Circadian control of tumor immunosuppression affects efficacy of immune checkpoint blockade.

The circadian clock is a critical regulator of immunity, and this circadian control of immune modulation has an essential function in host defense and tumor immunosurveillance. Here we use a single-cell RNA sequencing approach and a genetic model of colorectal cancer to identify clock-dependent changes to the immune landscape that control the abundance of immunosuppressive cells and consequent suppression of cytotoxic CD8+ T cells. Of these immunosuppressive cell types, PD-L1-expressing myeloid-derived suppressor cells (MDSCs) peak in abundance in a rhythmic manner. Disruption of the epithelial cell clock regulates the secretion of cytokines that promote heightened inflammation, recruitment of neutrophils and the subsequent development of MDSCs. We also show that time-of-day anti-PD-L1 delivery is most effective when synchronized with the abundance of immunosuppressive MDSCs. Collectively, these data indicate that circadian gating of tumor immunosuppression informs the timing and efficacy of immune checkpoint inhibitors.

A spatially resolved single-cell genomic atlas of the adult human breast.

The adult human breast is comprised of an intricate network of epithelial ducts and lobules that are embedded in connective and adipose tissue1-3. Although most previous studies have focused on the breast epithelial system4-6, many of the non-epithelial cell types remain understudied. Here we constructed the comprehensive Human Breast Cell Atlas (HBCA) at single-cell and spatial resolution. Our single-cell transcriptomics study profiled 714,331 cells from 126 women, and 117,346 nuclei from 20 women, identifying 12 major cell types and 58 biological cell states. These data reveal abundant perivascular, endothelial and immune cell populations, and highly diverse luminal epithelial cell states. Spatial mapping using four different technologies revealed an unexpectedly rich ecosystem of tissue-resident immune cells, as well as distinct molecular differences between ductal and lobular regions. Collectively, these data provide a reference of the adult normal breast tissue for studying mammary biology and diseases such as breast cancer.

Signalling by senescent melanocytes hyperactivates hair growth.

Niche signals maintain stem cells in a prolonged quiescence or transiently activate them for proper regeneration1. Altering balanced niche signalling can lead to regenerative disorders. Melanocytic skin nevi in human often display excessive hair growth, suggesting hair stem cell hyperactivity. Here, using genetic mouse models of nevi2,3, we show that dermal clusters of senescent melanocytes drive epithelial hair stem cells to exit quiescence and change their transcriptome and composition, potently enhancing hair renewal. Nevus melanocytes activate a distinct secretome, enriched for signalling factors. Osteopontin, the leading nevus signalling factor, is both necessary and sufficient to induce hair growth. Injection of osteopontin or its genetic overexpression is sufficient to induce robust hair growth in mice, whereas germline and conditional deletions of either osteopontin or CD44, its cognate receptor on epithelial hair cells, rescue enhanced hair growth induced by dermal nevus melanocytes. Osteopontin is overexpressed in human hairy nevi, and it stimulates new growth of human hair follicles. Although broad accumulation of senescent cells, such as upon ageing or genotoxic stress, is detrimental for the regenerative capacity of tissue4, we show that signalling by senescent cell clusters can potently enhance the activity of adjacent intact stem cells and stimulate tissue renewal. This finding identifies senescent cells and their secretome as an attractive therapeutic target in regenerative disorders.

Discoidin domain receptor 1 (DDR1) ablation promotes tissue fibrosis and hypoxia to induce aggressive basal-like breast cancers.

The discoidin domain receptor 1 (DDR1) is overexpressed in breast carcinoma cells. Low DDR1 expression is associated with worse relapse-free survival, reflecting its controversial role in cancer progression. We detected DDR1 on luminal cells but not on myoepithelial cells of DDR1+/+ mice. We found that DDR1 loss compromises cell adhesion, consistent with data that older DDR1-/- mammary glands had more basal/myoepithelial cells. Basal cells isolated from older mice exerted higher traction forces than the luminal cells, in agreement with increased mammary branches observed in older DDR1-/- mice and higher branching by their isolated organoids. When we crossed DDR1-/- mice with MMTV-PyMT mice, the PyMT/DDR1-/- mammary tumors grew faster and had increased epithelial tension and matricellular fibrosis with a more basal phenotype and increased lung metastases. DDR1 deletion induced basal differentiation of CD90+CD24+ cancer cells, and the increase in basal cells correlated with tumor cell mitoses. K14+ basal cells, including K8+K14+ cells, were increased adjacent to necrotic fields. These data suggest that the absence of DDR1 provides a growth and adhesion advantage that favors the expansion of basal cells, potentiates fibrosis, and enhances necrosis/hypoxia and basal differentiation of transformed cells to increase their aggression and metastatic potential.

Automated segmentation and tracking of mitochondria in live-cell time-lapse images.

Mitochondria display complex morphology and movements, which complicates their segmentation and tracking in time-lapse images. Here, we introduce Mitometer, an algorithm for fast, unbiased, and automated segmentation and tracking of mitochondria in live-cell two-dimensional and three-dimensional time-lapse images. Mitometer requires only the pixel size and the time between frames to identify mitochondrial motion and morphology, including fusion and fission events. The segmentation algorithm isolates individual mitochondria via a shape- and size-preserving background removal process. The tracking algorithm links mitochondria via differences in morphological features and displacement, followed by a gap-closing scheme. Using Mitometer, we show that mitochondria of triple-negative breast cancer cells are faster, more directional, and more elongated than those in their receptor-positive counterparts. Furthermore, we show that mitochondrial motility and morphology in breast cancer, but not in normal breast epithelia, correlate with metabolic activity. Mitometer is an unbiased and user-friendly tool that will help resolve fundamental questions regarding mitochondrial form and function.

Single-cell analysis reveals a stem-cell program in human metastatic breast cancer cells.

Despite major advances in understanding the molecular and genetic basis of cancer, metastasis remains the cause of >90% of cancer-related mortality. Understanding metastasis initiation and progression is critical to developing new therapeutic strategies to treat and prevent metastatic disease. Prevailing theories hypothesize that metastases are seeded by rare tumour cells with unique properties, which may function like stem cells in their ability to initiate and propagate metastatic tumours. However, the identity of metastasis-initiating cells in human breast cancer remains elusive, and whether metastases are hierarchically organized is unknown. Here we show at the single-cell level that early stage metastatic cells possess a distinct stem-like gene expression signature. To identify and isolate metastatic cells from patient-derived xenograft models of human breast cancer, we developed a highly sensitive fluorescence-activated cell sorting (FACS)-based assay, which allowed us to enumerate metastatic cells in mouse peripheral tissues. We compared gene signatures in metastatic cells from tissues with low versus high metastatic burden. Metastatic cells from low-burden tissues were distinct owing to their increased expression of stem cell, epithelial-to-mesenchymal transition, pro-survival, and dormancy-associated genes. By contrast, metastatic cells from high-burden tissues were similar to primary tumour cells, which were more heterogeneous and expressed higher levels of luminal differentiation genes. Transplantation of stem-like metastatic cells from low-burden tissues showed that they have considerable tumour-initiating capacity, and can differentiate to produce luminal-like cancer cells. Progression to high metastatic burden was associated with increased proliferation and MYC expression, which could be attenuated by treatment with cyclin-dependent kinase (CDK) inhibitors. These findings support a hierarchical model for metastasis, in which metastases are initiated by stem-like cells that proliferate and differentiate to produce advanced metastatic disease.

ZNF503/Zpo2 drives aggressive breast cancer progression by down-regulation of GATA3 expression.

The transcription factor GATA3 is the master regulator that drives mammary luminal epithelial cell differentiation and maintains mammary gland homeostasis. Loss of GATA3 is associated with aggressive breast cancer development. We have identified ZNF503/ZEPPO2 zinc-finger elbow-related proline domain protein 2 (ZPO2) as a transcriptional repressor of GATA3 expression and transcriptional activity that induces mammary epithelial cell proliferation and breast cancer development. We show that ZPO2 is recruited to GATA3 promoter in association with ZBTB32 (Repressor of GATA, ROG) and that ZBTB32 is essential for down-regulation of GATA3 via ZPO2. Through this modulation of GATA3 activity, ZPO2 promotes aggressive breast cancer development. Our data provide insight into a mechanism of GATA3 regulation, and identify ZPO2 as a possible candidate gene for future diagnostic and therapeutic strategies.

Microglia are not necessary for maintenance of blood-brain barrier properties in health, but PLX5622 alters brain endothelial cholesterol metabolism.

Microglia, the resident immune cells of the central nervous system, are intimately involved in the brain's most basic processes, from pruning neural synapses during development to preventing excessive neuronal activity throughout life. Studies have reported both helpful and harmful roles for microglia at the blood-brain barrier (BBB) in the context of disease. However, less is known about microglia-endothelial cell interactions in the healthy brain. To investigate the role of microglia at a healthy BBB, we used the colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 to deplete microglia and analyzed the BBB ultrastructure, permeability, and transcriptome. Interestingly, we found that, despite their direct contact with endothelial cells, microglia are not necessary for the maintenance of BBB structure, function, or gene expression in the healthy brain. However, we found that PLX5622 treatment alters brain endothelial cholesterol metabolism. This effect was independent from microglial depletion, suggesting that PLX5622 has off-target effects on brain vasculature.

Basal epithelial stem cells are efficient targets for prostate cancer initiation.

Prevailing theories suggest that luminal cells are the origin of prostate cancer because it is histologically defined by basal cell loss and malignant luminal cell expansion. We introduced a series of genetic alterations into prospectively identified populations of murine basal/stem and luminal cells in an in vivo prostate regeneration assay. Stromal induction of FGF signaling, increased expression of the ETS family transcription factor ERG1, and constitutive activation of PI3K signaling were evaluated. Combination of activated PI3K signaling and heightened androgen receptor signaling, which is associated with disease progression to androgen independence, was also performed. Even though luminal cells fail to respond, basal/stem cells demonstrate efficient capacity for cancer initiation and can produce luminal-like disease characteristic of human prostate cancer in multiple models. This finding provides evidence in support of basal epithelial stem cells as one target cell for prostate cancer initiation and demonstrates the propensity of primitive cells for tumorigenesis.

Microglia promote anti-tumour immunity and suppress breast cancer brain metastasis.

Breast cancer brain metastasis (BCBM) is a lethal disease with no effective treatments. Prior work has shown that brain cancers and metastases are densely infiltrated with anti-inflammatory, protumourigenic tumour-associated macrophages, but the role of brain-resident microglia remains controversial because they are challenging to discriminate from other tumour-associated macrophages. Using single-cell RNA sequencing, genetic and humanized mouse models, we specifically identify microglia and find that they play a distinct pro-inflammatory and tumour-suppressive role in BCBM. Animals lacking microglia show increased metastasis, decreased survival and reduced natural killer and T cell responses, showing that microglia are critical to promote anti-tumour immunity to suppress BCBM. We find that the pro-inflammatory response is conserved in human microglia, and markers of their response are associated with better prognosis in patients with BCBM. These findings establish an important role for microglia in anti-tumour immunity and highlight them as a potential immunotherapy target for brain metastasis.

A spatially resolved single cell genomic atlas of the adult human breast.

The adult human breast comprises an intricate network of epithelial ducts and lobules that are embedded in connective and adipose tissue. While previous studies have mainly focused on the breast epithelial system, many of the non-epithelial cell types remain understudied. Here, we constructed a comprehensive Human Breast Cell Atlas (HBCA) at single-cell and spatial resolution. Our single-cell transcriptomics data profiled 535,941 cells from 62 women, and 120,024 nuclei from 20 women, identifying 11 major cell types and 53 cell states. These data revealed abundant pericyte, endothelial and immune cell populations, and highly diverse luminal epithelial cell states. Our spatial mapping using three technologies revealed an unexpectedly rich ecosystem of tissue-resident immune cells in the ducts and lobules, as well as distinct molecular differences between ductal and lobular regions. Collectively, these data provide an unprecedented reference of adult normal breast tissue for studying mammary biology and disease states such as breast cancer.

ETS family transcription factors collaborate with alternative signaling pathways to induce carcinoma from adult murine prostate cells.

Chromosomal rearrangements involving erythroblast transformation specific (ETS) family transcription factors were recently defined as the most common genetic alterations in human prostate cancer. Despite their prevalence, it is unclear what quantitative role they play in either initiation or progression of the disease. Using a lentiviral transduction and dissociated cell prostate regeneration approach, we find that acutely increased expression of ETS proteins in adult murine prostate epithelial cells is sufficient to induce the formation of epithelial hyperplasia and focal prostatic intraepithelial neoplasia (PIN) lesions, but not progression to carcinoma. However, combined expression of ERG with additional genetic alternations associated with human prostate cancer can lead to aggressive disease. Although ERG overexpression does not cooperate with loss of the tumor suppressor p53, it does collaborate with alterations in PI3K signaling, such as Pten knockdown or AKT up-regulation, to produce a well-differentiated adenocarcinoma. Most striking is our finding that overexpression of androgen receptor (AR) does not give rise to any hyperplastic lesions, but when combined with high levels of ERG, it promotes the development of a more poorly differentiated, invasive adenocarcinoma. These findings suggest that in human prostate cancer, the most potent function of ETS gene fusions may be to synergize with alternative genetic events and provide different pathways for carcinoma production and invasive behavior. Our results provide direct evidence for selective cooperating events in ERG-induced prostate tumorigenesis and offer a rational basis for combined therapeutic interventions against multiple oncogenic pathways in prostate cancer.

Trop2 identifies a subpopulation of murine and human prostate basal cells with stem cell characteristics.

The epithelium of the adult prostate contains 3 distinct cell types: basal, luminal, and neuroendocrine. Tissue-regenerative activity has been identified predominantly from the basal cells, isolated by expression of CD49f and stem cell antigen-1 (Sca-1). An important question for the field is whether all basal cells have stem cell characteristics. Prostate-specific microarray databases were interrogated to find candidate surface antigens that could subfractionate the basal cell population. Tumor-associated calcium signal transducer 2 (TACSTD2/Trop2/M1S1/GA733-1) was identified because it was enriched after castration, in prostate sphere cells and in the basal fraction. In the murine prostate, Trop2 shows progenitor characteristics such as localization to the region of the gland proximal to the urethra and enrichment for sphere-forming and colony-forming cells. Trop2 subfractionates the basal cells into 2 populations, both of which express characteristic basal cell markers by quantitative PCR. However, only the basal cells expressing high levels of Trop2 were able to efficiently form spheres in vitro. In the human prostate, where Sca-1 is not expressed, sphere-forming progenitor cells were also isolated based on high expression of Trop2 and CD49f. Trop2-expressing murine basal cells could regenerate prostatic tubules in vivo, whereas the remaining basal cells had minimal activity. Evidence was found for basal, luminal, and neuroendocrine cells in prostatic tubules regenerated from Trop2(hi) basal cells. In summary, functionally distinct populations of cells exist within the prostate basal compartment and an epithelial progenitor can give rise to neuroendocrine cells in vivo.

Patient-derived xenograft culture-transplant system for investigation of human breast cancer metastasis.

Metastasis is a fatal disease where research progress has been hindered by a lack of authentic experimental models. Here, we develop a 3D tumor sphere culture-transplant system that facilitates the growth and engineering of patient-derived xenograft (PDX) tumor cells for functional metastasis assays in vivo. Orthotopic transplantation and RNA sequencing (RNA-seq) analyses show that PDX tumor spheres maintain tumorigenic potential, and the molecular marker and global transcriptome signatures of native tumor cells. Tumor spheres display robust capacity for lentiviral engineering and dissemination in spontaneous and experimental metastasis assays in vivo. Inhibition of pathways previously reported to attenuate metastasis also inhibit metastasis after sphere culture, validating our approach for authentic investigations of metastasis. Finally, we demonstrate a new role for the metabolic enzyme NME1 in promoting breast cancer metastasis, providing proof-of-principle that our culture-transplant system can be used for authentic propagation and engineering of patient tumor cells for functional studies of metastasis.

An in vitro vascularized micro-tumor model of human colorectal cancer recapitulates in vivo responses to standard-of-care therapy.

Around 95% of anti-cancer drugs that show promise during preclinical study fail to gain FDA-approval for clinical use. This failure of the preclinical pipeline highlights the need for improved, physiologically-relevant in vitro models that can better serve as reliable drug-screening and disease modeling tools. The vascularized micro-tumor (VMT) is a novel three-dimensional model system (tumor-on-a-chip) that recapitulates the complex human tumor microenvironment, including perfused vasculature, within a transparent microfluidic device, allowing real-time study of drug responses and tumor-stromal interactions. Here we have validated this microphysiological system (MPS) platform for the study of colorectal cancer (CRC), the second leading cause of cancer-related deaths, by showing that gene expression, tumor heterogeneity, and treatment responses in the VMT more closely model CRC tumor clinicopathology than current standard drug screening modalities, including 2-dimensional monolayer culture and 3-dimensional spheroids.

Stem cells in prostate cancer initiation and progression.

Peter Nowell and David Hungerford's discovery of the Philadelphia chromosome facilitated many critical studies that have led to a paradigm shift in our understanding of cancer as a disease of stem cells. This Review focuses on the application of these concepts to investigation of the role of stem cells in prostate cancer initiation and progression. Major strides in the development of in vitro and in vivo assays have enabled identification and characterization of prostate stem cells as well as functional evaluation of the tumorigenic effects of prostate cancer-related genetic alterations.

Transcriptional diversity and bioenergetic shift in human breast cancer metastasis revealed by single-cell RNA sequencing.

Although metastasis remains the cause of most cancer-related mortality, mechanisms governing seeding in distal tissues are poorly understood. Here, we establish a robust method for the identification of global transcriptomic changes in rare metastatic cells during seeding using single-cell RNA sequencing and patient-derived-xenograft models of breast cancer. We find that both primary tumours and micrometastases display transcriptional heterogeneity but micrometastases harbour a distinct transcriptome program conserved across patient-derived-xenograft models that is highly predictive of poor survival of patients. Pathway analysis revealed mitochondrial oxidative phosphorylation as the top pathway upregulated in micrometastases, in contrast to higher levels of glycolytic enzymes in primary tumour cells, which we corroborated by flow cytometric and metabolomic analyses. Pharmacological inhibition of oxidative phosphorylation dramatically attenuated metastatic seeding in the lungs, which demonstrates the functional importance of oxidative phosphorylation in metastasis and highlights its potential as a therapeutic target to prevent metastatic spread in patients with breast cancer.

Self-renewal and multilineage differentiation in vitro from murine prostate stem cells.

Murine prostate stem cells express integrin alpha 6, which modulates survival, proliferation, and differentiation signaling through its interaction with the extracellular protein laminin. When plated in vitro in laminin containing Matrigel medium, 1 of 500-1,000 murine prostate cells can grow and form clonogenic spheroid structures that we term prostate spheres. Prostate spheres can be serially passaged individually or in bulk to generate daughter spheres with similar composition, demonstrating that sphere-forming cells are capable of self-renewal. Spheres spontaneously undergo lineage specification for basal and transit-amplifying cell types. P63-expressing cells localized to the outer layers of prostate spheres possess higher self-renewal capacity, whereas cells toward the center display a more differentiated transit-amplifying phenotype, as demonstrated by the expression of the prostate stem cell antigen. When dihydrotestosterone is added to the medium, the androgen receptor is stabilized, is imported to the nucleus, and drives differentiation to a luminal cell-like phenotype. A fraction of sphere cells returned to an in vivo environment can undergo differentiation and morphogenesis to form prostate tubular structures with defined basal and luminal layers accompanied by prostatic secretions. This study demonstrates self-renewal and multilineage differentiation from single adult prostate stem/progenitor cells in a specific in vitro microenvironment.

Tumour heterogeneity and metastasis at single-cell resolution.

Tumours comprise a heterogeneous collection of cells with distinct genetic and phenotypic properties that can differentially promote progression, metastasis and drug resistance. Emerging single-cell technologies provide a new opportunity to profile individual cells within tumours and investigate what roles they play in these processes. This Review discusses key technological considerations for single-cell studies in cancer, new findings using single-cell technologies and critical open questions for future applications.