The bone marrow vascular niche: home of HSC differentiation and mobilization

Platelet generation in vivo and in vitro

Platelet (PLT) transfusion, which is the primary cell therapy for thrombocytopenia, has been a source of concern in recent years due to its limitations of donor-dependent supply and soaring costs. In vitro platelet generation on an industrial scale is a possible solution requiring exploration. The technology of platelet generation ex vivo has been widely studied across the world, though the mechanisms of physiological thrombopoiesis and platelet biology function in vivo still remain elusive today. Various culture systems have been studied, most of which proved quite inefficient in generating functional platelets ex vivo, so there is still a long way to reach our ultimate goal of generating a fully functional platelet in vitro on an industrial scale. This review integrates the latest research into physiological platelet biogenesis and ex vivo-platelet/megakaryocyte (MK) generation protocols with a focus on the ability to generate PLT/MK in large quantities, summarizes current culture systems based on induced human pluripotent stem cells and adipose-derived stem cells, and discusses significant challenges that must be overcome for these approaches to be perfected.

Endothelial progenitor cells promote tumor growth and progression by enhancing new vessel formation

Tumor growth and progression require new blood vessel formation to deliver nutrients and oxygen for further cell proliferation and to create a neovascular network exit for tumor cell metastasis. Endothelial progenitor cells (EPCs) are a bone marrow (BM)-derived stem cell population that circulates in the peripheral circulation and homes to the tumor bed to participate in new blood vessel formation. In addition to structural support to nascent vessels, these cells can also regulate the angiogenic process by paracrine secretion of a number of proangiogenic growth factors and cytokines, thus playing a crucial role in tumor neovascularization and development. Inhibition of EPC-mediated new vessel formation may be a promising therapeutic strategy in tumor treatment. EPC-mediated neovascularization is a complex process that includes multiple steps and requires a series of cytokines and modulators, thus understanding the underlying mechanisms may provide anti-neovasculogenesis targets that may be blocked for the prevention of tumor development. The present review stresses the process and contribution of EPCs to the formation of new blood vessels in solid tumors, in an attempt to gain an improved understanding of the underlying cellular and molecular mechanisms involved, and to provide a potential effective therapeutic target for cancer treatment.

Low Oxygen Modulates Multiple Signaling Pathways, Increasing Self-Renewal, While Decreasing Differentiation, Senescence, and Apoptosis in Stromal MIAMI Cells

Human bone marrow multipotent mesenchymal stromal cell (hMSC) number decreases with aging. Subpopulations of hMSCs can differentiate into cells found in bone, vasculature, cartilage, gut, and other tissues and participate in their repair. Maintaining throughout adult life such cell subpopulations should help prevent or delay the onset of age-related degenerative conditions. Low oxygen tension, the physiological environment in progenitor cell-rich regions of the bone marrow microarchitecture, stimulates the self-renewal of marrow-isolated adult multilineage inducible (MIAMI) cells and expression of Sox2, Nanog, Oct4a nuclear accumulation, Notch intracellular domain, notch target genes, neuronal transcriptional repressor element 1 (RE1)-silencing transcription factor (REST), and hypoxia-inducible factor-1 alpha (HIF-1α), and additionally, by decreasing the expression of (i) the proapoptotic proteins, apoptosis-inducing factor (AIF) and Bak, and (ii) senescence-associated p53 expression and β-galactosidase activity. Furthermore, low oxygen increases canonical Wnt pathway signaling coreceptor Lrp5 expression, and PI3K/Akt pathway activation. Lrp5 inhibition decreases self-renewal marker Sox2 mRNA, Oct4a nuclear accumulation, and cell numbers. Wortmannin-mediated PI3K/Akt pathway inhibition leads to increased osteoblastic differentiation at both low and high oxygen tension. We demonstrate that low oxygen stimulates a complex signaling network involving PI3K/Akt, Notch, and canonical Wnt pathways, which mediate the observed increase in nuclear Oct4a and REST, with simultaneous decrease in p53, AIF, and Bak. Collectively, these pathway activations contribute to increased self-renewal with concomitant decreased differentiation, cell cycle arrest, apoptosis, and/or senescence in MIAMI cells. Importantly, the PI3K/Akt pathway plays a central mechanistic role in the oxygen tension-regulated self-renewal versus osteoblastic differentiation of progenitor cells.

Niche heterogeneity in the bone marrow

In adult mammals, hematopoietic stem cells (HSCs) are defined by their abilities to self-renew and to differentiate to form all blood cell lineages. These rare multipotent cells occupy specific locations in the bone marrow (BM) microenvironment. The specific microenvironment regulating HSCs, commonly referred to as the niche, comprises multiple cell types whose exact contributions are under active investigation. Understanding cellular cross talk involving HSCs in the BM microenvironment is of fundamental importance for harnessing therapies against benign and malignant blood diseases. In this review, we summarize and evaluate recent advances in our understanding of niche heterogeneity and its influence on HSC function.

When the endothelium scores an own goal: endothelial cells actively augment metastatic extravasation through endothelial-mesenchymal transition

Endothelial-mesenchymal transition (EndMT) is an important mechanism during organ development and in certain pathological conditions. For example, EndMT contributes to myofibroblast formation during organ fibrosis, and it has been identified as an important source of cancer-associated fibroblasts, facilitating tumor progression. Recently, EndMT was proposed to modulate endothelial function during intravasation and extravasation of metastatic tumor cells. Evidence suggests that endothelial cells are not passive actors during transendothelial migration (TEM) of cancer cells, as there are profound changes in endothelial junctional protein expression, signaling, permeability, and contractility. This review describes these alterations in endothelial characteristics during TEM of metastatic tumor cells and discusses them in the context of EndMT. EndMT could play an important role during metastatic intravasation and extravasation, a novel hypothesis that may lead to new therapeutic approaches to tackle metastatic disease.

Challenges and Opportunities to Harnessing the (Hematopoietic) Stem Cell Niche

In our body, stem cells reside in a microenvironment termed the niche. While the exact composition and therefore the level of complexity of a stem cell niche can vary significantly tissue-to-tissue, the stem cell niche microenvironment is dynamic, typically containing spatial and temporal variations in both cellular, extracellular matrix, and biomolecular components. This complex flow of secreted or bound biomolecules, cytokines, extracellular matrix components, and cellular constituents all contribute to the regulation of stem cell fate specification events, making engineering approaches at the nano- and micro-scale of particular interest for creating an artificial niche environment in vitro. Recent advances in fabrication approaches have enabled biomedical researchers to capture and recreate the complexity of stem cell niche microenvironments in vitro. Such engineered platforms show promise as a means to enhance our understanding of the mechanisms underlying niche-mediated stem cell regulation as well as offer opportunities to precisely control stem cell expansion and differentiation events for clinical applications. While these principles generally apply to all adult stem cells and niches, in this review, we focus on recent developments in engineering synthetic niche microenvironments for one of the best-characterized stem cell populations, hematopoietic stem cells (HSC). Specifically, we highlight recent advances in platforms designed to facilitate the extrinsic control of HSC fate decisions.

Vascular niches: endothelial cells as tissue- and site-specific multifunctional team players in health and disease

The cardiovascular system is a circulatory system that transports the blood and reaches out to almost every site of the body. Endothelial cells (EC) as the innermost cell layer of blood vessels constitute a huge interface between the blood and nearly all other tissues and cells. Although blood vessels and their EC were considered as rather inert and passive conduits early in the history of vascular discoveries, progress in vascular research now suggests that EC are actively involved in a plethora of physiological or pathophysiological processes. Endothelial heterogeneity and the concept of vascular niches are in the spotlight of current research in vascular biology. Endothelial heterogeneity comprises morphologic, molecular and functional features. It allows EC to adapt to the organ- and site-specific requirements exhibited by different segments of the vascular tree. In organ-specific vascular niches, EC mutually interact with neighboring cells. EC-derived cytokines, called angiokines, acting in a paracrine (angiocrine) fashion have been shown to be decisively involved in various biological processes ranging from stem cell maintenance to organ regeneration as well as cancer progression and metastasis. In summary, EC should always be considered as teammates in the maintenance and regulation of complex multicellular network interactions in health and disease.

The impact of chronic intermittent hypoxia on hematopoiesis and the bone marrow microenvironment

Obstructive sleep apnea (OSA) is a highly prevalent sleep-related breathing disorder which is associated with patient morbidity and an elevated risk of developing hypertension and cardiovascular diseases. There is ample evidence for the involvement of bone marrow (BM) cells in the pathophysiology of cardiovascular diseases but a connection between OSA and modulation of the BM microenvironment had not been established. Here, we studied how chronic intermittent hypoxia (CIH) affected hematopoiesis and the BM microenvironment, in a rat model of OSA. We show that CIH followed by normoxia increases the bone marrow hypoxic area, increases the number of multipotent hematopoietic progenitors (CFU assay), promotes erythropoiesis, and increases monocyte counts. In the BM microenvironment of CIH-subjected animals, the number of VE-cadherin-expressing blood vessels, particularly sinusoids, increased, accompanied by increased smooth muscle cell coverage, while vWF-positive vessels decreased. Molecularly, we investigated the expression of endothelial cell-derived genes (angiocrine factors) that could explain the cellular phenotypes. Accordingly, we observed an increase in colony-stimulating factor 1, vascular endothelium growth factor, delta-like 4, and angiopoietin-1 expression. Our data shows that CIH induces vascular remodeling in the BM microenvironment, which modulates hematopoiesis, increasing erythropoiesis, and circulating monocytes. Our study reveals for the first time the effect of CIH in hematopoiesis and suggests that hematopoietic changes may occur in OSA patients.

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

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

VEGF-A/VEGFR Inhibition Restores Hematopoietic Homeostasis in the Bone Marrow and Attenuates Tumor Growth

Antiangiogenesis-based cancer therapies, specifically those targeting the VEGF-A/VEGFR2 pathway, have been approved for subsets of solid tumors. However, these therapies result in an increase in hematologic adverse events. We surmised that both the bone marrow vasculature and VEGF receptor-positive hematopoietic cells could be impacted by VEGF pathway-targeted therapies. We used a mouse model of spontaneous breast cancer to decipher the mechanism by which VEGF pathway inhibition alters hematopoiesis. Tumor-bearing animals, while exhibiting increased angiogenesis at the primary tumor site, showed signs of shrinkage in the sinusoidal bone marrow vasculature accompanied by an increase in the hematopoietic stem cell-containing Lin-cKit(+)Sca1(+) (LKS) progenitor population. Therapeutic intervention by targeting VEGF-A, VEGFR2, and VEGFR3 inhibited tumor growth, consistent with observed alterations in the primary tumor vascular bed. These treatments also displayed systemic effects, including reversal of the tumor-induced shrinkage of sinusoidal vessels and altered population balance of hematopoietic stem cells in the bone marrow, manifested by the restoration of sinusoidal vessel morphology and hematopoietic homeostasis. These data indicate that tumor cells exert an aberrant systemic effect on the bone marrow microenvironment and VEGF-A/VEGFR targeting restores bone marrow function.

Cancer associated fibroblasts in hematological malignancies

Tumor microenvironment plays an important role in cancer initiation and progression. In hematological malignancies, the bone marrow represents the paradigmatic anatomical site in which tumor microenvironment expresses its morphofunctional features. Among the cells participating in the composition of this microenvironment, cancer associated fibrobasts (CAFs) have received less attention in hematopoietic tumors compared to solid cancers. In this review article, we discuss the involvement of CAFs in progression of hematological malignancies and the potential targeting of CAFs in a therapeutic perspective.

Transforming growth factor-β signaling induced during prostate cancer cell death and neuroendocrine differentiation is mediated by bone marrow stromal cells

INTRODUCTION

Prostate cancer that has metastasized to bone undergoes critical interactions with bone marrow stromal cells (BMSCs), ultimately promoting tumor survival. Previous studies have shown that BMSCs secrete factors that promote prostate cancer apoptosis or neuroendocrine differentiation. Because of the significance of transforming growth factor-β (TGF-β) family cytokines in cytostasis and bone metastasis, the role of TGF-β signaling in the context of prostate cancer-BMSC interactions was investigated.

METHODS

The role of TGF-β family signaling in BMSC-induced apoptosis of lineage-related prostate cancer cells was investigated in live/dead assays. SMAD phosphorylation or activity during apoptosis and neuroendocrine differentiation was investigated using immunofluorescence, Western blotting, and luciferase reporter assays, along with the ALK-4, -5, -7 kinase inhibitor, SB-431542.

RESULTS

Treatment of castration-resistant prostate cancer cells with SB-431542 resulted in significant reduction of apoptosis mediated by HS-5 BMSCs, supporting the involvement of TGF-β/SMAD signaling during this event. Interestingly, however, pre-treatment of BMSCs with TGF-β1 (5 ng/mL) yielded a conditioned medium that elicited a marked reduction in prostate cancer death. Phosphorylated-SMAD2 (P-SMAD2) was activated in BMSC-triggered transdifferentiated prostate cancer cells, as demonstrated through immunoblotting and luciferase reporter assays. However, SB-431542 did not restore androgen receptor and prostate specific antigen levels down-regulated by BMSC-secreted factors. Prostate cancer cells induced to undergo neuroendocrine differentiation in a BMSC-independent mechanism also showed elevated levels of P-SMAD2.

DISCUSSION

Collectively, our findings indicate that: (1) TGF-β family cytokines or regulated factors secreted from BMSCs are involved in prostate cancer apoptosis; (2) TGF-β signaling in prostate cancer cells is induced during neuroendocrine differentiation; and (3) TGF-β1 stimulation of BMSCs alters paracrine signaling to create a permissive environment for prostate cancer survival, suggesting a mechanism for prostate cancer-mediated colonization of bone.

CONCLUSIONS

TGF-β signaling resulting in activation of SMAD2 in prostate cancer may be an indicator of cellular stress in the presence of toxic paracrine factors released from the bone marrow stroma, ultimately fostering prostate cancer colonization of bone.

Impairment of hematopoietic stem cell niches in patients with aplastic anemia

Aplastic anemia (AA) is a serious hematological disorder characterized by pancytopenia and defective bone marrow (BM) microenvironment. Previous investigations have demonstrated that there is a defect of angiogenesis in the BM of patients with AA. However, whether abnormalities of the BM microenvironment, particularly the overall specialization of niches, which have been classified into osteoblastic, vascular, and perivascular niches, are involved in the pathogenesis of AA is unknown. In the present study, 46 patients with AA and 15 controls were selected. The cellular elements of the BM microenvironment, including endosteal, vascular, and perivascular cells, were analyzed by immunohistochemical staining in situ. Patients with AA showed markedly fewer endosteal cells [0.33 vs 3.67 per high-power field (hpf); P < 0.05], vascular cells (8.00 vs 12.67 per hpf; P < 0.05), and perivascular cells (7.17 vs 10.67 per hpf; P < 0.05) compared with controls. These data indicate that AA is associated with impaired hematopoietic stem cell niches.

Engineering the hematopoietic stem cell niche: Frontiers in biomaterial science

Hematopoietic stem cells (HSCs) play a crucial role in the generation of the body's blood and immune cells. This process takes place primarily in the bone marrow in specialized 'niche' microenvironments, which provide signals responsible for maintaining a balance between HSC quiescence, self-renewal, and lineage specification required for life-long hematopoiesis. While our understanding of these signaling mechanisms continues to improve, our ability to engineer them in vitro for the expansion of clinically relevant HSC populations is still lacking. In this review, we focus on development of biomaterials-based culture platforms for in vitro study of interactions between HSCs and their local microenvironment. The tools and techniques used for both examining HSC-niche interactions as well as applying these findings towards controlled HSC expansion or directed differentiation in 2D and 3D platforms are discussed. These novel techniques hold the potential to push the existing boundaries of HSC cultures towards high-throughput, real-time, and single-cell level biomimetic approaches that enable a more nuanced understanding of HSC regulation and function. Their application in conjunction with innovative biomaterial platforms can pave the way for engineering artificial bone marrow niches for clinical applications as well as elucidating the pathology of blood-related cancers and disorders.

Advances in understanding the acute lymphoblastic leukemia bone marrow microenvironment: From biology to therapeutic targeting

The bone marrow (BM) microenvironment regulates the properties of healthy hematopoietic stem cells (HSCs) localized in specific niches. Two distinct microenvironmental niches have been identified in the BM, the "osteoblastic (endosteal)" and "vascular" niches. Nevertheless, these niches provide sanctuaries where subsets of leukemic cells escape chemotherapy-induced death and acquire a drug-resistant phenotype. Moreover, it is emerging that leukemia cells are able to remodel the BM niches into malignant niches which better support neoplastic cell survival and proliferation. This review focuses on the cellular and molecular biology of microenvironment/leukemia interactions in acute lymphoblastic leukemia (ALL) of both B- and T-cell lineage. We shall also highlight the emerging role of exosomes/microvesicles as efficient messengers for cell-to-cell communication in leukemia settings. Studies on the interactions between the BM microenvironment and ALL cells have led to the discovery of potential therapeutic targets which include cytokines/chemokines and their receptors, adhesion molecules, signal transduction pathways, and hypoxia-related proteins. The complex interplays between leukemic cells and BM microenvironment components provide a rationale for innovative, molecularly targeted therapies, designed to improve ALL patient outcome. A better understanding of the contribution of the BM microenvironment to the process of leukemogenesis and leukemia persistence after initial remission, may provide new targets that will allow destruction of leukemia cells without adversely affecting healthy HSCs. This article is part of a Special Issue entitled: Tumor Microenvironment Regulation of Cancer Cell Survival, Metastasis,Inflammation, and Immune Surveillance edited by Peter Ruvolo and Gregg L. Semenza.

Mapping Radiation Injury and Recovery in Bone Marrow Using 18F-FLT PET/CT and USPIO MRI in a Rat Model

We present and test the use of multimodality imaging as a topological tool to map the amount of the body exposed to ionizing radiation and the location of exposure, which are important indicators of survival and recovery. To achieve our goal, PET/CT imaging with 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) was used to measure cellular proliferation in bone marrow (BM), whereas MRI using ultra-small superparamagnetic iron oxide (USPIO) particles provided noninvasive information on radiation-induced vascular damage.

METHODS

Animals were x-ray-irradiated at a dose of 7.5 Gy with 1 of 3 radiation schemes-whole-body irradiation, half-body shielding (HBS), or 1-leg shielding (1LS)-and imaged repeatedly. The spatial information from the CT scan was used to segment the region corresponding to BM from the PET scan using algorithms developed in-house, allowing for quantification of proliferating cells, and BM blood volume was estimated by measuring the changes in the T2 relaxation rates (ΔR2) collected from MR scans.

RESULTS

(18)F-FLT PET/CT imaging differentiated irradiated from unirradiated BM regions. Two days after irradiation, proliferation of 1LS animals was significantly lower than sham (P = 0.0001, femurs; P < 0.0001, tibias) and returned to sham levels by day 10 (P = 0.6344, femurs; P = 0.3962, tibias). The degree of shielding affected proliferation recovery, showing an increase in the irradiated BM of the femurs, but not the tibias, of HBS animals when compared with 1LS (P = 0.0310, femurs; P = 0.5832, tibias). MRI of irradiated spines detected radiation-induced BM vascular damage, measured by the significant increase in ΔR2 2 d after whole-body irradiation (P = 0.0022) and HBS (P = 0.0003) with a decreasing trend of values, returning to levels close to baseline over 10 d. Our data were corroborated using γ-counting and histopathology.

CONCLUSION

We demonstrated that (18)F-FLT PET/CT and USPIO MRI are valuable tools in mapping regional radiation exposure and the effects of radiation on BM. Analysis of the (18)F-FLT signal allowed for a clear demarcation of exposed BM regions and elucidated the kinetics of BM recovery, whereas USPIO MRI was used to assess vascular damage and recovery.

Basic fibroblast growth factor induces VEGF expression in chondrosarcoma cells and subsequently promotes endothelial progenitor cell-primed angiogenesis

Chondrosarcoma, a common malignant tumour, develops in bone. Effective adjuvant therapy remains inadequate for treatment, meaning poor prognosis. It is imperative to explore novel remedies. Angiogenesis is a rate-limiting step in progression that explains neovessel formation for blood supply in the tumour microenvironment. Numerous studies indicate that EPCs (endothelial progenitor cells) promote angiogenesis and contribute to tumour growth. bFGF (basic fibroblast growth factor), a secreted cytokine, regulates biological activity, including angiogenesis, and correlates with tumorigenesis. However, the role of bFGF in angiogenesis-related tumour progression by recruiting EPCs in human chondrosarcoma is rarely discussed. In the present study, we found that bFGF induced VEGF (vascular endothelial growth factor) expression via the FGFR1 (fibroblast growth factor receptor 1)/c-Src/p38/NF-κB (nuclear factor κB) signalling pathway in chondrosarcoma cells, thereby triggering angiogenesis of endothelial progenitor cells. Our in vivo data revealed that tumour-secreted bFGF promotes angiogenesis in both mouse plug and chick CAM (chorioallantoic membrane) assays. Xenograft mouse model data, due to bFGF-regulated angiogenesis, showed the bFGF regulates angiogenesis-linked tumour growth. Finally, bFGF was highly expressed in chondrosarcoma patients compared with normal cartilage, positively correlating with VEGF expression and tumour stage. The present study reveals a novel therapeutic target for chondrosarcoma progression.

The bone marrow metastasis niche in retinoblastoma

BACKGROUND

Retinoblastoma (Rb) is a progressive cancer which mainly occurs in children, and which is caused by different genetic or epigenetic alterations that lead to inactivation of both alleles of the RB1 gene. Hereditary and non-hereditary forms of Rb do exist, and the hereditary form is associated with an increased risk of secondary malignancies. Metastasis to distant organs is a critical feature of many tumors, and may be caused by various molecular alterations at different stages. Recognition of these alterations and, thus, insight into the processes underlying the development of metastases may result in novel preventive as well as effective targeted treatment options. Rb is associated with metastases to various organs and tissues, including the bone marrow (BM).

METHODS

Here, we provide an overview of mutations and other molecular changes known to be involved in Rb development and metastasis to the BM. This overview is based on a literature search ranging from 1990 to 2015.

CONCLUSIONS

The various BM metastasis-related molecular changes identified to date may be instrumental for a better diagnosis, prognosis and classification of Rb patients, as well as for the development of novel comprehensive (targeted) therapies.

The Effect of Hypoxia on Mesenchymal Stem Cell Biology

Although physiological and pathological role of hypoxia have been appreciated in mammalians for decades however the cellular biology of hypoxia more clarified in the past 20 years. Discovery of the transcription factor hypoxia-inducible factor (HIF)-1, in the 1990s opened a new window to investigate the mechanisms behind hypoxia. In different cellular contexts HIF-1 activation show variable results by impacting various aspects of cell biology such as cell cycle, apoptosis, differentiation and etc. Mesenchymal stem cells (MSC) are unique cells which take important role in tissue regeneration. They are characterized by self-renewal capacity, multilineage potential, and immunosuppressive property. Like so many kind of cells, hypoxia induces different responses in MSCs by HIF- 1 activation. The activation of this molecule changes the growth, multiplication, differentiation and gene expression profile of MSCs in their niche by a complex of signals. This article briefly discusses the most important effects of hypoxia in growth kinetics, signalling pathways, cytokine secretion profile and expression of chemokine receptors in different conditions.

Endothelium-derived essential signals involved in pancreas organogenesis

Endothelial cells (ECs) are essential for pancreas differentiation, endocrine specification, and endocrine function. They are also involved in the physiopathology of type 1 and type 2 diabetes. During embryogenesis, aortic ECs provide specific factors that maintain the expression of key genes for pancreas development such as pancreatic and duodenal homeobox-1. Other unknown factors are also important for pancreatic endocrine specification and formation of insulin-producing beta cells. Endocrine precursors proliferate interspersed with ductal cells and exocrine precursors and, at some point of development, these endocrine precursors migrate to pancreatic mesenchyme and start forming the islets of Langerhans. By the end of the gestation and close to birth, these islets contain immature beta cells with the capacity to express vascular endothelial growth factor and therefore to recruit ECs from the surrounding microenvironment. ECs in turn produce factors that are essential to maintain insulin secretion in pancreatic beta cells. Once assembled, a cross talk between endocrine cells and ECs maintain the integrity of islets toward an adequate function during the whole life of the adult individual. This review will focus in the EC role in the differentiation and maturation of pancreatic beta cells during embryogenesis as well as the current knowledge about the involvement of endothelium to derive pancreatic beta cells in vitro from mouse or human pluripotent stem cells.

Perivascular deletion of murine Rac reverses the ratio of marrow arterioles and sinusoid vessels and alters hematopoiesis in vivo

Hematopoietic stem cells (HSCs) are localized within specialized microenvironments throughout the BM. Nestin-expressing (Nestin(+)) mesenchymal stromal cells (MSCs) are important in the perivascular space. Rac is critical for MSC cell shape in vitro, whereas its function in MSCs in vivo remains poorly characterized. We hypothesized that deletion of Rac in the Nestin(+) cells would perturb the perivascular space, altering HSC localization and hematopoiesis. Nestin-Cre-directed excision of Rac1 in Rac3(-/-) mice reduces Nestin(+) cells in the marrow. We observed a 2.7-fold decrease in homing of labeled wild-type hematopoietic cells into Rac1(Δ/Δ)Rac3(-/-) mice compared with control mice. Rac1(Δ/Δ)Rac3(-/-) mice demonstrated a marked decrease in arterioles and an increase in the number and volume of venous sinusoids in the marrow that was associated with a reduction in the numbers of immunophenotypically and functionally-defined long-term HSCs in the marrow, a decrease in colony-forming cells and a reduction in circulating progenitors. Rac-deleted animals demonstrated a significant increase in trabecular bone. These data demonstrate that Rac GTPases play an important role in the integrity of perivascular space. Increased trabecular bone and sinusoidal space and decreased arteriolar volume in this model were associated with decreased HSC, underscoring the complexity of regulation of hematopoiesis in the perivascular space.

Endothelial-mesenchymal transition of brain endothelial cells: possible role during metastatic extravasation

Cancer progression towards metastasis follows a defined sequence of events described as the metastatic cascade. For extravasation and transendothelial migration metastatic cells interact first with endothelial cells. Yet the role of endothelial cells during the process of metastasis formation and extravasation is still unclear, and the interaction between metastatic and endothelial cells during transendothelial migration is poorly understood. Since tumor cells are well known to express TGF-β, and the compact endothelial layer undergoes a series of changes during metastatic extravasation (cell contact disruption, cytoskeletal reorganization, enhanced contractility), we hypothesized that an EndMT may be necessary for metastatic extravasation. We demonstrate that primary cultured rat brain endothelial cells (BEC) undergo EndMT upon TGF-β1 treatment, characterized by the loss of tight and adherens junction proteins, expression of fibronectin, β1-integrin, calponin and α-smooth muscle actin (SMA). B16/F10 cell line conditioned and activated medium (ACM) had similar effects: claudin-5 down-regulation, fibronectin and SMA expression. Inhibition of TGF-β signaling during B16/F10 ACM stimulation using SB-431542 maintained claudin-5 levels and mitigated fibronectin and SMA expression. B16/F10 ACM stimulation of BECs led to phosphorylation of Smad2 and Smad3. SB-431542 prevented SMA up-regulation upon stimulation of BECs with A2058, MCF-7 and MDA-MB231 ACM as well. Moreover, B16/F10 ACM caused a reduction in transendothelial electrical resistance, enhanced the number of melanoma cells adhering to and transmigrating through the endothelial layer, in a TGF-β-dependent manner. These effects were not confined to BECs: HUVECs showed TGF-β-dependent SMA expression when stimulated with breast cancer cell line ACM. Our results indicate that an EndMT may be necessary for metastatic transendothelial migration, and this transition may be one of the potential mechanisms occurring during the complex phenomenon known as metastatic extravasation.

Microenvironment and multiple myeloma spread

In patients with multiple myeloma (MM), the bone marrow (BM) contains hematopoietic stem cells (HSCs) and non-hematopoietic cells. HSCs are able to give rise to all types of mature blood cells, while the non hematopoietic component includes mesenchymal stem cells (MSCs), fibroblasts, osteoblasts, osteoclasts, chondroclasts, endothelial cells, endothelial progenitor cells (EPCs), B and T lymphocytes, NK cells, erythrocytes, megakaryocytes, platelets, macrophages and mast cells. All of these cells form specialized "niches" in the BM microenvironment which are close to the vasculature ("vascular niche") or to the endosteum ("osteoblast niche"). The "vascular niche" is rich in blood vessels where endothelial cells and mural cells (pericytes and smooth muscle cells) create a microenvironment that affects the behavior of several stem and progenitor cells. The vessel wall serves as an independent niche for the recruitment of endothelial progenitor cells, MSCs and HSCs. The activation by angiogenic factors and inflammatory cytokines switch the "vascular niche" to promote MM tumor growth and spread. This review will focus on the mechanisms involved in the generation of signals released by endothelial cells in the "vascular niche" that promote tumor growth and spread in MM.

Identify multiple myeloma stem cells: Utopia?

Multiple myeloma (MM) is a hematologic malignancy of monoclonal plasma cells which remains incurable despite recent advances in therapies. The presence of cancer stem cells (CSCs) has been demonstrated in many solid and hematologic tumors, so the idea of CSCs has been proposed for MM, even if MM CSCs have not been define yet. The existence of myeloma CSCs with clonotypic B and clonotypic non B cells was postulated by many groups. This review aims to focus on these distinct clonotypic subpopulations and on their ability to develop and sustain MM. The bone marrow microenvironment provides to MM CSCs self-renewal, survival and drug resistance thanks to the presence of normal and cancer stem cell niches. The niches and CSCs interact each other through adhesion molecules and the interplay between ligands and receptors activates stemness signaling (Hedgehog, Wnt and Notch pathways). MM CSCs are also supposed to be responsible for drug resistance that happens in three steps from the initial cancer cell homing microenvironment-mediated to development of microenvironment-independent drug resistance. In this review, we will underline all these aspects of MM CSCs.

Bone Metastasis in Renal Cell Carcinoma is Preprogrammed in the Primary Tumor and Caused by AKT and Integrin α5 Signaling

PURPOSE

Bone metastasis develops in 30% of all patients with renal cell carcinoma. We elucidated the mechanisms that lead to and predict bone metastasis of renal cell carcinoma.

MATERIALS AND METHODS

Nine renal cell carcinoma primary cell lines and 30 renal cell carcinoma tissue specimens (normal and tumor tissue) were collected from 3 patients with no metastasis and 10 with lung or bone metastasis within 5 years after nephrectomy. Cell migration was analyzed in a Boyden chamber and proliferation was assessed by bromodeoxyuridine incorporation. Adhesion to fibronectin, and collagen I and IV was determined after cell staining. The expression and/or activity of cellular signaling molecules was quantified by Western blot.

RESULTS

Compared to renal cell carcinoma cells from patients without metastasis, the migration of cells from patients with bone metastasis was enhanced 13.5-fold (p = 0.034), and adhesion to fibronectin and collagen I was enhanced 5.8-fold and 6.1-fold (p = 0.002 and 0.014, respectively). In general proliferation was decreased in metastasizing cells. In accordance with these results we detected higher activity of AKT (p = 0.011) and FAK (p = 0.054), higher integrin α5 expression (p = 0.052) and lower PTEN expression in primary cells from patients with bone metastasis compared to nonmetastasizing cells. An almost similarly altered expression pattern was also observed in the renal cell carcinoma tissue specimens and the normal renal tissue of patients with bone metastasis.

CONCLUSIONS

We describe evidence that molecular predispositions determine the potential for bone metastasis to develop in renal cell carcinoma, which may serve as prognostic markers after initial tumor detection.

Bone marrow regeneration promoted by biophysically sorted osteoprogenitors from mesenchymal stromal cells

Human tissue repair deficiencies can be supplemented through strategies to isolate, expand in vitro, and reimplant regenerative cells that supplant damaged cells or stimulate endogenous repair mechanisms. Bone marrow-derived mesenchymal stromal cells (MSCs), a subset of which is described as mesenchymal stem cells, are leading candidates for cell-mediated bone repair and wound healing, with hundreds of ongoing clinical trials worldwide. An outstanding key challenge for successful clinical translation of MSCs is the capacity to produce large quantities of cells in vitro with uniform and relevant therapeutic properties. By leveraging biophysical traits of MSC subpopulations and label-free microfluidic cell sorting, we hypothesized and experimentally verified that MSCs of large diameter within expanded MSC cultures were osteoprogenitors that exhibited significantly greater efficacy over other MSC subpopulations in bone marrow repair. Systemic administration of osteoprogenitor MSCs significantly improved survival rates (>80%) as compared with other MSC subpopulations (0%) for preclinical murine bone marrow injury models. Osteoprogenitor MSCs also exerted potent therapeutic effects as "cell factories" that secreted high levels of regenerative factors such as interleukin-6 (IL-6), interleukin-8 (IL-8), vascular endothelial growth factor A, bone morphogenetic protein 2, epidermal growth factor, fibroblast growth factor 1, and angiopoietin-1; this resulted in increased cell proliferation, vessel formation, and reduced apoptosis in bone marrow. This MSC subpopulation mediated rescue of damaged marrow tissue via restoration of the hematopoiesis-supporting stroma, as well as subsequent hematopoiesis. Together, the capabilities described herein for label-freeisolation of regenerative osteoprogenitor MSCs can markedly improve the efficacy of MSC-based therapies.

Multiple myeloma as a model for the role of bone marrow niches in the control of angiogenesis

Bone marrow (BM) contains hematopoietic stem cells (HSCs) and nonhematopoietic cells. HSCs give rise to all types of mature blood cells, while the nonhematopoietic component includes osteoblasts/osteoclasts, endothelial cells (ECs), endothelial progenitor cells (EPCs), and mesenchymal stem cells (MSCs). These cells form specialized "niches" which are close to the vasculature ("vascular niche") or to the endosteum ("osteoblast niche"). The "vascular niche", rich in blood vessels where ECs and mural cells (pericytes and smooth muscle cells), create a microenvironment affecting the behavior of several stem and progenitor cells. The vessel wall acts as an independent niche for the recruitment of EPCs and MSCs. This chapter will focus on the description of the role of BM niches in the control of angiogenesis occurring during multiple myeloma progression.

Nitric oxide-induced murine hematopoietic stem cell fate involves multiple signaling proteins, gene expression, and redox modulation

There are a growing number of reports showing the influence of redox modulation in cellular signaling. Although the regulation of hematopoiesis by reactive oxygen species (ROS) and reactive nitrogen species (RNS) has been described, their direct participation in the differentiation of hematopoietic stem cells (HSCs) remains unclear. In this work, the direct role of nitric oxide (NO(•)), a RNS, in the modulation of hematopoiesis was investigated using two sources of NO(•) , one produced by endothelial cells stimulated with carbachol in vitro and another using the NO(•)-donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP) in vivo. Two main NO(•) effects were observed: proliferation of HSCs-especially of the short-term HSCs-and its commitment and terminal differentiation to the myeloid lineage. NO(•)-induced proliferation was characterized by the increase in the number of cycling HSCs and hematopoietic progenitor cells positive to BrdU and Ki-67, upregulation of Notch-1, Cx43, PECAM-1, CaR, ERK1/2, Akt, p38, PKC, and c-Myc. NO(•)-induced HSCs differentiation was characterized by the increase in granulocytic-macrophage progenitors, granulocyte-macrophage colony forming units, mature myeloid cells, upregulation of PU.1, and C/EBPα genes concomitantly to the downregulation of GATA-3 and Ikz-3 genes, activation of Stat5 and downregulation of the other analyzed proteins mentioned above. Also, redox status modulation differed between proliferation and differentiation responses, which is likely associated with the transition of the proliferative to differentiation status. Our findings provide evidence of the role of NO(•) in inducing HSCs proliferation and myeloid differentiation involving multiple signaling.

Exonuclease 1 is a critical mediator of survival during DNA double strand break repair in nonquiescent hematopoietic stem and progenitor cells

Hematopoietic stem cell (HSC) populations require DNA repair pathways to maintain their long-term survival and reconstitution capabilities, but mediators of these processes are still being elucidated. Exonuclease 1 (Exo1) participates in homologous recombination (HR) and Exo1 loss results in impaired 5' HR end resection. We use cultured Exo1(mut) fibroblasts and bone marrow to demonstrate that loss of Exo1 function results in defective HR in cycling cells. Conversely, in Exo1(mut) mice HR is not required for maintenance of quiescent HSCs at steady state, confirming the steady state HSC reliance on nonhomologous end joining (NHEJ). Exo1(mut) mice sustained serial repopulation, displayed no defect in competitive repopulation or niche occupancy, and exhibited no increased sensitivity to whole body ionizing radiation. However, when Exo1(mut) HSCs were pushed into cell cycle in vivo with 5-fluorouracil or poly IC, the hematopoietic population became hypersensitive to IR, resulting in HSC defects and animal death. We propose Exo1-mediated HR is dispensable for stem cell function in quiescent HSC, whereas it is essential to HSC response to DNA damage processing after cell cycle entry, and its loss is not compensated by intact NHEJ. In HSCs, the maintenance of stem cell function after DNA damage is dependent on the DNA repair capacity, segregated by active versus quiescent points in cell cycle.

Angiopoietins in haematopoietic stem cell mobilisation in patients with haematological malignancies

BACKGROUND

The bone marrow niche contains different types of cells including osteoblasts and endothelial progenitors, all of which interact and take part in the process of mobilisation. The aim of our study was to evaluate the levels of cytokines (osteopontin and angiopoietins 1 and 2) active in the bone marrow niche during the mobilisation of haematopoietic stem cells for autologous transplantation.

MATERIALS AND METHODS

Forty-eight patients (24 females, 24 males), median age 56.5 years, entered the study. The group consisted of patients with multiple myeloma (n=34), lymphoma (n=13) and acute myeloid leukaemia (n=1). Blood samples were collected before chemotherapy and on the day of the first apheresis. Cytokines were evaluated by enzyme-linked immunosorbent assays. Additionally, circulating endothelial cells were assessed by flow cytometry.

RESULTS

The median concentration of angiopoietin 1 at the time of apheresis was lower than that at baseline (2.7 vs 7.8 ng/mL, p<0.001). In contrast, the median level of angiopoietin 2 increased during the mobilisation procedure (3.6 vs 2.8 ng/mL, p=0.001). The patients were divided according to the number of days of granulocyte colony-stimulating factor treatment before the first apheresis into "early" (<median=11 days) and "late" (>median) mobilisers. The group of "early mobilisers" had higher baseline angiopoietin 1 levels (median=11.6 ng/mL) than those of the "late mobilisers" (median=6.0 ng/mL, p=0.05). An adverse correlation was observed between duration of granulocyte colony-stimulating factor treatment and baseline angiopoietin 1 level. Baseline angiopoietin 1 levels correlated with numbers of circulating endothelial cells. Low angiopoietin 2 level increased the chance of poor mobilisation.

CONCLUSIONS

The angiogenic processes can influence the timing of mobilisation. Angiopoietins 1 and 2 need further evaluation in the context of mobilisation.

MicroRNA-mRNA regulatory network study and apoptosis analysis on bone marrow endothelial cells induced by liver cirrhosis serum

BACKGROUND AND OBJECTIVE

As important components of bone marrow microenvironment, bone marrow endothelial cells (BMECs) have important roles in regulating haematopoietic functions of the bone marrow. In preliminary study, we found the humoral inhibitor in liver cirrhosis (LC) could lead to ultrastructure alterations of the bone marrow endothelium. The present study aimed to investigate functional changes occurred in BMECs during LC.

METHODS

A multi-step approach combining microarray microRNA (miRNA) and mRNA expression profiling and bioinformatics analysis was used to generate a specific miRNA-mRNA regulatory network in BMECs treated with supplemented medium with 20% pooled sera from 26 patients with LC or 10 healthy volunteers as a control group for 48h.

RESULTS

A total of 372 differentially expressed miRNAs and 1872 differentially expressed mRNAs were identified by miRNA and mRNA microarray, respectively. Twenty-one dysregulated miRNAs and their 23 dysregulated target mRNAs confirmed by experiment (31 miRNA-mRNA interactions) were screened based on databases miRecords and miRTarBase. Bioinformatics analysis revealed that the functions of these miRNA-mRNA interactions were involved in the positive regulation of apoptosis, negative regulation of cell proliferation, cell cycle arrest and so on. The results of flow cytometry analysis showed that the LC serum treated-BMECs had a higher apoptosis percentage (17.57±1.84%) compared to the control group (10.2±1.55%). (P<0.05) CONCLUSIONS: Thirty-one miRNA-mRNA pairs combined with 21 dysregulated miRNAs and their 23 target mRNAs identified by the multi-step approach are involved in BMECs treated with LC serum. The results indicated that LC serum-induced apoptosis in BMECs were regulated by a complicated miRNA-mRNA regulatory network.

SDF-1 dynamically mediates megakaryocyte niche occupancy and thrombopoiesis at steady state and following radiation injury

Megakaryocyte (MK) development in the bone marrow progresses spatially from the endosteal niche, which promotes MK progenitor proliferation, to the sinusoidal vascular niche, the site of terminal maturation and thrombopoiesis. The chemokine stromal cell-derived factor-1 (SDF-1), signaling through CXCR4, is implicated in the maturational chemotaxis of MKs toward sinusoidal vessels. Here, we demonstrate that both IV administration of SDF-1 and stabilization of endogenous SDF-1 acutely increase MK-vasculature association and thrombopoiesis with no change in MK number. In the setting of radiation injury, we find dynamic fluctuations in marrow SDF-1 distribution that spatially and temporally correlate with variations in MK niche occupancy. Stabilization of altered SDF-1 gradients directly affects MK location. Importantly, these SDF-1-mediated changes have functional consequences for platelet production, as the movement of MKs away from the vasculature decreases circulating platelets, while MK association with the vasculature increases circulating platelets. Finally, we demonstrate that manipulation of SDF-1 gradients can improve radiation-induced thrombocytopenia in a manner additive with earlier TPO treatment. Taken together, our data support the concept that SDF-1 regulates the spatial distribution of MKs in the marrow and consequently circulating platelet numbers. This knowledge of the microenvironmental regulation of the MK lineage could lead to improved therapeutic strategies for thrombocytopenia.

Bone marrow vascular niche: home for hematopoietic stem cells

Though discovered later than osteoblastic niche, vascular niche has been regarded as an alternative indispensable niche operating regulation on hematopoietic stem cells (HSCs). As significant progresses gained on this type niche, it is gradually clear that the main work of vascular niche is undertaking to support hematopoiesis. However, compared to what have been defined in the mechanisms through which the osteoblastic niche regulates hematopoiesis, we know less in vascular niche. In this review, based on research data hitherto we will focus on component foundation and various functions of vascular niche that guarantee the normal hematopoiesis process within bone marrow microenvironments. And the possible pathways raised by various research results through which this environment undergoes its function will be discussed as well.

Role of endothelial progenitor cells in cancer progression

Tumor-associated neovasculature is a critical therapeutic target; however, despite significant progress made in the clinical efficacy of anti-vessel drugs, the effect of these agents remains transient: over time, most patients develop resistance, which inevitably leads to tumor progression. To develop more effective treatments, it is imperative that we better understand the mechanisms involved in tumor vessel formation, how they participate to the tumor progression and metastasis, and the best way to target them. Several mechanisms contribute to the formation of tumor-associated vasculature: i) neoangiogenesis; ii) vascular co-option; iii) mosaicism; iv) vasculogenic mimicry, and v) postnatal vasculogenesis. These mechanisms can also play a role in the development of resistance to anti-angiogenic drugs, and could serve as targets for designing new anti-vascular molecules to treat solid as well as hematological malignancies. Bone marrow-derived endothelial progenitor cell (EPC)-mediated vasculogenesis represents an important new target, especially at the early stage of tumor growth (when EPCs are critical for promoting the "angiogenic switch"), and during metastasis, when EPCs promote the transition from micro- to macro-metastases. In hematologic malignancies, the EPC population could be related to the neoplastic clone, and both may share a common ontogeny. Thus, characterization of tumor-associated EPCs in blood cancers may provide clues for more specific anti-vascular therapy that has both direct and indirect anti-tumor effects. Here, we review the role of vasculogenesis, mediated by bone marrow-derived EPCs, in the progression of cancer, with a particular focus on the role of these cells in promoting progression of hematological malignancies.

Coupling of angiogenesis and osteogenesis by a specific vessel subtype in bone

The mammalian skeletal system harbours a hierarchical system of mesenchymal stem cells, osteoprogenitors and osteoblasts sustaining lifelong bone formation. Osteogenesis is indispensable for the homeostatic renewal of bone as well as regenerative fracture healing, but these processes frequently decline in ageing organisms, leading to loss of bone mass and increased fracture incidence. Evidence indicates that the growth of blood vessels in bone and osteogenesis are coupled, but relatively little is known about the underlying cellular and molecular mechanisms. Here we identify a new capillary subtype in the murine skeletal system with distinct morphological, molecular and functional properties. These vessels are found in specific locations, mediate growth of the bone vasculature, generate distinct metabolic and molecular microenvironments, maintain perivascular osteoprogenitors and couple angiogenesis to osteogenesis. The abundance of these vessels and associated osteoprogenitors was strongly reduced in bone from aged animals, and pharmacological reversal of this decline allowed the restoration of bone mass.

In vitro construction of 2D and 3D simulations of the murine hematopoietic niche

Hematopoietic stem cells (HSC) undergo multilineage differentiation or self-renewal to maintain normal hematopoiesis and to sustain the size of the HSC pool throughout life. These processes are determined by a complex interplay of molecular signals between HSC and other cellular components such as osteoblasts (OB), stromal cells, endothelial cells, and a number of extracellular matrix (ECM) proteins. Through changes in its physical properties within the bone marrow (BM) microenvironment, collagen, which is one of the most critical ECM proteins, can modulate HSC function and maintenance of the competence of the hematopoietic niche (HN). At present, there is no consensus as to how different cellular elements of the niche collaborate and interact to promote HSC self-renewal or differentiation to maintain hematopoiesis. Deciphering these interactions and the impact of mechanical properties of the collagen microstructures within the HN has critical clinical implications in the areas of stem cell homing, engraftment, and maintenance of HSC function. In this chapter, we describe several of the in vitro methodologies for establishing and maintaining HSC in vitro including the isolation of OB, stromal cells, and hematopoietic progenitor cells, as well as the establishment of both two-dimensional (2D) and three-dimensional (3D) coculture systems.

Microfluidic generation of gradient hydrogels to modulate hematopoietic stem cell culture environment

The bone marrow provides spatially and temporally variable signals that impact the behavior of hematopoietic stem cells (HSCs). While multiple biomolecular signals and bone marrow cell populations have been proposed as key regulators of HSC fate, new tools are required to probe their importance and mechanisms of action. Here, a novel method based on a microfluidic mixing platform to create small volume, 3D hydrogel constructs containing overlapping patterns of cell and matrix constituents inspired by the HSC niche is described. This approach is used to generate hydrogels containing opposing gradients of fluorescent microspheres, MC3T3-E1 osteoblasts, primary murine hematopoietic stem and progenitor cells (HSPCs), and combinations thereof in a manner independent of hydrogel density and cell/particle size. Three different analytical methods are described to characterize local properties of these hydrogels at multiple scales: 1) whole construct fluorescent analysis; 2) multi-photon imaging of individual cells within the construct; 3) retrieval of discrete sub-regions from the hydrogel post-culture. The approach reported here allows the creation of stable gradients of cell and material cues within a single, optically translucent 3D biomaterial to enable a range of investigations regarding how microenvironmental signals impact cell fate.

One more stem cell niche: how the sensitivity of chronic myeloid leukemia cells to imatinib mesylate is modulated within a "hypoxic" environment

This is a review (by no means comprehensive) of how the stem cell niche evolved from an abstract concept to a complex system, implemented with a number of experimental data at the cellular and molecular levels, including metabolic cues, on which we focused in particular. The concept was introduced in 1978 to model bone marrow sites suited to host hematopoietic stem cells (HSCs) and favor their self-renewal, while restraining clonal expansion and commitment to differentiation. Studies of the effects of low oxygen tension on HSC maintenance in vitro led us to hypothesize niches were located within bone marrow areas where oxygen tension is lower than elsewhere. We named these areas hypoxic stem cell niches, although a low oxygen tension is to be considered physiological for the environment where HSCs are maintained. HSCs were later shown to have the option of cycling in low oxygen, which steers this cycling to the maintenance of stem cell potential. Cell subsets capable of withstanding incubation in very low oxygen were also detected within leukemia cell populations, including chronic myeloid leukemia (CML). The oncogenetic Bcr/Abl protein is completely suppressed in these subsets, whereas Bcr/Abl messenger ribonucleic acid is not, indicating that CML cells resistant to low oxygen are independent of Bcr/Abl for persistence in culture but remain genetically leukemic. Accordingly, leukemia stem cells of CML selected in low oxygen are refractory to the Bcr/Abl inhibitor imatinib mesylate. Bcr/Abl protein suppression turned out to be actually determined when glucose shortage complicated the effects of low oxygen, indicating that ischemia-like conditions are the driving force of leukemia stem cell refractoriness to imatinib mesylate. These studies pointed to “ischemic” stem cell niches as a novel scenario for the maintenance of minimal residual disease of CML. A possible functional relationship of the “ischemic” with the “hypoxic” stem cell niche is discussed.

Extracellular matrix: a dynamic microenvironment for stem cell niche

Background

Extracellular matrix (ECM) is a dynamic and complex environment characterized by biophysical, mechanical and biochemical properties specific for each tissue and able to regulate cell behavior. Stem cells have a key role in the maintenance and regeneration of tissues and they are located in a specific microenvironment, defined as niche.

Scope of review

We overview the progresses that have been made in elucidating stem cell niches and discuss the mechanisms by which ECM affects stem cell behavior. We also summarize the current tools and experimental models for studying ECM–stem cell interactions.

Major conclusions

ECM represents an essential player in stem cell niche, since it can directly or indirectly modulate the maintenance, proliferation, self-renewal and differentiation of stem cells. Several ECM molecules play regulatory functions for different types of stem cells, and based on its molecular composition the ECM can be deposited and finely tuned for providing the most appropriate niche for stem cells in the various tissues. Engineered biomaterials able to mimic the in vivo characteristics of stem cell niche provide suitable in vitro tools for dissecting the different roles exerted by the ECM and its molecular components on stem cell behavior.

General significance

ECM is a key component of stem cell niches and is involved in various aspects of stem cell behavior, thus having a major impact on tissue homeostasis and regeneration under physiological and pathological conditions. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

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Stem cells have a key role in the maintenance and regeneration of tissues.

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The extracellular matrix is a critical regulator of stem cell function.

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Stem cells reside in a dynamic and specialized microenvironment denoted as niche.

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The extracellular matrix represents an essential component of stem cell niches.

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Bioengineered niches can be used for investigating stem cell–matrix interactions.

Genomics and proteomics approaches in understanding tumor angiogenesis

Functional genomic and proteomic approaches have begun to revolutionize cancer research. The advent of powerful technologies, such as DNA microarrays, serial analysis of gene expression, RNA interference and proteomics, has accelerated investigations of gene identification and function at a scale never before accomplished. Approaches integrating these technologies with high-throughput forward and reverse genetic screens, are already providing insights into the mechanistic understanding of angiogenesis, leading to the identification of proteins that can be used for selective targeting of tumor vessels.

Targeting the hypoxia-sensing pathway in clinical hematology

Hypoxia-inducible factors (HIFs) are oxygen-sensitive transcription factors regulated by oxygen-dependent prolyl hydroxylase domain (PHD) enzymes and are key to cell adaptation to low oxygen. The hematopoietic stem cell (HSC) niche in the bone marrow is highly heterogeneous in terms of microvasculature and thus oxygen concentration. The importance of hypoxia and HIFs in the hematopoietic environment is becoming increasingly recognized. Many small compounds that inhibit PHDs have been developed, enabling HIFs to be pharmacologically stabilized in an oxygen-independent manner. The use of PHD inhibitors for therapeutic intervention in hematopoiesis is being increasingly investigated. PHD inhibitors are well established to increase erythropoietin production to correct anemia in hemodialysis patients. Pharmacological stabilization of HIF-1α protein with PHD inhibitors is also emerging as an important regulator of HSC proliferation and self-renewal. Administration of PHD inhibitors increases quiescence and decreases proliferation of HSCs in the bone marrow in vivo, thereby protecting them from high doses of irradiation and accelerating hematological recovery. Recent findings also show that stabilization of HIF-1α increases mobilization of HSCs in response to granulocyte colony-stimulating factor and plerixafor, suggesting that PHD inhibitors could be useful agents to increase mobilization success in patients requiring transplantation. These findings highlight the importance of the hypoxia-sensing pathway and HIFs in clinical hematology.

A novel mechanism of abnormal hematological indices in liver cirrhosis: bone marrow endothelial cell dysfunction caused by humoral inhibitor affects the hematopoietic function of bone marrow

Abnormal hematological indices (HIs), a complication of liver cirrhosis (LC), present difficulties in the treatment of LC and pose a serious threat to the survival of patients. LC is a dynamic wound-healing process that occurs in response to repeated liver injury and is a chronic disorder associated with changes in various organs and tissues. It has been reported that humoral inhibitor in the formation of LC could affect the hematogenic functions of bone marrow (BM) by acting on erythroid differentiation. This indicates that the BM microenvironment is affected by humoral inhibitor in LC. Bone marrow endothelial cells (BMECs) are very important components of the BM microenvironment that function as the cytoskeleton to support the adhesion of hematopoietic stem cells (HSCs). In addition, they can secrete cytokines, which have important functions in regulating positioning, homing, proliferation, differentiation and other functions of HSCs on the BM microenvironment. These functions of BMECs may be affected due to direct contact with blood and long-term exposure to an environment with humoral inhibitor in the presence of LC. Multiple studies have shown that during the formation of LC, hepatic sinusoid endothelial cells were damaged and secreted cytokines and matrix proteins. Moreover, these cytokines and matrix proteins were involved in the formation and development of LC. Similar in function to mature-stage BM, liver at the embryonic stage also functions as a type of hematogenic organ. With similar anatomical position and functions to that of hepatic sinusoid endothelial cells, BMECs may undergo similar changes and impair hematogenic function of BM. More importantly, we found even more convincing evidence in that the humoral inhibitor in LC could lead to the ultrastructural damage of BMECs that were positively related to the degree of severity of LC. Therefore, we hypothesise the existence of a novel mechanism for abnormal HIs in LC: the continuous humoral inhibitor may lead to abnormal cytokine secretion of BMECs and attenuate their supporting functions, and such alterations of BMECs may lead to BM microenvironment disorder and dysfunction of HSCs, finally causing abnormal HIs.

Stem cell dynamics in the hair follicle niche

Hair follicles are appendages of the mammalian skin that have the ability to periodically and stereotypically regenerate in order to continuously produce new hair over our lifetime. The ability of the hair follicle to regenerate is due to the presence of stem cells that along with other cell populations and non-cellular components, including molecular signals and extracellular material, make up a niche microenvironment. Mounting evidence suggests that the niche is critical for regulating stem cell behavior and thus the process of regeneration. Here, we review the literature concerning past and current studies that have utilized mouse genetic models, combined with other approaches to dissect the molecular and cellular composition of the hair follicle niche. We also discuss our current understanding of how stem cells operate within the niche during the process of tissue regeneration and the factors that regulate their behavior.

Role of the Hypoxia-inducible factor-1 alpha induced autophagy in the conversion of non-stem pancreatic cancer cells into CD133+ pancreatic cancer stem-like cells

The initiation and progression of various solid tumors, including pancreatic carcinoma, are driven by a population of cells with stem cell properties, namely cancer stem cells (CSCs). Like their normal counterparts, CSCs are also believed to rely on their own microenvironment termed niches to sustain the population. Hypoxia-inducible factor-1α (HIF-1α) is a major actor in the cell survival response to hypoxia. Recently, several researchers proposed that non-stem cancer cells can convert to stem-like cells to maintain equilibrium. The present study focuses on whether non-stem pancreatic cancer cells can convert to stem-like cells and the role of HIF-1α and autophagy in modulating this conversation. The non-stem pancreatic cancer cells and pancreatic cancer stem-like cells were separated by magnetic sorting column. Intermittent hypoxia enhanced stem-like properties of non-stem pancreatic cancer cells and stimulated the levels of HIF-1α, LC3-II and Beclin. Enhanced autophagy was associated with the elevated level of HIF-1α. The conversation of non-stem pancreatic cancer cells into pancreatic cancer stem-like cells was induced by HIF-1α and autophagy. This novel finding may indicate the specific role of HIF-1α and autophagy in promoting the dynamic equilibrium between CSCs and non-CSCs. Also, it emphasizes the importance of developing therapeutic strategies targeting cancer stem cells as well as the microenvironmental influence on the tumor.

Hematopoietic stem and progenitor cells in inflammation and allergy

Hematopoietic stem and progenitor cells contribute to allergic inflammation. Pro-inflammatory cytokines that are generated following allergen challenge can impact the differentiation of hematopoietic progenitor cells leading to increased production of effector cells such as eosinophils and basophils, which are key cells involved in the pathogenesis of allergic airway inflammation. Homing of stem cells to the lungs is associated with inflammatory and remodeling changes in asthmatics. Factors that modulate the differentiation and increased migration of stem cells to the site of inflammation in asthma remain to be defined. Stem cells can mature at the site of inflammation in response to inflammatory mediators and other components in the milieu. While the available data suggest that hematopoietic cells traffic to target tissues, the molecular factors underlying in situ differentiation have yet to be specified. Here, we critically evaluate the potential role of hematopoietic progenitors in contributing to the increased immune cell infiltrate in allergic asthma and the factors that drive their differentiation.