Apoptosis in Postmitotic Cells: Increased Regulation and Novel Checkpoints

描述（由申请人提供）：凋亡通路的存在对生物体既重要又具有破坏性。虽然通过凋亡调控的细胞死亡在生物发育和维持体内平衡中是必不可少的，但凋亡的失调与许多病理状况有关，包括癌症、神经退行性疾病和心血管疾病。因此，细胞抑制凋亡通路的能力，仅在必要时激活它，确实是一种微妙的平衡。我们的假设是，在有丝分裂细胞和有丝分裂后细胞中，凋亡通路的调节方式存在根本差异。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，它们的再生潜力有限，并在生物体的一生中持续存在，细胞凋亡比有丝分裂细胞受到更严格的调节。我们将研究重点放在细胞色素c依赖性caspase激活的调控上，因为这一事件是哺乳动物细胞死亡的关键。我们发现细胞质细胞色素c在许多有丝分裂细胞中足以诱导凋亡，但在有丝分裂后神经元、心肌细胞和肌管中却不能，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水平限制有丝分裂后细胞的凋亡，而不是有丝分裂细胞。在Specific Aim 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而非有丝分裂细胞的凋亡。我们还将研究XIAP如何解除对半胱天冬酶的严格抑制，从而允许有丝分裂后细胞凋亡。在特异性目标2中，我们将研究内源性细胞色素c水平在有丝分裂后而非有丝分裂细胞中成为细胞凋亡限速的机制。我们还将验证有丝分裂后细胞凋亡信号将内源性细胞色素c水平提高到激活半胱天冬酶所需的阈值以上的假设。在Specific Aim 3中，我们将重点关注衰老细胞。我们将研究衰老的发展是否涉及细胞色素c依赖性凋亡的调节变化，如在终末分化细胞中所见。了解细胞凋亡在有丝分裂细胞和有丝分裂后细胞中的差异调控机制具有重要的临床意义，因为它可以确定抑制或激活选择性细胞类型凋亡的药物靶点。

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.