Week 5 - ECM and Cell junctions

Main structures of ECM

Proteoglycans and GAGs

Glycoproteins

Fibrous Proteins

Proteoglycans and GAGs

Function together as a protoglycan or isolated GAG molecule. Consists of:

Hyaluronan (only a gag)

Perlecan

Decorin

Aggrecan

Glycoproteins

Relatively short, branched oligosaccharides which are mostly protein and very little sugar. Consists of:

Laminin

Nidogens

Fibronectin

Fibrous Proteins

Consist of:

Elastin

Collegen

GAGs - Glycosaminoglycans

Unbranched polysaccharide chain with repeating disaccharide units

• Too stiff to fold into globular structures

• Strongly hydrophilic

• huge volume relative to the mass - space filler

• form hydrated gels even at the low concentrations

• have a compressive force

Types of GAGs

Types that form covalent bonds to protein:

1. chondroitin sulfate

2. dermatan sulfate

3. keratan sulfate

Non-covalent bonding

4. Hyaluronan

Proteoglycans

Protein + sugar

• at least 1 sugar but must be a GAG

  • which covalently bond to the protein

• hyaluronan DOES NOT covalently attach to proteins to form proteoglycans.

  • can be huge - aggrecan, >100 GAG chains

  • can be small - decorin, <10 GAG chains

Fibrous protein

Collagen and Elastin

Collagen

Alpha chain made of GLY-x-y

• x is usually proline

• y is usually hydroxyprolin

Further combines with amino acids to form triple helix

• Human genome contains 42 distinct genes coding for collagen alpha chain

• 20 types of triple helix collagen fibres.

Collagen assembly

In the ER lumen:

• Alpha chain combines to form triple helix

After secretion out of the cell

• Pro-collagen peptidase (cutting of loose ends)

• Assembly into collagen fibril

• Further assembly with other fibrils

Prominent collagen types

Type 1

• most common

• skin and bone

Type 9 & 12

• fibril-associated collagens

• Can connect type 1 and 2

• Mutation in this this collagen causes osteoarthritis

Elastin

Elastic fibre that can double its length

Laminin (glycoprotein)

Major for: basement membrane (lamina), brain & heart

• trimeric structure

• 16 combinations of alpha, beta and gamma chains

Basement Lamina

Laminin and type IV collagen are major components of BL

Laminin examples

Laminin 111

• myoblast proliferation and myotube formation

Laminin 332

• Epithelial cells and keratinocytes

Laminin 511

• in human placenta

Fibronectin

a dimer bound by disulphide bonds

• tension sensing

Has 2 types

• Cellular: insoluble in water, found within the cell

  • linearised by ionic strength or stretched by the cell

• Plasma: Water soluble, found in blood.

  • disk shaped and made in the liver

Fibronectin Domains

3 domains in the protein

Domain 1: 12 types

Domain 2: 2 types

Domain 3: 15-17 types

2 Main ways animal cells are bound together

Epithelial cells:

• Anchored cell to cell and cell to ECM

Mesenchymal cells (Cancer Type):

• cell-ECM dominant

Cell Junction types

1. tight junction

2. cell-cell anchoring junctions

3. channel-forming junctions

4. cell-matrix anchoring junctions

Tight junctions

seals gap between epithelial cells - prevent uncontrolled molecule movement

• contain strands of transmembrane adhesion proteins

Proteins:

• Claudin and Occludin

Mice with no claudin-1 gene

• fail to make tight junctions = lose water rapidly

Gap Junctions - channel forming

Allows passage of small water-soluble molecules from cell to cell

• connect cells electrically (ions) and metabolically (molecules)

• Diameter of gap dictates size of molecules that can pass

Gap junction structure

Connexins

• 2 types of proteins that make up the channel

Connexon

• Half og channel

• Heteromeric (mix of the two types of connexins)

• Homomeric (only one type of connexin)

Intercellular channel

• both sides of the channel from each cell combines

Cell-cell anchoring junction types

Adherens junction (actin filament)

Desmosomes (intermediate filaments)

Adherens junction

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

Adherens junction mechanism

Tension (extracellular) is required to expose the binding side of the actin fimalent and unfold the alpha-catenin molecule

• allows for binding of vinculin

• Junction is activated by tug of war tension between cadherins

Desmosomes

connects the intermediate filaments in one cell with that in the next cell

• utilise non-classic cadherin proteins

• Connected to hemidesmosomes via intermediate filament

Desmosome elements and their protein types

Cytoskeleton→ Intermediate filament

Intracellular adaptor protein

• plakoglobin, plakophilin, desmoplakin

Transmembrane Adhesion protein

• desmoglein, desmocollin (non-classical cadherin proteins)

Extracellular Ligand

• desmoglein, desmocollin

Cell-matrix anchoring junctions (types)

Actin linked cell-matrix junction

Hemidesmosome

Actin-linked cell matrix junction

Anchors actin filaments in cell to extracellular matrix

Adhesion proteins:

• Integrin heterodimer (with alpha (24) and beta (8) subunits)

Adaptor proteins:

• Talin & Vinculin

Hemidesmosome

Anchors intermediate filaments in a cell to extracellular matrix

adhesion proteins

• Integrin a6B4 + Collagen XVII

adaptor proteins

• BP230 + Plectin

Transmembrane adhesion proteins

Go through the cell membrane to adhere to something outside

• connects to intracellular adaptor proteins → which link to the filaments

(found in anchoring junctions, cell-cell or cell-ecm)

Intracellular adaptor proteins

Connect the cytoskeleton filaments to the transmembrane adhesion membrane.

cell-cell vs cell-ecm transmembrane adhesion proteins

Cell-cell = connect same kind of adhesion proteins

Cell-ECM = asymmetric

Classical Cadherins - adhesion molecules

A transmembrane membrane adhesion protein

• The extracellular part has 5 domains, separated by flexible hinge regions

• Each hinge has calcium channels that create a velcro-like mechanism holding cells together

Present homophilic bonding which allows for sorting by types and level of expression

What do Cadherins use to connect to cytoskeleton?

Catenins and Vinculin used to connect to actin filament

• happens via force feeling upon homophilic binding

Activation of integrin leads to...

intracellular signalling

(integrin beta subunit will be activated upon binding to ECM = recruits intracellular adaptor proteins)