Reference Work Entry

Encyclopedia of Neuroscience

pp 2862-2867

NF-κB – Potential Role in Adult Neural Stem Cells

  • Darius WideraAffiliated withDepartment of Cell Biology, Faculty of Biology, University of Bielefeld
  • , Christian KaltschmidtAffiliated withDepartment of Cell Biology, Faculty of Biology, University of Bielefeld
  • , Barbara KaltschmidtAffiliated withDepartment of Cell Biology, Faculty of Biology, University of Bielefeld

Synonyms

NF-κB: (NF-kappaB; NF-κB); DNA-binding subunits of NF- κB; p65 (RelA); p50 (NFKB1); p52 (NFKB2); c-Rel (REL); RelB (RELB); Inhibitory subunits of NF-κB: IκB-α (NFKBIA); IκB-β (NFKBIB); IκB-ε (NFKBIE); IκB-ζ (NFKBIZ)

Definition

Neural Stem Cells

Stem cells are defined as undifferentiated cells with the ability to (i) proliferate, (ii) exhibit self-maintenance, (iii) generate a large number of progeny, including the principal phenotypes of the tissue, (iv) retain their multilineage potential over time, and (v) generate new cells in response to injury or disease.

Neural stem cells can be found within their complex niche in the mammalian brain. Neural stem cells could be maintained in culture via propagation of floating cell clusters called “neurospheres.” Neurospheres contain committed progenitors, differentiated astrocytes, neurons and neural stem cells. A progenitor is defined as mitotic cell with a fast cell-devision cycle that retains the ability to proliferate and give rise to terminally differentiated cells but that is not capable of indefinite self-renewal.

Nuclear Factor-κB

Nuclear factor kappa B (NF-κB) is a transcription factor (TF) composed of homo- or heterodimeric DNA-binding subunits (e.g., p50 and p65). Inducible forms of NF-κB reside in the cytoplasm due to an interaction with one inhibitory subunit (e.g., IκB-α). Upon activation by growth factors, etc. (see Fig. 1), the IκB Kinase complex (IKK) catalyzes phosphorylation of the inhibitory subunit IκB, which leads to proteasomal degradation. This exposes the nuclear localization signals (see green dots in Fig. 1). Nuclear import of NF-κB via importins activates transcriptional target genes driving proliferation, such as Cyclin D1 (see Fig. 1). NF-κB is involved in many biological processes, such as inflammation and innate immunity, development, apoptosis and anti-apoptosis [1]. In the nervous system NF-κB plays a crucial role in neuronal plasticity, learning, neuroprotection and neurodegeneration. In addition, recent data suggest a crucial role of NF-κB on proliferation, migration and differentiation of neural stem cells.
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NF-κB – Potential Role in Adult Neural Stem Cells. Figure 1

Model for the involvement of NF-κB in the molecular machinery of the cell cycle essential for proliferation, differentiation and migration of neural stem cells. Activation of IKK complex and subsequent NF-κB activation via growth factors, chemokines or activating conditions leads to ubiquitination and proteasomal degradation of IκB. After nuclear translocation NF-κB binds to specific promoter regions of the target genes and activates their transcription. Depending on the cell cycle point these targets are genes regulating proliferation or migration and differentiation.

In the G1 phase, NF-κB activates cyclin D1 expression by direct binding to multiple sites in the cyclin D1 promotor. This promotes G1-to-S progression. In contrast, NF-κB action in the M-phase leads to differentiation or migration induction.

In this essay we suggest a model explaining the multiple action of NF-κB within neural stem cells.

CNS

The Central Nervous System (CNS, systema nervosum centrale), consisting of the brain and spinal cord is one of the two major parts of the nervous system. CNS integrates all nervous activities. The second part is the peripheral nervous system (PNS) which is outside the brain and spinal cord and regulates e.g., the heart muscle, the muscles in blood vessel walls or glands.

On the cellular level, CNS consists of a network of nerve cells, glial cells (e.g., astrocytes and microglia) and neural stem cells. Neurons, the primary cells of the CNS, are responsible for information processing and storage. Glial tissue surrounds and supports neurons and is important for response against infection and tissue repair (e.g., microglia). Novel data define astrocytes and radial glia as a potential stem cell pools.

Characteristics

Adult Neural Stem Cells

Until recently, the dogma existed that stem cells are not present in the adult CNS. Currently, there are many reports clearly demonstrating neurogenesis in different regions of the adult brain [2,3]. Stem cells within the adult brain were found within the subgranular zone of the hippocampus and in the subventricular zone (SVZ). The immunocytochemical markers expressed by NSCs include inter alia the intermediate filament Nestin, the transcription factors Sox1 and Sox2, the RNA binding protein Musashi and the transmembrane protein prominin-1(CD133) (see Table 1).
NF-κB – Potential Role in Adult Neural Stem Cells. Table 1

Examples for immunocytochemical markers for neural stem cells

Marker

Detected in species: m: mouse; r: rat; h: human

Expression in adult NSCs

Expression in fetal NSCs

 

Nestin

m/r/h

+

+

 

Sox1

m/r/h

+

+

 

Sox2

m/r/h

+

+

 

prominin-1(CD133)

m/h(CD133)

+

+

 

Musashi

m/r/h (MSI)

+

+

 

SSEA-1/LeX

m/r/h

+

+

 

L1

m/r/h

+

+

 

ABCG2 (Bcrp1)

r/h

+

+

 

PSA-NCAM

m/r/h

+

+

 

CD24

h

+

+

 

CD44

h

+

+

 

CD81

h

+

+

 

CD90

h

+

+

 

CD184

h

+

+

 

Dnmt3a

m

+

 

Vimentin

m/r/h

+

+

 

Isolated and cultured NSCs may have the ability to replace lost cells within the central nervous system, - an important issue for future therapy of neurodegenerative diseases as Parkinson’s and Alzheimer’s disease. Moreover NSCs also offer hope for fighting against cancer by the delivery of chemotherapy agents directly to tumor cells.

NF-κB in the Nervous System

In the nervous system, the most frequent form of NF-κB is a heterodimer composed of p50 and p65. Activating stimuli like Tumor Necrosis Factor (TNF) (see Fig. 2) or Erythropoietin (EPO) activate a kinase complex composed of two IκB-specific kinases (IKKα and IKKβ) and a modulatory subunit (IKKγ/NEMO). The IKK-α/β complex phosphorylates the inhibitory IκB, which is then ubiquitinilated and degraded via the proteasome. This degradation triggers the translocation of NF-κB into the nucleus followed by initiation of transcription. For a detailed discussion on the action of NF-κB in the CNS see also [4,5 and 6].
https://static-content.springer.com/image/prt%3A978-3-540-29678-2%2F14/MediaObjects/978-3-540-29678-2_14_Part_Fig2-3971_HTML.jpg
NF-κB – Potential Role in Adult Neural Stem Cells. Figure 2

TNF-induced nuclear localization of the transactivating NF-κB subunit p65 in neural stem cells. NSCs were fixed and stained with an antibody against the p65 subunit of NF-κB. Nuclei (DNA) were stained with SYTOX (green). The activation of the NF-κB pathway is shown as nuclear translocation of NF-κB visualised using an antibody against the p65 subunit. The nuclear translocation of NF-κB is followed by the transcription of target genes responsible for proliferation, migration and differentiation of neural stem cells.

Apart from the inducible NF-κB activity, there are reports on constitutively active NF-κB in several cell types, such as hippocampal neurons, or numerous brain-related cancer types such as gliobastomas.

NF-κB and Neural Stem Cell Proliferation

Most of the culture protocols for NSCs use bFGF (FGF-2) and EGF for keeping the cells in undifferentiated and proliferating state [23]. Over the years many additional molecules and cultivation conditions were identified to influence the NSC proliferation (see Table 2).
NF-κB – Potential Role in Adult Neural Stem Cells. Table 2

Examples for molecules and/or conditions inducing proliferation or migration of NSCs

Proliferation

 

 

Molecule or condition

Influence on proliferation + positive/- negative

 

EGF

+

 

bFGF

+

 

TNF

+

 

EPO

+

 

Hypoxia

+

 

L1

 

GM-CSF

+

 

ROS/Density

+

 

Neurofibromin

+

 

mAChR-stimulation

+

 

Cerebral infarction

+

 

Soluble amyloid precursor protein

+

 

Abeta

 

Sphingosine-1-phosphate

+

 

NO

 

Traumatic brain injury

+

 

Glutamate

+

 

Migration

 

MCP-1

 

SCF

 

SDF-1α

 

PDGF

 

Cerebral cortex injury

 

Microglia culture supernatants

 

ischemia stroke

 

Seizure

 

Here we summarize several evidences for a crucial involvement of NF-κB in proliferation control.

An enhanced proliferation of NSCs in vitro and in vivo after Erythropietin (EPO) treatment has been reported. Demonstrably, the authors provide evidence that EPO is a homeostatic autocrine-paracrine signaling molecule with actions mainly mediated by NF-κB [7].

EPO and EPO receptors are upregulated in the CNS after hypoxia. Similarly, hypoxia activates NF-κB in neonatal rat hippocampus and cortex. Under hypoxic conditions, hypoxia-inducible-factor1α (HIF-1α), an important transcription factor for regulation of the oxygen response, translocates into the nucleus and binds to promoter region of the epo gene leading to upregulated expression (for review see [8]). Induction of proliferation is also conceivable for culture density. The level of reactive oxygen species (ROS) is significantly elevated under low density conditions, leading to increased proliferation. Bonello et al. recently reported that ROS activates HIF-1α itself via a functional NF-κB binding site in pulmonary artery smooth muscle cells.

We and others demonstrated that TNF-α triggers the proliferation of NSCs. NF-κB has been identified as the main driving force of TNF-mediated proliferation [9].

In the nervous system glutamate is described as a potent activator of NF-κB. In respect of the influence on NSCs, also glutamate enhances survival and triggers the proliferation of SVZ derived NSC.

As another molecule which increases the proliferation of NSCs, Sphingosine-1-phosphate (S1P) was described. Studies investigating endothelial cells showed that S1P induces the activation of NF-κB-mediated transcriptional activity.

Amyloid beta-peptide (Aβ), a self aggregating peptide and responsible for Alzheimer’s disease, significantly decreases the proliferation of neural stem cells in vitro and in vivo. This result correlates with the fact that high amounts of Aβ acts as repressor of NF-κB. In contrast – the soluble secreted form of amyloid precursor protein, a well known NF-κB target gene, increases the proliferation of neural stem cells. In this context it is of importance that secreted beta-amyloid precursor protein counteracts the proapoptotic action of mutant presenilin-1 by activation of NF-κB.

Neurofibromin is able to increase the proliferation of NSCs. Neurofibromin, a product of the neurofibromatosis 1 (nf1) gene is one of the key regulators of the RAS oncogene. Noteworthy, expression of activated RAS stimulates NF-κB.

Cyclin dependent kinase 4 and 6 (CDK4/6) signaling is essential in cell cycle regulation in NSCs. In addition, the formation of the complex of CDKs 4 and 6 with Cyclin D1 is necessary for the cell cycle progression. Demonstrably, NF-κB controls growth and differentiation through transcriptional regulation of Cyclin D1 (see Fig. 1).

NO is a physiological inhibitor of neurogenesis. In addition, nitric oxide synthesis inhibition increases proliferation of neural precursors. It is noteworthy that NO is a well known repressor of NF-κB in neurons providing a link between NO dependent increase of progenitor proliferation and decreased NF-κB activity.

TGF-β1 is one of the well known inhibitors of NF-κB. According with our theory of proliferation control by NF-κB, TGF-β1 has been identified as a potent inhibitor of neurogenesis inducing a cell cycle arrest in the G0/G1 phase.

Taken together there are numerous evidences for a crucial role of NF-κB in control of neural stem cell proliferation.

NF-κB and Migration of Neural Stem Cells

In spite of many important proceedings on the field of neural stem cell biology, the factors that orchestrate homing of NSCs are largely unknown. There are only few reports identifying factors inducing migration of NSCs (see Table 2).

The expression of several chemokine receptors by NSCs such as CCR2, CXCR4 and c-kit (Stem Cell Factor Receptor) is well described.

MCP-1 is a very potent chemotactic factor for neural precursors [10]. Interestingly, MCP-1 expression can be strongly induced by TNF. In addition, the mcp-1 gene contains a functional NF-κB binding site in its promoter region which is necessary for response to TNF. In the hematopoietic system, binding of MCP-1 to its receptor strongly activates NF-κB, providing a further hint for NF-κB regulation of migration.

In another approach Sun et al. demonstrated potent induction of migration by neuronally expressed stem cell factor (c-kit Ligand, SCF) in vitro and in vivo. Analogous to mcp-1, also the scf gene contains a NF-κB binding site. Stromal derived factor 1α (SDF-1α), a well known ligand of the CXCR4, is described as a further chemokine, inducing migration of NSCs. Here, directed migration of neural stem cells induced by the SDF-1α secreted by astrocytes and endothelium was demonstrated. Furthermore, the over-expression of IκB results in loss of SDF-1α mediated migration of breast cancer cells in vivo.

All those results let us hypothesize that NF-κB is not only crucially involved in NSC proliferation, but also in control of migration.

NF-κB and Neural Stem Cell Differentiation

The IL-6 family of cytokines, including the leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) promote astrocytic differentiation by activating transcription factors, such as STAT 3, AP-1 and NF-κB. Both CNTF and LIF triggers the recruitment of glycoprotein 130 (gp 130) to their specific receptors leading to activation of the RAS-MAP kinase pathway. Downstream of RAS MAP kinases and PKC transduce the signals to their substrates activating nuclear transcription factors (NF-κB, AP-1 and NF-IL6). This cross-talk of those transcription factors and co-activators induce astrocytic fate specification in NSCs.

Recent reports demonstrated that NF-κB is required for neuronal differentiation of neuroblastoma cells. Cells induced to differentiate with retinoic acid, show nuclear NF-κB localization. In contrast, over-expression of NF-κB super-repressor suppressed neuronal differentiation.

NF-κB activity, induced via activation of the Rho family of small GTPases, regulates neurite outgrowth and dendritic spine formation in neuroblastoma cells.

Some studies demonstrated that bone morphogenetic proteins (BMPs) promote astroglial lineage commitment by mammalian subventricular zone progenitor cells. In contrast other approaches suggest that BMPs promote neuronal differentiation of NSCs in SVZ. These controversial findings can be explained by dose-dependent action and complex signaling via several cooperating transcription factors. Interestingly, it has been suggested, that NF-κB may positively regulate BMP-2 gene transcription and that overexpression of a NF-κB superrepressor may lead to changes in downstream signals including BMP-4.

All these results suggest a very complex control mechanism and clearly indicate an involvement of NF-κB in differentiation regulation. Further studies should investigate the involved mechanisms in detail.

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© Springer-Verlag GmbH Berlin Heidelberg 2009
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