Specific hybridization probes for mouse alpha 2(IX) and alpha 1(X) collagen mRNAs

Bone defect repair in immobilization-induced osteopenia: a pQCT, biomechanical, and molecular biologic study in the mouse femur

The present study was carried out to determine whether immobilization-induced (Im) osteopenic bone possesses the same reparative capacity as normal healthy bone. Furthermore, the effects of mechanical loading versus immobilization on bone defect healing were studied. Three-week cast-immobilization was used to induce local osteopenia in mice. A standardized metaphyseal bone defect of the distal femur was created unilaterally both in immobilization-induced (Im) osteopenic mice and in nonimmobilized (Mo) age-matched control animals. After creation of the bone defect, the animals in both groups were further divided into two groups: 3-week cast-immobilization (Im-Im and Mo-Im) groups, and unrestricted weight-bearing (Im-Mo and Mo-Mo) groups. The healing process was followed up to 3 weeks using RNA analysis, histomorphometry, biomechanical testing, and pQCT measurements. At 3 weeks of healing without immobilization, bone mineral density (BMD), as well as bone bending stiffness and strength were higher in normal (Mo-Mo) than in osteopenic (Im-Mo) bone. Although the levels of mRNAs characteristic to chondrocytes (Sox9 and type II collagen), hypertrophic chondrocytes (Type X collagen), osteoblasts (type I collagen and osteocalcin), and osteoclasts (cathepsin K) during the bone defect healing exhibited similarities in their expression profiles, mechanical loading conditions also caused characteristic differences. Mechanical loading during healing (Mo-Mo group) induced stronger expression of cartilage- and bone-specific genes and resulted in higher BMD than that seen in the cast-immobilized group (Mo-Im). In biomechanical analysis, increased bending stiffness and strength were also observed in animals that were allowed weight-bearing during healing. Thus, our study shows that bone healing follows the same molecular pathway both in osteopenic and normal bones and presents evidence for reduced or delayed regeneration of noncritical size defects in immobilization-induced osteopenic bone.

Abnormal craniofacial development and expression patterns of extracellular matrix components in transgenic Del1 mice harboring a deletion mutation in the type II collagen gene

OBJECTIVE

To analyze the effect of a type II collagen mutation on craniofacial development in transgenic Del1 mice.

DESIGN

Samples from homozygous (+/+) and heterozygous (+/-) transgenic Del1 mice harboring mutations in the type II collagen gene as well as non-transgenic (-/-) littermates were collected at days 12.5, 14.5, 16.5 and 18.5 of gestation. The cartilaginous and bony elements of the craniofacial skeleton were analyzed after staining with alcian blue, alizarin red S and von Kossa. The expression patterns of type II, IX and X collagens and aggrecan were analyzed by immunohistochemistry and in situ hybridization.

RESULTS

Several abnormalities were observed in the craniofacial skeleton of transgenic Del1 mice. These include an overall retardation of chondrogenesis and osteogenesis in Del1 +/+ mice, and to a lesser extent also in Del1+/- mice. Characteristic findings in Del1 +/+ mice included a reduced anterioposterior length, a smaller size of the mandible, a palatal cleft and a downward bending snout. We also detected retarded ossification of calvarial bones in Del1 +/+ and +/- mice when compared with Del1 -/- mice. A surprising finding was the presence of both type II and X collagens and their mRNAs in the periosteum of the cranial base.

CONCLUSION

The present study confirms the important role of type II collagen mutation in craniofacial development and growth. In addition to affecting endochondral ossification, the type II collagen mutation also disturbs intramembranous ossification in the developing craniofacial skeleton.

Expression patterns of cartilage collagens and Sox9 during mouse heart development

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

Anabolic actions of parathyroid hormone during bone growth are dependent on c-fos

PTH has anabolic and catabolic actions in bone that are not clearly understood. The protooncogene c-fos and other activating protein 1 family members are critical transcriptional mediators in bone, and c-fos is up-regulated by PTH. The purpose of this study was to examine the mechanisms of PTH and the role of c-fos in PTH-mediated anabolic actions in bone. Mice with ablation of c-fos (-/-) and their wild-type (+/+) and heterozygous (+/-) littermates were administered PTH for 17 d. The +/+ mice had increased femoral bone mineral density (BMD), whereas -/- mice had reduced BMD after PTH treatment. PTH increased the ash weight of +/+ and +/-, but not -/-, femurs and decreased the calcium content of -/-, but not +/+ or +/-, femurs. Histomorphometric analysis showed that PTH increased trabecular bone volume in c-fos +/+, +/- vertebrae, but, in contrast, decreased trabecular bone in -/- vertebrae. Serum calcium levels in +/+ mice were greater than those in -/- mice, and PTH increased calcium in -/- mice. Histologically, PTH resulted in an exacerbation of the already widened growth plate and zone of hypertrophic chondrocytes but not the proliferating zone in -/- mice. PTH also increased calvarial thickness in +/+ mice, but not -/- mice. The c-fos -/- mice had lower bone sialoprotein and osteocalcin (OCN), but unaltered PTH-1 receptor mRNA expression in calvaria, suggesting an alteration in extracellular matrix. Acute PTH injection (8 h) resulted in a decrease in osteocalcin mRNA expression in wild-type, but unaltered expression in -/-, calvaria. These data indicate that c-fos plays a critical role in the anabolic actions of PTH during endochondral bone growth.

Diminished callus size and cartilage synthesis in alpha 1 beta 1 integrin-deficient mice during bone fracture healing

Integrins mediate cell adhesion to extracellular matrix components. Integrin alpha 1 beta 1 is a collagen receptor expressed on many mesenchymal cells, but mice deficient in alpha 1 integrin (alpha1-KO) have no gross structural defects. Here, the regeneration of a fractured long bone was studied in alpha1-KO mice. These mice developed significantly less callus tissue than the wild-type (WT) mice, and safranin staining revealed a defect in cartilage formation. The mRNA levels of nine extracellular matrix genes in calluses were evaluated by Northern blotting. During the first 9 days the mRNA levels of cartilage-related genes, including type II collagen, type IX collagen, and type X collagen, were lower in alpha1-KO mice than in WT mice, consistent with the reduced synthesis of cartilaginous matrix appreciated in tissue sections. Histological observations also suggested a diminished number of chondrocytes in the alpha 1-KO callus. Proliferating cell nuclear antigen staining revealed a reduction of mesenchymal progenitors at the callus site. Although, the number of mesenchymal stem cells (MSCs) obtained from WT and alpha 1-KO whole marrow was equal, in cell culture the proliferation rate of the MSCs of alpha 1-KO mice was slower, recapitulating the in vivo observation of reduced callus cell proliferation. The results demonstrate the importance of proper collagen-integrin interaction in fracture healing and suggest that alpha1 integrin plays an essential role in the regulation of MSC proliferation and cartilage production.

Accelerated up-regulation of L-Sox5, Sox6, and Sox9 by BMP-2 gene transfer during murine fracture healing

Fracture repair is the best-characterized situation in which activation of chondrogenesis takes place in an adult organism. To better understand the mechanisms that regulate chondrogenic differentiation of mesenchymal progenitor cells during fracture repair, we have investigated the participation of transcription factors L-Sox5, Sox6, and Sox9 in this process. Marked up-regulation of L-Sox5 and Sox9 messenger RNA (mRNA) and smaller changes in Sox6 mRNA levels were observed in RNAse protection assays during early stages of callus formation, followed by up-regulation of type II collagen production. During cartilage expansion, the colocalization of L-Sox5, Sox6, and Sox9 by immunohistochemistry and type II collagen transcripts by in situ hybridization confirmed a close relationship of these transcription factors with the chondrocyte phenotype and cartilage production. On chondrocyte hypertrophy, production of L-Sox5, Sox9 and type II collagen were down-regulated markedly and that of type X collagen was up-regulated. Finally, using adenovirus mediated bone morphogenetic protein 2 (BMP-2) gene transfer into fracture site we showed accelerated up-regulation of the genes for all three Sox proteins and type II collagen in fractures treated with BMP-2 when compared with control fractures. These data suggest that L-Sox5, Sox6, and Sox9 are involved in the activation and maintenance of chondrogenesis during fracture healing and that enhancement of chondrogenesis by BMP-2 is mediated via an L-Sox5/Sox6/Sox9-dependent pathway.

Expression of Sox9 and type IIA procollagen during attempted repair of articular cartilage damage in a transgenic mouse model of osteoarthritis

OBJECTIVE

To determine the capacity of chondrocytes in aging and degenerating articular cartilage to produce major components of the extracellular matrix and maintain the normal structure of articular cartilage in a transgenic mouse model of osteoarthritis.

METHODS

Transcription factor Sox9 was used as an indicator of the activation and maintenance of the articular chondrocyte phenotype. Knee joints of Del1 mice carrying 6 copies of the pro alpha1(II) collagen transgene with a short deletion mutation were analyzed at the age of 10 days and at 2, 3, 4, 6, 9, and 15 months by Northern hybridization, RNase protection assay, quantitative reverse transcription-polymerase chain reaction, and immunohistochemistry. Nontransgenic littermates were used as controls.

RESULTS

We demonstrated the presence of Sox9 in articular chondrocytes during development, growth, and aging, with the highest messenger RNA levels during the period of rapid growth. With the appearance of degenerative lesions in articular cartilage, 2 repair processes were observed. Local proliferation and activation of chondrocytes rich in Sox9, surrounded by type IIA procollagen and proteoglycans, was seen in articular cartilage. In contrast, metabolically inactive chondrocytes were observed at the margins of the defects. They were devoid of Sox9 and were surrounded by a proteoglycan-poor matrix. Sometimes, the lesions were filled with repair tissue that contained type III collagen but little proteoglycan or type II collagen.

CONCLUSION

The results indicate that chondrocytes in mature articular cartilage are capable of inducing the production of Sox9 and type IIA procollagen, which is typical of early chondrogenesis. Degenerative defects in the knee joints of transgenic Del1 mice are associated with local activation of chondrocytes, which probably contributes to the repair process. In other areas, the repair process produces a noncartilaginous matrix, which is insufficient to maintain the integrity of articular cartilage and which allows degeneration to proceed.

Expression of extracellular matrix genes: transforming growth factor (TGF)-beta1 and ras in tibial fracture healing of lathyritic rats

Experimental osteolathyrism, induced by dietary aminoacetonitrile (AAN), was used to study the effect of altered extracellular matrix on the expression of connective tissue components in long bone healing. AAN inhibits lysyl oxidase, which is needed for the formation of collagen cross-link precursors, and is also shown to act as a regulator of Ras. Fractured tibias in lathyritic rats develop excessive amounts of mechanically weak callus tissue with irregular cartilage and reduced glycosaminoglycan accumulation. Cartilage-specific proteins (collagen types II, IX, and X and aggrecan) were expressed temporally much wider in lathyritic calluses than in the controls, and active transcription was observed even during the fibrous and ossifying stages. Soft connective tissue was still present in 2- and 3-week-old lathyritic calluses and could explain the elevated type III collagen, biglycan, and decorin mRNA levels. Both transforming growth factor (TGF)-beta1 and c-Ha-ras, which control cell growth and differentiation, were upregulated during the cartilaginous stage. The maximal expression of TGF-beta1 preceded that of ras in osteolathyrism.

A novel in vitro culture system for analysis of functional role of phosphate transport in endochondral ossification

In vivo expression of the type III sodium-dependent phosphate transporter (NaPiT) Glvr-1 during endochondral ossification, suggests a functional role for inorganic phosphate (Pi) transport in cartilage calcification. For further analysis of this relationship, an in vitro model of endochondral ossification is required. In this context, we investigated the characteristics of Pi transport in the new chondrogenic cell line ATDC5 in relation to extracellular matrix (ECM) formation and mineralization. Pi uptake in ATDC-5 cells and in isolated matrix vesicles (MVs) is mediated by an Na-dependent Pi transporter with a pH dependency characteristic of a type III Pi carrier (lower activity at alkaline pH). Northern blot analysis indicated that ATDC-5 cells express Glvr-1 transcripts during the various stages of their maturation with a maximal level during the proliferating stage. In isolated MVs, Pi transport activity was maximal at day 21, concomitant with the beginning of type X collagen messenger RNA expression. These events preceded the initiation of matrix mineralization, which was apparent at day 25, and then gradually increased until day 47. This temporal relationship between maximal Pi transport activity in MVs and the expression of a marker of mineralizing chondrocytes is compatible with the possible involvement of Pi transport in the ECM calcification observed in ATDC-5 cell cultures. In conclusion, these observations suggest that ATCD-5 cells in culture represent a promising model for the analysis of a functional role of Pi transport in the initial events of endochondral ossification.

Sox9 is required for cartilage formation

Chondrogenesis results in the formation of cartilages, initial skeletal elements that can serve as templates for endochondral bone formation. Cartilage formation begins with the condensation of mesenchyme cells followed by their differentiation into chondrocytes. Although much is known about the terminal differentiation products that are expressed by chondrocytes, little is known about the factors that specify the chondrocyte lineage. SOX9 is a high-mobility-group (HMG) domain transcription factor that is expressed in chondrocytes and other tissues. In humans, SOX9 haploinsufficiency results in campomelic dysplasia, a lethal skeletal malformation syndrome, and XY sex reversal. During embryogenesis, Sox9 is expressed in all cartilage primordia and cartilages, coincident with the expression of the collagen alpha1(II) gene (Col2a1) . Sox9 is also expressed in other tissues, including the central nervous and urogenital systems. Sox9 binds to essential sequences in the Col2a1 and collagen alpha2(XI) gene (Col11a2) chondrocyte-specific enhancers and can activate these enhancers in non-chondrocytic cells. Here, Sox9 is identified as a regulator of the chondrocyte lineage. In mouse chimaeras, Sox9-/- cells are excluded from all cartilages but are present as a juxtaposed mesenchyme that does not express the chondrocyte-specific markers Col2a1, Col9a2, Col11a2 and Agc. This exclusion occurred cell autonomously at the condensing mesenchyme stage of chondrogenesis. Moreover, no cartilage developed in teratomas derived from Sox9-/- embryonic stem (ES) cells. Our results identify Sox9 as the first transcription factor that is essential for chondrocyte differentiation and cartilage formation.

Production of cartilage collagens during metaphyseal bone healing in the mouse

Small defects of unfractured bone are believed to heal without a cartilaginous intermediate. We have determined the extent of cartilage production in an experimental model of metaphyseal bone repair involving defects in both cortical and cancellous bone, but no fracture. Northern analyses revealed the presence of mRNAs for type X and II collagens in the repair tissue. Immunohistology confirmed subperiosteal deposition of both collagen types adjacent to the defect. While the mRNAs for the two collagen types peaked by one week of defect healing, immunodetectable type X collagen was not observed until the second week. The data suggest that reactivity of periosteum and activation of chondrogenesis and subsequent endochondral ossification programs are involved in murine bone repair regardless of defect type.

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.

Absence of the alpha1(IX) chain leads to a functional knock-out of the entire collagen IX protein in mice

Cartilage fibrils contain collagen II as well as smaller amounts of collagens IX and XI. The three collagens are thought to co-assemble into cartilage-specific arrays. The precise role of collagen IX in cartilage has been addressed previously by generating mice harboring an inactivated Col9a1 gene encoding the alpha1(IX) chain, i.e. one of the three constituent chains of collagen IX (Fässler, R., Schnegelsberg, P. N. J., Dausman, J., Shinya, T., Muragaki, Y., McCarthy, M. T., Olsen, B. R., and Jaenisch, R. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 5070-5074). The animals did not produce alpha1(IX) mRNA or polypeptides and were born with no conspicuous skeletal abnormality but post-natally developed early onset osteoarthritis. Here we show that the deficiency in alpha1(IX) chains leads to a functional knock-out of all polypeptides of collagen IX, whereas the Col9a2 and Col9a3 genes were normally transcribed. Therefore, synthesis of alpha1(IX) polypeptides is essential for the assembly of heterotrimeric collagen IX molecules. Surprisingly, cartilage fibrils of all shapes and banding patterns found in normal newborn, adolescent, or adult mice were formed in transgenic animals, although they lacked collagen IX. Therefore, collagen IX is not essential, and may be functionally redundant, in fibrillogenesis in cartilage in vivo. The protein is required, however, for long term tissue stability, presumably by mediating interactions between fibrillar and extrafibrillar macromolecules.

Developmental regulation of mRNA species for types II, IX and XI collagens during mouse embryogenesis

Several techniques were used to study the co-ordination of mRNA levels for five constituent chains of cartilage collagen fibrils during mouse development. Short cDNA clones were first constructed for mouse and human alpha3(IX) and for mouse proalpha1(XI) collagen mRNA species. Northern analysis of developing mouse embryos revealed that the mRNA species for alpha1, alpha2 and alpha3 chains of type IX collagen peaked earlier than those for proalpha1(II) and proalpha1(XI) collagen chains. Quantification of these mRNA species by slot-blot hybridization confirmed this developmental regulation: the mRNA ratios for type II/type IX/type XI collagens changed from 5.7:1:0.6 (at embryonic day 12.5) to 10.6:1:0.9 (in newborn mice). However, the genes coding for the three chains of type IX collagen seemed to be under more co-ordinated regulation during mouse development. In addition to high mRNA levels in cartilages and the eye, low levels of type IX collagen transcripts were identified in brain and skin of newborn mouse using RNase protection and reverse transcriptase-PCR assays. Finally, hybridization in situ revealed identical tissue distributions of the three type IX collagen mRNA species during early chondrogenesis but somewhat more widespread expression of the alpha1(IX) and alpha3(IX) mRNA species during endochondral ossification at day 16.5 of embryonic development. These results suggest a relatively tight co-ordination of the alpha1(IX), alpha2(IX), and alpha3(IX) collagen mRNA species in chondrocytes, but a lack of co-ordination in several non-cartilaginous tissues.

Type IX collagen NC1 domain peptides can trimerize in vitro without forming a triple helix

Synthetic peptides of the three chains of type IX collagen consisting of the carboxyl-terminal end of the COL1 domain and the complete NC1 domain were characterized by circular dichroism spectroscopy and analyzed for their ability to assemble into trimers. In vitro association and oxidation result in disulfide-linked oligomers as shown by molecular sieve chromatography and SDS-polyacrylamide electrophoresis. Whereas the individual peptides show a tendency to self-associate, when an equimolar amount of the three peptides was oxidized, a heterotrimer of the three chains was observed. This heterotrimer is recognized by a monoclonal antibody against the disulfide-linked NC1 domain of chicken type IX collagen, indicating the correct formation of the disulfide bonds. Circular dichroism measurements show that under the association conditions used, a triple helix does not form between the chains. These results indicate that these peptides contain all the necessary information for chain selection and assembly.

Expression of type VI, IX and XI collagen genes and alternative splicing of type II collagen transcripts in fracture callus tissue in mice

The levels of six mRNAs coding for constituent alpha-chains of three minor collagens of cartilage were analyzed in an experimental fracture model in normal and transgenic Del1 mice harboring a deletion mutation of exon 7 in the type II collagen gene. Reduced and retarded chondrogenesis in Del1 mice was evident in callus samples as reduced mRNA levels for the cartilage specific type IX and XI collagens at days 7 and 9 of fracture healing. Analysis of the calluses for alternative splicing of pro alpha 1(II) collagen mRNA also suggested retarded chondrogenesis in Del1 calluses. Another developmentally regulated step in limb development, a switch between alternative promoters of the alpha 1(IX) collagen gene, was also seen during fracture healing but was less obvious in Del1 calluses. Finally, the current data suggest that the abnormality in bone remodelling in Del1 mice involves activation of the genes coding for alpha 1(XI) and alpha 2(VI) collagens.

Extended expression of cartilage components in experimental pseudoarthrosis

The healing of femoral fractures in an experimental rat pseudoarthrosis model was followed by studying the expression of cartilage specific genes coding for type II and X collagens and aggrecan, soft tissue and bone specific type I collagen, and decorin. Severe impairment of healing was observed with cartilage gene expression continuing until the seventh week and then declining rapidly. The abnormal healing pattern results in an inactive scar-like callus after the ninth week of healing even though house-keeping (e.g., GAPDH) genes are continuously expressed in the tissue. These results could be explained on the basis of continuous chondrogenic stimulus extending much beyond the normal range. If union is not achieved because of mechanical instability, signal of endochondral ossification persists until it becomes exhausted and callus at the fracture gap becomes an inactive fibrous scar. The disturbed matrix gene expression was confirmed by histology.

Characterization of primary cultures of chondrocytes from type II collagen/beta-galactosidase transgenic mice

Studies on the function of extracellular matrix components of cartilages and on chondrocyte-specific regulatory mechanisms will benefit from approaches in which transgenic mice and cell cultures will complement each other. We therefore established and extensively characterized primary cultures of mouse chondrocytes isolated from rib growth plates of newborn mice harboring a transgene in which type II collagen gene regulatory sequences were driving expression of an E. coli beta-galactosidase reporter gene. Primary chondrocytes expressed a fully differentiated phenotype in monolayer culture, producing mRNAs for the collagen types II, IX and X, and for the transgene. Transgenic cells also synthesized high levels of E. coli beta-galactosidase, easily quantifiable and also detectable in individual cells by X-gal staining. When chondrocytes were isolated from transgenic mice in which beta-galactosidase was fused to the product of the neomycin resistance gene, they displayed resistance to G418. After one to two weeks in culture, chondrocytes progressively lost expression of the transgenes, in parallel with that of cartilage-specific genes, and started expressing high levels of type I collagen RNA. The use of transgenic chondrocytes allowed us to easily score phenotypic changes by assaying beta-galactosidase activity and neomycin resistance. Cultures of mouse chondrocytes, such as those reported here, should also help characterize biochemically the phenotypes of other transgenic mice in studies of genetic diseases of cartilages and of mechanisms involved in chondrogenesis.

Retarded chondrogenesis in transgenic mice with a type II collagen defect results in fracture healing abnormalities

We have examined the biological and biomechanical consequences of defective type II collagen production for fracture repair employing a genetically engineered mouse line Del1 which was generated by microinjection of a 39-kb mouse pro alpha 1(II) collagen gene construct containing a deletion of exon 7 and intron 7 (Metsäranta et al. [1992] J. Cell Biol. 118:203-212). Standardized tibial fractures were produced in transgenic Del1 mice and their nontransgenic littermates were used as controls. The fracture callus tissues were analyzed at days 7, 9, 14, 28, and 42 using radiography, histomorphometry, biomechanical testing, and Northern analysis of mRNAs for several tissue-specific matrix components. Deficient production of cartilage in Del1 mice resulted in reduced radiographic callus size, smaller cross-sectional area, and impaired biomechanical properties when compared with fractures of nontransgenic control mice. The differences were most evident in 14-day fracture calluses. Consequently mRNAs for cartilage-specific type IX and X collagens and aggrecan were also reduced in Del1 calluses. Levels of type II collagen mRNAs were unaffected since the mutated transgene produced additional type II collagen mRNA molecules. Further abnormalities in the fracture repair process of Del1 mice were observed in callus remodeling. In the control animals a typical feature of external callus remodeling was reduction of callus size during endochondral ossification between days 14 and 28. Such reduction was not observed in the transgenic mice. Histological examination of fracture calluses suggested also a reduction in trabecular surface area, which was found to be even more pronounced in metaphyseal bone of Del1 mice. Despite these differences the biomechanical properties of the calluses in the two groups became similar by day 28 of fracture healing. The results thus suggest that reduced chondrogenesis due to the presence of mutated transgenes in Del1 mice not only causes a temporary impairment in biomechanical properties of healing fractures but also affects later stages of callus remodeling.

Molecular cloning of the human alpha 2(IX) collagen cDNA and assignment of the human COL9A2 gene to chromosome 1

Type IX collagen, a heterotrimer of alpha 1(IX), alpha 2(IX) and alpha 3(IX) chains, is a cartilage-specific fibril-associated collagen. In the process of characterizing genomic clones for the mouse alpha 2(IX) collagen gene four pairs of oligonucleotide primers designed for amplification of murine exon sequences were also utilized to construct cDNA clones for human alpha 2(IX) collagen spanning > 90% of the coding region. The amino acid and nucleotide sequence identities between human and chick are 78% and 71%, respectively. Localization of the COL9A2 gene to human chromosome 1 was subsequently performed using a panel of DNAs from human/rodent somatic cell hybrids.

The mouse collagen X gene: complete nucleotide sequence, exon structure and expression pattern

Overlapping genomic clones covering the 7.2 kb mouse alpha 1(X) collagen gene, 0.86 kb of promoter and 1.25 kb of 3'-flanking sequences were isolated from two genomic libraries and characterized by nucleotide sequencing. Typical features of the gene include a unique three-exon structure, similar to that in the chick gene, with the entire triple-helical domain of 463 amino acids coded by a single large exon. The highest degree of amino acid and nucleotide sequence conservation was seen in the coding region for the collagenous and C-terminal non-collagenous domains between the mouse and known chick, bovine and human collagen type X sequences. More divergence between the sequences occurred in the N-terminal non-collagenous domain. Similarity between the mammalian collagen X sequences extended into the 3'-untranslated sequence, particularly near the polyadenylation site. The promoter of the mouse collagen X gene was found to contain two TATAA boxes 159 bp apart; primer extension analyses of the transcription start site revealed that both were functional. The promoter has an unusual structure with a very low G + C content of 28% between positions -220 and -1 of the upstream transcription start site. Northern and in situ hybridization analyses confirmed that the expression of the alpha 1(X) collagen gene is restricted to hypertrophic chondrocytes in tissues undergoing endochondral calcification. The detailed sequence information of the gene is useful for studies on the promoter activity of the gene and for generation of transgenic mice.