ELAV/Hu RNA binding proteins determine multiple programs of neural alternative splicing

ELAV/Hu factors are conserved RNA binding proteins that play diverse roles in mRNA processing and regulation. The founding member, Drosophila Elav, was recognized as a vital neural factor 35 years ago. Nevertheless, still little is known about its impacts on the transcriptome, and potential functional overlap with its paralogs. Building on our recent findings that neural-specific lengthened 3' UTR isoforms are co-determined by ELAV/Hu factors, we address their impacts on splicing. While only a few splicing targets of Drosophila are known, we find that ectopic expression of the three family members (Elav, Fne and Rbp9) induces overlapping changes to hundreds of cassette exon and dozens of alternative last exon (ALE) splicing events. Reciprocally, double mutants of elav/fne, but not elav alone, have opposite effects on both classes of regulated mRNA processing events in the larval CNS. While manipulation of Drosophila ELAV/Hu factors induces both exon skipping and inclusion, motif analysis indicates their major direct effects are to suppress cassette exon usage. Moreover, the roles of ELAV/HU factors in global promotion of distal ALE splicing are mechanistically linked to terminal 3' UTR extensions in neurons, since both involve local suppression of proximal polyadenylation signals via ELAV/Hu binding sites downstream of cleavage sites. The phenotypic impact of combined ELAV/Hu activities in neural mRNA processing is overt, since fne loss strongly enhances neuronal differentiation phenotypes in elav mutants. Finally, we provide evidence for conservation in mammalian neurons, which undergo broad programs of distal ALE and APA lengthening, linked to ELAV/Hu motifs downstream of regulated polyadenylation sites. Overall, ELAV/Hu proteins orchestrate multiple conserved programs of neuronal mRNA processing by suppressing alternative exons and polyadenylation sites. Overall design: RNA-seq profiles from S2R+ cells with the overexpression of wildtype Elav, Rbp9, Fne and RRM mutant versions of Elav, Rbp9, Fne and BrdU-CLIP of wildtype Elav and RRM mutant variant.