Unloading Adaptation• Experimental models of decreased use

– (Immobilization)– (Hindlimb suspension)– Denervation– Spinal isolation

• Factors contributing to atrophy• Clinical consequences of immobilization

Denervation• Nerve transection

– Remove coordinated descending input– Potential mobility in surrounding muscles

• Repair processes– Nerve regrowth: Same fibers? Same junction?– Muscle-derived signals?

• Muscle remodeling– Inactivityatrophy– Neuromuscular junction remodeling

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

Schwann CellAxon

Synaptic cleft: Primary Secondary

Control 1 Week

3 Weeks (reinnervation

Axon dies rapidly, Schwann cell & ECM remain.Secondary synaptic clefts shrink & separate

Saito & Zacks, 1969

Muscle wasting• Myofiber size

decrease• Connective tissue

hypertrophy• Adipocyte invasion

Soleus, denervated 7 months

Soleus, denervated 7 weeks

Adipocytes

Myofiber degeneration• Dramatic loss of myofibrils & myofibril order

Soleus structure after 21 days denervation (Tomanek & Lund, 1973)

Fiber-type specific• Fast Fibers, esp in fast muscle, degenerate• Mass & function preserved

by electrical stim

Niederle & Mayr, 1978 Dow & al., 2004

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

Regeneration

Borisov & al., 2001

Laminin NCAMEmbMHCNew, small myofibers develop either as discrete structures outside the basal lamina (left), or as separate appendages inside the BL (right)

Regeneration

SlowMHC(mature fiber)

EmbMHC(regenerating fiber)

Laminin(fiber boundaries)

Borisov & al., 2001

Three relatively mature fibers with faint laminin boundaries within thicker laminin shell of (presumably) original fiber

Small, immature (EmbMHC+) fiber adjacent to (presumably) preserved original fiber

Reinnervation• Muscle-nerve match• Axon-fiber not matched• Loss of contractile specialization

– MU innervation ratio– Fiber size:phenotype

Motor Unit territories before & after reinnrvation (Bodine-Fowler & al 1993)

Twitch contraction records contralateral and reinnervated LG & Sol (Gillespie & al. 1986)

Electrical stim preserves morphology• Rat EDL, 2 mos; 200x 0.2 s @100 Hz/day

Kostrominova & al., 2005

Gene expression altered by ES• Degen/Regen

– AML1NCAM– Myogenin/MRF4/MyoD– Reduced by ES

• Myosin– Den: IIbIIa– Stim: IIaIIb

Kostrominova & al., 2005

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

NCAM influences nerve growth• Culture neurons on muscle

slices• Processes follow cell surface• Greater growth on denervated

(high NCAM)

Covault &al., 1987

NeuronNCAM

Axon growth stops on NCAM plaques

Electrical stimulation of damaged nerve

• Low intensity; no force• Retrograde transmission of AP• Improves reinnervation

Al-Majed & al., 2000

Denervation summary• Degeneration-Regeneration

– Increased protein degradation and synthesis– “Moderating” of phenotype (IIIa; IIbIIa)– 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

Spinal Isolation• Transect spinal cord

– 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

MU properties post-SI

FF-Pre

FF-Post

FR-Pre

FR-Post

Slower,Less sag,Less force,Larger Tw/Tet

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

SI response is weaker than denveration• Rate and extent of mass/force decline lower• Upregulation of MRFs lower & shorter

Hyatt & al., 2003

Tibialis Anterior Medial Gastrocnemius

• Less SC activationin SI than DEN

DAPI (nucleus)M-Cadherin (SC)BrDU (DNA synthesis)

Spinal isolation summary• Limited Degeneration-Regeneration

– “Moderating” of phenotype (IIIa; IIbIIa)– Loss of mass, but structure is preserved

• Spinal neurons don’t repair

Training and spinal transection• Careful training, tapering weight support

– Spontaneous weight support(standing)

– Treadmill-assisted leg motion(stepping)

Belanger & al., 1996

Post-mortem spinal cord, showing complete lesion

Pre/post step postures

0

500

1000

1500

2000

2500

Po

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Mass

Roy & al., 1998

Summary• Muscle wasting program: active degeneration

– FOXOMuRF/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