2011 - ENGINEERING A HEMATOPOIETIC STEM CELL NICHE BY REVITALIZING MESENCHYMAL STEM CELLS

Hematopoietic stem cells (HSCs) are maintained by bone marrow (BM) niches in vivo, but the ability of niche cells to maintain HSCs ex vivo is markedly diminished. Expression of niche factors by Nestin-GFP+ mesenchymal-derived stem cells (MSCs) is downregulated upon culture, suggesting that transcriptional rewiring may contribute to this reduced HSC maintenance potential. Using an RNA sequencing screen, we identified 5 transcription factors (Klf7, Ostf1, Xbp1, Irf3, Irf7) that restored HSC niche function in cultured BM-derived MSCs. These revitalized MSCs (rMSCs) exhibited enhanced synthesis of HSC niche factors while retaining their mesenchymal differentiation capacity. In contrast to HSCs co-cultured with control MSCs, HSCs expanded with rMSCs showed higher repopulation capacity and protected lethally irradiated recipient mice. Competitive reconstitution assays revealed 7-fold expansion of functional HSCs by rMSCs. rMSCs prevented the accumulation of DNA damage in cultured HSCs, a hallmark of ageing and replication stress. We have also identified Mef2c as an important transcription factor downstream of KOXII. Our results suggest that all five KOXII genes are necessary to fully restore the niche activity in MSCs ex vivo. Some of these genes may act singly, perhaps by preventing MSC differentiation or driving expansion of hematopoietic growth factors. Our results thus establish a new platform that provides critical insight in the regulatory network of the HSC niche and forms the basis toward the engineering of supportive niches for curative cell therapies.