Elizabeth Lacy, PhD

Fig.1: 3D mouse embryo

We are interested in the molecular and cellular mechanisms that mediate mouse gastrulation, a fundamental developmental process that coordinates complex cell movements with cell proliferation to reorganize and differentiate the embryonic ectoderm into the 3 definitive germ layers of the fetus: ectoderm, mesoderm, and endoderm. The defining structure of the gastrulating mouse embryo is the primitive streak.

Based on fate mapping experiments, the primitive streak can be divided into 3 functional domains: the proximal region, which gives rise to the extraembryonic mesoderm of the yolk sac; the distal region, which generates definitive endoderm and node derived mesoderm; and the middle streak, which produces lateral plate (kidney, limb) and paraxial (somitic) mesoderm. Currently little is known in the mouse about the mechanisms that mediate the assembly of the primitive streak into these functional domains or about the signaling pathways that specify the different types of mesoderm generated from the streak.

To gain insight into the process of gastrulation, we are studying amnionless (amn), a recessive transgene induced mouse mutation that specifically impairs the formation and/or specification of the middle streak. Our analyses of the amn mutant have revealed that middle streak assembly is mediated by a previously unknown pathway, one that is genetically separable from those directing the formation of the proximal and distal streak regions. Therefore, a disrupted gene at the amn locus must play a key role in this pathway.

Additionally, chimera analyses have shown that this gene, the amn gene, functions in an extraembryonic tissue (either visceral endoderm or extraembryonic ectoderm) to direct the formation of the middle streak by the overlying embryonic ectoderm.

Characterization of the mutant and wild type loci has recently identified one gene that is physically disrupted by the transgene insertion. The disrupted gene is an excellent candidate for amn as it is expressed during gastrulation exclusively in visceral endoderm, one of the extraembryonic tissues implicated in the chimera analysis as the site of amn function.

Furthermore, a BAC clone containing this gene rescues the gastrulation stage defects of the amn mutant. Consistent with the model that amn disrupts, a required interaction between the embryonic ectoderm and the extraembryonic visceral endoderm, this amn candidate encodes a novel gene product that is predicted to be a type I transmembrane protein. Gene targeting experiments are in progress to confirm that this candidate is, in fact, the amn gene.

Future studies will focus on biochemically and genetically characterizing the expression and function of the amn gene product. In particular, we want to determine whether amn interacts with known signaling pathways and whether it directs middle streak assembly by regulating epiblast cell proliferation, differentiation, and or migration. New projects are also being initiated to define amn's role later in development during organogenesis and in the adult mouse.