Wai Hang Cheng

Towards clinical management of traumatic brain injury: a review of models and mechanisms from a biomechanical perspective

Traumatic brain injury (TBI) is a major worldwide healthcare problem. Despite promising outcomes from many preclinical studies, the failure of several clinical studies to identify effective therapeutic and pharmacological approaches for TBI suggests that methods to improve the translational potential of preclinical studies are highly desirable. Rodent models of TBI are increasingly in demand for preclinical research, particularly for closed head injury (CHI), which mimics the most common type of TBI observed clinically. Although seemingly simple to establish, CHI models are particularly prone to experimental variability. Promisingly, bioengineering-oriented research has advanced our understanding of the nature of the mechanical forces and resulting head and brain motion during TBI. However, many neuroscience-oriented laboratories lack guidance with respect to fundamental biomechanical principles of TBI. Here, we review key historical and current literature that is relevant to the investigation of TBI from clinical, physiological and biomechanical perspectives, and comment on how the current challenges associated with rodent TBI models, particularly those involving CHI, could be improved.

Merging pathology with biomechanics using CHIMERA (Closed-Head Impact Model of Engineered Rotational Acceleration): a novel, surgery-free model of traumatic brain injury

BackgroundTraumatic brain injury (TBI) is a major health care concern that currently lacks any effective treatment. Despite promising outcomes from many preclinical studies, clinical evaluations have failed to identify effective pharmacological therapies, suggesting that the translational potential of preclinical models may require improvement. Rodents continue to be the most widely used species for preclinical TBI research. As most human TBIs result from impact to an intact skull, closed head injury (CHI) models are highly relevant, however, traditional CHI models suffer from extensive experimental variability that may be due to poor control over biomechanical inputs. Here we describe a novel CHI model called CHIMERA (Closed-Head Impact Model of Engineered Rotational Acceleration) that fully integrates biomechanical, behavioral, and neuropathological analyses. CHIMERA is distinct from existing neurotrauma model systems in that it uses a completely non-surgical procedure to precisely deliver impacts of prescribed dynamic characteristics to a closed skull while enabling kinematic analysis of unconstrained head movement. In this study, we characterized head kinematics as well as functional, neuropathological, and biochemical outcomes up to 14d following repeated TBI (rTBI) in adult C57BL/6 mice using CHIMERA.ResultsHead kinematic analysis showed excellent repeatability over two closed head impacts separated at 24h. Injured mice showed significantly prolonged loss of righting reflex and displayed neurological, motor, and cognitive deficits along with anxiety-like behavior. Repeated TBI led to diffuse axonal injury with extensive microgliosis in white matter from 2-14d post-rTBI. Injured mouse brains also showed significantly increased levels of TNF-α and IL-1β and increased endogenous tau phosphorylation.ConclusionsRepeated TBI using CHIMERA mimics many of the functional and pathological characteristics of human TBI with a reliable biomechanical response of the head. This makes CHIMERA well suited to investigate the pathophysiology of TBI and for drug development programs.

Reconstituted high-density lipoproteins acutely reduce soluble brain Aβ levels in symptomatic APP/PS1 mice.

Many lines of evidence suggest a protective role for high-density lipoprotein (HDL) and its major apolipoprotein (apo)A-I in Alzheimers Disease (AD). HDL/apoA-I particles are produced by the liver and intestine and, in addition to removing excess cholesterol from the body, are increasingly recognized to have vasoprotective functions. Here we tested the ability of reconstituted HDL (rHDL) consisting of human apoA-I reconstituted with soy phosphatidylcholine for its ability to lower amyloid beta (Aβ) levels in symptomatic APP/PS1 mice, a well-characterized preclinical model of amyloidosis. Animals were treated intravenously either with four weekly doses (chronic study) or a single dose of 60mg/kg of rHDL (acute study). The major finding of our acute study is that soluble brain Aβ40 and Aβ42 levels were significantly reduced within 24h of a single dose of rHDL. By contrast, no changes were observed in our chronic study with respect to soluble or deposited Aβ levels in animals assessed 7days after the final weekly dose of rHDL, suggesting that beneficial effects diminish as rHDL is cleared from the body. Further, rHDL-treated animals showed no change in amyloid burden, cerebrospinal fluid (CSF) Aβ levels, neuroinflammation, or endothelial activation in the chronic study, suggesting that the pathology-modifying effects of rHDL may indeed be acute and may be specific to the soluble Aβ pool. That systemic administration of rHDL can acutely modify brain Aβ levels provides support for further investigation of the therapeutic potential of apoA-I-based agents for AD. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Chronic Exposure to Androgenic-Anabolic Steroids Exacerbates Axonal Injury and Microgliosis in the CHIMERA Mouse Model of Repetitive Concussion.

Concussion is a serious health concern. Concussion in athletes is of particular interest with respect to the relationship of concussion exposure to risk of chronic traumatic encephalopathy (CTE), a neurodegenerative condition associated with altered cognitive and psychiatric functions and profound tauopathy. However, much remains to be learned about factors other than cumulative exposure that could influence concussion pathogenesis. Approximately 20% of CTE cases report a history of substance use including androgenic-anabolic steroids (AAS). How acute, chronic, or historical AAS use may affect the vulnerability of the brain to concussion is unknown. We therefore tested whether antecedent AAS exposure in young, male C57Bl/6 mice affects acute behavioral and neuropathological responses to mild traumatic brain injury (TBI) induced with the CHIMERA (Closed Head Impact Model of Engineered Rotational Acceleration) platform. Male C57Bl/6 mice received either vehicle or a cocktail of three AAS (testosterone, nandrolone and 17α-methyltestosterone) from 8–16 weeks of age. At the end of the 7th week of treatment, mice underwent two closed-head TBI or sham procedures spaced 24 h apart using CHIMERA. Post-repetitive TBI (rTBI) behavior was assessed for 7 d followed by tissue collection. AAS treatment induced the expected physiological changes including increased body weight, testicular atrophy, aggression and downregulation of brain 5-HT1B receptor expression. rTBI induced behavioral deficits, widespread axonal injury and white matter microgliosis. While AAS treatment did not worsen post-rTBI behavioral changes, AAS-treated mice exhibited significantly exacerbated axonal injury and microgliosis, indicating that AAS exposure can alter neuronal and innate immune responses to concussive TBI.

Defining the biomechanical and biological threshold of murine mild traumatic brain injury using CHIMERA (Closed Head Impact Model of Engineered Rotational Acceleration)

ABSTRACT CHIMERA (Closed Head Impact Model of Engineered Rotational Acceleration) is a recently described animal model of traumatic brain injury (TBI) that primarily produces diffuse axonal injury (DAI) characterized by white matter inflammation and axonal damage. CHIMERA was specifically designed to reliably generate a variety of TBI severities using precise and quantifiable biomechanical inputs in a nonsurgical user‐friendly platform. The objective of this study was to define the lower limit of single impact mild TBI (mTBI) using CHIMERA by characterizing the dose‐response relationship between biomechanical input and neurological, behavioral, neuropathological and biochemical outcomes. Wild‐type male mice were subjected to a single CHIMERA TBI using six impact energies ranging from 0.1 to 0.7 J, and post‐TBI outcomes were assessed over an acute period of 14 days. Here we report that single TBI using CHIMERA induces injury dose‐ and time‐dependent changes in behavioral and neurological deficits, axonal damage, white matter tract microgliosis and astrogliosis. Impact energies of 0.4 J or below produced no significant phenotype (subthreshold), 0.5 J led to significant changes for one or more phenotypes (threshold), and 0.6 and 0.7 J resulted in significant changes in all outcomes assessed (mTBI). We further show that linear head kinematics are the most robust predictors of duration of unconsciousness, severity of neurological deficits, white matter injury, and microgliosis following single TBI. Our data extend the validation of CHIMERA as a biofidelic animal model of DAI and establish working parameters to guide future investigations of the mechanisms underlying axonal pathology and inflammation induced by mechanical trauma. HIGHLIGHTSBiomechanical input energy predicts biological responses in mouse CHIMERA TBI.Impact energies of 0.4 J and below are subthreshold and produce no injury phenotype.Injury threshold is 0.5 J, where at least one biological outcome is altered.Mild TBI phenotypes are observed at 0.6 J and above.

High-density lipoproteins suppress Aβ-induced PBMC adhesion to human endothelial cells in bioengineered vessels and in monoculture

BackgroundAlzheimer’s Disease (AD), characterized by accumulation of beta-amyloid (Aβ) plaques in the brain, can be caused by age-related failures to clear Aβ from the brain through pathways that involve the cerebrovasculature. Vascular risk factors are known to increase AD risk, but less is known about potential protective factors. We hypothesize that high-density lipoproteins (HDL) may protect against AD, as HDL have vasoprotective properties that are well described for peripheral vessels. Epidemiological studies suggest that HDL is associated with reduced AD risk, and animal model studies support a beneficial role for HDL in selectively reducing cerebrovascular amyloid deposition and neuroinflammation. However, the mechanism by which HDL may protect the cerebrovascular endothelium in the context of AD is not understood.MethodsWe used peripheral blood mononuclear cell adhesion assays in both a highly novel three dimensional (3D) biomimetic model of the human vasculature composed of primary human endothelial cells (EC) and smooth muscle cells cultured under flow conditions, as well as in monolayer cultures of ECs, to study how HDL protects ECs from the detrimental effects of Aβ.ResultsFollowing Aβ addition to the abluminal (brain) side of the vessel, we demonstrate that HDL circulated within the lumen attenuates monocyte adhesion to ECs in this biofidelic vascular model. The mechanism by which HDL suppresses Aβ-mediated monocyte adhesion to ECs was investigated using monotypic EC cultures. We show that HDL reduces Aβ-induced PBMC adhesion to ECs independent of nitric oxide (NO) production, miR-233 and changes in adhesion molecule expression. Rather, HDL acts through scavenger receptor (SR)-BI to block Aβ uptake into ECs and, in cell-free assays, can maintain Aβ in a soluble state. We confirm the role of SR-BI in our bioengineered human vessel.ConclusionOur results define a novel activity of HDL that suppresses Aβ-mediated monocyte adhesion to the cerebrovascular endothelium.

Age at injury and genotype modify acute inflammatory and neurofilament-light responses to mild CHIMERA traumatic brain injury in wild-type and APP/PS1 mice

ABSTRACT Peak incidence of traumatic brain injury (TBI) occurs in both young and old individuals, and older age at injury is associated with worse outcome and poorer recovery. Moderate‐severe TBI is a reported risk factor for dementia, including Alzheimers disease (AD), but whether mild TBI (mTBI) alters AD pathogenesis is not clear. To delineate how age at injury and predisposition to amyloid formation affect the acute response to mTBI, we used the Closed Head Impact Model of Engineered Rotational Acceleration (CHIMERA) model of TBI to induce two mild injuries in wild‐type (WT) and APP/PS1 mice at either 6 or 13 months of age and assessed behavioural, histological and biochemical changes up to 14 days post‐injury. Age at injury did not alter acute behavioural responses to mTBI, including measures of neurological status, motor performance, spatial memory, fear, or anxiety, in either strain. Young APP/PS1 mice showed a subtle and transient increase in diffuse A&bgr; deposits after injury, whereas old APP/PS1 mice showed decreased amyloid deposits, without significant alterations in total soluble or insoluble A&bgr; levels at either age. Age at injury and genotype showed complex responses with respect to microglial and cytokine outcomes, where post‐injury neuroinflammation is increased in old WT mice but attenuated in old APP/PS1 mice. Intriguingly, silver staining confirmed axonal damage in both strains and ages, yet only young WT and APP/PS1 mice showed neurofilament‐positive axonal swellings after mTBI, as this response was almost entirely attenuated in old mice. Plasma neurofilament‐light levels were significantly elevated after injury only in young APP/PS1 mice. This study suggests that mild TBI has minimal effects on A&bgr; metabolism, but that age and genotype can each modify acute outcomes related to white matter injury. Graphical abstract Figure. No Caption available. HighlightsA&bgr; metabolism is not robustly affected by two mild concussive TBIs in APP/PS1 mice.Acute white matter inflammation after TBI is modified both by age and genotype.Age at injury markedly affects the neurofilament response to injury.

Defining an Analytic Framework to Evaluate Quantitative MRI Markers of Traumatic Axonal Injury: Preliminary Results in a Mouse Closed Head Injury Model.

Visual Abstract Diffuse axonal injury (DAI) is a hallmark of traumatic brain injury (TBI) pathology. Recently, the Closed Head Injury Model of Engineered Rotational Acceleration (CHIMERA) was developed to generate an experimental model of DAI in a mouse. The characterization of DAI using diffusion tensor magnetic resonance imaging (MRI; diffusion tensor imaging, DTI) may provide a useful set of outcome measures for preclinical and clinical studies. The objective of this study was to identify the complex neurobiological underpinnings of DTI features following DAI using a comprehensive and quantitative evaluation of DTI and histopathology in the CHIMERA mouse model. A consistent neuroanatomical pattern of pathology in specific white matter tracts was identified across ex vivo DTI maps and photomicrographs of histology. These observations were confirmed by voxelwise and regional analysis of DTI maps, demonstrating reduced fractional anisotropy (FA) in distinct regions such as the optic tract. Similar regions were identified by quantitative histology and exhibited axonal damage as well as robust gliosis. Additional analysis using a machine-learning algorithm was performed to identify regions and metrics important for injury classification in a manner free from potential user bias. This analysis found that diffusion metrics were able to identify injured brains almost with the same degree of accuracy as the histology metrics. Good agreement between regions detected as abnormal by histology and MRI was also found. The findings of this work elucidate the complexity of cellular changes that give rise to imaging abnormalities and provide a comprehensive and quantitative evaluation of the relative importance of DTI and histological measures to detect brain injury.

HIGH-DENSITY LIPOPROTEINS SUPPRESS Aβ-INDUCED BRAIN MICROVASCULAR ENDOTHELIAL CELL ACTIVATION

Background:Vascular endothelial growth factor (VEGF) is an angioneurin involved in the regulation of vascular and neural functions relevant for the pathophysiology of Alzheimer’s disease (AD), but the influence of AD severity and ApoE4 status on circulating VEGF and its relationship with cognition have not been investigated. Methods: Serum VEGF levels and cognitive performance were evaluated in AD, amnestic mild cognitive impairment (MCI) and control subjects. VEGF levels were measured in baseline serum samples by using specific ELISA kits for VEGF.Results:VEGF levelswere higher inAD patients than in MCI cases and controls (p<0.05), and showed a progressive increase with clinical severity in the whole study population (p<0.01). Among AD patients, severity-related VEGF elevations were significant in ApoE4 carriers (p<0.05), but not in non-carriers. Increased VEGF levels were associated with disease severity, and showed mild correlations with cognitive impairment that were only consistent for the ADAS-cog+ items remembering test instructions (memory) and maze task (executive functions) in the group of AD patients (p<0.05).On the other hand, higherVEGFvalueswere related to better memory and language performance in ApoE4 carriers with moderately-severe AD. Conclusions:VEGF serum levels are elevated and correlatewith cognitive impairment in AD patients; show a significant severity-related increase in ApoE4 carriers; and are associated to better memory and language performance in moderately-severe ApoE4cases.Thesefindings are pointing toVEGFas a relevantmolecular target in AD pathology and therapy; and suggest that increases in VEGF levels might represent an endogenous response driven by pathological factors and could entail cognitive benefits, particularly in AD ApoE4 carriers.

High-density lipoproteins at the interface between central nervous system and plasma lipoprotein metabolism

Abstract The brain is the most lipid-rich organ in the body and contains 25% of the body’s total cholesterol content. ApoE is the major apolipoprotein expressed in the brain and genetic variations in apoE underlie much of the genetically determined risk of late-onset Alzheimer’s disease. Regulation of lipid homeostasis in the CNS is therefore of great interest for healthy brain aging. The brain’s lipid transport system is built around lipoprotein particles that are similar in size and presumed function to circulating HDL. It is increasingly appreciated that many comorbidities that increase Alzheimer’s disease risk include aspects of aberrant HDL-metabolism, yet how circulating HDL may impact brain health is not fully understood. As comprehending the similarities and differences between CNS and peripheral lipid metabolism may reveal important relationships between cardiovascular and neurological diseases, here we review the fundamental properties of HDL metabolism in both peripheral and CNS compartments.