Mutation in osteoactivin decreases bone formation in vivo and osteoblast differentiation in vitro

GPNMB regulates the differentiation and transformation of monocyte-derived macrophages during MASLD

Metabolic dysfunction-associated steatotic liver disease (MASLD) is an increasingly concerning global health issue characterized by pronounced hepatic steatosis and liver fibrosis. Hepatic monocyte-derived macrophages (MDMs) are crucial in the pathogenesis of liver fibrosis under MASLD. Nevertheless, the precise functions of MDMs and the underlying mechanisms governing their differentiation remain inadequately elucidated. In this study, we revealed an orchestrator of this process: Glycoprotein Non-Metastatic Melanoma Protein B (GPNMB), one of the characteristic genes of MDMs. Notably, myeloid-specific Gpnmb-knockout contributed to the retention of resident Kupffer cells (KCs) and rerouted monocyte differentiation towards a monocyte-derived macrophage subset that occupies the Kupffer cell niche (MoKC subset, resembling resident KCs), thereby impeding the formation of hepatic lipid-associated macrophages (LAMs). This transition has a profound impact, manifested in significantly reduced steatosis and modestly decreased liver fibrosis in myeloid-specific Gpnmb-knockout mice. In conclusion, our research clarifies the complex interactions between Gpnmb and MDMs and underscores the therapeutic potential of targeting Gpnmb within MDMs to manage MASLD.

The glycoprotein GPNMB protects against oxidative stress through enhanced PI3K/AKT signaling in epidermal keratinocytes

Vitiligo, an autoimmune disease caused by environmental and genetic factors, is characterized by the specific loss of epidermal melanocytes (MCs). IFN-γ, predominantly derived from MC-targeting CD8+ T cells, plays a key role in vitiligo pathogenesis. Previously, we found that glycoprotein nonmetastatic melanoma protein B (GPNMB) is specifically lost in the basal epidermal layer of vitiligo lesions and downregulated by IFN-γ in normal human epidermal keratinocytes (KCs) (NHEKs). This study aimed to determine the role of KC GPNMB in normal and vitiligo epidermis and demonstrated that GPNMB plays a protective role against H2O2-induced oxidative stress due to its extracellular domain. In contrast, the NRF2/KEEP1 system was not involved in the anti-oxidative response in NHEKs but was active in MCs. GPNMB knockdown reduced the phosphorylation levels of AKTT308 and AKTS473 after H2O2 treatment, accompanied by reduced Dickkopf-1 (DKK1) mRNA and protein production and decreased FOXM1 mRNA expression. These results suggested that GPNMB protects KCs from H2O2-induced cell death through enhanced PI3K/AKT signaling, and WNT/β-catenin/FOXM1 and DKK1/CKAP4/AKT pathways. Furthermore, a significant increase in thioredoxin-interacting protein (TXNIP) following GPNMB knockdown was observed, indicating the enhanced phosphorylation of JNK and p38 and suppression of WNT/β-catenin signaling. These results suggest that the decreased expression of epidermal GPNMB in vitiligo lesions triggers increased sensitivity to H2O2-induced oxidative stress and decreased WNT/β-catenin signaling, consistent with the pathological features of the vitiligo epidermis. These findings may enhance our understanding of vitiligo pathogenesis, provide insights into the reduced risk of epidermal cancers, and highlight novel targets for treatment.

Preparation of genetically or chemically engineered exosomes and their therapeutic effects in bone regeneration and anti-inflammation

The treatment of bone or cartilage damage and inflammation-related diseases has been a long-standing research hotspot. Traditional treatments such as surgery and cell therapy have only displayed limited efficacy because they can't avoid potential deterioration and ensure cell activity. Recently, exosomes have become a favorable tool for various tissue reconstruction due to their abundant content of proteins, lipids, DNA, RNA and other substances, which can promote bone regeneration through osteogenesis, angiogenesis and inflammation modulation. Besides, exosomes are also promising delivery systems because of stability in the bloodstream, immune stealth capacity, intrinsic cell-targeting property and outstanding intracellular communication. Despite having great potential in therapeutic delivery, exosomes still show some limitations in clinical studies, such as inefficient targeting ability, low yield and unsatisfactory therapeutic effects. In order to overcome the shortcomings, increasing studies have prepared genetically or chemically engineered exosomes to improve their properties. This review focuses on different methods of preparing genetically or chemically engineered exosomes and the therapeutic effects of engineering exosomes in bone regeneration and anti-inflammation, thereby providing some references for future applications of engineering exosomes.

The Lymphatic Endothelial Cell Secretome Inhibits Osteoblast Differentiation and Bone Formation

Complex lymphatic anomalies (CLAs) are a set of rare diseases with unique osteopathic profiles. Recent efforts have identified how lymphatic-specific somatic activating mutations can induce abnormal lymphatic formations that are capable of invading bone and inducing bone resorption. The abnormal bone resorption in CLA patients has been linked to overactive osteoclasts in areas with lymphatic invasions. Despite these findings, the mechanism associated with progressive bone loss in CLAs remains to be elucidated. In order to determine the role of osteoblasts in CLAs, we sought to assess osteoblast differentiation and bone formation when exposed to the lymphatic endothelial cell secretome. When treated with lymphatic endothelial cell conditioned medium (L-CM), osteoblasts exhibited a significant decrease in proliferation, differentiation, and function. Additionally, L-CM treatment also inhibited bone formation through a neonatal calvaria explant culture. These findings are the first to reveal how osteoblasts may be actively suppressed during bone lymphatic invasion in CLAs.

Conditional Loss of Nmp4 in Mesenchymal Stem Progenitor Cells Enhances PTH-Induced Bone Formation

Activation of bone anabolic pathways is a fruitful approach for treating severe osteoporosis, yet FDA-approved osteoanabolics, eg, parathyroid hormone (PTH), have limited efficacy. Improving their potency is a promising strategy for maximizing bone anabolic output. Nmp4 (Nuclear Matrix Protein 4) global knockout mice exhibit enhanced PTH-induced increases in trabecular bone but display no overt baseline skeletal phenotype. Nmp4 is expressed in all tissues; therefore, to determine which cell type is responsible for driving the beneficial effects of Nmp4 inhibition, we conditionally removed this gene from cells at distinct stages of osteogenic differentiation. Nmp4-floxed (Nmp4fl/fl ) mice were crossed with mice bearing one of three Cre drivers including (i) Prx1Cre+  to remove Nmp4 from mesenchymal stem/progenitor cells (MSPCs) in long bones; (ii) BglapCre+  targeting mature osteoblasts, and (iii) Dmp1Cre+  to disable Nmp4 in osteocytes. Virgin female Cre+  and Cre- mice (10 weeks of age) were sorted into cohorts by weight and genotype. Mice were administered daily injections of either human PTH 1-34 at 30 μg/kg or vehicle for 4 weeks or 7 weeks. Skeletal response was assessed using dual-energy X-ray absorptiometry, micro-computed tomography, bone histomorphometry, and serum analysis for remodeling markers. Nmp4fl/fl ;Prx1Cre+  mice virtually phenocopied the global Nmp4-/- skeleton in the femur, ie, a mild baseline phenotype but significantly enhanced PTH-induced increase in femur trabecular bone volume/total volume (BV/TV) compared with their Nmp4fl/fl ;Prx1Cre- controls. This was not observed in the spine, where Prrx1 is not expressed. Heightened response to PTH was coincident with enhanced bone formation. Conditional loss of Nmp4 from the mature osteoblasts (Nmp4fl/fl ;BglapCre+ ) failed to increase BV/TV or enhance PTH response. However, conditional disabling of Nmp4 in osteocytes (Nmp4fl/fl ;Dmp1Cre+ ) increased BV/TV without boosting response to hormone under our experimental regimen. We conclude that Nmp4-/- Prx1-expressing MSPCs drive the improved response to PTH therapy and that this gene has stage-specific effects on osteoanabolism. © 2022 American Society for Bone and Mineral Research (ASBMR).

Stromal niche inflammation mediated by IL-1 signalling is a targetable driver of haematopoietic ageing

Haematopoietic ageing is marked by a loss of regenerative capacity and skewed differentiation from haematopoietic stem cells (HSCs), leading to impaired blood production. Signals from the bone marrow niche tailor blood production, but the contribution of the old niche to haematopoietic ageing remains unclear. Here we characterize the inflammatory milieu that drives both niche and haematopoietic remodelling. We find decreased numbers and functionality of osteoprogenitors at the endosteum and expansion of central marrow LepR+ mesenchymal stromal cells associated with deterioration of the sinusoidal vasculature. Together, they create a degraded and inflamed old bone marrow niche. Niche inflammation in turn drives the chronic activation of emergency myelopoiesis pathways in old HSCs and multipotent progenitors, which promotes myeloid differentiation and hinders haematopoietic regeneration. Moreover, we show how production of interleukin-1β (IL-1β) by the damaged endosteum acts in trans to drive the proinflammatory nature of the central marrow, with damaging consequences for the old blood system. Notably, niche deterioration, HSC dysfunction and defective regeneration can all be ameliorated by blocking IL-1 signalling. Our results demonstrate that targeting IL-1 as a key mediator of niche inflammation is a tractable strategy to improve blood production during ageing.

Improved Protocol to Study Osteoblast and Adipocyte Differentiation Balance

Adipogenesis-osteoblastogenesis balance-rupture is relevant in multiple diseases. Current human mesenchymal stem cells (hMSCs) in vitro differentiation models are expensive, and are hardly reproducible. Their scarcity and variability make an affordable and reliable method to study adipocyte-osteoblast-equilibrium difficult. Moreover, media composition has been inconstant throughout the literature. Our aims were to compare improved differentiation lab-made media with consensus/commercial media, and to identify a cell-line to simultaneously evaluate both MSCs differentiations. Lab-made media were compared with consensus and commercial media in C3H10T1/2 and hMSC, respectively. Lab-made media were tested on aged women primary pre-osteoblast-like cells. To determine the optimum cell line, C3H10T1/2 and hMSC-TERT cells were differentiated to both cell fates. Differentiation processes were evaluated by adipocytic and osteoblastic gene-markers expression and staining. Lab-made media significantly increased consensus medium induction and overcame commercial media in hMSCs differentiation to adipocytes and osteoblasts. Pre-osteoblast-like cells only properly differentiate to adipocyte. Lab-made media promoted adipocyte gene-markers expression in C3H10T1/2 and hMSC-TERT, and osteoblast gene-markers in C3H10T1/2. Oil Red O and Alizarin Red staining supported these findings. Optimized lab-made media were better at differentiating MSCs compared to consensus/commercial media, and evidenced the adipogenic commitment of pre-osteoblast-like cells from aged-women. C3H10T1/2 is an optimum MSC line by which to study adipocyte-osteoblast differentiation balance.

Osteopathy in Complex Lymphatic Anomalies

Complex Lymphatic Anomalies (CLA) are lymphatic malformations with idiopathic bone and soft tissue involvement. The extent of the abnormal lymphatic presentation and boney invasion varies between subtypes of CLA. The etiology of these diseases has proven to be extremely elusive due to their rarity and irregular progression. In this review, we compiled literature on each of the four primary CLA subtypes and discuss their clinical presentation, lymphatic invasion, osseous profile, and regulatory pathways associated with abnormal bone loss caused by the lymphatic invasion. We highlight key proliferation and differentiation pathways shared between lymphatics and bone and how these systems may interact with each other to stimulate lymphangiogenesis and cause bone loss.

Transcriptome of HPβCD-treated Niemann-pick disease type C1 cells highlights GPNMB as a biomarker for therapeutics

The rare, fatal neurodegenerative disorder Niemann-Pick disease type C1 (NPC1) arises from lysosomal accumulation of unesterified cholesterol and glycosphingolipids. These subcellular pathologies lead to phenotypes of hepatosplenomegaly, neurological degeneration and premature death. The timing and severity of NPC1 clinical presentation is extremely heterogeneous. This study analyzed RNA-Seq data from 42 NPC1 patient-derived, primary fibroblast cell lines to determine transcriptional changes induced by treatment with 2-hydroxypropyl-β-cyclodextrin (HPβCD), a compound currently under investigation in clinical trials. A total of 485 HPβCD-responsive genes were identified. Pathway enrichment analysis of these genes showed significant involvement in cholesterol and lipid biosynthesis. Furthermore, immunohistochemistry of the cerebellum as well as measurements of serum from Npc1m1N null mice treated with HPβCD and adeno-associated virus (AAV) gene therapy suggests that one of the identified genes, GPNMB, may serve as a useful biomarker of treatment response in NPC1 disease. Overall, this large NPC1 patient-derived dataset provides a comprehensive foundation for understanding the genomic response to HPβCD treatment.

Quantitative trait locus mapping identifies the Gpnmb gene as a modifier of mouse macrophage lysosome function

We have previously shown that the DBA/2J versus AKR/J mouse strain is associated with decreased autophagy-mediated lysosomal hydrolysis of cholesterol esters. Our objective was to determine differences in lysosome function in AKR/J and DBA/2J macrophages, and identify the responsible genes. Using a novel dual-labeled indicator of lysosome function, DBA/2J versus AKR/J bone marrow derived macrophages had significantly decreased lysosome function. We performed quantitative trait loci mapping of lysosome function in bone marrow macrophages from an AKR/J × DBA/2J strain intercross. Four distinct lysosome function loci were identified, which we named macrophage lysosome function modifier (Mlfm) Mlfm1 through Mlfm4. The strongest locus Mlfm1 harbors the Gpnmb gene, which has been shown to recruit autophagy protein light chain 3 to autophagosomes for lysosome fusion. The parental DBA/2J strain has a nonsense variant in Gpnmb. siRNA knockdown of Gpnmb in AKR/J macrophages decreased lysosome function, and Gpnmb deletion through CRISP/Cas9 editing in RAW 264.7 mouse macrophages also demonstrated a similar result. Furthermore, a DBA/2 substrain, called DBA/2J-Gpnmb+/SjJ, contains the wildtype Gpnmb gene, and macrophages from this Gpnmb-preserved DBA/2 substrain exhibited recovered lysosome function. In conclusion, we identified Gpnmb as a causal modifier gene of lysosome function in this strain pair.

The role of miR-150 regulates bone cell differentiation and function

BACKGROUND

mir-RNAs play a role in regulating bone homeostasis. In this study we assessed the functional role of mir-RNA 150 in bone homeostasis. We also assess the effects of miR-150 deficiency on osteoblast and osteoclast differentiation and function using in vivo and in vitro approaches.

METHODS

Wild type (WT) (C57BL/6J) and miR-150 KO mice were compared for a variety of parameters. Micro-CT imaging was conducted to quantify trabecular bone mass inferior to the distal growth plate of the femur. Von Kossa staining was performed for osteoblast culture mineralization. RT-qPCR, biochemical analysis and bone histomorphometry were utilized for quantification of relevant genes and serum protein measurements. Differentiation and function of osteoblasts and osteoclasts was performed using primarily cultures and assessed the cell autonomous response of mir-RNA-150 on cell differentiation and function.

RESULTS

Mir-150 exhibited expression in a variety of tissues and increases progressively with age. Through micro-CT imaging, we found that KO mice presented reduced bone mass at 4, 8, and 16 weeks of age compared to WT mice. Furthermore, histomorphometric analysis revealed increased trabecular separation, decreased bone thickness, and decreased osteoblast number in KO compared to WT mice. Mir-150 deficiency also correlated with higher bone resorption, accompanied with significant increases in CTX-1 serum levels, and a decrease in cell apoptotic rate ex vivo. Additionally, miR-150 KO mice showed increased osteoblast differentiation and decreased osteoclastogenesis ex vivo. Luciferase assay showed increased Osteoactivin/GPNMB expression in miR-150 KO osteoblasts compared to WT cells.

CONCLUSION

Our data suggests that miR-150 influences osteoblast and osteoclast functionality and differentiation; specifically, miR-150 serves as a negative regulator for osteoblasts and a positive regulator for osteoclasts by regulating, at least in part, Osteoactivin/GPNMB expression.

Extracellular vesicles from GPNMB-modified bone marrow mesenchymal stem cells attenuate bone loss in an ovariectomized rat model

AIMS

The efficacy of anti-osteoporotic treatments is still limited. Our study aimed to investigate the effect of extracellular vesicles (EVs) derived from bone marrow-derived MSCs (BMSCs) overexpressing glycoprotein non-melanoma clone B (GPNMB) on osteoporosis (OP).

MAIN METHODS

Lentiviral vector for GPNMB overexpression or its negative control was generated and transfected into BMSCs. EVs enriched with GPNMB (GPNMB-EVs) were extracted from GPNMB-modified BMSC-conditioned medium and then identified. Cellular uptake and proliferation were analyzed using the Dil-labeled assay and CCK-8 assay, respectively. Cytochemical staining, western blot, and RT-qPCR analysis were performed to assess the effect of GPNMB-EVs on osteogenic differentiation of BMSCs in vitro. Dickkopf-1 (DKK1) as the inhibitor was applied to explore the Wnt/β-catenin signaling pathway involved in the GPNMB-EV-induced osteogenic differentiation. In vivo experiments were conducted using an ovariectomized (OVX) rat model of postmenopausal osteoporosis, and then assessed the effect of GPNMB-EVs by micro-CT, and histological and immunohistochemical assays.

KEY FINDINGS

GPNMB-EVs were taken up by BMSCs, and they noticeably promoted the proliferation of BMSCs. Additionally, GPNMB-EVs activated the Wnt/β-catenin signaling to stimulate osteogenesis in BMSCs. In vivo examination showed that GPNMB-EVs remarkably improved trabecular bone regeneration and alleviated the osteoporotic phenotype in the OVX-induced rat model of OP.

SIGNIFICANCE

EVs derived from GPNMB-modified BMSCs significantly stimulated the proliferation and osteogenic differentiation of BMSCs via the activation of Wnt/β-catenin signaling and attenuated the bone loss in the OVX-induced rat model of OP. Our findings suggest the promising potential of GPNMB-EVs as cell-free therapy for the treatment of OP.

The soluble glycoprotein NMB (GPNMB) produced by macrophages induces cancer stemness and metastasis via CD44 and IL-33

In cancer, myeloid cells have tumor-supporting roles. We reported that the protein GPNMB (glycoprotein nonmetastatic B) was profoundly upregulated in macrophages interacting with tumor cells. Here, using mouse tumor models, we show that macrophage-derived soluble GPNMB increases tumor growth and metastasis in Gpnmb-mutant mice (DBA/2J). GPNMB triggers in the cancer cells the formation of self-renewing spheroids, which are characterized by the expression of cancer stem cell markers, prolonged cell survival and increased tumor-forming ability. Through the CD44 receptor, GPNMB mechanistically activates tumor cells to express the cytokine IL-33 and its receptor IL-1R1L. We also determined that recombinant IL-33 binding to IL-1R1L is sufficient to induce tumor spheroid formation with features of cancer stem cells. Overall, our results reveal a new paracrine axis, GPNMB and IL-33, which is activated during the cross talk of macrophages with tumor cells and eventually promotes cancer cell survival, the expansion of cancer stem cells and the acquisition of a metastatic phenotype.

GPNMB contributes to a vicious circle for chronic obstructive pulmonary disease

Osteoporosis (OP) is significant and debilitating comorbidity of chronic obstructive pulmonary disease (COPD). We hypothesize that genetic variance identified with OP may also play roles in COPD. We have conducted a large-scale relation data analysis to explore the genes implicated with either OP or COPD, or both. Each gene linked to OP but not to COPD was further explored in a mega-analysis and partial mega-analysis of 15 independently collected COPD RNA expression datasets, followed by gene set enrichment analysis (GSEA) and literature-based pathway analysis to explore their functional linked to COPD. A multiple linear regression (MLR) model was built to study the possible influence of sample size, population region, and study date on the gene expression data in COPD. At the first step of the analysis, we have identified 918 genes associated with COPD, 581 with OP, and a significant overlap (P<2.30e-140; 210 overlapped genes). Partial mega-analysis showed that, one OP gene, GPNMB presented significantly increased expression in COPD patients (P-value = 0.0018; log fold change = 0.83). GPNMB was enriched in multiple COPD pathways and plays roles as a gene hub formulating multiple vicious COPD pathways included gene MMP9 and MYC. GPNMB could be a novel gene that plays roles in both COPD and OP. Partial mega-analysis is valuable in identify case-specific genes for COPD.

Cycloastragenol prevents age-related bone loss: Evidence in d-galactose-treated and aged rats

BACKGROUND AND AIMS

Aging-induced bone loss is a multifactorial, age-related, and progressive phenomenon among the general population and may further progress to osteoporosis and increase the risk of fractures. Cycloastragenol (CAG), currently the only compound reported that activates human telomerase, is thought to be able to alleviate or delay the symptoms of aging and chronic diseases. Previous research has suggested that CAG may have the potential to alleviate age-related bone loss. However, to date, no research has specifically focused on this aspect. In this study, we aimed to investigate whether CAG could prevent senile osteoporosis, and further reveal its underlying mechanism.

METHODS

CAG treatment was administrated into two bone loss rat models (D-galactose administration and aging) for 20 weeks and 33 weeks, respectively. Serum biomarkers analyses, bone biomechanical tests, micro-computed tomography assessment, and bone histomorphometry analyses were performed on the bone samples collected at the endpoint, to determine whether CAG could prevent or alleviate age-related bone loss. Proteomic analysis was performed to reveal the changes in protein profiles of the bones, and western blot was used to further verify the identity of the key proteins. The viability, osteoblastic differentiation, and mineralization of MC3T3-E1 cells were also evaluated after CAG treatment in vitro.

RESULTS

The results suggest that CAG treatment improves bone formation, reduces osteoclast number, alleviates the degradation of bone microstructure, and enhances bone biomechanical properties in both d-galactose- and aging-induced bone loss models. CAG treatment promotes viability, osteoblastic differentiation, and mineralization in MC3T3-E1 cells. Proteomic and western blot analyses revealed that CAG treatment increases osteoactivin (OA) expression to alleviate bone loss.

CONCLUSION

The results revealed that CAG alleviates age-related bone loss and improves bone microstructure and biomechanical properties. This may due to CAG-induced increase in OA expression. In addition, the results support preclinical investigations of CAG as a potential therapeutic medicine for the treatment of senile osteoporosis.

Glycoprotein nonmetastatic melanoma protein B: A key mediator and an emerging therapeutic target in autoimmune diseases

The glycoprotein nonmetastatic melanoma protein B (GPNMB, also known as osteoactivin) is highly expressed in many cell types and regulates the homeostasis in various tissues. In different physiological contexts, it functions as a melanosome-associated protein, membrane-bound surface receptor, soluble ligand, or adhesion molecule. Therefore, GPNMB is involved in cell differentiation, migration, inflammation, metabolism, and neuroprotection. Because of its various involvement in different physiological conditions, GPNMB has been implicated in many diseases, including cancer, neurological disorders, and more recently immune-mediated diseases. This review summarizes the regulation and function of GPNMB in normal physiology, and discusses the involvement of GPNMB in disease conditions with a particular focus on its potential role and therapeutic implications in autoimmunity.

A novel regulatory role of TRAPPC9 in L-plastin-mediated osteoclast actin ring formation

Trafficking protein particle complex 9 (TRAPPC9) is a major subunit of the TRAPPII complex. TRAPPC9 has been reported to bind nuclear factor κB kinase subunit β (IKKβ) and NF-kB-inducing kinase (NIK) where it plays a role in the canonical and noncanonical of nuclear factor-κB (NF-kB) signaling pathways, receptively. The role of TRAPPC9 in protein trafficking and cytoskeleton organization in osteoclast (OC) has not been studied yet. In this study, we examined the mRNA expression of TRAPPC9 during OC differentiation. Next, we examined the colocalization of TRAPPC9 with cathepsin-K, known to mediate OC resorption suggesting that TRAPPC9 mediates the trafficking pathway within OC. To identify TRAPPC9 protein partners important for OC-mediated cytoskeleton re-organization, we conducted immunoprecipitation of TRAPPC9 in mature OCs followed by mass spectrometry analysis. Our data showed that TRAPPC9 binds various protein partners. One protein with high recovery rate is L-plastin (LPL). LPL localizes at the podosomes and reported to play a crucial role in actin aggregation thereby actin ring formation and OC function. Although the role of LPL in OC-mediated bone resorption has not fully reported in detail. Here, first, we confirmed the binding of LPL to TRAPPC9 and, then, we investigated the potential regulatory role of TRAPPC9 in LPL-mediated OC cytoskeleton reorganization. We assessed the localization of TRAPPC9 and LPL in OC and found that TRAPPC9 is colocalized with LPL at the periphery of OC. Next, we determined the effect of TRAPPC9 overexpression on LPL recruitment to the actin ring using a viral system. Interestingly, our data showed that TRAPPC9 overexpression promotes the recruitment of LPL to the actin ring when compared with control cultures. In addition, we observed that TRAPPC9 overexpression reorganizes actin clusters/aggregates and regulates vinculin recruitment into the OC periphery to initiate podosome formation.

Microglia express GPNMB in the brains of Alzheimer's disease and Nasu-Hakola disease

Glycoprotein non-metastatic melanoma protein B (GPNMB) is a type I transmembrane glycoprotein first identified in low-metastatic human melanoma cell lines as a regulator of tumor growth. GPNMB is widely expressed in various tissues, where it is involved in cell differentiation, migration, inflammation/anti-inflammation, tissue regeneration, and neuroprotection. GPNMB is identified in microglia of adult rat brains, neurons and astrocytes of GPNMB transgenic (Tg) mouse brains, and motor neurons of amyotrophic lateral sclerosis (ALS) patients. Nasu-Hakola disease (NHD) is a rare autosomal recessive disorder, characterized by progressive presenile dementia and formation of multifocal bone cysts, caused by genetic mutations of either TYROBP (DAP12) or TREM2. TREM2 and DAP12 constitute a receptor/adaptor signaling complex expressed exclusively on osteoclasts, dendritic cells, macrophages, and microglia. Pathologically, the brains of NHD patients exhibit leukoencephalopathy, astrogliosis, accumulation of axonal spheroids, and remarkable activation of microglia predominantly in the white matter of frontal and temporal lobes and the basal ganglia. At present, molecular mechanisms responsible for development of leukoencephaolpathy in NHD brains remain totally unknown. Recent evidence indicates that disease-associated microglia (DAM) that cluster around amyloid plaques express high levels of GPNMB in Alzheimer's disease (AD) brains. Because microglia act as a key regulator of leukoencephalopathy in NHD brains, it is proposed that GPNMB expressed on microglia might play a protective role in progression of leukoencephalopathy possibly via active phagocytosis of myelin debris. In the present study using immunohistochemistry, we have attempted to clarify the expression of GPNMB in NHD brains, compared with AD brains. We found that microglia accumulating in the white matter express an intense GPNMB immunoreactivity in both NHD and AD brains, suggesting that the accumulation of GPNMB-immunoreactive microglia is a general phenomenon in neurodegenerative brains.

The suppressive effects of miR-508-5p on the odontogenic differentiation of human dental pulp stem cells by targeting glycoprotein non-metastatic melanomal protein B

BACKGROUND

Although the involvement of glycoprotein non-metastatic melanomal protein B (GPNMB) in regulating the odontogenic differentiation of human dental pulp stem cells (hDPCs) has been identified, the underlying mechanisms are largely unknown. The purpose of this study is to investigate the effects of miR-508-5p on the GPNMB expression and the odontogenic differentiation of hDPCs.

METHODS

In this study, hDPCs were isolated and identified by flow cytometric analysis. Based on bioinformatics analysis, dual luciferase reporter assay was performed to verify GPNMB acting as a target of miR-508-5p. The regulatory roles of miR-508-5p in odontogenetic differentiation of hDPCs were investigated through its inhibition or overexpression (miRNA mimics and miRNA inhibitors). qRT-PCR and Western blot analysis were used to detect the expression of odontogenetic marker genes and proteins. The assays of alkaline phosphatase (ALP) activity and Alizarin Red S staining were performed to evaluate the odontogenetic phenotype.

RESULTS

We first found that the levels of miR-508-5p expression decreased gradually during odontogenesis of hDPCs, while the expressions of GPNMB were upregulated obviously. The suppressive effects of miR-508-5p on GPNMB were determined by oligonucleotide transfection in hDPCs and dual luciferase reporter assay in 293T cells. Subsequently, the significant inhibition of hDPC odontogenesis after the overexpression of miR-508-5p was observed, which is consistent with the decreased expression levels of several odontoblast-specific genes, such as dentin matrix protein 1 (DMP-1), dentin sialophosphoprotein (DSPP), and osteocalcin (OCN), as well as the decreased activity of ALP and weakened Alizarin Red S staining. Furthermore, ectopic expression of GPNMB (lacking 3'-UTR) rescued the effects of miR-508-5p on odontogenic differentiation.

CONCLUSIONS

Our study demonstrated that miR-508-5p regulated the osteogenesis of hDPCs by targeting GPNMB and provided novel insight into the critical roles of microRNAs in hDPC differentiation.

Screening Gene Knockout Mice for Variation in Bone Mass: Analysis by μCT and Histomorphometry

PURPOSE OF REVIEW

The international mouse phenotyping consortium (IMPC) is producing defined gene knockout mouse lines. Here, a phenotyping program is presented that is based on micro-computed tomography (μCT) assessment of distal femur and vertebra. Lines with significant variation undergo a computer-based bone histomorphometric analysis.

RECENT FINDINGS

Of the 220 lines examined to date, approximately 15% have a significant variation (high or low) by μCT, most of which are not identified by the IMPC screen. Significant dimorphism between the sexes and bone compartments adds to the complexity of the skeletal findings. The μCT information that is posted at www.bonebase.org can group KOMP lines with similar morphological features. The histological data is presented in a graphic form that associates the cellular features with a specific anatomic group. The web portal presents a bone-centric view appropriate for the skeletal biologist/clinician to organize and understand the large number of genes that can influence skeletal health. Cataloging the relative severity of each variant is the first step towards compiling the dataset necessary to appreciate the full polygenic basis of degenerative bone disease.

Loss of GPNMB Causes Autosomal-Recessive Amyloidosis Cutis Dyschromica in Humans

Amyloidosis cutis dyschromica (ACD) is a distinct form of primary cutaneous amyloidosis characterized by generalized hyperpigmentation mottled with small hypopigmented macules on the trunks and limbs. Affected families and sporadic case subjects have been reported predominantly in East and Southeast Asian ethnicities; however, the genetic cause has not been elucidated. We report here that the compound heterozygosity or homozygosity of GPNMB truncating alleles is the cause of autosomal-recessive ACD. Six nonsense or frameshift mutations were identified in nine individuals diagnosed with ACD. Immunofluorescence analysis of skin biopsies showed that GPNMB is expressed in all epidermal cells, with the highest staining observed in melanocytes. GPNMB staining is significantly reduced in the lesional skin of affected individuals. Hyperpigmented lesions exhibited significantly increased amounts of DNA/keratin-positive amyloid deposits in the papillary dermis and infiltrating macrophages compared with hypo- or depigmented macules. Depigmentation of the lesions was attributable to loss of melanocytes. Intracytoplasmic fibrillary aggregates were observed in keratinocytes scattered in the lesional epidermis. Thus, our analysis indicates that loss of GPNMB, which has been implicated in melanosome formation, autophagy, phagocytosis, tissue repair, and negative regulation of inflammation, underlies autosomal-recessive ACD and provides insights into the etiology of amyloidosis and pigment dyschromia.

Muscle-Bone Crosstalk: Emerging Opportunities for Novel Therapeutic Approaches to Treat Musculoskeletal Pathologies

Osteoporosis and sarcopenia are age-related musculoskeletal pathologies that often develop in parallel. Osteoporosis is characterized by a reduced bone mass and an increased fracture risk. Sarcopenia describes muscle wasting with an increasing risk of injuries due to falls. The medical treatment of both diseases costs billions in health care per year. With the impact on public health and economy, and considering the increasing life expectancy of populations, more efficient treatment regimens are sought. The biomechanical interaction between both tissues with muscle acting on bone is well established. Recently, both tissues were also determined as secretory endocrine organs affecting the function of one another. New exciting discoveries on this front are made each year, with novel signaling molecules being discovered and potential controversies being described. While this review does not claim completeness, it will summarize the current knowledge on both the biomechanical and the biochemical link between muscle and bone. The review will highlight the known secreted molecules by both tissues affecting the other and finish with an outlook on novel therapeutics that could emerge from these discoveries.

Macrophage-Associated Osteoactivin/GPNMB Mediates Mesenchymal Stem Cell Survival, Proliferation, and Migration Via a CD44-Dependent Mechanism

Although MSCs have been widely recognized to have therapeutic potential in the repair of injured or diseased tissues, it remains unclear how functional activities of mesenchymal stem cells (MSCs) are influenced by the surrounding inflammatory milieu at the site of tissue injury. Macrophages constitute an essential component of innate immunity and have been shown to exhibit a phenotypic plasticity in response to various stimuli, which play a central role in both acute inflammation and wound repair. Osteoactivin (OA)/Glycoprotein non-metastatic melanoma protein B (GPNMB), a transmembrane glycoprotein that plays a role in cell differentiation, survival, and angiogenesis. The objective of this study was to investigate the potential role of OA/GPNMB in macrophage-induced MSC function. We found that reparative M2 macrophages express significantly greater levels of OA/GPNMB than pro-inflammatory M1 macrophages. Furthermore, using loss of function and rescue studies, we demonstrated that M2 macrophages-secreted OA/GPNMB positively regulates the viability, proliferation, and migration of MSCs. More importantly, we demonstrated that OA/GPNMB acts through ERK and AKT signaling pathways in MSCs via CD44, to induce these effects. Taken together, our results provide pivotal insight into the mechanism by which OA/GPNMB contributes to the tissue reparative phenotype of M2 macrophages and positively regulates functional activities of MSCs. J. Cell. Biochem. 117: 1511-1521, 2016. © 2015 Wiley Periodicals, Inc.

Glycoprotein NMB: an Emerging Role in Neurodegenerative Disease

Neurodegeneration is characterized by severe neuronal loss leading to the cognitive and physical impairments that define various neurodegenerative diseases. Neuroinflammation is one hallmark of neurodegenerative diseases and can ultimately contribute to disease progression. Increased inflammatory cytokines, such as interleukin-6 (IL-6), interleukin-1β (IL-1 β), and tumor necrosis factor-α (TNF-α) are associated with Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). Unfortunately, current therapeutic options lack ability to stop or effectively slow progression of these diseases and are primarily aimed at alleviating symptoms. Thus, it is crucial to discover novel treatment candidates for neurodegenerative diseases. Glycoprotein nonmetastatic melanoma protein B (GPNMB) is a type-I transmembrane glycoprotein first identified in a melanoma cell line. GPNMB augments bone mineral deposition by stimulating osteoblast differentiation. Aside from its anabolic function in the bone, emerging evidence suggests that GPNMB has anti-inflammatory and reparative functions. GPNMB has also been demonstrated to be neuroprotective in an animal model of ALS, cerebral ischemia, and other disease models. Given these discoveries, GPNMB should be investigated as a potential therapeutic option for multiple neurodegenerative diseases.

Targeting GPNMB with glembatumumab vedotin: Current developments and future opportunities for the treatment of cancer

GPNMB has emerged as an immunomodulator and an important positive mediator of tumor progression and metastasis in numerous solid cancers. Tumor intrinsic GPNMB-mediated effects on cellular signaling, coupled with the ability of GPNMB to influence the primary tumor and metastatic microenvironments in a non-cell autonomous fashion, combine to augment malignant cancer phenotypes. In addition, GPNMB is often overexpressed in a variety of cancers, making it an attractive therapeutic target. In this regard, glembatumumab vedotin, an antibody-drug conjugate (ADC) that targets GPNMB, is currently in clinical trials as a single agent in multiple cancers. In this review, we will describe the physiological functions of GPNMB in normal tissues and summarize the processes through which GPNMB augments tumor growth and metastasis. We will review the pre-clinical and clinical development of glembatumumab vedotin, evaluate on-going clinical trials, explore emerging opportunities for this agent in new disease indications and discuss exciting possibilities for this ADC in the context of combination therapies.

Validating glycoprotein non-metastatic melanoma B (gpNMB, osteoactivin), a new biomarker of Gaucher disease

In the spleens of Gaucher disease mice and patients, there is a striking elevation of expression of glycoprotein non-Metastatic Melanoma B (gpNMB). We conducted a study in a large cohort of patients with Gaucher disease to assess the utility of serum levels of soluble fragment of gpNMB as a biomarker of disease activity. There was >15-fold elevation of gpNMB in sera of untreated patients with Gaucher disease. gpNMB levels correlated with overall disease severity as well as the severity of individual organ compartments: liver, spleen, bone and hematological disease. Imiglucerase enzyme replacement therapy resulted in significant reduction of gpNMB. Serum levels of gpNMB were highly correlated with accumulation of bioactive lipid substrate of Gaucher disease, glucosylsphingosine as well as established biomarkers, chitotriosidase and chemokine, CCL18. Our results suggest utility of gpNMB as a biomarker of Gaucher disease to monitor individual patients and cohorts of patients for disease progression or response to therapy. Investigation of gpNMB in Gaucher disease pathophysiology is likely to illuminate our understanding disease mechanisms.

Functional differences between AMPK α1 and α2 subunits in osteogenesis, osteoblast-associated induction of osteoclastogenesis, and adipogenesis

The endocrine role of the skeleton-which is impaired in human diseases including osteoporosis, obesity and diabetes-has been highlighted previously. In these diseases, the role of AMPK, a sensor and regulator of energy metabolism, is of biological and clinical importance. Since AMPK's main catalytic subunit α has two isoforms, it is unclear whether functional differences between them exist in the skeletal system. The current study overexpressed AMPKα1 and α2 in MC3T3-E1 cells, primary osteoblasts and mouse BMSCs by lentiviral transduction. Cells overexpressing AMPKα2 showed higher osteogenesis potential than AMPKα1, wherein androgen receptor (AR) and osteoactivin played important roles. RANKL and M-CSF were secreted at lower levels from cells overexpressing α2 than α1, resulting in decreased osteoblast-associated osteoclastogenesis. Adipogenesis was inhibited to a greater degree in 3T3-L1 cells overexpressing α2 than α1, which was modulated by AR. An abnormal downregulation of AMPKα2 was observed in human BMSCs exhibiting the fibrous dysplasia (FD) phenotype. Overexpression of AMPKα2 in these cells rescued the defect in osteogenesis, suggesting that AMPKα2 plays a role in FD pathogenesis. These findings highlight functional differences between AMPKα1 and α2, and provide a basis for investigating the molecular mechanisms of diseases associated with impaired functioning of the skeletal system.

Osteoactivin inhibition of osteoclastogenesis is mediated through CD44-ERK signaling

Osteoactivin is a heavily glycosylated protein shown to have a role in bone remodeling. Previous studies from our lab have shown that mutation in Osteoactivin enhances osteoclast differentiation but inhibits their function. To date, a classical receptor and a signaling pathway for Osteoactivin-mediated osteoclast inhibition has not yet been characterized. In this study, we examined the role of Osteoactivin treatment on osteoclastogenesis using bone marrow-derived osteoclast progenitor cells and identify a signaling pathway relating to Osteoactivin function. We reveal that recombinant Osteoactivin treatment inhibited osteoclast differentiation in a dose-dependent manner shown by qPCR, TRAP staining, activity and count. Using several approaches, we show that Osteoactivin binds CD44 in osteoclasts. Furthermore, recombinant Osteoactivin treatment inhibited ERK phosphorylation in a CD44-dependent manner. Finally, we examined the role of Osteoactivin on receptor activator of nuclear factor-κ B ligand (RANKL)-induced osteolysis in vivo. Our data indicate that recombinant Osteoactivin inhibits RANKL-induced osteolysis in vivo and this effect is CD44-dependent. Overall, our data indicate that Osteoactivin is a negative regulator of osteoclastogenesis in vitro and in vivo and that this process is regulated through CD44 and ERK activation.

Elevation of glycoprotein nonmetastatic melanoma protein B in type 1 Gaucher disease patients and mouse models

Gaucher disease is caused by inherited deficiency of lysosomal glucocerebrosidase. Proteome analysis of laser‐dissected splenic Gaucher cells revealed increased amounts of glycoprotein nonmetastatic melanoma protein B (gpNMB). Plasma gpNMB was also elevated, correlating with chitotriosidase and CCL18, which are established markers for human Gaucher cells. In Gaucher mice, gpNMB is also produced by Gaucher cells. Correction of glucocerebrosidase deficiency in mice by gene transfer or pharmacological substrate reduction reverses gpNMB abnormalities. In conclusion, gpNMB acts as a marker for glucosylceramide‐laden macrophages in man and mouse and gpNMB should be considered as candidate biomarker for Gaucher disease in treatment monitoring.

Prospects of osteoactivin in tissue regeneration

INTRODUCTION

Osteoactivin (OA) was first discovered in an osteopetrotic rat model using mRNA differential display a decade ago and has been studied recently. OA in bone tissue can directly or indirectly regulate the differentiation of osteoblasts by influencing cell behaviours, such as proliferation and adhesion, as well as inducing serial signal cascades, which would be of great importance in the field of tissue engineering. The results of recent studies have further demonstrated that OA plays a critical role in the differentiation and function of cells, especially in bone formation and fracture healing. Areas covered: The discovery, structure, and function of OA as well as its therapeutic potential in tissue regeneration of bone defects, kidney injury, liver damage, and muscle atrophy. Expert opinion: OA has great potential in promoting the regeneration of damaged tissues, particularly bone tissue, which is supported by a large body of data. Future studies should focus on exploring the underlying mechanism of OA as well as pursuing the ideal form of OA-related regenerative medicine.

The novel monoclonal antibody 9F5 reveals expression of a fragment of GPNMB/osteoactivin processed by furin-like protease(s) in a subpopulation of microglia in neonatal rat brain

To differentiate subtypes of microglia (MG), we developed a novel monoclonal antibody, 9F5, against one subtype (type 1) of rat primary MG. The 9F5 showed high selectivity for this cell type in Western blot and immunocytochemical analyses and no cross-reaction with rat peritoneal macrophages (Mφ). We identified the antigen molecule for 9F5: the 50- to 70-kDa fragments of rat glycoprotein nonmetastatic melanoma protein B (GPNMB)/osteoactivin, which started at Lys(170) . In addition, 9F5 immunoreactivity with GPNMB depended on the activity of furin-like protease(s). More important, rat type 1 MG expressed the GPNMB fragments, but type 2 MG and Mφ did not, although all these cells expressed mRNA and the full-length protein for GPNMB. These results suggest that 9F5 reactivity with MG depends greatly on cleavage of GPNMB and that type 1 MG, in contrast to type 2 MG and Mφ, may have furin-like protease(s) for GPNMB cleavage. In neonatal rat brain, amoeboid 9F5+ MG were observed in specific brain areas including forebrain subventricular zone, corpus callosum, and retina. Double-immunοstaining with 9F5 antibody and anti-Iba1 antibody, which reacts with MG throughout the CNS, revealed that 9F5+ MG were a portion of Iba1+ MG, suggesting that MG subtype(s) exist in vivo. We propose that 9F5 is a useful tool to discriminate between rat type 1 MG and other subtypes of MG/Mφ and to reveal the role of the GPNMB fragments during developing brain. GLIA 2016;64:1938-1961.

Transgenic Expression of Osteoactivin/gpnmb Enhances Bone Formation In Vivo and Osteoprogenitor Differentiation Ex Vivo

Initial identification of osteoactivin (OA)/glycoprotein non-melanoma clone B (gpnmb) was demonstrated in an osteopetrotic rat model, where OA expression was increased threefold in mutant bones, compared to normal. OA mRNA and protein expression increase during active bone regeneration post-fracture, and primary rat osteoblasts show increased OA expression during differentiation in vitro. To further examine OA/gpnmb as an osteoinductive agent, we characterized the skeletal phenotype of transgenic mouse overexpressing OA/gpnmb under the CMV-promoter (OA-Tg). Western blot analysis showed increased OA/gpnmb in OA-Tg osteoblasts, compared to wild-type (WT). In OA-Tg mouse femurs versus WT littermates, micro-CT analysis showed increased trabecular bone volume and thickness, and cortical bone thickness; histomorphometry showed increased osteoblast numbers, bone formation and mineral apposition rates in OA-Tg mice; and biomechanical testing showed higher peak moment and stiffness. Given that OA/gpnmb is also over-expressed in osteoclasts in OA-Tg mice, we evaluated bone resorption by ELISA and histomorphometry, and observed decreased serum CTX-1 and RANK-L, and decreased osteoclast numbers in OA-Tg, compared to WT mice, indicating decreased bone remodeling in OA-Tg mice. The proliferation rate of OA-Tg osteoblasts in vitro was higher, compared to WT, as was alkaline phosphatase staining and activity, the latter indicating enhanced differentiation of OA-Tg osteoprogenitors. Quantitative RT-PCR analysis showed increased TGF-β1 and TGF-β receptors I and II expression in OA-Tg osteoblasts, compared to WT. Together, these data suggest that OA overexpression has an osteoinductive effect on bone mass in vivo and stimulates osteoprogenitor differentiation ex vivo.

Targeting gene expression during the early bone healing period in the mandible: A base for bone tissue engineering

PURPOSE

Although bone tissue engineering techniques have become more and more sophisticated than in the past, natural bone healing mechanisms have not been sufficiently considered for further improvement of these techniques so far. We used an established animal model with transcriptome analysis to generate an unbiased picture of early bone healing to support tissue engineering concepts.

MATERIAL AND METHODS

In 30 Wistar rats, a 3-mm bone defect was created in the mandibular angle. Tissue was sampled at 5, 10, and 15 days, and the former defect area was excised to undergo transcriptome analysis after RNA extraction. Five differentially expressed genes were further evaluated with reverse transcription-polymerase chain reaction (rt-PCR).

RESULTS

Transcriptome analysis revealed 2467 significantly over- and under-expressed transcripts after 5 days and 2265 after 15 days of bone healing, respectively. Validation via rt-PCR confirmed overexpression of osteoactivin, angiopoietin-like factor-4, and metallomatrix proteinase-9 and underexpression of mastcellprotease-10 and proteoglycane-2 in comparison to values in the control group.

CONCLUSION

This systematic genome-wide transcriptome analysis helps to decipher the physiological mechanisms behind physiological bone healing. The exemplary depiction of 5 genes demonstrates the great complexity of metabolic processes during early bone healing. Here, BMP-2 signaling pathways and local hypoxia play decisive roles in bone formation.

Gene Expression Profiling in Peripheral Blood Cells and Synovial Membranes of Patients with Psoriatic Arthritis

Background

Psoriatic arthritis (PsA) is an inflammatory arthritis whose pathogenesis is poorly understood; it is characterized by bone erosions and new bone formation. The diagnosis of PsA is mainly clinical and diagnostic biomarkers are not yet available. The aim of this work was to clarify some aspects of the disease pathogenesis and to identify specific gene signatures in paired peripheral blood cells (PBC) and synovial biopsies of patients with PsA. Moreover, we tried to identify biomarkers that can be used in clinical practice.

Methods

PBC and synovial biopsies of 10 patients with PsA were used to study gene expression using Affymetrix arrays. The expression values were validated by Q-PCR, FACS analysis and by the detection of soluble mediators.

Results

Synovial biopsies of patients showed a modulation of approximately 200 genes when compared to the biopsies of healthy donors. Among the differentially expressed genes we observed the upregulation of Th17 related genes and of type I interferon (IFN) inducible genes. FACS analysis confirmed the Th17 polarization. Moreover, the synovial trascriptome shows gene clusters (bone remodeling, angiogenesis and inflammation) involved in the pathogenesis of PsA. Interestingly 90 genes are modulated in both compartments (PBC and synovium) suggesting that signature pathways in PBC mirror those of the inflamed synovium. Finally the osteoactivin gene was upregulared in both PBC and synovial biopsies and this finding was confirmed by the detection of high levels of osteoactivin in PsA sera but not in other inflammatory arthritides.

Conclusions

We describe the first analysis of the trancriptome in paired synovial tissue and PBC of patients with PsA. This study strengthens the hypothesis that PsA is of autoimmune origin since the coactivity of IFN and Th17 pathways is typical of autoimmunity. Finally these findings have allowed the identification of a possible disease biomarker, osteoactivin, easily detectable in PsA serum.

Mutation in Osteoactivin Promotes Receptor Activator of NFκB Ligand (RANKL)-mediated Osteoclast Differentiation and Survival but Inhibits Osteoclast Function

We previously reported on the importance of osteoactivin (OA/Gpnmb) in osteogenesis. In this study, we examined the role of OA in osteoclastogenesis, using mice with a nonsense mutation in the Gpnmb gene (D2J) and wild-type controls (D2J/Gpnmb(+)). In these D2J mice, micro-computed tomography and histomorphometric analyses revealed increased cortical thickness, whereas total porosity and eroded surface were significantly reduced in D2J mice compared with wild-type controls, and these results were corroborated by lower serum levels of CTX-1. Contrary to these observations and counterintuitively, temporal gene expression analyses supported up-regulated osteoclastogenesis in D2J mice and increased osteoclast differentiation rates ex vivo, marked by increased number and size. The finding that MAPK was activated in early differentiating and mature D2J osteoclasts and that survival of D2J osteoclasts was enhanced and mediated by activation of the AKT-GSK3β pathway supports this observation. Furthermore, this was abrogated by the addition of recombinant OA to cultures, which restored osteoclastogenesis to wild-type levels. Moreover, mix and match co-cultures demonstrated an induction of osteoclastogenesis in D2J osteoblasts co-cultured with osteoclasts of D2J or wild-type. Last, in functional osteo-assays, we show that bone resorption activity of D2J osteoclasts is dramatically reduced, and these osteoclasts present an abnormal ruffled border over the bone surface. Collectively, these data support a model whereby OA/Gpnmb acts as a negative regulator of osteoclast differentiation and survival but not function by inhibiting the ERK/AKT signaling pathways.

Role of inflammation in the aging bones

Chronic inflammation in aging is characterized by increased inflammatory cytokines, bone loss, decreased adaptation, and defective tissue repair in response to injury. Aging leads to inherent changes in mesenchymal stem cell (MSC) differentiation, resulting in impaired osteoblastogenesis. Also, the pro-inflammatory cytokines increase with aging, leading to enhanced myelopoiesis and osteoclastogenesis. Bone marrow macrophages (BMMs) play pivotal roles in osteoblast differentiation, the maintenance of hematopoietic stem cells (HSCs), and subsequent bone repair. However, during aging, little is known about the role of macrophages in the differentiation and function of MSC and HSC. Aged mammals have higher circulating pro-inflammatory cytokines than young adults, supporting the hypothesis of increased inflammation with aging. This review will aid in the understanding of the potential role(s) of pro-inflammatory (M1) and anti-inflammatory (M2) macrophages in differentiation and function of osteoblasts and osteoclasts in relation to aging.