00:00
So, Atrophy,
is nothing more than a
decrease in cell size,
as a result,
the organ will decrease in size, too.
00:08
So if we look at a normal
muscle on the right,
we have a certain size of the
individual skeletal myocytes,
and that gives the muscle
a certain appearance.
00:20
In the atrophy muscle,
we actually haven't lost any cells.
00:23
Just each cell has
become that much smaller,
so just cell size will
impact organ size, clearly.
00:33
The cells are smaller because
they've kind of retreated
to a less metabolically
active state.
00:38
They're still viable,
and it's potentially reversible.
00:41
If we restore blood fold,
we restore mechanical activity,
that atrophy muscle
will come back.
00:46
And those of you who may have
had a broken arm at some point
know that as you were in a
sling and not using that arm,
you had atrophy of the muscle.
00:54
And when you took it out of the
swing, you thought,
Oh my God, I've lost my muscle.
Well, no, it's still there.
00:59
It comes back when you use it, so that's
disuse atrophy of the skeletal muscle.
01:05
Also, the same thing will happen,
if we withdraw hormonal stimulation.
01:10
So we will see an example,
in a minute, where you will have
increased estrogen and progesterone,
causing increase of smooth muscle
mass in a gravit, a pregnant uterus.
01:25
And after the pregnancy,
we withdraw those at hormonal stimulus
and the uterus
returns to normal.
01:32
So, withdraw of hormone
stimulation can also
give rise to atrophy or to
restoration of normalcy.
01:42
It's important to note, too.
01:43
We can also get smaller
organs if we have fewer cells.
01:48
So the atrophy that occurs, say,
in a brain with Alzheimer's disease,
there is significant loss
of the the neuronal network,
and that's actually
because neurons have died.
02:00
As a result,
we get atrophy of the organ.
02:03
So we need to distinguish,
just briefly and in our own minds,
when we talk about
atrophy of a cell,
versus atrophy of an organ.
02:12
And you can have atrophy of an organ
due to individual cells getting smaller,
you can have atrophy of an
organ by having fewer cells.
02:21
All right, how does this happen?
How do we get atrophy?
Well, in fact,
there a couple different mechanisms,
and we're gonna discuss
those very briefly.
02:29
One is Proteasome Degradation.
02:31
In a previous talk, when we talked
about subcellular mechanisms
of degradation,
a breaking down small things.
02:39
We talked in detail about
proteasome degradation of proteins.
02:43
And we have a process by
which we identify a protein
is something that should be
targeted for destruction.
02:48
You see the target protein
in the right lower corner.
02:53
We are going to load up some E
ligases, E1, E2 and E3 ligases,
and there are a whole bunch of these and
they are going to transfer ubiquitin,
the green bubble to
the target protein,
and put multiple
ubiquitins on that protein.
03:09
And that's a label, it says,
I need to be broken down.
03:13
So we can have proteasome
degradation of proteins.
03:17
Once we've targeted the protein for
degradation, it goes into the proteasome,
whole bunch of these little
miniature garbage disposals
that are inside the cytosol,
and we break down the big
protein into little tiny peptides
that subsequently
get broken down into
constituent amino acids.
03:33
So proteasome catabolism
is a way to turn over.
03:36
We can activate this pathway.
03:39
If we need more proteins,
or we need more constituents and we will
break down those proteins causing atrophy.
03:49
A proteasome catabolism may
involve ubiquitin targeting
increasingly recognize that there are other
pathways that don't involve ubiquitin.
03:56
But for the most part it's
mostly through that targeting,
and maybe increased in hypercatabolic
states and contribute to atrophy.
04:04
Clearly, we're getting
increased protein turnover
by having them all break
down through the proteasome.
04:10
So that's one mechanism by
which we can get atrophy.
04:13
We can also get it through
breakdown using the lysosomes.
04:17
That's normal cell metabolism
through Heterophagy.
04:21
So it is eating of
other stuff hetero.</b>
So that's one way that we
have lysosomal catabolism,
that's the normal mechanisms by which
we take up a variety of large things.
04:34
You can also have,
as we've discussed previously,
autophagy,
so eating self-constituents.
04:41
This is actually a normal pathway
for normal cellular metabolism
and normal cell and
organelle are turnover.
04:49
As we've discussed many times,
the cells are constantly turning over,
large protein aggregates,
constantly turning over.
04:55
In order to degrade those we have
to, those are big things.
04:59
Mitochondria are big things.
05:01
We need to identify them as being
old, and then we break them down,
using the autophagic
or autophagy pathway,
that involves encasing them into
a membrane, the autophagosome,
fusing it with lysosomes that
contain various hydrolases.
05:18
And then we degrade
everything that's in there
and release it as the constituent
lipids in amino acids, ect.
05:26
This pathway can
be up regulated.
05:29
If we need to or up regulated
in hypercatabolic states
and lead to atrophy.
05:36
So autophagy, mainly used for throw over
of long lived proteins and organelles,
and we've talked
about this many times.
05:43
What's being shown on
the right hand side
is electron micrograph
of an autophagic vesicle,
that has ensnared a
senescent mitochondria
and the typical autophagic vesicles,
it doubled-layered membrane.
05:56
It derives from the rough
endoplasmic reticulum.
06:00
We can clear damage
constituents,
including protein aggregates
that have occurred.
06:05
If the cell is starving, it will
maintain its viability by eating itself
and getting down to a shrunken version
that's still alive and hanging in there.
06:17
It does so through autophagy.
06:20
Interestingly enough, it's also potentially
mechanism against intracellular pathogens.
06:25
So we can use the same
pathway, say a bacteria,
an intracellular bacteria
or a virus gets into a cell,
we recognize it is foreign.
06:33
We can encase it,
in an autophagic
vesicles and degrade it.
06:38
So it is a defense against
intracellular pathogens.
06:42
It may be a mechanism by
which tumors resist therapy,
so it's not just important that we
understand it in the context of atrophy.
06:50
But tumors...
06:51
under attack may hunker down,
and say, you know what?
I'm not gonna have any
metabolic activity.
06:56
I'm just going to kind
of eat myself here.
06:59
I'm gonna hang out in
a small inactive form
and wait for that nasty
chemotherapy to go away
and then I'll be
able to come back.
07:06
So tumor resistance to therapy has
actually also been related to autophagy.
07:11
It can also be too aggressive,
so every now and then,
actually more than every now and
then, cells will do too much of this
and they eat too
many mitochondria,
and there's not enough left
to even maintain viability,
and as a result of that,
it can be a mechanism of cell death.
07:27
So autophagy and proteasome
catabolism is a way by which we can
break down salary constituents and make
smaller and smaller and smaller cells.
07:35
And the cells get smaller and
smaller, the whole tissue atrophies.
07:39
So that's our mechanisms
for getting atrophy.
07:43
Let's get a little bit
more in detail on autophagy
because it's important to
understand for some metabolism
and some biochemistry that you're going
to see as part of your medical career.
07:52
And also because it has
an impact on certain drugs
that we will give our patients.
07:58
So autophagy, it happens as we
identify organelles that are senescent.
08:05
Again the mechanism by which this
happens, not entirely clear.
08:09
We surround them with a membrane
that's derived from them
rough endoplasmic reticulum,
that's our phagaphore.
08:16
We then take that phagaphore,
the autophagasome now,
that has trapped up senescent
things that we want to get rid of
and we take from the
cytosol, LC3B,
these are proteins that
are kind of like ubiquitin,
you can think of
them as ubiquitin.
08:32
But it's basically labeling this
autophagasome says, you know what?
I've got stuff in here
that needs to be degraded.
08:39
That is its signal for the lysosome
to come along, fuse with it.
08:44
The LC3B gets recycled for the next round
with another autophagasome someplace,
and the lysosome dumps
in its hydrolases
and degrades everything that's
inside the autophagasome.
08:56
And we get degradation,
we get amino acids
coming out, fatty acids,
all of that other stuff.
09:02
So that's the normal pathway.
09:04
This is actually very tightly
regulated in most cells.
09:07
So there are a number of genes,
they're called Atg genes for autophagy,
genes, Atg, good name.
09:13
And those products target organelles
for the new ritual nucleation.
09:17
And there are, I think,
eight different Atg genes
and protein products that
will control this process.
09:26
Okay, that's all well and good.
09:27
What does this have
to do with you?
Well, in fact,
there is a major regulator in
cells that you need to be aware of
because it's a very important
kind of linchpin in biochemistry
is the mammalian target of
rapamycin, or mTOR.
09:43
And mTOR levels,
very carefully regulate the transcription
and translation of Atg genes.
09:51
So the more mTOR we have,
the less atrophy
that we will have.
09:59
It's basically turning
off this process.
10:05
So what controls mTOR?
Well mTOR,
things like insulin and amino acids
and other gene products that
say, you know what?
We are in Fat City.
10:16
We've got all the nutrients we
need and more, they will act on.
10:21
They will up regulate mTOR,
which will then inhibit Atg.
10:25
And we'll turn off all
this autophagy stuff.
10:29
Cool.
10:29
So this, in fact, this is
biochemistry coming back to pathology,
and it's a nice kind
of intersection.
10:34
There's a drug that we give very
frequently for immune suppressing patients
who have had organ transplants,
and that's rapamycin.
10:43
And rapamycin will inhibit mTOR.
10:47
So, it also inhibits other processes
that are involved in the immune response,
that's why we give it for treating patients
who have received an organ transplant.
10:57
But that rapamycin
inhibits mTOR.
11:00
If we're inhibiting mTOR that means
we're not inhibiting Atg genes.
11:05
And now we're getting
more autophagy.
11:08
So in fact, when we give patients
rapamycin, we actually induce muscle,
skeletal muscle,
atrophy through this pathway.
11:16
Just a consequence.
11:17
So every drug has its target.
11:20
But it also has unintended targets as
well, just a good example of that.