The cell fate determinant Llgl1 influences HSC fitness and prognosis in AML

Cell polarity: cell type-specific regulators, common pathways, and polarized vesicle transport

Cell polarity, the asymmetric organization of cellular components, is evolutionarily conserved from unicellular and multicellular organisms and is crucial for many biological processes. Polarity is required to maintain cell and tissue integrity by regulating cell division, migration, orientation, cell-cell interactions, and morphogenesis. Impaired polarity leads to dysregulation of cellular functions and is associated with disease. Understanding how polarity is established, maintained, and regulated is thus critical to improving our knowledge of pathologies and devising novel therapies. Here, we explore the various manifestations of cell polarity across different model systems, tissues, and cell types and focus on known polarity mechanisms in hematopoietic stem and progenitor cells. We discuss how cells with vastly different functions utilize conserved molecular complexes to establish cell polarity while adapting polarity proteins to unique cell-type-specific functions. In this discussion, we attempt to extract common themes and concepts to improve our understanding of cell polarity in hematological malignancies and other diseases. Finally, we summarize, compare, and evaluate classical as well as recently developed methods to quantify cell polarity, highlight important advances in imaging and analytical techniques, and suggest critical next steps required to move the field forward.

KLF4 enhances transplantation-induced hematopoiesis by inhibiting TLRs and noncanonical NFκB signaling at a steady state

The transcription factor Krüppel-like factor 4 (KLF4) acts as a transcriptional activator and repressor. KLF4 plays a role in various cellular processes, including the dedifferentiation of somatic cells into induced pluripotent stem cells. Although it has been shown to enhance self-renewal in embryonic and leukemia stem cells, its role in adult hematopoietic stem cells (HSCs) remains underexplored. We demonstrate that conditional deletion of the Klf4 gene in hematopoietic cells led to an increased frequency of immunophenotypic HSCs in the bone marrow, along with a normal distribution of lymphoid and myeloid progenitor cells. Noncompetitive bone marrow transplants showed normal engraftment and multilineage reconstitution, except for monocytes and T cells. However, the loss of KLF4 hindered hematologic reconstitution in competitive serial bone marrow transplants, highlighting a critical role for KLF4 in stress-induced hematopoiesis. Transcriptome analysis revealed an upregulation of NFκB2 and toll-like receptors (e.g., TLR4) in Klf4-null HSCs during homeostasis. Flow cytometry and immunoblot analysis confirmed the increased cell surface expression of TLR4 and the activation of NFκB2 in HSCs under homeostatic conditions, whereas NFκB2 expression drops after radiation compared with steady-state levels. Our findings suggest that the constitutive activation of the TLR4-NFκB2 pathway inhibits the ability of HSCs to regenerate blood after transplantation in cytoablated bone marrow.

Monocytes in allo-HSCT with aged donors secrete IL-1/IL-6/TNF to increase the risk of GVHD and damage the aged HSCs

Aging is considered a critical factor of poor prognosis in allogenic hemopoietic stem cell transplantation (allo-HSCT). To elucidate the underlying mechanisms, we comprehensively reintegrated our clinical data from patients after allo-HSCT and public single-cell transcriptomic profile from post-allo-HSCT and healthy individuals, demonstrating that old donors were more prone to acute GVHD (aGVHD) with pronounced inflammation accumulation and worse overall survival (OS). We also found the presence of inflammation-related CXCL2+ HSC subpopulation during aging with significantly enriched pro-inflammatory pathways. Shifting attention to the HSC microenvironment, we deciphered that IL-1/IL-6 and TRAIL (i.e., TNFSF10) ligand‒receptor pair serves as the crucial bridge between CD14/CD16 monocytes and hematopoietic stem/progenitor cells (HSPCs). The profound upregulation of these signaling pathways during aging finally causes HSC dysfunction and lineage-biased differentiation. Our findings provide the theoretical basis for achieving tailored GVHD management and enhancing allo-HSCT regimens efficacy for aged donors.

Immunoproteasome function maintains oncogenic gene expression in KMT2A-complex driven leukemia

Pharmacologic targeting of chromatin-associated protein complexes has shown significant responses in KMT2A-rearranged (KMT2A-r) acute myeloid leukemia (AML) but resistance frequently develops to single agents. This points to a need for therapeutic combinations that target multiple mechanisms. To enhance our understanding of functional dependencies in KMT2A-r AML, we have used a proteomic approach to identify the catalytic immunoproteasome subunit PSMB8 as a specific vulnerability. Genetic and pharmacologic inactivation of PSMB8 results in impaired proliferation of murine and human leukemic cells while normal hematopoietic cells remain unaffected. Disruption of immunoproteasome function drives an increase in transcription factor BASP1 which in turn represses KMT2A-fusion protein target genes. Pharmacologic targeting of PSMB8 improves efficacy of Menin-inhibitors, synergistically reduces leukemia in human xenografts and shows preserved activity against Menin-inhibitor resistance mutations. This identifies and validates a cell-intrinsic mechanism whereby selective disruption of proteostasis results in altered transcription factor abundance and repression of oncogene-specific transcriptional networks. These data demonstrate that the immunoproteasome is a relevant therapeutic target in AML and that targeting the immunoproteasome in combination with Menin-inhibition could be a novel approach for treatment of KMT2A-r AML.

Cell fate determinant Llgl1 is required for propagation of acute myeloid leukemia

Scribble complex proteins can influence cell fate decisions and self-renewal capacity of hematopoietic cells. While specific cellular functions of Scribble complex members are conserved in mammalian hematopoiesis, they appear to be highly context dependent. Using CRISPR/Cas9-based genetic screening, we have identified Scribble complex-related liabilities in AML including LLGL1. Despite its reported suppressive function in HSC self-renewal, inactivation of LLGL1 in AML confirms its relevant role for proliferative capacity and development of AML. Its function was conserved in human and murine models of AML and across various genetic backgrounds. Inactivation of LLGL1 results in loss of stemness-associated gene-expression including HoxA-genes and induces a GMP-like phenotype in the leukemia stem cell compartment. Re-expression of HoxA9 facilitates functional and phenotypic rescue. Collectively, these data establish LLGL1 as a specific dependency and putative target in AML and emphasizes its cell-type specific functions.

Novel Insights into the Role of Kras in Myeloid Differentiation: Engaging with Wnt/β-Catenin Signaling

Cells of the HL-60 myeloid leukemia cell line can be differentiated into neutrophil-like cells by treatment with dimethyl sulfoxide (DMSO). The molecular mechanisms involved in this differentiation process, however, remain unclear. This review focuses on the differentiation of HL-60 cells. Although the Ras proteins, a group of small GTP-binding proteins, are ubiquitously expressed and highly homologous, each has specific molecular functions. Kras was shown to be essential for normal mouse development, whereas Hras and Nras are not. Kras knockout mice develop profound hematopoietic defects, indicating that Kras is required for hematopoiesis in adults. The Wnt/β-catenin signaling pathway plays a crucial role in regulating the homeostasis of hematopoietic cells. The protein β-catenin is a key player in the Wnt/β-catenin signaling pathway. A great deal of evidence shows that the Wnt/β-catenin signaling pathway is deregulated in malignant tumors, including hematological malignancies. Wild-type Kras acts as a tumor suppressor during DMSO-induced differentiation of HL-60 cells. Upon DMSO treatment, Kras translocates to the plasma membrane, and its activity is enhanced. Inhibition of Kras attenuates CD11b expression. DMSO also elevates levels of GSK3β phosphorylation, resulting in the release of unphosphorylated β-catenin from the β-catenin destruction complex and its accumulation in the cytoplasm. The accumulated β-catenin subsequently translocates into the nucleus. Inhibition of Kras attenuates Lef/Tcf-sensitive transcription activity. Thus, upon treatment of HL-60 cells with DMSO, wild-type Kras reacts with the Wnt/β-catenin pathway, thereby regulating the granulocytic differentiation of HL-60 cells. Wild-type Kras and the Wnt/β-catenin signaling pathway are activated sequentially, increasing the levels of expression of C/EBPα, C/EBPε, and granulocyte colony-stimulating factor (G-CSF) receptor.

Context-specific effects of NOX4 inactivation in acute myeloid leukemia (AML)

PURPOSE

Oxidative stress has been linked to initiation and progression of cancer and recent studies have indicated a potential translational role regarding modulation of ROS in various cancers, including acute myeloid leukemia (AML). Detailed understanding of the complex machinery regulating ROS including its producer elements in cancer is required to define potential translational therapeutic use. Based on previous studies in acute myeloid leukemia (AML) models, we considered NADPH oxidase (NOX) family members, specifically NOX4 as a potential target in AML.

METHODS

Pharmacologic inhibition and genetic inactivation of NOX4 in murine and human models of AML were used to understand its functional role. For genetic inactivation, CRISPR-Cas9 technology was used in human AML cell lines in vitro and genetically engineered knockout mice for Nox4 were used for deletion of Nox4 in hematopoietic cells via Mx1-Cre recombinase activation.

RESULTS

Pharmacologic NOX inhibitors and CRISPR-Cas9-mediated inactivation of NOX4 and p22-phox (an essential NOX component) decreased proliferative capacity and cell competition in FLT3-ITD-positive human AML cells. In contrast, conditional deletion of Nox4 enhanced the myeloproliferative phenotype of an FLT3-ITD induced knock-in mouse model. Finally, Nox4 inactivation in normal hematopoietic stem and progenitor cells (HSPCs) caused a minor reduction in HSC numbers and reconstitution capacity.

CONCLUSION

The role of NOX4 in myeloid malignancies appears highly context-dependent and its inactivation results in either enhancing or inhibitory effects. Therefore, targeting NOX4 in FLT3-ITD positive myeloid malignancies requires additional pre-clinical assessment.

Histone demethylase KDM4C is a functional dependency in JAK2-mutated neoplasms

Mutations of the JAK2 gene are frequent aberrations in the aging hematopoietic system and in myeloid neoplasms. While JAK-inhibitors efficiently reduce hyperinflammation induced by the constitutively active mutated JAK2 kinase, the malignant clone and abundance of mutated cells remains rather unaffected. Here, we sought to assess for genetic vulnerabilities of JAK2-mutated clones. We identified lysine-specific demethylase KDM4C as a selective genetic dependency that persists upon JAK-inhibitor treatment. Genetic inactivation of KDM4C in human and murine JAK2-mutated cells resulted in loss of cell competition and reduced proliferation. These findings led to reduced disease penetrance and improved survival in xenograft models of human JAK2-mutated cells. KDM4C deleted cells showed alterations in target histone residue methylation and target gene expression, resulting in induction of cellular senescence. In summary, these data establish KDM4C as a specific dependency and therapeutic target in JAK2-mutated cells that is essential for oncogenic signaling and prevents induction of senescence.

PLCG1 is required for AML1-ETO leukemia stem cell self-renewal

In an effort to identify novel drugs targeting fusion-oncogene-induced acute myeloid leukemia (AML), we performed high-resolution proteomic analysis. In AML1-ETO (AE)-driven AML, we uncovered a deregulation of phospholipase C (PLC) signaling. We identified PLCgamma 1 (PLCG1) as a specific target of the AE fusion protein that is induced after AE binding to intergenic regulatory DNA elements. Genetic inactivation of PLCG1 in murine and human AML inhibited AML1-ETO dependent self-renewal programs, leukemic proliferation, and leukemia maintenance in vivo. In contrast, PLCG1 was dispensable for normal hematopoietic stem and progenitor cell function. These findings are extended to and confirmed by pharmacologic perturbation of Ca++-signaling in AML1-ETO AML cells, indicating that the PLCG1 pathway poses an important therapeutic target for AML1-ETO+ leukemic stem cells.

YBX1 mediates translation of oncogenic transcripts to control cell competition in AML

Persistence of malignant clones is a major determinant of adverse outcome in patients with hematologic malignancies. Despite the fact that the majority of patients with acute myeloid leukemia (AML) achieve complete remission after chemotherapy, a large proportion of them relapse as a result of residual malignant cells. These persistent clones have a competitive advantage and can re-establish disease. Therefore, targeting strategies that specifically diminish cell competition of malignant cells while leaving normal cells unaffected are clearly warranted. Recently, our group identified YBX1 as a mediator of disease persistence in JAK2-mutated myeloproliferative neoplasms. The role of YBX1 in AML, however, remained so far elusive. Here, inactivation of YBX1 confirms its role as an essential driver of leukemia development and maintenance. We identify its ability to amplify the translation of oncogenic transcripts, including MYC, by recruitment to polysomal chains. Genetic inactivation of YBX1 disrupts this regulatory circuit and displaces oncogenic drivers from polysomes, with subsequent depletion of protein levels. As a consequence, leukemia cells show reduced proliferation and are out-competed in vitro and in vivo, while normal cells remain largely unaffected. Collectively, these data establish YBX1 as a specific dependency and therapeutic target in AML that is essential for oncogenic protein expression.

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.

A genetic screen in Drosophila uncovers the multifaceted properties of the NUP98-HOXA9 oncogene

Acute myeloid leukemia (AML) underlies the uncontrolled accumulation of immature myeloid blasts. Several cytogenetic abnormalities have been associated with AML. Among these is the NUP98-HOXA9 (NA9) translocation that fuses the Phe-Gly repeats of nucleoporin NUP98 to the homeodomain of the transcription factor HOXA9. The mechanisms enabling NA9-induced leukemia are poorly understood. Here, we conducted a genetic screen in Drosophila for modifiers of NA9. The screen uncovered 29 complementation groups, including genes with mammalian homologs known to impinge on NA9 activity. Markedly, the modifiers encompassed a diversity of functional categories, suggesting that NA9 perturbs multiple intracellular events. Unexpectedly, we discovered that NA9 promotes cell fate transdetermination and that this phenomenon is greatly influenced by NA9 modifiers involved in epigenetic regulation. Together, our work reveals a network of genes functionally connected to NA9 that not only provides insights into its mechanism of action, but also represents potential therapeutic targets.

Acute myeloid leukemia or AML is a cancer of blood cells. Despite significant progress in recent years, a majority of afflicted individuals still succumbs to the disease. A variety of genetic defects have been associated to AML. Among these are chromosomal translocations, which entail the fusion of two genes, leading to the production of cancer-inducing chimeric proteins. A representative example is the NUP98-HOXA9 oncoprotein, which results from the fusion of the NUP98 and HOXA9 genes. The mechanism of action of NUP98-HOXA9 remains poorly understood. Given the evolutionarily conservation of NUP98 and HOXA9 as well as basic cellular processes across multicellular organisms, we took advantage of Drosophila fruit flies as a genetic tool to identify genes that impinge on the activity of human NUP98-HOXA9. Surprisingly, this approach identified a relatively large spectrum of conserved genes that engaged in functional interplay with NUP98-HOXA9, which indicated the pervasive effects that this oncogene has on basic cellular events. While some genes have been previously linked to NUP98-HOXA9, thus validating our experimental approach, several others are novel and as such represent potentially new avenues for therapeutic intervention.

Predicting prognosis in acute myeloid leukemia patients by surface-enhanced Raman spectroscopy

Aim: To develop a timely and accurate method for predicting acute myeloid leukemia (AML) prognosis after chemotherapy treatment by surface-enhanced Raman spectroscopy (SERS). Methods: Biomolecular differences between AML patients with good and poor prognosis and individuals without AML were investigated based on SERS measurements of bone marrow supernatant fluid samples. Multivariate analysis was implemented on the SERS measurements to establish an AML prognostic model. Results: Significant differences in amino acid, saccharide and lipid levels were observed between AML patients with good and poor prognoses. The AML prognostic model achieved a prediction accuracy of 84.78%. Conclusion: The proposed method could be a potential diagnostic tool for timely and precise prediction of AML prognosis.

Yap1-Scribble polarization is required for hematopoietic stem cell division and fate

Yap1 and its paralogue Taz largely control epithelial tissue growth. We have identified that hematopoietic stem cell (HSC) fitness response to stress depends on Yap1 and Taz. Deletion of Yap1 and Taz induces a loss of HSC quiescence, symmetric self-renewal ability, and renders HSC more vulnerable to serial myeloablative 5-fluorouracil treatment. This effect depends on the predominant cytosolic polarization of Yap1 through a PDZ domain-mediated interaction with the scaffold Scribble. Scribble and Yap1 coordinate to control cytoplasmic Cdc42 activity and HSC fate determination in vivo. Deletion of Scribble disrupts Yap1 copolarization with Cdc42 and decreases Cdc42 activity, resulting in increased self-renewing HSC with competitive reconstitution advantages. These data suggest that Scribble/Yap1 copolarization is indispensable for Cdc42-dependent activity on HSC asymmetric division and fate. The combined loss of Scribble, Yap1, and Taz results in transcriptional upregulation of Rac-specific guanine nucleotide exchange factors, Rac activation, and HSC fitness restoration. Scribble links Cdc42 and the cytosolic functions of the Hippo signaling cascade in HSC fate determination.

Splicing factor YBX1 mediates persistence of JAK2-mutated neoplasms

Janus kinases (JAKs) mediate responses to cytokines, hormones and growth factors in haematopoietic cells^1,2. The JAK gene JAK2 is frequently mutated in the ageing haematopoietic system^3,4 and in haematopoietic cancers⁵. JAK2 mutations constitutively activate downstream signalling and are drivers of myeloproliferative neoplasm (MPN). In clinical use, JAK inhibitors have mixed effects on the overall disease burden of JAK2-mutated clones^6,7, prompting us to investigate the mechanism underlying disease persistence. Here, by in-depth phosphoproteome profiling, we identify proteins involved in mRNA processing as targets of mutant JAK2. We found that inactivation of YBX1, a post-translationally modified target of JAK2, sensitizes cells that persist despite treatment with JAK inhibitors to apoptosis and results in RNA mis-splicing, enrichment for retained introns and disruption of the transcriptional control of extracellular signal-regulated kinase (ERK) signalling. In combination with pharmacological JAK inhibition, YBX1 inactivation induces apoptosis in JAK2-dependent mouse and primary human cells, causing regression of the malignant clones in vivo, and inducing molecular remission. This identifies and validates a cell-intrinsic mechanism whereby differential protein phosphorylation causes splicing-dependent alterations of JAK2-ERK signalling and the maintenance of JAK2^V617F malignant clones. Therapeutic targeting of YBX1-dependent ERK signalling in combination with JAK2 inhibition could thus eradicate cells harbouring mutations in JAK2.

Wnt Signalling in Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is a group of malignant diseases of the haematopoietic system. AML occurs as the result of mutations in haematopoietic stem/progenitor cells, which upregulate Wnt signalling through a variety of mechanisms. Other mechanisms of Wnt activation in AML have been described such as Wnt antagonist inactivation through promoter methylation. Wnt signalling is necessary for the maintenance of leukaemic stem cells. Several molecules involved in or modulating Wnt signalling have a prognostic value in AML. These include: β-catenin, LEF-1, phosphorylated-GSK3β, PSMD2, PPARD, XPNPEP, sFRP2, RUNX1, AXIN2, PCDH17, CXXC5, LLGL1 and PTK7. Targeting Wnt signalling for tumour eradication is an approach that is being explored in haematological and solid tumours. A number of preclinical studies confirms its feasibility, albeit, so far no reliable clinical trial data are available to prove its utility and efficacy.

Attenuation of PKCδ enhances metabolic activity and promotes expansion of blood progenitors

A finely tuned balance of self‐renewal, differentiation, proliferation, and survival governs the pool size and regenerative capacity of blood‐forming hematopoietic stem and progenitor cells (HSPCs). Here, we report that protein kinase C delta (PKCδ) is a critical regulator of adult HSPC number and function that couples the proliferative and metabolic activities of HSPCs. PKCδ‐deficient mice showed a pronounced increase in HSPC numbers, increased competence in reconstituting lethally irradiated recipients, enhanced long‐term competitive advantage in serial transplantation studies, and an augmented HSPC recovery during stress. PKCδ‐deficient HSPCs also showed accelerated proliferation and reduced apoptosis, but did not exhaust in serial transplant assays or induce leukemia. Using inducible knockout and transplantation models, we further found that PKCδ acts in a hematopoietic cell‐intrinsic manner to restrict HSPC number and bone marrow regenerative function. Mechanistically, PKCδ regulates HSPC energy metabolism and coordinately governs multiple regulators within signaling pathways implicated in HSPC homeostasis. Together, these data identify PKCδ as a critical regulator of HSPC signaling and metabolism that acts to limit HSPC expansion in response to physiological and regenerative demands.

Stem cell fate in cancer growth, progression and therapy resistance

Although we have come a long way in our understanding of the signals that drive cancer growth, and how these signals can be targeted, effective control of this disease remains a key scientific and medical challenge. The therapy resistance and relapse that are commonly seen are driven in large part by the inherent heterogeneity within cancers that allows drugs to effectively eliminate some, but not all, malignant cells. Here, we focus on the fundamental drivers of this heterogeneity by examining emerging evidence that shows that these traits are often controlled by the disruption of normal cell fate and aberrant adoption of stem cell signals. We discuss how undifferentiated cells are preferentially primed for transformation and often serve as the cell of origin for cancers. We also consider evidence showing that activation of stem cell programmes in cancers can lead to progression, therapy resistance and metastatic growth and that targeting these attributes may enable better control over a difficult disease.

Diverging impact of cell fate determinants Scrib and Llgl1 on adhesion and migration of hematopoietic stem cells

PURPOSE

Cell fate determinants Scrib and Llgl1 influence self-renewal capacity of hematopoietic stem cells (HSCs). Scrib-deficient HSCs are functionally impaired and lack sufficient repopulation capacity during serial transplantation and stress. In contrast, loss of Llgl1 leads to increased HSC fitness, gain of self-renewal capacity and expansion of the stem cell pool. Here, we sought to assess for shared and unique molecular functions of Llgl1 and Scrib by analyzing their interactome in hematopoietic cells.

METHODS

Interactome analysis was performed by affinity purification followed by mass spectrometry. Motility, migration and adhesion were assessed on primary murine HSCs, which were isolated by FACS sorting following conditional deletion of Scrib or Llgl1, respectively. Imaging of Scrib-deficient HSCs was performed by intravital 2-photon microscopy.

RESULTS

Comparison of Scrib and Llgl1 interactome analyses revealed involvement in common and unique cellular functions. Migration and adhesion were among the cellular functions connected to Scrib but not to Llgl1. Functional validation of these findings confirmed alterations in cell adhesion and migration of Scrib-deficient HSCs in vitro and in vivo. In contrast, genetic inactivation of Llgl1 did not affect adhesion or migratory capacity of hematopoietic stem cells.

CONCLUSION

Our data provide first evidence for an evolutionarily conserved role of the cell fate determinant Scrib in HSC adhesion and migration in vitro and in vivo, a unique function that is not shared with its putative complex partner Llgl1.

Epithelial-mesenchymal transition in haematopoietic stem cell development and homeostasis

Epithelial-mesenchymal transition (EMT) is a morphogenetic process of cells that adopt an epithelial organization in their developmental ontogeny or homeostatic maintenance. Abnormalities in EMT regulation result in many malignant tumours in the human body. Tumours associated with the haematopoietic system, however, are traditionally not considered to involve EMT and haematopoietic stem cells (HSCs) are generally not associated with epithelial characteristics. In this review, we discuss the ontogeny and homeostasis of adult HSCs in the context of EMT intermediate states. We provide evidence that cell polarity regulation is critical for both HSC formation from embryonic dorsal aorta and HSC self-renewal and differentiation in adult bone marrow. HSC polarity is controlled by the same set of surface and transcriptional regulators as those described in canonical EMT processes. With an emphasis on partial EMT, we propose that the concept of EMT can be similarly applied in the study of HSC generation, maintenance and pathogenesis.

JAK2-V617F promotes venous thrombosis through β1/β2 integrin activation

JAK2-V617F-positive chronic myeloproliferative neoplasia (CMN) commonly displays dysfunction of integrins and adhesion molecules expressed on platelets, erythrocytes, and leukocytes. However, the mechanism by which the 2 major leukocyte integrin chains, β1 and β2, may contribute to CMN pathophysiology remained unclear. β1 (α4β1; VLA-4) and β2 (αLβ2; LFA-1) integrins are essential regulators for attachment of leukocytes to endothelial cells. We here showed enhanced adhesion of granulocytes from mice with JAK2-V617F knockin (JAK2+/VF mice) to vascular cell adhesion molecule 1- (VCAM1-) and intercellular adhesion molecule 1-coated (ICAM1-coated) surfaces. Soluble VCAM1 and ICAM1 ligand binding assays revealed increased affinity of β1 and β2 integrins for their respective ligands. For β1 integrins, this correlated with a structural change from the low- to the high-affinity conformation induced by JAK2-V617F. JAK2-V617F triggered constitutive activation of the integrin inside-out signaling molecule Rap1, resulting in translocation toward the cell membrane. Employing a venous thrombosis model, we demonstrated that neutralizing anti-VLA-4 and anti-β2 integrin antibodies suppress pathologic thrombosis as observed in JAK2+/VF mice. In addition, aberrant homing of JAK2+/VF leukocytes to the spleen was inhibited by neutralizing anti-β2 antibodies and by pharmacologic inhibition of Rap1. Thus, our findings identified cross-talk between JAK2-V617F and integrin activation promoting pathologic thrombosis and abnormal trafficking of leukocytes to the spleen.

The cell fate determinant Scribble is required for maintenance of hematopoietic stem cell function

Cell fate determinants influence self-renewal potential of hematopoietic stem cells. Scribble and Llgl1 belong to the Scribble polarity complex and reveal tumor-suppressor function in drosophila. In hematopoietic cells, genetic inactivation of Llgl1 leads to expansion of the stem cell pool and increases self-renewal capacity without conferring malignant transformation. Here we show that genetic inactivation of its putative complex partner Scribble results in functional impairment of hematopoietic stem cells (HSC) over serial transplantation and during stress. Although loss of Scribble deregulates transcriptional downstream effectors involved in stem cell proliferation, cell signaling, and cell motility, these effectors do not overlap with transcriptional targets of Llgl1. Binding partner analysis of Scribble in hematopoietic cells using affinity purification followed by mass spectometry confirms its role in cell signaling and motility but not for binding to polarity modules described in drosophila. Finally, requirement of Scribble for self-renewal capacity also affects leukemia stem cell function. Thus, Scribble is a regulator of adult HSCs, essential for maintenance of HSCs during phases of cell stress.

Regulation of cellular and PCP signalling by the Scribble polarity module

Since the first identification of the Scribble polarity module proteins as a new class of tumour suppressors that regulate both cell polarity and proliferation, an increasing amount of evidence has uncovered a broader role for Scribble, Dlg and Lgl in the control of fundamental cellular functions and their signalling pathways. Here, we review these findings as well as discuss more specifically the role of the Scribble module in PCP signalling.

PBX3 is essential for leukemia stem cell maintenance in MLL-rearranged leukemia

Interaction of HOXA9/MEIS1/PBX3 is responsible for hematopoietic system transformation in MLL-rearranged (MLL-r) leukemia. Of these genes, HOXA9 has been shown to be critical for leukemia cell survival, while MEIS1 has been identified as an essential regulator for leukemia stem cell (LSC) maintenance. Although significantly high expression of PBX3 was observed in clinical acute myeloid leukemia (AML) samples, the individual role of PBX3 in leukemia development is still largely unknown. In this study, we explored the specific role of PBX3 and its associated regulatory network in leukemia progression. By analyzing the clinical database, we found that the high expression of PBX3 is significantly correlated with a poor prognosis in AML patients. ChIP-Seq/qPCR analysis in MLL-r mouse models revealed aberrant epigenetic modifications with increased H3K79me2, and decreased H3K9me3 and H3K27me3 levels in LSCs, which may account for the high expression levels of Pbx3. To further examine the role of Pbx3 in AML maintenance and progression, we used the CRISPR/Cas9 system to delete Pbx3 in leukemic cells in the MLL-AF9 induced AML mouse model. We found that Pbx3 deletion significantly prolonged the survival of leukemic mice and decreased the leukemia burden by decreasing the capacity of LSCs and promoting LSC apoptosis. In conclusion, we found that PBX3 is epigenetically aberrant in the LSCs of MLL-r AML and is essential for leukemia development. Significantly, the differential expression of PBX3 in normal and malignant hematopoietic cells suggests PBX3 as a potential prognostic marker and therapeutic target for MLL-r leukemia.

Cell autonomous expression of CXCL-10 in JAK2V617F-mutated MPN

PURPOSE

Myeloproliferative neoplasms (MPN) are clonal disorders of hematopoietic stem- and progenitor cells. Mutation of Janus-Kinase 2 (JAK2) is the most frequent genetic event detected in Philadelphia-negative MPN. In advanced phases, the clinical hallmark of the disease is a striking inflammatory syndrome. So far, the cellular and molecular basis of inflammation is not fully understood. We, therefore, sought to investigate the relationship of activating JAK2 mutation and aberrant cytokine expression in MPN.

METHODS

Cytokine array was performed to identify Jak2V617F-related cytokine expression and secretion. CXCL10 mRNA expression was analyzed by qPCR in peripheral blood cells. To exclude paracrine/autocrine stimulation as a potential mechanism, we generated Ba/F3-EpoR-JAK2WT or EpoR-JAK2V617F cells lacking CXCL10 receptor. Pharmacologic inhibition of JAK2 kinase was achieved by JAK-Inhibitor treatment. Signaling pathways and downstream effectors were characterized by Western blotting, immunofluorescence microscopy, luciferase reporter assays, qPCR, and chromatin-immunoprecipitation studies.

RESULTS

We identified CXCL10 as the most highly induced cytokine in JAK2-mutated cell lines. In MPN patients, CXCL10 is highly expressed in JAK2V617F but not JAK2WT MPN or healthy donor controls. Moreover, CXCL10 expression correlates with JAK2V617F allelic burden. High CXCL10 correlates with the presence of clinical risk factors but not with clinical symptoms and quality of life. Pharmacologic inhibition of mutated JAK2 kinase inhibits CXCL10 expression. NFκB signaling is activated downstream of JAK2V617F receptor and directly induces CXCL10 expression.

CONCLUSIONS

Our data provide first evidence for a link between oncogenic JAK2V617F signaling and cell intrinsic induction of CXCL10 induced by activated NFkB signaling.

The progress of early growth response factor 1 and leukemia

Early growth response gene-1 (EGR1) widely exists in the cell nucleus of such as, zebrafish, mice, chimpanzees and humans, an it also can be observed in the cytoplasm of some tumors. EGR1 was named just after its brief and rapid expression of different stimuli. Accumulating studies have extensively demonstrated that the widespread dysregulation of EGR1 is involved in hematological malignancies such as human acute myeloid leukemia (AML), chronic myelogenous leukemia, chronic lymphocytic leukemia, multiple myeloma, and B cell lymphoma. With the deep research on EGR1, its expression, function and regulatory mechanism has been gradually elucidated, and provides more possibilities for treatment strategies of patients with leukemia. Herein, we summarize the roles of EGR1 in its biological function and relationship with leukemia.

Polarity and asymmetric cell division in the control of lymphocyte fate decisions and function

Polarity is important in several lymphocyte processes including lymphocyte migration, formation of the immunological synapse, and asymmetric cell division (ACD). While lymphocyte migration and immunological synapse formation are relatively well understood, the role of lymphocyte ACD is less clear. Recent advances in measuring polarity enable more robust analyses of asymmetric cell division. Use of these new methods has produced crucial quantification of ACD at precise phases of lymphocyte development and activation. These developments are leading to a better understanding of the drivers of fate choice during lymphocyte activation and provide a context within which to explain the effects of ACD.

Asymmetric Cell Division in T Lymphocyte Fate Diversification

Immunological protection against microbial pathogens is dependent on robust generation of functionally diverse T lymphocyte subsets. Upon microbial infection, naïve CD4(+) or CD8(+) T lymphocytes can give rise to effector- and memory-fated progeny that together mediate a potent immune response. Recent advances in single-cell immunological and genomic profiling technologies have helped elucidate early and late diversification mechanisms that enable the generation of heterogeneity from single T lymphocytes. We discuss these findings here and argue that one such mechanism, asymmetric cell division, creates an early divergence in T lymphocyte fates by giving rise to daughter cells with a propensity towards the terminally differentiated effector or self-renewing memory lineages, with cell-intrinsic and -extrinsic cues from the microenvironment driving the final maturation steps.

Lethal giant larvae-1 deficiency enhances the CD8(+) effector T-cell response to antigen challenge in vivo

Lethal giant larvae-1 (Lgl-1) is an evolutionary conserved protein that regulates cell polarity in diverse lineages; however, the role of Lgl-1 in the polarity and function of immune cells remains to be elucidated. To assess the role of Lgl-1 in T cells, we generated chimeric mice with a hematopoietic system deficient for Lgl-1. Lgl-1 deficiency did not impair the activation or function of peripheral CD8(+) T cells in response to antigen presentation in vitro, but did skew effector and memory T-cell differentiation. When challenged with antigen-expressing virus or tumor, Lgl-1-deficient mice displayed altered T-cell responses. This manifested in a stronger antiviral and antitumor effector CD8(+) T-cell response, the latter resulting in enhanced control of MC38-OVA tumors. These results reveal a novel role for Lgl-1 in the regulation of virus-specific T-cell responses and antitumor immunity.

Evolutionarily conserved signaling pathways: acting in the shadows of acute myelogenous leukemia's genetic diversity

Acute myelogenous leukemia stem cells (AML-LSC) give rise to the leukemic bulk population and maintain disease. Relapse can arise from residual LSCs that have distinct sensitivity and dependencies when compared with the AML bulk. AML-LSCs are driven by genetic and epigenomic changes, and these alterations influence prognosis and clonal selection. Therapies targeting these molecular aberrations have been developed and show promising responses in advanced clinical trials; however, so far success with LSCs has been limited. Besides the genetic diversity, AML-LSCs are critically influenced by the microenvironment, and a third crucial aspect has recently come to the fore: A group of evolutionarily conserved signaling pathways such as canonical Wnt signaling, Notch signaling, or the Hedgehog pathway can be essential for maintenance of AML-LSC but may be redundant for normal hematopoietic stem cells. In addition, early reports suggest also regulators of cell polarity may also influence hematopoietic stem cells and AML biology. Interactions between these pathways have been investigated recently and suggest a network of signaling pathways involved in regulation of self-renewal and response to oncogenic stress. Here, we review how recent discoveries on regulation of AML-LSC-relevant evolutionarily conserved pathways may open opportunities for novel treatment approaches eradicating residual disease.

The Phosphatases STS1 and STS2 Regulate Hematopoietic Stem and Progenitor Cell Fitness

FLT3 and c-KIT are crucial regulators of hematopoietic stem and progenitor cells. We investigated the role of STS1 and STS2 on FLT3 and c-KIT phosphorylation, activity, and function in normal and stress-induced hematopoiesis. STS1/STS2-deficient mice show a profound expansion of multipotent progenitor and lymphoid primed multipotent progenitor cells with elevated colony-forming capacity. Although long-term hematopoietic stem cells are not increased in numbers, lack of STS1 and STS2 significantly promotes long-term repopulation activity, demonstrating a pivotal role of STS1/STS2 in regulating hematopoietic stem and progenitor cell fitness. Biochemical analysis identified STS1/STS2 as direct phosphatases of FLT3 and c-KIT. Loss of STS1/STS2 induces hyperphosphorylation of FLT3, enhances AKT signaling, and confers a strong proliferative advantage. Therefore, our study reveals that STS1 and STS2 may serve as novel pharmaceutical targets to improve hematopoietic recovery after bone marrow transplantation.

Lethal (2) giant larvae: an indispensable regulator of cell polarity and cancer development

Cell polarity is one of the most basic properties of all normal cells and is essential for regulating numerous biological processes. Loss of polarity is considered a hallmark for cancer. Multiple polarity proteins are implicated in maintenance of cell polarity. Lethal (2) giant larvae (Lgl) is one of polarity proteins that plays an important role in regulating cell polarity, asymmetric division as well as tumorigenesis. Lgl proteins in different species have similar structures and conserved functions. Lgl acts as an indispensable regulator of cell biological function, including cell polarity and asymmetric division, through interplaying with other polarity proteins, regulating exocytosis, mediating cytoskeleton and being involved in signaling pathways. Furthermore, Lgl plays a role of a tumor suppressor, and the aberrant expression of Hugl, a human homologue of Lgl, contributes to multiple cancers. However, the exact functions of Lgl and the underlying mechanisms remain enigmatic. In this review, we will give an overview of the Lgl functions in cell polarity and cancer development, discuss the potential mechanisms underlying these functions, and raise our conclusion of previous studies and points of view about the future studies.

Fearful symmetry: subversion of asymmetric division in cancer development and progression

Asymmetric division is an evolutionarily conserved process that generates daughter cells with different fates through the unequal partitioning of fate determinants. While asymmetric division is critically important in generating diversity during development, its dysregulation can also promote oncogenesis. In particular, signals that shift the normal balance of symmetric and asymmetric division can lead to a differentiation arrest and trigger cancer progression. Here, we discuss the studies that have provided increasing support for this idea. Beginning with original work carried out in Drosophila, we trace more recent work in mammalian systems that suggest that the subversion of asymmetric division can contribute significantly to the development and progression of both hematologic malignancies and solid cancers.

Epo-induced erythroid maturation is dependent on Plcγ1 signaling

Erythropoiesis is a tightly regulated process. Development of red blood cells occurs through differentiation of hematopoietic stem cells (HSCs) into more committed progenitors and finally into erythrocytes. Binding of erythropoietin (Epo) to its receptor (EpoR) is required for erythropoiesis as it promotes survival and late maturation of erythroid progenitors. In vivo and in vitro studies have highlighted the requirement of EpoR signaling through Janus kinase 2 (Jak2) tyrosine kinase and Stat5a/b as a central pathway. Here, we demonstrate that phospholipase C gamma 1 (Plcγ1) is activated downstream of EpoR-Jak2 independently of Stat5. Plcγ1-deficient pro-erythroblasts and erythroid progenitors exhibited strong impairment in differentiation and colony-forming potential. In vivo, suppression of Plcγ1 in immunophenotypically defined HSCs (Lin(-)Sca1(+)KIT(+)CD48(-)CD150(+)) severely reduced erythroid development. To identify Plcγ1 effector molecules involved in regulation of erythroid differentiation, we assessed changes occurring at the global transcriptional and DNA methylation level after inactivation of Plcγ1. The top common downstream effector was H2afy2, which encodes for the histone variant macroH2A2 (mH2A2). Inactivation of mH2A2 expression recapitulated the effects of Plcγ1 depletion on erythroid maturation. Taken together, our findings identify Plcγ1 and its downstream target mH2A2, as a 'non-canonical' Epo signaling pathway essential for erythroid differentiation.

Relevance of leukemic stem cells in acute myeloid leukemia: heterogeneity and influence on disease monitoring, prognosis and treatment design

Acute myeloid leukemia is a bone marrow disease characterized by a block in differentiation of the myeloid lineage with a concomitant uncontrolled high proliferation rate. Development of acute myeloid leukemia from stem cells with specific founder mutations, leads to an oligoclonal disease that progresses into a very heterogeneous leukemia at diagnosis. Measurement of leukemic stem cell load and characterization of these cells are essential for prediction of relapse and target identification, respectively. Prediction of relapse by monitoring the disease during minimal residual disease detection is challenged by clonal shifts during therapy. To overcome this, characterization of the potential relapse-initiating cells is required using both flow cytometry and molecular analysis since leukemic stem cells can be targeted both on extracellular features and on stem-cell specific signal transduction pathways.

Influence of Egr-1 in cardiac tissue-derived mesenchymal stem cells in response to glucose variations

Mesenchymal stem cells (MSCs) represent a promising cell population for cell therapy and regenerative medicine applications. However, how variations in glucose are perceived by MSC pool is still unclear. Since, glucose metabolism is cell type and tissue dependent, this must be considered when MSCs are derived from alternative sources such as the heart. The zinc finger transcription factor Egr-1 is an important early response gene, likely to play a key role in the glucose-induced response. Our aim was to investigate how short-term changes in in vitro glucose concentrations affect multipotent cardiac tissue-derived MSCs (cMSCs) in a mouse model of Egr-1 KO (Egr-1^−/−). Results showed that loss of Egr-1 does not significantly influence cMSC proliferation. In contrast, responses to glucose variations were observed in wt but not in Egr-1^−/− cMSCs by clonogenic assay. Phenotype analysis by RT-PCR showed that cMSCs Egr-1^−/− lost the ability to regulate the glucose transporters GLUT-1 and GLUT-4 and, as expected, the Egr-1 target genes VEGF, TGFβ-1, and p300. Acetylated protein levels of H3 histone were impaired in Egr-1^−/− compared to wt cMSCs. We propose that Egr-1 acts as immediate glucose biological sensor in cMSCs after a short period of stimuli, likely inducing epigenetic modifications.

Rho GTPases control specific cytoskeleton-dependent functions of hematopoietic stem cells

The Rho family of guanosine triphosphatases (GTPases) is composed of members of the Ras superfamily of proteins. They are GTP-bound molecules with a modest intrinsic GTPase activity that can be accelerated upon activation/localization of specialized guanine nucleotide exchange factors. Members of this family act as molecular switches and are required for coordinated cytoskeletal rearrangements that are crucial in a set of specialized functions of mammalian stem cells. These functions include self-renewal, adhesion, and migration. Mouse gene-targeting studies have provided convincing evidence of the indispensable and dispensable roles of individual members of the Rho GTPase family and the putative upstream and downstream mediators in stem cell-specific functions. The role of Rho GTPases and related signaling pathways previously seen in other cell types and organisms have been confirmed in mammalian hematopoietic stem cells (HSCs), and new signaling pathways and unexpected functions unique to HSCs have been identified and dissected. This review summarizes our current understanding of the role of Rho family of GTPases on HSC and progenitor activity through cytoskeleton-mediated signaling pathways, providing insight about relevant signaling pathways that regulate mammalian stem cell self-renewal, adhesion, and migration.

Polarized cells, polarized views: asymmetric cell division in hematopoietic cells

It has long been recognized that alterations in cell shape and polarity play important roles in coordinating lymphocyte functions. In the last decade, a new aspect of lymphocyte polarity has attracted much attention, termed asymmetric cell division (ACD). ACD has previously been shown to dictate or influence many aspects of development in model organisms such as the worm and the fly, and to be disrupted in disease. Recent observations that ACD also occurs in lymphocytes led to exciting speculations that ACD might influence lymphocyte differentiation and function, and leukemia. Dissecting the role that ACD might play in these activities has not been straightforward, and the evidence to date for a functional role in lymphocyte fate determination has been controversial. In this review, we discuss the evidence to date for ACD in lymphocytes, and how it might influence lymphocyte fate. We also discuss current gaps in our knowledge, and suggest approaches to definitively test the physiological role of ACD in lymphocytes.

Lethal giant larvae 1 tumour suppressor activity is not conserved in models of mammalian T and B cell leukaemia

In epithelial and stem cells, lethal giant larvae (Lgl) is a potent tumour suppressor, a regulator of Notch signalling, and a mediator of cell fate via asymmetric cell division. Recent evidence suggests that the function of Lgl is conserved in mammalian haematopoietic stem cells and implies a contribution to haematological malignancies. To date, direct measurement of the effect of Lgl expression on malignancies of the haematopoietic lineage has not been tested. In Lgl1^−/− mice, we analysed the development of haematopoietic malignancies either alone, or in the presence of common oncogenic lesions. We show that in the absence of Lgl1, production of mature white blood cell lineages and long-term survival of mice are not affected. Additionally, loss of Lgl1 does not alter leukaemia driven by constitutive Notch, c-Myc or Jak2 signalling. These results suggest that the role of Lgl1 in the haematopoietic lineage might be restricted to specific co-operating mutations and a limited number of cellular contexts.

AML1-ETO triggers epigenetic activation of early growth response gene l, inducing apoptosis in t(8;21) acute myeloid leukemia

The t(8;21)(q22;q22) translocation is the most common chromosomal translocation in acute myeloid leukemia (AML), and it gives rise to acute myeloid gene 1 (AML1)-myeloid transforming gene 8 (ETO)-positive AML, which has a relatively favorable prognosis. However, the molecular mechanism related to a favorable prognosis in AML1-ETO-positive AML is still not fully understood. Our results show that the AML1-ETO fusion protein triggered activation of early growth response gene l (EGR1) by binding at AML1-binding sites on the EGR1 promoter and, subsequently, recruiting acetyltransferase P300, which is known to acetylate histones. However, AML1-ETO could not recruit DNA methyltransferases and histone deacetylases; therefore, EGR1 expression was affected by histone acetylation but not by DNA methylation. Both transcription and translation of EGR1 were higher in AML1-ETO-positive AML cell lines than in AML1-ETO-negative AML cell lines, owing to acetylation. Furthermore, when AML1-ETO-positive AML cell lines were treated with C646 (P300 inhibitor) and trichostatin A (histone deacetylase inhibitor), EGR1 expression was significantly decreased and increased, respectively. In addition, treatment with 5-azacytidine (methyltransferase inhibitor) did not cause any significant change in EGR1 expression. Overexpression of EGR1 inhibited cell proliferation and promoted apoptosis, and EGR1 knockout promoted cell proliferation. Thus, EGR1 could be a novel prognostic factor for a favorable outcome in AML1-ETO-positive AML. The results of our study may explain the molecular mechanisms underlying the favorable prognosis in AML1-ETO-positive AML.