Abstract

Cell fate specification in the mouse embryo depends on the interplay between intercellular signaling and morphogenesis. Despite the essential role of cell movements in the establishment of the body plan, molecular genetic analysis of mouse development has focused on the signaling pathways and transcription factors that control cell fate, and we understand much less about the genetic control of embryonic cell migration and tissue morphogenesis.

WAVE complexes are regulators of the actin cytoskeleton that couple extracellular signals to polarized cell movement. Here I 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. 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 EMT at gastrulation appear normal. A striking phenotype seen in approximately one quarter of Nap1 mutants is the duplication of the anterior-posterior body axis. The axis duplications arise because Nap1 is required for normal polarization and migration of cells of the anterior visceral endoderm (AVE), an early extraembryonic organizer tissue. Thus the Nap1 mutant phenotypes define the critical roles of Nap1/WAVE-mediated actin regulation in tissue organization and establishment of the body plan of the mammalian embryo.

The signals that control patterning and cell fate specification can be controlled by regulating both expression and post-translational modification of pathway components. Most secreted and transmembrane proteins are modified by addition of glycans to specific asparagine residues (N-glycosylation). However, the biological roles of N-glycans are not well understood. Here I show that embryos mutant for Alg5, one of the enzymes required to build the precursor of all N-glycans, fail to specify the left-right body axis. This identifies a previously unrecognized role for N-glycans in regulating signaling and specification in the mouse embryo.