Modulation of fibroblast growth factor signaling is essential for mammary epithelial morphogenesis

Tight Junction Component Occludin Binds to FIP5 to Regulate Endosome Trafficking and Mitotic Spindle Function

The genetic basis of vertebrate emergence during metazoan evolution has remained largely unknown. Understanding vertebrate-specific genes, such as the tight junction protein Occludin (Ocln), may help answer this question. Here, it is shown that mammary glands lacking Ocln exhibit retarded epithelial branching, owing to reduced cell proliferation and surface expansion. Interestingly, Ocln regulates mitotic spindle orientation and function, and its loss leads to a range of defects, including prolonged prophase and failed nuclear and/or cytoplasmic division. Mechanistically, Ocln binds to the RabGTPase-11 adaptor FIP5 and recruits recycling endosomes to the centrosome to participate in spindle assembly and function. FIP5 loss recapitulates Ocln null, leading to prolonged prophase, reduced cell proliferation, and retarded epithelial branching. These results identify a novel role in OCLN-mediated endosomal trafficking and potentially highlight its involvement in mediating membranous vesicle trafficking and function, which is evolutionarily conserved and essential.

Sprouty genes regulate activated fibroblasts in mammary epithelial development and breast cancer

Stromal fibroblasts are a major stem cell niche component essential for organ formation and cancer development. Fibroblast heterogeneity, as revealed by recent advances in single-cell techniques, has raised important questions about the origin, differentiation, and function of fibroblast subtypes. In this study, we show in mammary stromal fibroblasts that loss of the receptor tyrosine kinase (RTK) negative feedback regulators encoded by Spry1, Spry2, and Spry4 causes upregulation of signaling in multiple RTK pathways and increased extracellular matrix remodeling, resulting in accelerated epithelial branching. Single-cell transcriptomic analysis demonstrated that increased production of FGF10 due to Sprouty (Spry) loss results from expansion of a functionally distinct subgroup of fibroblasts with the most potent branching-promoting ability. Compared to their three independent lineage precursors, fibroblasts in this subgroup are "activated," as they are located immediately adjacent to the epithelium that is actively undergoing branching and invasion. Spry genes are downregulated, and activated fibroblasts are expanded, in all three of the major human breast cancer subtypes. Together, our data highlight the regulation of a functional subtype of mammary fibroblasts by Spry genes and their essential role in epithelial morphogenesis and cancer development.

Adipocyte-Derived Paracrine Factors Regulate the In Vitro Development of Bovine Mammary Epithelial Cells

The mammary gland is composed of epithelial tissue forming ducts and lobules, and the stroma, composed of adipocytes, connective tissue, and other cell types. The stromal microenvironment regulates mammary gland development by paracrine and cell-cell interactions. In the present study, primary cultures of bovine mammary epithelial cells (bMEC) and bovine adipose-derived stem cells (bASC) subjected to adipogenic differentiation were used to investigate the influence of paracrine factors secreted by preadipocytes and adipocytes on bMEC development. Four types of conditioned media (CM) were collected from undifferentiated preadipocytes (preA) and adipocytes on days: 8, 12, 14 of differentiation. Next, bMEC were cultured for 24 h in CM and cell viability, apoptosis, migratory activity, ability to form spheroids on Matrigel, and secretory activity (alpha S1-casein concentration) were evaluated. CM derived from fully differentiated adipocytes (12 d and 14 d) significantly decreased the number of apoptotic cells in bMEC population and increased the size of spheroids formed by bMEC on Matrigel. CM collected from preadipocytes significantly enhanced bMEC's migration, and stimulated bMEC to produce alpha S1-casein, but only in the presence of prolactin. These results confirm that preadipocytes and adipocytes are important components of the stroma, providing paracrine factors that actively regulate the development of bovine mammary epithelium.

FGFR2 Controls Growth, Adhesion and Migration of Nontumorigenic Human Mammary Epithelial Cells by Regulation of Integrin β1 Degradation

The role of fibroblast growth factor receptor 2 (FGFR2), an important mediator of stromal paracrine and autocrine signals, in mammary gland morphogenesis and breast cancer has been extensively studied over the last years. However, the function of FGFR2 signalling in the initiation of mammary epithelial oncogenic transformation remains elusive. Here, FGFR2-dependent behaviour of nontumorigenic model of mammary epithelial cells was studied. In vitro analyses demonstrated that FGFR2 regulates epithelial cell communication with extracellular matrix (ECM) proteins. Silencing of FGFR2 significantly changed the phenotype of cell colonies in three-dimensional cultures, decreased integrins α2, α5 and β1 protein levels and affected integrin-driven processes, such as cell adhesion and migration. More detailed analysis revealed the FGFR2 knock-down-induced proteasomal degradation of integrin β1. Analysis of RNA-seq databases showed significantly decreased FGFR2 and ITGB1 mRNA levels in breast tumour samples, when compared to non-transformed tissues. Additionally, high risk healthy individuals were found to have disrupted correlation profiles of genes associated with FGFR2 and integrin signalling, cell adhesion/migration and ECM remodelling. Taken together, our results strongly suggest that FGFR2 loss with concomitant integrin β1 degradation is responsible for deregulation of epithelial cell-ECM interactions and this process may play an important role in the initiation of mammary gland epithelial tumorigenesis.

LGL1 binds to Integrin β1 and inhibits downstream signaling to promote epithelial branching in the mammary gland

Branching morphogenesis is a fundamental process by which organs in invertebrates and vertebrates form branches to expand their surface areas. The current dogma holds that directional cell migration determines where a new branch forms and thus patterns branching. Here, we asked whether mouse Lgl1, a homolog of the Drosophila tumor suppressor Lgl, regulates epithelial polarity in the mammary gland. Surprisingly, mammary glands lacking Lgl1 have normal epithelial polarity, but they form fewer branches. Moreover, we find that Lgl1 null epithelium is unable to directionally migrate, suggesting that migration is not essential for mammary epithelial branching as expected. We show that LGL1 binds to Integrin β1 and inhibits its downstream signaling, and Integrin β1 overexpression blocks epithelial migration, thus recapitulating the Lgl1 null phenotype. Altogether, we demonstrate that Lgl1 modulation of Integrin β1 signaling is essential for directional migration and that epithelial branching in invertebrates and the mammary gland is fundamentally distinct.

Ma et al. show that Lgl1 is essential for mammary gland branching morphogenesis but not epithelial polarity. Lgl1 is required for directional migration by regulating Integrin β1 signaling levels and focal adhesion strengths. Finally, branching mechanisms are distinct between mammary gland and Drosophila systems where directional migration is indispensable.

Occludin is a target of Src kinase and promotes lipid secretion by binding to BTN1a1 and XOR

Lipid droplets (LDs) have increasingly been recognized as an essential organelle for eukaryotes. Although the biochemistry of lipid synthesis and degradation is well characterized, the regulation of LD dynamics, including its formation, maintenance, and secretion, is poorly understood. Here, we report that mice lacking Occludin (Ocln) show defective lipid metabolism. We show that LDs were larger than normal along its biogenesis and secretion pathway in Ocln null mammary cells. This defect in LD size control did not result from abnormal lipid synthesis or degradation; rather, it was because of secretion failure during the lactation stage. We found that OCLN was located on the LD membrane and was bound to essential regulators of lipid secretion, including BTN1a1 and XOR, in a C-terminus–dependent manner. Finally, OCLN was a phosphorylation target of Src kinase, whose loss causes lactation failure. Together, we demonstrate that Ocln is a downstream target of Src kinase and promotes LD secretion by binding to BTN1a1 and XOR.

Lipid droplets are an essential eukaryotic organelle, but how they are secreted has remained unclear. This study shows that the tight junction protein Occludin is a phosphorylation target of Src kinase; Occludin binds to BTN1A1 and XOR to facilitate lipid droplet secretion in mammary epithelial cells.

3D in vitro culture system to study collective migration in mammary organoid epithelium

We recently established an in vitro culture system in which mammary gland organoid undergoes directional migration in response to an FGF10 concentration gradient. Here, we describe a step-by-step protocol for preparing organoids, the setup of the 3D culture system, and the image acquisition approach. The technical difficulties in conducting the 3D migration assay are choosing epithelial organoids of appropriate sizes and manually paring organoids and beads pre-soaked in FGF10 within a desirable distance (∼100 μm). For complete details on the use and execution of this protocol, please refer to Lu et al. (2020).

Aging-Associated Alterations in Mammary Epithelia and Stroma Revealed by Single-Cell RNA Sequencing

Aging is closely associated with increased susceptibility to breast cancer, yet there have been limited systematic studies of aging-induced alterations in the mammary gland. Here, we leverage high-throughput single-cell RNA sequencing to generate a detailed transcriptomic atlas of young and aged murine mammary tissues. By analyzing epithelial, stromal, and immune cells, we identify age-dependent alterations in cell proportions and gene expression, providing evidence that suggests alveolar maturation and physiological decline. The analysis also uncovers potential pro-tumorigenic mechanisms coupled to the age-associated loss of tumor suppressor function and change in microenvironment. In addition, we identify a rare, age-dependent luminal population co-expressing hormone-sensing and secretory-alveolar lineage markers, as well as two macrophage populations expressing distinct gene signatures, underscoring the complex heterogeneity of the mammary epithelia and stroma. Collectively, this rich single-cell atlas reveals the effects of aging on mammary physiology and can serve as a useful resource for understanding aging-associated cancer risk.

Using single-cell RNA-sequencing, Li et al. compare mammary epithelia and stroma in young and aged mice. Age-dependent changes at cell and gene levels provide evidence suggesting alveolar maturation, functional deterioration, and potential pro-tumorigenic and inflammatory alterations. Additionally, identification of heterogeneous luminal and macrophage subpopulations underscores the complexity of mammary lineages.

Influence of Fibroblasts on Mammary Gland Development, Breast Cancer Microenvironment Remodeling, and Cancer Cell Dissemination

The stromal microenvironment regulates mammary gland development and tumorigenesis. In normal mammary glands, the stromal microenvironment encompasses the ducts and contains fibroblasts, the main regulators of branching morphogenesis. Understanding the way fibroblast signaling pathways regulate mammary gland development may offer insights into the mechanisms of breast cancer (BC) biology. In fact, the unregulated mammary fibroblast signaling pathways, associated with alterations in extracellular matrix (ECM) remodeling and branching morphogenesis, drive breast cancer microenvironment (BCM) remodeling and cancer growth. The BCM comprises a very heterogeneous tissue containing non-cancer stromal cells, namely, breast cancer-associated fibroblasts (BCAFs), which represent most of the tumor mass. Moreover, the different components of the BCM highly interact with cancer cells, thereby generating a tightly intertwined network. In particular, BC cells activate recruited normal fibroblasts in BCAFs, which, in turn, promote BCM remodeling and metastasis. Thus, comparing the roles of normal fibroblasts and BCAFs in the physiological and metastatic processes, could provide a deeper understanding of the signaling pathways regulating BC dissemination. Here, we review the latest literature describing the structure of the mammary gland and the BCM and summarize the influence of epithelial-mesenchymal transition (EpMT) and autophagy in BC dissemination. Finally, we discuss the roles of fibroblasts and BCAFs in mammary gland development and BCM remodeling, respectively.

Mammary stem cells, where art thou?

Tremendous progress has been made in the field of stem cell biology. This is in part due to the emergence of various vertebrate organs, including the mammary gland, as an amenable model system for adult stem cell studies and remarkable technical advances in single cell technology and modern genetic lineage tracing. In the current review, we summarize the recent progress in mammary gland stem cell biology at both the adult and embryonic stages. We discuss current challenges and controversies, and potentially new and exciting directions for future research. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cell Differentiation and Reversion Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration.

FGF signaling in mammary gland fibroblasts regulates multiple fibroblast functions and mammary epithelial morphogenesis

Fibroblast growth factor (FGF) signaling is crucial for mammary gland development. While multiple roles for FGF signaling in the epithelium were described, the function of FGF signaling in mammary stroma has not been elucidated. In this study, we investigated FGF signaling in mammary fibroblasts. We found that mammary fibroblasts express FGF receptors FGFR1 and FGFR2 and respond to FGF ligands. In particular, FGF2 and FGF9 induce sustained ERK1/2 signaling and promote fibroblast proliferation and migration in 2D. Intriguingly, only FGF2 induces fibroblast migration in 3D extracellular matrix (ECM) through regulation of actomyosin cytoskeleton and promotes force-mediated collagen remodeling by mammary fibroblasts. Moreover, FGF2 regulates production of ECM proteins by mammary fibroblasts, including collagens, fibronectin, osteopontin, and matrix metalloproteinases. Finally, using organotypic 3D co-cultures we show that FGF2 and FGF9 signaling in mammary fibroblasts enhances fibroblast-induced branching of mammary epithelium by modulating paracrine signaling and that knockdown of Fgfr1 and Fgfr2 in mammary fibroblasts reduces branching of mammary epithelium. Our results demonstrate a pleiotropic role for FGF signaling in mammary fibroblasts with implications for regulation of mammary stromal functions and epithelial branching morphogenesis.

SPRY1 regulates mammary epithelial morphogenesis by modulating EGFR-dependent stromal paracrine signaling and ECM remodeling

The role of the local microenvironment in influencing cell behavior is central to both normal development and cancer formation. Here, we show that sprouty 1 (SPRY1) modulates the microenvironment to enable proper mammary branching morphogenesis. This process occurs through negative regulation of epidermal growth factor receptor (EGFR) signaling in mammary stroma. Loss of SPRY1 resulted in up-regulation of EGFR-extracellular signal-regulated kinase (ERK) signaling in response to amphiregulin and transforming growth factor alpha stimulation. Consequently, stromal paracrine signaling and ECM remodeling is augmented, leading to increased epithelial branching in the mutant gland. By contrast, down-regulation of EGFR-ERK signaling due to gain of Sprouty function in the stroma led to stunted epithelial branching. Taken together, our results show that modulation of stromal paracrine signaling and ECM remodeling by SPRY1 regulates mammary epithelial morphogenesis during postnatal development.

FGF ligands of the postnatal mammary stroma regulate distinct aspects of epithelial morphogenesis

FGF signaling is essential for mammary gland development, yet the mechanisms by which different members of the FGF family control stem cell function and epithelial morphogenesis in this tissue are not well understood. Here, we have examined the requirement of Fgfr2 in mouse mammary gland morphogenesis using a postnatal organ regeneration model. We found that tissue regeneration from basal stem cells is a multistep event, including luminal differentiation and subsequent epithelial branching morphogenesis. Basal cells lacking Fgfr2 did not generate an epithelial network owing to a failure in luminal differentiation. Moreover, Fgfr2 null epithelium was unable to undergo ductal branch initiation and elongation due to a deficiency in directional migration. We identified FGF10 and FGF2 as stromal ligands that control distinct aspects of mammary ductal branching. FGF10 regulates branch initiation, which depends on directional epithelial migration. By contrast, FGF2 controls ductal elongation, requiring cell proliferation and epithelial expansion. Together, our data highlight a pleiotropic role of Fgfr2 in stem cell differentiation and branch initiation, and reveal that different FGF ligands regulate distinct aspects of epithelial behavior.