Ubiquitin E3 Ligase LNX2 is Critical for Osteoclastogenesis In Vitro by Regulating M-CSF/RANKL Signaling and Notch2

Molecular and functional mapping of Plekhm1-Rab7 interaction in osteoclasts

Mutations in PLEKHM1 cause osteopetrosis in humans and rats. The germline and osteoclast conditional deletions of Plekhm1 gene in mice lead to defective osteoclast bone resorption and increased trabecular bone mass without overt abnormalities in other organs. As an adaptor protein, pleckstrin homology and RUN domain containing M1 (PLEKHM1) interacts with the key lysosome regulator small GTPase RAB7 via its C-terminal RUBICON homologous (RH) domain. In this study, we have conducted a structural-functional study of the PLEKHM1 RH domain and RAB7 interaction in osteoclasts in vitro. The single mutations of the key residues in the Plekhm1 RH predicted from the crystal structure of the RUBICON RH domain and RAB7 interface failed to disrupt the Plekhm1-Rab7 binding, lysosome trafficking, and bone resorption. The compound alanine mutations at Y949-R954 and L1011-I1018 regions decreased Plekhm1 protein stability and Rab7-binding, respectively, thereby attenuated lysosome trafficking and bone resorption in osteoclasts. In contrast, the compound alanine mutations at R1060-Q1068 region were dispensable for Rab7-binding and Plekhm1 function in osteoclasts. These results indicate that the regions spanning Y949-R954 and L1011-I1018 of Plekhm1 RH domain are functionally important for Plekhm1 in osteoclasts and offer the therapeutic targets for blocking bone resorption in treatment of osteoporosis and other metabolic bone diseases.

Metallothionein 3 promotes osteoclast differentiation and survival by regulating the intracellular Zn2+ concentration and NRF2 pathway

In osteoclastogenesis, the metabolism of metal ions plays an essential role in controlling reactive oxygen species (ROS) production, mitochondrial biogenesis, and survival, and differentiation. However, the mechanism regulating metal ions during osteoclast differentiation remains unclear. The metal-binding protein metallothionein (MT) detoxifies heavy metals, maintains metal ion homeostasis, especially zinc, and manages cellular redox levels. We carried out tests using murine osteoclast precursors to examine the function of MT in osteoclastogenesis and evaluated their potential as targets for future osteoporosis treatments. MT genes were significantly upregulated upon differentiation from osteoclast precursors to mature osteoclasts in response to receptor activators of nuclear factor-κB (NF-κB) ligand (RANKL) stimulation, and MT3 expression was particularly pronounced in mature osteoclasts among MT genes. The knockdown of MT3 in osteoclast precursors demonstrated a remarkable inhibition of differentiation into mature osteoclasts. In preosteoclasts, MT3 knockdown suppressed the activity of mitogen-activated protein kinase (MAPK) and NF-κB signaling pathways upon RANKL stimulation, leading to affect cell survival through elevated cleaved Caspase 3 and poly (ADP-ribose) polymerase (PARP) levels. Additionally, ROS levels were decreased, and nuclear factor erythroid 2-related factor 2 (NRF2) (a suppressor of ROS) and the downstream antioxidant proteins, such as catalase (CAT) and heme oxygenase 1 (HO-1), were more highly expressed in the MT3 preosteoclast knockdowns. mitochondrial ROS, which is involved in mitochondrial biogenesis and the production of reactive oxygen species, were similarly decreased because cAMP response element-binding (CREB) and peroxisome proliferator-activated receptor γ coactivator 1β (PGC-1β) were less activated due to MT3 depletion. Thus, by modulating ROS through the NRF2 pathway, MT3 plays a crucial role in regulating osteoclast differentiation and survival, acting as a metabolic modulator of intracellular zinc ions.

The role of E3 ubiquitin ligases in bone homeostasis and related diseases

The ubiquitin-proteasome system (UPS) dedicates to degrade intracellular proteins to modulate demic homeostasis and functions of organisms. These enzymatic cascades mark and modifies target proteins diversly through covalently binding ubiquitin molecules. In the UPS, E3 ubiquitin ligases are the crucial constituents by the advantage of recognizing and presenting proteins to proteasomes for proteolysis. As the major regulators of protein homeostasis, E3 ligases are indispensable to proper cell manners in diverse systems, and they are well described in physiological bone growth and bone metabolism. Pathologically, classic bone-related diseases such as metabolic bone diseases, arthritis, bone neoplasms and bone metastasis of the tumor, etc., were also depicted in a UPS-dependent manner. Therefore, skeletal system is versatilely regulated by UPS and it is worthy to summarize the underlying mechanism. Furthermore, based on the current status of treatment, normal or pathological osteogenesis and tumorigenesis elaborated in this review highlight the clinical significance of UPS research. As a strategy possibly remedies the limitations of UPS treatment, emerging PROTAC was described comprehensively to illustrate its potential in clinical application. Altogether, the purpose of this review aims to provide more evidence for exploiting novel therapeutic strategies based on UPS for bone associated diseases.

Targeting proteostasis network in osteoporosis: Pathological mechanisms and therapeutic implications

As the most common bone disease, osteoporosis (OP) increases bone fragility and makes patients more vulnerable to the threat of osteoporotic fractures. With the ageing population in today's society, OP has become a huge and growing public health problem. Unfortunately, the clear pathogenesis of OP is still under exploration, and effective interventions are still scarce. Therefore, exploring new targets for pharmacological interventions to develop promising therapeutic drugs for OP is of great clinical value. Previous studies have shown that normal bone remodeling depends on proteostasis, whereas loss of proteostasis during ageing leads to the dysfunctional proteostasis network (PN) that fails to maintain bone homeostasis. Nevertheless, only a few studies have revealed the pathophysiological relationship between bone metabolism and a single component of PN, yet the role of PN as a whole in the pathogenesis of OP is still under investigation. This review comprehensively summarized the role of PN in the pathogenesis of OP and further discussed the potential of PN as innovative drug targets for the therapy of OP.

Impaired Blastocyst Formation in Lnx2-Knockdown Mouse Embryos

Ligand of Numb-protein X 2 (LNX2) is an E3 ubiquitin ligase that is known to regulate Notch signaling by participating in NUMB protein degradation. Notch signaling is important for differentiation and proliferation in mammals, and plays a significant role in blastocyst formation during early embryonic development. In this study, we investigated Lnx2 in mouse preimplantation embryos. Expression analysis showed that Lnx2 is expressed in oocytes and preimplantation embryos. Lnx2-knockdown embryos normally progress to the morula stage, but the majority of them do not develop into normal blastocysts. Transcript analysis revealed that the expression levels of genes critical for cell lineage specification, including octamer-binding transcription factor 4 (Oct4), are increased in Lnx2 knockdown embryos. Furthermore, the expression levels of Notch and Hippo signaling-related genes are also increased by Lnx2 knockdown. Collectively, our results show that Lnx2 is important for blastocyst formation in mice, suggest that this may act via lineage specification of inner cell mass, and further show that Lnx2 may be involved in transcriptionally regulating various genes implicated in early embryonic development.

E3 Ubiquitin Ligases: Potential Therapeutic Targets for Skeletal Pathology and Degeneration

The ubiquitination-proteasome system (UPS) is crucial in regulating a variety of cellular processes including proliferation, differentiation, and survival. Ubiquitin protein ligase E3 is the most critical molecule in the UPS system. Dysregulation of the UPS system is associated with many conditions. Over the past few decades, there have been an increasing number of studies focusing on the UPS system and how it affects bone metabolism. Multiple E3 ubiquitin ligases have been found to mediate osteogenesis or osteolysis through a variety of pathways. In this review, we describe the mechanisms of UPS, especially E3 ubiquitin ligases on bone metabolism. To date, many E3 ubiquitin ligases have been found to regulate osteogenesis or osteoclast differentiation. We review the classification of these E3 enzymes and the mechanisms that influence upstream and downstream molecules and transduction pathways. Finally, this paper reviews the discovery of the relevant UPS inhibitors, drug molecules, and noncoding RNAs so far and prospects the future research and treatment.

Identification of Genes with Altered Methylation in Osteoclast Differentiation and Its Roles in Osteoporosis

Osteoporosis is one of the most common metabolic skeletal diseases, which affects more than 200 million people worldwide, especially elderly and postmenopausal women. One of the main processes of osteoporosis is attenuated bone formation. Abundant evidence has confirmed that overactivated osteoclasts are responsible for the attenuated bone formation. This study aims at identifying novel methylation-associated biomarkers and therapeutic targets in osteoclasts by integrally analyzing methylation profiles and gene expression data. DNA methylation profile and gene expression data were obtained from the Gene Expression Omnibus (GEO) database. Subsequently, we integrated the two sets of data to screen for differentially expressed genes with differential methylation level (DM-DEGs) between osteoclasts and CD14⁺ monocytes from donors. Then, Gene Ontology (GO) analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were performed to uncover the enriched functions and pathways of identified DM-DEGs. In addition, by combining protein-protein interaction analysis and receiver-operator characteristic analysis, we finally identified four hub DM-DEGs. Gene Set Enrichment Analysis was utilized to validate and investigate the potential biological functions of the four hub DM-DEGs. Finally, Real-time quantitative PCR (QPCR) was performed to validate the mRNA expression level of the four identified hub DM-DEGs during osteoclast differentiation. CCRL2, CCL18, C1QB, and SELL were highly correlated with osteoclastic differentiation and osteoporosis phenotype. QPCR revealed that the expression of CCRL2, CCL18, and C1QB was increased during osteoclast differentiation, whereas the expression of SELL was decreased. The present study indicated a connection between gene expression and DNA methylation during osteoclast differentiation and that four hub DM-DEGs in osteoclastogenesis and osteoporosis pathogenesis might be potential candidates for intensive research and therapeutic targets for the treatment of osteoporosis.

Notch2 Blockade Mitigates Methotrexate Chemotherapy-Induced Bone Loss and Marrow Adiposity

Childhood cancer methotrexate (MTX) chemotherapy often causes bone growth impairments, bone loss, and increased risks of fractures during or after treatment, for which the pathobiology is unclear and there is a lack of specific treatment. Our time course analyses of long bones from rats receiving intensive MTX treatment (mimicking a clinical protocol) found decreased trabecular bone volume, increased osteoclast formation and activity, increased adipogenesis in the expense of osteogenesis from the bone marrow stromal cells at days 6 and 9 following the first of five daily MTX doses. For exploring potential mechanisms, PCR array expression of 91 key factors regulating bone homeostasis was screened with the bone samples, which revealed MTX treatment-induced upregulation of Notch receptor NOTCH2, activation of which is known to be critical in skeletal development and bone homeostasis. Consistently, increased Notch2 activation in bones of MTX-treated rats was confirmed, accompanied by increased expression of Notch2 intracellular domain protein and Notch target genes HEY1, HES1 and HEYL. To confirm the roles of Notch2 signalling, a neutralising anti-Notch2 antibody or a control IgG was administered to rats during MTX treatment. Microcomputed tomography analyses demonstrated that trabecular bone volume was preserved by MTX+anti-Notch2 antibody treatment. Anti-Notch2 antibody treatment ameliorated MTX treatment-induced increases in osteoclast density and NFATc1 and RANKL expression, and attenuated MTX-induced bone marrow adiposity via regulating Wnt/β-catenin signalling and PPARγ expression. Thus, Notch2 signalling plays an important role in mediating MTX treatment-induced bone loss and bone marrow adiposity, and targeting Notch2 could be a potential therapeutic option.

Genome-Wide CRISPR/Cas9-Based Screening for Deubiquitinase Subfamily Identifies Ubiquitin-Specific Protease 11 as a Novel Regulator of Osteogenic Differentiation

The osteoblast differentiation capacity of mesenchymal stem cells must be tightly regulated, as inadequate bone mineralization can lead to osteoporosis, and excess bone formation can cause the heterotopic ossification of soft tissues. The balanced protein level of Msh homeobox 1 (MSX1) is critical during normal osteogenesis. To understand the factors that prevent MSX1 protein degradation, the identification of deubiquitinating enzymes (DUBs) for MSX1 is essential. In this study, we performed loss-of-function-based screening for DUBs regulating MSX1 protein levels using the CRISPR/Cas9 system. We identified ubiquitin-specific protease 11 (USP11) as a protein regulator of MSX1 and further demonstrated that USP11 interacts and prevents MSX1 protein degradation by its deubiquitinating activity. Overexpression of USP11 enhanced the expression of several osteogenic transcriptional factors in human mesenchymal stem cells (hMSCs). Additionally, differentiation studies revealed reduced calcification and alkaline phosphatase activity in USP11-depleted cells, while overexpression of USP11 enhanced the differentiation potential of hMSCs. These results indicate the novel role of USP11 during osteogenic differentiation and suggest USP11 as a potential target for bone regeneration.

The Cytoplasmic Dynein Associated Protein NDE1 Regulates Osteoclastogenesis by Modulating M-CSF and RANKL Signaling Pathways

Cytoskeleton organization and lysosome secretion play an essential role in osteoclastogenesis and bone resorption. The cytoplasmic dynein is a molecular motor complex that regulates microtubule dynamics and transportation of cargos/organelles, including lysosomes along the microtubules. LIS1, NDE1, and NDEL1 belong to an evolutionary conserved pathway that regulates dynein functions. Disruption of the cytoplasmic dynein complex and deletion of LIS1 in osteoclast precursors arrest osteoclastogenesis. Nonetheless, the role of NDE1 and NDEL1 in osteoclast biology remains elusive. In this study, we found that knocking-down Nde1 expression by lentiviral transduction of specific shRNAs markedly inhibited osteoclastogenesis in vitro by attenuating the proliferation, survival, and differentiation of osteoclast precursor cells via suppression of signaling pathways downstream of M-CSF and RANKL as well as osteoclast differentiation transcription factor NFATc1. To dissect how NDEL1 regulates osteoclasts and bone homeostasis, we generated Ndel1 conditional knockout mice in myeloid osteoclast precursors (Ndel1ΔlysM) by crossing Ndel1-floxed mice with LysM-Cre mice on C57BL/6J background. The Ndel1ΔlysM mice developed normally. The µCT analysis of distal femurs and in vitro osteoclast differentiation and functional assays in cultures unveiled the similar bone mass in both trabecular and cortical bone compartments as well as intact osteoclastogenesis, cytoskeleton organization, and bone resorption in Ndel1ΔlysM mice and cultures. Therefore, our results reveal a novel role of NDE1 in regulation of osteoclastogenesis and demonstrate that NDEL1 is dispensable for osteoclast differentiation and function.

Role of Biomolecules in Osteoclasts and Their Therapeutic Potential for Osteoporosis

Osteoclasts (OCs) are important cells that are involved in the regulation of bone metabolism and are mainly responsible for coordinating bone resorption with bone formation to regulate bone remodeling. The imbalance between bone resorption and formation significantly affects bone metabolism. When the activity of osteoclasts exceeds the osteoblasts, it results in a condition called osteoporosis, which is characterized by reduced bone microarchitecture, decreased bone mass, and increased occurrences of fracture. Molecules, including transcription factors, proteins, hormones, nucleic acids, such as non-coding RNAs, play an important role in osteoclast proliferation, differentiation, and function. In this review, we have highlighted the role of these molecules in osteoclasts regulation and osteoporosis. The developed therapeutics targeting these molecules for the treatment of osteoporosis in recent years have also been discussed with challenges faced in clinical application.

Regulation of Notch signaling by E3 ubiquitin ligases

Notch signaling is an evolutionarily conserved pathway that is widely used for multiple cellular events during development. Activation of the Notch pathway occurs when the ligand from a neighboring cell binds to the Notch receptor and induces cleavage of the intracellular domain of Notch, which further translocates into the nucleus to activate its downstream genes. The involvement of the Notch pathway in diverse biological events is possible due to the complexity in its regulation. In order to maintain tight spatiotemporal regulation, the Notch receptor, as well as its ligand, undergoes a series of physical and biochemical modifications that, in turn, helps in proper maintenance and fine-tuning of the signaling outcome. Ubiquitination is the post-translational addition of a ubiquitin molecule to a substrate protein, and the process is regulated by E3 ubiquitin ligases. The present review describes the involvement of different E3 ubiquitin ligases that play an important role in the regulation and maintenance of proper Notch signaling and how perturbation in ubiquitination results in abnormal Notch signaling leading to a number of human diseases.

NUMB maintains bone mass by promoting degradation of PTEN and GLI1 via ubiquitination in osteoblasts

The adaptor protein NUMB is involved in asymmetric division and cell fate determination and recognized as an antagonist of Notch. Previous studies have proved that Notch activation in osteoblasts contributes to a high bone mass. In this study,  however, an osteopenic phenotype was found in 9-week-old mice using osteoblastic specific Col1a1–2.3-Cre to ablate both Numb and its homologue Numbl . The trabecular bone mass decreased dramatically while the cortical bone mass was unaffected. Here, the Notch signal was not activated, while the tensin homologue deleted on human chromosome 10 (PTEN), which dephosphorylates phosphatidylinositide 3-kinases, was elevated, attenuating protein kinase B (Akt). The ubiquitination assay revealed that NUMB may physiologically promote PTEN ubiquitination in the presence of neural precursor cell-expressed developmentally downregulated protein 4–1. In addition, the deficiency of Numb/Numbl also activated the Hedgehog pathway through GLI1. This process was found to improve the ratio of the receptor activator of nuclear factor-kB ligand to osteoprotegerin, which enhanced the differentiation of osteoclasts and bone resorption . In conclusion, this study provides an insight into  new functons of   NUMB and NUMBL on bone homeostasis.

The related proteins NUMB and NUMBL maintain the survival of bone-generating osteoblast cells. NUMB was previously recognized to antagonize Notch signaling pathway ; In this study, it observes  that genetically altered mice, unable to express the two proteins, suffered from degraded bone quality. This suggests that the two proteins play a more complex, nuanced role in the process of bone mass maintenance. The team’s studies showed that NUMB and NUMBL suppression inhibits a signaling pathway important to skeletal development and protein synthesis in osteoblasts, though raise that further investigations are essential to elucidate the exact mechanistic action of these proteins. The authors of this study suggest that NUMB constitutes a potential target for therapies targeting bone loss and reduced bone strength in patients with osteoporosis.

Highly Dispersed Fullerenols Hamper Osteoclast Ruffled Border Formation by Perturbing Ca2+ Bundles

Osteoporosis, a common and serious bone disorder affecting aged people and postmenopausal women, is characterized by osteoclast overactivity. One therapeutic strategy is suppressing the bone resorption function of hyperactive osteoclasts, but there is no effective drug in clinical practice so far. Herein, it is demonstrated that fullerenols suppress the bone resorption of osteoclasts by inhibiting ruffled borders (RBs) formation. The RBs formation, which is supported by well-aligned actin bundles (B-actins), is a critical event for osteoclast bone resorption. To facilitate this function, osteoclast RBs dynamics is regulated by variable microenvironments to bundle F-actins, protrude cell membrane, and so on. B-actin perturbation by fullerenols is determined here, offering an opportunity to regulate osteoclast function by destroying RBs. In vivo, the therapeutic effect of fullerenols on overactive osteoclasts is confirmed in a mouse model of lipopolysaccharide-induced bone erosion. Collectively, the findings suggest that fullerenols adhere to F-actin surfaces and inhibit RBs formation in osteoclasts, mainly through hampering Ca²⁺ from bundling F-actins, and this is likely due to the stereo-hindrance effect caused by adherent fullerenols.

Deubiquitinating Enzymes and Bone Remodeling

Bone remodeling, which is essential for bone homeostasis, is controlled by multiple factors and mechanisms. In the past few years, studies have emphasized the role of the ubiquitin-dependent proteolysis system in regulating bone remodeling. Deubiquitinases, which are grouped into five families, remove ubiquitin from target proteins and are involved in several cell functions. Importantly, a number of deubiquitinases mediate bone remodeling through regulating differentiation and/or function of osteoblast and osteoclasts. In this review, we review the functions and mechanisms of deubiquitinases in mediating bone remodeling.

The tamoxifen derivative ridaifen-B is a high affinity selective CB2 receptor inverse agonist exhibiting anti-inflammatory and anti-osteoclastogenic effects

Selective estrogen receptor modulators (SERMs) target estrogen receptors (ERs) to treat breast cancer and osteoporosis. Several SERMs exhibit anti-cancer activity not related to ERs. To discover novel anti-cancer drugs acting via ER-independent mechanisms, derivatives of the SERM tamoxifen, known as the "ridaifen" compounds, have been developed that exhibit reduced or no ER affinity, while maintaining cytotoxicity. Tamoxifen and other SERMs bind to cannabinoid receptors with moderate affinity. Therefore, ER-independent effects of SERMs might be mediated via cannabinoid receptors. This study determined whether RID-B, a first generation ridaifen compound, exhibits affinity and/or activity at CB₁ and/or CB₂ cannabinoid receptors. RID-B binds with high affinity (K_i = 43.7 nM) and 17-fold selectivity to CB₂ over CB₁ receptors. RID-B acts as an inverse agonist at CB₂ receptors, modulating G-protein and adenylyl cyclase activity with potency values predicted by CB₂ affinity. Characteristic of an antagonist, RID-B co-incubation produces a parallel-rightward shift in the concentration-effect curve of CB₂ agonist WIN-55,212-2 to inhibit adenylyl cyclase activity. CB₂ inverse agonists are reported to exhibit anti-inflammatory and anti-ostoeclastogenic effects. In LPS-activated macrophages, RID-B exhibits anti-inflammatory effects by reducing levels of nitric oxide (NO), IL-6 and IL-1α, but not TNFα. Only reduction of NO concentration by RID-B is mediated by cannabinoid receptors. RID-B also exhibits pronounced anti-osteoclastogenic effects, reducing the number of osteoclasts differentiating from primary bone marrow macrophages in a cannabinoid receptor-dependent manner. In summary, the tamoxifen derivative RID-B, developed with reduced affinity for ERs, is a high affinity selective CB₂ inverse agonist with anti-inflammatory and anti-osteoclastogenic properties.

LNX1/LNX2 proteins: functions in neuronal signalling and beyond

Ligand of NUMB Protein X1 and X2 (LNX1 and LNX2) are E3 ubiquitin ligases, named for their ability to interact with and promote the degradation of the cell fate determinant protein NUMB. On this basis they are thought to play a role in modulating NUMB/NOTCH signalling during processes such as cortical neurogenesis. However, LNX1/2 proteins can bind, via their four PDZ (PSD95, DLGA, ZO-1) domains, to an extraordinarily large number of other proteins besides NUMB. Many of these interactions suggest additional roles for LNX1/2 proteins in the nervous system in areas such as synapse formation, neurotransmission and regulating neuroglial function. Twenty years on from their initial discovery, I discuss here the putative neuronal functions of LNX1/2 proteins in light of the anxiety-related phenotype of double knockout mice lacking LNX1 and LNX2 in the central nervous system (CNS). I also review what is known about non-neuronal roles of LNX1/2 proteins, including their roles in embryonic patterning and pancreas development in zebrafish and their possible involvement in colorectal cancer (CRC), osteoclast differentiation and immune function in mammals. The emerging picture places LNX1/2 proteins as potential regulators of multiple cellular signalling processes, but in many cases the physiological significance of such roles remains only partly validated and needs to be considered in the context of the tight control of LNX1/2 protein levels in vivo.

Proteomic analysis reveals novel ligands and substrates for LNX1 E3 ubiquitin ligase

Ligand of Numb protein X1 (LNX1) is an E3 ubiquitin ligase that contains a catalytic RING (Really Interesting New Gene) domain and four PDZ (PSD-95, DlgA, ZO-1) domains. LNX1 can ubiquitinate Numb, as well as a number of other ligands. However, the physiological relevance of these interactions in vivo remain unclear. To gain functional insights into the LNX family, we have characterised the LNX1 interactome using affinity purification and mass spectrometry. This approach identified a large number of novel LNX1-interacting proteins, as well as confirming known interactions with NUMB and ERC2. Many of the novel interactions mapped to the LNX PDZ domains, particularly PDZ2, and many showed specificity for LNX1 over the closely related LNX2. We show that PPFIA1 (liprin-α1), KLHL11, KIF7 and ERC2 are substrates for ubiquitination by LNX1. LNX1 ubiquitination of liprin-α1 is dependent on a PDZ binding motif containing a carboxyl terminal cysteine that binds LNX1 PDZ2. Surprisingly, the neuronally-expressed LNX1p70 isoform, that lacks the RING domain, was found to promote ubiquitination of PPFIA1 and KLHL11, albeit to a lesser extent than the longer RING-containing LNX1p80 isoform. Of several E3-ligases identified in the LNX1 interactome we confirm interactions of LNX1 with MID2/TRIM1 and TRIM27. On this basis we propose a model whereby LNX1p70, despite lacking a catalytic RING domain, may function as a scaffold to promote ubiquitination of its ligands through recruitment of other E3-ligases. These findings provide functional insights into the LNX protein family, particularly the neuronal LNX1p70 isoform.

Fullerenol nanoparticles suppress RANKL-induced osteoclastogenesis by inhibiting differentiation and maturation

Bone health requires regulation of homeostatic equilibrium between osteoblasts and osteoclasts. The over-activation of osteoclasts can disrupt bone metabolism, resulting in osteoporosis and other bone-loss diseases. Fullerenol, a polyhydroxy derivative of fullerene, exhibits excellent biocompatibility. Here we show that fullerenol nanoparticles exert two functions: inhibition of osteoclastic differentiation and blockage of pre-osteoclast fusion to restructure osteoclast maturation and function. Experimentally, the nanoparticles reduced pre-osteoclast migration and inhibited ruffled border formation to block their maturation. In addition, fullerenol dose-dependently restricted the differentiation of bone marrow macrophage cells (BMMs) to form osteoclasts following treatment with macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor (NF)-κB (RANKL) to activate NF-κB and mitogen activating protein kinase (MAPK) signaling pathways. It is possible that the very small size of fullerenol allows it to directly cross the cellular membrane to access the cytoplasm and regulate osteoclastogenesis from BMMs. Our results suggest that fullerenol could be used to treat bone-loss diseases such as osteoporosis.

Inhibition of PRMT5 suppresses osteoclast differentiation and partially protects against ovariectomy-induced bone loss through downregulation of CXCL10 and RSAD2

Protein arginine methyltransferase 5 (PRMT5) is an arginine methylation methyltransferase that regulates various physiological processes. Abnormal PRMT5 activity has been reported in inflammation and various types of cancers. Because osteoclast differentiation is characterized by the activation of inflammation-related pathways, we speculated that PRMT5 may play a role in this process. In the present study, we found that PRMT5 was upregulated during osteoclast differentiation. Knockdown of PRMT5 with siRNA in bone marrow mononuclear cells (BMMs) resulted in inhibition of receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast formation. Consistent with the PRMT5 knockdown results, the PRMT5 inhibitor EPZ015666 (EPZ) suppressed osteoclast differentiation and bone resorption. Intraperitoneal administration of EPZ prevented ovariectomy-induced bone loss. Moreover, RANKL-induced NF-κB and MAPK activation was inhibited by EPZ. Expression microarrays showed that the expression of several osteoclast formation-related genes was altered by EPZ treatment, including chemokine C-X-C motif ligand 10 (CXCL10). Administration of recombinant CXCL10 partially reversed the osteoclastogenesis inhibition effect of the PRMT5 inhibitor. Intriguingly, RSAD2, which is a reported antiviral protein, was apparently suppressed when PRMT5 was inhibited. Knockdown of RSAD2 with siRNA in BMMs led to inhibition of osteoclast differentiation. Subsequent ChIP-qPCR identified that both PRMT5 inhibition and knockdown resulted in decreased H3R8 or/and H4R3 methylation at CXCL10 and RSAD2 promotors. In conclusion, our study found that PRMT5 is an activator of osteoclast differentiation and inhibition of PRMT5 partially suppressed osteoclastogenesis through downregulation of CXCL10 and RSAD2.

Expression profiling identifies Sertoli and Leydig cell genes as Fsh targets in adult zebrafish testis

Spermatogonial stem cells are quiescent, undergo self-renewal or differentiating divisions, thereby forming the cellular basis of spermatogenesis. This cellular development is orchestrated by follicle-stimulating hormone (FSH), through the production of Sertoli cell-derived factors, and by Leydig cell-released androgens. Here, we investigate the transcriptional events induced by Fsh in a steroid-independent manner on the restart of zebrafish (Danio rerio) spermatogenesis ex vivo, using testis from adult males where type A spermatogonia were enriched by estrogen treatment in vivo. Under these conditions, RNA sequencing preferentially detected differentially expressed genes in somatic/Sertoli cells. Fsh-stimulated spermatogonial proliferation was accompanied by modulating several signaling systems (i.e. Tgf-β, Hedgehog, Wnt and Notch pathways). In silico protein-protein interaction analysis indicated a role for Hedgehog family members potentially integrating signals from different pathways during fish spermatogenesis. Moreover, Fsh had a marked impact on metabolic genes, such as lactate and fatty acid metabolism, or on Sertoli cell barrier components. Fish Leydig cells express the Fsh receptor and one of the most robust Fsh-responsive genes was insulin-like 3 (insl3), a Leydig cell-derived growth factor. Follow-up work showed that recombinant zebrafish Insl3 mediated pro-differentiation effects of Fsh on spermatogonia in an androgen-independent manner. Our experimental approach allowed focusing on testicular somatic genes in zebrafish and showed that the activity of signaling systems known to be relevant in stem cells was modulated by Fsh, providing promising leads for future work, as exemplified by the studies on Insl3.

LIS1 Regulates Osteoclastogenesis through Modulation of M-SCF and RANKL Signaling Pathways and CDC42

We have previously reported that depletion of LIS1, a key regulator of microtubules and cytoplasmic dynein motor complex, in osteoclast precursor cells by shRNAs attenuates osteoclastogenesis in vitro. However, the underlying mechanisms remain unclear. In this study, we show that conditional deletion of LIS1 in osteoclast progenitors in mice led to increased bone mass and decreased osteoclast number on trabecular bone. In vitro mechanistic studies revealed that loss of LIS1 had little effects on cell cycle progression but accelerated apoptosis of osteoclast precursor cells. Furthermore, deletion of LIS1 prevented prolonged activation of ERK by M-CSF and aberrantly enhanced prolonged JNK activation stimulated by RANKL. Finally, lack of LIS1 abrogated M-CSF and RANKL induced CDC42 activation and retroviral transduction of a constitutively active form of CDC42 partially rescued osteoclastogenesis in LIS1-deficient macrophages. Therefore, these data identify a key role of LIS1 in regulation of cell survival of osteoclast progenitors by modulating M-CSF and RANKL induced signaling pathways and CDC42 activation.

RNA-binding protein Musashi2 induced by RANKL is critical for osteoclast survival

The Musashi family of RNA-binding proteins, Musashi1 and Musashi2, regulate self-renewal and differentiation of neuronal and hematopoietic stem cells by modulating protein translation. It has been recently reported that Musashi2, not Musashi1, regulates hematopoietic stem cells. Although osteoclasts are derived from hematopoietic cells, the expression and functions of Musashi proteins in osteoclast lineage cells remain unknown. In this study, we have uncovered that Musashi2 is the predominant isoform of Musashi proteins in osteoclast precursors and its expression is upregulated by receptor activator of NF-κB ligand (RANKL) during osteoclast differentiation. Knocking down the expression of Musashi2 in osteoclast lineage cells by shRNAs attenuates nuclear factor of activated T cells 1 (NFATc1) expression and osteoclast formation in vitro. Mechanistically, loss of Musashi2 inhibits Notch signaling during osteoclast differentiation and induces apoptosis in pre-osteoclasts. In contrast, depletion of Musashi2 has no effects on cell cycle progression and p21^WAF-1 protein expression in macrophages. Furthermore, depletion of Notch2 and its downstream target Hes1 in osteoclast precursors by shRNAs abrogates osteoclastogenesis by inhibiting NFATc1. Finally, absence of Musashi2 in osteoclast precursors promotes apoptosis and inhibits RANKL-induced nuclear factor-κB (NF-κB) activation, which is essential for osteoclast survival, Thus, Musashi2 is required for cell survival and optimal osteoclastogenesis by affecting Notch signaling and NF-κB activation.

Notch signaling promotes osteoclast maturation and resorptive activity

The role of Notch signaling in osteoclast differentiation is controversial with conflicting experimental evidence indicating both stimulatory and inhibitory roles. Differences in experimental protocols and in vivo versus in vitro models may explain the discrepancies between studies. In this study, we investigated cell autonomous roles of Notch signaling in osteoclast differentiation and function by altering Notch signaling during osteoclast differentiation using stimulation with immobilized ligands Jagged1 or Delta-like1 or by suppression with γ-secretase inhibitor DAPT or transcriptional inhibitor SAHM1. Stimulation of Notch signaling in committed osteoclast precursors resulted in larger osteoclasts with a greater number of nuclei and resorptive activity whereas suppression resulted in smaller osteoclasts with fewer nuclei and suppressed resorptive activity. Conversely, stimulation of Notch signaling in osteoclast precursors prior to induction of osteoclastogenesis resulted in fewer osteoclasts. Our data support a mechanism of context-specific Notch signaling effects wherein Notch stimulation inhibits commitment to osteoclast differentiation, but enhances the maturation and function of committed precursors.

Notch Signaling and the Skeleton

Notch 1 to 4 receptors are important determinants of cell fate and function, and Notch signaling plays an important role in skeletal development and bone remodeling. After direct interactions with ligands of the Jagged and Delta-like families, a series of cleavages release the Notch intracellular domain (NICD), which translocates to the nucleus where it induces transcription of Notch target genes. Classic gene targets of Notch are hairy and enhancer of split (Hes) and Hes-related with YRPW motif (Hey). In cells of the osteoblastic lineage, Notch activation inhibits cell differentiation and causes cancellous bone osteopenia because of impaired bone formation. In osteocytes, Notch1 has distinct effects that result in an inhibition of bone resorption secondary to an induction of osteoprotegerin and suppression of sclerostin with a consequent enhancement of Wnt signaling. Notch1 inhibits, whereas Notch2 enhances, osteoclastogenesis and bone resorption. Congenital disorders of loss- and gain-of-Notch function present with severe clinical manifestations, often affecting the skeleton. Enhanced Notch signaling is associated with osteosarcoma, and Notch can influence the invasive potential of carcinoma of the breast and prostate. Notch signaling can be controlled by the use of inhibitors of Notch activation, small peptides that interfere with the formation of a transcriptional complex, or antibodies to the extracellular domain of specific Notch receptors or to Notch ligands. In conclusion, Notch plays a critical role in skeletal development and homeostasis, and serious skeletal disorders can be attributed to alterations in Notch signaling.

Structural basis for the indispensable role of a unique zinc finger motif in LNX2 ubiquitination

LNX (Ligand of Numb Protein-X) proteins, LNX1 and LNX2, are RING- and PDZ-based E3-ubiquitin ligases known to interact with Numb. Silencing of LNX2 has been reported to down-regulate WNT and NOTCH, two key signaling pathways in tumorigenesis. Here we report the identification of the domain boundary of LNX2 to confer its ubiquitination activity, its crystal structure along with functional studies. We show that the RING domain in LNX2 is flanked by two Zinc-binding motifs (Zn-RING-Zn), in which the N-terminal Zinc-binding motif adopts novel conformation. Although this motif follows the typical Cys2His2-type zinc finger configuration, it is devoid of any secondary structure and forms an open circle conformation, which has not been reported yet. This unique N-terminal Zn-finger motif is indispensable for the activity and stability of LNX2, as verified using mutational studies. The Zn-RING-Zn domain of LNX2 is a dimer and assumes a rigid elongated structure that undergoes autoubiquitination and undergoes N-terminal polyubiquitination. The ubiquitin chains consist of all seven possible isopeptide linkages. These results were validated using full-length LNX2. Moreover we have demonstrated the ubiquitination of cell fate determinant protein, Numb by LNX2. Our study provides a structural basis for the functional machinery of LNX2 and thus provides the opportunity to investigate suitable drug targets against LNX2.

Identification of a new locus and validation of previously reported loci showing differential methylation associated with smoking. The REGICOR study

Smoking increases the risk of many diseases and could act through changes in DNA methylation patterns. The aims of this study were to determine the association between smoking and DNA methylation throughout the genome at cytosine-phosphate-guanine (CpG) site level and genomic regions. A discovery cross-sectional epigenome-wide association study nested in the follow-up of the REGICOR cohort was designed and included 645 individuals. Blood DNA methylation was assessed using the Illumina HumanMethylation450 BeadChip. Smoking status was self-reported using a standardized questionnaire. We identified 66 differentially methylated CpG sites associated with smoking, located in 38 genes. In most of these CpG sites, we observed a trend among those quitting smoking to recover methylation levels typical of never smokers. A CpG site located in a novel smoking-associated gene (cg06394460 in LNX2) was hypomethylated in current smokers. Moreover, we validated two previously reported CpG sites (cg05886626 in THBS1, and cg24838345 in MTSS1) for their potential relation to atherosclerosis and cancer diseases, using several different approaches: CpG site methylation, gene expression, and plasma protein level determinations. Smoking was also associated with higher THBS1 gene expression but with lower levels of thrombospondin-1 in plasma. Finally, we identified differential methylation regions in 13 genes and in four non-coding RNAs. In summary, this study replicated previous findings and identified and validated a new CpG site located in LNX2 associated with smoking.