Bioactive Glass-containing Alginate-RGD Hydrogel: Expression of Osteogenic Genes and Antimicrobial Properties

Objectives: Bone grafts may enhance healing processes by containing components that improve and speed-up osteogenesis. This study quantified osteogenic expression patterns within bioactive glass-containing (BAG) alginate-RGD bone graft materials and assessed their antimicrobial effects.
Methods: Rat calvarial-derived mesenchymal stem cells were obtained from neonatal rats and encapsulated into alginate beads (2M cells/mL). Beads also contained either 3% or 6% of two BAG compositions: 85mol% silica; or 65mol% silica. Only the 6% concentration was used for the antimicrobial tests.

Beads were cultured for 1 day in standard αMEM-culture media, and switched to osteogenic media for the remaining times. 3, 7, 14, 21, and 28 days later, mRNA was extracted (N=6/group). cDNA concentrations were measured using SYBR green and analyzed with the 2^-ΔΔCt method using GAPDH as a housekeeping-gene. We examined expression of osterix, osteocalcin, and runt-related transcription factor 2.

Streptococcus mutans (ATCC 25175) was grown aerobically (37°C, 5% CO2) and co-incubated with a 6% solution of bioactive glass or hydrogel beads for 12 and/or 24 hours. Viability of the bacteria was determined by colony counting. Student’s T-test looked for significant differences (α=0.05).
Results: Osteogenic expression levels increased at each time point. Runx2 expression peaked at day seven then plateaued across all groups. Osteocalcin and osterix increased at every time point. All groups showed a significant increase when compared to the gel + cells group by day seven and maintained this difference across the remainder of the times.

BAG-containing alginate-RGD beads showed significantly higher CFU formation compared to bioactive glass alone.
Conclusions: Gene expression profiles indicated that the RMSCs differentiated into osteoblasts. While the hydrogel beads did show a significantly reduced antimicrobial activity compared to the glass alone, the hydrogel does provide an extended release of the calcium and soluble silica into the bacterial solution.