The adhesion molecule esam1 is a novel hematopoietic stem cell marker

Pathophysiology in Brain Arteriovenous Malformations: Focus on Endothelial Dysfunctions and Endothelial-to-Mesenchymal Transition

Brain arteriovenous malformations (bAVMs) substantially increase the risk for intracerebral hemorrhage (ICH), which is associated with significant morbidity and mortality. However, the treatment options for bAVMs are severely limited, primarily relying on invasive methods that carry their own risks for intraoperative hemorrhage or even death. Currently, there are no pharmaceutical agents shown to treat this condition, primarily due to a poor understanding of bAVM pathophysiology. For the last decade, bAVM research has made significant advances, including the identification of novel genetic mutations and relevant signaling in bAVM development. However, bAVM pathophysiology is still largely unclear. Further investigation is required to understand the detailed cellular and molecular mechanisms involved, which will enable the development of safer and more effective treatment options. Endothelial cells (ECs), the cells that line the vascular lumen, are integral to the pathogenesis of bAVMs. Understanding the fundamental role of ECs in pathological conditions is crucial to unraveling bAVM pathophysiology. This review focuses on the current knowledge of bAVM-relevant signaling pathways and dysfunctions in ECs, particularly the endothelial-to-mesenchymal transition (EndMT).

The quest for the holy grail: overcoming challenges in expanding human hematopoietic stem cells for clinical use

Hematopoietic stem cell (HSC) transplantation has been the golden standard for many hematological disorders. However, the number of HSCs obtained from several sources, including umbilical cord blood (UCB), often is insufficient for transplantation. For decades, maintaining or even expanding HSCs for therapeutic purposes has been a "holy grail" in stem cell biology. Different methods have been proposed to improve the efficiency of cell expansion and enhance homing potential such as co-culture with stromal cells or treatment with specific agents. Recent progress has shown that this is starting to become feasible using serum-free and well-defined media. Some of these protocols to expand HSCs along with genetic modification have been successfully applied in clinical trials and some others are studied in preclinical and clinical studies. However, the main challenges regarding ex vivo expansion of HSCs such as limited growth potential and tendency to differentiate in culture still need improvements. Understanding the biology of blood stem cells, their niche and signaling pathways has provided possibilities to regulate cell fate decisions and manipulate cells to optimize expansion of HSCs in vitro. Here, we review the plethora of HSC expansion protocols that have been proposed and indicate the current state of the art for their clinical application.

Isolation and Characterization of Fetal Liver Hematopoietic Stem Cells

Hematopoietic stem cells (HSCs) are responsible for the generation and maintenance of pools of multipotent precursors that ultimately give rise to all fully differentiated blood and immune cells. Proper identification and isolation of HSCs for functional analysis has greatly facilitated our understanding of both normal and abnormal adult hematopoiesis. Whereas adult hematopoiesis in mice and humans is driven by quiescent HSCs that reside almost exclusively within the bone marrow (BM), developmental hematopoiesis is characterized by a series of transient progenitors driving waves of increasingly mature hematopoietic cell production that occur across multiple anatomical sites. These waves of hematopoietic cell production are also responsible for the generation of distinct immune cell populations during development that persist into adulthood and contribute uniquely to adult immunity. Therefore, methods to properly isolate and characterize fetal progenitors with high purity across development become increasingly important not only for defining developmental hematopoietic pathways, but also for understanding the contribution of developmental hematopoiesis to the immune system. Here, we describe and discuss methods and considerations for the isolation and characterization of HSCs from the fetal liver, the primary hematopoietic organ during fetal development.

Identification and characterization of in vitro expanded hematopoietic stem cells

Hematopoietic stem cells (HSCs) cultured outside the body are the fundamental component of a wide range of cellular and gene therapies. Recent efforts have achieved > 200-fold expansion of functional HSCs, but their molecular characterization has not been possible since the majority of cells are non-HSCs and single cell-initiated cultures have substantial clone-to-clone variability. Using the Fgd5 reporter mouse in combination with the EPCR surface marker, we report exclusive identification of HSCs from non-HSCs in expansion cultures. By directly linking single-clone functional transplantation data with single-clone gene expression profiling, we show that the molecular profile of expanded HSCs is similar to proliferating fetal HSCs and reveals a gene expression signature, including Esam, Prdm16, Fstl1, and Palld, that can identify functional HSCs from multiple cellular states. This "repopulation signature" (RepopSig) also enriches for HSCs in human datasets. Together, these findings demonstrate the power of integrating functional and molecular datasets to better derive meaningful gene signatures and opens the opportunity for a wide range of functional screening and molecular experiments previously not possible due to limited HSC numbers.

Dntt expression reveals developmental hierarchy and lineage specification of hematopoietic progenitors

Intrinsic and extrinsic cues determine developmental trajectories of hematopoietic stem cells (HSCs) towards erythroid, myeloid and lymphoid lineages. Using two newly generated transgenic mice that report and trace the expression of terminal deoxynucleotidyl transferase (TdT), transient induction of TdT was detected on a newly identified multipotent progenitor (MPP) subset that lacked self-renewal capacity but maintained multilineage differentiation potential. TdT induction on MPPs reflected a transcriptionally dynamic but uncommitted stage, characterized by low expression of lineage-associated genes. Single-cell CITE-seq indicated that multipotency in the TdT⁺ MPPs is associated with expression of the endothelial cell adhesion molecule ESAM. Stable and progressive upregulation of TdT defined the lymphoid developmental trajectory. Collectively, we here identify a new multipotent progenitor within the MPP4 compartment. Specification and commitment are defined by downregulation of ESAM which marks the progressive loss of alternative fates along all lineages.

Adhesion Molecules Involved in Stem Cell Niche Retention During Normal Haematopoiesis and in Acute Myeloid Leukaemia

In the bone marrow (BM) of adult mammals, haematopoietic stem cells (HSCs) are retained in micro-anatomical structures by adhesion molecules that regulate HSC quiescence, proliferation and commitment. During decades, researchers have used engraftment to study the function of adhesion molecules in HSC's homeostasis regulation. Since the 90's, progress in genetically engineered mouse models has allowed a better understanding of adhesion molecules involved in HSCs regulation by BM niches and raised questions about the role of adhesion mechanisms in conferring drug resistance to cancer cells nested in the BM. This has been especially studied in acute myeloid leukaemia (AML) which was the first disease in which the concept of cancer stem cell (CSC) or leukemic stem cells (LSCs) was demonstrated. In AML, it has been proposed that LSCs propagate the disease and are able to replenish the leukemic bulk after complete remission suggesting that LSC may be endowed with drug resistance properties. However, whether such properties are due to extrinsic or intrinsic molecular mechanisms, fully or partially supported by molecular crosstalk between LSCs and surrounding BM micro-environment is still matter of debate. In this review, we focus on adhesion molecules that have been involved in HSCs or LSCs anchoring to BM niches and discuss if inhibition of such mechanism may represent new therapeutic avenues to eradicate LSCs.

Syndecan-2 expression enriches for hematopoietic stem cells and regulates stem cell repopulating capacity

The discovery of novel hematopoietic stem cell (HSC) surface markers can enhance understanding of HSC identity and function. We have discovered a population of primitive bone marrow (BM) HSCs distinguished by their expression of the heparan sulfate proteoglycan, Syndecan-2, which serves as both a marker and regulator of HSC function. Syndecan-2 expression was increased 10-fold in CD150+CD48-CD34-c-Kit+Sca-1+Lineage- cells (long-term - HSCs, LT-HSCs) compared to differentiated hematopoietic cells. Isolation of BM cells based solely on Syndecan-2 surface expression produced a 24-fold enrichment for LT-HSCs, 6-fold enrichment for alpha-catulin+c-kit+ HSCs, and yielded HSCs with superior in vivo repopulating capacity compared to CD150+ cells. Competitive repopulation assays revealed the HSC frequency to be 17-fold higher in Syndecan-2+CD34-KSL cells compared to Syndecan-2-CD34-KSL cells and indistinguishable from CD150+CD34-KSL cells. Syndecan-2 expression also identified nearly all repopulating HSCs within the CD150+CD34-KSL population. Mechanistically, Syndecan-2 regulates HSC repopulating capacity through control of expression of Cdkn1c (p57) and HSC quiescence. Loss of Syndecan-2 expression caused increased HSC cell cycle entry, downregulation of Cdkn1c and loss of HSC long-term - repopulating capacity. Syndecan-2 is a novel marker of HSCs which regulates HSC repopulating capacity via control of HSC quiescence.

JAK2-V617F and interferon-α induce megakaryocyte-biased stem cells characterized by decreased long-term functionality

We studied a subset of hematopoietic stem cells (HSCs) that are defined by elevated expression of CD41 (CD41hi) and showed bias for differentiation toward megakaryocytes (Mks). Mouse models of myeloproliferative neoplasms (MPNs) expressing JAK2-V617F (VF) displayed increased frequencies and percentages of the CD41hi vs CD41lo HSCs compared with wild-type controls. An increase in CD41hi HSCs that correlated with JAK2-V617F mutant allele burden was also found in bone marrow from patients with MPN. CD41hi HSCs produced a higher number of Mk-colonies of HSCs in single-cell cultures in vitro, but showed reduced long-term reconstitution potential compared with CD41lo HSCs in competitive transplantations in vivo. RNA expression profiling showed an upregulated cell cycle, Myc, and oxidative phosphorylation gene signatures in CD41hi HSCs, whereas CD41lo HSCs showed higher gene expression of interferon and the JAK/STAT and TNFα/NFκB signaling pathways. Higher cell cycle activity and elevated levels of reactive oxygen species were confirmed in CD41hi HSCs by flow cytometry. Expression of Epcr, a marker for quiescent HSCs inversely correlated with expression of CD41 in mice, but did not show such reciprocal expression pattern in patients with MPN. Treatment with interferon-α further increased the frequency and percentage of CD41hi HSCs and reduced the number of JAK2-V617F+ HSCs in mice and patients with MPN. The shift toward the CD41hi subset of HSCs by interferon-α provides a possible mechanism of how interferon-α preferentially targets the JAK2 mutant clone.

Genomic Engineering in Human Hematopoietic Stem Cells: Hype or Hope?

Many gene editing techniques are developed and tested, yet, most of these are optimized for transformed cell lines, which differ from their primary cell counterparts in terms of transfectability, cell death propensity, differentiation capability, and chromatin accessibility to gene editing tools. Researchers are working to overcome the challenges associated with gene editing of primary cells, namely, at the level of improving the gene editing tool components, e.g., the use of modified single guide RNAs, more efficient delivery of Cas9 and RNA in the ribonucleoprotein of these cells. Despite these efforts, the low efficiency of proper gene editing in true primary cells is an obstacle that needs to be overcome in order to generate sufficiently high numbers of corrected cells for therapeutic use. In addition, many of the therapeutic candidate genes for gene editing are expressed in more mature blood cell lineages but not in the hematopoietic stem cells (HSCs), where they are tightly packed in heterochromatin, making them less accessible to gene editing enzymes. Bringing HSCs in proliferation is sometimes seen as a solution to overcome lack of chromatin access, but the induction of proliferation in HSCs often is associated with loss of stemness. The documented occurrences of off-target effects and, importantly, on-target side effects also raise important safety issues. In conclusion, many obstacles still remain to be overcome before gene editing in HSCs for gene correction purposes can be applied clinically. In this review, in a perspective way, we will discuss the challenges of researching and developing a novel genetic engineering therapy for monogenic blood and immune system disorders.

Proteomic analysis of young and old mouse hematopoietic stem cells and their progenitors reveals post-transcriptional regulation in stem cells

The balance of hematopoietic stem cell (HSC) self-renewal and differentiation is critical for a healthy blood supply; imbalances underlie hematological diseases. The importance of HSCs and their progenitors have led to their extensive characterization at genomic and transcriptomic levels. However, the proteomics of hematopoiesis remains incompletely understood. Here we report a proteomics resource from mass spectrometry of mouse young adult and old adult mouse HSCs, multipotent progenitors and oligopotent progenitors; 12 cell types in total. We validated differential protein levels, including confirmation that Dnmt3a protein levels are undetected in young adult mouse HSCs until forced into cycle. Additionally, through integrating proteomics and RNA-sequencing datasets, we identified a subset of genes with apparent post-transcriptional repression in young adult mouse HSCs. In summary, we report proteomic coverage of young and old mouse HSCs and progenitors, with broader implications for understanding mechanisms for stem cell maintenance, niche interactions and fate determination.

The transcription factor NFAT5 limits infection-induced type I interferon responses

Huerga Encabo et al. show that NFAT5, previously characterized as a pro-inflammatory transcription factor, limits the IFN-I response to control antiviral defenses and preserve HSC quiescence. NFAT5 represses IFN-I and ISG expression through an evolutionarily conserved DNA element that prevents IRF3 recruitment to the IFNB1 enhanceosome.

Type I interferon (IFN-I) provides effective antiviral immunity but can exacerbate harmful inflammatory reactions and cause hematopoietic stem cell (HSC) exhaustion; therefore, IFN-I expression must be tightly controlled. While signaling mechanisms that limit IFN-I induction and function have been extensively studied, less is known about transcriptional repressors acting directly on IFN-I regulatory regions. We show that NFAT5, an activator of macrophage pro-inflammatory responses, represses Toll-like receptor 3 and virus-induced expression of IFN-I in macrophages and dendritic cells. Mice lacking NFAT5 exhibit increased IFN-I production and better control of viral burden upon LCMV infection but show exacerbated HSC activation under systemic poly(I:C)-induced inflammation. We identify IFNβ as a primary target repressed by NFAT5, which opposes the master IFN-I inducer IRF3 by binding to an evolutionarily conserved sequence in the IFNB1 enhanceosome that overlaps a key IRF site. These findings illustrate how IFN-I responses are balanced by simultaneously opposing transcription factors.

Sox17-mediated expression of adherent molecules is required for the maintenance of undifferentiated hematopoietic cluster formation in midgestation mouse embryos

Hematopoietic stem cell-containing intra-aortic hematopoietic cell clusters (IAHCs) emerge in the dorsal aorta of the aorta-gonad-mesonephros (AGM) region during midgestation mouse embryos. We previously showed that transduction of Sox17 in CD45^lowc-Kit^high cells, which are one component of IAHCs, maintained the cluster formation and the undifferentiated state, but the mechanism of the cluster formation by Sox17 has not been clarified. By microarray gene expression analysis, we found that genes for vascular endothelial-cadherin (VE-cad) and endothelial cell-selective adhesion molecule (ESAM) were expressed at high levels in Sox17-transduced c-Kit⁺ cells. Here we show the functional role of these adhesion molecules in the formation of IAHCs and the maintenance of the undifferentiated state by in vitro experiments. We detected VE-cad and ESAM expression in endothelial cells of dorsal aorta and IAHCs in E10.5 embryos by whole mount immunohistochemistry. Cells with the middle expression level of VE-cad and the low expression level of ESAM had the highest colony-forming ability. Tamoxifen-dependent nuclear translocation of Sox17-ERT fusion protein induced the formation of cell clusters and the expression of Cdh5 (VE-cad) and ESAM genes. We showed the induction of the Cdh5 (VE-cad) and ESAM expression and the direct interaction of Sox17 with their promoter by luciferase assay and chromatin immunoprecipitation assay, respectively. Moreover, shRNA-mediated knockdown of either Cdh5 (VE-cad) or ESAM gene in Sox17-transduced cells decreased the multilineage-colony forming potential. These findings suggest that VE-cad and ESAM play an important role in the high hematopoietic activity of IAHCs and cluster formation.

Physiological Cues Involved in the Regulation of Adhesion Mechanisms in Hematopoietic Stem Cell Fate Decision

Hematopoietic stem cells (HSC) could have several fates in the body; viz. self-renewal, differentiation, migration, quiescence, and apoptosis. These fate decisions play a crucial role in maintaining homeostasis and critically depend on the interaction of the HSCs with their micro-environmental constituents. However, the physiological cues promoting these interactions in vivo have not been identified to a great extent. Intense research using various in vitro and in vivo models is going on in various laboratories to understand the mechanisms involved in these interactions, as understanding of these mechanistic would greatly help in improving clinical transplantations. However, though these elegant studies have identified the molecular interactions involved in the process, harnessing these interactions to the recipients' benefit would ultimately depend on manipulation of environmental cues initiating them in vivo: hence, these need to be identified at the earliest. HSCs reside in the bone marrow, which is a very complex tissue comprising of various types of stromal cells along with their secreted cytokines, extra-cellular matrix (ECM) molecules and extra-cellular vesicles (EVs). These components control the HSC fate decision through direct cell-cell interactions - mediated via various types of adhesion molecules -, cell-ECM interactions - mediated mostly via integrins -, or through soluble mediators like cytokines and EVs. This could be a very dynamic process involving multiple transient interactions acting concurrently or sequentially, and the adhesion molecules involved in various fate determining situations could be different. If the switch mechanisms governing these dynamic states in vivo are identified, they could be harnessed for the development of novel therapeutics. Here, in addition to reviewing the adhesion molecules involved in the regulation of HSCs, we also touch upon recent advances in our understanding of the physiological cues known to initiate specific adhesive interactions of HSCs with the marrow stromal cells or ECM molecules and EVs secreted by them.

Ubiquitous overexpression of CXCL12 confers radiation protection and enhances mobilization of hematopoietic stem and progenitor cells

C-X-C motif chemokine ligand 12 (CXCL12; aka SDF1α) is a major regulator of a number of cellular systems, including hematopoiesis, where it influences hematopoietic cell trafficking, proliferation, and survival during homeostasis and upon stress and disease. A variety of constitutive, temporal, ubiquitous, and cell-specific loss-of-function models have documented the functional consequences on hematopoiesis upon deletion of Cxcl12. Here, in contrast to loss-of-function experiments, we implemented a gain-of-function approach by generating a doxycycline-inducible transgenic mouse model that enables spatial and temporal overexpression of Cxcl12. We demonstrated that ubiquitous CXCL12 overexpression led to an increase in multipotent progenitors in the bone marrow and spleen. The CXCL12+ mice displayed reduced reconstitution potential as either donors or recipients in transplantation experiments. Additionally, we discovered that Cxcl12 overexpression improved hematopoietic stem and progenitor cell mobilization into the blood, and conferred radioprotection by promoting quiescence. Thus, this new CXCL12+ mouse model provided new insights into major facets of hematopoiesis and serves as a versatile resource for studying CXCL12 function in a variety of contexts.

S100A6 is a critical regulator of hematopoietic stem cells

The fate options of hematopoietic stem cells (HSCs) include self-renewal, differentiation, migration, and apoptosis. HSCs self-renewal divisions in stem cells are required for rapid regeneration during tissue damage and stress, but how precisely intracellular calcium signals are regulated to maintain fate options in normal hematopoiesis is unclear. S100A6 knockout (KO) HSCs have reduced total cell numbers in the HSC compartment, decreased myeloid output, and increased apoptotic HSC numbers in steady state. S100A6KO HSCs had impaired self-renewal and regenerative capacity, not responding to 5-Fluorouracil. Our transcriptomic and proteomic profiling suggested that S100A6 is a critical HSC regulator. Intriguingly, S100A6KO HSCs showed decreased levels of phosphorylated Akt (p-Akt) and Hsp90, with an impairment of mitochondrial respiratory capacity and a reduction of mitochondrial calcium levels. We showed that S100A6 regulates intracellular and mitochondria calcium buffering of HSC upon cytokine stimulation and have demonstrated that Akt activator SC79 reverts the levels of intracellular and mitochondrial calcium in HSC. Hematopoietic colony-forming activity and the Hsp90 activity of S100A6KO are restored through activation of the Akt pathway. We show that p-Akt is the prime downstream mechanism of S100A6 in the regulation of HSC self-renewal by specifically governing mitochondrial metabolic function and Hsp90 protein quality.

Mechanisms Ensuring Endothelial Junction Integrity Beyond VE-Cadherin

Endothelial junctions provide blood and lymph vessel integrity and are essential for the formation of a vascular system. They control the extravasation of solutes, leukocytes and metastatic cells from blood vessels and the uptake of fluid and leukocytes into the lymphatic vascular system. A multitude of adhesion molecules mediate and control the integrity and permeability of endothelial junctions. VE-cadherin is arguably the most important adhesion molecule for the formation of vascular structures, and the stability of their junctions. Interestingly, despite this prominence, its elimination from junctions in the adult organism has different consequences in the vasculature of different organs, both for blood and lymph vessels. In addition, even in tissues where the lack of VE-cadherin leads to strong plasma leaks from venules, the physical integrity of endothelial junctions is preserved. Obviously, other adhesion molecules can compensate for a loss of VE-cadherin and this review will discuss which other adhesive mechanisms contribute to the stability and regulation of endothelial junctions and cooperate with VE-cadherin in intact vessels. In addition to adhesion molecules, endothelial receptors will be discussed, which stimulate signaling processes that provide junction stability by modulating the actomyosin system, which reinforces tension of circumferential actin and dampens pulling forces of radial stress fibers. Finally, we will highlight most recent reports about the formation and control of the specialized button-like junctions of initial lymphatics, which represent the entry sites for fluid and cells into the lymphatic vascular system.

Interference With ESAM (Endothelial Cell-Selective Adhesion Molecule) Plus Vascular Endothelial-Cadherin Causes Immediate Lethality and Lung-Specific Blood Coagulation

Objective:

Vascular endothelial (VE)-cadherin is of dominant importance for the formation and stability of endothelial junctions, yet induced gene inactivation enhances vascular permeability in the lung but does not cause junction rupture. This study aims at identifying the junctional adhesion molecule, which is responsible for preventing endothelial junction rupture in the pulmonary vasculature in the absence of VE-cadherin.

Approach and Results:

We have compared the relevance of ESAM (endothelial cell-selective adhesion molecule), JAM (junctional adhesion molecule)-A, PECAM (platelet endothelial cell adhesion molecule)-1, and VE-cadherin for vascular barrier integrity in various mouse tissues. Gene inactivation of ESAM enhanced vascular permeability in the lung but not in the heart, skin, and brain. In contrast, deletion of JAM-A or PECAM-1 did not affect barrier integrity in any of these organs. Blocking VE-cadherin with antibodies caused lethality in ESAM −/− mice within 30 minutes but had no such effect in JAM-A −/− , PECAM-1 −/− or wild-type mice. Likewise, induced gene inactivation of VE-cadherin caused rapid lethality only in the absence of ESAM. Ultrastructural analysis revealed that only combined interference with VE-cadherin and ESAM disrupted endothelial junctions and caused massive blood coagulation in the lung. Mechanistically, we could exclude a role of platelet ESAM in coagulation, changes in the expression of other junctional proteins or a contribution of cytoplasmic signaling domains of ESAM.

Conclusions:

Despite well-documented roles of JAM-A and PECAM-1 for the regulation of endothelial junctions, only for ESAM, we detected an essential role for endothelial barrier integrity in a tissue-specific way. In addition, we found that it is ESAM which prevents endothelial junction rupture in the lung when VE-cadherin is absent.

Ex Vivo Expansion of Hematopoietic Stem Cells for Therapeutic Purposes: Lessons from Development and the Niche

Expansion of hematopoietic stem cells (HSCs) for therapeutic purposes has been a “holy grail” in the field for many years. Ex vivo expansion of HSCs can help to overcome material shortage for transplantation purposes and genetic modification protocols. In this review, we summarize improved understanding in blood development, the effect of niche and conservative signaling pathways on HSCs in mice and humans, and also advances in ex vivo culturing protocols of human HSCs with cytokines or small molecule compounds. Different expansion protocols have been tested in clinical trials. However, an optimal condition for ex vivo expansion of human HSCs still has not been found yet. Translating and implementing new findings from basic research (for instance by using genetic modification of human HSCs) into clinical protocols is crucial to improve ex vivo expansion and eventually boost stem cell gene therapy.

Analysis of Murine Hematopoietic Stem Cell Proliferation During Inflammation

Normally, quiescent hematopoietic stem cells (HSC) rapidly enter the cell cycle following exposure to inflammatory stimuli. The analysis of HSC cell cycle activity in murine bone marrow during inflammation is often complicated by the relative rarity of HSCs and shifts in Sca-1, a key cell surface marker used to identify HSCs. Here, we report a method to analyze HSC proliferation and cell cycle distribution under inflammatory conditions. Our approach uses EdU incorporation and Ki67 staining coupled with DNA content quantification by DAPI. We also incorporate the surface marker ESAM to help minimize the potential for contaminating events that may confound analysis in the HSC compartment.

Single Cell Resolution of Human Hematoendothelial Cells Defines Transcriptional Signatures of Hemogenic Endothelium

Endothelial-to-hematopoietic transition (EHT) is an important stage in definitive hematopoietic development. However, the genetic mechanisms underlying human EHT remain poorly characterized. We performed single cell RNA-seq using 55 hemogenic endothelial cells (HECs: CD31⁺ CD144⁺ CD41^- CD43^- CD45^- CD73^- RUNX1c⁺ ), 47 vascular endothelial cells without hematopoietic potential (non-HE: CD31⁺ CD144⁺ CD41^- CD43^- CD45^- CD73^- RUNX1c^- ), and 35 hematopoietic progenitor cells (HPCs: CD34⁺ CD43⁺ RUNX1c⁺ ) derived from human embryonic stem cells (hESCs). HE and HP were enriched in genes implicated in hemogenic endothelial transcriptional networks, such as ERG, GATA2, and FLI. We found transcriptional overlap between individual HECs and HPCs; however, these populations were distinct from non-HE. Further analysis revealed novel biomarkers for human HEC/HPCs, including TIMP3, ESAM, RHOJ, and DLL4. Collectively, we demonstrate that hESC-derived HE and HP share a common developmental pathway, while non-HE are more heterogeneous and transcriptionally distinct. Our findings provide a novel strategy to test new genetic targets and optimize the production of definitive hematopoietic cells from human pluripotent stem cells. Stem Cells 2018;36:206-217.

Single-cell analyses identify bioengineered niches for enhanced maintenance of hematopoietic stem cells

The in vitro expansion of long-term hematopoietic stem cells (HSCs) remains a substantial challenge, largely because of our limited understanding of the mechanisms that control HSC fate choices. Using single-cell multigene expression analysis and time-lapse microscopy, here we define gene expression signatures and cell cycle hallmarks of murine HSCs and the earliest multipotent progenitors (MPPs), and analyze systematically single HSC fate choices in culture. Our analysis revealed twelve differentially expressed genes marking the quiescent HSC state, including four genes encoding cell–cell interaction signals in the niche. Under basal culture conditions, most HSCs rapidly commit to become early MPPs. In contrast, when we present ligands of the identified niche components such as JamC or Esam within artificial niches, HSC cycling is reduced and long-term multipotency in vivo is maintained. Our approach to bioengineer artificial niches should be useful in other stem cell systems.

Haematopoietic stem cell (HSC) self-renewal is not sufficiently understood to recapitulate in vitro. Here, the authors generate gene signature and cell cycle hallmarks of single murine HSCs, and use identified endothelial receptors Esam and JamC as substrates to enhance HSC growth in engineered niches.

Niche Extracellular Matrix Components and Their Influence on HSC

Maintenance of hematopoietic stem cells (HSC) takes place in a highly specialized microenvironment within the bone marrow. Technological improvements, especially in the field of in vivo imaging, have helped unravel the complexity of the niche microenvironment and have completely changed the classical concept from what was previously believed to be a static supportive platform, to a dynamic microenvironment tightly regulating HSC homeostasis through the complex interplay between diverse cell types, secreted factors, extracellular matrix molecules, and the expression of different transmembrane receptors. To add to the complexity, non-protein based metabolites have also been recognized as a component of the bone marrow niche. The objective of this review is to discuss the current understanding on how the different extracellular matrix components of the niche regulate HSC fate, both during embryonic development and in adulthood. Special attention will be provided to the description of non-protein metabolites, such as lipids and metal ions, which contribute to the regulation of HSC behavior. J. Cell. Biochem. 118: 1984-1993, 2017. © 2017 Wiley Periodicals, Inc.

Liver Resident Macrophages (Kupffer Cells) Share Several Functional Antigens in Common with Endothelial Cells

The identification and specific functions of Kupffer cells (KCs), a liver resident macrophage subpopulation, are still unclear. We compared KCs with peritoneal macrophages using cDNA microarray analysis and found that these cells share some antigens with endothelial cells. KCs highly express VCAM-1 and VEGF receptors (VEGF-Rs) at transcriptional and protein levels. VCAM-1 mediates the functional binding of KCs with lymphocytes and induces KC activation. Among the VEGF receptors, VEGF-R2 and VEGF-R3 were expressed on the KCs, while VEGF-R1 was expressed on other tissue macrophage subsets. VEGF120, a ligand of both VEGF-R1 and VEGF-R2, transduced strong survival and chemotactic signals through the KCs, when compared to PIGF, a VEGF-R1 ligand, indicating that VEGF-R2 plays significant roles in regulating KC activities. Expression of the VEGF-Rs was regulated by TLR4 signalling. These results suggest that the function of KCs is partly regulated by the common antigens shared with endothelial cells.

Endothelial Cell-Selective Adhesion Molecule Expression in Hematopoietic Stem/Progenitor Cells Is Essential for Erythropoiesis Recovery after Bone Marrow Injury

Numerous red blood cells are generated every second from proliferative progenitor cells under a homeostatic state. Increased erythropoietic activity is required after myelo-suppression as a result of chemo-radio therapies. Our previous study revealed that the endothelial cell-selective adhesion molecule (ESAM), an authentic hematopoietic stem cell marker, plays essential roles in stress-induced hematopoiesis. To determine the physiological importance of ESAM in erythroid recovery, ESAM-knockout (KO) mice were treated with the anti-cancer drug, 5-fluorouracil (5-FU). ESAM-KO mice experienced severe and prolonged anemia after 5-FU treatment compared to wild-type (WT) mice. Eight days after the 5-FU injection, compared to WT mice, ESAM-KO mice showed reduced numbers of erythroid progenitors in bone marrow (BM) and spleen, and reticulocytes in peripheral blood. Megakaryocyte-erythrocyte progenitors (MEPs) from the BM of 5-FU-treated ESAM-KO mice showed reduced burst forming unit-erythrocyte (BFU-E) capacities than those from WT mice. BM transplantation revealed that hematopoietic stem/progenitor cells from ESAM-KO donors were more sensitive to 5-FU treatment than that from WT donors in the WT host mice. However, hematopoietic cells from WT donors transplanted into ESAM-KO host mice could normally reconstitute the erythroid lineage after a BM injury. These results suggested that ESAM expression in hematopoietic cells, but not environmental cells, is critical for hematopoietic recovery. We also found that 5-FU treatment induces the up-regulation of ESAM in primitive erythroid progenitors and macrophages that do not express ESAM under homeostatic conditions. The phenotypic change seen in macrophages might be functionally involved in the interaction between erythroid progenitors and their niche components during stress-induced acute erythropoiesis. Microarray analyses of primitive erythroid progenitors from 5-FU-treated WT and ESAM-KO mice revealed that various signaling pathways, including the GATA1 system, were impaired in ESAM-KO mice. Thus, our data demonstrate that ESAM expression in hematopoietic progenitors is essential for erythroid recovery after a BM injury.

Junctional Adhesion Molecule-A Is Highly Expressed on Human Hematopoietic Repopulating Cells and Associates with the Key Hematopoietic Chemokine Receptor CXCR4

Hematopoietic stem/progenitor cells (HSPCs) reside in specialized bone marrow microenvironmental niches, with vascular elements (endothelial/mesenchymal stromal cells) and CXCR4-CXCL12 interactions playing particularly important roles for HSPC entry, retention, and maintenance. The functional effects of CXCL12 are dependent on its local concentration and rely on complex HSPC-niche interactions. Two Junctional Adhesion Molecule family proteins, Junctional Adhesion Molecule-B (JAM)-B and JAM-C, are reported to mediate HSPC-stromal cell interactions, which in turn regulate CXCL12 production by mesenchymal stromal cells (MSCs). Here, we demonstrate that another JAM family member, JAM-A, is most highly expressed on human hematopoietic stem cells with in vivo repopulating activity (p < .01 for JAM-A(high) compared to JAM-A(Int or Low) cord blood CD34(+) cells). JAM-A blockade, silencing, and overexpression show that JAM-A contributes significantly (p < .05) to the adhesion of human HSPCs to IL-1β activated human bone marrow sinusoidal endothelium. Further studies highlight a novel association of JAM-A with CXCR4, with these molecules moving to the leading edge of the cell upon presentation with CXCL12 (p < .05 compared to no CXCL12). Therefore, we hypothesize that JAM family members differentially regulate CXCR4 function and CXCL12 secretion in the bone marrow niche. Stem Cells 2016;34:1664-1678.

Adhesion receptors involved in HSC and early-B cell interactions with bone marrow microenvironment

Hematopoiesis takes place in the bone marrow of adult mammals and is the process by which blood cells are replenished every day throughout life. Differentiation of hematopoietic cells occurs in a stepwise manner through intermediates of differentiation that could be phenotypically identified. This has allowed establishing hematopoietic cell classification with hematopoietic stem cells (HSCs) at the top of the hierarchy. HSCs are mostly quiescent and serve as a reservoir for maintenance of lifelong hematopoiesis. Over recent years, it has become increasingly clear that HSC quiescence is not only due to intrinsic properties, but is also mediated by cognate interactions between HSCs and surrounding cells within micro-anatomical sites called “niches”. This hematopoietic/stromal crosstalk model also applies to more mature progenitors such as B cell progenitors, which are thought to reside in distinct “niches”. This prompted many research teams to search for specific molecular mechanisms supporting leuko-stromal crosstalk in the bone marrow and acting at specific stage of differentiation to regulate hematopoietic homeostasis. Here, we review recent data on adhesion mechanisms involved in HSCs and B cell progenitors interactions with surrounding bone marrow stromal cells.

ESAM is a novel human hematopoietic stem cell marker associated with a subset of human leukemias

Reliable markers are essential to increase our understanding of the biological features of human hematopoietic stem cells and to facilitate the application of hematopoietic stem cells in the field of transplantation and regenerative medicine. We previously identified endothelial cell-selective adhesion molecule (ESAM) as a novel functional marker of hematopoietic stem cells in mice. Here, we found that ESAM can also be used to purify human hematopoietic stem cells from all the currently available sources (adult bone marrow, mobilized peripheral blood, and cord blood). Multipotent colony-forming units and long-term hematopoietic-reconstituting cells in immunodeficient mice were found exclusively in the ESAM(High) fraction of CD34(+)CD38(-) cells. The CD34(+)CD38(-) fraction of cord blood and collagenase-treated bone marrow contained cells exhibiting extremely high expression of ESAM; these cells are likely to be related to the endothelial lineage. Leukemia cell lines of erythroid and megakaryocyte origin, but not those of myeloid or lymphoid descent, were ESAM positive. However, high ESAM expression was observed in some primary acute myeloid leukemia cells. Furthermore, KG-1a myeloid leukemia cells switched from ESAM negative to ESAM positive with repeated leukemia reconstitution in vivo. Thus, ESAM is a useful marker for studying both human hematopoietic stem cells and leukemia cells.

How hematopoietic stem/progenitors and their niche sense and respond to infectious stress

Hematopoietic stem/progenitor cells (HSPCs) play important roles in fighting systemic infection as they supply immune cells in a demand-adapted manner. Various mechanisms govern HSPC responses to infection, including cytokine signaling, niche function, and direct sensing of pathogen-derived molecules by HSPCs themselves. Here we review recent advances in our understanding of HSPC responses to infection and also consider newly identified STING-mediated machinery recognizing bacteria-derived cyclic dinucleotides.

Characterization of rabbit limbal epithelial side population cells using RNA sequencing and single-cell qRT-PCR

Corneal epithelial stem cells reside in the limbus, a transitional zone between the cornea and conjunctiva, and are essential for maintaining homeostasis in the corneal epithelium. Although our previous studies demonstrated that rabbit limbal epithelial side population (SP) cells exhibit stem cell-like phenotypes with Hoechst 33342 staining, the different characteristics and/or populations of these cells remain unclear. Therefore, in this study, we determined the gene expression profiles of limbal epithelial SP cells by RNA sequencing using not only present public databases but also contigs that were created by de novo transcriptome assembly as references for mapping. Our transcriptome data indicated that limbal epithelial SP cells exhibited a stem cell-like phenotype compared with non-SP cells. Importantly, gene ontology analysis following RNA sequencing demonstrated that limbal epithelial SP cells exhibited significantly enhanced expression of mesenchymal/endothelial cell markers rather than epithelial cell markers. Furthermore, single-cell quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) demonstrated that the limbal epithelial SP population consisted of at least two immature cell populations with endothelial- or mesenchymal-like phenotypes. Therefore, our present results may propose the presence of a novel population of corneal epithelial stem cells distinct from conventional epithelial stem cells.

Ikaros mediates gene silencing in T cells through Polycomb repressive complex 2

T-cell development is accompanied by epigenetic changes that ensure the silencing of stem cell-related genes and the activation of lymphocyte-specific programmes. How transcription factors influence these changes remains unclear. We show that the Ikaros transcription factor forms a complex with Polycomb repressive complex 2 (PRC2) in CD4(-)CD8(-) thymocytes and allows its binding to more than 500 developmentally regulated loci, including those normally activated in haematopoietic stem cells and others induced by the Notch pathway. Loss of Ikaros in CD4(-)CD8(-) cells leads to reduced histone H3 lysine 27 trimethylation and ectopic gene expression. Furthermore, Ikaros binding triggers PRC2 recruitment and Ikaros interacts with PRC2 independently of the nucleosome remodelling and deacetylation complex. Our results identify Ikaros as a fundamental regulator of PRC2 function in developing T cells.

Functionally Distinct Subsets of Lineage-Biased Multipotent Progenitors Control Blood Production in Normal and Regenerative Conditions

Despite great advances in understanding the mechanisms underlying blood production, lineage specification at the level of multipotent progenitors (MPPs) remains poorly understood. Here, we show that MPP2 and MPP3 are distinct myeloid-biased MPP subsets that work together with lymphoid-primed MPP4 cells to control blood production. We find that all MPPs are produced in parallel by hematopoietic stem cells (HSCs), but with different kinetics and at variable levels depending on hematopoietic demands. We also show that the normally rare myeloid-biased MPPs are transiently overproduced by HSCs in regenerating conditions, hence supporting myeloid amplification to rebuild the hematopoietic system. This shift is accompanied by a reduction in self-renewal activity in regenerating HSCs and reprogramming of MPP4 fate toward the myeloid lineage. Our results support a dynamic model of blood development in which HSCs convey lineage specification through independent production of distinct lineage-biased MPP subsets that, in turn, support lineage expansion and differentiation.

ROBO4-mediated vascular integrity regulates the directionality of hematopoietic stem cell trafficking

Despite the use of hematopoietic stem cells (HSCs) in clinical therapy for over half a century, the mechanisms that regulate HSC trafficking, engraftment, and life-long persistence after transplantation are unclear. Here, we show that the vascular endothelium regulates HSC trafficking into and out of bone marrow (BM) niches. Surprisingly, we found that instead of acting as barriers to cellular entry, vascular endothelial cells, via the guidance molecule ROBO4, actively promote HSC translocation across vessel walls into the BM space. In contrast, we found that the vasculature inhibits the reverse process, as induced vascular permeability led to a rapid increase in HSCs in the blood stream. Thus, the vascular endothelium reinforces HSC localization to BM niches both by promoting HSC extravasation from blood-to-BM and by forming vascular barriers that prevent BM-to-blood escape. Our results uncouple the mechanisms that regulate the directionality of HSC trafficking and show that the vasculature can be targeted to improve hematopoietic transplantation therapies.

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Endothelial ROBO4 promotes unidirectional HSC trafficking across vessel walls

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Sinusoidal endothelial cells mediate HSC extravasation from blood to bone marrow

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Vascular integrity prevents HSC escape from bone marrow to blood

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Induced vascular permeability rapidly mobilizes HSCs to the blood stream

Fgd5 identifies hematopoietic stem cells in the murine bone marrow

Fdg5 identifies bone marrow cells with potent hematopoietic stem cell activity.

Hematopoietic stem cells (HSCs) are the best-characterized tissue-specific stem cells, yet experimental study of HSCs remains challenging, as they are exceedingly rare and methods to purify them are cumbersome. Moreover, genetic tools for specifically investigating HSC biology are lacking. To address this we sought to identify genes uniquely expressed in HSCs within the hematopoietic system and to develop a reporter strain that specifically labels them. Using microarray profiling we identified several genes with HSC-restricted expression. Generation of mice with targeted reporter knock-in/knock-out alleles of one such gene, Fgd5, revealed that though Fgd5 was required for embryonic development, it was not required for definitive hematopoiesis or HSC function. Fgd5 reporter expression near exclusively labeled cells that expressed markers consistent with HSCs. Bone marrow cells isolated based solely on Fgd5 reporter signal showed potent HSC activity that was comparable to stringently purified HSCs. The labeled fraction of the Fgd5 reporter mice contained all HSC activity, and HSC-specific labeling was retained after transplantation. Derivation of next generation mice bearing an Fgd5-CreERT2 allele allowed tamoxifen-inducible deletion of a conditional allele specifically in HSCs. In summary, reporter expression from the Fgd5 locus permits identification and purification of HSCs based on single-color fluorescence.

Re-entry into quiescence protects hematopoietic stem cells from the killing effect of chronic exposure to type I interferons

Quiescence acts as a safeguard mechanism to ensure survival of the HSC pool during chronic IFN-1 exposure

Type I interferons (IFN-1s) are antiviral cytokines that suppress blood production while paradoxically inducing hematopoietic stem cell (HSC) proliferation. Here, we clarify the relationship between the proliferative and suppressive effects of IFN-1s on HSC function during acute and chronic IFN-1 exposure. We show that IFN-1–driven HSC proliferation is a transient event resulting from a brief relaxation of quiescence-enforcing mechanisms in response to acute IFN-1 exposure, which occurs exclusively in vivo. We find that this proliferative burst fails to exhaust the HSC pool, which rapidly returns to quiescence in response to chronic IFN-1 exposure. Moreover, we demonstrate that IFN-1–exposed HSCs with reestablished quiescence are largely protected from the killing effects of IFNs unless forced back into the cell cycle due to culture, transplantation, or myeloablative treatment, at which point they activate a p53-dependent proapoptotic gene program. Collectively, our results demonstrate that quiescence acts as a safeguard mechanism to ensure survival of the HSC pool during chronic IFN-1 exposure. We show that IFN-1s can poise HSCs for apoptosis but induce direct cell killing only upon active proliferation, thereby establishing a mechanism for the suppressive effects of IFN-1s on HSC function.

SLAM family markers resolve functionally distinct subpopulations of hematopoietic stem cells and multipotent progenitors

Hematopoietic stem cells (HSCs) and multipotent hematopoietic progenitors (MPPs) are routinely isolated using various markers but remain heterogeneous. Here we show that four SLAM family markers, CD150, CD48, CD229, and CD244, can distinguish HSCs and MPPs from restricted progenitors and subdivide them into a hierarchy of functionally distinct subpopulations with stepwise changes in cell-cycle status, self-renewal, and reconstituting potential. CD229 expression largely distinguished lymphoid-biased HSCs from rarely dividing myeloid-biased HSCs, enabling prospective enrichment of these HSC subsets. Differences in CD229 and CD244 expression resolved CD150(-)CD48(-/low)Lineage(-/low)Sca-1(+)c-Kit(+) cells into a hierarchy of highly purified MPPs that retained erythroid and platelet potential but exhibited progressive changes in mitotic activity and reconstituting potential. Use of these markers, and reconstitution assays, showed that conditional deletion of Scf from endothelial cells and perivascular stromal cells eliminated the vast majority of bone marrow HSCs, including nearly all CD229(-/low) HSCs, demonstrating that quiescent HSCs are maintained by a perivascular niche.

Flk2/Flt3 promotes both myeloid and lymphoid development by expanding non-self-renewing multipotent hematopoietic progenitor cells

Defining differentiation pathways is central to understanding the pathogenesis of hematopoietic disorders, including leukemia. The function of the receptor tyrosine kinase Flk2 (Flt3) in promoting myeloid development remains poorly defined, despite being commonly mutated in acute myeloid leukemia. We investigated the effect of Flk2 deficiency on myelopoiesis, focusing on specification of progenitors between HSC and mature cells. We provide evidence that Flk2 is critical for proliferative expansion of multipotent progenitors that are common precursors for all lymphoid and myeloid lineages, including megakaryocyte/erythroid (MegE) cells. Flk2 deficiency impaired the generation of both lymphoid and myeloid progenitors by abrogating propagation of their common upstream precursor. At steady state, downstream compensatory mechanisms masked the effect of Flk2 deficiency on mature myeloid output, whereas transplantation of purified progenitors revealed impaired generation of all mature lineages. Flk2 deficiency did not affect lineage choice, thus dissociating the role of Flk2 in promoting cell expansion and regulating cell fate. Surprisingly, despite impairing myeloid development, Flk2 deficiency afforded protection against myeloablative insult. This survival advantage was attributed to reduced cell cycling and proliferation of progenitors in Flk2-deficient mice. Our data support the existence of a common Flk2(+) intermediate for all hematopoietic lineages and provide insight into how activating Flk2 mutations promote hematopoietic malignancy by non-Flk2-expressing myeloid cells.

Clonal precursor of bone, cartilage, and hematopoietic niche stromal cells

Organs are composites of tissue types with diverse developmental origins, and they rely on distinct stem and progenitor cells to meet physiological demands for cellular production and homeostasis. How diverse stem cell activity is coordinated within organs is not well understood. Here we describe a lineage-restricted, self-renewing common skeletal progenitor (bone, cartilage, stromal progenitor; BCSP) isolated from limb bones and bone marrow tissue of fetal, neonatal, and adult mice. The BCSP clonally produces chondrocytes (cartilage-forming) and osteogenic (bone-forming) cells and at least three subsets of stromal cells that exhibit differential expression of cell surface markers, including CD105 (or endoglin), Thy1 [or CD90 (cluster of differentiation 90)], and 6C3 [ENPEP glutamyl aminopeptidase (aminopeptidase A)]. These three stromal subsets exhibit differential capacities to support hematopoietic (blood-forming) stem and progenitor cells. Although the 6C3-expressing subset demonstrates functional stem cell niche activity by maintaining primitive hematopoietic stem cell (HSC) renewal in vitro, the other stromal populations promote HSC differentiation to more committed lines of hematopoiesis, such as the B-cell lineage. Gene expression analysis and microscopic studies further reveal a microenvironment in which CD105-, Thy1-, and 6C3-expressing marrow stroma collaborate to provide cytokine signaling to HSCs and more committed hematopoietic progenitors. As a result, within the context of bone as a blood-forming organ, the BCSP plays a critical role in supporting hematopoiesis through its generation of diverse osteogenic and hematopoietic-promoting stroma, including HSC supportive 6C3(+) niche cells.

Reviewing and updating the major molecular markers for stem cells

Stem cells (SC) are able to self-renew and to differentiate into many types of committed cells, making SCs interesting for cellular therapy. However, the pool of SCs in vivo and in vitro consists of a mix of cells at several stages of differentiation, making it difficult to obtain a homogeneous population of SCs for research. Therefore, it is important to isolate and characterize unambiguous molecular markers that can be applied to SCs. Here, we review classical and new candidate molecular markers that have been established to show a molecular profile for human embryonic stem cells (hESCs), mesenchymal stem cells (MSCs), and hematopoietic stem cells (HSCs). The commonly cited markers for embryonic ESCs are Nanog, Oct-4, Sox-2, Rex-1, Dnmt3b, Lin-28, Tdgf1, FoxD3, Tert, Utf-1, Gal, Cx43, Gdf3, Gtcm1, Terf1, Terf2, Lefty A, and Lefty B. MSCs are primarily identified by the expression of CD13, CD29, CD44, CD49e, CD54, CD71, CD73, CD90, CD105, CD106, CD166, and HLA-ABC and lack CD14, CD31, CD34, CD45, CD62E, CD62L, CD62P, and HLA-DR expression. HSCs are mainly isolated based on the expression of CD34, but the combination of this marker with CD133 and CD90, together with a lack of CD38 and other lineage markers, provides the most homogeneous pool of SCs. Here, we present new and alternative markers for SCs, along with microRNA profiles, for these cells.

Function of junctional adhesion molecules (JAMs) in leukocyte migration and homeostasis

Homeostasis is a word widely used in the scientific community to refer to the property of a system to maintain its uniformity and functionality. In living organisms, the word refers to the concept enunciated 150 years ago by C. Bernard by which external variations must be compensated for in order to maintain internal conditions compatible with life. This is especially true in the case of highly dynamic system such as the hematopoietic system that requires the coordinated control of cell proliferation and death within specialized microenvironments that are anatomically distinct. As a consequence, hematopoietic cell adhesion and migration must be tightly controlled in order for hematopoietic cells to reach and to be maintained in appropriate microenvironments. The junctional adhesion molecules (JAMs) are adhesion molecules that belong to the immunoglobulin superfamily (IgSf) and that have been initially identified as important players controlling vascular permeability and leukocyte transendothelial migration. This involves the regulated localization of the JAMs at lateral endothelial cell/cell borders and their interaction with leukocyte integrins. More recently, some of the JAM family members have also been found to be expressed by stromal cells and to regulate chemokine secretion within lymphoid organs, acting not only on leukocyte transendothelial migration, but also on hematopoietic cell retention within specialized microenvironments. This review summarizes recent progress in understanding the role of the JAMs in leukocyte adhesion and migration to tentatively draw an integrated view of the homeostatic function of the JAMs within the hematopoietic system.

JAM-B regulates maintenance of hematopoietic stem cells in the bone marrow

In adult mammals, hematopoietic stem cells (HSCs) reside in the bone marrow (BM) and are maintained in a quiescent and undifferentiated state through adhesive interactions with specialized microenvironmental niches. Although junctional adhesion molecule-C (JAM-C) is expressed by HSCs, its function in adult hematopoiesis remains elusive. Here, we show that HSCs adhere to JAM-B expressed by BM stromal cells in a JAM-C dependent manner. The interaction regulates the interplay between HSCs and BM stromal cells as illustrated by the decreased pool of quiescent HSCs observed in jam-b deficient mice. We further show that this is probably because of alterations of BM stromal compartments and changes in SDF-1α BM content in jam-b(-/-) mice, suggesting that JAM-B is an active player in the maintenance of the BM stromal microenvironment.

Robo4 cooperates with CXCR4 to specify hematopoietic stem cell localization to bone marrow niches

Specific bone marrow (BM) niches are critical for hematopoietic stem cell (HSC) function during both normal hematopoiesis and in stem cell transplantation therapy. We demonstrate that the guidance molecule Robo4 functions to specifically anchor HSCs to BM niches. Robo4-deficient HSCs displayed poor localization to BM niches and drastically reduced long-term reconstitution capability while retaining multilineage potential. Cxcr4, a critical regulator of HSC location, is upregulated in Robo4(-/-) HSCs to compensate for Robo4 loss. Robo4 deletion led to altered HSC mobilization efficiency, revealing that inhibition of both Cxcr4- and Robo4-mediated niche interactions are necessary for efficient HSC mobilization. Surprisingly, we found that WT HSCs express very low levels of Cxcr4 and respond poorly to Cxcr4 manipulation relative to other hematopoietic cells. We conclude that Robo4 cooperates with Cxcr4 to endow HSCs with competitive access to limited stem cell niches, and we propose Robo4 as a therapeutic target in HSC transplantation therapy.

Crebbp haploinsufficiency in mice alters the bone marrow microenvironment, leading to loss of stem cells and excessive myelopoiesis

CREB-binding protein (CREBBP) is important for the cell-autonomous regulation of hematopoiesis, including the stem cell compartment. In the present study, we show that CREBBP plays an equally pivotal role in microenvironment-mediated regulation of hematopoiesis. We found that the BM microenvironment of Crebbp(+/-) mice was unable to properly maintain the immature stem cell and progenitor cell pools. Instead, it stimulates myeloid differentiation, which progresses into a myeloproliferation phenotype. Alterations in the BM microenvironment resulting from haploinsufficiency of Crebbp included a marked decrease in trabecular bone that was predominantly caused by increased osteoclastogenesis. Although CFU-fibroblast (CFU-F) and total osteoblast numbers were decreased, the bone formation rate was similar to that found in wild-type mice. At the molecular level, we found that the known hematopoietic modulators matrix metallopeptidase-9 (MMP9) and kit ligand (KITL) were decreased with heterozygous levels of Crebbp. Lastly, potentially important regulatory proteins, endothelial cell adhesion molecule 1 (ESAM1) and cadherin 5 (CDH5), were increased on Crebbp(+/-) endothelial cells. Our findings reveal that a full dose of Crebbp is essential in the BM microenvironment to maintain proper hematopoiesis and to prevent excessive myeloproliferation.

Mechanisms controlling hematopoietic stem cell functions during normal hematopoiesis and hematological malignancies

Hematopoiesis, the process by which all mature blood cells are generated from multipotent hematopoietic stem cells (HSCs), is a finely tuned balancing act in which HSCs must constantly decide between different cell fates: to proliferate, to self-renew or differentiate, to stay quiescent in the bone marrow niche or migrate to the periphery, to live or die. These fates are regulated by a complex interplay between cell-extrinsic cues and cell-intrinsic regulatory pathways whose function is to maintain a homeostatic balance between HSC self-renewal and life-long replenishment of lost blood cells. Improper regulation of these competing cellular programs can transform HSCs and progenitor cells into disease-initiating leukemic stem cells (LSCs). Strikingly, many of the mechanisms required for maintenance of normal HSC fate decisions are equally critical for the aberrant functions of LSCs. Because of the inherent complexities of these molecular mechanisms, a systematic approach to understanding the regulatory networks underlying HSC self-renewal is critical for uncovering the similarities and differences between HSCs and LSCs. In this review, we focus on recent developments in elucidating the regulatory networks governing normal HSC self-renewal programs and their implications for leukemic transformation. We describe the current technical and methodological limitations in isolating and characterizing HSCs and LSCs, and the emerging approaches that may afford a better understanding of the regulation of normal and leukemic hematopoiesis. Finally, we discuss how such basic mechanistic information may be of use for the design of novel therapies that will selectively reprogram and/or eliminate LSCs.