G3BP1 restricts HIV-1 replication in macrophages and T-cells by sequestering viral RNA

Post-Transcriptional HIV-1 Latency: A Promising Target for Therapy?

Human Immunodeficiency Virus type 1 (HIV-1) latency represents a significant hurdle in finding a cure for HIV-1 infections, despite tireless research efforts. This challenge is partly attributed to the intricate nature of HIV-1 latency, wherein various host and viral factors participate in multiple physiological processes. While substantial progress has been made in discovering therapeutic targets for HIV-1 transcription, targets for the post-transcriptional regulation of HIV-1 infections have received less attention. However, cumulative evidence now suggests the pivotal contribution of post-transcriptional regulation to the viral latency in both in vitro models and infected individuals. In this review, we explore recent insights on post-transcriptional latency in HIV-1 and discuss the potential of its therapeutic targets, illustrating some host factors that restrict HIV-1 at the post-transcriptional level.

Role(s) of G3BPs in Human Pathogenesis

Ras-GTPase-activating protein (SH3 domain)-binding proteins (G3BP) are RNA binding proteins that play a critical role in stress granule (SG) formation. SGs protect critical mRNAs from various environmental stress conditions by regulating mRNA stability and translation to maintain regulated gene expression. Recent evidence suggests that G3BPs can also regulate mRNA expression through interactions with RNA outside of SGs. G3BPs have been associated with a number of disease states, including cancer progression, invasion, metastasis, and viral infections, and may be useful as a cancer therapeutic target. This review summarizes the biology of G3BP including their structure, function, localization, role in cancer progression, virus replication, mRNA stability, and SG formation. We will also discuss the potential of G3BPs as a therapeutic target. SIGNIFICANCE STATEMENT: This review will discuss the molecular mechanism(s) and functional role(s) of Ras-GTPase-activating protein (SH3 domain)-binding proteins in the context of stress granule formation, interaction with viruses, stability of RNA, and tumorigenesis.

HIV silencing and cell survival signatures in infected T cell reservoirs

Rare CD4 T cells that contain HIV under antiretroviral therapy represent an important barrier to HIV cure1-3, but the infeasibility of isolating and characterizing these cells in their natural state has led to uncertainty about whether they possess distinctive attributes that HIV cure-directed therapies might exploit. Here we address this challenge using a microfluidic technology that isolates the transcriptomes of HIV-infected cells based solely on the detection of HIV DNA. HIV-DNA+ memory CD4 T cells in the blood from people receiving antiretroviral therapy showed inhibition of six transcriptomic pathways, including death receptor signalling, necroptosis signalling and antiproliferative Gα12/13 signalling. Moreover, two groups of genes identified by network co-expression analysis were significantly associated with HIV-DNA+ cells. These genes (n = 145) accounted for just 0.81% of the measured transcriptome and included negative regulators of HIV transcription that were higher in HIV-DNA+ cells, positive regulators of HIV transcription that were lower in HIV-DNA+ cells, and other genes involved in RNA processing, negative regulation of mRNA translation, and regulation of cell state and fate. These findings reveal that HIV-infected memory CD4 T cells under antiretroviral therapy are a distinctive population with host gene expression patterns that favour HIV silencing, cell survival and cell proliferation, with important implications for the development of HIV cure strategies.

The roles of G3BP1 in human diseases (review)

Ras-GTPase-activating protein binding protein 1 (G3BP1) is a multifunctional binding protein involved in a variety of biological functions, including cell proliferation, metastasis, apoptosis, differentiation and RNA metabolism. It has been revealed that G3BP1, as an antiviral factor, can interact with viral proteins and regulate the assembly of stress granules (SGs), which can inhibit viral replication. Furthermore, several viruses have the ability to hijack G3BP1 as a cofactor, recruiting translation initiation factors to promote viral proliferation. However, many functions of G3BP1 are associated with other diseases. In various cancers, G3BP1 is a cancer-promoting factor, which can promote the proliferation, invasion and metastasis of cancer cells. Moreover, compared with normal tissues, G3BP1 expression is higher in tumor tissues, indicating that it can be used as an indicator for cancer diagnosis. In this review, the structure of G3BP1 and the regulation of G3BP1 in multiple dimensions are described. In addition, the effects and potential mechanisms of G3BP1 on various carcinomas, viral infections, nervous system diseases and cardiovascular diseases are elucidated, which may provide a direction for clinical applications of G3BP1 in the future.

Cytoplasmic RNA sensors and their interplay with RNA-binding partners in innate antiviral response: theme and variations

Sensing of pathogen-associated molecular patterns including viral RNA by innate immunity represents the first line of defense against viral infection. In addition to RIG-I-like receptors and NOD-like receptors, several other RNA sensors are known to mediate innate antiviral response in the cytoplasm. Double-stranded RNA-binding protein PACT interacts with prototypic RNA sensor RIG-I to facilitate its recognition of viral RNA and induction of host interferon response, but variations of this theme are seen when the functions of RNA sensors are modulated by other RNA-binding proteins to impinge on antiviral defense, proinflammatory cytokine production and cell death programs. Their discrete and coordinated actions are crucial to protect the host from infection. In this review, we will focus on cytoplasmic RNA sensors with an emphasis on their interplay with RNA-binding partners. Classical sensors such as RIG-I will be briefly reviewed. More attention will be brought to new insights on how RNA-binding partners of RNA sensors modulate innate RNA sensing and how viruses perturb the functions of RNA-binding partners.

Research Progress on the Structure and Function of G3BP

Ras-GTPase-activating protein (SH3 domain)-binding protein (G3BP) is an RNA binding protein. G3BP is a key component of stress granules (SGs) and can interact with many host proteins to regulate the expression of SGs. As an antiviral factor, G3BP can interact with viral proteins to regulate the assembly of SGs and thus exert antiviral effects. However, many viruses can also use G3BP as a proximal factor and recruit translation initiation factors to promote viral proliferation. G3BP regulates mRNA translation and attenuation to regulate gene expression; therefore, it is closely related to diseases, such as cancer, embryonic death, arteriosclerosis, and neurodevelopmental disorders. This review discusses the important discoveries and developments related G3BP in the biological field over the past 20 years, which includes the formation of SGs, interaction with viruses, stability of RNA, and disease progression.

Targeting of viral RNAs by Upf1-mediated RNA decay pathways

Viral RNAs are susceptible to co-translational RNA decay pathways mediated by the RNA helicase Upstream frameshift 1 (Upf1). Upf1 is a key component in nonsense-mediated decay (NMD), Staufen1-mediated mRNA decay (SMD), and structure-mediated RNA decay (SRD) pathways, among others. Diverse families of viruses have features that predispose them to Upf1 targeting, but have evolved means to escape decay through the action of cis-acting or trans-acting viral factors. Studies aimed at understanding how viruses are subjected to and circumvent NMD have increased our understanding of NMD target selection of host mRNAs. This review focuses on the knowledge gained from studying NMD in viral systems as well as related Upf1-dependent pathways and how these pathways restrict virus replication.

The Role of Macrophages in HIV-1 Persistence and Pathogenesis

Current antiretroviral therapy (ART) effectively suppresses Human Immunodeficiency Virus type 1 (HIV-1) in infected individuals. However, even long term ART does not eradicate HIV-1 infected cells and the virus persists in cellular reservoirs. Beside memory CD4⁺ T cells, cells of the myeloid lineage, especially macrophages, are believed to be an important sanctuary for HIV-1. Monocytes and macrophages are key players in the innate immune response to pathogens and are recruited to sites of infection and inflammation. Due to their long life span and ability to reside in virtually every tissue, macrophages have been proposed to play a critical role in the establishment and persistence of the HIV-1 reservoir. Current HIV-1 cure strategies mainly focus on the concept of “shock and kill” to purge the viral reservoir. This approach aims to reactivate viral protein production in latently infected cells, which subsequently are eliminated as a consequence of viral replication, or recognized and killed by the immune system. Macrophage susceptibility to HIV-1 infection is dependent on the local microenvironment, suggesting that molecular pathways directing differentiation and polarization are involved. Current latency reversing agents (LRA) are mainly designed to reactivate the HIV-1 provirus in CD4+ T cells, while their ability to abolish viral latency in macrophages is largely unknown. Moreover, the resistance of macrophages to HIV-1 mediated kill and the presence of infected macrophages in immune privileged regions including the central nervous system (CNS), may pose a barrier to elimination of infected cells by current “shock and kill” strategies. This review focusses on the role of monocytes/macrophages in HIV-1 persistence. We will discuss mechanisms of viral latency and persistence in monocytes/macrophages. Furthermore, the role of these cells in HIV-1 tissue distribution and pathogenesis will be discussed.

Strategies for Success. Viral Infections and Membraneless Organelles

Regulation of RNA homeostasis or “RNAstasis” is a central step in eukaryotic gene expression. From transcription to decay, cellular messenger RNAs (mRNAs) associate with specific proteins in order to regulate their entire cycle, including mRNA localization, translation and degradation, among others. The best characterized of such RNA-protein complexes, today named membraneless organelles, are Stress Granules (SGs) and Processing Bodies (PBs) which are involved in RNA storage and RNA decay/storage, respectively. Given that SGs and PBs are generally associated with repression of gene expression, viruses have evolved different mechanisms to counteract their assembly or to use them in their favor to successfully replicate within the host environment. In this review we summarize the current knowledge about the viral regulation of SGs and PBs, which could be a potential novel target for the development of broad-spectrum antiviral therapies.

Separate domains of G3BP promote efficient clustering of alphavirus replication complexes and recruitment of the translation initiation machinery

G3BP-1 and -2 (hereafter referred to as G3BP) are multifunctional RNA-binding proteins involved in stress granule (SG) assembly. Viruses from diverse families target G3BP for recruitment to replication or transcription complexes in order to block SG assembly but also to acquire pro-viral effects via other unknown functions of G3BP. The Old World alphaviruses, including Semliki Forest virus (SFV) and chikungunya virus (CHIKV) recruit G3BP into viral replication complexes, via an interaction between FGDF motifs in the C-terminus of the viral non-structural protein 3 (nsP3) and the NTF2-like domain of G3BP. To study potential proviral roles of G3BP, we used human osteosarcoma (U2OS) cell lines lacking endogenous G3BP generated using CRISPR-Cas9 and reconstituted with a panel of G3BP1 mutants and truncation variants. While SFV replicated with varying efficiency in all cell lines, CHIKV could only replicate in cells expressing G3BP1 variants containing both the NTF2-like and the RGG domains. The ability of SFV to replicate in the absence of G3BP allowed us to study effects of different domains of the protein. We used immunoprecipitation to demonstrate that that both NTF2-like and RGG domains are necessary for the formation a complex between nsP3, G3BP1 and the 40S ribosomal subunit. Electron microscopy of SFV-infected cells revealed that formation of nsP3:G3BP1 complexes via the NTF2-like domain was necessary for clustering of cytopathic vacuoles (CPVs) and that the presence of the RGG domain was necessary for accumulation of electron dense material containing G3BP1 and nsP3 surrounding the CPV clusters. Clustered CPVs also exhibited localised high levels of translation of viral mRNAs as detected by ribopuromycylation staining. These data confirm that G3BP is a ribosomal binding protein and reveal that alphaviral nsP3 uses G3BP to concentrate viral replication complexes and to recruit the translation initiation machinery, promoting the efficient translation of viral mRNAs.

In order to repel viral infections, cells activate stress responses. One such response involves inhibition of translation and restricted availability of the translation machinery via the formation of stress granules. However, the host translation machinery is absolutely essential for synthesis of viral proteins and consequently viruses have developed a broad spectrum of strategies to circumvent this restriction. Old World alphaviruses, such as Semliki Forest virus (SFV) and chikungunya virus (CHIKV), interfere with stress granule formation by sequestration of G3BP, a stress granule nucleating protein, mediated by the viral non-structural protein 3 (nsP3). Here we show that nsP3:G3BP complexes engage factors of the host translation machinery, which during the course of infection accumulate in the vicinity of viral replication complexes. Accordingly, we demonstrate that the nsP3:G3BP interaction is required for high localized translational activity around viral replication complexes. We find the RGG domain of G3BP to be essential for the recruitment of the host translation machinery. In cells expressing mutant G3BP lacking the RGG domain, SFV replication was attenuated, but detectable, while CHIKV was essentially non-viable. Our data demonstrate a novel mechanism by which viruses can recruit factors of the translation machinery in a G3BP-dependent manner.

The stress granule protein G3BP1 binds viral dsRNA and RIG-I to enhance interferon-β response

RIG-I senses viral RNA in the cytosol and initiates host innate immune response by triggering the production of type 1 interferon. A recent RNAi knockdown screen yielded close to hundred host genes whose products affected viral RNA-induced IFN-β production and highlighted the complexity of the antiviral response. The stress granule protein G3BP1, known to arrest mRNA translation, was identified as a regulator of RIG-I-induced IFN-β production. How G3BP1 functions in RIG-I signaling is not known, however. Here, we overexpress G3BP1 with RIG-I in HEK293T cells and found that G3BP1 significantly enhances RIG-I-induced ifn-b mRNA synthesis. More importantly, we demonstrate that G3BP1 binds RIG-I and that this interaction involves the C-terminal RGG domain of G3BP1. Confocal microscopy studies also show G3BP1 co-localization with RIG-I and with infecting vesicular stomatitis virus in Cos-7 cells. Interestingly, immunoprecipitation studies using biotin-labeled viral dsRNA or poly(I·C) and cell lysate-derived or in vitro translated G3BP1 indicated that G3BP1 could directly bind these substrates and again via its RGG domain. Computational modeling further revealed a juxtaposed interaction between G3BP1 RGG and RIG-I RNA-binding domains. Together, our data reveal G3BP1 as a critical component of RIG-I signaling and possibly acting as a co-sensor to promote RIG-I recognition of pathogenic RNA.

The Role of Macrophages in HIV-1 Persistence and Pathogenesis

Current antiretroviral therapy (ART) effectively suppresses Human Immunodeficiency Virus type 1 (HIV-1) in infected individuals. However, even long term ART does not eradicate HIV-1 infected cells and the virus persists in cellular reservoirs. Beside memory CD4⁺ T cells, cells of the myeloid lineage, especially macrophages, are believed to be an important sanctuary for HIV-1. Monocytes and macrophages are key players in the innate immune response to pathogens and are recruited to sites of infection and inflammation. Due to their long life span and ability to reside in virtually every tissue, macrophages have been proposed to play a critical role in the establishment and persistence of the HIV-1 reservoir. Current HIV-1 cure strategies mainly focus on the concept of "shock and kill" to purge the viral reservoir. This approach aims to reactivate viral protein production in latently infected cells, which subsequently are eliminated as a consequence of viral replication, or recognized and killed by the immune system. Macrophage susceptibility to HIV-1 infection is dependent on the local microenvironment, suggesting that molecular pathways directing differentiation and polarization are involved. Current latency reversing agents (LRA) are mainly designed to reactivate the HIV-1 provirus in CD4+ T cells, while their ability to abolish viral latency in macrophages is largely unknown. Moreover, the resistance of macrophages to HIV-1 mediated kill and the presence of infected macrophages in immune privileged regions including the central nervous system (CNS), may pose a barrier to elimination of infected cells by current "shock and kill" strategies. This review focusses on the role of monocytes/macrophages in HIV-1 persistence. We will discuss mechanisms of viral latency and persistence in monocytes/macrophages. Furthermore, the role of these cells in HIV-1 tissue distribution and pathogenesis will be discussed.

Rasputin a decade on and more promiscuous than ever? A review of G3BPs

Ras-GTPase-activating protein (SH3 domain)-binding proteins (G3BPs, also known as Rasputin) are a family of RNA binding proteins that regulate gene expression in response to environmental stresses by controlling mRNA stability and translation. G3BPs appear to facilitate this activity through their role in stress granules for which they are considered a core component, however, it should be noted that not all stress granules contain G3BPs and this appears to be contextual depending on the environmental stress and the cell type. Although the role of G3BPs in stress granules appears to be one of its major roles, data also strongly suggests that they interact with mRNAs outside of stress granules to regulate gene expression. G3BPs have been implicated in several diseases including cancer progression, invasion, and metastasis as well as virus survival. There is now a body of evidence that suggests targeting of G3BPs could be explored as a form of cancer therapeutic. This review discusses the important discoveries and advancements made in the field of G3BPs biology over the last two decades including their roles in RNA stability, translational control of cellular transcripts, stress granule formation, cancer progression and its interactions with viruses during infection. An emerging theme for G3BPs is their ability to regulate gene expression in response to environmental stimuli, disease progression and virus infection making it an intriguing target for disease therapies.

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Triage of many cellular mRNA occurs via stress granules in a G3BP-dependant manner.

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G3BPs control intra cellular responses to viral infection.

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Transcript stability, degradation and translation are controlled by G3BPs.

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G3BPs can control cancer progression.

The Interplay of Viral and Host Factors in Chikungunya Virus Infection: Targets for Antiviral Strategies

Chikungunya virus (CHIKV) has re-emerged as one of the many medically important arboviruses that have spread rampantly across the world in the past decade. Infected patients come down with acute fever and rashes, and a portion of them suffer from both acute and chronic arthralgia. Currently, there are no targeted therapeutics against this debilitating virus. One approach to develop potential therapeutics is by understanding the viral-host interactions. However, to date, there has been limited research undertaken in this area. In this review, we attempt to briefly describe and update the functions of the different CHIKV proteins and their respective interacting host partners. In addition, we also survey the literature for other reported host factors and pathways involved during CHIKV infection. There is a pressing need for an in-depth understanding of the interaction between the host environment and CHIKV in order to generate potential therapeutics.

Phospho-Rasputin Stabilization by Sec16 Is Required for Stress Granule Formation upon Amino Acid Starvation

Most cellular stresses induce protein translation inhibition and stress granule formation. Here, using Drosophila S2 cells, we investigate the role of G3BP/Rasputin in this process. In contrast to arsenite treatment, where dephosphorylated Ser142 Rasputin is recruited to stress granules, we find that, upon amino acid starvation, only the phosphorylated Ser142 form is recruited. Furthermore, we identify Sec16, a component of the endoplasmic reticulum exit site, as a Rasputin interactor and stabilizer. Sec16 depletion results in Rasputin degradation and inhibition of stress granule formation. However, in the absence of Sec16, pharmacological stabilization of Rasputin is not enough to rescue the assembly of stress granules. This is because Sec16 specifically interacts with phosphorylated Ser142 Rasputin, the form required for stress granule formation upon amino acid starvation. Taken together, these results demonstrate that stress granule formation is fine-tuned by specific signaling cues that are unique to each stress. These results also expand the role of Sec16 as a stress response protein.

Battling for Ribosomes: Translational Control at the Forefront of the Antiviral Response

In the early stages of infection, gaining control of the cellular protein synthesis machinery including its ribosomes is the ultimate combat objective for a virus. To successfully replicate, viruses unequivocally need to usurp and redeploy this machinery for translation of their own mRNA. In response, the host triggers global shutdown of translation while paradoxically allowing swift synthesis of antiviral proteins as a strategy to limit collateral damage. This fundamental conflict at the level of translational control defines the outcome of infection. As part of this special issue on molecular mechanisms of early virus-host cell interactions, we review the current state of knowledge regarding translational control during viral infection with specific emphasis on protein kinase RNA-activated and mammalian target of rapamycin-mediated mechanisms. We also describe recent technological advances that will allow unprecedented insight into how viruses and host cells battle for ribosomes.

Emerging Roles of N6-Methyladenosine on HIV-1 RNA Metabolism and Viral Replication

N⁶-methyladenosine (m⁶A) is the most abundant internal modification present in Eukaryotic mRNA. The functions of this chemical modification are mediated by m⁶A-binding proteins (m⁶A readers) and regulated by methyltransferases (m⁶A writers) and demethylases (m⁶A erasers), which together are proposed to be responsible of a new layer of post-transcriptional control of gene expression. Despite the presence of m⁶A in a retroviral genome was reported more than 40 years ago, the recent development of sequencing-based technologies allowing the mapping of m⁶A in a transcriptome-wide manner made it possible to identify the topology and dynamics of m⁶A during replication of HIV-1 as well as other viruses. As such, three independent groups recently reported the presence of m⁶A along the HIV-1 genomic RNA (gRNA) and described the impact of cellular m⁶A writers, erasers and readers on different steps of viral RNA metabolism and replication. Interestingly, while two groups reported a positive role of m⁶A at different steps of viral gene expression it was also proposed that the presence of m⁶A within the gRNA reduces viral infectivity by inducing the early degradation of the incoming viral genome. This review summarizes the recent advances in this emerging field and discusses the relevance of m⁶A during HIV-1 replication.

HIV-1 NC-induced stress granule assembly and translation arrest are inhibited by the dsRNA binding protein Staufen1

The nucleocapsid (NC) is an N-terminal protein derived from the HIV-1 Gag precursor polyprotein, pr55^Gag NC possesses key functions at several pivotal stages of viral replication. For example, an interaction between NC and the host double-stranded RNA-binding protein Staufen1 was shown to regulate several steps in the viral replication cycle, such as Gag multimerization and genomic RNA encapsidation. In this work, we observed that the overexpression of NC leads to the induction of stress granule (SG) assembly. NC-mediated SG assembly was unique as it was resistant to the SG blockade imposed by the HIV-1 capsid (CA), as shown in earlier work. NC also reduced host cell mRNA translation, as judged by a puromycylation assay of de novo synthesized proteins, and this was recapitulated in polysome profile analyses. Virus production was also found to be significantly reduced. Finally, Staufen1 expression completely rescued the blockade to NC-mediated SG assembly, global mRNA translation as well as virus production. NC expression also resulted in the phosphorylation of protein kinase R (PKR) and eIF2α, and this was inhibited with Staufen1 coexpression. This work sheds light on an unexpected function of NC in host cell translation. A comprehensive understanding of the molecular mechanisms by which a fine balance of the HIV-1 structural proteins NC and CA act in concert with host proteins such as Staufen1 to modulate the host stress response will aid in the development of new antiviral therapeutics.

G3BP1 interacts directly with the FMDV IRES and negatively regulates translation

RNA-protein interactions play a pivotal role in the function of picornavirus internal ribosome entry site (IRES) elements. Here we analysed the impact of Ras GTPase SH3 domain binding protein 1 (G3BP1) in the IRES activity of foot-and-mouth disease virus (FMDV). We found that G3BP1 interacts directly with three distinct sequences of the IRES element using RNA electrophoretic mobility-shift assays. Analysis of the interaction with domain 5 indicated that the G3BP1 binding-site is placed at the single-stranded region although it allows large sequence heterogeneity and the hairpin located upstream of this region enhances retarded complex formation. In addition, G3BP1 interacts directly with the polypyrimidine tract-binding protein and the translation initiation factor 4B (eIF4B) through the C-terminal region. Moreover, G3BP1 is cleaved during FMDV infection yielding two fragments, Ct-G3BP1 and Nt-G3BP1. Both fragments inhibit cap- and IRES-dependent translation, but the Ct-G3BP1 fragment shows a stronger effect on IRES-dependent translation. Assembly of complexes with G3BP1 results in a significantly reduced local flexibility of the IRES element, consistent with the negative effect of this protein. Our results highlight the IRES-binding capacity of G3BP1 and illustrate its function as a translation inhibitor.

Inhibition of type I interferon induction and signalling by mosquito-borne flaviviruses

The Flavivirus genus (Flaviviridae family) contains a number of important human pathogens, including dengue and Zika viruses, which have the potential to cause severe disease. In order to efficiently establish a productive infection in mammalian cells, flaviviruses have developed key strategies to counteract host immune defences, including the type I interferon response. They employ different mechanisms to control interferon signal transduction and effector pathways, and key research generated over the past couple of decades has uncovered new insights into their abilities to actively decrease interferon antiviral activity. Given the lack of antivirals or prophylactic treatments for many flaviviral infections, it is important to fully understand how these viruses affect cellular processes to influence pathogenesis and disease outcome. This review will discuss the strategies mosquito-borne flaviviruses have evolved to antagonise type I interferon mediated immune responses.

Interactions between the HIV-1 Unspliced mRNA and Host mRNA Decay Machineries

The human immunodeficiency virus type-1 (HIV-1) unspliced transcript is used both as mRNA for the synthesis of structural proteins and as the packaged genome. Given the presence of retained introns and instability AU-rich sequences, this viral transcript is normally retained and degraded in the nucleus of host cells unless the viral protein REV is present. As such, the stability of the HIV-1 unspliced mRNA must be particularly controlled in the nucleus and the cytoplasm in order to ensure proper levels of this viral mRNA for translation and viral particle formation. During its journey, the HIV-1 unspliced mRNA assembles into highly specific messenger ribonucleoproteins (mRNPs) containing many different host proteins, amongst which are well-known regulators of cytoplasmic mRNA decay pathways such as up-frameshift suppressor 1 homolog (UPF1), Staufen double-stranded RNA binding protein 1/2 (STAU1/2), or components of miRNA-induced silencing complex (miRISC) and processing bodies (PBs). More recently, the HIV-1 unspliced mRNA was shown to contain N⁶-methyladenosine (m⁶A), allowing the recruitment of YTH N⁶-methyladenosine RNA binding protein 2 (YTHDF2), an m⁶A reader host protein involved in mRNA decay. Interestingly, these host proteins involved in mRNA decay were shown to play positive roles in viral gene expression and viral particle assembly, suggesting that HIV-1 interacts with mRNA decay components to successfully accomplish viral replication. This review summarizes the state of the art in terms of the interactions between HIV-1 unspliced mRNA and components of different host mRNA decay machineries.

Prostaglandin E2 promotes M2 polarization of macrophages via a cAMP/CREB signaling pathway and deactivates granulocytes in teleost fish

The profile of prostaglandin (PG) production is determined by the differential expression of the enzymes involved in their production and degradation. Although the production of PGE2 by fish leukocytes has been relatively well studied in several fish species, knowledge of how its production is regulated, its biological activities and the signaling pathways activated by this PG is scant or even contradictory. In this work we show that in the teleost fish gilthead seabream (Sparus aurata L.) macrophages regulate PGE2 release mainly by inducing the expression of the genes encoding the enzymes responsible for its synthesis, while acidophilic granulocytes (AGs) not only induce these genes quickly after activation but also inhibit the expression of the genes encoding the enzymes responsible for PGE2 degradation at later time points. In addition, treatment of macrophages with PGE2 promoted their M2 polarization, which is characterized by high expression levels of interleukin-10, mannose-receptor c-type 1 and arginase 2 genes. In sharp contrast, PGE2 promoted the deactivation of AGs, since it decreased the production of reactive oxygen species and the expression of genes encoding pro-inflammatory cytokines. These differences are the result of the alternative signaling pathways used by PGE2 in macrophages and AGs, a cAMP/CREB signaling pathway operating in macrophages, but not in AGs, downstream of PGE2. Our data identify for the first time a role for professional phagocyte-derived-PGE2 in the resolution of inflammation in fish and highlight key differences in the PGE2 signaling pathway in macrophages and granulocytes.

Who Regulates Whom? An Overview of RNA Granules and Viral Infections

After viral infection, host cells respond by mounting an anti-viral stress response in order to create a hostile atmosphere for viral replication, leading to the shut-off of mRNA translation (protein synthesis) and the assembly of RNA granules. Two of these RNA granules have been well characterized in yeast and mammalian cells, stress granules (SGs), which are translationally silent sites of RNA triage and processing bodies (PBs), which are involved in mRNA degradation. This review discusses the role of these RNA granules in the evasion of anti-viral stress responses through virus-induced remodeling of cellular ribonucleoproteins (RNPs).