The pro-alpha 1(V) collagen chain. Complete primary structure, distribution of expression, and comparison with the pro-alpha 1(XI) collagen chain

Differential Expression of Extracellular Matrix and Adhesion Molecules in Fetal-Origin Amniotic Epithelial Cells of Preeclamptic Pregnancy

Preeclampsia is a common disease that can occur during human pregnancy and is a leading cause of both maternal and neonatal morbidity and mortality. Inadequate trophoblast invasion and deficient remodeling of uterine spiral arteries are associated with preeclampsia (PE). The development of this syndrome is thought to be related to multiple factors. Recently, we isolated patient-specific human amniotic epithelial cells (AECs) from the placentas of 3 women with normal pregnancy and 3 with preeclamptic pregnancy. Since the characteristics of human AECs in PE are different from those in normal pregnancy, we sought to confirm the genes differentially expressed between preeclamptic pregnancy and normal pregnancy. Therefore, we performed transcriptome analysis to investigate the candidate genes associated with the possible pathophysiology of preeclampsia. Pathway analysis was performed using the Database for Annotation, Visualization, and Integrated Discovery (DAVID) and Kyoto Encyclopedia of Genes and Genomes (KEGG) online resource. In this study, we selected a total of 12 pathways and focused on extracellular matrix-related and biological adhesion molecules. Using RT-PCR array and real-time PCR, we confirmed that COL16A1, ITGB2, and LAMA3 were significantly up-regulated, but ITGA1, ITGA3, ITGA6, MMP1, MMP3, MMP10 and MMP11 were significantly down-regulated in preeclamptic fetal origin cells. Taken together, we suggest that the genes and pathways identified here may be responsible for the occurrence and development of PE, and controlling their expression may play a role in communication with fetal-maternal placenta to keep normal pregnancy.

Collagen V localizes to pericellular sites during tendon collagen fibrillogenesis

During tendon development collagen fibrillogenesis occurs in extracellular micro-domains defined by the tenocytes. This permits cellular regulation of the extracellular steps involved in the tissue-specific matrix assembly required for function. The hypothesis tested here is that collagen V associates with the tenocyte surface where it functions in regulation of collagen assembly and cell-directed fibril deposition. The in vitro and in vivo data demonstrate that collagen V is a quantitatively minor component of the tendon. It is preferentially localized on the tenocyte surface as distinct foci in tendons and in cell culture. In vitro data indicate that this interaction with the tenocyte is not HSPG GAG-dependent. Collagen V is present as the mature, processed form, is absent from the media, and is a significant part of the detergent-insoluble cell layer, presumably as part of a membrane-associated complex. In contrast, procollagen I is not efficiently processed and is found predominantly in the culture media. Our data suggest that the regulatory role of collagen V requires collagen V to occupy a different cellular niche from the structural collagen I. In monolayer cultures, the conversion to the tissue form of collagen V and its deposition with the cell layer suggest efficient engagement of procollagen V with pericellular receptors and processing enzymes. The secretion of collagen I into the media and inefficient processing of procollagen I suggest reduced accessibility to these pericellular molecules due to disengagement from the cell surface. This all points to differential spatial localization of collagen V as a mechanism to optimize its regulatory roles during the cell-surface directed steps in tendon collagen fibril assembly.

Comprehensive mass spectrometric mapping of the hydroxylated amino acid residues of the α1(V) collagen chain

BACKGROUND

α1(V) is an extensively modified collagen chain important in disease.

RESULTS

Comprehensive mapping of α1(V) post-translational modifications reveals unexpectedly large numbers of X-position hydroxyprolines in Gly-X-Y amino acid triplets.

CONCLUSION

The unexpected abundance of X-position hydroxyprolines suggests a mechanism for differential modification of collagen properties.

SIGNIFICANCE

Positions, numbers, and occupancy of modified sites can provide insights into α1(V) biological properties. Aberrant expression of the type V collagen α1(V) chain can underlie the connective tissue disorder classic Ehlers-Danlos syndrome, and autoimmune responses against the α1(V) chain are linked to lung transplant rejection and atherosclerosis. The α1(V) collagenous COL1 domain is thought to contain greater numbers of post-translational modifications (PTMs) than do similar domains of other fibrillar collagen chains, PTMs consisting of hydroxylated prolines and lysines, the latter of which can be glycosylated. These types of PTMs can contribute to epitopes that underlie immune responses against collagens, and the high level of PTMs may contribute to the unique biological properties of the α1(V) chain. Here we use high resolution mass spectrometry to map such PTMs in bovine placental α1(V) and human recombinant pro-α1(V) procollagen chains. Findings include the locations of those PTMs that vary and those PTMs that are invariant between these α1(V) chains from widely divergent sources. Notably, an unexpectedly large number of hydroxyproline residues were mapped to the X-positions of Gly-X-Y triplets, contrary to expectations based on previous amino acid analyses of hydrolyzed α1(V) chains from various tissues. We attribute this difference to the ability of tandem mass spectrometry coupled to nanoflow chromatographic separations to detect lower-level PTM combinations with superior sensitivity and specificity. The data are consistent with the presence of a relatively large number of 3-hydroxyproline sites with less than 100% occupancy, suggesting a previously unknown mechanism for the differential modification of α1(V) chain and type V collagen properties.

Proteomic analysis of Col11a1-associated protein complexes

Cartilage plays an essential role during skeletal development within the growth plate and in articular joint function. Interactions between the collagen fibrils and other extracellular matrix molecules maintain structural integrity of cartilage, orchestrate complex dynamic events during embryonic development, and help to regulate fibrillogenesis. To increase our understanding of these events, affinity chromatography and liquid chromatography/tandem mass spectrometry were used to identify proteins that interact with the collagen fibril surface via the amino terminal domain of collagen α1(XI) a protein domain that is displayed at the surface of heterotypic collagen fibrils of cartilage. Proteins extracted from fetal bovine cartilage using homogenization in high ionic strength buffer were selected based on affinity for the amino terminal noncollagenous domain of collagen α1(XI). MS was used to determine the amino acid sequence of tryptic fragments for protein identification. Extracellular matrix molecules and cellular proteins that were identified as interacting with the amino terminal domain of collagen α1(XI) directly or indirectly, included proteoglycans, collagens, and matricellular molecules, some of which also play a role in fibrillogenesis, while others are known to function in the maintenance of tissue integrity. Characterization of these molecular interactions will provide a more thorough understanding of how the extracellular matrix molecules of cartilage interact and what role collagen XI plays in the process of fibrillogenesis and maintenance of tissue integrity. Such information will aid tissue engineering and cartilage regeneration efforts to treat cartilage tissue damage and degeneration.

Bilateral consecutive rupture of the quadriceps tendon in a man with BstUI polymorphism of the COL5A1 gene

A genetic component has been implicated in tendinopathies involving tendon rupture. Type V collagen, a quantitatively minor fibrillar collagen which forms heterotypic fibrils with type I collagen, plays a role in the regulation of the size and configuration of fibrils of the much more abundant component type I collagen. To date, no data on the genetic component of bilateral rupture of the quadriceps tendon have been reported. We describe the presence of BstUI polymorphism of the COL5A1 gene in a man with bilateral rupture of the quadriceps tendon. The COL5A1 (the variant rs12722, BstUI RFLP) can be a candidate gene associated with the development of bilateral quadriceps tendon rupture.

Characterization of the six zebrafish clade B fibrillar procollagen genes, with evidence for evolutionarily conserved alternative splicing within the pro-alpha1(V) C-propeptide

Genes for tetrapod fibrillar procollagen chains can be divided into two clades, A and B, based on sequence homologies and differences in protein domain and gene structures. Although the major fibrillar collagen types I-III comprise only clade A chains, the minor fibrillar collagen types V and XI comprise both clade A chains and the clade B chains pro-alpha1(V), pro-alpha3(V), pro-alpha1(XI) and pro-alpha2(XI), in which defects can underlie various genetic connective tissue disorders. Here we characterize the clade B procollagen chains of zebrafish. We demonstrate that in contrast to the four tetrapod clade B chains, zebrafish have six clade B chains, designated here as pro-alpha1(V), pro-alpha3(V)a and b, pro-alpha1(XI)a and b, and pro-alpha2(XI), based on synteny, sequence homologies, and features of protein domain and gene structures. Spatiotemporal expression patterns are described, as are conserved and non-conserved features that provide insights into the function and evolution of the clade B chain types. Such features include differential alternative splicing of NH(2)-terminal globular sequences and the first case of a non-triple helical imperfection in the COL1 domain of a clade B, or clade A, fibrillar procollagen chain. Evidence is also provided for previously unknown and evolutionarily conserved alternative splicing within the pro-alpha1(V) C-propeptide, which may affect selectivity of collagen type V/XI chain associations in species ranging from zebrafish to human. Data presented herein provide insights into the nature of clade B procollagen chains and should facilitate their study in the zebrafish model system.

Developmental roles of the BMP1/TLD metalloproteinases

The astacin family (M12A) of the metzincin subclan MA(M) of metalloproteinases has been detected in developing and mature individuals of species that range from hydra to humans. Functions of this family of metalloproteinase vary from digestive degradation of polypeptides, to biosynthetic processing of extracellular proteins, to activation of growth factors. This review will focus on a small subgroup of the astacin family; the bone morphogenetic protein 1 (BMP1)/Tolloid (TLD)-like metalloproteinases. In vertebrates, the BMP1/TLD-like metalloproteinases play key roles in regulating formation of the extracellular matrix (ECM) via biosynthetic processing of various precursor proteins into mature functional enzymes, structural proteins, and proteins involved in initiating mineralization of the ECM of hard tissues. Roles in ECM formation include: processing of the C-propeptides of procollagens types I-III, to yield the major fibrous components of vertebrate ECM; proteolytic activation of the enzyme lysyl oxidase, necessary to formation of covalent cross-links in collagen and elastic fibers; processing of NH2-terminal globular domains and C-propeptides of types V and XI procollagen chains to yield monomers that are incorporated into and control the diameters of collagen type I and II fibrils, respectively; processing of precursors for laminin 5 and collagen type VII, both of which are involved in securing epidermis to underlying dermis; and maturation of small leucine-rich proteoglycans. The BMP1/TLD-related metalloproteinases are also capable of activating the vertebrate transforming growth factor-beta (TGF-beta)-like "chalones" growth differentiation factor 8 (GDF8, also known as myostatin), and GDF11 (also known as BMP11), involved in negative feedback inhibition of muscle and neural tissue growth, respectively; by freeing them from noncovalent latent complexes with their cleaved prodomains. BMP1/TLD-like proteinases also liberate the vertebrate TGF-beta-like morphogens BMP2 and 4 and their invertebrate ortholog decapentaplegic, from latent complexes with the vertebrate extracellular antagonist chordin and its invertebrate ortholog short gastrulation (SOG), respectively. The result is formation of the BMP signaling gradients that form the dorsal-ventral axis in embryogenesis. Thus, BMP1/TLD-like proteinases appear to be key to regulating and orchestrating formation of the ECM and signaling by various TGF-beta-like proteins in morphogenetic and homeostatic events.

Type V collagen controls the initiation of collagen fibril assembly

Vertebrate collagen fibrils are heterotypically composed of a quantitatively major and minor fibril collagen. In non-cartilaginous tissues, type I collagen accounts for the majority of the collagen mass, and collagen type V, the functions of which are poorly understood, is a minor component. Type V collagen has been implicated in the regulation of fibril diameter, and we reported recently preliminary evidence that type V collagen is required for collagen fibril nucleation (Wenstrup, R. J., Florer, J. B., Cole, W. G., Willing, M. C., and Birk, D. E. (2004) J. Cell. Biochem. 92, 113-124). The purpose of this study was to define the roles of type V collagen in the regulation of collagen fibrillogenesis and matrix assembly. Mouse embryos completely deficient in pro-alpha1(V) chains were created by homologous recombination. The col5a1-/- animals die in early embryogenesis, at approximately embryonic day 10. The type V collagen-deficient mice demonstrate a virtual lack of collagen fibril formation. In contrast, the col5a1+/- animals are viable. The reduced type V collagen content is associated with a 50% reduction in fibril number and dermal collagen content. In addition, relatively normal, cylindrical fibrils are assembled with a second population of large, structurally abnormal collagen fibrils. The structural properties of the abnormal matrix are decreased relative to the wild type control animals. These data indicate a central role for the evolutionary, ancient type V collagen in the regulation of fibrillogenesis. The complete dependence of fibril formation on type V collagen is indicative of the critical role of the latter in early fibril initiation. In addition, this fibril collagen is important in the determination of fibril structure and matrix organization.

Biosynthetic Processing of the Pro-α1(V)Pro-α2(V)Pro-α3(V) Procollagen Heterotrimer

Type V collagen is a quantitatively minor fibrillar collagen comprised of different chain compositions in different tissues. The most widely distributed form, an alpha1(V)2alpha2(V) heterotrimer, regulates the physical properties of type I/V heterotypic collagen fibrils via partially processed NH2-terminal globular sequences. A less characterized alpha1(V)alpha2(V)alpha3(V) heterotrimer has a much more limited distribution of expression and unknown function(s). We characterized the biosynthetic processing of pro-alpha1(V)2pro-alpha2(V) procollagen previously and showed it to differ in important ways from biosynthetic processing of the major fibrillar procollagens I-III. Here we have successfully produced recombinant pro-alpha1(V)pro-alpha2(V)pro-alpha3(V) heterotrimers. We use these, and mouse embryo fibroblasts doubly homozygous null for the Bmp1 gene, which encodes the metalloproteinase bone morphogenetic protein-1 (BMP-1), and for a gene encoding the closely related metalloproteinase mammalian Tolloid-like 1, to characterize biosynthetic processing of pro-alpha1(V)pro-alpha2(V)pro-alpha3(V) heterotrimers, thus completing characterization of type V collagen biosynthetic processing. Whereas pro-alpha1(V) and pro-alpha2(V) processing in pro-alpha1(V)pro-alpha2(V)pro-alpha3(V) heterotrimers is similar to that which occurs in pro-alpha1(V)2pro-alpha2(V) heterotrimers, the processing of pro-alpha3(V) by BMP-1 occurs at an unexpected site within NH2-terminal globular sequences. We also demonstrate that, despite similarities in NH2-terminal domain structures, pro-alpha2(V) NH2-terminal globular sequences are not cleaved by ADAMTS-2, the metalloproteinase that cleaves the N-propeptides of the major fibrillar procollagen chains.

The molecular basis of classic Ehlers-Danlos syndrome: A comprehensive study of biochemical and molecular findings in 48 unrelated patients

Classic Ehlers-Danlos syndrome (EDS) is characterized by fragile and hyperextensible skin, atrophic scarring, and joint hypermobility. Mutations in the COL5A1 and the COL5A2 gene encoding the alpha1(V) and the alpha2(V) chains, respectively, of type V collagen have been shown to cause the disorder, but it is unknown what proportion of classic EDS patients carries a mutation in these genes. We studied fibroblast cultures from 48 patients with classic EDS by SDS-PAGE for the presence of type V collagen defects. An abnormal collagen pattern was detected in only 2 out of 48 cell lines, making this a poor method for routine diagnostic evaluation. A total of 42 out of 48 (88%) patients were heterozygous for an expressed polymorphic variant in COL5A1. cDNA from 18 (43%) of them expressed only one COL5A1 allele. In 37 patients, the COL5A1/A2 genes were then analyzed by SSCP and conformation sensitive gel electrophoresis (CSGE). A total of 26 patients that were mutation-negative after SSCP/CSGE screening were reanalyzed by dHPLC. In addition, 11 other patients were analyzed by dHPLC only. In total, 17 mutations leading to a premature stop codon and five structural mutations were identified in the COL5A1 and the COL5A2 genes. In three patients with a positive COL5A1 null-allele test, no causal mutation was found. Overall, in 25 out of 48 patients (52%) with classic EDS, an abnormality in type V collagen was confirmed. Variability in severity of the phenotype was observed, but no significant genotype-phenotype correlations emerged. The relatively low mutation detection rate suggests that other genes are involved in classic EDS. We excluded the COL1A1, COL1A2, and DCN gene as major candidate genes for classic EDS, since no causal mutation in these genes was found in a number of patients who tested negative for COL5A1 and COL5A2.

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

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

Mechanical stretching force promotes collagen synthesis by cultured cells from human ligamentum flavum via transforming growth factor-beta1

Although mechanical stress as a result of spinal instability is known to cause hypertrophy of the ligamentum flavum resulting in degenerative spinal canal stenosis, the mechanism of the ligament hypertrophy is not well understood. In the present study, we investigated the effect of mechanical stretching force on collagen synthesis and transforming growth factor-beta1 (TGF-beta1) production using ligament cells isolated from human ligamentum flavum in vitro. Ligamentum flavum cells (LFCs) were isolated from human ligamentum flavum obtained from patients who underwent lumbar spine surgery. The LFCs were subjected to a mechanical stretching force using a commercially available stretching device that physically deformed the cells. Collagen synthesis and TGF-beta1 production levels in the LFCs were then examined. Notable increases were observed in the gene expressions of collagen types I, III, and V in LFCs subjected to mechanical stretching force. The increase in collagen gene expression of LFCs was inhibited in the presence of anti-TGF-beta1 antibodies. Production of TGF-beta1 by the LFCs also increased significantly by the mechanical stretching force. Exogenous application of TGF-beta1 was confirmed to increase collagen synthesis of the LFCs. This data indicated that mechanical stretching force can promote TGF-beta1 production by LFCs, resulting in hypertrophy of the ligament.

Order of intron removal influences multiple splice outcomes, including a two-exon skip, in a COL5A1 acceptor-site mutation that results in abnormal pro-alpha1(V) N-propeptides and Ehlers-Danlos syndrome type I

Ehlers-Danlos syndrome (EDS) type I (the classical variety) is a dominantly inherited, genetically heterogeneous connective-tissue disorder. Mutations in the COL5A1 and COL5A2 genes, which encode type V collagen, have been identified in several individuals. Most mutations affect either the triple-helical domain of the protein or the expression of one COL5A1 allele. We identified a novel splice-acceptor mutation (IVS4-2A-->G) in the N-propeptide-encoding region of COL5A1, in one patient with EDS type I. The outcome of this mutation was complex: In the major product, both exons 5 and 6 were skipped; other products included a small amount in which only exon 5 was skipped and an even smaller amount in which cryptic acceptor sites within exon 5 were used. All products were in frame. Pro-alpha1(V) chains with abnormal N-propeptides were secreted and were incorporated into extracellular matrix, and the mutation resulted in dramatic alterations in collagen fibril structure. The two-exon skip occurred in transcripts in which intron 5 was removed rapidly relative to introns 4 and 6, leaving a large (270 nt) composite exon that can be skipped in its entirety. The transcripts in which only exon 5 was skipped were derived from those in which intron 6 was removed prior to intron 5. The use of cryptic acceptor sites in exon 5 occurred in transcripts in which intron 4 was removed subsequent to introns 5 and 6. These findings suggest that the order of intron removal plays an important role in the outcome of splice-site mutations and provide a model that explains why multiple products derive from a mutation at a single splice site.

Biosynthetic processing of the pro-alpha 1(V)2pro-alpha 2(V) collagen heterotrimer by bone morphogenetic protein-1 and furin-like proprotein convertases

The low abundance fibrillar collagen type V is incorporated into and regulates the diameters of type I collagen fibrils. Bone morphogenetic protein-1 (BMP-1) is a metalloprotease that plays key roles in regulating formation of vertebrate extracellular matrix; it cleaves the C-propeptides of the major fibrillar procollagens I-III and processes precursors to produce the mature forms of the cross-linking enzyme prolysyl oxidase, the proteoglycan biglycan, and the basement membrane protein laminin 5. Here we have successfully produced recombinant pro-alpha1(V)(2)pro-alpha2(V) heterotrimers, and we have used these to characterize biosynthetic processing of the most prevalent in vivo form of type V procollagen. In addition, we have compared the processing of endogenous pro-alpha1(V) chains by wild type mouse embryo fibroblasts and by fibroblasts derived from embryos doubly homozygous null for the Bmp-1 gene and for a gene encoding the closely related metalloprotease mammalian Tolloid-like 1. Together, results presented herein indicate that within pro-alpha1(V)(2)pro-alpha2(V) heterotrimers, pro-alpha1(V) N-propeptides and pro-alpha2(V) C-propeptides are processed by BMP-1-like enzymes, and pro-alpha1(V) C-propeptides are processed by furin-like proprotein convertases in vivo.

Bone morphogenetic protein-1 (BMP-1) mediates C-terminal processing of procollagen V homotrimer

The processing of the fibrillar procollagen precursors to mature collagens is an essential requirement for fibril formation. The enzymes involved in these events are known as the procollagen N and C proteinases. The latter, which cleaves the C-propeptides of the fibrillar procollagens I-III, is identical to the previously described bone morphogenetic protein-1 (BMP-1). Surprisingly, unlike the other fibrillar collagens, the processing of the C-propeptide domain of the procollagen V homotrimer was found to be mediated by furin rather than BMP-1. However, the presence of putative BMP-1 cleavage sites in the alpha1(V) C-propeptide sequence prompted us to reconsider the procollagen V C-propeptide cleavage by BMP-1. Using a recombinant system to produce substantial amounts of the proalpha1(V) homotrimer, we have previously shown that the C-propeptide is spontaneously released in the culture medium. The trimeric C-propeptide fragment, resulting from the furin cleavage, still encompassed the predicted BMP-1 cleavage sites. It was purified and tested as a substrate for BMP-1. In parallel, the release of the C-propeptide in the culture medium was inhibited by the addition of a specific furin inhibitor, allowing the re-examination of BMP-1 activity on the intact molecule. We showed that BMP-1 does cleave both substrates at one of the two predicted C-proteinase cleavage sites. Our results favor a role for PCP/BMP-1 in physiological C-terminal processing of procollagen V and imply a general mechanism for fibrillar collagen C-terminal processing.

Expression of type I and type V collagen mRNAs in the elasmoid scales of a teleost fish as revealed by in situ hybridization

The ability of scale-forming cells to produce both type I and type V collagens was investigated by in situ hybridization at the light and electron microscope levels. Biochemical analyses reported that type I collagen, the predominant component, was associated with the minor type V collagen in the collagenous matrix of the teleost scales where, thin and thick collagen fibrils formed distinct layers. Thin collagen fibrils of the external layer were produced by the episquamal scleroblasts scattered on the outer scale surface, while thick collagen fibrils forming the compact basal plate were produced by the hyposquamal scleroblasts lining the inner surface of the scale. We demonstrated that episquamal and hyposquamal scleroblasts contained mRNAs for alpha1(I) and alpha1(V) collagens. Quantification by image analysis of the relative amount of alpha1(I) and alpha1(V) mRNAs in episquamal and hyposquamal scleroblasts suggests that the gene expression of type V collagen was proportionally higher in episquamal scleroblasts. These results support our hypothesis that the diameter of the thin fibrils of the external layer is regulated by the significant amount of type V collagen that interacts with type I collagen.

Structure of a full-length cDNA clone for the pro-alpha1(V/XI) collagen chain of red seabream

The cDNA of type V/XI collagen alpha1 (rsCOL) chain has been isolated from cells established from eyed-period eggs of red seabream, Pagrus major, and sequenced. The amino acid sequence deduced from red seabream alpha1(V/XI) chain resembles that of type XI collagen alpha1 chain. On the other hand, tissue distribution of rsCOL resembles that of type V collagen based on RT-PCR analysis. This is the first report of the cloning of the full-length cDNA of type V/XI collagen alpha1 chain from fish.

Schwann cells synthesize type V collagen that contains a novel alpha 4 chain. Molecular cloning, biochemical characterization, and high affinity heparin binding of alpha 4(V) collagen

Previously, we reported the isolation of a heparan sulfate-binding collagenous protein, p200, that is expressed by Schwann cells in developing peripheral nerves ((1996) J. Biol. Chem. 271, 13844-13853; (1999) J. Neurosci. Res. 56, 284-294). Here, we report the cloning of p200 cDNA from a Schwann cell cDNA library. The deduced amino acid sequence identifies p200 as a novel member of the collagen type V gene family. This polypeptide, which we have named alpha4 type V (alpha4(V)) collagen, contains an uninterrupted Gly-X-X collagen domain of 1011 amino acids that shows 82% sequence identity to human alpha3(V) collagen and 71% identity to rat alpha1(V) collagen. alpha4(V) is secreted by Schwann cells as a collagen heterotrimer that also contains alpha1(V) chains. alpha4(V)-containing collagen molecules synthesized by Schwann cells retain their amino-terminal non-collagenous domains. alpha4(V) mRNA was detected by reverse transcriptase-linked polymerase chain reaction amplification in neonatal and adult brain and neonatal peripheral nerve. alpha4(V) mRNA and protein were not detected in most other tissues, including the placenta and heart, which are known to contain alpha3(V). This pattern of alpha4(V) expression contrasted with that of alpha1(V) mRNA and protein, which were ubiquitously expressed. The isolated alpha4(V) chain demonstrated an unusually high affinity for heparin. The restricted expression and unusual properties of alpha4(V)-containing collagen type V molecules suggest a unique and important role for these molecules in peripheral nerve development.

Cloning of cDNA for rat pro alpha1(V) collagen mRNA. Expression patterns of type I, type III and type V collagen genes in experimental granulation tissue

A cDNA clone for rat pro alpha1(V) collagen mRNA was constructed using PCR amplification, with primers based on human and hamster COL5A1 gene sequences. The clone pRCVA1 is 560 nucleotides long and it encodes for the carboxy propeptide of type V procollagen. Homology shared with type I collagen sequence was 64%, with type II collagen 65% and with type III collagen 61%. To evaluate the spatial and temporal expression of type V collagen mRNA in wound healing model, subcutaneously implanted viscose cellulose sponges in rats were used to induce granulation tissue formation. Analyses on granulation tissue were carried out on days 5, 8, 14, 21, 30, 59 and 84. Specific cDNA probes to pro alpha1(I), pro alpha1(III) and pro alpha1(V) collagen mRNA were used in slot blot, Northern and in situ hybridization. Type I collagen gene expression was upregulated at the initial stage of wound healing, type III collagen gene expression was constant and from the day 14 onwards type I and III collagen gene expressions were at the same level. Type V collagen gene expression was seen at every time point studied but at a considerably lower level than type I and III collagens. In situ hybridization showed that type V collagen was expressed in two different cell types. In conclusion, type V collagen was expressed in the wound healing model from at least day 5 onwards and it was synthesized by fibroblast-like and rounded cells.

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.

Null alleles of the COL5A1 gene of type V collagen are a cause of the classical forms of Ehlers-Danlos syndrome (types I and II)

Ehlers-Danlos syndrome (EDS) types I and II, which comprise the classical variety, are well characterized from the clinical perspective, but it has been difficult to identify the molecular basis of the disorder in the majority of affected individuals. Several explanations for this failure to detect mutations have been proposed, including genetic heterogeneity, failure of allele expression, and technical difficulties. Genetic heterogeneity has been confirmed as an explanation for such failure, since causative mutations have been identified in the COL5A1, COL5A2, and tenascin X genes and since they have been inferred in the COL1A2 gene. Nonetheless, in the majority of families with autosomal dominant inheritance of EDS, there appears to be linkage to loci that contain the COL5A1 or COL5A2 genes. To determine whether allele-product instability could explain failure to identify some mutations, we analyzed polymorphic variants in the COL5A1 gene in 16 individuals, and we examined mRNA for the expression of both alleles and for alterations in splicing. We found a splice-site mutation in a single individual, and we determined that, in six individuals, the mRNA from one COL5A1 allele either was not expressed or was very unstable. We identified small insertions or deletions in five of these cell strains, but we could not identify the mutation in the sixth individual. Thus, although as many as one-half of the mutations that give rise to EDS types I and II are likely to lie in the COL5A1 gene, a significant portion of them result in very low levels of mRNA from the mutant allele, as a consequence of nonsense-mediated mRNA decay.

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.

Lack of collagen type specificity for lysyl hydroxylase isoforms

Lysyl hydroxylase is the enzyme catalyzing the formation of hydroxylysyl residues in collagens. Large differences in the extent of hydroxylysyl residues are found among collagen types. Three lysyl hydroxylase isoenzymes (LH1, LH2, LH3) have recently been characterized from human and mouse tissues. Nothing is known about the distribution of these isoforms within cells or whether they exhibit collagen type specificity. We measured mRNA levels of the three isoforms, as well as the mRNAs of the main collagen types I, III, IV, and V and the alpha subunit of prolyl 4-hydroxylase, another enzyme involved in collagen biosynthesis, in different human cell lines. Large variations were found in mRNA expression of LH1 and LH2 but not LH3. Immunoblotting was utilized to confirm the results of Northern hybridization. The levels of mRNA of LH1, LH2, and the alpha subunit of prolyl 4-hydroxylase showed significant correlations with each other. The LH3 mRNA levels did not correlate with those of LH1, LH2, or the alpa subunit of prolyl 4-hydroxylase, clearly indicating a difference in the regulation of LH3. No correlation was observed between LH isoforms and individual collagen types, indicating a lack of collagen type specificity for lysyl hydroxylase isoforms. Our observations suggest that LH1, LH2, and the alpha subunit of prolyl 4-hydroxylase are coregulated together with total collagen synthesis but not with the specific collagen types and indicate that LH3 behaves differently from LH1 and LH2, implying a difference in their substrates. These observations set the basis for further studies to define the functions of lysyl hydroxylase isoforms.

Mutations in the COL5A1 coding sequence are not common in patients with spontaneous cervical artery dissections

BACKGROUND AND PURPOSE

The dermal connective tissue of most patients with spontaneous cervical artery dissections (sCAD) contains abnormal collagen fibers. This suggests a predisposing connective tissue defect. The ultrastructural abnormalities in the skin of patients with sCAD have similarity with the morphological alterations in patients with Ehlers-Danlos syndrome type II, a dominant hereditary disorder that has been correlated in some patients to mutations within the genes encoding type V collagen. The aim of this study was to assess the alpha 1 chain of type V collagen (COL5A1) as a candidate gene for sCAD.

METHODS

We searched for mutations in the COL5A1 gene in cDNA from cultured fibroblasts of 19 patients with sCAD using single-strand conformational polymorphism analysis and nucleotide sequence analysis of polymerase chain reaction-amplified fragments of the whole COL5A1 coding sequence.

RESULTS

We detected 1 missense mutation leading to a predicted amino acid (192D/N) substitution within the N-terminal propeptide in 2 siblings. All other patients showed regular COL5A1 sequences with some silent polymorphisms.

CONCLUSIONS

Mutations in the COL5A1 gene do not appear to be a major factor in the etiology of sCAD.

Bone morphogenetic protein-1 processes the NH2-terminal propeptide, and a furin-like proprotein convertase processes the COOH-terminal propeptide of pro-alpha1(V) collagen

Bone morphogenetic protein-1 (BMP-1) plays key roles in regulating the deposition of vertebrate extracellular matrix; it is the procollagen C-proteinase that processes the major fibrillar collagen types I-III, and it may process prolysyl oxidase to the mature enzyme necessary to the formation of covalent cross-links in collagen and elastic fibers. Type V collagen is a fibrillar collagen of low abundance that is incorporated into and helps regulate the shape and diameter of type I collagen fibrils. Here we show that, in contrast to its action on procollagens I-III, BMP-1 does not cleave the C-propeptide of pro-alpha1(V) homotrimers. Instead, the single BMP-1-specific cleavage site within pro-alpha1(V) chains, lies within the large globular N-propeptide. This cleavage site is immediately upstream of a glutamine, thus redefining the specificity of cleavage for BMP-1-like enzymes. It also produces an NH2 terminus that corresponds to an equivalent NH2 terminus on the processed matrix form of the similar alpha1(XI) chain, thus suggesting physiological significance. Cleavage of the C-propeptide occurs efficiently in recombinant pro-alpha1(V) homotrimers produced in 293-EBNA human embryonic kidney cells, and this cleavage is shown to occur immediately downstream of the sequence RTRR. This is similar to sites cleaved by subtilisin-like proprotein/prohormone convertases and is shown to be specifically cleaved by the recombinant subtilisin-like proprotein/prohormone convertase furin.

A point mutation in an intronic branch site results in aberrant splicing of COL5A1 and in Ehlers-Danlos syndrome type II in two British families

Ehlers-Danlos syndrome (EDS) is a heterogeneous group of connective-tissue disorders characterized by skin fragility, joint laxity, and skeletal deformities. Type V collagen appears to have a causal role in EDS types I and II, which show phenotypic overlap and may sometimes be allelic. Type V collagen can exist as a heterotrimer, [alpha1(V)]2alpha2(V), and it both coassembles with and regulates type I collagen-fibril diameter. Using an intragenic COL5A1 polymorphism, we have demonstrated linkage, at zero recombination, to the same allele in two large British EDS type II families (LOD scores 4.1 and 4.3). Affected members from each family were heterozygous for a point mutation in intron 32 (IVS32:T-25G), causing the 45-bp exon 33 to be lost from the mRNA in approximately 60% of transcripts from the mutant gene. This mutation lies only 2 bp upstream of a highly conserved adenosine in the consensus branch-site sequence, which is required for lariat formation. Although both families shared the same marker allele, we have been unable to identify a common genealogy. This is the first description of a mutation at the lariat branch site, which plays a pivotal role in the splicing mechanism, in a collagen gene. Very probably, the resulting in-frame exon skip has a dominant-negative effect due to incorporation of the mutant proalpha chain into the triple-helical molecule. These findings further confirm the importance of type V collagen in the causation of EDS type II, and the novel collagen mutation indicates the importance of the lariat branch site in splicing.

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.

Human recombinant alpha1(V) collagen chain. Homotrimeric assembly and subsequent processing

Human embryonic kidney cells (293-EBNA) have been transfected with the full-length human alpha1 chain of collagen V using an episomal vector. High yields (15 microgram/ml) of recombinant collagen were secreted in the culture medium. In presence of ascorbate, the alpha1(V) collagen is correctly folded into a stable triple helix as shown by electron microscopy and pepsin resistance. Circular dichroism data confirm the triple-helix conformation and indicate a melting temperature of 37.5 degrees C for the recombinant homotrimer. The major secreted form is a 250-kDa polypeptide (alpha1FL). N-terminal sequencing and collagenase digestion indicate that alpha1FL retains the complete N-propeptide but lacks the C-propeptide. However, alpha1FL might undergo a further N-terminal trimming into a form (alpha1TH) corresponding to the main triple-helix domain plus the major part of the NC2 domain. This processing is different from the one of the heterotrimeric (alpha1(V))2alpha2(V) and could have some physiological relevance. Analysis of cell homogenates indicates the presence of a 280-kDa polypeptide that is disulfide-linked through its C-terminal globular domain. This C-propeptide is rapidly cleaved after secretion in the medium, giving the first evidence of a C-terminal processing of recombinant fibrillar collagens. Rotary shadowing observations not only confirm the presence of a globular domain at the N-terminal end of the molecule but reveal the presence of a kink within the triple helix in a region poor in iminoacids. This region could represent a target for proteases. Together with the thermal stability data, these results might explain the low amount of (alpha1(V))3 recovered from tissues.

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.

Basement-membrane stromal relationships: interactions between collagen fibrils and the lamina densa

Collagens, the most abundant molecules in the extracellular space, predominantly form either fibrillar or sheet-like structures-the two major supramolecular conformations that maintain tissue integrity. In connective tissues, other than cartilage, collagen fibrils are mainly composed of collagens I, III, and V at different molecular ratios, exhibiting a D-periodic banding pattern, with diameters ranging from 30 to 150 nm, that can form a coarse network in the extracellular matrix in comparison with a fine meshwork of lamina densa. The lamina densa represents a stable sheet-like meshwork composed of collagen IV, laminin, nidogen, and perlecan compartmentalizing tissue from one another. We hypothesize that the interactions between collagen fibrils and the lamina densa are crucial for maintaining tissue-tissue interactions. A detailed analysis of these interactions forms the basis of this review article. Here, we demonstrate that there is a direct connection between collagen fibrils and the lamina densa and propose that collagen V may play a crucial role in this connection. Collagen V might also be involved in regulation of collagen fibril diameter and anchoring of epithelia to underlying connective tissues.

Processing of type XI collagen. Determination of the matrix forms of the alpha1(XI) chain

Type XI collagen is mainly found as a minor constituent in type II-containing fibrils and presents a alpha1(XI)alpha2(XI)alpha3(XI) stoichiometry. This molecule was shown to be partially processed in its intact tissue form. Moreover, alternative splicing has been demonstrated in the variable region of the N-terminal domain of alpha1(XI) and alpha2(XI) chains. In this work, the processing of a major intact form of alpha1(XI) from matrix laid down by chick chondrocytes in culture was identified using N-terminal sequencing and antibodies to synthetic peptides corresponding to the N-terminal propeptide cDNA-derived sequence. The results show that the fully processed form of alpha1(XI) begins at Gln254 of the N-terminal propeptide, seven residues before the end of the proline/arginine-rich protein region encoded by exon I (Zhidkova, N. I., Justice, S. K., and Mayne, R. (1995) J. Biol. Chem. 270, 9486-9493). This sequence is immediately followed by a sequence encoded by exon III. The processing takes place at an Ala-Gln sequence that corresponds to a consensus sequence for procollagen N-proteinase. The antibody raised against a sequence located within the region corresponding to exon IV (anti-P8) fails to recognize this fully processed form of the alpha1(XI) chain. It recognizes, however, two minor bands of high molecular mass. These results suggest that a major cartilage form of alpha1(XI) is the product of alternative splicing in which sequences encoded by both exons II and IV are skipped. The presence of a highly acidic subdomain encoded by exon III at the N terminus of the major form of the alpha1(XI) chain, as predicted by these data, provides potential sites for interaction of collagen XI with other molecules.

A translocation interrupts the COL5A1 gene in a patient with Ehlers-Danlos syndrome and hypomelanosis of Ito

Ehlers-Danlos syndrome (EDS) is a genetically and pathogenetically heterogeneous group of disorders of which at least 11 types have been described. All are connective tissue disorders characterized by defects of the skin, ligaments and blood vessels with the clinical spectrum ranging from innocuous findings to lethality. Mutations in the genes encoding the major fibrillar collagen types I and III have been demonstrated in EDS types VII and IV, respectively, while mutations in the lysyl hydroxylase and ATP7A genes, with roles in collagen cross-linking, are responsible for EDS types VI and IX. The biochemical and molecular bases for the most common forms of EDS (types I, II and III) are unknown. Here, we describe a balanced translocation between chromosome 9 and an X chromosome that disrupts the minor fibrillar collagen type V gene COL5A1 in a patient with both EDS type I and hypomelanosis of Ito. The breakpoint occurs at 9q34 within COL5A1 intron 24 and interestingly, within a LINE-1 (L1) element at Xp21.1. A fusion mRNA between COL5A1 and an Alu sequence is produced, but no aberrant protein is detectable. Rather, the amount of type V collagen is reduced in the patient's fibroblasts, suggesting haploinsufficiency as a cuase of the phenotype. This demonstrates that a mutation in a type V collagen gene, COL5A1, results in EDS type I, and shows the involvement of L1 sequences in a constitutional chromosomal translocation. Because collagen type V is a heteromorphic protein in which molecules may be composed of polypeptides encoded by three COL5A genes, this suggests all three genes as candidates for mutations in EDS.

The evolution of fibrillar collagens: a sea-pen collagen shares common features with vertebrate type V collagen

The extracellular matrix of marine primitive invertebrates (sponges, polyps and jellyfishes) contains collagen fibrils with narrow diameters. From various data, it has been hypothesized that these primitive collagens could represent ancestral forms of the vertebrate minor collagens, i.e., types V or XI. Recently we have isolated a primitive collagen from the soft tissues of the sea-pen Veretillum cynomorium. This report examines whether the sea-pen collagen shares some features with vertebrate type V collagen. Rotary shadowed images of acid-soluble collagen molecules extracted from beta-APN treated animals, positive staining of segment-long-spacing crystallites precipitated from pepsinized collagen, Western blots of the pepsinized alpha1 and alpha2 chains with antibodies to vertebrate types I, III and V collagens, and in situ gold immunolabeling of ECM collagen fibrils were examined. Our results showed that the tissue form of the sea-pen collagen is a 340-nm threadlike molecule, which is close to the vertebrate type V collagen with its voluminous terminal globular domain, the distribution of most of its polar amino-acid residues, and its antigenic properties.

Characterization of two genes coding for a similar four-cysteine motif of the amino-terminal propeptide of a sea urchin fibrillar collagen

We report the characterization of the 5' region of the gene coding for the 2 alpha fibrillar collagen chain of the sea urchin Paracentrotus lividus. This sequence analysis identified the intron/exon organization of the region of the gene coding for the signal peptide, the cysteine-rich domain and the 12 repeats of the four-cysteine module of the unusually long amino-propeptide. This still unknown four-cysteine motif is generally encoded by one exon, which confirms that the distinct amino-propeptide structures of the fibrillar collagens arise from the shuffling of several exon-encoding modules. Moreover, Southern-blot analysis of the sea urchin genome and sequencing of selected genomic clones allowed us to demonstrate that several sea urchin genes could potentially code for the four-cysteine module. Curiously, one of these genes lacks the exons coding for four repeats of this motif while, in another gene, the same exons are submitted to an alternative splicing event.

Structural analysis of cross-linking domains in cartilage type XI collagen. Insights on polymeric assembly

The collagen framework of hyaline cartilage is based on copolymers of types II, IX, and XI collagens. Previous studies have established specific covalent interactions between types II and IX collagens. The present study examined cross-linking sites in type XI collagen to define better the full heteropolymeric assembly. Pepsinsolubilized type XI collagen was purified from fetal bovine cartilage. The cross-linking amino acids in the preparation were primarily divalent, borohydride-reducible structures; pyridinoline residues were essentially absent. Individual alpha 1(XI), alpha 2(XI), and alpha 3(XI) chains were resolved by high performance liquid chromatography. Telopeptides still attached by cross-links to helical sites were released by periodate oxidation and identified by microsequencing. Analysis of cross-linked peptides isolated from trypsin digest of each alpha-chain identified the attachment helical sites for the telopeptides. A high degree of interchain specificity was evident in the cross-linking between N-telopeptides and the COOH terminus of the triple-helix, consistent with a head-to-tail interaction of molecules staggered by 4D (D = 67 nm) periods. In addition, alpha 1(II) C-telopeptide was linked to the amino-terminal site of the alpha 1(XI) triple helix. In summary, the results show that type XI collagen molecules are primarily cross-linked to each other in cartilage, implying that a homopolymer is initially formed. Links to type II collagen are also indicated, consistent with an eventual cofibrillar assembly. Analysis of cartilage extracts showed that all three chains, alpha 1(XI), alpha 2(XI), and alpha 3(XI), had at least in part retained their N-propeptides in cartilage matrix and that the alpha 3 (XI) chain was the IIB splicing variant product of the COL2A1 gene. Of particular note was the finding that the N-telopeptide cross-linking site in both alpha 1(XI) and alpha 2(XI) is located amino-terminal to the putative N-propeptidase cleavage site. This structural feature provides a potential mechanism for the proteolytic depolymerization of type XI collagen by proteases that can cleave between the cross-link and the triple helix (e.g. stromelysin).

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.

Alternative exon splicing within the amino-terminal nontriple-helical domain of the rat pro-alpha 1(XI) collagen chain generates multiple forms of the mRNA transcript which exhibit tissue-dependent variation

Type XI collagen is an integral, although minor component of cartilage collagen fibrils. We have established that alternative exon usage is a mechanism for increasing structural diversity within the amino-terminal nontriple helical domain of the pro-alpha 1(XI) collagen gene. cDNA clones spanning the amino-terminal domain were selected from a rat chondrosarcoma library, and were shown to contain two major sequence differences from the previously reported human sequence. The first difference was the replacement of sequence encoding an acidic domain of 39 amino acids in length by a sequence encoding a 51-amino acid basic domain with a predicted pI of 11.9. The second difference was the absence of a sequence that would translate into a highly acidic 85-amino acid sequence downstream from the first variation. These two changes, expressed together, result in the replacement of most of the acidic domain with one that is smaller and basic. These two sequence differences serve to identify subdomains of a variable region, designated V1 and V2, respectively. V1a is defined as the acidic 39-amino acid sequence element and V1b is defined as the 51-amino acid basic sequence. Analysis of genomic DNA revealed that both V1a and V1b are encoded by separate adjacent exons in the rat genome and V2 is also encoded in a single exon downstream. Analysis of mRNA from cartilage-derived sources revealed a complex pattern of alpha 1(XI) transcript expression due to differential exon usage. In non-cartilage sources, the pattern is less complex; the most prevalent form is the one containing the two acidic sequences, V1a and V2.

Alternative mRNA processing occurs in the variable region of the pro-alpha 1(XI) and pro-alpha 2(XI) collagen chains

An analysis was performed of differential splicing of primary transcripts in the noncollagenous variable region located in the amino terminus of the pro-alpha 1(XI) and pro-alpha 2(XI) collagen chains. The results for the pro-alpha 2(XI) chain showed that human cartilage or fibroblasts in culture contain transcripts in which a single highly acidic exon encoding for 21 amino acids is present or absent. For the chicken pro-alpha 1(XI) chain a more complex pattern of alternative splicing was detected with six possible variants. Of special interest was the alternative use of two exons (called IIA and IIB) in which IIA encodes for 39 amino acids and is highly acidic (estimated pI = 3.2), whereas IIB encodes for 49 amino acids and is highly basic (estimated pI = 10.6). A similar alternative use of exon IIA or exon IIB was also observed for human chondrocytes. Northern blotting with probes specific for IIA or IIB showed that both exons are present in transcripts from cartilage but exon IIB is preferentially utilized in transcripts from tendon. Present results suggest that both the pro-alpha 1(XI) and pro-alpha 2(XI) chains of type XI collagen undergo limited processing in vivo and that the noncollagenous variable region is initially retained on the surface of the fibrils. Differential splicing in the variable region may potentially influence the interaction of collagen fibrils with other molecules of the extracellular matrix and more specifically with sulfated glycosaminoglycan chains or with hyaluronan. Such interactions may play a key role in establishing both the organization of the collagen fibrils within the extracellular matrix and in limiting the diameter of collagen fibrils.

A fibrillar collagen gene, Col11a1, is essential for skeletal morphogenesis

Mice that are homozygous for the autosomal recessive chondrodysplasia (cho) mutation die at birth with abnormalities in cartilage of limbs, ribs, mandible, and trachea. Limb bones of newborn cho/cho mice are wider at the metaphyses than normal bones and only about half the normal length. By linkage analysis, the cho gene and the gene encoding the alpha 1 (XI) chain of cartilage collagen XI were mapped to the same region of chromosome 3. Deletion of a cytidine residue about 570 nt downstream of the translation initiation codon in cho alpha 1 (XI) mRNA causes a reading frame shift and introduces a premature stop codon. The data demonstrate that collagen XI is essential for normal formation of cartilage collagen fibrils and the cohesive properties of cartilage. The results also suggest that the normal differentiation and spatial organization of growth plate chondrocytes is critially dependent on the presence of type XI collagen in cartilage extracellular matrix.

Differential expression of an acidic domain in the amino-terminal propeptide of mouse pro-alpha 2(XI) collagen by complex alternative splicing

We isolated and sequenced genomic and cDNA clones encoding the complete amino-terminal portion and the 5'-untranslated region of mouse pro-alpha 2(XI) collagen mRNA. Fourteen exons encoded the amino-terminal propeptide, which was divided into three consecutive domains (a long globular domain, an amino-terminal triple helical domain, and a telopeptide domain). The long globular domain was further divided into an upstream basic subdomain and a downstream highly acidic subdomain, as is the case for the amino-terminal propeptides of pro-alpha 1(V) and pro alpha 1(XI) collagens. We also demonstrated that the primary transcript undergoes complex alternative splicing. Three consecutive exons (exons 6, 7, and 8) encoding most of the acidic subdomain showed alternative splicing which dramatically affected the structure of the amino-terminal propeptide of pro-alpha 2(XI) collagen. Using the reverse transcription-polymerase chain reaction, we analyzed the expression of these exons in various tissues and in developing limb buds of mice. The pro-alpha 2(XI) transcripts were abundant in cartilage, but most of them lacked the 3-exon sequences encoding the acidic domain. Most of other tissues also contained mRNAs that corresponded to longer splice variants, including exons 6-8. The differential expression of specific domains of pro-alpha 2(XI) collagen may be important in modulating interactions between various components of the extracellular matrix and/or may influence heterotypic collagen assembly.

Structural and functional analysis of the promoter of the human alpha 1(XI) collagen gene

In order to eventually elucidate the mechanisms regulating alpha 1(XI) collagen expression in cartilaginous and non-cartilaginous tissues, we performed an initial analysis of the structural-functional features of the promoter of the human gene (COL11A1). After cloning and sequencing the 5' portion of COL11A1, primer extension and nuclease protection assays identified several minor transcriptional start sites clustered around a major one located 318 base pairs from the ATG codon. Consistent with this finding, analysis of the upstream sequence revealed the absence of a TATA motif and the presence of several GC boxes. Transient transfection experiments delineated the smallest promoter sequence directing relatively high expression of a reporter gene in a cell type-specific manner. Nine nuclear protein-bound areas were located within this promoter sequence of the COL11A1 gene. Sequence homologies suggested that the majority of the footprints correspond to potential binding sites for ubiquitous nuclear proteins, such as AP2 and Sp1. Additional experimental evidence indicated that one of the protected areas may bind a transcriptional complex that is identical or closely related to the one that regulates tissue specificity in the coordinately expressed alpha 2(V) collagen gene.

Structural characteristics of cross-linking sites in type V collagen of bone. Chain specificities and heterotypic links to type I collagen

To understand the role of type V collagen and its spatial interrelationship with type I collagen in bone matrix, the molecule's covalent intermolecular cross-links were structurally characterized. Type V collagen containing alpha 1(V), alpha 2(V) and alpha 1(XI) chains was isolated from bovine bone and reacted with NaB3H4 to label the cross-linking residues. Radiolabeled native molecules and isolated alpha chains were treated with sodium metaperiodate to cleave the divalent cross-linking bonds. Sequence analysis of the periodate-released peptides matched two of them to alpha 1(V) and alpha 1(XI) aminopropeptide domains. A third peptide was derived from the alpha 1(I) carboxytelopeptide domain of type I collagen. This latter peptide, therefore, came from a site of heterotypic cross-linking between types I and V collagens and accounted for about 15% of the total cross-linked peptides. Sequence analysis of isolated cross-linked tryptic peptides defined the helical sites of attachment of the periodate-released telopeptides and revealed that the putative aminoproteinase-cleavage sites in the alpha 1(V) and alpha 1(XI) chains are located in the molecule interior to the cross-linking residue. These data imply that type V collagen molecules in the extracellular matrix are primarily cross-linked to each other in a head-to-tail linear polymer that is linked laterally to type I collagen molecules in copolymeric fibrils.

Diversity in the processing events at the N-terminus of type-V collagen

The processing of human collagen type-V chains was studied using anti-peptide polyclonal antibodies raised against peptide sequences at the N-terminal non-triple-helical region of pro-alpha 1(V) and pro-alpha 2(V) chains. The anti-peptide polyclonal antibody raised against positions 48-57 of the N-terminal alpha 2(V) sequence recognized the mature form of the human alpha 2(V) chain extracted without any proteolytic treatment from several tissues in the presence of a mixture of protease inhibitors. It also recognized the pro-alpha 2(V) and pN-alpha 2(V) collagen chains secreted in the cell-culture media of the rhabdomyosarcoma A204 cell line. The pN-alpha 2(V) collagen chain from this cell line migrated during electrophoresis with the alpha 2(V) chain obtained from tissues. This demonstrates that the alpha 2(V) chain in tissues is incompletely processed and is present as the pN-alpha 2(V) collagen chain which lacks the C-propeptide. In comparison, an anti-peptide polyclonal antibody raised against residues at positions 284-299 of the N-terminal alpha 1(V) human sequence failed to recognize the mature form of the alpha 1(V) chain while it reacted with the pN-alpha 1(V) collagen chain form. These results suggest that the alpha 1(V) chain undergoes a processing event in the N-terminal region that involves the removal of at least the first 284 residues. Amino acid sequence analysis was performed on cyanogen-bromide-generated or trypsin-generated peptides of the two electrophoretic bands obtained for the tissue form of collagen V. The slower-migrating band corresponding to the intact alpha 1(V) chain gave, as expected, only sequences corresponding to the alpha 1(V) chain. However, the band previously considered to be the intact alpha 2(V) chain also gave sequences for the alpha 1(V) chain in addition to the alpha 2(V) chain. This result indicates the presence in tissue extracts of a further processed form of alpha 1(V) chain which migrates with the intact alpha 2(V) chain. On further analysis, we observed that the two bands of the tissue form of collagen V occurred in a 1:1 ratio whereas, after the pepsin digestion to remove non-collagenous regions, two bands were observed with an alpha 1(V)/alpha 2(V) chain ratio of 3:1. These results indicate that the alpha 1(V) chain exists in an additional stoichiometry, different from [alpha 1(V)]2 alpha 2(V).(ABSTRACT TRUNCATED AT 400 WORDS)