Intron retention: regulation of S-endoglin and of cardiac ankyrin repeat domain 1-i8 (ANKRD1) during senescence. (a) S-endoglin. During endothelial cell senescence, the splicing factor SRSF1 supports generation of the S-endoglin mRNA, by its binding to a cis-element overlapping the branch point, that hampers splicing of intron 13. As a result, SRSF1 stabilizes intron retention and increases S-endoglin mRNA levels. (b) Cardiac ankyrin repeat domain 1 gene (ANKRD1 or CARP). In the ANKRD1-i7,8 splicing isoform retaining both intron 7 and 8, the fourth ankyrin repeat is absent. Higher levels of the ANKRD1-i8 splicing isoform retaining intron 8 were found in infant compared to adult heart tissues. This isoform maintains all conserved ankyrin repeat motifs crucial for its interactions with titin and cardiac calsequestrin. Yellow box: constitutive exon; solid line: intron.