# Evidence for a Novel Multipotent Mammary Progenitor with Pregnancy-Specific Activity

 Title: Evidence for a Novel Multipotent Mammary Progenitor with Pregnancy-Specific Activity Author: Kaanta, Alice Citation: Kaanta, Alice. 2012. Evidence for a Novel Multipotent Mammary Progenitor with Pregnancy-Specific Activity. Doctoral dissertation, Harvard University. Full Text & Related Files: Kaanta_gsas.harvard_0084L_10069.pdf (26.62Mb; PDF) Abstract: The mouse mammary gland has emerged as a model system for studying processes involved in the development of epithelial tissues. Current evidence suggests the existence of a differentiation hierarchy in the mammary gland, consisting of a stem cell capable of reconstituting the tissue, progenitors with the capacity to produce specific functional cell types, and differentiated cells with limited or no repopulation potential. Although markers for mammary stem cells and progenitors have been identified, these populations have not been isolated to purity and our understanding of how they function in different stages of mammary development remains incomplete. Many adult stem cells are mitotically quiescent and can therefore retain a DNA or histone label significantly longer than differentiated cells. In an attempt to identify mammary stem cells/progenitors by histone label retention, I crossed a mouse carrying the tetracycline-inducible histone 2b/eGFP (H2BGFP) gene with tetracycline transactivator strains expected to induce H2BGFP in the mammary gland. H2BGFP expression was induced in the mammary gland until puberty and then chased for 6-8 weeks; $$H2BGFP^+$$ label retaining cells were isolated and assayed. Transplantation experiments comparing MMTVrtTA/H2BGFP MECs isolated after induction to MMTVrtTA/H2BGFP MECs retaining label post-chase failed to prove that label retention enriches for stem cells/progenitors in the MMTVrtTA/H2BGFP system. During the course of these experiments, I unexpectedly discovered that MMTVrtTA induced H2BGFP expression exclusively in the $$CD24^+/CD29^+$$ and $$CD24^+/CD29^{lo}$$ populations, which contain stem cells and progenitors, respectively. Interestingly, I also discovered that H2BGFP+/CD24+/CD29lo MECs developed limited mammary outgrowths in vivo and that pregnancy increased the repopulation ability of these cells by 5-10-fold. H2BGFP+/CD24+/CD29lo outgrowths contained all mammary lineages and produced milk, but were unable to self-renew in serial transplant assays. Furthermore, $$H2BGFP^+/CD24^+/CD29^{lo}$$ and $$H2BGFP^-/CD24^+/CD29^{lo}$$ MECs had distinct gene expression profiles, with H2BGFP+/CD24+/CD29lo MECs expressing lower levels of transcripts involved in mammary development and differentiation. These data provide evidence for the existence of a multipotent, pregnancy-activated mammary progenitor and suggests that different progenitor populations are responsible for mammary expansion during puberty and pregnancy. Future studies may identify FACS markers for purification of pregnancy-activated progenitors and further elucidate the role of different mammary cell types during pregnancy. Terms of Use: This article is made available under the terms and conditions applicable to Other Posted Material, as set forth at http://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#LAA Citable link to this page: http://nrs.harvard.edu/urn-3:HUL.InstRepos:10087379 Downloads of this work: