Activation of Notch-Hes Signaling Pathway Impairs Osteogenic Differentiation and Hematopoietic Stem Cell-Supporting Capacitiy of Mesenchymal Stromal Cells Derived from Myelodysplastic Syndromes Patients

Ineffective hematopoiesis is a major characteristic of myelodysplastic syndromes (MDS). Bone marrow mesenchymal stromal cells(BMMSCs) and their progeny (i.e., osteoblasts, adipocytes, and reticular cells), which are considered as main cellular components of the bone marrow niche, have been shown to physiologically support hematopoiesis, but their contribution to the pathogenesis of MDS is controversially discussed.

In this study, we examined the osteogenic differentiation and hematopoietic stem cell-supporting capacitiy of BMMSCs in patients with MDS (n=67) and healthy donors (n=22). After 21 days osteogenic induction differentiation, osteogenesis potential of BMMSCs was significantly reduced in cases with RARS(83.3%), RCMD(75.0%), RAEB I(44.4%), RAEB II (40%), indicated by cytochemical stainings and reduced expressions of Runx2. Moreover, we observed that in co-cultures with normal hematopoietic stem cells(HSCs) and MDS-BMMSCs, the colony number (CFU-GM、BFU-E and CFU-GEMM) was significantly lower in the presence of MDS-BMMSCs in comparison to the normal counterpart.

Furthermore, in MDS-BMMSCs, we detected increased mRNA expression of several members of the Notch pathway, including Delta-like-1, Jagged-1, Notch1, Notch2, Hes1 and Hes5. Basically, the Notch-Hes pathway is the main regulator of the microenvironment dependent hematopoietic stem cell fate. Therefore we investigated if the activation of Notch-Hes pathway affected their osteogenesis and hematopoietic stem cell-supporting capacitiy of BMMSCs. By overexpression of Notch1 intracellular domain (NICD) in BMMSCs from healthy donors, we confirmed that Notch signaling negatively regulated BMMSCs osteogenesis through inhibition of Runx2 transcriptional activity. Importantly, treatment with the Notch1 inhibitor DAPT reversed the osteogenic differentiation and improved the hematopoiesis supporting capacitiy of MDS-BMMSCs.

Taken together, our findings suggest that the ineffective hematopoiesis typical of MDS may be partly due to the impaired osteogenic differentiation of BMMSCs, and the activation of Notch-Hes signaling is involved in the impaired osteogenic differentiation and diminished hematopoietic stem cell-supporting capacitiy of MDS-BMMSCs, restoring the adequate Notch-Hes signaling could represent a potential therapeutic approach to MDS.