The Guidance of Axons to Their Targets

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The Guidance of Axons to Their Targets

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Axon Growth Two Views

• molecular view
• random growth with selective survival
• stereotropism
• mechanical guidance
• along scratches, blood vessels or cartilage
• resonance
• congruent activity

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Retina-Tectum Experiment

• Roger Sperry, 1940s
• lower vertebrates
• regeneration of cut retinal axons
• frog, cut optic nerve, rotation of the eye

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Retina-Tectum Experiment

• chemical matching rather than functional
  validation of random connection
• chemospecificity hypothesis
• recognition molecules
• Molecular matching predominates during embryonic
  development.
• Activity (experience) modifies the circuits once
  they have been established.

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Axon Guidance Cues

• from the target (wrong)
• series of discrete steps
• retina optic fiber layer retinal basal lamina
  and end-feet of glia optic nerve head optic
  stalk (pioneer axons) optic chiasm - ventral
  diencephalon superior colliculus at different
  subregions - radial glial cells - synaptic
  partner a specific layer specific area of
  dendrites

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Axon Guidance Cues

• optic chiasm
• different responses to special midline cells
• intermediate targets
• terminal arbor
• interaction with target
• patterns of neural activity

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Positional Cues

• correction of mistaken fibers
• ephrins
• rotation of neural tube between the chiasm and
  the tectum
• markers of position or polarity

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Motor Axon Guidance

• series of discrete steps
• motor pool in the spinal cord segmental ventral
  roots by barriers in the somites rearrangement
  in plexus region large nerve target muscle
  synapse on a muscle fiber
• similar pattern to the retinal fibers

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Axon Growth Options

• grow / turn / stop
• positive and negative cues
• finer control over the direction of growth

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Pioneers

• short distance in a small embryo
• the first axons to exit the retina

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Intermediate Targets

• decision points
• optic chiasm, limb plexus

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Gradients

• cell surface molecules
• soluble molecules

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Ranges of Cues

• short-range cues
• cell membranes
• extracellular matrix
• precise contact guidance
• long-range cues
• soluble molecules
• less precise guidance

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Growth Cone

• Santiago Ramon y Cajal, 1890s
• both a sensory structure and a motor structure
• transduces positive and negative cues into
  signals that regulate the cytoskeleton and
  thereby determine the course and rate of axon
  outgrowth
• coupling between the sensory and motor
  capabilities

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Growth Cone

• central core
• microtubules, mitochondria, etc
• lamellipodia
• motile, ruffled appearance
• filopodia
• long slender extensions

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Filopodia

• sensory capability
• rod-like, actin-rich, membrane-limited
• highly motile advance, retract, turn
• length
• rapid movement
• flexibility

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FilopodiaSecond Messengers

• calcium
• set point optimal concentration
• gradient of calcium change of direction
• cyclic nucleotides
• modulate protein kinases, protein phosphatases,
  rho-family GTPases

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Pathway Guidance Cues

• promotion / inhibition
• cell surface / extracellular matrix / soluble
  form

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Pathway Guidance Cues

• extracellular matrix adhesion
• cell surface adhesion
• fasciculation
• chemoattraction
• contact inhibition
• chemorepulsion

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ECM Adhesion

• collagen, fibronectin, proteogylcans, etc
• laminins
• heterotrimer, at least 14 trimers, unique
  distribution, position- or stage-dependent
  signals
• integrins
• heterodimer, at least 16 alpha and 8 beta chains,
  specific ligands
• all cells in the body at least one integrin

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Cell-Cell Adhesion

• selective adhesive interaction
• cadherin calcium dependent
• immunoglobulin-like adhesion molecules calcium
  independent
• cell-cell binding
• short-range promoter of neurite growth

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Cadherins

• at least 100 related membrane-spanning
  glycoproteins
• extracellular calcium binding segments
• N-cadherin, proto-cadherins, cadherin-related
  neural receptors
• cells throughout the body

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Cadherins

• homophilic interaction
• prefers to bind to its own kind
• selective adhesion
• Cytoplasmic domain binds catenins, then affects
  cytoskeletal elements.

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Adhesion Molecules

• adhesion assay
• Initial adhesion triggers a cytoplasmic reaction
  that strengthens the adhesion.
• not just adhesion molecules but signaling
  molecules activated by membrane receptors

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Ig Superfamily

• disulfide bridges
• less ligand-specificity than cadherins
• intracellular domain protein tyrosine
  phosphatase or protein tyrosine kinase

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Chemoattractants

• soluble growth factors
• trophic factor
• chemotaxis tropism
• no clear examples of trophic and tropic factors
  in vivo
• neuronal chemoattractants
• two glycoproteins netrins

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Vertebrate vs. C. elegans

• netrin unc-6
• unc-5H unc-5
• DCC, neogenin unc-40
• Ig superfamily
• remarkably conserved during evolution

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Ephrins

• stripe assay
• axons from temporal retina to the anterior tectum
• heat treatment of each membranes
• heat treatment of posterior membrane random
  growth
• presence of inhibitory material in posterior
  membranes

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Ephrins

• repulsive axon guidance signal (RAGS) ephrin A5
• eph kinases receptor tyrosine kinases
• major group of inhibitory ligands and receptors
  in the developing nervous system

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Ephrins

• ephrin A2 and ephrin A5
• low-to high gradients in the rostral direction in
  the tectum
• eph A3 (a kinase receptor which binds ephrin A2
  and A5)
• low-to-high gradient in the temporal direction in
  retinal ganglion cells

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Somaphorins

• an inhibitory molecule
• at least 15 somaphorins
• in distinct types of neurons and nonneural cells
• key receptor neuropillins, plexins

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Chemorepulsion

• semaphorins
• some membrane-bound
• others soluble factor
• netrins
• DDC or neogenin attraction
• unc-5H repellant

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Chemorepulsion

• attract or inhibit according to the receptors or
  the level of intracellular messengers
• some neurotransmitters
• One synaptic transmission inhibits formation of
  another.