Cytokine signaling, lipid raft clustering, and HSC hibernation

Rapid activation of hematopoietic stem cells

Adult hematopoietic stem cells (HSCs) in the bone marrow (BM) are quiescent. Following perturbations, such as blood loss or infection, HSCs may undergo activation. Surprisingly, little is known about the earliest stages of HSCs activation. We utilize surface markers of HSCs activation, CD69 and CD317, revealing a response as early as 2 h after stimulation. The dynamic expression of HSCs activation markers varies between viral-like (poly-Inosinic-poly-Cytidylic) or bacterial-like (Lipopolysaccharide) immune stimuli. We further quantify dose response, revealing a low threshold, and similar sensitivity of HSCs and progenitors in the BM. Finally, we find a positive correlation between the expression of surface activation markers and early exit from quiescence. Our data show that the response of adult stem cells to immune stimulation is rapid and sensitive, rapidly leading HSCs out of quiescence.

The lipid rafts in cancer stem cell: a target to eradicate cancer

Cancer stem cells (CSCs) are a subpopulation of cancer cells with stem cell properties that sustain cancers, which may be responsible for cancer metastasis or recurrence. Lipid rafts are cholesterol- and sphingolipid-enriched microdomains in the plasma membrane that mediate various intracellular signaling. The occurrence and progression of cancer are closely related to lipid rafts. Emerging evidence indicates that lipid raft levels are significantly enriched in CSCs compared to cancer cells and that most CSC markers such as CD24, CD44, and CD133 are located in lipid rafts. Furthermore, lipid rafts play an essential role in CSCs, specifically in CSC self-renewal, epithelial-mesenchymal transition, drug resistance, and CSC niche. Therefore, lipid rafts are critical regulatory platforms for CSCs and promising therapeutic targets for cancer therapy.

Hypersensitivity response has negligible impact on Hematopoietic Stem Cells

Immune cells are generated from hematopoietic stem cells (HSCs) in the bone marrow (BM). Immune stimulation can rapidly activate HSCs out of their quiescent state to accelerate the generation of immune cells. HSCs' activation follows various viral or bacterial stimuli, and we sought to investigate the hypersensitivity immune response. Surprisingly, the Ova-induced hypersensitivity peritonitis model finds no significant changes in BM HSCs. HSC markers cKIT, SCA1, CD48, CD150, and the Fgd5-mCherry reporter showed no significant difference from control. Functionally, hypersensitivity did not alter HSCs' potency, as assayed by transplantation. We further characterized the possible impact of hypersensitivity using RNA-sequencing of HSCs, finding minor changes at the transcriptome level. Moreover, hypersensitivity induced no significant change in the proliferative state of HSCs. Therefore, this study suggests that, in contrast to other immune stimuli, hypersensitivity has no impact on HSCs.

Dynamic transcriptome profiling in DNA damage-induced cellular senescence and transient cell-cycle arrest

Cellular senescence is an irreversible cell cycle arrest process associated with aging and senescence-related diseases. DNA damage is an extensive feature of cellular senescence and aging. Different levels of DNA damage could lead to cellular senescence or transient cell-cycle arrest, but the genetic regulatory mechanisms determining cell fate are still not clear. In this work, high-resolution time course analysis of gene expression in DNA damage-induced cellular senescence and transient cell-cycle arrest was used to explore the transcriptomic differences between different cell fates after DNA damage response and to investigate the key regulatory factors affecting senescent cell fates. Pathways such as the cell cycle, DNA repair and cholesterol metabolism showed characteristic differential response. A number of key transcription factors were predicted to regulating cell cycle and DNA repair. Our study provides genome-wide insights into the molecular-level mechanisms of senescent cell fate decisions after DNA damage response.

Role of Lipid Rafts in Hematopoietic Stem Cells Homing, Mobilization, Hibernation, and Differentiation

Hematopoietic stem cells (HSCs) are multipotent, self-renewing cells that can differentiate into myeloid or lymphoid cells. The mobilization and differentiation processes are affected by the external environment, such as extracellular matrix and soluble molecules in the niche, where the lipid rafts (LRs) of the HSCs act as the receptors and control platforms for these effectors. LRs are membrane microdomains that are enriched in cholesterol, sphingolipid, and proteins. They are involved in diverse cellular processes including morphogenesis, cytokinesis, signaling, endocytic events, and response to the environment. They are also involved in different types of diseases, such as cancer, Alzheimer's, and prion disease. LR clustering and disruption contribute directly to the differentiation, homing, hibernation, or mobilization of HSCs. Thus, characterization of LR integrity may provide a promising approach to controlling the fate of stem cells for clinical applications. In this review, we show the critical role of LR modification (clustering, disruption, protein incorporation, and signal responding) in deciding the fate of HSCs, under the effect of soluble cytokines such as stem cell factor (SCF), transforming growth factor- β (TGF-β), hematopoietic-specific phospholipase Cβ2 (PLC-β2), and granulocyte colony-stimulating factor (G-CSF).

The Role of Glycosphingolipids in Immune Cell Functions

Glycosphingolipids (GSLs) exhibit a variety of functions in cellular differentiation and interaction. Also, they are known to play a role as receptors in pathogen invasion. A less well-explored feature is the role of GSLs in immune cell function which is the subject of this review article. Here we summarize knowledge on GSL expression patterns in different immune cells. We review the changes in GSL expression during immune cell development and differentiation, maturation, and activation. Furthermore, we review how immune cell GSLs impact membrane organization, molecular signaling, and trans-interactions in cellular cross-talk. Another aspect covered is the role of GSLs as targets of antibody-based immunity in cancer. We expect that recent advances in analytical and genome editing technologies will help in the coming years to further our knowledge on the role of GSLs as modulators of immune cell function.

TGF-β1 Negatively Regulates the Number and Function of Hematopoietic Stem Cells

Transforming growth factor β1 (TGF-β1) plays a role in the maintenance of quiescent hematopoietic stem cells (HSCs) in vivo. We asked whether TGF-β1 controls the cell cycle status of HSCs in vitro to enhance the reconstitution activity. To examine the effect of TGF-β1 on the HSC function, we used an in vitro culture system in which single HSCs divide with the retention of their short- and long-term reconstitution ability. Extensive single-cell analyses showed that, regardless of its concentration, TGF-β1 slowed down the cell cycle progression of HSCs but consequently suppressed their self-renewal potential. Cycling HSCs were not able to go back to quiescence with TGF-β1. This study revealed a negative role of TGF-β1 in the regulation of the HSC number and reconstitution activity.

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TGF-β1 slows down the cell cycle progression of HSCs

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Cycling HSCs were unable to go back to the G0 state with TGF-β1

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TGF-β1 suppresses the self-renewal potential in HSCs

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The reduced division rate with TGF-β1 is reversible

Reactive oxygen species mediates 50-Hz magnetic field-induced EGF receptor clustering via acid sphingomyelinase activation

PURPOSE

Exposure to extremely low frequency electromagnetic fields (ELF-EMFs) could elicit biological effects including carcinogenesis. However, the detailed mechanisms by which these ELF-EMFs interact with biological system are currently unclear. Previously, we found that a 50-Hz magnetic field (MF) exposure could induce epidermal growth factor receptor (EGFR) clustering and phosphorylation on cell membranes. In the present experiment, the possible roles of reactive oxygen species (ROS) in MF-induced EGFR clustering were investigated.

MATERIALS AND METHODS

Human amnion epithelial (FL) cells were exposed to a 50-Hz MF with or without N-acetyl-l-cysteine (NAC) or pyrrolidine dithiocarbamate (PDTC). EGFR clustering on cellular membrane surface was analyzed using confocal microscopy after indirect immunofluorescence staining. The intracellular ROS level and acid sphingomyelinase (ASMase) activity were detected using an ROS assay kit and an Amplex^® Red Sphingomyelinase Assay Kit, respectively.

RESULTS

Results showed that exposure of FL cells to a 50-Hz MF at 0.4 mT for 15 min significantly enhanced the ROS level, induced EGFR clustering and increased ASMase activity. However, pretreatment with NAC or PDTC, the scavenger of ROS, not only counteracted the effects of a 50-Hz MF on ROS level and AMS activity, but also inhibited the EGFR clustering induced by MF exposure.

CONCLUSIONS

The present and previous data suggest that ROS mediates the MF-induced EGFR clustering via ASMase activation.

Lovastatin enhances adenovirus-mediated TRAIL induced apoptosis by depleting cholesterol of lipid rafts and affecting CAR and death receptor expression of prostate cancer cells

Oncolytic adenovirus and apoptosis inducer TRAIL are promising cancer therapies. Their antitumor efficacy, when used as single agents, is limited. Oncolytic adenoviruses have low infection activity, and cancer cells develop resistance to TRAIL-induced apoptosis. Here, we explored combining prostate-restricted replication competent adenovirus-mediated TRAIL (PRRA-TRAIL) with lovastatin, a commonly used cholesterol-lowering drug, as a potential therapy for advanced prostate cancer (PCa). Lovastatin significantly enhanced the efficacy of PRRA-TRAIL by promoting the in vivo tumor suppression, and the in vitro cell killing and apoptosis induction, via integration of multiple molecular mechanisms. Lovastatin enhanced PRRA replication and virus-delivered transgene expression by increasing the expression levels of CAR and integrins, which are critical for adenovirus 5 binding and internalization. Lovastatin enhanced TRAIL-induced apoptosis by increasing death receptor DR4 expression. These multiple effects of lovastatin on CAR, integrins and DR4 expression were closely associated with cholesterol-depletion in lipid rafts. These studies, for the first time, show correlations between cholesterol/lipid rafts, oncolytic adenovirus infection efficiency and the antitumor efficacy of TRAIL at the cellular level. This work enhances our understanding of the molecular mechanisms that support use of lovastatin, in combination with PRRA-TRAIL, as a candidate strategy to treat human refractory prostate cancer in the future.

Liver maturation deficiency in p57(Kip2)-/- mice occurs in a hepatocytic p57(Kip2) expression-independent manner

Fetal hepatic stem/progenitor cells, hepatoblasts, are highly proliferative cells and the source of both hepatocytes and cholangiocytes. In contrast, mature hepatocytes have a low proliferative potency and high metabolic functions. Cell proliferation is regulated by cell cycle-related molecules. However, the correlation between cell cycle regulation and hepatic maturation are still unknown. To address this issue, we revealed that the cell cycle inhibitor p57(Kip2) was expressed in the hepatoblasts and mesenchymal cells of fetal liver in a spatiotemporal manner. In addition, we found that hepatoblasts in p57(Kip2)-/- mice were highly proliferative and had deficient maturation compared with those in wild-type (WT) mice. However, there were no remarkable differences in the expression levels of cell cycle- and bipotency-related genes except for Ccnd2. Furthermore, p57(Kip2)-/- hepatoblasts could differentiate into mature hepatocytes in p57(Kip2)-/- and WT chimeric mice, suggesting that the intrinsic activity of p57(Kip2) does not simply regulate hepatoblast maturation.

Role of p75 neurotrophin receptor in stem cell biology: more than just a marker

p75(NTR), the common receptor for both neurotrophins and proneurotrophins, has been widely studied because of its role in many tissues, including the nervous system. More recently, a close relationship between p75(NTR) expression and pluripotency has been described. p75(NTR) was shown to be expressed in various types of stem cells and has been used to prospectively isolate stem cells with different degrees of potency. Here, we give an overview of the current knowledge on p75(NTR) in stem cells, ranging from embryonic to adult stem cells, and cancer stem cells. In an attempt to address its potential role in the control of stem cell biology, the molecular mechanisms underlying p75(NTR) signaling in different models are also highlighted. p75(NTR)-mediated functions include survival, apoptosis, migration, and differentiation, and depend on cell type, (pro)neurotrophin binding, interacting transmembrane co-receptors expression, intracellular adaptor molecule availability, and post-translational modifications, such as regulated proteolytic processing. It is therefore conceivable that p75(NTR) can modulate cell-fate decisions through its highly ramified signaling pathways. Thus, elucidating the potential implications of p75(NTR) activity as well as the underlying molecular mechanisms of p75(NTR) will shed new light on the biology of both normal and cancer stem cells.

Humanized NOD-SCID IL2rg–/– mice as a preclinical model for cancer research and its potential use for individualized cancer therapies

Humanized mouse models have been developed and utilized in cancer research for decades. Newly developed combined immunodeficient NOD-SCID-IL2rg–/– mice are more permissive for human cells and tissue engraftment. In this review, we discuss the use of NOD-SCID-IL2rg(–/–) mice as a preclinical tool in cancer research and its potential use for individualized cancer therapies.

Concurrent measures of fusion and transduction efficiency of primary CD34+ cells with human immunodeficiency virus 1-based lentiviral vectors reveal different effects of transduction enhancers

Lentiviral vectors (LVs) are used for various gene transfer applications, notably for hematopoietic gene therapy, but methods are lacking for precisely evaluating parameters that control the efficiency of transduction in relation to the entry of vectors into target cells. We adapted a fluorescence resonance energy transfer-based human immunodeficiency virus-1 fusion assay to measure the entry of nonreplicative recombinant LVs in various cell types, including primary human hematopoietic stem progenitor cells (HSPCs), and to quantify the level of transduction of the same initially infected cells. The assay utilizes recombinant LVs containing β-lactamase (BLAM)-Vpr chimeric proteins (BLAM-LVs) and encoding a truncated form of the low-affinity nerve growth factor receptor (ΔNGFR). After infection of target cells with BLAM-LVs, the vector entry rapidly leads to BLAM-Vpr release into the cytoplasm, which is measured by cleavage of a fluorescent substrate using flow cytometry. Parallel cultures of the same infected cells show transduction efficiency resulting from ΔNGFR expression. This LV-based fusion/transduction assay is a dynamic and versatile tool, revealing, for instance, the postentry restrictions of LVs known to occur in cells of hematopoietic origin, especially human HSPCs. Furthermore, this BLAM-LV assay allowed us to evaluate the effect of cytokine prestimulation of HSPCs on the entry step of LVs. The assay also shows that transduction enhancers such as Vectofusin-1 or Retronectin can partially relieve the postentry block, but their effects differ in how they promote LV entry. In conclusion, one such assay should be useful to study hematopoietic postentry restrictions directed against LVs and therefore should allow improvements in various LV-based gene therapy protocols.

DNMT1-mediated PTEN hypermethylation confers hepatic stellate cell activation and liver fibrogenesis in rats

Hepatic stellate cell (HSC) activation is an essential event during liver fibrogenesis. Phosphatase and tension homolog deleted on chromosome 10 (PTEN), a tumor suppressor, is a negative regulator of this process. PTEN promoter hypermethylation is a major epigenetic silencing mechanism in tumors. The present study aimed to investigate whether PTEN promoter methylation was involved in HSC activation and liver fibrosis. Treatment of activated HSCs with the DNA methylation inhibitor 5-aza-2'-deoxycytidine (5-azadC) decreased aberrant hypermethylation of the PTEN gene promoter and prevented the loss of PTEN expression that occurred during HSC activation. Silencing DNA methyltransferase 1 (DNMT1) gene also decreased the PTEN gene promoter methylation and upregulated the PTEN gene expression in activated HSC-T6 cells. In addition, knockdown of DNMT1 inhibited the activation of both ERK and AKT pathways in HSC-T6 cells. These results suggest that DNMT1-mediated PTEN hypermethylation caused the loss of PTEN expression, followed by the activation of the PI3K/AKT and ERK pathways, resulting in HSC activation.

Identification of in vitro HSC fate regulators by differential lipid raft clustering

Most hematopoietic stem cells (HSC) in the bone marrow reside in a quiescent state and occasionally enter the cell cycle upon cytokine-induced activation. Although the mechanisms regulating HSC quiescence and activation remain poorly defined, recent studies have revealed a role of lipid raft clustering (LRC) in HSC activation. Here, we tested the hypothesis that changes in lipid raft distribution could serve as an indicator of the quiescent and activated state of HSCs in response to putative niche signals. A semi-automated image analysis tool was developed to map the presence or absence of lipid raft clusters in live HSCs cultured for just one hour in serum-free medium supplemented with stem cell factor (SCF). By screening the ability of 19 protein candidates to alter lipid raft dynamics, we identified six factors that induced either a marked decrease (Wnt5a, Wnt3a and Osteopontin) or increase (IL3, IL6 and VEGF) in LRC. Cell cycle kinetics of single HSCs exposed to these factors revealed a correlation of LRC dynamics and proliferation kinetics: factors that decreased LRC slowed down cell cycle kinetics, while factors that increased LRC led to faster and more synchronous cycling. The possibility of identifying, by LRC analysis at very early time points, whether a stem cell is activated and possibly committed upon exposure to a signaling cue of interest could open up new avenues for large-scale screening efforts.

Requirement of translocated lysosomal V1 H(+)-ATPase for activation of membrane acid sphingomyelinase and raft clustering in coronary endothelial cells

The activation of translocated lysosomal H⁺-ATPase is attributed to FasL-induced formation and maintenance of an acid microenvironment around the endothelial cell membrane, which facilitates the activation of ASM and production of ceramide, thereby leading to MR clustering and redox signaling platform formation.

Acid sphingomyelinase (ASM) mediates the formation of membrane raft (MR) redox signalosomes in a process that depends on a local acid microenvironment in coronary arterial endothelial cells (CAECs). However, it is not known how this local acid microenvironment is formed and maintained. The present study hypothesized that lysosomal V1 H⁺-ATPase provides a hospitable acid microenvironment for activation of ASM when lysosomes traffic and fuse into the cell membrane. Confocal microscopy showed that local pH change significantly affected MRs, with more fluorescent patches under low pH. Correspondingly, the ASM product, ceramide, increased locally in the cell membrane. Electron spin resonance assay showed that local pH increase significantly inhibited NADPH oxidase–mediated production of O₂^−. in CAECs. Direct confocal microscopy demonstrated that Fas ligand resulted in localized areas of decreased pH around CAEC membranes. The inhibitors of both lysosomal fusion and H⁺-ATPase apparently attenuated FasL-caused pH decrease. V1 H⁺-ATPase accumulation and activity on cell membranes were substantially suppressed by the inhibitors of lysosomal fusion or H⁺-ATPase. These results provide the first direct evidence that translocated lysosomal V1 H⁺-ATPase critically contributes to the formation of local acid microenvironment to facilitate activation of ASM and consequent MR aggregation, forming MR redox signalosomes and mediating redox signaling in CAECs.

Cell cycle regulation in hematopoietic stem cells

Hematopoietic stem cells (HSCs) give rise to all lineages of blood cells. Because HSCs must persist for a lifetime, the balance between their proliferation and quiescence is carefully regulated to ensure blood homeostasis while limiting cellular damage. Cell cycle regulation therefore plays a critical role in controlling HSC function during both fetal life and in the adult. The cell cycle activity of HSCs is carefully modulated by a complex interplay between cell-intrinsic mechanisms and cell-extrinsic factors produced by the microenvironment. This fine-tuned regulatory network may become altered with age, leading to aberrant HSC cell cycle regulation, degraded HSC function, and hematological malignancy.

Caveolin-1 and integrin β1 regulate embryonic stem cell proliferation via p38 MAPK and FAK in high glucose

The involvement of caveolin-1 (Cav-1) and integrin β1 (IN β1) in regulation of embryonic stem (ES) cell growth by high glucose is by no means clear cut. Therefore, the aim of this study was to examine the influence of high glucose on Cav-1 and IN β1 expression in mouse ES cells and their signaling pathways to modulate proliferation. High glucose significantly increased Cav-1 and IN β1 expression. In addition, increased IN β1 expression was inhibited by Cav-1 small interfering RNA (siRNA). High glucose caused reactive oxygen species generation and p38 mitogen-activated protein kinase (MAPK) phosphorylation. Inhibition of p38 MAPK blocked high glucose-induced Cav-1 and fibronectin (FN) expression. Moreover, phosphorylation of both Src and focal adhesion kinase (FAK) were increased by high glucose, which were inhibited by IN β1 antibody. In addition, high glucose increased the expression levels of PINCH1/2, integrin-linked kinase (ILK), and α-parvin [PIP] complex proteins, which were all inhibited by the FAK siRNA and Src specific inhibitor (PP2, 10(-7)  M). High glucose also increased F-actin expression, which was inhibited by ILK, PINCH1/2, and α-parvin siRNAs. Finally, high glucose-induced increase of ES cell proliferation was inhibited by TRIO and F-actin binding protein (TRIOBP) siRNA. The results demonstrate that high glucose-induced Cav-1 and IN β1 activation can stimulate ES cell proliferation through the modification of focal adhesion signaling pathways.

TIMP-1 deficiency subverts cell-cycle dynamics in murine long-term HSCs

In addition to the well-recognized role in extracellular matrix remodeling, the tissue inhibitor of metalloproteinases-1 (TIMP-1) has been suggested to be involved in the regulation of numerous biologic functions, including cell proliferation and survival. We therefore hypothesized that TIMP-1 might be involved in the homeostatic regulation of HSCs, whose biologic behavior is the synthesis of both microenvironmental and intrinsic cues. We found that TIMP-1(-/-) mice have decreased BM cellularity and, consistent with this finding, TIMP-1(-/-) HSCs display reduced capability of long-term repopulation. Interestingly, the cell cycle distribution of TIMP-1(-/-) stem cells appears distorted, with a dysregulation at the level of the G(1) phase. TIMP-1(-/-) HSCs also display increased levels of p57, p21, and p53, suggesting that TIMP-1 could be intrinsically involved in the regulation of HSC cycling dynamics. Of note, TIMP-1(-/-) HSCs present decreased levels of CD44 glycoprotein, whose expression has been proven to be controlled by p53, the master regulator of the G(1)/S transition. Our findings establish a role for TIMP-1 in regulating HSC function, suggesting a novel mechanism presiding over stem cell quiescence in the framework of the BM milieu.

Key roles for transforming growth factor beta in melanocyte stem cell maintenance

Melanocyte stem cells in the bulge area of hair follicles are responsible for hair pigmentation, and defects in them cause hair graying. Here we describe the process of melanocyte stem cell entry into the quiescent state and show that niche-derived transforming growth factor beta (TGF-beta) signaling plays important roles in this process. In vitro, TGF-beta not only induces reversible cell cycle arrest, but also promotes melanocyte immaturity by downregulating MITF, the master transcriptional regulator of melanocyte differentiation, and its downstream melanogenic genes. In vivo, TGF-beta signaling is activated in melanocyte stem cells when they reenter the quiescent noncycling state during the hair cycle and this process requires Bcl2 for cell survival. Furthermore, targeted TGF-beta type II receptor (TGFbRII) deficiency in the melanocyte lineage causes incomplete maintenance of melanocyte stem cell immaturity and results in mild hair graying. These data demonstrate that the TGF-beta signaling pathway is one of the key niche factors that regulate melanocyte stem cell immaturity and quiescence.

Lipid rafts play an important role for maintenance of embryonic stem cell self-renewal

Lipid rafts are cholesterol-rich microdomains of cell membranes that have a variety of roles in cellular processes including receptor-mediated signal transduction. Lipid rafts also occur in embryonic stem (ES) cells, but their role in ES cells is largely unknown. Therefore, we investigated the role of lipid rafts in the maintenance of ES cell self-renewal. In the present study, we observed that the presence of lipid rafts/caveolae. The results from sucrose gradient fractionation showed that the expression of glycoprotein 130 (gp130) and leukemia inhibitory factor receptor beta (LIFRbeta) was decreased by treatment with methyl-beta-cyclodextrin (Mbeta-CD) but, interestingly, was not affected by caveolin-1 small interfering RNA (siRNA). In addition, LIF increased phosphorylation of signal transducer and activator of transcription 3 (STAT3) and Akt, and the expression level of c-Myc, which were attenuated by the pretreatment with Mbeta-CD. However, caveolin-1 siRNA did not influence LIF-induced phosphorylation of STAT3 and Akt, and expression of c-Myc. Treatment with Mbeta-CD and caveolin-1 siRNA decreased expression levels of Oct4 protein and Oct4, Sox2, FoxD3, and Rex1 mRNAs in normal culture conditions. Additionally, Mbeta-CD and caveolin-1 siRNA decreased the expression levels of cyclin D1 and cyclin E, and the proliferation index [(S + G2/M)/(G0/G1 + S + G2/M)] of ES cells. We conclude that lipid raft/caveolae structures play important roles in the self-renewal of ES cells.

Insights into signaling and function of hematopoietic stem cells at the single-cell level

PURPOSE OF REVIEW

Development of a technique prospectively to isolate hematopoietic stem cells (HSCs) to near homogeneity has enabled clonal analysis and thus converted our understanding of HSCs from conceptual and qualitative to realistic and quantitative. Recent studies have revealed that despite their high proliferation potential, most HSCs are in G0 and enter cell cycle only after a long interval. This dormancy of HSCs, which seems to be important for long-term maintenance of 'stemness', appears to be regulated by the exchange of signals between HSCs and the bone marrow niche. Analysis of intersignaling and intrasignaling events in HSCs in and out of the bone marrow niche has begun.

RECENT FINDINGS

With the help of advances in confocal microscopy, laser scanning microscopy, and personal computer computational power over the last decade, it has become evident that thrombopoietin/c-Mpl signaling plays a role in HSC self-renewal and AKT-forkhead box O signaling in HSC dormancy. Furthermore, transforming growth factor-beta has been indicated as a candidate niche signal to induce hibernation in HSCs.

SUMMARY

Understanding of the signaling events between HSCs and niche is critical not only for stem cell biology in general and for transplantation medicine but also for the development of novel cancer therapy.

Membrane glycolipids in stem cells

Stem cells, such as embryonic stem cells, hematopoietic stem cells, neural stem cells, mesenchymal stem cells, and very small embryonic-like stem cells, are undifferentiated cells that are endowed with a high potential for proliferation and the capacity for self-renewal with retention of pluri/multipotency to differentiate into their progenies. Recently, studies regarding the biological functions of glycolipids and cell surface microdomains (caveolae, lipid rafts, or glycolipid-enriched microdomains) in stem cells are emerging. In this review, we introduce the expression patterns of glycolipids and the functional roles of cell surface microdomains in stem cells.

Beta-glycoglycosphingolipid-induced alterations of the STAT signaling pathways are dependent on CD1d and the lipid raft protein flotillin-2

Beta-glucosylceramide has been shown to affect natural killer T cell function in models of inflammation. We, therefore, investigated the effects of different beta-glycosphingolipids, including beta-glucosylceramide, on STAT (signal transducers and activators of transcription) signaling pathways and determined whether these effects were mediated by lipid raft microdomains and/or CD1d molecules. The effects of alpha- and beta-structured ligands on the lipid raft protein flotillin-2 were studied in both natural killer T hybridoma cells and leptin-deficient mice. To determine whether CD1d was involved in the effects of the beta-glycosphingolipids, an anti-CD1d blocking antibody was used in a cell proliferation assay system. The downstream effects on the protein phosphorylation levels of STAT1, STAT3, and STAT6 were examined in both immune-mediated hepatitis and hepatoma models. The effects of beta-glycosphingolipids on the STAT signaling pathways were found to be dependent on CD1d. Lipid rafts were affected by both the dose and ratio of the beta-glycosphingolipids and the acyl chain length, and these effects were followed by downstream effects on STAT proteins. Our results show that beta-glycosphingolipids have beneficial effects in natural killer T cell-dependent immune-mediated metabolic and malignant animal models in vivo.

Analysis of histone 2B-GFP retention reveals slowly cycling hematopoietic stem cells

Hematopoietic stem cells (HSCs) are thought to divide infrequently based on their resistance to cytotoxic injury targeted at rapidly cycling cells and have been presumed to retain labels such as the thymidine analog 5-bromodeoxyuridine (BrdU). However, BrdU retention is neither a sensitive nor specific marker for HSCs. Here we show that transient, transgenic expression of a histone 2B (H2B)-green fluorescent protein (GFP) fusion protein in mice has several advantages for label-retention studies over BrdU, including rapid induction of H2B-GFP in virtually all HSCs, higher labeling intensity and the ability to prospectively study label-retaining cells, which together permit a more precise analysis of division history. Mathematical modeling of H2B-GFP dilution in HSCs, identified with a stringent marker combination (L(-)K(+)S(+)CD48(-)CD150(+)), revealed unexpected heterogeneity in their proliferation rates and showed that approximately 20% of HSCs divide at an extremely low rate (< or =0.8-1.8% per day).

Lysosomal targeting and trafficking of acid sphingomyelinase to lipid raft platforms in coronary endothelial cells

OBJECTIVE

The purpose of this study was to determine whether lysosome trafficking and targeting of acid sphingomyelinase (ASMase) to this organelle contribute to the formation of lipid raft (LR) signaling platforms in the membrane of coronary arterial endothelial cells (CAECs).

METHODS AND RESULTS

By measurement of fluorescent resonance energy transfer (FRET), it was found that in FasL-stimulated CAECs, membrane lamp1 (a lysosome marker protein) or Fas and GM1 (a LR marker) were trafficking together. Cofocal colocalization assay showed that ceramide was enriched in these LR platforms. Further studies demonstrated that these ceramide molecules in LR platforms were colocalized with ASMase, a ceramide producing enzyme. Fluorescence imaging of living CAECs loaded with lysosomal specific dyes demonstrated that lysosomes fused with membrane on FasL stimulation. In the presence of lysosome function inhibitors, bafilomycin (Baf) or glycyl-L-phenylalanine-beta-naphthylamide (GPN), these FasL-induced changes were abolished. Moreover, this FasL-induced formation of LR platforms was also blocked in ECs transfected with siRNA of sortilin, an intracellular transporter for targeting of ASMase to lysosomes. Functionally, FasL-induced impairment of vasodilator response was reversed by lysosomal inhibitors or sortilin gene silencing.

CONCLUSIONS

Lysosomal trafficking and targeting of ASMase are importantly involved in LRs clustering in ECs membrane, leading to the formation of signaling platforms or signalosomes.

Protective effect of hyaluronate on oxidative DNA damage in WI-38 and A549 cells

Progressive DNA damage in live cells by oxidants is the key factor contributing to cell aging and preconditioning to neoplastic transformation. The strategies to slow aging or prevent cancer rely on protection of DNA from the damage. Since cells reside within intercellular matrix it is of interest to know whether matrix constituents possess properties of modulating oxidative DNA damage. We explored, therefore, the effect of hyaluronate (HA), the ubiquitous component of the matrix, on extent of DNA damage induced by exogenous and endogenously generated oxidants. WI-38 and A549 cells were exposed to 200 microM H2O2 in the absence or presence of HA and induction of histone H2AX phosphorylation and activation of ATM, the reporters of DNA damage, was assessed by multiparameter cytometry. Also explored was effect of HA on constitutive H2AX phosphorylation that reflects DNA damage caused by endogenous oxidants generated during aerobic metabolism. HA of average MW 5.4 million (high MW) and 2 million (medium MW) at 0.1% (w/v) in culture medium totally prevented the H2O2-induced H2AX phosphorylation in both cell types whereas effect of 60,000 average MW (low MW) HA was somewhat less pronounced. Constitutive H2AX phosphorylation in WI-38 cells growing in the presence of 0.1% HA of low MW and medium MW was reduced by about 35 and 30%, respectively; no reduction was observed in A549 cells. The data indicate that HA protected DNA from damage caused by the exogenous oxidant H2O2. In WI-38 fibroblasts, the cells that express the HA-receptor CD44, HA also protected DNA from damage caused by endogenous oxidants. We postulate that expression of CD44 in some cell types such as stem cells may provide the means to internalize HA by endocytosis and one of the functions of the internalized HA may be protection of DNA from oxidants. The mechanism of protective effect of HA may either: i) involve entrapment of iron ions thereby inhibiting the Fenton's reaction that produces secondary oxidative species, and/or: ii) directly scavenging of primary and secondary ROIs, as an antioxidant, resulting in HA degradation. Since no significant degradation of HA upon its exposure in tissue culture medium to H2O2 was detected the scavenging may occur intracellularly.

Ceramide signaling in cancer and stem cells

Most of the previous work on the sphingolipid ceramide has been devoted to its function as an apoptosis inducer. Recent studies, however, have shown that in stem cells, ceramide has additional nonapoptotic functions. In this article, ceramide signaling will be reviewed in light of 'systems interface biology': as an interconnection of sphingolipid metabolism, membrane biophysics and cell signaling. The focus will be on the metabolic interconversion of ceramide and sphingomyelin or sphingosine-1-phosphate. Lipid rafts and sphingolipid-induced protein scaffolds will be discussed as a membrane interface for lipid-controlled cell signaling. Ceramide/sphingomyelin and ceramide/sphingosine-1-phosphate-interdependent cell-signaling pathways are significant for the regulation of cell polarity, apoptosis and/or proliferation, and as novel pharmacologic targets in cancer and stem cells.

Pbx1 regulates self-renewal of long-term hematopoietic stem cells by maintaining their quiescence

Self-renewal is a defining characteristic of stem cells; however, the molecular pathways underlying its regulation are poorly understood. Here, we demonstrate that conditional inactivation of the Pbx1 proto-oncogene in the hematopoietic compartment results in a progressive loss of long-term hematopoietic stem cells (LT-HSCs) that is associated with concomitant reduction in their quiescence, leading to a defect in the maintenance of self-renewal as assessed by serial transplantation. Transcriptional profiling revealed that multiple stem cell maintenance factors are perturbed in Pbx1-deficient LT-HSCs, which prematurely express a large subset of genes, including cell-cycle regulators, normally expressed in non-self-renewing multipotent progenitors. A significant proportion of Pbx1-dependent genes is associated with the TGF-beta pathway, which serves a major role in maintaining HSC quiescence. Prospectively isolated, Pbx1-deficient LT-HSCs display altered transcriptional responses to TGF-beta stimulation in vitro, suggesting a possible mechanism through which Pbx1 maintenance of stem cell quiescence may in part be achieved.

Stem cells and aging in the hematopoietic system

The effector cells of the blood have limited lifetimes and must be replenished continuously throughout life from a small reserve of hematopoietic stem cells (HSCs) in the bone marrow. Although serial bone marrow transplantation experiments in mice suggest that the replicative potential of HSCs is finite, there is little evidence that replicative senescence causes depletion of the stem cell pool during the normal lifespan of either mouse or man. Studies conducted in murine genetic models defective in DNA repair, intracellular ROS management, and telomere maintenance indicate that all these pathways are critical to the longevity and stress response of the stem cell pool. With age, HSCs show an increased propensity to differentiate towards myeloid rather than lymphoid lineages, which may contribute to the decline in lymphopoiesis that attends aging. Challenges for the future include assessing the significance of 'lineage skewing' to immune dysfunction, and investigating the role of epigenetic dysregulation in HSC aging.

Neural cell adhesion molecule (NCAM) marks adult myogenic cells committed to differentiation

Although recent advances in broad-scale gene expression analysis have dramatically increased our knowledge of the repertoire of mRNAs present in multiple cell types, it has become increasingly clear that examination of the expression, localization, and associations of the encoded proteins will be critical for determining their functional significance. In particular, many signaling receptors, transducers, and effectors have been proposed to act in higher-order complexes associated with physically distinct areas of the plasma membrane. Adult muscle stem cells (satellite cells) must, upon injury, respond appropriately to a wide range of extracellular stimuli: the role of such signaling scaffolds is therefore a potentially important area of inquiry. To address this question, we first isolated detergent-resistant membrane fractions from primary satellite cells, then analyzed their component proteins using liquid chromatography-tandem mass spectrometry. Transmembrane and juxtamembrane components of adhesion-mediated signaling pathways made up the largest group of identified proteins; in particular, neural cell adhesion molecule (NCAM), a multifunctional cell-surface protein that has previously been associated with muscle regeneration, was significant. Immunohistochemical analysis revealed that not only is NCAM localized to discrete areas of the plasma membrane, it is also a very early marker of commitment to terminal differentiation. Using flow cytometry, we have sorted physically homogeneous myogenic cultures into proliferating and differentiating fractions based solely upon NCAM expression.

NOD/Shi-scid IL2rgamma(null) (NOG) mice more appropriate for humanized mouse models

"Humanized mice," in which various kinds of human cells and tissues can be engrafted and retain the same functions as in humans, are extremely useful because human diseases can be studied directly. Using the newly combined immunodeficient NOD-scid IL2rgamma(null) mice and Rag2(null) IL2rgamma(null) humanized mice, it has became possible to expand applications because various hematopoietic cells can be differentiated by human hematopoietic stem cell transplantation, and the human immune system can be reconstituted to some degree. This work has attracted attention worldwide, but the development and use of immunodeficient mice in Japan are not very well known or understood. This review describes the history and characteristics of the NOD/Shi-scid IL2rgamma(null) (NOG) and BALB/cA-Rag2(null) IL2rgamma(null) mice that were established in Japan, including our unpublished data from researchers who are currently using these mice. In addition, we also describe the potential development of new immunodeficient mice that can be used as humanized mice in the future.