Bone marrow as a source of endothelial cells for natural and iatrogenic vascular repair

Clonal Expansion in Cardiovascular Pathology

Clonal expansion refers to the proliferation and selection of advantageous "clones" that are better suited for survival in a Darwinian manner. In recent years, we have greatly enhanced our understanding of cell clonality in the cardiovascular context. However, our knowledge of the underlying mechanisms behind this clonal selection is still severely limited. There is a transpiring pattern of clonal expansion of smooth muscle cells and endothelial cells-and, in some cases, macrophages-in numerous cardiovascular diseases irrespective of their differing microenvironments. These findings indirectly suggest the possible existence of stem-like vascular cells which are primed to respond during disease. Subsequent clones may undergo further phenotypic changes to adopt either protective or detrimental roles. By investigating these clone-forming vascular cells, we may be able to harness this inherent clonal nature for future therapeutic intervention. This review comprehensively discusses what is currently known about clonal expansion across the cardiovascular field. Comparisons of the clonal nature of vascular cells in atherosclerosis (including clonal hematopoiesis of indeterminate potential), pulmonary hypertension, aneurysm, blood vessel injury, ischemia- and tumor-induced angiogenesis, and cerebral cavernous malformations are evaluated. Finally, we discuss the potential clinical implications of these findings and propose that proper understanding and specific targeting of these clonal cells may provide unique therapeutic options for the treatment of these cardiovascular conditions.

Piezoelectric Signals in Vascularized Bone Regeneration

The demand for bone substitutes is increasing in Western countries. Bone graft substitutes aim to provide reconstructive surgeons with off-the-shelf alternatives to the natural bone taken from humans or animal species. Under the tissue engineering paradigm, biomaterial scaffolds can be designed by incorporating bone stem cells to decrease the disadvantages of traditional tissue grafts. However, the effective clinical application of tissue-engineered bone is limited by insufficient neovascularization. As bone is a highly vascularized tissue, new strategies to promote both osteogenesis and vasculogenesis within the scaffolds need to be considered for a successful regeneration. It has been demonstrated that bone and blood vases are piezoelectric, namely, electric signals are locally produced upon mechanical stimulation of these tissues. The specific effects of electric charge generation on different cells are not fully understood, but a substantial amount of evidence has suggested their functional and physiological roles. This review summarizes the special contribution of piezoelectricity as a stimulatory signal for bone and vascular tissue regeneration, including osteogenesis, angiogenesis, vascular repair, and tissue engineering, by considering different stem cell sources entailed with osteogenic and angiogenic potential, aimed at collecting the key findings that may enable the development of successful vascularized bone replacements useful in orthopedic and otologic surgery.

Mechanisms of Endothelial Regeneration and Vascular Repair and Their Application to Regenerative Medicine

Endothelial barrier integrity is required for maintaining vascular homeostasis and fluid balance between the circulation and surrounding tissues and for preventing the development of vascular disease. Despite comprehensive understanding of the molecular mechanisms and signaling pathways that mediate endothelial injury, the regulatory mechanisms responsible for endothelial regeneration and vascular repair are incompletely understood and constitute an emerging area of research. Endogenous and exogenous reparative mechanisms serve to reverse vascular damage and restore endothelial barrier function through regeneration of a functional endothelium and re-engagement of endothelial junctions. In this review, mechanisms that contribute to endothelial regeneration and vascular repair are described. Targeting these mechanisms has the potential to improve outcome in diseases that are characterized by vascular injury, such as atherosclerosis, restenosis, peripheral vascular disease, sepsis, and acute respiratory distress syndrome. Future studies to further improve current understanding of the mechanisms that control endothelial regeneration and vascular repair are also highlighted.

miR-21 induces endothelial progenitor cells proliferation and angiogenesis via targeting FASLG and is a potential prognostic marker in deep venous thrombosis

BACKGROUND

Deep venous thrombosis (DVT) of lower extremities is a common thrombotic disease, occurring either in isolation or as a complication of other diseases or procedures. MiR-21 is one of important microRNAs which play critical role in various cellular function. This study aim to determine the effect of miR-21 on endothelial progenitor cells (EPCs) and its role in predicting prognosis of DVT.

METHODS

EPCs was isolated from DVT models and control subjects. miR-21 expression was confirmed by RT-PCR. Potential target mRNA was predicted by bioinformatics analysis. EPCs biological functions were examined by CCK-8 and tube formation assay. Besides, miR-21 expression was determined in DVT patients to investigate the correlation between miR-21 expression and prognosis of DVT. Cox proportional hazard regression analyses were also performed to reveal the risk factors associated with prognosis.

RESULTS

Here, we found miR-21 was downregulated in EPCs of DVT model rats. Increased miR-21 expression promoted proliferation and angiogenesis of EPCs. Moreover, we demonstrated that FASLG was a target of miR-21 and revealed that FASLG knockdown inhibited function of EPCs. Upregulation of miR-21 led to thrombus resolution in a rat model of venous thrombosis. In addition, lower expression level of miR-21 in DVT patients was associated with an increase of recurrent DVT and post thrombotic syndrome (PTS). Furthermore, Cox proportional hazard regression analyses demonstrated miR-21 expression level as an independent predictor of recurrence of DVT.

CONCLUSIONS

Our data revealed a role of miR-21 in regulating biological function of EPCs and could be a predictor for recurrent DVT or PTS.

Upregulation of miR-483-3p contributes to endothelial progenitor cells dysfunction in deep vein thrombosis patients via SRF

Background

Endothelial progenitor cells (EPCs) contribute to recanalization of deep vein thrombosis (DVT). This study aimed to detect miRNA expression profiles in EPCs from patients with DVT and characterize the role of miRNA in EPCs dysfunction.

Methods

EPCs was isolated from DVT patients and control subjects, and miRNA expression profiles were compared to screen differential miRNAs. The candidate miRNAs were confirmed by RT-PCR analysis. The targets of miRNA were identified by bioinformatics analyses, luciferase reporter assay and gene expression analyses. The apoptosis, migration and tube formation of EPCs were examined by flow cytometry, transwell assay and matrigel tube formation assay. A rat model of venous thrombosis was established as in vivo model.

Results

We identified miR-483-3p as a candidate miRNA upregulated in EPCs from DVT patients. By using miR-483-3p agomir and antagomir, we demonstrated that miR-483-3p decreased the migration and tube formation while increased the apoptosis of EPCs. Moreover, we identified serum response factor (SRF) as the target of miR-483-3p, and showed that SRF knockdown decreased the migration and tube formation while increased the apoptosis of EPCs. In addition, miR-483-3p inhibition led to enhanced ability of homing and thrombus resolution of EPCs in rat model of venous thrombosis.

Conclusions

miR-483-3p is upregulated in EPCs from DVT patients, and it targets SRF to decrease EPCs migration and tube formation and increase apoptosis in vitro, while decrease EPCs homing and thrombus resolution in vivo. MiR-483-3p is a potential therapeutic target in DVT treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-016-0775-2) contains supplementary material, which is available to authorized users.

Stimulation of vasculogenesis and leukopoiesis of embryonic stem cells by extracellular transfer RNA and ribosomal RNA

OBJECTIVE

Cell injury releases nucleic acids supporting inflammation and stem cell activation. Here, the impact of extracellular ribonucleic acid, especially transfer RNA (ex-tRNA), on vasculogenesis and leukopoiesis of mouse embryonic stem (ES) cells was investigated.

APPROACH AND RESULTS

ex-tRNA, whole cell RNA and ribosomal RNA (ex-rRNA) but not DNA increased CD31-positive vascular structures in embryoid bodies. Ex-tRNA and ex-rRNA increased numbers of VEGFR2(+), CD31(+) and VE-cadherin(+) vascular cells as well as CD18(+), CD45(+) and CD68(+) cells, indicating leukocyte/macrophage differentiation. This was paralleled by mRNA and protein expression of hypoxia-inducible factor-1α (HIF-1α), vascular endothelial growth factor-165 (VEGF165) and neuropilin 1 (NRP1), phosphorylation of phosphatidyl inositol 3-kinase (PI3K) and VEGF receptor 2 (VEGFR2) as well as mRNA expression of α-smooth muscle actin (α-SMA). ex-tRNA was taken up by endosomes, increased expression of the pro-angiogenic semaphorin B4 receptor plexin B1 as well as the ephrin-type B receptor 4 (EphB4) and ephrinB2 ligand and enhanced cell migration, which was inhibited by the VEGFR2 antagonist SU5614 and the PI3K inhibitor LY294002. This likewise abolished the effects of ex-tRNA on vasculogenesis and leukopoiesis of ES cells. Ex-tRNA increased NOX1, NOX2, NOX4 and DUOX2 mRNA and boosted the generation of superoxide and hydrogen peroxide which was inhibited by radical scavengers, the NADPH oxidase inhibitors apocynin, VAS2870, ML171, and plumbagin as well as shRNA silencing of NOX1 and NOX4.

CONCLUSIONS

Our findings indicate that ex-tRNA treatment induces vasculogenesis and leukopoiesis of ES cells via superoxide/hydrogen peroxide generated by NADPH oxidase and activation of VEGFR2 and PI3K.

Therapeutic manipulation of angiogenesis with miR-27b

BACKGROUND

Multiple studies demonstrated pro-angiogenic effects of microRNA (miR)-27b. Its targets include Notch ligand Dll4, Sprouty (Spry)-2, PPARγ and Semaphorin (SEMA) 6A. miR-27 effects in the heart are context-dependent: although it is necessary for ventricular maturation, targeted overexpression in cardiomyocytes causes hypertrophy and dysfunction during development. Despite significant recent advances, therapeutic potential of miR-27b in cardiovascular disease and its effects in adult heart remain unexplored. Here, we assessed the therapeutic potential of miR-27b mimics and inhibitors in rodent models of ischemic disease and cancer.

METHODS

We have used a number of models to demonstrate the effects of miR-27b mimicry and inhibition in vivo, including subcutaneous Matrigel plug assay, mouse models of hind limb ischemia and myocardial infarction and subcutaneous Lewis Lung carcinoma.

RESULTS

Using mouse model of myocardial infarction due to the coronary artery ligation, we showed that miR-27b mimic had overall beneficial effects, including increased vascularization, decreased fibrosis and increased ejection fraction. In mouse model of critical limb ischemia, miR-27b mimic also improved tissue re-vascularization and perfusion. In both models, miR-27b mimic clearly decreased macrophage recruitment to the site of hypoxic injury. In contrast, miR-27b increased the recruitment of bone marrow derived cells to the neovasculature, as was shown using mice reconstituted with fluorescence-tagged bone marrow. These effects were due, at least in part, to the decreased expression of Dll4, PPARγ and IL10. In contrast, blocking miR-27b significantly decreased vascularization and reduced growth of subcutaneous tumors and decreased BMDCs recruitment to the tumor vasculature.

CONCLUSIONS

Our study demonstrates the utility of manipulating miR-27b levels in the treatment of cardiovascular disease and cancer.

Adaptive learning can result in a failure to profit from good conditions: implications for understanding depression

BACKGROUND AND OBJECTIVES

Depression is a major medical problem diagnosed in an increasing proportion of people and for which commonly prescribed psychoactive drugs are frequently ineffective. Development of treatment options may be facilitated by an evolutionary perspective; several adaptive reasons for proneness to depression have been proposed. A common feature of many explanations is that depressive behaviour is a way to avoid costly effort where benefits are small and/or unlikely. However, this viewpoint fails to explain why low mood persists when the situation improves. We investigate whether a behavioural rule that is adapted to a stochastically changing world can cause inactivity which appears similar to the effect of depression, in that it persists after the situation has improved.

METHODOLOGY

We develop an adaptive learning model in which an individual has repeated choices of whether to invest costly effort that may result in a net benefit. Investing effort also provides information about the current conditions and rates of change of the conditions.

RESULTS

An individual following the optimal behavioural strategy may sometimes remain inactive when conditions are favourable (i.e. when it would be better to invest effort) when it is poorly informed about the current environmental state. Initially benign conditions can predispose an individual to inactivity after a relatively brief period of negative experiences.

CONCLUSIONS AND IMPLICATIONS

Our approach suggests that the antecedent factors causing depressed behaviour could go much further back in an individual s history than is currently appreciated. The insights from our approach have implications for the ongoing debate about best treatment options for patients with depressive symptoms.

Mononuclear cells and vascular repair in HHT

Hereditary hemorrhagic telangiectasia (HHT) or Rendu–Osler–Weber disease is a rare genetic vascular disorder known for its endothelial dysplasia causing arteriovenous malformations and severe bleedings. HHT-1 and HHT-2 are the most prevalent variants and are caused by heterozygous mutations in endoglin and activin receptor-like kinase 1, respectively. An undervalued aspect of the disease is that HHT patients experience persistent inflammation. Although endothelial and mural cells have been the main research focus trying to unravel the mechanism behind the disease, wound healing is a process with a delicate balance between inflammatory and vascular cells. Inflammatory cells are part of the mononuclear cells (MNCs) fraction, and can, next to eliciting an immune response, also have angiogenic potential. This biphasic effect of MNC can hold a promising mechanism to further elucidate treatment strategies for HHT patients. Before MNC are able to contribute to repair, they need to home to and retain in ischemic and damaged tissue. Directed migration (homing) of MNCs following tissue damage is regulated by the stromal cell derived factor 1 (SDF1). MNCs that express the C-X-C chemokine receptor 4 (CXCR4) migrate toward the tightly regulated gradient of SDF1. This directed migration of monocytes and lymphocytes can be inhibited by dipeptidyl peptidase 4 (DPP4). Interestingly, MNC of HHT patients express elevated levels of DPP4 and show impaired homing toward damaged tissue. Impaired homing capacity of the MNCs might therefore contribute to the impaired angiogenesis and tissue repair observed in HHT patients. This review summarizes recent studies regarding the role of MNCs in the etiology of HHT and vascular repair, and evaluates the efficacy of DPP4 inhibition in tissue integrity and repair.

Antiphospholipid antibodies attenuate endothelial repair and promote neointima formation in mice

Background

Antiphospholipid syndrome patients have antiphospholipid antibodies (aPLs) that promote thrombosis, and they have increased cardiovascular disease risk. Although the basis for the thrombosis has been well delineated, it is not known why antiphospholipid syndrome patients also have an increased prevalence of nonthrombotic vascular occlusion. The aims of this work were to determine if aPLs directly promote medial hypertrophy or neointima formation in mice and to identify the underlying mechanisms.

Methods and Results

Medial hypertrophy and neointima formation invoked by carotid artery endothelial denudation were evaluated in mice administered normal human IgG or aPLs. While aPLs had no effect on medial hypertrophy, they caused exaggerated neointima development. This was related to an aPL‐induced impairment in reendothelialization post denudation, and scratch assays in cell culture revealed that there are direct effects of aPLs on endothelium that retard cell migration. Further experiments showed that aPL antagonism of endothelial migration and repair is mediated by antibody recognition of β2‐glycoprotein I, apolipoprotein E receptor 2, and a decline in bioavailable NO. Consistent with these mechanisms, the adverse impacts of aPLs on reendothelialization and neointima formation were fully prevented by the NO donor molsidomine.

Conclusions

APLs blunt endothelial repair, and there is related aPL‐induced exaggeration in neointima formation after endothelial injury in mice. The initiating process entails NO deficiency mediated by β2‐glycoprotein I recognition by aPLs and apolipoprotein E receptor 2. The modulation of endothelial apolipoprotein E receptor 2 function or NO bioavailability may represent new interventions to prevent the nonthrombotic vascular occlusion and resulting cardiovascular disorders that afflict antiphospholipid syndrome patients.

Inhibitor of DNA binding 1 as a secreted angiogenic transcription factor in rheumatoid arthritis

Introduction

Rheumatoid arthritis (RA) is characterized by enhanced blood vessel development in joint synovium. This involves the recruitment of endothelial progenitor cells (EPCs), allowing for de novo vessel formation and pro-inflammatory cell infiltration. Inhibitor of DNA Binding 1 (Id1) is a transcription factor characteristic of EPCs that influences cell maturation.

Method

Enzyme-linked immunosorbant assay (ELISA) and polymerase chain reaction (PCR) were used to examine Id1 levels in synovial fluid (SF) and endothelial cells (ECs), respectively. Immunohistology was used to determine the expression of Id1 in synovial tissue (ST). Human dermal microvascular EC (HMVEC) migration and tube forming assays were used to determine if recombinant human Id1 (rhuId1) and/or RA SF immunodepleted Id1 showed angiogenic activity. We also utilized the RA ST severe combined immunodeficient (SCID) mouse chimera to examine if Id1 recruits EPCs to RA synovium.

Results

ST samples immunostained for Id1 showed heightened expression in RA compared to osteoarthritis (OA) and normal (NL) ST. By immunofluorescence staining, we found significantly more Id1 in RA compared to OA and NL vasculature, showing that Id1 expressing cells, and therefore EPCs, are most active in vascular remodeling in the RA synovium. We also detected significantly more Id1 in RA compared to OA and other arthritis SFs by ELISA, which correlates highly with Chemokine (C-X-C motif) ligand 16 (CXCL16) levels. In vitro chemotaxis assays showed that Id1 is highly chemotactic for HMVECs and can be attenuated by inhibition of Nuclear Factor κB and phosphoinositide 3-kinase. Using in vitro Matrigel assays, we found that HMVECs form tubes in response to rhuId1 and that Id1 immunodepleted from RA SF profoundly decreases tube formation in Matrigel in vitro. PCR showed that Id1 mRNA could be up-regulated in EPCs compared to HMVECs in response to CXCL16. Finally, using the K/BxN serum induced arthritis model, we found that EC CXCR6 correlated with Id1 expression by immunohistochemistry.

Conclusions

We conclude that Id1 correlates highly with CXCL16 expression, EPC recruitment, and blood vessel formation in the RA joint, and that Id1 is potently angiogenic and can be up-regulated in EPCs by CXCL16.

Bone marrow transplantation transfers age-related susceptibility to neovascular remodeling in murine laser-induced choroidal neovascularization

PURPOSE

Neovascular remodeling (NVR), the progression of small capillaries into large-caliber arterioles with perivascular fibrosis, represents a major therapeutic challenge in neovascular age-related macular degeneration (AMD). Neovascular remodeling occurs after laser-induced choroidal neovascularization (CNV) in aged but not young mice. Additionally, bone marrow-derived cells, including macrophages, endothelial precursor cells, and mesenchymal precursor cells, contribute to CNV severity. In this study, we investigated the impact of aged bone marrow transplantation (BMT) on the degree of fibrosis, size, and vascular morphology of CNV lesions in a mouse model of laser-induced CNV.

METHODS

Young (2 months) and old (16 months) mice were transplanted with green fluorescent protein (GFP)-labeled bone marrow isolated from either young or old donors. Laser CNV was induced 1 month following transplant, and eyes were analyzed via choroidal flat mounts and immunohistochemistry 1 month postlaser. The identity of cells infiltrating CNV lesions was determined using specific markers for the labeled transplanted cells (GFP+), macrophages (F4/80+), perivascular mesenchymal-derived cells (smooth muscle actin, SMA+), and endothelial cells (CD31+).

RESULTS

Bone marrow transplantation from aged mice transferred susceptibility to NVR into young recipients. Inversely, transplantation of young marrow into old mice prevented NVR, preserving small size and minimal fibrosis. Mice with NVR demonstrated a greater relative contribution of marrow-derived SMA+ perivascular mesenchymal cells as compared to other cells.

CONCLUSIONS

Our findings indicate that the status of bone marrow is an important determining factor of neovascular severity. Furthermore, we find that perivascular mesenchymal cells, rather than endothelial cells, derived from aged bone marrow may contribute to increased CNV severity in this murine model of experimental neovascularization.

Isolation and characterization of mouse bone marrow-derived Lin⁻/VEGF-R2⁺ progenitor cells

Circulating endothelial progenitor cells (EPCs) in the peripheral blood (PB) have physiological roles in the maintenance of the existing vascular beds and rescue of vascular injury. In this study, we have evaluated the properties of Lin⁻/VEGF-R2⁺ progenitor cells isolated from the mouse bone marrow (BM) and further studied their distribution and integration in an animal model of laser-induced retinal vascular injury. Lin⁻/VEGF-R2⁺ cells were enriched from C57BL/6 mice BM using magnetic cell sorting with hematopoietic lineage (Lin) depletion followed by VEGF-R2 positive selection. Lin⁻/VEGF-R2⁺ BM cells were characterized using flow cytometry and immunocytochemistry and further tested for colony formation during culture and tube formation on Matrigel®. Lin⁻/VEGF-R2⁺ BM cells possessed typical EPC properties such as forming cobble-stone shaped colonies after 3 to 4 weeks of culture, CD34⁺ expression, take up of Dil-acLDL and binding to Ulex europaeus agglutinin. However, they did not form tube-like structures on Matrigel®. The progenitor cells retained their phenotype over extended period of culture. After intravitreal transplantation in eyes subjected to the laser-induced retinal vascular injury, some Lin⁻/VEGF-R2⁺ cells were able to integrate into the damaged retinal vasculature but the level of cell integration seemed less efficient when compared with previous reports in which EPCs from the human PB were employed. Our results indicate that Lin⁻/VEGF-R2⁺ cells isolated from the mouse BM share some similarities to EPCs from the human PB but most of them are at a very early stage of maturation and remain quiescent during culture and after intravitreal transplantation.

Early onset of endothelial cell proliferation in coronary thrombi of patients with an acute myocardial infarction: implications for plaque healing

AIMS

Coronary thrombotic occlusion in ST-segment elevation myocardial infarction (STEMI) patients is often preceded by episodes of progressive growth of the thrombus mass. Similar to wound healing, the organization of thrombus could depend on ingrowth of microvessels in order to stabilize its structure. We investigated the patterns of neovascularization in different stages of coronary thrombus evolution.

MATERIAL AND METHODS

Thrombectomy materials obtained from STEMI patients were histologically classified according to thrombus age in three groups: fresh (< 1 day), lytic (1-5 days) or organized (> 5 days) thrombi. Forty thrombi of each group were randomly collected. Neovascularization in the thrombi was evaluated histomorphologically and with immunodouble stains to visualize various differentiation antigens of endothelial cells (ECs) and primitive cells.

RESULTS

Morphologically, ECs in the coronary thrombi manifested as: single cells, cell clusters or microvessels. CD31+/CD34+ ECs were present in 98% of all the thrombi. In addition, endothelial clusters were found in 63% of the fresh thrombi (< 1 day). CD105+, Ki67+, or C-kit+ ECs (active, proliferating cells) were observed in all the stages, but significantly more in organized thrombi (> 5 days) compared with fresh and lytic ones (< 5 days), and mainly as cell clusters (P ≤ 0.05 for all). CD133+ primitive cells were found only sporadically in 11% of all the samples.

CONCLUSION

EC proliferation is initiated very early, and gradually progresses during the organization process of thrombus after coronary plaque disruption, with only a limited contribution of primitive cells in this process.

Maintenance and repair of the lung endothelium does not involve contributions from marrow-derived endothelial precursor cells

Lung endothelium is believed to be a quiescent tissue with the potential to exhibit rapid and effective repair after injury. Endothelial progenitor cells derived from the bone marrow have been proposed as one source of new endothelial cells that may directly contribute to pulmonary endothelial cell homeostasis and repair. Here we use bone marrow transplantation models, using purified hematopoietic stem cells (HSCs) or unfractionated whole marrow, to assess engraftment of cells in the endothelium of a variety of tissues. We find scant evidence for any contribution of bone marrow-derived cells to the pulmonary endothelium in the steady state or after recovery from hyperoxia-induced endothelial injury. Although a rare population of CD45-/CD31+/VECadherin+ bone marrow-derived cells, originating from HSCs, can be found in lung tissue after transplantation, these cells are not readily found in anatomic locations that define the pulmonary endothelium. Moreover, by tracking transplanted bone marrow cells obtained from donor transgenic mice containing endothelial lineage-selective reporters (Tie2-GFP), no contribution of bone marrow-derived cells to the adult lung, liver, pancreas, heart, and kidney endothelium can be detected, even after prolonged follow-up periods of 11 months or after recovery from hyperoxic pulmonary endothelial injury. Our findings argue against any significant engraftment of bone marrow-derived cells in the pulmonary vascular endothelium.

Interaction of endothelial progenitor cells expressing cytosine deaminase in tumor tissues and 5-fluorocytosine administration suppresses growth of 5-fluorouracil-sensitive liver cancer in mice

The drug delivery system to tumors is a critical factor in upregulating the effect of anticancer drugs and reducing adverse events. Recent studies indicated selective migration of bone marrow-derived endothelial progenitor cells (EPC) into tumor tissues. Cytosine deaminase (CD) transforms nontoxic 5-fluorocytosine (5-FC) into the highly toxic 5-fluorouracil (5-FU). We investigated the antitumor effect of a new CD/5-FC system with CD cDNA transfected EPC for hepatocellular carcinoma (HCC) in mice. We used human hepatoma cell lines (HuH-7, HLF, HAK1-B, KYN-2, KIM-1) and a rat EPC cell line (TR-BME-2). Escherichia coli CD cDNA was transfected into TR-BME-2 (CD-TR-BME). The inhibitory effect of 5-FU on the proliferation of hepatoma cell lines and the inhibitory effect of 5-FU secreted by CD-TR-BME and 5-FC on the proliferation of co-cultured hepatoma cells were evaluated by a tetrazolium-based assay. In mouse subcutaneous xenograft models of KYN-2 and HuH-7, CD-TR-BME was transplanted intravenously followed by 5-FC injection intraperitoneally. HuH-7 cells were the most sensitive to 5-FU and KYN-2 cells were the most resistant. CD-TR-BME secreted 5-FU and inhibited HuH-7 proliferation in a 5-FC dose-dependent manner. CD-TR-BME were recruited into the tumor tissues and some were incorporated into tumor vessels. Tumor growth of HuH-7 was significantly suppressed during 5-FC administration. No bodyweight loss, ALT abnormality or bone marrow suppression was observed. These findings suggest that our new CD/5-FC system with CD cDNA transfected EPC could be an effective and safe treatment for suppression of 5-FU-sensitive HCC growth.

The impact of bioactive lipids on cardiovascular development

Lysophospholipids comprise a group of bioactive molecules with multiple biological functions. The cardinal members of this signalling molecule group are sphingosylphosphorylcholine (SPC), lysophosphatidic acid (LPA), and sphingosine 1-phosphate (S1P) which are, at least in part, homologous to each other. Bioactive lipids usually act via G-protein coupled receptors (GPCRs), but can also function as direct intracellular messengers. Recently, it became evident that bioactive lipids play a role during cellular differentiation development. SPC induces mesodermal differentiation of mouse ES cells and differentiation of promyelocytic leukemia cells, by a mechanism being critically dependent on MEK-ERK signalling. LPA stimulates the clonal expansion of neurospheres from neural stem/progenitor cells and induces c-fos via activation of mitogen- and stress-activated protein kinase 1 (MSK1) in ES cells. S1P acts on hematopoietic progenitor cells as a chemotactic factor and has also been found to be critical for cardiac and skeletal muscle regeneration. Furthermore, S1P promotes cardiogenesis and similarly activates Erk signalling in mouse ES cells. Interestingly, S1P may also act to maintain human stem cell pluripotency. Both LPA and S1P positively regulate the proliferative capacity of murine ES cells. In this paper we will focus on the differential and developmental impact of lysophospholipids on cardiovascular development.

Improved function of circulating angiogenic cells is evident in type 1 diabetic islet-transplanted patients

Circulating angiogenic cells (CACs) are vascular-committed bone marrow-derived cells that are dysfunctional in type 1 diabetes (T1D). Here we studied whether restoration of normoglycemia following islet transplantation is associated with better CAC function. We carried out a cross-sectional study of 18 T1D patients, 14 insulin-independent islet-transplanted patients (ITA) and 14 healthy controls (C) evaluating in vivo and in vitro CACs viability and function. We found that the percentage of CACs in vivo did not differ among the three groups while the number of CAC colonies obtained from T1D, but not from ITA, was reduced compared to C (C = 7.3 ± 1.9, T1D = 0.9 ± 0.4 and ITA = 4.7 ± 1.9; p < 0.05 T1D vs. all). In vitro CAC migration/differentiation were similar, while in vivo an improved angiogenic ability of ITA compared to T1D was shown (capillary density: C = 93.5 ± 22.1, T1D = 19.2 ± 2.8 and ITA = 44.0 ± 10.5, p < 0.05 T1D vs. all). Increased apoptosis and lesser IL-8 secretion were evident in CACs obtained from T1D compared to C and ITA. in vitro addition of anti-hIL-8 reduced the number of colonies obtained from C. Finally, T1D, but not ITA, had a lower endothelial-dependent dilatation (EDD) compared with C. These data suggest that CAC function is altered in T1D and may be improved after islet transplantation.

Erythropoietin augments the efficacy of therapeutic angiogenesis induced by allogenic bone marrow stromal cells in a rat model of limb ischemia

Transplantation of adult marrow stromal cells (MSCs) has been developed as a new method of treating severe ischemia diseases by therapeutic angiogenesis. Erythropoietin (EPO) is capable of inducing angiogenesis and inhibiting MSCs apoptosis. The effect of EPO on the therapeutic potency of MSCs transplantation in a rat model of limb ischemia was investigated in the current study. The results indicate that the combined treatment with MSC transplantation and EPO infusion is superior to MSC transplantation alone in the treatment of limb ischemia. MSCs transplantation and EPO infusion could enhance the angiogenic effect of each other to achieve a better therapeutic effect. This combination therapy may become a more effective approach for ischemia diseases of the limbs.

New insights in vascular development: vasculogenesis and endothelial progenitor cells

In the course of new blood vessel formation, two different processes--vasculogenesis and angiogenesis--have to be distinguished. The term vasculogenesis describes the de novo emergence of a vascular network by endothelial progenitors, whereas angiogenesis corresponds to the generation of vessels by sprouting from pre-existing capillaries. Until recently, it was thought that vasculogenesis is restricted to the prenatal period. During the last decade, one of the most fascinating innovations in the field of vascular biology was the discovery of endothelial progenitor cells and vasculogenesis in the adult. This review aims at introducing the concept of adult vasculogenesis and discusses the efforts to identify and characterize adult endothelial progenitors. The different sources of adult endothelial progenitors like haematopoietic stem cells, myeloid cells, multipotent progenitors of the bone marrow, side population cells and tissue-residing pluripotent stem cells are considered. Moreover, a survey of cellular and molecular control mechanisms of vasculogenesis is presented. Recent advances in research on endothelial progenitors exert a strong impact on many different disciplines and provide the knowledge for functional concepts in basic fields like anatomy, histology as well as embryology.

Capillary regeneration in scleroderma: stem cell therapy reverses phenotype?

BACKGROUND

Scleroderma is an autoimmune disease with a characteristic vascular pathology. The vasculopathy associated with scleroderma is one of the major contributors to the clinical manifestations of the disease.

METHODOLOGY/PRINCIPAL FINDINGS

We used immunohistochemical and mRNA in situ hybridization techniques to characterize this vasculopathy and showed with morphometry that scleroderma has true capillary rarefaction. We compared skin biopsies from 23 scleroderma patients and 24 normal controls and 7 scleroderma patients who had undergone high dose immunosuppressive therapy followed by autologous hematopoietic cell transplant. Along with the loss of capillaries there was a dramatic change in endothelial phenotype in the residual vessels. The molecules defining this phenotype are: vascular endothelial cadherin, a supposedly universal endothelial marker required for tube formation (lost in the scleroderma tissue), antiangiogenic interferon alpha (overexpressed in the scleroderma dermis) and RGS5, a signaling molecule whose expression coincides with the end of branching morphogenesis during development and tumor angiogenesis (also overexpressed in scleroderma skin. Following high dose immunosuppressive therapy, patients experienced clinical improvement and 5 of the 7 patients with scleroderma had increased capillary counts. It was also observed in the same 5 patients, that the interferon alpha and vascular endothelial cadherin had returned to normal as other clinical signs in the skin regressed, and in all 7 patients, RGS5 had returned to normal.

CONCLUSION/SIGNIFICANCE

These data provide the first objective evidence for loss of vessels in scleroderma and show that this phenomenon is reversible. Coordinate changes in expression of three molecules already implicated in angiogenesis or anti-angiogenesis suggest that control of expression of these three molecules may be the underlying mechanism for at least the vascular component of this disease. Since rarefaction has been little studied, these data may have implications for other diseases characterized by loss of capillaries including hypertension, congestive heart failure and scar formation.

Bone marrow-derived endothelial progenitor cells do not contribute significantly to new vessels during incisional wound healing

OBJECTIVE

To assess the contribution of bone marrow (BM)-derived endothelial progenitor cells (EPCs) to the neovascularisation of cutaneous incisional wounds.

METHODS

Lethally irradiated C57Bl/6 mice were transplanted with BM mononuclear cells from Tie2/lacZ mice, which constitutively overexpressed beta-galactosidase (beta-gal) in endothelial cells (ECs). Chimeras were wounded and the number of X-gal-stained (beta-gal(+)) BM-derived EPCs were calculated in histological wound sections.

RESULTS

EPCs were measured in skin sections from unwounded BM transplant (BMT) mice, or at day 1 and 3 postwounding, at the level of 0.1 +/- 0.1 (mean +/- SEM) per skin/wound section. In day-5 to day-14 wounds, the number of EPCs increased gradually (1.3 +/- 0.5 at day 5 and 4.8 +/- 0.9 at day 10), peaking at day 14, when there was a significant increase in the number of EPCs per wound section (6.5 +/- 1.7) when compared to unwounded skin. Between days 14 and 18 postwounding, there was a rapid fall-off in the number of beta-gal(+) EPCs (0.8 +/- 0.5 at day 18) and numbers returned to baseline by day 21 (0.1 +/- 0.1). No evidence of vascular structures derived from BM-derived EPCs ("in situ" vasculogenesis) was observed and it was calculated that these cells contributed only 4.4% +/- 1.5% to total wound ECs at their peak.

CONCLUSION

These findings indicate that the revascularization of dermal incisional wounds primarily occurs through angiogenesis because the low frequency and temporal expression of EPCs suggests that they do not make a significant contribution to the neovascularization process.

Granulocyte colony-stimulating factor and granulocyte macrophage colony-stimulating factor exacerbate atherosclerosis in apolipoprotein E-deficient mice

BACKGROUND

Recent studies have suggested a potential contribution of bone marrow-derived progenitor cells to vascular repair. Preliminary clinical studies have explored the possibility that mobilization of progenitor cells with granulocyte macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF) can affect vascular repair. However, it is not known whether the short-term administration of G-CSF or GM-CSF exerts beneficial effects on atherosclerosis.

METHODS AND RESULTS

Apolipoprotein E-deficient mice were treated with either GM-CSF or G-CSF at a dose of 10 microg x kg(-1) x d(-1) s.c. administered daily for 5 days per week on alternating weeks for a total of 20 doses over an 8-week treatment period. We found that in animals maintained on a high-fat diet, both G-CSF and GM-CSF actually demonstrated an increase in atherosclerotic lesion extent. The increase in atherosclerotic extent was not associated with an increase in either inflammatory cells or expression of proinflammatory genes. Interestingly, adventitial vascularity significantly increased, suggesting a mechanistic role for vasa vasorum neovascularization.

CONCLUSIONS

These findings demonstrate that in this animal model of atherosclerosis, not only did administration of G-CSF or GM-CSF fail to demonstrate any beneficial therapeutic effect, but both resulted in a worsening of atherosclerosis.

Niche-to-niche migration of bone-marrow-derived cells

During ontogenesis, haematopoietic stem cells (HSCs) relocate between extra-embryonic and embryonic compartments. Similarly, site-specific homing of HSCs is ongoing during adulthood. With the expanding knowledge of HSC physiology, a new paradigm emerges in which HSCs and haematopoietic progenitor cells (HPCs) migrate to defined microenvironments within the bone marrow (BM) and to 'activated' or 'inducible' niches elsewhere. Here, we summarize current understanding of HSC niche characteristics, and the physiological and pathological mechanisms that guide HSC homing both within the BM and to distant niches in the periphery, promoting new vessel growth in tumours and ischaemia. Recent observations suggest that features of the HSC niche might also be recapitulated in pre-metastatic sites. Clusters of BM-derived HPCs promote invasion of disseminating cancer cells. Clear clinical benefits can be foreseen by modulating HSCs and their microenvironments, in promoting tissue regeneration, and inhibiting tumourigenesis and cancer metastasis.

Overexpression of CXCR4 Increases Migration and Proliferation of Human Adipose Tissue Stromal Cells

Stromal-derived factor-1 (SDF-1)-mediated CXCR4 signaling plays important roles in migration, engraftment, and proliferation of stem cells. We report here that CXCR4 overexpression on human adipose tissue stromal cells (hADSCs) using a lentiviral gene transfer technique helped navigate these cells to the injured tissues in response to SDF-1 signaling. Transduced hADSCs, expressing high levels of CXCR4, displayed an increased capacity for cellular growth and protection against etoposide-induced cell death. CXCR4-overexpressed cells showed higher ERK activity than that of vector-transduced cells. U0126, an ERK inhibitor, and AMD3100, a CXCR4 antagonist, inhibited the proliferation of CXCR4 overexpression-induced proliferation and ERK phosphorylation. CXCR4-overexpressing cells showed increased level of beta-catenin and luciferase activity driven by the Tcf promoter. Our results suggest CXCR4 overexpression for improved hADSC motility, retention, and proliferation could be beneficial for in vivo navigation and expansion of stem cells.

Regenerative medicine for cardiovascular disorders-new milestones: adult stem cells

Cardiovascular disorders are the leading causes of mortality and morbidity in the developed world. Cell-based modalities have received considerable scientific attention over the last decade for their potential use in this clinical arena. This review was intended as a brief overview on the subject of therapeutic potential of adult stem cells in cardiovascular medicine with basic science findings and the current status of clinical applications. The historical perspective and basic concepts are reviewed and a description of current applications and potential adverse effects in cardiovascular medicine is given. Future improvements on cell-based therapies will likely provide remarkable improvement in survival and quality of life for millions of patients with cardiovascular disorders.

On metronomic chemotherapy: modulation of angiogenesis mediated by VEGE-A

Tumors are angiogenesis dependent. Preclinical studies have shown that well-tolerated continuous low dose, i.e. metronomic, chemotherapy can exert significant antiangiogenic effects per se and thereby a greater antitumor influence than conventional chemotherapy with high, spaced-out bolus doses. There are however, no means of quantitatively assessing the antiangiogenic effect of chemotherapy in tumors. We therefore used a surrogate tumor-free, non-surgical rat mesentery model and quantitatively studied the dose effect of metronomic treatment with cisplatin, cyclophosphamide, doxorubicin, fluorouracil and paclitaxel on VEGF-A-mediated angiogenesis, a characteristic of tumors. Cyclophosphamide and paclitaxel treatment exerted significant dose-dependent antiangiogenic effects, whereas doxorubicin treatment produced insignificant effects. By contrast, metronomic cisplatin and fluorouracil treatment occasionally significantly stimulated angiogenesis in a dose-dependent, non-linear manner. To our knowledge, this is the first report of metronomic chemotherapy stimulating angiogenesis in vivo. The data suggest that the angiogenic response to cisplatin, cyclophosphamide, fluorouracil and paclitaxel was significantly influenced by the presence of antioxidants in the vehicles or when co-treated with N-acetylcystein, a widely used free-radical scavenger. The data relating to the metronomic scheduling were compared with bolus treatment data for the identical agent formulations in the same experimental model. Cisplatin, cyclophosphamide and paclitaxel caused approximately the same overall, agent-specific angiogenesis-modulating effects following metronomic and bolus treatments. Moreover, apparently secondary delayed effects of chemotherapy affected capillary sprouting.

Mechanisms of endothelial cell guidance and vascular patterning in the developing mouse retina

The appropriate guidance and patterning of vessels during vascular development is critical for proper tissue function. The loss of these guidance mechanisms can lead to abnormal vascularization and a number of pathological conditions. The molecular basis of endothelial cell guidance and subsequent tissue specific vascular patterning remains largely unknown in spite of its clinical relevance and biological importance. In this regard, retinal vascular development offers many advantages for studying endothelial cell guidance and the mechanisms by which characteristic vascular patterns are formed. In this review, we will provide an overview of the known mechanisms that mediate vascular patterning during mouse retinal development, synthesizing these data to formulate a model of how growth factors, cellular adhesion molecules, and vascular-associated cells mediate directed endothelial cell migration and appropriate vascular remodeling. Finally, we will discuss the many aspects of retinal vascular development that remain unknown and cite evidence that many of these gaps may be addressed by further studying the guidance cues shared by vascular and neuronal elements in the retina and other parts of the central nervous system.

Endothelial cell proliferation in the choriocapillaris during human retinal differentiation

BACKGROUND

Differentiation patterns of the neural retina and its retinal vasculature are not well matched. The foveal region differentiates first, however the central retina is not vascularised until late in gestation. The authors explored the hypothesis that higher rates of endothelial cell proliferation in the choriocapillaris of the central retina might compensate for the slow growth of central retinal vessels, providing supplementary nutrients to the region during the early stages of neuronal maturation.

METHODS

Frozen sections of five human fetal eyes (14-18.5 weeks' gestation), were examined for Ki-67 and CD34 immunoreactivity using confocal microscopy. Measurements of choriocapillaris area and the number of proliferating choroidal endothelial cells were used to calculate the rate of choroidal endothelial proliferation at five different chorioretinal locations.

RESULTS

The choriocapillaris area is consistently greater in the foveal region than at other locations and increases progressively with age. A higher rate of endothelial cell proliferation was found in parts of the choriocapillaris associated with the undifferentiated (proliferating) neural retina, compared with the differentiated, central region.

CONCLUSION

The findings suggest that mechanisms regulating proliferation and growth of the choroidal vasculature are independent of differentiation in the neural retina, and are thus profoundly different from mechanisms that regulate formation of the retinal vasculature.

Thrombin and PAR-1 stimulate differentiation of bone marrow-derived endothelial progenitor cells

Endothelial progenitor cells (EPCs) from the bone marrow play an important role in vascular response to injury and ischemia. The mediators involved in the mobilization, recruitment, proliferation and differentiation of EPCs are not fully understood. In this study, the role of coagulation factor thrombin and protease-activated receptor-1 (PAR-1) on bone marrow-derived cell proliferation and differentiation was investigated. Bone marrow cells (BMCs) were isolated from C57/BL6 mice and plated on fibronectin-coated flasks. Cell characteristics, proliferation and the expression of endothelial cell markers were determined using immunohistochemistry, thymidine uptake and fluorescence activated-cell sorting (FACS), respectively. The results show that thrombin stimulated enrichment of bone marrow cells with endothelial morphology, exhibiting acetylated-low-density lipoprotein (LDL) uptake and isolectin staining. Thrombin or PAR-1-activating peptide produced a 2- to 3-fold increase in the total number of cells as well as an increase in vascular endothelial (VE)-cadherin-positive cells. Thrombin treatment of VE-cadherin-negative cells prepared after cell sorting resulted in the generation of 3- to 4-fold higher VE-cadherin-positive cells than the untreated cultures. Increase in VE-cadherin-positive cells was inhibited by hirudin and efegatran. These results provide first evidence for a novel activity of thrombin and PAR-1 on bone marrow progenitor cell proliferation and EPC differentiation, and suggest their potential role in vascular regeneration and recanalization of thrombus.

In vivo tissue-engineered small-caliber arterial graft prosthesis consisting of autologous tissue (biotube)

In this study, vascular-like tubular tissues called biotubes, consisting of autologous tissues, were prepared using in vivo tissue engineering. Their mechanical properties were evaluated for application as a small-caliber artificial vascular prosthesis. The biotubes were prepared by embedding six kinds of polymeric rods [poly(ethylene) (PE), poly(fluoroacetate) (PFA), poly(methyl methacrylate) (PMMA), segmented poly(urethane) (PU), poly(vinyl chloride) (PVC), and silicone (Si)] as a mold in six subcutaneous pouches in the dorsal skin of New Zealand White rabbits. For rods apart from PFA, biotubes were constructed after 1 month of implantation by encapsulation around the polymeric implants. The wall thickness of the biotubes ranged from about 50 to 200 microm depending on the implant material and were in the order PFA < PVC < PMMA < PU < PE. As for PE, PMMA, and PVC, the thickness increased after 3 months of implantation and ranged from 1.5-to 2-fold. None of the biotubes were ruptured when a hydrostatic pressure was gradually applied to their lumen up to 200 mmHg. The relationship between the intraluminal pressure and the external diameter, which was highly reproducible, showed a "J"-shaped curve similar to the native artery. The tissue mostly consisted of collagen-rich extracellular matrices and fibroblasts. Generally, the tissue was relatively firm and inelastic for Si and soft for PMMA. For PMMA, PE, and PVC the stiffness parameter (beta value; one of the indexes for compliance) of the biotubes obtained was similar to those of the human coronary, femoral, and carotid arteries, respectively. Biotubes, which possess the ability for wide adjustments in their matrices, mechanics, shape, and luminal surface design, can be applied for use as small-caliber blood vessels and are an ideal implant because they avoid immunological rejection.

Monocytes form a vascular barrier and participate in vessel repair after brain injury

Subpopulations of bone marrow-derived cells can be induced to assume a number of endothelial properties in vitro. However, their ability to form a functional vascular barrier has not been demonstrated. We report that human CD14+ peripheral blood monocytes cultured under angiogenic conditions develop a number of phenotypic and functional properties similar to brain microvascular endothelial cells. These cells express the tight junction proteins zonula occludens 1 (ZO-1) and occludin and form a barrier with a transcellular electrical resistance (TCER) greater than 100 ohm cm2 and low permeability to 4 kDa and 20 kDa dextrans. The TCER of the cellular barrier is decreased by bradykinin and histamine. We also demonstrate that these cells associate with repairing vasculature in areas of brain and skin injury. Our data suggest that CD14+ peripheral blood monocytes participate in the repair of the vascular barrier after brain injury.

Stem cells and cardiovascular and renal disease: today and tomorrow

The traditional view that organs have only limited regenerative capacity has been challenged in recent years as adult bone marrow stem cells as well circulating progenitor cells have been identified to retain the plasticity to participate in neovascularization, a process so far believed not to be possible after birth. An organ that is damaged by ischemia causes the release of cytokines; these act via the flowing blood and stimulate the bone marrow, which then mobilizes progenitor cells to the blood and directs them to adhere to and migrate into the damaged organ. Thus, these progenitor cells most likely constitute a natural repair mechanism that counteracts degenerative or aging processes. On the basis of encouraging experimental data, first clinical trials have been established to demonstrate the safety and the feasibility of progenitor cell therapy in case of peripheral artery disease or myocardial infarction. Trials investigating injection of bone marrow or circulating progenitor cells into the coronary artery after an acute myocardial infarction not only demonstrates safety of the procedure but also gave hints toward efficacy. Nevertheless, these findings have to be validated by subsequent larger, prospective, randomized, controlled trials. There are also potential topics in nephrology, where modification of progenitor cell activity might be of benefit, such as renal ischemic disease, glomerular disease, and renal transplant vasculopathy. Finding a way to integrate the principle of progenitor cell action into therapeutic efforts might provide a completely new therapeutic strategy that not only attempts to retard disease progression but furthermore targets to regenerate damaged organs.

Endothelial Progenitor Cells Are Recruited Into Resolving Venous Thrombi

Background— The purpose of this study was to determine whether endothelial cells of bone marrow origin are involved in thrombus recanalization.

Methods and Results— Irradiated mice were reconstituted with bone marrow from transgenic donors expressing green fluorescent protein (GFP) linked to the Tie2 promoter. Thrombi were formed in 2 groups of 6 mice. GFP-expressing cells were located and quantified in sections of the thrombi taken after 7 and 14 days. The cell markers Mac-3, F4/80, CD68 (macrophage), and vascular endothelial growth factor receptor 2 (VEGFR2; endothelial cells) were used to determine colocalization with GFP expression in tissue sections and peritoneal macrophages. The markers CD34 and VEGFR2 were used to quantify changes in circulating endothelial cells by flow cytometry of blood from 3 cohorts of wild-type animals that had either a thrombus induced (n=18), a sham operation (n=18), or no operation (n=10). The number of GFP-expressing cells was found to increase by ≈3-fold in thrombi formed in transplanted animals between 7 and 14 days after induction ( P =0.0022). No GFP-expressing cells were found lining the new vascular channels that formed at either time interval, but many of the GFP-expressing cells also expressed Mac-3, CD68, and VEGFR2. Approximately twice as many circulating CD34 + /VEGFR2 + cells were found by day 3 in animals with thrombus compared with sham controls (CD45 − , P =0.046 and CD45 + , P =0.016).

Conclusions— Bone marrow–derived, Tie2-expressing cells were recruited into the thrombus during resolution but did not line the new vessels. Many of these cells expressed a macrophage phenotype and may represent a population of plastic stem cells that orchestrate thrombus recanalization.

Trafficking of normal stem cells and metastasis of cancer stem cells involve similar mechanisms: pivotal role of the SDF-1-CXCR4 axis

The alpha-chemokine stromal-derived factor (SDF)-1 and the G-protein-coupled seven-span transmembrane receptor CXCR4 axis regulates the trafficking of various cell types. In this review, we present the concept that the SDF-1-CXCR4 axis is a master regulator of trafficking of both normal and cancer stem cells. Supporting this is growing evidence that SDF-1 plays a pivotal role in the regulation of trafficking of normal hematopoietic stem cells (HSCs) and their homing/retention in bone marrow. Moreover, functional CXCR4 is also expressed on nonhematopoietic tissue-committed stem/progenitor cells (TCSCs); hence, the SDF-1-CXCR4 axis emerges as a pivotal regulator of trafficking of various types of stem cells in the body. Furthermore, because most if not all malignancies originate in the stem/progenitor cell compartment, cancer stem cells also express CXCR4 on their surface and, as a result, the SDF-1-CXCR4 axis is also involved in directing their trafficking/metastasis to organs that highly express SDF-1 (e.g., lymph nodes, lungs, liver, and bones). Hence, we postulate that the metastasis of cancer stem cells and trafficking of normal stem cells involve similar mechanisms, and we discuss here the common molecular mechanisms involved in these processes. Finally, the responsiveness of CXCR4+ normal and malignant stem cells to an SDF-1 gradient may be regulated positively/primed by several small molecules related to inflammation which enhance incorporation of CXCR4 into membrane lipid rafts, or may be inhibited/blocked by small CXCR4 antagonist peptides. Consequently, strategies aimed at modulating the SDF-1-CXCR4 axis could have important clinical applications both in regenerative medicine to deliver normal stem cells to the tissues/organs and in clinical hematology/oncology to inhibit metastasis of cancer stem cells.

Preferential activity of Tie2 promoter in arteriolar endothelium

The tyrosine kinase Tie2/Tek (the receptor for angiopoietins) is considered one of the most reliable markers of the endothelial phenotype, across organisms, organs, and developmental stages. However, endothelium is intrinsically heterogeneous in origin, composition and function, presenting an arteriolar/venular asymmetry. In this regard, the expression of Tie2 along the vascular tree, although thought to be homogenous, has not been systematically investigated. Therefore we questioned whether the activity of Tie2 promoter is uniform in the microvascular endothelium. To this end, we analyzed in situ the expression of the markers beta-galactosidase [LacZ(Tie2)] and green fluorescent protein (GFP) [GFP(Tie2)], placed under the Tie2 promoter in transgenic mice, in whole mount tissue samples, which allow the simultaneous evaluation of its relative distribution in various microvascular compartments. In the mesenteries of LacZ(Tie2) and GFP(Tie2) mice, we found that the activity of Tie2 promoter is asymmetrically distributed, being much stronger in arteries and arterioles than on the venular side of the vascular tree. This observation was replicated in the diaphragm of LacZ(Tie2) mice. The capillaries presented a mosaic pattern of Tie2 promoter activity. Stimulation of angiogenesis either by wounding, or by intraperitoneal injection of Vascular Endothelial Growth Factor (VEGF), revealed that the arteriolar/venular asymmetry is established at endothelial cellular level early during new capillary formation, even before the starting of the microvascular blood flow. In conclusion, a strong Tie2 promoter activity qualifies as a novel marker of the arteriolar phenotype in microvascular endothelium.