Immunolocalization of CD44 and the ERM family in bone cells of mouse tibiae

Moesin activation controls bone resorption and tunneling nanotube-dependent osteoclast fusion

Osteoclasts are multinucleated cells unique in their ability to resorb bone. Osteoclastogenesis involves several steps of actin-driven rearrangements that participate not only in the cell-cell fusion process, but also in the formation of the sealing zone, the adhesive structure determining the resorption area. Despite the importance of these actin cytoskeleton-based processes, their precise mechanisms of regulation are still poorly characterized. Here, we found that moesin, a member of the Ezrin/Radixin/Moesin (ERM) protein family, is activated during osteoclast maturation and plays an instrumental role for both osteoclast fusion and function. In mouse and human osteoclast precursors, moesin is negatively regulated to potentiate their ability to fuse and degrade bone. Accordingly, we demonstrated that moesin depletion decreases membrane-to-cortex attachment and enhances formation of tunneling nanotubes (TNTs), F-actin-containing intercellular bridges that we revealed to trigger osteoclast fusion. In addition, via a β3-integrin/RhoA/SLK pathway and independently of its role in fusion, moesin regulates the number and organization of sealing zones in mature osteoclast, and thus participates in the control of bone resorption. Supporting these findings, we found that moesin-deficient mice are osteopenic with a reduced density of trabecular bones and increased osteoclast abundance and activity. These findings provide a better understanding of the regulation of osteoclast biology, and open new opportunities to specifically target osteoclast activity in bone disease therapy.

The Dibenzyl Isoquinoline Alkaloid Berbamine Ameliorates Osteoporosis by Inhibiting Bone Resorption

Postmenopausal osteoporosis (PMOP) is a kind of primary osteoporosis that is characterized by decreased bone density and strength. Berbamine is a nonbasic quaternary benzylisoquinoline plant alkaloid that has been widely used in the clinic to treat leukopenia in China. We found that berbamine inhibited RANKL-induced osteoclastogenesis of bone marrow-derived macrophages (BMMs) in vitro, which mainly occurred in the middle phase and late phase. The gene and protein expression levels of osteoclast-related molecules, including CTSK, MMP-9, NFATc1, CD44 and DC-STAMP, were also downregulated by berbamine. In vivo, we treated PMOP mice with berbamine for 8 weeks and found that the extent of osteoporosis was alleviated significantly according to micro-CT scanning, hematoxylin-eosin staining, DC-STAMP immunohistochemical staining and TRAP immunohistochemical staining in the distal femurs of the mice. Our findings demonstrate that berbamine has an inhibitory effect on the osteoclastogenesis of BMMs and can prevent bone loss after ovariectomy in vivo. This study provides evidence that berbamine is a potential drug for the prevention and treatment of PMOP.

Can impaired diaphyseal fracture healing be predicted early?: A cohort study of biomarkers

BACKGROUND

Currently, the clinico-radiological method was used to analyze the healing progression of fractures globally, but even they are also unable to presume the impaired healing early. Hence till date, no reliable methods are available to predict the impaired healing early, so that it could be interventionally managed as required within the time.

METHODS

In this prospective observational study, a total of 121 adults fractured patients and 108 healthy controls were analyzed. Peripheral blood samples were taken from controls (at once) and fractured cases (at different follow-ups) to quantify the Osteocalcin and Osteopontin mRNA and protein expression using qRT-PCR and western blotting assay respectively. In parallel to that the clinico-radiological follow-up examinations also done at various specific follow-up intervals up to 24th post-fracture weeks.

RESULTS

As per the clinico-radiological status at the 24th week, fracture patients were divided into normal healing (n = 102) and impaired healing (n = 19) groups. Mean RUST score between normal healing and the impaired healing group showed a significant statistical difference at each follow-up. In both groups, expressions of Osteocalcin (mRNA & protein) were gradually up-regulated from the baseline to end of follow-ups, whereas Osteopontin mRNA as well as protein gradually up-regulated from the baseline to a peak value at 10th day, then declined. In general, the Osteocalcin and Osteopontin mean fold expressions were higher in normal healing as compared to the impaired healing groups.A significant correlation was found between the mRNA expressions of Osteocalcin and Osteopontin with the RUST score at most of the follow-ups. However, the protein expressions were not shown any significant correlation.

CONCLUSIONS

The Osteocalcin and Osteopontin expression will provide an early prediction of the healing outcomes of tibial fractures. This may open a new horizon for innovations to deal with complications associated with impaired fracture healing, especially in tibial bone fractures.

Osteoactivin inhibition of osteoclastogenesis is mediated through CD44-ERK signaling

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

CD99 suppresses osteosarcoma cell migration through inhibition of ROCK2 activity

CD99, a transmembrane protein encoded by MIC2 gene is involved in multiple cellular events including cell adhesion and migration, apoptosis, cell differentiation and regulation of protein trafficking either in physiological or pathological conditions. In osteosarcoma, CD99 is expressed at low levels and functions as a tumour suppressor. The full-length protein (CD99wt) and the short-form harbouring a deletion in the intracytoplasmic domain (CD99sh) have been associated with distinct functional outcomes with respect to tumour malignancy. In this study, we especially evaluated modulation of cell-cell contacts, reorganisation of the actin cytoskeleton and modulation of signalling pathways by comparing osteosarcoma cells characterised by different metastasis capabilities and CD99 expression, to identify molecular mechanisms responsible for metastasis. Our data indicate that forced expression of CD99wt induces recruitment of N-cadherin and β-catenin to adherens junctions. In addition, transfection of CD99wt inhibits the expression of several molecules crucial to the remodelling of the actin cytoskeleton, such as ACTR2, ARPC1A, Rho-associated, coiled-coil containing protein kinase 2 (ROCK2) as well as ezrin, an ezrin/radixin/moesin family member that has been clearly associated with tumour progression and metastatic spread in osteosarcoma. Functional studies point to ROCK2 as a crucial intracellular mediator regulating osteosarcoma migration. By maintaining c-Src in an inactive conformation, CD99wt inhibits ROCK2 signalling and this leads to ezrin decrease at cell membrane while N-cadherin and β-catenin translocate to the plasma membrane and function as main molecular bridges for actin cytoskeleton. Taken together, we propose that the re-expression of CD99wt, which is generally present in osteoblasts but lost in osteosarcoma, through inhibition of c-Src and ROCK2 activity, manages to increase contact strength and reactivate stop-migration signals that counteract the otherwise dominant promigratory action of ezrin in osteosarcoma cells.

Isolation of primary avian osteocytes

Osteocytes can be isolated from chicken calvaria using mild EDTA treatment alternating with collagenase treatment. The cell population obtained contains both osteoblasts and osteocytes. A pure population of osteocytes is obtained following immunomagnetic separation with the osteocyte-specific monoclonal antibody MAb OB7.3.

Blood-derived human osteoclast resorption activity is impaired by Hyaluronan-CD44 engagement via a p38-dependent mechanism

The control of bone resorption is crucial in osteolytic diseases. Once attached to bone, osteoclasts (OCs) initiate the resorption process through the activation of a complex cascade of morphological and biochemical changes. Hyaluronan (HA), an extracellular glycosaminoglycan long non-branching polysaccharide, is expressed in bone matrices. Here we demonstrate that HA counter-balances the erosion activity of human mature OCs by significantly reducing their degradative potential. HA treatment of fully differentiated OCs derived from human peripheral blood monocytes inhibited migration on collagen as well as bone resorption. HA-mediated effects were primarily due to TRAcP, MMP-9, and cathepsin K down-regulation and to the increased levels of TIMP-1, a natural MMP-9 inhibitor. Binding of HA to mature OCs was entirely mediated by CD44: function-blocking anti-CD44 antibodies fully abrogated HA effects, and the engagement of HA receptor caused a rapid de-phosphorylation of Ser325 in the CD44 cytoplasmic tail. The inhibitory action by HA was associated with a transient up-phosphorylation of Pyk2, a novel persistent phosphorylation of p38 and the down-regulation of NFATc1 transcription factor. Our results provide a direct evidence for the involvement of CD44 in the HA-dependent regulation of OC activity and suggest a signaling pathway that could be unique in OC function inhibition.

Expression of CD44v6 as matrix-associated ectodomain in the bone development

This study describes the expression of CD44v6 in the bone development and is the first study of its kind to the authors' best knowledge. The CD44 family is a family of transmembrane glycoproteins that acts as cell adhesion molecules binding cells to other cells as well as cells to the extracellular matrix. It has been suggested that the CD44v6, a family member of CD44, is closely related to the osteosarcoma metastasis. In general, when cancer cells metastasize, they revert to their immature forms. In the present study, therefore, we have investigated CD44v6 and the standard form of CD44 (CD44st) in two types of immature forms of bone tissues: developmentally immature stages from fetuses to adults as well as experimentally immature stages using fracture models. CD44st expression was identified in osteoblasts, osteocytes, and in the peripheral portion of the bone matrix from the fetal to young ages of rats. Many more intense reactions for CD44v6 were observed in the bone matrix than CD44st in fetal stages. In experimental fracture models, positive immunoreactions to CD44st were clearly observed in the osteoblasts and osteocytes. CD44v6-positive immunoreactivity, however, was not detected in either osteoblasts or the bone matrix. In conclusion, CD44v6 is expressed in the embryonic stages and may be involved in the bone matrix formation as a matrix-associated ectodomain during normal ontogenetic development but not involved in the process of fracture healing.

Immunolocalization of proteoglycans in Meckel's cartilage of the rat

The aim of this study was to investigate the presence and distribution of proteoglycans within Meckel’s cartilage of rat embryos. A standard indirect immunoperoxidase technique was used on paraffin sections of rat heads. Sections were incubated with monoclonal antibodies recognising core protein epitopes in the proteoglycans versican and CD44. Polyclonal antibodies localized the proteoglycans decorin, biglycan and lumican. Versican was expressed by chondrocytes, but very weekly by the extracellular matrix. Decorin was strongly expressed by both of chondrocytes and the ECM. Both of biglycan and lumican were moderately expressed by chondrocytes, but weakly by the extracellular matrix. CD44 was weakly expressed by chondrocytes only, without staining of the ECM. It is concluded that Meckel’s cartilage chondrocytes express the proteoglycans versican, decorin, biglycan, lumican and CD44 at variable levels during development in the rat. Such data are important for a greater understanding of the changes that take place during mandibular development. Further studies are needed to elucidate the exact role of proteoglycans during Meckel’s cartilage and mandibular organogenesis.

LIF inhibits osteoblast differentiation at least in part by regulation of HAS2 and its product hyaluronan

LIF arrests osteogenesis in fetal rat calvaria cells in a differentiation stage-specific manner. Differential display identified HAS2 as a LIF-induced gene and its product, HA, modulated osteoblast differentiation similarly to LIF. Our data suggest that LIF arrests osteoblast differentiation by altering HA content of the extracellular matrix.

INTRODUCTION

Leukemia inhibitory factor (LIF) elicits both anabolic and catabolic effects on bone. We previously showed in the fetal rat calvaria (RC) cell system that LIF inhibits osteoblast differentiation at the late osteoprogenitor/early osteoblast stage.

MATERIALS AND METHODS

To uncover potential molecular mediators of this inhibitory activity, we used a positive-negative genome-wide differential display screen to identify LIF-induced changes in the developing osteoblast transcriptome.

RESULTS

Although LIF signaling is active throughout the RC cell proliferation-differentiation sequence, only a relatively small number of genes, in several different functional clusters, are modulated by LIF specifically during the LIF-sensitive inhibitory time window. Based on their known and predicted functions, most of the LIF-regulated genes identified are plausible candidates to be involved in the LIF-induced arrest of osteoprogenitor differentiation. To test this hypothesis, we further analyzed the function of one of the genes identified, hyaluronan synthase 2 (HAS2), in the LIF-induced inhibition. Synthesis of hyaluronan (HA), the product of HAS enzymatic activity, was stimulated by LIF and mimicked the HAS2 expression profile, with highest expression in early/proliferative and late/maturing cultures and lowest levels in intermediate/late osteoprogenitor-early osteoblast cultures. Exogenously added high molecular weight HA, the product of HAS2, dose-dependently inhibited osteoblast differentiation, with pulse-treatment effective in the same differentiation stage-specific inhibitory window as seen with LIF. In addition, however, pulse treatment with HA in early cultures slightly increased bone nodule formation. Treatment with hyaluronidase, on the other hand, stimulated bone nodule formation in early cultures but caused a small dose-dependent inhibition of osteoblast differentiation in the LIF- and HA-sensitive late time window.

CONCLUSIONS

Together the data suggest that osteoblast differentiation is acutely sensitive to HA levels and that LIF inhibits osteoblast development at least in part by stimulating high molecular weight HA synthesis through HAS2.

Identification and characterization of CPS1 as a hyaluronic acid synthase contributing to the pathogenesis of Cryptococcus neoformans infection

Cryptococcus neoformans is a pathogenic yeast that often causes devastating meningoencephalitis in immunocompromised individuals. We have previously identified the C. neoformans CPS1 gene, which is required for a capsular layer on the outer cell wall. In this report, we investigate the function of the CPS1 gene and its pathogenesis. We demonstrated that treatment of yeast with either 4-methylumbelliferone or hyaluronidase resulted in a reduction of the level of C. neoformans binding to human brain microvascular endothelial cells (HBMEC). Yeast extracellular structures were also altered accordingly in hyaluronidase-treated cells. Furthermore, observation of yeast strains with different hyaluronic acid contents showed that the ability to bind to HBMEC is proportional to the hyaluronic acid content. A killing assay with Caenorhabditis elegans demonstrated that the CPS1 wild-type strain is more virulent than the cps1Delta strain. When CPS1 is expressed in Saccharomyces cerevisiae and Escherichia coli, hyaluronic acid can be detected in the cells. Additionally, we determined by fluorophore-assisted carbohydrate electrophoretic analysis that hyaluronic acid is a component of the C. neoformans capsule. The size of hyaluronic acid molecules is evaluated by gel filtration and transmission electron microscopy studies. Together, our results support that C. neoformans CPS1 encodes hyaluronic acid synthase and that its product, hyaluronic acid, plays a role as an adhesion molecule during the association of endothelial cells with yeast.

Expression of the cytoskeleton linker protein ezrin in human cancers

Expression of the metastasis-associated protein, ezrin, in over 5,000 human cancers and normal tissues was analyzed using tissue microarray immunohistochemistry. Ezrin staining was compared between cancers and their corresponding normal tissues, between cancers of epithelial and mesenchymal origin, in the context of the putative inhibitor protein, merlin, and against clinicopathological data available for breast, lung, prostate cancers and sarcomas. Ezrin was found in most cancers and normal tissues at varying levels of intensity. In general ezrin was expressed at higher levels in sarcomas than in carcinomas. By normalizing the expression of ezrin in each cancer using ezrin expression found in the corresponding normal tissue, significant associations between ezrin were found in advancing histological grade in sarcomas (P = 0.02) and poor outcome in breast cancer (P = 0.025). Clinicopathologic associations were not changed by simultaneous assessment of ezrin and merlin in each patient sample for the cancer types examined. These data support a role for ezrin in the biology of human cancers and the need for additional studies in breast cancer and sarcoma patients that may validate ezrin as a marker of cancer progression and as a potential target for cancer therapy.

Secretory products of breast cancer cells upregulate hyaluronan production in a human osteoblast cell line

Hyaluronan (HA) has been implicated in breast cancer progression and metastasis to lymph nodes. Although breast cancer has a strong propensity for metastasis to the bone, the role of HA in the development of bone metastasis in breast cancer is not well delineated. In order to determine the role of HA in breast cancer induced osteolysis, we examined the effect of secretory products in the conditioned medium of breast cancer cell lines on the expression of hyaluronan synthases (HAS), accumulation of HA in pericellular matrix, secretion of HA in the culture media, and on expression of surface HA receptors, in a human immature osteoblast cell line (Hfob). Our results show that conditioned medium derived from breast cancer cells upregulate the expression of hyaluronan synthases, HAS1 and HAS2, followed by significant increase in pericellular and secreted HA in Hfob cells. Our results further demonstrate that both CD44 and receptor for hyaluronan-mediated motility (RHAMM) are involved in binding cell surface associated HA on Hfob cells. Analysis of the growth factors in the conditioned medium implicates TGF-beta1 in the modulation of HAS1 and HAS2, as well as in the increase in pericellular and secreted HA. This report is the first to show that soluble factors produced by breast cancer cells mediate increase in HA production in osteoblasts.

Effect of CD44 deficiency on in vitro and in vivo osteoclast formation

In vitro studies have shown that CD44 is involved in the fusion process of osteoclast precursor cells. Yet, in vivo studies do not support this, since an osteopetrotic phenotype has not been described for CD44 knock-out (CD44 k.o.) mice. This discrepancy may suggest that the role of CD44 in fusion may depend on the microenvironment of osteoclast formation. We investigated osteoclast formation of CD44 k.o. and wild-type mice under three conditions: in vitro, both on plastic and on bone and in vivo by analyzing osteoclast number, and size in long bones from wild-type and CD44 k.o. mice. Bone marrow cells from wild-type and CD44 k.o. mice were analyzed for their capacity to form osteoclasts on plastic and on bone in the presence of macrophage colony stimulating factor (M-CSF) and receptor activator of NF-kB ligand (RANKL). On plastic, the number of multinucleated tartrate resistant acid phosphatase (TRAP) positive cells in CD44 k.o. cultures was twofold higher than in wild-type cultures. On bone, however, equal numbers of osteoclasts were formed. Interestingly, the total number of osteoclasts formed on bone proved to be higher than on plastic for both genotypes, strongly suggesting that osteoclastogenesis was stimulated by the bone surface, and that CD44 is not required for osteoclast formation on bone. Functional analyses showed that bone resorption was similar for both genotypes. We further studied the osteoclastogenic potential of wild-type bone marrow cells in the presence of CD44 blocking antibodies. Osteoclastogenesis was not affected by these antibodies, a further indication that CD44 is not required for the formation of multinucleated cells. Finally, we analyzed the in vivo formation of osteoclasts by analyzing long bones from wild-type and CD44 k.o. mice. Morphometric analysis revealed no difference in osteoclast number, nor in number of nuclei per osteoclasts or in osteoclast size. Our in vitro experiments on plastic showed an enhanced formation of osteoclasts in the absence of CD44, thus suggesting that CD44 has an inhibitory effect on osteoclastogenesis. However, when osteoclasts were generated on bone, no differences in number of multinucleated cells nor in bone resorption were seen. These observations are in agreement with in vivo osteoclast characteristics, where no differences between wild-type and CD44 k.o. bones were encountered. Therefore, the modulating role of CD44 in osteoclast formation appears to depend on the microenvironment.

Effect of dexamethasone on moesin gene expression in rabbit bone marrow stromal cells

The influence of dexamethasone on rabbit bone marrow stromal cells differentiation was studied by screening the action of dexamethasone on gene expression. Using differential display, we observed some differential amplifications. The use of five of thirteen different primers combination allowed to identify one or more differential bands. One of them was identified as moesin gene. Real-time PCR confirmed a significant reduction of moesin gene expression following dexamethasone treatment. The decrease of expression for this protein, involved in cytoskeletal organization, could explain the effects of dexamethasone treatment on bone marrow stromal cells differentiation.

Localization of CD44 (hyaluronan receptor) and hyaluronan in rat mandibular condyle

CD44 is a multifunctional adhesion molecule that binds to hyaluronan (HA), type I collagen, and fibronectin. We investigated localization of CD44 and HA in mandibular condylar cartilage compared with the growth plate and the articular cartilage, to clarify the characteristics of chondrocytes. We also performed Western blotting using a lysate of mandibular condyle. In mandibular condyle, CD44-positive cells were seen in the surface region of the fibrous cell layer and in the proliferative cell layer. Western blotting revealed that the molecular weight of CD44 in condyle was 78 to 86 kD. Intense reactivity for HA was detected on the surface of the condyle and the lacunae of the hypertrophic cell layer. Moderate labeling was seen in cartilage matrix of the proliferative and maturative layer. Weak labeling was also seen in the fibrous cell layer. In growth plate and articular cartilage, HA was detected in all cell layers. However, chondrocytes of these cartilages did not exhibit reactivity for CD44. These results suggest that chondrocytes in the mandibular condylar cartilage differ in expression of CD44 from those in tibial growth plate and articular cartilage. Cell-matrix interaction between CD44 and HA may play an important role in the proliferation of chondrocytes in the mandibular condyle.

Hyaluronan increases RANKL expression in bone marrow stromal cells through CD44

HA activates CD44 to stimulate RANKL expression in bone marrow stromal cells. HA stimulation of RANKL is blocked by anti-CD44 antibody and is absent in cells from CD44(-/-) mice. CD44(-/-) mice exhibit thicker cortical bone and a smaller medullary cavity, but indices of bone resorption are not affected.

INTRODUCTION

Hyaluronan (HA), the major nonprotein glycosaminoglycan component of the extracellular matrix in mammalian bone marrow, functions in part through its receptor, CD44, to stimulate a series of intracellular signaling events that lead to cell migration, adhesion, and activation. To determine whether HA activation of CD44 influences RANKL and osteoprotegerin (OPG) expression and whether CD44 is functionally important in bone metabolism, we studied whole bone and bone marrow stromal cells (BMSCs) from wildtype and CD44(-/-) mice.

MATERIALS AND METHODS

BMSCs from wildtype and CD44(-/-) mice at 7 weeks of age were cultured and treated with either HA or anti-CD44 antibody. The levels of mRNA of RANKL, OPG, CD44, alkaline phosphatase (ALP), osteocalcin (OC), and alphaI collagen (COLL) were determined by quantitative real-time RT-PCR. Levels of RANKL and CD44 protein were measured by immunoblotting, and expression of CD44 in whole bone was determined by immunohistochemical staining. Double immunofluorescence staining and confocal microscopy were used to study colocalization of Cbfa1, CD44, and HA. Tibias were imaged using muCT, and cancellous and cortical parameters were measured. Osteoblast and osteoclast surface in the distal femoral metaphysis and osteoclast on the endocortical surface at the tibio-fibular junction were measured using quantitative histomorphometry. Differences were analyzed using ANOVA and the Newman-Keuls test.

RESULTS

Addition of HA dose-dependently increased RANKL mRNA (3.6-fold) and protein (3-fold) levels in BMSCs. Stimulation of RANKL by HA could be blocked with anti-CD44 antibody. Treatment of cells with HA or anti-CD44 antibody had no significant effect on OPG mRNA levels. Both CD44 and HA localized on the plasma membrane in cells expressing Cbfa1. HA localization on the cell membrane disappeared when cells were preincubated with anti-CD44 antibody. Compared with control mice, cortical bone of CD44(-/-) was thicker, and medullary area was smaller at both 7 and 17 weeks, but at 7 weeks, indices of bone resorption were normal. At 17 weeks of age, tibial mass of CD44(-/-) mice was higher than control mice. CD44(-/-) animals expressed less RANKL in whole bone (-30%) and in BMSCs (-50%). Cells from CD44(-/-) animals failed to respond to either HA or CD44 antibody treatment.

CONCLUSIONS

HA can increase RANKL expression in BMSCs through CD44.

Ultrastructure of the osteocyte process and its pericellular matrix

Osteocytes are believed to be the mechanical sensor cells in bone. One potential physical mechanism for the mechanosensing process is that osteocytes directly sense the deformation of the substrate to which they are attached. However, there is a fundamental paradox in this theory: tissue-level strains in whole bone are typically <0.2%, yet an extensive range of in vitro experiments show that dynamic substrate strains must be at least an order of magnitude larger in order for intracellular biochemical responses to occur. Recently, a theoretical model was developed (You et al. J. Biomech., 2001; 34:1375-1386) that provides a possible mechanism by which mechanical loading-induced fluid flow in the lacuno-canalicular system, under routine physical activity, can produce cellular-level strains on the osteocyte processes that are at least one order of magnitude larger than bone tissue deformations. This would resolve the fundamental paradox mentioned above. In this work we experimentally confirm and quantify the essential ultrastructural elements in this model: 1) the presence of the transverse elements that bridge the pericellular space surrounding the osteocyte process, which interact with the fluid flow and lead to an outward hoop tension on the process; and 2) the presence of bundled F-actin in the osteocyte processes, which resists the outward hoop tension and limits the cell process membrane deformation. Morphological data to support these assumptions are scant. Special staining techniques employing ruthenium III hexamine trichloride (RHT) were developed to elucidate these structures in the humeri of adult mice.

Rho-dependent Rho kinase activation increases CD44 surface expression and bone resorption in osteoclasts

Osteoclasts from osteopontin-deficient mice exhibit decreased CD44 surface expression [corrected]. Osteopontin (OPN)/alphavbeta3 generated Rho signaling pathway is required for the surface expression of CD44. In this work we show the Rho effector, Rho kinase (ROK-alpha), to be a potent activator of CD44 surface expression. ROK-alpha activation was associated with autophosphorylation, leading to its translocation to the plasma membrane, as well as its association with CD44. ROK-alpha promoted CD44 surface expression through phosphorylation of CD44 and ezrin-radixin-moesin (ERM) proteins and CD44.ERM.actin complex formation. Osteoclasts from OPN-/- mice exhibited an approximately 55-60% decrease in basal level ROK-alpha phosphorylation as compared with wild type osteoclasts. Furthermore, RhoVal-14 transduction was only partially effective in stimulating ROK-alpha/CD44 phosphorylation, as well as CD44 surface expression, in these osteoclasts. Studies on the inhibition of Rho by C3 transferase or ROK-alpha by the specific inhibitor, Y-27632, showed a decrease in the phosphorylation mediated by ROK-alpha and CD44 surface expression. Neutralizing antibodies to alphav, beta3, or CD44 inhibited the migration and bone resorption of wild type osteoclasts. However, only anti-alphav or -beta3 antibodies blocked OPN-induced phosphorylation of ROK-alpha, CD44, and the ERM proteins. Our results strongly suggest a role for ROK-alpha in alphavbeta3-mediated Rho signaling, which is required for the phosphorylation events and CD44 surface expression. The functional deficiencies in the Rho effector(s) because of the lack of OPN were associated with decreased CD44 surface expression and hypomotility in the OPN-/- osteoclasts. Finally, we find that cooperativity exists between alphavbeta3 and CD44 for osteoclast motility and bone resorption.

Crosslinking of CD44 on human osteoblastic cells upregulates ICAM-1 and VCAM-1

Cell adhesion of osteoblasts and osteoclastic precursors of hematopoietic origin is a prerequisite for osteoclast maturation. We have investigated the relevance of osteoblast-matrix binding and regulation of adhesion molecules to this process. Human osteoblastic cells highly expressed CD44, a major receptor for hyaluronan present in the surrounding bone matrix. Crosslinking of CD44 on osteoblastic cells with specific antibodies augmented the expression of intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1. Hyaluronan, the major ligand of CD44, also up-regulated ICAM-1 expression. Stimulation of CD44 on osteoblastic cells amplified their adhesion to monocytic cells through ICAM-1 and VCAM-1. These results suggest that such crosstalk among distinct adhesion molecules may be relevant to bone metabolism, including osteoclastogenesis.

Hyaluronan-CD44 interaction hampers migration of osteoclast-like cells by down-regulating MMP-9

Osteoclast (OC) precursors migrate to putative sites of bone resorption to form functionally active, multinucleated cells. The preOC FLG 29.1 cells, known to be capable of irreversibly differentiating into multinucleated OC-like cells, displayed several features of primary OCs, including expression of specific integrins and the hyaluronan (HA) receptor CD44. OC-like FLG 29.1 cells adhered to and extensively migrated through membranes coated with fibronectin, vitronectin, and laminins, but, although strongly binding to HA, totally failed to move on this substrate. Moreover, soluble HA strongly inhibited OC-like FLG 29.1 cell migration on the permissive matrix substrates, and this behavior was dependent on its engagement with CD44, as it was fully restored by function-blocking anti-CD44 antibodies. HA did not modulate the cell-substrate binding affinity/avidity nor the expression levels of the corresponding integrins. MMP-9 was the major secreted metalloproteinase used by OC-like FLG 29.1 cells for migration, because this process was strongly inhibited by both TIMP-1 and GM6001, as well as by MMP-9-specific antisense oligonucleotides. After HA binding to CD44, a strong down-regulation of MMP-9 mRNA and protein was detected. These findings highlight a novel role of the HA-CD44 interaction in the context of OC-like cell motility, suggesting that it may act as a stop signal for bone-resorbing cells.

Colocalization of intracellular osteopontin with CD44 is associated with migration, cell fusion, and resorption in osteoclasts

Although osteopontin (OPN) is recognized generally as a secreted protein, an intracellular form of osteopontin (iOPN), associated with the CD44 complex, has been identified in migrating fibroblastic cells. Because both OPN and CD44 are expressed at high levels in osteoclasts, we have used double immunofluorescence analysis and confocal microscopy to determine whether colocalization of these proteins has functional significance in the formation and activity of osteoclasts. Analysis of rat bone marrow-derived osteoclasts revealed strong surface staining for CD44 and beta1- and beta3-integrins, whereas little or no staining for OPN or bone sialoprotein (BSP) was observed in nonpermeabilized cells. In permeabilized perfusion osteoclasts and multinucleated osteoclasts, staining for OPN and CD44 was prominent in cell processes, including filopodia and pseudopodia. Confocal microscopy revealed a high degree of colocalization of OPN with CD44 in motile osteoclasts. In cells treated with cycloheximide (CHX), perinuclear staining for OPN and BSP was lost, but iOPN staining was retained within cell processes. In osteoclasts generated from the OPN-null and CD44-null mice, cell spreading and protrusion of pseudopodia were reduced and cell fusion was impaired. Moreover, osteoclast motility and resorptive activity were significantly compromised. Although the area resorbed by OPN-null osteoclasts could be rescued partially by exogenous OPN, the resorption depth was not affected. These studies have identified an intracellular form of OPN, colocalizing with CD44 in cell processes, that appears to function in the formation and activity of osteoclasts.

Ultrastructural study of cell-cell interaction between osteoclasts and osteoblasts/stroma cells in vitro

Many biochemical reports support cell-cell interaction between osteoclasts and osteoblasts/stroma cells in vitro, however there have been few morphological studies supporting this. Details of cell-cell interaction between osteoclasts and osteoblasts/stroma cells remain unclear. The present study examined cell-cell interaction between osteoclasts and osteoblasts/stroma cells by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Osteoclasts, osteoblasts/stroma cells, and bone marrow cells obtained from 10-day-old ddY mice were cultured on dentin slices for 72 hr. Specimens were fixed, and some were examined by SEM. Specimens were decalcified, embedded in Epon after determination of the tartrate-resistant acid phosphatase activity (TRAP), and TRAP-positive cells for investigation were serially sectioned by alternating semithin and ultrathin sections, and then examined by TEM. By SEM, many cellular contacts were seen between the cells cultured on the dentin, but by TEM there were few special structures on the cell membranes between osteoclasts and osteoblasts/stroma cells, or between osteoclasts and bone marrow cells. A special structure on the cell membranes of osteoclasts was observed between an osteoclast and a cytoplasmic process of osteoblast/stroma cells, and this cell membrane was coated with electron dense or bristle-like structures. These bristle-like structures were very similar to those of coated pits. The present results show that the coated pit-like structure plays an important role in cell-cell interaction between osteoclasts and osteoblasts/stroma cells in vitro, and suggest that macromolecules binding to the osteoclast-surface receptor via ligands, accumulate in the coated pits, and enter the osteoclast as receptor-macromolecule complexes in endocytic vesicles.

The distribution pattern of the hyaluronan receptor CD44 during human tooth development

The aim was to investigate the expression pattern of the major cell-surface hyaluronan receptor CD44, as there are no existing data on its presence or absence in human dental structures at different developmental stages. Immunohistochemical localization of CD44 was studied using a monoclonal antibody, H3, that specifically recognizes an epitope in the common backbone of all CD44 isoforms. The dental lamina displayed a strong CD44 signal; the external enamel epithelium was negative. In the coronal region of the tooth germ the presecretory ameloblasts showed an intense reaction whereas the less differentiated inner enamel epithelial cells showed no signal at the cervical loop where they meet the external enamel epithelium. In the stellate reticulum a moderate reaction was detected. The secretory ameloblasts and the stratum intermedium showed a strong cell-surface CD44 signal. A strong signal was also observed on the odontoblasts and their processes. In the pulp, close to the odontoblastic layer, weak labelling was seen in the walls of capillary vessels. The distribution of CD44 in the human tooth germ corresponds to that of hyaluronan in most locations, suggesting that during tooth development this transmembrane protein plays an important part in hyaluronan-mediated events.

Direct involvement of ezrin/radixin/moesin (ERM)-binding membrane proteins in the organization of microvilli in collaboration with activated ERM proteins

Ezrin/radixin/moesin (ERM) proteins have been thought to play a central role in the organization of cortical actin-based cytoskeletons including microvillar formation through cross-linking actin filaments and integral membrane proteins such as CD43, CD44, and ICAM-2. To examine the functions of these ERM-binding membrane proteins (ERMBMPs) in cortical morphogenesis, we overexpressed ERMBMPs (the extracellular domain of E-cadherin fused with the transmembrane/cytoplasmic domain of CD43, CD44, or ICAM-2) in various cultured cells. In cultured fibroblasts such as L and CV-1 cells, their overexpression significantly induced microvillar elongation, recruiting ERM proteins and actin filaments. When the ERM-binding domains were truncated from these molecules, their ability to induce microvillar elongation became undetectable. In contrast, in cultured epithelial cells such as MTD-1A and A431 cells, the overexpression of ERMBMPs did not elongate microvilli. However, in the presence of EGF, overexpression of ERMBMPs induced remarkable microvillar elongation in A431 cells. These results indicated that ERMBMPs function as organizing centers for cortical morphogenesis by organizing microvilli in collaboration with activated ERM proteins. Furthermore, immunodetection with a phosphorylated ERM-specific antibody and site-directed mutagenesis suggested that ERM proteins phosphorylated at their COOH-terminal threonine residue represent activated ERM proteins.

Moesin and CD44 expression in cutaneous melanocytic tumours

The ERM (ezrin, radixin and moesin) family members, located just beneath the plasma membranes, are thought to be involved in the association of action filaments with the plasma membrane. One of the family members, moesin, is reported to bind to CD44. Splice variants of CD44 are thought to be associated with tumour progression or differentiation. Our aim was to investigate immunohistochemically the expression of moesin together with CD44 on paraffin tissue sections of a series of melanocytic tumours. The material included 12 ordinary melanocytic naevi, six Spitz naevi, eight dysplastic naevi, six blue naevi, seven malignant melanomas in situ, 15 primary malignant melanomas, five metastatic melanomas to the skin and five lymph node metastases. In the normal skin and the melanocytic tumours the expression of moesin was largely similar to that of CD44 standard. Strong moesin staining was observed in benign melanocytic lesions and melanomas in situ. However, the expression was decreased in advanced malignant melanomas. The moesin labelling in melanoma cells was downregulated with the depth of dermal invasion. The immunoreactivity was also diminished in the skin metastases and the lymph node metastases of melanoma. These results suggest that in melanocytic tumours, the alternation in the expression of moesin may be involved in the progression of malignancy.

Ezrin/radixin/moesin (ERM) proteins bind to a positively charged amino acid cluster in the juxta-membrane cytoplasmic domain of CD44, CD43, and ICAM-2

CD44 has been identified as a membrane-binding partner for ezrin/radixin/moesin (ERM) proteins, plasma membrane/actin filament cross-linkers. ERM proteins, however, are not necessarily colocalized with CD44 in tissues, but with CD43 and ICAM-2 in some types of cells. We found that glutathione-S-transferase fusion proteins with the cytoplasmic domain of CD43 and ICAM-2, as well as CD44, bound to moesin in vitro. The regions responsible for the in vitro binding of CD43 and CD44 to moesin were narrowed down to their juxta-membrane 20-30-amino acid sequences in the cytoplasmic domain. These sequences and the cytoplasmic domain of ICAM-2 (28 amino acids) were all characterized by the positively charged amino acid clusters. When E-cadherin chimeric molecules bearing these positively charged amino acid clusters of CD44, CD43, or ICAM-2 were expressed in mouse L fibroblasts, they were co-concentrated with ERM proteins at microvilli, whereas those lacking these clusters were diffusely distributed on the cell surface. The specific binding of ERM proteins to the juxta-membrane positively charged amino acid clusters of CD44, CD43, and ICAM-2 was confirmed by immunoprecipitation and site-directed mutagenesis. From these findings, we conclude that ERM proteins bind to integral membrane proteins bearing a positively charged amino acid cluster in their juxta-membrane cytoplasmic domain.