Premed · Premed · Cell Biology
Lecture 24: The Extracellular Matrix
Cell Biology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the major components of the extracellular matrix (collagen, proteoglycans, glycoproteins)
- Explain the structure and function of basement membranes
- Describe the structure, activation, and signaling of integrins
- Explain how cells sense and respond to the mechanical properties of the ECM (mechanotransduction)
- Discuss ECM remodeling and its relevance to wound healing and disease
Lecture Content
I. Overview and Major Components
The extracellular matrix (ECM) is a complex network of macromolecules secreted by cells into the extracellular space. It serves multiple functions including structural support, tissue organization, cell signaling, growth factor storage, and providing a substrate for cell migration. The ECM can be divided into two major categories. The interstitial matrix fills the spaces between cells in connective tissue and is composed primarily of collagen types I and III, fibronectin, and proteoglycans. The basement membrane (basal lamina) is a thin, specialized ECM sheet that underlies epithelial, endothelial, and muscle cells.
II. Collagen
Collagen is the most abundant protein in the human body, constituting approximately 30% of total protein. Mammals produce 28 types of collagen, classified by their structural organization. Fibrillar collagens (types I, II, III, V, and XI) form long, banded fibrils that provide tensile strength. Network-forming collagens (type IV) assemble into sheet-like scaffolds and are a principal component of basement membranes. FACIT collagens (types IX, XII, and XIV) are fibril-associated collagens that modify fibril surfaces. Anchoring fibrils (type VII) anchor basement membranes to the underlying connective tissue.
The structure of fibrillar collagen is built around the triple helix, in which three polypeptide chains (alpha chains) wind into a right-handed superhelix, with each individual chain adopting a left-handed polyproline II helix. The amino acid sequence follows a characteristic Gly-X-Y repeat, with glycine at every third position because it is the smallest amino acid and fits in the center of the helix. The X position is often proline, and Y is often hydroxyproline, which is formed by post-translational hydroxylation of proline by prolyl hydroxylase, an enzyme that requires vitamin C as a cofactor. Hydroxyproline stabilizes the triple helix through hydrogen bonds, which is why vitamin C deficiency leads to scurvy: deficient hydroxylation produces unstable collagen, resulting in bleeding gums, poor wound healing, and vascular fragility. Hydroxylysine, produced by lysyl hydroxylase, provides sites for O-linked glycosylation and covalent cross-linking.
Collagen biosynthesis is a multi-step process. Preprocollagen is synthesized on rough ER ribosomes and the signal peptide is cleaved. In the ER lumen, prolyl and lysyl hydroxylation occurs (requiring vitamin C), followed by glycosylation of hydroxylysine residues. Three pro-alpha chains assemble via their C-propeptides, and the triple helix zips from the C-terminus to the N-terminus. Procollagen is then secreted because it is too large to form fibrils intracellularly. In the extracellular space, N- and C-propeptides are cleaved by procollagen proteinases to produce tropocollagen, which self-assembles into fibrils in a quarter-staggered array that generates the characteristic 67 nm D-period banding pattern. Finally, lysyl oxidase catalyzes covalent cross-links between lysine and hydroxylysine residues on adjacent tropocollagen molecules, providing the mature fibrils with their remarkable tensile strength.
Collagen diseases illustrate the importance of proper collagen structure. Osteogenesis imperfecta (OI) results from mutations in COL1A1 or COL1A2, which encode type I collagen. Most commonly, glycine substitutions in the Gly-X-Y repeat disrupt the triple helix, producing brittle bones. The effect is dominant negative because a single abnormal chain can disrupt the entire triple helix. Ehlers-Danlos syndrome encompasses a group of disorders with mutations in collagen genes or processing enzymes, characterized by hypermobile joints, hyperelastic skin, and vascular fragility. The vascular type (type IV), caused by COL3A1 mutations, carries a risk of arterial rupture. Alport syndrome is caused by mutations in type IV collagen genes (COL4A3, A4, or A5), producing defective basement membranes in the kidneys, eyes, and ears, and leading to progressive renal failure and hearing loss.
<image>Collagen structure and biosynthesis. Panel A: Collagen triple helix — three alpha chains with Gly-X-Y repeat, glycine in the center of the helix, hydroxyproline and hydroxylysine residues indicated, left-handed individual chains forming a right-handed superhelix. Panel B: Collagen biosynthesis pathway — (1) translation and hydroxylation in the ER, (2) triple helix assembly from C-propeptide, (3) secretion of procollagen, (4) extracellular cleavage of propeptides to form tropocollagen, (5) self-assembly into fibrils with quarter-staggered arrangement showing the 67 nm D-period banding pattern, (6) lysyl oxidase cross-linking. Panel C: Scurvy pathogenesis — vitamin C deficiency -> reduced prolyl hydroxylase activity -> underhydroxylated collagen -> unstable triple helix that denatures at body temperature -> symptoms (bleeding gums, poor wound healing).</image>
III. Proteoglycans and Glycosaminoglycans
Glycosaminoglycans (GAGs) are long, unbranched polysaccharide chains composed of repeating disaccharide units. They are highly negatively charged due to sulfate and carboxyl groups, which causes them to attract water and cations and form a hydrated gel. The major types include heparan sulfate, chondroitin sulfate, keratan sulfate, dermatan sulfate, and hyaluronan (hyaluronic acid). Hyaluronan is unique among GAGs in several respects: it is not sulfated, it is not attached to a core protein, it is synthesized at the plasma membrane rather than in the Golgi, and it can reach enormous molecular weights of up to millions of daltons.
Proteoglycans consist of a core protein with covalently attached GAG chains. Aggrecan is the major proteoglycan of cartilage, bearing approximately 100 chondroitin sulfate chains and 30 keratan sulfate chains on its core protein. Many aggrecan molecules bind to a single hyaluronan chain via link protein, forming massive aggregates with enormous water-holding capacity that allows cartilage to resist compression. Perlecan is a heparan sulfate proteoglycan and a major component of basement membranes. Syndecans are transmembrane proteoglycans that serve as co-receptors for growth factors such as FGF and VEGF. Glypicans are GPI-anchored proteoglycans that regulate Wnt and Hedgehog signaling. An important general function of GAGs is the sequestration of growth factors: heparan sulfate binds FGF, VEGF, and TGF-beta, creating local concentrations and presenting these factors to their receptors.
IV. Basement Membranes
Basement membranes are thin (approximately 50 to 100 nm), dense ECM sheets that underlie epithelia, endothelia, muscle cells, and Schwann cells. Their major components include type IV collagen, a network-forming collagen that assembles into a sheet-like scaffold; laminin, a large cross-shaped heterotrimeric glycoprotein (composed of alpha, beta, and gamma chains) that self-polymerizes into a network and binds integrins and dystroglycan on cell surfaces; nidogen (entactin), which cross-links the laminin and collagen IV networks; and perlecan, a heparan sulfate proteoglycan that contributes to charge-selective filtration. Laminin is essential for basement membrane assembly and is the first component deposited.
Basement membranes serve several critical functions. They provide structural support and cell attachment. In the kidney, the glomerular basement membrane serves as a filtration barrier with both charge selectivity (provided by heparan sulfate) and size selectivity. Defects in this barrier cause disease: Alport syndrome results from collagen IV mutations, while Goodpasture syndrome is caused by autoantibodies against the NC1 domain of the collagen IV alpha-3 chain. Basement membranes also guide cell migration during development and wound repair and serve as a barrier to cell invasion, which cancer cells must breach in order to metastasize.
<image>Basement membrane structure. Panel A: Schematic cross-section — epithelial cell layer sitting on the basement membrane (thin, dense sheet), with underlying interstitial connective tissue below. The basement membrane contains a laminin network (self-polymerized, bound to cell surface integrins and dystroglycan), a type IV collagen network (sheet-like scaffold), nidogen cross-linking the two networks, and perlecan (heparan sulfate proteoglycan) filling the space. Panel B: Laminin structure — cross-shaped trimer (alpha, beta, gamma chains) with the LG domains at the base binding integrins; self-polymerization via the short arms. Panel C: Glomerular filtration barrier — capillary endothelium (fenestrated), glomerular basement membrane (fused basement membranes of endothelium and podocytes), and podocyte foot processes with slit diaphragms. The basement membrane provides size and charge selectivity for filtration.</image>
V. Integrins and Cell-ECM Adhesion
Integrins are heterodimeric transmembrane receptors consisting of an alpha and a beta subunit, and they serve as the major receptors for ECM components. Mammals express 18 alpha and 8 beta subunits, which combine to form 24 known heterodimer pairs, each recognizing specific ECM ligands. For example, alpha5-beta1 is a fibronectin receptor that recognizes the RGD sequence, alpha6-beta4 is a laminin receptor found in hemidesmosomes, alphaIIb-beta3 is the platelet fibrinogen receptor essential for platelet aggregation, and alpha1-beta1 and alpha2-beta1 are collagen receptors.
A defining feature of integrins is their capacity for bidirectional signaling. In inside-out signaling, intracellular signals such as talin binding to the beta subunit tail activate integrins, driving a conformational change from a bent (inactive) state to an extended (active) and then ligand-bound (high-affinity) state. A clinically important example is chemokine-mediated activation of platelet integrins during hemostasis. In outside-in signaling, ECM binding triggers intracellular signaling cascades. Integrin clustering recruits talin, vinculin, paxillin, and focal adhesion kinase (FAK). FAK autophosphorylates on Tyr-397, recruiting Src kinase and activating downstream pathways including MAPK, PI3K, and Rho GTPases, which promote survival, proliferation, migration, and differentiation.
Focal adhesions are large, integrin-based multi-protein complexes that link the ECM to the actin cytoskeleton. They contain integrins, talin, vinculin, paxillin, zyxin, alpha-actinin, and FAK, and they transmit contractile force generated by actin and myosin to the ECM. Focal adhesions are also mechanosensors: they grow and strengthen under applied force. Hemidesmosomes are integrin alpha6beta4-based junctions that link cells to the basement membrane through connections to keratin intermediate filaments via plectin and BP230. They are critical in the skin, and mutations in their components cause junctional epidermolysis bullosa.
VI. Mechanotransduction and ECM Remodeling
Cells actively sense the stiffness (rigidity) of the ECM and adjust their behavior accordingly. Soft ECM resembling brain tissue (approximately 1 kPa) promotes neural differentiation, while stiff ECM resembling bone (approximately 100 kPa) promotes osteogenic differentiation. This was demonstrated in a landmark study by Engler and colleagues showing that mesenchymal stem cells differentiate based on substrate stiffness alone.
Several mechanotransduction pathways mediate these responses. YAP and TAZ are transcriptional coactivators whose nuclear localization is promoted by stiff substrates, driving pro-proliferative gene expression. FAK/Src signaling is activated by integrin-mediated force transmission. Mechanosensitive ion channels, notably Piezo1 and Piezo2, respond directly to membrane stretch.
Matrix metalloproteinases (MMPs) are zinc-dependent endopeptidases that degrade ECM components and are secreted as inactive zymogens (pro-MMPs) that are activated extracellularly. Collagenases (MMP-1, -8, -13) cleave the fibrillar collagen triple helix. Gelatinases (MMP-2, -9) degrade denatured collagen (gelatin) and basement membrane collagen IV. Stromelysins (MMP-3) have broad substrate specificity. MT-MMPs (membrane-type MMPs) are anchored to the cell surface and can activate other MMPs, as exemplified by MT1-MMP activating pro-MMP-2. MMP activity is regulated by TIMPs (tissue inhibitors of metalloproteinases), which comprise four family members (TIMP-1 through TIMP-4).
ECM remodeling in disease is a common pathological theme. Fibrosis involves excessive ECM deposition, particularly collagen types I and III, driven by TGF-beta-activated myofibroblasts, and manifests as liver cirrhosis, pulmonary fibrosis, or cardiac fibrosis. In cancer invasion and metastasis, cancer cells secrete MMPs to degrade the basement membrane and interstitial ECM. Lysyl oxidase (LOX) cross-links collagen and stiffens the tumor ECM, which paradoxically promotes further invasion through a positive feedback loop. Wound healing requires coordinated ECM degradation, formation of a provisional matrix from fibrin and fibronectin, and deposition of new ECM to restore tissue integrity.
