This first
opening post is hardly in the line of what I’m hoping to share on this blog
frequently, but it dawned on me, outlining things I wanted to post, that a lot
of it is hard to comprehend without a basic grasp of muscle molecular biology.
And that the effort in explaining everything over and over again as it relates
to each subject was a daunting task that sort of defeated the purpose of this
blog. So I decided to start by giving you, as reference material, a basic
outline of the hypertrophic process in skeletal muscle tissue. Now I’m not
going to go into great detail, mostly because I want to keep this information
simple and accessible to anyone, but also because its only about sharing the
relevant information to what follows.
Should there be an overwhelming demand for more detail, I’ll try and add
it to my slate of articles I still need to write.
Cell signalling in its most basic form
There are
cases where cell signaling is rather simple. As it is with steroid hormones
(estrogens, androgens, corticosteroids and mineralocorticoids) for example.
These are small lipophilic molecules that can freely cross cell membranes and
bind their receptors in the cell plasma. These bound receptors fold up and
become active, forming pair (homo-dimers). These pairs then migrate directly to
the nucleus where they act as initiators of transcription. That means they
recruit co-factors and begin copying parts of DNA to messenger RNA (mRNA) which
is then translated to primary protein (a long chain of amino acids) which is
then folded into secondary, tertiary and quaternary structures to form a 3D
structure with a particular function.
These proteins do pretty much everything in our body, from being
structural elements to being signaling molecules and hormones themselves.
In most
cases its more complex than thatn when the hormones are in fact proteins
themselves, like insulin, IGF-1 etc. These large hydrophilic molecules cannot
enter the cell, and must bind membrane-bound receptors to initiate their
effects. This causes conformational changes in the receptor on the inside of
the cell, which then initiates a new event on another protein. This usually
happens by either releasing or binding a bound molecule, or by phosphorylating
or dephosphorylating (adding or subtracting a phosphate from a specific
position on the protein) a molecule. All of these result in either activating
or de-activating the affected structure. This protein then affects another
protein in the same way and in this way long cascades of events are formed
until finally an initiator of transcription is activated who will transcribe the
DNA to mRNA. This seems overly complex, but this sort of redundancy allows for
a lot of external factors to control the cell signaling past simply the hormone
binding the receptor, which allows internal and external factors to intervene
if such signaling does not seem opportune for you and you’re body for some
reason. This is also the reason why corticosteroids, estrogens and androgens
have long been simple to use staples in medicine and sports, because they
basically do what they are supposed to do, with no intervention, and why
seemingly more powerful molecules like Growth Hormone and IGF-1 don’t seem to
always produce the desired results. Because of this long cascade of events that
can be influenced, they only work if the environment is right for them to work.
mTOR as the controller of muscular hypertrophy
The growth
of your muscle mass is known as skeletal muscle hypertrophy (yes 99% of you
know this, but like I said I wanted to make this as simple to understand as possible).
It has been discovered that the key factor in skeletal muscle hypertrophy is
the phosphorylation and activation of the mammalian target of rapamycin (mTOR).
All signals that influence hypertrophy, be they growth factors (insulin,
IGF-1), nutrients (leucine) or exercise, the control switch is mTOR. When
rapamycin, a known mTOR inhibitor, is added to a medium, it effectively blocks
the hypertrophy response from all these signals. mTOR mediates hypertrophy by forming the mTORC1 (mTOR complex 1) and influencing the phosphorylation of downstream targets, the most commonly known of which are S6K and 4EBP1.
All these triggers activate mTORC1 through a different pathway. Insulin, and to some extent IGF-1, work via their membrane bound receptors to activate receptor substrates who then activate PhosphatidylInositol-3-Kinase (PI3K) who in turn activates the PDK-Akt/PKB cascade which inhibits the primary inhibitor of mTOR, the TSC1/2 (Tuberous sclerosis complex 1/2) complex. This complex tends to inactivate a crucial element for the formation of mTORC1, Rheb. By preventing TSC1/2 effects it allows Rheb to incorporate into mTORC1. IGF-1 and other growth factors also seem to activate mTORC1 via the Ras/Raf/MEK/MAPK/RSK cascade. The last two effectors, MAPK and RSK seem to inhibit TSC1/2 as well, where RSK also directly stimulates mTORC1 formation and activation. Leucine, the primary neutrient trigger works via a poorly elucidated mechanism that starts with the class III PI3k isoform hVPS34, involves several GTPases, and ends with MAP4K3. However the precise mechanism hasn't been determined yet. All that is really known is that the trigger seems to require the influx of leucine into the cell, and glutamine out of the cell. Exercise does it through a different pathway. This was first discovered
when they noticed that adding wortmannin (a PI3K blocker) did not stop exercise
induced skeletal muscle hypertrophy. It was later discovered that
exercise-induced hypertrophic response is mediated through Phosphatidic acid
(PA). The initial response to is by activating PLD (phospholipase D) which then
catalyzes phsophatidylcholine (PC) to PA and Choline. Now PLD is also activated
by PKC, a downstream target of PI3K, but
plays no role in the effect of nutrients and growth factor signaling alone.
That is because PLD in the muscle is bound to α-actinine. In
this bound state PLD is completely inactive. α-actinine if part of the contractile mechanism of the muscle, forming
what is known as the Z-band. This part is the most vulnerable to mechanic load
and is likely the first to suffer damage when exercise is performed. This
releases the PLD in its active form, yielding PA to stimulate mTOR.
At least this is the
initial trigger, the PLD increase is transient and yet the PA increase is sustained,
which likely hints at more PA being created from Diacylglycerol (DAG) through
the DAGK (DAG-Kinase) pathway. Nutrient and growth factor signaling can support
this, causing not just an additive effect through PI3K-Akt-PKB in the form of
direct stimulation of mTOR, but also through PI3K-PKC which increases PLD
activity. That’s the key reason why the anabolic window surrounding exercise is
so crucial for eating and supplementation, because it forms a highly
synergistic window. Lastly DAG itself seems to cause an increase in PKC, and
DAG is created not only by PI3K, but also by PA being converted by Phosphatidic
acid Phosphatase (PAP), so it’s a transiently self-sustaining loop (it extinguishes
over time but manages to sustain itself for a while).
AMPK as the antagonist
Another protein
effector that you will read a lot about in exercise physiology is AMPK. AMPK
acts as an energy sensor in the cell. As most of you know from a million and
one creatine articles and ads, energy is stored in the form of
adenosine-triphosphate (ATP ) in the cell. When energy is used ATP is split
into ADP (adenosyl-diphosphate) and phosphate by breaking a highly energetic
phosphate bond. ADP is then usually quickly degraded to AMP (adenosyl
mono-phosphate) and when the AMP to ATP ratio increases, AMPK will sense it and
activate. This is typically what happens during endurance exercise. After the initial
burst the energy reserve depletes and AMPK signals to the cell that more energy
is needed. There are two isoforms of AMPK. AMPKα2 signals metabolic changes to
the muscle. Namely to recruit, sustain and increase the number of TypeI and
TypeIIa fibers for prolonged low intensity exercise, and to start generating
new energy through the oxidative krebs cycle. Repeated bouts of endurance
exercise cause changes in the muscle’s phenotype that support this. This is what you see in marathon runners,
their muscles are entirely adapted to their type of exercise and are extremely
efficient at burning anything you throw at it.
The other isoform AMPKα1
basically stops skeletal muscle hypertrophy. That’s all it does. It makes
sense. When energy is needed, and resources are low, you aren’t going to allow
it all to go to waste to build TypeII fibers for high intensity exercise when
you need all that energy to support low intensity exercise for really extended
periods of time. AMPK does this primarily by shutting down mTOR. AMPK is an antagonist of mTOR.
This may come as a
surprise to some, but its been scientifically established for some considerable
amount of time now that concurrent aerobic and resistance exercises will
attenuate the benefits bestowed by each. Meaning doing cardio before or after
your training basically kills your muscle growth. But even doing it at another
time of day can be counterinidicative since the hypertrophic response from
exercise lasts 24-48 hours. So with most people training 5-6 days a week,
unless you are doing cardio on your off days, chances are good you’ve been
sabotaging your growth.
DGAT1 and the athlete’s paradox
DGAT1 is the enzyme
that converts DAG to triacylglycerol (TAG). TAG is the form in which fat is
stored in skeletal muscle, adipose tissue and the liver. DGAT1 is also
upregulated by exercise. This may seem odd under times of energy demand, and
even odder when you know there is a large body of research examining how
reducing DGAT1 will serve as a treatment against obesity. But DGAT1 also seems
to increase fat oxidation in muscle. Basically athletes seem to have the
contradictory effect of gaining muscle while losing fat. Or on a molecular
level, of storing fat while also increasing the use of it. Mice with
constitutively overexpressed (always active) DGAT1 produced the athletes
paradox completely. But it makes sense on some level. The increase in fatty
acid oxidation provides extra energy, while the increase in TAG synhthesis increases
muscle demand for fatty acids, storing them there for future use, as opposed to
in adipose tissue, which would take some time for it to get to where its being
used. The preferential increase in TAG synthesis in muscle reduces the amount
of fat stored in fat tissue, without directly affecting that tissue. Now
increase in DGAT1 would mean a reduction in circulating DAG, which again, seems
counterintuitive since DAG supports PA levels. But DAG is also toxic for the cell.
That’s why it signals all forms of proteins that get rid of it, be it PKC-DAGK
to produce more PA, or whether its being reduced for fatty acid oxidation and
providing energy, or being made into TAG to be stored for future use.
Another benefit of DGAT1
is e reduction in another toxic substance, the sphingolipid ceramide. Ceramide
is created under the influence of excess fatty acids in the circulation, and
apart from being toxic, it has a direct inhibitory effect on mTOR. DGAT1 will
use ceramide with DAG to create TAG, or stimulate the use of ceramide for
energy, so that it cannot affect mTOR.
The takehome messages here are that A) one should avoid fatty foods
around exercise (not completely abolish fat, but avoid extensively fatty foods
that cause large increases in fatty acids at once) and B) that DGAT1 inhibitors, while promising
for fat people, are unlikely to be beneficial to active individuals and may
hurt your gains.
Conclusions
That’s the molecular
biology involved in skeletal muscle synthesis in a nutshell. I avoided going
into too much detail, or to explain what happens downstream of mTOR, because
its mainly meant to serve as a reference for future posts. This will help you
understand why certain things work the way they do. Should a detailed review of
these processes be in overwhelming demand of people reading this, I can always
add it to my list of articles I need to write sometime, and I can go into more
detail of how growth factors stimulate PI3K, how the downstream effectors of
mTOR lead to transcription initiation, etc. In any case I’m sure a future post
will also focus on how exercise, likely through mTOR, increases local secretion
of IGF1a and MGF (mechano-growth factor) because while IGF1a simply supports
the hypertrophic process, MGF stimulates proliferation of muscle satellite cells,
keeping the pool sufficiently large to keep repairing and growing muscle cells.
We also haven’t touched on myostatin and SIRT1 as negative regulators of
growth, or external factors like corticosteroids and androgens, but those are
all not so relevant to the immediate effects on skeletal muscle hypertrophy as
they relate to training and nutrition, so we will talk about those some other
time.
I hope this was
enlightening to some of you, questions and suggestions are always welcome. I’m
sure this first post provides you with fairly little insight, but it’s a necessary
step in making sure I can efficiently convey more useful information in future
posts.
Glossary of terms :
AMPK = AMP-activated protein Kinase
DAG = Diacylglycerol
DAGK = Diacylglycerol Kinase
DGAT1 = Diacylglycerol Acyl Transferase 1
DNA = Deoxyribonucleic acid
IGF-1 = Insulin-like Growth Factor 1
PI3K = Phosphatidylinositol-3-Kinase
PKB/PKC = protein kinase B/C
PLD = Phospholipase D
MGF = mechano growth factor
mRNA = messenger Ribonucleic acid
mTOR = mammalian target of rapamycin
PA = Phosphatidic Acid
PAP = Phosphatidic acid Phosphatase
PC = Phosphatidylcholine
TAG = Triacylglycerol
TSC1/2 = Tuberous Sclerosis Complex 1/2
References (on demand)
TSC1/2 = Tuberous Sclerosis Complex 1/2
References (on demand)
