**3.2 Amyloid β formation and amyloid plaques**

#### *3.2.1 Protein amyloid-β*

*Amyloid Diseases*

**3. Neuropathology of TBI: related proteins**

**3.1 Amyloid precursor protein (APP)**

TBI regulates the expression patterns of several proteins commonly associated with neurodegenerative diseases, such as α-synuclein, amyloid precursor protein, Aβ, TDP-43, and tau [37–40] (**Figure 1**). Besides, the ApoE4 gene and their cleaved products are implicated in neurodegenerative disorders, axonal pathology, and apoptosis following TBI [41, 42]. TBI also induces caspase-3, which is involved in APP processing, contributing to AD [43, 44]. This increase in APP expression and neuroinflammatory response following injury may contribute to a cycle of Aβ deposition and microglial activation that ultimately result in chronic neuropathology [45, 46]. In this section, I will summarize the principal proteins involved in TBI

APP and its proteolytic derivatives are important mediators of neuronal synaptogenesis and synapse maintenance [47]. APP functions in the axonal transport of vesicles and presenilin (PS) regulate intracellular protein trafficking, highlighting the role of APP as a synaptic vesicle protein [47]. TBI leads to overexpression

*Relationship between neuropathological proteins induced after brain injury and the onset of Alzheimer's disease. Schematic diagram showing that traumatic brain injury (TBI) and Alzheimer's disease (AD) share similar pathological pathways (such Aβ pathologies and TDP-43 proteinopathy) and neuroinflammatory responses that potentially could explain the vulnerability of TBI patients to the onset of dementia/AD.*

and AD and their associated factors in the neurodegenerative process.

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**Figure 1.**

Amyloid is a highly-ordered filamentous protein aggregate generally regarded as a misfolding event in which proteins that are soluble accumulate into fibrous structures [54]. However, determinants of amyloid formation and toxicity are largely unknown.

Edema, inflammatory response, vasculature changes, and deposition of Aβ have all been found to be localized pathological changes after TBI [55]. As such, an understanding of the mechanism promoting AD risk is important. Although TBI is typically believed to be a static pathological insult from a single event, new clinical unrecognized clinical symptoms can arise many years after the initial injury. In human studies, TBI has been shown to result in amyloid deposits reminiscent of AD pathology.

Aβ immunoreactivity and protein expression increase for as long as a year after injury, indicating that Aβ aggregation and plaques formation may continue long after APP gene expression returns to normal. Plaques found in TBI patients are strikingly similar to those observed in the early stages of AD [13, 14]. However, TBIassociated plaques can appear rapidly (within hours) after injury, whereas plaques in AD develop slowly and are found predominantly in the elderly [13].

Monomeric forms of Aβ can aggregate to form oligomers, protofibrils; these fibrils deposit as amyloid plaque (**Figure 1**), unaggregated oligomeric forms of Aβ may contribute to toxicity after TBI [56]. Aβ causes apoptotic cell death of neuronal cells in culture by the induction of caspases, known instigators of apoptotic cell death [57]. Accumulation of Aβ deposits, hippocampal damage, and chronic inflammation were found mainly in subcortical regions [18]. Early microglial accumulation in AD delays disease progression by promoting clearance of neurotoxic Aβ peptides before the formation of senile plaques. However, as AD mice age, microglia become dysfunctional, producing proinflammatory cytokines in response to Aβ aggregation downregulate genes involved in Aβ clearance [58].

#### *3.2.2 Mechanisms of post-traumatic amyloid-β formation*

The intracellular accumulation of Aβ, extracellular deposition of soluble Aβ plaques, and aggregation of tau protein have all been observed in patients, sometimes within hours after severe brain injury [59, 60]. Aβ accumulation and amyloid

#### **Figure 2.**

*Representative immunohistochemical images showing neurodegenerative markers in a mouse model of Alzheimer's disease after brain injury. (A) Aβ plaques detected using Aβ42 antibody in the cortex of AD model mice (3xTg-AD, 9 months old mice) (inset in A, high magnification in a). (B) Aβ plaques detected using Thioflavin-S staining in the cortex of old 3xTg-AD (B, high magnification in b). (C) Representative broken axons stained with APP showing axonal bulbs found acutely following TBI in wild-type mice. (D–F) Aβ42 diffuse plaques were identifying using an antibody specific for Aβ42 and were not detected by Thioflavin-S staining, in the CA1 hippocampal region (D), in the corpus callosum (E), and in the cortex (F) of wildtype mice. (G–H) Hippocampal neurons were stained using an antibody for phosphorylated tau in the CA1 hippocampus (G, high magnification in g) and cortical pyramidal layer (H, high magnification in h) after TBI in old 3xTg-AD mice. Scale bars: 200 μm (A), 50 μm (B–H), and 20 μm (a, b, f, g, and h).*

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*Neuropathology of Traumatic Brain Injury and Its Role in the Development of Alzheimer's Disease*

deposition precede the cognitive decline in Alzheimer's disease, with the pathology arriving later, and is associated with non-Alzheimer's disease dementia. Deposition of amyloid plaques from Aβ peptide in Alzheimer's disease or acute phase of TBI have previously been reported to involve either mononuclear phagocytes, endocytic uptake, or proteolytic processing of the APP during fibril formation [61, 62]. Levels of Aβ were found to be high days after TBI and then declined towards control levels in the subsequent 2 weeks. It has been suggested that a long-term process of Aβ metabolism is initiated by TBI, which can be cleaved to form Aβ. Both species of Aβ, Aβ40, and Aβ42, are increased in the first week after injury in the CSF of TBI patients; other studies have shown comparatively lower Aβ40 levels compared to high levels of Aβ42 [63]. Intracellular Aβ accumulation of non-plaque species of Aβ is more common than plaque deposition after TBI. Aβ is produced by sequential cleavage of the amyloid precursor protein APP via two enzymes, β- and γ-secretase. Depending on the cleavage point of γ-secretase, Aβ peptides of different amino acid length are produced. The two most closely linked to AD are Aβ40 and Aβ42 [64]. The accumulation of Aβ peptides is thought to be a major initiator event in AD pathogenesis (**Figure 1**). TBI leading to impaired axonal transport induces a long-term pathological co-accumulation of APP with β-site APP-cleavage enzyme 1 (BACE1), presenilin 1 and activated caspases, thus providing a possible mechanism for APP cleavage and production of Aβ within axons following TBI [65]. The release of Aβ (especially Aβ42) into tissue and plaque formation around damaged axons occurs after APP accumulation and Aβ production in damaged axons. Both presenilin-1 (PS1) and BACE were found in swollen axons in the swine model and in humans (**Table 2**). Targeting the APP secretase enzymes can prevent the increase in Aβ after TBI [19], specifically, Aβ42 was found to accumulate in the axonal bulbs of injured brains [22]. BACE1 and PS1 were increased in the damaged axons of TBI patients, and our previous studies have also shown that BACE1 and PS1 are consid-

*DOI: http://dx.doi.org/10.5772/intechopen.81945*

ered the promising targets for the treatment of TBI [19].

microglia, and reactive astrocytes [66, 67].

**3.3 Tau protein and neurofibrillary tangles**

Plaques have also been observed in pericontusional tissue surgically excised from survivors of TBI. Nevertheless, the key pathological similarity between TBI with AD is the observation that Aβ plaques are found in up to 30% of patients who die of acute TBI [14]. While TBI-associated plaques largely appear in the gray matter, they have also been identified in white matter. Amyloid plaques consist primarily of aggregated Aβ peptides, which are surrounded by dystrophic neurites,

The tau protein is associated with microtubules and plays a role in the outgrowth of neuronal processes and the development of neuronal polarity [68]. Misfolded and aggregated tau causes a gain of toxic function by hindering normal and axonal processes; axonal neurodegeneration due to the loss of tau is caused by a decrease in tau microtubule binding capabilities [69, 70]. Tau oligomerization is known as a critical mechanism in the development of NFTs, consisting of hyperphosphorylated tau proteins with pathological function [71]. AD is also characterized by intracellular hyperphosphorylated tau that constitutes the NFTs and senile plaques and is one of the most common tauopathies [72]. Furthermore, toxic tau proteins increase within hours after clinical brain injury [22], and their release and spreading effect may also contribute to the development of tauopathy following TBI [73]. It was described that the spatial pattern of the tau-immunoreactive pathology observed in chronic traumatic encephalopathy (CTE) is typical of the tauopathies [74]. The tau from both TBI and AD brains is phosphorylated at the same amino acids, resulting in the proteolytic cleavage of six isoforms known as

#### *Neuropathology of Traumatic Brain Injury and Its Role in the Development of Alzheimer's Disease DOI: http://dx.doi.org/10.5772/intechopen.81945*

deposition precede the cognitive decline in Alzheimer's disease, with the pathology arriving later, and is associated with non-Alzheimer's disease dementia. Deposition of amyloid plaques from Aβ peptide in Alzheimer's disease or acute phase of TBI have previously been reported to involve either mononuclear phagocytes, endocytic uptake, or proteolytic processing of the APP during fibril formation [61, 62]. Levels of Aβ were found to be high days after TBI and then declined towards control levels in the subsequent 2 weeks. It has been suggested that a long-term process of Aβ metabolism is initiated by TBI, which can be cleaved to form Aβ. Both species of Aβ, Aβ40, and Aβ42, are increased in the first week after injury in the CSF of TBI patients; other studies have shown comparatively lower Aβ40 levels compared to high levels of Aβ42 [63]. Intracellular Aβ accumulation of non-plaque species of Aβ is more common than plaque deposition after TBI. Aβ is produced by sequential cleavage of the amyloid precursor protein APP via two enzymes, β- and γ-secretase. Depending on the cleavage point of γ-secretase, Aβ peptides of different amino acid length are produced. The two most closely linked to AD are Aβ40 and Aβ42 [64]. The accumulation of Aβ peptides is thought to be a major initiator event in AD pathogenesis (**Figure 1**). TBI leading to impaired axonal transport induces a long-term pathological co-accumulation of APP with β-site APP-cleavage enzyme 1 (BACE1), presenilin 1 and activated caspases, thus providing a possible mechanism for APP cleavage and production of Aβ within axons following TBI [65]. The release of Aβ (especially Aβ42) into tissue and plaque formation around damaged axons occurs after APP accumulation and Aβ production in damaged axons. Both presenilin-1 (PS1) and BACE were found in swollen axons in the swine model and in humans (**Table 2**). Targeting the APP secretase enzymes can prevent the increase in Aβ after TBI [19], specifically, Aβ42 was found to accumulate in the axonal bulbs of injured brains [22]. BACE1 and PS1 were increased in the damaged axons of TBI patients, and our previous studies have also shown that BACE1 and PS1 are considered the promising targets for the treatment of TBI [19].

Plaques have also been observed in pericontusional tissue surgically excised from survivors of TBI. Nevertheless, the key pathological similarity between TBI with AD is the observation that Aβ plaques are found in up to 30% of patients who die of acute TBI [14]. While TBI-associated plaques largely appear in the gray matter, they have also been identified in white matter. Amyloid plaques consist primarily of aggregated Aβ peptides, which are surrounded by dystrophic neurites, microglia, and reactive astrocytes [66, 67].

### **3.3 Tau protein and neurofibrillary tangles**

The tau protein is associated with microtubules and plays a role in the outgrowth of neuronal processes and the development of neuronal polarity [68]. Misfolded and aggregated tau causes a gain of toxic function by hindering normal and axonal processes; axonal neurodegeneration due to the loss of tau is caused by a decrease in tau microtubule binding capabilities [69, 70]. Tau oligomerization is known as a critical mechanism in the development of NFTs, consisting of hyperphosphorylated tau proteins with pathological function [71]. AD is also characterized by intracellular hyperphosphorylated tau that constitutes the NFTs and senile plaques and is one of the most common tauopathies [72]. Furthermore, toxic tau proteins increase within hours after clinical brain injury [22], and their release and spreading effect may also contribute to the development of tauopathy following TBI [73]. It was described that the spatial pattern of the tau-immunoreactive pathology observed in chronic traumatic encephalopathy (CTE) is typical of the tauopathies [74]. The tau from both TBI and AD brains is phosphorylated at the same amino acids, resulting in the proteolytic cleavage of six isoforms known as

*Amyloid Diseases*

**142**

**Figure 2.**

*Representative immunohistochemical images showing neurodegenerative markers in a mouse model of Alzheimer's disease after brain injury. (A) Aβ plaques detected using Aβ42 antibody in the cortex of AD model mice (3xTg-AD, 9 months old mice) (inset in A, high magnification in a). (B) Aβ plaques detected using Thioflavin-S staining in the cortex of old 3xTg-AD (B, high magnification in b). (C) Representative broken axons stained with APP showing axonal bulbs found acutely following TBI in wild-type mice. (D–F) Aβ42 diffuse plaques were identifying using an antibody specific for Aβ42 and were not detected by Thioflavin-S staining, in the CA1 hippocampal region (D), in the corpus callosum (E), and in the cortex (F) of wildtype mice. (G–H) Hippocampal neurons were stained using an antibody for phosphorylated tau in the CA1 hippocampus (G, high magnification in g) and cortical pyramidal layer (H, high magnification in h) after* 

*TBI in old 3xTg-AD mice. Scale bars: 200 μm (A), 50 μm (B–H), and 20 μm (a, b, f, g, and h).*

cleaved tau (c-tau), including the AT8 epitope [75]. Hyperphosphorylated Tau has been shown to increase between 1 and 7 days after moderate TBI in triple transgenic AD mice [76] and at 3 weeks after repetitive mild TBI in the human Tau (hTau) tauopathy mouse model [77] (**Table 2**). Experimental studies in animal models suggest that intra-axonal tau accumulation and tau phosphorylation may be in fact the consequences of repeated brain trauma or dementia pugilistica/CTE [78]. Today, CTE is used to define the neurological sequelae and neuropathological changes that occur as a result of repeat concussive or subconcussive blows to the head. Besides, the pathology of CTE is also characterized as a tauopathy, a class of neurodegenerative disease caused by the pathological aggregation of tau protein [78]. In CTE, NFTs also consist of hyperphosphorylated and ubiquitinated tau [79, 80]. Tau degradation in boxers with CTE are structurally and chemically similar to those seen in AD and frontotemporal lobar degeneration (FTLD) [80]. Treatment with γ-secretase inhibitors diminishes amyloid pathology but does not affect TBI-induced tangle formation, suggesting that TBI-induced tau pathology is not a downstream event of Aβ and plaque formation [81].
