A retained intron in the 3'-UTR of Calm3 mRNA mediates its Staufen2- and activity-dependent localization to neuronal dendrites.

Dendritic localization and hence local mRNA translation contributes to synaptic plasticity in neurons. Staufen2 (Stau2) is a well-known neuronal double-stranded RNA-binding protein (dsRBP) that has been implicated in dendritic mRNA localization. The specificity of Stau2 binding to its target mRNAs remains elusive. Using individual-nucleotide resolution CLIP (iCLIP), we identified significantly enriched Stau2 binding to the 3-UTRs of 356 transcripts. In 28 (7.9%) of those, binding occurred to a retained intron in their 3-UTR. The strongest bound 3-UTR intron was present in the longest isoform of Calmodulin 3 (Calm3(L)) mRNA. Calm3(L) 3-UTR contains six Stau2 crosslink clusters, four of which are in this retained 3-UTR intron. The Calm3(L) mRNA localized to neuronal dendrites, while lack of the 3-UTR intron impaired its dendritic localization. Importantly, Stau2 mediates this dendritic localization via the 3-UTR intron, without affecting its stability. Also, NMDA-mediated synaptic activity specifically promoted the dendritic mRNA localization of the Calm3(L) isoform, while inhibition of synaptic activity reduced it substantially. Together, our results identify the retained intron as a critical element in recruiting Stau2, which then allows for the localization of Calm3(L) mRNA to distal dendrites.