Human Hematopoietic Stem Cells and Leukemic Cells Form Cadherin-Catenin Based Junctional Complexes with Mesenchymal Stromal Cells

The interaction between human hematopoietic stem cells (HSC) and their niche plays a key role in regulating maintenance of “stemness” and differentiation. We have demonstrated that a feeder layer of human mesenchymal stromal cells (MSC) can serve as a surrogate model for the niche for human HSC. We could also show, MSC are intimately connected to one another by a novel kind of adhering junction, consisting of villiformto-vermiform cell projections (processus adhaerentes). With this background, we have analyzed the intercellular junctional complexes between HSC and MSC. In comparison, we also studied the cell-cell contacts between leukemia cells (LC) and MSC.

MSC were derived from bone marrow aspirates from healthy voluntary donors. HSC were isolated from umbilical cord blood. Leukemia cells that were CD34+ were obtained from bone marrow aspirates from patients suffering from acute myeloid leukemia at the time point of initial diagnosis. After 24–48 hours of co-cultivation, we stained the cellular contacts with a panel of antibodies specific for various components of tight, gap and adherens junctions. 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 LC/MSC. To examine the specific function of N-cadherin, we analyzed the effect of siRNA knock down of N-cadherin in MSC upon co-cultures of HSC and MSC.

Intercellular connections between HSC and MSC are mainly characterized by podia formation of the HSC linking to the adjacent MSC. At the intimate contact zone to the MSC, we have identified the cytoplasmic plaque proteins alpha- and beta-catenin, co-localized with the transmembrane glycoprotein N-cadherin. Additionally, we compared these findings with a similar setting consisting of human LC co-cultured with feeder-layer of MSC. Our results demonstrated that in comparison to HSC, the proportion of leukemia cells adherent to the feeder-layer is significantly lower and podia formation is less frequent (ratio 1:3). However, the mechanism of adhesion through cadherin-catenin-complex has remained the same. At a functional level, we found that siRNA knock down of N-cadherin in MSC resulted in decreased adhesion of HSC to MSC and in a reduction of cell divisions of HSC. These results confirm that direct cellular contact via N-cadherin-based junctions is essential for homing and adhesion of HSC to the cellular niche and subsequently for the regulation of self-renewal versus differentiation in HSC.