SAMHD1 phosphorylation and cytoplasmic relocalization after human cytomegalovirus infection limits its antiviral activity

Host cell restriction factors of equine infectious anemia virus

Equine infectious anemia virus (EIAV) is a member of the lentivirus genus in the Retroviridae family and is considered an animal model for HIV/AIDS research. An attenuated EIAV vaccine, which was successfully developed in the 1970s by classical serial passage techniques, is the first and only lentivirus vaccine that has been widely used to date. Restriction factors are cellular proteins that provide an early line of defense against viral replication and spread by interfering with various critical steps in the viral replication cycle. However, viruses have evolved specific mechanisms to overcome these host barriers through adaptation. The battle between the viruses and restriction factors is actually a natural part of the viral replication process, which has been well studied in human immunodeficiency virus type 1 (HIV-1). EIAV has the simplest genome composition of all lentiviruses, making it an intriguing subject for understanding how the virus employs its limited viral proteins to overcome restriction factors. In this review, we summarize the current literature on the interactions between equine restriction factors and EIAV. The features of equine restriction factors and the mechanisms by which the EIAV counteract the restriction suggest that lentiviruses employ diverse strategies to counteract innate immune restrictions. In addition, we present our insights on whether restriction factors induce alterations in the phenotype of the attenuated EIAV vaccine.

BMS-265246, a Cyclin-Dependent Kinase Inhibitor, Inhibits the Infection of Herpes Simplex Virus Type 1

Herpes simplex virus type 1 (HSV-1) infections are prevalent illnesses that can cause mucocutaneous ulcerative disease, keratitis, and genital herpes. In patients with compromised immune systems, the infection can lead to serious problems, such as encephalitis. Additionally, neonatal infections can cause brain problems and even death. Current first-line antiviral drugs are nucleoside analog inhibitors that target viral polymerase, and resistant strains have emerged. As a result, new drugs with distinct action modes are needed. Recent research indicates that cyclin-dependent kinases (CDKs) are prospective antiviral targets. Thus, CDK inhibitors may be effective antiviral agents against HSV-1 infection. In this study, we examined a panel of CDK inhibitors that target CDKs in the present study. BMS-265246 (BMS), a CDK 1/2 inhibitor, was found to effectively limit HSV-1 multiplication in Vero, HepG2, and Hela cells. A mechanism of action study suggested that BMS inhibits the early stages of viral replication when added early in the viral infection. The suppression of multiple steps in viral replication by BMS was revealed when HSV-1 infected cells were treated at different time periods in the viral life cycle. Our results suggest that BMS is a potent anti-HSV-1 agent and unique in that it may interfere with multiple steps in HSV-1 replication.

Human cytomegalovirus UL24 and UL43 products participate in SAMHD1 subcellular localization

This report has analyzed the potential role of Human Cytomegalovirus (HCMV) UL24 and UL43 products in modulating the subcellular location of a host restriction factor, SAMHD1, in cells of human fibroblast origin. Recent studies have reported that the regulation of SAMHD1 is mediated by the HCMV UL97 product inside the nucleus, and by the CDK pathway when it is located in the cytoplasm of the infected cells but the viral gene products that may involve in cytosolic relocalization remain unknown yet. In the present report, we demonstrate that the HCMV UL24 product interacts with the SAMHD1 protein during infection based on mass spectrometry (MS) data and immunoprecipitation assay. The expression or depletion of the viral UL24 gene product did not affect the subcellular localization of SAMHD1 but when it coexpressed with the viral UL43 gene product, another member of the HCMV US22 family, induced the SAMHD1 cytosolic relocalization. Interestingly, the double deletion of viral UL24 and UL43 gene products impaired the cytosolic translocation and the SAMHD1 was accumulated in the nucleus of the infected cells, especially at the late stage post-infection. Our results provide evidence that the viral UL24 and UL43 gene products play a role in the SAMHD1 subcellular localization during HCMV infection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13337-022-00799-3.

Serine 13 of the human cytomegalovirus viral cyclin-dependent kinase UL97 is required for regulatory protein 14-3-3 binding and UL97 stability

The human cytomegalovirus (HCMV) UL97 protein is a conserved herpesvirus protein kinase (CHPK) and a viral cyclin-dependent kinase (v-CDK). However, mechanisms regulating its activity in the context of infection are unknown. Here, we identified several cellular regulatory 14-3-3 proteins as UL97-interacting partners that promote UL97 stability. Humans are known to encode seven isoforms of 14-3-3 proteins (β, ε, η, γ, σ, θ, and ζ) that bind phosphoserines or phosphothreonines to impact protein structure, stability, activity, and localization. Our proteomic analysis of UL97 identified 49 interacting partners, including 14-3-3 isoforms β, η, and γ. Furthermore, coimmunoprecipitation with Western blotting assays demonstrated that UL97 interaction with 14-3-3 isoforms β, ε, η, γ, and θ occurs in a kinase activity-dependent manner. Using mutational analysis, we determined the serine residue at amino acid 13 of UL97 is crucial for 14-3-3 interaction. We demonstrate UL97 S13A (serine to alanine substitution at residue 13) retains kinase activity but the mutant protein accumulated at lower levels than WT UL97. Finally, we show both laboratory (AD169) and clinical (TB40/E) strains of HCMV encoding UL97 S13A replicated with WT kinetics in fibroblasts but showed decreased UL97 accumulation. Taken together, we conclude that 14-3-3 proteins interact with and stabilize UL97 during HCMV infection.

IFI16 Impacts Metabolic Reprogramming during Human Cytomegalovirus Infection

Cellular lipid metabolism plays a pivotal role in human cytomegalovirus (HCMV) infection, as increased lipogenesis in HCMV-infected cells favors the envelopment of newly synthesized viral particles. As all cells are equipped with restriction factors (RFs) able to exert a protective effect against invading pathogens, we asked whether a similar defense mechanism would also be in place to preserve the metabolic compartment from HCMV infection. Here, we show that gamma interferon (IFN-γ)-inducible protein 16 (IFI16), an RF able to block HCMV DNA synthesis, can also counteract HCMV-mediated metabolic reprogramming in infected primary human foreskin fibroblasts (HFFs), thereby limiting virion infectivity. Specifically, we find that IFI16 downregulates the transcriptional activation of the glucose transporter 4 (GLUT4) through cooperation with the carbohydrate-response element-binding protein (ChREBP), thereby reducing HCMV-induced transcription of lipogenic enzymes. The resulting decrease in glucose uptake and consumption leads to diminished lipid synthesis, which ultimately curbs the de novo formation of enveloped viral particles in infected HFFs. Consistently, untargeted lipidomic analysis shows enhanced cholesteryl ester levels in IFI16 KO versus wild-type (WT) HFFs. Overall, our data unveil a new role of IFI16 in the regulation of glucose and lipid metabolism upon HCMV replication and uncover new potential targets for the development of novel antiviral therapies.

Schlafens Can Put Viruses to Sleep

The Schlafen gene family encodes for proteins involved in various biological tasks, including cell proliferation, differentiation, and T cell development. Schlafens were initially discovered in mice, and have been studied in the context of cancer biology, as well as their role in protecting cells during viral infection. This protein family provides antiviral barriers via direct and indirect effects on virus infection. Schlafens can inhibit the replication of viruses with both RNA and DNA genomes. In this review, we summarize the cellular functions and the emerging relationship between Schlafens and innate immunity. We also discuss the functions and distinctions of this emerging family of proteins as host restriction factors against viral infection. Further research into Schlafen protein function will provide insight into their mechanisms that contribute to intrinsic and innate host immunity.

Emerging Mechanisms of G1/S Cell Cycle Control by Human and Mouse Cytomegaloviruses

Cytomegaloviruses (CMVs) are among the largest pathogenic viruses in mammals. To enable replication of their long double-stranded DNA genomes, CMVs induce profound changes in cell cycle regulation. A hallmark of CMV cell cycle control is the establishment of an unusual cell cycle arrest at the G₁/S transition, which is characterized by the coexistence of cell cycle stimulatory and inhibitory activities. While CMVs interfere with cellular DNA synthesis and cell division, they activate S-phase-specific gene expression and nucleotide metabolism. This is facilitated by a set of CMV gene products that target master regulators of G₁/S progression such as cyclin E and A kinases, Rb-E2F transcription factors, p53-p21 checkpoint proteins, the APC/C ubiquitin ligase, and the nucleotide hydrolase SAMHD1. While the major themes of cell cycle regulation are well conserved between human and murine CMVs (HCMV and MCMV), there are considerable differences at the level of viral cell cycle effectors and their mechanisms of action. Furthermore, both viruses have evolved unique mechanisms to sense the host cell cycle state and modulate the infection program accordingly. This review provides an overview of conserved and divergent features of G₁/S control by MCMV and HCMV.

Comprehensive Analysis of Human Cytomegalovirus- and HIV-Mediated Plasma Membrane Remodeling in Macrophages

The plasma membrane (PM) must be overcome by viruses during entry and release. Furthermore, the PM represents the cellular communication compartment and the immune system interface. Hence, viruses have evolved sophisticated strategies to remodel the PM, for instance to avoid immune sensing and clearance of infected cells. We performed a comprehensive analysis of cell surface dysregulation by two human-pathogenic viruses, human cytomegalovirus (HCMV) and human immunodeficiency virus type 1 (HIV-1), in primary macrophages, which are classical antigen-presenting cells and orchestrators of the immune system. Scanning ion conductance microscopy revealed a loss of roughness and an overall smooth phenotype of HCMV-infected macrophages, in contrast to HIV-1 infection. This phenotype was also evident on the molecular level. When we screened for cell surface receptors modulated by HCMV, 42 of 332 receptors tested were up- or downregulated, whereas HIV-1 affected only 7 receptors. In particular CD164, CD84, and CD180 were targeted by HCMV. Mechanistically, HCMV induced transcriptional silencing of these receptors in an interferon (IFN)-independent manner, and expression was reduced not only by lab-adapted HCMV but also by clinical HCMV isolates. Altogether, our plasma membrane profiling of human macrophages provides clues to understand how viruses evade the immune system and identified novel cell surface receptors targeted by HCMV. IMPORTANCE The PM is a key component that viruses have to cope with. It is a barrier for infection and egress and is critically involved in antiviral immune signaling. We hence asked the question how two immunomodulatory viruses, HIV-1 and HCMV, dysregulate this compartment in infected macrophages, relevant in vivo targets of both viruses. We employed a contact-free microscopic technique to image the PM of infected cells and performed a phenotypic flow cytometry-based screen to identify receptor modulations on a molecular level. Our results show that HIV-1 and HCMV differentially manipulate the PM of macrophages. While HIV-1-mediated changes are relatively subtle, HCMV induces major alterations of the PM. We identify novel immune receptors manipulated by HCMV and define mechanisms of how HCMV interferes with receptor expression. Altogether, our study reveals differential strategies of how two human-pathogenic viruses manipulate infected cells and identifies potential novel pathways of HCMV immune evasion.

The balance between p53 isoforms modulates the efficiency of HIV-1 infection in macrophages

Several host factors influence HIV-1 infection and replication. The p53-mediated antiviral role in monocytes-derived macrophages (MDMs) was previously highlighted. Indeed, an increase in p53 level results in a stronger restriction against HIV-1 early replication steps through SAMHD1 activity. In this study, we investigated the potential role of some p53 isoforms in HIV-1 infection. Transfection of isoform-specific siRNA induces distinctive effects on the virus life cycle. For example, in contrast to a siRNA targeting all isoforms, a knockdown of Δ133p53 transcripts reduces virus replication in MDMs that is correlated with a decrease in phosphorylated inactive SAMHD1. Combination of Δ133p53 knockdown and Nutlin-3, a pharmacological inhibitor of MDM2 that stabilizes p53, further reduces susceptibility of MDMs to HIV-1 infection, thus suggesting an inhibitory role of Δ133p53 towards p53 antiviral activity. In contrast, p53β knockdown in MDMs increases the viral production independently of SAMHD1. Moreover, experiments with a Nef-deficient virus show that this viral protein plays a protective role against the antiviral environment mediated by p53. Finally, HIV-1 infection affects the expression pattern of p53 isoforms by increasing p53β and p53γ mRNA levels while stabilizing the protein level of p53α and some isoforms from the p53β subclass. The balance between the various p53 isoforms is therefore an important factor in the overall susceptibility of macrophages to HIV-1 infection, fine-tuning the p53 response against HIV-1. This study brings a new understanding of the complex role of p53 in virus replication processes in myeloid cells. Importance As of today, HIV-1 is still considered as a global pandemic without a functional cure, partly because of the presence of stable viral reservoirs. Macrophages constitute one of these cell reservoirs, contributing to the viral persistence. Studies investigating the host factors involved in cell susceptibility to HIV-1 infection might lead to a better understanding of the reservoir formation and will eventually allow the development of an efficient cure. Our team previously showed the antiviral role of p53 in macrophages, which acts by compromising the early steps of HIV-1 replication. In this study, we demonstrate the involvement of p53 isoforms, which regulates p53 activity and define the cellular environment influencing viral replication. In addition, the results concerning the potential role of p53 in antiviral innate immunity could be transposed to other fields of virology and suggest that knowledge in oncology can be applied to HIV-1 research.

PIAS1 potentiates the anti-EBV activity of SAMHD1 through SUMOylation

BACKGROUND

Sterile alpha motif and HD domain 1 (SAMHD1) is a deoxynucleotide triphosphohydrolase (dNTPase) that restricts the infection of a variety of RNA and DNA viruses, including herpesviruses. The anti-viral function of SAMHD1 is associated with its dNTPase activity, which is regulated by several post-translational modifications, including phosphorylation, acetylation and ubiquitination. Our recent studies also demonstrated that the E3 SUMO ligase PIAS1 functions as an Epstein-Barr virus (EBV) restriction factor. However, whether SAMHD1 is regulated by PIAS1 to restrict EBV replication remains unknown.

RESULTS

In this study, we showed that PIAS1 interacts with SAMHD1 and promotes its SUMOylation. We identified three lysine residues (K469, K595 and K622) located on the surface of SAMHD1 as the major SUMOylation sites. We demonstrated that phosphorylated SAMHD1 can be SUMOylated by PIAS1 and SUMOylated SAMHD1 can also be phosphorylated by viral protein kinases. We showed that SUMOylation-deficient SAMHD1 loses its anti-EBV activity. Furthermore, we demonstrated that SAMHD1 is associated with EBV genome in a PIAS1-dependent manner.

CONCLUSION

Our study reveals that PIAS1 synergizes with SAMHD1 to inhibit EBV lytic replication through protein-protein interaction and SUMOylation.

SAMHD1 … and Viral Ways around It

The SAM and HD domain-containing protein 1 (SAMHD1) is a dNTP triphosphohydrolase that plays a crucial role for a variety of different cellular functions. Besides balancing intracellular dNTP concentrations, facilitating DNA damage repair, and dampening excessive immune responses, SAMHD1 has been shown to act as a major restriction factor against various virus species. In addition to its well-described activity against retroviruses such as HIV-1, SAMHD1 has been identified to reduce the infectivity of different DNA viruses such as the herpesviruses CMV and EBV, the poxvirus VACV, or the hepadnavirus HBV. While some viruses are efficiently restricted by SAMHD1, others have developed evasion mechanisms that antagonize the antiviral activity of SAMHD1. Within this review, we summarize the different cellular functions of SAMHD1 and highlight the countermeasures viruses have evolved to neutralize the restriction factor SAMHD1.