EBF1-deficient bone marrow stroma elicits persistent changes in HSC potential

The role of the haematopoietic stem cell niche in development and ageing

Blood production depends on rare haematopoietic stem cells (HSCs) and haematopoietic stem and progenitor cells (HSPCs) that ultimately take up residence in the bone marrow during development. HSPCs and HSCs are subject to extrinsic regulation by the bone marrow microenvironment, or niche. Studying the interactions between HSCs and their niche is critical for improving ex vivo culturing conditions and genetic manipulation of HSCs, which is pivotal for improving autologous HSC therapies and transplantations. Additionally, understanding how the complex molecular network in the bone marrow is altered during ageing is paramount for developing novel therapeutics for ageing-related haematopoietic disorders. HSCs are unique amongst stem and progenitor cell pools in that they engage with multiple physically distinct niches during their ontogeny. HSCs are specified from haemogenic endothelium in the aorta, migrate to the fetal liver and, ultimately, colonize their final niche in the bone marrow. Recent studies employing single-cell transcriptomics and microscopy have identified novel cellular interactions that govern HSC specification and engagement with their niches throughout ontogeny. New lineage-tracing models and microscopy tools have raised questions about the numbers of HSCs specified, as well as the functional consequences of HSCs interacting with each developmental niche. Advances have also been made in understanding how these niches are modified and perturbed during ageing, and the role of these altered interactions in haematopoietic diseases. In this Review, we discuss these new findings and highlight the questions that remain to be explored.

The ATF4-RPS19BP1 axis modulates ribosome biogenesis to promote erythropoiesis

ABSTRACT

Hematopoietic differentiation is controlled by intrinsic regulators and the extrinsic hematopoietic niche. Activating transcription factor 4 (ATF4) plays a crucial role in the function of fetal and adult hematopoietic stem cell maintenance. However, the precise function of ATF4 in the bone marrow (BM) niche and the mechanism by which ATF4 regulates adult hematopoiesis remain largely unknown. Here, we used 4 cell-type-specific mouse Cre lines to achieve conditional knockout of Atf4 in Cdh5+ endothelial cells, Prx1+ BM stromal cells, Osx+ osteoprogenitor cells, and Mx1+ hematopoietic cells and uncovered the role of Atf4 in niche cells and hematopoiesis. Intriguingly, depletion of Atf4 in niche cells did not affect hematopoiesis; however, Atf4-deficient hematopoietic cells exhibited erythroid differentiation defects, leading to hypoplastic anemia. Mechanistically, ATF4 mediated direct regulation of Rps19bp1 transcription, which is, in turn, involved in 40 S ribosomal subunit assembly to coordinate ribosome biogenesis and promote erythropoiesis. Finally, we demonstrate that under conditions of 5-fluorouracil-induced stress, Atf4 depletion impedes the recovery of hematopoietic lineages, which requires efficient ribosome biogenesis. Taken together, our findings highlight the indispensable role of the ATF4-RPS19BP1 axis in the regulation of erythropoiesis.

Hematopoietic stem cell niche generation and maintenance are distinguishable by an epitranscriptomic program

The niche is typically considered as a pre-established structure sustaining stem cells. Therefore, the regulation of its formation remains largely unexplored. Whether distinct molecular mechanisms control the establishment versus maintenance of a stem cell niche is unknown. To address this, we compared perinatal and adult bone marrow mesenchymal stromal cells (MSCs), a key component of the hematopoietic stem cell (HSC) niche. MSCs exhibited enrichment in genes mediating m6A mRNA methylation at the perinatal stage and downregulated the expression of Mettl3, the m6A methyltransferase, shortly after birth. Deletion of Mettl3 from developing MSCs but not osteoblasts led to excessive osteogenic differentiation and a severe HSC niche formation defect, which was significantly rescued by deletion of Klf2, an m6A target. In contrast, deletion of Mettl3 from MSCs postnatally did not affect HSC niche. Stem cell niche generation and maintenance thus depend on divergent molecular mechanisms, which may be exploited for regenerative medicine.

Early B-Cell Factor 1: An Archetype for a Lineage-Restricted Transcription Factor Linking Development to Disease

The development of highly specialized blood cells from hematopoietic stem cells (HSCs) in the bone marrow (BM) is dependent upon a stringently orchestrated network of stage- and lineage-restricted transcription factors (TFs). Thus, the same stem cell can give rise to various types of differentiated blood cells. One of the key regulators of B-lymphocyte development is early B-cell factor 1 (EBF1). This TF belongs to a small, but evolutionary conserved, family of proteins that harbor a Zn-coordinating motif and an IPT/TIG (immunoglobulin-like, plexins, transcription factors/transcription factor immunoglobulin) domain, creating a unique DNA-binding domain (DBD). EBF proteins play critical roles in diverse developmental processes, including body segmentation in the Drosophila melanogaster embryo, and retina formation in mice. While several EBF family members are expressed in neuronal cells, adipocytes, and BM stroma cells, only B-lymphoid cells express EBF1. In the absence of EBF1, hematopoietic progenitor cells (HPCs) fail to activate the B-lineage program. This has been attributed to the ability of EBF1 to act as a pioneering factor with the ability to remodel chromatin, thereby creating a B-lymphoid-specific epigenetic landscape. Conditional inactivation of the Ebf1 gene in B-lineage cells has revealed additional functions of this protein in relation to the control of proliferation and apoptosis. This may explain why EBF1 is frequently targeted by mutations in human leukemia cases. This chapter provides an overview of the biochemical and functional properties of the EBF family proteins, with a focus on the roles of EBF1 in normal and malignant B-lymphocyte development.

Mesenchymal stromal cells, metabolism, and mitochondrial transfer in bone marrow normal and malignant hematopoiesis

Hematopoietic stem cell (HSC) transplantation-based treatments are in different phases of clinical development, ranging from current therapies to a promise in the repair and regeneration of diseased tissues and organs. Mesenchymal stromal/stem cells (MSCs), which are fibroblast-like heterogeneous progenitors with multilineage differentiation (osteogenic, chondrogenic, and adipogenic) and self-renewal potential, and exist in the bone marrow (BM), adipose, and synovium, among other tissues, represent one of the most widely used sources of stem cells in regenerative medicine. MSCs derived from bone marrow (BM-MSCs) exhibit a variety of traits, including the potential to drive HSC fate and anti-inflammatory and immunosuppressive capabilities via paracrine activities and interactions with the innate and adaptive immune systems. The role of BM-MSC-derived adipocytes is more controversial and may act as positive or negative regulators of benign or malignant hematopoiesis based on their anatomical location and functional crosstalk with surrounding cells in the BM microenvironment. This review highlights the most recent clinical and pre-clinical findings on how BM-MSCs interact with the surrounding HSCs, progenitors, and immune cells, and address some recent insights on the mechanisms that mediate MSCs and adipocyte metabolic control through a metabolic crosstalk between BM microenvironment cells and intercellular mitochondrial transfer in normal and malignant hematopoiesis.

Single-cell transcriptional profiling reveals immunomodulatory properties of stromal and epithelial cells in periodontal immune milieu with diabetes in rats

Periodontitis is the sixth major complication of diabetes. Gingiva, as an important component of periodontal tissues, serves as the first defense barrier against infectious stimuli. However, relatively little is known about cellular heterogeneity and cell-specific changes in gingiva in response to diabetes-associated periodontitis. To characterize molecular changes linking diabetes with periodontitis, we profiled single-cell transcriptome analyses of a total of 45,259 cells from rat gingiva with periodontitis under normoglycemic and diabetic condition. The single-cell profiling revealed that stromal and epithelial cells of gingiva contained inflammation-related subclusters enriched in functions of immune cell recruitment. Compared to normoglycemic condition, diabetes led to a reduction in epithelial basal cells, fibroblasts and smooth muscle cells in gingiva with periodontitis. Analysis of differentially expressed genes indicated that stromal and epithelial populations were reprogrammed towards pro-inflammatory phenotypes promoting immune cell recruitment in diabetes-related periodontitis. In aspect of immune cells, diabetes prominently enhanced neutrophil and M1 macrophage infiltration in periodontitis lesions. Cell-cell communications revealed enhanced crosstalk between stromal/epithelial cells and immune cells mediating by chemokine/chemokine receptor interplay in diabetes-associated periodontitis. Our findings deconvolved cellular heterogeneity of rat gingiva associated with periodontitis and diabetes, uncovered altered immune milieu caused by the disease, and revealed immunomodulatory functions of stromal and epithelial cells in gingival immune niche. The present study improves the understanding of the link between the diabetes and periodontitis and helps in formulating precise therapeutic strategies for diabetes-enhanced periodontitis.

Secondary bile acids function through the vitamin D receptor in myeloid progenitors to promote myelopoiesis

Metabolic products of the microbiota can alter hematopoiesis. However, the contribution and site of action of bile acids is poorly understood. Here, we demonstrate that the secondary bile acids, deoxycholic acid (DCA) and lithocholic acid (LCA), increase bone marrow myelopoiesis. Treatment of bone marrow cells with DCA and LCA preferentially expanded immunophenotypic and functional colony-forming unit-granulocyte and macrophage (CFU-GM) granulocyte-monocyte progenitors (GMPs). DCA treatment of sorted hematopoietic stem and progenitor cells (HSPCs) increased CFU-GMs, indicating that direct exposure of HSPCs to DCA sufficed to increase GMPs. The vitamin D receptor (VDR) was required for the DCA-induced increase in CFU-GMs and GMPs. Single-cell RNA sequencing revealed that DCA significantly upregulated genes associated with myeloid differentiation and proliferation in GMPs. The action of DCA on HSPCs to expand GMPs in a VDR-dependent manner suggests microbiome-host interactions could directly affect bone marrow hematopoiesis and potentially the severity of infectious and inflammatory disease.

Single-cell RNA sequencing of human non-hematopoietic bone marrow cells reveals a unique set of inter-species conserved biomarkers for native mesenchymal stromal cells

BACKGROUND

Native bone marrow (BM) mesenchymal stem/stromal cells (BM-MSCs) participate in generating and shaping the skeleton and BM throughout the lifespan. Moreover, BM-MSCs regulate hematopoiesis by contributing to the hematopoietic stem cell niche in providing critical cytokines, chemokines and extracellular matrix components. However, BM-MSCs contain a heterogeneous cell population that remains ill-defined. Although studies on the taxonomy of native BM-MSCs in mice have just started to emerge, the taxonomy of native human BM-MSCs remains unelucidated.

METHODS

By using single-cell RNA sequencing (scRNA-seq), we aimed to define a proper taxonomy for native human BM non-hematopoietic subsets including endothelial cells (ECs) and mural cells (MCs) but with a focal point on MSCs. To this end, transcriptomic scRNA-seq data were generated from 5 distinct BM donors and were analyzed together with other transcriptomic data and with computational biology analyses at different levels to identify, characterize and classify distinct native cell subsets with relevant biomarkers.

RESULTS

We could ascribe novel specific biomarkers to ECs, MCs and MSCs. Unlike ECs and MCs, MSCs exhibited an adipogenic transcriptomic pattern while co-expressing genes related to hematopoiesis support and multilineage commitment potential. Furthermore, by a comparative analysis of scRNA-seq of BM cells from humans and mice, we identified core genes conserved in both species. Notably, we identified MARCKS, CXCL12, PDGFRA, and LEPR together with adipogenic factors as archetypal biomarkers of native MSCs within BM. In addition, our data suggest some complex gene nodes regulating critical biological functions of native BM-MSCs together with a preferential commitment toward an adipocyte lineage.

CONCLUSIONS

Overall, our taxonomy for native BM non-hematopoietic compartment provides an explicit depiction of gene expression in human ECs, MCs and MSCs at single-cell resolution. This analysis helps enhance our understanding of the phenotype and the complexity of biological functions of native human BM-MSCs.

Osteoprogenitor-GMP crosstalk underpins solid tumor-induced systemic immunosuppression and persists after tumor removal

Remote tumors disrupt the bone marrow (BM) ecosystem (BME), eliciting the overproduction of BM-derived immunosuppressive cells. However, the underlying mechanisms remain poorly understood. Herein, we characterized breast and lung cancer-induced BME shifts pre- and post-tumor removal. Remote tumors progressively lead to osteoprogenitor (OP) expansion, hematopoietic stem cell dislocation, and CD41- granulocyte-monocyte progenitor (GMP) aggregation. The tumor-entrained BME is characterized by co-localization between CD41- GMPs and OPs. OP ablation abolishes this effect and diminishes abnormal myeloid overproduction. Mechanistically, HTRA1 carried by tumor-derived small extracellular vesicles upregulates MMP-13 in OPs, which in turn induces the alterations in the hematopoietic program. Importantly, these effects persist post-surgery and continue to impair anti-tumor immunity. Conditional knockout or inhibition of MMP-13 accelerates immune reinstatement and restores the efficacies of immunotherapies. Therefore, tumor-induced systemic effects are initiated by OP-GMP crosstalk that outlasts tumor burden, and additional treatment is required to reverse these effects for optimal therapeutic efficacy.

Mesenchymal stem cells: Emerging concepts and recent advances in their roles in organismal homeostasis and therapy

Stem cells play a crucial role in re-establishing homeostasis in the body, and the search for mechanisms by which they interact with the host to exert their therapeutic effects remains a key question currently being addressed. Considering their significant regenerative/therapeutic potential, research on mesenchymal stem cells (MSCs) has experienced an unprecedented advance in recent years, becoming the focus of extensive works worldwide to develop cell-based approaches for a variety of diseases. Initial evidence for the effectiveness of MSCs therapy comes from the restoration of dynamic microenvironmental homeostasis and endogenous stem cell function in recipient tissues by systemically delivered MSCs. The specific mechanisms by which the effects are exerted remain to be investigated in depth. Importantly, the profound cell-host interplay leaves persistent therapeutic benefits that remain detectable long after the disappearance of transplanted MSCs. In this review, we summarize recent advances on the role of MSCs in multiple disease models, provide insights into the mechanisms by which MSCs interact with endogenous stem cells to exert therapeutic effects, and refine the interconnections between MSCs and cells fused to damaged sites or differentiated into functional cells early in therapy.

Trained immunity and epigenetic memory in long-term self-renewing hematopoietic cells

Immunologic memory is a feature typically ascribed to the adaptive arm of the immune system. However, recent studies have demonstrated that hematopoietic stem cells (HSCs) and innate immune cells such as monocytes and macrophages can gain epigenetic signatures to enhance their response in the context of reinfection. This suggests the presence of long-term memory, a phenomenon referred to as trained immunity. Trained immunity in HSCs can occur via changes in the epigenetic landscape and enhanced chromatin accessibility in lineage-specific genes, as well as through metabolic alterations. These changes can lead to a skewing in lineage bias, particularly enhanced myelopoiesis and the generation of epigenetically modified innate immune cells that provide better protection against pathogens on secondary infection. Here, we summarize recent advancements in trained immunity and epigenetic memory formation in HSCs and self-renewing alveolar macrophages, which was the focus of the Spring 2022 International Society for Experimental Hematology (ISEH) webinar.

IL-13/IL-4 signaling contributes to fibrotic progression of the myeloproliferative neoplasms

Myelofibrosis (MF) is a disease associated with high unmet medical needs because allogeneic stem cell transplantation is not an option for most patients, and JAK inhibitors are generally effective for only 2 to 3 years and do not delay disease progression. MF is characterized by dysplastic megakaryocytic hyperplasia and progression to fulminant disease, which is associated with progressively increasing marrow fibrosis. Despite evidence that the inflammatory milieu in MF contributes to disease progression, the specific factors that promote megakaryocyte growth are poorly understood. Here, we analyzed changes in the cytokine profiles of MF mouse models before and after the development of fibrosis, coupled with the analysis of bone marrow populations using single-cell RNA sequencing. We found high interleukin 13 (IL-13) levels in the bone marrow of MF mice. IL-13 promoted the growth of mutant megakaryocytes and induced surface expression of transforming growth factor β and collagen biosynthesis. Similarly, analysis of samples from patients with MF revealed elevated levels of IL-13 in the plasma and increased IL-13 receptor expression in marrow megakaryocytes. In vivo, IL-13 overexpression promoted disease progression, whereas reducing IL-13/IL-4 signaling reduced several features of the disease, including fibrosis. Finally, we observed an increase in the number of marrow T cells and mast cells, which are known sources of IL-13. Together, our data demonstrate that IL-13 is involved in disease progression in MF and that inhibition of the IL-13/IL-4 signaling pathway might serve as a novel therapeutic target to treat MF.

EBF1 primes B-lymphoid enhancers and limits the myeloid bias in murine multipotent progenitors

Hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs) generate all cells of the blood system. Despite their multipotency, MPPs display poorly understood lineage bias. Here, we examine whether lineage-specifying transcription factors, such as the B-lineage determinant EBF1, regulate lineage preference in early progenitors. We detect low-level EBF1 expression in myeloid-biased MPP3 and lymphoid-biased MPP4 cells, coinciding with expression of the myeloid determinant C/EBPα. Hematopoietic deletion of Ebf1 results in enhanced myelopoiesis and reduced HSC repopulation capacity. Ebf1-deficient MPP3 and MPP4 cells exhibit an augmented myeloid differentiation potential and a transcriptome with an enriched C/EBPα signature. Correspondingly, EBF1 binds the Cebpa enhancer, and the deficiency and overexpression of Ebf1 in MPP3 and MPP4 cells lead to an up- and downregulation of Cebpa expression, respectively. In addition, EBF1 primes the chromatin of B-lymphoid enhancers specifically in MPP3 cells. Thus, our study implicates EBF1 in regulating myeloid/lymphoid fate bias in MPPs by constraining C/EBPα-driven myelopoiesis and priming the B-lymphoid fate.

Age-Progressive and Gender-Dependent Bone Phenotype in Mice Lacking Both Ebf1 and Ebf2 in Prrx1-Expressing Mesenchymal Cells

Ebfs are a family of transcription factors regulating the differentiation of multiple cell types of mesenchymal origin, including osteoblasts. Global deletion of Ebf1 results in increased bone formation and bone mass, while global loss of Ebf2 leads to enhanced bone resorption and decreased bone mass. Targeted deletion of Ebf1 in early committed osteoblasts leads to increased bone formation, whereas deletion in mature osteoblasts has no effect. To study the effects of Ebf2 specifically on long bone development, we created a limb bud mesenchyme targeted Ebf2 knockout mouse model by using paired related homeobox gene 1 (Prrx1) Cre. To investigate the possible interplay between Ebf1 and Ebf2, we deleted both Ebf1 and Ebf2 in the cells expressing Prrx1. Mice with Prrx1-targeted deletion of Ebf2 had a very mild bone phenotype. However, deletion of both Ebf1 and Ebf2 in mesenchymal lineage cells lead to significant, age progressive increase in bone volume. The phenotype was to some extent gender dependent, leading to an increase in both trabecular and cortical bone in females, while in males a mild cortical bone phenotype and a growth plate defect was observed. The phenotype was observed at both 6 and 12 weeks of age, but it was more pronounced in older female mice. Our data suggest that Ebfs modulate bone homeostasis and they are likely able to compensate for the lack of each other. The roles of Ebfs in bone formation appear to be complex and affected by multiple factors, such as age and gender.

Plasma EBF1 as a Novel Biomarker for Postmenopausal Osteoporosis

Postmenopausal osteoporosis (OPO) is one of the most common types of primary osteoporosis. There is currently lack of a plasma biomarker for sensitive and early diagnosis of OPO. Here we aimed to explore the potential of early B cell factor 1 (EBF1) as a new plasma biomarker of OPO. Quantitative real-time PCR was used to measure the plasma EBF1 levels. Absorptiometry markers, such as lumbar spine (LS) bone mineral density (BMD) and LS T score were obtained after X-ray scans. Biochemical analyses used to measure osteopontin (OPN), β-isomerized C-terminal telopeptides and total N-terminal procollagen of type-I collagen levels of patients with osteopenia (OPE, n = 81), osteoporosis (OPO, n = 98) as well as healthy subjects (NC, n = 110). Quantitative real-time PCR was used to measure the plasma levels of PAX5 and GSTP1, which are target genes of EBF1. EBF1 was downregulated in OPO patients. Levels of EBF1 were positively correlated to clinicopathological characteristics, including LS BMD and LS T scores, and negatively correlated to OPN and total N-terminal procollagen of type-I collagen levels. Increased PAX5 and GSTP1 levels also demonstrated strong correlations with higher EBF1, LS BMD and LS T score. Anti-osteoporotic treatment resulted in significant upregulation of EBF1, PAX5 and GSTP1 at 6 mo after treatment. Our study suggests that plasma EBF1 is a potential biomarker for diagnosing and assessing treatment outcome of OPO.

Understanding Chronic Venous Disease: A Critical Overview of Its Pathophysiology and Medical Management

Chronic venous disease (CVD) is a multifactorial condition affecting an important percentage of the global population. It ranges from mild clinical signs, such as telangiectasias or reticular veins, to severe manifestations, such as venous ulcerations. However, varicose veins (VVs) are the most common manifestation of CVD. The explicit mechanisms of the disease are not well-understood. It seems that genetics and a plethora of environmental agents play an important role in the development and progression of CVD. The exposure to these factors leads to altered hemodynamics of the venous system, described as ambulatory venous hypertension, therefore promoting microcirculatory changes, inflammatory responses, hypoxia, venous wall remodeling, and epigenetic variations, even with important systemic implications. Thus, a proper clinical management of patients with CVD is essential to prevent potential harms of the disease, which also entails a significant loss of the quality of life in these individuals. Hence, the aim of the present review is to collect the current knowledge of CVD, including its epidemiology, etiology, and risk factors, but emphasizing the pathophysiology and medical care of these patients, including clinical manifestations, diagnosis, and treatments. Furthermore, future directions will also be covered in this work in order to provide potential fields to explore in the context of CVD.

Categorization of lung mesenchymal cells in development and fibrosis

Pulmonary mesenchymal cells are critical players in both the mouse and human during lung development and disease states. They are increasingly recognized as highly heterogeneous, but there is no consensus on subpopulations or discriminative markers for each subtype. We completed scRNA-seq analysis of mesenchymal cells from the embryonic, postnatal, adult and aged fibrotic lungs of mice and humans. We consistently identified and delineated the transcriptome of lipofibroblasts, myofibroblasts, smooth muscle cells, pericytes, mesothelial cells, and a novel population characterized by Ebf1 expression. Subtype selective transcription factors and putative divergence of the clusters during development were described. Comparative analysis revealed orthologous subpopulations with conserved transcriptomic signatures in murine and human lung mesenchymal cells. All mesenchymal subpopulations contributed to matrix gene expression in fibrosis. This analysis would enhance our understanding of mesenchymal cell heterogeneity in lung development, homeostasis and fibrotic disease conditions.

Fibroblasts as confederates of the immune system

Fibroblastic stromal cells are as diverse, in origin and function, as the niches they fashion in the mammalian body. This cellular variety impacts the spectrum of responses elicited by the immune system. Fibroblast influence on the immune system keeps evolving our perspective on fibroblast roles and functions beyond just a passive structural part of organs. This review discusses the foundations of fibroblastic stromal-immune crosstalk, under the scope of stromal heterogeneity as a basis for tissue-specific tutoring of the immune system. Focusing on the skin as a relevant immunological organ, we detail the complex interactions between distinct fibroblast populations and immune cells that occur during homeostasis, injury repair, scarring, and disease. We further review the relevance of fibroblastic stromal cell heterogeneity and how this heterogeneity is central to regulate the immune system from its inception during embryonic development into adulthood.

Tmem100- and Acta2-Lineage Cells Contribute to Implant Osseointegration in a Mouse Model

Metal implants are commonly used in orthopedic surgery. The mechanical stability and longevity of implants depend on adequate bone deposition along the implant surface. The cellular and molecular mechanisms underlying peri-implant bone formation (ie, osseointegration) are incompletely understood. Herein, our goal was to determine the specific bone marrow stromal cell populations that contribute to bone formation around metal implants. To do this, we utilized a mouse tibial implant model that is clinically representative of human joint replacement procedures. Using a lineage-tracing approach, we found that both Acta2.creERT2 and Tmem100.creERT2 lineage cells are involved in peri-implant bone formation, and Pdgfra- and Ly6a/Sca1-expressing stromal cells (PαS cells) are highly enriched in both lineages. Single-cell RNA-seq analysis indicated that PαS cells are quiescent in uninjured bone tissue; however, they express markers of proliferation and osteogenic differentiation shortly after implantation surgery. Our findings indicate that PαS cells are mobilized to repair bone tissue and participate in implant osseointegration after surgery. Biologic therapies targeting PαS cells might improve osseointegration in patients undergoing orthopedic procedures. © 2021 American Society for Bone and Mineral Research (ASBMR).

Early B-cell Factor1 (Ebf1) promotes early osteoblast differentiation but suppresses osteoblast function

Early B cell factor 1 (Ebf1) is a transcription factor that regulates B cell, neuronal cell and adipocyte differentiation. We and others have shown that Ebf1 is expressed in osteoblasts and that global deletion of Ebf1 results in increased bone formation in vivo. However, as Ebf1 is expressed in multiple tissues and cell types, it has remained unclear, which of the phenotypic changes in bone are derived from bone cells. The aim of this study was to determine the cell-autonomous and differentiation stage-specific roles of Ebf1 in osteoblasts. In vitro, haploinsufficient Ebf1^+/- calvarial cells showed impaired osteoblastic differentiation indicated by lower alkaline phosphatase (ALP) activity and reduced mRNA expression of osteoblastic genes, while overexpression of Ebf1 in wild type mouse calvarial cells led to enhanced osteoblast differentiation with increased expression of Osterix (Osx). We identified a putative Ebf1 binding site in the Osterix promoter by ChIP assay in MC3T3-E1 osteoblasts and showed that Ebf1 was able to activate Osx-luc reporter construct that included this Ebf1 binding site, suggesting that Ebf1 indeed regulates osteoblast differentiation by inducing Osterix expression. To reconcile our previous data and that of others with our novel findings, we hypothesized that Ebf1 could have a dual role in osteoblast differentiation promoting early but inhibiting late stages of differentiation and osteoblast function. To test this hypothesis in vivo, we generated conditional Ebf1 knockout mice, in which Ebf1 deletion was targeted to early or late osteoblasts by crossing Ebf1^fl/fl mice with Osx- or Osteocalcin (hOC)-Cre mouse lines, respectively. Deletion of Ebf1 in early Ebf1_Osx^-/- osteoblasts resulted in significantly increased bone volume and trabecular number at 12 weeks by μCT analysis, while Ebf1_hOC^-/- mice did not have a bone phenotype. To conclude, our data demonstrate that Ebf1 promotes early osteoblast differentiation by regulating Osterix expression. However, Ebf1 inhibits bone accrual in the Osterix expressing osteoblasts in vivo but it is redundant in the maintenance of mature osteoblast function.

Determinants of Resident Tissue Macrophage Identity and Function

Resident tissue macrophages (RTMs) have a broad spectrum of immune- and non-immune-related tissue-supporting activities. The roots of this heterogeneity and versatility are only beginning to be understood. Here, we propose a conceptual framework for considering the RTM heterogeneity that organizes the factors shaping RTM identity within four cardinal points: (1) ontogeny and the view that adult RTM populations comprise a defined mixture of cells that arise from either embryonic precursors or adult monocytes; (2) local factors unique to the niche of residence, evolving during development and aging; (3) inflammation status; and (4) the cumulative effect of time spent in a specific tissue that contributes to the resilient adaptation of macrophages to their dynamic environment. We review recent findings within this context and discuss the technological advances that are revolutionizing the study of macrophage biology.

Emerging trends in chromatin remodeler plasticity in mesenchymal stromal cell function

Emerging evidences highlight importance of epigenetic regulation and their integration with transcriptional and cell signaling machinery in determining tissue resident adult pluripotent mesenchymal stem/stromal cell (MSC) activity, lineage commitment, and multicellular development. Histone modifying enzymes and large multi-subunit chromatin remodeling complexes and their cell type-specific plasticity remain the central defining features of gene regulation and establishment of tissue identity. Modulation of transcription factor expression gradient ex vivo and concomitant flexibility of higher order chromatin architecture in response to signaling cues are exciting approaches to regulate MSC activity and tissue rejuvenation. Being an important constituent of the adult bone marrow microenvironment/niche, pathophysiological perturbation in MSC homeostasis also causes impaired hematopoietic stem/progenitor cell function in a non-cell autonomous mechanism. In addition, pluripotent MSCs can function as immune regulatory cells, and they reside at the crossroad of innate and adaptive immune response pathways. Research in the past few years suggest that MSCs/stromal fibroblasts significantly contribute to the establishment of immunosuppressive microenvironment in shaping antitumor immunity. Therefore, it is important to understand mesenchymal stromal epigenome and transcriptional regulation to leverage its applications in regenerative medicine, epigenetic memory-guided trained immunity, immune-metabolic rewiring, and precision immune reprogramming. In this review, we highlight the latest developments and prospects in chromatin biology in determining MSC function in the context of lineage commitment and immunomodulation.

Stem cell quiescence and its clinical relevance

Quiescent state has been observed in stem cells (SCs), including in adult SCs and in cancer SCs (CSCs). Quiescent status of SCs contributes to SC self-renewal and conduces to averting SC death from harsh external stimuli. In this review, we provide an overview of intrinsic mechanisms and extrinsic factors that regulate adult SC quiescence. The intrinsic mechanisms discussed here include the cell cycle, mitogenic signaling, Notch signaling, epigenetic modification, and metabolism and transcriptional regulation, while the extrinsic factors summarized here include microenvironment cells, extracellular factors, and immune response and inflammation in microenvironment. Quiescent state of CSCs has been known to contribute immensely to therapeutic resistance in multiple cancers. The characteristics and the regulation mechanisms of quiescent CSCs are discussed in detail. Importantly, we also outline the recent advances and controversies in therapeutic strategies targeting CSC quiescence.

Epigenetic Regulators as the Gatekeepers of Hematopoiesis

Hematopoiesis is the process by which both fetal and adult organisms derive the full repertoire of blood cells from a single multipotent progenitor cell type, the hematopoietic stem cells (HSCs). Correct enactment of this process relies on a synergistic interplay between genetically encoded differentiation programs and a host of cell-intrinsic and cell-extrinsic factors. These include the influence of the HSC niche microenvironment, action of specific transcription factors, and alterations in intracellular metabolic state. The consolidation of these inputs with the genetically encoded program into a coherent differentiation program for each lineage is thought to rely on epigenetic modifiers. Recent work has delineated the precise contributions of different classes of epigenetic modifiers to HSC self-renewal as well as lineage specification and differentiation into various cell types. Here, we bring together what is currently known about chromatin status and the development of cells in the hematopoietic system under normal and abnormal conditions.

EBF1 and Pax5 safeguard leukemic transformation by limiting IL-7 signaling, Myc expression, and folate metabolism

In this study, Ramamoorthy et al. investigate EBF1 and PAX5 combined haploinsufficiency in the development of a B-ALL phenotype in mice. Using transcriptional and metabolomic profiling, the authors report that EBF1 and Pax5 may safeguard early stage B cells from transformation to B-ALL by limiting IL-7 signaling, folate metabolism, and Myc expression.

EBF1 and PAX5 mutations are associated with the development of B progenitor acute lymphoblastic leukemia (B-ALL) in humans. To understand the molecular networks driving leukemia in the Ebf1^+/−Pax5^+/− (dHet) mouse model for B-ALL, we interrogated the transcriptional profiles and chromatin status of leukemic cells, preleukemic dHet pro-B, and wild-type pro-B cells with the corresponding EBF1 and Pax5 cistromes. In dHet B-ALL cells, many EBF1 and Pax5 target genes encoding pre-BCR signaling components and transcription factors were down-regulated, whereas Myc and genes downstream from IL-7 signaling or associated with the folate pathway were up-regulated. We show that blockade of IL-7 signaling in vivo and methotrexate treatment of leukemic cells in vitro attenuate the expansion of leukemic cells. Single-cell RNA-sequencing revealed heterogeneity of leukemic cells and identified a subset of wild-type pro-B cells with reduced Ebf1 and enhanced Myc expression that show hallmarks of dHet B-ALL cells. Thus, EBF1 and Pax5 may safeguard early stage B cells from transformation to B-ALL by limiting IL-7 signaling, folate metabolism and Myc expression.