Cell Junctions and the Extracellular Matrix

what is the Epithelium and Connective tissue?

• how are mechanical stresses transmitted?

• what does the extracellular matrix do?

• 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

• mechanical stresses are transmitted from cell to cell by cytoskeletal filaments anchored to cell-matrix and cell-cell adhesion sites

• extracellular matrix directly bears mechanical stresses of tension and compression

Cell-Cell Junctions

• what do multicellular organisms do and form?

• what are cell-cell junctions?

• Multicellular organisms have means of joining cells in long-term associations to form tissues and organs

• the specialized structures where two cells come together are called cell-cell junctions

Animal VS Plant Cell-Cell Junctions

Animal Cells

1) Adhesive Junctions

• adherens junctions

• desmosomes

2) Tight Junctions

• Gap Junctions

Plant Cells

1. Plasmodesmata

Animal Cell Cell-Cell Junction Image

Cell-Cell Junction Chart

Adherens Junctions and Desmosomes

• what do they rely on?

• what are their traits?

• adhesion proteins are focal points for what?

• what is cell adhesion coordinated with?

Rely on:

• intracellular attachment proteins to link the junction to the cytoskeleton

• Cadherins on the outer surface to bind cells to each other

Traits:

• hold similar cells together

• basis for tissue formation

• adhesion proteins are focal points for signaling complexes and cytoskeletal structures

• many adhesion proteins are continuously recycled through endocytic and exocytic pathways

• cell adhesion is coordinated with cell signaling, cell movement, cell proliferation, and cell survival

Adherens Junctions

• what are they? what do they interact with?

• what are cadherins characterized by? (3)

• Adherens junctions: are cadherin-mediated junctions that interact with actin

Cadherins are characterized by:

• “repeats” in their extracellular domain that are structurally similar

• a transmembrane domain

• widely varying cytosolic ends

E-Cadherin

• what are the 3 domains in E-Cadherin

• characteristics

• Extracellular domain: 5 repeated segments

• Transmembrane domain

• cytoplasmic domain

• dimerizes

• extracellular repeats bind to each other (Ca2+ dependent)

Catenins

• name all 3 catenins, what do they do?

B-catenin:

• Links E-cadherin to a-catenin

A-catenin:

• Binds F-actin and B-catenin

• Links to a microfilament

p120 Catenin:

• just binds to E-cadherin

• regulates cadherin stability and turnover

Tissue Specificity of Cadherins

• where are E-cadherins found?

• the amount and type of cadherins on cell surfaces help segregrate cells into specific tissues

• Different types of cadherins are expressed in specific tissues

• E-cadherins are found on epithelial cells, N-cadherins in neurons (anbd cardiac muscle)

• The amount and type of cadherins on cell surfaces help segregate cells into specific tissues

Cadherins and the development of the vertebrate nervous system in a chick embryo

Desmosomes

• what are they?

• what do they provide?

• where are they abundant in?

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

• they provide a tissue with structural integrity

• they are especially abundant in cells that are under mechanical stress, like the skin, heart muscle, and the uterus

Desmosomes

• name all the different types of desmosomes and their subtype

1. Transmembrane Proteins

• Desmocollin (cadherin)

• Desmoglein

2. Adaptor Protein

• Plakoglobin (B-catenin)

• Desmoplakin (a-catenin)

• Plakophilin (p120 catenin)

3. Cytoskeleton

• Intermediate Filaments

Desmosomes

Consider the Differences between adherens junction and desmosome

• what are they?

• what do they link?

Adherens Junction:

• Continuous band of cell-cell connection

• links the MF network in one cell to the MF network in a neighboring cell

Desmosome:

• Localized disc of cell-cell connection

• links the IF network in one cell to the IF network in a neighboring cell

Tight Junctions, Adherens Junction, desmosome, gap junction, actin-linked cell-matrix junction, hemidesmosome

1. Tight junctions: seals gap between epithelial cells

2. Adherens Junction: connects actin filament bundle in one cell with that in the next cell

3. Desmosome: connects intermediate filaments in one cell to those in the next cell

4. Gap Junctions: allows the passage of small water-soluble molecules from ell to cell

5. actin-linked cell-matrix junction: anchors actin filaments in cell to extracellular matrix

Junction Images

C. Adherens Junctions

Tight Junctions

• what does it prevent?

• what do epithelial cells need?

• Prevent movement of molecules across cell layers

• epithelial cells need specialized structures to seal them tightly together to form a barrier between the internal cells and the outside world

• tight junctions serve this function, leaving no space between the plasma membranes of adjacent cells

• they form a continuous belt around the apical ends of lateral surfaces of each cell; molecules cross the cell layer by passing through the cells

• no connection to the cytoskeleton

Tight Junctions

• what are the functions of claudins

Claudins:

• 4 transmembrane domains with one large extracellular loop

• claudins from adjacent cells interlock to form a tight seal

• Ionic R group on the loops create ion-selective pores through the tight junction

• Paracellular Transport

Tight Junctions create what?

• what type of molecules can be seen by electron microscopy? where are they introduced to in the cell? what does it do?

• Tight Junctions create a permeability barrier

• Transmission electron microscopy

• “Tracer” molecules that can be seen by electron microscopy are introduced in solution on one side of an epithelial cell layer

• the tracer moves between the cells until it encounters the tight junctions and is stopped

Tight Junctions Permeability Barrier

Gap Junctions

• what is it?

• what is the gap lined by?

• what passes through one cell to another?

Direct electrical and chemical communication between the cytoplasm of adjacent cells

• A gap junction is a region where the plasma membranes of cells are aligned and brought into contact, with a very small gap between

• The gap is spanned by small pipelines or passages between the cells

• small molecules and ions can pass directly from one cell to another

What can and cannot pass through Gap Junctions?

CAN

• inorganic ions

• monomeric sugars

• amino acids

• nucleotides

• vitamins

• small signaling molecules

CANNOT

• proteins

• nucleic acids

• polysaccharides

Gap Junctions

• what transmembrane proteins are involved?

• how many connexin subunits join to for a connexon?

• what is the connexon?

• transmembrane proteins involved: connexin

• 6 connexin subunits join to form a connexon

• the connexon is a hollow protein cylinder with a 1.4 nm channel

Gap Junctions

• when are gap junctions formed?

• A Gap Junction is formed when a connexon on one cell lines up with a connexon on an adjacent cell

A. only adhesive junctions associate with microfilaments or intermediate filaments

3 Types of ECM

1. Bone: Bone consists mainly of a rigid extracellular matrix that contains a small number of interspersed cells

2. Cartilage: is a tissue constructed mostly of matrix materials that is more flexible than bones

3. Connective Tissue: surrounding glands and blood vessels is relatively gelatinous and contains interspersed fibroblast cells

What is the ECM made of?

3 Classes of Molecules with different functions

1. A gel-like matrix in which the other molecules are embedded: Made of Proteoglycans (combinations of proteins and sugar polymers)

2. Structural proteins embedded in the matrix

• Collagens: provide strength

• Elastins

3. Adhesive glycoproteins that attach cells to the matrix. They must bind to molecules in the matrix (above) as well as transmembrane proteins on cells

• Fibronectin: for most cell-ECM connections

• Laminins: for epithelial cell connection to the Basal Lamina (other specialized cells with basal lamina- muscle, fat, Schwann cells)

Proteoglycans and the ECM

• what are proteoglycans?

• what are glycosaminoglycans?

All Proteoglycans are combinations of core proteins and long polysaccharide chains

Glycosaminoglycans (GAGs)

• The polysaccharide chains are repeating disaccharides where: 1 monomer is an amino sugar (N-acetylglucosamine or N-acetylgalactosamine

• the other monomer is a sugar or sugar acid (galactose or glucuronate)

• the amino sugar generally also has sulfate groups attached

GAGs: Glycosaminoglycans

• GAG characteristics

• GAG and Proteoglycan Diversity (tissues vs cartilage)

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

GAG and Proteoglycan Diversity:

• In many tissues proteoglycans are present as individual molecules

• In cartilage, numerous proteoglycans become attached to long molecules of hyaluronate

Individual Proteoglycans and a proteoglycan aggregate along what

Individual Proteoglycans and a proteoglycan aggregate along a single hyaluronate molecule

Free Hyaluronate

• function, properties, where is it most abundant?

• how do GAGs in the ECM exists, what is the exception?

• Free hyaluronate LUBRICATES joints and FACILITATES cell migration

• Most GAGs in the ECM exist only as components of proteoglycans

• Hyaluronate is an exception that occurs both as a backbone of cartilage proteoglycans and as a free molecule

• It has lubricating properties and is most abundant where friction needs to be reduced

Collagens are responsible for what?

• what is the most abundant ECM component in animals?

• what are collagen fibers visible in?

• what is each fibril made out of?

• Collagens are responsible for the strength of the extracellular matrix

• The most abundant ECM component in animals is a family of closely related collagens, which form fibers with high tensile strength

• Collage fibers are visible in the ECM under scanning electron microscopy (EM)

• The fibers are enormously strong and are composed of numerous fibrils

• Each fibril is made of many collagen molecules, each composed of a 3 (a) chains twisted into a helix

Collagen Assembly Image

Elastins Impart Elasticity and Flexibility to the Extracellular Matrix

• what provides elasticity to the ECM

• what are they rich in?

• Elasticity is provided to the ECM by stretchable elastic fibers principally composed of elastins

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

Elastin Flexibility

• what does tension cause?

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

Adhesive Glycoproteins Anchor Cells to the Extracellular Matrix

• what are the most common types of domains?

• These glycoproteins have multiple domains to bind molecules in the ECM and receptors on membranes

• Laminins and fibronectins are the most common types

Fibronectin

• structure?

• domains?

• A fibronectin molecule has 2 large subunits linked near the C-terminals by two disulfide bonds

• each fibronectin subunit is folded into a series of rodlike domains

• several domains bind one or more ECM macromolecules, including several types of collagen

• other domains recognize and bind cells surface receptors via the RGD (arg-gly-asp) sequence

Fibronectin

• what are they?

• what happens to the RNA transcribed from the fibronectin gene?

• fibronectins are a family of closely related glycoproteins in the ECM

• RNA transcribed from the fibronectin gene is processed to produce many different mRNAs and thus many different variants of the protein

• Two main fibronectin variants

1. Insoluble Fibrils of fiibronectin in the ECM

2. Soluble fibronectin in blood and body fluids

1. Insoluble Fibrils of fibronectin in the ECM

• what does fibronectin act as?

• what requires fibronectin?

• what over or under produces fibronectin? what does this effect?

• fibronectin acts as a bridging molecule between cells and the ECM

• cell migration requires fibronectin

• many kinds of cancer cells over- or underproduce fibronectin compared to their tissue type

• in cancer cells, this affects uncontrolled migration

2. Soluble fibronectin in blood and body fluids

• what is the soluble form of fibronectin in blood?

• what does it promote?

• what can it attach to?

• The soluble form of fibronectin in blood is called plasma fibronectin.

• It promotes blood clotting because it has domains for binding fibrin, the blood-clotting protein.

• It can attach blood platelets to fibrin as the clot forms

Laminins Bind Cells to the Basal Lamina

• what is the basal lamina

• function? and location?

• what do basal laminae surround?

• what are laminins?

•The basal lamina is a thin sheet of specialized extracellular material.

•It underlies epithelial cells, separating them from connective tissues.

•Basal laminae also surround muscle cells, fat cells, and Schwann cells.

•The major adhesive glycoprotein in basal laminae are laminins.

Laminins Characteristics

• what do all forms of basal lamina contain?

• what do cells do here?

• what do MMPs do? why is this important?

•All forms of basal lamina contain type Ⅳ collagen, proteoglycans, laminins, and another glycoprotein called nidogen.

•Cells can alter the properties of the basal lamina by secreting enzymes that catalyze changes in the lamina

•Matrix metalloproteinases (M M Ps) degrade the E C M locally, allowing cells to pass through.

•This is important for leukocytes to invade injured tissues and may be a factor in cancer cell invasiveness

Laminin Image

• what is the major structural component of the basal lamina?

• where is the cell membrane located at?

• Network of collagen is the major structural component of the basal lamina

• the cell membrane is attached to the basal lamina through laminin

What are Integrins? Function?

• Integrins are cell surface receptors that bind fibronectin and laminin

• integrins integrate the cytoskelton with the ECM