Alternative splicing factor or splicing factor‐2 plays a key role in intron retention of the endoglin gene during endothelial senescence

Summary Alternative splicing involving intron retention plays a key role in the regulation of gene expression. We previously reported that the alternatively spliced short isoform of endoglin (S‐endoglin) is induced during the aging or senescence of endothelial cells by a mechanism of intron retention. In this work, we demonstrate that the alternative splicing factor or splicing factor‐2 (ASF/SF2) is involved in the synthesis of endoglin. Overexpression of ASF/SF2 in endothelial cells switched the balance between the two endoglin isoforms, favoring the synthesis of S‐endoglin. Using a minigene reporter vector and RNA immunoprecipitation experiments, it was shown that ASF/SF2 interacts with the nucleotide sequence of the endoglin minigene, suggesting the direct involvement of ASF/SF2. Accordingly, the sequence recognized by ASF/SF2 in the endoglin gene was identified inside the retained intron near the consensus branch point. Finally, the ASF/SF2 subcellular localization during endothelial senescence showed a preferential scattered distribution throughout the cytoplasm, where it interferes with the activity of the minor spliceosome, leading to an increased expression of S‐endoglin mRNA. In summary, we report for the first time the molecular mechanisms by which ASF/SF2 regulates the alternative splicing of endoglin in senescent endothelial cells, as well as the involvement of ASF/SF2 in the minor spliceosome.