Chapter 7
INTERACTIONS BETWEEN CELLS AND THEIR ENVIRONMENT
An overview of how cells are organized into tissues and how they interact with one another and with their extracellular
environment.
In this schematic diagram of a section
through human skin, the cells of the
epidermis are seen to adhere to one another
by specialized contacts. The basal layer of
epidermal cells also adheres to an underlying,
noncelluler layer (the basement membrane).
The dermis consists largely of extracellular
element that interact with each other and
with the surfaces of scattered cells. The cells
contain receptors that interact with
extracellular materials.
THE EXTRACELLULAR SPACE
Animal cells are sugar coated with the ogliosaccharides of lipids and proteins. The sugar coating is referred to as the
glycocalyx (or cell coat). The glycocalyx is made up of carbohydrate chains of lipids and proteins (mostly N-linked).
Removing the glycocalyx does not kill cells. Its purpose
Microvilli
Glycocalyx
is not entirely understood.
Electron micrograph of the apical
surface of an epithelial cell from the
lining of the intestine, showing the
glycocalyx, which has been stained with
the iron-containing protein ferritin.
THE EXTRACELLULAR MATRIX (ECM)
The extracellular matrix: an organized network of extracellular materials that is present beyond the immediate vicinity
of the plasma membrane.
Experimental determination of the thickness of the extracellular matrix:
RBCs
Chondrocyte
One of the best defined extracellular matrices is the
basement membrane (or basal lamina), a continuous 50 to
200 nm thick sheet that…
1) surrounds muscles and fat cells
2) underlies the basal surface of epithelial tissues (skin,
digestive lining, respiratory cell lining)
3) underlies the inner endothelial lining of blood
vessels
The basement membrane
(basal lamina) seen in a
scanning electron micrograph
of human skin.
Epidermis
Basal membrane
The epidermis has pulled
away from part of the
basement membrane.
Nucleus
Dermis
An unusually thick basement membrane is formed
between the blood vessels of the glomerulus and
proximal end of the renal tubules of the kidney.
Black dots within the glomerular basement membrane (GBM)
are gold particles attached to antibodies that are bound to
type IV collagen molecules in the basement membrane. CL:
capillary lumen (inside the capillary). P: podocyte of tubulespecialized cells associated with kidney glomeruli.
An overview of the macromolecular organization of the extracellular matrix:
The proteins depicted (fibronectin,
collagen, & laminin) contain binding
sites for one another as well as binding
sites for the receptors (integrins) that
are located at the cell surface. The
proteoglycans are huge proteinpolysaccharide complexes that occupy
much of the volume of the extracellular
space.
Intergrins interact with specific components of the extracellular matrix. Interactions with integrins send signals
inside the cell. These kinds of interactions are involved in cancer.
COLLAGEN
Collagens comprise a family of fibrous extracellular glycoproteins that are present only in
extracellular matrices. Collagens are found throughout the animal kingdom and are noted
for their high tensile strength, that is, their resistance to pulling forces.
Structure of collagen I: Triple alpha helix
The basic collagen monomer is a triple helix
composed of 3 identical chains, making
them homotimers. Other types have 2
or 3 different chains, making them
heterotrimers.
Collagen monomers become aligned in rows
in which the molecules of one row are
staggered relative to those in the
neighboring row.
An electron micrograph of human
collagen fibers showing their
characteristic banding pattern, which
results from the arrangement of
collagen monomers.
The distribution of type IV collagen is restricted to basement membranes. Type IV collagen molecules are organized into
a network that provides mechanical support and serves as a lattice for the deposition of other extracellular
materials. Type IV collagen is not fibrous.
The type IV collagen network of the
basement membrane following salt
extraction.
This lattice consists of type IV collagen
molecules interlinked with one
another in a complex 3D array.
If something is able to be extracted by a
salt extraction, it means it was not
very strongly attached.
PROTEOGLYCANS
In addition to collagen, extracellular matrices typically contain large amounts of a distinctive type of proteinpolysaccharide complex called proteoglycan.
Proteoglycan consists of a core protein molecule to which chains of glycosaminoglycans are attached. Proteoglycans are
capable of binding huge numbers of cations, which in turn draw huge numbers of water molecules. As a result,
polyglycans form a porous, hydrated gel that fills the extracellular space like a packing material and resists crushing
(compression forces). Together, collagen and proteoglycans give cartilage and other extracellular matrices strength
and resistance to deformation.
Schematic representation of a
single proteoglycan.
(a) Core protein is shown in black,
glycosaminoglycans are shown in
red.
(b) Lots of negatively charged sugars.
A proteoglycan from cartilage
matrix actually contains about 30
keratin sulfate and 100 chondroitin
sulfate chains.
(c) In the cartilage matrix, individual
proteoglycans are linked to a
nonsulfated glycosaminoglycan
FIBRONECTIN, LAMININ, AND OTHER PROTEINS OF THE EXTRACELLULAR MATRIX
The term “matrix” implies a structure made up of a network of interacting components, which is fitting for the
extracellular matrix. In addition to collagen and proteoglycans, there are many other proteins.
Fibronectin
—One of the best studied extracellular proteins
—Like other fibrous proteins of the ECM, fibronectin consists of a linear series of distinct domains that gives each
polypeptide a modular construction.
—Each of the 2 polypeptide chains that make up a fibronectin molecule contains:
1) Binding sites for other components of the ECM, such as collagen and proteoglycans. These binding sites
facilitate interactions that link these diverse molecules into a stable, interconnected network.
2) Binding sites for receptors on the cell surface (ex: integrins). These binding sites hold the ECM in a stable
attachment to the cell
The structure and function of fibronectin
A human fibronectin molecule consist of 2 similar, but
nonidentical, polypeptides joined by a pair of disulfide bonds
located near the C-termini. Each polypeptide is composed of a
linear series of 15-17 repeating modules which in turn are
organized into 5 or 6 larger domains.
RGD (arg-gly-asp) allows the fibronectin to bind to integrins
which can anchor the cell.
Micrograph of neural crest cells emigrating from a portion of the
developing chick nervous system onto a glass culture dish that contains
strips of fibronectin-coated surface alternating with strips of bare glass.
The cells outside the fibronectin trail do not grow—bare glass does not
support cell growth.
The role of cell migration during embryogenesis
A summary of cellular
migration traffic.
Interesting to note that the
origin of many cells in
mammals originate outside
the main body of the embryo
and migrate into the embryo.
The path that differentiating
cells take is largely
determined by extracellular
components.
Section through a 10 day old embryo in which
primordial germ cells (green) are migrating along
the dorsal mesentery on their way to the
developing gonad. The tissue has been stained
with antibodies against the protein laminin (red).
—Embryonic cells have to migrate to sites in order to develop
—The migration roadways are made up of extracellular matrix protein (in the case on the right, laminin)
Laminin
—Another major glycoprotein of the ECM
—Consists of 3 different polypeptide chains linked by disulfide bonds and organized into a molecule resembling a cross
with 3 short arms and 1 long arm.
—Like fibronectin, the polypeptides of a laminin molecule contain distinct domains that possess specific binding sites for
Entactin
other protein compounds
Collagen IV
Recognize this structure!
THE DYNAMIC PROPERTIES OF THE ECM
A model of the basement membrane scaffold
Basement membranes contain 2 network-forming
molecules: collagen IV (pink) and laminin (green).
The collagen and laminin networks are connected
by entactin (purple).
INTERACTIONS OF CELLS WITH EXTRACELLULAR MATERIAL
INTERGRINS
—A family of integral membrane proteins though to be present on the surface of all types of vertebrate cells
—Integrins have been implicated in 2 major types of activites:
1) Adhesion of cells to their substratum
2) Transmission of signals from the external environment to the cell interior
—Fig 7.14 (250): Activation of a heterodimeric integrin
Blood clots form when platelets adhere to one
another fibrinogen bridges that bind platelet
integrins
Platelet aggregation is triggered by the chemical
signal prostacyclin, which is a prostaglandin.
Prostaglandins are inhibited by cyclooxygenase
inhibitors. One effect of taking 81 mg of aspirin is to
block platelet aggregation.
The presence of synthetic RGD (arginine-glycineaspartic acid) peptides can inhibit blood clot
formation by competing with the fibrinogen
molecules for the RGD-binding sites on the
integrins. Shows that the specific binding site is RGD
blocked because aggregation can be blocked by
synthetic peptide mad of RGD.
FOCAL ADHESIONS AND HEMIDESMOSOMES
Focal adhesions are sites where cells adhere to their substratum
This cultured cell has been
stained with fluorescent
antibodies to reveal the
locations of the actin
filaments (gray-green) and
the integrins (red). The
integrins are localized in
small patched that
correspond to the sites of
focal adhesions
This is one way that integrins can signal transduction:
The cytoplasmic surface of a focal adhesionof a cultured
amphibian cell is shown here after the inner surface of
the membrane was processed for quick-freeze, deep etch
analysis. Bundles of microfilaments are seen to associate
with the inner surface of the membrane in the region of a
focal adhesion.
Schematic drawing of focal adhesion showing the
interactions of integrins molecules with other
proteins on both sides of the lipid bilayer.
Cytoplasmic domains of integrins are often associated
with actin filaments.
Linkage between actin and integrins is accomplished by
a number of proteins including: vinculin and talin.
Cytoplasmic domains are also associated with protein
kinases (ex: focal adhesion kinase, FAK) which can
ultimately trigger DNA synthesis.
The steps in the process of a cell spreading
Hemidesmosomes
—Focal adhesions occur primarily in vitro, where the cells attach to the underlying basement membrane. The
specialized adhesive structure is called a hemidesmosome.
—Hemidesmosomes anchor cells to the basement membrane or other surfaces
Electron micrograph or several
hemidesmosomes showing the
dense plaque on the inner surface
of the plasma membrane and the
intermediate filaments projecting
into the cytoplasm.
Schematic diagram
showing the major
components of a
hemidesosome connecting
the epidermis to the
underlying dermis
INTERACTIONS OF CELLS WITH OTHER CELLS
Experimental demonstration of cell recognition
Ectoderm
+
Mesoderm
2 regions of an early amphibian embryo (ectoderm and mesoderm) were dissociated into
single cells and combined. They eventually sorted out. The ectoderm (shown in red) moved to
the outer portion where it would be located in the embryo, and the mesodermal cells (purple)
move to the interior, the position they would occupy in the embryo. Both types of cells then
differentiate into the types of structures they would normally give rise to.
SELECTINS
Selectins are a family of integral membrane glycoproteins that recognize and bind to a particular arrangement of sugars
in the ogliosaccharides that project from the surface of other cells. The name of this class of cell-surface receptors is
derived from the word lectin.
There are 3 known selectins:
1. P-selectin: present on platelets and endothelial cells
2. E-selectin: present on endothelial cells
3. L-selectin: present on all types of leukocytes
All 3 recognize a similar ogliosaccharide
The receptor for all 3 is the same
← Removal of fuctose has a profound effect on binding
Potential prognostic significance of expression of the adhesion molecule L1 in ovarian cancer
Expression of L1-selectin bodes poorly for women with uterine and ovarian tumors
L1 expression as a predictor of progression and survival in patients with
uterine and ovarian carcinomas.
L-selectin is present in leukocytes.
Time after surgery (months)
← Woman has a tumor with the L1 selectin
← Woman does not have L1 selectin
IMMUNOGLOBULINS
Antibodies are the prototype of proteins termed Ig. The Ig-type domains are present in a wide variety of
proteins, which together constitute the immunoglobulin super-family, or IgSF.
Immunoglobulin superfamily (IgSF): a wide variety of proteins that contain domains 70 to 110 amino acids that
are homologous to domains that compose the polypeptide chains of blood-borne antibodies.
Most IgSF cell-adhesion molecules mediate the specific
interactions of lymphocytes with cells required for immune
response (ex: macrophages, other lymphocytes, and target cells).
However, some IgSF members, such as VCAM (vascular
cell-adhesion molecule), NCAM (neural cell-adhesion molecule),
and L1, mediate adhesion between non-immune cells.
L1 is a cell-adhesion molecule of the immunoglobulin (Ig) superfamily
CADHERINS
Cadherins: a large family of at least 30 related glycoproteins that mediate Ca2+-dependent cell-adhesion and transmit
signals from the extracellular matrix to the cytoplasm.
Cadherins and cell adhesion
Cadherins join cells of similar types to one
another by preferential binding to the same
cadherin present at the surface of the
neighboring cell
Cadherins and the epithelial-mesenchymal
transformation
There are periods during embryogenesis where embryonic
cells gain and lose specific adhesive properties. Cadherins
are important in these functions:
— loose largely nonsticking mesenchymal cells migrate
— form tight fitting somite epithelium
— become lose fitting again
Location of N-caherin is shown by
immunofluorescence to be where the tight fitting
somite cells are located
ADHERENS JUNCTIONS AND DEMOSOMES: ANCHORING CELLS TO OTHER CELLS
Adheren junctions are found in a variety of sites within the body. They are particularly common in epithelia, such as the
lining of the intestine, where they occur as a belt that encircles each of the cells near its apical surface. They keep
polar cells polar.
An intercellular junctional complex
The complex consists of a tight
junction, an adherens junction, and a
desmosome. Other desmosome and
gap junctions are located deeper
along the lateral surfaces of the cells.
Adherens junctions and tight
junctions encircle the cell, whereas
desmosomes and gap junctions are
restricted to a particular site
between adjacent cells.
The structure of an adherens junction
Schematic model for the molecular architecture of an adherens junction.
The cytoplasmic domain of the cadherin molecule is connected to the actin
filaments of the cytoskeleton by linking proteins, including β-Catenin. ΒCatenin has also been implicated as a key element in a signaling pathway
leading from the cell surface to the cell membrane.
A mutation in β-Catenin has been shown to be an important player in colon
cancer and arthritis. Higher levels of β-catenin have been associated with
osteoarthritis, which destroys cartilage.
Cadherins link the new cells across a narrow extracellular gap.
Desmosomes (maculae adherens): disk-shaped adhesive junctions approximately 1 µm in diameter that are found in a
variety of tissues, particularly those that are subjected to mechanical stress, such as the skin, gums, cardiac muscle, and
uterine cervix. Like adherens junctions, desmosomes contain cadherins that link the two cells across a narrow
extracellular gap.
An electron micrograph of a desmosome
from a newt epidermis
Intermediate filaments
Desmosome
Schematic model of the molecular
architecture of a desmosome
THE ROLE OF CELL-ADHESION RECEPTORS IN TRANSMEMBRANE SIGNALING
Transmembrane signaling: transfer of information across the cell membrane. This is one of the roles of integral
membrane proteins. All of the following types of cell-adhesion molecules have the potential to carry out this function.
An overview of the types of
interactions involving the cell
surface
4 types of cell-cell adhesive
interactions are shown, as
well as 2 types of
interactions between cells
and extracellular substrata
(IF, intermediate filament
and AF, actin filament)
For example, the engagement of an integrin with its ligand can induce a variety of responses within a cell, incuding
changes in cytoplasmic pH, Ca2+ concentration, protein phosphorylation, and gene expression. These changes, in turn,
can alter a cell’s growth potential, migratory activity, state of differentiation (illustrated below), or survival.
The role of extracellular proteins in maintaining the differentiated state of
cells
Cultured mammary epithelial gland cells
—In the absence of extracellular matrix: non-differentiated, flattened, not
engaged in the synthesis of milk proteins
—In the presence of extracellular matrix: cells regain their differentiated
appearance and synthesize milk proteins
Extracellular matrix and integrins allow the cells to retain their
differentiated characteristics and these cultured cells produce domes which
are capable of producing milk proteins.
TIGHT JUNCTIONS: SEALING THE EXTRACELLULAR SPACE
An electron micrograph of a section
through the apical region of 2 adjoining
epithelial cells, showing where the plasma
membranes of the 2 cells come together at
intermittent points with the tight junction.
A model of a tight
junction showing the
intermittent points of
contact between integral
proteins from 2 opposing
membranes. Each of
these contact sites
extends as a row of
proteins within the
membranes, forming a
barrier that blocks solutes
from penetrating the
space between the cells.
GAP JUNCTIONS: MEDIATING INTERCELLULAR COMMUNICATION
Gap junctions are sites between animal cells that are specialized for intercellular communication. Gap junctions are very
important in coordinated contractions of heart muscle.
Electron micrograph of a section through a gap junction
perpendicular to the plane of the 2 adjacent
membranes. The pipelines between the 2 cells are seen
as e- dense beads on the opposed plasma membranes.
Structure of the gap junction
A schematic model of a gap
junction shows the
arrangement of 6 connexin
subunits to form connexon,
which contains half of the
channel that connects the
cytoplasm of 2 adjoining cells.
Each connexin subunit is an
integral protein with 4
transmembrane domains.
Extracellular surface of a single connexon in
the open (left) and closed (right)
conformations. Closure is induced by
elevated Ca2+ concentration.
Gap junction channels are relatively non-selective; if the molecule is small enough, it passes through the open pipeline.
Gap junctions are thought to be gated probably by phosphorylation of connexin subunits.
Results of an experiment demonstrating
the passage of low molecular weight
solutes (X marks the spot where
fluorescein label was injected) through gap
junctions.