BIS 104 16-17: Cell-cell interaction
Cells interact with each other and surrounding environment in tissue and organ.
Cytoskeleton and extracellular matrix all are involved in cell-cell interaction. Generally, cells are
organized by two fundamental mechanisms:
 adhesion: Anchored cytoskeletal filaments transmit mechanical stresses from cell to
cell
 ECM: extracellular matrix directly bears mechanical stresses
I. Plant cell wall
Cell walls are polysaccharides that allow plant cells to acquire distinct morphologies during
differentiation.
Major component is cellulose: β 1-4 glucose polysaccharide regulates cell expansion.
Cellulose microfibrils are arranged in parallel to cortical microtubules beneath the plasma
membrane. Cellulose synthase is linked to microtubule by connector protein and tracks along
microtubules. They make cellulose molecule, assembles them into a microfibril, and adds them to
pre-existing cell wall. This may explain why deposition of cellulose is guided by microtubules.
II. Animals extracellular matrix (ECM)
a. Importance of ECM
Matrix proteases can destroy ECM proteins. ECM damage causes osteoarthritis and
rheumatoid arthritis. Also, ECM damage makes cancer cell easy to spread to other places.
b. Collagens
Collagens are the most abundant ECM proteins. A collagen chain is composed of repeated
Gly-X-Y pattern. 3 collagen chains can twist into a triple-stranded collagen molecule. And
collagens can have many different forms: fibrils, nonfibrils, network-forming and transmembrane.
Before assembling to fibrils, collagen need to be co-translated into ER and transported into Golgi
body. Then, collagen is secreted in the form of triple-strand procollagen molecule. Terminal
extensions of secreted procollagen molecules are cleaved by proteinase in the extracellular space
and become collagen molecules. Collagen molecules can self-assembly into fibrils outside of the
cell. Fault assembly of collagens can cause serious diseases.
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c. Elastins
Elastin is also a single-strand protein. Unlike collagens, elastin chains are covalently linked to
each other to form elastic fiber. Elastic fiber imparts elasticity and flexibility, as elastic fiber can
stretch and relax. As we grown older, collagen and elastin will gradually decrease in our skin,
causing wrinkle.
d. Fibronectin and other glycoproteins
Fibronectin is connected to each other by disulfide bond and forms oligomers. Fibronectin
and other glycoproteins connect collagens to fibroblasts, therefore adjust cell adhesion to ECM.
Fibronectin can bind to many ECM components, including collagen, integrin, heparin. The
orientation of fibronectin above the cell surface is organized by actin filaments of the cell.
e. Integrin
Integrin is the cell surface receptor for ECM. Integrin works in pairs of α and βsubunits. As a
transmembrane protein, integrin can bind to fibronectin outside the cell. Adaptor proteins link
actin filament and integrin dimer at the cytoplasmic side.
f. Proteoglycans
The ECM space is filled by proteoglycans, proteins covalently linked to glycosaminoglycans
(GAGs). Proteoglycans are really large, and their components include core protein, link protein,
and various form of GAGs. GAGs are repeats of disaccharides of N-acetylglucosamine and
glucuronic acid, a widely distributed form of which is called hyaluronic acid (or hyaluronan). GAGs
can be sulfated to become chondroitin sulfate GAGs or keratan sulfate GAGs. GAGs are linked
together by linker proteins. GAGs can be extremely large in human body, for example, synovial
fluid and vitreum.
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III. Cell-cell junctions
a. The big deal of cell-cell junctions
Cells are organized into tissues with ECM, yet dissociated cells show organ specific adhesion.
This is because cell-cell junctions are tissue specific. Different tissues organize cells differently and
certain arrangements are specific to certain tissues.
b. Tight junction
Tight junctions are sealed plasma membrane between cells. There is no space between the
junction point, so no exchange of molecule can occur. It creates permeability barriers between
cells.
c. Gap junctions
Gap junctions are 2-4 nm channels created by connexon. Connexon is composed of six
connexins, two connexons in register create a hydrophilic channel for molecules < 1000 Da to
pass. Ions, ATP, glucose, AAs, second messengers are examples for those can pass through gap
junction. Transportation between gap junctions is also regulated.
d. adhesive junctions
Adhesive junctions are connections between cytoskeletons of different cells. Adhesive
junctions are always loose, forming a 20-35 nm gap. Adhesive junctions are essential for tissue
and embryos forming.
Transmembrane proteins such as cadherins (except T-cadherin) mediate adhesion. The
extracellular parts of cadherins are composed of several cadherin domains. Interactions between
cadherin domains are calcium-binding triggered. Calcium binds to the hinge between two
cadherin domains. The interaction is homophilic, which means both participant of the interaction
is of the same. At the cytosol side, cadherin is attached to cytoskeleton by linker proteins such as
vinculin and catenin.
Based on cytoskeletons linked to cadherin, adhesive junctions can be categorized:
 adherens: cadherins linked to actin microfilaments
 desmosomes: cadherins linked to intermediate filaments
 hemidesomsomes: cadherins connects intermediate filaments to ECM
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IV. Plasmodesmata
Plasmodesmata are pores in the cell walls that allow cytoplasmic exchange. Smooth ER of
each cell forms desmotubule through the plasmodesmata, and membrane are lined along
plasmodesma, connecting two adjacent cells. Size of plasmodesmata differs according to cell
physiological state (20-200 nm)
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