Role of Flk-1 in mouse hematopoietic stem cells

Methods to label, image, and analyze the complex structural architectures of microvascular networks

Microvascular networks play key roles in oxygen transport and nutrient delivery to meet the varied and dynamic metabolic needs of different tissues throughout the body, and their spatial architectures of interconnected blood vessel segments are highly complex. Moreover, functional adaptations of the microcirculation enabled by structural adaptations in microvascular network architecture are required for development, wound healing, and often invoked in disease conditions, including the top eight causes of death in the Unites States. Effective characterization of microvascular network architectures is not only limited by the available techniques to visualize microvessels but also reliant on the available quantitative metrics that accurately delineate between spatial patterns in altered networks. In this review, we survey models used for studying the microvasculature, methods to label and image microvessels, and the metrics and software packages used to quantify microvascular networks. These programs have provided researchers with invaluable tools, yet we estimate that they have collectively attained low adoption rates, possibly due to limitations with basic validation, segmentation performance, and nonstandard sets of quantification metrics. To address these existing constraints, we discuss opportunities to improve effectiveness, rigor, and reproducibility of microvascular network quantification to better serve the current and future needs of microvascular research.

HIF prolyl hydroxylase inhibitor FG-4497 enhances mouse hematopoietic stem cell mobilization via VEGFR2/KDR

In normoxia, hypoxia-inducible transcription factors (HIFs) are rapidly degraded within the cytoplasm as a consequence of their prolyl hydroxylation by oxygen-dependent prolyl hydroxylase domain (PHD) enzymes. We have previously shown that hematopoietic stem and progenitor cells (HSPCs) require HIF-1 for effective mobilization in response to granulocyte colony-stimulating factor (G-CSF) and CXCR4 antagonist AMD3100/plerixafor. Conversely, HIF PHD inhibitors that stabilize HIF-1 protein in vivo enhance HSPC mobilization in response to G-CSF or AMD3100 in a cell-intrinsic manner. We now show that extrinsic mechanisms involving vascular endothelial growth factor receptor-2 (VEGFR2), via bone marrow (BM) endothelial cells, are also at play. PTK787/vatalanib, a tyrosine kinase inhibitor selective for VEGFR1 and VEGFR2, and neutralizing anti-VEGFR2 monoclonal antibody DC101 blocked enhancement of HSPC mobilization by FG-4497. VEGFR2 was absent on mesenchymal and hematopoietic cells and was detected only in Sca1+ endothelial cells in the BM. We propose that HIF PHD inhibitor FG-4497 enhances HSPC mobilization by stabilizing HIF-1α in HSPCs as previously demonstrated, as well as by activating VEGFR2 signaling in BM endothelial cells, which facilitates HSPC egress from the BM into the circulation.

sMEK1 inhibits endothelial cell proliferation by attenuating VEGFR-2-dependent-Akt/eNOS/HIF-1α signaling pathways

The suppressor of MEK null (sMEK1) protein possesses pro-apoptotic activities. In the current study, we reveal that sMEK1 functions as a novel anti-angiogenic factor by suppressing vascular endothelial growth factor (VEGF)-induced cell proliferation, migration, and capillary-like tubular structure in vitro. In addition, sMEK1 inhibited the phosphorylation of the signaling components up- and downstream of Akt, including phospholipase Cγ1 (PLC-γ1), 3-phosphoinositide-dependent protein kinase 1 (PDK1), endothelial nitric oxide synthetase (eNOS), and hypoxia-inducible factor 1 (HIF-1α) during ovarian tumor progression via binding with vascular endothelial growth factor receptor 2 (VEGFR-2). Furthermore, sMEK1 decreased tumor vascularity and inhibited tumor growth in a xenograft human ovarian tumor model. These results supply convincing evidence that sMEK1 controls endothelial cell function and subsequent angiogenesis by suppressing VEGFR-2-mediated PI3K/Akt/eNOS signaling pathway. Taken together, our results clearly suggest that sMEK1 might be a novel anti-angiogenic and anti-tumor agent for use in ovarian tumor.

Role of VEGF in organogenesis

The cardiovascular system, consisting of the heart, blood vessels and hematopoietic cells, is the first organ system to develop in vertebrates and is essential for providing oxygen and nutrients to the embryo and adult organs. Work done predominantly using the mouse and zebrafish as model systems has demonstrated that Vascular Endothelial Growth Factor (VEGF, also known as VEGFA) and its receptors KDR (FLK1/VEGFR2), FLT1 (VEGFR1), NRP1 and NRP2 play essential roles in many different aspects of cardiovascular development, including endothelial cell differentiation, migration and survival as well as heart formation and hematopoiesis. This review will summarize the approaches taken and conclusions reached in dissecting the role of VEGF signalling in vivo during the development of the early cardiovasculature and other organ systems. The VEGF-mediated assembly of a functional vasculature is also a prerequisite for the proper formation of other organs and for tissue homeostasis, because blood vessels deliver oxygen and nutrients and vascular endothelium provides inductive signals to other tissues. Particular emphasis will therefore be placed in this review on the cellular interactions between vascular endothelium and developing organ systems, in addition to a discussion of the role of VEGF in modulating the behavior of nonendothelial cell populations.

Engraftment and reconstitution of hematopoiesis is dependent on VEGFR2-mediated regeneration of sinusoidal endothelial cells

Myelosuppression damages the bone marrow (BM) vascular niche, but it is unclear how regeneration of bone marrow vessels contributes to engraftment of transplanted hematopoietic stem and progenitor cells (HSPCs) and restoration of hematopoiesis. We found that chemotherapy and sublethal irradiation induced minor regression of BM sinusoidal endothelial cells (SECs), while lethal irradiation induced severe regression of SECs and required BM transplantation (BMT) for regeneration. Within the BM, VEGFR2 expression specifically demarcated a continuous network of arterioles and SECs, with arterioles uniquely expressing Sca1 and SECs uniquely expressing VEGFR3. Conditional deletion of VEGFR2 in adult mice blocked regeneration of SECs in sublethally irradiated animals and prevented hematopoietic reconstitution. Similarly, inhibition of VEGFR2 signaling in lethally irradiated wild-type mice rescued with BMT severely impaired SEC reconstruction and prevented engraftment and reconstitution of HSPCs. Therefore, regeneration of SECs via VEGFR2 signaling is essential for engraftment of HSPCs and restoration of hematopoiesis.

Endothelial progenitor cells and their potential therapeutic applications

Endothelial progenitor cells (EPCs) are derived from the bone marrow (BM) and peripheral blood (PB), contributing to tissue repair in various pathological conditions via the formation of new blood vessels, that is, neovascularization. EPCs can be mobilized into the circulation in response to growth factors and cytokines released following stimuli such as vascular trauma, wounding and cancer. EPCs are involved in vasculogenesis during embryogenesis, but are now recognized to have a significant bearing upon disease outcome through their contribution to neovascularization in a variety of pathological states in adulthood. EPCs exist in very small numbers, especially in circulating blood in adults where they only account for 0.01% of all cells. We discuss the contribution and potential therapeutic applications of EPCs in disease, also noting the prognostic value of PB EPC numbers, especially in heart disease and cancer.

Distinct roles of VEGFR-1 and VEGFR-2 in the aberrant hematopoiesis associated with elevated levels of VEGF

Vascular endothelial growth factor (VEGF), a major factor in tumor-host interactions, plays a critical role in the aberrant hematopoiesis observed in cancer-bearing hosts. To dissect the roles of VEGF receptor (VEGFR)-1 and VEGFR-2 in cancer-associated hematopoiesis in vivo, we selectively stimulated VEGFR-1 and VEGFR-2 by continuous infusion of receptor-specific ligands or selective blockade with VEGF receptor-specific antibodies in mice infused with recombinant VEGF at levels observed in tumor-bearing animals. We found that the effect of VEGF on the accumulation of Gr1(+)CD11b(+) cells is mediated by VEGFR-2, but that the 2 receptors have opposite effects on lymphocyte development. Pathophysiologic levels of VEGF strongly inhibit T-cell development via VEGFR-2, whereas VEGFR-1 signaling decreases this inhibition. VEGFR-1, and not VEGFR-2, signaling is responsible for the observed increase of splenic B cells. Both receptors are capable of inhibiting dendritic cell function. These data suggest that most of observed aberrant hematopoiesis caused by excess tumor-derived VEGF is mediated by VEGFR-2, and VEGFR-1 alone has very limited independent effects but clearly both positively and negatively modulates the effects of VEGFR-2. Our findings suggest that selective blockade of VEGFR-2 rather than of both receptors may optimally overcome the adverse hematologic consequences of elevated VEGF levels found in malignancy.

Vascular endothelial growth factor in amyotrophic lateral sclerosis and other neurodegenerative diseases

The angiogenic activity of vascular endothelial growth factor (VEGF) is well known. Recently, it has become evident that VEGF is involved in central nervous system physiology and may play a role in the pathogenesis of neurological diseases. In particular, it may be involved in the mechanism of motor neuron degeneration in amyotrophic lateral sclerosis (ALS), and has been hypothesized to be implicated in the pathogenesis of peripheral neuropathies such as occur in the so-called POEMS syndrome and diabetes. VEGF is also being studied as a possible treatment option in some of these disorders. In this review we critically analyze the data supporting the notion that VEGF is a factor involved in motor neuron degeneration and review the studies linking VEGF to other diseases of the peripheral and central nervous systems.

Activation of vascular endothelial growth factor receptor-2 in bone marrow leads to accumulation of myeloid cells: role of granulocyte-macrophage colony-stimulating factor

Vascular endothelial growth factor (VEGF) is a secreted cytokine that plays a major role in the formation and maintenance of the hemopoietic and vascular compartments. VEGF and its receptors, VEGFR-1 and VEGFR-2, have been found to be expressed on subsets of normal and malignant hemopoietic cells, but the role of the individual receptors in hemopoiesis requires further study. Using a VEGFR-2 fusion protein that can be dimerized with a synthetic drug, we were able to specifically examine the effects of VEGFR-2 signaling in hemopoietic cells in vivo. Mice transplanted with bone marrow transduced with this inducible VEGFR-2 fusion protein demonstrated expansion of myeloid cells (Gr-1+, CD11b+). Levels of myeloid progenitors were also increased following VEGFR-2 activation, through autocrine and paracrine mechanisms, as measured by clonogenic progenitor assays. VEGFR-2 activation induced expression of GM-CSF and increased serum levels in vivo. Abrogation of GM-CSF activity, either with neutralizing Abs or by using GM-CSF-null hemopoietic cells, inhibited VEGFR-2-mediated myeloid progenitor activity. Our findings indicate that VEGF signaling through VEGFR-2 promotes myelopoiesis through GM-CSF-dependent and -independent mechanisms.

Hematopoietic stem cells do not engraft with absolute efficiencies

Hematopoietic stem cells (HSCs) can be isolated from murine bone marrow by their ability to efflux the Hoechst 33342 dye. This method defines an extremely small and hematopoietically potent subset of cells known as the side population (SP). Recent studies suggest that transplanted single SP cells are capable of lymphohematopoietic repopulation at near absolute efficiencies. Here, we carefully reevaluate the hematopoietic potential of individual SP cells and find substantially lower rates of reconstitution. Our strategy involved the cotransplantation of single SP cells along with different populations of competitor cells that varied in their self-renewal capacity. Even with minimized HSC competition, SP cells were only able to reconstitute up to 35% of recipient mice. Furthermore, through immunophenotyping and clonal in vitro assays we find that SP cells are virtually homogeneous. Isolation of HSCs on the basis of Hoechst exclusion and a single cell-surface marker allows enrichment levels similar to that obtained with complex multicolor strategies. Altogether, our results indicate that even an extremely homogeneous HSC population, based on phenotype and dye efflux, cannot reconstitute mice at absolute efficiencies.

Angiogenic factors reconstitute hematopoiesis by recruiting stem cells from bone marrow microenvironment

The mechanism by which angiogenic factors recruit bone marrow (BM)-derived quiescent endothelial and hematopoietic stem cells (HSCs) is not known. Here, we report that functional vascular endothelial growth factor receptor-1 (VEGFR1, Flt-1) is expressed on a subpopulation of human CD34(+) and mouse Lin-Sca-1(+)c-Kit(+) BM-repopulating stem cells, conveying signals for recruitment of HSCs and reconstitution of hematopoiesis. Inhibition of VEGFR1 signaling, but not VEGFR2 (Flk-1, KDR), blocked HSC cell cycling, differentiation and hematopoietic recovery after BM suppression, resulting in the demise of the treated mice. Plasma elevation of placental growth factor (PlGF), which signals through VEGFR1, but not VEGFR2, restored hematopoiesis during the early and late phases following BM suppression. The mechanism whereby PlGF enhanced early phases of BM recovery was mediated directly through rapid chemotaxis of readily available VEGFR1(+) BM-repopulating and progenitor cells. The late phase of hematopoietic recovery was driven by PlGF-induced upregulation of matrix metalloproteinase-9 (MMP-9) in the BM, mediating the release of soluble Kit-ligand (sKitL). sKitL increased proliferation and motility of HSCs and progenitor cells, thereby augmenting hematopoietic recovery. PlGF promotes recruitment of VEGFR1(+) HSCs from a quiescent to a proliferative microenvironment within the BM, favoring differentiation, mobilization, and reconstitution of hematopoiesis.

Embryonic and adult vasculogenesis

Two mechanisms account for the formation of blood vessels, vasculogenesis and angiogenesis. Unfortunately, the terms vasculogenesis and angiogenesis literally have the same meaning, i.e., the genesis of blood vessels, and thus do little to distinguish between the two processes. Despite the nomenclature, the two processes are clearly distinct. Vasculogenesis, the de novo formation of blood vessels from mesoderm, is driven by the recruitment of undifferentiated mesodermal cells to the endothelial lineage and the de novo assembly of such cells into blood vessels. Angiogenesis is the generation of new blood vessels from endothelial cells of existing blood vessels, a process driven by endothelial cell proliferation. Recent years have seen dramatic changes in our understanding of the process of vasculogenesis, expanding the scope of its occurrence beyond the earliest stages of development to include involvement in neovascular processes throughout development as well as in the adult. In this review, emphasis is placed on discussion of emerging perspectives on the process of vasculogenesis in both the embryo and the adult.

The role of VEGF in normal and neoplastic hematopoiesis

VEGF is a secreted growth factor that mediates its biological effects by binding to two transmembrane tyrosine kinase receptors, VEGFR-1 and VEGFR-2. The VEGF/receptor signaling system is involved in the regulation of two fundamental processes in vertebrates: the formation of blood vessels (angiogenesis) and of blood cells (hematopoiesis). Hematopoietic stem cells, capable of giving rise to all blood cell lineages, are often found in clusters with endothelial cells, the key cell type involved in the formation of blood vessels. Despite such proximity of VEGF-responsive cells, hematopoiesis occurs independently of neoangiogenesis in the adult bone marrow, suggesting that VEGF regulates the two processes by different mechanisms. In support of this hypothesis, the recently identified autocrine loop by which VEGF may control hematopoietic stem cell survival and repopulation, is fundamentally different from its paracrine effects regulating angiogenesis. Furthermore, coexpression of VEGF and its receptors, the prerequisite for autocrine loops, is frequently found in lymphomas and myelomas, suggesting that autocrine loops also play a role in hematological malignancies. Several therapeutic strategies blocking VEGF or VEGF-induced signaling are currently being investigated for the treatment of neoplastic diseases. They differ in their potential to interfere with the autocrine or paracrine effector functions of VEGF during angiogenesis, hematopoiesis, and tumor cell proliferation, properties which may ultimately determine their therapeutic potential.

Placental growth factor reconstitutes hematopoiesis by recruiting VEGFR1(+) stem cells from bone-marrow microenvironment

The mechanism by which angiogenic factors recruit bone marrow (BM)-derived quiescent endothelial and hematopoietic stem cells (HSCs) is not known. Here, we report that functional vascular endothelial growth factor receptor-1 (VEGFR1) is expressed on human CD34(+) and mouse Lin(-)Sca-1(+)c-Kit(+) BM-repopulating stem cells, conveying signals for recruitment of HSCs and reconstitution of hematopoiesis. Inhibition of VEGFR1, but not VEGFR2, blocked HSC cell cycling, differentiation and hematopoietic recovery after BM suppression, resulting in the demise of the treated mice. Placental growth factor (PlGF), which signals through VEGFR1, restored early and late phases of hematopoiesis following BM suppression. PlGF enhanced early phases of BM recovery directly through rapid chemotaxis of VEGFR1(+) BM-repopulating and progenitor cells. The late phase of hematopoietic recovery was driven by PlGF-induced upregulation of matrix metalloproteinase-9, mediating the release of soluble Kit ligand. Thus, PlGF promotes recruitment of VEGFR1(+) HSCs from a quiescent to a proliferative BM microenvironment, favoring differentiation, mobilization and reconstitution of hematopoiesis.