Localization of pro-alpha 2(V) collagen transcripts in the tissues of the developing mouse embryo

Plasticity in airway smooth muscle differentiation during mouse lung development

It has been proposed that smooth muscle differentiation may physically sculpt airway epithelial branches in mammalian lungs. Serum response factor (SRF) acts with its co-factor myocardin to activate the expression of contractile smooth muscle markers. In the adult, however, smooth muscle exhibits a variety of phenotypes beyond contractile, and these are independent of SRF/myocardin-induced transcription. To determine whether a similar phenotypic plasticity is exhibited during development, we deleted Srf from the mouse embryonic pulmonary mesenchyme. Srf-mutant lungs branch normally, and the mesenchyme displays mechanical properties indistinguishable from controls. scRNA-seq identified an Srf-null smooth muscle cluster, wrapping the airways of mutant lungs, which lacks contractile smooth muscle markers but retains many features of control smooth muscle. Srf-null embryonic airway smooth muscle exhibits a synthetic phenotype, compared with the contractile phenotype of mature wild-type airway smooth muscle. Our findings identify plasticity in embryonic airway smooth muscle and demonstrate that a synthetic smooth muscle layer promotes airway branching morphogenesis.

Mannose receptor C type 2 mediates 1,25(OH)2D3/vitamin D receptor-regulated collagen metabolism through collagen type 5, alpha 2 chain and matrix metalloproteinase 13 in murine MC3T3-E1 cells

Vitamin D plays an important role in maintaining skeletal development and bone homeostasis. Although vitamin D has been extensively researched, the direct effect of 1,25(OH)₂D₃ on osteoblasts is unclear. To explore the 1,25(OH)₂D₃ action on murine osteoblasts, we performed tandem mass tag experiments on MC3T3-E1 cells treated with and without 1,25(OH)₂D₃. Three up-regulated proteins (MRC2, WWTR1 and RASSF2) related to bone metabolism were confirmed in this study. 1,25(OH)₂D₃ up-regulated the expression of MRC2 through vitamin D receptor. MRC2 affects collagen metabolism in osteoblasts. Combined with bioinformatics and parallel reaction monitoring analysis, we inhibited the expression of MRC2 to explore the relationship between MRC2 and collagens. Then we found MRC2 down-regulated COL5A2 and up-regulated MMP13. This study provides a protein profile of 1,25(OH)₂D₃-treated murine osteoblasts, reveals a newly discovered signaling axis (1,25(OH)₂D₃/VDR/MRC2/COL5A2 and MMP13), and explains the effect of 1,25(OH)₂D₃ on bone metabolism from a new perspective.

Analysis of a gene co-expression network establishes robust association between Col5a2 and ischemic heart disease

Background

This study aims to expand knowledge of the complex process of myocardial infarction (MI) through the application of a systems-based approach.

Methods

We generated a gene co-expression network from microarray data originating from a mouse model of MI. We characterized it on the basis of connectivity patterns and independent biological information. The potential clinical novelty and relevance of top predictions were assessed in the context of disease classification models. Models were validated using independent gene expression data from mouse and human samples.

Results

The gene co-expression network consisted of 178 genes and 7298 associations. The network was dissected into statistically and biologically meaningful communities of highly interconnected and co-expressed genes. Among the most significant communities, one was distinctly associated with molecular events underlying heart repair after MI (P < 0.05). Col5a2, a gene previously not specifically linked to MI response but responsible for the classic type of Ehlers-Danlos syndrome, was found to have many and strong co-expression associations within this community (11 connections with ρ > 0.85). To validate the potential clinical application of this discovery, we tested its disease discriminatory capacity on independently generated MI datasets from mice and humans. High classification accuracy and concordance was achieved across these evaluations with areas under the receiving operating characteristic curve above 0.8.

Conclusion

Network-based approaches can enable the discovery of clinically-interesting predictive insights that are accurate and robust. Col5a2 shows predictive potential in MI, and in principle may represent a novel candidate marker for the identification and treatment of ischemic cardiovascular disease.

Genome-wide study identifies the regulatory gene networks and signaling pathways from chondrocyte and peripheral blood monocyte of Kashin-Beck disease

This investigation was designed to unravel gene networks in Kashin-Beck disease (KBD) and better identify target genes of KBD for gene therapy development. RNA was isolated separately from cartilage and peripheral blood samples of patients with KBD and healthy controls. Agilent 44K human whole-genome oligonucleotide microarrays were used to detect differentially expressed genes. Three significant canonical pathways and nine chondrocyte networks from chondrocytic gene expression profiles were screened using ingenuity pathway analysis (IPA), but only one network and no canonical pathways from peripheral blood monocytic gene profile were identified. Bak1, APAF-1, CASP6, IGFBP2, Col5a2 and TGFBI extracted from significant genes that involved in chondrocytic canonical pathways and networks may have closer relationship with the etiopathogenesis of KBD. Those genes may be potential targets for gene diagnosis and treatment. Six physiological functions were predominant and unique to the chondrocytic genes, whereas two were unique to peripheral blood monocytic genes. The identified genes may represent a source of potentially novel molecular targets, which may provide a better understanding of the molecular details in KBD pathogenesis and also provide useful pathways and network maps for the future research in osteochondrosis.

Minor fibrillar collagens, variable regions alternative splicing, intrinsic disorder, and tyrosine sulfation

Minor fibrillar collagen types V and XI, are those less abundant than the fibrillar collagen types I, II and III. The alpha chains share a high degree of similarity with respect to protein sequence in all domains except the variable region. Genomic variation and, in some cases, extensive alternative splicing contribute to the unique sequence characteristics of the variable region. While unique expression patterns in tissues exist, the functions and biological relevance of the variable regions have not been elucidated. In this review, we summarize the existing knowledge about expression patterns and biological functions of the collagen types V and XI alpha chains. Analysis of biochemical similarities among the peptides encoded by each exon of the variable region suggests the potential for a shared function. The alternative splicing, conservation of biochemical characteristics in light of low sequence conservation, and evidence for intrinsic disorder, suggest modulation of binding events between the surface of collagen fibrils and surrounding extracellular molecules as a shared function.

Origin of cardiac fibroblasts and the role of periostin

Cardiac fibroblasts are the most populous nonmyocyte cell type within the mature heart and are required for extracellular matrix synthesis and deposition, generation of the cardiac skeleton, and to electrically insulate the atria from the ventricles. Significantly, cardiac fibroblasts have also been shown to play an important role in cardiomyocyte growth and expansion of the ventricular chambers during heart development. Although there are currently no cardiac fibroblast-restricted molecular markers, it is generally envisaged that the majority of the cardiac fibroblasts are derived from the proepicardium via epithelial-to-mesenchymal transformation. However, still relatively little is known about when and where the cardiac fibroblasts cells are generated, the lineage of each cell, and how cardiac fibroblasts move to reside in their final position throughout all four cardiac chambers. In this review, we summarize the present understanding regarding the function of Periostin, a useful marker of the noncardiomyocyte lineages, and its role during cardiac morphogenesis. Characterization of the cardiac fibroblast lineage and identification of the signals that maintain, expand and regulate their differentiation will be required to improve our understanding of cardiac function in both normal and pathophysiological states.

Comparative analysis of gene expression profiles between the normal human cartilage and the one with endemic osteoarthritis

OBJECTIVE

To investigate the differences in gene expression profiles of adult articular cartilage with endemic osteoarthritis (OA), Kashin-Beck disease (KBD), and the same regions in the normal joint.

METHODS

The messenger RNA expression profiles of articular cartilage with KBD diagnosed according to "Diagnosing Criteria of Kashin-Beck Disease in China" were compared with the normal cartilage. Total RNA isolated separately from four pairs of the KBD and normal cartilage samples were evaluated by oligonucleotide microarray analysis. The microarray data were confirmed by quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) amplification and were compared with previously published experiments.

RESULTS

About 4100 transcripts, which corresponded to 35% of the expressed transcripts, showed >or=twofold differences in expression between the cartilage tissues in pairs. Approximately 2% of the expressed genes (79, 55 genes expressed in KBD>normal; 24 genes expressed in KBD<normal) were commonly expressed in the four pairs of samples. The expression of some genes related to the metabolism, apoptosis, cell proliferation and matrix degradation activity was significantly different in KBD cartilage than in the normal, similar to the findings for genes that inhibit matrix degradation. Comparisons of qRT-PCR data and the previously reported data with the result of gene chips support the validity of our microarray data.

CONCLUSION

Differences between KBD cartilage and the normal exhibited a similar pattern among the four pairs examined, indicating the presence of common mechanisms mainly including chondrocyte metabolism and apoptosis that contribute to cartilage destruction in KBD.

The introduction of an extracellular matrix-based scaffold to the marketplace

Continuous application of new scientific knowledge is a central characteristic of modern medical practice. The current pace of medical innovation creates an environment of rapid change, and the introduction of innovative treatments in the area of regenerative medicine in orthopaedics prompts health care providers, medical device companies, patient advocacy groups, and health insurance payors to study the most optimal method for introducing these treatments to clinical practice. Questions regarding the role and value of preclinical testing, clinical trials, and postmarketing surveillance are pertinent to this discussion, and answers to these questions should culminate in a strategy that benefits patient care.

A comprehensive study of the spatial and temporal expression of the col5a1 gene in mouse embryos: a clue for understanding collagen V function in developing connective tissues

Collagen V is a quantitatively minor component of collagen I fibrils and the defective product of classic Ehlers-Danlos syndrome (EDS). To provide new insights into its embryonic function, a continuous evaluation of the expression pattern of proalpha1(V), a chain common to all collagen V molecular forms, was performed by in situ hybridization of developing mouse from 7.5 days after conception (dpc) to birth. Proalpha1(V) transcripts were first detected at 8.5 dpc, signals being considerably augmented at 16.5 dpc and declining at birth. Hybridization signals were, at first, exclusively detected in the dorsal aorta wall, heart, and adnexa. At 10.5 dpc, col5a1 expression was found in the heart, dorsal aorta wall, branchial arches, mesonephrotic tubules, and intestinal mesenchyme and coincided with proalpha1(I) developmental expression. Later stages exhibited an intense signal in more restricted regions, notably the skin, the bones and vertebral column, the cornea, the tendons and ligaments, the peritoneal membranes, the umbilical cord, and the salivary gland. The data revealed the important contribution of collagen V to the development of functional connective tissues. Proalpha1(V) signals were exclusively detected in the flattened cells of the surface ectoderm at 10.5 dpc. By 12.5 dpc, when cells had become cuboidal, the signal switched to the dermal fibroblasts. Thus, type V collagen appears to contribute to epidermis differentiation. Our data also suggest that collagen V participates in bone formation and/or mineralization and in the renewal of stromal cells in the cornea. The results underscore the role of collagen V in developing embryos and provide important clues for analyzing the phenotype of mouse models for EDS.

Differential gene expression in the developing mouse ureter

In many instances, kidney dysgenesis results as a secondary consequence to defects in the development of the ureter. Through the use of mouse genetics a number of genes associated with such malformations have been identified, however, the cause of many other abnormalities remain unknown. In order to identify novel genes involved in ureter development we compared gene expression in embryonic day (E) 12.5, E15.5 and postnatal day (P) 75 ureters using the Compugen mouse long oligo microarrays. A total of 248 genes were dynamically upregulated and 208 downregulated between E12.5 and P75. At E12.5, when the mouse ureter is comprised of a simple cuboidal epithelium surrounded by ureteric mesenchyme, genes previously reported to be expressed in the ureteric mesenchyme, foxC1 and foxC2 were upregulated. By E15.5 the epithelial layer develops into urothelium, impermeable to urine, and smooth muscle develops for the peristaltic movement of urine towards the bladder. The development of these two cell types coincided with the upregulation of UPIIIa, RAB27b and PPARgamma reported to be expressed in the urothelium, and several muscle genes, Acta1, Tnnt2, Myocd, and Tpm2. In situ hybridization identified several novel genes with spatial expression within the smooth muscle, Acta1; ureteric mesenchyme and smooth muscle, Thbs2 and Col5a2; and urothelium, Kcnj8 and Adh1. This study marks the first known report defining global gene expression of the developing mouse ureter and will provide insight into the molecular mechanisms underlying kidney and lower urinary tract malformations.

Procollagen C proteinase enhancer 1 genes are important determinants of the mechanical properties and geometry of bone and the ultrastructure of connective tissues

Procollagen C proteinases (pCPs) cleave type I to III procollagen C propeptides as a necessary step in assembling the major fibrous components of vertebrate extracellular matrix. The protein PCOLCE1 (procollagen C proteinase enhancer 1) is not a proteinase but can enhance the activity of pCPs approximately 10-fold in vitro and has reported roles in inhibiting other proteinases and in growth control. Here we have generated mice with null alleles of the PCOLCE1 gene, Pcolce, to ascertain in vivo roles. Although Pcolce-/- mice are viable and fertile, Pcolce-/- male, but not female, long bones are more massive and have altered geometries that increase resistance to loading, compared to wild type. Mechanical testing indicated inferior material properties of Pcolce-/- male long bone, apparently compensated for by the adaptive changes in bone geometry. Male and female Pcolce-/- vertebrae both appeared to compensate for inferior material properties with thickened and more numerous trabeculae and had a uniquely altered morphology in deposited mineral. Ultrastructurally, Pcolce-/- mice had profoundly abnormal collagen fibrils in both mineralized and nonmineralized tissues. In Pcolce-/- tendon, 100% of collagen fibrils had deranged morphologies, indicating marked functional effects in this tissue. Thus, PCOLCE1 is an important determinant of bone mechanical properties and geometry and of collagen fibril morphology in mammals, and the human PCOLCE1 gene is identified as a candidate for phenotypes with defects in such attributes in humans.

Dermal connective tissue development in mice: an essential role for tenascin-X

Deficiency of the extracellular matrix protein tenascin-X (TNX) causes a recessive form of Ehlers-Danlos syndrome (EDS) characterized by hyperextensible skin and hypermobile joints. It is not known whether the observed alterations of dermal collagen fibrils and elastic fibers in these patients are caused by disturbed assembly and deposition or by altered stability and turnover. We used biophysical measurements and immunofluorescence to study connective tissue properties in TNX knockout and wild-type mice. We found that TNX knockout mice, even at a young age, have greatly disturbed biomechanical properties of the skin. No joint abnormalities were noted at any age. The spatio-temporal expression of TNX during normal mouse skin development, during embryonic days 13-19 (E13-E19), was distinct from tropoelastin and the dermal fibrillar collagens type I, III, and V. Our data show that TNX is not involved in the earliest phase (E10-E14) of the deposition of collagen fibrils and elastic fibers during fetal development. From E15 to E19, TNX starts partially to colocalize with the dermal collagens and elastin, and in adult mice, TNX is present in the entire dermis. In adult TNX knockout mice, we observed an apparent increase of elastin. We conclude that TNX knockout mice only partially recapitulate the phenotype of TNX-deficient EDS patients, and that TNX could potentially be involved in maturation and/or maintenance of the dermal collagen and elastin network.

Expression of fibroblast growth factor receptor-3 (FGFR3), signal transducer and activator of transcription-1, and cyclin-dependent kinase inhibitor p21 during endochondral ossification: differential role of FGFR3 in skeletal development and fracture repair

Increasing evidence suggests that fibroblast growth factor receptor-3 (FGFR3) is a negative regulator of endochondral bone growth; however, its role during skeletal repair is unknown. Using a rat model of closed femoral fracture healing, we analyzed the spatial and temporal expression of FGFR3. To assess a possible role for FGFR3 during healing, we also analyzed the spatial and temporal expression of signal transducer and activator of transcription-1 (STAT1) and cyclin-dependent kinase inhibitor p21, important mediators of FGFR3 signaling. Before these experiments, we studied the spatial expression of FGFR3 during skeletal development in mouse embryos. At 16.5 and 19.5 d post coitum, FGFR3 mRNA was strongly expressed in resting and proliferating chondrocytes but weakly in hypertrophic chondrocytes and not in osteoblasts. In contrast, during fracture repair, it was strongly expressed in prehypertrophic chondrocytes, and the expression level reached a maximum on d 14. Immunoreactivity for STAT1 was detected in the cytoplasm of chondrocytes on d 4 and 7 and both in the cytoplasm and nucleus of hypertrophic chondrocytes on d 14. Furthermore, FGFR3, STAT1, and p21 exhibited a similar temporal expression profile, suggesting that FGFR3-mediated STAT1-p21 signaling plays a role in fracture repair. These results indicate a differential role of FGFR3 in skeletal development and fracture repair.

Spatial and temporal gene expression in chondrogenesis during fracture healing and the effects of basic fibroblast growth factor

Chondrogenesis is an essential component of endochondral fracture healing, though the molecular and cellular events by which it is regulated have not been fully elucidated. In this study, we used a rat model of closed fracture healing to determine the spatial and temporal expression of genes for cartilage-specific collagens. Furthermore, to determine the effects of basic fibroblast growth factor (bFGF) on chondrogenesis in fracture healing, we injected 100 microg recombinant human bFGF into the fracture site immediately after fracture. In normal calluses, pro-alpha1(II) collagen mRNA (COL2A1) was detected in proliferative chondrocytes beginning on day 4 after the fracture, and pro-alpha1(X) collagen mRNA (COL10A1) in hypertrophic chondrocytes beginning on day 7. In FGF-injected calluses, the cartilage enlarged in size significantly. On day 14, both COL2A1- and COL10A1-expressing cells were more widely distributed, and the amounts of COL2A1 and COL10A1 mRNAs were both approximately 2-fold increased when compared with uninjected fractures. Temporal patterns of expression for these genes were, however, identical to those found in normal calluses. The number of proliferating cell nuclear antigen-positive cells was increased in the non-cartilaginous area in the bFGF-injected calluses by day 4. The present molecular analyses demonstrate that a single injection of bFGF enhances the proliferation of chondroprogenitor cells in fracture callus, and thus contributes to the formation of a larger cartilage. However, maturation of chondrocytes and replacement of the cartilage by osseous tissue are not enhanced by exogenous bFGF, and this results in the prolonged cartilaginous callus phase. We conclude that, in the healing of closed fractures of long bones, exogenous bFGF has a capacity to enlarge the cartilaginous calluses, but not to induce more rapid healing.

Embryonic expression of Krüppel-like factor 6 in neural and non-neural tissues

Mammalian Krüppel-like transcription factors include 12 zinc finger proteins (KLF1-12) that are involved in regulation of cell proliferation and differentiation during morphogenesis and development (Trends Biochem. Sci., 24 (1999) 236). Structural considerations have segregated KLF6 and KLF7 into a separate sub-group, whereas in situ hybridizations have revealed predominant expression of the mouse klf7 gene in the developing nervous system. We examined the embryonic pattern of mouse klf6 in order to assess whether close kinship between KLF6 and KLF7 reflects similar expression patterns of the genes. The results of the in situ hybridizations demonstrate that klf6 expression in the developing nervous system is more restricted than klf7. In contrast to klf7, we also identified several non-neural sites of strong klf6 expression; they include the developing hindgut, heart, lung, kidney, and autopod.

Embryonic expression of type XIX collagen is transient and confined to muscle cells

Type XIX collagen is a poorly characterized extracellular matrix component thought to be involved in the formation of specialized basement membrane zones. Here we examined the developmental expression of the mouse gene (Col19a1) by in situ hybridization. Col19a1 expression during embryogenesis commences at approximately E9.5 in the myotome and with a pattern that closely follows the myogenic regulatory factor myf-5. Like myf-5, Col19a1 transcription gradually decreases in differentiating skeletal muscle progenitors and concomitantly to increased myogenin gene expression. Transient expression of Col19a1 in muscular tissues is confined to a few sites of the developing embryo, such as limbs, tongue, and the smooth muscle layers of the stomach and esophagus. Additional non-muscular sites of Col19a1 activity include the skin of the E16.5 embryos and the cerebral cortex and hippocampus of the new born brain. Unlike all other tissues, expression of Col19a1 in the central nervous system gradually increases after birth.

Cooperative interactions between PBX, PREP, and HOX proteins modulate the activity of the alpha 2(V) collagen (COL5A2) promoter

Cell type-specific expression of the human alpha2(V) collagen (COL5A2) gene depends on a cis-acting element that consists of two contiguous protein binding sites (FPA and FPB) located between nucleotides -149 and -95, relative to the transcription start site. The present study focused on the characterization of the FPB-bound complex. DNA binding assays and cell transfection experiments revealed that the bipartite core sequence of FPB (5'-ATCAATCA-3') binds the PBX1/2, PREP1, and HOXB1 proteins, and this in turn leads to promoter transactivation. In the presence of all three nuclear factors, cooperative interactions between recombinant PBX1 and PREP1 or PBX1 and HOXB1 result in binding of the heterodimers to FPB in vitro. Similarly, overexpression of different combinations of PBX1, PREP1, and HOXB1 transactivates FPB-driven transcription. In contrast to the composition of the FPB complex purified from COL5A2-positive cells, the FPB complex from COL5A2-negative cells contains PBX2 and PREP1 but lacks PBX1. However, PBX1 exogenously introduced into COL5A2-negative cells cannot stimulate FPB-driven transcription unless co-expressed with PREP1. Within the intrinsic limitations of the experimental model, our results indicate that combinatorial interactions among PBX and PREP or HOX proteins are involved in regulating tissue-specific production of collagen V.

COL5A1 haploinsufficiency is a common molecular mechanism underlying the classical form of EDS

We have identified haploinsufficiency of the COL5A1 gene that encodes the proalpha1(V) chain of type V collagen in the classical form of the Ehlers-Danlos syndrome (EDS), a heritable connective-tissue disorder that severely alters the collagen-fibrillar structure of the dermis, joints, eyes, and blood vessels. Eight of 28 probands with classical EDS who were heterozygous for expressed polymorphisms in COL5A1 showed complete or nearly complete loss of expression of one COL5A1 allele. Reduced levels of proalpha1(V) mRNA relative to the levels of another type V collagen mRNA, proalpha2(V), were also observed in the cultured fibroblasts from EDS probands. Products of the two COL5A1 alleles were approximately equal after the addition of cycloheximide to the fibroblast cultures. After harvesting of mRNAs from cycloheximide-treated cultured fibroblasts, heteroduplex analysis of overlapping reverse transcriptase-PCR segments spanning the complete proalpha1(V) cDNA showed anomalies in four of the eight probands that led to identification of causative mutations, and, in the remaining four probands, targeting of CGA-->TGA mutations in genomic DNA revealed a premature stop at codon in one of them. We estimate that approximately one-third of individuals with classical EDS have mutations of COL5A1 that result in haploinsufficiency. These findings indicate that the normal formation of the heterotypic collagen fibrils that contain types I, III, and V collagen requires the expression of both COL5A1 alleles.

The pro-alpha3(V) collagen chain. Complete primary structure, expression domains in adult and developing tissues, and comparison to the structures and expression domains of the other types V and XI procollagen chains

The low abundance fibrillar collagen type V is widely distributed in tissues as an alpha1(V)(2)alpha2(V) heterotrimer that helps regulate the diameters of fibrils of the abundant collagen type I. Mutations in the alpha1(V) and alpha2(V) chain genes have been identified in some cases of classical Ehlers-Danlos syndrome (EDS), in which aberrant collagen fibrils are associated with connective tissue fragility, particularly in skin and joints. Type V collagen also exists as an alpha1(V)alpha2(V)alpha3(V) heterotrimer that has remained poorly characterized chiefly due to inability to obtain the complete primary structure or nucleic acid probes for the alpha3(V) chain or its biosynthetic precursor, pro-alpha3(V). Here we provide human and mouse full-length pro-alpha3(V) sequences. Pro-alpha3(V) is shown to be closely related to the alpha1(V) precursor, pro-alpha1(V), but with marked differences in N-propeptide sequences, and collagenous domain features that provide insights into the low melting temperature of alpha1(V)alpha2(V)alpha3(V) heterotrimers, lack of heparin binding by alpha3(V) chains and the possibility that alpha1(V)alpha2(V)alpha3(V) heterotrimers are incorporated into heterotypic fibrils. In situ hybridization of mouse embryos detects alpha3(V) expression primarily in the epimysial sheaths of developing muscles and within nascent ligaments adjacent to forming bones and in joints. This distribution, and the association of alpha1(V), alpha2(V), and alpha3(V) chains in heterotrimers, suggests the human alpha3(V) gene COL5A3 as a candidate locus for at least some cases of classical EDS in which the alpha1(V) and alpha2(V) genes have been excluded, and for at least some cases of the hypermobility type of EDS, a condition marked by gross joint laxity and chronic musculoskeletal pain. COL5A3 is mapped to 19p13.2 near a polymorphic marker that should be useful in analyzing linkage with EDS and other disease phenotypes.

The murine Bapx1 homeobox gene plays a critical role in embryonic development of the axial skeleton and spleen

Our previous studies in both mouse and human identified the Bapx1 homeobox gene, a member of the NK gene family, as one of the earliest markers for prechondrogenic cells that will subsequently undergo mesenchymal condensation, cartilage production and, finally, endochondral bone formation. In addition, Bapx1 is an early developmental marker for splanchnic mesoderm, consistent with a role in visceral mesoderm specification, a function performed by its homologue bagpipe, in Drosophila. The human homologue of Bapx1 has been identified and mapped to 4p16.1, a region containing loci for several skeletal diseases. Bapx1 null mice are affected by a perinatal lethal skeletal dysplasia and asplenia, with severe malformation or absence of specific bones of the vertebral column and cranial bones of mesodermal origin, with the most severely affected skeletal elements corresponding to ventral structures associated with the notochord. We provide evidence that the failure of the formation of skeletal elements in Bapx1 null embryos is a consequence of a failure of cartilage development, as demonstrated by downregulation of several molecular markers required for normal chondroblast differentiation (α 1(II) collagen, Fgfr3, Osf2, Indian hedgehog, Sox9), as well as a chondrocyte-specific alpha1 (II) collagen-lacZ transgene. The cartilage defects are correlated with failed differentiation of the sclerotome at the time when these cells are normally initiating chondrogenesis. Loss of Bapx1 is accompanied by an increase in apoptotic cell death in affected tissues, although cell cycling rates are unaltered.

The expression of the fibrillar collagen genes during fracture healing: heterogeneity of the matrices and differentiation of the osteoprogenitor cells

The cells that express the genes for the fibrillar collagens, types I, II, III and V, during callus development in rabbit tibial fractures healing under stable and unstable mechanical conditions were localized. The fibroblast-like cells in the initial fibrous matrix express types I, III and V collagen mRNAs. Osteoblasts, and osteocytes in the newly formed membranous bone under the periosteum, express the mRNAs for types I, III and V collagens, but osteocytes in the mature trabeculae express none of these mRNAs. Cartilage formation starts at 7 days in calluses forming under unstable mechanical conditions. The differentiating chondrocytes express both types I and II collagen mRNAs, but later they cease expression of type I collagen mRNA. Both types I and II collagens were located in the cartilaginous areas. The hypertrophic chondrocytes express neither type I, nor type II, collagen mRNA. Osteocalcin protein was located in the bone and in some cartilaginous regions. At 21 days, irrespective of the mechanical conditions, the callus consists of a layer of bone; only a few osteoblasts lining the cavities now express type I collagen mRNA. We suggest that osteoprogenitor cells in the periosteal tissue can differentiate into either osteoblasts or chondrocytes and that some cells may exhibit an intermediate phenotype between osteoblasts and chondrocytes for a short period. The finding that hypertrophic chondrocytes do not express type I collagen mRNA suggests that they do not transdifferentiate into osteoblasts during endochondral ossification in fracture callus.

cDNA sequence and expression of the mouse alpha1(V) collagen gene (Col5a1)

Several overlapping cDNA clones corresponding to the entire coding sequence of the mouse alpha1(V) collagen gene (Col5a1) were isolated. The conceptual amino acid translation indicated a high degree of sequence identity (94%) with the human alpha1(V) chain. All of the important structures previously noted in the human alpha1(V) chain were also conserved in the mouse chain. The alpha1(V) transcripts were easily detected in mouse embryos as early as 11 days post coitum (d.p.c.). The transcripts were widely distributed in non-cartilaginous and cartilaginous tissues. Finally, we calculated the ratio of transcripts of alpha1(V):alpha2(V):alpha1(XI) in the calvaria and tongue of 18 d.p.c. embryos using the competitive reverse transcription-polymerase chain reaction (RT-PCR) technique. The results raised the possibility that there are at least two different kind of types V/XI collagen heterotrimers in mouse embryonic tissues.

Spatial and temporal expression of fibril-forming minor collagen genes (types V and XI) during fracture healing

Skeletal development involves the coordinated participation of several types of collagen, including both major and minor fibrillar collagens. Although much is known about the major fibrillar collagens, such as types I and II, less is known about the minor fibrillar collagens, and their role in the repair and regeneration of bone has not been extensively studied. To clarify the role of minor fibrillar collagens in fracture repair, we examined the spatial and temporal expression of mRNAs for pro-alpha 2(V) collagen and pro-alpha 1(XI) collagen in healing fractures in the rat by in situ hybridization and compared their patterns of expression with those of mRNAs for pro-alpha 1(I) collagen, pro-alpha 1(II) collagen, and osteocalcin. A strong signal for pro-alpha 2(V) was detected in the periosteal osteoprogenitor cells, whereas osteocalcin mRNA was strongly expressed only in the deep layers of the hard callus. The distribution of the pro-alpha 2(V) signal was correlated with that of pro-alpha 1(I) but was mutually exclusive of that of pro-alpha 1(II). The expression of pro-alpha 1(XI) mRNA was synchronously regulated with that of pro-alpha 1(II) during chondrogenesis in the soft callus. In the hard callus, pro-alpha 1(XI) signal was found in osteoblastic cells at the site of intramembranous and endochondral ossification. These cells simultaneously expressed pro-alpha 2(V), although they were negative for pro-alpha 1(II). These findings suggest that the alpha 2(V) collagen chain participates in the formation of the noncartilaginous fibrillar network in the hard callus and preferentially contributes to the initial stage of the intramembranous bone formation. Recent reports have revealed that type-XI collagen, which had been classified as a cartilage-type collagen, is not necessarily specific for cartilage. The present results advanced this recognition and demonstrated a coexpression of alpha 1(XI) mRNA and alpha 2(V) mRNA in the noncartilaginous tissues in the fracture callus; this suggests the presence of tissue-specific and stage-specific heterotrimers consisting of alpha 1(XI) and alpha 2(V) collagen chains and the association of such hybrid trimers with the major fibrillar collagens in the process of fracture healing.

Distribution of minor collagens during skin development

The skin is a tissue containing a large number of collagen types. Several collagens are restricted at the dermo-epidermal junction, contrarily to others present throughout the dermis. However, the distribution of the dermal collagen varies during embryonic development. In this contribution, we have been interested in the collagen types associated with the major collagenous components of the dermis, which are the collagen types I and III. Type V collagen, which is mixed with collagen types I and III to form heterotypic fibrils, has been studied during mouse embryo development. Transcripts of the alpha 1 (V) gene have been localized by in situ hybridization, on flattened cells of the stratum germinativum first, and then only on dermal cells. The expression of the gene decreases at birth, while the expression of the alpha 1(I) gene remains constant, with, however, a ring of high intensity around hair follicles. Other collagen types (VI, and the fibril-associated collagens XII and XIV) have been studied during calf embryonic development by immunofluorescence and ultrastructural immunogold detection. Type VI collagen appears homogeneously distributed throughout the dermis. Type XII collagen is first widely distributed and becomes restricted in the upper, papillary dermis after 6 months of gestation. Type XIV collagen, on the contrary, is first located as a delicate framework around hair follicles (at 19 weeks of gestation), and progressively invades the whole dermis where it appears abundant just before birth. The different functions of all these collagens are discussed in terms of dermis architecture, mechanical properties and physiology.

Bapx1: an evolutionary conserved homologue of the Drosophila bagpipe homeobox gene is expressed in splanchnic mesoderm and the embryonic skeleton

In Drosophila, the visceral mesoderm giving rise to gut musculature is specified by the bagpipe homeobox gene. We have isolated, from both mouse and human, homologues of the bagpipe gene designated Bapx1 and BAPX1, respectively. Bapx1 encodes a predicted protein of 333 amino acids, and has significant regions of homology outside the homeodomain with members of the NK homeobox gene superfamily. Bapx1 maps to the proximal end of chromosome 5 in mouse, near the Msx1 gene. The syntenic region in human corresponds to a chromosomal region containing loci for several skeletal disorders. Bapx1 is first detectable in embryos just prior to axis rotation in lateral plate mesoderm (splanchnic mesoderm) adjacent to the endodermal lining of the prospective gut, and in the most newly formed somites in the region corresponding to the presclerotome, the precursor of the vertebrae. Thus, Bapx1 is one of the earliest developmental markers for the sclerotome portion of the somite and the gut mesentery. Bapx1 continues to be expressed well into organogenesis in lateral plate mesoderm surrounding the mid- and hindgut, and in essentially all cartilaginous condensations which will subsequently undergo endochondral bone formation. The expression pattern of Bapx1 in murine embryos suggests that there are evolutionary conserved mechanisms of visceral mesoderm development across the animal kingdom, and that the mammalian Bapx1 gene may have recently acquired an additional developmental role in skeletal patterning.

Murine FGF-12 and FGF-13: expression in embryonic nervous system, connective tissue and heart

The molecular cloning of cDNAs encoding murine fibroblast growth factor-13 (FGF-13/FHF-2) and three isoforms of murine FGF-12 (FHF-1) is described. Like their highly conserved human counterparts, murine FGF-12 and FGF-13 are part of a distinct subfamily of FGF-like proteins characterized by a greater degree of amino acid sequence cross-homology and by conserved N-terminal domains which do not include secretion signal sequences. In addition to their expression in several adult tissues, both of these FGF genes are prominently and regionally expressed in midgestation mouse embryos, as revealed by in situ hybridization. Fgf12 and fgf13. RNAs were detected in developing central nervous system in cells outside the proliferating ependymal layer, and fgf13 RNA was also found throughout the peripheral nervous system. Fgf12 is expressed in developing soft connective tissue of the limb skeleton and in presumptive connective tissue linking vertebrae and ribs. Both FGF genes are also expressed in the myocardium of the heart, with fgf12 RNA found only in the atrial chamber and fgf13 RNA detected in both atrium and ventricle. On the basis of their novel structure and patterns of expression, FGF-12 and FGF-13 are anticipated to perform embryonic functions distinct from other known FGF molecules.

Cloning of an annelid fibrillar-collagen gene and phylogenetic analysis of vertebrate and invertebrate collagens

Arenicola marina possesses cuticular and interstitial collagens, which are mostly synthesised by its epidermis. A cDNA library was constructed from the body wall. This annelid cDNA library was screened with a sea-urchin-collagen cDNA probe, and several overlapping clones were isolated. Nucleotide sequencing of these clones revealed an open reading frame of 2052 nucleotides. The translation product exhibits a triple helical domain of 138 Gly-Xaa-Yaa repeats followed by a 269-residue-long C-terminal non-collagenous domain (C-propeptide). The triple helical domain exhibits an imperfection that has been previously described in a peptide produced by cyanogen bromide digestion (CNBr peptide) of A. marina interstitial collagen. This imperfection occurs at the same place in the interstitial collagen of the vestimentiferan Riftia pachyptila. This identifies the clone as coding for the C-terminal part of a fibrillar collagen chain. It was called FAm1alpha, for fibrillar collagen 1alpha chain of A. marina. The non-collagenous domain possesses a structure similar to carboxy-terminal propeptides of fibrillar pro-alpha chains. Only six conserved cysteine residues are observed in A. marina compared with seven or eight in all other known C-propeptides. This provides information on the importance of disulfide bonds in C-propeptide interactions and in the collagen-assembly process. Phylogenetic studies indicate that the fibrillar collagen 1alpha chain of A. marina is homologous to the R. pachyptila interstitial collagen and that the FAm1alpha gene evolved independently from the other alpha-chain genes. Complementary analyses indicate that the vertebrate fibrillar collagen family is composed of two monophyletic subgroups with a specific position of the collagen type-V chains.

Disproportionate micromelia (Dmm) in mice caused by a mutation in the C-propeptide coding region of Col2a1

Mice that are homozygous for the autosomal semidominant disproportionate micromelia (Dmm) mutation are characterized by disproportionate micromelia, thoracic dysplasia, and cleft palate. Chondrocytes of the epiphyseal growth plates are not organized into columns, and ultrastructural analysis reveals excessive dilation of the endoplasmic reticulum and a paucity of collagen fibrils in the extracellular matrix. To map the Dmm locus, Dmm mice were crossed with the multiple ecotropic viral (MEV) linkage testing stock. Significant linkage of Dmm to the fourteen MEV linkage markers was not observed, thereby excluding approximately 50% of the genome as candidate regions encoding Dmm. Subsequently, microsatellite markers were used to assess linkage to the nonexcluded regions of the genome, revealing tight linkage to the locus of Col2a1, the gene encoding the alpha-chains of type II collagen. alpha 1(II) collagen cDNA, synthesized with RNA from homozygotes, was cloned and sequenced, revealing a three-nucleotide deletion in the region encoding the C-propeptide globular domain. The deletion leads to the substitution of one amino acid, Asn, in the mutant for two amino acids, Lys and Thr, in the wild type. Several human chondrodysplasias with similar phenotypes to that of Dmm are associated with defects in type II collagen. Thus, mice bearing the Dmm mutation serve as a model for studying the pathogenesis of these disorders while revealing novel insights into normal skeletal morphogenesis.

Developmental expression of MMP-9 (gelatinase B) mRNA in mouse embryos

Considerable remodeling of the extracellular matrix as well as cellular migration takes place during embryogenesis. Since the metalloproteinase MMP-9 is implicated in these functions in cancer cells, we studied the patterns of expression of MMP-9 mRNA during the development of post-implantation mouse embryos. MMP-9 mRNA was detected using the ribonuclease protection assay in poly A+RNA from 13 to 17 day embryos, but not at 11 days. In order to localize these transcripts, in situ hybridization was performed on sections of murine embryos from 7.5 to 15 days of gestation. At the time of implantation, MMP-9 mRNA was localized to the invading trophoblast cells. Strong signals were also seen in the yolk sac. No signal for MMP-9 mRNA was seen by in situ hybridization in the embryo until day 11 when detectable reaction was seen in the central nervous system. By day 15 strong signals were seen in the liver, in the developing bronchial epithelium of the lungs and in the primordial alveoli, in the epithelium of the thyroid gland, in the thymus, in the endochondrial plates of the bone, and in neural cells. The liver from day 15 embryos contained gelatinase activity at 105 kDa consistent with MMP-9. Thus, MMP-9 expression appears to be expressed in specific organs in a precise temporal sequence during development.

Developmental pattern of expression of the mouse alpha 1 (XI) collagen gene (Col11a1)

Fibrillar networks are intimately involved in several morphogenetic processes which underlie the harmonious development of the vertebrate embryo. Recent genetic evidence has demonstrated that the minor types V and XI collagen are key regulators of types I and II fibrillogenesis in non-cartilaginous and cartilaginous matrices, respectively. A comprehensive understanding of the expression and regulation of the genes coding for the chains of the minor collagen types is therefore relevant to animal morphogenesis and development. The present study was undertaken to elucidate the embryonic pattern of expression of the gene coding for the mouse alpha 1 chain of type XI colagen (Col11a1) using the technique of in situ hybridization. Transcripts of the Col11a1 gene were detected as early as 11 days of gestation. The alpha 1(XI) transcripts were found to accumulate mostly in cartilaginous tissues, such as the chondrocranium and the developing limbs. Like the major cartilage-specific collagen (type II), Col11a1 expression was also noted in the neuro-epithelium of the brain. However, alpha 1(XI) transcripts accumulated in several other non-cartilaginous sites. They include odontoblasts, trabecular bones, atrioventricular valve of the heart, the tongue, the intestine, and the otic vesicle. Altogether, the data confirm that Col11a1 has a broader spectrum of expression than previously thought. This finding raises the possibility that the alpha 1(XI) chain may participate in the formation of stage- and tissue-specific trimers with distinct functional properties.

Another look at collagen V and XI molecules

The fibrillar collagens are the most abundant proteins of extracellular matrices. Among them, collagens V and XI are quantitatively minor components which participate in the formation of the fibrillar collagen network. Since these collagens were discovered, studies have demonstrated that they may play a fundamental role in the control of fibrillogenesis, probably by forming a core within the fibrils. Another characteristic of these collagens is the partial retention of their N-propeptide extensions in tissue forms, an unusual observation in comparison to the other known fibrillar collagens. The tissue locations of collagens V and XI are different, but their structural and biological properties seem to be closely related. It has been shown that their primary structures are highly conserved at both the gene and protein levels, and that these conserved features are the bases of their similar biological properties. In particular, they are both resistant to mammalian collagenases, and surprisingly sensitive to trypsin treatment. Collagens V and XI are usually buried within the major collagen fibrils, although they have both cell adhesion and heparin binding sites which could be of crucial importance in physiological processes such as development and wound healing. It has became evident that several molecules are in fact heterotypic associations of chains from both collagens V and XI, demonstrating that these two collagens are not distinct types but a single type which can be called collagen V/XI.

Developmental expression of fibrillin genes suggests heterogeneity of extracellular microfibrils

Extracellular microfibrils, alone or in association with elastin, confer critical biomechanical properties on a variety of connective tissues. Little is known about the composition of the microfibrils or the factors responsible for their spatial organization into tissue-specific macroaggregates. Recent work has revealed the existence of two structurally related microfibrillar components, termed fibrillin-1 and fibrillin-2. The functional relationships between these glycoproteins and between them and other components of the microfibrils and elastic fibers are obscure. As a first step toward elucidating these important points, we compared the expression pattern of the fibrillin genes during mammalian embryogenesis. The results revealed that the two genes are differentially expressed, in terms of both developmental stages and tissue distribution. In the majority of cases, fibrillin-2 transcripts appear earlier and accumulate for a shorter period of time than fibrillin-1 transcripts. Synthesis of fibrillin-1 correlates with late morphogenesis and the appearance of well-defined organ structures; fibrillin-2 synthesis, on the other hand, coincides with early morphogenesis and, in particular, with the beginning of elastogenesis. The findings lend indirect support to our original hypothesis stating that fibrillins contribute to the compositional and functional heterogeneity of the microfibrils. The available evidence is also consistent with the notion that the fibrillins might have distinct, but related roles in microfibril physiology. Accordingly, we propose that fibrillin-1 provides mostly force-bearing structural support, whereas fibrillin-2 predominantly regulates the early process of elastic fiber assembly.

Targeted mutation in the col5a2 gene reveals a regulatory role for type V collagen during matrix assembly

The tissue-specific organization of collagen molecules into tridimensional macroaggregates determines the physiomechanical properties of most connective tissues, but the factors and mechanisms controlling this process are unknown. It has been postulated that quantitatively minor types V and XI collagen regulate the growth of type I and II collagen fibrils, respectively. To test this hypothesis, we created mice that produce a structurally abnormal alpha 2(V) collagen chain. Homozygous mutant mice survive poorly, possibly because of complications from spinal deformities, and exhibit skin and eye abnormalities caused by disorganized type I collagen fibrils. Our results demonstrate that type V collagen is a key determinant in the assembly of tissue-specific matrices, and provide an animal model for human connective tissue disorders.

Type II collagen is transiently expressed during avian cardiac valve morphogenesis

We present new evidence of the temporal and spatial expression of type II collagen in the embryonic chick heart during the very early stages of its development. In particular, we emphasize the distribution of its mRNA and protein during valve formation. Type II collagen as well as several other fibrillar collagens (types I, III, and V) are present in stage 18 endocardial cushion mesenchymal cells. At stage 23, alpha 1 (II) collagen transcripts and the cognate polypeptide colocalize in the atrioventricular valves. As development proceeds, the relative abundance of alpha 1 (II) collagen transcripts decreases during the stages studied (stages 22 to 45; day 3.5 to day 19) as assayed by RNA blotting of extracts of whole hearts. Type II collagen protein was immunologically undetectable in stage 38 (day 12) hearts, although collagens I, III, and V persisted and localize in the valve regions, in the endothelial lining of the heart, and in the epicardium. In keeping with other observations of type II collagen expression in non-chondrogenic regions of a variety of vertebrate embryos, the avian heart also exhibits transient type II collagen expression.