Chapter 19: The Extracellular Matrix

Cell Junctions

5 major cell to cell junction types

• desmosomes

• plasmodesmata

• tight junctions

• gap junctions

• adherens

connective and epithelial tissue junctions

• see that epithelial tissue has very little matrix

• the epithelial cells themselves resist stress via cell to cell and cell to matrix adhesions

image of diff cell junctions

tight junctions

seal the gap between epithelial cells

adherens

connect actin filament bundles in one cell to actin filament bundles in the next cell

gap junctions

allow the passage of small water-soluble molecules between cells

hemidesmosomes

anchor IFs in a cell to extracellular matrix and binds to basal lamina and provide stability to epithelial cells.

actin-linked cell-matrix junction

anchors actin filaments in the cell to the ECM

anchoring junctions

• hold two cells together

• each of the two cells has a transmembrane protein that connects to each other’s

things mediated by anchoring junctions summary table

• notice cell to cell vs cell to matrix

• cadherin and integrin superfamilies

cadherins

• have repeating extracellular domains

  • these domains are homophilic binders which bind to other ECs

  • ex: flamingo binding to another flamingo

• in all animals, but not all eukaryotes

• in some fungi, but not in plants

• has an extracellular and intracellular domain

structure of cadherins

• see how two cadherin EC domains bind to each other at junctions

• calcium maintains the structure of cadherins to they can extend far enough to interact

• calcium binds at the hinge regions

• the cadherins can only bind if calcium is bound to the cadherins

• cadherins are homophilic binders

cadherins bind to each other like

velcro

• the adhesions are weak alone but strong together

cadherin homophilic binding assists in development

• layers of homophiic binding cadherins form the layers of the 3 embryonic germ layers

• the layers reconstitute after being mixed up due to the cadherins, this shows which cell types go where

• cadherinds sort according to homophilicness

catenins

adherin junctions

• link cadherins to the actin cytoskeleton

• are adapter proteins

types of catenins

catenins: adapter proteins between cadherins and actin filaments

• p120

• beta-catenin

• alpha catenin

• alpha connects to beta, which connects t p120

adherens junction

• connection between two cells

• cadherin and catenin cluster between two cells

• areas closest to each other bind so if the cadherins are close enough to bind each other, they will

Rac at adherens junctions

• causes branched actin to form

• promotes additional protrusions to expand the zone of contact between two cells

Rho at adherens junctions

• inactivates Rac and stimulates the formation of linear contractile bundles

• this contractile actin and myosin bundles are what seals the cells together

steps of adherens junction formation

1) a cluster of cadherins and catenins forms, starting to bring the two adjacent cells together

2) more and more cadherins and catenins get recruited, bringing the two cells closer together and Rac promotes branched actin to form

3) Rho inactivates Rac and causes linear contractile bundles to form of actin and myosin

4) the two cells seal together

mechanotransduction in adherens junctions

• when the cells feel more tension, they form tighter adhesions in response to stay stable

• the cadherins get stretched out, pulling on the alpha catenin

• when the cell feels tension, the alpha catenin goes from folded to stretched out

• where the alpha catenin stretches out, vinculin builds more actin off of it and myosin pulls on the actin in the oppo direction to stabilize

adhesion belts

when adherens junctions in epithelial cells form a loop so that all of the cells can function as one unit to move

• from the ectodermal layer of cells

• forms a ring of actin bundles when the adherin belt contracts and pinches off

• these rings/belts form 2/3 the way up on epithelial cells

desmosomes

provide mechanical strength for the cell

• form “buttons”

• use nonclassical cadherins to seal between two cells

• cadherins connect intermediate filaments to desmosomes

• desmocollins and desmogleins are cadherins that help form desmosomes

• IFs are on the outside and connect to the button adpater proteins, which connect to desmogleins and desmocollins, the cadherins that hold the two sides of the cadherins together

tight junctions

• partitions the body and lines organs by forming sheets of epithelial cells

• act as permeability barriers in cells

• seal cells together so that molecules are moved selectively between cells (allows for regulation)

polarity of epithelial cells

• see how tight junctions separate the outside of the body from the area between two cells

• tight junctions stop proteins that belong in the apical side from going to the basal side and vice verse

experiment that shows the function of tight junctions

tiny dye molecules placed on either side of the tight junctions (inside or outside of the cell) could not get in to the other side

where GLUT1 and the sodium glucose pump are on epithelial cells

sealing strands of tight junctions

• water tight seals between two cell membranes

• like a woven material

• forms when claudins and occludins bind to each other (of themselves) on opposite sides of the tight junctions

claudins and occludins

claudins bind other claudins and vice versa

• transmembrane proteins that help form tight junctions

how scaffold proteins help form junction proteins

Zonula occludens (ZOs) bind to other ZOs

• ZOs are adapter and scaffold proteins and are next to other claudins in a line

gap junctions overview

• bridge gaps between cells to create and direct channels between two cells cytoplasms

• made up of connexin proteins in vertebrates

  • connexins are large enough to allow small molecules through, but not macromolecules

• gap junctions help move ATP and ions, so they “couple” cells metabolically and electrically

• gap junctions can function as channels

structure of gap junctions

• 6 connexins form a connexon

  • connexons can be homomeric or heteromeric

• two connexons combine to form homo or heterotypic intercullular channels

• diff combos of these channels selects for diff things, allowing the gap junctions to serve as a selectivity barrier

how many types of connexons are there?

2

homomeric and heteromeric

do plant cell walls have adhesion junctions?

NO, since they are already bound tightly and don’t need it

plasmodesmata

in between plant cells, similar to gap junctions in animal cells

plant cell walls are tightly

cemented together

• the plasma membrane of one plant cell is continuous with that of its neighbor via plasmodesmata

the plasma membrane of one plant cell is continuous with that of its neighbor via

plasmodesmata

plasmodesmata structure

the area between two plant cells

• allows plant cells next to each other to share their cell membranes and cytosol to allow solutes through, which cannot get through the cell wall

• there are large enough gaps that whole organelles can get through to adjacent cells

what forms the ECM

cells and their matrix

• the ECM is very dynamic and active

• the ECM is made up of macromolecules

• the ECM i has its own signals

things that are formed out of the ECM

• teeth and bone

• the transparent cornea of the eye

• tendons

• ALL of this is acellular

3 major classes of ECM macromolecules

• proteoglycans and GAGs

• fibrous proteins (collagens)

• glycoproteins

proteoglycans

• perlecan

• decorin

• aggrecan

• these three also contains GAGs

GAGs

ex: hyaluronan

• this is only a GAG, not a proteoglycan

• glycosaminoglycans: made up of repeating disaccharides of amino sugars

• made of unbranched polysaccharide chains

• mostly beta linkages between the dissacharides

• all are sulfated, which gives a neg charge

hyaluronan

• a space filler GAG that forms a hydrated gel that attracts nearby ions, and therefore water, forming a cushion that takes up a lot of space

• lacks sulfates, so it attracts a bit less water and is a bit more firm

• forms a cushion/jelly

• helps resist compressive forces

proteoglycans (that are also GAGs)

• GAGs attached to a protein

• have a linked tetrasaccharide, which differentiates them from glycoproteins

• must have at least one sugar chain that is a GAG to be a proteoglycan

proteoglycans can be

huge

• occurs when they form even larger polymeric complexes

• ex: aggrecan, which polymerizes and forms long branched propteoglycans

aggrecan

forms huge proteogylcan complexes

What is the most abundant protein in mammals?

collagens (fibrous proteins)

fibrous proteins/collagens

• resist tensile strength

• form long, stuff helical structures to provide strength for the ECM

• form triple helixes using proline and glycine which form kinks in the AA backbone to get a twisted shape

structure of collagen

long stiff fibers

• collagen backbone is formed of glycine, proline, and hydroxy proline (these three form a trimer)

• these trimers form a filament

elastin

• a form of fibrous protein that gives tissues their flexibility

• in the skin, uterus, bladder, aorta, lungs, etc for stretching

elastin forming a network

• elastin is intrinsically disordered so it forms random coils, which forms a network, stabilized by crosslinks of disulfide bonds (covalent bonds)

• this is why elastin has limited stretching, as these disulfide bonds only allow the elastin to go so far

• these S-S bonds are fine in the matrix, which is polar

glycoproteins

• ex: fibronectin

• scaffold proteins with multiple binding domains

• helps organize the ECM

• can respond to stretch by binding to other fibronectin molecules

fibronectin

• connective tissue in matrices

• forms V-shapes, connected in the middle by disulfide bonds

• when they are fully folded, only some of their binding sites are available

• unfolds to allow more binding sites to be accessible

basal lamina

• a specialized form of the ECM

• on the underside of all epithelial cells

• aka the basement membrane

• protects the outside of cells and adds additional filtration into cells (kidneys)

• it also gives survival signals to the cells on top of it, therefore is a part of cell survival/proliferation/etc

see basal lamina formation up close

• the basal lamina is formed by the cells on either side of it

laminin

a glycoprotein

• cross-shaped

• binds to things all around it (integrins, dystroglycan, perlecans)

• has a coiled domain and three NH2 domains that form the cross (alpha, beta, and gamma)

• alpha strand forms the tip of the cross

• laminin is the primary organizer of the basal lamina

What does the alpha chain of the laminin bind to?

other integrins (on both side of the alpha chain)

how are cell to matrix junctions mediated?

by matrix receptors that tie the matrix to the cytoskeleton

integrins

• transmit mechanical and molecular signals and convert them to help mediate the cell-matrix junctions

structure of integrins

• form heterodimers of 1 alpha and 1 beta integrin

• connect ECM proteins to things inside the cell by binding to adapter proteins that bind to actin

• adapter proteins that bind to actin to help integrin bind more actin: vinculin, talin, kindlin

How many integrins are in humans?

24

• 23 of these interact with actin

activation of integrins

• inactive state when folded up on themselves

• active state when they unfold and can reach out to protrusion during cell crawling

• stretched out region allows for strong ligand binding

What is the most prominent cell-matrix interaction in epithelia?

hemidesmosomes

hemidesmosomes

• half buttons

• attach epithelial cells to the basement membrane

• formed of BP230, which attaches to plectin, which attaches to integrin, which binds to collagen 12 (the adapter protein) that attaches to laminin, which attaches to collagen

• the BP230 is atatched to keratin on the other side of the cell

keratin

an IF that influences the strength of cells

integrin defects cause

extreme defects

FAK

focal adhesion kinase

• phosphorylates integrin focal adhesions

• binds to integrins to form the lamellopodia

the cell wall resists ___ forces

tensile and compressive

the plant ECM is made up of polymers that lack ____

nitrogen

• so fungi cant degrade plants for nitroigen fixation

What macromolecule do plants lack in the matrix?

proteins

• use sugars instead

cell walls are made out of

cellulose and pectin

• cellulose and pectin are made out of polysaccharides

• cellulose forms perpendicular fibers (like collagen in animals)

• pectin resists compression and is like the proteoglycans of animal cells

• pectin is like jelly and resists compressive forces (sucks up fluid)

turgor pressure

• the most solute is in the cytoplasm of plant cells, compared to the cell walls and externally, so water rushes inside the cell to accomodate, creating the pressure

• large internal hydrostatic pressure that pushes outward on the cell wall, allowing plants to stand up straight

• the cell wall resists the turgor pressure

what direction do cellulose microfibrils grow?

perpendicular to the axis of elongation of the plant

• creates strength for the plant