1. Extracellular Matrix
Cell Biology
Lecture 11

2. Readings and Objectives
Cooper: Chapter 14
Topics
The Extracellular Matrix
Composition
Cell-Matrix Interactions
Cell-Cell Interactions

3. Extracellular Matrix Introduction
Cell walls: bacteria, fungi, algae, and higher plants
Animal cell in tissues embedded in an extracellular matrix of proteins and polysaccharides
Function
Provides structural support to cells and tissues
Important role in regulating cell behavior
Cell to cell interaction, communication

4. General Structure of Extracellular Matrix
Animal cells embedded in an extracellular matrix
Basal laminae: thin layer on which epithelial cells rest. Also surrounds muscle cells, adipose cells, and peripheral nerves
most abundant in connective tissues
Connective tissue
loose connective tissue
Bone
tendon
cartilage

5. Composition of Extracellular Matrix
Fibrous proteins
Polysaccharides- gel like environment
Adhesion proteins- link components of the matrix to one another and to cells
Different matrices have different amounts of each component
Tendons, rich in fibrous proteins
Cartilage, high in polysaccharides
Bone, calcium phosphate crystal deposition

6. Matrix composition: Collagen
Collagen- major structural fibrous protein
Forms triple helices
Triple helix domains: repeats of the amino acid sequence Gly-X-Y
Glycine in every 3rd position
X=Pro, packs helices closely
Y= hydroxyproline, synthesized in ER
Pro, Hpro stabilizes by helping H-bonding

7. Matrix composition: Collagen
Type I collagen- the most abundant
polypeptide chains have about 330 Gly-X-Y repeats
Secreted through ER/golgi, form collagen fibrils
Triple helical molecules are associated in regular staggered arrays
Covalent cross-links: lysine and hydroxylysine side chains
strengthen the fibrils
Fibrils form collagen fibers, several µm in diameter

8. Types of Collagen Some are not fibril forming
Fibril-associated collagens: bind to collagen fibrils, link to others or to other matrix components
Network-forming collagens: have non helical interruption, cross-link to network
Anchoring fibrils: link basal laminae to underlying connective tissues
Transmembrane collagens: proteins that participate in cell-matrix interactions
Network-forming collagens

9. Matrix Polysaccharides
Extracellular matrix gels are polysaccharides called glycosaminoglycans (GAGs).
GAGs are repeating units of disaccharides: One sugar is either N-acetylglucosamine or N-acetylgalactosamine, the second is usually acidic (glucuronic acid or iduronic acid).

10. Matrix Polysaccharides
sulfate groups make GAGs negatively charged
bind positively charged ions and trap water molecules to form hydrated gel
GAGs are linked to proteins to form proteoglycans

11. Matrix Adhesion proteins: Fibronectin
Link matrix components
to each other
to cell surfaces
Fibronectin : main adhesion protein of connective tissues
A homodimeric protein (2500 aa/subunit), binds
collagen and GAGs
cells
Recognized by cell surface receptors
Attachment of cells to the extracellular matrix

12. Matrix Adhesion proteins: Laminins
Laminin: adehsion protein of basal laminae
Heterotrimeric: α, β, and γ-chains (5, 4, 3 genes, respectively)
have binding sites for
cell surface receptors, eg integrins
type IV collagen
Proteoglycans
Assemble to cross-linked network
Linking cells and matrix

13. Cell-Matrix Interactions
Integrins: major cell surface receptors, involved in attachment of cells to the extracellular matrix
Transmembrane proteins, heterodimer of α and β subunits (18α, 8β)
Bind to short aa in,
Collagen
Fibronectin
laminin
also anchor the cytoskeleton to the extracellular matrix

14. Cell-Matrix Junctions
Two types of cell-matrix junction
Focal adhesions: bundles of actin filaments are anchored to β subunits of integrins via
α-actinin
Vinculin via talin
Assembly of focal adhesions
Focal complex: small group of integrins
Recruite Talin, Vinculin, α-actinin and Formin
Formin initiates actin bundles

15. Focal adhesions are reversible
Integrins can reversibly bind matrix components
change conformation between active and inactive states
Inactive state: integrin heads turned close to cell surface
Cell signaling extends heads to matrix
Migrating cells: focal adhesions form at the leading edge

16. Cell-Matrix Junctions: Hemidesmosomes
Hemidesmosomes anchor epithelial cells to the basal lamina
α6β4 integrins bind to lamins
long cytoplasmic tail of β subunit binds to intermediate filaments via
Plectin and BP230 and BP180 (similar to transmembrane collagens)

17. Cell-Cell interactions
Interactions between cells are critical for development and function of multicellular organisms
Cell-cell interactions:
Transient: activation of immune cells; migration to injury site
Stable: role in the organization of tissues.
Cell-Cell junctions allow rapid communication between cells
During embryo development, cells from one tissue specifically adhere to cells of the same tissue rather than cells of a different tissue

18. Cell-Cell interactions
Cell-cell adhesion- mediated by four groups of cell adhesion molecules
Selectins, integrins, the immunoglobulin (Ig) superfamily, and cadherins
Many adhesions are divalent cation-dependent, requiring Ca2+, Mg2+ or Mn2+

19. Selectins
Selectins- transient interactions between leukocytes and endothelial cells
Leukocytes slow down, flattened, migrate from the circulation to sites of tissue inflammation
initial adhesion
stable adhesions binding of integrins to intercellular adhesion molecules (ICAMs) on endothelial cells

20. Cell to Cell Junctions
Four types of Cell-Cell connections in animal cells
Adherens Junctions
Desmosomes
Tight Junctions
Gap Junctions

21. Adherens Junctions
Cadherin form stable cell-cell connections involve actin filaments
Also include β-catenin, p120, and α-catenin,
β-catenin and p120 bind to cadherin and help maintain stability
β-catenin binds α-catenin that interacts with actin filament of cytoskeleton

22. Desmosomes link the intermediate filament of adjacent cells
Desmoglein and desmocollin (transmembrane cadherins) bind by heterophilic interactions across the junction
Plakoglobin and plakophilin bind to the cadherins and link to the intermediate filament binding protein, desmoplakin

23. Tight Junctions
Tight junctions provide minimal adhesive strength between the cells, usually associated with adherens junctions and desmosomes in a junctional complex

24. Tight Junctions
Tight junctions in epithelial cell form a seal that prevents free passage of molecules and ions between cells
separate apical and basolateral domains of the plasma membrane
prevent free diffusion of lipids and membrane proteins

25. Tight Junctions
transmembrane proteins, occludin, claudin, and junctional adhesion molecule (JAM), anchored on F-actin
Bind similar proteins on the adjacent cell
Sealing the space between cells

26. Gap Junctions open channels through the plasma membrane
allowing ions and small molecules to diffuse freely
Proteins and nucleic acids can not pass through
heart muscle cells, passage of ions through gap junctions synchronizes the contractions of neighboring cells
allow passage of some signaling molecules, such as cAMP and Ca2+, coordinating responses of cells in tissues

27. Gap Junctions
Gap junctions are made of transmembrane proteins in the connexin family
6 connexins form a cylinder with an open aqueous pore in its center, called a connexon
Connexons in the plasma membrane adjacent cells align
form open channels between the two cytoplasms

28. Gap Junctions
Specialized gap junctions occur on specific nerve cells and form an electrical synapse
Individual connexons can be opened or closed
When open, they allow rapid passage of ions between the two nerve cells