Dengue Virus Induced COX-2 Signaling Is Regulated Through Nutrient Sensor GCN2

GCN2 in Viral Defence and the Subversive Tactics Employed by Viruses

The recent SARS-CoV-2 pandemic and associated COVID19 disease illustrates the important role of viral defence mechanisms in ensuring survival and recovery of the host or patient. Viruses absolutely depend on the host's protein synthesis machinery to replicate, meaning that impeding translation is a powerful way to counteract viruses. One major approach used by cells to obstruct protein synthesis is to phosphorylate the alpha subunit of eukaryotic translation initiation factor 2 (eIF2α). Mammals possess four different eIF2α-kinases: PKR, HRI, PEK/PERK, and GCN2. While PKR is currently considered the principal eIF2α-kinase involved in viral defence, the other eIF2α-kinases have also been found to play significant roles. Unsurprisingly, viruses have developed mechanisms to counteract the actions of eIF2α-kinases, or even to exploit them to their benefit. While some of these virulence factors are specific to one eIF2α-kinase, such as GCN2, others target all eIF2α-kinases. This review critically evaluates the current knowledge of viral mechanisms targeting the eIF2α-kinase GCN2. A detailed and in-depth understanding of the molecular mechanisms by which viruses evade host defence mechanisms will help to inform the development of powerful anti-viral measures.

Cellular Stress Responses against Coronavirus Infection: A Means of the Innate Antiviral Defense

Cellular stress responses are crucial for maintaining cellular homeostasis. Stress granules (SGs), activated by eIF2α kinases in response to various stimuli, play a pivotal role in dealing with diverse stress conditions. Viral infection, as one kind of cellular stress, triggers specific cellular programs aimed at overcoming virus-induced stresses. Recent studies have revealed that virus-derived stress responses are tightly linked to the host's antiviral innate immunity. Virus infection-induced SGs act as platforms for antiviral sensors, facilitating the initiation of protective antiviral responses called "antiviral stress granules" (avSGs). However, many viruses, including coronaviruses, have evolved strategies to suppress avSG formation, thereby counteracting the host's immune responses. This review discusses the intricate relationship between cellular stress responses and antiviral innate immunity, with a specific focus on coronaviruses. Furthermore, the diverse mechanisms employed by viruses to counteract avSGs are described.

A little less aggregation a little more replication: Viral manipulation of stress granules

Recent exciting studies have uncovered how membrane-less organelles, also known as biocondensates, are providing cells with rapid response pathways, allowing them to re-organize their cellular contents and adapt to stressful conditions. Their assembly is driven by the phase separation of their RNAs and intrinsically disordered protein components into condensed foci. Among these, stress granules (SGs) are dynamic cytoplasmic biocondensates that form in response to many stresses, including activation of the integrated stress response or viral infections. SGs sit at the crossroads between antiviral signaling and translation because they concentrate signaling proteins and components of the innate immune response, in addition to translation machinery and stalled mRNAs. Consequently, they have been proposed to contribute to antiviral activities, and therefore are targeted by viral countermeasures. Equally, SGs components can be commandeered by viruses for their own efficient replication. Phase separation processes are an important component of the viral life cycle, for example, driving the assembly of replication factories or inclusion bodies. Therefore, in this review, we will outline the recent understanding of this complex interplay and tug of war between viruses, SGs, and their components. This article is categorized under: RNA in Disease and Development > RNA in Disease Translation > Regulation RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

Milk exosomes elicit a potent anti-viral activity against dengue virus

Background

Exosomes are nano-sized vesicles secreted by various cells into the intra and extracellular space and hence is an integral part of biological fluids including milk. In the last few decades, many research groups have proved the potential of milk exosomes as a sustainable, economical and non-immunogenic drug delivery and therapeutic agent against different pathological conditions. However, its anti-viral properties still remain to be unearthed.

Methods

Here, we have been able to isolate, purify and characterize the milk derived exosomes from Cow (CME) and Goat (GME) and further studied its antiviral properties against Dengue virus (DENV), Newcastle Disease Virus strain Komarov (NDV-K) and Human Immunodeficiency Virus (HIV-1) using an in-vitro infection system.

Results

TEM, NTA and DLS analysis validated the appropriate size of the isolated cow and goat milk exosomes (30–150 nm). Real-time PCR and immunoblotting results confirmed the presence of several milk exosomal miRNAs and protein markers. Our findings suggest that GME significantly decreased the infectivity of DENV. In addition, we confirmed that GME significantly reduces DENV replication and reduced the secretion of mature virions. Furthermore, heat inactivation of GME did not show any inhibition on DENV infection, replication, and secretion of mature virions. RNase treatment of GME abrogates the anti-viral properties indicating direct role of exosomes in DENV inhibition. In addition GME inhibited the infectivity of NDV-K, but not HIV-1, suggesting that the GME mediated antiviral activity might be virus specific.

Conclusion

This study demonstrates the anti-viral properties of milk exosomes and opens new avenues for the development of exosome-based therapies to treat viral diseases.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01496-5.

Immunogenic profiling of Mycobacterium tuberculosis DosR protein Rv0569 reveals its ability to switch on Th1 based immunity

Mycobacterium tuberculosis (M.tb) is a multifaceted bacterial pathogen known to infect more than 2 billion people globally. However, a majority of the individuals (>90%) show no overt clinical symptoms of active Tuberculosis (TB) and, it is reported that M.tb in these individuals resides in the latent form. Therefore, huge burden of latently infected population poses serious threat to human health. Inconsistent efficacy of BCG vaccine and poor understanding of latency-associated determinants contribute to the failure of combating M.tb. The discovery of DosR as the master regulator of dormancy, opened new avenues to understand the pathophysiology of the bacterium. Though the specific functions of various DosR genes are yet to be discovered, they have been reported as potent T-cell activators and could elicit strong protective immune responses. Rv0569 is a DosR-encoded conserved hypothetical protein overexpressed during dormancy. However, it is not clearly understood how this protein modulates the host immune response. In the present study, we have demonstrated that Rv0569 has a high antigenic index and induces enhanced secretion of Th1 cytokines IL-12p40 and TNF-α as compared to Th2 cytokine IL-10 in macrophages. Mechanistically, Rv0569 induced the transcription of these pro-inflammatory signatures through the activation of NF-κB pathway. Further, immunization of mice with DosR protein Rv0569 switched the immune response towards Th1-biased cytokine pattern, characterized by the enhanced production of IFN-γ, IL-12p40, and TNF-α. Rv0569 augmented the expansion of antigen-specific IFN-γ and IL-2 producing effector CD4⁺ and CD8⁺ T-cells which are hallmarks of Th1 biased protective immunity. Additionally, IgG2a/IgG1 and IgG2b/IgG1 ratio in the serum of immunized mice further confirmed the ability of Rv0569 to skew Th1 biased immune response. In conclusion, we emphasize that Rv0569 has the ability to generate signals to switch on Th1-dominated responses and further suggest that it could be a potential vaccine candidate against latent M.tb infection.

Amino acid metabolism and signalling pathways: potential targets in the control of infection and immunity

Defences to pathogens such as SarCoV2 in mammals involves interactions between immune functions and metabolic pathways to eradicate infection while preventing hyperinflammation. Amino acid metabolic pathways represent with other antimicrobial agent potential targets for therapeutic strategies. iNOS-mediated production of NO from Arg is involved in the innate inflammatory response to pathogens and NO overproduction can induce hyperinflammation. The two Arg- and Trp-catabolising enzymes Arg1 and IDO1 reduce the hyperinflammation by an immunosuppressive effect via either Arg starvation (for Arg1) or via the immunoregulatory activity of the Trp-derived metabolites Kyn (for IDO1). In response to amino acid abundance mTOR activates the host protein translation and Coronaviruses use this machinery for their own protein synthesis and replication. In contrast GCN2, the sensor of amino acid starvation, activates pathways that restrict inflammation and viral replication. Gln depletion alters the immune response that become more suppressive, by favouring a regulatory T phenotype rather than a Th1 phenotype. Proliferating activated immune cells are highly dependent on Ser, activation and differentiation of T cells need enough Ser and dietary Ser restriction can inhibit their proliferation. Cys is strictly required for T-cell proliferation because they cannot convert Met to Cys. Restricting Met inhibits both viral RNA cap methylation and replication, and the proliferation of infected cells with an increased requirement for Met. Phe catabolism produces antimicrobial metabolites resulting in the inhibition of microbial growth and an immunosuppressive activity towards T lymphocytes.

Amino acid metabolism and signalling pathways: potential targets in the control of infection and immunity

Defences to pathogens such as SarCoV2 in mammals involves interactions between immune functions and metabolic pathways to eradicate infection while preventing hyperinflammation. Amino acid metabolic pathways represent with other antimicrobial agent potential targets for therapeutic strategies. iNOS-mediated production of NO from Arg is involved in the innate inflammatory response to pathogens and NO overproduction can induce hyperinflammation. The two Arg-catabolising enzymes Arg1 and IDO1 reduce the hyperinflammation by an immunosuppressive effect via either Arg starvation (for Arg1) or via the immunoregulatory activity of the Arg-derived metabolites Kyn (for IDO1). In response to amino acid abundance mTOR activates the host protein translation and Coronaviruses use this machinery for their own protein synthesis and replication. In contrast GCN2, the sensor of amino acid starvation, activates pathways that restrict inflammation and viral replication. Gln depletion alters the immune response that become more suppressive, by favouring a regulatory T phenotype rather than a Th1 phenotype. Proliferating activated immune cells are highly dependent on Ser, activation and differentiation of T cells need enough Ser and dietary Ser restriction can inhibit their proliferation. Cys is strictly required for T-cell proliferation because they cannot convert Met to Cys. Restricting Met inhibits both viral RNA cap methylation and replication, and the proliferation of infected cells with an increased requirement for Met. Phe catabolism produces antimicrobial metabolites resulting in the inhibition of microbial growth and an immunosuppressive activity towards T lymphocytes.

Amino acid sensing pathway: A major check point in the pathogenesis of obesity and COVID-19

Obesity and obesogenic comorbidities have been associated with COVID-19 susceptibility and mortality. However, the mechanism of such correlations requires an in-depth understanding. Overnutrition/excess serum amino acid profile during obesity has been linked with inflammation and reprogramming of translational machinery through hyperactivation of amino acid sensor mammalian target of rapamycin (mTOR), which is exploited by SARS-CoV-2 for its replication. Conversely, we have shown that the activation of general control nonderepressible 2 (GCN2)-dependent amino acid starvation sensing pathway suppresses intestinal inflammation by inhibiting the production of reactive oxygen species (ROS) and interleukin-1 beta (IL-1β). While activation of GCN2 has shown to mitigate susceptibility to dengue infection, GCN2 deficiency increases viremia and inflammation-associated pathologies. These findings reveal that the amino acid sensing pathway plays a significant role in controlling inflammation and viral infections. The current fact is that obesity/excess amino acids/mTOR activation aggravates COVID-19, and it might be possible that activation of amino acid starvation sensor GCN2 has an opposite effect. This article focuses on the amino acid sensing pathways through which host cells sense the availability of amino acids and reprogram the host translation machinery to mount an effective antiviral response. Besides, how SARS-CoV-2 hijack and exploit amino acid sensing pathway for its replication and pathogenesis is also discussed.