The retroviral restriction factor TRIM5alpha

HIV-1 assembly, budding, and maturation

A defining property of retroviruses is their ability to assemble into particles that can leave producer cells and spread infection to susceptible cells and hosts. Virion morphogenesis can be divided into three stages: assembly, wherein the virion is created and essential components are packaged; budding, wherein the virion crosses the plasma membrane and obtains its lipid envelope; and maturation, wherein the virion changes structure and becomes infectious. All of these stages are coordinated by the Gag polyprotein and its proteolytic maturation products, which function as the major structural proteins of the virus. Here, we review our current understanding of the mechanisms of HIV-1 assembly, budding, and maturation, starting with a general overview and then providing detailed descriptions of each of the different stages of virion morphogenesis.

Epigenetic regulation of HIV-1 transcription

After entry into the target cell and reverse transcription, HIV-1 genes are integrated into the host genome. It is now well established that the viral promoter activity is directly governed by its chromatin environment. Nuc-1, a nucleosome located immediately downstream of the HIV-1 transcriptional initiation site directly impedes long-terminal repeat (LTR) activity. Epigenetic modifications and disruption of Nuc-1 are a prerequisite to the activation of LTR-driven transcription and viral expression. The compaction of chromatin and its permissiveness for transcription are directly dependent on the post-translational modifications of histones such as acetylation, methylation, phosphorylation and ubiquitination. Understanding the molecular mechanisms underlying HIV-1 transcriptional silencing and activation is thus a major challenge in the fight against AIDS and will certainly lead to new therapeutic tools.

p62/sequestosome-1 associates with and sustains the expression of retroviral restriction factor TRIM5alpha

TRIM5 proteins mediate a potent block to the cross-species transmission of retroviruses, the most well known being the TRIM5alpha protein from rhesus macaques, which potently inhibits human immunodeficiency virus type 1 (HIV-1) infection. This restriction occurs at an early stage in the replication cycle and is mediated by the binding of TRIM5 proteins to determinants present in the retroviral capsid. TRIM5alpha, as well as other TRIM family proteins, has been shown to be regulated by interferons (IFN). Here we show that TRIM5alpha associates with another IFN-induced gene, sequestosome-1/p62 (p62). p62 plays a role in several signal transduction cascades that are important for maintaining the antiviral state of cells. Here we demonstrate that p62 localizes to both human and rhesus macaque TRIM5alpha cytoplasmic bodies, and fluorescence resonance energy transfer (FRET) analysis demonstrates that these proteins closely associate in these structures. When p62 expression was knocked down via small interfering RNA (siRNA), the number of TRIM5alpha cytoplasmic bodies and the level of TRIM5alpha protein expression were reduced in cell lines stably expressing epitope-tagged versions of TRIM5alpha. In accordance with these data, p62 knockdown resulted in reduced TRIM5alpha-mediated retroviral restriction in cells expressing epitope-tagged TRIM5alpha or expressing endogenously expressed human TRIM5alpha. p62 may therefore operate to enhance TRIM5alpha-mediated retroviral restriction, contributing to the antiviral state of cells following IFN treatment.

Immunoelectron microscopic evidence for Tetherin/BST2 as the physical bridge between HIV-1 virions and the plasma membrane

Tetherin/BST2 was identified in 2008 as the cellular factor responsible for restricting HIV-1 replication at a very late stage in the lifecycle. Tetherin acts to retain virion particles on the plasma membrane after budding has been completed. Infected cells that express large amounts of tetherin display large strings of HIV virions that remain attached to the plasma membrane. Vpu is an HIV-1 accessory protein that specifically counteracts the restriction to virus release contributed by tetherin. Tetherin is an unusual Type II transmembrane protein that contains a GPI anchor at its C-terminus and is found in lipid rafts. The leading model for the mechanism of action of tetherin is that it functions as a direct physical tether bridging virions and the plasma membrane. However, evidence that tetherin functions as a physical tether has thus far been indirect. Here we demonstrate by biochemical and immunoelectron microscopic methods that endogenous tetherin is present on the viral particle and forms a bridge between virion particles and the plasma membrane. Endogenous tetherin was found on HIV particles that were released by partial proteolytic digestion. Immunoelectron microscopy performed on HIV-infected T cells demonstrated that tetherin forms an apparent physical link between virions and connects patches of virions to the plasma membrane. Linear filamentous strands that were highly enriched in tetherin bridged the space between some virions. We conclude that tetherin is the physical tether linking HIV-1 virions and the plasma membrane. The presence of filaments with which multiple molecules of tetherin interact in connecting virion particles is strongly suggested by the morphologic evidence.

Molecular control of HIV-1 postintegration latency: implications for the development of new therapeutic strategies

The persistence of HIV-1 latent reservoirs represents a major barrier to virus eradication in infected patients under HAART since interruption of the treatment inevitably leads to a rebound of plasma viremia. Latency establishes early after infection notably (but not only) in resting memory CD4⁺ T cells and involves numerous host and viral trans-acting proteins, as well as processes such as transcriptional interference, RNA silencing, epigenetic modifications and chromatin organization. In order to eliminate latent reservoirs, new strategies are envisaged and consist of reactivating HIV-1 transcription in latently-infected cells, while maintaining HAART in order to prevent de novo infection. The difficulty lies in the fact that a single residual latently-infected cell can in theory rekindle the infection. Here, we review our current understanding of the molecular mechanisms involved in the establishment and maintenance of HIV-1 latency and in the transcriptional reactivation from latency. We highlight the potential of new therapeutic strategies based on this understanding of latency. Combinations of various compounds used simultaneously allow for the targeting of transcriptional repression at multiple levels and can facilitate the escape from latency and the clearance of viral reservoirs. We describe the current advantages and limitations of immune T-cell activators, inducers of the NF-κB signaling pathway, and inhibitors of deacetylases and histone- and DNA- methyltransferases, used alone or in combinations. While a solution will not be achieved by tomorrow, the battle against HIV-1 latent reservoirs is well- underway.

HIV-1 antagonism of CD317 is species specific and involves Vpu-mediated proteasomal degradation of the restriction factor

Mammals encode proteins that inhibit viral replication at the cellular level. In turn, certain viruses have evolved genes that can functionally counteract these intrinsic restrictions. Human CD317 (BST-2/HM1.24/tetherin) is a restriction factor that blocks release of human immunodeficiency virus type 1 (HIV-1) from the cell surface and can be overcome by HIV-1 Vpu. Here, we show that mouse and rat CD317 potently inhibit HIV-1 release but are resistant to Vpu. Interspecies chimeras reveal that the rodent-specific resistance and human-specific sensitivity to Vpu antagonism involve all three major structural domains of CD317. To promote virus release, Vpu depletes cellular pools of human CD317, but not of the rodent orthologs, by accelerating its degradation via the 20S proteasome. Thus, HIV-1 Vpu suppresses the expression of the CD317 antiviral factor in human cells, and the species-specific resistance to this suppression may guide the development of small animal models of HIV infection.

Fragile X mental retardation protein restricts replication of human immunodeficiency virus type 1

Gag protein is the major structural component of human immunodeficiency virus type 1 (HIV-1) particles and drives virus assembly on cellular membranes. This function of Gag is attributed to its intrinsic self-multimerization ability as well as its interaction with cellular factors such as TSG101 that binds to the PTAP sequence in the p6 region of Gag and promotes HIV-1 budding through recruiting the ESCRT (endosomal sorting complex required for transport). As a result of its essential role in virus assembly, Gag also becomes the target of cellular restriction factors such as APOBEC3G and Trim5alpha. In this study, we report that the fragile X mental retardation protein (FMRP) interacts with HIV-1 Gag and is packaged into virus particles. Although knockdown of FMRP does not markedly affect production of virus particles, infectivity of HIV-1 virions was significantly decreased. Consistent with this observation, overexpression of the wild type FMRP, but not the FMRP mutants that lack the KH1 or the KH2 domains, led to 2- to 3-fold reduction of virus infectivity. Together, these results suggest that FMRP diminishes HIV-1 infectivity through association with viral Gag protein and virus particles.

Biochemical and biophysical characterization of a chimeric TRIM21-TRIM5alpha protein

The tripartite motif (TRIM) protein, TRIM5alpha, is an endogenous factor in primates that recognizes the capsids of certain retroviruses after virus entry into the host cell. TRIM5alpha promotes premature uncoating of the capsid, thus blocking virus infection. Low levels of expression and tendencies to aggregate have hindered the biochemical, biophysical, and structural characterization of TRIM proteins. Here, a chimeric TRIM5alpha protein (TRIM5(Rh)-21R) with a RING domain derived from TRIM21 was expressed in baculovirus-infected insect cells and purified. Although a fraction of the TRIM5(Rh)-21R protein formed large aggregates, soluble fractions of the protein formed oligomers (mainly dimers), exhibited a protease-resistant core, and contained a high percentage of helical secondary structure. Cross-linking followed by negative staining and electron microscopy suggested a globular structure. The purified TRIM5(Rh)-21R protein displayed E3-ligase activity in vitro and also self-ubiquitylated in the presence of ubiquitin-activating and -conjugating enzymes. The purified TRIM5(Rh)-21R protein specifically associated with human immunodeficiency virus type 1 capsid-like complexes; a deletion within the V1 variable region of the B30.2(SPRY) domain decreased capsid binding. Thus, the TRIM5(Rh)-21R restriction factor can directly recognize retroviral capsid-like complexes in the absence of other mammalian proteins.

Hepadnaviruses have a narrow host range — do they?

Host range describes the range of species that a virus can infect to productively propagate itself. Productive infection requires compatibility between virus and host molecules. Thus host range may be restricted by lack of appropriate permissivity factors;alternatively, hosts may actively counteract infection using restriction factors. Incompatibility between virus and host can manifest on the level of individual cells,of tissues or organs,and of the entire organism. All hepatitis B viruses are hepatotropic,but individual viruses infect the livers of only selected mammalian (orthohepadnaviruses) and avian (avihepadnaviruses) hosts. Hence a narrow host range is thought to be a salient feature of hepadnaviruses. Here we briefly review general mechanisms of host range restriction,and summarise older as well as recent data pertaining to hepadnaviral host range. Clearly,the term species-specific is inadequate for many hepadnaviruses because they can infect different species from one genus,and even species from different genera. For a few others,only a single species,or genus,has been identified that supports efficient infection;however,this could as well relate to the restricted number of experimentally addressable test species. Together with the uncertainty about quantitative phylogenetic relationships between species,still largely based on morphological rather than molecular criteria,this leaves the term narrow open to interpretation. Finally,few if any of the host molecules enabling productive infection by a hepadnavirus have unambiguously been identified,the role of restriction factors has not yet been assessed,and even on the virus side the so-called host determining regions in the PreS domains of the large envelope proteins appear to be relevant only under specialised experimental conditions. Hence this important aspect of hepadnavirus biology is still far from being understood.

Ataxin-1 fusion partners alter polyQ lethality and aggregation

Intranuclear inclusion bodies (IBs) are the histopathologic markers of multiple protein folding diseases. IB formation has been extensively studied using fluorescent fusion products of pathogenic polyglutamine (polyQ) expressing proteins. These studies have been informative in determining the cellular targets of expanded polyQ protein as well as the methods by which cells rid themselves of IBs. The experimental thrust has been to intervene in the process of polyQ aggregation in an attempt to alleviate cytotoxicity. However new data argues against the notion that polyQ aggregation and cytotoxicity are inextricably linked processes. We reasoned that changing the protein context of a disease causing polyQ protein could accelerate its precipitation as an IB, potentially reducing its cytotoxicity. Our experimental strategy simply exploited the fact that conjoined proteins influence each others folding and aggregation properties. We fused a full-length pathogenic ataxin-1 construct to fluorescent tags (GFP and DsRed1-E5) that exist at different oligomeric states. The spectral properties of the DsRed1-E5-ataxin-1 transfectants had the additional advantage of allowing us to correlate fluorochrome maturation with cytotoxicity. Each fusion protein expressed a distinct cytotoxicity and IB morphology. Flow cytometric analyses of transfectants expressing the greatest fluorescent signals revealed that the DsRed1-E5-ataxin-1 fusion was more toxic than GFP fused ataxin-1 (31.8±4.5% cell death versus 12.85±3%), although co-transfection with the GFP fusion inhibited maturation of the DsRed1-E5 fluorochrome and diminished the toxicity of the DsRed1-E5-ataxin-1 fusion. These data show that polyQ driven aggregation can be influenced by fusion partners to generate species with different toxic properties and provide new opportunities to study IB aggregation, maturation and lethality.

Transspecies transmissions of retroviruses: new cases

Transspecies transmission is common among retroviruses, although the consequences of the transmission are very different. Some transspecies transmissions have resulted in fatal diseases in the new host while others have remained asymptomatic. Some retroviruses are apathogenic in the original species, but pathogenic in a new host and others can be pathogenic or apathogenic in both species. In some cases, endogenization of the retrovirus in the new host has been observed but in others not, while some transmitted retroviruses exist in both forms. Although in most cases transspecies transmission has been observed in one direction only, bidirectional transmissions of caprine and ovine lentiviruses have recently been described. Studies on newly reported natural and experimental transspecies transmissions of the koala retrovirus (KoRV) may help to understand such events.