•We detected that RBFox1 influenced alternative splicing of APP, DLG3, and GNAO1.•RBFox proteins promoted APP exon 7 skipping and produced isoform APP E6-8-9.•APP E6-9 was slightly increased by overexpression of RBFox proteins.•We identified two cis-elements that regulate APP exon 7 skipping by RBFox proteins.RBFox proteins are well-known alternative splicing regulators. We have shown previously that during neuronal differentiation of P19 cells induced by all-trans retinoic acid and cell aggregation, RBFox1 shows markedly increased temporal expression. To find its key splicing regulation, we examined the effect of RBFox1 on 33 previously reported and validated neuronal splicing events of P19 cells. We observed that alternative splicing of three genes, specifically, amyloid precursor protein (APP), disks large homolog 3 (DLG3), and G protein, alpha activating activity polypeptide O (GNAO1), was altered by transient RBFox1 expression in HEK293 and HeLa cells. Moreover, an RBFox1 mutant (RBFox1FA) that was unable to bind the target RNA sequence ((U)GCAUG) did not induce these splicing events. APP generates amyloid beta peptides that are involved in the pathology of Alzheimer’s disease, and therefore we examined APP alternative splicing regulation by RBFox1 and other splicing regulators. Our results indicated that RBFox proteins promote the skipping of APP exon 7, but not the inclusion of exon 8. We made APP6789 minigenes and observed that two (U)GCAUG sequences, located upstream of exon 7 and in exon 7, functioned to induce skipping of exon 7 by RBFox proteins. Overall, RBFox proteins may shift APP from exon 7 containing isoforms, APP770 and APP751, toward the exon 7 lacking isoform, APP695, which is predominant in neural tissues.

The mechanisms by which huge human introns are spliced out precisely are poorly understood. We analyzed large intron 7 (110 199 nucleotides) generated from the human dystrophin (DMD) pre-mRNA by RT-PCR. We identified branching between the authentic 5′ splice site and the branch point; however, the sequences far from the branch site were not detectable. This RT-PCR product was resistant to exoribonuclease (RNase R) digestion, suggesting that the detected lariat intron has a closed loop structure but contains gaps in its sequence. Transient and concomitant generation of at least two branched fragments from nested introns within large intron 7 suggests internal nested splicing events before the ultimate splicing at the authentic 5′ and 3′ splice sites. Nested splicing events, which bring the authentic 5′ and 3′ splice sites into close proximity, could be one of the splicing mechanisms for the extremely large introns.Highlights► Huge intron 7 spliced from endogenous dystrophin pre-mRNA was analyzed by RT-PCR. ► The excised lariat intron 7 had a closed loop but it contained gaps in its sequence. ► Two presumed lariats generated from nested introns within intron 7 were detected. ► These RNA products were generated concomitantly with dystrophin pre-mRNA splicing. ► Proposed model: nested introns are spliced before ultimate splicing in huge intron.