Human mesenchymal stem cells (hMSCs), during in vitro expansion, gradually lose their distinct spindle morphology, self-renewal ability, multi-lineage differentiation potential and enter replicative senescence. This loss of cellular function is a major roadblock for clinical applications which demand cells in large numbers. Here, we demonstrate a novel role of substrate stiffness in the maintenance of hMSCs over long-term expansion. When serially passaged for 45 days from passage 3 to passage 18 on polyacrylamide gel of Young's modulus E =5 kPa, hMSCs maintained their proliferation rate and showed nine times higher population doubling in comparison to their counterparts cultured on plastic Petri-plates. They did not express markers of senescence, maintained their morphology and other mechanical properties such as cell stiffness and cellular traction, and were significantly superior in adipogenic differentiation potential. These results were demonstrated in hMSCs from two different sources, umbilical cord and bone marrow. In summary, our result shows that a soft gel is a suitable substrate to maintain the stemness of mesenchymal stem cells. As preparation of polyacrylamide gel is a well-established, and well-standardized protocol, we propose that this novel system of cell expansion will be useful in therapeutic and research applications of hMSCs.

High numbers of autologous human primary keratinocytes (HPKs) are required for patients with burns, wounds and for gene therapy of skin disorders.

The microenvironment of human mesenchymal stem cells (hMSCs) regulates their selfrenewal and differentiation properties. Previously it was shown that hMSCs remained quiescent on soft (0.25 kPa) polyacrylamide (PA) gels but re-entered into cell cycle on a stiff (7.5 kPa) gel. However, how cells behave on intermediate stiffness and what intracellular factors transmit mechanical changes to cell interior thereby regulating cell cycle remained unknown. In this work we demonstrated that PA gels between 1 and 5 kPa act as a mechanical switch in regulating cell cycle of hMSCs. By experiments on cell-cycle exit and re-entry, we found that hMSCs demonstrated a sharp transition from quiescence to proliferation between 1 and 5 kPa. Further studies with ROCK inhibitor Y-27632 revealed that contractile proteins, but not cell spread area, accounts for the sensitivity of hMSCs towards substrate stiffness and hence correlates with their changes in cell cycle. These observations therefore suggest that substrate stiffness regulates hMSC proliferation through contractile forces as generated by cellular contractile proteins in a unique pattern which is distinct from other cell types as studied.

No abstract

When human mesenchymal stem cells (hMSCs) are cultured on tissue culture plate (TCP) and go through in vitro, they spontaneously lose their proliferative capacity and multi-lineage differentiation potential.They also lose their distinct spindle morphology and become large and flat. After a certain number of population doubling, they enter into permanent cell cycle arrest, called senescence. This is a major roadblock for clinical use of hMSCs which demands large number of cells. A culture system is needed which can maintain the stemness of hMSCs over long term passages yet simple to use. In this study, we explore the role of substrate rigidity in maintaining stemness. hMSCs were serially passaged on TCP and 5.0 kPa poly-acrylamide gel for 20 population doubling. It was found that while on TCP, cell growth reached a plateau at cumulative population doubling (CPD) = 12.5, on 5 kPa gel, they continue to proliferate linearly till we monitored (CPD = 20). We also found that while on TCP, late passage MSCs lost their adipogenic potential, the same was maintained on soft gel. Cell surface markers related to MSCs were also unaltered. We demonstrated that this maintenance of stemness was correlated with delay in onset of senescence, which was confirmed by -gal assay and by differential expression of two nuclear markers Lamin A and Lamin B. As preparation of poly-acrylamide gel is a simple, well established, and well standardized protocol, we believe that this system of cell expansion will be useful in therapeutic and research applications of hMSCs.One Sentence Summary: hMSCs retain their stemness when expanded in vitro on soft polyacrylamide gel coated with collagen by delaying senescence. Main Text: Significance StatementFor clinical applications, mesenchymal stem cells (MSCs) are required in large numbers. As MSCs are available only in scarcity in vivo, to fulfill the need, extensive in vitro expansion is unavoidable.However, on expansion, they lose their replicative and multi-lineage differentiation potential and become senescent. A culture system that can maintain MSC stemness on long-term expansion, without compromising the stemness, is need of the hour. In this paper, we identified polyacrylamide (PAA) hydrogel of optimum stiffness that can be used to maintain stemness of MSCs during in vitro long term culture. Large quantity of MSCs thus grown can be used in regenerative medicine, cell therapy, and in treatment of inflammatory diseases.