HSC Diversity: Regulation by Clonal Selection vs Epigenetic Induction

描述（由申请人提供）：造血干细胞（HSC）被定义为既能自我更新又能产生所有血细胞谱系的克隆细胞。在过去的资助期内，我们证明了随着小鼠和人类年龄的增长，血细胞谱系的表现出现了倾斜，从年轻成年时淋巴细胞生成和骨髓单核细胞生成之间的平衡，到老年时骨髓单核细胞生成的优势。这种与年龄相关的造血分化结果的倾斜可能解释了晚年对髓系恶性肿瘤和贫血的易感性增加，以及对传染病的适应性免疫减弱。在这一建议中，我们考虑了两种可能的机制来解释这种与衰老相关的现象：(1)所有的HSC克隆型在年轻成人生活中产生平衡的血细胞谱系，但随着时间的推移，获得一系列的表观遗传变化，这些变化沉默了一些基因，诱导了其他基因，导致老年HSC偏向于骨髓单核细胞生成；或者(2)“平衡”和“骨髓偏倚”的HSC克隆型在年轻人中都存在，随着时间的推移，骨髓偏倚的HSC克隆型在环境、反馈和前馈调节的外部刺激下被优先选择。虽然我们的初步数据支持后一种解释，但目前的建议将通过对不同HSC克隆型的多样性和成熟度进行全面分析来明确评估这些替代方案。我们将首先确定不同类型的HSC克隆的数量，使用单细胞生物标记测定不同的基因表达模式，并评估从胚胎、胎儿、青年和老年小鼠获得的HSC。我们将用敏感的克隆标记技术标记不同类型的造血干细胞，然后研究它们在小鼠中的起源、成熟、迁移和调控。我们将从成人开始，追溯到胎儿或胚胎后期的发育阶段。我们将追踪到衰老和白血病的发生。我们将测试当缺乏或过量输入时，终端血细胞类型[红细胞、血小板、粒细胞、单核细胞和巨噬细胞，各种淋巴细胞]是否使内源性HSC克隆进入或退出细胞周期，从而增加或减少某些HSC克隆类型的数量。我们将评估是否同样的衰老相关的HSC分化结果发生在人类中。这项关于HSC克隆多样性是否由出生预先决定的研究，以及整个生命中的造血平衡是由响应多种生理事件的克隆选择塑造的，这将是推动本基金基础研究的主题。

DESCRIPTION (provided by applicant): Hematopoietic stem cells (HSC) are defined as clonogenic cells that can both self-renew and give rise to all blood cell lineages. In the past grant period we demonstrated a skewing in blood cell lineage representation as mice and humans age, from a balance between lymphopoiesis and myelomonocytopoiesis in young adult life, to a predominance of myelomonocytopoiesis in the elderly. This aging-related skewing of hematopoietic differentiation outcome potentially explains the increased susceptibility late in life to myeloid malignancies and anemia, as well as diminished adaptive immunity to infectious disease. In this proposal, we consider two possible mechanisms for this aging-related phenomenon (1) that all HSC clonotypes in young adult life produce a balanced repertoire of blood cell lineages, but acquire a series of epigenetic changes over time which silence some genes and induce others, resulting in elderly HSC which are biased to myelomonocytopoiesis; or (2) that "balanced" and "myeloid-biased" HSC clonotypes both exist in young adults, with myeloid-biased HSC clonotypes being preferentially clonally selected over time by environmental, feed- back, and feed-forward regulatory external stimuli. While our preliminary data favor the latter explanation, the current proposal will definitively evaluate these alternatives by undertaking a comprehensive analysis of the diversity and maturation of different HSC clonotypes. We will initially determine the number of different kinds of HSC clones, using the single cell Biomark assay for diverse gene expression patterns, and evaluating HSCs obtained from embryonic, fetal, young adult, and aged mice. We will label individual HSCs of different types with sensitive clone marking techniques, then study their origin, maturation, migration, and regulation in mice. We will start in adults, tracing them back to the fetal or late embryonic stages of development. We will trace them forward to aging and to leukemogenesis. We will test whether the end blood cell types [RBC, platelets, granulocytes, monocytes and macrophages, the various lymphocytes] when lacking or transfused in overabundance bring endogenous HSC clones into or out of cell cycle to expand or diminish the number of certain HSC clonotypes. We will evaluate whether the same aging-related skewing of HSC differentiation outcomes occurs in humans. This examination of whether HSC clonal diversity is pre-determined by birth, with hematopoietic balance throughout life being shaped by clonal selection in response to diverse physiological events, is the theme that will drive the basic research of this grant. PUBLIC HEALTH RELEVANCE: The results of this research project will help explain why people become more susceptible to certain hematologic malignancies, anemia, and infectious diseases as they age. Understanding the basis of this increased susceptibility is the first step to developing targeted therapies to prevent and/or reverse these public health issues.