Role of neuroinflammation in an amyloid-beta model of Alzheimer’s disease and the identification of possible biomarker.

Abstract

Introduction. Alzheimer's disease (AD) is the most common form of neurodegenerative disorder that results in dementia. It currently affects 75 million people worldwide and is predicted to affect as many as 135 million people by 2050. Despite considerable research, current medication provides only modest relief to symptoms and does not cure the underlying disease. The delay in identifying a definitive cure is probably due to the scant knowledge of the cellular and molecular mechanisms implicated in its pathogenesis. However, the role of neuroinflammation has been acknowledged. Neuroinflammation is generally due to sustained activation of the brain's resident immune cells, including microglia and astrocytes. Although the importance of amyloid-beta (Aβ) in the aetiology of AD has recently come into question, there is still consensus that Aβ is closely related to AD. Therefore, a reappraisal of the amyloid hypothesis focusing on the role of neuroinflammation will open new avenues for identifying novel targets for AD treatment. In this study, we sought to assess neuroinflammation and identify possible biomarkers in an Aβ(1-42) rat model of AD over a progressive period of time. Materials and Methods. Male Sprague-Dawley rats were used in all experiments. The animals were randomly divided into a vehicle group of rats that were infused with phosphate-buffered saline (PBS) and an Aβ(1–42) group that was lesioned with the Aβ(1–42) peptide. Each group was further sub-divided into four groups (Day 3 group: animals euthanised 3 days after infusion; Day 7 group: animals euthanised 7 days after infusion; Day 10 group: animals euthanised 10 days after infusion, and Day 14 group: animals euthanised 14 days after infusion). Animals were subjected to neurobehavioral tests pre and postinfusion.The Morris water maze test was used to assess spatial learning and memory and the fear conditioning test was used to assess associative fear learning and memory. After euthanisation, whole blood sample acquired aseptically from both the vehicle and Aβ(1–42) lesioned group of rats was collected into ethylenediaminetetraacetic acid (EDTA) coated tubes for cytokine, oxidative stress markers and microRNA assays using multiplex immunoassay, spectrophotometric and real-time polymerase chain reaction analysis respectively. The excised whole brain was post-fixed in 10% neutral buffered formalin (NBF) for immunofluorescence and immunohistochemical analysis. Other brain tissue was placed in frozen 0.9% saline slush before the hippocampus was carefully dissected out and placed in a bio-freezer at -80 ºC post dissection. The tissue was later used for messenger RNA analysis using the real-time polymerase chain reaction technique. Results. We observed impaired spatial and reduced contextual fear memory, which was exacerbated as the postlesion days increased. Our results also showed increased expression of ionized calcium-binding adaptor molecule 1 (IBA-1), glial fibrillary acidic protein (GFAP), and beta-site amyloid precursor protein cleaving enzyme1(BACE1) antibodies and upregulated mRNA expression levels of cluster of differentiation 33 (CD33) and triggering receptor expressed on myeloid cells 2 (TREM2) genes in the hippocampus, as well as downregulated expression of miRNA107 in the plasma. In addition, our results showed a positive relationship between the activated glial cell markers and lipid peroxidation. Furthermore, elevated plasma concentration of pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α) with a concomitantly lowered levels of the anti-inflammatory cytokine (IL-10) in the Aβ(1–42) lesioned rats was observed when compared to the vehicle groups. A negative correlation between the decline in spatial memory and plasma levels of the pro-inflammatory cytokines and a positive correlation between the decline in spatial memory and plasma concentration of the anti-inflammatory cytokine was observed. Conclusion. Our findings implicate cellular and molecular mechanisms, as shown by prolonged and progressive activation of the glial cells, resulting in a bidirectional interplay between neuroinflammation and oxidative stress. These interconnections result in the concomitant release of brain cytokines as a secondary response to the hallmarks of AD, which impacts both neural circuit activity and expression of microglial genes regulating neuroinflammation, indicating dynamic crosstalk between the immune and nervous systems. These interactions facilitate the understanding of AD's pathogenesis and provide the basis for an integrative approach to validate the role of neuroinflammation in memory processes and, importantly, identify a potential biomarker for the early diagnosis of Alzheimer's diseases. As a contribution to knowledge, this study unveils the connection between memory decline and plasma cytokine concentration, as well as the relationship between genes regulating neuroinflammation in AD. Therefore, it is incontrovertible that neuroinflammation holds a pivotal role in AD pathology.