Endothelial-specific inhibition of NF-κB enhances functional haematopoiesis

Endothelial heterogeneity in bone marrow: insights across development, adult life and leukemia

The central role of the endothelial microenvironment in orchestrating bone marrow (BM) homeostasis and hematopoietic support has been confirmed at various developmental stages and in adult life. The BM vasculature is crucial in mediating communication between BM parenchyma and circulating blood, displaying remarkable heterogeneity in structure and function. While vascular cell diversity in other tissues has long been recognized, the molecular basis of this phenomenon in BM is just now emerging. Over the past decade, single-cell approaches and microscopic observations have expanded our understanding of BM vasculature. While solely characterized for their paracrine properties in the past, recent advances have revolutionized our perception of endothelial function, revealing distinct anatomical locations associated with diverse endothelial cell states. The identification of phenotypic differences between normal and pathological conditions has therefore deepened our understanding of vascular dynamics and their impact on hematopoiesis in health and disease. In this review, we highlight key milestones and recent advances in understanding vascular heterogeneity within BM microenvironment during development, adulthood and aging. We also explore how leukemia affects this heterogeneity and how we can take this knowledge forward to improve clinical practices. By synthesizing existing literature, we aim to address unresolved questions and outline future research directions.

CCR2 cooperativity promotes hematopoietic stem cell homing to the bone marrow

Cross-talk between hematopoietic stem and progenitor cells (HSPCs) and bone marrow (BM) cells is critical for homing and sustained engraftment after transplantation. In particular, molecular and physical adaptation of sinusoidal endothelial cells (ECs) promote HSPC BM occupancy; however, signals that govern these events are not well understood. Extracellular vesicles (EVs) are mediators of cell-cell communication crucial in shaping tissue microenvironments. Here, we demonstrate that integrin α4β7 on murine HSPC EVs targets uptake into ECs. In BM ECs, HSPC EVs induce up-regulation of C-C motif chemokine receptor 2 (CCR2) ligands that synergize with CXCL12-CXCR4 signaling to promote BM homing. In nonirradiated murine models, marrow preconditioning with HSPC EVs or recombinant CCR2 ligands improves homing and early graft occupancy after transplantation. These findings identify a role for HSPC EVs in remodeling ECs, newly define CCR2-dependent graft homing, and inform novel translational conditioning strategies to improve HSPC transplantation.

The role of vascular and lymphatic networks in bone and joint homeostasis and pathology

The vascular and lymphatic systems are integral to maintaining skeletal homeostasis and responding to pathological conditions in bone and joint tissues. This review explores the interplay between blood vessels and lymphatic vessels in bones and joints, focusing on their roles in homeostasis, regeneration, and disease progression. Type H blood vessels, characterized by high expression of CD31 and endomucin, are crucial for coupling angiogenesis with osteogenesis, thus supporting bone homeostasis and repair. These vessels facilitate nutrient delivery and waste removal, and their dysfunction can lead to conditions such as ischemia and arthritis. Recent discoveries have highlighted the presence and significance of lymphatic vessels within bone tissue, challenging the traditional view that bones are devoid of lymphatics. Lymphatic vessels contribute to interstitial fluid regulation, immune cell trafficking, and tissue repair through lymphangiocrine signaling. The pathological alterations in these networks are closely linked to inflammatory joint diseases, emphasizing the need for further research into their co-regulatory mechanisms. This comprehensive review summarizes the current understanding of the structural and functional aspects of vascular and lymphatic networks in bone and joint tissues, their roles in homeostasis, and the implications of their dysfunction in disease. By elucidating the dynamic interactions between these systems, we aim to enhance the understanding of their contributions to skeletal health and disease, potentially informing the development of targeted therapeutic strategies.

ZMAT3 participated in benzene-caused disruption in self-renewal and differentiation of hematopoietic stem cells via TNF-α/NF-κB pathway

Benzene is a common environmental and occupational pollutant, benzene exposure causes damage to hematopoietic system. ZMAT3 is a zinc finger protein which has important biological functions. In this study, benzene-exposed mouse model and ZMAT3 overexpression and low expression hematopoietic stem cells (HSCs) models were constructed to explore the mechanism of ZMAT3 in benzene-induced hematopoietic toxicity. The results showed that benzene increased the expression of ZMAT3 in mouse bone marrow (BM) cells, HSCs and peripheral blood (PB) leukocyte, and the changes in HSCs were more sensitive than BM and PB cells. In addition, overexpression of ZMAT3 decreased the self-renewal ability of HSCs and reduced the HSCs differentiation into myeloid hematopoietic cells, while low expression has the opposite effect. Besides, over and low expression of ZMAT3 both increased the HSCs differentiation into lymphoid progenitor cells. Moreover, bioinformatics analysis suggested that ZMAT3 was associated with TNF-α signaling pathway, and the correlation was confirmed in mouse model. Meanwhile, the results indicated that ZMAT3 promoted TNF-α mRNA processing by binding to the ARE structural domain on TNF-α and interacting with hnRNP A2/B1 and hnRNP A1 proteins, ultimately activating the NF-κB signaling pathway. This study provides a new mechanism for the study of benzene toxicity.

Endothelial Robo4 suppresses endothelial-to-mesenchymal transition induced by irradiation and improves hematopoietic reconstitution

Bone marrow ablation is routinely performed before hematopoietic stem cell transplantation (HSCT). Hematopoietic stem and progenitor cells (HSPCs) require a stable bone marrow microenvironment to expand and refill the peripheral blood cell pool after ablation. Roundabout guidance receptor 4 (Robo4) is a transmembrane protein exclusive to endothelial cells and is vital in preserving vascular integrity. Hence, the hypothesis is that Robo4 maintains the integrity of bone marrow endothelial cells following radiotherapy. We created an endothelial cell injury model with γ-radiation before Robo4 gene manipulation using lentiviral-mediated RNAi and gene overexpression techniques. We demonstrate that Robo4 and specific mesenchymal proteins (Fibronectin, Vimentin, αSma, and S100A4) are upregulated in endothelial cells exposed to irradiation (IR). We found that Robo4 depletion increases the expression of endoglin (CD105), an auxiliary receptor for the transforming growth factor (TGF-β) family of proteins, and promotes endothelial-to-mesenchymal transition (End-MT) through activation of both the canonical (Smad) and non-canonical (AKT/NF-κB) signaling pathways to facilitate Snail1 activation and its nuclear translocation. Endothelial Robo4 overexpression stimulates the expression of immunoglobulin-like adhesion molecules (ICAM-1 and VCAM-1) and alleviates irradiation-induced End-MT. Our coculture model showed that transcriptional downregulation of endothelial Robo4 reduces HSPC proliferation and increases HSC quiescence and apoptosis. However, Robo4 overexpression mitigated the damaged endothelium's suppressive effects on HSC proliferation and differentiation. These findings indicate that by controlling End-MT, Robo4 preserves microvascular integrity after radiation preconditioning, protects endothelial function, and lessens the inhibitory effect of damaged endothelium on hematopoietic reconstitution.

2nd Window NIR Imaging of Radiation Injury Mitigation Provided by Reduced Notch-Dll4 Expression on Vasculature

PURPOSE

Vascular endothelium plays a central role in the pathogenesis of acute and chronic radiation injuries, yet the mechanisms which promote sustained endothelial dysfunction and contribute to late responding organ failure are unclear. We employed 2nd window (> 1100 nm emission) Near-Infrared (NIR) imaging using indocyanine green (ICG) to track and define the role of the notch ligand Delta-like ligand 4 (Dll4) in mediating vascular injury in two late-responding radiosensitive organs: the lung and kidney.

PROCEDURES

Consomic strains of female Salt Sensitive or SS (Dll4-high) and SS with 3rd chromosome inherited from Brown Norway, SS.BN3 (Dll4-low) rats at ages 11-12 weeks were used to demonstrate the impact of reduced Dll4 expression on long-term vascular integrity, renal function, and survival following high-dose 13 Gy partial body irradiation at 42- and 90 days post-radiation. 2nd window dynamic NIR fluorescence imaging with ICG was analyzed with physiology-based pharmacokinetic modeling and confirmed with assays of endothelial Dll4 expression to assess the role of endogenous Dll4 expression on radiation injury protection.

RESULTS

We show that SS.BN3 (Dll4-low) rats are relatively protected from vascular permeability disruption compared to the SS (Dll4-high) strain. We further demonstrated that SS.BN3 (Dll4-low) rats have reduced radiation induced loss of CD31+ vascular endothelial cells, and increased Dll4 vascular expression is correlated with vascular dysfunction.

CONCLUSIONS

Together, these data suggest Dll4 plays a key role in pathogenesis of radiation-induced vascular injury to the lung and kidney.

Cord Blood-Derived Endothelial Progenitor Cells Promote In Vivo Regeneration of Human Hematopoietic Bone Marrow

PURPOSE

Victims of acute radiation exposure are susceptible to hematopoietic toxicity due to bone marrow damage and loss of mature blood elements. Here, we evaluated cord blood-derived endothelial progenitor cells (CB-EPCs) as a potential cellular therapy for mitigation of hematologic acute radiation syndrome. CB-EPCs express endothelial cell markers and maintain their growth characteristics beyond 10+ passages without diminishing their doubling capacity. Further, CB-EPCs can be cryopreserved in vapor-phase liquid nitrogen and easily recovered for propagation, making them an attractive nonimmunogenic cellular therapy for off-the-shelf use. Importantly, we show CB-EPCs have the capacity to potently expand adult human bone marrow hematopoietic progenitor cells both in vitro and in vivo.

METHODS AND MATERIALS

To demonstrate the role of CB-EPCs in promoting in vivo human immune reconstitution after irradiation, we employed a novel humanized mouse model established by transplant of CD34+ bone marrow cells from 9 unique adult organ donors into immunocompromised NSG-SGM3 mice. The response of the humanized immune system to ionizing irradiation was then tested by exposure to 1 Gy followed by subcutaneous treatment of CB-EPCs, Food and Drug Administration-approved growth factor pegfilgrastim (0.3 mg/kg), or saline.

RESULTS

At day 7, total human bone marrow was decreased by 80% in irradiated controls. However, treatment with either growth factor pegfilgrastim or CB-EPCs increased recovery of total human bone marrow by 2.5-fold compared with saline. Notably, CB-EPCs also increased recovery of both human CD34+ progenitors by 5-fold and colony-forming capacity by 3-fold versus saline. Additionally, CB-EPCs promoted recovery of endogenous bone marrow endothelial cells as observed by both increased vessel area and length compared with saline.

CONCLUSIONS

These findings indicate the feasibility of using humanized mice engrafted with adult bone marrow for radiation research and the development of CB-EPCs as an off-the-shelf cellular therapy for mitigation of hematologic acute radiation syndrome.

Chronic inflammation decreases HSC fitness by activating the druggable Jak/Stat3 signaling pathway

Chronic inflammation represents a major threat to human health since long-term systemic inflammation is known to affect distinct tissues and organs. Recently, solid evidence demonstrated that chronic inflammation affects hematopoiesis; however, how chronic inflammation affects hematopoietic stem cells (HSCs) on the mechanistic level is poorly understood. Here, we employ a mouse model of chronic multifocal osteomyelitis (CMO) to assess the effects of a spontaneously developed inflammatory condition on HSCs. We demonstrate that hematopoietic and nonhematopoietic compartments in CMO BM contribute to HSC expansion and impair their function. Remarkably, our results suggest that the typical features of murine multifocal osteomyelitis and the HSC phenotype are mechanistically decoupled. We show that the CMO environment imprints a myeloid gene signature and imposes a pro-inflammatory profile on HSCs. We identify IL-6 and the Jak/Stat3 signaling pathway as critical mediators. However, while IL-6 and Stat3 blockage reduce HSC numbers in CMO mice, only inhibition of Stat3 activity significantly rescues their fitness. Our data emphasize the detrimental effects of chronic inflammation on stem cell function, opening new venues for treatment.

The glycolytic enzyme PFKFB3 determines bone marrow endothelial progenitor cell damage after chemotherapy and irradiation

Bone marrow (BM) endothelial progenitor cell (EPC) damage of unknown mechanism delays the repair of endothelial cells (EC) and recovery of hematopoiesis after chemo-radiotherapy. We found increased levels of the glycolytic enzyme PFKFB3 in the damaged BM EPC of patients with poor graft function, a clinical model of EPC damage-associated poor hematopoiesis after allogeneic hematopoietic stem cell transplantation. Moreover, in vitro the glycolysis inhibitor 3-(3-pyridinyl)- 1-(4-pyridinyl)-2-propen-1-one (3PO) alleviated the damaged BM EPC from patients with poor graft function. Consistently, PFKFB3 overexpression triggered BM EPC damage after 5-fluorouracil treatment and impaired hematopoiesis-supporting ability in vitro. Mechanistically, PFKFB3 facilitated pro-apoptotic transcription factor FOXO3A and expression of its downstream genes, including p21, p27, and FAS, after 5-fluorouracil treatment in vitro. Moreover, PFKFB3 induced activation of NF-κB and expression of its downstream adhesion molecule E-selectin, while it reduced hematopoietic factor SDF-1 expression, which could be rescued by FOXO3A silencing. High expression of PFKFB3 was found in damaged BM EC of murine models of chemo-radiotherapy-induced myelosuppression. Furthermore, a murine model of BM EC-specific PFKFB3 overexpression demonstrated that PFKFB3 aggravated BM EC damage, and impaired the recovery of hematopoiesis after chemotherapy in vivo, effects which could be mitigated by 3PO, indicating a critical role of PFKFB3 in regulating BM EC damage. Clinically, PFKFB3-induced FOXO3A expression and NF-κB activation were confirmed to contribute to the damaged BM EPC of patients with acute leukemia after chemotherapy. 3PO repaired the damaged BM EPC by reducing FOXO3A expression and phospho-NF-κB p65 in patients after chemotherapy. In summary, our results reveal a critical role of PFKFB3 in triggering BM EPC damage and indicate that endothelial-PFKFB3 may be a potential therapeutic target for myelosuppressive injury.

Multiple Nf1 Schwann cell populations reprogram the plexiform neurofibroma tumor microenvironment

To define alterations early in tumor formation, we studied nerve tumors in neurofibromatosis 1 (NF1), a tumor predisposition syndrome. Affected individuals develop neurofibromas, benign tumors driven by NF1 loss in Schwann cells (SCs). By comparing normal nerve cells to plexiform neurofibroma (PN) cells using single-cell and bulk RNA sequencing, we identified changes in 5 SC populations, including a de novo SC progenitor-like (SCP-like) population. Long after Nf1 loss, SC populations developed PN-specific expression of Dcn, Postn, and Cd74, with sustained expression of the injury response gene Postn and showed dramatic expansion of immune and stromal cell populations; in corresponding human PNs, the immune and stromal cells comprised 90% of cells. Comparisons between injury-related and tumor monocytes/macrophages support early monocyte recruitment and aberrant macrophage differentiation. Cross-species analysis verified each SC population and unique conserved patterns of predicted cell-cell communication in each SC population. This analysis identified PROS1-AXL, FGF-FGFR, and MIF-CD74 and its effector pathway NF-κB as deregulated in NF1 SC populations, including SCP-like cells predicted to influence other types of SCs, stromal cells, and/or immune cells in mouse and human. These findings highlight remarkable changes in multiple types of SCs and identify therapeutic targets for PN.

Evaluation of the antiviral effect of four plant polysaccharides against duck circovirus

Recently, outbreaks of duck circovirus (DuCV) are frequently occurring worldwide due to secondary infections caused by post infection-induced immunosuppression. Due to a lack of preventive drugs and vaccines, the waterfowl industry losses are ever increasing. In this study, we extracted Astragalus polysaccharides (APS), pine pollen polysaccharides (PPPS), Aloe vera polysaccharides (AVE), and Ficus carica polysaccharides (FCPS) from Astragalus, pine pollen, aloe, and F. carica leaves, respectively. We randomly divided 150 one-day-old Cherry Valley ducks into five groups, which were inoculated with the DuCV solution and orally administered APS, PPPS, AVE, FCPS, and phosphate buffer saline (PBS), respectively. We collected the duck immune organs and serum samples at 8, 16, 24, 32, 40, and 48 days post-infection (dpi). Using clinical symptom analysis, molecular biology experiments, and serological experiments, we proved that plant polysaccharides could (a) improve the duck immunity, (b) reduce the viral load, and (c) mitigate DuCV-induced damage to immune organs, with both APS and PPPS having significant effects. Moreover, we detected viral load and cytokines within the first 8 dpi. Since the body's innate immunity could inhibit viral replication within five days of virus infection, 1-5 dpi was the best treatment time. Among the four polysaccharides showing in vitro anti-apoptotic activity, APS and PPPS significantly inhibited the DuCV infection-induced apoptosis of peripheral blood lymphocytes. Overall, since our findings show APS and PPPS having significant anti-DuCV effects both in vivo and in vitro, they can be promising candidates for preventing DuCV infection in ducks.

A recombinant affitoxin derived from a HER3 affibody and diphteria-toxin has potent and selective antitumor activity

High expression of receptor tyrosine-protein kinase erbB-3 (HER3) has been found in several malignancies such as breast cancer. In this study, we designed, produced and evaluated a new affitoxin consisting of a truncated form of diphtheria toxin and a HER3-binding affibody domains. The new affitoxin was expressed in Escherichia coli and purified by affinity chromatography. We evaluated the suitability of affitoxin to kill HER3 positive breast cancer cells with MTT and apoptosis assays. The protein synthesis inhibition was also evaluated. The IC50 value in HER3 negative cells is about 10 times more than HER3 positive cells in new design of affitoxin. The specificity of affitoxin for binding to HER3 positive cells was also investigated with binding assay with flow cytometry. The results show that, the new affitoxin is an anti-cancer molecule with specific binding to HER3 positive cells and may open a new window for the treatment of HER3-positive cancers.

Dysfunctional bone marrow endothelial progenitor cells are involved in patients with myelodysplastic syndromes

BACKGROUND

Myelodysplastic syndromes (MDS) are a group of heterogeneous myeloid clonal disorders characterized by ineffective haematopoiesis and immune deregulation. Emerging evidence has shown the effect of bone marrow (BM) endothelial progenitor cells (EPCs) in regulating haematopoiesis and immune balance. However, the number and functions of BM EPCs in patients with different stages of MDS remain largely unknown.

METHODS

Patients with MDS (N = 30), de novo acute myeloid leukaemia (AML) (N = 15), and healthy donors (HDs) (N = 15) were enrolled. MDS patients were divided into lower-risk MDS (N = 15) and higher-risk MDS (N = 15) groups according to the dichotomization of the Revised International Prognostic Scoring System. Flow cytometry was performed to analyse the number of BM EPCs. Tube formation and migration assays were performed to evaluate the functions of BM EPCs. In order to assess the gene expression profiles of BM EPCs, RNA sequencing (RNA-seq) were performed. BM EPC supporting abilities of haematopoietic stem cells (HSCs), leukaemia cells and T cells were assessed by in vitro coculture experiments.

RESULTS

Increased but dysfunctional BM EPCs were found in MDS patients compared with HDs, especially in patients with higher-risk MDS. RNA-seq indicated the progressive change and differences of haematopoiesis- and immune-related pathways and genes in MDS BM EPCs. In vitro coculture experiments verified that BM EPCs from HDs, lower-risk MDS, and higher-risk MDS to AML exhibited a progressively decreased ability to support HSCs, manifested as elevated apoptosis rates and intracellular reactive oxygen species (ROS) levels and decreased colony-forming unit plating efficiencies of HSCs. Moreover, BM EPCs from higher-risk MDS patients demonstrated an increased ability to support leukaemia cells, characterized by increased proliferation, leukaemia colony-forming unit plating efficiencies, decreased apoptosis rates and apoptosis-related genes. Furthermore, BM EPCs induced T cell differentiation towards more immune-tolerant cells in higher-risk MDS patients in vitro. In addition, the levels of intracellular ROS and the apoptosis ratios were increased in BM EPCs from MDS patients, especially in higher-risk MDS patients, which may be therapeutic candidates for MDS patients.

CONCLUSION

Our results suggest that dysfunctional BM EPCs are involved in MDS patients, which indicates that improving haematopoiesis supporting ability and immuneregulation ability of BM EPCs may represent a promising therapeutic approach for MDS patients.

The Intercellular Communication Between Mesenchymal Stromal Cells and Hematopoietic Stem Cells Critically Depends on NF-κB Signalling in the Mesenchymal Stromal Cells

Mesenchymal stromal cells (MSCs) regulate the fate of the hematopoietic stem cells (HSCs) through both cell-cell interactions and paracrine mechanisms involving multiple signalling pathways. We have previously shown that co-culturing of HSCs with CoCl₂-treated MSCs expands functional HSCs. While performing these experiments, we had observed that the growth of CoCl₂-treated MSCs was significantly stunted. Here, we show that CoCl₂-treated MSCs possess activated NF-κB signalling pathway, and its pharmacological inhibition significantly relieves their growth arrest. Most interestingly, we found that pharmacological inhibition of NF-κB pathway in both control and CoCl₂-treated MSCs completely blocks their intercellular communication with the co-cultured hematopoietic stem and progenitor cells (HSPCs), resulting in an extremely poor output of hematopoietic cells. Mechanistically, we show that this is due to the down-regulation of adhesion molecules and various HSC-supportive factors in the MSCs. This loss of physical interaction with HSPCs could be partially restored by treating the MSCs with calcium ionophore or calmodulin, suggesting that NF-κB regulates intracellular calcium flux in the MSCs. Importantly, the HSPCs co-cultured with NF-κB-inhibited-MSCs were in a quiescent state, which could be rescued by re-culturing them with untreated MSCs. Our data underscore a critical requirement of NF-κB signalling in the MSCs in intercellular communication between HSCs and MSCs for effective hematopoiesis to occur ex vivo. Our data raises a cautionary note against excessive use of anti-inflammatory drugs targeting NF-κB.

Endothelial cell-derived angiopoietin-like protein 2 supports hematopoietic stem cell activities in bone marrow niches

Endothelial cell-derived ANGPTL2 is important for the maintenance of HSC activities in bone marrow niches.

ANGPTL2-mediated signaling pathways enhance PPARδ expression to transactivate G0s2 to sustain HSC activities.

Bone marrow niche cells have been reported to fine-tune hematopoietic stem cell (HSC) stemness via direct interaction or secreted components. Nevertheless, how niche cells control HSC activities remains largely unknown. We previously showed that angiopoietin-like protein 2 (ANGPTL2) can support the ex vivo expansion of HSCs by binding to human leukocyte immunoglobulin-like receptor B2. However, how ANGPTL2 from specific niche cell types regulates HSC activities under physiological conditions is still not clear. Herein, we generated an Angptl2-flox/flox transgenic mouse line and conditionally deleted Angptl2 expression in several niche cells, including Cdh5⁺ or Tie2⁺ endothelial cells, Prx1⁺ mesenchymal stem cells, and Pf4⁺ megakaryocytes, to evaluate its role in the regulation of HSC fate. Interestingly, we demonstrated that only endothelial cell-derived ANGPTL2 and not ANGPTL2 from other niche cell types plays important roles in supporting repopulation capacity, quiescent status, and niche localization. Mechanistically, ANGPTL2 enhances peroxisome-proliferator-activated receptor D (PPARD) expression to transactivate G0s2 to sustain the perinuclear localization of nucleolin to prevent HSCs from entering the cell cycle. These findings reveal that endothelial cell-derived ANGPTL2 serves as a critical niche component to maintain HSC stemness, which may benefit the understanding of stem cell biology in bone marrow niches and the development of a unique strategy for the ex vivo expansion of HSCs.

Vascular Regulation of Hematopoietic Stem Cell Homeostasis, Regeneration, and Aging

Purpose of Review

Hematopoietic stem cells (HSCs) sit at the top of the hierarchy that meets the daily burden of blood production. HSC maintenance relies on extrinsic cues from the bone marrow (BM) microenvironment to balance stem cell self-renewal and cell fate decisions. In this brief review, we will highlight the studies and model systems that define the centralized role of BM vascular endothelium in modulating HSC activity in health and stress.

Recent Findings

The BM microenvironment is composed of a diverse array of intimately associated vascular and perivascular cell types. Recent dynamic imaging studies, coupled with single-cell RNA sequencing (scRNA-seq) and functional readouts, have advanced our understanding of the HSC-supportive cell types and their cooperative mechanisms that govern stem cell fate during homeostasis, regeneration, and aging. These findings have established complex and discrete vascular microenvironments within the BM that express overlapping and unique paracrine signals that modulate HSC fate.

Summary

Understanding the spatial and reciprocal HSC-niche interactions and the molecular mechanisms that govern HSC activity in the BM vascular microenvironment will be integral in developing therapies aimed at ameliorating hematological disease and supporting healthy hematopoietic output.

Inflammatory Modulation of Hematopoiesis: Linking Trained Immunity and Clonal Hematopoiesis with Chronic Disorders

Inflammation-adapted hematopoietic stem and progenitor cells (HSPCs) have long been appreciated as key drivers of emergency myelopoiesis, thereby enabling the bone marrow to meet the elevated demand for myeloid cell generation under various stress conditions, such as systemic infection, inflammation, or myelosuppressive insults. In recent years, HSPC adaptations were associated with potential involvement in the induction of long-lived trained immunity and the emergence of clonal hematopoiesis of indeterminate potential (CHIP). Whereas trained immunity has context-dependent effects, protective in infections and tumors but potentially detrimental in chronic inflammatory diseases, CHIP increases the risk for hematological neoplastic disorders and cardiometabolic pathologies. This review focuses on the inflammatory regulation of HSPCs in the aforementioned processes and discusses how modulation of HSPC function could lead to novel therapeutic interventions. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Skeleton-vasculature chain reaction: a novel insight into the mystery of homeostasis

Angiogenesis and osteogenesis are coupled. However, the cellular and molecular regulation of these processes remains to be further investigated. Both tissues have recently been recognized as endocrine organs, which has stimulated research interest in the screening and functional identification of novel paracrine factors from both tissues. This review aims to elaborate on the novelty and significance of endocrine regulatory loops between bone and the vasculature. In addition, research progress related to the bone vasculature, vessel-related skeletal diseases, pathological conditions, and angiogenesis-targeted therapeutic strategies are also summarized. With respect to future perspectives, new techniques such as single-cell sequencing, which can be used to show the cellular diversity and plasticity of both tissues, are facilitating progress in this field. Moreover, extracellular vesicle-mediated nuclear acid communication deserves further investigation. In conclusion, a deeper understanding of the cellular and molecular regulation of angiogenesis and osteogenesis coupling may offer an opportunity to identify new therapeutic targets.

The Dynamic Interface Between the Bone Marrow Vascular Niche and Hematopoietic Stem Cells in Myeloid Malignancy

Hematopoietic stem cells interact with bone marrow niches, including highly specialized blood vessels. Recent studies have revealed the phenotypic and functional heterogeneity of bone marrow endothelial cells. This has facilitated the analysis of the vascular microenvironment in steady state and malignant hematopoiesis. In this review, we provide an overview of the bone marrow microenvironment, focusing on refined analyses of the marrow vascular compartment performed in mouse studies. We also discuss the emerging role of the vascular niche in “inflamm-aging” and clonal hematopoiesis, and how the endothelial microenvironment influences, supports and interacts with hematopoietic cells in acute myeloid leukemia and myelodysplastic syndromes, as exemplar states of malignant myelopoiesis. Finally, we provide an overview of strategies for modulating these bidirectional interactions to therapeutic effect in myeloid malignancies.

Endothelial mTOR maintains hematopoiesis during aging

Aging leads to a decline in hematopoietic stem and progenitor cell (HSPC) function. We recently discovered that aging of bone marrow endothelial cells (BMECs) leads to an altered crosstalk between the BMEC niche and HSPCs, which instructs young HSPCs to behave as aged HSPCs. Here, we demonstrate aging leads to a decrease in mTOR signaling within BMECs that potentially underlies the age-related impairment of their niche activity. Our findings reveal that pharmacological inhibition of mTOR using Rapamycin has deleterious effects on hematopoiesis. To formally determine whether endothelial-specific inhibition of mTOR can influence hematopoietic aging, we conditionally deleted mTOR in ECs (mTOR^(ECKO)) of young mice and observed that their HSPCs displayed attributes of an aged hematopoietic system. Transcriptional profiling of HSPCs from mTOR^(ECKO) mice revealed that their transcriptome resembled aged HSPCs. Notably, during serial transplantations, exposure of wild-type HSPCs to an mTOR^(ECKO) microenvironment was sufficient to recapitulate aging-associated phenotypes, confirming the instructive role of EC-derived signals in governing HSPC aging.

Overexpression of HOXA9 upregulates NF-κB signaling to promote human hematopoiesis and alter the hematopoietic differentiation potentials

Background

The HOX genes are master regulators of embryogenesis that are also involved in hematopoiesis. HOXA9 belongs to a cluster of HOX genes that play extensively studied roles in hematopoiesis and leukemogenesis.

Methods

We established HOXA9-inducible human embryonic stem cells (HOXA9/hESCs) with normal pluripotency and potential for hematopoiesis, which could be used to analyze gene function with high accuracy. HOXA9/hESCs co-cultured with aorta–gonad–mesonephros-derived stromal cells (AGM-S3) were induced to overexpress HOXA9 with doxycycline (DOX) at various times after hematopoiesis started and then subjected to flow cytometry.

Results

Induction of HOXA9 from Day 4 (D4) or later notably promoted hematopoiesis and also increased the production of CD34+ cells and derived populations. The potential for myelogenesis was significantly elevated while the potential for erythrogenesis was significantly reduced. At D14, a significant promotion of S phase was observed in green fluorescent protein positive (GFP+) cells overexpressing HOXA9. NF-κB signaling was also up-regulated at D14 following induction of HOXA9 on D4. All of these effects could be counteracted by addition of an NF-κB inhibitor or siRNA against NFKB1 along with DOX.

Conclusions

Overexpression of HOXA9 starting at D4 or later during hematopoiesis significantly promoted hematopoiesis and the production of myeloid progenitors while reduced the production of erythroid progenitors, indicating that HOXA9 plays a key role in hematopoiesis and differentiation of hematopoietic lineages.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13619-020-00066-0.

Bone Vasculature and Bone Marrow Vascular Niches in Health and Disease

Bone vasculature and bone marrow vascular niches supply oxygen, nutrients, and secrete angiocrine factors required for the survival, maintenance, and self-renewal of stem and progenitor cells. In the skeletal system, vasculature creates nurturing niches for bone and blood-forming stem cells. Blood vessels regulate hematopoiesis and drive bone formation during development, repair, and regeneration. Dysfunctional vascular niches induce skeletal aging, bone diseases, and hematological disorders. Recent cellular and molecular characterization of the bone marrow microenvironment has provided unprecedented insights into the complexity, heterogeneity, and functions of the bone vasculature and vascular niches. The bone vasculature is composed of distinct vessel subtypes that differentially regulate osteogenesis, hematopoiesis, and disease conditions in bones. Further, bone marrow vascular niches supporting stem cells are often complex microenvironments involving multiple different cell populations and vessel subtypes. This review provides an overview of the emerging vascular cell heterogeneity in bone and the new roles of the bone vasculature and associated vascular niches in health and disease. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Regeneration of the pulmonary vascular endothelium after viral pneumonia requires COUP-TF2

Acute respiratory distress syndrome is associated with a robust inflammatory response that damages the vascular endothelium, impairing gas exchange. While restoration of microcapillaries is critical to avoid mortality, therapeutic targeting of this process requires a greater understanding of endothelial repair mechanisms. Here, we demonstrate that lung endothelium possesses substantial regenerative capacity and lineage tracing reveals that native endothelium is the source of vascular repair after influenza injury. Ablation of chicken ovalbumin upstream promoter-transcription factor 2 (COUP-TF2) (Nr2f2), a transcription factor implicated in developmental angiogenesis, reduced endothelial proliferation, exacerbating viral lung injury in vivo. In vitro, COUP-TF2 regulates proliferation and migration through activation of cyclin D1 and neuropilin 1. Upon influenza injury, nuclear factor κB suppresses COUP-TF2, but surviving endothelial cells ultimately reestablish vascular homeostasis dependent on restoration of COUP-TF2. Therefore, stabilization of COUP-TF2 may represent a therapeutic strategy to enhance recovery from pathogens, including H1N1 influenza and SARS-CoV-2.

Targets for protection and mitigation of radiation injury

Protection of normal tissues against toxic effects of ionizing radiation is a critical issue in clinical and environmental radiobiology. Investigations in recent decades have suggested potential targets that are involved in the protection against radiation-induced damages to normal tissues and can be proposed for mitigation of radiation injury. Emerging evidences have been shown to be in contrast to an old dogma in radiation biology; a major amount of reactive oxygen species (ROS) production and cell toxicity occur during some hours to years after exposure to ionizing radiation. This can be attributed to upregulation of inflammatory and fibrosis mediators, epigenetic changes and disruption of the normal metabolism of oxygen. In the current review, we explain the cellular and molecular changes following exposure of normal tissues to ionizing radiation. Furthermore, we review potential targets that can be proposed for protection and mitigation of radiation toxicity.

Cancer Burden Is Controlled by Mural Cell-β3-Integrin Regulated Crosstalk with Tumor Cells

Enhanced blood vessel (BV) formation is thought to drive tumor growth through elevated nutrient delivery. However, this observation has overlooked potential roles for mural cells in directly affecting tumor growth independent of BV function. Here we provide clinical data correlating high percentages of mural-β3-integrin-negative tumor BVs with increased tumor sizes but no effect on BV numbers. Mural-β3-integrin loss also enhances tumor growth in implanted and autochthonous mouse tumor models with no detectable effects on BV numbers or function. At a molecular level, mural-cell β3-integrin loss enhances signaling via FAK-p-HGFR-p-Akt-p-p65, driving CXCL1, CCL2, and TIMP-1 production. In particular, mural-cell-derived CCL2 stimulates tumor cell MEK1-ERK1/2-ROCK2-dependent signaling and enhances tumor cell survival and tumor growth. Overall, our data indicate that mural cells can control tumor growth via paracrine signals regulated by β3-integrin, providing a previously unrecognized mechanism of cancer growth control.

Endothelial cells produce angiocrine factors to regulate bone and cartilage via versatile mechanisms

Blood vessels are conduits distributed throughout the body, supporting tissue growth and homeostasis by the transport of cells, oxygen and nutrients. Endothelial cells (ECs) form the linings of the blood vessels, and together with pericytes, are essential for organ development and tissue homeostasis through producing paracrine signalling molecules, called angiocrine factors. In the skeletal system, ECs - derived angiocrine factors, combined with bone cells-released angiogenic factors, orchestrate intercellular crosstalk of the bone microenvironment, and the coupling of angiogenesis-to-osteogenesis. Whilst the involvement of angiogenic factors and the blood vessels of the skeleton is relatively well established, the impact of ECs -derived angiocrine factors on bone and cartilage homeostasis is gradually emerging. In this review, we survey ECs - derived angiocrine factors, which are released by endothelial cells of the local microenvironment and by distal organs, and act specifically as regulators of skeletal growth and homeostasis. These may potentially include angiocrine factors with osteogenic property, such as Hedgehog, Notch, WNT, bone morphogenetic protein (BMP), fibroblast growth factor (FGF), insulin-like growth factor (IGF), and platelet-derived growth factor (PDGF). Understanding the versatile mechanisms by which ECs-derived angiocrine factors orchestrate bone and cartilage homeostasis, and pathogenesis, is an important step towards the development of therapeutic potential for skeletal diseases.

Chronic activation of endothelial MAPK disrupts hematopoiesis via NFKB dependent inflammatory stress reversible by SCGF

Inflammatory signals arising from the microenvironment have emerged as critical regulators of hematopoietic stem cell (HSC) function during diverse processes including embryonic development, infectious diseases, and myelosuppressive injuries caused by irradiation and chemotherapy. However, the contributions of cellular subsets within the microenvironment that elicit niche-driven inflammation remain poorly understood. Here, we identify endothelial cells as a crucial component in driving bone marrow (BM) inflammation and HSC dysfunction observed following myelosuppression. We demonstrate that sustained activation of endothelial MAPK causes NF-κB-dependent inflammatory stress response within the BM, leading to significant HSC dysfunction including loss of engraftment ability and a myeloid-biased output. These phenotypes are resolved upon inhibition of endothelial NF-κB signaling. We identify SCGF as a niche-derived factor that suppresses BM inflammation and enhances hematopoietic recovery following myelosuppression. Our findings demonstrate that chronic endothelial inflammation adversely impacts niche activity and HSC function which is reversible upon suppression of inflammation.

Developmental programming of adult haematopoiesis system

The Barker hypothesis of 'foetal origin of adult diseases' has led to emphasize the concept of 'developmental programming', based on the crucial role of epigenetic factors. Accordingly, it has been demonstrated that parental adversity (before conception and during pregnancy) and foetal factors (i.e., hypoxia, malnutrition and placental insufficiency) permanently modify the physiological systems of the progeny, predisposing them to premature ageing and chronic disease during adulthood. Thus, an altered functionality of the endocrine, immune, nervous and cardiovascular systems is observed in the progeny. However, it remains to be understood whether the haematopoietic system itself also represents a portrait of foetal programming. Here, we provide evidence, reporting and discussing related theories, and results of studies described in the literature. In addition, we have outlined our opinions and suggest how it is possible to intervene to correct foetal mal-programming. Some pro-health interventions and recommendations are proposed, with the hope of guarantee the health of future generations and trying to combat the continuous increase in age-related diseases in human populations.

Leukocyte Trafficking and Regulation of Murine Hematopoietic Stem Cells and Their Niches

Hematopoietic stem cells (HSC) are the most powerful type of adult stem cell found in the body. Hematopoietic stem cells are multipotent and capable of giving rise to all other types of hematopoietic cells found in the organism. A single HSC is capable of regenerating a functional hematopoietic system when transplanted into a recipient. Hematopoietic stem cells reside in the bone marrow in specific multicellular structures called niches. These niches are indispensable for maintaining and regulating HSC numbers and function. It has become increasingly clearer that HSC and their niches can also be regulated by migrating leukocytes. Here we will discuss the composition of murine bone marrow niches and how HSC and their niches are regulated by different types of leukocytes that traffic between the periphery and the niche. Unless otherwise indicated all the studies discussed below were performed in mouse models.

Constitutive Activation of NF-κB Pathway in Hematopoietic Stem Cells Causes Loss of Quiescence and Deregulated Transcription Factor Networks

Identifying physiological roles of specific signaling pathways that regulate hematopoietic stem cell (HSC) functions may lead to new treatment strategies and therapeutic interventions for hematologic disorders. Here, we provide genetic evidence that constitutive activation of NF-κB in HSCs results in reduced pool size, repopulation capacities, and quiescence of HSCs. Global transcriptional profiling and bioinformatics studies identified loss of ‘stemness’ and ‘quiescence’ signatures in HSCs with deregulated NF-κB activation. In particular, gene set enrichment analysis identified upregulation of cyclin dependent kinase- Ccnd1 and down regulation of cyclin dependent kinase inhibitor p57^kip2. Interestingly, constitutive activation of NF-κB is sufficient to alter the regulatory circuits of transcription factors (TFs) that are critical to HSC self-renewal and functions. Molecular studies identified Junb, as one of the direct targets of NF-κB in hematopoietic cells. In essence, these studies demonstrate that aberrant activation of NF-κB signals impairs HSC quiescence and functions and alters the ‘TF networks’ in HSCs.

The Instructive Role of the Bone Marrow Niche in Aging and Leukemia

Purpose of review

In this review, we aim to discuss the role of the bone marrow microenvironment in supporting hematopoiesis, with particular focus on the contribution of the endothelial niche in dictating hematopoietic stem cell (HSC) fate.

Recent findings

Evidence gathered in the past two decades revealed that specific cell types within the bone marrow niche influence the hematopoietic system. Endothelial cells have emerged as a key component of the HSC niche, directly affecting stem cell quiescence, self-renewal, and lineage differentiation. Physiological alterations of the bone marrow niche occurring in aging have been described to be sufficient to promote functional aging of young HSCs. Furthermore, a growing body of evidence suggests that aberrant activation of endothelial-derived signaling pathways can aid or trigger neoplastic transformation.

Summary

Several groups have contributed to the characterization of the different cell types that comprise the complex bone marrow environment, whose function was long perceived as an undiscernible sum of many parts. Further studies will need to uncover niche cell-type-specific pathways, in order to provide new targets and therapeutic options that aim at withdrawing the microenvironmental support to malignant cells while sparing normal HSCs.

Dynamic regulation of hematopoietic stem cells by bone marrow niches

Purpose of Review

Hematopoietic stem cells (HSC) reside in a specialized microenvironment called the HSC niche. While key components of the niche have been known for several years, recent advances have identified several additional cell types that regulate HSC in the bone marrow (BM). Here we review our current understanding of the components and dynamics of the HSC niche.

Recent Findings

While the niche has been considered a stable structure, recent advances clearly show that the niche is regulated in a dynamic manner to control HSC traffic and function. Moreover the niche can rapidly remodel in response to insults to the BM in a process controlled by positive and negative regulators.

Summary

Multiple niche cells have been shown to be dynamically regulated by systemic and local signals to influence how the niche controls HSC function. Elucidating how different components of the niche coordinate to orchestrate HSC behavior is essential to understand how the hematopoietic system adjusts blood cell production to the demands of the body.

Niches for Hematopoietic Stem Cells and Their Progeny

Steady-state hematopoietic stem cells' (HSCs) self-renewal and differentiation toward their mature progeny in the adult bone marrow is tightly regulated by cues from the microenvironment. Recent insights into the cellular and molecular constituents have uncovered a high level of complexity. Here, we review emerging evidence showing how HSCs and their progeny are regulated by an interdependent network of mesenchymal stromal cells, nerve fibers, the vasculature, and also other hematopoietic cells. Understanding the interaction mechanisms in these intricate niches will provide great opportunities for HSC-related therapies and immune modulation.

Functional Heterogeneity of Endothelial Cells Derived from Human Pluripotent Stem Cells

Specification of endothelial cells (ECs) into arterial, venous, and lymphatic cells is a crucial process of vascular development, and expanding our knowledge about EC specification from human pluripotent stem cells (hPSCs) will aid the design of optimal strategies for producing desired types of ECs for therapies. In our prior studies, we revealed that hPSC-derived VE-cadherin(V)⁺CD31⁺CD34⁺ ECs are heterogeneous and include at least three major subsets with distinct hemogenic properties: V⁺CD43/235a^-CD73^- hemogenic endothelial progenitors (HEPs), V⁺CD43^loCD235a⁺73^- angiogenic hematopoietic progenitors (AHPs), and V⁺CD43/235a^-73⁺ non-HEPs. In this study, using angiogenesis assays, we demonstrated that ECs within these subsets have distinct endothelial colony- and tube-forming properties, proliferative and migratory properties, and endothelial nitric oxide synthase and inflammatory cytokine production potentials. Culture of isolated subsets in arterial, venous, and lymphatic conditions revealed that AHPs are skewed toward lymphatic, HEPs toward arterial, and non-HEPs toward venous differentiation in vitro. These findings suggest that selection and enhancement of production of a particular EC subset may aid in generating desirable EC populations with arterial, venous, or lymphatic properties from hPSCs.

Bone Marrow Micro-Environment in Normal and Deranged Hematopoiesis: Opportunities for Regenerative Medicine and Therapies

Various cell types cooperate to create a highly organized and dynamic micro-environmental niche in the bone marrow. Over the past several years, the field has increasingly recognized the critical roles of the interplay between bone marrow environment and hematopoietic cells in normal and deranged hematopoiesis. These advances rely on several new technologies that have allowed us to characterize the identity and roles of these niches in great detail. Here, we review the progress of the last several years, list some of the outstanding questions in the field and propose ways to target the diseased environment to better treat hematologic diseases. Understanding the extrinsic regulation by the niche will help boost hematopoiesis for regenerative medicine. Based on natural development of hematologic malignancies, we propose that combinatory targeting the niche and hematopoietic intrinsic mechanisms in early stages of hematopoietic malignancies may help eliminate minimal residual disease and have the highest efficacy.

Endothelial transplantation rejuvenates aged hematopoietic stem cell function

Age-related changes in the hematopoietic compartment are primarily attributed to cell-intrinsic alterations in hematopoietic stem cells (HSCs); however, the contribution of the aged microenvironment has not been adequately evaluated. Understanding the role of the bone marrow (BM) microenvironment in supporting HSC function may prove to be beneficial in treating age-related functional hematopoietic decline. Here, we determined that aging of endothelial cells (ECs), a critical component of the BM microenvironment, was sufficient to drive hematopoietic aging phenotypes in young HSCs. We used an ex vivo hematopoietic stem and progenitor cell/EC (HSPC/EC) coculture system as well as in vivo EC infusions following myelosuppressive injury in mice to demonstrate that aged ECs impair the repopulating activity of young HSCs and impart a myeloid bias. Conversely, young ECs restored the repopulating capacity of aged HSCs but were unable to reverse the intrinsic myeloid bias. Infusion of young, HSC-supportive BM ECs enhanced hematopoietic recovery following myelosuppressive injury and restored endogenous HSC function in aged mice. Coinfusion of young ECs augmented aged HSC engraftment and enhanced overall survival in lethally irradiated mice by mitigating damage to the BM vascular microenvironment. These data lay the groundwork for the exploration of EC therapies that can serve as adjuvant modalities to enhance HSC engraftment and accelerate hematopoietic recovery in the elderly population following myelosuppressive regimens.

Regulation of the hematopoietic stem cell lifecycle by the endothelial niche

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) predominantly reside either in direct contact or in close proximity to the vascular endothelium throughout their lifespan. From the moment of HSC embryonic specification from hemogenic endothelium, endothelial cells (ECs) act as a critical cellular-hub that regulates a vast repertoire of biological processes crucial for HSC maintenance throughout its lifespan. In this review, we will discuss recent findings in endothelial niche-mediated regulation of HSC function during development, aging and regenerative conditions.

RECENT FINDINGS

Studies employing genetic vascular models have unequivocally confirmed that ECs provide the essential instructive cues for HSC emergence during embryonic development as well as adult HSC maintenance during homeostasis and regeneration. Aging of ECs may impair their ability to maintain HSC function contributing to the development of aging-associated hematopoietic deficiencies. These findings have opened up new avenues to explore the therapeutic application of ECs. ECs can be adapted to serve as an instructive platform to expand bona fide HSCs and also utilized as a cellular therapy to promote regeneration of the hematopoietic system following myelosuppressive and myeloablative injuries.

SUMMARY

ECs provide a fertile niche for maintenance of functional HSCs throughout their lifecycle. An improved understanding of the EC-HSC cross-talk will pave the way for development of EC-directed strategies for improving HSC function during aging.