A novel spheroidal progenitor cell differentiation and bone-like in vitro biomineralization model

Many age-related diseases affect our skeletal system, but bone health-targeting drug development strategies still largely rely on 2D in vitro screenings. We aimed at developing a scaffold-free progenitor cell-based 3D biomineralization model for more physiological high-throughput screenings. MC3T3-E1 pre-osteoblast spheroids were cultured in V-shaped plates for 28 days in alpha-MEM (10% FCS, 1% L-Gln, 1X NEAA) with 1% pen/strep, changed every two days, and differentiation was induced by 10mM b-glycerophosphate and 50µg/ml ascorbic-acid. Osteogenic cell differentiation was assessed through profiling mRNA expression of selected osteogenic markers by efficiency corrected normalized 2^DDCq RT-qPCR. Biomineralization in spheroids was evaluated by histochemistry (Alizarin Red/von Kossa staining), Alkaline phosphatase (Alp) activity, Fourier transform infrared spectroscopy (FTIR) analyses, micro-CT analyses, and scanning electron microscopy on critical point-dried samples. GraphPad Prism 9 analyses comprised Shapiro-Wilk and Brown-Forsythe tests as well as 2-way ANOVA with Tukey post-hoc and non-parametric Kruskal-Wallis with Dunn post-hoc tests. During mineralization, as opposed to non-mineralizing conditions, characteristic mRNA expression profiles of selected early and late osteoblast differentiation markers (e.g., RunX, Alp, Col1a1, Bglap) were observed between day 0 and 28 of culture; Alp was strongly upregulated (p<0.001) from day 7 on, followed by its enzymatic activity (p<0.001). Bglap and Col1a1 expression peaked on (p<0.001) and from day 14 on (p<0.05), respectively. IHC revealed osteocalcin staining in the spheroid core regions at day 14, while type I collagen staining of the cores was most prominent from day 21 on. Alizarin Red and Von Kossa confirmed central and radially outwards expanding mineralization patterns between day 14 and day 28, which was accompanied by a steady increase in extracellular calcium deposition over time (p<0.001). Micro-CT analyses allowed quantitative appreciation of the overall increase in mineral density over time (day21, p<0.05; d28, p<0.001), while SEM-EDX and FTIR ultimately confirmed a bone-like hydroxyapatite mineral deposition in 3D. A novel and thoroughly characterized versatile bone-like 3D biomineralization in vitro model was established, which allows for studying effects of pharmacological interventions on bone mineralization ex vivo under physiomimetic conditions. Ongoing studies currently aim at elucidating in how far it specifically recapitulates intramembranous ossification.