Muscle protein turnover

Question 1

structure of a muscle

Answer 1

1. muscle –> epimysium (dense irregular connective tissue)
2. fasicle –> perimysium (fibrous connective tissue)
3. muscle fiber –> endomysium (loose/areolar connective tissue)
4. myofibril
5. myofilament
6. sarcomere

Question 2

microstructure of a myofiber

Answer 2

• massive multinucleated cell –> fusion of myoblasts
• nuclei right beneeth the sarcolemma
• many glycosomes
• myoglobin
• sarcoplasmic reticulum (calcium)

Question 3

sarcomere

Answer 3

• sarcomere is area between 2 Z-discs: here the actin filaments attach
• in the middle is the H zone where the myosin filamants attach –> the M line bisects this here myomesin attaches
• the A band is of unchanging lenth –> the length of the myosin
• the H zone around the M line is the light zone where there is no actin myosin overlap –> shrinks during contraction
• the I-band is the same as the H zone but around the Z-disc

Question 4

embryonal myogenesis

Answer 4

• Skeletal muscle tissue is derived from the mesoderm –> somites
  • At the most outer part of the somites the
  dermomyotome this contains mononucleated
  skeletal muscle stem cells (pax3 and 7)
  • If they stay in the somite they will from the back
  muscles otherwise they have to migrate
  elsewhere to form different muscle groups

Question 5

myonuclear turnover

Answer 5

• The nuclei in myofibers are quiescent they no longer can divide still more nuclei are needed when cytoplasm volume increases (hypertrophy)
  • Multinucleated myofibers can sense the amount
  of cytoplasm per nucleus in the cytosol when this
  reaches the maximum more nuclei are produced
  by satellite cells –> to maintain myonuclear
  domain
• When the myofiber increases in size and more nuclei are needed satellite cells that are separate from the myofiber cytoplasm are activated (pax7) –> they divide asymmetrically and 1 of the daughter cells fuse into the existing fiber to add a nucleus while they other daughter satellite cell remain as a stem cell (repopulation)

Question 6

protein turnover

Answer 6

• Muscle hypertrophy can be stimulated by resistance training, growth hormones, testosterone, anabolic steroids, etc.
• A very important factor is IGF-1 (insulin-like growth factor 1) which can be produced by the muscle itself in response to resistance training (autocrine signalling) or by the liver as a hormone (influences more than just muscle tissue since it enters the blood stream)
  • IGF-1 binds to its cognate receptor on muscle
  cells which causes protein synthesis (anabolism)
  and inhibition of protein degradation
• IGF-1 causes hypertrophy via:
  • Satellite cell activation: addition of new
  myonuclei
  • mRNA translation: increased protein synthesis
  • UPS & autophagy downregulation: inhibition of
  protein synthesis
• IGF-1 initiates mRNA translation by activating eLF2 and eLF4 proteins: these are essential in first recognising and binding to met-tRNA (eLF-2) and recognising the cap on mRNA that has to be translated (eLF-4)
• IGF-1 binding to receptors causes activation of Akt which then phosphorylates GSK-3b, lifting the inhibition from eLF-2
  • Activated Akt also results in activation of mTOR
  which causes activation of eLF-4
  mTOR also leads to increased translational
  capacity by activating P70s6k which activates S6

Question 7

muscle plasticity

Answer 7

hypertrophy –> larger cell volume due to protein synthesis and followed by possible satellite cell fusion
atrophy –> protein degradation

Question 8

amino acids and muscle hypertrophy

Answer 8

additional to growth factors (insulin, IGF, etc.), steroids, etc. amino acids can induce protein sythesis by lifting inhibition of off Rheb and Rag proteins that inhibit mTOR and by translocating mTOR to a lysosome membrane

Question 9

muscle proteolysis (UPS)

Answer 9

• The ubiquitin proteasome pathway first processes the attachment of ubiquitin to a target protein which involves three critical enzymes:
  • The ubiquitin activating enzyme or E1 enzyme
  • The ubiquitin conjugating enzyme or E2 enzyme
  • The ubiquitin ligase or E3 ligase
  1. Ubiquitin needs to be activated from its precursor by adding to E1 through an ATP-dependent manner
  2. Activated ubiquitin is then transferred to the ubiquitin-conjugating enzyme E2
  3. E2 interacts with E3 to identify the substrate (very specific), and by which ubiquitin is attached to the target protein  this step is repeated to create a polyubiquitin tail in the target enzyme
  • In muscles MuRF1 and Atrogin-1 are very
  important E3s
  4. The ubiquitin conjugated protein is recognized by the 26S proteasome, and through which, the target protein is degraded to small peptides or amino acids by the proteasome enzymes
  • Of note, ubiquitin can be released by the
  deubiquitinating enzymes (DUBs), and therefore,
  ubiquitin conjugation to target substrates is a
  reversible process

Question 10

regulation of Murf1 and atrogin/MAFbx

Answer 10

• Inducible transcription factors regulate the UPS pathway:
  • Foxo: when active and when in the nucleus it
  will cause transcription of MuRF1 and Atrogin
  genes which cause muscle atrophy
  Under healthy conditions Foxo is phosphorylated
  (inhibited, because it cannot enter nucleus) by
  Akt which is activated via insulin or IGF-1
  signalling –> so Foxo is activated by starvation
  (no insuline) and disuse (no IGF)
  • NF-kB
  • Glucocorticoid receptors

Question 11

muscle proteolysis (autophagy)

Answer 11

• Energy crisis, growth factors (certain ones) nutrient deficiency and stress signals can:
  
  1. Initiate formation of a phagophore from the ER
     membrane
  2. This phagophore engulfs (when it is forming)
     some cellular components –> highly regulated
  3. The now called autophagosome fuses with a
     lysosome causing degradation of the engulfed
     cellular components (can be whole mitochondria
     so way larger scale than the UPS)
• mTOR can inhibit the initiation of autophagy
• FoxO stimulates all steps of autophagy