Chronic Traumatic Encephalopathy: The cellular sequela to repetitive brain injury

Single Versus Repetitive Traumatic Brain Injury: Current Knowledge on the Chronic Outcomes, Neuropathology and the Role of TDP-43 Proteinopathy

Traumatic brain injury (TBI) is one of the most important causes of death and disability in adults and thus an important public health problem. Following TBI, secondary pathophysiological processes develop over time and condition the development of different neurodegenerative entities. Previous studies suggest that neurobehavioral changes occurring after a single TBI are the basis for the development of Alzheimer's disease, while repetitive TBI is considered to be a contributing factor for chronic traumatic encephalopathy development. However, pathophysiological processes that determine the evolvement of a particular chronic entity are still unclear. Human post-mortem studies have found combinations of amyloid, tau, Lewi bodies, and TAR DNA-binding protein 43 (TDP-43) pathologies after both single and repetitive TBI. This review focuses on the pathological changes of TDP-43 after single and repetitive brain traumas. Numerous studies have shown that TDP-43 proteinopathy noticeably occurs after repetitive head trauma. A relatively small number of available preclinical research on single brain injury are not in complete agreement with the results from the human samples, which makes it difficult to draw specific conclusions. Also, as TBI is considered a heterogeneous type of injury, different experimental trauma models and injury intensities may cause differences in the cascade of secondary injury, which should be considered in future studies. Experimental and post-mortem studies of TDP-43 pathobiology should be carried out, preferably in the same laboratories, to determine its involvement in the development of neurodegenerative conditions after one and repetitive TBI, especially in the context of the development of new therapeutic options.

The Adult Neurogenesis Theory of Alzheimer's Disease

Alzheimer's disease starts in neural stem cells (NSCs) in the niches of adult neurogenesis. All primary factors responsible for pathological tau hyperphosphorylation are inherent to adult neurogenesis and migration. However, when amyloid pathology is present, it strongly amplifies tau pathogenesis. Indeed, the progressive accumulation of extracellular amyloid-β deposits in the brain triggers a state of chronic inflammation by microglia. Microglial activation has a significant pro-neurogenic effect that fosters the process of adult neurogenesis and supports neuronal migration. Unfortunately, this "reactive" pro-neurogenic activity ultimately perturbs homeostatic equilibrium in the niches of adult neurogenesis by amplifying tau pathogenesis in AD. This scenario involves NSCs in the subgranular zone of the hippocampal dentate gyrus in late-onset AD (LOAD) and NSCs in the ventricular-subventricular zone along the lateral ventricles in early-onset AD (EOAD), including familial AD (FAD). Neuroblasts carrying the initial seed of tau pathology travel throughout the brain via neuronal migration driven by complex signals and convey the disease from the niches of adult neurogenesis to near (LOAD) or distant (EOAD) brain regions. In these locations, or in close proximity, a focus of degeneration begins to develop. Then, tau pathology spreads from the initial foci to large neuronal networks along neural connections through neuron-to-neuron transmission.

Active immunotherapy against pathogenic Cis pT231-tau suppresses neurodegeneration in traumatic brain injury mouse models

Traumatic brain injury (TBI), characterized by acute neurological impairment, is associated with a higher incidence of neurodegenerative diseases, particularly chronic traumatic encephalopathy (CTE), Alzheimer's disease (AD), and Parkinson's disease (PD), whose hallmarks include hyperphosphorylated tau protein. Recently, phosphorylated tau at Thr231 has been shown to exist in two distinct cis and trans conformations. Moreover, targeted elimination of cis P-tau by passive immunotherapy with an appropriate mAb that efficiently suppresses tau-mediated neurodegeneration in severe TBI mouse models has proven to be a useful tool to characterize the neurotoxic role of cis P-tau as an early driver of the tauopathy process after TBI. Here, we investigated whether active immunotherapy can develop sufficient neutralizing antibodies to specifically target and eliminate cis P-tau in the brain of TBI mouse models. First, we explored the therapeutic efficacy of two different vaccines. C57BL/6 J mice were immunized with either cis or trans P-tau conformational peptides plus adjuvant. After rmTBI in mice, we found that cis peptide administration developed a specific Ab that precisely targeted and neutralized cis P-tau, inhibited the development of neuropathology and brain dysfunction, and restored various structural and functional sequelae associated with TBI in chronic phases. In contrast, trans P-tau peptide application not only lacked neuroprotective properties, but also contributed to a number of neuropathological features, including progressive TBI-induced neuroinflammation, widespread tau-mediated neurodegeneration, worsening functional deficits, and brain atrophy. Taken together, our results suggest that active immunotherapy strategies against pathogenic cis P-tau can halt the process of tauopathy and would have profound clinical implications.

Tau pathology, metal dyshomeostasis and repetitive mild traumatic brain injury: an unexplored link paving the way for neurodegeneration

Repetitive mild traumatic brain injury (r-mTBI), commonly experienced by athletes and military personnel, causes changes in multiple intracellular pathways, one of which involves the tau protein. Tau phosphorylation plays a role in several neurodegenerative conditions including chronic traumatic encephalopathy (CTE), a progressive neurodegenerative disorder linked to repeated head trauma. There is now mounting evidence suggesting that tau phosphorylation may be regulated by metal ions (such as iron, zinc and copper), which themselves are implicated in ageing and neurodegenerative disorders such as Alzheimer's disease (AD). Recent work has also shown that a single TBI can result in age-dependent and region-specific modulation of metal ions. As such, this review explores the link between TBI, CTE, ageing and neurodegeneration with a specific focus on the involvement of (and interaction between) tau pathology and metal dyshomeostasis. The authors highlight that metal dyshomeostasis has yet to be investigated in the context of repeat head trauma or CTE. Given the evidence that metal dyshomeostasis contributes to the onset and/or progression of neurodegeneration, and that CTE itself is a neurodegenerative condition, this brings to light an uncharted link that should be explored. The development of adequate models of r-mTBI and/or CTE will be crucial in deepening our understanding of the pathological mechanisms that drive the clinical manifestations in these conditions and also in the development of effective therapeutics targeted towards slowing progressive neurodegenerative disorders.

Differential Expression Patterns of TDP-43 in Single Moderate versus Repetitive Mild Traumatic Brain Injury in Mice

Traumatic brain injury (TBI) is a disabling disorder and a major cause of death and disability in the world. Both single and repetitive traumas affect the brain acutely but can also lead to chronic neurodegenerative changes. Clinical studies have shown some dissimilarities in transactive response DNA binding protein 43 (TDP-43) expression patterns following single versus repetitive TBI. We explored the acute cortical post-traumatic changes of TDP-43 using the lateral fluid percussion injury (LFPI) model of single moderate TBI in adult male mice and investigated the association of TDP-43 with post-traumatic neuroinflammation and synaptic plasticity. In the ipsilateral cortices of animals following LFPI, we found changes in the cytoplasmic and nuclear levels of TDP-43 and the decreased expression of postsynaptic protein 95 within the first 3 d post-injury. Subacute pathological changes of TDP-43 in the hippocampi of animals following LFPI and in mice exposed to repetitive mild TBI (rmTBI) were studied. Changes in the hippocampal TDP-43 expression patterns at 14 d following different brain trauma procedures showed pathological alterations only after single moderate, but not following rmTBI. Hippocampal LFPI-induced TDP-43 pathology was not accompanied by the microglial reaction, contrary to the findings after rmTBI, suggesting that different types of brain trauma may cause diverse pathophysiological changes in the brain, specifically related to the TDP-43 protein as well as to the microglial reaction. Taken together, our findings may contribute to a better understanding of the pathophysiological events following brain trauma.

Tau Is Elevated in Pediatric Patients on Extracorporeal Membrane Oxygenation

Neurologic injury is a known and feared complication of extracorporeal membrane oxygenation (ECMO). Neurologic biomarkers may have a role in assisting in early identification of such. Axonal biomarker tau has not been investigated in the pediatric ECMO population. The objective of this study is to evaluate plasma levels of tau in pediatric patients supported with ECMO. Eighteen patients requiring ECMO support in a quaternary pediatric intensive care unit at a university-affiliated children's hospital from October 2015 to February 2017 were enrolled. Patients undergoing extracorporeal cardiopulmonary resuscitation or recent history of bypass were excluded. Plasma tau was measured using enzyme-linked immunosorbent assay. Neuroimaging was reviewed for acute neurologic injury, and tau levels were analyzed to assess for correlation. Tau was significantly higher in ECMO patients than in control subjects. Sixty-one percent of subjects had evidence of acute brain injury on neuroimaging, but tau level did not correlate with injury. Subjects with multifocal injury all experienced infarction and had significantly higher tau levels on ECMO day 3 than patients with isolated injury. In addition, peak tau levels of neuro-injured subjects were compared with controls and noninjured ECMO subjects using receiver operating curve analysis. This study demonstrates preliminary evidence of axonal injury in pediatric ECMO patients.

Calpain-1 and Calpain-2 in the Brain: Dr. Jekill and Mr Hyde?

While the calpain system has now been discovered for over 50 years, there is still a paucity of information regard-ing the organization and functions of the signaling pathways regulated by these proteases, although calpains play critical roles in many cell functions. Moreover, calpain overactivation has been shown to be involved in numerous diseases. Among the 15 calpain isoforms identified, calpain-1 (aka µ-calpain) and calpain-2 (aka m-calpain) are ubiquitously distributed in most tissues and organs, including the brain. We have recently proposed that calpain-1 and calpain-2 play opposite functions in the brain, with calpain-1 activation being required for triggering synaptic plasticity and neuroprotection (Dr. Jekill), and calpain-2 limiting the extent of plasticity and being neurodegenerative (Mr. Hyde). Calpain-mediated cleavage has been ob-served in cytoskeleton proteins, membrane-associated proteins, receptors/channels, scaffolding/anchoring proteins, and pro-tein kinases and phosphatases. This review will focus on the signaling pathways related to local protein synthesis, cytoskele-ton regulation and neuronal survival/death regulated by calpain-1 and calpain-2, in an attempt to explain the origin of the op-posite functions of these 2 calpain isoforms. This will be followed by a discussion of the potential therapeutic applications of selective regulators of these 2 calpain isoforms.

Traumatic brain injury: neuropathological, neurocognitive and neurobehavioral sequelae

Traumatic brain injury (TBI) causes substantial neurological disabilities and mental distress. Annual TBI incidence is in magnitude of millions, making it a global health challenge. Categorization of TBI into severe, moderate and mild by scores on the Glasgow coma scale (GCS) is based on clinical grounds and standard brain imaging (CT). Recent research focused on repeated mild TBI (sport and non-sport concussions) suggests that a considerable number of patients have long-term disabling neurocognitive and neurobehavioral sequelae. These relate to subtle neuronal injury (diffuse axonal injury) visible only by using advanced neuroimaging distinguishing microstructural tissue damage. With advanced MRI protocols better characterization of TBI is achievable. Diffusion tensor imaging (DTI) visualizes white matter pathology, susceptibility weight imaging (SWI) detects microscopic bleeding while functional magnetic resonance imaging (fMRI) provides closer understanding of cognitive disorders etc. However, advanced imaging is still not integrated in the clinical care of patients with TBI. Patients with chronic TBI may experience many somatic disorders, cognitive disturbances and mental complaints. The underlying pathophysiological mechanisms occurring in TBI are complex, brain injuries are highly heterogeneous and include neuroendocrine dysfunctions. Post-traumatic neuroendocrine dysfunctions received attention since the year 2000. Occurrence of TBI-related hypopituitarism does not correlate to severity of the GCS scores. Complete or partial hypopituitarism (isolated growth hormone (GH) deficiency as most frequent) may occur after mild TBI equally as after moderate-to-severe TBI. Many symptoms of hypopituitarism overlap with symptoms occurring in patients with chronic TBI, i.e. they have lower scores on neuropsychological examinations (cognitive disability) and have more symptoms of mental distress (depression and fatigue). The great challenges for the endocrinologist are: (1) detection of hypopituitarism in patients with TBI prospectively (in the acute phase and months to years after TBI), (2) assessment of the extent of cognitive impairment at baseline, and (3) monitoring of treatment effects (alteration of cognitive functioning and mental distress with hormone replacement therapy). Only few studies recently suggest that with growth hormone (rhGH) replacement in patients with chronic TBI and with abnormal GH secretion, cognitive performance may not change while symptoms related to depression and fatigue improve. Stagnation in post-TBI rehabilitation progress is recommended as a signal for clinical suspicion of neuroendocrine dysfunction. This remains a challenging area for more research.

Diffuse Axonal Injury and Oxidative Stress: A Comprehensive Review

Traumatic brain injury (TBI) is one of the world’s leading causes of morbidity and mortality among young individuals. TBI applies powerful rotational and translational forces to the brain parenchyma, which results in a traumatic diffuse axonal injury (DAI) responsible for brain swelling and neuronal death. Following TBI, axonal degeneration has been identified as a progressive process that starts with disrupted axonal transport causing axonal swelling, followed by secondary axonal disconnection and Wallerian degeneration. These modifications in the axonal cytoskeleton interrupt the axoplasmic transport mechanisms, causing the gradual gathering of transport products so as to generate axonal swellings and modifications in neuronal homeostasis. Oxidative stress with consequent impairment of endogenous antioxidant defense mechanisms plays a significant role in the secondary events leading to neuronal death. Studies support the role of an altered axonal calcium homeostasis as a mechanism in the secondary damage of axon, and suggest that calcium channel blocker can alleviate the secondary damage, as well as other mechanisms implied in the secondary injury, and could be targeted as a candidate for therapeutic approaches. Reactive oxygen species (ROS)-mediated axonal degeneration is mainly caused by extracellular Ca²⁺. Increases in the defense mechanisms through the use of exogenous antioxidants may be neuroprotective, particularly if they are given within the neuroprotective time window. A promising potential therapeutic target for DAI is to directly address mitochondria-related injury or to modulate energetic axonal energy failure.

Understanding blast-induced neurotrauma: how far have we come?

Blast injuries, including blast-induced neurotrauma (BINT), are caused by blast waves generated during an explosion. Accordingly, their history coincides with that of explosives. Hence, it is intriguing that, after more than 1000 years of using explosives, our understanding of the pathological consequences of blast and body/brain interactions is extremely limited. Postconflict recovery mechanisms seemingly include the suppression of painful experiences, such as explosive injuries. Unfortunately, ignoring the knowledge generated by previous generations of scientists retards research progress, leading to superfluous and repetitive studies. This article summarizes clinical and experimental findings published about blast injuries and BINT following the wars of the 20th and 21th centuries. Moreover, it offers a personal view on potential factors interfering with the progress of BINT research working toward providing better diagnosis, treatment and rehabilitation for military personnel affected by blast exposure.

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.