Notchless affects the Wnt pathway during pre-implantation development in mice

In order to determine the genetic pathways required for early mammalian fetal development, a positional cloning study was performed using two embryonic lethal lines of mice, l11Jus1, and l11Jus4. These lines were derived from an ENU mutagenesis screen targeted to a 34Mb region of mouse chromosome 11. The mutations were identified for both lines in Notchless (Nle1), a regulator of the Notch pathway, and a gene expression study was initiated. RNA was isolated from wild type and l11Jus1 homozygous embryos for qRT- PCR analysis using a Notch pathway focused PCR array. Genes present on the array include Notch ligands, receptors, target genes, transcription factors, and genes from other related genetic pathways. Very few Notch target genes were misregulated in the mutants, while there was a non-significant down regulation of some of the ligands and receptors. Only Cdkn1a was significantly up-regulated in the mutants. Cdkn1a is a target gene of Notch signaling, but is also involved in apoptosis. Surprisingly, this study revealed a down-regulation of Wnt genes in the mutant embryos during pre-implantation development. Wnt and Notch are known to interact later in development during somitogenesis, but Notch is not required during pre-implantation development. This study shows a component of Notch signaling (i.e. Nle1) interacts with Wnt during peri-implantation in the developing embryo without affecting global Notch signaling. The over-expression of Cdkn1a could result from misregulation of Notch, disruption in another Nle1 dependent pathway during this developmental timepoint or because the embryo is undergoing apoptosis. Therefore, either Notch signaling interacts with Wnt during pre-implantation development, or Nle1 acts through a yet to be identified pathway that alters Wnt signaling