Neuronal stimulation changes U1 snRNP binding and intronic polyadenylation

Synaptic activity induces well-known changes in enhancer-promoter driven gene expression but also induces changes in splicing and polyadenylation that are understudied. Here, we investigate the mechanism of expression for alternative polyadenylation isoform Homer1a, an immediate early gene essential to synaptic plasticity. We report that neuronal activation, in neuronal cultures and in adult mouse brain, depletes the splice factor U1 snRNP from Homer1 pre-mRNA and that this causes shifted utilization of a cryptic polyadenylation signal within intron 5 resulting in Homer1a expression. Because U1 snRNP is a ubiquitous splice factor, we tested the generality of activity-driven U1 snRNP depletion as a mechanism for gene expression using RNA immunoprecipitation sequencing. Analysis reveals that neuronal activity changes U1 snRNP binding to ~2000 transcripts and for a subset of transcripts, a reduction in U1 snRNP binding was accompanied by utilization of a cryptic intronic polyadenylation site. This subset is enriched for transcripts encoding synaptic proteins involved in excitability control. Genes demonstrating activity-dependent reduced U1 snRNP binding often encode a binding motif for Sam68, a neuronal alternative polyadenylation factor. Findings reveal that activity-driven changes in intron utilization for transcript termination serves an important role in synaptic plasticity.