Essential and overlapping roles for laminin α chains in notochord and blood vessel formation

Laminins are major constituents of basement membranes and have wide ranging functions during development and in the adult. They are a family of heterotrimeric molecules created through association of an α, β and γ chain. We previously reported that two zebrafish loci, grumpy (gup) and sleepy (sly), encode laminin β1 and γ1, which are important both for notochord differentiation and for proper intersegmental blood vessel (ISV) formation. In this study we show that bashful (bal) encodes laminin α1 (lama1). Although the strongest allele, bal^m190, is fully penetrant, when compared to gup or sly mutant embryos, bal mutants are not as severely affected, as only anterior notochord fails to differentiate and ISVs are unaffected. This suggests that other α chains, and hence other isoforms, act redundantly to laminin 1 in posterior notochord and ISV development. We identified cDNA sequences for lama2, lama4 and lama5 and disrupted the expression of each alone or in mutant embryos also lacking laminin α1. When expression of laminin α4 and laminin α1 are simultaneously disrupted, notochord differentiation and ISVs are as severely affected as sly or gup mutants. Moreover, live imaging of transgenic embryos expressing enhanced green fluorescent protein in forming ISVs reveals that the vascular defects in these embryos are due to an inability of ISV sprouts to migrate correctly along the intersegmental, normally laminin-rich regions.