Project description:Hematopoietic stem cells (HSCs) primarily reside in the bone marrow where signals generated by stromal cells regulate their self-renewal, proliferation, and trafficking. Endosteal osteoblasts and perivascular stromal cells including endothelial cells3, CXCL12-abundant reticular (CAR) cells, leptin-receptor positive stromal cells, and nestin-GFP positive mesenchymal progenitors have all been implicated in HSC maintenance. However, it is unclear if specific hematopoietic progenitor cell (HPC) subsets reside in distinct niches defined by the surrounding stromal cells and the regulatory molecules they produce. CXCL12 (stromal-derived factor-1, SDF-1) regulates both HSCs and lymphoid progenitors and is expressed by all of these stromal cell populations. Here, we selectively deleted Cxcl12 from candidate niche stromal cell populations and characterized the effect on HPCs. Deletion of Cxcl12 from mineralizing osteoblasts has no effect on HSCs or lymphoid progenitors. Deletion of Cxcl12 from osterix-expressing stromal cells, which includes CAR cells and osteoblasts, results in constitutive HPC mobilization and a loss of B lymphoid progenitors, but HSC function is normal. Cxcl12 deletion in endothelial cells results in a modest loss of long-term repopulating activity. Strikingly, deletion of Cxcl12 in nestin-negative mesenchymal progenitors using Prx1-Cre is associated with a marked loss of HSCs, long-term repopulating activity, HSC quiescence, and common lymphoid progenitors. These data suggest that osterix-expressing stromal cells comprise a distinct niche that supports B lymphoid progenitors and retains HPC in the bone marrow, while expression of CXCL12 from stromal cells in the perivascular region, including endothelial cells and mesenchymal progenitors, support HSCs. Total of three samples of two groups analyzed. Replica samples of CXCL12-abundant reticular (CAR) cells from two CXCL12-GFP knock-in mice and a combined sample of PDGFRa+ Sca+ CD45- lineage- cells from three Prx1-Cre Rosa26Ai9/+ Cxcl12gfp/+ mice were used and analyzed.

Project description:Haematopoietic stem cells (HSCs) reside in bone marrow (BM) niches where they are maintained to replenish all blood cell lineages throughout life. Among the constituents of the murine HSC niches, various cell types such as CXCL12-abundant reticular (CAR) cells, Nestin-GFP+ perivascular cells, leptin receptor (LepR)+ cells, endothelial cells, osteoblasts, sympathetic nerves, macrophages and megakaryocytes in the BM have been proposed to contribute to HSC niche activity. When niche cells are removed from their natural habitat, the ability to maintain HSCs ex vivo is markedly diminished. Mesenchymal-derived stromal cells (MSCs) identified by Nestin-GFP express high levels of niche factors in vivo, but their expression is downregulated rapidly upon culture, suggesting that alterations in transcriptional rewiring may contribute to the reduced HSC maintenance potential. We found that the enforced expression of 5 genes (Klf7, Ostf1, Xbp1, Irf3 and Irf7) markedly restored HSC niche function in cultured BM-derived MSCs. These revitalized MSCs (rMSCs) exhibited boosted synthesis of HSC niche factors while retaining their mesenchymal lineage differentiation capacity. By contrast to HSCs co-cultured with control MSCs, HSCs expanded with rMSCs showed significantly higher repopulation capacity. To evaluate to what extent rMSCs are reprogrammed toward freshly isolated MSCs, we compared, by RNA-seq analysis, the transcriptome of freshly sorted CD45(-) Ter119(-) CD31(-) Scf-GFP(-) cells, CD45(-) Ter119(-) CD31(-) Scf-GFP(+) cells, rMSCs and control vector-transduced stroma. HSC niche-associated genes were highly expressed in both native Scf-GFP(+) stromal cells and rMSCs compared to the Scf-GFP cell fraction and control MSCs. With the motif analysis of ATAC-seq data, we found that myocyte enhancer factor 2c (Mef2c) is one of the key factor in the revitalization of MSCs. These results suggest that rMSC may provide a promising platform for curative transplantation therapies.

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The niche for extramedullar hematopoiesis in the spleen

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