The Relationship between Dendritic Branch Dynamics and CPEB-Labeled RNP Granules Captured in Vivo

Cytoplasmic Polyadenylation Element Binding protein (CPEB) is an RNA binding protein involved in dendritic delivery of mRNA and activity-dependent, polyadenylation-induced translation of mRNAs in the dendritic arbor. CPEB affects learning and memory and impacts neuronal morphological and synaptic plasticity. In neurons, CPEB is concentrated in ribonucleoprotein (RNP) granules that distribute throughout the dendritic arbor and localize near synapses, suggesting that the trafficking of RNP granules is important for CPEB function. We tagged full-length CPEB and an inactive mutant CPEB with fluorescent proteins, then imaged rapid dendritic branch dynamics and RNP distribution using two-photon time-lapse microscopy of neurons in the optic tectum of living Xenopus laevis tadpoles. Though the inactive CPEB mutant transports mRNA in the dendritic arbor, its expression interferes with CPEB-dependent translation because it is incapable of activity-triggered mRNA polyadenylation. In dendrites, the distributions of the active and inactive CPEB-containing RNP granules do not differ; the RNP granules are dense and their positions do not correlate with sites of rapid dendritic branch dynamics or the eventual fate of the dendritic branches. Because CPEB's sensitivity to activity-dependent signaling does not alter its dendritic distribution, it indicates that active sites in the dendritic arbor are not targeted for RNP granule localization. Nevertheless, inactive CPEB accumulates in granules in terminal dendritic branches, supporting the hypothesis that upon activation CPEB and its mRNA cargo are released from granules and are then available for dendritic translation.