Role of the inner mitochondrial membrane in innate immune signaling paradigms

线粒体在调节细胞代谢和协调细胞命运决定方面具有多种功能，对维持细胞和生物体的完整性至关重要。最近，线粒体被认为在协调先天免疫信号通路中起着关键作用。由于它们的运动性和动态性，促进了细胞间的交流，它们注定要完成这项任务。作为变形菌起源的细胞器，线粒体携带危险信号，需要保护免受免疫监视。另一方面，线粒体已被真核细胞所选择，以应对细胞损伤并促进有效的免疫反应。关于线粒体进化影响细胞信号传导的机制存在多个难题。在一些范例中，信号平台在线粒体外膜（OMM）组装，涉及OMM蛋白的翻译后修饰和特定蛋白的募集。然而，为了使线粒体功能适应信号传递过程中特定环境的细胞需求，信息需要从OMM传递到线粒体内膜（IMM）。OMM的变化是如何向IMM和线粒体基质发出信号的，这在很大程度上仍未得到探索。我们最近发现了一种先天免疫信号通路，它利用线粒体进行信号放大，并将活化的转录因子运送到细胞核。抗凋亡肿瘤坏死因子（TNF）在质膜上与其受体结合，诱导信号成分向OMM募集，并通过泛素化和磷酸化激活它们。这种OMM的重塑伴随着线粒体膜电位和呼吸活动的增加，以及在促凋亡条件下嵴完整性的维持，表明信号从OMM传递到IMM。在这里，我们的目标是识别与顺行（由外而内）线粒体信号有关的IMM成分，并确定其潜在机制（aim 1）。在这种情况下，一个有希望的目标是线粒体接触位点和嵴组织系统（MICOS），基于其在IMM和OMM之间形成接触位点的作用。在AIM 2中，我们将探讨线粒体作为危险信号的来源，如mtDNA，作为逆行（内向外）信号传导的一个例子，并表征在不同应激模式下阻止mtDNA释放的IMM完整性决定因素。这些实验结合了细胞生物学、高级细胞成像、生物化学、蛋白质组学和脂质组学等最先进的方法，我们希望揭示先天免疫信号中IMM结构和功能的关键方面。

Mitochondria are essential for the maintenance of cellular and organismal integrity, based on their manifold functions in regulating cellular metabolism and coordinating cell fate decisions. More recently, mitochondria have been recognized to play a pivotal role in orchestrating innate immune signaling pathways. They are predestined for this task by virtue of their motility and dynamics, facilitating interorganellar communication. As organelles of proteobacterial origin, mitochondria harbor danger signals and need to be protected from immune surveillance. On the other hand, mitochondria have been co-opted by eukaryotic cells to react to cellular damage and to promote efficient immune responses. There are multiple conundrums concerning the mechanisms mitochondria evolved to affect cellular signaling. In several paradigms, a signaling platform is assembled at the outer mitochondrial membrane (OMM), involving posttranslational modifications of OMM proteins and recruitment of specific proteins. However, to adapt mitochondrial functions to context-specific cellular demands during signaling, information needs to be transmitted from the OMM to the inner mitochondrial membrane (IMM). How alterations at the OMM are signaled to the IMM and the mitochondrial matrix has largely remained unexplored. We recently identified an innate immune signaling pathway that employs mitochondria for signal amplification and shuttling of activated transcription factors to the nucleus. Binding of the anti-apoptotic tumor necrosis factor (TNF) to its receptor at the plasma membrane induces recruitment of signaling components to the OMM and their activation by ubiquitination and phosphorylation. This remodeling of the OMM goes along with an increase in the mitochondrial membrane potential and respiratory activity and with the maintenance of cristae integrity under pro-apoptotic conditions, indicating signal transmission from the OMM to the IMM. Here, we aim at identifying components of the IMM implicated in anterograde (outside-in) mitochondrial signaling and to determine the underlying mechanisms (AIM 1). A promising target in this context is the mitochondrial contact site and cristae organizing system (MICOS), based on its role in forming contact sites between the IMM and OMM. In AIM 2 we will explore mitochondria as a source of danger signals, such as mtDNA, as an example for retrograde (inside-out) signaling and characterize determinants of IMM integrity that prevent the release of mtDNA in different stress paradigms. With these experiments combining state-of-the-art approaches from cell biology, advanced cellular imaging, biochemistry, proteomics and lipidomics, we expect to reveal key aspects of the IMM architecture and function in innate immune signaling.