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J Glaucoma. Author manuscript; available in PMC 2015 October 01.
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J Glaucoma. 2014 ; : S20–S23. doi:10.1097/IJG.0000000000000108.

Biology of the Extracellular Matrix: An Overview

Beatrice Yue, PhD
Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago College of
medicine, Chicago, Illinois

The extracellular matrix (ECM) is an intricate network composed of an array of

multidomain macromolecules organized in a cell/tissue-specific manner. Components of the
ECM link together to form a structurally stable composite, contributing to the mechanical
properties of tissues. The ECM is also a reservoir of growth factors and bioactive molecules.
It is a highly dynamic entity that is of vital importance, determining and controlling the most
fundamental behaviors and characteristics of cells such as proliferation, adhesion, migration,
polarity, differentiation, and apoptosis.1,2
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Major ECM elements

The “core matrisome”3 comprises approximately 300 proteins. Major components include
collagens, proteoglycans, elastin, and cell-binding glycoproteins, each with distinct physical
and biochemical properties.

Collagen is composed of 3 polypeptide α chains that form a triple helical structure. In

vertebrates, 46 distinct collagen chains assemble to form 28 collagen types2,4 that are
categorized into fibril-forming collagens (e.g., types I, II, III), network-forming collagens
(e.g., the basement membrane collagen type IV), fibril-associated collagens with
interruptions in their triple helices, or FACITs (e.g., types IX, XII), and others (e.g., type
VI). The fibril-forming collagens contain continuous triple-helix-forming domains flanked
by amino- and carboxyl-terminal noncollagenous domains. These noncollagenous domains
are proteolytically removed and triple helices formed are associated laterally into fibrils.
Nonfibril supramolecular structures, such as the networks of collagen IV in basement
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membranes and beaded filaments, are formed by nonfibrillar collagens. The FACITs do not
assemble into fibrils by themselves, but are associated with collagen fibrils.

Specific proline residues in collagens are hydroxylated by prolyl 4-hydroxylase and prolyl
3-hydroxylase. Selected lysine residues are also hydroxylated by lysyl hydroxylase. The
fibrillar procollagens, following processing, are secreted into the extracellular space where
their propeptides are removed. The resulting collagens then assemble into fibrils via
covalent cross-links formed between lysine residues of two collagen chains by a process
catalyzed by extracellular enzyme lysyl oxidases (LOX). The collagenous backbone dictates
the tissue architecture, shape, and organization.

Proteoglycans consist of a core protein to which glycosaminoglycan (GAG) side chains are
attached. GAGs are linear, anionic polysaccharides made up of repeating disaccharide units.
There are four groups of GAGs: hyaluronic acid, keratan sulfate; chondroitin/dermatan
sulfate; and heparan sulfate, including heparin. All except hyaluronic acid are sulfated. The
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highly negatively charged GAG chains allow the proteoglycans to sequester water and
divalent cations, conferring space-filling and lubrication functions. Secreted proteoglycans
include large proteoglycans, such as aggrecan and versican, small leucine-rich
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proteoglycans, such as decorin and lumican, and basement membrane proteoglycans, such as
perlecan. Syndecans are cell-surface-associated whereas serglycin is an intracellular
proteoglycan. The molecular diversity of proteoglycans provides structural basis for a
multitude of biological functions. For instance, aggrecan in cartilage generates elasticity and
high biomechanical resistance to pressure. Decorin and lumican have a regulatory role in
collagen fibril assembly. Proteoglycans also interact with growth factors and growth factor
receptors, and are implicated in cell signaling5 and biological processes, including
angiogenesis.

The laminin family comprises about 20 glycoproteins that are assembled into a cross-linked
web, interwoven with the type IV collagen network in basement membranes. They are
heterotrimers (400–800 kDa) consisting of one α, one β, and one γ chain. In vertebrates, five
α, three β, and three γ chains have been identified. Many laminins self-assemble to form
networks that remain in close association with cells through interactions with cell surface
receptors. Laminins are essential for early embryonic development and organogenesis.6
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Fibronectin is critical for the attachment and migration of cells, functioning as “biological
glue”. The fibronectin monomer (~250 kDa) is made of subunits which comprise three types
of repeats: I, II, and III. Fibronectin is secreted as dimers linked by disulfide bonds and has
binding sites to other fibronectin dimers, collagen, heparin, and cell surface receptors. In the
FNIII10 repeat, there is an important Arg-Gly-Asp cell-binding site. The fibronectin dimers
can form multimers. With continued deposition, the fibrils are lengthened and thickened and
the fibronectin fibrils can be further processed into a deoxycholate-insoluble matrix.7

Elastin imparts elasticity to tissue subjected to repeated stretch, such as vascular vessels and
the lung. It is encoded by a single gene in mammals and is secreted as a 60–70 kDa
tropoelastin monomer. Tropoelastin, with the assistance of fibulins, associates with
microfibrils to form elastic fibers. All tropoelastins share a characteristic domain
arrangement of hydrophobic sequences alternating with lysine-containing cross-linking
motifs. Microfibrillar proteins fibrillins and microfibril-associated glycoprotein-1 interact
directly with elastin and are important for its nucleation and assembly.2 A crucial feature of
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the elastic fiber, critical to its proper function, is the extensive cross-linking of tropoelastin
mediated by LOX, which oxidizes selective lysine residues in peptide linkage to allysine.
There are two major bifunctional cross-links in elastin: dehydrolysinonorleucine, formed
through the condensation of one residue of allysine and one of lysine, and allysine aldol,
formed through the association of two allysine residues. These two cross-links can further
condense with each other, or with other intermediates, to form desmosine or isodesmosine.
Electron microscopy and imaging studies show that tropoelastin is assembled into small
globular aggregates on the cell surface (microassembly). The cross-linking begins which
results in a loss of positive charges on the molecule, enabling the release of tropoelastin
from the cell and assisting globular fusion in the presence of microfibrils (macroassembly).
Fibulin-4 has a role in early stages of elastin assembly and fibulin-5 acts to bridge elastin
between the matrix and cells.8

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Cellular receptors for ECM molecules

ECM molecules connect to the cells through integrins, syndecans, and other receptors.
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Integrins are heterodimeric receptors composed of α and β subunits.9 In vertebrates, the

family encompasses 18 α and 8 β subunits that can assemble into 24 different integrins. Both
α and β subunits are transmembrane proteins with large modular extracellular domains,
single transmembrane helices, and short cytoplasmic regions that mediate cytoskeletal
interactions. The integrins can be grouped into subgroups based on ligand-binding properties
or based on their subunit composition. Major matrix-binding integrins are the β1 integrins
with affinity to fibronectin, collagens, and laminins.

Integrins function as links between the ECM and the cytoskeleton. Activation of integrins by
matrix ligands leads to conformational changes in the integrins, exposing their cytoplasmic
domains to binding of focal complex proteins such as the focal adhesion kinase (FAK) and
integrin–linked kinase (ILK). Clustering of integrins in the membrane follows, and
intracellular proteins including vinculin and talin along with actin stress fibers are assembled
into focal adhesion complexes. The integrins subsequently trigger phosphorylation cascades
and initiate signaling events including Rho and MAP kinase pathways to affect cell
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proliferation, differentiation, polarity, contractility, and gene expression in the so-called

“outside-in” signaling process. Conversely, intracellular signals from proteins such as FAK,
ILK and talin can induce changes in integrin conformation and activation that alter its ligand
binding activity in the “inside-out” signaling fashion. Integrins thus act as a bi-directional
conduit, transmitting signals and providing connections between intra- and extra-cellular
compartments.10,11

ECM remodeling and modulation

The ECM is constantly undergoing remodeling whereby its components are deposited,
degraded, or modified. Intermolecular cross-linking by LOX is a key posttranslational
modification for collagens and elastin. Expanded cross-linking due to excess LOX activity
increases tissue tensile strength and matrix stiffness, affecting thereby cellular behaviors.

Collagens and other ECM elements are substrates for matrix metalloproteinases (MMPs), a
disintegrin and metalloproteases (ADAMs), ADAM with thrombospondin motifs
(ADAMTS), as well as proteases such as cathepsin G and elastase.1 MMPs are produced in
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precursor forms and remain inactive until activated. Most of the 23 members of the MMP
family are secreted, but there are also membrane type MMPs. Their activities are
counteracted by tissue inhibitors of MMPs (TIMPs) and other inhibitors, and an imbalance
may lead to tissue fibrosis and diseases.1 Besides ECM molecules, MMPs and others can
also cleave precursor proteins, release ECM-bound growth factors, and release from ECM
proteins bioactive fragments with new bioactivities, such as endostatin.4

In addition to the turnover process, the ECM is also modulated by exogenous stimuli such as
cytokines, glucocorticoids, oxidative stress, pressure and mechanical stretch. The most
studied cytokine is transforming growth factor-β (TGF-β), which is known to enhance ECM
production and upregulate ECM-related genes.

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ECM as reservoir for bioactive molecules

The ECM has the capacity to store and sequester growth factors and cytokines, establishing
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concentration gradients and regulating spatially and temporally their bioavailability. In

particular, the fibroblast growth factor family strongly binds to heparan sulfate chains of
proteoglycans such as perlecan. Heparan sulfate proteoglycans are also involved in binding,
transporting, and activating developmental control factors including Wnt factors and
hedgehog. The ECM is furthermore a reservoir of bioactive fragments released upon limited
proteolysis. These fragments, with biological properties and activities of their own, regulate
physiological and pathological processes including angiogenesis. The ECM can additionally
participate in ligand maturation. TGF-β, secreted in latent form, is stored in the ECM and
remains inactive until activated by MMP-dependent proteolysis.1

ECM provides chemical and physical cues

The biochemical properties of the ECM allow cells to sense and interact with their
extracellular environment using various signal transduction pathways. The chemical cues are
provided by ECM components, especially the adhesive proteins such as fibronectin, integrin
and non-integrin receptors, as well as growth factors and associated signaling molecules.
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Interactions with different matrices via specific sets of receptors can trigger distinct cellular
responses.10,11

The ECM functions as a physical barrier, an anchorage site, or a movement track for cell
migration.1 The physical properties of the ECM, including its rigidity, density, porosity,
insolubility and topography (spatial arrangement and orientation), provide physical cues to
the cells. The mechanical properties are essentially sensed by integrins that connect
extracellular ECM to the actin cytoskeleton inside the cells. Stiff matrices induce integrin
clustering, robust focal adhesions, Rho and MAP kinase activation, leading to increased
proliferation and contractility.1 Matrix rigidity also regulates differentiation. For example,
on soft matrices, mesenchymal stem cells favor a neurogenic path, and on stiff ones they
favor an osteogenic path.

Concluding remarks
The ECM provides a structural scaffold via a network of protein–protein and protein–
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proteoglycan interactions. These interactions are involved in the formation of

supramolecular assemblies such as collagen fibrils and elastic fibers, in tissue architecture,
and in cell-matrix interactions that regulate cell growth and behavior.

The cells and the ECM have a two-way reciprocal relationship (Fig. 1).10,12 Cells produce,
secrete, deposit, and remodel ECM to mediate ECM composition and topography. The ECM
in turn transmits signals through ECM receptors to influence cell characteristics and
activities (Fig. 2).13 Such a feedback mechanism is essential for rapid response of cells to
surrounding environmental changes.

The multi-functional ECM is now recognized as a “niche” for stem cells (Fig. 3).13,14 ECM
biology also has wide implications in areas ranging from embryonic development, wound

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healing, fibrosis, cancer invasion and metastasis, diabetes, eye diseases including exfoliation
syndrome, to biomedical engineering.
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Acknowledgments
The author acknowledges support from the National Eye Institute (grant EY018828 and core grant EY01792),
Bethesda, MD; the Cless Family Foundation, Northbrook, IL; and Research to Prevent Blindness, New York, NY.

References
1. Lu P, Takai K, Weaver VM, Werb Z. Extracellular matrix degradation and remodeling in
development and disease. Cold Spring Harb Perspec Biol. 2011; 3:a005058.
2. Mecham RO. Overview of extracellular matrix. Curr Protoc Cell Biol. 2012; 57:10.1.1–10.1.16.
3. Hynes RO, Naba A. Overview of the matrisome - An inventory of extracellular matrix constituents
and functions. Cold Spring Harb Perspect Biol. 2012; 4:a004903. [PubMed: 21937732]
4. Ricard-Blum S. The collagen family. Cold Spring Harb Perspect Biol. 2011; 3:a004978. [PubMed:
21421911]
5. Esko, JD.; Kimata, K.; Lindahl, U. Proteoglycans and sulfated glycosaminoglycans. In: Varki, A.;
Cummings, RD.; Esko, JD., et al., editors. Essentials of Glycobiology. 2. Vol. Chapter 16. Cold
Spring Harbor: Cold Spring Harbor Laboratory Press; 2009.
6. Durbeej M. Laminins. Cell Tissue Res. 2010; 339:259–268. [PubMed: 19693542]
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7. Schwarzbauer JE, DeSimone DW. Fibronectins, their fibrillogenesis and in vivo functions. Cold
Spring Harb Perspect Biol. 2011; 3:a005041. [PubMed: 21576254]
8. Kozel, BA.; Mecham, RP.; Rosenbloom, J. Elastin. In: Mecham, RP., editor. The extracellular
matrix: An Overview. Berlin Heidelberg: Springer-Verlag; 2011. p. 267-299.
9. Barczyk M, Carracedo S, Gullberg D. Integrins. Cell Tissue Res. 2010; 339:269–280. [PubMed:
19693543]
10. Geiger B, Yamada KM. Molecular architecture and function of matrix adhesions. Cold Spring
Harb Perspect Biol. 2011; 3:a005033. [PubMed: 21441590]
11. Wolfenson H, Levelin I, Geiger B. Dynamic regulation of the structure and functions of integrin
adhesions. Dev Cell. 2013; 11:447–458. [PubMed: 23484852]
12. Daley WP, Yamada KM. ECM-mediated cellular dynamics as a driving force for tissue
morphogenesis. Curr Opin Genet Dev. 2013; 23:408–414. [PubMed: 23849799]
13. Gattazzo F, Urciuolo A, Bonaldo P. Extracellular matrix: A dynamic microenvironment for stem
cell niche. Biochim Biophys Acta. 2014; 1840:2506–2519. [PubMed: 24418517]
14. Brizzi MF, Tarone G, Defilippi P. Extracellular matrix, integrins, and growth factors as tailors of
the stem cell niche. Curr Opin Cell Biol. 2013; 24:645–651. [PubMed: 22898530]
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Figure 1.
Schematic illustration highlighting the dynamic cross-talk between cells and the
extracellular matrix (ECM). Cells secrete and remodel the ECM, and the ECM contributes to
the assembly of individual cells into tissues, affecting this process at both receptor and
cytoskeletal levels. Adhesion-mediated signaling, based on the cells’ capacity to sense the
chemical and physical properties of the matrix, affects both global cell physiology and local
molecular scaffolding of the adhesion sites. The molecular interactions within the adhesion
site stimulate, in turn, the signaling process, by clustering together the structural and
signaling components of the adhesome. (From Geiger B, Yamada KM. Molecular
architecture and function of matrix adhesions. Cold Spring Harb Perspect Biol
2011;3:a005033. Copyright 2014 Cold Spring Harbor.)
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Figure 2.
Regulation of cell behavior by ECM. The effects exerted on cells by ECM can be differently
mediated. The ECM can directly bind different types of cell surface receptors or co-
receptors (red, orange, black), thus mediating cell anchorage and regulating several
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pathways involved in intracellular signaling and mechanotransduction. Moreover, the ECM

can act by noncanonical growth factor (cyan) presentation and be remodeled by the action of
enzymes (yellow pie), which can release functional fragments (green). (From Gattazzo F,
Urciulo A, Bonaldo P. Extracellular matrix: A dynamic microenvironment for stem cell
niche. Biochim Biophys Acta 2014;1840:2506–2519. Copyright 2014 Elsevier.)
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Figure 3.
Hypothetical model of ECM, integrins and transmembrane receptors crosstalk in the stem
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cell niche. ECM/integrin interaction within the stem cell niche contributes to three main
functions: (1) Cell-matrix anchorage, in which integrin-mediated cell adhesion physically
anchors the stem cell to ECM proteins. Through these mechanisms, growth factor receptors
can be activated by an integrin-dependent mechanism as well. (2) Reservoir for growth
factors and morphogens: ECM binding of soluble growth factors and morphogens spatially
and temporally controls their interactions with transmembrane receptors. Integrin crosstalk
with other receptors regulates signaling (Erk1/2, Akt, SMADs) preserving stemness. (3)
Biomechanical stiffness. ECM/integrin interaction senses mechanical forces, leading to
cytoskeleton re-organization and stem cell homeostasis. , growth factors and their
receptors; , morphogens and their receptors; , extracellular matrix; , α and β
integrin subunits; , cytoskeletal filaments; SC: stem cell. (From Brizzi F, Tarone G,
Defilippi P. Extracellular matrix, integrins, and growth factors as tailors of the stem cell
niche. Curr Opin Cell Biol 2012;24:645–651. Copyright 2014 Elsevier.)
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