SR proteins regulate V(6) exon splicing of CD44 pre-mRNA

CD44 pre-mRNA includes 20 exons, of which exons 1–5 (C(1)–C(5)) and exons 16–20 (C(6)–C(10)) are constant exons, whereas exons 6–15 (V(1)–V(10)) are variant exons. V(6)-exon-containing isoforms have been known to be implicated in tumor cell invasion and metastasis. In the present study, we performed a SR protein screen for CD44 V(6) splicing using overexpression and lentivirus-mediated shRNA treatment. Using a CD44 V(6) minigene, we demonstrate that increased SRSF3 and SRSF4 expression do not affect V(6) splicing, but increased expression of SRSF1, SRSF6 and SRSF9 significantly inhibit V(6) splicing. In addition, using a constitutive exon-specific primer set, we could not detect alterations of CD44 splicing after SR protein-targeting shRNA treatment. However, using a V(6) specific primer, we identified that reduced SRSF2 expression significantly reduced the V(6) isoform, but increased V(6–10) and V(6,8–10) isoforms. Our results indicate that SR proteins are important regulatory proteins for CD44 V(6) splicing.