Collagen XII interacts with avian tenascin-X through its NC3 domain

Large oligomeric proteins often contain several binding sites for different molecules and can therefore induce formation of larger protein complexes. Collagen XII, a multidomain protein with a small collagenous region, interacts with fibrillar collagens through its C-terminal region. However, no interactions to other extracellular proteins have been identified involving the non-collagenous N-terminal NC3 domain. To further elucidate the components of protein complexes present close to collagen fibrils, different extracellular matrix proteins were tested for interaction in a solid phase assay. Binding to the NC3 domain of collagen XII was found for the avian homologue of tenascin-X that in humans is linked to Ehlers-Danlos disease. The binding was further characterized by surface plasmon resonance spectroscopy and supported by immunohistochemical co-localization in chick and mouse tissue. On the ultrastructural level, detection of collagen XII and tenascin-X by immunogold labeling confirmed this finding.

A soluble ectodomain of LRIG1 inhibits cancer cell growth by attenuating basal and ligand-dependent EGFR activity

Leucine-rich repeats and immunoglobulin-like domains-1 (LRIG1) is a transmembrane protein with an ectodomain containing 15 leucine-rich repeats (LRRs) homologous to mammalian decorin and the Drosophila kekkon1 gene. In this study, we demonstrate that a soluble ectodomain of LRIG1, containing only the LRRs, inhibits ligand-independent epidermal growth factor receptor (EGFR) activation and causes growth inhibition of A431, HeLa and MDA-468 carcinoma cells. In contrast, cells that do not express detectable levels of EGFR fail to respond to soluble LRIG1. However, when a functional EGFR gene is introduced in these cells, they become growth-inhibited by soluble LRIG1 protein. Furthermore, we demonstrate the existence of high-affinity (K(d)=10 nM) binding sites on the A431 cells that can be competitively displaced (up to 75%) by molar excess of EGF. Even more powerful effects are obtained with a chimeric proteoglycan harboring the N-terminus of decorin, substituted with a single glycosaminoglycan chain, fused to the LRIG1 ectodomain. Both proteins also inhibit ligand-dependent activation of the EGFR and extracellular signal-regulated protein kinase 1/2 signaling in a rapid and dose-dependent manner. These results suggest a novel mechanism of action evoked by a soluble ectodomain of LRIG1 protein that could modulate EGFR signaling and its growth-promoting activity. Attenuation of EGFR activity without physical downregulation of the receptor could represent a novel therapeutic approach toward malignancies in which EGFR plays a primary role in tumor growth and survival.

Binding of the low-density lipoprotein by streptococcal collagen-like protein Scl1 of Streptococcus pyogenes

Several bacterial genera express proteins that contain collagen-like regions, which are associated with variable (V) non-collagenous regions. The streptococcal collagen-like proteins, Scl1 and Scl2, of group A Streptococcus (GAS) are members of this 'prokaryotic collagen' family, and they too contain an amino-terminal non-collagenous V region of unknown function. Here, we use recombinant rScl constructs, derived from several Scl1 and Scl2 variants, and affinity chromatography to identify Scl ligands present in human plasma. First, we show that Scl1, but not Scl2, proteins from different GAS serotypes bind the same ligand identified as apolipoprotein B (ApoB100), which is a major component of the low-density lipoprotein (LDL). Scl1 binding to purified ApoB100 and LDL is specific and concentration-dependent. Furthermore, the non-collagenous V region of the Scl1 protein is responsible for LDL/ApoB100 binding because only those rScls, constructed by domain swapping, which contain the V region from Scl1 proteins, were able to bind to ApoB100 and LDL ligands, and this binding was inhibited by antibodies directed against the Scl1-V region. Electron microscopy images of Scl1-LDL complexes showed that the globular V domain of Scl1 interacted with spherical particles of LDL. Importantly, live M28-type GAS cells absorbed plasma LDL on the cell surface and this binding depended on the surface expression of the Scl1.28, but not Scl2.28, protein. Phylogenetic analysis showed that the non-collagenous globular domains of Scl1 and Scl2 evolved independently to form separate lineages, which differ in amino acid sequence, and these differences may account for the variations in binding patterns of Scl1 and Scl2 proteins. Present studies provide insight into the structure-function relationship of the Scl proteins and also underline the importance of lipoprotein binding by GAS.

Induction of Epidermolysis Bullosa Acquisita in Mice by Passive Transfer of Autoantibodies from Patients

Epidermolysis bullosa acquisita (EBA) is an autoimmune sub-epidermal blistering disease characterized by autoantibodies to type VII (anchoring fibril) collagen. To date, however, direct evidence for a pathogenic role of human EBA autoantibodies has not been demonstrated. In this study, we affinity-purified anti-type VII collagen antibodies from EBA patients' sera and then injected them into adult hairless immunocompetent mice. Mice injected with EBA autoantibodies developed skin fragility, blisters, erosions, and nail loss on their paws - all features of EBA patients. By clinical, histological, immunological, and ultrastructural parameters, the induced lesions were reminiscent of human EBA. Histology showed bullous lesions with an epidermal-dermal separation. IgG and C3 deposits were observed at the epidermal-dermal junction. All mice had serum antibodies that labeled the dermal side of salt-split human skin like EBA sera. Direct immunogold electron microscopy specifically localized deposits of human IgG to anchoring fibrils. (Fab')(2) fragments generated from EBA autoantibodies did not induce disease. We conclude that EBA human patient autoantibodies cause sub-epidermal blisters and induce EBA skin lesions in mice. These passive transfer studies demonstrate that human EBA autoantibodies are pathogenic. This novel EBA mouse model can be used to further investigate EBA autoimmunity and to develop possible therapies.

Identification, content, and distribution of type VI collagen in bovine tendons

Tendon composition changes according to differentiation, mechanical load, and aging. In this study, we attempted to identify, localize, and quantify type VI collagen in bovine tendons. Type VI collagen was identified by the electrophoretic behavior of the alpha chains and Western blotting, and by rotary shadowing. Type VI collagen was extracted from powdered tendon with three sequential 24-h extractions with 4 M guanidine-HCl. The amount of type VI collagen was determined by enzyme-linked immunosorbent assay for purely tensional areas and for the compressive fibrocartilage regions of the deep flexor tendon of the digits, for the corresponding fetal and calf tendons, and for the extensor digital tendon. The distal fibrocartilaginous region of the adult tendon was richer in type VI collagen than the tensional area, reaching as much as 3.3 mg/g (0.33%) of the wet weight. Calf tendons showed an accumulation of type VI at the fibrocartilage site. Immunocytochemistry demonstrated that type VI collagen was evenly distributed in the tensional areas of tendons but was highly concentrated around the fibrochondrocytes in the fibrocartilages. The results demonstrate that tendons are variable with regard to the presence and distribution of type VI collagen. The early accumulation of type VI collagen in the region of calf tendon that will become fibrocartilage in the adult suggests that it is a good marker of fibrocartilage differentiation. Furthermore, the distribution of type VI collagen in tendon fibrocartilage indicates that it organizes the pericellular environment and may represent a survival factor for these cells.

Fibrillins 1 and 2 perform partially overlapping functions during aortic development

Fibrillin-rich microfibrils are extracellular assemblies that impart structural properties to the connective tissue. To elucidate the contribution of fibrillin-rich microfibrils to organogenesis, we have examined the vascular phenotype of a newly created strain of mice that completely lacks fibrillin-1 and the consequences of combined deficiency of fibrillins 1 and 2 on tissue formation. The results demonstrated that fibrillins 1 and 2 perform partially overlapping functions during aortic development. Fbn1-/- mice died soon after birth from ruptured aortic aneurysm, impaired pulmonary function, and/or diaphragmatic collapse. Analysis of the neonatal Fbn1-/- aorta documented a disorganized and poorly developed medial layer but normal levels of elastin cross-links. Transcriptional profiling revealed that aneurysm progression in Fbn1 null mice is accompanied by unproductive up-regulation of gene products normally involved in tissue repair and vascular integrity, such as plasminogen activator inhibitor-1, activin A, and cysteine-rich angiogenic protein 61. In contrast to Fbn1-/- mice, Fbn2 null mice had a well developed and morphologically normal aortic wall. However, virtually all Fbn1-/-;Fbn2-/- embryos and about half of the Fbn1+/-;Fbn2-/- embryos died in utero and displayed a significantly more severe vascular phenotype than Fbn1-/- mice. Consistent with a specialized function of fibrillin-2, electron microscopy visualized ultrastructurally different microfibrils in Fbn1 null compared with control cell cultures. Collectively, these data demonstrate that involvement of fibrillin-2 in the initial assembly of the aortic matrix overlaps in part with fibrillin-1 and that continued fibrillin-1 deposition is absolutely required for the maturation and function of the vessel during neonatal life.

Assessment of prokaryotic collagen-like sequences derived from streptococcal Scl1 and Scl2 proteins as a source of recombinant GXY polymers

Collagen triple helix, composed of the repeating Gly-Xaa-Yaa (GXY) sequence, is a structural element found in all multicellular animals and also in some prokaryotes. Long GXY polymers are highly regarded components used in food, cosmetic, biomedical, and pharmaceutical industries. In this study, we explore a new concept for the production of recombinant GXY polymers which are based on the sequence of "prokaryotic collagens", the streptococcal collagen-like proteins Scl1 and Scl2. Analysis of 50 Scl variants identified the amino acid distribution and GXY-repeat usage that are involved in the stabilization of the triple helix in Scls. Using circular dichroism spectroscopy and electron microscopy, we show that significantly different recombinant rScl polypeptides form stable, unhydroxylated homotrimeric triple helices that can be produced both intra- and extracellularly in the Escherichia coli. These rScl constructs containing 20 to 129 GXY repeats had mid-point melting temperatures between 32 and 39 degrees C. Altogether, Scl-derived collagens, which are different from the mammalian collagens, can form stable triple helices under physiological conditions and can be used for the production of recombinant GXY polymers with a wide variety of potential applications.

The effect of matrix composition of 3D constructs on embryonic stem cell differentiation

The use of embryonic stem (ES) cells as unlimited cell source in tissue engineering has ignited the hope of regenerating any kind of tissue in vitro. However, the role of the material in control and guidance of their development and commitment into complex and viable three-dimensional (3D) tissues is still poorly understood. In this work, we investigate the role of material composition and structure on promoting ES cells growth and differentiation, by culturing mouse ES cell-derived embryoid bodies (EBs) in various semi-interpenetrating polymer networks (SIPNs), made of collagen, fibronectin (FN) and laminin (LM). We show that both composition and strength of the supportive matrix play an important role in EBs development. High collagen concentrations inhibit EBs cavitation and hence the following EBs differentiation, by inhibiting apoptosis. The presence of FN in 3D collagen constructs strongly stimulates endothelial cell differentiation and vascularization. Conversely, LM increases the ability of ES cells to differentiate into beating cardiomyocytes. Our data suggest that matrix composition has an important role in EBs development and that it is possible to influence stem cell differentiation toward preferential pattern, by modulating the physical and biochemical properties of the scaffold.

A 'Collagen Hug' model for Staphylococcus aureus CNA binding to collagen

The structural basis for the association of eukaryotic and prokaryotic protein receptors and their triple-helical collagen ligand remains poorly understood. Here, we present the crystal structures of a high affinity subsegment of the Staphylococcus aureus collagen-binding CNA as an apo-protein and in complex with a synthetic collagen-like triple helical peptide. The apo-protein structure is composed of two subdomains (N1 and N2), each adopting a variant IgG-fold, and a long linker that connects N1 and N2. The structure is stabilized by hydrophobic inter-domain interactions and by the N2 C-terminal extension that complements a beta-sheet on N1. In the ligand complex, the collagen-like peptide penetrates through a spherical hole formed by the two subdomains and the N1-N2 linker. Based on these two structures we propose a dynamic, multistep binding model, called the 'Collagen Hug' that is uniquely designed to allow multidomain collagen binding proteins to bind their extended rope-like ligand.

Collagen XXVIII, a novel von Willebrand factor A domain-containing protein with many imperfections in the collagenous domain

Here we describe a novel collagen belonging to the class of von Willebrand factor A (VWA) domain-containing proteins. This novel protein was identified by screening the EST data base and was subsequently recombinantly expressed and characterized as an authentic tissue component. The COL28A1 gene on human chromosome 7p21.3 and on mouse chromosome 6A1 encodes a novel protein that structurally resembles the beaded filament-forming collagens. The collagenous domain contains several very short interruptions arranged in a repeat pattern. As shown for other novel minor collagens, the expression of collagen XXVIII protein in mouse is very restricted. In addition to small amounts in skin and calvaria, the major signals were in dorsal root ganglia and peripheral nerves. By immunoelectron microscopy, collagen XXVIII was detected in the sciatic nerve, at the basement membrane of certain Schwann cells surrounding the nerve fibers. Even though the protein is present in the adult sciatic nerve, collagen XXVIII mRNA was only detected in sciatic nerve of newborn mice, indicating that the protein persists for an extended period after synthesis.

A novel binding site in collagen type III for integrins alpha1beta1 and alpha2beta1

Previously identified high affinity integrin-binding motifs in collagens, GFOGER and GLOGER, are not present in type III collagen. Here, we first characterized the binding of recombinant I domains from integrins alpha(1) and alpha(2) (alpha(1)I and alpha(2)I) to fibrillar collagen types I-III and showed that each I domain bound to the three types of collagens with similar affinities. Using rotary shadowing followed by electron microscopy, we identified a high affinity binding region in human type III collagen recognized by alpha(1)I and alpha(2)I. Examination of the region revealed the presence of two sequences that contain the critical GER motif, GROGER and GAOGER. Collagen-like peptides containing these two motifs were synthesized, and their triple helical nature was confirmed by circular dichroism spectroscopy. Experiments show that the GROGER-containing peptide was able to bind both alpha(1)I and alpha(2)I with high affinity and effectively inhibit the binding of alpha(1)I and alpha(2)I to type III and I collagens, whereas the GAOGER-containing peptide was considerably less effective. Furthermore, the GROGER-containing peptide supported adhesion of human lung fibroblast cells when coated on a culture dish. Thus, we have identified a novel high affinity binding sequence for the collagen-binding integrin I domains.

The prodomain of BMP-7 targets the BMP-7 complex to the extracellular matrix

Biochemical and biophysical methods are used to show that BMP-7 is secreted as a stable complex consisting of the processed growth factor dimer noncovalently associated with its two prodomain propeptide chains and that the BMP-7 complex is structurally similar to the small transforming growth factor beta (TGFbeta) complex. Because the prodomain of TGFbeta interacts with latent TGFbeta-binding proteins, a family of molecules homologous to the fibrillins, the prodomain of BMP-7 was tested for binding to fibrillin-1 or to LTBP-1. The BMP-7 prodomain and BMP-7 complex, but not the separated growth factor dimer, interact with N-terminal regions of fibrillin-1. This interaction may target the BMP-7 complex to fibrillin microfibrils in the extracellular matrix. Immunolocalization of BMP-7 in tissues like the kidney capsule and skin reveals co-localization with fibrillin. However, BMP-7 immunolocalization in other tissues known to be active sites for BMP-7 signaling is not apparent, suggesting that immunolocalization of BMP-7 in certain tissues represents specific extracellular storage sites. These studies suggest that the prodomains of TGFbeta-like growth factors are important for positioning and concentrating growth factors in the extracellular matrix. In addition, they raise the possibility that prodomains of other TGFbeta-like growth factors interact with fibrillins and/or LTBPs and are also targeted to the extracellular matrix.

A Streptococcal Collagen-like Protein Interacts with the α2β1 Integrin and Induces Intracellular Signaling

The streptococcal collagen-like proteins Scl1 and Scl2 are prokaryotic members of a large protein family with domains containing the repeating amino acid sequence (Gly-Xaa-Yaa)(n) that form a collagen-like triple-helical structure. Here, we test the hypothesis that Scl variant might interact with mammalian collagen-binding integrins. We show that the recombinant Scl protein p176 promotes adhesion and spreading of human lung fibroblast cells through an alpha2beta1 integrin-mediated interaction as shown in cell adhesion inhibition assays using anti-alpha2beta1 and anti-beta1 integrins monoclonal antibodies. Accordingly, C2C12 cells stably expressing alpha2beta1 integrin as the only collagen-binding integrin show productive cell adhesion activities on p176 that can be blocked by an anti-alpha2beta1 integrin antibody. In addition, p176 promotes tyrosine phosphorylation of p125(FAK) of C2C12 cells expressing alpha2beta1 integrin, whereas parental cells do not. Furthermore, C2C12 adhesion of human lung fibroblast cells to p176 induces phosphorylation of p125FAK, p130CAS, and p68Paxillin proteins. In a domain swapping experiment, we show that integrin binds to the collagenous domain of the Scl protein. Moreover, the recombinant inserted domain of the alpha2 integrin interacts with p176 with a relatively high affinity (K(D) = 17 nm). Attempts to identify the integrin sites in p176 suggest that more than one site may be involved. These studies, for the first time, suggest that the collagen-like proteins of prokaryotes retained not only structural but also functional characteristics of their eukaryotic counterparts.

Secretion of Cartilage Oligomeric Matrix Protein Is Affected by the Signal Peptide

Cartilage oligomeric matrix protein (COMP) is a secreted glycoprotein found in the extracellular matrices of skeletal tissues. Mutations associated with two human skeletal dysplasias, pseudoachondroplasia and multiple epiphyseal dysplasia, disturb COMP secretion leading to intracellular accumulation of mutant COMP, especially in chondrocytes. Here we show that the manifestation of this secretory defect is dramatically influenced by the signal peptide that targets COMP for secretion. The comparison of wild type and mutant COMP secretion directed by the COMP or BM40 signal peptide in HEK-293 cells and rat chondrosarcoma cells revealed that the BM40 signal peptide substantially enhances secretion of mutant COMP that accumulates in endoplasmic reticulum-like structures when targeted by its own signal peptide. Additionally, we demonstrate that mutant COMP forms mixed pentamers with wild type COMP. Our findings suggest that the secretory defect in pseudoachondroplasia and multiple epiphyseal dysplasia is not specific for chondrocytes, nor does it require interaction of mutant COMP with other matrix proteins prior to transport from the cell. They also imply a previously unappreciated role for the signal peptide in the regulation of protein secretion beyond targeting to the endoplasmic reticulum.

Intradermal injection of lentiviral vectors corrects regenerated human dystrophic epidermolysis bullosa skin tissue in vivo

Dystrophic epidermolysis bullosa (DEB) is a family of inherited mechanobullous disorders caused by mutations in the gene, COL7A1, that codes for type VII, (anchoring fibril), collagen, which is critical for epidermal-dermal adherence. Most gene therapy approaches have been ex vivo, involving cell culture and culture graft transplantation, which is logistically difficult. To develop a more simplified approach, we engineered a self-inactivating lentiviral vector expressing human type VII collagen and injected this vector intradermally into hairless, immunodeficient mice and into a human DEB composite skin equivalent grafted onto immunodeficient mice. In both situations, the vector transduced dermal cells, which in turn synthesized and exported type VII collagen into the extracellular space. Remarkably, the type VII collagen selectively adhered to and incorporated into the basement membrane zone (BMZ) between the dermis and the epidermis, where it formed anchoring fibril structures. In the case of the DEB skin equivalent, the newly expressed type VII collagen reversed the DEB phenotype characterized by poor epidermal-dermal adherence and anchoring fibril defects. A single lentiviral vector injection provided stable type VII collagen at the BMZ for at least 3 months. These data demonstrate efficient and long-term type VII collagen gene transfer in vivo using direct intradermal injection of an engineered lentiviral vector.

Differential expression of fibrillin-3 adds to microfibril variety in human and avian, but not rodent, connective tissues

The human genome contains three fibrillins: FBN1 and FBN2, both well characterized, and FBN3, reported only as a cDNA sequence. Like FBN2, the highest expression levels of FBN3 were found in fetal tissues, with only low levels in postnatal tissues. Immunolocalization demonstrated fibrillin-3 in extracellular microfibrils abundant in developing skeletal elements, skin, lung, kidney, and skeletal muscle. Unlike the other two fibrillins, FBN3 expression is high in brain, and FBN3 is alternatively spliced, removing the exon encoding cbEGF2. Like FBN1, FBN3 contains three alternate exons in the 5' UTR. While FBN3 orthologs were identified in cow and chicken, Fbn3 appears to have been inactivated in the mouse genome, perhaps during chromosome fission events. Located on chromosome 19p13.3-13.2, FBN3 is a candidate gene for Weill-Marchesani syndrome.

Evidence for a critical contribution of haploinsufficiency in the complex pathogenesis of Marfan syndrome

Marfan syndrome is a connective tissue disorder caused by mutations in the gene encoding fibrillin-1 (FBN1). A dominant-negative mechanism has been inferred based upon dominant inheritance, mulitimerization of monomers to form microfibrils, and the dramatic paucity of matrix-incorporated fibrillin-1 seen in heterozygous patient samples. Yeast artificial chromosome-based transgenesis was used to overexpress a disease-associated mutant form of human fibrillin-1 (C1663R) on a normal mouse background. Remarkably, these mice failed to show any abnormalities of cellular or clinical phenotype despite regulated overexpression of mutant protein in relevant tissues and developmental stages and direct evidence that mouse and human fibrillin-1 interact with high efficiency. Immunostaining with a human-specific mAb provides what we believe to be the first demonstration that mutant fibrillin-1 can participate in productive microfibrillar assembly. Informatively, use of homologous recombination to generate mice heterozygous for a comparable missense mutation (C1039G) revealed impaired microfibrillar deposition, skeletal deformity, and progressive deterioration of aortic wall architecture, comparable to characteristics of the human condition. These data are consistent with a model that invokes haploinsufficiency for WT fibrillin-1, rather than production of mutant protein, as the primary determinant of failed microfibrillar assembly. In keeping with this model, introduction of a WT FBN1 transgene on a heterozygous C1039G background rescues aortic phenotype.

Injection of recombinant human type VII collagen restores collagen function in dystrophic epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is a family of inherited mechano-bullous disorders that are caused by mutations in the type VII collagen gene and for which ex vivo gene therapy has been considered. To develop a simpler approach for treating DEB, we evaluated the feasibility of protein-based therapy by intradermally injecting human recombinant type VII collagen into mouse skin and a DEB human skin equivalent transplanted onto mice. The injected collagen localized to the basement membrane zone of both types of tissues, was organized into human anchoring fibril structures and reversed the features of DEB disease in the DEB skin equivalent.

A novel marker of tissue junctions, collagen XXII

Here we describe a novel specific component of tissue junctions, collagen XXII. It was first identified by screening an EST data base and subsequently expressed as a recombinant protein and characterized as an authentic tissue component. The COL22A1 gene on human chromosome 8q24.2 encodes a collagen that structurally belongs to the FACIT protein family (fibril-associated collagens with interrupted triple helices). Collagen XXII exhibits a striking restricted localization at tissue junctions such as the myotendinous junction in skeletal and heart muscle, the articular cartilage-synovial fluid junction, or the border between the anagen hair follicle and the dermis in the skin. It is deposited in the basement membrane zone of the myotendinous junction and the hair follicle and associated with the extrafibrillar matrix in cartilage. In situ hybridization of myotendinous junctions revealed that muscle cells produce collagen XXII, and functional tests demonstrated that collagen XXII acts as a cell adhesion ligand for skin epithelial cells and fibroblasts. This novel gene product, collagen XXII, is the first specific extracellular matrix protein present only at tissue junctions.

Fine tuning of growth factor signals depends on fibrillin microfibril networks

Growth factors, potent regulators of cell differentiation, tissue morphogenesis, tissue homeostasis, and cellular response to injury, reside in the extracellular matrix. Genetic evidence in humans and mice as well as biochemical data implicate fibrillins and LTBPs in the extracellular control of TGFbeta and BMP signaling. Fibrillins and LTBPs form tissue-specific and temporally regulated microfibril networks. In the developing embryo, three fibrillins and four LTBPs contribute molecular heterogeneity to microfibril networks, and provide different templates upon which TGFbeta-related growth factors can be positioned. By accommodating this molecular heterogeneity, microfibril architecture can orchestrate a variety of different signals in very specific tissue locations. Human fibrillinopathies display a broad phenotypic spectrum from tall to short stature, from hypermobile joints to joint contractures and stiffness, and from severe to mild or no cardiovascular manifestations. A spectrum of growth factor dysregulation may be caused by differential effects of mutations in fibrillins on microfibril architecture, thus altering appropriate targeting or positioning of growth factors within microfibril networks. Growth factor dysregulation may help to explain the broad phenotypic spectrum of the fibrillinopathies.

Collagen XVIII/endostatin is essential for vision and retinal pigment epithelial function

Age-related macular degeneration (ARMD) with abnormal deposit formation under the retinal pigment epithelium (RPE) is the major cause of blindness in the Western world. basal laminar deposits are found in early ARMD and are composed of excess basement membrane material produced by the RPE. Here, we demonstrate that mice lacking the basement membrane component collagen XVIII/endostatin have massive accumulation of sub-RPE deposits with striking similarities to basal laminar deposits, abnormal RPE, and age-dependent loss of vision. The progressive attenuation of visual function results from decreased retinal rhodopsin content as a consequence of abnormal vitamin A metabolism in the RPE. In addition, aged mutant mice show photoreceptor abnormalities and increased expression of glial fibrillary acidic protein in the neural retina. Our data demonstrate that collagen XVIII/endostatin is essential for RPE function, and suggest an important role of this collagen in Bruch's membrane. Consistent with such a role, the ultrastructural organization of collagen XVIII/endostatin in basement membranes, including Bruch's membrane, shows that it is part of basement membrane molecular networks.

Normal and Gene-Corrected Dystrophic Epidermolysis Bullosa Fibroblasts Alone Can Produce Type VII Collagen at the Basement Membrane Zone

Type VII collagen is synthesized and secreted by both human keratinocytes and fibroblasts. Although both cell types can secrete type VII collagen, it is thought that keratinocytes account for type VII collagen at the dermal-epidermal junction (DEJ). In this study, we examined if type VII collagen secreted solely by dermal fibroblasts could be transported to the DEJ. We established organotypic, skin-equivalent cultures composed of keratinocytes from patients with recessive dystrophic epidermolysis bullosa (RDEB) and normal dermal fibroblasts. Immuno-labeling of skin equivalent sections with the anti-type VII collagen antibody revealed tight linear staining at the DEJ. RDEB fibroblasts, were gene-corrected to make type VII collagen and used to regenerate human skin on immune-deficient mice. The human skin generated by gene-corrected RDEB fibroblasts or normal human fibroblasts combined with RDEB keratinocytes restored type VII collagen expression at the DEJ in vivo. Further, intradermal injection of normal human or gene-corrected RDEB fibroblasts into mouse skin resulted in the stable expression of human type VII collagen at the mouse DEJ. These data demonstrate that human dermal fibroblasts alone are capable of producing type VII collagen at the DEJ, and it is possible to restore type VII collagen gene expression in RDEB skin in vivo by direct intradermal injection of fibroblasts.

Use of Bmp1/Tll1 doubly homozygous null mice and proteomics to identify and validate in vivo substrates of bone morphogenetic protein 1/tolloid-like metalloproteinases

Bone morphogenetic protein 1 (BMP-1) and mammalian Tolloid (mTLD), two proteinases encoded by Bmp1, provide procollagen C-proteinase (pCP) activity that converts procollagens I to III into the major fibrous components of mammalian extracellular matrix (ECM). Yet, although Bmp1(-/-) mice have aberrant collagen fibrils, they have residual pCP activity, indicative of genetic redundancy. Mammals possess two additional proteinases structurally similar to BMP-1 and mTLD: the genetically distinct mammalian Tolloid-like 1 (mTLL-1) and mTLL-2. Mice lacking the mTLL-1 gene Tll1 are embryonic lethal but have pCP activity levels similar to those of the wild type, suggesting that mTLL-1 might not be an in vivo pCP. In vitro studies have shown BMP-1 and mTLL-1 capable of cleaving Chordin, an extracellular antagonist of BMP signaling, suggesting that these proteases might also serve to modulate BMP signaling and to coordinate the latter with ECM formation. However, in vivo evidence of roles for BMP-1 and mTLL-1 in BMP signaling in mammals is lacking. To remove functional redundancy obscuring the in vivo functions of BMP-1-related proteases in mammals, we here characterize Bmp1 Tll1 doubly null mouse embryos. Although these appear morphologically indistinguishable from Tll1(-/-) embryos, biochemical analysis of cells derived from doubly null embryos shows functional redundancy removed to an extent enabling us to demonstrate that (i) products of Bmp1 and Tll1 are responsible for in vivo cleavage of Chordin in mammals and (ii) mTLL-1 is an in vivo pCP that provides residual activity observed in Bmp1(-/-) embryos. Removal of functional redundancy also enabled use of Bmp1(-/-) Tll1(-/-) cells in a proteomics approach for identifying novel substrates of Bmp1 and Tll1 products.