Development and Remodelling of NMJ · Synaptic regeneration partly recapitulates development...
Transcript of Development and Remodelling of NMJ · Synaptic regeneration partly recapitulates development...
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Development and Remodelling of NMJ
Remodelling Neuromuscular SynapsesRemodelling Neuromuscular Synapses
Synapse formation
Presynaptic
Postsynaptic
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Synapse elimination
J.F. Tello Polyneuronal innervation in fetal human muscle
(1917)
PI= [(A+B)-AB]/A
AB
AB
A B A BPI= AB/A
Physiological methods of measuring PI
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Loss of motor neurones?Or elimination of connections?
Loss of motor neurones..XElimination of connections ✓
Keller-Peck, C. et al.(2001) Neuron 31,381-394
Neurones retract some of their synapses while stabilising others
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Nguyen QT, Parsadanian AS, Snider WD, Lichtman JW (1998) Science 279:1725–1729.
Transgenic expression of a growth factor, GDNF, delays elimination
Monoinnervated NMJ’s are stable throughout life
Balice-Gordon & Lichtman (1990) J Neurosci 10, 894
Quantal Content (variancemethod) at NMJ of rat HD
0 100 200 300 4000
50
100First EPPPlateau EPP (10 Hz)
Age
(Based on Kelly & Roberts, 1977 and Kelly, 1978)
Synapse elimination is competitive: or is it?
MPN LPN SN
4DL
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~ 1month after partial denervation
Compare motor unit sizes at the time of partialdenervation and a few weeks later
neonateBetz,W.J.,Caldwell,J.H. & Ribchester,R.R.(1979). The size of motor units during postnataldevelopment of rat lumbrical muscle. J.Physiol. 297,463-478.
Fladby & Jansen (1987) Acta Physiol Scand. 129,239-246.
Betz, Caldwell & Ribchester (1980) J.Physiol. 303,265-279
“Competition” or “Intrinsic Withdrawal”? We can determine motor unit size in thy1.2-YFP mice bycounting the number of muscle fibres that are innervated
Adrianna Teriakidis
Neonate~ 1 month after
partial denervation
Adrianna Teriakidis
Motor unit sizes 2 days after partial denervationare larger than motor unit sizes 4-6 weeks later.
Adrianna Teriakidis
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Synaptic regeneration: a recapitulation of development
Remodelling Neuromuscular SynapsesRemodelling Neuromuscular Synapses
J. Boeke, 1916
π
Sprouts
Axon
10 µm
µ
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J.A.Barry & R.R. Ribchester (1995) J.Neurosci. 15,6327-6339
FM1-43
RH414
Polyneuronal innervation in reinnervated muscle
MPN LPN SN
4DL
Ribchester (1988) J Physiol. 401, 53-75
WallerianDegeneration
X X
Collateralsprouting
Regeneration Synapseelimination
π µ
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Axonal sprouting is preceded by Schwann cell sprouting
Son et al (1996) TINS 19,280F.Court et al. J Cell Sci 121,3901-3911
Four cell-types at mammalian NMJ
NFSV2/AChR S100/AChR
Kranocytes sprout before terminal Schwann cells
Felipe Court
Paralysis and muscle atrophy provide strong stimulifor kranocyte proliferation and spread
Felipe Court
Synapse elimination is activity-dependent: or is it?
Synapse elimination is influenced by activity
0 10 20 300
20
40
60
80
100StimulationParalysisNormal
Age (days)
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Inactivity upregulates transmitter release and promotes sprouting
OsmoticPump(i.p.)
Tubing
Lumbrical(4DL)Muscle
MPN LPN Sural Nerve
SN
LPN
L+SCuff
TTX
Ribchester,R.R. & Taxt,T.(1983). Motor unit size and synaptic competition in rat
lumbrical muscle reinnervated by active and inactive motor axons. J.Physiol. 344,89-111.
LPN Blocked Control
TTX
Active motor units have a competitive advantage over inactive units
Is activity sufficient for synapse elimination?
J.A.Barry & R.R. RibchesterJ.Neurosci. 15,6327-6339(1995)
E.M. Costanzo, J.A. Barry & R.R. RibchesterJ.Physiol. 521.2,365-374 (1999)
0 2 4 6 8
20
40
60
0
%Dual
Weeks Recovery
Polyinnervation persists at some NMJ after activity resumes
Brown et al., (1981) J Physiol
π persists in some adult muscles after neonatal paralysis
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Synaptic degeneration
Remodelling Neuromuscular SynapsesRemodelling Neuromuscular Synapses
Augustus Waller, ca. 1860
“Wallerian” Degeneration
24 hrs
Slater, 1966, Nature 209,305-307.
Miledi & Slater, 1970: “It is suggested that end-plate degeneration istriggered by a signal which passes from the site of sciatic nerve injuryto the nerve terminal. The duration of the period after transcectionwhen end-plates appear to be normal would then reflect the timerequired for this signal to travel the length of the isolated nervestump.” J Physiol 270, 508
X X
Gillingwater
Synapses degenerate first in Wallerian Degeneration
24-48h
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Am not I a flylike thee?Or art not thouA man like me?
- William Blake
Drosophila life cycle
Firstinstar
SecondinstarThird instar
Drosophila neuromuscular junction
There are three types of There are three types of boutons boutons or varicosities: or varicosities: IbIb, Is and II. Glutamate, Is and II. Glutamateis the excitatory neurotransmitter mainly released by the type Iis the excitatory neurotransmitter mainly released by the type Iboutonsboutons. Peptides such as . Peptides such as octopamine octopamine are released by the type IIare released by the type IIboutons boutons and they have a and they have a modulatory modulatory role.role.
Anatomy of the Drosophila NMJ
Type II boutons
Type I boutons
Synaptogenesis
During lateembryogenesis themotor neuron growthcone contacts itstarget muscle.Filopodial processesare initially formed butas the junctionmatures, boutons thatresemble those inlarvae becomeapparent
Embryo
3rd instar larva
Watching a Synapse Grow…
By using a non invasive, confocal imaging of synapses in live, Zito etal., 1999 showed that boutons BUD from branches or from pre-existingboutons at the Drosophila NMJs.
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The UAS/GAL4 Binary SystemThe expression of the gene downstream to theUAS is driven by the yeast transcriptionalactivator GAL4 in a developmental and/ortissue-specific manner
~1 mm
Drosophila 3rd Instar Larva : Motor Unit
Soma diameter ~ 10 µm : Volume ~ 500 µm3
Axon length ~0.3-3 mm : Volume ~ 750 - 3000 µm3
NMJ length ~ 100 µm x 3 = 100 boutons: Volume ~ 500µm3
Segment A5: fibre 6/7 NMJSegment A3: fibre 7/6/ 13/ 12 NMJs
VNC
OK6-Gal4:UAS-myr-RFP
20 µm
VNC
T Mosca, T Schwartz& RR Ribchester (onhis sabbatical…)
SUMMARY Synapses are induced after growth cones contact muscle fibres but the steps differ in “FaSyn” and “DeSyn” muscles
Synapse elimination accompanies a postnatal reduction in average motor unit size
Synapse elimination is partly competitive
Monoinnervated NMJ’s are normally stable
Synaptic regeneration partly recapitulates development
Synapse elimination in neonatal muscle and in reinnervated adult muscle is partly activity-dependent.
Activity is not sufficient for synapse elimination at all NMJ
Studies of the Drosophila NMJ may yield molecular insights into mechanisms of synaptic degeneration.