Strict regulation of axon branching and synapse formation is critical for the correct development of the embryonic nervous system, and dysfunction in these processes can lead to neurodevelopmental and neurodegenerative disorders. Douglas Campbell and Hitoshi Okamoto from the RIKEN Brain Science Institute in Wako have now found that the activity of enzymes called caspases in the visual system of zebrafish embryos contributes to the dynamic addition and retraction of new branch tips at the ends of developing axons.
Caspases are involved in a form of programmed cell death known as apoptosis. However, their function in structurally complex cells such as neurons, which have multiple compartments including dendrites, an axon and a cell body, could vary depending on where in the cell they are localized.
Using a fluorescence tracking system that senses caspase activity, and taking advantage of the transparency of the zebrafish embryo for imaging, Campbell and Okamoto observed that high caspase activity occurred in axons at a point in development when visual-system axons are known to be highly dynamic. In embryos at a later developmental window when axons are known to be stable, axons were found to display lower caspase activity (Fig. 1).
By examining branching locations over time, Campbell and Okamoto found that the regions that would soon generate a new branch point exhibited increased caspase activation compared with regions that did not branch. They showed that reducing caspase expression in neurons led to enhanced stability, not only of axons but also of synaptic sites within the axons. These findings suggest that caspase expression allows the axons to remain morphologically flexible.
Many ligand–receptor systems, such as the ligand Slit, and its corresponding receptor, Robo, play an important role in regulating axon morphology during development. A previous study by Campbell and his colleagues showed that loss of Slit or Robo function could result in a more stable axonal morphology, suggesting that the Slit–Robo ligand–receptor system may modulate caspase signaling in order to affect the dynamics of axon branching and synapse formation. In zebrafish embryos with reduced Slit or Robo signaling, there was no caspase activation at axon branch points, providing evidence that caspases are activated after the induction of slit–robo signaling.
"Dysregulation of caspase activity could be crucial to the initiation of neurodegeneration. Identification of the mechanisms of caspase activation may therefore be relevant to understanding these disorders," explains Campbell.
Campbell, D. S. & Okamoto, H. "Local caspase activation interacts with Slit-Robo signaling to restrict axonal arborization." The Journal of Cell Biology 203, 657–672 (2013). dx.doi.org/10.1083/jcb.201303072