Isolation and Characterization of an Equine Foamy Virus

Abstract

Foamy viruses (FVs) are complex retroviruses which have been isolated from different animal species including nonhuman primates, cattle, and cats. Here, we report the isolation and characterization of a new FV isolated from blood samples of horses. Similar to other FVs, the equine foamy virus (EFV) exhibits a highly characteristic ultrastructure and induces syncytium formation and subsequent cell lysis on a large number of cell lines. Molecular cloning of EFV reveals that the general organization is that of other known FVs, whereas sequence similarity with its bovine FV counterpart is only 40%. Interestingly, EFV buds exclusively from the plasma membrane and not from the endoplasmic reticulum (ER), as previously shown for other FVs. The absence of the ER retrieval dilysine motif in EFV Env is likely responsible for this unexpected sorting pathway.Foamy viruses (FVs), also known as spumaviruses, are widespread complex retroviruses which have been isolated from nonhuman primates, cattle, and cats. Although highly lytic in vitro, these viruses, which are innocuous in vivo, are known to induce a persistent infection in their hosts (20). All FVs characterized to date have very large genomes (between 12 to 13 kb) with the classical gag, pol, and env structural genes and additional regulatory open reading frames (ORFs) located at the 3Ј end of the env gene which are under the control of both the 5Ј long terminal repeat (LTR) and an internal promoter (IP) (14). In the case of the human foamy virus (HFV), the prototype FV, two accessory proteins, Tas and Bet, have been described. While Tas (originally called Bel1) is the potent DNA binding transactivator of viral gene expression of both the LTR and the IP, Bet has been shown to play a key role in the establishment and control of viral persistence in vitro (1,19).The molecular biology of retroviruses was highly focused on human T-cell leukemia virus (HTLV) and human immunodeficiency virus (HIV), clearly associated with human pathologies. However, recent findings regarding FVs raise important issues of general interest. Indeed, some of their features, such as the formation of a specific pol mRNA and the infectivity of the incoming viral DNA contained in extracellular virions (26, 28), set FVs apart from all other retroviruses. By virtue of these two features, FVs resemble pararetroviruses.By analogy with lentiviruses, which were isolated from cattle, cats, primates, goats, and horses, we decided to look for the presence of an FV in horses. Here, we report the isolation of a new nonprimate FV from blood samples of naturally infected horses. The equine foamy virus (EFV) has been characterized by molecular cloning and nucleotide sequence analysis. The ultrastructure of the extracellular virion and the genomic organization were investigated and compared to those of other cloned FVs. Although displaying limited sequence similarities with primate FVs, EFV is phylogenetically closer to nonprimate FVs, especially to the bovine foamy virus (BFV). Interestingly, in contrast to other...

Identification of a Conserved Residue of Foamy Virus Gag Required for Intracellular Capsid Assembly

“…Interestingly, the equine and bovine FV possess sequences homologous to the intracellular assembly domain (Fig. 1A) but lack the ER retention signal on the Env protein and, correspondingly, bud virus solely from the PM (21,42). Recently, it has been reported that the N terminus of the leader peptide of SFVcpz(hu) Env, termed the budding domain, is required for efficient particle release (25).…”

“…Comparison of this region of FV Gag with the CTRS of MPMV reveals a number of identical residues, comprising a domain of FV Gag [GXWGX 3 RX 7 L (Q/V)D], centered on the conserved Arg (7,42). We predicted that if cytoplasmic assembly of FV capsids is mediated by sequences in this region, mutation of conserved residues might block assembly altogether, since the FV Gag protein is not myristylated and is not known to possess a membrane-targeting signal.…”

“…Although FVs cause substantial cytopathic effects in tissue culture (hence the name "foamy", referring to the highly vacuolated appearance of infected cells), an asymptomatic and persistent infection is seen in nature in a wide variety of organisms including nonhuman primates, cats, cattle, and horses (22,27,42,46). The genomic organization of the FVs, including the prototype molecular cloned simian foamy virus SFVcpz(hu), is similar to that of other complex retroviruses, with several additional open reading frames located 3Ј of the canonical gag, pol, and env genes, including the transcriptional transactivator gene, tas (23,29,36).…”

Complete nucleotide sequence and evolutionary analysis of a Gorilla foamy virus

“…Although FV cause a persistent infection, they are harmless, implying that the virus may be a valuable tool for viral vector development. FV have been isolated and cloned from a variety of primate (for references see below) and non-primate hosts (Holzschu et al, 1998;Tobaly-Tapiero et al, 2000;Winkler et al, 1997). Evolutionary studies would benefit from the characterization of the FV genomic diversity, but to date, only a few primate FVs from macaques (SFVmac) (Kupiec et al, 1991), African green monkey (SFVagm) (Renne et al, 1992), orang-utan (SFVora) (Verschoor et al, 2003), chimpanzees (SFVcpz) (Herchenröder et al, 1994(Herchenröder et al, , 1995, spider monkey (SFVspm) (Thümer et al, 2007), marmoset and squirrel monkey (Pacheco et al, 2010) have been cloned and completely sequenced.…”

Nuclear import of prototype foamy virus transactivator Bel1 is mediated by KPNA1, KPNA6 and KPNA7

“…FVs are found in primates, including humans, as well as in non-primates, including cows, cats and horses (1)(2)(3)(4)(5).…”