In vivo analysis of mouse mammary stem cell dynamics

Mammary stem cells play critical roles in the formation of the mammary gland and during its extensive expansion and involution during pregnancy. However, traditional markers to identify mammary stem cells have made it difficult to track mammary stem cell numbers in individual mice or to localize their position within the gland. To reconcile these difficulties, we have developed a novel mouse model in which mammary stem cells isolated from a transgenic mouse are the only mammary epithelial population to express appreciable levels of a luciferase reporter. Single cells isolated solely on transgene expression are able to reconstitute a functional mammary gland in vivo. By performing immunohistochemistry against the luciferase protein we localized mammary stem cells in both basal and luminal positions. Through the use of bioluminescent imaging, we non-invasively tracked mammary stem cell dynamics in individual mice throughout pregnancy. Using this approach, we observed a transient, extensive expansion of mammary stem cells during pregnancy before dropping back down to or below baseline levels after weaning. These findings are particularly interesting when considered in the context of pregnancy-dependent breast cancer protection. The changes in mammary stem/progenitor cell number observed closely mimic the changes in breast cancer risk during and after pregnancy, suggesting a possible mechanism for this protection. To better explore this connection we tracked mammary epithelial dynamics in MMTV-Neu mice in which pregnancy promotes rather than inhibits tumorigenesis. We observed a reduced expansion of mammary stem cells during pregnancy in this strain and ultimately reduced involution after weaning. To aid in future analyses of mammary stem cells, we analyzed the transcriptional profile of populations from pregnant and virgin mice using traditional markers and our transgenic marker to identify candidate surface proteins and transcriptional regulators of mammary stem cells. Two of these, PERP and GPR56 were validated to enrich for mammary stem cells with equal efficiency compared to published surface marker profiles. Use of this model in the future will significantly aid in further understanding how mammary stem cells are regulated by internal signaling pathways and external cues and in investigating what role mammary stem cells play in breast cancer tumorigenesis.