Excess BMP Signaling in Heterotopic Cartilage Forming in Prg4-null TMJ Discs

Heterotopic cartilage develops in certain pathologic conditions, including those affecting the human temporomandibular joint (TMJ), but the underlying molecular mechanisms remain obscure. This is in part due to the fact that a reliable animal model of such TMJ diseases is not available. Here, we show that aberrant chondrocyte differentiation and ectopic cartilage formation occur spontaneously in proteoglycan 4 (Prg4) mutant TMJ discs without further invasive procedure. By 2 mo of age, mutant disc cells displayed chondrocyte transdifferentiation, accompanied by strong expression of cartilage master gene Sox9 and matrix genes aggrecan and type II collagen. By 6 mo, heterotopic cartilage had formed in the discs and expressed cartilage hypertrophic markers Runx2 and ColX. The ectopic tissue grew in size over time and exhibited regional mineralization by 12 mo. Bone morphogenetic protein (BMP) signaling was activated with the ectopic chondrogenic cells and chondrocytes, as indicated by phosphorylated Smad 1/5/8 nuclear staining and by elevated expression of Bmp2, Bmpr1b, Bmpr2, and BMP signaling target genes. Likewise, we found that upon treatment with recombinant human BMP 2 in high-density micromass culture, mutant disc cells differentiated into chondrocytes and synthesized cartilage matrix more robustly than control cells. Importantly, a specific kinase inhibitor of BMP receptors drastically attenuated chondrogenesis in recombinant human BMP 2-treated mutant disc cultures. Unexpectedly, we found that Prg4 was expressed at joint-associated sites, including disc/muscle insertion and muscle/bone interface, and all these structures were abnormal in Prg4 mutants. Our data indicate that Prg4 is needed for TMJ disc integrity and function and that its absence leads to ectopic chondrogenesis and cartilage formation in conjunction with abnormal BMP signaling. Our findings imply that the BMP signaling pathway could be a potential therapeutic target for prevention or inhibition of ectopic cartilage formation in TMJ disease.

Lubricin is Required for the Structural Integrity and Post-natal Maintenance of TMJ

The Proteoglycan 4 (Prg4) product lubricin plays essential roles in boundary lubrication and movement in limb synovial joints, but its roles in temporomandibular joint (TMJ) are unclear. Thus, we characterized the TMJ phenotype in wild-type and Prg4(-/-) mouse littermates over age. As early as 2 weeks of age, mutant mice exhibited hyperplasia in the glenoid fossa articular cartilage, articular disc, and synovial membrane. By 1 month of age, there were fewer condylar superficial tenascin-C/Col1-positive cells and more numerous apoptotic condylar apical cells, while chondroprogenitors displayed higher mitotic activity, and Sox9-, Col2-, and ColX-expressing chondrocyte zones were significantly expanded. Mutant subchondral bone contained numerous Catepsin K-expressing osteoclasts at the chondro-osseous junction, increased invasive marrow cavities, and suboptimal subchondral bone. Mutant glenoid fossa, disc, synovial cells, and condyles displayed higher Hyaluronan synthase 2 expression. Mutant discs also lost their characteristic concave shape, exhibited ectopic chondrocyte differentiation, and occasionally adhered to condylar surfaces. A fibrinoid substance of unclear origin often covered the condylar surface. By 6 months of age, mutant condyles displayed osteoarthritic degradation with apical/mid-zone separation. In sum, lubricin exerts multiple essential direct and indirect roles to preserve TMJ structural and cellular integrity over post-natal life.

Muenke syndrome mutation, FgfR3P²⁴⁴R, causes TMJ defects

Muenke syndrome is characterized by various craniofacial deformities and is caused by an autosomal-dominant activating mutation in fibroblast growth factor receptor 3 (FGFR3(P250R) ). Here, using mice carrying a corresponding mutation (FgfR3(P244R) ), we determined whether the mutation affects temporomandibular joint (TMJ) development and growth. In situ hybridization showed that FgfR3 was expressed in condylar chondroprogenitors and maturing chondrocytes that also expressed the Indian hedgehog (Ihh) receptor and transcriptional target Patched 1(Ptch1). In FgfR3(P244R) mutants, the condyles displayed reduced levels of Ihh expression, H4C-positive proliferating chondroprogenitors, and collagen type II- and type X-expressing chondrocytes. Primary bone spongiosa formation was also disturbed and was accompanied by increased osteoclastic activity and reduced trabecular bone formation. Treatment of wild-type condylar explants with recombinant FGF2/FGF9 decreased Ptch1 and PTHrP expression in superficial/polymorphic layers and proliferation in chondroprogenitors. We also observed early degenerative changes of condylar articular cartilage, abnormal development of the articular eminence/glenoid fossa in the TMJ, and fusion of the articular disc. Analysis of our data indicates that the activating FgfR3(P244R) mutation disturbs TMJ developmental processes, likely by reducing hedgehog signaling and endochondral ossification. We suggest that a balance between FGF and hedgehog signaling pathways is critical for the integrity of TMJ development and for the maintenance of cellular organization.

Type IIA procollagen: expression in developing chicken limb cartilage and human osteoarthritic articular cartilage

Type IIA procollagen is an alternatively spliced product of the type II collagen gene and uniquely contains the cysteine (cys)-rich globular domain in its amino (N)-propeptide. To understand the function of type IIA procollagen in cartilage development under normal and pathologic conditions, the detailed expression pattern of type IIA procollagen was determined in progressive stages of development in embryonic chicken limb cartilages (days 5-19) and in human adult articular cartilage. Utilizing the antibodies specific for the cys-rich domain of the type IIA procollagen N-propeptide, we localized type IIA procollagen in the pericellular and interterritorial matrix of condensing pre-chondrogenic mesenchyme (day 5) and early cartilage (days 7-9). The intensity of immunostaining was gradually lost with cartilage development, and staining became restricted to the inner layer of perichondrium and the articular cap (day 12). Later in development, type IIA procollagen was re-expressed at the onset of cartilage hypertrophy (day 19). Different from type X collagen, which is expressed throughout hypertrophic cartilage, type IIA procollagen expression was transient and restricted to the zone of early hypertrophy. Immunoelectron microscopic and immunoblot analyses showed that a significant amount of the type IIA procollagen N-propeptide, but not the carboxyl (C)-propeptide, was retained in matrix collagen fibrils of embryonic limb cartilage. This suggests that the type IIA procollagen N-propeptide plays previously unrecognized roles in fibrillogenesis and chondrogenesis. We did not detect type IIA procollagen in healthy human adult articular cartilage. Expression of type IIA procollagen, together with that of type X collagen, was activated by articular chondrocytes in the upper zone of moderately and severely affected human osteoarthritic cartilage, suggesting that articular chondrocytes, which normally maintain a stable phenotype, undergo hypertrophic changes in osteoarthritic cartilage. Based on our data, we propose that type IIA procollagen plays a significant role in chondrocyte differentiation and hypertrophy during normal cartilage development as well as in the pathogenesis of osteoarthritis.

The roles of annexins and types II and X collagen in matrix vesicle-mediated mineralization of growth plate cartilage

Annexins II, V, and VI are major components of matrix vesicles (MV), i.e. particles that have the critical role of initiating the mineralization process in skeletal tissues. Furthermore, types II and X collagen are associated with MV, and these interactions mediated by annexin V stimulate Ca(2+) uptake and mineralization of MV. However, the exact roles of annexin II, V, and VI and the interaction between annexin V and types II and X collagen in MV function and initiation of mineralization are not well understood. In this study, we demonstrate that annexin II, V, or VI mediate Ca(2+) influx into phosphatidylserine (PS)-enriched liposomes, liposomes containing lipids extracted from authentic MV, and intact authentic MV. The annexin Ca(2+) channel blocker, K-201, not only inhibited Ca(2+) influx into fura-2-loaded PS-enriched liposomes mediated by annexin II, V, or VI, but also inhibited Ca(2+) uptake by authentic MV. Types II and X collagen only bound to liposomes in the presence of annexin V but not in the presence of annexin II or VI. Binding of these collagens to annexin V stimulated its Ca(2+) channel activities, leading to an increased Ca(2+) influx into the liposomes. These findings indicate that the formation of annexin II, V, and VI Ca(2+) channels in MV together with stimulation of annexin V channel activity by collagen (types II and X) binding can explain how MV are able to rapidly take up Ca(2+) and initiate the formation of the first crystal phase.

Transient chondrogenic phase in the intramembranous pathway during normal skeletal development

Calvarial and facial bones form by intramembranous ossification, in which bone cells arise directly from mesenchyme without an intermediate cartilage anlage. However, a number of studies have reported the emergence of chondrocytes from in vitro calvarial cell or organ cultures and the expression of type II collagen, a cartilage-characteristic marker, in developing calvarial bones. Based on these findings we hypothesized that a covert chondrogenic phase may be an integral part of the normal intramembranous pathway. To test this hypothesis, we analyzed the temporal and spatial expression patterns of cartilage characteristic genes in normal membranous bones from chick embryos at various developmental stages (days 12, 15 and 19). Northern and RNAse protection analyses revealed that embryonic frontal bones expressed not only the type I collagen gene but also a subset of cartilage characteristic genes, types IIA and XI collagen and aggrecan, thus resembling a phenotype of prechondrogenic-condensing mesenchyme. The expression of cartilage-characteristic genes decreased with the progression of bone maturation. Immunohistochemical analyses of developing embryonic chick heads indicated that type II collagen and aggrecan were produced by alkaline phosphatase activity positive cells engaged in early stages of osteogenic differentiation, such as cells in preosteogenic-condensing mesenchyme, the cambium layer of periosteum, the advancing osteogenic front, and osteoid bone. Type IIB and X collagen messenger RNAs (mRNA), markers for mature chondrocytes, were also detected at low levels in calvarial bone but not until late embryonic stages (day 19), indicating that some calvarial cells may undergo overt chondrogenesis. On the basis of our findings, we propose that the normal intramembranous pathway in chicks includes a previously unrecognized transient chondrogenic phase similar to prechondrogenic mesenchyme, and that the cells in this phase retain chondrogenic potential that can be expressed in specific in vitro and in vivo microenvironments.

Complete primary structure of the chicken alpha1(V) collagen chain

Chicken alpha1(V) collagen cDNAs have been cloned by a variety of methods and positively identified. We present here the entire translated sequence of the chick polypeptide and compare selected regions to other collagen chains in the type V/XI family.

Developmental restriction of embryonic calvarial cell populations as characterized by their in vitro potential for chondrogenic differentiation

The mechanism(s) by which the cells within the calvaria tissue are restricted into the osteogenic versus the chondrogenic lineage during intramembranous bone formation were examined. Cells were obtained from 12-day chicken embryo calvariae after tissue condensation, but before extensive osteogenic differentiation, and from 17-day embryo calvariae when osteogenesis is well progressed. Only cell populations from the younger embryos showed chondrogenic differentiation as characterized by the expression of collagen type II. The chondrocytes underwent a temporal progression of maturation and endochondral development, demonstrated by the expression of collagen type II B transcript and expression of collagen type X mRNA. Cell populations from both ages of embryos showed progressive osteogenic differentiation, based on the expression of osteopontin, bone sialoprotein, and osteocalcin mRNAs. Analysis using lineage markers for either chondrocytes or osteoblasts demonstrated that when the younger embryonic cultures were grown in conditions that were permissive for chondrogenesis, the number of chondrogenic cells increased from approximately 15 to approximately 50% of the population, while the number of osteogenic cells remained almost constant at approximately 35-40%. Pulse labeling of the cultures with BrdU showed selective labeling of the chondrogenic cells in comparison with the osteogenic cells. These data indicate that the developmental restriction of skeletal cells of the calvaria is not a result of positive selection for osteogenic differentiation but a negative selection against the progressive growth of chondrogenic cells in the absence of a permissive or inductive environment. These results further demonstrate that while extrinsic environmental factors can modulate the lineage progression of skeletal cells within the calvariae, there is a progressive restriction during embryogenesis in the number of cells within the calvaria with a chondrogenic potential. Finally, these data suggest that the loss of cells with chondrogenic potential from the calvaria may be related to the progressive limitation of the reparative capacity of the cranial bones.

The chick type III collagen gene contains two promoters that are preferentially expressed in different cell types and are separated by over 20 kb of DNA containing 23 exons

Type III collagen is present in prechondrogenic mesenchyme, but not in cartilages formed during endochondral ossification. However, cultured chick chondrocytes contain an unusual transcript of the type III collagen gene in which exons 1-23 are replaced with a previously undescribed exon, 23A; this alternative transcript does not encode type III collagen. This observation suggested that, although production of type III collagen mRNA is repressed in chondrocytes, transcription of the type III collagen gene may continue from an alternative promoter. To test this prediction, we isolated and characterized both the upstream and internal promoters of this gene and tested their ability to direct transcription in chondrocytes and skin fibroblasts. The upstream promoter is active in fibroblasts, but inactive in chondrocytes, indicating that repression of type III collagen synthesis during chondrogenesis is transcriptionally mediated. Additionally, sequences in intron 23, preceding exon 23A, function as a highly active promoter in chondrocytes; transcription from this promoter is repressed in fibroblasts. Thus transcriptional control of the type III collagen gene is highly complex, with two promoters separated by at least 20 kb of DNA that are preferentially expressed in different cell types and give rise to RNAs with different structures and functions.

Regulated production of mineralization-competent matrix vesicles in hypertrophic chondrocytes

Matrix vesicles have a critical role in the initiation of mineral deposition in skeletal tissues, but the ways in which they exert this key function remain poorly understood. This issue is made even more intriguing by the fact that matrix vesicles are also present in nonmineralizing tissues. Thus, we tested the novel hypothesis that matrix vesicles produced and released by mineralizing cells are structurally and functionally different from those released by nonmineralizing cells. To test this hypothesis, we made use of cultures of chick embryonic hypertrophic chondrocytes in which mineralization was triggered by treatment with vitamin C and phosphate. Ultrastructural analysis revealed that both control nonmineralizing and vitamin C/phosphatetreated mineralizing chondrocytes produced and released matrix vesicles that exhibited similar round shape, smooth contour, and average size. However, unlike control vesicles, those produced by mineralizing chondrocytes had very strong alkaline phosphatase activity and contained annexin V, a membrane-associated protein known to mediate Ca2+ influx into matrix vesicles. Strikingly, these vesicles also formed numerous apatite-like crystals upon incubation with synthetic cartilage lymph, while control vesicles failed to do so. Northern blot and immunohistochemical analyses showed that the production and release of annexin V-rich matrix vesicles by mineralizing chondrocytes were accompanied by a marked increase in annexin V expression and, interestingly, were followed by increased expression of type I collagen. Studies on embryonic cartilages demonstrated a similar sequence of phenotypic changes during the mineralization process in vivo. Thus, chondrocytes located in the hypertrophic zone of chick embryo tibial growth plate were characterized by strong annexin V expression, and those located at the chondro-osseous mineralizing border exhibited expression of both annexin V and type I collagen. These findings reveal that hypertrophic chondrocytes can qualitatively modulate their production of matrix vesicles and only when induced to initiate mineralization, will release mineralization-competent matrix vesicles rich in annexin V and alkaline phosphatase. The occurrence of type I collagen in concert with cartilage matrix calcification suggests that the protein may facilitate crystal growth after rupture of the matrix vesicle membrane; it may also offer a smooth transition from mineralized type II/type X collagen-rich cartilage matrix to type I collagen-rich bone matrix.

Annexin V-mediated calcium flux across membranes is dependent on the lipid composition: implications for cartilage mineralization

Annexin V is a major component of matrix vesicles and has a role in mediating the influx of Ca2+ into these vesicles, thus promoting the initiation of hypertrophic cartilage matrix mineralization. However, the mechanisms and factors regulating annexin V-mediated Ca2+ influx into these vesicles are not well understood. Since the lipid composition of matrix vesicles differs from that of the plasma membrane of chondrocytes and is rich in phosphatidylserine, we asked whether the lipid composition may regulate annexin V function. We prepared liposomes containing different concentrations of phosphatidylserine and determined how the lipid composition affected (a) the interactions between annexin V and liposomes and (b) annexin V-mediated Ca2+ influx into the liposomes. We found that annexin V was able to bind to every liposome tested. However, we observed the most prominent increases in tryptophan 187 emission intensity, a measure of the degree of interactions between annexin V and lipid bilayers, only with liposomes containing a high concentration of phosphatidylserine. In addition, a significant fraction of annexin V associated with phosphatidylserine-rich liposomes was not extractable by EDTA treatment but required a detergent, indicating that annexin V inserts into bilayers of these liposomes. Chemical cross-linking analysis revealed that matrix vesicles and phosphatidylserine-rich liposomes induced the formation of the annexin V hexamer. Interestingly, a significant Ca2+ influx in the presence of annexin V occurred only in liposomes containing a high phosphatidylserine content. Moreover, annexin V-mediated Ca2+ influx into these liposomes was inhibited (i) by anti-annexin V antibodies and (ii) by treatment with zinc and cadmium, indicating the essential role of the protein in Ca2+ influx. The results of this study indicate that phosphatidylserine-rich bilayers induce the formation of a hexameric annexin V, possibly leading to a Ca2+-dependent insertion of annexin V into the bilayer and establishment of annexin V-mediated Ca2+ influx into matrix vesicles or liposomes. The phosphatidylserine-rich membrane of matrix vesicles in vivo may thus offer an ideal specialized environment in which the biological function of annexin V is optimized, leading to rapid Ca2+ influx, intralumenal crystal growth, and cartilage matrix mineralization.

Syndecan-3 and the control of chondrocyte proliferation during endochondral ossification

During endochondral ossification, chondrocytes progress through several stages of maturation before they are replaced by bone cells. Chondrocyte proliferation, the first step in this complex multistage process, is strictly controlled both spatially and temporally but its underlying mechanisms of regulation remain unclear. In this study we asked whether chondrocytes produce syndecan-3, a cell surface receptor for growth factors such as fibroblast growth factor 2 (FGF-2), and whether syndecan-3 may play a role in proliferation during chondrocyte maturation. We found that proliferating immature cartilage from chick embryo tibia and sternum contained significant amounts of syndecan-3 mRNA, whereas mature hypertrophic cartilage contained markedly lower transcript levels. Immunohistochemical analyses on sections of Day 18 chick embryo tibia revealed that syndecan-3 was spatially restricted and indeed detectable only in immature proliferating chondrocytes in the top zone of growth plate. These syndecan-3-rich proliferating chondrocytes lay beneath developing articular chondrocytes rich in their typical matrix protein tenascin-C, resulting in a striking boundary between these two populations of chondrocytes. Immature proliferating chondrocyte populations reared in growth-promoting culture conditions displayed strong continuous syndecan-3 gene expression; upon induction of maturation by vitamin C treatment, syndecan-3 gene expression was markedly down-regulated. Treatment with FGF-2 for 24 h stimulated both syndecan-3 gene expression and chondrocyte proliferation; this growth stimulation was counteracted by cotreatment with heparinase I or III. The results of the study indicate that syndecan-3 participates in the maturation of chondrocytes during endochondral ossification and represents a regulator of the proliferative phase of this multistage process.

The chick alpha2(I) collagen gene contains two functional promoters, and its expression in chondrocytes is regulated at both transcriptional and post-transcriptional levels

Embryonic chick cartilages contain transcripts derived from the alpha2(I) collagen gene, although type I collagen is not normally found in these tissues; most of these RNAs are alternative transcripts initiating within intron 2. Use of the internal start site results in replacement of exons 1 and 2 with a previously undescribed exon and a change in the translational reading frame; thus, the alternative transcript cannot encode alpha2(I) collagen. We have demonstrated that production of the alternative transcript is due to activation of an internal promoter in chondrocytes and have identified a 179-base pair domain that is required for its activity. Furthermore, we have shown that the alternative transcript resulting from activation of the internal promoter turns over relatively rapidly; thus, the steady-state level of this transcript is less than predicted based on the transcription rate. The upstream promoter is only partially repressed in chondrocytes, suggesting that the lack of authentic alpha2(I) collagen mRNA may also be due in part to decreased mRNA stability. Thus, repression of alpha2(I) collagen synthesis in cartilage involves both transcriptional and post-transcriptional mechanisms. In contrast, repression of alpha1(I) collagen synthesis appears to be mediated primarily at the level of transcription.

Alternative transcript of the chick alpha 2(I) collagen gene is transiently expressed during endochondral bone formation and during development of the central nervous system

Endochondral bone formation is characterized by several transitions in the pattern of collagen gene expression, the best characterized of which occurs during chondrogenesis. Prechondrogenic mesenchymal cells synthesize predominantly type I collagen; during chondrogenesis, type I collagen synthesis ceases and production of cartilage-characteristic collagens is initiated. We previously identified the molecular mechanism that mediates cessation of alpha 2(I) collagen synthesis in chondrocytes (Bennett and Adams [1990] J. Biol. Chem. 265:2223-2230). This mechanism involves a change in the transcription initiation site, resulting in an alternative transcript that cannot encode alpha 2(I) collagen. In this report we demonstrate that the alternative transcript appears only transiently in cartilage. Its initial appearance is coincident with the onset of high levels of type II collagen synthesis in differentiated chondrocytes. However, it disappears in hypertrophic cartilage, and production of the authentic alpha 2(I) collagen mRNA is reinitiated, contributing to synthesis of a high level of type I collagen in hypertrophic chondrocytes at the chondro-osseous junction. We also show that the alternative transcript is not restricted to cartilage during embryonic development, since it initially appears in presomite embryos, well before the appearance of cartilage. At early stages of embryo-genesis the alternative transcript is restricted to tissues derived from neuroectoderm; its appearance in those tissues is also transient. These data suggest that production of the alternative transcript of the alpha 2(I) collagen gene may be required for cessation of alpha 2(I) collagen synthesis during chondrogenesis, but the alternative transcript may be involved in other important developmental programs as well.

Expression of syndecan-3 and tenascin-C: possible involvement in periosteum development

The development of cartilaginous elements of long bone during embryogenesis and postnatal bone repair processes is a complex process that involves skeletal cells and surrounding mesenchymal periosteal cells. Relatively little is known of the mechanisms underlying these processes. Previous studies from this and other laboratories have suggested that the extracellular matrix protein tenascin-C is involved in skeletogenesis. Using in situ hybridization and immunofluorescence, we extended those studies by comparing the expression of tenascin-C with that of syndecan-3, which belongs to a family of cell surface receptors with which tenascins are known to interact. We found that syndecan-3 transcripts at first were very abundant in the presumptive periosteum surrounding the diaphysis of early chondrocytic skeletal elements in chick limb. As the elements developed further, syndecan-3 gene expression decreased in the diaphyseal periosteum, whereas it became stronger around the early epiphysis and within the forming articular cells. However, as the diaphyseal periosteum initiated osteogenesis and gave rise to the intramembranous bone collar, syndecan-3 gene expression increased again. At early stages of skeletogenesis: the tenascin-C gene exhibited patterns of expression that were similar to and temporally followed, those of the syndecan-3 gene. At later stages, however, tenascin-C gene expression was markedly reduced during intramembranous osteogenesis around the diaphysis. In addition, although syndecan-3 gene expression was low in osteoblasts and osteocytes located deep into trabecular bone, tenascin-C gene expression remained strong. Thus, tenascin-C and syndecan-3 display distinct temporal and spatial patterns of expression in periosteum and during the development of long bone. Given their multidomain structure and specific patterns of expression, these macromolecules may regulate site-specific skeletal processes, including interactions between developing periosteum and chondrocytes and delineation of the early cartilaginous skeletal elements.

Syndecan-3, tenascin-C, and the development of cartilaginous skeletal elements and joints in chick limbs

The mechanisms by which the early limb cell condensations and interzone mesenchyme give rise to skeletal elements and joints are poorly understood. Previous work from this laboratory has shown that the extracellular matrix protein tenascin-C is associated with articular cartilage and joint tissue development; others have shown that tenascin-C may exert its biological activities via interactions with cell surface receptors, such as syndecans. To further analyze the roles of tenascin-C and its putative receptors in skeletal development, we carried out a detailed in situ hybridization analysis of tenascin-C and syndecan-3 gene expression during development of chick limb skeletal elements and joints. We found that as the early mesenchymal condensations chondrify around day 5 (E5) of development, they become surrounded by a thick syndecan-3 rich perichondrium while tenascin-C transcripts are much fewer and restricted to diaphyseal perichondrium and developing interzones. Similar patterns were observed as distal carpal and digit condensations formed in older embryos. As the cartilaginous long bone models elongated proximo-distally and joint formation proceeded with age, we observed that syndecan-3 transcripts decrease significantly along the diaphysis and remain very abundant along the metaphysis and in the epiphyseal articular cap and interzone. Conversely, tenascin-C RNAs remain abundant along the diaphysis and begin to increase at the epiphysis and in interzone-derived tissues, such as menisci and joint capsule. By E10, the skeletal elements have well-defined morphologies, endochondral ossification has initiated in their diaphysis, and diaphyseal perichondrium has become periosteum. These developmental changes were accompanied by equally marked changes in gene expression; these included a marked increase in tenascin-C gene expression in articular cap, fragmentation of tenascin-C gene expression along the periosteum, reinitiation of syndecan-3 gene expression in periosteum, and differential gene expression in osteoprogenitor cells. The sheer complexity of the gene expression patterns documented in this study attests to the complexity of processes that bring about normal skelatogenesis. Clearly, tenascin-C and syndecan-3 appear to be closely associated with several of these processes, particularly in establishing tissue boundaries (perichondrium and periosteum) between condensations and surrounding mesenchymal cells, in regulating perichondral cell differentiation and incorporation into the growing skeletal elements, and in the genesis of epiphyseal chondrocytes and associated joint tissues.

An alternative transcript of the chick type III collagen gene that does not encode type III collagen

Type III collagen, a ubiquitous protein found in most connective tissues, is not present in hyaline cartilage. However, we have identified an alternative transcript of the type III collagen gene in cultured chondrocytes from several embryonic chick cartilages. This RNA contains exons 24-52, but exons 1-23 are replaced by 70 nucleotides of unique sequence, suggesting that transcription initiates at an alternative promoter. Two of the open reading frames in the alternative transcript are out of frame with the collagen coding sequence; a third open reading frame encodes the carboxyl-terminal two-thirds of the collagen sequence. Thus, this RNA cannot serve as a template for synthesis of normal type III collagen and may encode noncollagenous proteins and/or a truncated collagen. The alternative transcript has been detected as early as 2.5 days of embryogenesis and at later stages is present at low levels in many tissues, including limb mesenchyme and cartilage. These results, together with our previous identification of an alternative transcript of the chick alpha 2(I) collagen gene (Bennett, V. D., and Adams, S. L. (1990) J. Biol. Chem. 265, 2223-2230), suggest that some collagen genes may have alternative functions that are independent of their roles in collagen production.

Detection of a type IX collagen-related mRNA in an invertebrate, the marine annelid Nereis virens

Fibrous and non-fibrous collagens have been described in both vertebrate and invertebrate animals. However, there has been limited characterization of non-fibrous collagens and their corresponding genes in invertebrate animals. In the present study we have used as a probe an avian cDNA clone which encompasses the COL3, NC3 and part of the COL2 domain of the collagen alpha 3(IX) subunit. This probe hybridized to mRNA obtained from the cuticle and body of the marine annelid, Nereis virens. Northern blot hybridization exhibited an mRNA of ca. 7.5-8 kilobases which in situ hybridization shows to be most abundant over cuticle-associated cells. Dot-blot hybridization, comparing cuticle mRNA and body mRNA, indicates that this collagen mRNA is five times more abundant in the cuticle. The composite data suggest evolutionary conservation, in both vertebrate and invertebrate animals, of a non-fibrillar collagen.

The chicken alpha 1 (XI) collagen gene is widely expressed in embryonic tissues

Complementary DNA and genomic DNA clones corresponding to the chicken alpha 1 (XI) collagen gene were isolated and characterized. These recombinant DNA clones covered 2667 base pairs of the mRNA and encode 624 amino acids of the triple helical region plus the entire carboxyl-terminal propeptide. Northern blot analysis showed a major band of approximately 6.5 kilobases and a minor band of approximately 7.5 kilobases. A combination of Northern blot and in situ hybridization analyses showed that, in addition to its presence in cartilage, this mRNA also is present in a wide variety of chicken noncartilaginous embryonic tissues including brain, heart, skeletal muscle, calvaria, and skin, but was not detected in liver. Type II collagen mRNA has also been detected at low levels in these same tissues. Also, similar to the mRNA for the alpha 1 chain for type II collagen, the alpha 1 (XI) collagen mRNA is detected in limb mesenchyme prior to condensation and differentiation of the core mesenchyme into cartilage.

Cloning and developmental expression of the alpha 3 chain of chicken type IX collagen

Fibrous and nonfibrous collagens comprise two major groups within the collagen family and both groups are found in a diverse variety of tissue fabrics. Type IX collagen is in the nonfibrous group; three different subunits of type IX collagen have been identified and the alpha 1 and alpha 2 subunits have been cloned. Using molecular cloning methods we have isolated, from an embryonic chicken cartilage library, cDNA clones which code for the entire alpha 3 chain of chicken type IX collagen. The cDNA clones encompass 2416 base pairs which have a conceptual open reading frame for a protein containing 675 amino acids including 193 Gly-X-Y repeats. These collagen repeats are in three separate domains which are interspersed with four major noncollagen domains. The collagen repeats also have four minor interruptions. This chain organization directly aligns with both the alpha 1 and alpha 2 chains of chicken type IX collagen. Comparison of the deduced amino acid sequence with peptide sequences of type IX collagens shows identity with 95 of the 96 known residues of the chicken alpha 3 chain and 81 of the 98 known residues of the bovine alpha 3 chain. The identical residues match those in five peptide fragments, two from the bovine protein and three from the chicken protein. The chicken and bovine alpha 3 chains have conserved cross-linking sites, separated by 137 residues which span 40 nm, the length of the hole zone in a collagen fibril. The NC3 domain of the chicken alpha 3 chain contains a repeat Cys-Pro motif which is present in both vertebrate and invertebrate nonfibrillar collagens. Northern blot hybridization exhibits a major mRNA of about 3.3 kilobases; this transcript is found in cartilaginous tissues in the embryo, including the developing limb and is not detected in other tissues or in the precondensation stage of limb development. The composite data delineate the primary structure of the alpha 3 chain of chicken type IX collagen, show its close relationship to the alpha 1 and alpha 2 chains, demonstrate its mRNA transcript, and show the appearance of that transcript in tissues of the developing chick embryo.

Type II collagen mRNA containing an alternatively spliced exon predominates in the chick limb prior to chondrogenesis

A series of cDNA clones corresponding to the 5' end of the chicken type II collagen mRNA were generated using a single-sided polymerase chain reaction technique. Analysis of these cDNAs showed that the second exon of the gene is alternatively spliced such that it is either present or absent in the mRNA. This exon encodes a 70-amino acid cysteine-rich globular domain which is present in the amino-terminal propeptides of alpha 1(I), alpha 1(III), and alpha 2(V) procollagen chains but which was previously thought to be absent from type II procollagen. Analysis of the expression of the two alternatively spliced forms of the chicken type II collagen mRNA showed that the mRNA without the second exon was the predominant form (approximately 90%) in sternal cartilage from 14-day embryos, but in precartilage limb mesenchyme only the form including the second exon was detected. This later form was also present in a number of non-cartilage tissues including embryonic calvaria, skin, heart, skeletal muscle, and brain; no type II collagen mRNA was detected in liver. Studies of developing limbs from progressive embryonic stages suggest that the appearance of the mRNA lacking the second exon is a relatively late event during chondrogenesis.

In situ hybridization analysis of the expression of the type II collagen gene in the developing chicken limb bud

In situ hybridization with [32P]- or [35S]-labeled double-stranded DNA or single-stranded RNA probes was used to investigate the temporal and spatial distribution of cartilage-characteristic type II collagen mRNA during embryonic chick limb development and cartilage differentiation in vivo. When the type II collagen probes were hybridized to sections through embryonic limb buds at the earliest stages of their development (stages 18-25), an accumulation of silver grains representing type II collagen mRNA first became detectable in the proximal central core of the limb coincident with the prechondrogenic condensation of mesenchymal cells that characterizes the onset of cartilage differentiation. At later stages of development (stage 32; 7 days) intense hybridization signals with the type II collagen probes were localized over the well differentiated cartilage rudiments, whereas few or no silver grains above background were observed over the non-chondrogenic tissues. In contrast, sections hybridized with a probe complementary to mRNA for the alpha 1 chain of type I collagen exhibited an intense hybridization signal over the perichondrium and little or no signal over the cartilage primordia. At all stages of development examined, [32P]-labeled double-stranded DNA probes or single-stranded RNA probes labeled with either [32P] or [35S] provided adequate hybridization signals. Several experimental protocols were employed to control for the potential cross-hybridization and non-specific hybridization of the type II collagen probes. These included the utilization of labeled noncomplementary "sense-strand" type II collagen RNA as a control probe for nonspecific background, and prehybridization with a large excess of appropriate unlabeled RNA to block sequences in heterologous collagen RNAs that might cross-hybridize to the specific labeled probe.

Persistence of taste buds in denervated fungiform papillae

Taste buds in hamster fungiform papillae persist in an atrophic state for as long as 330 days after chorda tympani denervation or 50 days after combined chorda tympani-lingual nerve resection. Although taste bud structure depends on innervation, there is no absolute neural requirement for taste bud survival.