What Is the Extracellular Matrix?

• Extracellular Matrix Types
• ECM Composition
• Glycosaminoglycans (GAGs)
• Proteoglycans
• Glycoproteins
• Fibrous Proteins

The extracellular matrix (ECM) is secreted by cells and surrounds them in tissues. It has long been understood to be the structural support for cells since its characteristics set the characteristics of the tissue (i.e. bone compared to cartilage compared to brain)¹. However, instead of simply being a passive, mechanical support for cells, it is in fact an extraordinarily complex scaffold composed of a variety of biologically active molecules that are highly regulated and critical for determining the action and fate of the cells that it surrounds⁴.

Extracellular Matrix Types

The ECM has two basic forms:

1. Basement membrane: ECM between epithelial and stromal layers of cells⁵
2. Interstitial matrix: ECM surrounding cells forming a porous 3D lattice⁵

Basement Membrane

The basement membrane (BM) is a thin layer of ECM that forms between the epithelia and endothelia. It surrounds muscle, fat, and nerve cells. It provides mechanical structure, separates different cell types, and signals for cell differentiation, migration, and survival⁶. A summary of the proteins in the basement membrane and their characterized cell surface receptors are given in Table 1⁶.

The BM structure is thought to be set by laminin and collagen IV. Laminin has been shown to self assemble into polygonal lattices in vitro⁶. Likewise, collagen IV also self assembles in vitro through interactions between the carboxy-terminal NC1 (non-collagenase) domains and some slight interactions between the collagen triple helices⁶. The other protein components of the BM are thought to be non-covalently immobilized in the matrix formed by laminin and collagen IV⁶.

A form of the basement membrane is commercially available. It is derived from the Engelbreth-Holm-Swarm (EHS) mouse sarcoma, a tumor that is rich in ECM proteins. Its major components are laminin, collagen IV, heparan sulfate proteoglycan, and nidogen/entactin. In addition to these proteins, there are a variety of growth factors present including bFGF, EGF, IGF-1, PDGF, NGF and TGF-ß1. This same tumor is the primary source for commercially available mouse laminin.

ECM Composition

The ECM is a complex mixture of proteins and glycosaminoglycans (a class of negatively charged polysaccharides). It is composed of three categories of materials:

1. Glycosaminoglycans and their proteoglycansthat resist compressive forces
2. Adhesive glycoproteins (laminin, fibronectin, tenascin, nidogen)
3. Fibrous proteinsthat provide tensile strength (collagens, elastin)

Glycosaminoglycans (GAGs)

GAGs were originally primarily known for being “space fillers” in the ECM. More recently they have been shown to be active signaling molecules whose roles in a variety of cellular processes (including cytokine production, leukocyte recruitment and inflammatory response) are important for controlling cell fate⁷.

Glycosaminoglycans (GAGs) are linear polysaccharides composed of two basic saccharides: an amino sugar and an uronic acid ^2,3. The amino sugar is typically either N-acetyl-D-glucosamine (D-GlcNAc) or N-acetyl-D-galactosamine (D-GalNAc). The uronic acid is either D-glucuronic acid (D-GlcA) or L-iduronic acid (L-IdoA)⁷. These basic components are further varied by epimerization, sulfation, and deacetylation. The order of the carbohydrate chain and the other chemical modifications determine their specificity and functionality⁷.

Hyaluronan is the simplest GAG since it is non-sulfated, doesn’t undergo epimerization, and is composed of an unmodified disaccharide repeat. It is also not typically covalently linked to any proteins. The other GAGs are chondroitin sulfate (CS), dermatan sulfate (DS), keratan sulfate (KS), and heparan sulfate (HS). These four GAGs are typically covalently attached to proteins to form proteoglycans. Chondroitin and heparan sulfate are extensively modified by sulfation ^1,2,3.

Proteoglycans

A proteoglycan is composed of a core protein with one or more covalently attached GAGs. They are stored in secretory granules, inserted into the plasma membrane or secreted into the ECM⁸. Table 2 lists several proteoglycans along with their molecular weight, GAG chains, and tissue localization.

Glycoproteins

Laminin

Currently there are 15 known heterotrimeric laminins⁶. Laminin is composed of α, β, and γ chains of which there are 5 α, 4 β, and 6 γ chains². It has a cross-like structure with 3 short arms and 1 long arm. The α chains possess a large globular domain known as the G domain at the C-termini. This domain is composed of 3 LG domains (LG1-LG3) connected by a linker region to two more LG domains (LG4-LG5). Integrins bind to LG1-3 and dystroglycan to LG4-LG5. Heparan sulfate has been shown to bind to LG4 of the α1 chain6. Laminin is a component of the basement matrix; however, the isoform of laminin present varies with the tissue2. The laminin-1 form is the most prominent in early development and is comprised of α1, β1, and γ1 chains⁶.

Fibronectin

Fibronectin is a dimer with a molecular weight of ~270 kDa. It exists as a soluble form in blood and body fluids and in fibrils in the ECM. It binds collagen, heparin, other fibronectin proteins, and cell surface integrins. Fibronectin binds integrins through the tri-peptide motif of arginine, glycine, and aspartic acid (RGD)^2,3.

Fibrous Proteins

Collagen

Collagens are the major structural component of the ECM¹. They are the most prevalent protein in the skin and bone, making up 25% of the total protein mass². Collagens provide scaffolding for the attachment of laminin, proteoglycans and cell surface receptors¹. Twenty-eight types of collagens (I–XXVIII) have been identified so far in vertebrates¹. Collagens are triple helical proteins that are formed from either homotrimers or heterotrimers of polypeptide chains, referred to as α-chains. α-chains have a three amino acid repeat of Gly-X-Y, where X is typically proline and Y is 4-hydroxyproline (post-translationally modified proline)^1,2.

Based on their supramolecular architectures, these types are divided in:

• Fibrillar (I, II, III, V, XI, XXIV and XXVII) – The most common type of collagen, which accounts for 90% of the collagen in the body; it is prominent in bone, skin, tendons, ligaments, and cartilage . Fibrils have a diameter of 10-30 nm in diameter and assembled collagen fibers can have a diameter of 500-3000 nm.
• Fibril-associated (FACIT) (IX, XII, XIV, XVI, XIX, XX, XXI, XXII) – These collagens do not form fibrils; instead they attach to fibril-forming collagens; they are thought to order collagen fibrils within the matrix
• Beaded filament (VI)
• Network-forming (IV, VIII and X) – These form net-like structures such as in the basement membrane; they also interact with anchoring fibrils (type VII) which link the basement membrane to collagen and laminin in the ECM. Collagen IV self assembles and is instrumental in the formation of the basement membrane.
• Transmembrane collagens (XIII, XVII, XXIII, XXV) – These suprastructures have functional roles in cell adhesion, differentiation, tissue development and structural integrity¹.

Elastin

Elastin, as its name suggests, provides elasticity to the ECM. It is produced as tropoelastin, a 72 kDa precursor protein and is secreted from the cell. In the extracellular space, it crosslinks with other elastin molecules to form sheets and fibers. Elastin is the primary ECM protein present in arteries where it composes ~50% of their dry weight².