- Selectins play a critical role in white blood cell migration to sites of infection,
this is one main thing they do.
- So what we’re looking at in this diagram is a blood vessel, so this endothelial
cell layer on one side, this is a type of epithelium – in this case it is called an
endothelial cell layer since it lines a blood vessel but it is an epithelium just like
other epithelium we discussed about. Here is the white blood cell, normally it
would be flowing through the blood stream so going through all of our blood
vessels, surveying the system looking for infections. When an infection happens
in the tissue outside of this blood vessel, it starts sending signals to the blood
vessels saying there is a problem, that there is a bacterial infection or something
like that. This infection leads to these epithelial cells changing their properties.
They express different things on their cell surface and one key thing they express
is the selectin, the cell adhesion molecule. These specific cells here begin to
express selectin so that they can stick to that white blood cell. Instead of this
white blood cell flowing by in the blood stream, it can now be captured by the
endothelium so that it can start to attack this infection.
- The first thing that happens in this selectin dependent step, here it is illustrated
with this flash of yellow and so, when this cell starts to express selectin, the white
blood cell travelling through the bloodstream will all of a sudden get attached to
one side with these selectins and that will start that cell to actually start to roll as it
grabs hold to one side and the rest of it starts going so it rolls over on itself. This
will make more selectin contacts and more selectin contacts and that starts to lead
to further signalling which induces a second step where there are much stronger
adhesion and this is integrin dependent and we’ll talk about integrins next week,
how they function so we will revisit this slide later. This leads to tight adhesion to
the endothelial layer and the white blood cell will pass through the epithelium to
fight the infection. The selectins play a key role in the initial binding of white
blood cells to target these white blood cells to sites of infection.
! Ig-like extracellular domains
! Calcium-independent, homophilic and heterophilic receptor interactions
- Here is another main class of anchoring junctions. These junctions here, they
involve this other type of receptor & these receptors are called immunoglobulin
superfamily & it is called this b/c the receptors contain multiple Ig-like
extracellular domains. These Ig-like domains, these are the same domains that are
found in antibodies so that is why it is called Ig immunoglobulin-like, same as in
antibodies. So it is this little domain in red, the one that is connected by this
disulfide bond that is one of these Ig-like domains so in this receptor you have 5
of these Ig-like domains but this is all one polypeptide chain running from the
extracellular, crossing the membrane and having the C terminus of this protein in
- These Ig-like domains can mediate homophilic or heterophilic receptor
interactions – they can bind like with like or different receptors and the key
difference here with the cadherins is that these guys are calcium independent.
They bind independent of calcium. But like the other adhesion complex, these
also link to the cytoskeleton and they function in many tissues.
- He wants to point out in the handout that there is a typo mentioning the NCAM
but we see the NCAM receptor is the neural cell adhesion molecule so it is a
receptor that works in neural cells to mediate cell-cell adhesion. The other one is
ICAM and so note down that this is intercellular cell adhesion molecule – this is a
molecule that works in the immune system to mediate cell-cell adhesion. These
are a couple of examples of two different receptors acting to mediate cell adhesion
between different types of cells in the body.