UC Santa Cruz

The breast (mammary gland) is a dynamic organ that undergoes multiple stages of expansion during puberty and pregnancy. During pubertal growth, the mammary gland undergoes branching morphogenesis to generate a bilayered-branched epithelium. Driving branching morphogenesis are extracellular cues that regulate cell proliferation and tissue expansion. The WNT superfamily of secreted signaling molecules governs many of the developmental processes that occur during mammary gland morphogenesis through interactions with specific receptors. While canonical WNT receptors have been investigated in the mammary gland, the role of receptors that function solely in non-canonical WNT signaling remain elusive. In Chapter 1, I review current knowledge of extracellular cues that regulate asymmetric cell division, an important aspect of development. Next, I show that non-canonical WNT receptors ROR2 and VANGL2 regulate branching and proliferation of the mammary gland. In the first study, I characterize Ror2-/- mice and perform serial transplantation experiments that reveal hyperbranching and tissue longevity phenotypes. In vivo experiments in which WT and Ror2-/- mammary outgrowths are exposed to exogenous WNT5A signals reveal that ROR2 is required for restricting the process of branching morphogenesis. In a second study, I show that the non-canonical WNT/Planar cell polarity protein VANGL2 regulates multiple types of growth in the mammary gland. Mammary reconstitution assays, in which I transplanted fragments of WT or VANGL2 mutant tissue into mammary fat pads that have their endogenous epithelia removed, revealed that loss of VANGL2 function generates smaller mammary outgrowths compared to WT. These mutants also display defects in branching. Using lentiviral- mediated knockdown of Vangl2 in cultured primary mammary epithelial cells, I stratified the function of VANGL2 by cell-type. Lastly, I found that loss of VANGL2 function leads to down regulation of the transcriptional repressor Bmi1. Together, the studies provide insight into the mechanisms that drive morphogenesis during mammary gland development.