Amyloid-β induced apoptosis has also been ascribed to dyshomeostasis of intracellular Ca2+ and oxidative stress [106-108], two critical biochemical derangements known to activate NF-κB
In primary neuronal cultures, Amyloid-β has been shown to elicit oxidative stress and evoke NF-κB activation
Amyloid-β has also been demonstrated to induce apoptosis via the JNK1/c-Jun/Fas ligand signaling cascade [110], which results in NF-κB activation
Recent evidence has cogently shown that Amyloid-β induces apoptosis in rat primary neurons and human post-mitotic neuronal cells by reducing Bcl-XL expression level and evoking the release of cytochrome c from the mitochondria in a NF-κB – dependent manner
Furthermore, fibrillar Amyloid-β has been shown to activate NF-κB via the assembly pf the C5b-MAC complex
Furthermore, Amyloid-β actuates NF-κB – dependent pro-inflammatory pathways in microglia culminating in TNFα expression and subsequently TNFα effectuated neurotoxicity
Furthermore, Amyloid-β –induced NF-κB also results in the up-regulation of the antioxidant mitochondrial membrane enzyme – MnSOD (superoxide dismutase 2) [328] which is well known to combat oxidative stress and apoptosis
Indeed, Checler and colleagues have shown in a recent study that, NF-κB mediates the Amyloid-β – induced increase in expression of AβPP in HEK293 cells
There is evidence that Amyloid-β causes the activation of Ca2+/calmodulin/CamKII pathway [332-334], thereby potentially leading to NF-κB activation.
Moreover, there is preponderance of data implicating Amyloid-β in the modulation of PKC signaling pathway [335-338] and the PI3K/Akt/mTOR signaling pathway [339-341], which are known to activate NF-κB signaling pathway
Furthermore, negative regulators of NF-κB such as the NAD+- dependent histone deacetylase – SIRT1, abolish the deleterious neurotoxic effects of Amyloid-β
Emerging evidence has implicated Amyloid-β in augmenting cytosolic Ca2+ levels and causing NF- κB activation via calcineurin in astrocytes
Amyloid-β also induces microglial activation that results in NF-κB – induced expression of pro-inflammatory cytokines such as TNFα, IL1β, IL6, and IL8 from the microglia resulting in neuronal death
Additionally, NF-κB activity is also actuated by glutamate- mediated excitatory neurotransmission in the hippocampus, cerebral cortex, and the cerebellar granule cells
Furthermore, nitric oxide (NO), a well characterized repressor of NF-κB activation and signaling [174, 175], causes a mitigation in neurogenesis
Physiological, pathophysiological, and biochemical stimuli known to induce proliferation of NPC via NF-κB activation include cerebral infarction [165], traumatic brain injury [166], reactive oxygen species [167], hypoxia [168-172], sAPPα [147], and sphingosine-1-phosphate [173]
Overexpression of the p65 subunit of NF-κB confers resistance to etoposide-induced apoptosis in cortical neurons
NF-κB activation also protects hippocampal neurons from oxidative stress-induced apoptosis by inducing manganese superoxide dismutase (MnSOD) expression and mitigating peroxynitrite-induced protein nitration
Consequently, NF-κB is constitutively activated in the excitatory neurons of the cerebral cortex (layers 2, 4, and 5), hippocampus (granule and pyramidal neurons of CA1 and CA3), and cerebellar granule cells and this constitutive activity is indispensable for neuronal survival in response to glutamate-induced excitotoxicity
The molecular mechanism underlying constitutive activation of NF-κB by glutamate-induced excitatory synaptic neurotransmission has been ascribed to NMDA receptor mediated Ca2+ influx and subsequent activation of CaMKII
Other species of NF-κB have not been found at the synapses suggesting that the synapses contain the p65-p50 heterodimer exclusively
Moreover, NF-κB mediates the NSC migration in response to physiological and pathophysiological stimuli such as cerebral cortex injury [181], seizure [182], and ischemic stroke
NF-κB positively regulates the transcription of interleukin 6 (IL6) family of cytokines such as IL6, leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) [184], which in-turn induce the differentiation of NSC into astrocytes by activating transcription factors STAT3, AP-1, and NF-κB itself
NF-κB also plays a vital role in axon guidance subsequent to neurogenesis and axon growth in response to neurotrophins, resulting in the integration of the nascent neurons
In addition to CamKII, other kinases activated in response to a rise in cytosolic Ca2+ such as protein kinase C (PKC), phosphatidylinositol-3-kinase (PI3K), and Akt, also activate NF-κB signaling pathway by increasing the phosphorylation and activation of IKK
The expression of p65 and p50 is induced in response to long-term changes in synaptic strength and transmission such as LTP [39] and LTD
Moreover, NF-κB – induced IL1 genesis has been shown to precipitate tau phosphorylation at Ser202 and Thr205 (AT8 epitopes) via the activation of the p38- MAPK pathway
The three well characterized sensors of intracellular calcium – calmodulin/CamKII pathway, PI3K/ Akt pathway, and protein kinase C (PKC) pathway – are known to induce NF-κB activation and couple upstream signal transduction pathways that induce calcium dyshomeostasis to NF-κB activation
This paradoxical incongruity in the effects of NF-κB emanate from the prevailing phosphorylation status of the p65 subunit (Ser536) and coinciding physiological and biochemical stimuli, with the phosphorylation status of p65 being inversely related to the NF-κB – induced neurite outgrowth
NF-κB acts as a sensor of oxidative stress and it is well established that oxidative stress results in the activation of NF-κB
The indispensable role of NF-κB in synaptic transmission is corroborated by the fact that the mice that are deficient in neuronal p65, exhibit severe deficits in hippocampal basal synaptic transmission and long term potentiation (LTP)
Given the indispensable role of NF-κB proteins in synaptic transmission and synaptic plasticity, it is not surprising that NF-κB is plays an indispensable role in cognition and behavior as well
Preponderance of evidence has implicated NF-κB as a survival factor in neurons [24, 114, 115] primarily because of the ability of NF-κB to suppress apoptosis in response to excitotoxic and apoptotic stimuli
Inhibition of NF-κB activity using a κB decoy DNA that results in the sequestration of NF-κB, increases the susceptibility of hippocampal neurons to noxious and apoptotic stimuli
NF-κB is also indispensable for longterm spatial memory as assessed by radial arm maze [25] and Morris water maze paradigms in mice
The molecular mechanisms underlying the aforementioned involvement of NF-κB proteins in learning and memory have not been completely comprehended, although recent evidence has implicated NF-κB – induced transcriptional activation of Protein Kinase A (PKA) catalytic subunit that culminates in activation of CREB (cyclic AMP-response element binding protein) signaling [77] which is regarded as the molecular switch that converts short-term memory to long-term memory
This is counter-intuitive as it is established that NF-κB positively regulates the expression of Bcl-XL and other members of the Bcl2 family
The anti-apoptotic effects of NF-κB have been ascribed to its ability to trans-activate the expression of a multitude of anti-apoptotic genes such as Bcl-2, Bcl-XL, and Bfl-1/A1 in the neurons of the amygdala, olfactory bulb, and the CA1/CA3 region of the hippocampus
NF-κB activity abrogates Amyloid-β peptide-induced toxicity in dissociated hippocampal cultures from C57Bl/6 mice
The trophic factors that evoke proliferation of NPC by inducing activation of NF-κB include epidermal growth factor (EGF) [155-161], basic fibroblast growth factor (bFGF) [155-161], vascular endothelial growth factor (VEGF) [162], tumor necrosis factor α (TNFα) [163], and erythropoietin (EPO)
NF-κB also plays a pivotal role in the differentiation of neural stem cells
IL6 is another NF-κB – induced [256-260] pro-inflammatory cytokine up-regulated and integrally involved in the etio-pathogenesis of Alzheimer’s disease
Furthermore, NF-κB - induced IL6 has been demonstrated to evoke hyperphosphorylation of tau at Ser202 and Thr205 via the activation of the cdk5/p35 complex
NF-κB is also indispensable in the neuronal differentiation of neuroblastoma cells [186] as its transcriptional activity directly orchestrates dendritic spine formation and neurite outgrowth
Multiple studies have implicated NF-κB in the regulation of post-natal axonal growth or neurite outgrowth.
Both, brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor (CNTF) promote neurite outgrowth, in a NF- κB - dependent manner, in primary cultures of nodose ganglion sensory neurons from developing mice
NF-κB positively regulates the expression of extracellular matrix protein involved in cell adhesion, β1-integrin [199], a well characterized pivotal player in axon growth initiation and guidance
Furthermore, NF-κB directly regulates the expression of a plethora of cell adhesion molecules such as neural cell adhesion molecule (NCAM) [202], slit and Trk-like family member 1 (SLITRK1) [203], glial cell-derived neurotrophic factor receptor α1 (GFRα1) [204], and T-lymphoma and metastasis 1 (TLAM1) [203], key players in directing axon growth.
NF-κB is integrally involved in the orchestration of growth of dendritic arbors and critically regulates dendritic morphology because of its indispensable role in dendritic growth and branching
NF-κB activation directly induces the transcription of Hes1 and Hes5 [206, 207], two key transcription factors involved in the regulation of expression of proteins involved in dendritic growth, morphology, and branching
NF-κB activates the expression of the two microtubule-associated proteins, microtubule-associated protein 1B (MAP1B) and microtubule-associated protein 2 (MAP2) [209], two major proteins known to play a pivotal role in the growth, elongation, and arborization of dendrites
NF-κB directly regulates the transcription of a multitude of proteins of the complement pathway that are involved in antigen presenting such as C3 (complement component 3) [241], Bf (complement factor B) [242], and CR2 (complement receptor 2) [243] as well as acute phase proteins such as C4 (complement factor 4) [244] and C4BPA (complement factor 4 binding protein)
It is well established that NF-κB is one of the most prominent transcription factors that regulates IL1β production
Another known target of NF-κB, TNFα [266, 267] is also up-regulated in the cortex [268], cerebrospinal fluid, and the serum of Alzheimer’s disease patients
Multiple studies have cogently shown that the inhibition of NF-κB activity results in the mitigation of secreted Amyloid-β by cultured cells in vitro
NF-κB is also widely implicated in the engenderment of Amyloid-β in vivo in a multitude of mouse models
A multitude of studies have demonstrated that NF-κB directly regulates the transcription and expression of BACE1, thereby eliciting profound effects on AβPP processing and engenderment of Amyloid-β
NF-κB induced up-regulation in BACE1 expression is contingent on the nature of the NF-κB heterodimer
Recent studies have unveiled a novel role of NF-κB in the regulation of the γ-secretase activity mediated processing of the C99 (CTFβ) fragment
In addition to augmenting γ-secretase activity, NF-κB also regulates the expression of the PS1 subunit of the γ-secretase complex
Furthermore, two κB-binding sites have been identified in the proximal promoter region of AβPP, suggesting the potential regulation of AβPP expression by NF-κB
The binding of p65/p50 or p65/p52 NF- κB heterodimer in the BACE1 promoter results in transcriptional activation whereas the binding of c-Rel/p52 heterodimer results in repression of BACE1 transcription
p50 knock-out mice exhibit deficits in late LTP resulting in compromised spatial memory
p50 knock-out mice exhibit severe deficits in learning as assessed by an active avoidance assay [93] in addition to displaying lack of anxiety-like behavior in well-established tests and paradigms that assess exploratory drive as a measure of anxiety
c-Rel null mice exhibit deficits in hippocampal long term depression (LTD), despite the lack of presence of c-Rel at the synapses in wild-type mic
Multiple studies have demonstrated the presence of NF-κB in the synapses and synaptosomal preparations [25, 35, 39] and the presynaptic protein synaptophysin has been shown to co-localize with the p65 and p50 subunits of NF- κB in synaptosomal preparations [25, 35, 39], also been observed in the dendritic spines of the post-synaptic densities of the synaptosomes isolated from the hippocampus and the cerebral cortex
NF-κB induction and activation mediates the neuroprotection bestowed by pre-conditioning with a sub-optimal dose of kainic acid and linolenic acid against severe ischemia
The mechanism underlying the effect of BDNF and CNTF on NF-κB activation has been attributed to the activation of Src and Lck non-receptor tyrosine kinases which phosphorylate IκBα on Tyr42 resulting in subsequent NF-κB activation
BEL Commons is developed and maintained in an academic capacity by Charles Tapley Hoyt and Daniel Domingo-Fernández at the Fraunhofer SCAI Department of Bioinformatics with support from the IMI project, AETIONOMY. It is built on top of PyBEL, an open source project. Please feel free to contact us here to give us feedback or report any issues. Also, see our Publishing Notes and Data Protection information.
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.