Interleukin-1β promotes the LC3-mediated secretory function of osteoclast precursors by stimulating the Ca2+-dependent activation of ERK

Hecogenin alleviates LPS-induced osteolysis via regulating pyroptosis and ROS involved Nrf2 activation

Reactive oxidative species (ROS) generation triggers pyroptosis and induces development of inflammatory osteolysis. Hecogenin (HG) has anti-inflammatory and antioxidative property, but its effects on inflammatory osteolysis remains unclear. In our study, we investigated the mechanism of HG on pyroptosis and its effect on inflammatory osteolysis in vitro and in vivo. The impact of HG on osteoclastogenesis was evaluated using cytotoxicity, TRAcP staining and bone resorption assays. The RNA-sequencing was employed to identify potential signaling pathways, and then RT-qPCR, western blot, immunofluorescence, and ELISA were used to verify. To determine the protective effect of HG in vivo, Lipopolysaccharide (LPS)-induced animal models were utilized, along with micro-CT and histological examination. HG suppressed RANKL-induced osteoclast differentiation, bone resorption, NFATc1 activity and downstream factors. RNA-sequencing results showed that HG inhibited osteoclastogenesis by modulating the inflammatory response and macrophage polarization. Furthermore, HG inhibited the NF-κB pathway, and deactivated the NLRP3 inflammasome. HG activated the expression of nuclear factor E2-related factor 2 (Nrf2) to eliminate ROS generation. Importantly, the inhibitory effect of HG on NLRP3 inflammasome could be reversed by treatment with the Nrf2 inhibitor ML385. In vivo, HG prevented the mice against LPS-induced osteolysis by suppressing osteoclastogenesis and inflammatory factors. In conclusion, HG could activate Nrf2 to eliminate ROS generation, inactivate NLRP3 inflammasome and inhibit pyroptosis, thereby suppressing osteoclastogenesis in vitro and alleviating inflammatory osteolysis in vivo, which indicating that HG might be a promising candidate to treat inflammatory osteolysis.

Extracellular Signal-Regulated Kinases Play Essential but Contrasting Roles in Osteoclast Differentiation

Bone homeostasis is regulated by the balanced actions of osteoblasts that form the bone and osteoclasts (OCs) that resorb the bone. Bone-resorbing OCs are differentiated from hematopoietic monocyte/macrophage lineage cells, whereas osteoblasts are derived from mesenchymal progenitors. OC differentiation is induced by two key cytokines, macrophage colony-stimulating factor (M-CSF), a factor essential for the proliferation and survival of the OCs, and receptor activator of nuclear factor kappa-B ligand (RANKL), a factor for responsible for the differentiation of the OCs. Mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinases (ERKs), p38, and c-Jun N-terminal kinases, play an essential role in regulating the proliferation, differentiation, and function of OCs. ERKs have been known to play a critical role in the differentiation and activation of OCs. In most cases, ERKs positively regulate OC differentiation and function. However, several reports present conflicting conclusions. Interestingly, the inhibition of OC differentiation by ERK1/2 is observed only in OCs differentiated from RAW 264.7 cells. Therefore, in this review, we summarize the current understanding of the conflicting actions of ERK1/2 in OC differentiation.

Dysfunction in parkin aggravates inflammatory bone erosion by reinforcing osteoclast activity

BACKGROUND

Parkin dysfunction associated with the progression of parkinsonism contributes to a progressive systemic skeletal disease characterized by low bone mineral density. However, the role of parkin in bone remodeling has not yet been elucidated in detail.

RESULT

We observed that decreased parkin in monocytes is linked to osteoclastic bone-resorbing activity. siRNA-mediated knockdown of parkin significantly enhanced the bone-resorbing activity of osteoclasts (OCs) on dentin without any changes in osteoblast differentiation. Moreover, Parkin-deficient mice exhibited an osteoporotic phenotype with a lower bone volume accompanied by increased OC-mediated bone-resorbing capacity displaying increased acetylation of α-tubulin compared to wild-type (WT) mice. Notably, compared to WT mice, the Parkin-deficient mice displayed increased susceptibility to inflammatory arthritis, reflected by a higher arthritis score and a marked bone loss after arthritis induction using K/BxN serum transfer, but not ovariectomy-induced bone loss. Intriguingly, parkin colocalized with microtubules and parkin-depleted-osteoclast precursor cells (Parkin^-/- OCPs) displayed augmented ERK-dependent acetylation of α-tubulin due to failure of interaction with histone deacetylase 6 (HDAC6), which was promoted by IL-1β signaling. The ectopic expression of parkin in Parkin^-/- OCPs limited the increase in dentin resorption induced by IL-1β, accompanied by the reduced acetylation of α-tubulin and diminished cathepsin K activity.

CONCLUSION

These results indicate that a deficiency in the function of parkin caused by a decrease in parkin expression in OCPs under the inflammatory condition may enhance inflammatory bone erosion by altering microtubule dynamics to maintain OC activity.

STAT3/Mitophagy Axis Coordinates Macrophage NLRP3 Inflammasome Activation and Inflammatory Bone Loss

Signal transducer and activator of transcription 3 (STAT3), a cytokine-responsive transcription factor, is known to play a role in immunity and bone remodeling. However, whether and how STAT3 impacts macrophage NLR family pyrin domain containing 3 (NLRP3) inflammasome activation associated with inflammatory bone loss remains unknown. Here, STAT3 signaling is hyperactivated in macrophages in the context of both non-sterile and sterile inflammatory osteolysis, and this was highly correlated with the cleaved interleukin-1β (IL-1β) expression pattern. Strikingly, pharmacological inhibition of STAT3 markedly blocks macrophage NLRP3 inflammasome activation in vitro, thereby relieving inflammatory macrophage-amplified osteoclast formation and bone-resorptive activity. Mechanistically, STAT3 inhibition in macrophages triggers PTEN-induced kinase 1 (PINK1)-dependent mitophagy that eliminates dysfunctional mitochondria, reverses mitochondrial membrane potential collapse, and inhibits mitochondrial reactive oxygen species release, thus inactivating the NLRP3 inflammasome. In vivo, STAT3 inhibition effectively protects mice from both infection-induced periapical lesions and aseptic titanium particle-mediated calvarial bone erosion with potent induction of PINK1 and downregulation of inflammasome activation, macrophage infiltration, and osteoclast formation. This study reveals the regulatory role of the STAT3/mitophagy axis at the osteo-immune interface and highlights a potential therapeutic intervention to prevent inflammatory bone loss. © 2022 American Society for Bone and Mineral Research (ASBMR).

Cysteine cathepsins: A long and winding road towards clinics

Biomedical research often focuses on properties that differentiate between diseased and healthy tissue; one of the current focuses is elevated expression and altered localisation of proteases. Among these proteases, dysregulation of cysteine cathepsins can frequently be observed in inflammation-associated diseases, which tips the functional balance from normal physiological to pathological manifestations. Their overexpression and secretion regularly exhibit a strong correlation with the development and progression of such diseases, making them attractive pharmacological targets. But beyond their mostly detrimental role in inflammation-associated diseases, cysteine cathepsins are physiologically highly important enzymes involved in various biological processes crucial for maintaining homeostasis and responding to different stimuli. Consequently, several challenges have emerged during the efforts made to translate basic research data into clinical applications. In this review, we present both physiological and pathological roles of cysteine cathepsins and discuss the clinical potential of cysteine cathepsin-targeting strategies for disease management and diagnosis.

A Review of Signaling Transduction Mechanisms in Osteoclastogenesis Regulation by Autophagy, Inflammation, and Immunity

Osteoclastogenesis is an ongoing rigorous course that includes osteoclast precursors fusion and bone resorption executed by degradative enzymes. Osteoclastogenesis is controlled by endogenous signaling and/or regulators or affected by exogenous conditions and can also be controlled both internally and externally. More evidence indicates that autophagy, inflammation, and immunity are closely related to osteoclastogenesis and involve multiple intracellular organelles (e.g., lysosomes and autophagosomes) and certain inflammatory or immunological factors. Based on the literature on osteoclastogenesis induced by different regulatory aspects, emerging basic cross-studies have reported the emerging disquisitive orientation for osteoclast differentiation and function. In this review, we summarize the partial potential therapeutic targets for osteoclast differentiation and function, including the signaling pathways and various cellular processes.

FAM20C plays a critical role in the development of mouse vertebra

BACKGROUND CONTEXT

Family with sequence similarity 20-member C (FAM20C) is a protein kinase that is responsible for the phosphorylation of many secretory proteins; however, its roles in spine or vertebra development have not be studied.

PURPOSE

The aim of this investigation is to analyze the roles of FAM20C in vertebra development.

STUDY DESIGN/SETTING

A mouse study of the Fam20c gene using conditional knockout to assess the effects of its inactivation on vertebra development.

METHODS

By breeding Sox2-Cre mice with Fam20c^flox/flox mice, Sox2-Cre;Fam20c^flox/flox mice (abbreviated as cKO mice) are created. X-ray radiography, resin-casted scanning electron microscopy, Hematoxylin and Eosin staining, safranin O staining, Goldner's Masson trichrome staining, Von Kossa staining, tartrate-resistant alkaline phosphatase staining, immunohistochemistry staining, Western Immunoblotting and real-time PCR were employed to characterize the vertebrae of cKO mice compared to the normal control mice.

RESULTS

Inactivation of Fam20c in mice results in remarkable spine deformity, severe morphology and mineralization defects, altered levels of osteoblast differentiation markers, reduction of activity of the Wnt/β-catenin signaling pathway and reduced level of osteoclastogenesis in the vertebrae.

CONCLUSIONS

FAM20C plays an essential role in vertebral development; it may regulate vertebral formation through the Wnt/β-catenin signaling pathway.

CLINICAL SIGNIFICANCE

Mutations in the human FAM20C gene are associated with Raine syndrome. The findings of this study provide valuable clues for the clinical management of Raine syndrome regarding spine manifestations in patients.

DNA damage-inducible transcript 3 restrains osteoclast differentiation and function

DNA damage-inducible transcript 3 (DDIT3), a member of the CCAAT/enhancer-binding protein (C/EBP) family, is involved in cellular apoptosis and differentiation. DDIT3 participates in the regulation of adipogenesis and osteogenesis in vitro and in vivo. However, the role of DDIT3 in osteoclastogenesis is not yet known. In this study, the involvement of DDIT3 in osteoclast differentiation and function was reported for the first time. CRISPR/Cas9-mediated DDIT3 knockout (KO) mice were generated for functional assessment. Tartrate-resistant acid phosphatase (TRAP) staining of distal femurs showed increased positive cells in DDIT3 KO mice. DDIT3 expression was downregulated during the receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclast differentiation of bone marrow-derived macrophages (BMMs). The loss of DDIT3 increased the expression of osteoclast-specific markers, including nuclear factor of activated T-cells cytoplasmic 1 (NFATc1), TRAP, cathepsin K (CTSK), and dendritic cell-specific transmembrane protein (DC-STAMP) and promoted the formation of TRAP-positive multinucleated osteoclasts. The actin ring number and resorption area of bone slices were also increased in DDIT3 KO BMMs. Lentivirus-mediated DDIT3 overexpression significantly inhibited the osteoclast differentiation of RAW264.7 cells. In the tumor necrosis factor-α-induced osteolysis model, DDIT3 deficiency enhanced osteoclast formation and aggravated bone resorption. DDIT3 inhibited osteoclast differentiation by regulating the C/EBPα-CTSK axis. Furthermore, DDIT3 KO intensified the RANKL-triggered activation of the MAPKs and Akt signaling pathways. Taken together, the results revealed the essential role of DDIT3 in osteoclastogenesis in vitro and in vivo and its close relationship with osteoclast-associated transcription factors and pathways.

Effects of Probiotic Culture Supernatant on Cariogenic Biofilm Formation and RANKL-Induced Osteoclastogenesis in RAW 264.7 Macrophages

Postbiotics are a promising functional ingredient that can overcome the limitations of viability and storage stability that challenge the production of probiotics. To evaluate the effects of postbiotics on oral health, eight spent culture supernatants (SCSs) of probiotics were prepared, and the effects of SCSs on Streptococcus mutans-induced cariogenic biofilm formation and the receptor activator of the nuclear factor κB ligand (RANKL)-induced osteoclastogenesis were evaluated in RAW 264.7 macrophages. SCS of Lactobacillus salivarius MG4265 reduced S. mutans-induced biofilm formation by 73% and significantly inhibited tartrate-resistant acid phosphatase (TRAP) activity, which is a biomarker of mature osteoclasts in RAW 264.7 macrophages. The suppression of RANKL-induced activation of mitogen activated the protein kinases (c-Jun N-terminal kinase, extracellular signal-regulated kinase, and p38) and nuclear factor κB pathways, as well as the upregulation of heme oxygenase-1 expression. The suppression of RANK-L-induced activation of mitogen also inhibited the expression of transcriptional factors (c-fos and nuclear factor of activated T cells cytoplasmic 1) and, subsequently, osteoclastogenesis-related gene expression (tartrate-resistant acid phosphatase-positive (TRAP), cathepsin K, and matrix metalloproteinase-9).Therefore, SCS of L. salivarius MG4265 has great potential as a multifunctional oral health ingredient that inhibits biofilm formation and suppresses the alveolar bone loss that is associated with periodontitis.

Autophagy and its significance in periodontal disease

Autophagy is an evolutionarily conserved process essential for cellular homeostasis and human health. As a lysosome-dependent degradation pathway, autophagy acts as a modulator of the pathogenesis of diverse diseases. The relationship between autophagy and oral diseases has been explored in recent years, and there is increasing interest in the role of autophagy in periodontal disease. Periodontal disease is a prevalent chronic inflammatory disorder characterized by the destruction of periodontal tissues. It is initiated through pathogenic bacterial infection and interacts with the host immune defense, leading to inflammation and alveolar bone resorption. In this review, we outline the machinery of autophagy and present an overview of work on the significance of autophagy in regulating pathogen invasion, the immune response, inflammation, and alveolar bone homeostasis of periodontal disease. Existing data provide support for the importance of autophagy as a multi-dimensional regulator in the pathogenesis of periodontal disease and demonstrate the importance of future research on the potential roles of autophagy in periodontal disease.

Nepetin inhibits osteoclastogenesis by inhibiting RANKL-induced activation of NF-κB and MAPK signalling pathway, and autophagy

Aseptic prosthetic loosening due to wear particle-induced inflammatory osteolysis is the main cause of failure for artificial joint replacement. The inflammatory response and the production of pro-osteoclastic factors lead to elevation of osteoclast formation and excessive activity results in extensive bone destruction around the bone-implant interface. Here we showed that Nepetin, a natural bioactive flavonoid with proven anti-inflammatory and anti-proliferative properties, potently inhibited RANKL-induced osteoclast differentiation, formation and bone resorption in vitro, and protected mice against the deleterious effects of titanium particle-induced calvarial osteolysis in vivo. Mechanistically, Nepetin attenuated RANKL-induced activation of NF-κB and MAPK signalling pathways and TRAF6-dependent ubiquitination of Beclin 1 which is necessary for the induction of autophagy. In brief, our study demonstrates the potential therapeutic application of Nepetin against osteoclast-mediated osteolytic diseases.

Interleukin‑17A facilitates osteoclast differentiation and bone resorption via activation of autophagy in mouse bone marrow macrophages

Interleukin 17A (IL-17A) exerts pleiotropic effects on periodontitis, partially through enhancement of alveolar bone loss. Osteoclasts are the main culprits that absorb alveolar bone. However, studies describing the correlation between IL-17A and osteoclasts are not conclusive. Previously, autophagy was revealed to be involved in osteoclast differentiation and bone resorption. However, the role of autophagy in IL-17A-mediated osteoclast formation is yet to be clarified. In the present study, bone marrow macrophages (BMMs) were treated with or without IL-17A. 3-Methyladenine (3-MA) was applied to inhibit autophagy. Osteoclast formation was detected by tartrate-resistant acid phosphatase (TRAP) staining, immunofluorescence, and scanning electron microscope. The effects of IL-17A on osteoclast-specific genes and autophagy-related genes during osteoclast differentiation were examined by real-time quantitative polymerase chain reaction and western blot analysis. Autophagosomes were observed by transmission electron microscope. Hematoxylin and eosin (H&E), and TRAP staining was adopted to assess alveolar bone destruction and the number of osteoclasts, respectively in a rat periodontitis model. Consequently, IL-17A stimulated osteoclast differentiation and bone resorption of BMMs accompanied by an increase in the mRNA expression of osteoclast-specific genes. Furthermore, IL-17A increased the levels of autophagy-related genes and proteins, and inhibition of autophagy with 3-MA attenuated the IL-17A-mediated osteoclastogenesis. In addition, there was an increase in the number of osteoclasts and alveolar bone resorption with IL-17A treatment in the periodontitis rat model. Collectively, these findings indicated that IL-17A facilitated osteoclast differentiation and bone resorption in vitro and in vivo, which may contribute to the understanding of the molecular basis of IL-17A in alveolar bone destruction and provide insight on the clinical therapeutic targets for periodontitis.

The Autophagy in Osteoimmonology: Self-Eating, Maintenance, and Beyond

It has been long realized that the immune and skeletal systems are closely linked. This crosstalk, also known as osteoimmunology, is a primary process required for bone health. For example, the immune system acts as a key regulator in osteoclasts-osteoblasts coupling to maintain the balanced bone remodeling. Osteoimmunology is achieved through many cellular and molecular processes, among which autophagy has recently been found to play an indispensable role. Autophagy is a highly conserved process in eukaryotic cells, by which the cytoplasm components such as dysfunctional organelles are degraded through lysosomes and then returned to the cytosol for reuse. Autophagy is present in all cells at basal levels to maintain homeostasis and to promote cell survival in response to cellular stress conditions such as nutrition deprivation and hypoxia. Autophagy is a required process in immune cell activation/polarization and osteoclast differentiation, which protecting cells from oxidative stress. The essential of autophagy in osteogenesis is its involvement in osteoblast differentiation and mineralization, especially the role of autophagosome in extracellular calcium transportation. The modulatory feature of autophagy in both immune and skeleton systems suggests its crucial roles in osteoimmunology. Furthermore, autophagy also participates in the maintenance of bone marrow hematopoietic stem cell niche. The focus of this review is to highlight the role of autophagy in the immune-skeleton interactions and the effects on bone physiology, as well as the future application in translational research.

Prostate transmembrane protein androgen induced 1 is induced by activation of osteoclasts and regulates bone resorption

Osteoclasts derived from hematopoietic cells are activated on bone surface. To resorb bone, osteoclasts release acid and lysosome acid hydrolase via membrane transport. Prostate transmembrane protein androgen induced 1 (Pmepa1) is a type I transmembrane protein that regulates proliferation, migration, and metastasis of cancer cells. Because recent reports showed that Pmepa1 is involved in membrane transport in cancer cells, we investigated the role of Pmepa1 in osteoclast function. Pmepa1 expression was barely detected in osteoclasts formed on plastic surfaces in vitro, but was markedly increased in activated osteoclasts formed on calcified matrix. Inhibitors of bone resorption, such as alendronate, bafilomycin A1, and the PI3K inhibitor LY294002, suppressed the expression of Pmepa1 in osteoclasts. Knockdown of Pmepa1 expression impaired bone resorption activity and inhibited formation of a ring-like, actin-rich podosome belt that is essential for osteoclast function. Pmepa1 protein localized to lysosomes in osteoclasts. In addition, in sites of bone destruction observed in rats with adjuvant-induced arthritis, a marked high level of Pmepa1 expression was associated with the osteoclasts' resorbing bone. Our results suggest that Pmepa1 is a critical regulator of bone resorption and is a promising marker for activated osteoclasts and a potential therapeutic target in pathologic bone destruction.-Xu, X., Hirata, H., Shiraki, M., Kamohara, A., Nishioka, K., Miyamoto, H., Kukita, T., Kukita, A. Prostate transmembrane protein androgen induced 1 is induced by activation of osteoclasts and regulates bone resorption.

Roles of Mitogen-Activated Protein Kinases in Osteoclast Biology

Bone undergoes continuous remodeling, which is homeostatically regulated by concerted communication between bone-forming osteoblasts and bone-degrading osteoclasts. Multinucleated giant osteoclasts are the only specialized cells that degrade or resorb the organic and inorganic bone components. They secrete proteases (e.g., cathepsin K) that degrade the organic collagenous matrix and establish localized acidosis at the bone-resorbing site through proton-pumping to facilitate the dissolution of inorganic mineral. Osteoporosis, the most common bone disease, is caused by excessive bone resorption, highlighting the crucial role of osteoclasts in intact bone remodeling. Signaling mediated by mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38, has been recognized to be critical for normal osteoclast differentiation and activation. Various exogenous (e.g., toll-like receptor agonists) and endogenous (e.g., growth factors and inflammatory cytokines) stimuli contribute to determining whether MAPKs positively or negatively regulate osteoclast adhesion, migration, fusion and survival, and osteoclastic bone resorption. In this review, we delineate the unique roles of MAPKs in osteoclast metabolism and provide an overview of the upstream regulators that activate or inhibit MAPKs and their downstream targets. Furthermore, we discuss the current knowledge about the differential kinetics of ERK, JNK, and p38, and the crosstalk between MAPKs in osteoclast metabolism.

IL-1β Induces Pathologically Activated Osteoclasts Bearing Extremely High Levels of Resorbing Activity: A Possible Pathological Subpopulation of Osteoclasts, Accompanied by Suppressed Expression of Kindlin-3 and Talin-1

As osteoclasts have the central roles in normal bone remodeling, it is ideal to regulate only the osteoclasts performing pathological bone destruction without affecting normal osteoclasts. Based on a hypothesis that pathological osteoclasts form under the pathological microenvironment of the bone tissues, we here set up optimum culture conditions to examine the entity of pathologically activated osteoclasts (PAOCs). Through searching various inflammatory cytokines and their combinations, we found the highest resorbing activity of osteoclasts when osteoclasts were formed in the presence of M-CSF, receptor activator of NF-κB ligand, and IL-1β. We have postulated that these osteoclasts are PAOCs. Analysis using confocal laser microscopy revealed that PAOCs showed extremely high proton secretion detected by the acid-sensitive fluorescence probe Rh-PM and bone resorption activity compared with normal osteoclasts. PAOCs showed unique morphology bearing high thickness and high motility with motile cellular processes in comparison with normal osteoclasts. We further examined the expression of Kindlin-3 and Talin-1, essential molecules for activating integrin β-chains. Although normal osteoclasts express high levels of Kindlin-3 and Talin-1, expression of these molecules was markedly suppressed in PAOCs, suggesting the abnormality in the adhesion property. When whole membrane surface of mature osteoclasts was biotinylated and analyzed, the IL-1β-induced cell surface protein was detected. PAOCs could form a subpopulation of osteoclasts possibly different from normal osteoclasts. PAOC-specific molecules could be an ideal target for regulating pathological bone destruction.

Deletion of Opg Leads to Increased Neovascularization and Expression of Inflammatory Cytokines in the Lumbar Intervertebral Disc of Mice

STUDY DESIGN

Neovascularization and expression of inflammatory cytokines were examined in Osteoprotegerin (Opg) knockout (KO) mice that show intervertebral disc (IVD) degeneration.

OBJECTIVE

The aim of this study was to clarify the pathological changes in lumbar IVD degeneration in Opg KO mice.

SUMMARY OF BACKGROUND DATA

Osteoporosis is a controversial risk factor for IVD degeneration. Deletion of Opg resulted in IVD degeneration in mice. Neovascularization and inflammatory cytokines are key factors in IVD degeneration.

METHODS

Opg KO mice and their wild-type (WT) littermates were euthanized. Lumbar IVDs were harvested. Safranin O/Fast Green staining was performed to examine the pathological changes. Microcomputed tomographic (micro-CT) analysis was performed to determine the structural changes at the junction of lumbar IVD cartilage and vertebrae. Tartrate-resistant acid phosphatase (TRAP) staining was performed to evaluate osteoclast formation. Protein expression of vascular endothelial growth factor A (VEGF-A), CD31, VE-cadherin, CD 34, interleukin-1β (IL-1β), and tumor necrosis factors α (TNF-α) were analyzed by immunohistochemistry (IHC) assays. Gene expressions of IL-1β, IL-6, and TNF-α were analyzed by real-time polymerase chain reaction (RT-PCR).

RESULTS

In 12-week-old Opg KO mice, new bone was formed in the endplate cartilage of lumbar IVDs and this became more obvious in 24-week-old Opg KO mice. Three-dimensional (3D) μCT reconstruction analyses showed that the edges of the L4 and L5 vertebrae were rugged with bone marrow cavities in it. Protein expression of VEGF-A, CD31, VE-cadherin, and CD34 was increased in the endplate and growth plate of lumbar IVDs of Opg KO mice. Gene expression of IL-1β, IL-6, and TNF-α as well as protein expression of IL-1β and TNF-α were highly expressed in the lumbar IVDs of Opg KO mice.

CONCLUSION

Deletion of Opg leads to increased neovascularization and expression of inflammatory cytokines in the lumbar disc in Opg KO mice, which may play important roles in IVD degeneration.

LEVEL OF EVIDENCE

N/A.

The osteoimmunology of alveolar bone loss

The mineralized structure of bone undergoes constant remodeling by the balanced actions of bone-producing osteoblasts and bone-resorbing osteoclasts (OCLs). Physiologic bone remodeling occurs in response to the body's need to respond to changes in electrolyte levels, or mechanical forces on bone. There are many pathological conditions, however, that cause an imbalance between bone production and resorption due to excessive OCL action that results in net bone loss. Situations involving chronic or acute inflammation are often associated with net bone loss, and research into understanding the mechanisms regulating this bone loss has led to the development of the field of osteoimmunology. It is now evident that the skeletal and immune systems are functionally linked and share common cells and signaling molecules. This review discusses the signaling system of immune cells and cytokines regulating aberrant OCL differentiation and activity. The role of these cells and cytokines in the bone loss occurring in periodontal disease (PD) (chronic inflammation) and orthodontic tooth movement (OTM) (acute inflammation) is then described. The review finishes with an exploration of the emerging role of Notch signaling in the development of the immune cells and OCLs that are involved in osteoimmunological bone loss and the research into Notch signaling in OTM and PD.

Microtubule-associated protein light chain 3 is involved in melanogenesis via regulation of MITF expression in melanocytes

Although autophagy plays a role in melanogenesis by regulating melanosome degradation and biogenesis in melanocytes, a detailed understanding of the regulatory functions of autophagy factors is lacking. Here, we report a mechanistic link between microtubule-associated protein light chain 3 (LC3) activation and melanogenesis. We observed high expression of LC3 in melanosome-associated pigment-rich melanocytic nevi of sun-exposed skin, as indicated by patterns of melanosomal protein MART1 expression. Rapamycin-induced autophagy significantly increased the melanin index, tyrosinase activity and expression of several proteins linked to melanosome biogenesis, including microphthalmia transcription factor (MITF), pre-melanosome protein and tyrosinase, in Melan-a melanocytes. siRNA-mediated knockdown of LC3, but not beclin-1 or ATG5, decreased melanin content and tyrosinase activity. LC3 knockdown also markedly inhibited MITF expression and subsequent rapamycin-induced melanosome formation. More importantly, LC3 knockdown suppressed α-MSH-mediated melanogenesis by attenuating cAMP response element-binding protein (CREB) phosphorylation and MITF expression in Melan-a cells via decreased extracellular signal-regulated kinase (ERK) activity. Overexpression of constitutively active ERK reversed the effect of LC3 knockdown on CREB phosphorylation and MITF expression. These findings demonstrate that LC3 contributes to melanogenesis by increasing ERK-dependent MITF expression, thereby providing a mechanistic insight into the signaling network that links autophagy to melanogenesis.

Metformin Improves Diabetic Bone Health by Re-Balancing Catabolism and Nitrogen Disposal

Objective

Metformin, a leading drug used to treat diabetic patients, is reported to benefit bone homeostasis under hyperglycemia in animal models. However, both the molecular targets and the biological pathways affected by metformin in bone are not well identified or characterized. The objective of this study is to investigate the bioengergeric pathways affected by metformin in bone marrow cells of mice.

Materials and Methods

Metabolite levels were examined in bone marrow samples extracted from metformin or PBS -treated healthy (Wild type) and hyperglycemic (diabetic) mice using liquid chromatography-mass spectrometry (LC-MS)-based metabolomics. We applied an untargeted high performance LC-MS approach which combined multimode chromatography (ion exchange, reversed phase and hydrophilic interaction (HILIC)) and Orbitrap-based ultra-high accuracy mass spectrometry to achieve a wide coverage. A multivariate clustering was applied to reveal the global trends and major metabolite players.

Results

A total of 346 unique metabolites were identified, and they are grouped into distinctive clusters that reflected general and diabetes-specific responses to metformin. As evidenced by changes in the TCA and urea cycles, increased catabolism and nitrogen waste that are commonly associated with diabetes were rebalanced upon treatment with metformin. In particular, we found glutamate and succinate whose levels were drastically elevated in diabetic animals were brought back to normal levels by metformin. These two metabolites were further validated as the major targets of metformin in bone marrow stromal cells.

Conclusion

Overall using limited sample size, our study revealed the metabolic pathways modulated by metformin in bones which have broad implication in our understanding of bone remodeling under hyperglycemia and in finding therapeutic interventions in mammals.

Interleukin-1β promotes the neurogenesis of carotid bodies by stimulating the activation of ERK1/2

The carotid body (CB) is a complex sensory organ that functions to sense homeostatic O2 in the blood. Previous studies have shown that CBs express interleukin (IL)-1 receptor type I and that the chemosensitivity of CBs is increased following stimulation with pro-inflammatory cytokines. However, the effects of pro-inflammatory cytokines, such as IL-1β, on the neurogenesis of CB are unclear. Thus, in this study, we aimed to assess the effects of IL-1β and intermittent hypobaric hypoxia (IHH) plus IL-1β on the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2, tyrosine hydroxylase (TH) and the expression of nestin, a well-established stem cell marker in the nervous system. The results showed that TH, nestin expression and ERK1/2 phosphorylation were increased in the rat CB following intraperitoneal injection of IL-1β. Moreover, IL-1β had additive effects on IHH. These results suggested that the plasticity of CB was increased following treatment with IL-1β and that ERK1/2 may be involved in neurogenic signaling in CBs.