EphB4 Expressing Stromal Cells Exhibit an Enhanced Capacity for Hematopoietic Stem Cell Maintenance

Excess glucose alone depress young mesenchymal stromal/stem cell osteogenesis and mitochondria activity within hours/days via NAD+/SIRT1 axis

BACKGROUND

The impact of global overconsumption of simple sugars on bone health, which peaks in adolescence/early adulthood and correlates with osteoporosis (OP) and fracture risk decades, is unclear. Mesenchymal stromal/stem cells (MSCs) are the progenitors of osteoblasts/bone-forming cells, and known to decrease their osteogenic differentiation capacity with age. Alarmingly, while there is correlative evidence that adolescents consuming greatest amounts of simple sugars have the lowest bone mass, there is no mechanistic understanding on the causality of this correlation.

METHODS

Bioinformatics analyses for energetics pathways involved during MSC differentiation using human cell information was performed. In vitro dissection of normal versus high glucose (HG) conditions on osteo-/adipo-lineage commitment and mitochondrial function was assessed using multi-sources of non-senescent human and murine MSCs; for in vivo validation, young mice was fed normal or HG-added water with subsequent analyses of bone marrow CD45- MSCs.

RESULTS

Bioinformatics analyses revealed mitochondrial and glucose-related metabolic pathways as integral to MSC osteo-/adipo-lineage commitment. Functionally, in vitro HG alone without differentiation induction decreased both MSC mitochondrial activity and osteogenesis while enhancing adipogenesis by 8 h' time due to depletion of nicotinamide adenine dinucleotide (NAD+), a vital mitochondrial co-enzyme and co-factor to Sirtuin (SIRT) 1, a longevity gene also involved in osteogenesis. In vivo, HG intake in young mice depleted MSC NAD+, with oral NAD+ precursor supplementation rapidly reversing both mitochondrial decline and osteo-/adipo-commitment in a SIRT1-dependent fashion within 1 ~ 5 days.

CONCLUSIONS

We found a surprisingly rapid impact of excessive glucose, a single dietary factor, on MSC SIRT1 function and osteogenesis in youthful settings, and the crucial role of NAD+-a single molecule-on both MSC mitochondrial function and lineage commitment. These findings have strong implications on future global OP and disability risks in light of current worldwide overconsumption of simple sugars.

Targeting Acute Myeloid Leukemia Using Sphingosine Kinase 1 Inhibitor-Loaded Liposomes

Acute myeloid leukemia (AML) kills 75% of patients and represents a major clinical challenge with a need to improve on current treatment approaches. Targeting sphingosine kinase 1 with a novel ATP-competitive-inhibitor, MP-A08, induces cell death in AML. However, limitations in MP-A08's "drug-like properties" (solubility, biodistribution, and potency) hinder its pathway to the clinic. This study demonstrates a liposome-based delivery system of MP-A08 that exhibits enhanced MP-A08 potency against AML cells. MP-A08-liposomes increased MP-A08 efficacy against patient AML cells (>140-fold) and significantly prolonged overall survival of mice with human AML disease (P = 0.03). The significant antileukemic property of MP-A08-liposomes could be attributed to its enhanced specificity, bioaccessibility, and delivery to the bone marrow, as demonstrated in the pharmacokinetic and biodistribution studies. Our findings indicate that MP-A08-liposomes have potential as a novel treatment for AML.

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.

The roles of bone remodeling in normal hematopoiesis and age-related hematological malignancies

Prior research establishing that bone interacts in coordination with the bone marrow microenvironment (BMME) to regulate hematopoietic homeostasis was largely based on analyses of individual bone-associated cell populations. Recent advances in intravital imaging has suggested that the expansion of hematopoietic stem cells (HSCs) and acute myeloid leukemia cells is restricted to bone marrow microdomains during a distinct stage of bone remodeling. These findings indicate that dynamic bone remodeling likely imposes additional heterogeneity within the BMME to yield differential clonal responses. A holistic understanding of the role of bone remodeling in regulating the stem cell niche and how these interactions are altered in age-related hematological malignancies will be critical to the development of novel interventions. To advance this understanding, herein, we provide a synopsis of the cellular and molecular constituents that participate in bone turnover and their known connections to the hematopoietic compartment. Specifically, we elaborate on the coupling between bone remodeling and the BMME in homeostasis and age-related hematological malignancies and after treatment with bone-targeting approaches. We then discuss unresolved questions and ambiguities that remain in the field.

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.

Harnessing the Power of Eph/ephrin Biosemiotics for Theranostic Applications

Comprehensive basic biological knowledge of the Eph/ephrin system in the physiologic setting is needed to facilitate an understanding of its role and the effects of pathological processes on its activity, thereby paving the way for development of prospective therapeutic targets. To this end, this review briefly addresses what is currently known and being investigated in order to highlight the gaps and possible avenues for further investigation to capitalize on their diverse potential.

Conditional knockout of ephrinB1 in osteogenic progenitors delays the process of endochondral ossification during fracture repair

The Eph receptor tyrosine kinase ligand, ephrinB1 (EfnB1) is important for correct skeletal and cartilage development, however, the role of EfnB1 in fracture repair is unknown. This study investigated the role of EfnB1 during fracture repair where EfnB1 expression increased significantly at 1 and 2 weeks post fracture in C57Bl/6 wildtype mice, coinciding with the haematoma, soft callus formation/remodelling stages, respectively. To investigate the specific role of EfnB1 within the osteogenic lineage during fracture repair, male mice with a conditional deletion of EfnB1 in the osteogenic lineage (EfnB1_OB^fl/O), driven by the Osterix (Osx) promoter, and their male Osx:Cre counterparts were subject to a femoral fracture with internal fixation. Two weeks post fracture micro computed tomography (μCT) analysis revealed that EfnB1_OB^fl/O mice displayed a significant decrease in bone volume relative to tissue volume within the fracture callus. This was attributed to an alteration in the distribution of osteoclasts within the fracture site, a significant elevation in cartilaginous tissue and reduction in the osteoprogenitor population and calcein labelled bone within the fracture site of EfnB1_OB^fl/O mice. Supportive in vitro studies demonstrated that under osteogenic conditions, cultured EfnB1_OB^fl/O stromal cells derived from the 2 week fracture site exhibited a reduced capacity to produce mineral and decreased expression of the osteogenic gene, Osterix, when compared to Osx:Cre controls. These findings suggest that the loss of EfnB1 delays the fracture repair process. The present study confirmed that EFNB1 activation in human BMSC, following stimulation with soluble-EphB2 resulted in de-phosphorylation of TAZ, demonstrating similarities in EfnB1 signalling between human and mouse stromal populations. Overall, the present study provides evidence that loss of EfnB1 in the osteo/chondrogenic lineages delays the soft callus formation/remodelling stages of the fracture repair process.

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.

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.

Osteoblast lineage-specific cell-surface antigen (A7) regulates osteoclast recruitment and calcification during bone remodeling

Bone remodeling is a continuous process characterized by highly coordinated cell-cell interactions in distinct multi-cellular units. Osteoclasts, which are specialized bone resorbing cells, play a central role in bone remodeling. Although the RANKL/RANK axis determines the gross number of osteoclasts present in bone tissue, detailed molecular events regulating bone remodeling related to osteoclast recruitment, initiation of bone remodeling, and coupling of bone resorption and bone formation are still ambiguous. We hypothesized that osteoblast-specific cell-surface molecules contribute to the molecular modulation of bone remodeling. Therefore, we searched for regulatory cell-surface molecules expressed on osteoblasts by use of B-cell hybridoma technology. We obtained a monoclonal antibody A7 (A7 MAb) highly specific to cells of osteoblast-lineage. Here we describe the expression pattern and possible role of A7 antigen specifically recognized by A7 MAb. In vitro, A7 antigen was expressed on cell-surface of osteoblasts and osteoblast-like bone marrow stromal cells. In vivo, A7 antigen was detected in a subset of bone surface osteoblasts and in osteocytes, with a typical cell membrane expression pattern. Tissue array analysis showed only a limited expression of A7 antigen in osteocytes close to the bone surface. Immunoblotting and immunoprecipitation analysis showed that A7 antigen is a lineage-specific cell-surface protein with an approximate molecular weight of 45 KDa. Cross-linking of cell-surface A7 antigen in cultures of osteoclastogenesis showed stimulation of osteoclast formation. Marked suppression of calcification in primary osteoblast cultures was observed when A7 antigen was cross-linked with anti-A7 antigen MAb, A7 MAb. These data suggest that A7 antigen regulates recruitment of osteoclasts and triggering of calcification. A7 antigen may be an important molecule involved in the precise regulation of bone remodeling.

Specific functions of TET1 and TET2 in regulating mesenchymal cell lineage determination

Background

The 5 hydroxymethylation (5hmC) mark and TET DNA dioxygenases play a pivotal role in embryonic stem cell differentiation and animal development. However, very little is known about TET enzymes in lineage determination of human bone marrow-derived mesenchymal stem/stromal cells (BMSC). We examined the function of all three TET DNA dioxygenases, responsible for DNA hydroxymethylation, in human BMSC cell osteogenic and adipogenic differentiation.

Results

We used siRNA knockdown and retroviral mediated enforced expression of TET molecules and discovered TET1 to be a repressor of both osteogenesis and adipogenesis. TET1 was found to recruit the co-repressor proteins, SIN3A and the histone lysine methyltransferase, EZH2 to osteogenic genes. Conversely, TET2 was found to be a promoter of both osteogenesis and adipogenesis. The data showed that TET2 was directly responsible for 5hmC levels on osteogenic and adipogenic lineage-associated genes, whereas TET1 also played a role in this process. Interestingly, TET3 showed no functional effect in BMSC osteo-/adipogenic differentiation. Finally, in a mouse model of ovariectomy-induced osteoporosis, the numbers of clonogenic BMSC were dramatically diminished corresponding to lower trabecular bone volume and reduced levels of TET1, TET2 and 5hmC.

Conclusion

The present study has discovered an epigenetic mechanism mediated through changes in DNA hydroxymethylation status regulating the activation of key genes involved in the lineage determination of skeletal stem cells, which may have implications in BMSC function during normal bone regulation. Targeting TET molecules or their downstream targets may offer new therapeutic strategies to help prevent bone loss and repair following trauma or disease.

Electronic supplementary material

The online version of this article (10.1186/s13072-018-0247-4) contains supplementary material, which is available to authorized users.

Loss of EfnB1 in the osteogenic lineage compromises their capacity to support hematopoietic stem/progenitor cell maintenance

The bone marrow stromal microenvironment contributes to the maintenance and function of hematopoietic stem/progenitor cells (HSPCs). The Eph receptor tyrosine kinase family members have been implicated in bone homeostasis and stromal support of HSPCs. The present study examined the influence of EfnB1-expressing osteogenic lineage on HSPC function. Mice with conditional deletion of EfnB1 in the osteogenic lineage (EfnB1_OB^-/-), driven by the Osterix promoter, exhibited a reduced prevalence of osteogenic progenitors and osteoblasts, correlating to lower numbers of HSPCs compared with Osx:Cre mice. Long-term culture-initiating cell (LTC-IC) assays confirmed that the loss of EfnB1 within bone cells hindered HSPC function, with a significant reduction in colony formation in EfnB1_OB^-/- mice compared with Osx:Cre mice. Human studies confirmed that activation of EPHB2 on CD34⁺ HSPCs via EFNB1-Fc stimulation enhanced myeloid/erythroid colony formation, whereas functional blocking of either EPHB1 or EPHB2 inhibited the maintenance of LTC-ICs. Moreover, EFNB1 reverse signaling in human and mouse stromal cells was found to be required for the activation of the HSPC-promoting factor CXCL12. Collectively, the results of this study confirm that EfnB1 contributes to the stromal support of HSPC function and maintenance and may be an important factor in regulating the HSPC niche.

The osteoprogenitor-specific loss of ephrinB1 results in an osteoporotic phenotype affecting the balance between bone formation and resorption

The present study investigated the effects of conditional deletion of ephrinB1 in osteoprogenitor cells driven by the Osterix (Osx) promoter, on skeletal integrity in a murine model of ovariectomy-induced (OVX) osteoporosis. Histomorphometric and μCT analyses revealed that loss of ephrinB1 in sham Osx:cre-ephrinB1^fl/fl mice caused a reduction in trabecular bone comparable to OVX Osx:Cre mice, which was associated with a significant reduction in bone formation rates and decrease in osteoblast numbers. Interestingly, these observations were not exacerbated in OVX Osx:cre-ephrinB1^fl/fl mice. Furthermore, sham Osx:cre-ephrinB1^fl/fl mice displayed significantly higher osteoclast numbers and circulating degraded collagen type 1 compared to OVX Osx:Cre mice. Confirmation studies found that cultured monocytes expressing EphB2 formed fewer TRAP⁺ multinucleated osteoclasts and exhibited lower resorption activity in the presence of soluble ephrinB1-Fc compared to IgG control. This inhibition of osteoclast formation and function induced by ephrinB1-Fc was reversed in the presence of an EphB2 chemical inhibitor. Collectively, these observations suggest that ephrinB1, expressed by osteoprogenitors, influences bone loss during the development of osteoporosis, by regulating both osteoblast and osteoclast formation and function, leading to a loss of skeletal integrity.

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.

Mechanisms of Hematopoietic Stem Cell Ageing and Targets for Hematopoietic Tumour Prevention

Hematopoietic stem cells represent a rare population in the bone marrow, with the capacity of generating all blood lineage and themselves at the same time. With aging, the reconstitution capacity of hematopoietic stem cells decreases accompanying with differentiation skewing wherein the myeloid branch dominates in both mouse and human. In recent years, various molecular mechanisms that induce functional decline of HSC during aging were disclosed including DNA damage accumulation, metabolic alteration, defects in protein homeostasis, and aging-induced changes in the blood circulatory environment. Deciphering the nature of HSC aging could improve our knowledge of HSC aging-related diseases and furthermore promote the developing of therapeutic interventions for human HSC aging and diseases.

Current Understanding of the Pathways Involved in Adult Stem and Progenitor Cell Migration for Tissue Homeostasis and Repair

With the advancements in the field of adult stem and progenitor cells grows the recognition that the motility of primitive cells is a pivotal aspect of their functionality. There is accumulating evidence that the recruitment of tissue-resident and circulating cells is critical for organ homeostasis and effective injury responses, whereas the pathobiology of degenerative diseases, neoplasm and aging, might be rooted in the altered ability of immature cells to migrate. Furthermore, understanding the biological machinery determining the translocation patterns of tissue progenitors is of great relevance for the emerging methodologies for cell-based therapies and regenerative medicine. The present article provides an overview of studies addressing the physiological significance and diverse modes of stem and progenitor cell trafficking in adult mammalian organs, discusses the major microenvironmental cues regulating cell migration, and describes the implementation of live imaging approaches for the exploration of stem cell movement in tissues and the factors dictating the motility of endogenous and transplanted cells with regenerative potential.

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.

EphA5 and EphA7 forward signaling enhances human hematopoietic stem and progenitor cell maintenance, migration, and adhesion via Rac1 activation

The proliferation, differentiation, adhesion, and migration of hematopoietic stem and progenitor cells (HSPCs) are dependent upon bone marrow stromal cells (BMSCs). In this study, we found that human primitive HSPCs (CD34⁺CD38^-), but not lineage-committed hematopoietic cell populations, express the tyrosine kinase receptors EphA5 and EphA7. Moreover, we found that the ephrinA5 ligand, the high-affinity binding partner of EphA5 and EphA7, is highly expressed by primary human BMSCs. Previous studies have reported that interactions between EphA and ephrinA play important roles in hematopoietic cell trafficking; however, their role in BMSC support of hematopoiesis had not been described previously. Herein, we show that stimulating EphA5 and/or EphA7 forward signaling in HSPCs using soluble ephrinA5-Fc molecules promoted human HSPC-derived colony formation significantly and was associated with increased expression of granulocyte macrophage colony-stimulating factor receptor on HSPCs. Studies using functional blocking peptides to EphA5/7 found that disruption of EphA5/ephrinA5 and/or EphA7/ephrinA5 interactions inhibited HSPC function in BMSC-dependent long-term culture-initiating cell assays. Furthermore, the adhesion and migration of HSPCs was increased significantly in the presence of ephrinA5-Fc molecules compared with human immunoglobulin G-treated controls. Conversely, blocking EphA5 activation led to a reduction of HSPC adhesion, whereas inhibiting EphA5 and/or EphA7 activation hindered HSPC migration. Analysis of HSPC cultured in the presence of ephrinA5-Fc showed that EphA forward signaling stimulated Rac1 gene and protein expression and the Rac1 target molecule WAVE1. Moreover, a significant reduction of ephrinA5-mediated HSPC adhesion and migration was observed in the presence of Rac1 inhibitor. These findings suggest that interactions between EphA and ephrinA5 are important in maintaining the HSPC niche mediated in part by activation of Rac1 signaling.

The role of Eph/ephrin molecules in stromal–hematopoietic interactions

Bone marrow mesenchymal stromal/stem cells(BMSC) are fundamental regulatory elements of the hematopoietic stem cell niche; however, the molecular signals that mediate BMSC support of hematopoiesis are poorly understood. Recent studies indicate that BMSC and hematopoietic stem/progenitors cells differentially express the Eph cell surface tyrosine kinase receptors, and their ephrinligands. Eph/ephrin interactions are thought to mediate cross-talk between BMSC and different hematopoietic cell populations to influence cell development, migration and function. This review summarizes Eph/ephrin interactions in the regulation of BMSC communication with hematopoietic stem/progenitor cells and discusses Eph/ephrintargeted therapeutic strategies that are currently being pursued or various hematotological malignancies.

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

Hematopoietic stem and progenitor cells (HSPCs) reside in the bone marrow. Stress signals from cancer and other conditions promote HSPC mobilization into circulation and subsequent homing to tissue microenvironments. HSPC infiltration into tissue microenvironments can influence disease progression; notably, in cancer, HSPCs encourage tumor growth. Here we have uncovered a mutually exclusive distribution of EPHB4 receptors in bone marrow sinusoids and ephrin B2 ligands in hematopoietic cells. We determined that signaling interactions between EPHB4 and ephrin B2 control HSPC mobilization from the bone marrow. In mice, blockade of the EPHB4/ephrin B2 signaling pathway reduced mobilization of HSPCs and other myeloid cells to the circulation. EPHB4/ephrin B2 blockade also reduced HSPC infiltration into tumors as well as tumor progression in murine models of melanoma and mammary cancer. These results identify EPHB4/ephrin B2 signaling as critical to HSPC mobilization from bone marrow and provide a potential strategy for reducing cancer progression by targeting the bone marrow.

Bidirectional ephrin signaling in bone

The interaction between ephrin ligands (efn) and their receptors (Eph) is capable of inducing forward signaling, from ligand to receptor, as well as reverse signaling, from receptor to ligand. The ephrins are widely expressed in many tissues, where they mediate cell migration and adherence, properties that make the efn-Eph signaling critically important in establishing and maintaining tissue boundaries. The efn-Eph system has also received considerable attention in skeletal tissues, as ligand and receptor combinations are predicted to mediate interactions between the different types of cells that regulate bone development and homeostasis. This review summarizes our current understanding of efn-Eph signaling with a particular focus on the expression and functions of ephrins and their receptors in bone.

Membrane-mediated regulation of vascular identity

Vascular diseases span diverse pathology, but frequently arise from aberrant signaling attributed to specific membrane-associated molecules, particularly the Eph-ephrin family. Originally recognized as markers of embryonic vessel identity, Eph receptors and their membrane-associated ligands, ephrins, are now known to have a range of vital functions in vascular physiology. Interactions of Ephs with ephrins at cell-to-cell interfaces promote a variety of cellular responses such as repulsion, adhesion, attraction, and migration, and frequently occur during organ development, including vessel formation. Elaborate coordination of Eph- and ephrin-related signaling among different cell populations is required for proper formation of the embryonic vessel network. There is growing evidence supporting the idea that Eph and ephrin proteins also have postnatal interactions with a number of other membrane-associated signal transduction pathways, coordinating translation of environmental signals into cells. This article provides an overview of membrane-bound signaling mechanisms that define vascular identity in both the embryo and the adult, focusing on Eph- and ephrin-related signaling. We also discuss the role and clinical significance of this signaling system in normal organ development, neoplasms, and vascular pathologies.

Twist-1 Enhances Bone Marrow Mesenchymal Stromal Cell Support of Hematopoiesis by Modulating CXCL12 Expression

Twist-1 encodes a basic helix-loop-helix transcription factor, known to contribute to mesodermal and skeletal tissue development. We have reported previously that Twist-1 maintains multipotent human bone marrow-derived mesenchymal stem/stromal cells (BMSC) in an immature state, enhances their life-span, and influences cell fate determination. In this study, human BMSC engineered to express high levels of Twist-1 were found to express elevated levels of the chemokine, CXCL12. Analysis of the CXCL12 proximal promoter using chromatin immunoprecipitation analysis identified several E-box DNA sites bound by Twist-1. Functional studies using a luciferase reporter construct showed that Twist-1 increased CXCL12 promoter activity in a dose dependent manner. Notably, Twist-1 over-expressing BMSC exhibited an enhanced capacity to maintain human CD34 + hematopoietic stem cells (HSC) in long-term culture-initiating cell (LTC-IC) assays. Moreover, the observed increase in HSC maintenance by Twist-1 over-expressing BMSC was blocked in the presence of the CXCL12 inhibitor, AMD3100. Supportive studies, using Twist-1 deficient heterozygous mice demonstrated a significant decrease in the frequency of stromal progenitors and increased numbers of osteoblasts within the bone. These observations correlated to a decreased incidence in the number of clonogenic stromal progenitors (colony forming unit-fibroblasts) and lower levels of CXCL12 in Twist-1 mutant mice. Furthermore, Twist-1 deficient murine stromal feeder layers, exhibited a significant decrease in CXCL12 levels and lower numbers of hematopoietic colonies in LTC-IC assays, compared with wild type controls. These findings demonstrate that Twist-1, which maintains BMSC at an immature state, endows them with an increased capacity for supporting hematopoiesis via direct activation of CXCL12 gene expression.

Navigating the bone marrow niche: translational insights and cancer-driven dysfunction

The bone marrow niche consists of stem and progenitor cells destined to become mature cells such as haematopoietic elements, osteoblasts or adipocytes. Marrow cells, influenced by endocrine, paracrine and autocrine factors, ultimately function as a unit to regulate bone remodelling and haematopoiesis. Current evidence highlights that the bone marrow niche is not merely an anatomic compartment; rather, it integrates the physiology of two distinct organ systems, the skeleton and the marrow. The niche has a hypoxic microenvironment that maintains quiescent haematopoietic stem cells (HSCs) and supports glycolytic metabolism. In response to biochemical cues and under the influence of neural, hormonal, and biochemical factors, marrow stromal elements, such as mesenchymal stromal cells (MSCs), differentiate into mature, functioning cells. However, disruption of the niche can affect cellular differentiation, resulting in disorders ranging from osteoporosis to malignancy. In this Review, we propose that the niche reflects the vitality of two tissues - bone and blood - by providing a unique environment for stem and stromal cells to flourish while simultaneously preventing disproportionate proliferation, malignant transformation or loss of the multipotent progenitors required for healing, functional immunity and growth throughout an organism's lifetime. Through a fuller understanding of the complexity of the niche in physiologic and pathologic states, the successful development of more-effective therapeutic approaches to target the niche and its cellular components for the treatment of rheumatic, endocrine, neoplastic and metabolic diseases becomes achievable.