Lef1 is required for the transition of Wnt signaling from mesenchymal to epithelial cells in the mouse embryonic mammary gland

Inductive reciprocal signaling between mesenchymal and adjacent epithelia gives rise to skin appendages such as hair follicles and mammary glands. Lef1-mediated canonical Wnt signaling is required for morphogenesis of these skin appendages during embryogenesis. In order to define the role of canonical Wnt signaling during early embryonic mammary gland development, we determined the temporal and spatial changes in Wnt signaling during embryogenesis in wild-type and Lef1-deficient embryos harboring a Tcf/Lef1-βgal reporter (TOPGAL) transgene. In contrast to previous studies using TOPGAL mice from a distinct founder, we observe that Wnt signaling acts initially on mesenchymal cells associated with the sequential appearance of mammary placodes. As placode development progresses between 12.5 and 15.5 dpc, Wnt signaling progressively accumulates in the mammary epithelial compartment. By 18.5 dpc, βgal activity is confined to mesenchymal and epithelial cells near the nipple region. In Lef1-deficient embryos, the transition of Wnt signaling from mesenchyme to the mammary epithelia is blocked for placodes #1, 4 and 5 despite the expression of Tcf1 in epithelial cells. These placodes ultimately disappear by 15.5 dpc, while placodes 2 and 3 typically did not form in the absence of Lef1. Progressive loss of placodes 1, 4, and 5 is accompanied by increased apoptosis in mesenchymal cells adjacent to the mammary epithelial placodes. While factors important for embryonic mammary gland development, such as FGF7, are expressed normally in Lef1-deficient animals, one mediator of the Hedgehog (Hh)-signaling pathway is aberrantly expressed. Specifically, Shh, Ihh, and Gli2 are expressed in mammary epithelial cells at levels in Lef1-deficient animals similar to wild-type littermates. However, the signal for Ptc-1 is strongly reduced in mesenchymal cells surrounding the mammary placode in Lef1 mutants relative to wild-type embryos. The loss of Ptc-1, both a receptor for and transcriptional target of Hh signaling, suggests that Hh signaling is blocked in Lef1-deficient embryos. Thus, these data reveal distinct requirements of different mammary placodes for Lef1-dependent Wnt signaling. They further define dynamic changes in which cells integrate Lef1-dependent Wnt signaling during progression of embryonic mammary gland development