Effect of calcium on the proliferation kinetics of synovium-derived mesenchymal stromal cells

Background aims. Synovium-derived mesenchymal stromal cells (S-MSCs) have potential utility in clinical joint repair applications. However, their scarcity in tissues means S-MSCs cannot be isolated in large quantities and need to be expanded in culture. Because synovial tissues in vivo are exposed to higher calcium (Ca2+) levels than typically found in culture media, this study examined the impact of Ca2+ supplementation on the rate of S-MSC proliferation in culture. Methods. S-MSCs were serially cultured with or without Ca2+ supplementation. The effect of inhibiting Ca2+ uptake was assessed using Ca2+ channel blockers. After extended exposure to elevated Ca2+ concentrations, S-MSCs were characterized by evaluating surface marker profiles, performing reverse transcriptase quantitative polymerase chain reaction and carrying out tri-lineage differentiation assays. Results. Elevated Ca2+ concentrations resulted in enhanced S-MSC proliferation. Peak growth occurred at 5.0 mmol/L Ca2+, with an average fold increase of 4.52 +/- 0.65 per passage over 8 passages compared with 2.03 +/- 0.46 in un-supplemented medium. Proliferation was inhibited by Ca2+ channel blockers. Ca2+-supplemented cells showed enhanced capacity toward osteogenesis (17.82 +/- 4.21 mu g Ca2+ deposited/sample vs. 12.70 +/- 2.11 mu g Ca2+ deposited/sample) and adipogenesis (0.47 +/- 0.04 mg oil red O/sample vs. 0.352 +/- 0.005 mg oil red O/sample) and retained their capacity to undergo chondrogenesis (1.37 +/- 0.07 mu g glycosaminoglycan/pellet vs. 1.33 +/- 0.17 mu g glycosaminoglycan/pellet). S-MSCs cultured in elevated Ca2+ expressed enhanced messenger RNA levels for SOX-9 and peroxisome proliferator activated receptor gamma and depressed levels for collagen I. Conclusions. S-MSC sensitivity to Ca2+ has not been reported previously. These findings indicate that S-MSC population expansion rates may be up-regulated by Ca2+ supplementation without compromising defining cell characteristics. This study exemplifies the need to consider medium composition when culturing stem cells.