Chapter 19: Cell junctions and the
extracellular matrix
Know the terminology:
Cadherins, integrins, tight junction, desmosome,
adhesion belt, hemidesmosome, gap junction,
connexin/connexan,
Basal lamina, connective tissue, fibroblast,
proteoglycan, GAG (glycosaminoglycan), hyaluronin,
aggrecan, collagen, fibronectin,
Overview
Multicellular organisms combine cells into tissues.
Extracellular matrix is a complex network of
macromolecules (proteins, glycosaminoglycans,
proteoglycans) secreted by cells
Specialized ECMs include connective tissue, basal
lamina, exoskeletons, cartilage/bones,
Tissues are formed from cell-cell connections and
cell-matrix connections.
Tissue composition
Outline
How are cells organized into tissues?
I. Cell junctions
-types of cell junctions
-tight junctions, adherans junctions,
desmosomes, focal adhesions, hemidesmosomes,
gap junctions
II. Extracellular matrix proteins
-types of ECM macromolecules
-synthesis and properties of hyaluronan,
aggrecan, collagen, fibronectin
Cell junctions
Three types of cell junctions:
1. Occluding junctions: seal cells together into
sheets (forming an impermeable barrier)
2. Anchoring junctions: attach cells (and their
cytoskeleton) to other cells or extracellular matrix
(providing mechanical support)
3. Communicating junctions: allow exchange of
chemical/electrical information between cells
Occluding junctions
Example: Tight junctions
of intestinal epithelium
Tight junction
Each cell
possesses
integral
membrane
proteins that
bind to
similar
proteins in
the adjacent,
forming a
continuous
“weld”
Anchoring junctions
Integral membrane proteins connect a cell’s
cytoskeleton to another cell or extracellular matrix
Anchoring junctions
Integral membrane proteins connect a cell’s
cytoskeleton to another cell or extracellular matrix
Anchoring junctions
Cytoskeletal fibers (MF, intermediate filaments)
connect to a
Membrane protein receptor
which attaches to another protein in either:
-the extracellular matrix
or
-another cell membrane
Cadherins and desmosomes
Cell to cell connections
are mediated by
cadherins.
These receptors
extend out from the
cell, binding to other
cadherens
Cadherins participiate in adherens junctions
Under the cell
membrane,
contractile fibers of
microfilaments
connect to cell
membrane proteins
called cadherins
They surround the
cell, forming a belt
Desmosomes
Cadherins can also form localized spot connections
Cadherins
attach to
intermediate
filaments via
anchoring
proteins: a
desmosome
Cells-to-ECM attachments:
Focal adhesions and hemidesmosomes
Cytoskeletal fibers
attach to
transmembrane
receptors (integrins)
that are attached to
extracellular matrix
components
•Focal adhesions use
MF
•Hemidesmosomes use
IF
Gap junctions
Gap junctions allow cells to exchange electrical
and/or chemical signals
Composed of proteins that form channels that allow
small molecules to pass.
Subunits of these channels are connexins that are
assembled together to make connexons. The
connexons from 2 cells join together to make a gap
junction.
Gap junctions
Regulation of connectivity
When might a cell want to alter its connections to
other cells?
How do cells alter these connections?
-alter the profile of cytoskeletal connections,
receptors, and extracellular matrix
-alter the binding affinity of receptors
-many are Ca2+ dependent
-many are affected by protein kinases
Summary
Summary
Extracellular matrix
•A network of proteins, glycosoaminoglycans (GAGs)
and combinations of the two (proteoglycan) found in
the extracellular space
•All ECM components are secreted by cells
•Most cells can secrete elements of the ECM but
many ECMs are built by fibroblasts
Examples of ECM:
-connective tissue, basal lamina, cartilage, bone,
plant/fungi cell walls, myelin sheath,
Basal lamina
Connective tissue
Connective tissue
Components of the extracellular matrix
Proteins/glycoproteins (secreted by cotranslational import) differ in physical properties
(e.g. size, flexibility) and are able to bind to
different combinations of macromolecules on cell
membranes (e.g. integrins, cadherins) and other
ECM elements
Examples include:
-collagen, elastin, fibronectin, laminin
Components of the extracellular matrix
Proteins can provide
elastic properties in
many tissues (e.g.
elastin)
Components of the extracellular matrix
GAGs differ in physical properties (e.g. size,
flexibility, hydration).
Composed of repeating sugar + amino sugar units
(e.g. N-acetylglucosamine, N-acetylgalactosamine)
They occur in long strings, often attached to
proteins
Examples include:
-hyaluronan, chondroitin sulfate, heparan sulfate,
keratan sulfate
GAGs
GAGs attract a great deal of water. Hydroxyl groups
form hydrogen bonds, and the many negative charges
attract clouds of cations (Na+) that induce an osmotic
movement of water. These hydrated gels resist
compression (useful for joints).
Components of the extracellular matrix
Proteoglycans (made of both proteins and GAGs)
also differ in physical properties
Synthesized in Golgi prior to secretion
In addition to structural roles, proteoglycans can
also bind hormones (e.g., inflammatory chemokines,
FGF, TGFb) to alter cell signaling pathways
Examples include:
-decoran, aggrecan (the main component of
cartilage)
ECM proteins, GAGs, and proteoglycans
Green-protein
Red-GAG
Hyaluronan
Very simple GAG, consisting of 10,000+ repeats of
glucuronic acid and N-acetylglucosamine
Spun directly from cell membranes by a surface
enzyme complex
Attracts water and fills spaces between cells with
non-compressible gel (found around joints)
Some cells secrete it to isolate themselves from
other cells (e.g. myoblasts). These cells can secrete
hyaluronidase to break it down, allowing contact
Hyaluronan
Very long
macromolecule
that hydrates and
fills enormous
volumes
Aggrecan
One of the largest macromolecules, consisting of a
core protein with GAGs attached to form a
feather-like appearance.
Aggrecan
An aggrecan core protein is very large but also
binds many (different) GAGs (shown in red)
Aggrecan aggregates
Each aggrecan
can be attached
to a hyaluronan
backbone to form
an aggrecan
aggregate
Collagen
All multicellular animals possess collagens, often with
many different collagen genes
Synthesized as pro-collagen monomers (pro-a
collagen)
Peptide modified by hydroxylation and glycosylation
Prior to secretion they self- assemble into trimers
Upon secretion the trimers are processed by
proteolytic enzymes then assemble into fibrils
Collagen fibers
Collagen proteins (trimers) are then cross-linked to
form collagen fibers (stiff, not very elastic)
Fibronectin
Animals have a single fibronectin gene that can be
alternatively spiced into 40+ forms.
Different exons are able to bind different
proteins/GAGs (e.g. integrins, collagen, etc)
Fibronectin dimerizes using 2 similar (not
identical) monomers
Fibronectin
Control of ECM
Cells control the synthesis of ECM by altering
gene expression and co-translational import and
secretion
They also control the degradation of the ECM by
secreting and activating/inactivating extracellular
enzymes.
Most important are a group of serine proteases
called matrix metalloproteinases
MMPs
Cells that need to migrate must first break down
the connections to the ECM (e.g. tissue repair,
metastasis of tumors)
MMPs can be:
-secreted in active form (collagenase)
-secreted as inactive form (e.g. plasminogen is
inactive until it is modified by “plasminogen
activators)
-activated when cells stop secreting TIMPs (tissue
inhibitors of metalloproteinases)