Importance of the short cytoplasmic domain of the feline immunodeficiency virus transmembrane glycoprotein for fusion activity and envelope glycoprotein incorporation into virions

The life cycle of feline immunodeficiency virus

Feline immunodeficiency virus (FIV) is a retrovirus of worldwide distribution that can cause an acquired immunodeficiency disease in domestic cats. FIV and the primate lentiviruses, human and simian immunodeficiency viruses (HIV and SIV, respectively) share structural and biological features but also exhibit important differences, which reflect both their evolutionary relationship and divergence. Given that FIV is not only an important cat pathogen but also a useful model for certain aspects of HIV-1 infections in humans, the study of FIV biology is highly relevant. In this review we provide an updated description of the molecular mechanisms involved in each stage of the FIV life cycle.

In vitro inhibitory effect of maraviroc on the association of the simian immunodeficiency virus envelope glycoprotein with CCR5

Asian macaques infected with simian immunodeficiency viruses (SIVs) isolated from African non-human primates develop a disease similar to human AIDS. SIV enters its target cells by binding to CD4 and a coreceptor, typically CCR5. Maraviroc is an entry inhibitor of human immunodeficiency virus type 1 (HIV-1) that prevents the interaction between CCR5 and the surface subunit gp120 of the viral envelope glycoprotein (Env). Thus far, the activity of maraviroc on SIV entry has been poorly studied. Here, we determined in vitro pharmacological parameters of the effect of maraviroc on the SIV Env association with CCR5. Cell-to-cell fusion inhibition assays were used to compare the susceptibility to maraviroc of the SIVsmmPBj Env-CCR5 interaction with that of HIV-1BaL Env. Analysis of dose-response curves and determination of IC₅₀ values demonstrate that increasing concentrations of maraviroc inhibit the membrane fusion activity of SIVsmmPBj Env in a manner and to an extent similar to that of HIV-1BaL Env.

The Conserved Tyr176/Leu177 Motif in the α-Helix 9 of the Feline Immunodeficiency Virus Capsid Protein Is Critical for Gag Particle Assembly

The capsid domain (CA) of the lentiviral Gag polyproteins has two distinct roles during virion morphogenesis. As a domain of Gag, it mediates the Gag–Gag interactions that drive immature particle assembly, whereas as a mature protein, it self-assembles into the conical core of the mature virion. Lentiviral CA proteins are composed of an N-terminal region with seven α-helices and a C-terminal domain (CA-CTD) formed by four α-helices. Structural studies performed in HIV-1 indicate that the CA-CTD helix 9 establishes homodimeric interactions that contribute to the formation of the hexameric Gag lattice in immature virions. Interestingly, the mature CA core also shows inter-hexameric associations involving helix 9 residues W184 and M185. The CA proteins of feline immunodeficiency virus (FIV) and equine infectious anemia virus (EIAV) exhibit, at equivalent positions in helix 9, the motifs Y176/L177 and L169/F170, respectively. In this paper, we investigated the relevance of the Y176/L177 motif for FIV assembly by introducing a series of amino acid substitutions into this sequence and studying their effect on in vivo and in vitro Gag assembly, CA oligomerization, mature virion production, and viral infectivity. Our results demonstrate that the Y176/L177 motif in FIV CA helix 9 is essential for Gag assembly and CA oligomerization. Notably, mutations converting the FIV CA Y176/L177 motif into the HIV-1 WM and EIAV FL sequences allow substantial particle production and viral replication in feline cells.

Properties and Functions of Feline Immunodeficiency Virus Gag Domains in Virion Assembly and Budding

Feline immunodeficiency virus (FIV) is an important cat pathogen worldwide whose biological and pathophysiological properties resemble those of human immunodeficiency virus type 1 (HIV-1). Therefore, the study of FIV not only benefits its natural host but is also useful for the development of antiviral strategies directed against HIV-1 infections in humans. FIV assembly results from the multimerization of a single but complex viral polypeptide, the Gag precursor. In this review, we will first give an overview of the current knowledge of the proteins encoded by the FIV pol, env, rev, vif, and orf-A genes, and then we will describe and discuss in detail the critical roles that each of the FIV Gag domains plays in virion morphogenesis. Since retroviral assembly is an attractive target for therapeutic interventions, gaining a better understanding of this process is highly desirable.

Differences in Env and Gag protein expression patterns and epitope availability in feline immunodeficiency virus infected PBMC compared to infected and transfected feline model cell lines

Env and Gag are key components of the FIV virion that are targeted to the plasma membrane for virion assembly. They are both important stimulators and targets of anti-FIV immunity. To investigate and compare the expression pattern and antigenic changes of Gag and Env in various research models, infected PBMC (the natural FIV host cells) and GFox, and transfected CrFK were stained over time with various Env and Gag specific MAbs. In FIV infected GFox and PBMC, Env showed changes in epitope availability for antibody binding during processing and trafficking, which was not seen in transfected CrFK. Interestingly, epitopes exposed on intracellular Env and Env present on the plasma membrane of CrFK and GFox seem to be hidden on plasma membrane expressed Env of FIV infected PBMC. A kinetic follow up of Gag and Env expression showed a polarization of both Gag and Env expression to specific sites at the plasma membrane of PBMC, but not in other cell lines. In conclusion, mature trimeric cell surface expressed Env might be antigenically distinct from intracellular monomeric Env in PBMC and might possibly be unrecognizable by feline humoral immunity. In addition, Env expression is restricted to a small area on the plasma membrane and co-localizes with a large moiety of Gag, which may represent a preferred FIV budding site, or initiation of virological synapses with direct cell-to-cell virus transmission.

Processing, fusogenicity, virion incorporation and CXCR4-binding activity of a feline immunodeficiency virus envelope glycoprotein lacking the two conserved N-glycosylation sites at the C-terminus of the V3 domain

The process of feline immunodeficiency virus (FIV) entry into its target cells is initiated by the association of the surface (SU) subunit of the viral envelope glycoprotein (Env) with the cellular receptors CD134 and CXCR4. This event is followed by the fusion of the viral and cellular membranes, which is mediated by the transmembrane (TM) subunit of Env. We and others have previously demonstrated that the V3 domain of the SU subunit of Env is essential for CXCR4 binding. Of note, there are two contiguous and highly conserved potential N-glycosylation sites ((418)NST(420) and (422)NLT(424)) located at the C-terminal side of the V3 domain. We therefore decided to study the relevance for Env functions of these N-glycosylation motifs and found that disruption of both of them by introducing the N418Q/N422Q double amino acid substitution drastically impairs Env processing into the SU and TM subunits. Moreover, the simultaneous mutation of these N-glycosylation sites prevents Env incorporation into virions and Env-mediated cell-to-cell fusion. Notably, a recombinant soluble version of the SU glycoprotein carrying the double amino acid replacement N418Q/N422Q at the V3 C-terminal side binds to CXCR4 with an efficiency similar to that of wild-type SU.

Lentiviral Gag assembly analyzed through the functional characterization of chimeric simian immunodeficiency viruses expressing different domains of the feline immunodeficiency virus capsid protein

To gain insight into the functional relationship between the capsid (CA) domains of the Gag polyproteins of simian and feline immunodeficiency viruses (SIV and FIV, respectively), we constructed chimeric SIVs in which the CA-coding region was partially or totally replaced by the equivalent region of the FIV CA. The phenotypic characterization of the chimeras allowed us to group them into three categories: the chimeric viruses that, while being assembly-competent, exhibit a virion-associated unstable FIV CA; a second group represented only by the chimeric SIV carrying the N-terminal domain (NTD) of the FIV CA which proved to be assembly-defective; and a third group constituted by the chimeric viruses that produce virions exhibiting a mature and stable FIV CA protein, and which incorporate the envelope glycoprotein and contain wild-type levels of viral genome RNA and reverse transcriptase. Further analysis of the latter group of chimeric SIVs demonstrated that they are non-infectious due to a post-entry impairment, such as uncoating of the viral core, reverse transcription or nuclear import of the preintegration complex. Furthermore, we show here that the carboxyl-terminus domain (CTD) of the FIV CA has an intrinsic ability to dimerize in vitro and form high-molecular-weight oligomers, which, together with our finding that the FIV CA-CTD is sufficient to confer assembly competence to the resulting chimeric SIV Gag polyprotein, provides evidence that the CA-CTD exhibits more functional plasticity than the CA-NTD. Taken together, our results provide relevant information on the biological relationship between the CA proteins of primate and nonprimate lentiviruses.

Replacement of the V3 domain in the surface subunit of the feline immunodeficiency virus envelope glycoprotein with the equivalent region of a T cell-tropic human immunodeficiency virus type 1 results in a chimeric surface protein that efficiently binds to CXCR4

Feline immunodeficiency virus (FIV) and the T cell-tropic strains of human immunodeficiency virus type 1 (HIV-1) share the use of the chemokine receptor CXCR4 for cell entry. To study this process further we developed a cell surface binding assay based on the expression of a soluble version of the FIV SU C-terminally tagged with the influenza virus hemagglutinin epitope (HA). The specificity of the assay was demonstrated by the following evidence: (1) the SU-HA protein bound to HeLa cells that express CXCR4 but not to MDCK cells that lack this chemokine receptor; and (2) binding of the SU-HA to HeLa cells was blocked by incubation with the CXCR4 antagonist AMD3100 as well as with the anti-CXCR4 monoclonal antibody (MAb) 12G5. Deletion of the V3 region from the FIV SU glycoprotein abolished its ability to bind CXCR4-expressing cells. Remarkably, substitution of the V3 domain of the FIV SU by the equivalent region of the HIV-1 NL4-3 isolate resulted in efficient cell surface binding of the chimeric SU protein to CXCR4. Moreover, transfection of MDCK cells with a plasmid encoding human CXCR4 allowed the association of the chimeric SU-HA glycoprotein to the transfected cells. Interestingly, while cell binding of the chimeric FIV-HIV SU was inhibited by an anti-HIV-1 V3 MAb, its association with CXCR4 was found to be resistant to AMD3100. Of note, the chimeric FIV-HIV Env glycoprotein was capable of promoting CXCR4-dependent cell-to-cell fusion.

Understanding the process of envelope glycoprotein incorporation into virions in simian and feline immunodeficiency viruses

The lentiviral envelope glycoproteins (Env) mediate virus entry by interacting with specific receptors present at the cell surface, thereby determining viral tropism and pathogenesis. Therefore, Env incorporation into the virions formed by assembly of the viral Gag polyprotein at the plasma membrane of the infected cells is a key step in the replication cycle of lentiviruses. Besides being useful models of human immunodeficiency virus (HIV) infections in humans and valuable tools for developing AIDS therapies and vaccines, simian and feline immunodeficiency viruses (SIV and FIV, respectively) are relevant animal retroviruses; the study of which provides important information on how lentiviral replication strategies have evolved. In this review, we discuss the molecular mechanisms underlying the incorporation of the SIV and FIV Env glycoproteins into viral particles.

Sequence variation of the feline immunodeficiency virus genome and its clinical relevance

The ongoing evolution of feline immunodeficiency virus (FIV) has resulted in the existence of a diverse continuum of viruses. FIV isolates differ with regards to their mutation and replication rates, plasma viral loads, cell tropism and the ability to induce apoptosis. Clinical disease in FIV-infected cats is also inconsistent. Genomic sequence variation of FIV is likely to be responsible for some of the variation in viral behaviour. The specific genetic sequences that influence these key viral properties remain to be determined. With knowledge of the specific key determinants of pathogenicity, there is the potential for veterinarians in the future to apply this information for prognostic purposes. Genomic sequence variation of FIV also presents an obstacle to effective vaccine development. Most challenge studies demonstrate acceptable efficacy of a dual-subtype FIV vaccine (Fel-O-Vax FIV) against FIV infection under experimental settings; however, vaccine efficacy in the field still remains to be proven. It is important that we discover the key determinants of immunity induced by this vaccine; such data would compliment vaccine field efficacy studies and provide the basis to make informed recommendations on its use.

Palmitoylation of the feline immunodeficiency virus envelope glycoprotein and its effect on fusion activity and envelope incorporation into virions

The feline immunodeficiency virus (FIV) envelope glycoprotein (Env) possesses a short cytoplasmic domain of 53 amino acids containing four highly conserved cysteines at Env positions 804, 811, 815 and 848. Since palmitoylation of transmembrane proteins occurs at or near the membrane anchor, we investigated whether cysteines 804, 811 and 815 are acylated and analyzed the relevance of these residues for Env functions. Replacement of cysteines 804, 811 and 815 individually or in combination by serine residues resulted in Env glycoproteins that were efficiently expressed and processed. However, mutations C804S and C811S reduced Env fusogenicity by 93% and 84%, respectively, compared with wild-type Env. By contrast, mutant C815S exhibited a fusogenic capacity representing 50% of the wild-type value. Remarkably, the double mutation C804S/C811S abrogated both Env fusion activity and Env incorporation into virions. Finally, by means of Click chemistry assays we demonstrated that the four FIV Env cytoplasmic cysteines are palmitoylated.

The molecular biology of feline immunodeficiency virus (FIV)

Feline immunodeficiency virus (FIV) is widespread in feline populations and causes an AIDS-like illness in domestic cats. It is highly prevalent in several endangered feline species. In domestic cats FIV infection is a valuable small animal model for HIV infection. In recent years there has been sa significant increase in interest in FIV, in part to exploit this, but also because of the potential it has as a human gene therapy vector. Though much less studied than HIV there are many parallels in the replication of the two viruses, but also important differences and, despite their likely common origin, the viruses have in some cases used alternative strategies to overcome similar problems. Recent advances in understanding the structure and function of FIV RNA and proteins and their interactions has enhanced our knowledge of FIV replication significantly, however, there are still many gaps. This review summarizes our current knowledge of FIV molecular biology and its similarities with, and differences from, other lentiviruses.

FIV Gag: virus assembly and host-cell interactions

Infection of domestic cats with virulent strains of the feline immunodeficiency virus (FIV) leads to an acquired immunodeficiency syndrome (AIDS), similar to the pathogenesis induced in humans by infection with human immunodeficiency virus type 1 (HIV-1). Thus, FIV is a highly relevant model for anti-HIV therapy and vaccine development. FIV is not infectious in humans, so it is also a potentially effective non-toxic gene therapy vector. To make better use of this model, it is important to define the cellular machinery utilized by each virus to produce virus particles so that relevant similarities can be identified. It is well understood that all replication-competent retroviruses encode gag, pol, and env genes, which provide core elements for virus replication. As a result, most antiretroviral therapy targets pol-derived enzymes (protease, reverse transcriptase, and integrase) orenv-derived glycoproteins that mediate virus attachment and entry. However, resistance to drugs against these targets is a persistent problem, and novel targets must be identified to produce more effective drugs that can either substitute or be combined with current therapy. Elements of the gag gene (matrix, capsid, nucleocapsid, and "late" domains) have yet to be exploited as antiviral targets, even though the Gag precursor polyprotein is self-sufficient for the assembly and release of virus particles from cells. This process is far better understood in primate lentiviruses, especially HIV-1. However, there has been significant progress in recent years in defining how FIV Gag is targeted to the cellular plasma membrane, assembles into virions, incorporates FIV Env glycoproteins, and utilizes host cell machinery to complete virus release. Recent discoveries of intracellular restriction factors that target HIV-1 and FIV capsids after virus entry have also opened exciting new areas of research. This review summarizes currently known interactions involving HIV-1 and FIV Gag that affect virus release, infectivity, and replication.

Reconstitution of the ancestral glycoprotein of human endogenous retrovirus k and modulation of its functional activity by truncation of the cytoplasmic domain

Endogenous retroviruses present in the human genome provide a rich record of ancient infections. All presently recognized elements, including the youngest and most intact proviruses of the human endogenous retrovirus K(HML-2) [HERV-K(HML-2)] family, have suffered postinsertional mutations during their time of chromosomal residence, and genes encoding the envelope glycoprotein (Env) have not been spared these mutations. In this study, we have, for the first time, reconstituted an authentic Env of a HERV-K(HML-2) provirus by back mutation of putative postinsertional amino acid changes of the protein encoded by HERV-K113. Aided by codon-optimized expression, we demonstrate that the reconstituted Env regained its ability to be incorporated into retroviral particles and to mediate entry. The original ancient HERV-K113 Env was synthesized as a moderately glycosylated gp95 precursor protein cleaved into surface and transmembrane (TM) subunits. Of the nine N-linked oligosaccharides, four are part of the TM subunit, contributing 15 kDa to its apparent molecular mass of 41 kDa. The carbohydrates, as well as the cytoplasmic tail, are critical for efficient intracellular trafficking, processing, stability, and particle incorporation. Whereas deletions of the carboxy-terminal 6 residues completely abrogated cleavage and virion association, more extensive truncations slightly enhanced incorporation but dramatically increased the ability to mediate entry of pseudotyped lentiviruses. Although the first HERV-K(HML-2) elements infected human ancestors about 30 million years ago, our findings indicate that their glycoproteins are in most respects remarkably similar to those of classical contemporary retroviruses and can still mediate efficient entry into mammalian cells.

Multifaceted sequence-dependent and -independent roles for reovirus FAST protein cytoplasmic tails in fusion pore formation and syncytiogenesis

Fusogenic reoviruses utilize the FAST proteins, a novel family of nonstructural viral membrane fusion proteins, to induce cell-cell fusion and syncytium formation. Unlike the paradigmatic enveloped virus fusion proteins, the FAST proteins position the majority of their mass within and internal to the membrane in which they reside, resulting in extended C-terminal cytoplasmic tails (CTs). Using tail truncations, we demonstrate that the last 8 residues of the 36-residue CT of the avian reovirus p10 FAST protein and the last 20 residues of the 68-residue CT of the reptilian reovirus p14 FAST protein enhance, but are not required for, pore expansion and syncytium formation. Further truncations indicate that the membrane-distal 12 residues of the p10 and 47 residues of the p14 CTs are essential for pore formation and that a residual tail of 21 to 24 residues that includes a conserved, membrane-proximal polybasic region present in all FAST proteins is insufficient to maintain FAST protein fusion activity. Unexpectedly, a reextension of the tail-truncated, nonfusogenic p10 and p14 constructs with scrambled versions of the deleted sequences restored pore formation and syncytiogenesis, while reextensions with heterologous sequences partially restored pore formation but failed to rescue syncytiogenesis. The membrane-distal regions of the FAST protein CTs therefore exert multiple effects on the membrane fusion reaction, serving in both sequence-dependent and sequence-independent manners as positive effectors of pore formation, pore expansion, and syncytiogenesis.

In vitro binding of simian immunodeficiency virus matrix protein to the cytoplasmic domain of the envelope glycoprotein

Incorporation of the envelope (Env) glycoprotein into budding virions is a key step in the replication cycle of lentiviruses. Previously, we provided genetic and biochemical evidence indicating that Env packaging into simian immunodeficiency virus (SIV) particles is mediated by the association of the Env cytoplasmic domain (CD) with the matrix (MA) domain of Gag. In this study, we developed an in vitro binding assay that, based on recombinant proteins expressed in bacteria, allowed us to demonstrate the physical interaction between the SIV Env CD and the MA in the absence of other viral or cellular proteins. We show that this association is blocked by mutations in each of the interacting domains that have been reported to interfere in vivo with the incorporation of Env into SIV virions. Moreover, we determined that the binding of SIV MA to the Env CD is saturable with a dissociation constant of 7x10(-7) M. Interestingly, the SIV MA is capable of specifically interacting in vitro with the human immunodeficiency virus type 1 Env CD, but not with that of the distantly related feline immunodeficiency virus. Our results strongly support the notion that the association between the SIV MA and Env CD plays a central role in the process of SIV Env incorporation into Gag-made particles.