Dynamics of axonal beta-actin mRNA in live hippocampal neurons

Localization of mRNA facilitates spatiotemporally controlled protein expression in neurons. In axons, mRNA transport followed by local protein synthesis plays a critical role in axonal growth and guidance. However, it is not yet clearly understood how mRNA is transported to axonal subcellular sites and what regulates axonal mRNA localization. Using a transgenic mouse model in which endogenous beta-actin mRNA is fluorescently labeled, we investigated beta-actin mRNA movement in axons of hippocampal neurons. We cultured neurons in microfluidic devices to separate axons from dendrites and performed single-particle tracking of axonal beta-actin mRNA. Compared with dendritic beta-actin mRNA, axonal beta-actin mRNA showed less directed motion and exhibited mostly subdiffusive motion, especially near filopodia and boutons in mature dissociated hippocampal neurons. We found that axonal beta-actin mRNA was likely to colocalize with actin patches (APs), regions that have a high density of filamentous actin (F-actin) and are known to have a role in branch initiation. Moreover, simultaneous imaging of F-actin and axonal beta-actin mRNA in live neurons revealed that moving beta-actin mRNA tended to be docked in the APs. Our findings reveal that axonal beta-actin mRNA localization is facilitated by actin networks and suggest that localized beta-actin mRNA plays a potential role in axon branch formation.