Abstract
Fibroblasts have been shown to be one of the essential players for mammary gland organization. Here, we identify two major types of mouse mammary gland fibroblasts through single-cell RNA sequencing analysis: Dpp4 + fibroblasts and Dpp4 - fibroblasts. Each population exhibits unique functional characteristics as well as discrete localization in normal mouse mammary glands. Remarkably, estrogen, a crucial mediator of mammary gland organization, alters the gene expression profiles of fibroblasts in a population-specific manner, without distinct activation of estrogen receptor signaling. Further integrative analysis with the inclusion of five other publicly available datasets reveals a directional differentiation among the mammary gland fibroblast populations. Moreover, the combination with the mouse mammary epithelium atlas allows us to infer multiple potential interactions between epithelial cells and fibroblasts in mammary glands. This study provides a comprehensive view of mouse mammary gland fibroblasts at the single-cell level.
Highlights
The mammary gland is a dynamic organ that undergoes major morphological changes after birth to develop its fully functional structure and is continuously modulated by ovarian hormones, such as estrogen and progesterone, with cyclic changes in their levels
Menopausal transition has been recognized as a window of susceptibility in a woman’s life because significant structural and functional changes occur in the mammary gland, as well as alterations in the mammary micro-environment and hormone signaling, that may influence breast cancer risk (Terry et al, 2019)
We previously reported that Ccl2 was a fibroblast-derived and estrogen-induced factor which led to macrophage recruitment (Kanaya et al, 2019), and we found it to be upregulated in the E2-treated Dpp4+ fibroblasts in our current study as well (Supplementary Figure S6H), suggesting that Ccl2 might be important in regard to fibroblastfibroblast interactions, especially in the presence of estrogen
Summary
The mammary gland is a dynamic organ that undergoes major morphological changes after birth to develop its fully functional structure and is continuously modulated by ovarian hormones, such as estrogen and progesterone, with cyclic changes in their levels. The mammary gland is comprised of two major components: mammary epithelium, which forms mammary ductal and lobular structures, and mammary stroma, which includes fibroblasts, adipocytes, preadipocytes, endothelial cells, and/or immune cells. The epithelium has been recognized to be an essential player in the mammary gland, the involvement of fibroblasts in mammary gland organization has been extensively discussed (Wiseman and Werb, 2002; Sumbal et al, 2021). Since ECM undergoes significant modifications during mammary gland organization such as ductal elongation and branching morphogenesis, fibroblasts are considered essential for proper gland development (Maller et al, 2010; Schedin and Keely, 2011). Mammary gland fibroblasts contribute towards many aspects of mammary gland organization, and carcinogenesis as well
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