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Extracellular Matrix
Readings and Objectives
• Reading
– Cooper: Chapter 14
• Topics
• The Extracellular Matrix
• Composition
• Cell-Matrix Interactions
• Cell-Cell Interactions
2
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
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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
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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
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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
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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
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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 cellmatrix interactions
Network-forming collagens
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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).
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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
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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
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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
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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
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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, αactinin and Formin
Formin initiates actin bundles
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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
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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)
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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
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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+
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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
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Cell to Cell Junctions
Four types of Cell-Cell connections in
animal cells
• Adherens Junctions
• Desmosomes
• Tight Junctions
• Gap Junctions
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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
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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
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Tight Junctions
• Tight junctions provide
minimal adhesive strength
between the cells, usually
associated with adherens
junctions and desmosomes
in a junctional complex
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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
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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
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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
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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
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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
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