Explore the vast range of titles available.

The mammary epithelium undergoes profound morphogenetic changes during development. Architecturally, it comprises two primary lineages, the inner luminal and outer myoepithelial cell layers. Two opposing concepts on the nature of mammary stem cells (MaSCs) in the postnatal gland have emerged. One model, based on classical transplantation assays, postulates that bipotent MaSCs have a key role in coordinating ductal epithelial expansion and maintenance in the adult gland, whereas the second model proposes that only unipotent MaSCs identified by lineage tracing contribute to these processes. Through clonal cell-fate mapping studies using a stochastic multicolour cre reporter combined with a new three-dimensional imaging strategy, we provide evidence for the existence of bipotent MaSCs as well as distinct long-lived progenitor cells. The cellular dynamics at different developmental stages support a model in which both stem and progenitor cells drive morphogenesis during puberty, whereas bipotent MaSCs coordinate ductal homeostasis and remodelling of the mouse adult gland. Through the use of a novel three-dimensional imaging technique, used in conjunction with a multicolour reporter that allows lineage tracing and cell tracking of entire mammary ducts in vivo , bipotent stem cells are shown to have a central role in both puberty and long-term maintenance; in addition, long-lived luminal progenitor cells with a prominent role in ductal expansion are identified. Active bipotent stem cells in the mammary gland The respective roles of bipotent and unipotent stem cells in the mammalian mammary gland are a focus of much current research. Jane Visvader and colleagues have developed a three-dimensional imaging technique to use in combination with a stochastic multicolour reporter that allows lineage tracing at a clonal level and cell tracking in vivo of entire mammary ducts. Using this approach the authors show that bipotent mammary stem cells have a central role in the morphogenesis of the gland during puberty and in long-term maintenance. They also identify long-lived luminal progenitor cells in situ that have a prominent role in ductal expansion.

A new study by Jane Visvader, Geoff Lindeman and colleagues reports on the potential role of RANK signaling in pre-neoplastic breast tissue from BRCA1 -mutation carriers and in a mouse model of Brca1 -deficient mammary cancer, suggesting that targeting RANK could be explored as an approach to prevent growth of tumors harboring mutated BRCA1 . Individuals who have mutations in the breast-cancer-susceptibility gene BRCA1 (hereafter referred to as BRCA1 -mutation carriers) frequently undergo prophylactic mastectomy to minimize their risk of breast cancer. The identification of an effective prevention therapy therefore remains a 'holy grail' for the field. Precancerous BRCA1 mut/+ tissue harbors an aberrant population of luminal progenitor cells 1 , and deregulated progesterone signaling has been implicated in BRCA1- associated oncogenesis 2 , 3 , 4 , 5 . Coupled with the findings that tumor necrosis factor superfamily member 11 (TNFSF11; also known as RANKL) is a key paracrine effector of progesterone signaling 6 , 7 , 8 , 9 , 10 and that RANKL and its receptor TNFRSF11A (also known as RANK) contribute to mammary tumorigenesis 11 , 12 , 13 , we investigated a role for this pathway in the pre-neoplastic phase of BRCA1 -mutation carriers. We identified two subsets of luminal progenitors (RANK + and RANK − ) in histologically normal tissue of BRCA1 -mutation carriers and showed that RANK + cells are highly proliferative, have grossly aberrant DNA repair and bear a molecular signature similar to that of basal-like breast cancer. These data suggest that RANK + and not RANK − progenitors are a key target population in these women. Inhibition of RANKL signaling by treatment with denosumab in three-dimensional breast organoids derived from pre-neoplastic BRCA1 mut/+ tissue attenuated progesterone-induced proliferation. Notably, proliferation was markedly reduced in breast biopsies from BRCA1 -mutation carriers who were treated with denosumab. Furthermore, inhibition of RANKL in a Brca1- deficient mouse model substantially curtailed mammary tumorigenesis. Taken together, these findings identify a targetable pathway in a putative cell-of-origin population in BRCA1 -mutation carriers and implicate RANKL blockade as a promising strategy in the prevention of breast cancer.

The mammary epithelium comprises two primary cellular lineages, but the degree of heterogeneity within these compartments and their lineage relationships during development remain an open question. Here we report single-cell RNA profiling of mouse mammary epithelial cells spanning four developmental stages in the post-natal gland. Notably, the epithelium undergoes a large-scale shift in gene expression from a relatively homogeneous basal-like program in pre-puberty to distinct lineage-restricted programs in puberty. Interrogation of single-cell transcriptomes reveals different levels of diversity within the luminal and basal compartments, and identifies an early progenitor subset marked by CD55. Moreover, we uncover a luminal transit population and a rare mixed-lineage cluster amongst basal cells in the adult mammary gland. Together these findings point to a developmental hierarchy in which a basal-like gene expression program prevails in the early post-natal gland prior to the specification of distinct lineage signatures, and the presence of cellular intermediates that may serve as transit or lineage-primed cells. The mammary epithelium comprises two cell lineages but the heterogeneity amongst these during development is unclear. Here, the authors report single-cell RNA sequencing of the mouse mammary epithelium at four developmental stages, revealing diversity in both compartments and a transcriptional shift with puberty onset.

Bone marrow is a preferred metastatic site for multiple solid tumours and is associated with poor prognosis and significant morbidity. Accumulating evidence indicates that cancer cells colonise specialised niches within the bone marrow to support their long-term propagation, but the precise location and mechanisms that mediate niche interactions are unknown. Using breast cancer as a model of solid tumour metastasis to the bone marrow, we applied large-scale quantitative three-dimensional imaging to characterise temporal changes in the bone marrow microenvironment during disease progression. We show that mouse mammary tumour cells preferentially home to a pre-existing metaphyseal domain enriched for type H vessels. Metastatic lesion outgrowth rapidly remodelled the local vasculature through extensive sprouting to establish a tumour-supportive microenvironment. The evolution of this tumour microenvironment reflects direct remodelling of the vascular endothelium through tumour-derived granulocyte-colony stimulating factor (G-CSF) in a hematopoietic cell-independent manner. Therapeutic targeting of the metastatic niche by blocking G-CSF receptor inhibited pathological blood vessel remodelling and reduced bone metastasis burden. These findings elucidate a mechanism of ‘host’ microenvironment hijacking by mammary tumour cells to subvert the local microvasculature to form a specialised, pro-tumorigenic niche. The visualisation of the bone metastasis process in a spatial temporal manner is lacking. Here, the authors use three-dimensional quantitative imaging and show that mouse mammary tumour cells preferentially home to endothelial subtype type H vessels within the bone marrow and remodel this vasculature by producing granulocyte-colony stimulating factor.

The mammary gland represents a unique tissue to study organogenesis as it predominantly develops in the post-natal animal and undergoes dramatic morphogenetic changes during puberty and the reproductive cycle. The physiological function of the mammary gland is to produce milk to sustain the newborn. Here we view the lactating gland through three-dimensional confocal imaging of intact tissue. We observed that the majority of secretory alveolar cells are binucleated. These cells first arise in very late pregnancy due to failure of cytokinesis and are larger than mononucleated cells. Augmented expression of Aurora kinase-A and Polo-like kinase-1 at the lactogenic switch likely mediates the formation of binucleated cells. Our findings demonstrate an important physiological role for polyploid mammary epithelial cells in lactation, and based on their presence in five different species, suggest that binucleated cells evolved to maximize milk production and promote the survival of offspring across all mammalian species. The main function of the mammary gland is to produce milk to sustain offspring. Here, the authors show that secretory alveolar cells in the lactating gland in several species are binucleated, which increases milk production, and that binucleation is regulated by Aurora kinase-A and Polo-like kinase-1.

Background Heterogeneity within the mouse mammary epithelium and potential lineage relationships have been recently explored by single-cell RNA profiling. To further understand how cellular diversity changes during mammary ontogeny, we profiled single cells from nine different developmental stages spanning late embryogenesis, early postnatal, prepuberty, adult, mid-pregnancy, late-pregnancy, and post-involution, as well as the transcriptomes of micro-dissected terminal end buds (TEBs) and subtending ducts during puberty. Methods The single cell transcriptomes of 132,599 mammary epithelial cells from 9 different developmental stages were determined on the 10x Genomics Chromium platform, and integrative analyses were performed to compare specific time points. Results The mammary rudiment at E18.5 closely aligned with the basal lineage, while prepubertal epithelial cells exhibited lineage segregation but to a less differentiated state than their adult counterparts. Comparison of micro-dissected TEBs versus ducts showed that luminal cells within TEBs harbored intermediate expression profiles. Ductal basal cells exhibited increased chromatin accessibility of luminal genes compared to their TEB counterparts suggesting that lineage-specific chromatin is established within the subtending ducts during puberty. An integrative analysis of five stages spanning the pregnancy cycle revealed distinct stage-specific profiles and the presence of cycling basal, mixed-lineage, and 'late' alveolar intermediates in pregnancy. Moreover, a number of intermediates were uncovered along the basal-luminal progenitor cell axis, suggesting a continuum of alveolar-restricted progenitor states. Conclusions This extended single cell transcriptome atlas of mouse mammary epithelial cells provides the most complete coverage for mammary epithelial cells during morphogenesis to date. Together with chromatin accessibility analysis of TEB structures, it represents a valuable framework for understanding developmental decisions within the mouse mammary gland.

Contrary to the belief that basal-like breast cancers develop from mammary stem cells in BRCA1 mutation carriers, an aberrant luminal progenitor population might be the target for transformation in basal tumors in these individuals ( pages 842–844 ). Basal-like breast cancers arising in women carrying mutations in the BRCA1 gene, encoding the tumor suppressor protein BRCA1, are thought to develop from the mammary stem cell. To explore early cellular changes that occur in BRCA1 mutation carriers, we have prospectively isolated distinct epithelial subpopulations from normal mammary tissue and preneoplastic specimens from individuals heterozygous for a BRCA1 mutation. We describe three epithelial subsets including basal stem/progenitor, luminal progenitor and mature luminal cells. Unexpectedly, we found that breast tissue from BRCA1 mutation carriers harbors an expanded luminal progenitor population that shows factor-independent growth in vitro . Moreover, gene expression profiling revealed that breast tissue heterozygous for a BRCA1 mutation and basal breast tumors were more similar to normal luminal progenitor cells than any other subset, including the stem cell–enriched population. The c-KIT tyrosine kinase receptor (encoded by KIT ) emerged as a key marker of luminal progenitor cells and was more highly expressed in BRCA1 -associated preneoplastic tissue and tumors. Our findings suggest that an aberrant luminal progenitor population is a target for transformation in BRCA1 -associated basal tumors .

Mammary stem cells The existence of mammary stem cells has been inferred from genetic analysis of human breast tissue, and from the fact that mouse mammary gland can regenerate from tissue fragments. Two groups now report the isolation of mammary stem cells from mice. Shackleton et al . use a technique based on the introduction of a marker for stem-cell function. A single cell from this population then showed its potency by regenerating an entire mammary gland in vivo . This population of cells is expanded in a pre-malignant mammary tumour model, providing support for the concept of a mammary cancer stem cell. In a paper published online, Stingl et al . report the use of a powerful limiting dilution transplant procedure to purify to near homogeneity a rare subset of adult mouse mammary cells that can individually regenerate an entire mammary gland within six weeks. The existence of mammary stem cells (MaSCs) has been postulated from evidence that the mammary gland can be regenerated by transplantation of epithelial fragments in mice 1 , 2 , 3 . Interest in MaSCs has been further stimulated by their potential role in breast tumorigenesis 4 . However, the identity and purification of MaSCs has proved elusive owing to the lack of defined markers. We isolated discrete populations of mouse mammary cells on the basis of cell-surface markers and identified a subpopulation (Lin - CD29 hi CD24 + ) that is highly enriched for MaSCs by transplantation. Here we show that a single cell, marked with a LacZ transgene, can reconstitute a complete mammary gland in vivo . The transplanted cell contributed to both the luminal and myoepithelial lineages and generated functional lobuloalveolar units during pregnancy. The self-renewing capacity of these cells was demonstrated by serial transplantation of clonal outgrowths. In support of a potential role for MaSCs in breast cancer, the stem-cell-enriched subpopulation was expanded in premalignant mammary tissue from MMTV- wnt-1 mice and contained a higher number of MaSCs. Our data establish that single cells within the Lin - CD29 hi CD24 + population are multipotent and self-renewing, properties that define them as MaSCs.

Hair follicles cycle through expansion, regression and quiescence. To investigate the role of MCL‑1, a BCL‑2 family protein with anti‑apoptotic and apoptosis‑unrelated functions, we delete Mcl‑1 within the skin epithelium using constitutive and inducible systems. Constitutive Mcl‑1 deletion does not impair hair follicle organogenesis but leads to gradual hair loss and elimination of hair follicle stem cells. Acute Mcl‑1 deletion rapidly depletes activated hair follicle stem cells and completely blocks depilation‑induced hair regeneration in adult mice, while quiescent hair follicle stem cells remain unaffected. Single‑cell RNA‑seq profiling reveals the engagement of P53 and DNA mismatch repair signaling in hair follicle stem cells upon depilation‑induced activation. Trp53 deletion rescues hair regeneration defects caused by acute Mcl‑1 deletion, highlighting a critical interplay between P53 and MCL‑1 in balancing proliferation and death. The ERBB pathway plays a central role in sustaining the survival of adult activated hair follicle stem cells by promoting MCL‑1 protein expression. Remarkably, the loss of a single Bak allele, a pro‑apoptotic Bcl‑2 effector gene, rescues Mcl‑1 deletion‑induced defects in both hair follicles and mammary glands. These findings demonstrate the pivotal role of MCL‑1 in inhibiting proliferation stress‑induced apoptosis when quiescent stem cells activate to fuel tissue regeneration. Here the authors unveil the essential role of MCL-1 for adult hair follicle regeneration and inhibition of proliferation stress-induced apoptosis in mice. They also identify a P53/MCL-1/BAK axis balancing proliferation and death of activated hair follicle stem cells to ensure proper hair growth.

Overexpression of the prosurvival protein BCL-2 is common in breast cancer. Here we have explored its role as a potential therapeutic target in this disease. BCL-2, its anti-apoptotic relatives MCL-1 and BCL-XL, and the proapoptotic BH3-only ligand BIM were found to be coexpressed at relatively high levels in a substantial proportion of heterogeneous breast tumors, including clinically aggressive basal-like cancers. To determine whether the BH3 mimetic ABT-737 that neutralizes BCL-2, BCL-XL, and BCL-W had potential efficacy in targeting BCL-2–expressing basal-like triple-negative tumors, we generated a panel of primary breast tumor xenografts in immunocompromised mice and treated recipients with either ABT-737, docetaxel, or a combination. Tumor response and overall survival were significantly improved by combination therapy, but only for tumor xenografts that expressed elevated levels of BCL-2. Treatment with ABT-737 alone was ineffective, suggesting that ABT-737 sensitizes the tumor cells to docetaxel. Combination therapy was accompanied by a marked increase in apoptosis and dissociation of BIM from BCL-2. Notably, BH3 mimetics also appeared effective in BCL-2–expressing xenograft lines that harbored p53 mutations. Our findings provide in vivo evidence that BH3 mimetics can be used to sensitize primary breast tumors to chemotherapy and further suggest that elevated BCL-2 expression constitutes a predictive response marker in breast cancer.