Glycosaminoglycans inhibit the adherence and the spreading of osteoclasts and their precursors: Role in osteoclastogenesis and bone resorption

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.

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.

Heparin and Derivatives for Advanced Cell Therapies

Heparin and its derivatives are saving thousands of human lives annually, by successfully preventing and treating thromboembolic events. Although the mode of action during anticoagulation is well studied, their influence on cell behavior is not fully understood as is the risk of bleeding and other side effects. New applications in regenerative medicine have evolved supporting production of cell-based therapeutics or as a substrate for creating functionalized matrices in biotechnology. The currently resurgent interest in heparins is related to the expected combined anti-inflammatory, anti-thrombotic and anti-viral action against COVID-19. Based on a concise summary of key biochemical and clinical data, this review summarizes the impact for manufacturing and application of cell therapeutics and highlights the need for discriminating the different heparins.

New insights into the role of glycosaminoglycans in the endosteal bone microenvironment

The bone microenvironment is a complex tissue in which heterogeneous cell populations of hematopoietic and mesenchymal origin interact with environmental cues to maintain tissue integrity. Both cellular and matrix components are subject to physiologic challenges and can dynamically respond by modifying cell/matrix interactions. When either component is impaired, the physiologic balance is lost. Here, we review the current state of knowledge of how glycosaminoglycans - organic components of the bone extracellular matrix - influence the bone micromilieu. We point out how they interact with mediators of distinct signaling pathways such as the RANKL/OPG axis, BMP and WNT signaling, and affect the activity of bone remodeling cells within the endosteal niche summarizing their potential for therapeutic intervention.

Trabecular Bone Deficit and Enhanced Anabolic Response to Re-Ambulation after Disuse in Perlecan-Deficient Skeleton

Perlecan/Hspg2, a large monomeric heparan sulfate proteoglycan, is found in the basement membrane and extracellular matrix, where it acts as a matrix scaffold, growth factor depot, and tissue barrier. Perlecan deficiency leads to skeletal dysplasia in Schwartz-Jampel Syndrome (SJS) and is a risk factor for osteoporosis. In the SJS-mimicking murine model (Hypo), inferior cortical bone quality and impaired mechanotransduction in osteocytes were reported. This study focused on trabecular bone, where perlecan deficiency was hypothesized to result in structural deficit and altered response to disuse and re-loading. We compared the Hypo versus WT trabecular bone in both axial and appendicular skeletons of 8-38-week-old male mice, and observed severe trabecular deficit in Hypo mice, approximately 50% reduction of Tb.BV/TV regardless of skeletal site and animal age. Defects in endochondral ossification (e.g., accelerated mineralization), increases in osteoclast activity, and altered differentiation of bone progenitor cells in marrow contributed to the Hypo phenotype. The Hypo trabecular bone deteriorated further under three-week hindlimb suspension as did the WT. Re-ambulation partially recovered the lost trabecular bone in Hypo, but not in WT mice. The novel finding that low-impact loading could counter detrimental disuse effects in the perlecan-deficient skeleton suggests a strategy to maintain skeletal health in SJS patients.

Nanoparticulate mineralized collagen glycosaminoglycan materials directly and indirectly inhibit osteoclastogenesis and osteoclast activation

The ability of the extracellular matrix (ECM) to direct cell fate has generated the potential for developing a materials-only strategy for tissue regeneration. Previously, we described a nanoparticulate mineralized collagen glycosaminoglycan (MC-GAG) material that efficiently induced osteogenic differentiation of human mesenchymal stem cells (hMSCs) and calvarial bone healing without exogenous growth factors or progenitor cell expansion. In this work, we evaluated the interactions between MC-GAG and primary human osteoclasts (hOCs). In the absence of hMSCs, mineralized Col-GAG materials directly inhibited hOC viability, proliferation, and resorption in contrast to nonmineralized Col-GAG, which demonstrated a modest inhibition of resorptive activity only. Cocultures containing differentiating hMSCs with hOCs demonstrated increased hOC-mediated resorption only on Col-GAG while MC-GAG cocultures continued to inhibit resorption. Unlike Col-GAG, hMSCs on MC-GAG expressed increased amounts of osteoprotegerin (OPG) protein, the major endogenous osteoclast inhibitor. Interestingly, OPG expression was found to be antagonized by small mothers against decapentaplegic1/5 (Smad1/5) phosphorylation, an obligate pathway for osteogenic differentiation of hMSCs on MC-GAG, and potentiated by extracellular signal-regulated kinase (ERK1/2) phosphorylation. Collectively, these results suggested that the MC-GAG material both directly inhibited the osteoclast viability, proliferation, and resorptive activity as well as induced hMSCs to secrete osteoprotegerin, an antiosteoclastogenic factor, via a signalling pathway distinct from osteogenic differentiation.

Suppressive effect of syndecan ectodomains and N-desulfated heparins on osteoclastogenesis via direct binding to macrophage-colony stimulating factor

Syndecans, a family of cell surface heparan sulfate proteoglycans, regulate cell differentiation via binding of their heparan sulfate chains to growth factors and cytokines and play a role in tumor growth and progression, wound repair, and intestinal mucosal damage. However, the functional and mechanistic roles of syndecans in osteoclast differentiation and bone metabolism are yet unclear. Here, we demonstrated that post-translationally glycosylated ectodomains of syndecan-1 to 4 obtained from mammalian cells efficiently suppressed osteoclast differentiation compared to those obtained from Escherichia coli with no systems for glycosylation. A concomitant decrease in the expression of osteoclast markers such as nuclear factor of activated T cells 1 (NFATc1), c-Fos, and ATP6V0D2 was observed. In addition, heparan sulfate and selectively N-desulfated heparin derivatives with 2-O- and 6-O-sulfate groups and no anticoagulant activity in blood inhibited osteoclast differentiation. The inhibitory effects of syndecan ectodomains, heparan sulfate, and N-desulfated heparin derivatives on osteoclast differentiation were attributed to their direct binding to the macrophage-colony stimulating factor (M-CSF), resulting in the blocking of M-CSF-mediated downstream signals such as extracellular signal-regulated kinase (ERK), c-JUN N-terminal kinase (JNK), p38, and Akt. Furthermore, mice injected with syndecan ectodomains, heparan sulfate, and N-desulfated heparin derivatives into periosteal regions of calvaria showed reduction in the formation of tartrate-resistant acid phosphatase (TRAP)-positive mature osteoclasts on the calvarial bone surface, thereby exhibiting decreased bone resorption. Together, these results revealed a novel role of heparan sulfate chains of syndecan ectodomains in the regulation of osteoclast differentiation.

Dimerization interface of osteoprotegerin revealed by hydrogen-deuterium exchange mass spectrometry

Previous structural studies of osteoprotegerin (OPG), a crucial negative regulator of bone remodeling and osteoclastogenesis, were mostly limited to the N-terminal ligand-binding domains. It is now known that the three C-terminal domains of OPG also play essential roles in its function by mediating OPG dimerization, OPG-heparan sulfate (HS) interactions, and formation of the OPG-HS-receptor activator of nuclear factor κB ligand (RANKL) ternary complex. Employing hydrogen-deuterium exchange MS methods, here we investigated the structure of full-length OPG in complex with HS or RANKL in solution. Our data revealed two noteworthy aspects of the OPG structure. First, we found that the interconnection between the N- and C-terminal domains is much more rigid than previously thought, possibly because of hydrophobic interactions between the fourth cysteine-rich domain and the first death domain. Second, we observed that two hydrophobic clusters located in two separate C-terminal domains directly contribute to OPG dimerization, likely by forming a hydrophobic dimerization interface. Aided by site-directed mutagenesis, we further demonstrated that an intact dimerization interface is essential for the biological activity of OPG. Our study represents an important step toward deciphering the structure-function relationship of the full-length OPG protein.

Periodontal reconstruction by heparan sulfate mimetic-based matrix therapy in Porphyromonas gingivalis-infected mice

BACKGROUND

Periodontitis is a set of chronic inflammatory diseases affecting the supporting structures of the teeth, during which a persistent release of lytic enzymes and inflammatory mediators causes a self-perpetuating vicious cycle of tissue destruction and repair. A matrix-based therapy using a heparan sulfate (HS) analogue called ReGeneraTing Agent (RGTA) replaces destroyed HS by binding to available heparin-binding sites of structural molecules, leading to restoration of tissue homeostasis in several inflammatory tissue injuries, including a hamster periodontitis model.

METHODS

The ability of RGTA to restore the periodontium was tested in a model of Porphyromonas gingivalis-infected Balb/cByJ mice. After 12 weeks of disease induction, mice were treated weekly with saline or RGTA (1.5 mg/kg) for 8 weeks. Data were analyzed by histomorphometry.

RESULTS

RGTA treatment restored macroscopic bone loss. This was related to (1) a significant reduction in gingival inflammation assessed by a decrease in infiltrated connective tissue, particularly in cells expressing interleukin 1ß, an inflammatory mediator selected as a marker of inflammation; (2) a normalization of bone resorption parameters, i.e. number, activation and activity of osteoclasts, and number of preosteoclasts; (3) a powerful bone formation reaction. The Sharpey's fibers of the periodontal ligament recovered their alkaline phosphatase coating. This was obtained while P. gingivalis infection was maintained throughout the treatment period.

CONCLUSIONS

RGTA treatment was able to control the chronic inflammation characteristic of periodontitis and blocked destruction of periodontal structures. It ensured tissue regeneration with recovery of the periodontium's anatomy.

Injectable Gel Constructs with Regenerative and Anti-Infective Dual Effects Based on Assembled Chitosan Microspheres

There is increasing demand for biomaterials that both assist with bone regeneration and have anti-infection qualities in clinical applications. To achieve this goal, chitosan microspheres with either positive or negative charges were fabricated and then assembled as a gel for bone healing. The positively charged chitosan microspheres (CSM; ∼35.5 μm) and negatively charged O-carboxymethyl chitosan microspheres (CMCSM; ∼13.5 μm) were loaded, respectively, with bone morphogenetic protein (BMP-2) and berberine (Bbr) via swollen encapsulation and physical adsorption without a significant change in the electric charges. The release kinetics of BMP-2 and Bbr from the microspheres were also studied in vitro. The results showed that the Bbr/CMCSM microsphere group possessed high antibacterial activity against Staphylococcus aureus; the BMP-2/CSM microsphere group also had excellent cytocompatibility and improved osteoinductivity with the assistance of BMP-2. The assembled gel group consisting of Bbr/CMCSM and BMP-2/CSM had a porous structure that allowed biological signal transfer and tissue infiltration and exhibited significantly enhanced bone reconstruction compared with that of the respective microsphere groups, which should result from the osteoconductivity of the porous structure and the osteoinduction of the BMP-2 growth factor. The oppositely charged microspheres and their assembled gel provide a promising prospect for making injectable tissue-engineered constructs with regenerative and anti-infective dual effects for biomedical applications.

RGTA®-based matrix therapy - A new branch of regenerative medicine in locomotion

Matrix therapy is an innovative, minimally invasive approach in the field of regenerative medicine, that aims to promote tissue regeneration by reconstructing the cellular microenvironment following tissue injury. This approach has significant therapeutic potential in the treatment of pathologies characterized by tissue inflammation and damage, or following injury, conditions which can be incapacitating and cost-consuming. Heparan sulfate mimics, termed ReGeneraTing Agents (RGTA^®s) have emerged as a unifying approach to treat these diverse pathologies. Today, skin and corneal healing topical products have already been used in clinics, demonstrating a proof of concept in humans. In this review, we present key evidence that RGTA^®s regenerate damaged tissue in bone, muscle, tendon and nerve, with astonishing results. In animal models of bone surgical defects and inflammatory bone loss, RGTA^® induced healing of injured bones by controlling inflammation and bone resorption, and stimulated bone formation by coordinating vascularization, recruitment and differentiation of competent cells from specific niches, restoring tissue quality to that of uninjured tissue, evoking true regeneration. In models of muscle injury, RGTA^® had marked effects on healing speed and quality, evidenced by increased muscle fiber density, maturation, vascularization and reduced fibrosis, more mature motor endplates and functional recovery. Applications merging RGTA^®-based matrix therapy and cell therapy, combining Extra-Cellular Matrix reconstruction with cells required for optimal tissue repair show significant promise. Hence restoration of the proper microenvironment is a new paradigm in regenerative medicine. Harnessing the potential of RGTA^® in this brave, new vision of regenerative therapy will therefore be the focus of future studies.

Bioinspired Collagen/Glycosaminoglycan-Based Cellular Microenvironments for Tuning Osteoclastogenesis

Replicating the biocomplexity of native extracellular matrices (ECM) is critical for a deeper understanding of biochemical signals influencing bone homeostasis. This will foster the development of bioinspired biomaterials with adjustable bone-inducing properties. Collagen-based coatings containing single HA derivatives have previously been reported to promote osteogenic differentiation and modulate osteoclastogenesis and resorption depending on their sulfation degree. However, the potential impact of different GAG concentrations as well as the interplay of multiple GAGs in these coatings is not characterized in detail to date. These aspects were addressed in the current study by integrating HA and different sulfate-modified HA derivatives (sHA) during collagen in vitro fibrillogenesis. Besides cellular microenvironments with systematically altered single-GAG concentrations, matrices containing both low and high sHA (sHA1, sHA4) were characterized by biochemical analysis such as agarose gel electrophoresis, performed for the first time with sHA derivatives. The morphology and composition of the collagen coatings were altered in a GAG sulfation- and concentration-dependent manner. In multi-GAG microenvironments, atomic force microscopy revealed intermediate collagen fibril structures with thin fibrils and microfibrils. GAG sulfation altered the surface charge of the coatings as demonstrated by ζ-potential measurements revealed for the first time as well. This highlights the prospect of GAG-containing matrices to adjust defined surface charge properties. The sHA4- and the multi-GAG coatings alike significantly enhanced the viability of murine osteoclast-precursor-like RAW264.7 cells. Although in single-GAG matrices there was no dose-dependent effect on cell viability, osteoclastogenesis was significantly suppressed only on sHA4-coatings in a dose-dependent fashion. The multi-GAG coatings led to an antiosteoclastogenic effect in-between those with single-GAGs which cannot simply be attributed to the overall content of sulfate groups. These data suggest that the interplay of sGAGs influences bone cell behavior. Whether these findings translate into favorable biomaterial properties needs to be validated in vivo.

Injectable Bone Substitute Based on β-TCP Combined With a Hyaluronan-Containing Hydrogel Contributes to Regeneration of a Critical Bone Size Defect Towards Restitutio ad Integrum

In the present in vivo study, the regenerative potential of a new injectable bone substitute (IBS) composed of beta-tricalcium phosphate (β-TCP) and hyaluronan was tested in a rabbit distal femoral condyle model. To achieve this, 2 defects of 6 mm in diameter and 10 mm in length were drilled into each femur condyle in a total of 12 animals. For each animal, 1 hole was filled with the substitute material, and the other was left empty to serve as the control. After 1, 3, and 6 months, the regenerative process was analyzed by radiography as well as by histological and histomorphometrical analysis. The results revealed that bone tissue formation took place through osteoconductive processes over time, starting from the defect borders to the center. Both the β-TCP content and the hydrogel support bone tissue growth. The histomorphometrical measurements showed that the amount of bone formation in the experimental group was significantly higher compared with that found in the control group after 3 months (19.51 ± 5.08% vs. 1.96 ± 0.77%, P < .05) and 6 months (4.57 ± 1.56% vs. 0.23 ± 0.21%, P < .05). The application of the IBS gave a restitutio ad integrum result after 6 months and was associated with its nearly complete degradation, in contrast to the results found in the control group. In conclusion, the results of the present study demonstrate that the IBS contributes to sufficient bone regeneration by serving as a scaffold-like structure, combined with its degradation within 6 months.

Glycosaminoglycan mimetic associated to human mesenchymal stem cell-based scaffolds inhibit ectopic bone formation, but induce angiogenesis in vivo

Tissue engineering approaches to stimulate bone formation currently combine bioactive scaffolds with osteocompetent human mesenchymal stem cells (hMSC). Moreover, osteogenic and angiogenic factors are required to promote differentiation and survival of hMSC through improved vascularization through the damaged extracellular matrix (ECM). Glycosaminoglycans (GAGs) are ECM compounds acting as modulators of heparin-binding protein activities during bone development and regenerative processes. GAG mimetics have been proposed as ECM stabilizers and were previously described for their positive effects on bone formation and angiogenesis after local treatment. Here, we developed a strategy associating the GAG mimetic [OTR4120] with bone substitutes to optimize stem cell-based therapeutic products. We showed that [OTR4120] was able to potentiate proliferation, migration, and osteogenic differentiation of hMSC in vitro. Its link to tricalcium phosphate/hydroxyapatite scaffolds improved their colonization by hMSC. Surprisingly, when these combinations were tested in an ectopic model of bone formation in immunodeficient mice, the GAG mimetics inhibit bone formation induced by hMSC and promoted an osteoclastic activity. Moreover, the inflammatory response was modulated, and the peri-implant vascularization stimulated. All together, these findings further support the ability of GAG mimetics to organize the local ECM to coordinate the host response toward the implanted biomaterial, and to inhibit the abnormal bone formation process on a subcutaneous ectopic site.

Multifunctional properties of RANKL/RANK in cell differentiation, proliferation and metastasis

It is known that there are close relationships between bone destruction and tumor growth in bone metastasis. RANKL is a central factor in bone metastasis, inducing osteoclastogenesis mediated by its receptor RANK. Recent reports demonstrate that RANKL has important roles in organogenesis stimulating proliferation and differentiation of epithelial and stroma cells. RANKL is induced not only by cytokines and hormones but also by UV-irradiation, inflammation and carcinogens. Expression of RANK and RANKL is found in several human cancer cell lines, and RANK signaling stimulates proliferation, migration and epithelial–mesenchymal transition of cancer cells, which may be involved in metastasis via an autocrine/paracrine mechanism. RANKL regulates the number of Tregs that produce RANKL, which may affect cancer metastasis. In this review we discuss the multifunctional roles of RANKL/RANK in osteoclastogenesis, organogenesis, and the metastasis and tumorigenesis of cancer cells.

Osteoprotegerin: multiple partners for multiple functions

Osteoprotegerin (OPG) is an essential secreted protein in bone turnover due to its role as a decoy receptor for the Receptor Activator of Nuclear Factor-kB ligand (RANKL) in the osteoclasts, thus inhibiting their differentiation. However, there are additional ligands of OPG that confer various biological functions. OPG can promote cell survival, cell proliferation and facilitates migration by binding TNF-related apoptosis inducing ligand (TRAIL), glycosaminoglycans or proteoglycans. A large number of in vitro, pre-clinical and clinical studies provide evidences of OPG involvement in vascular, bone, immune and tumor biology. This review describes an overview of the different OPG ligands regulating its biological functions.

Metaphyseal bone formation induced by a new injectable β-TCP-based bone substitute: a controlled study in rabbits

PURPOSE

Adequate filling of bone defects still poses a challenge in every day clinical work. As many bone defects are irregularly shaped the need for appropriate scaffolds reaching the complete defect surface are great. The purpose of this pre-clinical pilot study was to investigate the handling, biocompatibility, biodegradation and osteoconductivity of a new pasty bone substitute (pure phase β-TCP, hyaluronic acid, methylcellulose) in bone tissue.

METHODS

In an unilateral tibial defect model the peri-implant and bone tissue response to the new pasty bone substitute was tested in New Zealand white rabbits for up to 24 weeks compared to empty controls. Analysis included HR-pQCT scans, histomorphometric evaluation and quantification of vascularization of un-decalcified histological slices.

RESULTS

After 1 week the experimental group presented significantly higher new bone volume fraction (p = 0.021) primarily consisting of immature bone matrix and higher vessel density compared to controls (p = 0.013). After 4 weeks bone formation was not significantly different to controls but was distributed more evenly throughout the defect. Bone matrix was now mineralized and trabeculae were thicker than in controls (p = 0.002) indicating faster intramedullary bone maturation. Controls presented extensive periosteal bone formation, major fibrous tissue influx and high vascularization. After 12 and 24 weeks there was no new bone detectable. There were no severe signs of inflammation at all time points.

CONCLUSION

The substitute showed an early induction of bone formation. It promoted accelerated intramedullary bone repair and maturation and prevented periosteal bone formation indicating its potential use for reconstructive surgery of bone defects.

Expression of osteoprotegerin, receptor activator of nuclear factor kappa-B ligand, tumor necrosis factor-related apoptosis-inducing ligand, stromal cell-derived factor-1 and their receptors in epithelial metastatic breast cancer cell lines

BACKGROUND

While breast cancer (BC) is the major cause of death among women worldwide, there is no guarantee of better patient survival because many of these patients develop primarily metastases, despite efforts to detect it in its early stages. Bone metastasis is a common complication that occurs in 65-80 % of patients with disseminated disease, but the molecular basis underlying dormancy, dissemination and establishment of metastasis is not understood. Our objective has been to evaluate simultaneously osteoprotegerin (OPG), receptor activator of nuclear factor kappa B ligand (RANKL), tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), stromal cell-derived factor-1 (SDF-1), and their receptors (R) in 2 human BC cell lines, MDA-MB-231 and MCF-7.

METHODS

OPG, RANKL, TRAIL and SDF-1 expression and release, in addition to the expression of their receptors has been investigated using immunofluorescence, immunocytochemistry and ELISA analyses.

RESULTS

MCF-7 cells released higher levels of OPG in conditioned media (CM) than MDA-MB-231 cells; 100 % of both types of cell expressed OPG, RANKL, TRAIL and SDF-1. Moreover, 100 % in both lines expressed membrane RANKL and RANK, whereas only 50 % expressed CXCR4. Furthermore, 100 % expressed TRAIL-R1 and R4, 30-50 % TRAIL-R2, and 40-55 % TRAIL-R3.

CONCLUSIONS

MCF-7 and MDA-MB-231 cells not only released OPG, but expressed RANKL, TRAIL and SDF-1. The majority of the cells also expressed RANK, CXCR4 and TRAIL-R. Since these ligands and their receptors are implicated in the regulation of proliferation, survival, migration and future bone metastasis during breast tumor progression, assessment of these molecules in tumor biopsies of BC patients could be useful in identifying patients with more aggressive tumors that are also at risk of bone metastasis, which may thus improve the available options for therapeutic intervention.

Proteoglycans and osteolysis

Osteolysis is a complex mechanism resulting from an exacerbated activity of osteoclasts associated or not with a dysregulation of osteoblast metabolism leading to bone loss. This bone defect is not compensated by bone apposition or by apposition of bone matrix with poor mechanical quality. Osteolytic process is regulated by mechanical constraints, by polypeptides including cytokines and hormones, and by extracellular matrix components such as proteoglycans (PGs) and glycosaminoglycans (GAGs). Several studies revealed that GAGs may influence osteoclastogenesis, but data are very controversial: some studies showed a repressive effect of GAGs on osteoclastic differentiation, whereas others described a stimulatory effect. The controversy also affects osteoblasts which appear sometimes inhibited by polysaccharides and sometimes stimulated by these compounds. Furthermore, long-term treatment with heparin leads to the development of osteoporosis fueling the controversy. After a brief description of the principal osteoclastogenesis assays, the present chapter summarizes the main data published on the effect of PGs/GAGs on bone cells and their functional incidence on osteolysis.