Neuronal Synapses & Synaptogenesis

The Growth Cone

• On the tips of axons to guide neurons (flattened fan shape)

• Express specific receptors to sense environmental cues and transform them into steering decisions

• They require

  • Solid substrates

  • Extracellular cues (attractive or repulsive) act together to ensure accurate guidance

• They use intermediate targets provide guidance information and prepare for the next stage of their journey.

• Neurons can extinguish their response to to cues and acquire responsiveness to others by changing their surface receptors after reaching these points.

Mechanisms Guiding Growth Cones

Long Range	Short Range
Chemorepulsion Chemattraction	Contact attraction Contact repulsion

4 Major Classes of Guidance Cues

• Semaphorins

• Netrins

• Slits

• Ephrins

• Morphogens

• Cell adhesion molecules

Semaphorins

• Can be secreted and transmembrane guidance cues for long and short range communication

• 20 different semaphorins contain a sema domain

• Domain allows them to bind directly to plexins or the co-receptors Neurophilin-1/2.

• Neurophilins will interact with a plexin receptor to form a different receptor complex to allow them to attract and repel.

• Sema3A is a repellant in tissues surrounding peripheral nerves acting as a repellant so they grow around the tissue.

Netrins

• Secreted and tethered to the cell surface through an G protein coupled anchor that they cleaved for short range communication.

• For long range communicate the can diffuse from their source.

• Attractive through DCC and repel using UNC5

Slits

• Large secreted proteins

• Repulsive actions that are mediated by the robo receptor family which controls axon guidance across the midline of the corpus callosum to prevent re crossing of axons.

• Slit 2 can be processed and cleaved by protease to generate 2 active fragments with different receptors

  • N terminal fragment: Binds to robo receptors

  • C terminal fragment: Binds to plexin A1 (same as semaphorin)

• Netrins can act as attractant or repellant depending on the netrin gradient and the receptors expressed by the neuron

Ephrins

• Cell surface signaling proteins embedded in the plasma membrane that can bind to F receptors

• Important role in axonal guidance and axonal branching

• Two types

  • Class A which are tethered to the cell surface via GPI anchors

  • Class B are transmembrane proteins

• Not active if they are released from the surface so they only do short range communication

Morphogens

• Wnt - attractant & repellant

• Sonic hedgehog- attractant & repellant

• Transforming growth factor beta - Attractant?

• Bone morphogenic protein

Cell Adhesion Molecules

• Interact with guidance cues

• DsCAM

  • attractant with DCC

  • repellant with Unc5 co-receptor for netrin-1

Binding of these molecles

• Ligand binds to receptor

• Downstream signalling causes

• Regulation of cytoskeleton causing

• Growth cone retraction/extension, neuronal branching or pruning, altered morphology, substrate adhesion or attachment.

• Steer neuron to reach target

Laminar, Cellular, Subcellular & Synapse-type Specificity

• Laminar Specificity: Axons are guided to the appropriate target area

• Cellular Specificity: Within the area axons choose a particular cell as a partner

• Subcellular Specificity: The portion f the target cells surface it forms a synapse with such as the dendrites, somata or axonal segments.

• Synapse-type specificity: Differentiation and maturation into a specific type of synapse

Mechanisms of cellular specificity

• Target recognition (lock-and-key).

• Axon guidance (guidance cues).

• Interactions among axons.

• Inhibition of inappropriate synapses (repel).

• Matchmaking by intermediate target cells to form transient contact (priming (changing receptors to ensure it responds to the correct signals)).

• Elimination of inappropriate synapses (pruning only).

• Death of cells (excess synapses).

• Conversion of partner after forming random synapses (slow muscle fibres to fast muscle fibres when need).

• Guidance by dendrites.

• Delivery at stereotyped times and places

Synaptic Specificity

• Target recognition is mediated by signaling molecules that help with target recognition and contact stabilization

• Two main classed implicated are

  • Cadherins

  • Immunoglobin

• Act as attractants and repellants

• Inner plexiform layer of the retina

  • Axons must find their target in a very crowded space

  • Neurites are stratified within in the plexiform and neurons will form connections within distinct sublayers

  • Each neuron expresses a different immunoglobin adherent molecule

  • Pre and post synaptic cells express the proteins for a match up in the same sublamina

  • Suggests homophilic recognition tested on chicken eggs

    • overexpression of descam only form in layer 5 on retina and synapse on layer 2

Subcellular Specificity

• Axons selective innervate certain domains on the partener

• Extrinsic cues guide them to particular potentials of cells

• Repellent/attracting molecules

• Timing - excludes from synapsing on synapses/layers that haven’t formed yet

• Interactions with partners can cause remodelling of the target cell (spines in excitatory)

What’s involved in synaptogenesis?

• Priming factors from surrounding cells that help initiate synapse formation

  • Diffusible molecules

  • From surrounding cells or glial

  • Induce accumulation of vesicles, promote axonal and dendritic maturation and facilitate the ability of the processes to initiate synapse formation

• Adhesive factors help neurons survive and help stabilize them

• Inducing factors can directly induce synapse formation

  • Clustering of post-synaptic proteins

  • Triggering formation of pre-synaptic buttons

  • Recruitment of receptors and vesicles

  • NARP and FMB1 increase synaptic clustering of AMPA or NMDA receptors and initiate presynaptic specialisation

• Stabilising and destabilising factors that regulate synaptic strength and stability that help regulate neuronal activity in synaptogenesis

  • Neuronal activity can trigger neuronal clustering

  • ?Can can protein degradation to promote synapse elimination

    

Whisker to Barrel Pathway in the Mouse

• Stimulated whiskers of mouse for 24h 36% increase in synapses both excitatory and inhibitory

• 4 days later it returned to normal

  

Synaptic maturation

• Recruitment of essential proteins and components to the synapse (both pre- and postsynapse).

• Pre-synapse:

  • Vesicles deliver critical proteins

  • Establishment of docking and fusion sites.

• Post-synapse:

  • Gradual accumulation of essential proteins e.g. PSD95, NMDA and AMPA receptors, scaffolding.

    

Synaptic Pruning

• Starts at 2 and continues until 10 years old (around 50% removed)

• Not enough pruning in autism and too much in schizophrenia

• Support from astrocytes is needed to strength remaining synapses and prevent other fibres connecting

  

Regulation of Synaptic Pruning

• Microglial cells engulf synapses in response to fractalkine secreted from the synapse

• Signaling from the complement cascade can also cause pruning from micro glial

• Astrocytes prune synapses in response to phagocytic receptors Megf10 and MERTK

• Astrocytes regulate microglial elimination by secreting transforming growth factor beta or interleukin 33 causing complementary signaling on microglial resulting in pruning of the synapse.

Sequential and Concurrent Formation-Elimination Models

Sequential Formation-Elimination Models	Concurrent Formation-Elimination Models
Excessive formation Then elimination	Formation and elimination at the same time Correct model

Example

• Inputs from the two eyes projecting to the same target are segregated into eye-specific adomains in the LGN.

• In each projection system, the two monocular inputs are initially overlapped and gradually segregate from one another by the selective local pruning of overlapping parts of axonal arbors to form the adult pattern of connections through a competitive process driven by neural activity.