Human-mouse interspecies collagen I heterotrimer is functional during embryonic development of Mov13 mutant mouse embryos

Variable impairment of wound healing in the heterozygous collagenase-resistant mouse

Collagen undergoes dramatic reorganization during wound repair. Matrix metalloproteinases degrade and remodel collagen in a tightly controlled process. The collagenase-resistant mouse, Col1a1(tm1Jae), produces type I collagen, which is resistant to degradation by human matrix metalloproteinase 1. These mice grow normally but develop thickened skin with age. We have previously reported that the early wound repair response in homozygous mutant (Col1a1(r/r)) mice is delayed compared to wild type (Col1a1(+/+)). However, the late-stage scar of Col1a1(r/r) wounds was not significantly altered compared to Col1a1(+/+). Here we have investigated the response of heterozygous mice (Col1a1(+/r)) to wounding, not previously reported. Wound reepithelialization was delayed to a similar degree to wounds in the Col1a1(r/r) mice. However, the recovery of impaired wound contraction was faster in Col1a1(+/r) than in Col1a1(r/r) mice, but still slower than in wild-type animals. Analysis of wound protein extracts showed expression of some matrix metalloproteinases was prolonged in both the Col1a1(r/r) and Col1a1(+/r) wounds compared to wild type. We suggest the partial resistance of collagen to collagenase-mediated degradation in the heterozygous animals causes equivalent impairment of keratinocyte migration compared to homozygous collagenase-resistant mice, but that wound contraction during late-stage healing is only partially retarded.

Severely impaired wound healing in the collagenase-resistant mouse

Collagen in the skin undergoes dramatic reorganization during wound repair. Matrix metalloproteinases degrade and remodel the collagen in a tightly controlled process. The collagenase-resistant mouse, Col1a1(tm1Jae), has been developed to produce collagen type I, which is resistant to degradation by human matrix metalloproteinase 1. These mice grow normally but develop thickened skin with age. We investigated the effect of this mutant collagen on wound repair. Incisional wounds were made on Col1a1(tm1Jae) homozygous mutant (Col1a1(r/r)) and wild-type (Col1a1+/+) mice and these wounds were harvested at 1 and 6 h, 1, 2, 3, 7, 10, 14, and 70 d post wounding. Wound healing was severely delayed in Col1a1(r/r) wounds, with wounds remaining significantly wider than wild-type for the first 2 wk after injury. Reepithelialization of the Col1a1(r/r) wounds took 7 d longer than in the wild-type. The Col1a1(r/r) wounds had a prolonged early inflammatory response. Immunostaining for matrix metalloproteinases revealed significant upregulation of matrix metalloproteinase 13 in Col1a1(r/r) wounds, but minimal changes in other matrix metalloproteinases. There was no significant difference in scarring between Col1a1(r/r) and Col1a1+/+ wounds after 70 d.

Restoration of normal bone development by human homologue of collagen type II (COL2A1) gene in Col2a1 null mice

Development of the vertebrate skeleton is a highly complex process in which collagen type II plays a vital role in the formation of long bones via endochondral ossification. Collagen type II, which is encoded by a single COL2A1/ Col2a1 gene, is the most abundant structural protein in the cartilage matrix, where it undergoes complex interactions with several other proteins. The sequence of mature collagen type II chains, each with about 1,100 amino acids, is conserved between different mammalian species. There are 37 amino acid positions that are different between mouse and human collagen type II. Previously, we have demonstrated that transgenic mice, in which Col2a1 gene is knocked out, exhibit a lethal phenotype due to the absence of endochondral bone formation. To investigate whether the biological role of collagen type II is conserved between the species, human COL2A1 gene was expressed in Col2a1 null mice by crossing with transgenic mice in which human COL2A1 gene was integrated. The collagen type II from human gene rescued the lethal phenotype in null mice, indicating that the biological function of collagen type II is conserved between human and mouse. The animals exhibited normal endochondral bone formation and a normal growth plate in tibio-tarsal joint. Chondrocytes isolated from the cartilage of these mice secreted human protein, suggesting that the animals incorporated heterologous protein to form cartilage which is essentially "humanized." The animals reached puberty and produced normal progeny. A completely normal phenotype in newborns indicates that human COL2A1 gene is expressed properly both temporally and spatially. These animals may be useful to generate models to study the effect of COL2A1 mutations on skeletal development in humans by introducing mutated gene constructs either into embryos or by crossing with transgenic animals with COL2A1 mutations.

Molecular cloning and chromatin structure analysis of the murine alpha1(I) collagen gene domain

We have isolated molecular clones of genomic mouse DNA spanning 55 kb, including the entire coding region of the murine alpha1(I) collagen (Col1a1) gene and 24 kb of 5' and 13 kb of 3'-flanking sequences, and have performed a detailed chromatin structure analysis of these sequences. Several new DNase-I-hypersensitive sites were identified. The distal 5'-flanking region contains two clusters of DNase-I-hypersensitive sites located between 7 and 8 kb and between 15 and 20 kb upstream of the start site of transcription, respectively. Several of these sites were shown to be present in collagen-producing, but not in non-producing cells, indicating that they are associated with transcription of the gene and may function in its regulation. One strong constitutive DNase-I-hypersensitive site at -18.5 kb was also cleaved by endogenous nucleases. The 3'-flanking region of the gene contains a DNase-I-hypersensitive site located 6 kb downstream of the end of the gene, as well as sequences that can induce a non-B DNA structure. Because these latter sequences coincide with DNase-I-hypersensitive sites in the homologous human gene, our results suggest that some regulatory elements may play a role in gene regulation, not by specific protein-DNA interactions but by virtue of their ability to induce a non-B DNA structure and/or an alternate chromatin conformation. A comparison of the murine and human Col1a1 domains shows a similar, although not identical, distribution of DNase-I-hypersensitive sites, indicating a conserved arrangement of regulatory elements. Our results strongly suggest that these new sites constitute regulatory elements which are involved in the transcriptional regulation and/or chromatin loop organization of the Col1a1 gene, and they are now amenable for functional analyses.

Binding of upstream stimulatory factor to an E-box in the 3'-flanking region stimulates alpha1(I) collagen gene transcription

Since several lines of evidence implicate the 3'-flanking region in regulating alpha1(I) collagen gene transcription, we analyzed 12. 4-kilobase pairs of 3'-flanking sequence of the murine alpha1(I) collagen gene for transcriptional elements. A region of the 3'-flanking region stimulated expression of the heterologous beta-globin gene promoter in an enhancer trap plasmid and of the alpha1(I) collagen gene promoter in a collagen-luciferase reporter gene construct when located 3' to the luciferase reporter gene. DNase I footprinting analysis demonstrated the presence of three regions where DNA binding proteins specifically interact within this 3'-stimulatory region. Inspection of the DNA sequence revealed a consensus E-box, a binding site for basic helix-loop-helix proteins, in one of the protein binding sites. Mobility shift assays demonstrated that upstream stimulatory factors (USF) USF-1 and USF-2 bind to this E-box. Mutating the E-box in the context of the 3'-flanking region confirmed that it contributes to the enhancement of transcriptional activity of the alpha1(I) collagen gene promoter. Mutations in all three protein binding sites abolished transcriptional activation by the 3'-flanking region, suggesting a complex interaction among the trans-acting factors in enhancing transcriptional activity. Thus, a region of the 3'-flanking region of the alpha1(I) collagen gene stimulates transcription of the alpha1(I) collagen gene promoter, and USF-1 and USF-2 contribute to this transcriptional stimulation.

Type I collagen-deficient Mov-13 mice do not retain SPARC in the extracellular matrix: implications for fibroblast function

The Mov-13 strain of mice was created by the insertion of the murine Moloney leukemia virus into the first intron of the alpha 1 (I) collagen gene. Consequently, Mov-13 embryos do not transcribe alpha 1 (I) collagen mRNA and lack type I collagen protein in the extracellular matrix (ECM). Homozygotes die within 12-14 days of embryonic development, in part from the rupture of large blood vessels, and also exhibit deficiencies in hematopoesis and assembly of the ECM (Lohler et al. [1984] Cell 38:597-607). Several matricellular proteins, proteoglycans, and growth factors bind to type I collagen, e.g., fibronectin, secreted protein acidic and rich in cysteine (SPARC), decorin, and transforming growth factor-beta. Here we investigate the expression and function of SPARC in the absence of type I collagen. We show that fibroblasts isolated from Mov-13 homozygous, heterozygous, and wild-type embryos transcribed and translated SPARC mRNA in vitro. However, accumulation of extracellular SPARC was severely affected in the tissues of Mov-13 homozygotes, whereas extracellular deposition of the secreted glycoproteins fibronectin and type III collagen was not altered. Since SPARC has been shown to be a regulator of cell shape, the functional consequences of the absence of extracellular SPARC were evaluated in collagen gel contraction assays. Fibroblasts isolated from homozygous Mov-13 mice did not contract native type I collagen gels as efficiently as fibroblasts from heterozygous littermates; however, addition of exogenous SPARC enhanced the contraction of collagen by homozygous Mov-13 fibroblasts. The stimulatory effect of SPARC was blocked by antibodies specific for the amino terminus of the protein. These results provide evidence that type I collagen is one of the major extracellular proteins that binds SPARC in vivo. Furthermore, the capacity of fibroblasts to contract ECM in vitro is enhanced by extracellular SPARC. We therefore propose that the remodeling of ECM by cells in vivo is regulated in part by a specific interaction between SPARC and type I collagen.

Endoplasmic reticulum-mediated quality control of type I collagen production by cells from osteogenesis imperfecta patients with mutations in the pro alpha 1 (I) chain carboxyl-terminal propeptide which impair subunit assembly

A heterozygous single base change in exon 49 of COL1A1, which converted the codon for pro alpha 1(I) carboxyl-terminal propeptide residue 94 from tryptophan (TGG) to cysteine (TGT) was identified in a baby with lethal osteogenesis imperfecta (OI64). The C-propeptide mutations in OI64 and in another lethal osteogenesis imperfecta cell strain (OI26), which has a frameshift mutation altering the sequence of the carboxyl-terminal half of the propeptide (Bateman, J. F., Lamande, S. R., Dahl, H.-H. M., Chan, D., Mascara, T. and Cole, W. G. (1989) J. Biol. Chem. 264, 10960-10964), disturbed procollagen folding and retarded the formation of disulfide-linked trimers. Although assembly was delayed, the presence of slowly migrating, overmodified alpha 1(I) and alpha 2(I) chains indicated that mutant pro alpha 1(I) could associate with normal pro alpha 1(I) and pro alpha 2(I) to form pepsin-resistant triple-helical molecules, a proportion of which were secreted. Further evidence of the aberrant folding of mutant procollagen in OI64 and OI26 was provided by experiments demonstrating that the endoplasmic reticulum resident molecular chaperone BiP, which binds to malfolded proteins, was specifically bound to type I procollagen and was coimmunoprecipitated in the osteogenesis imperfecta cells but not control cells. Experiments with brefeldin A, which inhibits protein export from the endoplasmic reticulum, demonstrated that unassembled mutant pro alpha 1(I) chains were selectively degraded within the endoplasmic reticulum resulting in reduced collagen production by the osteogenesis imperfecta cells. This biosynthetic deficiency was reflected in the inability of OI64 and OI26 cells to produce a substantial in vitro collagenous matrix when grown in the continuous presence of ascorbic acid to allow collagen matrix formation. Both these carboxyl-terminal propeptide mutants showed a marked reduction in collagen accumulation to 20% (or less) of control cultures, comparable to the reduced collagen content of tissues from OI26.

Insertional mutagenesis in transgenic mice

Increasing numbers of transgenic mouse lines have resulted in several dozens of mutants created by insertional mutagenesis. The advantages of different vector systems and the problems associated with the analysis of mutations and the cloning of the affected genes are discussed in this review.

Regulation of expression of the type I collagen genes

The identification and functional analysis of DNA-protein interactions in the intronic and 5' flanking regions of the type I collagen genes has begun to define a series of cis-elements and trans-acting factors which regulate transcription of these genes. Studies such as these will eventually be expected to elucidate the mechanisms responsible for coordinate transcription of the alpha 1 and alpha 2 genes, a question which remains central to the field of collagen research. Although it is relatively straightforward to define sites of DNA-protein binding, interpretation of the functional importance of such interactions can be extremely complex. Furthermore, while mutation or deletion of a particular binding site may alter the functional activity of a construct transfected into cultured cells, there is no guarantee that a similar change will have the same effect in vivo, where the entire gene locus is present in its native chromosomal context. Nevertheless, these kinds of in vitro studies offer the best current approach to defining and isolating transcription factors that control expression of the alpha 1 and alpha 2 genes. Ultimately, it will be necessary to test the activity of such factors (and their respective cis-elements) in defined systems in vivo.

An X-linked human collagen transgene escapes X inactivation in a subset of cells

Transgenic mice carrying one complete copy of the human alpha 1(I) collagen gene on the X chromosome (HucII mice) were used to study the effect of X inactivation on transgene expression. By chromosomal in situ hybridization, the transgene was mapped to the D/E region close to the Xce locus, which is the controlling element. Quantitative RNA analyses indicated that transgene expression in homozygous and heterozygous females was about 125% and 62%, respectively, of the level found in hemizygous males. Also, females with Searle's translocation carrying the transgene on the inactive X chromosome (Xi) expressed about 18% transgene RNA when compared to hemizygous males. These results were consistent with the transgene being subject to but partially escaping from X inactivation. Two lines of evidence indicated that the transgene escaped X inactivation or was reactivated in a small subset of cells rather than being expressed at a lower level from the Xi in all cells, (i) None of nine single cell clones carrying the transgene on the Xi transcribed transgene RNA. In these clones the transgene was highly methylated in contrast to clones carrying the transgene on the Xa. (ii) In situ hybridization to RNA of cultured cells revealed that about 3% of uncloned cells with the transgene on the Xi expressed transgene RNA at a level comparable to that on the Xa. Our results indicate that the autosomal human collagen gene integrated on the mouse X chromosome is susceptible to X inactivation. Inactivation is, however, not complete as a subset of cells carrying the transgene on Xi expresses the transgene at a level comparable to that when carried on Xa.

NF-I/Sp1 switch elements regulate collagen alpha 1(I) gene expression

The expression of type I collagen is regulated developmentally and tissue specifically. Two sets of binding sites for nuclear factor I (NF-I) and Sp1 transcription factors arrayed as an imperfect tandem repeat are critical for high activity of the murine alpha 1(I) collagen gene in NIH-3T3 fibroblasts and are conserved in evolution. Gel retardation analysis combined with methylation interference studies show that NF-I and Sp1 bind to overlapping sites in a mutually exclusive manner. Cotransfection studies using Drosophila Schneider L2 cells, which lack both transcription factors, demonstrate that each factor alone trans-activates the gene, while cotransfection of both factors results in the inhibition of the strong Sp1 trans-activation. In contrast, the herpes simplex virus thymidine kinase promoter, which contains functionally independent NF-I and Sp1 binding sites, is maximally transactivated by the cotransfection of both factors. Because the two NF-I/Sp1 binding sites overlap, the ratio of the activities of the two factors rather than their absolute concentrations determine alpha 1(I) gene expression, characterizing these promoter sequences as transcription factor switch elements.