Primary Cilia Regulate Branching Morphogenesis during Mammary Gland Development

During mammary gland development, an epithelial bud undergoes branching morphogenesis to expand into a continuous tree-like network of branched ducts [ 1]. The process involves multiple cell types that are coordinated by hormones and growth factors coupled with signaling events including Wnt and Hedgehog [ 2–5]. Primary cilia play key roles in the development of many organs by coordinating extracellular signaling (of Wnt and Hedgehog) with cellular physiology [ 6–8]. During mammary development, we find cilia on luminal epithelial, myoepithelial, and stromal cells during early branching morphogenesis when epithelial ducts extend into the fat pad and undergo branching morphogenesis. When branching is complete, cilia disappear from luminal epithelial cells but remain on myoepithelial and stromal cells. Ciliary dysfunction caused by intraflagellar transport defects results in branching defects. These include decreased ductal extension and decreased secondary and tertiary branching, along with reduced lobular-alveolar development during pregnancy and lactation. We find increased canonical Wnt and decreased Hedgehog signaling in the mutant glands, which is consistent with the role of cilia in regulating these pathways [ 6–11]. In mammary gland and other organs, increased canonical Wnt [ 12–14] and decreased Hedgehog [ 15, 16] signaling decrease branching morphogenesis, suggesting that Wnt and Hedgehog signaling connect ciliary dysfunction to branching defects. ► Cilia are present on all cell types during mammary gland development ► Cilia are lost from luminal epithelia when mammary gland development is complete ► Loss of cilia decreases branching during mammary gland development and pregnancy ► Primary cilia regulate Wnt and Hedgehog signaling during mammary gland development