Migration Potential of Human Mesenchymal Stromal Cells Is Determined by Origin of Tissue Rather Than Their Developmental Stage

It is thought that adult mesenchymal stromal cells (MSC) are important for tissue repair and maintenance. Crucial in these processes is the presence of MSC at the site of injury, however the recruitment and migration of MSC towards their destiny is poorly understood. With respect to future cell therapy, we are studying the process of migration of various human mesenchymal stem cell sources, and hypothesize that only a subpopulation of ex vivo expanded mesenchymal stem cells is capable of specific homing. For this purpose, MSC from different sources i.e. fetal lung (FL), fetal bone marrow (FBM), adult bone marrow (ABM) and adult adipose tissue (AT) were derived by plastic adherence and subsequently expanded. All MSC sources were characterized as CD73⁺, CD90⁺, CD105⁺, CD34⁻ and CD45⁻. MSC (P4-9) were allowed to migrate for 4h towards SDF-1a, PDGF-BB, HGF, bFGF or FCS over fibronectin-coated 12 mm pore size transwell plates. FL-MSC migrated significantly better towards SDF-1a as compared to ABM-MSC or AT-MSC. This enhanced migration capacity towards SDF-1a is specific for FL-MSC since AT-MSC migrated better towards FCS as compared to FL-MSC. Even ABM-MSC responded better to FCS than FL-MSC. This suggests that MSC originating from all sources are able to migrate but require different triggers to induce migration. In order to elucidate whether the observed differences in migration potential were due to developmental stage, cultured MSC derived from fetal bone marrow were tested as well. No significant differences in migration capacity were observed between adult and fetal BM- MSC for any of the (chemotactic) stimuli evaluated. Interestingly, FL-MSC had a significant increased migration capacity as compared to FBM-MSC towards SDF-1a, PDGF-BB and HGF, suggesting that the origin of tissue may determine migration capacity of ex vivo expanded MSC. Since it was observed that only a small percentage of the cultured MSC were able to migrate towards the various stimuli, checkerboard migration was performed to elucidate whether a synergistic effect could be observed. No synergistic effect was observed between SDF and PDGF, SDF and FCS or PDGF and FCS in FL-MSC, suggesting that there may be one subpopulation of MSC that possesses migratory capacities. When studying the SDF-1-induced migratory subpopulation in more detail, it was observed that, after migration, migratory MSC originating from all tissue sources maintain their proliferation and differentiation capacity and express CXCR4 at a higher level than MSC that did not migrate. To be able to migrate, cells have to rearrange their actin cytoskeleton and focal adhesions. These processes can be initiated by various chemokines and growth factors. In response to SDF or FCS, morphological changes were observed in ABM-MSC by confocal microscopy. Cells became smaller and membrane protrusions appeared, whereas this was absent in the control. Furthermore, upon stimulation with SDF, PDGF and FCS, tyrosine-phosphorylation of the adapter protein paxillin that links the actin cytoskeleton to focal adhesions was increased. In conclusion, our results suggest that migration potential of ex vivo expanded MSC derived from various adult and fetal tissues have different migratory capacity towards growth factor and chemokine stimuli and may involve paxillin phosphorylation. Our data indicate that further studies on the migratory subpopulation(s) within the heterogeneous population of culture expanded MSC will contribute to unravel how and which MSC will be of interest for future cellular therapies.