#18 Biochemistry Signaling I Lecture for Kevin Ahern's BB 450/550

Uploaded by oharow on 07.11.2011

Kevin Ahern: Well, sorry about that.
I have one last thing to finish up saying about carbohydrates,
in general, and then we're going to turn our attention to
a very interesting phenomenon known as signaling.
When we get into signaling, we start beginning to see how
controls of cell division and processes like that lead to,
ultimately, important health considerations
for cancer and other things.
The one thing I want to mention today about
carbohydrates that I didn't finish up with last time also has
some significant health considerations, as well,
and it actually has to do with viral receptors.
The example I have for you is that of the flu virus.
It's the time of the year where the flu is moving around and many
people have not gotten vaccinated against it.
For students that want to go into health professions, I think
that's pretty outrageous if you haven't
gotten inoculated against the flu.
But, in any event, that aside,
flu is an important health consideration.
What you see on the screen is an interesting depiction about how
a flu virus affects a blood celló"infects,"
not "affects"óinfects a blood cell.
The flu virus is a virus that contains RNA, not DNA.
If you look inside the virus right here, you see several,
one, two, three, four, five, six, seven, eight, actually,
in this caseófragments of RNA that consist of the
entire coding information for the flu virus.
One of the questions people commonly ask about the flu virus is,
"How come there are so many different types?"
and so forth, and it's partly because of mixing and matching
of different strands of RNA that can occur
when you start mixing infections from different organisms.
So flu virus is a very important, as I say, health consideration,
and also very, very, variable in terms of the different
forms that it can come up with.
Well, a common feature of many of the forms of the flu virus is
what you see on the screen.
They infect blood cells by
attaching to an extracellular component.
You can see this extracellular component is actually here.
The virus has, on its outside coat
it has projections sticking out,
and two of them are of interest to us.
The first one is hemagglutinin.
Hemagglutinin, again, as its name implies, is what's responsible
for the virus agglutinatingóthat is, attaching itselfóto a blood
cell, "hem - " referring to the blood cell.
So hemagglutinin is a protein that recognizes and binds to a
specific carbohydrate residue on the surface of a red blood cell.
So this protein hemagglutinin, you can see
it's projecting all around this virus.
It's just basically waiting to latch onto the appropriate
carbohydrate residue on the surface of a blood cell.
Once it has latched onto that specific carbohydrate,
then the virus has to get its RNA into the blood cell.
It turns out that, in order for this to happen,
that there has to be an opening created in the blood cell
for the entry of the viral RNA.
The opening creation requires action
of this enzyme known as neuraminidase.
What neuraminidase does is it cleaves a residue, a modified
carbohydrate residue known as neuraminic acid,
and that cleavage is necessary we can see it depicted over here
for the entry of the viral RNA.
So the combination of the hemagglutinin binding
to a specific carbohydrate residue and the neuraminidase
cleaving a neuraminic acid containing residue
on the surface of the red blood cell
allows the viral RNA of the flu virus to enter the cell,
infect the cell and cause many more copies
of the virus to be made as a result of that.
Interestingly, this neuraminidase is a target of anti-flu drugs.
When you hear of the anti-flu drug known as Tamiflu,
it works because it is a neuraminidase inhibitor.
It inhibits the action of neuraminidase.
If the neuraminidase can't cleave that residue, there's no entry,
there's no way for the viral RNAs to enter the blood cell
and the flu virus is pretty much left waiting out there.
So that's one place where cellular carbohydrate residues on the
surface obviously play important roles in human health.
Student: [unintelligible]
Kevin Ahern: Sorry?
Student: Do inoculations actually work against the cell or just
boost your immune system?
Kevin Ahern: Inoculations always boost your immune system and they
are targeted at recognizing specific proteins on the surface
of flu viruses, as they are for any virus.
There are many other strategies for viruses, as well,
but Tamiflu is a very cool one.
That's what I want to say about carbohydrates.
I want to turn our attention now
to talking about cellular signaling,
and I think you'll find some interesting and important
considerations of signaling for human health.
Signaling is essential for multicellular life.
When we have differentiated cells of an organism,
it's important that those cells of an organism
all pull their oars in conjunction with each other,
and that is coordinated by the action of
small molecules that move through the body
that basically communicate.
Those small molecules are known as hormones, and hormones are
basically produced in one part of the body,
by cells in one part of the body.
They travel, usually through the bloodstream, and get to their
target tissues, where they bind to specific receptors and cause,
inside of the cells of those target tissues, a response.
That response might be, "Let's activate a bunch of enzymes,"
"Let's inactivate a bunch of enzymes,"
"Let's tell the cell to divide,"
"Let's tell the cell not to divide."
All kinds of possible responses can happen as a result
of the binding of hormones to the cell surface receptors.
So we're going to spend some time talking about those receptors,
as well as about a few of the signaling pathways that are there.
I will caution you as I go into this,
that this lecture is the beginning of a series of
"This goes to this, goes to this, goes to this, goes to this,"
and it is important for you to understand
and know what those pathways are.
So if I talk about them here, yes,
you will be responsible for them.
Before we get to the "This goes to this,
goes to this, goes to this,"
let's take a look at three receptors that we'll be talking about
over the next day and a half or so.
The three receptors are: the beta-adrenergic receptor that,
as we will see, is very, very important in, ultimately,
in controlling levels of glucose in the body;
the insulin receptor, which plays a very important role in also
controlling levels of lucose in the body, but it works
in opposite fashion to the beta-adrenergic receptor.
The insulin receptor also is involved in many other processes.
It's not only involved in blood glucose.
And a third receptor, the EGF receptor,
which stands for "epidermal growth factor receptor,"
which is intimately involved in helping
cells to decide, "Do I divide or do I not divide?"
That decision of dividing or not dividing
is a very important one, as we can imagine.
If the cells are continually getting
signals telling them to divide and they shouldn't be dividing,
we may have uncontrolled growth and, of course,
the definition of an uncontrolled growth is a cancer.
This very simple figure shows what happens
in signaling in the body.
I told you that the tissues in one part of the body communicate
by making a molecule that they release, called a hormone.
They release that into the bloodstream of the body.
It goes and travels to the place where it encounters other cells
that have a receptor specific for binding to that.
So that's the reception part of the process.
As we will see, the reception part of the process
invlolves not only binding of the molecule that was released
óthat is, the hormoneó but that binding
induces some very important structural changes
inside of the receptor protein,
the receptor protein being located in the
membrane of the target cells, and the changes in
that structure of the receptor protein
results in a process we call "transduction."
So when you hear the term "signal transduction"
what we're talking about is the communication of information
outside the cell to mediate a response inside of the cell.
So that's the phenomenon of transduction.
Transduction, in turn, causes those responses
to happen that I talked about earlier.
It might be activating an enzyme.
It might be inactivating an enzyme.
It might be activating entire classes of enzymes, proteins,
quite a wide variety of things that can happen.
Then, it's very important that cells
be able to turn that process off.
Cells are not one-way machines, as it were.
They turn something on,
they need to have the ability to turn it off.
Student: Could you say the definition of
"signal transduction" one more time?
Kevin Ahern: So signal transduction is the phenomenon whereby
information outside the cell is communicated inside the cell.
An example is a hormone binding to a receptor.
That receptor has some shape changes, as we will see, that will
cause several things to happen inside the cell
as a result of that.
But the transduction is just that general phenomenon.
Now, turning this process off is also important.
I will spend more time talking about turning
the processes on, but I will point out to you some places
where turning them off is important considerations.
Now I want to introduce a term to you, "second messengers,"
before I introduce the term to you, "first messengers."
That's kind of odd, but I need to do that.
What is a second messenger?
A second messenger is a molecule inside of a cell, and it's a
molecule inside of a cell that is made as part of that signal
transduction process.
So it's made as a result of that signal transduction process.
Well, now you ask the question, "What's the first messenger?"
In the scheme that I've been depicting for you,
the first messenger is the hormone.
The hormone is extracellular.
The first messenger is extracellular.
"Hormone," "first messenger,"
I will use those terms interchangeably.
It never makes it, at least in the scheme we'll be talking about
here, it never makes it into the cell.
It binds to a protein receptor on the cell surface.
That protein changes shape.
That shape change causes some things to happen,
and those things happen inside the cell,
but the hormone does not make it into the cell.
Second messengers, in general, are very small molecules.
Second messengers are not proteins.
Second messengers, one we'll talk about a lot is cyclic AMP.
Next term we'll talk briefly about cyclic GMP, because cyclic GMP
plays a very important role in our vision.
Cyclic AMP is a much more generic second messenger.
It occurs in a lot of cells
and is used for a lot of signaling purposes.
Calcium, as we will see today, or if I don't finish today then
certainly on Wednesday, plays an important role in this signal
transduction, that is, the signaling process inside of cells,
and it happens, it is not something that is made,
but it is something that is released from various
stores that cells have inside of them.
Inositol 1,4,5-triphosphate
or as you're more likely to know it, IP3,
is an important messenger that is made as a result of action
on a bigger molecule that also makes diacylglycerol,
and I'll show you that later.
So the four messengers that we will be concerned with in the
lectures here are shown on the screen.
Let's think about those receptors.
Receptors are important because receptors, we remember, have to
basically bind to that first messenger.
They have to change shape upon binding, and that change of shape
has to, somehow, result in the production of a second messenger.
We will see that, in most cases, that production of a second
messenger does not occur immediately but, instead,
occurs after several steps later.
There are different classes of receptors.
One of the more common classes that we have on our cells
are called "7TM receptors" and, no, you don't need
to memorize this table. But you can see that 7TM receptors
play some very important roles in a wide variety of processes:
neurotransmission, hormone secretion, smell, taste,
vision, embryogenesis, control of blood pressure.
All of these things are very, very important processes
that are mediated by 7TMs.
So 7TMs play very, very important roles in all of these processes.
Well, why do we call them "7TMs"?
The reason we call them 7TM is if we schematically examine them,
first of all, we remember that they are located
in the membranes of target cells.
Schematically, they look like this.
They project through the lipid bilayer.
This is the membrane, the outer membrane of the cell.
They cross. here's the start.
This is the N-terminus and here is the C-terminus.
Here's the end, over here.
They cross the membrane seven times.
So the "TM" part of it doesn't stand for "transcendental
meditation." In fact, it stands for "transmembrane."
"Seven transmembrane domain," that's what it's called.
Yes, sir?
Student: Is the amino or carboxyl end preferentially
on the inside or outside of the cell?
Kevin Ahern: Yes. It will generally be as you see it here.
So this arrangement is common among many, many
different receptors involved in these
processes that I'll be describing to you.
Now, this is a very simplistic way of depicting the way they look.
A more realistic way of how they actually appear in three
dimensions is something like what you see on the screen.
There are some similarities.
Here's one involved in rhodopsin.
Rhodopsin is light sensitive, for example.
Here's one that is a beta-adrenergic receptor
that we'll be talking about, here.
But we see some similarities in terms of the organization of the
seven transmembrane domains.
You'll notice in the middle of these 7TMs that there is a
binding site for a molecule.
That binding site for the molecule is, in fact,
the first messenger.
So the 7TM has a binding site for the first messenger
to come and do its thing.
The one I'll be talking about first
is the beta-adrenergic receptor,
as I said earlier, and the beta-adrenergic receptor
is sensitive to, that means it binds to, the hormone epinephrine.
Epinephrine is shown on the screen.
No, you don't need to know the structure of it,
but I will tell you that epinephrine is derived from tyrosine,
and epinephrine is also known as "adrenaline."
So epinephrine is the thing that we produce
when we get scared, we get anxious or whatever.
It's produce in other conditions, as well, but a big dump of
epinephrine can have an enormous effect on our bodies,
as we shall see a little bit today, but much more
in the next couple of weeks.
So epinephrine is a first messenger.
It's a hormone produced by the adrenal glands.
It is essential for the flight-or-fight response, basically.
Now, what happens in this response?
I'm going to show you the first half of that today
and I'm going to allude to the second half of it,
and then I'll show you more detail about
the second half of it in about two weeks.
Let's imagine that I am out going for a hike in the woods
and I discover that there's a grizzly bear that is on my tail.
So the grizzly bear starts to chase me
and I realize that I'd better get my butt moving
or I'm going to be in trouble.
I get scared and my body produces epinephrine.
Epinephrine goes and binds to target cells.
Now, these target cells that I'll be describing to you
here we can think of as muscle and/or liver cells,
because these are both important for us to produce glucose,
in the case of the liver, and get away,
in the case of the muscle cells.
So epinephrine is released into the bloodstream.
It travels. It hits the receptor, and when it
hits the receptor, as I've said previously,
what happens is the receptor goes through a slight
change of shape upon binding.
So this guy has bound to epinephrine.
It's the little yellow ball inside of there.
That schlight. "schlight." That slight change of shape
what did I have to drink before I came to class today, right?
Some Schlitz.
The slight change of shape causes the interaction of the 7TM
that is, the beta-adrenergic receptoróit causes
the interaction between it and a cellular protein
known as a G protein to change.
So this G protein is normally just sitting here,
right next to the 7TM.
It's sitting here right next to the 7TM.
Binding of epinephrine changes
the interaction between these two.
You can see the result of this change is that this G protein
which has three proteins in it,
known as alpha, beta and gamma,
changes from holding GDP to holding GTP.
That's number one.
Now, I will tell you that, first of all,
that is a replacement reaction.
That is, the receptor does not make GTP.
It causes this guy to dump its GDP and pick up GTP.
That action causes a change in the shape of the G protein.
This whole complex is known as the G protein, by the way.
It causes a change in the shape of this G protein, such that,
when GTP is bound, the beta and the gamma subunits
no longer bind to the alpha subunit.
[student sneezes]
[student sneezes]
Gesundheit, again.
Why is that important?
Well, it turns out that the beta and the gamma subunits,
when they bind to the alpha, they cover up
a region of the alpha that would otherwise bind to an enzyme.
The enzyme is known as adenylate cyclase.
So we can see this process happening.
When this guy has GTP, it sheds its beta gamma subunits
and now can interact with this enzyme
known as adenylate cyclase again, the "-ase"
telling us it's an enzyme.
Adenylate cyclase, you can see, is also a membrane protein.
But it's a membrane protein that is an enzyme.
When the alpha subunit of the G protein binds to
adenylate cyclase, we see that adenylate cyclase
catalyzes the formation,
of cyclic AMP from ATP.
ATP is converted into cyclic AMP.
Now we've seen several steps happening in this process:
binding of the hormone, alteration of the
interaction with the G protein,
replacement of the GDP on the G protein with GTP,
the interaction of the GTP with the alpha subunit of the
7TM with the adenylate cyclase,
and now adenylate cyclase is activated to make cyclic AMP.
Well, you remember from our discussion last week that protein
kinase A is allosterically activated by cyclic AMP,
and that's what's depicted on the screen here.
So we see this being activated, and, if you recall what I said
about protein kinase A, I said its name told you what it does:
"kinase" means it puts phosphate onto,
"protein" means it's putting phosphates onto proteins.
So what the result of this entire action of the screen is,
is that protein kinase A has now been converted from
an inactive form to an active form.
It will start putting phosphates onto serines and threonines
of target proteins.
What we will see in a couple of weeks
is really interesting with that.
I'm going to give you just sort of a preview of that right here.
Putting phosphates onto target proteins is going to affect those
proteins, and it generally has an effect of either
turning them way on or turning them way off.
A really good example, in our liver,
for example, our liver has glycogen.
I told you last week that glycogen
was a storage carbohydrate for glucose.
We store it so when we need glucose we can make it.
We can release it from glycogen.
We have glycogen in our liver because
we have enzymes that make it.
We also have enzymes in our liver that can break it down.
Now, what protein kinase A does is it puts phosphates onto
both the enzymes that make glycogen as well
as the enzymes that break down glycogen.
Why is that important?
Well, it has opposite effects on them.
Putting phosphates onto the enzymes
that break down glycogen activates them.
Putting phosphates onto enzymes that
make glycogen inactivates them.
That's kind of important.
We don't want to be making glycogen as quickly as we're
breaking it down at the same time.
Student: Say that again?
Kevin Ahern: Okay.
So putting phosphates onto enzymes
that break down glycogen, activates them,
and putting phosphates onto enzymes
that make glycogen inactivates them.
I'll talk about that, as I said, in the next couple of weeks,
so don't panic on that.
I'm just giving you a broad view here.
Let's think about what's happened with this hormone action.
I got scared.
My adrenal glands produced epinephrine.
Epinephrine went out into the bloodstream.
It bound to target receptors.
Let's think about these for the moment in the liver.
That caused a G protein to be activated,
and putting a GTP in it is what activates it.
That activation allows it to interact with adenylate cyclase.
Adenylate cyclase makes cyclic AMP.
Cyclic AMP activates protein kinase,
and what is protein kinase doing in the liver?
It's stimulating the breakdown of glycogen, and breakdown of
glycogen gives me, ultimately, glucose.
I got scared.
My blood supply gets a giant infusion of glucose.
When you hear the stories about people who see a baby under an
automobile and they go out and they grab the automobile up
and they pick it up because they got scared,
those are real, because of the enormous dump of glucose that's
happening as a result of this hormone action.
So this pathway is pretty phenomenal.
You look at this, wow, there's a lot of steps to this pathway.
This process happens in seconds and it actually
would happen faster if the hormone itself
didn't have to travel through the bloodstream.
So this is a pretty phenomenal process
and it happens really rapidly.
This process is the one I usually start with in talking about
signaling because it gives us a good taste
of what signaling pathways are like.
We like to think about, well, one thing happens
and then all of a sudden the cell responds.
But, in fact, we saw several things that had to happen here,
sequentially, in order for the signal to be communicated.
There was our second messenger, right there.
We had to go through all of this before
we made this second messenger.
As we will see, cyclic AMP has many effects inside of cells.
One of them is activating protein kinase.
There are other effects that it has, as well.
It might be a good place for me
to tell you a story about cyclic AMP.
Cyclic AMP, when I say we turn this process on, we also like to
have ways of turning this process off.
Well, let me show you one of them.
One of the processes that we have to turn off is shown right here.
Actually, I'll leave that there.
Here's my activated G protein.
I've got GTP in there.
I want to and I need to not leave that thing in the active state
because if I leave the G protein with its GTP,
what's going to happen?
It's going to stimulate the production of a lot of cyclic AMP,
and the production of a lot of cyclic AMP is going to
activate a lot of protein kinase, and that's going to
activate a lot of glycogen breakdown enzymes, and, bang!
all of a sudden, I burn up all my glycogen.
I need to control that fairly readily.
For G proteins, cells have a very interesting
but a very odd way of controlling them.
G proteins get their name from the fact that you
find them carrying guanine nucleotides,
either a GDP, where it's inactive, which is what
we see over here, or a GTP, where it's active.
How do we get the GDP from the GTP?
Well, you can see right here that there's a hydrolysis
reaction that occurs, and it turns out
that G proteins are really bad enzymes.
I'll repeat that.
G proteins are really bad enzymes.
Bad in what sense?
They're terribly inefficient.
What do they catalyze?
They catalyze the breakdown of GTP.
They catalyze the breakdown of the very thing
that activates them. They bind to it
and over the course of minutes, they'll say, "Okay,
I'm going to break you down," and when they break it down,
they basically turn themselves off.
So G proteins are self-regulating.
They turn themselves off over time.
That keeps the cell from making too much
activated enzyme for breaking down glycogen.
That's very important.
We don't want to be doing that.
So G proteins are very inefficient enzymes.
How about some other considerations?
Well, what happens if I have a receptor that binds to
epinephrine, but the epinephrine gets stuck?
In the normal scheme of things, it just goes backwards this way,
dissociates, the epinephrine is gone.
The cell doesn't go on and do its thing.
So it stops activating G proteins and the process stops
because the G proteins, in turn, inactivate themselves.
But what happens if that gets stuck in there?
Well, one of the considerations if it gets stuck in there,
is cells have yet another way of turning off
the beta-adrenergic receptor.
That's by action of this enzyme known as receptor kinase.
What receptor kinase does is it puts phosphates onto
that C-terminus of the G protein.
I'm sorry. Not the G protein, the C-terminus of the 7TM.
That's important because now those phosphorylated residues
on the C-terminus are a target for binding
by the enzyme known as arrestin.
This now binds the 7TM and stops the 7TM from
activating G proteins. So the cell has a way of turning
off that signal if there's a problem with the receptor.
Again, the fact that this machinery is built into the cell
says something very important about the need to control
signaling processes. If I don't control signaling processes,
just like I don't control enzymes, I'm in deep doodoo.
Well, that's good.
That's all fine and dandy.
Student: The arrestin is attracted to the phosphates?
Kevin Ahern: Beta arrestin binds to the phosphates
on the beta-adrenergic receptor.
Well, that's fine and dandy, but there's one thing
I haven't told you.
I've shown you how we can knock out the epinephrine,
I've shown you that the G protein turns itself on.
What about this guy over here?
Once I've made it, isn't it just going to sit there forever?
If I have this cyclic AMP, isn't it just going to sit there?
Even if I turn everything else off, isn't cyclic AMP
going to be a problem?
Well, it turns out, no.
Cells have an enzyme floating around inside of them,
ubiquitously, known as phosphodiesterase.
What does phosphodiesterase do?
Well, it breaks down cyclic AMP.
That means that cyclic AMP, if we look at the cyclic AMP
levels in the cell, we see that when signaling happens,
they go up, but they fairly quickly come back down.
The reason that they come back down is the phosphodiesterase
starts catching up and starts breaking down that cyclic AMP.
So now we've seen three things here that can help to shut down
this signal when cells don't want to have it going all the time.
I tell you this because it's very interesting that a critical
player in this process is phosphodiesterase.
Phosphodiesterase is a target for a very important drug.
It's known as caffeine.
Caffeine inhibits phosphodiesterase.
Now, I'd like you to think about the buzz you get
from drinking your coffee.
The buzz is real, and, by the way,
the buzz occurs at a couple of levels.
I'm only describing one level to you.
But one of the levels at which it occurs is you are
inhibiting phosphodiesterase, therefore
you have less breakdown of cyclic AMP.
Less breakdown of cyclic AMP means more breakdown of glycogen.
More breakdown of glycogen means more blood glucose.
I just got a buzz!
Kind of cool.
Questions about that?
I'll slow down.
Student: And that's why you crash, right?
Kevin Ahern: That's why you crash?
You will crash, yeah, and the other reason that you
crash is people don't just drink coffee.
They drink what I describe as chocolate milk
syrup cream macchiato latte espresso.
They've got all this sugar crap that's in there,
that not only is their body dumping sugar out there,
but they've got all this stuff that they've put
into their system, so the blood glucose levels go, "Bo-ing!"
As we will see, when the body sees blood glucose levels
going "Bo-ing!" glucose is a poison, so the body acts
to take it out of the bloodstream and there's your crash.
I'll talk about that in a bit.
Student: Without all the sugar and stuff you might add to
your coffee, would just the caffeine be bad for a diabetic,
then, because it can alter.
Kevin Ahern: It's a very common question.
She's asked if caffeine is bad for a diabetic.
It's one of the most common questions I get and I don't know.
I suspect for some people it could be a problem,
but in general, diabetics have problems more with
what they ingest than what their body is producing.
Student: What about synthetic sugars?
Would your body recognize those and crash?
Kevin Ahern: What about synthetic,
you mean like artificial sweeteners?
Student: Yeah.
Kevin Ahern: Does your body crash from artificial sweeteners?
The idea of the artificial sweetener is that you
stimulate the sense receptors that taste sweet
and there's no calories that's contained in them.
For a long time, it was felt that in fact,
that there was very little response that happened to that.
But there have been recent studies now that have suggested
that artificial sweeteners, in fact, actually
are inducing the production of insulin,
which is what happens in the consumption of sugar.
It's not as pronounced, but there is some production
that's there, not because of the process that
I'll describe to you, but probably more likely
because of a learned response in your brain.
Okay, so you learned something about how your body works, there.
There's adenylate cyclase.
It is not a 7TM, but you see it's bound in the membrane.
Blah, blah.
What else do I want to say here?
There's words telling you what I've showed you on a figure there.
That's what I want to say about the beta-adrenergic receptor.
I've got some time, so I want to talk now about another
7TM system. I'm not going to talk about the first messenger,
and that's not really important for our purposes.
The process I want to talk about here is another 7TM system.
It involves a receptor.
It involves a G protein.
The receptor that most commonly is associated with this
is called "angiotensin," which is an important receptor for
modulating blood pressure, so some of the things that
I have to say here will have effects,
ultimately, on blood pressure.
The angiotensin receptor is involved in using a
different kind of second messenger.
The second messenger that it uses is called "PIP2."
PIP2 is basically a compound that's found in the
membrane of cells. You know that cells have a lipid bilayer.
That lipid bilayer tends to have molecules that are long
and nonpolar stuck into the layer, and then
a polar portion that is projecting out of the layer.
In this case, "out of" means on the inner portion of the membrane.
What this signaling pathway does is it activates
an enzyme known as phospholipase C.
Phospholipase C acts by cleaving this guy
in the membrane into two pieces.
Both pieces are second messengers.
So when the angiotensin receptor gets stimulated, it activates
phospholipase C by action of a G protein.
Phospholipase C then catalyzes the breakdown of PIP2 to DAG,
D-A-G, and this long name which you can call IP3.
So that's what up in this signaling pathway.
Now, if we look at it at the level of the cell,
this is what it looks like.
Here's the cell membrane.
You'll notice that we're not depicting, in this case,
the angiotensin receptor out there, at all.
But we have, in this membrane, we have some PIP2.
When the angiotensin receptor activates the G protein,
the G protein activates phospholipase C, and phospholipase C
now cleaves PIP2 and makes two things.
One is, it makes DAG, which remains in the membrane,
and second, it makes IP3, which is water soluble,
and IP3 leaves the membrane and travels into,
in this case, a calcium storage reservoir.
It's labeled as "ER" here, endoplasmic reticulum.
Sometimes you'll see it labeled as "sarcoplasmic reticulum."
They're both involved in sequestering calcium ions.
So what's happened?
Receptor has activated a G protein.
G protein activates phospholipase C.
Phospholipase C makes DAG and it makes IP3.
IP3 travels to a receptoróthis is a second receptor, now
in the endoplasmic reticulum and binds to it.
When it binds to it, it causes the receptor
to open up and let calcium out.
That, then, increases the concentration of
calcium in the cytoplasm and an increased concentration
of calcium in the cytoplasm goes and binds to this
protein called "protein kinase C."
Protein kinase C requires two things to be active.
It requires calcium and it requires DAG.
The combination of these activate, now,
a different protein kinase.
This different protein kinase then is active in phosphorylating
a variety of target proteins that mediate a cell's response.
I always like to point out, when I talk about this, that though
we don't really talk much about muscular contraction in
this class, that you should know, ultimately from your
basic biology classes, that calcium is
a signal to initiate muscular contraction.
Calcium is a signal to initiate muscular contraction.
Angiotensin is favoring the forming of tension
by stimulating the release of calcium in these target cells,
and we could imagine how this tension in target blood vessels,
for example, could affect blood pressure.
So calcium is described in this system as a second messenger.
I would describe it, actually, as a third messenger,
and you can call it either one, as far as I'm concerned,
for the exam. But I think you can make a case for it being
a third messenger, because here it is produced as
a result of action of a second messenger, IP3.
IP3 is a second messenger.
DAG is a second messenger.
Calcium, if you call it a second or a third,
it really doesn't matter, but it happens only
after the action of a second messenger.
Student: Couldn't you also think of IP3,
then, as a first messenger?
Kevin Ahern: No, it's not a first messenger because
first messengers will always be outside the cell.
So that is the phospholipase C system.
Now calcium, it turns out, in the cell is
a bit of a problem for cells.
The reason it's a bit of a problem for cells, you saw that
the cell had it sequestered in the endoplasmic reticulum.
There's a reason it keeps most of the
calcium in the endoplasmic reticulum.
A, it allows it to release it and signal
so that's kind of important,
but even more importantly
is the fact that calcium really likes to bind to DNA.
Calcium binding to DNA can actually cause
your chromosomes to precipitate.
So you don't want that calcium concentration to be too high.
So it's let out in little batches.
One of the ways that cells keep the calcium concentration low,
even when calcium is released, is by using
calcium binding proteins to help communicate the signal that,
"Hey, calcium's been released."
Now, when we examine the structure of these calcium binding
proteins in the cell, we discover they all have a common shape,
and the common shape is that they're known as "EF hands."
It's depicted here.
You can see the finger in yellow.
You can see the sort of fist of the hand down here below,
and you can see that the calcium site is right here
in where these fingers are curled around.
This common feature is found in many calcium binding proteins.
One of the most abundant calcium binding proteins
that we have inside of cells is known as calmodulin,
Calmodulin binds to calcium.
It has EF hands, and the binding of calcium
by calmodulin induces, not surprisingly,
a big structural change in calmodulin.
Here's calmodulin without calcium bound to it.
Here's calmodulin with calcium bound to it.
We see that the binding of calcium induces
a big structural change and that big structural change
allows calmodulin to interact with, in this case,
something called "CaM kinase"
that it couldn't interact with before.
Why is that important?
Well, if calcium is a signaling ion and calcium is a problem in
concentration, what if I have a protein that gobbles
up the calcium but still communicates the signal?
That's what calmodulin is doing.
It's saying, "Hey, calcium has been released.
Do your thing." In this case, it's activating a kinase by
binding to it, and that activation happens only
because this protein has bound to calcium.
So tricks that cells use to counter the effects of
high concentrations of calcium arise because of this binding,
in this case, of calcium by calmodulin.
Let's see here.
I'll start and I won't finish this.
I'll start one last thing.
The last thing I want to say very briefly about is insulin.
When I introduced the hormones originally, I said to you that
insulin is important because it really counteracts
the effects of the beta-adrenergic receptor.
The beta-adrenergic receptor, when it's activated,
stimulates an increase in blood glucose,
by the process I described to you.
Blood glucose levels go very high.
Well, glucose in our bloodstream is a poison.
If there's one message I want you to take
out of this class, that's it.
It's a poison.
In high levels, glucose is a problem.
So our body has a defense against glucose being a poison.
It's insulin.
Insulin is also a hormone.
Insulin is a first messenger.
What insulin does is it stimulates target cells
to take up glucose, thereby lowering
the glucose concentration in the bloodstream.
You're sitting there saying, "But you said it was a poison.
Cells are taking up a poison."
If cells did nothing with it, they would have a problem.
But cells do things with glucose.
They may burn it.
They may store it in the form of glycogen.
They may do other things to it, but the important thing is,
the blood glucose levels are falling.
It's glucose in the bloodstream that is the real problem.
That's how we get kidney damage.
That's why some diabetics have to have
limbs amputated, for example.
That's why they may go blind.
Because they've got too much glucose in their bloodstream.
Now, next time I will tell you how insulin does all of that.
It's a pretty cool processóand remind you again about
the poisonous nature of that compound.
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