Lecture 32 Flashcards
Where are satellite cells placed?
At the periphery where they can easily recieve signals from both the muscle fibre, and from the blood supply
It colocalises around the capillaries.
What phase do satellite cells normally remain in when unneeded?
G0 phase
Upregulates Pax7 in response to injury to enter G1
Define epigenetics
Heritable changes in the absence of a change to the genetic sequence
Describe 3 mechanisms of epigenetics
DNA methylation (and demethylation)
Histone modification
RNA based modification (not assessed)
In ehich nucleotide does methylation occur almost exclusively?
CpG dinucleotide (in CG rich domains, called CpG islands, as well as promoter regions)
Describe the mechanism of DNA methylation
S-Adenosyl Methionine (SAM) converts to S-Adenosyl Homocysteine (SAH) after donating its methyl group to the DNA.
It is mediated by DNA methyl transferases, called dmnts
They attach to SAM and bind to the DNA via a binding partner, attaches SAM to the DNA and releases SAH, methylating the DNA.
Folate/serine is needed for SAM production
Why is folate over the RDI recommended in women?
Folate is needed for SAM production, which methylates DNA - important for correct embryogenesis
Describe the mechanism of DNA demethylation
Mediated by DNA demethylases, tets (1-3)
a-Kg (alpha ketogluterate) needed by tet for correct action.
Similar to methylation, tet binds to a-Kg, binds to the methylated DNA, and releases succinate, demethylating it.
Why is serine so important for methylation?
Serine is an intermediate of glycolysis, and able to enter the 1-carbon cycle
The product of the 1-carbon cycle is SAM
What is the effect of glycolytic flux or folate increase?
Increased global DNA methylation, provided dmnt is in sufficient quantity
Describe what can happen to a-Kg
a-Kg can export from the mitochondria and import to the nucleus with enough glucose and oxygen. If sufficient tet is present, increased levels of global demthylation is seen.
How does dmnt affect proliferating satellite cells?
Dmnt concentrations are 4-5 fold in proliferating satellite cells versus quiescent cells, indicating DNA methylation is neccessary.
Tet is underexpressed.
How do proliferating satellite cells ensure methylation?
Serine production via glycolysis and SAM via the 1-carbon cycle - all enzymes necessary are upregulated 10-15 fold in activated satellite cells.
Which genes are methylated in satellite cells?
Quiescent genes, as well as non-myogenic genes
What is a histone?
An octomer of 2 each - H2A, H2B, H3 and H4
Which amino acid does histone methylation most often involve?
Lysiine (~80%), occasionally arginine
results in tightly packed chromatin, preventing transcription factor binding (there are exceptions)
Describe the nomenclature of histone modifying agents
histone unit - amino acid - action
ie. H3K27me - methylates H3 at lysine 27
What is histone methylation mediated by, and describe its mechanism.
Mediated by histone methyl transferases, over 20 present.
Mechanism
Ezh2 (a transferase) binds to SAM, binds to the histone tail, transfers the methyl group to release SAH, inhibiting transcription.
What is histone demethylation mediated by, and describe its mechanism.
Mediated by lysine demethylases (KDMs)
Two families - HDM (similar to tet, requires a-Kg, releases succiniate)
- LSD (lol) - requires FAD+, releases FADH2
How does increased histone methylation in satellite cells occur?
Ezh2 upregulation, not expressed in quiescence.
What effect does histone acetylation have on chromatin and transcription?
It increases transcription by resulting in less packed/dense chromatin.
What is histone acetylation mediated by, and describe its mechanism.
Mediated by histone acetyl transferases (HATs) - major two: p300 & GCN5
Requires Acetyl-CoA, and releases CoA.
Acetyl-CoA is derived from citrate, off the TCA cycle, which is converted to Acetyl-CoA in the nucleus.
What is histone de-acetylation mediated by, and describe its mechanism.
Mediated by HDAC and SIRT
HDAC requires water and releases acetate
SIRT requires NAD+ and releases NAM (nicotinamide)
NAD+ is sourced from the ETC, and charges SIRT
Describe how satellite cells can control how and when methylation and acetylation occurs, whilst inhibiting demthylation and deacetylation
Satellite cells become highly glycolytic
Glycolysis can be used to reduce NAD+ availability, and deacetylation is reduced.
It also increases serine production and conversion to SAM via the 1-carbon pathway, resultng in more methylation.
