Atmospheric oxygen inhibits growth and differentiation of marrow-derived mouse mesenchymal stem cells via a p53-dependent mechanism: Implications for long-term culture expansion

[[abstract]]Large scale expansion of human mesenchymal stem cells (MSCs) is routinely performed for clinical therapy. In contrast, developing protocols for large scale expansion of primary mouse MSCs has been more difficult due to unique aspects of rodent biology. Currently, established methods to isolate mouse MSCs select for rapidly dividing subpopulations that emerge from bone marrow cultures following long-term (months) expansion in atmospheric oxygen. Herein, we demonstrate that exposure to atmospheric oxygen rapidly induced p53, TOP2A, and BCL2-associated X protein (BAX) expression and mitochondrial reactive oxygen species (ROS) generation in primary mouse MSCs resulting in oxidative stress, reduced cell viability, and inhibition of cell proliferation. Alternatively, procurement and culture in 5% oxygen supported more prolific expansion of the CD45-ve/CD44+ve cell fraction in marrow, produced increased MSC yields following immunodepletion, and supported sustained MSC growth resulting in a 2,300-fold increase in cumulative cell yield by fourth passage. MSCs cultured in 5% oxygen also exhibited enhanced trilineage differentiation. The oxygen-induced stress response was dependent upon p53 since siRNA-mediated knockdown of p53 in wild-type cells or exposure of p53-/- MSCs to atmospheric oxygen failed to induce ROS generation, reduce viability, or arrest cell growth. These data indicate that long-term culture expansion of mouse MSCs in atmospheric oxygen selects for clones with absent or impaired p53 function, which allows cells to escape oxygen-induced growth inhibition. In contrast, expansion in 5% oxygen generates large numbers of primary mouse MSCs that retain sensitivity to atmospheric oxygen, and therefore a functional p53 protein, even after long-term expansion in vitro

MSCs: science and trials

Although the field of regenerative medicine is rife with unsubstantiated claims of benefit and is often biased by strong commercial interests, it is important that the legitimate scientific enterprise does not allow this noise to overshadow meaningful advances in the field. By providing a skeletal system\u2013centric view of MSCs, the Perspective by Bianco et al. relegates the rapidly growing body of literature dedicated to exploring nonprogenitor functions of MSCs, including immunomodulation, to obscurity and undermines the efforts of legitimate and dedicated scientists to understand these functions and exploit them to achieve a therapeutic benefit in human patients. Indeed, our committee believes that the existing scientific data are sufficiently mature to warrant MSC-based clinical trials for disease indications beyond the skeletal system. However, we acknowledge that the sound establishment of MSC-based therapies requires controlled, randomized, prospective clinical trials that incorporate mechanistic-based studies to fully assess the pharmaceutical underpinnings of such therapy