An antibody against Siglec-15 promotes bone formation and fracture healing by increasing TRAP^(+)mononuclear cells and PDGF-BB secretion

Osteoporosis(OP)is a common age-related disease characterized by a deterioration of bone mass and structure that predisposes patients to fragility fractures.Pharmaceutical therapies that promote anabolic bone formation in OP patients and OP-induced fracture are needed.We investigated whether a neutralizing antibody against Siglec-15 can simultaneously inhibit bone resorption and stimulate bone formation.We found that the multinucleation of osteoclasts was inhibited in SIGLEC-15 conditional knockout mice and mice undergoing Siglec-15 neutralizing antibody treatment.The secretion of platelet-derived growth factor-BB(PDGF-BB),the number of tartrate-resistant acid phosphatase-positive(TRAP+)mononuclear cells,and bone formation were significantly increased in the SIGLEC-15 conditional knockout mice and antibody-treated mice.The anabolic effect of the Siglec-15 neutralizing antibody on bone formation was blunted in mice with Pdgfb deleted in TRAP-1"cells.These findings showed that the anabolic effect of the Siglec-15 neutralizing antibody was mediated by elevating PDGF-BB production of TRAP4 mononuclear cells.To test the therapeutic potential of the Siglec-15 neutralizing antibody,we injected the antibody in an ovariectomy-induced osteoporotic mouse model,which mimics postmenopausal osteoporosis in women,and in two fracture healing models because fracture is the most serious health consequence of osteoporosis.The Siglec-15 neutralizing antibody effectively reduced bone resorption and stimulated bone formation in estrogen deficiency-induced osteoporosis.Of note,the Siglec-15 neutralizing antibody promoted intramembranous and endochondral ossification at the damaged area of cortical bone in fracture healing mouse models.Thus,the Siglec-15 neutralizing antibody shows significant translational potential as a novel therapy for OP and bone fracture.

Osteoclast-derived apoptotic bodies couple bone resorption and formation in bone remodeling

Bone remodeling is precisely coordinated by bone resorption and formation.Apoptotic osteoclasts generate large amounts of apoptotic bodies(ABs)marking the end of the bone resorption phase,whereas the functions of osteoclast-derived ABs remain largely unknown.Here,we identified the molecular profile of ABs derived from osteoclasts at distinct differentiation stages and investigated their corresponding functions.ABs were isolated from apoptotic bone marrow macrophages,preosteoclasts,and mature osteoclasts induced by staurosporine.Proteomic signature analysis with liquid chromatography-tandem mass spectrometry suggested marked protein cargo differences among the different ABs.Further bioinformatic analysis showed that the proteomic signatures of the ABs were highly similar to those of their parental cells.Functionally,pOC-ABs induced endothelial progenitor cell differentiation and increased CD31hiEmcnhi endothelial cell formation in a murine bone defect model via their PDGF-BB cargo.mOC-ABs induced osteogenic differentiation of mesenchymal stem cells and facilitated osteogenesis via RANKL reverse signaling.In summary,we mapped the detailed proteomic landscapes of ABs derived from osteoclasts and showed that their potential biological roles are important in coupling bone formation with resorption during bone remodeling.

Sialylation of TLR2 initiates osteoclast fusion

The molecular control of osteoclast formation is still not clearly elucidated. Here, we show that a process of cell recognition mediated by Siglec15-TLR2 binding is indispensable and occurs prior to cell fusion in RANKL-mediated osteoclastogenesis. Siglec15 has been shown to regulate osteoclastic bone resorption. However, the receptor for Siglec15 has not been identified, and the signaling mechanism involving Siglec15 in osteoclast function remains unclear. We found that Siglec15 bound sialylated TLR2 as its receptor and that the binding of sialylated TLR2 to Siglec15 in macrophages committed to the osteoclast-lineage initiated cell fusion for osteoclast formation, in which sialic acid was transferred by the sialyltransferase ST3 Gal1. Interestingly, the expression of Siglec15 in macrophages was activated by M-CSF, whereas ST3 Gal1 expression was induced by RANKL. Both Siglec15-specific deletion in macrophages and intrafemoral injection of sialidase abrogated cell recognition and reduced subsequent cell fusion for the formation of osteoclasts, resulting in increased bone formation in mice. Thus, our results reveal that cell recognition mediated by the binding of sialylated TLR2 to Siglec15 initiates cell fusion for osteoclast formation.

Bone-targeted pH-responsive cerium nanoparticles for anabolic therapy in osteoporosis

Antiresorptive drugs are widely used for treatment of osteoporosis and cancer bone metastasis,which function mainly through an overall inhibition of osteoclast.However,not all osteoclasts are“bone eaters”;preosteoclasts(pOCs)play anabolic roles in bone formation and angiogenesis through coupling with osteoblasts and secreting platelet derived growth factor-BB(PDGF-BB).In this study,a bone-targeted pH-responsive nanomaterial was designed for selectively eliminating mature osteoclasts(mOCs)without affecting pOCs.Biocompatible cerium nano-system(CNS)was guided to the acidic extracellular microenvironment created by mOCs and gained oxidative enzymatic activity.Oxidative CNS decreased the viability of mOCs through accumulating intracellular reactive oxygen species and enhancing calcium oscillation.Non-acid secreting anabolic pOCs were thus preserved and kept producing PDGF-BB,which lead to mesenchymal stem cell osteogenesis and endothelial progenitor cell angiogenesis via PI3K-Akt activated focal adhesion kinase.In treating osteoporotic ovariectomized mice,CNS showed better protective effects compare with the current first line antiresorptive drug due to the better anabolic effects marked by higher level of bone formation and vascularization.We provided a novel anabolic therapeutic strategy in treating bone disorders with excessive bone resorption.

Modulation of SIRT6 activity acts as an emerging therapeutic implication for pathological disorders in the skeletal system

The skeletal system is a dynamically balanced system, which undergoes continuous bone resorption and formation to maintain bone matrix homeostasis. As an important ADP-ribosylase and NAD+-dependent deacylase, SIRT6 (SIR2-like protein 6) is widely expressed on various kinds of bone cells, such as chondrocytes, osteoblasts, osteoclasts. The aberration of SIRT6 impairs gene expression (e.g., NF-κB and Wnt target genes) and cellular functions (e.g., DNA repair, glucose and lipid metabolism, telomeric maintenance), which disturbs the dynamic balance and ultimately leads to several bone-related diseases. In this review, we summarize the critical roles of SIRT6 in the onset and progression of bone-related diseases including osteoporosis, osteoarthritis, rheumatoid arthritis, and intervertebral disc degeneration, as well as the relevant signaling pathways. In addition, we discuss the advances in the development of SIRT6 activators and elucidate their pharmacological profiles, which may provide novel treatment strategies for these skeletal diseases.

Long non-coding RNA HCAR promotes endochondral bone repair by upregulating VEGF and MMP13 in hypertrophic chondrocyte through sponging miR-15b-5p

Endochondral bone formation is an important route for bone repair.Although emerging evidence has revealed the functions of long non-coding RNAs(lncRNAs)in bone and cartilage development,the effect of lncRNAs in endochondral bone repair is still largely unknown.Here,we identified a lncRNA,named Hypertrophic Chondrocyte Angiogenesis-related lncRNA(HCAR),and proved it to promote the endochondral bone repair by upregulating the expression of matrix metallopeptidase 13(Mmp13)and vascular endothelial growth factorα(Vegfa)in hypertrophic chondrocytes.Lnc-HCAR knockdown in hypertrophic chondrocytes restrained the cartilage matrix remodeling and decrease the CD31hiEmcnhi vessels number in a bone repair model.Mechanistically,we proved that lnc-HCAR was mainly enriched in the cytoplasm using fluorescence in situ hybridization(FISH)assay,and it acted as a molecular sponge for miR-15b-5p.Further,in hypertrophic chondrocytes,lnc-HCAR competitively bound to miR-15b-5p to increase Vegfa and Mmp13 expression.Our results proved that lncRNA is deeply involved in endochondral bone repair,which will provide a new theoretical basis for future strategies for promoting fracture healing.