Absence of P21 Gene Augments Proliferation of Mesenchymal Stem Cells.

Bone marrow derived mesenchymal stem cells (MSCs) have the capacity to differentiate into cells of the various mesodermal lineages. Efficiency of engraftment, however, remains a significant obstacle in MSC transplantation. P21 is a G1 checkpoint regulator and cyclin-dependent kinase inhibitor. In its absence, hematopoieitc stem cell (HSC) proliferation increases under normal homeostatic conditions. The aim of the study is to evaluate the effects of p21 deficiency on the proliferative capacity of MSCs. Therefore, MSCs from p21 KO and wildtype mice were maintained in culture in hypoxic and normoxic conditions. Cell counts and respective days in cultures were recorded from which population doubling (PD) times were calculated and compared. 1:1 Competition assays were performed between p21 KO MSCs transduced with DsRed and eGFP transduced WT MSCs as well as between DsRed transduced p21 KO MSCs and non-transduced WT MSCs. Osteogenic, chondrogenic, and adipogenic differentiation assays were performed on both cell populations using standard protocols. Sca-1, CD34, CD45, CD90, MHC Class I and II and other surface antigens were assessed using FACS analysis. In normoxic conditions, the p21 KO MSCs went through 25.68 population doublings in 60 days versus 25.43 population doublings in 103 days for the p21 WT MSCs. Under hypoxic conditions, the KO MSC population doubled 23.3 times in 58 days versus 14.78 times in 60 days for the WT MSCs. All population doubling times were taken from passage 10 cells. When the DsRed transduced KO MSCs (96.6-98.5% DsRed+) was placed in a 1:1 competition assay with eGFP transduced WT MSCs (99.3-99.9% eGFP+) or non-transduced WT MSCs (99.9–100% eGFP−), the normoxic results are as shown in Table 1. Similarly, the competition assay result between DsRed transduced KO MSCs (86.9–94.1% DsRed+) and eGFP transduced WT MSCs (99.7–100% eGFP+) or non-transduced WT MSCs (100% eGFP−) for the hypoxic condition are shown in Table 2. Both the p21 KO and wildtype MSCs populations were able to differentiate into ostenogenic, adipogenic, and chondrogenic lineages. No significant surface marker differences were observed between the 2 populations on FACS analysis. Our results clearly showed that p21 deficient MSCs have increased proliferative ability in vitro compared to normal MSCs. These findings have implications for expansion of MSC populations in vitro , and for the enhancement of competitive capacity of MSCs following in vivo administration. Table 1 - Normoxic Competition Assay KO MSC (DsRed) WT MSC (eGFP) KO MSC (DsRed) WT MSC (none) results analyzed by FACS Day 6 64.2±3.7% 34.5±4.3% 70.3±0.4% 19.7±0.6% Day 11 84.0±4.5% 13.6±4.6% 92.0±0.8% 6.4±0.7% Table 2 - Hypoxic Competition Assay KO MSC (DsRed) WT MSC (eGFP) KO MSC (DsRed) WT MSC (none) results analyzed by FACS Day 4 70.3±1.7% 30.0±1.7% 83.0±0.8% 18.6±0.9% Day 8 75.7±0.7% 25.8±0.7% 81.3±1.2% 20.6±1.2%