Notch Signaling Pathway in Hematopoietic Stem Cells Is Activated through Interactive Communication with Mesenchymal Stem Cells.

Mesenchymal stem cells (MSCs), which are key elements of hematopoietic microenvironment in bone marrow, are known to play a critical role in supporting hematopoiesis. A variety of hematopoietic growth factors are produced from MSCs, and cell-to-cell contact is also believed to be crucial in the interaction between hematopoietic stem cells (HSCs) and MSCs. However, the molecular mechanisms of hematopoiesis-supporting ability of MSCs are still unclear. In particular, there is little information regarding the effects of HSCs on MSC function. In the present study, we investigated the cellular and molecular events in the interactive communication between HSCs and MSCs using a differentiation-inducible MSC model; i.e. parent C3H10T1/2 cells and 10T1/2-derived cell lines, A54 preadipocytes and M1601 myoblasts. These cells were co-cultured with murine HSCs (Lin-Sca1+) isolated from bone marrow. There was 9-fold increase in the number of hematopoietic progenitors after co-culture with A54 preadipocytes, whereas there was no increase when co-cultured with parent 10T1/2 or M1601 cells. More intriguingly, cobblestone areas were observed only when HSCs were co-cultured with A54 cells. Quantitative RT-PCR showed that A54 cells express significantly higher levels of SCF, SDF-1, and angiopoietin-1 (Ang-1) compared with parent 10T1/2 cells and M1601 cells, although these cytokines were not up-regulated when co-cultured with HSCs. To search for the genes involved in the interaction between HSCs and MSCs, we compared gene expression profiles before and after the co-culture by using a microarray analysis. Among the candidate genes with up-regulation after the co-culture, we paid attention to the Notch system, because Notch ligands are considered to play an important role in nurturing HSCs within the hematopoietic microenvironment. As a result, the expression of Notch ligands, Jagged1 and Dll3, increased in A54 cells after the coculture with HSCs. On the other hand, the expression of Notch1 and Hes-1 also increased in HSCs upon co-culture with A54 cells. These data were confirmed by quantitative RT-PCR. Moreover, when HSCs were co-cultured with A54 cells without cell-to-cell contact using Transwell permeable supports, there was neither increase in the number of progenitors in the upper wells, nor the up-regulation of Notch ligands in A54 cells in the lower wells. These findings support the idea that HSCs act on MSCs to induce the expression of Notch ligands via direct cell-to-cell contact and that the Notch ligands derived from MSCs act on HSCs in turn to activate Notch signaling pathway, possibly leading to the cobblestone formation with the maintenance of immature state of HSCs. The Notch system may be one of the critical elements in the interactive communication between HSCs and MSCs.