Osteoclast adhesion and resorption: the role of podosomes

Parent-of-origin effects propagate through networks to shape metabolic traits

Parent-of-origin effects are unexpectedly common in complex traits, including metabolic and neurological traits. Parent-of-origin effects can be modified by the environment, but the architecture of these gene-by-environmental effects on phenotypes remains to be unraveled. Previously, quantitative trait loci (QTL) showing context-specific parent-of-origin effects on metabolic traits were mapped in the F16 generation of an advanced intercross between LG/J and SM/J inbred mice. However, these QTL were not enriched for known imprinted genes, suggesting another mechanism is needed to explain these parent-of-origin effects phenomena. We propose that non-imprinted genes can generate complex parent-of-origin effects on metabolic traits through interactions with imprinted genes. Here, we employ data from mouse populations at different levels of intercrossing (F0, F1, F2, F16) of the LG/J and SM/J inbred mouse lines to test this hypothesis. Using multiple populations and incorporating genetic, genomic, and physiological data, we leverage orthogonal evidence to identify networks of genes through which parent-of-origin effects propagate. We identify a network comprised of three imprinted and six non-imprinted genes that show parent-of-origin effects. This epistatic network forms a nutritional responsive pathway and the genes comprising it jointly serve cellular functions associated with growth. We focus on two genes, Nnat and F2r, whose interaction associates with serum glucose levels across generations in high-fat-fed females. Single-cell RNAseq reveals that Nnat expression increases and F2r expression decreases in pre-adipocytes along an adipogenic trajectory, a result that is consistent with our observations in bulk white adipose tissue.

L-Plastin deficiency produces increased trabecular bone due to attenuation of sealing ring formation and osteoclast dysfunction

Bone resorption requires the formation of complex, actin-rich cytoskeletal structures. During the early phase of sealing ring formation by osteoclasts, L-plastin regulates actin-bundling to form the nascent sealing zones (NSZ). Here, we show that L-plastin knockout mice produce osteoclasts that are deficient in the formation of NSZs, are hyporesorptive, and make superficial resorption pits in vitro. Transduction of TAT-fused full-length L-plastin peptide into osteoclasts from L-plastin knockout mice rescued the formation of nascent sealing zones and sealing rings in a time-dependent manner. This response was not observed with mutated full-length L-plastin (Ser-5 and -7 to Ala-5 and -7) peptide. In contrast to the observed defect in the NSZ, L-plastin deficiency did not affect podosome formation or adhesion of osteoclasts in vitro or in vivo. Histomorphometry analyses in 8- and 12-week-old female L-plastin knockout mice demonstrated a decrease in eroded perimeters and an increase in trabecular bone density, without a change in bone formation by osteoblasts. This decrease in eroded perimeters supports that osteoclast function is attenuated in L-plastin knockouts. Micro-CT analyses confirmed a marked increase in trabecular bone mass. In conclusion, female L-plastin knockout mice had increased trabecular bone density due to impaired bone resorption by osteoclasts. L-plastin could be a potential target for therapeutic interventions to treat trabecular bone loss.

Asperosaponin VI protects against bone destructions in collagen induced arthritis by inhibiting osteoclastogenesis

BACKGROUND

Bone destructive diseases like rheumatoid arthritis (RA), osteoporosis and bone metastatic tumors are mainly mediated by over-activated osteoclasts. Asperosaponin VI (AVI), isolated from the rhizome of Dipsacus asper, belongs to triterpenoid saponins. It has multiple physiological activities but its effects on RA, especially on osteoclast differentiation and activation are still unclear.

PURPOSE

Explore the protective role of AVI on collagen induced arthritis (CIA) in vivo and RANKL induced osteoclastogenesis in vitro.

METHODS

The effects of AVI on cell viability and RANKL-induced osteoclastogenesis, actin ring formation, bone resorption activity as well as on osteoclast specific gene and protein expression were tested using bone marrow derived monocytes (BMMs). Paws from CIA mice were used for micro-CT, HE and TRAP staining, real-time PCR and western blot. Sera were used for cytokine analysis by ELISA. The signaling pathways were detected using western blot, real-time PCR and immunofluorescence assay.

RESULTS

AVI significantly inhibited RANKL-induced osteoclast formation and bone resorption activity by suppressing the formation of actin ring. It also inhibited the expression of various osteoclatogenesis marker genes and signaling pathways. AVI protected arthritis in vivo by suppressing inflammation and bone loss.

CONCLUSION

AVI exerts its anti-osteoclastogenic activity both in vitro and in vivo by inhibiting RANKL-induced osteoclast differentiation and function. Thus, our studies demonstrate a potential therapeutic role for AVI in preventing or inhibiting RANKL-mediated osteolytic bone diseases.

Incorporation and effects of mesoporous SiO2-CaO nanospheres loaded with ipriflavone on osteoblast/osteoclast cocultures

Mesoporous nanospheres in the system SiO₂-CaO (NanoMBGs) with a hollow core surrounded by a radial arrangement of mesopores were characterized, labeled with FITC (FITC-NanoMBGs) and loaded with ipriflavone (NanoMBG-IPs) in order to evaluate their incorporation and their effects on both osteoblasts and osteoclasts simultaneously and maintaining the communication with each other in coculture. The influence of these nanospheres on macrophage polarization towards pro-inflammatory M1 or reparative M2 phenotypes was also evaluated in basal and stimulated conditions through the expression of CD80 (as M1 marker) and CD206 (as M2 marker) by flow cytometry and confocal microscopy. NanoMBGs did not induce the macrophage polarization towards the M1 pro-inflammatory phenotype, favoring the M2 reparative phenotype and increasing the macrophage response capability against stimuli as LPS and IL-4. NanoMBG-IPs induced a significant decrease of osteoclast proliferation and resorption activity after 7 days in coculture with osteoblasts, without affecting osteoblast proliferation and viability. Drug release test demonstrated that only a fraction of the payload is released by diffusion, whereas the rest of the drug remains within the hollow core after 7 days, thus ensuring the local long-term pharmacological treatment beyond the initial fast IP release. All these data ensure an appropriate immune response to these nanospheres and the potential application of NanoMBG-IPs as local drug delivery system in osteoporotic patients.

Effects of a mesoporous bioactive glass on osteoblasts, osteoclasts and macrophages

A mesoporous bioactive glass (MBG) of molar composition 75SiO₂-20CaO-5P₂O₅ (MBG-75S) has been synthetized as a potential bioceramic for bone regeneration purposes. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), nitrogen adsorption studies and transmission electron microscopy (TEM) demonstrated that MBG-75S possess a highly ordered mesoporous structure with high surface area and porosity, which would explain the high ionic exchange rate (mainly calcium and silicon soluble species) with the surrounded media. MBG-75S showed high biocompatibility in contact with Saos-2 osteoblast-like cells. Concentrations up to 1 mg/ml did not lead to significant alterations on either morphology or cell cycle. Regarding the effects on osteoclasts, MBG-75S allowed the differentiation of RAW-264.7 macrophages into osteoclast-like cells but exhibiting a decreased resorptive activity. These results point out that MBG-75S does not inhibit osteoclastogenesis but reduces the osteoclast bone-resorbing capability. Finally, in vitro studies focused on the innate immune response, evidenced that MBG-75S allows the proliferation of macrophages without inducing their polarization towards the M1 pro-inflammatory phenotype. This in vitro behavior is indicative that MBG-75S would just induce the required innate immune response without further inflammatory complications under in vivo conditions. The overall behavior respect to osteoblasts, osteoclasts and macrophages, makes this MBG a very interesting candidate for bone grafting applications in osteoporotic patients.

Nanocrystalline silicon substituted hydroxyapatite effects on osteoclast differentiation and resorptive activity

In the present study, the effects of nanocrystalline hydroxyapatite (nano-HA) and nanocrystalline Si-substituted hydroxyapatite (nano-SiHA) on osteoclast differentiation and resorptive activity have been evaluated in vitro using osteoclast-like cells. The action of these materials on proinflammatory and reparative macrophage populations was also studied. Nano-SiHA disks delayed the osteoclast differentiation and decreased the resorptive activity of these cells on their surface, as compared to nano-HA samples, without affecting cell viability. Powdered nano-SiHA also induced an increase of the reparative macrophage population. These results along with the beneficial effects on osteoblasts previously observed with powdered nano-SiHA suggest the potential of this biomaterial for modulating the fundamental processes of bone formation and turnover, preventing bone resorption and enhancing bone formation at implantation sites in treatment of osteoporotic bone and in bone repair and regeneration.

V-ATPases in osteoclasts: structure, function and potential inhibitors of bone resorption

The vacuolar-type H(+)-ATPase (V-ATPase) proton pump is a macromolecular complex composed of at least 14 subunits organized into two functional domains, V(1) and V(0). The complex is located on the ruffled border plasma membrane of bone-resorbing osteoclasts, mediating extracellular acidification for bone demineralization during bone resorption. Genetic studies from mice to man implicate a critical role for V-ATPase subunits in osteoclast-related diseases including osteopetrosis and osteoporosis. Thus, the V-ATPase complex is a potential molecular target for the development of novel anti-resorptive agents useful for the treatment of osteolytic diseases. Here, we review the current structure and function of V-ATPase subunits, emphasizing their exquisite roles in osteoclastic function. In addition, we compare several distinct classes of V-ATPase inhibitors with specific inhibitory effects on osteoclasts. Understanding the structure-function relationship of the osteoclast V-ATPase may lead to the development of osteoclast-specific V-ATPase inhibitors that may serve as alternative therapies for the treatment of osteolytic diseases.

Erk1 positively regulates osteoclast differentiation and bone resorptive activity

The extracellular signal-regulated kinases (ERK1 and 2) are widely-expressed and they modulate proliferation, survival, differentiation, and protein synthesis in multiple cell lineages. Altered ERK1/2 signaling is found in several genetic diseases with skeletal phenotypes, including Noonan syndrome, Neurofibromatosis type 1, and Cardio-facio-cutaneous syndrome, suggesting that MEK-ERK signals regulate human skeletal development. Here, we examine the consequence of Erk1 and Erk2 disruption in multiple functions of osteoclasts, specialized macrophage/monocyte lineage-derived cells that resorb bone. We demonstrate that Erk1 positively regulates osteoclast development and bone resorptive activity, as genetic disruption of Erk1 reduced osteoclast progenitor cell numbers, compromised pit formation, and diminished M-CSF-mediated adhesion and migration. Moreover, WT mice reconstituted long-term with Erk1^−/− bone marrow mononuclear cells (BMMNCs) demonstrated increased bone mineral density as compared to recipients transplanted with WT and Erk2^−/− BMMNCs, implicating marrow autonomous, Erk1-dependent osteoclast function. These data demonstrate Erk1 plays an important role in osteoclast functions while providing rationale for the development of Erk1-specific inhibitors for experimental investigation and/or therapeutic modulation of aberrant osteoclast function.

Multiple increased osteoclast functions in individuals with neurofibromatosis type 1

Skeletal abnormalities including scoliosis, tibial dysplasia, sphenoid wing dysplasia, and decreased bone mineral density (BMD) are associated with neurofibromatosis type 1 (NF1). We report the cellular phenotype of NF1 human-derived osteoclasts and compare the in vitro findings with the clinical phenotype. Functional characteristics (e.g., osteoclast formation, migration, adhesion, resorptive capacity) and cellular mechanistic alterations (e.g., F-actin polymerization, MAPK phosphorylation, RhoGTPase activity) from osteoclasts cultured from peripheral blood of individuals with NF1 (N = 75) were assessed. Osteoclast formation was compared to phenotypic, radiologic, and biochemical data. NF1 osteoprogenitor cells demonstrated increased osteoclast forming capacity. Human NF1-derived osteoclasts demonstrated increased migration, adhesion, and in vitro bone resorption. These activities coincided with increased actin belt formation and hyperactivity in MAPK and RhoGTPase pathways. Although osteoclast formation was increased, no direct correlation of osteoclast formation with BMD, markers of bone resorption, or the clinical skeletal phenotype was observed suggesting that osteoclast formation in vitro cannot directly predict NF1 skeletal phenotypes. While NF1 haploinsufficiency produces a generalized osteoclast gain-in-function and may contribute to increased bone resorption, reduced BMD, and focal skeletal defects associated with NF1, additional and perhaps local modifiers are likely required for the development of skeletal abnormalities in NF1.

Advances in osteoclast biology: old findings and new insights from mouse models

The maintenance of adequate bone mass is dependent upon the controlled and timely removal of old, damaged bone. This complex process is performed by the highly specialized, multinucleated osteoclast. Over the past 15 years, a detailed picture has emerged describing the origins, differentiation pathways and activation stages that contribute to normal osteoclast function. This information has primarily been obtained by the development and skeletal analysis of genetically modified mouse models. Mice harboring mutations in specific genetic loci exhibit bone defects as a direct result of aberrations in normal osteoclast recruitment, formation or function. These findings include the identification of the RANK-RANKL-OPG system as a primary mediator of osteoclastogenesis, the characterization of ion transport and cellular attachment mechanisms and the recognition that matrix-degrading enzymes are essential components of resorptive activity. This Review focuses on the principal observations in osteoclast biology derived from genetic mouse models, and highlights emerging concepts that describe how the osteoclast is thought to contribute to the maintenance of adequate bone mass and integrity throughout life.

Healing of periodontal tissues following transplantation of cells in a rat orthodontic tooth movement model

OBJECTIVE

To determine the fate and differentiation of transplanted periodontal ligament (PL) precursor cells and mouse embryonic stem (ES) cells and their relative capacity to regenerate wounded periodontium.

MATERIALS AND METHODS

Orthodontic tooth movement was introduced 24 hours before transplantation of PL or ES cells, and rats were euthanized either 24 hours or 72 hours after cell transplantation. The control rats received either no tooth movement and no cell transplantation or tooth movement and no cell transplantation. Differentiation of transplanted cells was assessed from mandibular periodontal histological tissue sections by immunohistochemical methods using monoclonal antibodies against PL cell differentiation markers. Data were analyzed using Student's t-test at a significance level of P = .05.

RESULTS

Transplantation of PL and ES cells resulted in a higher number of osteopontin, bone sialoprotein, and alpha-smooth muscle actin labeled transplanted cells, predominantly around the blood vessels of the periodontium in study rats compared with control rats (cell transplantation but no orthodontic tooth movement, P = .05). Combined treatments of tooth movement and cell transplantation resulted in enhanced regeneration of the periodontium as a result of tooth movement. Transplantation of PL cells induced a higher number of differentiating cells in the PL and alveolar bone than did transplantation of ES cells.

CONCLUSIONS

Orthodontic tooth movement promotes the differentiation of transplanted cells, and the differentiation occurs predominantly in the paravascular areas of the periodontium. In terms of regeneration of wounded periodontium, transplantation of PL cells produced a higher level of regeneration than ES cells, possibly because of PL cell plasticity and the capacity to undergo effective differentiation in the periodontal cellular microenvironment.

Osteoporosis

Effective therapies are available for the patient who has arthritic osteoporosis. The approach is critical to ensure an optimal quality of life in these individuals who suffer with a disease that is clearly treatable.

Hyperactivation of p21ras and PI3K cooperate to alter murine and human neurofibromatosis type 1-haploinsufficient osteoclast functions

Individuals with neurofibromatosis type 1 (NF1) have a high incidence of osteoporosis and osteopenia. However, understanding of the cellular and molecular basis of these sequelae is incomplete. Osteoclasts are specialized myeloid cells that are the principal bone-resorbing cells of the skeleton. We found that Nf1(+/-) mice contain elevated numbers of multinucleated osteoclasts. Both osteoclasts and osteoclast progenitors from Nf1(+/-) mice were hyperresponsive to limiting concentrations of M-CSF and receptor activator of NF-kappaB ligand (RANKL) levels. M-CSF-stimulated p21(ras)-GTP and Akt phosphorylation was elevated in Nf1(+/-) osteoclasts associated with gains of function in survival, proliferation, migration, adhesion, and lytic activity. These gains of function are associated with more severe bone loss following ovariectomy as compared with that in syngeneic WT mice. Intercrossing Nf1(+/-) mice and mice deficient in class 1(A) PI3K (p85alpha) restored elevated PI3K activity and Nf1(+/-) osteoclast functions to WT levels. Furthermore, in vitro-differentiated osteoclasts from NF1 patients also displayed elevated Ras/PI3K activity and increased lytic activity analogous to those in murine Nf1(+/-) osteoclasts. Collectively, our results identify a what we believe to be a novel cellular and biochemical NF1-haploinsufficient phenotype in osteoclasts that has potential implications for the pathogenesis of NF1 bone disease.

LIGHT (TNFSF14), a novel mediator of bone resorption, is elevated in rheumatoid arthritis

OBJECTIVE

Human osteoclast formation from mononuclear phagocyte precursors involves interactions between tumor necrosis factor (TNF) ligand superfamily members and their receptors. LIGHT is a transmembrane protein expressed and shed from the surface of activated T cells. Since activated T cells have been implicated in osteoclastogenesis in rheumatoid arthritis (RA), this study sought to determine whether LIGHT can regulate RANKL/cytokine-induced osteoclast formation, to identify the mechanism by which LIGHT influences osteoclastogenesis, and to investigate the presence of LIGHT in the serum of RA patients.

METHODS

The effect of LIGHT on human and murine osteoclast formation was assessed in the presence and absence of neutralizing reagents to known osteoclastogenic factors. Serum levels of LIGHT in RA patients were measured by enzyme-linked immunosorbent assay.

RESULTS

In the presence and absence of RANKL, LIGHT induced osteoclast formation from both human peripheral blood mononuclear cells and murine macrophage precursors, in a dose-dependent manner, whereas no inhibition was observed by adding osteoprotegerin, RANK:Fc, TNFalpha, or interleukin-8 or by blocking the LIGHT receptors herpesvirus entry mediator or lymphotoxin beta receptor. However, formation of osteoclasts was significantly decreased by the soluble decoy receptor for LIGHT, DcR3, and by blocking antibodies to the p75 component of the TNF receptor. A significant increase in LIGHT levels in the serum of RA patients compared with normal controls was also noted.

CONCLUSION

Our results indicate that LIGHT promotes RANKL-mediated osteoclastogenesis and that it can induce osteoclast formation by a mechanism independent of RANKL. The increased concentration of LIGHT in patients with RA raises the possibility that LIGHT may play a role in immunopathogenic conditions that are associated with localized or systemic bone loss.

Synovial fluid macrophages are capable of osteoclast formation and resorption

To determine whether synovial fluid (SF) macrophages isolated from the SF of osteoarthritis (OA), rheumatoid arthritis (RA) and pyrophosphate arthropathy (PPA) joints are capable of osteoclast formation, and to investigate the cellular and humoral factors required for this to occur, SF macrophages (CD14+) were isolated from the knee joint SF from patients with OA, RA and PPA and cultured for up to 14 days with macrophage-colony stimulating factor (M-CSF) and soluble receptor activator for nuclear factor-kappaB ligand (RANKL) or tumour-necrosis factor-alpha (TNFalpha) and interleukin-1alpha (IL-1alpha). Osteoclast differentiation was assessed by expression of tartrate-resistant acid phosphatase (TRAP) and vitronectin receptor (VNR), F-actin ring formation and lacunar resorption. Osteoclast formation and lacunar resorption was seen in RANKL-treated cultures of SF macrophages isolated from OA, RA and PPA joints with the largest amount of resorption noted in RA and PPA SF macrophage cultures. In TNFalpha/IL-1alpha-treated RA and PPA SF macrophage cultures, osteoclasts capable of lacunar resorption were also formed. Lacunar resorption was more extensive in RANKL than TNFalpha/IL-1alpha-treated cultures. These findings indicate that SF macrophages are capable of differentiating into mature osteoclasts capable of lacunar resorption. M-CSF in combination with RANKL or TNFalpha/IL-1alpha was required for osteoclast formation. As inflammatory synovial fluids contain an increase in the number of macrophages and an increase in the amounts of RANKL, TNFalpha and IL-1alpha, these findings suggest that one means whereby bone erosions may form in rheumatoid or crystal arthritis is by differentiation of synovial fluid macrophages into osteoclasts.

Adhesions that mediate invasion

Infiltration of new tissue areas requires that a mammalian cell overcomes the physical and biochemical barrier of the surrounding extracellular matrix. Cell migration during embryonic development, and growth, invasion and dispersal of metastatic tumor cells depend to a large extent on the controlled degradation of extracellular matrix components. Localized degradation of the surrounding matrix is seen at defined adhesive (podosomes) and/or protrusive (invadopodia) locations in a variety of normal cells and aggressive carcinoma cells, suggesting that these membrane-associated cellular devices have a central role in mediating polarized migration in cells that cross-tissue boundaries. Here, we will discuss the recent advances and developments in this field, and provide our provisional outlook into the future understanding of the principles of focal extracellular matrix degradation by podosomes and invadopodia.

Features of the clear zone of odontoclasts in the Chinook salmon (Oncorhynchus tshawytscha)

This study aims to clarify the features of the clear zone of odontoclasts on shedding teeth of a teleost fish, Chinook salmon, Oncorhynchus tshawytscha (Walbaum), using a light microscope to determine the orientation between a cell body and a resorptive lacuna, followed by transmission electron microscopy. Ultrathin sections of LR White embedded material were incubated in rabbit anti-actin polyclonal antibody and then were incubated with 15 nm gold-conjugated goat anti-rabbit IgG. The clear zones of odontoclasts showed a variable structure with electron-dense structures on sections, but distinct clear zones were not always seen on odontoclasts. In odontoclasts sectioned in the direction perpendicularly to the surface of a resorptive lacuna, some cells showed a wide clear zone, but two types of clear zones were usually observed: a part composed of some cytoplasmic processes and one composed of several complicatedly interwoven processes. Gold particles were localized on the clear zones, especially in electron-dense structures; very few gold particles were detected in ruffled borders. These results show that the clear zone of odontoclasts in Chinook salmon contains actin. Our results suggest that the clear zone of an odontoclast in Chinook salmon is not always a wide annular structure.

Osteoporosis

Bone is a complex organ which contains an organic matrix which serves as scaffolding, includes mineral as calcium distributed in a pattern providing structure and serves as an ion reservoir for the body. Throughout life it dynamically changes in response to changes in activity, body mass, and weight bearing. It is important to define patients at risk for bone loss, since accrued bone loss leading to osteoporosis in the older population of both men and women is unacceptable. There are many different therapies including biphosphonates which can decrease loss of bone and decrease fracture risk in patients who already have had sustained a fracture. Newer therapies such as parathyroid hormone may improve the fracture risk even more than biphosphonates over a shorter period of time.

(Pre-)osteoclasts induce retraction of osteoblasts before their fusion to osteoclasts

Precursors of osteoclasts seeded on top of a confluent layer of osteoblasts/bone lining cells induced retraction of the latter cells. The (pre)osteoclasts then migrated in the formed cell-free areas and fused to form osteoclast-like cells. Retraction of the osteoblasts/bone lining cells proved to depend on activity of matrix metalloproteinases, and TGF-beta1 prevented the retraction.

INTRODUCTION

It is well known that osteoblasts have a profound effect on (pre)osteoclasts in inducing the formation of bone-resorbing osteoclasts. Whether, on the other hand, (pre)osteoclasts also modulate osteoblast activity is largely unknown. Because osteoblasts/bone lining cells have to retract from the surface before resorption of bone by osteoclasts, we addressed the question of whether (pre)osteoclasts have the capacity to induce such an activity.

MATERIALS AND METHODS

Rabbit calvarial osteoblasts/bone lining cells or periosteal fibroblasts were cultured until confluency, after which rabbit peripheral blood mononuclear cells (PBMCs) were seeded on top of them. The co-cultures were maintained for up to 15 days in the presence or absence of the cytokines transforming growth factor (TGF)-beta1 and TNF-alpha and selective inhibitors of matrix metalloproteinases and serine proteinases. The formation of cell-free areas and the number of TRACP+ multinucleated osteoclast-like cells were analyzed. In addition, formation of cell-free areas was analyzed in co-cultures of osteoblasts with mature osteoclasts.

RESULTS

The seeding of PBMCs on a confluent layer of osteoblasts/bone lining cells resulted in the following sequence of events. (1) A low number of PBMCs strongly attached to osteoblasts. 2) At these sites of contact, the osteoblasts retracted, thus forming cell-free areas. (3) The PBMCs invaded these areas and attached to the surface of the well, after which they fused and formed multinucleated TRACP+ osteoclast-like cells. Retraction was only seen if the cells were in direct contact; conditioned media from cultured PBMCs added to osteoblasts had no effect. Mature osteoclasts seeded on osteoblasts similarly induced retraction, but this retraction occurred at a much faster rate (within 2 days) than the retraction effectuated by the osteoclast precursors (after 8 days in co-culture). Inhibition of matrix metalloproteinase activity, but not of serine proteinases, strongly reduced retraction of the osteoblasts, thus indicating that this type of cell movement depends on the activity of matrix metalloproteinases. A similar inhibitory effect was found with TGF-beta1. TNF-alpha had no effect on osteoblast retraction but enhanced the formation of multinucleated osteoclast-like cells. Addition of PBMCs to confluent layers of periosteal fibroblasts resulted in similar phenomena as observed in co-cultures with osteoblasts. However, the cell-free areas proved to be significantly smaller, and the number of multinucleated cells formed within cell-free areas was three to four times lower.

CONCLUSION

Our results indicate that osteoclast precursors and mature osteoclasts have the capacity to modulate the activity of osteoblasts and that, yet unknown, membrane-bound signaling molecules are essential in inducing retraction of osteoblasts and the subsequent formation of cell-free areas.

Polyphosphoinositides-dependent regulation of the osteoclast actin cytoskeleton and bone resorption

BACKGROUND

Gelsolin, an actin capping protein of osteoclast podosomes, has a unique function in regulating assembly and disassembly of the podosome actin filament. Previously, we have reported that osteopontin (OPN) binding to integrin alphavbeta3 increased the levels of gelsolin-associated polyphosphoinositides, podosome assembly/disassembly, and actin filament formation. The present study was undertaken to identify the possible role of polyphosphoinositides and phosphoinositides binding domains (PBDs) of gelsolin in the osteoclast cytoskeletal structural organization and osteoclast function.

RESULTS

Transduction of TAT/full-length gelsolin and PBDs containing gelsolin peptides into osteoclasts demonstrated: 1) F-actin enriched patches; 2) disruption of actin ring; 3) an increase in the association polyphosphoinositides (PPIs) with the transduced peptides containing PBDs. The above-mentioned effects were more pronounced with gelsolin peptide containing 2 tandem repeats of PBDs (PBD (2)). Binding of PPIs to the transduced peptides has resulted in reduced levels of PPIs association with the endogenous gelsolin, and thereby disrupted the actin remodeling processes in terms of podosome organization in the clear zone area and actin ring formation. These peptides also exhibited a dominant negative effect in the formation of WASP-Arp2/3 complex indicating the role of phosphoinositides in WASP activation. The TAT-PBD gelsolin peptides transduced osteoclasts are functionally defective in terms of motility and bone resorption.

CONCLUSIONS

Taken together, these data demonstrate that transduction of PBD gelsolin peptides into osteoclasts produced a dominant negative effect on actin assembly, motility, and bone resorption. These findings indicate that phosphoinositide-mediated signaling mechanisms regulate osteoclast cytoskeleton, podosome assembly/disassembly, actin ring formation and bone resorption activity of osteoclasts.

Type II benign osteopetrosis (Albers-Schönberg disease) caused by a novel mutation in CLCN7 presenting with unusual clinical manifestations

A 16-year-old male patient with type II autosomal dominant benign osteopetrosis (ADO) was genotyped and found to harbor a novel mutation in exon 25 of the gene encoding for the osteoclast-specific chloride channel, CLCN7, inherited from the father, who was asymptomatic. The patient had normal biochemical findings and acid-base balance, except for increased serum levels of creatine kinase, lactic dehydrogenase, and the bone formation markers bone alkaline phosphatase isoenzyme, osteocalcin and N-terminal type I collagen telopeptide/creatinine ratio. Unusual generalized osteosclerosis was observed together with a canonical increase in vertebral and pelvis bone mass. An affected first grade cousin presented with normal biochemical findings and a milder osteosclerotic pattern of the pelvis. At the cellular level, cultured osteoclasts from the patient showed increased motility, with lamellipodia, membrane ruffling and motile pattern of podosome distribution, all of which could have contributed to functional impairment of bone resorption. The present report documents a novel mutation of the CLCN7 gene causing osteopetrosis in a radiologically uncertain form of the diseases, with apparent incomplete penetrance.

Matrix metalloproteinases (MMP) and cathepsin K contribute differently to osteoclastic activities

The best established proteolytic event of osteoclasts is bone matrix solubilization by the cysteine proteinase cathepsin K. Here, however, we draw the attention on osteoclastic activities depending on matrix metalloproteinases (MMPs). We discuss the observations supporting that MMPs contribute significantly to bone matrix solubilization in specific areas of the skeleton and in some developmental and pathological situations. Our discussion takes into account (1) the characteristics of the bone remodeling persisting in the absence of cathepsin K, (2) the ultrastructure of the resorption zone in response to inactivation of MMPs and of cathepsin K in different bone types, (3) bone resorption levels in MMP knockout mice compared to wild-type mice, (4) the identification of MMPs in osteoclasts and surrounding cells, and (5) the effect of different bone pathologies on the serum concentrations of specific collagen fragments believed to discriminate between cathepsin K and MMP cleavage. Next, we provide evidence that MMPs are very critical for osteoclast migration, thereby controlling also the cell-matrix interactions required for cell attachment/detachment. The evidence supporting this role is based on a model of osteoclast recruitment in primitive long bones, an assay of osteoclast invasion through collagen gel, and the effect of proteinase inhibitors/knockouts in these models. Furthermore, we mention observations indicating a role of MMPs in initiation of bone resorption. Finally, we emphasize the many distinct ways MMPs may alter focally the extracellular environment thereby regulating the osteoclast behavior. Although the understanding of MMPs in osteoclast biology is rapidly expanding, it is suspected that important roles remain to be discovered.

Genetic regulation of osteoclast development and function

Osteoclasts are the principal, if not exclusive, bone-resorbing cells, and their activity has a profound impact on skeletal health. So, disorders of skeletal insufficiency, such as osteoporosis, typically represent enhanced osteoclastic bone resorption relative to bone formation. Prevention of pathological bone loss therefore depends on an appreciation of the mechanisms by which osteoclasts differentiate from their precursors and degrade the skeleton. The past five years have witnessed important insights into osteoclast formation and function. Many of these discoveries have been made through genetic experiments that involved the rare hereditary disorder osteopetrosis.

Three-dimensional distribution of the clear zone of migrating osteoclasts on dentin slices in vitro

Osteoclasts are cells that dynamically alternate resorption and migration on bone surfaces, and have the special structure called ruffled borders and clear zones by transmission electron microscopy (TEM). However, TEM features, especially the distribution of the clear zone of osteoclasts during migration, remains unclear. This study aimed to examine osteoclasts cultured on dentin slices by TEM and clarify the features of migrating osteoclasts, especially the three-dimensional distribution of clear zones. Osteoclasts obtained from mice were cultured with dentin slices for 72 h, and then cells were fixed and the tartrate-resistant acid phosphatase (TRAP) activity was detected. Specimens were embedded in Epon, then TRAP-positive cells were serially sectioned by alternating semithin and ultrathin sections. The cells were examined by TEM and the three-dimensional structures were reconstructed by computer. By TEM, most TRAP-positive cells were resorbing osteoclasts with ruffled borders and a clear zone. There were osteoclasts without ruffled borders, and these cells had clear zone-like structures and lamellipodia. The three-dimensional reconstruction showed that resorbing osteoclasts had rounded contours and ring-shaped clear zones encircling ruffled borders, and that osteoclasts without ruffled borders had irregular and flat shapes; the clear zone-like structures showed a dot or patch-like distribution. The presence of lamellipodia of the osteoclasts without ruffled borders shows that the cells are migrating osteoclasts. These results suggest that dot or patch-like distribution is the feature of the clear zone of osteoclasts during migration, and that these structures play the role of focal contacts and adhesion to the dentin surfaces during cell migration.

Leupaxin is a critical adaptor protein in the adhesion zone of the osteoclast

Leupaxin is a cytoskeleton adaptor protein that was first identified in human macrophages and was found to share homology with the focal adhesion protein, paxillin. Leupaxin possesses several protein-binding domains that have been implicated in targeting proteins such as focal adhesion kinase (pp125FAK) to focal adhesions. Leupaxin can be detected in monocytes and osteoclasts, both cells of hematopoietic origin. We have identified leupaxin to be a component of the osteoclast podosomal signaling complex. We have found that leupaxin in murine osteoclasts is associated with both PYK2 and pp125FAK in the osteoclast. Treatment of osteoclasts with TNF-alpha and soluble osteopontin were found to stimulate tyrosine phosphorylation of both leupaxin and leupaxin-associated PYK2. Leupaxin was found to co-immunoprecipitate with the protein tyrosine phosphatase PTP-PEST. The cellular distribution of leupaxin, PYK2, and protein tyrosine phosphorylation-PEST co-localized at or near the osteoclast podosomal complex. Leupaxin was also found to associate with the ARF-GTPase-activating protein, paxillin kinase linker p95PKL, thereby providing a link to regulators of cytoskeletal dynamics in the osteoclast. Overexpression of leupaxin by transduction into osteoclasts evoked numerous cytoplasmic projections at the leading edge of the cell, resembling a motile phenotype. Finally, in vitro inhibition of leupaxin expression in the osteoclast led to a decrease in resorptive capacity. Our data suggest that leupaxin may be a critical nucleating component of the osteoclast podosomal signaling complex.

Adhesion, cytoskeletal architecture and activation status of primary human macrophages on a diamond-like carbon coated surface

Diamond-like carbon is a promising surface coating for biomedicinal implants like coronary stents or hip joints. Before widespread clinical use of this material, its biocompatibility has to be thoroughly assessed. Cells likely to encounter a diamond-like coated implant in the human body are cells of the monocytic lineage. Their interaction with the diamond-like carbon coated surface will probably critically influence the fate of the implant, as monocytes orchestrate inflammatory reactions and also affect osseointegration of implants. We therefore investigated adhesion, cytoarchitecture and activation status of primary human monocytes and their differentiated derivatives, macrophages, on diamond-like coated glass coverslips using immunofluorescence technique. We show that adhesion of primary monocytes to a diamond-like-coated coverslip is slightly, but not significantly, enhanced in comparison to uncoated coverslips, while the actin and microtubule cytoskeletons of mature macrophages show a normal development. The activation status of macrophages, as judged by polarization of the cell body, was not affected by growth on a diamond-like carbon surface. We conclude that diamond-like carbon shows good indications for biocompatibility to blood monocytes in vitro. It is therefore unlikely that contact with a diamond-like carbon coated surface in the human body will elicit inflammatory signals by these cells.

Organization of cytoskeletal F-actin, G-actin, and gelsolin in the adhesion structures in cultured osteoclast

Immunofluorescence using Gc protein (group-specific component or vitamin D binding protein [DBP]) as a marker of G-actin showed that nonfilamentous, monomeric G-actin is a component of the podosomes of osteoclasts cultured on glass plates or bone slices. Typical individual podosomes of the well-spread cells on glass plates were rosette in form. When viewed from the basolateral surface, the core portion of the dotlike podosomes was associated with packed F-actin filaments surrounded by G-actin organized in a ringlike structure. The podosomes, when viewed perpendicular to the substrate, showed a conical shape as a bundle of short F-actin core and a ring of G-actin. With cell spreading on glass plates, the clustering of the podosomes formed a continuous belt of tightly packed podosomes as an adhesion structure at the paramarginal area. In addition, these structures were seen on the ventral cell surface. Similar changes in cell shape were seen in the osteoclasts when they were plated on bone slices. With the loss of dotlike podosomes, a continuous band of F-actin was formed around the resorption lacunae. It became evident then that F- and G-actin dissociated from each other in the podosomes. The staining patterns of G-actin varied from a discrete dot to a diffuse one. Toward the nonresorption phase, the osteoclasts lost their continuous F-actin band but dotlike podosomes appeared in the leading and the trailing edges. In such a cell undergoing translational movements, G-actin was located diffusely in the cytoplasm behind the lamellipodia and along some segments of the leading edge. Cytochalasin B treatment caused cells to disorganize the actin cytoskeletal architecture, which indicated the disassembling of F-actin into G-actin in podosomes and disappearance of actin-ring of cultured osteoclasts. Staining with polyclonal actin antibody or monoclonal beta-actin was overlapped with the distribution pattern of G- and F-actin. Gelsolin was detected in the region of the adhesion area corresponding to the podosome. The observation that F-actin, G-actin, and gelsolin were detected in the osteoclastic adhesion structures suggests that the podosomes may represent sites where a rapid polymerization/depolymerization of actin occurs. These dynamic changes in cytoskeletal organization and reorganization of G-actin may reflect changes in the functional polarization of the osteoclast during the bone resorption cycle and suggest the important role of G-actin in the regulation of osteoclast adhesion.

Fimbrin in podosomes of monocyte-derived osteoclasts

Fimbrin, an actin-bundling protein, is a component of the osteoclast adhesion complexes called podosomes. In this study, we (1) determined the localization of fimbrin in the mature rabbit osteoclast as well as in differentiating osteoclasts using the avian monocyte-derived osteoclast differentiation model, (2) characterized the distribution and accumulation of three fimbrin isotypes (T, L, and I) in avian monocytes as they fused to form multinucleate osteoclast-like cells, and (3) report for the first time, a close spatial relationship between podosomes and microtubules using fimbrin as a marker of the podosome. Immunofluorescence using anti-T-fimbrin, anti-L-fimbrin, and pan-isotype-anti-fimbrin antibodies, showed that fimbrin is an integral component of the podosome core in the mature rabbit osteoclast and in the monocyte-derived osteoclast throughout differentiation. Anti-I-fimbrin, however, did not show immunoreactivity in these cultures. These studies also show that in the avian model of monocyte-derived osteoclast differentiation, day 2 cells (D2) are predominantly mononucleate and have few podosomes. By days 4 and 6 in culture (D4 and D6), many cells have fused and punctate rows of podosomes are commonly observed at cell margins. Analysis by Western blot of protein accumulation showed that after an initial small rise from D2 to D4, L-fimbrin levels remained relatively constant from D4 to D6. However, T-fimbrin protein levels increase steadily from D2 to D6, suggesting that it may be related to the increase in podosome formation as monocytes fuse to form osteoclasts. Finally, we examined the distribution of podosomes relative to other cytoskeletal elements such as microtubules and intermediate filaments. Double immunofluorescence labeling using anti-fimbrin and anti-tubulin showed podosomes lying adjacent to microtubules at cell margins. When osteoclasts were treated with nocodazole (1 X 10(-6) M) to disrupt microtubules, the distribution of podosomes became more random and was no longer confined to the cell periphery. These results suggest that microtubule-podosome interactions may play a role in osteoclast adhesion.

Compactin suppresses bone resorption by inhibiting the fusion of prefusion osteoclasts and disrupting the actin ring in osteoclasts

Compactin (mevastatin), which inhibits 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, and thus biosynthesis of cholesterol and the prenylation of proteins, inhibits osteoclastic bone resorption. Although it has been suggested that compactin inhibits bone resorption by inducing apoptosis of osteoclasts, the pathway by which compactin inhibits resorption has not been established. We investigated the effect of compactin on the differentiation of osteoclasts and the relationship between the morphological changes elicited by compactin and its inhibitory effect on bone resorption. Compactin inhibited the differentiation of osteoclasts, interfering with the fusion process by which prefusion osteoclasts (pOCs) develop into multinucleated osteoclast-like cells (OCLs), and also disrupted the actin ring of OCLs. The potency of compactin to inhibit fusion of pOCs and to disrupt the actin ring of OCLs corresponded to that of compactin to inhibit bone resorption. The effects of compactin were prevented by the addition of MVA lactone or its downstream products farnesylpyrophosphate (FPP) and geranylgeranyl-pyrophosphate (GGPP) but not by squalene. Apoptosis of OCLs was not induced by the concentration of compactin that inhibited fusion of pOCs and disrupted the actin ring. The normal process of pOC fusion and the integrity of the actin ring were restored by the withdrawal of compactin from the cultures after they had been treated with compactin for 24 h, but they were not restored by the addition of zVAD-fmk, a caspase inhibitor. Compactin also reversibly inhibited interleukin-1beta (IL-1beta)-, 1alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3)-, and parathyroid hormone (PTH)-stimulated 45Ca release in bone organ cultures. Our results indicate that the inhibitory effects of compactin on bone resorption result from the inhibition of fusion of pOCs into OCLs and disruption of actin ring in OCLs and that apoptosis of OCLs is not necessary for these inhibitory effects of compactin. These effects of compactin are likely to be a consequence of the inhibition of prenylation of proteins that play an important role in the fusion of pOCs and in maintaining actin ring integrity in OCLs.

Specific antagonists of NMDA receptors prevent osteoclast sealing zone formation required for bone resorption

N-Methyl-d-aspartate (NMDA) glutamate receptors, widely distributed in the nervous system, have recently been identified in bone. They are expressed and are functional in osteoclasts. In the present work, we have studied the effects of specific antagonists of NMDA receptors on osteoclast activation and bone resorption. Using an in vitro assay of bone resorption, we showed that several antagonists of NMDA receptors binding to different sites of the receptor inhibit bone resorption. Osteoclast activation requires adhesion to the bone surface, cytoskeletal reorganization and survival. We demonstrated by autoradiography that the specific NMDA receptor channel blocker, MK 801, binds to osteoclasts. This antagonist had no effect on osteoclast attachment to bone and did not induce osteoclast apoptosis. In contrast, MK 801 rapidly decreased the percentage of osteoclasts with actin ring structures that are associated with actively resorbing osteoclasts. These results suggest that NMDA receptors expressed by osteoclasts may be involved in adhesion-induced formation of the sealing zone required for bone resorption.

Extensive clear zone and defective ruffled border formation in osteoclasts of osteopetrotic (ia/ia) rats: implications for secretory function

The cellular distribution of osteoclast integrin subunits alpha(v) and beta(3), the tissue distribution, and level of the apparent ligand osteopontin (OPN) as well as of the putative regulatory enzyme tartrate-resistant acid phosphatase (TRAP) were studied along with the intracellular distribution of the activation marker c-src in osteopetrotic ia/ia (incisors-absent) mutant rats and their normal littermates. In ia/ia rats, the osteoclasts are incapable of bone matrix resorption. Ultrastructurally the cells exhibit extended clear zones at the expense of ordinary ruffled borders. A secretory dysfunction in the mutant is strongly suggested by the absence of detectable extracellular TRAP, concomitant with an accumulation of the enzyme in abundant small cytoplasmic vesicles. Moreover, TRAP mRNA, protein content, as well as enzymatic activity were elevated. Furthermore, increased levels of integrin subunits alpha(v) and beta(3) were detected at the clear zone of mutant osteoclasts. OPN mRNA levels were elevated in long bones from mutants. In ia/ia rats, immunolabeling for OPN was homogeneously distributed at the surface facing osteoclasts, while in normal littermates it was concentrated at the clear zone area and barely detectable at ruffled borders. The absence of OPN labeling in the abundant, putative intracellular secretory vesicles in mutant osteoclasts suggests that these cells do not produce OPN. The osteoclasts of ia/ia rats appeared to produce and translocate the c-src protein to the cell membrane. In ia/ia a defect ruffled border-formation is observed along with extensive clear zone formation and decreased secretory function. The lesion may be due to a signaling defect, but in that case the defect seems to be located downstream to or not involving the c-src pathway. Our results illustrate the close relationship between secretory function and ruffled border formation in osteoclasts, a relationship that appears to be necessary for proper resorptive function.

Synergetic activation of outwardly rectifying Cl- currents by hypotonic stress and external Ca2+ in murine osteoclasts

1. An outwardly rectifying Cl- (ORCl) current of murine osteoclasts was activated by hypotonic stimulation. The current was characterized by rapid activation, little inactivation, strong outward rectification, blockage by DIDS and permeability to organic acids (pyruvate and glutamate). 2. The hypotonically activated ORCl current was inhibited by intracellular dialysis with an ATP-free pipette solution, but not by replacement of ATP with a poorly hydrolysable ATP analogue adenosine 5'-O-(3-thiotriphosphate). The current amplitude was reduced when intracellular alkalinity increased over the pH range 6.6-8.0. 3. Intracellular application of cytochalasin D occasionally activated the ORCl current without hypotonic stress, but inhibited activation of the ORCl current by hypotonic stimulation. The hypotonically activated ORCl current was unaffected by a non-actin-depolymerizing cytochalasin, chaetoglobosin C, but partially inhibited by deoxyribonuclease I. 4. Removal of extracellular Ca2+ inhibited activation of the ORCl current by hypotonic shock, but did not reduce the current once activated. The hypotonically activated ORCl current was partially decreased by intracellular dialysis with 20 mM EGTA. 5. With 10 mM Ca2+ in the extracellular medium, the ORCl current was activated in response to more minor decreases in osmolarity than with 1 mM Ca2+. The increased sensitivity to hypotonicity was mimicked by increasing the intracellular Ca2+ level (pCa 6.5). 6. These results suggest that hypotonic stimulation and a rise in the extracellular Ca2+ level synergistically activate the ORCl channel of murine osteoclasts, and that the activating process is modified by multiple intracellular factors (pH, ATP and actin cytoskeletal organization).

Identification of a novel marker for primordial smooth muscle and its differential expression pattern in contractile vs noncontractile cells

The assembly of the vessel wall from its cellular and extracellular matrix components is an essential event in embryogenesis. Recently, we used the descending aorta of the embryonic quail to define the morphological events that initiate the formation of a multilayered vessel wall from a nascent endothelial cell tube (Hungerford, J.E., G.K. Owens, W.S. Argraves, and C.D. Little. 1996. Dev. Biol. 178:375-392). We generated an mAb, 1E12, that specifically labels smooth muscle cells from the early stages of development to adulthood. The goal of our present study was to characterize further the 1E12 antigen using both cytological and biochemical methods. The 1E12 antigen colocalizes with the actin cytoskeleton in smooth muscle cells grown on planar substrates in vitro; in contrast, embryonic vascular smooth muscle cells in situ contain 1E12 antigen that is distributed in threadlike filaments and in cytoplasmic rosette-like patterns. Initial biochemical analysis shows that the 1E12 mAb recognizes a protein, Mr = 100,000, in lysates of adult avian gizzard. An additional polypeptide band, Mr = 40,000, is also recognized in preparations of lysate, when stronger extraction conditions are used. We have identified the 100-kD polypeptide as smooth muscle alpha-actinin by tandem mass spectroscopy analysis. The 1E12 antibody is an IgM isotype. To prepare a more convenient 1E12 immunoreagent, we constructed a single chain antibody (sFv) using recombinant protein technology. The sFv recognizes a single 100-kD protein in gizzard lysates. Additionally, the recombinant antibody recognizes purified smooth muscle alpha-actinin. Our results suggest that the 1E12 antigen is a member of the alpha-actinin family of cytoskeletal proteins; furthermore, the onset of its expression defines a primordial cell restricted to the smooth muscle lineage.

Tiludronate inhibits protein tyrosine phosphatase activity in osteoclasts

Signaling pathways mediated by tyrosine phosphorylation and dephosphorylation have been reported to be involved in the regulation of cytoskeletal organization in osteoclasts, the principal cells responsible for bone resorption. We examined the effects of tiludronate [(4-chlorophenyl)thiomethylene bisphosphonate] on the cytoskeleton and the balance of phosphotyrosine levels in osteoclast-like multinucleated cells (OCLs) formed in cocultures of mouse osteoblastic cells and bone marrow cells. When OCLs were placed on plastic dishes in the presence of 10% fetal bovine serum, they formed a ringed structure of F-actin dots (actin ring) within 2 h. Tiludronate did not inhibit the process of actin ring formation, but it disrupted preformed actin rings in a time- and a dose-dependent manner. Western blot analysis using an antiphosphotyrosine antibody revealed that tyrosine phosphorylation of certain proteins in OCLs was stimulated by tiludronate added to the purified OCLs. Tyrosine kinase activity of the p60c-src immunoprecipitated from cell lysates of the purified OCLs was not affected by tiludronate directly added to the kinase assay. OCL lysates stimulated dephosphorylation of tyrosine-phosphorylated substrates such as phosphoneuroprotein 14 and epidermal growth factor receptors. Like sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, tiludronate dose-dependently inhibited tyrosine dephosphorylation of those substrates induced by OCL lysates. These findings suggest that tiludronate disrupts the preformed actin rings and suppresses bone-resorbing activity by inhibiting protein tyrosine phosphatases in osteoclasts.

Identification of the membrane-type matrix metalloproteinase MT1-MMP in osteoclasts

The osteoclasts are the cells responsible for bone resorption. Matrix metalloproteinases (MMPs) appear crucial for this process. To identify possible MMP expression in osteoclasts, we amplified osteoclast cDNA fragments having homology with MMP genes, and used them as a probe to screen a rabbit osteoclast cDNA library. We obtained a cDNA of 1,972 bp encoding a polypeptide of 582 amino acids that showed more than 92% identity to human, mouse, and rat membrane-type 1 MMP (MT1-MMP), a cell surface proteinase believed to trigger cancer cell invasion. By northern blotting, MT1-MMP was found to be highly expressed in purified osteoclasts when compared with alveolar macrophages and bone stromal cells, as well as with various tissues. In situ hybridization on bone sections showed that MT1-MMP is expressed also in osteoclasts in vivo. Antibodies recognizing MT1-MMP reacted with specific plasma membrane areas corresponding to lamellipodia and podosomes involved, respectively, in migratory and attachment activities of the osteoclasts. These observations highlight how cells might bring MT1-MMP into contact with focal points of the extracellular matrix, and are compatible with a role of MT1-MMP in migratory and attachment activities of the osteoclast.

Molecular aspects of osteoclast function

In this article we have overviewed recent important advances in understanding the molecular mechanisms involved in osteoclastic bone resorption. Specifically, new findings relating to osteoclast activation and the process of bone resorption are reviewed and a current overall model of how osteoclasts resorb bone is presented. Controversial research topics concerning the regulation of osteoclast activity are also critically discussed.

A possible mechanism of the specific action of bisphosphonates on osteoclasts: tiludronate preferentially affects polarized osteoclasts having ruffled borders

The mechanism of action of tiludronate [(4-chlorophenyl)-thiomethylene bisphosphonate] on osteoclastic bone resorption was examined in mouse culture systems. Tiludronate did not inhibit the formation of osteoclast-like multinucleated cells (OCLs) induced by 1 alpha,25-dihydroxyvitamin D3 in cocultures of mouse osteoblastic cells and bone marrow cells. OCLs obtained from cocultures on collagen gel-coated dishes, treated with tiludronate, formed as many resorption pits on dentine slices as those obtained from the control cocultures. However, pit formation by OCLs was dose-dependently inhibited when tiludronate was added directly to the pit formation assay. Other bisphosphonates such as alendronate and etidronate dose-dependently inhibited pit formation according to the in vivo potencies of the respective bisphosphonates to inhibit bone resorption. However, they had no inhibitory effect on the recruitment of OCLs induced by 1 alpha,25-dihydroxyvitamin D3 in the cocultures. When OCLs were placed on dentine slices, they formed the ringed structure of F-actin-containing podosomes and ruffled borders (polarized OCLs) even in the presence of tiludronate. However, the actin rings in OCLs were disrupted by the addition of tiludronate soon after they began to resorb dentine. In contrast, OCLs placed on collagen gel formed neither actin rings nor ruffled borders (nonpolarized OCLs), and showed no response to tiludronate. OCLs formed from the spleen cells of osteosclerotic (oc/oc) mice developed the ringed structure of podosomes, but not ruffled borders, on dentine slices. The actin ring in the oc/oc spleen cell-derived OCLs placed on dentine slices was not disrupted by the addition of tiludronate.(ABSTRACT TRUNCATED AT 250 WORDS)

The small GTP-binding protein, rho p21, is involved in bone resorption by regulating cytoskeletal organization in osteoclasts

Rho protein (rho p21), a p21ras-related small guanine nucleotide binding protein, regulates cytoskeletal organization in a number of different types of cells. Evidence has indicated that Clostridium botulinum-derived ADP-ribosyltransferase (C3 exoenzyme) specifically ADP-ribosylates rho p21 at Asn41 and renders it functionally inactive. In this study, we examined the involvement of rho p21 in osteoclastic bone resorption using the C3 exoenzyme. When osteoclast-like multinucleated cells obtained from cocultures of mouse osteoblastic cells and bone marrow cells were placed on dentine slices, they formed ringed structures of podosomes containing F-actin (corresponding to the clear zone) within 8 hours. Many resorption pits were formed on dentine slices after culture for 24 hours. The C3 exoenzyme at 0.15-10 micrograms/ml added to the culture medium disrupted the ringed structure of podosomes in osteoclast-like cells in a dose-dependent manner. Correspondingly, pit formation by osteoclast-like cells on dentine slices was dose-dependently inhibited also by adding the C3 exoenzyme. Microinjection of the C3 exoenzyme into osteoclast-like cells placed on culture dishes completely disrupted the ringed podosome structure within 20 minutes. The amount of the rho p21 which was ADP-ribosylated by the C3 exoenzyme in vitro was much greater in purified osteoclast-like cells than in osteoblastic cells. Prior exposure of the purified osteoclast-like cell preparation to the C3 exoenzyme in vivo markedly decreased the amount of unribosylated rho p21. This indicated that the C3 exoenzyme incorporated into osteoclast-like cells effectively ADP-ribosylates rho p21 in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Osteopontin and the bone remodeling sequence. Colloidal-gold immunocytochemistry of an interfacial extracellular matrix protein

Relative to other noncollagenous, extracellular matrix proteins in mineralized tissues, colloidal-gold immunocytochemistry has demonstrated that the ultrastructural distribution of osteopontin (OPN) is unique in that this protein preferentially accumulates at mineralized tissue interfaces. In bone, this protein is present as a major component of cell-matrix and matrix-matrix interfacial structures called cement lines and laminae limitantes. In the present article, the implications of this distinct tissue distribution are discussed in terms of the bone remodeling sequence, and a detailed account of the secretion, accumulation and potential role of OPN is presented and related to current theory on the cellular and extracellular matrix events associated with basic multicellular unit (BMU)-based bone remodeling. In this context, a proposal is made describing the production of this protein as one of the earliest, and latest, secretory activities of the osteoblastic lineage, and that this activity manifests itself morphologically as a cement line ('plane') and a lamina limitans, respectively, at bone matrix interfaces. When integrated with other, known functional characteristics of this protein, the present morphological and compositional data indicate that OPN in cement lines and laminae limitantes may participate in initial and late extracellular matrix organization and mineralization, matrix-matrix/mineral adhesion and/or cell adhesion at bone interfaces.

Interaction of osteopontin with osteoclast integrins

Mammalian osteoclasts express three integrin receptors--alpha v beta 3 (vitronectin receptor), alpha 2 beta 1 and alpha v beta 1. The vitronectin receptor recognizes bone matrix proteins, including bone sialoproteins, in an RGD-dependent manner, whereas adhesion to collagen involves beta 1 integrins. Interference with integrin function, by anti-receptor antibodies or RGD-peptides, blocks bone resorption. Data on the mechanism of osteoclast adhesion to sialoproteins and the differential synthesis of osteopontin and bone sialoprotein by osteoclasts is presented. Thus, osteoclasts adhere to both osteopontin and bone sialoprotein with a characteristic irregular morphology with numerous, peripherally placed, actin-rich podosomes. Adhesion is predominantly RGD and beta 3 dependent, though alpha v beta 1 may also be involved in adhesion to bone sialoprotein. KQAGD and AGDV, but not H12, fibrinogen peptides induce osteoclast 'rounding' on osteopontin suggesting there is an alternative anti-adhesive signal to 'RGD.' However, adhesion is not completely inhibited and is not specific for osteopontin as equivalent effects are seen with adhesion to serum. The role of sialoproteins in osteoclast adhesion in situ in the skeleton is complicated by the finding of endogenous synthesis of osteopontin, but not bone sialoprotein, by osteoclasts. The disposition of osteoclast integrins during resorption and the role of integrins and sialoprotein-derived peptides in osteoclast adhesion and function is also reviewed.

Displacement and translocation of osteoblast-like cells by osteoclasts

Rabbit osteoclasts and rabbit osteoblast-like stroma cells (OB cells) were placed onto plastic surfaces and the migration patterns of individual osteoclasts and osteoclast-OB interactions were analyzed with time-lapse recording. To induce directed migration, the cultures were exposed to an electrical field of 0.01 or 0.1 V/mm. At 0.1 V/mm, osteoclasts moved directly toward the anode in some cases, clearing OB cells from their path of migration. In other cases, osteoclasts migrated toward the anode for part of the time but then changed direction and moved toward groups of OB cells. Observations were made on osteoclasts interacting with single OB cells or small colonies and on osteoclasts interacting with OB monolayers, at both field strengths; the results were independent of field strength. There were several characteristic behaviors. With single OB cells and small OB colonies, retraction of OB cells upon contact with the osteoclast was the predominant mechanism whereby these cells begin to move out of the path of the osteoclast. A pronounced ruffling or blebbing of the OB cell membrane often followed retraction. When osteoclasts displaced OB cells that were part of a monolayer, extension of an osteoclast lamellipodium underneath the edge of the OB cell layer generally preceded partial retraction of the OB cells involved. It sometimes appeared as if the detached or partially detached OB cells were "pushed" by the osteoclast, which in some cases resulted in OB cells being moved hundreds of microns in a period of a few hours, at rates comparable to the normal speed for osteoclast migration (congruent to 100 microns/h), much faster than the normal speed for OB cells (congruent to 10 microns/h).(ABSTRACT TRUNCATED AT 250 WORDS)

Signal transduction by HLA class II molecules in human T cells: induction of LFA-1-dependent and independent adhesion

Crosslinking HLA-DR molecules by monoclonal antibodies (moAbs) induces protein tyrosine phosphorylation and results in a secondary elevation of free cytoplasmic calcium concentrations in activated human T cells. Binding of bacterial superantigens or moAbs to DR molecules on activated T cells was recently reported to induce homotypic aggregation through activation of protein kinase C (PKC) and mediated by CD11a/CD54 (LFA-1/CAM-1) adhesion molecules. Here, we report that moAbs directed against framework DR, but neither DR1, 2- and DRw52- nor DQ- and DP-specific moABs induced homotypic aggregation of antigen- and alloantigen-activated T cells, antigen-specific CD4+ T-cell lines, a CD8+ T-cytotoxic cell line, and T-leukemia cells (HUT78). Protein tyrosine kinase (PTK) inhibitor herbimycin A partly blocked class-II-induced aggregation responses. In contrast, phorbol ester (PMA)-induced aggregation was essentially unaffected. A potent inhibitor of PKC, staurosporin, inhibited both moAb- and PMA-induced aggregation responses. The aggregation responses were completely inhibited by low temperatures, cytochalasins B and E, and partly inhibited by EDTA and CD18 moAbs, but unaffected by aphidicolin, mitomycin C, an adenylate cyclase inhibitor (2'5'-dideoxyadenosine), and moAbs against other adhesion molecules (CD2/CD58 [LFA-3], CD28/CD28 ligand B7, CD4, and CD44). In conclusion, HLA class-II-induced aggregation responses in activated T cells appear to involve PTK and PKC activation and to be mediated through CD11a-dependent and independent adhesion pathways.