Lecture 13 Flashcards
What are the main roles of skeletal muscle
Control of coordinated movement and posture, communication through speech expression and writing, maintenance of temperature by heat release during contraction, and respiration
Recall the pathway of muscle development
Stem cells –> myoblasts –> myotubes –> myofibres
Describe the mature muscle cell
The mature muscle cell or myofibre is a multinucleate contractile cell
Muscle cells aren’t differentiated until they become myofibres, T or F
F – the differentiated muscle cell is called a myotube, the myofibre is a mature muscle cell
Myotubes are mononucleate, T or F
F – they are multinucleate
How do myotube form
Fusion of mononucleate myoblasts
At what stage in muscle development do the cells become committed to the muscle cell lineage
When they become myoblasts
What are C3H10T1/2 cells
These are a population of fibroblast cells that are already capable of giving rise to myocytes under certain conditions
What is the result of exposure of certain types of fibroblasts to 5aza and how does this occur
Exposure of the C3H10T1/2 cells to 5aza causes the fibroblasts to commit to a muscle cell fate. This is because 5Aza is a demethylating agent that demethylates histones and CpG nucleotides which results in a change in conformation of the chromatin. This provides a favourable environment for transcription and gene expression
What does MyoD stand for
Myogenic determinant factor
Explain the experimentation that lead to the isolation of MyoD
The transcriptomes of fibroblasts cells that have been exposed to 5Aza were compared to a population of the same cells that hadn’t been exposed to it to identify the different expression profiles of the cells that conferred their commitment to the muscle cell lineage. These mRNA transcripts were used to create cDNA which allowed for the isolation of MyoD
What is meant by MyoD being a master regulatory gene and how was this discovered
MyoD can induce a transition of a fully differentiated cell into a fully differentiated cell. This was carried out by placing the MyoD gene under a constitutively active promoter and introducing the construct into terminally differentiated cells. This results in the cells switching to a fully differentiated muscle cell fate
What type of protein is MyoD and how does it interact with DNA
MyoD is a basic helix-loop-helix protein that acts as a transcription factor. Its basic amino acid domain is responsible for binding to the DNA whilst the helix is involved in dimerization with E12 and E47 proteins. MyoD binds to E-box regions contained in the promoter and enhancer regions upstream of muscle related genes
Other than MyoD what 3 other genes in the same family act to confer muscle cell fate
Myf5, myogenin, MRF4
What is meant by the term myotome
The myotome is the dorsal region of the somite that will give rise to the skeletal muscles of the ventral trunk
Initially the somitic mesoderm is pluripotent, T or F
F – it is multipotent and can give rise to cartilage, bone and skeletal muscle
What is meant by the sclerotome and how is this region formed in the somites
The sclerotome is the region in the ventral somite that will give rise to the axial skeleton. It is formed when cells in the ventral somite undergo an EMT and delaminate
How is the myotome formed
The myotome forms when dorsal epithelial cells in the somites undergo an EMT and drop beneath the somite
What structures do the myotome give rise to
Skeletal muscle of the trunk (and limbs)
Myotome cells become sandwiched between cells that remain epithelial in the dorsal somite called the dermomyotome and the cells of the ventral sclerotome, T or F
T
What are the two definitive regions of the myotome and which specific structures do these give rise to
Cells in the medial myotome give rise to the epaxial muscles of the back. Cells in the lateral myotome will give rise to the hypaxial muscles of the abdomen and limbs
MRFs are factors expressed in the myoblasts during embryogenesis with important functions in the formation of skeletal muscle, what region is their expression restricted to
The myotome
What happens when you knockout Myf5 in mice
Mice produced are still viable but do show delayed myotome formation until MyoD is expressed
What is seen in MyoD knockouts
Mice are born viable with no obvious muscle defects at birth. There is a slight delay in limb muscle development and a deficit in muscle regeneration in adults
What is meant by functional redundancy
One gene can restore the function of another gene if that gene is knocked out
What can be done to determine if Myf5 and MyoD act in functional redundancy
Double knockout mice
What is seen in the MyoD and Myf5 -/- mice
Complete absence of skeletal muscle and no presence of myoblast cells
What can be determined about how Myf5 and MyoD interact and dictate muscle generation
Myf5 and MyoD act in functional redundancy and are required to generate myoblast cells
What is seen in myogenin knockout mice
The offspring die shortly after birth from a diaphragm defect. They also have reduced density of myofibres. The mice are capable of producing myoblasts but not myotubes
What can be inferred about myogenin from its knockout in mice
Myogenin is required downstream of MyoD and Myf5 to confer muscle differentiation from myoblasts to myotubes
Which genes are expressed during each stage of muscle cell development
MyoD, Myf5 and MRF4 expressed during specification of stem cells to a muscle lineage. Myogenin expressed during differentation. MRF4 expressed again during maturation
Which region of the myotome gives rise to the muscles of the back
Epaxial myotome
Which region of the myotome gives rise the muscles of the abdominal wall
Hypaxial myotome
Which cell signalling events are required for formation of the epaxial myotome
Sonic hedgehog signalling from the notochord and floor plate and wnt signalling from the dorsal neural tube
Which cell signalling events are required for formation of the hypaxial myotome
Wnt signalling from the ectoderm and BMP4 from the lateral mesoderm
Which region contains cells that will give rise to the muscles of the limbs
Lateral myotome
MyoD and Myf5 expression if delayed in the cells that become the muscles of the limb, explain the events that lead up to this delay
Cells in the lateral myotome undergo and EMT to initiate their migration. It is only once they have reached the dorsal and ventral positions in the limb bud where they begin to differentiate
Describe the splotch mouse phenotype
Splotch mice have a loss of function of the Pax3 transcription factor and have neural tube and brain defects. Importantly for muscle development however, they have muscles of the trunk that develop normally but have a complete lack of skeletal muscles in the limbs
What can be inferred about the action of Pax3 from splotch mice data
Pax3 acts upstream of MyoD in the cells of the somite that are to migrate and give rise to muscles of the limb
How does Pax3 act
Pax3 is a transcription factor that is expressing in somitic cells and drives the expression of the c-Met receptor. c-Met binds to its ligand hepatocyte growth factor (HGF) which is synthesised by the mesenchymal cells of the limb. This acts as a chemoattractive signal to direct these cells to migrate to the limb bud
Depletion of Pax3 results in a failure of limb skeletal muscle cell progenitors to migrate to the limb bud, T or F
T
What is the name given to muscle stem cells that are retained in adult muscles and dictate muscle regeneration
Satellite cells
What is significant about the number of muscle stem cells during development and what role does this indicate
Satellite cells make up 32% of muscle nuclei at birth but this decreases to only 5% by adulthood. This indicates the role of these cells in directing postnatal muscle growth
What stimuli can trigger the induction of muscle stem cells in adults
Injury, exercise, denervation and stretching. They can also become more active in muscle wasting disease such as Duchene muscular dystrophy
What is significant about the gene expression by muscle specific stem cells in muscle regeneration
They express the same sequence of genes as is seen in development and embryogenesis firstly expressing Pax7(duplicate of Pax3) then Myf5, MyoD and Myogenin
What signalling pathway is responsible for induces MyoD expression and causing muscle differentiation
Sonic hedgehog
