VEGF delivery by smart polymeric PNIPAM nanoparticles affects both osteogenic and angiogenic capacities of human bone marrow stem cells

Bone tissue engineering (BTE) faces a major challenge with cell viability after implantation of a construct due to lack of functional vasculature within the implant. Human bone marrow derived mesenchymal stem cells (hBMSCs) have the potential to undergo transdifferentiation towards an endothelial cell phenotype, which may be appropriate for BTE in conjunction with the appropriate scaffolds and microenvironment.We hypothesized that slow delivery of vascular endothelial growth factor (VEGF) by using nanoparticles in combination with osteogenic stimuli might enhance both osteogenic and angiogenic differentiation of angiogenic primed hBMSCs cultured in an osteogenic microenvironment. Therefore, we developed a new strategy to enhance vascularization in BTE in vitro by synthesis of smart temperature sensitive poly(N‑isopropylacrylamide) (PNIPAM) nanoparticles. We used PNIPAM nanoparticles loaded with collagen to investigate their ability to deliver VEGF for both angiogenic and osteogenic differentiation.We used the free radical polymerization technique to synthesize PNIPAM nanoparticles, which had particle sizes of approximately 100 nm at 37 °C and LCST of 30-32 °C. The cumulative VEGF release after 72 h for VEGF loaded PNIPAM (VEGF-PNIPAM) nanoparticles was 70%; for VEGF-PNIPAM loaded collagen hydrogels, it was 23%, which indicated slower release of VEGF in the VEGF-PNIPAM loaded collagen system. Immunocytochemistry (ICC) and inverted microscope visualization confirmed endothelial differentiation and capillary-like tube formation in the osteogenic culture medium after 14 days. Quantitative real-time polymerase chain reaction (QRT-PCR) also confirmed expressions of collagen type I (Col I), runt-related transcription factor 2 (RUNX2), and osteocalcin (OCN) osteogenic markers along with expressions of platelet-endothelial cell adhesion molecule-1 (CD31), von Willebrand factor (vWF), and kinase insert domain receptor (KDR) angiogenic markers. Our data clearly showed that VEGF released from PNIPAM nanoparticles and VEGF-PNIPAM loaded collagen hydrogel could significantly contribute to the quality of engineered bone tissue.