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Synapse Formation II
April 20, 2007 Mu-ming Poo
1. Activity-dependent synapse maturation
2. Activity-dependent regulation of muscle fibers
3. Polyneuronal innervation and synapse elimination
4. Denervation and Reinnervation of muscle fibers
-- Denervation effects in muscle
-- Specificity in muscle reinnervation
5. Specificity of neuromuscular synapse formation during
development
6. Formation of central synapses
Innervation-induced changes in AChR Properties
Embryonic AChRs:
-- low conductance and long opening time (pentamer
consisting of 2a, b, e, d subunits), resulting in slow
synaptic current
-- mobile (diffusely distributed), short half-life (1 day)
Mature AChRs:
-- high conductance and short opening time (pentamer
consisting 2a, b, g, d subunits, resulting fast synaptic
currents
-- stable (clustered), long half-life (5 days)
Summary on the Role of Synaptic (Electrical) Activity on
Synapse formation
1. Initial clustering of AChR activity-independent (by unknown factor)
2. Activity-dependent processes at developing synapses
-- Increased AChR lifetime (from 1 day to 1 week)
-- Down-regulation of extrasynaptic AChRs
-- Maturation of AChR clusters (pretzel-shaped)
-- Switch of AChR subunit from α2βγδ to α2βεδ (change in kinetics)
-- Synaptic competition and elimination of polyneuronal innervation
3. Activity-dependent mechanisms
-- Factors secreted as a result of activity?
-- Depolarization-induced Ca2+ influx and downstream effectors.
Innervation-dependent muscle contractile properties
-- Embryonic: mostly slow (S) motor units
-- Mature: more fast (FF) motor units
-- The transition is induced by innervation, change in expression of different
types of myosin isoforms (which different ATPase activities)
-- Cross-innervation of the muscle (in animal experiments) results in
switching of muscle properties according to the nerve type.
-- Altering the pattern of activity in the nerve has similar effects, i.e.,
applying phasic stimulation (transient high-frequency bursts) change the
slow muscle into fast muscle
Elimination of Polyneuronal Innervation at NMJs
Each muscle fiber innervated by several motor axons at birth
1. Establishment of several inputs results in refractory of
muscle (extrasynaptic) surface to further innervation
2. Within 2 postnatal weeks, all but one motor axon remains
3. Competition of postsynaptic territory occurs at the endplate
among several terminals. Synapse becomes weakened as it
looses the territory, and eventually retracts.
4. Competition mediated by muscle, two inputs can compete
even separated by up to 1 mm.
5. Competition activity dependent – blockade of synaptic
activity and excessive stimulation slow down and speed up
elimination, respectively
Innervation of individual neuromuscular
junctions by axonal branch trimming
• Each muscle fiber change from poly to single innervation
• Number of fibers innervated by each motor neuron is reduced
• This is mediated by retraction of axonal branches
Green motor neuron.
Red – AChR
(rhodamine
tagged α–
BTX)
Synapse elimination: activity-dependent competition
Competition
Remove some motor neurons, all muscle
fibers are still innervated; remove the
original innervation, other motor neurons
take over
Activity-dependent
• TTX (blocks Na channels, action
potentials) blocks elimination (the same
muscle fiber remain innervated by
multiple motor neurons)
• a-bungarotoxin (blocks AChR) blocks
elimination
Mechanism of Synaptic Competition
1. Trophic factor hypothesis: Competition for muscle derived
maintenance factor (“synaptotrophin”), and the active nerve
terminal takes most of the factor and becomes stabilized, while
inactive terminals are eliminated due to the lack of the factor.
(Secreted factors added to the NMJs can retard elimination: BDNF,
leukemia inhibitory factor, CNTF, FGF-2, GDNF, but in vivo
evidence of secretion of these factors lacking)
pre-
post-
Mechanism of Synaptic Competition
2. Toxic factor hypothesis: Elimination due to muscle-derived
toxic substance, and active nerve terminal becomes immune.
-- Protease inhibitor slows down elimination process
-- Calcium activated neutral protease destablized nerve terminal
-- Thrombin, a serine protease, speeds up elimination
-- Agrin can act as protease inhibitor
3. Retrograde factor hypothesis: Intracellular factors
(“synaptomedins”) in the muscle cell triggered by active nerve
terminal selective stablizes and destabilizes active and inactive
nerve teriminals, respectively, through transmembrane actions on
the nerve terminals
-- Levels of AChRs and rapsin begin to decline at the synaptic site
before nerve terminal withdrawal.
Ann. Rev. Neurosci. (1999)
Denervation and
Reinnervation of Muscle
Effects of denervation in skeletal muscles
1. Paralysis (immediately): loss of sensation and motility
2. Fasiculation (immediately): spontaneous firing of the
injured axon, causing twitching of motor units.
3. Fibrillation (days) : spontaneous twitching of individual
muscle fibers due changes in muscle excitability (e.g.,
Na channels)
4. Supersensitivity to ACh (days): increased AChRs in the
muscle membrane, due to elevated extrasynaptic AChRs
(increased AChR synthesis).
5. Muscle atrophy (>1 week): loss of muscle proteins
6. Receptiveness to innervation: allows reinnervation of
the muscle by motor axons
Regeneration of NMJs after denervation
1. Regenerating motor nerve may be guided by remaining
perineurial tube to the original endplate to form synapse
2. Ectopic synaptic sites may be formed by reinnervating axons.
(Denervated muscle becomes hypersensitive to ACh, and loses
refractoriness to innervation)
3. Reinnervation is NOT muscle-specific
-- uncontrolled facial expression
-- “crocodile tear syndrome” (Spontaneous tearing during normal
salivation of eating, due to mis-innervation of the tear gland by
axons destined for salivary gland following facial nerve damage)
-- importance of intrafascicular nerve repair after peripheral
nerve transection