Lecture 19 - Myogenesis: molecular bases of muscle development Flashcards
Muscle cells are very ____ when compared to most other cells
large
up to 12 cm long
Each muscle fibre is a single muscle cell
Muscle cells are very ____ when comared to most other cells
large
up to 12 cm long
Each muscle fibre is a single muscle cell
Zygote –> ____–> _____
Zygote –> Blastula –> Gastrula –> Mesoderm
The mesoderm goes on to develop the ____ and ____
notochord and somites
Somites differentiate further to form the…
Myotome - from which our muscles arise
True or False
Early in embryo development, the mesoderm segments into somites
True
Where do somites form?
Somites are bilaterally paired blocks of mesoderm that form along the head to tail axis of a developing embryo
give rise to vertebrae, ribs, occipital bone, skeletal muscle, cartilage, tendons and skin
ALL skeltal muscle from the neck down is derived from ____
somites
Myoblasts migrate out from somite
What does the medial and laterla somite form?
Medial somite (near neural tube) forms the axial (trunk) muscles
Lateral somite (distal to neural tube) for the limb muscles
Some head and neck muscles are derived form the somite, such as the ____
tongue
True or False
The extra-ocular and jaw/facial muscles are derivied from somites
False
derived from the head mseoderm
A muscle cell is a syncytium, meaning..
A large mass of cytoplasm not separated into individual cells and contains many nuclei
A muscle cell is a syncytium, meaning..
A large mass of cytoplasm not separated into individual cells and contains many nuclei
Embryonic stem cell –> ____________—> ________—> _________—> mature myofibre
Embryonic stem cell –> Muscle progenitor cell —> Myoblast —> myotube —> mature myofibre
_______s exit the cell cycle, align and fuse together
Myoblasts
Myotubes recruit more myoblasts/nuclei and increase size to become ___ ______
Mature Myofibres
Myotubes recruit more myoblasts/nuclei and increase size to become ___ ______
Mature Myofibres
Muscle progenitor cells (MPCs) are what type of stem cell?
Multipotent
Myoblasts are committed to becoming muscle but do retain some characteristics of stem cells, including….
still being highly proliferative
What is the morphology of the myoblast?
Spindle shaped
Centrally nucleated
Highly proliferative
What is the morphology of the Myotube?
long structures
Terminally differentiated
Central nuclear chain (line up along the middle)
Can contract but lack specialised structures of muscle fibres
What happens to Myotubes to become mature myofibres?
Nuclei move the periphery
Specialised structures develop (e.g t-tubules, DGC becomes properly expressed and functional)
____ migrate to specific areas of the embryo where they proliferate
Myoblasts
Myogenesis is regulated by a complex network of …
Transcription factors
growth factors
microRNA (miRNA)
For an embryonic stem cells to become a muscle progenitor cell which two key genes must be turned on and expressed?
Pax7+
Pax3+
What is additionally expressed in myoblasts?
MyoD+
In addition to:
Pax7+
Pax3+
What are the changes in gene expression from a Myoblast to a Myotube?
Pax genes need to be downregulated
These genes are upregulated:
Myogenin +
SRF +
MyoD
What are the specialised proteins for the maturation to the Myofibre from Myotubes?
MHC+ve
Myogenin+
Factors controlling expression of Pax3/Pax7 are very complex but involve:
Sonic hedgehog Wnts Notch noggin BMP4 - particularly
Pax3 and Pax7 are highly homologous genes but only have overlapping fucntions during ____ ______-
myogenic specification
True or false
Forced expression of either Pax3 or Pax7 can make an embryonic stem cell commit to becoming a muscle precursor cell
True
What does loss of Pax3 only result in?
absense of limb muscles
What does loss of Pax7 only result in?
Results in normal embryonic myogenesis but lack of adult ‘satellite cells
What does loss of Pax7 and Pax3 result in?
complete absense of musculature
Expression of Pax3 and Pax7 is essential for the regulation of ____ expression
Myogenic regulatory factors (MRF)
Myogenic regulatory factors (MRF) are a family of very powerful transcription factors
All are similarly structured
what are the MRFs to remember?
MyoD, Myf5, Myogenin, MRF4
Myogenic regulatory factors (MRF) are a family of very powerful transcription factors
All are similarly structured
what are the MRFs to remember?
Primary: MyoD, Myf5,
Myogenin, MRF4
Primary and secondary RMFs having different roles in..
myogenesis
MyoD, Myf5 are expresssed ___in muscle development
What is their key role?
MyoD, Myf5 are expresssed early in muscle development
promote expression of Myogenin and beginning of differentiaton
What happens when either MyoD or Myf5 are knocked out?
muscles develop fine
They have redundancy with each other
What happens wgen MyoD and Myf5 are both knocked out?
no skeletal muscle
true or false
The secondary MRFs are reduntant to each other
False
Knockout either Myogenin and MRF4 and myoblast differentiation is severely affected
What does Myogenin and MRF4 do?
promote expression of muscle-specific genes
e.g Myosin heavy chains
What are the transcription factors that are key to myogenesis but aren’t MRFs?
MEF2 and SRF
True or False
MEF2 and SRF can’t initiate myogenesis on their own but contribute to differentiation and maturation of muscle fibres
true
What regulates the actual transcription factors during myogenesis?
Growth factors
Bind to cell surface receptors and induce cellular effects
Involve a variety of signalling cascade
What are the two stages in which Growth Factors can affect both myogenesis and mature muscle?
Myogenesis: bu affecting proliferation and/or differentiation
Mature myofibres - by affecting protein metabolism
- Hyper trophy (muscle fibre growth) or atrophy (muscle fibre shrinking)
What are the growth factors of interest?
Insulin, IGF-1, IGF-2, HGF, FGF
IL-6, IL-15, LIF
TGF-beta family
Can have positive or negative effects depending on what they’re acting on
What is the effect on proliferation and differentiation for each of these growth factors
IGF-1 LLIF, IL-6 FGF, HGF Insulin, IGF-2 TGF-B1
IGF-1 - increase proliferation, increase differentiation
LLIF, IL-6 - increase proliferation,
FGF, HGF - increase proliferation, decrease differentiation
Insulin, IGF-2- increase differentiation
TGF-B1 - decrease proliferation, increase differentiation
Myostatin is traditionally recognised as an inhibitor of differentiation as inhibition of myostatin does what?
increases muscle mass
Myostatin is an essential regulator of the balance between…
proliferation vs. differentiation of embryonic muscle progenitors
Increased myostatin =
favoured myoblast differentiation = smaller muscles
Decreased myostatin =
favoured myoblast proliferation = larger muscles
Decreased myostatin =
favoured myoblast proliferation = larger muscles
What are microRNA?
Small, non-coding RNAs that play a major role in cell development and differentiation by negatively regulating gene function
several miRNAs have been shown to be reguolated by MRFs
myomiRs =
Muscle specific miRNAs
miR-1/206 family enhances…
myoblast differentiation
miR-133 family drives…
myoblast proliferation
What is follistatin’s role?
antagonise myostation
What effect will miR-1 and miR-206 have on follistatin?
inhibits follistatin
blocking the ‘‘blocker’’
What does miR-133 inhibit?
SRF
blocking differentiation to favour proliferation
From birth to adulthood muscle size increases ___ fold
20
But there is no significant change in muscle fibre number after birth
