Collagen type IX: evidence for covalent linkages to type II collagen in cartilage

Exercise builds the scaffold of life: muscle extracellular matrix biomarker responses to physical activity, inactivity, and aging

Skeletal muscle extracellular matrix (ECM) is critical for muscle force production and the regulation of important physiological processes during growth, regeneration, and remodelling. ECM remodelling is a tightly orchestrated process, sensitive to multi-directional tensile and compressive stresses and damaging stimuli, and its assessment can convey important information on rehabilitation effectiveness, injury, and disease. Despite its profound importance, ECM biomarkers are underused in studies examining the effects of exercise, disuse, or aging on muscle function, growth, and structure. This review examines patterns of short- and long-term changes in the synthesis and concentrations of ECM markers in biofluids and tissues, which may be useful for describing the time course of ECM remodelling following physical activity and disuse. Forces imposed on the ECM during physical activity critically affect cell signalling while disuse causes non-optimal adaptations, including connective tissue proliferation. The goal of this review is to inform researchers, and rehabilitation, medical, and exercise practitioners better about the role of ECM biomarkers in research and clinical environments to accelerate the development of targeted physical activity treatments, improve ECM status assessment, and enhance function in aging, injury, and disease.

ATDC5 cells as a model of cartilage extracellular matrix neosynthesis, maturation and assembly

Fibrillar collagens and proteoglycans (PGs) are quantitatively the major constituents of extracellular matrices (ECM). They carry numerous crucial post-translational modifications (PTMs) that tune the resulting biomechanical properties of the corresponding tissues. The mechanisms determining these PTMs remain largely unknown, notably because available established cell lines do not recapitulate much of the complexity of the machineries involved. ATDC5 cells are a model of chondrogenesis widely used for decades, but it remains described mostly at histological and transcriptional levels. Here, we asked to what extent this model recapitulates the events of ECM synthesis and processing occurring in cartilage. Insulin-stimulated ATDC5 cells exhibit up- or down-regulation of more than one-hundred proteins, including a number of known participants in chondrogenesis and major markers thereof. However, they also lack several ECM components considered of significant, yet more subtle, function in cartilage. Still, they assemble the large PG aggrecan and type II collagen, both carrying most of their in vivo PTMs, into an ECM. Remarkably, collagen crosslinking is fully lysyl oxidase (LOX)-dependent. The ATDC5 model recapitulates critical aspects of the cartilage ECM-processing machinery and should be useful to decipher the mechanisms involved. Proteomics data are available via ProteomeXchange with identifier PXD014121. SIGNIFICANCE: The present work provides the first proteome characterization of the ATDC5 chondrogenesis model, which has been used for decades in the field of cartilage biology. The results demonstrate the up- and down-regulation of more than one hundred proteins. Overall, specific drawbacks of the model are pointed out, that will be important to take into consideration for future studies. However, major cartilage components are massively assembled into an extracellular matrix and carry most of their post-translational modifications occurring in cartilage tissue. Unlike other available established cell lines, the ATDC5 model recapitulates major aspects of cartilage biosynthesis and should be useful in investigating the mechanisms that regulate collagen maturation events.

The minor collagens in articular cartilage

Articular cartilage is a connective tissue consisting of a specialized extracellular matrix (ECM) that dominates the bulk of its wet and dry weight. Type II collagen and aggrecan are the main ECM proteins in cartilage. However, little attention has been paid to less abundant molecular components, especially minor collagens, including type IV, VI, IX, X, XI, XII, XIII, and XIV, etc. Although accounting for only a small fraction of the mature matrix, these minor collagens not only play essential structural roles in the mechanical properties, organization, and shape of articular cartilage, but also fulfil specific biological functions. Genetic studies of these minor collagens have revealed that they are associated with multiple connective tissue diseases, especially degenerative joint disease. The progressive destruction of cartilage involves the degradation of matrix constituents including these minor collagens. The generation and release of fragmented molecules could generate novel biochemical markers with the capacity to monitor disease progression, facilitate drug development and add to the existing toolbox for in vitro studies, preclinical research and clinical trials.

Synchrotron DUV luminescence micro-imaging to identify and map historical organic coatings on wood

Collagen-based materials in historical coatings were characterised and imaged at the sub-micrometer scale using synchrotron DUV luminescence microspectroscopy and spectro-imaging.

Molecular adhesion between cartilage extracellular matrix macromolecules

In this study, we investigated the molecular adhesion between the major constituents of cartilage extracellular matrix, namely, the highly negatively charged proteoglycan aggrecan and the type II/IX/XI fibrillar collagen network, in simulated physiological conditions. Colloidal force spectroscopy was applied to measure the maximum adhesion force and total adhesion energy between aggrecan end-attached spherical tips (end radius R ≈ 2.5 μm) and trypsin-treated cartilage disks with undamaged collagen networks. Studies were carried out in various aqueous solutions to reveal the physical factors that govern aggrecan–collagen adhesion. Increasing both ionic strength and [Ca²⁺] significantly increased adhesion, highlighting the importance of electrostatic repulsion and Ca²⁺-mediated ion bridging effects. In addition, we probed how partial enzymatic degradation of the collagen network, which simulates osteoarthritic conditions, affects the aggrecan–collagen interactions. Interestingly, we found a significant increase in aggrecan–collagen adhesion even when there were no detectable changes at the macro- or microscales. It is hypothesized that the aggrecan–collagen adhesion, together with aggrecan–aggrecan self-adhesion, works synergistically to determine the local molecular deformability and energy dissipation of the cartilage matrix, in turn, affecting its macroscopic tissue properties.

Maternal obesity enhances collagen accumulation and cross-linking in skeletal muscle of ovine offspring

Maternal obesity (MO) has harmful effects on both fetal development and subsequent offspring health. We previously demonstrated that MO enhances collagen accumulation in fetal skeletal muscle, but its impact on mature offspring muscle collagen accumulation is unknown. Ewes were fed either a control diet (Con, fed 100% of NRC nutrient recommendations) or obesogenic diet (OB, fed 150% of NRC nutrient recommendations) from 60 days before conception to birth. All ewes received the Con diet during lactation. Male offspring were euthanized at 2.5 years (mean) and the left Longissimus dorsi (LD) muscle and semitendinosus (ST) muscle were sampled. Collagen concentration increased by 37.8±19.0% (P<0.05) in LD and 31.2±16.0% (P<0.05) in ST muscle of OB compared to Con offspring muscle. Mature collagen cross-linking (pyridinoline concentration) was increased for 22.3±7.4% and 36.3±9.9% (P<0.05) in LD and ST muscle of OB group respectively. Expression of lysyl oxidase, lysyl hydroxylase-2b (LH2b) and prolyl 4-hydroxylase (P4HA) was higher in OB LD and ST muscle. In addition, the expression of metalloproteinases (MMPs) was lower but tissue inhibitor of metalloproteinases (TIMPs) was higher in OB offspring muscle, indicating reduced collagen remodeling. MO enhanced collagen content and cross-linking in offspring muscle, which might be partially due to reduced collagen remodeling. Our observation that the collagen content and cross-linking are enhanced in MO offspring muscle is significant, because fibrosis is known to impair muscle functions and is a hallmark of muscle aging.

Nanomechanics of the Cartilage Extracellular Matrix

Cartilage is a hydrated biomacromolecular fiber composite located at the ends of long bones that enables proper joint lubrication, articulation, loading, and energy dissipation. Degradation of extracellular matrix molecular components and changes in their nanoscale structure greatly influence the macroscale behavior of the tissue and result in dysfunction with age, injury, and diseases such as osteoarthritis. Here, the application of the field of nanomechanics to cartilage is reviewed. Nanomechanics involves the measurement and prediction of nanoscale forces and displacements, intra- and intermolecular interactions, spatially varying mechanical properties, and other mechanical phenomena existing at small length scales. Experimental nanomechanics and theoretical nanomechanics have been applied to cartilage at varying levels of material complexity, e.g., nanoscale properties of intact tissue, the matrix associated with single cells, biomimetic molecular assemblies, and individual extracellular matrix biomolecules (such as aggrecan, collagen, and hyaluronan). These studies have contributed to establishing a fundamental mechanism-based understanding of native and engineered cartilage tissue function, quality, and pathology.

Actinotrichia collagens and their role in fin formation

The skeleton of zebrafish fins consists of lepidotrichia and actinotrichia. Actinotrichia are fibrils located at the tip of each lepidotrichia and play a morphogenetic role in fin formation. Actinotrichia are formed by collagens associated with non-collagen components. The non-collagen components of actinotrichia (actinodins) have been shown to play a critical role in fin to limb transition. The present study has focused on the collagens that form actinotrichia and their role in fin formation. We have found actinotrichia are formed by Collagen I plus a novel form of Collagen II, encoded by the col2a1b gene. This second copy of the collagen II gene is only found in fishes and is the only Collagen type II expressed in fins. Both col1a1a and col2a1b were found in actinotrichia forming cells. Significantly, they also expressed the lysyl hydroxylase 1 (lh1) gene, which encodes an enzyme involved in the post-translational processing of collagens. Morpholino knockdown in zebrafish embryos demonstrated that the two collagens and lh1 are essential for actinotrichia and fin fold morphogenesis. The col1a1 dominant mutant chihuahua showed aberrant phenotypes in both actinotrichia and lepidotrichia during fin development and regeneration. These pieces of evidences support that actinotrichia are composed of Collagens I and II, which are post-translationally processed by Lh1, and that the correct expression and assembling of these collagens is essential for fin formation. The unique collagen composition of actinotrichia may play a role in fin skeleton morphogenesis.

Articular cartilage: structure and regeneration

Articular cartilage (AC) has no or very low ability of self-repair, and untreated lesions may lead to the development of osteoarthritis. One method that has been proven to result in long-term repair or isolated lesions is autologous chondrocyte transplantation. However, first generation of these cells' implantation has limitations, and introducing new effective cell sources can improve cartilage repair. AC provides a resilient and compliant articulating surface to the bones in diarthrodial joints. It protects the joint by distributing loads applied to it, so preventing potentially damaging stress concentrations on the bone. At the same time it provides a low-friction-bearing surface to enable free movement of the joint. AC may be considered as a visco- or poro-elastic fiber-composite material. Fibrils of predominantly type II collagen provide tensile reinforcing to a highly hydrated proteoglycan gel. The tissue typically comprises 70% water and it is the structuring and retention of this water by the proteoglycans and collagen that is largely responsible for the remarkable ability of the tissue to support compressive loads.

Dynamic mechanical loading enhances functional properties of tissue-engineered cartilage using mature canine chondrocytes

OBJECTIVE

The concept of cartilage functional tissue engineering (FTE) has promoted the use of physiologic loading bioreactor systems to cultivate engineered tissues with load-bearing properties. Prior studies have demonstrated that culturing agarose constructs seeded with primary bovine chondrocytes from immature joints, and subjected to dynamic deformation, produced equilibrium compressive properties and proteoglycan content matching the native tissue. In the process of translating these results to an adult canine animal model, it was found that protocols previously successful with immature bovine primary chondrocytes did not produce the same successful outcome when using adult canine primary chondrocytes. The objective of this study was to assess the efficacy of a modified FTE protocol using adult canine chondrocytes seeded in agarose hydrogel and subjected to dynamic loading.

METHOD

Two modes of dynamic loading were applied to constructs using custom bioreactors: unconfined axial compressive deformational loading (DL; 1 Hz, 10% deformation) or sliding contact loading (Slide; 0.5 Hz, 10% deformation). Loading for 3 h daily was initiated on day 0, 14, or 28 (DL0, DL14, DL28, and Slide14).

RESULTS

Constructs with applied loading (both DL and Slide) exhibited significant increases in Young's modulus compared with free-swelling control as early as day 28 in culture (p < 0.05). However, glycosaminoglycan, collagen, and DNA content were not statistically different among the various groups. The modulus values attained for engineered constructs compare favorably with (and exceed in some cases) those of native canine knee (patella groove and condyle) cartilage.

CONCLUSION

Our findings successfully demonstrate an FTE strategy incorporating clinically relevant, adult chondrocytes and gel scaffold for engineering cartilage replacement tissue. These results, using continuous growth factor supplementation, are in contrast to our previously reported studies with immature chondrocytes where the sequential application of dynamic loading after transient transforming growth factor-beta3 application was found to be a superior culture protocol. Sliding, which simulates aspects of joint articulation, has shown promise in promoting engineered tissue development and provides an alternative option for FTE of cartilage constructs to be further explored.

Theoretical and uniaxial experimental evaluation of human annulus fibrosus degeneration

The highly organized structure and composition of the annulus fibrosus provides the tissue with mechanical behaviors that include anisotropy and nonlinearity. Mathematical models are necessary to interpret and elucidate the meaning of directly measured mechanical properties and to understand the structure-function relationships of the tissue components, namely, the fibers and extrafibrillar matrix. This study models the annulus fibrosus as a combination of strain energy functions describing the fibers, matrix, and their interactions. The objective was to quantify the behavior of both nondegenerate and degenerate annulus fibrosus tissue using uniaxial tensile experimental data. Mechanical testing was performed with samples oriented along the circumferential, axial, and radial directions. For samples oriented along the radial direction, the toe-region modulus was 2x stiffer with degeneration. However, no other differences in measured mechanical properties were observed with degeneration. The constitutive model fit well to samples oriented along the radial and circumferential directions (R(2)> or =0.97). The fibers supported the highest proportion of stress for circumferential loading at 60%. There was a 70% decrease in the matrix contribution to stress from the toe-region to the linear-region of both the nondegenerate and degenerate tissue. The shear fiber-matrix interaction (FMI) contribution increased by 80% with degeneration in the linear-region. Samples oriented along the radial and axial direction behaved similarly under uniaxial tension (modulus=0.32 MPa versus 0.37 MPa), suggesting that uniaxial testing in the axial direction is not appropriate for quantifying the mechanics of a fiber reinforcement in the annulus. In conclusion, the structurally motivated nonlinear anisotropic hyperelastic constitutive model helps to further understand the effect of microstructural changes with degeneration, suggesting that remodeling in the subcomponents (i.e., the collagen fiber, matrix and FMI) may minimize the overall effects on mechanical function of the bulk material with degeneration.

Type III collagen, a fibril network modifier in articular cartilage

The collagen framework of hyaline cartilages, including articular cartilage, consists largely of type II collagen that matures from a cross-linked heteropolymeric fibril template of types II, IX, and XI collagens. In the articular cartilages of adult joints, type III collagen makes an appearance in varying amounts superimposed on the original collagen fibril network. In a study to understand better the structural role of type III collagen in cartilage, we find that type III collagen molecules with unprocessed N-propeptides are present in the extracellular matrix of adult human and bovine articular cartilages as covalently cross-linked polymers extensively cross-linked to type II collagen. Cross-link analyses revealed that telopeptides from both N and C termini of type III collagen were linked in the tissue to helical cross-linking sites in type II collagen. Reciprocally, telopeptides from type II collagen were recovered cross-linked to helical sites in type III collagen. Cross-linked peptides were also identified in which a trifunctional pyridinoline linked both an alpha1(II) and an alpha1(III) telopeptide to the alpha1(III) helix. This can only have arisen from a cross-link between three different collagen molecules, types II and III in register staggered by 4D from another type III molecule. Type III collagen is known to be prominent at sites of healing and repair in skin and other tissues. The present findings emphasize the role of type III collagen, which is synthesized in mature articular cartilage, as a covalent modifier that may add cohesion to a weakened, existing collagen type II fibril network as part of a chondrocyte healing response to matrix damage.

Collagens

The collagens represent a family of trimeric extracellular matrix molecules used by cells for structural integrity and other functions. The three alpha chains that form the triple helical part of the molecule are composed of repeating peptide triplets of glycine-X-Y. X and Y can be any amino acid but are often proline and hydroxyproline, respectively. Flanking the triple helical regions (i.e., Col domains) are non-glycine-X-Y regions, termed non-collagenous domains. These frequently contain recognizable peptide modules found in other matrix molecules. Proper tissue function depends on correctly assembled molecular aggregates being incorporated into the matrix. This review highlights some of the structural characteristics of collagen types I-XXVIII.

Differences in chain usage and cross-linking specificities of cartilage type V/XI collagen isoforms with age and tissue

Collagen type V/XI is a minor but essential component of collagen fibrils in vertebrates. We here report on age- and tissue-related variations in isoform usage in cartilages. With maturation of articular cartilage, the alpha1(V) chain progressively replaced the alpha2(XI) chain. A mix of the molecular isoforms, alpha1(XI)alpha1(V)alpha3(XI) and alpha1(XI)alpha2(XI)alpha3(XI), best explained this finding. A prominence of alpha1(V) chains is therefore characteristic and a potential biomarker of mature mammalian articular cartilage. Analysis of cross-linked peptides showed that the alpha1(V) chains were primarily cross-linked to alpha1(XI) chains in the tissue and hence an integral component of the V/XI polymer. From nucleus pulposus of the intervertebral disc (in which the bulk collagen monomer is type II as in articular cartilage), type V/XI collagen consisted of a mix of five genetically distinct chains, alpha1(XI), alpha2(XI), alpha3(XI), alpha1(V), and alpha2(V). These presumably were derived from several different molecular isoforms, including alpha1(XI)alpha2(XI)alpha3(XI), (alpha1(XI))(2)alpha2(V), and others. Meniscal fibrocartilage shows yet another V/XI phenotype. The findings support and extend the concept that the clade B subfamily of COL5 and COL11 gene products should be considered members of the same collagen subfamily, from which, in combination with clade A gene products (COL2A1 or COL5A2), a range of molecular isoforms has evolved into tissue-dependent usage. We propose an evolving role for collagen V/XI isoforms as an adaptable polymeric template of fibril macro-architecture.

Neuronal differentiation of synovial sarcoma and its therapeutic application

Synovial sarcoma is a rare sarcoma of unknown histologic origin. We previously reported the gene expression profile of synovial sarcoma was closely related to that of malignant peripheral nerve sheath tumors, and the fibroblast growth factor (FGF) signal was one of the main growth signals in synovial sarcoma. Here we further demonstrate the neural origin of synovial sarcoma using primary tumors and cell lines. The expression of neural tissue-related genes was confirmed in synovial sarcoma tumor tissues, but the expression of some genes was absent in synovial sarcoma cell lines. Treatment of synovial sarcoma cell lines with BMP4 or FGF2 enhanced or restored the expression of neural tissue-related genes and induced a neuron-like morphology with positive Tuj-1 expression. Treatment with all-trans-retinoic acid also induced the expression of neural tissue-related genes in association with growth inhibition, which was not observed in other cell lines except a malignant peripheral nerve sheath tumor cell line. A growth-inhibitory effect of all-trans-retinoic acid was also observed for xenografted tumors in athymic mice. The simultaneous treatment with FGF signal inhibitors enhanced the growth-inhibitory effect of all-trans-retinoic acid, suggesting the combination of growth signaling inhibition and differentiation induction could be a potential molecular target for treating synovial sarcoma.

Low-intensity pulsed ultrasound activates the phosphatidylinositol 3 kinase/Akt pathway and stimulates the growth of chondrocytes in three-dimensional cultures: a basic science study

Introduction

The effect of low-intensity pulsed ultrasound (LIPUS) on cell growth was examined in three-dimensional-cultured chondrocytes with a collagen sponge. To elucidate the mechanisms underlying the mechanical activation of chondrocytes, intracellular signaling pathways through the Ras/mitogen-activated protein kinase (MAPK) and the integrin/phosphatidylinositol 3 kinase (PI3K)/Akt pathways as well as proteins involved in proliferation of chondrocytes were examined in LIPUS-treated chondrocytes.

Methods

Articular cartilage tissue was obtained from the metatarso-phalangeal joints of freshly sacrificed pigs. Isolated chondrocytes mixed with collagen gel and culture medium composites were added to type-I collagen honeycomb sponges. Experimental cells were cultured with daily 20-minute exposures to LIPUS. The chondrocytes proliferated and a collagenous matrix was formed on the surface of the sponge. Cell counting, histological examinations, immunohistochemical analyses and western blotting analysis were performed.

Results

The rate of chondrocyte proliferation was slightly but significantly higher in the LIPUS group in comparison with the control group during the 2-week culture period. Western blot analysis showed intense staining of type-IX collagen, cyclin B₁ and cyclin D₁, phosphorylated focal adhesion kinase, and phosphorylated Akt in the LIPUS group in comparison with the control group. No differences were detected, however, in the MAPK, phosphorylated MAPK and type-II collagen levels.

Conclusion

LIPUS promoted the proliferation of cultured chondrocytes and the production of type-IX collagen in a three-dimensional culture using a collagen sponge. In addition, the anabolic LIPUS signal transduction to the nucleus via the integrin/phosphatidylinositol 3-OH kinase/Akt pathway rather than the integrin/MAPK pathway was generally associated with cell proliferation.

Trimerization of collagen IX alpha-chains does not require the presence of the COL1 and NC1 domains

Collagen IX is a heterotrimer of three alpha-chains, which consists of three COL domains (collagenous domains) (COL1-COL3) and four NC domains (non-collagenous domains) (NC1-NC4), numbered from the C-terminus. Although collagen IX chains have been shown to associate via their C-terminal NC1 domains and form a triple helix starting from the COL1 domain, it is not known whether chain association can occur at other sites and whether other collagenous and non-collagenous regions are involved. To address this question, we prepared five constructs, two long variants (beginning at the NC4 domain) and three short variants (beginning at the COL2 domain), all ending at the NC2 domain (or NC2 replaced by NC1), to study association and selection of collagen IX alpha-chains. Both long variants were able to associate with NC1 or NC2 at the C-terminus and form various disulfide-bonded trimers, but the specificity of chain selection was diminished compared with full-length chains. Trimers of the long variant ending at NC2 were shown to be triple helical by CD. Short variants were not able to assemble into disulfide-bonded trimers even in the presence of both conserved cysteine residues from the COL1-NC1 junction. Our results demonstrate that collagen IX alpha-chains can associate in the absence of COL1 and NC1 domains to form a triple helix, but the COL2-NC2 region alone is not sufficient for trimerization. The results suggest that folding of collagen IX is a co-operative process involving multiple COL and NC domains and that the COL1-NC1 region is important for chain specificity.

Skeletal dysplasias and the growth plate

Skeletal dysplasias are disorders in which there is derangement in the growth or shape of the skeleton. Long bone grows from cartilage that persists near the ends until skeletal maturity as the growth plate. Developmental biology work has identified the major regulatory proteins in growth plate chondroyte function. There are hundreds of skeletal dysplasias, and the molecular genetic etiology of many was defined in the past decade and a half. Now that the causative genes for these disorders have been identified, they can be broadly classified by the function of the protein that these genes encode for into disorders caused by extracellular structural proteins, proteins that regulate normal growth plate chondrocyte differentiation and patterning, and enzymes that process these proteins. There are clinical similarities within each group, and the phenotype can be predicted based on the role of the mutated protein in normal growth plate function. As such, this framework to classify the skeletal dysplasias has practical clinical implications.

Nanostructure of the neurocentral growth plate: Insight from scanning small angle X-ray scattering, atomic force microscopy and scanning electron microscopy

In this study, the experimental techniques scanning electron microscopy (SEM) including energy-dispersive X-ray analysis, atomic force microscopy (AFM) and scanning small angle X-ray scattering (SAXS) have been exploited to characterize the organization of large molecules and nanocrystallites in and around the neurocentral growth plate (NGP) of a pig vertebrae L4. The techniques offer unique complementary information on the nano- to micrometer length scale and provide new insight in the changes in the matrix structure during endochondral bone formation. AFM and SEM imaging of the NGP reveal a fibrous network likely to consist of collagen type II and proteoglycans. High-resolution AFM imaging shows that the fibers have a diameter of approximately 100 nm and periodic features along the fibers with a periodicity of 50-70 nm. This is consistent with the SAXS analysis that yields a cross-sectional diameter of the fibers in the range of 90 to 112 nm and a predominant orientation in the longitudinal direction of the NGP. Furthermore, we find inhomogeneities around 7 nm in the NGP by SAXS analysis. Moving towards the bone in the direction perpendicular to the growth plate, a systematic change in apparent thickness is observed, while the large-scale structural features remain constant. In the region of bone, the apparent thickness equals the mean mineral thickness and increases from 2 nm to approximately 3.5 nm as a function distance from the NGP. The mineral particles are organized as plates in a rather compact network structure. We have demonstrated that SEM, AFM and SAXS are valuable tools for the investigation of the organization of large molecules and nanocrystallites in the NGP and adjacent trabecular bone. Our findings will be an important basis for future work into identifying the defects on nanometer length scale responsible for idiopathic scoliosis and other growth-plate-related diseases.

Nanomechanical properties of individual chondrocytes and their developing growth factor-stimulated pericellular matrix

The nanomechanical properties of individual cartilage cells (chondrocytes) and their aggrecan and collagen-rich pericellular matrix (PCM) were measured via atomic force microscope nanoindentation using probe tips of two length scales (nanosized and micron-sized). The properties of cells freshly isolated from cartilage tissue (devoid of PCM) were compared to cells that were cultured for selected times (up to 28 days) in 3-D alginate gels which enabled PCM assembly and accumulation. Cells were immobilized and kept viable in pyramidal wells microfabricated into an array on silicon chips. Hertzian contact mechanics and finite element analyses were employed to estimate apparent moduli from the force versus depth curves. The effects of culture conditions on the resulting PCM properties were studied by comparing 10% fetal bovine serum to medium containing a combination of insulin growth factor-1 (IGF-1)+osteogenic protein-1 (OP-1). While both systems showed increases in stiffness with time in culture between days 7 and 28, the IGF-1+OP-1 combination resulted in a higher stiffness for the cell-PCM composite by day 28 and a higher apparent modulus of the PCM which is compared to the FBS cultured cells. These studies give insight into the temporal evolution of the nanomechanical properties of the pericellar matrix relevant to the biomechanics and mechanobiology of tissue-engineered constructs for cartilage repair.

Genome-wide linkage analysis of inguinal hernia in pigs using affected sib pairs

BACKGROUND

Inguinal and scrotal hernias are of great concern to pig producers, and lead to poor animal welfare and severe economic loss. Selection against these conditions is highly preferable, but at this time no gene, Quantitative Trait Loci (QTL), or mode of inheritance has been identified in pigs or in any other species. Therefore, a complete genome scan was performed in order to identify genomic regions affecting inguinal and scrotal hernias in pigs. Records from seedstock breeding farms were collected. No clinical examinations were executed on the pigs and there was therefore no distinction between inguinal and scrotal hernias. The genome scan utilised affected sib pairs (ASP), and the data was analysed using both an ASP test based on Non-parametric Linkage (NPL) analysis, and a Transmission Disequilibrium Test (TDT).

RESULTS

Significant QTLs (p < 0.01) were detected on 8 out of 19 porcine chromosomes. The most promising QTLs, however, were detected in SSC1, SSC2, SSC5, SSC6, SSC15, SSC17 and SSCX; all of these regions showed either statistical significance with both statistical methods, or convincing significance with one of the methods. Haplotypes from these suggestive QTL regions were constructed and analysed with TDT. Of these, six different haplotypes were found to be differently transmitted (p < 0.01) to healthy and affected pigs. The most interesting result was one haplotype on SSC5 that was found to be transmitted to hernia pigs with four times higher frequency than to healthy pigs (p < 0.00005).

CONCLUSION

For the first time in any species, a genome scan has revealed suggestive QTLs for inguinal and scrotal hernias. While this study permitted the detection of chromosomal regions only, it is interesting to note that several promising candidate genes, including INSL3, MIS, and CGRP, are located within the highly significant QTL regions. Further studies are required in order to narrow down the suggestive QTL regions, investigate the candidate genes, and to confirm the suggestive QTLs in other populations. The haplotype associated with inguinal and scrotal hernias may help in achieving selection against the disorder.

The anchorless adhesin Eap (extracellular adherence protein) from Staphylococcus aureus selectively recognizes extracellular matrix aggregates but binds promiscuously to monomeric matrix macromolecules

Besides a number of cell wall-anchored adhesins, the majority of Staphylococcus aureus strains produce anchorless, cell wall-associated proteins, such as Eap (extracellular adherence protein). Eap contains four to six tandem repeat (EAP)-domains. Eap mediates diverse biological functions, including adherence and immunomodulation, thus contributing to S. aureus pathogenesis. Eap binding to host macromolecules is unusually promiscuous and includes matrix or matricellular proteins as well as plasma proteins. The structural basis of this promiscuity is poorly understood. Here, we show that in spite of the preferential location of the binding epitopes within triple helical regions in some collagens there is a striking specificity of Eap binding to different collagen types. Collagen I, but not collagen II, is a binding substrate in monomolecular form. However, collagen I is virtually unrecognized by Eap when incorporated into banded fibrils. By contrast, microfibrils containing collagen VI as well as basement membrane-associated networks containing collagen IV, or aggregates containing fibronectin bound Eap as effectively as the monomeric proteins. Therefore, Eap-binding to extracellular matrix ligands is promiscuous at the molecular level but not indiscriminate with respect to supramolecular structures containing the same macromolecules. In addition, Eap bound to banded fibrils after their partial disintegration by matrix-degrading proteinases, including matrix metalloproteinase 1. Therefore, adherence to matrix suprastructures by S. aureus can be supported by inflammatory reactions.

Collagen and proteoglycan turnover in focally damaged human ankle cartilage: evidence for a generalized response and active matrix remodeling across the entire joint surface

OBJECTIVE

Although cartilage lesions occur in the ankles, osteoarthritis rarely develops in the ankles, suggesting that ankle cartilage can up-regulate mechanisms to repair the damaged matrix. To define these processes, we compared cartilage samples obtained from normal tali and from lesional sites of damaged tali.

METHODS

Cartilage samples were obtained from the tali of normal ankles and from 3 sites on tali with lesions (the lesion, adjacent to the lesion, and far removed from the lesion). Cartilage was analyzed for type II collagen (CII) messenger RNA, C-terminal type II procollagen propeptide (CPII), the collagenase cleavage neoepitope (Col2-3/4C(short)), and the denaturation epitope (Col2-3/4m). For the assessment of type IX collagen, the COL2 and NC4 domains were evaluated. The cartilage samples were also assayed for glycosaminoglycans, epitope 846 of aggrecan, and DNA.

RESULTS

The DNA content, epitope 846, COL2(IX), and the denaturation epitope were significantly increased in lesional cartilage. Although there was a tendency toward an increase in CII content and CPII, the increase did not reach significance. Neither the NC4(IX) domain nor Col2-3/4C was elevated. Surprisingly, changes in cartilage both adjacent to and remote from the lesion were similar to those in the lesion.

CONCLUSION

The changes observed in cartilage obtained from the lesion and from sites adjacent to the lesion were not surprising; however, the changes in cartilage obtained from sites remote from the lesion were unexpected. This up-regulation of matrix turnover in ankles with degenerative lesions may indicate a physiologic response of the entire articular surface to repair the damaged matrix, which is not restricted to the lesion site. This suggests that there may be some mechanism of communication across the cartilage. The response by ankle cartilage obtained from a site remote from the lesion has not been observed in the knee.

Gene expression changes in an early stage of intervertebral disc degeneration induced by passive cigarette smoking

STUDY DESIGN

This study attempts to determine the molecular changes in intervertebral disc degeneration of rats induced by passive cigarette smoking.

OBJECTIVES

To quantitate and compare the gene expression levels in intervertebral discs from passively cigarette smoking rats and nonsmoking rats.

SUMMARY OF BACKGROUND DATA

The molecular mechanism of intervertebral disc degeneration has been investigated mainly in vitro but little in vivo, and gene expression analysis has been performed in a few studies. The cigarette smoking is a risk factor of low back pain. We developed a smoking box to create a rat model of intervertebral disc degeneration induced by passive cigarette smoking.

METHODS

Total RNA was extracted from intervertebral discs of rats that were raised in a cigarette-smoking box for 2 to 7 weeks. After synthesis of cDNA, the quantitative analysis of gene expression was performed by the real-time PCR. The remaining spines were subjected to the histologic examination.

RESULTS

Histologic changes of the nucleus pulposus and the anulus fibrosus were detected after 2 weeks of smoking and were frequently found after 7 weeks. Collagen genes were downregulated remarkably after 7 weeks of smoking. No significant increase was observed in the expressions of matrix metalloproteinase-3, but the expression of tissue inhibitor of metalloproteinases-1 started to increase at 4 weeks of smoking. Aggrecan also started to be up-regulated at 4 weeks.

CONCLUSIONS

Changes in gene expression by passive cigarette smoking precede the histologic changes in the intervertebral discs. Reactions to suppress the destruction of tissue matrix and to regenerate the intervertebral discs are occurring at the same time as the degenerative histologic changes.

The role of computational models in the search for the mechanical behavior and damage mechanisms of articular cartilage

Articular cartilage plays a vital role in the function of diarthrodial joints. Due to osteoarthritis degeneration of articular cartilage occurs. The initial event that triggers the pathological process of cartilage degeneration is still unknown. Cartilage damage due to osteoarthritis is believed to be mechanically induced. Hence, to investigate the initiation of osteoarthritis the stresses and strains in the cartilage must be determined. So far the most common method to accomplish that is finite element analysis. This paper provides an overview of computational descriptions developed for this purpose, and what they can be used for. Articular cartilage composition and structure are discussed in relation with degenerative changes, and how these affect mechanical properties.

Position of single amino acid substitutions in the collagen triple helix determines their effect on structure of collagen fibrils

Collagen II fibrils are a critical structural component of the extracellular matrix of cartilage providing the tissue with its unique biomechanical properties. The self-assembly of collagen molecules into fibrils is a spontaneous process that depends on site-specific binding between specific domains belonging to interacting molecules. These interactions can be altered by mutations in the COL2A1 gene found in patients with a variety of heritable cartilage disorders known as chondrodysplasias. Employing recombinant procollagen II, we studied the effects of R75C or R789C mutations on fibril formation. We determined that both R75C and R789C mutants were incorporated into collagen assemblies. The effects of the R75C and R789C substitutions on fibril formation differed significantly. The R75C substitution located in the thermolabile region of collagen II had no major effect on the fibril formation process or the morphology of fibrils. In contrast, the R789C substitution located in the thermostable region of collagen II caused profound changes in the morphology of collagen assemblies. These results provide a basis for identifying pathways leading from single amino acid substitutions in collagen II to changes in the structure of individual fibrils and in the organization of collagenous matrices.

The role of type X collagen in facilitating and regulating endochondral ossification of articular cartilage

AUTHOR: Shen G Objective -This review was compiled to explore the role of type X collagen in growth, development and remodeling of articular cartilage by elucidating the linkage between the synthesis of this protein and the phenotypic changes in chondrogenesis and the onset of endochondral ossification.

DESIGN

The current studies closely dedicated to elucidating the role of type X collagen incorporating into chondrogenesis and endochondral ossification of articular cartilage were assessed and analyzed to allow for obtaining the mainstream consensus on the bio-molecular mechanism with which type X collagen functions in articular cartilage.

RESULTS

There are spatial and temporal correlations between synthesis of type X collagen and occurrence of endochondral ossification. The expression of type X collagen is confined within hypertrophic condrocytes and precedes the embark of endochondral bone formation. Type X collagen facilitates endochondral ossification by regulating matrix mineralization and compartmentalizing matrix components.

CONCLUSION

Type X collagen is a reliable marker for new bone formation in articular cartilage. The future clinical application of this collagen in inducing or mediating endochondral ossification is perceived, e.g. the fracture healing of synovial joints and adaptive remodeling of madibular condyle.

The Fibril-associated Collagen IX Provides a Novel Mechanism for Cell Adhesion to Cartilaginous Matrix

Collagen IX is the prototype fibril-associated collagen with interruptions in triple helix. In human cartilage it covers collagen fibrils, but its putative cellular receptors have been unknown. The reverse transcription-PCR analysis of human fetal tissues suggested that based on their distribution all four collagen receptor integrins, namely alpha1beta1, alpha2beta1, alpha10beta1, and alpha11beta1, are possible receptors for collagen IX. Furthermore primary chondrocytes and chondrosarcoma cells express the four integrins simultaneously. Chondrosarcoma cells, as well as Chinese hamster ovary cells transfected to express alpha1beta1, alpha2beta1, or alpha10beta1 integrin as their only collagen receptor, showed fast attachment and spreading on human recombinant collagen IX indicating that it is an effective cell adhesion protein. To further study the recognition of collagen IX we produced recombinant alphaI domains in Escherichia coli. For each of the four alphaI domains, collagen IX was among the best collagenous ligands, making collagen IX exceptional compared with all other collagen subtypes tested so far. Rotary shadowing electron microscopy images of both alpha1I- and alpha2I-collagen IX complexes unveiled only one binding site located in the COL3 domain close to the kink between it and the COL2 domain. The recognition of collagen IX by alpha2I was considered to represent a novel mechanism for two reasons. First, collagen IX has no GFOGER motif, and the identified binding region lacks any similar sequences. Second, the alpha2I domain mutations D219R and H258V, which both decreased binding to collagen I and GFOGER, had very different effects on its binding to collagen IX. D219R had no effect, and H258V prevented type IX binding. Thus, our results indicate that collagen IX has unique cell adhesion properties when compared with other collagens, and it provides a novel mechanism for cell adhesion to cartilaginous matrix.

Fibronectin fragments cause release and degradation of collagen-binding molecules from equine explant cultures

OBJECTIVE

Previous experiments have shown that addition of fragmented fibronectin can induce cartilage chondrolysis. In this study we investigated the fate of the collagen- and cell-binding molecules Cartilage oligomeric matrix protein (COMP) and chondroadherin.

DESIGN

Equine articular cartilage explants were stimulated with the C-terminal and the N-terminal heparin-binding fragments of fibronectin respectively, and the conditioned media were analysed by both quantitative (ELISA) and qualitative (mass spectrometry, Western blots) methods.

RESULTS

Both COMP and chondroadherin were released in a dose-dependent manner upon stimulation with the Hep II (C-terminal heparin-binding) fragment of fibronectin. The kinetics of release for the two components differed. Moreover, COMP was degraded while no fragments of chondroadherin could be detected. Stimulation with Hep II also induced production of nitric oxide in a dose-dependent manner. We compared effects of the Hep II fragment with that of Hep I (the N-terminal heparin-binding fragment of fibronectin) and found that while Hep I did indeed elicit release of COMP and chondroadherin, the response was less potent, and production of nitric oxide was negligible. The responses to both fragments were elicited within 24h.

CONCLUSIONS

We suggest that the events described here may be early, critical stages in cartilage destruction preceding collagen destruction.

Covalent cross-linking of the NC1 domain of collagen type IX to collagen type II in cartilage

From a study to understand the mechanism of covalent interaction between collagen types II and IX, we present experimental evidence for a previously unrecognized molecular site of cross-linking. The location relative to previously defined cross-linking sites predicts a specific manner of interaction and folding of collagen IX on the surface of nascent collagen II fibrils. The initial evidence came from Western blot analysis of type IX collagen extracted by pepsin from fetal human cartilage, which showed a molecular species that had properties indicating an adduct between the alpha1(II) chain and the C-terminal domain (COL1) of type IX collagen. A similar component was isolated from bovine cartilage in sufficient quantity to confirm this identity by N-terminal sequence analysis. Using an antibody that recognized the putative cross-linking sequence at the C terminus of the alpha1(IX) chain, cross-linked peptides were isolated by immunoaffinity chromatography from proteolytic digests of human cartilage collagen. They were characterized by immunochemistry, N-terminal sequence analysis, and mass spectrometry. The results establish a link between a lysine near the C terminus (in the NC1 domain) of alpha1(IX) and the known cross-linking lysine at residue 930 of the alpha1(II) triple helix. This cross-link is speculated to form early in the process of interaction between collagen IX molecules and collagen II polymers. A model of molecular folding and further cross-linking is predicted that can spatially accommodate the formation of all six known cross-linking interactions to the collagen IX molecule on a fibril surface. Of particular biological significance, this model can accommodate potential interfibrillar as well as intrafibrillar links between the collagen IX molecules themselves, so providing a mechanism whereby collagen IX could stabilize a collagen fibril network.

Assembly of collagen types II, IX and XI into nascent hetero-fibrils by a rat chondrocyte cell line

The cell line, RCS-LTC (derived from the Swarm rat chondrosarcoma), deposits a copious extracellular matrix in which the collagen component is primarily a polymer of partially processed type II N-procollagen molecules. Transmission electron microscopy of the matrix shows no obvious fibrils, only a mass of thin unbanded filaments. We have used this cell system to show that the type II N-procollagen polymer nevertheless is stabilized by pyridinoline cross-links at molecular sites (mediated by N- and C-telopeptide domains) found in collagen II fibrils processed normally. Retention of the N-propeptide therefore does not appear to interfere with the interactions needed to form cross-links and mature them into trivalent pyridinoline residues. In addition, using antibodies that recognize specific cross-linking domains, it was shown that types IX and XI collagens, also abundantly deposited into the matrix by this cell line, become covalently cross-linked to the type II N-procollagen. The results indicate that the assembly and intertype cross-linking of the cartilage type II collagen heteropolymer is an integral, early process in fibril assembly and can occur efficiently prior to the removal of the collagen II N-propeptides.

Cloning and expression of a type IX-like collagen in tissues of the ascidian Ciona intestinalis

Collagens are highly preserved proteins in invertebrates and vertebrates. To identify the collagens in urochordates, the total RNA extracted from the pharynx of the ascidian Ciona intestinalis was hybridized with a heterologous probe specific for the echinoderm Paracentrotus lividus fibrillar type I-like larval collagen. Using this probe, two main bands (i.e. 6 and 2.8 kb mRNA) were observed on Northern blot hybridization. The cDNA library prepared from poly(A)+RNA extracted from pharyngeal tissue was screened and a cDNA that specifies a type IX-like collagen was identified. This molecule presents a conceptual open reading frame for a protein containing 734 amino acids. In particular, we showed a 1 alpha chain type IX-like collagen characterized by three short triple-helical domains interspersed with four non-triple-helical sequences, with structural features of fibril-associated collagens with interrupted triple-helices (FACIT) collagens. Northern blot hybridizations indicate a 2.8 kb transcript size. Sequence comparison indicated homology (47.64%, 48.95%) between the type IX-like collagen of C. intestinalis and mouse and human type IX collagen. In situ hybridization of tunic and pharynx tissues shows the presence of transcripts in connective tissue cells.

Genomic organization and characterization of the human type XXI collagen (COL21A1) gene

We cloned a 4.1-kb full-length cDNA based on a reported human genomic clone containing a partial open reading frame (ORF) coding for a novel collagen-like protein. Sequence analysis indicated that the ORF codes for the alpha(1)-chain of type XXI collagen. Assembly of the genomic data reveals a complete sequence of the human gene COL21A1. COL21A1 is localized to chromosome 6p11.2-12.3, spanning 337 kb in size. The gene contains 31 exons, in which the 5'-untranslated exons 1 and 1a are alternatively spliced. The exon/domain organization of COL21A1 resembles that of the reported FACIT collagen genes, including COL9A1, COL9A2, COL9A3, and COL19A1, suggesting that these genes may have derived from the same ancestor FACIT gene by duplication. The expression of COL21A1 in human tissues is developmentally regulated, with a higher level at fetal stages. Type XXI collagen is an extracellular matrix component of the blood vessel walls, secreted by smooth-muscle cells. Platelet-derived growth factor (PDGF) has a pronounced effect on the stimulation of COL21A1 expression in cultured aortic smooth-muscle cells, suggesting that alpha1(XXI) collagen may contribute to the extracellular matrix assembly of the vascular network during blood vessel formation.

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.

A degeneration-based hypothesis for interpreting fibrillar changes in the osteoarthritic cartilage matrix

The collagen fibrillar architectures in the general matrix of cartilage slices removed from both normal and osteoarthritic femoral heads were examined by both differential interference light microscopy and scanning electron microscopy. Whereas the normal general matrix contained a finely differentiated pseudo-random weave of fibrils developed from an interconnected array of radial elements, the osteoarthritic general matrix was characterised by the presence of structurally distinct regions consisting of strongly aligned radial bundles of fibrils and associated intense tangles or 'knotted' features. Simple structural models were developed to explore possible transformation structures based on two different types of interconnectivity in the three-dimensional fibrillar network. These models support the hypothesis that the distinctive ultrastructural features of the osteoarthritic general matrix can develop as a consequence of largely passive degradative changes occurring in the fibrillar weave originally present in the normal matrix. This could, in principle, occur independently of any new structure that might develop as a consequence of any upregulation of collagen associated with the osteoarthritic process.

Combination of reduced oxygen tension and intermittent hydrostatic pressure: a useful tool in articular cartilage tissue engineering

Cartilage cells are normally studied under atmospheric pressure conditions and without loading. However, since cartilage exists in a condition of reduced oxygen and intermittent hydrostatic pressure we hypothesized lower partial oxygen pressures (PO2) and different intermittent hydrostatic pressures (IHP) would increase articular chondrocyte proliferation and matrix production and to stabilize chondrocyte phenotype in vitro. Monolayers of adult bovine articular chondrocytes were cultured under 5% or 21% PO2 in combination with IHP (0.2 MPa amplitude, frequencies 5/5s = 0.1 Hz, 30/2 or 2/30 min on/off loading). We measured proliferation (3H-thymidine incorporation) and collagen secretion (protein-binding assay, collagen type II-ELISA and immunocytochemical staining of pericellular collagen types I, II and IX). Reduced PO2 stimulated proliferation and collagen type II and IX secretion of chondrocytes in comparison to 21% PO2. Additionally, collagen type I expression was delayed by low PO2, indicating a stabilization of the cell phenotype. IHP 5/5s and 30/2 min inhibited proliferation but increased collagen secretion (pericellular collagen type IX was decreased). IHP 30/2 min delayed first expression of collagen type I. In contrast, IHP 2/30 min increased proliferation, but lowered collagen expression. All stimulating or inhibiting effects of PO2 and IHP were additive and vice versa. Reduced PO2 and different settings of IHP increased proliferation, collagen secretion, and phenotype stability of chondrocytes. The oxygen- and IHP-induced effects were additive, suggesting that a combination of these parameters might be a useful tool in cartilage tissue engineering.

Anionic sites in articular cartilage revealed by polyethyleneimine staining

Articular cartilage is a unique tissue that contains neither blood vessels nor nerves, and that performs mechanical loading during joint movement. These properties are endowed by abundant glycosaminoglycans (GAGs), which are capable of retaining water-soluble substances. The GAGs attach to core proteins and form proteoglycans. Although many studies have focused on proteoglycans and collagen fibrils in cartilage, little is known about the nature of the negative charge of GAGs. Recently, we investigated this subject using a cationic dye, polyethyleneimine (PEI), with several different techniques such as pre-embedding, post-embedding, and quick-freezing and deep-etching methods. In addition, we investigated whether the anionic charge is altered at low pH, using PEI and cationic colloidal gold (CCG) labeling. The shapes of PEI-positive structures revealed by the pre-embedding method varied at different pHs. Three-dimensional analysis using the quick-freezing and deep-etching method demonstrated that meshwork structures composed of fine filaments were decorated with tiny PEI granules. Additionally, the meshwork structure was broken down after chondroitinase ABC digestion. These data indicate that the large PEI deposits observed in pre-embedding preparations are, at least in part, artificial images, and that the meshwork structure consists of chondroitin sulfate-retaining anionic sites. Low pH conditions changed PEI or CCG labeling patterns, showing that negative charges of GAGs in articular cartilage are altered under environmental pH conditions. These findings demonstrate that binding capacities of anionic sites to water-soluble or ionic substances are greatly affected by pH alterations without actually decreasing the number of anionic sites. Therefore, to understand cartilage dynamics and the pathogenesis of joint diseases in greater detail, alterations of anionic charge during mechanical loading or under pathological conditions should be examined in future studies.

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.

Two-dimensional peptide mapping of cross-linked type IX collagen in human cartilage

Type IX collagen is a quantitatively minor component of hyaline cartilage that is essential for the normal structural integrity of the tissue. Purification and analysis are difficult because the mature protein is insoluble as a cross-linked integral component of the fibrillar matrix. In order to view a peptide map of the total pool of type IX collagen in a cartilage sample, a selective method based on Western blot analysis was developed for displaying collagen IX peptides in a cyanogen bromide digest of tissue. Digests were partially resolved by reverse-phase HPLC, individual fractions were run on SDS-PAGE and then transblotted to membrane, and the collagen IX fragments were revealed using an anti-collagen IX rabbit antiserum. All major CB-peptides from alpha1(IX), alpha2(IX), and alpha3(IX) chains in the resulting two-dimensional display were identified by amino-terminal sequence analysis. Cross-linked peptides originating from sites of covalent interaction between collagen types IX and II and between IX and IX were also defined. By comparison with an analysis of soluble type IX collagen from chondrocyte culture medium, the results showed that the pool of type IX collagen molecules in fetal and adult human cartilage is extensively cross-linked intermolecularly at sites previously revealed by other methods using purified protein. This sensitive, direct method has the potential to screen for abnormalities in the content and properties of type IX collagen in tissue samples, for example, in the study of heritable chondrodysplasia syndromes and the pathogenesis of cartilage destruction in osteoarthritis.

Collagen XI nucleates self-assembly and limits lateral growth of cartilage fibrils

Fibrils of embryonic cartilage are heterotypic alloys formed by collagens II, IX, and XI and have a uniform diameter of approximately 20 nm. The molecular basis of this lateral growth control is poorly understood. Collagen II subjected to fibril formation in vitro produced short and tapered tactoids with strong D-periodic banding. The maximal width of these tactoids varied over a broad range. By contrast, authentic mixtures of collagens II, IX, and XI yielded long and weakly banded fibrils, which, strikingly, had a uniform width of about 20 nm. The same was true for mixtures of collagens II and XI lacking collagen IX as long as the molar excess of collagen II was less than 8-fold. At higher ratios, the proteins assembled into tactoids coexisting with cartilage-like fibrils. Therefore, diameter control is an inherent property of appropriate mixtures of collagens II and XI. Collagen IX is not essential for this feature but strongly increases the efficiency of fibril formation. Therefore, this protein may be an important stabilizing factor of cartilage fibrils.

Mammalian skeletogenesis and extracellular matrix: what can we learn from knockout mice?

Formation of the vertebrate skeleton and the proper functions of bony and cartilaginous elements are determined by extracellular, cell surface and intracellular molecules. Genetic and biochemical analyses of human heritable skeletal disorders as well as the generation of knockout mice provide useful tools to identify the key players of mammalian skeletogenesis. This review summarises our recent work with transgenic animals carrying ablated genes for cartilage extracellular matrix proteins. Some of these mice exhibit a lethal phenotype associated with severe skeletal defects (type II collagen-null, perlecan-null), whereas others show mild (type IX collagen-null) or no skeletal abnormalities (matrilin-1-null, fibromodulin-null, tenascin-C-null). The appropriate human genetic disorders are discussed and contrasted with the knockout mice phenotypes.

The alpha1(VIII) and alpha2(VIII) collagen chains form two distinct homotrimeric proteins in vivo

The short chain collagen variant, type VIII, is considered to be comprised of two distinct gene products, the alpha1 and alpha2 polypeptide chains. However, recent in vitro translation studies suggest that these chains can form homotrimers. We report here data from biochemical, immunohistochemical and molecular biological experiments, which together provide evidence that alpha1 and alpha2 polypeptides of type VIII collagen exist as homotrimers in cells and tissues. High-performance liquid chromatographic separation of type VIII collagen isolated from Descemet's membrane consistently demonstrated equimolar quantities of the two chains (alpha1:alpha2 1. 03+/-0.02 (S.E.M.); n=41). The availability of highly specific antibodies for the two polypeptides has assisted the in vivo characterisation of type VIII collagen. Immunoprecipitation of trimeric type VIII collagen from Descemet's membrane with purified anti-alpha1(VIII) and anti-alpha2(VIII) yielded fractions that contained only the alpha1(VIII) and alpha2(VIII) chains, respectively. Cultured human mesangial cells synthesised both polypeptides, but the alpha1(VIII) chain was found exclusively in the cell pellet, while the media contained only the alpha2(VIII) chain. The RNA from human mesangial cells and cornea showed message for both chains. However, in peritoneal fibroblast and mesothelial cell RNA, only alpha1(VIII) mRNA was detectable, demonstrating that the transcription of these two genes was not always co-ordinated. Immunohistochemistry showed that both polypeptides were present in cornea, optic nerve, aorta and umbilical cord but did not always co-localise. These results indicate the alpha1(VIII) and alpha2(VIII) chains preferentially form pepsin-resistant, homotrimeric molecules and so can exist as two distinct proteins.

A short isoform of Col9a1 supports alveolar bone repair

Bone wound created in intramembranous alveolar bone heals without the formation of cartilage precursor tissue. However, the expression of cartilage collagen mRNAs has been suggested. In this report, we examined the expression and the potential role of type IX collagen in bone restoration and remodeling. The sequence specific polymerase chain reaction demonstrated the exclusive expression of short transcriptional isoform of alpha1(IX) collagen (Col9a1) in alveolar bone wound healing, while the long isoform of Col9a1 transcript was absent. Type IX collagen was immunolocalized in the preliminary matrix organized in granulation tissue before trabecular bone formation in tooth extraction socket. In Col9a1-null mutant mice, there were considerable variations in alveolar bone wound healing with the absence of or abnormally organized trabecular bone. Occasionally, unusual apposition of cortical-bone-like layers in bone marrow space was observed. The Col9a1-null mice indicated no growth retardation, and their facial and long bones maintained the normal size and shape. However, the primary spongiosa region of adult Col9a1 mutant mice showed an abnormal trabecular bone structure associated with abnormal immunostaining with the hypertrophic cartilage specific type X collagen antibody. These data suggest that type IX collagen short transcriptional variant is involved in the restoration and remodeling processes of trabecular bone.

Cleavage of aggrecan at the Asn341-Phe342 site coincides with the initiation of collagen damage in murine antigen-induced arthritis: a pivotal role for stromelysin 1 in matrix metalloproteinase activity

OBJECTIVE

The destruction of articular cartilage during arthritis is due to proteolytic cleavage of the extracellular matrix components. This study investigates the kinetic involvement of metalloproteinases (MMPs) in the degradation of the 2 major cartilage components, aggrecan and type II collagen, during murine antigen-induced arthritis (AIA). In addition, the role of stromelysin 1 (SLN-1) induction of MMP-induced neoepitopes was studied.

METHODS

VDIPEN neoepitopes in aggrecan and collagenase-induced COL2-3/4C neoepitopes in type II collagen were identified by immunolocalization. Stromelysin 1-deficient knockout (SLN1-KO) mice were used to study SLN-1 involvement.

RESULTS

In AIA, the VDIPEN epitopes in aggrecan appeared after initial proteoglycan (PG) depletion. The collagenase-induced type II collagen neoepitopes colocalized with VDIPEN epitopes. Remarkably, cartilage from arthritic SLN1-KO mice showed neither the induction of VDIPEN nor collagen cleavage-site neoepitopes during AIA, suggesting that stromelysin is a pivotal mediator in this process. PG depletion, as measured by the loss of Safranin O staining, was similar in SLN1-KO mice and wild-type strains. Furthermore, in vitro induction of VDIPEN epitopes in aggrecan and COL2-3/4C epitopes in type II collagen, on exposure of cartilage to interleukin-1, could not be accomplished in SLN1-KO mice, whereas intense staining was achieved for both epitopes in cartilage of wild-type strains.

CONCLUSION

This study emphasizes that SLN-1 is essential in the induction of MMP-specific aggrecan and collagen cleavage sites during AIA. It suggests that SLN-1 is not a dominant enzyme in PG breakdown, but that it activates procollagenases and is crucial in the initiation of collagen damage.