Progressive neuropathological changes following traumatic brain injury

Whilst the acute effects of traumatic brain injury (TBI) may be devastating, growing evidence suggests TBI may initiate long-term neurodegenerative processes. Indeed, an epidemiological association between TBI and Alzheimer’s disease (AD) has been demonstrated. Here the link between TBI and chronic neurodegeneration is explored by examining associated pathologies in material from patients surviving TBI for varying intervals. Furthermore, the Aβ clearing enzyme, neprilysin, has emerged as important with regards to Aβ dynamics in AD and thus may be relevant to Aβ-plaque formation in TBI. Its influence on Aβ dynamics in TBI was explored in an animal model of TBI and in human TBI material. First, a potential association between acute-Aβ plaques post-TBI and a GT repeat polymorphism of the Aβ-degrading enzyme, neprilysin was investigated using DNA fragment sizing analysis (n=81). Results show there was an increased risk of Aβ plaques for patients with greater than 41 total repeats (p<0.0001, OR:10.1). In addition, the presence of 22 repeats in at least one allele was independently associated with plaque deposition (p=0.03, OR: 5.2). In contrast, the presence of 20 GT repeats in one allele was independently associated with a reduced incidence of Aβ plaques (p=0.003). These data suggest a genetically linked mechanism that determines which TBI patients will rapidly form Aβ plaques. The role of neprilysin in TBI was further examined using immunohistochemistry (IHC) to evaluate plaque formation in neprilysin knock-out mice at 2h, 48h and 4mo following a single controlled cortical impact in mice. Young (2mo) and older (8mo) mice were examined at each time point and compared to age-matched injured and uninjured wild-type mice (n=4 for all groups). No plaques were observed at any time point or in age group indicating that neprilysin deficiency alone is insufficient to recapitulate the plaque pathology observed in humans using this model. Next, post-mortem brains from long-term survivors of just a single TBI (1 to 47 years survival; n=39) versus uninjured, age-matched controls (n=47) were examined for hallmark AD pathologies, neurofibrillary tangles (NFTs) and Aβ plaques using IHC and thioflavin-S staining. NFTs were exceptionally rare in young, uninjured controls, yet were abundant and widely distributed in approximately one third of TBI cases. Moreover, thioflavin-S staining revealed that while plaque-positive control cases displayed predominantly diffuse plaques, 64% of plaque-positive TBI cases, displayed predominantly thioflavin-S positive plaques or a mixed positive / diffuse pattern. These data indicate that TBI may induce or accelerate the presence of neurodegeneration-associated pathologies many years following just a single injury. To explore potential mechanisms driving plaque formation post-TBI, IHC was used to examine amyloid-β’s precursor, APP in TBI cases with acute survival (10h to 1 week; n=20), moderate survival (1 week to 1 year; n=17) and long-term survival (>1 year; n=33), versus uninjured controls (n=47). Following TBI, increased somatic APP immunoreactivity (IR) was observed in all TBI cases (n=70; 100%) versus (n=47; 70%) of controls (Chi Sq; p=0.00001). Moreover, increased APP IR was present in all survival intervals examined, (10h to 10 days, p=0.001; 10 days to 1 year, p=0.0001; > 1 year; p=0.001; all Chi Sq) versus controls, and an increased extent / distribution of pathology was demonstrated in the TBI group regardless of survival time. Interestingly, APP IR was not associated with the presence of plaques suggesting increased APP alone is insufficient to drive post-traumatic plaque formation. The Tar-DNA binding protein, TDP-43 has emerged as an important pathological feature in various chronic neurodegenerative diseases including frontotemporal lobe dementia, and more recently in dementia pugilistica caused by repetitive TBI. Here we examined survivors of a single TBI using IHC specific for TDP-43. TBI cases were acute (n=23: Survival