3 - Axonal Growth & Synaptogenesis

Question 0

Describe in general terms the 4 different Axon Guidance Molecules

Answer 0

Non-diffusable (short range)
     -Contact Attraction
     -Contact Repulsion
Diffusable (long range)
     -Chemoattraction
     -Chemorepulsion

Binding of axon guidance molecule to receptors on growth cones activates signaling cascades resulting in reorganization of the growth cone cytoskeleton, which controls the direction and rate at which growth cone moves

Fasciculcated Growth Cones = side-by-side growth of axons

Question 1

Describe the Axonal Growth Cone

Answer 1

• Motile tip of extending axon
• Explores extracellular environment, determines direction of growth, and guides extension of axon
• Lamellapodium = fan-shaped sheet at tip of axon that contains actin filaments and microtubules
• Filopodia = fine processes extending from the lamellapodium. Contains actin filaments, and the form/disappear rapidly
  
  • G-actin is incorporated into F-actin at the leading edge of filopodium in response to environmental cues
  • Growth turning is via F-actin binding proteins that bind F-actin, regulating retrograde flow.
  • When encountering an attractive cue, assembly is increased and retrograde flow is slowed, causing the filopodium to turn towards the attractive cue
• Microtubules makes the core of axon all cytoskeleton, very strong
• Microtubules responsible for axon elongation, F-actin just dictates direction

Question 2

CNS ECM molecules

Answer 2

Largely Repulsive
Chemorepulsion despite non-diffusable
Hyaluronan, Proteoglycans, Glycoproteins

Question 3

Cell Adhesion Molecules (CAMs)

Answer 3

Attractive
Non-diffusable
Act as ligands via homophobic binding involving kinases
Calcium-independent
L1 CAM associated with fasciculation of axons

Question 4

Cadherins

Answer 4

Attractive
Same as CAMs but Calcium-dependent
Ligand/receptor binding induces actin binding and organization

Question 5

Semaphorins

Answer 5

Repellent (sometimes attractive)
Non-diffusible
Ligand/receptor interaction results in growth cone collapse and inhibition of axon extension via intracellular signaling molecules

Question 6

Ephrins

Answer 6

Repellent
Non-diffusible
Ephrin A = GPI-linked to cell surface
Ephrin B = Single pass transmembrane proteins
Receptors are tyrosine kinases, collapses growth cone

Question 7

Netrins and Slits

Answer 7

Neutrons - Attractive and Repellent
Slits - Repellent
Diffusible
Netrins secreted by target cells in midline of embryo
-receptor type determines whether attractive (DCC) or Repellent (UNC5)
Slits secreted and bind to Robo receptors for repulsion

Netrins allow axons to cross the midline w/o crossing back by regulating receptor expression
-DCC to cross midline, UNC5 to prevent crossing back. Slits with Robo help in inhibiting DCC once crossed-over

Question 8

Synaptogenesis at the Neuromuscular Junction

Answer 8

• Motor axon randomly makes contact with a myotube on muscle
• Both nerve terminal and myotube differentiate, induced by Agrin, which activates MuSKc, causing clustering and increased local expression of ACh receptors through Rapsyn

Question 9

Synaptogenesis in Superior Cervical Ganglia

Answer 9

The correct pre- and post- synaptic neurons have higher affinity for each other, so incoming axons preferentially synapse on huge correct target

Question 10

Synaptogenesis in the CNS

Answer 10

• Nascent Presynaptic Process (from atonal growth cone) recognizes an appropriate site on the target cell via Cadherin/Protocadherin adhesion molecule interaction
• synaptic vesicles and active zone components accumulate
• Additional adhesion molecules are recruited to the developing synapse: Neurexin (presynaptic) and Neuroligin (postsynaptic)
• Neurexin helps localize cytoskeletal elements, vesicles, and active zone proteins
• Neuroligin recruits NT receptors to postsynaptic membrane

Question 11

Tropic Interactions between Neurons and Target Cells

Answer 11

• Neurons are dependent on their targets for survival and differentiation
• Target cells secrete neurotrophic factors (Neurotrophins) for which all synapses neurons compete
• Neurons receiving insufficient support from the target cell will degenerate and die via apoptosis

Question 12

Synaptic Rearrangement in PNS

Answer 12

PNS: Synaptic Refinement via Competition (Synapse Elimination)

• Synaptic inputs are gradually eliminated via competition for tropic resources until only a single neuron synapse remains
• Competition is dependent on electrical activity in both pre-/post- synaptic cells

Question 13

Neurotrophins

Answer 13

• Functions include survival of subset of neurons, formation/maintenance of appropriate number of connections, and elaboration of atonal and dendritic branches to support connections
• Nerve Growth Factor (NGF) acts on peripheral neurons:
  
  • Neuronal death in absence of NGF
  • Survival of excess neurons with increased NGF
• Neutrophin Receptors
  
  • Trk receptors are tyrosine kinases with high affinity for only processed (cleaved) neutrophins
  • p75 receptors are activated by all neutrophins but have high affinity for unprocessed neutrophins and low affinity for processed neutrophins
• Activation of each class of receptor triggers a distinct intracellular signaling cascade, which determines the cellular response:
  
  • Cell survival vs death, cell growth & differentiation, and stabilization vs elimination

Neurotrophic Interactions:
Neutrophin type Determines # of neurons
Neutrophin receptor Determines Shape of neurons
Intracellular Signaling Determines # of synapses