An RNAi screen identifies Msi2 and Prox1 as having opposite roles in the regulation of hematopoietic stem cell activity

Signaling proteins in HSC fate determination are unequally segregated during asymmetric cell division

Hematopoietic stem cells (HSCs) continuously replenish mature blood cells with limited lifespans. To maintain the HSC compartment while ensuring output of differentiated cells, HSCs undergo asymmetric cell division (ACD), generating two daughter cells with different fates: one will proliferate and give rise to the differentiated cells' progeny, and one will return to quiescence to maintain the HSC compartment. A balance between MEK/ERK and mTORC1 pathways is needed to ensure HSC homeostasis. Here, we show that activation of these pathways is spatially segregated in premitotic HSCs and unequally inherited during ACD. A combination of genetic and chemical perturbations shows that an ERK-dependent mechanism determines the balance between pathways affecting polarity, proliferation, and metabolism, and thus determines the frequency of asymmetrically dividing HSCs. Our data identify druggable targets that modulate HSC fate determination at the level of asymmetric division.

RNA binding protein-directed control of leukemic stem cell evolution and function

Strict control over hematopoietic stem cell decision making is essential for healthy life-long blood production and underpins the origins of hematopoietic diseases. Acute myeloid leukemia (AML) in particular is a devastating hematopoietic malignancy that arises from the clonal evolution of disease-initiating primitive cells which acquire compounding genetic changes over time and culminate in the generation of leukemic stem cells (LSCs). Understanding the molecular underpinnings of these driver cells throughout their development will be instrumental in the interception of leukemia, the enabling of effective treatment of pre-leukemic conditions, as well as the development of strategies to target frank AML disease. To this point, a number of precancerous myeloid disorders and age-related alterations are proving as instructive models to gain insights into the initiation of LSCs. Here, we explore this myeloid dysregulation at the level of post-transcriptional control, where RNA-binding proteins (RBPs) function as core effectors. Through regulating the interplay of a myriad of RNA metabolic processes, RBPs orchestrate transcript fates to govern gene expression in health and disease. We describe the expanding appreciation of the role of RBPs and their post-transcriptional networks in sustaining healthy hematopoiesis and their dysregulation in the pathogenesis of clonal myeloid disorders and AML, with a particular emphasis on findings described in human stem cells. Lastly, we discuss key breakthroughs that highlight RBPs and post-transcriptional control as actionable targets for precision therapy of AML.

RNA-binding MSI proteins and their related cancers: A medicinal chemistry perspective

Musashi1 and Musashi2 are RNA-binding proteins originally found in drosophila, in which they play a crucial developmental role. These proteins are pivotal in the maintenance and differentiation of stem cells in other organisms. Research has confirmed that the Musashi proteins are highly involved in cell signal-transduction pathways such as Notch and TGF-β. These signaling pathways are related to the induction and development of cancers, such as breast cancer, leukemia, hepatoma and liver cancer. In this review we focus on how Musashi proteins interact with molecules in different signaling pathways in various cancers and how they affect the physiological functions of these pathways. We further illustrate the status quo of Musashi proteins-targeted therapies and predict the target RNA regions that Musashi proteins interact with, in the hope of exploring the prospect of the design of Musashi protein-targeted medicines.

Down-regulation of Musashi-2 exerts antileukemic effects on acute lymphoblastic leukemia cells and increases sensitivity to dexamethasone

Musashi-2 (MSI2), implicated in the oncogenesis and propagation of a broad array of malignancies, inclusive of certain leukemia, remains a nascent field of study within the context of acute lymphoblastic leukemia (ALL). Using lentiviral transfection, ALL cells with stable MSI2 knockdown were engineered. A suite of analytic techniques - a CCK-8 assay, flow cytometry, qRT-PCR, and western blotting - were employed to evaluate cellular proliferation, cell cycle arrest, and apoptosis and to confirm differential gene expression. The suppression of MSI2 expression yielded significant results: inhibition of cell proliferation, G0/G1 cell cycle arrest, and induced apoptosis in ALL cell lines. Furthermore, it was noted that MSI2 inhibition heightened the responsiveness of ALL cells to dexamethasone. Significantly, the depletion of MSI2 prompted the translocation of GR from the cytoplasm to the nucleus upon dexamethasone treatment, consequently leading to enhanced sensitivity. Additionally, the FOXO1/4 signaling pathway contributed to the biological effects of ALL cells evoked by MSI2 silencing. Our study offers novel insight into the inhibitory effects of MSI2 suppression on ALL cells, positing MSI2 as a promising therapeutic target in the treatment of ALL.

Post-Transplant Administration of G-CSF Impedes Engraftment of Gene Edited Human Hematopoietic Stem Cells by Exacerbating the p53-Mediated DNA Damage Response

Granulocyte colony stimulating factor (G-CSF) is commonly used as adjunct treatment to hasten recovery from neutropenia following chemotherapy and autologous transplantation of hematopoietic stem and progenitor cells (HSPCs) for malignant disorders. However, the utility of G-CSF administration after ex vivo gene therapy procedures targeting human HSPCs has not been thoroughly evaluated. Here, we provide evidence that post-transplant administration of G-CSF impedes engraftment of CRISPR-Cas9 gene edited human HSPCs in xenograft models. G-CSF acts by exacerbating the p53-mediated DNA damage response triggered by Cas9- mediated DNA double-stranded breaks. Transient p53 inhibition in culture attenuates the negative impact of G-CSF on gene edited HSPC function. In contrast, post-transplant administration of G-CSF does not impair the repopulating properties of unmanipulated human HSPCs or HSPCs genetically engineered by transduction with lentiviral vectors. The potential for post-transplant G-CSF administration to aggravate HSPC toxicity associated with CRISPR-Cas9 gene editing should be considered in the design of ex vivo autologous HSPC gene editing clinical trials.

In Vivo Screening Unveils Pervasive RNA-Binding Protein Dependencies in Leukemic Stem Cells and Identifies ELAVL1 as a Therapeutic Target

Acute myeloid leukemia (AML) is fueled by leukemic stem cells (LSC) whose determinants are challenging to discern from hematopoietic stem cells (HSC) or uncover by approaches focused on general cell properties. We have identified a set of RNA-binding proteins (RBP) selectively enriched in human AML LSCs. Using an in vivo two-step CRISPR-Cas9 screen to assay stem cell functionality, we found 32 RBPs essential for LSCs in MLL-AF9;NrasG12D AML. Loss-of-function approaches targeting key hit RBP ELAVL1 compromised LSC-driven in vivo leukemic reconstitution, and selectively depleted primitive malignant versus healthy cells. Integrative multiomics revealed differentiation, splicing, and mitochondrial metabolism as key features defining the leukemic ELAVL1-mRNA interactome with mitochondrial import protein, TOMM34, being a direct ELAVL1-stabilized target whose repression impairs AML propagation. Altogether, using a stem cell-adapted in vivo CRISPR screen, this work demonstrates pervasive reliance on RBPs as regulators of LSCs and highlights their potential as therapeutic targets in AML.

SIGNIFICANCE

LSC-targeted therapies remain a significant unmet need in AML. We developed a stem-cell-adapted in vivo CRISPR screen to identify key LSC drivers. We uncover widespread RNA-binding protein dependencies in LSCs, including ELAVL1, which we identify as a novel therapeutic vulnerability through its regulation of mitochondrial metabolism. This article is highlighted in the In This Issue feature, p. 171.

RNA binding protein SYNCRIP maintains proteostasis and self-renewal of hematopoietic stem and progenitor cells

Tissue homeostasis is maintained after stress by engaging and activating the hematopoietic stem and progenitor compartments in the blood. Hematopoietic stem cells (HSCs) are essential for long-term repopulation after secondary transplantation. Here, using a conditional knockout mouse model, we revealed that the RNA-binding protein SYNCRIP is required for maintenance of blood homeostasis especially after regenerative stress due to defects in HSCs and progenitors. Mechanistically, we find that SYNCRIP loss results in a failure to maintain proteome homeostasis that is essential for HSC maintenance. SYNCRIP depletion results in increased protein synthesis, a dysregulated epichaperome, an accumulation of misfolded proteins and induces endoplasmic reticulum stress. Additionally, we find that SYNCRIP is required for translation of CDC42 RHO-GTPase, and loss of SYNCRIP results in defects in polarity, asymmetric segregation, and dilution of unfolded proteins. Forced expression of CDC42 recovers polarity and in vitro replating activities of HSCs. Taken together, we uncovered a post-transcriptional regulatory program that safeguards HSC self-renewal capacity and blood homeostasis.

GATA2 regulates blood/lymph separation in a platelet-dependent and lymphovenous valve-independent manner

INTRODUCTION

Lymphatic vessels collect interstitial fluid, immune cells, and digested lipids and return these bodily fluids to blood through two pairs of lymphovenous valves (LVVs). Like other cardiovascular valves LVVs prevent the backflow of blood into the lymphatic vessels. In addition to LVVs, platelets are necessary to prevent the entry of blood into the lymphatic vessels. Platelet thrombi are observed at LVVs suggesting that LVVs and platelets function in synergy to regulate blood/lymphatic separation.

OBJECTIVES

The primary objective of this work is to determine whether platelets can regulate blood/lymph separation independently of LVVs.

METHODS

The transcription factor GATA2 is necessary for the development of both LVVs and hematopoietic stem cells. Using various endothelial- and hematopoietic cell expressed Cre-lines, we conditionally deleted Gata2. We hypothesized that this strategy would identify the tissue- and time-specific roles of GATA2 and reveal whether platelets and LVVs can independently regulate blood/lymph separation.

RESULTS

Lymphatic vasculature-specific deletion of Gata2 results in the absence of LVVs without compromising blood/lymph separation. In contrast, deletion of GATA2 from both lymphatic vasculature and hematopoietic cells results in the absence of LVVs, reduced number of platelets and blood-filled lymphatic vasculature.

CONCLUSION

GATA2 promotes blood/lymph separation through platelets. Furthermore, LVVs are the only known sites of interaction between blood and lymphatic vessels. The fact that blood is able to enter the lymphatic vessels of mice lacking LVVs and platelets indicates that under these circumstances the lymphatic and blood vessels are connected at yet to be identified sites.

A Prox1 enhancer represses haematopoiesis in the lymphatic vasculature

Transcriptional enhancer elements are responsible for orchestrating the temporal and spatial control over gene expression that is crucial for programming cell identity during development^1-3. Here we describe a novel enhancer element that is important for regulating the expression of Prox1 in lymphatic endothelial cells. This evolutionarily conserved enhancer is bound by key lymphatic transcriptional regulators including GATA2, FOXC2, NFATC1 and PROX1. Genome editing of the enhancer to remove five nucleotides encompassing the GATA2-binding site resulted in perinatal death of homozygous mutant mice due to profound lymphatic vascular defects. Lymphatic endothelial cells in enhancer mutant mice exhibited reduced expression of genes characteristic of lymphatic endothelial cell identity and increased expression of genes characteristic of haemogenic endothelium, and acquired the capacity to generate haematopoietic cells. These data not only reveal a transcriptional enhancer element important for regulating Prox1 expression and lymphatic endothelial cell identity but also demonstrate that the lymphatic endothelium has haemogenic capacity, ordinarily repressed by Prox1.

MSI2 deficiency in ILC3s attenuates DSS-induced colitis by affecting the intestinal microbiota

Background

The etiology and pathogenesis of inflammatory bowel disease (IBD), including ulcerative colitis (UC) and Crohn's disease (CD), are generally believed to be related to immune dysfunction and intestinal microbiota disorder. However, the exact mechanism is not yet fully understood. The pathological changes associated with dextran sodium sulfate (DSS)-induced colitis are similar to those in human UC. As a subgroup of the innate immune system, group 3 innate lymphoid cells (ILC3s) are widely distributed in the lamina propria of the intestinal mucosa, and their function can be regulated by a variety of molecules. Musashi2 (MSI2) is a type of evolutionarily conserved RNA-binding protein that maintains the function of various tissue stem cells and is essential for postintestinal epithelial regeneration. The effect of MSI2 deficiency in ILC3s on IBD has not been reported. Thus, mice with conditional MSI2 knockout in ILC3s were used to construct a DSS-induced colitis model and explore its effects on the pathogenesis of IBD and the species, quantity and function of the intestinal microbiota.

Methods

Msi2^flox/flox mice (Msi2^fl/fl ) and Msi2^flox/floxRorc^Cre mice (Msi2^ΔRorc ) were induced by DSS to establish the IBD model. The severity of colitis was evaluated by five measurements: body weight percentage, disease activity index, colon shortening degree, histopathological score and routine blood examination. The species, quantity and function of the intestinal microbiota were characterized by high-throughput 16S rRNA gene sequencing of DNA extracted from fecal samples.

Results

MSI2 was knocked out in the ILC3s of Msi2^ΔRorc mice. The Msi2^ΔRorc mice exhibited reductions in body weight loss, the disease activity index, degree of colon shortening, tissue histopathological score and immune cells in the peripheral blood compared to those of Msi2^fl/fl mice after DSS administration. The 16S rRNA sequencing results showed that the diversity of the intestinal microbiota in DSS-treated Msi2^ΔRorc mice changed, with the abundance of Firmicutes increasing and that of Bacteroidetes decreasing. The linear discriminant analysis effect size (LEfSe) approach revealed that Lactobacillaceae could be the key bacteria in the Msi2^ΔRorc mouse during the improvement of colitis. Using PICRUST2 to predict the function of the intestinal microbiota, it was found that the functions of differential bacteria inferred by modeling were mainly enriched in infectious diseases, immune system and metabolic functions.

Conclusions

MSI2 deficiency in ILC3s attenuated DSS-induced colonic inflammation in mice and affected intestinal microbiota diversity, composition, and function, with Lactobacillaceae belonging to the phylum Firmicutes possibly representing the key bacteria. This finding could contribute to our understanding of the pathogenesis of IBD and provide new insights for its clinical diagnosis and treatment.

Combinatorial gene targeting in primary human hematopoietic stem and progenitor cells

The CRISPR/Cas9 system offers enormous versatility for functional genomics but many applications have proven to be challenging in primary human cells compared to cell lines or mouse cells. Here, to establish a paradigm for multiplexed gene editing in primary human cord blood-derived hematopoietic stem and progenitor cells (HSPCs), we used co-delivery of lentiviral sgRNA vectors expressing either Enhanced Green Fluorescent Protein (EGFP) or Kusabira Orange (KuO), together with Cas9 mRNA, to simultaneously edit two genetic loci. The fluorescent markers allow for tracking of either single- or double-edited cells, and we could achieve robust double knockout of the cell surface molecules CD45 and CD44 with an efficiency of ~ 70%. As a functional proof of concept, we demonstrate that this system can be used to model gene dependencies for cell survival, by simultaneously targeting the cohesin genes STAG1 and STAG2. Moreover, we show combinatorial effects with potential synergy for HSPC expansion by targeting the Aryl Hydrocarbon Receptor (AHR) in conjunction with members of the CoREST complex. Taken together, our traceable multiplexed CRISPR/Cas9 system enables studies of genetic dependencies and cooperation in primary HSPCs, and has important implications for modelling polygenic diseases, as well as investigation of the underlying mechanisms of gene interactions.

Epigenetic modifier SMCHD1 maintains a normal pool of long-term hematopoietic stem cells

SMCHD1 (structural maintenance of chromosomes hinge domain containing 1) is a noncanonical SMC protein that mediates long-range repressive chromatin structures. SMCHD1 is required for X chromosome inactivation in female cells and repression of imprinted and clustered autosomal genes, with SMCHD1 mutations linked to human diseases facioscapulohumeral muscular dystrophy (FSHD) and bosma arhinia and micropthalmia syndrome (BAMS). We used a conditional mouse model to investigate SMCHD1 in hematopoiesis. Smchd1-deleted mice maintained steady-state hematopoiesis despite showing an impaired reconstitution capacity in competitive bone marrow transplantations and age-related hematopoietic stem cell (HSC) loss. This phenotype was more pronounced in Smchd1-deleted females, which showed a loss of quiescent HSCs and fewer B cells. Gene expression profiling of Smchd1-deficient HSCs and B cells revealed known and cell-type-specific SMCHD1-sensitive genes and significant disruption to X-linked gene expression in female cells. These data show SMCHD1 is a regulator of HSCs whose effects are more profound in females.

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SMCHD1 is not required to maintain steady-state hematopoiesis

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Smchd1-deletion leads to loss of adult hematopoietic stem cells

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Smchd1-deleted female mice are more severely affected than males

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SMCHD1 maintains cellular quiescence in female hematopoietic stem cells

Hematopoietic stem cell regeneration through paracrine regulation of the Wnt5a/Prox1 signaling axis

The crosstalk between the BM microenvironment (niche) and hematopoietic stem cells (HSCs) is critical for HSC regeneration. Here, we show that in mice, deletion of the Fanconi anemia (FA) genes Fanca and Fancc dampened HSC regeneration through direct effects on HSCs and indirect effects on BM niche cells. FA HSCs showed persistent upregulation of the Wnt target Prox1 in response to total body irradiation (TBI). Accordingly, lineage-specific deletion of Prox1 improved long-term repopulation of the irradiated FA HSCs. Forced expression of Prox1 in WT HSCs mimicked the defective repopulation phenotype of FA HSCs. WT mice but not FA mice showed significant induction by TBI of BM stromal Wnt5a protein. Mechanistically, FA proteins regulated stromal Wnt5a expression, possibly through modulating the Wnt5a transcription activator Pax2. Wnt5a treatment of irradiated FA mice enhanced HSC regeneration. Conversely, Wnt5a neutralization inhibited HSC regeneration after TBI. Wnt5a secreted by LepR⁺CXCL12⁺ BM stromal cells inhibited β-catenin accumulation, thereby repressing Prox1 transcription in irradiated HSCs. The detrimental effect of deregulated Wnt5a/Prox1 signaling on HSC regeneration was also observed in patients with FA and aged mice. Irradiation induced upregulation of Prox1 in the HSCs of aged mice, and deletion of Prox1 in aged HSCs improved HSC regeneration. Treatment of aged mice with Wnt5a enhanced hematopoietic repopulation. Collectively, these findings identified the paracrine Wnt5a/Prox1 signaling axis as a regulator of HSC regeneration under conditions of injury and aging.

Decoding Human Hematopoietic Stem Cell Self-Renewal

Purpose of Review

Hematopoietic stem cells (HSCs) maintain blood and immune cell homeostasis by balancing quiescence, self-renewal, and differentiation. HSCs can be used in lifesaving transplantation treatments to create a healthy hematopoietic system in patients suffering from malignant or inherited blood diseases. However, lack of matching bone marrow donors, and the low quantity of HSCs in a single cord blood graft, are limitations for successful transplantation. The enormous regenerative potential of HSCs has raised the hope that HSC self-renewal could be recapitulated in culture to achieve robust expansion of HSCs for therapeutic use. Yet, when HSCs are cultured ex vivo their function becomes compromised, limiting successful expansion.

Recent Findings

After decades of efforts to expand human HSCs ex vivo that resulted in minimal increase in transplantable units, recent studies have helped define culture conditions that can increase functional HSCs. These studies have provided new insights into how HSC stemness can be controlled from the nucleus by transcriptional, posttranscriptional and epigenetic regulators, or by improving the HSC microenvironment using 3D scaffolds, niche cells, or signaling molecules that mimic specific aspects of human HSC niche. Recent studies have also highlighted the importance of mitigating culture induced cellular stress and balancing mitochondrial, endoplasmic reticulum, and lysosomal functions. These discoveries have provided better markers for functional human HSCs and new insights into how HSC self-renewal and engraftment ability may be controlled ex vivo.

Summary

Uncovering the mechanisms that control the human HSC self-renewal process may help improve the ex vivo expansion of HSCs for clinical purposes.

Small Molecule Palmatine Targeting Musashi-2 in Colorectal Cancer

Musashi-2 (MSI2) is an evolutionally conserved RNA-binding protein and recently considered as an attractive therapeutic target in a wide spectrum of malignancies. However, MSI2-engaged mRNAs are not well profiled, and no MSI2-dependent antagonist is available so far. In the study, we created MSI2 knockout cancer cells and demonstrated that MSI2 is required for the survival of colorectal cancer HCT116 cells but not non-small cell lung cancer A549 cells. In addition, the global profiling of the transcriptome and proteomics of MSI2 knockout colorectal cells revealed 38 candidate MSI2-targeted genes. In a loss–rescue screening, palmatine was identified as a functional MSI2 antagonist inhibiting the MSI2-dependent growth of colorectal cancer cells. Finally, we confirmed that palmatine is directly bound to MSI2 at its C-terminal. Our findings not only indicated MSI2 as a promising therapeutic target of colorectal cancer but also provided a small molecule palmatine as a direct and functional MSI2 antagonist for cancer therapy.

The androgen receptor messenger RNA: what do we know?

The Androgen Receptor (AR), transcriptionally activated by its ligands, testosterone and dihydrotestosterone (DHT), is widely expressed in cells and tissues, influencing normal biology and disease states. The protein product of the AR gene is involved in the regulation of numerous biological functions, including the development and maintenance of the normal prostate gland and of the cardiovascular, musculoskeletal and immune systems. Androgen signalling, mediated by AR protein, plays a crucial role in the development of prostate cancer (PCa), and is presumed to be involved in other cancers including those of the breast, bladder, liver and kidney. Significant research and reviews have focused on AR protein function; however, inadequate research and literature exist to define the function of AR mRNA in normal and cancer cells. The AR mRNA transcript is nearly 11 Kb long and contains a long 3’ untranslated region (UTR), suggesting its biological role in post-transcriptional regulation, consequently affecting the overall functions of both normal and cancer cells. Research has demonstrated that many biological activities, including RNA stability, translation, cellular trafficking and localization, are associated with the 3’ UTRs of mRNAs. In this review, we describe the potential role of the AR 3’ UTR and summarize RNA-binding proteins (RBPs) that interact with the AR mRNA to regulate post-transcriptional metabolism. We highlight the importance of AR mRNA as a critical modulator of carcinogenesis and its important role in developing therapy-resistant prostate cancer.

A global screening identifies chromatin-enriched RNA-binding proteins and the transcriptional regulatory activity of QKI5 during monocytic differentiation

BACKGROUND

Cellular RNA-binding proteins (RBPs) have multiple roles in post-transcriptional control, and some are shown to bind DNA. However, the global localization and the general chromatin-binding ability of RBPs are not well-characterized and remain undefined in hematopoietic cells.

RESULTS

We first provide a full view of RBPs' distribution pattern in the nucleus and screen for chromatin-enriched RBPs (Che-RBPs) in different human cells. Subsequently, by generating ChIP-seq, CLIP-seq, and RNA-seq datasets and conducting combined analysis, the transcriptional regulatory potentials of certain hematopoietic Che-RBPs are predicted. From this analysis, quaking (QKI5) emerges as a potential transcriptional activator during monocytic differentiation. QKI5 is over-represented in gene promoter regions, independent of RNA or transcription factors. Furthermore, DNA-bound QKI5 activates the transcription of several critical monocytic differentiation-associated genes, including CXCL2, IL16, and PTPN6. Finally, we show that the differentiation-promoting activity of QKI5 is largely dependent on CXCL2, irrespective of its RNA-binding capacity.

CONCLUSIONS

Our study indicates that Che-RBPs are versatile factors that orchestrate gene expression in different cellular contexts, and identifies QKI5, a classic RBP regulating RNA processing, as a novel transcriptional activator during monocytic differentiation.

The Wnt signaling pathway in tumorigenesis, pharmacological targets, and drug development for cancer therapy

Wnt signaling was initially recognized to be vital for tissue development and homeostasis maintenance. Further studies revealed that this pathway is also important for tumorigenesis and progression. Abnormal expression of signaling components through gene mutation or epigenetic regulation is closely associated with tumor progression and poor prognosis in several tissues. Additionally, Wnt signaling also influences the tumor microenvironment and immune response. Some strategies and drugs have been proposed to target this pathway, such as blocking receptors/ligands, targeting intracellular molecules, beta-catenin/TCF4 complex and its downstream target genes, or tumor microenvironment and immune response. Here we discuss the roles of these components in Wnt signaling pathway in tumorigenesis and cancer progression, the underlying mechanisms that is responsible for the activation of Wnt signaling, and a series of drugs targeting the Wnt pathway provide multiple therapeutic values. Although some of these drugs exhibit exciting anti-cancer effect, clinical trials and systematic evaluation should be strictly performed along with multiple-omics technology.

Arhgef2 regulates mitotic spindle orientation in hematopoietic stem cells and is essential for productive hematopoiesis

How hematopoietic stem cells (HSCs) coordinate their divisional axis and whether this orientation is important for stem cell-driven hematopoiesis is poorly understood. Single-cell RNA sequencing data from patients with Shwachman-Diamond syndrome (SDS), an inherited bone marrow failure syndrome, show that ARHGEF2, a RhoA-specific guanine nucleotide exchange factor and determinant of mitotic spindle orientation, is specifically downregulated in SDS hematopoietic stem and progenitor cells (HSPCs). We demonstrate that transplanted Arhgef2-/- fetal liver and bone marrow cells yield impaired hematopoietic recovery and a production deficit from long-term HSCs, phenotypes that are not the result of differences in numbers of transplanted HSCs, their cell cycle status, level of apoptosis, progenitor output, or homing ability. Notably, these defects are functionally restored in vivo by overexpression of ARHGEF2 or its downstream activated RHOA GTPase. By using live imaging of dividing HSPCs, we show an increased frequency of misoriented divisions in the absence of Arhgef2. ARHGEF2 knockdown in human HSCs also impairs their ability to regenerate hematopoiesis, culminating in significantly smaller xenografts. Together, these data demonstrate a conserved role for Arhgef2 in orienting HSPC division and suggest that HSCs may divide in certain orientations to establish hematopoiesis, the loss of which could contribute to HSC dysfunction in bone marrow failure.

Musashi 2 influences chronic lymphocytic leukemia cell survival and growth making it a potential therapeutic target

Progression of chronic lymphocytic leukemia (CLL) results from the expansion of a small fraction of proliferating leukemic B cells. When comparing the global gene expression of recently divided CLL cells with that of previously divided cells, we found higher levels of genes involved in regulating gene expression. One of these was the oncogene Musashi 2 (MSI2), an RNA-binding protein that induces or represses translation. While there is an established role for MSI2 in normal and malignant stem cells, much less is known about its expression and role in CLL. Here we report for the first time ex vivo and in vitro experiments that MSI2 protein levels are higher in dividing and recently divided leukemic cells and that downregulating MSI2 expression or blocking its function eliminates primary human and murine CLL and mature myeloid cells. Notably, mature T cells and hematopoietic stem and progenitor cells are not affected. We also confirm that higher MSI2 levels correlate with poor outcome markers, shorter time-to-first-treatment, and overall survival. Thus, our data highlight an important role for MSI2 in CLL-cell survival and proliferation and associate MSI2 with poor prognosis in CLL patients. Collectively, these findings pinpoint MSI2 as a potentially valuable therapeutic target in CLL.

Ex Vivo Analysis of Primary Tumor Specimens for Evaluation of Cancer Therapeutics

The use of ex vivo drug sensitivity testing to predict drug activity in individual patients has been actively explored for almost 50 years without delivering a generally useful predictive capability. However, extended failure should not be an indicator of futility. This is especially true in cancer research where ultimate success is often preceded by less successful attempts. For example, both immune- and genetic-based targeted therapies for cancer underwent numerous failed attempts before biological understanding, improved targets, and optimized drug development matured to facilitate an arsenal of transformational drugs. Similarly, the concept of directly assessing drug sensitivity of primary tumor biopsies-and the use of this information to help direct therapeutic approaches-has a long history with a definitive learning curve. In this review, we will survey the history of ex vivo testing as well as the current state of the art for this field. We will present an update on methodologies and approaches, describe the use of these technologies to test cutting-edge drug classes, and describe an increasingly nuanced understanding of tumor types and models for which this strategy is most likely to succeed. We will consider the relative strengths and weaknesses of predicting drug activity across the broad biological context of cancer patients and tumor types. This will include an analysis of the potential for ex vivo drug sensitivity testing to accurately predict drug activity within each of the biological hallmarks of cancer pathogenesis.

Impact of AHR Ligand TCDD on Human Embryonic Stem Cells and Early Differentiation

Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor, which mediates the effects of a variety of environmental stimuli in multiple tissues. Recent advances in AHR biology have underlined its importance in cells with high developmental potency, including pluripotent stem cells. Nonetheless, there is little data on AHR expression and its role during the initial stages of stem cell differentiation. The purpose of this study was to investigate the temporal pattern of AHR expression during directed differentiation of human embryonic stem cells (hESC) into neural progenitor, early mesoderm and definitive endoderm cells. Additionally, we investigated the effect of the AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the gene expression profile in hESCs and differentiated cells by RNA-seq, accompanied by identification of AHR binding sites by ChIP-seq and epigenetic landscape analysis by ATAC-seq. We showed that AHR is differentially regulated in distinct lineages. We provided evidence that TCDD alters gene expression patterns in hESCs and during early differentiation. Additionally, we identified novel potential AHR target genes, which expand our understanding on the role of this protein in different cell types.

Spatial Genome Re-organization between Fetal and Adult Hematopoietic Stem Cells

Fetal hematopoietic stem cells (HSCs) undergo a developmental switch to become adult HSCs with distinct functional properties. To better understand the molecular mechanisms underlying the developmental switch, we have conducted deep sequencing of the 3D genome, epigenome, and transcriptome of fetal and adult HSCs in mouse. We find that chromosomal compartments and topologically associating domains (TADs) are largely conserved between fetal and adult HSCs. However, there is a global trend of increased compartmentalization and TAD boundary strength in adult HSCs. In contrast, intra-TAD chromatin interactions are much more dynamic and wide-spread, involving over a thousand gene promoters and distal enhancers. These developmental-stage-specific enhancer-promoter interactions are mediated by different sets of transcription factors, such as TCF3 and MAFB in fetal HSCs, versus NR4A1 and GATA3 in adult HSCs. Loss-of-function studies of TCF3 confirm the role of TCF3 in mediating condition-specific enhancer-promoter interactions and gene regulation in fetal HSCs.

A developmental transition occurs between fetal and adult hematopoietic stem cells. How the 3D genome folding contributes to this transition is poorly understood. Chen et al. show global genome organization is largely conserved, but a large fraction of enhancer-promoter interactions is reorganized and regulate genes contributing to the phenotypic differences.

RNA-Binding Proteins in Acute Leukemias

Acute leukemias are genetic diseases caused by translocations or mutations, which dysregulate hematopoiesis towards malignant transformation. However, the molecular mode of action is highly versatile and ranges from direct transcriptional to post-transcriptional control, which includes RNA-binding proteins (RBPs) as crucial regulators of cell fate. RBPs coordinate RNA dynamics, including subcellular localization, translational efficiency and metabolism, by binding to their target messenger RNAs (mRNAs), thereby controlling the expression of the encoded proteins. In view of the growing interest in these regulators, this review summarizes recent research regarding the most influential RBPs relevant in acute leukemias in particular. The reported RBPs, either dysregulated or as components of fusion proteins, are described with respect to their functional domains, the pathways they affect, and clinical aspects associated with their dysregulation or altered functions.

HyperTRIBE uncovers increased MUSASHI-2 RNA binding activity and differential regulation in leukemic stem cells

The cell-context dependency for RNA binding proteins (RBPs) mediated control of stem cell fate remains to be defined. Here we adapt the HyperTRIBE method using an RBP fused to a Drosophila RNA editing enzyme (ADAR) to globally map the mRNA targets of the RBP MSI2 in mammalian adult normal and malignant stem cells. We reveal a unique MUSASHI-2 (MSI2) mRNA binding network in hematopoietic stem cells that changes during transition to multipotent progenitors. Additionally, we discover a significant increase in RNA binding activity of MSI2 in leukemic stem cells compared with normal hematopoietic stem and progenitor cells, resulting in selective regulation of MSI2's oncogenic targets. This provides a basis for MSI2 increased dependency in leukemia cells compared to normal cells. Moreover, our study provides a way to measure RBP function in rare cells and suggests that RBPs can achieve differential binding activity during cell state transition independent of gene expression.

Immunohistochemical Analysis Revealed a Correlation between Musashi-2 and Cyclin-D1 Expression in Patients with Oral Squamous Cells Carcinoma

Aim: Musashi 2 (MSI2), which is an RNA-binding protein, plays a fundamental role in the oncogenesis of several cancers. The aim of this study is to investigate the expression of MSI2 in Oral Squamous Cell Carcinoma (OSCC) and evaluate its correlation to clinic-pathological variables and prognosis. Materials and Methods: A bioinformatic analysis was performed on data downloaded from The Cancer Genome Atlas (TCGA) database. The MSI2 expression data were analysed for their correlation with clinic-pathological and prognostic features. In addition, an immmunohistochemical evaluation of MSI2 expression on 108 OSCC samples included in a tissue microarray and 13 healthy mucosae samples was performed. Results: 241 patients’ data from TCGA were included in the final analysis. No DNA mutations were detected for the MSI2 gene, but a hyper methylated condition of the gene emerged. MSI2 mRNA expression correlated with Grading (p = 0.009) and overall survival (p = 0.045), but not with disease free survival (p = 0.549). Males presented a higher MSI2 mRNA expression than females. The immunohistochemical evaluation revealed a weak expression of MSI2 in both OSCC samples and in healthy oral mucosae. In addition, MSI2 expression directly correlated with Cyclin-D1 expression (p = 0.022). However, no correlation has been detected with prognostic outcomes (overall and disease free survival). Conclusions: The role of MSI2 expression in OSCC seems to be not so closely correlated with prognosis, as in other human neoplasms. The correlation with Cyclin-D1 expression suggests an indirect role that MSI2 might have in the proliferation of OSCC cells, but further studies are needed to confirm such results.

Aberrant Clonal Hematopoiesis following Lentiviral Vector Transduction of HSPCs in a Rhesus Macaque

Lentiviral vectors (LVs) are used for delivery of genes into hematopoietic stem and progenitor cells (HSPCs) in clinical trials worldwide. LVs, in contrast to retroviral vectors, are not associated with insertion site-associated malignant clonal expansions and, thus, are considered safer. Here, however, we present a case of markedly abnormal dysplastic clonal hematopoiesis affecting the erythroid, myeloid, and megakaryocytic lineages in a rhesus macaque transplanted with HSPCs that were transduced with a LV containing a strong retroviral murine stem cell virus (MSCV) constitutive promoter-enhancer in the LTR. Nine insertions were mapped in the abnormal clone, resulting in overexpression and aberrant splicing of several genes of interest, including the cytokine stem cell factor and the transcription factor PLAG1. This case represents the first clear link between lentiviral insertion-induced clonal expansion and a clinically abnormal transformed phenotype following transduction of normal primate or human HSPCs, which is concerning, and suggests that strong constitutive promoters should not be included in LVs.

Signaling pathways involved in chronic myeloid leukemia pathogenesis: The importance of targeting Musashi2-Numb signaling to eradicate leukemia stem cells

OBJECTIVES

Chronic myeloid leukemia (CML) is a myeloid clonal proliferation disease defining by the presence of the Philadelphia chromosome that shows the movement of BCR-ABL1. In this study, the critical role of the Musashi2-Numb axis in determining cell fate and relationship of the axis to important signaling pathways such as Hedgehog and Notch that are essential for self-renewal pathways in CML stem cells will be reviewed meticulously.

MATERIALS AND METHODS

In this review, a PubMed search using the keywords of Leukemia, signaling pathways, Musashi2-Numb was performed, and then we summarized different research works .

RESULTS

Although tyrosine kinase inhibitors such as Imatinib significantly kill and remove the cell with BCR-ABL1 translocation, they are unable to target BCR-ABL1 leukemia stem cells. The main problem is stem cells resistance to Imatinib therapy. Therefore, the identification and control of downstream molecules/ signaling route of the BCR-ABL1 that are involved in the survival and self-renewal of leukemia stem cells can be an effective treatment strategy to eliminate leukemia stem cells, which supposed to be cured by Musashi2-Numb signaling pathway.

CONCLUSION

The control of molecules /pathways downstream of the BCR-ABL1 and targeting Musashi2-Numb can be an effective therapeutic strategy for treatment of chronic leukemia stem cells. While Musashi2 is a poor prognostic marker in leukemia, in treatment and strategy, it has significant diagnostic value.

A stemness screen reveals C3orf54/INKA1 as a promoter of human leukemia stem cell latency

There is a growing body of evidence that the molecular properties of leukemia stem cells (LSCs) are associated with clinical outcomes in acute myeloid leukemia (AML), and LSCs have been linked to therapy failure and relapse. Thus, a better understanding of the molecular mechanisms that contribute to the persistence and regenerative potential of LSCs is expected to result in the development of more effective therapies. We therefore interrogated functionally validated data sets of LSC-specific genes together with their known protein interactors and selected 64 candidates for a competitive in vivo gain-of-function screen to identify genes that enhanced stemness in human cord blood hematopoietic stem and progenitor cells. A consistent effect observed for the top hits was the ability to restrain early repopulation kinetics while preserving regenerative potential. Overexpression (OE) of the most promising candidate, the orphan gene C3orf54/INKA1, in a patient-derived AML model (8227) promoted the retention of LSCs in a primitive state manifested by relative expansion of CD34+ cells, accumulation of cells in G0, and reduced output of differentiated progeny. Despite delayed early repopulation, at later times, INKA1-OE resulted in the expansion of self-renewing LSCs. In contrast, INKA1 silencing in primary AML reduced regenerative potential. Mechanistically, our multidimensional confocal analysis found that INKA1 regulates G0 exit by interfering with nuclear localization of its target PAK4, with concomitant reduction of global H4K16ac levels. These data identify INKA1 as a novel regulator of LSC latency and reveal a link between the regulation of stem cell kinetics and pool size during regeneration.

PLAG1 and USF2 Co-regulate Expression of Musashi-2 in Human Hematopoietic Stem and Progenitor Cells

MSI2, which is expressed predominantly in hematopoietic stem and progenitor cells (HSPCs), enforces HSPC expansion when overexpressed and is upregulated in myeloid leukemias, indicating its regulated transcription is critical to balanced self-renewal and leukemia restraint. Despite this, little is understood of the factors that enforce appropriate physiological levels of MSI2 in the blood system. Here, we define a promoter region that reports on endogenous expression of MSI2 and identify USF2 and PLAG1 as transcription factors whose promoter binding drives reporter activity. We show that these factors co-regulate, and are required for, efficient transactivation of endogenous MSI2. Coincident overexpression of USF2 and PLAG1 in primitive cord blood cells enhanced MSI2 transcription and yielded cellular phenotypes, including expansion of CD34+ cells in vitro, consistent with that achieved by direct MSI2 overexpression. Global chromatin immunoprecipitation sequencing analyses confirm a preferential co-binding of PLAG1 and USF2 at the promoter of MSI2, as well as regulatory regions corresponding to genes with roles in HSPC homeostasis. PLAG1 and USF2 cooperation is thus an important contributor to stem cell-specific expression of MSI2 and HSPC-specific transcriptional circuitry.

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We define a regulatory region governing physiological MSI2 expression in human HSPCs

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USF2 and PLAG1 collaboratively control endogenous HSPC-specific MSI2 expression

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MSI2 expression and stemness is maintained in culture upon USF2 and PLAG1 co-overexpression

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USF2 and PLAG1 exhibit genomic co-localization and associate with autophagy genes

Musashi-2 is a prognostic marker for the survival of patients with cervical cancer

Cervical cancer is one of the most common gynecological malignancies. Mousasi 2 (Msi2) is a RNA-binding protein that regulates various key cellular functions and has emerged as a crucial regulator of cancer development. However, the clinical significance and biological functions of Msi2 in cervical cancer remain unknown. The current study assessed the expression of Msi2 mRNA using reverse transcription-quantitative polymerase chain reaction. Furthermore, the expression of Msi2 was examined in 162 cervical cancer samples using immunohistochemistry and the association between Msi2 expression and patient clinicopathological features was analyzed. The overall survival (OS) and progression-free survival (PFS) of patients were estimated using the Kaplan-Meier method and Cox regression analysis was performed to investigate the clinicopathological significance of Msi2 expression. In vitro migration and invasion assays were performed in Sinha and Caskie cells. The results demonstrated that, compared with normal cervical tissues, the expression of Msi2 was increased in cervical cancer tissues. The expression of Msi2 was significantly correlated with International Federation of Gynaecology and Obstetrics (FIGO) stage (P=0.049) and lymph node metastasis (P=0.036). Furthermore, patients with higher Msi2 expression exhibited significantly poorer OS (P=0.013) and PFS (P=0.006) than patients with low Msi2 expression. Notably, high Msi2 expression was correlated with poorer OS in patients with a FIGO stage ≤I (P=0.015), a smaller tumor size (P=0.043) and grade 3 tumor (P=0.002). High Msi2 expression was also correlated with a poorer PFS in patients with a FIGO stage ≤I (P=0.016) and grade 3 tumor (P=0.001). Multivariate analysis suggested that Msi2 expression was an independent prognostic marker of the OS (P=0.027) and PFS (P=0.013) of patients with cervical cancer. Furthermore, Msi2 knockdown significantly (P<0.05) inhibited the invasion and migration of cervical cancer cells. The results of the current study demonstrate that Msi2 may act as a prognostic biomarker in patients with cervical cancer. Targeting Msi2 may therefore offer a promising therapeutic strategy for the treatment of patients with cervical cancer.

Highly Efficient Genome Editing of Murine and Human Hematopoietic Progenitor Cells by CRISPR/Cas9

Our understanding of the mechanisms that regulate hematopoietic stem/progenitor cells (HSPCs) has been advanced by the ability to genetically manipulate mice; however, germline modification is time consuming and expensive. Here, we describe fast, efficient, and cost-effective methods to directly modify the genomes of mouse and human HSPCs using the CRISPR/Cas9 system. Using plasmid and virus-free delivery of guide RNAs alone into Cas9-expressing HSPCs or Cas9-guide RNA ribonucleoprotein (RNP) complexes into wild-type cells, we have achieved extremely efficient gene disruption in primary HSPCs from mouse (>60%) and human (∼75%). These techniques enabled rapid evaluation of the functional effects of gene loss of Eed, Suz12, and DNMT3A. We also achieved homology-directed repair in primary human HSPCs (>20%). These methods will significantly expand applications for CRISPR/Cas9 technologies for studying normal and malignant hematopoiesis.

Mapping genomic and transcriptomic alterations spatially in epithelial cells adjacent to human breast carcinoma

Almost all genomic studies of breast cancer have focused on well-established tumours because it is technically challenging to study the earliest mutational events occurring in human breast epithelial cells. To address this we created a unique dataset of epithelial samples ductoscopically obtained from ducts leading to breast carcinomas and matched samples from ducts on the opposite side of the nipple. Here, we demonstrate that perturbations in mRNA abundance, with increasing proximity to tumour, cannot be explained by copy number aberrations. Rather, we find a possibility of field cancerization surrounding the primary tumour by constructing a classifier that evaluates where epithelial samples were obtained relative to a tumour (cross-validated micro-averaged AUC = 0.74). We implement a spectral co-clustering algorithm to define biclusters. Relating to over-represented bicluster pathways, we further validate two genes with tissue microarrays and in vitro experiments. We highlight evidence suggesting that bicluster perturbation occurs early in tumour development.

Studying the spatial mutational and gene expression alterations in breast cancer could impact our understanding of breast cancer development. Here, the authors analyse a unique dataset of epithelial samples that highlight potential field cancerisation surrounding the primary tumour.

Msi2 plays a carcinogenic role in esophageal squamous cell carcinoma via regulation of the Wnt/β-catenin and Hedgehog signaling pathways

Msi2 has been widely reported to be upregulated and strongly associated with fast progress and poor prognosis in many cancers. However, the expression and role of Msi2 in esophageal squamous cell carcinoma (ESCC) remain unknown. In this study, we found that Msi2 was upregulated in ESCC clinical samples, and was significantly associated with tumor size, differentiation status, and lymph node metastasis in ESCC patients. Multivariate Cox regression analysis showed that Msi2 was an independent predictor for disease-free survival (DFS) and overall survival (OS). Moreover, knockdown of Msi2 impaired ESCC cell proliferation, epithelial-mesenchymal transition (EMT) and migration, while overexpression of Msi2 promoted ESCC cell proliferation, EMT and migration in vitro. Animal experiments also confirmed that Msi2 promoted ESCC cell proliferation in vivo. Mechanistically, Msi2 promoted ESCC cell proliferation, EMT and migration via regulation of the Wnt/β-catenin and Hedgehog (Hh) signaling pathways. Taken together, our study suggested that Msi2 could serve as a candidate for diagnosis and prognosis and as a potential therapeutic target in ESCC.

Single-cell analyses identify bioengineered niches for enhanced maintenance of hematopoietic stem cells

The in vitro expansion of long-term hematopoietic stem cells (HSCs) remains a substantial challenge, largely because of our limited understanding of the mechanisms that control HSC fate choices. Using single-cell multigene expression analysis and time-lapse microscopy, here we define gene expression signatures and cell cycle hallmarks of murine HSCs and the earliest multipotent progenitors (MPPs), and analyze systematically single HSC fate choices in culture. Our analysis revealed twelve differentially expressed genes marking the quiescent HSC state, including four genes encoding cell–cell interaction signals in the niche. Under basal culture conditions, most HSCs rapidly commit to become early MPPs. In contrast, when we present ligands of the identified niche components such as JamC or Esam within artificial niches, HSC cycling is reduced and long-term multipotency in vivo is maintained. Our approach to bioengineer artificial niches should be useful in other stem cell systems.

Haematopoietic stem cell (HSC) self-renewal is not sufficiently understood to recapitulate in vitro. Here, the authors generate gene signature and cell cycle hallmarks of single murine HSCs, and use identified endothelial receptors Esam and JamC as substrates to enhance HSC growth in engineered niches.

Cooperation of Musashi-2, Numb, MDM2, and P53 in drug resistance and malignant biology of pancreatic cancer

Our earlier work showed that Musashi (MSI)-2 promoted the development of pancreatic cancer (PC) by down-regulating Numb, which prevented murine double-minute (MDM)-2-mediated p53 ubiquitin degradation. Thus, we investigate the relationship among MSI2, Numb, MDM2, and p53 in PC in vitro and invivo, an association that has not been reported to our knowledge. MSI2 had no relationship with mutant p53 (mtp53) and wild-type p53 (wtp53) in normal PC cells. However, in response to gemcitabine or cisplatin treatment, MSI2 silencing simultaneously down-regulated MDM2 and up-regulated Numb and wtp53 protein levels. Moreover, these 4 endogenous proteins can be coimmunoprecipitated as a quaternary complex. Numb small interfering RNA (siRNA) reversed the MSI2 silencing-induced p53 increase. During treatment with chemical agents, MSI2 silencing decreased drug resistance and cell motility in vitro and inhibited tumor growth in vivo, all of which were significantly reversed by p53 siRNA. MSI2 was also negatively associated with Numb and positively associated with MDM2 expression in tissue. Overexpression of MSI2, MDM2, and mtp53 and weak expression of Numb were closely associated with aggressive clinicopathologic characteristics and poor prognosis for patients with PC. MSI2 negatively regulates wtp53 protein by up-regulating MDM2 and down-regulating Numb after treatment with chemical agents. MSI2 promotes drug resistance and malignant biology of PC in a p53-dependent manner.-Sheng, W., Dong, M., Chen, C., Wang, Z., Li, Y., Wang, K., Li, Y., Zhou, J. Cooperation of Musashi-2, Numb, MDM2, and P53 in drug resistance and malignant biology of pancreatic cancer.

Differential DNA methylation of MSI2 and its correlation with diabetic traits

Differential DNA methylation with hyperglycemia is significantly associated with Type 2 Diabetes (T2D). Longtime extended exposure to high blood glucose levels can affect the epigenetic signatures in all organs. However, the relevance of the differential DNA methylation changes with hyperglycemia in blood with pancreatic islets remains unclear. We investigated differential DNA methylation in relation to glucose homeostasis based on the Oral Glucose Tolerance Test (OGTT) in a population-based cohort. We found a total of 382 differential methylation sites from blood DNA in hyperglycemia and type 2 diabetes subgroups using a longitudinal and cross-sectional approach. Among them, three CpG sites were overlapped; they were mapped to the MSI2 and CXXC4 genes. In a DNA methylation replication study done by pyrosequencing (n = 440), the CpG site of MSI2 were shown to have strong associations with the T2D group (p value = 2.20E-16). The differential methylation of MSI2 at chr17:55484635 was associated with diabetes-related traits, in particular with insulin sensitivity (QUICKI, p value = 2.20E-16) and resistance (HOMA-IR, p value = 1.177E-07). In human pancreatic islets, at the single-base resolution (using whole-genome bisulfite sequencing), the 292 CpG sites in the ±5kb at chr17:55484635 were found to be significantly hypo-methylated in donors with T2D (average decrease = 13.91%, 95% confidence interval (CI) = 4.18~ 17.06) as compared to controls, and methylation patterns differed by sex (-9.57%, CI = -16.76~ -6.89) and age (0.12%, CI = -11.17~ 3.77). Differential methylation of the MSI2 gene (chr17:55484635) in blood and islet cells is strongly related to hyperglycemia. Our findings suggest that epigenetic perturbation on the target site of MSI2 gene in circulating blood and pancreatic islets should represent or affect hyperglycemia.

Msi2 Maintains Quiescent State of Hair Follicle Stem Cells by Directly Repressing the Hh Signaling Pathway

Hair follicles (HFs) undergo precisely regulated cycles of active regeneration (anagen), involution (catagen), and relative quiescence (telogen). Hair follicle stem cells (HFSCs) play important roles in regenerative cycling. Elucidating mechanisms that govern HFSC behavior can help uncover the underlying principles of hair development, hair growth disorders, and skin cancers. RNA-binding proteins of the Musashi (Msi) have been implicated in the biology of different stem cell types, yet they have not been studied in HFSCs. Here we utilized gain- and loss-of-function mouse models to demonstrate that forced MSI2 expression retards anagen entry and consequently delays hair growth, whereas loss of Msi2 enhances hair regrowth. Furthermore, our findings show that Msi2 maintains quiescent state of HFSCs in the process of the telogen-to-anagen transition. At the molecular level, our unbiased transcriptome profiling shows that Msi2 represses Hedgehog signaling activity and that Shh is its direct target in the hair follicle. Taken together, our findings reveal the importance of Msi2 in suppressing hair regeneration and maintaining HFSC quiescence. The previously unreported Msi2-Shh-Gli1 pathway adds to the growing understanding of the complex network governing cyclic hair growth.

In Vivo RNAi Screen Unveils PPARγ as a Regulator of Hematopoietic Stem Cell Homeostasis

Hematopoietic stem cell (HSC) defects can cause repopulating impairment leading to hematologic diseases. To target HSC deficiency in a disease setting, we exploited the repopulating defect of Fanconi anemia (FA) HSCs to conduct an in vivo short hairpin RNA (shRNA) screen. We exposed Fancd2^−/− HSCs to a lentiviral shRNA library targeting 947 genes. We found enrichment of shRNAs targeting genes involved in the PPARγ pathway that has not been linked to HSC homeostasis. PPARγ inhibition by shRNA or chemical compounds significantly improves the repopulating ability of Fancd2^−/− HSCs. Conversely, activation of PPARγ in wild-type HSCs impaired hematopoietic repopulation. In mouse HSCs and patient-derived lymphoblasts, PPARγ activation is manifested in upregulating the p53 target p21. PPARγ and co-activators are upregulated in total bone marrow and stem/progenitor cells from FA patients. Collectively, this work illustrates the utility of RNAi technology coupled with HSC transplantation for the discovery of novel genes and pathways involved in stress hematopoiesis.

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In vivo screening identifies of deleterious Pparγ effect on HSCs

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Pharmacological activation of Pparγ impaired normal HSC repopulation

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Inhibition of Pparγ improves Fancd2-deficient HSC repopulation ability

Musashi2 promotes the development and progression of pancreatic cancer by down-regulating Numb protein

Musashi2-Numb interaction plays a vital role in the progression of myeloid leukemia. However, its potential role in solid cancers has rarely been reported. We investigated the coordinate function of Musashi2-Numb in the development of pancreatic cancer (PC) in vitro and vivo. Both Musashi2 protein and mRNA levels were higher in PC tissues than that in paired normal pancreas (P<0.05). Musashi2 overexpression and Numb positive expression were positively and negatively associated with tumor size and UICC stage, respectively (P<0.05). Multivariate analysis identified Musashi2 and Numb as adverse and favorable independent indicators for the survival of PC patients. Moreover, patients with high Musashi2 expression combining with negative Numb expression had a significantly worse overall survival (P=0.001). The negative relationship between Musashi2 and Numb was found at both PC tissue and cell levels. These two endogenous proteins can be co-immunoprecipitated from PC cell lines, and Musashi2 silence up-regulated Numb protein in vitro and vivo. Meanwhile, its silence decreased cell invasion and migration in vitro and inhibited the growth of subcutaneous tumors and the frequency of liver metastasis in vivo. However, Numb knockdown significantly reversed the decrease of cell invasion and migration induced by Musashi2 silence. Musashi2 promotes the development and progression of pancreatic cancer by down-regulating Numb protein. The interaction of Musashi2-Numb plays a significant role in the development and progression of PC.

RNA binding protein MSI2 positively regulates FLT3 expression in myeloid leukemia

FLT3 is frequently mutated and overexpressed in acute myelogenous leukemia (AML) and other hematologic malignancies. Although signaling events downstream of FLT3 receptor tyrosine kinase have been studied in depth, molecular mechanisms of how FLT3 expression is regulated at the post-transcriptional level in particular remain elusive. In this study, we investigated the roles of an RNA binding protein MSI2 as a regulator of FLT3 expression. MSI2 and FLT3 are significantly co-regulated in human AML and chronic myelogenous leukemia in blast crisis (BC-CML). Genetic loss of MSI2 leads to down-regulation of the FLT3 receptor in both AML and BC-CML cells and concomitant impairment of clonogenic growth potential. Furthermore, we demonstrate that MSI2 protein is physically bound to FLT3 mRNA transcripts, suggesting post-transcriptional control of FLT3 expression. Collectively, these results reveal a novel mode of FLT3 regulation essential for leukemia growth, which may aid in designing a targeted therapy to treat human myeloid leukemia.

Forward RNAi Screens in Human Hematopoietic Stem Cells

Identifying the genes and pathways that regulate self-renewal and differentiation in somatic stem cells is a central goal in stem cell and cancer biology. Here, we describe a method for RNA interference (RNAi)-based screens combined with next-generation sequencing (NGS) in primary human hematopoietic stem and progenitor cells (HSPCs). These cells are suitable targets for complex, selection-based screens using pooled lentiviral short hairpin RNA (shRNA) libraries. The screening approach presented in this chapter is a promising tool to dissect regulatory mechanisms in hematopoietic stem cells (HSCs) and somatic stem cells in general, and may be particularly useful to identify gene targets and modifiers that can be further exploited in strategies for ex vivo stem cell expansion.

Msi RNA-binding proteins control reserve intestinal stem cell quiescence

Regeneration of the intestinal epithelium is driven by multiple intestinal stem cell (ISC) types, including an active, radiosensitive Wnthigh ISC that fuels turnover during homeostasis and a reserve, radioresistant Wntlow/off ISC capable of generating active Wnthigh ISCs. We examined the role of the Msi family of oncoproteins in the ISC compartment. We demonstrated that Msi proteins are dispensable for normal homeostasis and self-renewal of the active ISC, despite their being highly expressed in these cells. In contrast, Msi proteins are required specifically for activation of reserve ISCs, where Msi activity is both necessary and sufficient to drive exit from quiescence and entry into the cell cycle. Ablation of Msi activity in reserve ISCs rendered the epithelium unable to regenerate in response to injury that ablates the active stem cell compartment. These findings delineate a molecular mechanism governing reserve ISC quiescence and demonstrate a necessity for the activity of this rare stem cell population in intestinal regeneration.

The Novel KLF4/MSI2 Signaling Pathway Regulates Growth and Metastasis of Pancreatic Cancer

PURPOSE

Musashi 2 (MSI2) is reported to be a potential oncoprotein in cases of leukemia and several solid tumors. However, its expression, function, and regulation in pancreatic ductal adenocarcinoma (PDAC) cases have yet to be demonstrated. Therefore, in the current study, we investigated the clinical significance and biologic effects of MSI2 expression in PDAC cases and sought to delineate the clinical significance of the newly identified Krüppel-like factor 4 (KLF4)/MSI2 regulatory pathway.

EXPERIMENTAL DESIGN

MSI2 expression and its association with multiple clinicopathologic characteristics in human PDAC specimens were analyzed immunohistochemically. The biological functions of MSI2 regarding PDAC cell growth, migration, invasion, and metastasis were studied using gain- and loss-of-function assays both in vitro and in vivo Regulation of MSI2 expression by KLF4 was examined in several cancer cell lines, and the underlying mechanisms were studied using molecular biologic methods.

RESULTS

MSI2 expression was markedly increased in both PDAC cell lines and human PDAC specimens, and high MSI2 expression was associated with poor prognosis for PDAC. Forced MSI2 expression promoted PDAC proliferation, migration, and invasion in vitro and growth and metastasis in vivo, whereas knockdown of MSI2 expression did the opposite. Transcriptional inhibition of MSI2 expression by KLF4 occurred in multiple PDAC cell lines as well as mouse models of PDAC.

CONCLUSIONS

Lost expression of KLF4, a transcriptional repressor of MSI2 results in overexpression of MSI2 in PDACs, which may be a biomarker for accurate prognosis. A dysregulated KLF4/MSI2 signaling pathway promotes PDAC progression and metastasis. Clin Cancer Res; 23(3); 687-96. ©2016 AACR.

Image-based detection and targeting of therapy resistance in pancreatic adenocarcinoma

Pancreatic intraepithelial neoplasia (PanIN) is a premalignant lesion that can progress to pancreatic ductal adenocarcinoma, a highly lethal malignancy marked by its late stage at clinical presentation and profound drug resistance¹. The genomic alterations that commonly occur in pancreatic cancer include activation of KRAS2 and inactivation of p53, and SMAD4^2-4. To date, however, it has been challenging to target these pathways therapeutically; thus the search for other key mediators of pancreatic cancer growth remains an important endeavor. Here we show that the stem cell determinant Musashi (Msi) is a critical element of pancreatic cancer progression in both genetic models and patient derived xenografts. Specifically, we developed Msi reporter mice that allowed image based tracking of stem cell signals within cancers, revealing that Msi expression rises as PanIN progresses to adenocarcinoma, and that Msi-expressing cells are key drivers of pancreatic cancer: they preferentially harbor the capacity to propagate adenocarcinoma, are enriched in circulating tumor cells, and are markedly drug resistant. This population could be effectively targeted by deletion of either Msi1 or Msi2, which led to a striking defect in PanIN progression to adenocarcinoma and an improvement in overall survival. Msi inhibition also blocked the growth of primary patient-derived tumors, suggesting that this signal is required for human disease. To define the translational potential of this work we developed antisense oligonucleotides against Msi; these showed reliable tumor penetration, uptake and target inhibition, and effectively blocked pancreatic cancer growth. Collectively, these studies highlight Msi reporters as a unique tool to identify therapy resistance, and define Msi signaling as a central regulator of pancreatic cancer.

Musashi-2 attenuates AHR signalling to expand human haematopoietic stem cells

Umbilical cord blood (CB)-derived hematopoietic stem cells (HSCs) are essential in many life saving regenerative therapies, but their low number in CB units has significantly restricted their clinical use despite the advantages they provide during transplantation¹. Select small molecules that enhance hematopoietic stem and progenitor cell (HSPC) expansion in culture have been identified^2,3, however, in many cases their mechanisms of action or the nature of the pathways they impinge on are poorly understood. A greater understanding of the molecular pathways that underpin the unique human HSC self-renewal program will facilitate the development of targeted strategies that expand these critical cell types for regenerative therapies. Whereas transcription factor networks have been shown to influence the self-renewal and lineage decisions of human HSCs^4,5, the post-transcriptional mechanisms guiding HSC fate have not been closely investigated. Here we show that overexpression of the RNA-binding protein (RBP) Musashi-2 (MSI2) induces multiple pro-self-renewal phenotypes, including a 17-fold increase in short-term repopulating cells and a net 23-fold ex vivo expansion of long-term repopulating HSCs. By performing a global analysis of MSI2-RNA interactions, we determined that MSI2 directly attenuates aryl hydrocarbon receptor (AHR) signaling through post-transcriptional downregulation of canonical AHR pathway components in CB HSPCs. Our study provides new mechanistic insight into RBP-controlled RNA networks that underlie the self-renewal process and give evidence that manipulating such networks ex vivo can provide a novel means to enhance the regenerative potential of human HSCs.