The role of BET proteins in pathological cardiac remodeling

摘要 在对持续的神经激素/血液动力学应激或损伤的反应中，心脏经历病理性 重塑过程通常以心肌细胞肥大和纤维化为特征。心脏纤维化， 定义为心脏中过量的细胞外基质沉积，是心脏收缩和舒张功能的主要贡献者。 心脏舒张功能不全。损伤后，静止的成纤维细胞经历了一个 细胞状态转变为过度分泌细胞外基质的活化成纤维细胞和肌成纤维细胞 蛋白质，可以收缩。纤维化重塑导致左心室僵硬，舒张受损， 顺应性降低和电导率改变。这会推动疾病进展并恶化 病理尽管众所周知心脏纤维化会导致与心脏相关的不良结局， 然而，尽管治疗失败，靶向心脏纤维化的有效疗法仍然难以捉摸。BET（溴结构域）的抑制 具有化学小分子（例如JQ 1）的表观遗传阅读器蛋白的（例如，末端外结构域）家族具有 已证实可有效阻断体内心脏肥大、炎症和纤维化， 在啮齿动物心力衰竭模型中的功能。虽然这些抑制剂阻断心脏中的所有BET蛋白（BRD2， BRD3，BRD4），在细胞培养实验中单个BET蛋白的遗传靶向支持以下观点： BRD4是主要的BET蛋白质，负责激活病理基因表达程序， 心肌细胞和心脏成纤维细胞。然而，最近的工作已经深入了解了细胞类型的特定作用， BRD4在心肌细胞中的作用，揭示了BRD4在维持线粒体稳态中的重要作用。 成年心肌细胞中BRD4的条件性缺失导致进行性心功能不全， 线粒体功能，并最终导致小鼠死亡。结合以前的研究，这表明， 小分子BET抑制剂在心力衰竭小鼠模型中的有益作用不太可能是介导的 通过抑制心肌细胞中的BRD4。我们假设，BRD4的体内遗传破坏可能与BRD4的表达有关。 活化的成纤维细胞将安全地阻断纤维化，并且多种BET蛋白（即BRD2和BRD3） 介导小分子泛BET抑制的有益效果。在目标1中，我们将确定体内作用 BRD4在活化的成纤维细胞中的作用，在目标2中，我们将阐明以前未探索的作用， BET蛋白在心肌细胞基因表达调控中的作用这项工作将在 病理性心脏重塑的表观遗传调控专家Matthew Stratton博士的实验室， 在心脏生理学的世界领导者洛伦·沃尔德博士的共同监督下进行。我们的长期目标是 开发安全有效的治疗方法，以改善心脏纤维化和HF患者的临床结局。

ABSTRACT In response to persistent neurohormonal/hemodynamic stress or injury, the heart undergoes a pathologic remodeling process typically characterized by cardiomyocyte hypertrophy and fibrosis. Cardiac fibrosis, defined as excess extracellular matrix deposition in the heart, is a major contributor to both systolic and diastolic dysfunction for millions of heart failure patients. Upon injury, resident quiescent fibroblasts undergo a cell state transition to become activated fibroblasts and myofibroblasts that hyper-secrete extracellular matrix proteins and can be contractile. Fibrotic remodeling causes stiffening of the left ventricle, impaired relaxation, reduced compliance, and altered electrical conductance. This drives disease progression and worsens pathology. Despite the well-known contribution of cardiac fibrosis to adverse outcomes associated with heart failure, effective therapies targeting fibrosis in the heart remain elusive. Inhibition of the BET (bromodomain and extra-terminal domain) family of epigenetic reader proteins with chemical small molecules (e.g. JQ1) has proven effective at blocking in vivo cardiac hypertrophy, inflammation, and fibrosis and preserving cardiac function in rodent models of heart failure. Though these inhibitors block all BET proteins in the heart (BRD2, BRD3, BRD4), genetic targeting of individual BET proteins in cell culture experiments support the notion that BRD4 is the main BET protein responsible for activation of pathologic gene expression programs in cardiomyocytes and cardiac fibroblasts. However, recent work has given insight on the cell-type specific role of BRD4 in cardiomyocytes, uncovering an essential role for BRD4 in maintaining mitochondrial homeostasis. Conditional deletion of BRD4 in adult cardiomyocytes resulted in progressive cardiac dysfunction, impaired mitochondrial function, and eventual death in mice. Taken together with previous studies, this suggests that the beneficial effects of small molecule BET inhibitors in mouse models of heart failure are unlikely to be mediated exclusively by inhibition of BRD4 in cardiomyocytes. We hypothesize that genetic in vivo disruption of BRD4 in activated fibroblasts will safely block fibrosis and that multiple BET proteins (i.e. also BRD2 and BRD3) mediate the beneficial effects of small-molecule pan-BET inhibition. In Aim 1, we will determine the in vivo role of BRD4 in activated fibroblasts, and in Aim 2 we will elucidate the previously unexplored role of individual BET proteins in cardiac cell gene expression regulation through ChIP-Seq. This work will be carried out in the laboratory of Dr. Matthew Stratton, an expert in epigenetic regulation of pathologic cardiac remodeling, and under the co-supervision of Dr. Loren Wold, a world-leader in cardiac physiology. Our long-term goal is to develop safe and effective therapeutics to improve clinical outcomes for patients with cardiac fibrosis and HF.