cells Review The Transcription Factor NF-κB in Stem Cells and Development Christian Kaltschmidt 1, Johannes F. W. Greiner 1 and Barbara Kaltschmidt 1,2,* 1 Department of Cell Biology, Bielefeld University, Universitätsstrasse 25, 33615 Bielefeld, Germany; [email protected] (C.K.); [email protected] (J.F.W.G.) 2 Molecular Neurobiology, Bielefeld University, Universitätsstrasse 25, 33615 Bielefeld, Germany * Correspondence: [email protected]; Tel.: +49-521-106-5624 Abstract: NF-κB (nuclear factor kappa B) belongs to a family of transcription factors known to regulate a broad range of processes such as immune cell function, proliferation and cancer, neuropro- tection, and long-term memory. Upcoming fields of NF-κB research include its role in stem cells and developmental processes. In the present review, we discuss one role of NF-κB in development in Drosophila, Xenopus, mice, and humans in accordance with the concept of evo-devo (evolutionary developmental biology). REL domain-containing proteins of the NF-κB family are evolutionarily conserved among these species. In addition, we summarize cellular phenotypes such as defective B- and T-cell compartments related to genetic NF-κB defects detected among different species. While NF-κB proteins are present in nearly all differentiated cell types, mouse and human embryonic stem cells do not contain NF-κB proteins, potentially due to miRNA-dependent inhibition. However, the mesodermal and neuroectodermal differentiation of mouse and human embryonic stem cells is ham- pered upon the repression of NF-κB. We further discuss NF-κB as a crucial regulator of differentiation in adult stem cells such as neural crest-derived and mesenchymal stem cells. In particular, c-REL seems to be important for neuronal differentiation and the neuroprotection of human adult stem cells, while RELA plays a crucial role in osteogenic and mesodermal differentiation. Citation: Kaltschmidt, C.; Greiner, J.F.W.; Kaltschmidt, B. The κ Transcription Factor NF-κB in Stem Keywords: NF- B; REL; development; adult stem cells; embryonic stem cells; differentiation Cells and Development. Cells 2021, 10, 2042. https://doi.org/10.3390/ cells10082042 1. Introduction to Canonical and Non-Canonical NF-kB Signaling Academic Editor: Alexander NF-κB (nuclear factor kappa B) was discovered in the laboratory of the Nobel Prize E. Kalyuzhny laureate David Baltimore as a latent transcription factor with inducible binding activity [1]. Later, Baeuerle and Baltimore described an inhibitory protein family named inhibitor κB Received: 7 July 2021 (IκB), which could interact with NF-κB in the cytoplasm and was responsible for the latent Accepted: 6 August 2021 form [2]. Molecular cloning revealed that the NF-κB family consisted of five DNA-binding Published: 10 August 2021 members in humans: two without transactivation domains, NFKB1 (p50) and NFKB2 (p52); three with transactivation domains, REL (c-REL), RELA (p65), and RELB (RELB). In the Publisher’s Note: MDPI stays neutral canonical signaling pathway, NF-κB is formed by a heteromeric DNA-binding dimer, e.g., with regard to jurisdictional claims in RELA (p65) or NFKB1 (p50), which is in its cytoplasmic (latent) form complexed with published maps and institutional affil- inhibitor of nuclear factor κB (IκB) [3]. This results in the allosteric induction of the closed iations. non-DNA-binding conformation. IκB binding to the heterodimer RELA/p50 induces an alpha-helical conformation of the nuclear translocation signal sequence (NLS) on RELA, inhibiting the interaction with the nuclear import machinery [4]. For nuclear import, a disordered NLS sequence is important. IκB itself can translocate to the nucleus and Copyright: © 2021 by the authors. might interact with DNA-bound NF-κB to induce nuclear export [5]. In its latent state, the Licensee MDPI, Basel, Switzerland. RELA/p50 heterodimer exists in a stable cytosolic complex with a member of the IκB family This article is an open access article (e.g., NFKBIA) (Figure1, canonical pathway); see [ 6] for a comprehensive review. There distributed under the terms and are more than 300 proteins with IκB activity [7]. Extracellular stimuli for NF-κB include conditions of the Creative Commons bacterial and viral products, inflammatory cytokines, reactive oxygen species, ultraviolet Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ light, and even neurotransmitters such as glutamate [8,9]. For instance, inflammatory 4.0/). cytokines such as TNFα bind to TNFR1 inducing receptor trimerisation (Figure1). Cells 2021, 10, 2042. https://doi.org/10.3390/cells10082042 https://www.mdpi.com/journal/cells Cells 2021, 10, x FOR PEER REVIEW 2 of 18 ber of the IκB family (e.g., NFKBIA) (Figure 1, canonical pathway); see [6] for a compre- hensive review. There are more than 300 proteins with IκB activity [7]. Extracellular stimuli for NF-κB include bacterial and viral products, inflammatory cytokines, reactive oxygen Cells 2021, 10, 2042 species, ultraviolet light, and even neurotransmitters such as glutamate [8,9]. For instance,2 of 17 inflammatory cytokines such as TNFα bind to TNFR1 inducing receptor trimerisation (Fig- ure 1). FigureFigure 1.1.Schematic Schematic view view of of canonical canonical and and non-canoncial non-canoncial NF- NFκB- signalingκB signaling in mammals. in mammals. In canonical In canonical signaling, signaling, NF-κB NF exists-κB inexists two in forms: two forms: a latent a latent form complexedform complexed with IwithκB retained IκB retained within within the cytoplasm, the cytoplasm and, activatedand activated NF-κ NFB without-κB without inhibitory inhib- subunits,itory subunits binding, binding to DNA to inDNA the nucleus.in the nucleus. Various Various extracellular extracellular stimuli activatestimuli NF-activateκB, such NF- asκB, bacterial such as andbacterial viral products,and viral inflammatoryproducts, inflammatory cytokines, cytokines, reactive oxygen reactive species, oxygen ultraviolet species, ultraviolet light, and light even, neurotransmitters.and even neurotransmitters. Intracellularly, Intracellularly, a kinase a kinase complex (IKK, IκB kinase) is activated, in turn, resulting in phosphorylation and linear ubiquitination of the complex (IKK, IκB kinase) is activated, in turn, resulting in phosphorylation and linear ubiquitination of the heterodimer- heterodimer-associated IκB. Rapid degradation of IκB through ubiquitin-mediated proteasomal degradation is followed associated IκB. Rapid degradation of IκB through ubiquitin-mediated proteasomal degradation is followed by nuclear by nuclear import of NF-κB and binding to target gene promoters such as IκB. Later, IκB can enter the nucleus and termi- importnate the of expression NF-κB and of binding NF-κB-target to target genes. gene In promoters non-canonical such signaling, as IκB. Later, binding IκB of can ligands enter theto respective nucleus and receptors terminate such the as expressionBAFFR leads of NF-to NFκB-target-κB-inducing genes. Inkinase non-canonical (NIK)-mediated signaling, phosphorylation binding of ligands of IKK1, to respective which in receptors turn results such in as processing BAFFR leads of top100 NF- toκB-inducing p52 and enables kinase (NIK)-mediatedthe translocation phosphorylation of p52/RELB into of IKK1,the nucleus which and in turn the results initiation in processing of target gene of p100 transcription. to p52 and enablesExpression the translocationpatterns in human of p52/RELB single cel intols measur the nucleused by and RNA the s initiationequencing of differed target gene considerably. transcription. Whereas Expression RELA patternshas low incell human type-specificity, single cells c- measuredREL is predominantly by RNA sequencing expressed differed in monocytes considerably. and keratinocytes Whereas RELA and RELB has low in monocytes cell type-specificity, and glan- c-RELdular iscells predominantly (The Human expressed Protein Atlas in monocytes, www.proteinatlas. and keratinocytesorg (acces andsed RELB on 23 in J monocytesuly 2021)). and glandular cells (The Human Protein Atlas, www.proteinatlas.org (accessed on 23 July 2021)). They activate a kinase complex (IKK, IκB kinase; see Figure 1) containing two related catalyticThey subunits, activate aIKK kinase-1 (or complex -α) and (IKK,IKK-2 I κ(orB kinase; -β) [10] see. An Figure additional1) containing component, two relatedNEMO catalytic(NF-κB essential subunits, modulator IKK-1 (or or -α )IKKγ), and IKK-2 with (orno enzymatic -β)[10]. An activ additionality has been component, cloned through NEMO (NF-the geneticκB essential complementation modulator or of IKK anγ ),NF with-κB noactivation enzymatic-defective activity cell has line been [11] cloned. Biochemical through thepurification genetic complementation identified IKK-γ of as an an NF- essentialκB activation-defective regulatory subunit cell of line the [11 IκB]. Biochemicalkinase complex pu- rification[12]. The identifiedactivated IKK-IKK γcomplexas an essential catalyzes regulatory the sequence subunit-specific of the phosphorylation IκB kinase complex (serine [12]. The32 and activated 36) and IKK ubiquitination complex catalyzes of the the heterodimer sequence-specific-associated phosphorylation IκBα (Figure (serine1) [6,13] 32. This and 36) and ubiquitination of the heterodimer-associated IκBα (Figure1)[ 6,13]. This leads to the rapid degradation of IκBα through ubiquitin-mediated proteasomal degradation [14]. κ α κ κ The removal of I B activates the