Collagen of articular cartilage

Chondroconductive potential of tantalum trabecular metal

Mesenchymal stem cells or chondrocytes have been implanted into joints in biodegradable matrices in order to improve the quality of healing cartilage defects; however, insufficient biomechanical strength of the construct at implantation is a limiting factor for clinical application. Logically, a construct with better biomechanical characteristics would provide better results. Tantalum trabecular metal (TTM) is osteoconductive and mechanically similar to subchondral bone. The objective of this pilot study was to determine if TTM is also chondroconductive. Small sections of TTM were cultured with emu and canine chondrocytes in static and dynamic culture environments. The sections cultured in dynamic bioreactors were diffusely covered with a cartilaginous matrix. Sections cultured in static conditions had no growth. Histologic evaluation from emu and canine dynamic cultures showed tissue that was heavily populated with mesenchymal cells that resembled chondrocytes, and glycosaminoglycan staining that was distributed throughout the matrix. Type II collagen content in the canine dynamic culture was 84% by SDS-PAGE. Tantalum trabecular metal is chondroconductive in vitro in a dynamic environment when cultured with adult canine or emu chondrocytes. This technology could be expanded to determine if cartilaginous-metallic constructs may be used for joint resurfacing of osteoarthritic joints.

BMP receptor signaling is required for postnatal maintenance of articular cartilage

Articular cartilage plays an essential role in health and mobility, but is frequently damaged or lost in millions of people that develop arthritis. The molecular mechanisms that create and maintain this thin layer of cartilage that covers the surface of bones in joint regions are poorly understood, in part because tools to manipulate gene expression specifically in this tissue have not been available. Here we use regulatory information from the mouse Gdf5 gene (a bone morphogenetic protein [BMP] family member) to develop new mouse lines that can be used to either activate or inactivate genes specifically in developing joints. Expression of Cre recombinase from Gdf5 bacterial artificial chromosome clones leads to specific activation or inactivation of floxed target genes in developing joints, including early joint interzones, adult articular cartilage, and the joint capsule. We have used this system to test the role of BMP receptor signaling in joint development. Mice with null mutations in Bmpr1a are known to die early in embryogenesis with multiple defects. However, combining a floxed Bmpr1a allele with the Gdf5-Cre driver bypasses this embryonic lethality, and leads to birth and postnatal development of mice missing the Bmpr1a gene in articular regions. Most joints in the body form normally in the absence of Bmpr1a receptor function. However, articular cartilage within the joints gradually wears away in receptor-deficient mice after birth in a process resembling human osteoarthritis. Gdf5-Cre mice provide a general system that can be used to test the role of genes in articular regions. BMP receptor signaling is required not only for early development and creation of multiple tissues, but also for ongoing maintenance of articular cartilage after birth. Genetic variation in the strength of BMP receptor signaling may be an important risk factor in human osteoarthritis, and treatments that mimic or augment BMP receptor signaling should be investigated as a possible therapeutic strategy for maintaining the health of joint linings.

Collagens and cartilage matrix homeostasis

At least 27 types of collagen, the products of forty genes, are expressed in the tissues of higher vertebrates. Cartilage has a distinctive collagen phenotype. Two-thirds of the dry weight of adult articular cartilage is collagen. Proteolysis of this collagen framework is integral to the process of cartilage destruction and joint failure in osteoarthritis. Molecular studies are revealing the mechanisms of assembly of cartilage collagen fibrils. The nascent Type II collagen fibril is a heteropolymer, with collagen IX molecules covalently linked to the surface and collagen XI forming a filamentous template at the core, which regulates fibril diameter through its retained N-propeptide domains. This structure presents a challenge to understanding how fibril growth and collagen network maturation are brought about. Proteolytic remodeling, other than that mediated by collagenases, would appear to be involved, but the proteases and molecular mechanisms are still undefined. Valuable insights and predictions on the function of the individual collagen types in cartilage continue to come from the study of skeletal dysplasia syndromes caused by mutations in genes for collagens and associated matrix proteins.

Characterization of recombinant amino-terminal NC4 domain of human collagen IX: interaction with glycosaminoglycans and cartilage oligomeric matrix protein

The N-terminal NC4 domain of collagen IX is a globular structure projecting away from the surface of the cartilage collagen fibril. Several interactions have been suggested for this domain, reflecting its location and its characteristic high isoelectric point. In an attempt to characterize the NC4 domain in more detail, we set up a prokaryotic expression system to produce the domain. The purified 27.5-kDa product was analyzed for its glycosaminoglycan-binding potential by surface plasmon resonance and solid-state assays. The results show that the NC4 domain of collagen IX specifically binds heparin with a K(d) of 0.6 microm, and the full-length recombinant collagen IX has an even stronger interaction with heparin, with an apparent K(d) of 3.6 nm. The heparin-binding site of the NC4 domain was located in the extreme N terminus, containing a heparin-binding consensus sequence, whereas electron microscopy suggested the presence of at least three additional heparin-binding sites on full-length collagen IX. The NC4 domain was also shown to bind cartilage oligomeric matrix protein. This interaction and the association of cartilage oligomeric matrix protein with other regions of collagen IX were found to be heparin-competitive. Circular dichroism analyses of the NC4 domain indicated the presence of stabilizing disulfide bonds and a thermal denaturation point of about 80 degrees C. The pattern of disulfide bond formation within the NC4 domain was identified by tryptic peptide mass mapping of the NC4 in native and reduced states. A similar pattern was demonstrated for the NC4 domain of full-length recombinant collagen IX.

Distribution of Endo180 receptor and ligand in developing articular cartilage

OBJECTIVE

To investigate the expression of a novel member of the mannose receptor family, Endo180 (also known as uPARAP), and the distribution of Endo180 ligand(s) in the articular cartilage and growth plate of normal CBA mice and STR/ort mice, a well characterized model of spontaneous osteoarthritis.

DESIGN

A polyclonal anti-Endo180 antibody was used to determine receptor expression. The Endo180 extracellular domain fused to a human immunoglobulin Fc tail was used to detect ligand.

RESULTS

Endo180 receptor was strongly expressed in chondrocytes both in vitro and throughout the articular cartilage of young CBA and STR/ort mice. Expression decreased in older animals. In STR/ort mice with osteoarthritic lesions, no upregulation of Endo180 was detected. In the developing growth plate, Endo180 was expressed strongly by the proliferating chondrocytes. In contrast, Endo180 ligand was detected most strongly in hypertrophic zone of the growth plate and only at low levels in articular cartilage. In cultured chondrocytes, Endo180 was localized on the cell surface and in intracellular vesicles.

CONCLUSION

Constitutively recycling endocytic receptors function to internalize ligand from the extracellular milieu and the ability of Endo180 to bind both glycosylated ligands and collagens suggests a role in extracellular matrix remodeling. Expression of Endo180 in articular cartilage chondrocytes of young, but not old, mice and the reciprocal expression of Endo180 and its ligands in the growth plate suggest that this receptor is involved in cartilage development but not in cartilage homeostasis. In addition, our data indicates that Endo180 does not appear to play a role in the development or progression of murine osteoarthritis.

The Application of Recombinant Human Collagen in Tissue Engineering

Collagen is the main structural protein in vertebrates. It plays an essential role in providing a scaffold for cellular support and thereby affecting cell attachment, migration, proliferation, differentiation, and survival. As such, it also plays an important role in numerous approaches to the engineering of human tissues for medical applications related to tissue, bone, and skin repair and reconstruction. Currently, the collagen used in tissue engineering applications is derived from animal tissues, creating concerns related to the quality, purity, and predictability of its performance. It also carries the risk of transmission of infectious agents and precipitating immunological reactions. The recent development of recombinant sources of human collagen provides a reliable, predictable and chemically defined source of purified human collagens that is free of animal components. The triple-helical collagens made by recombinant technology have the same amino acid sequence as human tissue-derived collagen. Furthermore, by achieving the equivalent extent of proline hydroxylation via coexpression of genes encoding prolyl hydroxylase with the collagen genes, one can produce collagens with a similar degree of stability as naturally occurring material. The recombinant production process of collagen involves the generation of single triple-helical molecules that are then used to construct more complex three-dimensional structures. If one loosely defines tissue engineering as the use of a biocompatible scaffold combined with a biologically active agent (be it a gene or gene construct, growth factor or other biologically active agent) to induce tissue regeneration, then the production of recombinant human collagen enables the engineering of human tissue based on a human matrix or scaffold. Recombinant human collagens are an efficient scaffold for bone repair when combined with a recombinant bone morphogenetic protein in a porous, sponge-like format, and when presented as a membrane, sponge or gel can serve as a basis for the engineering of skin, cartilage and periodontal ligament, depending on the specific requirements of the chosen application.

Extracellular matrix glycoproteins and diffusion barriers in human astrocytic tumours

The extracellular matrix (ECM) and changes in the size and geometry of the extracellular space (ECS) in tumour tissue are thought to be of critical importance in influencing the migratory abilities of tumour cells as well as the delivery of therapeutic agents into the tumour. In 21 astrocytic neoplasms, the ECM composition was investigated in situ by the immunohistochemical detection of ECM glycoproteins (tenascin, laminin, vitronectin, fibronectin, collagen types I-VI). To explain the changes in ECS size and to detect barriers to diffusion in the tumour tissue, the ECM composition, the cellularity, the density of glial fibrillary acidic protein (GFAP)-positive tumour cell processes and the proliferative activity of the tumours were compared with the size and geometry of the ECS. The ECS volume fraction and the complex of hindrances to diffusion in the ECS (i.e. the tortuosity) were revealed by the real-time iontophoretic tetramethylammonium method. Increased proliferative activity of the tumours correlated with increased ECS volume fraction and tortuosity. The tortuosity of the tumour tissue was not significantly influenced by tumour cell density. Higher tortuosity was found in low-grade astrocytomas associated with the presence of a dense net of GFAP-positive fibrillary processes of the tumour cells. The increase in tortuosity in high-grade tumours correlated with an increased accumulation of ECM molecules, particularly of tenascin. We conclude that the increased malignancy of astrocytic tumours correlates with increases in both ECS volume and ECM deposition.

Accelerated, aging-dependent development of osteoarthritis in alpha1 integrin-deficient mice

OBJECTIVE

Cell-matrix interactions regulate chondrocyte differentiation and survival. The alpha1beta1 integrin is a major collagen receptor that is expressed on chondrocytes. Mice with targeted inactivation of the integrin alpha1 gene (alpha1-KO mice) provide a model that can be used to address the role of cell-matrix interactions in cartilage homeostasis and osteoarthritis (OA) pathogenesis.

METHODS

Knee joints from alpha1-KO and wild-type (WT) BALB/c mice were harvested at ages 4-15 months. Knee joint sections were examined for inflammation, cartilage degradation, and loss of glycosaminoglycans (by Safranin O staining). Immunohistochemistry was performed to detect the distribution of alpha1 integrin, matrix metalloproteinases (MMPs), and chondrocyte apoptosis.

RESULTS

In WT mice, the alpha1 integrin subunit was detected in hypertrophic chondrocytes in the growth plate and in a subpopulation of cells in the deep zone of articular cartilage. There was a marked increase in alpha1-positive chondrocytes in the superficial and upper mid-zones in OA-affected areas in joints from old WT mice. The alpha1-KO mice showed more severe cartilage degradation, glycosaminoglycan depletion, and synovial hyperplasia as compared with the WT mice. MMP-2 and MMP-3 expression was increased in the OA-affected areas. In cartilage from alpha1-KO mice, the cellularity was reduced and the frequency of apoptotic cells was increased. These results suggest that the alpha1 integrin subunit is involved in the early remodeling process in OA cartilage.

CONCLUSION

Deficiency in the alpha1 integrin subunit is associated with an earlier deregulation of cartilage homeostasis and an accelerated, aging-dependent development of OA.

The release of crosslinked peptides from type II collagen into human synovial fluid is increased soon after joint injury and in osteoarthritis

OBJECTIVE

To determine concentrations of crosslinked C-telopeptide fragments of type II collagen (CTX-II) in synovial fluid (SF) from patients with joint injury, osteoarthritis (OA), or other knee arthritides.

METHODS

Two study groups were used: a cross-sectional group, which included healthy-knee volunteers (reference group [REF]) and patients with pseudogout (PPA), an anterior cruciate ligament tear with or without a meniscus tear (INJ), or primary knee OA (POA); and a longitudinal group, which included patients with arthroscopic cartilage changes or septic arthritis. CTX-II was quantified by competition enzyme-linked immunosorbent assay (ELISA) based on a monoclonal antibody that recognized the C-terminus of the peptide EKGPDP as a proteolytic neoepitope. Aggrecan fragments, matrix metalloproteinases 1 and 3, and tissue inhibitor of metalloproteinases 1 were determined by ELISAs.

RESULTS

Concentrations of CTX-II in SF were higher in patients with PPA, INJ, and POA than in the REF group (P < 0.001). After joint injury, mean levels of CTX-II in SF were increased above REF levels at all time intervals (P < 0.001), and were highest within hours after trauma. In those in the longitudinal study group with joint cartilage damage, variation coefficients for CTX-II were 81% (between patients) and 64% (within patient), monitored over 1 year. In a patient with septic arthritis, SF CTX-II increased at the onset of symptoms, and peaked 30-fold higher than the baseline. Concentrations of all biomarkers decreased with successful treatment.

CONCLUSION

This is the first report to describe the release into SF of soluble molecular fragments specific for the degradation of mature, crosslinked, type II collagen (CII) in human OA and joint injury. The results provide strong evidence that the integrity of the CII network of cartilage is compromised soon after joint injury and in arthritis. This early degradation of CII may represent an important treatment target.

Macromolecular specificity of collagen fibrillogenesis: fibrils of collagens I and XI contain a heterotypic alloyed core and a collagen I sheath

Suprastructures of the extracellular matrix, such as banded collagen fibrils, microfibrils, filaments, or networks, are composites comprising more than one type of macromolecule. The suprastructural diversity reflects tissue-specific requirements and is achieved by formation of macromolecular composites that often share their main molecular components alloyed with minor components. Both, the mechanisms of formation and the final macromolecular organizations depend on the identity of the components and their quantitative contribution. Collagen I is the predominant matrix constituent in many tissues and aggregates with other collagens and/or fibril-associated macromolecules into distinct types of banded fibrils. Here, we studied co-assembly of collagens I and XI, which co-exist in fibrils of several normal and pathologically altered tissues, including fibrous cartilage and bone, or osteoarthritic joints. Immediately upon initiation of fibrillogenesis, the proteins co-assembled into alloy-like stubby aggregates that represented efficient nucleation sites for the formation of composite fibrils. Propagation of fibrillogenesis occurred by exclusive accretion of collagen I to yield composite fibrils of highly variable diameters. Therefore, collagen I/XI fibrils strikingly differed from the homogeneous fibrillar alloy generated by collagens II and XI, although the constituent polypeptides of collagens I and II are highly homologous. Thus, the mode of aggregation of collagens into vastly diverse fibrillar composites is finely tuned by subtle differences in molecular structures through formation of macromolecular alloys.

Membrane traffic fuses with cartilage development

The ability of cells to synthesize and secrete proteins is essential for numerous cellular functions. Therefore, when mutations in one component of the secretory pathway result in a tissue-specific defect, a unique opportunity arises to examine the molecular mechanisms at play. The recent finding that a defect in the protein sedlin, whose yeast counterpart is involved in the first step of the secretory pathway, leads to a cartilage-specific disorder in humans raises numerous questions and interesting possibilities for understanding both the pathobiology involved and the role of membrane traffic in normal cartilage development.

Establishing the protein MIA (melanoma inhibitory activity) as a marker for chondrocyte differentiation

Melanoma inhibitory activity (MIA), also referred to as cartilage derived retinoic acid-sensitive protein (CD-RAP), is detected physiologically in cartilage tissue and pathologically in malignant melanomas. To measure MIA/CD-RAP quantitatively we developed a sensitive ELISA system. Recently, we described diagnostic applications of the MIA-ELISA in patients with cartilage diseases. The study described herein was performed to determine whether there is any relation between MIA/CD-RAP levels and the degree of chondrocyte differentiation in tissue culture and to analyse whether MIA/CD-RAP may serve as a useful marker to control chondrocyte differentiation in in vitro tissue engineering. Our data provide evidence that measuring MIA in tissue culture supernatant by a quantitative ELISA can be used as a marker for differentiated chondrocytes.

Intervertebral disc collagen. Usage of the short form of the alpha1(IX) chain in bovine nucleus pulposus

Nucleus pulposus, the central zone of the intervertebral disc, is gel-like and has a similar collagen phenotype to that of hyaline cartilage. Amino-terminal protein sequence analysis of the alpha1(IX)COL3 domain purified from bovine nucleus pulposus gave a different sequence to that of the long alpha1(IX) transcript expressed in hyaline cartilage and matched the predicted sequence of short alpha1(IX). The findings indicate that the matrix of bovine nucleus pulposus contains only the short form of alpha1(IX) that lacks the NC4 domain. The sequence encoded by exon 7, predicted from human COL9A1, is absent from both short and long forms of alpha1(IX) from bovine nucleus pulposus and articular cartilage. A structural analysis of the cross-linking sites occupied in type IX collagen from nucleus pulposus showed that usage of the short alpha1(IX) transcript in disc tissue had no apparent effect on cross-linking behavior. As in cartilage, type IX collagen of nucleus pulposus was heavily cross-linked to type II collagen and to other molecules of type IX collagen with a similar site occupancy.

Phenotype discovery by gene expression profiling: mapping of biological processes linked to BMP-2-mediated osteoblast differentiation

Understanding physiological control of osteoblast differentiation necessitates characterization of the regulatory signals that initiate the events directing a cell to lineage commitment and establishing competency for bone formation. The bone morphogenetic protein, BMP-2, a member of the TGFbeta superfamily, induces osteoblast differentiation and functions through the Smad signal transduction pathway during in vivo bone formation. However, the molecular targets of BMP-mediated gene transcription during the process of osteoblast differentiation have not been comprehensively identified. In the present study, BMP-2 responsive factors involved in the early stages of commitment and differentiation to the osteoblast phenotype were analyzed by microarray gene expression profiling in samples ranging from 1 to 24 h following BMP-2 dependent differentiation of C2C12 premyoblasts into the osteogenic lineage. A total of 1,800 genes were responsive to BMP-2 and expression was modulated from 3- to 14-fold for less than 100 genes during the time course. Approximately 50% of these 100 genes are either up- or downregulated. Major events associated with phenotypic changes towards the osteogenic lineage were identified from hierarchical and functional clustering analyses. BMP-2 immediately responsive genes (1-4 h), which exhibited either transient or sustained expression, reflect activation and repression of non-osseous BMP-2 developmental systems. This initial response was followed by waves of expression of nuclear proteins and developmental regulatory factors including inhibitors of DNA binding, Runx2, C/EBP, Zn finger binding proteins, forkhead, and numerous homeobox proteins (e.g., CDP/cut, paired, distaless, Hox) which are expressed at characterized stages during osteoblast differentiation. A sequential profile of genes mediating changes in cell morphology, cell growth, and basement membrane formation is observed as a secondary transient early response (2-8 h). Commitment to the osteogenic phenotype is recognized by 8 h, reflected by downregulation of most myogenic-related genes and induction of a spectrum of signaling proteins and enzymes facilitating synthesis and assembly of an extracellular skeletal environment. These genes included collagens Type I and VI and the small leucine rich repeat family of proteoglycans (e.g., decorin, biglycan, osteomodulin, fibromodulin, and osteoadherin/osteoglycin) that reached peak expression at 24 h. With extracellular matrix development, the bone phenotype was further established from 16 to 24 h by induction of genes for cell adhesion and communication and enzymes that organize the bone ECM. Our microarray analysis resulted in the discovery of a class of genes, initially described in relation to differentiation of astrocytes and oligodendrocytes that are functionally coupled to signals for cellular extensions. They include nexin, neuropilin, latexin, neuroglian, neuron specific gene 1, and Ulip; suggesting novel roles for these genes in the bone microenvironment. This global analysis identified a multistage molecular and cellular cascade that supports BMP-2-mediated osteoblast differentiation.

Synovial fluid features and their relations to osteoarthritis severity: new findings from sequential studies

OBJECTIVE

Many factors are involved in the osteoarthritic process. It is not yet known which are initiators, promoters or simply results. Thus, we have evaluated some of those potentially important factors in osteoarthritis (OA) as observed sequentially for the first time in synovial fluids.

DESIGN

Synovial fluids (SF) obtained between 1992-2002 were all routinely evaluated for gross appearance, leukocyte counts and microscopic examination of wet drop preparations. We used regular and polarized light and alizarin red s stains. We separated out all OA patients, then we looked for patients who had more than two synovial fluid analyses to get sequential information. Time between first and final aspiration ranged from 2 to 7 (3.6+/-1.6) years and number of analyses per patients from 3 to 6 (3.3+/-0.7). We related synovial fluid crystals, fibrils and white blood cell count (WBC) to age, sex, disease duration and radiographic assessment according to the Kellgren-Lawrence radiographic rating system.

RESULTS

Of 4523 synovial fluid examinations, we found 855 in patients with knee OA; 330 patients with adequate clinical details for comparison were included in our study. Twenty-six patients (one woman and 25 men) had sequentially examined SF. We found that 52% of those OA patients with effusions studied had crystals identified in their synovial fluid. Twenty-one percent of all the patients had CPPD crystals, 47% had hydroxyapatite, also called basic calcium phosphate (BCP) crystals and 16% had both types of crystals. Microscopically identifiable fibrils were found in 60% of SF. In sequentially examined patients, CPPD crystals and apatite (BCP) were found in 19% and 23%, respectively, at the first aspiration and, in 34% and 58% at the final aspiration. Fibrils were seen in 54% at first examination and 85% later. Apatite and fibrils showed more significant correlation with time (r=0.51,r =0.92) than did CPPD (r=0.32). SF WBC correlated only with CPPD crystals and did not increase with OA duration or severity. CPPD, apatite and fibrils all correlated with higher radiographic grades of OA.

CONCLUSIONS

As noted before CPPD and apatite crystals were more common in patients with more severe OA. New findings are that our sequential cases showed that there were some patients with no crystals at onset but that crystals appeared with progression of the disease. Fibril presence in SF also correlated with progression of the disease.

Identification and initial characterization of 6,000 expressed sequenced tags (ESTs) from rat normal-growing cartilage and swarm rat chondrosarcoma cDNA libraries

Chondrosarcoma is the second most common type of skeletal malignancy with a survival rate at five years for histological grade III of only 29 percent. The development of a reliable chondrosarcoma animal model could enable the study of tumor growth and progression, the effect of the host on tumor behavior, and the effectiveness of various therapeutic modalities. The Swarm rat chondrosarcoma is a tumor tissue line that has been maintained through the years by serial subcutaneous injections, and the histochemical characteristics of the tumor have remained essentially similar in all transplants over the years. This study was designed to initiate the characterization of the Swarm rat chondrosarcoma model by gene expression profiling as compared to normal-growing rat cartilage. Analysis of the gene expression from both libraries revealed a complex pattern of gene expression, including many genes not yet reported to be expressed by chondrocytes. It suggests that the biochemical characterization of growing cartilage and chondrosarcoma reported to date has only begun to describe the complexity of these tissues.