Human Hematopoietic and Mesenchymal Stem Cells Are Interconnected by Cadherin-Catenin Based Junctions.

The direct cell-cell contact between human hematopoietic stem cells (HSC) and their niche has been shown to control stem cell fate by regulating self-renewal versus differentiation. In previous studies we have demonstrated gap and adherens junctions arranged in complex villiform, vermiform, intercalating protrusions and invaginations among mesenchymal stem cells (MSC). Specific cadherin-catenin-molecules, among other junction proteins, were found in these contact systems. Using MSC-feeder-layer as an in vitro surrogate model for the hematopoietic stem cell niche, we have analyzed the intercellular junctional complexes between HSC and MSC in this study. Additionally, we characterized cell-cell-connections between leukemia stem cells (LSC) and MSC. MSC were derived from bone marrow aspirates from healthy voluntary donors. HSC were isolated from umbilical cord blood. Leukemia stem cells were obtained from bone marrow aspirates from patients suffering from acute myeloid leukemia either at the time of initial diagnosis or at relapse after chemotherapy. After 24–48 hours of co-cultivation, we have stained the cell-cell contacts with a panel of antibodies specific for various components of tight, gap and adherens junctions including alpha-/beta-catenin, N-cadherin, Cadherin 11, E-Cadherin, p120, Vinculin and Connexin 43. Using advanced confocal laser scanning microscopy in combination with deconvolution and volume rendering software, we were able to produce 3D-images of intercellular junctions between HSC/MSC as well as between LSC/MSC. We have confirmed previous observations that HSC demonstrated directed locomotion towards a MSC gradient. HSC then migrated around the MSC and some of them are then interconnected by podia formation to the MSC. These podia are directly linked to the MSC feeder layer and at the intimate contact zone we have identified N-cadherin as well as alpha-/beta-catenin as the main junction proteins. The specific function of this cadherin/catenin-complex is probably associated with redistribution of beta-catenin and hence to self-renewing division, similar to the findings in stem cells of drosophila model systems. Functional analyses are currently underway to confirm this hypothesis. We have also compared these findings with a similar setting consisting of human LSC co-cultured with MSC-feeder-layer.