Microtubules are not required to generate a nascent axon in embryonic spinal neurons in vivo

• Published in: EMBO reports Vol. 23; no. 11; pp. e52493 - n/a
• Main Authors: Moore, Rachel E, Pop, Sînziana, Alleyne, Caché, Clarke, Jonathan D W
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 07.11.2022; John Wiley and Sons Inc
• Subjects: actin; Actins; Animals; axon initiation; Axons - physiology; EMBO05; EMBO11; EMBO27; Microtubules; Neurites; neuronal differentiation; Neurons; Zebrafish; actin; axon initiation; microtubules; zebrafish; neuronal differentiation
• ISSN: 1469-221X; 1469-3178
• DOI: 10.15252/embr.202152493

Our understanding of the cell behaviours and cytoskeletal requirements of axon formation is largely derived from in vitro models but how these relate to axon formation in vivo is not clear. In vitro , neurons progress through a well‐defined multineurite stage to form an axon and both actin and microtubules cooperate to drive the first steps in neurite and axon morphogenesis. However, these steps are not recapitulated in vivo , and it is not clear whether the underlying cell biological mechanisms may differ also. Here, we investigate the mechanisms that regulate axon formation in embryonic zebrafish spinal neurons in vivo . We find microtubule organising centres are located distant from the site of axon initiation, and microtubule plus‐ends are not enriched in the axon during axon initiation. Focal F‐actin accumulation precedes axon formation, and we find that nocodazole‐treated neurons with no detectable microtubules are still able to form nascent axonal protrusions that are approximately 10‐μm long, dilated and relatively long‐lived. We suggest spinal axon formation in vivo is fundamentally different from axon formation in in vitro models. Synopsis In vivo imaging of embryonic zebrafish neurons defines early morphological events of axon initiation and shows focal F‐actin is likely the major cytoskeletal player, while microtubules are not required for nascent axon formation. In vivo imaging reveals stereotypical behaviours of nascent axon formation in embryonic spinal neurons. Nascent axon formation is preceded by local F‐actin accumulation but no local enrichment of microtubules. Local F‐actin accumulation and nascent axon formation still occur in the presence of nocodazole treatment that eliminates detectable microtubules. Graphical Abstract In vivo imaging of embryonic zebrafish neurons defines early morphological events of axon initiation and shows focal F‐actin is likely the major cytoskeletal player, while microtubules are not required for nascent axon formation.