Dendritically Localized Transcripts Are Sorted into Distinct Ribonucleoprotein Particles That Display Fast Directional Motility along Dendrites of Hippocampal Neurons

Localization of mRNAs to postsynaptic sites and their subsequent translation is thought to contribute to synapse-specific plasticity. However, the direct visualization of dendritic RNA transport in living neurons remains a major challenge. Here, we analyze the transport of Alexa-labeled RNAs microinjected into mature hippocampal neurons. We show that microinjected MAP2 and CaMKIIalpha RNAs form particles that localize into dendrites as their endogenous counterparts. In contrast, nonlocalizing RNAs or truncated CaMKIIalpha, lacking the dendritic targeting element, remain in the cell body. Furthermore, our microinjection approach allowed us to identify a novel dendritically localized RNA, Septin7. Time-lapse videomicroscopy of neurons injected with CaMKIIalpha and Septin7 RNAs demonstrates fast directional movement along the dendrites of hippocampal neurons, with similar kinetics to Staufen1 ribonucleoprotein particles (RNPs). Coinjection and simultaneous visualization of two RNAs, as well as double detection of the corresponding endogenous RNAs, reveal that neuronal transcripts are differentially sorted in dendritic RNPs.