Home › Biochemistry › Extracellular Matrix

Extracellular Matrix

Q: Which enzymes degrade extracellular matrix proteins?

ECM proteins are degraded by proteolytic enzymes, notably: Matrix metalloproteinases (MMPs). Collagenase MMP-1 is a $Zn^{2+}$-dependent ECM enzyme that performs the initial cleavage of native collagen molecules into two fragments (1/4 and 3/4 length). It also degrades other matrix proteins such as laminin and fibronectin.Gelatinases and other proteases, which further break down collagen fragments into amino acids.Lysosomal proteases, which degrade triple-helical collagen fragments.Serine proteases, which work alongside MMPs in pathological remodeling, such as the weakening of the fibrous cap in atherosclerotic plaques.

For medical students2 min readUpdated 2026-10-10

Extracellular matrix (ECM) is a highly specialized molecular framework of connective tissues (such as bone, teeth, cartilage, tendons, and basement membranes). It provides mechanical support to cells while ensuring tensile strength, shock absorption, and selective macromolecular filtration.

Two componentsThe matrix is formed by two main groups of substances: structural proteins and heteropolysaccharides.

Water dynamicsIn the morning, intervertebral discs contain up to 75% water, losing up to 20% under daily mechanical load.

Selective filterIn the kidneys, the basement membrane lies between two cell layers, filtering plasma.

Anchoring fixationType VII collagen firmly "stitches" the epidermis to the underlying dermis by forming loops.

Biochemical Composition

The properties of any tissue depend on the proportions and organization of the two fundamental components of the extracellular matrix:

Proteins
Collagen — forms the primary structural scaffold.
Elastin — provides tissue extensibility and elasticity.
Fibronectin — acts as a universal adhesive protein.
Laminin — a crucial adhesive component linking cells to sub-cellular structures (characteristic of basement membranes).

Heteropolysaccharides
Glycosaminoglycans (GAGs).
Proteoglycans — complex macromolecules consisting of a core protein bonded to GAG chains.

Basement Membranes

Basement membranes are thin sheets of specialized ECM that isolate various cells (epithelial, endothelial, fat, muscle, and Schwann cells) from the surrounding connective tissue.

Under the electron microscope, a bilayered structure is visible:

Lamina rara (lucida) — lies directly adjacent to the cell membrane.
Lamina densa — contacts the underlying connective tissue.

Molecular Organization: The structural backbone is formed by type IV collagen, creating an elastic three-dimensional network. Laminin and heparan sulfate proteoglycans (HSPGs) attach to this network. A vital role is played by the sulfated glycoprotein nidogen (entactin). Its polypeptide chain features three globular domains: one firmly binds laminin, and another binds type IV collagen. Consequently, nidogen acts as a linker molecule, forming a ternary complex of "laminin – nidogen – collagen IV".

Functions of Basement Membranes:

Structural: Cell attachment and strict spatial polarization.
Filtration: In renal glomeruli, the membrane acts as a semipermeable filter, preventing plasma macromolecules from entering the primary urine.
Electrostatic barrier: Proteoglycan chains carry a high negative charge, physically repelling negatively charged molecules (such as plasma proteins) and erythrocytes.
Regulatory: Involvement in tissue regeneration and embryogenesis.

Organization of Subepithelial Layers

To prevent the epithelial basement membrane from separating from the underlying connective tissue (stroma/dermis), an anchoring mechanism is required. The primary organizing element here is type VII collagen.

Dimers of this protein form bundles known as anchoring fibrils. Their C-termini attach to the lamina densa of the basement membrane and loop down into the connective tissue matrix.

In the subepidermal zone, these loops are fixed by two main mechanisms:

Attaching to anchoring plaques — specialized matrix densities composed of type IV collagen.
Surrounding interstitial fibrils (types I and III collagen) that pass through their loops.

Biomechanics of Articular Cartilage

Articular cartilage must withstand immense mechanical loads. Its fibrillar endoskeleton is formed by types II, IX, and XI collagens. However, the shock-absorbing function relies entirely on water-binding macromolecules.

Cartilage contains massive high-molecular-weight aggregates of aggrecan and hyaluronic acid. The GAG chains within them act as polyanions, possessing numerous acidic groups and a strong negative charge. This property allows the matrix to attract and retain large amounts of water (high hydration).

Mechanism of Action:

During compression, water is physically squeezed out of the matrix until the internal swelling pressure balances the applied load.
Upon load removal, decompression occurs, and water rushes back into the cartilage matrix.

Mnemonic

The layers of the basement membrane can be remembered by density: first comes the clear and sparse lamina rara (Latin for rare/thin, adjacent to the cell), followed by the dense lamina densa (Latin for dense/thick), which rests on the connective tissue.

Frequently asked questions

What are the main classes of glycosaminoglycans (GAGs) in the matrix?

The main classes of glycosaminoglycans (GAGs) in the ECM include hyaluronic acid and sulfated GAGs:

Hyaluronic acid — nonsulfated.
Chondroitin sulfates — sulfated (typically at C4 or C6).
Dermatan sulfate — an epimer of chondroitin sulfate, sulfated at C4.
Keratan sulfate — the only GAG lacking uronic acid, sulfated at C6.
Heparin — a heavily sulfated GAG.
Heparan sulfate — contains N- and O-sulfate groups.

All listed GAGs except hyaluronic acid contain sulfate groups and act as polyanions.

What are the stages of collagen biosynthesis?

Collagen biosynthesis is divided into two primary phases: intracellular (synthesis and modification) and extracellular (maturation and assembly).

Intracellular stages:

Synthesis of preprocollagen on rough endoplasmic reticulum ribosomes.
Cleavage of the signal peptide.
Hydroxylation of proline and lysine residues (requires prolyl hydroxylase and vitamin C as a cofactor).
Glycosylation.
Formation of the triple helix (procollagen).
Secretion into the extracellular space.

Extracellular stages:

Cleavage of N- and C-terminal propeptides by procollagen peptidases in the ECM.
Formation of tropocollagen.
Self-assembly of tropocollagen into collagen fibrils (via microfibrils).
Formation of covalent cross-links.
Aggregation of fibrils into large bundles forming mature collagen fibers.

Which cell surface receptors bind to fibronectin?

Fibronectin binds to integrins, which are transmembrane cell surface receptors. Specifically, fibronectin acts as a ligand for the following leukocyte $\alpha\beta$-integrins:

VLA-4 ($\alpha_4\beta_1$)
VLA-5 ($\alpha_5\beta_1$)
LPAM-1 ($\alpha_4\beta_7$)

Which enzymes degrade extracellular matrix proteins?

ECM proteins are degraded by proteolytic enzymes, notably:

Matrix metalloproteinases (MMPs). Collagenase MMP-1 is a $Zn^{2+}$-dependent ECM enzyme that performs the initial cleavage of native collagen molecules into two fragments (1/4 and 3/4 length). It also degrades other matrix proteins such as laminin and fibronectin.
Gelatinases and other proteases, which further break down collagen fragments into amino acids.
Lysosomal proteases, which degrade triple-helical collagen fragments.
Serine proteases, which work alongside MMPs in pathological remodeling, such as the weakening of the fibrous cap in atherosclerotic plaques.

Why does human height change throughout the day?

This is a clinical example of the hydrated matrix function in intervertebral discs. During the day, gravity squeezes water out of the cartilage (a loss of about 20%), reducing height by 1–2 cm. During sleep, without axial load, water reabsorbs. In microgravity (spaceflight), height can increase by up to 5 cm.

How does the basement membrane function as an electrostatic barrier?

The barrier is provided by proteoglycans carrying a high negative density charge. They physically repel negatively charged molecules and cells, preventing the loss of plasma proteins and blood cells.

What is nidogen and what is its function?

Nidogen is a sulfated glycoprotein with three globular domains. It serves as a molecular linker: one domain binds to laminin, and the other binds to type IV collagen.

Go deeper

Minor cartilage matrix components: small leucine-rich proteoglycans, growth factors, and non-collagenous proteins (anchorin, chondroadherin).
Structure and functional domains of the laminin molecule in cell-matrix interactions.
Supramolecular organization of aggrecan and hyaluronic acid aggregates in cartilage.
The role of the extracellular matrix in embryonic development and tissue regeneration.