Regulation of Erythropoiesis by BMP4 and Smad5

成人骨髓中的红细胞生成主要是动态平衡的，在成人一生中产生恒定水平的红细胞。这种情况在胚胎发育期间的胎肝和急性红系应激后的成人脾中有很大的不同。在这两种情况下，红细胞生成都会迅速产生更多的红细胞。鉴于胎肝和脾都是扩张性红细胞生成的部位，因此有人认为，胚胎肝脏在发育过程中调节红细胞生成的分子机制与在红系应激期间调节脾的分子机制相似，但不同于成人骨髓的稳态红细胞生成。这种联系在弯曲尾巴(F)基因突变的小鼠中很明显。F/f突变小鼠表现出严重的胎儿贫血，出生后2周就会消失。成年f/f小鼠表现出正常的稳态血液参数，但它们对急性红系应激的反应严重延迟。我们已经在f/f小鼠身上发现了Smad5基因的突变。Smad5是一种信号蛋白转录因子，作用于BMP4受体的下游。BMP4在造血细胞的发育，特别是红系的发育中起着关键作用。该方案概述了旨在研究BMP4/Smad5信号通路在胎儿肝脏发育期间和脾对红细胞生成应激反应中扩张性红细胞生成特征中的作用的实验。我们将利用f/f小鼠作为一种手段来剖析BMP4和Smad5在造血和红细胞生成中的作用。首先，我们确定了BMP4和Smad5在急性应激后脾中应激红系祖细胞的扩张和分化中的作用。其次，我们将分析BMP4和Smad5在胚胎中腹主动脉-性腺-中肾(AGM)区造血干细胞和多潜能细胞发育中的作用。第三，我们将分析BMP4和Smad5在胎肝红系祖细胞发育和分化中的作用。这些分析将提供重要的基本信息，可用于开发贫血治疗和创伤性失血治疗。

Erythropoiesis in the adult bone marrow is primarily homeostatic, producing a constant level of erythrocytes throughout adult life. This situation is dramatically different in the fetal liver during embryogenesis and in the adult spleen following acute erythroid stress. In both of these cases, erythropoiesis rapidly produces larger numbers of erythrocytes. Given that both the fetal liver and the spleen are sites of expansive erythropoiesis it has been suggested that the molecular mechanisms that regulate erythropoiesis in the fetal liver during development and the spleen during erythroid stress are similar, but distinct from steady state adult bone marrow erythropoiesis. This connection is evident in mice mutated at the flexed-tail (f) locus. f/f mutant mice exhibit a severe fetal anemia that resolves by 2 weeks after birth. Adult f/f mice exhibit normal steady state blood parameters, however, they exhibit a severe delay in the response to acute erythroid stress. We have identified a mutation in the Smad5 gene in f/f mice. Smad5 is a signaling protein-transcription factor that acts downstream of the BMP4 receptor. BMP4 plays a key role in the development of the development of hematopoietic cells and in particular the development of the erythroid lineage. This proposal outlines experiments designed to investigate the role of the BMP4/Smad5 signaling pathway in the expansive erythropoiesis characteristic of the fetal liver during development and the spleen during the response to erythropoietic stress. We will utilize the f/f mice as a means to dissect the role of BMP4 and Smad5 in hematopoesis and erythropoiesis. First, we determine the role of BMP4 and Smad5 in the expansion and differentiation of stress erythroid progenitors in the spleen following acute stress. Second we will analyze the role of BMP4 and Smad5 in the development of hematopoietic stem cells and multipotential cells in the Aorta- Gonad-Mesonephros (AGM) region of the embryo. Third, we will analyze the role of BMP4 and Smad5 in the development and differentiation of fetal liver erythroid progenitors. These analyses will provide important basic information that could be used to develop therapies for anemia and the treatment of traumatic blood loss.