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

Collagen alpha1(XI) in normal and malignant breast tissue

Little is known about collagen XI expression in normal and malignant breast tissue. Tissue microarrays, constructed from 72 patients with breast carcinoma and matched normal tissue, were immunohistochemically stained with five antisera against isoform-specific regions of collagen alpha1(XI) N-terminal domain. Staining intensity was graded on a 0-3 scale in epithelial cytoplasm, stroma, and endothelial staining of the vasculature of each tissue core. The staining was compared to known pathologic parameters: age, tumor size, overall tumor grade, nuclear grade, tubule formation, mitotic counts, angiolymphatic invasion, node status, estrogen receptor status, progesterone receptor status, and HER-2/neu status. Estrogen and progesterone receptor status were used as a control for comparison. With antisera V1a and amino propeptide (Npp), stroma surrounding cancerous cells was found to have decreased collagen alpha1(XI) staining compared to stroma adjacent to normal epithelium (P=0.0006, P<0.0001). Collagen alpha1(XI) staining with V1a antiserum in cytoplasm of cancer cells demonstrated decreased intensity in metastasized primary tumors when compared to nonmetastasized primary tumors (P=0.009). Cytoplasmic staining with Npp antiserum in cancer demonstrated an inverse relationship to positive estrogen receptor status in cancer (P=0.012) and to progesterone receptor status (P=0.044). Stromal staining for Npp in cancerous tissue demonstrated an inverse relationship with tubule formation score (P=0.015). This is the first study to localize collagen XI within normal and malignant breast tissue. Collagen alpha1(XI) appears to be downregulated in stroma surrounding breast cancer. Detection of collagen XI in breast tissue may help predict women who have lymph node metastases.

Collagen fibrillogenesis: fibronectin, integrins, and minor collagens as organizers and nucleators

Collagens are triple helical proteins that occur in the extracellular matrix (ECM) and at the cell–ECM interface. There are more than 30 collagens and collagen-related proteins but the most abundant are collagens I and II that exist as D-periodic (where D = 67 nm) fibrils. The fibrils are of broad biomedical importance and have central roles in embryogenesis, arthritis, tissue repair, fibrosis, tumor invasion, and cardiovascular disease. Collagens I and II spontaneously form fibrils in vitro, which shows that collagen fibrillogenesis is a selfassembly process. However, the situation in vivo is not that simple; collagen I-containing fibrils do not form in the absence of fibronectin, fibronectin-binding and collagen-binding integrins, and collagen V. Likewise, the thin collagen II-containing fibrils in cartilage do not form in the absence of collagen XI. Thus, in vivo, cellular mechanisms are in place to control what is otherwise a protein self-assembly process. This review puts forward a working hypothesis for how fibronectin and integrins (the organizers) determine the site of fibril assembly, and collagens V and XI (the nucleators) initiate collagen fibrillogenesis.

Walking the interactome for prioritization of candidate disease genes

The identification of genes associated with hereditary disorders has contributed to improving medical care and to a better understanding of gene functions, interactions, and pathways. However, there are well over 1500 Mendelian disorders whose molecular basis remains unknown. At present, methods such as linkage analysis can identify the chromosomal region in which unknown disease genes are located, but the regions could contain up to hundreds of candidate genes. In this work, we present a method for prioritization of candidate genes by use of a global network distance measure, random walk analysis, for definition of similarities in protein-protein interaction networks. We tested our method on 110 disease-gene families with a total of 783 genes and achieved an area under the ROC curve of up to 98% on simulated linkage intervals of 100 genes surrounding the disease gene, significantly outperforming previous methods based on local distance measures. Our results not only provide an improved tool for positional-cloning projects but also add weight to the assumption that phenotypically similar diseases are associated with disturbances of subnetworks within the larger protein interactome that extend beyond the disease proteins themselves.

Gene expression patterns underlying proximal-distal skeletal segmentation in late-stage zebrafish, Danio rerio

Timing and pattern of expression of ten candidate segmentation genes or gene pairs were reviewed or examined in developing median fins of late-stage zebrafish, Danio rerio. We found a general correspondence in timing and pattern of expression between zebrafish fin radial segmentation and tetrapod joint development, suggesting that molecular mechanisms underlying radial segmentation have been conserved over 400 million years of evolution. Gene co-expression during segmentation (5.5-6.5 mm SL) is similar between tetrapods and zebrafish: bmp2b, bmp4, chordin, and gdf5 in interradial mesenchyme and ZS; bapx1, col2a1, noggin3, and sox9a in chondrocytes. Surprisingly, wnt9a is not expressed in the developing median fins, though wnt9b is detected. In contrast to all other candidate segmentation genes we examined, bapx1 is not expressed in the caudal fin, which does not segment. Together, these data suggest a scenario of gene interactions underlying radial segmentation based on the patterns and timing of candidate gene expression.

Modification of the composition of articular cartilage collagen fibrils with increasing age

Recent studies have identified a range of interactions between type IX collagen and other cartilage matrix components. To determine the extent to which these interactions are important in maintaining the integrity of ageing articular cartilage, we analyzed an age range of normal healthy articular cartilage samples by Western blotting, immunohistochemical, and PCR analyses. Reduced levels of type IX collagen were detected in post adolescence cartilage. Type IX collagen epitopes were evident throughout the matrix in all cartilage samples up to 19 years of age. Post adolescence, however, the pattern of immunoreactivity revealed territorial staining only. Type IX collagen expression at the transcriptional level is maintained at all ages. Type IX collagen fragments were extracted from young tissue, supporting the hypothesis that young cartilage is continually remodelled, while mature cartilage maintains relatively low levels of collagen turnover. Clearly the age changes we observed may have significant effects on the integrity of the tissue as the chondrocytes in ageing articular cartilage have limited capacity to turnover the interterritorial matrix. However, this study provides evidence that even in old age, the chondrocyte attempts to maintain its pericellular environment and hence its mechanical role. Therefore, the potential of type IX collagen to interact with other matrix components continues to be of importance in the territorial environment, and these interactions may have significant roles in mechanotransduction.

Immunohistochemical localization of collagen type XI alpha1 and alpha2 chains in human colon tissue

In previous studies, collagen XI mRNA has been detected in colon cancer, but its location in human colon tissue has not been determined. The heterotrimeric collagen XI consists of three alpha chains. While it is known that collagen XI plays a regulatory role in collagen fibril formation, its function in the colon is unknown. The characterization of normal human colon tissue will allow a better understanding of the variance of collagen XI in abnormal tissues. Grossly normal and malignant human colon tissue was obtained from pathology archives. Immunohistochemical staining with a 58K Golgi marker and alpha1(XI) and alpha2(XI) antisera was used to specifically locate their presence in normal colon tissue. A comparative bright field microscopic analysis showed the presence of collagen XI in human colon. The juxtanuclear, dot-like collagen XI staining in the Golgi apparatus of goblet cells in normal tissue paralleled the staining of the 58K Golgi marker. Ultra light microscopy verified these results. Staining was also confirmed in malignant colon tissue. This study is the first to show that collagen XI is present in the Golgi apparatus of normal human colon goblet cells and localizes collagen XI in both normal and malignant tissue. Although the function of collagen XI in the colon is unknown, our immunohistochemical characterization provides the foundation for future immunohistopathology studies of the colon.

Type XXVII collagen at the transition of cartilage to bone during skeletogenesis

COL27A1 is a member of the collagen fibrillar gene family and is expressed in cartilaginous tissues including the anlage of endochondral bone. To begin to understand its role in skeletogenesis, the temporospatial distributions of its RNA message and protein product, type XXVII collagen, were determined in developing human skeletal tissues. Laser capture microdissection and quantitative reverse-transcription polymerase chain reaction demonstrated that gene expression occurred throughout the growth plate and that it was higher in the resting and proliferative zones than in hypertrophic cartilage. Immunohistochemical analyses showed that type XXVII collagen was most evident in hypertrophic cartilage at the primary ossification center and at the growth plate and that it accumulated in the pericellular matrix. Synthesis of type XXVII collagen overlapped partly with that of type X collagen, a marker of chondrocyte hypertrophy, preceded the transition of cartilage to bone, and was associated with cartilage calcification. Immunogold electron microscopy of extracted ECM components from mouse growth plate showed that type XXVII collagen was a component of long non-banded fibrous structures, filamentous networks, and thin banded fibrils. The timing and location of synthesis suggest that type XXVII collagen plays a role during the calcification of cartilage and the transition of cartilage to bone.

Soluble signalling factors derived from differentiated cartilage tissue affect chondrogenic differentiation of rat adult marrow stromal cells

BACKGROUND

Chondral defects show lack of proper regeneration whereas osteochondral lesions display limited regeneration capacity. Latter is probably due to immigration of chondroprogenitor cells from the subchondral bone. Known chondroprogenitor cells for cartilage tissues are multi-potent adult marrow stromal or mesenchymal stem cells (MSCs). In vitro chondrogenic differentiation of these precursor cells usually require cues from growth and signalling factors provided in vivo by surrounding tissues and cells. We hypothesise that signalling factors secreted by differentiated cartilage tissue can initiate and maintain chondrogenic differentiation status of MSCs.

METHODS

To study such paracrine communication between allogenic rat articular cartilage and rat MSCs embedded in alginate beads a novel coculture system without addition of external growth factors has been established.

RESULTS

Impact of cartilage on differentiating MSCs was observed at two different time points. Firstly, sustained expression of Sox9 was observed at an early stage which indicated induction of chondrogenic differentiation. Secondly, late stage repression of collagen X indicated pre-hypertrophic arrest of differentiation. In the culture supernatant we have identified vascular endothelial growth factor alpha (VEGF-164 alpha), matrix metalloproteinase (MMP) -13 and tissue inhibitors of MMPs (TIMP-1 and TIMP-2) which could be traced back either to the cartilage explant or to the MSCs under the influence of cartilage.

CONCLUSION

The identified factors might be involved in regulation of collagen X gene and protein expression and therefore, may have an impact on the control and regulation of MSCs differentiation.

Collagen XI chain misassembly in cartilage of the chondrodysplasia (cho) mouse

Molecular mechanisms controlling the assembly of cartilage-specific types II, IX and XI collagens into a heteropolymeric network of uniformly thin, unbanded fibrils are not well understood, but collagen XI has been implicated. The present study on cartilage from the homozygous chondrodysplasia (cho/cho) mouse adds support to this concept. In the absence of alpha1(XI) collagen chains, thick, banded collagen fibrils are formed in the extracellular matrix of cho/cho cartilage. A functional knock-out of the type XI collagen molecule has been assumed. We have re-examined this at the protein level to see if, rather than a complete knock-out, alternative type XI chain assemblies were formed. Mass spectrometry of purified triple-helical collagen from the rib cartilage of cho/cho mice identified alpha1(V) and alpha2(XI) chains. These chains were recovered in roughly equal amounts based on Coomassie Blue staining of SDS-PAGE gels, in addition to alpha1(II)/alpha3(XI) collagen chains. Using telopeptide-specific antibodies and Western blot analysis, it was further shown that type V/XI trimers were present in the matrix cross-linked to each other and to type II collagen molecules to form heteropolymers. Cartilage from heterozygous (cho/+) mice contained a mix of alpha1(V) and alpha1(XI) chains and a mix of thin and thick fibrils on transmission electron microscopy. In summary, the results imply that native type XI collagen molecules containing an alpha1(XI) chain are required to form uniformly thin fibrils and support a role for type XI collagen as the template for the characteristic type II collagen fibril network of developing cartilage.

The bone morphogenetic protein 1/Tolloid-like metalloproteinases

A decade ago, bone morphogenetic protein 1 (BMP1) was shown to provide the activity necessary for proteolytic removal of the C-propeptides of procollagens I-III: precursors of the major fibrillar collagens. Subsequent studies have shown BMP1 to be the prototype of a small group of extracellular metalloproteinases that play manifold roles in regulating formation of the extracellular matrix (ECM). Soon after initial cloning of BMP1, genetic studies showed the related Drosophila proteinase Tolloid (TLD) to be necessary for the formation of the dorsal-ventral axis in early embryogenesis. It is now clear that the BMP1/TLD-like proteinases, conserved in species ranging from Drosophila to humans, act in dorsal-ventral patterning via activation of transforming growth factor beta (TGFbeta)-like proteins BMP2, BMP4 (vertebrates) and decapentaplegic (arthropods). More recently, it has become apparent that the BMP1/TLD-like proteinases are activators of a broader subset of the TGFbeta superfamily of proteins, with implications that these proteinases may be key in orchestrating the formation of ECM with growth factor activation and BMP signaling in morphogenetic processes.

A proteomic analysis of adult rat bone reveals the presence of cartilage/chondrocyte markers

The non-mineral component of bone matrix consists of 90% collagenous, 10% non-collagenous proteins. These proteins regulate mineralization, growth, cell signaling and differentiation, and provide bone with its tensile strength. Expression of bone matrix proteins have historically been studied individually or in small numbers owing to limitations in analytical technologies. Current mass-spectrometric and separations technologies allow a global view of protein expression patterns in complex samples. To our knowledge, no proteome profile of bone matrix has yet been reported. Therefore, we have used mass spectrometry as a tool to generate a profile of proteins present in the extracellular matrix of adult rat bone. Overall, 108 and 25 proteins were identified with high confidence in the metaphysis and diaphysis, respectively, using a bottom up proteomic technique. Twenty-one of these proteins were present in both the metaphysis and diaphysis including the bone specific proteins, osteocalcin, type I collagen, osteopontin, osteoregulin, and bone sialoprotein. Interestingly, type II collagen, a protein thought to be exclusively expressed in cartilage, was identified in both the metaphysis and diaphysis. This observation was validated by Western blot. Additionally, the presence of aggrecan, another protein expressed in cartilage was identified in the bone matrix extracts by Western blot. The proteome profile generated using this technology represents an initial survey of the acid soluble proteins of bone matrix which provides a reference for the analysis of deviations from the normal composition due to perturbations or disease states.

ColVa1 and ColXIa1 are required for myocardial morphogenesis and heart valve development

Genetic mutations in minor fibrillar collagen types Va1 (ColVa1) and XIa1 (ColXI) have been identified in connective tissue disorders including Ehlers-Danlos syndrome and chondrodysplasias. ColVa1+/- and ColXIa1-/- mutant mice recapitulate these human disorders and show aberrations in collagen fiber organization in connective tissue of the skin, cornea, cartilage, and tendon. In the heart, fibrous networks of collagen fibers form throughout the ventricular myocardium and heart valves, and alterations in collagen fiber homeostasis are apparent in many forms of cardiac disease associated with myocardial dysfunction and valvular insufficiency. There is increasing evidence for cardiac dysfunction in connective tissue disorders, but the mechanisms have not been addressed. ColVa1+/- and ColXIa1-/- mutant mice were used to identify roles for ColVa1 and ColXIa1 in ventricular myocardial morphogenesis and heart valve development. These affected cardiac structures show a compensatory increase in type I collagen deposition, similar to that previously described in valvular and cardiomyopathic disease. Morphological cardiac defects associated with changes in collagen fiber homeostasis identified in ColVa1+/- and ColXIa1-/- mice provide an insight into previously unappreciated forms of cardiac dysfunction associated with connective tissue disorders.

Collagen IX is indispensable for timely maturation of cartilage during fracture repair in mice

Fracture repair recapitulates in adult organisms the sequence of cell biological events of endochondral ossification during skeletal development and growth. After initial inflammation and deposition of granulation tissue, a cartilaginous callus is formed which, subsequently, is remodeled into bone. In part, bone formation is influenced also by the properties of the extracellular matrix of the cartilaginous callus. Deletion of individual macromolecular components can alter extracellular matrix suprastructures, and hence stability and organization of mesenchymal tissues. Here, we took advantage of the collagen IX knockout mouse model to better understand the role of this collagen for organization, differentiation and maturation of a cartilaginous template during formation of new bone. Although a seemingly crucial component of cartilage fibrils is missing, collagen IX-deficient mice develop normally, but are predisposed to premature joint cartilage degeneration. However, we show here that lack of collagen IX alters the time course of callus differentiation during bone fracture healing. The maturation of cartilage matrix was delayed in collagen IX-deficient mice calli as judged by collagen X expression during the repair phase and the total amount of cartilage matrix was reduced. Entering the remodeling phase of fracture healing, Col9a1(-/-) calli retained a larger percentage of cartilage matrix than in wild type indicating also a delayed formation of new bone. We concluded that endochondral bone formation can occur in collagen IX knockout mice but is impaired under conditions of stress, such as the repair of an unfixed fractured long bone.

Cell instructive polymers

Polymeric materials used in tissue engineering were initially used solely as delivery vehicles for transplanting cells. However, these materials are currently designed to actively regulate the resultant tissue structure and function. This control is achieved through spatial and temporal regulation of various cues (e.g., adhesion ligands, growth factors) provided to interacting cells from the material. These polymeric materials that control cell function and tissue formation are termed cell instructive polymers, and recent trends in their design are outlined in this chapter.

The 10+4 microfibril structure of thin cartilage fibrils

Determining the structure of cartilage collagen fibrils will provide insights into how mutations in collagen genes affect cartilage formation during skeletal morphogenesis and understanding the mechanism of fibril growth. The fibrils are indeterminate in size, heteropolymeric, and highly cross-linked, which make them refractory to analysis by conventional high-resolution structure determination techniques. Electron microscopy has been limited to making simple measurements of fibril diameter and immunolocalizing certain molecules at the fibril surface. Consequently, structural information on the fibrils is limited. In this study we have used scanning transmission electron microscopic mass mapping, analysis of axial stain exclusion pattern, and r-weighted back-projection techniques to determine the intermediate resolution (to approximately 4 nm) structure of thin collagen fibrils from embryonic cartilage. The analyses show that the fibrils are constructed from a 10+4 microfibrillar arrangement in which a core of four microfibrils is surrounded by a ring of 10 microfibrils. Accurate mass measurements predict that each microfibril contains five collagen molecules in cross-section. Based on the proportion of collagen II, IX, and XI in the fibrils, the fibril core comprises two microfibrils each of collagen II and collagen XI. Single molecules of collagen IX presumably occur at the fibril surface between the extended N-terminal domains of collagen XI. The 10+4 microfibril structure explains the mechanism of diameter limitation in the narrow fibrils and the absence of narrow collagen fibrils in cartilage lacking collagen XI.

Gene expression in human keloids is altered from dermal to chondrocytic and osteogenic lineage

Keloids are a dermal fibrotic disease whose etiology remains totally unknown and for which there is no successful treatment. Here, we employed cDNA microarray analysis to examine gene expression in keloid lesions and control skin. We found that 32 genes among the 9000 tested were strongly up-regulated in keloid lesions, of which 21 were confirmed by Northern blotting. These included at least seven chondrocyte/osteoblast marker genes, and RT-PCR analysis revealed that transcription factors specific for these genes, SOX9 and CBFA1, were induced. Immunostaining and in situ hybridization further supported that these markers are expressed in keloid lesions. Intriguingly, scleraxis, a transcription factor known as a marker of tendons and ligaments, was also induced in keloid fibroblasts. We propose that reprogramming of gene expression or disordered differentiation from a dermal pattern to that of a chondrocytic/osteogenic lineage, probably closer to that of tendon/ligament lineage, may be involved in the etiology of keloids.

A new autosomal recessive form of Stickler syndrome is caused by a mutation in the COL9A1 gene

Stickler syndrome is characterized by ophthalmic, articular, orofacial, and auditory manifestations. It has an autosomal dominant inheritance pattern and is caused by mutations in COL2A1, COL11A1, and COL11A2. We describe a family of Moroccan origin that consists of four children with Stickler syndrome, six unaffected children, and two unaffected parents who are distant relatives (fifth degree). All family members were clinically investigated for ear, nose, and throat; ophthalmologic; and radiological abnormalities. Four children showed symptoms characteristic of Stickler syndrome, including moderate-to-severe sensorineural hearing loss, moderate-to-high myopia with vitreoretinopathy, and epiphyseal dysplasia. We considered the COL9A1 gene, located on chromosome 6q13, to be a candidate gene on the basis of the structural association with collagen types II and XI and because of the high expression in the human inner ear indicated by cDNA microarray. Mutation analysis of the coding region of the COL9A1 gene showed a homozygous R295X mutation in the four affected children. The parents and four unaffected children were heterozygous carriers of the R295X mutation. Two unaffected children were homozygous for the wild-type allele. None of the family members except the homozygous R295X carriers had any signs of Stickler syndrome. Therefore, COL9A1 is the fourth identified gene that can cause Stickler syndrome. In contrast to the three previously reported Stickler syndrome-causing genes, this gene causes a form of Stickler syndrome with an autosomal recessive inheritance pattern. This finding will have a major impact on the genetic counseling of patients with Stickler syndrome and on the understanding of the pathophysiology of collagens. Mutation analysis of this gene is recommended in patients with Stickler syndrome with possible autosomal recessive inheritance.

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.

Influence of 17beta-estradiol and insulin on type II collagen and protein synthesis of articular chondrocytes

Clinical observations have suggested a relationship between osteoarthritis and a changed estrogen metabolism in menopausal women. Type II collagen is one main structural protein of articular cartilage matrix and its synthesis is increased by insulin in growth plate cartilage. Therefore, it was investigated if [(3)H]-proline incorporation and type II collagen synthesis (immunocytochemistry, ELISA) in female bovine articular chondrocytes are affected by 17beta-estradiol and/or insulin. Articular chondrocytes were cultured in monolayers at 5% O(2) in medium containing serum for 5-9 days, followed by application of 10(-13) to 10(-9) M estradiol or 5 microg/ml insulin during a serum-free culture phase of 2-3 days. Immunostaining for type II collagen was strong in the serum-free culture phase whereas it was negative for type I collagen, indicating that cells did not dedifferentiate to fibroblast-like cells during culture in serum-free medium. Whereas insulin raised the proline incorporation and the type II collagen synthesis significantly, physiological doses of estradiol did not show significant effects. The stimulating effect of insulin on the [(3)H]-proline incorporation or the type II collagen synthesis was significantly suppressed after preincubation of cells with 10(-11) to 10(-9) M estradiol resembling an unfavorable effect for articular cartilage. The suppression was reversed if cells were incubated with 10(-11) to 10(-7) M tamoxifen or ICI 182,780 combined with 10(-11) or 10(-9) M estradiol followed by incubation with 5 microg/ml insulin, indicating an estrogen receptor-mediated process. Because the articular cartilage of diabetic patients is biomechanically less stable, further experiments are needed to clarify the role of estradiol and insulin in the metabolism of articular chondrocytes.

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.

Sp1 family of transcription factors regulates the human alpha2 (XI) collagen gene (COL11A2) in Saos-2 osteoblastic cells

Genes encoding type XI collagen, normally associated with chondrogenesis, are also expressed by osteoblasts. By studying Saos-2 cells, we showed that the transcription factors, Sp1, Sp3, and Sp7 (Osterix), regulate COL11A2 expression through its proximal promoter. The findings indicate both ubiquitous and osteoblast-specific mechanisms of collagen gene regulation.

INTRODUCTION

Type XI collagen is essential for skeletal morphogenesis. Collagen XI gene regulation has been studied in chondrocytes but not in osteoblasts.

MATERIALS AND METHODS

We cultured Saos-2 cells, a human osteosarcoma-derived line of osteoblasts, and analyzed them for alpha2(XI) protein and COL11A2 regulatory mechanisms.

RESULTS AND CONCLUSIONS

Although types I and V were the dominant collagens deposited by Saos-2 cells, they expressed COL11A2 mRNA, and alpha2(XI) chains were present in the extracellular matrix. The COL11A2 promoter region (from -149 to -40) containing three Sp1 binding sites was required for promoter activity in transient transfection assays. All three Sp1 sites were critical for binding by nuclear proteins in electrophoretic mobility shift assays. Further analysis using consensus oligonucleotides and specific antibodies as well as chromatin immunoprecipitation assay implicated Sp1 and Sp3 in binding to this promoter region. Overexpressing Sp1 or Sp3 significantly increased COL11A2 promoter activity and endogenous COL11A2 gene expression, an effect that was suppressed by the Sp1-binding inhibitor mithramycin A. Further experiments showed that Sp1, Sp3, CREB-binding protein (CBP), p300, and histone deacetylase (HDAC) were physically associated and HDAC inhibitors (trichostatin A or NaB) upregulated COL11A2 promoter activity and endogenous gene expression. Another Sp1 family member, Sp7 (Osterix), was expressed in Saos-2 cells, but not in chondrocytes, and was shown by chromatin immunoprecipitation to occupy the COL11A2 promoter. Overexpressing Sp7 increased COL11A2 promoter activity and endogenous gene expression, an effect also blocked by mithramycin A. Using siRNA to knockdown Sp1, Sp3, or Sp7, it was shown that depression of any of them decreased COL11A2 promoter activity and endogenous gene expression. Finally, primary cultures of osteoblasts expressed COL11A2 and Sp7, upregulated COL11A2 promoter activity and endogenous gene expression when Sp1, Sp3, or Sp7 were overexpressed, and downregulated them when Sp1, Sp3, or Sp7 were selectively depressed. The results establish that Sp1 proteins regulate COL11A2 transcription by binding to its proximal promoter and directly interacting with CBP, p300, and HDAC.

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.

An introductory review of cell mechanobiology

Mechanical loads induce changes in the structure, composition, and function of living tissues. Cells in tissues are responsible for these changes, which cause physiological or pathological alterations in the extracellular matrix (ECM). This article provides an introductory review of the mechanobiology of load-sensitive cells in vivo, which include fibroblasts, chondrocytes, osteoblasts, endothelial cells, and smooth muscle cells. Many studies have shown that mechanical loads affect diverse cellular functions, such as cell proliferation, ECM gene and protein expression, and the production of soluble factors. Major cellular components involved in the mechanotransduction mechanisms include the cytoskeleton, integrins, G proteins, receptor tyrosine kinases, mitogen-activated protein kinases, and stretch-activated ion channels. Future research in the area of cell mechanobiology will require novel experimental and theoretical methodologies to determine the type and magnitude of the forces experienced at the cellular and sub-cellular levels and to identify the force sensors/receptors that initiate the cascade of cellular and molecular events.

Collagen types I, III, and V constitute the thick collagen fibrils of the mouse decidua

A mammal's endometrium is deeply remodeled while receiving and implanting an embryo. In addition to cell proliferation and growth, endometrial remodeling also comprises synthesis and degradation of several molecular components of the extracellular matrix. All of these events are orchestrated by a precise sequence of ovarian hormones and influenced by several types of cytokines. As we have previously reported, an intriguing and rapid increase in collagen fibril diameter occurs in the decidualized areas of the endometrium, surrounding the implantation crypt, whereas collagen fibrils situated far from the embryo remain unchanged. Collagen fibrilogenesis is a complex molecular process coordinated by a number of factors, such as the types and amounts of glycosaminoglycans and proteoglycans associated with collagen molecules. Collagen genetic type, mechanical stress, aging, and other factors not yet identified also contribute to this development. A recent study suggests that thick fibrils from mouse decidua are formed, at least in part, by aggregation of thin fibrils existing in the stroma before the onset of decidualization. In the present ultrastructural study using single and double immunogold localization, we showed that both thin and thick collagen fibrils present in the mouse pregnant endometrium endometrium are heterotypic structures formed at least by type I, type III, and type V collagens. However, type V collagen predominates in the thick collagen fibrils, whereas it is almost absent of the thin collagen fibrils. The putative role of type V homotrimer in the rapid increase of the diameter of collagen fibrils of the mouse decidua is discussed.

Arrangement and fine structure of collagen fibrils in the decidualized mouse endometrium

The adaptations of the mouse uterus to pregnancy include extensive modifications of the cells and extracellular matrix of the endometrial connective tissue that surround the embryos. Around each implanted embryo this tissue redifferentiates into a transient structure called decidua, which is formed by polygonal cells joined by intercellular junctions. In the mouse, thick collagen fibrils with irregular profile appear in decidualized areas of the endometrium but not in the nondecidualized stroma and interimplantation sites. The fine organization of these thick fibrils has not yet been established. This work was addressed to understand the arrangement and fine structure of collagen fibrils of the decidua of pregnant mice during the periimplantation stage. Major modifications occurred in collagen fibrils that surrounded decidual cells: (1) the fibrils, which were arranged in parallel bundles in nonpregnant animals, became organized as baskets around decidual cells; (2) very thick collagen fibrils with very irregular profiles appeared around decidual cells. Analysis of replicas and serial sections suggests that the thick collagen fibrils form by the lateral aggregation of thinner fibrils to a central fibril resulting in very irregular profile observed in cross sections of thick fibrils. The sum of modifications of the collagen fibrils seem to represent an adaptation of the endometrium to better support the decidual cells while they hold the embryos during the beginning of their development. The deposition of thick collagen fibrils in the decidua may contribute to form a barrier that impedes leukocyte migration within the decidua, preventing immunological rejection of genetically dissimilar embryonic tissues.

Developmental roles of the BMP1/TLD metalloproteinases

The astacin family (M12A) of the metzincin subclan MA(M) of metalloproteinases has been detected in developing and mature individuals of species that range from hydra to humans. Functions of this family of metalloproteinase vary from digestive degradation of polypeptides, to biosynthetic processing of extracellular proteins, to activation of growth factors. This review will focus on a small subgroup of the astacin family; the bone morphogenetic protein 1 (BMP1)/Tolloid (TLD)-like metalloproteinases. In vertebrates, the BMP1/TLD-like metalloproteinases play key roles in regulating formation of the extracellular matrix (ECM) via biosynthetic processing of various precursor proteins into mature functional enzymes, structural proteins, and proteins involved in initiating mineralization of the ECM of hard tissues. Roles in ECM formation include: processing of the C-propeptides of procollagens types I-III, to yield the major fibrous components of vertebrate ECM; proteolytic activation of the enzyme lysyl oxidase, necessary to formation of covalent cross-links in collagen and elastic fibers; processing of NH2-terminal globular domains and C-propeptides of types V and XI procollagen chains to yield monomers that are incorporated into and control the diameters of collagen type I and II fibrils, respectively; processing of precursors for laminin 5 and collagen type VII, both of which are involved in securing epidermis to underlying dermis; and maturation of small leucine-rich proteoglycans. The BMP1/TLD-related metalloproteinases are also capable of activating the vertebrate transforming growth factor-beta (TGF-beta)-like "chalones" growth differentiation factor 8 (GDF8, also known as myostatin), and GDF11 (also known as BMP11), involved in negative feedback inhibition of muscle and neural tissue growth, respectively; by freeing them from noncovalent latent complexes with their cleaved prodomains. BMP1/TLD-like proteinases also liberate the vertebrate TGF-beta-like morphogens BMP2 and 4 and their invertebrate ortholog decapentaplegic, from latent complexes with the vertebrate extracellular antagonist chordin and its invertebrate ortholog short gastrulation (SOG), respectively. The result is formation of the BMP signaling gradients that form the dorsal-ventral axis in embryogenesis. Thus, BMP1/TLD-like proteinases appear to be key to regulating and orchestrating formation of the ECM and signaling by various TGF-beta-like proteins in morphogenetic and homeostatic events.

Altered integration of matrilin-3 into cartilage extracellular matrix in the absence of collagen IX

The matrilins are a family of four noncollagenous oligomeric extracellular matrix proteins with a modular structure. Matrilins can act as adapters which bridge different macromolecular networks. We therefore investigated the effect of collagen IX deficiency on matrilin-3 integration into cartilage tissues. Mice harboring a deleted Col9a1 gene lack synthesis of a functional protein and produce cartilage fibrils completely devoid of collagen IX. Newborn collagen IX knockout mice exhibited significantly decreased matrilin-3 and cartilage oligomeric matrix protein (COMP) signals, particularly in the cartilage primordium of vertebral bodies and ribs. In the absence of collagen IX, a substantial amount of matrilin-3 is released into the medium of cultured chondrocytes instead of being integrated into the cell layer as in wild-type and COMP-deficient cells. Gene expression of matrilin-3 is not affected in the absence of collagen IX, but protein extraction from cartilage is greatly facilitated. Matrilin-3 interacts with collagen IX-containing cartilage fibrils, while fibrils from collagen IX knockout mice lack matrilin-3, and COMP-deficient fibrils exhibit an intermediate integration. In summary, the integration of matrilin-3 into cartilage fibrils occurs both by a direct interaction with collagen IX and indirectly with COMP serving as an adapter. Matrilin-3 can be considered as an interface component, capable of interconnecting macromolecular networks and mediating interactions between cartilage fibrils and the extrafibrillar matrix.

Evidence for a role of the genomic region of the gene encoding for the α1 chain of type IX collagen (COL9A1) in hip osteoarthritis: A population-based study

OBJECTIVE

Type IX collagen proteoglycan is an important protein in collagen networks and has been implicated in hip osteoarthritis (OA). We studied 2 COL9A1 markers (509-8B2 and 509-12B1) in relation to radiographic OA, within the framework of the Rotterdam Study, a population-based study of 7,983 subjects ages 55 years and older.

METHODS

We used 2 different designs, as follows: 1) a linkage study of 83 probands with multiple joints affected with radiographic OA and their 221 siblings, yielding 445 sibpairs who participated in the study, and 2) an association study in a series of 71 patients with radiographic hip OA and 269 controls without radiographic OA. All subjects were characterized for the 2 COL9A1 markers, 509-8B2 and 509-12B1. The mean test was used to assess the proportion of alleles shared in concordantly affected and unaffected sibpairs. The chi-square test was used to compare the allele distributions in patients and controls.

RESULTS

Affected sibpairs with radiographic hip OA shared alleles identical by descent at markers 8B2 and 12B1 significantly more often than expected (mean +/- SD 0.66 +/- 0.07 and 0.65 +/- 0.08, respectively; P < 0.05). No excess sharing for radiographic OA was observed at other joint sites. When comparing the frequency of marker 8B2 and 12B1 alleles in subjects with radiographic OA and controls, the frequency of 8B2 alleles in subjects with radiographic OA differed significantly(P = 0.01) from that in controls.

CONCLUSION

Our data suggest that susceptibility for hip OA is conferred within or close to the COL9A1 gene in linkage disequilibrium with the COL9A1 509-8B2 marker.

Procollagen trafficking, processing and fibrillogenesis

Collagen fibrils in the extracellular matrix allow connective tissues such as tendon, skin and bone to withstand tensile forces. The fibrils are indeterminate in length, insoluble and form elaborate three-dimensional arrays that extend over numerous cell lengths. Studies of the molecular basis of collagen fibrillogenesis have provided insight into the trafficking of procollagen (the precursor of collagen) through the cellular secretory pathway, the conversion of procollagen to collagen by the procollagen metalloproteinases, and the directional deposition of fibrils involving the plasma membrane and late secretory pathway. Fibril-associated molecules are targeted to the surface of collagen fibrils, and these molecules play an important role in regulating the diameter and interactions between the fibrils.

Disease- and cell-type-specific transcriptional targeting of vectors for osteoarthritis gene therapy: further development of a clinical canine model

OBJECTIVES

The potential for undesirable systemic effects related to constitutive expression of certain therapeutic transgenes may be limited through the development of transcriptionally targeted disease- and cell-type-specific vectors. The objective of this study was to analyse the canine matrix metalloproteinase-9 (MMP-9) promoter and deletion constructs for its ability to drive expression in response to pro-inflammatory cytokines (interleukin-1beta and tumour necrosis factor-alpha).

METHODS

Initial analysis of MMP-9 deletion constructs was made using a luciferase reporter system. The promoter was subsequently engineered to incorporate multiple NF-kappaB sites. In parallel experiments we used the mouse collagen type XI promoter to study cell-type-specific promoter activity in chondrocyte-specific cells (SW1353) and undifferentiated chondroprogenitor cells (ATDC5).

RESULTS

Incorporation of multiple NF-kappaB sites into the MMP-9 promoter enhanced activity while maintaining disease specificity. Further, manipulation of the mouse collagen type XI (mColXI) promoter by the incorporation of SOX9 enhancer sites downstream of a reporter gene, increased gene activity while maintaining cell type specificity.

CONCLUSIONS

Manipulation of promoter and enhancer regions can improve transcriptionally targeted genes. A combination of these systems, in the context of the canine model, has the potential to improve the safety of osteoarthritis gene therapy vectors.

Comparative ultrastructure of collagen fibrils in uterine leiomyomas and normal myometrium

OBJECTIVE

To examine the ultrastructural characteristics of extracellular matrix and mature collagen fibrils in uterine leiomyomas and compare them with those in adjacent normal myometrium.

DESIGN

Analysis of paired leiomyoma-myometrium in surgical specimens.

SETTING

Research center and tertiary care center.

SUBJECT(S)

Women undergoing medically indicated hysterectomy for symptomatic uterine leiomyomas.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Appearance and spatial orientation of the collagen fibrils in leiomyomas compared with myometrium.

RESULT(S)

Observation of specimens at x 12,500 magnification indicated that collagen fibrils were more abundant, loosely packed, and arrayed in a nonparallel manner in leiomyomas compared with myometrium. Random areas were examined at x 6,500 to x 64,000 magnification and revealed collagen fibrils of equal diameter in both leiomyomas and myometrium. However, an ordered and regular barbed appearance was present in collagen fibrils from myometrium but was lacking in leiomyomas.

CONCLUSION(S)

Leiomyomas contain an abnormal collagen fibril structure and orientation, which suggests that the well-regulated fibril formation in myometrium is altered in leiomyomas. Alterations in collagen genes may play a role in the pathogenesis of leiomyomas.

Collagens and collagen-related matrix components in the human and mouse eye

The three-dimensional structure of the eye plays an important role in providing a correct optical environment for vision. Much of this function is dependent on the unique structural features of ocular connective tissue, especially of the collagen types and their supramolecular structures. For example, the organization of collagen fibrils is largely responsible for transparency and refraction of cornea, lens and vitreous body, and collagens present in the sclera are largely responsible for the structural strength of the eye. Phylogenetically, most of the collagens are highly conserved between different species, which suggests that collagens also share similar functions in mice and men. Despite considerable differences between the mouse and the human eye, particularly in the proportion of the different tissue components, the difficulty of performing systematic histologic and molecular studies on the human eye has made mouse an appealing alternative to studies addressing the role of individual genes and their mutations in ocular diseases. From a genetic standpoint, the mouse has major advantages over other experimental animals as its genome is better known than that of other species and it can be manipulated by the modern techniques of genetic engineering. Furthermore, it is easy, quick and relatively cheap to produce large quantities of mice for systematic studies. Thus, transgenic techniques have made it possible to study consequences of specific mutations in genes coding for structural components of ocular connective tissues in mice. As these changes in mice have been shown to resemble those in human diseases, mouse models are likely to provide efficient tools for pathogenetic studies on human disorders affecting the extracellular matrix. This review is aimed to clarify the role of collagenous components in the mouse and human eye with a closer look at the new findings of the collagens in the cartilage and the eye, the so-called "cartilage collagens".

The transcription factor CCAAT-binding factor CBF/NF-Y and two repressors regulate the core promoter of the human pro-alpha3(V) collagen gene (COL5A3)

To elucidate the mechanisms underlining alpha3(V) collagen chain expression, we performed an initial analysis of the structure and function of the core promoter of the human COL5A3 gene. The core promoter, which lacks a typical TATA motif and has a high GC content, was defined within the -129 bp immediately upstream from the major transcription start site by transient transfection experiments. In this region, we identified four DNA-protein complexes, named A, B, C, and D, by a combination of DNase I footprinting and electrophoretic mobility shift assays. Electrophoretic mobility shift assays using mutant oligonucleotide revealed that the complexes A, B, C, and D bind to -122 to -117, the -101 to -96, the -83 to -78, and the -68 to -57 bp, respectively. The competition assays using consensus oligonucleotides and supershift assays with specific antibodies showed that complex A consists of CBF/NF-Y. In a chromatin immunoprecipitation assay, CBF/NF-Y protein directly bound to this region, in vivo. Functional analysis showed that CBF/NF-Y activated the gene, whereas the proteins of complexes B and C repressed its activity. Furthermore, overexpression of a mutant form of the CBF-B/NF-YA subunit, which forms CBF/NF-Y with CBF-A/NF-YB and CBF-C/NF-YC subunits, inhibited promoter activity.

Biosynthetic Processing of the Pro-α1(V)Pro-α2(V)Pro-α3(V) Procollagen Heterotrimer

Type V collagen is a quantitatively minor fibrillar collagen comprised of different chain compositions in different tissues. The most widely distributed form, an alpha1(V)2alpha2(V) heterotrimer, regulates the physical properties of type I/V heterotypic collagen fibrils via partially processed NH2-terminal globular sequences. A less characterized alpha1(V)alpha2(V)alpha3(V) heterotrimer has a much more limited distribution of expression and unknown function(s). We characterized the biosynthetic processing of pro-alpha1(V)2pro-alpha2(V) procollagen previously and showed it to differ in important ways from biosynthetic processing of the major fibrillar procollagens I-III. Here we have successfully produced recombinant pro-alpha1(V)pro-alpha2(V)pro-alpha3(V) heterotrimers. We use these, and mouse embryo fibroblasts doubly homozygous null for the Bmp1 gene, which encodes the metalloproteinase bone morphogenetic protein-1 (BMP-1), and for a gene encoding the closely related metalloproteinase mammalian Tolloid-like 1, to characterize biosynthetic processing of pro-alpha1(V)pro-alpha2(V)pro-alpha3(V) heterotrimers, thus completing characterization of type V collagen biosynthetic processing. Whereas pro-alpha1(V) and pro-alpha2(V) processing in pro-alpha1(V)pro-alpha2(V)pro-alpha3(V) heterotrimers is similar to that which occurs in pro-alpha1(V)2pro-alpha2(V) heterotrimers, the processing of pro-alpha3(V) by BMP-1 occurs at an unexpected site within NH2-terminal globular sequences. We also demonstrate that, despite similarities in NH2-terminal domain structures, pro-alpha2(V) NH2-terminal globular sequences are not cleaved by ADAMTS-2, the metalloproteinase that cleaves the N-propeptides of the major fibrillar procollagen chains.

Molecular Structure and Interaction of Recombinant Human Type XVI Collagen

Collagen XVI is a minor component of at least two different extracellular fibrillar networks of specialized regions of skin and cartilage. In skin, collagen XVI is integrated into particular fibrillin-rich microfibrils lacking an amorphous elastin core. In cartilage, collagen XVI is a component of small heterotypic D-banded fibrils, mainly occurring in the territorial matrix of chondrocytes. Here, we present the first direct evidence for the molecular structure and functional properties of these fibril-associated collagens with interrupted triple helices (FACIT). We have expressed recombinantly the full-length alpha1 chain of human collagen XVI in HEK 293 EBNA cells in large quantities using an episomal expression system. Secreted full-length recombinant collagen XVI forms stable disulfide-bonded homotrimers and is rapidly proteolytically processed to distinct fragments at specific protease sequence motifs, one resembling an aggrecanase recognition site. Limited trypsin digestion assays and thermal transition curves imply sequential thermal denaturation of individual triple helical domains of this recombinant collagen, similar to authentic collagen XVI. Molecular images of collagen XVI reveal rod-like molecules which harbor multiple sharp kinks attributing a highly flexible structure presumably introduced by non-collagenous (NC) regions. Terminally located cloverleaf-shaped nodules correspond to the large NC NC11 domain of trimeric collagen XVI. The total length of individual trimeric recombinant collagen XVI molecules constitutes about 240 nm as calculated by atomic force and negative staining electron microscopy. Recombinant collagen XVI interacts with fibrillin-1 and with fibronectin indicating multiple molecular interactions in which this ubiquitously expressed and versatile FACIT-collagen can participate. In vitro generated collagen XVI provides an indispensable tool for future determination of its function during supramolecular assembly of matrix aggregates and its role in maintenance, organization and interaction of fibrillar structures.

A novel marker of tissue junctions, collagen XXII

Here we describe a novel specific component of tissue junctions, collagen XXII. It was first identified by screening an EST data base and subsequently expressed as a recombinant protein and characterized as an authentic tissue component. The COL22A1 gene on human chromosome 8q24.2 encodes a collagen that structurally belongs to the FACIT protein family (fibril-associated collagens with interrupted triple helices). Collagen XXII exhibits a striking restricted localization at tissue junctions such as the myotendinous junction in skeletal and heart muscle, the articular cartilage-synovial fluid junction, or the border between the anagen hair follicle and the dermis in the skin. It is deposited in the basement membrane zone of the myotendinous junction and the hair follicle and associated with the extrafibrillar matrix in cartilage. In situ hybridization of myotendinous junctions revealed that muscle cells produce collagen XXII, and functional tests demonstrated that collagen XXII acts as a cell adhesion ligand for skin epithelial cells and fibroblasts. This novel gene product, collagen XXII, is the first specific extracellular matrix protein present only at tissue junctions.

Immunophenotypic analysis of human articular chondrocytes: Changes in surface markers associated with cell expansion in monolayer culture

Cartilage tissue engineering relies on in vitro expansion of primary chondrocytes. Monolayer is the chosen culture model for chondrocyte expansion because in this system the proliferative capacity of chondrocytes is substantially higher compared to non-adherent systems. However, human articular chondrocytes (HACs) cultured as monolayers undergo changes in phenotype and gene expression known as "dedifferentiation." To gain a better understanding of the cellular mechanisms involved in the dedifferentiation process, our research focused on the characterization of the surface molecule phenotype of HACs in monolayer culture. Adult HACs were isolated by enzymatic digestion of cartilage samples obtained post-mortem. HACs cultured in monolayer for different time periods were analyzed by flow cytometry for the expression of cell surface markers with a panel of 52 antibodies. Our results show that HACs express surface molecules belonging to different categories: integrins and other adhesion molecules (CD49a, CD49b, CD49c, CD49e, CD49f, CD51/61, CD54, CD106, CD166, CD58, CD44), tetraspanins (CD9, CD63, CD81, CD82, CD151), receptors (CD105, CD119, CD130, CD140a, CD221, CD95, CD120a, CD71, CD14), ectoenzymes (CD10, CD26), and other surface molecules (CD90, CD99). Moreover, differential expression of certain markers in monolayer culture was identified. Up-regulation of markers on HACs regarded as distinctive for mesenchymal stem cells (CD10, CD90, CD105, CD166) during monolayer culture suggested that dedifferentiation leads to reversion to a primitive phenotype. This study contributes to the definition of HAC phenotype, and provides new potential markers to characterize chondrocyte differentiation stage in the context of tissue engineering applications.

Bone Morphogenetic Protein-1/Tolloid-like Proteinases Process Dentin Matrix Protein-1

Bone morphogenetic protein-1 (BMP-1)/Tolloid-like metalloproteinases play key roles in formation of mammalian extracellular matrix (ECM), through the biosynthetic conversion of precursor proteins into their mature functional forms. These proteinases probably play a further role in formation of bone through activation of transforming growth factor beta-like BMPs. Dentin matrix protein-1 (DMP1), deposited into the ECM during assembly and involved in initiating mineralization of bones and teeth, is thought to undergo proteolysis in vivo to generate functional cleavage fragments found in extracts of mineralized tissues. Here, we have generated recombinant DMP1 and demonstrate that it is cleaved, to varying extents, by all four mammalian BMP-1/Tolloid-like proteinases, to generate fragments similar in size to those previously isolated from bone. Consistent with possible roles for the BMP-1/Tolloid-like proteinases in the physiological processing of DMP1, NH2-terminal sequences of products generated by BMP-1 cleavage of DMP1 match those predicted from processing at the predicted DMP1 site that shows greatest cross-species conservation of sequences. Moreover, fibroblasts derived from mouse embryos homozygous null for genes encoding three of the four mammalian BMP-1/Tolloid-like proteinases appear to be deficient in processing of DMP1. Thus, a further role for BMP-1-Tolloid-like proteinases in formation of mineralized tissues is indicated, via proteolytic processing of DMP1.

Interaction between amino propeptides of type XI procollagen alpha1 chains

Type XI collagen is a quantitatively minor yet essential constituent of the cartilage extracellular matrix. The amino propeptide of the alpha1 chain remains attached to the rest of the molecule for a longer period of time after synthesis than the other amino propeptides of type XI collagen and has been localized to the surface of thin collagen fibrils. Yeast two-hybrid system was used to demonstrate that a homodimer of alpha1(XI) amino propeptide (alpha1(XI)Npp) could form in vivo. Interaction was also confirmed using multi-angle laser light scattering, detecting an absolute weight average molar mass ranging from the size of a monomer to the size of a dimer (25,000-50,000 g/mol), respectively. Binding was shown to be saturable by ELISA. An interaction between recombinant alpha1(XI)Npp and the endogenous alpha1(XI)Npp was observed, and specificity for alpha1(XI)Npp but not alpha2(XI)Npp was demonstrated by co-precipitation. The interaction between the recombinant form of alpha1(XI)Npp and the endogenous alpha1(XI)Npp resulted in a stable association during the regeneration of cartilage extracellular matrix by fetal bovine chondrocytes maintained in pellet culture, generating a protein that migrated with an apparent molecular mass of 50-60 kDa on an SDS-polyacrylamide gel.

Complexes of matrilin-1 and biglycan or decorin connect collagen VI microfibrils to both collagen II and aggrecan

Native supramolecular assemblies containing collagen VI microfibrils and associated extracellular matrix proteins were isolated from Swarm rat chondrosarcoma tissue. Their composition and spatial organization were characterized by electron microscopy and immunological detection of molecular constituents. The small leucine-rich repeat (LRR) proteoglycans biglycan and decorin were bound to the N-terminal region of collagen VI. Chondroadherin, another member of the LRR family, was identified both at the N and C termini of collagen VI. Matrilin-1, -3, and -4 were found in complexes with biglycan or decorin at the N terminus. The interactions between collagen VI, biglycan, decorin, and matrilin-1 were studied in detail and revealed a biglycan/matrilin-1 or decorin/matrilin-1 complex acting as a linkage between collagen VI microfibrils and aggrecan or alternatively collagen II. The complexes between matrilin-1 and biglycan or decorin were also reconstituted in vitro. Colocalization of collagen VI and the different ligands in the pericellular matrix of cultured chondrosarcoma cells supported the physiological relevance of the observed interactions in matrix assembly.

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

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

The transcription factor CCAAT-binding factor CBF/NF-Y regulates the proximal promoter activity in the human alpha 1(XI) collagen gene (COL11A1)

We have characterized the proximal promoter region of the human COL11A1 gene. Transient transfection assays indicate that the segment from -199 to +1 is necessary for the activation of basal transcription. Electrophoretic mobility shift assays (EMSAs) demonstrated that the ATTGG sequence, within the -147 to -121 fragment, is critical to bind nuclear proteins in the proximal COL11A1 promoter. We demonstrated that the CCAAT binding factor (CBF/NF-Y) bound to this region using an interference assay with consensus oligonucleotides and a supershift assay with specific antibodies in an EMSA. In a chromatin immunoprecipitation assay and EMSA using DNA-affinity-purified proteins, CBF/NF-Y proteins directly bound this region in vitro and in vivo. We also showed that four tandem copies of the CBF/NF-Y-binding fragment produced higher transcriptional activity than one or two copies, whereas the absence of a CBF/NF-Y-binding fragment suppressed the COL11A1 promoter activity. Furthermore, overexpression of a dominant-negative CBF-B/NF-YA subunit significantly inhibited promoter activity in both transient and stable cells. These results indicate that the CBF/NF-Y proteins regulate the transcription of COL11A1 by directly binding to the ATTGG sequence in the proximal promoter region.

Expression patterns of cartilage collagens and Sox9 during mouse heart development

A majority of congenital heart defects are due to abnormal development of the valves and membranous septa, i.e., connective tissue components of the heart. During development, an interesting feature of cardiac connective tissue is transient expression of collagens typical for cartilage. To better understand the role of these collagens in the heart, we have performed a systematic study on the temporospatial expression of type II and IX collagen isoforms during mouse heart development employing northern hybridization and RNase protection assay. The mRNAs for alpha1(II) and alpha1(IX) collagens were expressed transiently between embryonic days 10.5 and 14.5 in embryonic mouse heart. RNase protection assays revealed that for both transcripts the embryonic ("prechondrogenic") variants of the alternatively spliced mRNA isoforms dominated. Immunohistochemistry demonstrated that type IIA collagen and Sox9, its key transcriptional regulator, were expressed in the epithelial-mesenchymal areas of the developing heart, with partially overlapping patterns particularly in valvular and septal regions. In addition, Sox9 expression was detected widely in the developing heart. These observations support the hypothesis that cartilage collagens, especially the long isoform of type II collagen, participate in the morphogenesis of cardiac valves and septa.

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.