Cartilage contains mixed fibrils of collagen types II, IX, and XI

Multiphoton microscopy analysis of extracellular collagen I network formation by mesenchymal stem cells

Collagen I is the major fibrous extracellular component of bone responsible for its ultimate tensile strength. In tissue engineering, one of the most important challenges for tissue formation is to get cells interconnected via a strong and functional extracellular matrix (ECM), mimicking as closely as possible the natural ECM geometry. Still missing in tissue engineering are: (a) a versatile, high-resolution and non-invasive approach to evaluate and quantify different aspects of ECM development within engineered biomimetic scaffolds online; and (b) deeper insights into the mechanism whereby cellular matrix production is enhanced in 3D cell-scaffold composites, putatively via enhanced focal adhesion linkage, over the 2D setting. In this study, we developed sensitive morphometric detection methods for collagen I-producing and bone-forming mesenchymal stem cells (MSCs), based on multiphoton second harmonic generation (SHG) microscopy, and used those techniques to compare collagen I production capabilities in 2D- and 3D-arranged cells. We found that stimulating cells with 1% serum in the presence of ascorbic acid is superior to other medium conditions tested, including classical osteogenic medium. In contrast to conventional 2D culture, having MSCs packed closely in a 3D environment presumably stimulates cells to produce strong and complex collagen I networks with defined network structures (visible in SHG images) and improves collagen production. Copyright © 2015 John Wiley & Sons, Ltd.

Multiscale Biofabrication of Articular Cartilage: Bioinspired and Biomimetic Approaches

Articular cartilage is the load-bearing tissue found inside all articulating joints of the body. It vastly reduces friction and allows for smooth gliding between contacting surfaces. The structure of articular cartilage matrix and cellular composition is zonal and is important for its mechanical properties. When cartilage becomes injured through trauma or disease, it has poor intrinsic healing capabilities. The spectrum of cartilage injury ranges from isolated areas of the joint to diffuse breakdown and the clinical appearance of osteoarthritis. Current clinical treatment options remain limited in their ability to restore cartilage to its normal functional state. This review focuses on the evolution of biomaterial scaffolds that have been used for functional cartilage tissue engineering. In particular, we highlight recent developments in multiscale biofabrication approaches attempting to recapitulate the complex 3D matrix of native articular cartilage tissue. Additionally, we focus on the application of these methods to engineering each zone of cartilage and engineering full-thickness osteochondral tissues for improved clinical implantation. These methods have shown the potential to control individual cell-to-scaffold interactions and drive progenitor cell differentiation into a chondrocyte lineage. The use of these bioinspired nanoengineered scaffolds hold promise for recreation of structure and function on the whole tissue level and may represent exciting new developments for future clinical applications for cartilage injury and restoration.

Sp1 upregulates the proximal promoter activity of the mouse collagen α1(XI) gene (Col11a1) in chondrocytes

Type XI collagen is a cartilage-specific extracellular matrix, and is important for collagen fibril formation and skeletal morphogenesis. We have previously reported that NF-Y regulated the proximal promoter activity of the mouse collagen α1(XI) gene (Col11a1) in chondrocytes (Hida et. al. In Vitro Cell. Dev. Biol. Anim. 2014). However, the mechanism of the Col11a1 gene regulation in chondrocytes has not been fully elucidated. In this study, we further characterized the proximal promoter activity of the mouse Col11a1 gene in chondrocytes. Cell transfection experiments with deletion and mutation constructs indicated that the downstream region of the NF-Y binding site (-116 to +1) is also necessary to regulate the proximal promoter activity of the mouse Col11a1 gene. This minimal promoter region has no TATA box and GC-rich sequence; we therefore examined whether the GC-rich sequence (-96 to -67) is necessary for the transcription regulation of the Col11a1 gene. Luciferase assays using a series of mutation constructs exhibited that the GC-rich sequence is a critical element of Col11a1 promoter activity in chondrocytes. Moreover, in silico analysis of this region suggested that one of the most effective candidates was transcription factor Sp1. Consistent with the prediction, overexpression of Sp1 significantly increased the promoter activity. Furthermore, knockdown of Sp1 expression by siRNA transfection suppressed the proximal promoter activity and the expression of endogenous transcript of the mouse Col11a1 gene. Taken together, these results indicate that the transcription factor Sp1 upregulates the proximal promoter activity of the mouse Col11a1 gene in chondrocytes.

Development of novel real-time PCR methodology for quantification of COL11A1 mRNA variants and evaluation in breast cancer tissue specimens

Background

Collagen XI is a key structural component of the extracellular matrix and consists of three alpha chains. One of these chains, the α1 (XI), is encoded by the COL11A1 gene and is transcribed to four different variants at least (A, B, C and E) that differ in the propensity to N-terminal domain proteolysis and potentially in the way the extracellular matrix is arranged. This could affect the ability of tumor cells to invade the remodeled stroma and metastasize. No study in the literature has so far investigated the expression of these four variants in breast cancer nor does a method for their accurate quantitative detection exist.

Methods

We developed a conventional PCR for the general detection of the general COL11A1 transcript and real-time qPCR methodologies with dual hybridization probes in the LightCycler platform for the quantitative determination of the variants. Data from 90 breast cancer tissues with known histopathological features were collected.

Results

The general COL11A1 transcript was detected in all samples. The developed methodologies for each variant were rapid as well as reproducible, sensitive and specific. Variant A was detected in 30 samples (33 %) and variant E in 62 samples (69 %). Variants B and C were not detected at all. A statistically significant correlation was observed between the presence of variant E and lymph nodes involvement (p = 0.037) and metastasis (p = 0.041).

Conclusions

With the newly developed tools, the possibility of inclusion of COL11A1 variants as prognostic biomarkers in emerging multiparameter technologies examining tissue RNA expression should be further explored.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1725-8) contains supplementary material, which is available to authorized users.

Defective Proteolytic Processing of Fibrillar Procollagens and Prodecorin Due to Biallelic BMP1 Mutations Results in a Severe, Progressive Form of Osteogenesis Imperfecta

Whereas the vast majority of osteogenesis imperfecta (OI) is caused by autosomal dominant defects in the genes encoding type I procollagen, mutations in a myriad of genes affecting type I procollagen biosynthesis or bone formation and homeostasis have now been associated with rare autosomal recessive OI forms. Recently, homozygous or compound heterozygous mutations in BMP1, encoding the metalloproteases bone morphogenetic protein-1 (BMP1) and its longer isoform mammalian Tolloid (mTLD), were identified in 5 children with a severe autosomal recessive form of OI and in 4 individuals with mild to moderate bone fragility. BMP1/mTLD functions as the procollagen carboxy-(C)-proteinase for types I to III procollagen but was also suggested to participate in amino-(N)-propeptide cleavage of types V and XI procollagens and in proteolytic trimming of other extracellular matrix (ECM) substrates. We report the phenotypic characteristics and natural history of 4 adults with severe, progressive OI characterized by numerous fractures, short stature with rhizomelic shortening, and deformity of the limbs and variable kyphoscoliosis, in whom we identified novel biallelic missense and frameshift mutations in BMP1. We show that BMP1/mTLD-deficiency in humans not only results in delayed cleavage of the type I procollagen C-propeptide but also hampers the processing of the small leucine-rich proteoglycan prodecorin, a regulator of collagen fibrillogenesis. Immunofluorescent staining of types I and V collagen and transmission electron microscopy of the dermis show impaired assembly of heterotypic type I/V collagen fibrils in the ECM. Our study thus highlights the severe and progressive nature of BMP1-associated OI in adults and broadens insights into the functional consequences of BMP1/mTLD-deficiency on ECM organization.

COL9A1 gene polymorphism is associated with Kashin-Beck disease in a northwest Chinese Han population

Objective

We sought to determine whether genomic polymorphism in collagen IX genes (COL9A) was associated with Kashin-Beck disease (KBD).

Methods

Twenty seven single nucleotide polymorphisms (SNPs) in COL9AI, COL9A2 and COL9A3 were genotyped in 274 KBD cases and 248 healthy controls using the Sequenom MassARRAY system. Associations between the COL9A polymorphism and KBD risk were detected using an unconditional logistic regression model. Linkage disequilibrium (LD) and haplotypes analysis were performed with the Haploview software.

Results

After Bonferroni correction, the frequency distribution of genotypes in rs6910140 in COL9A1 was significantly different between the KBD and the control groups (X² = 16.74, df = 2, P = 0.0002). Regression analysis showed that the allele “C” in SNP rs6910140 had a significant protective effect on KBD [odds ratio (OR) = 0.49, 95% confidence interval (CI) = 0.34–0.70, P = 0.0001]. The frequencies of alleles and genotypes in rs6910140 were significantly different among subjects of different KBD stages (allele: X² = 7.82, df = 2, P = 0.02, genotype: X² = 14.81, df = 4, P = 0.005). However, haplotype analysis did not detect any significant association between KBD and COL9A1, COL9A2 and COL9A3.

Conclusions

We observed a significant association between rs6910140 of COL9A1 and KBD, suggesting a role of COL9A1 in the development of KBD.

β1 Integrins Mediate Attachment of Mesenchymal Stem Cells to Cartilage Lesions

Mesenchymal stem cells (MSC) may have great potential for cell-based therapies of osteoarthritis. However, after injection in the joint, only few cells adhere to defective articular cartilage and contribute to cartilage regeneration. Little is known about the molecular mechanisms of MSC attachment to defective articular cartilage. Here, we developed an ex vivo attachment system, using rat osteochondral explants with artificially created full-thickness cartilage defects in combination with genetically labeled MSC isolated from bone marrow of human placental alkaline phosphatase transgenic rats. Binding of MSC to full-thickness cartilage lesions was improved by serum, but not hyaluronic acid, and was dependent on the presence of divalent cations. Additional in vitro tests showed that rat MSC attach, in a divalent cation-dependent manner, to collagen I, collagen II, and fibronectin, but not to collagen XXII or cartilage oligomeric matrix protein (COMP). RGD peptides partially blocked the adhesion of MSC to fibronectin in vitro and to cartilage lesions ex vivo. Furthermore, the attachment of MSC to collagen I and II in vitro and to cartilage lesions ex vivo was almost completely abolished in the presence of a β1 integrin blocking antibody. In conclusion, our data suggest that attachment of MSC to ex vivo full-thickness cartilage lesions is almost entirely β1 integrin-mediated, whereby both RGD- and collagen-binding integrins are involved. These findings suggest a key role of integrins during MSC attachment to defective cartilage and may pave the way for improved MSC-based therapies in the future.

Osteoarthritic cartilage explants affect extracellular matrix production and composition in cocultured bone marrow-derived mesenchymal stem cells and articular chondrocytes

Introduction

In the present study, we established a novel in vitro coculture model to evaluate the influence of osteoarthritis (OA) cartilage explants on the composition of newly produced matrix and chondrogenic differentiation of human bone marrow-derived mesenchymal stem cells (BMSCs) and the phenotype of OA chondrocytes. In addition, we included a “tri-culture” model, whereby a mixture of BMSCs and chondrocytes was cultured on the surface of OA cartilage explants.

Methods

Gene expression analysis, protein and glycosaminoglycan (GAG) assays, dot-blot, immunofluorescence, and biomechanical tests were used to characterize the properties of newly generated extracellular matrix (ECM) from chondrocytes and chondrogenically differentiated BMSCs and a mix thereof. We compared articular cartilage explant cocultures with BMSCs, chondrocytes, and mixed cultures (chondrocytes and BMSCs 1:1) embedded in fibrin gels with fibrin gel-embedded cells cultured without cartilage explants (monocultures).

Results

In general, co- and tri-cultured cell regimens exhibited reduced mRNA and protein levels of collagens I, II, III, and X in comparison with monocultures, whereas no changes in GAG synthesis were observed. All co- and tri-culture regimens tended to exhibit lower Young’s and equilibrium modulus compared with monocultures. In contrast, aggregate modulus and hydraulic permeability seemed to be higher in co- and tri-cultures. Supernatants of cocultures contained significant higher levels of interleukin-1 beta (IL-1β), IL-6, and IL-8. Stimulation of monocultures with IL-1β and IL-6 reduced collagen gene expression in BMSCs and mixed cultures in general but was often upregulated in chondrocytes at late culture time points. IL-8 stimulation affected BMSCs only.

Conclusions

Our results suggest an inhibitory effect of OA cartilage on the production of collagens. This indicates a distinct modulatory influence that affects the collagen composition of the de novo-produced ECM from co- and tri-cultured cells and leads to impaired mechanical and biochemical properties of the matrix because of an altered fibrillar network. We suggest that soluble factors, including IL-1β and IL-6, released from OA cartilage partly mediate these effects. Thus, neighbored OA cartilage provides inhibitory signals with respect to BMSCs’ chondrogenic differentiation and matrix composition, which need to be accounted for in future cell-based OA treatment strategies.

Gene expression analysis of murine and human osteoarthritis synovium reveals elevation of transforming growth factor β-responsive genes in osteoarthritis-related fibrosis

OBJECTIVE

Synovial fibrosis is a major contributor to joint stiffness in osteoarthritis (OA). Transforming growth factor β (TGFβ), which is elevated in OA, plays a key role in the onset and persistence of synovial fibrosis. However, blocking of TGFβ in OA as a therapeutic intervention for fibrosis is not an option since TGFβ is crucial for cartilage maintenance and repair. Therefore, we undertook the present study to seek targets downstream of TGFβ for preventing OA-related fibrosis without interfering with joint homeostasis.

METHODS

Experiments were performed to determine whether genes involved in extracellular matrix turnover were responsive to TGFβ and were elevated in OA-related fibrosis. We analyzed gene expression in TGFβ-stimulated human OA synovial fibroblasts and in the synovium of mice with TGFβ-induced fibrosis, mice with experimental OA, and humans with end-stage OA. Gene expression was determined by microarray, low-density array, or quantitative polymerase chain reaction analysis.

RESULTS

We observed an increase in expression of procollagen genes and genes encoding collagen crosslinking enzymes under all of the OA-related fibrotic conditions investigated. Comparison of gene expression in TGFβ-stimulated human OA synovial fibroblasts, synovium from mice with experimental OA, and synovium from humans with end-stage OA revealed that the genes PLOD2, LOX, COL1A1, COL5A1, and TIMP1 were up-regulated in all of these conditions. Additionally, we confirmed that these genes were up-regulated by TGFβ in vivo in mice with TGFβ-induced synovial fibrosis.

CONCLUSION

Most of the up-regulated genes identified in this study would be poor targets for therapy development, due to their crucial functions in the joint. However, the highly up-regulated gene PLOD2, responsible for the formation of collagen crosslinks that make collagen less susceptible to enzymatic degradation, is an attractive and promising target for interference in OA-related synovial fibrosis.

Changes in type II procollagen isoform expression during chondrogenesis by disruption of an alternative 5' splice site within Col2a1 exon 2

This study describes a new mechanism controlling the production of alternatively spliced isoforms of type II procollagen (Col2a1) in vivo. During chondrogenesis, precursor chondrocytes predominantly produce isoforms containing alternatively spliced exon 2 (type IIA and IID) while Col2a1 mRNA devoid of exon 2 (type IIB) is the major isoform produced by differentiated chondrocytes. We previously identified an additional Col2a1 isoform containing a truncated exon 2 and premature termination codons in exon 6 (type IIC). This transcript is produced by utilization of another 5' splice site present in exon 2. To determine the role of this IIC splicing event in vivo, we generated transgenic mice containing silent knock-in mutations at the IIC 5' splice site (Col2a1-mIIC), thereby inhibiting production of IIC transcripts. Heterozygous and homozygous knock-in mice were viable and display no overt skeletal phenotype to date. However, RNA expression profiles revealed that chondrocytes in cartilage from an age range of Col2a1-mIIC mice produced higher levels of IIA and IID mRNAs and decreased levels of IIB mRNAs throughout pre-natal and post-natal development, when compared to chondrocytes from littermate control mice. Immunofluorescence analyses showed a clear increase in expression of embryonic type II collagen protein isoforms (i.e. containing the exon 2-encoded cysteine-rich (CR) protein domain) in cartilage extracellular matrix (ECM). Interestingly, at P14, P28 and P56, expression of embryonic Col2a1 isoforms in Col2a1-mIIC mice persisted in the pericellular domain of the ECM in articular and growth plate cartilage. We also show that persistent expression of the exon 2-encoded CR domain in the ECM of post-natal cartilage tissue may be due, in part, to the embryonic form of type XI collagen (the α3 chain of which is also encoded by the Col2a1 gene). In conclusion, expression of the Col2a1 IIC splice form may have a regulatory function in controlling alternative splicing of exon 2 to generate defined proportions of IIA, IID and IIB procollagen isoforms during cartilage development. Future studies will involve ultrastructural and biomechanical analysis of the collagen ECM to determine the effects of persistent mis-expression of embryonic collagen isoforms in mature cartilage tissue.

Lateral growth limitation of corneal fibrils and their lamellar stacking depend on covalent collagen cross-linking by transglutaminase-2 and lysyl oxidases, respectively

Corneal stroma contains an extracellular matrix of orthogonal lamellae formed by parallel and equidistant fibrils with a homogeneous diameter of ~35 nm. This is indispensable for corneal transparency and mechanical functions. However, the mechanisms controlling corneal fibrillogenesis are incompletely understood and the conditions required for lamellar stacking are essentially unknown. Under appropriate conditions, chick embryo corneal fibroblasts can produce an extracellular matrix in vitro resembling primary corneal stroma during embryonic development. Among other requirements, cross-links between fibrillar collagens, introduced by tissue transglutaminase-2, are necessary for the self-assembly of uniform, small diameter fibrils but not their lamellar stacking. By contrast, the subsequent lamellar organization into plywood-like stacks depends on lysyl aldehyde-derived cross-links introduced by lysyl oxidase activity, which, in turn, only weakly influences fibril diameters. These cross-links are introduced at early stages of fibrillogenesis. The enzymes are likely to be important for a correct matrix deposition also during repair of the cornea.

Nuclear factor Y (NF-Y) regulates the proximal promoter activity of the mouse collagen α1(XI) gene (Col11a1) in chondrocytes

Type XI collagen, a heterotrimer composed of α1(XI), α2(XI), and α3(XI), plays a critical role in cartilage formation and in skeletal morphogenesis. However, the transcriptional regulation of α1(XI) collagen gene (Col11a1) in chondrocyte is poorly characterized. In this study, we investigated the proximal promoter of mouse Col11a1 gene in chondrocytes. Major transcription start site was located at -299 bp upstream of the translation start site, and the proximal promoter lacks a TATA sequence but has a high guanine-cytosine (GC) content. Cell transfection experiments demonstrated that the segment from -116 to -256 is necessary for activation of the proximal Col11a1 promoter, and an electrophoretic mobility shift assay showed that a nuclear protein is bound to the segment from -116 to -176 in this promoter. Additional comparative and in silico analyses demonstrated that an ATTGG sequence, which is critical for binding to nuclear factor Y (NF-Y), is within the highly conserved region from -135 to -145. Interference assays using wild-type and mutant oligonucleotide or with specific antibody revealed that NF-Y protein is bound to this region. Cell transfection experiments with reporter constructs in the absence of NF-Y binding sequence exhibited the suppression of the promoter activity. Furthermore, chromatin immunoprecipitation assay demonstrated that NF-Y protein is directly bound to this region in vivo, and overexpression of dominant-negative NF-Y A mutant also inhibited the proximal promoter activity. Taken together, these results indicate that the transcription factor NF-Y regulates the proximal promoter activity of mouse Col11a1 gene in chondrocytes.

Canine chondrodysplasia caused by a truncating mutation in collagen-binding integrin alpha subunit 10

The skeletal dysplasias are disorders of the bone and cartilage tissues. Similarly to humans, several dog breeds have been reported to suffer from different types of genetic skeletal disorders. We have studied the molecular genetic background of an autosomal recessive chondrodysplasia that affects the Norwegian Elkhound and Karelian Bear Dog breeds. The affected dogs suffer from disproportionate short stature dwarfism of varying severity. Through a genome-wide approach, we mapped the chondrodysplasia locus to a 2-Mb region on canine chromosome 17 in nine affected and nine healthy Elkhounds (p_raw = 7.42×10⁻⁶, p_genome-wide = 0.013). The associated locus contained a promising candidate gene, cartilage specific integrin alpha 10 (ITGA10), and mutation screening of its 30 exons revealed a nonsense mutation in exon 16 (c.2083C>T; p.Arg695*) that segregated fully with the disease in both breeds (p = 2.5×10⁻²³). A 24% mutation carrier frequency was indicated in NEs and an 8% frequency in KBDs. The ITGA10 gene product, integrin receptor α10-subunit combines into a collagen-binding α10β1 integrin receptor, which is expressed in cartilage chondrocytes and mediates chondrocyte-matrix interactions during endochondral ossification. As a consequence of the nonsense mutation, the α10-protein was not detected in the affected cartilage tissue. The canine phenotype highlights the importance of the α10β1 integrin in bone growth, and the large animal model could be utilized to further delineate its specific functions. Finally, this study revealed a candidate gene for human chondrodysplasias and enabled the development of a genetic test for breeding purposes to eradicate the disease from the two dog breeds.

A unique tool to selectively detect the chondrogenic IIB form of human type II procollagen protein

Type II collagen, the major fibrillar collagen of cartilage, is synthesized as precursor forms (procollagens) containing N- and C-terminal propeptides. Three splice variants are thought to be translated to produce procollagen II isoforms (IIA/D and IIB) which differ in their amino propeptide parts. The IIA and IID are transient embryonic isoforms that include an additional cysteine-rich domain encoded by exon 2. The IIA and IID transcripts are co-expressed during chondrogenesis then decline and the IIB isoform is the only one expressed and synthesized in fully differentiated chondrocytes. Additionally, procollagens IIA/D can be re-expressed by dedifferentiating chondrocytes and in osteoarthritic cartilage. Therefore, it is an important point to determine which isoform(s) is (are) synthesized in vivo in normal and pathological situations and in vitro, to fully assess the phenotype of cells producing type II collagen protein. Antibodies directed against the cysteine-rich extra domain found in procollagens IIA and IID are already available but antibodies detecting only the chondrogenic IIB form of type II procollagen were missing so far. A synthetic peptide encompassing the junction between exon 1 and exon 3 of the human sequence was used as immunogen to produce rabbit polyclonal antibodies to procollagen IIB. After affinity purification on immobilized peptide their absence of crossreaction with procollagens IIA/D and with the fibrillar procollagens I, III and V was demonstrated by Western blotting. These antibodies were used to reveal at the protein level that the treatment of dedifferentiated human chondrocytes by bone morphogenic protein (BMP)-2 induces the synthesis of the IIB (chondrocytic) isoform of procollagen II. In addition, immunohistochemical staining of bovine cartilage demonstrates the potential of these antibodies in the analysis of the differential spatiotemporal distribution of N-propeptides of procollagens IIA/D and IIB during normal development and in pathological situations.

Analysis of the cartilage proteome from three different mouse models of genetic skeletal diseases reveals common and discrete disease signatures

Pseudoachondroplasia and multiple epiphyseal dysplasia are genetic skeletal diseases resulting from mutations in cartilage structural proteins. Electron microscopy and immunohistochemistry previously showed that the appearance of the cartilage extracellular matrix (ECM) in targeted mouse models of these diseases is disrupted; however, the precise changes in ECM organization and the pathological consequences remain unknown. Our aim was to determine the effects of matrilin-3 and COMP mutations on the composition and extractability of ECM components to inform how these detrimental changes might influence cartilage organization and degeneration.

Cartilage was sequentially extracted using increasing denaturants and the extraction profiles of specific proteins determined using SDS-PAGE/Western blotting. Furthermore, the relative composition of protein pools was determined using mass spectrometry for a non-biased semi-quantitative analysis.

Western blotting revealed changes in the extraction of matrilins, COMP and collagen IX in mutant cartilage. Mass spectrometry confirmed quantitative changes in the extraction of structural and non-structural ECM proteins, including proteins with roles in cellular processes such as protein folding and trafficking. In particular, genotype-specific differences in the extraction of collagens XII and XIV and tenascins C and X were identified; interestingly, increased expression of several of these genes has recently been implicated in susceptibility and/or progression of murine osteoarthritis.

We demonstrated that mutation of matrilin-3 and COMP caused changes in the extractability of other cartilage proteins and that proteomic analyses of Matn3 V194D, Comp T585M and Comp DelD469 mouse models revealed both common and discrete disease signatures that provide novel insight into skeletal disease mechanisms and cartilage degradation.

Disruption of the developmentally-regulated Col2a1 pre-mRNA alternative splicing switch in a transgenic knock-in mouse model

The present study describes the generation of a knock-in mouse model to address the role of type II procollagen (Col2a1) alternative splicing in skeletal development and maintenance. Alternative splicing of Col2a1 precursor mRNA is a developmentally-regulated event that only occurs in chondrogenic tissue. Normally, chondroprogenitor cells synthesize predominantly exon 2-containing mRNA isoforms (type IIA and IID) while Col2a1 mRNA devoid of exon 2 (type IIB) is the major isoform produced by differentiated chondrocytes. Another isoform, IIC, has also been identified that contains a truncated exon 2 and is not translated into protein. The biological significance of this IIA/IID to IIB splicing switch is not known. Utilizing a splice site targeting knock-in approach, a 4 nucleotide mutation was created to convert the 5' splice site of Col2a1 exon 2 from a weak, non-consensus sequence to a strong, consensus splice site. This resulted in apparent expression of only the IIA mRNA isoform, as confirmed in vitro by splicing of a type II procollagen mini-gene containing the 5' splice site mutation. To test the splice site targeting approach in vivo, homozygote mice engineered to retain IIA exon 2 (Col2a1(+ex2)) were generated. Chondrocytes from hindlimb epiphyseal cartilage of homozygote mice were shown to express only IIA mRNA and protein at all pre- and post-natal developmental stages analyzed (E12.5, E16.5, P0, P3, P7, P14, P28 and P70). As expected, type IIB procollagen was the major isoform produced in wild type cartilage at all post-natal time points. Col2a1(+ex2) homozygote mice are viable, appear healthy and display no overt phenotype to date. However, research is currently underway to investigate the biological consequence of persistent expression of the exon 2-encoded conserved cysteine-rich domain in post-natal skeletal tissues.

Effect of exercise on articular cartilage

This review primarily focuses on how the macromolecular composition and architecture of articular cartilage and its unique biomechanical properties play a pivotal role in the ability of articular cartilage to withstand mechanical loads several magnitudes higher than the weight of the individual. Current findings on short-term and long-term effects of exercise on human articular cartilage are reviewed, and the importance of appropriate exercises for individuals with normal and diseased or aberrated cartilage is discussed.

AGE-DEPENDENT EXPRESSION OF TRANSFORMING GROWTH FACTOR β1 (TGF-β1) AND ITS RECEPTORS, AND AGE-RELATED STIMULATORY EFFECT OF TGF-β1 ON PROTEOGLYCAN SYNTHESIS IN RAT INTERVERTEBRAL DISCS

Age-related alterations of gene expression of transforming growth factor β1 (TGF-β1) and its receptors ( T β Rs ) in tissues derived from rat intervertebral discs were assessed together with TGF-β1-dependent proteoglycan synthesis by the cultured disc cells. Disc tissues and cells were individually harvested from two sites of the coccygeal vertebrae, namely the nucleus pulposus (NP) and annulus fibrosus (AF), which are major distinct components of the intervertebral discs. Semi-quantitative RT-PCR analysis indicated that the level of gene expression of TGF-β1/TGF-β1 receptor type I (TβR-I) of NP decreased with age. In AF, the level of TGF-β1/ T β Rs gene expression did not apparently differ with age. Consistent with the RT-PCR results, stimulation of proteoglycan synthesis by TGF-β1 in NP cells decreased with age. Proteoglycan synthesis by AF cells was also stimulated by TGF-β1. However, levels of this stimulation by AF cells were identical. The present findings indicate that the genetic expression of TGF-β1/ T β R -I and TGF-β1-dependent proteoglycan synthesis decreased with age in NP cells, and further suggest that a loss of proteoglycan synthesis with age in the intervertebral disc is at least in part due to the transcriptional down regulation of TGF-β1/ T βR-I and decreased synthetic ability of proteoglycans in response to TGF-β1 by NP cells.

Metalloproteinases in Drosophila to humans that are central players in developmental processes

Many secreted proteins are synthesized as precursors with propeptides that must be cleaved to yield the mature functional form of the molecule. In addition, various growth factors occur in extracellular latent complexes with protein antagonists and are activated upon cleavage of such antagonists. Research in the separate fields of embryonic patterning and extracellular matrix formation has identified members of the BMP1/Tolloid-like family of metalloproteinases as key players in these types of biosynthetic processing events in species ranging from Drosophila to humans.

Type IX collagen interacts with fibronectin providing an important molecular bridge in articular cartilage

Type IX collagen is covalently bound to the surface of type II collagen fibrils within the cartilage extracellular matrix. The N-terminal, globular noncollagenous domain (NC4) of the α1(IX) chain protrudes away from the surface of the fibrils into the surrounding matrix and is available for molecular interactions. To define these interactions, we used the NC4 domain in a yeast two-hybrid screen of a human chondrocyte cDNA library. 73% of the interacting clones encoded fibronectin. The interaction was confirmed using in vitro immunoprecipitation and was further characterized by surface plasmon resonance. Using whole and pepsin-derived preparations of type IX collagen, the interaction was shown to be specific for the NC4 domain with no interaction with the triple helical collagenous domains. The interaction was shown to be of high affinity with nanomolar K(d) values. Analysis of the fibronectin-interacting clones indicates that the constant domain is the likely site of interaction. Type IX collagen and fibronectin were shown to co-localize in cartilage. This novel interaction between the NC4 domain of type IX collagen and fibronectin may represent an in vivo interaction in cartilage that could contribute to the matrix integrity of the tissue.

Hydroureternephrosis due to loss of Sox9-regulated smooth muscle cell differentiation of the ureteric mesenchyme

Congenital ureter anomalies, including hydroureter, affect up to 1% of the newborn children. Despite the prevalence of these developmental abnormalities in young children, the underlying molecular causes are only poorly understood. Here, we show that the high mobility group domain transcription factor Sox9 plays an important role in ureter development in the mouse. Transient Sox9 expression was detected in the undifferentiated ureteric mesenchyme and inactivation of Sox9 in this domain resulted in strong proximal hydroureter formation due to functional obstruction. Loss of Sox9 did not affect condensation, proliferation and apoptosis of the undifferentiated mesenchyme, but perturbed cyto-differentiation into smooth muscle cells (SMCs). Expression of genes encoding extracellular matrix (ECM) components was strongly reduced, suggesting that deficiency in ECM composition and/or signaling may underlie the observed defects. Prolonged expression of Sox9 in the ureteric mesenchyme led to increased deposition of ECM components and SMC dispersal. Furthermore, Sox9 genetically interacts with the T-box transcription factor 18 gene (Tbx18) during ureter development at two levels--as a downstream mediator of Tbx18 function and in a converging pathway. Together, our results argue that obstructive uropathies in campomelic dysplasia patients that are heterozygous for mutations in and around SOX9 arise from a primary requirement of Sox9 in the development of the ureteric mesenchyme.

Noninvasive evaluation of tissue-engineered cartilage with time-resolved laser-induced fluorescence spectroscopy

Regenerative medicine requires noninvasive evaluation. Our objective is to investigate the application of time-resolved laser-induced fluorescence spectroscopy (TR-LIFS) using a nano-second-pulsed laser for evaluation of tissue-engineered cartilage (TEC). To prepare scaffold-free TEC, articular chondrocytes from 4-week-old Japanese white rabbits were harvested, and were inoculated at a high density in a mold. Cells were cultured for 5 weeks by rotating culture (RC) or static culture (SC). The RC group and SC group at each week (n = 5), as well as normal articular cartilage and purified collagen type II (as controls), were analyzed by TR-LIFS. The peak wavelength was compared with those of type II collagen immunostaining and type II collagen quantification by enzyme-linked immunosorbent assay and tensile testing. The fluorescence peak wavelength of the TEC analyzed by this method shifted significantly in the RC group at 3 weeks, and in the SC group at 5 weeks (p < 0.01). These results correlated with changes in type II collagen (enzyme-linked immunosorbent assay) and changes in Young's modulus on tensile testing. The results were also supported by immunohistologic findings (type II collagen staining). Our findings show that TR-LIFS is useful for evaluating TEC.

Gene expression profiling of primary human articular chondrocytes in high-density micromasses reveals patterns of recovery, maintenance, re- and dedifferentiation

The high-density micromass culture has been widely applied to study chondrocyte cell physiology and pathophysiological mechanisms. Since an integrated image has not been established so far, we analyzed the phenotypic alterations of human articular chondrocytes in this model on the broad molecular level. Freshly isolated chondrocytes were assembled as micromasses and maintained up to 6 weeks in medium containing human serum. Formation of cartilaginous extracellular matrix (ECM) was evaluated by histological and immunohistochemical staining. At 0, 3 and 6 weeks, chondrocyte micromasses were subjected to gene expression analysis using oligonucleotide microarrays and real-time RT-PCR. Micromasses developed a cartilaginous ECM rich in proteoglycans and type II collagen. On gene expression level, time-dependent expression patterns was observed. The induction of genes associated with cartilage-specific ECM (COL2A1 and COL11A1) and developmental signaling (GDF5, GDF10, ID1, ID4 and FGFR1-3) indicated redifferentiation within the first 3 weeks. The repression of genes related to stress response (HSPA1A and HSPA4), apoptotic events (HYOU1, NFKBIA and TRAF1), and degradation (MMP1, MMP10 and MMP12) suggested a recovery of chondrocytes. Constant expression of other chondrogenic (ACAN, FN1 and MGP) and hypertrophic markers (COL10A1, ALPL, PTHR1 and PTHR2) indicated a pattern of phenotypic maintenance. Simultaneously, the expression of chondrogenic growth (BMP6, TGFA, FGF1 and FGF2) and transcription factors (SOX9, EGR1, HES1 and TGIF1), and other cartilage ECM-related genes (COMP and PRG4) was consistently repressed and expression of collagens related to dedifferentiation (COL1A1 and COL3A1) was steadily induced indicating a progressing loss of cartilage phenotype. Likewise, a steady increase of genes associated with proliferation (GAS6, SERPINF1, VEGFB and VEGFC) and apoptosis (DRAM, DPAK1, HSPB, GPX1, NGFRAP1 and TIA1) was observed. Sequence and interplay of identified expression patterns suggest that chondrocyte micromass cultures maintain a differentiated phenotype up to 3 weeks in vitro and might be useful for studying chondrocyte biology, pathophysiology and differentiation. Cultivation longer than 6 weeks leads to progressing dedifferentiation of chondrocytes that should be considered on long-term evaluations.

Osteochondritis dissecans (OCD), an endoplasmic reticulum storage disease?: a morphological and molecular study of OCD fragments

Osteochondritis dissecans (OCD) fragments, cartilage and blood from four patients were used for morphological and molecular analysis. Controls included articular cartilage and blood samples from healthy individuals. Light microscopy and transmission electron microscopy (TEM) showed abnormalities in chondrocytes and extracellular matrix of cartilage from OCD patients. Abnormal type II collagen heterofibrils in "bundles" and chondrocytes with abnormal accumulation of matrix proteins in distended rough endoplasmic reticulum were typical findings. Further, Von Kossa staining and TEM showed empty lacunae close to mineralized "islands" in the cartilage and hypertrophic chondrocytes containing accumulated matrix proteins. Immunostaining revealed: (1) that types I, II, VI and X collagens and aggrecans were deposited intracellulary and (2) co-localization within the islands of types I, II, X collagens and aggrecan indicating that hypertrophic chondrocytes express a phenotype of bone cells during endochondral ossification. Types I, VI and X collagens were also present across the entire dissecates suggesting that chondrocytes were dedifferentiated. DNA sequencings were non-conclusive, only single nucleotide polymorphism was found within the COL2A1 gene for one patient. We suggest that OCD lesions are caused by an alteration in chondrocyte matrix synthesis causing an endoplasmic reticulum storage disease phenotype, which disturbs or abrupts endochondral ossification.

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.

Comprehensive profiling of cartilage extracellular matrix formation and maturation using sequential extraction and label-free quantitative proteomics

Articular cartilage is indispensable for joint function but has limited capacity for self-repair. Engineering of neocartilage in vitro is therefore a major target for autologous cartilage repair in arthritis. Previous analysis of neocartilage has targeted cellular organization and specific molecular components. However, the complexity of extracellular matrix (ECM) development in neocartilage has not been investigated by proteomics. To redress this, we developed a mouse neocartilage culture system that produces a cartilaginous ECM. Differential analysis of the tissue proteome of 3-week neocartilage and 3-day postnatal mouse cartilage using solubility-based protein fractionation targeted components involved in neocartilage development, including ECM maturation. Initially, SDS-PAGE analysis of sequential extracts revealed the transition in protein solubility from a high proportion of readily soluble (NaCl-extracted) proteins in juvenile cartilage to a high proportion of poorly soluble (guanidine hydrochloride-extracted) proteins in neocartilage. Label-free quantitative mass spectrometry (LTQ-Orbitrap) and statistical analysis were then used to filter three significant protein groups: proteins enriched according to extraction condition, proteins differentially abundant between juvenile cartilage and neocartilage, and proteins with differential solubility properties between the two tissue types. Classification of proteins differentially abundant between NaCl and guanidine hydrochloride extracts (n = 403) using bioinformatics revealed effective partitioning of readily soluble components from subunits of larger protein complexes. Proteins significantly enriched in neocartilage (n = 78) included proteins previously not reported or with unknown function in cartilage (integrin-binding protein DEL1; coiled-coil domain-containing protein 80; emilin-1 and pigment epithelium derived factor). Proteins with differential extractability between juvenile cartilage and neocartilage included ECM components (nidogen-2, perlecan, collagen VI, matrilin-3, tenascin and thrombospondin-1), and the relationship between protein extractability and ECM ultrastructural organization was supported by electron microscopy. Additionally, one guanidine extract-specific neocartilage protein, protease nexin-1, was confirmed by immunohistochemistry as a novel component of developing articular cartilage in vivo. The extraction profile and matrix-associated immunostaining implicates protease nexin-1 in cartilage development in vitro and in vivo.

Regulation of the chondrogenic phenotype in culture

In recent years, there has been a great deal of interest in the development of regenerative approaches to produce hyaline cartilage ex vivo that can be utilized for the repair or replacement of damaged or diseased tissue. It is clinically imperative that cartilage engineered in vitro mimics the molecular composition and organization of and exhibits biomechanical properties similar to persistent hyaline cartilage in vivo. Experimentally, much of our current knowledge pertaining to the regulation of cartilage formation, or chondrogenesis, has been acquired in vitro utilizing high-density cultures of undifferentiated chondroprogenitor cells stimulated to differentiate into chondrocytes. In this review, we describe the extracellular matrix molecules, nuclear transcription factors, cytoplasmic protein kinases, cytoskeletal components, and plasma membrane receptors that characterize cells undergoing chondrogenesis in vitro and regulate the progression of these cells through the chondrogenic differentiation program. We also provide an extensive list of growth factors and other extracellular signaling molecules, as well as chromatin remodeling proteins such as histone deacetylases, known to regulate chondrogenic differentiation in culture. In addition, we selectively highlight experiments that demonstrate how an understanding of normal hyaline cartilage formation can lead to the development of novel cartilage tissue engineering strategies. Finally, we present directions for future studies that may yield information applicable to the in vitro generation of hyaline cartilage that more closely resembles native tissue.

Characterization of the six zebrafish clade B fibrillar procollagen genes, with evidence for evolutionarily conserved alternative splicing within the pro-alpha1(V) C-propeptide

Genes for tetrapod fibrillar procollagen chains can be divided into two clades, A and B, based on sequence homologies and differences in protein domain and gene structures. Although the major fibrillar collagen types I-III comprise only clade A chains, the minor fibrillar collagen types V and XI comprise both clade A chains and the clade B chains pro-alpha1(V), pro-alpha3(V), pro-alpha1(XI) and pro-alpha2(XI), in which defects can underlie various genetic connective tissue disorders. Here we characterize the clade B procollagen chains of zebrafish. We demonstrate that in contrast to the four tetrapod clade B chains, zebrafish have six clade B chains, designated here as pro-alpha1(V), pro-alpha3(V)a and b, pro-alpha1(XI)a and b, and pro-alpha2(XI), based on synteny, sequence homologies, and features of protein domain and gene structures. Spatiotemporal expression patterns are described, as are conserved and non-conserved features that provide insights into the function and evolution of the clade B chain types. Such features include differential alternative splicing of NH(2)-terminal globular sequences and the first case of a non-triple helical imperfection in the COL1 domain of a clade B, or clade A, fibrillar procollagen chain. Evidence is also provided for previously unknown and evolutionarily conserved alternative splicing within the pro-alpha1(V) C-propeptide, which may affect selectivity of collagen type V/XI chain associations in species ranging from zebrafish to human. Data presented herein provide insights into the nature of clade B procollagen chains and should facilitate their study in the zebrafish model system.

Matrilin-3 activates the expression of osteoarthritis-associated genes in primary human chondrocytes

Here, we tested the matrilin-3-dependent induction of osteoarthritis-associated genes in primary human chondrocytes. Matrilin stimulation leads to the induction of MMP1, MMP3, MMP13, COX-2, iNOS, IL-1beta, TNFalpha, IL-6 and IL-8. Furthermore, we show the participation of ADAMTS4 and ADAMTS5 in the in vitro degradation of matrilin-3. We provide evidence for a matrilin-3-dependent feed-forward mechanism of matrix degradation, whereby proteolytically-released matrilin-3 induces pro-inflammatory cytokines as well as ADAMTS4 and -5 indirectly via IL-1beta. ADAMTS4 and ADAMTS5, in turn, cleave matrilin-3 and may release more matrilin-3 from the matrix, which could lead to further release of pro-inflammatory cytokines and proteases in cartilage.

Chondrogenic differentiation potential of osteoarthritic chondrocytes and their possible use in matrix-associated autologous chondrocyte transplantation

Introduction

Autologous chondrocyte transplantation (ACT) is a routine technique to regenerate focal cartilage lesions. However, patients with osteoarthritis (OA) are lacking an appropriate long-lasting treatment alternative, partly since it is not known if chondrocytes from OA patients have the same chondrogenic differentiation potential as chondrocytes from donors not affected by OA.

Methods

Articular chondrocytes from patients with OA undergoing total knee replacement (Mankin Score > 3, Ahlbäck Score > 2) and from patients undergoing ACT, here referred to as normal donors (ND), were isolated applying protocols used for ACT. Their chondrogenic differentiation potential was evaluated both in high-density pellet and scaffold (Hyaff-11) cultures by histological proteoglycan assessment (Bern Score) and immunohistochemistry for collagen types I and II. Chondrocytes cultured in monolayer and scaffolds were subjected to gene expression profiling using genome-wide oligonucleotide microarrays. Expression data were verified by using real-time PCR.

Results

Chondrocytes from ND and OA donors demonstrated accumulation of comparable amounts of cartilage matrix components, including sulphated proteoglycans and collagen types I and II. The mRNA expression of cartilage markers (ACAN, COL2A1, COMP, CRTL1, SOX9) and genes involved in matrix synthesis (BGN, CILP2, COL9A2, COL11A1, TIMP4) was highly induced in 3D cultures of chondrocytes from both donor groups. Genes associated with hypertrophic or OA cartilage (ALPL, COL1A1, COL3A1, COL10A1, MMP13, POSTN, PTH1R, RUNX2) were not significantly regulated between the two groups of donors. The expression of 661 genes, including COMP, FN1, and SOX9, was differentially regulated between OA and ND chondrocytes cultured in monolayer. During scaffold culture, the differences diminished between the OA and ND chondrocytes, and only 184 genes were differentially regulated.

Conclusions

Only few genes were differentially expressed between OA and ND chondrocytes in Hyaff-11 culture. The risk of differentiation into hypertrophic cartilage does not seem to be increased for OA chondrocytes. Our findings suggest that the chondrogenic capacity is not significantly affected by OA, and OA chondrocytes fulfill the requirements for matrix-associated ACT.

Characterization of collagenous matrix assembly in a chondrocyte model system

Collagen is a major component of the newly synthesized pericellular microenvironment of chondrocytes. Collagen types II, IX, and XI are synthesized and assembled into higher ordered complexes by a mechanism in which type XI collagen plays a role in nucleation of new fibrils, and in limiting fibril diameter. This study utilizes a cell line derived from the Swarm rat chondrosarcoma that allows the accumulation and assembly of pericellular matrix. Immunofluorescence and atomic force microscopy were used to assess early intermediates of fibril formation. Results indicate that this cell line synthesizes and secretes chondrocyte-specific pericellular matrix molecules including types II, IX, and XI collagen and is suitable for the study of newly synthesized collagen matrix under the experimental conditions used. AFM data indicate that small fibrils or assemblies of microfibrils are detectable and may represent precursors of the approximately 20 nm thin fibrils reported in cartilage. Treatment with hyaluronidase indicates that the dimensions of the small fibrils may be dependent upon the presence of hyaluronan within the matrix. This study provides information on the composition and organization of the newly synthesized extracellular matrix that plays a role in establishing the material properties and performance of biological materials such as cartilage.

Craniofacial cartilage morphogenesis requires zebrafish col11a1 activity

The zebrafish ortholog of the human COL11A1 gene encoding the cartilage collagen XI proalpha1 chain was characterized to explore its function in developing zebrafish using the morpholino-based knockdown strategy. We showed that its expression in zebrafish is developmentally regulated. A low expression level was detected by real-time PCR during the early stages of development. At 24 hpf, a sharp peak of expression was observed. At that stage, in situ hybridization indicated that col11a1 transcripts are restricted to notochord. At 48 hpf, they were exclusively detected in the craniofacial skeleton, endoskeleton of pectoral fins and in otic vesicles. Collagen XI alpha1-deficient zebrafish embryos developed defects in craniofacial cartilage formation and in notochord morphology. Neural crest specification and mesenchymal condensation occurred normally in morpholino-injected embryos. Col11a1 depletion affected the spatial organization of chondrocytes, the shaping of cartilage elements, and the maturation of chondrocytes to hypertrophy. Knockdown of col11a1 in embryos stimulated the expression of the marker of chondrocyte differentiation col2a1, resulting in the deposit of abnormally thick and sparse fibrils in the cartilage extracellular matrix. The extracellular matrix organization of the perichordal sheath was also altered and led to notochord distortion. The data underscore the importance of collagen XI in the development of a functional cartilage matrix. Moreover, the defects observed in cartilage formation resemble those observed in human chondrodysplasia such as the Stickler/Marshall syndrome. Zebrafish represent a novel reliable vertebrate model for collagen XI collagenopathies.

Col11a2 deletion reveals the molecular basis for tectorial membrane mechanical anisotropy

The tectorial membrane (TM) has a significantly larger stiffness in the radial direction than other directions, a prominent mechanical anisotropy that is believed to be critical for the proper functioning of the cochlea. To determine the molecular basis of this anisotropy, we measured material properties of TMs from mice with a targeted deletion of Col11a2, which encodes for collagen XI. In light micrographs, the density of TM radial collagen fibers was lower in Col11a2 -/- mice than wild-types. Tone-evoked distortion product otoacoustic emission and auditory brainstem response measurements in Col11a2 -/- mice were reduced by 30-50 dB independent of frequency as compared with wild-types, showing that the sensitivity loss is cochlear in origin. Stress-strain measurements made using osmotic pressure revealed no significant dependence of TM bulk compressibility on the presence of collagen XI. Charge measurements made by placing the TM as an electrical conduit between two baths revealed no change in the density of charge affixed to the TM matrix in Col11a2 -/- mice. Measurements of mechanical shear impedance revealed a 5.5 +/- 0.8 dB decrease in radial shear impedance and a 3.3 +/- 0.3 dB decrease in longitudinal shear impedance resulting from the Col11a2 deletion. The ratio of radial to longitudinal shear impedance fell from 1.8 +/- 0.7 for TMs from wild-type mice to 1.0 +/- 0.1 for those from Col11a2 -/- mice. These results show that the organization of collagen into radial fibrils is responsible for the mechanical anisotropy of the TM. This anisotropy can be attributed to increased mechanical coupling provided by the collagen fibrils. Mechanisms by which changes in TM material properties may contribute to the threshold elevation in Col11a2 -/- mice are discussed.

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.