feeding behavior are compiled, and their roles in learning and memory formation are discussed. In addition, diseases are described in which functional changes of cerebral glucose transporters are relevant. These are GLUT1 deficiency syndrome (GLUT1-SD), diabetes mellitus, Alzheimer’s disease (AD), stroke, and traumatic brain injury (TBI). GLUT1-SD is caused by defect mutations in GLUT1. Diabetes and AD are associated with changed expression of glucose transporters in brain, and transporter-related energy deficiency of neurons may contribute to pathogenesis of AD. Stroke and TBI are associated with changes of glucose transporter expression that influence clinical outcome.

Subjective Cognitive Decline and Related Cognitive Deficits

Since late stage dementia, including Alzheimer's disease (AD), cannot be reversed by any available drugs, there is increasing research interest in the preclinical stage of AD, i.e., subjective cognitive decline (SCD). SCD is characterized by self-perceptive cognitive decline but is difficult to detect using objective tests. At SCD stage, the cognitive deficits can be more easily reversed compared to that of mild cognitive impairment (MCI) and AD only if accurate diagnosis of SCD and early intervention can be developed. In this paper, we review the recent progress of SCD research including current assessment tools, biomarkers, neuroimaging, intervention and expected prognosis, and the potential relevance to traumatic brain injury (TBI)-induced cognitive deficits.

Traumatic brain injury triggers APP and Tau cleavage by delta-secretase, mediating Alzheimer's disease pathology

Traumatic brain injury (TBI) is associated in some studies with clinical dementia, and neuropathological features, including amyloid plaque deposition and Tau neurofibrillary degeneration commonly identified in Alzheimer's disease (AD). However, the molecular mechanisms linking TBI to AD remain unclear. Here we show that TBI activates transcription factor CCAAT/Enhancer Binding Protein Beta (C/EBPβ), increasing delta-secretase (AEP) expression. Activated AEP cleaves both APP and Tau at APP N585 and Tau N368 sites, respectively, which mediate AD pathogenesis by promoting Aβ production and Tau hyperphosphorylation and inducing neuroinflammation and neurotoxicity. Knockout of AEP or C/EBPβ diminishes TBI-induced AD-like pathology and cognitive impairment in the 3xTg AD mouse model. Remarkably, viral expression of AEP-resistant Tau N368A in the hippocampus of 3xTg mice also ameliorates the pathological and cognitive consequences of TBI. Finally, clinical TBI activates C/EBPβ and escalates AEP expression, leading to APP N585 and Tau N368 proteolytic cleavage in TBI patient brains. Hence, our findings support a potential role for AEP in linking TBI exposure with AD pathogenesis.

Activation of both transforming growth factor-β and bone morphogenetic protein signalling pathways upon traumatic brain injury restrains pro-inflammatory and boosts tissue reparatory responses of reactive astrocytes and microglia

Various ligands and receptors of the transforming growth factor-β superfamily have been found upregulated following traumatic brain injury; however, the role of this signalling system in brain injury pathophysiology is not fully characterized. To address this, we utilized an acute stab wound brain injury model to demonstrate that hallmarks of transforming growth factor-β superfamily system activation, such as levels of phosphorylated Smads, ligands and target genes for both transforming growth factor-β and bone morphogenetic protein pathways, were upregulated within injured tissues. Using a bone morphogenetic protein-responsive reporter mouse model, we showed that activation of the bone morphogenetic protein signalling pathway involves primarily astrocytes that demarcate the wound area. Insights regarding the potential role of transforming growth factor-β superfamily activation in glia cells within the injured tissues were obtained indirectly by treating purified reactive astrocytes and microglia with bone morphogenetic protein-4 or transforming growth factor-β1 and characterizing changes in their transcriptional profiles. Astrocytes responded to both ligands with considerably overlapping profiles, whereas, microglia responded selectively to transforming growth factor-β1. Novel pathways, crucial for repair of tissue-injury and blood-brain barrier, such as activation of cholesterol biosynthesis and transport, production of axonal guidance and extracellular matrix components were upregulated by transforming growth factor-β1 and/or bone morphogenetic protein-4 in astrocytes. Moreover, both ligands in astrocytes and transforming growth factor-β1 in microglia shifted the phenotype of reactive glia cells towards the anti-inflammatory and tissue reparatory 'A2'-like and 'M0/M2'-like phenotypes, respectively. Increased expression of selected key components of the in vitro modulated pathways and markers of 'A2'-like astrocytes was confirmed within the wound area, suggesting that these processes could also be modulated in situ by the integrated action of transforming growth factor-β and/or bone morphogenetic protein-mediated signalling. Collectively, our study provides a comprehensive comparative analysis of transforming growth factor-β superfamily signalling in reactive astrocytes and microglia and points towards a crucial role of both transforming growth factor-β and bone morphogenetic protein pathways in modulating the inflammatory and brain injury reparatory functions of activated glia cells.

Time-Dependent Changes in Microglia Transcriptional Networks Following Traumatic Brain Injury

The neuroinflammatory response to traumatic brain injury (TBI) is critical to both neurotoxicity and neuroprotection, and has been proposed as a potentially modifiable driver of secondary injury in animal and human studies. Attempts to broadly target immune activation have been unsuccessful in improving outcomes, in part because the precise cellular and molecular mechanisms driving injury and outcome at acute, subacute, and chronic time points after TBI remain poorly defined. Microglia play a critical role in neuroinflammation and their persistent activation may contribute to long-term functional deficits. Activated microglia are characterized by morphological transformation and transcriptomic changes associated with specific inflammatory states. We analyzed the temporal course of changes in inflammatory genes of microglia isolated from injured brains at 2, 14, and 60 days after controlled cortical impact (CCI) in mice, a well-established model of focal cerebral contusion. We identified a time dependent, injury-associated change in the microglial gene expression profile toward a reduced ability to sense tissue damage, perform housekeeping, and maintain homeostasis in the early stages following CCI, with recovery and transition to a specialized inflammatory state over time. This later state starts at 14 days post-injury and is characterized by a biphasic pattern of IFNγ, IL-4, and IL-10 gene expression changes, with concurrent proinflammatory and anti-inflammatory gene changes. Our transcriptomic data sets are an important step to understand microglial role in TBI pathogenesis at the molecular level and identify common pathways that affect outcome. More studies to evaluate gene expression at the single cell level and focusing on subacute and chronic timepoint are warranted.

Mammalian Models of Traumatic Brain Injury and a Place for Drosophila in TBI Research

Traumatic brain injury (TBI), caused by a sudden blow or jolt to the brain that disrupts normal function, is an emerging health epidemic with ∼2.5 million cases occurring annually in the United States that are severe enough to cause hospitalization or death. Most common causes of TBI include contact sports, vehicle crashes and domestic violence or war injuries. Injury to the central nervous system is one of the most consistent candidates for initiating the molecular and cellular cascades that result in Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS). Not every TBI event is alike with effects varying from person to person. The majority of people recover from mild TBI within a short period of time, but repeated incidents can have deleterious long-lasting effects which depend on factors such as the number of TBIs sustained, time till medical attention, age, gender and genetics of the individual. Despite extensive research, many questions still remain regarding diagnosis, treatment, and prevention of long-term effects from TBI as well as recovery of brain function. In this review, we present an overview of TBI pathology, discuss mammalian models for TBI and focus on current methods using Drosophila melanogaster as a model for TBI study. The relatively small brain size (∼100,000 neurons and glia), conserved neurotransmitter signaling mechanisms and sophisticated genetics of Drosophila allows for cell biological, molecular and genetic analyses that are impractical in mammalian models of TBI.

Traumatic Brain Injury, Chronic Traumatic Encephalopathy, and Alzheimer Disease

Traumatic brain injury (TBI) is a major health and economic burden. With increasing aging population, this issue is expected to continue to rise. Neurodegenerative disorders are more common with aging population in general regardless of history of TBI. Recent evidence continues to support a relation between a TBI and neurocognitive decline later in life (such as in athletes and military). This article summarizes the pathologic and clinical effects of TBI (regardless of severity) on the later development of dementia in individuals 65 years or older.

Delayed Hypoxemia after Traumatic Brain Injury Exacerbates Long-Term Behavioral Deficits

Hypoxemia during initial stabilization of patients with severe traumatic brain injury (TBI) has been associated with poorer outcomes. However, the effects of delayed hypoxemia occurring during intensive care post-TBI on outcome is unclear. Pre-clinical models of TBI have rarely shown cognitive or behavioral deficits beyond 6 weeks post-injury and commonly have not included modeling of secondary insults. We have previously developed a murine model of TBI followed by delayed hypoxemia to model the secondary insult of hypoxemia and brain hypoxia occurring in the intensive care setting. Understanding long-term effects of delayed hypoxemia post-TBI in our murine model is critical for future testing of candidate therapeutics targeting secondary brain hypoxia. For this study, forty 5-week-old male mice were randomized to controlled cortical impact (CCI; N = 24) or sham surgery (N = 16). One day later, awake animals were randomized to 60 min of hypoxemia or normoxemia. Six months after initial injury, animals underwent behavior testing (Morris water maze, social interaction, and tail suspension) before euthanasia for immunohistochemistry (IHC) assessments. At 6 months post-injury, mice experiencing CCI and hypoxemia (CCI+H) had longer swim distances to the hidden platform (51 cm) compared to CCI alone (26 cm) or sham animals (22 cm). During social interaction assessments, CCI + H mice spent less time interacting with novel stimulus mice (79 sec) than CCI alone (101 sec) or sham animals (139 sec). CCI + H had larger lesion volumes compared to CCI alone (14.0% vs. 9.9%; p < 0.003). Glial fibrillary acidic protein IHC at 6 months post-injury demonstrated increased astrogliosis in the ipsilateral white matter of CCI + H compared to CCI alone. To summarize, this clinically relevant model of delayed hypoxia post-TBI resulted in long-term behavioral deficits and evidence of exacerbated structural injury.

Elevated concentrations of hypoxia-inducible factor-1α in patients with fracture and concomitant traumatic brain injury

Background Compelling evidence indicate that traumatic brain injury is highly related to accelerated bone fracture repair, but the underlying mechanism still remains elusive. Fracture repair process relies greatly on the formation of new blood vessels in fracture site, and angiogenic factors have been confirmed to be essential for the initiation and maintenance of the fracture healing. Hypoxia-inducible factor-1α was demonstrated to be a critical regulator of angiogenic-osteogenic coupling during bone development and regeneration. The aim of the present study was to investigate the local and circulating concentrations of hypoxia-inducible factor-1α in patients with long-bone fractures and concomitant traumatic brain injury and to determine the potential role of hypoxia-inducible factor-1α in fracture healing. Methods Twenty-five patients with a long-bone fracture and concomitant traumatic brain injury (FT group) and 33 without a brain injury (Fr group) were enrolled in this study. Healthy subjects donated serum samples as control. Serum samples were collected over a period of six months, following a standardized time schedule. Hypoxia-inducible factor-1α concentrations were measured in fracture haematoma and serum of patients in both groups using enzyme-linked immunosorbent assay. Results Patients in FT group had a short time to union. Serum hypoxia-inducible factor-1α concentrations elevated in the early healing period and reached the maximum level during intramembranous bone formation phase in both groups. Thereafter, it decreased continuously and approached to the minimum levels until the end of the observation period. Serum hypoxia-inducible factor-1α concentrations in both groups were significantly higher compared with controls and hypoxia-inducible factor-1α concentrations in both serum and fracture haematoma were higher in FT group than that in Fr group. Fracture haematoma contained significantly higher hypoxia-inducible factor-1α concentrations compared with hypoxia-inducible factor-1α concentrations in serum. Serum hypoxia-inducible factor-1α concentrations had a positive correlation with hypoxia-inducible factor-1α concentrations in fracture haematoma in patients with fractures. Conclusions These findings suggest the local and systemic involvement of hypoxia-inducible factor-1α in fracture healing and the accelerated fracture repair in patients with traumatic brain injury might be associated with elevated hypoxia-inducible factor-1α concentrations in fracture haematoma and serum.

Delayed Hypoxemia Following Traumatic Brain Injury Exacerbates White Matter Injury

Hypoxemia immediately following traumatic brain injury (TBI) has been observed to exacerbate injury. However, it remains unclear whether delayed hypoxemia beyond the immediate postinjury period influences white matter injury. In a retrospective clinical cohort of children aged 4-16 years admitted with severe TBI, 28/74 (35%) patients were found to experience delayed normocarbic hypoxemia within 7 days of admission. Based on these clinical findings, we developed a clinically relevant mouse model of TBI with delayed hypoxemia by exposing 5-week old (adolescent) mice to hypoxic conditions for 30 minutes starting 24 hours after moderate controlled cortical impact (CCI). Injured mice with hypoxemia had increased axonal injury using both β-amyloid precursor protein and NF200 immunostaining in peri-contusional white matter compared with CCI alone. Furthermore, we detected increased peri-contusional white matter tissue hypoxia with pimonidazole and augmented astrogliosis with anti-glial fibrillary acidic protein staining in CCI + delayed hypoxemia compared with CCI alone or sham surgery + delayed hypoxemia. Microglial activation as evidenced by Iba1 staining was not significantly altered by delayed hypoxemia. These clinical and experimental data indicate the prevention or amelioration of delayed hypoxemia effects following TBI may provide a unique opportunity for the development of therapeutic interventions to reduce axonal injury and improve clinical outcomes.

Cerebral and extracerebral vulnerability to hypoxic insults after diffuse traumatic brain injury in rats

The post-traumatic brain vulnerability suggests that after traumatic brain injury (TBI), the brain may be more susceptible to posttraumatic hypoxic insults. This concept could be extended to 'peripheral' organs, as non-neurologic organ failure is common after TBI. This study aims to characterize and quantify cerebral and extracerebral tissue hypoxia with pimonidazole resulting from a standardized hypoxia-hypotension (HH) phase occurring after a diffuse experimental TBI in rats. Rats were allocated to Sham (n=5), TBI (n=7), HH (n=7) and TBI+HH (n=7) groups. Then, pimonidazole was injected and brain, liver, heart and kidneys were analysed. In the cerebral cortex, post-treatment hypoxia was higher in TBI+HH group than Sham group (p=0.003), HH group (p=0.003) and TBI group (p=0.002). Large trends in thalamus, hippocampus and striatum for the TBI+HH group compared to the other groups were observed. For the heart and liver, the 4 groups were comparable. For the kidneys, post-treatment hypoxia was higher in the TBI group compared to the Sham and HH groups, but not more than TBI+HH group. This study reveals that a posttraumatic hypoxic insult occurring after a severe TBI has major hypoxic consequences on brain structures. However, TBI by itself appears to induce renal hypoxia that is not enhanced by posttraumatic hypoxic insult.

Endogenous TGFβ1 Plays a Crucial Role in Functional Recovery After Traumatic Brain Injury Associated with Smad3 Signal in Rats

Transforming growth factor-β 1 (TGFβ1) has a diverse role in astrogliosis and neuronal survival, but the underlying mechanism remains to be elucidated, especially in traumatic brain injury (TBI). Here, we show that the expression of TGFβ1 was increased in the pericontusional region, accompanied with astrogliosis and neuronal loss in TBI rats. Moreover, TGFβ1 knockdown not only reduced the number of neurons and inhibited astrogliosis but also resulted in a significant neurological dysfunction in rats with TBI. Subsequently, Smad3, a key downstream signal of TGFβ1, was involved in pericontusional region after TBI. These findings therefore indicate that TGFβ1 is involved in neuroprotection and astrogliosis, via activation of down stream Smad3 signal in the brain after injury.

Biomarkers in traumatic brain injury

Traumatic brain injury (TBI) is a common cause of neurological morbidity globally, and neurologic sequelae may occur even in the setting of mild injury. At present, the tools that guide diagnostic and prognostic evaluation of patients who suffer from TBI remain limited, especially for prehospital evaluation. Biomarkers of brain injury hold promise in facilitating early management and triage decisions in the civilian and military settings. The identification of biomarkers of brain injury may also be helpful in guiding end-of-life decision making and may facilitate the design of neuroprotective trials.

Hypoxia-inducible factor 1 is essential for spontaneous recovery from traumatic brain injury and is a key mediator of heat acclimation induced neuroprotection

Heat acclimation (HA), a well-established preconditioning model, confers neuroprotection in rodent models of traumatic brain injury (TBI). It increases neuroprotective factors, among them is hypoxia-inducible factor 1α (HIF-1α), which is important in the response to postinjury ischemia. However, little is known about the role of HIF-1α in TBI and its contribution to the establishment of the HA protecting phenotype. Therefore, we aimed to explore HIF-1α role in TBI defense mechanisms as well as in HA-induced neuroprotection. Acriflavine was used to inhibit HIF-1 in injured normothermic (NT) or HA mice. After TBI, we evaluated motor function recovery, lesion volume, edema formation, and body temperature as well as HIF-1 downstream transcription targets, such as glucose transporter 1 (GLUT1), vascular endothelial growth factor, and aquaporin 4. We found that HIF-1 inhibition resulted in deterioration of motor function, increased lesion volume, hypothermia, and reduced edema formation. All these parameters were significantly different in the HA mice. Western blot analysis and enzyme-linked immunosorbent assay showed reduced levels of all HIF-1 downstream targets in HA mice, however, only GLUT1 was downregulated in NT mice. We conclude that HIF-1 is a key mediator in both spontaneous recovery and HA-induced neuroprotection after TBI.

Traumatic brain injury: a risk factor for Alzheimer's disease

Traumatic brain injury (TBI) constitutes a major global health and socio-economic problem with neurobehavioral sequelae contributing to long-term disability. It causes brain swelling, axonal injury and hypoxia, disrupts blood brain barrier function and increases inflammatory responses, oxidative stress, neurodegeneration and leads to cognitive impairment. Epidemiological studies show that 30% of patients, who die of TBI, have Aβ plaques which are pathological features of Alzheimer's disease (AD). Thus TBI acts as an important epigenetic risk factor for AD. This review focuses on AD related genes which are expressed during TBI and its relevance to progression of the disease. Such understanding will help to diagnose the risk of TBI patients to develop AD and design therapeutic interventions.