Lecture 20 - Cell-to-cell interactions: Part 2

what is a tissue made of?

cells + ECM

four major tissue types

epithelial, connective, muscle, nervous

ECM in epithelial tissues

not abundant in epithelial tissue (is only a thin sheet of basal lamina)

ECM in connective tissues

very abundant in connective tissue (cells are more sparse and don't necessarily contact each other)

what provides mechanical strength to epithelial tissue?

cell-cell contacts/cytoskeleton

what provides mechanical strength to connective tissue?

the ECM, as cells are spare (connective tissue ECM has collagen fibers)

the basal lamina of epithelial cells is located between...

epithelial (or muscle) cells and the connective tissue

basal lamina

sheet-like meshwork of ECM (basal lamina is a type of ECM)

connective tissue

isolated cells, with a large amount of ECM

True or False: the basal lamina is structured differently in different tissues

True

basal lamina and muscle cells

the basal lamina surrounds the muscle cells

basal lamina and epithelial cells

the basal lamina is sheet-like, one surface of the cells rests on it

how does cancer metastasize?

cells invade and migrate through barriers between tissues

cancer cells have mechanisms to break through ECM so they can invade other parts of the body

integrins

link the cell to the ECM

integrins are transmembrane proteins on the surface of the cell

cluster into adhesive structures that can mediate cell-ECM interactions

components of focal adhesions and hemidesmosomes

integrin structure

consist of two subunits (alpha and beta)

integrin conformation changes

integrins activate by changing from bent to extended

integrin activation can be triggered by ECM components outside the cell or by proteins inside the cell

collagen

provides structural integrity, mechanical strength

proteoglycans

proteins with attached sugar molecules; provide cushioning (they have a gel-like quality)

multi-adhesive matrix proteins

proteins that cross-link proteins and create the ECM meshwork; these proteins can also bind integrins (to link ECM to the cell)

collagen, proteoglycans, and multi-adhesive matrix proteins are found in...

both basal lamina and in the ECM of connective tissues, though the relative abundance of components can differ in these two types of ECM

structure of basal lamina

cross-linked networks of collagen, proteoglycans, and multi-adhesive matrix proteins

integrins (transmembrane proteins in the plasma membrane) bind to ECM components, linking the ECM to cells

two major types of collagen in the ECM

1. "sheet forming" collagen in basal lamina (collagen IV)

2. "fibrillar" collagen in the ECM of connective tissue

"sheet forming" collagens

three collagen IV molecules come together to form a triple helix - this is the subunit

these collagen IV triple helices interact via their N- and C- terminal globular domains

subunits associate into a 2D network (not into fibers)

"fibrillar" collagens

form fibers and are present in connective tissue

assemble into triple-stranded molecules (no globular domains)

subunits associate along their lengths (not via N- or C- termini) to form fibrils

these assemble into the fibers found in connective tissue

levels of "fibrillar collagen" assembly from smallest to largest

1. single collagen polypeptide chain

2. triple stranded molecule molecule (subunit)

3. collagen fibril

4. collagen fiber

the basal lamina has ______ collagens

"sheet forming"

the connective tissue has _____ collagens

"fibrillar"

True or False: collagen fibers are stronger than steel

True

True or False: collagen is secreted from cells in connective in a form that is ready for fiber assembly

False

procollagen

cells secrete collagen in a form that cannot assemble into fibrils - this "procollagen" has extra domains on the ends

what happens after procollagen is secreted?

outside the cell, the procollagen is processed by a protease in the ECM

the cleaved collagen molecules can then form fibers outside the cell

defects in processing collagen lead to...

hyperflexible skin

how do defects in processing collagen occur?

1. lack of the enzyme that converts procollagen to collagen

2. defect in procollagen itself

structure of proteoglycans

proteoglycans have attached polysaccharide (sugar) chains called GAGs (glycosaminoglycans)

GAGs are covalently-linked polysaccharide chains

proteoglycans give the ECM its gel-like properties, which resists compression on tissues

GAGs are assembled on the proteoglycans in the...

Golgi

proteoglycans in the ECM

proteoglycans with GAGs can form large aggregates

GAGs tend to adopt highly extended conformations, which occupy a large volume

they can therefore act as "space fillers" that provide cushioning to the ECM

what do multi-adhesive matrix proteins have their name?

"multi-adhesive" because they bind to lots of other proteins

"matrix" because they help form the extracellular matrix, by crosslinking ECM proteins

multi-adhesive matrix proteins bind _____ to link the ECM to the cell

integrins

examples of multi-adhesive matrix proteins

laminin and fibronectin

laminin

component of the basal lamina

cross-shaped protein with binding domains for other ECM proteins and for integrins

interacts with other proteins through the globular domains

fibronectin

fibronectin has a domain that binds ECM components (collagens)

can bind fibrillar collagens (in connective tissue) or sheet-forming collagens (in basal lamina)

also has a domain the binds integrins

fibronectin therefore helps anchor collagens in the ECM to the cell via integrins

extravasation

movement of leukocytes (white blood cells) to areas of infection until they ultimately invade the underlying tissue (extravasation)

if there is an infection, leukocytes respond to inflammatory signals (e.g. chemokines) and start associating with the endothelial cells - this is loose adhesion

cells "roll" along the endothelial cells of the blood vessel

when reach the site of infection, this switches to strong adhesion

loose adhesion in extravasation - glycoproteins

cell rolling is mediated by loose adhesion

glycoproteins with attached sugar residues are on the surface of the leukocyte (white blood cell)

similar to the proteoglycans of the ECM, these proteins have sugar residues added in the Golgi

loose adhesion in extravasation - P-selectin

cell rolling is mediated by loose adhesion

P-selectin (a protein with a "lectin" domain that binds sugars) is on the surface of the cells lining the blood vessel

selectins bind to the sugar residues of the glycoproteins

this binding is weak, so it allows the rolling of the leukocytes along the surface of the blood vessel

strong adhesion in extravasation

integrins mediate the switch to tight adhesion

when the cell reaches the site of inflammation, there is a signal to switch to tight adhesion

this signal activates integrin (on the leukocyte) - switches from bent to straight conformation

activated integrin tightly binds a protein ("iCAM") that is on the surface of the endothelial cells

this stops the rolling

summary of extravasation initiation and adhesion

1. inflammatory signals (e.g. chemokines) cause leukocytes to associate with the endothelial cells on the walls of the blood vessel

2. binding of selectins to glycoproteins recruits the leukocytes

3. this is a loose adhesion - blood flow pushes the cell and it rolls

4. activation of integrins at site of infection triggers tight binding to the blood vessel

5. leukocyte stops rolling and then can crawl between the cells of the blood vessel onto the infected tissue