Satellite Cells 1 Flashcards
Why are stem cells(skeletal cells) important for skeletal muscle
skeletal cells are the stem cells of the skeletal cells cells
Genetically and metabolically very active and have a high degree of plasticity -
Hypertrophy
Hyperplasia
Fibre type switch (contraction type)
Metabolic adaptation
High regenerative capacity
What makes the skeletal muscle cell cycle different to other cell cycles
Mitotically inactive but have many nuclei
Meaning they can’t divide anymore
What is myogenesis
The formation of skeletal muscle tissue
What is the main stages of myogenesis
Mesodermal stem cells –> determined myoblasts –> terminal differentiation occurs (with growth factors) –> differentiated multinucleated myotubes
What is the process of embryonic myogenesis
Myotome derived myoblasts migrate into the limb buds
Proliferation and initiation of the myogenic program starts the formation of limb and skeletal muscle development
What controls the myogenic process
myogenic regulatory factors- they are a family of proteins
What are the two myogenic regulatory factors (MRF) seen at the early stages of development from mesodermal stem cells into determined myoblasts
MyoD
Myf 5
What are the myogenic regulatory factors (MRF) seen at fusion
Myogenin
What are the myogenic regulatory factors (MRF) seen during terminal differentiation (determined myoblasts to differentiated myotubes)
Myogenin
MyoD
MyF5
MRF4 (expressed in myofibril)
What is the role of the MRF family
They are BHLH proteins (helix-loop-helix) that bind to DNA sequences (E box) in muscle promoters
What regulates MRFs
They autoregulate
What are E-boxes
Specific DNA sequence -
CANNTG
N - can be any base
Why are bHLH proteins important
Needed for dimerisation (2 smaller similar molecules combining to form a dimer)
Needed for DNA binding
Therefore the The bHLH domain is essential for activation of myogenesis.
What can bHLH proteins formed dimers with
Myogenic bHLH can form dimers with other myogenic bHLH factors or with bHLH factors of the E12 or E47 family
E12 and E47 increase myogenesis
Dimerisation with bHLH and what causes inhibition of DNA binding
Factor Id
How can bHLH factors be inhibited
Phosphorylation inactivates myogenic transcription factors by inhibition of DNA binding
Phosphorylation is activated by growth factor signals which activate protein kinase C to cause phosphorylation
How does factor Id disrupt bHLH activity
Id disrupts dimerisation and DNA binding of myogenic bHLH proteins by ‘blocking’ E12 and E47 from binding. By blocking these from binding this results in no myogenic regulatory genes being transcribed.
Id is expressed on a high level in response to growth factors
How do bHLH proteins cause myogenesis
E12/E47 is activated by inhibition of growth factors which allows it to bind with myo (Myogenic regulatory gene promoters).
Myo and E12/E47 then binds downstream to genes of terminal differentation
What is MEF2 and its role
A myogenic transcription factor - During development and differentiation MEF2 transcription factors amplify and maintain the expression of other myogenic transcription factors. Essentially regulates myogenesis
How is terminal differentiation (determined myoblasts to differentiated myotubes) started
Removing growth factors
What has been found out on MRFs knock-out studies of mice regarding their ability to compensate
MyoD -/- = normal muscle
Myf5 -/- = muscle delayed but normal
Both knocked out - no muscle present
Shows level of compensation between both MRFs
Define commitment
move cell to new location, still maintains its original fate
Define determination
in context of muscle, expression of Myf 5/MyoD
Means the cell becomes a specific muscle cell - e/g actin
What are satellite cells
Mononucleated cells which rest between the muscle fibre and basal lamina
How are satellite cells formed
Determined myoblasts that have just undergone determination from mesodermal stem cells are put into a quiescent (rest) stage by MyoD and myf5 to form a satellite cell
They are formed during embryogenesis but are activated and divide at various points in life.
What is the role of satellite cells
Used for muscle fibre repair and reconstruction by differentiating and proliferating when growth factors are removed.
They are very dominate, but as soon as become activated
What activates increased satellite cells
Removal of growth factors
How do satellite cells go into the quiescent stage
Pax7 (paired box gene 7) is crucial for satellite cell formation
How do we know pax7 is crucial for satellite cell formation
Pax7 -/- transgenic animals die soon after birth due to missing satellite cells
What is the role of Pax7
Paired box gene Transctiption factors are important for embryonic development
Pax7 is also essential for post-natal muscle development and maintenance
What did Mansouri et al 1996 show about pax7 -/- mice
50% smaller and died within 2 weeks
Had thinner diaphragm development
What three genes activate a quiescent satellite cell
CD34
Pax7
Myf5 - shows that it is destined to become a muscle cell
What genes are important when satellite cells become a myoblast
Myf5
MyoD
(Pax7 becomes less important)
This makes them harder to study as more difficult to track with the constant change of markers
What gene is important for the maturation of a myofibre/fusion
Myogenin
How do satellite cells become activated and differentiated with regard to growth factors
Downregulate growth factor receptors when growth factor decreases and then differentiate from there.
What happens in satillete cell quiensence
Dormant, small cytoplasm resulting in very little metabolism occurring.
What has been shown about the link between satellite cells and myofibre nuclei and its link to age-related muscular atrophy
EXTRA READING - Brack 2005
Using myoD-knockout mice, it was found that a satellite cell defect causes inadequate nuclear replacement within myofibres. This triggers cytoplasmic atrophy in the muscle fibres because of an increase in area covered per nuclei.
It may partially explain muscle atrophy during ageing.
When were satellite cells discovered
1961 by mauro looking at frog myofibres.
What is believed to inhibit satellite cells, causing them to become quiscent.
WIDER READING - McCroskery 2003
Myostatin - up-regulate a cyclin-dependent kinase inhibitor called p21 and thereby inhibit the differentiation of satellite cells
How has light exercise been shown to counteract muscle atrophy
EXTRA READING - KADI 2005
light, endurance training regimen may be useful to counteract the age-correlated satellite cell decrease therefore help retain muscle
What is the role of mechano growth factor (IGF subtype)
Extra reading - Yang 2002
Activation and proliferation of satellite cells
What is the role of IGF-IEa (IGF subtype)
Extra reading - Yang 2002
Differentiation of the proliferated satellite cells
What is the role of HGF in satellite cells
EXTRA READING - Allen 1995
HGF is released during muscle injury
Causes satellite cell proliferation
How does FGF-6 play a role in satellite cells
EXTRA READING - Floss 1997
FGF-6 knockout in mice has impaired satellite cell proliferation and a subsequent defect in muscle regeneration in response to a crush injury.
How are satellite cells formed?
Formation of the muscle involves progression of precursor cells through different stages of development:
determination-> quiescence
-> proliferation-> differentiation
