Survival and dissemination of enteric pathogens through activation and subsequent inhibition of programmed cell death pathways

在被感染的食物摄入后，致病菌被肠道细胞吸收，进入细胞并试图生长。被感染的细胞意识到危险后，会经历一种被称为凋亡的严格控制的细胞自杀形式。因此，细胞在向免疫系统发出警告信号的同时，消除了自身和危险。大肠杆菌和沙门氏菌是英国最常见的两种与食物中毒相关的病原体。根据我们对身体对感染的反应的了解，我们预计当细胞接触到大肠杆菌和沙门氏菌时，它们会发生凋亡。然而，在这些病原体的情况下，被感染的细胞在感染后存活了一段时间，允许细菌在被感染的细胞内生长。这段额外的生长时间至关重要，让这些病原体有时间繁殖，导致更严重的长期感染。通过这一建议，我们旨在了解这些病原体是如何延长感染细胞的寿命的。这项工作将与许多细菌病原体相关，我们现在知道其中许多病原体也试图在感染期间干扰这种凋亡过程。我们之前的工作对沙门氏菌感染细胞中发生的复杂相互作用有了新的认识。在感染期间，细菌通过诱导细胞自杀，故意瞄准细胞中对感染作出反应的蛋白质。特别是一种非常有效的宿主细胞酶，称为caspase-3，或“刽子手caspase”。这是诱导细胞自杀的关键酶，能以可控的方式快速杀死细胞。由于caspase-3的破坏力，它的激活受到严格调控，这使得它被细菌病原体靶向更加令人惊讶。然而，这些细菌最初并没有试图阻止caspase-3发挥作用，而是试图利用它的活性，绕过严格的控制措施，确保caspase-3的活性得到控制。我们现在知道沙门氏菌感染通过一种独特的机制激活caspase-3，了解和利用这一机制是本提案的关键目标。激活像caspase-3这样具有破坏性的酶对于只有有限时间在被感染细胞内生长的病原体来说是一种危险的策略。然而，沙门氏菌和大肠杆菌已经发展出一种机制来控制这种酶的破坏力。宿主细胞有一个叫做蛋白酶体的自然循环系统，它用来吸收旧的或受损的蛋白质，将它们分解，并使用构建块来形成新的蛋白质。这些细菌病原体利用这个循环系统，用它们自己的蛋白质模仿细胞中的蛋白质，对宿主蛋白质进行分类以进行循环利用。利用这种策略，细菌转移了caspase-3的正常作用，导致宿主细胞无意中将其送去回收。这延缓了细胞死亡，意味着细菌现在可以在细胞内繁殖。我们相信这种策略在感染过程中会被许多其他细菌病原体复制，这意味着这一建议将对许多微生物感染的研究产生影响。

After being ingested on infected food pathogenic bacteria are taken up by cells in the intestine, entering the cells and attempting to grow. An infected cell recognizing the danger undergoes a tightly controlled form of cell suicide known as apoptosis. Thus the cell removes itself and the danger whilst simultaneously sending out warning signals to the immune system. E. coli and Salmonella are two of the most common food poisoning associated pathogens in the U.K. Based on our knowledge of the body's response to infection we would expect cells to undergo apoptosis when they come into contact with E. coli and Salmonella. In the case of these pathogens however the infected cells survive for some time following infection allowing the bacteria to grow within the infected cell. This extra time for growth is crucial, allowing these pathogens time to multiply and cause a more serious prolonged infection. Through this proposal we aim to understand how these pathogens are prolonging the life of infected cells. This work will have relevance for numerous bacterial pathogens, many of which we now know also attempt to interfere with this process of apoptosis during infection. Our previous work has shed new light on the complex interactions occurring in Salmonella infected cells. During infection the bacteria deliberately target proteins in the cell that respond to the infection by inducing cell suicide. In particular one extremely potent host cell enzyme called caspase-3, or the 'executioner caspase' is targeted. This is the key enzyme in inducing cell suicide, killing the cell quickly but in a controlled fashion. Due to the destructive power of caspase-3 its activation is tightly regulated, making its targeting by bacterial pathogens all the more surprising. However instead of trying to prevent caspase-3 from working these bacteria initially try to harness its activity bypassing the stringent controls put in place to ensure caspase-3 activity is kept in check. We now know Salmonella infection is activating caspase-3 by a unique mechanism and understanding and exploiting this is a key objective for this proposal. Activation of an enzyme as destructive as caspase-3 is a risky strategy for a pathogen that only has a limited time to try to grow within an infected cell. Salmonella and E. coli however have developed a mechanism to control the destructive power of the enzyme. Host cells have a natural recycling system called the proteasome that is used to take old or damaged proteins and break them down and use the building blocks to form new proteins. These bacterial pathogens tap into this recycling system, using their own proteins to mimic proteins from the cell that sort host proteins for recycling. Using this tactic the bacteria divert caspase-3 from its normal role, causing the host cell to inadvertently send it for recycling. This delays cell death meaning the bacteria can now multiply within the cell. This tactic we believe is replicated by numerous other bacterial pathogens during infection, meaning this proposal will have repercussions for the study of numerous microbial infections.