Zebrafish embryonic neurons transport messenger RNA to axons and growth cones in vivo
Baraban, M., Anselme, I., Schneider-Maunoury, S., and Giudicelli, F.
ABSTRACT
Although mRNA was once thought to be excluded from the axonal compartment, the existence of protein synthesis in growing or
regenerating axons in culture is now generally accepted. However, its extent and functional importance remain a subject of
intense investigation. Furthermore, unambiguous evidence of mRNA axonal transport and local translation in vivo, in the context of a whole developing organism is still lacking. Here, we provide direct evidence of the presence of mRNAs
of the tubb5, nefma, and stmnb2 genes in several types of axons in the developing zebrafish (Danio rerio) embryo, with frequent accumulation at the growth cone. We further show that axonal localization of mRNA is a specific property
of a subset of genes, as mRNAs of the huc and neurod genes, abundantly expressed in neurons, were not found in axons. We set up a reporter system in which the 32 untranslated
region (UTR) of candidate mRNA, fused to a fluorescent protein coding sequence, was expressed in isolated neurons of the zebrafish
embryo. Using this reporter, we identified in the 32UTR of tubb5 mRNA a motif necessary and sufficient for axonal localization. Our work thus establishes the zebrafish as a model system
to study axonal transport in a whole developing vertebrate organism, provides an experimental frame to assay this transport
in vivo and to study its mechanisms, and identifies a new zipcode involved in axonal mRNA localization.
