Bmi1 suppresses adipogenesis in the hematopoietic stem cell niche: RNA-Seq

Bone marrow mesenchymal stromal cells (MSCs) that express high levels of stem cell factor (SCF) and CXC chemokine ligand 12 (CXCL12) are one crucial component of the hematopoietic stem cell (HSC) niche. While the secreted factors produced by MSCs to support HSCs have been well described, little is known regarding the transcriptional regulators controlling the cell fate of MSCs and thus indirectly maintaining HSCs. Bmi1 is a polycomb group protein that regulates HSCs both cell intrinsically and extrinsically, but it is unknown in which cell type and how Bmi1 functions to maintain HSCs extrinsically. Here we show that Bmi1 maintains HSCs by preventing adipogenic differentiation of MSCs. Bmi1 is highly expressed in MSCs but becomes downregulated upon adipogenic differentiation and during aging. Deleting Bmi1 from MSCs increased marrow adipocytes, induced HSC quiescence and depletion, and impaired hematopoiesis. We found that Bmi1 repressed multiple developmental programs in MSCs by safeguarding the repressive epigenetic marks histone H2A ubiquitylation and H3 lysine 27 trimethylation. We identified a novel adipogenic program governed by Pax3, which Bmi1 repressed in MSCs. Our results establish Bmi1 as a critical regulator of MSC cell fate that suppresses marrow adipogenesis to create a supportive niche for HSCs. Overall design: RNA-Seq of two treatments (Ctrl, KO) with three replications per treatment.

6

Hu T, Kitano A, Luu V, Dawson B, Hoegenauer KA, Lee BH, Nakada D