Lecture 4 - Cell adhesion and communication

Question 1

What is epithelial tissue?

Answer 1

covers the surfaces of internal and external organs
contain a layer, or layers of cells on top of a basement membrane. Cells adhere to each other laterally within these layers and also to the basement membrane.

Question 2

What is cell-cell adhesion?

Answer 2

Connections between
cells can be established via:
-Tight junctions
-Adherens junctions
-Desmosomes
-Gap Junctions
-“Non junctional adhesions”
Lateral connections between cells are facilitated through cell-cell junctions; specialized modifications of the cell surface in neighbouring cells.  Neighbouring cells can also interact through non-junctional mechanisms using proteins such as cadherins, selectins and integrins.

Question 3

What is cell-matrix adhesion?

Answer 3

Connections between
cells & their underlying
matrix can be established via:

Hemidesmosomes
Focal adhesions
“Non junctional adhesions” - integrins

Question 4

Name the 3 different classes of cell adhesion junctions

Answer 4

There are 3 distinct classes of cell adhesion junctions.

Occluding (tight) Junctions form a barrier between epithelial layers.

Attachment junctions link a cell’s cytoskeleton to that of its neighbouring cells and also to the extracellular matrix.

Communication junctions directly connect the cytoplasm of adjacent cells.

Question 5

What are tight junctions?

Answer 5

(occluding junctions) seal adjacent epithelial cells in a narrow band just beneath their apical surface. They are made up from interconnected strands of integral membrane proteins that encircle the cell.

Question 6

Describe the components of tight junctions

Answer 6

Sealing strands of tight junctions contain 3 different types of transmembrane proteins: claudins, occludins and junctional adhesion molecules (JAMs) that wrap around the cell. Proteins in the sealing strands attach stably to structural proteins to interconnect the sealing strands and also transiently to signalling proteins.

Claudins are the core proteins in the tight junction fibrils.

Tight junctions allow the passage of small ions, solutes and electric currents between epithelial layers - different tight junctions allow different solutes through varying in size, ~4-40 angstroms, and charge depending on the tissue in which they are found. These selectivities come from different claudins.

Question 7

Describe how tight junctions regulate paracellular transport

Answer 7

transfer of substances across an epithelium by passing through the intercellular space between the cells.
Act as molecular sieves (with different tissues having different selectivities) rather than completely impermeable ‘occluding junctions’ -
Which substances can cross an epithelial layer depends on the claudins present in the tight junctions.

Question 8

What is transcellular transport?

Answer 8

transfer of substances across an epithelium by passing through cells, crossing both apical and basolateral membranes
Tight junctions play a critical role in maintaining the separate identity of the apical and basolateral membranes and membrane proteins can’t cross the barrier that they create – in this way tight junctions act as a fence to maintain the characteristic molecular composition of the two membrane domains.

Question 9

How are tight junctions formed and what is their role?

Answer 9

Formed by interconnected sealing strands of claudin, occludin and junctional adhesion molecules that encircle the cell.

Act as molecular filters thus regulating paracellular transport across epithelial tissue.

Maintain identity of apical and basolateral plasma membranes by preventing diffusion of proteins and lipids between the two.

Question 10

What are adherens junctions?

Answer 10

Adherens junctions serve as bridges to connect actin cytoskeletons of adjacent cells.
Contain cadherins
Link to actin filaments (through catenins – role in signalling)

Question 11

What are desmosomes?

Answer 11

link adjacent cells together through cadherins (in this case desmocollins and desmogleins) that bridge the intracellular space on one side of the membrane and serve as docking sites for cytosolic proteins such as desmoplakin that binds intermediate filaments on the other. Attachment to intermediate filaments through these adaptors can be regulated by cell signalling events initiated by activated growth factor receptors – allowing coordination of cell growth and adhesion.

Question 12

What are attachment junctions?

Answer 12

connect cells to the underlying matrix, e.g. hemidesmosomes which linking the extracellular matrix to the intermediate filament network via transmembrane proteins – in this case integrins - providing structural stability to epithelial sheets,

Question 13

What are hemidesmosomes?

Answer 13

Located on basal surface of epithelial cells

Anchor cells to underlying basement membrane through cytoplasmic “plaques” that connect to intermediate filaments Plaques composed of integrins and the adapter protein plectin

Extracellular region interacts with components of the basement membrane.

Question 14

What are integrins?

Answer 14

Heterodimers composed of alpha & beta subunits

18 different a subunits and 8 b subunits

24 different integrin heterodimers known

Integrins bind to extracellular matrix (ECM) components
Integrins are also components of focal adhesions, linking the actin cytoskeleton to the extracellular matrix.
This specificity of which extracellular matrix component(s) a particular integrin binds is conferred by the alpha subunits.

Most integrin heterodimers can bind to more than one component of the ECM, and each ECM protein can bind to more than one integrin.

Question 15

What are gap junctions?

Answer 15

regions where adjacent plasma membranes are closer together than in the surrounding areas with gaps (2-4nm wide) bridged by channels projecting out of the plasma membrane.
contain anywhere between tens to many thousand channels forming small or large gap junctions.

Gap junction channels consist of two halves (hemichannels or connexons) docked together in the intracellular gap.

Each connexon is composed of six protein subunits (connexins).

Question 16

Describe connexin structure

Answer 16

Connexins are integral tetrapsan membrane that form hexameric complexes generating a pore of between 2 and 4 nm diameter. Two of these connexons present on opposing plasma membranes of adjacent cells dock together to form a gap junction through which ions and small molecules can pass.
Connexin subunits: tetraspan - 4 trans membrane alpha helices
6 Connexin subunits
Channel cluster
Intercellular gap 2-4nm
Connexon hemichannel in each membrane
Allows transport of small molecules between cells: electrically couples neighbouring cells

Question 17

How are gap junctions involved in transport?

Answer 17

Can transport inorganic ions, sugars, amino acids, nucleotides, vitamins, cAMP and IP3
But not macromolecules (proteins, nucleic acids, and polysaccharides)
Gap junctions can have different permeability properties depending on connexins present.
Vary in composition - there are at least 20 different connexin proteins and these can be combined in different ways to generate gap junctions with different selectivities

Question 18

What is the function of a gap junction?

Answer 18

Utilised where v. rapid communication is required (e.g. reflex reactions in the brain mediated by neurons linking by gap junctions allowing action potential to be spread rapidly, avoiding the delay at chemical synapses.

Synchronisation of cardiomyocytes signal to contract communicated through gap junction (mutations in connexins -> arrythmia).
Gap junctions function where extremely rapid communication is required – e.g. rapid reflex reactions in the brain are mediated by neurones linked by gap junctions allowing nearly instantaneous exchange of ions, as is the synchronised contraction of heart muscle fibers.

Question 19

How are gap junctions regulated?

Answer 19

Alternate between open and closed states.
Large increases in intracellular calcium ion concentrations and/or pH induce gap junctions closure (through phosphorylation of connexins).

Example: cell damage cause calcium ion elevation within the cell and release of damaging metabolites.

Gap junction closure prevents further damage to neighbouring cells.
Gap junctions are NOT permanently open – they can be opened and closed as required–controlled by phosphorylation of connexins in response to changes in intracellular pH - this can be protective e.g. prevent spread of apoptotic signals.