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The Mechanism of Apoptosis.
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This is actually a
really beautiful story.
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I'm only going to be able to
kind of touch along the surface.
00:08
But I would encourage you if
you're very interested in this,
that there are many very excellent
reviews that talk about this.
00:16
So, again just touching the surface, but
so that you understand how this happens.
00:21
There are many different ways to
get into the apoptotic pathway.
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And we talked about, you know,
the physiologic roles, etc.
00:29
But there are lots of ways to get in
there also via injury and other things.
00:32
So, first we're gonna start with
injury, driving apoptosis.
00:36
So radiation or toxins
or free radicals
can actually, not necessarily induced
necrosis but induce apoptosis,
by causing DNA damage.
00:48
One of the mechanisms that cells
have a protective mechanism
against cancer is that if they recognize
that there's DNA damage was like, "dang"
we don't want to have
that DNA damage turn into
permanent mutation
that leads to a cancer.
01:02
So if I have DNA damage,
I'm going to activate P53.
01:06
The guardian of the genome will come back
to this when we talked about malignancy.
01:11
But if there's DNA damage
P53 gets activated, and then,
if it's not able to fix the
damage, it sends that cell
towards apoptosis.
01:22
It will then have activated series of
what are called execution caspaces.
01:28
Caspases,
great name for a protein.
01:31
And there's a family of these caspases
that have a cysteine site that's the C,
and cleave it aspartates,
that's the ASP.
01:39
So caspaces.
01:41
There's a reason for the name.
01:44
P53 activation in the setting of
inadequate DNA repair will give rise
to starting the sequence of events that
will lead to that cell committing suicide.
01:54
And it will do so bravely so that
it never turns into a cancer.
01:58
That's what's supposed to happen
if we have irreversible injury.
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It may not be necrosis.
02:03
This is now another safeguard.
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Okay, so that's one way in to
starting the a apoptosis pathway,
and we'll come back to the
number three in a minute.
02:14
Okay,
we can withdraw growth factors.
02:17
So remember I told you about
that lactational breast.
02:20
It's got all that estrogen, progesterone,
and it's it's got all the other hormones
that are driving lactation, and now
we're no longer breast feeding the baby.
02:27
We want that all to revert.
02:29
We want the cells in that hyperplastic
lactational breast to go away.
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So we withdraw those hormones.
02:36
Another way into the system,
it will act on mitochondria,
to induce release
of cytochrome C.
02:46
And remember, we've talked about
Cytochrome C is one of the major things
that mitochondria can
release that will control
activation of apoptotic pathway.
02:57
It's actually a whole
lot more complicated.
02:59
This is where the story gets really
beautiful but more complicated
and not necessarily
worth your time.
03:04
I've just listed some of the regulators
of that cytochrome C release,
and there are certain
proteins in the mitochondria.
03:13
BCL-2, BCL-x, there are others,
the names they're not great names,
but they will inhibit the release of
cytochrome C, so I don't get apoptosis.
03:25
And there are others that
will promote release.
03:28
And it's a nice kind of conversation
between the promoters and the inhibitors,
whether or not cytochrome
C gets released.
03:35
So when the mitochondria
are part of the equation
and they are for many
of these pathways,
we tightly regulate
whether or not we let out
that cytochrome C and
start the process.
03:46
So, the cytochrome C then will get to
the actual formation of the apoptosome,
which is the seven
spoked wheel of death
we have talked about at
least two or three times,
which will get us into the
execution caspase pathway.
04:01
All right, another way
to get into this pathway.
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Intrinsic embryogenic signals,
and this is gets very complicated as well.
04:09
But it's that pole point about me
having fingers and not a flipper.
04:13
There are intrinsic signals
that say during development,
this cell, after having grown and made a
tissue, will now undergo apoptosis and die.
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So there are intrinsic embryogenic signals
that also thrive execution caspases.
04:31
There are various
receptor-ligand interactions.
04:34
So there are a variety of
factors, tumor necrosis factor,
acting on receptors for tumor
necrosis on the cell surface.
04:42
There's FAS/FAS ligand don't need to
necessarily know what that stands for,
but there are ways
that exogenous signals
can interact with receptors on particular
cells and drive their cell death.
04:55
These are mainly used
in the immune response.
04:59
So they're going to be important
players in that particular pathway,
so they're all number ones
getting into this pathway.
05:07
And so this is just showing that
receptor-ligand interaction.
05:11
And they can drive either
interaction with proteins
that get us into the
execution caspase.
05:17
Or they can actually even
have initiator caspases,
things that are
even more upstream.
05:22
As they say,
it gets complicated.
05:24
Don't get too bogged down.
05:25
But just realize
there are many ways
into this final pathway
beginning with number three.
05:31
Okay.
05:32
And finally, cytotoxic T
cells, they kill.
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A natural killer cells
to kill by introducing
Granzyme B which will
start execution caspases.
05:46
So all these things kind of
come down to number three,
the execution caspases.
05:52
These are going to be
a series of enzymes
that have a cysteine
interactive site
and they cleave it aspartates, and they
will cleave a whole variety of proteins.
06:01
Okay.
06:02
Next slide is that next step.
06:04
So the executioner caspases
will involve breaking
down the cytoskeleton.
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They cleave acting polymers.
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They cleave
intermediate filaments.
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They cleave a whole
variety of proteins.
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They also have Endonuclease
activity, so they will break down
in a very rigorous way
around his stones, the DNA.
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So we'll get nice bite size
bits of nuclear material
that has also been digested
by the executioner caspases.
06:37
The net result of breaking
down the cytoskeleton
and that results of breaking down
the in the nucleus is that we end up
with a series of signals
driven again by the caspases
that allow a little escape
pod, if you will,
of cytosol and organelles and membrane
and nuclear fragments to be formed.
06:56
That's the cytoplasmic bud,
or a apoptotic bud or a apoptotic body.
07:02
So we have a little
escape pod that's taken up
some bits of cytoplasm and all
the organelles and some DNA,
fragment of DNA,
and we have that cytoplasmic bud,
that is a separate
now apoptotic body.
07:16
It pinches off from the cell
that's undergoing apoptosis,
and there will be many of these,
but it pinches off,
along with its little bodies
and those apoptotic bodies
now have a important change
from the internal side of the
plasma membrane phosphatidylserine.
07:36
Those fossil lipids that we've talked
about previously in the cell biology topics
will flip from the
interphase to the outerphase
of the plasma membrane on
these apoptotic bodies.
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And that's an 'eat me' signal,
for the macrophages or
adjacent epithelial cells,
or whatever happens
to be around.
07:54
And that adjacent cell can recognize that
phosphatidylserine on the outerphase.
07:58
And it will eat and degrade and break
up this apoptotic body completely.
08:05
And so in a very
controlled passion,
without a whole lot of damage,
we can get rid of an entire cell,
and it just gets gobbled
up by its neighbors.
08:14
And the constituents get turned into
lipids and amino acids and sugars.
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So, apoptosis.
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Now you see it,
and now you don't.
08:25
This is again
speaking to the fact
that it's really hard to
see by light microscopy.
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We have some...
08:32
tests and acids that we could do that
kind of highlighted, but it's hard.
08:36
So anyway, it's usually single
cells or small clusters of cells.
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It's not big areas of tissue,
it's little tiny things.
08:45
The cytoplasm as it gets
condensed and crosslink
because of the effects
of the caspases
will get very pyk and the
nucleus becomes fragmented,
so it may become pyknotic,
little condensed areas of chromatin
or actually fragmented
karyorrhectic.
09:02
We make cytoplasmic buds that
turned into apoptotic bodies,
that are rapidly phagocytosed
by adjacent cells.
09:09
And that's why it kind
of, it's hard to see it.
09:11
It just happens kind
of under the radar.
09:14
There's no inflammatory
response, really important point.
09:18
In apoptosis...
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there is no inflammation.
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In necrosis,
there's lots of inflammation.
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And that's because
when there's necrosis,
we want to make sure that
there's no infection.
09:30
So that cell death of necrosis, we wanna
make sure that we clean up everything
and make sure
there's no infection.
09:36
And we'll revisit that in a
subsequent series of talks.
09:39
But in apoptosis,
we want no inflammation,
so it's a kinder, gentler,
suicide of the cells,
And it's really hard to
see by light microscopy
without the special studies,
and we're not going to get into that.