Of these, the NLRP3 inflammasome can be acti- vated via bacterial RNA species [3].
In AD, microglial cells and astrocytes express NLRP3, which in turn can detect A beta plaques and act by secreting caspase-1 to activate IL-1 beta and IL- 18 [23–25].
It is noteworthy that IL-1 beta and IL-18 can activate various cell types, par- ticularly astrocytes and microglia to induce additional cytokine release involving IL-1 beta , IL-6, and IL-18, and also nitric oxide (NO) synthase that can stimulate production of free radical NO, leading to the forma- tion of peroxynitrite that denatures DNA and impairs cellular energy pathways [48, 49].
NO can also bring about apoptosis of hippocampal neurons via caspase- 3 activity [50] whereas astrocyte-secreted IL-1 beta can increase the production of APP and A beta from neu- rons [51–53] (Fig. 1).
Additionally, it can induce phosphorylation of the tau protein and promote for- mation of neurofibrillary tangles through the mitogen activated protein kinases-p38 (MAPK-p38) stress pathway [22, 54].
Pattern recognition receptors such as the TLR4 receptor are expressed in the brain’s own immune cells like microglia and astrocytes that induce inflammation via cytokine secretion [38].
This family serves several biological functions but if inappropriately secreted can lead to manifestation of depressive behaviors typ- ically associated with dementia onset and chronic neuroinflammation.
P2X7 expressed by microglial cells will also activate the NLP3 inflammasome [30, 32] and the expression of P2X7 is likely to be increased in AD brains [35].
The NLRP3 inflammasome has a role in AD by increas- ing caspase-1 expression levels in AD brains [13, 23].
Inflammasomes are responsible for the maturation of pro-inflammatory cytokines such as interleukin (IL)-1beta, IL-18, and IL-33 and activation of inflammatory cell death, pyroptosis.
They appear to be involved in several pathological processes activated by microbes including Alzheimer’s disease (AD).
Post activation of the inflammasome, caspase 1 enzyme initiates the maturation of pro-inflammatory cytokines particularly interleukin (IL)-1beta, IL-18, and IL-33 [4] (Fig. 1),and inflammation mediated cell death occurs via the nucleotide-binding domain and leucine-rich repeat(NLR) family of proteins [5].
Both IL-1 beta and IL-18 are generated in their mature secreted form by caspase-1 through activa- tion of the inflammasome.
In the NLRP3 inflammasome, the NLR pro- tein recruits the inflammasome-adaptor protein apoptosis-associated speck-like protein containing CARD (ASC), which in turn interacts with caspase- 1 leading to its activation [7].
Once activated, caspase-1 promotes the maturation of the proin- flammatory cytokines IL-1 beta , IL-18, and IL-33.
However, IL- 18 can be derived as a byproduct from the activities of various extracellular enzymes such as protease 3, serine protease, elastase and cathepsin G [62–65].
Pro-inflammatory IL-18 increases AD-associated A beta deposition in human neuron-like cells in culture [55]. IL-18 also increases the expression of glycogen synthase kinase 3 beta (GSK-3 beta ) and cyclin-dependent kinase 5, both of which are involved in hyperphos- phorylation of the tau protein [56].
Increased levels of IL-1 beta and IL-18 have been detected in serum, cerebrospinal fluid, and brains of patients with AD and in other forms of dementia [42–46].
Both these studies indicated an important role of IL-18 in AD.
P2X7 activation is followed by a number of downstream events, including release of pro-inflammatory mediators, cell death, and proliferation.
Longitudinal studies including that by Ide et al. [8] all agree that in AD, the presence of periodonti- tis appears to be associated with a marked increase in cognitive decline.
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If you find BEL Commons useful in your work, please consider citing: Hoyt, C. T., Domingo-Fernández, D., & Hofmann-Apitius, M. (2018). BEL Commons: an environment for exploration and analysis of networks encoded in Biological Expression Language. Database, 2018(3), 1–11.