Understanding the mechanisms by which the different fetal hematopoietic niches support the development of undifferentiated HSPCs versus promote lineage differentiation is a major goal for the field. We document that loss of PDGF-B or its receptor, PDGFRβ, induces premature differentiation of hematopoietic progenitors into definitive erythroid cells in the placenta. Ectopic erythropoiesis was caused by upregulation of Epo in placental trophoblasts due to a direct requirement of PDGF-B signaling in regulating Epo levels. These studies identify the trophoblasts and PDGF-B signaling as key components of the unique placental hematopoietic niche.
Trophoblasts are tropho-ectoderm derived epithelial-like cells that facilitate implantation and maternal blood flow, and secrete growth factors and hormones (Simmons and Cross, 2005
). We show that loss of PDGF-B signaling provokes upregulation of Epo in sinusoidal trophoblast giant cells (S-TGCs), which secrete factors in fetal circulation (Simmons et al., 2007
). We discovered that S-TGCs express PDGFRβ (receptor), but not PDGF-B (ligand), which is mainly expressed in the endothelium during mid/late gestation, suggesting paracrine signaling between endothelium and trophoblasts. Moreover, mouse embryos that lack PDGF-B in the endothelium evidence similar placental structural defects as in Pdgf-b-/-
embryos, including reduction in trophoblasts, further indicating that the endothelium is a critical source of PDGF-B in the placenta (Bjarnegard et al., 2004
). Trophoblasts also express other growth factors and hormones that support hematopoiesis, such as VEGF, PLP-E and PLP-F (Breier et al., 1995
; Lin et al., 1997
; Simmons et al., 2008
). However, unlike Epo, these factors are not expressed in labyrinth S-TGCs (Lin et al., 1997
; Simmons et al., 2008
and data not shown), implying a different cellular origin and regulation. Future studies will be needed to elucidate how the different trophoblast subtypes contribute to the placental hematopoietic microenvironment.
Our study revealed that the ectopic erythropoiesis in placentas lacking PDGF-B signaling was induced by upregulation of Epo in trophoblasts. Epo promotes erythropoiesis by regulating proliferation and differentiation of erythroid progenitors (Wu et al., 1995
). Althobe drastically upregulated during anemia and hypoxia (Ebert and Bunn, 1999
), we excluded these cugh Epo levels can onditions as a major cause of Epo upregulation in PDGF-B deficient embryos. Furthermore, although co-existence of placental defects, hypocellular liver and anemia has been described also in other knockout mouse models such as p38α MAPK, SOCS3, or c-Met (Adams et al., 2000
; Marine et al., 1999
; Roberts et al., 2001
; Sasaki et al., 2000
; Tamura et al., 2000
), Epo upregulation and ectopic definitive erythropoiesis in the placenta has been described so far only in embryos deficient for PDGF-B signaling.
The role of Epo in normal placental biology is largely unknown. Epo signaling acts also in endothelial cells, supporting their survival, proliferation and migration (Anagnostou et al., 1990
; Carlini et al., 1995
), and it has been suggested that Epo signaling can regulate PDGF-B expression in the endothelium (Janmaat et al., 2010
). EpoR, the receptor for Epo, is expressed both in placental vasculature and trophoblasts (Fairchild Benyo and Conrad, 1999
; Sawyer et al., 1989
), suggesting that Epo may also have an autocrine function in trophoblasts. Although the physiological role of Epo signaling in the different placental cell types is still largely undefined, the embryos lacking Epo signaling possess smaller placentas (personal communication - Hong Wu, UCLA). Our data also indicates that the placental Epo levels have to be tightly regulated, as the overexpression of Epo in the trophoblasts is sufficient to induce ectopic definitive erythropoiesis in the placenta.
There are major differences in the regulation of Epo in the fetus and adult. While Epo production in the adult is restricted to the kidney, during development, the sites of Epo production change temporally. Epo is first expressed in the yolk sac and the placenta (Conrad et al., 1996
; Ebert and Bunn, 1999
; Yasuda et al., 2002
), after which its expression transitions to the fetal liver (Lee et al., 2001
) and at around birth to the kidney (Dame et al., 1998
; Koury et al., 1988
). Systemic Epo in the adult does not pose a risk for ectopic erythropoiesis as HSPCs reside in their niche in the bone marrow, whereas during fetal life, HSPCs traffic between the different hematopoietic niches, exposing HSPCs to signals they encounter during migration. Furthermore, it has been suggested that fetal progenitors are more sensitive to Epo than their adult counterparts (Linch et al., 1982
). Hence, the niche restricted Epo production may be essential to facilitate HSPC migration between the fetal hematopoietic organs without exposing them to premature differentiation.
The discovery of PDGF-B signaling in the trophoblasts as an important regulator of local Epo levels in the placenta reveals a developmental stage and niche specific mechanism for regulating Epo expression, which is critical for governing the fate of HSPCs during their developmental journey. This work gives new insights into the goal of recreating the different types of hematopoietic niches in vitro, as well as furthers our understanding of the etiology of developmental defects originating from the placenta.