Myeloid Deletion of Nemo Causes Osteopetrosis in Mice Owing to Upregulation of Transcriptional Repressors

Molecular Mechanisms of Craniofacial and Dental Abnormalities in Osteopetrosis

Osteopetrosis is a group of genetic bone disorders characterized by increased bone density and defective bone resorption. Osteopetrosis presents a series of clinical manifestations, including craniofacial deformities and dental problems. However, few previous reports have focused on the features of craniofacial and dental problems in osteopetrosis. In this review, we go through the clinical features, types, and related pathogenic genes of osteopetrosis. Then we summarize and describe the characteristics of craniofacial and dental abnormalities in osteopetrosis that have been published in PubMed from 1965 to the present. We found that all 13 types of osteopetrosis have craniomaxillofacial and dental phenotypes. The main pathogenic genes, such as chloride channel 7 gene (CLCN7), T cell immune regulator 1 (TCIRG1), osteopetrosis-associated transmembrane protein 1 (OSTM1), pleckstrin homology domain-containing protein family member 1 (PLEKHM1), and carbonic anhydrase II (CA2), and their molecular mechanisms involved in craniofacial and dental phenotypes, are discussed. We conclude that the telltale craniofacial and dental abnormalities are important for dentists and other clinicians in the diagnosis of osteopetrosis and other genetic bone diseases.

Osteoclasts, Master Sculptors of Bone

Osteoclasts are multinucleated cells with the unique ability to resorb bone matrix. Excessive production or activation of osteoclasts leads to skeletal pathologies that affect a significant portion of the population. Although therapies that effectively target osteoclasts have been developed, they are associated with sometimes severe side effects, and a fuller understanding of osteoclast biology may lead to more specific treatments. Along those lines, a rich body of work has defined essential signaling pathways required for osteoclast formation, function, and survival. Nonetheless, recent studies have cast new light on long-held views regarding the origin of these cells during development and homeostasis, their life span, and the cellular sources of factors that drive their production and activity during homeostasis and disease. In this review, we discuss these new findings in the context of existing work and highlight areas of ongoing and future investigation.

Critical Roles of NF-κB Signaling Molecules in Bone Metabolism Revealed by Genetic Mutations in Osteopetrosis

The nuclear factor-κB (NF-κB) transcription factor family consists of five related proteins, RelA (p65), c-Rel, RelB, p50/p105 (NF-κB1), and p52/p100 (NF-κB2). These proteins are important not only for inflammation and the immune response but also for bone metabolism. Activation of NF-κB occurs via the classic and alternative pathways. Inflammatory cytokines, such as tumor necrosis factor (TNF)-α and interleukin (IL)-1β, activate the former, and cytokines involved in lymph node formation, such as receptor activator of NF-κB ligand (RANKL) and CD40L, activate the latter. p50 and p52 double-knockout mice revealed severe osteopetrosis due to the total lack of osteoclasts, which are specialized cells for bone resorption. This finding suggests that the activation of NF-κB is required for osteoclast differentiation. The NF-κB signaling pathway is controlled by various regulators, including NF-κB essential modulator (NEMO), which is encoded by the IKBKG gene. In recent years, mutant forms of the IKBKG gene have been reported as causative genes of osteopetrosis, lymphedema, hypohidrotic ectodermal dysplasia, and immunodeficiency (OL-EDA-ID). In addition, a mutation in the RELA gene, encoding RelA, has been reported for the first time in newborns with high neonatal bone mass. Osteopetrosis is characterized by a diffuse increase in bone mass, ranging from a lethal form observed in newborns to an asymptomatic form that appears in adulthood. This review describes the genetic mutations in NF-κB signaling molecules that have been identified in patients with osteopetrosis.

Methods to Analyze the Roles of TAK1, TRAF6, and NEMO in the Regulation of NF-κB Signaling by RANK Stimulation During Osteoclastogenesis

The skeletal system is constantly undergoing turnover in order to create strong, organized structures, requiring the bone breakdown and building properties by osteoclasts and osteoblasts, respectively. However, in pathological disease states, excessive osteoclast activity can cause bone loss leading to increase in morbidity and mortality. Osteoclasts differentiate from macrophages in the presence of various factors. M-CSF is a cytokine that is required to maintain the survival of macrophages. However, RANKL is the critical factor required for differentiation of osteoclasts. RANKL is produced from a variety of different cell types such as osteoblasts and osteocytes. RANKL binds to RANK, its receptor, on the surface of osteoclast precursors, which activates various signaling pathways to drive the transcription and production of genes important for osteoclast formation. The major signaling pathway activated by RANKL-RANK interaction is the NF-κB pathway. The NF-κB pathway is the principle inflammatory response pathway activated by a variety of stimuli such as inflammatory cytokines, genotoxic stress, and other factors. This likely explains the finding that inflammatory diseases often present with some component of increased osteoclast formation and activity, driving bone loss. Determining the signaling mechanisms downstream of RANKL can provide valuable therapeutic targets for the treatment of bone loss in various disease states.

Renal proximal tubular NEMO plays a critical role in ischemic acute kidney injury

We determined that renal proximal tubular (PT) NF-κB essential modulator (NEMO) plays a direct and critical role in ischemic acute kidney injury (AKI) using mice lacking renal PT NEMO and by targeted renal PT NEMO inhibition with mesoscale nanoparticle-encapsulated NEMO binding peptide (NBP MNP). We subjected renal PT NEMO-deficient mice, WT mice, and C57BL/6 mice to sham surgery or 30 minutes of renal ischemia and reperfusion (IR). C57BL/6 mice received NBP MNP or empty MNP before renal IR injury. Mice treated with NBP MNP and mice deficient in renal PT NEMO were protected against ischemic AKI, having decreased renal tubular necrosis, inflammation, and apoptosis compared with control MNP-treated or WT mice, respectively. Recombinant peptidylarginine deiminase type 4 (rPAD4) targeted kidney PT NEMO to exacerbate ischemic AKI in that exogenous rPAD4 exacerbated renal IR injury in WT mice but not in renal PT NEMO-deficient mice. Furthermore, rPAD4 upregulated proinflammatory cytokine mRNA and NF-κB activation in freshly isolated renal proximal tubules from WT mice but not from PT NEMO-deficient mice. Taken together, our studies suggest that renal PT NEMO plays a critical role in ischemic AKI by promoting renal tubular inflammation, apoptosis, and necrosis.

Inflammatory osteolysis is regulated by site-specific ISGylation of the scaffold protein NEMO

Inflammatory osteolysis is governed by exacerbated osteoclastogenesis. Ample evidence points to central role of NF-κB in such pathologic responses, yet the precise mechanisms underpinning specificity of these responses remain unclear. We propose that motifs of the scaffold protein IKKγ/NEMO partly facilitate such functions. As proof-of-principle, we used site-specific mutagenesis to examine the role of NEMO in mediating RANKL-induced signaling in mouse bone marrow macrophages, known as osteoclast precursors. We identified lysine (K)270 as a target regulating RANKL signaling as K270A substitution results in exuberant osteoclastogenesis in vitro and murine inflammatory osteolysis in vivo. Mechanistically, we discovered that K270A mutation disrupts autophagy, stabilizes NEMO, and elevates inflammatory burden. Specifically, K270A directly or indirectly hinders binding of NEMO to ISG15, a ubiquitin-like protein, which we show targets the modified proteins to autophagy-mediated lysosomal degradation. Taken together, our findings suggest that NEMO serves as a toolkit to fine-tune specific signals in physiologic and pathologic conditions.

The human skeleton contains over 200 bones that together act as an internal framework for the body. Over our lifetime, the body constantly removes older bone tissue from the skeleton and replaces it with new bone tissue. This “bone remodeling” also controls how bones are repaired after being damaged by injuries, disease or normal wear and tear.

Cells known as osteoclasts are responsible for breaking down old bone tissue and participate in repairing damaged bone. A cellular pathway known as NF-kB signaling stimulates other cells called “bone marrow macrophages” to become osteoclasts. A certain level of NF-kB signaling is required to maintain a healthy skeleton. However, under certain inflammatory conditions, the level of NF-kB signaling becomes too high causing hyperactive osteoclasts to accumulate and inflict severe bone breakdown. This abnormal osteoclast activity leads to eroded and fragile bones and joints, as is the case in diseases such as rheumatoid arthritis and osteoporosis.

Previous studies have shown that a protein called NEMO is a core component of the NF-kB signal pathway, but the precise role of NEMO in the diseased response remained unclear. Adapala, Swarnkar, Arra et al. have now used site-directed mutagenesis approach to study the role of NEMO in bone marrow macrophages in mice. The experiments showed that one specific site within the NEMO protein, referred to as lysine 270, is crucial for its role in controlling osteoclasts and the breakdown of bone tissue. Mutating NEMO at lysine 270 led to uncontrolled NF-kB signaling in the bone marrow macrophages. Further experiments showed that lysine 270 served as a sensor to allow NEMO to bind another protein called ISG15, which in turn helped to decrease NF-kB signaling and slow down the erosion of the bone.

These findings suggest that site-specific targeting of NEMO, rather than inhibiting the whole NF-kB pathway, may help to reduce the symptoms of bone disease while maintaining the beneficial roles of this essential pathway. However, additional research is required to identify NEMO sites responsible for controlling the inflammatory component.

Absence of an osteopetrosis phenotype in IKBKG (NEMO) mutation-positive women: A case-control study

BACKGROUND

NF-κB essential modulator (NEMO), encoded by IKBKG, is necessary for activation of the ubiquitous transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Animal studies suggest NEMO is required for NF-κB mediated bone homeostasis, but this has not been thoroughly studied in humans. IKBKG loss-of-function mutation causes incontinentia pigmenti (IP), a rare X-linked disease featuring linear hypopigmentation, alopecia, hypodontia, and immunodeficiency. Single case reports describe osteopetrosis (OPT) in boys carrying hypomorphic IKBKG mutations.

METHOD

We studied the bone phenotype in women with IP with evaluation of radiographs of the spine and non-dominant arm and leg; lumbar spine and femoral neck aBMD using DXA; μ-CT and histomorphometry of trans-iliac crest biopsy specimens; bone turnover markers; and cellular phenotype in bone marrow skeletal (stromal) stem cells (BM-MSCs) in a cross-sectional, age-, sex-, and BMI-matched case-control study. X-chromosome inactivation was measured in blood leucocytes and BM-MSCs using a PCR method with methylation of HpaII sites. NF-κB activity was quantitated in BM-MSCs using a luciferase NF-κB reporter assay.

RESULTS

Seven Caucasian women with IP (age: 24-67 years and BMI: 20.0-35.2 kg/m²) and IKBKG mutation (del exon 4-10 (n = 4); c.460C>T (n = 3)) were compared to matched controls. The IKBKG mutation carriers had extremely skewed X-inactivation (>90:10%) in blood, but not in BM-MSCs. NF-κB activity was lower in BM-MSCs from IKBKG mutation carriers (n = 5) compared to controls (3094 ± 679 vs. 5422 ± 1038/μg protein, p < 0.01). However, no differences were identified on skeletal radiographics, aBMD, μ-architecture of the iliac crest, or bone turnover markers. The IKBKG mutation carriers had a 1.7-fold greater extent of eroded surfaces relative to osteoid surfaces (p < 0.01), and a 2.0-fold greater proportion of arrested reversal surface relative to active reversal surface (p < 0.01).

CONCLUSION

Unlike mutation-positive males, the IKBKG mutation-positive women did not manifest OPT.

C/EBPα and PU.1 exhibit different responses to RANK signaling for osteoclastogenesis

The transcription factors C/EBPα and PU.1 are upregulated by RANKL through activation of its receptor RANK during osteoclastogenesis and are critical for osteoclast differentiation. Herein we investigated the mechanisms underlying how C/EBPα and PU.1 regulate osteoclast differentiation in response to RANK signaling. We showed that C/EBPα or PU.1 overexpression could initiate osteoclastogenesis and upregulate the expressions of the osteoclast genes encoding the nuclear factor of activated T-cells, C1, cathepsin K, and tartrate-resistant acid phosphatase independently of RANKL. However, while PU.1 upregulated C/EBPα, C/EBPα could not upregulate PU.1. RANK has a unique cytoplasmic domain, 535IVVY538 motif, which is crucial for osteoclast differentiation. We demonstrated that mutational inactivation of RANK IVVY motif blocked osteoclast differentiation and significantly attenuated C/EBPα, but not PU.1, expression, indicating that RANK-IVVY-induced signaling is dispensable to PU.1 upregulation during osteoclastogenesis. However, C/EBPα or PU.1 overexpression failed to promote osteoclastogenesis in cells expressing mutated RANK IVVY motif. We noted that RANK-IVVY-motif inactivation significantly repressed osteoclast genes as compared with a vector control, suggesting that IVVY motif might also negatively regulate osteoclast inhibitors during osteoclastogenesis. Consistently, IVVY-motif inactivation triggered upregulation of RBP-J, a potent osteoclast inhibitor, during osteoclastogenesis. Notably, C/EBPα or PU.1 overexpression in cells expressing mutated RANK IVVY motif failed to control the deregulated RBP-J expression, resulting in repression of osteoclast genes. Accordingly, RBP-J silencing in the mutant cells rescued osteoclastogenesis with C/EBPα or PU.1 overexpression. In conclusion, we revealed that while PU.1 and C/EBPα are critical for osteoclastogenesis, they respond differently to RANKL-induced activation of RANK IVVY motif.

NUMBL Interacts with TAK1, TRAF6 and NEMO to Negatively Regulate NF-κB Signaling During Osteoclastogenesis

NF-κB signaling is essential for osteoclast differentiation and skeletal homeostasis. We have reported recently that NUMB-like (NUMBL) protein modulates osteoclastogenesis by down regulating NF-κB activation. Herein, we decipher the mechanism underlying this phenomenon. We found that whereas NUMBL mRNA expression decreases upon stimulation of wild type (WT) bone marrow macrophages (BMMs) with RANKL, TAK1 deficiency in these cells leads to increased NUMBL and decreased TRAF6 and NEMO expression. These changes were restored upon WT-TAK1 expression, but not with catalytically inactive TAK1-K63W, suggesting that TAK1 enzymatic activity is required for these events. Forced expression of NUMBL inhibits osteoclast differentiation and function as evident by reduction in all hallmarks of osteoclastogenesis. Conversely, NUMBL-null BMMs, show increased osteoclast differentiation and mRNA expression of osteoclast marker genes. Post-translationally, K48-linked poly-ubiquitination of NUMBL is diminished in TAK1-null BMMs compared to elevated K48-poly-ubiquitination in WT cells, indicating increased stability of NUMBL in TAK1-null conditions. Further, our studies show that NUMBL directly interacts with TRAF6 and NEMO, and induces their K48-poly-ubiquitination mediated proteasomal degradation. Collectively, our data suggest that NUMBL and TAK1 are reciprocally regulated and that NUMBL acts as an endogenous regulator of NF-κB signaling and osteoclastogenesis by targeting the TAK1-TRAF6-NEMO axis.

Osteoimmunology: The Conceptual Framework Unifying the Immune and Skeletal Systems

The immune and skeletal systems share a variety of molecules, including cytokines, chemokines, hormones, receptors, and transcription factors. Bone cells interact with immune cells under physiological and pathological conditions. Osteoimmunology was created as a new interdisciplinary field in large part to highlight the shared molecules and reciprocal interactions between the two systems in both heath and disease. Receptor activator of NF-κB ligand (RANKL) plays an essential role not only in the development of immune organs and bones, but also in autoimmune diseases affecting bone, thus effectively comprising the molecule that links the two systems. Here we review the function, gene regulation, and signal transduction of osteoimmune molecules, including RANKL, in the context of osteoclastogenesis as well as multiple other regulatory functions. Osteoimmunology has become indispensable for understanding the pathogenesis of a number of diseases such as rheumatoid arthritis (RA). We review the various osteoimmune pathologies, including the bone destruction in RA, in which pathogenic helper T cell subsets [such as IL-17-expressing helper T (Th17) cells] induce bone erosion through aberrant RANKL expression. We also focus on cellular interactions and the identification of the communication factors in the bone marrow, discussing the contribution of bone cells to the maintenance and regulation of hematopoietic stem and progenitors cells. Thus the time has come for a basic reappraisal of the framework for understanding both the immune and bone systems. The concept of a unified osteoimmune system will be absolutely indispensable for basic and translational approaches to diseases related to bone and/or the immune system.