Trigger 9: Acquired brain injury and neuroinflammation

TBI

'' Injury to the brain caused by a trauma to the head Not of degenerative or congenital of nature requires external physical force'

Dementia Pugilistica

''Boxing Dementia'' Associated with slurred speech Memory impairment and Parkinson-like Syndrome ''Punch-drunk syndrome'' Thought to be the same as Chronic Traumatic Encephalopathy (CTE) orm of chronic traumatic encephalopathy.​ Repeated trauma to head.​ Most common in contact-heavy sports.​ Rapid mental and cognitive deterioration.​ Occurs after some time.​ Can be years later that symptoms manifest.​ Common symptoms are:​ Slurred speech​ Short-term memory loss​ Confusion​ Likelihood of development increases with number of impacts.

Closed traumatic

(blunt injury)has focal or diffuse injury classification and includes brain contusions, lacerations, intracerebral haemorrhage, diffuse axonal injury from blast related, falls, injuries, vehicle and sports accidents . an injury where no object penetrates the skull. The injury may or may not be associated with a fracture of the skull. If there is a skull fracture, the broken bone does not directly impinge upon the brain tissue. Significant brain damage may be present without any obvious injury to the skull.

Open traumatic

(penetrating injury) has primary focal disease classification includes epidural and subdural hematomas, intecerebral haemorrhage and infections caused by gunshot, stabbing, falls, vehicle and sports accidents describes an injury in which an object(s) penetrates the skull and impinges upon brain tissue. Examples include Pieces of bone from a fracture directly causing brain injury and bullets causing high velocity brain injury. This can provide a route for bacteria to enter leading to serious infection.

Blast injury models

A blast wave e.g. from detonation is used to induce brain injury Military personnel exposed to a blast often suffer from TBIs even without any external injuries. Blast injury mouse models are ideal for investigating how such blast waves affect the brain by inducing mainly diffuse injuries. This could in turn help develop treatments to mitigate blast-induced TBIs. Notably, even mild injuries from blast waves lead to spatial memory and social recognition impairments, and motor coordination deficits.​ The effects of blast injury on mice are variable and tend to be different from TBIs induced mechanically at a focal point. For instance, where the mouse is placed along the shock tube alone greatly affects the type and severity of injury.​ Given the experimental variation in current blast injury models, results from this model may require further characterization to improve reproducibility of data and facilitate comparisons between studies​

Fluid percussion injury models

A fluid pulse is driven rapidly into the epidural space The Fluid Percussion Injury model mimics non fracture TBIs adding intracranial pressure can be regional and depend on amount of pressure you want to add. A pressure pulse targeted at the intact dura via a craniotomy induces the TBI. This pulse (or "percussion") is generated when a pendulum strikes the piston of a reservoir of fluid.​ Fluid percussion injuries lead to brief brain tissue displacement and deformation, and the strength of the pressure pulse and the site of the craniotomy are the main factors determining the severity of the injury.​ Fluid percussion injury mice can mimic clinical TBIs without skull fracture and often present pathophysiological features such as brain swelling and intracranial haemorrhage. Cognitive and motor coordination deficits have been reported using this technique. Results obtained are highly reproducible with fine-tuning of the pendulum height

Glial Scar

A glial scar can form after CNS injury To protect the brain and stop neurodegeneration Containing hypertrophic astrocytes and ependymal and NG2+ cells that have undergone gliogenesis Migration and then proliferation of reactive astrocytes results in formation of the glial scar and has neighbouring microglia, endothelial cells, fibroblasts and a basal membrane layer In neurodegeneration it can be of a detriment or benefit to the brain for various reasons.

Levels of Brain Injury

ABI has been classified by mechanism, clinical severity, and structural damage. The Glasgow Coma Scale has evolved into a universal classification system for the severity of ABI and consists of the sum score (range 3-15) of three components (eye, motor, and verbal scales).

CNS entry routes

ACE2 receptor is concentrated in brain stem among regions associated with regulation of CV and respiratory systems. These are highly vascularised and some lack BBB, In absence of BBB the makes the circumventricular organs, ,more vulnerable to peripheral neurotoxic molecules or viruses.Neuro-invasion 2 types: Retrograde neuronal routes and Hematogenic access

COVID-19 and epilepsy CNS inflammation

After the invasion, the virus triggers reactive astrogliosis and activates the microglia to induce a large inflammatory cascade. Virus entry into the central nervous system leads to the release of pro-inflammatory cytokines (TNF-α, IL-6, IL-1B), nitric oxide, prostaglandin E2, and free radicals, and causes chronic inflammation neural hyper-excitability, seizure, and death (4) Inflammatory cytokines exacerbate apoptosis and neuronal necrosis in the central nervous system, specifically in different parts of the hippocampus, and these pro-inflammatory cytokines play a key role in epileptic pathogenesis.

Controlled cortical impact injury models

An air/electromagnetic driven piston is used to penetrate the brain The Controlled Cortical Impact model is characterized by consistent data with specific parameters. It is a type of animal model that delivers a highly controlled focal impact to the mouse at a specific time, velocity and impact depth means you can focus when concussion occurs. This is typically done using an impactor targeted to the dura.​ This model is easily reproducible and mimics conditions such as concussion, axonal injury, and blood-brain barrier dysfunction. Neuropathological damage can be widespread, involving degeneration of different brain regions such as the cortex, hippocampus and thalamus. The minimal risk of rebound injury helps to increase the reliability of results obtained. However, a craniotomy (surgical opening into the skull) is needed, and this would require some degree of technical expertise.​

Oligodendrocytes

Arise from precursors (NG2-glia) found in the subventricular zone and the ventral neural tube of the brain and spinal cord respectively (1)(2)(3) NG2-glia are highly proliferative cells in the brain's parenchyma (1) NG2-glia can form functional synapses with neurons - but are not able to create their own action potentials and propagate them on, they just receive neuronal signals (1) Regulates axonal myelination - a single oligodendrocyte can contact and myelinate many axons (3) Myelin is a lipid-rich wrapping around the axon - allow rapid saltatory conduction (1)(2)(8) Provide a variety of metabolites to axons - cholesterol, lactate, glycogen etc. (3) Loss of their lactate transporters results in axonal damage (3) Oligodendrocyte death leads to axonal damage and demyelination (1) Accompanied by severe motor dysfunctions like tremor and ataxia (loss of control of body movements)

neurovascular unit.

BBB is formed by the endothelial cells lining the brain vessels together with cells like pericytes, astrocytes and microglia and extracellular components such as the basement membrane in a neurovascular unit.

Neuroinflammation

CNS's response to infections, diseases and injuries. leaky BBB, immune cells, cytokine storm, glial and macrophage polarisation, swelling, gliosis, not always bad can be, neuronal death. Has a vast entity of things that can impact it i.e. immune cells, cytokines and glial cells. Comes from activation of the brains innate system, dependent on phenotype astrocytes and microglia take can protect neurons of brain or if exacerbated in their function can cause neuronal damage and death

COVID-19 infection

COVID-19 infection directly invades neural cells and also effects the neurovascular unit. It enters the CNS through systemic inflammation which is characterised by increased proinflammatory factors in the blood known as a cytokine storm which causes lung injury, kidney and heart failure and this large increase in chemokines and interleukins compromises the BBB so enters CNS. in systemic infections by COVID, the blood brain barrier can actually undergo disruptive and non disruptive remodelling.

Acquired brain injury

Covers all scenarios where brain injury has happened since birth. There are two types which do have key differences making treating them different. Main causes of brain injury are stroke, hypoxia, poison such as alcohol, virus, tumour, fall, struck by an object, sport and IED, infections such as meningitis and encephalitis, degenerative or metabolic disorders such as MS, AD and PD and Huntingdon's. White matter axonal injury is a common feature of all injuries

Effects of TBI on daily life

Divided by where the TBI is located: frontal lack of focus, irritability, language difficulty, parietal difficult reading, spatial misconception, occipital blurred vision temporal problems with short and long term memory, cerebellum difficulty walking or slurred speech, brainstem changes in breathing and difficulty swallowing = essential to survival unlikely would survive

Gliosis

During infection and neuroinflammation divided into Astrogliosis A1/A2 Microgliosis M1/M2 Where normal activity of either cell type causes them to become reactive and response to multiple different signals change morphology and secrete different growth factors and cytokines and neutrophils. A spectrum of molecular, cellular and function changes in glial cells (most notably astrocytes) as a response to various brain diseases and injury Degree of gliosis depends on the severity of the stimuli Can be beneficial (protective) or detrimental (maladaptive) Triggers of gliosis include: TBI Ischemia Infection Neuronal hyperactivity Neurodegeneration Metabolic disorders Main functions of gliosis are to handle acute stress by limiting tissue damage (by glutamate toxicity and oxidative stress), restoring homeostasis and limiting the spread of inflammatory cells/infectious agents

Synaptic pruning and role in neuroinflammation

During normal neurodevelopment and continuous synaptic plasticity, microglia has the role of synaptic pruning - getting rid of excess, unnecessary neuronal connections. During neuroinflammation, signals are causing microglia to start excessively prune away neurons, therefore less synapses are available and neurodegeneration is the end result. Region-specific loss of synaptic integrity and aberrant neuronal networks have been linked to many developmental and neurodegenerative diseaseInflammation results in excessive synaptic pruning Reactive microglia through priming Altered complement system activity The detrimental roles of complement in CNS disorders include apoptosis, leukocyte and glial chemotaxis, opsonization, neurotoxicity, and BBB/BSB dysfunction.

COVID-19 and epilepsy BBB breakdown

Endothelial cells in blood vessels play an important role in the mechanism of blood-brain barrier (BBB) permeability. COVID-19 infection breaks down the integrity of the BBB, which severely impairs brain homeostasis and leads to neuronal apoptosis and death. Activated glia are not the only source for the production of pro-inflammatory cytokines in SARS-CoV-2 infections in the brain. Cytokines, such as IL-6 and TNF-α, can enter the brain through passive or active transmission. The other cause of BBB disruption and seizure-inducement by COVID-19 is fever and hyperthermia. Brain damage during extreme hyperthermia increases the acute activation of glial cells and BBB permeability

Endothelial cells

Form the capillary Connected by tight junctions Contain glucose transporters for delivery to astrocytes Contain aquaporins for movement of H2O

M2 state

From activation by IL-4, IL-10, IL-13, IL-25, TGF-B, glucocorticoid, M-CSF, BMP-7, substance P and galectin-1. Anti-inflammatory role where secrets antiinflammation cytokines to enhance phagocytosis, release neurotrophic factors, reverse damage and protect the brain from inflammation and demyelination in alternatively activated state. Increasing research in taregtting cell type and polarisation states to protect CNS from further damage driving microglia into neuroprotective state in acquired brain injury. causes Attenuation of inflammatory response Phagocytosis Tissue remodelling, repair and healing Recruitment and differentiation of Th2 and Treg cells Overall, contribute to neuroprotection

Glial cells

Glial cells play a defensive and supportive role. Includes Neuroglia Astrocytes and microglia (immune cells in brain). Play pivotal roles in the brain, response to neuro inflammatory insults and neurodegenerative diseases and further accumulating evidence has actually shown that some of those cells are targeted by several neurotropic viruses and several of them can actually impact their function.

Cascade of secondary neurochemical events

Glutamate Chloride Potassium and Sodium Swelling and inflammation from tissue damage Increasing the pressure, producing more damageBlood-brain barrier disruption Excitotoxicity Inflammation: plays an essential role in secondary stages. Oxidative Stress Cell death Mitochondrial disfunction

HDAC inhibitors

Histone deacetylases (HDAC) remove acetyl groups on a histone, allowing for tighter DNA wrapping around the histones, thus suppressing gene transcription. HDAC inhibitors have been widely developed and utilized to activate gene transcription Both grey matter and white matter tracts are significantly preserved by HDAC inhibition after TBI The expression of HDAC3 is upregulated in the innate immune cells (microglia/macrophages) The inhibition of microglia/macrophage activation is the major mechanism underlying the neuroprotective benefits from HDAC inhibition therapy. Valproate or vorinostat These HDAC inhibitors preferentially upregulate the transcriptional expression of many neuroprotective genes involved in cell survival, proliferation, and differentiation. Both grey matter and white matter tracts are significantly preserved by HDAC inhibition after TBI. HDAC inhibitors suppress inflammatory responses, promote neurogenesis and stimulate axonal regeneration.

Neuroinflammation in glial scar

INF-g, IL-1, IL-2, IL-6, TNF-a, and macrophage colony stimulating factors are involved in neuroinflammation leading to activation of astrocytic activity IL-6 promotes differentiation of neural stem/ progenitor cells (NSPCs) into astrocytes via JAK/STAT IL-1 promotes expression of GFAP and astrocyte hypertrophy Activated macrophages can up-regulate the expression of chondroitin sulfate proteoglycans (CSPGs) Type I interferon (anti-inflammatory cytokine) can decrease activation of astrocytes by deactivating MEK/ERK pathway Glial scar formation depends on the interactions between CNS cells and non-CNS cells including hematogenous macrophages and fibroblasts

mechanism for excitotoxicity from leaky BBB

In acquired brain injury and associated leaky disrupted BBB see astrocyte swelling in early response to neurovascular unit damage which increases uptake of glutamate and water, this compresses vessels in region of injury to exacerbate hypoperfusion. Microglia and macrophages under phenotypic to go into inflammatory state. Also involves changes to pericytes.

BBB disruption

In acquired brain injury the BBB is disrupted implicates many cascades which affect different neuroglia and neurones. Type of BBB disruption depend on severity of systemic infection, COVID-19 BBB disruption is highly likely. Under disrupted conditions BBB is remodelled either in Non disruptive pathology: through molecular and cellular mechanisms compromise the permeability of the barrier disruptive pathology in anatomical changes and loss of tight junction apoptotic death of endothelial cells and break down of astrocytes increased vesicular transport. This is exacerbated in conditions where things like sleep deprivation, metabolic syndromes, ageing, that allows the central nervous system entrance of various different pathologies, including viral particles and DAMPs

Glial cell therapeutic strategies

Inhibition of NFKB signaling in astrocytes reduces inflammation in mice. SPGs Extracellular protein molecules that are highly concentrated in spinal injury scarsThey restrict neuronal plasticity and may prevent neuronal regrowth

Blood brain barrier function

Is lined with endothelial tissue, with tight junctions between membranes, made up of capillaries surrounded by glial network of astrocytes. Allows certain substances like water, ions and glucose whilst blocking larger components. highly specialised structure, formed by a tight mono-layer of endothelial cells which maintain the bloodstream cells and neurotoxic components and microorganisms outside of the CNS so that things can't actually penetrate from the blood. Other interfaces with blood and neural tissue include the retinal barrier and blood spinal cord barrier and protect against pathogens e.g. viruses and control the immunological status of the brain. Highly selective semi-permeable membrane that acts as a barrier between the cerebral capillary blood and neural tissues/its fluid spaces Protects the brain from any acute changes in the periphery (chemical or hormonal) and keeps neurotoxic compounds and microorganisms outside of the CNS The BBB forms part of the NVU (also called the gliovascular unit)

Linear vs Angular Injury

Linear acceleration injuries result from straight line forces due to a sudden deceleration of the brain moving through space. Rotational acceleration injuries result from nonlinear forces that twist the brain within the skull. "The human brain is most sensitive to rotational motion. The bulk modulus of brain tissue is roughly five to six orders of magnitude larger than the shear modulus so that for a given impact it tends to deform predominantly in shear. This gives a large sensitivity of the strain in the brain to rotational loading and a small sensitivity to linear kinematics. Therefore, rotational kinematics should be a better indicator of traumatic brain injury risk than linear acceleration".

How does the BBB protect the brain?

Materials from the blood enter the interstitial fluid via capillaries that are made of a single layer of endothelial cells Endothelial cells are joined by tight junctions to limit movement of materials between cells Materials therefore have to pass through the endothelial cells to enter the CNS Lipid soluble molecules (e.g. O2, CO2, steroids) can easily pass through the plasma membrane Non-lipid soluble molecules such as glucose, amino acids and ions pass through via highly selective membrane-bound carriers

Treatments of TBI

Mild traumatic injury No specific treatments - though healthcare specialist may give advice. May suggest avoiding alcohol or other drugs as may slow recovery and increase chance of re-injury. May need to reduce time spent that require hard concentration - e.g. on computer. Medications Some medication can be used to reduce symptoms of TBI and lower risk of further effects: Anticoagulants - prevent blood clots. Anticonvulsants - prevent seizures. Diuretics - remove fluids that increases pressure inside brain. Muscle relaxants - reduce muscle spasms. Anti-depression/anxiety - prevent mood instability and feelings of fear. Some medication used straight after TBI, others used whilst recovering to reduce later symptoms. Surgery and emergency treatments Focussed on keeping patient alive & stable Control blood pressure Prevent further injury to head Maintaining O2 supply to brain. Examples of surgeries: Removing blood clots or pools (haematomas); put pressure on brain and cause damage. Repair skull fractures. Relieving intracranial pressure; hole made in skull or shunt/drain inserted - relieves pressure or drains excess fluid. Rehabilitation therapy Help TBI patients relearn functions and skills. Various different therapies. E.g. physical, cognitive, occupational or speech therapy. Similarly, counselling and coping skills offered to TBI sufferers.

Therapeutic Targeting of Glial Polarisation

Most treatment are focussing on driving M1 into M2 Immunotherapy at the innate immune response after CNS injury Ameliorate the pro-inflammatory M1-like response Promote the anti-inflammatory M2-like positive tissue remodelling. Examples: Minocycline targeting microglia after TBI Lipid-lowering drugs for immunomodulation HDAC inhibitors, Interferon Regulatory Factors, Lipocalin-2

Secondary damage

Mostly associated with traumatic brain injury.​ Tearing, bleeding and sheering associated with TBI can have longer-term effects on brain health.​ Initial damage can include white matter (axonal) damage and haematoma (bleeding).​ Can occur hours-days after primary injury ​ Usually results from injuries such as contrecoup injuries that result in bleeding, tearing and sheering of the brain (subdural haematoma, axonal tearing etc) ​ Also results in neurotoxicity as the brain releases a large amount of neurotransmitters in an attempt to restore homeostasis after injury ​ Chemicals are also released from the tearing of the axons ​

Optical coherence tomography for early diagnosis

One of the key areas that is really highlighted in acquired brain injury and particular traumatic brain injury is how can we diagnose that an injury early on in life can actually lead to dementia later in life? Non invasive test for early diagnosis which can followed bu neuroimaging studies using MRI, PET and MRI. Uses near-infrared light to produce high-resolution images of tissue up to 2mm in depth. Images produced in real time. Used routinely in ophthalmology - looking at retinal pathologies. Used for ABI; detect markers of trauma/injury physiology. Diagnosis of damage established from specific injuries.

COVID-19 and epilepsy Abnormal coagulation and stroke

Persistent inflammatory status in COVID-19 patients acts as an important stimulus for a coagulation cascade. Certain cytokines, including IL-6, activates the coagulation cascade and suppresses the fibrinolytic system.. Endothelial cell damage can activate the coagulation system. Moreover, the immune response can be increased by coagulation disorders. These two processes may act as a vicious cycle to worsen this situation.

Maladaptive/Detrimental effects of Gliosis

Prevents neuronal regrowth Usually following injury, axon sprouts attempt to repair damaged regions Scarring prevents axon extension via physical and chemical barriers Astrocytes create dense network of tight junctions and secrete growth-inhibitory molecules which prevent extensions

COVID-19 and epilepsy Mitochondria disturbance and oxidative stress

Production of reactive oxygen species (ROS), a part of oxidative stress, is a key feature of mitochondrial dysfunction Disturbance to mitochondria has been evident in COVID-19 pathophysiology: link has been seen between mitochondria, oxidative stress and inflammation ROS production in mitochondria is exacerbated via the generation of inflammatory cytokines- TNF-alpha and IL-6 These pro-inflammatory cytokines have been found in blood samples from COVID-19 patients

Second impact syndrome

Rare but potentially serious common in sports players Life-threatening condition Back to back concussions Results in a loss of autoregulation of the blood supply vascular engorgement increased intracranial pressure rapid brain stem failure which will lead to death Occurs when individual suffers back to back concussions.​ Seen in sports players.​ Second impact must occur before recovery from previous one - can be minutes, days or weeks.​ Rapid brain swelling.​ Vascular engorgement.​ Increased intracranial pressure.​ Rapid brain stem failure.​ Brain herniation.​

Benefits of Gliosis

Re-establishes physical and chemical integrity of the CNS Generates a physical barrier around injury site - regeneration of the barrier prevents further microbial infections/spread of cellular damage Glial scar promotes revascularisation to increase homeostatic support to damaged tissue

Astrocyte end feet

Receives glucose from the blood and provides to neurons Supports blood vessel Deposits waste/metabolites from neurons into capillary

A2 astrocytes

Release neurotrophic factors to aid in brain repair and promote M2 polarisation Overall, contribute to neuroprotection

A1 astrocytes

Release proinflammatory signals to activate microglia Proinflammatory Signals & Neurotoxins LPS, IFN-γ, IL-1, C1q, Lcn2 Overall, contribute to neurotoxicity and neurodegeneration A2 anti-inflammatory morphology activated by IL-10 and IL-4 resulting in alternative activation and neuroprotection Anti-inflammatory Signals IL-4, IL-13, IL-10, BDNF, VEGF, bFGF

Secondary injury Astrogliosis

Secondary injury processes of TBI include neuroinflammation, excitatory, BBB disruption, metabolic disturbance, apoptosis, ischemia, and oxidative stress. Within extracranial injury can have inflammatory cytokines IL-1B, IL-6 and TNF-a and sepsis systemically that can last for weeks after which causes inflammatory cytokines and leukocyte infiltration in the CNS which causes neuroinflammation with microglial activation and astrogliosis

Microglia

Smaller cells, derived from hematopoietic stem cells (2) Ramified (offshoots) cells with thin processes and branches (3) Known as the resident immunocompetent and phagocytic cells of the NS (1)(3) High capacity to repopulate - within 1 week of depletion the whole population is restored (1) Share similar properties with tissue macrophages - express same cellular markers (1)(2) Primary role in defense mechanisms (3): Remove cellular debris from sites of injury (2) Remove cells from cell turnover (2) Play a role in normal brain development and function (3) Role in synaptic modulation and in synaptic pruning during development through the complement system (1)(4)(5) Following brain damage (either ABI or TBI)... The number of microglia at the site of injury dramatically increases - either proliferate from resident brain microglia, or come from migrating macrophages (from the circulation) (2) Release cytokines and phagocytose debris (dead/dying cells) from injury site Also undergo changes in shape and protein expression (protects the brain) (3) Dysfunction/ loss of microglia can result in behavioral deficits and impaired synaptic plasticity (3

Maintaining brain homeostasis and circumventricular organs

Some areas of the brain, such as parts of the hypothalamus, do not have a BBB and instead have highly fenestrated capillaries This is to sample the blood to monitor the periphery and adjust function to maintain homeostasis Includes functions such as feeding/fasting - Trigger 7 on obesity, the arcuate nucleus is a circumventricular organ

COVID-19 and epilepsy Electrolyte imbalances

Studies have reported various electrolyte abnormalities in patients with COVID-19 with decreased serum concentrations of sodium, potassium, magnesium, and calcium, leading to hyponatremia, hypokalemia, hypocalcemia, and hypomagnesemia. These disorders, especially hypokalemia, may have severe clinical consequences for the infected patient as can leads to exacerbation of ARDS and acute heart damage Electrolyte imbalance is triggered by elevated potassium excretion and GI symptoms such as diarrhoea and vomiting Decreased ACE2 expression increases angiotensin II levels, promoting potassium excretion via kidneys elevated angiotensin II mediates acute lung damage Diarrhoea and vomiting lead to loss of other electrolytes Seizures are a symptom of electrolyte imbalances Early detection and treatment essential to avoid brain damage and stop seizures Anti-epileptic drugs (AEDs) will NOT work unless electrolyte imbalance is corrected Should be administered after electrolyte treatmen

Bleeding, tearing and sheering

Subdural haematoma Bleeding under the dura mater White matter injury Axonal damage twisting and tearing of the brain inside the skull, skull moves faster than brain so it accelerates movement and this tearing and shearing effect effects axons causing brain damage

Functional polarisation of microglia

Switch between M1 and M2. Upon activation by toxic proteins microglia will secrete high levels of toxic cytokines and membrane receptors thus activating other cells including astrocytes and overall priming microglia.

TBI mechanisms

TBI results as development of complex neurological deficits and is caused by both the primary and the secondary injury mechanism. Primary mechanism leads to mechanical damage of the neurones and that external damage of the sheering and tearing. Secondary event results in changes in biochemical, metabolic and cellular events which results into longer term damage that you can see or even neurodegeneration and cognitive decline. Longer-term effects of chemical secondary damage can include degeneration of neural networks.​ Patients with chronic concussion have similar degenerative pathologies to Alzheimer's patients.

Maintenance of the BBB and beyond

The homeostasis is critically dependent on the function of these glial cells includes astrocytes which regulate the vascular system, BBB, metabolic control, water distribution, synaptogenesis and synaptic pruning e.g. in tripartite synapse, oligodendrocytes nourish axons increase conductance 100x and form myelin sheath and maintain BBB and microglia for immunological responses, phagocytes dying cells and debris, synaptic pruning through the complement system Glial cells can influence BBB physiology by their secretion of several factors (9) During ischemic injury, interactions between the astrocyte and the ECM are disrupted - results in the incomplete reconstruction of the damaged BBB (10) In ABI/TBI the BBB becomes leaky, and this disrupts the tight junctions (TJ) of the EC membrane (9) See a loss of TJ integrity, an increase in apoptotic death of the ECs and astrocytic damage Microglia undergo phenotypic polarization into pro- and anti-inflammatory phenotypes Also increase BBB permeability by secretion of MMPs (9) Astrocyte's swell (can be triggered by DAMP infiltration following viral infection) - this increases glutamate and water uptake and the swelling compresses the vessels in the region of brain injury (11) Therefore, astrocytes can advance the hyper-perfusion This astrocyte dysfunction along with pinocytosis in the ECs, can further contribute to the BBB breakdown following excitotoxic injury (10)

Astrocytes

The most abundant Glial cells in the brain (also present in the spinal cord) (1)(2)(6) Elaborate local processes - starlike appearance (2) Many functions: Regulation of local blood flow (as part of the neurovascular unit) - NT release from neurons activates receptors on astrocytes, this then triggers Ca2+ release that in turn promotes the release of vasoactive substances from the end-feet of astrocytes (that contact the blood vessels). Results in an increase in blood flow (3) Essential role in maintaining appropriate neuronal excitability and water/ ion homeostasis - regulates brain volume and K+ concentration (3) (1)(2) Participate in synaptic function - development, refinement and modulation of activity by release of syntactic factors (3) Therefore, are contributors to cognition, learning and memory (3) Astrocytes can advance the further neurodegeneration of the CNS (7) They can act as viral hosts giving non-productive infection and a mild-inflammatory response. They can form glial scars and synthesise various inhibitory regeneration molecules (including MMPs). This causes the inflammatory response which then contributes to the neurodegeneration following viral infection. Therefore astrocytes can also be detrimental to neuronal functioning.

COVID-19 and epilepsy

The potential mechanism of COVID-19 an seizures is still uncertain but do know have these key cytokines of TNF-a, IL-6 and IL-1B and increase in neurotransmitters of glutamate and aspartate cause altered ion channel functions such as Kir4.1 and overall reduction in GABA inhibitory neurotransmitters. Involves astrocytes and disruption in tripartite synapse due to viral penetration can alter ion channel function which alters MAPK and ERK which disrupted BDNF synthesis and secretion. BDNK is important normally for neural sprouting, synaptogenesis, neurogenesis and reactive gliosis which means more excitability is occurring and calcium release which results in neuronal excitation , epilepsy and mood dirsorders and eventually neuronal death,

Glial scarring

Under these stages using GFAP marker of astrocytes can see few healthy astrocyte in healthy tissue then in severe astroglioses see large amount fo astrocyte's forming glial scar. Time line of this is very interesting where can go on for hours to months. Initially get BBB disruption and leakage, get immune cell infiltration and microglial movement which results in inflammation and angiogenesis in days then in weeks get gliosis and asto glial scar formation eventually formed by axonal sprouting and neurogenesis so get repair where structure and tissue regeneration

What are the roles of different cell types within the Glio vascular unit?

Vascular endothelial cells: Core anatomical unit of the BBB Pericytes: Communicate directly with cerebral endothelial cells and other pericytes through gap junctions Induce polarisation of astrocyte endfeet around vessels In disease - pericyte degeneration leads to increased blood vessel permeability Stem cell like properties - potentially capable of differentiating into other cell types in the NVU Astrocytes: Centrally positioned between neurons and endothelial cells Respond to synaptic activity and neuronal metabolism to help regulate cerebral blood flow Cover 99% of cerebral blood vessels Express high levels of aquaporin-4 to allow H2O entry Secrete growth factors from end feet to induce tight junction formation and upregulate transport proteins in vascular endothelial cells Neurons: Neuronal processes are in contact with the vasculature and can mediate local increases in cerebral blood flow in response to metabolic demand Microglia: Primary immune cells of the brain, lie near the vasculature to detect invading pathogens and signals from astrocytes to initiate an appropriate response

Pericytes

Vascular smooth muscle cells Aid in BBB maintenance Regulate structure and function of tight junctions Regulates cerebral blood flow through constriction/relaxation

Diffuse Axonal Injury

When the brain is subject to significant acceleration-deceleration and rotatory shearing forces, these forces can disrupt the white matter of the neurons, disrupting further cell signaling, often extending/affecting the white matter. This type of injury may not be visible on a brain scan; however it may lead to significant brain injury associated with long term disability.

Other ways to detect TBI

X-ray CT scan, MRI, EEG, PET, DTI

experimental models used to investigate TBI

blast injury, fluid percussion, weight drop, controlled cortical impact, in vitro

Closed head injury

blunt injury movement within skull e.g. fall or car accident. Concussion or cerebral contusion where theirs bruise on brain tissue.

Traumatic Brain injury

can be divided further contact injury were head struck and this can be an open or closed primary injury mechanism or against an object and non contact where brain moves within skull. Damage to the brain caused by an external physical force such as a motor vehicle accident, assault, or a fall. This term can also be applied to brain injury caused by cranial surgery and any subsequent complications (e.g. following surgery for brain tumour removal).

In vitro models

can be used to study specific pathophysiological cascades — individually and without confounding factors — and to test potential neuroprotective strategies.​ These in vitro models include transection, compression, barotrauma, acceleration, hydrodynamic, chemical injury and cell-stretch methodologies.​ arious cell culture systems can also be utilised, including brain-on-a-chip, immortalised cell lines, primary cultures, acute preparations and organotypic cultures.

Infections by viruses

can trigger multiple neurological anomalies including non-traumatic brain injury ranges from general cognition and motor deficit to wide spectrum of CNS disorders e.g. meningitis, encephalitis, guillian barre and anxiety and visual disabilities. Peripheral viruses including measles virus, influenza virus and the SARS CoV-2 virus, for are reported to cause various neurological manifestations in patients and are proven to be a neuro pathogenic even in cellular and animal model systems

Chronic traumatic encephalopathy

causes from mild repetitive TBI effects athletes in contact sports, due to pathology of tau protein tangles which accumulate in brain collects around blood vessels and deep in cortical sulci no treatments and results in suicidal thoughts depression, changes in behaviour and speaking shows Parkinson's like symptoms like boxing dementia all through this is process of reactive astroglioses and glial scar formation

Glia vascular unit

facilitates a cross talk between CNS and periphery through the BBB, is a complex that surrounds most blood vessels in barrier and is a barrier between the bloodstream and extracellular space. Made up of pericytes, astrocytes, microglia. The BBB does not function independently but as part of the NVU Functional unit comprised of many cells including: neurons, interneurons, astrocytic endfeet, microglia, oligodendrocytes, smooth muscle cells, pericytes, endothelial cells and ECM Reacts in response to physiological stimuli to regulate vascular permeability, cerebral blood flow and activating the neuroimmune response to maintain CNS homeostasis Facilitates the crosstalk between the CNS and periphery via the BBB

Glial cell dysfunction

have been associated with several neuro inflammatory diseases, suggesting that a virus like COVID has a primary effect on these cells. In addition to a secondary effect from the neuronal damage. Both lead to viral dissemination through the CNS as COVID replicates by glial cells. It is unclear how it does this, although astrocytes have actually been suggested to be acting as a reservoir for taking up the virus and contributing to the viral spread.

Non contact traumatic injury

is primary diffuse (multifocal) primary injury mechanism characterised by diffuse axonal injury, white matter lesions and haemorrhage caused by falls, vehicle and sports accidents

Non-traumatic brain injury

is severe reductions in blood flow and haemorrhage due to clotting as primary injury mechanism, can be primary or focal disease classification and injury pathophysiology is white matter lesions or haemorrhage and is caused by stroke, neurotoxic poisoning (CO), hypoxia (drowning, smoke inhalation, heart attacks, severe asthma, suffocation, periods of prolonged epilepsy, diabetes and associated CNS metabolic disturbance) and anoxia (no o2 in brain), ischemia, infection, tumours, Infections, Strokes, Metabolic disorders, Anoxia, Autoimmune causes of brain injury. Happens for often then one may thin and if continues to happen will result in neurodegeneration. Any sort of systemic inflammation exacerbates the central immune system can lead to non-traumatic injury.

Glial Priming

is the process by which the primary insult sensitises the cell to exhibit an exaggerated response to subsequent stimuli. So further injury exacerbate that type of glia and causes further damage and neurotoxic effects i.e. M1 hyperactivation of synapse engulfment, more neurotoxic factors. Acts in reciprocal activation and positive feedback . Can be neuroprotective to increase microglia neuroprotective cytokine and synaptic pruning Involves cytokines, interleukins and also receptors and ion channels they bind to and microRNA

CNS defensive system

made up of glial cells such as astrocytes and microglia which are critical in defining neurological damage and outcome in COVID-19. Cytokines, chemokines and colony stimulated factors released by astrocytes and microglia in cytokine storm lead to neurotoxicity, synapse loss and consequently contribute to long and short term brain damage.

Perivascular astrocytes

may incorporate viral particles by direct contact with BBB endothelial cells and or BBB breakdown lead to viral infection of non-perivascular astrocytes so play an important role as a viral host. Gives rise to either non productive infection or productive infection with destruction of astral glial network which is less likely. Astrocytes are not primary targets of viral infection, but they are responsive to pro-inflammatory signals from endothelial cells, macrophages, microglia and/or neurons. In such case, astrocytes may polarize to A1 pro-inflammatory - neurodegenerative phenotype expanding neuroinflammation and stopping being protective and supportive to neurons which also degenerate by lack of nutrients and neurotrofic factors. In addition to astrocytes being potential targets for SARS-CoV-2, both astrocytes and microglia are highly sensitive to systemic pro-inflammatory cytokines Indeed, the massive release of inflammatory cytokines described for severe COVID-19 patients could be enough to destabilize the tight junctions of the BBB endothelial cells and astrocytes, thus facilitating viral entry. It has been shown that the presence of reactive pro-inflammatory microglia, the exposure to pro-inflammatory cytokines such as IL-1b, TNFa, IL-6, or the exposure to PAMP/DAMP can induce in astrocytes the polarization to the A1 phenotype, which facilitates neuroinflammation and neurodegeneration and BBB disruption

Viral cell response

microglial cells trigger further activation and recruitment of T cells, monocytes and macrophages and APC's which enter brain. APC conduct antigen presentation to develop an immune response, and there is a large release of inflammatory mediators of cytokines and interleukins in the cytokine storm. Short term effects are headache, anosmia, mood disorders, seizure, vision impairment, dizziness and ataxia. Long term effects are demyelination, cognitive impairment, premature aging anf neurodegenerative disorders.

TBI astrocytes

migrate to site of injury form this reactive site and start to proliferate to form glial scarring. Glial scarring is a structural formation of reactive glia around an area of severe tissue damage. Formed by migration of myelin associated inhibitors, astrocytes, oligodendrocytes, oligodendrocytes precursors and microglia as a wound. Stages of astrocyte reactivity sees increased homeostasis and trophic factors, then proliferation and migration of these astrocytes to site of activity secretion of cytokines, growth factors, and toxic molecules for glial scar formation repair of damage at BBB site

Mechanisms of neuroinflammation

mmune cells release cytokines and inflammatory factors to activate M1 polarisation state, leads to activation of other primary glial cells of astrocytes again polarising to A1 neuroinflammatory phenotype to damage neuron. Can be A2, M2 neuroprotective. If inflammation isn't stopped leads to execration of cytokine storm and damage.Activated microglia induce the activation of astrocytes by releasing cytokines such as IL1α, TNFα, and C1q. (4) Astrocytes have a regulatory effect on microglia by releasing cytokines such as IL-33. (5) Activated microglia and astrocytes can damage neurons. (6) Astrocytes may have neuroprotective functions by producing several types of neurotrophic factors. (7) Activated astrocytes have damaging effects on oligodendrocytes. (8) Microglia may have a dual role in the regulation of oligodendrocytes: inflammatory factors produced by activated microglia cause impairment of oligodendrocyte/OPCs, whereas VEGF-C produced by microglia stimulates OPC proliferation and M2 microglia can drive oligodendrocyte differentiation during remyelination.

blood

n hematogenous route where endothelial cells become infected followed by perivascular astrocytes and peripheral macrophages. Secondly viral particles may reach microglia and neurons. During severe case of COVID viral particles can reach Choriod Plexus and the ventricular organs,which lack this blood brain barrier and subsequently enters to directly into the brain parenchyma. Thirdly a possible mechanism of CNS infection may actually arise from the breakdown of the BBB. from systemic inflammation and this cytokine release, and it increases BBB permeability, which could then also further facilitate the entry of the virus into the central nervous system.

Blast TBI

occurs in people exposed to bombing and warfare. The brain injury is caused by a combination of factors: Contact injuries which may cause closed or open/penetrating blast injuries, High pressure caused by shock waves that are significantly above normal atmospheric pressure. These can result in direct trauma to the brain.

Diffusion Tensor Imaging

only way to determine extent of damage, visualises the white matter and under TBI can see gaps under neuronal tracts provides lots more info into mechanisms of TBI. allows you to look at axon fibre tracts made up of white matter to look at important axonal tearing and shearing primary injury and extent of damage which then result in cytokine release as opposed to other imaging techniques which just look at general injury or swelling.

SARS-COV-2

ost research is focussed on it as we do not know the neuroinflammatory impact it may have. It does result in neurological damage likely through hypoxic brain injury and immune mediated change to the CNS. It enters CNS via binding ACE2 receptor and this is expressed in CNS by epithelial cells also neurons and neuron glial cells. Regional variability in the distribution of ACE2 receptors in the human brain has been reported. The highest ACE2 expression level was detected in the brainstem containing the medullary respiratory centers, an observation that could be relevant to the respiratory distress experienced by many COVID-19 patients. Upregulation of ACE2 in the brain has been linked to oxidative stress, apoptosis, and neuroinflammation leading to neurodegeneration in several brain disorders

Open head injury

penetrating injury Involves penetration into skull severity depends on areas affected, if involvement and damage to both hemispheres, ventricles or multiple lobes usually fatal, research evolved from Phineas Gage who had metal rod in brain survived could walk and talk but underwent personality changes. Can survive if only partial damage.

BBB

plays a key role in maintaining the specialised microenvironment of the central nervous system and enabling communication with the systemic compartment. BBB changes occur in several CNS pathologies including acquired brain injury. Can undergo both disruptive and non-disruptive changes due to systemic inflammation and infections and this has downstream effects to CNS and glia vascular unit. 2 key mechanisms involved in neuroinflammatory responses are gliosis and functional polarisation of glia

Weight drop injury models

ree weight is dropped onto the exposed dura​ Contact sports, military activities and child abuse commonly result in mild TBIs. In the lab, weight drop models in mice can be used to mimic the clinical consequences of such TBIs. This model is simple and easy to perform. Here, the skull of the mouse is exposed to a guided free-falling weight - this can occur either with or without a craniotomy. The mass of the weight and height of dropping it determines how severe the injury is. Additionally, the type of injury induced depends on the type of weight drop model used depends on type of injury whether you want focussed or global brain. ​ 1) Shohami's weight drop model​ Mainly focal injury​ i.e. focal strokes or certain area in brain i.e. frontal temporal responses Weight drop is delivered to one side of the unprotected skull​ ​ 2) Marmarou's weight drop model​ Mainly diffuse injury​ A metal disk placed over the skull prevents bone fractures from the weight drop​ This technique can induce severe morphological changes in the brain, behavioural abnormalities such as delayed spatial learning as well as various degrees of neurodegeneration

Focal brain injury

refers to areas of localised damage and includes contusions and lacerations. Contusions are multiple small haemorrhages in the surface layers of the brain (i.e. bruises). They can occur at the coup site of impact and/or at the contra-coup site of impact. Contusions are commonly found at the tips of the frontal, temporal and occipital lobes. A contra-coup injury may result in diffuse axonal injury. Lacerations are tears to the brain tissue caused by penetrating objects or the sharp edges of fractured skull bones. Haemorrhage from arteries or veins can result in a haematoma within the cranium. Intracranial haematomas can push the brain to the side causing midline shift and/or herniation.

Functional polarisation of astrocytes

role in the tripartite synapse. However, astrocytes are also well equipped to participate in the CNS immune and inflammatory responses alongside microglia A1 proinflammatory phenotype activated by IFN-y, LPS, IL-1, LCN2 classically activated to have neurotoxic effects in brain injury Upon activation astrocytes express interleukins, ROS but in astrogliosis production of TGF-a and TGF-B to activate MAPK cascade and ERK which helps in neuronal regeneration and repair. For developing therapies want to target proinflammatory. As soon as there is a damage response in brain these astrocytes prime and phenotype switch dependent on what cytokines are released in environment and multiple different cytokine pathways generally is loss of notch signalling after glia senses environment which means JAK/STAT pathway (STAT3) which drives A1 phenotype to activate NfKB which alters again to A1 inflammatory

M1 state

rom activation by LPS, TNF-a, IL-1B, IL-17A, IFN-y, GM-CSF and LCN2 leads to classically activated microglia, which leads to neurotoxicity neurodegeneration and Other neurological deficits. If this pathway continue microglia will be primed and exacerbate conditions further. causes Inflammation Phagocytosis Blood-brain barrier permeabilization Neural death Recruitment and differentiation of Th1 and Th17 cells Antigen presentation

Concussion

symptoms of Nausea and vomiting, confusion and vision problems. don't necessarily appear straight away, should have 72 hour monitoring period can even be a week after dependent on severity of injury facing. You can get concussion without hitting your head for example rotational and acceleratory forces in brain injury e.g. baby shaking and any sort of impact can result in concussion, hit, shaking or swelling so pressure on skull. A sports player subject to concussion can Return to play After the athlete is seen by a health care provider and declared free of symptoms. although most continue to play shorter then should of which can result to secondary impact syndrome and even death due to repetitive injury.

Barriers of the brain

there are three main interfaces in the brain that basically protects neurones from blood borne substances and to help maintain water, homeostasis and an appropriate environment for full neuronal functioning. Includes Blood-brain, blood-CSF and arachnoid and secondary barriers blood retinal and blood spinal.

moderate hypothermia

there hasn't actually been any drugs that have shown to be beneficial to this secondary cascade of events. One therapy, which is inducing moderate hypothermia, has shown use because the events that occur from the damage in the brain are highly temperature sensitive. Cooling slows down the rate of damage that is occurring. And it means that the injury due to the lack of oxygen and glucose in the brain and excited toxicity that causes cell death basically slows down. It thereby reduces that inflammation and swelling.

Positive aspects of neuroinflammation

transient low inflammation in immune surveillance, low neuroinflammation for development, memory and learning, transient inflammation for injury induced remodelling and immune preconditioning synaptic plasticity and learning, remodelling and glial scar formation, gliogenesis and astrogenesis before repair of neurons,

Negative aspects of neuroinflammation

where goes bad in ABI, systemic inflammation - anything that causes overactivation of central immune cells in brain leading to cytokine storm, causes traumtic CNS injury, repeated social defect stress, aging, TBI and neurodegeneration

olfactory epithelium

where particles have actual access to the CNS using trans neuronal and synaptic pathways and following neuronal infection, virus particles basically released may infect microglia and astrocytes

Tight junctions

within the BBB do not allow ions to move passively into the brain and thus prevents fluctuations in electrolytes that occur within the blood. Makes it different to systemic barriers Create small gaps between endothelial cells Only allows passage of lipid soluble molecules such as O2 and CO2