09-P089 Mosaic invalidation of the mouse Notchless gene in the epiblast results in multiple developmental abnormalities

Notchless (Nle) has been identified in Drosophila as a direct regulator of Notch activity. In the mouse, we have previously demonstrated that mNle is instrumental for the maintenance of pluripotent cells at the blastocyst stage. To characterize the function of mNle in post-implantation embryos, we used mosaic epiblast-specific inactivation of a conditional mNle allele (mNle) taking advantage of the Cre recombinase expression in the Mox2-Cre transgenic line. Mox2-Cre;Nle mutant embryos were recovered at expected Mendelian ratio between E14.5 and E18.5. At these stages, mutant embryos exhibit severe developmental defects including the quasi absence of axial skeleton, neural tube defects, oedema, hemorrhages and abnormal urogenital tract development. First anomalies can be detected by E9.5. At this stage, we observed a dramatically upregulated apoptosis in neural tube and somites. At E10.5, mutant embryos exhibit segmented but irregular and compact somites. Somitic lineage analyses reveal a primary defect in the most dorsal part of somites. To reveal an eventual disruption of Notch signalling in these mutant embryos, we use the Notch Activity Sensor transgenic line that we recently developed to monitor Notch activity. We observed that Notch reporter transgene was downregulated in the presomitic mesoderm and upregulated in the dorsal region of mutant embryos. Altogether, these analyses will allow to better understand the role of mNle during mouse development and its possible interaction with the Notch pathway.