Discrete integration of collagen XVI into tissue-specific collagen fibrils or beaded microfibrils

Investigating the interfaces of the epiphyseal plate: An integrated approach of histochemistry, microtomography and SEM

We investigated the interfaces of the epiphyseal plate with over- and underlying bone segments using an integrated approach of histochemistry, microtomography and scanning electron microscopy (SEM) to overcome the inherent limitations of sections-based techniques. Microtomography was able to provide an unobstructed, frontal view of large expanses of the two bone surfaces facing the growth plate, while SEM observation after removal of the soft matrix granted an equally unhindered access with a higher resolution. The two interfaces appeared widely dissimilar. On the diaphyseal side the hypertrophic chondrocytes were arranged in tall columns packed in a sort of compact palisade; the interposed extracellular matrix was actively calcifying into a thick mineralized crust growing towards the epiphysis. Behind the mineralization front, histochemical data revealed a number of surviving cartilage islets which were being slowly remodelled into bone. In contrast, the epiphyseal side of the cartilage consisted of a relatively quiescent reserve zone whose mineralization was marginal in amount and discontinuous in extension; the epiphyseal bone consisted of a loose trabecular meshwork, with ample vascular spaces opening directly into the non-mineralized cartilage. On both sides the calcification process took place through the formation of spheroidal bodies 1-2 μm wide which gradually grew by apposition and coalesced into a solid mass, in a way distinctly different from that of bone and other calcified tissues.

Cartilage-Related Collagens in Osteoarthritis and Rheumatoid Arthritis: From Pathogenesis to Therapeutics

Collagens serve essential mechanical functions throughout the body, particularly in the connective tissues. In articular cartilage, collagens provide most of the biomechanical properties of the extracellular matrix essential for its function. Collagen plays a very important role in maintaining the mechanical properties of articular cartilage and the stability of the ECM. Noteworthily, many pathogenic factors in the course of osteoarthritis and rheumatoid arthritis, such as mechanical injury, inflammation, and senescence, are involved in the irreversible degradation of collagen, leading to the progressive destruction of cartilage. The degradation of collagen can generate new biochemical markers with the ability to monitor disease progression and facilitate drug development. In addition, collagen can also be used as a biomaterial with excellent properties such as low immunogenicity, biodegradability, biocompatibility, and hydrophilicity. This review not only provides a systematic description of collagen and analyzes the structural characteristics of articular cartilage and the mechanisms of cartilage damage in disease states but also provides a detailed characterization of the biomarkers of collagen production and the role of collagen in cartilage repair, providing ideas and techniques for clinical diagnosis and treatment.

The chondro-osseous junction of articular cartilage

In the synovial joints the transition between the soft articular cartilage and the subchondral bone is mediated by a layer of calcified cartilage of structural and mechanical characteristics closer to those of bone. This layer, buried in the depth of articular cartilage, is not directly accessible and is mostly visualized in histological sections of decalcified tissue, where it appears as a darker strip in contact with the subchondral bone. In this study conventional histology and scanning electron microscopy (SEM) with secondary electron imaging (SE) or backscattered electron imaging (BSE) were used to discriminate the calcified and the uncalcified cartilage in high resolution on native, untreated tissue as well as in deproteinated or demineralized tissue. This approach evidenced a high heterogeneity of the calcified layer of articular cartilage. High resolution pictures revealed that the mineralization process originates by progressive accretion and confluence of individual, small mineral clusters, in a very different way from other hard tissues such as bone, dentin and mineralized tendons. Finally, selective removal of the soft matrix by thermal treatment allowed for the first time a face-on, unrestricted 3D view of the mineralization front.

Systematic Analysis of Actively Transcribed Core Matrisome Genes Across Tissues and Cell Phenotypes

The extracellular matrix (ECM) is a highly dynamic, well-organized acellular network of tissue-specific biomolecules, that can be divided into structural or core ECM proteins and ECM-associated proteins. The ECM serves as a blueprint for organ development and function and, when structurally altered through mutation, altered expression, or degradation, can lead to debilitating syndromes that often affect one tissue more than another. Cross-referencing the FANTOM5 SSTAR (Semantic catalog of Samples, Transcription initiation And Regulators) and the defined catalog of core matrisome ECM (glyco)proteins, we conducted a comprehensive analysis of 511 different human samples to annotate the context-specific transcription of the individual components of the defined matrisome. Relative log expression normalized SSTAR cap analysis gene expression peak data files were downloaded from the FANTOM5 online database and filtered to exclude all cell lines and diseased tissues. Promoter-level expression values were categorized further into eight core tissue systems and three major ECM categories: proteoglycans, glycoproteins, and collagens. Hierarchical clustering and correlation analyses were conducted to identify complex relationships in promoter-driven gene expression activity. Integration of the core matrisome and curated FANTOM5 SSTAR data creates a unique tool that provides insight into the promoter-level expression of ECM-encoding genes in a tissue- and cell-specific manner. Unbiased clustering of cap analysis gene expression peak data reveals unique ECM signatures within defined tissue systems. Correlation analysis among tissue systems exposes both positive and negative correlation of ECM promoters with varying levels of significance. This tool can be used to provide new insight into the relationships between ECM components and tissues and can inform future research on the ECM in human disease and development. We invite the matrix biology community to continue to explore and discuss this dataset as part of a larger and continuing conversation about the human ECM. An interactive web tool can be found at matrixpromoterome.github.io along with additional resources that can be found at dx.doi.org/10.6084/m9.figshare.19794481 (figures) and https://figshare.com/s/e18ecbc3ae5aaf919b78 (python notebook).

Main and Minor Types of Collagens in the Articular Cartilage: The Role of Collagens in Repair Tissue Evaluation in Chondral Defects

Several collagen subtypes have been identified in hyaline articular cartilage. The main and most abundant collagens are type II, IX and XI collagens. The minor and less abundant collagens are type III, IV, V, VI, X, XII, XIV, XVI, XXII, and XXVII collagens. All these collagens have been found to play a key role in healthy cartilage, regardless of whether they are more or less abundant. Additionally, an exhaustive evaluation of collagen fibrils in a repaired cartilage tissue after a chondral lesion is necessary to determine the quality of the repaired tissue and even whether or not this repaired tissue is considered hyaline cartilage. Therefore, this review aims to describe in depth all the collagen types found in the normal articular cartilage structure, and based on this, establish the parameters that allow one to consider a repaired cartilage tissue as a hyaline cartilage.

Collagen- and hyaluronic acid-based hydrogels and their biomedical applications

Hydrogels have been widely investigated in biomedical fields due to their similar physical and biochemical properties to the extracellular matrix (ECM). Collagen and hyaluronic acid (HA) are the main components of the ECM in many tissues. As a result, hydrogels prepared from collagen and HA hold inherent advantages in mimicking the structure and function of the native ECM. Numerous studies have focused on the development of collagen and HA hydrogels and their biomedical applications. In this extensive review, we provide a summary and analysis of the sources, features, and modifications of collagen and HA. Specifically, we highlight the fabrication, properties, and potential biomedical applications as well as promising commercialization of hydrogels based on these two natural polymers.

Collagen Biosynthesis, Processing, and Maturation in Lung Ageing

In addition to providing a macromolecular scaffold, the extracellular matrix (ECM) is a critical regulator of cell function by virtue of specific physical, biochemical, and mechanical properties. Collagen is the main ECM component and hence plays an essential role in the pathogenesis and progression of chronic lung disease. It is well-established that many chronic lung diseases, e.g., chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) primarily manifest in the elderly, suggesting increased susceptibility of the aged lung or accumulated alterations in lung structure over time that favour disease. Here, we review the main steps of collagen biosynthesis, processing, and turnover and summarise what is currently known about alterations upon lung ageing, including changes in collagen composition, modification, and crosslinking. Recent proteomic data on mouse lung ageing indicates that, while the ER-resident machinery of collagen biosynthesis, modification and triple helix formation appears largely unchanged, there are specific changes in levels of type IV and type VI as well as the two fibril-associated collagens with interrupted triple helices (FACIT), namely type XIV and type XVI collagens. In addition, levels of the extracellular collagen crosslinking enzyme lysyl oxidase are decreased, indicating less enzymatically mediated collagen crosslinking upon ageing. The latter contrasts with the ageing-associated increase in collagen crosslinking by advanced glycation endproducts (AGEs), a result of spontaneous reactions of protein amino groups with reactive carbonyls, e.g., from monosaccharides or reactive dicarbonyls like methylglyoxal. Given the slow turnover of extracellular collagen such modifications accumulate even more in ageing tissues. In summary, the collective evidence points mainly toward age-induced alterations in collagen composition and drastic changes in the molecular nature of collagen crosslinks. Future work addressing the consequences of these changes may provide important clues for prevention of lung disease and for lung bioengineering and ultimately pave the way to novel targeted approaches in lung regenerative medicine.

Slippery when wet: The free surface of the articular cartilage

The free surface of the articular cartilage must withstand compressive and shearing forces, maintain a low friction coefficient and allow oxygen and metabolites to reach the underlying matrix. In many ways it is critical to the physiology of the whole tissue and its disruption always involves deep pathological alterations and loss of the joint integrity. Being very difficult to image with section-based conventional techniques, it was often described by previous research in conflicting terms or entirely overlooked. High-magnification face-on observations with high resolution scanning electron microscopy and with scanning probe microscopy revealed a very thin, delicate superficial layer rich in glycoconjugates, which may explain the very low friction coefficient of the tissue but which was very easily altered and/or dissolved in the preparation. Beneath this superficial sheet lies a thicker coat of thin, highly uniform, slightly wavy collagen fibrils lying parallel to the surface and mutually interconnected by a huge number of interfibrillar glycosaminoglycan bridges. These bridges and the collagen fibrils form an extended reticular structure able to redistribute tensile and compressive stress across a larger area of the surface and hence a greater volume of tissue.

Collagen: quantification, biomechanics, and role of minor subtypes in cartilage

Collagen is a ubiquitous biomaterial in vertebrate animals. Although each of its 28 subtypes contributes to the functions of many different tissues in the body, most studies on collagen or collagenous tissues have focussed on only one or two subtypes. With recent developments in analytical chemistry, especially mass spectrometry, significant advances have been made toward quantifying the different collagen subtypes in various tissues; however, high-throughput and low-cost methods for collagen subtype quantification do not yet exist. In this Review, we introduce the roles of collagen subtypes and crosslinks, and describe modern assays that enable a deep understanding of tissue physiology and disease states. Using cartilage as a model tissue, we describe the roles of major and minor collagen subtypes in detail; discuss known and unknown structure-function relationships; and show how tissue engineers may harness the functional characteristics of collagen to engineer robust neotissues.

Analysis of opticin binding to collagen fibrils identifies a single binding site in the gap region and a high specificity towards thin heterotypic fibrils containing collagens II, and XI or V/XI

Opticin is a class III member of the extracellular matrix small leucine-rich repeat protein/proteoglycan (SLRP) family found in vitreous humour and cartilage. It was first identified associated with the surface of vitreous collagen fibrils and several other SLRPs are also known to bind collagen fibrils and it some cases alter fibril morphology. The purpose of this study was to investigate the binding of opticin to the collagen II-containing fibrils found in vitreous and cartilage. Electron microscopic studies using gold labelling demonstrated that opticin binds vitreous and thin cartilage collagen fibrils specifically at a single site in the gap region of the collagen D-period corresponding to the e2 stain band; this is the first demonstration of the binding site of a class III SLRP on collagen fibrils. Opticin did not bind thick cartilage collagen fibrils from cartilage or tactoids formed in vitro from collagen II, but shows high specificity for thin, heterotypic collagen fibrils containing collagens II, and XI or V/XI. Vitreous collagen fibrils from opticin null and wild-type mice were compared and no difference in fibril morphology or diameter was observed. Similarly, in vitro fibrillogenesis experiments showed that opticin did not affect fibril formation. We propose that when opticin is bound to collagen fibrils, rather than influencing their morphology it instead hinders the binding of other molecules to the fibril surfaces and/or act as an intermediary bridge linking the collagen fibrils to other non-collagenous molecules.

The Role of microRNAs in Organismal and Skin Aging

The aging process starts directly after birth and lasts for the entire lifespan; it manifests itself with a decline in an organism’s ability to adapt and is linked to the development of age-related diseases that eventually lead to premature death. This review aims to explore how microRNAs (miRNAs) are involved in skin functioning and aging. Recent evidence has suggested that miRNAs regulate all aspects of cutaneous biogenesis, functionality, and aging. It has been noted that some miRNAs were down-regulated in long-lived individuals, such as let-7, miR-17, and miR-34 (known as longevity-related miRNAs). They are conserved in humans and presumably promote lifespan prolongation; conversely, they are up-regulated in age-related diseases, like cancers. The analysis of the age-associated cutaneous miRNAs revealed the increased expression of miR-130, miR-138, and miR-181a/b in keratinocytes during replicative senescence. These miRNAs affected cell proliferation pathways via targeting the p63 and Sirtuin 1 mRNAs. Notably, miR-181a was also implicated in skin immunosenescence, represented by the Langerhans cells. Dermal fibroblasts also expressed increased the levels of the biomarkers of aging that affect telomere maintenance and all phases of the cellular life cycle, such as let-7, miR-23a-3p, 34a-5p, miR-125a, miR-181a-5p, and miR-221/222-3p. Among them, the miR-34 family, stimulated by ultraviolet B irradiation, deteriorates collagen in the extracellular matrix due to the activation of the matrix metalloproteinases and thereby potentiates wrinkle formation. In addition to the pro-aging effects of miRNAs, the plausible antiaging activity of miR-146a that antagonized the UVA-induced inhibition of proliferation and suppressed aging-related genes (e.g., p21WAF-1, p16, and p53) through targeting Smad4 has also been noticed. Nevertheless, the role of miRNAs in skin aging is still not fully elucidated and needs to be further discovered and explained.

Quantitative proteomic profiling of extracellular matrix and site-specific collagen post-translational modifications in an in vitro model of lung fibrosis

Lung fibrosis is characterized by excessive deposition of extracellular matrix (ECM), in particular collagens, by fibroblasts in the interstitium. Transforming growth factor-β1 (TGF-β1) alters the expression of many extracellular matrix (ECM) components produced by fibroblasts, but such changes in ECM composition as well as modulation of collagen post-translational modification (PTM) levels have not been comprehensively investigated. Here, we performed mass spectrometry (MS)-based proteomics analyses to assess changes in the ECM deposited by cultured lung fibroblasts from idiopathic pulmonary fibrosis (IPF) patients upon stimulation with transforming growth factor β1 (TGF-β1). In addition to the ECM changes commonly associated with lung fibrosis, MS-based label-free quantification revealed profound effects on enzymes involved in ECM crosslinking and turnover as well as multiple positive and negative feedback mechanisms of TGF-β1 signaling. Notably, the ECM changes observed in this in vitro model correlated significantly with ECM changes observed in patient samples. Because collagens are subject to multiple PTMs with major implications in disease, we implemented a new bioinformatic platform to analyze MS data that allows for the comprehensive mapping and site-specific quantitation of collagen PTMs in crude ECM preparations. These analyses yielded a comprehensive map of prolyl and lysyl hydroxylations as well as lysyl glycosylations for 15 collagen chains. In addition, site-specific PTM analysis revealed novel sites of prolyl-3-hydroxylation and lysyl glycosylation in type I collagen. Interestingly, the results show, for the first time, that TGF-β1 can modulate prolyl-3-hydroxylation and glycosylation in a site-specific manner. Taken together, this proof of concept study not only reveals unanticipated TGF-β1 mediated regulation of collagen PTMs and other ECM components but also lays the foundation for dissecting their key roles in health and disease.

The proteomic data has been deposited to the ProteomeXchange Consortium via the MassIVE partner repository with the data set identifier MSV000082958.

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Quantitative proteomics of TGF-β-induced changes in ECM composition and collagen PTM in pulmonary fibroblasts

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TGF-β promotes crosslinking and turnover as well as complex feedback mechanisms that alter fibroblast ECM homeostasis.

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A novel bioinformatic workflow for MS data analysis enabled global mapping and quantitation of known and novel collagen PTMs

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Quantitative assessment of prolyl-3-hydroxylation site occupancy and lysine-O-glycosylation microheterogeneity

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TGF-β1 modulates collagen PTMs in a site-specific manner that may favor collagen accumulation in lung fibrosis

Serum type XVI collagen is associated with colorectal cancer and ulcerative colitis indicating a pathological role in gastrointestinal disorders

Altered extracellular matrix (ECM) remodeling is an important part of the pathology of gastrointestinal (GI) disorders. In the intestine, type XVI collagen (col‐16) plays a role in pathogenesis by affecting ECM architecture and induce cell invasion. Measuring col‐16 in serum may therefore have biomarker potential in GI disorders such as colorectal cancer (CRC) and ulcerative colitis (UC). The aim of this study was to determine whether col‐16 can serve as a biomarker for altered ECM remodeling in patients with CRC and UC. A monoclonal antibody was raised against the C‐terminal end of col‐16 (PRO‐C16), and a competitive enzyme‐linked immunosorbent assay (ELISA) was developed and technically validated. Levels of PRO‐C16 were measured in serum from patients with CRC (before (n = 50) and 3 months after (n = 23) tumor resections), UC (n = 39) and healthy controls (n = 50). The PRO‐C16 ELISA was specific toward the C‐terminal of col‐16. PRO‐C16 was significantly elevated both in serum from patients with CRC (P = 0.0026) and UC (P < 0.0001) compared to controls. No difference was detected in levels of PRO‐C16 between patients with CRC at baseline and 3 months after tumor resections (P > 0.999). Levels of PRO‐C16 identified patients with a GI disorder with a positive predictive value of 0.9 and an odds ratio of 12 (95%CI = 4.5‐29.5, P < 0.0001). The newly developed assay detected significantly elevated levels of PRO‐C16 in serum from patients with GI disorders compared to controls suggesting its potential as a biomarker in this setting. Future studies are needed to validate these findings.

Age-related evolutions of the dermis: Clinical signs, fibroblast and extracellular matrix dynamics

Ageing is today a major societal concern that is intrinsically associated with the increase of life expectancy. Outside the context of severe degenerative diseases that affect the elderly populations, normal visible signs of ageing, notably skin sagging and wrinkles, influence the social and individual perception of peoples. Accordingly, there is a strong demand for researches on skin ageing. Deciphering the cellular and molecular processes of skin evolution through ageing is thus an active scientific domain, at the frontier of tissue developmental and ageing biology. The focus of the present article is to provide an overview of the current knowledge concerning the evolution of dermis characteristics at different life stages, from intra-uterine to post-natal life. The description will integrate stage-specific and age-related changes in dermis characteristics at the tissue, cell, and molecular levels.

Manipulation of Panx1 Activity Increases the Engraftment of Transplanted Lacrimal Gland Epithelial Progenitor Cells

Purpose

Sjögren's syndrome is a systemic chronic autoimmune inflammatory disease that primarily targets the salivary and lacrimal glands (LGs). Currently there is no cure; therefore, cell-based regenerative therapy may be a viable option. LG inflammation is facilitated by extracellular ATP and mediated by the Pannexin-1 (Panx1) membrane channel glycoprotein. We propose that suppression of inflammation through manipulation of Panx1 activity can stimulate epithelial cell progenitor (EPCP) engraftment.

Methods

The expression of pannexins in the mouse and human LG was assayed by qRT-PCR and immunostaining. Acute LG inflammation was induced by interleukin-1α (IL1α) injection. Prior to EPCP transplantation, IL1α-injured or chronically inflamed LGs of thrombospondin-1–null mice (TSP-1^−/−) were treated with the Panx1-specific blocking peptide (¹⁰panx) or the self-deliverable RNAi (sdRNAi). The efficacy of cell engraftment and the area of inflammation were analyzed by microscopy.

Results

Panx1 and Panx2 were detected in the mouse and human LGs. Panx1 and proinflammatory factors were upregulated during acute inflammation at days 1 to 3 after the IL1α injection. The analysis of EPCP engraftment demonstrated a significant and reproducible positive correlation between the ¹⁰panx peptide or Panx1 sdRNAi treatment and the number of engrafted cells. Similarly, treatment of the LG of the TSP-1^−/− mouse (mouse model of chronic LG inflammation) by either Panx1 or Caspase-4 (also known as Casp11) sdRNAi showed a significant decrease in expression of proinflammatory markers and the lymphocyte infiltration.

Conclusions

Our results suggest that blocking Panx1 and/or Casp4 activities is a beneficial strategy to enhance donor cell engraftment and LG regeneration through the reduction of inflammation.

Role of the extracellular matrix in cancer-associated epithelial to mesenchymal transition phenomenon

The epithelial to mesenchymal transition (EMT) program is a crucial component in the processes of morphogenesis and embryonic development. The transition of epithelial to mesenchymal phenotype is associated with numerous structural and functional changes, including loss of cell polarity and tight cell-cell junctions, the acquisition of invasive abilities, and the expression of mesenchymal proteins. The switch between the two phenotypes is involved in human pathology and is crucial for cancer progression. Extracellular matrices (ECMs) are multi-component networks that surround cells in tissues. These networks are obligatory for cell survival, growth, and differentiation as well as tissue organization. Indeed, the ECM suprastructure, in addition to its supportive role, can process and deliver a plethora of signals to cells, which ultimately regulate their behavior. Importantly, the ECM derived signals are critically involved in the process of EMT during tumorigenesis. This review discusses the multilayer interaction between the ECM and the EMT process, focusing on contributions of discrete mediators, a strategy that may identify novel potential target molecules. Developmental Dynamics 247:368-381, 2018. © 2017 Wiley Periodicals, Inc.

Unraveling the role of Calcium ions in the mechanical properties of individual collagen fibrils

Collagen, the dominating material in the extracellular matrix, provides the strength, elasticity and mechanical stability to the organisms. The mechanical property of collagen is mainly dominated by its surrounding environments. However, the variation and origin of the mechanics of collagen fibril under different concentrations of calcium ions (χ_Ca) remains unknown. By using the atomic force microscopy based nanoindentation, the mechanics and structure of individual type II collagen fibril were first investigated under different χ_Ca in this study. The results demonstrate that both of the mechanical and structural properties of the collagen fibril show a prominent dependence on χ_Ca. The mechanism of χ_Ca-dependence of the collagen fibril was attributed to the chelation between collagen molecules and the calcium ions. Given the role of calcium in the pathology of osteoarthritis, the current study may cast new light on the understanding of osteoarthritis and other soft tissue hardening related diseases in the future.

The minor collagens in articular cartilage

Articular cartilage is a connective tissue consisting of a specialized extracellular matrix (ECM) that dominates the bulk of its wet and dry weight. Type II collagen and aggrecan are the main ECM proteins in cartilage. However, little attention has been paid to less abundant molecular components, especially minor collagens, including type IV, VI, IX, X, XI, XII, XIII, and XIV, etc. Although accounting for only a small fraction of the mature matrix, these minor collagens not only play essential structural roles in the mechanical properties, organization, and shape of articular cartilage, but also fulfil specific biological functions. Genetic studies of these minor collagens have revealed that they are associated with multiple connective tissue diseases, especially degenerative joint disease. The progressive destruction of cartilage involves the degradation of matrix constituents including these minor collagens. The generation and release of fragmented molecules could generate novel biochemical markers with the capacity to monitor disease progression, facilitate drug development and add to the existing toolbox for in vitro studies, preclinical research and clinical trials.

Effects of a Hydroxyapatite-based Biomaterial on Gene Expression in Osteoblast-like Cells

Biostite® is a hydroxyapatite-derived biomaterial that is used in periodontal and bone reconstructive procedures due to its osteoconductive properties. Since the molecular effects of this biomaterial on osteoblasts are still unknown, we decided to assess whether it may specifically modulate osteoblast functions in vitro. We found that a brief exposure to Biostite® significantly reduced the proliferation of MG-63 and SaOS-2 osteoblast-like cells to ~ 50% of the plateau value. Furthermore, gene array analysis of MG-63 cells showed that Biostite® caused a differential expression of 37 genes which are involved in cell proliferation and interaction, and related to osteoblast differentiation and tissue regeneration. Results were confirmed by RT-PCR, Western blot, and by an increase in alkaline phosphatase (ALP) specific activity. Biostite® also increased levels of polycystin-2, a mechano-sensitive Ca2+ channel, a promising new marker of bone cell differentiation. Biostite®, therefore, may directly affect osteoblasts by enhancing chondro/osteogenic gene expression and cytoskeleton-related signaling pathways, which may contribute to its clinical efficacy.

Fibroblast heterogeneity and its implications for engineering organotypic skin models in vitro

Advances in cell culture methods, multidisciplinary research, clinical need to replace lost skin tissues and regulatory need to replace animal models with alternative test methods has led to development of three dimensional models of human skin. In general, these in vitro models of skin consist of keratinocytes cultured over fibroblast-populated dermal matrices. Accumulating evidences indicate that mesenchyme-derived signals are essential for epidermal morphogenesis, homeostasis and differentiation. Various studies show that fibroblasts isolated from different tissues in the body are dynamic in nature and are morphologically and functionally heterogeneous subpopulations. Further, these differences seem to be dictated by the local biological and physical microenvironment the fibroblasts reside resulting in "positional identity or memory". Furthermore, the heterogeneity among the fibroblasts play a critical role in scarless wound healing and complete restoration of native tissue architecture in fetus and oral mucosa; and excessive scar formation in diseased states like keloids and hypertrophic scars. In this review, we summarize current concepts about the heterogeneity among fibroblasts and their role in various wound healing environments. Further, we contemplate how the insights on fibroblast heterogeneity could be applied for the development of next generation organotypic skin models.

Microdeletion 1p35.2: a recognizable facial phenotype with developmental delay

We describe two patients with microdeletion 1p35.2, intrauterine growth retardation, small stature, hypermetropia, hearing impairment and developmental delay. Both patients have long, myopathic facies, with fine eyebrows, small mouths and micrognathia. We postulate a role for the histone deacetylase HDAC1 in the facial phenotype and suggest that deletion of KPNA6 may prevent transmission of the 1p35.2 deletion from affected girls to any offspring through impaired zygotic genome activation.

Collagen XVI induces expression of MMP9 via modulation of AP-1 transcription factors and facilitates invasion of oral squamous cell carcinoma

Collagen XVI belongs to the family of fibril-associated collagens with interrupted triple helices (FACIT). It is overexpressed during the progression of oral squamous cell carcinoma (OSCC). The present data show a strong collagen XVI-dependent induction of MMP9 and an increase in OSCC cell invasion. We found activated integrin-linked kinase (ILK) in a complex with kindlin-1 and activation of protein kinase B (PKB/Akt) to be responsible for MMP9 induction. Inhibition of the formation of focal adhesions reduced MMP9 expression. Moreover, collagen XVI overexpressing OSCC cell clones (COLXVI cell clones) transfected with vectors containing different MMP9 promoter fragments adjacent to a luciferase reporter revealed an increase in luciferase signal dependent on AP-1 binding sites. Deletion of the AP-1 binding site 98 bp upstream of the reported transcription start site and inhibition of AP-1 with Tanshinone IIA resulted in decreased MMP9 expression. The AP-1 subunit JunB showed differential expression between COLXVI cell clones and mock control cells. Additionally, mass spectrometric analysis of immunoprecipitates revealed that c-Fos interacted strongly with dyskerin in COLXVI cell clones compared to mock controls.

MicroRNA-152 and -181a participate in human dermal fibroblasts senescence acting on cell adhesion and remodeling of the extra-cellular matrix

Ageing of human skin is associated with phenotypic changes in the cutaneous cells; the major functional markers of ageing occur as consequences of dermal and epidermal cell senescence and of structural and compositional remodeling of normally long-lived dermal extracellular matrix proteins. Understanding the contribution of the dermal cells in skin ageing is a key question, since this tissue is particularly important for skin integrity and its properties can affect the epidermis. Several microRNAs have been shown to be involved in the regulation of pathways involved in cellular senescence and exerted important effects on tissues ageing. In this study, we demonstrate that the expression of miR-152 and miR-181a increased during the human dermal fibroblasts senescence and that their overexpression, is sufficient to induce cellular senescence in early-passage cells. The increase of these miRNAs during cells senescence was accompanied by a decrease in integrin α5 and collagen XVI expression at mRNA and/or protein levels resulting in reduced cellular adhesion and suggesting extracellular matrix remodeling. These findings indicate that changes in miRNAs expression, by modulating the levels of adhesion proteins and extra-cellular matrix components, such as integrin α5 and collagen XVI, could contribute to the compositional remodelling of the dermis and epidermis occurring during skin aging.

WARP interacts with collagen VI-containing microfibrils in the pericellular matrix of human chondrocytes

Collagen VI and WARP are extracellular structural macromolecules present in cartilage and associated with BM suprastructures in non-skeletal tissues. We have previously shown that in WARP-deficient mice, collagen VI is specifically reduced in regions of the peripheral nerve ECM where WARP is expressed, suggesting that both macromolecules are part of the same suprastructure. The object of this study was to conduct a detailed analysis of WARP-collagen VI interactions in vitro in cartilage, a tissue rich in WARP and collagen VI. Immunohistochemical analysis of mouse and human articular cartilage showed that WARP and collagen VI co-localize in the pericellular matrix of superficial zone articular chondrocytes. EM analysis on extracts of human articular cartilage showed that WARP associates closely with collagen VI-containing suprastructures. Additional evidence of an interaction is provided by immunogold EM and immunoblot analysis showing that WARP was present in collagen VI-containing networks isolated from cartilage. Further characterization were done by solid phase binding studies and reconstitution experiments using purified recombinant WARP and isolated collagen VI. Collagen VI binds to WARP with an apparent K(d) of approximately 22 nM and the binding site(s) for WARP resides within the triple helical domain since WARP binds to both intact collagen VI tetramers and pepsinized collagen VI. Together, these data confirm and extend our previous findings by demonstrating that WARP and collagen VI form high affinity associations in vivo in cartilage. We conclude that WARP is ideally placed to function as an adapter protein in the cartilage pericellular matrix.

Epidermolysis bullosa - a group of skin diseases with different causes but commonalities in gene expression

Epidermolysis bullosa (EB) is a group of hereditary skin disorders. Although each subtype is caused by mutations in genes encoding differentially located components of the skin, the resulting phenotype is similar. In this study, we investigated similarities in the gene expression profiles of each subtype on mRNA level. Type XVI collagen (COL16A1), G0/G1 switch 2 (G0S2), fibronectin (FN1), ribosomal protein S27A (RPS27A) and low density lipoprotein receptor (LDLR) were shown to exhibit corresponding changes in gene expression in all three EB subtypes. While COL16A1, G0S2 and FN1 are up-regulated, LDLR and RPS27A mRNA levels are decreased. These data indicate that EB cells seem to take measures increasing their mechanical stability. Apoptosis is likely to be exacerbated, and migratory potential appears to be elevated. Protein degradation is hampered, and the release of fatty acids and glycerol is restricted, probably to save energy. These commonalities might benefit existing EB treatment strategies or could help to reveal new starting points for the treatment of EB in the future.

Collagen XVI in health and disease

Collagen XVI, by structural analogy a member of the FACIT- (fibril-associated collagens with interrupted triple helices) family of collagens, is described as a minor collagen component of connective tissues. Collagen XVI is expressed in various cells and tissues without known occurrence of splice variants or isoforms. For skin and cartilage tissues its suprastructure is known. Presumably, there it acts as an adaptor protein connecting and organizing large fibrillar networks and thus modulates integrity and stability of the extracellular matrix (ECM). Collagen XVI is produced by myofibroblasts in the normal intestine and its synthesis is increased in the inflamed bowel wall where myofibroblasts develop increased numbers of focal adhesion contacts on collagen XVI. Consequently, recruitment of α1 integrin into the focal adhesions at the tip of the cells is induced followed by increased cell spreading on collagen XVI. This presumably adds to the maintenance of myofibroblasts in the inflamed intestinal regions and thus promotes fibrotic responses of the tissue. Notably, α1/α2 integrins interact with collagen XVI through an α1/α2β1 integrin binding site located in the COL 1-3 domains. Collagen XVI may act as a substrate for adhesion and invasion of connective tissue tumor cells. In glioblastoma it induces tumor invasiveness by modification of the β1-integrin activation pattern. Thus, altering the cell-matrix interaction through collagen XVI might be a molecular mechanism to further augment the invasive phenotype of glioma cells. In this line, in oral squamous cell carcinoma collagen XVI expression is induced which results in an upregulation of Kindlin-1 followed by an increased interaction with beta1-integrin. Consequently, collagen XVI induces a proliferative tumor phenotype by promoting an early S-phase entry. In summary, collagen XVI plays a decisive role in the interaction of connective tissue cells with their ECM, which is impaired in pathological situations. Alteration of tissue location and expression level of collagen XVI appears to promote tumorigenesis and to perpetuate inflammatory reactions.

The epidermal basement membrane is a composite of separate laminin- or collagen IV-containing networks connected by aggregated perlecan, but not by nidogens

The basement membrane between the epidermis and the dermis is indispensable for normal skin functions. It connects, and functionally separates, the epidermis and the dermis. To understand the suprastructural and functional basis of these connections, heterotypic supramolecular aggregates were isolated from the dermal-epidermal junction zone of human skin. Individual suprastructures were separated and purified by immunomagnetic beads, each recognizing a specific, molecular component of the aggregates. The molecular compositions of the suprastructures were determined by immunogold electron microscopy and immunoblotting. A composite of two networks was obtained from fibril-free suspensions by immunobeads recognizing either laminin 332 or collagen IV. After removal of perlecan-containing suprastructures or after enzyme digestion of heparan sulfate chains, a distinct network with a diffuse electron-optical appearance was isolated with magnetic beads coated with antibodies to collagen IV. The second network was more finely grained and comprised laminin 332 and laminins with α5-chains. The core protein of perlecan was an exclusive component of this network whereas its heparan sulfate chains were integrated into the collagen IV-containing network. Nidogens 1 and 2 occurred in both networks but did not form strong molecular cross-bridges. Their incorporation into one network appeared to be masked after their incorporation into the other one. We conclude that the epidermal basement membrane is a composite of two structurally independent networks that are tightly connected in a spot-welding-like manner by perlecan-containing aggregates.

Integrin-mediated cell-matrix interaction in physiological and pathological blood vessel formation

Physiological as well as pathological blood vessel formation are fundamentally dependent on cell-matrix interaction. Integrins, a family of major cell adhesion receptors, play a pivotal role in development, maintenance, and remodeling of the vasculature. Cell migration, invasion, and remodeling of the extracellular matrix (ECM) are integrin-regulated processes, and the expression of certain integrins also correlates with tumor progression. Recent advances in the understanding of how integrins are involved in the regulation of blood vessel formation and remodeling during tumor progression are highlighted. The increasing knowledge of integrin function at the molecular level, together with the growing repertoire of integrin inhibitors which allow their selective pharmacological manipulation, makes integrins suited as potential diagnostic markers and therapeutic targets.

Cartilage intermediate layer protein 2 (CILP-2) is expressed in articular and meniscal cartilage and down-regulated in experimental osteoarthritis

Using transcriptome profiling to determine differential gene expression between the permanent mouse articular cartilage and the transient growth plate cartilage, we identified a highly expressed gene, Cilp2, which is expressed differentially by articular chondrocytes. CILP-2 is highly homologous to CILP-1 (cartilage intermediate layer protein 1), which is expressed in the intermediate zone of articular cartilage and has been linked to cartilage degenerative diseases. We demonstrated that Cilp2 has a restricted mRNA distribution at the surface of the mouse articular cartilage during development, becoming localized to the intermediate zone of articular cartilage and meniscal cartilage with maturity. Although the extracellular CILP-2 protein localization is broadly similar to CILP-1, CILP-2 appears to be more localized in the deeper intermediate zone of the articular cartilage extracellular matrix at maturity. CILP-2 was shown to be proteolytically processed, N-glycosylated, and present in human articular cartilage. In surgically induced osteoarthritis in mice, Cilp1 and Cilp2 gene expression was dysregulated. However, whereas Cilp1 expression was increased, Cilp2 gene expression was down-regulated demonstrating a differential response to mechanically induced joint destabilization. CILP-2 protein was reduced in the mouse osteoarthritic cartilage. Ultrastructural analysis also suggested that CILP-2 may be associated with collagen VI microfibrils and thus may mediate interactions between matrix components in the territorial and inter-territorial articular cartilage matrix. mRNA expression analysis indicated that whereas Cilp1 and Cilp2 are expressed most abundantly in cartilaginous tissues, expression can be detected in muscle and heart.

Inhibition of collagen XVI expression reduces glioma cell invasiveness

Glioblastomas are characterized by an intense local invasiveness that limits surgical resection. One mechanism by which glioma cells enforce their migration into brain tissue is reorganization of tumour associated extracellular matrix (ECM). Collagen XVI is a minor component of connective tissues. However, in glioblastoma tissue it is dramatically upregulated compared to the ECM of normal cortex. The aim of this study is to delineate tumour cell invasion and underlying mechanisms involving collagen XVI by using a siRNA mediated collagen XVI knockdown model in U87MG human glioblastoma cells. Knockdown of collagen XVI resulted in decreased invasiveness in Boyden chamber assays, and in a reduction of focal adhesion contact numbers per cell. Gene expression was upregulated for protocadherin 18 and downregulated for kindlin-1 and -2. Proliferation was not affected while flow cytometric analysis demonstrated reduced β1-integrin activation in collagen XVI knockdown cells. We suggest that in glioblastoma tissue collagen XVI may impair the cell-cell interaction in favour of enhancement of invasion. The modification of the β1-integrin activation pattern through collagen XVI might be a molecular mechanism to further augment the invasive phenotype of glioma cells. Elucidating the underlying mechanisms of glioma cell invasion promoted by collagen XVI may provide novel cancer therapeutic approaches in neurooncology.

Induction of type XVI collagen expression facilitates proliferation of oral cancer cells

Type XVI collagen belongs to the family of fibril-associated collagens with interrupted triple helices (FACIT). Recently, high affinity to integrin alpha1beta1 has been shown allowing cells expressing those integrins to attach and spread on recombinant type XVI collagen. Here, we show that type XVI collagen is overexpressed in dysplastic areas of mucosal epithelium from oral squamous cell carcinoma (OSCC) patients. Induction of its expression in OSCC cell lines (COLXVI cells) leads to an increased expression of Kindlin-1. Moreover, we demonstrate a significantly increased Kindlin-1/beta1-integrin interaction. Additionally, we detected a higher number of activated beta1-integrins in COLXVI cells and found a neo-expression of alpha1 integrin subunit on these cells. FACS analysis revealed a significantly higher amount of COLXVI cells in S-phase and G2/M-phase 6h after synchronisation leading to a markedly higher proliferation activity. Blocking beta1-integrins with a specific antibody resulted in reduced proliferation of COLXVI cells. In summary, we demonstrate that overexpression of type XVI collagen in aberrant oral keratinocytes leads to Kindlin-1 induction, increased Kindlin-1/beta1-integrin interaction, integrin activation and subsequently to a proliferative cellular phenotype.

The collagen family

Collagens are the most abundant proteins in mammals. The collagen family comprises 28 members that contain at least one triple-helical domain. Collagens are deposited in the extracellular matrix where most of them form supramolecular assemblies. Four collagens are type II membrane proteins that also exist in a soluble form released from the cell surface by shedding. Collagens play structural roles and contribute to mechanical properties, organization, and shape of tissues. They interact with cells via several receptor families and regulate their proliferation, migration, and differentiation. Some collagens have a restricted tissue distribution and hence specific biological functions.

Phospholipid scramblase 1 is secreted by a lipid raft-dependent pathway and interacts with the extracellular matrix protein 1 in the dermal epidermal junction zone of human skin

We examined the interaction of ECM1 (extracellular matrix protein 1) using yeast two-hybrid screening and identified the type II transmembrane protein, PLSCR1 (phospholipid scramblase 1), as a binding partner. This interaction was then confirmed by in vitro and in vivo co-immunoprecipitation experiments, and additional pull-down experiments with GST-tagged ECM1a fragments localized this interaction to occur within the tandem repeat region of ECM1a. Furthermore, immunohistochemical staining revealed a partial overlap of ECM1 and PLSCR1 in human skin at the basal epidermal cell layer. Moreover, in human skin equivalents, both proteins are expressed at the basal membrane in a dermal fibroblast-dependent manner. Next, immunogold electron microscopy of ultrathin human skin sections showed that ECM1 and PLSCR1 co-localize in the extracellular matrix, and using antibodies against ECM1 or PLSCR1 cross-linked to magnetic immunobeads, we were able to demonstrate PLSCR1-ECM1 interaction in human skin extracts. Furthermore, whereas ECM1 is secreted by the endoplasmic/Golgi-dependent pathway, PLSCR1 release from HaCaT keratinocytes occurs via a lipid raft-dependent mechanism, and is deposited in the extracellular matrix. In summary, we here demonstrate that PLSCR1 interacts with the tandem repeat region of ECM1a in the dermal epidermal junction zone of human skin and provide for the first time experimental evidence that PLSCR1 is secreted by an unconventional secretion pathway. These data suggest that PLSCR1 is a multifunctional protein that can function both inside and outside of the cell and together with ECM1 may play a regulatory role in human skin.

Nanostructure of collagen fibrils in human nucleus pulposus and its correlation with macroscale tissue mechanics

Collagen fibrils are the main structural components of the nucleus pulposus tissue in the intervertebral discs. The structure-property relationship of the nucleus pulposus (NP) tissues is still unclear. We investigated the structure of individual collagen fibrils of the NP and evaluated its correlation with the bulk mechanical properties of the tissue. Collagen fibrils were extracted from the NP of discs retrieved from adolescents during scoliosis correction surgery, and the extracts were confirmed by SDS-PAGE. The diameters of the individual collagen fibrils were measured through atomic force microscopy, and the compressive mechanical properties of the tissues were evaluated by confined compression. The correlations between the nanoscale morphology of the collagen fibrils and the macroscale mechanical properties of the tissues were evaluated by linear regression. The SDS-PAGE results showed that the fibril extracts were largely composed of type II collagen. The mean diameter of the collagen fibrils was 92.1 +/- 26.54 nm; the mean swelling pressure and compressive modulus of the tissues were 6.15 +/- 4.3 kPa and 1.23 +/- 0.7 MPa, respectively. The mean fibril diameter had no linear correlation (R(2) = 0.30) with the swelling pressure of the tissues. However, it had a mild linear correlation with the compressive modulus (p = 0.023, R(2) = 0.68). This is the first study, to our knowledge, to evaluate the nanostructure of the individual collagen fibrils of the nucleus pulposus and its relationship with macroscale mechanical properties of the NP tissues.

Collagens

The collagens represent a family of trimeric extracellular matrix molecules used by cells for structural integrity and other functions. The three alpha chains that form the triple helical part of the molecule are composed of repeating peptide triplets of glycine-X-Y. X and Y can be any amino acid but are often proline and hydroxyproline, respectively. Flanking the triple helical regions (i.e., Col domains) are non-glycine-X-Y regions, termed non-collagenous domains. These frequently contain recognizable peptide modules found in other matrix molecules. Proper tissue function depends on correctly assembled molecular aggregates being incorporated into the matrix. This review highlights some of the structural characteristics of collagen types I-XXVIII.

Collagen XVI expression is upregulated in glioblastomas and promotes tumor cell adhesion

The poor prognosis of glioblastoma patients is related to diffuse brain invasion and interaction of tumor cells with extracellular matrices (ECM). We describe expression and function of the FACIT-collagen XVI in glioblastomas. We found upregulation of collagen XVI mRNA as well as protein in glioblastomas as compared to normal cortex. SiRNA knockdown resulted in decreased cell adhesion whereas increased adhesion was observed on surfaces coated with collagen XVI. The migration of glioblastoma cells on this substrate remained unchanged. Our results demonstrate de-novo expression of collagen XVI in glioblastomas as part of the tumor specific remodeling of the ECM.

Evolution of collagen-based adhesion systems

Collagens are large, triple-helical proteins that form fibrils and network-like structures in the extracellular matrix. The collagens may have participated in the evolution of the metazoans from their very earliest origins. Cell adhesion receptors, such as the integrins, are at least as old as the collagens. Still, the early metazoan cells might not have been able to anchor directly to collagen fibrils, since the integrin-type collagen receptors have only been identified in vertebrates. Instead, the early metazoans may have used integrin-type receptors in the recognition of collagen-binding glycoproteins. It is possible that specialized, high-avidity collagen-receptor integrins have become instrumental for the evolution of bone, cartilage, circulatory and immune systems in the chordates. In vertebrates, specific collagen-binding receptor tyrosine kinases send signals into cells after adhesion to collagen. These receptors are members of the discoidin domain receptor (DDR) group. The evolutionary history of DDRs is poorly known at this time. DDR orthologs have been found in many invertebrates, but their ability to function as collagen receptors has not yet been tested. The two main categories of collagens, fibrillar and non-fibrillar, already exist in the most primitive metazoans, such as the sponges. Interestingly, both integrin and DDR families seem to have members that favor either one or the other of these two groups of collagens.

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.

Three novel collagen VI chains, alpha4(VI), alpha5(VI), and alpha6(VI)

We report the identification of three new collagen VI genes at a single locus on human chromosome 3q22.1. The three new genes are COL6A4, COL6A5, and COL6A6 that encode the alpha4(VI), alpha5(VI), and alpha6(VI) chains. In humans, the COL6A4 gene has been disrupted by a chromosome break. Each of the three new collagen chains contains a 336-amino acid triple helix flanked by seven N-terminal von Willebrand factor A-like domains and two (alpha4 and alpha6 chains) or three (alpha5 chain) C-terminal von Willebrand factor A-like domains. In humans, mRNA expression of COL6A5 is restricted to a few tissues, including lung, testis, and colon. In contrast, the COL6A6 gene is expressed in a wide range of fetal and adult tissues, including lung, kidney, liver, spleen, thymus, heart, and skeletal muscle. Antibodies to the alpha6(VI) chain stained the extracellular matrix of human skeletal and cardiac muscle, lung, and the territorial matrix of articular cartilage. In cell transfection and immunoprecipitation experiments, mouse alpha4(VI)N6-C2 chain co-assembled with endogenous alpha1(VI) and alpha2(VI) chains to form trimeric collagen VI molecules that were secreted from the cell. In contrast, alpha5(VI)N5-C1 and alpha6(VI)N6-C2 chains did not assemble with alpha1(VI) and alpha2(VI) chains and accumulated intracellularly. We conclude that the alpha4(VI)N6-C2 chain contains all the elements necessary for trimerization with alpha1(VI) and alpha2(VI). In summary, the discovery of three additional collagen VI chains doubles the collagen VI family and adds a layer of complexity to collagen VI assembly and function in the extracellular matrix.

Interaction of extracellular matrix protein 1 with extracellular matrix components: ECM1 is a basement membrane protein of the skin

The extracellular matrix protein 1 (ECM1) is a secreted glycoprotein, which plays an important role in the structural and functional biology of the skin as demonstrated by the identification of loss-of-function mutations in ECM1 as cause of the genodermatosis lipoid proteinosis, characterized by reduplication of the skin basement membrane and hyalinization of the underlying dermis. To search for binding partner(s) of ECM1, we tested the in vitro binding activity of ECM1a, a major isoform of four ECM1 splice variants, to different skin extracellular matrix proteins (such as laminin 332, collagen type IV, and fibronectin) and polysaccharides (such as hyaluronan, heparin, and chondroitin sulfate A) with solid-phase binding assay. We demonstrated that ECM1a utilizes different regions to bind to a variety of extracellular matrix components. Ultrastructurally, ECM1 is a basement membrane protein in human skin and is part of network-like suprastructures containing perlecan, collagen type IV, and laminin 332 as constituents. Furthermore, ECM1a enhanced the binding of collagen IV to laminin 332 dose-dependently, showing its involvement in the dermal-epidermal junction and interstitial dermis and making the functional link to the pathophysiology of lipoid proteinosis. To our knowledge, this is previously unreported.

The collagen family members as cell adhesion proteins

The collagen family of extracellular matrix proteins has played a fundamental role in the evolution of multicellular animals. At the present, 28 triple helical proteins have been named as collagens and they can be divided into several subgroups based on their structural and functional properties. In tissues, the cells are anchored to collagenous structures. Often the interaction is indirect and mediated by matrix glycoproteins, but cells also express receptors, which have the ability to directly bind to the triple helical domains in collagens. Some receptors bind to sites that are abundant in all collagens. However, increasing evidence indicates that the coevolution of collagens and cell adhesion mechanisms has given rise to receptors that bind to specific motifs in collagens. These receptors may also recognize the different members of the large collagen family in a selective manner. This review summarizes the present knowledge about the properties of collagen subtypes as cell adhesion proteins.

Type XXVII collagen at the transition of cartilage to bone during skeletogenesis

COL27A1 is a member of the collagen fibrillar gene family and is expressed in cartilaginous tissues including the anlage of endochondral bone. To begin to understand its role in skeletogenesis, the temporospatial distributions of its RNA message and protein product, type XXVII collagen, were determined in developing human skeletal tissues. Laser capture microdissection and quantitative reverse-transcription polymerase chain reaction demonstrated that gene expression occurred throughout the growth plate and that it was higher in the resting and proliferative zones than in hypertrophic cartilage. Immunohistochemical analyses showed that type XXVII collagen was most evident in hypertrophic cartilage at the primary ossification center and at the growth plate and that it accumulated in the pericellular matrix. Synthesis of type XXVII collagen overlapped partly with that of type X collagen, a marker of chondrocyte hypertrophy, preceded the transition of cartilage to bone, and was associated with cartilage calcification. Immunogold electron microscopy of extracted ECM components from mouse growth plate showed that type XXVII collagen was a component of long non-banded fibrous structures, filamentous networks, and thin banded fibrils. The timing and location of synthesis suggest that type XXVII collagen plays a role during the calcification of cartilage and the transition of cartilage to bone.