Genomic, Proteomic and Functional Screens Identify PAI-1 as an Important Factor Influencing Mesenchymal Stromal Cell Survival Post-Transplantation In Vivo.

MSCs have robust reparative properties through their ability to limit apoptosis, enhance angiogenesis and direct positive tissue remodelling. However, a major limitation on the widespread application of MSC transplantation is that transplanted cells have low survival rates in vivo. Thus, enhancing MSCs survival post-transplantation is critical for the successful implementation of cellular therapy. One explanation for their low survival is that MSCs are often transplanted into ischemic tissue - such as infarcted myocardium - where there is poor blood supply and low oxygen tension. How these factors influence MSC survival is still unclear, therefore the objective of this project was to better understand how hypoxia under low nutrient conditions affects the behaviour and survival of MSCs. To answer this question, we performed the following series of experiments:

1. Isolate and immunophenotype murine MSCs.

2. Assess impact of in vitro ischemic conditions (hypoxia and serum deprivation) on MSCs by microarray analysis and ITRAQ proteomics for secreted proteins.

3. Confirm changes of specific factors in vitro.

4. Isolate and immunophenotype MSCs from knock-out mice for specific factors.

5. Determine whether knock-out MSCs have altered survival in vitro and in vivo.

As confirmed by PCR and Western blotting, microarray and proteomic screens, identified plasminogen activator inhibitor-1 (PAI-1) as one factor consistently upregulated in hypoxia (3% oxygen) treated MSCs. In vitro migration studies demonstrated that PAI-1 blocks migration of MSCs towards a chemotactic gradient, while isolation of PAI-1 knock-out MSCs, revealed an enhanced survival of PAI-1null MSCs over wild-type MSCs. In vivo subcutaneous transplantation of beta-galactosidase engineered wild-type and PAI-1null MSCs, recapitulated our in vitro results, such that 14 days post-implantation PAI-1null MSCs had a five fold increase in survival compared to wild-type MSCs. In conclusion, PAI-1 is a multifunctional protein. As an inhibitor, PAI-1 acts as ‘bait’ regulating the conversion of plasminogen to plasmin thus functioning as a major control point in the regulation of systemic fibrinolysis and local growth factor availability. In addition, PAI-1 can also act as a ligand for vitronectin to influence cellular migration/adhesion. Up-regulation of PAI-1 by MSCs after transplantation impacts MSC survival. Determining how to block PAI-1 following transplantation of MSCs is currently under investigation.