Objective: It is poorly understood what kind of factors are involved in lineage commitment and maturation of mesenchymal stem cells. The present study investigates whether mechanical stress promotes expression of smooth muscle cell (SMC)-specific cytoskeletal protein in marrow stromal cells.
Methods: Fibroblast-like stromal cells expressing STRO-1 antigen were isolated from rat bone marrow by density gradient separation. After preincubation for 7, 14, or 21 days in static condition, cells were exposed to one of three types of fluid flow-induced mechanical forces (flow dominant, pressure dominant, or combined) for 36 hours. The expression of SMC-specific cytoskeletal protein [alpha smooth muscle actin (alphaSMA) and smooth muscle myosin heavy chain (SMMHC)] was evaluated by immunofluorescence staining and Western blotting.
Results: The proportion of SMMHC-positive cells was increased with longer preincubation periods (p < 0.01 vs 7-day incubation) and by any types of mechanical stimulation (p < 0.01 vs static control condition). The SMMHC-positive fraction after exposure to pressure-dominant forces (0.9% +/- 0.2%, 2.9% +/- 0.9%, and 12.6% +/- 0.8% for 7, 14, and 21 days of preincubation) or to combined forces (1.2% +/- 0.2%, 3.1% +/- 1.6%, and 15.5% +/- 2.8%) was higher than after flow-dominant stimulation (0, 1.2% +/- 0.1%, and 7.2% +/- 2.0%) (p < 0.01). In Western blotting, pressure-dominant or combined stimulation upregulated alphaSMA and SMMHC expression compared to static control condition.
Conclusion: The long-term cell incubation and subsequent mechanical stimulation, especially compressive strain, promote expression of SMC-specific cytoskeletal protein in marrow stromal cells.