Intron retention (IR) is now recognized as a dominant splicing event during motor neuron (MN) development, however the role and regulation of intron-retaining transcripts (IRTs) localized to the cytoplasm remain particularly understudied. Here we show that IR is a physiological process that is spatiotemporally regulated during MN lineage restriction and that IRTs in the cytoplasm are detected in as many as 13% (n=2297) of the genes expressed during this process. We identify a major class of cytoplasmic IRTs, which are not associated with reduced expression of their own genes, but instead show a high capacity for RNA-binding protein and miRNA occupancy. Finally, we show that ALS-causing VCP mutations lead to a selective increase in cytoplasmic abundance of this particular class of IRTs, which in turn temporally coincides with an increase in the nuclear expression level of predicted miRNA target genes. Altogether, our study identifies a previously unrecognized class of cytoplasmic intronic sequences with potential regulatory function beyond gene expression.

SUMMARYIntron retention (IR) is now recognized as a dominant splicing event during motor neuron (MN) development, however the role and regulation of intron-retaining transcripts (IRTs) localized to the cytoplasm remain particularly understudied. By resolving the spatiotemporal dynamics of IR underlying distinct stages of MN lineage restriction, we identify a cytoplasmic group of IRTs that is not associated with reduced expression of their own genes but instead with an upregulation of predicted target genes of specific miRNAs, the motifs of which are enriched within the intronic sequences of this group. Next, we show that ALS-causing VCP mutations lead to a selective increase in IR of this particular class of introns. This in turn temporally coincides with an increase in the expression level of predicted target genes of these miRNAs, providing a potential mechanistic insight into ALS pathogenesis. Altogether, we propose a novel role for the cytoplasmic intronic sequences in regulating miRNA activity through miRNA sequestration, which potentially contributes to ALS pathogenesis.