Sinusoidal ephrin receptor EPHB4 controls hematopoietic progenitor cell mobilization from bone marrow

Bone marrow niches for hematopoietic stem cells

Hematopoietic stem cells (HSCs) are the cornerstone of the hematopoietic system. HSCs sustain the continuous generation of mature blood derivatives while self-renewing to preserve a relatively constant pool of progenitors throughout life. Yet, long-term maintenance of functional HSCs exclusively takes place in association with their native tissue microenvironment of the bone marrow (BM). HSCs have been long proposed to reside in fixed and identifiable anatomical units found in the complex BM tissue landscape, which control their identity and fate in a deterministic manner. In the last decades, tremendous progress has been made in the dissection of the cellular and molecular fabric of the BM, the structural organization governing tissue function, and the plethora of interactions established by HSCs. Nonetheless, a holistic model of the mechanisms controlling HSC regulation in their niche is lacking to date. Here, we provide an overview of our current understanding of BM anatomy, HSC localization, and crosstalk within local cellular neighborhoods in murine and human tissues, and highlight fundamental open questions on how HSCs functionally integrate in the BM microenvironment.

A Multifunctional Therapeutic Strategy Using P7C3 as A Countermeasure Against Bone Loss and Fragility in An Ovariectomized Rat Model of Postmenopausal Osteoporosis

By 2060, an estimated one in four Americans will be elderly. Consequently, the prevalence of osteoporosis and fragility fractures will also increase. Presently, no available intervention definitively prevents or manages osteoporosis. This study explores whether Pool 7 Compound 3 (P7C3) reduces progressive bone loss and fragility following the onset of ovariectomy (OVX)-induced osteoporosis. Results confirm OVX-induced weakened, osteoporotic bone together with a significant gain in adipogenic body weight. Treatment with P7C3 significantly reduced osteoclastic activity, bone marrow adiposity, whole-body weight gain, and preserved bone area, architecture, and mechanical strength. Analyses reveal significantly upregulated platelet derived growth factor-BB and leukemia inhibitory factor, with downregulation of interleukin-1 R6, and receptor activator of nuclear factor kappa-B (RANK). Together, proteomic data suggest the targeting of several key regulators of inflammation, bone, and adipose turnover, via transforming growth factor-beta/SMAD, and Wingless-related integration site/be-catenin signaling pathways. To the best of the knowledge, this is first evidence of an intervention that drives against bone loss via RANK. Metatranscriptomic analyses of the gut microbiota show P7C3 increased Porphyromonadaceae bacterium, Candidatus Melainabacteria, and Ruminococcaceae bacterium abundance, potentially contributing to the favorable inflammatory, and adipo-osteogenic metabolic regulation observed. The results reveal an undiscovered, and multifunctional therapeutic strategy to prevent the pathological progression of OVX-induced bone loss.

Ephs in cancer progression: complexity and context-dependent nature in signaling, angiogenesis and immunity

Eph receptors constitute the largest family of receptor tyrosine kinases, comprising 14 distinct members classified into two subgroups: EphAs and EphBs.. Despite their essential functions in normal physiological processes, accumulating evidence suggests that the involvement of the Eph family in cancer is characterized by a dual and often contradictory nature. Research indicates that Eph/ephrin bidirectional signaling influences cell-cell communication, subsequently regulating cell migration, adhesion, differentiation and proliferation. The contradictory functionalities may arise from the diversity of Eph signaling pathways and the heterogeneity of different cancer microenvironment. In this review, we aim to discuss the dual role of the Eph receptors in tumor development, attempting to elucidate the paradoxical functionality through an exploration of Eph receptor signaling pathways, angiogenesis, immune responses, and more. Our objective is to provide a comprehensive understanding of the molecular mechanisms underlying tumor development. Additionally, we will explore the evolving landscape of utilizing Eph receptors as potential targets for tumor therapy and diagnostic tools.

Inhibition of Ephrin B2 Reverse Signaling Abolishes Multiple Myeloma Pathogenesis

Bone marrow vascular endothelial cells (BM EC) regulate multiple myeloma pathogenesis. Identification of the mechanisms underlying this interaction could lead to the development of improved strategies for treating multiple myeloma. Here, we performed a transcriptomic analysis of human ECs with high capacity to promote multiple myeloma growth, revealing overexpression of the receptor tyrosine kinases, EPHB1 and EPHB4, in multiple myeloma-supportive ECs. Expression of ephrin B2 (EFNB2), the binding partner for EPHB1 and EPHB4, was significantly increased in multiple myeloma cells. Silencing EPHB1 or EPHB4 in ECs suppressed multiple myeloma growth in coculture. Similarly, loss of EFNB2 in multiple myeloma cells blocked multiple myeloma proliferation and survival in vitro, abrogated multiple myeloma engraftment in immune-deficient mice, and increased multiple myeloma sensitivity to chemotherapy. Administration of an EFNB2-targeted single-chain variable fragment also suppressed multiple myeloma growth in vivo. In contrast, overexpression of EFNB2 in multiple myeloma cells increased STAT5 activation, increased multiple myeloma cell survival and proliferation, and decreased multiple myeloma sensitivity to chemotherapy. Conversely, expression of mutant EFNB2 lacking reverse signaling capacity in multiple myeloma cells increased multiple myeloma cell death and sensitivity to chemotherapy and abolished multiple myeloma growth in vivo. Complementary analysis of multiple myeloma patient data revealed that increased EFNB2 expression is associated with adverse-risk disease and decreased survival. This study suggests that EFNB2 reverse signaling controls multiple myeloma pathogenesis and can be therapeutically targeted to improve multiple myeloma outcomes.

SIGNIFICANCE

Ephrin B2 reverse signaling mediated by endothelial cells directly regulates multiple myeloma progression and treatment resistance, which can be overcome through targeted inhibition of ephrin B2 to abolish myeloma.

Eph receptors and ephrins in cancer progression

Evidence implicating Eph receptor tyrosine kinases and their ephrin ligands (that together make up the 'Eph system') in cancer development and progression has been accumulating since the discovery of the first Eph receptor approximately 35 years ago. Advances in the past decade and a half have considerably increased the understanding of Eph receptor-ephrin signalling mechanisms in cancer and have uncovered intriguing new roles in cancer progression and drug resistance. This Review focuses mainly on these more recent developments. I provide an update on the different mechanisms of Eph receptor-ephrin-mediated cell-cell communication and cell autonomous signalling, as well as on the interplay of the Eph system with other signalling systems. I further discuss recent advances in elucidating how the Eph system controls tumour expansion, invasiveness and metastasis, supports cancer stem cells, and drives therapy resistance. In addition to functioning within cancer cells, the Eph system also mediates the reciprocal communication between cancer cells and cells of the tumour microenvironment. The involvement of the Eph system in tumour angiogenesis is well established, but recent findings also demonstrate roles in immune cells, cancer-associated fibroblasts and the extracellular matrix. Lastly, I discuss strategies under evaluation for therapeutic targeting of Eph receptors-ephrins in cancer and conclude with an outlook on promising future research directions.

An interactive resource of molecular signalling in the developing human haematopoietic stem cell niche

The emergence of definitive human haematopoietic stem cells (HSCs) from Carnegie Stage (CS) 14 to CS17 in the aorta-gonad-mesonephros (AGM) region is a tightly regulated process. Previously, we conducted spatial transcriptomic analysis of the human AGM region at the end of this period (CS16/CS17) and identified secreted factors involved in HSC development. Here, we extend our analysis to investigate the progression of dorso-ventral polarised signalling around the dorsal aorta over the entire period of HSC emergence. Our results reveal a dramatic increase in ventral signalling complexity from the CS13-CS14 transition, coinciding with the first appearance of definitive HSCs. We further observe stage-specific changes in signalling up to CS17, which may underpin the step-wise maturation of HSCs described in the mouse model. The data-rich resource is also presented in an online interface enabling in silico analysis of molecular interactions between spatially defined domains of the AGM region. This resource will be of particular interest for researchers studying mechanisms underlying human HSC development as well as those developing in vitro methods for the generation of clinically relevant HSCs from pluripotent stem cells.

EPH/Ephrin Signaling in Normal Hematopoiesis and Hematologic Malignancies: Deciphering Their Intricate Role and Unraveling Possible New Therapeutic Targets

Erythropoietin-producing hepatocellular carcinoma receptors (EPHs) represent the largest family of receptor tyrosine kinases (RTKs). EPH interaction with ephrins, their membrane-bound ligands, holds a pivotal role in embryonic development, while, though less active, it is also implicated in various physiological functions during adult life. In normal hematopoiesis, different patterns of EPH/ephrin expression have been correlated with hematopoietic stem cell (HSC) maintenance and lineage-committed hematopoietic progenitor cell (HPC) differentiation, as well as with the functional properties of their mature offspring. Research in the field of hematologic malignancies has unveiled a rather complex involvement of the EPH/ephrinsignaling pathway in the pathophysiology of these neoplasms. Aberrations in genetic, epigenetic, and protein levels have been identified as possible players implicated both in tumor progression and suppression, while correlations have also been highlighted regarding prognosis and response to treatment. Initial efforts to therapeutically target the EPH/ephrin axis have been undertaken in the setting of hematologic neoplasia but are mainly confined to the preclinical level. To this end, deciphering the complexity of this signaling pathway both in normal and malignant hematopoiesis is necessary.

Diverse roles for axon guidance pathways in adult tissue architecture and function

Classical axon guidance ligands and their neuronal receptors were first identified due to their fundamental roles in regulating connectivity in the developing nervous system. Since their initial discovery, it has become clear that these signaling molecules play important roles in the development of a broad array of tissue and organ systems across phylogeny. In addition to these diverse developmental roles, there is a growing appreciation that guidance signaling pathways have important functions in adult organisms, including the regulation of tissue integrity and homeostasis. These roles in adult organisms include both tissue-intrinsic activities of guidance molecules, as well as systemic effects on tissue maintenance and function mediated by the nervous and vascular systems. While many of these adult functions depend on mechanisms that mirror developmental activities, such as regulating adhesion and cell motility, there are also examples of adult roles that may reflect signaling activities that are distinct from known developmental mechanisms, including the contributions of guidance signaling pathways to lineage commitment in the intestinal epithelium and bone remodeling in vertebrates. In this review, we highlight studies of guidance receptors and their ligands in adult tissues outside of the nervous system, focusing on in vivo experimental contexts. Together, these studies lay the groundwork for future investigation into the conserved and tissue-specific mechanisms of guidance receptor signaling in adult tissues.

The EPH/Ephrin System in Colorectal Cancer

The EPH/ephrin system constitutes a bidirectional signaling pathway comprised of a family of tyrosine kinase receptors in tandem with their plasma membrane-bound ligand (ephrins). Its significance in a wide variety of physiologic and pathologic processes has been recognized during the past decades. In carcinogenesis, EPH/ephrins coordinate a wide spectrum of pathologic processes, such as angiogenesis, vessel infiltration, and metastasis. Despite the recent advances in colorectal cancer (CRC) diagnosis and treatment, it remains a leading cause of death globally, accounting for 9.2% of all cancer deaths. A growing body of literature has been published lately revitalizing our scientific interest towards the role of EPH/ephrins in pathogenesis and the treatment of CRC. The aim of the present review is to present the recent CRC data which might lead to clinical practice changes in the future.

Targeting the SHP2 phosphatase promotes vascular damage and inhibition of tumor growth

The tyrosine phosphatase SHP2 is oncogenic in cancers driven by receptor-tyrosine-kinases, and SHP2 inhibition reduces tumor growth. Here, we report that SHP2 is an essential promoter of endothelial cell survival and growth in the remodeling tumor vasculature. Using genetic and chemical approaches to inhibit SHP2 activity in endothelial cells, we show that SHP2 inhibits pro-apoptotic STAT3 and stimulates proliferative ERK1/2 signaling. Systemic SHP2 inhibition in mice bearing tumor types selected for SHP2-independent tumor cell growth promotes degeneration of the tumor vasculature and blood extravasation; reduces tumor vascularity and blood perfusion; and increases tumor necrosis. Reduction of tumor growth ensues, independent of SHP2 targeting in the tumor cells, blocking immune checkpoints, or recruiting macrophages. We also show that inhibiting the Angiopoietin/TIE2/AKT cascade magnifies the vascular and anti-tumor effects of SHP2 inhibition by blocking tumor endothelial AKT signaling, not a target of SHP2. Since the SHP2 and Ang2/TIE2 pathways are active in vascular endothelial cells of human melanoma and colon carcinoma, SHP2 inhibitors alone or with Ang2/TIE2 inhibitors hold promise to effectively target the tumor endothelium.

Eph-Ephrin Signaling Mediates Cross-Talk Within the Bone Microenvironment

Skeletal integrity is maintained through the tightly regulated bone remodeling process that occurs continuously throughout postnatal life to replace old bone and to repair skeletal damage. This is maintained primarily through complex interactions between bone resorbing osteoclasts and bone forming osteoblasts. Other elements within the bone microenvironment, including stromal, osteogenic, hematopoietic, endothelial and neural cells, also contribute to maintaining skeletal integrity. Disruption of the dynamic interactions between these diverse cellular systems can lead to poor bone health and an increased susceptibility to skeletal diseases including osteopenia, osteoporosis, osteoarthritis, osteomalacia, and major fractures. Recent reports have implicated a direct role for the Eph tyrosine kinase receptors and their ephrin ligands during bone development, homeostasis and skeletal repair. These membrane-bound molecules mediate contact-dependent signaling through both the Eph receptors, termed forward signaling, and through the ephrin ligands, referred to as reverse signaling. This review will focus on Eph/ ephrin cross-talk as mediators of hematopoietic and stromal cell communication, and how these interactions contribute to blood/ bone marrow function and skeletal integrity during normal steady state or pathological conditions.

Eph Receptors in the Immunosuppressive Tumor Microenvironment

The tumor microenvironment (TME) promotes tumor development via complex intercellular signaling, aiding tumor growth and suppressing immunity. Eph receptors (Eph) and their ephrin ligands control cell interactions during normal development, and reemerge in tumors and the TME, where they are implicated in invasion, metastasis, and angiogenesis. Recent studies also indicate roles for Ephs in suppressing immune responses by controlling tumor interactions with innate and adaptive immune cells within the TME. Accordingly, inhibiting these functions can promote immune response and efficacy of immune checkpoint inhibition. This research highlights Ephs as potential targets to enhance efficacy of immune-based therapies in patients with cancer.

Ephrin-B2-EphB4 communication mediates tumor-endothelial cell interactions during hematogenous spread to spinal bone in a melanoma metastasis model

Metastases account for the majority of cancer deaths. Bone represents one of the most common sites of distant metastases, and spinal bone metastasis is the most common source of neurological morbidity in cancer patients. During metastatic seeding of cancer cells, endothelial-tumor cell interactions govern extravasation to the bone and potentially represent one of the first points of action for antimetastatic treatment. The ephrin-B2-EphB4 pathway controls cellular interactions by inducing repulsive or adhesive properties, depending on forward or reverse signaling. Here, we report that in an in vivo metastatic melanoma model, ephrin-B2-mediated activation of EphB4 induces tumor cell repulsion from bone endothelium, translating in reduced spinal bone metastatic loci and improved neurological function. Selective ephrin-B2 depletion in endothelial cells or EphB4 inhibition increases bone metastasis and shortens the time window to hind-limb locomotion deficit from spinal cord compression. EphB4 overexpression in melanoma cells ameliorates the metastatic phenotype and improves neurological outcome. Timely harvesting of bone tissue after tumor cell injection and intravital bone microscopy revealed less tumor cells attached to ephrin-B2-positive endothelial cells. These results suggest that ephrin-B2-EphB4 communication influences bone metastasis formation by altering melanoma cell repulsion/adhesion to bone endothelial cells, and represents a molecular target for therapeutic intervention.

Lactate released by inflammatory bone marrow neutrophils induces their mobilization via endothelial GPR81 signaling

Neutrophils provide first line of host defense against bacterial infections utilizing glycolysis for their effector functions. How glycolysis and its major byproduct lactate are triggered in bone marrow (BM) neutrophils and their contribution to neutrophil mobilization in acute inflammation is not clear. Here we report that bacterial lipopolysaccharides (LPS) or Salmonella Typhimurium triggers lactate release by increasing glycolysis, NADPH-oxidase-mediated reactive oxygen species and HIF-1α levels in BM neutrophils. Increased release of BM lactate preferentially promotes neutrophil mobilization by reducing endothelial VE-Cadherin expression, increasing BM vascular permeability via endothelial lactate-receptor GPR81 signaling. GPR81^-/- mice mobilize reduced levels of neutrophils in response to LPS, unless rescued by VE-Cadherin disrupting antibodies. Lactate administration also induces release of the BM neutrophil mobilizers G-CSF, CXCL1 and CXCL2, indicating that this metabolite drives neutrophil mobilization via multiple pathways. Our study reveals a metabolic crosstalk between lactate-producing neutrophils and BM endothelium, which controls neutrophil mobilization under bacterial infection.

Daily Ethanol Drinking Followed by an Abstinence Period Impairs Bone Marrow Niche and Mitochondrial Function of Hematopoietic Stem/Progenitor Cells in Rhesus Macaques

BACKGROUND

Unhealthy consumption of alcohol is a major public health crisis with strong associations between immunological dysfunctions, high vulnerability to infectious disease, anemia, and an increase in the risk of hematological malignancies. However, there is a lack of studies addressing alcohol-induced changes in bone marrow (BM) and hematopoiesis as fundamental aspects of immune system function.

METHODS

To address the effect of chronic alcohol consumption on hematopoietic stem and progenitor cells (HSPCs) and the BM niche, we used an established rhesus macaque model of voluntary alcohol drinking. A cohort of young adult male rhesus macaques underwent a standard ethanol self-administration protocol that allowed a choice of drinking alcohol or water 22 hours/day with periods of forced abstinence that elevated subsequent intakes when alcohol availability resumed. Following the last month of forced abstinence, the monkeys were euthanized. HSPCs and bone samples were collected and analyzed in functional assays and by confocal microscopy.

RESULTS

HSPCs from alcohol animals exhibited reduced ability to form granulocyte-monocyte and erythroid colonies in vitro. HSPCs also displayed a decrease in mitochondrial oxygen consumption linked to ATP production and basal respiratory capacity. Chronic alcohol use led to vascular remodeling of the BM niche, a reduction in the number of primitive HSPCs, and a shift in localization of HSPCs from an adipose to a perivascular niche.

CONCLUSIONS

Our study demonstrates, for the first time, that chronic voluntary alcohol drinking in rhesus macaque monkeys leads to the long-term impairment of HSPC function, a reduction in mitochondrial respiratory activity, and alterations in the BM microenvironment. Further studies are needed to determine whether these changes in hematopoiesis are persistent or adaptive during the abstinent period and whether an initial imprinting to alcohol primes BM to become more vulnerable to future exposure to alcohol.

Spatial and biochemical interactions between bone marrow adipose tissue and hematopoietic stem and progenitor cells in rhesus macaques

Recent developments in in situ microscopy have enabled unparalleled resolution of the architecture of the bone marrow (BM) niche for murine hematopoietic stem and progenitor cells (HSPCs). However, the extent to which these observations can be extrapolated to human BM remains unknown. In humans, adipose tissue occupies a significant portion of the BM medullary cavity, making quantitative immunofluorescent analysis difficult due to lipid-mediated light scattering. In this study, we employed optical clearing, confocal microscopy and nearest neighbor analysis to determine the spatial distribution of CD34+ HSPCs in the BM in a translationally relevant rhesus macaque model. Immunofluorescent analysis revealed that femoral BM adipocytes are associated with the branches of vascular sinusoids, with half of HSPCs localizing in close proximity of the nearest BM adipocyte. Immunofluorescent microscopy and flow cytometric analysis demonstrate that BM adipose tissue exists as a multicellular niche consisted of adipocytes, endothelial cells, granulocytes, and macrophages. Analysis of BM adipose tissue conditioned media using liquid chromatography-tandem mass spectrometry revealed the presence of multiple bioactive proteins involved in regulation of hematopoiesis, inflammation, and bone development, with many predicted to reside inside microvesicles. Pretreatment of purified HSPCs with BM adipose tissue conditioned media, comprising soluble and exosomal/microvesicle-derived factors, led to enhanced proliferation and an increase in granulocyte-monocyte differentiation potential ex vivo. Our work extends extensive studies in murine models, indicating that BM adipose tissue is a central paracrine regulator of hematopoiesis in nonhuman primates and possibly in humans.

Eph/ephrin Signaling and Biology of Mesenchymal Stromal/Stem Cells

Mesenchymal stromal/stem cells (MSCs) have emerged as important therapeutic agents, owing to their easy isolation and culture, and their remarkable immunomodulatory and anti-inflammatory properties. However, MSCs constitute a heterogeneous cell population which does not express specific cell markers and has important problems for in vivo homing, and factors regulating their survival, proliferation, and differentiation are largely unknown. Accordingly, in the present article, we review the current evidence on the relationships between Eph kinase receptors, their ephrin ligands, and MSCs. These molecules are involved in the adult homeostasis of numerous tissues, and we and other authors have demonstrated their expression in human and murine MSCs derived from both bone marrow and adipose tissue, as well as their involvement in the MSC biology. We extend these studies providing new results on the effects of Eph/ephrins in the differentiation and immunomodulatory properties of MSCs.

Identification of Eph receptor signaling as a regulator of autophagy and a therapeutic target in colorectal carcinoma

Advanced colorectal carcinoma is currently incurable, and new therapies are urgently needed. We report that phosphotyrosine‐dependent Eph receptor signaling sustains colorectal carcinoma cell survival, thereby uncovering a survival pathway active in colorectal carcinoma cells. We find that genetic and biochemical inhibition of Eph tyrosine kinase activity or depletion of the Eph ligand EphrinB2 reproducibly induces colorectal carcinoma cell death by autophagy. Spautin and 3‐methyladenine, inhibitors of early steps in the autophagic pathway, significantly reduce autophagy‐mediated cell death that follows inhibition of phosphotyrosine‐dependent Eph signaling in colorectal cancer cells. A small‐molecule inhibitor of the Eph kinase, NVP‐BHG712 or its regioisomer NVP‐Iso, reduces human colorectal cancer cell growth in vitro and tumor growth in mice. Colorectal cancers express the EphrinB ligand and its Eph receptors at significantly higher levels than numerous other cancer types, supporting Eph signaling inhibition as a potential new strategy for the broad treatment of colorectal carcinoma.

Emerging Roles for Eph Receptors and Ephrin Ligands in Immunity

Eph receptors are the largest family of receptor tyrosine kinases and mediate a myriad of essential processes in humans from embryonic development to adult tissue homeostasis through interactions with membrane-bound ephrin ligands. The ubiquitous expression of Eph receptors and ephrin ligands among the cellular players of the immune system underscores the importance of these molecules in orchestrating an optimal immune response. This review provides an overview of the various roles of Eph receptors and ephrin ligands in immune cell development, activation, and migration. We also discuss the role of Eph receptors in disease pathogenesis as well as the implications of Eph receptors as future immunotherapy targets. Given the diverse and critical roles of Eph receptors and ephrin ligands throughout the immune system during both resting and activated states, this review aims to highlight the critical yet underappreciated roles of this family of signaling molecules in the immune system.

Ephrin/Eph receptor interaction facilitates macrophage recognition of differentiating human erythroblasts

Erythropoiesis is one of the most efficient cellular processes in the human body producing approximately 2.5 million red blood cells every second. This process occurs in a bone marrow niche comprised of a central resident macrophage surrounded by differentiating erythroblasts, termed an erythroblastic island. It is not known what initially attracts the macrophage to erythroblasts to form these islands. The ephrin/Eph receptor family are known to regulate heterophilic cell-cell adhesion. We find that human VCAM1⁺ and VCAM1^- bone marrow macrophages and in vitro cultured macrophages are ephrin-B2 positive, whereas differentiating human erythroblasts express EPHB4, EPHB6 and EPHA4. Furthermore, we detect a rise in integrin activation on erythroblasts at the stage at which the cells bind which is independent of EPH receptor presence. Using a live cell imaging assay, we show that specific inhibitory peptides or shRNA depletion of EPHB4 cause a significant reduction in the ability of macrophages to interact with erythroblasts but do not affect integrin activation. This study demonstrates for the first time that EPHB4 expression is required on erythroblasts to facilitate the initial recognition and subsequent interaction with macrophages, alongside the presence of active integrins.

Inhibition of EphB4-Ephrin-B2 Signaling Reprograms the Tumor Immune Microenvironment in Head and Neck Cancers

Identifying targets present in the tumor microenvironment that contribute to immune evasion has become an important area of research. In this study, we identified EphB4-ephrin-B2 signaling as a regulator of both innate and adaptive components of the immune system. EphB4 belongs to receptor tyrosine kinase family that interacts with ephrin-B2 ligand at sites of cell-cell contact, resulting in bidirectional signaling. We found that EphB4-ephrin-B2 inhibition alone or in combination with radiation (RT) reduced intratumoral regulatory T cells (Tregs) and increased activation of both CD8⁺ and CD4⁺Foxp3^- T cells compared with the control group in an orthotopic head and neck squamous cell carcinoma (HNSCC) model. We also compared the effect of EphB4-ephrin-B2 inhibition combined with RT with combined anti-PDL1 and RT and observed similar tumor growth suppression, particularly at early time-points. A patient-derived xenograft model showed reduction of tumor-associated M2 macrophages and favored polarization towards an antitumoral M1 phenotype following EphB4-ephrin-B2 inhibition with RT. In vitro, EphB4 signaling inhibition decreased Ki67-expressing Tregs and Treg activation compared with the control group. Overall, our study is the first to implicate the role of EphB4-ephrin-B2 in tumor immune response. Moreover, our findings suggest that EphB4-ephrin-B2 inhibition combined with RT represents a potential alternative for patients with HNSCC and could be particularly beneficial for patients who are ineligible to receive or cannot tolerate anti-PDL1 therapy. SIGNIFICANCE: These findings present EphB4-ephrin-B2 inhibition as an alternative to anti-PDL1 therapeutics that can be used in combination with radiation to induce an effective antitumor immune response in patients with HNSCC.

Cytokine-induced hematopoietic stem and progenitor cell mobilization: unraveling interactions between stem cells and their niche

Peripheral blood hematopoietic stem and progenitor cells (HSPCs), mobilized by granulocyte colony‐stimulating factor, are widely used as a source for both autologous and allogeneic stem cell transplantation. The use of mobilized HSPCs has several advantages over traditional bone marrow–derived HSPCs, including a less invasive harvesting process for the donor, higher HSPC yields, and faster hematopoietic reconstitution in the recipient. For years, the mechanisms by which cytokines and other agents mobilize HSPCs from the bone marrow were not fully understood. The field of stem cell mobilization research has advanced significantly over the past decade, with major breakthroughs in the elucidation of the complex mechanisms that underlie stem cell mobilization. In this review, we provide an overview of the events that underlie HSPC mobilization and address the relevant cellular and molecular components of the bone marrow niche. Furthermore, current and future mobilizing agents will be discussed.

Pancreatic Tumor Microenvironment Modulation by EphB4-ephrinB2 Inhibition and Radiation Combination

PURPOSE

A driving factor in pancreatic ductal adenocarcinoma (PDAC) treatment resistance is the tumor microenvironment, which is highly immunosuppressive. One potent immunologic adjuvant is radiotherapy. Radiation, however, has also been shown to induce immunosuppressive factors, which can contribute to tumor progression and formation of fibrotic tumor stroma. To capitalize on the immunogenic effects of radiation and obtain a durable tumor response, radiation must be rationally combined with targeted therapies to mitigate the influx of immunosuppressive cells and fibrosis. One such target is ephrinB2, which is overexpressed in PDAC and correlates negatively with prognosis.Experimental Design: On the basis of previous studies of ephrinB2 ligand-EphB4 receptor signaling, we hypothesized that inhibition of ephrinB2-EphB4 combined with radiation can regulate the microenvironment response postradiation, leading to increased tumor control in PDAC. This hypothesis was explored using both cell lines and in vivo human and mouse tumor models.

RESULTS

Our data show this treatment regimen significantly reduces regulatory T-cell, macrophage, and neutrophil infiltration and stromal fibrosis, enhances effector T-cell activation, and decreases tumor growth. Furthermore, our data show that depletion of regulatory T cells in combination with radiation reduces tumor growth and fibrosis.

CONCLUSIONS

These are the first findings to suggest that in PDAC, ephrinB2-EphB4 interaction has a profibrotic, protumorigenic role, presenting a novel and promising therapeutic target.

EPHA4 regulates vascular smooth muscle cell contractility and is a sex-specific hypertension risk gene in individuals with type 2 diabetes

OBJECTIVE

We investigated the association of genetic variants of EPHA4, a receptor tyrosine kinase, with hypertension, and its role in vascular smooth muscle cell (VSMC) contractility.

METHODS

Data from two human genetic studies, ADVANCE and HCHS/SOL, were analyzed for association of EPHA4 single nucleotide variants (SNVs) with hypertension risks. The effect of EPHA4 signalling on mouse VSMC contractility was assessed.

RESULTS

We identified a SNV (rs75843691 hg19 chr2:g.222395371 C>G), located in the third intron of EPHA4 gene, being significantly associated with hypertension in human female patients (P value = 8.3 × 10, below the Bonferroni-corrected critical P value) but not male patients with type 2 diabetes from the ADVANCE clinical trial. We found that EPHA4 was expressed in VSMCs and its stimulation by anti-EPHA4 antibody led to reduced VSMC contractility. Estrogen enhanced the contractility-lowering effect of EPHA4 stimulation. Conversely, siRNA knockdown of Epha4 expression in VSMCs resulted in increased contractility of VSMCs from female mice but not from male mice.

CONCLUSION

EPHA4 appears to be a sex-specific hypertension risk gene in type 2 diabetic patients. Forward EPHA4 signalling reduces VSMC contractility, and estrogen is a modifier of this effect. The effect of EPHA4 on VSMCs contractility explains the association of EPHA4 gene with hypertension risks in female patients.

HIF prolyl hydroxylase inhibitor FG-4497 enhances mouse hematopoietic stem cell mobilization via VEGFR2/KDR

In normoxia, hypoxia-inducible transcription factors (HIFs) are rapidly degraded within the cytoplasm as a consequence of their prolyl hydroxylation by oxygen-dependent prolyl hydroxylase domain (PHD) enzymes. We have previously shown that hematopoietic stem and progenitor cells (HSPCs) require HIF-1 for effective mobilization in response to granulocyte colony-stimulating factor (G-CSF) and CXCR4 antagonist AMD3100/plerixafor. Conversely, HIF PHD inhibitors that stabilize HIF-1 protein in vivo enhance HSPC mobilization in response to G-CSF or AMD3100 in a cell-intrinsic manner. We now show that extrinsic mechanisms involving vascular endothelial growth factor receptor-2 (VEGFR2), via bone marrow (BM) endothelial cells, are also at play. PTK787/vatalanib, a tyrosine kinase inhibitor selective for VEGFR1 and VEGFR2, and neutralizing anti-VEGFR2 monoclonal antibody DC101 blocked enhancement of HSPC mobilization by FG-4497. VEGFR2 was absent on mesenchymal and hematopoietic cells and was detected only in Sca1+ endothelial cells in the BM. We propose that HIF PHD inhibitor FG-4497 enhances HSPC mobilization by stabilizing HIF-1α in HSPCs as previously demonstrated, as well as by activating VEGFR2 signaling in BM endothelial cells, which facilitates HSPC egress from the BM into the circulation.

Changes in ephrin gene expression during bone healing identify a restricted repertoire of ephrins mediating fracture repair

To identify the repertoire of ephrin genes that might regulate endochondral bone fracture repair, we examined changes in ephrin ligand and receptor (Eph) gene expression in fracture callus tissues during bone fracture healing. Ephrin and Eph proteins were then localized in the fracture callus tissues present when changes in gene expression were observed. Ephrin gene expression was widespread in fracture tissues, but the repertoire of ephrin genes with significant changes in expression that might suggest a regulatory role in fracture callus development was restricted to the ephrin A family members Epha4, Epha5 and the ephrin B family member Efnb1. After 3 weeks of healing, Epha4 fracture expression was downregulated from 1.3- to 0.8-fold and Epha5 fracture expression was upregulated from 1.2- to 1.5-fold of intact contralateral femur expression, respectively. Efnb1 expression was downregulated from 1.5- to 1.2-fold after 2 weeks post-fracture. These ephrin proteins were localized to fracture callus prehypertrophic chondrocytes and osteoblasts, as well as to the periosteum and fibrous tissues. The observed positive correlation between mRNA levels of EfnB1 with Col10 and Epha5 with Bglap, together with colocalized expression with their respective proteins, suggest that EfnB1 is a positive mediator of prehypertrophic chondrocyte development and that Epha5 contributes to osteoblast-mediated mineralization of fracture callus. In contrast, mRNA levels of Epha4 and Efnb1 correlated negatively with Bglap, thus suggesting a negative role for these two ephrin family members in mature osteoblast functions. Given the number of family members and widespread expression of the ephrins, a characterization of changes in ephrin gene expression provides a basis for identifying ephrin family members that might regulate the molecular pathways of bone fracture repair. This approach suggests that a highly restricted repertoire of ephrins, EfnB1 and EphA5, are the major mediators of fracture callus cartilage hypertrophy and ossification, respectively, and proposes candidates for additional functional study and eventual therapeutic application.

Role of IGF1 and EFN-EPH signaling in skeletal metabolism

Insulin-like growth factor 1(IGF1) and ephrin ligand (EFN)-receptor (EPH) signaling are both crucial for bone cell function and skeletal development and maintenance. IGF1 signaling is the major mediator of growth hormone-induced bone growth, but a host of different signals and factors regulate IGF1 signaling at the systemic and local levels. Disruption of the Igf1 gene results in reduced peak bone mass in both experimental animal models and humans. Additionally, EFN-EPH signaling is a complex system which, particularly through cell-cell interactions, contributes to the development and differentiation of many bone cell types. Recent evidence has demonstrated several ways in which the IGF1 and EFN-EPH signaling pathways interact with and depend upon each other to regulate bone cell function. While much remains to be elucidated, the interaction between these two signaling pathways opens a vast array of new opportunities for investigation into the mechanisms of and potential therapies for skeletal conditions such as osteoporosis and fracture repair.

Eph/Ephrin-mediated stimulation of human bone marrow mesenchymal stromal cells correlates with changes in cell adherence and increased cell death

Background

Mesenchymal stromal cells (MSC) are components of connective tissues and, in vitro, cell entities characterized by cell adhesion and immunophenotyping, although specific markers for their identification are lacking. Currently, MSC derived from either human bone marrow (BM-MSC) or adipose tissue (Ad-MSC) are considered the main sources of MSC for cell therapy. Eph receptors and their ligands, Ephrins, are molecules involved in cell adhesion and migration in several tissues and organs. In the current study, we analyze the pattern of Eph/Ephrin expression in MSC and evaluate the effects of blockade and stimulation of these receptor/ligand pairs on their biology.

Methods

Eph/Ephrin expression was analyzed in both BM-MSC and Ad-MSC by qRT-PCR. Then, we supplied BM-MSC cultures with either blocking or activating compounds to evaluate their effects on MSC proliferation, survival, and cell cycle by FACS. Changes in cytoskeleton and integrin α5β1 expression were studied in stimulated BM-MSC by immunofluorescence microscopy and FACS, respectively.

Results

Higher numbers of Eph/Ephrin transcripts occurred in BM-MSC than in Ad-MSC. In addition, the blocking of Eph/Ephrin signaling correlated with decreased numbers of BM-MSC due to increased proportions of apoptotic cells in the cultures but without variations in the cycling cells. Unexpectedly, activation of Eph/Ephrin signaling by clustered Eph/Ephrin fusion proteins also resulted in increased proportions of apoptotic MSC. In this case, MSC underwent important morphological changes, associated with altered cytoskeleton and integrin α5β1 expression, which did not occur under the blocking conditions.

Conclusions

Taken together, these results suggest that Eph/Ephrin activation affects cell survival through alterations in cell attachment to culture plates, affecting the biology of BM-MSC.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0912-3) contains supplementary material, which is available to authorized users.

Stem cell factor is selectively secreted by arterial endothelial cells in bone marrow

Endothelial cells (ECs) contribute to haematopoietic stem cell (HSC) maintenance in bone marrow, but the differential contributions of EC subtypes remain unknown, owing to the lack of methods to separate with high purity arterial endothelial cells (AECs) from sinusoidal endothelial cells (SECs). Here we show that the combination of podoplanin (PDPN) and Sca-1 expression distinguishes AECs (CD45⁻ Ter119⁻ Sca-1^bright PDPN⁻) from SECs (CD45⁻ Ter119⁻ Sca-1^dim PDPN⁺). PDPN can be substituted for antibodies against the adhesion molecules ICAM1 or E-selectin. Unexpectedly, prospective isolation reveals that AECs secrete nearly all detectable EC-derived stem cell factors (SCF). Genetic deletion of Scf in AECs, but not SECs, significantly reduced functional HSCs. Lineage-tracing analyses suggest that AECs and SECs self-regenerate independently after severe genotoxic insults, indicating the persistence of, and recovery from, radio-resistant pre-specified EC precursors. AEC-derived SCF also promotes HSC recovery after myeloablation. These results thus uncover heterogeneity in the contribution of ECs in stem cell niches.

Endothelial cells (EC) are known to contribute to haematopoietic stem cell (HSC) maintenance in the bone marrow (BM). Here the authors demonstrate that arterial ECs can be distinguished from sinusoidal ECs by podoplanin and Sca-1 expression, and that specifically arterial, but not sinusoidal ECs maintain HSCs by secreting SCF.

Inhibition of EphB4-Ephrin-B2 Signaling Enhances Response to Cetuximab-Radiation Therapy in Head and Neck Cancers

Purpose: The clinical success of targeted therapies such as cetuximab and radiotherapy (RT) is hampered by the low response rates and development of therapeutic resistance. In the current study, we investigated the involvement of EphB4-ephrin-B2 protumorigenic signaling in mediating resistance to EGFR inhibition and RT in head and neck cancers.Experimental Design: We used patient-derived xenograft (PDX) models of head and neck squamous cell carcinoma (HNSCC) and HNSCC cell lines to test our hypothesis. Tumor tissues were subjected to PhosphoRTK array, and Western blotting to detect changes in EphB4-ephrin-B2 targets. mRNA sequencing and microarray data analysis were performed on PDX tumors and HNSCC cell lines, respectively, to determine differences in gene expression of molecules involved in tumor cell growth, proliferation, and survival pathways. Effects on cell growth were determined by MTT assay on HNSCC cells downregulated for EphB4/ephrin-B2 expression, with and without EGFR inhibitor and radiation.Results: Our data from locally advanced HNSCC patients treated with standard-of-care definitive chemo-RT show elevated EphB4 and ephrin-B2 levels after failure of treatment. We observed significant response toward cetuximab and RT following EphB4-ephrin-B2 inhibition, resulting in improved survival in tumor-bearing mice. Tumor growth inhibition was accompanied by a decrease in the levels of proliferation and prosurvival molecules and increased apoptosis.Conclusions: Our findings underscore the importance of adopting rational drug combinations to enhance therapeutic effect. Our study documenting enhanced response of HNSCC to cetuximab-RT with EphB4-ephrin-B2 blockade has the potential to translate into the clinic to benefit this patient population. Clin Cancer Res; 24(18); 4539-50. ©2018 AACR.

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.

Ephrin ligands and Eph receptors contribution to hematopoiesis

Hematopoietic stem and progenitor cells reside predominantly in the bone marrow. They supply billions of mature blood cells every day during life through maturation into multilineage progenitors and self-renewal. Newly produced mature cells serve to replenish the pool of circulating blood cells at the end of their life-span. These mature blood cells and a few hematopoietic progenitors normally exit the bone marrow through the sinusoidal vessels, a specialized venous vascular system that spreads throughout the bone marrow. Many signals regulate the coordinated mobilization of hematopoietic cells from the bone marrow to the circulation. In this review, we present recent advances on hematopoiesis and hematopoietic cell mobilization with a focus on the role of Ephrin ligands and their Eph receptors. These constitute a large family of transmembrane ligands and receptors that play critical roles in development and postnatally. New insights point to distinct roles of ephrin and Eph in different aspects of hematopoiesis.

Current Developments in Mobilization of Hematopoietic Stem and Progenitor Cells and Their Interaction with Niches in Bone Marrow

The clinical application of hematopoietic stem and progenitor cells (HSPCs) has evolved from a highly experimental stage in the 1980s to a currently clinically established treatment for more than 20,000 patients annually who suffer from hematological malignancies and other severe diseases. Studies in numerous murine models have demonstrated that HSPCs reside in distinct niches within the bone marrow environment. Whereas transplanted HSPCs travel through the bloodstream and home to sites of hematopoiesis, HSPCs can be mobilized from these niches into the blood either physiologically or induced by pharmaceutical drugs. Firstly, this review aims to give a synopsis of milestones defining niches and mobilization pathways for HSPCs, including the identification of several cell types involved such as osteoblasts, adventitial reticular cells, endothelial cells, monocytic cells, and granulocytic cells. The main factors that anchor HSPCs in the niche, and/or induce their quiescence are vascular cell adhesion molecule(VCAM)-1, CD44, hematopoietic growth factors, e.g. stem cell factor (SCF) and FLT3 Ligand, chemokines including CXCL12, growth-regulated protein beta and IL-8, proteases, peptides, and other chemical transmitters such as nucleotides. In the second part of the review, we revise the current understanding of HSPC mobilization. Here, we discuss which mechanisms found to be active in HSPC mobilization correspond to the mechanisms relevant for HSPC interaction with niche cells, but also deal with other mediators and signals that target individual cell types and receptors to mobilize HSPCs. A multitude of questions remain to be addressed for a better understanding of HSPC biology and its implications for therapy, including more comprehensive concepts for regulatory circuits such as calcium homeostasis and parathormone, metabolic regulation such as by leptin, the significance of autonomic nervous system, the consequences of alteration of niches in aged patients, or the identification of more easily accessible markers to better predict the efficiency of HSPC mobilization.

Evidence from single nucleotide polymorphism analyses of ADVANCE study demonstrates EFNB3 as a hypertension risk gene

EPH kinases and their ligands, ephrins (EFNs), have vital and diverse biological functions. We recently reported that Efnb3 gene deletion results in hypertension in female but not male mice. These data suggest that EFNB3 regulates blood pressure in a sex- and sex hormone-dependent way. In the present study, we conducted a human genetic study to assess the association of EFNB3 single nucleotide polymorphisms with human hypertension risks, using 3,448 patients with type 2 diabetes from the ADVANCE study (Action in Diabetes and Vascular Disease: Peterax and Diamicron MR Controlled Evaluation). We have observed significant association between 2 SNPs in the 3′ untranslated region or within the adjacent region just 3′ of the EFNB3 gene with hypertension, corroborating our findings from the mouse model. Thus, our investigation has shown that EFNB3 is a hypertension risk gene in certain individuals.

Cellular players of hematopoietic stem cell mobilization in the bone marrow niche

Hematopoietic stem cells (HSC) reside in perivascular regions of the bone marrow (BM) embedded within a complex regulatory unit called the niche. Cellular components of HSC niches include vascular endothelial cells, mesenchymal stromal progenitor cells and a variety of mature hematopoietic cells such as macrophages, neutrophils, and megakaryocytes-further regulated by sympathetic nerves and complement components as described in this review. Three decades ago the discovery that cytokines induce a large number of HSC to mobilize from the BM into the blood where they are easily harvested, revolutionised the field of HSC transplantation-curative for immune-deficiencies and some malignancies. However, despite now routine use of granulocyte-colony stimulating factor (G-CSF) to mobilise HSC for transplant, only in last 15 years has research on the mechanisms behind why and how HSC can be induced to move into the blood began. These studies have revealed the complexity of the niche that retains HSC in the BM. This review describes how BM niches and HSC themselves change during administration of G-CSF-or in the recovery phase of chemotherapy-to facilitate movement of HSC into the blood, and research now leading to development of novel therapeutics to further boost HSC mobilization and transplant success.