Rab5 and Rab4 regulate axon elongation in the Xenopus visual system
Julien Falk1, Filip A Konopacki, Krishna H Zivraj
1Department of Physiology, Development, and Neuroscience, Anatomy Building, University of Cambridge, Cambridge CB2 3DY, United Kingdom.
Summary
Small GTPases Rab5 and Rab4 regulate axon growth by controlling endosome recycling within the growth cone. This process is essential for axon elongation but not pathfinding in Xenopus retinal axons.
Area of Science:
- Neuroscience
- Cell Biology
- Developmental Biology
Background:
- Axon elongation is a critical process during neural development, tightly regulated by various cellular mechanisms.
- Plasma membrane recycling is recognized as a key regulator of axon extension, but the specific molecules involved in growth cone recycling remain incompletely understood.
Purpose of the Study:
- To investigate the role of small GTPases Rab4 and Rab5, known regulators of short-loop recycling, in the extension of Xenopus retinal axons.
- To elucidate the specific molecular mechanisms of endosome trafficking within the growth cone that support axon growth.
Main Methods:
- Localization studies of Rab4 and Rab5 proteins in Xenopus retinal axon growth cones.
- Fluorescence recovery after photo-bleaching (FRAP) to assess the dynamics of Rab4 and Rab5 recruitment to endosomes.
- Dynamic image analysis to track Rab4-positive carriers.
- Functional inhibition using dominant-negative Rab4 or Rab4 morpholino, and constitutive activation of Rab5.
- In vitro and in vivo assays to measure axon elongation and pathfinding.
Main Results:
- Rab5 and Rab4 proteins localize to endosomes within the growth cone, accumulating constitutively recycled markers.
- FRAP experiments confirmed local recruitment of Rab5 and Rab4 to growth cone endosomes.
- Rab4-positive carriers were observed budding from Rab5 endosomes and moving towards the growth cone periphery.
- Inhibition of Rab4 or constitutive activation of Rab5 impaired axon elongation in vitro and in vivo.
- Crucially, these manipulations did not disrupt axon pathfinding, highlighting a specific role in growth.
Conclusions:
- Rab5- and Rab4-mediated endosome trafficking is essential for regulating axon elongation.
- Recycling from Rab5-positive endosomes via Rab4 occurs locally within the growth cone, directly supporting axon extension.
- This localized recycling pathway is critical for axon growth but separable from axon guidance mechanisms.
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