Vascular endothelial growth factor (VEGF-A) expression in human mesenchymal stem cells: autocrine and paracrine role on osteoblastic and endothelial differentiation

VEGF is a mediator of the renoprotective effects of multipotent marrow stromal cells in acute kidney injury

Adult stem cell treatment of complex disorders is a promising therapeutic approach and multipotent marrow stromal cells (MSCs) have been shown to be effective in various animal models of diseases. Acute kidney injury (AKI) is a common and serious problem in hospitalized patients and bone marrow derived multipotent MSCs have been shown to be effective in different models of AKI. The mechanism of action of MSCs is complex but involves paracrine actions including growth factor secretion. Knockdown of vascular enthothelial growth factor (VEGF) by siRNA reduced effectiveness of MSCs in the treatment of ischemic AKI in a rat model. Animals treated with MSCs had increased renal microvessel density compared to VEGF knockdown MSC-treated and vehicle-treated animals. These results show that VEGF is an important mediator of the early and late phase of renoprotective action after AKI in the context of stem cell treatment.

Mesenchymal stem cells and cardiac repair

Accumulating clinical and experimental evidence indicates that mesenchymal stem cells (MSCs) are promising cell types in the treatment of cardiac dysfunction. They may trigger production of reparative growth factors, replace damaged cells and create an environment that favours endogenous cardiac repair. However, identifying mechanisms which regulate the role of MSCs in cardiac repair is still at work. To achieve the maximal clinical benefits, ex vivo manipulation can further enhance MSC therapeutic potential. This review focuses on the mechanism of MSCs in cardiac repair, with emphasis on ex vivo manipulation.

Desferrioxamine-driven upregulation of angiogenic factor expression by human bone marrow stromal cells

Bone marrow stromal cells (BMSCs) are the subject of intense research because of their biological properties and potential use for the repair of damaged tissues. Success of BMSC-based therapies, however, relies on a number of methodological improvements, including the establishment of a vascular network providing nutrients and oxygen to the transplanted cells and ensuring their immediate survival and long-term functionality. We described a method to enhance the autocrine expression of angiogenic factors by BMSCs. For this purpose, human BMSCs were treated with desferrioxamine (DFX). No PDGF-BB, VEGF-R1 or -R2 mRNA expression was detected under any of the conditions tested. mRNA and protein expression levels of TGFbeta1 were similar in BMSCs, whether they were exposed to DFX (50 microM) or to control conditions under normoxia for 48 h. In comparison with the results obtained with control conditions under normoxia, exposure of BMSCs to DFX for 48 h resulted in upregulation of bFGF at the protein (26-fold) but not at the mRNA levels and VEGF at both the mRNA (1.5-fold) and protein levels (4.5-fold). In comparison with the results obtained with control conditions under hypoxia, DFX induced a 50% increase in VEGF secretion but led to the same level of hypoxia inducible factor-1alpha protein expression (a transduction factor involved in angiogenic factor expression and known to be activated by DFX). Exposure of BMSCs to DFX resulted in oversecretion of angiogenic factors, suggesting that DFX-treated BMSCs could be used to supply angiogenic factors.

The effect of human osteoblasts on proliferation and neo-vessel formation of human umbilical vein endothelial cells in a long-term 3D co-culture on polyurethane scaffolds

Angiogenesis is a key element in early wound healing and is considered important for tissue regeneration and for directing inflammatory cells to the wound site. The improvement of vascularization by implementation of endothelial cells or angiogenic growth factors may represent a key solution for engineering bone constructs of large size. In this study, we describe a long-term culture environment that supports the survival, proliferation, and in vitro vasculogenesis of human umbilical vein endothelial cells (HUVEC). This condition can be achieved in a co-culture model of HUVEC and primary human osteoblasts (hOB) employing polyurethane scaffolds and platelet-rich plasma in a static microenvironment. We clearly show that hOB support cell proliferation and spontaneous formation of multiple tube-like structures by HUVEC that were positive for the endothelial cell markers CD31 and vWF. In contrast, in a monoculture, most HUVEC neither proliferated nor formed any apparent vessel-like structures. Immunohistochemistry and quantitative PCR analyses of gene expression revealed that cell differentiation of hOB and HUVEC was stable in long-term co-culture. The three-dimensional, FCS-free co-culture system could provide a new basis for the development of complex tissue engineered constructs with a high regeneration and vascularization capacity.

Protecting axons in multiple sclerosis

Typically patients with multiple sclerosis (MS) experience acute episodes of neurological dysfunction, which recover followed, at a later stage, by slow and insidious accumulation of disability (disease progression). Disease progression reflects axon damage and loss within the central nervous system. However, the precise mechanism of axon injury in MS is not clear. Inflammation occurring during acute relapses undoubtedly causes some degree of acute axon damage, but epidemiological data and treatment studies have suggested that inflammation alone is not the sole cause of axonopathy. Indeed, there appears to be dissociation between inflammation and disease progression once a certain level of clinical disability has been reached because immune suppression in patients who have established disease progression does not halt the slow decrease of function. The slow and insidious loss of neurological function that occurs during the progressive phase of the disease implies a degenerative process. Whether axon drop-out occurs at these later stages because of previous inflammatory damage to axons; because of low grade inflammation causing damage to already vulnerable demyelinated axons; because of loss of trophic environment for axons to survive; or as part of a completely independent neurodegenerative process is not clear. Understanding disease mechanisms involved in the axonopathy of MS allows for the development of rational therapies for disease progression.

Loss of Thy-1 (CD90) antigen expression on mesenchymal stromal cells from hematologic malignancies is induced by in vitro angiogenic stimuli and is associated with peculiar functional and phenotypic characteristics

BACKGROUND

Little is known about human mesenchymal stromal cell (hMSC) phenotypic and functional subsets in response to environmental stimuli. The strategy used in this study focused on defining hMSC functional subpopulations based in particular on their Thy-1 (CD90) antigen (Ag) surface expression.

METHODS

The effect of different in vitro microenvironmental conditions on the isolation and expansion of bone marrow-derived (BM) hMSC from hematologic malignancies (HM) and normal samples (NS) was assayed. hMSC clonogenic and differentiation potential, phenotypic profile and long-term capacity to sustain in vitro hemopoiesis were considered in relation to the different expansion protocols.

RESULTS

The results showed that angiogenic supplements used in combination with low serum content gave rise to the appearance of Thy-1(-) HM-MSC with high proliferative potential, capable of restoring the typical HM stromal impairment. The expression of the CD271 was partially maintained. We further report an enhancement towards the osteogenic and adipogenic differentiation capacity by the Thy-1(-) HM-MSC subset. Despite the angiogenic treatment, the Thy-1(-) MSC stopped short of full endothelial differentiation.

DISCUSSION

In this paper we provide evidence that in vitro angiogenic stimuli generate HM-MSC lacking CD90 Ag expression. The Thy-1(-) MSC subset is characterized by peculiar functional and phenotypic characteristics, thus supporting the role played by the microenvironment in selecting particular hMSC subsets maintaining normal tissue homeostasis or inducing pathologic processes.

Bone formation during distraction osteogenesis is dependent on both VEGFR1 and VEGFR2 signaling

INTRODUCTION

Distraction osteogenesis (DO) is characterized by the induction of highly vascularized new bone formation through an intramembranous process largely devoid of the formation of cartilage.

MATERIALS AND METHODS

To test the hypothesis that DO is strictly dependent on vascualrization, we inhibited vascular endothelial growth factor (VEGF) activity by antibody blockade of both receptors VEGFR1 (Flt-1) and VEGFR2 (Flk-1) or only VEGFR2 (Flk-1) in a previously developed murine tibia DO model. During normal DO, VEGFR1 (Flt-1), VEGFR2 (Flk-1), VEGFR3 (Flt4) and all four VEGF ligand (A, B, C, and D) mRNAs are induced.

RESULTS

The expression of mRNA for the receptors generally paralleled those of the ligands during the period of active distraction. Bone formation, as assessed by muCT, showed a significant decrease with the double antibody treatment and a smaller decrease with single antibody treatment. Vessel volume, number, and connectivity showed progressive and significant inhibition in all of these of parameters between the single and double antibody blockade. Molecular analysis showed significant inhibition in skeletal cell development with the single and double antibody blockade of both VEGFR1 and 2. Interestingly, the single antibody treatment led to selective early development of chondrogenesis, whereas the double antibody treatment led to a failure of both osteogenesis and chondrogenesis.

CONCLUSIONS

Both VEGFR1 and VEGFR2 are functionally essential in blood vessel and bone formation during DO and are needed to promote osteogenic over chondrogenic lineage progression.

Mesenchymal stem cells and platelet-rich plasma enhance bone formation in sinus grafting: a histomorphometric study in minipigs

OBJECTIVES

Autologous, allogenic, and alloplastic materials for sinus augmentation have specific drawbacks, which has stimulated an ongoing search for new materials and tissue-engineering constructs. We investigated whether mesenchymal stem cells (MSCs) and platelet-rich plasma (PRP) seeded on a fluorohydroxyapatite (FH) scaffold can improve bone formation and bone-to-implant contact (BIC) in maxillary sinus grafting.

MATERIAL AND METHODS

Bilateral sinus augmentation procedures were performed in eight minipigs. MSCs, PRP, and FH scaffold (test site) or FH alone (control site) were grafted in each maxillary sinus. Distal to the osteotomy, one dental implant per sinus was placed in the grafting material through the facial sinus wall. The animals were killed 3 months after grafting, and block sections of the implant sites were harvested and prepared for histomorphometric analysis.

RESULTS

After 12 weeks, a significant increase in bone formation occurred in the test sites compared with the control sites (42.51%versus 18.98%; p=0.001). In addition, BIC was significantly greater in the test sites compared with the control sites in the regenerated area (23.71%versus 6.63%; p=0.028).

CONCLUSIONS

These findings show that sinus augmentation with MSCs-PRP, combined with FH may enhance bone formation and osseointegration of dental implants compared with FH alone in minipigs.

Paracrine factors of mesenchymal stem cells recruit macrophages and endothelial lineage cells and enhance wound healing

Bone marrow derived mesenchymal stem cells (BM-MSCs) have been shown to enhance wound healing; however, the mechanisms involved are barely understood. In this study, we examined paracrine factors released by BM-MSCs and their effects on the cells participating in wound healing compared to those released by dermal fibroblasts. Analyses of BM-MSCs with Real-Time PCR and of BM-MSC-conditioned medium by antibody-based protein array and ELISA indicated that BM-MSCs secreted distinctively different cytokines and chemokines, such as greater amounts of VEGF-alpha, IGF-1, EGF, keratinocyte growth factor, angiopoietin-1, stromal derived factor-1, macrophage inflammatory protein-1alpha and beta and erythropoietin, compared to dermal fibroblasts. These molecules are known to be important in normal wound healing. BM-MSC-conditioned medium significantly enhanced migration of macrophages, keratinocytes and endothelial cells and proliferation of keratinocytes and endothelial cells compared to fibroblast-conditioned medium. Moreover, in a mouse model of excisional wound healing, where concentrated BM-MSC-conditioned medium was applied, accelerated wound healing occurred compared to administration of pre-conditioned or fibroblast-conditioned medium. Analysis of cell suspensions derived from the wound by FACS showed that wounds treated with BM-MSC-conditioned medium had increased proportions of CD4/80-positive macrophages and Flk-1-, CD34- or c-kit-positive endothelial (progenitor) cells compared to wounds treated with pre-conditioned medium or fibroblast-conditioned medium. Consistent with the above findings, immunohistochemical analysis of wound sections showed that wounds treated with BM-MSC-conditioned medium had increased abundance of macrophages. Our results suggest that factors released by BM-MSCs recruit macrophages and endothelial lineage cells into the wound thus enhancing wound healing.

Vascular endothelial growth factor expression in cultured periosteal-derived cells

The purpose of this study was to examine the expression of vascular endothelial growth factor (VEGF) during osteoblastic differentiation of cultured human periosteal-derived cells. Periosteal tissues were obtained from mandible during surgical extraction of lower impacted third molars. Periosteal-derived cells were introduced into cell culture. After passage 3, the periosteal-derived cells were further cultured for 42 days in an osteogenic-inductive culture medium containing dexamethasone, ascorbic acid, and beta-glycerophosphate. The alkaline phosphatase activity in the cultured periosteal-derived cells increased rapidly up to day 14, followed by decrease in activity. The Runx2 protein was expressed at day 7 and day 14, and its expression was not observed thereafter. Both VEGF(165) and VEGF(121) were expressed strongly at days 35 and 42 of culture, particularly during the later stages of differentiation. Alizarin red S-positive nodules were first observed on day 14 and then increased in number during the entire culture period. Osteocalcin and VEGF were first detected in the culture medium on day 14, and their levels increased thereafter in a time-dependent manner. These results suggest that VEGF secretion from cultured human periosteal-derived cells increases along with the mineralization process of the extracellular matrix.

Mesenchymal stem cells and neovascularization: role of platelet-derived growth factor receptors

There is now accumulating evidence that bone marrow-derived mesenchymal stem cells (MSCs) make an important contribution to postnatal vasculogenesis, especially during tissue ischaemia and tumour vascularization. Identifying mechanisms which regulate the role of MSCs in vasculogenesis is a key therapeutic objective, since while increased neovascularization can be advantageous during tissue ischaemia, it is deleterious during tumourigenesis. The potent angiogenic stimulant vascular endothelial growth factor (VEGF) is known to regulate MSC mobilization and recruitment to sites of neovascularization, as well as directing the differentiation of MSCs to a vascular cell fate. Despite the fact that MSCs did not express VEGF receptors, we have recently identified that VEGF-A can stimulate platelet-derived growth factor (PDGF) receptors, which regulates MSC migration and proliferation. This review focuses on the role of PDGF receptors in regulating the vascular cell fate of MSCs, with emphasis on the function of the novel VEGF-A/PDGF receptor signalling mechanism.

Changes in the expression of CD106, osteogenic genes, and transcription factors involved in the osteogenic differentiation of human bone marrow mesenchymal stem cells

Mesenchymal stem cells (MSCs) are well known to possess multipotential differentiation and are becoming a good tool for clinical research. However, specific markers for their purification and the mechanism of their osteogenic differentiation remain to be elucidated. In the present study, we compared the expression of CD106, and osteogenic differentiation-related proteins and genes in human bone marrow (BM)-derived MSCs, before and after differentiation by FACS, histochemical staining, immunohistochemical staining, RT-PCR, and real-time PCR. It was found that MSCs were positive for CD13, CD29, CD44, CD73, CD90, CD105, and CD166, but negative for CD14, CD31, CD34, CD62E, CD45, and GlyA. Notably, CD106 was detected before osteogenic induction, but its expression was downregulated 10 fold after 2 weeks of osteogenic differentiation as determined by flow cytometry. The results of RT-PCR and real-time PCR revealed that the expression of CD106 mRNA in MSCs significantly decreased by 7.1-, 4.2-, and 5.1-fold, respectively after osteogenic, chondrogenic, and adipogenic differentiation. In contrast, other MSC-positive markers described above did not change significantly even after differentiation. Compared to levels in control cells, after 2 weeks of osteogenic differentiation, mRNA levels of alkaline phosphatase, bone sialoprotein, osteocalcin, and transcript factors RUNX2 and Osterix showed more than 2-fold, 5-fold, 1.5-fold, 2-fold, and 5-fold increase, respectively. Thus, we speculate that CD106 might be a useful surface marker for BMMSCs. Moreover, alkaline phosphatase, type I collagen, osteonectin, osteopontin, and biglycin were expressed in the early stages of osteogenic differentiation before bone sialoprotein and osteocalcin. The present study should help to provide a novel marker for isolating purified MSCs and characterizing osteogenic differentiation.

Coordinated vascular endothelial growth factor expression and signaling during skeletal myogenic differentiation

Angiogenesis is largely controlled by hypoxia-driven transcriptional up-regulation and secretion of vascular endothelial growth factor (VEGF) and its binding to the endothelial cell tyrosine receptor kinases, VEGFR1 and VEGFR2. Recent expression analysis suggests that VEGF is expressed in a cell-specific manner in normoxic adult tissue; however, the transcriptional regulation and role of VEGF in these tissues remains fundamentally unknown. In this report we demonstrate that VEGF is coordinately up-regulated during terminal skeletal muscle differentiation. We reveal that this regulation is mediated in part by MyoD homo- and hetero-dimeric transcriptional mechanisms. Serial deletions of the VEGF promoter elucidated a region containing three tandem CANNTG consensus MyoD sites serving as essential sites of direct interaction for MyoD-mediated up-regulation of VEGF transcription. VEGF-null embryonic stem (ES) cells exhibited reduced myogenic differentiation compared with wild-type ES cells, suggesting that VEGF may serve a role in skeletal muscle differentiation. We demonstrate that VEGFR1 and VEGFR2 are expressed at low levels in myogenic precursor cells and are robustly activated upon VEGF stimulation and that their expression is coordinately regulated during skeletal muscle differentiation. VEGF stimulation of differentiating C2C12 cells promoted myotube hypertrophy and increased myogenic differentiation, whereas addition of sFlt1, a VEGF inhibitor, resulted in myotube hypotrophy and inhibited myogenic differentiation. We further provide evidence indicating VEGF-mediated myogenic marker expression, mitogenic activity, migration, and prosurvival functions may contribute to increased myogenesis. These data suggest a novel mechanism whereby VEGF is coordinately regulated as part of the myogenic differentiation program and serves an autocrine function regulating skeletal myogenesis.

Angiogenesis in bone fracture healing: a bioregulatory model

The process of fracture healing involves the action and interaction of many cells, regulated by biochemical and mechanical signals. Vital to a successful healing process is the restoration of a good vascular network. In this paper, a continuous mathematical model is presented that describes the different fracture healing stages and their response to biochemical stimuli only (a bioregulatory model); mechanoregulatory effects are excluded here. The model consists of a system of nonlinear partial differential equations describing the spatiotemporal evolution of concentrations and densities of the cell types, extracellular matrix types and growth factors indispensable to the healing process. The model starts after the inflammation phase, when the fracture callus has already been formed. Cell migration is described using not only haptokinetic, but also chemotactic and haptotactic influences. Cell differentiation is controlled by the presence of growth factors and sufficient vascularisation. Matrix synthesis and growth factor production are controlled by the local cell and matrix densities and by the local growth factor concentrations. Numerical simulations of the system, using parameter values based on experimental data obtained from literature, are presented. The simulation results are corroborated by comparison with experimental data from a standardised rodent fracture model. The results of sensitivity analyses on the parameter values as well as on the boundary and initial conditions are discussed. Numerical simulations of compromised healing situations showed that the establishment of a vascular network in response to angiogenic growth factors is a key factor in the healing process. Furthermore, a correct description of cell migration is also shown to be essential to the prediction of realistic spatiotemporal tissue distribution patterns in the fracture callus. The mathematical framework presented in this paper can be an important tool in furthering the understanding of the mechanisms causing compromised healing and can be applied in the design of future fracture healing experiments.

Regulation of in vitro vascular calcification by BMP4, VEGF and Wnt3a

Vascular calcification is a common clinical complication of cardiovascular disease, diabetes and end-stage renal failure, associated with significant morbidity and mortality. In this study we demonstrate that factors secreted by the hypertrophic chondrocytes induce matrix mineralization and osteoblastic transformation in cultured mouse vascular smooth muscle cells (VSMCs). In addition, these factors render VSMCs responsive to BMP4 and Wnt3a ligands. Neither BMP-4 nor Wnt3a could induce mineralization in short-term (up to 8 days) cultures of primary mouse VSMCs. However, both ligands act synergistically with the chondrocyte-conditioned medium causing a further increase in VSMC calcification. Finally, we show that commitment of VSMCs towards the BMP-regulated mineralization can be induced by the chondrocyte-secreted bone anabolic factor VEGF. In addition, expression profiling suggests a novel role in vascular calcification for the matrix proteins previously known to regulate bone formation and mineralization (including MMP3, fibulin, 11betahydroxysteroid dehydrogenase 1 and retinoic acid receptor responder 2). The results of this study may contribute to further understanding of the cellular mechanisms responsible for vascular calcification and provide important information for the treatment of this pathology.

Microvessel-Like Structures from Outgrowth Endothelial Cells from Human Peripheral Blood in 2-Dimensional and 3-Dimensional Co-Cultures with Osteoblastic Lineage Cells

Tissue regeneration involves complex processes in the interaction between different cell types that control the process of neo-vascularization. In bone, osteoblasts and bone marrow stem cells provide cue elements for the proliferation of endothelial cells, differentiation of endothelial precursors, and the maturation of a vascular network. In this study, we investigated outgrowth endothelial cells (OECs), a potential source of autologous endothelial cells derived from human peripheral blood, in direct 2-dimensional (2-D) and 3-D co-culture systems with cells relevant for the regeneration of bone tissue, such as osteoblasts. In the co-cultures, OECs were evaluated in terms of their stability as an endothelial population at the single cell level using flow cytometry and their ability to establish a pre-vascular network at the light-microscopical and ultra-structural level. In co-cultures with the osteoblast cell line MG63 and with human primary osteoblasts (pOBs), OECs, in contrast to human umbilical vein endothelial cells, formed highly organized microvessel-like structures. These microvessel-like structures included the formation of a vascular lumen with tight junctional complexes at intercellular contacts of endothelial cells. In the co-culture, the formation of this vascular network was achieved in the standard growth medium for OECs. Furthermore, using a rotating culture vessel system, 3-D co-cultures consisting of OECs and pOBs were generated. Based on these observations, we conclude that OECs could provide a valuable source of autologous endothelial cells for the generation of complex tissue-engineered tissues.

Biological responses in osteoblast-like cell line according to thin layer hydroxyapatite coatings on anodized titanium

Several features of the implant surface, such as roughness, topography and composition play a relevant role in implant integration with bone. This study was conducted in order to determine the effects of various thin layer hydroxyapatite (HA) coatings on anodized Ti surfaces on the biological responses of a human osteoblast-like cell line (MG63). MG63 cells were cultured on 100 nm HA (100 nm HA coating on anodized surface), 500-700 nm HA (500-700 nm HA coating on anodized surface), 1 mum HA (1 mum HA coating on anodized surface) and anodize (non-HA coating on anodized surface) Ti. The morphology of these cells was assessed by scanning electron microscopy (SEM). The cDNAs prepared from the total RNAs of the MG63 were hybridized into a human cDNA microarray (1152 elements). The appearances of the surfaces observed by SEM were different on each of the four dental substrate types. MG63 cells cultured on 100 nm HA, 1 mum HA and anodize exhibited cell-matrix interactions. It was 500-700 nm HA surface showing cell-cell interaction. In the expression of genes involved in osseointegration, several genes, including bone morphogenetic protein 2, latent transforming growth factor beta binding protein 1, catenin (cadherin-associated protein), integrin, PDGFRB and GDF-1 growth differentiation factor 1 were up-regulated on the different surfaces. Several genes, including fibroblast growth factor receptor 3, fibroblast growth factor 12 and CD4 were down-regulated on the different surfaces. The attachment and expression of key osteogenic regulatory genes were enhanced by the surface morphology of the dental materials used.

Evaluation of chemokine and cytokine profiles in osteoblast progenitors from umbilical cord blood stem cells by BIO-PLEX technology

We have used cytokine protein array to analyze the secretion of cytokines from an osteoblastic clone derived from human umbilical cord blood mesenchymal stem cells (MSCs) cultured in an osteogenic differentiation medium. The analysis demonstrated the unexpected ability of osteoblast committed cells and their early progenitors to produce significant amounts of a range of soluble immune mediators without in vitro exposure to clinically relevant bacterial pathogens. The cells were expanded and their osteogenic potential analyzed over 45 days of culture was revealed by the expression of osteoblast-specific markers (alkaline phosphatase and Runx2), and by matrix mineralization. Over this culture period, the cells secreted particularly high levels of IL-8, MCP-1 and VEGF, but did not express IL-2, IL-7, IL-17, eotaxin, G-CSF and IFN-gamma. These findings should encourage the use of human umbilical cord blood as a potential stem cells source for bone regeneration.

Intrinsic Axial Vascularization of an Osteoconductive Bone Matrix by Means of an Arteriovenous Vascular Bundle

BACKGROUND

The purpose of this study was to generate an autonomously vascularized hard-tissue construct suitable for microsurgical transfer. The effector of vascularization was an arteriovenous bundle inserted into a specially designed channel in the matrix. The authors also evaluated corrosion cast and intravital magnetic resonance angiography as methods for monitoring and quantifying the angiogenic response.

METHODS

Thirty inbred male Lewis rats were divided into two groups. In both groups (n = 15), a disk of processed bovine cancellous bone matrix was placed into an isolation chamber. In group A, a ligated arteriovenous bundle was inserted into the biogenic matrix as a vascular carrier. In group B, there was no vascular carrier. At 2, 4, and 8 weeks after implantation, four constructs per group were evaluated by means of histology and histomorphometry and one by scanning electron microscopy of vascular corrosion casts. Micro-magnetic resonance angiography was used for intravital evaluation of the vascularized matrices.

RESULTS

Vascular density was higher in group A. The capillary network in group A displayed a higher degree of maturation, with organization into vessels of different orders. Both the sprouting and intussusceptive modes of angiogenesis could be documented. Micro-magnetic resonance angiography showed a patency rate of approximately 75 percent in the bundle.

CONCLUSIONS

The authors zeroed in on the issue of vascularization. The results might provide a basis for further investigations on induction of bone formation in axially prevascularized matrices. Axially vascularized bone substitutes might solve issues of availability in mass and form and provide perfusion autonomy in sites of impaired circulation.

Impairment of endothelial cell differentiation from bone marrow-derived mesenchymal stem cells: new insight into the pathogenesis of systemic sclerosis

OBJECTIVE

Systemic sclerosis (SSc) is a disorder characterized by vascular damage and fibrosis of the skin and internal organs. Despite marked tissue hypoxia, there is no evidence of compensatory angiogenesis. The ability of mesenchymal stem cells (MSCs) to differentiate into endothelial cells was recently demonstrated. The aim of this study was to determine whether impaired differentiation of MSCs into endothelial cells in SSc might contribute to disease pathogenesis by decreasing endothelial repair.

METHODS

MSCs obtained from 7 SSc patients and 15 healthy controls were characterized. The number of colony-forming unit-fibroblastoid colonies was determined. After culture in endothelial-specific medium, the endothelial-like MSC (EL-MSC) phenotype was assessed according to the surface expression of vascular endothelial growth factor receptors (VEGFRs). Senescence, chemoinvasion, and capillary morphogenesis studies were also performed.

RESULTS

MSCs from SSc patients displayed the same phenotype and clonogenic activity as those from controls. In SSc MSCs, a decreased percentage of VEGFR-2+, CXCR4+, VEGFR-2+/CXCR4+ cells and early senescence was detected. After culturing, SSc EL-MSCs showed increased expression of VEGFR-1, VEGFR-2, and CXCR4, did not express CD31 or annexin V, and showed significantly decreased migration after specific stimuli. Moreover, the addition of VEGF and stromal cell-derived factor 1 to cultured SSc EL-MSCs increased their angiogenic potential less than that in controls.

CONCLUSION

Our data strongly suggest that endothelial repair may be affected in SSc. The possibility that endothelial progenitor cells could be used to increase vessel growth in chronic ischemic tissues may open up new avenues in the treatment of vascular damage caused by SSc.

Calcium phosphate bone cements, functionalized with VEGF: release kinetics and biological activity

Calcium phosphate bone cements are of great interest for bone replacement since the nanocrystalline structure allows their remodelling into native bone tissue. A strategy to accelerate vascularization of the implant region is the functionalization with vascular endothelial growth factor (VEGF), which is known to mediate angiogenesis in vivo. In this study, the release of recombinant human VEGF (rhVEGF(165)) following physical adsorption to Biocement D (BioD) and several modifications were investigated. Our data demonstrate a high VEGF binding capacity of BioD and a sustained release with a moderate initial burst. A proliferation assay using endothelial cells revealed maintenance of biological activity of VEGF after release from BioD. Release behavior of BioD was not improved by modification with mineralized collagen type I, as well as with a combination of mineralized collagen with O-phospho-L-serine and sodium citrate, respectively. In contrast, a positive impact of these modifications on the activity of released VEGF was observed; in case of the phosphoserine- and sodium citrate-modified cements, the biological efficacy of released VEGF was even higher than that of nonreleased control VEGF. We conclude that the bone implant material BioD and, especially, the phosphoserine modification may support activation of angiogenesis by delivery of VEGF in a local and sustained manner.

Review: ex vivo engineering of living tissues with adult stem cells

Adult stem cells have the potential to revolutionize regenerative medicine with their unique abilities to self-renew and differentiate into various phenotypes. This review examines progress and challenges in ex vivo tissue engineering with adult stem cells. These rare cells are harvested from a variety of tissues, including bone marrow, adipose, skeletal muscle, and placenta, and differentiate into cells of their own lineage and in some cases atypical lineages. Insight into the stem cell niche leads to the identification of matrix components, soluble factors, and physiological conditions that enhance the ex vivo amplification and differentiation of stem cells. Scaffolds composed of metals, naturally occurring materials, and synthetic polymers organize stem cells into complex spatial groupings that mimic native tissue. Cell signals from covalently bound ligands and slowly released regulatory factors in scaffolds direct stem cell fate. Future advances in stem cell biology and scaffold design will ultimately improve the efficacy of tissue substitutes as implants, in research, and as extracorporeal devices.

Elucidating the secretion proteome of human embryonic stem cell-derived mesenchymal stem cells

Transplantation of mesenchymal stem cells (MSCs) has been used to treat a wide range of diseases, and the mechanism of action is postulated to be mediated by either differentiation into functional reparative cells that replace injured tissues or secretion of paracrine factors that promote tissue repair. To complement earlier studies that identified some of the paracrine factors, we profiled the paracrine proteome to better assess the relevance of MSC paracrine factors to the wide spectrum of MSC-mediated therapeutic effects. To evaluate the therapeutic potential of the MSC paracrine proteome, a chemically defined serum-free culture medium was conditioned by MSCs derived from human embryonic stem cells using a clinically compliant protocol. The conditioned medium was analyzed by multidimensional protein identification technology and cytokine antibody array analysis and revealed the presence of 201 unique gene products. 86-88% of these gene products had detectable transcript levels by microarray or quantitative RT-PCR assays. Computational analysis predicted that these gene products will significantly drive three major groups of biological processes: metabolism, defense response, and tissue differentiation including vascularization, hematopoiesis, and skeletal development. It also predicted that the 201 gene products activate important signaling pathways in cardiovascular biology, bone development, and hematopoiesis such as Jak-STAT, MAPK, Toll-like receptor, transforming growth factor-beta, and mTOR (mammalian target of rapamycin) signaling pathways. This study identified a large number of MSC secretory products that have the potential to act as paracrine modulators of tissue repair and replacement in diseases of the cardiovascular, hematopoietic, and skeletal tissues. Moreover our results suggest that human embryonic stem cell-derived MSC-conditioned medium has the potency to treat a variety of diseases in humans without cell transplantation.

The hypoxia-inducible factor is stabilized in circulating hematopoietic stem cells under normoxic conditions

The hypoxia-inducible factor (HIF) transcriptional system enables cell adaptation to limited O(2) availability, transducing this signal into patho-physiological responses such as angiogenesis, erythropoiesis, vasomotor control, and altered energy metabolism, as well as cell survival decisions. However, other factors beyond hypoxia are known to activate this pleiotropic transcription factor. The aim of this study was to characterize HIF in human hematopoietic stem cells (HSCs) and evidence is provided that granulocyte colony stimulating factor-mobilized CD34+- and CD133+-HSCs express a stabilized cytoplasmic form of HIF-1alpha under normoxic conditions. It is shown that HIF-1alpha stabilization correlates with down-regulation of the tumour suppressor von Hippel-Lindau protein (pVHL) and is positively controlled by NADPH-oxidase-dependent production of reactive oxygen species, indicating a specific O(2)-independent post-transcriptional control of HIF in mobilized HSCs. This novel finding is discussed in the context of the proposed role of HIF as a mediator of progenitor cell recruitment to injured ischemic tissues and/or in the control of the maintenance of the undifferentiated state.

High efficient adenoviral-mediated VEGF and Ang-1 gene delivery into osteogenically differentiated human mesenchymal stem cells

Survival of ex vivo constructed tissues after transplantation is limited by insufficient oxygen and nutrient supply. Therefore, strategies aiming at improvement of neovascularization of engineered tissues are a key issue in tissue engineering applications. This in vitro study aimed at exploring the usability of osteogenically differentiated human mesenchymal stem cells (MSCs) as carriers of the angiogenic growth factor genes vascular endothelial growth factor (VEGF) and angiopoietin-1 (Ang-1) for therapeutic angiogenesis in bone tissue engineering. The ex vivo adenoviral vector mediated transduction into osteogenically differentiated MSCs revealed a highly efficient and long lasting expression of the transgenes. Biological activity of VEGF and Ang-1 secreted from transduced cells was confirmed by analyzing the sprouting, proliferation and apoptosis of human umbilical vein endothelial cells (HUVECs) in response to conditioned medium obtained from transduced cells. The transduced osteogenically differentiated MSCs described in this report may be suitable for inducing neovascularization in bone tissue engineering applications.

Angiogenic inhibition reduces germinal matrix hemorrhage

The germinal matrix of premature infants is selectively vulnerable to hemorrhage within the first 48 h of life. To assess the role of vascular immaturity in germinal matrix hemorrhage (GMH), we evaluated germinal matrix angiogenesis in human fetuses and premature infants, as well as in premature rabbit pups, and noted active vessel remodeling in all three. Vascular endothelial growth factor (VEGF), angiopoietin-2 and endothelial cell proliferation were present at consistently higher levels in the germinal matrix relative to the white matter anlagen and cortical mantle. On that basis, we asked whether prenatal treatment with either of two angiogenic inhibitors, the COX-2 inhibitor celecoxib, or the VEGFR2 inhibitor ZD6474, could suppress the incidence of GMH in premature rabbit pups. Celecoxib treatment decreased angiopoietin-2 and VEGF levels as well as germinal matrix endothelial proliferation. Furthermore, treatment with celecoxib or ZD6474 substantially decreased the incidence of GMH. Thus, by suppressing germinal matrix angiogenesis, prenatal celecoxib or ZD6474 treatment may be able to reduce both the incidence and severity of GMH in susceptible premature infants.

Expression of vascular endothelial growth factor and its receptors after mandibular distraction osteogenesis

During distraction osteogenesis, angiogenic activity is essential for new bone formation. This study examined the expression of vascular endothelial growth factor (VEGF) and two of its receptors, Flt-1 (VEGFR-1) and Flk-1 (VEGFR-2), in cellular components after mandibular distraction osteogenesis. Unilateral mandibular distraction (0.5 mm twice per day for 10 days) was performed in six mongrel dogs. Two animals each were killed on days 7, 14 and 28 after completion of distraction. The distracted mandibular segments and contralateral undistracted control segments were harvested and processed for immunohistochemical examination. Seven days after distraction, there was a significant increase in the expression levels of VEGF and its receptors in the osteoblasts, osteocytes and immature fibroblast-like cells compared to control specimens. These levels were maintained for 14 days after distraction in the osteoblasts and fibroblast-like cells. Twenty-eight days after distraction, VEGF and VEGFR-1 were expressed only moderately/weakly in the osteoblasts, and no VEGFR-2 expression was detected in the cellular component of the distracted bone. Throughout the observation period, VEGFR-1 expression was stronger than that of VEGFR-2. The expression patterns of VEGF and its receptors suggest that it plays an important role in osteogenesis, and that osteoblasts and immature fibroblast-like cells of the distracted bone may have an autocrine growth effect during distraction osteogenesis.

Hyaluronan mixed esters of butyric and retinoic Acid drive cardiac and endothelial fate in term placenta human mesenchymal stem cells and enhance cardiac repair in infarcted rat hearts

We have developed a mixed ester of hyaluronan with butyric and retinoic acid (HBR) that acted as a novel cardiogenic/vasculogenic agent in human mesenchymal stem cells isolated from bone marrow, dental pulp, and fetal membranes of term placenta (FMhMSCs). HBR remarkably enhanced vascular endothelial growth factor (VEGF), KDR, and hepatocyte growth factor (HGF) gene expression and the secretion of the angiogenic, mitogenic, and antiapoptotic factors VEGF and HGF, priming stem cell differentiation into endothelial cells. HBR also increased the transcription of the cardiac lineage-promoting genes GATA-4 and Nkx-2.5 and the yield of cardiac markerexpressing cells. These responses were notably more pronounced in FMhMSCs. FMhMSC transplantation into infarcted rat hearts was associated with increased capillary density, normalization of left ventricular function, and significant decrease in scar tissue. Transplantation of HBR-preconditioned FMhM-SCs further enhanced capillary density and the yield of human vWF-expressing cells, additionally decreasing the infarct size. Some engrafted, HBR-pretreated FMhMSCs were also positive for connexin 43 and cardiac troponin I. Thus, the beneficial effects of HBR-exposed FMhMSCs may be mediated by a large supply of angiogenic and antiapoptotic factors, and FMhMSC differentiation into vascular cells. These findings may contribute to further development in cell therapy of heart failure.

Murine bone marrow stromal progenitor cells elicit an in vivo cellular and humoral alloimmune response

Stromal progenitor cells (SPC) exhibit immunosuppressive effects in vitro that have led to speculation regarding their capacity to evade host immune recognition and to treat autoimmune diseases and gravt-versus-host disease. However, there is little in vivo experimental data to support these immunologic claims. To assess immune recognition of SPC in vivo, we evaluated the immune response of animals transplanted with SPC. C57BL/6 (B6) or Balb/c adult, murine, bone marrow-derived SPC (AmSPC) were administered by intraperitoneal injection into B6 recipients. T cell proliferation and alloantibody response was assessed from spleens and peripheral blood harvested from transplanted animals and analyzed by cell proliferation assay and flow cytometry. To assess tolerance induction, transplanted animals also received allogeneic skin grafts. Animals injected with allogeneic AmSPC mounted an accelerated CD4 response to alloantigen compared to syngeneic AmSPC injected and uninjected controls. Allogeneic AmSPC-injected animals also demonstrated high titers (> or =1:1000) of antibody directed against allogeneic AmSPC targets. Animals primed with donor or host-matched AmSPC also failed to induce tolerance, and all animals exhibited rejection of allogeneic skin grafts (n = 7, P < .0001). In contrast to their in vitro behavior, our data demonstrate that AmSPC are recognized by the host immune system in vivo, elicit a cellular and humoral immune response, and fail to induce tolerance. These findings have significant implications for all allogeneic SPC-based therapeutic strategies.

Cord blood in regenerative medicine: do we need immune suppression?

Cord blood is currently used as an alternative to bone marrow as a source of stem cells for hematopoietic reconstitution after ablation. It is also under intense preclinical investigation for a variety of indications ranging from stroke, to limb ischemia, to myocardial regeneration. A major drawback in the current use of cord blood is that substantial morbidity and mortality are associated with pre-transplant ablation of the recipient hematopoietic system. Here we raise the possibility that due to unique immunological properties of both the stem cell and non-stem cell components of cord blood, it may be possible to utilize allogeneic cells for regenerative applications without needing to fully compromise the recipient immune system. Issues raised will include: graft versus host potential, the immunogenicity of the cord blood graft, and the parallels between cord blood transplantation and fetal to maternal trafficking. The previous use of unmatched cord blood in absence of any immune ablation, as well as potential steps for widespread clinical implementation of allogeneic cord blood grafts will also be discussed.

Mesenchymal stem cells regulate angiogenesis according to their mechanical environment

In fracture and bone defect healing, MSCs largely drive tissue regeneration. MSCs have been shown to promote angiogenesis both in vivo and in vitro. Angiogenesis is a prerequisite to large tissue reconstitution. The present study investigated how mechanical loading of MSCs influences their proangiogenic capacity. The results show a significant enhancement of angiogenesis by conditioned media from mechanically stimulated compared with unstimulated MSCs in two-dimensional tube formation and three-dimensional spheroid sprouting assays. In particular, proliferation but not migration or adhesion of endothelial cells was elevated. Promotion of angiogenesis was dependent upon fibroblast growth factor receptor 1 (FGFR1) signaling. Moreover, stimulation of tube formation was inhibited by vascular endothelial growth factor receptor (VEGFR) tyrosine kinase blocking. Screening for the expression levels of different soluble regulators of angiogenesis revealed an enrichment of matrix metalloprotease 2, transforming growth factor beta1, and basic fibroblast growth factor but not of vascular endothelial growth factor in response to mechanical stimulation. In conclusion, mechanical loading of MSCs seems to result in a paracrine stimulation of angiogenesis, most likely by the regulation of a network of several angiogenic molecules. The underlying mechanism appears to be dependent on the FGFR and VEGFR signaling cascades and might be mediated by an additional cross-talk with other pathways.

Remodeling and vascular spaces in bone

In recent years, we have come to appreciate that the close association between bone and vasculature plays a pivotal role in the regulation of bone remodeling and fracture repair. In 2001, Hauge et al. characterized a specialized vascular structure, the bone remodeling compartment (BRC), and showed that the outer lining of this compartment was made up of flattened cells, displaying all the characteristics of lining cells in bone. A decrease in bone turnover leads to a decrease in surfaces covered with remodeling compartments, whereas increased turnover causes an increase. Immunoreactivity for all major osteotropic growth factors and cytokines including osteoprotegerin (OPG) and RANKL has been shown in the cells lining the BRC, which makes the BRC the structure of choice for coupling between resorption and formation. The secretion of these factors inside a confined space separated from the bone marrow would facilitate local regulation of the remodeling process without interference from growth factors secreted by blood cells in the marrow space. The BRC creates an environment where cells inside the structure are exposed to denuded bone, which may enable direct cellular interactions with integrins and other matrix factors known to regulate osteoclast/osteoblast activity. However, the denuded bone surface inside the BRC also constitutes an ideal environment for the seeding of bone metastases, known to have high affinity for bone matrix. Reduction in BRC space brought about by antiresorptive therapies such as bisphosphonates reduce the number of skeletal events in advanced cancer, whereas an increase in BRC space induced by remodeling activators like PTH may increase the bone metastatic burden. The BRC has only been characterized in detail in trabecular bone; there is, however, evidence that a similar structure may exist in cortical bone, but further characterization is needed.

Transcriptional profiling of human cord blood CD133+ and cultured bone marrow mesenchymal stem cells in response to hypoxia

Umbilical cord blood (UCB) and bone marrow (BM)-derived stem and progenitor cells possess two characteristics required for successful tissue regeneration: extensive proliferative capacity and the ability to differentiate into multiple cell lineages. Within the normal BM and in pathological conditions, areas of hypoxia may have a role in maintaining stem cell fate or determining the fine equilibrium between their proliferation and differentiation. In this study, the transcriptional profiles and proliferation and differentiation potential of UCB CD133(+) cells and BM mesenchymal cells (BMMC) exposed to normoxia and hypoxia were analyzed and compared. Both progenitor cell populations responded to hypoxic stimuli by stabilizing the hypoxia inducible factor (HIF)-1alpha protein. Short exposures to hypoxia increased the clonogenic myeloid capacity of UCB CD133(+) cells and promoted a significant increase in BMMC number. The differentiation potential of UCB CD133(+) clonogenic myeloid cells was unaltered by short exposures to hypoxia. In contrast, the chondrogenic differentiation potential of BMMCs was enhanced by hypoxia, whereas adipogenesis and osteogenesis were unaltered. When their transcriptional profiles were compared, 183 genes in UCB CD133(+) cells and 45 genes in BMMC were differentially regulated by hypoxia. These genes included known hypoxia-responsive targets such as BNIP3, PGK1, ENO2, and VEGFA, and other genes not previously described to be regulated by hypoxia. Several of these genes, namely CDTSPL, CCL20, LSP1, NEDD9, TMEM45A, EDG-1, and EPHA3 were confirmed to be regulated by hypoxia using quantitative reverse transcriptase polymerase chain reaction. These results, therefore, provide a global view of the signaling and regulatory network that controls oxygen sensing in human adult stem/progenitor cells derived from hematopoietic tissues.

Derivation of clinically compliant MSCs from CD105+, CD24- differentiated human ESCs

Adult tissue-derived mesenchymal stem cells (MSCs) have demonstrated therapeutic efficacy in treating diseases or repairing damaged tissues through mechanisms thought to be mediated by either cell replacement or secretion of paracrine factors. Characterized, self-renewing human ESCs could potentially be an invariable source of consistently uniform MSCs for therapeutic applications. Here we describe a clinically relevant and reproducible manner of generating identical batches of hESC-derived MSC (hESC-MSC) cultures that circumvents exposure to virus, mouse cells, or serum. Trypsinization and propagation of HuES9 or H1 hESCs in feeder- and serum-free selection media generated three polyclonal, karyotypically stable, and phenotypically MSC-like cultures that do not express pluripotency-associated markers but displayed MSC-like surface antigens and gene expression profile. They differentiate into adipocytes, osteocytes, and chondrocytes in vitro. Gene expression and fluorescence-activated cell sorter analysis identified CD105 and CD24 as highly expressed antigens on hESC-MSCs and hESCs, respectively. CD105+, CD24- monoclonal isolates have a typical MSC gene expression profiles and were identical to each other with a highly correlated gene expression profile (r(2) > .90). We have developed a protocol to reproducibly generate clinically compliant and identical hESC-MSC cultures.

Microvascular adaptation to growth in rat humeral head

The aim of this work is to investigate the growth of the vasculature in the rat humeral head cartilage after the initial development of the secondary ossification centre until the adult organization. Rats aging from 5 weeks to 12 months were used. Histological observations on humeral heads were implemented with morphometrical analysis. Subsequently, vascular corrosion cast, that permits a three-dimensional observation of the vasculature, were prepared and observed by scanning electron microscopy. In young animals the epiphysis contains thin bone trabeculae and most of the epiphysis is occupied by bone marrow spaces. With age, the bone trabeculae progressively enlarge up to double their thickness. The percentage of bone tissue increases from 33.6 to 58.6% of the entire epiphysis, while the bone marrow spaces tend to increase very little in their mean dimension. Vascular corrosion casts show that the epiphyseal microcirculation is well distinguished from that of the diaphysis, and arises from the vessels present in the capsule and the periosteal networks. In young animals the only capillaries are bone marrow sinusoids and few subchondral capillaries. In adult animals small vessels run between the clusters of sinusoids forming the trabecular circulation. Capillary sprouts from sinusoids are always observed both in the young and adult animals. Thus, in adult rats different proper microcirculatory districts can be distinguished in the epiphysis: (a) the sinusoidal network, that supplies the hematopoiesis of the bone marrow and the adjacent osteogenic tissue; (b) the bone tissue microcirculation, limited to small vessels that supply the metabolism and the remodelling of the bone tissue. The reported microvascular organization and its adaptation to the epiphyseal growth represent the morphological basis for understanding the reciprocal interaction among the different tissues in developing and adult rat epiphysis.