Asymmetric microtubule nucleation from Golgi stacks promotes opposite microtubule polarity in axons and dendrites
Résumé
The neuronal microtubule cytoskeleton is highly polarised, with most microtubules growing away from the soma in axons (plus-end-out) but many microtubules growing towards the soma in dendrites (minus-end-out). This differential microtubule polarity allows directional trafficking of specific organelle, vesicles and molecules into either axons or dendrites but how it is established and maintained remains unclear. We showed previously that microtubules are nucleated asymmetrically from Golgi stacks within the soma of Drosophila neurons, with their plus ends growing preferentially towards and into axons and away from dendrites. Here, we show that this microtubule nucleation asymmetry correlates with a cis-to-trans orientation of specific Golgi stacks towards the axon and depends on microtubule nucleating γ-TuRCs at the cis-Golgi and the plus-end-stabilising protein CLASP at the trans-Golgi. Depleting CLASP or reducing γ-TuRC localisation to the Golgi by depleting the Golgin protein GMAP perturbs asymmetric microtubule nucleation and growth within the soma and results in polarity changes in proximal axons and dendrites. We propose that the plus ends of microtubules nucleated by γ-TuRCs at the cis-Golgi are stabilised by CLASP at the trans-Golgi to promote the growth of microtubules along the cis-to-trans Golgi axis. This, coupled with orientated Golgi stacks, promotes microtubule growth towards and into axons and away from dendrites, helping promote plus-end-out microtubule polarity in axons and maintain minus-end-out microtubule polarity in dendrites.
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