BMP4, SCF, and hypoxia cooperatively regulate the expansion of murine stress erythroid progenitors

Abstract

The erythroid response to acute anemia relies on the rapid expansion in the spleen of a specialized population of erythroid progenitors termed stress BFU-E. This expansion requires BMP4/Madh5-dependent signaling in vivo; however, in vitro, BMP4 alone cannot recapitulate the expansion of stress BFU-E observed in vivo, which suggests that other signals are required. In this report we show that mutation of the Kit receptor results in a severe defect in the expansion of stress BFU-E, indicating a role for the Kit/SCF signaling pathway in stress erythropoiesis. In vitro analysis showed that BMP4 and SCF are necessary for the expansion of stress BFU-E, but only when spleen cells were cultured in BMP4 ؉ SCF at low-oxygen concentrations did we recapitulate the expansion of stress BFU-E observed in vivo. Culturing spleen cells in BMP4, SCF under hypoxic conditions resulted in the preferential expansion of erythroid progenitors characterized by the expression of Kit, CD71, and TER119. This expression pattern is also seen in stress erythroid progenitors isolated from patients with sickle cell anemia and patients with ␤-thalassemia. Taken IntroductionAcute anemia induces a systemic response that involves the induction of erythropoietin (Epo) expression in the kidney and the rapid mobilization and differentiation of erythroid progenitors. Previous work from our laboratory led to the development of a new model for the recovery from acute anemia where BMP4 expression in the spleen drives the expansion of a specialized population of stress erythroid progenitors, which we termed stress BFU-E. 1 Our analysis of flexed-tail (f) mutant mice, which exhibit a defect in BMP4-dependent signaling, demonstrated that BMP4 is required for the recovery from acute anemia. 1 BMP4 is required only transiently and acts on an earlier progenitor, the BMP4 responsive (BMP4 R ) cell, causing it to differentiate into a stress BFU-E. BMP4 R cells are contained in the spleen megakaryocyte erythroid progenitor (MEP) population but are not observed in bone marrow MEPs. We define a stress BFU-E as a progenitor that rapidly forms a large burst colony in 5 days when cultured in media containing only Epo. If stress BFU-Es are cultured for 7 days as steady state bone marrow BFU-Es are, they produce significantly larger colonies. 1 We showed that treatment of spleen cells with BMP4 in vitro resulted in the expansion of stress BFU-Es, which demonstrated that BMP4 R cells are resident in the spleens of normal mice. However, culturing spleen cells in media containing BMP4 failed to recapitulate the 45-fold increase in stress BFU-Es observed in vivo during the recovery from acute anemia. This observation suggests that additional signals present in the spleen microenvironment were also required for the expansion of stress BFU-Es.Stem cell factor (SCF) is known to play a key role in the development and expansion of erythroid progenitors. [2][3][4] SCF is encoded by the murine Steel (Sl) locus 5-7 and its receptor; Kit is encoded by the murine dominant white spotti...

The role of Smad signaling in hematopoiesis and translational hematology

“…44 Subsequently, it was shown that stem cell factor and hypoxia synergize with BMP4 to drive effective recovery upon stress anemia. 45 Although acute anemia has never been studied in the context of complete deletion of Smad5 in the mouse, these studies stand in sharp contrast to studies performed by Singbrant et al In a series of experiments, it was shown that Cre-mediated deletion of Smad1 and/or Smad5 did not impair adult hematopoiesis in the mouse and mutant HSCs displayed normal self-renewal and differentiation capacities upon transplantation. 67,68 Similarly, using a fetal liver-specific Cre-driver to induce deletion of Smad5 or Smad1/Smad5 together, hematopoiesis was shown to occur normally upon transplantation into wild-type hosts.…”

“…It is, however, interesting to note that the BMP pathway has been previously linked to stress erythropoiesis, as will be discussed below. 44,45 The mechanism by which TIF1g cooperates with Smads continues to be a controversial issue. Developmental studies from a variety of species have suggested that TIF1g, also known as Ectodermin, acts as a ubiquitin ligase for Smad4, thus functioning as a direct inhibitor of Smad4 downstream of TGF-b and BMP signaling.…”

Hemolytic anemia repressed hepcidin level without hepatocyte iron overload: lesson from Günther disease model

“…In adult spleen, stress erythropoiesis may be induced during acute anemia and hypoxia by BMP4 (Bone Morphogenic Protein 4) factor. 24,25 We thus analyzed BMP4 expression level in CEP mice and show its strong increase in the red pulp of the spleen ( Figure 2E), likely contributing to the rapid formation of stress burst-forming unit erythroid progenitors (BFU-Es) as a consequence of the high levels of erythropoietin (Epo) in these mice (Table 1). Therefore, hemolytic anemia in CEP mice activates a compensatory stress erythropoiesis with no sign of ineffective erythropoiesis.…”

-thalassemias: paradigmatic diseases for scientific discoveries and development of innovative therapies

“…It has been shown that in SE some of the signals that regulate this process are Hedgehog, Bmp4, stem cell factor and hypoxia. [84][85][86] The Bmp4-dependent SE pathway plays a key role in the recovery from acute anemia. 87 Bmp4 induces, through Smad5 signaling, the proliferation of stress erythroid progenitors, which are phenotypically different from steady-state progenitors.…”