Synaptogenesis

Synaptogenesis

1. 'Battle of the synapses'

   • Synapse formation is competitive
   • Not all neurons / axons / branches make synapses
   • Not all synapses persist
   • Synapse formation can dictate neuronal or target survival
   • Not all synapses are made the same way e.g. new vs regeneration

2. Targets: Dependant/Independent development

   • Muscle spindles need sensory input to differentiate
   • Merkel cells
     • Specialised epithelium overlying free nerve endings forming Merkel discs
     • Present before sensory innervation

3. Morphological changes in synaptogenesis

   • The growth cone turns into a presynapse
     • Post-synapse develops specializations
     • When does it occur?
       • When axons reach targets
         • ∴Variable
     • Pre-synaptic differentiation
       • Induced on abnormal substrates e.g. poly-lysine
       • Pre-synaptic neurexin post-synaptic neuroligin
       • Soluble factors e.g. WNT-7a and cerebellar mossy fibres

4. What dictates synaptic sites?

   • Approaching growth cone ‘talks’ to target
   • Site availability may be restricted
     • Astrocytes may cover cell body – dendrites free (olive)
   • Post synaptic cells may have pre-prepared sites
     • e.g. cadherin / other adhesion molecules? High variability

5. The adhesive synapse

   • Find the table

6. Neuromuscular junction

   • Most early work on NMJ
   • Multiple → single innervation
   • Early effect is receptor clustering
     • Diffuse distribution (1,000 μm-2)
     • Focused distribution
       • 10,000 μm-2 junctional
       • 10 μm-2 extrajunctional
     • Re-distribution of existing receptors
     • Transcription ↑ in adjacent nuclei ↓ further away
     • What evokes clustering?
       • Denervated and destroyed muscle
       • NMJ’s form where ‘synaptic’ basal lamina persists
       • Torpedo californica used to investigate clustering factor
         • Electric organ used to generate 50V discharge to stun prey / predator
         • Agrin
           • Agrin purified from T. californica clusters AChRs
           • Multiple binding sites for ECM / adhesion
             • Made by motorneuron and muscle
           • Role
             • Neural agrin stimulates clusters (chick ab exp.)
               • Agrin KO mice die with malformed NMJ’s
             • Agrin variants found in the CNS
               • Role unsure
               • Agrin antisense mRNA ↓ hippocampal synapses
           • Mechanism
             • Agrin binds to muscle-specific kinase (MUSK)
             • MUSK KO mice are agrin insensitive

7. What induces receptor synthesis?

   • AChR receptor-inducing activity
   • ARIA
     • 42kd protein
     • Released by motorneurons
     • ↑ AChR subunit mRNA (especially ε - subunit)
     • ε - subunit replaces γ in the mature (junctional) receptor
     • ARIA is a member of the neuregulin family (nrg-1 gene)
       • Multiple NRGs in developing brain
       • Neuregulin can ↑ NMDA receptors in developing cerebellum

8. Rapsyn

   • Clustering needs rapsyn
   • Transfection studies
     • AchR alone – no clusters
     • AchR + rapsyn – clusters
     • Rapsyn – clusters
     • Rapsyn KO – no clusters