Innate antiviral immunity is the first line of the host defense system that rapidly detects invading viruses. Mitochondria function as platforms for innate antiviral signal transduction in mammals through the adaptor protein, MAVS. Excessive activation of MAVS-mediated antiviral signaling leads to dysfunction of mitochondria and cell apoptosis that likely causes the pathogenesis of autoimmunity. However, the mechanism of how MAVS is regulated at mitochondria remains unknown. Here we show that the Cytochrome c Oxidase (CcO) complex subunit COX5B physically interacts with MAVS and negatively regulates the MAVS-mediated antiviral pathway. Mechanistically, we find that while activation of MAVS leads to increased ROS production and COX5B expression, COX5B down-regulated MAVS signaling by repressing ROS production. Importantly, our study reveals that COX5B coordinates with the autophagy pathway to control MAVS aggregation, thereby balancing the antiviral signaling activity. Thus, our study provides novel insights into the link between mitochondrial electron transport system and the autophagy pathway in regulating innate antiviral immunity. Pattern recognition receptors are vital to innate immunity. In the antiviral innate immunity, retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), such as RIG-I and MDA5, sense viral RNAs through their C-terminal helicase domains, then initiate the antiviral response through interaction with the essential adaptor protein MAVS, which is located in mitochondrial outer membrane. Although cumulative studies have showed that mitochondria-associated MAVS plays an important role in antiviral signaling, much remains unknown about the mechanism of MAVS activity related to mitochondrial membrane localization. In this article we demonstrate that the CcO complex subunit COX5B negatively regulates the MAVS-mediated antiviral pathway through interaction with MAVS. At the mechanistic level, we show that COX5B inhibits MAVS-mediated antiviral pathway by suppressing ROS production, and coordinating with the autophagy pathway to control MAVS aggregation. Our data support a notion that mitochondrial electron transport system coordinates with the autophagy pathway to regulate MAVS-mediated signaling for a tight control of innate antiviral immunity.

Stimulator of interferon genes (STING, also known as MITA and ERIS) is critical in protecting the host against DNA pathogen invasion. However, the molecular mechanism underlying the regulation of STING remains unclear. Here, we show that PPM1A negatively regulates antiviral signaling by targeting STING in its phosphatase activity-dependent manner, and in a line with this, PPM1A catalytically dephosphorylates STING and TBK1 in vitro. Importantly, we provide evidence that whereas TBK1 promotes STING aggregation in a phosphorylation-dependent manner, PPM1A antagonizes STING aggregation by dephosphorylating both STING and TBK1, emphasizing that phosphorylation is crucial for the efficient activation of STING. Our findings demonstrate a novel regulatory circuit in which STING and TBK1 reciprocally regulate each other to enable efficient antiviral signaling activation, and PPM1A dephosphorylates STING and TBK1, thereby balancing this antiviral signal transduction. Innate antiviral immunity is essential for the host defense system that rapidly detects and eliminates invading viruses. STING, an endoplasmic reticulum (ER)-associated protein, plays important roles in the activation of type I IFN in response to DNA virus infection. Whereas excessive activation of STING can potentially cause lethal inflammatory diseases, STING activity must thus be precisely controlled to ensure the proper antiviral signaling transduction. However, the mechanisms of how STING activation is regulated are not fully understood. In this study, we find that PPM1A physically interacts with STING and negatively regulates STING-mediated antiviral signaling. Mechanistically, we find that PPM1A functions as a phosphatase that targets both STING and TBK1 for their dephosphorylation. Moreover, our study demonstrates that while TBK1 enhances STING aggregation in a kinase activity-dependent manner, PPM1A suppresses STING aggregation by dephosphorylating both STING and TBK1. Collectively, our study not only reveals that STING and TBK1 reciprocally regulate each other’s activity to elicit antiviral signaling, but also shows that PPM1A antagonizes STING aggregation by targeting both STING and TBK1, thereby maintaining proper antiviral responses.

Maternal RNA degradation is critical for embryogenesis and is tightly controlled by maternal RNA-binding proteins. Fragile X mental-retardation protein (FMR1) binds target mRNAs to form ribonucleoprotein (RNP) complexes/granules that control various biological processes, including early embryogenesis. However, how FMR1 recognizes target mRNAs and how FMR1-RNP granule assembly/disassembly regulates FMR1-associated mRNAs remain elusive. Here we show that Drosophila FMR1 preferentially binds mRNAs containing m6A-marked “AGACU” motif with high affinity to contributes to maternal RNA degradation. The high-affinity binding largely depends on a hydrophobic network within FMR1 KH2 domain. Importantly, this binding greatly induces FMR1 granule condensation to efficiently recruit unmodified mRNAs. The degradation of maternal mRNAs then causes granule de-condensation, allowing normal embryogenesis. Our findings reveal that sequence-specific mRNAs instruct FMR1-RNP granules to undergo a dynamic phase-switch, thus contributes to maternal mRNA decay. This mechanism may represent a general principle that regulated RNP-granules control RNA processing and normal development.

Innate immunity mediated by Toll signalling has been extensively studied, but how Toll signalling is precisely controlled in balancing innate immune responses remains poorly understood. It was reported that the plasma membrane localization of Drosophila MyD88 is necessary for the recruitment of cytosolic adaptor Tube to the cell surface, thus contributing to Toll signalling transduction. Here we demonstrate that Drosophila Pellino functions as a negative regulator in Toll-mediated signalling. We show that Pellino accumulates at the plasma membrane upon the activation of Toll signalling in a MyD88-dependent manner. Moreover, we find that Pellino is associated with MyD88 via its CTE domain, which is necessary and sufficient to promote Pellino accumulation at the plasma membrane where it targets MyD88 for ubiquitination and degradation. Collectively, our study uncovers a mechanism by which a feedback regulatory loop involving MyD88 and Pellino controls Toll-mediated signalling, thereby maintaining homeostasis of host innate immunity.

Mitochondrial antiviral signalling protein (MAVS) acts as a critical adaptor protein to transduce antiviral signalling by physically interacting with activated RIG-I and MDA5 receptors. MAVS executes its functions at the outer membrane of mitochondria to regulate downstream antiviral signalling, indicating that the mitochondria provides a functional platform for innate antiviral signalling transduction. However, little is known about whether and how MAVS-mediated antiviral signalling contributes to mitochondrial homeostasis. Here we show that the activation of MAVS is sufficient to induce autophagic signalling, which may mediate the turnover of the damaged mitochondria. Importantly, we find MAVS directly interacts with LC3 through its LC3-binding motif ‘YxxI’, suggesting that MAVS might act as an autophagy receptor to mediate mitochondrial turnover upon excessive activation of RLR signalling. Furthermore, we provide evidence that both MAVS self-aggregation and its interaction with TRAF2/6 proteins are important for MAVS-mediated mitochondrial turnover. Collectively, our findings suggest that MAVS acts as a potential receptor for mitochondria-associated autophagic signalling to maintain mitochondrial homeostasis.

Aberrant activation of the cyclic guanosine monophosphate–adenosine monophosphate synthase–stimulator of interferon genes (cGAS-STING) pathway causes autoimmunity in humans and mice; however, the exact mechanism by which the cGAS-STING pathway initiates adaptive immunity and tissue pathology is still not fully understood. Here, we used a cGAS knockin (KI) mouse model that develops systemic autoimmunity. In the lungs of cGAS-KI mice, blood vessels were enclosed by organized lymphoid tissues that resemble tertiary lymphoid structures (TLSs). Cell-intrinsic cGAS induction promoted up-regulation of CCR5 in CD8 ⁺ T cells and led to CCL5 production in vascular endothelial cells. Peripheral CD8 ⁺ T cells were recruited to the lungs and produced CXCL13 and interferon-γ. The latter triggered endothelial cell death, potentiated CCL5 production, and was essential for TLS establishment. Blocking CCL5 or CCR5, or depleting CD8 ⁺ T cells, impaired TLS formation. cGAS-mediated TLS formation also enhanced humoral and antitumor responses. These data demonstrate that cGAS signaling drives a specialized lymphoid structure that underlies autoimmune tissue pathology.