Gene structure for the alpha 1 chain of a human short-chain collagen (type XIII) with alternatively spliced transcripts and translation termination codon at the 5' end of the last exon

Collagens at the vertebrate neuromuscular junction, from structure to pathologies

The extracellular matrix at the neuromuscular junction is built upon components secreted by the motoneuron, the muscle cell and terminal Schwann cells, the cells constituting this specific synapse. This compartment contains glycoproteins, proteoglycans and collagens that form a dense and specialized layer, the synaptic basal lamina. A number of these molecules are known to play a crucial role in anterograde and retrograde signalings that are active in neuromuscular junction formation, maintenance and function. Here, we focus on the isoforms of collagens which are enriched at the synapse. We summarize what we know of their structure, their function and their interactions with transmembrane receptors and other components of the synaptic basal lamina. A number of neuromuscular diseases, congenital myastenic syndromes and myasthenia gravis are caused by human mutations and autoantibodies against these proteins. Analysis of these diseases and of the specific collagen knock-out mice highlights the roles of some of these collagens in promoting a functional synapse.

Congenital Myasthenic Syndrome Type 19 Is Caused by Mutations in COL13A1, Encoding the Atypical Non-fibrillar Collagen Type XIII α1 Chain

The neuromuscular junction (NMJ) consists of a tripartite synapse with a presynaptic nerve terminal, Schwann cells that ensheathe the terminal bouton, and a highly specialized postsynaptic membrane. Synaptic structural integrity is crucial for efficient signal transmission. Congenital myasthenic syndromes (CMSs) are a heterogeneous group of inherited disorders that result from impaired neuromuscular transmission, caused by mutations in genes encoding proteins that are involved in synaptic transmission and in forming and maintaining the structural integrity of NMJs. To identify further causes of CMSs, we performed whole-exome sequencing (WES) in families without an identified mutation in known CMS-associated genes. In two families affected by a previously undefined CMS, we identified homozygous loss-of-function mutations in COL13A1, which encodes the alpha chain of an atypical non-fibrillar collagen with a single transmembrane domain. COL13A1 localized to the human muscle motor endplate. Using CRISPR-Cas9 genome editing, modeling of the COL13A1 c.1171delG (p.Leu392Sfs^∗71) frameshift mutation in the C2C12 cell line reduced acetylcholine receptor (AChR) clustering during myotube differentiation. This highlights the crucial role of collagen XIII in the formation and maintenance of the NMJ. Our results therefore delineate a myasthenic disorder that is caused by loss-of-function mutations in COL13A1, encoding a protein involved in organization of the NMJ, and emphasize the importance of appropriate symptomatic treatment for these individuals.

Altered expression of type XIII collagen in keratoconus and scarred human cornea: Increased expression in scarred cornea is associated with myofibroblast transformation

PURPOSE

Type XIII collagen (ColXIII) is a transmembrane protein thought to be involved in cell-cell and cell-matrix interactions. We report here on its presence in the normal human cornea and compare the results for keratoconus and scarred corneas.

METHODS

Immunohistochemistry and in situ hybridization were applied to human corneal samples obtained by penetrating keratoplasty.

RESULTS

In the normal human cornea, ColXIII was immunolocalized to the corneal epithelial cells, and to a lesser degree to the stromal keratocytes. The keratoconus cases showed otherwise similar results, but in areas containing Bowman membrane disruptions showed thinned epithelial cells reduced immunostaining for ColXIII, whereas occasionally pronounced immunoreactivity was seen in the stromal keratocytes. The corneal scar samples contained highly increased ColXIII immunostaining by stromal cells in the fibrotic foci, whereas the peripheral areas showed less intense immunostaining. In situ hybridization confirmed that the corneal epithelium and keratocytes actively synthesize the transcript. Immunostaining with alphaSMA revealed that a substantial proportion of the ColXIII mRNA-expressing cells in the stromal scar tissues was myofibroblasts and that these areas lack CD34 immunoreactivity.

CONCLUSIONS

The results indicate that ColXIII, which is predominantly confined to the basal corneal cells in the normal cornea, may have a role in the adhesion of corneal epithelial cells to each other and to the underlying basement membrane. Additionally, highly increased expression in scarred corneas suggests that it participates in the corneal wound healing process.

Expression of type XXIII collagen mRNA and protein

Collagen XXIII is a member of the transmembranous subfamily of collagens containing a cytoplasmic domain, a membrane-spanning hydrophobic domain, and three extracellular triple helical collagenous domains interspersed with non-collagenous domains. We cloned mouse, chicken, and humanalpha1(XXIII) collagen cDNAs and showed that this non-abundant collagen has a limited tissue distribution in non-tumor tissues. Lung, cornea, brain, skin, tendon, and kidney are the major sites of expression. In contrast, five transformed cell lines were tested for collagen XXIII expression, and all expressed the mRNA. In vivo the alpha1(XXIII) mRNA is found in mature and developing organs, the latter demonstrated using stages of embryonic chick cornea and mouse embryos. Polyclonal antibodies were generated in guinea pig and rabbit and showed that collagen XXIII has a transmembranous form and a shed form. Comparison of collagen XXIII with its closest relatives in the transmembranous subfamily of collagens, types XIII and XXV, which have the same number of triple helical and non-collagenous regions, showed that there is a discontinuity in the alignment of domains but that striking similarities remain despite this.

Type XIII collagen is widely expressed in the adult and developing human eye and accentuated in the ciliary muscle, the optic nerve and the neural retina

The distribution of mRNAs coding for type XIII collagen, a novel nonfibril-forming collagen, was studied by Northern and in situ hybridizations of adult and fetal human eyes and the corresponding protein was localized by indirect immunofluorescence in frozen sections of 12 and 17 week human fetal eyes using a polyclonal antipeptide antibody to type XIII collagen. Type XIII collagen was found to be widely expressed in ocular tissues when studied at both the mRNA and protein levels in fetal and adult human tissues. No major differences were observed in the expression patterns between fetal and adult tissues. Surprisingly, the strongest signals seen in in situ hybridizations and immunofluorescence stainings occurred in the optic nerve bundles and in the ganglion cell layer of the retina. Other notable locations containing type XIII collagen included the developing ciliary smooth muscle, the posterior two-thirds of the corneal stroma and the striated extraocular muscles. Low level signals were also detected in the blood vessel walls and mesenchymal cells of the other ocular tissues. All immunosignals detected were adherent to cells, and the extracellular matrices appeared to be devoid of type XIII collagen. Our results are in concert with the presumed plasma membrane location of type XIII collagen, and it is hypothesized that this molecule could be involved in cell-matrix and perhaps cell-cell interactions. The wide expression of type XIII collagen in the eye, and especially in the neural structures, warrants future studies on type XIII collagen in other nerve structures and in pathological conditions affecting the eye. Due to its wide expression, type XIII collagen is likely to be an important factor for the normal development and functioning of the eye.

Complete exon-intron organization and chromosomal location of the gene for mouse type XIII collagen (col13a1) and comparison with its human homologue

Recent findings indicate that type XIII collagen is a transmembrane protein with a short N-terminal sytocsolic domain, a single transmembrane domain and a large, mainly collagenous ectodomain. The complete exon-intron structure of the gene coding for the mouse alpha1(XIII) collagen chain, col13a1, has now been characterized from genomic clones spanning over 180 kilobases (kb) and shown to be approximately 135 kb in size and to contain 42 exons varying between 8 base pairs (bp), the shortest exon in the genes encoding the various collagens, and 836 bp. Nuclease S1 mapping and 5'RACE resulted in identification of multiple transcription initiation points in the mouse gene, ranging between 470 and 548 bp upstream from the initiation methionine. This is in good agreement with a recently identified human EST clone extending 537 bp upstream from the initiation methionine. The 836-bp first exon of the mouse gene covers both the long 5' untranslated region and also a 36-residue cytosolic portion, a 23-residue transmembrane domain, and 37 residues of the 60-residue non-collagenous ectodomain immediately adjacent to the plasma membrane. One striking feature of the exons encoding solely collagenous sequences is the abundance of 27-bp exons, half the ancestral 54-bp size characteristic of fibrillar collagen genes, while the others vary between 8 and 144 bp, including instances of 36-, 45- and 54-bp exons. Determination of approximately 2.6 kb of sequences upstream of the initiation methionine of both the mouse and human genes and the identification of a clone containing four exons and spanning a gap in the previously characterized human clones allowed detailed comparison of the two genes. The exon-intron structures were found to be completely conserved between the species, and both genes have their 5' untranslated region preceded by a highly homologous apparent promoter region of approximately 350 bp containing a modified TATAA motif and several GC boxes. The chromosomal location of the mouse gene was determined by SSCP and fluorescence in situ hybridization and found to be at chromosome 10, band 4, between markers D1OMit5 -2.3 +/- 1.6 cM -col13a1 - 3.4+/-1.9 cM - D1OMit15. This result indicates that the mouse type XIII collagen gene and its human counterpart are located in chromosomal segments with conserved syntenies (The GenBank accession numbers for the mouse gene are AF063666-AF063693. The new GenBank accession number for the 5' end of the human type XIII collagen gene is AF071009).

Characterization of the gene for human EMMPRIN, a tumor cell surface inducer of matrix metalloproteinases

EMMPRIN (extracellular matrix metalloproteinase inducer) also known as CD147 and basigin, is a member of the immunoglobulin family that is present on the surface of tumor cells and stimulates nearby fibroblasts to synthesize matrix metalloproteinases. Using our EMMPRIN cDNA, we have isolated a cosmid clone that contains the human EMMPRIN gene. S1 analysis with a fragment of the gene clone and primer extension of the mRNA was performed to determine the transcription start site. PCR and sequence analysis have defined the exon/intron organization of the gene and show that it is highly conserved with the mouse EMMPRIN/basigin gene. About 950 bases of the 5'-flanking region were examined for transcription factor consensus binding sites, locating three SP1 sites and two AP2 sites. The transcription start site was found to be located in a CpG island. Elements in the proximal promoter region were conserved in the human and mouse genes.

Type XIII collagen is identified as a plasma membrane protein

The complete primary structure of the mouse type XIII collagen chain was determined by cDNA cloning. Comparison of the mouse amino acid sequences with the previously determined human sequences revealed a high identity of 90%. Surprisingly, the mouse cDNAs extended further in the 5' direction than the previously identified human clones. The 5' sequences contained a new in-frame ATG codon for translation initiation which resulted in elongation of the N-terminal noncollagenous domain by 81 residues. These N-terminal sequences lack a typical signal sequence but include a highly hydrophobic segment that clearly fulfills the criteria for a transmembrane domain. The sequence data thus unexpectedly suggested that type XIII collagen may be located on the plasma membrane, with a short cytosolic N-terminal portion and a long collagenous extracellular portion. These sequence data prompted us to generate antipeptide antibodies against type XIII collagen in order to study the protein and its subcellular location. Western blotting of human tumor HT-1080 cell extract revealed bands of over 180 kDa. These appeared to represent disulfide-bonded multimeric polypeptide forms that resolved upon reduction into 85-95-kDa bands that are likely to represent a mixture of splice forms of monomeric type XIII collagen chains. These chains were shown to contain the predicted N-terminal extension and thus also the putative transmembrane segment. Immunoprecipitation of biotinylated type XIII collagen from surface-labeled HT-1080 cells, subcellular fractionation, and immunofluorescence staining were used to demonstrate that type XIII collagen molecules are indeed located in the plasma membranes of these cells.

Ubiquitous expression of the alpha1(XIX) collagen gene (Col19a1) during mouse embryogenesis becomes restricted to a few tissues in the adult organism

Type XIX collagen is a poorly characterized member of the fibril-associated collagens with an interrupted triple helices (FACIT) class of collagen molecules. As a first step toward elucidating its function, we have isolated full size cDNA clones from the mouse alpha1(XIX) collagen gene (Col19a1) and established its pattern of expression in the developing embryo and adult organism. Col19a1 transcripts can be detected as early as 11 days of gestation and in all embryonic tissues, except the liver, of an 18-day postcoitum mouse. In contrast, only a few adult tissues, brain, eye, and testis, seem to accumulate Col19a1 mRNA. Col19a1 transcripts are at least 10 times more abundant in adult than fetal brain and significantly less in adult than fetal muscle and skin. Consistent with the RNA data, polyclonal antibodies for alpha1(XIX) collagen reacted with a 150-kDa protein in the neutral salt extraction of adult mouse brain tissues. We therefore propose that type XIX collagen plays a distinct role from the other FACIT molecules, particularly in the assembly of embryonic matrices and in the maintenance of specific adult tissues.

Alternative splicing of mouse alpha1(XIII) collagen RNAs results in at least 17 different transcripts, predicting alpha1(XIII) collagen chains with length varying between 651 and 710 amino acid residues

The alpha1(XIII) collagen chain has three collagenous domains (COL1-COL3) and four noncollagenous domains (NC1-NC4). A hydrophobic sequence in the extreme amino-terminal noncollagenous domain suggests that type XIII collagen is a transmembrane protein. The alpha1(XIII) collagen RNA is characterized by complex alternative splicing. In this study, expression of the alpha1(XIII) collagen chain was detected in 12 mouse tissues using reverse transcription (RT) and the polymerase chain reaction (PCR). Alternative splicings affecting the COL1, NC2, and COL3 domains were first evaluated separately. Subsequently, sequences spanning from the NC1 domain to the NC4 domain were studied for the first time to elucidate how the alternative splicing of type XIII collagen transcripts affects the structures of the entire mRNAs. A total of 10 alternatively spliced exons, which were freely combinatory, and 9 new exon combinations encoding parts of the COL1, NC2, and COL3 domains have been found. The sequences for the COL1 domain involved two common variants, one containing all the known COL1 exons and the other lacking exon 4B. Exons 12 and 13, encoding most of the NC2 domain, were subject to an alternative splicing that was found to display marked tissue-specific differences. The most common variant of the COL3 sequences lacked exons 28B and 33, or only the exon 33, which was found to be 100% identical to the corresponding human sequences. A total of 17 splice combinations of nine exons were characterized. The results suggest that the predicted length of the corresponding polypeptide varies between 710 and 651 residues.

Cloning of the human type XVII collagen gene (COL17A1), and detection of novel mutations in generalized atrophic benign epidermolysis bullosa

Generalized atrophic benign epidermolysis bullosa (GABEB) is a nonlethal variant of junctional epidermolysis bullosa (JEB). Previous findings have suggested that type XVII collagen is the candidate gene for mutations in this disease. We now have cloned the entire human type XVII collagen gene (COL17A1) and have elucidated its intron-exon organization. The gene comprises 56 distinct exons, which span approximately 52 kb of the genome, on the long arm of chromosome 10. It encodes a polypeptide, the alpha1(XVII) chain, consisting of an intracellular globular domain, a transmembrane segment, and an extracellular domain that contains 15 separate collagenous subdomains, the largest consisting of 242 amino acids. We also have developed a strategy to identify mutations in COL17A1 by use of PCR amplification of genomic DNA, using primers placed on the flanking introns. The PCR products are scanned for sequence variants by heteroduplex analysis using conformation-sensitive gel electrophoresis and then are subjected to direct automated sequencing. We have identified several intragenic polymorphisms in COL17A1, as well as mutations, in both alleles, in two Finnish families with GABEB. The probands in both families showed negative immunofluorescence staining with an anti-type XVII collagen antibody. In one family, the proband was homozygous for a 5-bp deletion, 2944del5, which resulted in frameshift and a premature termination codon of translation. The proband in the other family was a compound heterozygote, with one allele containing the 2944del5 mutation and the other containing a nonsense mutation, Q1023X. These results expand the mutation database in different variants of JEB, and they attest to the functional importance of type XVII collagen as a transmembrane component of the hemidesmosomes at the dermal/epidermal junction.

Tenascin mRNA isoforms in the developing mouse brain

The extracellular matrix glycoprotein tenascin is expressed in the developing mouse cerebellum as a group of four protein species of different molecular weights. The difference is most likely due to alternative splicing which is known to occur in tenascin mRNA within the region of the fibronectin type III repeats. In order to systematically analyze tenascin mRNA isoforms that would account for this heterogeneity, tenascin splice variants were isolated from mouse brain by the polymerase chain reaction (PCR). In agreement with Northern blot analysis, amplification by PCR revealed a general decrease in tenascin mRNA expression during development from embryonic and early postnatal to adult stages. This decrease was more pronounced for isoforms of high molecular weight compared to those of low molecular weight. In accord with the observations at the protein level, four splice variants were found to be predominantly expressed, containing insertions of either six, five, or one fibronectin type III repeat, or comprising no insertion. In addition, a minor splice variant with an insertion of four fibronectin type III repeats was isolated. Three of the isolated mRNA splice variants have not yet been described for mouse tenascin. Among them, an isoform containing six alternatively spliced repeats was found to include a novel fibronectin type III repeat. The sequence of this repeat displays 96.7% similarity to a corresponding type III repeat in human tenascin, revealing a strict evolutionary conservation between tenascin molecules from different species in the region of alternative splicing. Southern blot analysis of the amplified mRNA isoforms showed that the novel mouse type III repeat is confined to splice variants with an insertion of six fibronectin type III repeats. Furthermore, in situ hybridization on sections from mouse embryos indicated that tenascin-specific mRNAs containing the novel type III repeat are predominantly expressed in the central nervous system.

Collagen genes: mutations affecting collagen structure and expression

It is to be expected that more collagen genes will be identified and that additional heritable connective tissue diseases will be shown to arise from collagen mutations. Further progress will be fostered by the coordinated study of naturally occurring and induced heritable connective tissues diseases. In some instances, human mutations will be studied in more detail using transgenic mice, while in others, transgenic studies will be used to determine the type of human phenotype that is likely to result from mutations of a given collagen gene. Further studies of transcriptional regulation of the collagen genes will provide the prospect for therapeutic control of expression of specific collagen genes in patients with genetically determined collagen disorders as well as in a wide range of common human diseases in which abnormal formation of the connective tissues is a feature.

Alternate exon usage is a commonly used mechanism for increasing coding diversity within genes coding for extracellular matrix proteins

Extracellular matrix proteins are a diverse family of secreted proteins and glycoproteins that are responsible for a variety of critical functions in different tissues. A large number of multiexon genes encode these proteins of the extracellular matrix. Over the last few years, it has become evident that the processing of the pre-mRNA from several of these genes involves alternative splicing. This review summarizes the known examples of alternative splicing in genes coding for the extracellular matrix and attempts to relate the increase in coding diversity generated by alternate exon usage to the function(s) of individual extracellular matrix proteins.

Structural comparison of the chicken genes for alpha 1(VI) and alpha 2(VI) collagen

The chicken alpha 1(VI) polypeptide is encoded by a single gene spanning 21 kbp of genomic DNA. This gene is composed of 34 exons and 33 introns. Its structure is closely related to that of the alpha 2(VI) collagen gene, suggesting that the two genes evolved by gene duplication. Both genes contain 19 exons coding for the triple-helical domain. These exons are multiples of 9 bp (27, 36, 45, 54, 63 and 90 bp) and encode an integral number of collagenous Gly-Xaa-Yaa triplets. Since there is no convincing correlation to a building block of 54 bp, it is unlikely that type VI collagen has evolved from a primordial 54-bp module as suggested for all fibrillar collagens.

Alternative splicing of rat tropoelastin mRNA is tissue-specific and developmentally regulated

Sequence analysis of cDNA clones coding for rat tropoelastin previously has identified two variants that potentially corresponded to alternatively spliced tropoelastin mRNAs (Pierce et al., 1990). We have now used S1 nuclease protection analysis of total RNA from aorta, skin and lungs of 10-day and 6-week old rats to localize all sites of alternative splicing in the tropoelastin mRNA and to examine tissue-specific and developmental regulation of the use of these sites. This analysis revealed multiple sites of alternative splicing involving rat tropoelastin coding sequences corresponding to exons 12 through 15 of the bovine tropoelastin gene and a single site of alternative splicing at sequences corresponding to exon 33. Messenger RNAs from all three tissues at both developmental stages were alternatively spliced at the same sites; there was no evidence for the use of an alternative splice site unique to a particular tissue or developmental stage. However, both tissue-specific and developmentally regulated differences were apparent in the proportion of rat tropoelastin mRNA alternatively spliced at exon 33. Tropoelastin mRNA from the aorta and lungs of neonatal rats was alternatively spliced at exon 33 ten time more frequently than tropoelastin mRNA from skin. Between 10 days and 6 weeks of development, the use of this site of alternative splicing decreased by twenty-fold in RNA from skin, ten-fold in RNA from lungs and two-fold in RNA from aorta. In contrast, alternative splicing at exons 12 through 15 occurred in a small percentage of the mRNA and use of these sites exhibited minimal tissue-specific differences or developmental regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Interodontoblastic collagen (von Korff fibers) and circumpulpal dentin formation: an ultrathin serial section study in the cat

The collagenous fibers of von Korff pass from the dentin matrix between the odontoblasts into the dental pulp. Although collagen fibrils are known to be present between odontoblasts, the existence of von Korff fibers has remained controversial. This may be because their continuity between the dentin matrix and the pulp has not been demonstrated ultrastructurally. In this study we have examined the odontoblast layer in the middle to apical regions of perfusion-fixed permanent canine teeth of cats by using transmission electron microscopy. Ultrathin sections of demineralized specimens revealed frequent bundles of collagen fibrils 1) entering the odontoblast layer from the predentin, 2) present between odontoblast cell bodies, and 3) passing from between the odontoblasts into the pulp. The question of continuity of these bundles from the predentin, across the odontoblast layer into the pulp was examined in ultrathin serial sections. Unbroken continuity of a collagen bundle from the predentin between the odontoblasts into the pulp was established in a reconstruction of one series of 22 serial sections and was very strongly suggested by a number of other series in which the numbers of available sections restricted their full visibility. This investigation has shown, therefore, that classical von Korff fibers are present and that these fibers are present in fully erupted teeth with closed apices, i.e., at a time when secondary circumpulpal dentinogenesis is in progress. The findings call for a reexamination of the question of von Korff fibers during mantle dentinogenesis and primary circumpulpal dentinogenesis. Resolution of their existence at the earlier stages of dentinogenesis should be possible by using the ultrathin serial-sectioning technique.

Multiple forms of chicken alpha 3(VI) collagen chain generated by alternative splicing in type A repeated domains

Type VI collagen is a structurally unique component widely distributed in connective tissues. Its molecular structure consists of monomers that have the potential to assemble intracellularly into dimers and tetramers which, once secreted, can form microfilaments by end-to-end association. Individual monomers are composed of chains of Mr = approximately 140,000 (alpha 1 and alpha 2) and greater than 300,000 (alpha 3). Type VI collagen molecules contain a short triple helix with large globular domains at both ends. These domains are made for their greatest part of repetitive units similar to type A repeats of von Willebrand Factor. The alpha 3(VI) chain, contributing most of the mass of the NH2-terminal globule, appeared heterogenous both at the mRNA and protein level. Several alpha 3(VI)-specific clones that lack the sequences corresponding to repeats A8 and A6 were isolated from a chicken aorta cDNA library. Northern blot hybridization of poly (A+)-enriched RNA from chicken gizzard with cDNA fragments corresponding to several individual type A repeats showed that A8- and A6-specific probes did not hybridize to the lower Mr transcripts. Clones spanning approximately 20 kb of the 5'-end of the alpha 3(VI) gene were isolated from a chicken genomic library and subjected to analysis by restriction mapping, Southern blotting, and selective sequencing of the intron-exon boundaries. At the most 5'-end of the gene an additional type A repeat (A9), previously undetected in cDNA clones, was identified. Furthermore, it was determined that the presumed signal peptide and repeats A9 through A6 are encoded within individual exons. Reverse transcription and polymerase chain reaction of aorta RNA suggested that a mechanism of alternative mRNA splicing by a phenomenon of exon skipping generates alpha 3(VI) isoform variants that contain different numbers of type A repeats. Immunohistochemistry of frozen sections of chicken embryo tissues with repeat-specific mAbs showed that an antibody directed against a conditional exon has a more restricted tissue distribution compared to an antibody against a constitutive exon.

Characterization of a fibrillar collagen gene in sponges reveals the early evolutionary appearance of two collagen gene families

We have characterized cDNA and genomic clones coding for a sponge collagen. The partial cDNA has an open reading frame encoding 547 amino acid residues. The conceptual translation product contains a probably incomplete triple-helical domain (307 amino acids) with one Gly-Xaa-Yaa-Zaa imperfection in the otherwise perfect Gly-Xaa-Yaa repeats and a carboxyl propeptide (240 amino acids) that includes 7 cysteine residues. Amino acid sequence comparisons indicate that this sponge collagen is homologous to vertebrate and sea urchin fibrillar collagens. Partial characterization of the corresponding gene reveals an intron-exon organization clearly related to the fibrillar collagen gene family. The exons coding for the triple-helical domain are 54 base pairs (bp) or multiples thereof, except for a 57-bp exon containing the Gly-Xaa-Yaa-Zaa coding sequence and for two unusual exons of 126 and 18 bp, respectively. This latter 18-bp exon marks the end of the triple-helical domain, contrary to the other known fibrillar collagen genes that contain exons coding for the junction between the triple-helical domain and the carboxyl propeptide. Compared to other fibrillar collagen genes, the introns are remarkably small. Hybridization to blotted RNAs established that the gene transcript is 4.9 kilobases. Together with previous results that showed the existence of a nonfibrillar collagen in the same species, these data demonstrate that at least two collagen gene families are represented in the most primitive metazoa.

Cloning and sequencing of a Porifera partial cDNA coding for a short-chain collagen

Collagen is present in Porifera, the lowest multicellular animals, but there is no information available on the primary structure of the collagen chains in this phylum. Developing fresh-water sponges have been used to extract total RNA in order to study in vitro translation products and to construct a cDNA library. Four translated proteins were collagenase-sensitive (200 kDa, 160 kDa, 81 kDa and 48 kDa). The cDNA library was screened with a human collagen probe and a clone, EmC4, covering 1.2 kb was isolated. Nucleotide sequencing of EmC4 revealed a conceptual open reading frame coding for 366 amino acids terminated by a stop codon TGA with 103 nucleotides downstream. The presumed translation product encoded contained several domains: a non-collagenous C-terminal domain of 156 amino acids with 9 cysteines, an uninterrupted collagenous domain of 171 amino acids, a non-collagenous domain of 16 amino acids with 3 cysteines and a probably incomplete N-terminal collagenous domain of 23 amino acids. Comparison with other sequences suggested that this collagen chain might belong to a non-fibrillar collagen family which evolved into several sub-families giving rise to nematode cuticular collagens, and type IV collagens.

Short-chain basement membrane collagen. Further characterization and its biosynthesis by F-9 embryonal carcinoma cells

The paper describes further characterization of the 55-kDa short-chain collagen from lens capsule. Lens capsules were extracted with 5.5 M guanidine.HCl and the extracted material was fractionated on agarose A-5M followed by high-pressure liquid chromatography (HPLC). By amino acid composition, the major fraction obtained from HPLC was found to be different than type-IV collagen fragments. The 55-kDa short-chain collagen on pepsin digestion produced a 45-kDa pepsin-resistant fragment. The undifferentiated embryonal carcinoma (F-9) cells were found to synthesize increased amounts of 55-kDa short-chain collagen. The identity of this biosynthesized molecule with 55-kDa short-chain collagen from lens capsules was established by immunoprecipitation experiments. The results indicated a close similarity or identical nature of the short-chain collagens from these two sources.

Expression of mRNAs coding for the alpha 1 chain of type XIII collagen in human fetal tissues: comparison with expression of mRNAs for collagen types I, II, and III

This paper describes the topographic distribution of the multiple mRNAs coding for a novel human short-chain collagen, the alpha 1 chain of type XIII collagen. To identify the tissues and cells expressing these mRNAs, human fetal tissues of 15-19 gestational wk were studied by Northern and in situ hybridizations. The distribution pattern of the type XIII collagen mRNAs was compared with that of fibrillar collagen types I, II, and III using specific human cDNA probes for each collagen type. Northern hybridization showed the bone, cartilage, intestine, skin, and striated muscle to contain mRNAs for type XIII collagen. An intense in situ hybridization signal was obtained with the type XIII collagen cDNAs in the epidermis, hair follicles, and nail root cells of the skin, whereas the fibrillar collagen mRNAs were detected in the dermis. Cells in the intestinal mucosal layer also appeared to contain high levels of alpha 1(XIII) collagen mRNAs, but contained none of the fibrillar collagen mRNAs. In the bone and striated muscle, alpha 1(XIII) collagen mRNAs were detected in the mesenchymal cells forming the reticulin fibers of the bone marrow and endomycium. The hybridization signal obtained with the alpha 1(XIII) collagen cDNA probe in cartilaginous areas of the growth plates was similar, but less intense, to that obtained with the type II collagen probe. A clear hybridization signal was also detected at the (pre)articular surfaces and at the margins of the epiphyses, whereas it was weaker in the resting chondrocytes in the middle of the epiphyses. The brain, heart, kidney, liver, lung, placenta, spleen, testis, tendon, and thymus did not appear to contain alpha 1(XIII) collagen mRNAs.

Increased deposition of types III and V collagen in neurofibroma tissue from patients with von Recklinghausen disease

Collagen components in neurofibroma tissue from patients with von Recklinghausen disease were investigated, in comparison with those in normal skin and peripheral nerve tissue. Biochemical analysis of collagen isolated from the tissues by limited pepsin digestion indicated that the neurofibroma tissue contained type I collagen as the major constituent and increased amounts of types III and V collagen. The relative ratios of alpha 1(III)/alpha 1(I) and alpha 1(V) + alpha 2(V)/alpha 1(I) in the tissue were 0.87-0.92 and 0.16-0.17, respectively, while in normal skin, these ratios were 0.36-0.45 and less than 0.024, respectively. Amino acid analysis and circular dichroism studies of types I, III and V collagen purified from the tissue showed that these collagens were essentially the same as the corresponding types of collagen isolated from fetal human skin and placenta. The increased deposition of types III and V collagen suggested that alternation of collagen metabolism had occurred in the neurofibroma tissue.