Lecture 12 Flashcards
What is polarity?
This is a difference in the structure, composition or function between the two polls of a cell, such as the apical/basolateral in an epithelial cell or the axon/dendrite in a neuron. In epithelial cells, this also means location of a protein in a specific location
The polarity affects what?
where proteins go
Epithelial cells develop distinct ______ and ______ domains as an epithelia forms
apical
basolateral
Proteins must be sorted and directed to apical and basolateral membrane for the ____ transport function of epithelial cells to function
ion
What is the first step to forming an epithelia?
- Cell basement membrane and cell-cell interactions
Describe the first step to forming an epithelia in a lab
You start off with a cell that doesn’t have the typical cell shape. It is not connected to the basal lamina and the Na+/K+ ATPases and ENaC are not in any specific location. These single cells when they want to form an epithelia is they bind to the basement membrane through hemidesmosomes but the ion channels are still in random places. This binding to the BM helps to orient the channels. Once they are bound to the BM, they start to make connections with each other and we get intercellular connections forming. *the ion channels are at their appropriate membrane
Describe how an epithelia is formed in the body
In the body epithelia, there are founder cells (a type of stem cell) and if we get (eg.) a scratch, the cells divide and new cells form with the BM through hemidesmosomes. The new cells then form intercellular connections with neighbouring epithelial cells, helping to form an enlarged epithelium
Which intercellular connection forms first after the cells bind to the BM?
adherens junctions
Describe how adherens junctions are formed
- nectins form to make initial cell to cell contact
- when bring the cells in close contact which allows E-cadherin to form a homodimer with E-cadherin on another cell. This requires Ca2+
- the cytoplasmic tails of the E-cadherin bind catenins which links to the actin cytoskeleton
- Catenins also link nectin and cadherin complexes to pull all the proteins together to make the adherens junction
Which processes are required for establishing epithelial polarity?
A. Neighbouring cells interacting with each other through adherens junctions.
B. Connections between epithelial cells and their basement membrane.
C. Nectin proteins from neighbouring cells connecting with each other.
D. All of the above.
D. All of the above.
What is the third step to forming an epithelia, after forming a connection with the basement membrane, and then forming adherens junctions?
step 3: small GTP binding proteins are activated and step 4: activation of polarity complexes
Describe step 3: small GTP binding proteins are activated
and set 4: activation of the polarity complexes
Adherens junctions form and the adherens junctions recruit a number of other proteins including small GTP binding proteins (CDC42). CDC42 activates PKC to help form polarity complexes.
What are the four polarity complexes that are formed by PKC?
- PKC
- PAR
- CRB
- SCRIB
What do polarity complexes do? What is the advantage of this?
form tight junctions
these are right close to the apical surface so it helps to form apical and basolateral domains
What is the importance of the polarity complexes?
they maintain the apical and basolateral polarity
Describe step 5: forming tight junctions
these are formed from claudins, occludins and JAMs and they are connected to the cytoskeleton through ZO proteins and it prevents the movement of proteins between apical and basolateral domains
What are the three roles of the tight junction?
Describe these
- barrier: limits the passage of ions and molecules between cells in the paracellular pathway
- gate: can allow certain solutes to flow through the paracellular pathway
- fence: prevents the movement of proteins between apical and basolateral domains
Describe step 6: positioning of three protein polarity complexes
Atypical PKC is activated by GTP binding proteins such as CDC42. This aPKC is the only polarity protein that has any enzymatic activity (the other ones just serve as scaffolding, holding the cell in place to maintain a polarity). PKC phosphorylates CRBS and SCRB. This is the signal for these to move to their respective parts of the cell (apical and basolateral respectively). When this happens, we get mutual exclusion
Which protein polarity complexes sits apically?
PAR and CRB
Which protein polarity complexes sit basolaterally?
SCRIB
What is mutual exclusion?
When SCRB is in the basolateral bit, and CRB is in the apical bit, it prevents the basolateral proteins moving to the apical membrane and vice versa
Summarise the of formation of TJs and epithelial polarity (7 steps)
- Interactions between neighbouring cells, and between cells and basement membrane.
- Interaction between cells forms adherens junction.
- Small GTP proteins (cdc42) activated.
- Cdc42 activates aPKC - a polarity complex protein.
- Tight junctions start to form.
- Positioning of PAR and CRB polarity complexes to apical domain, and SCRIB complex to basolateral domain.
- Apical-basolateral polarity established.
Crumbs defines the basolateral domain and scribble defines the apical domain BECAUSE aPKC is the only polarity complex with enzymatic activity
D – First false, second true
When epithelial cells need to divide, we need to pull apart these junctions and polarity complexes and we have to be able to reform them if needed. How does this happen?
through TGF-β
What does TGF-β do?
TGF-β binds to its receptor in the membrane which will disrupt these adherens junctions, tight junctions and polarity complexes in one of two ways:
- it will directly inhibit the PAR complex (which helps to define apical domain)
- it activates SMADs
what are SMADs?
These are transcription factors which, when activated, will translocated into the nucleus, so there will be an increase in the expression of Snail, Zeb and Twist.
What do Snail, Zeb and Twist do?
These down regulate a huge number of proteins involved in tight junction and adherens junction formation which causes the down regulation of cadherins, claudins, occludins and ZO proteins which means we can’t have adherens or tight junctions
What is the relevance of breaking up adherens and tight junctions?
it is one of the first steps in epithelial mesenchymal transition (EMT)
What is epithelial mesenchymal transition (EMT)?
How a cell can go from an epithelial cell to a mesenchymal cell to allow it to divide and then come all the way back to an epithelial cell again
How does a cell go from an epithelial cell to a mesenchymal cell to allow it to divide and then come all the way back to an epithelial cell again?
________ inhibits/downregulates _________ complexes so the ______ junctions and _________ junctions are ________ _______. The cells start to lose their _________. You also lose your connection to the ________ _________ as we lose the expression of ___________. We also get rearrangement of the ____________ filaments towards the front end of the cell
TGF-β inhibits/downregulates polarity complexes so the tight junctions and adherens junctions are falling apart. The cells start to lose their connections. You also lose your connection to the basement membrane as we lose the expression of hemidesmosomes. We also get rearrangement of the actin filaments towards the front end of the cell
Why is EMT important when it comes to cancer?
If cells go through EMT but don’t go back again, they remain as mesenchymal cells. Their growth rate is really quick and they are able to migrate and invade
What four things do mutations in polarity complex proteins cause?
- Decreased formation or lack of tight junctions so barrier, gate and fence functions compromised.
- Changes in cell-cell adhesion and cell movement.
- Changes in location of apical and basolateral proteins.
- Cancer
When epithelia are fully polarised with AJs and TJs formed this structure inhibits ________ ________ pathways, and promotes differentiation (epithelia takes on specialised function):
ZO can sense there is a _______ cell density there and so the ZO stop __________ ________ that promote cell division close to the ________ junctions so they can’t _________ into the ___________ and help initiate cell _________. This means the epithelia can carry out normal functions such as _______ and ____________
cell division
ZO can sense there is a high cell density there and so the ZO stop transcription factors that promote cell division close to the tight junctions so they can’t translocate into the nucleus and help initiate cell division. This means the epithelia can carry out normal functions such as absorption and secretion
If we have some sort of damage to the epithelia (eg. we cut ourselves) What do the ZO proteins do?
They can sense that we have a low cell density and they release transcription factors that promote cell division which allows them to translocate into the nucleus and increase the expression of genes that will help initiate cell division.
What is important about cancer?
Genetic changes in cancer cells of epithelia may promote EMT: cell division increases and loss of epithelial polarity.
If cancer cells are mesenchymal through genetic changes in the epithelial cells, the tumour cells can leave the epithelial tumour, invade into the blood stream, migrate around the body, invade a new site and they can become epithelial again
