A first-in-class NAE inhibitor, MLN4924, blocks lentiviral infection in myeloid cells by disrupting neddylation-dependent Vpx-mediated SAMHD1 degradation

CRL4-DCAF1 Ubiquitin Ligase Dependent Functions of HIV Viral Protein R and Viral Protein X

The Human Immunodeficiency Virus (HIV) encodes several proteins that contort the host cell environment to promote viral replication and spread. This is often accomplished through the hijacking of cellular ubiquitin ligases. These reprogrammed complexes initiate or enhance the ubiquitination of cellular proteins that may otherwise act to restrain viral replication. Ubiquitination of target proteins may alter protein function or initiate proteasome-dependent destruction. HIV Viral Protein R (Vpr) and the related HIV-2 Viral Protein X (Vpx), engage the CRL4-DCAF1 ubiquitin ligase complex to target numerous cellular proteins. In this review we describe the CRL4-DCAF1 ubiquitin ligase complex and its interactions with HIV Vpr and Vpx. We additionally summarize the cellular proteins targeted by this association as well as the observed or hypothesized impact on HIV.

Effects of neddylation on viral infection: an overview

Neddylation is a post-translational modification that plays an important role not only in cancer development but also in regulating viral infection and replication. Upregulation of neddylation occurs in viral infections, and inhibition of neddylation can suppress viral replication. Neddylation is thought to enhance viral protein stability and replication. Neddylation has been reported to enhance the stability of the regulatory hepatitis B virus (HBV) X protein, modulate viral replication, and enhance hepatocarcinogenesis. Inhibition of neddylation using the NEDD8-activating enzyme E1 inhibitor MLN4924 inhibits viral replication, including that of HBV. Understanding of the role of neddylation in viral infections is critical for developing new therapeutic targets and potential treatment strategies. In this review, we discuss recent progress in the understanding of the effects of neddylation during viral infection, particularly in HBV infection, and strategies for curing viral infection by targeting the neddylation pathway.

Host Cell Neddylation Facilitates Alphaherpesvirus Entry in a Virus-Specific and Cell-Dependent Manner

Herpes simplex virus 1 (HSV-1) commandeers the host cell proteasome at several steps of its replication cycle, including entry. Here we demonstrate that HSV-2, pseudorabies virus (PRV), and bovine herpesvirus 1 (BoHV-1) entry are blocked by bortezomib, a proteasome inhibitor that is an FDA-approved cancer drug. Proteasome-dependent entry of HSV-1 is thought to be ubiquitin-independent. To interrogate further the proteasomal mechanism of entry, we determined the involvement of the ubiquitin-like molecule NEDD8 and the neddylation cascade in alphaherpesvirus entry and infection. MLN4924 is a small-molecule inhibitor of neddylation that binds directly to the NEDD8-activating enzyme. Cell treatment with MLN4924 inhibited plaque formation and infectivity by HSV-1, PRV, and BoHV-1 at noncytotoxic concentrations. Thus, the neddylation pathway is broadly important for alphaherpesvirus infection. However, the neddylation inhibitor had little effect on entry of the veterinary viruses but had a significant inhibitory effect on entry of HSV-1 and HSV-2 into seven different cell types. Washout experiments indicated that MLN4924's effect on viral entry was reversible. A time-of-addition assay suggested that the drug was acting on an early step in the entry process. Small interfering RNA (siRNA) knockdown of NEDD8 significantly inhibited HSV entry. In probing the neddylation-dependent step in entry, we found that MLN4924 dramatically blocked endocytic uptake of HSV from the plasma membrane by >90%. In contrast, the rate of HSV entry into cells that support direct fusion of HSV with the cell surface was unaffected by MLN4924. Interestingly, proteasome activity was less important for the endocytic internalization of HSV from the cell surface. The results suggest that the NEDD8 cascade is critical for the internalization step of HSV entry. IMPORTANCE Alphaherpesviruses are ubiquitous pathogens of humans and veterinary species that cause lifelong latent infections and significant morbidity and mortality. Host cell neddylation is important for cell homeostasis and for the infection of many viruses, including HSV-1, HSV-2, PRV, and BoHV-1. Inhibition of neddylation by a pharmacologic inhibitor or siRNA blocked HSV infection at the entry step. Specifically, the NEDD8 pathway was critically important for HSV-1 internalization from the cell surface by an endocytosis mechanism. The results expand our limited understanding of cellular processes that mediate HSV internalization. To our knowledge, this is the first demonstration of a function for the neddylation cascade in virus entry.

Involvement of the neddylation modification system in Bombyx mori nucleopolyhedrovirus replication

Neddylation is a posttranslational modification that is similar to ubiquitination, and involved in some critical biological processes, such as DNA repair, transcription regulation, and ubiquitin-proteasome pathway. Recently, it was found that neddylation inhibitor MLN4924 has potent antiviral activity against human viruses including herpes simplex virus (HSV)-1, HSV-2, and influenza viruses. Here, we report that MLN4924 could dramatically and dose-dependently inhibits the propagation, formation of budding virus (BV) and occlusion body (OB) of a lepidopteran virus-Bombyx mori nucleopolyhedrovirus (BmNPV), impaired OB assembly. In addition, the neddylation modification protein NEDD8 is colocalized with aggresome and autophagosome. Our findings suggest that inhibiting neddylation could be an antibaculovirus strategy and MLN4924 may be used as candidate drug for that purpose.

Association Between Neddylation and Immune Response

Neddylation is a ubiquitin-like post-translational protein modification. It occurs via the activation of the neural precursor cell expressed, developmentally downregulated protein 8 (NEDD8) by three enzymes: activating enzyme, conjugating enzyme, and ligase. NEDD8 was first isolated from the mouse brain in 1992 and was initially considered important for the development and differentiation of the central nervous system. Previously, the downregulation of neddylation was associated with some human diseases, such as neurodegenerative disorders and cancers. In recent years, neddylation has also been proven to be pivotal in various processes of the human immune system, including the regulation of inflammation, bacterial infection, viral infection, and T cell function. Additionally, NEDD8 was found to act on proteins that can affect viral transcription, leading to impaired infectivity. Here, we focused on the influence of neddylation on the innate and adaptive immune responses.

Specific Deubiquitinating Enzymes Promote Host Restriction Factors Against HIV/SIV Viruses

Hijacking host ubiquitin pathways is essential for the replication of diverse viruses. However, the role of deubiquitinating enzymes (DUBs) in the interplay between viruses and the host is poorly characterized. Here, we demonstrate that specific DUBs are potent inhibitors of viral proteins from HIVs/simian immunodeficiency viruses (SIVs) that are involved in viral evasion of host restriction factors and viral replication. In particular, we discovered that T cell-functioning ubiquitin-specific protease 8 (USP8) is a potent and specific inhibitor of HIV-1 virion infectivity factor (Vif)-mediated apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3)G (A3G) degradation. Ectopic expression of USP8 inhibited Vif-induced A3G degradation and suppressed wild-type HIV-1 infectivity even in the presence of Vif. In addition, specific DUBs repressed Vpr-, Vpu-, and Vpx-triggered host restriction factor degradation. Our study has revealed a previously unrecognized interplay between the host's DUBs and viral replication. Enhancing the antiviral activity of DUBs therefore represents an attractive strategy against HIVs/SIVs.

Diverse and pivotal roles of neddylation in metabolism and immunity

Neddylation is one type of protein post-translational modification by conjugating a ubiquitin-like protein neural precursor cell-expressed developmentally downregulated protein 8 to substrate proteins via a cascade involving E1, E2, and E3 enzymes. The best-characterized substrates of neddylation are cullins, essential components of cullin-RING E3 ubiquitin-ligase complexes. The discovery of noncullin neddylation targets indicates that neddylation may have diverse biological functions. Indeed, neddylation has been implicated in various cellular processes including cell cycle progression, metabolism, immunity, and tumorigenesis. Here, we summarized the reported neddylation substrates and also discuss the functions of neddylation in the immune system and metabolism.

Adenovirus oncoprotein E4orf6 triggers Cullin5 neddylation to activate the CLR5 E3 ligase for p53 degradation

The human adenovirus oncoprotein E4orf6 hijacks intracellular Cullin 5-based E3 ubiquitin ligases (CRL5s) to induce the degradation of host proteins, including p53, that impede efficient viral replication. The complex also relies on another viral protein, E1B55K, to recruit substrates for ubiquitination. However, the determinants of adenoviral E4orf6-CRL5 E3 ligase-mediated p53 degradation in the scaffolding protein Cullin5 remain rarely investigated. Here, we demonstrated that the viral protein E4orf6 triggered relocalization of the Cullin5 protein from the cytoplasm to the nucleus and induced activation of the CRL5 E3 ligase via facilitating neddylation. The expression of the deneddylase SENP8/Den1 was significantly downregulated by E4orf6. We then identified SENP8 as a natural restriction factor for E4orf6-induced p53 degradation. Furthermore, our results indicated that the NEDD8-conjugating E2 enzyme UBE2M was essential for E4orf6-mediated p53 degradation and that its dominant negative mutant UBE2M C111S dramatically blocked E4orf6 functions. The Nedd8-activating enzyme inhibitor MLN4924 decreased E4orf6-induced neddylation of the cullin5 protein and subsequently suppressed p53 degradation. Collectively, our findings illuminate the strategy by which this viral oncoprotein specifically utilizes the neddylation pathway to activate host CRL E3 ligases to degrade host restriction factors. Disrupting this post-translational modification is an attractive pharmacological intervention against human adenoviruses.

Cellular Cullin RING Ubiquitin Ligases: Druggable Host Dependency Factors of Cytomegaloviruses

Human cytomegalovirus (HCMV) is a ubiquitous betaherpesvirus that frequently causes morbidity and mortality in individuals with insufficient immunity, such as transplant recipients, AIDS patients, and congenitally infected newborns. Several antiviral drugs are approved to treat HCMV infections. However, resistant HCMV mutants can arise in patients receiving long-term therapy. Additionally, side effects and the risk to cause birth defects limit the use of currently approved antivirals against HCMV. Therefore, the identification of new drug targets is of clinical relevance. Recent work identified DNA-damage binding protein 1 (DDB1) and the family of the cellular cullin (Cul) RING ubiquitin (Ub) ligases (CRLs) as host-derived factors that are relevant for the replication of human and mouse cytomegaloviruses. The first-in-class CRL inhibitory compound Pevonedistat (also called MLN4924) is currently under investigation as an anti-tumor drug in several clinical trials. Cytomegaloviruses exploit CRLs to regulate the abundance of viral proteins, and to induce the proteasomal degradation of host restriction factors involved in innate and intrinsic immunity. Accordingly, pharmacological blockade of CRL activity diminishes viral replication in cell culture. In this review, we summarize the current knowledge concerning the relevance of DDB1 and CRLs during cytomegalovirus replication and discuss chances and drawbacks of CRL inhibitory drugs as potential antiviral treatment against HCMV.

Vif-CBFβ interaction is essential for Vif-induced cell cycle arrest

Interaction between HIV-1 Vif and host factor CBFβ leads to the assembly of the Vif-Cul5-EloB/C ubiquitin ligase (E3 complex). By inducing the formation of E3 complex, Vif depletes host APOBEC3 restriction factors and promotes HIV-1 infection. In addition, Vif is known to arrest host cells at G2/M phase (G2 arrest), benefiting HIV-1 replication and contributing to the depletion of CD4⁺ T cells. However, whether CBFβ is also involved in Vif-induced cell cycle arrest remains unclear. In the present study, we report that CBFβ is an essential factor for Vif-induced G2 arrest. Reducing endogenous CBFβ expression significantly compromised Vif's potency in cell cycle regulation. In addition, tests with CBFβ and Vif mutants indicated that Vif-CBFβ interaction is crucial for Vif to induce G2 arrest. Furthermore, suppressors against Vif-hijacked E3 complex or proteasome-mediated proteolysis also abolished Vif's ability to cause G2 arrest. In general, our data indicated that Vif induces G2 arrest through depletion of a yet-unknown cellular factor, where the involvement of CBFβ is essential. On the other hand, our data also suggested that, antiviral drugs targeting the Vif-CBFβ interaction have the potential to abolish Vif's ability to cause APOBEC3 degradation as well as G2 arrest in host cells, thus reducing both HIV-1 replication and Vif-induced CD4⁺ T-cell depletion.

A Mass Spectrometry-Based Profiling of Interactomes of Viral DDB1- and Cullin Ubiquitin Ligase-Binding Proteins Reveals NF-κB Inhibitory Activity of the HIV-2-Encoded Vpx

Viruses and hosts are situated in a molecular arms race. To avoid morbidity and mortality, hosts evolved antiviral restriction factors. These restriction factors exert selection pressure on the viruses and drive viral evolution toward increasingly efficient immune antagonists. Numerous viruses exploit cellular DNA damage-binding protein 1 (DDB1)-containing Cullin RocA ubiquitin ligases (CRLs) to induce the ubiquitination and subsequent proteasomal degradation of antiviral factors expressed by their hosts. To establish a comprehensive understanding of the underlying protein interaction networks, we performed immuno-affinity precipitations for a panel of DDB1-interacting proteins derived from viruses such as mouse cytomegalovirus (MCMV, Murid herpesvirus [MuHV] 1), rat cytomegalovirus Maastricht MuHV2, rat cytomegalovirus English MuHV8, human cytomegalovirus (HCMV), hepatitis B virus (HBV), and human immunodeficiency virus (HIV). Cellular interaction partners were identified and quantified by mass spectrometry (MS) and validated by classical biochemistry. The comparative approach enabled us to separate unspecific interactions from specific binding partners and revealed remarkable differences in the strength of interaction with DDB1. Our analysis confirmed several previously described interactions like the interaction of the MCMV-encoded interferon antagonist pM27 with STAT2. We extended known interactions to paralogous proteins like the interaction of the HBV-encoded HBx with different Spindlin proteins and documented interactions for the first time, which explain functional data like the interaction of the HIV-2-encoded Vpr with Bax. Additionally, several novel interactions were identified, such as the association of the HIV-2-encoded Vpx with the transcription factor RelA (also called p65). For the latter interaction, we documented a functional relevance in antagonizing NF-κB-driven gene expression. The mutation of the DDB1 binding interface of Vpx significantly impaired NF-κB inhibition, indicating that Vpx counteracts NF-κB signaling by a DDB1- and CRL-dependent mechanism. In summary, our findings improve the understanding of how viral pathogens hijack cellular DDB1 and CRLs to ensure efficient replication despite the expression of host restriction factors.

The poly-proline tail of SIVmac Vpx provides gain of function for resistance to a cryptic proteasome-dependent degradation pathway

The lentiviral accessory protein Vpx is critical for viral infection of myeloid cells and acts by hijacking CRL4(DCAF1) E3 ubiquitin ligase to induce the degradation of the host restriction factor SAMHD1. It has been observed that the sequences from HIV-2 and SIVsmm/SIVmac Vpx contain a poly-proline tail which is distinct from other SIV Vpx proteins. However, the role of this region in Vpx function is controversial. Herein, we found proteasome-dependent degradation of a Vpx mutant lacking the poly-proline tail in the nucleus in a CRL4(DCAF1) E3 ligase-independent fashion. Unlike wild-type Vpx, the poly-proline tail mutant Vpx is partly defective in enhancing viral infection in macrophages. Our findings suggest that during Vpx evolution, Vpx of the HIV-2/SIVsm/SIVmac lineage is targeted by a CRL4(DCAF1) E3 ligase-independent ubiquitination pathway, and have gained this interesting region, allowing them to maintain nuclear accumulation as part of their adaptation to host cell regulation.

Roles of SAMHD1 in antiviral defense, autoimmunity and cancer

The enzyme, sterile α motif and histidine-aspartic acid domain-containing protein 1 (SAMHD1) diminishes infection of human immunodeficiency virus type 1 (HIV-1) by hydrolyzing intracellular deoxynucleotide triphosphates (dNTPs) in myeloid cells and resting CD4+ T cells. This dNTP degradation reduces the dNTP concentration to a level insufficient for viral cDNA synthesis, thereby inhibiting retroviral replication. This antiviral enzymatic activity can be inhibited by viral protein X (Vpx). The HIV-2/SIV Vpx causes degradation of SAMHD1, thus interfering with the SAMHD1-mediated restriction of retroviral replication. Recently, SAMHD1 has been suggested to restrict HIV-1 infection by directly digesting genomic HIV-1 RNA through a still controversial RNase activity. Here, we summarize the current knowledge about structure, antiviral mechanisms, intracellular localization, interferon-regulated expression of SAMHD1. We also describe SAMHD1-deficient animal models and an antiviral drug on the basis of disrupting proteasomal degradation of SAMHD1. In addition, the possible roles of SAMHD1 in regulating innate immune sensing, Aicardi-Goutières syndrome and cancer are discussed in this review.

Inhibition of Vpx-Mediated SAMHD1 and Vpr-Mediated Host Helicase Transcription Factor Degradation by Selective Disruption of Viral CRL4 (DCAF1) E3 Ubiquitin Ligase Assembly

The lentiviral accessory proteins Vpx and Vpr are known to utilize CRL4 (DCAF1) E3 ligase to induce the degradation of the host restriction factor SAMHD1 or host helicase transcription factor (HLTF), respectively. Selective disruption of viral CRL4 (DCAF1) E3 ligase could be a promising antiviral strategy. Recently, we have determined that posttranslational modification (neddylation) of Cullin-4 is required for the activation of Vpx-CRL4 (DCAF1) E3 ligase. However, the mechanism of Vpx/Vpr-CRL4 (DCAF1) E3 ligase assembly is still poorly understood. Here, we report that zinc coordination is an important regulator of Vpx-CRL4 E3 ligase assembly. Residues in a conserved zinc-binding motif of Vpx were essential for the recruitment of the CRL4 (DCAF1) E3 complex and Vpx-induced SAMHD1 degradation. Importantly, altering the intracellular zinc concentration by treatment with the zinc chelator N,N,N'-tetrakis-(2'-pyridylmethyl)ethylenediamine (TPEN) potently blocked Vpx-mediated SAMHD1 degradation and inhibited wild-type SIVmac (simian immunodeficiency virus of macaques) infection of myeloid cells, even in the presence of Vpx. TPEN selectively inhibited Vpx and DCAF1 binding but not the Vpx-SAMHD1 interaction or Vpx virion packaging. Moreover, we have shown that zinc coordination is also important for the assembly of the HIV-1 Vpr-CRL4 E3 ligase. In particular, Vpr zinc-binding motif mutation or TPEN treatment efficiently inhibited Vpr-CRL4 (DCAF1) E3 ligase assembly and Vpr-mediated HLTF degradation or Vpr-induced G₂ cell cycle arrest. Collectively, our study sheds light on a conserved strategy by the viral proteins Vpx and Vpr to recruit host CRL4 (DCAF1) E3 ligase, which represents a target for novel anti-human immunodeficiency virus (HIV) drug development.IMPORTANCE The Vpr and its paralog Vpx are accessory proteins encoded by different human immunodeficiency virus (HIV)/simian immunodeficiency virus (SIV) lentiviruses. To facilitate viral replication, Vpx has evolved to induce SAMHD1 degradation and Vpr to mediate HLTF degradation. Both Vpx and Vpr perform their functions by recruiting CRL4 (DCAF1) E3 ligase. In this study, we demonstrate that the assembly of the Vpx- or Vpr-CRL4 E3 ligase requires a highly conserved zinc-binding motif. This motif is specifically required for the DCAF1 interaction but not for the interaction of Vpx or Vpr with its substrate. Selective disruption of Vpx- or Vpr-CRL4 E3 ligase function was achieved by zinc sequestration using N,N,N'-tetrakis-(2'-pyridylmethyl)ethylenediamine (TPEN). At the same time, zinc sequestration had no effect on zinc-dependent cellular protein functions. Therefore, information obtained from this study may be important for novel anti-HIV drug development.

Broad and potent antiviral activity of the NAE inhibitor MLN4924

In terms of infected human individuals, herpesviruses range among the most successful virus families. Subclinical herpesviral infections in healthy individuals contrast with life-threatening syndromes under immunocompromising and immunoimmature conditions. Based on our finding that cytomegaloviruses interact with Cullin Roc ubiquitin ligases (CRLs) in the context of interferon antagonism, we systematically assessed viral dependency on CRLs by utilizing the drug MLN4924. CRL activity is regulated through the conjugation of Cullins with the ubiquitin-like molecule Nedd8. By inhibiting the Nedd8-activating Enzyme (NAE), MLN4924 interferes with Nedd8 conjugation and CRL activity. MLN4924 exhibited pronounced antiviral activity against mouse and human cytomegalovirus, herpes simplex virus (HSV)- 1 (including multi-drug resistant clinical isolates), HSV-2, adeno and influenza viruses. Human cytomegalovirus genome amplification was blocked at nanomolar MLN4924 concentrations. Global proteome analyses revealed that MLN4924 blocks cytomegaloviral replication despite increased IE1 amounts. Expression of dominant negative Cullins assigned this IE regulation to defined Cullin molecules and phenocopied the antiviral effect of MLN4924.

HD domain of SAMHD1 influences Vpx-induced degradation at a post-interaction step

Primate SAMHD1 proteins are potent inhibitors of viruses, including retroviruses such as HIV-1, HIV-2, and SIV. Vpx, a distinctive viral protein expressed by HIV-2 and some SIVs, induces SAMHD1 degradation by forming a Vpx-DCAF1-based ubiquitin ligase complex. Either the N- or the C-terminus of SAMHD1 is critical for Vpx-induced degradation, depending on the types of SAMHD1 and Vpx proteins. However, it was not fully understood whether other regions of SAMHD1 also contribute to its depletion by Vpx. In the present study, we report that SAMHD1 from chicken (SAMHD1GG) was not degraded by SIVmac Vpx, in contrast with results for human SAMHD1 (SAMHD1HS). Results regarding to SAMHD1HS and SAMHD1GG fusion proteins supported previous findings that the C-terminus of SAMHD1HS is essential for Vpx-induced degradation. Internal domain substitution, however, revealed that the HD domain also contributes to Vpx-mediated SAMHD1 degradation. Interestingly, the HD domain influenced Vpx-mediated SAMHD1 degradation without affecting Vpx-SAMHD1 interaction. Therefore, our findings revealed that factors in addition to Vpx-SAMHD1 binding influence the efficiency of Vpx-mediated SAMHD1 degradation.

Pharmacologic Inhibition of Nedd8 Activation Enzyme Exposes CD4-Induced Epitopes within Env on Cells Expressing HIV-1

HIV-1 Vpu decreases the exposure of epitopes within the viral envelope glycoprotein (Env) on the surface of infected cells by downregulating both BST2 and CD4. To test the hypothesis that inhibiting Vpu activity would increase the exposure of these epitopes and sensitize infected cells to antibody-dependent cellular cytotoxicity (ADCC), we treated cells with the Nedd8 activation enzyme (NAE) inhibitor MLN4924, which inhibits the cullin1-based ubiquitin ligase complex coopted by Vpu to degrade cellular targets. Treatment of HeLa cells with MLN4924 or expression of a dominant negative mutant of cullin1 inhibited the Vpu-mediated downregulation of CD4 but not the downregulation of BST2. NAE inhibition also increased the surface exposure of CD4-induced epitopes within Env on HEK293 cells containing an inducible HIV genome, on infected CEM T cells, and on infected primary T cells. In contrast, the Vpu-mediated downregulation of BST2 was substantially inhibited by MLN4924 only when T cells were treated with alpha interferon (IFN-α) to induce high levels of BST2 expression. As reported previously, the absence of vpu or nef and even more so the combined absence of these two genes sensitized infected cells to ADCC. However, NAE inhibition affected ADCC minimally. Paradoxically, even in infected, IFN-treated cells in which NAE inhibition substantially rescued the surface level of BST2, the surface level of Env detected with an antibody recognizing a CD4-independent epitope (2G12) was minimally increased. Mutation of the C-terminal Vpu residue W76, which supports the ability of Vpu to stimulate virion release by displacing BST2 from assembly sites on the plasma membrane by a cullin1-independent mechanism, increased the exposure of Env detected by 2G12 on infected T cells. Thus, inhibiting the displacement function of Vpu together with its ability to degrade CD4 and BST2 may be required to sensitize infected cells to ADCC.

IMPORTANCE

Pathogenic viruses encode gene products that enable evasion of host immune surveillance mechanisms. One such mechanism is antibody-dependent cellular cytotoxicity (ADCC), whereby host antibodies bind envelope glycoproteins of the virus that are inserted into the cellular membrane and direct the destruction of infected cells. Targeting pharmacologically the activity of HIV-1 Vpu, which contributes to evasion of ADCC, could potentially sensitize infected cells to this immune surveillance mechanism, an outcome that would have therapeutic implications with respect to the goal of curing HIV-1 infection. The Nedd8 activation enzyme inhibitor MLN4924 blocks the activity of the host ubiquitin ligase that Vpu coopts to direct the degradation of CD4 and BST2. We observed that while MLN4924 partially reverses the activity of Vpu and could become part of a therapeutic approach by virtue of CD4-induced epitope exposure, sufficient Vpu activity as an antagonist of BST2 persists despite this drug to allow escape from ADCC.

HIV-1 Vpu utilizes both cullin-RING ligase (CRL) dependent and independent mechanisms to downmodulate host proteins

Background

Hijacking of the cullin-RING E3 ubiquitin ligase (CRL) machinery is a common mechanism employed by diverse groups of viruses for the efficient counteraction and degradation of host proteins. In particular, HIV-1 Vpu usurps the SCF^β-TrCP E3 ubiquitin ligase complex to mark CD4 for degradation by the 26S proteasome. Vpu also interacts with and downmodulates a number of other host proteins, including the restriction factor BST-2. However, whether Vpu primarily relies on a cullin-dependent or -independent mechanism to antagonize its cellular targets has not been fully elucidated.

Results

We utilized a sulphamate AMP analog, MLN4924, to effectively block the activation of CRLs within infected primary CD4⁺ T cells. MLN4924 treatment, in a dose dependent manner, efficiently relieved surface downmodulation and degradation of CD4 by NL4-3 Vpu. MLN4924 inhibition was highly specific, as this inhibitor had no effect on Nef’s ability to downregulate CD4, which is accomplished by a CRL-independent mechanism. In contrast, NL4-3 Vpu’s capacity to downregulate BST-2, NTB-A and CCR7 was not inhibited by the drug. Vpu’s from both a transmitted founder (T/F) and chronic carrier (CC) virus preserved the ability to downregulate BST-2 in the presence of MLN4924. Finally, depletion of cellular pools of cullin 1 attenuated Vpu’s ability to decrease CD4 but not BST-2 surface levels.

Conclusions

We conclude that Vpu employs both CRL-dependent and CRL-independent modes of action against host proteins. Notably, we also establish that Vpu-mediated reduction of BST-2 from the cell surface is independent of β-TrCP and the CRL- machinery and this function is conserved by Vpu’s from primary isolates. Therefore, potential therapies aimed at antagonizing the activities of Vpu may need to address these distinct mechanisms of action in order to achieve a maximal effect.

HIV-1 Vpr suppresses the cytomegalovirus promoter in a CRL4(DCAF1) E3 ligase independent manner

Although the Vpr protein of human immunodeficiency virus type 1 (HIV-1) has been shown to act as a transcriptional activator of the HIV-1 LTR and certain host genes, the current study demonstrates that it can also function as a potent inhibitor of the cytomegalovirus (CMV) promoter. Previous studies have shown that the cell cycle arrest and apoptotic functions of Vpr required recruitment of the CRL4(DCAF1) E3 ligase, but this complex is shown not to be required for inhibition of the CMV promoter. We identified conserved sites (A30/V31) from diverse Vpr from HIV/SIV that were critical for blocking the CMV promoter activity. Interestingly, the Vpr mutant A30S/V31S protein also impaired the ability of Vpr to down-regulate transcription of the host UNG2 gene. Our findings shed light on the dual functions of Vpr on the transcription of HIV-1, other viruses and host genes which may contribute to viral replication and disease progression in vivo.

MLN4924 sensitizes monocytes and maturing dendritic cells for TNF-dependent and -independent necroptosis

BACKGROUND AND PURPOSE

MLN4924 prevents the formation of active cullin-RING ubiquitin ligase complexes and thus inhibits NF-κB signalling. Here, we evaluated the effects of this compound on monocytes and dendritic cells (DCs).

EXPERIMENTAL APPROACH

Monocytes and DCs were challenged with TNF or LPS in the presence and absence of MLN4924. The effects of MLN4924 on cellular viability, pro-inflammatory gene induction and DC maturation were investigated using the MTT assay, elisa and FACS analysis. Mechanisms of cell death induction were evaluated by using inhibitors of caspases, RIPK1 and MLKL.

KEY RESULTS

MLN4924 inhibited NF-κB activation and sensitized monocytes and immature DCs (iDCs) for TNFR1-induced cell death. Neither the caspase inhibitor zVAD-fmk, the RIPK1 inhibitor necrostatin-1 (nec-1) nor the MLKL inhibitor necrosulfonamide (NSA) alone prevented TNF-induced cell death. A combination of zVAD-fmk and nec-1 or NSA, however, rescued monocytes and iDCs from MLN4924/TNF-induced cell death indicating that MLN4924 affects anti-apoptotic and anti-necrotic activities in TNFR1 signalling. MLN4924 also converted the response of iDCs to LPS from maturation to cell death. LPS-induced cell death in MLN4924-treated iDCs was again only effectively blocked by cotreatment with zVAD-fmk and nec-1 or NSA. Noteworthy, MLN4924/LPS-induced cell death was almost completely independent of endogenous TNF. MLN4924 also strongly inhibited maturation and activation of iDCs that were rescued from cell death by zVAD-fmk and nec-1.

CONCLUSIONS AND IMPLICATIONS

Our data reveal a strong dual suppressive effect of MLN4924 on DC activity. The targeting of NAE by MLN4924 could be a new way to treat inflammatory diseases.

Cullin4A and cullin4B are interchangeable for HIV Vpr and Vpx action through the CRL4 ubiquitin ligase complex

Human immunodeficiency virus (HIV) seizes control of cellular cullin-RING E3 ubiquitin ligases (CRLs) to promote viral replication. HIV-1 Vpr and HIV-2/simian immunodeficiency virus (SIV) Vpr and Vpx engage the cullin4 (CUL4)-containing ubiquitin ligase complex (CRL4) to cause polyubiquitination and proteasomal degradation of host proteins, including ones that block infection. HIV-1 Vpr engages CRL4 to trigger the degradation of uracil-N-glycosylase 2 (UNG2). Both HIV-1 Vpr and HIV-2/SIV Vpr tap CRL4 to initiate G2 cell cycle arrest. HIV-2/SIV Vpx secures CRL4 to degrade the antiviral protein SAMHD1. CRL4 includes either cullin4A (CUL4A) or cullin4B (CUL4B) among its components. Whether Vpr or Vpx relies on CUL4A, CUL4B, or both to act through CRL4 is not known. Reported structural, phenotypic, and intracellular distribution differences between the two CUL4 types led us to hypothesize that Vpr and Vpx employ these in a function-specific manner. Here we determined CUL4 requirements for HIV-1 and HIV-2/SIV Vpr-mediated G2 cell cycle arrest, HIV-1 Vpr-mediated UNG2 degradation, and HIV-2 Vpx-mediated SAMHD1 degradation. Surprisingly, CUL4A and CUL4B are exchangeable for CRL4-dependent Vpr and Vpx action, except in primary macrophages, where Vpx relies on both CUL4A and CUL4B for maximal SAMHD1 depletion. This work highlights the need to consider both CUL4 types for Vpr and Vpx functions and also shows that the intracellular distribution of CUL4A and CUL4B can vary by cell type.

IMPORTANCE

The work presented here shows for the first time that HIV Vpr and Vpx do not rely exclusively on CUL4A to cause ubiquitination through the CRL4 ubiquitin ligase complex. Furthermore, our finding that intracellular CUL4 and SAMHD1 distributions can vary with cell type provides the basis for reconciling previous disparate findings regarding the site of SAMHD1 depletion. Finally, our observations with primary immune cells provide insight into the cell biology of CUL4A and CUL4B that will help differentiate the functions of these similar proteins.

SAMHD1 is mutated recurrently in chronic lymphocytic leukemia and is involved in response to DNA damage

SAMHD1 is a deoxynucleoside triphosphate triphosphohydrolase and a nuclease that restricts HIV-1 in noncycling cells. Germ-line mutations in SAMHD1 have been described in patients with Aicardi-Goutières syndrome (AGS), a congenital autoimmune disease. In a previous longitudinal whole genome sequencing study of chronic lymphocytic leukemia (CLL), we revealed a SAMHD1 mutation as a potential founding event. Here, we describe an AGS patient carrying a pathogenic germ-line SAMHD1 mutation who developed CLL at 24 years of age. Using clinical trial samples, we show that acquired SAMHD1 mutations are associated with high variant allele frequency and reduced SAMHD1 expression and occur in 11% of relapsed/refractory CLL patients. We provide evidence that SAMHD1 regulates cell proliferation and survival and engages in specific protein interactions in response to DNA damage. We propose that SAMHD1 may have a function in DNA repair and that the presence of SAMHD1 mutations in CLL promotes leukemia development.