Synaptogenesis Flashcards
3 stages of development in synaptogenesis
1) synapse formation and differentiation (forms pre/post synaptic cells)
2) circuit refinement
3) synaptic growth and maintenance (columnar organisation occurs after refinement)
synapse density
number of synapses created and the number of synapses removed
turnover
developmental changes in the synapse
birth - 6months (synaptic contact > pruning)
6 months - 3 years (ASD feature detection)
3 years onwards - synaptic pruning > contact
15-20 yrs have large scale changes
what does the postsynaptic density contain
clustered receptors (few nm wide)
e.g. AChR (coded receptors), codes 4 aspects of the stimulus (modality/intensity/location/duration)
Transgenic mice expressing fluorescent proteins (CFP/YFP)
Young et al., 2003
in-vivo innervation
a-bungarotoxin - snake venom blocks AChR function and labels post-synaptic membrane (permanent- forms synapse)
each MN innervates many target muscles
NMJ adult vs embryonic
MN growth cone releases ACh before myotube contraction - transmission at NMJ is weak/subthreshold (embryogenesis)
terminal, broad AP in adult pre-terminal axons are myelinated
post synaptic differentiation in NMJ
spontaneous AChR clusters form on muscle (absence of light)
neurites synapse on AChR (cause further clustering)
spontaneous clusters remain even if no nerve is present
Receptors not hit by clusters disappear
innervation causes synthesis of new AChR
what is agrin
secreted factor
induces AChR clustering in cultured myotubes (C2C12 muscle cells)
agrin KO
impairs postsynaptic function
no clusters/no axon guidance
what do clusters activates
MuSK (muscle specific kinase) (TRK) - agrin R
Rapsyn (binds to AChR) downstream effector of MuSK
Musk/rapsyn/agrin mutants
musk mutant - no NMJ/no clustering/agrin signalling required for clustering
rapsyn mutant - needed for clustering/diffuse clusters
agrin mutant - needed for maintenance/growth/clusters formed but not sustained
how does NT release suppress AChR clustering
Misgeld et al., 2005
ChAT -> choline acetyltransferase (enzyme used in ACh synthesis)
ACh prevents AChR expression
agrin prevents ACh release, causes AChR clusters
Chat-/- clustering
agrn-/- declustering, no synapses
molecules involved in synaptic maturation
agrin (frog/mouse) - increase AChR clustering/Musk activation
ACh - decreases AChR synthesis/clustering
both ACh and agrin are post synaptic
what are boutons
presynaptic terminals which form synapses
3 factors needed for synapse formation
1) priming factors - neuronal(FGF/wnt) glial (cholesterol/TSP)
2) adhesive factors - cadherins/protocadherins inductive factors - Syn CAM/NXN/NLG/narp/EphrinB/EphBR
3) de/stabilising factors - activity/ubiquitination for pruning
is agrin essential for CNS synaptogenesis and why
no
Agrin -/- causes no change in PSD-95 quantity
types of synapses
excitatory (glutamate synapses) on spine
inhibitory (GABAergic) on shaft
neurexin is common in both
Priming diffusable factors
Terauchi et al., 2010
FGF 7/22 released post synaptically and binds presynaptically
VGLUT - measures excitatory synapses VGAT - measures inhibitory synapses
FGF22 KO in CA3 neurons - decreased vGLUT/decreases excitatory synapses/decreases stochastic events
FGF7 KO - decreased VGAT/decreases inhibitory synapses
cell adhesion factors (transmembrane proteins)
NRX
NLG
CASK
neurexin beads
neurexin has no concept of identity
neuroligins (1-4)
NLG1/3 - induces glutamatergic synapse formation (co-localised with PSD-95)
NLG 2 - induces GABAergic synapse formation (co-localised with gephyrin)
NLG triple KO (1/2/3)
still forms synapses
no significant changes in PSD length/synaptic cleft width/no. of docked vesicles
NLG-1 competition
co-culture Nlg1-/- and Nlg+/+ causes an imbalance
glutamate uncaging
caged glutamate is inert and cannot interact with the receptor
UV light releases Glutamate
new spine formed which matures
in young networks - glutamate acts at a distance
GABA uncaging
GABA is excitatory and causes depolarisation (early in development)
at site of uncaging - GABA forms inhibitory postsynaptic receptors and new spines
spine activation formed inhibitory synapses
pruning
many neurons compete for 1 postsynaptic receptor
occurs in: NMJ/visual system/cerebellum
synapse elimination
occurs in first 2 weeks post-natal
occupies less synaptic space
inverse correlation between relative motor unit size (competition sites) and synaptic occupancy
competition between axons
synaptically active axons out-compete silent axons
synapses with no NT are retracted e.g. AChRs
axon thickness and ACh
ChAT+/+ - ACh release and thicker axons
ChAT-/- - no release and thinner axons
no ACh release - similar diameter
CNS refinement at the retinogeniculate synapse
LGN receives input from RGCs
axon recruitment : if little current - fewer axons recruited if increased current - more axon excitation, bigger postsynaptic response - graded
