Neuron-Target interactions

Question 1

What is an active zone? What is a postsynaptic density?

Answer 1

An active zone is a presynaptic site that is composed of electron dense proteins in which synaptic vesicles are fused. The postsynaptic density is involved in clustering postsynaptic receptors and ion channels.

Question 2

Where is found the basal lamina in a synapse?

Answer 2

In the synaptic cleft.

Question 3

Why would a drug like tetrodotoxin not prevent target selection in synapse formation?

Answer 3

Target selection minimally relies on neuronal activity. Instead, it further concerns adhesive and cell-cell recognition mechanisms.

Question 4

In synapse formation, what is the process of address selection referring to?

Answer 4

In this process, rough synaptic connections established during the first stages of synapse formation are refined through activity-dependent mechanisms.

Question 5

What axon guidance cue involved in the establishment of the amphibian visual system also serves target selection?

Answer 5

Ephrin signaling.

Question 6

What is bungarotoxin (BTX)?

Answer 6

Snake toxin binding specifically to AChRs.

Question 7

What causes synapse formation regarding the distribution of AChRs on the myotube?

Answer 7

Synapse formation involves important clustering of AChRs.

Question 8

What morphological change is parallel to AChR clustering when the myotube is innervated?

Answer 8

Switch from γ to ε AChR subunits.

Question 9

Name the four mechanisms at play in AChR clustering.

Answer 9

1. Redistribution of existing receptors in the plane of the myotube membrane.
2. Increased stability (life expectancy) on membrane.
3. Synaptic myonuclei selectively upregulate AChR gene transcription.
4. Suppression of AChR gene transcription in non-synaptic myonuclei.

Question 10

Why is it that exclusively synaptic myonuclei are upregulating AChR gene transcription?

Answer 10

The presynaptic terminal releases neuregulin (Nr), which binds to tyrosine kinase erbB receptor on the post-synaptic side and leads to an intracellular cascade upregulating AChR transcription. This process is therefore spatially restricted.

Question 11

Three experiments involving the basal lamina of the myotube and motor nerves show that the basal lamina directs clustering of AChR and attracts the motor neurons. What are the three experiments? What is each of one demonstrating? What are the three of them supporting?

Answer 11

A: If a mature myotube synapse is denervated from its presynaptic motor neuron, the regenerated motor neuron will contact the myotube exactly at the same site as before. This shows that the myotube attracts the nerve.

B: If a mature myotube synapse is denervated from its presynaptic motor neuron and the myotube is killed, leaving behind a “ghost” basal lamina, the regenerated motor neuron will contact the myotube exactly at the same site as before. This shows that it is specifically the basal lamina of the myotube that attracts the nerve.

C: On a myotube regenerating into the shell of an old denervated basal lamina, AChRs will cluster again at the same site where the synapse used to be. This shows that the presynaptic terminal is not necessary once a synapse has been formed to trigger the AChR clustering.

In sum, these experiments suggest that the motor neuron innervating the myotube secretes a cue that marks the basal lamina at its very position, which directs AChR clustering.

Question 12

What is the cue secreted by the presynatic motor neuron onto the synaptic basal lamina of the myotube? What is its receptor?

Answer 12

Agrin, binding to MuSK.

Question 13

To what family of receptors does belong the MuSK receptor?

Answer 13

RTK.

Question 14

How could MuSK receptors be extracellularly activated without Agrin?

Answer 14

With antibodies that crosslink MuSK receptors (dimerization activating the intracellular domains).

Question 15

What is Rapsyn involved in?

Answer 15

Adaptor protein between intracellular domain of AChR and F-actin (cytoskeleton).

Question 16

True or false: Rapsyn is indispensable for AChR clustering.

Answer 16

True.

Question 17

Why would a MuSK -/- mutation produce a worse phenotype in terms of AChR clustering than Agrin -/-?

Answer 17

MuSK receptors have a low intrinsic/spontaneous activity.

Question 18

List a few differences between CNS and PNS synapses?

Answer 18

CNS synapses are;
much smaller.
sometimes inhibitory.
without obvious basal lamina.
sometimes many onto a single postsynaptic neuron.
more likely required to be multiple to trigger the postsynaptic cell.

Question 19

What are postsynaptic polysomes?

Answer 19

Ribosomes mounted with mRNA regulating the transcription of proteins in the dendritic spine.

Question 20

What functionally important characteristic is spine motility regulating?

Answer 20

Strength of synaptic inputs.

Question 21

In synapse modelling, how is netrin involved?

Answer 21

Recruits AMPA receptors to the surface of dendritic spines.

Question 22

If CNS synapses do not have an obvious basal lamina, what is driving synapse formation?

Answer 22

SynCAMs expressed on the postsynaptic and presynaptic sides bring growth cones and dendrites together and differentiate the postsynaptic side.

Question 23

Apart from SynCAM, what other adhesion molecule plays a role in synaptogenesis?

Answer 23

Cadherins, bridging the cytoskeletons of the pre- and post- synaptic terminals.

Question 24

What is happening to the morphology of a CNS synapse if cadherin molecules are disrupted?

Answer 24

Loss of mature synaptic morphology:
loss of synaptic vesicle clusters.
reduced PSD.
immature dendritic spines.

Question 25

What imagery technique can be used to investigate whether different neurotransmitters are stored in separate clusters?

Answer 25

Fluorescent labelling.

Question 26

What is synaptophysin?

Answer 26

Presynaptic fluorescent tag for both inhibitory and excitatory synapses.

Question 27

What is responsible for receptor clustering in CNS synapses? How is it different for excitatory and inhibitory synapses?

Answer 27

Adaptor proteins:
Gephyrin (inhibitory) -> linking inhibitory receptors to microtubules.
PSD95 (excitatory) -> linking excitatory receptors to each other + cytoskeleton.

Question 28

What simple experiment could be done in vitro to show that gephyrin is essential for glycine receptors clustering and that glutamate receptors rely on a different adaptor protein to cluster?

Answer 28

In a gephyrin mutant, you would expect to observe abnormal dispersion of glycine receptors, but intact glutamate receptor clusters.

Question 29

If intracellular adaptor proteins hold receptors, what is holding adaptor proteins?

Answer 29

Heterophilic synaptic adhesion molecules.

Question 30

Give an example of heterophilic synaptic adhesion molecules. State which is presynaptic and which is postsynaptic.

Answer 30

Neurexin (presynaptic) + neuroligin (postsynaptic).

Question 31

What are PDZ domains? Why are they essential?

Answer 31

They are intracellular protein-protein recognition modules. They allow to organize multi-protein signaling complexes that are indispensable at synapses.

Question 32

A synaptic transport vesicle the lacks synaptobrevin, synaptophysin, and synaptotagmin is considered an active zone precursor vesicle, why?

Answer 32

It lacks the machinery to fuse with the plasma membrane of an active zone and release neurotransmitters.

Question 33

Give an overview of synapse formation in a few steps.

Answer 33

1. Adhesion molecules linking pre-post terminals (+ differentiation).
2. Recruitment of precursor vesicles.
3. Recruitment of adaptor proteins (PDZ domains).
4. Recruitment of postsynaptic receptors