Module 1 Lecture 6: Synaptogenesis Flashcards
starburst amacrine function
detecting direction of motion
characteristics of fly Dscam1isoforms
- homophilic
- 4 alternatively spliced exons; 3 are for the ectodomain
- 19008 ectodomain isoforms possible
what happens if dendritic arborization neurons don’t have Dscam1
spread out process of dendritic arborization (da) neurons collapse and cross over
is Dscam1 splicing selective?
no, nearly random
how is the Dscam1 identity developed
each neuron expresses several to a few tens of different isoforms
what does homophilic binding result in in Dscam1
repulsion
how is self-avoidance designed in Dscam1
each neuron can only bind its own set of Dscam1 isoforms
what results from reduced Dscam1 diversity (by overexpressing 1 isoform)
2 different da neurons that normally ignore each other now avoid crossing
how is ectodomain diversity in vertebrate Protocadherin clusters achieved
through alternative transcription start sites (contrast to fly Dscam1)
- each isoform is homophilic
function of domains EC2 and EC3 in Protocadherin clusters
mediate the specificity of homophilic binding
similarities between Protocadherin EC1 and Dscam Ig2
domains have similar folds
the question of synaptic specificity
after having arrived at the correct target area and layer, how do neurons choose the correct synaptic partners from among many contact partners?
example of stringent specificity
C3 neurons in fly visual system contacts both Tm1 and Tm2 neurons but only makes synapses with Tm1
biased specificity example
an On-SAC in the mouse retina has similar amount of contact with N- and V- ooDSGCs but makes more synapses with the V-type
types of synaptic specificity
adhesion, repulsion, elimination, and subcellular localization
first hypothesis for synaptic specificity
sequential: partner choice, then synapse formation
second hypothesis for synaptic specificity
package deal: combination of same cell adhesion molecules on pre- and post-synaptic cell mediate both partner choice and synapse formation
third hypothesis for synaptic specificity
make, then break: non-specific (promiscuous) synapses selectively eliminated after partner choice is made
why are in vitro studies of neurons less relevant to in vivo studies
neurons are synapse-happy and synapse-willing; they will readily make synapses with most sticky surfaces
what happens after neurogenesis and neuronal migration
axon guidance and dendritic arborization
what happens after axon guidance and dendritic arborization
nascent synapse formation
what happens after nascent synapse formation
either mature synapses or synapse elimination
what happens after mature synapses form
synapse elimination
what happens after synapse elimination
nascent synapse formation
first action from nascent synapses
engaging trans-cellular SAM complexes
- eg FLRTs/Teneurins <–> Lphns
second action from nascent synapses
pre- and postsynaptic intracellular signals
third action from nascent synapses
initial synapse assembly
first action from mature synapses
signaling via diverse trans-synaptic SAM complexes
- eg Neurexins <–> Nlgns and Cblns/GluDs
second action from mature synapses
pre- and postsynaptic protein activation (eg CASK)
third action from mature synapses
specification of synapse properties
first action of synapse elimination
disassembly of SAM complexes
second action of synapse elimination
pre- and postsynaptic disruption signals (unknown)
third action of synapse elimination
synapse elimination
what happens if you disrupt gene/process involved in earlier steps in synapse formation
will not get to synapse formation
what happens if you disrupt a gene/process involved in later steps of synapse development
may look like synapse formation was affected
what do axons from CA3 region of the hippocampus form synapses with
CA1 pyramidal neurons in two of three possible layers (stratum oriens and s. radiatum and not s. lacunosum-moleculare
where are Lphn2 and Lphn3 targeted
to distinct and non-overlapping regions of the post-synaptic CA1 neuron dendrites
what does Lphn3 mediated synapse formation require
simultaneous binding of two pre-synaptic cell surface molecules
how is specificity of synapse formation achieved?
specific compartmentalization of Lphn3 and co-incidence detection of correct pre-synaptic axons
what is Lphn3 necessary for
- formation of subset of excitatory synapses in cultured hippocampal neurons
- synapse formation in s. oriens and s. radiatum but not s. lacunosum-moleculare
- CA1 excitation by the Schaffer collateral fibers
what happens without Lph3
- loss in excitatory post-synapses (spines)
- in vivo hippocampal neuron excitatory synapse function compromised
- number of pre-synaptic CA3 neurons (origin of Schaffer collaterals) drop significantly without Lphn3
characteristic of Lphn3 4A mutatnt
cannot bind FLRT
characteristic of delta Lec
cannot bind Teneurin
can binding mutants rescue loss of excitatory synapses
no
what does Lphn3 dependent synapse formation in cultured hippocampal neurons require
both teneurin binding and FLRT binding by Lphn3
can binding mutants rescue loss of excitatory synapses
no
what is necessary for excitatory synapse function
Lphn ability to bind both pre-synaptic partners
what are Schaffer collaterals
axons from CA3
under what conditions do Schaffer collaterals come through
where Lphn3 is expressed
what happens where Lphn2 is expressed
axons from the entorhinal cortex come through (perforant pathway)
significance of Lphn3 capable of binding both pre-synaptic partners
is sufficient to rescue CA1 excitation in response to Schaffer collateral stimulation
can Lphn2 expression rescue Lphn3-loss dependent increase in CA1 excitation in response to Schaffer collateral stimulation
no
is Lphn3 necessary for CA1 excitation in response to perforant pathway stimulation
no
rabies virus function
spreads from post-synaptic to pre-synaptic neuron; used for retrograde tracing
first step of rabies virus experiment
introduce Cre to delete Lphn3
second step of rabies virus experiment
introduce rescue constructs (eg WT or mutant Lphn3)
third step of rabies virus experiment
set up retrograde tracing with rabies virus, engineered to ‘jump’ only 1 synapse (replication-deficient) and to carry GFP for marking the infected pre-synaptic neurons
end result of the rabies virus experiment
Cre (which deletes the Lpn3 cKO allele), GFP, mCherry (red)
what do pre-synaptic neurons express in the rabies virus experiment
GFP
what is not affected by loss of Lphn3
number of pre-synaptic entorhinal cortex neurons (perforant pathway)
significance of post-synaptic expression of Lphn3 in CA1 neurons
necessary and sufficient to form synapses specifically with Schaffer collaterals from CA3 neurons
