Contribution of Rat Endothelial Progenitor Cells on Three-Dimensional Network FormationIn Vitro

Effects of PB-EPCs on Homing Ability of Rabbit BMSCs via Endogenous SDF-1 and MCP-1

Traumas, infections, tumors, and some congenital malformations can lead to bone defects or even bone loss. The goal of the present study was to investigate whether inclusion of endothelial progenitor cells derived from peripheral blood (PB–EPCs) in cell-seeded partially deproteinized bone (PDPB) implants would stimulate recruitment of systemically injected bone marrow stromal cells (BMSCs) to the implant. Methods: BMSCs were injected intravenously with lentiviral expression vector expressing enhanced green fluorescent protein (eGFP) for tracing. Recruitment of eGFP-positive BMSCs was tested for the following implant configurations: 1) seeded with both BMSC and PB-EPC, 2) BMSC alone, 3) PB-EPC alone, and 4) unseeded PDPB. Protein and mRNA levels of endogenous stromal-derived factor-1 (SDF-1) and its receptor CXCR4, as well as monocyte chemotactic protein-1 (MCP-1) and its receptor CCR2, were evaluated on the 8th week. Immunohistochemical staining was performed to determine eGFP-positive areas at the defective sites. Masson’s trichrome staining was conducted to observe the distribution of collagen deposition and evaluate the extent of osteogenesis. Results: The mRNA and protein levels of SDF-1 and CXCR4 in the co-culture group were higher than those in other groups (p < 0.05) 8 weeks after the surgery. MCP-1 mRNA level in the co-culture group was also higher than that in the other groups (p < 0.05). Immunohistochemical assays revealed that the area covered by eGFP-positive cells was larger in the co-culture group than in the other groups (p < 0.05) after 4 weeks. Masson’s trichrome staining revealed better osteogenic potential of the co-culture group compared to the other groups (p < 0.05). Conclusion: These experiments demonstrate an association between PB-EPC and BMSC recruitment mediated by the SDF-1/CXCR4 axis that can enhance repair of bone defects.

Photoinitiator-free synthesis of endothelial cell-adhesive and enzymatically degradable hydrogels

We report on a photoinitiator-free synthetic method of incorporating bioactivity into poly(ethylene glycol) (PEG) hydrogels in order to control physical properties, enzymatic biodegradability and cell-specific adhesiveness of the polymer network, while eliminating the need for UV-mediated photopolymerization. To accomplish this, hydrogel networks were polymerized using Michael addition with four-arm PEG acrylate (10 kDa), using a collagenase-sensitive peptide (CSP) as a crosslinker, and introducing an endothelial cell-adhesive peptide either terminally (RGD) or attached to the crosslinking peptide sequence (CSP-RGD). The efficiency of the Michael addition reactions were determined by nuclear magnetic resonance and Ellman's assay. Successful decoupling of cell adhesivity and physical properties was demonstrated by quantifying and comparing the swelling ratios and Young's moduli of various hydrogel formulations. Degradation profiles were established by incubating functionalized hydrogels in collagenase solutions (0.0-1.0 μg ml(-1)), demonstrating that functionalized hydrogels degraded at a rate dependent upon collagenase concentration. Moreover, it was shown that the degradation rate was independent of CSP-RGD concentration. Cell attachment and proliferation on functionalized hydrogels were compared for various RGD concentrations, providing evidence that cell attachment and proliferation were directly related to relative amounts of the CSP-RGD combination peptide. An increase in cell viability was achieved using Michael addition techniques when compared to UV polymerization, and was assessed by a LIVE/DEAD fluorescence assay. This photoinitiator-free method shows promise in creating hydrogel-based tissue engineering scaffolds allow for decoupled cell adhesivity and physical properties and that render greater cell viability.

Endothelial progenitor cells promote directional three-dimensional endothelial network formation by secreting vascular endothelial growth factor

Endothelial progenitor cell (EPC) transplantation induces the formation of new blood-vessel networks to supply nutrients and oxygen, and is feasible for the treatment of ischemia and cardiovascular diseases. However, the role of EPCs as a source of proangiogenic cytokines and consequent generators of an extracellular growth factor microenvironment in three-dimensional (3D) microvessel formation is not fully understood. We focused on the contribution of EPCs as a source of proangiogenic cytokines on 3D microvessel formation using an in vitro 3D network model. To create a 3D network model, EPCs isolated from rat bone marrow were sandwiched with double layers of collagen gel. Endothelial cells (ECs) were then cultured on top of the upper collagen gel layer. Quantitative analyses of EC network formation revealed that the length, number, and depth of the EC networks were significantly enhanced in a 3D model with ECs and EPCs compared to an EC monoculture. In addition, conditioned medium (CM) from the 3D model with ECs and EPCs promoted network formation compared to CM from an EC monoculture. We also confirmed that EPCs secreted vascular endothelial growth factor (VEGF). However, networks cultured with the CM were shallow and did not penetrate the collagen gel in great depth. Therefore, we conclude that EPCs contribute to 3D network formation at least through indirect incorporation by generating a local VEGF gradient. These results suggest that the location of EPCs is important for controlling directional 3D network formation in the field of tissue engineering.

Endothelial progenitor cells: current development of their paracrine factors in cardiovascular therapy

Endothelial progenitor cells were initially considered to radically alter the concepts of adult tissue angiogenesis for their contribution of incorporation into new blood vessels. Nevertheless, controversy arises over their mechanism of action due to rare cell population and decreased number and impaired activity under pathological changes. Recent studies show that endothelial progenitor cells also function in a paracrine manner by secreting multiple cytokines and growth factors, but the beneficial paracrine signals remain partially unidentified. In this review, we provide an overview of varieties and signal pathways of factors secreted by endothelial progenitor cells and further present the prospect of new ways to encourage cardiovascular protection such as neovascularization, reendothelialization of larger vessels, and myocardial remodeling based on the paracrine factors.

Endothelial invasive response in a co-culture model with physically-induced osteodifferentiation

Manipulation of stem cells using physicochemical stimuli has emerged as an important tool in regenerative medicine. While 2D substrates with tunable elasticity have been studied for control of stem cell differentiation, we recently developed a stratified co-culture model of angiogenesis of human mesenchymal stem cells (hMSCs) that differentiate on a tunable polydimethylsiloxane (PDMS) substrate, thereby creating a physiologic context for elasticity-induced differentiation. Endothelial cells (EC) were cultured on top of the hMSC construct on a collagen gel to monitor network formation. Media composition influenced EC invasion due to the conditioning media, the reduction of serum and supplemental growth factors, and the addition of recombinant growth factors. Conditioned media, recombinant growth factors and direct co-culture were compared for endothelial cell invasive response using quantitative image analysis. As anticipated, use of recombinant vascular endothelial growth factor (VEGF) induced the deepest EC invasions while direct co-culture caused shallow invasions compared to other conditions. However, endothelial cells displayed lumen-like morphology, suggesting that cell-cell interaction in the co-culture model could mimic sprouting behaviour. In summary, an engineered suitable biochemical and physical environment facilitated endothelial cells to form 3D vessel structures onto hMSCs. These structures were plated on a stiff surface known to induce osteodifferentiation of stem cells. This low cost co-culture system, with its minimal chemical supplementation and physically controllable matrix, could potentially model in vivo potential in engineered and pre-vascularized bone grafts.

Combined effect of osteopontin and BMP-2 derived peptides grafted to an adhesive hydrogel on osteogenic and vasculogenic differentiation of marrow stromal cells

The objective of this work was to investigate the combined effect of grafting the peptide corresponding to amino acid residues 162-168 of osteopontin (OPD peptide) and the peptide corresponding to amino acid residues 73-92 of bone morphogenetic protein-2 (BMP peptide) to an RGD-conjugated inert hydrogel on osteogenic and vasculogenic differentiation of bone marrow stromal (BMS) cells. RGD-conjugated three-dimensional (3D) porous hydrogel scaffolds with well-defined cylindrical pore geometry were produced from sacrificial wax molds fabricated by fused deposition modeling rapid prototyping system. Propargyl acrylate and 4-pentenal were conjugated to the hydrogel for orthogonal grafting of BMP and OPD peptides by click reaction and oxime ligation, respectively. The OPD peptide was grafted by the reaction between aminooxy moiety of aminooxy-mPEG-OPD (mPEG = mini-poly(ethylene glycol)) and the aldehyde moiety in the hydrogel. The BMP peptide was grafted by the reaction between the azide moiety of Az-mPEG-BMP and the propargyl moiety in the hydrogel. The hydrogels seeded with BMS cells were characterized by biochemical, immunocytochemical, and mRNA analyses. Groups included RGD control hydrogel (RGD), RGD and BMP peptides without OPD (RGD+BMP), RGD and BMP peptides with mutant OPD (RGD+BMP+mOPD), and RGD and BMP peptides with OPD (RGD+BMP+OPD) grafted hydrogels. The extent of mineralization of RGD, RGD+BMP, RGD+BMP+mOPD, and RGD+BMP+OPD groups after 28 days was 650 ± 70, 990 ± 30, 850 ± 30, and 1150 ± 40 mg/(mg of DNA), respectively, indicating that the BMP and OPD peptides enhanced osteogenic differentiation of the BMS cells. The BMS cells seeded on RGD+BMP+OPD grafted hydrogels stained positive for vasculogenic markers α-SMA, PECAM-1, and VE-cadherin while the groups without OPD peptide (RGD+BMP and RGD+BMP+mOPD) stained only for α-SMA but not PECAM-1 or VE-cadherin. These results were consistent with the significantly higher PECAM-1 mRNA expression for RGD+BMP+OPD group after 21 and 28 days, compared to the groups without OPD. These findings suggest that the RGD+BMP+OPD peptides provide a favorable microenvironment for concurrent osteogenic and vasculogenic differentiation of progenitor marrow-derived cells.

Biomimetic poly(ethylene glycol)-based hydrogels as scaffolds for inducing endothelial adhesion and capillary-like network formation

The extracellular matrix (ECM) is an attractive model for designing synthetic scaffolds with a desirable environment for tissue engineering. Here, we report on the synthesis of ECM-mimetic poly(ethylene glycol) (PEG) hydrogels for inducing endothelial cell (EC) adhesion and capillary-like network formation. A collagen type I-derived peptide GPQGIAGQ (GIA)-containing PEGDA (GIA-PEGDA) was synthesized with the collagenase-sensitive GIA sequence attached in the middle of the PEGDA chain, which was then copolymerized with RGD capped-PEG monoacrylate (RGD-PEGMA) to form biomimetic hydrogels. The hydrogels degraded in vitro with the rate dependent on the concentration of collagenase and also supported the adhesion of human umbilical vein ECs (HUVECs). Biomimetic RGD/GIA-PEGDA hydrogels with incorporation of 1% RGD-PEGDA into GIA-PEGDA hydrogels induced capillary-like organization when HUVECs were seeded on the hydrogel surface, while RGD/PEGDA and GIA-PEGDA hydrogels did not. These results indicate that both cell adhesion and biodegradability of scaffolds play important roles in the formation of capillary-like networks.

The cytochrome P450 4A/F-20-hydroxyeicosatetraenoic acid system: a regulator of endothelial precursor cells derived from human umbilical cord blood

Endothelial progenitor cells (EPCs) contribute to physiological and pathological neovascularization. Previous data have suggested that the cytochrome P450 4A/F (CYP4A/F)-20-hydroxyeicosatetraenoic acid (20-HETE) system regulates neovascularization. Therefore, we studied whether the angiogenic effects of the CYP4A/F-20-HETE system involve regulation of EPC function. We extracted human umbilical cord blood and isolated EPCs, which express AC133(+)CD34(+) and kinase insert domain receptor (KDR) surface markers and contain mRNA and protein for CYP4A11 and CYP4A22 enzymes, as opposed to mesenchymal stem cells, which only express negligible amounts of CYP4A11/22. When EPCs were incubated with arachidonic acid, they produced 20-HETE, which stimulated the cells to proliferate and migrate, as did vascular endothelial growth factor. Incubation with 1 μM N-hydroxy-N'-(4-butyl-2-methylphenyl)formamidine (HET0016), a selective inhibitor of 20-HETE synthesis, reduced the proliferative and migratory effects of vascular endothelial growth factor and also significantly abolished EPC migration mediated by stroma-derived factor-1α, as did (6,15) 20-hydroxyeicosadienoic acid. Coculturing EPCs and endothelial cells on a Matrigel matrix led to tube formation, which in turn was inhibited by both HET0016 and 20-hydroxyeicosadienoic acid. We concluded that the CYP4A/F-20-HETE system is expressed in EPCs and can act as both an autocrine and a paracrine regulatory factor.