Analysis of collagen synthesis and assembly in culture by immortalized mouse chondrocytes in the presence or absence of alpha 1(IX) collagen chains

Control of collagen production in mouse chondrocytes by using a combination of bone morphogenetic protein-2 and small interfering RNA targeting Col1a1 for hydrogel-based tissue-engineered cartilage

Because articular cartilage does not self-repair, tissue-engineering strategies should be considered to regenerate this tissue. Autologous chondrocyte implantation is already used for treatment of focal damage of articular cartilage. Unfortunately, this technique includes a step of cell amplification, which results in dedifferentiation of chondrocytes, with expression of type I collagen, a protein characteristic of fibrotic tissues. Therefore, the risk of producing a fibrocartilage exists. The aim of this study was to propose a new strategy for authorizing the recovery of the differentiated status of the chondrocytes after their amplification on plastic. Because the bone morphogenetic protein (BMP)-2 and the transforming growth factor (TGF)-β1 are cytokines both proposed as stimulants for cartilage repair, we undertook a detailed comparative analysis of their biological effects on chondrocytes. As a cellular model, we used mouse chondrocytes after their expansion on plastic and we tested the capability of BMP-2 or TGF-β1 to drive their redifferentiation, with special attention given to the nature of the proteins synthesized by the cells. To prevent any fibrotic character of the newly synthesized extracellular matrix, we silenced type I collagen by transfecting small interfering RNA (siRNA) into the chondrocytes, before their exposure to BMP-2 or TGF-β1. Our results showed that addition of siRNA targeting the mRNA encoded by the Col1a1 gene (Col1a1 siRNA) and BMP-2 represents the most efficient combination to control the production of cartilage-characteristic collagen proteins. To go one step further toward scaffold-based cartilage engineering, Col1a1 siRNA-transfected chondrocytes were encapsulated in agarose hydrogel and cultured in vitro for 1 week. The analysis of the chondrocyte-agarose constructs by using real-time polymerase chain reaction, Western-blotting, immunohistochemistry, and electron microscopy techniques demonstrated that the BMP-2/Col1a1 siRNA combination is effective in reinitializing correct production and assembly of the cartilage-characteristic matrix in agarose hydrogel, without production of type I collagen. Because agarose is known to favor long-term expression of the chondrocyte phenotype and agarose-based hydrogels are approved for clinical trials, this strategy appears very promising to repair hyaline cartilage.

Essential modification of the Sircol Collagen Assay for the accurate quantification of collagen content in complex protein solutions

Collagen contains the unique imino acid hydroxyproline (HyPro), which is involved in the stabilization of this triple helical molecule. The concentration of HyPro is customarily used to calculate the total collagen content in a cell culture environment and in acid hydrolysates of normal and pathophysiological tissues. Radiolabelling, chromatographic and calorimetric assays have been developed over the years for the accurate determination of collagen content through HyPro estimation. Recently, the Sircol Collagen Assay (SCA) has been almost exclusively adopted as the fastest and simplest colorimetric method for the determination of collagen concentration in complex protein solutions. We show here that the colorimetric SCA, which is based on the binding of Sirius red (SR) to collagen, is flawed by interference of non-collagenous proteins (e.g. serum). In fact, we demonstrate that SCA in cell culture systems and tissue hydrolysates results in a dramatic overestimation of collagen content ranging from 3- to 24-fold. In order to rescue this otherwise very practical assay, we introduce a simple purification procedure that allows the removal of interfering non-collagenous proteins from culture media and tissue samples so that accurate measurements with SCA are now possible.

Investigating conversion of mechanical force into biochemical signaling in three-dimensional chondrocyte cultures

The culture of chondrocytes embedded within agarose hydrogels maintains chondrocytic phenotype over extended periods and allows analysis of the chondrocyte response to mechanical forces. The mechanisms involved in the transduction of a mechanical stimulus to a physiological process are not completely deciphered. We present protocols to prepare and characterize constructs of murine chondrocytes and agarose (1 week pre-culture period), to analyze the effect of compression on mRNA level by RT-PCR (2-3 d), gene transcription by gene reporter assay (3 d) and phosphorylation state of signaling molecules by western blotting (3-4 d). The protocols can be carried out with a limited number of mouse embryos or newborns and this point is particularly important regarding genetically modified mice.

The role of collagen antibodies in mediating arthritis

This review examines evidence that rheumatoid arthritis (RA) depends on autoimmunity to articular collagen, and mechanisms whereby autoantibodies to type II collagen contribute to disease development. Three major autoantigenic reactants have been identified in RA; the corresponding autoantibodies are rheumatoid factor (RF), antibodies to citrullinated peptide antigens (ACPA), citrullinated peptides (anti-CCP), and anti-type II collagen (anti-CII). Both RF and ACPA are well-validated and predictive markers of severe erosive RA, but cannot be linked to pathogenesis. By contrast, in various animal species immunized with CII there occurs an erosive inflammatory arthritis resembling that seen in human RA, together with antibodies to CII with an epitope specificity similar to that in RA. We discuss the well-known role of immune complexes in the induction of inflammation within the joint, and present recent data showing, additionally, that antibodies to CII cause direct damage to cartilage in vitro. The close resemblances between human RA and collagen-induced arthritis in animals suggest that autoimmunity, and particularly autoantibodies to CII, are important for both the initiation and perpetuation of RA in a dual manner: as contributors to the inflammation associated with immune complex deposition, and as agents with direct degradative effects on cartilage integrity and its repair.

Labeling of mesenchymal stem cells by bioconjugated quantum dots

Long-term labeling of stem cells during self-replication and differentiation benefits investigations of development and tissue regeneration. We report the labeling of human mesenchymal stem cells (hMSCs) with RGD-conjugated quantum dots (QDs) during self-replication, and multilineage differentiations into osteogenic, chondrogenic, and adipogenic cells. QD-labeled hMSCs remained viable as unlabeled hMSCs from the same subpopulation. These findings suggest the use of bioconjugated QDs as an effective probe for long-term labeling of stem cells.

Integrin alpha1beta1 mediates collagen induction of MMP-13 expression in MC615 chondrocytes

During endochondral ossification, type I collagen is synthesized by osteoblasts together with some hypertrophic chondrocytes. Type I collagen has also been reported to be progressively synthesized in degenerative joints. Because Matrix Metalloproteinase-13 (MMP-13) plays an active role in remodeling cartilage in fetal development and osteoarthritic cartilage, we investigated whether type I collagen could activate MMP-13 expression in chondrocytes. We used a well-established chondrocytic cell line (MC615) and we found that MMP-13 expression was induced in MC615 cells cultured in type I collagen gel. We also found that alpha1beta1 integrin, a major collagen receptor, was expressed by MC615 cells and we further assessed the role of alpha1beta1 integrin in conducting MMP-13 expression. Induction of MMP-13 expression by collagen was potently and synergistically inhibited by blocking antibodies against alpha1 and beta1 integrin subunits, indicating that alpha1beta1 integrin mediates the MMP-13-inducing cellular signal generated by three-dimensional type I collagen. We also determined that activities of tyrosine kinase and ERK and JNK MAP kinases were required for this collagen-induced MMP-13 expression. Interestingly, bone morphogenetic protein (BMP)-2 opposed this induction, an effect that may be related to a role of BMP-2 in the maintenance of cartilage matrix.

A common pathway in differentiation and inflammation: p38 mediates expression of the acute phase SIP24 iron binding lipocalin in chondrocytes

SIP24 is an acute phase iron binding lipocalin physiologically expressed in vivo in developing cartilage by prehypertrophic/hypertrophic chondrocytes. Taking advantage of the chondrocytic cell line MC615 and using SIP24 as a marker we investigated the pathways active in cartilage differentiation and inflammation. MC615 cells were cultured as: (i) proliferating prechondrogenic cells expressing type I collagen (ii) differentiated hyperconfluent cells expressing Sox9 and type II collagen. In proliferating cells the pathway PKC/ERK1, ERK2 was activated and SIP24 was not expressed while in differentiated cells the pathway p38/NF-kappaB was activated and SIP24 was expressed. Proliferating cells treated with inflammatory agents expressed a large amount of SIP24 and showed activation of p38/NF-kappaB pathway and inhibition of PKC/ERK1, ERK2 pathway indicating that in inflammation and differentiation the same factors are activated (p38, NF-kappaB) or inactivated (PKC, ERKs). Treatment of proliferating cells with the p38 specific inhibitor SB203580 inhibited the inflammation induced activation of p38 and the synthesis of SIP24. PMA treatment induced activation of PKC, inactivation of p38 and suppression of SIP24 synthesis, suggesting that PKC activation inhibits p38 activation. In differentiated hyperconfluent cells the same factors (p38/NF-kappaB/SIP24) are constitutively activated: treatment with inflammatory agents does not increase synthesis of SIP24 while treatment with SB203580 and with PMA does not repress activation of p38 nor synthesis of SIP24. We propose that the SIP24 stress related protein is expressed via p38 activation/NF-kappaB recruitment both in chondrocyte differentiation and inflammation and that a signaling pathway active in the acute phase response is physiologically activated in differentiation.

Arthritogenic anti-type II collagen antibodies are pathogenic for cartilage-derived chondrocytes independent of inflammatory cells

OBJECTIVE

Some monoclonal antibodies (mAb) to type II collagen (CII) are arthritogenic upon passive transfer to mice. We undertook this study to investigate whether such mAb are pathogenic in the absence of mediators of inflammation.

METHODS

The arthritogenic mAb CIIC1 and M2139, and the nonarthritogenic mAb CIIF4, each reactive with a distinct and well-defined conformational epitope on CII, were compared with control mAb GAD6. Bovine chondrocytes were cultured with one of the mAb, and on days 3, 6, and 9, antibody binding by chondrocytes and newly synthesized extracellular matrix (ECM) was examined by immunofluorescence, morphologic effects were studied by electron microscopy, and synthesis of matrix components was determined by metabolic labeling with (3)H-proline for collagen and (35)S-sulfate for proteoglycans.

RESULTS

All 3 mAb to CII bound to the matrix. CIIC1 and M2139 adversely affected the cultures, whereas CIIF4 did not. CIIC1 caused disorganization of CII fibrils in the ECM without affecting chondrocyte morphology, and increased matrix synthesis. M2139 caused thickening and aggregation of CII fibrils in the ECM and abnormal chondrocyte morphology but matrix synthesis was unaffected.

CONCLUSION

The unique arthritogenic capacity of particular anti-CII mAb upon passive transfer could be explained by their adverse, albeit differing, effects in primary cultures of chondrocytes. Such effects occur independent of inflammation mediators and are related to the epitope specificity of the mAb. Interference with the structural integrity of CII could precede, and even initiate, the inflammatory expression of disease.

Collagen XXIV, a vertebrate fibrillar collagen with structural features of invertebrate collagens: selective expression in developing cornea and bone

Tissue-specific assembly of fibers composed of the major collagen types I and II depends in part on the formation of heterotypic fibrils, using the quantitatively minor collagens V and XI. Here we report the identification of a new fibrillar-like collagen chain that is related to the fibrillar alpha1(V), alpha1(XI), and alpha2(XI) collagen polypeptides and which is coexpressed with type I collagen in the developing bone and eye. The new collagen was designated the alpha1(XXIV) chain and consists of a long triple helical domain flanked by typical propeptide-like sequences. The carboxyl propeptide is classic, with 8 conserved cysteine residues. The amino-terminal peptide contains a thrombospodin-N-terminal-like (TSP) motif and a highly charged segment interspersed with several tyrosine residues, like the fibril diameter-regulating collagen chains alpha1(V) and alpha1(XI). However, a short imperfection in the triple helix makes alpha1(XXIV) unique from other chains of the vertebrate fibrillar collagen family. The triple helical interruption and additional select features in both terminal peptides are common to the fibrillar chains of invertebrate organisms. Based on these data, we propose that collagen XXIV is an ancient molecule that may contribute to the regulation of type I collagen fibrillogenesis at specific anatomical locations during fetal development.

Dynamics of the cell and its extracellular matrix-a simple mathematical approach

The extracellular matrix (ECM) is produced by the cells and secreted into the surrounding medium, and consists of a complex mixture of structural and functional proteins. It has been recently observed that the ECM can influence the behavior of cell growth in vitro quite remarkably. A simple mathematical model has been constructed based on negative feedback control mechanisms to represent the dynamics of ECM deposition and cellular differentiation. The model analysis shows a strong relationship between the numerical solution and the experimental observations in cell-polymer constructs for the design of engineered cartilage. The current paper may be a useful guide for those who want to explore the studies on cell-matrix interactions.

Modeling the dynamic composition of engineered cartilage

Mathematical models to describe extracellular matrix (ECM) deposition and scaffold degradation in cell-polymer constructs for the design of engineered cartilage were developed and validated. The ECM deposition model characterized a product-inhibition mechanism in the concentration of cartilage molecules, collagen and glycosaminoglycans (GAG). The scaffold degradation model used first-order kinetics to describe hydrolysis (not limited by diffusion) of biodegradable polyesters, polyglycolic acid and polylactic acid. Each model was fit to published accumulation and degradation data. As experimental validation, cell-polymer constructs (n=24) and unseeded scaffolds (n=24) were cultured in vitro. Biochemical assays for ECM content and measurements of scaffold mass were performed at 1, 2, 4, 6, 8, or 10 weeks (n=8 per time point). The models demonstrated a strong fit with published data and experimental results (R(2)=0.75 to 0.99) and predicted the temporal total construct mass with reasonable accuracy (30% RMS error). This approach can elucidate mechanisms governing accumulation/degradation and may be coupled with structure-function relationships to describe time-dependent changes in construct elastic properties.

Functions of transforming growth factor-beta family type I receptors and Smad proteins in the hypertrophic maturation and osteoblastic differentiation of chondrocytes

We investigated the effects of bone morphogenetic protein (BMP)-2, a member of the transforming growth factor-beta superfamily, on the regulation of the chondrocyte phenotype, and we identified signaling molecules involved in this regulation. BMP-2 triggers three concomitant responses in mouse primary chondrocytes and chondrocytic MC615 cells. First, BMP-2 stimulates expression or synthesis of type II collagen. Second, BMP-2 induces expression of molecular markers characteristic of pre- and hypertrophic chondrocytes, such as Indian hedgehog, parathyroid hormone/parathyroid hormone-related peptide receptor, type X collagen, and alkaline phosphatase. Third, BMP-2 induces osteocalcin expression, a specific trait of osteoblasts. Constitutively active forms of transforming growth factor-beta family type I receptors and Smad proteins were overexpressed to address their role in this process. Activin receptor-like kinase (ALK)-1, ALK-2, ALK-3, and ALK-6 were able to reproduce the hypertrophic maturation of chondrocytes induced by BMP-2. In addition, ALK-2 mimicked further the osteoblastic differentiation of chondrocytes induced by BMP-2. In the presence of BMP-2, Smad1, Smad5, and Smad8 potentiated the hypertrophic maturation of chondrocytes, but failed to induce osteocalcin expression. Smad6 and Smad7 impaired chondrocytic expression and osteoblastic differentiation induced by BMP-2. Thus, our results indicate that Smad-mediated pathways are essential for the regulation of the different steps of chondrocyte and osteoblast differentiation and suggest that additional Smad-independent pathways might be activated by ALK-2.

Differential effects of osteogenic protein-1 (BMP-7) on gene expression of BMP and GDF family members during differentiation of the mouse MC615 chondrocyte cells

The mRNA expression patterns of several bone morphogenetic proteins (BMPs) and growth differentiation factors (GDFs) in long-term cultures of the clonal mouse chondrocyte cell line MC615 were examined. Distinct spatial and temporal patterns of expression of BMPs and GDFs were observed. The temporal orders of expression were correlated with those of several biochemical markers characteristic of chondrocytic cell differentiation. BMP-1, -2, -5, and -6 mRNA expression increased throughout the chondrogenic process and BMP-4 mRNA expression was not changed. GDF-1 and -3 mRNA expression increased throughout the chondrogenic process, and GDF-5, -6, -8, and -9 mRNA expressions were not changed. Effects of osteogenic protein-1 (OP-1, BMP-7) on the expression patterns of several other members of the BMP family and of the GDF family were also examined. OP-1 downregulated the BMP-1, -4, -5, and -6 mRNA expression by a maximal 3-, 5-, 2.5-, and 3-fold, respectively. The BMP-2 mRNA expression was not changed significantly by a low concentration of OP-1, but was increased at 200 ng/ml at day 7 of treatment. In contrast to the BMPs, OP-1 upregulated significantly the six GDF members examined (GDF-1, -3, -5, -6, -8, and -9) by three- to four-fold. Our findings demonstrate that OP-1 differentially regulates the mRNA expression of several related members of the BMP family and upregulates the mRNA expression of several members of the GDF family. The observations suggest that OP-1 action on cartilage differentiation involves a complex regulation of gene expression of several members of the BMP and the GDF family.

Establishment and characterization of chondrocyte cell lines from the costal cartilage of SV40 large T antigen transgenic mice

Complete understanding of the physiology and pathology of the cartilage is essential to establish treatments for a variety of cartilage disorders and defects such as rheumatoid arthritis, congenital malformations, and tumors of cartilage. Although synthetic materials have been used in many cases, they possess inherent problems including wear of the materials and low mechanical strength. Autograft has been considered very effective to overcome these problems. However, the limitation of the transplant volume is a major problem in autograft to be overcome. The costal cartilage is the most serious candidate for donor site transplantation, since it is the largest permanent hyaline cartilage in the body. To investigate the possibility using the costal cartilage as a transplant source, we have established and characterized three mouse chondrocyte cell lines (MCC-2, MCC-5, and MCC-35) derived from the costal cartilage of 8-week-old male SV40 large T-antigen transgenic mice. At confluence, all the cell lines formed nodules that could be positively stained with alcian blue (pH 2.5). The size of nodules gradually increased during culturing time. After 2 and 6 weeks of culture, RT-PCR analysis demonstrated that all three cell lines expressed mRNA from the cartilage-specific genes for type II collagen, type XI collagen, aggrecan, and link protein. Furthermore, type X collagen expression was detected in MCC-5 and MCC-35 but not in MCC-2. Any phenotypic changes were not observed over 31 cell divisions. Immunocytochemistry showed further that MCC-2, MCC-5, and MCC-35 produced cartilage-specific proteins type II collagen and type XI collagen, while in addition MCC-5 and MCC-35 produced type X collagen. Treatment with 1alpha, 25-dihydroxyvitamin D(3) inhibited cell proliferation and differentiation of the three cell lines in a dose-dependent manner. These phenotypic characteristics have been found consistent with chondrocyte cell lines established from cartilage tissues other than costal cartilage. In conclusion, costal cartilage shows phenotypic similarities to other cartilages, i.e., articular cartilage and embryonic limbs, suggesting that costal cartilage may be very useful as the donor transplantation site for the treatment of cartilage disorders. Furthermore, the cell lines established in this study are also beneficial in basic research of cartilage physiology and pathology.

Signal-selectivity of parathyroid hormone (PTH)/PTH-related peptide receptor-mediated regulation of differentiation in conditionally immortalized growth-plate chondrocytes

Type-1 PTH/PTH-related peptide receptors (PTH1Rs), which activate both adenylyl cyclase and phospholipase C (PLC), control endochondral bone development by regulating chondrocyte differentiation. To directly analyze PTH1R function in such cells, we isolated conditionally transformed clonal chondrocytic cell lines from tibial growth plates of neonatal mice heterozygous for PTH1R gene ablation. Among 104 cell lines isolated, messenger RNAs for PTH1R, collagen II, and collagen X were detected in 28%, 90%, and 29%, respectively. These cell lines were morphologically diverse. Some appeared large, rounded, and enveloped by abundant extracellular matrix; whereas others were smaller, flattened, and elongated. Two PTH1R-expressing clones showed similar PTH1R binding and cAMP responsiveness to PTH and PTH-related peptide but disparate morphologic features, characteristic of hypertrophic (hC1--5) or nonhypertrophic (nhC2--27) chondrocytes, respectively. hC1--5 cells expressed messenger RNAs for collagen II and X, alkaline phosphatase (ALP), and matrix GLA protein, whereas nhC2--27 cells expressed collagen II and Indian hedgehog but not collagen X or ALP. In hC1--5 cells, PTH and cAMP analog, but not phorbol ester, inhibited both ALP and mineralization. PTH1R-null hC1--5 subclones were isolated by in vitro selection and then reconstituted by stable transfection with wild-type PTH1Rs or mutant (DSEL) PTH1Rs defective in PLC activation. ALP and mineralization were inhibited similarly via both forms of the receptor. These results indicate that PLC activation is not required for PTH1R regulation of mineralization or ALP in hypertrophic chondrocytes and are consistent with a major role for cAMP in regulating differentiation of hypertrophic chondrocytes.

Proteoglycan production by immortalized human chondrocyte cell lines cultured under conditions that promote expression of the differentiated phenotype

Large and small proteoglycans are essential components of articular cartilage. How to induce chondrocytes to repair damaged cartilage with normal ratios of matrix components after their loss due to degenerative joint disease has been a major research focus. We have developed immortalized human chondrocyte cell lines for examining the regulation of cartilage-specific matrix gene expression. However, the decreased synthesis and deposition of cartilage matrix associated with a rapid rate of proliferation has presented difficulties for further examination at the protein level. In these studies, proteoglycan synthesis was characterized in two chondrocyte cell lines, T/C-28a2 and tsT/AC62, derived, respectively, from juvenile costal and adult articular cartilage, under culture conditions that either promoted or decreased cell proliferation. Analysis of proteo[36S]glycans by Sepharose CL-4B chromatography and SDS-PAGE showed that the large proteoglycan aggrecan and the small, leucine-rich proteoglycans, decorin and biglycan, were produced under every culture condition studied. In monolayer cultures, a high initial cell density and conditions that promoted proliferation (presence of serum for T/C-28a2 cells or permissive temperature for the temperature-sensitive tsT/AC62 cells) favored cell survival and ratios of proteoglycans expected for differentiated chondrocytes. However, the tsT/AC62 cells produced more proteoglycans at the nonpermissive temperature. Culture of cells suspended in alginate resulted in a significant decrease in proteoglycan production in all culture conditions. While the tsT/AC62 cells continued to produce a larger amount of aggrecan than small proteoglycans, the T/C-28a2 cells lost the ability to produce significant amounts of aggrecan in alginate culture. In addition, our data indicate that immortalized chondrocytes may alter their ability to retain pericellular matrix under changing culture conditions, although the production of the individual matrix components does not change. These findings provide critical information that will assist in the development of a reproducible chondrocyte culture model for the study of regulation of proteoglycan biosynthesis in cartilage.

A short isoform of Col9a1 supports alveolar bone repair

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

Isolation, cloning, and expression of a new murine zinc finger encoding gene

With the aim of identifying genes involved in cartilage differentiation, we have used a subtractive hybridization strategy with cDNAs from a chondrocytic cell line (MC615) and mRNAs from a mesenchymal precursor cell line (10T1/2). We have isolated a cDNA clone representing a novel mouse gene. The predicted 368-amino acid protein, designated ZF-12, contains four C(2)H(2)-type zinc finger motifs and one region homologous to the LeR domain, a finger-associated structural domain. ZF-12 mRNAs are expressed during embryonic development and in different organs in adult, including rib cartilage. These data suggest that ZF-12 might play an important role not only in cartilage differentiation, but also in basic cellular processes.

Different effects of bone morphogenetic proteins 2, 4, 12, and 13 on the expression of cartilage and bone markers in the MC615 chondrocyte cell line

In order to study the lineage leading to chondrocyte and osteoblast phenotype in vertebrate development, we examined the effect of recombinant human bone morphogenetic protein (BMP)-2, BMP-4, BMP-12 [or growth and differentiation factor (GDF)-7], and BMP-13 (or GDF-6) on the phenotypic expression of the mouse chondrocyte cell line MC615, grown for 1 or 2 weeks in monolayer. Protein synthesis rates were monitored after incubation with [(14)C]proline. BMP-2 and BMP-4 increased protein synthesis, in agreement with our observation by phase-contrast microscopy of a highly refractile matrix around MC615 cells treated with BMP-2 and -4. Markers of the chondrocytic and osteoblastic differentiation were analyzed at mRNA level. Expression of the type II collagen gene, a marker of the cartilage phenotype, was up-regulated in the presence of low concentration of BMP-2 or -4 (50 ng/ml) and down-regulated at higher concentrations (100-400 ng/ml). In parallel, this expression was stable in the presence of BMP-12 or -13 in the dose range tested (50-400 ng/ml). Expression of the matrix Gla protein (MGP) gene, another marker of cartilage, was also reduced in the presence of 100 ng/ml BMP-2 or -4, while it remained stable in the presence of BMP-12 or -13 at the same concentration. In contrast, expression of the bone Gla protein (BGP) gene, or osteocalcin, a marker of the bone phenotype, was induced when the cells were treated with BMP-2 or -4 but was not detected when the cells were treated with BMP-12 or -13. At the same time, BMP-2 or -4 markedly up-regulated expression of type X collagen mRNA, indicating that MC615 cells possess the ability to express traits associated with endochondral ossification, when exposed to specific BMPs. Furthermore, detailed analysis of type II collagen expression showed that the alternatively spliced transcript collagen IIB, specific for cartilage, is expressed concomitantly with BGP. Therefore, MC615 chondrocytes can simultaneously express chondrocytic and osteoblastic markers, in response to BMP-2 or -4, but show minimal response to BMP-12 (or GDF-7) or to BMP-13 (or GDF-6). These results raise the possibility that chondrocytes in vivo can express osteoblastic properties, provided they are induced by BMP-2 or -4.

Collagen in tissue-engineered cartilage: types, structure, and crosslinks

The function of articular cartilage as a weight-bearing tissue depends on the specific arrangement of collagen types II and IX into a three-dimensional organized collagen network that can balance the swelling pressure of the proteoglycan/water gel. To determine whether cartilage engineered in vitro contains a functional collagen network, chondrocyte-polymer constructs were cultured for up to 6 weeks and analyzed with respect to the composition and ultrastructure of collagen by using biochemical and immunochemical methods and scanning electron microscopy. Total collagen content and the concentration of pyridinium crosslinks were significantly (57% and 70%, respectively) lower in tissue-engineered cartilage that in bovine calf articular cartilage. However, the fractions of collagen types II, IX, and X and the collagen network organization, density, and fibril diameter in engineered cartilage were not significantly different from those in natural articular cartilage. The implications of these findings for the field of tissue engineering are that differentiated chondrocytes are capable of forming a complex structure of collagen matrix in vitro, producing a tissue similar to natural articular cartilage on an ultrastructural scale.

Differences between the thermal stabilities of the three triple-helical domains of type IX collagen

Fibre-forming collagens in dilute solution show highly co-operative helix-coil transitions at temperatures that are remarkably close to the body temperature of the animal from which the collagen was extracted. This close correlation holds across animal Phyla and the transition temperatures, which range from 5 degrees C to 40 degrees C, are adjusted to suit by changing the primary structure, especially the concentration of the water-bridge-enhancing hydroxyproline residue. Fibril-forming collagens are thermally stabilised by fibrillogenesis, which causes a loss of random coil configurational entropy by intermolecular and intramolecular cross-linking and by spacial confinement of the molecule within the lattice of the fibre. But this mechanism cannot apply to the full length of the type IX collagen molecule, since its COL3 arm, according to current models, projects out from the stabilising influence of the type II fibre. In this paper we examine the thermal stability of the type IX collagen molecule and its three triple-helical domains, thereby demonstrating that the COL3 arm is much more stable than the rest of the molecule. At a scanning rate of 60 deg. C/h COL3 exhibited an unfolding endotherm with a tmax at 49.0 degrees C, well above body temperature. Corresponding peak maxima for COL1 and COL2 were seen at 40.6 degrees C and 39.6 degrees C, respectively. The sizes of the thermally labile units of COL1, COL2 and COL3, calculated from the measured activation enthalpies, were 24, 28 and 28 residues, respectively, much smaller than type I (65 residues) because of the relatively short lengths of triple helix to be unfolded. However, unlike type I collagen, no regions of the required size were found completely devoid of hydroxyproline. Consequently, the intrinsic stabilities of these thermally labile units were higher than that of type I with DeltaH updownarrow DeltaS updownarrow for COL1, COL2 and COL3 being, respectively, 385 K, 371 K and 384 K, contrasting with the much lower 349 K of type I collagen. We therefore speculate that the increased thermal stability of the thermally labile units was caused by the presence of the water-bridge-enhancing residue, hydroxyproline. Finally the stabilisation of type IX collagen tissue is considered and an alternative structural organisation of the type IX molecule on the type II fibre is proposed.

Three-dimensional cell cultures: from molecular mechanisms to clinical applications

This article reviews actual advances in the development and application of three-dimensional (3-D) cell culture systems. Recent therapeutically oriented studies include characterization of multicellular-mediated drug resistance, novel ways of quantifying hypoxia, and new approaches to more efficient immunotherapy. Recent progress toward understanding the development of necrosis in tumor spheroids has been made using novel spheroid models. 3-D cultures have been used for studies on molecular mechanisms involved in invasion and metastasis, with a major focus on the role of E-cadherin. Similarly, tumor angiogenesis and the significance of vascular endothelial growth factor have been investigated in a variety of 3-D culture systems. There are many ongoing developments in tissue modeling or remodeling that promise significant progress toward the development of bioartificial liver support and artificial blood. Perhaps one of the most interesting areas of basic research with 3-D cultures is the characterization of embryoid bodies obtained from stable embryonic stem cells. These models have greatly increased the understanding of embryonic development, in particular through the notable exceptional advances in cardiogenesis.

Assembly in vitro of thin and thick fibrils of collagen II from recombinant procollagen II. The monomers in the tips of thick fibrils have the opposite orientation from monomers in the growing tips of collagen I fibrils

Human type II procollagen was prepared in a recombinant system and cleaved to pC-collagen II by procollagen N-proteinase. The pC-collagen II was then used as a substrate to generate collagen II fibrils by cleavage with procollagen C-proteinase at 37 degrees C. Electron microscopy of the fibrils demonstrated that, at the early stages of fibril assembly, very thin fibrils were formed. As the system approached equilibrium over 7-12 h, however, the thin fibrils were largely but not completely replaced by thick fibrils that had diameters of about 240 nm and a distinct D-period banding pattern. One typical fibril was photographed and analyzed in its entirety. The fibril was 776 D-periods (52 microM) long. It had a central shaft with a uniform diameter that was about 516 D-periods long and two tips of about 100 D-periods each. Most of the central shaft had a symmetrical banding pattern flanked by two transition regions of about 30 D-periods each. Measurements by scanning transmission electron microscopy demonstrated that the mass per unit length from the tips to the shafts increased linearly over approximately 100 D-periods from the fibril end. The linear increase in mass per unit length was consistent with previous observations for collagen I fibrils and established that the tips of collagen II also had a near paraboloidal shape. However, the orientation of monomers in the tips differed from the tips of collagen I fibrils in that the C termini instead of the N termini were directed toward the tips. The thin fibrils that were present at early stages of assembly and at equilibrium were comparable to the collagen II fibrils seen in embryonic tissues and probably represented intermediates on the pathway of thick fibrils formation. The results indicated that the molecular events in the self-assembly of collagen II fibrils are apparently similar to those in self-assembly of collagen I fibrils, but that there are also important differences in the structural information contained in collagen I and collagen II monomers.