Biomechanical Activation of Yap Induces Hematopoietic Stem Cell Production

总结 造血干细胞（HSC），首先在发育中的脊椎动物胚胎中产生， 血液和免疫系统的基础。HSC作为HSC移植（HSCT）具有治疗价值， 将供体HSC给予免疫功能低下的受体是许多人的护理标准， 血液病然而，由于免疫不相容性，治疗的可用性仍然存在问题， 捐助者短缺。同样，虽然众所周知移植的HSC的数量直接影响植入， 效率，目前还没有建立的临床方案来成功扩增供体收获的HSC， 也不能在体外将胚胎或诱导多能干细胞（iPSC）分化为功能性HSC。因此，我们认为， 鉴定具有长期自我更新和分化的HSC从头产生的新修饰剂 容量是主要的未满足的临床需要。尽管经过十多年的研究，目前的协议主要依赖于 在转录因子的强制表达以帮助引导细胞进入“HSC样”转录程序时， 然而，将这些体外衍生的HSC移植到受辐射的小鼠中说明了有限的长期 移植和多谱系潜力。这些观察结果表明，目前的体外分化 这些策略缺少关键线索，这些线索对于释放或维持体内全部HSC潜力至关重要。 在发育中的胚胎中，永久性HSC来自于独特的中胚层前体细胞群 称为生血内皮（HEC），通过称为内皮细胞向造血细胞转化的过程 （EHT）。转录因子RUNX 1在脊椎动物HSC从头形成的所有位点表达， HSC规范和EHT要求。我们先前的工作显示Runx 1表达强烈上调， 在胚胎心跳开始后，HSC的产生与强有力的心跳的开始相协调。 循环流动和剪切应力。而壁龛的机械性能，包括剪切应力和 周向伸展，越来越多地被认为是重要的干细胞线索在许多情况下， 机械转导驱动造血命运和HSC产生的机制 脊椎动物的发育在很大程度上还未被探索。本研究旨在描述生物力学的作用 在体内和体外HSC形成中调节生血血管生态位，总体目标是 鉴定将选择的生物物理力连接到基因调控网络的信号通路 控制HSC承诺。我们的初步数据表明，圆周拉伸有一个新的，但必不可少的作用- 刺激转录因子Yap 1的活化，调节Runx 1 + HEC特化和HSC 生产确定体内介导生产性HSC形成的分子信号通路将揭示 用于优化成人型HSC的定向扩增和/或分化以供临床使用的新靶点。

SUMMARY Hematopoietic stem cells (HSCs), first produced in the developing vertebrate embryo, supply the lifelong foundation of the blood and immune systems. HSCs are therapeutically valuable as HSC transplantation (HSCT), the administration of donor HSCs to an immunocompromised recipient, is the standard of care for many hematological diseases. However, treatment availability remains problematic due to immune incompatibility and donor shortage. Likewise, while the number of transplanted HSCs is well known to directly impact engraftment efficiency, there are currently no established clinical protocols to successfully expand donor-harvested HSCs, nor to differentiate embryonic or induced pluripotent stem cells (iPSCs) into functional HSCs in vitro. Therefore, the identification of novel modifiers of de novo production of HSCs with long-term self-renewal and differentiation capacity is a major unmet clinical need. Despite more than a decade of research, current protocols rely primarily on enforced expression of transcription factors to help steer cells into an “HSC-like” transcriptional program, however transplantation of these in vitro-derived HSCs into irradiated mice illustrates both limited long-term engraftment and multilineage potential. These observations imply that current in vitro differentiation strategies are missing critical cues which are essential to unlock or maintain full HSC potential in vivo. In the developing embryo, definitive HSCs arise from a unique population of mesodermal precursors termed hemogenic endothelium (HEC) through a process known as endothelial-to-hematopoietic transition (EHT). The transcription factor RUNX1, expressed in all sites of de novo HSC formation across vertebrates, is required for HSC specification and EHT. Our prior work revealed that Runx1 expression is strongly upregulated after initiation of the embryonic heartbeat, and HSC production is coordinated with the onset of vigorous circulatory flow and sheer stress. While mechanical properties of the niche, including sheer stress and circumferential stretch, are increasingly recognized as important stem cell cues in many contexts, the mechanism(s) by which mechanotransduction drives commitment to hemogenic fate and HSC productionduring vertebrate development remain largely unexplored. This study aims to characterize the role of biomechanical modulation of the hemogenic vascular niche in HSC formation in vivo and in vitro, with the overall goal of identifying the signaling pathway(s) connecting select biophysical forces to the gene regulatory network controlling HSC commitment. Our preliminary data indicate a novel, yet essential, role for circumferential stretch- stimulated activation of the transcription factor Yap1 in regulation of Runx1+ HEC specification and HSC production. Defining the molecular signaling pathways that mediate productive HSC formation in vivo will reveal new targets for optimizing the directed expansion and/or differentiation of adult-type HSCs for clinical use.