Transcript Calcium Signaling - Georgia Institute of Technology

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
– Inactivityatrophy
– Neuromuscular junction remodeling
Degeneration-Regeneration
• Initial insult
– Reduced protein synthesis/Elevated degradation
– Fiber deconstruction/death
• Recovery
– SC activation
– Restored
protein syn
Degradation/mg
• Reinnervation
Synthesis/mg
– Fiber reorg
– Relative
hypertrophy
Degradation/muscle
Synthesis/muscle
Goldspink, 1976
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
Muscle wasting
• Myofiber size
decrease
• Connective tissue
hypertrophy
• Adipocyte invasion
Soleus, denervated 7 months
Adipocytes
Soleus, denervated 7 weeks
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
New, small
myofibers
develop either
as discrete
structures
outside the
basal lamina
(left), or as
separate
appendages
inside the BL
(right)
EmbMHC
Laminin
NCAM
Borisov & al., 2001
Regeneration
Small, immature (EmbMHC+) fiber
adjacent to (presumably) preserved
original fiber
EmbMHC
(regenerating fiber)
Three relatively mature fibers with
faint laminin boundaries within
thicker laminin shell of (presumably)
original fiber
Laminin
(fiber boundaries)
SlowMHC
(mature fiber)
Borisov & al., 2001
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)
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
– AML1NCAM
– Myogenin/MRF4/MyoD
– Reduced by ES
• Myosin
– Den: IIbIIa
– Stim: IIaIIb
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)
Neuron
NCAM
Axon growth stops on
NCAM plaques
Covault &al., 1987
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 (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
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
Slower,
Less sag,
Less force,
Larger Tw/Tet
FF-Post
FR-Pre
FR-Post
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
Tibialis Anterior
Medial Gastrocnemius
Hyatt & al., 2003
• Less SC activation
in SI than DEN
DAPI (nucleus)
M-Cadherin (SC)
BrDU (DNA synthesis)
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
Training and spinal transection
• Careful training, tapering weight support
– Spontaneous weight support
(standing)
– Treadmill-assisted leg motion
(stepping)
Mass
Po
Post-mortem spinal cord,
showing complete lesion
Pre/post step postures
4.0
2500
3.5
2000
3.0
2.5
1500
2.0
1000
1.5
1.0
500
0.5
0.0
0
Roy & al., 1998
Belanger & al., 1996
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