Bone Remodeling Study using Strontium enriched Hydroxyapatite nanoparticles (nHAPs)

Microgravity (MG) significantly modifies the metabolism of bone leading to site-specific alterations in remodeling of the bone tissue. A decrement in bone formation and an increase in bone resorption determine a significant loss of bone mass causing bone fragility and therefore a greater risk of fractures [1]. The proposed study, is focused on the development of the countermeasures to be taken in order to reduce the process of bone demineralization, while promoting a greater deposition of bone matrix. Strontium (Sr) is present in the mineral phase of bone, in particular in regions with high metabolic activity turn-over [2]. Recently, main in vitro and in vivo effects of Sr showed the reduction of bone resorption and the promotion of bone formation. Therefore, powders of hydroxyapatite (HAP) and HAP enriched with Sr were synthetized by sol-gel method. Hydroxyapatites powder were firstly characterized by ICP, XRD, N2 sorption, SEM and TEM analysis. Ultrasound sonication treatment was used to produce HAP nanoparticles (nHAPs) and nHAPs enriched with Sr (nHAP-S 5% and nHAP-Sr 10%). The nHAPs were dispersed in Bovine Serum Albumin (BSA) and characterized by using FT-IR to investigate the interaction between materials and BSA, by dynamic light scattering (DLS) to study hydrodynamic radius, by Z-potential for the surface charges, and by TEM to measure size and visualize the shape. In vitro studies were performed by using cell line precursor of osteoblasts (SAOS-2) and human mesenchymal stem cells isolated from bone marrow (hBM-MSCs) to evaluate nanoparticles biocompatibility at different time of incubation (1, 3 and 7 days of culture, respectively). The size of the nanostructures measured by DLS is around 100 nm. Higher values were observed for the nHAPs enriched with Sr due to the greater atomic radius of Strontium if compared to Calcium. The FT-IR spectra shows the modifications of the amino-groups bands of the protein and phosphate groups of HAP, suggesting an interaction at the molecular level of these groups. Regarding in vitro studies, a live count with Trypan Blue solution was performed; a 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyl-tetrazolium bromide test (MTT test) was performed to evaluate the cells' mitochondrial activity; and the release of the lactate dehydrogenase (LDH) enzyme in the culture medium was measured to evaluate membrane damages.