Lecture 20

Cells organize into tissues

• cells secrete proteins to build the ECM

• a tissue = cells + ECM

• 4 major tissue types

  • epithelial

  • connective

  • muscle

  • nervous

• tissues are held together by cell—cell interactions, by ECM, or both

tissue organization

• ECM is not abundant in epithelial tissue (is only a thin sheet of basal lamina): cell-cell contacts/cytoskeleton provide strength

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

  • has many collagen fibers

  • sparse cells so ECM bears mechanical load

basil lamina = thin sheet of ECM

ECM in connective tissue

• same basic composition as basal lamina, with some different components

LOOK at pic

the basil lamina is located between epithelial (or muscle cells) and connective tissue

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

connective tissue = isolated cells, with a large amount of ECM

The basal lamina is structured differently in different tissues

muscle: basal lamina surrounds the cells

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

both cases: basal lamina is adjacent to the plasma membrane, between cells and connective tissue

Aberrant cell adhesion contributes to metastasis

• cells invade and migrate through barriers between tissues

• cancer cells have mechanisms to break through ECM

• this is a contributor to metastasis because cells can invade other parts of the body

Aberrant cell adhesion contributes to metastasis

ex.

ex. tumor cell (red) migrating in blood vessels (green)

Integrins link the cell to the ECM

• integrins are transmembrane proteins on the surface of the cell

• integrins cluster into adhesive structures that can mediate cell-ECM interactions

Reminder: integrins are components of___

integrins are components of focal adhesions and hemidesmosomes

pics

reminder: Integrins can link the cell to ECM

hemi-desmosomes pic

note: integrins also form the cell-ECM connection in focal adhesions

Integrins switch between inactive and active states

• integrins consist of two subunits (alpha and beta)

• conformatinal changes (bent to extended) activate integrin

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

binding to the ECM outside the cell

binding to linker proteins inside the cell

note: sometimes integrins can bind to proteins on the surface of other cells; they are not only used for cell-ECM contacts

major components of the ECM

1. collagen: provides structural ibntegrity, mechanical strength

2. proteoglycans: proteins with attached sugar molecules: provide cushioning(they have a gel-like quality)

3. 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)

these 3 major components are found in both basal lamina and in the ECM of connective tissue, through the relative abundance of components can differ in these 2 types of ECM

structure of the 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

ECM components: collagen in basal lamina

the basal lamina has “sheet-forming” collagens (collagen IV) that assemble into a network

• 3 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)

• Collagen IV proteins form a meshwork by associating through their N- and C- terminal globular domains

ECM components: collagen in connective tissue

• image shows fibroblasts surrounded by connective tissue

• connective tissue has collagen fibers (not a sheet-like network)

ECM components: collagen in connective tissue

assembly

• “fibrillar collagens” form fibers and are present in connective tissue

• fibrillar collagens assemble into triple-stranded molecules

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

• these assemble into the fibers found in connective tissue

collagen fibers are stronger than ___

steel

ECM components are made through the secretory pathway

why don’t collagen fibers form inside cells before they are secreted?

• cells secrete collagen in a form that can’t assemble into fibrils - this “procollagen” has extra domains on the ends

• 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

condition cana rise from either

• lack of the enzyme that converts procollagen to collagen

• defect in procollagen itself

ECM components: proteoglycans part 1

• 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

Reminder: modication of proteins occurs in the __

Reminder: modication of proteins occurs in the golgi apparatus

ECM components: proteoglycans part 2

• 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

ECM components: multi-adhesive matrix proteins

• called “multi-adhesive” because they bind to lots of other proteins

• called “matrix” because they hel[ formthe extracellular matrix, by crosslinking ECM proteins

• also bind integrins to link the ECM to the cell

  • ex. 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

ECM components: multi-adhesive matrix proteins

ex. fibronectin

• fibronectin has a domain that binds ECM components (collagens)

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

• also has a domain that binds integrins

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

regulation of adhesion strength allows cells to have complex behaviors 1

• 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 (ex. chemokines)

• cells “roll” along the endothelial cells of the blood vessel

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

cell-cell adhesion during extravasation 1

loose adhesion:

• 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 to the golgi

terminology

• glycoproteins are proteins with attached sugar residues

• proteoglycans are a specific type of glycoprotein that contain GAGs

cell-cell adhesion during extravasation 1

loose adhesion

• 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 bind is weak, so allows the rolling of leukocytes along the surface of the blood vessel

cell-cell adhesion during extravasation

integrins mediate the switch to tight adhesion

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

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

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

• this stops the rolling

this is a cell-cell interaction

regulation of adhesion strength allows cells to have complex behaviors 2

• inflammatory signals (ex. chemokines) cause leukocytes to associate with the endothelial cells on the walls of the blood vessel

• binding of selectins to glycoproteins recruits the leukocytes

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

• activation of integrins at site of infection tirggers tight binding to the blood vessel

• leukocyte stops rolling and then can crawl between the cells of the blood vessel into the infected tissue