Genetic analysis ofrab7mutants in zebrafish

Abstract

Vascular network formation requires the fusion of newly formed blood vessels and the emergence of a patent lumen between the newly established connections so that blood flow can start. Lumen formation has been shown to depend on the late endosomal/lysosomal pathway in various organs of animal tubular systems. Here, we identified a late endosomal/lysosomal vesicular fraction (Rab7/Lamp2) in early zebrafish angiogenic sprouts, which appears to contribute to apical membrane growth during lumen formation. To study the effect of the late endocytic pathway on vascular development, we generated mutant alleles for all threerab7genes in zebrafish (rab7a, rab7ba, rab7bb). Allrab7genes are expressed in wild-type zebrafish and we did not detect any compensatory effects by the otherrab7isoforms in single knockout mutants, which were all viable. Only the triple mutant was lethal suggesting some functional redundancy. However, the differentrab7isoforms fulfil also at least partially independent functions because eggs laid from mothers lacking tworab7(rab7a and/or rab7bb). showed reduced survival and contained enlarged yolk granules, suggesting maternal contribution of these tworab7. Finally, we observed minor effects on lumen formation in embryos which still express one copy ofrab7. Our results support the notion that the late endocytic/lysosomal compartment contributes to lumen expansion.