Cripto regulates hematopoietic stem cells as a hypoxic-niche-related factor through cell surface receptor GRP78

Hypoxic Preconditioning Promotes the Bioactivities of Mesenchymal Stem Cells via the HIF-1α-GRP78-Akt Axis

Mesenchymal stem cells (MSC) are ideal materials for stem cell-based therapy. As MSCs reside in hypoxic microenvironments (low oxygen tension of 1% to 7%), several studies have focused on the beneficial effects of hypoxic preconditioning on MSC survival; however, the mechanisms underlying such effects remain unclear. This study aimed to uncover the potential mechanism involving 78-kDa glucose-regulated protein (GRP78) to explain the enhanced MSC bioactivity and survival in hindlimb ischemia. Under hypoxia (2% O₂), the expression of GRP78 was significantly increased via hypoxia-inducible factor (HIF)-1α. Hypoxia-induced GRP78 promoted the proliferation and migration potential of MSCs through the HIF-1α-GRP78-Akt signal axis. In a murine hind-limb ischemia model, hypoxic preconditioning enhanced the survival and proliferation of transplanted MSCs through suppression of the cell death signal pathway and augmentation of angiogenic cytokine secretion. These effects were regulated by GRP78. Our findings indicate that hypoxic preconditioning promotes survival, proliferation, and angiogenic cytokine secretion of MSCs via the HIF-1α-GRP78-Akt signal pathway, suggesting that hypoxia-preconditioned MSCs might provide a therapeutic strategy for MSC-based therapies and that GRP78 represents a potential target for the development of functional MSCs.

Extrinsic regulation of hematopoietic stem cells in development, homeostasis and diseases

Lifelong generation of blood and immune cells depends on hematopoietic stem cells (HSCs). Their function is precisely regulated by complex molecular networks that integrate and respond to ever changing physiological demands of the body. Over the past several years, significant advances have been made in understanding the extrinsic regulation of HSCs during development and in homeostasis. Propelled by technical advances in the field, the cellular and molecular components of the microenvironment that support HSCs in vivo are emerging. In addition, the interaction of HSCs with their niches is appreciated as a critical contributor to the pathogenesis of a number of hematologic disorders. Here, we review these advances in detail and highlight the extrinsic regulation of HSCs in the context of development, homeostasis, and diseases. WIREs Dev Biol 2017, 6:e279. doi: 10.1002/wdev.279 For further resources related to this article, please visit the WIREs website.

Metabolic regulation of hematopoietic and leukemic stem/progenitor cells under homeostatic and stress conditions

Hematopoietic stem cells (HSCs) exhibit multilineage differentiation and self-renewal activities that maintain the entire hematopoietic system during an organism's lifetime. These abilities are sustained by intrinsic transcriptional programs and extrinsic cues from the microenvironment or niche. Recent studies using metabolomics technologies reveal that metabolic regulation plays an essential role in HSC maintenance. Metabolic pathways provide energy and building blocks for other factors functioning at steady state and in stress. Here we review recent advances in our understanding of metabolic regulation in HSCs relevant to cell cycle quiescence, symmetric/asymmetric division, and proliferation following stress and lineage commitment, and discuss the therapeutic potential of targeting metabolic factors or pathways to treat hematological malignancies.

Metabolic regulation of hematopoietic stem cell commitment and erythroid differentiation

PURPOSE OF REVIEW

Hematopoietic stem cell (HSC) renewal and lineage differentiation are finely tuned processes, regulated by cytokines, transcription factors and cell-cell contacts. However, recent studies have shown that fuel utilization also conditions HSC fate. This review focuses on our current understanding of the metabolic pathways that govern HSC self-renewal, commitment and specification to the erythroid lineage.

RECENT FINDINGS

HSCs reside in a hypoxic bone marrow niche that favors anaerobic glycolysis. Although this metabolic pathway is required for stem cell maintenance, other pathways also play critical roles. Fatty acid oxidation preserves HSC self-renewal by promoting asymmetric division, whereas oxidative phosphorylation induces lineage commitment. Committed erythroid progenitors support the production of 2.4 million erythrocytes per second in human adults via a synchronized regulation of iron, amino acid and glucose metabolism. Iron is indispensable for heme biosynthesis in erythroblasts; a process finely coordinated by at least two hormones, hepcidin and erythroferrone, together with multiple cell surface iron transporters. Furthermore, hemoglobin production is promoted by amino acid-induced mTOR signaling. Erythropoiesis is also strictly dependent on glutamine metabolism; under conditions where glutaminolysis is inhibited, erythropoietin-signaled progenitors are diverted to a myelomonocytic fate. Indeed, the utilization of both glutamine and glucose in de-novo nucleotide biosynthesis is a sine qua non for erythroid differentiation.

SUMMARY

Diverse metabolic networks function in concert with transcriptional, translational and epigenetic programs to regulate HSC potential and orient physiological as well as pathological erythroid differentiation.

A zebrafish xenograft model for studying human cancer stem cells in distant metastasis and therapy response

Lethal and incurable bone metastasis is one of the main causes of death in multiple types of cancer. A small subpopulation of cancer stem/progenitor-like cells (CSCs), also known as tumor-initiating cells from heterogenetic cancer is considered to mediate bone metastasis. Although over the past decades numerous studies have been performed in different types of cancer, it is still difficult to track small numbers of CSCs during the onset of metastasis. With use of noninvasive high-resolution imaging, transparent zebrafish embryos can be employed to dynamically visualize cancer progression and reciprocal interaction with stroma in a living organism. Recently we established a zebrafish CSC-xenograft model to visually and functionally analyze the role of CSCs and their interactions with the microenvironment at the onset of metastasis. Given the highly conserved human and zebrafish genome, transplanted human cancer cells are able to respond to zebrafish cytokines, modulate the zebrafish microenvironment, and take advantage of the zebrafish stroma during cancer progression. This chapter delineates the zebrafish CSC-xenograft model as a useful tool for both CSC biological study and anticancer drug screening.

New insights into the unfolded protein response in stem cells

The unfolded protein response (UPR) is an evolutionarily conserved adaptive mechanism to increase cell survival under endoplasmic reticulum (ER) stress conditions. The UPR is critical for maintaining cell homeostasis under physiological and pathological conditions. The vital functions of the UPR in development, metabolism and immunity have been demonstrated in several cell types. UPR dysfunction activates a variety of pathologies, including cancer, inflammation, neurodegenerative disease, metabolic disease and immune disease. Stem cells with the special ability to self-renew and differentiate into various somatic cells have been demonstrated to be present in multiple tissues. These cells are involved in development, tissue renewal and certain disease processes. Although the role and regulation of the UPR in somatic cells has been widely reported, the function of the UPR in stem cells is not fully known, and the roles and functions of the UPR are dependent on the stem cell type. Therefore, in this article, the potential significances of the UPR in stem cells, including embryonic stem cells, tissue stem cells, cancer stem cells and induced pluripotent cells, are comprehensively reviewed. This review aims to provide novel insights regarding the mechanisms associated with stem cell differentiation and cancer pathology.

Metabolic regulation of stem cell function in tissue homeostasis and organismal ageing

Many tissues and organ systems in metazoans have the intrinsic capacity to regenerate, which is driven and maintained largely by tissue-resident somatic stem cell populations. Ageing is accompanied by a deregulation of stem cell function and a decline in regenerative capacity, often resulting in degenerative diseases. The identification of strategies to maintain stem cell function and regulation is therefore a promising avenue to allay a wide range of age-related diseases. Studies in various organisms have revealed a central role for metabolic pathways in the regulation of stem cell function. Ageing is associated with extensive metabolic changes, and interventions that influence cellular metabolism have long been recognized as robust lifespan-extending measures. In this Review, we discuss recent advances in our understanding of the metabolic control of stem cell function, and how stem cell metabolism relates to homeostasis and ageing.

p38α Activates Purine Metabolism to Initiate Hematopoietic Stem/Progenitor Cell Cycling in Response to Stress

Hematopoietic stem cells (HSCs) maintain quiescence by activating specific metabolic pathways, including glycolysis. We do not yet have a clear understanding of how this metabolic activity changes during stress hematopoiesis, such as bone marrow transplantation. Here, we report a critical role for the p38MAPK family isoform p38α in initiating hematopoietic stem and progenitor cell (HSPC) proliferation during stress hematopoiesis in mice. We found that p38MAPK is immediately phosphorylated in HSPCs after a hematological stress, preceding increased HSPC cycling. Conditional deletion of p38α led to defective recovery from hematological stress and a delay in initiation of HSPC proliferation. Mechanistically, p38α signaling increases expression of inosine-5'-monophosphate dehydrogenase 2 in HSPCs, leading to altered levels of amino acids and purine-related metabolites and changes in cell-cycle progression in vitro and in vivo. Our studies have therefore uncovered a p38α-mediated pathway that alters HSPC metabolism to respond to stress and promote recovery.

Carbon monoxide improves neuronal differentiation and yield by increasing the functioning and number of mitochondria

The process of cell differentiation goes hand-in-hand with metabolic adaptations, which are needed to provide energy and new metabolites. Carbon monoxide (CO) is an endogenous cytoprotective molecule able to inhibit cell death and improve mitochondrial metabolism. Neuronal differentiation processes were studied using the NT2 cell line, which is derived from human testicular embryonic teratocarcinoma and differentiates into post-mitotic neurons upon retinoic acid treatment. CO-releasing molecule A1 (CORM-A1) was used do deliver CO into cell culture. CO treatment improved NT2 neuronal differentiation and yield, since there were more neurons and the total cell number increased following the differentiation process. CO supplementation enhanced the mitochondrial population in post-mitotic neurons derived from NT2 cells, as indicated by an increase in mitochondrial DNA. CO treatment during neuronal differentiation increased the extent of the classical metabolic change that occurs during neuronal differentiation, from glycolytic to more oxidative metabolism, by decreasing the ratio of lactate production and glucose consumption. The expression of pyruvate and lactate dehydrogenases was higher, indicating an augmented oxidative metabolism. Moreover, these findings were corroborated by an increased percentage of (13) C incorporation from [U-(13) C]glucose into the tricarboxylic acid cycle metabolites malate and citrate, and also glutamate and aspartate in CO-treated cells. Finally, under low levels of oxygen (5%), which enhances glycolytic metabolism, some of the enhancing effects of CO on mitochondria were not observed. In conclusion, our data show that CO improves neuronal and mitochondrial yield by stimulation of tricarboxylic acid cycle activity, and thus oxidative metabolism of NT2 cells during the process of neuronal differentiation. The process of cell differentiation is coupled with metabolic adaptations. Carbon monoxide (CO) is an endogenous cytoprotective gasotransmitter able to prevent cell death and improve mitochondrial metabolism. Herein CO supplementation improved neuronal differentiation yield, by enhancing mitochondrial population and promoting the classical metabolic change that occurs during neuronal differentiation, from glycolytic to oxidative metabolism.

Donor age and long-term culture do not negatively influence the stem potential of limbal fibroblast-like stem cells

BACKGROUND

In regenerative medicine the maintenance of stem cell properties is of crucial importance. Ageing is considered a cause of reduced stemness capability. The limbus is a stem niche of easy access and harbors two stem cell populations: epithelial stem cells and fibroblast-like stem cells. Our aim was to investigate whether donor age and/or long-term culture have any influence on stem cell marker expression and the profiles in the fibroblast-like stem cell population.

METHODS

Fibroblast-like stem cells were isolated and digested from 25 limbus samples of normal human corneo-scleral rings and long-term cultures were obtained. SSEA4 expression and sphere-forming capability were evaluated; cytofluorimetric assay was performed to detect the immunophenotypes HLA-DR, CD45, and CD34 and the principle stem cell markers ABCG2, OCT3/4, and NANOG. Molecular expression of the principal mesenchymal stem cell genes was investigated by real-time PCR. Two-dimensional gel electrophoresis and mass spectrometric sequencing were performed and a stable proteomic profile was identified. The proteins detected were explored by gene ontology and STRING analysis. The data were reported as means ± SD, compared by Student's unpaired t test and considering p < 0.05 as statistically significant.

RESULTS

The isolated cells did not display any hematopoietic surface marker (CD34 and CD45) and HLA-DR and they maintained these features in long-term culture. The expression of the stemness genes and the multilineage differentiation under in-vitro culture conditions proved to be well maintained. Proteomic analysis revealed a fibroblast-like stem cell profile of 164 proteins with higher expression levels. Eighty of these showed stable expression levels and were involved in maintenance of "the stem gene profile"; 84 were differentially expressed and were involved in structural activity.

CONCLUSIONS

The fibroblast-like limbal stem cells confirmed that they are a robust source of adult stem cells and that they have good plasticity, good proliferative capability, and long-term maintenance of stem cell properties, independently of donor age and long-term culture conditions. Our findings confirm that limbal fibroblast-like stem cells are highly promising for application in regenerative medicine and that in-vitro culture steps do not influence their stem cell properties. Moreover, the proteomic data enrich our knowledge of fibroblast-like stem cells.

Hypoxia: Signaling the Metastatic Cascade

Hypoxia is a potent microenvironmental factor that promotes tumor metastasis. Recent studies have revealed mechanisms by which hypoxia and activation of hypoxia inducible factor (HIF)-dependent signaling promotes metastasis through the regulation of metabolic reprogramming, the stem cell phenotype, invasion, angiogenesis, immune suppression, the premetastatic niche, intravasation and/or extravasation, and resistance to apoptosis. These discoveries suggest novel paradigms in tumor metastasis and identify new opportunities for therapeutic intervention in the prevention and treatment of metastatic disease. Here, we review the impact of hypoxia and hypoxic signaling pathways in tumor and stromal cells on each step of the metastatic cascade.

Disruption of Glut1 in Hematopoietic Stem Cells Prevents Myelopoiesis and Enhanced Glucose Flux in Atheromatous Plaques of ApoE(-/-) Mice

RATIONALE

Inflamed atherosclerotic plaques can be visualized by noninvasive positron emission and computed tomographic imaging with (18)F-fluorodeoxyglucose, a glucose analog, but the underlying mechanisms are poorly understood.

OBJECTIVE

Here, we directly investigated the role of Glut1-mediated glucose uptake in apolipoprotein E-deficient (ApoE(-/-)) mouse model of atherosclerosis.

METHODS AND RESULTS

We first showed that the enhanced glycolytic flux in atheromatous plaques of ApoE(-/-) mice was associated with the enhanced metabolic activity of hematopoietic stem and multipotential progenitor cells and higher Glut1 expression in these cells. Mechanistically, the regulation of Glut1 in ApoE(-/-) hematopoietic stem and multipotential progenitor cells was not because of alterations in hypoxia-inducible factor 1α signaling or the oxygenation status of the bone marrow but was the consequence of the activation of the common β subunit of the granulocyte-macrophage colony-stimulating factor/interleukin-3 receptor driving glycolytic substrate utilization by mitochondria. By transplanting bone marrow from WT, Glut1(+/-), ApoE(-/-), and ApoE(-/-)Glut1(+/-) mice into hypercholesterolemic ApoE-deficient mice, we found that Glut1 deficiency reversed ApoE(-/-) hematopoietic stem and multipotential progenitor cell proliferation and expansion, which prevented the myelopoiesis and accelerated atherosclerosis of ApoE(-/-) mice transplanted with ApoE(-/-) bone marrow and resulted in reduced glucose uptake in the spleen and aortic arch of these mice.

CONCLUSIONS

We identified that Glut1 connects the enhanced glucose uptake in atheromatous plaques of ApoE(-/-) mice with their myelopoiesis through regulation of hematopoietic stem and multipotential progenitor cell maintenance and myelomonocytic fate and suggests Glut1 as potential drug target for atherosclerosis.

Energy metabolism in the acquisition and maintenance of stemness

Energy metabolism is traditionally considered a reactive homeostatic system addressing stage-specific cellular energy needs. There is however growing appreciation of metabolic pathways in the active control of vital cell functions. Case in point, the stem cell lifecycle--from maintenance and acquisition of stemness to lineage commitment and specification--is increasingly recognized as a metabolism-dependent process. Indeed, metabolic reprogramming is an early contributor to the orchestrated departure from or reacquisition of stemness. Recent advances in metabolomics have helped decipher the identity and dynamics of metabolic fluxes implicated in fueling cell fate choices by regulating the epigenetic and transcriptional identity of a cell. Metabolic cues, internal and/or external to the stem cell niche, facilitate progenitor pool restitution, long-term tissue renewal or ensure adoption of cytoprotective behavior. Convergence of energy metabolism with stem cell fate regulation opens a new avenue in understanding primordial developmental biology principles with future applications in regenerative medicine practice.

The role of the endoplasmic reticulum stress in stemness, pluripotency and development

The molecular machinery of endoplasmic reticulum (ER) integrates various intracellular and extracellular cues to maintain homeostasis in diverse physiological or pathological scenarios. ER stress and the unfolded protein response (UPR) have been found to mediate molecular and biochemical mechanisms that affect cell proliferation, differentiation, and apoptosis. Although a number of reviews on the ER stress response have been published, comprehensive reviews that broadly summarize ER physiology in the context of pluripotency, embryonic development, and tissue homeostasis are lacking. This review complements the current ER literature and provides a summary of the important findings on the role of the ER stress and UPR in embryonic development and pluripotent stem cells.

Effect of hypoxia-inducible factors in normal and leukemic stem cell regulation and their potential therapeutic impact

INTRODUCTION

Hypoxia inducible factors (HIF-1α and HIF-2α) are the main mediators of hypoxic responses that operate in both normal and pathological conditions. Recent evidence indicates that HIF-1α and HIF-2α could have overlapping, unique and even sometimes opposing activities in both normal physiology and disease. Despite an increase in our understanding of the different pathways regulated by HIF-1α and HIF-2α, the role played by each factor in HSC maintenance and leukemogenesis is still controversial.

AREAS COVERED

This review summarizes our current understanding of HIF-1α and HIF-2α activities and discusses the implications and challenges of using HIF inhibitors therapeutically in blood malignancies.

EXPERT OPINION

As HIF inhibitors are currently under clinical evaluation in different cancers, including hematological malignancies, a more thorough understanding of the unique roles performed by HIF-1α and HIF-2α in human neoplasia is warranted.

Hypoxia regulates the hematopoietic stem cell niche

Bone marrow, the site of hematopoiesis throughout adulthood, is a physiologically hypoxic organ. Thus, various biological oxygen sensors and their signaling cascades play a pivotal role in hematopoietic systems in the bone marrow under both physiologic and pathologic conditions. Hypoxia-inducible factors (HIFs) are hypoxic stress sensor proteins that are stabilized under homeostatic or stress-induced hypoxia. In the hypoxic bone marrow, HIFs play crucial roles in hematopoietic stem cells (HSCs) and in the cells of the HSC niche. The signals downstream of the HIFs maintain HSC quiescence, survival, and metabolic homeostasis through both cell-autonomous and non-cell-autonomous mechanisms. Leukemic stem cells (LSCs) hijack these delicate hypoxia-sensing mechanisms to sustain their self-renewal potential, promoting disease progression and drug resistance even under normoxic conditions. This review focuses on HIF-mediated oxygen-sensing mechanisms of adult HSCs and LSCs and their niche cells in the hypoxic bone marrow.

Cripto-1 ablation disrupts alveolar development in the mouse mammary gland through a progesterone receptor-mediated pathway

Cripto-1, a member of the epidermal growth factor-Cripto-1/FRL-1/Cryptic family, is critical for early embryonic development. Together with its ligand Nodal, Cripto-1 has been found to be associated with the undifferentiated status of mouse and human embryonic stem cells. Several studies have clearly shown that Cripto-1 is involved in regulating branching morphogenesis and epithelial-mesenchymal transition of the mammary gland both in vitro and in vivo and together with the cofactor GRP78 is critical for the maintenance of mammary stem cells ex vivo. Our previous studies showed that mammary-specific overexpression of human Cripto-1 exhibited dramatic morphological alterations in nulliparous mice mammary glands. The present study shows a novel mechanism for Cripto-1 regulation of mammary gland development through direct effects on progesterone receptor expression and pathways regulated by progesterone in the mammary gland. We demonstrate a strict temporal regulation of mouse Cripto-1 (mCripto-1) expression that occurs during mammary gland development and a stage-specific function of mCripto-1 signaling during mammary gland development. Our data suggest that Cripto-1, like the progesterone receptor, is not required for the initial ductal growth but is essential for subsequent side branching and alveologenesis during the initial stages of pregnancy. Dissection of the mechanism by which this occurs indicates that mCripto-1 activates receptor activator NF-κB/receptor activator NF-κB ligand, and NF-κB signaling pathways.

Regulation of mitochondrial function and cellular energy metabolism by protein kinase C-λ/ι: a novel mode of balancing pluripotency

Pluripotent stem cells (PSCs) contain functionally immature mitochondria and rely upon high rates of glycolysis for their energy requirements. Thus, altered mitochondrial function and promotion of aerobic glycolysis are key to maintain and induce pluripotency. However, signaling mechanisms that regulate mitochondrial function and reprogram metabolic preferences in self-renewing versus differentiated PSC populations are poorly understood. Here, using murine embryonic stem cells (ESCs) as a model system, we demonstrate that atypical protein kinase C isoform, PKC lambda/iota (PKCλ/ι), is a key regulator of mitochondrial function in ESCs. Depletion of PKCλ/ι in ESCs maintains their pluripotent state as evident from germline offsprings. Interestingly, loss of PKCλ/ι in ESCs leads to impairment in mitochondrial maturation, organization, and a metabolic shift toward glycolysis under differentiating condition. Our mechanistic analyses indicate that a PKCλ/ι-hypoxia-inducible factor 1α-PGC1α axis regulates mitochondrial respiration and balances pluripotency in ESCs. We propose that PKCλ/ι could be a crucial regulator of mitochondrial function and energy metabolism in stem cells and other cellular contexts.

Hypoxic niche-mediated regeneration of hematopoiesis in the engraftment window is dominantly affected by oxygen tension in the milieu

The bone marrow (BM) microenvironment or the hematopoietic stem cell (HSC) niche is normally hypoxic, which maintains HSC quiescence. Paradoxically, transplanted HSCs rapidly proliferate in this niche. Pretransplant myelosuppression results in a substantial rise in oxygen levels in the marrow microenvironment due to reduced cellularity and consequent low oxygen consumption. Therefore, it may be construed that the rapid proliferation of the engrafted HSCs in the BM niche is facilitated by the transiently elevated oxygen tension in this milieu during the “engraftment window.” To determine whether oxygen tension dominantly affects the regeneration of hematopoiesis in the BM niche, we created an “oxygen-independent hypoxic niche” by treating BM-derived mesenchymal stromal cells (BMSCs) with a hypoxia-mimetic compound, cobalt chloride (CoCl₂) and cocultured them with BM-derived HSC-enriched cells under normoxic conditions (HSCs; CoCl₂-cocultures). Cocultures with untreated BMSCs incubated under normoxia (control- cocultures) or hypoxia (1% O₂; hypoxic-cocultures) were used as comparators. Biochemical analyses showed that though, both CoCl₂ and hypoxia evoked comparable signals in the BMSCs, the regeneration of hematopoiesis in their respective cocultures was radically different. The CoCl₂-BMSCs supported robust hematopoiesis, while the hypoxic-BMSCs exerted strong inhibition. The hematopoiesis-supportive ability of CoCl₂-BMSCs was abrogated if the CoCl₂-cocultures were incubated under hypoxia, demonstrating that the prevalent oxygen tension in the milieu dominantly affects the outcome of the HSC-BM niche interactions. Our data suggest that pharmacologically delaying the reestablishment of hypoxia in the BM may boost post-transplant regeneration of hematopoiesis.

Mitochondrial metabolism in hematopoietic stem cells requires functional FOXO3

Hematopoietic stem cells (HSC) are primarily dormant but have the potential to become highly active on demand to reconstitute blood. This requires a swift metabolic switch from glycolysis to mitochondrial oxidative phosphorylation. Maintenance of low levels of reactive oxygen species (ROS), a by-product of mitochondrial metabolism, is also necessary for sustaining HSC dormancy. Little is known about mechanisms that integrate energy metabolism with hematopoietic stem cell homeostasis. Here, we identify the transcription factor FOXO3 as a new regulator of metabolic adaptation of HSC. ROS are elevated in Foxo3(-/-) HSC that are defective in their activity. We show that Foxo3(-/-) HSC are impaired in mitochondrial metabolism independent of ROS levels. These defects are associated with altered expression of mitochondrial/metabolic genes in Foxo3(-/-) hematopoietic stem and progenitor cells (HSPC). We further show that defects of Foxo3(-/-) HSC long-term repopulation activity are independent of ROS or mTOR signaling. Our results point to FOXO3 as a potential node that couples mitochondrial metabolism with HSC homeostasis. These findings have critical implications for mechanisms that promote malignant transformation and aging of blood stem and progenitor cells.

The role of novel and known extracellular matrix and adhesion molecules in the homeostatic and regenerative bone marrow microenvironment

Maintenance of haematopoietic stem cells and differentiation of committed progenitors occurs in highly specialized niches. The interactions of haematopoietic stem and progenitor cells (HSPCs) with cells, growth factors and extracellular matrix (ECM) components of the bone marrow (BM) microenvironment control homeostasis of HSPCs. We only start to understand the complexity of the haematopoietic niche(s) that comprises endosteal, arterial, sinusoidal, mesenchymal and neuronal components. These distinct niches produce a broad range of soluble factors and adhesion molecules that modulate HSPC fate during normal hematopoiesis and BM regeneration. Adhesive interactions between HSPCs and the microenvironment will influence their localization and differentiation potential. In this review we highlight the current understanding of the functional role of ECM- and adhesion (regulating) molecules in the haematopoietic niche during homeostatic and regenerative hematopoiesis. This knowledge may lead to the improvement of current cellular therapies and more efficient development of future cellular products.

Role of the unfolded protein response, GRP78 and GRP94 in organ homeostasis

The endoplasmic reticulum (ER) is a cellular organelle where secretory and membrane proteins, as well as lipids, are synthesized and modified. When cells are subjected to ER stress, an adaptive mechanism referred to as the Unfolded Protein Response (UPR) is triggered to allow the cells to restore homeostasis. Evidence has accumulated that the UPR pathways provide specialized and unique roles in diverse development and metabolic processes. The glucose regulated proteins (GRPs) are traditionally regarded as ER proteins with chaperone and calcium binding properties. The GRPs are constitutively expressed at basal levels in all organs, and as stress-inducible ER chaperones, they are major players in protein folding, assembly and degradation. This conventional concept is augmented by recent discoveries that GRPs can be actively translocated to other cellular locations such as the cell surface, where they assume novel functions that regulate signaling, proliferation, apoptosis and immunity. Recent construction and characterization of mouse models where the gene encoding for the UPR components and the GRPs is genetically altered provide new insights on the physiological contribution of these proteins in vivo. This review highlights recent progress towards the understanding of the role of the UPR and two major GRPs (GRP78 and GRP94) in regulating homeostasis of organs arising from the endoderm, mesoderm and ectoderm. GRP78 and GRP94 exhibit shared and unique functions, and in specific organs their depletion elicits adaptive responses with physiological consequences.

CD44, Hyaluronan, the Hematopoietic Stem Cell, and Leukemia-Initiating Cells

CD44 is an adhesion molecule that varies in size due to glycosylation and insertion of so-called variant exon products. The CD44 standard isoform (CD44s) is highly expressed in many cells and most abundantly in cells of the hematopoietic system, whereas expression of CD44 variant isoforms (CD44v) is more restricted. CD44s and CD44v are known as stem cell markers, first described for hematopoietic stem cells and later on confirmed for cancer- and leukemia-initiating cells. Importantly, both abundantly expressed CD44s as well as CD44v actively contribute to the maintenance of stem cell features, like generating and embedding in a niche, homing into the niche, maintenance of quiescence, and relative apoptosis resistance. This is surprising, as CD44 is not a master stem cell gene. I here will discuss that the functional contribution of CD44 relies on its particular communication skills with neighboring molecules, adjacent cells and, last not least, the surrounding matrix. In fact, it is the interaction of the hyaluronan receptor CD44 with its prime ligand, which strongly assists stem cells to fulfill their special and demanding tasks. Recent fundamental progress in support of this “old” hypothesis, which may soon pave the way for most promising new therapeutics, is presented for both hematopoietic stem cell and leukemia-initiating cell. The contribution of CD44 to the generation of a stem cell niche, to homing of stem cells in their niche, to stem cell quiescence and apoptosis resistance will be in focus.

Snail-mediated Cripto-1 repression regulates the cell cycle and epithelial-mesenchymal transition-related gene expression

Transcription factor Snail mediates epithelial to mesenchymal transitions (EMT) by coordinate repression of epithelial markers, facilitating mass cell movement during germ layer formation. Aberrant reprogramming in its signaling pathways causes metastatic cancer. Snail's involvement in "fate-changing" decisions is however not understood. Cripto-1 shares a common temporal expression pattern with Snail during development. While Cripto-1 is required for mammary morphogenesis and hematopoietic stem cell renewal, its unregulated expression causes metastatic cancers. Therefore, we suspected that Snail regulates the expression of Cripto-1 controlling decisions such as motility, transformation and differentiation. We demonstrate that Snail represses Cripto-1 gene by direct transcriptional interaction and propose a novel mechanism by which it co-ordinately regulates cell fate decisions during development and could be causal of cancers.

Reduced levels of Hspa9 attenuate Stat5 activation in mouse B cells

HSPA9 is located on chromosome 5q31.2 in humans, a region that is commonly deleted in patients with myeloid malignancies [del(5q)], including myelodysplastic syndrome (MDS). HSPA9 expression is reduced by 50% in patients with del(5q)-associated MDS, consistent with haploinsufficient levels. Zebrafish mutants and knockdown studies in human and mouse cells have implicated a role for HSPA9 in hematopoiesis. To comprehensively evaluate the effects of Hspa9 haploinsufficiency on hematopoiesis, we generated an Hspa9 knockout mouse model. Although homozygous knockout of Hspa9 is embryonically lethal, mice with heterozygous deletion of Hspa9 (Hspa9(+/-)) are viable and have a 50% reduction in Hspa9 expression. Hspa9(+/-) mice have normal basal hematopoiesis and do not develop MDS. However, Hspa9(+/-) mice have a cell-intrinsic reduction in bone marrow colony-forming unit-PreB colony formation without alterations in the number of B-cell progenitors in vivo, consistent with a functional defect in Hspa9(+/-) B-cell progenitors. We further reduced Hspa9 expression (<50%) using RNA interference and observed reduced B-cell progenitors in vivo, indicating that appropriate levels (≥50%) of Hspa9 are required for normal B lymphopoiesis in vivo. Knockdown of Hspa9 in an interleukin 7 (IL-7)-dependent mouse B-cell line reduced signal transducer and activator of transcription 5 (Stat5) phosphorylation following IL-7 receptor stimulation, supporting a role for Hspa9 in Stat5 signaling in B cells. Collectively, these data imply a role for Hspa9 in B lymphopoiesis and Stat5 activation downstream of IL-7 signaling.

Genetic variants modulating CRIPTO serum levels identified by genome-wide association study in Cilento isolates

Cripto, the founding member of the EGF-CFC genes, plays an essential role in embryo development and is involved in cancer progression. Cripto is a GPI-anchored protein that can interact with various components of multiple signaling pathways, such as TGF-β, Wnt and MAPK, driving different processes, among them epithelial-mesenchymal transition, cell proliferation, and stem cell renewal. Cripto protein can also be cleaved and released outside the cell in a soluble and still active form. Cripto is not significantly expressed in adult somatic tissues and its re-expression has been observed associated to pathological conditions, mainly cancer. Accordingly, CRIPTO has been detected at very low levels in the plasma of healthy volunteers, whereas its levels are significantly higher in patients with breast, colon or glioblastoma tumors. These data suggest that CRIPTO levels in human plasma or serum may have clinical significance. However, very little is known about the variability of serum levels of CRIPTO at a population level and the genetic contribution underlying this variability remains unknown. Here, we report the first genome-wide association study of CRIPTO serum levels in isolated populations (n = 1,054) from Cilento area in South Italy. The most associated SNPs (p-value<5*10-8) were all located on chromosome 3p22.1-3p21.3, in the CRIPTO gene region. Overall six CRIPTO associated loci were replicated in an independent sample (n = 535). Pathway analysis identified a main network including two other genes, besides CRIPTO, in the associated regions, involved in cell movement and proliferation. The replicated loci explain more than 87% of the CRIPTO variance, with 85% explained by the most associated SNP. Moreover, the functional analysis of the main associated locus identified a causal variant in the 5’UTR of CRIPTO gene which is able to strongly modulate CRIPTO expression through an AP-1-mediate transcriptional regulation.

Cripto gene has a fundamental role in embryo development and is also involved in cancer. The protein is bound to the cell membrane through an anchor, that can be cleaved, causing the secretion of the protein, in a still active form. In the adult, CRIPTO is detected at very low levels in normal tissues and in the blood, while its increase in both tissues and blood is associated to pathological conditions, mainly cancer. As other GPI linked proteins such as the carcinoembryonic antigen (CEA), one of the most used tumor markers, CRIPTO is able to reach the bloodstream. Therefore, CRIPTO represents a new promising biomarker and potential therapeutic target, and blood CRIPTO levels might be associated to clinical features. Here we examined the variability of blood CRIPTO levels at a population level (population isolates from the Cilento region in South Italy) and we investigated the genetic architecture underlying this variability. We reported the association of common genetic variants with the levels of CRIPTO protein in the blood and we identified a main locus on chromosome 3 and additional five associated loci. Moreover, through functional analyses, we were able to uncover the mechanism responsible for the variation in CRIPTO levels, which is a regulation mediated by the transcriptional factor AP-1.

Related-to-receptor tyrosine kinase receptor regulates hematopoietic stem and progenitor sensitivity to myelosuppressive injury in mice

Maintaining a careful balance between quiescence and proliferation of hematopoietic stem and progenitor cells (HSPCs) is necessary for lifelong blood formation. Previously, we demonstrated that the Wnt5a ligand inhibits HSPC proliferation through a functional interaction with a noncanonical Wnt ligand receptor termed 'related-to-receptor tyrosine kinase' (Ryk). Expression of Ryk on HSPCs in vivo is associated with a lower rate of proliferation, and, following treatment with fluorouracil (5-FU), the percentage of Ryk(+/high) HSPCs increased and the percentage of Ryk(-/low) HSPCs decreased. Based on these data, we hypothesized that one function of the Ryk receptor is to protect HSPCs from the effects of myeloablative agents. We found that Ryk expression on HSPCs is associated with lower rates of apoptosis following 5-FU and radiation. Transient inhibition of Ryk signaling in vivo resulted in increased hematopoietic-stem-cell proliferation and decreased hematopoietic-stem-cell function in bone marrow transplant assays. Furthermore, inhibition of Ryk signaling sensitized HSPCs to 5-FU treatment in association with increased levels of reactive oxygen species. Together, these results demonstrated an association between Ryk expression and survival of HSPCs following suppressive injury.

Induction of autophagy supports the bioenergetic demands of quiescent muscle stem cell activation

The exit of a stem cell out of quiescence into an activated state is characterized by major metabolic changes associated with increased biosynthesis of proteins and macromolecules. The regulation of this transition is poorly understood. Using muscle stem cells, or satellite cells (SCs), we found that autophagy, which catabolizes intracellular contents to maintain proteostasis and to produce energy during nutrient deprivation, was induced during SC activation. Inhibition of autophagy suppressed the increase in ATP levels and delayed SC activation, both of which could be partially rescued by exogenous pyruvate as an energy source, suggesting that autophagy may provide nutrients necessary to meet bioenergetic demands during this critical transition from quiescence to activation. We found that SIRT1, a known nutrient sensor, regulates autophagic flux in SC progeny. A deficiency of SIRT1 led to a delay in SC activation that could also be partially rescued by exogenous pyruvate. These studies suggest that autophagy, regulated by SIRT1, may play an important role during SC activation to meet the high bioenergetic demands of the activation process.

Hematopoietic stem cell niche maintenance during homeostasis and regeneration

The bone marrow niche has mystified scientists for many years, leading to widespread investigation to shed light into its molecular and cellular composition. Considerable efforts have been devoted toward uncovering the regulatory mechanisms of hematopoietic stem cell (HSC) niche maintenance. Recent advances in imaging and genetic manipulation of mouse models have allowed the identification of distinct vascular niches that have been shown to orchestrate the balance between quiescence, proliferation and regeneration of the bone marrow after injury. Here we highlight the recently discovered intrinsic mechanisms, microenvironmental interactions and communication with surrounding cells involved in HSC regulation, during homeostasis and in regeneration after injury and discuss their implications for regenerative therapy.

The multifaceted role of the embryonic gene Cripto-1 in cancer, stem cells and epithelial-mesenchymal transition

Cripto-1 (CR-1)/Teratocarcinoma-derived growth factor1 (TDGF-1) is a cell surface glycosylphosphatidylinositol (GPI)-linked glycoprotein that can function either in cis (autocrine) or in trans (paracrine). The cell membrane cis form is found in lipid rafts and endosomes while the trans acting form lacking the GPI anchor is soluble. As a member of the epidermal growth factor (EGF)/Cripto-1-FRL-1-Cryptic (CFC) family, CR-1 functions as an obligatory co-receptor for the transforming growth factor-β (TGF-β) family members, Nodal and growth and differentiation factors 1 and 3 (GDF1/3) by activating Alk4/Alk7 signaling pathways that involve Smads 2, 3 and 4. In addition, CR-1 can activate non-Smad-dependent signaling elements such as PI3K, Akt and MAPK. Both of these pathways depend upon the 78kDa glucose regulated protein (GRP78). Finally, CR-1 can facilitate signaling through the canonical Wnt/β-catenin and Notch/Cbf-1 pathways by functioning as a chaperone protein for LRP5/6 and Notch, respectively. CR-1 is essential for early embryonic development and maintains embryonic stem cell pluripotentiality. CR-1 performs an essential role in the etiology and progression of several types of human tumors where it is expressed in a population of cancer stem cells (CSCs) and facilitates epithelial-mesenchymal transition (EMT). In this context, CR-1 can significantly enhance tumor cell migration, invasion and angiogenesis. Collectively, these facts suggest that CR-1 may be an attractive target in the diagnosis, prognosis and therapy of several types of human cancer.

Glucose and glutamine metabolism regulate human hematopoietic stem cell lineage specification

The metabolic state of quiescent hematopoietic stem cells (HSCs) is an important regulator of self-renewal, but it is unclear whether or how metabolic parameters contribute to HSC lineage specification and commitment. Here, we show that the commitment of human and murine HSCs to the erythroid lineage is dependent upon glutamine metabolism. HSCs require the ASCT2 glutamine transporter and active glutamine metabolism for erythroid specification. Blocking this pathway diverts EPO-stimulated HSCs to differentiate into myelomonocytic fates, altering in vivo HSC responses and erythroid commitment under stress conditions such as hemolytic anemia. Mechanistically, erythroid specification of HSCs requires glutamine-dependent de novo nucleotide biosynthesis. Exogenous nucleosides rescue erythroid commitment of human HSCs under conditions of limited glutamine catabolism, and glucose-stimulated nucleotide biosynthesis further enhances erythroid specification. Thus, the availability of glutamine and glucose to provide fuel for nucleotide biosynthesis regulates HSC lineage commitment under conditions of metabolic stress.

Regulation of high-density lipoprotein on hematopoietic stem/progenitor cells in atherosclerosis requires scavenger receptor type BI expression

OBJECTIVE

Recently, we demonstrated that scavenger receptor type BI (SR-BI), a high-density lipoprotein (HDL) receptor, was expressed on murine hematopoietic stem/progenitor cells (HSPC) and infusion of reconstituted HDL and purified human apolipoprotein A-I (apoA-I) suppressed HSPC proliferation. We hypothesized that SR-B1 expression is required for the observed antiproliferative effects of HDL on HSPC.

APPROACH AND RESULTS

SR-BI-deficient (SR-BI(-/-)) mice and wild-type controls were fed on chow or high-fat diet (HFD) for 8 to 10 weeks. Under chow diet, a significant increase in Lin(-) Sca1(+) cKit(+) cells (LSK cells, so-called HSPC) was found in the bone marrow of SR-BI(-/-) mice when compared with wild-type mice. HFD induced a further expansion of CD150(+)CD48(-) LSK cells (HSC), HSPC, and granulocyte monocyte progenitors in SR-BI(-/-) mice. Injection of reactive oxygen species inhibitor N-acetylcysteine attenuated HFD-induced HSPC expansion, leukocytosis, and atherosclerosis in SR-BI(-/-) mice. ApoA-I infusion inhibited HSPC cell proliferation, Akt phosphorylation and reactive oxygen species production in HSPC and plaque progression in low-density lipoprotein receptor knockout (LDLr(-/-)) apoA-I(-/-) mice on HFD but had no effect on SR-BI(-/-) mice on HFD. Transplantation of SR-BI(-/-) bone marrow cells into irradiated LDLr(-/-) recipients resulted in enhanced white blood cells reconstitution, inflammatory cell production, and plaque development. In patients with coronary heart disease, HDL levels were negatively correlated with white blood cells count and HSPC frequency in the peripheral blood. By flow cytometry, SR-BI expression was detected on human HSPC.

CONCLUSIONS

SR-BI plays a critical role in the HDL-mediated regulation HSPC proliferation and differentiation, which is associated with atherosclerosis progression.

In vivo hypobaric hypoxia performed during the remodeling process accelerates bone healing in mice

We investigated the effects of respiratory hypobaric hypoxia on femoral bone-defect repair in mice because hypoxia is believed to influence both mesenchymal stromal cell (MSC) and hematopoietic stem cell mobilization, a process involved in the bone-healing mechanism. To mimic conditions of non-weight-bearing limb immobilization in patients suffering from bone trauma, our hypoxic mouse model was further subjected to hind-limb unloading. A hole was drilled in the right femur of adult male C57/BL6J mice. Four days after surgery, mice were subjected to hind-limb unloading for 1 week. Seven days after surgery, mice were either housed for 4 days in a hypobaric room (FiO2 at 10%) or kept under normoxic conditions. Unsuspended control mice were housed in either hypobaric or normoxic conditions. Animals were sacrificed on postsurgery day 11 to allow for collection of both contralateral and lesioned femurs, blood, and spleen. As assessed by microtomography, delayed hypoxia enhanced bone-healing efficiency by increasing the closing of the cortical defect and the newly synthesized bone volume in the cavity by +55% and +35%, respectively. Proteome analysis and histomorphometric data suggested that bone-repair improvement likely results from the acceleration of the natural bone-healing process rather than from extended mobilization of MSC-derived osteoprogenitors. Hind-limb unloading had hardly any effect beyond delayed hypoxia-enhanced bone-healing efficiency.

Metabolic requirements for the maintenance of self-renewing stem cells

A distinctive feature of stem cells is their capacity to self-renew to maintain pluripotency. Studies of genetically-engineered mouse models and recent advances in metabolomic analysis, particularly in haematopoietic stem cells, have deepened our understanding of the contribution made by metabolic cues to the regulation of stem cell self-renewal. Many types of stem cells heavily rely on anaerobic glycolysis, and stem cell function is also regulated by bioenergetic signalling, the AKT-mTOR pathway, Gln metabolism and fatty acid metabolism. As maintenance of a stem cell pool requires a finely-tuned balance between self-renewal and differentiation, investigations into the molecular mechanisms and metabolic pathways underlying these decisions hold great therapeutic promise.

Bone marrow endosteal mesenchymal progenitors depend on HIF factors for maintenance and regulation of hematopoiesis

Maintenance and differentiation of hematopoietic stem cells (HSCs) is regulated through cell-autonomous and non-cell-autonomous mechanisms within specialized bone marrow microenvironments. Recent evidence demonstrates that signaling by HIF-1α contributes to cell-autonomous regulation of HSC maintenance. By investigating the role of HIF factors in bone marrow mesenchymal progenitors, we found that murine endosteal mesenchymal progenitors express high levels of HIF-1α and HIF-2α and proliferate preferentially in hypoxic conditions ex vivo. Inactivation of either HIF-1α or HIF-2α dramatically affects their phenotype, propagation, and differentiation. Also, downregulation of HIF factors provokes an increase in interferon-responsive genes and triggers expansion and differentiation of hematopoietic progenitors by a STAT1-mediated mechanism. Interestingly, in conditions of demand-driven hematopoiesis HIF factors are specifically downregulated in mesenchymal progenitors in vivo. In conclusion, our findings indicate that HIF factors also regulate hematopoiesis non-cell-autonomously by preventing activation of a latent program in mesenchymal progenitors that promotes hematopoiesis.

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Endosteal mesenchymal progenitors exhibit a hypoxic state

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HIF factors cell autonomously regulate the biology of mesenchymal progenitors

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HIF factors repress interferon-responsive programs in mesenchymal progenitors

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HIF factors regulate hematopoiesis through non-cell-autonomous mechanisms

Surviving change: the metabolic journey of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are a rare population of somatic stem cells that maintain blood production and are uniquely wired to adapt to diverse cellular fates during the lifetime of an organism. Recent studies have highlighted a central role for metabolic plasticity in facilitating cell fate transitions and in preserving HSC functionality and survival. This review summarizes our current understanding of the metabolic programs associated with HSC quiescence, self-renewal, and lineage commitment, and highlights the mechanistic underpinnings of these changing bioenergetics programs. It also discusses the therapeutic potential of targeting metabolic drivers in the context of blood malignancies.

Hypoxia and metabolic properties of hematopoietic stem cells

SIGNIFICANCE

The effect of redox signaling on hematopoietic stem cell (HSC) function is not clearly understood.

RECENT ADVANCES

A growing body of evidence suggests that adult HSCs reside in the hypoxic bone marrow microenvironment or niche during homeostasis. It was recently shown that primitive HSCs in the bone marrow prefer to utilize anaerobic glycolysis to meet their energy demands and have lower rates of oxygen consumption and lower ATP levels. Hypoxia-inducible factor-α (Hif-1α) is a master regulator of cellular metabolism. With hundreds of downstream target genes and crosstalk with other signaling pathways, it regulates various aspects of metabolism from the oxidative stress response to glycolysis and mitochondrial respiration. Hif-1α is highly expressed in HSCs, where it regulates their function and metabolic phenotype. However, the regulation of Hif-1α in HSCs is not entirely understood. The homeobox transcription factor myeloid ecotropic viral integration site 1 (Meis1) is expressed in the most primitive HSCs populations, and it is required for primitive hematopoiesis. Recent reports suggest that Meis1 is required for normal adult HSC function by regulating the metabolism and redox state of HSCs transcriptionally through Hif-1α and Hif-2α.

CRITICAL ISSUES

Given the profound effect of redox status on HSC function, it is critical to fully characterize the intrinsic, and microenvironment-related mechanisms of metabolic and redox regulation in HSCs.

FUTURE DIRECTIONS

Future studies will be needed to elucidate the link between HSC metabolism and HSC fates, including quiescence, self-renewal, differentiation, apoptosis, and migration.

CRIPTO/GRP78 signaling maintains fetal and adult mammary stem cells ex vivo

Little is known about the extracellular signaling factors that govern mammary stem cell behavior. Here, we identify CRIPTO and its cell-surface receptor GRP78 as regulators of stem cell behavior in isolated fetal and adult mammary epithelial cells. We develop a CRIPTO antagonist that promotes differentiation and reduces self-renewal of mammary stem cell-enriched populations cultured ex vivo. By contrast, CRIPTO treatment maintains the stem cell phenotype in these cultures and yields colonies with enhanced mammary gland reconstitution capacity. Surface expression of GRP78 marks CRIPTO-responsive, stem cell-enriched fetal and adult mammary epithelial cells, and deletion of GRP78 from adult mammary epithelial cells blocks their mammary gland reconstitution potential. Together, these findings identify the CRIPTO/GRP78 pathway as a developmentally conserved regulator of fetal and adult mammary stem cell behavior ex vivo, with implications for the stem-like cells found in many cancers.

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CRIPTO/GRP78 signaling activates PI3K/AKT in fetal mammary epithelial cells ex vivo

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Cell-surface GRP78 marks a CRIPTO-responsive adult mammary stem cell population

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An antagonist, ALK4^L75A-Fc, blocks soluble CRIPTO growth-factor-like activity

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CRIPTO promotes and ALK4^L75A-Fc inhibits mammary stem cell maintenance ex vivo

To breathe or not to breathe: the haematopoietic stem/progenitor cells dilemma

Adult haematopoietic stem/progenitor cells (HSPCs) constitute the lifespan reserve for the generation of all the cellular lineages in the blood. Although massive progress in identifying the cluster of master genes controlling self-renewal and multipotency has been achieved in the past decade, some aspects of the physiology of HSPCs still need to be clarified. In particular, there is growing interest in the metabolic profile of HSPCs in view of their emerging role as determinants of cell fate. Indeed, stem cells and progenitors have distinct metabolic profiles, and the transition from stem to progenitor cell corresponds to a critical metabolic change, from glycolysis to oxidative phosphorylation. In this review, we summarize evidence, reported in the literature and provided by our group, highlighting the peculiar ability of HSPCs to adapt their mitochondrial oxidative/bioenergetic metabolism to survive in the hypoxic microenvironment of the endoblastic niche and to exploit redox signalling in controlling the balance between quiescence versus active cycling and differentiation. Especial prominence is given to the interplay between hypoxia inducible factor-1, globins and NADPH oxidases in managing the mitochondrial dioxygen-related metabolism and biogenesis in HSPCs under different ambient conditions. A mechanistic model is proposed whereby 'mitochondrial differentiation' is a prerequisite in uncommitted stem cells, paving the way for growth/differentiation factor-dependent processes. Advancing the understanding of stem cell metabolism will, hopefully, help to (i) improve efforts to maintain, expand and manipulate HSPCs ex vivo and realize their potential therapeutic benefits in regenerative medicine; (ii) reprogramme somatic cells to generate stem cells; and (iii) eliminate, selectively, malignant stem cells.

LINKED ARTICLES

This article is part of a themed section on Emerging Therapeutic Aspects in Oncology. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.169.issue-8.

Mortalin and DJ-1 coordinately regulate hematopoietic stem cell function through the control of oxidative stress

Mitochondrial heat shock protein, mortalin, is essential for the maintenance of HSCs via the control of oxidative stress. Mortalin directly interact with DJ-1 to regulate ROS levels in the mitochondria of HSCs.

Profilin 1 is essential for retention and metabolism of mouse hematopoietic stem cells in bone marrow

How stem cells interact with the microenvironment to regulate their cell fates and metabolism is largely unknown. Here we demonstrated that the deletion of the cytoskeleton-modulating protein profilin 1 (pfn1) in hematopoietic stem cell (HSCs) led to bone marrow failure, loss of quiescence, and mobilization and apoptosis of HSCs in vivo. A switch from glycolysis to mitochondrial respiration with increased reactive oxygen species (ROS) level was also observed in HSCs on pfn1 deletion. Importantly, treatment of pfn1-deficient mice with the antioxidant N-acetyl-l-cysteine reversed the ROS level and loss of quiescence of HSCs, suggesting that the metabolism is mechanistically linked to the cell cycle quiescence of stem cells. The actin-binding and proline-binding activities of pfn1 are required for its function in HSCs. Our study provided evidence that pfn1 at least partially acts through the axis of pfn1/Gα13/EGR1 to regulate stem cell retention and metabolism in the bone marrow.

Hypoxia and HIFs in regulating the development of the hematopoietic system

Many physiologic processes during the early stages of mammalian ontogeny, particularly placental and vascular development, take place in the low oxygen environment of the uterus. Organogenesis is affected by hypoxia inducible factor (HIF) transcription factors that are sensors of hypoxia. In response to hypoxia, HIFs activate downstream target genes - growth and metabolism factors. During hematopoietic system ontogeny, blood cells and hematopoietic progenitor/stem cells are respectively generated from mesodermal precursors, hemangioblasts, and from a specialized subset of endothelial cells that are hemogenic. Since HIFs are known to play a central role in vascular development, and hematopoietic system development occurs in parallel to that of the vascular system, several studies have examined the role of HIFs in hematopoietic development. The response to hypoxia has been examined in early and mid-gestation mouse embryos through genetic deletion of HIF subunits. We review here the data showing that hematopoietic tissues of the embryo are hypoxic and express HIFs and HIF downstream targets, and that HIFs regulate the development and function of hematopoietic progenitor/stem cells.

Cited2 is required for the maintenance of glycolytic metabolism in adult hematopoietic stem cells

Mammalian adult hematopoietic stem cells (HSCs) reside in the hypoxic bone marrow microenvironment and display a distinct metabolic phenotype compared with their progenitors. It has been proposed that HSCs generate energy mainly through anaerobic glycolysis in a pyruvate dehydrogenase kinase (Pdk)-dependent manner. Cited2 is an essential regulator for HSC quiescence, apoptosis, and function. Herein, we show that conditional deletion of Cited2 in murine HSCs results in elevated levels of reactive oxygen species, decreased cellular glutathione content, increased mitochondrial activity, and decreased glycolysis. At the molecular level, Cited2 deficiency significantly reduced the expression of genes involved in metabolism, such as Pdk2, Pdk4, and lactate dehydrogenases B and D (LDHB and LDHD). Cited2-deficient HSCs also exhibited increased Akt signaling, concomitant with elevated mTORC1 activity and phosphorylation of FoxOs. Further, inhibition of PI3/Akt, but not mTORC1, partially rescued the repression of Pdk4 caused by deletion of Cited2. Altogether, our results suggest that Cited2 is required for the maintenance of adult HSC glycolytic metabolism likely through regulating Pdk2, Pdk4, LDHB, LDHD, and Akt activity.

c-Kit-positive cardiac stem cells nested in hypoxic niches are activated by stem cell factor reversing the aging myopathy

RATIONALE

Hypoxia favors stem cell quiescence, whereas normoxia is required for stem cell activation, but whether cardiac stem cell (CSC) function is regulated by the hypoxic/normoxic state of the cell is currently unknown.

OBJECTIVE

A balance between hypoxic and normoxic CSCs may be present in the young heart, although this homeostatic control may be disrupted with aging. Defects in tissue oxygenation occur in the old myocardium, and this phenomenon may expand the pool of hypoxic CSCs, which are no longer involved in myocyte renewal.

METHODS AND RESULTS

Here, we show that the senescent heart is characterized by an increased number of quiescent CSCs with intact telomeres that cannot re-enter the cell cycle and form a differentiated progeny. Conversely, myocyte replacement is controlled only by frequently dividing CSCs with shortened telomeres; these CSCs generate a myocyte population that is chronologically young but phenotypically old. Telomere dysfunction dictates their actual age and mechanical behavior. However, the residual subset of quiescent young CSCs can be stimulated in situ by stem cell factor reversing the aging myopathy.

CONCLUSIONS

Our findings support the notion that strategies targeting CSC activation and growth interfere with the manifestations of myocardial aging in an animal model. Although caution has to be exercised in the translation of animal studies to human beings, our data strongly suggest that a pool of functionally competent CSCs persists in the senescent heart and that this stem cell compartment can promote myocyte regeneration effectively, partly correcting the aging myopathy.