Purna A. Joshi

Stromal-epithelial interactions underlie fundamental tissue morphogenesis in normal development, tissue regeneration, and cancer. Peripheral nerves are increasingly implicated as important stromal drivers of normal and malignant epithelial tissue biology. In the mammary gland, epithelial cell fate is highly dependent on stromal cues during distinct stages of postnatal growth, which are triggered by ovarian sex hormones estrogen and progesterone. However, the effects of sex hormones on peripheral nerves in the postnatal mammary gland are largely unknown. Here, we uncover extensive changes in peripheral sympathetic innervation in the mammary stromal microenvironment during puberty and pregnancy, which are periods of active postnatal mammary epithelial growth. We find that sex hormones induce sympathetic axonal branching through an intricate hormone-epithelial-nerve cross-talk. Specifically, estrogen and progesterone stimulate the expression of brain-derived neurotrophic factor (BDNF) in mammary hormone receptor-expressing luminal epithelial cells, which acts on Tropomyosin receptor kinase B (TrkB)-expressing sympathetic nerves to activate BDNF-TrkB signaling. Our findings illustrate a previously unrecognized capacity of hormone-sensing epithelial cells to modulate sympathetic innervation, providing a framework for understanding nerve dynamics during tissue regeneration and cancer.

Stromal-epithelial interactions underlie fundamental tissue morphogenesis in normal development, tissue regeneration and cancer. Peripheral nerves are increasingly implicated as important stromal drivers of normal and malignant epithelial tissue biology. In the mammary gland, epithelial cell fate is highly dependent on stromal cues during distinct stages of postnatal growth which are triggered by ovarian sex hormones estrogen and progesterone. However, the effects of sex hormones on peripheral nerves in the postnatal mammary gland is largely unknown. Here, we uncover extensive changes in peripheral sympathetic innervation in the mammary stromal microenvironment during puberty and pregnancy which are periods of active postnatal mammary epithelial growth. We find that sex hormones induce sympathetic axonal branching through an intricate hormone-epithelial-nerve cross-talk. Specifically, estrogen and progesterone stimulate the expression of brain-derived neurotrophic factor (BDNF) in mammary hormone receptor- expressing luminal epithelial cells which acts on Tropomyosin receptor kinase B (TrkB)- expressing sympathetic nerves to activate BDNF-TrkB signaling. Our findings illustrate a previously unrecognized capacity of hormone-sensing epithelial cells to modulate sympathetic innervation, providing a framework for understanding nerve dynamics during tissue regeneration and cancer.

Summary: Organ development is attributed to stem/progenitor cells which self-renew and differentiate into mature cells critical for tissue form and function. The prostate is an epithelial organ that participates in the production of seminal fluid. Bipotent and unipotent stem/progenitor cells resident in prostatic epithelia are shown to support its development. While the stroma is crucial for prostate organogenesis, precise stromal cells including immature stromal subsets involved in prostate biology are unknown. Utilizing genetic reporter and lineage tracing mouse models, we identify a PDGFRα ⁺ mesenchymal population in the fibromuscular stroma that harbors progenitors which undergo a mesenchymal-to-epithelial transition, generating prostatic luminal and basal epithelial lineages during postnatal development. Further, these mesenchymal progenitors and their epithelial progeny persist in the adult, demonstrating their self-renewal potential. Our findings unveil a prostatic progenitor beyond the epithelium, laying down a framework for probing the contribution of stromal progenitors to the normal and diseased prostate. Graphical

Excess fat gain culminating in obesity is a mounting global pandemic associated with a myriad of diseases including a higher risk of developing multiple cancers such as breast cancer. The underlying biological basis for obesity-linked breast cancer is attributed to alterations in adipose tissue-derived endocrine, metabolic and inflammatory factors. However, the precise cell types responsible for generating a cancer-susceptible mammary gland in obesity are not well understood. Using diet-induced obesity in conjunction with genetic reporter mouse models, we reveal elevated mammary adipocyte progenitors (MAPs) expressing Platelet-Derived Growth Factor Receptor alpha (PDGFRα) in the mammary tissue microenvironment during obesity. Single-cell RNA sequencing analysis demonstrate intensified obesity-driven trajectories of MAPs to immune and epithelial progenitor cells. Further, lineage tracing indicates an invasive cellular state of MAPs that confers greater access of MAP descendants into the mammary epithelium concomitant with their transition into immune and epithelial cell fates. MAPs in obesity exhibit heightened activation of cancer-associated and inflammatory pathways where Egfr is identified as a common target upregulated in MAPs. Mechanistic studies on purified MAPs with a major obesity diet component and EGFR stimulation corroborate dynamic EGFR-mediated MAP responses. Our findings uncover MAPs as key contributors to forging an aberrant mammary gland in obesity, providing insight into the potential utility of targeting this cell lineage for eradicating cancer risk related to augmented adiposity.

Excess fat gain culminating in obesity is a mounting global pandemic associated with a myriad of diseases including a higher risk of developing multiple cancers such as breast cancer. The underlying biological basis for obesity-linked breast cancer is attributed to alterations in adipose tissue-derived endocrine, metabolic and inflammatory factors. However, the precise cell types responsible for generating a cancer-susceptible mammary gland in obesity are not well understood. Using diet-induced obesity in conjunction with genetic reporter mouse models, we reveal elevated mammary adipocyte progenitors (MAPs) expressing Platelet-Derived Growth Factor Receptor alpha (PDGFRα) in the mammary tissue microenvironment during obesity. Single-cell RNA sequencing analysis demonstrate intensified obesity-driven trajectories of MAPs to immune and epithelial progenitor cells. Further, lineage tracing indicates an invasive cellular state of MAPs that confers greater access of MAP descendants into the mammary epithelium concomitant with their transition into immune and epithelial cell fates. MAPs in obesity exhibit heightened activation of cancer-associated and inflammatory pathways where Egfr is identified as a common target upregulated in MAPs. Mechanistic studies on purified MAPs with a major obesity diet component and EGFR stimulation corroborate dynamic EGFR-mediated MAP responses. Our findings uncover MAPs as key contributors to forging an aberrant mammary gland in obesity, providing insight into the potential utility of targeting this cell lineage for eradicating cancer risk related to augmented adiposity.

The mammary gland experiences substantial remodeling and regeneration during development and reproductive life, facilitated by stem cells and progenitors that act in concert with physiological stimuli. While studies have focused on deciphering regenerative cells within the parenchymal epithelium, cell lineages in the stroma that may directly contribute to epithelial biology is unknown. Here we identify, in mouse, the transition of a PDGFRα⁺ mesenchymal cell population into mammary epithelial progenitors. In addition to being adipocyte progenitors, PDGFRα⁺ cells make a de novo contribution to luminal and basal epithelia during mammary morphogenesis. In the adult, this mesenchymal lineage primarily generates luminal progenitors within lobuloalveoli during sex hormone exposure or pregnancy. We identify cell migration as a key molecular event that is activated in mesenchymal progenitors in response to epithelium-derived chemoattractant. These findings demonstrate a stromal reservoir of epithelial progenitors and provide insight into cell origins and plasticity during mammary tissue growth.

Breast cancer is the most common female cancer, affecting approximately one in eight women during their life-time. Besides environmental triggers and hormones, inherited mutations in the breast cancer 1 (BRCA1) or BRCA2 genes markedly increase the risk for the development of breast cancer. Here, using two different mouse models, we show that genetic inactivation of the key osteoclast differentiation factor RANK in the mammary epithelium markedly delayed onset, reduced incidence, and attenuated progression of Brca1;p53 mutation-driven mammary cancer. Long-term pharmacological inhibition of the RANK ligand RANKL in mice abolished the occurrence of Brca1 mutation-driven pre-neoplastic lesions. Mechanistically, genetic inactivation of Rank or RANKL/RANK blockade impaired proliferation and expansion of both murine Brca1;p53 mutant mammary stem cells and mammary progenitors from human BRCA1 mutation carriers. In addition, genome variations within the RANK locus were significantly associated with risk of developing breast cancer in women with BRCA1 mutations. Thus, RANKL/RANK control progenitor cell expansion and tumorigenesis in inherited breast cancer. These results present a viable strategy for the possible prevention of breast cancer in BRCA1 mutant patients.

The Women’s Health Initiative (WHI) randomized trials of menopausal hormone therapy have provided insights that have dramatically changed clinical practice and