The relationship of the biophysical and biochemical characteristics of type VII collagen to the function of anchoring fibrils

Designing collagens to shed light on the multi-scale structure-function mapping of matrix disorders

Collagens are the most abundant structural proteins in the extracellular matrix of animals and play crucial roles in maintaining the structural integrity and mechanical properties of tissues and organs while mediating important biological processes. Fibrillar collagens have a unique triple helix structure with a characteristic repeating sequence of (Gly-X-Y)n. Variations within the repetitive sequence can cause misfolding of the triple helix, resulting in heritable connective tissue disorders. The most common variations are single-point missense mutations that lead to the substitution of a glycine residue with a bulkier amino acid (Gly → X). In this review, we will first discuss the importance of collagen's triple helix structure and how single Gly substitutions can impact its folding, structure, secretion, assembly into higher-order structures, and biological functions. We will review the role of "designer collagens," i.e., synthetic collagen-mimetic peptides and recombinant bacterial collagen as model systems to include Gly → X substitutions observed in collagen disorders and investigate their impact on structure and function utilizing in vitro studies. Lastly, we will explore how computational modeling of collagen peptides, especially molecular and steered molecular dynamics, has been instrumental in probing the effects of Gly substitutions on structure, receptor binding, and mechanical stability across multiple length scales.

Sequence-dependent mechanics of collagen reflect its structural and functional organization

Extracellular matrix mechanics influence diverse cellular functions, yet surprisingly little is known about the mechanical properties of their constituent collagen proteins. In particular, network-forming collagen IV, an integral component of basement membranes, has been far less studied than fibril-forming collagens. A key feature of collagen IV is the presence of interruptions in the triple-helix-defining (Gly-X-Y) sequence along its collagenous domain. Here, we used atomic force microscopy to determine the impact of sequence heterogeneity on the local flexibility of collagen IV and of the fibril-forming collagen III. Our extracted flexibility profile of collagen IV reveals that it possesses highly heterogeneous mechanics, ranging from semiflexible regions as found for fibril-forming collagens to a lengthy region of high flexibility toward its N-terminus. A simple model in which flexibility is dictated only by the presence of interruptions fit the extracted profile reasonably well, providing insight into the alignment of chains and demonstrating that interruptions, particularly when coinciding in multiple chains, significantly enhance local flexibility. To a lesser extent, sequence variations within the triple helix lead to variable flexibility, as seen along the continuously triple-helical collagen III. We found this fibril-forming collagen to possess a high-flexibility region around its matrix-metalloprotease binding site, suggesting a unique mechanical fingerprint of this region that is key for matrix remodeling. Surprisingly, proline content did not correlate with local flexibility in either collagen type. We also found that physiologically relevant changes in pH and chloride concentration did not alter the flexibility of collagen IV, indicating such environmental changes are unlikely to control its compaction during secretion. Although extracellular chloride ions play a role in triggering collagen IV network formation, they do not appear to modulate the structure of its collagenous domain.

QR-313, an Antisense Oligonucleotide, Shows Therapeutic Efficacy for Treatment of Dominant and Recessive Dystrophic Epidermolysis Bullosa: A Preclinical Study

Dystrophic epidermolysis bullosa (DEB) is a blistering skin disease caused by mutations in the gene COL7A1 encoding collagen VII. DEB can be inherited as recessive DEB (RDEB) or dominant DEB (DDEB) and is associated with a high wound burden. Perpetual cycles of wounding and healing drive fibrosis in DDEB and RDEB, as well as the formation of a tumor-permissive microenvironment. Prolonging wound-free episodes by improving the quality of wound healing would therefore confer substantial benefit for individuals with DEB. The collagenous domain of collagen VII is encoded by 82 in-frame exons, which makes splice-modulation therapies attractive for DEB. Indeed, antisense oligonucleotide-based exon skipping has shown promise for RDEB. However, the suitability of antisense oligonucleotides for treatment of DDEB remains unexplored. Here, we developed QR-313, a clinically applicable, potent antisense oligonucleotide specifically targeting exon 73. We show the feasibility of topical delivery of QR-313 in a carbomer-composed gel for treatment of wounds to restore collagen VII abundance in human RDEB skin. Our data reveal that QR-313 also shows direct benefit for DDEB caused by exon 73 mutations. Thus, the same topically applied therapeutic could be used to improve the wound healing quality in RDEB and DDEB.

Structural and biophysical characterization of the type VII collagen vWFA2 subdomain leads to identification of two binding sites

Type VII collagen is an extracellular matrix protein, which is important for skin stability; however, detailed information at the molecular level is scarce. The second vWFA (von Willebrand factor type A) domain of type VII collagen mediates important interactions, and immunization of mice induces skin blistering in certain strains. To understand vWFA2 function and the pathophysiological mechanisms leading to skin blistering, we structurally characterized this domain by X-ray crystallography and NMR spectroscopy. Cell adhesion assays identified two new interactions: one with β1 integrin via its RGD motif and one with laminin-332. The latter interaction was confirmed by surface plasmon resonance with a K_D of about 1 mm. These data show that vWFA2 has additional functions in the extracellular matrix besides interacting with type I collagen.

Molecular pathology of the basement membrane zone in heritable blistering diseases:: The paradigm of epidermolysis bullosa

Epidermolysis bullosa (EB), a phenotypically heterogeneous group of skin fragility disorders, is characterized by blistering and erosions with considerable morbidity and mortality. Mutations in as many as 18 distinct genes expressed at the cutaneous basement membrane zone have been shown to be associated with the blistering phenotype, attesting to the role of the corresponding proteins in providing stable association of the epidermis to the dermis through adhesion at the dermo-epidermal basement membrane zone. Thus, different forms of EB have been highly instructive in providing information on the physiological functions of these proteins as integral components of the supramolecular adhesion complexes. In addition, precise information of the underlying genes and distinct mutations in families with EB has been helpful in subclassification of the disease with prognostic implications, as well as for prenatal testing and preimplantation genetic diagnosis. Furthermore, knowledge of the types of mutations is a prerequisite for application of allele-specific treatment approaches that have been recently developed, including read-through of premature termination codon mutations and chaperone-facilitated intracellular transport of conformationally altered proteins to proper physiologic subcellular location. Collectively, EB serves as a paradigm of heritable skin diseases in which significant progress has been made in identifying the underlying genetic bases and associated aberrant pathways leading from mutations to the phenotype, thus allowing application of precision medicine for this, currently intractable group of diseases.

Chemical shift assignments of the fibronectin III like domains 7-8 of type VII collagen

Type VII collagen (Col7) is important for skin stability. This is underlined by the severe skin blistering phenotype in the Col7 related diseases dystrophic epidermolysis bullosa and epidermolysis bullosa acquisita (EBA). Col7 has a large N-terminal non-collagenous domain (NC1) that is followed by the triple helical collagenous domain. The NC1 domain has subdomains with homology to adhesion molecules and mediates important interactions within the extracellular matrix. An 185 amino acid long part of the NC1-subdomain termed fibronectin III like domains 7 and 8 (FNIII7-8) was investigated. Antibodies against this region are pathogenic in a mouse model of EBA and one reported missense mutations of Col7 lies within these domains. The nearly complete NMR resonance assignment of recombinant FNIII7-8 of Col7 is reported.

Type VII Collagen Expression in the Human Vitreoretinal Interface, Corpora Amylacea and Inner Retinal Layers

Type VII collagen, as a major component of anchoring fibrils found at basement membrane zones, is crucial in anchoring epithelial tissue layers to their underlying stroma. Recently, type VII collagen was discovered in the inner human retina by means of immunohistochemistry, while proteomic investigations demonstrated type VII collagen at the vitreoretinal interface of chicken. Because of its potential anchoring function at the vitreoretinal interface, we further assessed the presence of type VII collagen at this site. We evaluated the vitreoretinal interface of human donor eyes by means of immunohistochemistry, confocal microscopy, immunoelectron microscopy, and Western blotting. Firstly, type VII collagen was detected alongside vitreous fibers6 at the vitreoretinal interface. Because of its known anchoring function, it is likely that type VII collagen is involved in vitreoretinal attachment. Secondly, type VII collagen was found within cytoplasmic vesicles of inner retinal cells. These cells resided most frequently in the ganglion cell layer and inner plexiform layer. Thirdly, type VII collagen was found in astrocytic cytoplasmic inclusions, known as corpora amylacea. The intraretinal presence of type VII collagen was confirmed by Western blotting of homogenized retinal preparations. These data add to the understanding of vitreoretinal attachment, which is important for a better comprehension of common vitreoretinal attachment pathologies.

Epidermal Basement Membrane in Health and Disease

Skin, as the organ protecting the individual from environmental aggressions, constantly meets external insults and is dependent on mechanical toughness for its preserved function. Accordingly, the epidermal basement membrane (BM) zone has adapted to enforce tissue integrity. It harbors anchoring structures created through unique organization of common BM components and expression of proteins exclusive to the epidermal BM zone. Evidence for the importance of its correct assembly and the nonredundancy of its components for skin integrity is apparent from the multiple skin blistering disorders caused by mutations in genes coding for proteins associated with the epidermal BM and from autoimmune disorders in which autoantibodies target these molecules. However, it has become clear that these proteins not only provide mechanical support but are also critically involved in tissue homeostasis, repair, and regeneration. In this chapter, we provide an overview of the unique organization and components of the epidermal BM. A special focus will be given to its function during regeneration, and in inherited and acquired diseases.

The TrpA protein of Trichodesmium erythraeum IMS101 is a non-fibril-forming collagen and a component of the outer sheath

Collagen molecules are structural in nature and primarily found in eukaryotic, multicellular organisms. Recently, a collagen-like protein, TrpA, was identified and characterized in the marine cyanobacterium Trichodesmium erythraeum IMS 101, and it was shown to be involved in maintaining the structural integrity of the trichomes. The TrpA protein contains one glycine interruption in the otherwise perfectly uninterrupted collagenous domain. In this study, we used phylogenetic analysis to determine that the TrpA protein sequence is most closely associated with non-fibril-forming collagen proteins. Structural modelling and circular dichroism data suggest that the glycine insertion decreases the stability of TrpA compared to uninterrupted collagen sequences. Additionally, scanning electron microscopy revealed that TrpA is expressed entirely on the surface of the trichomes, with no specific pattern of localization. These data indicate that the TrpA protein is part of the outer sheath of this organism. As such, this protein may function to promote adhesion between individual T. erythraeum trichomes, and between this organism and heterotrophic bacteria found in the same environment.

The hinge region of type VII collagen is intrinsically disordered

Type VII collagen (Col7) is important for skin integrity. As a major component of the anchoring fibrils, Col7 is essential for linking different skin layers together. The central collagenous domain of Col7 contains several interruptions of the collagen triple helix. The longest interruption is 39 amino acids long and referred to as the hinge region. The hinge region is highly conserved between species. This region was predicted to adopt a coiled coil structure and to serve as the trimerization domain of Col7. To gain insight into the potential function of the hinge region we investigated a heterologous expressed peptide by CD and NMR spectroscopy. CD spectroscopy implies that the hinge region is intrinsically disordered. Resonance assignment was performed and allowed secondary structure analysis based on the chemical shift values. Seven amino acids in the N-terminal moiety show residual α-helical conformation. Subsequent investigation of temperature dependency of amide chemical shifts indicated participation in hydrogen bonding of amino acid residues in the C-terminal moiety of the hinge region. Therefore, the hinge region does not form a coiled coil structure under the employed experimental conditions. The intrinsic disorder of the hinge region might be desired for flexibility to serve as a "hinge" or the hinge region is an important interaction site as typically observed for intrinsically disordered proteins.

A case of recalcitrant epidermolysis bullosa acquisita responsive to rituximab therapy

Epidermolysis bullosa acquisita (EBA) is an autoimmune blistering disorder that occurs infrequently in children. Although typically associated with a good prognosis in children, there are rare cases that are refractory to treatment with conventional immunosuppressive therapy. Rituximab, an anti-CD20 monoclonal antibody, has been reported to be effective in the adult form of EBA. We report a case of a child with severe and recalcitrant EBA exhibiting a sustained response to treatment with rituximab.

Superficial dermal fibroblasts enhance basement membrane and epidermal barrier formation in tissue-engineered skin: implications for treatment of skin basement membrane disorders

Basement membrane is a highly specialized structure that binds the dermis and the epidermis of the skin, and is mainly composed of laminins, nidogen, collagen types IV and VII, and the proteoglycans, collagen type XVIII and perlecan, all of which play critical roles in the function and resilience of skin. Both dermal fibroblasts and epidermal keratinocytes contribute to the development of the basement membrane, and in turn the basement membrane and underlying dermis influence the development and function of the epidermal barrier. Disruption of the basement membrane results in skin fragility, extensive painful blistering, and severe recurring wounds as seen in skin basement membrane disorders such as epidermolysis bullosa, a family of life-threatening congenital skin disorders. Currently, there are no successful strategies for treatment of these disorders; we propose the use of tissue-engineered skin as a promising approach for effective wound coverage and to enhance healing. Fibroblasts and keratinocytes isolated from superficial and deep dermis and epidermis, respectively, of tissue from abdominoplasty patients were independently cocultured on collagen-glycosaminoglycan matrices, and the resulting tissue-engineered skin was assessed for functional differences based on the underlying specific dermal fibroblast subpopulation. Tissue-engineered skin with superficial fibroblasts and keratinocytes formed a continuous epidermis with increased epidermal barrier function and expressed higher levels of epidermal proteins, keratin-5, and E-cadherin, compared to that with deep fibroblasts and keratinocytes, which had an intermittent epidermis. Further, tissue-engineered skin with superficial fibroblasts and keratinocytes formed better basement membrane, and produced more laminin-5, nidogen, collagen type VII, compared to that with deep fibroblasts and keratinocytes. Overall, our results demonstrate that tissue-engineered skin with superficial fibroblasts and keratinocytes forms significantly better basement membrane with higher expression of dermo-epidermal adhesive and anchoring proteins, and superior epidermis with enhanced barrier function compared to that with deep fibroblasts and keratinocytes, or with superficial fibroblasts, deep fibroblasts, and keratinocytes. The specific use of superficial fibroblasts in tissue-engineered skin may thus be more beneficial to promote adhesion of newly formed skin and wound healing, and is therefore promising for the treatment of patients with basement membrane disorders and other skin blistering diseases.

Type VII collagen associated with the basement membrane of amniotic epithelium forms giant anchoring rivets which penetrate a massive lamina reticularis

In human amnion a simple cuboidal epithelium and underlying fibroblast layer are separated by an almost acellular compact layer rich in collagen types I and III. This (>10 μm) layer, which may be a thick lamina reticularis, apparently presents an unusual set of conditions. Integration of the multilaminous tissue across it is apparently achieved by waisted structures which we have observed with the light microscope in frozen, paraffin-wax and semi-thin resin sections. We have also captured transmission and scanning electron micrographs of the structures. These structures which cross the compact layer we call "rivets". The composition of these "rivets" has been examined immunocytochemically and in three dimensions using the confocal laser scanning epi-fluorescence microscope. The rivets contain type VII collagen and an α6 integrin. They associate with type IV collagen containing structures (basement membrane lamina densa and spongy coils) and a special population of fibroblasts which may generate, maintain or anchor rivets to the underlying mesenchymal layer. Although type VII collagen is well known to anchor basal lamina to underlying mesodermal collagen fibres these "rivets" are an order of magnitude larger than any previously described type VII collagen containing anchoring structures. Intriguing possible functions of these features include nodes for growth of fibrous collagen sheets and sites of possible enzymatic degradation during regulated amnion weakening approaching term. If these sites are confirmed to be involved in amnion degradation and growth they may represent important targets for therapeutic agents that are designed to delay preterm premature rupture of the membranes a major cause of fetal morbidity and mortality.

Folding delay and structural perturbations caused by type IV collagen natural interruptions and nearby Gly missense mutations

The standard collagen triple helix requires Gly as every third residue in the amino acid sequence, yet all nonfibrillar collagens contain sites where this repeating pattern is interrupted. To explore the effects of such natural interruptions on the triple helix, a 4- or 15-residue sequence from human basement membrane type IV collagen was introduced between (Gly-Xaa-Yaa)(n) domains within a recombinant bacterial collagen. The interruptions had little effect on melting temperature, consistent with the high thermal stability reported for nonfibrillar collagens. Although the 4-residue interruption cannot be accommodated within a standard triple helix, trypsin and thermolysin resistance indicated a tightly packed structure. Central residues of the 15-residue interruption were protease-susceptible, whereas residues near the (Gly-Xaa-Yaa)(n) boundary were resistant, supporting a transition from an alternate conformation to a well packed triple helix. Both interruptions led to a delay in triple-helix folding, with the 15-residue interruption causing slower folding than the 4-residue interruption. These results suggest that propagation through interruptions represents a slow folding step. To clarify the relation between natural interruptions and pathological mutations, a Gly to Ser missense mutation was placed three triplets away from the 4-residue interruption. As a result of this mutation, the 4-residue interruption and nearby triple helix became susceptible to protease digestion, and an additional folding delay was observed. Because Gly missense mutations that cause disease are often located near natural interruptions, structural and folding perturbations arising from such proximity could be a factor in collagen genetic diseases.

Molecular organization of the basement membrane zone

The dermal-epidermal basement membrane is a complex assembly of proteins that provide adhesion and regulate many important processes such as development, wound healing, and cancer progression. This contribution focuses on the structure and function of individual components of the basement membrane, how they assemble together, and how they participate in human tissues and diseases, with an emphasis on skin involvement. Understanding the composition and structure of the basement membrane provides insight into the pathophysiology of inherited blistering disorders, such as epidermolysis bullosa, and acquired bullous diseases, such as the pemphigoid group of autoimmune diseases and epidermolysis bullosa acquisita.

Interruptions in the collagen repeating tripeptide pattern can promote supramolecular association

The standard collagen triple-helix requires a perfect (Gly-Xaa-Yaa)(n) sequence, yet all nonfibrillar collagens contain interruptions in this tripeptide repeating pattern. Defining the structural consequences of disruptions in the sequence pattern may shed light on the biological role of sequence interruptions, which have been suggested to play a role in molecular flexibility, collagen degradation, and ligand binding. Previous studies on model peptides with 1- and 4-residue interruptions showed a localized perturbation within the triple-helix, and this work is extended to introduce natural collagen interruptions up to nine residue in length within a fixed (Gly-Pro-Hyp)(n) peptide context. All peptides in this set show decreases in triple-helix content and stability, with greater conformational perturbations for the interruptions longer than five residue. The most stable and least perturbed structure is seen for the 5-residue interruption peptide, whose sequence corresponds to a Gly to Ala missense mutation, such as those leading to collagen genetic diseases. The triple-helix peptides containing 8- and 9-residue interruptions exhibit a strong propensity for self-association to fibrous structures. In addition, a small peptide modeling only the 9-residue sequence within the interruption aggregates to form amyloid-like fibrils with antiparallel beta-sheet structure. The 8- and 9-residue interruption sequences studied here are predicted to have significant cross-beta aggregation potential, and a similar propensity is reported for approximately 10% of other naturally occurring interruptions. The presence of amyloidogenic sequences within or between triple-helix domains may play a role in molecular association to normal tissue structures and could participate in observed interactions between collagen and amyloid.

Role of dermal-epidermal basement membrane zone in skin, cancer, and developmental disorders

The dermal-epidermal basement membrane zone is an important epithelial and stromal interface, consisting of an intricately organized collection of intracellular, transmembrane, and extracellular matrix proteins. The basement membrane zone has several main functions including acting as a permeability barrier, forming an adhesive interface between epithelial cells and the underlying matrix, and controlling cellular organization and differentiation. This article identifies key molecular players of the dermal-epidermal membrane zone, and highlights recent research studies that have identified structural and functional roles of these components in the context of various blistering, neoplastic, and developmental syndromes.

Bone marrow cell transfer into fetal circulation can ameliorate genetic skin diseases by providing fibroblasts to the skin and inducing immune tolerance

Recent studies have shown that skin injury recruits bone marrow-derived fibroblasts (BMDFs) to the site of injury to accelerate tissue repair. However, whether uninjured skin can recruit BMDFs to maintain skin homeostasis remains uncertain. Here, we investigated the appearance of BMDFs in normal mouse skin after embryonic bone marrow cell transplantation (E-BMT) with green fluorescent protein-transgenic bone marrow cells (GFP-BMCs) via the vitelline vein, which traverses the uterine wall and is connected to the fetal circulation. At 12 weeks of age, mice treated with E-BMT were observed to have successful engraftment of GFP-BMCs in hematopoietic tissues accompanied by induction of immune tolerance against GFP. We then investigated BMDFs in the skin of the same mice without prior injury and found that a significant number of BMDFs, which generate matrix proteins both in vitro and in vivo, were recruited and maintained after birth. Next, we performed E-BMT in a dystrophic epidermolysis bullosa mouse model (col7a1(-/-)) lacking type VII collagen in the cutaneous basement membrane zone. E-BMT significantly ameliorated the severity of the dystrophic epidermolysis bullosa phenotype in neonatal mice. Type VII collagen was deposited primarily in the follicular basement membrane zone in the vicinity of the BMDFs. Thus, gene therapy using E-BMT into the fetal circulation may offer a potential treatment option to ameliorate genetic skin diseases that are characterized by fibroblast dysfunction through the introduction of immune-tolerated BMDFs.

Supramolecular interactions in the dermo-epidermal junction zone: anchoring fibril-collagen VII tightly binds to banded collagen fibrils

The dermis and the epidermis of normal human skin are functionally separated by a basement membrane but, together, form a stable structural continuum. Anchoring fibrils reinforce this connection by insertion into the basement membrane and by intercalation with banded collagen fibrils of the papillary dermis. Structural abnormalities in collagen VII, the major molecular constituent of anchoring fibrils, lead to a congenital skin fragility condition, dystrophic epidermolysis bullosa, associated with skin blistering. Here, we characterized the molecular basis of the interactions between anchoring fibrils and banded collagen fibrils. Suprastructural fragments of the dermo-epidermal junction zone were generated by mechanical disruption and by separation with magnetic Immunobeads. Anchoring fibrils were tightly attached to banded collagen fibrils. In vitro binding studies demonstrated that a von Willebrand factor A-like motif in collagen VII was essential for binding of anchoring fibrils to reconstituted collagen I fibrils. Since collagen I and VII molecules reportedly undergo only weak interactions, the attachment of anchoring fibrils to collagen fibrils depends on supramolecular organization of their constituents. This complex is stabilized in situ and resists dissociation by strong denaturants.

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.

Single Amino Acid Substitutions in Procollagen VII Affect Early Stages of Assembly of Anchoring Fibrils

Procollagen VII is a homotrimer of 350-kDa pro-alpha1(VII) chains, each consisting of a central collagenous domain flanked by the noncollagenous N-terminal NC1 domain and the C-terminal NC2 domain. After secretion from cells, procollagen VII molecules form anti-parallel dimers with a C-terminal 60-nm overlap. Characteristic alignment of procollagen VII monomers forming a dimer depends on site-specific binding between the NC2 domain and the triple-helical region adjacent to Cys-2634 of the interacting procollagen VII molecules. Formation of the intermolecular disulfide bonds between Cys-2634 and either Cys-2802 or Cys-2804 is promoted by the cleavage of the NC2 domain by procollagen C-proteinase. By employing recombinant procollagen VII variants harboring G2575R, R2622Q, or G2623C substitutions previously disclosed in patients with dystrophic epidermolysis bullosa, we studied how these amino acid substitutions affect intermolecular interactions. Binding assays utilizing an optical biosensor demonstrated that the G2575R substitution increased affinity between mutant molecules. In contrast, homotypic binding between the R2622Q or G2623C molecules was not detected. In addition, kinetics of heterotypic binding of all analyzed mutants to wild type collagen VII were different from those for binding between wild type molecules. Moreover, solid-state binding assays demonstrated that R2622Q and G2623C substitutions prevent formation of stable assemblies of procollagen C-proteinase-processed mutants. These results indicate that single amino acid substitutions in procollagen VII alter its self-assembly and provide a basis for understanding the pathomechanisms leading from mutations in the COL7A1 gene to fragility of the dermal-epidermal junction seen in patients with dystrophic forms of epidermolysis bullosa.

Stretching type II collagen with optical tweezers

Collagens are the most abundant proteins of vertebrates and they provide mechanical and supportive functions in a wide range of connective tissues. Knowledge of the mechanical properties of single collagen molecules is essential in studying the self-assembly of collagen, the interaction between cells and extracellular matrix, the etiology of tissue degeneration and mechanism of regeneration, and the relationship between the structures and mechanical properties of tissues. Here we stretched single type II collagen molecules in neutral pH solution using optical tweezers. The molecular parameters of collagen were obtained by fitting force-extension curves into worm-like chain elasticity model. The molecule length of type II collagen monomer was 295.8 nm. The persistence length of type II collagen monomer was 11.2 nm. These observations indicate that collagen molecules are flexible rather than rigid rod molecules at neutral pH solution.

Procollagen VII self-assembly depends on site-specific interactions and is promoted by cleavage of the NC2 domain with procollagen C-proteinase

Procollagen VII is a homotrimer of 350-kDa proalpha1(VII) chains. Each chain has a central collagenous domain flanked by a noncollagenous amino-terminal NC1 domain and a carboxy-terminal NC2 domain. After secretion from cells, procollagen VII molecules form antiparallel dimers with a 60 nm overlap. These dimers are stabilized by disulfide bonds formed between cysteines present in the NC2 domain and cysteines present in the triple-helical domain. Electron microscopy has provided direct evidence for the existence of collagen VII dimers, but the dynamic process of dimer formation is not well understood. In the present study, we tested the hypothesis that, during dimer formation, the NC2 domain of one procollagen VII molecule specifically recognizes and binds to the triple-helical region adjacent to Cys-2625 of another procollagen VII molecule. We also investigated the role of processing of the NC2 domain by the procollagen C-proteinase/BMP-1 in dimer assembly. We engineered mini mouse procollagen VII variants consisting of intact NC1 and NC2 domains and a shortened triple helix in which the C-terminal region encompassing Cys-2625 was either preserved or substituted with the region encompassing Cys-1448 derived from the N-terminal part of the triple-helical domain. The results indicate that procollagen VII self-assembly depends on site-specific interactions between the NC2 domain and the triple-helical region adjacent to Cys-2625 and that this process is promoted by the cleavage of the NC2 by procollagen C-proteinase/BMP1.

Immunomapping of EBA sera to multiple epitopes on collagen VII: further evidence that anchoring fibrils originate and terminate in the lamina densa

Epidermolysis bullosa acquisita (EBA) is an autoimmune blistering disease with circulating antibodies to type VII collagen, a major component of anchoring fibrils located at the dermal-epidermal junction. The purpose of this study was to further confirm the ultrastructural organisation of anchoring fibrils and to assess the relationship between the clinical phenotype of EBA and target site of their autoantibodies on anchoring fibrils. We studied the ultrastructural binding site of circulating autoantibodies from two patients with atypical clinical features who predominantly presented with oral lesions, and compared this with two patients with clinically typical forms of EBA. Immunoblotting of whole dermal extracts showed labelling of 290-kDa bands consistent with that of type VII collagen as well as the non-collagenous (NC-1) domain fusion protein in three out of four patients' sera. Postembedding immunoelectron microscopy (IEM) using Lowicryl K11M embedded normal human skin and patients' sera demonstrated the majority of labelling within the lamina densa, not below the lamina densa. We conclude that EBA autoantibodies in these patient's sera bind to the NC-1 domain of collagen VII situated in the lamina densa of the epidermal basement membrane, regardless of the EBA clinical phenotype. This confirms the previous notion that anchoring fibrils originate and terminate in the lamina densa.

Direct quantification of the flexibility of type I collagen monomer

Collagens are the most abundant structural proteins found in the extracellular matrix of vertebrates. Knowledge of the mechanical behavior of collagen monomers is essential for understanding the mechanical properties of collagen fibrils that constitute the main architectural framework of skin, bone, cartilage, and other connective tissues. In this study, the flexibility of type I collagen monomer was studied by stretching type I collagen monomers directly. The force-extension relationship was measured and analyzed by fitting the data into a worm-like chain elasticity model. The persistence length of collagen I monomer was determined to be 14.5 nm and the contour length was 309 nm. The results confirm that type I collagen monomer is flexible rather than rigid, rod-like molecule. Such flexibility may possibly be a consequence of the micro-unfolding of discrete domains of single collagen molecule.

Collagen VII expression in glomerular sclerosis

Glomerular sclerosis is the final stage of a variety of kidney diseases and matrix molecules not normally expressed in the extracellular matrix are synthesized and accumulate during the sclerotic process. Collagen type VII is the major component of the anchoring fibrils at the dermal-epidermal junction, but it is usually not present in normal glomeruli. The aim of this study was to investigate whether this type of fibrillary collagen, different from types I and III, is expressed in chronically diseased glomerular extracellular matrix. The presence and distribution of collagen VII have been examined in 50 renal biopsies by indirect immunofluorescence staining, standard electron microscopy, and immuno-electron microscopy. In selected cases, collagen VII mRNA expression was also measured by RT-PCR on isolated glomeruli. Cases included focal segmental glomerulosclerosis, minimal change disease, membranous glomerulonephritis, IgA nephropathy, SLE nephritis, diabetic glomerulosclerosis, ischaemic renal disease, extracapillary glomerulonephritis, and end-stage renal disease. Collagen VII protein and mRNA expression was absent or present in trace amounts in normal kidneys or in disorders with only a mild increase of mesangial matrix, without scarring of the tuft. Maximal expression was evident in the presence of adhesions between the glomerular tuft and Bowman's capsule or fibrous crescents. The results showed that collagen VII is actively synthesized and laid down in areas of glomerular and/or tubular scarring, irrespective of the underlying disease, confirming the de novo expression of fibrillary collagens in diseased renal extracellular matrix. The appearance of an anchoring collagen may be a response to support mechanical stress and it takes part in the process of cell proliferation and tissue repair.

The architecture of a collagen coating on a synthetic polymer influences epithelial adhesion

The current study sought to identify a collagen coating methodology for application to polymer surfaces that would provide for the development of adhesive structures responsible for the sustained adhesion of corneal epithelial tissue. We compared an uncoated microporous polycarbonate surface and equivalent surfaces coated with either covalently immobilized collagen I or chemically crosslinked collagen I gel in a corneal explant outgrowth assay over 21 days. Electron microscopy was used to examine the formation of hemidesmosomes, basal lamina, and anchoring fibrils at the tissue-polymer interface. The crosslinked collagen gel preparation supported the overlying epithelial tissue across the pore openings and allowed for the formation of identifiable basal lamina, hemidesmosomes, and anchoring fibrils between the epithelial tissue and the polymer surface. Hemidesmosomal plaque, but no basal lamina or anchoring fibril formation, occurred on the uncoated surface or on that coated with covalently immobilized collagen I. We propose that the collagen matrix provided by the crosslinked collagen gel was reorganized by the epithelial tissue and that this, combined with the secretion of ECM molecules, served to limit the diffusion of basement membrane components, which permitted an increase in the local concentration of these molecules, which favored the assembly of epithelial adhesive structures.

Basement membrane formation during wound healing is dependent on epidermal transplants

The purpose of the study was to compare directly the effect of healing and the formation of the basement membrane during wound healing from two autologous primary keratinocyte cultures in the liquid environment in full-thickness wounds in pigs. Wounds were either transplanted with cultured epidermal autografts (n = 26) or autologous keratinocyte suspensions (n = 24) or treated with saline alone (n = 40) and covered with a chamber. All wounds transplanted with cultured epidermal autografts and keratinocyte cell suspensions had positive "take" after transplantation. Healing times were significantly shorter for wounds treated with either cultured epidermal autografts or keratinocyte suspensions (p = 0.0001) compared with saline-treated wounds but were not different from each other (p = 0.1835). There were no differences in cytokeratin and laminin expression; however, staining with monoclonal antibody against collagen type VII showed a lower signal for cultured epidermal autografts only on days 8 and 16 compared with keratinocyte suspensions. Electron microscope evaluation showed a higher incidence of anchoring fibrils and a more mature dermal-epidermal junction in wounds treated with keratinocyte cell suspensions at day 8. These findings may be due to the single, noncontact-inhibited cells and the early formation of an in vivo neodermis to the wet wound environment. These data suggest that wounds transplanted with autologous keratinocyte suspensions in a wet environment may be an alternative method in the treatment of wounds.

Bullous systemic lupus erythematosus

A 19-year-old woman with a 6 month history of systemic lupus erythematosus (SLE) developed a widespread urticated, erythematous eruption associated with tense, fluid-filled blisters, erosions and crusting. Biopsy showed subepidermal blistering with a prominent neutrophilic infiltrate. Direct immunofluorescence showed markedly positive granular IgG deposition with weak IgM, IgA and C3 at the dermoepidermal junction. No circulating antibodies were detected on indirect immunofluorescence. A diagnosis of bullous systemic erythematosus was made. Treatment with prednisone was ineffective. Subsequent treatment with dapsone led to rapid sustained remission of skin symptoms. Bullous SLE is a rare manifestation of SLE. We review the recent literature and discuss the distinctive features of this condition and contrast them with cutaneous SLE with blisters and the subepidermal blistering disorders.

Alpha 2 beta 1 integrin mediates dermal fibroblast attachment to type VII collagen via a 158-amino-acid segment of the NC1 domain

Dermal fibroblasts are in apposition to type VII (anchoring fibril) collagen in both unwounded and wounded skin. The NC1 domain of type VII collagen contains multiple submodules with homology to known adhesive molecules, including fibronectin type III-like repeats and a potential RGD cell attachment site. We previously reported the structure and matrix binding properties of authentic and recombinant NC1. In this study, we examined the interaction between dermal fibroblasts and the NC1 domain of type VII collagen. We found that both recombinant and authentic NC1 vigorously promoted human fibroblast attachment. Adhesion of fibroblasts to NC1 was dose dependent, saturable, and abolished by both polyclonal and monoclonal antibodies to NC1. Cell adhesion to NC1 was divalent cation dependent and specifically inhibited by a monoclonal antibody directed against the alpha2 or beta1 integrin subunits, but not by the presence of RGD peptides. Furthermore, the cell-binding activity of NC1 was not conformation dependent, since heat-denatured NC1 still promoted cell adhesion. Using a series of recombinant NC1 deletion mutant proteins, the cell binding site of NC1 was mapped to a 158-aa (residues 202-360) subdomain. We conclude that human dermal fibroblasts interact with the NC1 domain of type VII collagen and this cell-matrix interaction is mediated by the alpha2beta1 integrin and is RGD independent.

Hereditary skin diseases of anchoring fibrils

Remarkable progress has been made in the last few years in understanding the functions of the anchoring fibrils, polymers of collagen VII, that connect the epidermal basement membrane with the dermal connective tissue. Novel insights into the biology of these fibrils have been gained from studies on dystrophic epidermolysis bullosa (DEB), a group of inherited blistering disorders caused by abnormalities of the anchoring fibrils. Mutations in the COL7A1 gene encoding collagen VII have been disclosed in a number of DEB families, and the mutation analyses and studies on genotype-phenotype correlations in DEB have revealed an unusual complexity of the gene defects and their biological consequences. In analogy to heritable disorders of other collagen genes, predictable phenotypes of COL7A1 mutations causing premature termination codons (PTC) or dominant negative interference have been observed. However, collagen VII seems to be unique among collagens in that many mutations lead to minimal phenotypes, or to no phenotype at all. Furthermore, the mild DEB phenotypes can be severely modulated by a second mutation in individuals compound heterozygous for two different COL7A1 defects. Therefore, not only definition of mutations with diagnostic analyses, but also cell biological, protein chemical and suprastructural studies of the mutated molecules are required for understanding the pathomechanisms underlying DEB.

NC1 domain of type VII collagen binds to the beta3 chain of laminin 5 via a unique subdomain within the fibronectin-like repeats

Type VII collagen, the major component of anchoring fibrils, consists of a central collagenous triple-helical domain flanked by two noncollagenous, globular domains, NC1 and NC2. Approximately 50% of the molecular mass of the molecule is consumed by the NC1 domain. We previously demonstrated that NC1 binds to various extracellular matrix components including a complex of laminin 5 and laminin 6 (Chen et al, 1997a). In this study, we examined the interaction of NC1 with laminin 5 (a component of anchoring filaments). Both authentic and purified recombinant NC1 bound to human and rat laminin 5 as measured by enzyme-linked immunosorbant assay and by binding of 125I-radiolabeled NC1 to laminin 5-coated wells, but not to laminin 1 or albumin. NC1 bound predominantly to the beta3 chain of laminin 5, but also to the gamma2 chain when examined by a protein overlay assay. The binding of 125I-NC1 to laminin 5 was inhibited by a 50-fold excess of unlabeled NC1 or de-glycosylated NC1, as well as a polyclonal antibody to laminin 5 or a monoclonal antibody to the beta3 chain. In contrast, the NC1-laminin 5 interaction was not affected by a monoclonal antibody to the alpha3 chain. Using NC1 deletion mutant recombinant proteins, a 285 AA (residues 760-1045) subdomain of NC1 was identified as the binding site for laminin 5. IgG from an epidermolysis bullosa acquisita serum containing autoantibodies to epitopes within NC1 that colocalized with the laminin 5 binding site inhibited the binding of NC1 to laminin 5. Thus, perturbation of the NC1-laminin 5 interaction may contribute to the pathogenesis of epidermolysis bullosa acquisita.

Biology of anchoring fibrils: lessons from dystrophic epidermolysis bullosa

Anchoring fibrils are adhesive suprastructures that ensure the connection of the epidermal basement membrane with the dermal extracellular matrix. The fibrils represent polymers of collagen VII, the major structural fibril component, but may also contain other proteins. Remarkable progress has been made in the last few years in understanding the functions of skin basement membrane components including the anchoring fibrils. Novel insights into the biology of the anchoring fibrils have been gained from experimental studies on dystrophic epidermolysis bullosa (DEB), a group of inherited blistering disorders caused by mutations in the gene for collagen VII, COL7A1. Mutation analyses of DEB families have disclosed more than 100 COL7A1 gene defects so far, but the unusual complexity of the mutation constellations and their biological consequences are only beginning to emerge. In analogy to heritable disorders of other collagen genes, predictable phenotypes of COL7A1 mutations causing premature termination codons or dominant negative interference have been observed. However, collagen VII seems to represent a remarkable exception among collagens in that many mutations, including heterozygous glycine substitutions and deletions, lead to minimal phenotypes, or to no phenotype at all. In contrast to fibrillar collagens, structural abnormalities of collagen VII molecules in anchoring fibrils appear to be tolerated to a certain extent. However, the mild DEB phenotypes can be severely modulated by a second aberration in individuals compound heterozygous for two different COL7A1 mutations. Therefore, not only definition of mutation(s) but also cell biological, protein chemical and suprastructural studies of the mutated molecules yield novel insight into the molecular pathomechanisms underlying disease.

Bullous systemic lupus erythematosus: detection of antibodies against noncollagenous domain of type VII collagen

A 9-year-old girl with systemic lupus erythematosus developed bullous eruption. Histopathologic examination showed subepidermal blistering and immunoglobulins (IgG, IgM, IgA) and complement components (C1q, C3) deposited linearly at the basement membrane zone. IgG in this patient's serum bound to the dermal side of 1 mol/L sodium chloride-split normal human skin, and a 290 kd protein was identified in the dermal extract. The patient's serum recognized various epitopes on the noncollagenous domain as demonstrated by the use of fusion proteins of type VII collagen. Resolution of the bullous eruption occurred after treatment with dapsone.

Laminin 5 binds the NC-1 domain of type VII collagen

Mutational analyses of genes that encode components of the anchoring complex underlying the basolateral surface of external epithelia indicate that this structure is the major element providing for resistance to external friction. Ultrastructurally, laminin 5 (alpha3beta3gamma2; a component of the anchoring filament) appears as a thin filament bridging the hemidesmosome with the anchoring fibrils. Laminin 5 binds the cell surface through hemidesmosomal integrin alpha6beta4. However, the interaction of laminin 5 with the anchoring fibril (type VII collagen) has not been elucidated. In this study we demonstrate that monomeric laminin 5 binds the NH2-terminal NC-1 domain of type VII collagen. The binding is dependent upon the native conformation of both laminin 5 and type VII collagen NC-1. Laminin 6 (alpha3beta1gamma1) has no detectable affinity for type VII collagen NC-1, indicating that the binding is mediated by the beta3 and/or gamma2 chains of laminin 5. Approximately half of the laminin 5 solubilized from human amnion or skin is covalently complexed with laminins 6 or 7 (alpha3beta2gamma1). The adduction occurs between the NH2 terminus of laminin 5 and the branch point of the short arms of laminins 6 or 7. The results are consistent with the presumed orientation of laminin 5, having the COOH-terminal G domain apposed to the hemidesmosomal integrin, and the NH2-terminal domains within the lamina densa. The results also support a model predicting that monomeric laminin 5 constitutes the anchoring filaments and bridges integrin alpha6beta4 with type VII collagen, and the laminin 5-6/7 complexes are present within the interhemidesmosomal spaces bound at least by integrin alpha3beta1 where they may mediate basement membrane assembly or stability, but contribute less significantly to epithelial friction resistance.

Interactions of the amino-terminal noncollagenous (NC1) domain of type VII collagen with extracellular matrix components. A potential role in epidermal-dermal adherence in human skin

Type VII collagen, the major component of anchoring fibrils, consists of a central collagenous triple-helical domain flanked by two noncollagenous domains, NC1 and NC2. The NC1 domain contains multiple submodules with homology to known adhesive molecules including fibronectin type III-like repeats and the A domain of von Willebrand factor. In this study, we produced the entire NC1 domain of human type VII collagen in the stably transfected human kidney 293 cell clones and purified large quantities of the recombinant NC1 protein from serum-free culture media. The recombinant NC1 formed interchain disulfide-bonded dimers and trimers and was N-linked glycosylated. Tunicamycin inhibited the cellular secretion of NC1, suggesting that N-linked glycosylation may play a role in NC1 secretion. The recombinant NC1 was indistinguishable from the authentic NC1 obtained from human amnions or WISH cells with respect to N-linked sugar content, electrophoretic mobility, rotary shadow imaging, and binding affinity to type IV collagen. Purified recombinant NC1, like authentic NC1, also bound specifically to fibronectin, collagen type I, and a laminin 5/6 complex. Both monomeric and trimeric forms of NC1 exhibited equal affinity for these extracellular matrix components, suggesting that the individual arms of NC1 can function independently. The multiple interactions of NC1 with other extracellular matrix components may support epidermal-dermal adhesion.

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.

Production of basement membrane components by a reconstructed epidermis cultured in the absence of serum and dermal factors

A fully differentiated epithelium displaying features of human epidermis was obtained in vitro by culturing second-passage normal human keratinocytes for 14 days in defined medium and on an inert polycarbonate filter substratum at the air-liquid interface. Vertical sections stained for histology and indirect immunofluorescence studies showed that the 'basal' cells synthesize and secrete all major markers of hemidesmosomes and the lamina lucida. Components of the lamina densa are also expressed. Collagen VII is synthesized, but not secreted. Ultrastructural studies showed the presence of hemidesmosomes with major dense plaques and anchoring filaments, and a basement membrane-like structure was clearly identified. These results show that epidermal cells are able to produce hemidesmosomes and to secrete the major components of the dermo-epidermal junction in the absence of serum and dermal factors, suggesting that basement membrane synthesis and hemidesmosome assembly are not dependent on the presence of dermis.

Type VII collagen, anchoring fibrils, and epidermolysis bullosa

The anchoring fibrils at the dermal-epidermal junction have been well characterized as ultrastructural entities. From their appearance, it was proposed that they fortified the attachment of the epidermis to the dermis. This hypothesized function was strengthened by observations indicating that the anchoring fibrils were abnormal, diminished, or absent from individuals with dystrophic epidermolysis bullosa. Therefore, characterization of the molecular constituents of the anchoring fibrils and their interactions with other basement membrane and dermal components might lead to identification of the gene defects underlying at least some forms of epidermolysis bullosa. Type VII collagen was identified as the protein component of anchoring fibrils in 1986. Since then, the major characteristics of the molecule have been described. These are consistent with a model wherein secreted type VII collagen molecules form disulfide-bond stabilized antiparallel dimers. The dimers then condense laterally into unstaggered arrays that are the anchoring fibrils. This arrangement allows for the protrusion of large globular domains (NC-1) from both ends of the fibrils. The aggregated triple-helical domains extend into the papillary dermis and entrap fibrous dermal components. The NC-1 domains are believed to interact with components of the basement membrane and thus to mediate the attachment of the basement membrane to the dermis. This model predicts that mutations in the type VII collagen gene that prevent the secretion of the molecule will be the most devastating, whereas mutations in the regions encoding the globular domains may show more variable phenotype. Ultimately, understanding the function of type VII collagen at the molecular level will be the key to devising strategies to moderate the pathophysiology of dystrophic epidermolysis bullosa.

Autoantibodies to type VII collagen recognize epitopes in a fibronectin-like region of the noncollagenous (NC1) domain

Autoantibodies to type VII collagen are characteristic of the blistering diseases epidermolysis bullosa acquisita and bullous systemic lupus erythematosus (SLE). Blisters in those diseases are due to defective adhesion of the lamina densa subregion of the epithelial basement membrane to the underlying dermis. Previous studies indicating that type VII collagen contributes to lamina densa-dermal adhesion by cross-linking lamina densa and dermal matrix proteins suggests that autoantibodies may contribute to blisters by interfering with type VII collagen function. That hypothesis is supported by previous studies showing autoantibodies from a small number of epidermolysis bullosa acquisita patients recognize proteolytic fragments containing the 145-kD noncollagenous domain of type VII collagen. In this study, we examined reactivity of autoantibodies from a large number of epidermolysis bullosa acquisita and bullous SLE patients with fusion proteins representing most of the noncollagenous domain of type VII collagen and that those regions are homologous to type III repeats of fibronectin. These results suggest autoantibodies binding to fibronectin homology regions within the 145-kD noncollagenous domain may interfere with the adhesion function of type VII collagen and contribute to lamina densa-dermal dysadhesion in epidermolysis bullous acquisita and bullous SLE.

Noncollagenous (NC1) domain of collagen VII resembles multidomain adhesion proteins involved in tissue-specific organization of extracellular matrix

Type VII collagen (C7) is a stratified squamous epithelial basement membrane protein composed of three identical alpha chains, each consisting of a 145-kDa amino-terminal noncollagenous (NC1) domain and a 145-kDa carboxyl-terminal collagenous domain. Morphologic and biochemical studies have shown that tissue-specific aggregates of C7 dimers called anchoring fibrils may contribute to epithelial basement membrane organization and adherence by interacting with extracellular matrix (ECM) proteins such as type IV collagen. In this study, we cloned a cDNA encoding most of the NC1 domain of C7. The deduced amino acid sequence revealed motifs characteristic of multidomain ECM proteins that contribute to the tissue-specific organization of ECM including a region of 7 1/2 sequential fibronectin type III (Fn III) homology repeats, a potential collagen-binding region homologous to the A domain of von Willebrand factor (vWf) and an RGD sequence. A purified C7 fusion protein containing these motifs specifically bound to type IV collagen in a functional assay. These results suggest that regions within the NC1 domain of C7 mediate interactions with lamina densa and dermal ECM proteins including type IV collagen. Structural mutations and autoepitopes in these regions may represent mechanisms for the development of defective basement membrane organization and adherence in genetic and autoimmune forms of epidermolysis bullosa.

Molecular biology and pathology of type VII collagen*

Type VII collagen is a genetically distinct member of the collagen family of proteins. Type VII collagen has been shown to be the major component of anchoring fibrils, attachment complexes which secure the cutaneous basement membrane of the skin to the underlying dermis. Understanding of the structure of type VII collagen has been advanced by recent cloning of the corresponding gene. Chromosomal mapping of the gene to the short arm of chromosome 3 and identification of intragenic polymorphic markers have allowed demonstration of strong genetic linkage between the type VII collagen locus and the dystrophic forms of EB (epidermolysis bullosa). This overview summarizes the progress made in the molecular genetics of type VII collagen.

Basement membrane proteins: structure, assembly, and cellular interactions

Basement membranes are thin layers of a specialized extracellular matrix that form the supporting structure on which epithelial and endothelial cells grow, and that surround muscle and fat cells and the Schwann cells of peripheral nerves. One common denominator is that they are always in close apposition to cells, and it has been well demonstrated that basement membranes do not only provide a mechanical support and divide tissues into compartments, but also influence cellular behavior. The major molecular constituents of basement membranes are collagen IV, laminin-entactin/nidogen complexes, and proteoglycans. Collagen IV provides a scaffold for the other structural macromolecules by forming a network via interactions between specialized N- and C-terminal domains. Laminin-entactin/nidogen complexes self-associate into less-ordered aggregates. These two molecular assemblies appear to be interconnected, presumably via binding sites on the entactin/nidogen molecule. In addition, proteoglycans are anchored into the membrane by an unknown mechanism, providing clusters of negatively charged groups. Specialization of different basement membranes is achieved through the presence of tissue-specific isoforms of laminin and collagen IV and of particular proteoglycan populations, by differences in assembly between different membranes, and by the presence of accessory proteins in some specialized basement membranes. Many cellular responses to basement membrane proteins are mediated by members of the integrin class of transmembrane receptors. On the intracellular side some of these signals are transmitted to the cytoskeleton, and result in an influence on cellular behavior with respect to adhesion, shape, migration, proliferation, and differentiation. Phosphorylation of integrins plays a role in modulating their activity, and they may therefore be a part of a more complex signaling system.