Eph receptors and ephrin ligands are key players in many developmental processes including embryo patterning, angiogenesis, and axon guidance. Eph/ephrin interactions lead to the generation of a bidirectional signal, in which both the Eph receptors and the ephrins activate downstream signaling cascades simultaneously. To understand the role of ephrin-B1 and the importance of ephrin-B1-induced reverse signaling during embryonic development, we have generated mouse lines carrying mutations in the efnb1 gene. Complete ablation of ephrin-B1 resulted in perinatal lethality associated with a range of phenotypes, including defects in neural crest cell (NCC)-derived tissues, incomplete body wall closure, and abnormal skeletal patterning. Conditional deletion of ephrin-B1 demonstrated that ephrin-B1 acts autonomously in NCCs, and controls their migration. Last, a mutation in the PDZ binding domain indicated that ephrin-B1-induced reverse signaling is required in NCCs. Our results demonstrate that ephrin-B1 acts both as a ligand and as a receptor in a tissue-specific manner during embryogenesis.

To uncover roles for the Hoxa-5 gene during embryogenesis, we have focused on identifying structural and functional defects in organ systems underlying the perinatal lethality in Hoxa-5 homozygous mutants. Analysis of the mutant phenotype shows that Hoxa-5 is essential for normal organogenesis and function of the respiratory tract. In homozygous newborn mutants, improper tracheal and lung morphogenesis can lead to tracheal occlusion, and to respiratory distress associated with a marked decrease in the production of surfactant proteins. Collectively, these defects likely underlie the pronounced mortality of homozygous mutant pups. Furthermore, the loss of Hoxa-5 function results in altered TTF-1, HNF-3 beta, and N-myc gene expression in the pulmonary epithelium. Since expression of Hoxa-5 is confined to the mesenchymal component of the developing trachea and lung, the effects observed in epithelial cells may result from a disruption of normal epithelial-mesenchymal interactions.

Respiration in mammals relies on the rhythmic firing of neurons within the Phrenic Motor Column (PMC), a motor neuron group that provides the sole source of diaphragm innervation. Despite their essential role in breathing, the specific determinants of PMC identity and patterns of connectivity are largely unknown. We show that two Hox genes, Hoxa5 and Hoxc5, control diverse aspects of PMC development including their clustering, intramuscular branching, and survival. In mice lacking Hox5 genes in motor neurons, axons extend to the diaphragm but fail to arborize, leading to respiratory failure. Genetic rescue of cell death fails to restore columnar organization and branching patterns, indicating these defects are independent of neuronal loss. Unexpectedly, late Hox5 removal preserves columnar organization but depletes PMC number and branches, demonstrating a continuous requirement for Hox function in motor neurons. These findings indicate that Hox5 genes orchestrate PMC development through deployment of temporally distinct wiring programs.