Unloading Adaptation• Experimental models of decreased use
– Immobilization– Hindlimb suspension– Spaceflight– (Denervation)
• Factors contributing to atrophy• Clinical consequences of immobilization
Immobilization• Mechanical fixation
– External (cast)– Internal (pins)– Mixed (bone-mounted external clamps)
• Posture• Muscle activity
– Animal models: length-dependent activity– Human/clinical
Fournier study• ‘Residual’ muscle activity depends
on length• Muscle mass preserved at long
length• Reduced activity (short) without
extra atrophy
Lieber study• External Fixator
– Immobilize only one joint– No wiggling
• Quadriceps– Vasti: single joint knee
extensors– Rectus femoris: biarticular KE
and hip flexor
Muscle-specific atrophyVastus Medialis Rectus Femoris
Dark: fastLight: slow
Use and mechanics influence atrophy• RF is relatively spared (biarticular)• Fiber type
– Slow fibers in slow VM sensitive– Fast fibers in fast VL sensitive
Ubiquitin/Proteasome• Predominant pathway for protein degradation• Anti-ribosome• Ubiquitin• Poly-Ub• Proteasome
EM of proteasome
Pollard & Earnshaw, 2008
“Atrogene” signaling• MuRF + Atrogin/MafBx
– Muscle specific E3 ligases– Seem to drive atrophy
Transgenic HSP70 expression reduces immobilization-atrophy Senf & al., 2008
FOXO1/3a
Akt
HSP70
MuRF Atrogin
Protein Degradation
Growth Factors
“Stress”
Unloading• Reduce force, maintain mobility• Spaceflight
– Maintains mobility, decreases ROM– Inertial loading– Rapid loss of bone and muscle
• 6° head-down bed rest– Space-mimetic– Cardiovascular & hemodynamic
• Hindlimb suspension
Space: Loss of function• Rapid loss of strength (20%
3 weeks)• Slower, variable loss of
mass ~15% 5 weeks
Adams & al., 2003
Spaceflight muscle disruption• SLS-1 (1991)• 9 days• 25% atrophy• Expanded
interstitia
Riley & al., 1996
Ground control 9 days SLS-1 + 3h
Spaceflight muscle disruption• Sarcomere disruption• Z-disk streaming
Spaceflight: fiber adaptation• Sandona & al 2012
– Mice Drawer System (MDS)– 91 days on ISS
• Fiber properties• Transcriptional
profiling
Image: NASA
Muscle-specific atrophy• EDL: fast muscle
doesn’t care (much)• Soleus: postural
muscleA few type 2b fibersA few type 1 fibers
No atrophyAtrophy
Spaceflight-induced genes• “Stress Response”
– PERK– HSP70– NFkB
• Atrophy– MuRF– Atrogin
• Channels
Fold induction with 90 day spaceflight
Ubiquitin ligases
6° head-down bedrest• 30-90 days
– Blood draws– Biopsies/scans
• Space-mimetic– Fluid shift– Cardiorespiratory
• Similar magnitude muscle/bone strength loss
Photo: NASA Ames
Muscle atrophy during bedrest• Nitrogen balance
– Net amino acid intake-excretion– Protein accretion
estimate
• Strength loss:selective
Stein & Schulter 1997
Negative nitrogen balanceatrophy
-60
-40
-20
0
20
40
600 5 10 15 20 25
Knee Ext
Knee Flex
Weeks (16 wk bed+recovery)
Ste
rngt
h C
hang
e (%
)
Muscle-specific atrophy• By MRI volume
Miokovic, & al.,2012
Acute ‘atrophy’ with bed rest• 24 hours BR/HDT• 0.5, 2, 5 hour upright• 15% apparent
atrophy overnight• Apparent
hypertrophy inneck muscles
• Full recovery in0.5-2 hours
• Fluid shiftConley & al., 1996
Calf, horizontal
Calf, head-down
Neck, horizontalNeck, head-down
Hindlimb suspension• Rodent model
– Capture tail in low stress mesh/friction tape– Suspend by runner system– Hindlimbs just elevated
• Fluid shift• Unload, esp anti-grav• Stretch flexors
Shimano & Volpon, 2007
ControlPair-fed
Suspended
Suspendedand casted
Time (weeks)
Suspension Atrophy• Young rats (~100g)• Soleus
– 40% atrophy– 100% loss-of-growth– Mass preserved by
casting
• Protein accretion– Control: +13%/-8%/day– Suspended:+11%/-28%
Goldspink & al., 1986
Atrogene signaling during HS• Rat Medial Gastroc
– Rapid muscle mass loss– Preceded by MuRF/MAFbx
• Transgenic MAFbx– Smaller cells
Bodine & al., 2001
Proteolytic systems during HS• Lysosomes
– Acidic, autophagic compartment– Cathepsin proteases
• Calpains– Calcium-activated cytosolic
Taillandier & al 1996Enns & al., 2007
Calpain action during HS• cp mice express calpain
inhibitor• Doesn’t (much) change
loss of mass• Substantial sparing of
force production
Salazaar & al., 2010
Calpain Targets• Structural: Desmin, nebulin, utrophin• Suspension disrupts
sarcomere structure• Calpastatin (cp)
mice retain struct &force capacity
• Calpains ‘release’sarcomere matrix tofacilitate digestion
Salazaar & al., 2010
Summary• Models of decreased use• Atrophy rules
– Immobility, inactivity atrophy– Strength loss precedes mass loss– Large fibers are more sensitive
• Active degradation pathways– Proteasome (MuRF/MAFbx)– Lysosomes (cathepsin)– Calpains (sarcomere stability)
