Author Correction: Inositol phosphates are assembly co-factors for HIV-1.

In this Letter, the Protein Data Bank (PDB) accessions were incorrectly listed as '6BH5, 6BHT and 6BHS' instead of '6BHR, 6BHT and 6BHS'; this has been corrected online.

Inositol phosphates are assembly co-factors for HIV-1.

A short, 14-amino-acid segment called SP1, located in the Gag structural protein1, has a critical role during the formation of the HIV-1 virus particle. During virus assembly, the SP1 peptide and seven preceding residues fold into a six-helix bundle, which holds together the Gag hexamer and facilitates the formation of a curved immature hexagonal lattice underneath the viral membrane2,3. Upon completion of assembly and budding, proteolytic cleavage of Gag leads to virus maturation, in which the immature lattice is broken down; the liberated CA domain of Gag then re-assembles into the mature conical capsid that encloses the viral genome and associated enzymes. Folding and proteolysis of the six-helix bundle are crucial rate-limiting steps of both Gag assembly and disassembly, and the six-helix bundle is an established target of HIV-1 inhibitors4,5. Here, using a combination of structural and functional analyses, we show that inositol hexakisphosphate (InsP6, also known as IP6) facilitates the formation of the six-helix bundle and assembly of the immature HIV-1 Gag lattice. IP6 makes ionic contacts with two rings of lysine residues at the centre of the Gag hexamer. Proteolytic cleavage then unmasks an alternative binding site, where IP6 interaction promotes the assembly of the mature capsid lattice. These studies identify IP6 as a naturally occurring small molecule that promotes both assembly and maturation of HIV-1.

Primate lentiviruses require Inositol hexakisphosphate (IP6) or inositol pentakisphosphate (IP5) for the production of viral particles.

Inositol hexakisphosphate (IP6) potently stimulates HIV-1 particle assembly in vitro and infectious particle production in vivo. However, knockout cells lacking inositol-pentakisphosphate 2-kinase (IPPK-KO), the enzyme that produces IP6 by phosphorylation of inositol pentakisphosphate (IP5), were still able to produce infectious HIV-1 particles at a greatly reduced rate. HIV-1 in vitro assembly can also be stimulated to a lesser extent with IP5, but until recently, it was not known if IP5 could also function in promoting assembly in vivo. Here we addressed whether there is an absolute requirement for IP6 or IP5 in the production of infectious HIV-1 particles. IPPK-KO cells expressed no detectable IP6 but elevated IP5 levels and displayed a 20-100-fold reduction in infectious particle production, correlating with lost virus release. Transient transfection of an IPPK expression vector stimulated infectious particle production and release in IPPK-KO but not wildtype cells. Several attempts to make IP6/IP5 deficient stable cells were not successful, but transient expression of the enzyme multiple inositol polyphosphate phosphatase-1 (MINPP1) into IPPK-KOs resulted in near ablation of IP6 and IP5. Under these conditions, we found that HIV-1 infectious particle production and virus release were essentially abolished (1000-fold reduction) demonstrating an IP6/IP5 requirement. However, other retroviruses including a Gammaretrovirus, a Betaretrovirus, and two non-primate Lentiviruses displayed only a modest (3-fold) reduction in infectious particle production from IPPK-KOs and were not significantly altered by expression of IPPK or MINPP1. The only other retrovirus found to show a clear IP6/IP5 dependence was the primate (macaque) Lentivirus Simian Immunodeficiency Virus, which displayed similar sensitivity as HIV-1. We were not able to determine if producer cell IP6/IP5 is required at additional steps beyond assembly because viral particles devoid of both molecules could not be generated. Finally, we found that loss of IP6/IP5 in viral target cells had no effect on permissivity to HIV-1 infection.

Structures of immature EIAV Gag lattices reveal a conserved role for IP6 in lentivirus assembly.

Retrovirus assembly is driven by the multidomain structural protein Gag. Interactions between the capsid domains (CA) of Gag result in Gag multimerization, leading to an immature virus particle that is formed by a protein lattice based on dimeric, trimeric, and hexameric protein contacts. Among retroviruses the inter- and intra-hexamer contacts differ, especially in the N-terminal sub-domain of CA (CANTD). For HIV-1 the cellular molecule inositol hexakisphosphate (IP6) interacts with and stabilizes the immature hexamer, and is required for production of infectious virus particles. We have used in vitro assembly, cryo-electron tomography and subtomogram averaging, atomistic molecular dynamics simulations and mutational analyses to study the HIV-related lentivirus equine infectious anemia virus (EIAV). In particular, we sought to understand the structural conservation of the immature lentivirus lattice and the role of IP6 in EIAV assembly. Similar to HIV-1, IP6 strongly promoted in vitro assembly of EIAV Gag proteins into virus-like particles (VLPs), which took three morphologically highly distinct forms: narrow tubes, wide tubes, and spheres. Structural characterization of these VLPs to sub-4Å resolution unexpectedly showed that all three morphologies are based on an immature lattice with preserved key structural components, highlighting the structural versatility of CA to form immature assemblies. A direct comparison between EIAV and HIV revealed that both lentiviruses maintain similar immature interfaces, which are established by both conserved and non-conserved residues. In both EIAV and HIV-1, IP6 regulates immature assembly via conserved lysine residues within the CACTD and SP. Lastly, we demonstrate that IP6 stimulates in vitro assembly of immature particles of several other retroviruses in the lentivirus genus, suggesting a conserved role for IP6 in lentiviral assembly.

Optimized Pseudotyping Conditions for the SARS-COV-2 Spike Glycoprotein.

The severe acute respiratory syndrome coronavirus 2 (SARS-COV-2) Spike glycoprotein is solely responsible for binding to the host cell receptor and facilitating fusion between the viral and host membranes. The ability to generate viral particles pseudotyped with SARS-COV-2 Spike is useful for many types of studies, such as characterization of neutralizing antibodies or development of fusion-inhibiting small molecules. Here, we characterized the use of a codon-optimized SARS-COV-2 Spike glycoprotein for the generation of pseudotyped HIV-1, murine leukemia virus (MLV), and vesicular stomatitis virus (VSV) particles. The full-length Spike protein functioned inefficiently with all three systems but was enhanced over 10-fold by deleting the last 19 amino acids of the cytoplasmic tail. Infection of 293FT target cells was possible only if the cells were engineered to stably express the human angiotensin-converting enzyme 2 (ACE2) receptor, but stably introducing an additional copy of this receptor did not further enhance susceptibility. Stable introduction of the Spike-activating protease TMPRSS2 further enhanced susceptibility to infection by 5- to 10-fold. Replacement of the signal peptide of the Spike protein with an optimal signal peptide did not enhance or reduce infectious particle production. However, modifications D614G and R682Q further enhanced infectious particle production. With all enhancing elements combined, the titer of pseudotyped HIV-1 particles reached almost 106 infectious particles/ml. Finally, HIV-1 particles pseudotyped with SARS-COV-2 Spike were successfully used to detect neutralizing antibodies in plasma from coronavirus disease 2019 (COVID-19) patients, but not in plasma from uninfected individuals.IMPORTANCE In work with pathogenic viruses, it is useful to have rapid quantitative tests for viral infectivity that can be performed without strict biocontainment restrictions. A common way of accomplishing this is to generate viral pseudoparticles that contain the surface glycoprotein from the pathogenic virus incorporated into a replication-defective viral particle that contains a sensitive reporter system. These pseudoparticles enter cells using the glycoprotein from the pathogenic virus, leading to a readout for infection. Conditions that block entry of the pathogenic virus, such as neutralizing antibodies, will also block entry of the viral pseudoparticles. However, viral glycoproteins often are not readily suited for generating pseudoparticles. Here, we describe a series of modifications that result in the production of relatively high-titer SARS-COV-2 pseudoparticles that are suitable for the detection of neutralizing antibodies from COVID-19 patients.

Multiple Gag domains contribute to selective recruitment of murine leukemia virus (MLV) Env to MLV virions.

Retroviruses, like all enveloped viruses, must incorporate viral glycoproteins to form infectious particles. Interactions between the glycoprotein cytoplasmic tail and the matrix domain of Gag are thought to direct recruitment of glycoproteins to native virions for many retroviruses. However, retroviruses can also incorporate glycoproteins from other viruses to form infectious virions known as pseudotyped particles. The glycoprotein murine leukemia virus (MLV) Env can readily form pseudotyped particles with many retroviruses, suggesting a generic mechanism for recruitment. Here, we sought to identify which components of Gag, particularly the matrix domain, contribute to recruitment of MLV Env into retroviral particles. Unexpectedly, we discovered that the matrix domain of HIV-1 Gag is dispensable for generic recruitment, since it could be replaced with a nonviral membrane-binding domain without blocking active incorporation of MLV Env into HIV virions. However, MLV Env preferentially assembles with MLV virions. When MLV and HIV particles are produced from the same cell, MLV Env is packaged almost exclusively by MLV particles, thus preventing incorporation into HIV particles. Surprisingly, the matrix domain of MLV Gag is not required for this selectivity, since MLV Gag containing the matrix domain from HIV is still able to outcompete HIV particles for MLV Env. Although MLV Gag is sufficient for selective incorporation to occur, no single Gag domain dictates the selectivity. Our findings indicate that Env recruitment is more complex than previously believed and that Gag assembly/budding sites have fundamental properties that affect glycoprotein incorporation.

CRM1-dependent trafficking of retroviral Gag proteins revisited.

We analyzed the nuclear trafficking ability of Gag proteins from six retroviral genera. Contrary to a previous report, human immunodeficiency virus type 1 (HIV-1) Gag showed no propensity to cycle through the nucleus. The only Gag protein that displayed CRM1-dependent nuclear cycling was that of Rous sarcoma virus (RSV). Surprisingly, this cycling could be eliminated without compromising infectivity by replacing the RSV Gag N-terminal matrix (MA) domain with HIV MA.

Novel Compound Inhibitors of HIV-1NL4-3 Vpu.

HIV-1 Vpu targets the host cell proteins CD4 and BST-2/Tetherin for degradation, ultimately resulting in enhanced virus spread and host immune evasion. The discovery and characterization of small molecules that antagonize Vpu would further elucidate the contribution of Vpu to pathogenesis and lay the foundation for the study of a new class of novel HIV-1 therapeutics. To identify novel compounds that block Vpu activity, we have developed a cell-based 'gain of function' assay that produces a positive signal in response to Vpu inhibition. To develop this assay, we took advantage of the viral glycoprotein, GaLV Env. In the presence of Vpu, GaLV Env is not incorporated into viral particles, resulting in non-infectious virions. Vpu inhibition restores infectious particle production. Using this assay, a high throughput screen of >650,000 compounds was performed to identify inhibitors that block the biological activity of Vpu. From this screen, we identified several positive hits but focused on two compounds from one structural family, SRI-41897 and SRI-42371. We developed independent counter-screens for off target interactions of the compounds and found no off target interactions. Additionally, these compounds block Vpu-mediated modulation of CD4, BST-2/Tetherin and antibody dependent cell-mediated toxicity (ADCC). Unfortunately, both SRI-41897 and SRI-42371 were shown to be specific to the N-terminal region of NL4-3 Vpu and did not function against other, more clinically relevant, strains of Vpu; however, this assay may be slightly modified to include more significant Vpu strains in the future.

Tracking cryptic SARS-CoV-2 lineages detected in NYC wastewater.

Tracking SARS-CoV-2 genetic diversity is strongly indicated because diversifying selection may lead to the emergence of novel variants resistant to naturally acquired or vaccine-induced immunity. To monitor New York City (NYC) for the presence of novel variants, we deep sequence most of the receptor binding domain coding sequence of the S protein of SARS-CoV-2 isolated from the New York City wastewater. Here we report detecting increasing frequencies of novel cryptic SARS-CoV-2 lineages not recognized in GISAID's EpiCoV database. These lineages contain mutations that had been rarely observed in clinical samples, including Q493K, Q498Y, E484A, and T572N and share many mutations with the Omicron variant of concern. Some of these mutations expand the tropism of SARS-CoV-2 pseudoviruses by allowing infection of cells expressing the human, mouse, or rat ACE2 receptor. Finally, pseudoviruses containing the spike amino acid sequence of these lineages were resistant to different classes of receptor binding domain neutralizing monoclonal antibodies. We offer several hypotheses for the anomalous presence of these lineages, including the possibility that these lineages are derived from unsampled human COVID-19 infections or that they indicate the presence of a non-human animal reservoir.

Pseudotyping incompatibility between HIV-1 and gibbon ape leukemia virus Env is modulated by Vpu.

The Env protein from gibbon ape leukemia virus (GaLV) has been shown to be incompatible with human immunodeficiency virus type 1 (HIV-1) in the production of infectious pseudotyped particles. This incompatibility has been mapped to the C-terminal cytoplasmic tail of GaLV Env. Surprisingly, we found that the HIV-1 accessory protein Vpu modulates this incompatibility. The infectivity of HIV-1 pseudotyped with murine leukemia virus (MLV) Env was not affected by Vpu. However, the infectivity of HIV-1 pseudotyped with an MLV Env with the cytoplasmic tail from GaLV Env (MLV/GaLV Env) was restricted 50- to 100-fold by Vpu. A Vpu mutant containing a scrambled membrane-spanning domain, Vpu(RD), was still able to restrict MLV/GaLV Env, but mutation of the serine residues at positions 52 and 56 completely alleviated the restriction. Loss of infectivity appeared to be caused by reduced MLV/GaLV Env incorporation into viral particles. The mechanism of this downmodulation appears to be distinct from Vpu-mediated CD4 downmodulation because Vpu-expressing cells that failed to produce infectious HIV-1 particles nonetheless continued to display robust surface MLV/GaLV Env expression. In addition, if MLV and HIV-1 were simultaneously introduced into the same cells, only the HIV-1 particle infectivity was restricted by Vpu. Collectively, these data suggest that Vpu modulates the cellular distribution of MLV/GaLV Env, preventing its recruitment to HIV-1 budding sites.

Diphtheria Toxin A-Resistant Cell Lines Enable Robust Production and Evaluation of DTA-Encoding Lentiviruses.

Suicide genes have been widely investigated for their utility as therapeutic agents and as tools for in vitro negative selection strategies. Several methods for delivery of suicide genes have been explored. Two important considerations for delivery are the quantity of delivered cargo and the ability to target the cargo to specific cells. Delivery using a lentiviral vector is particularly attractive due to the ability to encode the gene within the viral genome, as well as the ability to limit off-target effects by using cell type-specific glycoproteins. Here, we present the design and validation of a diphtheria toxin A (DTA)-encoding lentiviral vector expressing DTA under the control of a constituitive promoter to allow for expression of DTA in a variety of cell types, with specificity provided via selection of glycoproteins for pseudotyping of the lentiviral particles. DTA exerts its toxic activity through inhibition of eukaryotic translation elongation factor 2 (eEF2) via adenosine diphosphate (ADP)-ribosylation of a modified histidine residue, diphthamide, at His715, which blocks protein translation and leads to cell death. Thus, we also detail development of DTA-resistant cell lines, engineered through CRISPR/Cas9-mediated knockout of the diphthamide 1 (DPH1) gene, which enable both robust virus production by transfection and evaluation of DTA-expressing virus infectivity.

ßTrCP is Required for HIV-1 Vpu Modulation of CD4, GaLV Env, and BST-2/Tetherin.

The Human immunodeficiency virus-1 (HIV-1) accessory protein Vpu modulates numerous proteins, including the host proteins CD4 and BST-2/tetherin. Vpu interacts with the Skp, Cullin, F-Box (SCF) ubiquitin ligase through interactions with the F-Box protein ßTrCP (1 and/or 2). This interaction is dependent on phosphorylation of S52,56 in Vpu. Mutation of S52,56, or inhibition of the SCF, abolishes most Vpu activity against CD4 and partly reduces activity against BST-2/tetherin. Recently, Vpu has also been reported to interact with the clathrin adapter proteins AP-1 and AP-2, and these interactions were also found to be required for BST-2/tetherin antagonism in an S52,56 -dependent manner. In assays where HIV-1 is pseudotyped with gibbon ape leukemia virus (GaLV Env), Vpu has also been found to prevent GaLV Env from being incorporated into viral particles, but the mechanism for this antagonism is not fully understood. To clarify the role of the ßTrCPs in Vpu function we used CRISPR/Cas9 to generate a clonal cell line lacking both ßTrCP-1 and -2. Vpu activity against CD4 and GaLV Env was abolished in this cell line, and activity against BST-2/tetherin reduced significantly. Mutation of the S52,56 residues no longer affected Vpu activity against BST-2/tetherin in this cell line. These data suggest that the primary role of the S52,56 residues in antagonism of CD4, GaLV Env, and BST-2/tetherin is to recruit the SCF/ßTrCP ubiquitin ligase.

Human endometrial epithelial cells cyclically express Toll-like receptor 3 (TLR3) and exhibit TLR3-dependent responses to dsRNA.

Toll-like receptor 3 (TLR3) responds to dsRNA, a product of most viral life cycles, and initiates production of proinflammatory and antiviral cytokines. The role of TLR3 in human mucosal immunity of the endometrium has not been examined. The effects of TLR3 ligation in endometrial epithelium could be significant as the endometrium is a significant site for viral entry and infection. Additionally, the cytokine milieu plays an essential role in normal functions of the endometrium such as uterine cycle progression, epithelial proliferation and shedding, and embryo implantation. In this study, we demonstrated cycle dependent expression of functional TLR3 in primary endometrial epithelial tissue and expression of intracellular TLR3 in human endometrial epithelial cell lines. We established that stimulation of TLR3-positive cell lines and primary human endometrial epithelial cells with dsRNA leads to TLR3-dependent expression of interleukin (IL)-6, IL-8, interferon (IFN)-inducible protein 10, RANTES, and IFN-beta. These results indicate that the cytokine profile of human endometrial epithelial cells can be modified through TLR3 stimulation. Our findings suggest that TLR3 is involved in the immune responses of endometrial epithelial cells after exposure to dsRNA and has the potential to alter the cytokine milieu and influence the outcome and consequences of infection.

Two distinct mechanisms regulate recruitment of murine leukemia virus envelope protein to retroviral assembly sites.

The cytoplasmic tail domain (CTD) of retroviral envelope (Env) proteins has been implicated in modulating Env incorporation into viral particles. We generated a panel of murine leukemia virus (MLV) Env mutants and analyzed their ability to be recruited to human immunodeficiency virus-1 (HIV-1) assembly sites. Surprisingly, the entire CTD was dispensable for recruitment to assembly sites, but a mutation that disrupted the furin cleavage site in Env abolished recruitment. To determine if MLV Env can show selectivity for homologous assembly sites, cells were co-transfected with both HIV-1 and MLV assembly components along with each MLV Env construct and assayed for infectious particle production. MLV Env selectively formed infectious particles with the MLV components at the expense of infectious HIV-1 infectious particle production, but truncation of the CTD progressively reduced this selectivity. Collectively these data suggest that there are two separable mechanisms that govern MLV Env recruitment to viral assembly sites.

Expression of Toll-like receptors in human endometrial epithelial cells and cell lines.

PROBLEM: Are toll-like receptors (TLRs) expressed by human endometrium and endometrial cell lines? METHODS OF STUDY: Expression of each TLR mRNA species was determined by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of proliferative-phase human endometrium, separated endometrial epithelial cells, and the Ishikawa and RL95-2 endometrial epithelial cell lines. U-937 and SKW 6.4 cell lines were used as positive controls. Functional relevance of PCR findings was tested by enzyme-linked immunsorbent assay (ELISA)analysis of IL-8 production after stimulating cell lines with ligands for TLR2-5. RESULTS: TLR1-6 and 9 mRNA species were detected in both whole endometrium and separated endometrial epithelial cells. Ishikawa cells expressed TLR2 and TLR5, while RL95-2 cells expressed TLR3, 5, and 9. Response of RL95-2, Ishikawa, and U-937 cells to TLR2-5 ligands was consistent with RT-PCR findings except response to flagellin by Ishikawa cells. CONCLUSION: These studies provide the first evidence of TLR expression in the endometrium of any species and suggest the usefulness of endometrial cell lines to study TLR function.