Robust Neuronal Symmetry Breaking by Ras-Triggered Local Positive Feedback

Marc Fivaz, Samuel Bandara, Takanari Inoue, Tobias Meyer

Research output: Contribution to journalArticlepeer-review

77 Scopus citations


Neuronal polarity is initiated by a symmetry-breaking event whereby one out of multiple minor neurites undergoes rapid outgrowth and becomes the axon [1]. Axon formation is regulated by phosphatidylinositol 3-kinase (PI3K)-related signaling elements [2-10] that drive local actin [11] and microtubule reorganization [3, 12], but the upstream signaling circuit that causes symmetry breaking and guarantees the formation of a single axon is not known. Here, we use live FRET imaging in hippocampal neurons and show that the activity of the small GTPase HRas, an upstream regulator of PI3K, markedly increases in the nascent axonal growth cone upon symmetry breaking. This local increase in HRas activity results from a positive feedback loop between HRas and PI3K, locally reinforced by vesicular transport of HRas to the axonal growth cone. Recruitment of HRas to the axonal growth cone is paralleled by a decrease in HRas concentration in the remaining neurites, suggesting that competition for a limited pool of HRas guarantees that only one axon forms. Mathematical modeling demonstrates that local positive feedback between HRas and PI3K, coupled to recruitment of a limited pool of HRas, generates robust symmetry breaking and formation of a single axon in the absence of extrinsic spatial cues.

Original languageEnglish (US)
Pages (from-to)44-50
Number of pages7
JournalCurrent Biology
Issue number1
StatePublished - Jan 8 2008
Externally publishedYes



ASJC Scopus subject areas

  • Biochemistry, Genetics and Molecular Biology(all)
  • Agricultural and Biological Sciences(all)


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