Tuesday, March 3, 2009
- This week we’re going to be discussing cell junctions & adhesion b/w
- They connect cells together or they connect cells to the matrix that
Mechanically attach cells to cells or the extracellular matrix
Seal the contacts b/w the cells
Allow chemical or electrical signals to pass from cell to cell (through
- Today we’ll focus on anchoring junctions.
- Anchoring junctions hold cells together mechanically – it attaches them
to one another or to the matrix.
- On this diagram, he’s drawn to scale a typical size of a mammalian cell,
~50 microns wide & the tiny little dot/speck is a 50 nm long cell adhesion
molecule. How can these individual molecules deal with these relatively
- We heard about this a little bit when we talked about the actin
cytoskeleton & how actin molecules, which are also very small, similar to
the size in the diagram, we saw that they could impact cell structure by
assembling huge networks inside the cell & pushing against the plasma
membrane, pushing the plasma membrane outwards to change cell shape.
Here that same principle applies.
- These cell adhesion molecules, these tiny cell adhesion molecules,
assemble larger complexes & these complexes have 2 features that
overcome this challenge of size (as listed in the slide).
- Diagram: Here is one single cell & here is a cell-cell adhesion complex
– these would be a large number of cell adhesion molecules clustered
together in that complex & they connect to the cytoskeletal networks.
- So you can follow protein-protein interactions b/w the cell adhesion
complex through the cytoskeleton to another cell adhesion complex
through the cytoskeleton of that cell, through another cell adhesion
complex & so on so you have a huge assembly of proteins within these
cells supporting the tissue support.
- How can clustering cell adhesion molecules increase the strength of
their binding? The binding strength b/w 2 molecules via a single binding site
The total binding strength b/w 2 molecules or complexes involving
multiple binding sites
- So you might think that this total binding strength here in the avidity
would be just all of those individual interaction affinities added together
to create the overall binding strength. So if there were say 5 interactions
here & you knew the affinity of each individual one, you could add those
5 together & that would give the total. This is not the case.
The binding strength b/w 2 molecules via a single binding site
The total binding strength b/w 2 molecules or complexes involving
multiple binding sites
- You might imagine that there would be one interaction interface, if this
was sitting on its own, it might have a tendency to break apart at a certain
frequency so it may have the tendency to break apart but if a
neighbouring interaction is still holding on at a different site, that’s going
to hold that first one in place & it’s going to reduce its tendency to break
apart so this is this co-operativity.
- If you have 5 interaction sites all beside one another, if one is about to
break but the other 4 aren’t about to break those 4 sites will keep that first
one in place & keep it bound. So that is why this avidity is higher than the
sum of the individual interaction affinities so you get this real boast of
binding strength b/w the molecules.
- Another feature of this is that the pulling forces are distributed among
many proteins & this is significant if you think about 2 cells that might be
trying to crawl away from one another, or move away from one another
or being pushed by physical stress to separate from one another but these
many molecules will hold those cells together.
- This is where you can see the analogy of a Velcro type of effect where
you’d have these tiny little loops, right here would be one single cell
adhesion molecule, one thread over here would be the other single cell
adhesion molecule so if those 2 interacting with one another, they’re not
going to be very strong at all but then when we have them all clustered
together into this large cluster here, those 2 Velcro patches will adhere to
one another very strongly.
- So that is one key aspect of cell adhesion complexes is this clustering of
receptors. The other aspect is the linkage to the cytoskeleton inside the
Adherens junctions (cell-cell)
Focal adhesions (cell-matrix)
- There are linkage sites for actin & microtubules. So actin &
microtubules can interact with adherens junctions which are cell-cell
adhesion complexes & also with focal adhesions which are cell-matrix
- Then intermediate filaments, they can attach to specific cell adhesion
complexes – the desmosomes that mediate cell-cell & hemidesmosomes
which mediate cell-matrix interactions.
- These are the basic properties of any adhesion complex that are going to
be clustered together & they’re going to be attached to the cytoskeleton
on the other side of the plasma membrane of the cell & this allows those
adhesion complexes to adhere cells together into a larger tissue.
- Now he’s going to talk more specificity about adherens junctions. - These epithelia are sheets of cells that line our body compartments so
it’s the most common architecture in our body, it lines all of our organs, it
forms our skin.
- Diagram: can see 2 epithelial cells – what you can think of here is these
individual cells are sort of like bricks in a wall, they add up together to
form this sheet of cells & this separates one body compartment from the
other. Here, for example, would be one body compartment, say the lumen
of the gut & on the other side of this would be the underlying tissue. The
adherens junctions are forming right b/w these cells – this cell is cutting
cross-section so this adherens junction is actually forming a ring right
around this cell & connecting that cell to all of its neighbours so you can
think of these adherens junctions as acting as mortar in b/w the bricks so
they’re found all around the cells, connecting all the cells together in the
Clustered cadherin adhesion receptors
- If we look close up at the molecular structure of these adherens
junctions we can see these 2 basic properties of a cell adhesion complex
as listed in the slide.
- Diagram: So we can see that right over here in this diagram so this is
one cadherin receptor in green so it has 5 blobs, a transmembrane domain
in the cytoplasmic tail so you can see this one cadherin forms a dimer
with another one which interacts with another dimer from the other cell &
another one & another one. This is just in 2 dimensions here but this will
form a 3D plaque of all of these receptors interacting with one another in
a cluster. Then these adhesion receptors then run through the plasma
membrane, on the other side they interact with adaptor proteins here in
blue & these adaptor proteins link the adhesion complex to actin
cytoskeleton shown by these red filaments in the diagram.
Five extracellular domains
A transmembrane domain
A cytoplasmic tail
- Here is the structure of the cadherin receptor in more detail, at the
- So for an adhesion molecule or adhesion receptor to mediate adhesion
b/w one cell & another, that receptor has to reach out from one cell &
make contact to another cell in the extracellular space. So these
molecules, they have to be a transmembrane proteins, they have to cross
the plasma membrane so they can reach out into the extracellular space &
contact other cells.
- Diagram: Here in the cadherin structure, there are 5 of these
extracellular domains so here again is one individual cadherin receptor,
this is a 2 receptor so this is a dimer of the 2. So if we look in the
extracellular space here, we can see 5 of these repeats & each of these
repeats has a specific folded structure of the amino acid residues. So this
would be the N terminus of the protein, this folds into a specific structure
making up that domain & then the C terminus of this then connects to the
N terminus of the next one & this is all one continuous polypeptide all the
way through to the cytoplasm where the C terminus is. So to run through
to the cytosol, there has to be a transmembrane domain then there is a
cytoplasmic tail on the inside. Five extracellular domains
A transmembrane domain
A cytoplasmic tail
Dimers b/w cadherin proteins along the plasma membrane of the same
- There are 2 key features of this molecule for it to build larger
complexes. One feature is that these receptors bind to Ca & they do so in
b/w each of these extracellular repeats & that’s what these red dots are in
the diagram. The other thing is that they form cis homodimers so these
are dimers b/w cadherin proteins along the same plasma membrane of the
same cell. So cis means it’s on the same PM. In trans is b/w the cells – it
runs across, b/w 2 cells & then the homo here is b/c it is the same
molecule. So we have the same molecule interacting along the same PM
forming dimers – that is a cis homodimer. So this Ca binding & the
formation of these cis homodimers plays a key role in making larger
clusters of these adhesion complexes or of this adhesion receptor.
- With Ca binding this straightens the receptor up so if there is no Ca
present, people have done electromicroscopy studies & have seen that the
adhesion receptor is folded over on itself so it’s not reaching out to other
cells, it’s not available to reach out & grab hold of other cells. But upon
Ca binding this will straighten this molecule up & this then will promote
trans homodimerization so if it’s straight & reaching out towards the
other side, this will promote a trans interaction b/w the cells.
- Diagram: Here we have our cadherin dimer that’s extended out, it then
starts to interact in trans b/w its extracellular domains so then we have
this one dimer interacting with another dimer & now another cis
homodimer can come in & insert right here & then another one from the
other side can insert here & so on & walk along & then basically you can
view this cis dimer, you can see it almost like a tooth in a zipper so you
have the teeth from the zipper zippering up along the plasma membrane
producing a large clustered array of cell adhesion complexes.
- So this is the idea of creating these clustered cell adhesion complexes
from the individual molecules, this increases avidity of these interactions,
increases strength of binding b/w the 2 huge cells.
- So that’s the basic molecular & cellular mechanism associated with
cadherins in terms of receptor binding. Now we have to shift into the cell.
- We just talked about the clustering that occurs out here but they also
link to actin filaments. This is mediated by these adaptor proteins so
they’re called the catenins, so there’s beta-catenin, there’s another called
alpha-catenins & these catenins can bind to the cytoplasmic tails of the
cadherin & they can also link to actin so that is why they are called
adaptors – one part of the complex binds to the cadherin & the other part
of the complex binds to the actin so they can connect the receptor to the
- In this sort of model right here, this looks rather static but in fact, these
adaptors can be quite dynamic & recent work is shown that it’s not really
clear how this is working & this is a subject of active research right now
in the cadherin field. **Don’t need to know the details of this**
- So for example we can see this all connected up to actin, but recent
papers have suggested that one of these key adaptors, alpha-catenin may
not actually bind to the cadherin & the actin at the same time & it might
have to release from this & then bind to actin separately.
- One thing that this does highlight is that these complexes can be quite
dynamic so you can have a protein releasing from it, binding to actin,
maybe binding back to the complex so there is a lot of molecular
dynamics going on.
- Another thing is that in a developing organism or in our bodies, the cells
that are there, we often think of epithelia or tissues as being quite static,
however in development these sheets of cells move quite dramatically so
it’s a question of how these cell adhesion complexes can deal with that
sort of movement.
- Diagram: Here we’re looking at a developing Drosophila embryo.
We’re looking at individual epithelial cells. And we can see how this
sheet of cell moves around in a very orderly way, they’re maintaining
contact with one another, it looks like an intact sheet but it’s shifting
through a process that we’ll talk about in the last couple of lectures
during our discussion of morphogenesis & development. So it’s amazing,
it’s being held intact but it’s undergoing this dramatic movement so
what’s holding this all together? If you take these embryos & you remove
adherens junctions from them by looking at an adherens junction mutant,
you see something quite different.
- So this is the wild-type embryo, the normal one.
- In the mutant, it will look like this to start with but something very
different will happen after that. Here is an adherens junction mutant – it
starts off relatively normal & it starts to try to extend & then basically the
tissues start exploding, the cells start separating from one another,
rounding up & that overall organized tissue structure is lost, you don’t see
this orchestrated normal movement of the tissue. Basically all these cells
just lose contact with one another & are all wiggling around. This
illustrates a key role for adherens junctions in maintaining tissue structure
& in this case, during development.
- Here we’ve taken a normal embryo & experimentally removed adherens
junctions from it by looking at a mutant & we see a loss of this epithelial