Nitta et al. Retrovirology (2015) 12:68 DOI 10.1186/s12977-015-0193-1 RESEARCH Open Access Human and murine APOBEC3s restrict replication of koala retrovirus by different mechanisms Takayuki Nitta1,2,3*, Dat Ha1,2, Felipe Galvez1,2, Takayuki Miyazawa4 and Hung Fan1,2* Abstract Background: Koala retrovirus (KoRV) is an endogenous and exogenous retrovirus of koalas that may cause lym- phoma. As for many other gammaretroviruses, the KoRV genome can potentially encode an alternate form of Gag protein, glyco-gag. Results: In this study, a convenient assay for assessing KoRV infectivity in vitro was employed: the use of DERSE cells (initially developed to search for infectious xenotropic murine leukemia-like viruses). Using infection of DERSE and other human cell lines (HEK293T), no evidence for expression of glyco-gag by KoRV was found, either in expression of glyco-gag protein or changes in infectivity when the putative glyco-gag reading frame was mutated. Since glyco-gag mediates resistance of Moloney murine leukemia virus to the restriction factor APOBEC3, the sensitivity of KoRV (wt or putatively mutant for glyco-gag) to restriction by murine (mA3) or human APOBEC3s was investigated. Both mA3 and hA3G potently inhibited KoRV infectivity. Interestingly, hA3G restriction was accompanied by extensive G A hypermutation during reverse transcription while mA3 restriction was not. Glyco-gag status did not affect the→ results. Conclusions: These results indicate that the mechanisms of APOBEC3 restriction of KoRV by hA3G and mA3 differ (deamination dependent vs. independent) and glyco-gag does not play a role in the restriction. Keywords: KoRV, APOBEC3, Glyco-gag Background endogenous KoRV proviruses have not become fixed Koala retrovirus (KoRV) is a recently discovered retrovi- into the koala population. This contrasts with the situa- rus that infects koalas [1], and it is a likely candidate for tion for endogenous retroviruses in many other species, the causative agent of lymphomas and other hematologic including humans, where most individual proviruses diseases in these animals. There are several interest- are shared by all members of the species. Second, KoRV ing and unique features of KoRV. First, some but not all infection appears to be spreading from north to south in koalas carry endogenous KoRV-related proviruses, and Australia, with virtually all animals in the north showing among different unrelated animals the patterns of endog- evidence for infection (both endogenous and potentially enous proviral integration sites are distinct [2]. It has also exogenous KoRVs); in southern regions and on some off- been reported that endogenization of KoRV into koa- shore islands, the incidences of KoRV infection and/or las is an ongoing process [3]. Thus it appears that KoRV endogenization are lower [4]. has infected koalas recently, to the extent that individual KoRV is a member of the gammaretrovirus genus. Gammaretroviruses encode the standard three genes of retroviruses, gag, pol and env, but no accessory proteins *Correspondence: [email protected]; [email protected] such as those of lentiviruses, deltaretroviruses, epsilon- 2 Cancer Research Institute, University of California, Irvine, Irvine, CA 92697‑3905, USA retroviruses and betaretroviruses. KoRV’s closest rela- 3 Department of Biology, Savannah State University, 3219 College St, tive is gibbon-ape leukemia virus (GaLV), and it is more Savannah, GA 31404‑5254, USA distantly related to the murine leukemia viruses (MuLVs) Full list of author information is available at the end of the article © 2015 Nitta et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Nitta et al. Retrovirology (2015) 12:68 Page 2 of 12 [1]. The predominant KoRV (endogenous and exogenous) 17]. DERSE cells are derivatives of human LNCaP pros- is now referred to as KoRV-A, that infects cells by bind- tate cancer cells that express an XMRV viral sequence ing to the Pit-1 phosphate transporter as a receptor [5]. containing an inverted, intron-interrupted GFP reporter Other KoRV isolates (e.g. KoRV-B and others) differ from cassette. GFP protein is not expressed in DERSE cells, KoRV-A in the envelope gene [6], and they infect cells but if they are infected with a replication-competent ret- via different receptors (e.g. the thiamine transporter for rovirus that can package spliced XMRV RNA (with the KoRV-B) [7]. It has been suggested that KoRV-B infection inverted intron removed) into virus particles, spread is associated with development of lymphoma [7]. KoRV of infection to secondarily infected cells results in tran- may result from relatively recent cross-species infection scription of the uninterrupted inverted GFP RNA from of an MuLV-like virus of a southeast Asian rodent into integrated XMRV DNA, resulting in expression of GFP koalas. that can be easily monitored by fluorescence microscopy. We and others have shown that many gammaretrovi- Since human cells have functional KoRV receptors, and ruses encode an alternate form of Gag polyprotein, gly- KoRV is a gammaretrovirus phylogenetically related to cosylated Gag or glyco-gag [8–10]. We have shown that murine gammaretroviruses (including XMRV), it seemed glyco-gag of Moloney MuLV enhances virus replication possible that KoRV could package XMRV RNA and func- by facilitating virus release through lipid rafts [11] and tion in the DERSE cell assay. 293T cells were transiently antagonizing a host restriction factor, mouse APOBEC3 transfected with the KoRV molecular clone, pKoRV522, (mA3) [12]. Glyco-gags in MuLVs are translated from and the viruses released from the cells were collected as unspliced viral RNA from an upstream CUG initiation a source of infectious virus (Fig. 1). Similar to XMRV, codon in the same reading frame as the AUG codon used DERSE cells infected with KoRV showed individual GFP for initiation of Gag and Gag-Pol polyproteins. However fluorescent cells 4 days post-infection, indicating that some endogenous MuLVs (of the class A xenotropic, KoRV can package the spliced XMRV-GFP reporter polytropic and modified polytropic classes) do not have RNA and transfer it to neighboring DERSE cells in a sec- the capacity to encode glyco-gag [13]. The KoRV genome ond round of infection (Fig. 1). The infectivity of KoRV contains an in-frame ORF upstream of Gag characteristic released from the infected DERSE cells was also demon- of glyco-gags of exogenous and endogenous MuLVs [13]. strated by using supernatants from these cells to infect In this report, we tested if KoRV expresses a glyco-gag, fresh 293T cells (Fig. 1). After serial transfer the infected and if glyco-gag is important for infectivity in vitro. In 293T cells showed strong signals for KoRV Gag protein, light of the role of glyco-gag in counteracting restriction indicating that they were also infected with the XMRV- by APOBEC3 for M-MuLV we also investigated restric- GFP reporter construct (Fig. 1). tion of wild-type and a putative glyco-gag negative ver- To test if DERSE cells could be used to quantify KoRV sion of KoRV by murine and human APOBEC3s. infection, the relationship between the amount of KoRV input and levels of GFP or KoRV Gag proteins in infected Results DERSE cells was assessed. Different amounts of KoRV Use of DERSE cells to detect KoRV infection from 293T cells transiently transfected with pKoRV522 Previous experiments demonstrated that different KoRV were used to infect DERSE cells. As expected, GFP isolates can infect human cells through the sodium- expression monitored by fluorescence microscopy was dependent phosphate transporter 1 (PiT1) or the thia- correlated with the doses of KoRV (Fig. 2a). In addition, mine transporter 1 (THTR1) that are present on certain western blots demonstrated that the amounts of KoRV human cell lines, but not on PBMCs [7, 14]. Starting Gag protein and GFP in the infected DERSE cells also with the infectious KoRV522 clone, we confirmed that were well correlated with the doses of the infecting KoRV KoRV522 can infect cell lines such as human 293T cells (Fig. 2b). These results demonstrated that DERSE cells and monkey COS7 cells (data not shown). To develop a are a rapid and convenient method to assay and quantify convenient tool for assessing KoRV infection, we tested KoRV infectivity. KoRV infection of DERSE (Detectors of Exogenous Retroviral Sequence Elements) cells. DERSE cells were Absence of glycosylated gag expression in human cells originally developed for detecting xenotropic murine infected by KoRV leukemia virus-related virus (XMRV) infection [15]. As mentioned in the introduction, the KoRV genome XMRV was initially considered a potential infectious contains sequences that could potentially encode a human retrovirus, but it was subsequently found to be typical glyco-gag. Examination of the KoRV (J group) a recombinant between two murine endogenous xeno- RNA sequence indicated that KoRV has three potential tropic retroviruses that occurred during passage of a upstream CUG codons in the same reading frame as the human prostate cancer in nude mouse xenografts [16, Gag polyprotein Pr60gag (Fig. 3). Moreover one protein Nitta et al. Retrovirology (2015) 12:68 Page 3 of 12 nearly identical nucleic acid and protein sequences beginning with the conserved LGDVP motif in the leader peptide sequence (Additional file 1: Figs. S1, S2). To assess whether KoRV produces functional glyco-gag pro- tein analogous to those in MuLVs, we introduced a muta- tion that would disrupt expression of putative glyco-gag protein in the plasmid containing the full-length KoRV molecular clone, pKoRV522 (Fig.