Splicing limits the spread of Crumbs

Vichas et al. reveal that an RNA helicase controls epithelial cell polarity by regulating alternative splicing of the apical membrane protein Crumbs.

In a genetic screen, Vichas et al. found that mutations in a gene called obelus (obe) perturbed epithelial cell polarity and intercellular junctions in early Drosophila embryos. obe mutant cells displayed an expanded apical membrane domain, and their intercellular adherens junctions clustered into single large aggregates instead of becoming evenly distributed across cell–cell interfaces. In addition, centrosomes were in aberrantly close proximity to the clustered junctions.

obe encodes an RNA helicase homologous to human and yeast proteins that are core components of the spliceosome, which catalyzes pre-mRNA splicing. Surprisingly, however, only a few transcripts were differentially spliced in obe mutant embryos. One of these encoded the apical membrane determinant Crumbs. In obe mutants, crumbs transcripts frequently retained an additional exon, increasing the expression of a Crumbs isoform with an extra EGF-like repeat in its extracellular domain. Overexpressing this isoform induced apical expansion, junction aggregation, and centrosome mispositioning similar to obe mutants. In contrast, overexpressing the shorter Crumbs isoform that usually predominates in early embryos had only mild effects on centrosome location and a distinct effect on junction organization.

Obelus therefore controls epithelial polarity and adherens junction organization by regulating crumbs splicing. It remains to be determined how the inclusion of an additional EGF-like repeat has such a marked affect on Crumbs activity.