Widespread expression of tartrate-resistant acid phosphatase (Acp 5) in the mouse embryo

Effect of nanoemulsion carbonated hydroxyapatite-statin administration on Acp 5 and Runx-2 expression during orthodontic relapse in rats

Background

Retention is an essential element of orthodontic treatment. In the past two decades, numerous biological treatments have been developed to alleviate orthodontic relapse. Pharmacologic bone modulation is a viable approach to mitigate relapse. The aim of this investigation was to determine whether administering nanoemulsion carbonated hydroxyapatite-statin (CHA-statin) would increase Runx-2 expression and decrease Acp 5 expression in rats experiencing experimental orthodontic relapse.

Materials & methods

Forty-eight rats (n = 48) were assigned to four groups: control, CHA, statin, and CHA-statin, with 12 rats in each group. A 30 g mesial traction was applied for 7 days via a closed-coil spring that connected the first maxillary molar to the maxillary incisor. To maintain the moved teeth, CHA, statin hydrogel, and nanoemulsion CHA-statin were administered intrasulcularly every 3 days for a period of 7 days. The removal of the devices facilitated the occurrence of relapse. The expression of Acp 5 and Runx-2 was evaluated using the immunohistochemistry method. The collected data were evaluated using analysis of variance and post hoc tests, with p < 0.05.

Results

A significant reduction in Acp 5-positive cells in the CHA-statin group was observed on days 7 and 14 of relapse movement compared with the other groups (p < 0.05), whereas the average Runx-2 expressions in the CHA-statin groups were significantly higher than in the other groups on days 1, 7, and 14 during the relapse phase (p < 0.05).

Conclusion

The nanoemulsion CHA-statin increased Runx-2 expression and decreased Acp 5 expression, thereby potentially preventing orthodontic relapse in rats.

Bone Targeting Nanoparticles for the Treatment of Osteoporosis

Osteoporosis (OP) affects millions of people worldwide, especially postmenopausal women and the elderly. Although current available anti-OP agents can show promise in slowing down bone resorption, most are not specifically delivered to the hard tissue, causing significant toxicity. A bone-targeted nanodrug delivery system can reduce side effects and precisely deliver drug candidates to the bone. This review focuses on the progress of bone-targeted nanoparticles in OP therapy. We enumerate the existing OP medications, types of bone-targeted nanoparticles and categorize pairs of the most common bone-targeting functional groups. Finally, we summarize the potential use of bone-targeted nanoparticles in OP treatment. Ongoing research into the development of targeted ligands and nanocarriers will continue to expand the possibilities of OP-targeted therapies into clinical application.

Biocompatibility of calcium phosphate bone cement with optimised mechanical properties: an in vivo study

This work establishes the in vivo performance of modified calcium phosphate bone cements for vertebroplasty of spinal fractures using a lapine model. A non-modified calcium phosphate bone cement and collagen-calcium phosphate bone cements composites with enhanced mechanical properties, utilising either bovine collagen or collagen from a marine sponge, were compared to a commercial poly(methyl methacrylate) cement. Conical cement samples (8 mm height × 4 mm base diameter) were press-fit into distal femoral condyle defects in New Zealand White rabbits and assessed after 5 and 10 weeks. Bone apposition and tartrate-resistant acid phosphatase activity around cements were assessed. All implants were well tolerated, but bone apposition was higher on calcium phosphate bone cements than on poly(methyl methacrylate) cement. Incorporation of collagen showed no evidence of inflammatory or immune reactions. Presence of positive tartrate-resistant acid phosphatase staining within cracks formed in calcium phosphate bone cements suggested active osteoclasts were present within the implants and were actively remodelling within the cements. Bone growth was also observed within these cracks. These findings confirm the biological advantages of calcium phosphate bone cements over poly(methyl methacrylate) and, coupled with previous work on enhancement of mechanical properties through collagen incorporation, suggest collagen-calcium phosphate bone cement composite may offer an alternative to calcium phosphate bone cements in applications where low setting times and higher mechanical stability are important.

Response of knee fibrocartilage to joint destabilization

OBJECTIVE

A major challenge to understanding osteoarthritis (OA) pathology is identifying the cellular events that precede the onset of cartilage damage. The objective of this study is to determine the effect of joint destabilization on early changes to fibrocartilage in the joint.

DESIGN/METHODS

The anterior cruciate ligament was transected in collagen reporter mice (Col1CFP and ColXRFP). Mineralization labels were given every 2 weeks to measure new mineralized cartilage apposition. Novel fluorescent histology of mineralized tissue was used to characterize the changes in fibrocartilage at 2 and 4 weeks post-injury.

RESULTS

Changes in fibrocartilaginous structures of the joint occur as early as 2 weeks after injury and are well developed by 4 weeks. The alterations are seen in multiple entheses and in the medial surface of the femoral and tibial condyles. In the responding entheses, mineral apposition towards the ligament midsubstance results in thickening of the mineralize fibrocartilage. These changes are associated with increases in ColX-RFP, Col1-CFP reporter activity and alkaline phosphatase enzyme activity. Mineral apposition also occurs in the fibrocartilage of the non-articular regions of the medial condyles by 2 weeks and develops into osteophytes by 4 weeks post-injury. An unexpected observation is punctate expression of tartrate resistant acid phosphatase activity in unmineralized fibrochondrocytes adjacent to active appositional mineralization.

DISCUSSION

These observations suggest that fibrocartilage activates prior to degradation of the articular cartilage. Thus clinical and histological imaging of fibrocartilage may be an earlier indicator of disease initiation and may indicate a more appropriate time to start preventative treatment.

Role of tartrate-resistant acid phosphatase (TRAP) in long bone development

Tartrate resistant acid phosphatase (TRAP) was shown to be critical for skeleton development, and TRAP deficiency leads to a reduced resorptive activity during endochondral ossification resulting in an osteopetrotic phenotype and shortened long bones in adult mice. A proper longitudinal growth depends on a timely, well-coordinated vascularization and formation of the secondary ossification center (SOC) of the long bones epiphysis. Our results demonstrate that TRAP is not essential for the formation of the epiphyseal vascular network. Therefore, in wild type (Wt) controls as well as TRAP deficient (TRAP^−/−) mutants vascularised cartilage canals are present from postnatal day (P) five. However, in the epiphysis of the TRAP^−/− mice cartilage mineralization, formation of the marrow cavity and the SOC occur prematurely compared with the controls. In the mutant mice the entire growth plate is widened due to an expansion of the hypertrophic zone. This is not seen in younger animals but first detected at week (W) three and during further development. Moreover, an enhanced number of thickened trabeculae, indicative of the osteopetrotic phenotype, are observed in the metaphysis beginning with W three. Epiphyseal excavation was proposed as an important function of TRAP, and we examined whether TRAP deficiency affects this process. We therefore evaluated the marrow cavity volume (MCV) and the epiphyseal volume (EV) and computed the MCV to EV ratio (MCV/EV). We investigated developmental stages until W 12. Our results indicate that both epiphyseal excavation and establishment of the SOC are hardly impaired in the knockouts. Furthermore, no differences in the morphology of the epiphyseal bone trabeculae and remodeling of the articular cartilage layers are noted between Wt and TRAP^−/− mice. We conclude that in long bones, TRAP is critical for the development of the growth plate and the metaphysis but apparently not for the epiphyseal vascularization, excavation, and establishment of the SOC.

Interaction between marrow-derived human mesenchymal stem cells and peripheral blood mononuclear cells in endothelial cell differentiation

BACKGROUND AND AIMS

In adult connective tissues, mesenchymal stem cells (MSCs) play a key role in normal tissue turnover and repair. MSCs can participate in these processes not only through proliferation and differentiation but also through paracrine/autocrine functions. These characteristics make MSCs the optimal target in the development of cell-based therapies. This study describes a novel interaction between human MSC and blood mononuclear cells (MNCs), resulting in formation of blood vessel-like structures.

MATERIALS AND METHODS

Human marrow-derived MSCs and peripheral blood MNCs were co-cultured in monolayer cultures as well as in bovine collagen sponge up to 20 days. No exogenously supplied growth factors were applied. Morphological changes and formations of three dimensional structures were detected by light microscopy. The process was further stu-died for the expression of different endothelial cell markers. The expression of PECAM-1 and endoglin was studied by immunohistochemistry and the expression of vascular endothelial growth factor receptors 1 and 2 using quantitative real time PCR.

RESULTS

In co-cultures of human MSCs and MNCs, the previously nonadherent cells attached and started to elongate and formed tube-like structures within one week. At day 10, elongated PECAM-1 and endoglin expressing cells were detected in co-cultures. At day 20, PECAM-1 and endoglin-positive vessel-like structures were observed. VEGFR1 was up-regulated in co-cultures after 10 days, and expression levels increased with time. No PECAM-1, endoglin or VEGFR1 expressing cells were discovered in MSC-cultures without MNCs at any time point.

CONCLUSIONS

This study demonstrates induction of endothelial differentiation in co-cultures of human MSCs and MNCs, indicating a mechanism by which local application of MSCs could induce angiogenesis in vivo.

Genetic mouse models for bone studies--strengths and limitations

Mice have become a preferred model system for bone research because of their genetic and pathophysiological similarities to humans: a relatively short reproductive period, leading to relatively low cost of maintenance and the availability of the entire mouse genome sequence information. The success in producing the first transgenic mouse line that expressed rabbit β-globin protein in mouse erythrocytes three decades ago marked the beginning of the use of genetically engineered mice as model system to study human diseases. Soon afterward the development of cultured pluripotent embryonic stem cells provided the possibility of gene replacement or gene deletion in mice. These technologies have been critical to identify new genes involved in bone development, growth, remodeling, repair, and diseases, but like many other approaches, they have limitations. This review will introduce the approaches that allow the generation of transgenic mice and global or conditional (tissue-specific and inducible) mutant mice. A list of the various promoters used to achieve bone-specific gene deletion or overexpression is included. The limitations of these approaches are discussed, and general guidelines related to the analysis of genetic mouse models are provided.

Development of articular cartilage and the metaphyseal growth plate: the localization of TRAP cells, VEGF, and endostatin

During long bone development the original cartilaginous model in mammals is replaced by bone, but at the long bone endings the avascular articular cartilage remains. Before the articular cartilage attains structural maturity it undergoes reorganization, and molecules such as vascular endothelial growth factor (VEGF) and endostatin could be involved in this process. VEGF attracts blood vessels, whereas endostatin blocks their formation. The present study therefore focused on the spatio-temporal localization of these two molecules during the development of the articular cartilage. Furthermore, we investigated the distribution of the chondro/osteoclasts at the chondro-osseous junction of the articular cartilage with the subchondral bone. Mice served as our animal model, and we examined several postnatal stages of the femur starting with week (W) 4. Our results indicated that during the formation of the articular cartilage, VEGF and endostatin had an overlapping localization. The former molecule was, however, down-regulated, whereas the latter was uniformly intensely localized until W12. At the chondro-osseous junction, the number of tartrate-resistant acid phosphatase (TRAP)-positive chondro/osteoclasts declined with increasing age. Until W3 the articular cartilage was not well organized but at W8 it appeared structurally mature. At that time only a few TRAP cells were present, indicative of a low resorptive activity at the chondro-osseous junction. Subsequently, we examined the metaphyseal growth plate that is closed when skeletal maturity is attained. Within its hypertrophic zone, localization of endostatin and VEGF was observed until W6 and W8, respectively. At the chondro-osseous junction of the growth plate, chondro/osteoclasts remained numerous until W12 to allow for its complete resorption. According to former findings, VEGF is critical for a normal skeleton development, whereas endostatin has almost no effect on this process. In conclusion, our findings suggest that both VEGF and endostatin play a role in the structural reorganization of the articular cartilage and endostatin may be involved in the maintenance of its avascularity. In the growth plate, however, endostatin does not appear to counteract VEGF, allowing vascular invasion of hypertrophic cartilage and bone growth.

VEGF and its role in the early development of the long bone epiphysis

In long bones of murine species, undisturbed development of the epiphysis depends on the generation of vascularized cartilage canals shortly after birth. Despite its importance, it is still under discussion how this event is exactly regulated. It was suggested previously that, following increased hypoxia in the epiphyseal core, angiogenic factors are expressed and hence stimulate the ingrowth of the vascularized canals. In the present study, we tested this model and examined the spatio-temporal distribution of two angiogenic molecules during early development in mice. In addition, we investigated the onset of cartilage hypertrophy and mineralization. Our results provide evidence that the vascular endothelial growth factor is expressed in the epiphyseal resting cartilage prior to the moment of canal formation and is continuously expressed until the establishment of a large secondary ossification centre. Interestingly, we found no expression of secretoneurin before the establishment of the canals although this factor attracts blood vessels under hypoxic conditions. Epiphyseal development further involves maturation of the resting chondrocytes into hypertrophic ones, associated with the mineralization of the cartilage matrix and eventual death of the latter cells. Our results suggest that vascular endothelial growth factor is the critical molecule for the generation of the epiphyseal vascular network in mice long bones. Secretoneurin, however, does not appear to be a player in this event. Hypertrophic chondrocytes undergo cell death by a mechanism interpreted as chondroptosis.

Genome-wide expression analyses establish dendritic cells as a new osteoclast precursor able to generate bone-resorbing cells more efficiently than monocytes

Dendritic cells (DCs), mononuclear cells that initiate immune responses, and osteoclasts (OCs), multinucleated bone-resorbing cells, are hematopoietic cells derived from monocytic precursor cells. Using in vitro generated dendritic cells, we previously showed that human and murine DCs could transdifferentiate into resorbing osteoclasts in the presence of macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-kappaB ligand (RANKL). In this study we globally compared by transcriptomic profiling this new osteoclast differentiation pathway from DCs with the canonical differentiation pathway from monocytes. DNA chip data revealed that starting from two very distinct cell types, treatment with M-CSF and RANKL generated two highly similar types of osteoclast. In particular, DC-derived osteoclasts expressed all the characteristic marker genes of monocyte-derived osteoclasts. Two major molecular events could be observed during osteoclastogenesis: downregulation of a large set of monocyte or DC specific markers, together with upregulation of characteristic osteoclast marker genes. Most interestingly, our transcriptomic data showed a closer molecular profile between DCs and OCs than between monocytes and OCs. Our data establish DCs as a new osteoclast precursor able to generate OCs more efficiently than monocytes.

Genome-wide analyses of gene expression during mouse endochondral ossification

Background

Endochondral ossification is a complex process involving a series of events that are initiated by the establishment of a chondrogenic template and culminate in its replacement through the coordinated activity of osteoblasts, osteoclasts and endothelial cells. Comprehensive analyses of in vivo gene expression profiles during these processes are essential to obtain a complete understanding of the regulatory mechanisms involved.

Methodology/Principal Findings

To address these issues, we completed a microarray screen of three zones derived from manually segmented embryonic mouse tibiae. Classification of genes differentially expressed between each respective zone, functional categorization as well as characterization of gene expression patterns, cytogenetic loci, signaling pathways and functional motifs both confirmed reported data and provided novel insights into endochondral ossification. Parallel comparisons of the microdissected tibiae data set with our previously completed micromass culture screen further corroborated the suitability of micromass cultures for modeling gene expression in chondrocyte development. The micromass culture system demonstrated striking similarities to the in vivo microdissected tibiae screen; however, the micromass system was unable to accurately distinguish gene expression differences in the hypertrophic and mineralized zones of the tibia.

Conclusions/Significance

These studies allow us to better understand gene expression patterns in the growth plate and endochondral bones and provide an important technical resource for comparison of gene expression in diseased or experimentally-manipulated cartilages. Ultimately, this work will help to define the genomic context in which genes are expressed in long bones and to understand physiological and pathological ossification.

The role of Akt1 in terminal stages of endochondral bone formation: angiogenesis and ossification

Longitudinal bone growth is the result of endochondral bone formation which takes place in the growth plate. The rate of chondrocyte proliferation and hypertrophy, vascular invasion with the formation of primary ossification centers and cartilage replacement by bone tissue are all important processes required for normal growth. We have shown a role for the PI3K signaling pathway in chondrocyte hypertrophy and bone growth in tibia explant cultures. In this current study, we aimed to investigate the role of Akt1, an important target of PI3K, in endochondral ossification. Akt1 KO mice showed reduced size compared to their littermates throughout life, but the largest difference in body size was observed around 1 week of age. Focusing on this specific developmental stage, we discovered delayed secondary ossification in the long bones of Akt1 KO mice. A delay in formation of a structure resembling a secondary ossification center was also seen in tibia organ cultures treated with the PI3K inhibitor LY294002. The expression of matrix metalloproteinase-14 (MMP-14), the main protease responsible for development of secondary ossification centers, was decreased in the epiphysis of Akt1 KO mice, possibly explaining the delay in secondary ossification centers seen in the Akt1 KO mice. Bone mineral density (BMD) and bone mineral content (BMC) measured in the proximal tibia of 1-year-old mice were decreased in Akt1 KO mice, suggesting that the original delay in ossification might affect bone quality in older animals.

Biology and clinical significance of tartrate-resistant acid phosphatases: new perspectives on an old enzyme

Type 5 tartrate-resistant acid phosphatase (TRAP) has been a clinically relevant biomarker for about 50 years. It has always been a reliable and specific cytochemical marker for hairy cell leukemia and for differentiated cells of monocytic lineage. Only recently has the test for serum TRAP activity been accepted as sensitive and specific enough for clinical use as a marker of osteoclasts and bone resorption. This has come about through steady advances in knowledge about TRAP enzymology, structure, function, and molecular regulation and a consequent appreciation that TRAP isoforms 5a and 5b have very different clinical significance. As a measure of osteoclast number and bone resorption, TRAP 5b has diagnostic and prognostic applications in osteoporosis, cancers with bone metastasis, chronic renal failure, and perhaps other metabolic and pathologic bone diseases. Serum TRAP 5a, on the other hand, has no relationship to bone metabolism but seems instead to be a measure of activated macrophages and chronic inflammation. Exploration of the real clinical usefulness of serum TRAP 5a for diagnosis and disease management in a wide variety of chronic inflammatory diseases is only now beginning. This perspective traces the important basic scientific developments that have led up to the refinement of serum TRAP isoform immunoassays and their validation as biomarkers of disease. Many unanswered questions remain, providing a wealth of opportunity for continued research of this multifaceted enzyme.

Localization of tartrate-resistant acid phosphatase (TRAP), membrane type-1 matrix metalloproteinases (MT1-MMP) and macrophages during early endochondral bone formation

Endochondral bone formation, the process by which most parts of our skeleton evolve, leads to the establishment of the diaphyseal primary (POC) and epiphyseal secondary ossification centre (SOC) in long bones. An essential event for the development of the SOC is the early generation of vascularized cartilage canals that requires the proteolytic cleavage of the cartilaginous matrix. This in turn will allow the canals to grow into the epiphysis. In the present study we therefore initially investigated which enzymes and types of cells are involved in this process. We have chosen the mouse as an animal model and focused our studies on the distal part of the femur during early stages after birth. The formation of the cartilage canals was promoted by tartrate-resistant acid phosphatase (TRAP) and membrane type-1 matrix metalloproteinases (MT1-MMP). In addition, macrophages and cells containing numerous lysosomes contributed to the establishment of the canals and enabled their further advancement into the epiphysis. As development continued, the SOC was formed, and in mice aged 10 days a distinct layer of type I collagen (= osteoid) was laid down onto the cartilage scaffold. The events leading to the establishment of the SOC were compared with those of the POC. Basically these processes were quite similar, and in both ossification centers, TRAP-positive chondroclasts resorbed the cartilage matrix. However, occasionally co-expression of TRAP and MT1-MMP was noted in a small subpopulation of this cell type. Furthermore, numerous osteoblasts expressed MT1-MMP from the start of endochondral ossification, whereas others did not. In osteocytogenesis, MT1-MMP has been shown to be critical for the establishment of the cytoplasmic processes mediating the communication between osteocytes and bone-lining cells. Considering the well-known fact that not all osteoblasts transform into osteocytes, and in accordance with the present data, we suggest that MT1-MMP is needed at the very beginning of osteocytogenesis and may additionally determine whether an osteoblast further differentiates into an osteocyte.

Structure, formation and role of cartilage canals in the developing bone

In the long bones, endochondral bone formation proceeds via the development of a diaphyseal primary ossification centre (POC) and an epiphyseal secondary ossification centre (SOC). The growth plate, the essential structure for longitudinal bone growth, is located between these two sites of ossification. Basically, endochondral bone development depends upon neovascularization, and the early generation of vascularized cartilage canals is an initial event, clearly preceding the formation of the SOC. These canals form a discrete network within the cartilaginous epiphysis giving rise to the formation of the marrow space followed by the establishment of the SOC. These processes require excavation of the provisional cartilaginous matrix which is eventually replaced by permanent bone matrix. In this review, we discuss the formation of the cartilage canals and the importance of their cells in the ossification process. Special attention is paid to the enzymes required in disintegration of the cartilaginous matrix which, in turn, will allow for the invasion of new vessels. Furthermore, we show that the mesenchymal cells of the cartilage canals express bone-relevant proteins and transform into osteocytes. We conclude that the canals are essential for normal epiphyseal bone development, the establishment of the growth plate and ultimately longitudinal growth of the bones.

Tartrate-resistant acid phosphatase (TRAP) and the osteoclast/immune cell dichotomy

Tartrate-resistant acid phosphatase (TRAP), once considered to be just a histochemical marker of osteoclasts is now recognised to be a molecule of widespread occurrence with functions in both the skeleton and the immune system. TRAP is expressed by osteoclasts, macrophages, dendritic cells and a number of other cell types. It has a critical role in many biological processes including skeletal development, collagen synthesis and degradation, the mineralisation of bone, cytokine production by macrophages and dendritic cells, macrophage recruitment, dendritic cell maturation and a role in the development of Th1 responses. TRAP is able to degrade skeletal phosphoproteins including osteopontin (OPN), identical to the T-cell cytokine, Eta-1. In this review, we discuss the role of TRAP in bone and immune cells and suggest that TRAP may be implicated in autoimmune disorders regulated by Th1 inflammatory responses as well as certain cancers.

Altered collagen in tartrate-resistant acid phosphatase (TRAP)-deficient mice: a role for TRAP in bone collagen metabolism

Tartrate-resistant acid phosphatase (TRAP) is an iron-containing protein that is highly expressed by osteoclasts, macrophages, and dendritic cells. The enzyme is secreted by osteoclasts during bone resorption, and serum TRAP activity correlates with resorptive activity in disorders of bone metabolism. TRAP is essential for normal skeletal development. In knockout mice lacking TRAP, bone shape and modeling is altered with increased mineral density. Here, we report the effect of TRAP on the biochemical and biomechanical properties of collagen, the major protein constituting the bone matrix, using these mice. Femurs from TRAP-/- and wild-type mice were used in these studies. The biomechanical properties were investigated using a three-point bending technique. Collagen synthesis was determined by measuring cross-link content using high-performance liquid chromatography and amino acid analysis. Collagen degradation was determined by measuring matrix metalloproteinase-2 (MMP-2) activity. The rates of collagen synthesis and degradation were significantly greater in bones from TRAP-/- mice compared with wild type. At 8 weeks, there was an increase in the intermediate cross-links but no significant difference in animals aged 6 months. There was a significant increase in mature cross-links at both ages. A significant increase in MMP-2 production both pro and active was observed. A significant increase in ultimate stress and Young's modulus of elasticity was needed to fracture the bones from mice deficient in TRAP. We conclude that both synthesis as well as degradation of collagen are increased when TRAP is absent in mice at 8 weeks and 6 months of age, showing that TRAP has an important role in the metabolism of collagen.

Development of a novel fragments absorbed immunocapture enzyme assay system for tartrate-resistant acid phosphatase 5b

BACKGROUND

Osteoclastic activity is mainly assessed by measuring urinary markers. To correct for differences in renal clearance, the levels of urinary markers are usually corrected by the urine creatinine concentration. Therefore, alternative serum markers to evaluate osteoclastic activity are required. We developed a novel system for the determination of serum tartrate-resistant acid phosphatase 5b (TRACP5b) activity to evaluate osteoclastic activity.

METHODS

Two unique monoclonal antibodies were generated and the specificity was tested using a surface enhanced laser desorption/ionization time-of-flight mass spectroscopy (SELDI TOF-MS). A novel fragments absorbed immunocapture enzymatic assay (FAICEA) method was developed using 2 monoclonal antibodies.

RESULTS

FAICEA gave a sensitivity 0.1 U/l, linearity of 0.1-28 U/l, recovery 92-103%, inter-assay CV 2.95% and intra-assay CV 2.15%. Unlike other TRACP5b assay systems, FAICEA avoided interference from TRACP 5a.

CONCLUSIONS

According to the FAICEA, postmenopausal women had higher TRACP5b concentrations than younger women. The results show that TRACP5b is a novel bone resorption marker in serum.

TRACP Influences Th1 pathways by affecting dendritic cell function

TRACP, a marker of osteoclasts, is also expressed by cells of the immune system. We identified a novel function for TRACP in the dendritic cell. DCs from TRACP knockout mice have impaired maturation and trigger reduced Th1 responses in vivo. We postulate that TRACP has an important role in the presentation of antigens to T cells.

INTRODUCTION

TRACP is highly expressed by osteoclasts, activated macrophages, and dendritic cells (DCs). Knockout mice lacking TRACP have an intrinsic defect in osteoclastic resorption and macrophages that display abnormal immunomodulatory responses and cytokine secretion profiles. Our aim in this study was to investigate the significance of TRACP in the inductive phase of the immune response by examining dendritic cells from TRACP(-/-) mice.

MATERIALS AND METHODS

Maturational state and function of leukocyte subsets in mice was assessed by flow cytometry. The ability of the immune system to respond to nonspecific activation and to specific antigen was assessed by delayed type hypersensitivity and the presence of isotype-specific serum antibody in vivo and T-cell proliferation and cytokine production in vitro.

RESULTS

The ability of lipopolysaccharide (LPS) to upregulate MHC II and CD80 in DCs from TRACP(-/-) mice was reduced compared with wildtype mice, although production of IL-10 by DCs from TRACP-deficient animals was increased. T- and B-cell responses not involving antigen presentation (anti-CD3, TNP-ficoll) were normal in TRACP(-/-) mice, but responses to T-dependent antigens were impaired. Specifically, TRACP(-/-) mice had defective delayed hypersensitivity responses to picryl chloride and reduced proliferative responses to ovalbumin compared with wildtype mice. In response to ovalbumin, but not anti-CD3, T cells from TRACP(-/-) mice produced less interferon-gamma (IFN-gamma), but there was no difference in IL-4 production: TRACP(-/-) mice also produced less ovalbumin (OVA)-specific IgG2a after immunization.

CONCLUSIONS

The finding that DCs from TRACP(-/-) mice have impaired maturation and defective Th1 responses shows that TRACP is important for polarizing responses in naïve T cells to antigen-presented dendritic cells.

Substrate positioning by His92 is important in catalysis by purple acid phosphatase

Proteolysis of single polypeptide mammalian purple acid phosphatases (PAPs) results in the loss of an interaction between the loop residue Asp146 and the active site residues Asn91 and/or His92. While Asn91 is a ligand to the divalent metal of the mixed-valent di-iron center, the role of His92 in the catalytic mechanism is unknown. Site-directed mutagenesis of His92 was performed to examine the role of this residue in single polypeptide PAP. Conversion of His92 into Ala, which eliminates polar interactions of this residue with the active site, resulted in a 10-fold decrease in catalytic activity at the optimal pH. Conversely, conversion of this residue into Asn, which cannot function as either a proton donor or acceptor, but can provide hydrogen-bonding interactions, resulted in a three-fold increase in activity at the optimal pH. Both mutant enzymes had more acidic pH optima, with pK(es,1) values consistent with the involvement of an iron(III) hydroxide unit or a hydroxide in the second coordination sphere in catalysis. These results, together with EPR data, support a role of His92 in positioning either the nucleophile or the substrate, rather than directly in acid or base catalysis. The existence of an extensive hydrogen-bonding network that could fine-tune the position of His92 is consistent with this proposal.

Quantification of tartrate resistant acid phosphatase distribution in mouse tibiae using image analysis

Tartrate resistant acid phosphatase (TRAP) activity of bone is a suitable biochemical marker for osteoclastic bone resorption. Qualitatively, the histochemical distribution of TRAP has been used to identify osteoclasts responsible for bone resorption; however, there have been few attempts to quantify TRAP localization. We describe a method for evaluating bone resorption by quantifying area percentages of positive TRAP localization using image analysis. Mouse tibiae were paraffin embedded following demineralization in disodium ethylenediamine tetraacetic acid. Longitudinal sections of tibia were cut from 15 levels in the left and the right limbs of six mice (180 sections total) and stained for TRAP distribution. Positive TRAP localization was quantified by pixel area count and reported as a percentage of the total tissue area specified. The 1.85 mm2 region of interest was placed at the midpoint of the epiphyseal growth plate containing the provisional calcification layer and the primary spongiosa, while excluding cortical bone of each mouse tibia. The percentage of TRAP localization ranged from 0.95 to 1.31% and was not significantly different from level to level or limb to limb in each mouse (p > 0.100). Within the same region of interest, an osteoclast count along the bone perimeter also was performed. We demonstrated a strong correlation (r2 = 0.903) between the conventional histomorphometric osteoclast index and positive TRAP localization, validating the latter as an alternative method to assess bone resorption. Quantitative analysis of TRAP is significant because it allows statistical comparisons between treatment groups, promotes precise pathological diagnoses and facilitates a reference data base that may aid the study of bone related diseases involving increased bone resorption.

Tartrate-resistant acid phosphatase knockout mice

TRACP is a lysosomal enzyme found in diverse tissues, where it is expressed in dendritic cells as well as osteoclasts and macrophages. To investigate the function of TRACP in vivo, we have generated mice in which the gene-encoding TRACP has been selectively disrupted by targeted homologous recombination in murine embryonic stem cells. Homozygous TRACP "knockout" mice have progressive foreshortening and deformity of the long bones and axial skeleton suggesting a role for TRACP in endochondral ossification. There is increased mineralization reflecting a mild osteopetrosis caused by reduced osteoclast modeling activity. These knockout mice also display an impairment of macrophage function with abnormal immunomodulatory cytokine responses. Superoxide formation and nitrite production were enhanced in stimulated macrophages lacking TRACP as was the secretion of the proinflammatory cytokines TNF-alpha, interleukin (IL)-1beta, and IL-12. TRACP knockout mice showed delayed clearance of the microbial pathogen Staphylococcus aureus after sublethal intraperitoneal inoculation. The macrophages lacking TRACP showed an increase in tartrate-sensitive lysosomal acid phosphatase activity (LAP). The TRACP knockout mice were bred with mice lacking LAP. Mice lacking both TRACP and LAP had even shorter bones than the TRACP single knockouts. Osteopontin, identical to the T-cell cytokine eta-1, accumulated adjacent to actively resorbing osteoclasts suggesting that both phosphatases are important for processing this protein. We propose that TRACP may be an important regulator of osteopontin/eta-1 activity common to both the immune system and skeleton.

A phage display technique identifies a novel regulator of cell differentiation

The formation of new bone during the process of bone remodeling occurs almost exclusively at sites of prior bone resorption. In an attempt to discover what regulatory pathways are utilized by osteoblasts to effect this site-specific formation event we probed components of an active bone resorption surface with an osteoblast phage expression library. In these experiments primary cultures of rat osteoblasts were used to construct a phage display library in T7 phage. Tartrate-resistant acid phosphatase (type V) (TRAP) was used as the bait in a biopanning procedure. 40 phage clones with very high affinity for TRAP were sequenced, and of the clones with multiple consensus sequences we identified a regulatory protein that modulates osteoblast differentiation. This protein is the TGFbeta receptor-interacting protein (TRIP-1). Our data demonstrate that TRAP activation of TRIP-1 evokes a TGFbeta-like differentiation process. Specifically, TRIP-1 activation increases the activity and expression of osteoblast alkaline phosphatase, osteoprotegerin, collagen, and Runx2. Moreover, we show that TRAP interacts with TRIP intracellularly, that activation of the TGFbeta type II receptor by TRIP-1 occurs in the presence of TRAP and that the differentiation process is mediated through the Smad2/3 pathway. A final experiment demonstrates that osteoblasts, when cultured in osteoclast lacunae containing TRAP, rapidly and specifically differentiate into a mature bone-forming phenotype. We hypothesize that binding to TRAP may be one mechanism by which the full osteoblast phenotype is expressed during the process of bone remodeling.

Acid phosphatases as markers of bone metabolism

Various biochemical markers have been used to assess bone metabolism and to monitor the effects of treatments. Tartrate resistant acid phosphatase (TRAP; EC 3.1.3.2) has often been used to assess bone absorption. Although osteoclasts contain abundant TRAP and they are responsible for bone resorption, the total TRAP activities in the serum measured by colorimetric methods little reflect the bone turnover. TRAP 5 is further separated into 5a and 5b by electrophoresis. Type 5b is considered to be derived from the osteoclast, and therefore attempts are being made to measure exclusively serum TRAP 5b by kinetic methods, immunological methods, and chromatographic methods including ion-exchange and heparin column chromatography.

Characterization of four monoclonal antibodies to recombinant human tartrate-resistant acid phosphatase

In this study we produced a recombinant human Tartrate-resistant acid phosphatase (TRAP) enzyme from baculovirus-infected insect cells, generated four monoclonal antibodies (MAbs) 15A4, 13B9, 1C6 and 3G7, to the enzyme, and characterized these antibodies. In the human serum and lung specimen, all four antibodies appeared to have a high specificity for native TRAP enzyme in western blot analysis, immunohistochemical analysis and enzyme immunoassay. These antibodies may react with respective conformational determinants, therefore, they may be useful for detection of active TRAP. Only one of the antibodies, 15A4 also reacted with a denatured epitope, therefore, it is suitable for western blot analysis, enzyme immunoassay and for immunohistochemistry in the rat. Taken together, having characterized properties of four monoclonal antibodies against recombinant human TRAP enzyme may be useful for development of TRAP specific immunoassays in pathology and hematology of the bone. They will certainly be of use for the study of biosynthesis, regulation and function of the TRAP enzyme.

Use of a phage display technique to identify potential osteoblast binding sites within osteoclast lacunae

There is a temporal coupling between the processes of bone resorption and bone formation in normal skeletal remodeling. That is, osteoblastic activity usually follows episodes of osteoclastic activity. However, what has not been universally appreciated is that there also is a spatial coupling between these processes. Bone formation only occurs in the immediate vicinity of the resorptive event. In this study, we describe a phage display technique that has been used to identify the mechanisms by which osteoblasts recognize components of the prior resorbed lacunar surface. Using a type V tartrate-resistant acid phosphatase (TRAP) as the bait and a random peptide M13 phage display library as the probe, we have identified specific sequences that show a very high affinity for TRAP. One of these peptides, designated clone 5, has a subnanomolar Kd for TRAP, interacts with TRAP in a Far-Western assay, binds exclusively to TRAP within osteoclast lacunae, is present in osteoblasts, and can effectively block osteoblast binding to resorption surfaces. The clone 5 peptide shows a high homology to glypican 4 (GPC4), a proteoglycan attachment receptor found in a number of cell types.

Tartrate-resistant acid phosphatase (Acp 5): identification in diverse human tissues and dendritic cells

Histochemical demonstration of tartrate-resistant acid phosphatase (TRAP) is used for the specific identification of osteoclasts. The enzyme, which we have shown to be critical for normal bone development in mice, is also characteristic of monohistiocytes, including alveolar macrophages, and is associated with diverse pathological conditions such as Gaucher's disease and hairy cell leukemia. TRAP activity is enhanced in serum when bone resorption is increased, and the activity is used routinely to monitor treatment responses in Gaucher's disease. We have lately shown widespread expression of the enzyme in murine tissues with particular reference to the skin, thymus, gut epithelia, and isolated dendritic cells, suggesting a possible role in immunity. To further clarify the significance of TRAP in human physiology, we have examined its distribution in non-skeletal human tissues and in CD34+ -derived human dendritic cells. TRAP mRNA determined by Northern blotting analysis was expressed abundantly in spleen, liver, colon, lung, small intestine, kidney, stomach, testis, placenta, lymph node, thymus, peripheral blood leukocyte, bone marrow, and fetal liver. Expression of TRAP protein was investigated by immunohistochemistry, with which the enzyme was identified in multiple tissues. Histochemical staining detected enzymatically active protein in spleen, lung, skin, colon, stomach, and ileum. Active TRAP was identified in CD34+ -derived immature dendritic cells and co-localized to intracellular CD63 positive organelles. When these cells were matured by induction with LPS, the TRAP activity increased fivefold and remained within the cell during the phase associated with CD63 surface expression. Our findings demonstrate widespread expression of TRAP in human tissues. Its abundant expression in epithelia and dendritic cells suggests a potential role in antigen processing and in immune responses.