TGF-β Signaling Plays an Essential Role in the Lineage Specification of Mesenchymal Stem/Progenitor Cells in Fetal Bone Marrow

摘要

class="head no_bottom_margin" id="sec1title">IntroductionThe bone marrow microenvironment is uniquely adapted to support hematopoiesis. A complex network of stromal cells in the bone marrow provides key signals that support the proliferation and survival of hematopoietic stem/progenitor cells (). CXCL12-abundant reticular (CAR) cells are perivascular mesenchymal stromal cells that express high levels of CXCL12 and stem cell factor (); they overlap considerably with Leptin-receptor⁺ stromal cells () and Nestin-GFP⁺ stromal cells in the bone marrow (). CAR cells and NG2⁺ arteriolar pericytes produce cytokines and chemokines that play crucial roles in regulating hematopoietic stem cells (HSCs), including CXCL12 and stem cell factor (, , , ). Adipocytes are rare in the bone marrow at birth but increase with aging and after myeloablation (, ). The presence of adipocytes in the bone marrow negatively correlates with hematopoietic activity (). However, a recent study showed that adipocytes promote hematopoietic recovery following myeloablation through production of stem cell factor (), suggesting a context-specific role for adipocytes in regulating hematopoiesis.The development and maintenance of mesenchymal stromal cells in the bone marrow is not well characterized. Lineage tracing studies show that Sp7 (osterix)-cre-targeted mesenchymal stem/progenitor cells (MSPCs) are present in the perichondrium of the future hindlimb by embryonic day 12.5 (E12.5) (, ). These fetal MSPCs transiently give rise to all mesenchymal stromal cells in the bone marrow, including osteoblasts, CAR cells, arteriolar pericytes, and adipocytes. However, these stromal cells are gradually replaced during adulthood. Indeed, a distinct osterix-Cre-targeted MSPC population is present in neonatal bone marrow and gives rise to long-lived mesenchymal stromal cells (). The signals regulating lineage specification of MSPCs also are not well characterized. Omatsu and colleagues, in two separate studies, showed that Foxc1 and Ebf1/Ebf3 contribute to the lineage specification of postnatal MSPCs. Specifically, the Foxc1 transcription factor negative regulates adipocyte differentiation of postnatal MSPCs, while the Ebf1/Ebf3 transcription factors inhibit osteoblast differentiation (href="#bib23" rid="bib23" class=" bibr popnode">Omatsu et al., 2014, href="#bib26" rid="bib26" class=" bibr popnode">Seike et al., 2018).Transforming growth factor β (TGF-β) is an inflammatory cytokine that also may contribute to MSPC differentiation. Cell culture studies show that TGF-β negatively regulates adipocyte and terminal osteoblast differentiation, while stimulating osteoblast progenitor proliferation (href="#bib2" rid="bib2" class=" bibr popnode">Alliston et al., 2001, href="#bib13" rid="bib13" class=" bibr popnode">Ignotz and Massague, 1985, href="#bib28" rid="bib28" class=" bibr popnode">Sparks et al., 1992). Studies examining the role of TGF-β signaling in MSPC differentiation in vivo are limited. Loss of Tgfb1 is associated with bone loss and a deficiency of osteoblasts (href="#bib30" rid="bib30" class=" bibr popnode">Tang et al., 2009). Tgfbr2, encoding TGF-β receptor 2, is required for TGF-β signaling. Deletion of Tgfbr2 using Prx1-Cre, which is active in early limb bud mesenchyme, results in severe skeletal defects and embryonic lethality (href="#bib27" rid="bib27" class=" bibr popnode">Seo and Serra, 2007). href="#bib32" rid="bib32" class=" bibr popnode">Wang et al. (2013) used an osterix-Cre (Osx-Cre) transgene to delete Tgfbr2 in mesenchymal progenitors. They showed that Osx-Cre, Tgfbr2^fl/fl mice have impaired tooth development and reduced mineralization of the mandible due to reduced osteoblast differentiation. In humans, genetic alterations leading to enhanced TGF-β signaling are associated with bone dysplasia in Camurati-Engelmann disease (href="#bib31" rid="bib31" class=" bibr popnode">Wallace and Wilcox, 1993). Of note, TGF-β regulates HSC quiescence and hematopoietic recovery following myeloablation (href="#bib5" rid="bib5" class=" bibr popnode">Brenet et al., 2013, href="#bib34" rid="bib34" class=" bibr popnode">Yamazaki et al., 2011, href="#bib38" rid="bib38" class=" bibr popnode">Zhao et al., 2014). Whether TGF-β signaling in mesenchymal stromal cells contributes to these hematopoietic responses is an open question.In this study, we characterize the contribution of TGF-β signaling in MSPCs on the development of mesenchymal stromal cells that comprise the bone marrow hematopoietic niche. We show that loss of TGF-β signaling in Osx-Cre-targeted fetal MSPCs results in alterations in mesenchymal stromal cells, including marked expansions of CAR cells and adipocytes. Both canonical and noncanonical TGF-β signaling in fetal MSPCs contribute to this phenotype. The resulting alterations in mesenchymal stromal cells are associated with a reduced capacity to support HSCs and a shift in hematopoiesis from lymphopoiesis to myelopoiesis. Together, these data suggest that TGF-β plays a key role in the lineage specification of MSPCs and is required for the emergence of a normal hematopoietic niche during fetal bone marrow development.