When I went to update this section of the lecture,
and I started searching through all of literature,
I realized I could run a whole entire course
on cytokine signaling and the immune system.
And so rather than drive me into boredom, and you as well,
I have chosen a couple of examples that will give you
a review of the principles and some of the vocabulary, that
will allow you to follow up on it if it's something you're interested in.
And do my due diligence, in case it's something
you're not particularly interested in.
What we're going to start out with is the IFN gamma signaling pathway.
And in this case, we have a signal that's produced by Th1 cells,
in order to coordinate a very serious response against things like virus,
and possibly intracellular bacteria, or any other intracellular threat.
So this is the response that is a very severe response,
and one that involves damaging your own tissues if you find out
that they're harboring bad guys.
If you look here, you can see that the receptor is a heterodimer.
And attached to the cytoplasmic part are two kinases.
These are Janus kinases, because they're named
after the Roman god that faces in two directions at once.
And of course the month of January is also named for this god.
And sometimes they're called "just another kinase"
because there are so many kinases that put phosphates on and off molecules
as part of a signaling pathway.
And in fact, adding and removing phosphates is one of the commonest ways
to activate, deactivate, change the activity
of a protein-signaling molecule in the cytoplasm.
And so this is going to be an example of that process.
Now if you look at this here, you can see that the two receptor peptides will
be shown starting out separate from each other.
If either one of them binds the IFN gamma, shown here
as this cute purple heart, it'll stick to one side
and when they bump into the other one, the two of them will stick together.
And so in that case we will set off the signal.
Now notice this signal is going to be transduced by a STAT.
That is a signal transducer and activation of transcription.
And a STAT is basically a transcription factor
that sits waiting in the cytoplasm.
When it's phosphorylated it goes to the nucleus and it turns on genes.
But depending on the cell and how the cell is set up,
it can turn on different batteries of genes in different cells.
For example, IFN gamma upregulates both macrophages and B cells.
In the B cells one thing it will do is lead
to class switching to the complement activating antibodies, IgG1 and 3.
It also upregulates the activity of macrophages.
In macrophages it upregulates the ability to phagocytise,
and it upregulates the compounds that kill off
pathogens that have been trapped in a phagolysosome.
That is, it upregulates the enzymes that make reactive oxygen and nitrogen
species.
And it upregulates the Golgi production and distribution
of various kinds of hydrolytic enzymes so that the macrophage
becomes primed to attack effectively.
Again, it also will feed back on the Th cells to promote a Th response
and continue down this particular developmental pathway.
Now notice what we have here is when the receptors get together
the dimer will bring together the two kinases,
and that essentially will activate them.
Here they are phosphorylating each other.
When they phosphorylated each other they're
providing a docking site for that STAT protein.
And when the protein docks, it picks up a phosphate.
That activates it.
And so when the protein is activated, it dimerizes-- here it
is-- and heads into the nucleus.
Now for the IFN gamma, the particular STAT is something called STAT1.
And interestingly enough, the STAT that causes the Th1 development that
will produce the IFN gamma is a STAT4.
So you have one set of signals that causes
the Th1 cells to develop and produce their signals.
And then the signals also use STATS to effect the response,
but they use a slightly different one.
So that's a bit of an interesting complication.
But let's look at the next example, which has even further complications.
This is a picture of the different subunits of the IL-2 receptor.
And also here's the IL-2 in green.
Now notice this far left one looks suspiciously like the IFN gamma
receptor.
And in fact, this is a Class 1 receptor.
The IFN gamma is a Class 2.
But they have in common that both of them
use Janus kinases attached to the cytoplasmic signaling peptides,
and they both set off STATs.
All right.
So what's the difference?
In many cases, especially true of the IL-2 receptor, just these two beta
and gamma subunits by themselves will not set off a very strong response.
They don't bind the IL-2 particularly well.
There's this third subunit, the alpha subunit,
that doesn't bind the IL-2 all that well by itself, either.
But when I put the three of them together, as I have here in the middle,
I have a receptor that is highly effective at binding IL-2,
and therefore highly effective at setting off the phosphorylation events
that lead to the activation of those STATs.
So here it is-- the complete structure.
Notice this is the alpha chain off here to the right.
This is also called CD25.
And one thing that happens in the course of activating immune cells
is that they upregulate the production of this alpha subunit.
So that it means that prior to activation,
they don't really respond very well to IL-2.
After activation, they do.
Again, these have the motifs we talked about before-- the CCCC at the end
terminal, which we found in the previous one.
The signaling motifs at the cytoplasmic or C terminal.
And in this one, we have that special WSXWS motif that we looked at before.
So the resting T cells don't have the CD25.
Activation causes synthesis, and when that happens
the three chains get together.
And if any IL-2 is around, they're going to pick it up and begin
signaling by the JAK-STAT pathway.
So here is a picture of the complete IL-2 receptor with the kinases--
Janus kinases-- attached to the cytoplasmic domains.
In this one, they had been phosphorylated, as we saw before.
And so this one is ready to dock the STAT.
This picture shows the STATs docking, and these
are going to dock to this molecule, and they're going to pick up a phosphate.
And so then, what we get next is going to look suspiciously familiar.
Because basically, I use the same illustration.
OK.
So once we've done that, it's something we've seen before.
The STATs have picked up a phosphate.
They have formed a dimer.
They have entered the nucleus and they're turning on banks of genes.
Now what makes this thing different is that the IL-2 receptors, especially
on T cells, will up upregulate the production of more than one STAT.
And in particular, they'll be-- I think 3 and 5 are the biggies,
and there's different versions of those.
And it gets all very complicated.
But what we're going to get from IL-2 is a more complex set of activations.
More STATs, more reinforcing pathways.
So here we have it in the next one.
Oh no, look at all of that.
OK.
Well, what's important here?
Well, first of all, one thing that happens with IL-2 activation
is that you use those phospholipids for signaling.
So up here, we have a phospholipid.
We have, again, the two fatty acids and the polar group.
And as we do in many signaling pathways, we're
going to clip the polar group off of the fatty acids.
And the polar group, the IP-3, will be a signal for the endoplasmic reticulum
to release calcium.
OK.
So here's the calcium coming out.
The calcium will be picked up by calmodulin and then activated.
So here's the inactive calmodulin.
And here is the active calmodulin with its calciums.
It's coming down and activating another-- in this case, phosphatase,
which is going to remove a phosphate from the NFAD.
So in this case, we're actually upregulating the activity
of a transcription factor by taking the phosphate off.
So again, don't ever become too rigid with your thinking.
So that's one pathway.
The other pathway involves the polar group, the diacylglycerol,
which is coming over here.
And, not that I'm going to go through all of these particular stages--
but again, we're going to activate another kinase.
And we're going to activate the NF-kappaB.
And that's another one of those transcription factors
that upregulates the inflammatory response.
In particular, these two pathways also converge to prevent apoptosis
and to promote cell division.
So one of the biggest signals you get from an IL-2 promotion of cell
development is the signal to be fruitful and multiply.
So this is not only a particular prod to a development process,
but it's also a prod to that clonal expansion
process that is so basic to upregulating the adaptive immune system.
All right.
So what have we done here?
We've got some basic patterns.
For a take-home message, we have the insight
that the basic mechanism of having a signal on the outside,
transducing it to the inside, using kinases, upregulating
certain kinds of gene expressions from those kinases,
and possibly setting up other pathways, as well.
That's a good general pattern we're going to take.
One last thing before we go into the next lecture
is I want to have you pay attention to this phospholipid here.
We developed two different signals by clipping it.
When we go into lecture 13, one of the later clips in that
will show you a number of other kinds of ways
we can process these phospholipids to generate signals.
So again, it's all about the signals.
It's all about the information.
And this is some of the basis that you can take into those further lectures.