Hepatitis C Virus NS5A Targets the Nucleosome Assembly Protein NAP1L1 to Control the Interferon Response

Hepatitis C virus (HCV) is a hepatotropic virus affecting more than 150 million people worldwide. HCV establishes a chronic infection in the majority of cases, which leads to severe liver complications such as cirrhosis and hepatocellular carcinoma. Fortunately, recent potent direct acting antivirals can now cure the infection. However, such treatments can induce resistance, are extremely costly limiting their use to wealthier countries and are ineffective for the complications of the infection. Therefore, a better understanding of the interaction of HCV with the host cell remains a priority both to increase the armamentarium of antiviral drugs and to define the relationship between infection and malignant transformation. My study focuses on the mechanisms governing the evasion of the innate immune system, which are required to establish a chronic infection. The non-structural viral protein NS5A has the capacity to interact with a large number of cellular factors involved in promoting viral replication/assembly and in the cell antiviral response to HCV. Interaction of NS5A with the nucleosome assembly protein NAP1L1 has been recently characterized in my laboratory. NAP1L1 is a histone chaperone protein with various functions related to nuclear chromatin remodelling that impact on the regulation of cell cycle, on cell differentiation and on transcription. I have confirmed the interaction of NS5A with NAP1L1 in the cytoplasm and demonstrated the NS5A-dependent impairment of NAP1L1 nuclear translocation. Whole genome transcription analysis performed in NAP1L1 depleted hepatocytes indicated that its nuclear function might be essential for the transcriptional control of several interferon stimulated genes and the function of key innate immunity pathways. Indeed, I was able to demonstrate that NAP1L1 is a novel factor involved in the interferon response and specifically modulates TBK1/IKKε mediated IRF-3 phosphorylation and NF-κB levels. Hence, both the TLR3 and RIG-I/MDA5 pathways are affected by NAP1L1 depletion. I could further demonstrate that NAP1L1 controls the basal transcription of genes involved in the immune pathway and that it interacts with the adaptor protein MAVS, which is required for RIG-I/MDA5 signalling. In conclusion, by studying the interaction of the viral protein NS5A with the cellular factor NAP1L1 I could discover a novel mechanism of regulation of the innate response mediated by NAP1L1. These findings have wider implications for HCV and beyond, further highlighting the importance of studying viruses to uncover cellular functions

Hepatitis C virus NS5A targets the nucleosome assembly protein NAP1L1 to control the innate cellular response

Hepatitis C virus (HCV) is a single-stranded positive-sense RNA hepatotropic virus. Despite cellular defenses, HCV is able to replicate in hepatocytes and to establish a chronic infection that could lead to severe complications and hepatocellular carcinoma. An important player in subverting the host response to HCV infection is the viral non-structural protein NS5A that, in addition to its role in replication and assembly, targets several pathways involved in the cellular response to viral infection. Several unbiased screens identified the nucleosome-assembly protein 1-like 1 (NAP1L1) as an interaction partner of HCV NS5A. Here we confirm this interaction and map it to the C-terminus of NS5A of both genotype 1 and 2. NS5A sequesters NAP1L1 in the cytoplasm blocking its nuclear translocation. However, only NS5A from genotype 2 HCV, but not from genotype 1, targets NAP1L1 for proteosomal-mediated degradation. NAP1L1 is a nuclear chaperone involved in chromatin remodeling and we demonstrate the NAP1L1-dependent regulation of specific pathways involved in cellular responses to viral infection and cell survival. Among those we show that lack of NAP1L1 leads to a decrease of RELA protein levels and a strong defect of IRF3 TBK1/IKKϵ-mediated phosphorylation leading to inefficient RIG-I and TLR3 responses. Hence, HCV is able to modulate the host cell environment by targeting NAP1L1 through NS5A

HSV-1 miRNAs are post-transcriptionally edited in latently infected human ganglia

Herpes simplex virus 1 is an important human pathogen that has been intensively studied for many decades. Nevertheless, the molecular mechanisms regulating its establishment, maintenance, and reactivation from latency are poorly understood. Here, we show that HSV-1-encoded miR-H2 is post-transcriptionally edited in latently infected human tissues. Hyperediting of viral miRNAs increases the targeting potential of these miRNAs and may play an important role in regulating latency. We show that the edited miR-H2 can target ICP4, an essential viral protein. Interestingly, we found no evidence of hyperediting of its homolog, miR-H2, which is expressed by the closely related virus HSV-2. The discovery of post-translational modifications of viral miRNA in the latency phase suggests that these processes may also be important for other non-coding viral RNA in the latency phase, including the intron LAT, which in turn may be crucial for understanding the biology of this virus