A bone marrow niche-derived molecular switch between osteogenesis and hematopoiesis.

Hematopoietic stem cells (HSCs) reside and are maintained in specialized microenvironments within the bone marrow known as niches, which are comprised of various cell types. Among them, leptin receptor (LepR)-expressing CXC chemokine ligand 12 (CXCL12)-abundant reticular (CAR) cells are known to create a niche for HSCs and at the same time to give rise to osteoblasts. These two functions of CAR/LepR+ cells appear to be tightly but inversely regulated to ensure adequate physical space for HSCs. However, how osteogenesis is prevented in CAR cells to maintain spaces available for HSCs and hematopoiesis remains unclear. In this issue of Genes & Development , Seike and colleagues (pp. 359-372) report that the transcription factor early B-cell factor ( Ebf3 ) is preferentially expressed by CAR/LepR+ cells and inhibits CAR cell differentiation into osteoblasts while at the same time maintaining self-renewal of CAR/LepR+ cells. Using conditional knockout and retroviral systems, the investigators show that loss of Ebf3 in CAR cells impairs HSC numbers and leads to osteosclerosis. This study provides novel insights into transcriptional requirements for CAR cell bone formation by identifying Ebf3 as a niche factor secreted from CAR/Lepr+ cells that regulates the interplay between osteogenesis and hematopoiesis.