Cell junctions and the Extracellular Matrix

epithelium

sheets of cells that are polarized, with discrete functional domains at opposite ends (apical and basal) of the cells

connective tissue

more loosely organized, in which cells are attached to each other, a rigid scaffold, or both

cell-cell junctions

specialized structure where two cells come together in long term associations to form tissues and organs

List the types of cell-cell junctions in animal cells

1. adhesive juncitons

   1. adherens junctions

   2. desmosomes

2. tight junctions

3. gap junctions

list the types of cell-cell junctions in plant cells

plamodesmata

Tight junctions, adherens junctions, gap junctions, and desmosomes are all a part of which region?

the basolateral region

what two things do adherens junctions and desmosomes rely on?

1. intracellular attachment proteins to link the junction to the cytoskeleton

2. cadherins on the outer surface to bind cells to each other

which types of junctions are the basis for tissue formation?

adherens junctions and desmosomes

What are focal points for signaling complexes and cytoskeletal structures in adherens junctions and desmosomes?

adhesion proteins

How are many adhesion proteins recycled?

through endocytic and exocytic pathways

what four processes is cell adhesion coordinated with?

1. cell signaling

2. cell movement

3. cell proliferation

4. cell survival

adherens junctions

are cadherin-mediated junctions that interact with actin

what three things characterize cadherins?

1. repeats in their extracellulardomain that are structurally similar

2. a transmembrane domain

3. widely varying cytosolic ends

3 domains of E-cadherin

1. extracellular domain

2. transmembrane domain

3. cytoplasmic domain

E-cadherins ___ and extracellular repeats bind to each other, which is ___ dependent

dimerizes, ca2+

3 types od catenins

1. Beta catenin

2. alpha catenin

3. p120 catenin

Beta catenin

links E-cadherin to aloha catenin

alpha catenin

binds F-actin and Beta catenin, links to a microfilament

p120 catenin

only binds E-cadherin and regulates cadherin stability/turnover

cadherins are ____ specific

tissue

Where are E-cadherins found?

epithelial cells

Where are N-cadherins found?

in neurons and cardiac muscle

What helps segregate cells into specific tissues?

the amount and type of cadherins on cell surfaces

desmosomes

button-like points of strong adhesion between adjacent cells in a tissue

desmosomes function

provide tissues with structural integrity

Where are desmosomes particularly abundant?

in cells that are under mechanical stress like the skin, heart muscle, and uterus

Two types of transmembrane proteins found in desmosomes

1. desmocollin

2. desmoglein

Desmocollins are

cadherins

three types of adaptor proteins found in desmosomes

1. plakoglobin

2. desmoplakin

3. plakophilin

what type of catenin is plakoglobin?

beta-catenin

what type of catenin is desmoplakin

alpha catenin

what type of catenin is plakophilin

p120 catenin

the cytoskeleton is made up of

intermediate filaments

name the three classes of proteins found in. desmosomes

1. transmembrane proteins

2. adaptor proteins

3. cytoskeletal proteins

Adherens junctions are a ____ of cell-cell connection and link ____

continuous band; the MF network in one cell to the MF network in a neighboring cell

Desmosomes are a ___ of cell-cell connection and link____

localized disc; IF network of one cell to the IF network in a neighboring cell `

tight junction function

seals gap between epithelial cells

adherens junction functions

connects actin filament bundle in one cell with that in the next cell

hemidesmosome function

anchors intermediate filaments in cell to extracellular matrix

actin-linked cell-matrix junction function

anchors actin filaments in cell to extracellular matrix

what do tight junctions prevent?

movement of molecules across cell layers

Tight junctions have ___ connection to the cytoskeleton

no

Claudins

4 transmembrane domains with one large extracellular loop

claudin function

interlock with claudins from adjacent cells to form a tight seal

Paracellular transport

ionic R groups on loops of claudins create ion-selective pores through the tight junction

gap junctions

a region where the plasma membrane of cells are aligned and brought into contact, with a very small gap inbetween

what spans the gap in gap junctions?

small pipelines or passages between the cells

name 6 things that can pass through gap junctions

1. inorganic ions

2. monomeric sugars

3. amino acids

4. nucleotides

5. vitamins

6. small signaling molecules

name three things that cannot pass through gap junctions

1. proteins

2. nucleic acids

3. polysaccarides

transmembrane protein involved in gap junctions

connexin

What makes up a connexon?

6 connexin subunits in a holoprotein cylinder

When is a gap junction formed?

when a connexon on one cell lines up with a connexon on an adjacent cell

Three types of extracellular matrix

1. bone

2. cartilage

3. connective tissue

bone mainly consists of

a ridgid extracellular matrix that contains a small number of interspersed cells

cartilage

a tissue constructed mostly o matrix materials that is more felxible than bone

connective tissue surrounding glands and blood vessels is

relatively gelatinous and contains interspersed fibroblast cells

3 classes of ECM

1. a gel like matrix in which other molecules are embedded

2. structural proteins embedded in the matrix

3. adhesive glycoproteins that attach cells to the matrix. the must bind to molecules in the matrix as well as transmembrane proteins on cells

ECM made up of a gel like matrix in which other molecules are embedded is made up of what?

proteoglycans (combinations of sugar polymers and proteins

ECM made up of structural proteins embedded in the matrix includes

collage, which provides strength, and elastins, which provides flexibility.

ECM made up of adhesive glycoproteins that attach cells to the matrix includes what 2 things

fibronectin and laminins

Fibronectin

for most cell-ECM connections

Laminins

for epithelial cell connections to the Basal Lamina

what three cell types aside from epithelial cells have basal lamina?

1. muscle cells

2. fat cells

3. schwann cells

Glycosaminoglycans (GAGs)

the polysaccharide chains are repeating disaccharides where one monomer is an amino sugar and the other is a sugar or sugar acid

the amino sugar in glycosaminoglycans

N-acetylglucosamine or N-acetylgalactosamine

the sugar or sugar acid in glycosaminoglycans

galactose or glucuronate

GAGs and water

they are hydrophilic and attract water and cations forming a gelatinous matrix where collagen and elastin are embedded

in many tissues, proteoglycans are present as

individual molecules

in cartilage, numerous proteoglycans…

become attached to long molecules of hyaluronate

What lubricates joints and facilitates cell migration?

free hyaluronate

Most GAGs in the ECM exist as…

only as components of proteoglycans

How is hyaluronate an exception to most GAGs?

it occurs both as a backbone of cartilage proteoglycans and as a free molecule, while most GAGs only exist as components of proteoglycans

What is responsible for the strength of the extracellular matrix?

collagens

the most abundant ECM component in animals is a family of

closely related collagens, which forms fibers with high tensile strength

collagen fibers are composed of many

fibrils

each fibril is made of

many collagen molecules, each composed of three alpha chains twisted into a helix

list the part of collagen assembly that occurs in the ER lumen

three precursor chains assemble into a procollagen

what part of collagen assembly occurs after secretion from cell

the collagen molecule assembles into a collagen fibril, and then that assembles into a collagen fiber

elastin structure

rich in glycine and proline, and the molecules are crosslinked by bonds between lysine residues

elastin flexibility

tension causes the overall network of elastin to stretch, and release of tension causes individual molecules to relax

How are cells anchored to the extracellular matrix?

adhesive glycoproteins, which have multiple domains to bind molecules in the ECM and receptors on membranes

2 most common types of adhesive glycoproteins

laminins and fibronectins

integrins

a protein family that forms the cell surface receptors that bind fibronectin

fibronectin structure

two large subunits linked near the C-terminals by two disulfide bonds, with each fibronectin subunit is folded into a series of rodlike domains

what do several fibronectin domains bind?

one or more ECM macromolecules, including several types of collagen. others recognize and bind cell surface receptors via the RDG (arg-gly-asp) sequence

RNA transcribed from the fibronectin gene is processed to produce what?

many different mRNAs and thus many different variants of the protein

2 main fibronectin variants

1. insoluble fibrils of fibronectin in the ECM

2. soluble fibronectin in blood and body fluids

plasma fibronectin

the soluble form of fibronectin in blood; promotes blood clotting

fibrin

the blood-clotting protein

basal lamina

a thin sheet of specialized extracellular material

basal lamina underlies …

epithelial cells, seperating them from connective tissue

laminins

the major adhesive glycoprotein in basal laminae

all forms of basal lamina contain

type IV collagen, proteoglycansm laminins, and another glycoprotein called nidogen

How can cells alter the properties of the basal lamina?

by secreting enzymes that catalyze changes in the lamina

Matrix metalloproteinases (MMPs)

degrade the ECM locally, allowing cells to pass through