Analysis of concentration-dependent functions of PU.1 in hematopoiesis using mouse models

Epigallocatechin gallate regulates the myeloid-specific transcription factor PU.1 in macrophages

Our previous research demonstrated that PU.1 regulates expression of the genes involved in inflammation in macrophages. Selective knockdown of PU.1 in macrophages ameliorated LPS-induced acute lung injury (ALI) in bone marrow chimera mice. Inhibitors that block the transcriptional activity of PU.1 in macrophages have the potential to mitigate the pathophysiology of LPS-induced ALI. However, complete inactivation of PU.1 gene disrupts normal myelopoiesis. Although the green tea polyphenol Epigallocatechin gallate (EGCG) has been shown to regulate inflammatory genes in various cell types, it is not known if EGCG alters the transcriptional activity of PU.1 protein. Using Schrodinger Glide docking, we have identified that EGCG binds with PU.1 protein, altering its DNA-binding and self-dimerization activity. In silico analysis shows that EGCG forms Hydrogen bonds with Glutamic Acid 209, Leucine 250 in DNA binding and Lysine 196, Tryptophan 193, and Leucine 182 in the self-dimerization domain of the PU.1 protein. Experimental validation using mouse bone marrow-derived macrophages (BMDM) confirmed that EGCG inhibits both DNA binding by PU.1 and self-dimerization. Importantly, EGCG had no impact on expression of the total PU.1 protein levels but significantly reduced expression of various inflammatory genes and generation of ROS. In summary, we report that EGCG acts as an inhibitor of the PU.1 transcription factor in macrophages.

Investigation of the Expression and Regulation of SCG5 in the Context of the Chromogranin-Secretogranin Family in Malignant Tumors

The SCG5 gene has been demonstrated to play an essential role in the development and progression of a range of malignant neoplasms. The regulation of SCG5 expression involves multiple biological pathways. According to relevant studies, SCG5 is differentially expressed in different cancers, and its up- or down-regulation may even affect tumour growth, invasion, and migration, which caught our attention. Therefore, we summarise the regulatory roles played by the SCG5 gene in a variety of cancers and the biological regulatory mechanisms associated with its possible promotion or inhibition of tumour biological behavior, to further explore the potential of SCG5 as a new tumour marker and hopefully provide theoretical guidance for subsequent disease research and treatment.

PU.1 is required to restrain myelopoiesis during chronic inflammatory stress

Chronic inflammation is a common feature of aging and numerous diseases such as diabetes, obesity, and autoimmune syndromes and has been linked to the development of hematological malignancy. Blood-forming hematopoietic stem cells (HSC) can contribute to these diseases via the production of tissue-damaging myeloid cells and/or the acquisition of mutations in epigenetic and transcriptional regulators that initiate evolution toward leukemogenesis. We previously showed that the myeloid "master regulator" transcription factor PU.1 is robustly induced in HSC by pro-inflammatory cytokines such as interleukin (IL)-1β and limits their proliferative activity. Here, we used a PU.1-deficient mouse model to investigate the broader role of PU.1 in regulating hematopoietic activity in response to chronic inflammatory challenges. We found that PU.1 is critical in restraining inflammatory myelopoiesis via suppression of cell cycle and self-renewal gene programs in myeloid-biased multipotent progenitor (MPP) cells. Our data show that while PU.1 functions as a key driver of myeloid differentiation, it plays an equally critical role in tailoring hematopoietic responses to inflammatory stimuli while limiting expansion and self-renewal gene expression in MPPs. These data identify PU.1 as a key regulator of "emergency" myelopoiesis relevant to inflammatory disease and leukemogenesis.

Myeloid cell-specific mutation of Spi1 selectively reduces M2-biased macrophage numbers in skeletal muscle, reduces age-related muscle fibrosis and prevents sarcopenia

Intramuscular macrophages play key regulatory roles in determining the response of skeletal muscle to injury and disease. Recent investigations showed that the numbers and phenotype of intramuscular macrophages change during aging, suggesting that those changes could influence the aging process. We tested that hypothesis by generating a mouse model that harbors a myeloid cell-specific mutation of Spi1, which is a transcription factor that is essential for myeloid cell development. The mutation reduced the numbers of macrophages biased to the CD163+/CD206+ M2 phenotype in muscles of aging mice without affecting the numbers of CD68-expressing macrophages and reduced the expression of transcripts associated with the M2-biased phenotype. The mutation did not affect the colony-forming ability or the frequency of specific subpopulations of bone marrow hematopoietic cells and did not affect myeloid/lymphoid cell ratios in peripheral blood leukocyte populations. Cellularity of most myeloid lineage cells was not influenced by the mutation. The Spi1 mutation in bone marrow-derived macrophages in vitro also did not affect expression of transcripts that indicate the M2-biased phenotype. Thus, myeloid cell-targeted mutation of Spi1 influences macrophage phenotype in muscle but did not affect earlier stages of differentiation of cells in the macrophage lineage. The mutation reduced age-related muscle fibrosis, which is consistent with the reduction of M2-biased macrophages, and reduced expression of the pro-fibrotic enzyme arginase. Most importantly, the mutation prevented sarcopenia. Together, our observations indicate that intramuscular, M2-biased macrophages play significant roles in promoting detrimental, age-related changes in muscle.

Identification of Metastasis-Associated Genes in Cutaneous Squamous Cell Carcinoma Based on Bioinformatics Analysis and Experimental Validation

INTRODUCTION

Cutaneous squamous cell carcinoma (cSCC) is a global malignant tumor with a high degree of malignancy. Once metastasis occurs, it will lead to poor prognosis and even death. This study attempts to find out the central genes closely related to cSCC metastasis, so as to clarify the molecular regulatory mechanism of cSCC metastasis and open up new ideas for clinical treatment.

METHODS

Firstly, cSCC data set GSE98767 was used to establish a tumor metastasis model via clustering analysis. The key module and hub genes associated with cSCC metastasis were analyzed by weighted gene co-expression analysis (WGCNA). Next, the prognostic functions of hub genes were identified by functional and pathway enrichment analysis, pan-cancer analysis, and receiver operating characteristic-area under the curve (ROC-AUC) validation. Finally, the key genes were verified by clinical sample detection and biological in vitro test.

RESULTS

A total of 19 hub genes related to cSCC metastasis were identified. They were highly expressed in cSCC metastatic tissues and were mainly enriched in cellular material and energy metabolism pathways. Overall survival (OS) and disease-free survival (DFS) results from pan-cancer analysis showed that eight and six highly expressed genes, respectively, with PAPSS2 and SCG5 had highly reliable ROC-AUC validation values and were poor prognostic factors. Clinical and biological tests also confirmed the upregulation of PAPSS2 and SCG5 in cSCC. Deletion of PAPSS2 and SCG5 resulted in decreased viability, migration, and invasion of A-431 cells.

CONCLUSION

PAPSS2 and SCG5 may be important factors for cSCC metastasis, and they are involved in the regulation of cSCC cell viability, migration, and invasion.

Single-cell RNA sequencing uncovers the nuclear decoy lincRNA PIRAT as a regulator of systemic monocyte immunity during COVID-19

SARS-CoV-2–infected patients often display characteristic changes in the production of immune mediators that trigger life-threatening courses of COVID-19. The underlying molecular mechanisms are not yet fully understood. Here, we used single-cell RNA sequencing to investigate the involvement of the emerging class of long regulatory RNA in COVID-19. Our data reveal that a previously unknown regulatory RNA in the nucleus of immune cells is altered after SARS-CoV-2 infection. The degradation of this RNA removes a natural brake on the production of critical immune mediators that can promote the development of severe COVID-19. We believe that therapeutic intervention in this nuclear RNA circuit could counteract the overproduction of disease-causing immune mediators and protect against severe COVID-19.

The systemic immune response to viral infection is shaped by master transcription factors, such as NF-κB, STAT1, or PU.1. Although long noncoding RNAs (lncRNAs) have been suggested as important regulators of transcription factor activity, their contributions to the systemic immunopathologies observed during SARS-CoV-2 infection have remained unknown. Here, we employed a targeted single-cell RNA sequencing approach to reveal lncRNAs differentially expressed in blood leukocytes during severe COVID-19. Our results uncover the lncRNA PIRAT (PU.1-induced regulator of alarmin transcription) as a major PU.1 feedback-regulator in monocytes, governing the production of the alarmins S100A8/A9, key drivers of COVID-19 pathogenesis. Knockout and transgene expression, combined with chromatin-occupancy profiling, characterized PIRAT as a nuclear decoy RNA, keeping PU.1 from binding to alarmin promoters and promoting its binding to pseudogenes in naïve monocytes. NF-κB–dependent PIRAT down-regulation during COVID-19 consequently releases a transcriptional brake, fueling alarmin production. Alarmin expression is additionally enhanced by the up-regulation of the lncRNA LUCAT1, which promotes NF-κB–dependent gene expression at the expense of targets of the JAK-STAT pathway. Our results suggest a major role of nuclear noncoding RNA networks in systemic antiviral responses to SARS-CoV-2 in humans.

I SPI1 something needed for B cells

In this issue, Le Coz et al. (2021. J. Exp. Med.https://doi.org/10.1084/jem.20201750) describe a novel immunodeficiency syndrome caused by mutations in SPI1. Through a series of in-depth studies, the authors provide insights into how SPI1 affects blood lineage specification, highlighting the important role of master transcription factors as cellular fate determinants.

Constrained chromatin accessibility in PU.1-mutated agammaglobulinemia patients

The pioneer transcription factor (TF) PU.1 controls hematopoietic cell fate by decompacting stem cell heterochromatin and allowing nonpioneer TFs to enter otherwise inaccessible genomic sites. PU.1 deficiency fatally arrests lymphopoiesis and myelopoiesis in mice, but human congenital PU.1 disorders have not previously been described. We studied six unrelated agammaglobulinemic patients, each harboring a heterozygous mutation (four de novo, two unphased) of SPI1, the gene encoding PU.1. Affected patients lacked circulating B cells and possessed few conventional dendritic cells. Introducing disease-similar SPI1 mutations into human hematopoietic stem and progenitor cells impaired early in vitro B cell and myeloid cell differentiation. Patient SPI1 mutations encoded destabilized PU.1 proteins unable to nuclear localize or bind target DNA. In PU.1-haploinsufficient pro–B cell lines, euchromatin was less accessible to nonpioneer TFs critical for B cell development, and gene expression patterns associated with the pro– to pre–B cell transition were undermined. Our findings molecularly describe a novel form of agammaglobulinemia and underscore PU.1’s critical, dose-dependent role as a hematopoietic euchromatin gatekeeper.

Immunological Role and Prognostic Value of APBB1IP in Pan-Cancer Analysis

Objective: APBB1IP is a Rap1-binding protein that mainly acts as a regulator of leukocyte recruitment and pathogen clearance through complement-mediated phagocytosis. However, the role of APBB1IP in tumor immunity remains unclear. This study was carried out to evaluate the prognostic landscape of APBB1IP in pan-cancer analysis and investigate the relationship between APBB1IP expression and immune infiltration.

Methods: We explored the expression pattern and prognostic value of APBB1IP in pan-cancer analysis through Kaplan-Meier Plotter and multiple databases, including TCGA, Oncomine. We then assessed the correlation between APBB1IP expression and immune cell infiltration using the TIMER database. Furthermore, we identified the proteins that interact with APBB1IP and performed epigenetic and transcriptional analyses. Multivariate Cox regression analyses were applied to construct a prognostic model, which consisted of APBB1IP and its interacting proteins, based on the lung cancer cohorts from the Gene Expression Omnibus (GEO) database.

Results: The expression of APBB1IP was correlated with the prognosis of several types of cancer. APBB1IP upregulation was found to be associated with increased immune cell infiltration, especially for CD8⁺ T cells, natural killer (NK) cells, and immune regulators. A link was found between APBB1IP and immune-related proteins including RAP1A/B, TLN1/2 and VCL in the interaction network.

Conclusion: APBB1IP can serve as a prognostic biomarker in pan-cancer analysis. APBB1IP upregulation was correlated with increased immune-cell infiltration, and the expression APBB1IP in different tumors might be related to the tumor immune microenvironment.

Myelopoiesis specific gene expression profiling in human CD34+ hematopoietic stem cells

Differentiation of the HSCs into myeloid lineage is primarily monitored by transcription factor PU.1. GATA1 acts as a negative regulator by antagonizing the function of PU.1 by bindings its β3/β4 domain. In this study, a mutation was induced in PU.1 which blocks its interaction with GATA1. The pure form of this mutant protein i.e Y244D was loaded on poly (lactic-co-glycolic acid) nanoparticles to transfect CD34⁺ cells, which act as a selective tool to enhance the myelopoiesis, as confirmed by flow cytometry analysis. Further, microarray data analysis was performed to gather information on myelopoiesis specific signaling pathways and genes involved in myelopoiesis like CCL2, S100A8, and S100A9, which were also found to be significantly upregulated in the mutant form. Different molecular functions like antioxidant activity, signal transduction, developmental processes, and biological adhesion were analyzed. This study potentially signifies that PU.1 mutant is highly efficient in myelopoiesis.

Immunohistological staining of unknown chemokine RANTES/CCL5 expression in jawbone marrow defects-osteoimmunology and disruption of bone remodeling in clinical case studies targeting on predictive preventive personalized medicine

Background

Fatty degenerative osteonecrosis in the medullary spaces of the jawbone (FDOJ) may be identified as a lesser known source of RANTES/CCL5 (R/C) overexpression. The chemokine R/C also interferes with bone metabolism leading to osteolysis in areas affected by FDOJ. Many dental surgeries require functioning repair mechanisms and these may be disrupted by R/C overexpression.

Objective

To clarify the way in which R/C expression from adipocytes in FDOJ causes a disturbance in osteogenesis and impacts on medullary stem cells by investigating the detection of R/C expression with immunochemical staining.

Materials and methods

We examined the tissue samples of 449 patients with FDOJ to assess the level of the chemokine R/C using bead-based Luminex® analysis. In six clinical case studies of FDOJ, we compared bone density, histological findings, R/C expression, and immunohistochemical staining.

Results

R/C is overexpressed by up to 30-fold in the 449 FDOJ cases when compared with healthy jawbone samples. The comparison of the six clinical cases consistently shows greatly reduced bone density, (i.e., osteolysis), but varies in terms of the level of agreement across the other three parameters.

Discussion

R/C from FDOJ sources may be implicated in several immune responses and considered a key pathogenetic pathway for increased adipogenesis rather than desirable osteogenesis. Adipocytes pathogenetically act via R/C expression in local FDOJ and systemically on the immune system.

Conclusion

R/C may be regarded as an important trigger for possible pathological developments in the fate of hematopoietic stem cells. FDOJ is not a rigidly uniform process but reflects changing stages of development. The absence of correlating findings should not be interpreted as a misdiagnosis. It seems appropriate to direct further research in the field of “maxillo–mandibular osteoimmunology” focusing on R/C overexpression in FDOJ areas. This may contribute to the development of personalized strategies in preventive medicine.

Leukemogenic nucleophosmin mutation disrupts the transcription factor hub that regulates granulomonocytic fates

Nucleophosmin (NPM1) is among the most frequently mutated genes in acute myeloid leukemia (AML). It is not known, however, how the resulting oncoprotein mutant NPM1 is leukemogenic. To reveal the cellular machinery in which NPM1 participates in myeloid cells, we analyzed the endogenous NPM1 protein interactome by mass spectrometry and discovered abundant amounts of the master transcription factor driver of monocyte lineage differentiation PU.1 (also known as SPI1). Mutant NPM1, which aberrantly accumulates in cytoplasm, dislocated PU.1 into cytoplasm with it. CEBPA and RUNX1, the master transcription factors that collaborate with PU.1 to activate granulomonocytic lineage fates, remained nuclear; but without PU.1, their coregulator interactions were toggled from coactivators to corepressors, repressing instead of activating more than 500 granulocyte and monocyte terminal differentiation genes. An inhibitor of nuclear export, selinexor, by locking mutant NPM1/PU.1 in the nucleus, activated terminal monocytic fates. Direct depletion of the corepressor DNA methyltransferase 1 (DNMT1) from the CEBPA/RUNX1 protein interactome using the clinical drug decitabine activated terminal granulocytic fates. Together, these noncytotoxic treatments extended survival by more than 160 days versus vehicle in a patient-derived xenotransplant model of NPM1/FLT3-mutated AML. In sum, mutant NPM1 represses monocyte and granulocyte terminal differentiation by disrupting PU.1/CEBPA/RUNX1 collaboration, a transforming action that can be reversed by pharmacodynamically directed dosing of clinical small molecules.

Cancer-induced systemic myeloid dysfunction: Implications for treatment and a novel nanoparticle approach for its correction

Unlike other regulatory circuits, cancer-induced myeloid dysfunction involves more than an accumulation of impaired dendritic cells, protumoral macrophages, and myeloid derived suppressor cells in the tumor microenvironment. It is also characterized by "aberrant" myelopoiesis that results in the accumulation and expansion of immature myeloid precursors with a suppressive phenotype in the systemic circulation. The first part of this review briefly describes the evidence for and consequences of this systemic dysfunctional myelopoiesis and the possible reinforcement of this phenomenon by conventional treatments used in patients with cancer, in particular chemotherapy and granulocyte-colony stimulating factor. The second half of this review describes very small size particles, a novel immune-modulatory nanoparticle, and the evidence indicating a possible role of this agent in correcting or re-programming the dysfunctional myelopoiesis in different scenarios.

Senescence is a Spi1-induced anti-proliferative mechanism in primary hematopoietic cells

Transcriptional deregulation caused by epigenetic or genetic alterations is a major cause of leukemic transformation. The Spi1/PU.1 transcription factor is a key regulator of many steps of hematopoiesis, and limits self-renewal of hematopoietic stem cells. The deregulation of its expression or activity contributes to leukemia, in which Spi1 can be either an oncogene or a tumor suppressor. Herein we explored whether cellular senescence, an anti-tumoral pathway that restrains cell proliferation, is a mechanism by which Spi1 limits hematopoietic cell expansion, and thus prevents the development of leukemia. We show that Spi1 overexpression triggers cellular senescence both in primary fibroblasts and hematopoietic cells. Erythroid and myeloid lineages are both prone to Spi1-induced senescence. In hematopoietic cells, Spi1-induced senescence requires its DNA-binding activity and a functional p38MAPK14 pathway but is independent of a DNA-damage response. In contrast, in fibroblasts, Spi1-induced senescence is triggered by a DNA-damage response. Importantly, using our well-established Spi1 transgenic leukemia mouse model, we demonstrate that Spi1 overexpression also induces senescence in erythroid progenitors of the bone marrow in vivo before the onset of the pre-leukemic phase of erythroleukemia. Remarkably, the senescence response is lost during the progression of the disease and erythroid blasts do not display a higher expression of Dec1 and CDKN1A, two of the induced senescence markers in young animals. These results bring indirect evidence that leukemia develops from cells which have bypassed Spi1-induced senescence. Overall, our results reveal senescence as a Spi1-induced anti-proliferative mechanism that may be a safeguard against the development of acute myeloid leukemia.

C/EBP-Induced Transdifferentiation Reveals Granulocyte-Macrophage Precursor-like Plasticity of B Cells

The lymphoid-myeloid transdifferentiation potentials of members of the C/EBP family (C/EBPα, β, δ, and ε) were compared in v-Abl-immortalized primary B cells. Conversion of B cells to macrophages was readily induced by the ectopic expression of any C/EBP, and enhanced by endogenous C/EBPα and β activation. High transgene expression of C/EBPβ or C/EBPε, but not of C/EBPα or C/EBPδ, also induced the formation of granulocytes. Granulocytes and macrophages emerged in a mutually exclusive manner. C/EBPβ-expressing B cells produced granulocyte-macrophage progenitor (GMP)-like progenitors when subjected to selective pressure to eliminate lymphoid cells. The GMP-like progenitors remained self-renewing and cytokine-independent, and continuously produced macrophages and granulocytes. In addition to their suitability to study myelomonocytic lineage bifurcation, lineage-switched GMP-like progenitors could reflect the features of the lympho-myeloid lineage switch observed in leukemic progression.

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Transactivating C/EBP family members transdifferentiate B cells to myeloid cells

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C/EBPβ or C/EBPε transdifferentiate B cells to macrophages and granulocytes

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Transgene dosage determines granulocyte versus macrophage cell-type outcome

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C/EBP-mediated B cell conversion elicits GMP-like potential

Systemic inoculation of Escherichia coli causes emergency myelopoiesis in zebrafish larval caudal hematopoietic tissue

Emergency granulopoiesis occurs in response to severe microbial infection. However, whether and how other blood components, particularly monocytes/macrophages and their progenitors, including hematopoietic stem/progenitor cells (HSPCs), participate in the process and the underlying molecular mechanisms remain unknown. In this study, we challenged zebrafish larvae via direct injection of Escherichia coli into the bloodstream, which resulted in systemic inoculation with this microbe. The reaction of hematopoietic cells, including HSPCs, in the caudal hematopoietic tissue was carefully analysed. Both macrophages and neutrophils clearly expanded following the challenge. Thus, emergency myelopoiesis, including monopoiesis and granulopoiesis, occurred following systemic bacterial infection. The HSPC reaction was dependent on the bacterial burden, manifesting as a slight increase under low burden, but an obvious reduction following the administration of an excessive volume of bacteria. Pu.1 was important for the effective elimination of the microbes to prevent excessive HSPC apoptosis in response to stress. Moreover, Pu.1 played different roles in steady and emergency monopoiesis. Although Pu.1 was essential for normal macrophage development, it played suppressive roles in emergency monopoiesis. Overall, our study established a systemic bacterial infection model that led to emergency myelopoiesis, thereby improving our understanding of the function of Pu.1 in this scenario.

Myeloid Growth Factors Promote Resistance to Mycobacterial Infection by Curtailing Granuloma Necrosis through Macrophage Replenishment

The mycobacterial ESX-1 virulence locus accelerates macrophage recruitment to the forming tuberculous granuloma. Newly recruited macrophages phagocytose previously infected apoptotic macrophages to become new bacterial growth niches. Granuloma macrophages can then necrose, releasing mycobacteria into the extracellular milieu, which potentiates their growth even further. Using zebrafish with genetic or pharmacologically induced macrophage deficiencies, we find that global macrophage deficits increase susceptibility to mycobacterial infection by accelerating granuloma necrosis. This is because reduction in the macrophage supply below a critical threshold decreases granuloma macrophage replenishment to the point where apoptotic infected macrophages, failing to get engulfed, necrose. Reducing macrophage demand by removing bacterial ESX-1 offsets the susceptibility of macrophage deficits. Conversely, increasing macrophage supply in wild-type fish by overexpressing myeloid growth factors induces resistance by curtailing necrosis. These findings may explain the susceptibility of humans with mononuclear cytopenias to mycobacterial infections and highlight the therapeutic potential of myeloid growth factors in tuberculosis.

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Myeloid deficiencies increase innate immune susceptibility to mycobacterial infection

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Depletion of macrophage supply below a critical threshold hastens granuloma necrosis

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Increasing macrophage supply to the granuloma delays granuloma necrosis

Reprogramming of human peripheral blood monocytes to erythroid lineage by blocking of the PU-1 gene expression

In hematopoietic system development, PU.1 and GATA-1 as lineage-specific transcription factors (TF) are expressed in common myeloid progenitors. The cross antagonism between them ascertains gene expression programs of monocytic and erythroid cells, respectively. This concept in transdifferentiation approaches has not been well considered yet, especially in intralineage conversion systems. To demonstrate whether PU.1 suppression induces monocyte lineage conversion into red blood cells, a combination of three PU.1-specific siRNAs was implemented to knock down PU.1 gene expression and generate the balance in favor of GATA-1 expression to induce erythroid differentiation. For this purpose, monocytes were isolated from human peripheral blood and transfected by PU.1 siRNAs. In transfected monocytes, the rate of PU.1 expression in mRNA level was significantly decreased until 0.38 ± 0.118 when compared to untreated monocytes at 72 h (p value ≤0.05) which resulted in significant overexpression of GATA1 of 16.1 ± 0.343-fold compared to the untreated group (p value ≤0.01). Subsequently, overexpression of hemoglobin (α 13.26 ± 1.34-fold; p value≤0.0001) and β-globin (37.55 ± 16.56-fold; p value≤0.0001) was observed when compared to control groups. The results of western immunoblotting confirm those findings too. While, reduced expression of monocyte, CD14 gene, was observed in qRT-PCR and flow cytometry results. Our results suggest that manipulating the ratio of the two TFs in bifurcation differentiation pathways via applying siRNA technology can possibly change the cells' fate as a safe way for therapeutics application.

The chromatin remodeler Brg1 activates enhancer repertoires to establish B cell identity and modulate cell growth

Early B cell development is orchestrated by the combined activities of the transcriptional regulators E2A, EBF1, Foxo1 and Ikaros. However, how the genome-wide binding patterns of these regulators are modulated during B lineage development remains to be determined. Here we found that in lymphoid progenitor cells, the chromatin remodeler Brg1 specified the B cell fate. In committed pro-B cells, Brg1 regulated contraction of the locus encoding the immunoglobulin heavy chain (Igh) and controlled expression of the gene encoding the transcription factor c-Myc (Myc) to modulate the expression of genes encoding products that regulate ribosome biogenesis. In committed pro-B cells, Brg1 suppressed a pre-B lineage-specific pattern of gene expression. Finally, we found that Brg1 acted mechanistically to establish B cell fate and modulate cell growth by facilitating access of lineage-specific transcription factors to enhancer repertoires.

Megakaryopoiesis and platelet production: insight into hematopoietic stem cell proliferation and differentiation

Hematopoietic stem cells (HSCs) undergo successive lineage commitment steps to generate megakaryocytes (MKs) in a process referred to as megakaryopoiesis. MKs undergo a unique differentiation process involving endomitosis to eventually produce platelets. Many transcription factors participate in the regulation of this complex progress. Chemokines and other factors in the microenvironment where megakaryopoiesis and platelet production occur play vital roles in the regulation of HSC lineage commitment and MK maturation; among these factors, thrombopoietin (TPO) is the most important. Endomitosis is a vital process of MK maturation, and granules that are formed in MKs are important for platelet function. Proplatelets are firstly generated from mature MKs and then become platelets. The proplatelet production process was verified by novel studies that revealed that the mechanism is partially regulated by the invaginated membrane system (IMS), microtubules and Rho GTPases. The extracellular matrices (ECMs) and shear stress also affect and regulate the process while the mature MKs migrate from the marrow to the sub-endothelium region near the venous sinusoids leading to the release of platelets into the circulation. This review describes the entire process of megakaryopoiesis in detail, illustrates both the transcriptional and microenvironmental regulation of MKs and provides insight into platelet biogenesis.

Mechanisms of in vivo binding site selection of the hematopoietic master transcription factor PU.1

The transcription factor PU.1 is crucial for the development of many hematopoietic lineages and its binding patterns significantly change during differentiation processes. However, the 'rules' for binding or not-binding of potential binding sites are only partially understood. To unveil basic characteristics of PU.1 binding site selection in different cell types, we studied the binding properties of PU.1 during human macrophage differentiation. Using in vivo and in vitro binding assays, as well as computational prediction, we show that PU.1 selects its binding sites primarily based on sequence affinity, which results in the frequent autonomous binding of high affinity sites in DNase I inaccessible regions (25-45% of all occupied sites). Increasing PU.1 concentrations and the availability of cooperative transcription factor interactions during lineage differentiation both decrease affinity thresholds for in vivo binding and fine-tune cell type-specific PU.1 binding, which seems to be largely independent of DNA methylation. Occupied sites were predominantly detected in active chromatin domains, which are characterized by higher densities of PU.1 recognition sites and neighboring motifs for cooperative transcription factors. Our study supports a model of PU.1 binding control that involves motif-binding affinity, PU.1 concentration, cooperativeness with neighboring transcription factor sites and chromatin domain accessibility, which likely applies to all PU.1 expressing cells.

Cotargeting signaling pathways driving survival and cell cycle circumvents resistance to Kit inhibitors in leukemia

Oncogenic mutations leading to persistent kinase activities are associated with malignancies. Therefore, deciphering the signaling networks downstream of these oncogenic stimuli remains a challenge to gather insights into targeted therapy. To elucidate the biochemical networks connecting the Kit mutant to leukemogenesis, in the present study, we performed a global profiling of tyrosine-phosphorylated proteins from mutant Kit-driven murine leukemia proerythroblasts and identified Shp2 and Stat5 as proximal effectors of Kit. Shp2 or Stat5 gene depletion by sh-RNA, combined with pharmacologic inhibition of PI3kinase or Mek/Erk activities, revealed 2 distinct and independent signaling pathways contributing to malignancy. We demonstrate that cell survival is driven by the Kit/Shp2/Ras/Mek/Erk1/2 pathway, whereas the G(1)/S transition during the cell cycle is accelerated by both the Kit/Stat5 and Kit/PI3K/Akt pathways. The combined use of the clinically relevant drugs NVP-BEZ235, which targets the cell cycle, and Obatoclax, which targets survival, demonstrated synergistic effects to inhibit leukemia cell growth. This synergy was confirmed with a human mast leukemia cell line (HMC-1.2) that expresses mutant Kit. The results of the present study using liquid chromatography/tandem mass spectrometry analysis have elucidated signaling networks downstream of an oncogenic kinase, providing a molecular rationale for pathway-targeted therapy to treat cancer cells refractory to tyrosine kinase inhibitors.

Rantes/Ccl5 influences hematopoietic stem cell subtypes and causes myeloid skewing

HSCs undergo dramatic changes with aging. An increase in absolute numbers of HSCs along with a functional deficit in reconstitution potential and a shift toward production of myeloid cells are the hallmarks of murine hematopoietic aging. Here, we show that high levels of the inflammatory cytokine Rantes are found in the aging stem cell milieu. Forced overproduction of Rantes by retroviral expression in BM progenitors resulted in a deficit of T-cell output, and brief ex vivo exposure of HSCs to Rantes resulted in a decrease in T-cell progeny concomitant with an increase in myeloid progenitors. In contrast, Rantes knockout (KO) animals exhibit a decrease in myeloid-biased HSCs and myeloid progenitors and an increase in T cells and lymphoid-biased HSCs. KO HSCs retained their HSC subtype distribution and they produced more lymphoid-biased HSCs in transplantations. Rantes deficiency also resulted in a decreased mammalian target of rapamycin (mTOR) activity in KLS cells. In a heterochronic transplantation setting, we further show that aged HSCs placed in a young environment generate less myeloid cells. These data establish a critical role for environmental factors in the establishment of the aged-associated myeloid skewing phenotype, which may contribute to age-associated immune deficiency.

The prolyl isomerase Pin1 modulates development of CD8+ cDC in mice

Background

Pin1 has previously been described to regulate cells that participate in both innate and adaptive immunity. Thus far, however, no role for Pin1 has been described in modulating conventional dendritic cells, innate antigen presenting cells that potently activate naïve T cells, thereby bridging innate and adaptive immune responses.

Methodology/Principal Findings

When challenged with LPS, Pin1-null mice failed to accumulate spleen conventional dendritic cells (cDC). Analysis of steady-state spleen DC populations revealed that Pin1-null mice had fewer CD8+ cDC. This defect was recapitulated by culturing Pin1-null bone marrow with the DC-instructive cytokine Flt3 Ligand. Additionally, injection of Flt3 Ligand for 9 days failed to induce robust expansion of CD8+ cDC in Pin1-null mice. Upon infection with Listeria monocytogenes, Pin1-null mice were defective in stimulating proliferation of adoptively transferred WT CD8+ T cells, suggesting that decreases in Pin1 null CD8+ cDC may affect T cell responses to infection in vivo. Finally, upon analyzing expression of proteins involved in DC development, elevated expression of PU.1 was detected in Pin1-null cells, which resulted from an increase in PU.1 protein half-life.

Conclusions/Significance

We have identified a novel role for Pin1 as a modulator of CD8+ cDC development. Consistent with reduced numbers of CD8+ cDC in Pin1-null mice, we find that the absence of Pin1 impairs CD8+ T cell proliferation in response to infection with Listeria monocytogenes. These data suggest that, via regulation of CD8+ cDC production, Pin1 may serve as an important modulator of adaptive immunity.

PU.1 is a major transcriptional activator of the tumour suppressor gene LIMD1

LIMD1 is a tumour suppressor gene (TSG) down regulated in ∼80% of lung cancers with loss also demonstrated in breast and head and neck squamous cell carcinomas. LIMD1 is also a candidate TSG in childhood acute lymphoblastic leukaemia. Mechanistically, LIMD1 interacts with pRB, repressing E2F-driven transcription as well as being a critical component of microRNA-mediated gene silencing. In this study we show a CpG island within the LIMD1 promoter contains a conserved binding motif for the transcription factor PU.1. Mutation of the PU.1 consensus reduced promoter driven transcription by 90%. ChIP and EMSA analysis demonstrated that PU.1 specifically binds to the LIMD1 promoter. siRNA depletion of PU.1 significantly reduced endogenous LIMD1 expression, demonstrating that PU.1 is a major transcriptional activator of LIMD1.

Irf8 regulates macrophage versus neutrophil fate during zebrafish primitive myelopoiesis

In vertebrates, myeloid cells comprise polymorphonuclear and mononuclear lineages that arise from 2 successive waves of development: a transitory primitive wave giving rise to limited myeloid cells during embryonic stage and a definitive wave capable of producing myeloid cells throughout the fetal and adult life. One key unresolved question is what factors dictate polymorphonuclear versus mononuclear lineage fates during myelopoiesis. Here we show that during zebrafish embryogenesis interferon regulatory factor-8 (irf8) is expressed specifically in macrophages but not neutrophils. Suppression of Irf8 function in zebrafish causes a depletion of macrophages and an enhanced output of neutrophils but does not affect the overall number, proliferation, and survival of primitive myeloid cells. These data indicate that the skewed myeloid lineage development in Irf8 knockdown embryos results from a cell-fate switching. Such a conclusion is further supported by the observation showing that overexpression of Irf8 promotes macrophage formation at the expense of neutrophil development. Genetic epistasis analysis reveals that Irf8 acts downstream of Pu.1 but is insufficient to promote macrophage development in the absence of Pu.1. Our findings demonstrate that Irf8 is a critical determinant for neutrophil versus macrophage fate choice during zebrafish primitive myelopoiesis.

Cell-specific determinants of peroxisome proliferator-activated receptor gamma function in adipocytes and macrophages

The nuclear receptor peroxisome proliferator activator receptor gamma (PPARgamma) is the target of antidiabetic thiazolidinedione drugs, which improve insulin resistance but have side effects that limit widespread use. PPARgamma is required for adipocyte differentiation, but it is also expressed in other cell types, notably macrophages, where it influences atherosclerosis, insulin resistance, and inflammation. A central question is whether PPARgamma binding in macrophages occurs at genomic locations the same as or different from those in adipocytes. Here, utilizing chromatin immunoprecipitation and high-throughput sequencing (ChIP-seq), we demonstrate that PPARgamma cistromes in mouse adipocytes and macrophages are predominantly cell type specific. In thioglycolate-elicited macrophages, PPARgamma colocalizes with the hematopoietic transcription factor PU.1 in areas of open chromatin and histone acetylation, near a distinct set of immune genes in addition to a number of metabolic genes shared with adipocytes. In adipocytes, the macrophage-unique binding regions are marked with repressive histone modifications, typically associated with local chromatin compaction and gene silencing. PPARgamma, when introduced into preadipocytes, bound only to regions depleted of repressive histone modifications, where it increased DNA accessibility, enhanced histone acetylation, and induced gene expression. Thus, the cell specificity of PPARgamma function is regulated by cell-specific transcription factors, chromatin accessibility, and histone marks. Our data support the existence of an epigenomic hierarchy in which PPARgamma binding to cell-specific sites not marked by repressive marks opens chromatin and leads to local activation marks, including histone acetylation.

Heterozygous deletion of the PU.1 locus in human AML

The transcription factor PU.1 is essential for myeloid development. Targeted disruption of an upstream regulatory element (URE) decreases PU.1 expression by 80% and leads to acute myeloid leukemia (AML) in mice. Here, we sequenced the URE sequences of PU.1 in 120 AML patients. Four polymorphisms (single nucleotide polymorphisms [SNPs]) in the URE were observed, with homozygosity in all SNPs in 37 patients. Among them, we compared samples at diagnosis and remission, and one patient with cytogenetically normal acute myeloid leukemia M2 was identified with heterozygosity in 3 of the SNPs in the URE at remission. Loss of heterozygosity was further found in this patient at 2 polymorphic sites in the 5′ promoter region and in 2 intronic sites flanking exon 4, thus suggesting loss of heterozygosity covering at least 40 kb of the PU.1 locus. Consistently, PU.1 expression in this patient was markedly reduced. Our study suggests that heterozygous deletion of the PU.1 locus can be associated with human AML.

PU.1 activation relieves GATA-1-mediated repression of Cebpa and Cbfb during leukemia differentiation

Hematopoietic transcription factors GATA-1 and PU.1 bind each other on DNA to block transcriptional programs of undesired lineage during hematopoietic commitment. Murine erythroleukemia (MEL) cells that coexpress GATA-1 and PU.1 are blocked at the blast stage but respond to molecular removal (downregulation) of PU.1 or addition (upregulation) of GATA-1 by inducing terminal erythroid differentiation. To test whether GATA-1 blocks PU.1 in MEL cells, we have conditionally activated a transgenic PU.1 protein fused with the estrogen receptor ligand-binding domain (PUER), resulting in activation of a myeloid transcriptional program. Gene expression arrays identified components of the PU.1-dependent transcriptome negatively regulated by GATA-1 in MEL cells, including CCAAT/enhancer binding protein alpha (Cebpa) and core-binding factor, beta subunit (Cbfb), which encode two key hematopoietic transcription factors. Inhibition of GATA-1 by small interfering RNA resulted in derepression of PU.1 target genes. Chromatin immunoprecipitation and reporter assays identified PU.1 motif sequences near Cebpa and Cbfb that are co-occupied by PU.1 and GATA-1 in the leukemic blasts. Significant derepression of Cebpa and Cbfb is achieved in MEL cells by either activation of PU.1 or knockdown of GATA-1. Furthermore, transcriptional regulation of these loci by manipulating the levels of PU.1 and GATA-1 involves quantitative increases in a transcriptionally active chromatin mark: acetylation of histone H3K9. Collectively, we show that either activation of PU.1 or inhibition of GATA-1 efficiently reverses the transcriptional block imposed by GATA-1 and leads to the activation of a myeloid transcriptional program directed by PU.1.

NDRG1 contributes to retinoic acid-induced differentiation of leukemic cells

N-Myc downstream-regulated gene 1 (NDRG1) protein has been shown to be up-regulated during leukemic cell differentiation induced by some differentiation-inducing agents such as all-trans retinoic acid (ATRA). However, the potential role of up-regulated NDRG1 in the event is greatly unknown. In this work, we show that inducible NDRG1 expression can drive leukemic U937 cells to undergo differentiation, while the knock-down of NDRG1 expression by specific small interfering RNA significantly antagonizes ATRA-induced differentiation of leukemic cells, proposing the role of NDRG1 in leukemic cell differentiation. Furthermore, our work shows that CCAAT/enhancer-binding protein beta (C/EBPbeta) and PU.1, which are important hematopoiesis-related transcription factors, may act as downstream effectors of NDRG1 in leukemic cell differentiation. Taking together, this study provides direct evidence for the role of NDRG1 protein in myeloid leukemic cell differentiation.

Disruption of NAD(P)H:quinone oxidoreductase 1 gene in mice leads to radiation-induced myeloproliferative disease

NAD(P)H:quinone oxidoreductase 1 null (NQO1(-/-)) mice exposed to 3 Gy of gamma-radiation showed an increase in neutrophils, bone marrow hypercellularity, and enlarged lymph nodes and spleen. The spleen showed disrupted follicular structure, loss of red pulp, and granulocyte and megakarocyte invasion. Blood and histologic analysis did not show any sign of infection in mice. These results suggested that exposure of NQO1(-/-) mice to gamma-radiation led to myeloproliferative disease. Radiation-induced myeloproliferative disease was observed in 74% of NQO1(-/-) mice as compared with none in wild-type (WT) mice. NQO1(-/-) mice exposed to gamma-radiation also showed lymphoma tissues (32%) and lung adenocarcinoma (84%). In contrast, only 11% WT mice showed lymphoma and none showed lung adenocarcinoma. Exposure of NQO1(-/-) mice to gamma-radiation resulted in reduced apoptosis in granulocytes and lack of induction of p53, p21, and Bax. NQO1(-/-) mice also showed increased expression of myeloid differentiation factors CCAAT/enhancer binding protein alpha (C/EBPalpha) and Pu.1. Intriguingly, exposure of NQO1(-/-) mice to gamma-radiation failed to induce C/EBPalpha and Pu.1, as was observed in WT mice. These results suggest that decreased p53/apoptosis and increased Pu.1 and C/EBPalpha led to myeloid hyperplasia in NQO1(-/-) mice. The lack of induction of apoptosis and differentiation contributed to radiation-induced myeloproliferative disease in NQO1(-/-) mice.

Monocytes from cystic fibrosis patients are locked in an LPS tolerance state: down-regulation of TREM-1 as putative underlying mechanism

Cystic Fibrosis (CF) is an inherited pleiotropic disease that results from abnormalities in the gene that codes for the chloride channel, Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). CF patients are frequently colonized by several pathogens, but the mechanisms that allow colonization in spite of apparently functional immune systems are incompletely understood. In this paper we show that blood peripheral monocytes isolated from CF patients are found in an endotoxin tolerance state, yet this is not due to a deficient TLR activation. On the other hand, levels of the amplifier of inflammatory responses, TREM-1 (Triggering Receptor Expressed on Myeloid cells), are notably down-regulated in monocytes from patients, in comparison to those extracted from healthy volunteers. Furthermore, the soluble form of TREM-1 (sTREM-1) was not detected in the sera of patients. Additionally, and in strict contrast to patients who suffer from Chronic Obstructive Pulmonary Disease (COPD), CF monocytes challenged ex vivo with LPS neither up-regulated membrane-anchored TREM-1 nor sTREM-1. Finally, similar levels of PGE₂ expression and p65 translocation into the nucleus were found in both patients and healthy volunteers, thus suggesting that TREM-1 regulation is neither controlled by PGE₂ levels nor by p65 activation in this case. However, PU.1 translocation into the nucleus was significantly higher in CF monocytes than in controls, suggesting a role for this transcription factor in the control of TREM-1 expression. We conclude that down-regulation of TREM-1 expression in cystic fibrosis patients is at least partly responsible for the endotoxin tolerance state in which their monocytes are locked.

Evolving views on the genealogy of B cells

Many fundamental concepts about immune system development have changed substantially in the past few years, and rapid advances with animal models are presenting prospects for further discovery. However, continued progress requires a clearer understanding of the relationships between haematopoietic stem cells and the progenitors that replenish each type of lymphocyte pool. Blood-cell formation has traditionally been described in terms of discrete developmental branch points, and a single route is given for each major cell type. As we discuss in this Review, recent findings suggest that the process of B-cell formation is much more dynamic.