The Cleared Mammary Fat Pad Transplantation Assay for Mammary Epithelial Organogenesis

Spatiotemporal modulation of growth factors directs the generation of multilineage mouse embryonic stem cell-derived mammary organoids

Ectodermal appendages, such as the mammary gland (MG), are thought to have evolved from hair-associated apocrine glands to serve the function of milk secretion. Through the directed differentiation of mouse embryonic stem cells (mESCs), here, we report the generation of multilineage ESC-derived mammary organoids (MEMOs). We adapted the skin organoid model, inducing the dermal mesenchyme to transform into mammary-specific mesenchyme via the sequential activation of Bone Morphogenetic Protein 4 (BMP4) and Parathyroid Hormone-related Protein (PTHrP) and inhibition of hedgehog (HH) signaling. Using single-cell RNA sequencing, we identified gene expression profiles that demonstrate the presence of mammary-specific epithelial cells, fibroblasts, and adipocytes. MEMOs undergo ductal morphogenesis in Matrigel and can reconstitute the MG in vivo. Further, we demonstrate that the loss of function in placode regulators LEF1 and TBX3 in mESCs results in impaired skin and MEMO generation. In summary, our MEMO model is a robust tool for studying the development of ectodermal appendages, and it provides a foundation for regenerative medicine and disease modeling.

Chronic Interferon Stimulated Gene Transcription Promotes Oncogene Induced Breast Cancer

The Mre11 complex (comprising Mre11, Rad50, Nbs1) is integral to the maintenance of genome stability. We previously showed that a hypomorphic Mre11 mutant mouse strain ( Mre11 ATLD1/ATLD1 ) was highly susceptible to oncogene induced breast cancer. Here we used a mammary organoid system to examine which Mre11 dependent responses are tumor suppressive. We found that Mre11 ATLD1/ATLD1 organoids exhibited an elevated interferon stimulated gene (ISG) signature and sustained changes in chromatin accessibility. This Mre11 ATLD1/ATLD1 phenotype depended on DNA binding of a nuclear innate immune sensor, IFI205. Ablation of Ifi205 in Mre11 ATLD1/ATLD1 organoids restored baseline and oncogene-induced chromatin accessibility patterns to those observed in WT . Implantation of Mre11 ATLD1/ATLD1 organoids and activation of oncogene led to aggressive metastatic breast cancer. This outcome was reversed in implanted Ifi205 -/- Mre11 ATLD1/ATLD1 organoids. These data reveal a connection between innate immune signaling and tumor suppression in mammary epithelium. Given the abundance of aberrant DNA structures that arise in the context of genome instability syndromes, the data further suggest that cancer predisposition in those contexts may be partially attributable to tonic innate immune transcriptional programs.

Experimental and spontaneous metastasis assays can result in divergence in clonal architecture

Intratumoural heterogeneity is associated with poor outcomes in breast cancer. To understand how malignant clones survive and grow in metastatic niches, in vivo models using cell lines and patient-derived xenografts (PDX) have become the gold standard. Injections of cancer cells in orthotopic sites (spontaneous metastasis assays) or into the vasculature (experimental metastasis assays) have been used interchangeably to study the metastatic cascade from early events or post-intravasation, respectively. However, less is known about how these different routes of injection impact heterogeneity. Herein we directly compared the clonality of spontaneous and experimental metastatic assays using the human cell line MDA-MB-231 and a PDX model. Genetic barcoding was used to study the fitness of the subclones in primary and metastatic sites. Using spontaneous assays, we found that intraductal injections resulted in less diverse tumours compared to other routes of injections. Using experimental metastasis assays via tail vein injection of barcoded MDA-MB-231 cells, we also observed an asymmetry in metastatic heterogeneity between lung and liver that was not observed using spontaneous metastasis assays. These results demonstrate that these assays can result in divergent clonal outputs in terms of metastatic heterogeneity and provide a better understanding of the biases inherent to each technique.

Mammary duct luminal epithelium controls adipocyte thermogenic programme

Sympathetic activation during cold exposure increases adipocyte thermogenesis via the expression of mitochondrial protein uncoupling protein 1 (UCP1)1. The propensity of adipocytes to express UCP1 is under a critical influence of the adipose microenvironment and varies between sexes and among various fat depots2-7. Here we report that mammary gland ductal epithelial cells in the adipose niche regulate cold-induced adipocyte UCP1 expression in female mouse subcutaneous white adipose tissue (scWAT). Single-cell RNA sequencing shows that glandular luminal epithelium subtypes express transcripts that encode secretory factors controlling adipocyte UCP1 expression under cold conditions. We term these luminal epithelium secretory factors 'mammokines'. Using 3D visualization of whole-tissue immunofluorescence, we reveal sympathetic nerve-ductal contact points. We show that mammary ducts activated by sympathetic nerves limit adipocyte UCP1 expression via the mammokine lipocalin 2. In vivo and ex vivo ablation of mammary duct epithelium enhance the cold-induced adipocyte thermogenic gene programme in scWAT. Since the mammary duct network extends throughout most of the scWAT in female mice, females show markedly less scWAT UCP1 expression, fat oxidation, energy expenditure and subcutaneous fat mass loss compared with male mice, implicating sex-specific roles of mammokines in adipose thermogenesis. These results reveal a role of sympathetic nerve-activated glandular epithelium in adipocyte UCP1 expression and suggest that mammary duct luminal epithelium has an important role in controlling glandular adiposity.

Growth factor dependency in mammary organoids regulates ductal morphogenesis during organ regeneration

Signaling pathways play an important role in cell fate determination in stem cells and regulate a plethora of developmental programs, the dysregulation of which can lead to human diseases. Growth factors (GFs) regulating these signaling pathways therefore play a major role in the plasticity of adult stem cells and modulate cellular differentiation and tissue repair outcomes. We consider murine mammary organoid generation from self-organizing adult stem cells as a tool to understand the role of GFs in organ development and tissue regeneration. The astounding capacity of mammary organoids to regenerate a gland in vivo after transplantation makes it a convenient model to study organ regeneration. We show organoids grown in suspension with minimal concentration of Matrigel and in the presence of a cocktail of GFs regulating EGF and FGF signaling can recapitulate key epithelial layers of adult mammary gland. We establish a toolkit utilizing in vivo whole animal imaging and ultrasound imaging combined with ex vivo approaches including tissue clearing and confocal imaging to study organ regeneration and ductal morphogenesis. Although the organoid structures were severely impaired in vitro when cultured in the presence of individual GFs, ex vivo imaging revealed ductal branching after transplantation albeit with significantly reduced number of terminal end buds. We anticipate these imaging modalities will open novel avenues to study mammary gland morphogenesis in vivo and can be beneficial for monitoring mammary tumor progression in pre-clinical and clinical settings.

Evolution of gene expression signature in mammary gland stem cells from neonatal to old mice

During the lifetime of females, mammary epithelial cells undergo cyclical expansion and proliferation depending on the cyclical activation of mammary gland stem/progenitor cells (MaSCs) in response to the change of hormone level. The structural shrink of mammary duct tree and the functional loss of mammary gland occur along with inactivation of MaSCs in old females, even leading to breast cancer occasionally. However, the gene expression signature in MaSCs across the lifespan remains unclear. Herein, we tested the tissue regeneration ability of CD24⁺CD49f^high MaSCs over six time points from neonatal (4-day-old) to aged mice (360-day-old). Further RNA-seq analyses identified four clusters of gene signatures based on the gene expression patterns. A subset of stemness-related genes was identified, showing the highest level at day 4 of the neonatal age, and the lowest level at the old age. We also identified an aging-related gene signature showing significant change in the old mice, in which an association between aging process and stemness loss was indicated. The aging-related gene signature showed regulation of cancer signaling pathways, as well as aging-related diseases including Huntington disease, Parkinson disease, and Alzheimer disease. Moreover, 425, 1056, 418, and 1107 gene variants were identified at D20, D40, D90, and D180, respectively, which were mostly reported to associated with tumorigenesis and metastasis in cancer. In summary, the current study is the first to demonstrate the gene expression shift in MaSCs from neonatal to aging, which leads to stemness loss, aging, aging-related diseases, and even breast cancer in old mice.

Human iPSC-derived fallopian tube organoids with BRCA1 mutation recapitulate early-stage carcinogenesis

Germline pathogenic mutations in BReast CAncer (BRCA1) genes are thought to drive normal fallopian tube epithelial (FTE) cell transformation to high-grade serous ovarian cancer. No human models capture the sequence of events for disease initiation and progression. Here, we generate induced pluripotent stem cells (iPSCs) from healthy individuals and young ovarian cancer patients with germline pathogenic BRCA1 mutations (BRCA1^mut). Following differentiation into FTE organoids, BRCA1^mut lines exhibit cellular abnormalities consistent with neoplastic transformation compared to controls. BRCA1^mut organoids show an increased production of cancer-specific proteins and survival following transplantation into mice. Organoids from women with the most aggressive ovarian cancer show the greatest pathology, indicating the potential value to predict clinical severity prior to disease onset. These human FTE organoids from BRCA1^mut carriers provide a faithful physiological in vitro model of FTE lesion generation and early carcinogenesis. This platform can be used for personalized mechanistic and drug screening studies.

Yucer et al. generate a human BRCA1 mutant iPSC-derived fallopian tube organoid model, which recapitulates BRCA1 mutant ovarian carcinogenesis in vitro and shows tumors in vivo. This model provides a biologically relevant platform to validate drugs and a basis for personalized early detection and preventative strategies for women carrying BRCA1 mutations.

Systemic alterations play a dominant role in epigenetic predisposition to breast cancer in offspring of obese fathers and is transmitted to a second generation

We previously showed that environmentally-induced epigenetic inheritance of cancer occurs in rodent models. For instance, we reported that paternal consumption of an obesity-inducing diet (OID) increased breast cancer susceptibility in the offspring (F1). Nevertheless, it is still unclear whether programming of breast cancer in daughters is due to systemic alterations or mammary epithelium-specific factors and whether the breast cancer predisposition in F1 progeny can be transmitted to subsequent generations. In this study, we show that mammary glands from F1 control (CO) female offspring exhibit enhanced growth when transplanted into OID females compared to CO mammary glands transplanted into CO females. Similarly, carcinogen-induced mammary tumors from F1 CO female offspring transplanted into OID females has a higher proliferation/apoptosis rate. Further, we show that granddaughters (F2) from the OID grand-paternal germline have accelerated tumor growth compared to CO granddaughters. This between-generation transmission of cancer predisposition is associated with changes in sperm tRNA fragments in OID males. Our findings indicate that systemic and mammary stromal alterations are significant contributors to programming of mammary development and likely cancer predisposition in OID daughters. Our data also show that breast cancer predisposition is transmitted to subsequent generations and may explain some familial cancers, if confirmed in humans.

3D Collagen Vascular Tumor-on-a-Chip Mimetics for Dynamic Combinatorial Drug Screening

Disease models, including in vitro cell culture and animal models, have contributed significantly to developing diagnostics and treatments over the past several decades. The successes of traditional drug screening methods were generally hampered by not adequately mimicking critical in vivo features, such as a 3D microenvironment and dynamic drug diffusion through the extracellular matrix (ECM). To address these issues, we developed a 3D dynamic drug delivery system for cancer drug screening that mimicks drug dissemination through the tumor vasculature and the ECM by creating collagen-embedded microfluidic channels. Using this novel 3D ECM microsystem, we compared viability of tumor pieces with traditionally used 2D methods in response to three different drug combinations. Drug diffusion profiles were evaluated by simulation methods and tested in the 3D ECM microsystem and a 2D 96-well setup. Compared with the 2D control, the 3D ECM microsystem produced reliable data on viability, drug ratios, and combination indeces. This novel approach enables higher throughput and sets the stage for future applications utilizing drug sensitivity predicting algorithms based on dynamic diffusion profiles requiring only minimal patient tissue. Our findings moved drug sensitivity screening closer to clinical implications with a focus on testing combinatorial drug effects, an option often limited by the amount of available patient tissues.

Depletion of the Transcriptional Coactivator Amplified in Breast Cancer 1 (AIB1) Uncovers Functionally Distinct Subpopulations in Triple-Negative Breast Cancer

The transcriptional coactivator Amplified in Breast Cancer 1 (AIB1) plays a major role in the progression of hormone and HER2-dependent breast cancers but its role in triple negative breast cancer (TNBC) is undefined. Here, we report that established TNBC cell lines, as well as cells from a TNBC patient-derived xenograft (PDX) that survive chemotherapy treatment in vitro express lower levels of AIB1 protein. The surviving cell population has an impaired tube-formation phenotype when cultured onto basement membrane, a property shared with TNBC cells that survive shRNA-mediated depletion of AIB1 (AIB1^LOW cells). DNA analysis by exome sequencing revealed that AIB1^LOW cells represent a distinct subpopulation. Consistent with their in vitro phenotype AIB1^LOW cells implanted orthotopically generated slower growing tumors with less capacity for pulmonary metastases. Gene expression analysis of cultured cells and tumors revealed that AIB1^LOW cells display a distinct expression signature of genes in pro-inflammatory pathways, cell adhesion, proteolysis and tissue remodeling. Interestingly, the presence of this AIB1^LOW expression signature in breast cancer specimens is associated with shorter disease free survival of chemotherapy treated patients. We concluded that TNBC cell lines contain heterogeneous populations with differential dependence on AIB1 and that the gene expression pattern of AIB1^LOW cells may represent a signature indicative of poor response to chemotherapy in TNBC patients.

MiR-203a is differentially expressed during branching morphogenesis and EMT in breast progenitor cells and is a repressor of peroxidasin

MicroRNAs regulate developmental events such as branching morphogenesis, epithelial to mesenchymal transition (EMT) and its reverse process mesenchymal to epithelial transition (MET). In this study, we performed small RNA sequencing of a breast epithelial progenitor cell line (D492), and its mesenchymal derivative (D492M) cultured in three-dimensional microenvironment. Among the most downregulated miRNAs in D492M was miR-203a, a miRNA that plays an important role in epithelial differentiation. Increased expression of miR-203a was seen in D492, concomitant with increased complexity of branching. When miR-203a was overexpressed in D492M, a partial reversion towards epithelial phenotype was seen. Gene expression analysis of D492M and D492MmiR-203a revealed peroxidasin, a collagen IV cross-linker, as the most significantly downregulated gene in D492MmiR-203a. Collectively, we demonstrate that miR-203a expression temporally correlates with branching morphogenesis and is suppressed in D492M. Overexpression of miR-203a in D492M induces a partial MET and reduces the expression of peroxidasin. Furthermore, we demonstrate that miR-203a is a novel repressor of peroxidasin. MiR-203-peroxidasin axis may be an important regulator in branching morphogenesis, EMT/MET and basement membrane remodeling.

Pink Adipocytes

Adipocytes are lipid-rich parenchymal cells contained in a very plastic organ, whose composition can undergo striking physiologic changes. In standard conditions the organ contains white and brown adipocytes which play opposite roles: lipid storage to meet metabolic requirements and lipid burning for thermogenesis, respectively. During chronic cold exposure, white adipocytes transdifferentiate to brown, to increase thermogenesis, whereas in conditions of chronic positive energy balance brown adipocytes transdifferentiate to white, to increase energy stores. During pregnancy, lactation, and post-lactation, subcutaneous white adipocytes convert to milk-producing glands formed by lipid-rich elements that can be defined as pink adipocytes. Recent fate-mapping data support the conversion of pink to brown adipocytes and the reversible conversion of brown adipocytes to myoepithelial cells of alveoli.

Tissue Recombination Models for the Study of Epithelial Cancer

Animal models of cancer provide fundamental insight into the cellular and molecular mechanisms of human cancer development. As an alternative to genetically engineered mouse models, increasing evidence shows that tissue recombination and transplantation models represent an efficient approach to faithfully recapitulate solid epithelial cancer in mice. Cancer can be rapidly initiated through lentiviral delivery of defined genetic alterations into target cells that are grown in a physiological milieu with an appropriate epithelial-stromal interaction. Through genetic manipulation of distinct subpopulations of epithelial cells and mesenchymal cells, this powerful system can readily test both cell-autonomous roles of genetic events in the epithelial compartment and the paracrine effects of the microenvironment. Here we review the recent advances in mouse models of several epithelial cancers achieved using orthotopic transplantation and tissue recombination strategies, with an emphasis on the dissociated cell in vivo prostate regeneration model to investigate prostate cancer.