Junctions and Adhesions

Question 1

Types of Cell-Cell
Interactions

Answer 1

Cell-Type Recognition
1) Homotypic
2) Heterotypic
Molecular Interactions
1) Homophilic
2) Heterophilic
Timing
1) Permanent
2) Transient

Question 2

Homotypic Adhesion – An
experimental example

Answer 2

 Developing embryo
 Dissociated into single cells
 Allowed to re-associate
 Cells of a particular type e.g. liver or kidney form aggregates
consisted only of liver and kidney cells

Question 3

Sponge

Answer 3

 If cells of a sponge are
separated, they will eventually
reassemble into a whole
sponge via homotypic
interactions.
 Species specific
 2 different sponges would
reassemble into 2 separate
sponges.
 Membrane glycoproteins
recognise each other

Question 4

Examples of heterotypic
adhesion

Answer 4

Fertilisation Embryo implantation
Leukocyte extravasation

Question 5

TJ Proteins

Answer 5

 Three types of transmembrane
proteins found in the tight
junction:
 Claudins (CLDN)
 Occludins (OCLN)
 Junctional adhesion
molecule (JAM)
 Bind to actin filaments
 Maintain cell polarity –
contribute to cell shape, position
and location of proteins in
membrane

Question 6

Cell Polarity

Answer 6

The spatial difference in
shape, structure and
function
* Different expression
characteristics
* Examples
- Epithelial cell (Diagram)
- Migratory cells

Question 7

Cell polarity dictates distribution of
proteins

Answer 7

• Confocal microscopy – imaging through cells a thin
  slice at a time
• Tight junctions regulate this

Question 8

Tight Junctions form selective barriers
between cells

Answer 8

 Paracellular transport – intercellular
movement of substance between cells
Dynamically regulated to control paracellular
solute and ion transport
- Example – reabsorption of Cl- ions in urine
formation
- Example – Glucose absorption in the gut

Question 9

Fast

Answer 9

Fast:
Through ion
channels
embedded
within
junction

Question 10

Slow

Answer 10

Slow:
Solutes that can’t
pass, requires
strands to
gradually break
sequentially

Question 11

Gates and Fences

Answer 11

A gate for ions and other
solutes.
 A physical barrier against
water, allergens, pollutants and
bacteria – role in innate
immunity.
 A fence to prevent apical and
basolateral transmembrane proteins
from moving into the wrong domain.

Question 12

Adherens Junction = adherence

Answer 12

 Hold epithelial and endothelial cells together.
 Links to the actin cytoskeleton.
 E-cadherin joins to a range of adaptor proteins

Question 13

Different types of Adherens Junction

Answer 13

Serve to hold cells together:
* Epithelium
* Synapses
* Cardiac muscle cells
* Between layers of myelin
sheath

Question 14

Cell adhesion molecules found
within the Adherens Junction

Answer 14

Cell adhesion molecules (CAMS) comprise of two classes:
 Those requiring Ca2+ for adhesion – most important
 Cadherins
 Ca2+ - independent – N-CAMS are neural cell adhesion
molecules

Question 15

Cadherins

Answer 15

 Family of Ca2+ dependent cell-cell adhesion molecules
 Cadherins are divided into several classes:
 E-cadherins (classical cadherins)
 P-cadherins
 N-cadherins
 R-cadherins
 VE-cadherins
 All integral membrane proteins (720-750 amino-acids)

Question 16

Experiment

Answer 16

• Epithelial cells with or
  without Calcium
• Stain for E-cadherin
• Calcium required for
  correct folding and
  localization of protein

Question 17

Calcium-dependent Cadherins Mediate Adhesion between Cells

Answer 17

• Ca2+ ions are
  situated between
  cadherin molecules
• Provide rigidity to
  the extracellular
  portion of the
  protein

Question 18

β-catenin - A linker

Answer 18

Links Cadherins to actin
* Integral part of cell signalling pathways
* Can also act as a transcription factor
* Mutated in many cancers
Adherens junctions: Catenin interacts with Cadherin

Question 19

Cell adhesion molecules (CAMS) comprise of two classes:

Answer 19

 Those requiring Ca2+ for adhesion – most important
 Cadherins
 Ca2+
- independent – N-CAMS are neural cell adhesion
molecules

Question 20

Calcium-independent NCAMs Mediate Adhesion between Neural
Cells

Answer 20

• Do not require
  calcium binding to
  promote adhesion
• Expressed only in
  neural cells –
  homophilic adhesion
• Different splice
  variants

Question 21

Continued

Answer 21

 Permits strong adhesion in
adult neurons.
 Polysialic acid (PSA) binding
is a post-translational
modification.
 Changes shape and function
of NCAMs – less adhesive
 Results in a low-affinity
interactions – important for
development

Question 22

A closer look at Desmosomes

Answer 22

 Desmoglein and
Desmocolin
 Attach to adaptor
proteins
 Weld cells together!
 Resist stress so are
abundant in skin and
cardiac muscle

Question 23

What happens when desmosomes
go wrong?

Answer 23

Patients with damaged or
missing desmosomes
have blistering diseases.
Epithelia become badly
disorganised and sheets
of cells are easily
dislodged.
Tissue engineering
approaches offer
promising treatment
options.

Question 24

Gap Junctions are composed of
Connexin

Answer 24

Allow direct transfer
of molecules between
adjacent cells
 Connexin
 Six Connexins form
hydrophilic channel =
Connexon

Question 25

Connexons make Gap junctions

Answer 25

Six Connexins form hydrophilic channel = Connexon  More than 20 Connexin family members  Adjacent cells form intercellular channels

Question 26

Gap Junctions Allow Direct Transfer of Molecules between Adjacent Cells

Answer 26

Molecules up to 1200 Da can pass through. Proteins and nucleic acids do not pass through Electrical signals can pass through

Question 27

Continued

Answer 27

Cluster into ‘plaques’ Can contain 1000s of channels At least 20 different connexin proteins in humans – Make a multitude of connexons Can form homotypic or heterotypic junctions with neighboring cells important role in cell-cell communication – electrical signals, second messenger molecules e.g. cAMP

Question 28

Cell movement

Answer 28

Cells that participate in defense against foreign organisms (white blood cells) must move from blood to tissues  Free floating unattached cells move into tissues in a process called EXTRAVASATION

Question 29

How do circulating Immune Cells attach?

Answer 29

Selectins – the biological equivalent of hand grabbing a paddle  Schematic of white blood cell in vessel  Selectins expressed by vascular cells (paddle)  Carrier protein/Sialyl Lewis (your arm) * A series of broken adhesions bring the leukocyte to a ‘rolling stop’.

Question 30

Extravasation

Answer 30

Successive formation and breakage of cell-cell contacts between white blood cells and endothelial cells lining blood vessels – discontinuous cell-cell adhesion  Key protein in contact process – P-selectins (blood facing surface)  Passing white blood cells (WBCs) adhere weakly to the endothelium but the force of the blood flow means that WBCs are “slowed” as opposed to being “trapped”  They roll along the surface of the endothelium