L15 - Synaptogensis Flashcards
What is synaptogenesis?
Functional connection between nerve cells
Synaptogenesis method?
- Axonal growth cones follow cues to target tissue
- Growth cone contacts post-synaptic cell
- Neuron stops growing and differentiates to pre-synaptic terminal
- Target cell specialises to create postsynaptic site
Final adult phenotype reached only after weeks/month
Synapse formation is competitive
Not all neurons/axons/branches make synapses
Not all synapses persist or are made the same way
Synapse formation dictates neuronal or target survival
To form functional synapses you need?
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
Target - dependent development
Muscle spindles - need sensory input to differentiate
Target - independent development
Don’t need sensory input to differentiate
Specialised epithelium overlying free nerve endings
Features of synaptic specialization
Small vesicles at the presynaptic membrane
Narrow cleft between pre- and post-synaptic membranes
Postsynaptic membrane appears thickened
Changes when a growth cone turns into a presynapse
Filopodia retraction and tight junction formation
Membrane and extracellular glycoproteins added
Presynaptic vesicles form, dense ECM and receptors accumulate in cleft
When does synaptogenesis occur?
When axons reach targets – highly variable
Synaptogenesis in cat visual cortex
Synapse density increases - postnatal day 10
Synaptogenesis in Cf moue olfactory bulb
Synapse density increase - postnatal week 1
What dictates synaptic sites?
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
Neuromuscular junction - before growth cones arrive?
Diffuse distribution of receptors
- 1,000 um-2
Neuromuscular junction - after growth cones arrive?
Focused distribution
- 10,000 um-2 junctional
- 10 um-2 extrajunctional
Why does the neuromuscular junction go from multiple to single innervation?
Early effect is receptor clustering into the correct region
Redistribution of existing receptors
Transcription increases in adjacent nuclei decreases in further away nuclei
Neurotransmitter receptor clustering
AChRs cluster in developing myotubes
Glycine, GABA and glutamate receptors cluster
What induces receptor synthesis - ARIA
AChR receptor-inducing activity
- 42kd protein
- Released by motorneurons
- Causes increased AChR subunit mRNA
- E subunit replaces gamma in the mature junctional receptor
ARIA is a member of what family?
Neuregulin family
- Many NRGs in developing brain
- Neuregulin can increase NMDA receptors in developing cerebellum
What evokes clustering?
Denervated and destroyed muscle
NMJ’s form where synaptic basal lamina persists
What is Agrin?
Purified from T. californica
Helps cluster AChRs
Made by motorneuron and muscle
What is Agrins role?
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
What is Agrins mechanism of action?
Agrin binds to muscle-specific kinase
Muscle-specific kinase KO mice - agrin insensitive
Agrin binding sites
Multiple binding sites for ECM / adhesion proteins
- Laminin
- FGF2
- Heparin sulfate
What is Rapsyns role?
Required for clustering
Rapsyn transfection studies?
AchR alone – no clusters
AchR + rapsyn – clusters
Rapsyn – clusters
Rapsyn KO – no clusters
Synaptic refinement - motorneurons axon loss
Muscle fibres initially receive multiple inputs
- Mature muscle - one input per muscle fibre
- This is non-random loss which is competitive
Synaptic refinement -cerebellum climbing fibre loss
Mature cerebellum – 1 climbing fibre per cell
During development - 4 climbing fibres per cell
Synaptic refinement – focusing
Climbing fibre re-organization
- One climbing fibre is chosen by activity to strengthen it connections to Pukinje cell
Contacts of the climbing fibre cell
Initial contacts - soma
Mature contacts – dendrites
Synaptic refinement – transmitter choice
Parasympathetic/ sympathetic neurons
- Sympathetic - ADR+
- Parasympathetic - ACh+
Transmitter choice - transplantation of cell bod y
Parasympathetic to sympathetic = ADR+
Sympathetic to parasympathetic = ACh+
Changing transmitters
Target-dependent transmitter choice
Some sympathetic neurons normally become ACh+
- E.g. sweat glands / transplantation studies
Synaptic refinement – turning on synapses
Silent synapses in developing and mature systems - intact, non-functional synapses
Seen in NMJ’s but focus on glutamatergic
Activating silent synapses
Like LTP in mature hippocampus - AMPAfication
