Department of Biology
Memorial University of Newfoundland
Extracellular Structures
The cell does not end at the cell membrane.
Many animal cells are intrinsically linked
to other cells and to the
extracellular matrix (ECM).
Bone and cartilage are mostly ECM plus a very
few cells.
Connective tissue, that surrounds glands and blood vessels, is a gelatinous
matrix containing many fibroblast cells.
The ECM contains three classes of molecules:
1) structural proteins (collagens and elastins);
2) protein-polysachharide complexes to embed the structural proteins
(proteoglycans);
3) adhesive glycoproteins to attach cells to matrix (fibronectins and
laminins),
N-CAMs and cadherins mediate cell-cell recognition and cell-cell
adhesion.
Neural cell adhesion molecules (N-CAMs) are large plasma membrane glycoproteins
that all share a similar extacellular domain that contains a binding site
involved in cell-cell adhesion.
When embryonic tissue is exposed to antibodies that interact with N-CAMs,
the cells do not bind to each other and neural tissue is not formed.
Cadherins are a group of adhesive glycoproteins
that are similar to the N-CAMs that require extracellular Calcium ions
to function and are required for development.
E-cadherin (epithethial tissue), N-cadherin (nervous tissues) and P-cadherin
(placental tissue) act to drive the adhesion of cells
of particular tissue
type.
The carbohydrate groups of N-CAMs and caherins determine the strength
and specificity of cell-cell recognition and adhesion.
N-CAMs have repeating chains of negatively charged sialic acid which
changes during development.
Vesicles with N-CAMs having little sialic acid bind tighter than those
with large amounts.
The loss of sialic acid groups from glycophorin may target old erythrocytes
for destruction in the spleen.
The enzyme neuraminidase can cleave the terminal sialic acid groups
as a mechanism to identify old red blood cells for retirement.
During inflamation, leukocytes initiate attachment
to the endothelial cell surface through the selectins then stabilize the
adhesion through the interaction of an integrin and an ICAM.
Cell Junctions
The cell junction are the structures where long term association between
neighbouring cells are established.
In animals, the three most common kinds of cell junctions are adhesive
junctions, tight junctions and gap junctions.
Adhesive junctions (desmosomes, hemidesmosomes and adherens junctions)
link adjoining cell to each other and to the ECM.
Although adhesive junction types are similar in structure and function,
they contain distinct 1) intracellular attachment proteins and 2) transmembrane
linker proteins.
The intracellular attachment proteins form a thick layer of fibrous
material on the cytoplasmic side of the plasma membrane called a plaque
which binds actin microfilaments in adherens junctions and intermediate
filaments in desmosomes and hemidesmosomes.
The transmembrane linker proteins is anchored to the plaque by the
cytoplasmic domain and binds the ECM or to the same proteins on other cells.
Desmosomes form strong points of adhesion between
cells in a tissue such that two adjoining cells are separated by a thin
space of 25-35 nm, the desmosome core, in which cadherin molecules mediate
cell-cell adhesion.
The plaques on the inner surfaces of cells joined by desmosomes have
a mixture of intracellular attachment proteins (desmoplakins and plakoglobin)
which interact with the tonofilament intermediate filaments.
Hemidesmosomes connect a cell, through a plaque, to the basal lamina
(ECM) by integrins.
As in desmosomes, hemidesmosomes interact with tonofilament intermediate
filaments.
Adherens junctions resemble desmosomes except two adjoining cells are
separated by a thin space of 20-25 nm and connect to actin microfilaments
in the cytoplasm.
Some of the transmembrane glycoproteins are cadherins.
Adherens junctions called focal contacts can join a cell to the ECM,
primarily through fibronectin receptors.
Tight junctions leave no space between plasma
membranes of adjacent cells to prevent the movement
of molecules across cell layers.
The structure of tight junctions consists of fused ridges of tightly
packed transmembrane junctional proteins.
Tight junctions block lateral movement of lipids and membrane
proteins to keep a cell polarized.
In intestinal epithelial cells transport of glucose from the intestinal
lumen through the cell to the blood stream requires the uptake of glucose
through apical surface sodium/glucose symport proteins and export by glucose
transport proteins on the basalateral surface and tight junctions prevent
the lateral movement of these transport proteins.
Gap junctions separate cells by 2-3 nm and allow
direct electrical and chemical communication.
Connexons are tightly packed 7 nm wide hollow cylinders in two adjacent
cell membranes that form a 3 nm thin hydrophilic channel that allows the
passage small molecules and ions.
Collagens & Elastins
Collagens, the most abundant proteins in the ECM
(25-30% of total protein in vertebrates), are secreted by cells, such as
fibroblasts.
Collagens are responsible for the strength of the ECM and form high
tensile strength fibres and are prominent in tendons and ligaments.
Collagens occur in a triple helix of three
polypeptide chains and are high in glycine, hydroxylysine and hydroxyproline.
Collagen fibres are bundles of collagen fibrils which are, in turn,
bundles of collagen molecules which consist of three alpha chains of collagen
polypeptides.
Procollagen forms many types of tissue-specific collagens.
Many tissues require flexibility and strength (lung tissues, arteries,
skin and intestines) constantly change shape.
The elastins impart elasticity and flexibility
to the ECM and can stretch several times their length.
Elastins are rich in glycine and proline and are crosslinked by covalent
bonds between lysines.
The crosslinks allow elastin fibres to recoil back to original shape
after extension.
During aging, collagens become more crosslinked and elastins are lost
resulting in bones, joints and skin losing flexibility.
Proteoglycans
A matrix of proteoglycans (many glycosaminoglycans
attached to each protein) embed collagen and elastin fibres
Glycosaminoglycans (GAGs), the major carbohydrate part of proteoglycans,
consist of repeating disaccharide subunits.
One of the two sugars in the disaccharide is often an amino sugar (N-acetylglucosamine
or N-acetylgalactosamine; usually with an attached sulfate group) and the
other is a sugar or sugar acid (galactose or glucuronate).
GAGs, which are hydrophilic due to the negative sulfate and carboxyl
groups, attract water and cations to form the hydrated gelatinous matrix.
Chondroitin sulfate, keratan sulfate and hyaluronate are the most common
GAGs.
Most GAGs in the ECM are bound to proteins to form proteoglycans (mucoproteins).
Numberous GAGs (1-200 per molecule, average length of 800 monosaccharide
units) are attached to a core protein and different kinds of proteoglycans
can be made by varing the combination of core proteins and GAGs.
These large proteoglycans (MW of~ 1 million) can be individual or attached
to long hyaluronate molecules to form complexes (as in cartilage).
Proteoglycans trap water (up to 50 times their weight) to act as extracellular
sponges resistant to physical forces in cartilage and joints.
Proteoglycans can be embedded in the plasma membrane or covalently
linked to membrane phospholipids or bound to receptor proteins.
Proteoglycans and collagen may bind to receptor proteins (often integrins)
which are reinforced by adhesive glycoproteins, such as fibronectins and
laminins, to anchor cells to the ECM.
Fibronectins and Laminins
Fibronectins, a family of closely related glycoproteins,
are soluble in body fluids (blood), insoluble in the ECM and partially
soluble at the cell surface.
The fibronectins bind cells to the matrix and guide cellular movement.
The RGD (arginine-glycine-aspartate) sequence binds to the integrin
fibronectin receptor.
The fibronectins bind cells to the ECM by bridging cell-surface receptors
to the ECM.
The intracellular cytoskeleton will align with the extracellular fibronectin
to detemine cell shape.
In many kinds of cancer, cells unable to make fibronectins loose shape
and detach from the ECM to become malignant.
During cell movement (as during embryogenesis),
pathways of fibronectins guide cells to their destinations.
Soluble plasma fibronectin promotes blood clotting by direct binding
of fibrin.
Fibronectins guide immune cells to wounded areas and thus promote wound
healing.
Laminins bind cells to the basal lamina.
Laminins are found mostly in the basal laminae, the ~50 nm thick ECM
layer between epithelial cells and connective tissue, and surrounding muscle
cells, fat cells, and Schwann cells.
The basal lamina serves as a structural support for tissues and as
a permeability barrier to regulate movement of both cell and molecules.
Laminin is a very large protein comprised of
three proteins that form a cross.
The domains of laminin bind type IV collagen, heparin, heparin sulfate,
entactin and laminin receptor proteins in overlying cells to allow bridging
between the cells and the ECM.
Integrins, N-CAMs & Cadherins
Integrins are cell surface receptors that bind
the ECM.
Fibronectins, laminins and other ECM components bind specific receptor
glycoproteins on the cells’ surfaces known as the integrins.
The fibronectin receptor is the best characterized integrin.
Integrins act to integrate the cytoskeleton
and the extracellular matrix.
Integrins consist of two large non-covalently bound transmembrane proteins
(alpha and beta subunits).
A number of both alpha and beta subunits combine to produce a large
variety of heterodimeric integrins.
On the outer surface, the subunits interact to form a binding site
for the adhesive glycoprotein, the RGD sequence of the ECM glycoprotein.
Most of the binding specificity depend upon the alpha subunit.
On the cytosolic side, the receptor binds components of the cytoskeleton
to enable the ECM to communicate through the plasma membrane to the cytoskeleton.
Notes prepared from Becker's World of the Cell, 8th edition
Hardin, Bertoni & Kleinsmith, 2012
Figures copyright of Pearson Education Inc.
email me at bestave@mun.ca
