1. Unloading Adaptation Experimental models of decreased use
(Immobilization)
(Hindlimb suspension)
Denervation
Spinal isolation
Factors contributing to atrophy
Clinical consequences of immobilization

2. Denervation Nerve transection Repair processes Muscle remodeling
Remove coordinated descending input
Potential mobility in surrounding muscles
Repair processes
Nerve regrowth: Same fibers? Same junction?
Muscle-derived signals?
Muscle remodeling
Inactivityatrophy
Neuromuscular junction remodeling

3. Degeneration-Regeneration
Initial insult
Reduced protein synthesis/Elevated degradation
Fiber deconstruction/death
Recovery
SC activation
Restored protein syn
Reinnervation
Fiber reorg
Relative hypertrophy
Goldspink, 1976
Degradation/mg
Synthesis/mg
Degradation/muscle
Synthesis/muscle

4. Schwann Cell
Axon
Control
1 Week
Synaptic cleft: Primary
Secondary
Axon dies rapidly, Schwann cell & ECM remain.
Secondary synaptic clefts shrink & separate
Saito & Zacks, 1969
3 Weeks (reinnervation

5. Muscle wasting Myofiber size decrease Connective tissue hypertrophy
Adipocyte invasion
Soleus, denervated 7 months
Adipocytes
Soleus, denervated 7 weeks

6. Myofiber degeneration
Dramatic loss of myofibrils & myofibril order
Soleus structure after 21 days denervation (Tomanek & Lund, 1973)

7. Fiber-type specific Fast Fibers, esp in fast muscle, degenerate
Mass & function preserved by electrical stim
Niederle & Mayr, 1978
Dow & al., 2004

8. Mechanisms of degeneration
Increased proteolysis
Increase MuRF/MAFbx & proteasome
Increase cathepsins
Decrease PGC-1a
Reduced metabolic capacity
Decrease glycolysis (LDH, PK, triose isomerase)
Decrease ETC (NADH, malate dehydrogenase, ATP synthase)
Increase ECM
Collagen, fibronectin, fibrillin

9. Regeneration
New, small myofibers develop either as discrete structures outside the basal lamina (left), or as separate appendages inside the BL (right)
Borisov & al., 2001
Laminin
NCAM
EmbMHC

10. Regeneration
Small, immature (EmbMHC+) fiber adjacent to (presumably) preserved original fiber
Three relatively mature fibers with faint laminin boundaries within thicker laminin shell of (presumably) original fiber
EmbMHC
(regenerating fiber)
Laminin
(fiber boundaries)
SlowMHC
(mature fiber)
Borisov & al., 2001

11. Reinnervation Muscle-nerve match Axon-fiber not matched
Loss of contractile specialization
MU innervation ratio
Fiber size:phenotype
Twitch contraction records contralateral and reinnervated LG & Sol (Gillespie & al. 1986)
Motor Unit territories before & after reinnrvation (Bodine-Fowler & al 1993)

12. Electrical stim preserves morphology
Rat EDL, 2 mos; 200x 0.2 Hz/day
Kostrominova & al., 2005

13. Gene expression altered by ES
Degen/Regen
AML1NCAM
Myogenin/MRF4/MyoD
Reduced by ES
Myosin
Den: IIbIIa
Stim: IIaIIb
Kostrominova & al., 2005

14. Electrical stimulation of denervated muscle
Neural cell adhesion molecule
Normal: only NMJ nuclei
Denervated: all nuclei
Potential benefits
Increased ‘receptivity’ of muscle
Increase axonal branching/guidance
Normal
Denervated
Denervated+ES

15. NCAM influences nerve growth
Culture neurons on muscle slices
Processes follow cell surface
Greater growth on denervated (high NCAM)
Neuron
NCAM
Axon growth stops on NCAM plaques
Covault &al., 1987

16. Electrical stimulation of damaged nerve
Low intensity; no force
Retrograde transmission of AP
Improves reinnervation
Al-Majed & al., 2000

17. Denervation summary Degeneration-Regeneration Reinnervation
Increased protein degradation and synthesis
“Moderating” of phenotype (IIIa; IIbIIa)
Loss of mass and order
Loss of myonuclear specialization (NMJ)
Reinnervation
Usually original MEP
Muscle-specific, not fiber-specific
Disrupts Size Principle
Loss of proprioception

18. Spinal Isolation Transect spinal cord Transect dorsal roots
Proximal to muscle of interest: no descending input
Distal: no ascending reflex
Transect dorsal roots
Sensory
Reduce reflex hyperactivity
Muscle inactive, nerve intact
Spinal cord injury model
Hyatt & al., 2003

19. MU properties post-SI FF-Pre Slower, Less sag, FF-Post Less force,
Larger Tw/Tet
FF-Post
FR-Pre
FR-Post

20. Physiological Response to SI
Grossly similar to denervation
Slow muscle  fast
Fast muscle  slow
Moderating of metabolic processes
Lower SDH in slow muscles
Higher GPDH in slow muscles
Inactive muscles revert to a ‘neutral’ phenotype

21. SI response is weaker than denveration
Rate and extent of mass/force decline lower
Upregulation of MRFs lower & shorter
Tibialis Anterior
Medial Gastrocnemius
Hyatt & al., 2003

22. Less SC activation in SI than DEN
DAPI (nucleus)
M-Cadherin (SC)
BrDU (DNA synthesis)

23. Spinal isolation summary
Limited Degeneration-Regeneration
“Moderating” of phenotype (IIIa; IIbIIa)
Loss of mass, but structure is preserved
Spinal neurons don’t repair

24. Training and spinal transection
Careful training, tapering weight support
Spontaneous weight support (standing)
Treadmill-assisted leg motion (stepping)
Post-mortem spinal cord, showing complete lesion
Pre/post step postures
Belanger & al., 1996
Roy & al., 1998

25. Summary Muscle wasting program: active degeneration
FOXOMuRF/Atrogin-1
Proteasome proteins (ubiquitin, S26)
Autophagy proteins (cathepsin)
Decreased metabolic capacity
Mitochondrial apoptosis
Reduced PGC-1a
Loss of fiber type specialization
Atrophy is its own program, separate from absence of hypertrophy