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Neuroinflammation leads to N/A, Neurodegeneration
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding||Point of Contact||Author Status||OECD Status|
|Binding of agonists to ionotropic glutamate receptors in adult brain causes excitotoxicity that mediates neuronal cell death, contributing to learning and memory impairment.||adjacent||Moderate||Allie Always (send email)||Open for citation & comment||WPHA/WNT Endorsed|
|Chronic binding of antagonist to N-methyl-D-aspartate receptors (NMDARs) during brain development leads to neurodegeneration with impairment in learning and memory in aging||adjacent||Low||Arthur Author (send email)||Open for citation & comment||WPHA/WNT Endorsed|
|Binding of Sars-CoV-2 spike protein to ACE 2 receptors expressed on brain cells (neuronal and non-neuronal) leads to neuroinflammation resulting in encephalitis||adjacent||High||Not Specified||Agnes Aggy (send email)||Under development: Not open for comment. Do not cite||Under Development|
Life Stage Applicability
|During brain development, adulthood and aging||High|
Key Event Relationship Description
It is well accepted that chronic neuroinflammation is involved in the pathogenesis of neurodegenerative diseases (McNaull et al., 2010; Tansey and Goldberg, 2009; Thundyil and Lim, 2015 ). Chronic neuroinflammation can cause secondary damage (Kraft and Harry, 2011). The mechanisms by which neuroinflammation (i.e. activated microglia and astrocytes) can kill neurons and induce/exacerbate the neurodegenerative process has been suggested to include the release of nitric oxide that causes inhibition of neuronal respiration, ROS and RNS production, and rapid glutamate release resulting in excitotoxic death of neurons (Brown & Bal-Price, 2003; Kraft & Harry, 2011; Taetzsch & Block, 2013). Glial reactivity is also associated with excessive production and release of pro-inflammatory cytokines that not only affect neurons, but also have detrimental feedback effects on microglia (Heneka et al., 2014). For example, sustained exposure to bacterial lipopolysaccharide (LPS) or to other pro-inflammatory mediators was shown to restrict microglial phagocytosis of misfiled and aggregated proteins (Sheng et al., 2003). Systemic immune challenge during pregnancy leading to microglial activation caused increased deposition of amyloid plaques and tau hyperphosphorylation in aged mice (Krstic et al., 2012, 2013), suggesting that neuroinflammation is involved in the amyloid plaques and neurofibrillary tangles formation. There is further evidence that the formation of neurofibrillary tangles is caused by microglial cell-driven neuroinflammation, since LPS-induced systemic inflammation increased tau pathology (Kitazawa et al., 2005).
Sars-CoV-2 specific evidence:
Studies on post-mortem cases indicate that lymphocytes and monocytes infiltrate in brain vessel walls, exacerbating the neuronal degeneration and demyelination process (Wu et al., 2020)
The aberrant immune response characterized by a surge in cytokine levels (e.g., IL-6) derived by SARS-CoV-2 accelerates the process of neurodegeneration that may contribute to the development of neurodegenerative diseases (Debnath et al. 2020).
SARS-CoV-2 can infect human brain organoids resulting in unique metabolic changes and the death of infected and neighbouring neurons. This phenotype is accompanied by impaired synaptogenesis (Song et al., 2020) (Mesci et al, 2020).
Moreover, it is hypothesized that an autoimmune reaction mediated by the cross-reaction between viral particles and myelin basic protein may provide the driving force for neural demyelination, as part of the neurodegenerative process. This hypothesis is supported by the fact that the genome of other coronaviruses like CoV-OC43 and CoV-229E, as well as their antibodies, has been isolated from the CNS of Multiple Sclerosis (MS) patients, and coronavirus-like particles have been found in perivascular cuffs of human MS brain (Montalvan et al. 2020). In fact, the virus might lie dormant in astrocytes and oligodendrocytes and trigger the autoimmunity mediated by molecular mimicry (Mohammadi et al., 2020).
Neurons are the target cells undergoing degeneration during infection, in part due to apoptosis (de Assis et al. 2020). Intracerebral inoculation with CoV-OC43 in susceptible mice led to an acute encephalitis, with neuronal cell death by necrosis and apoptosis (Jacomy H, et al. 2006).
SARS-CoV infection causes neuronal death (even in the absence of encephalitis) in mice transgenic for human ACE2. Death of the animal likely results from dysfunction and/or death of infected neurons, especially those located in cardiorespiratory centres in the medulla. The absence of the host cell receptor prevents severe murine brain disease (Netland J, et al. 2008).
Evidence Collection Strategy
Evidence Supporting this KER
Neuroinflammation is a component of neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease (Neumann, 2001) which may play a secondary or an active primary role in the disease process (Hirsch and Hunot, 2009). McNaull and coworkers (McNaull et al., 2010) suggested that early developmental onset of brain inflammation could be linked with late onset of Alzheimer’s disease. A recent paper by Krstic and coworkers (2012) showed that a systemic immune challenge during late gestation predispose mice to develop Alzheimer’s like pathology when aging, suggesting a causal link between systemic inflammation, neuroinflammation, and the onset of Alzheimer’s disease. Regarding toxicant-induced neuroinflammation, microglial/astrocyte activation and chronic neuron damage may continue for years after initial exposure (Taetsch and Block, 2013), suggesting that chronical neuroinflammation and neurodegeneration have a slow long-term temporal evolution. Ongoing neuroinflammation can be visualized in patients using the positron emission tomography (PET) ligand [11C] (R)-PK11195 (Cagnin et al., 2001). Recent genome-wide association study (GWAS) analyses of sporadic Alzheimer's disease revealed a set of genes that point to a pathogenic role of neuroinflammation in Alzheimer's disease (for review, see Heneka et al., 2014). High levels of pro-inflammatory cytokines produced by activated microglia and astrocytes are detected in the brain of Alzheimer's subjects and animal models (McGeer and McGeer, 1998; Janelsins et al., 2005).
Uncertainties and Inconsistencies
Long-term treatments with NSAIDs (non-steroidal anti-inflammatory drugs) have a preventive effect on Alzheimer's disease development (Piertrzick and Behl, 2005; Wang et al., 2015), but such treatment has no effect or is even detrimental if administered once the disease is at an advanced stage (Lichtenstein et al., 2010), This may be due to the dual protective/destructive effects of neuroinflammation and to its complexity.
Serum Pb level negatively correlates with verbal memory score, but not with abnormal cognition in Alzheimer's disease (Park et al., 2014). Epidemiologic studies are not well-suited to accomodate the long latency period between exposures during early life and late onset of Alzheimer's disease, even if bone Pb content is an accurate measurement of historical Pb exposure in adult (Bakulski et al., 2012).
Besides neuroinflammation or effects associated with neuroinflammation, other mechanisms may be involved in neurodegeneration with Abeta and tau accumulation: Pb-induced epigenetic modifications of genes involved in the amyloid cascade or tau expression may contribute to the accumulation of Abeta and tau accumulation following developmental exposure to Pb (Zawia and Basha, 2005; Basha and Reddy, 2010). Also oxidative damage to DNA was shown to be involved in delayed effects observed in old rats (PD 600), if exposed early postanatally (PD 1 to 20) (Bolin et al., 2006)
Gap of knowledge: there are no studies showing that glufosinate-induced neuroinflammation leads to neurodegeneration.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
The hypotheisis of developmental origin of Pb-induced neurodegeneration was tested and observed in Zebra fish by Lee and Freeman (2014).
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