Ameloblastin attenuates RANKL-mediated osteoclastogenesis by suppressing activation of nuclear factor of activated T-cell cytoplasmic 1 (NFATc1)

Early evolution of enamel matrix proteins is reflected by pleiotropy of physiological functions

Highly specialized enamel matrix proteins (EMPs) are predominantly expressed in odontogenic tissues and diverged from common ancestral gene. They are crucial for the maturation of enamel and its extreme complexity in multiple independent lineages. However, divergence of EMPs occured already before the true enamel evolved and their conservancy in toothless species suggests that non-canonical functions are still under natural selection. To elucidate this hypothesis, we carried out an unbiased, comprehensive phenotyping and employed data from the International Mouse Phenotyping Consortium to show functional pleiotropy of amelogenin, ameloblastin, amelotin, and enamelin, genes, i.e. in sensory function, skeletal morphology, cardiovascular function, metabolism, immune system screen, behavior, reproduction, and respiratory function. Mice in all KO mutant lines, i.e. amelogenin KO, ameloblastin KO, amelotin KO, and enamelin KO, as well as mice from the lineage with monomeric form of ameloblastin were affected in multiple physiological systems. Evolutionary conserved motifs and functional pleiotropy support the hypothesis of role of EMPs as general physiological regulators. These findings illustrate how their non-canonical function can still effect the fitness of modern species by an example of influence of amelogenin and ameloblastin on the bone physiology.

A Review on the Molecular Mechanisms of Action of Natural Products in Preventing Bone Diseases

The drugs used for treating bone diseases (BDs), at present, elicit hazardous side effects that include certain types of cancers and strokes, hence the ongoing quest for the discovery of alternatives with little or no side effects. Natural products (NPs), mainly of plant origin, have shown compelling promise in the treatments of BDs, with little or no side effects. However, the paucity in knowledge of the mechanisms behind their activities on bone remodeling has remained a hindrance to NPs’ adoption. This review discusses the pathological development of some BDs, the NP-targeted components, and the actions exerted on bone remodeling signaling pathways (e.g., Receptor Activator of Nuclear Factor κ B-ligand (RANKL)/monocyte/macrophage colony-stimulating factor (M-CSF)/osteoprotegerin (OPG), mitogen-activated protein kinase (MAPK)s/c-Jun N-terminal kinase (JNK)/nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), Kelch-like ECH-associated protein 1 (Keap-1)/nuclear factor erythroid 2–related factor 2 (Nrf2)/Heme Oxygenase-1 (HO-1), Bone Morphogenetic Protein 2 (BMP2)-Wnt/β-catenin, PhosphatidylInositol 3-Kinase (PI3K)/protein kinase B (Akt)/Glycogen Synthase Kinase 3 Beta (GSK3β), and other signaling pathways). Although majority of the studies on the osteoprotective properties of NPs against BDs were conducted ex vivo and mostly on animals, the use of NPs for treating human BDs and the prospects for future development remain promising.

Mechanisms involved in suppression of osteoclast supportive activity by transforming growth factor-β1 via the ubiquitin-proteasome system

Orthodontic treatment requires the regulation of bone remodeling in both compression and tension sides. Transforming growth factor-β1 (TGF-β1) is an important coupling factor for bone remodeling. However, the mechanism underlying the TGF-β1-mediated regulation of the osteoclast-supporting activity of osteoblasts and stromal cells remain unclear. The current study investigated the effect of TGF-β1 on receptor activator of nuclear factor kappa-B ligand (RANKL) expression in stromal cells induced by 1α,25(OH)2D3 (D3) and dexamethasone (Dex). TGF-β1 downregulated the expression of RANKL induced by D3 and Dex in mouse bone marrow stromal lineage, ST2 cells. Co-culture system revealed that TGF-β1 suppressed osteoclast differentiation from bone marrow cell induced by D3 and Dex-activated ST2 cells. The inhibitory effect of TGF-β1 on RANKL expression was recovered by inhibiting the interaction between TGF-β1 and the TGF-β type I/activin receptor or by downregulating of smad2/3 expression. Interestingly, TGF-β1 degraded the retinoid X receptor (RXR)-α protein which forms a complex with vitamin D receptor (VDR) and regulates transcriptional activity of RANKL without affecting nuclear translocation of VDR and phosphorylation of signal transducer and activator of transcription3 (STAT3). The degradation of RXR-α protein by TGF-β1 was recovered by a ubiquitin-proteasome inhibitor. We also observed that poly-ubiquitination of RXR-α protein was induced by TGF-β1 treatment. These results indicated that TGF-β1 downregulates RANKL expression and the osteoclast-supporting activity of osteoblasts/stromal cells induced by D3 and Dex through the degradation of the RXR-α protein mediated by ubiquitin-proteasome system.

Driving β2- While Suppressing α-Adrenergic Receptor Activity Suppresses Joint Pathology in Inflammatory Arthritis

Objective

Hypersympathetic activity is prominent in rheumatoid arthritis, and major life stressors precede onset in ~80% of patients. These findings and others support a link between stress, the sympathetic nervous system and disease onset and progression. Here, we extend previous research by evaluating how selective peripherally acting α/β2-adrenergic drugs affect joint destruction in adjuvant-induced arthritis.

Methods

Complete Freund's adjuvant induced inflammatory arthritis in male Lewis rats. Controls received no treatment. Arthritic rats then received vehicle or twice-daily treatment with the α-adrenergic antagonist, phentolamine (0.5 mg/day) and the β2-adrenergic agonist, terbutaline (1200 µg/day, collectively named SH1293) from day (D) of disease onset (D12) through acute (D21) and severe disease (D28). Disease progression was assessed in the hind limbs using dorsoplantar widths, X-ray analysis, micro-computed tomography, and routine histology on D14, D21, and D28 post-immunization.

Results

On D21, SH1293 significantly attenuated arthritis in the hind limbs, based on reduced lymphocytic infiltration, preservation of cartilage, and bone volume. Pannus formation and sympathetic nerve loss were not affected by SH1293. Bone area and osteoclast number revealed high- and low-treatment-responding groups. In high-responding rats, treatment with SH1293 significantly preserved bone area and decreased osteoclast number, data that correlated with drug-mediated joint preservation. SH1293 suppressed abnormal bone formation based on reduced production of osteophytes. On D28, the arthritic sparing effects of SH1293 on lymphocytic infiltration, cartilage and bone sparing were maintained at the expense of bone marrow adipocity. However, sympathetic nerves were retracted from the talocrural joint.

Conclusion and Significance

Our findings support a significant delay in early arthritis progression by treatment with SH1293. Targeting sympathetic neurotransmission may provide a strategy to slow disease progression.

Dectin-1-mediated suppression of RANKL-induced osteoclastogenesis by glucan from baker's yeast

Immunoreceptors expressed on osteoclast precursor cells modify osteoclast differentiation and bone resorption activity. Dectin-1 is a lectin receptor of β-glucan and is specifically expressed in osteoclast precursor cells. In this study, we evaluated the bioactivity of β-glucan on receptor activator of nuclear factor-kappa B ligand (RANKL)-induced osteoclastogenesis and observed that glucan from baker's yeast inhibited this process in mouse bone marrow cells and dectin-1-overexpressing RAW264.7 (d-RAW) cells. In conjunction, RANKL-induced nuclear factor of activated T cell c1 expression was suppressed, subsequently downregulating TRAP and Oc-stamp. Additionally, nuclear factor-kappa B activation and the expression of c-fos and Blimp1 were reduced in d-RAW cells. Furthermore, glucan from baker's yeast induced the degradation of Syk protein, essential factor for osteoclastogenesis. These results suggest that glucan from baker's yeast suppresses RANKL-induced osteoclastogenesis and can be applied as a new treatment strategy for bone-related diseases.

Pathogenic Mechanisms of Myeloma Bone Disease and Possible Roles for NRF2

Osteolytic bone lesions are one of the central features of multiple myeloma (MM) and lead to bone pain, fractures, decreased quality of life, and decreased survival. Dysfunction of the osteoclast (OC)/osteoblast (OB) axis plays a key role in the development of myeloma-associated osteolytic lesions. Many signaling pathways and factors are associated with myeloma bone diseases (MBDs), including the RANKL/OPG and NF-κB pathways. NRF2, a master regulator of inflammatory signaling, might play a role in the regulation of bone metabolism via anti-inflammatory signaling and decreased reactive oxygen species (ROS) levels. The loss of NRF2 expression in OCs reduced bone mass via the RANK/RANKL pathway and other downstream signaling pathways that affect osteoclastogenesis. The NRF2 level in OBs could interfere with interleukin (IL)-6 expression, which is associated with bone metabolism and myeloma cells. In addition to direct impact on OCs and OBs, the activity of NRF2 on myeloma cells and mesenchymal stromal cells influences the inflammatory stress/ROS level in these cells, which has an impact on OCs, OBs, and osteocytes. The interaction between these cells and OCs affects the osteoclastogenesis of myeloma bone lesions associated with NRF2. Therefore, we have reviewed the effects of NRF2 on OCs and OBs in MBDs.

CaMKII(δ) regulates osteoclastogenesis through ERK, JNK, and p38 MAPKs and CREB signalling pathway

Calcium/calmodulin-dependent protein kinases (CaMKs) are a group of important molecules mediating calcium signal transmission and have been proved to participate in osteoclastogenesis regulation. CaMKII, a subtype of CaMKs is expressed during osteoclast differentiation, but its role in osteoclastogenesis regulation remains controversial. In the present study, we identified that both mRNA and protein levels of CaMKII (δ) were upregulated in a time-dependent manner during osteoclast differentiation. CaMKII (δ) gene silencing significantly inhibited osteoclast formation, bone resorption, and expression of osteoclast-related genes, including nuclear factor of activated T cells c1 (NFATc1), tartrate-resistant acid phosphatase (TRAP), and c-Src. Furthermore, CaMKII (δ) gene silencing downregulated phosphorylation of mitogen-activated protein kinases (MAPKs), including JNK, ERK, and p38, which were transiently activated by RANKL. Specific inhibitors of ERK, JNK, and p38 also markedly inhibited expression of osteoclast-related genes, osteoclast formation, and bone resorption like CaMKII (δ) gene silencing. Additionally, CaMKII (δ) gene silencing also suppressed RANKL-triggered CREB phosphorylation. Collectively, these data demonstrate the important role of CaMKII (δ) in osteoclastogenesis regulation through JNK, ERK, and p38 MAPKs and CREB pathway.

New missense variants in RELT causing hypomineralised amelogenesis imperfecta

Amelogenesis imperfecta (AI) is a heterogeneous group of genetic diseases characterised by dental enamel malformation. Pathogenic variants in at least 33 genes cause syndromic or non-syndromic AI. Recently variants in RELT, encoding an orphan receptor in the tumour necrosis factor (TNF) superfamily, were found to cause recessive AI, as part of a syndrome encompassing small stature and severe childhood infections. Here we describe four additional families with autosomal recessive hypomineralised AI due to previously unreported homozygous mutations in RELT. Three families carried a homozygous missense variant in the fourth exon (c.164C>T, p.(T55I)) and a fourth family carried a homozygous missense variant in the 11th exon (c.1264C>T, p.(R422W)). We found no evidence of additional syndromic symptoms in affected individuals. Analyses of tooth microstructure with computerised tomography and scanning electron microscopy suggest a role for RELT in ameloblasts' coordination and interaction with the enamel matrix. Microsatellite genotyping in families segregating the T55I variant reveals a shared founder haplotype. These findings extend the RELT pathogenic variant spectrum, reveal a founder mutation in the UK Pakistani population and provide detailed analysis of human teeth affected by this hypomineralised phenotype, but do not support a possible syndromic presentation in all those with RELT-variant associated AI.

Creation of RANKL mutants with low affinity for decoy receptor OPG and their potential anti-fibrosis activity

Fibrosis is characterized by the progressive alteration of the tissue structure due to the excessive production of extracellular matrix (ECM). The signaling system encompassing Receptor Activator of Nuclear factor NF‐κB Ligand (RANKL)/RANK/Osteoprotegerin (OPG) was discovered to play an important role in the regulation of ECM formation and degradation in bone tissue. However, whether and how this signaling pathway plays a role in liver or pulmonary ECM degradation is unclear up to now. Interestingly, increased decoy receptor OPG levels are found in fibrotic tissues. We hypothesize that RANKL can stimulate RANK on macrophages and initiate the process of ECM degradation. This process may be inhibited by highly expressed OPG in fibrotic conditions. In this case, RANKL mutants that can bind to RANK without binding to OPG might become promising therapeutic candidates. In this study, we built a structure‐based library containing 44 RANKL mutants and found that the Q236 residue of RANKL is important for OPG binding. We show that RANKL_Q236D can activate RAW cells to initiate the process of ECM degradation and is able to escape from the obstruction by exogenous OPG. We propose that the generation of RANKL mutants with reduced affinity for OPG is a promising strategy for the exploration of new therapeutics against fibrosis.

Accumulation of hyaluronic acid in stromal cells modulates osteoclast formation by regulation of receptor activator of nuclear factor kappa-B ligand expression

Hyaluronic acid (HA) has a pivotal role in bone and cartilage metabolism. In this study, we investigated the effect and underlying mechanisms of HA accumulation on the expression of receptor activator of nuclear factor kappa-B ligand (RANKL) induced by 1α,25(OH)₂D₃ and dexamethasone in stromal cells, which support osteoclastogenesis. Degradation of HA by hyaluronidase (HA'ase) treatment enhanced the expression of RANKL in ST2 cells stimulated with 1α,25(OH)₂D₃ and dexamethasone. Down-regulation of hyaluronan synthase 2 (HAS2) expression by siRNA also stimulated RANKL expression induced by 1α,25(OH)₂D₃ and dexamethasone. Results from a cell co-culture system with bone marrow cell showed that 1α,25(OH)₂D₃ and dexamethasone-induced RANKL expression in HA'ase treated- and HAS2 siRNA transfected-ST2 cells was down-regulated by treatment of cells with high molecular weight HA. In contrast, transforming growth factor-β1 (TGF-β1), which stimulates HAS2 expression and HA synthesis, down-regulated RANKL expression induced by 1α,25(OH)₂D₃ and dexamethasone. Interestingly, knockdown of has2 gene enhanced the expression of vitamin D receptor (VDR) and phosphorylation of signal transducers and activator of transcription 3 (STAT3) in ST2 cells stimulated by 1α,25(OH)₂D₃ and dexamethasone. These results indicate that accumulation of HA in bone marrow cells may affect RANKL-mediated osteoclast-supporting activity via regulation of VDR and STAT3 signaling pathways.

Roles of Cyclic AMP Response Element Binding Activation in the ERK1/2 and p38 MAPK Signalling Pathway in Central Nervous System, Cardiovascular System, Osteoclast Differentiation and Mucin and Cytokine Production

There are many downstream targets of mitogen-activated protein kinase (MAPK) signalling that are involved in neuronal development, cellular differentiation, cell migration, cancer, cardiovascular dysfunction and inflammation via their functions in promoting apoptosis and cell motility and regulating various cytokines. It has been reported that cyclic AMP response element-binding protein (CREB) is phosphorylated and activated by cyclic AMP signalling and calcium/calmodulin kinase. Recent evidence also points to CREB phosphorylation by the MAPK signalling pathway. However, the specific roles of CREB phosphorylation in MAPK signalling have not yet been reviewed in detail. Here, we describe the recent advances in the study of this MAPK-CREB signalling axis in human diseases. Overall, the crosstalk between extracellular signal-related kinase (ERK) 1/2 and p38 MAPK signalling has been shown to regulate various physiological functions, including central nervous system, cardiac fibrosis, alcoholic cardiac fibrosis, osteoclast differentiation, mucin production in the airway, vascular smooth muscle cell migration, steroidogenesis and asthmatic inflammation. In this review, we focus on ERK1/2 and/or p38 MAPK-dependent CREB activation associated with various diseases to provide insights for basic and clinical researchers.

Vitexin suppresses RANKL-induced osteoclastogenesis and prevents lipopolysaccharide (LPS)-induced osteolysis

Osteolytic diseases are characterized by an increase in the number and/or activity of bone-resorbing osteoclasts. Identification of natural compounds that can suppress osteoclast formation and function is crucial for the prevention and treatment of osteolytic diseases. Vitexin, a naturally-derived flavonoid extracted from various medicinal plant species, demonstrates a broad range of pharmacological properties including anticancer and anti-inflammatory effects. Here in this study, we showed that vitexin exerts antiosteoclastogenic effects by directly inhibiting receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast formation and bone resorption in vitro and protected against lipopolysaccharide (LPS)-induced inflammatory osteolysis in vivo. Vitexin suppressed the early activation of ERK and p38 MAPK pathways in response to RANKL thereby attenuating the downstream induction of c-Fos and NFATc1, and abrogating the expression of osteoclast marker genes. Collectively, these results provide evidence for the therapeutic application of vitexin in the treatment of osteoclast-mediated bone lytic diseases.