Defining the Hematopoietic Stem Cell Niche in the Fetal Liver

干细胞移植是治疗几种血液疾病的重要工具11。 患者 淋巴瘤或多发性骨髓瘤接受动员的祖细胞自体移植后， 高剂量化疗12. 动员的祖细胞数量可能不足，特别是当 需要重复自体移植13。 这些数据迫使我们制定协议， 造血干细胞在胎儿肝脏（FL）内，造血干细胞急剧扩增， 因此，我们把我们的努力集中在这个机构上。 HSC群体在成人和胎儿中已被高度表征，并且很明显， 在成人骨髓（BM）中，HSC主要是非循环/静止的。在开发过程中，整个 血液谱系的等级必须增长以满足成长中的胚胎的需要。 在实物中，所有HSC都在循环 并且HSC数量增加：HSC的自我更新是明显的。胎儿和新生儿造血干细胞的差异 成年人可能归因于不同的微环境，或生态位，信号从FL和BM。 我们希望 确定胎儿HSC的生态位，并剖析调控HSC增殖的机制。 在BM中鉴定了巢蛋白+间充质干细胞群体，其充当HSC小生境。 通过应用 利用我们用来鉴定骨髓中HSC小生境的工具，我们已经在骨髓中鉴定了一个候选HSC小生境细胞。 胎儿肝脏（FL）是胎儿造血的主要部位。 也许这并不奇怪，FL中的Nestin+细胞具有 与BM中的那些惊人的相似。在这里，我们建议用几种体内方法来描述这一人群的特征。 和体外测定。然后，我们将证明FL Nestin+细胞在维持HSC中的重要性， 包括体内和体外方法。 我们将使用Cre重组酶删除候选基因CXCL 12， 我们的Nestin+群体来测试这种趋化因子在维持FL小生境中的HSC中的重要性。 我们 将对FL和BM Nestin+细胞进行RNA测序，以定量BM和FL之间的差异 壁龛 比较转录组学可能有助于了解FL和BM Nestin+细胞 调节HSC自我更新、静止和分化。 对利基市场的进一步研究将有助于确定 HSC可以扩增用于治疗目的。成虫和胚胎中的小生境识别 这给了我们一个独特的机会来比较干细胞处于静止状态和 在生理环境中增殖，也可以洞察非生理生长， 骨髓增生异常综合征我们的初步证据表明，我们已经确定了一种罕见的细胞类型在FL 其富含支持HSC所必需的生长因子。 通过进一步表征该细胞群 在其他潜在的候选者中，我们希望研究HSC扩展的机制。

Stem cell transplantation is an important tool to treat several hematological disorders11. Patients with lymphomas or multiple myeloma receive autologous transplantation of mobilized progenitors after receiving high-dose chemotherapy12. The numbers of mobilized progenitors may not be sufficient, especially when repeat autologous transplantations are required13. These data compel us to develop protocols to expand hematopoietic stem cells. Within the fetal liver (FL), hematopoietic stem cells expand dramatically and for this reason, we have focused our efforts on this organ. The HSC population has been highly characterized in the adult and the fetus, and it is clear that in the adult bone marrow (BM), HSCs are primarily non-cycling/quiescent. During development however, the entire hierarchy of blood lineages must grow to serve the needs of the growing embryo. In kind, all HSCs are cycling and the HSC population increases: self renewal of HSCs is evident. The differences in HSCs in the fetus and adult may be attributed to distinct microenvironmental, or niche, signals from the FL and BM. We wish to identify the fetal HSC niche and dissect the mechanisms employed to regulate HSC proliferation.We previously identified a Nestin+mesenchymal stem cell population in the BM that served as a HSC niche. By applying the tools we have used to identify the HSC niche in the BM, we have identified a candidate HSC niche cell in the Fetal Liver (FL), the major site of fetal hematopoiesis. Perhaps not-surprisingly, Nestin+ cells in the FL bear a striking resemblance to those in the BM. Here, we propose to characterize this population with several in vivo and in vitro assays. We will then demonstrate the importance of the FL Nestin+ cells in maintaining HSCs with both in vivo and in vitro approaches. We will use Cre-recombinase to delete a candidate gene, CXCL12, within our Nestin+ population to test for the importance of this chemokine in maintaining HSCs in the FL niche. We will perform RNA-sequencing of FL and BM Nestin+ cells to quantify the differences between BM and FL niches. Comparative transcriptomicsmay give insight into mechanisms by which FL and BM Nestin+ cells regulate HSC self-renewal, quiescence, and differentiation. Further study of the niche will help define how HSCs may be expanded for therapeutic purposes. The identification of niches in the adult and in the embryo allows us a unique opportunity to compare niches where stem cells are quiescent and where they are proliferating in physiological settings and may also give insight into non-physiological growths such as myelodysplastic syndrome. Our preliminary evidence suggests that we have identified a rare cell type in the FL that is enriched for growth factors necessary to support HSCs. By further characterizing this cell population among other potential candidates, we wish to study the mechanisms of HSC expansion.