Chondrocytes Directly Transform into Bone Cells in Mandibular Condyle Growth

For decades, it has been widely accepted that hypertrophic chondrocytes undergo apoptosis prior to endochondral bone formation. However, very recent studies in long bone suggest that chondrocytes can directly transform into bone cells. Our initial in vivo characterization of condylar hypertrophic chondrocytes revealed modest numbers of apoptotic cells but high levels of antiapoptotic Bcl-2 expression, some dividing cells, and clear alkaline phosphatase activity (early bone marker). Ex vivo culture of newborn condylar cartilage on a chick chorioallantoic membrane showed that after 5 d the cells on the periphery of the explants had begun to express Col1 (bone marker). The cartilage-specific cell lineage-tracing approach in triple mice containing Rosa 26(tdTomato) (tracing marker), 2.3 Col1(GFP) (bone cell marker), and aggrecan Cre(ERT2) (onetime tamoxifen induced) or Col10-Cre (activated from E14.5 throughout adult stage) demonstrated the direct transformation of chondrocytes into bone cells in vivo. This transformation was initiated at the inferior portion of the condylar cartilage, in contrast to the initial ossification site in long bone, which is in the center. Quantitative data from the Col10-Cre compound mice showed that hypertrophic chondrocytes contributed to ~80% of bone cells in subchondral bone, ~70% in a somewhat more inferior region, and ~40% in the most inferior part of the condylar neck (n = 4, P < 0.01 for differences among regions). This multipronged approach clearly demonstrates that a majority of chondrocytes in the fibrocartilaginous condylar cartilage, similar to hyaline cartilage in long bones, directly transform into bone cells during endochondral bone formation. Moreover, ossification is initiated from the inferior portion of mandibular condylar cartilage with expansion in one direction.

Cartilage regeneration by gene therapy

Damage of articular cartilage is a frequent clinical problem and is commonly considered to be irreversible. Full-thickness defects may lead to the formation of fibrous repair tissue of minor mechanical quality, while partial-thickness lesions hardly show any repair response. Surgical approaches often fail to restore the articular surface, facing the problem of incomplete chondrogenesis or rapid degradation of the repair tissue. However, advances in molecular biology have revealed the potential of growth factors, differentiation factors, and cytokines in directing cellular differentiation and metabolic activity. Anabolic factors including members of the TGF-beta superfamily, IGF-1, FGF, or HGF have proven their potential to stimulate chondrogenesis and synthesis of cartilage-specific matrix components, allowing the formation of a hyaline cartilage-like repair tissue in experimental studies. In addition, anti-catabolic or anti-inflammatory molecules, such as IL-4, IL-10, IL-1Ra, and TNFsR may also exert beneficial effects by inhibiting excessive cartilage degradation. Although it is questionable whether regeneration of hyaline cartilage implying a complete restoration of the articular surface by a tissue that is identical with the original can ever be achieved, all these molecules have been considered as suitable tools for cartilage repair. The transfer of the respective genes into the joint, possibly in combination with the supply of chondroprogenitor cells, might be an elegant method to achieve a sustained delivery of such therapeutic factors at the required location in vivo. This review focuses on the therapeutic molecules, the suitability of different viral and non-viral vectors for intraarticular gene transfer and the lessons learned from gene therapy studies on various animal models.

Bone regeneration in critical size defects by cell-mediated BMP-2 gene transfer: a comparison of adenoviral vectors and liposomes

Large bone defects resulting from nonunion fractures or tumour resections are common clinical problems. Recent studies have shown bone morphogenetic protein-2 (BMP-2) gene transfer using adenoviral vectors to be a promising new therapeutic approach. However, comparative studies of different vectors are required to identify the optimal system for possible clinical trials. This study compares the use of liposome-mediated and adenoviral gene transfer for the generation of autologous BMP-2-producing bone marrow stromal cells (BMSC). Primary BMSC isolated from the rat femur were treated ex vivo with either an adenovirus or a liposome carrying human BMP-2 cDNA. The genetically modified cells were evaluated in vitro and transplanted into critical size defects in the rat mandible in vivo. BMSC treated with a reporter gene vector or untreated BMSC served as controls. The newly formed tissue was analysed by in situ hybridization, radiography and immunohistochemistry. Both groups of genetically modified cells produced BMP-2 for at least 2 weeks, and markers of new bone matrix such as osteopontin and osteocalcin were observed within 2 weeks following gene transfer. In the liposome group, the critical size defects were found completely healed at 6 weeks after the gene transfer, whereas the more efficient adenoviral gene transfer allowed for complete bone healing within 4 weeks. None of the three control groups showed bone healing, not even after 8 weeks. Thus, both liposome-mediated and adenoviral BMP-2 gene transfer to primary BMSC are suitable methods to achieve the healing of critical size bone defects in rats. As liposomes have proven sufficient for this purpose and offer several advantages over any other vector, such as ease of preparation, theoretically no limitation of the size of the DNA, and less immunological and safety problems, they may represent the best vector system for future clinical trials of bone regeneration by BMP-2 gene therapy.

Isolation and characterization of a collagen-binding glycoprotein from chondrocyte membranes

A collagen-binding glycoprotein was isolated from purified chick chondrocyte surface membranes by affinity chromatography on type II collagen-Sepharose. The purified glycoprotein has an apparent mol. wt. of 31,000 and binds to native chick collagen types I, II, III, V and M. Although it contains 30% carbohydrates, the majority of which is fucose, it is hydrophobic and soluble only in detergents. The integral membrane protein character of the 31-K protein became apparent from its ability to insert into lecithin vesicles. Liposome-inserted 31-K protein binds 125I-labelled type II collagen in the presence of 0.5 M NaCl, while detergent-solubilized 31-K protein is dissociated from type II collagen by 0.05-0.1 M NaCl. Electron microscopic studies employing the rotary shadowing technique indicate that 31-K protein particles bind to the ends of collagen molecules. We propose that this glycoprotein serves as anchorage site for extracellular collagen to the chondrocyte membrane and thus may be involved in cell-matrix interactions in cartilage.

Changes in the expression of annexin A5 gene during in vitro chondrocyte differentiation: influence of cell attachment

Several lines of evidence indicate that annexin A5, a membrane-associated protein with calcium-channel activity, plays a key role in cartilage calcification during endochondral ossification. As a major constituent of cartilage matrix vesicles, which are released from microvilli of hypertrophic chondrocytes, it is involved in calcium uptake necessary for the initial stages of cartilage calcification. Little is known, however, concerning transcriptional regulation of the annexin A5 gene during chondrocyte differentiation. Here, we report on changes in annexin A5 expression by measuring mRNA and protein levels during in vitro differentiation of chick sternal chondrocytes to the hypertrophic phenotype. Terminal differentiation of mature sternal chondrocytes was achieved in the presence of sodium ascorbate in high-density cultures growing either in monolayer or over agarose as cell aggregates. Differentiation of chondrocytes to hypertrophic cells was followed by morphological analysis and by the onset of type X collagen expression. High expression levels of annexin A5 mRNA were detected in chondrocytes freshly isolated from the sterna by enzymatic digestion and subsequently in cells growing in monolayer, but annexin A5 gene transcription was rapidly downregulated when cells were grown in suspension as aggregates over agarose. However, protein levels did not decrease probably due to its low turnover rate. In suspension culture, annexin A5 mRNA reappeared after 3 weeks concomitantly with segregation of the aggregates into single cells and onset of chondrocyte hypertrophy. The downregulation of annexin A5 expression in cells growing as matrix-rich aggregates was reverted when extracellular matrix components were removed and cells were reseeded onto tissue culture plastic, suggesting that cell spreading, formation of focal contacts and stress fibers stimulated annexin A5 expression in proliferating as well as in hypertrophic chondrocytes.

Role of growth factors in rabbit articular cartilage repair by chondrocytes in agarose

OBJECTIVE

Novel approaches to intervention in joint diseases consist of the replacement of diseased cartilage by in vitro engineered, viable cells or graft tissues. Two major obstacles remain to be overcome: (1) Hyaline cartilage in vitro often loses differentiated traits. (2) Grafts frequently are not integrated satisfactorily into host cartilage and/or the tissue is remodelled in situ into functionally inferior fibrocartilage. Therefore, we have explored the possibility whether chondrocytes embedded into agarose gels provided better graft tissues in a repair model of full thickness defects in rabbit joint cartilage.

DESIGN

Experimental defects of knee joint cartilage was filled with articular chondrocytes cultured in agarose gels. Chondrocytes in vitro either remained unstimulated or were treated with several growth factors. Repair of the defects was assessed by histology and was scored between 0 (no healing) and 1 (perfect healing) as judged by the follwing parameters: intensity of proteoglycan staining, organization of the superficial zone, ossification at the border between repair cartilage and subchondral bone, tidemark formation in the repaired area, arrangement of chondrocytes, and integration of repair cartilage into host.

RESULTS

Treatment of chondrocyte cultures with bFGF had a stabilizing effect on the differentiated state of the cells in implanted grafts whereas bone morphogenetic proteins stimulated ingrowth of subchondral bone reducing repair cartilage thickness and preventing normal tide mark formation; TGF-beta did not significantly affect evaluation parameters in comparison with untreated controls.

CONCLUSION

Growth factor treatment resulted in an ambiguous quality of graft development. Only FGF had a clear beneficial effect to the graft tissues after 1 month. Further studies are required to define the precise conditions and sequence of growth factor treatment of in vitro engineered cartilage which benefits graft quality.

Sequential expression of annexin A5 in the vasculature and skeletal elements during mouse development

Annexin A5 (annexin V, anchorin CII) represents the prototype member of the large annexin family, characterized by its ability to interact with phospholipids in a calcium-dependent manner and to form calcium-specific ion channels. Despite intense biochemical analysis, the in vivo expression and function of this annexin during mouse development, still remains unclear. Immunohistochemistry, in situ hybridization and reporter gene expression were used to define expression of annexin A5 during early mouse development. First, annexin A5 expression is associated with the developing vascular system. Later, expression is detected within the notochord and found in parallel to the differentiation of cartilage and bone. Therefore, expression of the Anxa5 gene may represent a novel marker characterizing cell lineages involved in the development of the vascular as well as the skeletal system.

Fibroblast-mediated delivery of growth factor complementary DNA into mouse joints induces chondrogenesis but avoids the disadvantages of direct viral gene transfer

OBJECTIVE

To assess the advantages and disadvantages of a direct adenoviral and a cell-mediated approach to the induction of cartilage formation in joints by transfer of growth factor genes.

METHODS

Adenoviral vectors carrying insulin-like growth factor 1 (IGF-1) or bone morphogenetic protein 2 (BMP-2) complementary DNA were constructed and applied to primary human and murine chondrocytes or fibroblasts. Transgene expression was quantified by enzyme-linked immunosorbent assay. Direct injection of these vectors or AdLacZ, a reporter gene vector, into mouse knee joints was compared with the transplantation of syngeneic fibroblasts (infected ex vivo with the same vectors) with respect to virus spread, immune response, and cartilage formation by use of histologic, immunohistochemical, and molecular analyses.

RESULTS

AdIGF-1 and AdBMP-2 efficiently infected all cell types tested. Human cells secreted biologically relevant levels of protein over a period of at least 28 days. Direct transfer of AdLacZ into mouse knee joints resulted in positively stained synovial tissues, whereas AdLacZ-infected fibroblasts settled on the surface of the synovial membranes. Inadvertent spread of vector DNA into the liver, lung, and spleen was identified by nested polymerase chain reaction in all mice that had received the vector directly; this rarely occurred following fibroblast-mediated gene transfer. Direct injection of AdBMP-2 induced the synthesis of new cartilage in periarticular mesenchyme, accompanied by extensive osteophyte formation. When AdBMP-2 was administered by injecting ex vivo-infected fibroblasts, cartilage formation was observed only in regions near the injected cells. AdIGF-1 treatment did not lead to morphologic changes. Importantly, fibroblast-mediated gene transfer avoided the strong immune response to adenovirus that was elicited following direct application of the vector.

CONCLUSION

Our results indicate that cell-mediated gene transfer provides sufficient BMP-2 levels in the joint to induce cartilage formation while avoiding inadvertent vector spread and immune reactions.

Coexpression of alpha and beta subunits of prolyl 4-hydroxylase stabilizes the triple helix of recombinant human type X collagen

We have reported previously on the expression of recombinant human type X collagen (hrColX) in HEK 293 and HT 1080 cells by using the eukaryotic expression vector pCMVsis (in which CMV stands for cytomegalovirus). Several stably transfected clones secreted full-length triple-helical hrColX molecules in large amounts, but the secreted collagen was underhydroxylated, with a hydroxyproline-to-proline ratio of 0.25 and a melting temperature (T(m)) of 31 degrees C. By comparison, native chicken type X procollagen has a T(m) of 46 degrees C. To stabilize the triple helix of hrColX, an hrColX-expressing clone (A6/16) was co-transfected with both alpha and beta subunits of human prolyl 4-hydroxylase. Clones were selected that secreted proalpha1(X) collagen chains with an apparent molecular mass of 75 kDa and an increased hydroxyproline-to-proline ratio of close to 0.5. As a result of enhanced prolyl hydroxylation, the T(m) of the hrColX was increased to 41 degrees C as measured by CD analysis at various temperatures. The CD spectra indicated a minimum ellipticity at 198 nm and a peak at 225 nm at 20 degrees C, confirming the presence of a triple helix. The same T(m) of 41 degrees C was measured for the triple-helical core fragments of hrColX of 60-65 kDa that were retained after brief digestion with chymotrypsin/trypsin at increasing temperatures. This shows that the human cell line HEK-293 is suitable for the simultaneous expression of three genes and the stable production of substantial amounts of recombinant, fully hydroxylated type X collagen over several years.

Laminin alpha4 and integrin alpha6 are upregulated in regenerating dy/dy skeletal muscle: comparative expression of laminin and integrin isoforms in muscles regenerating after crush injury

The expression of laminin isoforms and laminin-binding integrin receptors known to occur in muscle was investigated during myogenic regeneration after crush injury. Comparisons were made between dystrophic 129ReJ dy/dy mice, which have reduced laminin alpha2 expression, and their normal littermates. The overall histological pattern of regeneration after crush injury was similar in dy/dy and control muscle, but proceeded faster in dy/dy mice. In vitro studies revealed a greater yield of mononuclear cells extracted from dy/dy muscle and a reduced proportion of desmin-positive cells upon in vitro cultivation, reflecting the presence of inflammatory cells and "preactivated" myoblasts due to ongoing regenerative processes within the endogenous dystrophic lesions. Laminin alpha1 was not detectable in skeletal muscle. Laminin alpha2 was present in basement membranes of mature myofibers and newly formed myotubes in control and dy/dy muscles, albeit weaker in dy/dy. Laminin alpha2-negative myogenic cells were detected in dy/dy and control muscle, suggesting the involvement of other laminin alpha chains in early myogenic differentiation, such as laminin alpha4 and alpha5 which were both transiently expressed in basement membranes of newly formed myotubes of dy/dy and control mice. Integrin beta1 was expressed on endothelial cells, muscle fibers, and peripheral nerves in uninjured muscle and broadened after crush injury to the interstitium where it occurred on myogenic and nonmyogenic cells. Integrin alpha3 was not expressed in uninjured or regenerating muscle, while integrin alpha6 was expressed mainly on endothelial cells and peripheral nerves in uninjured muscle. Upon crush injury integrin alpha6 increased in the interstitium mainly on nonmyogenic cells, including infiltrating leukocytes, endothelial cells, and fibroblasts. In dy/dy muscle, integrin alpha6 occurred on some newly formed myotubes. Integrin alpha7 was expressed on muscle fibers at the myotendinous junction and showed weak and irregular expression on muscle fibers. After crush injury, integrin alpha7 expression extended to the newly formed myotubes and some myoblasts. However, many myoblasts and newly formed myotubes were integrin alpha7 negative. No marked difference was observed in integrin alpha7 expression between dy/dy and control muscle, either uninjured or after crush injury. Only laminin alpha4 and integrin alpha6 expression patterns were notably different between dy/dy and control muscle. Expression of both molecules was more extensive in dy/dy muscle, especially in the interstitium of regenerating areas and on newly formed myotubes. In view of the faster myogenic regeneration observed in dy/dy mice, the data suggest that laminin alpha4 and integrin alpha6 support myogenic regeneration. However, whether these accelerated myogenic effects are a direct consequence of the reduced laminin alpha2 expression in dy/dy mice, or an accentuation of the ongoing regenerative events in focal lesions in the muscle, requires further investigation.

The role of extracellular and cytoplasmic splice domains of alpha7-integrin in cell adhesion and migration on laminins

The major laminin-binding integrin of skeletal, smooth, and heart muscle is alpha7beta1-integrin, which is structurally related to alpha6beta1. It occurs in three cytoplasmic splice variants (alpha7A, -B, and -C) and two extracellular forms (X1 and X2) which are developmentally regulated and differentially expressed in skeletal muscle. Previously, we have shown that ectopic expression of the alpha7beta-integrin splice variant in nonmotile HEK293 cells specifically induced cell locomotion on laminin-1 but not on fibronectin. To investigate the specificity and the mechanism of the alpha7-mediated cell motility, we expressed the three alpha7-chain cytoplasmic splice variants, as well as alpha6A- and alpha6B-integrin subunits in HEK293 cells. Here we show that all three alpha7 splice variants (containing the X2 domain), as well as alpha6A and alpha6B, promote cell attachment and stimulate cell motility on laminin-1 and its E8 fragment. Deletion of the cytoplasmic domain (excluding the GFFKR consensus sequence) from alpha7B resulted in a loss of the motility-enhancing effect. On laminin-2/4 (merosin), the predominant isoform in mature skeletal muscle, only alpha7-expressing cells showed enhanced motility, whereas cells transfected with alpha6A and alpha6B neither attached nor migrated on laminin-2. Adhesion of alpha7-expressing cells to both laminin-1 and laminin-2 was specifically inhibited by a new monoclonal antibody (6A11) specific for alpha7. Expression of the two extracellular splice variants alpha7X1 and alpha7X2 in HEK293 cells conferred different motilities on laminin isoforms: Whereas alpha7X2B promoted cell migration on both laminin-1 and laminin-2, alpha7X1B supported motility only on laminin-2 and not on laminin-1, although both X1 and X2 splice variants revealed similar adhesion rates to laminin-1 and -2. Fluorescence-activated cell sorter analysis revealed a dramatic reduction of surface expression of alpha6-integrin subunits after alpha7A or -B transfection; also, surface expression of alpha1-, alpha3-, and alpha5-integrins was significantly reduced. These results demonstrate selective responses of alpha6- and alpha7-integrins and of the alpha7 splice variants to laminin-1 and -2 and indicate differential roles in laminin-controlled cell adhesion and migration.

[Current status of autologous chondrocyte transplantation]

PURPOSE

Autogenous chondrocyte transplantation (ACT) is a promising but disputable new method for the treatment of full thickness hyaline cartilage defects. Neither long-term follow-up studies nor prospective randomised comparative clinical trials exist to measure the outcome.

METHODS

Several bioengeneering companies have emerged (Genzyme, Verigen, Co.don, etc.) and commercialised this new method of ACT which has generated enormous interest. Each company uses its own (secret) protocol for culturing human cartilage cells. These protocols vary considerably from each other with regard to culture media, enzymes and other additives (e.g. antibiotics) used. However, no quality control requirements for the culturing process of cartilage cells have been proposed which the companies have to fulfill. At the time of surgery the treating surgeon has to assume (and hope) that the cultured cells are viable, sterile, active and potent but a control mechanism does not exist.

RESULTS

Considering the enormous price for ACT which the patients and the insurance companies are asked to pay quality control standards should be developed with the following informations given: cell viability, grade of differentiation, cell morphology, secretion levels of essential matrix components (collagen type II, IX, X and XI, Aggrecan) and the total cell number. None of the above information is currently provided by any of the companies involved.

CONCLUSION

The widespread clinical use of this technique cannot be recommended at this stage as the scientific proof for reproducibility of ACT has not been given so far. The operative technique of ACT is demanding and should only be used by surgeons having attended special practical training courses and workshops to minimise technical failure rates. Clinical results should be documented in a uniform standardised way using established outcome scores and modern diagnostic measures (e.g. intraoperative biomechanical testing of repair tissue). Further controlled clinical trials, experimental research and the establishment of quality control standards are required.

Immunohistochemical analysis of type-X-collagen expression in osteoarthritis of the hip joint

Conflicting data have been reported on the spatial distribution of type X-collagen expression in osteoarthritis, and no concise data exist on a possible correlation between type X-collagen expression and clinical and radiological alterations. Well defined clinical and radiological data were compared with histopathological and immunohistochemical findings to investigate the expression of type-X collagen in osteoarthritis of the hip joint. Femoral heads were obtained in toto from 11 patients undergoing routine hip arthroplasty for femoral neck fractures (n = 3) or osteoarthritis (n = 8) and from 13 patients (age: 12 days to 69 years) without any evidence of hip-joint pathology. Whole coronal sections from the femoral head were decalcified for routine histology and immunohistochemical analysis with use of type-specific monoclonal antibodies to type-X collagen. Our results demonstrate that type-X collagen is consistently found in osteoarthritic cartilage and is absent from normal adult cartilage (including the region of calcified cartilage). Except for the occurrence of type-X collagen in the middle zone of articular cartilage in advanced stages of osteoarthritis, there is no specific change in the staining pattern or intensity for the collagen during osteoarthritis, particularly when the staining is related to clinical and radiological parameters. Hardly more than 20% of the extracellular matrix stained for type-X collagen; therefore, we suggest that, in most cases, this type of collagen may not play a direct biomechanical role in the weakening of osteoarthritic cartilage but rather may contribute indirectly to a disturbance of the disc biomechanics by altering matrix-molecule interaction. However, expression of type-X collagen may indicate a change in chondrocyte phenotype that consistently coincides with the formation of chondrocyte clusters, one of the first alterations in osteoarthritis visible on histologic examination.

Localization of type X collagen in the intervertebral disc of mature beagle dogs

Type X collagen expression in intervertebral disc of young adult beagle dogs (n = 10) was studied. Type X collagen was immunostained mainly pericellularly in the central area of the vertebral endplate, but interterritorial staining there was also present. Annulus fibrosus and nucleus pulposus did not usually stain for type X collagen. However, immunostaining of nucleus pulposus for type X collagen with a simultaneous expression of collagen alpha1(X) mRNA was observed in one dog. A weak staining was observed in two other animals with a weak collagen alpha1(X) mRNA signal. In annulus fibrosus, lamellar staining was observed in two dogs. In three animals, type X collagen mRNAs were observed in the outer edge of the annulus fibrosus, but immunohistochemical staining did not always correlate with in situ hybridization signals. In conclusion, intervertebral disc type X collagen was mainly expressed in the cartilaginous endplate. In some apparently healthy animals there was type X collagen expression in the nucleus pulposus and also in the annulus fibrosus.

Variation with age in the pattern of type X collagen expression in normal and scoliotic human intervertebral discs

The distribution and expression of type X collagen, a calcium-binding collagen, which is a marker of hypertrophic chondrocytes and thought to be involved in cartilage calcification, was examined in situ in nondegenerate (grade I or II) human discs taken at autopsy over a wide age range (fetal->80 years) and also in scoliotic discs removed at surgery. In the fetal vertebral column, type X collagen was strongly expressed in the hypertrophic chondrocytes of the endplate, but was not seen in other areas. In the cartilaginous endplate of adults, it was found over the whole age range examined, with intensity increasing with age. In the disc matrix itself, type X collagen was demonstrated around individual cells from all individuals older than 50 years, but not in any fetal or autopsy disc from individuals younger than 40 years. In scoliotic discs, however, focal type X collagen expression was seen in 3/8 patients younger than 40 years including one 12-year-old. No type X collagen was found in the outer annulus in any autopsy or scoliotic disc, supporting the idea that cells of the outer annulus are phenotypically distinct from cells of the inner annulus and the nucleus. Our results demonstrate for the first time that type X collagen is a possible gene product of the intervertebral disc cells and a potential biochemical component of the disc matrix. They indicate that with age or in scoliosis, some cells from the inner annulus or nucleus of the disc differentiate to the hypertrophic chondrocyte phenotype. This might be the initiating event for the abnormal calcification described in aged and scoliotic discs in other studies.

Type X collagen, a natural component of mouse articular cartilage: association with growth, aging, and osteoarthritis

OBJECTIVE

To perform a systematic study on the production and deposition of type X collagen in developing, aging, and osteoarthritic (OA) mouse articular cartilage.

METHODS

Immunohistochemistry was employed to define the distribution of type X collagen and Northern analyses to determine the messenger RNA levels as an indicator of the synthetic activity of the protein.

RESULTS

Type X collagen was observed in the epiphyseal and articular cartilage of mouse knee joints throughout development and growth. Type X collagen deposition in the transitional zone of articular cartilage became evident toward cessation of growth, at the age of 2-3 months. The most intense staining for type X collagen was limited to the tidemark, the border between uncalcified and calcified cartilage. Northern analysis confirmed that the type X collagen gene is also transcribed by articular cartilage chondrocytes. Intense immunostaining was observed in the areas of OA lesions, specifically, at sites of osteophyte formation and surface fibrillation. Type X collagen deposition was also seen in degenerating menisci.

CONCLUSION

This study demonstrates that type X collagen is a natural component of mouse articular cartilage throughout development, growth, and aging. This finding and the deposition of type X collagen at sites of OA lesions suggest that type X collagen may have a role in providing structural support for articular cartilage.

Mapping of a carboxyl-terminal active site of parathyroid hormone by calcium-imaging

We recently showed that the C-terminal fragment PTH (52-84) effectively increases intracellular free calcium ([Ca2+]i) in a subset of growth plate chondrocytes not activated by the N-terminal PTH fragment (1-34). Here we characterize the active site on C-terminal PTH (52-84) with respect to calcium (Ca2+)-signaling and the mechanism involved by using synthetic PTH-subfragments in digital CCD ratio-imaging experiments. Our results show amino acids 73-76 to be the core region for increasing [Ca2+]i. Ryanodine (1 microM), caffeine (10 mM), lithium (2 mM), or cyclopiazonic acid (2-5 microM), agents that interfere with intracellular Ca2+ release, all failed to block PTH (52-84) induced [Ca2+]i increases. Depletion of extracellular calcium ([Ca2+]o) blocked PTH (52-84) induced [Ca2+]i increases, indicating a transmembrane Ca2+ influx. In contrast to voltage-gated and Ca2+ release activated Ca2+ influx, PTH (52-84) evoked Ca2+ influx was not blocked by nickel (1 mM). We conclude that PTH amino acids 73-76 are essential for activation of a nickel-insensitive Ca2+ influx pathway in growth plate chondrocytes that is likely to be of relevance for matrix calcification, a key step in endochondral bone formation.

Characterization of human type X procollagen and its NC-1 domain expressed as recombinant proteins in HEK293 cells

Type X collagen is a short-chain, network-forming collagen found in hypertrophic cartilage in the growth zones of long bones, vertebrae, and ribs. To obtain information about the structure and assembly of mammalian type X collagen, we generated recombinant human type collagen X by stable expression of full-length human alpha1(X) cDNA in the human embryonal kidney cell line HEK293 and the fibrosarcoma cell line HT1080. Stable clones were obtained secreting recombinant human type X collagen (hrColX) in amounts of 50 microg/ml with alpha1(X)-chains of apparent molecular mass of 75 kDa. Pepsin digestion converted the native protein to a molecule migrating as one band at 65 kDa, while bands of 55 and 43 kDa were generated by trypsin digestion. Polyclonal antibodies prepared against purified hrColX reacted specifically with type X collagen in sections of human fetal growth cartilage. Circular dichroism spectra and trypsin/chymotrypsin digestion experiments of hrColX at increasing temperatures indicated triple helical molecules with a reduced melting temperature of 31 degrees C as a result of partial underhydroxylation. Ultrastructural analysis of hrColX by rotary shadowing demonstrated rodlike molecules with a length of 130 nm, assembling into aggregates via the globular noncollagenous (NC)-1 domains as reported for chick type X collagen. NC-1 domains generated by collagenase digestion of hrColX migrated as multimers of apparent mass of 40 kDa on SDS-polyacrylamide gel electrophoresis, even after reduction and heat denaturation, and gave rise to monomers of 18-20 kDa after treatment with trichloroacetic acid. The NC-1 domains prepared by collagenase digestion comigrated with NC-1 domains prepared as recombinant protein in HEK293 cells, both in the multimeric and monomeric form. These studies demonstrate the potential of the pCMVsis expression system to produce recombinant triple helical type X collagens in amounts sufficient for further studies on its structural and functional domains.

Arthritis-related B cell epitopes in collagen II are conformation-dependent and sterically privileged in accessible sites of cartilage collagen fibrils

In collagen-induced arthritis, a murine autoimmune model for rheumatoid arthritis, immunization with native but not heat-denatured cartilage-specific collagen type II (CII) induces a B cell response that largely contributes to arthritogenicity. Previously, we have shown that monoclonal antibodies established from arthritis prone DBA/1 mice require the triple-helical conformation of their epitopes for antigen recognition. Here, we present a novel approach to characterize arthritis-related conformational epitopes by preparing a panel of 130 chimeric collagen X/CII molecules. The insertion of a series of CII cassettes into the triple-helical recombinant collagen X allowed for the first time the identification of five triple-helical immunodominant domains of 5-11 amino acid length, to which 75% of 36 monoclonal antibodies bound. A consensus motif, "R G hydrophobic," was found in all immunodominant epitopes. The antibodies were encoded by a certain combination of V-genes in germline configuration, indicating a role of the consensus motif in V-gene selection. The immunodominant domains are spread over the entire monomeric CII molecule with no apparent order; however, a highly organized arrangement became apparent when the CII molecules were displayed in the quarter-staggered assembly within a fibril. This discrete epitope organization most likely reflects structural constraints that restrict the exposure of CII epitopes on the surface of heterotypically assembled cartilage fibrils. Thus, our data suggest a preimmune B cell selection process that is biased by the accessibility of CII determinants in the intact cartilage tissue.

Transplantation of allograft chondrocytes embedded in agarose gel into cartilage defects of rabbits

OBJECTIVE

Durable healing of full-thickness articular cartilage defects has been considered for a long time as a highly desirable, but unlikely event to occur. In recent years, conflicting reports on the outcome of in vitro and in vivo studies on chondrocyte and cartilage grafting into animal and human joints have raised new arguments for and against controlled repair of articular cartilage following injury. Some of the problems result from insufficient characterization of implant and repair tissue, and from too short follow up phases. Here we describe a new approach to repair articular cartilage defects in rabbit knees by allografting chondrocytes cultured in agarose gels.

DESIGN

The implants were monitored over 6-18 months and graded histologically, immunohistochemically, and electron microscopically, using a grading scale based on seven evaluation criteria. Control implants of pure agarose produced poor fibrous substitute tissue, insufficient healing and incomplete filling of the cartilage defects. After transplantation of allogenic chondrocytes embedded in agarose, the quality of the newly formed repair cartilage was superior with respect to type II collagen and proteoglycan content and cellular architecture when compared with untreated defects. Superficial fibrillation and degradation were significantly diminished or prevented.

RESULTS

New subchondral bone formed at the level of the previous subchondral bone. In most cases the repair tissue merged with the host articular cartilage; normal calcified cartilage was the only tissue zone that did not participate in the integration of the transplant. By gross evaluation 24% of grafts showed an extent of recovery never observed in controls. The best results were obtained after 18 months when 47% of the grafts (N = 15) developed a morphologically stable hyaline cartilage.

CONCLUSION

These studies demonstrate that agarose-embedded chondrocyte may prove a valuable tool for controlled repair of articular cartilage defects.

Immunolocalization of type X collagen in human lumbar intervertebral discs during ageing and degeneration

Type X collagen has so far not been reported to occur in human intervertebral discs. The objective of this study was therefore to investigate the occurrence of type X collagen in human lumbar intervertebral discs during ageing and degeneration. Ninety intervertebral discs with adjacent endplates were excised in toto from individuals (0-86 years) without known spinal disease and were processed for routine decalcified histology. Appropriate slices of each disc were processed for immunohistochemistry using a type-specific, monoclonal antibody raised against human type X collagen. Each intervertebral disc was examined for macroscopic and histomorphological features of disc degeneration. Immunohistochemically, a positive specific type X staining was observed in the hypertrophic zone of the growth plate and only in the interstitial matrix of juvenile (<2 years) nucleus pulposus. In adult discs, type X collagen could be localized in conjunction with advanced disc degeneration and first occurred in the disc matrix (i.e., pericellular region) of a 47-year-old specimen. Positive type X staining of the disc matrix was more frequently found in senile (>70 years) discs with end stages of disc degeneration. This study provides the first evidence for the occurrence of type X collagen in human lumbar intervertebral discs and it appears that type X collagen is re-expressed in late stages of disc degeneration.

Synthesis of collagen by bovine chondrocytes cultured in alginate; posttranslational modifications and cell-matrix interaction

The extracellular matrix synthesized by articular chondrocytes cultured in alginate beads was investigated. Collagen levels increased sigmoidally with time and remained constant after 2 weeks of culture. The presence of cartilage-specific type II collagen was confirmed immunohistochemically. Predominantly type II collagen was present in the alginate bead, as reflected by the unique extent of lysyl hydroxylation, glycosylation, and pyridinoline crosslink formation measured. Collagen crosslinks, predominantly hydroxylysylpyridinoline (> 93%), were observed after 7 to 11 days of culture and their formation was effectively blocked by beta-aminopropionitrile (BAPN). Unexpectedly, BAPN treatment resulted in a 100% increase of collagen levels, without influencing cell proliferation and proteoglycan levels. In control cultures 90% of the synthesized collagen was retained in the cell-associated matrix, while in BAPN-treated cultures half of the collagen was found in the interterritorial matrix compartment further removed from the cells. This suggests that impaired crosslinking of collagen interferes with pericellular collagen deposition, causing upregulation of collagen synthesis by impaired cell-matrix interactions. Integrins are likely to be involved in this feedback inhibition by extracellular collagen since the cyclic RGD-containing peptide CGRGDSPC downregulated collagen synthesis by 37%.

Absence of integrin alpha 7 causes a novel form of muscular dystrophy

Integrin alpha 7 beta 1 is a specific cellular receptor for the basement membrane protein laminin-1 (refs 1,2), as well as for the laminin isoforms -2 and -4 (ref. 3). The alpha 7 subunit is expressed mainly in skeletal and cardiac muscle and has been suggested to be involved in differentiation and migration processes during myogenesis. Three cytoplasmic and two extracellular splice variants that have been described are developmentally regulated and expressed in different sites in the muscle. In adult muscle, the alpha 7A and alpha 7B subunits are concentrated in myotendinous junctions but can also be detected in neuromuscular junctions and along the sarcolemmal membrane. To study the potential involvement of alpha 7 integrin, during myogenesis and its role in muscle integrity and function, we generated a null allele of the alpha 7 gene (Itga7) in the germline of mice by homologous recombination in embryonic stem (ES) cells. Surprisingly, mice homozygous for the mutation are viable and fertile, indicating that the alpha 7 beta 1 integrin is not essential for myogenesis. However, histological analysis of skeletal muscle revealed typical symptoms of a progressive muscular dystrophy starting soon after birth, but with a distinct variability in different muscle types. The observed histopathological changes strongly indicate an impairment of function of the myotendinous junctions. These findings demonstrate that alpha 7 beta 1 integrin represents an indispensable linkage between the muscle fibre and the extracellular matrix that is independent of the dystrophin-dystroglycan complex-mediated interaction of the cytoskeleton with the muscle basement membrane.

Localization of silencer and enhancer elements in the human type X collagen gene

Collagen type X is a short, network-forming collagen expressed temporally and spatially tightly controlled in hypertrophic chondrocytes during endochondral ossification. Studies on chicken chondrocytes indicate that the regulation of type X collagen gene expression is regulated at the transcriptional level. In this study, we have analyzed the regulatory elements of the human type X collagen (Col10a1) by reporter gene constructs and transient transfections in chondrogenic and nonchondrogenic cells. Four different promoter fragments covering up to 2,864 bp of 5'-flanking sequences, either including or lacking the first intron, were linked to luciferase reporter gene and transfected into 3T3 fibroblasts, HT1080 fibrosarcoma cells, prehypertrophic chondrocytes from the resting zone, hypertrophic chondrocytes, and chondrogenic cell lines. The results indicated the presence of three regulatory elements in the human Col10a1 gene besides the proximal promoter. First, a negative regulatory element located between 2.4 and 2.8 kb upstream of the transcription initiation site was active in all nonchondrogenic cells and in prehypertrophic chondrocytes. Second, a positive, but also non-tissue-specific positive regulatory element was present in the first intron. Third, a cell-type-specific enhancer element active only in hypertrophic chondrocytes was located between -2.4 and -0.9 kb confirming a previous report by Thomas et al. [(1995): Gene 160:291-296]. The enhancing effect, however, was observed only when calcium phosphate was either used for transfection or included in the culture medium after lipofection. These findings demonstrate that the rigid control of human Col10a1 gene expression is achieved by both positive and negative regulatory elements in the gene and provide the basis for the identification of factors binding to those elements.

The B cell repertoire of patients with rheumatoid arthritis. II. Increased frequencies of IgG+ and IgA+ B cells specific for mycobacterial heat-shock protein 60 or human type II collagen in synovial fluid and tissue

OBJECTIVE

A qualitative and quantitative analysis of the functional, antigen-specific B cell receptor repertoire of patients with rheumatoid arthritis (RA) in synovial and peripheral compartments.

METHODS

B cells were activated to grow and differentiate at high efficiency in vitro under limiting-dilution conditions. Isotype and specificity of the secreted Ig were tested by enzyme-linked immunosorbent assay.

RESULTS

In contrast to peripheral B cells, most synovial B cells had already switched to IgG/IgA in vivo. The frequencies of B cells specifically recognizing foreign antigens were decreased within the synovial population, whereas the frequencies of B cells specific for type II collagen, mycobacterial heat-shock protein 60 (hsp60), or IgG Fc fragments were significantly increased, revealing a negative correlation in terms of frequencies.

CONCLUSION

B cells specific for human type II collagen, hsp60, and IgG Fc fragments are produced and/or expanded locally within the affected joints of RA patients. Thus, the specific immune system is definitely involved in the local inflammatory and destructive processes.

Annexin V interactions with collagen

Annexin V was originally identified as a collagen-binding protein called anchorin CII and was isolated from chondrocyte membranes by affinity chromatography on native type II collagen. The binding of annexin V to native collagen type II is stable at physiological ionic strength when annexin V is reconstituted in liposomes. The binding to native collagen types II and X, and to some extent to type I as well, was confirmed using recombinant annexin V. A physiological role for annexin V interactions with extracellular collagen is consistent with the localization of annexin V on the outer cell surface of chondrocytes, microvilli of hypertrophic chondrocytes, fibroblasts and osteoblasts. A breakthrough in our understanding of the function of annexin V was made with the discovery of its calcium channel activity. At least one of several putative functions of annexin V became obvious from studies on matrix vesicles derived from calcifying cartilage. It was found that calcium uptake by matrix vesicles depend on collagen type II and type X binding to annexin V in the vesicles and was lost when collagens were digested with collagenase: calcium influx was reconstituted after adding back native collagen II or V. These findings indicate that annexin V plays a major role in matrix vesicle-initiated cartilage calcification as a collagen-regulated calcium channel.

Type X collagen expression and hypertrophic differentiation in chondrogenic neoplasias

Little is known about matrix biochemistry and cell differentiation patterns in chondrogenic neoplasms. This is the first description of the focal expression of collagen type X by neoplastic chondrocytes in situ and its incorporation into the extracellular matrix of cartilaginous tumors. This shows that neoplastic chondrocytes have the potential to undergo the full program of cell differentiation, including hypertrophy, comparable to their physiological counterparts in the growth plate. However, only in benign osteochondromas was a zonal expression of type X collagen found similar to that observed in the growth plate, where the cells immediately above the ossification frontier are selectively positive for type X collagen. In enchondromas and chondrosarcomas, the expression was randomly distributed within the tumors. Surprisingly, in less differentiated chondrosarcomas with spindle-shaped cells and non-cartilaginous extracellular matrix, exceptional expression of collagen type X was observed, which indicates potential uncoupling of collagen type X expression from the differentiated chondrocytic phenotype in neoplastic chondrocytes in vivo.

Phenotypic diversity of neoplastic chondrocytes and extracellular matrix gene expression in cartilaginous neoplasms

Chondrocyte differentiation is characterized by distinct cellular phenotypes, which can be identified by specific extracellular matrix gene expression profiles. By applying in situ analysis on the mRNA and protein level in a series of benign and malignant human chondrogenic neoplasms, we were able to identify for the first time different phenotypes of neoplastic chondrocytes in vivo: 1) mature chondrocytes, which synthesized the characteristic cartilaginous extracellular tumor matrix, 2) cells resembling hypertrophic chondrocytes of the fetal growth plate, 3) cells resembling so-called dedifferentiated chondrocytes, and 4) well differentiated chondrocytic cells, which expressed type I collagen, indicating the presence of post-hypertrophic differentiated neoplastic chondrocytes. Chondrocytes exhibiting a range of phenotypes were found to be present in the same neoplasm. The different observed phenotypes, including the dedifferentiated phenotype, were in contrast to the anaplastic cells of high-grade chondrosarcomas. Comparison of expression data with tumor morphology revealed a relationship between the cellular phenotypes, the tumor matrix composition, and the matrix and cell morphology within the neoplasms. The distinctly different phenotypes of neoplastic chondrocytes are the basis of the characteristic high biochemical and morphological heterogeneity of chondroid neoplasms and shed light on their biological and clinical behavior.

Activation of fibrillar collagen synthesis and phenotypic modulation of chondrocytes in early human osteoarthritic cartilage lesions

The objective of this study was to investigate the expression and extracellular distribution of fibrillar collagen types I, II, and III in early stage osteoarthritic cartilage in order to elucidate matrix gene expression and cell differentiation in early phases of the disease. Arthroscopically, derived specimens of early stage osteoarthritic articular cartilage were analyzed by histochemistry, immunohistochemistry and by in situ hybridization and compared with normal articular cartilage samples. In normal articular cartilage no significant mRNA expression of any of the investigated collagen types was found. In early stage osteoarthritic specimens, a strongly enhanced mRNA expression of the major cartilage matrix component type II collagen was detected. Additionally, a focal onset of type III, but not type I collagen expression was observed. Thus, besides activation of matrix synthesis, the modulation of the chondrocytic phenotype is likely to play a distinct role in the cellular response in the early phases of the degenerative process in osteoarthritis.

Increased extracellular deposition of fibrillin-containing fibrils in pseudoexfoliation syndrome

PURPOSE

To localize the distribution of fibrillin-containing microfibrils in normal human anterior segment tissues and to characterize the role of fibrillin in the pathogenesis of pseudoexfoliation syndrome.

METHODS

Anterior segment tissues were obtained from 10 eyes with pseudoexfoliation syndrome and 10 normal eyes and investigated by indirect immunofluorescence and electron microscopic immunogold labeling using a monoclonal antibody to fibrillin-1.

RESULTS

In addition to labeling of zonular fibers, fibrillin-immunoreactive microfibrillar bundles generally were found in the corneal stroma; the stromal connective tissues of conjunctiva, ciliary body, and iris, especially in the iris root area; the periphery of Schlemm's canal, the scleral spur, and the most anterior portion of the trabecular meshwork; the ciliary muscle, and the dilator and sphincter muscles of the iris; the basement membranes of peripheral corneal epithelium, conjunctival epithelium, ciliary pigmented epithelium, and the lens capsule. The microfibrillar bundles were found to be isolated or in association with elastic fibers and cellular basement membranes. In pseudoexfoliation eyes, an additional strong immunoreaction was localized to pseudoexfoliation fibers and their microfibrillar subunits in close proximity to surfaces of cells involved in pseudoexfoliation fiber production.

CONCLUSIONS

The fibrillin-containing microfibrillar system in normal ocular tissues is suggested to have a substantial role in the maintenance of tissue integrity by providing tensile strength and flexibility to mechanically strained tissues. The findings further provide evidence for fibrillin as an intrinsic component of pseudoexfoliation fibers, suggesting the possibility that enhanced expression of fibrillin or abnormal aggregation of fibrillin-containing microfibrils may be involved in the pathogenesis of pseudoexfoliation syndrome.

Suppression of cartilage matrix gene expression in upper zone chondrocytes of osteoarthritic cartilage

OBJECTIVE

To evaluate the anabolic activity of osteoarthritic chondrocytes in situ by investigating the messenger RNA (mRNA) expression of 3 major cartilage components, type II collagen, aggrecan, and link protein:

METHODS

In situ hybridization experiments and histochemical analysis for proteoglycan content were performed on parallel sections of normal and osteoarthritic (OA) cartilage specimens.

RESULTS

Most chondrocytes in the deeper.zones of OA cartilage showed an increase in mRNA expression, in particular, of type II collagen and to a lesser extent, aggrecan, compared with normal specimens. However, chondrocytes of the upper zone were largely negative for aggrecan or type II collagen mRNA. The expression of link protein mRNA was low in normal and OA specimens.

CONCLUSION

These observations suggest that suppression of the anabolic activity of chondrocytes in the upper zones contributes to the metabolic imbalance observed in OA cartilage. Stimulation of matrix anabolism in superficial chondrocytes might be a suitable target for therapeutic intervention.

Variability in the upstream promoter and intron sequences of the human, mouse and chick type X collagen genes

The type X collagen gene is specifically expressed in hypertrophic chondrocytes during endochondral ossification. Transcription of the type X collagen gene by these differentiated cells is turned on at the same time as transcription of several other cartilage specific genes is switched off and before mineralization of the matrix begins. Analysis of type X collagen promoters for regulatory regions in different cell culture systems and in transgenic mice has given contradictory results suggesting major differences among species. To approach this problem, we have determined the nucleotide sequences of the two introns and upstream promoter sequences of the human and mouse type X collagen genes and compared them with those of bovine and chick. Within the promoter regions, we found three boxes of homology which are nearly continuous in the human gene but have interruptions in the murine gene. One of these interruptions was identified as a complex 1.9 kb repetitive element with homology to LINE, B1, B2 and long terminal repeat sequences. Regulatory elements of the human type X collagen gene are located upstream of the region where the repetitive element is inserted in the mouse gene, making it likely that the repetitive element is inserted between the coding region and regulatory sequences of the murine gene without interfering with its expression pattern. We also compared the sequences of the introns of both genes and found strong conservation. Comparisons of the mammalian sequences with promoter and first intron sequences of the chicken type X collagen gene revealed that only the proximal 120 nucleotides of the promoter were conserved, whereas all other sequences displayed no obvious homology to the murine and human sequences.

Differential effects of parathyroid hormone fragments on collagen gene expression in chondrocytes

The effect of parathyroid hormone (PTH) in vivo after secretion by the parathyroid gland is mediated by bioactive fragments of the molecule. To elucidate their possible role in the regulation of cartilage matrix metabolism, the influence of the amino-terminal (NH2-terminal), the central, and the carboxyl-terminal (COOH-terminal) portion of the PTH on collagen gene expression was studied in a serum free cell culture system of fetal bovine and human chondrocytes. Expression of alpha1 (I), alpha1 (II), alpha1 (III), and alpha1 (X) mRNA was investigated by in situ hybridization and quantified by Northern blot analysis. NH2-terminal and mid-regional fragments containing a core sequence between amino acid residues 28-34 of PTH induced a significant rise in alpha1 (II) mRNA in proliferating chondrocytes. In addition, the COOH-terminal portion (aa 52-84) of the PTH molecule was shown to exert a stimulatory effect on alpha1 (II) and alpha1 (X) mRNA expression in chondrocytes from the hypertrophic zone of bovine epiphyseal cartilage. PTH peptides harboring either the functional domain in the central or COOH-terminal region of PTH can induce cAMP independent Ca2+ signaling in different subsets of chondrocytes as assessed by microfluorometry of Fura-2/AM loaded cells. These results support the hypothesis that different hormonal effects of PTH on cartilage matrix metabolism are exerted by distinct effector domains and depend on the differentiation stage of the target cell.

Specific induction of cell motility on laminin by alpha 7 integrin

Laminin, the major glycoprotein of basement membranes, actively supports cell migration in development, tissue repair, tumor growth, metastasis, and other pathological processes. Previously we have shown that the locomotion of murine skeletal myoblasts is specifically and significantly enhanced on laminin but not on other matrix proteins. One of the major laminin receptors of myoblasts is the alpha 7 beta 1 integrin, which was first described in human MeWo melanoma cells and Rugli glioblastoma cells. In order to investigate and directly test the role of the alpha 7 integrin in cell migration on laminin, we expressed the murine alpha 7B splice variant in human 293 kidney cells and 530 melanoma cells which cannot migrate on laminin and are devoid of endogenous alpha 7. Northern blotting of the transfected cells showed that the alpha 7 mRNA was expressed efficiently, and the protein was detected on the cell surface by immunofluorescence and fluorescence-activated cell sorter analysis. Cell motility measurements by computer-assisted time-lapse videomicroscopy of the alpha 7-transfected cells revealed an 8-10-fold increase in motility on laminin-1 and its E8 fragment, but not on fibronectin. Mock-transfected cells did not migrate significantly of alpha 7-transfected 293 cells through laminin-coated filters in a Boyden chamber assay was significantly enhanced in comparison to mock transfected cells. These findings prove that alpha 7 integrin expression confers a gain of function-motile phenotype to immobile cells and may be responsible for transduction of the laminin-induced cell motility.

Identification and immunolocalization of laminin in cartilage

In a previous study on the role of integrins in the interaction of human chondrocytes with extracellular collagen and fibronectin (Dürr et al., (1993) Exp. Cell Res. 207, 235) we showed that chondrocytes adhere to laminin-1 (LN-1) in a beta 1-integrin-dependent manner. FACS analysis with various integrin antibodies including the monoclonal antibody GOH3 indicated the presence of the alpha 6 beta 1-laminin receptor on the chondrocyte surface. Anti-alpha 6 antibodies inhibited adhesion to the LN-1/E8 fragment, but not to whole laminin or heat-denatured Laminin-1, indicating that chondrocytes utilize at least two beta 1-integrins for laminin adhesion, one of which is alpha 6 beta 1 recognizing the LN-1/E8 fragment. The presence of alpha 6 beta 1-integrin on the chondrocyte surface also suggested the existence of laminin-like molecules in cartilage. Here we provide immunological and biochemical evidence in support of this possibility. Several polyclonal antibodies raised against laminin-1 or the LN-1/E8 fragment revealed a strong pericellular reaction in sections of human fetal epiphyseal cartilage and adult articular cartilage. In the fetal epiphysis laminin staining was most prominent in mature, large chondrocytes appearing in the secondary ossification zone, in particular, in the vicinity of invading capillary sprouts. Chondrocytes in the proliferating and hypertrophic zone of the growth plate and perichondrium cells were negative. All chondrocytes that stained for alpha 6-integrin also stained for laminin-1. A laminin-1-like molecule was extracted from hyaline cartilage with two bands migrating slightly faster than the alpha 1 and beta 1/gamma 1 subunits of laminin on SDS-gel electrophoresis. The two bands stained with anti-laminin-1 antibodies and could be immunoprecipitated with the same antibodies from metabolically labeled chondrocyte cultures. These findings suggests a role for laminin in developing cartilage and thus additional roles for laminins outside basement membranes.

[Inflammatory cytokine mediated anti-anabolic effects: a potential mechanism in rheumatoid cartilage degeneration]

Increased levels of inflammatory cytokines such as II-1 and TNF-alpha are described in rheumatoid and osteoarthritic synovial fluid. These mediators are also very well established anti-anabolic modulators of chondrocyte synthetic activity in vitro. Our study aimed to investigate, whether chondrocytes in rheumatoid and osteoarthritic cartilage in situ show reaction pattern compatible with the putative effects of these modulatory agents. Immunohistochemical analysis using type II collagen specific antibodies showed considerable loss of staining in many sites of osteoarthritic and rheumatoid articular cartilage. mRNA analysis showed besides an overall activation of synthetic activity in rheumatoid and osteoarthritic cartilage, a decreased expression of cartilage matrix proteins in the upper zone. The cease of the anabolic activity of rheumatoid and osteoarthritic chondrocytes and the increased catabolism of matrix components contributes to the anabolic-catabolic imbalance in rheumatoid and osteoarthritic cartilage and is suggestive to be a crucial event in the progress of the disease. It correlates well to the putative anti-anabolic effect of inflammatory cytokines such as II-1 and TNF-alpha and could indicate a potential role of these mediators in rheumatoid and osteoarthritic cartilage destruction.

Characterization of the binding region for the Yersinia enterocolitica adhesin YadA on types I and II collagen

OBJECTIVE

The plasmid-encoded adhesin YadA confers pathogenic functions on Yersinia enterocolitica, a microorganism associated with reactive arthritis. While emerging evidence has indicated that the persistence of the bacteria in individuals with reactive arthritis is a prerequisite for the development of the disease, the tissue specificity of this immunologic disease sequela remains elusive. The present study was undertaken to investigate YadA-mediated binding of Y enterocolitica to the most abundant collagens in joints, types I and II collagen.

METHODS

Binding studies were performed with recombinant Y enterocolitica strains and highly purified type II collagen and the alpha 1(I) chain of type I collagen, or fragments of these collagens generated by various enzymatic and nonenzymatic cleavage procedures. Interactions of bacteria with the proteins were determined in binding assays with radiolabeled proteins.

RESULTS

Binding regions for YadA were identified at the 181-amino acid fragment alpha 1(I)78CBN of type I collagen and the CB10 fragment of type II collagen. From binding and blocking experiments with alpha 1(I) fragments, cyanogen bromide-derived or mammalian collagenase-derived type II collagen fragments, and synthetic peptides with collagen-like structures, it was concluded that the binding site for YadA on collagen is determined by a restricted amino acid sequence and is defined within a highly homologous 134-amino acid region. Furthermore, the binding site is not affected by mammalian collagenase digest. Binding of YadA-positive yersiniae to collagen could be inhibited by an antiserum specific for YadA.

CONCLUSION

This study provides the first evidence of a binding site for bacterial proteins on collagens which is not determined by the repetitive sequence Gly-X-Y of collagens. We speculate that the binding region is conserved between types I and II collagen, the most abundant collagens in the joints. Specific binding of Yersinia products to joint collagens might contribute to the arthritogenic potential of enteropathogenic yersiniae.

Differential expression of collagen types I, II, III, and X in human osteophytes

BACKGROUND

Osteophytes are neoplastic cartilaginous and osseous protrusions growing at the margins of osteoarthritic joints. Their formation involves complex patterns of cellular proliferation, differentiation, as well as matrix synthesis and turnover that are poorly understood.

EXPERIMENTAL DESIGN

Here we report on an experimental approach using in situ hybridization and immunohistology to elucidate pathways of chrondrocyte differentiation in human osteophytes. Ab and cDNA probes for collagen types were used as specific parameters for chondrocyte phenotypes.

RESULTS

In early precartilaginous mesenchymal tissue, cytoplasmic mRNA for alpha 1(I) and alpha 1(III) collagen genes (Col1A1 and Col3A1) were found by in situ hybridization, correlating with the distribution of type I and III collagen as revealed by Ab staining. Strong expression of type II collagen both at mRNA and protein levels was the hallmark of chondrogenic differentiation in the cartilaginous zone of osteophytes. Type II collagen expression increased in all cartilaginous and fibrocartilaginous areas with growth and maturation of osteophytes. The signal intensity obtained after in situ hybridization with a COL2A1 probe was high and corresponded to that obtained in fetal cartilage, whereas normal adult articular cartilage usually did not show measurable type II collagen expression. In fibrocartilaginous areas, the most abundant, but heterogeneous tissue type seen in osteophytes, type II and III collagen mRNA expression overlapped considerably. Type III collagen was scattered, both pericellularly and interterritorially, over the whole osteophyte, excluding bone and chondrocytic cells of the deep zone. The strongest type I collagen expression was seen in bone and in the superficial fibrous layer. In areas of endochondral ossification, large chondrocytes were found expressing type X collagen, a specific marker for hypertrophic chondrocytes.

CONCLUSIONS

These results show that discrete stages of cartilage differentiation can be precisely followed in osteophytes using collagen type-specific cDNA probes and Ab as markers. In addition, a fibrocartilaginous chondrocyte phenotype was identified that expresses type II and III, but not type I collagen.

Collagen binding activity of recombinant and N-terminally modified annexin V (anchorin CII)

We have cloned the full coding cDNA sequence of chicken annexin V and of a mutant lacking 8 amino acid residues of the N-terminal tail for prokaryotic expression. Both proteins were synthesized in Escherichia coli upon induction with isopropyl thio-beta-D-galactoside, and were purified following two different protocols: one based on the ability of these proteins to interact reversibly with liposomes in the presence of calcium, and the other based on two sequential ion-exchange chromatographic steps. Spectroscopical analysis of recombinant annexin V revealed that binding of calcium did not change the circular dichroism spectra indicating no significant changes on the secondary structure; however, a conformational change affecting the exposition to the solvent of the tryptophan residue 187 was detected by analysis of fluorescence emission spectra. Recombinant annexin V binds with high affinity to collagen types II and X, and with lower affinity to collagen type I in a calcium-independent manner. Heat denaturing of collagen decreases this interaction while pepsin-treatment of collagen almost completely abolishes annexin V binding. Mutated annexin V interacts with collagen in a similar way as the nonmutated recombinant protein, indicating that the N-terminal tail of annexin V is not essential for collagen binding.

Polar distribution of annexin-like proteins during phytochrome-mediated initiation and growth of rhizoids in the ferns Dryopteris and Anemia

Although the calcium requirement of phytochrome-mediated fern spore germination and early rhizoid growth is well established, the calcium-binding proteins that serve as transducers for these responses are not known. Here we report the presence of annexin-like proteins in germinating spores of Dryopteris filix-mas (L.) Schott and Anemia phyllitidis (L.) Sw. and evidence that they may be important participants in early photomorphogenic changes in gametophytes. Immunolocalization and immunoblot assays of these proteins were carried out using polyclonal antibodies raised either against a 35-kDa annexin-like protein from pea or against anchorin CII from chicken. Western-blot analysis showed that crude protein extracts obtained from both species after red-light treatment contained two cross-reactive protein bands with molecular weights around 70 kDa. These proteins were annexin-like in that they bound to a phosphatidylserine affinity column in a calcium-dependent fashion. Using this column, two protein bands around 70 kDa, i.e. 67 and 73 kDa, were partially purified together with proteins at 36 kDa and a doublet at 54 kDa. Proteins of these latter molecular weights are suggested to be members of the annexin family, but no cross-reactivity could be found between these and the two antibodies used in our investigations. Immunodetectable levels of these proteins were observed only after light-mediated induction of spore germination. Imaging of the immuno-localization patterns observed with both antibodies showed that the annexin-like proteins are concentrated at the extreme tips of the rhizoids in D. filix-mas and A. phyllitidis during rhizoid initiation and all stages of elongation. We suggest that these proteins may play a major role in the tip-oriented exocytosis events that are critical for the initiation and growth of fern rhizoids.

Interaction of enteropathogenic Yersinia enterocolitica with complex basement membranes and the extracellular matrix proteins collagen type IV, laminin-1 and -2, and nidogen/entactin

The plasmid-encoded virulence factor yersinia adhesin A (YadA) contributes to pathogenicity of Yersinia enterocolitica which might be related to its adhesive potential. Therefore, we have investigated the interaction of Y. enterocolitica with basement membrane (BM) and with the major BM proteins collagen type IV, laminin, and nidogen/entactin. Recombinant YadA-positive but not YadA-negative yersiniae bound specifically to lens capsule BM tissue, as well as purified collagen type IV and the laminin-1 and -2 (formally known as merosin) isoforms. Binding sites are located on the alpha 1 chain of the 58-nm amino-terminal 7sL fragment of collagen type IV and on the elastase-fragment E1 of laminin-1. YadA-mediated binding of yersiniae to collagen type IV was rapid and saturable, it was independent of divalent cations, stable over a wide pH range, and not influenced by higher salt concentrations. D-Glucose and D-galactose did not interfere with binding, indicating a protein-protein interaction. In contrast, adhesion of yersiniae to the laminin-2 isoform occurred also independent of YadA expression and no binding was observed to nidogen/entactin. The results support the hypothesis that adhesion of Y. enterocolitica could contribute to pathogenicity of enteropathogenic yersiniae. Further definition of binding sites for YadA on BM proteins might allow determination of the relevance of Yersinia-BM interactions to infection.

Organisation of the chicken annexin V gene and its correlation with the tertiary structure of the protein

Chicken annexin V (anchorin CII) is a collagen binding, membrane-associated molecule with Ca2+ channel activity. Here we report on the coding sequences, promoter region, size and distribution of exons, and exon-intron junctions of the chicken annexin V gene. It is about 25 kb long and codes for 13 short exons between 50 and 581 bp length. Exon sizes and locations of splice sites are almost completely homologous to those of the human and mouse annexin II or pigeon annexin I genes, although there is only 50-60% homology in the sequence of the corresponding proteins. The four repeat structure and symmetry of the annexin V as evident from sequence and X-ray analysis studies is only partially reflected in this highly conserved exon distribution. In the first two repeats of chicken annexin V the exons correlate with protein domains containing one, two, or three alpha-helices, while in the repeats 3 and 4 exon junctions and alpha-helical domains do not correlate. The analysis of the promoter structure revealed the absence of a typical TATA-box, but a GC-rich region which may possibly promote transcription from several start sites.

Immunohistochemical analysis of interstitial collagens in cartilage of different stages of osteoarthrosis

The distribution of the interstitial collagens I, II and III was analyzed immunohistochemically in cartilage and bone samples from 32 patients with degenerative osteoarthrosis at various morphological stages. The alterations observed showed a very patchy, focal distribution demonstrating significant heterogeneity in the tissue reaction. In minor osteoarthrotic lesions a focal pericellular deposition of collagens III and I was seen, while the majority of the interterritorial matrix reacted exclusively with collagen II antibodies. These changes were first seen in the superficial cartilage layer. At the more advanced stages of osteoarthrosis, particularly when osteophytic bone spur formation was present, extensive changes in the expression of collagen types in the pericellular matrix was revealed with extensive and overlapping localization of collagens I, II and III in the whole cartilage. These observations support the suggestion that degenerative cartilage shows a collagen type "switch". In addition, it was demonstrated that the interterritorial cartilage matrix was still mainly composed of collagen II even in advanced lesions. These observations may explain some of the previous discrepancies reported.