The molecular genetics of collagen

In their Bioessays review ‘Current views of collagen degradation’, Gillian Murphy and John Reynolds gave an outline of the molecular structure of the members of the collagen family and described their traditional role in providing stable tissue frameworks.¹ This short review considers the relationship between the different members of that family and what gene structure reveals about their evolution. Mutation of the collagen structural genes has been discovered in patients suffering from brittle-bone syndrome and other inherited connective tissue disorders, and here I attempt to rationalize these results into an overall concept of collagen gene mutation and evolution.     

Chromosomal assignment of a gene encoding a new collagen type (COL15A1) to 9q21 --> q22.

The collagens constitute a large family of extracellular matrix components primarily responsible for maintaining the structure and biological integrity of connective tissue. These proteins exhibit considerable diversity size, sequence, tissue distribution, and molecular composition. Fourteen types of homo- and/or heterotrimeric molecules, thus far reported, are encoded by a minimum of 27 genes. Nineteen of these genes, including several that are closely linked, have been assigned to 10 separate autosomes, and one collagen gene has been mapped to the X chromosome. We have isolated a 2.1-kb human cDNA clone coding for a collagen molecule different in sequence and structure from types I-XIV collagens. This polypeptide has been designated the alpha 1 chain of type XV collagen. To determine the location of the corresponding gene, the cDNA clone was hybridized to rodent-human hybrid DNAs and to human metaphase chromosomes. The results obtained using the hybrid cell lines showed that this newly identified collagen gene, COL15A1, is present in the pter --> q34 region of chromosome 9. In situ hybridization allowed sublocalization to 9q21 --> q22, a region to which no other collagen genes had previously been assigned. Our data further demonstrate the complex arrangement of the many collagen genes in the human genome.     

Effect of silicoorganic compounds on protein biosynthesis in granulation-fibrous tissue]

The authors studied the effect of propoxysilatran (POS) on the biosynthesis of the main connective tissue biopolymeres. The use of POS in the form of 0.5 and 2.0% ointments on lanolin-vaseline base caused stimulation of cellular proliferation in the granulation fibrous tissue developing in the open skin defects of albino rats. Stimulation of cellular proliferation in these animals was accompanied by increase of collagen biosynthesis and of noncollagen proteins. In concentrations of 10(-3)--10(-4) M POS caused intensification of collagen biosynthesis (formation of peptide-bound nondialyzed 14C-oxyproline) in vitro in the cartilage tissue of chick embryos. Thus, silatrans are biologically-active substances producing a regulating effect on the course of the reparative-proliferative processes in the connective tissue.     

Signaling pathways affected by mutations causing osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous connective tissue disorder characterized by bone fragility and skeletal deformity. To maintain skeletal strength and integrity, bone undergoes constant remodeling of its extracellular matrix (ECM) tightly controlled by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. There are at least 20 recognized OI-forms caused by mutations in the two collagen type I-encoding genes or genes implicated in collagen folding, posttranslational modifications or secretion of collagen, osteoblast differentiation and function, or bone mineralization. The underlying disease mechanisms of non-classical forms of OI that are not caused by collagen type I mutations are not yet completely understood, but an altered ECM structure as well as disturbed intracellular homeostasis seem to be the main defects. The ECM orchestrates local cell behavior in part by regulating bioavailability of signaling molecules through sequestration, release and activation during the constant bone remodeling process.Here, we provide an overview of signaling pathways that are associated with known OI-causing genes and discuss the impact of these genes on signal transduction. These pathways include WNT-, RANK/RANKL-, TGFβ-, MAPK- and integrin-mediated signaling as well as the unfolded protein response.     

Ultrastructural changes in the intestinal connective tissue of Xenopus laevis during metamorphosis

Ultrastructural changes in the intestinal connective tissue of Xenopus laevis during metamorphosis have been studied. Throughout the larval period to stage 60, the connective tissue consists of a few immature fibroblasts surrounded by a sparse extracellular matrix: few collagen fibrils are visible except close to the thin basal lamina. At the beginning of the transition from larval to adult epithelial form around stage 60, extensive changes are observed in connective tissue. The cells become more numerous and different types appear as the collagen fibrils increase in number and density. Through gaps in the thickened and extensively folded basal lamina, frequent contacts between epithelial and connective tissue cells are established. Thereafter, with the progression of fold formation, the connective tissue cells become oriented according to their position relative to the fold structure. The basal lamina beneath the adult epithelium becomes thin after stage 62, while that beneath the larval epithelium remains thick. Upon the completion of metamorphosis, the connective tissue consists mainly of typical fibroblasts with definite orientation and numerous collagen fibrils. These observations indicate that developmental changes in the connective tissue, especially in the region close to the epithelium, are closely related spatiotemporarily to the transition from the larval to the adult epithelial form. This suggests that tissue interactions between the connective tissue and the epithelium play important roles in controlling the epithelial degeneration, proliferation, and differentiation during metamorphic climax.     

Bacterial adhesion to animal tissues: protein determinants for recognition of extracellular matrix components

The extracellular matrix (ECM) is present within all animal tissues and organs. Actually, it surrounds the eukaryotic cells composing the four basic tissue types, i.e. epithelial, muscle, nerve and connective. ECM does not solely refer to connective tissue but composes all tissues where its composition, structure and organization vary from one tissue to another. Constituted of the four main fibrous proteins, i.e. collagen, fibronectin, laminin and elastin, ECM components form a highly structured and functional network via specific interactions. From the basement membrane to interstitial matrix, further heterogeneity exists in the organization of the ECM in various tissues and organs also depending on their physiological state. Back to a molecular level, bacterial proteins represent the most significant part of the microbial surface components recognizing adhesive matrix molecules (MSCRAMM). These cell surface proteins are secreted and localized differently in monoderm and diderm–LPS bacteria. While one collagen‐binding domain (CBD) and different fibronectin‐binding domains (FBD1 to 8) have been registered in databases, much remains to be learned on specific binding to other ECM proteins via single or supramolecular protein structures. Besides theinteraction of bacterial proteins with individual ECM components, this review aims at stressing the importance of fully considering the ECM at supramolecular, cellular, tissue and organ levels. This conceptual view should not be overlooked to rigorously comprehend the physiology of bacterial interaction from commensal to pathogenic species.     

Immunohistochemical study of types I, III and IV collagen in diseased human gingiva of patients with rapidly progressive periodontitis: a light and electron microscopic study

The distribution of type I, III and IV collagens and their ultrastructural organization have been studied in diseased gingival connective tissue of patients with rapidly progressive periodontitis. This disease is characterized by acute destruction of the gingival collagenous components. The use of an immunofluorescent procedure has shown that the diseased connective tissue was made up of both type I and III collagens but that type III collagen was less resistant to acute inflammation. Ultrastructural immunolabelling, using the peroxidase procedure has shown that the large, dense bundles of type I collagen of PI, the main pattern of organization of the gingival connective tissue offered a better resistance to acute destruction than PII, a loose pattern of organization mainly composed of type III collagen. Type IV collagen was exclusively located in degraded lamina densa of basement membrane.     

Functional role of periostin in development and wound repair: implications for connective tissue disease

Integrity of the extracellular matrix (ECM) is essential for maintaining the normal structure and function of connective tissues. ECM is secreted locally by cells and organized into a complex meshwork providing physical support to cells, tissues, and organs. Initially thought to act only as a scaffold, the ECM is now known to provide a myriad of signals to cells regulating all aspects of their phenotype from morphology to differentiation. Matricellular proteins are a class of ECM related molecules defined through their ability to modulate cell-matrix interactions. Matricellular proteins are expressed at high levels during development, but typically only appear in postnatal tissue in wound repair or disease, where their levels increase substantially. Members of the CCN family, tenascin-C, osteopontin, secreted protein acidic rich in cysteine (SPARC), bone sialoprotein, thrombospondins, and galectins have all been classed as matricellular proteins. Periostin, a 90 kDa secreted homophilic cell adhesion protein, was recently added to matricellular class of proteins based on its expression pattern and function during development as well as in wound repair. Periostin is expressed in connective tissues including the periodontal ligament, tendons, skin and bone, and is also prominent in neoplastic tissues, cardiovascular disease, as well as in connective tissue wound repair. This review will focus on the functional role of periostin in tissue physiology. Fundamentally, it appears that periostin influences cell behaviour as well as collagen fibrillogenesis, and therefore exerts control over the structural and functional properties of connective tissues in both health and disease. Periostin is a novel matricellular protein with close homology to Drosophila fasciclin 1. In this review, the functional role of periostin is discussed in the context of connective tissue physiology, in development, disease, and wound repair.     

The mouse alpha 1(XII) and human alpha 1(XII)-like collagen genes are localized on mouse chromosome 9 and human chromosome 6.

Type XII collagen is a member of the FACIT (fibril-associated collagens with interrupted triple helices) group of extracellular matrix proteins. Like the other members of this group, collagen types IX and XIV, type XII has alternating triple-helical and non-triple-helical domains. Because of its structure, its association with collagen fibrils, and its distribution in dense connective tissues, type XII is thought possibly to act as a cross-bridge between fibrils and resist shear forces caused by tension. A portion of the ffuse gene was isolated by screening a genomic library with a chicken alpha 1 (XII) cDNA probe, followed by subcloning and sequence analysis. Comparison of exon sequences with the sequence of a mouse cDNA clone allowed the mouse gene to be identified as the alpha 1 (XII) collagen gene. In the mouse, Col12a1 is located on chromosome 9, as determined by linkage analysis using DNA from interspecific backcrosses with Mus spretus. Screening of a human genomic library also allowed the isolation of a human alpha 1(XII)-like gene (CoL12A1). This gene was mapped to chromosome 6 by blot hybridization to DNA from human/hamster hybrid cell lines. This information should prove useful in determining the role of type XII collagen genes as candidate genes in inheritable connective tissue diseases.     

Interactions of heat shock protein 47 with collagen and the stress response: an unconventional chaperone model?

Heat shock proteins (HSPs) are upregulated and manifested upon cellular stress and possess chaperoning functions. HSP47 is an endoplasmic reticulum (ER)-resident, collagen-specific chaperone and plays a key role in collagen biosynthesis and its structural assembly. The collagen scaffold is a primary structural target of recent interest due to its applications in tissue engineering and drug delivery and in treatment of clinical disorders. This review highlights the fundamental aspects of HSPs in protein folding and quality control, in the elicitation of a stress response in connective tissue and in the characterization of HSP47 in collagen folding and assembly. The significant features of HSP47 which are distinct in its cellular capabilities are discussed. We propose that targeting the stress response is a key factor in identifying connective tissue biomarkers. We also address the issues and strategies involved in the stress response of connective tissue diseases. In conclusion, we describe the prospects of collagen biochemistry in correlation to the science of HSPs.     

Particle agglutination assays to identify fibronectin and collagen cell surface receptors and lectins in Aeromonas and Vibrio species

A rapid particle agglutination assay (PAA) utilizing latex beads coated with connective tissue and serum proteins was evaluated for its ability to identify fibronectin, collagen (types I and IV), fibrinogen, and transferrin cell surface receptors on Vibrio and Aeromonas strains isolated from diseased fish, human infections, and the environment. Similar tests were performed to screen for cell surface lectins. Vibrio as well as Aeromonas strains were found to bind connective tissue proteins (collagen types I, II, and IV and fibronectin), serum proteins (i.e., fibrinogen), and glycoproteins (bovine submaxillary mucin, hog gastric mucin, orosomucoid, and fetuin) immobilized on the latex particles. The specificity of the agglutination reaction was studied by particle agglutination inhibition assays performed by preincubating bacterial suspensions in solutions containing either gelatin (for the various connective tissue protein PAA reagents) or sialic acid-rich glycoproteins (for the various glycoprotein PAA reagents). Expression of cell surface receptors for connective tissue proteins was found to depend on culture methods.     

Mechanical model for a collagen fibril pair in extracellular matrix

In this paper, we model the mechanics of a collagen pair in the connective tissue extracellular matrix that exists in abundance throughout animals, including the human body. This connective tissue comprises repeated units of two main structures, namely collagens as well as axial, parallel and regular anionic glycosaminoglycan between collagens. The collagen fibril can be modeled by Hooke’s law whereas anionic glycosaminoglycan behaves more like a rubber-band rod and as such can be better modeled by the worm-like chain model. While both computer simulations and continuum mechanics models have been investigated for the behavior of this connective tissue typically, authors either assume a simple form of the molecular potential energy or entirely ignore the microscopic structure of the connective tissue. Here, we apply basic physical methodologies and simple applied mathematical modeling techniques to describe the collagen pair quantitatively. We found that the growth of fibrils was intimately related to the maximum length of the anionic glycosaminoglycan and the relative displacement of two adjacent fibrils, which in return was closely related to the effectiveness of anionic glycosaminoglycan in transmitting forces between fibrils. These reveal the importance of the anionic glycosaminoglycan in maintaining the structural shape of the connective tissue extracellular matrix and eventually the shape modulus of human tissues. We also found that some macroscopic properties, like the maximum molecular energy and the breaking fraction of the collagen, were also related to the microscopic characteristics of the anionic glycosaminoglycan.     

Particle agglutination assays to identify fibronectin and collagen cell surface receptors and lectins in Aeromonas and Vibrio species.

A rapid particle agglutination assay (PAA) utilizing latex beads coated with connective tissue and serum proteins was evaluated for its ability to identify fibronectin, collagen (types I and IV), fibrinogen, and transferrin cell surface receptors on Vibrio and Aeromonas strains isolated from diseased fish, human infections, and the environment. Similar tests were performed to screen for cell surface lectins. Vibrio as well as Aeromonas strains were found to bind connective tissue proteins (collagen types I, II, and IV and fibronectin), serum proteins (i.e., fibrinogen), and glycoproteins (bovine submaxillary mucin, hog gastric mucin, orosomucoid, and fetuin) immobilized on the latex particles. The specificity of the agglutination reaction was studied by particle agglutination inhibition assays performed by preincubating bacterial suspensions in solutions containing either gelatin (for the various connective tissue protein PAA reagents) or sialic acid-rich glycoproteins (for the various glycoprotein PAA reagents). Expression of cell surface receptors for connective tissue proteins was found to depend on culture methods.     

Abnormal deposition of basement membrane and connective tissue components in dimethylbenzanthracene-induced rat mammary tumours: an immunocytochemical and ultrastructural study

Summary Dimethylbenzanthracene-induced rat mammary tumours consist of lobules of tumours cells surrounded by connective tissue. The interstitial connective tissue proteins, collagen types I, III and V, fibronectin and elastin are largely restricted to the interlobular connective tissue. The tumour lobules are surrounded by a basement membrane that stains with antiserum to laminin. Electron microscopy reveals a greatly thickened basement membrane to which striated interstitial collagen fibres are closely juxtaposed. The lumina within the tumour lobules are of two types. In the first type, the luminal surface is characterized by the presence of microvilli and tight junctions are reacts with antiserum to rat milk fat globule membrane. In the second type, the luminal surface is flattened and lined by a thickened basement membrane that stains with antiserum to laminin and type IV collagen. These abnormal patterns of growth and differentiation may be partly a consequence of the disorganization of extracellular matrix components at the interface between the tumour epithelial cells and the surrounding stroma.     

Ultrastructural and cytochemical studies on the connective tissue of chaetognaths

The hydroskeleton plays a central role in the architecture of the trunk of the Chaetognath. Its fibrous part is composed by a ‘basement membrane’ which separates the epithelial and nervous level from the locomotory muscle and other tissues which surround the general cavity. This structure corresponds to a dense connective tissue sheath; together with the aqueous phase of the general cavity it constitutes the main part of the hydroskeleton. The axes of the lateral and caudal fins are extensions of this connective tissue; they are rich in ground substance and contain several kinds of fibrils and granules.The ‘basement membrane’ is made of a network of densely packed parallel layers of collagen fibrils which form helices which wrap around the trunk. The collagen fibrils of this connective stratum are sandwiched between two basal lamina; they are embedded in a reduced extracellular matrix whose components are closely related to the architecture of the collagen fibrils. In the core of the fin, the ground substance is very abundant and classical cross-striated collagen fibrils are not to be found. A compact fibrillar transition zone is to be noted between the dense connective stratum surrounding the body and the hyaline axis of the fins. In this zone, no crossbanded collagen fibrils are to be seen.The hydroskeleton and the fins show variations within the phylum. They could be related to speciation, and the ancestral pathway of the phylum. Furthermore these variations are related to the general problem of the evolution of the extracellular matrices and collagen molecule itself.     

Stress and matrix‐responsive cytoskeletal remodeling in fibroblasts

In areolar “loose” connective tissue, fibroblasts remodel their cytoskeleton within minutes in response to static stretch resulting in increased cell body cross‐sectional area that relaxes the tissue to a lower state of resting tension. It remains unknown whether the loosely arranged collagen matrix, characteristic of areolar connective tissue, is required for this cytoskeletal response to occur. The purpose of this study was to evaluate cytoskeletal remodeling of fibroblasts in, and dissociated from, areolar and dense connective tissue in response to 2 h of static stretch in both native tissue and collagen gels of varying crosslinking. Rheometric testing indicated that the areolar connective tissue had a lower dynamic modulus and was more viscous than the dense connective tissue. In response to stretch, cells within the more compliant areolar connective tissue adopted a large “sheet‐like” morphology that was in contrast to the smaller dendritic morphology in the dense connective tissue. By adjusting the in vitro collagen crosslinking, and the resulting dynamic modulus, it was demonstrated that cells dissociated from dense connective tissue are capable of responding when seeded into a compliant matrix, while cells dissociated from areolar connective tissue can lose their ability to respond when their matrix becomes stiffer. This set of experiments indicated stretch‐induced fibroblast expansion was dependent on the distinct matrix material properties of areolar connective tissues as opposed to the cells' tissue of origin. These results also suggest that disease and pathological processes with increased crosslinks, such as diabetes and fibrosis, could impair fibroblast responsiveness in connective tissues. J. Cell. Physiol. 228: 50–57, 2013. © 2012 Wiley Periodicals, Inc.     

THE FIBROBLAST AND WOUND REPAIR

This review of connective tissue repair has attempted to place into historical perspective information obtained by newer approaches. The literature review is incomplete, as it was unfortunately necessary to leave many interesting studies out of the discussion. Emphasis has been placed upon what is known of the inflammatory response, the fine structure of the connective tissue cells in healing wounds and with correlated chemical findings in these tissues. An optimal inflammatory response appears to be an important, rapid, non-specific stimulus for fibroplasia. It is not clear how inflammation exerts this effect. The inflammatory cells and their enzymes markedly alter the extracellular matrix of injured tissue. The matrix of connective tissue may itself participate in the control of its own synthesis and degradation. It is possible that modification of this environment by injury and/or inflammation with ensuing matrix alteration may provide a stimulus for cell migration and protein synthesis. The converse may also be true, that is, a given level of matrix concentration may have an inhibitory effect upon the connective tissue cells. The inter-relationships between the connective tissue matrix and the cells, and the possibilities of feedback mechanisms playing a role in maintaining a balance between these two are important areas for future investigation. In this regard, additional questions may be asked concerning the role of the fibroblast in remodelling and degradation of connective tissue. It is not yet clear how important a balance between collagenolytic enzymes and the solubility states, or stability, of collagen are in each connective tissue. It will be interesting to determine which cells make collagenolytic and/or proteolytic enzymes upon appropriate stimulus. It is possible to distinguish between the fibroblast and the monocyte, or potential macrophage with the electron microscope. The rough endoplasmic reticulum with its large numbers of attached ribosomes is extensively developed in the fibroblast in contrast to the monocyte. The endoplasmic reticulum sequesters collagen precursors and other secretory proteins for transport either directly to the extracellular space, as appears to be the case for collagen, or to the Golgi complex as is the case for other exportable proteins. Collagen precursors are secreted into the environment and are not shed from within the cell surface. A number of cytoplasmic alterations have been described for fibroblasts and other cells during various pathological states. The significance of these alterations is not clear. It will be important to distinguish between specific and non-specific responses to injury, if these alterations are to aid us in understanding the various cellular responses. The source of the fibroblasts in granulation tissue appears to be mesenchymal cells from adjacent tissues rather than blood-borne precursors. Although contact inhibition can be demonstrated in vitro, it is not clear how important this phenomenon is in vivo, nor are the reasons for the ability of some tissues to heal by regeneration rather than by scar tissue formation understood. These and many other questions remain to be answered. The healing wound is multifaceted and presents the opportunity for systematic investigation into the problems of cell proliferation, cell and matrix interactions, and protein synthesis in vivo and it also can help to further our understanding of the ubiquitous fibroblast and its complex extracellular matrix.     

The role of small leucine-rich proteoglycans in collagen fibrillogenesis

Small leucine-rich proteoglycans/proteins (SLRPs) are associated with collagen fibril formation, and therefore important for the proper formation of extracellular matrices. SLRPs are differentially expressed in tissues and during pathological conditions, contributing to the development of connective tissue properties. The binding of SLRPs to collagens have recently been characterized, and may give some clues to the significance of these interactions. In this mini review, we summarize published work in this field, and propose several mechanisms for how SLRPs can control collagen matrix structure and function. SLRPs appear to influence collagen cross-linking patterns. We also propose that the SLRP-collagen interactions can assist in the process of juxtaposing the collagen monomers by steric hindrance or by directly connecting two collagen monomers during the fibril growth.     

Distribution of type-VII collagen in xenografted human carcinomas

The distribution of type-VII collagen, the main molecular component of the anchoring fibrils (AF) attaching the basal lamina (BL, lamina densa of the basement membrane) to the surrounding connective tissue, was investigated in four xenografted human carcinomas of the hypopharynx (H-Stg 1), the lung (L 261), the sigmoid colon (CA 1), and the rectum (R 85). The studies were performed with a recently prepared, affinity-purified and highly specific antibody to type-VII collagen by using the indirect immunofluorescence and the APAAP (alkaline phosphatase anti-alkaline phosphatase) techniques. For comparison, the localization of the intrinsic BL components laminin and type-IV collagen were additionally analyzed in all four carcinomas. It was shown that type-VII collagen usually colocalized to laminin and type-IV collagen and was deposited at the borderline between carcinoma cell clusters and the surrounding strands of connective tissue in a similar, but more diffuse and less continuous distribution than both intrinsic BL components. In the squamous cell carcinoma H-Stg 1 and the adenocarcinoma L 261, type-VII collagen was additionally accumulated in enlarged extracellular spaces between carcinoma cells, away from the contact zone to the connective tissue and again colocalized to laminin and type-IV collagen. Numerous carcinoma cells of both xenografts showed remarkable intracytoplasmic immunoreactivity for the antibody to type-VII collagen. Even in the case of the gastrointestinal carcinomas CA 1 and R 85, faint immunoreactivity for type-VII collagen was found at the contact zone between the mucosal epithelium and the surrounding connective tissue. These results confirm that epithelial carcinoma cells are obviously involved with the synthesis of the main molecular component of AF usally attaching the BL to the adjacent connective tissue and hint at a possible correlation between the localization of type-VII collagen and the observed pattern of the BL. However, it cannot be decided whether there is a direct causal relation between both phenomena or whether they are both the consequence of an independent but common cause, such as abnormal cellular differentiation of carcinoma cells. In no case, can the discontinuities in the distribution of type-VII collagen be explained by active tumor cell invasion since xenografted human carcinomas neither invade nor metastasize.