Neuroscience lectures 6-7 - Synapse Formation Flashcards
Describe the development of the neuromuscular junction
1) Motorneuron growth cone approaches developing muscle
2) Nerve-muscle contact formed
3) Growth cone begins to differentiate into a presynaptic terminal
- accumulates neurotransmitter-containing vesicles
PRESYNAPTIC DIFFERENTIATION
4) Initially many axons may bind to one muscle fibre, but only one will stabilise, others will be eliminated
- Schwann cells start to myelinate axon
5) Post-synaptic muscle membrane invaginates to form post-junctional folds that accumulate neurotransmitter receptors
POSTSYNAPTIC DIFFERENTIATION
What is secreted by muscle and schwann cells in early synaptogenesis?
- Synaptic basal lamina
- lies between nerve and muscle at forming synapse
What is the role of the synaptic basal lamina?
It contains signals for both pre- and post- synaptic differentiation
What is a good source for purifying molecules involved in synapse development?
Electrocytes found in the electric organ of electric fish such as eels
- Rich source of synaptic material
Where is neural agrin secreted and what is its main function?
- Secreted by motorneurons during synaptic development
- Induces ACH receptor clustering
How are different isoforms of agrin produced?
Alternative splicing
Where are some other forms of agrin present?
In the muscle (mAgrin) and the CNS
What splice site gives the most active form of neural agrin?
The B/z splice site
What is the mechanism of Agrin action in terms of receptor binding?
2 parts of the nAgrin receptor:
1) Muscle-Specific Kinase (MuSK)
2) Low-density lipoprotein receptor-related protein 4 (Lrp4)
- MuSK expressed on developing myotubes
- Agrin binds to Lrp4, which causes myotubes to autophosphorylate MuSK
- Results in signalling that induces ACHR signalling
What in the downstream signalling pathway of MuSK/Lrp4?
- MuSK autophosphorylation recruits Dok-7 adaptor protein that becomes tyrosine phosphorylated
- Dok-7 recruits Crk and Crk-L that interact with a yet unknown pathway resulting in ACH receptor phosphorylation
- ACHRs cluster
What is the role of Rapsyn in MuSK-Lrp4 signalling?
- Peripheral membrane protein - attached by lipid anchor
- Clustered in response to nAgrin-MuSK/Lrp4 signalling
- Interacts directly with ACH receptors in 1:1 ratio
- Also interacts with cytoskeletal proteins, dystrophin, utrophin etc:
> Regulates clustering of these proteins
> Anchors clustered receptors to cytoskeleton
What are the 3 ways that ACH receptors accumulate in the muscle membrane directly opposite a nerve terminal?
1) Redistribution of existing receptors (nAgrin/MuSK/Lrp4/Rapsyn)
2) Synthesis of new receptor proteins by subsynaptic nuclei
3) Switching off expression of extrasynaptic receptors
What pathway regulates synthesis of new receptor proteins?
Neuregulin-ErbB
What type of molecule is Neuregulin-1 and where is it found?
Member of the epidermal growth factor family
- secreted by motorneuron into the synaptic basal lamina
What type of ErbB receptor binds neuregulin-1 and where is it found?
A ErbB2 tyrosine kinase receptor
- on muscle membrane
What does Neuregulin-1/ErbB2 signalling cause?
- Induces transcription of ACH receptor subunits in subsynaptic nuclei by regulating specific transcription factors
What binding is also required for neuregulin-1/ErbB2 signalling?
nAgrin/MuSK/Lrp4
How are ACH receptors in extrasynaptic nuclei repressed during early electrical activity?
- ACH binds to ACHR
- Depolarisation
- Ca2+ channels open
- Ca2+ entry
- Activation of Ca2+ dependant kinase pathway resultins in silencing on myogenin
- This is a transcription factors that regulates ACH receptor subunit gene expression
What signalling is required for pre-synaptic differentiation?
nAgrin/MuSK/Lrp4 signalling
What does this suggest about the signalling pathway that induces pre-synpatic differentiation?
nAgrin activation of MuSK must trigger a retrograde signalling pathway from muscle back to nerve
What are some candidate retrograde signalling molecules?
- Laminin-beta2
- Collagen IV
- Fibroblast growth factors (FGFs)
What experimental evidence suggest there are other molecules involved?
Loss of any of the above molecules only has mild effects
What disorders are related to the neuromuscular junction? What are the common symptoms?
- Broad range
> Myasthenic disorders e.g myasthenia gravis
> Neuromuscular junction protein disorders - Weakness and fatigability of skeletal muscle
What are neurmuscular disorders usually caused by?
- Gene mutations
- Autoimmune attack of proteins (pre- or post synaptic, basal lamina)
Dysfunctions in which proteins make up the majority of cases?
- ACHR, rapsyn and Dok7
What is Synapse elimination?
Elimination of excess synapses formed in early development
Why is synaptic elimination necessary?
- Many more connections are present during development than are needed (each muscle fibre innervated by multiple motor neurons)
- Excess connections therefore need to be eliminated so that each muscle fibre has a strong connection to one motor neuron
What is the mechanism of synapse elimination?
Competition between axons
What are the 2 types of axon competition?
Direct - for trophic factors produced by muscle
Indirect - the muscle chooses one axon/synapse over another according to relative synaptic activity at synapse (synapse strength)
Evidence shows that which type of synapse competition is the most significant?
Indirect
How do differences in synaptic activity cause synaptic elimination
If one synapse is less active than another, the ACHR will be lost
- This causes the loss of the associated synapse
Can be shown experimentally using alpha-bungarotoxin (nicotinic ACH receptor antagonist)
