Why is matrix extracellular

Tissue homeostasis is mediated by the coordinated secretion of fibroblast metalloproteinases MMPs Mott and Werb, ; this is counterbalanced by tissue inhibitors of metalloproteinases TIMPs Cruz-Munoz and Khokha, and the controlled activity of other enzymes, such as LOX, and also transglutaminases that crosslink and, consequently, stiffen the ECM Lucero and Kagan, These ECM-bound GFs differentially modulate cell growth and migration and, when released, comprise part of a tightly controlled feedback circuit that is essential for normal tissue homeostasis Hynes, As a tissue ages the levels of junctional proteins such as cadherin, catenin or occludin decrease and this loss can compromise junctional integrity as revealed by the appearance of gaps between the epithelial cells Akintola et al.

Old tissue is also characterized by a thinning of the BM, probably. To date, 28 types of collagen have been identified in vertebrates Gordon and Hahn, The majority of collagen molecules form a triple-stranded helix that subsequently can assemble into supramolecular complexes, such as fibrils and networks, depending on the type of collagen.

Fibrous collagens form the backbone of the collagen fibril bundles within the interstitial tissue stroma, whereas network collagens are incorporated into the basal membrane BM.

Synthesis of collagen type I involves a number of enzymatic post-translational modifications Gordon and Hahn, ; Myllyharju and Kivirikko, , mainly the hydroxylation of proline and lysine residues, glycosylation of lysine and the cleavage of N- and C-terminal propeptides. Following their cleavage, collagen fibrils are strengthened by the covalent crosslinking between lysine residues of the constituent collagen molecules by lysyl oxidases LOX Myllyharju and Kivirikko, ; Robins, FN is secreted as a dimer joined by two C-terminal disulfide bonds and has several binding sites to other FN dimers, to collagen, to heparin and also to cell-surface integrin receptors Pankov and Yamada, Cell-surface binding of the soluble FN dimer is essential for its assembly into longer fibrils.

Moreover, cell contraction through the actomyosin cytoskeleton and the resulting integrin clustering promotes FN—fibril assembly by exposing cryptic binding sites, thus allowing them to bind one another Leiss et al.

Moreover, the resident fibroblasts in aged tissues are growth-arrested and resistant to apoptotic cues, which is indicative of senescence Campisi and d'Adda di Fagagna, Nevertheless, and somewhat paradoxically, in an aging tissue, collagen fibers are frequently — inappropriately — crosslinked through glycation, by byproducts of lipid oxidation and through exposure to UV light Robins, The combination of elevated and inappropriate collagen crosslinking contributes to tissue stiffening so that an aged tissue is mechanically weaker and less elastic but also more rigid than a young tissue Calleja-Agius et al.

This aberrant mechanical state can severely compromise ECM organization, and modify epithelial organization and function, potentially promoting age-related diseases such as cancer Coppe et al. Acute injury activates the fibrogenic machinery and induces wound healing. One of the first events that characterize a wound response is vascular damage and the formation of a fibrin clot, which stimulates monocyte infiltration to the damaged ECM.

Upon binding to ECM-degradation products and cytokines, monocytes rapidly differentiate into macrophages Clark, These activated macrophages, in turn, secrete and release multiple GFs, MMPs and cytokines that promote angiogenesis and stimulate fibroblast migration and proliferation Schultz and Wysocki, The elevated mechanical stress associated with this profound ECM deposition can induce the transdifferentiation of fibroblasts and other tissue-resident cells — i.

Myofibroblasts, which have a high capacity to synthesize ECM components and are highly contractile, can promote the formation of large, rigid collagen bundles that, if crosslinked by LOX enzymes, mechanically strengthen and stiffen the tissue Szauter et al. The remodeled ECM also promotes the directional migration of cells within the tissue towards the wound site Schafer and Werner, In a healthy tissue, once the wound has been repopulated, strict feedback mechanisms are initiated that ensure restoration of tissue homeostasis and resolution of fibrosis Schultz and Wysocki, ; Velnar et al.

Under extreme conditions, such as repeated injury or when normal feedback mechanisms are compromised, continuous ECM synthesis, deposition and remodeling ensue and myofibroblasts remain, in which TIMP production prevails over MMP synthesis. These aberrant conditions promote chronic vascular remodeling and enhanced ECM crosslinking that eventually leads to aberrant fibrosis and an inability of the tissue to heal properly.

This aberrant wound healing scenario is characterized by the altered mechanical stability and reduced elasticity that is typical of scarred tissue Kisseleva and Brenner, In extreme cases, a chronic wound can also promote a tumor phenotype De Wever et al. Cancer is the loss of tissue organization and aberrant behavior of the cellular components. Cell transformation results from genetic mutations and epigenetic alterations. Yet, tumors have also been likened to wounds that fail to heal Schafer and Werner, Thus, the tumor stroma exhibits some of the characteristics found in an unresolved wound Bissell and Radisky, For example, tumors are characteristically stiffer than the surrounding normal tissue.

The stiffening of tumors is induced by ECM deposition and remodeling by resident fibroblasts, and by increased contractility of the transformed epithelium Butcher et al. Moreover, chemokines and GFs De Wever et al. Tissue inflammation potentiates stromal fibroblast activation and induces their transdifferentiation into myofibroblasts, thus exacerbating and promoting tissue desmoplasia De Wever et al.

As a consequence, newly deposited and remodeled collagen and elastin fibers are reoriented and, thereafter, crosslinked by LOX and transglutaminase, thus generating larger, more-rigid fibrils that further stiffen the tissue ECM Butcher et al.

MMPs, which are secreted and activated by tumor cells and by myofibroblasts De Wever et al. The release of GFs, including vascular endothelial growth factor VEGF , enhances vascular permeability and promotes new vessel growth, which generates interstitial tissue pressure. Thus, an amplifying circuitry between tumor-associated ECM stiffening, an ensuing reciprocal ECM resistance that is induced by resident tumor cells, and myoepithelial and cell-generated contractility act as a vicious, positive-feedback loop to potentiate tumor growth and survival.

This induces angiogenesis and invasion and, eventually, fosters metastasis Butcher et al. Considering the importance of the ECM to so many fundamental cellular processes, a myriad of tissue-culture models have been developed to study the interplay between its biochemical and biophysical properties, and to understand the molecular origins of cellular behaviors regulated by ECM ligation.

With respect to assessing the fundamental nature of cell adhesion and its effects on cell behavior, the majority of cancer researchers have relied on coating tissue culture dishes whether plastic or glass with purified preparations or mixtures of ECM proteins in order to obtain 2D monolayers Kuschel et al. Yet, none of these strategies faithfully recapitulates the behavior of cells within tissues, which demand not only a 3D format, but an ECM that can be readily remodeled.

To address the aspect of 3D and ECM remodeling, researchers have used natural ECM and reconstituted ECM gels to recapitulate specific aspects of tissue-specific differentiation and architecture see poster, panel 3. For instance, the rBM, which mimics some of the biochemical and biophysical properties of endogenous epithelial basement membranes, has been used frequently in 3D organotypic culture assays, for xenograft manipulations or tissue engineering, and to study tissue-specific morphogenesis e.

Fibrin has also been used as natural biodegradable scaffold with reasonable success in vascular tissue engineering, but lacks the mechanical strength and durability of native interstitial ECM Blomback and Bark, ; Shaikh et al. By contrast, type I collagen is reasonably useful and can be combined with rBM, purified laminin or FN to reconstitute some of the biological aspects of normal and diseased interstitial ECM Friess, ; Gudjonsson et al.

Moreover, collagen type I readily assembles into a mechanically tense network of fibrils that can be oriented, functionally modified, and enzymatically or chemically crosslinked and stiffened. Thus collagen I gels are useful substrates to assess the role of collagen and FN stiffness, and organization on the pathogenesis of tumor progression and invasion Levental et al.

Nevertheless, collagen gels are quite heterogeneous, and modifying their architecture changes their organization, pore size and ligand concentration, thereby complicating the interpretation of data generated from experiments conducted by using this natural scaffold Johnson et al.

To overcome this issue, tissue engineers and biomaterial specialists have generated denuded ECM scaffold from various tissues Macchiarini et al. These scaffolds, combined with colonies of seeded stem cells, can reconstitute normal tissues with reasonable fidelity Lutolf et al. ECMs have also been isolated and extracted from various tissues, such as small intestine, skin from cadavers , pancreas and breast Rosso et al.

One such example is given by porcine-derived small intestinal submucosa SIS , which has proven clinical success for treating patients with hernias Franklin et al. Although these purified ECMs certainly have useful applications, their use is limited in scope owing to the need for well-defined microenvironments in tissue regeneration and stem cell transplantation in which animal byproducts and contaminants are limited.

Moreover, to understand the molecular and biophysical mechanisms by which the ECM elicits diverse effects on cellular differentiation and morphogenesis it is crucial to use chemically and physically defined, modular ECMs that can be reliably reproduced.

In this respect, synthetic matrices have been developed that feature defined and tunable compositions, organization, mechanics and ECM remodeling capabilities. Indeed, in response to this need there has been literally an explosion of publications describing the generation and application of synthetic ECMs for tissue regeneration, and the reader is referred to some excellent reviews on these topics Ayres et al.

One example is given by polyethylene glycol PEG hydrogels — frequently used biologically compatible synthetic matrices that support cell adhesion, viability and growth Lutolf and Hubbell, Although these matrices can be covalently modified with ECM ligands and collagenase-degradable peptides and GFs Ehrbar et al. By contrast, peptide-based hydrogels, such as peptideamphiphiles, assemble into secondary structures that recapitulate the collagen triple helix, and readily support stem cell growth and viability, and direct multicellular morphogenesis Hauser and Zhang, ; Sieminski et al.

These peptides-amphiphiles are amenable to modification by covalent binding of native proteins and MMP-degradable ECM peptides. Indeed, one of the most exciting recent advances in the field has been the development of modular biocompatible ECMs, which contain ligand-binding cassettes and have tunable stiffness features that permit a precise patterning of cell adhesion in 2D and 3D formats Serban and Prestwich, The realization that ECM organization is a crucial aspect of cellular behavior has led to the development of new methodologies and generated ECMs whose fiber size, orientation, stiffness, ligand-binding function and remodeling potential can be strictly controlled and monitored — including electrospun silk, and lactic-acid polymer PLLA and PLGA scaffolds Zhang et al.

Anisotropically nanofabricated substrates formed from scalable biocompatible PEG Kim et al. Although only time can tell whether this new generation of biomaterials will indeed prove useful, it is an appealing time to be an ECM biologist and our next challenge will be to embrace this smorgasbord of enticing new tools — which hopefully will at last allow us to decipher the language of the matrix.

Deposited in PMC for release after 12 months. Read more about our commitment to Open Access. Submit your essay by 1 December for a chance to be published in Journal of Cell Science. Journal of Cell Science is pleased to welcome submissions for consideration for an upcoming special issue, Cell Biology of Motors , which will be guest edited by Anne Straube University of Warwick, UK. Submission deadline: 15 July Our resident insectivore, Mole, continues his latest series — The Corona Files.

Sign In or Create an Account. Advanced Search. User Tools. Sign in. Skip Nav Destination Article Navigation. Close mobile search navigation Article navigation. Volume , Issue Next Article. Article contents. Bits and pieces — molecular composition of the ECM. The definition of normal — the ECM and tissue homeostasis. Stiffening up — the ECM and tissue aging. Tensional homeostasis and fibrosis. Tumors — a tough situation. Where do we go from here? Challenges encountered with natural and synthetic ECMs.

Article Navigation. The extracellular matrix at a glance Christian Frantz , Christian Frantz. This site. Google Scholar. Kathleen M. Stewart , Kathleen M. Valerie M. Weaver Valerie M. Author and article information. Christian Frantz. Online Issn: J Cell Sci 24 : — Cite Icon Cite. View large Download slide. Box 1. Structure and function of proteoglycans Proteoglycans PGs are composed of glycosaminoglycan GAG chains covalently linked to a specific protein core with the exception of hyaluronic acid Iozzo and Murdoch, ; Schaefer and Schaefer, Box 2.

Collagen and fibronectin synthesis To date, 28 types of collagen have been identified in vertebrates Gordon and Hahn, The extracellular region is primarily occupied by a complicated network of macromolecules constituent called as extracellular matrix ECM.

The composition of ECM is varied, depends on the species and also developing or ground molecules Figures 1 and 2. Commonly, the ECM is composed of three major classes of biomolecules; there are glycosaminoglycans GAGs , linked to a protein known as the proteoglycans, and also fibrous proteins, including collagen, elastin, fibronectin, vitronectin, and laminin.

The structure of the extracellular matrix. The ECM mainly contained collagen fibers. There are also some glycoproteins as an adhesion molecule, such as integrin family fibronectin and laminin, which conduct cell attachments to the ECM by binding to collagen in the ECM and integrin. The intracellular part of integrin highly associated with the cytoskeleton thus may promote to anchoring the cell.

In the end, there are various proteoglycans in the ECM that act as primary proteins and are profoundly modified by the addition of sugars. The extracellular matrix of hyaline cartilage found in abundant collagen fibril and proteoglycan aggregates. The chemical analysis of the ground substance reveals that it contains a few glycoproteins and a high concentration of three types of glycosaminoglycans: hyaluronic acid, chondroitin sulfate, and keratan sulfate.

Adapted from Crammer and Bakkum [ 1 ]. In addition, connective tissue Figure 3 is also composed of the matrix of ECM. One of the essential components of connective tissue is fibroblasts and ground substance.

Ground substance is a mixing complex between GAGs, proteoglycans, and glycoproteins mainly laminin and fibronectin. In most connective tissues, the matrix constituents are secreted by fibroblasts, but in several certain specialized types of connective tissues, like cartilage and bone, these components are secreted by chondroblasts and osteoblasts Table 1.

The components of connective tissue. In addition to the extracellular matrix, connective tissues are characterized by a lot of cells, mainly the fibroblasts, all of which are surrounded by the ground substance. Modified from Mescher [ 2 ]. Source: Michael W. All rights reserved. Modified from Crammer and Bakkum [ 1 ]. In general, all the cells need to attach to the extracellular matrix to grow and multiply.

Extracellular matrix provides support and anchorage for the shape of the cells, regulates and determines cells dynamic and behavior including cell survival, cell proliferation, cell polarity, cell differentiation, cell adhesion, and cell migration. Moreover, the ECM, also gives the mechanical support for tissues and is involved in the growth mechanism, regenerative, and healing processes.

GAGs are unbranched chains of polysaccharides; GAGs are composed of repeating disaccharide units and are heterogeneous groups in negatively charged polysaccharide chains that are covalently linked to proteins to form proteoglycan molecules. The name GAGs is because in this polysaccharide, one of the two sugars in a repetitive disaccharide is always an amino sugar such as N-acetylglucosamine or N-acetylgalactosamine [ 3 ]. The second sugar of GAGs usually is the uronic acid like glucuronic or iduronate.

GAG molecules are negatively charged, because there are sulfate or carboxyl groups in most of the sugar. The five main groups of GAGs are differentiated based on the sugar type including 1 hyaluronan or hyaluronic acid, 2 chondroitin sulfate, 3 dermatan sulfate, 4 heparan sulfate, and 5 keratin sulfate. Hyaluronan is the simplest GAGs.

Hyaluronan does not contain sulfate sugars; all disaccharides units are the same, and the chain length is extensively big thousands of sugar monomers. The hyaluronan is not connected covalently to some core proteins. Proteoglycans are composed of GAG chains that are covalently linked to the core protein and considered to have a significant role in chemical signaling among cells Figure 4. The structure of glycosaminoglycan A structure of a proteoglycan monomer.

Several glycosaminoglycan chains chondroitin sulfphate and keratan sulfate attached to a core protein. The protein molecule can connect to a long hyaluronic acid molecule to help form a proteoglycan aggregate. B An example of an individual glycosaminoglycan chain, in this case, chondroitin 6-sulphate, and its attachment to the core protein. C The morphological of a proteoglycan monomer. Collagen is a major abundant fibrous protein in the extracellular matrix.

Collagens, which constitute the primary structural element of the ECM, provide tensile strength, regulate cell adhesion, support chemotaxis and migration, and direct tissue development [ 4 ].

Recently, there have been already described 28 types of collagen. After secretion, the fibrillar procollagen molecule divides to become collagen molecules, which converge into fibrils [ 5 ]. Fibronectin is an extracellular protein that makes cells adhere to the matrix.

Fibronectin is considered as a large glycoprotein found in all vertebrates. Fibronectin is a ligand member of the integrin receptor family. Integrins are structurally and functionally related to the cell surface as heterodimeric receptors that link the ECM with the intracellular cytoskeleton.

The primary type of fibronectin is known as type III fibronectin replica cylinder , which binds to integrins. This model has a length of about 90 amino acids. Fibronectin appears in a soluble and fibrillar form. There are two others fibronectin isoforms, which are fibronectin type I hexagon and fibronectin type II square [ 6 ].

Fibronectin is not only crucial for attaching cells to matrices but also to guiding cell migration in vertebrate embryos. Fibronectin has many functions, which allow it to interact with many extracellular substances, such as collagen, fibrin and heparin, and with specific membrane receptors in responsive cells.

Extracellular matrix is the primary factor required in the process of forming a new network and tissue. Along with the development found, many different factors can trigger the growth of ECM or used to create a synthetic ECM. The process of wound healing is strongly influenced by the role of migration and proliferation of fibroblasts in the injury site.

Indeed fibroblast is one part of ECM. The proliferation of fibroblasts determines the outcome of wound healing. Fibroblasts will produce collagen that will link to the wound, and fibroblasts will also affect the process of reepithelialization that will close the wound. Fibroblasts will produce type III collagen during proliferation and facilitate wound closure.

During proliferation stage, fibroblasts proliferation activity is higher due to the presence of TGF-stimulated fibroblasts to secrete bFGF.

The higher number of fibroblasts also induces increasing of collagen synthesis. Collagen fiber is the major protein secreted by fibroblast, composed of extracellular matrix to replace wound tissue strength and function.

Collagen fibers deposition was significant on 8—10 days after injury. The number of fibroblasts increases significantly, in correlation with the presence of an abundance of bFGF on 8—10 days after wounding.