Antiresorptive activity of osteoprotegerin requires an intact heparan sulfate-binding site

Heparan sulfate selectively inhibits the collagenase activity of cathepsin K

Cathepsin K (CtsK) is a cysteine protease with potent collagenase activity. CtsK is highly expressed by bone-resorbing osteoclasts and plays an essential role in resorption of bone matrix. Although CtsK is known to bind heparan sulfate (HS), the structural details of the interaction, and how HS regulates the biological functions of CtsK, remains largely unknown. In this report, we discovered that HS is a multifaceted regulator of the structure and function of CtsK. Structurally, HS forms a highly stable complex with CtsK and induces its dimerization. Co-crystal structures of CtsK with bound HS oligosaccharides reveal the location of the HS binding site and suggest how HS may support dimerization. Functionally, HS plays a dual role in regulating the enzymatic activity of CtsK. While it preserves the peptidase activity of CtsK by stabilizing its active conformation, it inhibits the collagenase activity of CtsK in a sulfation level-dependent manner. These opposing effects can be explained by our finding that the HS binding site is remote from the active site, which allows HS to specifically inhibit the collagenase activity without affecting the peptidase activity. At last, we show that structurally defined HS oligosaccharides effectively block osteoclast resorption of bone in vitro without inhibiting osteoclast differentiation, which suggests that HS-based oligosaccharide might be explored as a new class of selective CtsK inhibitor for many diseases involving exaggerated bone resorption.

Osteocyte-mediated mechanical response controls osteoblast differentiation and function

Low bone mass is a pervasive global health concern, with implications for osteoporosis, frailty, disability, and mortality. Lifestyle factors, including sedentary habits, metabolic dysfunction, and an aging population, contribute to the escalating prevalence of osteopenia and osteoporosis. The application of mechanical load to bone through physical activity and exercise prevents bone loss, while sufficient mechanical load stimulates new bone mass acquisition. Osteocytes, cells embedded within the bone, receive mechanical signals and translate these mechanical cues into biological signals, termed mechano-transduction. Mechano-transduction signals regulate other bone resident cells, such as osteoblasts and osteoclasts, to orchestrate changes in bone mass. This review explores the mechanisms through which osteocyte-mediated response to mechanical loading regulates osteoblast differentiation and bone formation. An overview of bone cell biology and the impact of mechanical load will be provided, with emphasis on the mechanical cues, mechano-transduction pathways, and factors that direct progenitor cells toward the osteoblast lineage. While there are a wide range of clinically available treatments for osteoporosis, the majority act through manipulation of the osteoclast and may have significant disadvantages. Despite the central role of osteoblasts to the deposition of new bone, few therapies directly target osteoblasts for the preservation of bone mass. Improved understanding of the mechanisms leading to osteoblastogenesis may reveal novel targets for translational investigation.

Control of tissue homeostasis by the extracellular matrix: Synthetic heparan sulfate as a promising therapeutic for periodontal health and bone regeneration

Proteoglycans are core proteins associated with carbohydrate/sugar moieties that are highly variable in disaccharide composition, which dictates their function. These carbohydrates are named glycosaminoglycans, and they can be attached to proteoglycans or found free in tissues or on cell surfaces. Glycosaminoglycans such as hyaluronan, chondroitin sulfate, dermatan sulfate, keratan sulfate, and heparin/heparan sulfate have multiple functions including involvement in inflammation, immunity and connective tissue structure, and integrity. Heparan sulfate is a highly sulfated polysaccharide that is abundant in the periodontium including alveolar bone. Recent evidence supports the contention that heparan sulfate is an important player in modulating interactions between damage associated molecular patterns and inflammatory receptors expressed by various cell types. The structure of heparan sulfate is reported to dictate its function, thus, the utilization of a homogenous and structurally defined heparan sulfate polysaccharide for modulation of cell function offers therapeutic potential. Recently, a chemoenzymatic approach was developed to allow production of many structurally defined heparan sulfate carbohydrates. These oligosaccharides have been studied in various pathological inflammatory conditions to better understand their function and their potential application in promoting tissue homeostasis. We have observed that specific size and sulfation patterns can modulate inflammation and promote tissue maintenance including an anabolic effect in alveolar bone. Thus, new evidence provides a strong impetus to explore heparan sulfate as a potential novel therapeutic agent to treat periodontitis, support alveolar bone maintenance, and promote bone formation.

Heparan sulfate selectively inhibits the collagenase activity of cathepsin K

Cathepsin K (CtsK) is a cysteine protease with potent collagenase activity. CtsK is highly expressed by bone-resorbing osteoclasts and plays an essential role in bone remodeling. Although CtsK is known to bind heparan sulfate (HS), the structural details of the interaction, and how HS ultimately regulates the biological functions of CtsK, remains largely unknown. In this report, we determined that CtsK preferably binds to larger HS oligosaccharides, such as dodecasaccharides (12mer), and that the12mer can induce monomeric CtsK to form a stable dimer in solution. Interestingly, while HS has no effect on the peptidase activity of CtsK, it greatly inhibits the collagenase activity of CtsK in a manner dependent on sulfation level. By forming a complex with CtsK, HS was able to preserve the full peptidase activity of CtsK for prolonged periods, likely by stabilizing its active conformation. Crystal structures of Ctsk with a bound 12mer, alone and in the presence of the endogenous inhibitor cystatin-C reveal the location of HS binding is remote from the active site. Mutagenesis based on these complex structures identified 6 basic residues of Ctsk that play essential roles in mediating HS-binding. At last, we show that HS 12mers can effectively block osteoclast resorption of bone in vitro. Combined, we have shown that HS can function as a multifaceted regulator of CtsK and that HS-based oligosaccharide might be explored as a new class of selective CtsK inhibitor in many diseases that involve exaggerated bone resorption.

The molecular mechanisms of glycosaminoglycan biosynthesis regulating chondrogenesis and endochondral ossification

Disorders of chondrocyte differentiation and endochondral osteogenesis are major underlying factors in skeletal developmental disorders, including tibial dysplasia (TD), osteoarthritis (OA), chondrodysplasia (ACH), and multiple epiphyseal dysplasia (MED). Understanding the cellular and molecular pathogenesis of these disorders is crucial for addressing orthopedic diseases resulting from impaired glycosaminoglycan synthesis. Glycosaminoglycan is a broad term that refers to the glycan component of proteoglycan macromolecules. It is an essential component of the cartilage extracellular matrix and plays a vital role in various biological processes, including gene transcription, signal transduction, and chondrocyte differentiation. Recent studies have demonstrated that glycosaminoglycan biosynthesis plays a regulatory role in chondrocyte differentiation and endochondral osteogenesis by modulating various growth factors and signaling molecules. For instance, glycosaminoglycan is involved in mediating pathways such as Wnt, TGF-β, FGF, Ihh-PTHrP, and O-GlcNAc glycosylation, interacting with transcription factors SOX9, BMPs, TGF-β, and Runx2 to regulate chondrocyte differentiation and endochondral osteogenesis. To propose innovative approaches for addressing orthopedic diseases caused by impaired glycosaminoglycan biosynthesis, we conducted a comprehensive review of the molecular mechanisms underlying chondrocyte glycosaminoglycan biosynthesis, which regulates chondrocyte differentiation and endochondral osteogenesis. Our analysis considers the role of genes, glycoproteins, and associated signaling pathways during chondrogenesis and endochondral ossification.

Glycobiology in osteoclast differentiation and function

Glycans, either alone or in complex with glycan-binding proteins, are essential structures that can regulate cell biology by mediating protein stability or receptor dimerization under physiological and pathological conditions. Certain glycans are ligands for lectins, which are carbohydrate-specific receptors. Bone is a complex tissue that provides mechanical support for muscles and joints, and the regulation of bone mass in mammals is governed by complex interplay between bone-forming cells, called osteoblasts, and bone-resorbing cells, called osteoclasts. Bone erosion occurs when bone resorption notably exceeds bone formation. Osteoclasts may be activated during cancer, leading to a range of symptoms, including bone pain, fracture, and spinal cord compression. Our understanding of the role of protein glycosylation in cells and tissues involved in osteoclastogenesis suggests that glycosylation-based treatments can be used in the management of diseases. The aims of this review are to clarify the process of bone resorption and investigate the signaling pathways mediated by glycosylation and their roles in osteoclast biology. Moreover, we aim to outline how the lessons learned about these approaches are paving the way for future glycobiology-focused therapeutics.

Multiple Factors Involved in Bone Damage Caused by Chikungunya Virus Infection

Chronic cases of chikungunya fever represent a public health problem in countries where the virus circulates. The disease is prolonged, in some cases, for years, resulting in disabling pain and bone erosion among other bone and joint problems. As time progresses, tissue damage is persistent, although the virus has not been found in blood or joints. The pathogenesis of these conditions has not been fully explained. Additionally, it has been considered that there are multiple factors that might intervene in the viral pathogenesis of the different conditions that develop. Other mechanisms involved in osteoarthritic diseases of non-viral origin could help explain how damage is produced in chronic conditions. The aim of this review is to analyze the molecular and cellular factors that could be involved in the tissue damage generated by different infectious conditions of the chikungunya virus.

Dietary Effects of Nanopowder Eggshells on Mineral Contents, Bone Turnover Biomarkers, and Regulators of Bone Resorption in Healthy Rats and Ovariectomy-Induced Osteoporosis Rat Model

Postmenopausal osteoporosis is a critical issue for female health worldwide. This current study was designed to evaluate the role of nanopowder eggshell (NPES) in healthy and ovariectomy-induced osteoporosis rats. Fifty-six female rats were divided into healthy rats (35) and ovariectomized rats (21). The healthy rats were subdivided into five groups (G1-G5) and received one of the following treatments: saline, 20 or 40 mg/kg of calcium carbonate, and 20 or 40 mg/kg of NPES. The 21 ovariectomized rats were divided into three groups (G6-G8) and received either saline, 40 mg/kg of calcium carbonate, or 40 mg/kg of NPES. Biochemical and histopathological assessments of bone formation and resorption were performed. Biomarkers of bone formation (calcium and osteocalcin (OCN)) and calcium content in left femur ashes were significantly higher in healthy rats given 40-mg/kg NPES than in healthy control rats and healthy rats given 40-mg/kg calcium carbonate. The ovariectomized groups had significantly lower levels of vitamin D3, OCN, and osteoprotegerin (OPG) than the healthy control. Alanine transaminase (ALT), alkaline phosphatase (ALP), and receptor activator of nuclear factor-κB ligand (RANKL) were significantly increased in the ovariectomized group than in the healthy control group. Treatment with NPES and calcium carbonate reduced liver enzymes in ovariectomized rats. NPES treatment significantly increased Vit D3, OCN, OPG, and bone ash mineral content (calcium, magnesium, zinc, and phosphorus) in ovariectomized rats. NPES also increased femur cortical thickness, osteoblast number, and collagen fiber. The current study suggests that NPES can modulate bone turnover biomarkers and increase bone trace elements. Moreover, NPES alleviates bone resorption in ovariectomy-induced osteoporosis.

Targeting heparan sulfate-protein interactions with oligosaccharides and monoclonal antibodies

Heparan sulfate-binding proteins (HSBPs) are structurally diverse extracellular and membrane attached proteins that interact with HS under normal physiological conditions. Interactions with HS offer an additional level of control over the localization and function of HSBPs, which enables them to behave in a more refined manner. Because all cell signaling events start at the cell membrane, and cell-cell communication relies on translocation of soluble factors across the extracellular matrix, HS occupies an apical position in cellular signal transduction by interacting with hundreds of growth factors, cytokines, chemokines, enzymes, enzyme inhibitors, receptors and adhesion molecules. These extracellular and membrane proteins can play important roles in physiological and pathological conditions. For most HS-binding proteins, the interaction with HS represents an essential element in regulating their normal physiological functions. Such dependence on HS suggests that manipulating HS-protein interactions could be explored as a therapeutic strategy to selectively antagonize/activate HS-binding proteins. In this review, we will discuss current understanding of the diverse nature of HS-HSBP interactions, and the latest advancements in targeting the HS-binding site of HSBPs using structurally-defined HS oligosaccharides and monoclonal antibodies.

Osteoclasts, Master Sculptors of Bone

Osteoclasts are multinucleated cells with the unique ability to resorb bone matrix. Excessive production or activation of osteoclasts leads to skeletal pathologies that affect a significant portion of the population. Although therapies that effectively target osteoclasts have been developed, they are associated with sometimes severe side effects, and a fuller understanding of osteoclast biology may lead to more specific treatments. Along those lines, a rich body of work has defined essential signaling pathways required for osteoclast formation, function, and survival. Nonetheless, recent studies have cast new light on long-held views regarding the origin of these cells during development and homeostasis, their life span, and the cellular sources of factors that drive their production and activity during homeostasis and disease. In this review, we discuss these new findings in the context of existing work and highlight areas of ongoing and future investigation.

Biochemical characterization of a disease-causing human osteoprotegerin variant

Recently, a human mutation of OPG was identified to be associated with familial forms of osteoarthritis. This missense mutation (c.1205A =  > T; p.Stop402Leu) occurs on the stop codon of OPG, which results in a 19-residue appendage to the C-terminus (OPG+19). The biochemical consequence of this unusual sequence alteration remains unknown. Here we expressed OPG+19 in 293 cells and the mutant OPG was purified to homogeneity by heparin affinity chromatography and size exclusion chromatography. We found that in sharp contrast to wildtype OPG, which mainly exists in dimeric form, OPG+19 had a strong tendency to form higher-order oligomers. To our surprise, the hyper-oligomerization of OPG+19 had no impact on how it binds cell surface heparan sulfate, how it inhibits RANKL-induced osteoclastogenesis and TRAIL-induced chondrocytes apoptosis. Our data suggest that in biological contexts where OPG is known to play a role, OPG+19 functions equivalently as wildtype OPG. The disease-causing mechanism of OPG+19 likely involves an unknown function of OPG in cartilage homeostasis and mineralization. By demonstrating the biochemical nature of this disease-causing OPG mutant, our study will likely help elucidating the biological roles of OPG in cartilage biology.

Mutation in the CCAL1 locus accounts for bidirectional process of human subchondral bone turnover and cartilage mineralization

OBJECTIVES

To study the mechanism by which the readthrough mutation in TNFRSF11B, encoding osteoprotegerin (OPG) with additional 19 amino acids at its C-terminus (OPG-XL), causes the characteristic bidirectional phenotype of subchondral bone turnover accompanied by cartilage mineralization in chondrocalcinosis patients.

METHODS

OPG-XL was studied by human induced pluripotent stem cells expressing OPG-XL and two isogenic CRISPR/Cas9-corrected controls in cartilage and bone organoids. Osteoclastogenesis was studied with monocytes from OPG-XL carriers and matched healthy controls followed by gene expression characterization. Dual energy X-ray absorptiometry scans and MRI analyses were used to characterize the phenotype of carriers and non-carriers of the mutation.

RESULTS

Human OPG-XL carriers relative to sex- and age-matched controls showed, after an initial delay, large active osteoclasts with high number of nuclei. By employing hiPSCs expressing OPG-XL and isogenic CRISPR/Cas9-corrected controls to established cartilage and bone organoids, we demonstrated that expression of OPG-XL resulted in excessive fibrosis in cartilage and high mineralization in bone accompanied by marked downregulation of MGP, encoding matrix Gla protein, and upregulation of DIO2, encoding type 2 deiodinase, gene expression, respectively.

CONCLUSIONS

The readthrough mutation at CCAL1 locus in TNFRSF11B identifies an unknown role for OPG-XL in subchondral bone turnover and cartilage mineralization in humans via DIO2 and MGP functions. Previously, OPG-XL was shown to affect binding between RANKL and heparan sulphate (HS) resulting in loss of immobilized OPG-XL. Therefore, effects may be triggered by deficiency in the immobilization of OPG-XL Since the characteristic bidirectional pathophysiology of articular cartilage calcification accompanied by low subchondral bone mineralization is also a hallmark of OA pathophysiology, our results are likely extrapolated to common arthropathies.

Biological role of heparan sulfate in osteogenesis: A review

Heparan sulfate (HS) is extensively expressed in cells, for example, cell membrane and extracellular matrix of most mammalian cells and tissues, playing a key role in the growth and development of life by maintaining homeostasis and implicating in the etiology and diseases. Recent studies have revealed that HS is involved in osteogenesis via coordinating multiple signaling pathways. The potential effect of HS on osteogenesis is a complicated and delicate biological process, which involves the participation of osteocytes, chondrocytes, osteoblasts, osteoclasts and a variety of cytokines. In this review, we summarized the structural and functional characteristics of HS and highlighted the molecular mechanism of HS in bone metabolism to provide novel research perspectives for the further medical research.

Small leucine-rich proteoglycans in physiological and biomechanical function of bone

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Proteoglycans (PGs) and glycosaminoglycans (GAGs) play vital roles in key signaling pathways to regulate bone homeostasis.

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The highly negatively charged GAGs are crucial in retaining bound water and modulating mechanical properties of bone.

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Age-related changes of PGs, GAGs, and bound water contribute to deterioration of bone quality during aging.

Proteoglycans (PGs) contain long unbranched glycosaminoglycan (GAG) chains attached to core proteins. In the bone extracellular matrix, PGs represent a class of non-collagenous proteins, and have high affinity to minerals and collagen. Considering the highly negatively charged character of GAGs and their interfibrillar positioning interconnecting with collagen fibrils, PGs and GAGs play pivotal roles in maintaining hydrostatic and osmotic pressure in the matrix. In this review, we will discuss the role of PGs, especially the small leucine-rich proteoglycans, in regulating the bioactivity of multiple cytokines and growth factors, and the bone turnover process. In addition, we focus on the coupling effects of PGs and GAGs in the hydration status of bone extracellular matrix, thus modulating bone biomechanical properties under physiological and pathological conditions.

Bi-directional regulation functions of lanthanum-substituted layered double hydroxide nanohybrid scaffolds via activating osteogenesis and inhibiting osteoclastogenesis for osteoporotic bone regeneration

Rationale: Osteoporotic patients suffer symptoms of excessive osteoclastogenesis and impaired osteogenesis, resulting in a great challenge to treat osteoporosis-related bone defects. Based on the positive effect of rare earth elements on bone metabolism and bone regeneration, we try to prove the hypothesis that the La³⁺ dopants in lanthanum-substituted MgAl layered double hydroxide (La-LDH) nanohybrid scaffolds simultaneously activate osteogenesis and inhibit osteoclastogenesis.

Methods: A freeze-drying technology was employed to construct La-LDH nanohybrid scaffolds. The in vitro osteogenic and anti-osteoclastogenic activities of La-LDH nanohybrid scaffolds were evaluated by using ovariectomized rat bone marrow stromal cells (rBMSCs-OVX) and bone marrow-derived macrophages (BMMs) as cell models. The in vivo bone regeneration ability of the scaffolds was investigated by using critical-size calvarial bone defect model of OVX rats.

Results: La-LDH nanohybrid scaffolds exhibited three-dimensional macroporous structure, and La-LDH nanoplates arranged perpendicularly on chitosan organic matrix. The La³⁺ dopants in the scaffolds promote proliferation and osteogenic differentiation of rBMSCs-OVX by activating Wnt/β-catenin pathway, leading to high expression of ALP, Runx-2, COL-1 and OCN genes. Moreover, La-LDH scaffolds significantly suppressed RANKL-induced osteoclastogenesis by inhibiting NF-κB signaling pathway. As compared with the scaffolds without La³⁺ dopants, La-LDH scaffolds provided more favourable microenvironment to induce new bone in-growth along macroporous channels.

Conclusion: La-LDH nanohybrid scaffolds possessed the bi-directional regulation functions on osteogenesis and osteoclastogenesis for osteoporotic bone regeneration. The modification of La³⁺ dopants in bone scaffolds provides a novel strategy for osteoporosis-related bone defect healing.

The heparan sulfate proteoglycan Syndecan-1 influences local bone cell communication via the RANKL/OPG axis

The heparan sulfate proteoglycan Syndecan-1, a mediator of signals between the extracellular matrix and cells involved is able to interact with OPG, one of the major regulators of osteoclastogenesis. The potential of osteoblasts to induce osteoclastogenesis is characterized by a switch of OPG (low osteoclastogenic potential) towards RANKL production (high osteoclastogenic potential). In the present study, we investigated the influence of endogenous Syndecan-1 on local bone-cell-communication via the RANKL/OPG-axis in murine osteoblasts and osteoclasts in wild type and Syndecan-1 lacking cells. Syndecan-1 expression and secretion was increased in osteoblasts with high osteoclastogenic potential. Syndecan-1 deficiency led to increased OPG release by osteoblasts that decreased the availability of RANKL. In co-cultures of Syndecan-1 deficient osteoblasts with osteoclast these increased OPG in supernatant caused decreased development of osteoclasts. Syndecan-1 and RANKL level were increased in serum of aged WT mice, whereas Syndecan-1 deficient mice showed high serum OPG concentration. However, bone structure of Syndecan-1 deficient mice was not different compared to wild type. In conclusion, Syndecan-1 could be regarded as a new modulator of bone-cell-communication via RANKL/OPG axis. This might be of high impact during bone regeneration or bone diseases like cancer where Syndecan-1 expression is known to be even more prevalent.