Mammary stem cells have myoepithelial cell properties

Fibroblasts direct differentiation of human breast epithelial progenitors

BACKGROUND

Breast cancer arises within specific regions in the human breast referred to as the terminal duct lobular units (TDLUs). These are relatively dynamic structures characterized by sex hormone driven cyclic epithelial turnover. TDLUs consist of unique parenchymal entities embedded within a fibroblast-rich lobular stroma. Here, we established and characterized a new human breast lobular fibroblast cell line against its interlobular counterpart with a view to assessing the role of region-specific stromal cues in the control of TDLU dynamics.

METHODS

Primary lobular and interlobular fibroblasts were transduced to express human telomerase reverse transcriptase (hTERT). Differentiation of the established cell lines along lobular and interlobular pathways was determined by immunocytochemical staining and genome-wide RNA sequencing. Their functional properties were further characterized by analysis of mesenchymal stem cell (MSC) differentiation repertoire in culture and in vivo. The cells' physiological relevance for parenchymal differentiation was examined in heterotypic co-culture with fluorescence-activated cell sorting (FACS)-purified normal breast primary luminal or myoepithelial progenitors. The co-cultures were immunostained for quantitative assessment of epithelial branching morphogenesis, polarization, growth, and luminal epithelial maturation. In extension, myoepithelial progenitors were tested for luminal differentiation capacity in culture and in mouse xenografts. To unravel the significance of transforming growth factor-beta (TGF-β)-mediated crosstalk in TDLU-like morphogenesis and differentiation, fibroblasts were incubated with the TGF-β signaling inhibitor, SB431542, prior to heterotypic co-culture with luminal cells.

RESULTS

hTERT immortalized fibroblast cell lines retained critical phenotypic traits in culture and linked to primary fibroblasts. Cell culture assays and transplantation to mice showed that the origin of fibroblasts determines TDLU-like and ductal-like differentiation of epithelial progenitors. Whereas lobular fibroblasts supported a high level of branching morphogenesis by luminal cells, interlobular fibroblasts supported ductal-like myoepithelial characteristics. TDLU-like morphogenesis, at least in part, relied on intact TGF-β signaling.

CONCLUSIONS

The significance of the most prominent cell type in normal breast stroma, the fibroblast, in directing epithelial differentiation is largely unknown. Through establishment of lobular and interlobular fibroblast cell lines, we here demonstrate that epithelial progenitors are submitted to stromal cues for site-specific differentiation. Our findings lend credence to considering stromal subtleties of crucial importance in the development of normal breast and, in turn, breast cancer.

CXCL12γ induces human prostate and mammary gland development

BACKGROUND

Epithelial stem cells (ESCs) demonstrate a capacity to maintain normal tissues homeostasis and ESCs with a deregulated behavior can contribute to cancer development. The ability to reprogram normal tissue epithelial cells into prostate or mammary stem-like cells holds great promise to help understand cell of origin and lineage plasticity in prostate and breast cancers in addition to understanding normal gland development. We previously showed that an intracellular chemokine, CXCL12γ induced cancer stem cells and neuroendocrine characteristics in both prostate and breast adenocarcinoma cell lines. However, its role in normal prostate or mammary epithelial cell fate and development remains unknown. Therefore, we sought to elucidate the functional role of CXCL12γ in the regulation of ESCs and tissue development.

METHODS

Prostate epithelial cells (PNT2) or mammary epithelial cells (MCF10A) with overexpressed CXCL12γ was characterized by quantitative real-time polymerase chain reaction, Western blots, and immunofluorescence for lineage marker expression, and fluorescence activated cell sorting analyses and sphere formation assays to examine stem cell surface phenotype and function. Xenotransplantation animal models were used to evaluate gland or acini formation in vivo.

RESULTS

Overexpression of CXCL12γ promotes the reprogramming of cells with a differentiated luminal phenotype to a nonluminal phenotype in both prostate (PNT2) and mammary (MCF10A) epithelial cells. The CXCL12γ-mediated nonluminal type cells results in an increase of epithelial stem-like phenotype including the subpopulation of EPCAM^Lo /CD49f^Hi /CD24^Lo /CD44^Hi cells capable of sphere formation. Critically, overexpression of CXCL12γ promotes the generation of robust gland-like structures from both prostate and mammary epithelial cells in in vivo xenograft animal models.

CONCLUSIONS

CXCL12γ supports the reprogramming of epithelial cells into nonluminal cell-derived stem cells, which facilitates gland development.

Polyisocyanide Hydrogels as a Tunable Platform for Mammary Gland Organoid Formation

In the last decade, organoid technology has developed as a primary research tool in basic biological and clinical research. The reliance on poorly defined animal-derived extracellular matrix, however, severely limits its application in regenerative and translational medicine. Here, a well-defined, synthetic biomimetic matrix based on polyisocyanide (PIC) hydrogels that support efficient and reproducible formation of mammary gland organoids (MGOs) in vitro is presented. Only decorated with the adhesive peptide RGD for cell binding, PIC hydrogels allow MGO formation from mammary fragments or from purified single mammary epithelial cells. The cystic organoids maintain their capacity to branch for over two months, which is a fundamental and complex feature during mammary gland development. It is found that small variations in the 3D matrix give rise to large changes in the MGO: the ratio of the main cell types in the MGO is controlled by the cell-gel interactions via the cell binding peptide density, whereas gel stiffness controls colony formation efficiency, which is indicative of the progenitor density. Simple hydrogel modifications will allow for future introduction and customization of new biophysical and biochemical parameters, making the PIC platform an ideal matrix for in depth studies into organ development and for application in disease models.

H2AX Promoter Demethylation at Specific Sites Plays a Role in STAT5-Induced Tumorigenesis

Deregulated STAT5 activity in the mammary gland of transgenic mice results in parity-dependent latent tumorigenesis. The trigger for cell transformation was previously associated with hyperactivation of the H2AX proximal promoter in a small basal cell population during pregnancy. The current study focuses on the latent activation of tumor development. H2AX was highly expressed in carcinoma and adenocarcinoma as compared to the multiparous mammary gland, whereas pSTAT5 expression decreased in a tumor type-dependent manner. In contrast to the pregnant gland, no positive correlation between H2AX and pSTAT5 expression could be defined in carcinoma and adenocarcinoma. Using targeted methylation analysis, the methylation profile of the H2AX promoter was characterized in the intact gland and tumors. Average H2AX promoter methylation in the tumors was relatively high (~90%), but did not exceed that of the multiparous gland; 5mC methylation was higher in the differentiated tumors and negatively correlated with its oxidative product 5hmC and H2AX expression. Individual analysis of 25 H2AX promoter-methylation sites revealed two consecutive CpGs at positions -77 and - 54 that were actively demethylated in the multiparous gland, but not in their age-matched virgin counterpart. The different methylation profiles at these sites distinguished tumor types and may assume a prognostic role. In-silico and ChIP analyses revealed overlapping methylation-independent SP1-binding and methylation-dependent p53-binding to these sites. We propose that interference with SP1-assisted p53-binding to these sites abrogates H2AX's ability to arrest the cell cycle upon DNA damage, and contributes to triggering latent development of STAT5-induced tumors in estrapausal multiparous mice.

Heterotypic cell-cell communication regulates glandular stem cell multipotency

Glandular epithelia, including the mammary and prostate glands, are composed of basal cells (BCs) and luminal cells (LCs)^1,2. Many glandular epithelia develop from multipotent basal stem cells (BSCs) that are replaced in adult life by distinct pools of unipotent stem cells^1,3-8. However, adult unipotent BSCs can reactivate multipotency under regenerative conditions and upon oncogene expression^3,9-13. This suggests that an active mechanism restricts BSC multipotency under normal physiological conditions, although the nature of this mechanism is unknown. Here we show that the ablation of LCs reactivates the multipotency of BSCs from multiple epithelia both in vivo in mice and in vitro in organoids. Bulk and single-cell RNA sequencing revealed that, after LC ablation, BSCs activate a hybrid basal and luminal cell differentiation program before giving rise to LCs-reminiscent of the genetic program that regulates multipotency during embryonic development⁷. By predicting ligand-receptor pairs from single-cell data¹⁴, we find that TNF-which is secreted by LCs-restricts BC multipotency under normal physiological conditions. By contrast, the Notch, Wnt and EGFR pathways were activated in BSCs and their progeny after LC ablation; blocking these pathways, or stimulating the TNF pathway, inhibited regeneration-induced BC multipotency. Our study demonstrates that heterotypic communication between LCs and BCs is essential to maintain lineage fidelity in glandular epithelial stem cells.

Stromal cell diversity associated with immune evasion in human triple-negative breast cancer

The tumour stroma regulates nearly all stages of carcinogenesis. Stromal heterogeneity in human triple-negative breast cancers (TNBCs) remains poorly understood, limiting the development of stromal-targeted therapies. Single-cell RNA sequencing of five TNBCs revealed two cancer-associated fibroblast (CAF) and two perivascular-like (PVL) subpopulations. CAFs clustered into two states: the first with features of myofibroblasts and the second characterised by high expression of growth factors and immunomodulatory molecules. PVL cells clustered into two states consistent with a differentiated and immature phenotype. We showed that these stromal states have distinct morphologies, spatial relationships and functional properties in regulating the extracellular matrix. Using cell signalling predictions, we provide evidence that stromal-immune crosstalk acts via a diverse array of immunoregulatory molecules. Importantly, the investigation of gene signatures from inflammatory-CAFs and differentiated-PVL cells in independent TNBC patient cohorts revealed strong associations with cytotoxic T-cell dysfunction and exclusion, respectively. Such insights present promising candidates to further investigate for new therapeutic strategies in the treatment of TNBCs.

Parity reduces mammary repopulating activity but does not affect mammary stem cells defined as CD24 + CD29/CD49fhi in mice

BACKGROUND

Breast cancer (BCa) mortality is decreasing with early detection and improvement in therapies. The incidence of BCa, however, continues to increase, particularly estrogen-receptor-positive (ER +) subtypes. One of the greatest modifiers of ER + BCa risk is childbearing (parity), with BCa risk halved in young multiparous mothers. Despite convincing epidemiological data, the biology that underpins this protection remains unclear. Parity-induced protection has been postulated to be due to a decrease in mammary stem cells (MaSCs); however, reports to date have provided conflicting data.

METHODS

We have completed rigorous functional testing of repopulating activity in parous mice using unfractionated and MaSC (CD24^midCD49f^hi)-enriched populations. We also developed a novel serial transplant method to enable us to assess self-renewal of MaSC following pregnancy. Lastly, as each pregnancy confers additional BCa protection, we subjected mice to multiple rounds of pregnancy to assess whether additional pregnancies impact MaSC activity.

RESULTS

Here, we report that while repopulating activity in the mammary gland is reduced by parity in the unfractionated gland, it is not due to a loss in the classically defined MaSC (CD24⁺CD49f^hi) numbers or function. Self-renewal was unaffected by parity and additional rounds of pregnancy also did not lead to a decrease in MaSC activity.

CONCLUSIONS

Our data show instead that parity impacts on the stem-like activity of cells outside the MaSC population.

Flow Cytometry Definition of Rat Mammary Epithelial Cell Populations and Their Distinct Radiation Responses

Breast tissue is very susceptible to radiation-induced carcinogenesis, and mammary stem/progenitor cells are potentially important targets of this. The mammary epithelium is maintained as two mostly independent lineages of luminal and basal cells. To elucidate their immediate radiation responses, we analyzed the mammary glands of female Sprague-Dawley rats, a radiation carcinogenesis model, using colony formation, flow cytometry and immunofluorescence. The results revealed that flow cytometry successfully fractionates rat mammary cells into CD49fhi CD24lo basal, CD49fmed CD24hi luminal progenitor, and CD49flo CD24hi mature luminal populations, resembling human breast, rather than mouse tissues. The colony-forming ability of the basal cells was more radiosensitive than the luminal progenitor cells. Flow cytometry and immunofluorescence showed more efficient cell cycle arrest, γ-H2AX responses, and apoptosis in the irradiated luminal progenitor cells, than in the basal cells. These results provide important insights into the early phase of radiation-induced breast cancer.

In vivo Mouse Mammary Gland Formation

For years, the mammary gland serves as a perfect example to study the self-renew and differentiation of adult stem cells, and the regulatory mechanisms of these processes as well. To assess the function of given genes and/or other factors on stemness of mammary cells, several in vitro assays were developed, such as mammospheres formation assay, detection of stem cell markers by mRNA expression or flow cytometry and so on. However, the capacity of reconstruction of whole mount in the cleared fat pad of recipient female mice is a golden standard to estimate the stemness of the cells. Here we described a step-by-step protocol for in vivo mammary gland formation assay, including preparation of "cleared" recipients and mammary cells for implantation, the surgery process and how to assess the experimental results. Combined with manipulation of mammary cells via gene editing and /or drug treatment, this protocol could be very useful in the researches of mammary stem cells and mammary development.

Origin and Lineage Plasticity of Endogenous Lacrimal Gland Epithelial Stem/Progenitor Cells

The lacrimal gland (LG) is an exocrine organ responsible for the secretion of aqueous tear film. Regenerative and stem cell therapies that target LG repair are coming to the fore, although our understanding of LG cell lineage hierarchy is still incomplete. We utilize the analysis of label-retaining cells (LRCs) and genetic lineage tracing to define LG cell lineage hierarchy. Our study suggests that embryonic LG contains unique long-lived multipotent stem cells that give rise to all postnatal epithelial cell types. Following birth, lineages become established and the fate of progenitor cell descendants becomes restricted. However, some cell lineages retain plasticity after maturation and can trans-differentiate into other cell types upon injury. The demonstration that the LG contains progenitor cells with different levels of plasticity has profound implications for our understanding of LG gland function in homeostasis and disease and will be helpful for developing stem cell-based therapies in the future.

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Multipotent stem cells differentiate into distal Sox10+ and proximal Sox10− lineages

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Lineage-restricted progenitor cells sustain the long-term lacrimal gland maintenance

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Label-retaining cells are localized in the intercalated ducts and excretory ducts

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Some cell lineages in the adult lacrimal gland retain plasticity

Obesity reduces mammary epithelial cell TGFβ1 activity through macrophage-mediated extracellular matrix remodeling

Obesity is a risk factor for breast cancer in postmenopausal and high‐risk premenopausal women. Changes within the obese breast microenvironment may increase breast cancer risk. Transforming growth factor beta‐1 (TGFβ1) is a major regulator of mammary epithelial stem/progenitor cells, and its activity is dysregulated under conditions of obesity. Using a high‐fat diet model of obesity in mice and breast tissue from women, we observed that TGFβ1 activity is reduced in breast epithelial cells in obesity. Breast ducts and lobules demonstrated increased decorin in the extracellular matrix (ECM) surrounding epithelial cells, and we observed that decorin and latent TGFβ1 complexed together. Under conditions of obesity, macrophages expressed higher levels of decorin and were significantly increased in number surrounding breast epithelial cells. To investigate the relationship between macrophages and decorin expression, we treated obese mice with either IgG control or anti‐F4/80 antibodies to deplete macrophages. Mice treated with anti‐F4/80 antibodies demonstrated reduced decorin surrounding mammary ducts and enhanced TGFβ1 activity within mammary epithelial cells. Given the role of TGFβ1 as a tumor suppressor, reduced epithelial TGFβ1 activity and enhanced TGFβ1 within the ECM of obese mammary tissue may enhance breast cancer risk.

Organoid cultures from normal and cancer-prone human breast tissues preserve complex epithelial lineages

Recently, organoid technology has been used to generate a large repository of breast cancer organoids. Here we present an extensive evaluation of the ability of organoid culture technology to preserve complex stem/progenitor and differentiated cell types via long-term propagation of normal human mammary tissues. Basal/stem and luminal progenitor cells can differentiate in culture to generate mature basal and luminal cell types, including ER+ cells that have been challenging to maintain in culture. Cells associated with increased cancer risk can also be propagated. Single-cell analyses of matched organoid cultures and native tissues by mass cytometry for 38 markers provide a higher resolution representation of the multiple mammary epithelial cell types in the organoids, and demonstrate that protein expression patterns of the tissue of origin can be preserved in culture. These studies indicate that organoid cultures provide a valuable platform for studies of mammary differentiation, transformation, and breast cancer risk.

Organoid technology has enabled the generation of several breast cancer organoids. Here, the authors combine propagation of normal human mammary tissues with mass cytometry to evaluate the ability of organoid culture technologies to preserve stem cells and differentiated cell types.

Membrane expression of the estrogen receptor ERα is required for intercellular communications in the mammary epithelium

17β-Estradiol induces the postnatal development of mammary gland and influences breast carcinogenesis by binding to the estrogen receptor ERα. ERα acts as a transcription factor but also elicits rapid signaling through a fraction of ERα expressed at the membrane. Here, we have used the C451A-ERα mouse model mutated for the palmitoylation site to understand how ERα membrane signaling affects mammary gland development. Although the overall structure of physiological mammary gland development is slightly affected, both epithelial fragments and basal cells isolated from C451A-ERα mammary glands failed to grow when engrafted into cleared wild-type fat pads, even in pregnant hosts. Similarly, basal cells purified from hormone-stimulated ovariectomized C451A-ERα mice did not produce normal outgrowths. Ex vivo, C451A-ERα basal cells displayed reduced matrix degradation capacities, suggesting altered migration properties. More importantly, C451A-ERα basal cells recovered in vivo repopulating ability when co-transplanted with wild-type luminal cells and specifically with ERα-positive luminal cells. Transcriptional profiling identified crucial paracrine luminal-to-basal signals. Altogether, our findings uncover an important role for membrane ERα expression in promoting intercellular communications that are essential for mammary gland development.

Single-Cell Transcriptomes Distinguish Stem Cell State Changes and Lineage Specification Programs in Early Mammary Gland Development

The mammary gland consists of cells with gene expression patterns reflecting their cellular origins, function, and spatiotemporal context. However, knowledge of developmental kinetics and mechanisms of lineage specification is lacking. We address this significant knowledge gap by generating a single-cell transcriptome atlas encompassing embryonic, postnatal, and adult mouse mammary development. From these data, we map the chronology of transcriptionally and epigenetically distinct cell states and distinguish fetal mammary stem cells (fMaSCs) from their precursors and progeny. fMaSCs show balanced co-expression of factors associated with discrete adult lineages and a metabolic gene signature that subsides during maturation but reemerges in some human breast cancers and metastases. These data provide a useful resource for illuminating mammary cell heterogeneity, the kinetics of differentiation, and developmental correlates of tumorigenesis.

Single-cell RNA sequencing of developing mouse mammary epithelia reveals the timing of lineage specification. Giraddi et al. find that fetal mammary stem cells co-express factors that define distinct lineages in their progeny and bear functionally relevant metabolic program signatures that change with differentiation and are resurrected in human breast cancers and metastases.

Long-Term In Vitro Expansion of Epithelial Stem Cells Enabled by Pharmacological Inhibition of PAK1-ROCK-Myosin II and TGF-β Signaling

Despite substantial self-renewal capability in vivo, epithelial stem and progenitor cells located in various tissues expand for a few passages in vitro in feeder-free condition before they succumb to growth arrest. Here, we describe the EpiX method, which utilizes small molecules that inhibit PAK1-ROCK-Myosin II and TGF-β signaling to achieve over one trillion-fold expansion of human epithelial stem and progenitor cells from skin, airway, mammary, and prostate glands in the absence of feeder cells. Transcriptomic and epigenomic studies show that this condition helps epithelial cells to overcome stresses for continuous proliferation. EpiX-expanded basal epithelial cells differentiate into mature epithelial cells consistent with their tissue origins. Whole-genome sequencing reveals that the cells retain remarkable genome integrity after extensive in vitro expansion without acquiring tumorigenicity. EpiX technology provides a solution to exploit the potential of tissue-resident epithelial stem and progenitor cells for regenerative medicine.

Stem Cells and the Differentiation Hierarchy in Mammary Gland Development

The mammary gland is a highly dynamic organ that undergoes profound changes within its epithelium during puberty and the reproductive cycle. These changes are fueled by dedicated stem and progenitor cells. Both short- and long-lived lineage-restricted progenitors have been identified in adult tissue as well as a small pool of multipotent mammary stem cells (MaSCs), reflecting intrinsic complexity within the epithelial hierarchy. While unipotent progenitor cells predominantly execute day-to-day homeostasis and postnatal morphogenesis during puberty and pregnancy, multipotent MaSCs have been implicated in coordinating alveologenesis and long-term ductal maintenance. Nonetheless, the multipotency of stem cells in the adult remains controversial. The advent of large-scale single-cell molecular profiling has revealed striking changes in the gene expression landscape through ontogeny and the presence of transient intermediate populations. An increasing number of lineage cell-fate determination factors and potential niche regulators have now been mapped along the hierarchy, with many implicated in breast carcinogenesis. The emerging diversity among stem and progenitor populations of the mammary epithelium is likely to underpin the heterogeneity that characterizes breast cancer.

Remodeling of Murine Mammary Adipose Tissue during Pregnancy, Lactation, and Involution

White adipocytes in the mammary gland stroma comprise the majority of the mammary gland mass. White adipocytes regulate numerous hormonal and metabolic processes and exhibit compositional and phenotypic plasticity. This plasticity is exemplified by the ability of mammary adipocytes to regress during lactation, when mammary epithelial cells expand to establish sufficient milk-producing alveoli. Upon weaning, the process reverses through mammary involution, during which adipocytes extensively regenerate, and alveolar epithelial cells disappear through cell death, returning the mammary gland to the non-lactating state. Despite intensive studies on the development and involution of the mammary alveolar epithelium, the fate of mammary adipocytes during pregnancy and lactation, and the origins of mammary adipocytes regenerated during mammary involution, is poorly understood. Here, we discuss the recent discoveries of the fate of mammary adipocytes during pregnancy and lactation in a number of different mouse models, and the lineage origin of mammary adipocytes regenerated during involution.

Mammary Epithelial Cell Lineage Changes During Cow's Life

Milk production is highly dependent on the optimal development of the mammary epithelium. It is therefore essential to better understand mammary epithelial cell growth and maintenance from the related epithelial lineage during the animal life. Here, we characterized the epithelial lineage at puberty, lactation and dry-off in bovine using the cell surface markers CD49_f, CD24, and CD10. The pubertal period was characterized by a high proportion of CD49_f^pos cells corresponding to various epithelial subpopulations, notably the CD24^pos subpopulations. The proportion of CD49_f^pos cells was weaker during lactation and dry-off, and CD24^pos cells were relatively few. Of note, the (sub)population profile at dry-off appeared close to that during lactation. Using a targeted gene approach, we associated specific genes with epithelial subpopulations, their expression level varying, or not, according to physiological stages. Caseins were only expressed in the CD49_f^medCD24^neg subpopulation. Basal marker genes (keratin(KRT)5, KRT14 and αSMA) were found in the CD49_f^highCD24^neg subpopulations. Luminal gene markers (KRT7, KRT8 and KRT19, CDH1 and the PRLR) were expressed in the CD49_f^lowCD24^neg subpopulation. The CD49_f^lowCD24^pos subpopulation, only abundant at puberty, expressed luminal gene markers and KI67 at high level. In contrast to others, the CD49_f^highCD24^pos cells accounted for a small proportion of total cells, decreasing from puberty to dry-off. They were characterized by expression of luminal and basal gene markers and low KI67 level. Interestingly, this subpopulation showed a remarkable stability of gene expression profile throughout physiological stages and bear the hallmark of quiescence that designate them as the potential bovine mammary stem cells.

Sox9 regulates the luminal stem/progenitor cell properties of salivary glands

Exocrine glands share a common morphology consisting of ductal, acinar, and basal/myoepithelial cells, but their functions and mechanisms of homeostasis differ among tissues. Salivary glands are an example of exocrine glands, and they have been reported to contain multipotent stem cells that differentiate into other tissues. In this study, we purified the salivary gland stem/progenitor cells of adult mouse salivary glands using the cell surface marker CD133 by flow cytometry. CD133⁺ cells possessed stem cell capacity, and the transplantation of CD133⁺ cells into the submandibular gland reconstituted gland structures, including functional acinar. CD133⁺ cells were sparsely distributed in the intercalated and exocrine ducts and expressed Sox9 at higher levels than CD133^- cells. Moreover, we demonstrated that Sox9 was required for the stem cell properties CD133⁺ cells, including colony and sphere formation. Thus, the Sox9-related signaling may control the regeneration salivary glands.

Identifying the murine mammary cell target of metformin exposure

The heterogeneity of breast cancer makes current therapies challenging. Metformin, the anti-diabetic drug, has shown promising anti-cancer activities in epidemiological studies and breast cancer models. Yet, how metformin alters the normal adult breast tissue remains elusive. We demonstrate metformin intake at a clinically relevant dose impacts the hormone receptor positive (HR+) luminal cells in the normal murine mammary gland. Metformin decreases total cell number, progenitor capacity and specifically reduces DNA damage in normal HR+ luminal cells, decreases oxygen consumption rate and increases cell cycle length of luminal cells. HR+ luminal cells demonstrate the lowest levels of mitochondrial respiration and capacity to handle oxidative stress compared to the other fractions, suggesting their intrinsic susceptibility to long-term metformin exposure. Uncovering HR+ luminal cells in the normal mammary gland as the major cell target of metformin exposure could identify patients that would most benefit from repurposing this anti-diabetic drug for cancer prevention/therapy purposes.

Deciphering the Mammary Stem Cell Niche: A Role for Laminin-Binding Integrins

Integrins, which bind laminin, a major component of the mammary basement membrane, are strongly expressed in basal stem cell-enriched populations, but their role in controlling mammary stem cell function remains unclear. We found that stem cell activity, as evaluated in transplantation and mammosphere assays, was reduced in mammary basal cells depleted of laminin receptors containing α3- and α6-integrin subunits. This was accompanied by low MDM2 levels, p53 stabilization, and diminished proliferative capacity. Importantly, disruption of p53 function restored the clonogenicity of α3/α6-integrin-depleted mammary basal stem cells, while inhibition of RHO or myosin II, leading to decreased p53 activity, rescued the mammosphere formation. These data suggest that α3/α6-integrin-mediated adhesion plays an essential role in controlling the proliferative potential of mammary basal stem/progenitor cells through myosin II-mediated regulation of p53 and indicate that laminins might be important components of the mammary stem cell niche.

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α3- and α6-integrins are required for mammary basal stem cell function

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p53 is activated in mammary basal cells depleted of α3- and α6-integrins

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RHO and myosin II mediate p53 activation in α3- and α6-integrin-depleted cells

Assays for functionally defined normal and malignant mammary stem cells

The discovery of rare, heterogeneous self-renewing stem cells with shared developmental and molecular features within epithelial components of mammary gland and breast cancers has provided a conceptual framework to understand cellular composition of these tissues and mechanisms that control their number. These normal mammary epithelial stem cells (MaSCs) and breast cancer stem cells (BCSCs) were identified and analyzed using transplant assays (namely mammary repopulating unit (MRU) assay, mammary tumor-initiating cell (TIC) assay), which reveal their latent ability to regenerate respective normal and malignant epithelial tissues with self-renewing units displaying hierarchical cellular differentiation over multiple generations in recipient mice. "Next-generation" methods using "barcoded" normal and malignant mammary cells, with the help of next-generation sequencing (NGS) technology, have revealed hidden complexity and heterogeneous growth potential of MaSCs and BCSCs. Several single markers or combinations of markers have been reported to prospectively enrich MaSCs and BCSCs. Such markers and the extent to which they enrich for MaSCs and BCSCs activity require a critical appraisal. Also, knowledge of the functional assays and their limitations and harmonious reporting of results is a prerequisite to improve our understanding of MaSCs and BCSCs. This chapter describes evolution of the concept of MaSCs and BCSCs, and specific methodologies to investigate them.

p53 controls the plasticity of mammary luminal progenitor cells downstream of Met signaling

Background

The adult mammary epithelium is composed of basal and luminal cells. The luminal lineage comprises two major cell populations, positive and negative for estrogen and progesterone receptors (ER and PR, respectively), both containing clonogenic progenitor cells. Deregulated ER/PR⁻ luminal progenitor cells are suspected to be at the origin of basal-type triple-negative (TNBC) breast cancers, a subtype frequently associated with loss of P53 function and MET signaling hyperactivation. Using mouse models, we recently reported that p53 restricts luminal progenitor cell amplification whereas paracrine Met activation stimulates their growth and favors a luminal-to-basal switch. Here, we analyzed how these two critical pathways interact to control luminal progenitor function.

Methods

We have (i) established and analyzed the gene expression profile of luminal progenitors isolated by ICAM-1, a robust surface marker we previously identified; (ii) purified luminal progenitors from p53-deficient and p53-proficient mouse mammary epithelium to compare their functional and molecular characteristics; and (iii) analyzed their response to HGF, the major Met ligand, in three-dimensional cultures.

Results

We found that luminal progenitors, compared to non-clonogenic luminal cells, overexpress Trp53 and numerous p53 target genes. In vivo, loss of Trp53 induced the expansion of luminal progenitors, affecting expression of several important p53 target genes including those encoding negative regulators of cell cycle progression. Consistently, p53-deficient luminal progenitors displayed increased proliferative and self-renewal activities in culture. However, they did not exhibit perturbed expression of luminal-specific markers and major regulators, such as Hey1, Elf5, and Gata3. Moreover, although expressing Met at higher level than p53-proficient luminal progenitors, p53-deficient luminal progenitors failed to acquire basal-specific features when stimulated by HGF, showing that p53 promotes the plastic behavior of luminal progenitors downstream of Met activation.

Conclusions

Our study reveals a crosstalk between Met- and p53-mediated signaling pathways in the regulation of luminal progenitor function. In particular, it shows that neither p53 loss alone nor p53 loss combined with Met signaling activation caused an early detectable cell fate alteration in luminal progenitors. Conceivably, additional events are required to confer basal-specific characteristics to luminal-derived TNBCs.

Electronic supplementary material

The online version of this article (10.1186/s13058-019-1101-8) contains supplementary material, which is available to authorized users.

Cancer stem cell mobilization and therapeutic targeting of the 5T4 oncofetal antigen

Cancer stem cells (CSCs) can act as the cellular drivers of tumors harnessing stem cell properties that contribute to tumorigenesis either as founder elements or by the gain of stem cell traits by the malignant cells. Thus, CSCs can self-renew and generate the cellular heterogeneity of tumors including a hierarchical organization similar to the normal tissue. While the principle tumor growth contribution is often from the non-CSC components, it is the ability of small numbers of CSCs to avoid the effects of therapeutic strategies that can contribute to recurrence after treatment. However, identifying and characterizing CSCs for therapeutic targeting is made more challenging by their cellular potency being influenced by a particular tissue niche or by the capacity of more committed cells to regain stem cell functions. This review discusses the properties of CSCs including the limitations of the available cell surface markers, the assays that document tumor initiation and clonogenicity, the roles of epithelial mesenchymal transition and molecular pathways such as Notch, Wnt, Hippo and Hedgehog. The ability to target and eliminate CSCs is thought to be critical in the search for curative cancer treatments. The oncofetal tumor-associated antigen 5T4 (TBGP) has been linked with CSC properties in several different malignancies. 5T4 has functional attributes that are relevant to the spread of tumors including through EMT, CXCR4/CXCL12, Wnt, and Hippo pathways which may all contribute through the mobilization of CSCs. There are several different immunotherapies targeting 5T4 in development including antibody-drug conjugates, antibody-targeted bacterial super-antigens, a Modified Vaccinia Ankara-basedvaccine and 5T4-directed chimeric antigen receptor T-cells. These immune therapies would have the advantage of targeting both the bulk tumor as well as mobilized CSC populations.

Hormones induce the formation of luminal-derived basal cells in the mammary gland

In the mammary gland, it is widely believed that the luminal cells are unipotent after birth, contributing only to the luminal compartment in normal development. Here, by lineage tracing, we uncovered an unexpected potential of luminal cells that can give rise to basal cells during pregnancy. These luminal-derived basal cells (LdBCs) persisted through mammary regression and generated more progeny in successive rounds of pregnancies. LdBCs express basal markers as well as estrogen receptor α (ERα). In ovariectomized (OVX) mice, stimulation with estrogen and progesterone promoted the formation of LdBCs. In serial transplantation assays, LdBCs were able to reconstitute new mammary glands in a hormone-dependent manner. Transcriptome analysis and genetic experiments suggest that Wnt/β-catenin signaling is essential for the formation and maintenance of LdBCs. Our data uncover an unexpected bi-potency of luminal cells in a physiological context. The discovery of ERα⁺ basal cells, which can respond to hormones and are endowed with stem cell-like regenerative capacity in parous mammary gland, provides new insights into the association of hormones and breast cancer.

Stem Cells and Cellular Origins of Mammary Gland: Updates in Rationale, Controversies, and Cancer Relevance

Evidences have supported the pivotal roles of stem cells in mammary gland development. Many molecular markers have been identified to characterize mammary stem cells. Cellular fate mapping of mammary stem cells by lineage tracing has put unprecedented insights into the mammary stem cell biology, which identified two subtypes of mammary stem cells, including unipotent and multipotent, which specifically differentiate to luminal or basal cells. The emerging single-cell sequencing profiles have given a more comprehensive understanding on the cellular hierarchy and lineage signatures of mammary epithelium. Besides, the stem cell niche worked as an essential regulator in sustaining the functions of mammary stem cells. In this review, we provide an overview of the characteristics of mammary stem cells. The cellular origins of mammary gland are discussed to understand the stem cell heterogeneity and their diverse differentiations. Importantly, current studies suggested that the breast cancer stem cells may originate from the mammary stem cells after specific mutations, indicating their close relationships. Here, we also outline the recent advances and controversies in the cancer relevance of mammary stem cells.

A method of producing genetically manipulated mouse mammary gland

Background

To obtain a deep understanding of the mechanism by which breast cancer develops, the genes involved in tumorigenesis should be analyzed in vivo. Mouse mammary gland can regenerate completely from a mammary stem cell (MaSC), which enables us to analyze the effect of gene expression and repression on tumorigenesis in mammary gland regenerated from genetically manipulated MaSCs. Although lentiviral and retroviral systems have usually been applied for gene transduction into MaSCs, they are associated with difficulty in introducing long, repeated, or transcriptional termination sequences. There is thus a need for an easier and quicker gene delivery system.

Methods

We devised a new system for gene delivery into MaSCs using the piggyBac transposon vectors and electroporation. Compared with viral systems, this system enables easier and quicker transfection of even long, repeated, or transcriptional termination DNA sequences. We designed gene expression vectors of the transposon system, equipped with a luciferase (Luc) expression cassette for monitoring gene transduction into regenerative mammary gland in mice by in-vivo imaging. A doxycycline (Dox)-inducible system was also integrated for expressing the target gene after mammary regeneration to mimic the actual mechanism of tumorigenesis.

Results

With this new gene delivery system, genetically manipulated mammary glands were successfully reconstituted even though the vector size was > 200 kb and even in the presence of DNA elements such as promoters and transcription termination sequences, which are major obstacles to viral vector packaging. They differentiated correctly into both basal and luminal cells, and showed normal morphological change and milk production after pregnancy, as well as self-renewal capacity. Using the Tet-On system, gene expression can be controlled by the addition of Dox after mammary reconstitution. In a case study using polyoma-virus middle T antigen (PyMT), oncogene-induced tumorigenesis was achieved. The histological appearance of the tumor was highly similar to that of the mouse mammary tumor virus-PyMT transgenic mouse model.

Conclusions

With this system, gene transduction in the mammary gland can be easily and quickly achieved, and gene expression can be controlled by Dox administration. This system for genetic manipulation could be useful for analyzing genes involved in breast cancer.

Electronic supplementary material

The online version of this article (10.1186/s13058-018-1086-8) contains supplementary material, which is available to authorized users.

Heterogeneity of Mammary Stem Cells

Adult female mammals are endowed with the unique ability to produce milk for nourishing their newborn offspring. Milk is secreted on demand by the mammary gland, an organ which develops during puberty, further matures during pregnancy and lactation, but reverts to a resting state after weaning. The glandular tissue (re)generated through this series of structural and functional changes is finely sourced by resident stem cells under the control of systemic hormones and local stimuli.Over the past decades a plethora of studies have been carried out in order to identify and characterize mammary stem cells, primarily in mice and humans. Intriguingly, it is now emerging that multiple mammary stem cell pools (co)exist and are characterized by distinctive molecular markers and context-dependent functions.This chapter reviews the heterogeneity of the mammary stem cell compartment with emphasis on the key properties and molecular regulators of distinct stem cell populations in both the mouse and human glands.

Molecular signature of the putative stem/progenitor cells committed to the development of the bovine mammary gland at puberty

Milk production is highly dependent on the extensive development of the mammary epithelium, which occurs during puberty. It is therefore essential to distinguish the epithelial cells committed to development from the related epithelial hierarchy. Using cell phenotyping and sorting, we highlighted four cell sub-populations within the bovine mammary gland at puberty. The CD49_f^highCD24^neg cells expressing CD10, KRT14, vimentin and PROCR corresponded to cells committed to the basal lineage. The CD49_f^low sub-population contained two cell subsets (CD49_f^lowCD24^neg and CD49_f^lowCD24^pos). Both subsets expressed hormone receptors including ER, PR and PRLR, as well as ALDH1 activity but only the CD49_f^lowCD24^pos subset expressed ELF5. These data indicated that the CD49_f^low sub-population is mainly composed of cells displaying a luminal phenotype and that this population comprises two luminal cell subsets, namely the CD24^neg and CD24^pos cells, likely committed to ductal and alveolar lineage, respectively. The putative mammary stem cell (MaSC) fraction was recovered in the CD49_f^highCD24^pos sub-population which were shown to form mammospheres in vitro. These cells differentially expressed CD10, KRT14 and KRT7, suggesting the existence of several putative MaSC sub-fractions. In-depth characterization of these epithelial sub-populations provides new insights into the bovine mammary epithelial cell lineage and suggests a common developmental lineage in mammals.

Cellular Plasticity of Mammary Epithelial Cells Underlies Heterogeneity of Breast Cancer

The hierarchical relationships between stem cells, lineage-committed progenitors, and differentiated cells remain unclear in several tissues, due to a high degree of cell plasticity, allowing cells to switch between different cell states. The mouse mammary gland, similarly to other tissues such as the prostate, the sweat gland, and the respiratory tract airways, consists of an epithelium exclusively maintained by unipotent progenitors throughout adulthood. Such unipotent progenitors, however, retain a remarkable cellular plasticity, as they can revert to multipotency during epithelial regeneration as well as upon oncogene activation. Here, we revise the current knowledge on mammary cell hierarchies in light of the most recent lineage tracing studies performed in the mammary gland and highlight how stem cell differentiation or reversion to multipotency are at the base of tumor development and progression. In addition, we will discuss the current knowledge about the interplay between tumor cells of origin and defined genetic mutations, leading to different tumor types, and its implications in choosing specific therapeutic protocols for breast cancer patients.

Genetic and scRNA-seq Analysis Reveals Distinct Cell Populations that Contribute to Salivary Gland Development and Maintenance

Stem and progenitor cells of the submandibular salivary gland (SMG) give rise to, maintain, and regenerate the multiple lineages of mature epithelial cells including those belonging to the ductal, acinar, basal and myoepithelial subtypes. Here we have exploited single cell RNA-sequencing and in vivo genetic lineage tracing technologies to generate a detailed map of the cell fate trajectories and branch points of the basal and myoepithelial cell populations of the mouse SMG during embryonic development and in adults. Our studies show that the transcription factor p63 and alpha-smooth muscle actin (SMA) serve as faithful markers of the basal and myoepithelial cell lineages, respectively and that both cell types are endowed with progenitor cell properties. However, p63⁺ basal and SMA⁺ myoepithelial cells exhibit distinct cell fates by virtue of maintaining different cellular lineages during morphogenesis and in adults. Collectively, our results reveal the dynamic and complex nature of the diverse SMG cell populations and highlight the distinct differentiation potential of the p63 and SMA expressing subtypes in the stem and progenitor cell hierarchy. Long term these findings have profound implications towards a better understanding of the molecular mechanisms that dictate lineage commitment and differentiation programs during development and adult gland maintenance.

Epigenomics of mammary gland development

Differentiation of stem cells into highly specialised cells requires gene expression changes brought about by remodelling of the chromatin architecture. During this lineage-commitment process, the majority of DNA needs to be packaged into inactive heterochromatin, allowing only a subset of regulatory elements to remain open and functionally required genes to be expressed. Epigenetic mechanisms such as DNA methylation, post-translational modifications to histone tails, and nucleosome positioning all potentially contribute to the changes in higher order chromatin structure during differentiation. The mammary gland is a particularly useful model to study these complex epigenetic processes since the majority of its development is postnatal, the gland is easily accessible, and development occurs in a highly reproducible manner. Inappropriate epigenetic remodelling can also drive tumourigenesis; thus, insights into epigenetic remodelling during mammary gland development advance our understanding of breast cancer aetiology. We review the current literature surrounding DNA methylation and histone modifications in the developing mammary gland and its implications for breast cancer.

Proliferative heterogeneity of murine epithelial cells in the adult mammary gland

Breast cancer is the most common cancer in females. The number of years menstruating and length of an individual menstrual cycle have been implicated in increased breast cancer risk. At present, the proliferative changes within an individual reproductive cycle or variations in the estrous cycle in the normal mammary gland are poorly understood. Here we use Fucci2 reporter mice to demonstrate actively proliferating mammary epithelial cells have shorter G1 lengths, whereas more differentiated/non-proliferating cells have extended G1 lengths. We find that cells enter into the cell cycle mainly during diestrus, yet the expansion is erratic and does not take place every reproductive cycle. Single cell expression analyses feature expected proliferation markers (Birc5, Top2a), while HR+ luminal cells exhibit fluctuations of key differentiation genes (ER, Gata3) during the cell cycle. We highlight the proliferative heterogeneity occurring within the normal mammary gland during a single-estrous cycle, indicating that the mammary gland undergoes continual dynamic proliferative changes.

GPR56/ADGRG1 Inhibits Mesenchymal Differentiation and Radioresistance in Glioblastoma

A mesenchymal transition occurs both during the natural evolution of glioblastoma (GBM) and in response to therapy. Here, we report that the adhesion G-protein-coupled receptor, GPR56/ADGRG1, inhibits GBM mesenchymal differentiation and radioresistance. GPR56 is enriched in proneural and classical GBMs and is lost during their transition toward a mesenchymal subtype. GPR56 loss of function promotes mesenchymal differentiation and radioresistance of glioma initiating cells both in vitro and in vivo. Accordingly, a low GPR56-associated signature is prognostic of a poor outcome in GBM patients even within non-G-CIMP GBMs. Mechanistically, we reveal GPR56 as an inhibitor of the nuclear factor kappa B (NF-κB) signaling pathway, thereby providing the rationale by which this receptor prevents mesenchymal differentiation and radioresistance. A pan-cancer analysis suggests that GPR56 might be an inhibitor of the mesenchymal transition across multiple tumor types beyond GBM.

Neutral lineage tracing of proliferative embryonic and adult mammary stem/progenitor cells

Mammary gland development occurs over multiple phases, beginning in the mammalian embryo and continuing throughout reproductive life. The remarkable morphogenetic capacity of the mammary gland at each stage of development is attributed to the activities of distinct populations of mammary stem cells (MaSCs) and progenitor cells. However, the relationship between embryonic and adult MaSCs, and their fate during different waves of mammary gland morphogenesis, remains unclear. By employing a neutral, low-density genetic labelling strategy, we characterised the contribution of proliferative stem/progenitor cells to embryonic, pubertal and reproductive mammary gland development. Our findings further support a model of lineage restriction of MaSCs in the postnatal mammary gland, and highlight extensive redundancy and heterogeneity within the adult stem/progenitor cell pool. Furthermore, our data suggest extensive multiplicity in their foetal precursors that give rise to the primordial mammary epithelium before birth. In addition, using a single-cell labelling approach, we revealed the extraordinary capacity of a single embryonic MaSC to contribute to postnatal ductal development. Together, these findings provide tantalising new insights into the disparate and stage-specific contribution of distinct stem/progenitor cells to mammary gland development.

Summary: Neutral, low-density lineage tracing of proliferative mammary stem and progenitor cells during embryonic, pubertal and reproductive mammary gland development reveal the disparate and stage-specific contribution of distinct stem/progenitor cells.

Early lineage segregation of multipotent embryonic mammary gland progenitors

The mammary gland (MG) is composed of basal cells (BCs) and luminal cells (LCs). While it is generally believed that MG arises from embryonic multipotent progenitors (EMPs), it remains unclear when lineage restriction occurs and what are the mechanisms responsible for the switch from multipotency to unipotency during MG morphogenesis. Here, we performed multicolor lineage tracing and assessed the fate of single progenitors and demonstrated the existence of a developmental switch from multipotency to unipotency during embryonic MG development. Molecular profiling and single cell RNA-seq revealed that EMPs express a unique hybrid basal and luminal signature and the factors associated with the different lineages. Sustained p63 expression in EMPs promotes unipotent BC fate and was sufficient to reprogram adult LCs into BCs by promoting an intermediate hybrid multipotent like state. Altogether, this study identifies the timing and the mechanisms mediating the early lineage segregation of multipotent progenitors during MG development.

Clonal analysis of Notch1-expressing cells reveals the existence of unipotent stem cells that retain long-term plasticity in the embryonic mammary gland

Recent lineage tracing studies have revealed that mammary gland homeostasis relies on unipotent stem cells. However, whether and when lineage restriction occurs during embryonic mammary development, and which signals orchestrate cell fate specification, remain unknown. Using a combination of in vivo clonal analysis with whole mount immunofluorescence and mathematical modelling of clonal dynamics, we found that embryonic multipotent mammary cells become lineage-restricted surprisingly early in development, with evidence for unipotency as early as E12.5 and no statistically discernable bipotency after E15.5. To gain insights into the mechanisms governing the switch from multipotency to unipotency, we used gain-of-function Notch1 mice and demonstrated that Notch activation cell autonomously dictates luminal cell fate specification to both embryonic and basally committed mammary cells. These functional studies have important implications for understanding the signals underlying cell plasticity and serve to clarify how reactivation of embryonic programs in adult cells can lead to cancer.

Mammary Stem Cells and Breast Cancer Stem Cells: Molecular Connections and Clinical Implications

Cancer arises from subpopulations of transformed cells with high tumor initiation and repopulation ability, known as cancer stem cells (CSCs), which share many similarities with their normal counterparts. In the mammary gland, several studies have shown common molecular regulators between adult mammary stem cells (MaSCs) and breast cancer stem cells (bCSCs). Cell plasticity and self-renewal are essential abilities for MaSCs to maintain tissue homeostasis and regenerate the gland after pregnancy. Intriguingly, these properties are similarly executed in breast cancer stem cells to drive tumor initiation, tumor heterogeneity and recurrence after chemotherapy. In addition, both stem cell phenotypes are strongly influenced by external signals from the microenvironment, immune cells and supportive specific niches. This review focuses on the intrinsic and extrinsic connections of MaSC and bCSCs with clinical implications for breast cancer progression and their possible therapeutic applications.

Interpreting heterogeneity in intestinal tuft cell structure and function

Intestinal tuft cells are a morphologically unique cell type, best characterized by striking microvilli that form an apical tuft. These cells represent approximately 0.5% of gut epithelial cells depending on location. While they are known to express chemosensory receptors, their function has remained unclear. Recently, numerous groups have revealed startling insights into intestinal tuft cell biology. Here, we review the latest developments in understanding this peculiar cell type's structure and function. Recent advances in volumetric microscopy have begun to elucidate tuft cell ultrastructure with respect to its cellular neighbors. Moreover, single-cell approaches have revealed greater diversity in the tuft cell population than previously appreciated and uncovered novel markers to characterize this heterogeneity. Finally, advanced model systems have revealed tuft cells' roles in mucosal healing and orchestrating type 2 immunity against eukaryotic infection. While much remains unknown about intestinal tuft cells, these critical advances have illuminated the physiological importance of these previously understudied cells and provided experimentally tractable tools to interrogate this rare cell population. Tuft cells act as luminal sensors, linking the luminal microbiome to the host immune system, which may make them a potent clinical target for modulating host response to a variety of acute or chronic immune-driven conditions.

Lineage-Restricted Mammary Stem Cells Sustain the Development, Homeostasis, and Regeneration of the Estrogen Receptor Positive Lineage

The mammary gland (MG) is composed of different cell lineages, including the basal and the luminal cells (LCs) that are maintained by distinct stem cell (SC) populations. LCs can be subdivided into estrogen receptor (ER)⁺ and ER⁻ cells. LCs act as the cancer cell of origin in different types of mammary tumors. It remains unclear whether the heterogeneity found in luminal-derived mammary tumors arises from a pre-existing heterogeneity within LCs. To investigate LC heterogeneity, we used lineage tracing to assess whether the ER⁺ lineage is maintained by multipotent SCs or by lineage-restricted SCs. To this end, we generated doxycycline-inducible ER-rtTA mice that allowed us to perform genetic lineage tracing of ER⁺ LCs and study their fate and long-term maintenance. Our results show that ER⁺ cells are maintained by lineage-restricted SCs that exclusively contribute to the expansion of the ER⁺ lineage during puberty and their maintenance during adult life.

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ER⁺ stem cells mediate expansion and maintenance of the ER⁺ lineage

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ER⁺ stem cells expand and differentiate into ER⁺ cells following transplantation

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ER⁺ stem cells survive involution and repopulate the ER⁺ lineage

Single-cell RNA-Seq reveals cell heterogeneity and hierarchy within mouse mammary epithelia

The mammary gland is very intricately and well organized into distinct tissues, including epithelia, endothelia, adipocytes, and stromal and immune cells. Many mammary gland diseases, such as breast cancer, arise from abnormalities in the mammary epithelium, which is mainly composed of two distinct lineages, the basal and luminal cells. Because of the limitation of traditional transcriptome analysis of bulk mammary cells, the hierarchy and heterogeneity of mammary cells within these two lineages remain unclear. To this end, using single-cell RNA-Seq coupled with FACS analysis and principal component analysis, we determined gene expression profiles of mammary epithelial cells of virgin and pregnant mice. These analyses revealed a much higher heterogeneity among the mammary cells than has been previously reported and enabled cell classification into distinct subgroups according to signature gene markers present in each group. We also identified and verified a rare CDH5⁺ cell subpopulation within a basal cell lineage as quiescent mammary stem cells (MaSCs). Moreover, using pseudo-temporal analysis, we reconstructed the developmental trajectory of mammary epithelia and uncovered distinct changes in gene expression and in biological functions of mammary cells along the developmental process. In conclusion, our work greatly refines the resolution of the cellular hierarchy in developing mammary tissues. The discovery of CDH5⁺ cells as MaSCs in these tissues may have implications for our understanding of the initiation, development, and pathogenesis of mammary tumors.

Submucosal Gland Myoepithelial Cells Are Reserve Stem Cells That Can Regenerate Mouse Tracheal Epithelium

The mouse trachea is thought to contain two distinct stem cell compartments that contribute to airway repair-basal cells in the surface airway epithelium (SAE) and an unknown submucosal gland (SMG) cell type. Whether a lineage relationship exists between these two stem cell compartments remains unclear. Using lineage tracing of glandular myoepithelial cells (MECs), we demonstrate that MECs can give rise to seven cell types of the SAE and SMGs following severe airway injury. MECs progressively adopted a basal cell phenotype on the SAE and established lasting progenitors capable of further regeneration following reinjury. MECs activate Wnt-regulated transcription factors (Lef-1/TCF7) following injury and Lef-1 induction in cultured MECs promoted transition to a basal cell phenotype. Surprisingly, dose-dependent MEC conditional activation of Lef-1 in vivo promoted self-limited airway regeneration in the absence of injury. Thus, modulating the Lef-1 transcriptional program in MEC-derived progenitors may have regenerative medicine applications for lung diseases.

Myoepithelial Cells of Submucosal Glands Can Function as Reserve Stem Cells to Regenerate Airways after Injury

Cells demonstrate plasticity following injury, but the extent of this phenomenon and the cellular mechanisms involved remain underexplored. Using single-cell RNA sequencing (scRNA-seq) and lineage tracing, we uncover that myoepithelial cells (MECs) of the submucosal glands (SMGs) proliferate and migrate to repopulate the airway surface epithelium (SE) in multiple injury models. Specifically, SMG-derived cells display multipotency and contribute to basal and luminal cell types of the SMGs and SE. Ex vivo expanded MECs have the potential to repopulate and differentiate into SE cells when grafted onto denuded airway scaffolds. Significantly, we find that SMG-like cells appear on the SE of both extra- and intra-lobular airways of large animal lungs following severe injury. We find that the transcription factor SOX9 is necessary for MEC plasticity in airway regeneration. Because SMGs are abundant and present deep within airways, they may serve as a reserve cell source for enhancing human airway regeneration.

Conditional reprogrammed human limbal epithelial cells represent a novel in vitro cell model for drug responses

In this study, we established human limbal epithelial cells (LECs) from normal limbal tissues by using Conditional Reprogramming (CR) technology (refer to CR-LEC cells in this study). We have successfully established CR-LEC cell strains from three human donors (3 out of 3), and normal rabbits (2 out of 2) and pig (1 out of 1) as well. CR-LEC cells sustained a continuous and stable proliferation status with a normal karyotype, normal response to DNA damage, well-defined structured spheres in matrigel 3D culture. Responses of CR-LEC cells to IFN α2b, Ganciclovir and 5-Fluorouracil were different, suggesting that these drugs had different toxicities to these cells as expected. More important, there was no significant difference of responses to drugs between early and late passages of CR-LEC cells (p>0.05), indicating CR-LEC cells can serve a stable normal human cell model for toxicity assessment. Toxicity tests with monolayer cultures of CR-LEC cells were measured by staining the F-actin and Dsg-1 expression. Toxicity of three drugs at LD50 concentration resulted in a gradually increased destruction of monolayer, which is, in accordance with the irritation grade of three drugs on human cornea epithelium. Therefore, CR-LEC cells provide a novel and reliable in vitro physiological cell model for corneal toxicity assessment.

Podoplanin regulates mammary stem cell function and tumorigenesis by potentiating Wnt/β-catenin signaling

Stem cells (SCs) drive mammary development, giving rise postnatally to an epithelial bilayer composed of luminal and basal myoepithelial cells. Dysregulation of SCs is thought to be at the origin of certain breast cancers; however, the molecular identity of SCs and the factors regulating their function remain poorly defined. We identified the transmembrane protein podoplanin (Pdpn) as a specific marker of the basal compartment, including multipotent SCs, and found Pdpn localized at the basal-luminal interface. Embryonic deletion of Pdpn targeted to basal cells diminished basal and luminal SC activity and affected the expression of several Wnt/β-catenin signaling components in basal cells. Moreover, Pdpn loss attenuated mammary tumor formation in a mouse model of β-catenin-induced breast cancer, limiting tumor-initiating cell expansion and promoting molecular features associated with mesenchymal-to-epithelial cell transition. In line with the loss-of-function data, we demonstrated that mechanistically Pdpn enhances Wnt/β-catenin signaling in mammary basal cells. Overall, this study uncovers a role for Pdpn in mammary SC function and, importantly, identifies Pdpn as a new regulator of Wnt/β-catenin signaling, a key pathway in mammary development and tumorigenesis.

Luminal STAT5 mediates H2AX promoter activity in distinct population of basal mammary epithelial cells

Deregulated STAT5 activity in the mammary gland causes parity-dependent tumorigenesis. Epithelial cell cultures transfected with constitutively active STAT5 express higher levels of the histone H2AX than their non-transfected counterparts. Higher H2AX expression may be involved in tumorigenesis. Here, we aimed to link high STAT5 activity to H2AX–GFP expression by looking for distinct types of mammary cells that express these proteins. In vitro and in transgenic mice, only 0.2 and 0.02%, respectively, of the cells expressed the H2AX–GFP hybrid gene. Its expression correlated with that of the endogenous H2AX gene, suggesting that detectable H2AX–GFP expression marks high levels of H2AX transcript. Methylation of the H2AX promoter characterized non-GFP-expressing H2AX–GFP cells and was inversely correlated with promoter activity. Administration of 5-azacytidine increased H2AX promoter activity in an activated STAT5-dependent manner. In transgenic mice, H2AX–GFP expression peaked at pregnancy. The number of H2AX–GFP-expressing cells and GFP expression decreased in a Stat5a-null background and increased in mice expressing the hyperactivated STAT5. Importantly, H2AX–GFP activity was allocated to basal mammary cells lacking stem-cell properties, whereas STAT5 hyperactivity was detected in the adjacent luminal cells. Knockdown of RANKL by siRNA suggested its involvement in signaling between the two layers. These results suggest paracrine activation of H2AX via promoter demethylation in specific populations of basal mammary cells that is induced by a signal from neighboring luminal cells with hyper STAT5 activity. This pathway provides an alternative route for the luminally confined STAT5 to affect basal mammary cell activity.

Galectin-7 in Epithelial Homeostasis and Carcinomas

Galectins are small unglycosylated soluble lectins distributed both inside and outside the cells. They share a conserved domain for the recognition of carbohydrates (CRD). Although galectins have a common affinity for β-galatosides, they exhibit different binding preferences for complex glycans. First described twenty years ago, galectin-7 is a prototypic galectin, with a single CRD, able to form divalent homodimers. This lectin, which is mainly expressed in stratified epithelia, has been described in epithelial tissues as being involved in apoptotic responses, in proliferation and differentiation but also in cell adhesion and migration. Most members of the galectins family have been associated with cancer biology. One of the main functions of galectins in cancer is their immunomodulating potential and anti-angiogenic activity. Indeed, galectin-1 and -3, are already targeted in clinical trials. Another relevant function of galectins in tumour progression is their ability to regulate cell migration and cell adhesion. Among these galectins, galectin-7 is abnormally expressed in various cancers, most prominently in carcinomas, and is involved in cancer progression and metastasis but its precise functions in tumour biology remain poorly understood. In this issue, we will focus on the physiological functions of galectin-7 in epithelia and present the alterations of galectin-7 expression in carcinomas with the aim to describe its possible functions in tumour progression.