Calcium homeostasis and cellular fitness in sepsis

摘要 每年有200万美国人因脓毒症住院，三分之一的人死亡。然而，那些幸存下来的人 是无法治愈的。神经认知障碍的发生率高达50%，认知能力下降的情况会持续长达8年。 脓毒症住院在计划外再入院中所占比例高于心肌梗死患者 脑梗塞、心力衰竭和慢性阻塞性肺病。脓毒症幸存者的五年死亡率超过心力衰竭和 卒中。这种持续的健康损失背后的机制仍有待确定。我们假设 在脓毒症早期，线粒体被重组为一种适应机制，以保护细胞免受任何 未来的环境应激，如反复出现的脓毒症。这些结构性变化使 支持细胞表型所必需的线粒体钙(Ca~(2+))稳态和代谢， 对于多细胞有机体来说，这些都是不能容忍的，并导致了持续的健康损失。 我们实验室花了近20年的时间研究脓毒症，以阐明钙依赖的机制 调节线粒体生物学，平衡钙动态平衡和三磷酸腺苷生成，维护细胞健康。 我们已经证明，脓毒症后早期，线粒体去极化产生一个钙信号。委员会成员 钙/钙调蛋白依赖的蛋白激酶家族(CaMK)转导这些钙信号，并在 串联介导线粒体分裂、有丝分裂和氧化代谢的适应性变化以减少 细胞损伤。最近，我们观察到败血症改变了线粒体钙单转运蛋白。 (MCU)复合体，对线粒体和细胞内钙稳态和 扰乱整个生物体的细胞和组织功能的代谢。我们建议，作为一种 对脓毒症的“习得性”反应，细胞重组MCU复合体以对抗潜在的钙离子 因未来的侮辱而超负荷；这会导致钙离子稳态的长期变化，氧化 新陈代谢和组织表型。利用下呼吸道和腹膜腔感染模型 针对相关的人体样本，我们提出了以下目标： 目的1.在小鼠和人身上研究重组的MCU复合体如何改变钙动态平衡和 氧化代谢，因此，组成有机体的每个组织的表型。 目的2.明确通过溶酶体吞噬有丝分裂和蛋白质降解的机制。 蛋白酶体是小鼠MICU1表达持续缺失的根本原因 败血症和人类败血症幸存者。 这项新的实验工作将提供有关机制如何管理的基础知识 脓毒症期间线粒体钙离子和新陈代谢发生重构，从而导致细胞持续丢失 导致健康逐渐丧失和生存时间缩短的表型。

ABSTRACT Two million Americans are hospitalized for sepsis each year, and 1 in 3 die. Those that survive, however, are not cured. Neurocognitive disorders occur in up to 50%, and cognitive decline continues for up to 8 years. Sepsis hospitalizations account for a higher proportion of unplanned readmissions than those for myocardial infarction, heart failure, and COPD. Five-year mortality for sepsis survivors exceeds that for heart failure and stroke. The mechanisms underlying this persistent loss of health remain to be defined. We hypothesize that early during sepsis the mitochondrion is restructured as an adaptive mechanism to protect the cell against any future environmental stress, such as recurrent sepsis. These structural changes impart lasting alterations to the mitochondrial calcium (Ca2+) homeostasis and metabolism necessary to support a cellular phenotype, which for a multicellular organism are poorly tolerated and underlie a persistent loss of health. Our lab has spent nearly two decades studying sepsis to elucidate the Ca2+-dependent mechanisms that regulate mitochondrial biology to balance Ca2+ homeostasis and ATP generation and preserve cellular health. We have shown that early after sepsis, mitochondrial depolarization generates a Ca2+ signal. Members of the family of Ca2+ /calmodulin-dependent protein kinases (CaMK) transduce these Ca2+ signals and work in tandem to mediate adaptive changes in mitochondrial fission, mitophagy, and oxidative metabolism to lessen cellular damage. More recently, we observed that sepsis restructures the mitochondrial calcium uniporter (MCU) complex, imposing long-lasting changes to mitochondrial and cellular Ca2+ homeostasis and metabolism that perturb cellular and tissue function across the entire organism. We propose that as a ‘learned’ response to sepsis, the cell restructures the MCU complex to counter the potential for Ca2+ overload with future insult; this imparts long-lasting alterations in Ca2+ homeostasis, oxidative metabolism, and tissue phenotype. Using models of lower-respiratory tract and intraperitoneal infection and correlative human samples, we propose the following aims: Aim 1. To study in mice and humans how a restructured MCU complex alters Ca2+ homeostasis and oxidative metabolism and thereby, the phenotype of each tissue comprising the organism. Aim 2. To define the mechanisms of mitophagy and protein degradation through the lysosome and proteasome as underlying causes of the persistent loss of MICU1 expression in murine models of sepsis and in human sepsis survivors. This new experimental work will provide foundational knowledge as to how the mechanisms governing mitochondrial Ca2+ and metabolism are restructured during sepsis to underlie a persistent loss of cellular phenotype that leads to a progressive loss of health and shortened survival.