BMP signaling: at the gate between activated melanocyte stem cells and differentiation

Through recurrent bouts synchronous with the hair cycle, quiescent melanocyte stem cells (McSCs) become activated to generate proliferative progeny that differentiate into pigment-producing melanocytes. The signaling factors orchestrating these events remain incompletely understood. Here, we use single-cell RNA sequencing with comparative gene expression analysis to elucidate the transcriptional dynamics of McSCs through quiescence, activation, and melanocyte maturation. Unearthing converging signs of increased WNT and BMP signaling along this progression, we endeavored to understand how these pathways are integrated. Employing conditional lineage-specific genetic ablation studies in mice, we found that loss of BMP signaling in the lineage leads to hair graying due to a block in melanocyte maturation. We show that interestingly, BMP signaling functions downstream from activated McSCs and maintains WNT effector, transcription factor LEF1. Employing pseudotime analysis, genetics, and chromatin landscaping, we show that following WNT-mediated activation of McSCs, BMP and WNT pathways collaborate to trigger the commitment of proliferative progeny by fueling LEF1- and MITF-dependent differentiation. Our findings shed light upon the signaling interplay and timing of cues that orchestrate melanocyte lineage progression in the hair follicle and underscore a key role for BMP signaling in driving complete differentiation.

Significant Left Main Coronary Artery Disease Does Not Incur Any Additional Risk to the Short Term Outcome of Off Pump Coronary Artery Bypass Grafting Surgery

Coronary artery disease is the most common form of heart disease and single most important cause of premature death in developed countries. Off pump coronary artery bypass grafting surgery has recently became widespread internationally and has produced good clinical outcome even in left main coronary artery disease following off pump coronary artery bypass grafting surgery. A prospective observational study was conducted in the department of Cardiac Surgery, National Heart Foundation Hospital and Research Institute, Dhaka, Bangladesh from January 2015 to September 2016 after fulfillment of enrollment criteria, 428 patients were studied for the purpose of the study and they were grouped in two, significant left main coronary artery disease in group A and non left main coronary artery disease requiring surgery in group B. Comparison of risk factors between groups demonstrates that diabetes mellitus and smoking habit between two groups were almost identical. 69.2% of patient with significant left main coronary artery group were hypertensive and 25.2% were dyslipidaemic in comparison to non left main coronary artery disease group which were 50.9% and 6.5% respectively. Requirement of per operative inotropic support was significantly higher in left main coronary artery group. 33.6% of patient of left main coronary artery group required per operative inotropic support whereas non left main coronary artery group require 24.8%. Postoperative inotropic support, mechanical ventilation time, ICU stay, hospital stay and complication were similar in both these groups. So, we can say that off pump coronary artery bypass grafting surgery for significant left main coronary artery disease is as safe as non left main coronary artery disease.

An RNAi screen unravels the complexities of Rho GTPase networks in skin morphogenesis

During mammalian embryogenesis, extensive cellular remodeling is needed for tissue morphogenesis. As effectors of cytoskeletal dynamics, Rho GTPases and their regulators are likely involved, but their daunting complexity has hindered progress in dissecting their functions. We overcome this hurdle by employing high throughput in utero RNAi-mediated screening to identify key Rho regulators of skin morphogenesis. Our screen unveiled hitherto unrecognized roles for Rho-mediated cytoskeletal remodeling events that impact hair follicle specification, differentiation, downgrowth and planar cell polarity. Coupling our top hit with gain/loss-of-function genetics, interactome proteomics and tissue imaging, we show that RHOU, an atypical Rho, governs the cytoskeletal-junction dynamics that establish columnar shape and planar cell polarity in epidermal progenitors. Conversely, RHOU downregulation is required to remodel to a conical cellular shape that enables hair bud invagination and downgrowth. Our findings underscore the power of coupling screens with proteomics to unravel the physiological significance of complex gene families.

Distinct modes of cell competition shape mammalian tissue morphogenesis

Cell competition (CC)—the sensing and elimination of less fit “loser” cells by neighbouring “winner” cells—was first described in Drosophila. Although proposed as a selection mechanism to optimize tissue and organ development, its evolutionary generality remains unclear. Here, by employing live-imaging, lineage-tracing, single cell transcriptomics and genetics, we unearth two intriguing CC mechanisms that sequentially shape and maintain stratified tissue architecture during mouse skin development. In early embryonic epidermis, winner progenitors within the single-layered epithelium kill and clear neighbouring losers by engulfment. Upon stratification and skin barrier formation, the basal layer instead expels losers through a homeostatic upward flux of differentiating progeny. This CC switch is physiologically relevant: when perturbed, so too is barrier formation. Our findings establish CC as a selective force to optimize vertebrate tissue function, and illuminate how a tissue dynamically adjusts CC strategies to preserve fitness as it encounters increased architectural complexity during morphogenesis.

Inflammatory memory sensitizes skin epithelial stem cells to tissue damage

The body’s first line of defense against environmental assaults, the skin barrier is maintained by epithelial stem cells (EpSCs). Despite EpSCs’ vulnerability to inflammatory pressures, neither the primary response nor its enduring consequences are understood. Here, we unearth a prolonged memory to acute inflammation that enables EpSCs to hasten barrier restoration following subsequent tissue damage. This functional adaptation does not require skin resident macrophages or T cells. Rather, EpSCs maintain chromosomal accessibility at key stress response genes that are activated by the primary stimulus. Upon a secondary challenge, genes governed by these domains are transcribed rapidly. Fueling this memory is Aim2, encoding an activator of the inflammasome. Absence of AIM2 or its downstream effectors, Caspase-1 and Interleukin-1β, erases EpSCs’ ability to recollect inflammation. While EpSCs benefit from inflammatory tuning by heightening their responsiveness to subsequent stressors, this enhanced sensitivity likely increases their susceptibility to autoimmune and hyperproliferative disorders, including cancer.

Abstract 4766: Translation from unconventional 5' start sites drives tumor initiation

How translational control impacts tumor-initiation and malignancy is just beginning to unfold. Here, we devise an epidermis-specific, in vivo ribosome profiling strategy to interrogate the translational landscape during the transition from normal homeostasis to malignancy. Inducing SOX2, broadly expressed in oncogenic RAS-associated cancers, we find that despite widespread reductions in translation and protein synthesis, certain oncogenic mRNAs are spared. Seeking mechanism, we find that during tumor-initiation, the translational apparatus is redirected towards unconventional upstream initiation sites, enhancing translational efficiency of oncogenic mRNAs. An in vivo RNAi screen of translational regulators revealed that dampening conventional EIF2 complexes has dire consequences for normal but not oncogenic growth. Conversely, we identify alternative initiation factors essential for cancer progression, where they mediate initiation at these upstream sites, differentially skewing translation and protein expression. Our findings unveil a hitherto unappreciated role of 5’UTR translation in cancer, and expose new targets for therapeutic intervention.

Citation Format: Ataman Sendoel, Joshua G. Dunn, Edwin H. Rodriguez, Shruti Naik, Nicholas C. Gomez, Brian Hurwitz, John Levorse, Brian D. Dill, Daniel Schramek, Henrik Molina, Jonathan S. Weissman, Elaine Fuchs. Translation from unconventional 5' start sites drives tumor initiation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4766. doi:10.1158/1538-7445.AM2017-4766

Widespread Chromatin Accessibility at Repetitive Elements Links Stem Cells with Human Cancer

Chromatin regulation is critical for differentiation and disease. However, features linking the chromatin environment of stem cells with disease remain largely unknown. We explored chromatin accessibility in embryonic and multipotent stem cells and unexpectedly identified widespread chromatin accessibility at repetitive elements. Integrating genomic and biochemical approaches, we demonstrate that these sites of increased accessibility are associated with well-positioned nucleosomes marked by distinct histone modifications. Differentiation is accompanied by chromatin remodeling at repetitive elements associated with altered expression of genes in relevant developmental pathways. Remarkably, we found that the chromatin environment of Ewing sarcoma, a mesenchymally derived tumor, is shared with primary mesenchymal stem cells (MSCs). Accessibility at repetitive elements in MSCs offers a permissive environment that is exploited by the critical oncogene responsible for this cancer. Our data demonstrate that stem cells harbor a unique chromatin landscape characterized by accessibility at repetitive elements, a feature associated with differentiation and oncogenesis.

The Cardiac TBX5 Interactome Reveals a Chromatin Remodeling Network Essential for Cardiac Septation

Human mutations in the cardiac transcription factor gene TBX5 cause congenital heart disease (CHD), although the underlying mechanism is unknown. We report characterization of the endogenous TBX5 cardiac interactome and demonstrate that TBX5, long considered a transcriptional activator, interacts biochemically and genetically with the nucleosome remodeling and deacetylase (NuRD) repressor complex. Incompatible gene programs are repressed by TBX5 in the developing heart. CHD mis-sense mutations that disrupt the TBX5-NuRD interaction cause depression of a subset of repressed genes. Furthermore, the TBX5-NuRD interaction is required for heart development. Phylogenetic analysis showed that the TBX5-NuRD interaction domain evolved during early diversification of vertebrates, simultaneous with the evolution of cardiac septation. Collectively, this work defines a TBX5-NuRD interaction essential to cardiac development and the evolution of the mammalian heart, and when altered may contribute to human CHD.

PTEN deficiency mediates a reciprocal response to IGFI and mTOR inhibition

Recent evidence implicates the insulin-like growth factor (IGF) pathway in development of Ewing sarcoma, a highly malignant bone and soft-tissue tumor that primarily affects children and young adults. Despite promising results from preclinical studies of therapies that target this pathway, early-phase clinical trials have shown that a significant fraction of patients do not benefit, suggesting that cellular factors determine tumor sensitivity. Using FAIRE-seq, a chromosomal deletion of the PTEN locus in a Ewing sarcoma cell line was identified. In primary tumors, PTEN deficiency was observed in a large subset of cases, although not mediated by large chromosomal deletions. PTEN loss resulted in hyperactivation of the AKT signaling pathway. PTEN rescue led to decreased proliferation, inhibition of colony formation, and increased apoptosis. Strikingly, PTEN loss decreased sensitivity to IGF1R inhibitors but increased responsiveness to temsirolimus, a potent mTOR inhibitor, as marked by induction of autophagy. These results suggest that PTEN is lost in a significant fraction of primary tumors, and this deficiency may have therapeutic consequences by concurrently attenuating responsiveness to IGF1R inhibition while increasing activity of mTOR inhibitors. The identification of PTEN status in the tumors of patients with recurrent disease could help guide the selection of therapies.

IMPLICATIONS

PTEN status in Ewing sarcoma affects cellular responses to IGFI and mTOR-directed therapy, thus justifying its consideration as a biomarker in future clinical trials.

EWS and RE1-Silencing Transcription Factor Inhibit Neuronal Phenotype Development and Oncogenic Transformation in Ewing Sarcoma

The gene encoding EWS (EWSR1) is involved in various chromosomal translocations that cause the production of oncoproteins responsible for multiple cancers including Ewing sarcoma, myxoid liposarcoma, soft tissue clear cell sarcoma, and desmoplastic small round cell sarcoma. It is well known that EWS fuses to FLI to create EWS/FLI, which is the abnormal transcription factor that drives tumor development in Ewing sarcoma. However, the role of wild-type EWS in Ewing sarcoma pathogenesis remains unclear. In the current study, we identified EWS-regulated genes and cellular processes through RNA interference combined with RNA sequencing and functional annotation analyses. Interestingly, we found that EWS and EWS/FLI co-regulate a significant cluster of genes, indicating an interplay between the 2 proteins in regulating cellular functions. We found that among the EWS-down-regulated genes are a subset of neuronal genes that contain binding sites for the RE1-silencing transcription factor (REST or neuron-restrictive silencer factor [NRSF]), neuron-restrictive silencer element (NRSE), suggesting a cooperative interaction between REST and EWS in gene regulation. Co-immunoprecipitation analysis demonstrated that EWS interacts directly with REST. Genome-wide binding analysis showed that EWS binds chromatin at or near NRSE. Furthermore, functional studies revealed that both EWS and REST inhibit neuronal phenotype development and oncogenic transformation in Ewing sarcoma cells. Our data implicate an important role of EWS in the development of Ewing sarcoma phenotype and highlight a potential value in modulating EWS function in the treatment of Ewing sarcoma and other EWS translocation-based cancers.