Isolation and Propagation of Mammary Epithelial Stem and Progenitor Cells

Several methods of mammary gland dissociation have been described that utilize a combined strategy of mechanical and enzymatic dissociation to isolate mammary epithelial cells (MECs) from intact tissue (Smalley et al., J Mammary Gland Biol Neoplasia 17:91-97, 2012). Here we detail a robust method that enables the isolation of all major stem and progenitor MEC populations, which has been successfully used to study stem cell behavior when coupled with transplantation and in vitro assays (Shackleton et al., Nature 439:84-88, 2006; Bouras et al., Cell Stem Cell 3:429-441, 2008; Sheridan et al., BMC Cancer 15:221, 2015; Jamieson et al., Development 144:1065-1071, 2017). Furthermore, we outline two prominent methods for culturing MECs for the purposes of ex vivo manipulation or study: 2D feeder layer cultures and 3D Matrigel colony assays. Importantly, all outlined methods retain stem and progenitor cell behaviors and can be used in combination with downstream in vivo, in vitro, or in silico analyses.

Breast tumour organoids: promising models for the genomic and functional characterisation of breast cancer

Until recently, established cancer cell lines have been used extensively in breast cancer research, due largely to the difficulties associated with the manipulation and long-term maintenance in culture of primary tumour cells from patients. The recent development of organoid cultures has provided new opportunities to model and analyse patient samples, allowing the propagation of malignant cells under conditions that resemble the three-dimensional growth of breast tumours. They have proved efficacious in preserving the heterogeneity of primary samples and are emerging as a new model to further characterise the molecular features of breast cancer. Organoids formed from patient-derived cells are now in use for the evaluation of drug sensitivity and to validate disease-causing genomic variations. Here, the advantages and limitations of organoid cultures will be discussed and compared with the parallel development of other two- and three-dimensional culture strategies and with patient-derived xenografts. In particular, we will focus on the molecular characterisation of breast cancer organoids and provide some examples of how they have been used in functional studies.

From single cells to tissue self-organization

Self-organization is a process by which interacting cells organize and arrange themselves in higher order structures and patterns. To achieve this, cells must have molecular mechanisms to sense their complex local environment and interpret it to respond accordingly. A combination of cell-intrinsic and cell-extrinsic cues are decoded by the single cells dictating their behaviour, their differentiation and symmetry-breaking potential driving development, tissue remodeling and regenerative processes. A unifying property of these self-organized pattern-forming systems is the importance of fluctuations, cell-to-cell variability, or noise. Cell-to-cell variability is an inherent and emergent property of populations of cells that maximize the population performance instead of the individual cell, providing tissues the flexibility to develop and maintain homeostasis in diverse environments. In this review, we will explore the role of self-organization and cell-to-cell variability as fundamental properties of multicellularity-and the requisite of single-cell resolution for its understanding. Moreover, we will analyze how single cells generate emergent multicellular dynamics observed at the tissue level 'travelling' across different scales: spatial, temporal and functional.

3D culture models for studying branching morphogenesis in the mammary gland and mammalian lung

The intricate architecture of branched tissues and organs has fascinated scientists and engineers for centuries. Yet-despite their ubiquity-the biophysical and biochemical mechanisms by which tissues and organs undergo branching morphogenesis remain unclear. With the advent of three-dimensional (3D) culture models, an increasingly powerful and diverse set of tools are available for investigating the development and remodeling of branched tissues and organs. In this review, we discuss the application of 3D culture models for studying branching morphogenesis of the mammary gland and the mammalian lung in the context of normal development and disease. While current 3D culture models lack the cellular and molecular complexity observed in vivo, we emphasize how these models can be used to answer targeted questions about branching morphogenesis. We highlight the specific advantages and limitations of using 3D culture models to study the dynamics and mechanisms of branching in the mammary gland and mammalian lung. Finally, we discuss potential directions for future research and propose strategies for engineering the next generation of 3D culture models for studying tissue morphogenesis.

Tumour cell invasiveness and response to chemotherapeutics in adipocyte invested 3D engineered anisotropic collagen scaffolds

Breast cancers are highly heterogeneous and their metastatic potential and response to therapeutic drugs is difficult to predict. A tool that could accurately gauge tumour invasiveness and drug response would provide a valuable addition to the oncologist’s arsenal. We have developed a 3-dimensional (3D) culture model that recapitulates the stromal environment of breast cancers by generating anisotropic (directional) collagen scaffolds seeded with adipocytes and culturing tumour fragments therein. Analysis of tumour cell invasion in the presence of various therapeutic drugs, by immunofluorescence microscopy coupled with an optical clearing technique, demonstrated the utility of this approach in determining both the rate and capacity of tumour cells to migrate through the stroma while shedding light also on the mode of migration. Furthermore, the response of different murine mammary tumour types to chemotherapeutic drugs could be readily quantified.

Canonical PRC2 function is essential for mammary gland development and affects chromatin compaction in mammary organoids

Distinct transcriptional states are maintained through organization of chromatin, resulting from the sum of numerous repressive and active histone modifications, into tightly packaged heterochromatin versus more accessible euchromatin. Polycomb repressive complex 2 (PRC2) is the main mammalian complex responsible for histone 3 lysine 27 trimethylation (H3K27me3) and is integral to chromatin organization. Using in vitro and in vivo studies, we show that deletion of Suz12, a core component of all PRC2 complexes, results in loss of H3K27me3 and H3K27 dimethylation (H3K27me2), completely blocks normal mammary gland development, and profoundly curtails progenitor activity in 3D organoid cultures. Through the application of mammary organoids to bypass the severe phenotype associated with Suz12 loss in vivo, we have explored gene expression and chromatin structure in wild-type and Suz12-deleted basal-derived organoids. Analysis of organoids led to the identification of lineage-specific changes in gene expression and chromatin structure, inferring cell type-specific PRC2-mediated gene silencing of the chromatin state. These expression changes were accompanied by cell cycle arrest but not lineage infidelity. Together, these data indicate that canonical PRC2 function is essential for development of the mammary gland through the repression of alternate transcription programs and maintenance of chromatin states.

The Central Role of Wnt Signaling and Organoid Technology in Personalizing Anticancer Therapy

The Wnt pathway is at the heart of organoid technology, which is set to revolutionize the cancer field. We can now predetermine a patient's response to any given anticancer therapy by exposing tumor organoids established from the patient's own tumor. This cutting-edge biomedical platform translates to patients being treated with the correct drug at the correct dose from the outset, a truly personalized and precise medical approach. A high throughput drug screen on organoids also allows drugs to be tested in limitless combinations. More recently, the tumor cells that are resistant to the therapy given to a patient were selected in culture using the patient's organoids. The resistant tumor organoids were then screened empirically to identify drugs that will kill the resistant cells. This information allows diagnosis in real-time to either prevent tumor recurrence or effectively treat the recurring tumor. Furthermore, the ability to culture stem cell-derived epithelium as organoids has enabled us to begin to understand how a stem cell becomes a cancer cell or to pin-point the genetic alteration that underlies a given genetic syndrome. Here we summarize these advances and the central role of Wnt signaling, and identify the next challenges for organoid technology.

Organoids: a Special Issue

Summary: This Editorial provides an overview of the entire contents of the Special Issue, highlighting some of the important findings and major themes therein.

Next generation organoids for biomedical research and applications

Organoids are in vitro cultures of miniature fetal or adult organ-like structures. Their potentials for use in tissue and organ replacement, disease modeling, toxicology studies, and drug discovery are tremendous. Currently, major challenges facing human organoid technology include (i) improving the range of cellular heterogeneity for a particular organoid system, (ii) mimicking the native micro- and matrix-environment encountered by cells within organoids, and (iii) developing robust protocols for the in vitro maturation of organoids that remain mostly fetal-like in cultures. To tackle these challenges, we advocate the principle of reverse engineering that replicates the inner workings of in vivo systems with the goal of achieving functionality and maturation of the resulting organoid structures with the input of minimal intrinsic (cellular) and environmental (matrix and niche) constituents. Here, we present an overview of organoid technology development in several systems that employ cell materials derived from fetal and adult tissues and pluripotent stem cell cultures. We focus on key studies that exploit the self-organizing property of embryonic progenitors and the role of designer matrices and cell-free scaffolds in assisting organoid formation. We further explore the relationship between adult stem cells, niche factors, and other current developments that aim to enhance robust organoid maturation. From these works, we propose a standardized pipeline for the development of future protocols that would help generate more physiologically relevant human organoids for various biomedical applications.

Wnt/β-Catenin Signaling, Disease, and Emerging Therapeutic Modalities

The WNT signal transduction cascade is a main regulator of development throughout the animal kingdom. Wnts are also key drivers of most types of tissue stem cells in adult mammals. Unsurprisingly, mutated Wnt pathway components are causative to multiple growth-related pathologies and to cancer. Here, we describe the core Wnt/β-catenin signaling pathway, how it controls stem cells, and contributes to disease. Finally, we discuss strategies for Wnt-based therapies.