Synapse development and pruning

What are the basic steps of synapse formation?

Target selection → growth cone contacts target → cell adhesion stabilises contact and anchors pre and post synaptic elements together→ presynaptic organisation + postsynaptic specialisation (clustering of specific ion channels/receptors depending on needs of neruon) → mature synapse.

What are the main specialisations of the neuromuscular junction (NMJ)? (3)

• Presynaptic active zones + synaptic vesicles

• Postsynaptic receptor-rich invaginations

• Schwann cell surrounding the NMJ.

What is the basal lamina and how does it help NMJ formation?

It is an extracellular matrix layer containing proteins such as laminin produced by mytotubules (post synaptic target). Synaptic laminin acts as a retrograde signal that helps organize synapse formation for adhesion and clustering and organization.

Synaptic basal lamina rich in acetylcholinesterase

What do laminin knockout experiments show with 2 examples?

Loss of laminin causes abnormal synapse formation, including reduced postsynaptic specialisation and abnormal Schwann-cell invasion, showing laminin is an important retrograde synaptic organiser.

When alpaha 2 is knocked out: synapse is not formed well with less invaginations and less post synaptic receptors , schwann cell grows into synapse

When beta 2 knocked out, schwann cell invaginates entire synapse preventing synapse communication, can cause death is beta 2 knocked out in development

How are ACh receptors clustered at the NMJ?

Four mechanisms:

1. Redistribution in the plane of the membrane and anchoring of pre and post synaptic elemnets

2. ↑ AChR metabolic stability (in embryonic mytobutubles, receptors live 1 day but in adults 14)

3. ↑ AChR gene expression at the synapse

4. ↓ AChR gene expression away from the synapse.

What is the role of agrin in AChR clustering?

Agrin acts through postsynaptic MuSK and rapsyn to cluster ACh receptors at the developing NMJ.

• initially cell nuclei and RNA for post synaptic receptors are spread out throughout the myofibre

• during maturation, there is increased gene expression for post synaptic receptors under the synapse

Agrin released from pre synaptic membrane binds to MuSK receptors on the post synaptic membrane anchroing pre and post synaptic elements. Increased gene expression for AchR immediately after synapse to cause clustering

What is the role of neuregulin in AChR expression?

Presynaptic neurons release neuregulin → binds postsynaptic ErbB receptors → interaction with nucelus and ER to cause more transcription and translation of AchR (↑ AChR gene expression)

What is the difference between agrin and neuregulin at the NMJ?

Agrin → clusters/anchors AChRs

Neuregulin → ↑ AChR gene expression

How does neural activity suppress extrasynaptic ACh receptors?

Postsynaptic muscle activity suppresses AChR expression outside the synapse.

Blocking activity or denervating muscle → ↑ extrasynaptic AChRs.

Electrical stimulation after denervation prevents this increase, showing postsynaptic activity itself is important.

What features do NMJ and CNS synapse development share?

Both undergo:

• Receptor clustering

• Synapse formation/maturation

• Synaptic elimination/refinement.

What is an important difference between NMJ and CNS synapse formation?

CNS neurons do not have synaptic basal lamina/laminin like the NMJ. It has laminin for the BBB but not for synapses. Instead, matched pre- and postsynaptic adhesion molecules help organise the synapse.

What is the role of agrin in CNS?

Clusters synapsin 1 pre synaptically. When released, it binds to targets on post synaptic terminal causing upregulation in gene expression of the specific receptor that we need at the synapse.

How does a CNS synapse begin to form?

(Proto)cadherins recognise the pre and post synaptic membranes → synaptic vesicles accumulate with cadherin → additional adhesion molecules are recruited (neuregulin and neurexin) → synapse stabilises

How are receptors clustered at excitatory CNS synapses?

Neurexin pre synaptically binds to post synaptic Neuroligin 1 →helps anchor PSD-95 → clusters and anchors AMPA + NMDA receptors.

How are receptors clustered at inhibitory CNS synapses?

Neurexin pre synaptically binds to post synaptic Neuroligin 2 → anchors gephyrin → clusters and anchors GABA receptors.

What happens to synapse density during development?

Synapses are initially overproduced, followed by substantial synaptic elimination/refinement, mainly through development into adolescence/young adulthood.

Why are synapses overproduced during development?

Proposed reasons include:

Allows connections to be fine-tuned to experience/circumstances

Helps ensure targets such as muscle fibres are innervated

May partly result from initially over-eager neuronal growth.

What happens to NMJ innervation during development?

Immature muscle fibres have polyneural innervation (multiple inputs), which is selectively refined so mature muscle fibres have one motor input.

What are the structural steps of synapse elimination?

Synaptic segregation (only innervate specific parts) → territory declines/invasion-retraction (close proximity to competing branch pruned first) → receptor loss → axon thinning → retraction bulb → terminal removed, decreased synaptic strength, reduced quantal content

What is a retraction bulb?

The swollen tip of a motor axon terminal as it retracts during synaptic elimination.

What happens to synaptic strength during synapse elimination?

Synaptic strength decreases, with reduced quantal content, as the connection is progressively eliminated

Does AChR loss occur before or after complete axon retraction?

Before. Postsynaptic ACh receptors are mostly lost before the presynaptic axon completely retracts.

Is synaptic elimination random?

No. Multiple terminals compete for the target, with weaker/"losing" inputs retracting while innervation is maintained.

How does activity influence synaptic elimination?

Active synapses can destabilise competing inactive synapses, causing the inactive inputs to weaken and eventually retract.

What are maintenance and punishment signals during synaptic competition?

Active inputs receive/support maintenance signals, while competing inactive inputs can receive a punishment signal that promotes their elimination.

If 2 receptors are getting maintenance signals and sending out punishment signals to each other this cancels out

Why is synchronised activity important during synapse development?

Synchronous firing protects synapses from retraction and can maintain polyneural innervation. Differences in firing between competing inputs promote remodelling.

How may JAK2 signalling contribute to synaptic elimination?

A punishment signal can activate JAK2 in the inactive synapse, which is proposed to recruit microglia involved in synaptic refinement.

How can microglia contribute to synaptic pruning? (4)

DCME

They can:

• Destabilise synapses by removing extracellular matrix, microglia end meet can destablise matrix causing synapse to bub off forming an exopher which can be removed by microglia

• Clean up cellular debris, cellular material broken down and shed from cell by forming axosomes which is taken up by microglia

• Modify synapses, end feet can slide in the synapse and modify activity or microglia can perform trogocytosis which removes small parts of the synapse which can affect signalling

• Engulf membrane compartments, engulf full pre or post synaptic membrane which can stop signalling

What is Hebbian modification during synapse development?

“Neurons that fire together, wire together.”

↑ coordinated pre + postsynaptic activity → synapse strengthens

↓ activity → synapse weakens and may be removed.

What factors contribute to Hebbian modification?

NMDA receptors + Ca²⁺ influx

Competition

Synchronous pre/postsynaptic activity

Postsynaptic feedback via neurotrophic factors.

What are ocular dominance columns an example of?

Activity-dependent synaptic refinement during visual-system development.

How do ocular dominance columns develop?

What happens if one eye is occluded during the critical period?

Initially, inputs from both eyes overlap throughout layer 4 of V1. During the critical period, neural activity causes the inputs to segregate into eye-specific regions.

Inputs from the non-occluded eye gain a larger cortical representation, while the deprived eye loses representation, potentially causing long-lasting visual deficits.

Why is the critical period important for visual development?

Changes in activity during this short developmental period can cause large and potentially permanent changes to cortical organisation; similar changes in adulthood have much smaller effects.