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Gene Therapy in Improving
Muscle Mass and Strength
Effects of Follistatin and
Myostatin on Muscle Growth
Follistatin
also known as activin-binding protein is a
protein that in humans is encoded by the FST
gene
an autocrine glycoprotein that is expressed in
nearly all tissues of higher animals
a naturally occurring protein that inhibits
myostatin, a growth factor expressed specifically
in skeletal muscle.
FST gene
the promotion of cellular differentiation of the
estrogen producing granulosa cells (GC) of the
dominant follicle into the progesterone
producing large lutein cells (LLC) of the
corpus luteum.
Structure
Structure of the
follistatin/activin
complex
FS Isoforms
Three reported isoforms, FS-288, FS-300, and
FS-315
FS-288 and FS-315, are known to be created by
alternative splicing of the primary mRNA
transcript
FS-300 (porcine follistatin) is thought to be the
product of posttranslational modification via
truncation of the C-terminal domain from the
primary amino-acid chain
FS Isoforms
FS is ubiquitous its highest concentration has
been found to be in the female ovary, followed
by the skin
The activin-binding protein follistatin is produced
by folliculostellate (FS) cells of the anterior
pituitary
FS cells make numerous contacts with the
classical endocrine cells of the anterior pituitary
including gonadotrophs
FS Isoforms
In the tissues activin has a strong role in
cellular proliferation, and follistatin the
controlled cellular proliferation and allowing it
to function as an instrument of cellular
differentiation
In the blood, activin and follistatin are both
known to be involved in the inflammatory
response following tissue injury or pathogenic
incursion
FS Production
The source of follistatin in circulating blood
plasma has yet to be determined, but due to its
autocrine nature speculation suggests the
endothelial cells lining all blood vessels, or the
macrophages and monocytes also circulating
within the whole blood, may be sources.
Functions
Follistatin is being studied for its role in
regulation of muscle growth in mice, as an
antagonist to myostatin (also known as
GDF-8, a TGF superfamily member) which
inhibits excessive muscle growth.
Functions
Lee & McPherron demonstrated that inhibition
of GDF-8, either by genetic elimination
(knockout mice) or by increasing the amount
of follistatin, resulted in greatly increased
muscle mass
In 2009, research with Macaque monkeys
demonstrated that regulating follistatin via
gene therapy also resulted in muscle growth
and increases in strength
Myostatin
also known as growth differentiation factor 8
(GDF-8)
is a secreted TGF beta protein family member
that inhibits muscle differentiation and growth.
Myostatin is produced primarily in skeletal
muscle cells, circulates in the blood and acts
on muscle tissue, by binding a cell-bound
receptor called the activin type II receptor
Myostatin
In humans, myostatin is encoded by the MSTN
gene.
The myostatin gene is expressed almost
exclusively in cells of skeletal-muscle lineage
throughout embryonic development as well as
in adult animals and functions as a negative
regulator of muscle growth
Myostatin
Animals lacking myostatin or animals treated
with substances such as follistatin that block
the binding of myostatin to its receptor have
significantly larger muscles
MSTN gene
Mighty mouse and Belgian Blue" with the "no
myostatin" mutation
Functions
Myostatin is active in muscles used for
movement (skeletal muscles) both before and
after birth.
This protein normally restrains muscle growth,
ensuring that muscles do not grow too large
Functions
Mutations that reduce the production of
functional myostatin lead to an overgrowth of
muscle tissue
Myostatin-related muscle hypertrophy has a
pattern of inheritance known as incomplete
autosomal dominance
Functions
People with a mutation in both copies of the
MSTN gene in each cell (homozygotes) have
significantly increased muscle mass and
strength.
People with a mutation in one copy of the
MSTN gene in each cell (heterozygotes) also
have increased muscle bulk, but to a lesser
degree
Functions
A German boy was diagnosed with a mutation
in both copies of the myostatin-producing gene,
making him considerably stronger than his
peers. His mother, a former sprinter, has a
mutation in one copy of the gene
An American boy defect in his myostatin
receptor makes his muscles not respond to the
myostatin signal
Biochemistry/Physiology
Myostatin is a member of the TGF beta
superfamily of proteins.
Human myostatin consists of two identical
subunits, each consisting of 109 amino acid
residues
Biochemistry/Physiology
Its total molecular weight is 25.0 kDa. The
protein is made in an inactive form
For it to be activated, a protease cleaves the
NH3-terminal, or "pro-domain" portion of the
molecule, resulting in the now-active COOHterminal dimer
Biochemistry/Physiology
Myostatin binds to the activin type II receptor,
resulting in a recruitment of a coreceptor
called Alk-3 or Alk-4 (a transducer of activin
or activin like ligands (e.g., inhibin) signals
This coreceptor then initiates a cell signaling
cascade in the muscle, which includes the
activation of transcription factors in the SMAD
family - SMAD2 and SMAD3.
Biochemistry/Physiology
These factors then induce myostatin-specific
gene regulation. When applied to myoblasts,
myostatin inhibits their differentiation into
mature muscle fibers
Myostatin has also been shown to inhibit Akt,
a kinase which is sufficient to cause muscle
hypertrophy, in part through the activation of
protein synthesis.
Biochemistry/Physiology
Therefore myostatin acts in two ways, by
inhibiting muscle differentiation, and by
inhibiting Akt-induced protein synthesis
Recent study showed that a two-week
treatment of normal mice with soluble activin
type IIB receptor, a molecule that is normally
attached to cells and binds to myostatin, leads
to a significantly increased muscle mass (up to
60%)
Biochemistry/Physiology
It is thought that binding of myostatin to the
soluble activin receptor prevents it from
interacting with the cell-bound receptors
Exercise Inhibits Myostatin RNA
Expression
Fold changes in myostatin
mRNA expression
following an acute bout of
RE or RUN normalized to
GAPDH mRNA and
relative to preexercise levels.
Total RNA was extracted
from the muscle biopsies of
the vastus lateralis (RE) or
gastrocnemius (RUN).
Values are means ± SE. *P
< 0.05 from preexercise
mRNA expression.
Louis E et al. J Appl Physiol 2007;103:1744-1751
It remains unclear whether long term treatment
of muscular dystrophy with myostatin
inhibitors is beneficial
Perspective in Sports Science
Discussion
For the purpose to promote muscle function, can
you use the myostain’s knowledge to treat the
people having weak muscle.
In your opinion, an adequate strength training
would produce a muscle hypertrophy may be a
result of follistatin stimulation or myostain
inhibition or both.
Considering exercises performance, why
athletes do not use follistain to improve their
muscle strength, what is the reason?