Endothelial progenitor cells: characterization, in vitro expansion, and prospects for autologous cell therapy

Generation of transgene-free hematopoietic stem cells from human induced pluripotent stem cells

Hematopoietic stem cells (HSCs) are the rare cells responsible for the lifelong curative effects of hematopoietic cell (HC) transplantation. The demand for clinical-grade HSCs has increased significantly in recent decades, leading to major difficulties in treating patients. A promising but not yet achieved goal is the generation of HSCs from pluripotent stem cells. Here, we have obtained vector- and stroma-free transplantable HSCs by differentiating human induced pluripotent stem cells (hiPSCs) using an original one-step culture system. After injection into immunocompromised mice, cells derived from hiPSCs settle in the bone marrow and form a robust multilineage hematopoietic population that can be serially transplanted. Single-cell RNA sequencing shows that this repopulating activity is due to a hematopoietic population that is transcriptionally similar to human embryonic aorta-derived HSCs. Overall, our results demonstrate the generation of HSCs from hiPSCs and will help identify key regulators of HSC production during human ontogeny.

Endothelial Progenitor Cells as Biomarkers of Cardiovascular Pathologies: A Narrative Review

Endothelial progenitor cells (EPC) may influence the integrity and stability of the vascular endothelium. The association of an altered total EPC number and function with cardiovascular diseases (CVD) and risk factors (CVF) was discussed; however, their role and applicability as biomarkers for clinical purposes have not yet been defined. Endothelial dysfunction is one of the key mechanisms in CVD. The assessment of endothelial dysfunction in vivo remains a major challenge, especially for a clinical evaluation of the need for therapeutic interventions or for primary prevention of CVD. One of the main challenges is the heterogeneity of this particular cell population. Endothelial cells (EC) can become senescent, and the majority of circulating endothelial cells (CEC) show evidence of apoptosis or necrosis. There are a few viable CECs that have properties similar to those of an endothelial progenitor cell. To use EPC levels as a biomarker for vascular function and cumulative cardiovascular risk, a correct definition of their phenotype, as well as an update on the clinical application and practicability of current isolation methods, are an urgent priority.

Endothelial Colony-Forming Cells Dysfunctions Are Associated with Arterial Hypertension in a Rat Model of Intrauterine Growth Restriction

Infants born after intrauterine growth restriction (IUGR) are at risk of developing arterial hypertension at adulthood. The endothelium plays a major role in the pathogenesis of hypertension. Endothelial colony-forming cells (ECFCs), critical circulating components of the endothelium, are involved in vasculo-and angiogenesis and in endothelium repair. We previously described impaired functionality of ECFCs in cord blood of low-birth-weight newborns. However, whether early ECFC alterations persist thereafter and could be associated with hypertension in individuals born after IUGR remains unknown. A rat model of IUGR was induced by a maternal low-protein diet during gestation versus a control (CTRL) diet. In six-month-old offspring, only IUGR males have increased systolic blood pressure (tail-cuff plethysmography) and microvascular rarefaction (immunofluorescence). ECFCs isolated from bone marrow of IUGR versus CTRL males displayed a decreased proportion of CD31+ versus CD146+ staining on CD45- cells, CD34 expression (flow cytometry, immunofluorescence), reduced proliferation (BrdU incorporation), and an impaired capacity to form capillary-like structures (Matrigel test), associated with an impaired angiogenic profile (immunofluorescence). These dysfunctions were associated with oxidative stress (increased superoxide anion levels (fluorescent dye), decreased superoxide dismutase protein expression, increased DNA damage (immunofluorescence), and stress-induced premature senescence (SIPS; increased beta-galactosidase activity, increased p16INK4a, and decreased sirtuin-1 protein expression). This study demonstrated an impaired functionality of ECFCs at adulthood associated with arterial hypertension in individuals born after IUGR.

Impaired Vps34 complex activity-mediated autophagy inhibition contributes to endothelial progenitor cells damage in the ischemic conditions

AIMS

Endothelial progenitor cells (EPCs) are widely accepted to be applied in ischemic diseases. However, the therapeutic potency is largely impeded because of its inviability in these ischemic conditions. Autophagy is recognized to be vital in cell activity. Therefore, we explore the role and the mechanism of autophagy in ischemic EPCs.

METHODS AND RESULTS

We applied 7d-cultured bone marrow EPCs to investigate the autophagy status under the oxygen and glucose deprivation (OGD) conditions in vitro, mimicking the in-vivo harsh ischemia and anoxia microenvironment. We found increased EPC apoptosis, accompanied by an impaired autophagy activation. Intriguingly, mTOR inhibitor Rapamycin was incapable to reverse this damped autophagy and EPC damage. We further found that autophagy pathway downstream Vps34-Beclin1-Atg14 complex assembly and activity were impaired in OGD conditions, and an autophagy-inducing peptide Tat-Beclin1 largely recovered the impaired complex activity and attenuated OGD-stimulated EPC injury through restoring autophagy activation.

CONCLUSIONS

The present study discovered that autophagy activation is inhibited when EPCs located in the ischemia and anoxia conditions. Restoration of Vps34 complex activity obtains sufficient autophagy, thus promoting EPC survival, which will provide a potential target and advance our understanding of autophagy manipulation in stem cell transplantation.

Potential role of Peroxisome Proliferator Activated Receptor gamma analogues in regulation of endothelial progenitor cells in diabetes mellitus: An overview

Endothelial progenitor cells are recognized as the potential targets for the revascularization and angiogenesis because of their ability to get themselves transformed into mature endothelial cells. Underlying pathophysiology in diabetes mellitus leads to decrease in circulatory endothelial progenitor cells, resulting in diabetic macro-vascular and micro-vascular complications. Peroxisome Proliferator Activated Receptor (PPAR) gamma analogues serves as an effective therapy for controlling blood sugar levels and preventing its complications. Reports of clinical trials and meta-analysis of clinical trial suggests the beneficial aspects of PPAR gamma therapy in increasing the number and function of circulating endothelial progenitor cells. This review highlights the pleotropic effect of PPAR gamma analogs, apart from their antidiabetic action via reduction of oxidative stress, increasing expression of eNOS, reducing level of miR 22, miR 222 levels and positive modulation of rapamycin/Protein kinase B/phosphoinoside3-kinase pathways, preventing the early apoptosis, enhanced mobility proliferation and transformation into mature endothelial cells. PPAR gamma therapy in diabetes regulates endothelial progenitor cells, reduces complications of diabetes like retinopathy, nephropathy, neuropathy, cardiomyopathy, deep vein thrombosis, and maintains the healthy vasculature.

Transplantation of bone marrow-derived endothelial progenitor cells and hepatocyte stem cells from liver fibrosis rats ameliorates liver fibrosis

AIM

To explore the effectiveness for treating liver fibrosis by combined transplantation of bone marrow-derived endothelial progenitor cells (BM-EPCs) and bone marrow-derived hepatocyte stem cells (BDHSCs) from the liver fibrosis environment.

METHODS

The liver fibrosis rat models were induced with carbon tetrachloride injections for 6 wk. BM-EPCs from rats with liver fibrosis were obtained by different rates of adherence and culture induction. BDHSCs from rats with liver fibrosis were isolated by magnetic bead cell sorting. Tracing analysis was conducted by labeling EPCs with PKH26 in vitro to show EPC location in the liver. Finally, BM-EPCs and/or BDHSCs transplantation into rats with liver fibrosis were performed to evaluate the effectiveness of BM-EPCs and/or BDHSCs on liver fibrosis.

RESULTS

Normal functional BM-EPCs from liver fibrosis rats were successfully obtained. The co-expression level of CD133 and VEGFR2 was 63.9% ± 2.15%. Transplanted BM-EPCs were located primarily in/near hepatic sinusoids. The combined transplantation of BM-EPCs and BDHSCs promoted hepatic neovascularization, liver regeneration and liver function, and decreased collagen formation and liver fibrosis degree. The VEGF levels were increased in the BM-EPCs (707.10 ± 54.32) and BM-EPCs/BDHSCs group (615.42 ± 42.96), compared with those in the model group and BDHSCs group (P < 0.05). Combination of BM-EPCs/BDHSCs transplantation induced maximal up-regulation of PCNA protein and HGF mRNA levels. The levels of alanine aminotransferase (AST), aspartate aminotransferase, total bilirubin (TBIL), prothrombin time (PT) and activated partial thromboplastin time in the BM-EPCs/BDHSCs group were significantly improved, to be equivalent to normal levels (P > 0.05) compared with those in the BDHSC (AST, TBIL and PT, P < 0.05) and BM-EPCs (TBIL and PT, P < 0.05) groups. Transplantation of BM-EPCs/BDHSCs combination significantly reduced the degree of liver fibrosis (staging score of 1.75 ± 0.25 vs BDHSCs 2.88 ± 0.23 or BM-EPCs 2.75 ± 0.16, P < 0.05).

CONCLUSION

The combined transplantation exhibited maximal therapeutic effect compared to that of transplantation of BM-EPCs or BDHSCs alone. Combined transplantation of autogenous BM-EPCs and BDHSCs may represent a promising strategy for the treatment of liver fibrosis, which would eventually prevent cirrhosis and liver cancer.

Specific recruitment of circulating angiogenic cells using biomaterials as filters

Endogenous recruitment of circulating angiogenic cells (CACs) is an emerging strategy to induce angiogenesis within a defect site, and multiple recent strategies have deployed soluble protein releasing biomaterials for this purpose. However, the way in which the design of biomaterials affects CAC recruitment and invasion are poorly understood. Here we used an enhanced-throughput cell invasion assay to systematically examine the effects of biomaterial design on CAC recruitment. The screens co-optimized hydrogel presentation of a stromal-derived factor-1α (SDF-1α) gradient, hydrogel degradability, and hydrogel stiffness for maximal CAC invasion. We also examined the specificity of this invasion by assessing dermal fibroblast, mesenchymal stem cell, and lymphocyte invasion individually and in co-culture with CACs to identify hydrogels specific to CAC invasion. These screens suggested a subset of MMP-degradable hydrogels presenting a specific range of SDF-1α gradient slopes that induced specific invasion of CACs, and we posit that the design parameters of this subset of hydrogels may serve as instructive templates for the future design of biomaterials to specifically recruit CACs. We also posit that this design concept may be applied more broadly in that it may be possible to utilize these specific subsets of biomaterials as "filters" to control which types of cell populations invade into and populate the biomaterial.

STATEMENT OF SIGNIFICANCE

The recruitment of specific cell types for cell-based therapies in vivo is of great interest to the regenerative medicine community. Circulating angiogenic cells (CACs), CD133+ cells derived from the blood stream, are of particular interest for induction of angiogenesis in ischemic tissues, and recent studies utilizing soluble-factor releasing biomaterials to recruit these cells in vivo show great promise. However, these studies are largely "proof of concept" and are not systematic in nature. Thus, little is currently known about how biomaterial design affects the recruitment of CACs. In the present work, we use a high throughput cell invasion screening platform to systematically examine the effects of biomaterial design on circulating angiogenic cell (CAC) recruitment, and we successfully screened 263 conditions at 3 replicates each. Our results identify a particular subset of conditions that robustly recruit CACs. Additionally, we found that these conditions also specifically recruited CACs and excluded the other tested cells types of dermal fibroblasts, mesenchymal stem cells, and lymphocytes. This suggests an intriguing new role for biomaterials as "filters" to control the types of cells that invade and populate that biomaterial.

Infarcted Myocardium-Primed Dendritic Cells Improve Remodeling and Cardiac Function After Myocardial Infarction by Modulating the Regulatory T Cell and Macrophage Polarization

BACKGROUND

Inflammatory responses play a critical role in left ventricular remodeling after myocardial infarction (MI). Tolerogenic dendritic cells (tDCs) can modulate immune responses, inducing regulatory T cells in a number of inflammatory diseases.

METHODS

We generated tDCs by treating bone marrow-derived dendritic cells with tumor necrosis factor-α and cardiac lysate from MI mice. We injected MI mice, induced by a ligation of the left anterior descending coronary artery in C57BL/6 mice, twice with tDCs within 24 hours and at 7 days after the ligation.

RESULTS

In vivo cardiac magnetic resonance imaging and ex vivo histology confirmed the beneficial effect on postinfarct left ventricular remodeling in MI mice treated with tDCs. Subcutaneously administered infarct lysate-primed tDCs near the inguinal lymph node migrated to the regional lymph node and induced infarct tissue-specific regulatory T-cell populations in the inguinal and mediastinal lymph nodes, spleen, and infarcted myocardium, indicating that a local injection of tDCs induces a systemic activation of MI-specific regulatory T cells. These events elicited an inflammatory-to-reparative macrophage shift. The altered immune environment in the infarcted heart resulted in a better wound remodeling, preserved left ventricular systolic function after myocardial tissue damage, and improved survival.

CONCLUSIONS

This study showed that tDC therapy in a preclinical model of MI was potentially translatable into an antiremodeling therapy for ischemic tissue repair.

Sphingosine 1-phosphate elicits RhoA-dependent proliferation and MRTF-A mediated gene induction in CPCs

Although c-kit(+) cardiac progenitor cells (CPCs) are currently used in clinical trials there remain considerable gaps in our understanding of the molecular mechanisms underlying their proliferation and differentiation. G-protein coupled receptors (GPCRs) play an important role in regulating these processes in mammalian cell types thus we assessed GPCR mRNA expression in c-kit(+) cells isolated from adult mouse hearts. Our data provide the first comprehensive overview of the distribution of this fundamental class of cardiac receptors in CPCs and reveal notable distinctions from that of adult cardiomyocytes. We focused on GPCRs that couple to RhoA activation in particular those for sphingosine-1-phosphate (S1P). The S1P2 and S1P3 receptors are the most abundant S1P receptor subtypes in mouse and human CPCs while cardiomyocytes express predominantly S1P1 receptors. Treatment of CPCs with S1P, as with thrombin and serum, increased proliferation through a pathway requiring RhoA signaling, as evidenced by significant attenuation when Rho was inhibited by treatment with C3 toxin. Further analysis demonstrated that both S1P- and serum-induced proliferation are regulated through the S1P2 and S1P3 receptor subtypes which couple to Gα12/13 to elicit RhoA activation. The transcriptional co-activator MRTF-A was activated by S1P as assessed by its nuclear accumulation and induction of a RhoA/MRTF-A luciferase reporter. In addition S1P treatment increased expression of cardiac lineage markers Mef2C and GATA4 and the smooth muscle marker GATA6 through activation of MRTF-A. In conclusion, we delineate an S1P-regulated signaling pathway in CPCs that introduces the possibility of targeting S1P2/3 receptors, Gα12/13 or RhoA to influence the proliferation and commitment of c-kit(+) CPCs and improve the response of the myocardium following injury.

Thrombin receptor PAR-1 activation on endothelial progenitor cells enhances chemotaxis-associated genes expression and leukocyte recruitment by a COX-2-dependent mechanism

BACKGROUND

Endothelial colony forming cells (ECFC) represent a subpopulation of endothelial progenitor cells involved in endothelial repair. The activation of procoagulant mechanisms associated with the vascular wall's inflammatory responses to injury plays a crucial role in the induction and progression of atherosclerosis. However, little is known about ECFC proinflammatory potential.

AIMS

To explore the role of the thrombin receptor PAR-1 proinflammatory effects on ECFC chemotaxis/recruitment capacity.

METHODS AND RESULTS

The expression of 30 genes known to be associated with inflammation and chemotaxis was quantified in ECFC by real-time qPCR. PAR-1 activation with the SFLLRN peptide (PAR-1-ap) resulted in a significant increase in nine chemotaxis-associated genes expression, including CCL2 and CCL3 whose receptors are present on ECFC. Furthermore, COX-2 expression was found to be dramatically up-regulated consequently to PAR-1 activation. COX-2 silencing with the specific COX-2-siRNA also triggered down-regulation of the nine target genes. Conditioned media (c.m.) from control-siRNA- and COX-2-siRNA-transfected ECFC, stimulated or not with PAR-1-ap, were produced and tested on ECFC capacity to recruit leukocytes in vitro as well in the muscle of ischemic hindlimb in a preclinical model. The capacity of the c.m. from ECFC stimulated with PAR-1-ap to recruit leukocytes was abrogated when COX-2 gene expression was silenced in vitro (in terms of U937 cells migration and adhesion to endothelial cells) as well as in vivo. Finally, the postnatal vasculogenic stem cell derived from infantile hemangioma tumor (HemSC) incubated with PAR-1-ap increased leukocyte recruitment in Matrigel(®) implant.

CONCLUSIONS

PAR-1 activation in ECFC increases chemotactic gene expression and leukocyte recruitment at ischemic sites through a COX-2-dependent mechanism.

The Role of Stem Cells in Wound Angiogenesis

Significance: Revascularization plays a critical role in wound healing and is regulated by a complex milieu of growth factors and cytokines. Deficiencies in revascularization contribute to the development of chronic nonhealing wounds. Recent Advances: Stem-cell-based therapy provides a novel strategy to enhance angiogenesis and improve wound healing. With bioethical concerns associated with embryonic stem cells, focus has shifted to different populations of vascular precursors, isolated from adult somatic tissue. Three main populations have been identified: endothelial progenitor cells, mesenchymal stem cells, and induced-pluripotent stem cells. These populations demonstrate great promise to positively influence neovascularization and wound repair. Critical Issues: Further studies to more definitively define each population are necessary to efficiently translate stem-cell-based therapeutic angiogenesis to the bedside. Better understanding of the physiologic pathways of how stem cells contribute to angiogenesis in normal tissue repair will help identify targets for successful therapeutic angiogenesis. Future Directions: Active studies in both animal models and clinical trials are being conducted to develop effective delivery routes, including dosing, route, and timing. Stem-cell-based therapy holds significant potential as a strategy for therapeutic angiogenesis in the care of patients with chronic nonhealing wounds.

Targeting VEGFR1 on endothelial progenitors modulates their differentiation potential

OBJECTIVES

We studied whether plasma levels of angiogenic factors VEGF and placental growth factor (PlGF) in coronary artery disease patients or undergoing cardiac surgery are modified, and whether those factors modulate endothelial progenitor's angiogenic potential.

METHODS AND RESULTS

A total of 143 patients' plasmas from two different studies were analyzed (30 coronary artery disease patients, 30 patients with stable angina, coupled with 30 age and sex-matched controls; 53 patients underwent cardiac surgery). Among factors screened, only PlGF was found significantly increased in these pathological populations. PlGF-1 and PlGF-2 were then tested on human endothelial-colony-forming cells (ECFCs). We found that PlGF-1 and PlGF-2 induce VEGFR1 phosphorylation and potentiate ECFCs tubulogenesis in vitro. ECFCs VEGFR1 was further inhibited using a specific small interfering RNA (siRNA) and the chemical compound 4321. We then observed that the VEGFR1-siRNA and the compound 4321 decrease ECFCs tubulogenesis potential in vitro. Finally, we tested the compound 4321 in the preclinical Matrigel(®)-plug model with C57Bl/6J mice as well as in the murine hindlimb ischemia model. We found that 4321 inhibited the plug vascularization, attested by the hemoglobin content and the VE-Cadherin expression level and that 4321 inhibited the post-ischemic revascularization.

CONCLUSION

PlGF plasma levels were found increased in cardiovascular patients. Disrupting PlGF/VEGFR1 pathway could modulate ECFC-induced tubulogenesis, the cell type responsible for newly formed vessels in vivo.

Postischemic revascularization: from cellular and molecular mechanisms to clinical applications

After the onset of ischemia, cardiac or skeletal muscle undergoes a continuum of molecular, cellular, and extracellular responses that determine the function and the remodeling of the ischemic tissue. Hypoxia-related pathways, immunoinflammatory balance, circulating or local vascular progenitor cells, as well as changes in hemodynamical forces within vascular wall trigger all the processes regulating vascular homeostasis, including vasculogenesis, angiogenesis, arteriogenesis, and collateral growth, which act in concert to establish a functional vascular network in ischemic zones. In patients with ischemic diseases, most of the cellular (mainly those involving bone marrow-derived cells and local stem/progenitor cells) and molecular mechanisms involved in the activation of vessel growth and vascular remodeling are markedly impaired by the deleterious microenvironment characterized by fibrosis, inflammation, hypoperfusion, and inhibition of endogenous angiogenic and regenerative programs. Furthermore, cardiovascular risk factors, including diabetes, hypercholesterolemia, hypertension, diabetes, and aging, constitute a deleterious macroenvironment that participates to the abrogation of postischemic revascularization and tissue regeneration observed in these patient populations. Thus stimulation of vessel growth and/or remodeling has emerged as a new therapeutic option in patients with ischemic diseases. Many strategies of therapeutic revascularization, based on the administration of growth factors or stem/progenitor cells from diverse sources, have been proposed and are currently tested in patients with peripheral arterial disease or cardiac diseases. This review provides an overview from our current knowledge regarding molecular and cellular mechanisms involved in postischemic revascularization, as well as advances in the clinical application of such strategies of therapeutic revascularization.

The challenges for cardiac vascular precursor cell therapy: lessons from a very elusive precursor

There is compelling evidence that cardiovascular disorders arise and/or progress due mainly to endothelial dysfunction. Novel therapeutic strategies aim to generate new myocardial tissue using cells with regenerative potential, either alone or in combination with biomaterials, cytokines and advanced monitoring devices. Among the human adult progenitor cells used in such methods, those historically termed 'endothelial progenitor cells' show promise for vascular growth and repair. Asahara et al. [Science 1997;275:964-967] initially described putative endothelial cell precursors in 1997. Subsequently, distinct cell populations termed endothelial colony-forming units-Hill, circulating angiogenic cells and endothelial colony-forming cells were identified that varied in terms of phenotype, vascular homeostasis contribution and purity. Notably, most of these cells are not genuine vascular precursor cells belonging to the endothelial lineage. This review provides a broad overview of the main properties of the endothelium, focusing on the basis governing its growth and repair. We discuss efforts to identify true vascular precursors, a matter of debate for the past 15 years, as well as recent methodological advances in identifying new hierarchies of more homogeneous, clonogenic and proliferative vascular endothelial-lineage precursors. Consideration of these issues provides insights that may help develop more effective therapies against human diseases that involve vascular deficits.

The Role of Endothelial Progenitor Cells in Postnatal Vasculogenesis: Implications for Therapeutic Neovascularization and Wound Healing

SIGNIFICANCE

Postnatal vasculogenesis mediated via endothelial progenitor cells (EPCs) contributes to re-endothelialization and augments neovascularization after ischemia and tissue injury, providing a novel therapeutic application. However, controversy exists with respect to the origin, identification, and contributions of the EPCs to neovascularization, necessitating further study.

RECENT ADVANCES

Bone marrow (BM) or circulating cells expressing cd133/vascular endothelial growth factor receptor 2 include those with endothelial progenitor capacity. Increasing evidence suggests that there are additional BM-derived (myeloid; mesenchymal cells) and non-BM-derived (peripheral and cord-blood; tissue-resident) cell populations which also give rise to endothelial cells (ECs) and contribute to re-endothelialization and growth factor release after ischemia and tissue injury. Currently, EPCs are being used as diagnostic markers for the assessment of cardiovascular and tumor risk/progression. Techniques aimed at enhancing ex vivo expansion and the therapeutic potential of these cells are being optimized.

CRITICAL ISSUES

Mobilization and EPC-mediated neovascularization are critically regulated. Stimulatory (growth factors, statins, and exercise) or inhibitory factors (obesity, diabetes, and other cardiovascular diseases) modulate EPC numbers and function. Recruitment and incorporation of EPCs require a coordinated sequence of signaling events, including adhesion, migration (by integrins), and chemoattraction. Finally, EPCs differentiate into ECs and/or secrete angiogenic growth factors. These cells are highly plastic, and depending on the microenvironment and presence of other cells, EPCs transdifferentiate and/or undergo cell fusion and become cells of a different lineage. Therefore, in vitro culture conditions should be optimized to mimic the in vivo milieu to fully characterize the biological function and contribution of EPCs to postnatal vasculogenesis.

FUTURE DIRECTIONS

Advances in characterization of the EPC biology and enhancement of EPC functions are required. In addition, innovative tissue-engineered carrier matrices that permit embedding of EPCs and provide optimal conditions for EPC survival and endothelial outgrowth will further contribute to EPC-mediated therapeutic applications in wound healing and ischemia repair.

Comparative characteristics of endothelial-like cells derived from human adipose mesenchymal stem cells and umbilical cord blood-derived endothelial cells

Adult peripheral blood contains a limited number of endothelial progenitor cells that can be isolated for treatment of ischemic diseases. The adipose tissue became an interesting source of stem cells for regenerative medicine. This study aimed to investigate the phenotype of cells obtained by culturing adipose-derived mesenchymal stem cells (ad-MSCs) in the presence of endothelial growth supplements compared to endothelial cells obtained from umbilical cord blood (UCB). Passage 3 ad-MSCs and mononuclear layer from UCB were cultured in presence of endothelial growth media for 3 weeks followed by their characterization by flow cytometry and polymerase chain reaction. After culture in endothelial inductive media, ad-MSCs expressed endothelial genes and some endothelial marker proteins as CD31 and CD34, respectively. Adipose tissue could be a reliable source for easy obtaining, expanding and differentiating MSCs into endothelial-like cells for autologous cell-based therapy.

[Genic and cellular therapy for peripheral arterial diseases]

Late evolution of peripheral arterial disease consists in the apparition of critical limb ischemia. Surgical treatments allow to treat these patients during long time; however, in most patients, especially the diabetic ones, there a very few options and the clinical evolution is rapidly dramatic. For these reasons, the critical limb ischemia is one of the first diseases treated by genic or cellular therapies aiming to improve blood flow perfusion in the lower-limbs. In this short review, we describe the main clinical trials of genic therapy; most of them have been abandoned because serious side effects, modest effects and major risks. Different types of stem cells are now used for cell therapy: endothelial progenitor cells, early or late, activated or not, mesenchymal stem cells, embryonic stem cells and human induced pluripotent stem cells. Problems of characterization are described and the results of the most important clinical trials are reported.

Decreased pool of mesenchymal stem cells is associated with altered chemokines serum levels in atrophic nonunion fractures

Nonunion fractures can cause severe dysfunction and are often difficult to treat mainly due to a poor understanding of their physiopathology. Although many aspects of impaired fracture healing have been extensively studied, little is known about the cellular and molecular mechanisms leading to atrophic nonunion. Therefore, the aim of the present study was to assess the pools and biological functions of bone marrow-derived mesenchymal stem cells (hMSCs) and circulating endothelial progenitor cells (EPCs) in atrophic nonunion patients compared to healthy subjects, and the systemic levels of growth factors involved in the recruitment, proliferation and differentiation of these cells. In nonunions, the pool of hMSCs was decreased and their proliferation delayed. However, once committed, hMSCs from nonunions were able to proliferate, differentiate into osteoblastic cells and mineralize in vitro as efficiently as hMSCs from healthy subjects. In parallel, we found altered serum levels of chemokines and growth factors involved in the chemotaxis and proliferation of hMSCs such as leptin, interleukin-6 (IL-6) and its soluble receptor, platelet-derived growth factor-BB (PDGF-BB), stem cell factor (SCF) and insulin-like growth factor-1 (IGF-1). Moreover, we showed that the number of EPCs and their regulating growth factors were not affected in nonunion patients. If nonunion is generally attributed to a vascular defect, our results also support a role for a systemic mesenchymal and osteogenic cell pool defect that might be related to alterations in systemic levels of factors implicated in their chemotaxis and proliferation.

Differential expression of Tie2 receptor and VEGFR2 by endothelial clones derived from isolated bovine mononuclear cells

The purpose of these experiments was to evaluate the expression of endothelial markers, such as Tie2 and VEGFR2 in endothelial cells derived from blood mononuclear endothelial progenitor cells. Bovine mononuclear cells were isolated using separation by centrifugation and were grown in endothelial specific media supplemented with growth factors. Isolation of the whole cell population of mononuclear cells (MNC) from bovine peripheral blood gave rise to progenitor-like cells (CD45⁻) that, although morphologically similar, have different phenotypes revealed by expression of endothelial specific markers Tie2 and VEGFR2. Plating of MNCs on collagen and fibronectin gave rise to more colonies than non-coated dishes. Occasional colonies from MNC isolations had a mural cell phenotype, negative for Tie2 and VEGFR2 but positive for smooth muscle actin and PDGFRβ. Although cells expressing high levels of VEGFR2 and low levels of Tie2, and vice versa were both able to form cords on Matrigel, cells with higher expression of Tie2 migrate faster in a scratch assay than ones with lower expression of Tie2. When these different clones of cells were introduced in mice through tail vein injections, they retained an ability to home to angiogenesis occurring in a subcutaneous Matrigel plug, regardless of their Tie2/VEGFR2 receptor expression patterns, but cells with high VEGFR2/low Tie2 were more likely to be CD31 positive. Therefore, we suggest that active sites of angiogenesis (such as wounds, tumors, etc.) can attract a variety of endothelial cell precursors that may differentially express Tie2 and VEGFR2 receptors, and thus affect our interpretation of EPCs as biomarkers or therapies for vascular disease.

Local Transplantation of Ex Vivo Expanded Bone Marrow-Derived CD34-Positive Cells Accelerates Fracture Healing

Transplantation of bone marrow (BM) CD34+ cells, an endothelial/hematopoietic progenitor-enriched cell population, has shown therapeutic efficiency in the treatment of ischemic diseases enhancing neovascularization. However, the number of CD34+ cells obtained from bone marrow is not sufficient for routine clinical application. To overcome this issue, we developed a more efficient and clinically applicable CD34+ cell expansion method. Seven-day ex vivo expansion culture of BM CD34+ cells with a cocktail of five growth factors containing VEGF, SCF, IL-6, Flt-3 ligand, and TPO resulted in reproducible more than 20-fold increase in cell number. The favorable effect of the local transplantation of culture expanded (cEx)-BM CD34+ cells on rat unhealing fractures was equivalent or higher than that of nonexpanded (fresh) BM CD34+ cells exhibiting sufficient therapeutic outcome with frequent vasculogenic/osteogenic differentiation of transplanted cEx-BM CD34+ cells and fresh BM CD34+ cells as well as intrinsic enhancement of angiogenesis/osteogenesis at the treated fracture sites. Specifically, cEx-BM CD34+ cell treatment demonstrated the best blood flow recovery at fracture sites compared with the nonexpanded BM CD34+ cells. In vitro, cEx-BM CD34+ cells showed higher colony/tube-forming capacity than nonexpanded BM CD34+ cells. Both cells demonstrated differentiation potential into osteoblasts. Since fresh BM CD34+ cells can be easily collected from fracture sites at the time of primary operation and stored for future use, autologous cEx-BM CD34+ cell transplantation would be not only a simple but also a promising therapeutic strategy for unhealing fractures in the field of orthopedic trauma surgery.

Effect of transplantation of bone marrow monomuclears on angiogenesis in rats

We studied the effect of non-selective intracoronary transplantation of bone marrow mononuclears on day 30 after acute coronary infarction on angiogenesis in rats. On days 14 and 30 after transplantation of mononuclear cells, stable formation of new vessels was observed. The number of venules considerably increased after transplantation of mononuclear cells, which was seen from increased volume density of blood vessels and their caliber. Stable vascularization after transplantation of mononuclear cells improves blood supply, which is essential for reparation of the myocardium.

Progenitor cell mobilizing treatments prevent experimental transplant arteriosclerosis

OBJECTIVE

Vascular rejection after organ transplantation is characterized by an arterial occlusive lesion, resulting from intimal proliferation occurring in response to arterial wall immune aggression. Our hypothesis is that an early endothelial repair may prevent vascular graft rejection. The aim of the current study was to compare different pharmacologic progenitor cell mobilizing treatments for their protective effects against vascular rejection.

METHODS AND RESULTS

Aortic transplants were made from balb/c donor to C57Bl/6 recipient mice. Three different mobilizing pharmacologic agents were used: low molecular weight fucoidan (LMWF), simvastatin, and AMD3100. The circulating levels of progenitor cells were found to be increased by all three treatments, as determined by flow cytometry. For each treatment, the design was: treated allografts, nontreated allografts, treated isografts, and nontreated isografts. After 21 d, morphometric and immunohistochemical analyses were performed. We found that the three treatments significantly reduced intimal proliferation, compared with nontreated allografts. This was associated with intimal re-endothelialization of the grafts. Further, in chimeric mice that had previously received GFP-transgenic bone marrow transplantation, GFP-positive cells were found in the vascular allograft intima, indicating that re-endothelialization was, at least partly, due to the recruitment of bone marrow-derived, presumably endothelial progenitor circulating cells.

CONCLUSIONS

In this aortic allograft model, three different mobilizing treatments were found to partially prevent vascular transplant rejection. Bone marrow-derived progenitor cells mobilized by the three treatments may play a direct role in the endothelial repair process and in the suppression of intimal proliferation.

Effective mobilisation of bone marrow-derived cells through proteolytic activity: a new treatment strategy for age-related macular degeneration

Selective targeting of bone marrow-derived cells (BMCs) has been heralded as a promising avenue for age-related macular degeneration (AMD) therapeutics. Many researchers have demonstrated that the function of circulating BMCs is related to disease severity in patients with AMD. Transplanted BMCs are able to transdifferentiate into retina-specific cells to replace those lost due to damage or degeneration in the pathologic process of experimental models of AMD, which may provide beneficial effects in patients with AMD. However, a major barrier to transferring the use of BMCs into clinical practice is the limited quantity of BMCs in the peripheral circulation. Technology has not yet reached a stage where ex vivo-expanded BMCs can be routinely used for cell therapy. A feasible strategy to circumvent this issue of BMC scarcity is to increase the mobilisation of autologous BMCs from the patient's bone marrow into the blood circulation. Extensive studies have demonstrated that the SDF-1/CXCR4 axis is a key regulator for BMC mobilisation. Moreover, abrogation of the SDF-1/CXCR4 axis by proteolytic modification can efficiently increase BMC mobilisation. We speculate that BMC mobilisation by proteolytic enzymes may supply a sufficient amount of autologous cells to repair and regenerate injured and degenerated the retinal pigment epithelium (RPE), photoreceptors, or other retina-specific cells, which could prevent AMD progression. If the BMC mobilisation strategy is used to treat AMD, it may overcome the existing problems of transferring BMC-based therapy into the clinic and become a particularly feasible therapeutic approach for AMD.

Early endothelial progenitor cells in bone marrow are a biomarker of cell therapy success in patients with critical limb ischemia

BACKGROUND AIMS

Endothelial progenitor cells (EPC) have been proposed for autologous angiogenic therapy. The objectives of this study were to quantify EPC in the peripheral blood and bone marrow mononuclear cells (BM-MNC) of patients with critical limb ischemia that had received BM-MNC as a cell therapy product, and to study the putative relationship between the presence of EPC and the process of neovascularization in toe or transmetatarsal amputation specimens.

METHODS

Early and late endothelial progenitor cells (CFU-EC and ECFC) were cultivated and quantified according to published methods in peripheral blood and BM-MNC from patients with critical limb ischemia (CLI; n = 11) enrolled in the OPTIPEC trial ( http://clinicaltrials.gov/ct2/show/NCT00377897 ) to receive BM-MNC as a cell therapy product.

RESULTS

Eight out of the 11 patients had undergone amputations. Three of the patients displayed a neoangiogenic process that was associated with a higher number of CFU-EC in BM-MNC, while CD3+ , CFU-GM and CD34+ in BM-MNC, and EPC in peripheral blood, did not correlate with the appearance of newly formed vessels. As expected, circulating CFU-EC and ECFC counts were significantly lower in CLI patients compared with age-matched controls.

CONCLUSIONS

In patients with critical limb ischemia, EPC in peripheral blood were decreased compared with healthy individuals. However, in BM-MNC we found that relative numbers of CFU-EC could be used as an indicator to discriminate patients with neoangiogenic processes. These results need to be confirmed in a randomized study.

Cellular players in angiogenesis during the course of systemic sclerosis

Vascular endothelial injury in Systemic Sclerosis (SSc) leads to pathological changes in the blood vessels that adversely impact the physiology of many organs, resulting in chronic tissue ischemia. The response to hypoxia induces complex cellular and molecular mechanisms in the attempt to recover endothelial cell function and tissue perfusion. The progressive losses of capillaries on one hand, and the vascular remodeling of arteriolar vessels on the other, result in insufficient blood flow, causing severe and chronic hypoxia. Hypoxia is a major stimulus of angiogenesis, leading to the expression of pro-angiogenic molecules, mainly of Vascular Endothelial Growth Factor (VEGF), which triggers the angiogenic process. Nevertheless, in SSc patients there is no evidence of adaptive angiogenesis. Failure of the angiogenic process in SSc largely depends on alteration in the balance between pro- and anti-angiogenic factors, as well as on functional alterations of the cellular players involved in the angiogenic and vasculogenic program. A decreased urokinase plasminogen activator (uPA) dependent invasion, proliferation, and capillary morphogenesis, was showed in SSc endothelial cells (EC). Although hematopoietic endothelial progenitor cells (EPC) count in the peripheral blood of SSc patients is still a matter of controversy, alterations in mobilization process, an excessive immune-mediated EPC destruction in the peripheral circulation or in the bone marrow, a progressive depletion of EPCs following homing to ischemic tissues under persistent peripheral vascular injury, an intrinsic functional impairment could lead to poor vasculogenesis. Human mesenchymal stem cells represent an alternative source of endothelial progenitor cells and it has been observed that their angiogenic potential is reduced in SSc. Targeting autologous stem and progenitor cells could be an ideal tool to counteract and repair dysfunctional angiogenesis.

Stem cells in the treatment of patients with coronary heart disease. Part I

Heart failure (HF) is one of the leading death causes in patients with myocardial infarction (MI). The modern methods of reperfusion MI therapy, such as thrombolysis, surgery and balloon revascularization, even when performed early, could fail to prevent the development of large myocardial damage zones, followed by HF. Therefore, the researches have been searching for the methods which improve functional status of damaged myocardium. This review is focused on stem cell therapy, a method aimed at cardiac function restoration. The results of experimental and clinical studies on stem cell therapy in coronary heart disease are presented. Various types of stem cells, used for cellular cardiomyoplasty, are characterised. The methods of cell transplantation into myocardium and potential adverse effects of stem cell therapy are discussed.

Thrombospondin-1 is a plasmatic marker of peripheral arterial disease that modulates endothelial progenitor cell angiogenic properties

OBJECTIVE

We examined whether plasma levels of angiogenic factors are altered in plasma of patients with peripheral arterial disease (PAD) and whether these factors affect endothelial progenitor cell-induced angiogenesis.

METHODS AND RESULTS

Plasma was collected from 184 patients with PAD and 330 age-matched healthy controls. Vascular endothelial growth factor and placental growth factor concentrations did not differ between the groups, whereas we found a linear correlation between PAD disease and thrombospondin (TSP)-1 plasma level. TSP-1 was expressed in newly formed vessels in PAD patients having received local injections of bone marrow mononuclear cells. To analyze the functional role of TSP-1 during neoangiogenesis, we used a Matrigel-plug assay and showed that vascularization of implanted Matrigel-plugs was increased in TSP-1(-/-) mice. Moreover, injections of TSP-1 in C57Bl6/J mice after hindlimb ischemia induced a significant decrease of blood flow recovery. To investigate the effects of TSP-1 on human endothelial colony-forming cell (ECFC) angiogenic potential, recombinant human TSP-1 and a small interfering RNA were used. In vitro, TSP-1 N-terminal part significantly enhanced ECFC adhesion, whereas recombinant human TSP-1 had a negative effect on ECFC angiogenic potential. This effect, mediated by CD47 binding, modulated stromal cell-derived factor 1/CXC chemokine receptor 4 pathway.

CONCLUSIONS

TSP-1 is a potential biomarker of PAD and ECFC-induced angiogenesis, suggesting that TSP-1 modulation might improve local tissue ischemia in this setting. (

CLINICAL TRIAL REGISTRATION

NCT00377897.).

Endothelial progenitor cells as therapeutic agents in the microcirculation: an update

This review evaluates novel beneficial effects of circulating endothelial progenitor cells (EPCs) as shown by several preclinical studies and clinical trials carried out to test the safety and feasibility of using EPCs. There are 31 registered clinical trials (and many others still ongoing) and 19 published studies. EPCs originate in the bone marrow and migrate into the bloodstream where they undergo a differentiation program leading to major changes in their antigenic characteristics. EPCs lose typical progenitor markers and acquire endothelial markers, and two important receptors, (VEGFR and CXCR-4), which recruit circulating EPCs to damaged or ischemic microcirculatory (homing to damaged tissues) beds. Overall, therapeutic angiogenesis will likely change the face of regenerative medicine in the next decade with many patients worldwide predicted to benefit from these treatments.

Are purified or expanded cord blood-derived CD133+ cells better at improving cardiac function?

Endothelial progenitor cells (EPCs), which express the CD133 marker, can differentiate into mature endothelial cells (ECs) and create new blood vessels. Normal angiogenesis is unable to repair the injured tissues that result from myocardial infarction (MI). Patients who have high cardiovascular risks have fewer EPCs and their EPCs exhibit greater in vitro senescence. Human umbilical cord blood (HUCB)-derived EPCs could be an alternative to rescue impaired stem cell function in the sick and elderly. The aim of this study was to purify HUCB-derived CD133(+) cells, expand them in vitro and evaluate the efficacy of the purified and expanded cells in treating MI in rats. CD133(+) cells were selected for using CD133-coupled magnetic microbeads. Purified cells stained positive for EPC markers. The cells were expanded and differentiated in media supplemented with fetal calf serum and basic fibroblast growth factor, insulin-like growth factor-I and vascular endothelial growth factor (VEGF). Differentiation was confirmed by lack of staining for EPC markers. These expanded cells exhibited increased expression of mature EC markers and formed tubule-like structures in vitro. Only the expanded cells expressed VEGF mRNA. Cells were expanded up to 70-fold during 60 days of culture, and they retained their functional activity. Finally, we evaluated the therapeutic potential of purified and expanded CD133(+) cells in treating MI by intramyocardially injecting them into a rat model of MI. Rats were divided into three groups: A (purified CD133(+) cells-injected); B (expanded CD133(+) cells-injected) and C (saline buffer-injected). We observed a significant improvement in left ventricular ejection fraction for groups A and B. In summary, CD133(+) cells can be purified from HUCB, expanded in vitro without loosing their biological activity, and both purified and expanded cells show promising results for use in cellular cardiomyoplasty. However, further pre-clinical testing should be performed to determine whether expanded CD133(+) cells have any clinical advantages over purified CD133(+) cells.

Endothelial precursors in vascular repair

The endothelium is an essential component of the cardiovascular system, playing a vital role in blood vessel formation, vascular homeostasis, permeability and the regulation of inflammation. The integrity of the endothelial monolayer is also critical in the prevention of atherogenesis and as such, restoration of the monolayer is essential following damage or cell death. Over the past decade, data has suggested that progenitor cells from different origins within the body are released into the circulation and contribute to re-endothelialisation. These cells, termed endothelial progenitor cells (EPCs), also gave rise to the theory of new vessel formation within adults (vasculogenesis) without proliferation and migration of mature endothelial cells (angiogenesis). As such, intense research has been carried out identifying how these cells may be mobilised and contribute to vascular repair, either encouraging vasculogenesis into regions of ischemia or the re-endothelialisation of vessels with a dysfunctional endothelium. However, classification and isolation procedures have been a major problem in this area of research and beneficial use for therapeutic application has been controversial. In the present review we focus on the role of EPCs in vascular repair. We also provide an update on EPC classification and discuss autologous stem cell-derived endothelial cell (EC) as a functional source for therapy.

Circulating endothelial cells, microparticles and progenitors: key players towards the definition of vascular competence

The balance between lesion and regeneration of the endothelium is critical for the maintenance of vessel integrity. Exposure to cardiovascular risk factors (CRF) alters the regulatory functions of the endothelium that progresses from a quiescent state to activation, apoptosis and death. In the last 10 years, identification of circulating endothelial cells (CEC) and endothelial-derived microparticles (EMP) in the circulation has raised considerable interest as non-invasive markers of vascular dysfunction. Indeed, these endothelial-derived biomarkers were associated with most of the CRFs, were indicative of a poor clinical outcome in atherothrombotic disorders and correlated with established parameters of endothelial dysfunction. CEC and EMP also behave as potential pathogenic vectors able to accelerate endothelial dysfunction and promote disease progression. The endothelial response to injury has been enlarged by the discovery of a powerful physiological repair process based on the recruitment of circulating endothelial progenitor cells (EPC) from the bone marrow. Recent studies indicate that reduction of EPC number and function by CRF plays a critical role in the progression of cardiovascular diseases. This EPC-mediated repair to injury response can be integrated into a clinical endothelial phenotype defining the ‘vascular competence’ of each individual. In the future, provided that standardization of available methodologies could be achieved, multimarker strategies combining CEC, EMP and EPC levels as integrative markers of ‘vascular competence’ may offer new perspectives to assess vascular risk and to monitor treatment efficacy.

[Characterization of endothelial progenitor cells and putative strategies to improve their expansion]

Injection of endothelial progenitor cells (EPC) expanded ex vivo has been shown to increase neovascularization in preclinical models of ischemia and in adult patients, but the precise origin and identity of the cell population responsible for these clinical benefits are controversial. Given the potential usefulness of EPC as a cell therapy product, their thorough characterization is of major importance. This review describes the two cell populations currently called EPC and the means to find differential phenotypic markers. We have shown that BMP2/4 are specific markers of late EPC and play a key role in EPC commitment and outgrowth during neovascularization. Several authors have attempted to expand EPC ex vivo in order to obtain a homogeneous cell therapy product. One possible mean of expanding EPC ex vivo is to activate the thrombin receptor PAR-1 with the specific peptide SFLLRN. Indeed, PAR-1 activation increases angiogenic properties of EPC through activation of SDF-1, angiopoietin and IL-8 pathways. This review summarizes the characterization of EPC and different methods of ex vivo expansion.

Capability of human umbilical cord blood progenitor-derived endothelial cells to form an efficient lining on a polyester vascular graft in vitro

One of the goals of vascular tissue engineering is to create functional conduits for small-diameter bypass grafting. The present biocompatibility study was undertaken to check the ability of cord blood progenitor-derived endothelial cells (PDECs) to take the place of endothelial cells in vascular tissue engineering. After isolation, culture and characterization of endothelial progenitor cells, the following parameters were explored, with a commercial knitted polyester prosthesis (Polymaille C, Laboratoires Pérouse, France) impregnated with collagen: cell adhesion and proliferation, colonization, cell retention on exposure to flow, and the ability of PDECs to be regulated by arterial shear stress via mRNA levels. PDECs were able to adhere to commercial collagen-coated vascular grafts in serum-free conditions, and were maintained but did not proliferate when seeded at 2.0 x 10(5) cm(-2). Cellularized conduits were analyzed by histology and histochemical staining, demonstrating collagen impregnation and the endothelial characteristics of the colonizing cells. Thirty-six hours after cell seeding the grafts were maintained for 6 h of either static conditions (controls) or application of pulsatile laminar shear stress, which restored the integrity of the monolayer. Finally, quantitative real-time RT-PCR analysis performed at 4 and 8 h from cells lining grafts showed that MMP1 mRNA only was increased at 4h whereas vWF, VE-cadherin and KDR were not significantly modified at 4 and 8 h. Our results show that human cord blood PDECs are capable of forming an efficient lining and to withstand shear stress.

Circulating endothelial cells: a new candidate biomarker of irreversible pulmonary hypertension secondary to congenital heart disease

BACKGROUND

Congenital heart disease can be complicated by pulmonary arterial hypertension (PAH), the reversibility of which is often difficult to predict. We recently reported a lung biopsy study showing impaired apoptotic regulation of endothelial cells in irreversible PAH. The objective of the present study was to identify noninvasive biomarkers of endothelial turnover that could be used to identify congenital heart disease patients at risk of irreversible PAH.

METHODS AND RESULTS

Circulating endothelial cells (CECs) isolated with CD146-coated beads and circulating CD34(+)CD133(+) progenitor cells (CPCs) were quantified in peripheral vein, pulmonary artery, and pulmonary vein blood samples from 26 patients with congenital heart disease (16 with reversible PAH [median age 2 years] and 10 with irreversible PAH [median age 9 years]) and 5 control patients. Surgical lung biopsy was performed in 19 cases. As expected, endothelial remodeling was observed in irreversible PAH but not in reversible PAH. CEC and CPC numbers were each similar in the 3 types of blood samples. CEC numbers were significantly higher in patients with irreversible PAH (median 57 CEC/mL) than in patients with reversible PAH and control subjects (median 3 CEC/mL in the 2 groups). In contrast, CPC numbers did not differ among patients with irreversible or reversible PAH and control subjects (median 84, 64, and 44 CPC/10(5) lymphocytes, respectively, in the 3 groups).

CONCLUSIONS

Irreversible PAH in congenital heart disease is associated with endothelial damage and with increased circulating endothelial cell counts. The present study suggests that CECs could be a valuable tool to define therapeutic strategies in congenital heart disease patients with PAH.

Thrombin and brain recovery after intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is a common and often fatal subtype of stroke and produces severe neurological deficits in survivors. At present, there is lack of effective treatments that improve outcome in ICH. A neglected aspect of ICH research is the development of approaches that can be effectively used to improve recovery. Although previous studies have showed that thrombin induces blood-brain barrier leakage, brain edema, and neuronal death after ICH, our recent studies have shown that thrombin may have a role in brain recovery after ICH. An understanding of the mechanisms by which thrombin affects neurogenesis, angiogenesis, and plasticity may facilitate brain recovery after ICH.

Biofunctionalization of biomaterials for accelerated in situ endothelialization: a review

The higher patency rates of cardiovascular implants, including vascular bypass grafts, stents, and heart valves are related to their ability to inhibit thrombosis, intimal hyperplasia, and calcification. In native tissue, the endothelium plays a major role in inhibiting these processes. Various bioengineering research strategies thereby aspire to induce endothelialization of graft surfaces either prior to implantation or by accelerating in situ graft endothelialization. This article reviews potential bioresponsive molecular components that can be incorporated into (and/or released from) biomaterial surfaces to obtain accelerated in situ endothelialization of vascular grafts. These molecules could promote in situ endothelialization by the mobilization of endothelial progenitor cells (EPC) from the bone marrow, encouraging cell-specific adhesion (endothelial cells (EC) and/or EPC) to the graft and, once attached, by controlling the proliferation and differentiation of these cells. EC and EPC interactions with the extracellular matrix continue to be a principal source of inspiration for material biofunctionalization, and therefore, the latest developments in understanding these interactions will be discussed.

Bone morphogenetic proteins 2 and 4 are selectively expressed by late outgrowth endothelial progenitor cells and promote neoangiogenesis

OBJECTIVE

Endothelial progenitor cells are currently identified either by their surface antigen expression or by their generation of early colonies in culture (CFU-Hill). Another population, endothelial colony-forming cells (ECFCs), has strong vessel-forming capacity but is less well characterized. Given the potential usefulness of CFU-Hill and ECFCs as cell therapy products, their thorough characterization is of major importance.

METHODS AND RESULTS

CFU-Hill and ECFCs were expanded from human cord and adult blood. Bone morphogenetic proteins 2 and 4 (BMP2/4) were selectively expressed by ECFCs but not by CFU-Hill. The BMP pathway was involved in ECFC commitment and angiogenic potential in vitro. In vivo, BMP inhibition strongly reduced plug vascularization in bFGF-containing Matrigel plugs implanted in C57/Bl6 mice. Moreover, ECFC exposure to BMP increased their therapeutic potential in a nude mouse model of hindlimb ischemia. In amputation specimens from patients with critical leg ischemia who had received a local therapeutic injection of bone marrow mononuclear cells, newly formed vessels were strongly positive for BMP2/4, suggesting that endothelial cells involved in neovascularization have an ECFC-like phenotype.

CONCLUSIONS

BMP2/4 are a marker of ECFCs and play a key role in ECFC commitment and outgrowth during neovascularization.

Thrombin bound to a fibrin clot confers angiogenic and haemostatic properties on endothelial progenitor cells

Recent data suggest that endothelial progenitor cells (EPCs) are involved in recanalizing venous thrombi. We examined the impact of a fibrin network, and particularly of adsorbed thrombin, on EPCs derived from cord blood CD34(+) cells. Fibrin networks generated in microplates by adding CaCl(2) to platelet-depleted plasma retained adsorbed thrombin at the average concentration of 4.2 nM per well. EPCs expressed high levels of endothelial cell protein C receptor and thrombomodulin, allowing the generation of activated protein C on the fibrin matrix in the presence of exogenous human protein C. The fibrin matrix induced significant EPC proliferation and, when placed in the lower chamber of a Boyden device, strongly enhanced EPC migration. These effects were partly inhibited by hirudin by 41% and 66%, respectively), which suggests that fibrin-adsorbed thrombin interacts with EPCs via the thrombin receptor PAR-1. Finally, spontaneous lysis of the fibrin network, studied by measuring D-dimer release into the supernatant, was inhibited by EPCs but not by control mononuclear cells. Such an effect was associated with a 10-fold increase in plasminogen activator inhibitor-1 (PAI-1) secretion by EPCs cultivated in fibrin matrix. Overall, our data show that EPCs, in addition to their angiogenic potential, have both anticoagulant and antifibrinolytic properties. Thrombin may modulate these properties and contribute to thrombus recanalization by EPCs.

Circulating progenitor cells and scleroderma

Scleroderma (systemic sclerosis) is a disease of unknown origins that involves tissue ischemia and fibrosis in the skin and internal organs such as the lungs. The tissue ischemia is due to a lack of functional blood vessels and an inability to form new blood vessels. Bone marrow--derived circulating endothelial progenitor cells play a key role in blood vessel repair and neovascularization. Scleroderma patients appear to have defects in the number and function of circulating endothelial progenitor cells. Scleroderma patients also develop fibrotic lesions, possibly as the result of tissue ischemia. Fibroblast-like cells called fibrocytes that differentiate from a different pool of bone marrow-derived circulating progenitor cells seem to be involved in this process. Manipulating the production, function, and differentiation of circulating progenitor cells represents an exciting new possibility for treating scleroderma.

Interleukin 8 is differently expressed and modulated by PAR-1 activation in early and late endothelial progenitor cells

The proinflammatory chemokine interleukin 8 exerts potent angiogenic effects on endothelial cells by interacting with its receptors CXCR1 and CXCR2. As thrombin is also a potent inflammatory factor, and as endothelial progenitor cells (EPC) express functional PAR-1 thrombin receptor, we examined whether PAR-1 stimulation interferes with the IL-8 pathway in EPC. EPC were obtained from adult blood (AB) and cord blood (CB). The effect of PAR-1 stimulation by the peptide SFLLRN on IL-8, CXCR1 and CXCR2 expression was examined by RTQ-PCR and at the protein level in AB and CB late EPC and in AB early EPC. Specific siRNA was used to knock down PAR-1 expression. The IL-8 gene was expressed strongly in AB early EPC and moderately in late EPC. In contrast, CXCR1 and CXCR2 gene expression was restricted to AB early EPC. The IL-8 level in AB early EPC conditioned medium was high in basal conditions and did not change after PAR-1 activation. By contrast, IL-8 secretion by late EPC was low in basal conditions and strongly up-regulated upon PAR-1 activation. PAR-1 activation induced a number of genes involved in activating protein-1 (AP-1) and nuclear factor (NF)-kappaB pathways. Conditioned medium of PAR-1-activated late EPC enhanced the migratory potential of early EPC, and this effect was abrogated by blocking IL-8. Target-specific siRNA-induced PAR-1 knockdown, and fully inhibited PAR-1-induced IL-8 synthesis. In conclusion, PAR-1 activation induces IL-8 synthesis by late EPC. This could potentially enhance cooperation between late and early EPC during neovascularization, through a paracrine effect.

Identification of a rare subset of adipose tissue-resident progenitor cells, which express CD133 and TRPC3 as a VEGF-regulated Ca2+ entry channel

VEGF-induced Ca2+ signalling was investigated in CD133+/VEGFR-2+ progenitor cells isolated from human adipose stroma. Colonies derived from CD133+ immunoselected cells displayed inhomogenous Ca2+ signals, with variable magnitude of VEGF-induced Ca2+ entry, which positively correlated with expression of the Ca2+ channel protein TRPC3. High levels of VEGF-induced Ca2+ entry and TRPC3 expression were preferentially detected in rim areas of expanding colonies. Dominant negative suppression of TRPC3 inhibited VEGF-induced Ca2+ entry into CD133+ cells. Our results identify TRPC3 as a key Ca2+ entry channel in a subset of CD133+ stem cells. We suggest TRPC3 as an essential determinant of cell fate in CD133+ progenitor-derived colonies.

Effects of thrombin on neurogenesis after intracerebral hemorrhage

BACKGROUND AND PURPOSE

Neurogenesis in intracerebral hemorrhage (ICH) has not been investigated. Thrombin formation causes acute brain injury after ICH, but thrombin also can stimulate cell proliferation. The present study examined whether neurogenesis takes place in ICH and the role of thrombin in ICH-related neurogenesis.

METHODS

This study was divided into four parts. (1) Rats received either an ICH or a needle insertion (sham). The rats were killed for doublecortin (DCX) Western blot analysis and immunohistochemistry. (2) Rats had an ICH or a sham operation, and then received intraperitoneal injections of 5-bromo-2'-deoxyuridine (BrdU) at day-7 and day-9 later. Brains were perfused to identify BrdU-positive cells. (3) Rats had an intracaudate injection of thrombin (1 U) and brains were sampled for Western blots. (4) Rats had an ICH with or without a thrombin inhibitor, hirudin. The brains were sampled for DCX quantitation.

RESULTS

DCX levels in the ipsilateral basal ganglia started to increase as early as 7 days after ICH, peaked at 14 days, and then gradually decreased at 1 month. Immunohistochemistry also demonstrated that DCX immunoreactivity was increased in the ipsilateral subventricular zone and basal ganglia at 2 weeks after ICH. Some DCX-positive cells were BrdU-positive. One unit thrombin, which does not cause marked brain injury, was injected into the caudate. Thrombin increased DCX levels in the ipsilateral basal ganglia and hirudin blocked ICH-induced upregulation of DCX.

CONCLUSIONS

Our results demonstrated that neurogenesis occurs in the brain after ICH and that thrombin may play a role in ICH-induced neurogenesis.