Dendritically localized RNAs are packaged as diversely composed ribonucleoprotein particles with heterogeneous copy number states
Authors: Tarannum Renesa , Mun Grace , Quddos Fatima , Swanger Sharon A. , Steward Oswald , Farris Shannon 
Published: July 2024

Localization of mRNAs to dendrites is a fundamental mechanism by which neurons achieve spatiotemporal control of gene expression. Translationally repressed neuronal mRNA transport granules, also referred to as ribonucleoprotein particles (RNPs), have been shown to be trafficked as single or low copy number RNPs and as larger complexes with multiple copies and/or species of mRNAs. However, there is little evidence of either population in intact neuronal circuits. Using single molecule fluorescence in situ hybridization studies in the dendrites of adult rat and mouse hippocampus, we provide evidence that supports the existence of multi-transcript RNPs with the constituents varying in amounts for each RNA species. By competing-off fluorescently labeled probe with serial increases of unlabeled probe, we detected stepwise decreases inArcRNP number and fluorescence intensity, suggestingArcRNAs localize to dendrites in both low- and multiple-copy number RNPs. When probing for multiple mRNAs, we find that localized RNPs are heterogeneous in size and colocalization patterns that vary per RNA. Further, localized RNAs that are targeted by the same trans-acting element (FMRP) display greater levels of colocalization compared to an RNA not targeted by FMRP. Simultaneous visualization of a dozen FMRP-targeted mRNA species using highly multiplexed imaging demonstrates that dendritic RNAs are mostly trafficked as heteromeric cargoes of multiple types of RNAs (at least one or more RNAs). Moreover, the composition of these RNA cargoes, as assessed by colocalization, correlates with the abundance of the transcripts even after accounting for the expected differences in colocalization based on expression. Collectively, these results suggest that dendritic RNPs are packaged as heterogeneous co-assemblies of different mRNAs and that RNP contents may be driven, at least partially, by highly abundant dendritic RNAs; a model that favors efficiency over fine-tuned control for sustaining long-distance trafficking of thousands of messenger molecules.

MAIN POINTS

Dendritic RNAs exist as diversely composed RNPs that contain variable quantities of an individual transcript as well as combination of multiple different species of transcripts.

RNAs targeted by the same RBP display a bias to localize together as opposed to being colocalized with a non-target RNA.

Dendritically localized RNAs, regardless of their individual abundance, colocalize more with highly abundant RNAs compared to less abundant RNAs, even after correcting for random colocalization based on the abundance of RNAs being compared. These data are consistent with the possibility that RNA abundance plays a role in driving the assembly of heteromeric RNA cargos, facilitating the transport of thousands of transcripts to distal compartments

There is limited evidence of dendritic RNPs being composed of a single RNA species, suggesting that dendritic RNA cargoes are multimeric complexesin vivo.