Concise review: Multidimensional regulation of the hematopoietic stem cell state

Bidirectional interplay between metabolism and epigenetics in hematopoietic stem cells and leukemia

During the last decades, remarkable progress has been made in further understanding the complex molecular regulatory networks that maintain hematopoietic stem cell (HSC) function. Cellular and organismal metabolisms have been shown to directly instruct epigenetic alterations, and thereby dictate stem cell fate, in the bone marrow. Epigenetic regulatory enzymes are dependent on the availability of metabolites to facilitate DNA- and histone-modifying reactions. The metabolic and epigenetic features of HSCs and their downstream progenitors can be significantly altered by environmental perturbations, dietary habits, and hematological diseases. Therefore, understanding metabolic and epigenetic mechanisms that regulate healthy HSCs can contribute to the discovery of novel metabolic therapeutic targets that specifically eliminate leukemia stem cells while sparing healthy HSCs. Here, we provide an in-depth review of the metabolic and epigenetic interplay regulating hematopoietic stem cell fate. We discuss the influence of metabolic stress stimuli, as well as alterations occurring during leukemic development. Additionally, we highlight recent therapeutic advancements toward eradicating acute myeloid leukemia cells by intervening in metabolic and epigenetic pathways.

Cell-intrinsic factors governing quiescence vis-à-vis activation of adult hematopoietic stem cells

Hematopoiesis is a highly complex process, regulated by both intrinsic and extrinsic factors. Often, these two regulatory arms work in tandem to maintain the steady-state condition of hematopoiesis. However, at times, certain intrinsic attributes of hematopoietic stem cells (HSCs) override the external stimuli and dominate the outcome. These could be genetic events like mutations or environmentally induced epigenetic or transcriptomic changes. Since leukemic stem cells (LSCs) share molecular pathways that also regulate normal HSCs, identifying specific, dominantly acting intrinsic factors could help in the development of novel therapeutic approaches. Here we have reviewed such dominantly acting intrinsic factors governing quiescence vis-à-vis activation of the HSCs in the face of external forces acting on them. For brevity, we have restricted our review to the articles dealing with adult HSCs of human and mouse origin that have been published in the last 10 years. Hematopoietic stem cells (HSCs) are closely associated with various stromal cells in their microenvironment and, thus, constantly receive signaling cues from them. The illustration depicts some dominantly acting intrinsic or cell-autonomous factors operative in the HSCs. These fall into various categories, such as epigenetic regulators, transcription factors, cell cycle regulators, tumor suppressor genes, signaling pathways, and metabolic regulators, which counteract the outcome of extrinsic signaling exerted by the HSC niche.

High expression of BCL6 inhibits the differentiation and development of hematopoietic stem cells and affects the growth and development of chickens

Background

B-cell CLL/lymphoma 6 (BCL6) is a transcriptional master regulator that represses more than 1200 potential target genes. Our previous study showed that a decline in blood production in runting and stunting syndrome (RSS) affected sex-linked dwarf (SLD) chickens compared to SLD chickens. However, the association between BCL6 gene and hematopoietic function remains unknown in chickens.

Methods

In this study, we used RSS affected SLD (RSS-SLD) chickens, SLD chickens and normal chickens as research object and overexpression of BCL6 in hematopoietic stem cells (HSCs), to investigate the effect of the BCL6 on differentiation and development of HSCs.

Results

The results showed that comparison of RSS-SLD chickens with SLD chickens, the BCL6 was highly expressed in RSS-SLD chickens bone marrow. The bone marrow of RSS-SLD chickens was exhausted and red bone marrow was largely replaced by yellow bone marrow, bone density was reduced, and the levels of immature erythrocytes in peripheral blood were increased. At the same time, the hematopoietic function of HSCs decreased in RSS-SLD chickens, which was manifested by a decrease in the hematopoietic growth factors (HGFs) EPO, SCF, TPO, and IL-3, as well as hemoglobin α₁ and hemoglobin β expression. Moreover, mitochondrial function in the HSCs of RSS-SLD chickens was damaged, including an increase in ROS production, decrease in ATP concentration, and decrease in mitochondrial membrane potential (ΔΨm). The same results were also observed in SLD chickens compared with normal chickens; however, the symptoms were more serious in RSS-SLD chickens. Additionally, after overexpression of the BCL6 in primary HSCs, the secretion of HGFs (EPO, SCF, TPO and IL-3) was inhibited and the expression of hemoglobin α₁ and hemoglobin β was decreased. However, cell proliferation was accelerated, apoptosis was inhibited, and the HSCs entered a cancerous state. The function of mitochondria was also abnormal, ROS production was decreased, and ATP concentration and ΔΨm were increased, which was related to the inhibition of apoptosis of stem cells.

Conclusions

Taken together, we conclude that the high expression of BCL6 inhibits the differentiation and development of HSCs by affecting mitochondrial function, resulting in impaired growth and development of chickens. Moreover, the abnormal expression of BCL6 might be a cause of the clinical manifestations of chicken comb, pale skin, stunted growth and development, and the tendency to appear RSS in SLD chickens.

Normal and leukemic stem cell niche interactions

PURPOSE OF REVIEW

Normal hematopoietic stem cells (HSCs) and leukemic stem cells (LSCs) interact with the stem cell niche bone marrow in different ways. Understanding the potentially unique microenvironmental regulation of LSCs is key to understanding in-vivo leukemogenic mechanisms and developing novel antileukemic therapies.

RECENT FINDINGS

When leukemic cells are engrafted in the stem cell niche, the cellular nature of the niche - including mesenchymal stromal cells - is reprogramed. Altered mesenchymal cells selectively support leukemic cells and reinforce the pro-leukemic environment. As the niche plays an active role in leukemogenesis, its remodeling may significantly influence the leukemogenic pattern, and cause differences in clinical prognosis. Notably, niche cells could be stimulated to revert to a pronormal/antileukemic state, creating potential for niche-based antileukemic therapy.

SUMMARY

Bone marrow microenvironments are under dynamic regulation for normal and leukemic cells, and there is bi-directional control of leukemic cells in the niche. Leukemic cells are both protected by stroma and able to reprogram stromal cells to transform the niche to a state, which reinforces leukemogenesis. Because of its dynamic nature, the niche could be converted to an environment with antileukemic properties, making it an attractive target for therapy.

Ryk modulates the niche activity of mesenchymal stromal cells by fine-tuning canonical Wnt signaling

The importance of modulating the intensity of Wnt signaling has been highlighted in various biological models, but their mechanisms remain unclear. In this study, we found that Ryk—an atypical Wnt receptor with a pseudokinase domain—has a Wnt-modulating effect in bone marrow stromal cells to control hematopoiesis-supporting activities. We first found that Ryk is predominantly expressed in the mesenchymal stromal cells (MSCs) of the bone marrow (BM) compared with hematopoietic cells. Downregulation of Ryk in MSCs decreased their clonogenic activity and ability to support self-renewing expansion of primitive hematopoietic progenitors (HPCs) in response to canonical Wnt ligands. In contrast, under high concentrations of Wnt, Ryk exerted suppressive effects on the transactivation of target genes and HPC-supporting effects in MSCs, thus fine-tuning the signaling intensity of Wnt in BM stromal cells. This ability of Ryk to modulate the HPC-supporting niche activity of MSCs was abrogated by induction of deletion mutants of Ryk lacking the intracellular domain or extracellular domain, indicating that the pseudokinase-containing intracellular domain mediates the Wnt-modulating effects in response to extracellular Wnt ligands. These findings indicate that the ability of the BM microenvironment to respond to extracellular signals and support hematopoiesis may be fine-tuned by Ryk via modulation of Wnt signaling intensity to coordinate hematopoietic activity.

Steady production of immune and blood cells depends on a signaling protein that helps maintain stable stem cell populations within the bone marrow. Hematopoietic stem cells (HSCs), which give rise to blood cells, reside within a supportive “niche” surrounded by mesenchymal stromal cells (MSCs), with extensive communication between the two populations. Researchers led by Il-Hoan Oh at The Catholic University of Korea, Seoul, have now identified a mechanism that MSCs employ to stabilize the niche environment through fine-tuning the signaling intensity of Wnt. Oh and colleagues focused on a signaling pathway that controls the undifferentiated state of HSCs, and showed that these signals are specifically modulated by an MSC protein known as Ryk. Without Ryk, MSCs can no longer promote HSC proliferation. However, when these signals are excessively strong, Ryk helps suppress proliferation to keep HSC numbers under control.

BMP4 and perivascular cells promote hematopoietic differentiation of human pluripotent stem cells in a differentiation stage-specific manner

The efficient and reproducible derivation and maturation of multipotent hematopoietic progenitors from human pluripotent stem cells (hPSCs) requires the recapitulation of appropriate developmental stages and the microenvironment. Here, using serum-, xeno-, and feeder-free stepwise hematopoietic induction protocols, we showed that short-term and high-concentration treatment of hPSCs with bone morphogenetic protein 4 (BMP4) strongly promoted early mesoderm induction followed by increased hematopoietic commitment. This method reduced variations in hematopoietic differentiation among hPSC lines maintained under chemically defined Essential 8 medium compared to those maintained under less-defined mTeSR medium. We also found that perivascular niche cells (PVCs) significantly augmented the production of hematopoietic cells via paracrine signaling mechanisms only when they were present during the hematopoietic commitment phase. A protein array revealed 86 differentially expressed (>1.5-fold) secretion factors in PVC-conditioned medium compared with serum-free control medium, of which the transforming growth factor-β inducible gene H3 significantly increased the number of hematopoietic colony-forming colonies. Our data suggest that BMP4 and PVCs promote the hematopoietic differentiation of hPSCs in a differentiation stage-specific manner. This will increase our understanding of hematopoietic development and expedite the development of hPSC-derived blood products for therapeutic use.

Adding a vital regulatory molecule and support cells to the culture medium can help in the derivation of blood products from stem cells. A team led by Seok-Ho Hong from Kangwon National University in Chuncheon, South Korea, followed a clinical-grade protocol for converting embryonic stem cells or induced pluripotent stem cells from adults into blood cell precursors. The researchers showed that incorporating high doses of a growth factor called bone morphogenetic protein 4 into the standard culture medium for a short period promoted early differentiation toward blood cells. Incorporating so-called perivascular cells taken from umbilical cord blood also enhanced the process through the secretion of signaling molecules that further pushed the stem cells toward differentiating into blood cells. The findings could help improve protocols for making blood products from stem cells for therapeutic purposes.

Evaluation of changes arising in the pig mesenchymal stromal cells transcriptome following cryopreservation and Trichostatin A treatment

Cryopreservation is an important procedure in maintenance and clinical applications of mesenchymal stem/stromal cells (MSCs). Although the methods of cell freezing using various cryoprotectants are well developed and allow preserving structurally intact living cells, the freezing process can be considered as a severe cellular stress associated with ice formation, osmotic damage, cryoprotectants migration/cytotoxicity or rapid cell shrinkage. The cellular response to freezing stress is aimed at the restoring of homeostasis and repair of cell damage and is crucial for cell viability. In this study we evaluated the changes arising in the pig mesenchymal stromal cell transcriptome following cryopreservation and showed the vast alterations in cell transcriptional activity (5,575 genes with altered expression) suggesting the engagement in post-thawing cell recovery of processes connected with cell membrane tension regulation, membrane damage repair, cell shape maintenance, mitochondria-connected energy homeostasis and apoptosis mediation. We also evaluated the effect of known gene expression stimulator—Trichostain A (TSA) on the frozen/thawed cells transcriptome and showed that TSA is able to counteract to a certain extent transcriptome alterations, however, its specificity and advantages for cell recovery after cryopreservation require further studies.

Wnt Signaling in Stem Cells and Cancer Stem Cells: A Tale of Two Coactivators

Wnt signaling in stem cells plays critical roles in development, normal adult physiology, and disease. In this chapter, we focus on the role of the Wnt signaling pathway in somatic stem cell biology and its critical role in normal tissue homeostasis and cancer. Wnt signaling can both maintain potency and initiate differentiation in somatic stem cells, depending on the cellular and environmental context. Based principally on studies from our lab, we will explain the dichotomous behavior of this signaling pathway in determining stem cell fate decisions, placing special emphasis on the interaction of β-catenin with either of the two highly homologous Kat3 coactivator proteins, CBP and p300. We will also discuss our results, both preclinical and clinical, demonstrating that small molecule modulators of the β-catenin/Kat3 coactivator interaction can be safely utilized to shift the balance between maintenance of potency and initiation of differentiation.

A small-molecule/cytokine combination enhances hematopoietic stem cell proliferation via inhibition of cell differentiation

Background

Accumulated evidence supports the potent stimulating effects of multiple small molecules on the expansion of hematopoietic stem cells (HSCs) which are important for the therapy of various hematological disorders. Here, we report a novel, optimized formula, named the SC cocktail, which contains a combination of three such small molecules and four cytokines.

Methods

Small-molecule candidates were individually screened and then combined at their optimal concentration with the presence of cytokines to achieve maximum capacity for stimulating the human CD34⁺ cell expansion ex vivo. The extent of cell expansion and the immunophenotype of expanded cells were assessed through flow cytometry. The functional preservation of HSC stemness was confirmed by additional cell and molecular assays in vitro. Subsequently, the expanded cells were transplanted into sublethally irradiated NOD/SCID mice for the assessment of human cell viability and engraftment potential in vivo. Furthermore, the expression of several genes in the cell proliferation and differentiation pathways was analyzed through quantitative polymerase chain reaction (qPCR) during the process of CD34⁺ cell expansion.

Results

The SC cocktail supported the retention of the immunophenotype of hematopoietic stem/progenitor cells remarkably well, by yielding purities of 86.6 ± 11.2% for CD34⁺ cells and 76.2 ± 10.5% for CD34⁺CD38^– cells, respectively, for a 7-day culture. On day 7, the enhancement of expansion of CD34⁺ cells and CD34⁺CD38^– cells reached a maxima of 28.0 ± 5.5-fold and 27.9 ± 4.3-fold, respectively. The SC cocktail-expanded CD34⁺ cells preserved the characteristics of HSCs by effectively inhibiting their differentiation in vitro and retained the multilineage differentiation potential in primary and secondary in vivo murine xenotransplantation trials. Further gene expression analysis suggested that the small-molecule combination strengthened the ability of the cytokines to enhance the Notch pathway for the preservation of HSC stemness, and inhibited the ability of the cytokines to activate the Wnt pathway for HSC differentiation.

Conclusions

We developed an optimal small-molecule/cytokine combination for the enhancement of HSC expansion via inhibition of differentiation. This approach indicates promising application for preparation of both the HSCs and the mature, functional hematopoietic cells for clinical transplantation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0625-z) contains supplementary material, which is available to authorized users.

Epigenetic Guardian: A Review of the DNA Methyltransferase DNMT3A in Acute Myeloid Leukaemia and Clonal Haematopoiesis

Acute myeloid leukaemia (AML) is a haematological malignancy characterized by clonal stem cell proliferation and aberrant block in differentiation. Dysfunction of epigenetic modifiers contributes significantly to the pathogenesis of AML. One frequently mutated gene involved in epigenetic modification is DNMT3A (DNA methyltransferase-3-alpha), a DNA methyltransferase that alters gene expression by de novo methylation of cytosine bases at CpG dinucleotides. Approximately 22% of AML and 36% of cytogenetically normal AML cases carry DNMT3A mutations and around 60% of these mutations affect the R882 codon. These mutations have been associated with poor prognosis and adverse survival outcomes for AML patients. Advances in whole-exome sequencing techniques have recently identified a large number of DNMT3A mutations present in clonal cells in normal elderly individuals with no features of haematological malignancy. Categorically distinct from other preleukaemic conditions, this disorder has been termed clonal haematopoiesis of indeterminate potential (CHIP). Further insight into the mutational landscape of CHIP may illustrate the consequence of particular mutations found in DNMT3A and identify specific “founder” mutations responsible for clonal expansion that may contribute to leukaemogenesis. This review will focus on current research and understanding of DNMT3A mutations in both AML and CHIP.

Marrow-inspired matrix cues rapidly affect early fate decisions of hematopoietic stem and progenitor cells

Hematopoiesis is the physiological process where hematopoietic stem cells (HSCs) continuously generate the body's complement of blood and immune cells within unique regions of the bone marrow termed niches. Although previous investigations have revealed gradients in cellular and extracellular matrix (ECM) content across the marrow, and matrix elasticity and ligand type are believed to be strong regulators of stem cell fate, the impact of biophysical signals on HSC response is poorly understood. Using marrow-inspired ECM ligand-coated polyacrylamide substrates that present defined stiffness and matrix ligand cues, we demonstrate that the interplay between integrin engagement and myosin II activation processes affects the morphology, proliferation, and myeloid lineage specification of primary murine HSCs within 24 hours ex vivo. Notably, the impact of discrete biophysical signals on HSC fate decisions appears to be correlated to known microenvironmental transitions across the marrow. The combination of fibronectin and marrow matrix-associated stiffness was sufficient to maintain hematopoietic progenitor populations, whereas collagen and laminin enhanced proliferation and myeloid differentiation, respectively. Inhibiting myosin II-mediated contraction or adhesion to fibronectin via specific integrins (α₅β₁ and α_νβ₃) selectively abrogated the impact of the matrix environment on HSC fate decisions. Together, these findings indicate that adhesive interactions and matrix biophysical properties are critical design considerations in the development of biomaterials to direct HSC behavior in vitro.

Phage Display Technology in Biomaterials Engineering: Progress and Opportunities for Applications in Regenerative Medicine

The field of regenerative medicine has been gaining momentum steadily over the past few years. The emphasis in regenerative medicine is to use various in vitro and in vivo approaches that leverage the intrinsic healing mechanisms of the body to treat patients with disabling injuries and chronic diseases such as diabetes, osteoarthritis, and degenerative disorders of the cardiovascular and central nervous system. Phage display has been successfully employed to identify peptide ligands for a wide variety of targets, ranging from relatively small molecules (enzymes, cell receptors) to inorganic, organic, and biological (tissues) materials. Over the past two decades, phage display technology has advanced tremendously and has become a powerful tool in the most varied fields of research, including biotechnology, materials science, cell biology, pharmacology, and diagnostics. The growing interest in and success of phage display libraries is largely due to its incredible versatility and practical use. This review discusses the potential of phage display technology in biomaterials engineering for applications in regenerative medicine.

Kat3 coactivators in somatic stem cells and cancer stem cells: biological roles, evolution, and pharmacologic manipulation

Long-lived somatic stem cells regenerate adult tissues throughout our lifetime. However, with aging, there is a significant deterioration in the function of stem and progenitor cells, which contribute to diseases of aging. The decision for a long-lived somatic stem cell to become activated and subsequently to undergo either a symmetric or an asymmetric division is a critical cellular decision process. The decision to preferentially divide symmetrically or asymmetrically may be the major fundamental intrinsic difference between normal somatic stem cells and cancer stem cells. Based upon work done primarily in our laboratory over the past 15 years, this article provides a perspective on the critical role of somatic stem cells in aging. In particular, we discuss the importance of symmetric versus asymmetric divisions in somatic stem cells and the role of the differential usage of the highly similar Kat3 coactivators, CREB-binding protein (CBP) and p300, in stem cells. We describe and propose a more complete model for the biological mechanism and roles of these two coactivators, their evolution, and unique roles and importance in stem cell biology. Finally, we discuss the potential to pharmacologically manipulate Kat3 coactivator interactions in endogenous stem cells (both normal and cancer stem cells) to potentially ameliorate the aging process and common diseases of aging.

Effect of Chromatin Structure on the Extent and Distribution of DNA Double Strand Breaks Produced by Ionizing Radiation; Comparative Study of hESC and Differentiated Cells Lines

Chromatin structure affects the extent of DNA damage and repair. Thus, it has been shown that heterochromatin is more protective against DNA double strand breaks (DSB) formation by ionizing radiation (IR); and that DNA DSB repair may proceed differently in hetero- and euchromatin regions. Human embryonic stem cells (hESC) have a more open chromatin structure than differentiated cells. Here, we study the effect of chromatin structure in hESC on initial DSB formation and subsequent DSB repair. DSB were scored by comet assay; and DSB repair was assessed by repair foci formation via 53BP1 antibody staining. We found that in hESC, heterochromatin is confined to distinct regions, while in differentiated cells it is distributed more evenly within the nuclei. The same dose of ionizing radiation produced considerably more DSB in hESC than in differentiated derivatives, normal human fibroblasts; and one cancer cell line. At the same time, the number of DNA repair foci were not statistically different among these cells. We showed that in hESC, DNA repair foci localized almost exclusively outside the heterochromatin regions. We also noticed that exposure to ionizing radiation resulted in an increase in heterochromatin marker H3K9me3 in cancer HT1080 cells, and to a lesser extent in IMR90 normal fibroblasts, but not in hESCs. These results demonstrate the importance of chromatin conformation for DNA protection and DNA damage repair; and indicate the difference of these processes in hESC.

Exosomes and Their Therapeutic Potentials of Stem Cells

Exosomes, a group of vesicles originating from the multivesicular bodies (MVBs), are released into the extracellular space when MVBs fuse with the plasma membrane. Numerous studies indicate that exosomes play important roles in cell-to-cell communication, and exosomes from specific cell types and conditions display multiple functions such as exerting positive effects on regeneration in many tissues. It is widely accepted that the therapeutic potential of stem cells may be mediated largely by the paracrine factors, so harnessing the paracrine effects of stem and progenitor cells without affecting these living, replicating, and potentially pluripotent cell populations is an advantage in terms of safety and complexity. Ascending evidence indicated that exosomes might be the main components of paracrine factors; thus, understanding the role of exosomes in each subtype of stem cells is far-reaching. In this review, we discuss the functions of exosomes from different types of stem cells and emphasize the therapeutic potentials of exosomes, providing an alternative way of developing strategies to cure diseases.

Concise review: Exploring miRNAs--toward a better understanding of hematopoiesis

Hematopoiesis is governed by a multidimensional regulatory network involving both intrinsic and extrinsic factors that control self-renewal and differentiation of hematopoietic stem cells (HSCs) through the coordination of influences that affect cell fate. Increasing evidence indicates that microRNAs (miRNAs), short noncoding RNAs of approximately 22 nucleotides, play a central role in orchestrating these regulatory mechanisms to modulate the multiple entities of hematopoietic function in a cell-type specific manner, including self-renewal, lineage commitment, and survival of HSCs as well as their microenvironmental crosstalk. Here, we summarize the current understanding regarding the regulatory effects of miRNA on hematopoietic cells, thus enlightening their role in fine-tuning HSC function and hematopoietic homeostasis.

The use of covalently immobilized stem cell factor to selectively affect hematopoietic stem cell activity within a gelatin hydrogel

Hematopoietic stem cells (HSCs) are a rare stem cell population found primarily in the bone marrow and responsible for the production of the body's full complement of blood and immune cells. Used clinically to treat a range of hematopoietic disorders, there is a significant need to identify approaches to selectively expand their numbers ex vivo. Here we describe a methacrylamide-functionalized gelatin (GelMA) hydrogel for in vitro culture of primary murine HSCs. Stem cell factor (SCF) is a critical biomolecular component of native HSC niches in vivo and is used in large dosages in cell culture media for HSC expansion in vitro. We report a photochemistry based approach to covalently immobilize SCF within GelMA hydrogels via acrylate-functionalized polyethylene glycol (PEG) tethers. PEG-functionalized SCF retains the native bioactivity of SCF but can be stably incorporated and retained within the GelMA hydrogel over 7 days. Freshly-isolated murine HSCs cultured in GelMA hydrogels containing covalently-immobilized SCF showed reduced proliferation and improved selectivity for maintaining primitive HSCs. Comparatively, soluble SCF within the GelMA hydrogel network induced increased proliferation of differentiating hematopoietic cells. We used a microfluidic templating approach to create GelMA hydrogels containing gradients of immobilized SCF that locally direct HSC response. Together, we report a biomaterial platform to examine the effect of the local presentation of soluble vs. matrix-immobilized biomolecular signals on HSC expansion and lineage specification. This approach may be a critical component of a biomaterial-based artificial bone marrow to provide the correct sequence of niche signals to grow HSCs in the laboratory.

Immuno-metabolism and adipose tissue: The key role of hematopoietic stem cells

The field of immunometabolism has come a long way in the past decade, leading to the emergence of a new role for white adipose tissue (WAT) that is now recognized to stand at the junction of immune and metabolic regulations. Interestingly, a crucial role of the abundant and heterogeneous immune population present in WAT has been proposed in the induction and development of metabolic diseases. Although a large body of data focused on mature immune cells, only few scattered studies are dedicated to leukocyte production, and the activity of hematopoietic stem cells (HSC) in these pathological states. Considering that blood cell production and the differentiation of HSCs and their progeny is orchestrated, in part, by complex interacting signals emanating from their microenvironment, it thus seems worth to better understand the relationships between metabolism and HSC. This review discusses the alterations of hematopoietic process described in metabolic diseases and focused on the emerging data concerning HSC present in WAT.

Regulation of mesenchymal stromal cells through fine tuning of canonical Wnt signaling

Mesenchymal stromal cells (MSCs) have been extensively utilized for various cell therapeutic trials, but the signals regulating their stromal function remain largely unclear. Here, we show that canonical Wnt signals distinctively regulate MSCs in a biphasic manner depending on signal intensity, i.e., MSCs exhibit proliferation and progenitor self-renewal under low Wnt/β-catenin signaling, whereas they exhibit enhanced osteogenic differentiation with priming to osteoblast-like lineages under high Wnt/β-catenin signaling. Moreover, low or high levels of β-catenin in MSCs distinctly regulated the hematopoietic support of MSCs to promote proliferation or the undifferentiated state of hematopoietic progenitors, respectively. A gene expression study demonstrated that different intracellular levels of β-catenin in MSCs induce distinct transcriptomic changes in subsets of genes belonging to different gene function categories. Different β-catenin levels also induced differences in intracellular levels of the β-catenin co-factors, Tcf-1 and Lef-1. Moreover, nano-scale mass spectrometry of proteins that co-precipitated with β-catenin revealed distinctive spectra of proteins selectively interacting with β-catenin at specific expression levels. Together, these results show that Wnt/β-catenin signals can coax distinct transcription milieu to induce different transcription profiles in MSCs depending on the signal intensity and that fine-tuning of the canonical Wnt signaling intensity can regulate the phase-specific functionality of MSCs.

Molecular integration of HoxB4 and STAT3 for self-renewal of hematopoietic stem cells: a model of molecular convergence for stemness

The upregulation of HoxB4 promotes self-renewal of hematopoietic stem cells (HSCs) without overriding the normal stem cell pool size. A similar enhancement of HSC self-renewal occurs when signal transducer and activator of transcription 3 (STAT3) is activated in HSCs. In this study, to gain insight into the functional organization of individual transcription factors (TFs) that have similar effects on HSCs, we investigated the molecular interplay between HoxB4 and STAT3 in the regulation of HSC self-renewal. We found that while STAT3-C or HoxB4 similarly enhanced the in vitro self-renewal and in vivo repopulating activities of HSCs, simultaneous transduction of both TFs did not have additive effects, indicating their functional redundancy in HSCs. In addition, activation of STAT3 did not cause changes in the expression levels of HoxB4. In contrast, the inhibition of STAT3 activity in HoxB4-overexpressing hematopoietic cells significantly abrogated the enhancing effects of HoxB4, and the upregulation of HoxB4 caused a ligand-independent Tyr-phosphorylation of STAT3. Microarray analysis revealed a significant overlap of the transcriptomes regulated by STAT3 and HoxB4 in undifferentiated hematopoietic cells. Moreover, a gene set enrichment analysis showed significant overlap in the candidate TFs that can recapitulate the transcriptional changes induced by HoxB4 or STAT3. Interestingly, among these common TFs were the pluripotency-related genes Oct-4 and Nanog. These results indicate that tissue-specific TFs regulating HSC self-renewal are functionally organized to play an equivalent role in transcription and provide insights into the functional convergence of multiple entries of TFs toward a conserved transcription program for the stem cell state.

The heart: mostly postmitotic or mostly premitotic? Myocyte cell cycle, senescence, and quiescence

The concept of myocyte division and myocyte-mediated regeneration has re-emerged in the past 5 years through development of sophisticated transgenic mice and carbon-dating of cells. Although recently, a couple of studies have been conducted as an attempt to intervene in myocyte division, the efficiency in adult animals remains discouragingly low. Re-enforcing myocyte division is a vision that has been desired for decades, leading to years of experience in myocyte resistance to proproliferative stimuli. Previous attempts have indeed provided a platform for basic knowledge on molecular players and signalling in myocytes. However, natural biological processes such as hypertrophy and binucleation provide layers of complexity in interpretation of previous and current findings. A major hurdle in mediating myocyte division is a lack of insight in the myocyte cell cycle. To date, no knowledge is gained on myoycte cell cycle progression and/or duration. This review will include an overview of previous and current literature on myocyte cell cycle and division. Furthermore, the limitations of current approaches and basic questions that might be essential in understanding myocardial resistance to division will be discussed.

Epigenetic reprogramming induces the expansion of cord blood stem cells

Cord blood (CB) cells that express CD34 have extensive hematopoietic capacity and rapidly divide ex vivo in the presence of cytokine combinations; however, many of these CB CD34+ cells lose their marrow-repopulating potential. To overcome this decline in function, we treated dividing CB CD34+ cells ex vivo with several histone deacetylase inhibitors (HDACIs). Treatment of CB CD34+ cells with the most active HDACI, valproic acid (VPA), following an initial 16-hour cytokine priming, increased the number of multipotent cells (CD34+CD90+) generated; however, the degree of expansion was substantially greater in the presence of both VPA and cytokines for a full 7 days. Treated CD34+ cells were characterized based on the upregulation of pluripotency genes, increased aldehyde dehydrogenase activity, and enhanced expression of CD90, c-Kit (CD117), integrin α6 (CD49f), and CXCR4 (CD184). Furthermore, siRNA-mediated inhibition of pluripotency gene expression reduced the generation of CD34+CD90+ cells by 89%. Compared with CB CD34+ cells, VPA-treated CD34+ cells produced a greater number of SCID-repopulating cells and established multilineage hematopoiesis in primary and secondary immune-deficient recipient mice. These data indicate that dividing CB CD34+ cells can be epigenetically reprogrammed by treatment with VPA so as to generate greater numbers of functional CB stem cells for use as transplantation grafts.

Human bone marrow mesenchymal progenitors: perspectives on an optimized in vitro manipulation

When it comes to regenerative medicine, mesenchymal stem cells (MSCs) are considered one of the most promising cell types for use in many cell therapies and bioengineering protocols. The International Society of Cellular Therapy recommended minimal criteria for defining multipotential MSC is based on adhesion and multipotency in vitro, and the presence or absence of select surface markers. Though these criteria help minimize discrepancies and allow some comparisons of data generated in different laboratories, the conditions in which cells are isolated and expanded are often not considered. Herein, we propose and recommend a few procedures to be followed to facilitate the establishment of quality control standards when working with mesenchymal progenitors isolation and expansion. Following these procedures, the classic Colony-Forming Unit-Fibroblast (CFU-f) assay is revisited and three major topics are considered to define conditions and to assist on protocol optimization and data interpretation. We envision that the creation of a guideline will help in the identification and isolation of long-term stem cells and short-term progenitors to better explore their regenerative potential for multiple therapeutic purposes.

The Ah receptor in stem cell cycling, regulation, and quiescence

Processes that regulate quiescence, self-renewal, and differentiation of hematopoietic stem cells (HSCs) are not well understood. Owing, in part, to the ability of xenobiotic ligands to have persistent effects on the immune system in experimental animals, there has been much work to define a physiological role of the aryl hydrocarbon receptor (AhR) and its relationship to human disease. Persistent AhR activation by dioxin, a potent agonist, results in altered numbers and function of HSCs in mice. HSCs from AhR(-/-) knockout (KO) mice are hyperproliferative and have an altered cell cycle. Aging KO mice show characteristics consistent with premature bone marrow exhaustion. We propose that the increased proliferation of HSCs lacking AhR expression or activity is a result of loss of quiescence, and as such, AhR normally acts as a negative regulator to curb excessive or unnecessary proliferation. Similarly, prolonged and/or inappropriate stimulation of AhR activity may compromise the ability of HSCs to sense environmental signals that allow these cells to balance quiescence, proliferation, migration, and differentiation. These data and others support a hypothesis that deregulation of AhR function has an important role in HSC regulation and in the etiology and/or progression of certain hematopoietic diseases, many of which are associated with aging.

Drosophila as a model to study the role of blood cells in inflammation, innate immunity and cancer

Drosophila has a primitive yet effective blood system with three types of haemocytes which function throughout different developmental stages and environmental stimuli. Haemocytes play essential roles in tissue modeling during embryogenesis and morphogenesis, and also in innate immunity. The open circulatory system of Drosophila makes haemocytes ideal signal mediators to cells and tissues in response to events such as infection and wounding. The application of recently developed and sophisticated genetic tools to the relatively simple genome of Drosophila has made the fly a popular system for modeling human tumorigensis and metastasis. Drosophila is now used for screening and investigation of genes implicated in human leukemia and also in modeling development of solid tumors. This second line of research offers promising opportunities to determine the seemingly conflicting roles of blood cells in tumor progression and invasion. This review provides an overview of the signaling pathways conserved in Drosophila during haematopoiesis, haemostasis, innate immunity, wound healing and inflammation. We also review the most recent progress in the use of Drosophila as a cancer research model with an emphasis on the roles haemocytes can play in various cancer models and in the links between inflammation and cancer.

Murine adipose tissue-derived stromal cell apoptosis and susceptibility to oxidative stress in vitro are regulated by genetic background

Adipose tissue-derived stromal cells (ADSCs) are of interest for regenerative medicine as they are isolated easily and can differentiate into multiple cell lineages. Studies of their in vitro proliferation, survival, and differentiation are common; however, genetic effects on these phenotypes remain unknown. To test if these phenotypes are genetically regulated, ADSCs were isolated from three genetically diverse inbred mouse strains- C57BL/6J (B6), BALB/cByJ (BALB), and DBA/2J (D2)- in which genetic regulation of hematopoietic stem function is well known. ADSCs from all three strains differentiated into osteogenic and chondrogenic lineages in vitro. ADSCs from BALB grew least well in vitro, probably due to apoptotic cell death after several days in culture. BALB ADSCs were also the most susceptible to the free radical inducers menadione and H₂O₂. ADSCs from the three possible F1 hybrids were employed to further define genetic regulation of ADSC phenotypes. D2, but not B6, alleles stimulated ADSC expansion in BALB cells. In contrast, B6, but not D2, alleles rescued BALB H₂O₂ resistance. We conclude that low oxidative stress resistance does not limit BALB ADSC growth in vitro, as these phenotypes are genetically regulated independently. In addition, ADSCs from these strains are an appropriate model system to investigate genetic regulation of ADSC apoptosis and stress resistance in future studies. Such investigations are essential to optimize cell expansion and differentiation and thus, potential for regenerative medicine.

Proliferation-dependent alterations of the DNA methylation landscape underlie hematopoietic stem cell aging

The functional potential of hematopoietic stem cells (HSCs) declines during aging, and in doing so, significantly contributes to hematopoietic pathophysiology in the elderly. To explore the relationship between age-associated HSC decline and the epigenome, we examined global DNA methylation of HSCs during ontogeny in combination with functional analysis. Although the DNA methylome is generally stable during aging, site-specific alterations of DNA methylation occur at genomic regions associated with hematopoietic lineage potential and selectively target genes expressed in downstream progenitor and effector cells. We found that age-associated HSC decline, replicative limits, and DNA methylation are largely dependent on the proliferative history of HSCs, yet appear to be telomere-length independent. Physiological aging and experimentally enforced proliferation of HSCs both led to DNA hypermethylation of genes regulated by Polycomb Repressive Complex 2. Our results provide evidence that epigenomic alterations of the DNA methylation landscape contribute to the functional decline of HSCs during aging.

Bone marrow cell death and proliferation: Controlling mechanisms in normal and leukemic state

Bone marrow cell death and proliferation are regulated by multiple factors including genetic and epigenetic alterations of hematopoietic cells, crosstalk of hematopoietic cells with bone marrow mesenchymal cells through direct cell-cell interaction or cytokine/chemokine production, vascularity of the bone marrow, and interactions of sympathetic nerve system with hematopoiesis. Cell proliferation usually predominates over cell death in neoplastic processes such as leukemia and myeloproliferative neoplasms, while apoptotic processes also have a significant role in the pathogenesis of myelodysplastic syndromes. Recently, hematopoietic stem cells (HSCs) and leukemia stem cells (LSCs) have been identified and their characters on self renewal process, differentiation, cell dynamics and drug resistance have been implicated. Although most leukemia cells are initially sensitive to chemo- or radiotherapy, LSCs are resistant and considered to be the basis for disease relapse after initial response. HSCs and LSCs may use similar interactions with bone marrow microenvironment. However, bone marrow microenvironment called niche should influence the normal as well as malignant hematopoiesis in different manners. Recent studies have expanded the number of cell types constituting bone marrow niche and made the issue more complex. Since the majority of excellent and contributing studies on bone marrow niches have been performed in animal models, niches in human tissues are beginning to be localized and characterized. In this article, we summarize the relation of hematopoietic cells with niches and hope to point a hint to the novel strategy for treatment of malignant proliferation of hematopoietic cells.

Cytofluorometric assessment of cell cycle progression

One of the most prominent features of cellular senescence, a stress response that prevents the propagation of cells that have accumulated potentially oncogenic alterations, is a permanent loss of proliferative potential. Thus, at odds with quiescent cells, which resume proliferation when stimulated to do so, senescent cells cannot proceed through the cell cycle even in the presence of mitogenic factors. Here, we describe a set of cytofluorometric techniques for studying how chemical and/or physical stimuli alter the cell cycle in vitro, in both qualitative and quantitative terms. Taken together, these methods allow for the identification of bona fide cytostatic effects as well as for a refined characterization of cell cycle distributions, providing information on proliferation, DNA content as well as on the presence of cell cycle phase-specific markers. At the end of the chapter, a set of guidelines is offered to assist researchers that approach the study of the cell cycle with the interpretation of results.

SyStemCell: a database populated with multiple levels of experimental data from stem cell differentiation research

Elucidation of the mechanisms of stem cell differentiation is of great scientific interest. Increasing evidence suggests that stem cell differentiation involves changes at multiple levels of biological regulation, which together orchestrate the complex differentiation process; many related studies have been performed to investigate the various levels of regulation. The resulting valuable data, however, remain scattered. Most of the current stem cell-relevant databases focus on a single level of regulation (mRNA expression) from limited stem cell types; thus, a unifying resource would be of great value to compile the multiple levels of research data available. Here we present a database for this purpose, SyStemCell, deposited with multi-level experimental data from stem cell research. The database currently covers seven levels of stem cell differentiation-associated regulatory mechanisms, including DNA CpG 5-hydroxymethylcytosine/methylation, histone modification, transcript products, microRNA-based regulation, protein products, phosphorylation proteins and transcription factor regulation, all of which have been curated from 285 peer-reviewed publications selected from PubMed. The database contains 43,434 genes, recorded as 942,221 gene entries, for four organisms (Homo sapiens, Mus musculus, Rattus norvegicus, and Macaca mulatta) and various stem cell sources (e.g., embryonic stem cells, neural stem cells and induced pluripotent stem cells). Data in SyStemCell can be queried by Entrez gene ID, symbol, alias, or browsed by specific stem cell type at each level of genetic regulation. An online analysis tool is integrated to assist researchers to mine potential relationships among different regulations, and the potential usage of the database is demonstrated by three case studies. SyStemCell is the first database to bridge multi-level experimental information of stem cell studies, which can become an important reference resource for stem cell researchers. The database is available at http://lifecenter.sgst.cn/SyStemCell/.

B cell maintenance and function in aging

In this review we discuss the changes that occur in the B lymphocyte compartment of mice and humans as they progress to old age, focusing on recent advances in this important area of research. Primary areas considered include increased morbidity and mortality in the elderly following infection, and decreased responsiveness to vaccines that evoke primary humoral immune responses, as well as those that evoke responses by memory B cells generated following vaccination and natural infection earlier in life. We then consider some of the mechanisms that may underlie these observed declines in humoral immune function. This includes a discussion of alterations in B cell repertoire and subcompartment distribution, as well as defects in B lymphopoiesis, cell development and homeostasis that may contribute to these alterations, and ultimately to declining protective quality of antibodies produced in the elderly.