L12 - Myogenesis Flashcards
What are the 4 functions of skeletal muscle?
Movement and posture - Simple of coordinated movement – allows standing Communication - Speech, expression and writing Maintains body temperature - Heat released through muscle contraction participates in control of body temperature Respiration - The importance of the diaphragm
What are some examples of muscle wasting disease?
Injuries
Ageing
Muscle degenerating disease – dystrophy
What are the 4 steps the produce muscle?
- Stem cells
- Muscle progenitor cells – myoblast
- Differentiated cells – myotubules
- Myofibres
What are the 3 process required to produce muscle?
Specification/determination
Differentiation – specific to skeletal muscle. Also, activation of a series of gene
Maturation
When was MYOD discovered?
Discovered in 1987 – myogenesis determination gene
What was the first step in the discovery of MyoD?
Started with a fibroblast cell line – C3H10T1/2
- Under conditions can give rise to a variety of cells
- Need to be cultured in the presence of the drug – 5Aza
- Demethylating agent
- Release chromatin from a silencing state
- Methylations allows cell to control availability of DNA for transcription
- Demethylating agent
Differential sequencing method for the discovery of MyoD?
- Untreated fibroblast cell line and fibroblast cell line treated with 5Aza
- Gave two pools of cDNA
- Subtracted cDNA enriched in muscle-specific genes
- Screening using myoblast specific probes isolated MyoD cDNA
What is MyoD?
A master regulatory gene
How was the role of MyoD discovered?
- Introduced the gene into a viral vector
- Vector then introduced into a variety of cells
- The cells rapidly lost their differentiation characteristics and were converting into myoblasts
- This MYOD gene can control the fates of the cells
- Want to know if this gene is expressed in progenitor cells and when
What is the structure of the MyoD protein family?
Structure of bHLH proteins
- Basic domain – binds to DNA
- Helix-loop-helix domain – forms dimers with other proteins - E12 and E47 proteins
What are some examples of the MyoD protein family?
MyoD
Myf5
Myogenin
MRF4
What is the function of MyoD proteins?
Transcription activators
Form heterodimers with E12 or E47
Binds to E box sequence – CANNTG
What are the four subdivisions of mesoderm?
Intermediate mesoderm
Axial mesoderm
Paraxial mesoderm
Lateral plate mesoderm
What does paraxial mesoderm form?
Segmentation into somites which then form
- Myotome – skeletal muscle
- Sclerotome – cartilage
- Syndrotome – tendons
- Dermatome – skeletal muscle progenitors
Where do skeletal muscles originate from?
Dermatome
Where are the different sub-divisions of paraxial mesoderm found in the embryo?
Ventrally - epithelial-mesenchymal transition – sclerotome
Dorsally – cells stay as epithelial – dermatome
What does the dermatome contain?
Dermatome contains progenitor cells for skeletal muscles of the trunk and limbs
Skeletal muscle progenitors express Pax3
In the trunk, Pax3-positive cells contribute to the myotome
What are the two myotome domains?
Epaxial - medial
Hypaxial - lateral
Where are MRFs expressed?
They are expressed in myoblasts during embryogenesis in
- Head muscles
- Branchial arch
- Wing bud
- Epaxial
- Hypaxial
What is the timing of MRF activation during embryonic development?
All expressed in muscle progenitors
All expressed at the time of formation of skeletal muscle
What happens in the Myf5 knockout?
Mice are viable
No obvious muscle defect at birth
During embryogenesis delay in myotome formation until the onset of MyoD expression
Myf5 -/- cells migrate aberrantly into sclerotome and dermatome
What happens in the MyoD knockout?
Mice are viable
No obvious muscle defect at birth
During embryogenesis increased Myf5 expression in somites compensated for lack of MyoD
Slight delay in limb muscle development and deficit in muscle regeneration in adult mice
What happens in the Myf5/MyoD knockout?
Complete absence of skeletal muscles
No presence of myoblasts
Result - Myf5 or MyoD required to generate myoblasts
What happens in the myogenin knockout?
Mice die shortly after birth from diaphragm defect
Reduced density of myofibres replaced by myoblasts
Result - Myogenin is requires for muscle differentiation
What is muscle commitment and differentiation mediated by?
Mediated by the myogenic regulatory factors
- Myf5 and MyoD – determination
- Myogenin – differentiation
- MRF4 – maturation
What mouse mutant showed the migration of hyperaxial muscle cells in the limb?
Splotch mouse
- Naturally occurring mutation
- Deletion in Pax3 gene
- Loss of Pax3 function
What happens if their is weak regeneration of satellite cells?
Muscular dystrophies
Sarcopenia
Cachexia
What happens if their is perturbed development of satellite cells?
Cancer
Hyperplasia
What are satellite cells?
Skeletal muscle specific stem cells
Where do satellite cells originate from?
Somites
What % of muscle cells do satellite cells represent?
32% of muscle cells in embryo
5% of muscle cells in adult muscle
What do satellite cells do during embryonic development?
During embryonic development they do not engage in the process all other progenitors do
- Position themselves under the basal lamina of muscle fibre
- They are quiescent
Why are satellite cells known as stem cells?
Will always have a subset of these cells that do not differentiate and return to quiescent
- Allow us to continue to repair muscle
What are satellite cells activated by?
Muscle injury, exercise, stretch
What happens within satellite cells following activation?
- Induction of Myf5 or MyoD
- Expression of both Myf5 and MyoD
- Proliferation
- Differentiation and fusion to existing fibres
What are the two signalling pathways controlling muscle gene activation?
Epaxial muscle
Hypaxial muscle
How is epaxial muscle generated?
All progenitors express Pax3
Cooperation between Shh and Wnt from neural tube and notochord to drive expression of Myf5 and MyoD
How is hypaxial muscle generated?
All progenitors express Pax3
Wnt signals induces Myf5 and MyoD in cells entering the lateral myotome
BMP4 induces Pax3, and represses Myf5 and MyoD in cells fated to migrate in the limb bud
How is Pax3 involved in the migration of muscle cells into the limb bud?
Pax3 drives expression of cMET in limb muscle progenitor cells
What is the role of cMET?
cMET is the receptor for growth factor – HGF/SF
1. HGF is a chemoattractant in the limb mesenchyme
2 Triggers migration of cells into the limb mesenchyme
3 Follow these routes then proliferate – ventral or dorsal
4 Only once proliferated do they begin to be determined and express Myf5 and MyoD
