Apoptosis in Postmitotic Cells: Increased Regulation and Novel Checkpoints

描述(由申请人提供)：凋亡途径的存在对生物体来说既是至关重要的，也是毁灭性的。虽然通过细胞凋亡调节细胞死亡在发育过程中和维持生物体内稳态是必不可少的，但细胞凋亡的失调与包括癌症、神经退行性变和心血管疾病在内的许多病理疾病有关。因此，细胞抑制凋亡途径的能力，只有在必要时才会激活，这确实是一个微妙的平衡。我们的假设是，在有丝分裂细胞和有丝分裂后细胞中，细胞凋亡途径的调控方式存在根本差异。我们认为，在有丝分裂后的细胞中，如神经元、心肌细胞和肌管，这些细胞的再生潜力有限，并且可以持续到生物体的整个生命周期，因此，细胞凋亡的调控比有丝分裂细胞更严格。我们的研究集中在细胞色素c依赖的caspase激活的调控上，因为这一事件是哺乳动物细胞死亡的一个关键点。我们发现，尽管胞浆细胞色素c本身足以诱导许多有丝分裂细胞的凋亡，但它不能在有丝分裂后的神经元、心肌细胞和肌管中做到这一点，因为XIAP严格控制caspase的激活。我们还发现了另一个意想不到的检查点，在那里我们发现内源性细胞色素c水平限制了有丝分裂后细胞的凋亡，但不是有丝分裂细胞。在特定的目标1中，我们将研究内源性XIAP如何能够选择性地调节有丝分裂后细胞的凋亡，而不是有丝分裂后细胞的凋亡。我们还将研究XIAP对caspase的严格抑制是如何解除的，从而允许有丝分裂后细胞的凋亡。在特定的目标2中，我们将研究内源性细胞色素c水平成为有丝分裂后细胞凋亡的限速机制，但不是有丝分裂后细胞。我们还将测试这一假设，即有丝分裂后细胞的凋亡信号将内源性细胞色素c水平提高到激活caspase所需的阈值以上。在具体目标3中，我们将注意力集中在衰老细胞上。我们将研究衰老的发展是否像在终末分化的细胞中看到的那样，参与细胞色素c依赖的细胞凋亡调节的变化。了解细胞凋亡在有丝分裂和有丝分裂后细胞中的差异调控具有重要的临床意义，因为它识别了可以抑制或激活选择性细胞类型中的凋亡的药物靶点。

DESCRIPTION (provided by applicant): The existence of the apoptotic pathway can be both vital and devastating for organisms. While regulated cell death by apoptosis is essential during development and for maintaining homeostasis in organisms, dysregulation of apoptosis is associated with numerous pathological conditions including cancer, neurodegeneration and cardiovascular diseases. Therefore, a cell's ability to suppress the apoptotic pathway, activating it only when necessary, is indeed a delicate balance. Our hypothesis is that there are fundamental differences in how the apoptotic pathway is regulated in mitotic versus postmitotic cells. We propose that in postmitotic cells such as neurons, cardiomyocytes, and myotubes, which have limited regenerative potential and last for the lifetime of organisms, apoptosis is regulated more strictly than in mitotic cells. We have focused our studies on the regulation of cytochrome c- dependent caspase activation because this event is a crucial point of no return for cell death in mammalian cells. We find that whereas cytosolic cytochrome c alone is sufficient to induce apoptosis in many mitotic cells, it is not capable of doing so in the postmitotic neurons, cardiomyocytes and myotubes because of strict control of caspase activation by XIAP. We have also identified another unexpected checkpoint where we find levels of endogenous cytochrome c to be limiting for apoptosis in postmitotic but not mitotic cells. In Specific Aim 1, we will investigate how endogenous XIAP is able to selectively regulate apoptosis in postmitotic but not mitotic cells. We will also examine how XIAP's strict inhibition of caspases is relieved to permit apoptosis in postmitotic cells. In Specific Aim 2, we will examine the mechanism by which endogenous levels of cytochrome c become rate-limiting for apoptosis in postmitotic but not mitotic cells. We will also test the hypothesis that apoptotic signals in postmitotic cells elevate endogenous cytochrome c levels above the threshold necessary for activating caspases. In Specific Aim 3, we focus our attention on senescent cells. We will examine whether the development of senescence engages changes in the regulation of cytochrome c-dependent apoptosis as seen in the terminally-differentiated cells. Understanding how apoptosis is differentially regulated in mitotic and postmitotic cells is clinically significant because it identifies drug targets that could inhibit or activate apoptosis in selective cell types.