Lecture 8 - Cell Junctions Flashcards
Features of epithelial cells:
They are polarised/asymmetrical
All epithelial cells though each other as they have junctions, they also anchor the ECM
By doing this they create a barrier, and allow the strength to withstand outside forces
What are the different types of epithelial tissues?
Columnar, Cuboidal, Squamous, Stratified
Different epithelial tissues are organised in different ways depending on the function and location of the tissue
What are the types of cell junction?
Adherens, Desmosomes, Tight Junction, Gap Junctions
What do Adherens use? Is there more than one type?
Cadherins Can either be classical or non-classical Classical = E, N, P E = Epithelial N = Nervous, Lens, Muscle P = Placenta, Epidermis
How does cadherin interact with actin?
Cadherin passes through the membrane and binds to actin via catenins and other adapter molecules, cadherin does not bind actin directly, actin is then bound to myosin, so when cadherin on one cell binds to cadherin on another cell it creates a structure that can now resist tension, using the internal structure of actin binding to myosin
What are the two main features of cadherin binding?
It is homophilic, cadherin only binds to itself on other cells (E will only bind E, and N will only bind N)
Catherine binding is also Ca2+ dependent, cadherin is made of 5 copies of extracellular domain separated by flexible hinge regions, Ca2+ binding prevents flexing so promoting homophilic binding to another cadherin, this isn’t permanent
Actin and cadherin macro assembly:
Macro-assembly allows cell junctions to withstand mechanical force
Initially when two cells come together the cadherins interact, then over time in the presence of calcium, the sticking together becomes stronger, then actin moves towards the membrane, which is when the actin is said to be polarised
The actin is now lined up at right angles to the cadherin molecules
What are desmosome junctions made of?
Similar to adheren, but contain specialised cadherins (tend to be non-classical) that connect with intermediate filaments
What does desmosome macro assembly do?
Also allows cells junctions to withstand mechanical force
Where are tight junctions found?
Cells in the lumen of the gut
What are tight junctions made of?
Claudin and occludin
What are the function of tight junctions?
Prevent leakage of transported molecules from cells, seals the gap between epithelial cells
Tight junctions with example of glucose
Can allow selective transport across the epithelia
Place two different glucose transporters on either side of the tight junction meaning glucose won’t leave the same side it came in
What are gap junctions made of? Whats their size?
Connexions and Innexins
They are very small, won’t allow macromolecule through
Structure of gap junctions:
They are water filled channels with allow the passage of small water-soluble molecules from cell to cell, which can be opened and closed in response to signals
