Repopulating activity of ex vivo-expanded murine hematopoietic stem cells resides in the CD48-c-Kit+Sca-1+lineage marker- cell population

Controlling genetic heterogeneity in gene-edited hematopoietic stem cells by single-cell expansion

Gene editing using engineered nucleases frequently produces unintended genetic lesions in hematopoietic stem cells (HSCs). Gene-edited HSC cultures thus contain heterogeneous populations, the majority of which either do not carry the desired edit or harbor unwanted mutations. In consequence, transplanting edited HSCs carries the risks of suboptimal efficiency and of unwanted mutations in the graft. Here, we present an approach for expanding gene-edited HSCs at clonal density, allowing for genetic profiling of individual clones before transplantation. We achieved this by developing a defined, polymer-based expansion system and identifying long-term expanding clones within the CD201+CD150+CD48-c-Kit+Sca-1+Lin- population of precultured HSCs. Using the Prkdcscid immunodeficiency model, we demonstrate that we can expand and profile edited HSC clones to check for desired and unintended modifications, including large deletions. Transplantation of Prkdc-corrected HSCs rescued the immunodeficient phenotype. Our ex vivo manipulation platform establishes a paradigm to control genetic heterogeneity in HSC gene editing and therapy.

CD244 expression represents functional decline of murine hematopoietic stem cells after in vitro culture

Isolation of long-term hematopoietic stem cell (HSC) is possible by utilizing flow cytometry with multiple cell surface markers. However, those cell surface phenotypes do not represent functional HSCs after in vitro culture. Here we show that cultured HSCs express mast cell-related genes including Cd244. After in vitro culture, phenotypic HSCs were divided into CD244^- and CD244⁺ subpopulations, and only CD244^- cells that have low mast cell gene expression and maintain HSC-related genes sustain reconstitution potential. The result was same when HSCs were cultured in an efficient expansion medium containing polyvinyl alcohol. Chemically induced endoplasmic reticulum (ER) stress signal increased the CD244⁺ subpopulation, whereas ER stress suppression using a molecular chaperone, TUDCA, decreased CD244⁺ population, which was correlated to improved reconstitution output. These data suggest CD244 is a potent marker to exclude non-functional HSCs after in vitro culture thereby useful to elucidate mechanism of functional decline of HSCs during ex vivo treatment.

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Murine HSCs up-regulate mast cell-related genes including Cd244 during in vitro culture

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Long-term HSCs after in vitro culture are enriched in CD244⁻CD48⁻KSL population

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Induction of unfolded protein response is involved in the increase of CD244⁺HSC

An improved lentiviral fluorescent genetic barcoding approach distinguishes hematopoietic stem cell properties in multiplexed in vivo experiments

Hematopoietic stem cells (HSC) represent a rare cell population of particular interest for biomedical research and regenerative medicine. Various marker combinations enable the isolation of HSCs but fail to reach purity in transplantation assays. To reduce animal consumption, we developed a multiplexing system based on lentiviral fluorescent genetic barcoding (FGB) to enable the parallel characterization of multiple HSC samples within single animals. While previous FGB-mediated HSC multiplexing experiments achieved high in vitro gene marking rates, in vivo persistence of transduced cells remained sub-optimal. Thus, we aimed to optimize vector design and gene transfer protocols to demonstrate the applicability of FGB for functional characterization of two highly similar HSC populations in a reduced number of mice. We developed a set of 6 new lentiviral FGB vectors, utilizing individual and combinatorial expression of Azami Green, mCherry, and YFP derivatives. Gene transfer rates were optimized by overnight transduction of pre-stimulated HSCs with titrated vector doses. Populations for competitive transplantation experiments were identified by immunophenotyping murine HSCs. This identified an LSK-SLAM^- (Lin^-Sca-1⁺cKit⁺CD48^-CD150⁺EPCR^-) cell subpopulation that lacks EPCR expression and exhibits prospectively reduced self-renewal potential compared with prototypical ESLAM (CD45⁺EPCR⁺CD48^-CD150⁺) HSCs. We monitored 30 data points per HSC-subpopulation in two independent experiments (each n=5) after co-transplantation of 3 uniquely color-coded ESLAM and LSK-SLAM^- samples per recipient. While the first experiment was hampered by data fluctuations, increasing cell numbers and exchange of the internal promoter in the second experiment led to 74.4% chimerism, with 87.1% of fluorescent cells derived from ESLAM HSCs. Furthermore, ESLAM-derived cells produced 88.1% of myeloid cells, which is indicative of their origin from long-term repopulating HSCs. This work verifies the importance of EPCR for long-term repopulating HSCs and demonstrates the applicability of our optimized FGB-driven multiplexing approach for the efficient characterization of blood cell populations in biomedical research.

Small molecule-mediated modulation of ubiquitination and neddylation improves HSC function ex vivo

Hematopoietic stem cells (HSCs) are particularly characterized by their quiescence and self-renewal. Cell cycle regulators tightly control quiescence and self-renewal capacity. Studies suggest that modulation of ubiquitination and neddylation could contribute to HSC function via cyclin-dependent kinase inhibitors (CDKIs). S-phase kinase-associated protein 2 (SKP2) is responsible for ubiquitin-mediated proteolysis of CDKIs. Here, we modulated overall neddylation and SKP2-associated ubiquitination in HSCs by using SKP2-C25, an SKP2 inhibitor, and MLN4924 (Pevonedistat) as an inhibitor of the NEDD8 system. Treatments of SKP2-C25 and MLN4924 increased both murine and human stem and progenitor cell (HSPC) compartments. This is associated with the improved quiescence of murine HSC by upregulation of p27 and p57 CDKIs. A colony-forming unit assay showed an enhanced in vitro self-renewal potential post inhibition of ubiquitination and neddylation. In addition, MLN4924 triggered the mobilization of bone marrow HSPCs to peripheral blood. Intriguingly, MLN4924 treatment could decrease the proliferation of murine bone marrow mesenchymal stem cells or endothelial cells. These findings shed light on the contribution of SKP2, and associated ubiquitination and neddylation in HSC maintenance, self-renewal, and expansion.

High Canonical Wnt/β-Catenin Activity Sensitizes Murine Hematopoietic Stem and Progenitor Cells to DNA Damage

Aging is characterized by the accumulation of DNA damage and a decrease in stem cell functionality, yet molecular mechanisms that limit the maintenance of stem cells in response to DNA damage remain to be delineated. Here we show in mouse models that DNA damage leads to a transient over-activation of Wnt signaling in hematopoietic stem cells (HSCs), and that high activity of canonical Wnt/β-catenin signaling sensitizes HSCs to DNA damage induced by X-irradiation which results in preferential maintenance of HSCs with low levels of Wnt signaling. The study shows that genetic or chemical activation of canonical Wnt signaling enhances radiosensitivity of HSCs while inhibition of Wnt signaling decreases it. Together, these results indicate that levels of Wnt signaling activity mediate heterogeneity in the sensitivity of HSCs to DNA damage induced depletion. These findings could be relevant for molecular alterations and selection of stem cells in the context of DNA damage accumulation during aging and cancer formation.

Interferon gamma inhibits the differentiation of mouse adult liver and bone marrow hematopoietic stem cells by inhibiting the activation of notch signaling

BACKGROUND

The paradigm of hematopoietic stem and progenitor cells (HSPCs) has become accepted ever since the discovery of adult mouse liver hematopoietic stem cells and their multipotent characteristics that give rise to all blood cells. However, differences between bone marrow (BM) and liver hematopoietic stem cells and the hematopoietic microenvironment remain poorly understood. In addition, the regulation of the liver hematopoietic system remains unknown.

METHODS

Clone formation assays were used to confirm that the proliferation of adult mouse liver and bone marrow HSPCs. Model mice with different interferon gamma (IFN-γ) levels and a co-culture system were used to detect the differentiation of liver HSPCs. The γ-secretase inhibitor (GSI) and the JAK/STAT inhibitor ruxolitinib and cell culture assays were used to explore the molecular mechanism by which IFN-γ impairs HSPC proliferation and differentiation.

RESULTS

The colony-forming activity of liver and bone marrow HSPCs was inhibited by IFN-γ. Model mice with different IFN-γ levels showed that the differentiation of liver HSPCs was impaired by IFN-γ. Using a co-culture system comprising liver HSPCs, we found that IFN-γ inhibited the development of liver hematopoietic stem cells into γδT cells. We then demonstrated that IFN-γ might impair liver HSPC differentiation by inhibiting the activation of the notch signaling via the JAK/STAT signaling pathway.

CONCLUSIONS

IFN-γ inhibited the proliferation of liver-derived HSPCs. IFN-γ also impaired the differentiation of long-term hematopoietic stem cells (LT-HSCs) into short-term hematopoietic stem cells (ST-HSCs) and multipotent progenitors (MPPs) and the process from LSK (Lineage^-Sca-1⁺c-Kit⁺) cells to γδT cells. Importantly, we proposed that IFN-γ might inhibit the activation of notch signaling through the JAK/STAT signaling pathway and thus impair the differentiation process of mouse adult liver and BM hematopoietic stem cells.

Histological study of postnatal development of mouse tongues

Numerous factors, including trauma, tumors and myophagism, may lead to tongue defects, which are mostly repaired via muscular flaps. However, these methods cannot restore the muscular function and gustation function of the tongue. Intensive research on tongue development may offer useful clues for tongue regeneration based on tissue engineering or stem cell therapy. In the present study, staining results revealed that tongue muscle fibers became larger, mature and stronger, and the foliate and fungiform papillae also became mature from newborn to adult C57BL/6J genetic background mice. Immunofluorescence staining and polymerase chain reaction results revealed that C-kit was dynamically expressed in muscle cells, as well as in foliate and fungiform papilla cells from newborn to adult stages. The expression level decreased from P1 to P15 and increased at P90. The immunofluorescence staining results revealed that Ki-67 was expressed in muscle cells and papilla cells from newborn to adult stages, and high expression was observed at P6 and P15. In addition, the immunofluorescence staining results also demonstrated that msh homeobox 2 (Msx2) was dynamically expressed in postnatal tongue muscle cells; however, almost no expression was detected in papilla cells. There was relative high expression level of Msx2 at P1 and P6 stages, but this gradually decreased from P15, and it was expressed primarily in the muscle cells located in the marginal zone of the tongue at P90. These findings suggest that the amount of c-kit-expressing precursor cells in tongue muscle and papilla cells increases to promote tongue development at the early postnatal stage and to maintain homeostasis and functional adaptation of the tongue in the adult stage. Furthermore, Msx2 may serve an important role in postnatal tongue muscle development. The present study also suggests that C-kit and Msx2 may be used as cell markers for postnatal tongue regeneration and self-repair, and may provide an approach for developing treatment methods for tongue diseases with a postnatal onset.

SATB2-Nanog axis links age-related intrinsic changes of mesenchymal stem cells from craniofacial bone

Bone mesenchymal stem cells (BMSCs) senescence contributes to age-related bone loss. The alveolar bone in jaws originates from neural crest cells and possesses significant site- and age-related properties. However, such intrinsic characteristics of BMSCs from alveolar bone (AB-BMSCs) and the underlying regulatory mechanisms still remain unknown. Here, we found that the expression of special AT-rich binding protein 2 (SATB2) in human AB-BMSCs significantly decreased with aging. SATB2 knockdown on AB-BMSCs from young donors displayed these aging-related phenotypes in vitro. Meanwhile, enforced SATB2 overexpression could rejuvenate AB-BMSCs from older donors. Importantly, satb2 gene- modified BMSCs therapy could prevent the alveolar bone loss during the aging of rats. Mechanistically, the stemness regulator Nanog was identified as the direct transcriptional target of SATB2 in BMSCs and functioned as a downstream mediator of SATB2. Collectively, our data reveal that SATB2 in AB-BMSCs associates with their age-related properties, and prevents AB-BMSCs senescence via maintaining Nanog expression. These findings highlight the translational potential of transcriptional factor-based cellular reprogramming for anti-aging therapy.

Integrin αvβ3 enhances the suppressive effect of interferon-γ on hematopoietic stem cells

Hematopoietic homeostasis depends on the maintenance of hematopoietic stem cells (HSCs), which are regulated within a specialized bone marrow (BM) niche. When HSC sense external stimuli, their adhesion status may be critical for determining HSC cell fate. The cell surface molecule, integrin αvβ3, is activated through HSC adhesion to extracellular matrix and niche cells. Integrin β3 signaling maintains HSCs within the niche. Here, we showed the synergistic negative regulation of the pro-inflammatory cytokine interferon-γ (IFNγ) and β3 integrin signaling in murine HSC function by a novel definitive phenotyping of HSCs. Integrin αvβ3 suppressed HSC function in the presence of IFNγ and impaired integrin β3 signaling mitigated IFNγ-dependent negative action on HSCs. During IFNγ stimulation, integrin β3 signaling enhanced STAT1-mediated gene expression via serine phosphorylation. These findings show that integrin β3 signaling intensifies the suppressive effect of IFNγ on HSCs, which indicates that cell adhesion via integrin αvβ3 within the BM niche acts as a context-dependent signal modulator to regulate the HSC function under both steady-state and inflammatory conditions.

Multiple allogeneic progenitors in combination function as a unit to support early transient hematopoiesis in transplantation

Cord blood (CB) is a valuable donor source in hematopoietic cell transplantation. However, the initial time to engraftment in CB transplantation (CBT) is often delayed because of low graft cell numbers. This limits the use of CB. To overcome this cell dose barrier, we modeled an insufficient dose CBT setting in lethally irradiated mice and then added hematopoietic stem/progenitor cells (HSCs/HPCs; HSPCs) derived from four mouse allogeneic strains. The mixture of HSPCs rescued recipients and significantly accelerated hematopoietic recovery. Including T cells from one strain favored single-donor chimerism through graft versus graft reactions, with early hematopoietic recovery unaffected. Furthermore, using clinically relevant procedures, we successfully isolated a mixture of CD34(+) cells from multiple frozen CB units at one time regardless of HLA-type disparities. These CD34(+) cells in combination proved transplantable into immunodeficient mice. This work provides proof of concept that when circumstances require support of hematopoiesis, combined multiple units of allogeneic HSPCs are capable of early hematopoietic reconstitution while allowing single-donor hematopoiesis by a principal graft.

Energy Metabolism Plays a Critical Role in Stem Cell Maintenance and Differentiation

Various stem cells gradually turned to be critical players in tissue engineering and regenerative medicine therapies. Current evidence has demonstrated that in addition to growth factors and the extracellular matrix, multiple metabolic pathways definitively provide important signals for stem cell self-renewal and differentiation. In this review, we mainly focus on a detailed overview of stem cell metabolism in vitro. In stem cell metabolic biology, the dynamic balance of each type of stem cell can vary according to the properties of each cell type, and they share some common points. Clearly defining the metabolic flux alterations in stem cells may help to shed light on stemness features and differentiation pathways that control the fate of stem cells.

Ex vivo expansion of hematopoietic stem cells

Ex vivo expansion of hematopoietic stem cells (HSCs) would benefit clinical applications in several aspects, to improve patient survival, utilize cord blood stem cells for adult applications, and selectively propagate stem cell populations after genetic manipulation. In this review we summarize and discuss recent advances in the culture systems of mouse and human HSCs, which include stroma/HSC co-culture, continuous perfusion and fed-batch cultures, and those supplemented with extrinsic ligands, membrane transportable transcription factors, complement components, protein modification enzymes, metabolites, or small molecule chemicals. Some of the expansion systems have been tested in clinical trials. The optimal condition for ex vivo expansion of the primitive and functional human HSCs is still under development. An improved understanding of the mechanisms for HSC cell fate determination and the HSC culture characteristics will guide development of new strategies to overcome difficulties. In the future, development of a combination treatment regimen with agents that enhance self-renewal, block differentiation, and improve homing will be critical. Methods to enhance yields and lower cost during collection and processing should be employed. The employment of an efficient system for ex vivo expansion of HSCs will facilitate the further development of novel strategies for cell and gene therapies including genome editing.

β2-Microglobulin is an appropriate reference gene for RT-PCR-based gene expression analysis of hematopoietic stem cells

Real-time reverse transcription polymerase chain reaction (RT-PCR) is regarded as one of the most useful and powerful tools for characterizing hematopoietic stem cells (HSCs), because samples of extremely small cell numbers can be analyzed. The expression levels determined by RT-PCR are based on relative quantification; therefore, the selection of an appropriate reference gene with a relatively stable expression level under most conditions is crucial. Here, we determined that beta2-microglobulin (B2m) is an appropriate reference gene for analyzing mouse HSCs by a novel method using single-cell RT-PCR. Clonally sorted HSCs were subjected to RT reactions with exogenous RNA fragments and then to real-time PCR. Next, the relative gene expression levels of 4 well-known housekeeping genes were quantified in each single cell sample based on the threshold cycle of exogenous RNA. The analysis revealed that B2m expression was reproducibly detected in almost all HSCs and that B2m had the most stable expression level among the compared genes, even after the cells had been cultured under various conditions. Thus, our results indicate that B2m can reliably be used as a reference gene for the relative quantification of expression levels in HSCs across various conditions. Furthermore, our work proposes a novel method for the selection of appropriate reference genes.

Role of flow-sensitive microRNAs in endothelial dysfunction and atherosclerosis: mechanosensitive athero-miRs

Atherosclerosis preferentially occurs in arterial regions exposed to disturbed flow, in part, due to alterations in gene expression. MicroRNAs (miRNAs) are small, noncoding genes that post-transcriptionally regulate gene expression by targeting messenger RNA transcripts. Emerging evidence indicates that alteration of flow conditions regulate expression of miRNAs in endothelial cells both in vitro and in vivo. These flow-sensitive miRNAs, known as mechano-miRs, regulate endothelial gene expression and can regulate endothelial dysfunction and atherosclerosis. MiRNAs such as, miR-10a, miR-19a, miR-23b, miR-17-92, miR-21, miR-663, miR-92a, miR-143/145, miR-101, miR-126, miR-712, miR-205, and miR-155, have been identified as mechano-miRs. Many of these miRNAs were initially identified as flow sensitive in vitro and were later found to play a critical role in endothelial function and atherosclerosis in vivo through either gain-of-function or loss-of-function approaches. The key signaling pathways that are targeted by these mechano-miRs include the endothelial cell cycle, inflammation, apoptosis, and nitric oxide signaling. Furthermore, we have recently shown that the miR-712/205 family, which is upregulated by disturbed flow, contributes to endothelial inflammation and vascular hyperpermeability by targeting tissue inhibitor of metalloproteinase-3, which regulates metalloproteinases and a disintegrin and metalloproteinases. The mechano-miRs that are implicated in atherosclerosis are termed as mechanosensitive athero-miRs and are potential therapeutic targets to prevent or treat atherosclerosis. This review summarizes the current knowledge of mechanosensitive athero-miRs and their role in vascular biology and atherosclerosis.

Nov/CCN3 regulates long-term repopulating activity of murine hematopoietic stem cells via integrin αvβ3

Throughout life, hematopoietic stem cells (HSCs) sustain the blood cell supply through their capacities for self-renewal and multilineage differentiation. These processes are regulated within a specialized microenvironment termed the 'niche'. Here, we show a novel mechanism for regulating HSC function that is mediated by nephroblastoma overexpressed (Nov/CCN3), a matricellular protein member of the CCN family. We found that Nov contributes to the maintenance of long-term repopulating (LTR) activity through association with integrin αvβ3 on HSCs. The resultant β3 integrin outside-in signaling is dependent on thrombopoietin (TPO), a crucial cytokine involved in HSC maintenance. TPO was required for Nov binding to integrin αvβ3, and stimulated Nov expression in HSCs. However, in the presence of IFNγ, a cytokine known to impair HSC function, not only was TPO-induced expression of Nov suppressed, but the LTR activity was conversely impaired by TPO-mediated ligation of integrin αvβ3 with exogenous ligands, including Nov, as well. Thus, Nov/integrin αvβ3-mediated maintenance of HSCs appears to be modulated by simultaneous stimulation by other cytokines. Our finding suggests that this system contributes to the regulation of HSCs within the bone marrow niche.

Spatial and temporal expression of c-Kit in the development of the murine submandibular gland

The c-Kit pathway is important in the development of many mammalian cells and organs and is indispensable for the development of hematopoiesis, melanocytes, and primordial germ cells. Loss-of-function mutations in c-Kit lead to perinatal death in mouse embryos. Previously, c-Kit has been used as one of salivary epithelial stem or progenitor cell markers in mouse, its specific temporo-spatial expression pattern and function in developing murine submandibular gland (SMG) is still unclear. Here we used quantitative real-time PCR, in situ hybridization, and immunohistochemistry analysis to detect c-Kit expression during the development of the murine SMG. We found that c-Kit was expressed in the epithelia of developing SMGs from embryonic day 11.5 (E11.5; initial bud stage) to postnatal day 90 (P90; when the SMG is completely mature). c-Kit expression in the end bud epithelium increased during prenatal development and then gradually decreased after birth until its expression was undetectable in mature acini at P30. Moreover, c-Kit was expressed in the SMG primordial cord at the initial bud, pseudoglandular, canacular, and terminal end bud stages. c-Kit was also expressed in the presumptive ductal cells adjacent to the developing acini. By the late terminal end bud stage on P14, c-Kit expression could not be detected in ductal cells. However, c-Kit expression was detected in ductal cells at P30, and its expression had increased dramatically at P90. Taken together, these findings describe the spatial and temporal expression pattern of c-Kit in the developing murine SMG and suggest that c-Kit may play roles in epithelial histo-morphogenesis and in ductal progenitor cell homeostasis in the SMG.

Rapamycin enhances long-term hematopoietic reconstitution of ex vivo expanded mouse hematopoietic stem cells by inhibiting senescence

BACKGROUND

The mammalian target of rapamycin (mTOR) is an important regulator of hematopoietic stem cell (HSC) self-renewal and its overactivation contributes to HSC premature exhaustion in part via induction of HSC senescence. Inhibition of mTOR with rapamycin has the potential to promote long-term hematopoiesis of ex vivo expanded HSCs to facilitate the clinical application of HSC transplantation for various hematologic diseases.

METHODS

A well-established ex vivo expansion system for mouse bone marrow HSCs was used to investigate whether inhibition of overactivated mTOR with rapamycin can promote long-term hematopoiesis of ex vivo expanded HSCs and to elucidate the mechanisms of action of rapamycin.

RESULTS

HSC-enriched mouse bone marrow LSK cells exhibited a time-dependent activation of mTOR after ex vivo expansion in a serum-free medium supplemented with stem cell factor, thrombopoietin, and Flt3 ligand. The overactivation of mTOR was associated with induction of senescence but not apoptosis in LSK cells and a significant reduction in the ability of HSCs to produce long-term hematopoietic reconstitution. Inhibition of overactivated mTOR with rapamycin promoted ex vivo expansion and long-term hematopoietic reconstitution of HSCs. The increase in long-term hematopoiesis of expanded HSCs is likely attributable in part to rapamycin-mediated up-regulation of Bmi1 and down-regulation of p16, which prevent HSCs from undergoing senescence during ex vivo expansion.

CONCLUSIONS

These findings suggest that mTOR plays an important role in the regulation of HSC self-renewal in vitro and inhibition of mTOR hyperactivation with rapamycin may represent a novel approach to promote ex vivo expansion and their long-term hematopoietic reconstitution of HSCs.

Visualizing developmentally programmed endoreplication in mammals using ubiquitin oscillators

The majority of mammalian somatic cells maintain a diploid genome. However, some mammalian cell types undergo multiple rounds of genome replication (endoreplication) as part of normal development and differentiation. For example, trophoblast giant cells (TGCs) in the placenta become polyploid through endoreduplication (bypassed mitosis), and megakaryocytes (MKCs) in the bone marrow become polyploid through endomitosis (abortive mitosis). During the normal mitotic cell cycle, geminin and Cdt1 are involved in ‘licensing’ of replication origins, which ensures that replication occurs only once in a cell cycle. Their protein accumulation is directly regulated by two E3 ubiquitin ligase activities, APCCdh1 and SCFSkp2, which oscillate reciprocally during the cell cycle. Although proteolysis-mediated, oscillatory accumulation of proteins has been documented in endoreplicating Drosophila cells, it is not known whether the ubiquitin oscillators that control normal cell cycle transitions also function during mammalian endoreplication. In this study, we used transgenic mice expressing Fucci fluorescent cell-cycle probes that report the activity of APCCdh1 and SCFSkp2. By performing long-term, high temporal-resolution Fucci imaging, we were able to visualize reciprocal activation of APCCdh1 and SCFSkp2 in differentiating TGCs and MKCs grown in our custom-designed culture wells. We found that TGCs and MKCs both skip cytokinesis, but in different ways, and that the reciprocal activation of the ubiquitin oscillators in MKCs varies with the polyploidy level. We also obtained three-dimensional reconstructions of highly polyploid TGCs in whole, fixed mouse placentas. Thus, the Fucci technique is able to reveal the spatiotemporal regulation of the endoreplicative cell cycle during differentiation.

Regulation of glycolysis by Pdk functions as a metabolic checkpoint for cell cycle quiescence in hematopoietic stem cells

Defining the metabolic programs that underlie stem cell maintenance will be essential for developing strategies to manipulate stem cell capacity. Mammalian hematopoietic stem cells (HSCs) maintain cell cycle quiescence in a hypoxic microenvironment. It has been proposed that HSCs exhibit a distinct metabolic phenotype under these conditions. Here we directly investigated this idea using metabolomic analysis and found that HSCs generate adenosine-5'-triphosphate by anaerobic glycolysis through a pyruvate dehydrogenase kinase (Pdk)-dependent mechanism. Elevated Pdk expression leads to active suppression of the influx of glycolytic metabolites into mitochondria. Pdk overexpression in glycolysis-defective HSCs restored glycolysis, cell cycle quiescence, and stem cell capacity, while loss of both Pdk2 and Pdk4 attenuated HSC quiescence, glycolysis, and transplantation capacity. Moreover, treatment of HSCs with a Pdk mimetic promoted their survival and transplantation capacity. Thus, glycolytic metabolic status governed by Pdk acts as a cell cycle checkpoint that modulates HSC quiescence and function.

Establishment of monoclonal antibodies against a novel eosinophil-specific cell surface molecule, major facilitator super family domain containing 10

Eosinophilic inflammation is the prominent feature of bronchial asthma, though the importance of eosinophils in the pathogenesis of this disease is controversial. We here established monoclonal antibodies against a newly identified cell surface molecule specifically expressed on mouse eosinophils. Eosinophils were highly purified from small intestine lamina propria and thymus as CD11c(+)Gr1(low)F4/80(+)B220(-) cells. Upon comparative microarray analysis for mRNA expressed in eosinophils and other leukocytes, major facilitator super family domain containing 10 (Mfsd10) was identified as a novel eosinophil-specific cell surface molecule. Hybridomas were established from spleen cells of rats immunized with Mfsd10-introduced Ba/F3 cells. One of three monoclonal antibodies against Mfsd10 displayed selective binding activity against eosinophils recovered in bronchoalveolar lavage fluid of ovalbumin-immunized and -challenged mice. Administration of this antibody in vivo induced a significant reduction of eosinophils recruited in the allergic lungs. Anti-Mfsd10 antibody is useful for investigating the pathophysiological roles of eosinophils with its selective binding and neutralizing activity for mouse eosinophils.

Low level of c-kit expression marks deeply quiescent murine hematopoietic stem cells

Although c-kit is expressed highly on murine hematopoietic stem cells (HSCs) and essential for bone marrow (BM) hematopoiesis, the significance of the high level of expression of c-kit on HSCs was not well determined. We show here that CD150(+) CD48(-) Lineage(-) Sca-1(+) c-kit(+) HSCs in adult BM are distributed within the range of roughly a 20-fold difference in the expression level of c-kit, and that c-kit density correlates with the cycling status of the HSC population. This predisposition is more evident in the BM of mice older than 30 weeks. The HSCs in G(0) phase express a lower level of c-kit both on the cell surface and inside the cells, which cannot be explained by ligand receptor binding and internalization. It is more likely that the low level of c-kit expression is a unique property of HSCs in G(0). Despite functional differences in the c-kit gradient, the HSCs are uniformly hypoxic and accessible to blood perfusion. Therefore, our data indicate the possibility that the hypoxic state of the HSCs is actively regulated, rather than them being passively hypoxic through a simple anatomical isolation from the circulation.

Ex vivo expanded hematopoietic stem cells overcome the MHC barrier in allogeneic transplantation

The lack of understanding of the interplay between hematopoietic stem cells (HSCs) and the immune system has severely hampered the stem cell research and practice of transplantation. Major problems for allogeneic transplantation include low levels of donor engraftment and high risks of graft-versus-host disease (GVHD). Transplantation of purified allogeneic HSCs diminishes the risk of GVHD but results in decreased engraftment. Here we show that ex vivo expanded mouse HSCs efficiently overcame the major histocompatibility complex barrier and repopulated allogeneic-recipient mice. An 8-day expansion culture led to a 40-fold increase of the allograft ability of HSCs. Both increased numbers of HSCs and culture-induced elevation of expression of the immune inhibitor CD274 (B7-H1 or PD-L1) on the surface of HSCs contributed to the enhancement. Our study indicates the great potential of utilizing ex vivo expanded HSCs for allogeneic transplantation and suggests that the immune privilege of HSCs can be modulated.

Osteohematopoietic stem cell niches in bone marrow

In adult mammals, maturation of blood and bone cells from their respective progenitors occurs in the bone marrow. The marrow region contains many progenitor and stem cell types that are confined by their biochemical and cellular microenvironments, referred to as stem cell niches. The unique properties of each niche assist the survival, proliferation, migration, and differentiation of that particular stem or progenitor cell type. Among the different niches of the bone marrow, our understanding of the osteohematopoietic niche is the most complete. Its properties, described in this chapter, are a model for studying adult stem cell differentiation, but a lot remains unknown. Our improved understanding of hematopoietic stem cell biology and its relationship with the properties of these niches are critical in the effective and safe use of these cells in regenerative medicine. Here, we review the current knowledge on the properties of these niches and suggest how the potential of hematopoietic progenitors can be utilized in regenerative medicine.

Integrin-αvβ3 regulates thrombopoietin-mediated maintenance of hematopoietic stem cells

Throughout life, one's blood supply depends on sustained division of hematopoietic stem cells (HSCs) for self-renewal and differentiation. Within the bone marrow microenvironment, an adhesion-dependent or -independent niche system regulates HSC function. Here we show that a novel adhesion-dependent mechanism via integrin-β3 signaling contributes to HSC maintenance. Specific ligation of β3-integrin on HSCs using an antibody or extracellular matrix protein prevented loss of long-term repopulating (LTR) activity during ex vivo culture. The actions required activation of αvβ3-integrin "inside-out" signaling, which is dependent on thrombopoietin (TPO), an essential cytokine for activation of dormant HSCs. Subsequent "outside-in" signaling via phosphorylation of Tyr747 in the β3-subunit cytoplasmic domain was indispensable for TPO-dependent, but not stem cell factor-dependent, LTR activity in HSCs in vivo. This was accompanied with enhanced expression of Vps72, Mll1, and Runx1, 3 factors known to be critical for maintaining HSC activity. Thus, our findings demonstrate a mechanistic link between β3-integrin and TPO in HSCs, which may contribute to maintenance of LTR activity in vivo as well as during ex vivo culture.

Forced expression of the histone demethylase Fbxl10 maintains self-renewing hematopoietic stem cells

OBJECTIVE

The methylation status of histones changes dramatically depending on cellular context and defines cell type-specific gene expression profiles. Histone demethylases have recently been implicated in this process. However, it is unknown how histone demethylases function in the maintenance of self-renewing hematopoietic stem cells (HSCs).

MATERIALS AND METHODS

We profiled the expression of histone demethylase genes in mouse hematopoietic cells and listed genes preferentially expressed in HSCs. We analyzed the impact of a selected gene by transducing CD34(-)c-Kit(+)Sca-1(+)lineage marker(-) (CD34(-)KSL) HSCs using retroviral system followed by in vitro methylcellulose colony assays and in vivo competitive repopulation assays.

RESULTS

We found that F-box and leucine-rich repeat protein 10 (Fbxl10, also known as Jhdm1b or Kdm2b), is highly expressed in CD34(-)KSL HSCs. Fbxl10 encodes a demethylase specific to the histone H3 mono/di-methylated at lysine 36 (H3K36me1/me2) and forms complexes with polycomb-group proteins, essential regulators of HSCs. Forced expression of Fbxl10 in HSCs expanded numbers of colony-forming cells with multilineage differentiation potential in culture and prevented exhaustion of the long-term repopulating potential of HSCs following serial transplantation. Fbxl10 tightly repressed the expression of cyclin-dependent kinase inhibitor genes, including Ink4a, Ink4b, and Ink4c, through direct binding to their promoters and gene bodies and demethylation at H3K36. Increased levels of mono-ubiquitylation of H2A at target loci also suggested the collaboration of Fbxl10 with polycomb-group proteins.

CONCLUSIONS

Our findings implicate Fbxl10 in the maintenance of self-renewal capacity of HSCs, thus highlight a role of histone demethylation for the first time in the epigenetic regulation of HSCs.

Polyclonal fluctuation of lentiviral vector–transduced and expanded murine hematopoietic stem cells

Gene therapy has proven its potential to cure diseases of the hematopoietic system. However, severe adverse events observed in clinical trials have demanded improved gene-transfer conditions. Whereas progress has been made to reduce the genotoxicity of integrating gene vectors, the role of pretransplantation cultivation is less well investigated. We observed that the STIF (stem cell factor [SCF], thrombopoietin [TPO], insulin-like growth factor-2 [IGF-2], and fibroblast growth factor-1 [FGF-1]) cytokine cocktail developed to effectively expand murine hematopoietic stem cells (HSCs) also supports the expansion of leukemia-initiating insertional mutants caused by gammaretroviral gene transfer. We compared 4 protocols to examine the impact of prestimulation and posttransduction culture in STIF in the context of lentiviral gene transfer. Observing 56 transplanted mice for up to 9.5 months, we found consistent engraftment and gene-marking rates after prolonged ex vivo expansion. Although a lentiviral vector with a validated insertional-mutagenic potential was used, longitudinal analysis identifying > 7000 integration sites revealed polyclonal fluctuations, especially in “expanded” groups, with de novo detection of clones even at late time points. Posttransduction expansion in STIF did not enrich clones with insertions in proto-oncogenes but rather increased clonal diversity. Our data indicate that lentiviral transduction in optimized media mediates intact polyclonal hematopoiesis without selection for growth-promoting hits by posttransduction expansion.

Histone deacetylase controls adult stem cell aging by balancing the expression of polycomb genes and jumonji domain containing 3

Aging is linked to loss of the self-renewal capacity of adult stem cells. Here, we observed that human multipotent stem cells (MSCs) underwent cellular senescence in vitro. Decreased expression of histone deacetylases (HDACs), followed by downregulation of polycomb group genes (PcGs), such as BMI1, EZH2 and SUZ12, and by upregulation of jumonji domain containing 3 (JMJD3), was observed in senescent MSCs. Similarly, HDAC inhibitors induced cellular senescence through downregulation of PcGs and upregulation of JMJD3. Regulation of PcGs was associated with HDAC inhibitor-induced hypophosphorylation of RB, which causes RB to bind to and decrease the transcriptional activity of E2F. JMJD3 expression regulation was dependant on histone acetylation status at its promoter regions. A histone acetyltransferase (HAT) inhibitor prevented replicative senescence of MSCs. These results suggest that HDAC activity might be important for MSC self-renewal by balancing PcGs and JMJD3 expression, which govern cellular senescence by p16^INK4A regulation.

The aryl hydrocarbon receptor has an important role in the regulation of hematopoiesis: implications for benzene-induced hematopoietic toxicity

The aryl hydrocarbon receptor (AhR) belongs to the basic helix-loop-helix (bHLH) Per-Arnt-Sim (PAS) family of transcription factors. Many of these proteins are involved in regulating responses to signals in the tissue environment such as hypoxia, oxidation-reduction status, and circadian rhythms. Although the AhR is well studied as a mediator of the toxicity of certain xenobiotics, the normal physiological function remains unknown. However, accumulating data support a hypothesis that the AhR has an important function in the regulation of hematopoietic stem cells (HSCs). Persistent AhR activation by dioxin, a potent xenobiotic AhR agonist, results in altered numbers and function of HSCs in mouse bone marrow. Analysis of HSCs from AhR null-allele mice also indicates that lack of AhR expression results in altered characteristics and function of these cells. HSCs from these animals are hyperproliferative and have altered cell cycle. In addition, aging AhR-KO mice show characteristics consistent with premature bone marrow senescence and are prone to hematopoietic disease. Finally, some data suggest that the expression of the Ahr gene is regulated under conditions that control HSC proliferation. The presence of a normal functioning AhR may provide an important advantage to organisms by regulating the balance between quiescence and proliferation and preventing the premature exhaustion of HSCs and sensitivity to genetic alterations. This function assists in the preservation of HSC function and long-term multi-lineage generation over the lifespan of the organism. This also implicates a role for the AhR in the aging process. Furthermore, these functions may affect the sensitivity of HSCs to certain xenobiotics, including benzene. Defining the molecular mechanisms by which these events occur may lead to the identification of previously undefined roles of this transcription factor in human diseases, particularly those caused or affected by xenobiotics.

Surface antigen phenotypes of hematopoietic stem cells from embryos and murine embryonic stem cells

Surface antigens on hematopoietic stem cells (HSCs) enable prospective isolation and characterization. Here, we compare the cell-surface phenotype of hematopoietic repopulating cells from murine yolk sac, aorta-gonad-mesonephros, placenta, fetal liver, and bone marrow with that of HSCs derived from the in vitro differentiation of murine embryonic stem cells (ESC-HSCs). Whereas c-Kit marks all HSC populations, CD41, CD45, CD34, and CD150 were developmentally regulated: the earliest embryonic HSCs express CD41 and CD34 and lack CD45 and CD150, whereas more mature HSCs lack CD41 and CD34 and express CD45 and CD150. ESC-HSCs express CD41 and CD150, lack CD34, and are heterogeneous for CD45. Finally, although CD48 was absent from all in vivo HSCs examined, ESC-HSCs were heterogeneous for the expression of this molecule. This unique phenotype signifies a developmentally immature population of cells with features of both primitive and mature HSC. The prospective fractionation of ESC-HSCs will facilitate studies of HSC maturation essential for normal functional engraftment in irradiated adults.

The aryl hydrocarbon receptor has a normal function in the regulation of hematopoietic and other stem/progenitor cell populations

The aryl hydrocarbon receptor (AhR) is known mainly as the mediator for the toxicity of certain xenobiotics. However, there is also much information to indicate that this transcription factor has important biological functions. Here we review the evidence that the AhR has a significant role in the regulation of hematopoietic stem cells (HSCs). Data to support this come from studies with xenobiotic AhR ligands, phenotypic analyses of mice lacking AhR, examining the presence and regulation of the AhR within HSCs, knowledge of genes and signaling pathways regulated by the AhR, and investigations of hematopoietic disorders. Based on this information, we hypothesize that AhR expression is necessary for the proper maintenance of quiescence in HSCs, and that AhR down-regulation is essential for "escape" from quiescence and subsequent proliferation of these cells. This implicates the AhR as a negative regulator of hematopoiesis with a function of curbing excessive or unnecessary proliferation. This provides an important advantage by preventing the premature exhaustion of HSCs and sensitivity to genetic alterations, thus preserving HSC function and long-term multi-lineage generation over the lifespan of the organism. This also implicates a role of the AhR in aging processes. AhR dysregulation may result in the altered ability of HSCs to sense appropriate signals in the bone marrow microenvironment leading to hematopoietic disease. It is also reasonable to hypothesize that this protein has an important function in the regulation of other tissue stem cell populations. Suggestive evidence is consistent with a role in skin and neural stem cells.