L11.2 Transcriptional regulation of skeletal muscle stem cells

Question 1

Basic level of transcriptional regulations

Answer 1

• Binding of transcription factors to 5’ promoter region
• Binding of RNA polymerase to transcriptional start sites
• Epigenetic regulations
• *(not until a later stage) 3’ UTR regulation

Question 2

3 epigenetic regulations of transcription

Answer 2

• DNA methylation
• Histone modification
• RNA-based mechanism (not covered)

Question 3

DNA methylation

Answer 3

• Adding methyl groups to cytosine, often at promoter regions (repression)
• Generally at C-G rich sites (CpG islands)
• S-adenosylmethionine (SAM) donates methyl group
• Produced from serine and folate (from diet)
• S-adenosylhomocysteine (SAH) released

​Facilitated by:

• DNA methyl transferases (Dnmt)
• Attaches SAM to DNA → release SAH

Demethylation:

• Targeted DNA demethylation via DNA demethylases (TeT)
• Dependent on α-KG → release methyl group → produces succinate

Question 4

How is DNA methylation balanced?

Answer 4

• Glycolysis produces serine → enter 1 carbon cycle → produce SAM → ↑ global DNA methylation
  
  • Glycolysis also produces α-KG in TCA cycle → ↓DNA methylation
  • Leads to balance of DNA methylation (carefully regulated)

Question 5

Satellite cells relevance to DNA methylation

Answer 5

• Sat cell activation → enter cell cycle → ↑ DNA methylation
  
  • As cells become more committed → ↑methylation (silence ‘stemness’ of cells)

Question 6

Histone modification

Answer 6

• Core histone: H2A, H2B, H3, H4
• Each have long tails → undergo post-translational modification

Question 7

Histone methylation

Answer 7

• Add methyl group to specific lysine (80%) or sometimes arginine
• Forms tightly packed chromatin → prevents transcription factors binding & inhibits transcription (some exceptions with targetted modifications)
• Nomenclature: H3K27me (H3=histone 3; K27=lysine 27; me=methylated)

​Facilitated by:

• Histone methyltransferase (HMT - Ezh2)
• Requires SAM → binds to histone tail, deposits CH3 → releases SAH

Demethylation:

• Lysine demethylase (KDM - LSD & JHDM)
• JHDM requires α-KG & releases succinate
• LSD requires FAD+ (from ETC) → produces FADH2

Question 8

Satellite cell relevance to histone methylation

Answer 8

• ↑ expression of HMT Ezh2 & H3K27me3 marker as sat cells go from quiescent to prolif

Question 9

Histone acetylation

Answer 9

• Results in loosely packed chromatin → allows transcription factor binding & promotes transcription

Facilitated by:

• Histone acetyl transferase (HAT - p300 & GLN5)
• Requires acetly-CoA (from citrate) → CoA

Deacetylation:

• Histone deacetylases (HDACs & SIRT)
• HDAC: H2O (present in nucleus) → acetate (HDAC)
• SIRT: NAD+ (From ETC) → NAM
• SIRT becomes charged as NAD+ binds → able to release acetyl group

Question 10

Satellite cell relevance to histone acetylation

Answer 10

• NAD+ consumed by glycolysis: ∴↑glycolytic cells → ↓[NAD+]
• Quiescent sat cells: predominantly oxidative
  
  • Once activated → becomes more glycolytic → ↓[NAD+] → ↑acetylation