Traumatic Brain Injury

tlrs and ligands

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Traumatic Brain Injury (TBI) is defined as an injury to the brain caused by trauma to the head, with car accidents, firearms injuries and falls being amongst the main causes. Clinical prognosis is directly related to the type of injury, its location and its severity. The effects can be wide ranging and include cognitive issues (the way an individual thinks, learns and remembers), communication problems and emotional difficulties.

TBI often leads to dysfunction of the blood brain barrier (BBB), which is the key interface between the vascular system and the brain. The existence of the blood brain barrier was discovered over a century ago, when Paul Ehrlich reported that the brain tissue and spinal cord of rats remained unstained following the injection of water soluble dyes in to the bloodstream (1). Historically the blood brain barrier has been defined as the layer of endothelial cells which line the cerebral capillaries, with the permeability of this endothelium being restricted by tight junctions that selectively exclude most blood-borne substances from entering the brain. Tight junctions play a pivotal role in the polarisation of many epithelial cells, and their formation is dependent on interactions between the extracellular domains of specialised membrane proteins. Tight junction proteins include claudins, occludin, tricellulin and Junctional Adhesion Molecules (JAMs), and these are attached to various cytoskeletal proteins including zonula occludens ZO-1/2/3, MAGI-1, MAGI-3, CASK/LIN-2, MUPP1, AF6, ASIP, PALS1, PATJ and cingulin (2). Highly regulated processes allow the transport of water, solutes and ions across the blood brain barrier, with astrocytes playing a key role in brain water homeostasis. Astrocytes express water channel proteins known as aquaporins at their end-feet processes, which can bi-directionally transport water, ions and solutes across a cell membrane. At least fourteen different aquaporin proteins have so far been identified, with at least six of these reported in the brain (3). Taking into consideration the role of astrocytes and other cell types in maintaining and regulating the BBB, the concept of the neurovascular unit (NVU) has more recently been introduced. The NVU is a dynamic structure consisting of endothelial cells ensheathed by a basal lamina, and surrounded by astrocytic feet processes, pericytes, and neurons (4).

TBI is often associated with the release of various factors across the blood-brain barrier, and an indicator of the integrity of the BBB is the cerebrospinal fluid to serum albumin ratio of these proteins (5, 6). There are many potential TBI biomarkers, and these include amyloid-β, Amyloid Precursor Protein (APP), Brain Derived Neurotrophic Factor (BDNF), Connexins, Creatine Kinase B-type (CKBB), Glial Fibrillary Acidic Protein (GFAP), Lactate Dehydrogenase (LDH), Myelin Basic Protein (MBP), Neurofilament Light polypeptide (NFL), Neurofilament Heavy polypeptide (NFH), Neuron-Specific Enolase (NSE), Protein S100B, Spectrin breakdown products (SBDP), Tau and Ubiquitin Carboxyl-terminal Hydrolase isozyme L1 (UCH-L1).

  • The Amyloid Precursor Protein (APP) is a transmembrane receptor that is concentrated at neuronal synapses. Its function is not fully understood, however it exists as multiple isoforms and is cleaved by secretases and lysosomal proteases to give rise to products that include the amyloid-β (Aβ) peptides Aβ-40 and Aβ-42 (7).
  • Amyloid-β (Aβ) peptides are found in the human cerebrospinal fluid (CSF). The majority of these peptides exist as a 40 amino acid-long form known as Aβ-40, whilst the longer variant Aβ-42 is more hydrophobic and tends to aggregate in to plaques. Levels of Aβ-42 are elevated in the CSF following TBI, and can be correlated with other markers to aid determination of the degree of injury (8). The Aβ peptides have been extensively studied, since extracellular plaque formation is one of the characteristics of Alzheimer’s-Disease (AD). Low CSF Aβ42 concentrations have been shown to correlate with a high number of brain plaques, while the Aβ42/ Aβ40 ratio can also be used to aid the diagnosis of AD (9).
  • Brain Derived Neurotrophic Factor (BDNF) is a member of the neurotrophin family, which also includes Nerve Growth Factor (NGF), Neurotrophin-3 (NT-3) and Neurotrophin-4/5 (NT-4/5) (10). These proteins all have distinct effects on neuronal sub-populations, with BDNF being a major regulator of synaptic connections, synaptic plasticity and neural growth, and showing high expression levels in the brain (11).
  • Connexins are gap-junction proteins. Gap junctions connect neighbouring cells via channels, allowing communication and the transfer of ions and small molecules. Cells express hemi-channels at their surface, with each hemi-channel being composed of six connexin subunits; a gap junction is formed when these hemi-channels come together. There are many different connexin proteins, all named according to their molecular weight, and those found in neurons include Cx36, Cx30.2 and Cx40 (12). More recently another family of proteins, named pannexins, has been identified. These operate as channels rather than forming gap junctions, and one member of this family, Px1, has been found to be expressed on neurons and glial cells and has been implicated as a biomarker of TBI (13).
  • Creatine kinases are critical for energy transduction in tissues with large, fluctuating energy demands such as the brain, heart and skeletal muscle. They exist as homodimers or heterodimers of B-type and M-type chains, with the BB homodimer (CKBB) predominant in the brain. Levels of CKBB have been found to be elevated in the CSF immediately following TBI (14).
  • Glial Fibrillary Acidic Protein (GFAP) is a major component of cytoskeletal intermediate filaments and is strongly expressed by astrocytes; it is a structural protein that is involved in processes related to cell movement, and it is also thought to play a role in astrocyte-neuron communication (15).
  • Lactate dehydrogenase (LDH) exists as a tetramer composed of two different subunits, LDHA and LDHB, and mediates the bidirectional conversion of pyruvate and lactate (16). LDHA is the predominant subunit form found in glycolytic tissues such as neurons, and its release from these cells following TBI can be used as a marker of neuronal damage (17).
  • Myelin Basic Protein (MBP) is one of the major myelin proteins within the Central Nervous System (CNS), alongside myelin proteolipid protein (PLP), myelin-associated glycoprotein (MAG) and 2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNP) (18). MBP exists as multiple isoforms, with molecular weights ranging from 14-21.5kDa. The major role of MBP is to insulate nerve fibres, facilitating the rapid conduction of electrical nerve impulses.
  • Neurofilaments are the major structural proteins of neurons, playing a key role in the maintenance of neuronal shape and in the conduction of nerve impulses along the axons (19). They consist of four subunits – the light, medium and heavy neurofilament polypeptides, as well as alpha-internexin. Numerous studies have been performed to utilise NFL and NFH as biomarkers for neuronal damage (20).
  • Neuron-Specific Enolase (NSE) is a glycolytic enzyme that exists as isozyme homodimers or heterodimers which are cell-type or development specific. It is composed of α, β and γ sub-units, with the αγ and γγ dimers being highly abundant in the brain and at low levels in all other tissues. The αγ and γγ dimers are primarily located in the neuronal cytoplasm, and are only detected in serum following disruption of the BBB (21). The αα homodimer is expressed in the embryo and in most adult tissues, while the αβ heterodimer and the ββ homodimer are found in striated muscle.
  • The calcium-binding protein S100B is a member of the S100 protein family that regulates both intracellular and extracellular processes (22). Its intracellular effects include the stimulation of cellular proliferation and migration, and the inhibition of apoptosis; its extracellular effects are dependent on its concentration (23). S100B is located in the cytoplasm of astrocytes and oligodendrocytes, from which it can be actively released (24).
  • Spectrins are enriched in the neuronal cytoplasm. They exist as heterotetramers that are composed of various α and β subunit isoforms, and are multifunctional proteins with roles that include the maintenance of cell membrane integrity and its mechanical properties (25). The αII Spectrin protein in particular has been extensively researched in the area of TBI, with a number of studies demonstrating that the protein is processed by calpain and caspase proteases to produce signature breakdown products indicative of neuronal cell death. Calpain degradation gives rise to proteins of 150 and 145kDa, known as SBDP150 and SBDP145 respectively, while caspase degradation generates a 150kDa protein that is further cleaved to a 120kDa fragment (26).
  • The microtubule-associated protein Tau (MAPT) is localised to axons, where it plays a significant role in axonal growth, the development of neuronal polarity and the maintenance of microtubule dynamics (27). Phosphorylation of Tau is developmentally regulated, with high levels of phosphorylation during embryogenesis and early development, and lower levels of phosphorylation detectable in adult brain (28). Hyper-phosphorylated Tau is a major component of neurofibrillary tangles, which occur in tauopathies such as Alzheimer’s-Disease (AD) and some forms of dementia (29). TBI may pre-dispose individuals to later development of conditions such as AD.
  • Ubiquitin Carboxyl-terminal Hydrolase isozyme L1 (UCH-L1) is another abundant neuronal cytoplasmic protein, and makes up 1-5% of total soluble brain protein (30). UCH-L1 is expressed almost exclusively in neurons, with mice lacking the functional protein having been reported to exhibit neuronal dysfunction. More recently the potential oncogenic role of UCH-L1 has been the studied since the protein has been detected in many non-neuronal tumours (31).

The effects of TBI vary widely on a case-to-case basis, and may cause transient or lifelong impairments in physical, cognitive and behavioural function. Recovery from TBI is a slow and complex process, requiring alternative parts of the brain to take over the function of damaged regions, and new nerve pathways to be established by existing, undamaged brain cells. These pathways can be developed through rehabilitation activities that enable the brain to learn alternative ways of working, and which include retraining in daily living activities, as well as cognitive and behavioural therapies. In addition to studying the levels of various biomarkers, information regarding the effects of TBI on brain function and recovery can be aided by neuroimaging. This is an extremely powerful tool, although a more comprehensive understanding of an individual’s TBI can often be gained by using multiple datasets, especially since some forms of imaging are qualitative and others quantitative (32). For example, Susceptibility-Weighted Imaging (SWI) is significantly more sensitive to brain haemorrhages than Computerised Tomography (CT) scanning (33).
TBI is a multi-faceted condition, with research ongoing to establish methods of evaluation, as well as potential therapies to aid recovery.

PDF download Download the PDF version of the Traumatic Brain Injury White paper

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