Skeletal muscle plasticity frommolecules to training protocols

Carlo ReggianiUniversity of Padova

Flexibility

The same muscles used for differenttasks thanks to neural control (m.u.recruitment and discharge pattern)

Plasticity

Skeletal muscles change theirstructure and function to bettercope with their tasks

Plasticity is regulation of muscle fibre type, size (and possibly number )

Plasticity is regulation of muscle fibre type, size (and possibly number )

Adaptative change of muscle fibre size

atrophy

hypertrophy

Disuse/inactivity

Fasting

Cachexia

Cytochines

Myostatin

Load/resistance training

IGF-1

Androgens

Resistance training

load

Nervous system learning

Improved MU recruitment and MN discharge

Muscle fibre hypertrophy

Resistance training

load

Nervous system learning

Improved MU recruitment and MN discharge

Muscle fibre hypertrophy

Load sensors

Signallingpathways

↑Protein synthesis

↓Protein degradation

Load sensors: transducers of tension into chemical signals

Costameric proteins: integrins - FAK

Sarcomeric proteins: titin – titin kinase

Signalling pathways for hypertrophy

Three models of hypertrophy induced without load:

�IGF 1 overexpression

�AKT overexpression

�Myostatin knock out

v

Spontaneous mutations resulting in muscle hypertrophy

Myostatin mutation in cattle

Myostatin mutation in humans

AKT power !Hypertrophy induced in muscle fibres * transfected in

vivo with a plasmid over-expressing AKT

*

*

*

**

*

*

*

IGF-1 and AKT pathways are linked together

plasma membrane

SR

myofibrils

cytosol,mitochondria

nucleus

AKT

IGF-1

IGF-1 receptor +

PI3 K

AKT

FOXO- P

FOXO

Atrogin (MAFbx)

4E-BP1

eIF-4E

Proteinsynthesis in ribosomes

mTOR

S6K

S6

Proteindegradation in proteasome

AKT increases protein synthesis at ribosomes and inhibits ubiquitinizationand protein degradation

Inducible AKT overexpression

No increase in proportion of new myonuclei generated duringhypertrophy development

Only protein accumulation or also myonucleiaddition to achieve hypertrophy ?

Only protein accumulation or also myonucleiaddition to achieve hypertrophy ?

8 weeks resistance training – upper limbs, young subjects (19-29 y)

pre post 0

500

1000

1500

myo

nucl

ei /

mm

3

16 weeks knee extension resistance training

Only protein accumulation or also myonucleiaddition to achieve hypertrophy ?

Plasticity is regulation of muscle fibre type, size (and possibly number)

Myosin isoforms as markers of fibre type

Myosin isoforms can be identified with ATPase staining, antibodies, gel electrophoresis, PCR

Myosin is the molecular motor of muscle contraction

Myosin isoforms are associated with distinct values of speed of contraction, power output, ATP consumption rate

Fibre types in skeletal muscle

Slowfibres

Fastfibres

2A

2X

2B

Contractionspeed

Resistance tofatigue Metabolism

low

high low

oxidative

glycolytic

high

In human muscles only 1, 2A and 2X fibres

Effects of motor neuron silencing in human skeletal muscle

normal

spinal cordinjury

slow myosin fast myosin

J. Andersen – S. Schiaffino

Activity and innervation are necessary for slow fibre expression in regenerating muscles

Inn Den

Stainingfor slow myosin(soleus)

When denervated, regenerating muscles becomehomogeneously fast

myc-cain MyHC-slow

*

* *

*

If calcineurin is blocked by expression of the peptide inhibitor cain/cabin-1,the expression of MyHC-slow is blocked in regenerating rat soleus

plasma membrane

SR

myofibrils

Gene transcription

cytosol,mitochondria

nucleus

Searching for transcription factors involved in excitation-transcription coupling: calcium released from SR can control transcription ?

calcineurin

NFAT

fiber type

calmodulin

calcium

Calcineurin

P

NFAT

P

Kinases

Cytoplasm Nucleus

TranscriptionNFAT NFAT

NFAT

Tibialis anterior

Fused tetanus at 150 Hz and unfused tetanus at 20 Hz

Only repeated trains at 20 Hz (slow stimulation pattern) induces NFAT nuclear translocation

Nuclear translocation of NFATc1-GFP induced by 20Hz (slow-like) but not 150Hz (fast-like) stimulation

20Hz 150Hz

conclusion

• Muscle plasticity is highly specific• Depending on load and activation pattern

distinct intracellular pathways are activated

• This leads to the required changes in muscle fibre size and type

acknowledgments

• VIMM – Padova– Stefano Schiaffino

– Marco Sandri– Bert Blaauw

• Dept of Anatomy and Physiology (University of Padova)– Luana Toniolo

– Marta Canato

Supported by

MYOAGE – VII EU framework

MIUR – Italian Ministry of University and Research

ASI – Italian Space Agency