The biology of muscle damage and remodelling

Question 1

Why is acute repair from muscle injury important?

Answer 1

o Normal part of hypertrophic response to exercise
o Mechanisms in reverse might cause muscle atrophy
o Recovery from sports injuries
o Repair of injury sustained in old age
o Recovering from acute trauma
o Disease

Question 2

What is actually meant by muscle ‘damage’?

Answer 2

o Commonly used when describing ‘post-eccentric exercise’ DOMS
o Usually peaks 24-48 h post-ECC, and can last 5-7d
o But… DOMS does not = damage

Question 3

Is muscle damage / DOMS required for muscle growth?

Answer 3

Studies of pre-trained (gradually ‘ramped up’ exercise) vs naïve individuals show that pre-trained group avoided DOMS but both groups displayed comparable hypertrophic responses to ECC training

Question 4

What molecular mechanism(s) co-ordinate this regeneration from muscle damage?

Answer 4

o Muscle regeneration is a highly orchestrated and highly ordered process, characterised by a predictable pattern of cellular processes, aimed at removing damaged muscle and replacing with new muscle
o i) Mild muscle injury > ii) inflammation/degeneration > iii) regeneration > iv) growth
o Starts within 1-2 h following injury and lasts for up to 2 weeks
o Site of EIMD is invaded by leukocytes
o Actively secrete cytokines and growth factors
o These amplify the inflammatory response and take part in the second phase of muscle regeneration

Question 5

Early event(s) in muscle repair?

Answer 5

o So, membranes can be damaged and repaired MUCH, MUCH quicker than whole sarcomeres / myofibres
o Raises the possibility that repairing membranes is a preceding event in muscle regeneration

Question 6

What proteins might influence/ regulate the early regeneration process?

Answer 6

 Dysferlin: i. Ca2+ increases affinity of dysferlin for phospholipids ii. Calpains also activated iii. Encourages recruitment of internal vesicles for membrane repair
 Mitsugumin 53: MG53 is a striated muscle-specific tripartite motif (TRIM) family protein; MG53 required for cell membrane resealing; mg53-/- mice display progressive dystrophy

Question 7

There’s a mechanical stimulus, right!!??

Answer 7

Teleologically, muscle regeneration from exercise likely involves activation of mechanically sensitive signalling pathways

Question 8

Mechanotransduction at centre stage?

Answer 8

 Link between mechanosensation and muscle growth/ atrophy is not new
 Transmembrane proteins at Z-lines and M-lines (e.g. integrins) sense mechanical stretch from ECM…
 …then signals to protein synthetic and breakdown machinery to alter protein turnover (and thus muscle size, chronically)
 Shown for a very limited number of mechanotransduction signalling proteins

Question 9

Can we simplify the adhesome?

Answer 9

This likely represents those proteins that permanently reside at the cell membrane

Question 10

What adhesome-associated pathways might regulate recovery from EIMD?

Answer 10

Exercise > disrupts adhesomes > signalling response(s) + concurrent sarcomere disruption > adhesomes repair > sarcomeres then repair

Question 11

Do adhesomes co-ordinate / initiate the muscle repair process?

Answer 11

Certain adhesome-associated enzymes are specialised at degrading proteins - provides a mechanism for bulk degradation of muscle proteins irreparably damaged during exercise

Question 12

Transcriptional regulation by adhesomes in an exercise-responsive manner?

Answer 12

o A number of adhesome components are zinc finger containing LIM domain proteins – Zyxin, PINCH, Paxillin
o Includes: transcription factors; cytoskeletal proteins; kinases; adaptor proteins
o Involved in gene transcription and cytoskeletal organisation

Question 13

Role for MEF2?

Answer 13

MEF2A mediated resynthesis of adhesome proteins might be an early event in muscle regeneration from EIMD

Question 14

So what might be the physiological relevance of adhesome function translating in regulation of human muscle repair?

Answer 14

DE genes almost identical between contraction modes, despite known divergent chronic adaptation to CON and ECC training

Question 15

In muscle generation, where do molecular regulators of repair go, and why?

Answer 15

E.g. regulators of MPS likely translocate to where they are needed (i.e. sites of active protein synthesis), mRNA of specific genes being translated into new proteins must translocate from the nuclear site of transcription, to sites of active MPS

Question 16

Does immunofluorescent microscopy analysis precede metabolic / functional muscle recovery from EIMD?

Answer 16

o	Fluorescently-tagged antibodies against predicted regulators of regeneration 
o	Examine (e.g.) disruption and subsequent repair of adhesomes

Question 17

Muscle hypertrophy induced by myostatin inhibition accelerates degeneration in dysferlinopathy (Lee et al., 2015)

Answer 17

These findings suggest that depending on the disease context, inducing muscle hypertrophy by myostatin blockade may have detrimental effects, which need to be weighed against the potential gains in muscle growth and decreased fibrosis.

Question 18

Calpains Mediate Integrin Attachment Complex Maintenance of Adult Muscle in Caenorhabditis elegans (Etheridge et al., 2012)

Answer 18

These results demonstrate that calpains are required for proper assembly and maintenance of integrin attachment complexes. Taken together our data provide in vivo evidence that a calpain-based molecular repair mechanism exists for dealing with attachment complex disruption in adult muscle. Since C. elegans lacks satellite cells, this mechanism is intrinsic to the muscles and raises the question if such a mechanism also exists in higher metazoans.

Question 19

Dynamic distribution of muscle-specific calpain in mice has a key role in physical-stress adaptation and is impaired in muscular dystrophy (Ojima et al., 2010)

Answer 19

These findings indicate that the stretch-induced dynamic redistribution of p94 is dependent on its protease activity and essential to protect muscle from degeneration, particularly under conditions of physical stress. Furthermore, our data provide direct evidence that loss of p94 protease activity can result in LGMD2A and molecular insight into how this could occur.

Question 20

Rapid muscle-specific gene expression changes after a single bout of eccentric contractions in the mouse (Barash et al., 2004)

Answer 20

In conclusion, this experiment developed and validated a model for ECs in the mouse and completed microarray analysis of the TA muscle 48 h after the EC bout. The analysis uncovered three interesting muscle-specific genes (MARP1, MARP2, and MLP) whose time-dependent regulation was later confirmed by QPCR analysis to be specific to the EC treatment as opposed to either isometric activation of the muscle or passive stretch. The role of these genes in the biological response to ECs in skeletal muscle remains to be determined, but their rapid regulation indicates that the role is an important upstream event associated with muscle response to EC.