L15 - Synaptogensis

Question 1

What is synaptogenesis?

Answer 1

Functional connection between nerve cells

Question 2

Synaptogenesis method?

Answer 2

1. Axonal growth cones follow cues to target tissue
2. Growth cone contacts post-synaptic cell
3. Neuron stops growing and differentiates to pre-synaptic terminal
4. Target cell specialises to create postsynaptic site
   Final adult phenotype reached only after weeks/month

Question 3

Synapse formation is competitive

Answer 3

Not all neurons/axons/branches make synapses
Not all synapses persist or are made the same way
Synapse formation dictates neuronal or target survival

Question 4

To form functional synapses you need?

Answer 4

Correct receptors being expressed
Synapses at correct locations 
Correct part of the membrane to differentiate into synapse 
Receptors match the target tissue 
Correct number of synapses made

Question 5

Target - dependent development

Answer 5

Muscle spindles - need sensory input to differentiate

Question 6

Target - independent development

Answer 6

Don’t need sensory input to differentiate

Specialised epithelium overlying free nerve endings

Question 7

Features of synaptic specialization

Answer 7

Small vesicles at the presynaptic membrane
Narrow cleft between pre- and post-synaptic membranes
Postsynaptic membrane appears thickened

Question 8

Changes when a growth cone turns into a presynapse

Answer 8

Filopodia retraction and tight junction formation
Membrane and extracellular glycoproteins added
Presynaptic vesicles form, dense ECM and receptors accumulate in cleft

Question 9

When does synaptogenesis occur?

Answer 9

When axons reach targets – highly variable

Question 10

Synaptogenesis in cat visual cortex

Answer 10

Synapse density increases - postnatal day 10

Question 11

Synaptogenesis in Cf moue olfactory bulb

Answer 11

Synapse density increase - postnatal week 1

Question 12

What dictates synaptic sites?

Answer 12

Approaching growth cones communicates with target
Site availability may be restricted
- Astrocytes may cover cell body
Post synaptic cells may have pre-prepared sites
- Cadherin/adhesion molecules

Question 13

Neuromuscular junction - before growth cones arrive?

Answer 13

Diffuse distribution of receptors

- 1,000 um-2

Question 14

Neuromuscular junction - after growth cones arrive?

Answer 14

Focused distribution

• 10,000 um-2 junctional
• 10 um-2 extrajunctional

Question 15

Why does the neuromuscular junction go from multiple to single innervation?

Answer 15

Early effect is receptor clustering into the correct region
Redistribution of existing receptors
Transcription increases in adjacent nuclei decreases in further away nuclei

Question 16

Neurotransmitter receptor clustering

Answer 16

AChRs cluster in developing myotubes

Glycine, GABA and glutamate receptors cluster

Question 17

What induces receptor synthesis - ARIA

Answer 17

AChR receptor-inducing activity

• 42kd protein
• Released by motorneurons
• Causes increased AChR subunit mRNA
• E subunit replaces gamma in the mature junctional receptor

Question 18

ARIA is a member of what family?

Answer 18

Neuregulin family

• Many NRGs in developing brain
• Neuregulin can increase NMDA receptors in developing cerebellum

Question 19

What evokes clustering?

Answer 19

Denervated and destroyed muscle

NMJ’s form where synaptic basal lamina persists

Question 20

What is Agrin?

Answer 20

Purified from T. californica
Helps cluster AChRs
Made by motorneuron and muscle

Question 21

What is Agrins role?

Answer 21

Neural agrin stimulates clusters - chick ablation experiment
Agrin KO mice die with malformed NMJ’s
Agrin variants found in the CNS
Agrin antisense mRNA  hippocampal synapses

Question 22

What is Agrins mechanism of action?

Answer 22

Agrin binds to muscle-specific kinase

Muscle-specific kinase KO mice - agrin insensitive

Question 23

Agrin binding sites

Answer 23

Multiple binding sites for ECM / adhesion proteins

• Laminin
• FGF2
• Heparin sulfate

Question 24

What is Rapsyns role?

Answer 24

Required for clustering

Question 25

Rapsyn transfection studies?

Answer 25

AchR alone – no clusters
AchR + rapsyn – clusters
Rapsyn – clusters
Rapsyn KO – no clusters

Question 26

Synaptic refinement - motorneurons axon loss

Answer 26

Muscle fibres initially receive multiple inputs

• Mature muscle - one input per muscle fibre
• This is non-random loss which is competitive

Question 27

Synaptic refinement -cerebellum climbing fibre loss

Answer 27

Mature cerebellum – 1 climbing fibre per cell

During development - 4 climbing fibres per cell

Question 28

Synaptic refinement – focusing

Answer 28

Climbing fibre re-organization

- One climbing fibre is chosen by activity to strengthen it connections to Pukinje cell

Question 29

Contacts of the climbing fibre cell

Answer 29

Initial contacts - soma

Mature contacts – dendrites

Question 30

Synaptic refinement – transmitter choice

Answer 30

Parasympathetic/ sympathetic neurons

• Sympathetic - ADR+
• Parasympathetic - ACh+

Question 31

Transmitter choice - transplantation of cell bod y

Answer 31

Parasympathetic to sympathetic = ADR+

Sympathetic to parasympathetic = ACh+

Question 32

Changing transmitters

Answer 32

Target-dependent transmitter choice
Some sympathetic neurons normally become ACh+
- E.g. sweat glands / transplantation studies

Question 33

Synaptic refinement – turning on synapses

Answer 33

Silent synapses in developing and mature systems - intact, non-functional synapses
Seen in NMJ’s but focus on glutamatergic

Question 34

Activating silent synapses

Answer 34

Like LTP in mature hippocampus - AMPAfication