Role of the inner mitochondrial membrane in innate immune signaling paradigms

线粒体对于维持细胞和生物体的完整性是必不可少的，基于其在调节细胞代谢和协调细胞命运决定中的多种功能。最近，线粒体已被认为在协调先天免疫信号通路中发挥关键作用。它们的运动性和动态性促进了细胞器间的交流，因此注定要完成这项任务。作为蛋白细菌起源的细胞器，线粒体含有危险信号，需要保护免受免疫监视。另一方面，线粒体已被真核细胞吸收以对细胞损伤作出反应并促进有效的免疫应答。关于线粒体进化以影响细胞信号传导的机制，存在多种难题。在几个范例中，信号传导平台在线粒体外膜（OMM）处组装，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导过程中特定于环境的细胞需求，需要将信息从OMM传递到线粒体内膜（IMM）。OMM的改变如何向IMM和线粒体基质发出信号，这在很大程度上仍未被探索。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激活的转录因子穿梭到细胞核。抗凋亡肿瘤坏死因子（TNF）与其质膜受体的结合诱导信号传导组分向OMM的募集及其通过泛素化和磷酸化的活化。OMM的这种重塑沿着线粒体膜电位和呼吸活性的增加以及在促凋亡条件下嵴完整性的维持，表明从OMM到IMM的信号传递。在这里，我们的目的是确定组件的IMM牵连顺行（外向内）线粒体信号传导，并确定潜在的机制（AIM 1）。在这种情况下，一个有前途的目标是线粒体接触位点和嵴组织系统（MICOS），基于其在形成IMM和OMM之间的接触位点的作用。在AIM 2中，我们将探索线粒体作为危险信号的来源，例如mtDNA，作为逆行（由内而外）信号传导的例子，并描述IMM完整性的决定因素，这些决定因素阻止mtDNA在不同的压力范例中释放。这些实验结合了来自细胞生物学、先进细胞成像、生物化学、蛋白质组学和脂质组学的最先进方法，我们期望揭示IMM结构和先天免疫信号传导功能的关键方面。

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.