Cell polarity is a fundamental characteristic of many cell types and essential for their function. We hypothesized that the polarity protein Scrib acts also in endothelial cells as a scaffolding protein controlling the localization and function of other proteins and therefore regulating endothelial cell function.

By immunoprecipitation and proximity-ligation assay of Scrib we identified integrin α5 as a new Scrib interacting protein. A GST-pull down with the intracellular domain of integrin α5 revealed that only the wild type protein, but not mutants of Scrib either missing the LRR or the PDZ domains, interacts with integrin α5. Scrib siRNA reduces integrin α5 protein abundance and surface expression as shown by Western blot and FACS analysis, respectively. Scrib silencing lowered the half life of integrin α5 from about 30 to 15 min. Antibody feeding assays and cell surface biotinylation showed that downregulation of Scrib reduces integrin α5 recycling but not internalization. Analysis of the integrin α5-interacting proteom and proximity-ligation assay revealed that the interaction of integrin α5 and Rab7a (regulating sorting from late endosomes into lysosomes) is increased by Scrib silencing. Downregulation of Rab7a indeed rescued integrin α5 protein expression in Scrib missing cells. This mechanism contributed to a disturbed directed migration of endothelial cells lacking Scrib on fibronectin but not collagen. Furthermore, integrin α5 overexpression partially rescued the reduced directed cell migration after silencing Scrib. In flkGFP zebrafish mutants, Scrib as well as integrin α5 morphants show a delayed sprouting of intersegmental vessels and the knock down of both has synergistic effects.

In conclusion, Scrib represents a new endothelial protein protecting integrin α5 from Rab7a-dependent lysosomal degradation. This mechanism contributes to disturbed directed cell migration and angiogenesis in the absence of Scrib.