The extracellular matrix of cartilage in the growth plate before and during calcification: changes in composition and degradation of type II collagen

Vertebrate mineralized tissues: A modular structural analysis

Vertebrate mineralized tissues, present in bones, teeth and scales, have complex 3D hierarchical structures. As more of these tissues are characterized in 3D using mainly FIB SEM at a resolution that reveals the mineralized collagen fibrils and their organization into collagen fibril bundles, highly complex and diverse structures are being revealed. In this perspective we propose an approach to analyzing these tissues based on the presence of modular structures: material textures, pore shapes and sizes, as well as extents of mineralization. This modular approach is complimentary to the widely used hierarchical approach for describing these mineralized tissues. We present a series of case studies that show how some of the same structural modules can be found in different mineralized tissues, including in bone, dentin and scales. The organizations in 3D of the various structural modules in different tissues may differ. This approach facilitates the framing of basic questions such as: are the spatial relations between modular structures the same or similar in different mineralized tissues? Do tissues with similar sets of modules carry out similar functions or can similar functions be carried out using a different set of modular structures? Do mineralized tissues with similar sets of modules have a common developmental or evolutionary pathway? STATEMENT OF SIGNIFICANCE: 3D organization studies of diverse vertebrate mineralized tissues are revealing detailed, but often confusing details about the material textures, the arrangements of pores and differences in the extent of mineralization within a tissue. The widely used hierarchical scheme for describing such organizations does not adequately provide a basis for comparing these tissues, or addressing issues such as structural components thought to be characteristic of bone, being present in dermal tissues and so on. The classification scheme we present is based on identifying structural components within a tissue that can then be systematically compared to other vertebrate mineralized tissues. We anticipate that this classification approach will provide insights into structure-function relations, as well as the evolution of these tissues.

ENPP1 downregulation and FGF23 upregulation in growth-related calcification of the tibial tuberosity in rats

During tibial tuberosity growth, superficial and deep portions can be observed; however, the deep portion is not observed after the growth period, as it develops into bone tissues. Calcification in vivo is known to be constitutively suppressed by ectonucleotide pyrophosphatase/phosphodiesterase-1 (ENPP1) but promoted by tissue-nonspecific alkaline phosphatase (TNAP). FGF23 promotes calcification of enthesis. Gene expression of FGF23 increased rapidly at 13W in this study. Therefore, the tibial tuberosity is speculated to develop via Enpp1 downregulation and Tnap upregulation; however, the understanding of these processes remains unclear. Hence, in the present study, we aimed to explore the age-related structural changes and underlying gene expression changes in the tibial tuberosity of rats. Male Wistar rats were divided into three groups (3-, 7-, and 13-week-old; eight each). The tibial tuberosity superficial and deep portions were clearly observed in 3- and 7-week-old rats, but the presence of the deep portion was not confirmed in 13-week-old rats. The extracellular matrix of hypertrophic chondrocytes was calcified. Furthermore, the Enpp1 expression was the highest in 3-week-old rats and decreased with growth. The TNAP expression did not differ significantly among the groups. The deep portion area was significantly lower in 3-week-old rats than in 7-week-old rats. Generally, the extracellular matrix of the immature chondrocytes is not calcified. Therefore, we speculated that the cartilaginous tibial tuberosity calcifies and ossifies with growth. The Enpp1 expression decreased with growth, whereas the Tnap expression remained unchanged. Thus, we surmise that the tibial tuberosity calcifies with growth and that this process involves Enpp1 downregulation and FGF23 upregulation. As Osgood-Schlatter disease is closely related to the calcification of the tibial tuberosity, these findings may help clarify the pathogenesis of this disease.

Chondrocyte targeting gold nanoparticles protect growth plate against inflammatory damage by maintaining cartilage balance

Cartilage destruction caused by inflammation is a clinical challenge. Many studies have investigated cartilage destruction in adults, but little research was conducted on children. In this study, the protective effect of gold nanoparticles (AuNPs) on the cartilage of children was realized by counteracting chondrocyte apoptosis and extracellular matrix (ECM) degradation in a young mouse model of lipopolysaccharide (LPS)-induced growth plate (GP) cartilage damage. Initially, engineered AuNPs can be efficiently absorbed by chondrocytes, approximately 20 times the amount absorbed by macrophages, resulting in a 29% ± 0.05% increase in chondrocyte viability. Then, AuNPs exposure significantly reduced the release of inflammatory cytokines and secretion of ECM degradation factors induced by LPS. Subsequently, AuNPs were applied to resist LPS-induced cartilage destruction in young mice. AuNPs inhibited the formation of gaps, without chondrocytes and extracellular matrix, between the proliferative and hypertrophy zones of the GP cartilage, and the gaps were noticeable in the LPS group. This finding can be attributed to the capability of AuNPs to reduce the LPS-induced apoptosis rate of mouse chondrocytes by 72.38% and the LPS-induced ECM degradation rate by 70.89%. Further analysis demonstrated that remission is partly due to AuNPs' role in maintaining the balance of catabolic and anabolic factors in the ECM. Altogether, these findings indicate that AuNPs can partially protect the cartilage of children from inflammatory damage by suppressing chondrocyte apoptosis and ECM degradation.

Properties of Cartilage-Subchondral Bone Junctions: A Narrative Review with Specific Focus on the Growth Plate

OBJECTIVE

The purpose of this narrative review is to summarize what is currently known about the structural, chemical, and mechanical properties of cartilage-bone interfaces, which provide tissue integrity across a bimaterial interface of 2 very different structural materials. Maintaining these mechanical interfaces is a key factor for normal bone growth and articular cartilage function and maintenance.

MATERIALS AND METHODS

A comprehensive search was conducted using Google Scholar and PubMed/Medline with a specific focus on the growth plate cartilage-subchondral bone interface. All original articles, reviews in journals, and book chapters were considered. Following a review of the overall structural and functional characteristics of the physis, the literature on histological studies of both articular and growth plate chondro-osseous junctions is briefly reviewed. Next the literature on biochemical properties of these interfaces is reviewed, specifically the literature on elemental analyses across the cartilage-subchondral bone junctions. The literature on biomechanical studies of these junctions at the articular and physeal interfaces is also reviewed and compared.

RESULTS

Unlike the interface between articular cartilage and bone, growth plate cartilage has 2 chondro-osseous junctions. The reserve zone of the mature growth plate is intimately connected to a plate of subchondral bone on the epiphyseal side. This interface resembles that between the subchondral bone and articular cartilage, although much less is known about its makeup and formation.

CONCLUSION

There is a notably paucity of information available on the structural and mechanical properties of reserve zone-subchondral epiphyseal bone interface. This review reveals that further studies are needed on the microstructural and mechanical properties of chondro-osseous junction with the reserve zone.

Characterization of the growth plate-bone interphase region using cryo-FIB SEM 3D volume imaging

The interphase region at the base of the growth plate includes blood vessels, cells and mineralized tissues. In this region, cartilage is mineralized and replaced with bone. Blood vessel extremities permeate this space providing nutrients, oxygen and signaling factors. All these different components form a complex intertwined 3D structure. Here we use cryo-FIB SEM to elaborate this 3D structure without removing the water. As it is challenging to image mineralized and unmineralized tissues in a hydrated state, we provide technical details of the parameters used. We obtained two FIB SEM image stacks that show that the blood vessels are in intimate contact not only with cells, but in some locations also with mineralized tissues. There are abundant red blood cells at the extremities of the vessels. We also documented large multinucleated cells in contact with mineralized cartilage and possibly also with bone. We observed membrane bound mineralized particles in these cells, as well as in blood serum, but not in the hypertrophic chondrocytes. We confirm that there is an open pathway from the blood vessel extremities to the mineralizing cartilage. Based on the sparsity of the mineralized particles, we conclude that mainly ions in solution are used for mineralizing cartilage and bone, but these are augmented by the supply of mineralized particles.

Observation of Solute Transport between Articular Cartilage and Subchondral Bone in Live Mice

OBJECTIVE

To establish a method for investigating the permeability of calcified cartilage zone (CCZ) and to observe solute transport between articular cartilage (AC) and subchondral bone (SB) through intact CCZ in vivo.

DESIGN

We developed a novel fixing device combined with un-decalcified fluorescence observation method to address the permeability of CCZ in live mice. Twenty-four Balb/c female mice aged 1 to 8 months were used to observe the development of CCZ. Eighty-four Balb/c female mice (aged 1 or 6 months) with mature or immature CCZ of distal femur were used to investigate the permeability of intact CCZ in vivo. Diffusivity of rhodamine B (476 Da) and tetramethyl-rhodamine isothicyanate-dextran (TRITC-Dextran, 20 kDa) was tested from AC to SB in 0 minutes, 1 minute, 15 minutes, 30 minutes, 1 hour, and 2 hours. None diffused knee joints (0 minutes) served as blank control, while in vitro immersion of distal femurs in rhodamine B or TRITC-Dextran for 72 hours served as positive control.

RESULTS

CCZ was well developed in 6-month mice. Both tracers penetrated immature CCZ down to SB in less than 1 hour in live mice, while the diffusion of both tracers decreased rapidly at tidemark in all testing time points.

CONCLUSION

Current study provided direct evidence of blocking effect of CCZ in solute transportation during short diffusion period in live animal, indicating the important role of CCZ in joint development and microenvironment maintenance.

Immunohistochemical Analysis on Cortex-to-Cortex Healing After Mandibular Vertical Ramus Osteotomy: A Preliminary Study

PURPOSE

The present study analyzed the expression of specific cytokines in the transforming growth factor (TGF)-β superfamily postoperatively after mandibular vertical ramus osteotomy (VRO).

MATERIALS AND METHODS

Four beagle dogs were enrolled and euthanized at 1, 2, 4, and 8 weeks postoperatively for immunohistochemical analysis using 6 specific antibodies (bone morphogenetic protein [BMP]-2/4, BMP-7, TGF-β2, TGF-β3, matrix metalloproteinase-3, and vascular endothelial growth factor [VEGF]). The results from the surgical site and control (adjacent area) were compared.

RESULTS

Generalized upregulation of BMP-2/4 was observed in all healing periods, and the strongest expression of BMP-7 was observed at 1 week postoperatively. The strongest expression of TGF-β2 was observed at 8 weeks with increasing pattern. The strong expression of TGF-β3 was observed at 1 and 4 weeks, with the strongest expression of VEGF at 1 week, with a decreasing pattern. No notable uptake was detected with the 6 specific antibodies in the adjacent bone (control).

CONCLUSIONS

The absence of internal fixation after VRO led to dynamic healing with a specific expression pattern of BMP-7 and TGF-β2. The anatomic factors, including sufficient preexisting vascularity, led to the earlier expression pattern of VEGF.

Multiphasic construct studied in an ectopic osteochondral defect model

In vivo osteochondral defect models predominantly consist of small animals, such as rabbits. Although they have an advantage of low cost and manageability, their joints are smaller and more easily healed compared with larger animals or humans. We hypothesized that osteochondral cores from large animals can be implanted subcutaneously in rats to create an ectopic osteochondral defect model for routine and high-throughput screening of multiphasic scaffold designs and/or tissue-engineered constructs (TECs). Bovine osteochondral plugs with 4 mm diameter osteochondral defect were fitted with novel multiphasic osteochondral grafts composed of chondrocyte-seeded alginate gels and osteoblast-seeded polycaprolactone scaffolds, prior to being implanted in rats subcutaneously with bone morphogenic protein-7. After 12 weeks of in vivo implantation, histological and micro-computed tomography analyses demonstrated that TECs are susceptible to mineralization. Additionally, there was limited bone formation in the scaffold. These results suggest that the current model requires optimization to facilitate robust bone regeneration and vascular infiltration into the defect site. Taken together, this study provides a proof-of-concept for a high-throughput osteochondral defect model. With further optimization, the presented hybrid in vivo model may address the growing need for a cost-effective way to screen osteochondral repair strategies before moving to large animal preclinical trials.

The effect of dexamethasone and triiodothyronine on terminal differentiation of primary bovine chondrocytes and chondrogenically differentiated mesenchymal stem cells

The newly evolved field of regenerative medicine is offering solutions in the treatment of bone or cartilage loss and deficiency. Mesenchymal stem cells, as well as articular chondrocytes, are potential cells for the generation of bone or cartilage. The natural mechanism of bone formation is that of endochondral ossification, regulated, among other factors, through the hormones dexamethasone and triiodothyronine. We investigated the effects of these hormones on articular chondrocytes and chondrogenically differentiated mesenchymal stem cells, hypothesizing that these hormones would induce terminal differentiation, with chondrocytes and differentiated stem cells being similar in their response. Using a 3D-alginate cell culture model, bovine chondrocytes and chondrogenically differentiated stem cells were cultured in presence of triiodothyronine or dexamethasone, and cell proliferation and extracellular matrix production were investigated. Collagen mRNA expression was measured by real-time PCR. Col X mRNA and alkaline phosphatase were monitored as markers of terminal differentiation, a prerequisite of endochondral ossification. The alginate culture system worked well, both for the culture of chondrocytes and for the chondrogenic differentiation of mesenchymal stem cells. Dexamethasone led to an increase in glycosaminoglycan production. Triiodothyronine increased the total collagen production only in chondrocytes, where it also induced signs of terminal differentiation, increasing both collagen X mRNA and alkaline phosphatase activity. Dexamethasone induced terminal differentiation in the differentiated stem cells. The immature articular chondrocytes used in this study seem to be able to undergo terminal differentiation, pointing to their possible role in the onset of degenerative osteoarthritis, as well as their potential for a cell source in bone tissue engineering. When chondrocyte-like cells, after their differentiation, can indeed be moved on towards terminal differentiation, they can be used to generate a model of endochondral ossification, but this limitation must be kept in mind when using them in cartilage tissue engineering application.

Tissue and cellular morphological changes in growth plate explants under compression

The mechanisms by which mechanical loading may alter bone development within growth plates are still poorly understood. However, several growth plate cell or tissue morphological parameters are associated with both normal and mechanically modulated bone growth rates. The aim of this study was to quantify in situ the three-dimensional morphology of growth plate explants under compression at both cell and tissue levels. Growth plates were dissected from ulnae of immature swine and tested under 15% compressive strain. Confocal microscopy was used to image fluorescently labeled chondrocytes in the three growth plate zones before and after compression. Quantitative morphological analyses at both cell (volume, surface area, sphericity, minor/major radii) and tissue (cell/matrix volume ratio) levels were performed. Greater chondrocyte bulk strains (volume decrease normalized to the initial cell volume) were found in the proliferative (35.4%) and hypertrophic (41.7%) zones, with lower chondrocyte bulk strains (24.7%) in the reserve zone. Following compression, the cell/matrix volume ratio decreased in the reserve and hypertrophic zones by 24.3% and 22.6%, respectively, whereas it increased by 35.9% in the proliferative zone. The 15% strain applied on growth plate explants revealed zone-dependent deformational states at both tissue and cell levels. Variations in the mechanical response of the chondrocytes from different zones could be related to significant inhomogeneities in growth plate zonal mechanical properties. The ability to obtain in situ cell morphometry and monitor the changes under compression will contribute to a better understanding of mechanisms through which abnormal growth can be triggered.

Hyaluronan production by means of Has2 gene expression in chondrocytes is essential for long bone development

Mice possessing no Has2 expression in chondrocytes died near birth and displayed abnormalities throughout their skeleton. By embryonic day 18.5, the long bones were short and wide, and possessed excessive mineralization within their diaphysis, with little evidence of diaphyseal bone modeling. However, this does not appear to be associated with an absence of blood vessel invasion or the reduced presence of osteoclasts. There was no evidence for the formation of an organized growth plate between the epiphysis and diaphysis, and while hypertrophic chondrocytes were present in this region they were abnormal in both appearance and organization. There was also increased cellularity in the epiphyseal cartilage and a corresponding decrease in the abundance of extracellular matrix, but aggrecan was still present. Thus, hyaluronan production by chondrocytes is not only essential for formation of an organized growth plate and subsequent long bone growth but also for normal modeling of the diaphyseal bone.

Matrix metalloprotease inhibitors suppress initiation and progression of chondrogenic differentiation of mesenchymal stromal cells in vitro

Mesenchymal stromal cells (MSC) are an attractive source for cell therapy and tissue engineering of joint cartilage. Common chondrogenic in vitro protocols, however, induce hypertrophic markers like COL10A1, matrix metalloproteinase 13 (MMP13), and alkaline phosphatase (ALP) reminiscent of endochondral bone formation. To direct MSC toward articular chondrocytes more specifically, a better understanding of the regulatory steps is desirable. Proteases are important players in matrix remodeling, display inhibitory effects on growth plate development and MMP13 inhibition prevented hypertrophy of bovine chondrocytes. The aim of this study was to evaluate whether the activity of proteases and MMPs, especially MMP13, is crucial for the transition of MSC toward mature chondrocytes and could allow to selectively influence aspects of early and late chondrogenic differentiation. Protease inhibitors were added during MSC chondrogenesis and stage-specific markers were assessed by histology, qPCR, and ALP quantification. Chondrogenesis was little affected by leupeptin, pepstatin, or aprotinin. In contrast, broad spectrum pan-MMP inhibitors dose dependently suppressed proteoglycan deposition, collagen type II and type X staining, ALP activity, and reduced SOX9 and COL2A1 expression. A selective MMP13 inhibitor allowed chondrogenesis and showed only weak effects on ALP activity. In conclusion, transition of MSC toward mature chondrocytes in vitro depended on molecules suppressed by pan-MMP inhibitors identifying chondrogenic differentiation of MSC as a sophistically regulated process in which catabolic enzymes are capable to directly influence cellular fate. In future therapeutic applications of diseased joints, the tested MMP13-specific inhibitor promises suppression of collagen type II degradation without imposing a risk to impair MSC-driven regeneration processes.

Collagen/annexin V interactions regulate chondrocyte mineralization

Physiological mineralization in growth plate cartilage is highly regulated and restricted to terminally differentiated chondrocytes. Because mineralization occurs in the extracellular matrix, we asked whether major extracellular matrix components (collagens) of growth plate cartilage are directly involved in regulating the mineralization process. Our findings show that types II and X collagen interacted with cell surface-expressed annexin V. These interactions led to a stimulation of annexin V-mediated Ca(2+) influx resulting in an increased intracellular Ca(2+) concentration, [Ca(2+)](i), and ultimately increased alkaline phosphatase activity and mineralization of growth plate chondrocytes. Consequently, stimulation of these interactions (ascorbate to stimulate collagen synthesis, culturing cells on type II collagen-coated dishes, or overexpression of full-length annexin V) resulted in increase of [Ca(2+)](i), alkaline phosphatase activity, and mineralization of growth plate chondrocytes, whereas inhibition of these interactions (3,4-dehydro-l-proline to inhibit collagen secretion, K-201, a specific annexin channel blocker, overexpression of N terminus-deleted mutant annexin V that does not bind to type II collagen and shows reduced Ca(2+) channel activities) decreased [Ca(2+)](i), alkaline phosphatase activity, and mineralization. In conclusion, the interactions between collagen and annexin V regulate mineralization of growth plate cartilage. Because annexin V is up-regulated during pathological mineralization events of articular cartilage, it is possible that these interactions also regulate pathological mineralization.

In situ localization of gelatinolytic activity during development and resorption of Meckel's cartilage in mice

Degradation of Meckel's cartilage in the middle portion is accompanied by hypertrophy and death of chondrocytes, calcification of the cartilaginous matrix, and chondroclastic resorption. We hypothesize that the gelatinolytic activity of matrix metalloproteinases (MMPs) largely contributes to the degradation of extracellular matrix (ECM) in the process. The activity in Meckel's cartilage of mouse mandibular arches at embryonic days 14-16 (E14-E16) was examined by a combination of in situ zymography (ISZ), using quenched fluorescent dye-labeled gelatin as a substrate, with CTT (a selective inhibitor of MMP-2 and -9) or with EDTA (a general MMP inhibitor). On E14 and E15, ISZ showed fluorescence in the perichondrium, in the intercellular septa between chondrocytes, and in the nucleus of chondrocytes. CTT attenuated fluorescence, and EDTA eliminated it. On E16, calcified cartilaginous matrix showed intense fluorescence, and dot-like fluorescence was observed in as-yet uncalcified intercellular septa, even after CTT treatment. EDTA inhibited fluorescence, but unexpectedly intense fluorescence was found in the cytoplasm of hypertrophic chondrocytes facing the resorption front. MMP-2, -9, and -13 immunoreactivity was detected in the perichondrium and chondrocytes of Meckel's cartilage. These findings suggest that MMPs and other proteinases capable of degrading gelatin play an integral role in the development, calcification, and resorption of Meckel's cartilage through ECM reconstitution.

Immunohistochemical Examination of Epiphyseal Growth Plates of Japanese Brown Cattle with Chondrodysplasia

A new type of inherited chondrodysplasia is described in Japanese Brown cattle, but the basic defects of the epiphyseal growth plate (EGP) in the limb long bones, and proliferation and differentiation of the chondrocytes in the EGP, are not yet understood. In the present study, the EGPs of the limb long bones in eight cases of chondrodysplasia and four normal (control) cattle were examined histologically and immunohistochemically. In the control cattle, proliferative chondrocytes (PCs) and hypertrophic chondrocytes (HCs) were arranged in columns parallel to the long axis of the bone, and HCs were situated on the metaphyseal side of the EGP. In all the affected cattle, many chondrocytes with a hypertrophic appearance were detected in the inner areas of the central portion of the EGP. The PC columns were short and arranged irregularly. Bone tissue and small blood vessels were found frequently in these areas. Six affected cattle showed complete EGP-closure. Backscattered electron (BSE) imaging showed that the calcified cartilage matrix was restricted to the lower region of the hypertrophic zone (HZ) of the EGP in the control cattle, while the calcified cartilage matrix and bone tissue were scattered in the inner areas of the EGP in all the chondrodysplastic cattle. Immunohistochemistry revealed type X collagen in the HCs and cartilage matrix of the HZ in the control cattle. In all the affected cattle, type X collagen was detected in apparently hypertrophic chondrocytes in the inner areas of the EGP. Type II collagen was detected in the entire EGP in all the affected cattle, as in the controls. BrdU (5-bromo-2'-deoxyuridine), injected intravenously 1h before euthanasia was detected in many PCs in the EGP in the control cattle; none, however, was detected in the central portion of the EGP in any affected animal. These observations indicate that differentiation into HCs and calcification of cartilage matrix occur in the inner areas of the central portion of the EGP in chondrodysplasia of Japanese Brown cattle. Differentiation into the HCs at this abnormal site may be caused by the inadequate proliferation and disorganization of the PCs. Premature EGP-closure, observed commonly in chondrodysplasia of Japanese Brown cattle, was thought to be caused by replacement of the calcified cartilage in the inner areas of the EGP by bone tissue.

Matrix metalloproteinases are not essential for aggrecan turnover during normal skeletal growth and development

The growth plate is a transitional region of cartilage and highly diversified chondrocytes that controls long bone formation. The composition of growth plate cartilage changes markedly from the epiphysis to the metaphysis, notably with the loss of type II collagen, concomitant with an increase in MMP-13; type X collagen; and the C-propeptide of type II collagen. In contrast, the fate of aggrecan in the growth plate is not clear: there is biosynthesis and loss of aggrecan from hypertrophic cartilage, but the mechanism of loss is unknown. All matrix metalloproteinases (MMPs) cleave aggrecan between amino acids N341 and F342 in the proteinase-sensitive interglobular domain (IGD), and MMPs in the growth plate are thought to have a role in aggrecanolysis. We have generated mice with aggrecan resistant to proteolysis by MMPs in the IGD and found that the mice develop normally with no skeletal deformities. The mutant mice do not accumulate aggrecan, and there is no significant compensatory proteolysis occurring at alternate sites in the IGD. Our studies reveal that MMP cleavage in this key region is not a predominant mechanism for removing aggrecan from growth plate cartilage.

Altered endochondral bone development in matrix metalloproteinase 13-deficient mice

The assembly and degradation of extracellular matrix (ECM) molecules are crucial processes during bone development. In this study, we show that ECM remodeling is a critical rate-limiting step in endochondral bone formation. Matrix metalloproteinase (MMP) 13 (collagenase 3) is poised to play a crucial role in bone formation and remodeling because of its expression both in terminal hypertrophic chondrocytes in the growth plate and in osteoblasts. Moreover, a mutation in the human MMP13 gene causes the Missouri variant of spondyloepimetaphyseal dysplasia. Inactivation of Mmp13 in mice through homologous recombination led to abnormal skeletal growth plate development. Chondrocytes differentiated normally but their exit from the growth plate was delayed. The severity of the Mmp13- null growth plate phenotype increased until about 5 weeks and completely resolved by 12 weeks of age. Mmp13-null mice had increased trabecular bone, which persisted for months. Conditional inactivation of Mmp13 in chondrocytes and osteoblasts showed that increases in trabecular bone occur independently of the improper cartilage ECM degradation caused by Mmp13 deficiency in late hypertrophic chondrocytes. Our studies identified the two major components of the cartilage ECM, collagen type II and aggrecan, as in vivo substrates for MMP13. We found that degradation of cartilage collagen and aggrecan is a coordinated process in which MMP13 works synergistically with MMP9. Mice lacking both MMP13 and MMP9 had severely impaired endochondral bone, characterized by diminished ECM remodeling, prolonged chondrocyte survival, delayed vascular recruitment and defective trabecular bone formation (resulting in drastically shortened bones). These data support the hypothesis that proper ECM remodeling is the dominant rate-limiting process for programmed cell death, angiogenesis and osteoblast recruitment during normal skeletal morphogenesis.

Extracellular matrix changes in early osteochondrotic defects in foals: a key role for collagen?

Osteochondrosis (OC) is the most important developmental orthopaedic disease in the horse. Despite some decades of research, much of the pathogenesis of the disorder remains obscure. Increasing knowledge of articular cartilage development in juvenile animals led to the presumption that the role of collagen in OC might be more important than previously thought. To study collagen characteristics of both cartilage and subchondral bone in young (5 and 11 months of age) horses, samples were taken of subchondral bone and articular cartilage from a group of 43 Dutch Warmblood foals and yearlings that suffered from varying degrees of OC. Based on a histological classification, lesions were graded as early, middle and end stage. Collagen content and some posttranslational modifications (lysyl hydroxylation, hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP) cross-links) were determined, as was proteoglycan content. Data were compensated for site effects and analysed for differences due to the stage of the lesion. In early lesions total collagen was significantly decreased in both cartilage and subchondral bone of 5- and 11-month-old foals. Also in cartilage, HP cross-linking was reduced in the early lesions of 5- and 11-month-old foals, while LP cross-linking was decreased in subchondral bone of the end-stage lesions of both 5- and 11-month-old foals. Hydroxylysine content was unaffected. Collagen content remained reduced in cartilage from middle- and end-stage lesions, but returned to normal in subchondral bone. In cartilage there was a decrease in proteoglycan content in the end-stage lesions of both age groups. Thus, alterations of the collagen component, but not of the proteoglycan component, of the extracellular matrix might play a role in early OC. More severe lesions show a more general picture of an unspecific repair reaction. Biomarkers of collagen metabolism can be expected to be good candidates for early detection of OC.

A bit of give and take: the relationship between the extracellular matrix and the developing chondrocyte

The extracellular matrix (ECM), once thought to be a static structural component of tissues, is now known to play a complex and dynamic role in a variety of cellular functions in a number of diverse tissues. A significant body of literature attests to the ability of the ECM to communicate both spatial and temporal information to adherent cells, thereby directing cell behavior via interactions between the ECM and cell-surface receptors. Moreover, volumes of experimental data show that a great deal of communication travels in the opposite direction, from the cell to the ECM, allowing for regulation of the cues transmitted by the ECM. As such, the ECM, with respect to its components and their organization, is not a fixed reflection of the state the local microenvironment in which a cell finds itself at a particular time, but rather is able to respond to and effect changes in its local microenvironment. As an example of the developmental consequences of ECM interactions, this review gives an overview of the 'give and take' relationship between the ECM and the cells of the developing skeletal elements, in particular, the chondrocyte.

Distinct phases of coordinated early and late gene expression in growth plate chondrocytes in relationship to cell proliferation, matrix assembly, remodeling, and cell differentiation

Although much has been learned about growth plate development and chondrocyte gene expression during cellular maturation and matrix remodeling in the mouse, there has been a limited study of the interrelationships of gene expression between proteinases, growth factors, and other regulatory molecules in the mouse and in other species. Here we use RT-PCR of sequential transverse sections to examine the expression profiles of genes involved in chondrocyte growth, differentiation, matrix assembly, remodeling, and mineralization in the bovine proximal tibial growth plate. Specifically, we studied the expression of genes encoding COL2A1 and COL10A1, the latter a marker of cellular hypertrophy, the matrix metalloproteinases (MMP), MMP-13 and MMP-9, as well as the transcriptional factors, Sox9 and Cbfa1, the growth factors basic fibroblast growth factor (bFGF), parathyroid hormone-related peptide (PTHrP), transforming growth factor (TGF)beta1, and beta2, Indian hedgehog (Ihh), and the matrix protein osteocalcin. These were analyzed in relationship to cell division defined by cyclin B2 expression. Two peaks of gene expression activity were observed. One was transient, limited, and located immediately before and at the onset of cyclin B2 expression in the early proliferative zone. The other was generally much more pronounced and was located in the early hypertrophic zone. The upregulation of expression of COL2A1, its transcriptional activator Sox9, osteocalcin, MMP-13, and TGFbeta2 was observed immediately before and at the onset of cyclin B2 expression and also in the hypertrophic zones. The upregulation of COL10A1, Cbfa1, MMP-9, TGFbeta-1, and Ihh gene expression was associated exclusively with the terminal differentiation of chondrocytes at the time of mineral formation in the extracellular matrix. In contrast, bFGF and PTHrP expression was observed in association with the onset of cyclin B2 expression and hypertrophy. This initial cluster of gene expression associated predominantly with matrix assembly and onset of cell proliferation is therefore characterized by expression of regulatory molecules distinct from those involved at hypertrophy. Together these results identify separate phases of coordinated gene expression associated with the development of the physis in endochondral bone formation.

Differential distribution of cathepsins B and L in articular cartilage during skeletal development in the horse

REASONS FOR PERFORMING STUDY

This study was designed to examine a new role for cysteine proteinases in the process of endochondral ossification.

OBJECTIVES

The aim of the present study was to investigate the presence and distribution of cathepsin B and cathepsin L in equine articular cartilage during development.

METHODS

Full-depth cartilage samples from a total of 40 horses (age range: 4 month fetuses to 2 years) were examined and enzymes detected by immunocytochemical localisation.

RESULTS

Observations on the presence of cathepsins B and L revealed significant age-related differences, resulting in clear division of the animals into 2 age groups: i) fetuses and neonates; ii) young growing horses (age 4 weeks to 2 years). Cathepsin B was not detected in cartilage from the majority of fetuses and neonates but was located characteristically in chondrocytes at the articular surface and hypertrophic zone in all growing horses. In contrast, cathepsin L was predominantly present in fetal and neonatal cartilage, located primarily in proliferating chondrocytes.

CONCLUSIONS

This study is the first to demonstrate differential and site-specific roles for cathepsin B and cathepsin L in skeletal development in the horse.

POTENTIAL RELEVANCE

The demonstrated involvement of cathepsins B and L in endochondral ossification is of relevance to developmental orthopaedic diseases such as osteochondrosis in which there is a focal failure of bone formation.

New functional roles for non-collagenous domains of basement membrane collagens

Collagens IV, XV and XVIII are major components of various basement membranes. In addition to the collagen-specific triple helix, these collagens are characterized by the presence of several non-collagenous domains. It is clear now that these ubiquitous collagen molecules are involved in more subtle and sophisticated functions than just the molecular architecture of basement membranes, particularly in the context of extracellular matrix degradation. Degradation of the basement membrane collagens occurs during numerous physiological and pathological processes such as embryonic development or tumorigenesis and generates collagen fragments. These fragments are involved in the regulation of functions differing from those of their original intact molecules. The non-collagenous C-terminal fragment NC1 of collagen IV, XV and XVIII have been recently highlighted in the literature because of their potential in reducing angiogenesis and tumorigenesis, but it is clear that their biological functions are not limited to these processes. Proteolytic release of soluble NC1 fragments stimulates migration, proliferation, apoptosis or survival of different cell types and suppresses various morphogenetic events.

Immunolocalization of carboxy-terminal type II procollagen peptide in regenerated articular cartilage of osteoarthritic knees after reduction of mechanical stress

OBJECTIVE

The purpose of this study was to investigate the immunolocalization of carboxy-terminal type II procollagen peptide (pCOL-II-C) in the regenerated articular cartilage grown 1-2 years after reduction of mechanical stress by correction of varus deformity with high tibial osteotomy (HTO) for knees with medial compartmental osteoarthritis.

DESIGN

The series included 24 knees of 16 patients with a mean age of 70 (56-79) years. Synovial fluid and tissue specimens of the regenerated articular cartilage were obtained at the time of plate removal with arthrotomy. Tissue specimens were decalcified and stained with toluidine blue, safranin O, anti-type I and type II collagen and anti-pCOL-II-C. Pineda's histological grading of articular cartilage repair and Okada's grade of immunostaining were employed to assess the regenerated articular cartilage.

RESULTS

In knees with regeneration of articular cartilage, there was a positive linear correlation between the grade of immunostaining and the concentration of synovial fluid pCOL-II-C (r=0.652; P< 0.001). Similarly, a positive linear correlation was observed between the grade of immunostaining and the histological grading score (r=0.683; P< 0.001).

CONCLUSIONS

The immunostaining and synovial fluid concentration of pCOL-II-C decreased in accordance with the progression of articular cartilage regeneration observed after reduction of mechanical stress by correction of deformity with HTO.

The influence of pulsed low-intensity ultrasound on matrix production of chondrocytes at different stages of differentiation: an explant study

The proximal and distal parts of sterna of chick embryos represent cartilage undergoing endochondral ossification and hyaline cartilage, respectively. Cartilage explants from both regions were exposed for 20 min to pulsed low-intensity ultrasound (PLIUS) with an intensity of 30 mW. cm(-2) (spatial average-temporal average) at a frequency of 1.5 MHz, with a pulse burst frequency of 1 kHz and burst duration of 200 micros. Histological and immunohistochemical analysis was performed on days 1, 3, 5 and 7 after treatment. An anabolic effect of PLIUS on matrix production was shown by an increase of up to 10% to 20% in quantitative immunohistochemical staining for type II collagen and aggrecan in the two parts of the sternum. PLIUS also increased type X collagen staining by up to 10% in certain regions of the proximal part of the sternum. Staining for type X collagen was negative in the distal part of the sternum in both PLIUS and control groups. These results suggest that PLIUS may stimulate bone formation by increasing hypertrophy of chondrocytes directed to terminal differentiation. However, PLIUS did not induce hypertrophy in hyaline cartilage; moreover, increased matrix synthesis indicates a potential role in cartilage repair.

Excessive degradation of type II collagen in articular cartilage in equine osteochondrosis

Articular osteochondrosis (OCD) occurs in both man and animals. The etiology remains to be determined. Studies of OCD lesions in animals may provide clues as to its pathogenesis. The aim of our study was to determine whether there was evidence for increased degradation namely proteoglycan (PG) release and type II collagen cleavage in articular cartilage harvested from OCD lesions. We examined ex vivo explants at post-mortem from equine OCD lesions and macroscopically normal site and age matched cartilage. These were cultured over a 10 day period in serum-free medium. Type II collagen cleavage was measured in articular cartilage and media using an Elisa assay to detect the COL2-3/4C(short) epitope, which is generated on cleavage of the triple helix of type II collagen by collagenases. PG release was measured by a dye-binding assay. Cumulative release of PG and COL2-3/4C(short) and their contents in cartilage at the end of the culture period were determined. In OCD lesions there was a significant increase in type II collagen cleavage by collagenase but no evidence for increase of PG degradation. These findings point to a selective increase in type II collagen cleavage by collagenases, in OCD lesions of the kind observed in osteoarthritis. Further work is needed to determine whether changes represent primary or secondary events in the pathogenesis of OCD.

Proteolysis involving matrix metalloproteinase 13 (collagenase-3) is required for chondrocyte differentiation that is associated with matrix mineralization

Collagenases are involved in cartilage matrix resorption. Using bovine fetal chondrocytes isolated from physeal cartilages and separated into a distinct prehypertrophic subpopulation, we show that in serum-free culture they elaborate an extracellular matrix and differentiate into hypertrophic chondrocytes. This is characterized by expression of type X collagen and the transcription factor Cbfal and increased incorporation of 45Ca2+ in the extracellular matrix, which is associated with matrix calcification. Collagenase activity, attributable only to matrix metalloproteinase (MMP) 13 (collagenase-3), is up-regulated on differentiation. A nontoxic carboxylate inhibitor of MMP-13 prevents this differentiation; it suppresses expression of type X collagen, Cbfal, and MMP-13 and inhibits increased calcium incorporation in addition to inhibiting degradation of type II collagen in the extracellular matrix. General synthesis of matrix proteins is unaffected. These results suggest that proteolysis involving MMP-13 is required for chondrocyte differentiation that occurs as part of growth plate development and which is associated with matrix mineralization.

Identification of the metalloproteinase stromelysin in the physis

For long bone growth to occur, calcification of the matrix must commence in the lower hypertrophic zone of the growth plate. It is generally accepted that physeal proteoglycans help regulate mineralization, and that at least in vitro, intact proteoglycans can inhibit mineralization. Thus degradation of proteoglycan may be a necessary step prior to calcification. Previous work in our laboratory has demonstrated the presence of neutral metallo-proteases in the growth plate with highest levels in the hypertrophic zone, where calcification occurs. Stromelysin (MMP-3) is a connective tissue matrix-degrading enzyme. It was formerly known as proteoglycanase and is generally considered to be one of the major proteoglycan degrading enzymes in cartilage. Stromelysin is implicated in cartilage destruction in osteoarthritis and may also be involved in tissue remodeling in the physis. Our goal was to determine if the neutral protease previously reported by the authors in the physis was stromelysin. In this study we used Western blots and antibodies to stromelysin and to the stromelysin cleavage site in aggrecan, the most common form of proteoglycan, to demonstrate the presence of stromelysin in the bovine physis. When an antibody raised against the stromelysin cleavage site of aggrecan (FVDIPEN) was incubated with a Western blot, which had been run with aggrecan extracted from bovine physes, a positive reaction resulted. This suggests that there is stromelysin degradation in vivo in the physis. Two different polyclonal antibodies to stromelysin gave positive results on Western blots of purified media from growth plate cultures indicating that stromelysin is produced in vitro in culture. These antibodies also reacted with active stromelysin. The presence of stromelysin in the physis implicates it in physeal physiology. The concentration of its activity in the lower hypertrophic zone and zone of provisional calcification suggests that it may be particularly important in mineralization.

The assembly and remodeling of the extracellular matrix in the growth plate in relationship to mineral deposition and cellular hypertrophy: an in situ study of collagens II and IX and proteoglycan

The recent development of new specific immunoassays has provided an opportunity to study the assembly and resorption of type II and IX collagens of the extracellular matrix in relationship to endochondral calcification in situ. Here, we describe how in the bovine fetal physis prehypertrophic chondrocytes deposit an extensive extracellular matrix that, initially, is rich in both type II and type IX collagens and proteoglycan (PG; principally, aggrecan). The majority of the alpha1(IX)-chains lack the NC4 domain consistent with our previous studies with cultured chondrocytes. During assembly, the molar ratio of type II/COL2 domain of the alpha1(IX)-chain varied from 8:1 to 25:1. An increase in the content of Ca2+ and inorganic phosphate (Pi) was initiated in the prehypertrophic zone when the NC4 domain was removed selectively from the alpha1(IX)-chain. This was followed by the progressive loss of the alpha1(IX) COL2 domain and type II collagen. In the hypertrophic zone, the Ca2+/Pi molar ratio ranged from 1.56 to a maximum of 1.74, closely corresponding to that of mature hydroxyapatite (1.67). The prehypertrophic zone had an average ratio Ca2+/Pi ranging from 0.25 to 1, suggesting a phase transformation. At hypertrophy, when mineral content was maximal, type II collagen was reduced maximally in content coincident with a peak of cleavage of this molecule by collagenase when matrix metalloproteinase 13 (MMP-13) expression was maximal. In contrast, PG (principally aggrecan) was retained when hydroxyapatite was formed consistent with the view that this PG does not inhibit and might promote calcification in vivo. Taken together with earlier studies, these findings show that matrix remodeling after assembly is linked closely to initial changes in Ca2+ and Pi to subsequent cellular hypertrophy and mineralization. These changes involve a progressive and selective removal of types II and IX collagens with the retention of the PG aggrecan.

Apoptosis in neural crest cells by functional loss of APC tumor suppressor gene

Apc is a gene associated with familial adenomatous polyposis coli (FAP) and its inactivation is a critical step in colorectal tumor formation. The protein product, adenomatous polyposis coli (APC), acts to down-regulate intracellular levels of beta-catenin, a key signal transducer in the Wnt signaling. Conditional targeting of Apc in the neural crest of mice caused massive apoptosis of cephalic and cardiac neural crest cells at about 11.5 days post coitum, resulting in craniofacial and cardiac anomalies at birth. Notably, the apoptotic cells localized in the regions where beta-catenin had accumulated. In contrast to its role in colorectal epithelial cells, inactivation of APC leads to dysregulation of beta-catenin/Wnt signaling with resultant apoptosis in certain tissues including neural crest cells.

Anti-TNF-alpha antibody allows healing of joint damage in polyarthritic transgenic mice

Anti-tumor-necrosis-factor-alpha (TNF-alpha) monoclonal antibody was used to treat Tg197 transgenic mice, which constitutively produce human TNF-alpha (hTNF-alpha) and develop a progressive polyarthritic disease. Treatment of both young (7- or 8-week-old) and aged (27- or 28-week-old) mice commenced when at least two limbs showed signs of moderate to severe arthritis. The therapeutic efficacy of anti-TNF-alpha antibody was assessed using various pathological indicators of disease progression. The clinical severity of arthritis in Tg197 mice was significantly reduced after anti-TNF-alpha treatment in comparison with saline-treated mice and in comparison with baseline assessments in both young and aged mice. The treatment with anti-TNF-alpha prevented loss of body weight. Inflammatory pathways as reflected by elevated circulating hTNF-alpha and local expression of various proinflammatory mediators were all diminished by anti-TNF-alpha treatment, confirming a critical role of hTNF-alpha in this model of progressive polyarthritis. More importantly, the amelioration of the disease was associated with reversal of existing structural damage, including synovitis and periosteal bone erosions evident on histology. Repair of cartilage was age dependent: reversal of cartilage degradation after anti-TNF-alpha treatment was observed in young mice but not in aged mice.

UDP-glucose pyrophosphorylase: up-regulation in hypertrophic cartilage and role in hyaluronan synthesis

Previously, we have performed subtractive hybridization to identify genes up-regulated in hypertrophic chondrocytes of the avian epiphyseal growth plate. In the present study, we report the identification of one of the clones as UDP-glucose pyrophosphorylase (UDPG-PPase) and propose a possible function for this enzyme in regulating hyaluronan (HA) synthesis in hypertrophic cartilage. We have cloned the 2.6 kb full-length cDNA for avian UDPG-PPase and confirmed its up-regulation in hypertrophic versus non-hypertrophic cartilage by Northern-blot analysis. The 6-fold increase in mRNA was paralleled by an equivalent increase in enzymic activity. The enzyme catalyses the conversion of glucose 1-phosphate into UDP-glucose, which is used to synthesize a number of cellular components, including HA. Overexpression of enzymically active UDPG-PPase in non-hypertrophic chondrocytes resulted in a 2-3-fold increase in total HA, as determined by a competitive binding assay and immunohistochemistry. In the developing growth plate, HA synthesis was elevated in the hypertrophic zone along with the up-regulation of the HA synthase (HAS)-2 gene. Our data suggest that an increase in both activities, UDPG-PPase and HAS-2, is required for non-hypertrophic chondrocytes to synthesize an amount of HA comparable with that in hypertrophic chondrocytes. Therefore we conclude that HA synthesis during chondrocyte differentiation is regulated at the level of the substrate-provider gene, UDPG-PPase, as well as the HAS genes.

Relationships of matrix metalloproteinases and their inhibitors to cartilage proteoglycan and collagen turnover and inflammation as revealed by analyses of synovial fluids from patients with rheumatoid arthritis

OBJECTIVE

To determine interrelationships in matrix turnover in articular cartilage and joint inflammation in rheumatoid arthritis (RA).

METHODS

Synovial fluid was obtained from the knees of 63 RA patients; radiographs were evaluated to determine the RA stage. Concentrations of matrix metalloproteinases (MMPs), tissue inhibitors of metalloproteinases (TIMPs), the 846 epitope, and the keratan sulfate (KS) epitope of aggrecan, the C-propeptide of cartilage type II procollagen (CPII; biosynthetic marker), the cleavage of type II collagen by collagenase (CIIC; generated neoepitope), and polymorphonuclear leukocyte elastase (PMNE; inflammation marker) were measured by immunoassay. Concentrations of the unsaturated disaccharides of hyaluronic acid (delta di-HA) and the proteoglycan glycosaminoglycan disaccharides of chondroitins 4 and 6 sulfate (delta di-C4S and delta di-C6S) were determined by high-performance liquid chromatography.

RESULTS

MMP-3 was markedly increased in RA compared with osteoarthritis. Increases in TIMP-1 in RA were less pronounced and were inversely correlated with MMP-3 levels. CIIC was reduced in RA, as was the release of the KS epitope and delta di-C6S. In contrast, delta di-C4S and the 846 epitope were up-regulated. PMNE levels correlated with the 846 epitope and delta di-C4S, and more strongly with TIMPs 1 and 2. The changes may signify attempts at control of proteolysis in parallel with increased aggrecan turnover, which would favor matrix assembly. PMNE also correlated with MMP-9, and MMP-9 correlated with CPII. The delta di-HA level was decreased in RA and was inversely correlated with CPII and MMP-9 as well as with MMPs 2 and 3. In contrast, delta di-HA was directly correlated with TIMP-1 and the 846 epitope. These observations suggest that HA and PMNs may be involved in the control of proteolysis and cartilage proteoglycan assembly.

CONCLUSION

Our observations provide new insights into the complex changes in cartilage turnover and PMN influx in RA joints.

Developmental distribution of collagen type XII in cartilage: association with articular cartilage and the growth plate

Collagen type XII is a member of the fibril-associated collagens and is characterized by a short triple-helical domain with three extended noncollagenous NC3 domains. Previous studies suggested that collagen XII is a component of cartilage but little is known about its spatial-temporal distribution. This study uses a polyclonal antibody to the purified NC3 domain to investigate its developmental distribution in rat forelimb. Collagen XII was present at the joint interzone on embryonic day 16 (E16d) and restricted to the presumptive articular cartilage by E18d. Labeling of the articular surface intensified as development progressed postnatally (day 1 [1d] to 28d) and extended approximately six cell diameters deep. In juvenile rats, collagen XII antibodies also labeled the longitudinal and transverse septa of stacked chondrocytes in the growth plate. However, collagen XII was not associated at any developmental stage with the cartilaginous secondary ossification center and was only weakly expressed in epiphyseal cartilage. Ultrastructural localization of the NC3 domain epitope showed labeling of the surface of collagen II fibrils both in tissue and in isolated fibrils. The results presented provide further evidence that articular cartilage differs substantially from the underlying epiphyseal cartilage and that different chondrocytic developmental fates are reflected in the composition of their extracellular matrix starting early in development. In addition, collagen XII was distributed in areas of cartilage with more organized fibril orientation and may have a role in promoting alignment or stabilizing such an organization, thereby creating a matrix capable of withstanding load-bearing forces.

Type II collagen degradation in articular cartilage fibrillation after anterior cruciate ligament transection in rats

OBJECTIVE

To investigate the kinetics of early cartilage changes in mechanically induced osteoarthritis (OA) and the association of these changes with damage to the type II collagen network.

METHODS

Experimental OA was induced by anterior cruciate ligament transsection in the rat knee joint (ACLT-OA). Animals were sacrificed after 2, 7, 14, 28 and 70 days. Knee joints were evaluated using routine histology and immunohistochemistry for denatured (unwound) type II collagen to detect collagen damage. An antibody recognizing the collagenase cleavage site in type II collagen was used to study the role of collagenase in this process.

RESULTS

The first changes of the articular cartilage after anterior cruciate ligament transection occurred in the superficial zone. These changes included loss of superficial chondrocytes, swelling of the remaining chondrocytes and superficial fibrillation. The swelling of the chondrocytes did not result from a change towards the hypertrophic phenotype, since these cells did not stain for type X collagen. A marked increase in denatured type II collagen staining was present in the fibrillated areas. Staining of the collagenase cleavage site showed the same distribution as denatured collagen but was clearly less intense. Collagen damage could never be detected before fibrillation occurred and was not present in non-fibrillated areas.

CONCLUSIONS

These results indicate that in this model cartilage degeneration starts at the articular surface and that this degeneration is associated with a localized expression of type II collagen degradation products.

Expression of type X collagen in young and old C57Bl/6 and Balb/c mice. Relation with articular cartilage degeneration

OBJECTIVE

To investigate whether the development of osteoarthritic lesions in the knee joints of mice is associated with increased immunostaining of type X collagen.

METHODS

Sections of total knee joints in combination with immunohistochemistry were used to study the distribution of type X collagen in the cartilage of young and old mice of two mouse strains, Balb/c and C57Bl/6, known to develop osteoarthritic lesions at different locations. Expression of type X collagen and PTH/PTHrP-receptor mRNA were studied by RT-PCR.

RESULTS

Young adult Balb/c and C57Bl/6 mice both expressed type X collagen in the non-calcified cartilage of the tibia-femoral joint. Old mice of both strains had a strongly increased deposition of type X collagen in the patella-femoral but not in the tibia-femoral joint. The locations in the murine knee joints prone to develop osteoarthritis (OA) did not preferentially express increased amounts of type X collagen. Thus, whereas increased type X was observed in both strains in the patella-femoral joints, only Balb/c mice preferentially developed osteoarthritic lesions in these joints. Also cartilage degeneration was usually seen only in the lateral compartment of the knee joints of C57Bl/6 mice but this was not accompanied by increased type X collagen immunostaining. Increased deposition of type X collagen was not associated with elevated levels of type X collagen mRNA or with decreased levels of PTH/PTHrP-receptor mRNA.

CONCLUSION

Type X collagen expression and spontaneous OA in mice are not necessarily related since OA prone locations in the murine knee joint do not preferentially express type X collagen.

Changes in leucine-rich repeat proteoglycans during maturation of the bovine growth plate

The primary growth plate of the fetal bovine tibia was studied in order to determine whether changes in the structure, abundance and expression of the leucine-rich repeat proteoglycans were occurring during tissue maturation from reserve cartilage to hypertrophic cartilage. The proteoglycans under study were decorin, biglycan, fibromodulin and lumican. Decorin was readily detectable in both the reserve and proliferating zones of the growth plate, but its abundance decreased markedly in the zones of maturation and hypertrophy where it could not be detected under the same conditions of analysis. In contrast to decorin, fibromodulin and biglycan could be detected throughout the growth plate, though their abundance was decreased in the proliferative and hypertrophic zones. Unlike the other proteoglycans, lumican could not be detected throughout the growth plate. At the message level, the expression of decorin shows a similar trend to that of protein abundance in the extracellular matrix, with its expression dropping markedly in the proliferative and hypertrophic zones. In the case of both biglycan and fibromodulin, message expression continued at a similar level throughout the growth plate. Thus, the leucine-rich repeat proteoglycans are different in the way they behave during growth plate maturation.

Trimer carboxyl propeptide of collagen I produced by mature osteoblasts is chemotactic for endothelial cells

During the second phase of osteogenesis in vitro, rat osteoblasts secrete inducer(s) of chemotaxis and chemoinvasion of endothelial and tumor cells. We report here the characterization and purification from mature osteoblast conditioned medium of the agent chemotactic for endothelial cells. The chemoactive conditioned medium specifically induces directional migration of endothelial cells, not affecting the expression and activation of gelatinases, cell proliferation, and scattering. Directional migration induced in endothelial cells by conditioned medium from osteoblasts is inhibited by pertussis toxin, by blocking antibodies to integrins alpha(1), beta(1), and beta(3), and by antibodies to metalloproteinase 2 and 9. The biologically active purified protein has two sequences, coincident with the amino-terminal amino acids, respectively, of the alpha(1) and of the alpha(2) carboxyl propeptides of type I collagen, as physiologically produced by procollagen C proteinase. Antibodies to type I collagen and to the carboxyl terminus of alpha(1) or alpha(2) chains inhibit chemotaxis. The chemoattractant is the propeptide trimer carboxyl-terminal to type I collagen, and its activity is lost upon reduction. These data illustrate a previously unknown function for the carboxyl-terminal trimer, possibly relevant in promoting endothelial cell migration and vascularization of tissues producing collagen type I.

Quantitative histomorphometric analysis of the human growth plate from birth to adolescence

Longitudinal bone growth occurs via the transformation of growth plate cartilage into bone through a series of cell and matrix changes, termed endochondral ossification. In this study, we characterize the development of trabecular bone from growth plate cartilage in the human rib from birth to adolescence. The height of the proliferative and hypertrophic zones within the growth plate and the primary bone spongiosa decreased with increasing age, with the greatest change observed in the first year of postnatal life. Within these zones, an internal rearrangement of tissue structure occurred. The matrix volume fraction (either cartilage or bone) increased with age in each of the zones. A concomitant increase in cartilage septae thickness and bone trabecular thickness was observed. A decrease in cartilage septae number was seen in the proliferative zone and a decrease in bone trabeculae number was also observed in the primary spongiosa. However, no difference in cartilage septae number was noted in the hypertrophic zone, the region at which cartilage is transformed into bone. Together the proliferative and hypertrophic regions of the growth plate and the bone primary spongiosa appear to constitute the active growth region, with concomitant changes observed that result in longitudinal growth. In contrast, bone mineral volume in the secondary spongiosa was stable over the ages examined; however, trabecular architecture underwent consolidation as trabecular number decreased and trabecular thickness increased. The integration of the structural transformation from cartilage to bone is crucial in achieving the dual purposes of longitudinal growth and peak bone mass. The structure developed during childhood will have an important bearing on the response to bone-altering disease in later life.

Selective assembly and remodelling of collagens II and IX associated with expression of the chondrocyte hypertrophic phenotype

The assembly and resorption of the extracellular matrix in the physis of the growth plate are poorly understood. By examining isolated fetal growth plate chondrocytes in culture and using immunochemical methods we show that type II collagen, proteoglycan aggrecan, and type IX collagen are assembled into a matrix that is initially enriched in type II collagen over proteoglycan and type IX collagen. When compared to the content of the COL2 domain in the alpha(1)(IX) chain it is evident that the majority ( 90%) of type IX molecules lack the NC4 domain unlike in articular cartilage. During matrix assembly the molar ratio of type II/COL2 of alpha(1)(IX) varied from 25:1 to 2.5:1. Following expression of the hypertrophic phenotype (initiation of type X collagen synthesis) there are parallel changes in both collagen and proteoglycan contents (inversely related to collagenase cleavage of type II collagen). The NC4 domain is then selectively, rapidly and irreversibly removed as mineralization is initiated, leaving the alpha(1)(IX) chain COL2 domain. Subsequently as mineralization progresses type II and type IX collagen (COL2 domain), but not the proteoglycan aggrecan, are resorbed coincident with a markedly increased cleavage of type II collagen by collagenase as mineral is deposited in the matrix. This study, therefore reveals a carefully orchestrated series of events in matrix assembly and resorption that prepares the extracellular matrix for mineralization.