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

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 (Ca(2+)) levels than typically found in culture media, this study examined the impact of Ca(2+) supplementation on the rate of S-MSC proliferation in culture.

S-MSCs were serially cultured with or without Ca(2+) supplementation. The effect of inhibiting Ca(2+) uptake was assessed using Ca(2+) channel blockers. After extended exposure to elevated Ca(2+) concentrations, S-MSCs were characterized by evaluating surface marker profiles, performing reverse transcriptase quantitative polymerase chain reaction and carrying out tri-lineage differentiation assays.

Elevated Ca(2+) concentrations resulted in enhanced S-MSC proliferation. Peak growth occurred at 5.0 mmol/L Ca(2+), 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 Ca(2+) channel blockers. Ca(2+)-supplemented cells showed enhanced capacity toward osteogenesis (17.82 ± 4.21 μg Ca(2+) deposited/sample vs. 12.70 ± 2.11 μg Ca(2+) 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 μg glycosaminoglycan/pellet vs. 1.33 ± 0.17 μg glycosaminoglycan/pellet). S-MSCs cultured in elevated Ca(2+) expressed enhanced messenger RNA levels for SOX-9 and peroxisome proliferator activated receptor gamma and depressed levels for collagen I.

S-MSC sensitivity to Ca(2+) has not been reported previously. These findings indicate that S-MSC population expansion rates may be up-regulated by Ca(2+) supplementation without compromising defining cell characteristics. This study exemplifies the need to consider medium composition when culturing stem cells.