Syk and Hrs Regulate TLR3-Mediated Antiviral Response in Murine Astrocytes

TLR3-mediated Astrocyte Responses in High and Normal Glucose Adaptation Differently Regulated by Metformin

Toll-like receptors 3 (TLR3) are innate immune receptors expressed on a wide range of cell types, including glial cells. Inflammatory responses altered by hyperglycemia highlight the need to explore the molecular underpinnings of these changes in cellular models. Therefore, here we estimated TLR3-mediated response of astrocytes cultured at normal (NG, 5 mM) and high (HG, 22.5 mM) glucose concentrations for 48 h before stimulation with polyinosinic:polycytidylic acid Poly(I:C) (PIC) for 6 h. Seahorse Extracellular Flux Analyzer (Seahorse XFp) was used to estimate the extracellular acidification rate (ECAR) and oxygen consumption rate (OCR). Although adaptation to HG affected ECAR and OCR, the stimulation of cells with PIC had no effect on ECAR. PIC reduced maximal OCR, but this effect disappeared upon adaptation to HG. PIC-stimulated release of cytokines IL-1β, IL-10 was reduced, and that of IL-6 and iNOS was increased in the HG model. Adaptation to HG reduced PIC-stimulated synthesis of COX-derived oxylipins measured by ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Adaptation to HG did not alter PIC-stimulated p38 activity, ERK mitogen-activated protein kinase, STAT3 and ROS production. Metformin exhibited anti-inflammatory activity, reducing PIC-stimulated synthesis of cytokines and oxylipins. Cell adaptation to high glucose concentration altered the sensitivity of astrocytes to TLR3 receptor activation, and the hypoglycemic drug metformin may exert anti-inflammatory effects under these conditions.

Virus-Mediated Inhibition of Apoptosis in the Context of EBV-Associated Diseases: Molecular Mechanisms and Therapeutic Perspectives

Epstein-Barr virus (EBV), the representative of the Herpesviridae family, is a pathogen extensively distributed in the human population. One of its most characteristic features is the capability to establish latent infection in the host. The infected cells serve as a sanctuary for the dormant virus, and therefore their desensitization to apoptotic stimuli is part of the viral strategy for long-term survival. For this reason, EBV encodes a set of anti-apoptotic products. They may increase the viability of infected cells and enhance their resistance to chemotherapy, thereby contributing to the development of EBV-associated diseases, including Burkitt’s lymphoma (BL), Hodgkin’s lymphoma (HL), gastric cancer (GC), nasopharyngeal carcinoma (NPC) and several other malignancies. In this paper, we have described the molecular mechanism of anti-apoptotic actions of a set of EBV proteins. Moreover, we have reviewed the pro-survival role of non-coding viral transcripts: EBV-encoded small RNAs (EBERs) and microRNAs (miRNAs), in EBV-carrying malignant cells. The influence of EBV on the expression, activity and/or intracellular distribution of B-cell lymphoma 2 (Bcl-2) protein family members, has been presented. Finally, we have also discussed therapeutic perspectives of targeting viral anti-apoptotic products or their molecular partners.

Syk Facilitates Influenza A Virus Replication by Restraining Innate Immunity at the Late Stage of Viral Infection

Spleen tyrosine kinase (Syk) has recently come forth as a critical regulator of innate immune response. Previous studies identify Syk as a key kinase for STAT1 activation at the early stage of influenza A virus (IAV) infection that is involved in initial antiviral immunity. However, the involvement of Syk in host antiviral immunity during the late phase of IAV infection and its effect on pathogenesis of the virus remain unknown. Here, we found through time course studies that Syk restrained antiviral immune response at the late stage of IAV infection, thereby promoting viral replication. Depletion of Syk suppressed IAV replication in vitro, whereas ectopic expression of Syk facilitated viral replication. Moreover, Syk-deficient mice were employed, and we observed that knockout of Syk rendered mice more resistant to IAV infection, as evidenced by a lower degree of lung injury, slower body weight loss, and an increased survival rate of Syk knockout mice challenged with IAV. Furthermore, we revealed that Syk repressed the interferon response at the late stage of viral infection. Loss of Syk potentiated the expression of type I and III interferons in both Syk-depleted cells and mice. Mechanistically, Syk interacted with TBK1 and modulated its phosphorylation status, thereby impeding TBK1 activation and restraining innate immune signaling that governs interferon response. Together, these findings unveil a role of Syk in temporally regulating host antiviral immunity and advance our understanding of complicated mechanisms underlying regulation of innate immunity against viral invasion. IMPORTANCE Innate immunity must be tightly controlled to eliminate invading pathogens while avoiding autoimmune or inflammatory diseases. Syk is essential for STAT1 activation at the early stage of IAV infection, which is critical for initial antiviral responses. Surprisingly, here a time course study showed that Syk suppressed innate immunity during late phases of IAV infection and thereby promoted IAV replication. Syk deficiency enhanced the expression of type I and III interferons, inhibited IAV replication, and rendered mice more resistant to IAV infection. Syk impaired innate immune signaling through impeding TBK1 activation. These data reveal that Syk participates in the initiation of antiviral defense against IAV infection and simultaneously contributes to the restriction of innate immunity at the late stage of viral infection, suggesting that Syk serves a dual function in regulating antiviral responses. This finding provides new insights into complicated mechanisms underlying interaction between virus and host immune system.

Participation of Endosomes in Toll-Like Receptor 3 Transportation Pathway in Murine Astrocytes

TLR3 provides immediate type I IFN response following entry of stimulatory PAMPs into the CNS, as it is in HSV infection. The receptor plays a vital role in astrocytes, contributing to rapid infection sensing and suppression of viral replication, precluding the spread of virus beyond neurons. The route of TLR3 mobilization culminating in the receptor activation remains unexplained. In this research, we investigated the involvement of various types of endosomes in the regulation of the TLR3 mobility in C8-D1A murine astrocyte cell line. TLR3 was transported rapidly to early EEA1-positive endosomes as well as LAMP1-lysosomes following stimulation with the poly(I:C). Later, TLR3 largely associated with late Rab7-positive endosomes. Twenty-four hours after stimulation, TLR3 co-localized with LAMP1 abundantly in lysosomes of astrocytes. TLR3 interacted with poly(I:C) intracellularly from 1 min to 8 h following cell stimulation. We detected TLR3 on the surface of astrocytes indicating constitutive expression, which increased after poly(I:C) stimulation. Our findings contribute to the understanding of cellular modulation of TLR3 trafficking. Detailed analysis of the TLR3 transportation pathway is an important component in disclosing the fate of the receptor in HSV-infected CNS and may help in the search for rationale therapeutics to control the replication of neuropathic viruses.

The Late Domain of Prototype Foamy Virus Gag Facilitates Autophagic Clearance of Stress Granules by Promoting Amphisome Formation

Prototype foamy virus (PFV), a complex retrovirus belonging to Spumaretrovirinae, maintains lifelong latent infection. The maintenance of lifelong latent infection by viruses relies on the repression of the type I interferon (IFN) response. However, the mechanism involving PFV latency, especially regarding the suppression of the IFN response, is poorly understood. Our previous study showed that PFV promotes autophagic flux. However, the underlying mechanism and the role of PFV-induced autophagy in latent infection have not been clarified. Here, we report that the PFV viral structural protein Gag induced amphisome formation and triggered autophagic clearance of stress granules (SGs) to attenuate type I IFN production. Moreover, the late domain (L-domain) of Gag played a central role in Alix recruitment, which promoted endosomal sorting complex required for transport I (ESCRT-I) formation and amphisome accumulation by facilitating late endosome formation. Our data suggest that PFV Gag represses the host IFN response through autophagic clearance of SGs by activating the endosome-autophagy pathway. More importantly, we found a novel mechanism by which a retrovirus inhibits the SG response to repress the type I IFN response.IMPORTANCE Maintenance of lifelong latent infection for viruses relies on repression of the type I IFN response. Autophagy plays a double-edged sword in antiviral immunity. However, the role of autophagy in the regulation of the type I IFN response and the mechanism involving virus-promoted autophagy have not been fully elucidated. SGs are an immune complex associated with the antiviral immune response and are critical for type I IFN production. Autophagic clearance of SGs is one means of degradation of SGs and is associated with regulation of immunity, but the detailed mechanism remains unclear. In this article, we demonstrate that PFV Gag recruits ESCRT-I to facilitate amphisome formation. Our data also suggest that amphisome formation is a critical event for autophagic clearance of SGs and repression of the type I IFN response. More importantly, we found a novel mechanism by which a retrovirus inhibits the SG response to repress the type I IFN response.