Vascularization is a prerequisite for bone formation. Endothelial progenitor cells (EPCs) stimulate bone formation by creating a vascular network. Moreover, EPCs secrete various bioactive molecules that may regulate bone formation. The aim of this research was to shed light on the pathways of EPCs in bone formation. In a subcutaneous nude mouse ectopic bone model, the transplantation of human EPCs onto β-TCP scaffold increased angiogenesis (p < 0.001) and mineralization (p < 0.01), compared to human neonatal dermal fibroblasts (HNDF group) and a-cellular scaffold transplantation (β-TCP group). Human EPCs were lining blood vessels lumen; however, the majority of the vessels originated from endogenous mouse endothelial cells at a higher level in the EPC group (p < 01). Ectopic mineralization was mostly found in the EPCs group, and can be attributed to the recruitment of endogenous mesenchymal cells ten days after transplantation (p < 0.0001). Stromal derived factor-1 gene was expressed at high levels in EPCs and controlled the migration of mesenchymal and endothelial cells towards EPC conditioned medium in vitro. Blocking SDF-1 receptors on both cells abolished cell migration. In conclusion, EPCs contribute to osteogenesis mainly by the secretion of SDF-1, that stimulates homing of endothelial and mesenchymal cells. This data may be used to accelerate bone formation in the future.

Clinical trials have demonstrated the safety and efficacy of autologous endothelial progenitor cell (EPC) therapy in various diseases. Since EPCs’ functions are influenced by genetic, systemic and environmental factors, the therapeutic potential of each individual EPCs is unknown and may affect treatment outcome. Therefore, our aim was to compare EPCs function among healthy donors in order to predict blood vessel formation (angiogenesis) before autologous EPC transplantation. Human EPCs were isolated from the blood of ten volunteers. EPCs proliferation rate, chemoattractant ability, and CXCR4 mRNA levels were different among donors (p < 0.0001, p < 0.01, p < 0.001, respectively). A positive correlation was found between SDF-1, CXCR4, and EPCs proliferation (R = 0.736, p < 0.05 and R = 0.8, p < 0.01, respectively). In-vivo, blood vessels were counted ten days after EPCs transplantation in a subcutaneous mouse model. Mean vessel density was different among donors (p = 0.0001); nevertheless, donors with the lowest vessel densities were higher compared to control (p < 0.05). Finally, using a linear regression model, a mathematical equation was generated to predict blood vessel density relying on: (i) EPCs chemoattractivity, and (ii) VEGFR-2 mRNA levels. Results reveal differences in EPCs functions among healthy individuals, emphasizing the need for a potency assay to pave the way for standardized research and clinical use of human EPCs.

Recurrent pregnancy loss (RPL) is a clinically heterogeneous reproductive disorder, and in many women the underlying mechanism remains unexplained by standard thrombophilia evaluation. Pregnancy is characterized by a tightly regulated hemostatic balance at the maternal–placental interface. Heparanase, tissue factor pathway inhibitor 2 (TFPI-2), and heparan sulfate (HS) are expressed in placental and hemostatic pathways and may contribute to coagulation-related pregnancy complications. We investigated whether the heparanase/TFPI-2/HS axis is altered in women with RPL. Blood samples were obtained from 69 women with RPL and 47 control women with at least two normal deliveries and no history of pregnancy-related vascular complications. Samples were collected at least three months after pregnancy and in the absence of hormonal therapy. Plasma and white blood cell samples were analyzed using an enzyme-linked immunosorbent assay, Western blotting, real-time polymerase chain reaction, co-immunoprecipitation, and Sanger sequencing. Mechanistic experiments using heparanase-derived peptide 16 and TFPI-2-derived peptide 6 were performed in MCF-7 and JAR cells. We found that women with RPL had increased plasma heparanase levels, heparanase procoagulant activity, HS levels, and heparanase mRNA expression compared with controls. TFPI-2 protein and mRNA expression were also elevated. The heparanase rs4693608 AA genotype was more frequent in women with RPL and, together with rs4364254 TT, was associated with increased heparanase expression and procoagulant activity. Although TFPI-2 levels were higher in RPL, co-immunoprecipitation showed reduced heparanase/TFPI-2 complex formation, suggesting impaired inhibitory regulation. HS modulated heparanase binding to TF and TFPI-2 in a concentration-dependent, bell-shaped manner. Heparanase-derived peptide 16 increased TFPI-2 expression, supporting a regulatory feedback loop. In conclusion, RPL is associated with dysregulation of the heparanase/TFPI-2/HS axis, including increased heparanase expression and activity, altered heparanase/TFPI-2 interaction, and heparanase-related genetic variation. These findings suggest a coagulation-related reproductive mechanism that may contribute to pregnancy loss and warrants validation in larger prospective studies.