Cell-Cell Junction

Question 1

Cell Junctions

Answer 1

Occluding junctions - tight junctions seal gap between epithelial cells
Cell-cell anchoring junctions type 1 - adherens junction connects actin filament bundle in one cell with that in the next cell
Cell cell anchoring junctions type 2 - dsmosome connects inter-mediate filaments in one cell to those in the next
Channel-forming junctions - gap junctions allow the passage of small water-soluble molecules from cell to cell
Cell-matrix anchoring junctions type 1 - actin-linked cell-matrix adhesion anchors actin filaments in cell to extracellular matrix
Cell-matrix anchoring junctions type 2 - hemidesmosome anchors intermediate filaments in a cell to extracellular matrix

Question 2

Adherens Junctions

Answer 2

Initially referred to as belt desmosomes
Now called cadherins e.g. E(pithelial)-cadherin
Ca2+ dependent
homophilic interaction
Makes strong links between cells
Link to actin cytoskeleton and causes tension between cells
Go back over differences between adhesion junctions and focal adhesions

Question 3

Adhesion belt in morphogenesis

Answer 3

Cadherin -> Catenin -> Actin -> Myosin
Invagination epithelial sheet caused by organised tightening along adhesion belts in selected regions of cell sheet
Then epithelial tube pinches off from overlying sheet of cells
Basal lamina will stay same length even if epithelial cells change (check this)

Question 4

Desmosomes and hemidesmosomes

Answer 4

Focal adhesions and adherens junctions link to actin cytoskeleton
Hemidesmosomes purpose:
- Attach cells to basal lamina
- Integrins and intermediate filaments, type of cytoskeleton
Desmosomes purpose:
- Cell-cell junctions
- Cadherins and intermediate filaments
- Plentiful in heart muscle and epidermis
Will find these structures in places that are prone to damage

look at the table

Question 5

Desmosomes

Answer 5

Doesn’t provide the same tension as actin but runs across cell to give it some tensile strength
Keratin filaments visible on light microscope as they are so big

Question 6

Pemphigus

Answer 6

Defects in desmosome can lead to pemphigus
Mutation in desmoglein, more in autoimmune system as they perceive it as threat so destroy it
By destroying it it makes skin fall apart which is what causes very nasty blisters
Hold together keratinocytes in epidermis
Autoimmune skin blistering

Question 7

Tight junctions

Answer 7

Introduced electron dye at tight junctions on apical side which shows concept that (something)
Occluding junctions
Zonula occludens

Question 8

Role of tight junctions

Answer 8

Locations
- Epithelia e.g. Gut
Roles
- To prevent fluid, ion and membrane flow
- Excluding movement to variable extent
- Transcellular transport, has energy burden, clearly counterproductive to use energy to get energy from the gut
- Paracellular transport, allows leaky tight junction to allow some things through, only works if things moving down concentration gradient so only works in certain situations, but saves lot of energy

Question 9

Tight junction structure

Answer 9

Look at diagram

Question 10

Tight junction proteins

Answer 10

Zonula occludens proteins bind to claudin, occludin and actin
Occludin and claudin proteins are most important proteins for binding in tight junctions

Question 11

Defining membrane compartments

Answer 11

What’s going on in microvilli very different to what’s occuring at other points in cell membrane so has two different cell faces
Specialisation of membrane regions
Lipids and proteins segregated
Apical outer membrane
- Glycolipid
- Cholesterol
Basolateral
- Phosphatidylcholine

Question 12

Gap junctions

Answer 12

Tend to be puncta, form together to make then very porous
100-500 nm long
Close apposition of membranes
- Gap = 2-4 nm
Wide distribution
- Connective tissue, epithelia, neurons, heart muscle
Allows regulated and direct cell-cell communication e.g. like in heart
Form 1.5 nm diameter pores
What can pass through - water, inorganic ions, sugars, amino acids, ATP, cAMP, IP3
Anything from size 100-1000 doltons can pass through but anything larger can’t

Question 13

Gap junction structure

Answer 13

Gap junction made of connexins (these are pores)
6 connexins makes 1 connexon
2 connexons make intercellular channel
Different formation of connexins make different forms connexons make different varieties intercellular channels
Are 24 different types connexins

Question 14

Gap junctions regulation

Answer 14

Membrane potential, pH, Ca2+ and cell signals
Limit damage caused by calcium influx

Question 15

Key points

Answer 15

Integrity
Cytoskeleton
Compartments
Communication
Protein families