Downloaded from genesdev.cshlp.org on October 5, 2021 - Published by Cold Spring Harbor Laboratory Press OUTLOOK A bone marrow niche-derived molecular switch between osteogenesis and hematopoiesis Marta Galán-Díez and Stavroula Kousteni Department of Physiology and Cellular Biophysics, College of Physicians and Surgeons, Columbia University, New York, New York 10032, USA Hematopoietic stem cells (HSCs) reside and are main- tions of HSCs such as homing, mobilization, quiescence, tained in specialized microenvironments within the self-renewal, or lineage commitment. Instructive cues are bone marrow known as niches, which are comprised of provided as essential autocrine, endocrine, and paracrine various cell types. Among them, leptin receptor (LepR)-ex- signals as well as direct cell–cell interactions between pressing CXC chemokine ligand 12 (CXCL12)-abundant the stromal niches and the HSCs. reticular (CAR) cells are known to create a niche for Numerous studies have examined the role of the osteo- HSCs and at the same time to give rise to osteoblasts. blast lineage in HSCs and point toward the notion that os- These two functions of CAR/LepR+ cells appear to be teoblast regulation of hematopoiesis depends on the stage tightly but inversely regulated to ensure adequate physical of osteoblast differentiation (Sacchetti et al. 2007; Wu space for HSCs. However, how osteogenesis is prevented et al. 2008; Méndez-Ferrer et al. 2010; Calvi et al. 2012; in CAR cells to maintain spaces available for HSCs and he- Kunisaki et al. 2013). Whereas the most immature un- matopoiesis remains unclear. In this issue of Genes & De- committed progenitor subset influences HSC mainte- velopment, Seike and colleagues (pp. 359–372) report that nance and proliferation, committed progenitors or the transcription factor early B-cell factor (Ebf3) is prefer- mature osteoblasts tailor HSC differentiation along the entially expressed by CAR/LepR+ cells and inhibits CAR lymphoid, myeloid, and erythroid lineages (Calvi et al. cell differentiation into osteoblasts while at the same 2003; Zhu et al. 2007; Ding and Morrison 2013; Krevvata time maintaining self-renewal of CAR/LepR+ cells. Using et al. 2014). Indeed, self-renewing osteoprogenitor cells in conditional knockout and retroviral systems, the investi- the marrow can form supportive HSC niches (Sacchetti gators show that loss of Ebf3 in CAR cells impairs HSC et al. 2007). Mesenchymal stem cells (MSCs) of the endos- numbers and leads to osteosclerosis. This study provides teal niche in closest proximity to HSCs express secreted or novel insights into transcriptional requirements for CAR cell surface molecules that regulate HSC quiescence cell bone formation by identifying Ebf3 as a niche factor se- (Nilsson et al. 2005; Silberstein et al. 2016). Nestin-posi- creted from CAR/Lepr+ cells that regulates the interplay tive MSCs produce soluble factors involved in HSC main- between osteogenesis and hematopoiesis. tenance, such as CXC chemokine ligand 12 (CXCL12) and stem cell factor (SCF), and colocalize with HSCs, and their depletion results in HSC mobilization (Méndez-Ferrer One of the critical and unique functions of the skeleton is et al. 2010). These observations suggested that osteogene- to provide the anatomical spaces for maintaining hemato- sis and HSC maintenance in the marrow may be inversely poiesis. Bone marrow is the main site of residence for he- related. matopoietic stem cells (HSCs), where they self-renew, Perivascular niches regulate HSC maintenance and qui- + proliferate, or become progressively restricted to several escence. An arteriolar niche marked by rare NG2 (CSPG4 ) committed precursors and/or single lineages that give pericytes inhibits HSC cycling and reduces functional rise to the different types of mature blood cells. In the long-term repopulating HSCs in the bone marrow (Kuni- bone marrow, HSCs interact dynamically with an intri- saki et al. 2013). A distinct perisinusoid niche containing cate network of stromal cells. Such groups of cells com- the majority of dividing and nondividing HSCs appears prise a collection of several specialized “microniches” to regulate the interplay between osteogenesis and hema- that sustain hematopoiesis by influencing different func- topoiesis. Cells within the perisinusoid niche that express © 2018 Galán-Díez and Kousteni This article is distributed exclusively [Keywords: Ebf3; stem cell niche; hematopoietic stem cell; mesenchymal by Cold Spring Harbor Laboratory Press for the first six months after the stem cell; bone] full-issue publication date (see http://genesdev.cshlp.org/site/misc/ Corresponding author: [email protected] terms.xhtml). After six months, it is available under a Creative Commons Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.314013. License (Attribution-NonCommercial 4.0 International), as described at 118. http://creativecommons.org/licenses/by-nc/4.0/. 324 GENES & DEVELOPMENT 32:324–326 Published by Cold Spring Harbor Laboratory Press; ISSN 0890-9369/18; www.genesdev.org Downloaded from genesdev.cshlp.org on October 5, 2021 - Published by Cold Spring Harbor Laboratory Press A bone marrow niche-derived molecular switch high levels of CXCL12 and SCF (CXCL12-abundant retic- of age, double-knockout mice showed an occlusion of the ular [CAR] cells) make up the majority of perivascular cells marrow cavity and reduced CAR cell number. Therefore, (Omatsu et al. 2010) and overlap with perisinusoidal cells, Ebf1/Ebf3 do not appear to define CAR cell fate during identified on the basis of their expression of the leptin re- embryogenesis but are required to inhibit differentiation ceptor (LepR) (Zhou et al. 2014). CAR and CAR/LepR+ of CAR cells into the osteoblast lineage to maintain the MSCs, representing ∼0.3% of bone marrow cells, include marrow spaces. nearly all of the colony-forming unit fibroblasts (CFU- To investigate the respective contributions of Ebf3 and Fs), are the major source of adipocytes and osteoblasts in Ebf1 in HSC maintenance and osteoblast differentiation, adult mouse bone marrow, and are required to support the investigators studied the skeletal and hematopoietic HSC maintenance. However, the majority of CAR/LepR+ phenotypes of adult CAR/LepR+ cell-specific Ebf1−/−; cells might remain undifferentiated in the bone marrow Ebf3+/− and Ebf1−/+;Ebf3−/− mice. A severe reduction of cavity, and it remains unclear how osteogenesis is prevent- HSC progenitors in the marrow resulted in extramedullary ed in most CAR/LepR+ cells to maintain the spaces avail- hematopoiesis and a marked increase in trabecular bone able for HSCs and hematopoiesis. mass. This was due to increased osteoblast numbers with- To address this question, Seike et al. (2018) sought to out changes in osteoclasts, which was observed in Ebf1−/+; identify transcriptional regulators of CAR cells and fo- Ebf3−/− mice but not in Ebf1−/−;Ebf3+/− mice at 34 wk of cused on transcription factors of the early B-cell factor age. CAR cells from CAR/LepR+ cell-specific Ebf1−/+; (Ebf) family. They found that Ebf3 is preferentially ex- Ebf3−/− mice, but not Ebf1−/−;Ebf3+/− mice, had higher ex- pressed in CAR cells, as compared with endothelial, he- pression of osteoblast markers. Therefore, the activity of matopoietic, and stromal cells of adult marrow. Lineage one functional Ebf3 allele is sufficient to maintain marrow tracing of Ebf3-expressing CAR/LepR+ cells was per- spaces and HSCs and suppress osteoblastogenesis, where- formed using triple-mutant mice expressing CreERT2 as one Ebf1 allele is not. Finally, overexpression of Ebf1 knocked into the endogenous Ebf3 locus, the Cxcl12- and Ebf3 in sorted CAR cells increased expression of GFP transgene, and Cre-activated Rosa26 tandem dimer Cxcl12 and Scf and decreased their osteogenic potential Tomato (tdTomato). The number of Ebf3-expressing in culture. This effect was dependent on the DNA-binding CAR cells, which give rise to adipocytes and osteoblasts domain of Ebf3. in the bone marrow, remained stable over the course of In conclusion, this study identifies Ebf3 as the molecu- 13 mo, suggesting that they have the capacity to self-re- lar trigger that acts in both a cell-autonomous and a cell- new. Conditional deletion of Ebf3 in CAR/LepR+ cells or nonautonomous manner in CAR/LepR+ cells—to sup- in all MSCs of developing limbs resulted in decreased press their differentiation to the osteoblast lineage so as numbers of long-term repopulating HSCs (LT-HSCs), to maintain their HSC-supporting function (Fig. 1). The megakaryocyte/erythrocyte progenitors (MEPs) proery- precise characteristics of the osteolineage cells responsi- throblasts, common lymphoid progenitors (CLPs), and ble for HSC support are of great interest, since they could granulocyte/macrophage progenitors (GMPs). Moreover, be used to predict whether therapeutic strategies that reg- in aged mice, the number of functional HSCs decreased. ulate osteoblastic commitment could also be exploited to This compromised hematopoietic phenotype correlated enhance the formation of HSC niches from nonniche with increased osteoblast numbers and osteogenesis and cells. lower expression of Cxcl12 and Scf in CAR/LepR+ cells. As a result, aged mice became osteosclerotic. Interesting- ly, residual platelet-derived growth factor receptor β-posi- tive (PDGFRβ+)Sca-1−CD31−CD45− and Ter119− CAR References cells had much higher expression levels of osteoblast Calvi LM, Adams GB, Weibrecht KW, Weber JM, Olson DP, markers. Collectively, these results showed