Extracellular Matrix & Cell Adhesion Flashcards
- Discuss the contributions of the ECM to cell and tissue function.
Extracellular Matrix (ECM):
-A significant fraction of tissue and organ volume is extracellular space filled by an intricate network of macromolecules, the extracellular matrix (ECM).
-The ECM consists of a variety of proteins and polysaccharides that are secreted and assembled in close association with the cellsthat synthesized them.
-The relative amount of ECM in different tissues varies greatly, from
connective tissues, where the ECM may occupy the bulk of the volume, to brain, where it is a very minor component only.
-The ECM serves not only as a scaffold for cells but also participates
in regulating various cell functions:
- migration
- survival
- differentiation
- proliferation
- shape
(ms. dps)
- Define the four major classes of ECM components and their properties.
GENERAL
a)-Glycosaminoglycans (GAGS)
(usually covalently lied to proteins to form proteoclycans)
The proteoglycan molecules from a highly hydrated gel in which the fibrous and multidomain
proteins are embedded
b)-Fibrous proteins
(Collagen and elastin)
Nutrients, metabolites, hormones, etc. readily can diffuse through the
ECM. The fibrous proteins give it its mechanical properties.
c) -Multidomain adapter proteins (fibronectin and laminin)
d) -Water and many solutes
The ECM typically is amorphous, but it may form a distinct structure, the basal lamina (see
Epithelia). Two of the basal lamina’s characteristic components are collagen IV and laminin (see
below).
- Define two types of fibrillar proteins and at least two types of multidomain adapter proteins of the ECM.
bull shit card.
look at the cards for
fibrous proteins (Collagen and elastin) and adapter card (fibronectin and laminin)
- Discuss the role of MMPs in ECM remodeling.
-Like any other structure in the body, the ECM must be turned over in an ordered manner.
-This is achieved by the secretion of extracellular proteases.
-Typically these proteases are either “matrix
metalloproteases” (MMP).
-These may be substrate-selective (such as collagenases) or quite
promiscuous.
-Extracellular protease activity is particularly important in tissue remodeling during
development and in cell invasion of tissues:
-leukocytes reaching an inflammation site
-cell migration in development
-neurite outgrowth
-cancer cell metastasis.
-Advancing cells are known
to secrete proteases to clear the migration path.
-However, functions are more complex.
Protease activity may:
-unmask cryptic cell binding sites to promote cell binding or migration,
-promote cell detachment,
-activate (proteolytically) growth factors,
-release ECMbound
extracellular signals.
Extracellular protease activity thus must be highly regulated.
- Discuss the role of adhesion in cell function and survival.
As stated before, cell adhesion molecules are not simply adhesive devices:
-For adhesion and
force generation against the ECM or other cells, they form a trans-membrane link with the cytoskeleton.
-When bound to a ligand, CAMs signal their engagement via conformational change to the cell interior, thus affecting cell function.
-There are many different known CAMs.
-The majority of these are sizable transmembrane glycoproteins (MW ~ 100-150 kDa)
- Define and describe at least three different types of cell adhesion molecules (CAMs) and their ligands.
1) Cadherins
-Cadherins are single-pass transmembrane glycoproteins that operate as homodimers.
-The extracellular domains contain 5 repeats that are stabilized by Ca2+
to form a rod-like
structure.
-This means, cadherin binding is Ca2+ dependent.
-Cadherins bind to other cells via cadherins on the juxtaposed cell surface (homophilic binding) in a zipper-like fashion.
-They are very common in intercellular junctional complexes.
2) Ig Superfamily (IgSF) CAMS:
-IgSF CAMs also are single-pass transmembrane
glycoproteins engaged in a usually homophilic binding mechanism (there are important
exceptions).
-However, IgSF members do not form dimers, and binding does not require Ca2+.
-Binding is mediated by multiple Ig domains.
-Closer to the membrane, IgSF CAMs usually
contain a couple of fibronectin type III domains.
3) Integrins
-These are the “classic” adhesion molecules interacting with the ECM.
-Thus, their binding is heterophilic. I
-ntegrins are composed of α/β heterodimers.
-Many different α and β subunits exist (both are transmembrane glycoproteins).
-They are mixed and matched to some
degree.
-Since both subunits participate in ligand binding, this results in a large variety of integrins with distinct binding selectivities.
-Typical ligands are the ECM proteins laminin,
fibronectin, collagen, etc. (many involving the RGD sequence; see fibronectin).
- Discuss the role of CAMs in signaling.
8. Describe proteins associated intracellularly with CAMs
-Because of the relative structural weakness of membranes, CAMs must be linked to the cytoskeleton.
-This is achieved by a variety of proteins associated with the cytoplasmic tails of
CAMs.
-Such proteins include, in all cases, actin-binding proteins (vinculin, talin, α-actinin, etc.).
-Cadherins have very specific additional associated proteins, the catenins.
-In addition to providing
the cytoskeletal link, CAM-associated proteins also are involved in the regulation of adhesion, in the control of actin polymerization, and in cell signaling mediated by CAMs.
-For example, assembly of adhesion sites requires the action of protein tyrosine kinases, whereas disassembly
and detachment appears to require protein kinase C activity.
-Members of the Rho family of small GTPases involved in the regulation of F-actin polymerization (see Cell Motility) also are
associated with certain CAMs.
-It follows that CAMs are paired on the cytoplasmic face of the membrane with complex protein assemblies that serve mechanical, controlling, and signaling functions.
-It is not surprising, therefore, that these adhesion sites play major roles in cell
differentiation/development and cancer.
-In development, CAMs change as cells differentiate.
- Discuss the ECM and cell adhesion in the context of disease processes.
a) Examples of ECM involvement in disease.
-Collagen I mutations are relatively frequent. They interfere with osteogenesis and cause skeletal
dysplasias.
-Fibronectin null mutant mice are early embryonic lethal.
-Some laminin mutations have been linked to nephrotic syndrome (glomerular filtration defect)
and to neuromuscular junction/muscle innervation problems in children.
-Loss-of-function mutations of matrix metalloproteases (MMP) 2 or 13 cause inherited
osteolysis/arthritis syndromes and bone dysplasias.
-Overexpression of some MMPs, specifically MMP2, MMP9 and MMP14, correlates with high
invasiveness and poor prognosis in many cancers.
b) Cell adhesion involvement in disease.
-Because of the severe consequences (developmental and other) loss-of-function of a CAM may
result in embryonic lethality (see fibronectin, above).
-Two types of leukocyte adhesion deficiencies have been identified:
Type I, affecting a particular integrin.
-Type II, affecting a selectin, a CAM involved in leukocyte rolling (the initial contact with
vascular endothelial cells, prior to adhesion and extravasation).
Cancer:
- One of the early signs of carcinogenesis is a change in CAM(s). This is accompanied by (and may be the reason for) perturbed cell polarity and cytoarchitecture.
- Cadherin down-regulation appears to be a prerequisite for the dispersal of epithelium derived
cancer cells.
- Anchorage independence of cancer cells results from mutations in proteins of the signaling apparatus associated with cell adhesions.
- Certain catenins are known to be tumor suppressors. This means that loss of catenin function promotes carcinogenesis.
- a) Define the four major classes of ECM components and their properties.
- Glycosaminoclycans (GAGs) and proteoglycans
Glycosaminoglycans (GAGs):
-GAGs consist of unbranched polysaccharide chains composed of disaccharide repeats.
-One of the sugars is always an amino sugar (such as N-acetyl-glucosamine), usually sulfated.
-The second sugar typically is uronic acid (e.g., glucuronic acid).
-Sulfation and carboxyl groups convey to the GAGs a high negative charge and, thus, the capacity to become highly hydrated.
-GAGs typically exhibit an extended conformation, fill very large volumes relative to their mass,
and readily form gels.
-Some common GAGs are:
*hyaluronan
*chondroitin sulfate
*dermatan sulfate
*heparan sulfate
*keratan sulfate.
They differ in disaccharide composition, sugar
linkage, and location of sulfate groups.
Example: hyaluronan (or hyaluronic acid) is composed of glucuronic-acid–N-acetyl-glucosamine disaccharides. Single chains may contain up to 25,000 disaccharide units. Hyaluronan is atypical, however, in that it is generally not linked to protein.
Proteoglycans (PGs):
-PGs are covalently linked complexes of GAGs and protein, typically of very high molecular
weight.
-The “core protein” has attached to serines special link tetrasaccharides, and these serve as the primers for polysaccharide assembly.
-These post-translational modifications take place
chiefly in the Golgi complex.
-The polymerized sugars maybe modified further (e.g., sulfation).
-PGs contain at least one GAG chain but may carry many, and the GAG chains can be very long,
typically about 80 monosaccharides.
-The PG aggrecan, for example, contains over 100 GAG chains.
-As a result, the total mass of an aggrecan molecule is about 3x10^6 daltons.
-PGs are likely to play an important role, e.g., in the filtering function of the kidney glomerulus and as
“reservoirs” of growth factors and proteases, which they may bind, and whose activity they may
modify.
-Thus, they are not simply space-filling, inert components of the ECM.
-Some PGs are membrane-bound molecules, anchored via a transmembrane core protein tail or linked via a GPI anchor.
-PGs and GAGs may assemble to form higher-order aggregates.
-Aggrecan is an example:
About 100 aggrecan monomers (each ~3x10^6
MW) may be bound non-covalently to a
hyaluronan chain via pairs of link proteins. Such complexes may exceed MW 10^8.
.
- b) Define the four major classes of ECM components and their properties.
- Fibrous proteins; collagen and elastin.
Collagens:
-Collagens are fibrous proteins and the most abundant proteins in mammals (about
25% of protein mass).
-About 25 distinct collagen subunits have been identified.
-They are known
to assemble into about 20 different collagens.
-Collagen I is the most common form, abundant in
connective tissues (see Connective Tissue).
-Collagen IV is characteristic of the basal lamina.
Elastin:
- Elasticity, an important property of many tissues (skin, lungs, blood vessels, etc.), is provided by a network of elastic fibers in the ECM.
- Elastin is a fibrous protein of distinct chemistry and function.
- c) Define the four major classes of ECM components and their properties.
- Multidomain adapter proteins; fibronectin, laminin, tenascin
The ECM contains several proteins with multiple domains that act as binding sites for other matrix macromolecules and for adhesion molecules on the surfaces of cells. These proteins include, e.g., fibronectin, laminin and tenascin. Thus, such proteins help to organize the matrix and to attach cells to it.
Fibronectin:
-Fibronectin is a large, dimeric glycoprotein whose two large subunits are linked together by
disulfide bonds.
-Each subunit is folded into a series of functionally distinct binding domains.
-Serially repeated, smaller modules make up each of these domains.
-The main module is the “type III fibronectin repeat”, which binds to integrins.
-This repeat contains the
characteristic Arg-Gly-Asp (RGD) binding sequence.
-Other domains bind to collagen, heparin or
serve self-association.
-Thus, secreted fibronectin assembles in the ECM into highly insoluble
fibronectin fibrils.
Laminin:
-Laminin is another very large protein, composed of three subunits (α,β,γ) that form anasymmetric, disulfide-linked cross with the longer arm formed by a helical structure containing
long stretches of all three subunits.
-Laminin is found in the basal lamina only (hence its name).
-Several isoforms of the laminin subunits assemble in different combinations to form a large protein family.
-Many laminins can self-assemble into a network via interactions between ends of their arms.
-Laminins, like fibronectins, have numerous binding sites for cells (integrins) and other ECM proteins that link them to collagen (type IV).
