Axis specification and morphogenesis in the mouse embryo requireNap1, a regulator of WAVE-mediated actin branching

Abstract

Dynamic cell movements and rearrangements are essential for the generation of the mammalian body plan, although relatively little is known about the genes that coordinate cell movement and cell fate. WAVE complexes are regulators of the actin cytoskeleton that couple extracellular signals to polarized cell movement. Here, we show that mouse embryos that lack Nap1, a regulatory component of the WAVE complex, arrest at midgestation and have defects in morphogenesis of all three embryonic germ layers. WAVE protein is not detectable in Nap1 mutants, and other components of the WAVE complex fail to localize to the surface of Nap1 mutant cells; thus loss of Nap1 appears to inactivate the WAVE complex in vivo. Nap1 mutants show specific morphogenetic defects: they fail to close the neural tube, fail to form a single heart tube (cardia bifida), and show delayed migration of endoderm and mesoderm. Other morphogenetic processes appear to proceed normally in the absence of Nap1/WAVE activity: the notochord, the layers of the heart, and the epithelial-to-mesenchymal transition (EMT) at gastrulation appear normal. A striking phenotype seen in approximately one quarter of Nap1 mutants is the duplication of the anteroposterior body axis. The axis duplications arise because Nap1 is required for the normal polarization and migration of cells of the Anterior Visceral Endoderm (AVE), an early extraembryonic organizer tissue. Thus, the Nap1 mutant phenotypes define the crucial roles of Nap1/WAVE-mediated actin regulation in tissue organization and establishment of the body plan of the mammalian embryo.

Genetic ablation of FLRT3 reveals a novel morphogenetic function for the anterior visceral endoderm in suppressing mesoderm differentiation

“…Support for this idea can be found in embryos deficient in Nap1, a regulator of WAVE-mediated actin dynamics. These embryos display AVE migration defects, and, as a consequence, inhibitors of posteriorizing signals are not delivered to the right place, leading to axis duplication (Rakeman and Anderson 2006). Importantly, Nap1 −/− embryos do not show the kind of ectopic T expression or BM breakdown that we detect in FLRT3 −/− embryos.…”

Essential role for Abi1 in embryonic survival and WAVE2 complex integrity

“…It has been shown that Nap1 is essential for a series of actin-mediated cell-migration events during early mouse embryogenesis (32). These events include migration of the anterior visceral endoderm, required for specification of the anteriorposterior body axis of the animal, and migration of the mesoderm and endoderm germ layers (32). As a WAVE complex component, Abi1 is involved in the regulation of cell migration; therefore it was expected that Abi1 also might regulate morphogenetic events during early mouse embryogenesis.…”

“…For example, inactivation of murine WAVE2 is lethal by E12.5 (during mid-gestation) (16); inactivation of N-WASP is lethal at E11.0 (during organogenesis) (33,41); inactivation of Cdc42 is lethal at E3.5 (during implantation) (42); inactivation of Nap1 is lethal at E9.0 (during gastrulation); and inactivation of Rac1 is lethal at E7.5 (during gastrulation) (32,43,44). Abi2 mutants are viable and without morphological defects (27).…”

Abl-interactor-1 (Abi1) has a role in cardiovascular and placental development and is a binding partner of the α4 integrin

“…WAVE2-deficient mice exhibit impaired vascular remodeling in response to growth factor stimulation (24). Further, mouse mutants of Nap1, a direct binding partner of Abi1 in the WAVE complex, also exhibit defective chorioallantoic fusion, and other developmental phenotypes associated with defects in cell migration and adhesion (34). Interestingly, heterozygous insertional mutants of Nap1, which display neural tube closure defects, also have dramatic reductions in vessel diameter (35).…”