Distinct bone marrow blood vessels differentially regulate haematopoiesis

Loss of sphingosine kinase 2 promotes the expansion of hematopoietic stem cells by improving their metabolic fitness

Hematopoietic stem cells (HSCs) have reduced capacities to properly maintain and replenish the hematopoietic system during myelosuppressive injury or aging. Expanding and rejuvenating HSCs for therapeutic purposes has been a long-sought goal with limited progress. Here, we show that the enzyme Sphk2 (sphingosine kinase 2), which generates the lipid metabolite sphingosine-1-phosphate, is highly expressed in HSCs. The deletion of Sphk2 markedly promotes self-renewal and increases the regenerative potential of HSCs. More importantly, Sphk2 deletion globally preserves the young HSC gene expression pattern, improves the function, and sustains the multilineage potential of HSCs during aging. Mechanistically, Sphk2 interacts with prolyl hydroxylase 2 and the Von Hippel-Lindau protein to facilitate HIF1α ubiquitination in the nucleus independent of the Sphk2 catalytic activity. Deletion of Sphk2 increases hypoxic responses by stabilizing the HIF1α protein to upregulate PDK3, a glycolysis checkpoint protein for HSC quiescence, which subsequently enhances the function of HSCs by improving their metabolic fitness; specifically, it enhances anaerobic glycolysis but suppresses mitochondrial oxidative phosphorylation and generation of reactive oxygen species. Overall, targeting Sphk2 to enhance the metabolic fitness of HSCs is a promising strategy to expand and rejuvenate functional HSCs.

The emergence of the calvarial hematopoietic niche in health and disease

The diploë region of skull has recently been discovered to act as a myeloid cell reservoir to the underlying meninges. The presence of ossified vascular channels traversing the inner skull of cortex provides a passageway for the cells to traffic from the niche, and CNS-derived antigens traveling through cerebrospinal fluid in a perivascular manner reaches the niche to signal myeloid cell egress. This review will highlight the recent findings establishing this burgeoning field along with the known role this niche plays in CNS aging and disease. It will further highlight the anatomical routes and physiological properties of the vascular structures these cells use for trafficking, spanning from skull to brain parenchyma.

How Does Lifestyle Affect Hematopoiesis and the Bone Marrow Microenvironment?

Lifestyle factors are modifiable behavioral factors that have a significant impact on health and longevity. Diet-induced obesity and physical activity/exercise are two prevalent lifestyle factors that have strong relationships to overall health. The mechanisms linking obesity to negative health outcomes and the mechanisms linking increased participation in physical activity/exercise to positive health outcomes are beginning to be elucidated. Chronic inflammation, due in part to overproduction of myeloid cells from hematopoietic stem cells (HSCs) in the bone marrow, is an established mechanism responsible for the negative health effects of obesity. Recent work has shown that exercise training can reverse the aberrant myelopoiesis present in obesity in part by restoring the bone marrow microenvironment. Specifically, exercise training reduces marrow adipose tissue, increases HSC retention factor expression, and reduces pro-inflammatory cytokine levels in the bone marrow. Other, novel mechanistic factors responsible for these exercise-induced effects, including intercellular communication using extracellular vesicles (EVs), is beginning to be explored. This review will summarize the recent literature describing the effects of exercise on hematopoiesis in individuals with obesity and introduce the potential contribution of EVs to this process.

Cooperative ETS Transcription Factors Enforce Adult Endothelial Cell Fate and Cardiovascular Homeostasis

Current dogma dictates that during adulthood, endothelial cells (ECs) are locked in an immutable stable homeostatic state. By contrast, herein we show that maintenance of EC fate and function are linked and active processes, which depend on the constitutive cooperativity of only two ETS-transcription factors (TFs) ERG and Fli1. While deletion of either Fli1 or ERG manifest subtle vascular dysfunction, their combined genetic deletion in adult EC results in acute vasculopathy and multiorgan failure, due to loss of EC fate and integrity, hyperinflammation, and spontaneous thrombosis, leading to death. ERG and Fli1 co-deficiency cause rapid transcriptional silencing of pan- and organotypic vascular core genes, with dysregulation of inflammation and coagulation pathways. Vascular hyperinflammation leads to impaired hematopoiesis with myeloid skewing. Accordingly, enforced ERG and FLI1 expression in adult human mesenchymal stromal cells activates vascular programs and functionality enabling engraftment of perfusable vascular network. GWAS-analysis identified vascular diseases are associated with FLI1/Erg mutations. Constitutive expression of ERG and Fli1 uphold EC fate, physiological function, and resilience in adult vasculature; while their functional loss can contribute to systemic human diseases.

Dysregulated stem cell niches and altered lymphocyte recirculation cause B and T cell lymphopenia in WHIM syndrome

Gain-of-function (GOF) mutations in CXCR4 cause WHIM (warts, hypogammaglobulinemia, infections, and myelokathexis) syndrome, characterized by infections, leukocyte retention in bone marrow (BM), and blood leukopenias. B lymphopenia is evident at early progenitor stages, yet why do CXCR4 GOF mutations that cause B (and T) lymphopenia remain obscure? Using a CXCR4 R334X GOF mouse model of WHIM syndrome, we showed that lymphopoiesis is reduced because of a dysregulated mesenchymal stem cell (MSC) transcriptome characterized by a switch from an adipogenic to an osteolineage-prone program with limited lymphopoietic activity. We identify lymphotoxin beta receptor (LTβR) as a critical pathway promoting interleukin-7 (IL-7) down-regulation in MSCs. Blocking LTβR or CXCR4 signaling restored IL-7 production and B cell development in WHIM mice. LTβR blocking also increased production of IL-7 and B cell activating factor (BAFF) in secondary lymphoid organs (SLOs), increasing B and T cell numbers in the periphery. These studies revealed that LTβR signaling in BM MSCs and SLO stromal cells limits the lymphocyte compartment size.

Meteorin links the bone marrow hypoxic state to hematopoietic stem/progenitor cell mobilization

Hematopoietic stem/progenitor cells (HSPCs) are supported and regulated by niche cells in the bone marrow with an important characterization of physiological hypoxia. However, how hypoxia regulates HSPCs is still unclear. Here, we find that meteorin (Metrn) from hypoxic macrophages restrains HSPC mobilization. Hypoxia-induced factor 1α and Yin Yang 1 induce the high expression of Metrn in macrophages, and macrophage-specific Metrn knockout increases HSPC mobilization through modulating HSPC proliferation and migration. Mechanistically, Metrn interacts with its receptor 5-hydroxytryptamine receptor 2b (Htr2b) to regulate the reactive oxygen species levels in HSPCs through targeting phospholipase C signaling. The reactive oxygen species levels are reduced in HSPCs of macrophage-specific Metrn knockout mice with activated phospholipase C signaling. Targeting the Metrn/Htr2b axis could therefore be a potential strategy to improve HSPC mobilization for stem cell-based therapy.

Murine fetal bone marrow does not support functional hematopoietic stem and progenitor cells until birth

While adult bone marrow (BM) hematopoietic stem and progenitor cells (HSPCs) and their extrinsic regulation is well studied, little is known about the composition, function, and extrinsic regulation of the first HSPCs to enter the BM during development. Here, we functionally interrogate murine BM HSPCs from E15.5 through P0. Our work reveals that fetal BM HSPCs are present by E15.5, but distinct from the HSPC pool seen in fetal liver, both phenotypically and functionally, until near birth. We also generate a transcriptional atlas of perinatal BM HSPCs and the BM niche in mice across ontogeny, revealing that fetal BM lacks HSPCs with robust intrinsic stem cell programs, as well as niche cells supportive of HSPCs. In contrast, stem cell programs are preserved in neonatal BM HSPCs, which reside in a niche expressing HSC supportive factors distinct from those seen in adults. Collectively, our results provide important insights into the factors shaping hematopoiesis during this understudied window of hematopoietic development.

Relatively little is known about the first hematopoietic stem and progenitor cells to arrive in the fetal bone marrow. Here they characterize the frequency, function, and molecular identity of fetal BM HSPCs and their bone marrow niche, and show that most BM HSPCs have little hematopoietic function until birth.

Nestin-GFP transgene labels immunoprivileged bone marrow mesenchymal stem cells in the model of ectopic foci formation

Immune privileges are demonstrated for different types of quiescent stem cells of adult mammalian organisms. Mesenchymal stem cells (MSCs) are believed to have immune privileges; however, an accurate experimental confirmation hasn’t been presented. Here, we provide direct experimental evidence that MSCs of C57Black/6J murine bone marrow (BM) are immune privileged in vivo and retain their functionality after prolonged exposure to the uncompromised immune system. The BM of Nes-Gfp transgenic mice was implanted as a tissue fragment under the kidney capsule in isogenic C57Black/6J immunocompetent recipients. Nestin-Gfp strain provides a fluorescent immunogenic marker for a small fraction of BM cells, including GFP+CD45– MSCs. Despite the exposure of xenogenically marked MSCs to the fully-functional immune system, primary ectopic foci of hematopoiesis formed. Six weeks after implantation, multicolor fluorescence cytometry revealed both GFP+CD45– and GFP+CD45+ cells within the foci. GFP+CD45– cells proportion was 2.0 × 10–5 ×÷9 and it didn’t differ significantly from syngenic Nes-GFP transplantation control. According to current knowledge, the immune system of the recipients should eliminate GFP+ cells, including GFP+ MSCs. These results show that MSCs evade immunity. Primary foci were retransplanted into secondary Nes-GFP recipients. The secondary foci formed, in which CD45–GFP+ cells proportion was 6.7 × 10–5 ×÷2.2, and it didn’t differ from intact Nes-GFP BM. The results demonstrate that MSCs preserve self-renewal and retain their functionality after prolonged immune exposure. The success of this study relied on the implantation of BM fragments without prior dissociation of cells and the fact that the vast majority of implanted cells were immunologically equivalent to the recipients.

The cellular composition and function of the bone marrow niche after allogeneic hematopoietic cell transplantation

Allogeneic hematopoietic cell transplantation (HCT) is a potentially curative therapy for patients with a variety of malignant and non-malignant diseases. Despite its life-saving potential, HCT is associated with significant morbidity and mortality. Reciprocal interactions between hematopoietic stem cells (HSCs) and their surrounding bone marrow (BM) niche regulate HSC function during homeostatic hematopoiesis as well as regeneration. However, current pre-HCT conditioning regimens, which consist of high-dose chemotherapy and/or irradiation, cause substantial short- and long-term toxicity to the BM niche. This damage may negatively affect HSC function, impair hematopoietic regeneration after HCT and predispose to HCT-related morbidity and mortality. In this review, we summarize current knowledge on the cellular composition of the human BM niche after HCT. We describe how pre-HCT conditioning affects the cell types in the niche, including endothelial cells, mesenchymal stromal cells, osteoblasts, adipocytes, and neurons. Finally, we discuss therapeutic strategies to prevent or repair conditioning-induced niche damage, which may promote hematopoietic recovery and improve HCT outcome.

Exploring the origin of the cancer stem cell niche and its role in anti-angiogenic treatment for glioblastoma

Cancer stem cells are thought to be the main drivers of tumorigenesis for malignancies such as glioblastoma (GBM). They are maintained through a close relationship with the tumor vasculature. Previous literature has well-characterized the components and signaling pathways for maintenance of this stem cell niche, but details on how the niche initially forms are limited. This review discusses development of the nonmalignant neural and hematopoietic stem cell niches in order to draw important parallels to the malignant environment. We then discuss what is known about the cancer stem cell niche, its relationship with angiogenesis, and provide a hypothesis for its development in GBM. A better understanding of the mechanisms of development of the tumor stem cell niche may provide new insights to potentially therapeutically exploit.

Defining the ultrastructure of the hematopoietic stem cell niche by correlative light and electron microscopy

The blood system is supported by hematopoietic stem and progenitor cells (HSPCs) found in a specialized microenvironment called the niche. Many different niche cell types support HSPCs, however how they interact and their ultrastructure has been difficult to define. Here, we show that single endogenous HSPCs can be tracked by light microscopy, then identified by serial block-face scanning electron microscopy (SBEM) at multiscale levels. Using the zebrafish larval kidney marrow (KM) niche as a model, we followed single fluorescently labeled HSPCs by light sheet microscopy, then confirmed their exact location in a 3D SBEM dataset. We found a variety of different configurations of HSPCs and surrounding niche cells, suggesting there could be functional heterogeneity in sites of HSPC lodgement. Our approach also allowed us to identify dopamine beta-hydroxylase (dbh) positive ganglion cells as a previously uncharacterized functional cell type in the HSPC niche. By integrating multiple imaging modalities, we could resolve the ultrastructure of single rare cells deep in live tissue and define all contacts between an HSPC and its surrounding niche cell types.

Mechanisms involved in hematopoietic stem cell aging

Hematopoietic stem cells (HSCs) undergo progressive functional decline over time due to both internal and external stressors, leading to aging of the hematopoietic system. A comprehensive understanding of the molecular mechanisms underlying HSC aging will be valuable in developing novel therapies for HSC rejuvenation and to prevent the onset of several age-associated diseases and hematological malignancies. This review considers the general causes of HSC aging that range from cell-intrinsic factors to cell-extrinsic factors. In particular, epigenetics and inflammation have been implicated in the linkage of HSC aging, clonality, and oncogenesis. The challenges in clarifying mechanisms of HSC aging have accelerated the development of therapeutic interventions to rejuvenate HSCs, the major goal of aging research; these details are also discussed in this review.

Inhibitor of DNA binding proteins revealed as orchestrators of steady state, stress and malignant hematopoiesis

Adult mammalian hematopoiesis is a dynamic cellular process that provides a continuous supply of myeloid, lymphoid, erythroid/megakaryocyte cells for host survival. This process is sustained by regulating hematopoietic stem cells (HSCs) quiescence, proliferation and activation under homeostasis and stress, and regulating the proliferation and differentiation of downstream multipotent progenitor (MPP) and more committed progenitor cells. Inhibitor of DNA binding (ID) proteins are small helix-loop-helix (HLH) proteins that lack a basic (b) DNA binding domain present in other family members, and function as dominant-negative regulators of other bHLH proteins (E proteins) by inhibiting their transcriptional activity. ID proteins are required for normal T cell, B cell, NK and innate lymphoid cells, dendritic cell, and myeloid cell differentiation and development. However, recent evidence suggests that ID proteins are important regulators of normal and leukemic hematopoietic stem and progenitor cells (HSPCs). This chapter will review our current understanding of the function of ID proteins in HSPC development and highlight future areas of scientific investigation.

In vivo observation of multi-phase spatiotemporal cellular dynamics of transplanted HSPCs during early engraftment

Hematopoietic stem cell transplantation (HSCT) is commonly used to treat patients with various blood disorders, genetic and immunological diseases, and solid tumors. Several systemic complications following HSCT are critical limiting factors for achieving a successful outcome. These systemic complications are mainly due to the lack of initial engraftment after transplantation. However, the detailed underlying cellular dynamics of early engraftment have not been fully characterized yet. We performed in vivo longitudinal visualization of early engraftment characteristics of transplanted hematopoietic stem and progenitor cells (HSPCs) in the mouse calvarial bone marrow (BM). To achieve this, we utilized an in vivo laser-scanning confocal microscopy imaging system with a cranial BM imaging window and stereotaxic device. We observed two distinct cellular behaviors of HSPCs in vivo, cluster formation and cluster dissociation, early after transplantation. Furthermore, we successfully identified three cellular phases of engraftment with distinct cellular distances which are coordinated with cell proliferation and cell migration dynamics during initial engraftment.

CXCR4high megakaryocytes regulate host-defense immunity against bacterial pathogens

Megakaryocytes (MKs) continuously produce platelets to support hemostasis and form a niche for hematopoietic stem cell maintenance in the bone marrow. MKs are also involved in inflammatory responses; however, the mechanism remains poorly understood. Using single-cell sequencing, we identified a CXCR4 highly expressed MK subpopulation, which exhibited both MK-specific and immune characteristics. CXCR4high MKs interacted with myeloid cells to promote their migration and stimulate the bacterial phagocytosis of macrophages and neutrophils by producing TNFα and IL-6. CXCR4high MKs were also capable of phagocytosis, processing, and presenting antigens to activate T cells. Furthermore, CXCR4high MKs also egressed circulation and infiltrated into the spleen, liver, and lung upon bacterial infection. Ablation of MKs suppressed the innate immune response and T cell activation to impair the anti-bacterial effects in mice under the Listeria monocytogenes challenge. Using hematopoietic stem/progenitor cell lineage-tracing mouse lines, we show that CXCR4high MKs were generated from infection-induced emergency megakaryopoiesis in response to bacterial infection. Overall, we identify the CXCR4high MKs, which regulate host-defense immune response against bacterial infection.

Resveratrol's neural protective effects for the injured embryoid body and cerebral organoid

OBJECTIVE

Resveratrol (RSV) is a polyphenol compound found in grapes, veratrum and other plants. It has been reported that RSV has anti-inflammatory, anti-oxidant, anti-cancer and other pharmacological effects. However, the impacts of RSV on development of nervous system are not understood well. The study aims to investigate RSV's neuroprotective effect during development and to provide a health care for pregnant women and their fetuses with RSV supplementation.

METHODS

In this study, we induced human induced pluripotent stem cells (hiPSCs) to form the embryoid bodies (EBs) and cerebral organoids (COs) with 3 dimensional (3D) culture. In the meantime, D-galactose (D-gal, 5 mg/ml) was used to make nervous injury model, and on the other hand, RSV with various doses, such as 2 μm/L, 10 μm/L, 50 μm/L, were applied to understand its neuroprotection. Therefore, the cultures were divided into control group, D-gal nervous injury group and RSV intervention groups. After that, the diameters of EBs and COs were measured regularly under a reverted microscope. In the meantime, the neural proliferation, cell apoptosis and the differentiation of germ layers were detected via immunofluorescence.

RESULTS

(1) D-gal could delay the development of EBs and COs; (2) RSV could rescue the atrophy of EBs and COs caused by D-gal; (3) RSV showed its neuroprotection, through promoting the neural cell proliferation, inhibiting apoptosis and accelerating the differentiation of germ layers.

CONCLUSION

RSV has a neuroprotective effect on the development of the nervous system, suggesting RSV supplementation may be necessary during the health care of pregnancy and childhood.

Recent advances in "sickle and niche" research - Tribute to Dr. Paul S Frenette

In this retrospective, we review the two research topics that formed the basis of the outstanding career of Dr. Paul S. Frenette. In the first part, we focus on sickle cell disease (SCD). The defining feature of SCD is polymerization of the deoxygenated mutant hemoglobin, which leads to a vicious cycle of hemolysis and vaso-occlusion. We survey important discoveries in SCD pathophysiology that have led to recent advances in treatment of SCD. The second part focuses on the hematopoietic stem cell (HSC) niche, the complex microenvironment within the bone marrow that controls HSC function and homeostasis. We detail the cells that constitute this niche, and the factors that these cells use to exert control over hematopoiesis. Here, we trace the scientific paths of Dr. Frenette, highlight key aspects of his research, and identify his most important scientific contributions in both fields.

Hematopoietic stem cell mobilization

Hematopoietic stem cell (HSC) transplantation has been used to treat hematopoietic diseases for over 50 years. HSCs can be isolated from bone marrow (BM), umbilical cord blood, or peripheral blood. Because of lower costs, shorter hospitalization, and faster engraftment, peripheral blood has become the predominant source of HSCs for transplantation. The major factors determining the rate of successful HSC transplantation include the degree of human leukocyte antigen matching between the donor and recipient and the number of HSCs for transplantation. Administration of granulocyte colony-stimulating factor (G-CSF) alone or combined with plerixafor (AMD3100) are clinical used methods to promote HSC mobilization from BM to the peripheral blood for HSC transplantations. However, a significant portion of healthy donors or patients may be poor mobilizers of G-CSF, resulting in an insufficient number of HSCs for the transplantation and necessitating alternative strategies to increase the apheresis yield. The detailed mechanisms underlying G-CSF-mediated HSC mobilization remain to be elucidated. This review summarizes the current research on deciphering the mechanism of HSC mobilization.

Hematopoietic Stem and Progenitor Cells (HSPCs) and Hematopoietic Microenvironment: Molecular and Bioinformatic Studies of the Zebrafish Models

Hematopoietic stem cells (HSCs) reside in a specialized microenvironment in a peculiar anatomic location which regulates the maintenance of stem cells and controls its functions. Recent scientific progress in experimental technologies have enabled the specific detection of epigenetic factors responsible for the maintenance and quiescence of the hematopoietic niche, which has improved our knowledge of regulatory mechanisms. The aberrant role of RNA-binding proteins and their impact on the disruption of stem cell biology have been reported by a number of recent studies. Despite recent modernization in hematopoietic microenvironment research avenues, our comprehension of the signaling mechanisms and interactive pathways responsible for integration of the hematopoietic niche is still limited. In the past few decades, zebrafish usage with regards to exploratory studies of the hematopoietic niche has expanded our knowledge for deeper understanding of novel cellular interactions. This review provides an update on the functional roles of different genetic and epigenetic factors and molecular signaling events at different sections of the hematopoietic microenvironment. The explorations of different molecular approaches and interventions of latest web-based tools being used are also outlined. This will help us to get more mechanistic insights and develop therapeutic options for the malignancies.

Niche Regulation of Hematopoiesis: The Environment Is "Micro," but the Influence Is Large

Immune cell production is governed by a process known as hematopoiesis, where hematopoietic stem cells (HSCs) differentiate through progenitor cells and ultimately to the mature blood and immune cells found in circulation. While HSCs are capable of cell-autonomous regulation, they also rely on extrinsic factors to balance their state of quiescence and activation. These cues can, in part, be derived from the niche in which HSCs are found. Under steady-state conditions, HSCs are found in the bone marrow. This niche is designed to support HSCs but also to respond to external factors, which allows hematopoiesis to be a finely tuned and coordinated process. However, the niche, and its regulation, can become dysregulated to potentiate inflammation during disease. This review will highlight the architecture of the bone marrow and key regulators of hematopoiesis within this niche. Emphasis will be placed on how these mechanisms go awry to exacerbate hematopoietic contributions that drive cardiovascular disease.

Metalloproteinase inhibition reduces AML growth, prevents stem cell loss, and improves chemotherapy effectiveness

Acute myeloid leukemia (AML) is a blood cancer of the myeloid lineage. Its prognosis remains poor, highlighting the need for new therapeutic and precision medicine approaches. AML symptoms often include cytopenias linked to loss of healthy hematopoietic stem and progenitor cells (HSPCs). The mechanisms behind HSPC decline are complex and still poorly understood. Here, intravital microscopy (IVM) of a well-established experimental model of AML allows direct observation of the interactions between healthy and malignant cells in the bone marrow (BM), suggesting that physical dislodgment of healthy cells by AML through damaged vasculature may play an important role. Multiple matrix metalloproteinases (MMPs), known to remodel extracellular matrix, are expressed by AML cells and the BM microenvironment. We reason MMPs could be involved in cell displacement and vascular leakiness; therefore, we evaluate the therapeutic potential of MMP pharmacological inhibition using the broad-spectrum inhibitor prinomastat. IVM analyses of prinomastat-treated mice reveal reduced vascular permeability and healthy cell clusters in circulation and lower AML infiltration, proliferation, and cell migration. Furthermore, treated mice have increased retention of healthy HSPCs in the BM and increased survival following chemotherapy. Analysis of a human AML transcriptomic database reveals widespread MMP deregulation, and human AML cells show susceptibility to MMP inhibition. Overall, our results suggest that MMP inhibition could be a promising complementary therapy to reduce AML growth and limit HSPC loss and BM vascular damage caused by MLL-AF9 and possibly other AML subtypes.

Deconvolution of the hematopoietic stem cell microenvironment reveals a high degree of specialization and conservation

Understanding the regulation of normal and malignant human hematopoiesis requires comprehensive cell atlas of the hematopoietic stem cell (HSC) regulatory microenvironment. Here, we develop a tailored bioinformatic pipeline to integrate public and proprietary single-cell RNA sequencing (scRNA-seq) datasets. As a result, we robustly identify for the first time 14 intermediate cell states and 11 stages of differentiation in the endothelial and mesenchymal BM compartments, respectively. Our data provide the most comprehensive description to date of the murine HSC-regulatory microenvironment and suggest a higher level of specialization of the cellular circuits than previously anticipated. Furthermore, this deep characterization allows inferring conserved features in human, suggesting that the layers of microenvironmental regulation of hematopoiesis may also be shared between species. Our resource and methodology is a stepping-stone toward a comprehensive cell atlas of the BM microenvironment.

Impaired haematopoietic stem / progenitor cell traffic and multi-organ damage in diabetes

During antenatal development, hematopoietic stem/progenitor cells (HSPCs) arise from a specialized endothelium and migrate from the extraembryonic mesoderm to the fetal liver before establishing hematopoiesis in the bone marrow (BM). It is still debated whether, in adulthood, HSPCs display such ontologic overlap with vascular cells and capacity for endothelial differentiation. Yet, adult HSPCs retain a prominent migratory activity and traffic in the bloodstream to secondary lymphoid organs and all peripheral tissues, before eventually returning to the BM. While patrolling parenchymatous organs, HSPCs locate close to the vasculature, where they establish local hematopoietic islands and contribute to tissue homeostasis by paracrine signals. Solid evidence shows that diabetes mellitus jeopardizes the traffic of HSPCs from BM to the circulation and peripheral tissues, a condition called “mobilopathy.” A reduction in the levels of circulating HSPCs is the most immediate and apparent consequence, which has been consistently observed in human diabetes, and is strongly associated with future risk for multi-organ damage, including micro- and macro-angiopathy. But the shortage of HSPCs in the blood is only the visible tip of the iceberg. Abnormal HSPC traffic results from a complex interplay among metabolism, innate immunity, and hematopoiesis. Notably, mobilopathy is mechanistically connected with diabetes-induced myelopoiesis. Impaired traffic of HSPCs and enhanced generation of pro-inflammatory cells synergize for tissue damage and impair the resolution of inflammation. We herein summarize the current evidence that diabetes affects HSPC traffic, which are the causes and consequences of such alteration, and how it contributes to the overall disease burden.

Targeting the Hematopoietic Stem Cell Niche in β-Thalassemia and Sickle Cell Disease

In the last decade, research on pathophysiology and therapeutic solutions for β-thalassemia (BThal) and sickle cell disease (SCD) has been mostly focused on the primary erythroid defect, thus neglecting the study of hematopoietic stem cells (HSCs) and bone marrow (BM) microenvironment. The quality and engraftment of HSCs depend on the BM microenvironment, influencing the outcome of HSC transplantation (HSCT) both in allogeneic and in autologous gene therapy settings. In BThal and SCD, the consequences of severe anemia alter erythropoiesis and cause chronic stress in different organs, including the BM. Here, we discuss the recent findings that highlighted multiple alterations of the BM niche in BThal and SCD. We point out the importance of improving our understanding of HSC biology, the status of the BM niche, and their functional crosstalk in these disorders towards the novel concept of combined therapies by not only targeting the genetic defect, but also key players of the HSC–niche interaction in order to improve the clinical outcomes of transplantation.

Bone Marrow Aging and the Leukaemia-Induced Senescence of Mesenchymal Stem/Stromal Cells: Exploring Similarities

Bone marrow aging is associated with multiple cellular dysfunctions, including perturbed haematopoiesis, the propensity to haematological transformation, and the maintenance of leukaemia. It has been shown that instructive signals from different leukemic cells are delivered to stromal cells to remodel the bone marrow into a supportive leukemic niche. In particular, cellular senescence, a physiological program with both beneficial and deleterious effects on the health of the organisms, may be responsible for the increased incidence of haematological malignancies in the elderly and for the survival of diverse leukemic cells. Here, we will review the connection between BM aging and cellular senescence and the role that these processes play in leukaemia progression. Specifically, we discuss the role of mesenchymal stem cells as a central component of the supportive niche. Due to the specificity of the genetic defects present in leukaemia, one would think that bone marrow alterations would also have particular changes, making it difficult to envisage a shared therapeutic use. We have tried to summarize the coincident features present in BM stromal cells during aging and senescence and in two different leukaemias, acute myeloid leukaemia, with high frequency in the elderly, and B-acute lymphoblastic leukaemia, mainly a childhood disease. We propose that mesenchymal stem cells are similarly affected in these different leukaemias, and that the changes that we observed in terms of cellular function, redox balance, genetics and epigenetics, soluble factor repertoire and stemness are equivalent to those occurring during BM aging and cellular senescence. These coincident features may be used to explore strategies useful to treat various haematological malignancies.

Prophylactic NAC promoted hematopoietic reconstitution by improving endothelial cells after haploidentical HSCT: a phase 3, open-label randomized trial

BACKGROUND

Poor graft function (PGF) or prolonged isolated thrombocytopenia (PT), which are characterized by pancytopenia or thrombocytopenia, have become serious complications after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Our previous single-arm trial suggests that N-acetyl-L-cysteine (NAC) prophylaxis reduced PGF or PT after allo-HSCT. Therefore, an open-label, randomized, phase 3 trial was performed to investigate the efficacy and tolerability of NAC prophylaxis to reduce PGF or PT after allo-HSCT.

METHODS

A phase 3, open-label randomized trial was performed. Based on the percentage of CD34+VEGFR2 (CD309)+ endothelial cells (ECs) in bone marrow (BM) detected by flow cytometry at 14 days before conditioning, patients aged 15 to 60 years with acute leukemia undergoing haploidentical HSCT were categorized as low-risk (EC ≥ 0.1%) or high-risk (EC < 0.1%); patients at high risk were randomly assigned (2:1) to receive NAC prophylaxis or nonprophylaxis. The primary endpoint was PGF and PT incidence at +60 days post-HSCT.

RESULTS

Between April 18, 2019, and June 24, 2021, 120 patients with BM EC <0.1% were randomly assigned for NAC (group A, N = 80) or nonprophylaxis (group B, N = 40), and 105 patients with EC≥0.1% (group C) were also analyzed. The +60 days incidence of PGF and PT was 7.5% (95% CI, 1.7 to 13.3%) and 22.5% (95% CI, 9.1 to 35.9%) in group A and group B (hazard ratio, 0.317; 95% CI, 0.113 to 0.890; P = 0.021) and 11.4% (95% CI, 5.2 to 17.6%) in group C (hazard ratio, 0.643; 95% CI, 0.242 to 1.715; P = 0.373). Consistently, NAC prophylaxis gradually improved BM ECs and CD34+ cells in group A, whereas reduced their reactive oxygen species (ROS) levels post-HSCT. Within 60 days post-HSCT, the most common grade 3 to 5 adverse events for the NAC and control groups were infections (19/80 [24%] vs. 10/40 [25%]) and gastrointestinal adverse events (16/80 [20%] vs. 7/40 [18%]). There were no treatment-related deaths.

CONCLUSIONS

N-Acetyl-L-cysteine prophylaxis can prevent the occurrence of poor hematopoietic function and is well tolerated in haploidentical HSCT. It may offer a potential pathogenesis-oriented therapeutic approach for patients with poor hematopoietic function.

TRIAL REGISTRATION

This trial was registered at ClinicalTrials.gov as #NCT03967665.

The Bone Marrow Microenvironment in B-Cell Development and Malignancy

B lymphopoiesis is characterized by progressive loss of multipotent potential in hematopoietic stem cells, followed by commitment to differentiate into B cells, which mediate the humoral response of the adaptive immune system. This process is tightly regulated by spatially distinct bone marrow niches where cells, including mesenchymal stem and progenitor cells, endothelial cells, osteoblasts, osteoclasts, and adipocytes, interact with B-cell progenitors to direct their proliferation and differentiation. Recently, the B-cell niche has been implicated in initiating and facilitating B-cell precursor acute lymphoblastic leukemia. Leukemic cells are also capable of remodeling the B-cell niche to promote their growth and survival and evade treatment. Here, we discuss the major cellular components of bone marrow niches for B lymphopoiesis and the role of the malignant B-cell niche in disease development, treatment resistance and relapse. Further understanding of the crosstalk between leukemic cells and bone marrow niche cells will enable development of additional therapeutic strategies that target the niches in order to hinder leukemia progression.

3D bioprinting of osteon-mimetic scaffolds with hierarchical microchannels for vascularized bone tissue regeneration

The integration of three-dimensional (3D) bioprinted scaffold's structure and function for critical-size bone defect repair is of immense significance. Inspired by the basic component of innate cortical bone tissue--osteons, many studies focus on biomimetic strategy. However, the complexity of hierarchical microchannels in the osteon, the requirement of mechanical strength of bone, and the biological function of angiogenesis and osteogenesis remain challenges in the fabrication of osteon-mimetic scaffolds. Therefore, we successfully built mimetic scaffolds with vertically central medullary canals, peripheral Haversian canals, and transverse Volkmann canals structures simultaneously by 3D bioprinting technology using polycaprolactone and bioink loading with bone marrow mesenchymal stem cells (BMSCs) and bone morphogenetic protein-4 (BMP-4). Subsequently, endothelial progenitor cells (EPCs) were seeded into the canals to enhance angiogenesis. The porosity and compressive properties of bioprinted scaffolds could be well controlled by altering the structure and canal numbers of the scaffolds. The osteon-mimetic scaffolds showed satisfactory biocompatibility and promotion of angiogenesis and osteogenesis in vitro and prompted the new blood vessels and new bone formation in vivo. In summary, this study proposes a biomimetic strategy for fabricating structured and functionalized 3D bioprinted scaffolds for vascularized bone tissue regeneration.

Deletion of Vhl in Dmp1-Expressing Cells Causes Microenvironmental Impairment of B Cell Lymphopoiesis

The contributions of skeletal cells to the processes of B cell development in the bone marrow (BM) have not been completely described. The von-Hippel Lindau protein (VHL) plays a key role in cellular responses to hypoxia. Previous work showed that Dmp1-Cre;Vhl conditional knockout mice (VhlcKO), which deletes Vhl in subsets of mesenchymal stem cells, late osteoblasts and osteocytes, display dysregulated bone growth and reduction in B cells. Here, we investigated the mechanisms underlying the B cell defects using flow cytometry and high-resolution imaging. In the VhlcKO BM, B cell progenitors were increased in frequency and number, whereas Hardy Fractions B-F were decreased. VhlcKO Fractions B-C cells showed increased apoptosis and quiescence. Reciprocal BM chimeras confirmed a B cell-extrinsic source of the VhlcKO B cell defects. In support of this, VhlcKO BM supernatant contained reduced CXCL12 and elevated EPO levels. Intravital and ex vivo imaging revealed VhlcKO BM blood vessels with increased diameter, volume, and a diminished blood-BM barrier. Staining of VhlcKO B cells with an intracellular hypoxic marker indicated the natural existence of distinct B cell microenvironments that differ in local oxygen tensions and that the B cell developmental defects in VhlcKO BM are not initiated by hypoxia. Our studies identify novel mechanisms linking altered bone homeostasis with drastic BM microenvironmental changes that dysregulate B cell development.

Loading-driven PI3K/Akt signaling and erythropoiesis enhanced angiogenesis and osteogenesis in a postmenopausal osteoporosis mouse model

Bone vasculature influences osteogenesis and haematopoiesis in the bone microenviroment. Mechanical loading has been shown to stimulate the formation of osteogenesis-related type H vessels in an ovariectomy (OVX)-induced osteoporosis mouse model. To determine the loading-driven mechanism of angiogenesis and the formation of type H vessels in bone, we evaluated the roles of PI3K/Akt signaling and erythropoiesis in the bone marrow. The daily application of mechanical loading (1 N at 5 Hz for 6 min/day) for 2 weeks on OVX mice inhibited osteoclast activity, associated with an increase in the number of osteoblasts and trabecular volume ratio. Mechanical loading enhanced bone vasculature and vessel formation, as well as PI3K/Akt phosphorylation and erythropoiesis in the bone marrow. Notably, LY294002, an inhibitor of PI3K signaling, blocked the tube formation by endothelial progenitor cells, as well as their migration and wound healing. The conditioned medium, derived from erythroblasts, also promoted the function of HUVECs with elevated levels of VEGF, CD31, and Emcn. Collectively, this study demonstrates that mechanical loading prevents osteoporotic bone loss by promoting angiogenesis and type H vessel formation. This load-driven preventing effect is in part mediated by PI3K/Akt signaling and erythropoiesis in the bone marrow.

The long and winding road: homeostatic and disordered haematopoietic microenvironmental niches: a narrative review

Haematopoietic microenvironmental niches have been described as the 'gatekeepers' for the blood and immune systems. These niches change during ontogeny, with the bone marrow becoming the predominant site of haematopoiesis in post-natal life under steady state conditions. To determine the structure and function of different haematopoietic microenvironmental niches, it is essential to clearly define specific haematopoietic stem and progenitor cell subsets during ontogeny and to understand their temporal appearance and anatomical positioning. A variety of haematopoietic and non-haematopoietic cells contribute to haematopoietic stem and progenitor cell niches. The latter is reported to include endothelial cells and mesenchymal stromal cells (MSCs), skeletal stem cells and/or C-X-C motif chemokine ligand 12-abundant-reticular cell populations, which form crucial components of these microenvironments under homeostatic conditions. Dysregulation or deterioration of such cells contributes to significant clinical disorders and diseases worldwide and is associated with the ageing process. A critical appraisal of these issues and of the roles of MSC/C-X-C motif chemokine ligand 12-abundant-reticular cells and the more recently identified skeletal stem cell subsets in bone marrow haematopoietic niche function under homeostatic conditions and during ageing will form the basis of this research review. In the context of haematopoiesis, clinical translation will deal with lessons learned from the vast experience garnered from the development and use of MSC therapies to treat graft versus host disease in the context of allogeneic haematopoietic transplants, the recent application of these MSC therapies to treating emerging and severe coronavirus disease 2019 (COVID-19) infections, and, given that skeletal stem cell ageing is one proposed driver for haematopoietic ageing, the potential contributions of these stem cells to haematopoiesis in healthy bone marrow and the benefits and challenges of using this knowledge for rejuvenating the age-compromised bone marrow haematopoietic niches and restoring haematopoiesis.

Upregulation of SIRT1 Contributes to dmPGE2-dependent Radioprotection of Hematopoietic Stem Cells

Exposure to potentially lethal high-dose ionizing radiation results in bone marrow suppression, known as the hematopoietic acute radiation syndrome (H-ARS), which can lead to pancytopenia and possible death from hemorrhage or infection. Medical countermeasures to protect from or mitigate the effects of radiation exposure are an ongoing medical need. We recently reported that 16,16 dimethyl prostaglandin E₂ (dmPGE₂) given prior to lethal irradiation protects hematopoietic stem (HSCs) and progenitor (HPCs) cells and accelerates hematopoietic recovery by attenuating mitochondrial compromise, DNA damage, apoptosis, and senescence. However, molecular mechanisms responsible for the radioprotective effects of dmPGE₂ on HSCs are not well understood. In this report, we identify a crucial role for the NAD⁺-dependent histone deacetylase Sirtuin 1 (Sirt1) downstream of PKA and CREB in dmPGE₂-dependent radioprotection of hematopoietic cells. We found that dmPGE₂ increases Sirt1 expression and activity in hematopoietic cells including HSCs and pharmacologic and genetic suppression of Sirt1 attenuates the radioprotective effects of dmPGE₂ on HSC and HPC function and its ability to reduce DNA damage, apoptosis, and senescence and stimulate autophagy in HSCs. DmPGE₂-mediated enhancement of Sirt1 activity in irradiated mice is accompanied by epigenetic downregulation of p53 activation and inhibition of H3K9 and H4K16 acetylation at the promoters of the genes involved in DNA repair, apoptosis, and autophagy, including p53, Ku70, Ku80, LC3b, ATG7, and NF-κB. These studies expand our understanding of intracellular events that are induced by IR but prevented/attenuated by dmPGE₂ and suggest that modulation of Sirt1 activity may facilitate hematopoietic recovery following hematopoietic stress. Graphical Abstract.

Fundamentals of bone vasculature: Specialization, interactions and functions

Angiogenesis, hematopoiesis and osteogenesis are fundamental processes mediating complex and essential biological functions. In the bone marrow, endothelial cells (ECs) are a principal mediator of regulatory signals that govern hematopoietic and mesenchymal stem cells. EC and osteoblast interactions and niche functions of ECs are fundamental in maintaining bone health and coordinating repair and regeneration following injury. These cellular interactions are subject to dysregulation and deterioration under stress, aging, chronic disease states and malignancy. Thus, the prospect of manipulating the bone vasculature has tremendous potential to advance therapeutic interventions for the management of bone diseases. This review discusses the current state of vascular-skeletal tissue interactions focusing on osteoblast and hematopoietic stem cells interaction with ECs.

DT7 peptide-modified lecithin nanoparticles co-loaded with γ-secretase inhibitor and dexamethasone efficiently inhibit T-cell acute lymphoblastic leukemia and reduce gastrointestinal toxicity

T-cell acute lymphoblastic leukemia (T-ALL) is a serious hematologic malignancy and glucocorticoid resistance is the main recurrent cause for a high relapsed and death rate. Here, we proposed an effective therapeutic regimen of combining gamma-secretase inhibitors (GSIs) with dexamethasone (DEX) to overcome glucocorticoid resistance. Moreover, the bone marrow targeting DT7 peptide-modified lecithin nanoparticles co-loaded with DEX and GSI (TLnp/D&G) were developed to enhance T-ALL cells recognition and endocytosis. In vitro cytotoxicity studies showed that TLnp/D&G significantly inhibited cell survival and promoted apoptosis of T-ALL cells. Mechanically, we found that GSIs promoted DEX-induced cell apoptosis by two main synergetic mechanisms: 1) GSIs significantly upregulated glucocorticoid receptor (GR) expression in T-ALL and restored the glucocorticoid-induced pro-apoptotic response. 2) Both DEX and GSI synergistically inhibited BCL2 and suppressed the survival of T-ALL cells. Furthermore, in vivo studies demonstrated that TLnp/D&G showed high bone marrow accumulation and better antileukemic efficacy both in leukemia bearing models and in systemic Notch1-induced T-ALL models, with excellent biosafety and reduced gastrointestinal toxicity. Overall, our study provides new strategies for the treatment of T-ALL and promising bone marrow targeting systems with high transformation potential.

Hematopoietic Progenitors and the Bone Marrow Niche Shape the Inflammatory Response and Contribute to Chronic Disease

It is now well understood that the bone marrow (BM) compartment can sense systemic inflammatory signals and adapt through increased proliferation and lineage skewing. These coordinated and dynamic alterations in responding hematopoietic stem and progenitor cells (HSPCs), as well as in cells of the bone marrow niche, are increasingly viewed as key contributors to the inflammatory response. Growth factors, cytokines, metabolites, microbial products, and other signals can cause dysregulation across the entire hematopoietic hierarchy, leading to lineage-skewing and even long-term functional adaptations in bone marrow progenitor cells. These alterations may play a central role in the chronicity of disease as well as the links between many common chronic disorders. The possible existence of a form of “memory” in bone marrow progenitor cells is thought to contribute to innate immune responses via the generation of trained immunity (also called innate immune memory). These findings highlight how hematopoietic progenitors dynamically adapt to meet the demand for innate immune cells and how this adaptive response may be beneficial or detrimental depending on the context. In this review, we will discuss the role of bone marrow progenitor cells and their microenvironment in shaping the scope and scale of the immune response in health and disease.

Control of immunity via nutritional interventions

Nutrition affects all physiological processes including those linked to the development and function of our immune system. Here, we discuss recent evidence and emerging concepts supporting the idea that our newfound relationship with nutrition in industrialized countries has fundamentally altered the way in which our immune system is wired. This will be examined through the lens of studies showing that mild or transient reductions in dietary intake can enhance protective immunity while also limiting aberrant inflammatory responses. We will further discuss how trade-offs and priorities begin to emerge in the context of severe nutritional stress. In those settings, specific immunological functions are heightened to re-enforce processes and tissue sites most critical to survival. Altogether, these examples will emphasize the profound influence nutrition has over the immune system and highlight how a mechanistic exploration of this cross talk could ultimately lead to the design of novel therapeutic approaches that prevent and treat disease.

The microbiota regulates hematopoietic stem cell fate decisions by controlling iron availability in bone marrow

Host microbiota crosstalk is essential for the production and functional modulation of blood-cell lineages. Whether, and if so how, the microbiota influences hematopoietic stem cells (HSCs) is unclear. Here, we show that the microbiota regulates HSC self-renewal and differentiation under stress conditions by modulating local iron availability in the bone marrow (BM). In microbiota-depleted mice, HSC self-renewal was enhanced during regeneration, while the commitment toward differentiation was dramatically compromised. Mechanistically, microbiota depletion selectively impaired the recycling of red blood cells (RBCs) by BM macrophages, resulting in reduced local iron levels without affecting systemic iron homeostasis. Limiting iron availability in food (in vivo) or in culture (ex vivo), or by CD169⁺ macrophage depletion, enhanced HSC self-renewal and expansion. These results reveal an intricate interplay between the microbiota, macrophages, and iron, and their essential roles in regulating critical HSC fate decisions under stress.

Spred1 deficit promotes treatment resistance and transformation of chronic phase CML

Spred1 is highly expressed in normal hematopoietic stem cells (HSCs). Lack of Spred1 function has been associated with aberrant hematopoiesis and acute leukemias. In chronic myelogenous leukemia (CML), Spred1 is reduced in patients with accelerated phase (AP) or blast crisis (BC) CML, thereby suggesting that deficit of this protein may contribute to disease transformation. In fact, Spred1 knockout (KO) in SCLtTA/BCR-ABL CML mice either globally, or restricted to hematopoietic cells (i.e., HSCs) or to endothelial cells (ECs), led to transformation of chronic phase (CP) CML into AP/BC CML. Upon BCR-ABL induction, all three Spred1 KO CML models showed AP/BC features. However, compared with global Spred1 KO, the AP/BC phenotypes of HSC-Spred1 KO and EC-Spred1 KO CML models were attenuated, suggesting a concurrent contribution of Spred1 deficit in multiple compartments of the leukemic bone marrow niche to the CML transformation. Spred1 KO, regardless if occurred in HSCs or in ECs, increased miR-126 in LSKs (Lin-Sca-1+c-Kit+), a population enriched in leukemic stem cells (LSCs), resulting in expansion of LSCs, likely through hyperactivation of the MAPK/ERK pathway that augmented Bcl-2 expression and stability. This ultimately led to enhancement of Bcl-2-dependent oxidative phosphorylation that supported homeostasis, survival and activity of LSCs and drove AP/BC transformation.

The extracellular matrix of hematopoietic stem cell niches

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Comprehensive overview of different classes of ECM molecules in the HSC niche.

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Overview of current knowledge on role of biophysics of the HSC niche.

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Description of approaches to create artificial stem cell niches for several application.

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Importance of considering ECM in drug development and testing.

Hematopoietic stem cells (HSCs) are the life-long source of all types of blood cells. Their function is controlled by their direct microenvironment, the HSC niche in the bone marrow. Although the importance of the extracellular matrix (ECM) in the niche by orchestrating niche architecture and cellular function is widely acknowledged, it is still underexplored. In this review, we provide a comprehensive overview of the ECM in HSC niches. For this purpose, we first briefly outline HSC niche biology and then review the role of the different classes of ECM molecules in the niche one by one and how they are perceived by cells. Matrix remodeling and the emerging importance of biophysics in HSC niche function are discussed. Finally, the application of the current knowledge of ECM in the niche in form of artificial HSC niches for HSC expansion or targeted differentiation as well as drug testing is reviewed.

Cholinergic signals preserve haematopoietic stem cell quiescence during regenerative haematopoiesis

The sympathetic nervous system has been evolutionary selected to respond to stress and activates haematopoietic stem cells via noradrenergic signals. However, the pathways preserving haematopoietic stem cell quiescence and maintenance under proliferative stress remain largely unknown. Here we found that cholinergic signals preserve haematopoietic stem cell quiescence in bone-associated (endosteal) bone marrow niches. Bone marrow cholinergic neural signals increase during stress haematopoiesis and are amplified through cholinergic osteoprogenitors. Lack of cholinergic innervation impairs balanced responses to chemotherapy or irradiation and reduces haematopoietic stem cell quiescence and self-renewal. Cholinergic signals activate α7 nicotinic receptor in bone marrow mesenchymal stromal cells leading to increased CXCL12 expression and haematopoietic stem cell quiescence. Consequently, nicotine exposure increases endosteal haematopoietic stem cell quiescence in vivo and impairs hematopoietic regeneration after haematopoietic stem cell transplantation in mice. In humans, smoking history is associated with delayed normalisation of platelet counts after allogeneic haematopoietic stem cell transplantation. These results suggest that cholinergic signals preserve stem cell quiescence under proliferative stress.

Microfluidic device engineered to study the trafficking of multiple myeloma cancer cells through the sinusoidal niche of bone marrow

Multiple myeloma (MM) is an incurable B cell malignancy characterized by the accumulation of monoclonal abnormal plasma cells in the bone marrow (BM). It has been a significant challenge to study the spatiotemporal interactions of MM cancer cells with the embedded microenvironments of BM. Here we report a microfluidic device which was designed to mimic several physiological features of the BM niche: (1) sinusoidal circulation, (2) sinusoidal endothelium, and (3) stroma. The endothelial and stromal compartments were constructed and used to demonstrate the device's utility by spatiotemporally characterizing the CXCL12-mediated egression of MM cells from the BM stroma and its effects on the barrier function of endothelial cells (ECs). We found that the egression of MM cells resulted in less organized and loosely connected ECs, the widening of EC junction pores, and increased permeability through ECs, but without significantly affecting the number density of viable ECs. The results suggest that the device can be used to study the physical and secreted factors determining the trafficking of cancer cells through BM. The sinusoidal flow feature of the device provides an integral element for further creating systemic models of cancers that reside or metastasize to the BM niche.

Quantification of bone marrow interstitial pH and calcium concentration by intravital ratiometric imaging

The fate of hematopoietic stem cells (HSCs) can be directed by microenvironmental factors including extracellular calcium ion concentration ([Ca2+]e), but the local [Ca2+]e around individual HSCs in vivo remains unknown. Here we develop intravital ratiometric analyses to quantify the absolute pH and [Ca2+]e in the mouse calvarial bone marrow, taking into account the pH sensitivity of the calcium probe and the wavelength-dependent optical loss through bone. Unexpectedly, the mean [Ca2+]e in the bone marrow (1.0 ± 0.54 mM) is not significantly different from the blood serum, but the HSCs are found in locations with elevated local [Ca2+]e (1.5 ± 0.57 mM). With aging, a significant increase in [Ca2+]e is found in M-type cavities that exclusively support clonal expansion of activated HSCs. This work thus establishes a tool to investigate [Ca2+]e and pH in the HSC niche with high spatial resolution and can be broadly applied to other tissue types.

Megakaryocyte-stromal cell interactions: effect on megakaryocyte proliferation, proplatelet production, and survival

Bone marrow stromal cells provide a proper environment for the development of hematologic lineages. The incorporation of different stromal cells to in vitro culture systems would be an attractive model to study megakaryopoiesis and thrombopoiesis. Our objective was to evaluate the participation of different types of stromal cells on in vitro megakaryopoiesis, thrombopoiesis and megakaryocyte (MK) survival. CD34-positive progenitors from umbilical cord blood were differentiated into MK precursors and then co-cultured with umbilical vein endothelial cells (HUVEC), bone marrow mesenchymal stem cells (MSCs), skin fibroblasts (SF) (all human) or mouse fibroblast cell line (L929). The number of MKs (CD61-positive cells) was increased in the presence of HUVEC and SF while L929 cells decreased total and mature MK count. Concerning thrombopoiesis, HUVEC increased proplatelet (PP)-producing MKs, while MSCs, L929 and SF had the opposite effect (immunofluorescence staining and microscopic analysis).  MK survival was enhanced in MSC and SF co-cultures, as assessed by evaluation of pyknotic nuclei. However, HUVEC and L929 did not prevent apoptosis of MKs. Reciprocally, the cloning efficiency of MSCs was decreased in the presence of MKs, while the ability of stromal cells (either MSCs or SF) to produce the extracellular matrix proteins type III collagen, fibronectin, dermatan sulfate, heparan sulfate and P4HB was preserved. These data indicate that each stromal cell type performs distinctive functions, which differentially modulate MK growth and platelet production, and, at the same time, that MKs also modify stromal cells behavior. Overall, our results highlight the relevance of considering the influence of stromal cells in MK research as well as the close interplay of different cell types within the bone marrow milieu.

Ginsenoside Rg1 as a Potential Regulator of Hematopoietic Stem/Progenitor Cells

Ginsenoside Rg1 (Rg1), a purified, active component of the root or stem of ginseng, exerts positive effects on mesenchymal stem cells (MSCs). Many recent studies have found that hematopoietic stem cells (HSCs), which can develop into hematopoietic progenitor cells (HPCs) and mature blood cells, are another class of heterogeneous adult stem cells that can be regulated by Rg1. Rg1 can affect HSC proliferation and migration, regulate HSC/HPC differentiation, and alleviate HSC aging, and these findings potentially provide new strategies to improve the HSC homing rate in HSC transplantation and for the treatment of graft-versus-host disease (GVHD) or other HSC/HPC dysplasia-induced diseases. In this review, we used bioinformatics methods, molecular docking verification, and a literature review to systematically explore the possible molecular pharmacological activities of Rg1 through which it regulates HSCs/HPCs.

High content Image Analysis to study phenotypic heterogeneity in endothelial cell monolayers

Endothelial cells (EC) are heterogeneous across and within tissues, reflecting distinct, specialised functions. EC heterogeneity has been proposed to underpin EC plasticity independently from vessel microenvironments. However, heterogeneity driven by contact-dependent or short-range cell-cell crosstalk cannot be evaluated with single cell transcriptomic approaches as spatial and contextual information is lost. Nonetheless, quantification of EC heterogeneity and understanding of its molecular drivers is key to developing novel therapeutics for cancer, cardiovascular diseases and for revascularisation in regenerative medicine. Here, we developed an EC profiling tool (ECPT) to examine individual cells within intact monolayers. We used ECPT to characterise different phenotypes in arterial, venous and microvascular EC populations. In line with other studies, we measured heterogeneity in terms of cell cycle, proliferation, and junction organisation. ECPT uncovered a previously under-appreciated single-cell heterogeneity in NOTCH activation. We correlated cell proliferation with different NOTCH activation states at the single cell and population levels. The positional and relational information extracted with our novel approach is key to elucidating the molecular mechanisms underpinning EC heterogeneity.

Thorny ground, rocky soil: Tissue-specific mechanisms of tumor dormancy and relapse

Disseminated tumor cells (DTCs) spread systemically yet distinct patterns of metastasis indicate a range of tissue susceptibility to metastatic colonization. Distinctions between permissive and suppressive tissues are still being elucidated at cellular and molecular levels. Although there is a growing appreciation for the role of the microenvironment in regulating metastatic success, we have a limited understanding of how diverse tissues regulate DTC dormancy, the state of reversible quiescence and subsequent awakening thought to contribute to delayed relapse. Several themes of microenvironmental regulation of dormancy are beginning to emerge, including vascular association, co-option of pre-existing niches, metabolic adaptation, and immune evasion, with tissue-specific nuances. Conversely, DTC awakening is often associated with injury or inflammation-induced activation of the stroma, promoting a proliferative environment with DTCs following suit. We review what is known about tissue-specific regulation of tumor dormancy on a tissue-by-tissue basis, profiling major metastatic organs including the bone, lung, brain, liver, and lymph node. An aerial view of the barriers to metastatic growth may reveal common targets and dependencies to inform the therapeutic prevention of relapse.