Alternative splicing ofMef2cpromoted by Fox‐1 during neural differentiation in P19 cells

Mef2c protein is one of the MADS-box type transcription factors involved in muscular differentiation and synaptic formation. Previously, it has been reported that the Mef2c gene is responsible for three alternative splicing regulations. Here, we investigated the alternative splicing variants of Mef2c during neural differentiation of P19 cells and during cardio muscular differentiation of P19 clone 6 (P19CL6). We detected that two Mef2c mRNA isoforms, using exon α1 with and without the γ region at exon 10, are mainly produced in immature P19 cells. Remarkably, Mef2c isoforms containing exon β specifically appeared in the neural cell stage. Because most transcripts contain exon β in the neural cell stage and in the brain, this suggests that the alternative splicing of exon β is highly regulated. Among known regulators, Fox-1 was specifically expressed in the neural cell stage in correlation with Mef2c exon β. Fox-1 promoted exon β inclusion in transfection experiments using Mef2cβ minigene. Moreover, we found that the promotion required RNA-binding activity of Fox-1 and GCAUG sequence located in adjacent intron of exon β. Taken together, our results suggest that Fox-1, expressed specifically in the neural cell stage, promoted Mef2c exon β inclusion via the GCAUG.