Characterization of intracellular reverse transcription complexes of Moloney murine leukemia virus

Capsid-host interactions for HIV-1 ingress

The HIV-1 capsid, composed of approximately 1,200 copies of the capsid protein, encases genomic RNA alongside viral nucleocapsid, reverse transcriptase, and integrase proteins. After cell entry, the capsid interacts with a myriad of host factors to traverse the cell cytoplasm, pass through the nuclear pore complex (NPC), and then traffic to chromosomal sites for viral DNA integration. Integration may very well require the dissolution of the capsid, but where and when this uncoating event occurs remains hotly debated. Based on size constraints, a long-prevailing view was that uncoating preceded nuclear transport, but recent research has indicated that the capsid may remain largely intact during nuclear import, with perhaps some structural remodeling required for NPC traversal. Completion of reverse transcription in the nucleus may further aid capsid uncoating. One canonical type of host factor, typified by CPSF6, leverages a Phe-Gly (FG) motif to bind capsid. Recent research has shown these peptides reside amid prion-like domains (PrLDs), which are stretches of protein sequence devoid of charged residues. Intermolecular PrLD interactions along the exterior of the capsid shell impart avid host factor binding for productive HIV-1 infection. Herein we overview capsid-host interactions implicated in HIV-1 ingress and discuss important research questions moving forward. Highlighting clinical relevance, the long-acting ultrapotent inhibitor lenacapavir, which engages the same capsid binding pocket as FG host factors, was recently approved to treat people living with HIV.

The Role of Capsid in the Early Steps of HIV-1 Infection: New Insights into the Core of the Matter

In recent years, major advances in research and experimental approaches have significantly increased our knowledge on the role of the HIV-1 capsid in the virus life cycle, from reverse transcription to integration and gene expression. This makes the capsid protein a good pharmacological target to inhibit HIV-1 replication. This review covers our current understanding of the role of the viral capsid in the HIV-1 life cycle and its interaction with different host factors that enable reverse transcription, trafficking towards the nucleus, nuclear import and integration into host chromosomes. It also describes different promising small molecules, some of them in clinical trials, as potential targets for HIV-1 therapy.

Post-mitotic BET-induced reshaping of integrase quaternary structure supports wild-type MLV integration

The Moloney murine leukemia virus (MLV) is a prototype gammaretrovirus requiring nuclear disassembly before DNA integration. In the nucleus, integration site selection towards promoter/enhancer elements is mediated by the host factor bromo- and extraterminal domain (BET) proteins (bromodomain (Brd) proteins 2, 3 and 4). MLV-based retroviral vectors are used in gene therapy trials. In some trials leukemia occurred through integration of the MLV vector in close proximity to cellular oncogenes. BET-mediated integration is poorly understood and the nature of integrase oligomers heavily debated. Here, we created wild-type infectious MLV vectors natively incorporating fluorescent labeled IN and performed single-molecule intensity and Förster resonance energy transfer experiments. The nuclear localization of the MLV pre-integration complex neither altered the IN content, nor its quaternary structure. Instead, BET-mediated interaction of the MLV intasome with chromatin in the post-mitotic nucleus reshaped its quaternary structure.

A Novel Phenotype Links HIV-1 Capsid Stability to cGAS-Mediated DNA Sensing

The HIV-1 capsid executes essential functions that are regulated by capsid stability and host factors. In contrast to increasing knowledge on functional roles of capsid-interacting host proteins during postentry steps, less is known about capsid stability and its impact on intracellular events. Here, using the antiviral compound PF-3450074 (PF74) as a probe for capsid function, we uncovered a novel phenotype of capsid stability that has a profound effect on innate sensing of viral DNA by the DNA sensor cGAS. A single mutation, R143A, in the capsid protein conferred resistance to high concentrations of PF74, without affecting capsid binding to PF74. A cell-free assay showed that the R143A mutant partially counteracted the capsid-destabilizing activity of PF74, pointing to capsid stabilization as a resistance mechanism for the R143A mutant. In monocytic THP-1 cells, the R143A virus, but not the wild-type virus, suppressed cGAS-dependent innate immune activation. These results suggest that capsid stabilization improves the shielding of viral DNA from innate sensing. We found that a naturally occurring transmitted founder (T/F) variant shares the same properties as the R143A mutant with respect to PF74 resistance and DNA sensing. Imaging assays revealed delayed uncoating kinetics of this T/F variant and the R143A mutant. All these phenotypes of this T/F variant were controlled by a genetic polymorphism located at the trimeric interface between capsid hexamers, thus linking these capsid-dependent properties. Overall, this work functionally connects capsid stability to innate sensing of viral DNA and reveals naturally occurring phenotypic variation in HIV-1 capsid stability.IMPORTANCE The HIV-1 capsid, which is made from individual viral capsid proteins (CA), is a target for a number of antiviral compounds, including the small-molecule inhibitor PF74. In the present study, we utilized PF74 to identify a transmitted/founder (T/F) strain that shows increased capsid stability. Interestingly, PF74-resistant variants prevented cGAS-dependent innate immune activation under a condition where the other T/F strains induced type I interferon. These observations thus reveal a new CA-specific phenotype that couples capsid stability to viral DNA recognition by cytosolic DNA sensors.

Multiple Roles of HIV-1 Capsid during the Virus Replication Cycle

Human immunodeficiency virus-1 capsid (HIV-1 CA) is involved in different stages of the viral replication cycle. During virion assembly, CA drives the formation of the hexameric lattice in immature viral particles, while in mature virions CA monomers assemble in cone-shaped cores surrounding the viral RNA genome and associated proteins. In addition to its functions in late stages of the viral replication cycle, CA plays key roles in a number of processes during early phases of HIV-1 infection including trafficking, uncoating, recognition by host cellular proteins and nuclear import of the viral pre-integration complex. As a result of efficient cooperation of CA with other viral and cellular proteins, integration of the viral genetic material into the host genome, which is an essential step for productive viral infection, successfully occurs. In this review, we will summarize available data on CA functions in HIV-1 replication, describing in detail its roles in late and early phases of the viral replication cycle.

Cellular and molecular mechanisms of HIV-1 integration targeting

Integration is central to HIV-1 replication and helps mold the reservoir of cells that persists in AIDS patients. HIV-1 interacts with specific cellular factors to target integration to interior regions of transcriptionally active genes within gene-dense regions of chromatin. The viral capsid interacts with several proteins that are additionally implicated in virus nuclear import, including cleavage and polyadenylation specificity factor 6, to suppress integration into heterochromatin. The viral integrase protein interacts with transcriptional co-activator lens epithelium-derived growth factor p75 to principally position integration within gene bodies. The integrase additionally senses target DNA distortion and nucleotide sequence to help fine-tune the specific phosphodiester bonds that are cleaved at integration sites. Research into virus-host interactions that underlie HIV-1 integration targeting has aided the development of a novel class of integrase inhibitors and may help to improve the safety of viral-based gene therapy vectors.

Murine leukemia virus p12 tethers the capsid-containing pre-integration complex to chromatin by binding directly to host nucleosomes in mitosis

The murine leukaemia virus (MLV) Gag cleavage product, p12, is essential for both early and late steps in viral replication. The N-terminal domain of p12 binds directly to capsid (CA) and stabilises the mature viral core, whereas defects in the C-terminal domain (CTD) of p12 can be rescued by addition of heterologous chromatin binding sequences (CBSs). We and others hypothesised that p12 tethers the pre-integration complex (PIC) to host chromatin ready for integration. Using confocal microscopy, we have observed for the first time that CA localises to mitotic chromatin in infected cells in a p12-dependent manner. GST-tagged p12 alone, however, did not localise to chromatin and mass-spectrometry analysis of its interactions identified only proteins known to bind the p12 region of Gag. Surprisingly, the ability to interact with chromatin was conferred by a single amino acid change, M63I, in the p12 CTD. Interestingly, GST-p12_M63I showed increased phosphorylation in mitosis relative to interphase, which correlated with an increased interaction with mitotic chromatin. Mass-spectrometry analysis of GST-p12_M63I revealed nucleosomal histones as primary interactants. Direct binding of MLV p12_M63I peptides to histones was confirmed by biolayer-interferometry (BLI) assays using highly-avid recombinant poly-nucleosomal arrays. Excitingly, using this method, we also observed binding between MLV p12_WT and nucleosomes. Nucleosome binding was additionally detected with p12 orthologs from feline and gibbon ape leukemia viruses using both pull-down and BLI assays, indicating that this a common feature of gammaretroviral p12 proteins. Importantly, p12 peptides were able to block the binding of the prototypic foamy virus CBS to nucleosomes and vice versa, implying that their docking sites overlap and suggesting a conserved mode of chromatin tethering for different retroviral genera. We propose that p12 is acting in a similar capacity to CPSF6 in HIV-1 infection by facilitating initial chromatin targeting of CA-containing PICs prior to integration.

CRISPR/Cas9 therapeutics for liver diseases

The development of innovative genome editing techniques in recent years has revolutionized the field of biomedicine. Among the novel approaches, the clustered regularly interspaced short palindromic repeat/CRISPR-associated protein (CRISPR/Cas9) technology has become the most popular, in part due to its matchless ability to carry out gene editing at the target site with great precision. With considerable successes in animal and preclinical studies, CRISPR/Cas9-mediated gene editing has paved the way for its use in human trials, including patients with a variety of liver diseases. Gene editing is a logical therapeutic approach for liver diseases because many metabolic and acquired disorders are caused by mutations within a single gene. In this review, we provide an overview on current and emerging therapeutic strategies for the treatment of liver diseases using the CRISPR/Cas9 technology.

X-ray crystal structure of the N-terminal region of Moloney murine leukemia virus integrase and its implications for viral DNA recognition

The retroviral integrase (IN) carries out the integration of a dsDNA copy of the viral genome into the host DNA, an essential step for viral replication. All IN proteins have three general domains, the N-terminal domain (NTD), the catalytic core domain, and the C-terminal domain. The NTD includes an HHCC zinc finger-like motif, which is conserved in all retroviral IN proteins. Two crystal structures of Moloney murine leukemia virus (M-MuLV) IN N-terminal region (NTR) constructs that both include an N-terminal extension domain (NED, residues 1-44) and an HHCC zinc-finger NTD (residues 45-105), in two crystal forms are reported. The structures of IN NTR constructs encoding residues 1-105 (NTR_1-105 ) and 8-105 (NTR_8-105 ) were determined at 2.7 and 2.15 Å resolution, respectively and belong to different space groups. While both crystal forms have similar protomer structures, NTR_1-105 packs as a dimer and NTR_8-105 packs as a tetramer in the asymmetric unit. The structure of the NED consists of three anti-parallel β-strands and an α-helix, similar to the NED of prototype foamy virus (PFV) IN. These three β-strands form an extended β-sheet with another β-strand in the HHCC Zn²⁺ binding domain, which is a unique structural feature for the M-MuLV IN. The HHCC Zn²⁺ binding domain structure is similar to that in HIV and PFV INs, with variations within the loop regions. Differences between the PFV and MLV IN NEDs localize at regions identified to interact with the PFV LTR and are compared with established biochemical and virological data for M-MuLV. Proteins 2017; 85:647-656. © 2016 Wiley Periodicals, Inc.

Phosphorylation Requirement of Murine Leukemia Virus p12

The p12 protein of murine leukemia virus (MLV) Gag is associated with the preintegration complex (PIC), and mutants of p12 (PM14) exhibit defects in nuclear entry/retention. Mutants of the phosphorylated serine 61 also have been reported to have defects in the early life cycle. Here we show that a phosphorylated peptide motif derived from human papillomavirus 8 (HPV-8), the E2 hinge region including residues 240 to 255, can functionally replace the main phosphorylated motif of MLV p12 and can rescue the viral titer of a strain with the lethal p12-PM14 mutation. Complementation with the HPV-8 E2 hinge motif generated multiple second-site mutations in live viral passage assays. Additional p12 phosphorylation sites were detected, including the late domain of p12 (PPPY) as well as the late domain/protease cleavage site of matrix (LYPAL), by mass spectrometry and Western blotting. Chromatin binding of p12-green fluorescent protein (GFP) fusion protein and functional complementation of p12-PM14 occurred in a manner independent of the E2 hinge region phosphorylation. Replacement of serine 61 by alanine within the minimal tethering domain (₆₁SPMASRLRGRR₇₁) maintained tethering, but in the context of the full-length p12, mutants with substitutions in S61 remained untethered and lost infectivity, indicating phosphorylation of p12 serine 61 functions to temporally regulate early and late p12 functions.

IMPORTANCE

The p12 protein, required for both early and late viral functions, is the predominant phosphorylated viral protein of Moloney MLV and is required for virus viability. Our studies indicate that the N terminus of p12 represses the early function of the chromatin binding domain and that deletion of the N terminus activates chromatin binding in the wild-type Moloney MLV p12 protein. Mass spectrometry and mutagenesis studies suggest that phosphorylation of both the repression domain and the chromatin binding domain acts to temporally regulate this process at the appropriate stages during infection.

Histones Are Rapidly Loaded onto Unintegrated Retroviral DNAs Soon after Nuclear Entry

Chromosomal structure of nuclear DNA is usually maintained by insertion of nucleosomes into preexisting chromatin, both on newly synthesized DNA at replication forks and at sites of DNA damage. But during retrovirus infection, a histone-free DNA copy of the viral genome is synthesized that must be loaded with nucleosomes de novo. Here, we show that core histones are rapidly loaded onto unintegrated Moloney murine leukemia virus DNAs. Loading of nucleosomes requires nuclear entry, but does not require viral DNA integration. The histones associated with unintegrated DNAs become marked by covalent modifications, with a delay relative to the time of core histone loading. Expression from unintegrated DNA can be enhanced by modulation of the histone-modifying machinery. The data show that histone loading onto unintegrated DNAs occurs very rapidly after nuclear entry and does not require prior establishment of an integrated provirus.

Structure of the Brd4 ET domain bound to a C-terminal motif from γ-retroviral integrases reveals a conserved mechanism of interaction

The bromodomain and extraterminal domain (BET) protein family are promising therapeutic targets for a range of diseases linked to transcriptional activation, cancer, viral latency, and viral integration. Tandem bromodomains selectively tether BET proteins to chromatin by engaging cognate acetylated histone marks, and the extraterminal (ET) domain is the focal point for recruiting a range of cellular and viral proteins. BET proteins guide γ-retroviral integration to transcription start sites and enhancers through bimodal interaction with chromatin and the γ-retroviral integrase (IN). We report the NMR-derived solution structure of the Brd4 ET domain bound to a conserved peptide sequence from the C terminus of murine leukemia virus (MLV) IN. The complex reveals a protein-protein interaction governed by the binding-coupled folding of disordered regions in both interacting partners to form a well-structured intermolecular three-stranded β sheet. In addition, we show that a peptide comprising the ET binding motif (EBM) of MLV IN can disrupt the cognate interaction of Brd4 with NSD3, and that substitutions of Brd4 ET residues essential for binding MLV IN also impair interaction of Brd4 with a number of cellular partners involved in transcriptional regulation and chromatin remodeling. This suggests that γ-retroviruses have evolved the EBM to mimic a cognate interaction motif to achieve effective integration in host chromatin. Collectively, our findings identify key structural features of the ET domain of Brd4 that allow for interactions with both cellular and viral proteins.

SJP-L-5, a novel small-molecule compound, inhibits HIV-1 infection by blocking viral DNA nuclear entry

Background

Small-molecule compounds that inhibit human immunodeficiency virus type 1 (HIV-1) infection can be used not only as drug candidates, but also as reagents to dissect the life cycle of the virus. Thus, it is desirable to have an arsenal of such compounds that inhibit HIV-1 infection by various mechanisms. Until now, only a few small-molecule compounds that inhibit nuclear entry of viral DNA have been documented.

Results

We identified a novel, small-molecule compound, SJP-L-5, that inhibits HIV-1 infection. SJP-L-5 is a nitrogen-containing, biphenyl compound whose synthesis was based on the dibenzocyclooctadiene lignan gomisin M2, an anti-HIV bioactive compound isolated from Schisandra micrantha A. C. Smith. SJP-L-5 displayed relatively low cytotoxicity (50 % cytoxicity concentrations were greater than 200 μg/ml) and high antiviral activity against a variety of HIV strains (50 % effective concentrations (EC₅₀)) of HIV-1 laboratory-adapted strains ranged from 0.16–0.97 μg/ml; EC₅₀s of primary isolates ranged from 1.96–5.33 μg/ml). Analyses of the viral DNA synthesis indicated that SJP-L-5 specifically blocks the entry of the HIV-1 pre-integration complex (PIC) into the nucleus. Further results implicated that SJP-L-5 inhibits the disassembly of HIV-1 particulate capsid in the cytoplasm of the infected cells.

Conclusions

SJP-L-5 is a novel small-molecule compound that inhibits HIV-1 nuclear entry by blocking the disassembly of the viral core.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0605-3) contains supplementary material, which is available to authorized users.

HIV-1 capsid: the multifaceted key player in HIV-1 infection

In a mature, infectious HIV-1 virion, the viral genome is housed within a conical capsid core made from the viral capsid (CA) protein. The CA protein and the structure into which it assembles facilitate virtually every step of infection through a series of interactions with multiple host cell factors. This Review describes our understanding of the interactions between the viral capsid core and several cellular factors that enable efficient HIV-1 genome replication, timely core disassembly, nuclear import and the integration of the viral genome into the genome of the target cell. We then discuss how elucidating these interactions can reveal new targets for therapeutic interactions against HIV-1.

Liver-targeted gene therapy: Approaches and challenges

The liver plays a major role in many inherited and acquired genetic disorders. It is also the site for the treatment of certain inborn errors of metabolism that do not directly cause injury to the liver. The advancement of nucleic acid-based therapies for liver maladies has been severely limited because of the myriad untoward side effects and methodological limitations. To address these issues, research efforts in recent years have been intensified toward the development of targeted gene approaches using novel genetic tools, such as zinc-finger nucleases, transcription activator-like effector nucleases, and clustered regularly interspaced short palindromic repeats as well as various nonviral vectors such as Sleeping Beauty transposons, PiggyBac transposons, and PhiC31 integrase. Although each of these methods uses a distinct mechanism of gene modification, all of them are dependent on the efficient delivery of DNA and RNA molecules into the cell. This review provides an overview of current and emerging therapeutic strategies for liver-targeted gene therapy and gene repair.

Retroviral Integrase Structure and DNA Recombination Mechanism

Due to the importance of human immunodeficiency virus type 1 (HIV-1) integrase as a drug target, the biochemistry and structural aspects of retroviral DNA integration have been the focus of intensive research during the past three decades. The retroviral integrase enzyme acts on the linear double-stranded viral DNA product of reverse transcription. Integrase cleaves specific phosphodiester bonds near the viral DNA ends during the 3' processing reaction. The enzyme then uses the resulting viral DNA 3'-OH groups during strand transfer to cut chromosomal target DNA, which simultaneously joins both viral DNA ends to target DNA 5'-phosphates. Both reactions proceed via direct transesterification of scissile phosphodiester bonds by attacking nucleophiles: a water molecule for 3' processing, and the viral DNA 3'-OH for strand transfer. X-ray crystal structures of prototype foamy virus integrase-DNA complexes revealed the architectures of the key nucleoprotein complexes that form sequentially during the integration process and explained the roles of active site metal ions in catalysis. X-ray crystallography furthermore elucidated the mechanism of action of HIV-1 integrase strand transfer inhibitors, which are currently used to treat AIDS patients, and provided valuable insights into the mechanisms of viral drug resistance.

Quantitative microscopy of functional HIV post-entry complexes reveals association of replication with the viral capsid

The steps from HIV-1 cytoplasmic entry until integration of the reverse transcribed genome are currently enigmatic. They occur in ill-defined reverse-transcription- and pre-integration-complexes (RTC, PIC) with various host and viral proteins implicated. In this study, we report quantitative detection of functional RTC/PIC by labeling nascent DNA combined with detection of viral integrase. We show that the viral CA (capsid) protein remains associated with cytoplasmic RTC/PIC but is lost on nuclear PIC in a HeLa-derived cell line. In contrast, nuclear PIC were almost always CA-positive in primary human macrophages, indicating nuclear import of capsids or capsid-like structures. We further show that the CA-targeted inhibitor PF74 exhibits a bimodal mechanism, blocking RTC/PIC association with the host factor CPSF6 and nuclear entry at low, and abrogating reverse transcription at high concentrations. The newly developed system is ideally suited for studying retroviral post-entry events and the roles of host factors including DNA sensors and signaling molecules.

The N-terminus of murine leukaemia virus p12 protein is required for mature core stability

The murine leukaemia virus (MLV) gag gene encodes a small protein called p12 that is essential for the early steps of viral replication. The N- and C-terminal regions of p12 are sequentially acting domains, both required for p12 function. Defects in the C-terminal domain can be overcome by introducing a chromatin binding motif into the protein. However, the function of the N-terminal domain remains unknown. Here, we undertook a detailed analysis of the effects of p12 mutation on incoming viral cores. We found that both reverse transcription complexes and isolated mature cores from N-terminal p12 mutants have altered capsid complexes compared to wild type virions. Electron microscopy revealed that mature N-terminal p12 mutant cores have different morphologies, although immature cores appear normal. Moreover, in immunofluorescent studies, both p12 and capsid proteins were lost rapidly from N-terminal p12 mutant viral cores after entry into target cells. Importantly, we determined that p12 binds directly to the MLV capsid lattice. However, we could not detect binding of an N-terminally altered p12 to capsid. Altogether, our data imply that p12 stabilises the mature MLV core, preventing premature loss of capsid, and that this is mediated by direct binding of p12 to the capsid shell. In this manner, p12 is also retained in the pre-integration complex where it facilitates tethering to mitotic chromosomes. These data also explain our previous observations that modifications to the N-terminus of p12 alter the ability of particles to abrogate restriction by TRIM5alpha and Fv1, factors that recognise viral capsid lattices.

All retroviral genomes contain a gag gene that codes for the Gag polyprotein. Gag is cleaved upon viral maturation to release individual proteins, including matrix, capsid and nucleocapsid, providing the structural components of the virion. In murine leukaemia virus (MLV), Gag cleavage releases an additional protein, named p12, required for both early and late stages of the viral life cycle. The role of p12 during early events is poorly understood, and it is the only MLV protein without a function-associated name. Here, we show that p12 binds to the capsid shell of the viral core and stabilises it. Mutations that give rise to N-terminally altered p12 proteins result in a rapid loss of both p12 and capsid from viral cores, leading to abnormal core morphologies and abolishing the ability of particles to abrogate restriction by cellular factors that target viral capsid lattices. Understanding how the mature retroviral core forms and how it disassembles during infection is important as this determines the infectivity of all retroviruses, including HIV-1. Furthermore, altering core stability has recently become a novel target for HIV-1 therapeutics.

HIV-1 uncoating is facilitated by dynein and kinesin 1

Following entry into the target cell, human immunodeficiency virus type 1 (HIV-1) must reverse transcribe its RNA genome to DNA and traffic to the nuclear envelope, where the viral genome is translocated into the nucleus for subsequent integration into the host cell chromosome. During this time, the viral core, which houses the genome, undergoes a poorly understood process of disassembly, known as uncoating. Collectively, many studies suggest that uncoating is tightly regulated to allow nuclear import of the genome while minimizing the exposure of the newly synthesized DNA to cytosolic DNA sensors. However, whether host cellular proteins facilitate this process remains poorly understood. Here we report that intact microtubules facilitate HIV-1 uncoating in target cells. Disruption of microtubules with nocodazole substantially delays HIV-1 uncoating, as revealed with three different assay systems. This defect in uncoating did not correlate with defective reverse transcription at early times postinfection, demonstrating that microtubule-facilitated uncoating is distinct from the previously reported role of viral reverse transcription in the uncoating process. We also find that pharmacological or small interfering RNA (siRNA)-mediated inhibition of cytoplasmic dynein or the kinesin 1 heavy chain KIF5B delays uncoating, providing detailed insight into how microtubules facilitate the uncoating process. These studies reveal a previously unappreciated role for microtubules and microtubule motor function in HIV-1 uncoating, establishing a functional link between viral trafficking and uncoating. Targeted disruption of the capsid motor interaction may reveal novel mechanisms of inhibition of viral infection or provide opportunities to activate cytoplasmic antiviral responses directed against capsid or viral DNA.

IMPORTANCE

During HIV-1 infection, fusion of viral and target cell membranes dispenses the viral ribonucleoprotein complex into the cytoplasm of target cells. During this time, the virus must reverse transcribe its RNA genome, traffic from the location of fusion to the nuclear membrane, and undergo the process of uncoating, whereby the viral capsid core disassembles to allow the subsequent nuclear import of the viral genome. Numerous cellular restriction factors target the viral capsid, suggesting that perturbation of the uncoating process represents an excellent antiviral target. However, this uncoating process, and the cellular factors that facilitate uncoating, remains poorly understood. The main observation of this study is that normal uncoating requires intact microtubules and is facilitated by dynein and kinesin motors. Targeting these factors may either directly inhibit infection or delay it enough to trigger mediators of intrinsic immunity that recognize cytoplasmic capsid or DNA and subsequently induce an antiviral state in these cells.

Retroviral transduction of murine and human hematopoietic progenitors and stem cells

Genetic modification of cells using retroviral vectors is the method of choice when the cell population is difficult to transfect and/or requires persistent transgene expression in progeny cells. There are innumerable potential applications for these procedures in laboratory research and clinical therapeutic interventions. One paradigmatic example is the genetic modification of hematopoietic stem and progenitor cells (HSPCs). These are rare nucleated cells which reside in a specialized microenvironment within the bone marrow, and have the potential to self-renew and/or differentiate into all hematopoietic lineages. Due to their enormous regenerative capacity in steady state or under stress conditions these cells are routinely used in allogeneic bone marrow transplantation to reconstitute the hematopoietic system in patients with metabolic, inflammatory, malignant, and other hematologic disorders. For patients lacking a matched bone marrow donor, gene therapy of autologous hematopoietic stem cells has proven to be an alternative as highlighted recently by several successful gene therapy trials. Genetic modification of HSPCs using retrovirus vectors requires ex vivo manipulation to efficiently introduce the new genetic material into cells (transduction). Optimal culture conditions are essential to facilitate this process while preserving the stemness of the cells. The most frequently used retroviral vector systems for the genetic modifications of HSPCs are derived either from Moloney murine leukemia-virus (Mo-MLV) or the human immunodeficiency virus-1 (HIV-1) and are generally termed according to their genus gamma-retroviral (γ-RV) or lentiviral vectors (LV), respectively. This chapter describes in a step-by-step fashion some techniques used to produce research grade vector supernatants and to obtain purified murine or human hematopoietic stem cells for transduction, as well as follow-up methods for analysis of transduced cell populations.

Viral and cellular requirements for the nuclear entry of retroviral preintegration nucleoprotein complexes

Retroviruses integrate their reverse transcribed genomes into host cell chromosomes as an obligate step in virus replication. The nuclear envelope separates the chromosomes from the cell cytoplasm during interphase, and different retroviral groups deal with this physical barrier in different ways. Gammaretroviruses are dependent on the passage of target cells through mitosis, where they are believed to access chromosomes when the nuclear envelope dissolves for cell division. Contrastingly, lentiviruses such as HIV-1 infect non-dividing cells, and are believed to enter the nucleus by passing through the nuclear pore complex. While numerous virally encoded elements have been proposed to be involved in HIV-1 nuclear import, recent evidence has highlighted the importance of HIV-1 capsid. Furthermore, capsid was found to be responsible for the viral requirement of various nuclear transport proteins, including transportin 3 and nucleoporins NUP153 and NUP358, during infection. In this review, we describe our current understanding of retroviral nuclear import, with emphasis on recent developments on the role of the HIV-1 capsid protein.

Viruses challenge selectivity barrier of nuclear pores

Exchange between the nucleus and the cytoplasm occurs through nuclear pore complexes (NPCs) embedded in the double membrane of the nuclear envelope. NPC permeability barrier restricts the entry of inert molecules larger than 5 nm in diameter but allows facilitated entry of selected cargos, whose size can reach up to 39 nm. The translocation of large molecules is facilitated by nuclear transport receptors (NTRs) that have affinity to proteins of NPC permeability barrier. Viruses that enter the nucleus replicate evolved strategies to overcome this barrier. In this review, we will discuss the functional principles of NPC barrier and nuclear transport machinery, as well as the various strategies viruses use to cross the selective barrier of NPCs.

Viral DNA tethering domains complement replication-defective mutations in the p12 protein of MuLV Gag

The p12 protein of murine leukemia virus (MuLV) group-specific antigen (Gag) is associated with the preintegration complex, and mutants of p12 (PM14) show defects in nuclear entry or retention. Here we show that p12 proteins engineered to encode peptide sequences derived from known viral tethering proteins can direct chromatin binding during the early phase of viral replication and rescue a lethal p12-PM14 mutant. Peptides studied included segments of Kaposi sarcoma herpesvirus latency-associated nuclear antigen (LANA)(1-23), human papillomavirus 8 E2, and prototype foamy virus chromatin-binding sequences. Amino acid substitutions in Kaposi sarcoma herpesvirus LANA and prototype foamy virus chromatin-binding sequences that blocked nucleosome association failed to rescue MuLV p12-PM14. Rescue by a larger LANA peptide, LANA(1-32), required second-site mutations that are predicted to reduce peptide binding affinity to chromosomes, suggesting that excessively high binding affinity interfered with Gag/p12 function. This is supported by confocal microscopy of chimeric p12-GFP fusion constructs showing the reverted proteins had weaker association to condensed mitotic chromosomes. Analysis of the integration-site selection of these chimeric viruses showed no significant change in integration profile compared with wild-type MuLV, suggesting release of the tethered p12 post mitosis, before viral integration.

The importance of becoming double-stranded: Innate immunity and the kinetic model of HIV-1 central plus strand synthesis

Central initiation of plus strand synthesis is a conserved feature of lentiviruses and certain other retroelements. This complication of the standard reverse transcription mechanism produces a transient "central DNA flap" in the viral cDNA, which has been proposed to mediate its subsequent nuclear import. This model has assumed that the important feature is the flapped DNA structure itself rather than the process that produces it. Recently, an alternative kinetic model was proposed. It posits that central plus strand synthesis functions to accelerate conversion to the double-stranded state, thereby helping HIV-1 to evade single-strand DNA-targeting antiviral restrictions such as APOBEC3 proteins, and perhaps to avoid innate immune sensor mechanisms. The model is consistent with evidence that lentiviruses must often synthesize their cDNAs when dNTP concentrations are limiting and with data linking reverse transcription and uncoating. There may be additional kinetic advantages for the artificial genomes of lentiviral gene therapy vectors.

Fates of retroviral core components during unrestricted and TRIM5-restricted infection

TRIM5 proteins can restrict retroviral infection soon after delivery of the viral core into the cytoplasm. However, the molecular mechanisms by which TRIM5α inhibits infection have been elusive, in part due to the difficulty of developing and executing biochemical assays that examine this stage of the retroviral life cycle. Prevailing models suggest that TRIM5α causes premature disassembly of retroviral capsids and/or degradation of capsids by proteasomes, but whether one of these events leads to the other is unclear. Furthermore, how TRIM5α affects the essential components of the viral core, other than capsid, is unknown. To address these questions, we devised a biochemical assay in which the fate of multiple components of retroviral cores during infection can be determined. We utilized cells that can be efficiently infected by VSV-G-pseudotyped retroviruses, and fractionated the cytosolic proteins on linear gradients following synchronized infection. The fates of capsid and integrase proteins, as well as viral genomic RNA and reverse transcription products were then monitored. We found that components of MLV and HIV-1 cores formed a large complex under non-restrictive conditions. In contrast, when MLV infection was restricted by human TRIM5α, the integrase protein and reverse transcription products were lost from infected cells, while capsid and viral RNA were both solubilized. Similarly, when HIV-1 infection was restricted by rhesus TRIM5α or owl monkey TRIMCyp, the integrase protein and reverse transcription products were lost. However, viral RNA was also lost, and high levels of preexisting soluble CA prevented the determination of whether CA was solubilized. Notably, proteasome inhibition blocked all of the aforementioned biochemical consequences of TRIM5α-mediated restriction but had no effect on its antiviral potency. Together, our results show how TRIM5α affects various retroviral core components and indicate that proteasomes are required for TRIM5α-induced core disruption but not for TRIM5α-induced restriction.

The TRIM5 proteins found in primates are inhibitors of retroviral infection that act soon after delivery of the viral core into the cytoplasm. It has been difficult to elucidate how TRIM5 proteins work, because techniques that can be applied to this step of the viral life cycle are cumbersome. We developed an experimental approach in which we can monitor TRIM5-induced changes in the viral core at early times after infection, when TRIM5 exerts its effects. Specifically, we monitored the fate of the viral capsid protein, the integrase enzyme and the viral genome. We show that TRIM5 induces disassembly of each of these core components, and while some core components simply dissociate, others are degraded. These dissociation and degradation events all appear to be dependent on the activity of the proteasome. However, we also find that each of these TRIM5-induced effects events are not necessary for inhibition. The assay developed herein provides important insight into the mechanism of TRIM5α restriction and can, in principle, be applied to other important processes that occur at this point in the retrovirus life cycle.

p12 tethers the murine leukemia virus pre-integration complex to mitotic chromosomes

The p12 protein of the murine leukemia virus (MLV) is a constituent of the pre-integration complex (PIC) but its function in this complex remains unknown. We developed an imaging system to monitor MLV PIC trafficking in live cells. This allowed the visualization of PIC docking to mitotic chromosomes and its release upon exit from mitosis. Docking occurred concomitantly with nuclear envelope breakdown and was impaired for PICs of viruses with lethal p12 mutations. Insertion of a heterologous chromatin binding module into p12 of one of these mutants restored PICs attachment to the chromosomes and partially rescued virus replication. Capsid dissociated from wild type PICs in mitotic cells but remained associated with PICs harboring tethering-negative p12 mutants. Altogether, these results explain, in part, MLV restriction to dividing cells and reveal a role for p12 as a factor that tethers MLV PIC to mitotic chromosomes.

Retroviruses, including the murine leukemia virus (MLV), reverse transcribe their RNA genome to a DNA copy, which travels from the cytoplasm to the nucleus as part of a ‘pre-integration complex’ (PIC), to integrate into cellular chromosomes. The viral p12 protein is a constituent of the MLV PIC, but its function in this complex has remained unknown. We developed a real-time imaging system to detect p12 and MLV PICs in live cells. This revealed that p12 tethers the MLV PIC to mitotic chromosomes. Accordingly, PICs derived from viruses with specific lethal mutations in p12 failed to attach to the chromosomes, and insertion of a heterologous chromatin binding module into p12 restored PICs attachment to the chromosomes and rescued virus replication. In addition, docking of wild type PICs to chromosomes coincided with nuclear envelope breakdown during mitosis, and detachment occurred upon exit from mitosis. Capsid - another viral component of the PIC - dissociated from wild type PICs in mitotic cells but remained associated with PICs harboring tethering-negative p12 mutants, suggesting interplay between these two proteins in regulating targeting of mitotic chromosomes by the PIC. These results highlight steps contributing to the high tropism of MLV to dividing cells.

Multiple roles of the capsid protein in the early steps of HIV-1 infection

The early steps of HIV-1 infection starting after virus entry into cells up to integration of its genome into host chromosomes are poorly understood. From seminal work showing that HIV-1 and oncoretroviruses follow different steps in the early stages post-entry, significant advances have been made in recent years and an important role for the HIV-1 capsid (CA) protein, the constituent of the viral core, has emerged. CA appears to orchestrate several events, such as virus uncoating, recognition by restriction factors and the innate immune system. It also plays a role in nuclear import and integration of HIV-1 and has become a novel target for antiretroviral drugs. Here we describe the different functions of CA and how they may be integrated into one or more coherent models that illuminate the early events in HIV-1 infection and their relations with the host cell.

Interaction of the HIV-1 intasome with transportin 3 protein (TNPO3 or TRN-SR2)

Transportin 3 (TNPO3 or TRN-SR2) has been shown to be an important cellular factor for early steps of lentiviral replication. However, separate studies have implicated distinct mechanisms for TNPO3 either through its interaction with HIV-1 integrase or capsid. Here we have carried out a detailed biophysical characterization of TNPO3 and investigated its interactions with viral proteins. Biophysical analyses including circular dichroism, analytical ultracentrifugation, small-angle x-ray scattering, and homology modeling provide insight into TNPO3 architecture and indicate that it is highly structured and exists in a monomer-dimer equilibrium in solution. In vitro biochemical binding assays argued against meaningful direct interaction between TNPO3 and the capsid cores. Instead, TNPO3 effectively bound to the functional intasome but not to naked viral DNA, suggesting that TNPO3 can directly engage the HIV-1 IN tetramer prebound to the cognate DNA. Mass spectrometry-based protein footprinting and site-directed mutagenesis studies have enabled us to map several interacting amino acids in the HIV-1 IN C-terminal domain and the cargo binding domain of TNPO3. Our findings provide important information for future genetic analysis to better understand the role of TNPO3 and its interacting partners for HIV-1 replication.

The HIV-1 passage from cytoplasm to nucleus: the process involving a complex exchange between the components of HIV-1 and cellular machinery to access nucleus and successful integration

The human immunodeficiency virus 1 (HIV-1) synthesizes its genomic DNA in cytoplasm as soon as it enters the cell. The newly synthesized DNA remains associated with viral/cellular proteins as a high molecular weight pre-integration complex (PIC), which precludes passive diffusion across intact nuclear membrane. However, HIV-1 successfully overcomes nuclear membrane barrier by actively delivering its DNA into nucleus with the help of host nuclear import machinery. Such ability allows HIV-1 to productively infect non-dividing cells as well as dividing cells at interphase. Further, HIV-1 nuclear import is also found important for the proper integration of viral DNA. Thus, nuclear import plays a crucial role in establishment of infection and disease progression. While several viral components, including matrix, viral protein R, integrase, capsid, and central DNA flap are implicated in HIV-1 nuclear import, their molecular mechanism remains poorly understood. In this review, we will elaborate the role of individual viral factors and some of current insights on their molecular mechanism(s) associated with HIV-1 nuclear import. In addition, we will discuss the importance of nuclear import for subsequent step of viral DNA integration. Hereby we aim to further our understanding on molecular mechanism of HIV-1 nuclear import and its potential usefulness for anti-HIV-1 strategies.

Transportin 3 promotes a nuclear maturation step required for efficient HIV-1 integration

The HIV/AIDS pandemic is a major global health threat and understanding the detailed molecular mechanisms of HIV replication is critical for the development of novel therapeutics. To replicate, HIV-1 must access the nucleus of infected cells and integrate into host chromosomes, however little is known about the events occurring post-nuclear entry but before integration. Here we show that the karyopherin Transportin 3 (Tnp3) promotes HIV-1 integration in different cell types. Furthermore Tnp3 binds the viral capsid proteins and tRNAs incorporated into viral particles. Interaction between Tnp3, capsid and tRNAs is stronger in the presence of RanGTP, consistent with the possibility that Tnp3 is an export factor for these substrates. In agreement with this interpretation, we found that Tnp3 exports from the nuclei viral tRNAs in a RanGTP-dependent way. Tnp3 also binds and exports from the nuclei some species of cellular tRNAs with a defective 3'CCA end. Depletion of Tnp3 results in a re-distribution of HIV-1 capsid proteins between nucleus and cytoplasm however HIV-1 bearing the N74D mutation in capsid, which is insensitive to Tnp3 depletion, does not show nucleocytoplasmic redistribution of capsid proteins. We propose that Tnp3 promotes HIV-1 infection by displacing any capsid and tRNA that remain bound to the pre-integration complex after nuclear entry to facilitate integration. The results also provide evidence for a novel tRNA nucleocytoplasmic trafficking pathway in human cells.

Gammaretroviral vectors: biology, technology and application

Retroviruses are evolutionary optimized gene carriers that have naturally adapted to their hosts to efficiently deliver their nucleic acids into the target cell chromatin, thereby overcoming natural cellular barriers. Here we will review—starting with a deeper look into retroviral biology—how Murine Leukemia Virus (MLV), a simple gammaretrovirus, can be converted into an efficient vehicle of genetic therapeutics. Furthermore, we will describe how more rational vector backbones can be designed and how these so-called self-inactivating vectors can be pseudotyped and produced. Finally, we will provide an overview on existing clinical trials and how biosafety can be improved.

The requirement for nucleoporin NUP153 during human immunodeficiency virus type 1 infection is determined by the viral capsid

Lentiviruses likely infect nondividing cells by commandeering host nuclear transport factors to facilitate the passage of their preintegration complexes (PICs) through nuclear pore complexes (NPCs) within nuclear envelopes. Genome-wide small interfering RNA screens previously identified karyopherin β transportin-3 (TNPO3) and NPC component nucleoporin 153 (NUP153) as being important for infection by human immunodeficiency virus type 1 (HIV-1). The knockdown of either protein significantly inhibited HIV-1 infectivity, while infection by the gammaretrovirus Moloney murine leukemia virus (MLV) was unaffected. Here, we establish that primate lentiviruses are particularly sensitive to NUP153 knockdown and investigate HIV-1-encoded elements that contribute to this dependency. Mutants lacking functional Vpr or the central DNA flap remained sensitive to NUP153 depletion, while MLV/HIV-1 chimera viruses carrying MLV matrix, capsid, or integrase became less sensitive when the latter two elements were substituted. Two capsid missense mutant viruses, N74D and P90A, were largely insensitive to NUP153 depletion, as was wild-type HIV-1 when cyclophilin A was depleted simultaneously or when infection was conducted in the presence of cyclosporine A. The codepletion of NUP153 and TNPO3 yielded synergistic effects that outweighed those calculated based on individual knockdowns, indicating potential interdependent roles for these factors during HIV-1 infection. Quantitative PCR revealed normal levels of late reverse transcripts, a moderate reduction of 2-long terminal repeat (2-LTR) circles, and a relatively large reduction in integrated proviruses upon NUP153 knockdown. These results suggest that capsid, likely by the qualities of its uncoating, determines whether HIV-1 requires cellular NUP153 for PIC nuclear import.

The inside out of lentiviral vectors

Lentiviruses induce a wide variety of pathologies in different animal species. A common feature of the replicative cycle of these viruses is their ability to target non-dividing cells, a property that constitutes an extremely attractive asset in gene therapy. In this review, we shall describe the main basic aspects of the virology of lentiviruses that were exploited to obtain efficient gene transfer vectors. In addition, we shall discuss some of the hurdles that oppose the efficient genetic modification mediated by lentiviral vectors and the strategies that are being developed to circumvent them.

Interplay between HIV entry and transportin-SR2 dependency

BACKGROUND

Transportin-SR2 (TRN-SR2, TNPO3, transportin 3) was previously identified as an interaction partner of human immunodeficiency virus type 1 (HIV-1) integrase and functions as a nuclear import factor of HIV-1. A possible role of capsid in transportin-SR2-mediated nuclear import was recently suggested by the findings that a chimeric HIV virus, carrying the murine leukemia virus (MLV) capsid and matrix proteins, displayed a transportin-SR2 independent phenotype, and that the HIV-1 N74D capsid mutant proved insensitive to transportin-SR2 knockdown.

RESULTS

Our present analysis of viral specificity reveals that TRN-SR2 is not used to the same extent by all lentiviruses. The DNA flap does not determine the TRN-SR2 requirement of HIV-1. We corroborate the TRN-SR2 independent phenotype of the chimeric HIV virus carrying the MLV capsid and matrix proteins. We reanalyzed the HIV-1 N74D capsid mutant in cells transiently or stably depleted of transportin-SR2 and confirm that the N74D capsid mutant is independent of TRN-SR2 when pseudotyped with the vesicular stomatitis virus glycoprotein (VSV-G). Remarkably, although somewhat less dependent on TRN-SR2 than wild type virus, the N74D capsid mutant carrying the wild type HIV-1 envelope required TRN-SR2 for efficient replication. By pseudotyping with envelopes that mediate pH-independent viral uptake including HIV-1, measles virus and amphotropic MLV envelopes, we demonstrate that HIV-1 N74D capsid mutant viruses retain partial dependency on TRN-SR2. However, this dependency on TRN-SR2 is lost when the HIV N74D capsid mutant is pseudotyped with envelopes mediating pH-dependent endocytosis, such as the VSV-G and Ebola virus envelopes.

CONCLUSION

Here we discover a link between the viral entry of HIV and its interaction with TRN-SR2. Our data confirm the importance of TRN-SR2 in HIV-1 replication and argue for careful interpretation of experiments performed with VSV-G pseudotyped viruses in studies on early steps of HIV replication including the role of capsid therein.

How viruses access the nucleus

Many viruses depend on nuclear proteins for replication. Therefore, their viral genome must enter the nucleus of the host cell. In this review we briefly summarize the principles of nucleocytoplasmic transport, and then describe the diverse strategies used by viruses to deliver their genomes into the host nucleus. Some of the emerging mechanisms include: (1) nuclear entry during mitosis, when the nuclear envelope is disassembled, (2) viral genome release in the cytoplasm followed by entry of the genome through the nuclear pore complex (NPC), (3) capsid docking at the cytoplasmic side of the NPC, followed by genome release, (4) nuclear entry of intact capsids through the NPC, followed by genome release, and (5) nuclear entry via virus-induced disruption of the nuclear envelope. Which mechanism a particular virus uses depends on the size and structure of the virus, as well as the cellular cues used by the virus to trigger capsid disassembly and genome release. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

The Gag cleavage product, p12, is a functional constituent of the murine leukemia virus pre-integration complex

The p12 protein is a cleavage product of the Gag precursor of the murine leukemia virus (MLV). Specific mutations in p12 have been described that affect early stages of infection, rendering the virus replication-defective. Such mutants showed normal generation of genomic DNA but no formation of circular forms, which are markers of nuclear entry by the viral DNA. This suggested that p12 may function in early stages of infection but the precise mechanism of p12 action is not known. To address the function and follow the intracellular localization of the wt p12 protein, we generated tagged p12 proteins in the context of a replication-competent virus, which allowed for the detection of p12 at early stages of infection by immunofluorescence. p12 was found to be distributed to discrete puncta, indicative of macromolecular complexes. These complexes were localized to the cytoplasm early after infection, and thereafter accumulated adjacent to mitotic chromosomes. This chromosomal accumulation was impaired for p12 proteins with a mutation that rendered the virus integration-defective. Immunofluorescence demonstrated that intracellular p12 complexes co-localized with capsid, a known constituent of the MLV pre-integration complex (PIC), and immunofluorescence combined with fluorescent in situ hybridization (FISH) revealed co-localization of the p12 proteins with the incoming reverse transcribed viral DNA. Interactions of p12 with the capsid and with the viral DNA were also demonstrated by co-immunoprecipitation. These results imply that p12 proteins are components of the MLV PIC. Furthermore, a large excess of wt PICs did not rescue the defect in integration of PICs derived from mutant p12 particles, demonstrating that p12 exerts its function as part of this complex. Altogether, these results imply that p12 proteins are constituent of the MLV PIC and function in directing the PIC from the cytoplasm towards integration.

All retroviruses reverse transcribe their RNA genome to a DNA copy in the cytoplasm of the infected cell. To be expressed, the viral genomic DNA has to travel to the cell nucleus and to integrate into the cellular chromosomes. This trafficking is governed by cellular and viral proteins that associate with the viral genome to form a ‘pre-integration complex’ (PIC), yet the full composition of this complex is unknown. Former studies showed that for the murine leukemia virus (MLV), mutations in a viral protein named p12 abrogate MLV infection, after reverse transcription and prior to the integration step, suggesting a role for this protein in early stages of infection. However, the precise mechanism of p12 action is not known. We combined microscopic, genetic and biochemical techniques to provide evidence that the p12 protein is part of the MLV PIC and that it exerts its function from within this complex. These analyses also suggest a role for p12 in the trafficking of the PIC from the cytoplasm to the chromosomes of the infected cell. Altogether, these findings highlight an important ‘building block’ of a complex that is essential for MLV infection.

Strand transfer and elongation of HIV-1 reverse transcription is facilitated by cell factors in vitro

Recent work suggests a role for multiple host factors in facilitating HIV-1 reverse transcription. Previously, we identified a cellular activity which increases the efficiency of HIV-1 reverse transcription in vitro. Here, we describe aspects of the activity which shed light on its function. The cellular factor did not affect synthesis of strong-stop DNA but did improve downstream DNA synthesis. The stimulatory activity was isolated by gel filtration in a single fraction of the exclusion volume. Velocity-gradient purified HIV-1, which was free of detectable RNase activity, showed poor reverse transcription efficiency but was strongly stimulated by partially purified cell proteins. Hence, the cell factor(s) did not inactivate an RNase activity that might degrade the viral genomic RNA and block completion of reverse transcription. Instead, the cell factor(s) enhanced first strand transfer and synthesis of late reverse transcription suggesting it stabilized the reverse transcription complex. The factor did not affect lysis of HIV-1 by Triton X-100 in the endogenous reverse transcription (ERT) system, and ERT reactions with HIV-1 containing capsid mutations, which varied the biochemical stability of viral core structures and impeded reverse transcription in cells, showed no difference in the ability to be stimulated by the cell factor(s) suggesting a lack of involvement of the capsid in the in vitro assay. In addition, reverse transcription products were found to be resistant to exogenous DNase I activity when the active fraction was present in the ERT assay. These results indicate that the cell factor(s) may improve reverse transcription by facilitating DNA strand transfer and DNA synthesis. It also had a protective function for the reverse transcription products, but it is unclear if this is related to improved DNA synthesis.

The requirement for cellular transportin 3 (TNPO3 or TRN-SR2) during infection maps to human immunodeficiency virus type 1 capsid and not integrase

Recent genome-wide screens have highlighted an important role for transportin 3 in human immunodeficiency virus type 1 (HIV-1) infection and preintegration complex (PIC) nuclear import. Moreover, HIV-1 integrase interacted with recombinant transportin 3 protein under conditions whereby Moloney murine leukemia virus (MLV) integrase failed to do so, suggesting that integrase-transportin 3 interactions might underscore active retroviral PIC nuclear import. Here we correlate infectivity defects in transportin 3 knockdown cells with in vitro protein binding affinities for an expanded set of retroviruses that include simian immunodeficiency virus (SIV), bovine immunodeficiency virus (BIV), equine infectious anemia virus (EIAV), feline immunodeficiency virus (FIV), and Rous sarcoma virus (RSV) to critically address the role of integrase-transportin 3 interactions in viral infection. Lentiviruses, with the exception of FIV, display a requirement for transportin 3 in comparison to MLV and RSV, yielding an infection-based dependency ranking of SIV > HIV-1 > BIV and EIAV > MLV, RSV, and FIV. In vitro pulldown and surface plasmon resonance assays, in contrast, define a notably different integrase-transportin 3 binding hierarchy: FIV, HIV-1, and BIV > SIV and MLV > EIAV. Our results therefore fail to support a critical role for integrase binding in dictating transportin 3 dependency during retrovirus infection. In addition to integrase, capsid has been highlighted as a retroviral nuclear import determinant. Accordingly, MLV/HIV-1 chimera viruses pinpoint the genetic determinant of sensitization to transportin 3 knockdown to the HIV-1 capsid protein. We therefore conclude that capsid, not integrase, is the dominant viral factor that dictates transportin 3 dependency during HIV-1 infection.

Stabilization of gammaretroviral and lentiviral vectors: from production to gene transfer

BACKGROUND

The low stability of gammaretroviral and lentiviral vectors affects their production, making high quality clinical preparations a difficult goal to achieve. Recently, our laboratory has shown that the main inactivation mechanism for both these vectors is the loss of their capacity to perform reverse transcription. The present study aimed to increase the stability of gammaretroviral and lentiviral at 37 degrees C and at 4 degrees C.

METHODS

Inactivation studies were performed with gammaretroviral and lentiviral vectors at 37 and 4 degrees C, with and without several stabilizing compounds. The residual viral infectivity and reverse transcription capacity of these samples were tested.

RESULTS

The results obtained demonstrate that it is possible to increase the stability of reverse transcription and the infectivity stability of purified gammaretroviral vectors by adding recombinant human albumin (rHSA) to the storage buffer, both at 37 degrees C and at 4 degrees C. For lentiviral vectors, it was observed that further protection was needed. This was achieved by adding lipids to the storage buffer, using a mixture of lipoproteins and rHSA. The difference of stabilization between gammaretroviral and lentiviral vectors was validated by performing stabilization tests with vectors possessing different envelope proteins and produced by different cell lines.

CONCLUSIONS

The presented study reveals that it is possible to increase the half-life of purified gammaretroviral and lentiviral vectors at 37 degrees C and at 4 degrees C, but the two vectors have different stabilization requirements: for retroviral vectors, the addition of rHSA is enough and, for lentiviral vectors, it is necessary to add both lipoproteins and rHSA. The increase of the stability of the reverse transcription process was shown to have a high impact with respect to the increase of the stability of infectivity.

Methods of preparation and analysis of intracellular reverse transcription complexes

Shortly after penetration into the cell, HIV-1 must reverse transcribe its genome into a double-stranded DNA molecule and must gain access to the nucleus of nondividing cells for productive infection. There is limited knowledge of these early events in HIV-1 life cycle. We have developed methods, which allow the study of postpenetration events and in particular allow the isolation and purification of intracellular complexes mediating reverse transcription. Such complexes can be examined for their structure, function, protein composition and ability to access the nucleus.

HIV-1 exploits importin 7 to maximize nuclear import of its DNA genome

BACKGROUND

Nuclear import of the HIV-1 reverse transcription complex (RTC) is critical for infection of non dividing cells, and importin 7 (imp7) has been implicated in this process. To further characterize the function of imp7 in HIV-1 replication we generated cell lines stably depleted for imp7 and used them in conjunction with infection, cellular fractionation and pull-down assays.

RESULTS

Imp7 depletion impaired HIV-1 infection but did not significantly affect HIV-2, simian immunodeficiency virus (SIVmac), or equine infectious anemia virus (EIAV). The lentiviral dependence on imp7 closely correlated with binding of the respective integrase proteins to imp7. HIV-1 RTC associated with nuclei of infected cells with remarkable speed and knock down of imp7 reduced HIV-1 DNA nuclear accumulation, delaying infection. Using an HIV-1 mutant deficient for reverse transcription, we found that viral RNA accumulated within nuclei of infected cells, indicating that reverse transcription is not absolutely required for nuclear import. Depletion of imp7 impacted on HIV-1 DNA but not RNA nuclear import and also inhibited DNA transfection efficiency.

CONCLUSION

Although imp7 may not be essential for HIV-1 infection, our results suggest that imp7 facilitates nuclear trafficking of DNA and that HIV-1 exploits imp7 to maximize nuclear import of its DNA genome. Lentiviruses other than HIV-1 may have evolved to use alternative nuclear import receptors to the same end.

An invariant surface patch on the TRIM5alpha PRYSPRY domain is required for retroviral restriction but dispensable for capsid binding

TRIM5alpha is a retrovirus restriction factor in the host cell cytoplasm that blocks infection before provirus establishment. Restriction activity requires capsid (CA)-specific recognition by the PRYSPRY domain of TRIM5alpha. To better understand the restriction mechanism, nine charge-cluster-to-triple-alanine mutants in the TRIM5alpha PRYSPRY domain were assessed for CA-specific restriction activity. Five mutants distributed along the TRIM5alpha PRYSPRY primary sequence disrupted restriction activity against N-tropic murine leukemia virus and equine infectious anemia virus. Modeling of the TRIM5alpha PRYSPRY domain based on the crystal structures of PRYSPRY-19q13.4.1, GUSTAVUS, and TRIM21 identified a surface patch where disruptive mutants clustered. All mutants in this patch retained CA-binding activity, a reticular distribution in the cytoplasm, and steady-state protein levels comparable to those of the wild type. Residues in the essential patch are conserved in TRIM5alpha orthologues and in closely related paralogues. The same surface patch in the TRIM18 and TRIM20 PRYSPRY domains is the site of mutants causing Opitz syndrome and familial Mediterranean fever. These results indicate that, in addition to CA-specific binding, the PRYSPRY domain possesses a second function, possibly binding of a cofactor, that is essential for retroviral restriction activity by TRIM5alpha.

Cyclophilin A-dependent restriction of human immunodeficiency virus type 1 capsid mutants for infection of nondividing cells

Among retroviruses, lentiviruses are unusual in their ability to efficiently infect both dividing and nondividing cells, such as activated T cells and macrophages, respectively. Recent studies implicate the viral capsid protein (CA) as a key determinant of cell-cycle-independent infection by human immunodeficiency virus type 1 (HIV-1). We investigated the effects of the host cell protein cyclophilin A (CypA), which binds to HIV-1 CA, on HIV-1 infection of nondividing cells. The HIV-1 CA mutants A92E, T54A, and R132K were impaired for infection of aphidicolin-arrested HeLa cells, but not HOS cells. The mutants synthesized normal quantities of two-long-terminal-repeat circles in arrested HeLa cells, indicating that the mutant preintegration complexes can enter the nuclei of both dividing and nondividing cells. The impaired infectivity of the CA mutants on both dividing and nondividing HeLa cells was relieved by either pharmacological or genetic disruption of the CypA-CA interaction or by RNA interference-mediated depletion of CypA expression in target cells. A second-site suppressor of the CypA-restricted phenotype also restored the ability of CypA-restricted HIV-1 mutants to infect growth-arrested HeLa cells. These results indicate that CypA-restricted mutants are specifically impaired at a step between nuclear import and integration in nondividing HeLa cells. This study reveals a novel target cell-specific restriction of HIV-1 CA mutants in nondividing cells that is dependent on CypA-CA interactions.

Evidence for direct involvement of the capsid protein in HIV infection of nondividing cells

HIV and other lentiviruses can productively infect nondividing cells, whereas most other retroviruses, such as murine leukemia virus, require cell division for efficient infection. However, the determinants for this phenotype have been controversial. Here, we show that HIV-1 capsid (CA) is involved in facilitating HIV infection of nondividing cells because amino acid changes on CA severely disrupt the cell-cycle independence of HIV. One mutant in the N-terminal domain of CA in particular has lost the cell-cycle independence in all cells tested, including primary macrophages. The defect in this mutant appears to be at a stage past nuclear entry. We also find that the loss of cell-cycle independence can be cell-type specific, which suggests that a cellular factor affects the ability of HIV to infect nondividing cells. Our data suggest that CA is directly involved at some step in the viral life cycle that is important for infection of nondividing cells.

HIV and related viruses are unusual among retroviruses in their ability to replicate independently of cell-cycle progression of target cells. However, the determinants of this phenotype have been controversial. Here, we identified mutations on the surface of the capsid (CA) protein that reduce the ability of HIV to infect nondividing cells. These mutations also confer cell-cycle dependency on HIV, even in dividing cells. Interestingly, some CA mutants lose cell-cycle independence only in certain cell types. Thus, these findings suggest that a cellular factor targeting CA regulates HIV-1 infection in nondividing cells. Surprisingly, these mutations do not appear to affect nuclear localization of viral genomes, which points to a novel regulation of the cell-cycle independence of HIV by the CA protein.

Host cell factors and HIV-1 integration

Retroviral replication hinges on the formation of the provirus, the integrated product of the linear DNA that is made during reverse transcription. Integration is catalyzed by the viral recombinase integrase, yet a number of studies indicate that other viral or cellular proteins play important cofactor roles during HIV-1 integration. Some of these factors bind directly to integrase, whereas others gain access to the integration machinery by binding to the DNA or other viral proteins. This article reviews recent advances on the roles of cellular proteins in HIV-1 integration. As a number of studies have highlighted a particularly important role for the integrase interactor lens epithelium-derived growth factor (LEDGF), much of the focus will be on its mechanism of action and the potential to develop inhibitors of this crucial virus–host interaction.

The road to chromatin - nuclear entry of retroviruses

Human immunodeficiency virus 1 (HIV-1) and other retroviruses synthesize a DNA copy of their genome after entry into the host cell. Integration of this DNA into the host cell's genome is an essential step in the viral replication cycle. The viral DNA is synthesized in the cytoplasm and is associated with viral and cellular proteins in a large nucleoprotein complex. Before integration into the host genome can occur, this complex must be transported to the nucleus and must cross the nuclear envelope. This Review summarizes our current knowledge of how this journey is accomplished.