Lecture 32

Question 1

Where are satellite cells placed?

Answer 1

At the periphery where they can easily recieve signals from both the muscle fibre, and from the blood supply
It colocalises around the capillaries.

Question 2

What phase do satellite cells normally remain in when unneeded?

Answer 2

G0 phase

Upregulates Pax7 in response to injury to enter G1

Question 3

Define epigenetics

Answer 3

Heritable changes in the absence of a change to the genetic sequence

Question 4

Describe 3 mechanisms of epigenetics

Answer 4

DNA methylation (and demethylation)
Histone modification
RNA based modification (not assessed)

Question 5

In ehich nucleotide does methylation occur almost exclusively?

Answer 5

CpG dinucleotide (in CG rich domains, called CpG islands, as well as promoter regions)

Question 6

Describe the mechanism of DNA methylation

Answer 6

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

Question 7

Why is folate over the RDI recommended in women?

Answer 7

Folate is needed for SAM production, which methylates DNA - important for correct embryogenesis

Question 8

Describe the mechanism of DNA demethylation

Answer 8

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.

Question 9

Why is serine so important for methylation?

Answer 9

Serine is an intermediate of glycolysis, and able to enter the 1-carbon cycle
The product of the 1-carbon cycle is SAM

Question 10

What is the effect of glycolytic flux or folate increase?

Answer 10

Increased global DNA methylation, provided dmnt is in sufficient quantity

Question 11

Describe what can happen to a-Kg

Answer 11

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.

Question 12

How does dmnt affect proliferating satellite cells?

Answer 12

Dmnt concentrations are 4-5 fold in proliferating satellite cells versus quiescent cells, indicating DNA methylation is neccessary.
Tet is underexpressed.

Question 13

How do proliferating satellite cells ensure methylation?

Answer 13

Serine production via glycolysis and SAM via the 1-carbon cycle - all enzymes necessary are upregulated 10-15 fold in activated satellite cells.

Question 14

Which genes are methylated in satellite cells?

Answer 14

Quiescent genes, as well as non-myogenic genes

Question 15

What is a histone?

Answer 15

An octomer of 2 each - H2A, H2B, H3 and H4

Question 16

Which amino acid does histone methylation most often involve?

Answer 16

Lysiine (~80%), occasionally arginine

results in tightly packed chromatin, preventing transcription factor binding (there are exceptions)

Question 17

Describe the nomenclature of histone modifying agents

Answer 17

histone unit - amino acid - action

ie. H3K27me - methylates H3 at lysine 27

Question 18

What is histone methylation mediated by, and describe its mechanism.

Answer 18

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.

Question 19

What is histone demethylation mediated by, and describe its mechanism.

Answer 19

Mediated by lysine demethylases (KDMs)
Two families - HDM (similar to tet, requires a-Kg, releases succiniate)
- LSD (lol) - requires FAD+, releases FADH2

Question 20

How does increased histone methylation in satellite cells occur?

Answer 20

Ezh2 upregulation, not expressed in quiescence.

Question 21

What effect does histone acetylation have on chromatin and transcription?

Answer 21

It increases transcription by resulting in less packed/dense chromatin.

Question 22

What is histone acetylation mediated by, and describe its mechanism.

Answer 22

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.

Question 23

What is histone de-acetylation mediated by, and describe its mechanism.

Answer 23

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

Question 24

Describe how satellite cells can control how and when methylation and acetylation occurs, whilst inhibiting demthylation and deacetylation

Answer 24

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.