What are cell adhesion molecules?
Well they have adhesion molecules that are present on the
surface of cells that recognize other molecules in the body.
And sometimes these cell adhesion
molecules or CAMs as we often call
them, will promote cell-cell and
cell-extracellular matrix interactions.
So it may be that the molecule they
recognize is on the surface of another cell
or it may be that the molecule they recognize
is part of the extracellular matrix.
They can be expressed constitutively,
in other words they’re already there,
or they can be induced by pathogens and
host products during inflammation.
So following encounter with a pathogen or some host substance,
you might induce expression of the
gene for an adhesion molecule.
And an adhesion molecule that wasn’t there
previously now becomes expressed on the cell surface.
Sometimes adhesion molecules recognize the same adhesion
molecule on another cell, in other
words homophilic interactions.
But alternatively, an adhesion
molecule may recognize a different
adhesion molecule on another cell, heterophilic interaction.
There are five main groups of cell adhesion molecules: The
cadherins, the immunoglobulin-superfamily
cell adhesion molecules,
mucin-like cell adhesion molecules, group of adhesion molecules
called selectins and another quite
large family called the integrins.
Integrins are interesting because
they can signal in two different ways.
They are often present on the membrane of the cell in a
particular conformation, or the way that they’re folded, which
means that they are a very low affinity for the ligands
that they recognize and therefore are essentially inactive.
However, signaling from within the cell
involving the molecule talin interacting with
the actin network within the cell, can lead
to what is called inside-out signaling.
So the signal here is being
initiated from within the cell.
And that signal, being delivered
through talin allows a change in the
conformation, so that now you have a
high affinity active conformation.
And as you can see here these integrins are heterodimers, they
consist of two chains, an alpha (α) chain and a beta (β) chain.
And now, following this change in
conformation, the integrin is able
to interact with cell or extracellular
matrix adhesion molecules.
In contrast to inside-out signaling,
integrins can also interact
in a way that is perhaps more
typical when we think of signaling.
They can interact with an external ligand,
in other words outside-in signaling.
And in this situation, what happens is that ligands
lead to a cross linking of several integrin
heterodimers and molecules such as the kinase Src
and the molecule FAK are involved in this process.
And you get a clustering of several
integrin heterodimers together.
So there are multiple copies, that’s what the end there is
representing - multiple copies of
the integrins clustered together.
And this can lead to cell polarity
changes, to survival and migration
of the cells, due to changes in the
cytoskeleton and gene expression.