Neuronal polarity

Neurons have polarity
1. Axons and dendrites are distinct
  1. Functionally distinct
    1. Dendrites integrate incoming information
    2. Axons carry information away from cell body in action potentials
  2. Structurally
    1. Axons have highly polarised microtubules`
    2. Dendrites also have microtubules, but these are less ordered and have mixed orientations
  3. Molecularly
    1. Microtubules are stabilised through cross-linking by microtubule-associated proteins, which are distinct between axons (Tau) and dendrites (MAP2)
    2. The plasma membrane is also compartmentalised
      1. Actin-dependent diffusion barrier maintains membrane compartments
        Beads coated with antibodies to L1 cannot easily be dragged (using optical tweezers) across the boundary into the cell body, whereas GluR1-coated can be dragged into dendrites from cell body.
        From Winckler et al., 1999 Nature v397, p698
        Somatodendritic vs axonal domains
        Cell surface molecules appear to be held in particular sections by the underlying actin cytoskeleton
    3. L1, like other axonal components, is added to the axon  at the growth cone
      1. L1 is a cell surface adhesion molecule that is restricted to axons
        whereas the glutamate receptor component, GluR1, is restricted to the cell body and dendrites
How is neuronal polarity set up?
1. Neurite selection in hippocampal neurons
  1. Dotti et al., 1988 J Neurosci v8, p1454
  2. Because microtubules are polarised in axons, GFP-labelled, plus (+) end directed kinesins (e.g. Kif-1) mark where axons are forming:
    1. Choice of neurite appears to be ‘stochastic’ after different neurites are ‘tried out’
      1. How are neurites selected?
        Microtubule stabilisation is critical for axon initiation
        What underlies axon choice?
        Growth cones and neurites contain dynamic MTs (tyrosinated)
        Stabilised MTs (acetylated) are present in newly polarised axon
        Artificial stabilisation of MTs by localised taxol treatment of one neurite selects for axon formation:
        Suggests competition between axons to stabilise MTs and some kind of feedback loop to suppress other neurites (only one axon)
        What is the feedback loop?
        Conceptual Mechanisms for Long-Range Inhibition - ‘The Feedback Loop’
        (A) Diffusible inhibitor
        An autocatalytic activator (A, green) produces an inhibitory molecule (I, red) that diffuses throughout the cytoplasm to act as a long-range inhibitor of leading-edge formation.
        (B) Limiting component
        An autocatalytic activator in the front inhibits activation elsewhere by consuming essential substrates (S, gold) of the positive feedback loop, rather than generating a diffusible inhibitor (as in A).
        Houk et al. Cell (2012) vol. 148 (1-2) pp. 175-88
PI3 Kinase, GSK3ß and SAD kinases
1. HRas<=>PI3K +ve feedback loop = activator?
  1. Overexpression of Hras or PI3K activation results in multiple axons
  2. HRas accumulates in GCs as symmetry is broken in a PI3K-dependent manner (LY294002 = PI3K inhibitor)
  3. Activation of PI3K (iRAP) activates HRas
  4. Fivaz et al. Robust neuronal symmetry breaking by Ras-triggered local positive feedback. Curr Biol (2008) vol. 18 (1) pp. 44-50
2. Downstream of many signalling receptors, PI3K elevates PIP3 which phosphorylates Akt
  1. Evidence of elevated PIP3 (phosphorylated Akt) is found in nascent axons (but not naïve neurites).
    1. GSK3ß is inhibited by PIP3
      1. GSK3ß also regulates MAPs.
      2. Inhibition of GSK3ß results in multiple axons (A,B)
3. SAD kinases are related to Partitioning-defective (PAR) kinases, which are involved in many polarising events (e.g. asymmetric division in C. elegans) and phosphorylate MAPs.
  1. Loss of SAD kinases -> loss of axons (E)
  2. LKB1 regulates SAD kinases
    1. Ser/Thr kinase LKB1 = mammalian Par4.
      1. Can polarise non-polarised cells
      2. Phosphorylated LKB1 activates SAD kinases
        Loss of LKB1 in cortex -> loss of axon initiation
        But what are the ‘polarised extracellular cues’?
Semaphorins
1. a family of inhibitory guidance cues
2. Sema3A attracts dendrites and suppresses axons in the developing cortex
  1. Sema3A is expressed in a gradient from basal to apical and attracts dendrites basally
  2. Sema3A also promotes dendrite formation at the expense of axons in vitro
  3. Sema3A increases [cGMP] and suppresses [cAMP], thus inhibiting phosphorylation of LKB1 and GSK3ß*
    1. * Protein Kinase A (PKA) and PKG affect intracellular levels of cAMP and cGMP respectively. See LKB1 slide to see how this feeds into the pathway
  4. Manipulation of cAMP and cGMP have opposite effects on axon and dendrite formation.

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Neuronal polarity

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	Created by Kristi Brogden about 10 years ago