Xenotropic retrovirus Bxv1 in human pancreatic ß cell lines.

It has been reported that endogenous retroviruses can contaminate human cell lines that have been passaged as xenotransplants in immunocompromised mice. We previously developed and described 2 human pancreatic ß cell lines (EndoC-ßH1 and EndoC-ßH2) that were generated in this way. Here, we have shown that B10 xenotropic virus 1 (Bxv1), a xenotropic endogenous murine leukemia virus (MuLV), is present in these 2 recently described cell lines. We determined that Bxv1 was also present in SCID mice that were used for in vivo propagation of EndoC-ßH1/2 cells, suggesting that contamination occurred during xenotransplantation. EndoC-ßH1/2 cells released Bxv1 particles that propagated to human 293T and Mus dunni cells. Mobilization assays demonstrated that Bxv1 transcomplements defective MuLV-based retrovectors. In contrast, common rodent ß cell lines, rat INS-1E and RIN-5F cells and mouse MIN6 and ßTC3 cells, displayed either no or extremely weak xenotropic helper activity toward MuLV-based retrovectors, although xenotropic retrovirus sequences and transcripts were detected in both mouse cell lines. Bxv1 propagation from EndoC-ßH1/2 to 293T cells occurred only under optimized conditions and was overall poorly efficient. Thus, although our data imply that MuLV-based retrovectors should be cautiously used in EndoC-ßH1/2 cells, our results indicate that an involuntary propagation of Bxv1 from these cells can be easily avoided with good laboratory practices.

Gammaretrovirus mRNA expression is mediated by a novel, bipartite post-transcriptional regulatory element.

Post-transcriptional regulatory mechanisms of several complex and simple retroviruses and retroelements have been elucidated, with the exception of the gammaretrovirus family. We found that, similar to the other retroviruses, gag gene expression of MuLV and XMRV depends on post-transcriptional regulation mediated via an RNA sequence overlapping the pro-pol open reading frame, termed the Post-Transcriptional Element (PTE). PTE function can be replaced by heterologous RNA export elements, e.g. CTE of simian type D retroviruses. Alternatively, Gag particle production is achieved using an RNA/codon optimized gag gene. PTE function is transferable and can replace HIV Rev-RRE-regulated expression of HIV gag. Analysis of PTE by SHAPE revealed a highly structured RNA comprising seven stem-loop structures, with the 5' and 3' stem-loops forming an essential bipartite signal. MuLV and XMRV PTE share 98% identity and have highly similar RNA structures, with changes mostly located to single-stranded regions. PTE identification strongly suggests that all retroviruses and retroelements share common strategies of post-transcriptional gene regulation to produce Gag. Expression depends on complex RNA structures embedded within retroviral mRNA, in coding regions or the 3' untranslated region. These specific structures serve as recognition signals for either cellular or viral proteins.

Detection of viral proteins in human cells lines by xeno-proteomics: elimination of the last valid excuse for not testing every cellular proteome dataset for viral proteins.

Cell cultures used routinely in proteomic experiments may contain proteins from other species because of infection, transfection or just contamination. Since infection or contamination may affect the results of a biological experiment, it is important to test the samples for the presence of "alien" proteins. Usually cells are tested only for the most common infections, and most of the existing tests are targeting specific contaminations. Here we describe a three-step procedure for reliable untargeted detection of viral proteins using proteomics data, and recommend this or similar procedure to be applied to every proteomics dataset submitted for publication.

Cholesterol depletion inactivates XMRV and leads to viral envelope protein release from virions: evidence for role of cholesterol in XMRV infection.

Membrane cholesterol plays an important role in replication of HIV-1 and other retroviruses. Here, we report that the gammaretrovirus XMRV requires cholesterol and lipid rafts for infection and replication. We demonstrate that treatment of XMRV with a low concentration (10 mM) of 2-hydroxypropyl-ß-cyclodextrin (2OHpßCD) partially depleted virion-associated cholesterol resulting in complete inactivation of the virus. This effect could not be reversed by adding cholesterol back to treated virions. Further analysis revealed that following cholesterol depletion, virus-associated Env protein was significantly reduced while the virions remained intact and retained core proteins. Increasing concentrations of 2OHpßCD (≥20 mM) resulted in loss of the majority of virion-associated cholesterol, causing disruption of membrane integrity and loss of internal Gag proteins and viral RNA. Depletion of cholesterol from XMRV-infected cells significantly reduced virus release, suggesting that cholesterol and intact lipid rafts are required for the budding process of XMRV. These results suggest that unlike glycoproteins of other retroviruses, the association of XMRV glycoprotein with virions is highly dependent on cholesterol and lipid rafts.

Moloney murine leukemia virus glyco-gag facilitates xenotropic murine leukemia virus-related virus replication through human APOBEC3-independent mechanisms.

BACKGROUND: One of the unique features of gammaretroviruses is that they contain an additional extended form of Gag, glyco-gag, which initiates in the leader sequence. MuLV glyco-gag, gPr80Gag, promotes retrovirus replication and disease progression. Although virtually all infectious MuLVs encode glyco-gag, XMRV (xenotropic murine leukemia virus-related virus) lacks the classical gPr80Gag sequence. We examined XMRV to determine if its leader sequence contains glyco-gag activity, whether the presence of conventional gPr80Gag affects replication of XMRV, and we describe the evolution of glyco-gag-deficient MuLVs in Mus. RESULTS: We introduced several mutations disrupting two putative but noncanonical glyco-gag proteins in the leader sequence region in XMRV and found that those mutations did not affect virus release nor susceptibility to the antiviral activity of hA3G (human APOBEC3G). A chimeric XMRV encoding the Moloney MuLV (M-MuLV) leader sequence (MXMRV) demonstrated that M-MuLV glyco-gag facilitated MXMRV release and increased infectivity. Infectivity assays with several cell lines showed that glyco-gag increases XMRV infectivity in all cell lines tested, but the level of this increase varies in different cell lines. Because MuLV glyco-gag counteracts mouse APOBEC3, we investigated whether M-MuLV glyco-gag enhances XMRV infection by counteracting human APOBEC3. Comparison of hAPOBEC3 isoforms expressed in different cell lines indicated that hA3B was the most likely candidate for a restrictive hA3. However over-expression of hA3B showed no enhanced restriction of infection by XMRV compared to MXMRV. Endogenous MuLVs in the sequenced mouse genome were screened for canonical glyco-gag, which was identified in two clades of xenotropic MuLVs (X-MuLVs) and ecotropic MuLVs, but not in other X-MuLVs or in any polytropic MuLVs. CONCLUSIONS: M-MuLV glyco-gag facilitates XMRV replication, and the leader sequence region in XMRV does not encode proteins equivalent to M-MuLV glyco-gag. The fact that the ability of glyco-gag to enhance XMRV infection varies in different cell lines suggests a glyco-gag sensitive restrictive factor that further reduces XMRV infectivity. The M-MuLV glyco-gag enhancement for XMRV replication is through a hAPOBEC3 independent mechanism. The absence of glyco-gag in MuLVs carried by western European mice suggests that loss of this sequence is a relatively recent event with limited subspecies distribution.

Membrane fusion and cell entry of XMRV are pH-independent and modulated by the envelope glycoprotein's cytoplasmic tail.

Xenotropic murine leukemia virus-related virus (XMRV) is a gammaretrovirus that was originally identified from human prostate cancer patients and subsequently linked to chronic fatigue syndrome. Recent studies showed that XMRV is a recombinant mouse retrovirus; hence, its association with human diseases has become questionable. Here, we demonstrated that XMRV envelope (Env)-mediated pseudoviral infection is not blocked by lysosomotropic agents and cellular protease inhibitors, suggesting that XMRV entry is not pH-dependent. The full length XMRV Env was unable to induce syncytia formation and cell-cell fusion, even in cells overexpressing the viral receptor, XPR1. However, truncation of the C-terminal 21 or 33 amino acid residues in the cytoplasmic tail (CT) of XMRV Env induced substantial membrane fusion, not only in the permissive 293 cells but also in the nonpermissive CHO cells that lack a functional XPR1 receptor. The increased fusion activities of these truncations correlated with their enhanced SU shedding into culture media, suggesting conformational changes in the ectodomain of XMRV Env. Noticeably, further truncation of the CT of XMRV Env proximal to the membrane-spanning domain severely impaired the Env fusogenicity, as well as dramatically decreased the Env incorporations into MoMLV oncoretroviral and HIV-1 lentiviral vectors resulting in greatly reduced viral transductions. Collectively, our studies reveal that XMRV entry does not require a low pH or low pH-dependent host proteases, and that the cytoplasmic tail of XMRV Env critically modulates membrane fusion and cell entry. Our data also imply that additional cellular factors besides XPR1 are likely to be involved in XMRV entry.

Susceptibility of human primary neuronal cells to xenotropic murine leukemia virus-related (XMRV) virus infection.

BACKGROUND: Xenotropic Murine Leukemia Virus-related (XMRV) virus is a recently identified mouse gammaretrovirus that has the ability to infect certain human cells. In this study, we investigated the susceptibility of primary neuronal cell types to infection with XMRV. FINDINGS: We observed that the human primary progenitors, progenitor-derived neurons, and progenitor-derived astrocytes supported XMRV multiplication. Interestingly, both progenitors and progenitor-derived neurons were more susceptible compared with progenitor-derived astrocytes. In addition, XMRV-infected Jurkat cells were able to transmit infection to neuronal cells. CONCLUSIONS: These data suggest that neuronal cells are susceptible for XMRV infection.

Antibody responses against xenotropic murine leukemia virus-related virus envelope in a murine model.

BACKGROUND: Xenotropic murine leukemia virus-related virus (XMRV) was recently discovered to be the first human gammaretrovirus that is associated with chronic fatigue syndrome and prostate cancer (PC). Although a mechanism for XMRV carcinogenesis is yet to be established, this virus belongs to the family of gammaretroviruses well known for their ability to induce cancer in the infected hosts. Since its original identification XMRV has been detected in several independent investigations; however, at this time significant controversy remains regarding reports of XMRV detection/prevalence in other cohorts and cell type/tissue distribution. The potential risk of human infection, coupled with the lack of knowledge about the basic biology of XMRV, warrants further research, including investigation of adaptive immune responses. To study immunogenicity in vivo, we vaccinated mice with a combination of recombinant vectors expressing codon-optimized sequences of XMRV gag and env genes and virus-like particles (VLP) that had the size and morphology of live infectious XMRV. RESULTS: Immunization elicited Env-specific binding and neutralizing antibodies (NAb) against XMRV in mice. The peak titers for ELISA-binding antibodies and NAb were 1:1024 and 1:464, respectively; however, high ELISA-binding and NAb titers were not sustained and persisted for less than three weeks after immunizations. CONCLUSIONS: Vaccine-induced XMRV Env antibody titers were transiently high, but their duration was short. The relatively rapid diminution in antibody levels may in part explain the differing prevalences reported for XMRV in various prostate cancer and chronic fatigue syndrome cohorts. The low level of immunogenicity observed in the present study may be characteristic of a natural XMRV infection in humans.

Purify First: rapid expression and purification of proteins from XMRV.

Purifying proteins from recombinant sources is often difficult, time-consuming, and costly. We have recently instituted a series of improvements in our protein purification pipeline that allows much more accurate choice of expression host and conditions and purification protocols. The key elements are parallel cloning, small scale parallel expression and lysate preparation, and small scale parallel protein purification. Compared to analyzing expression data only, results from multiple small scale protein purifications predict success at scale-up with greatly improved reliability. Using these new procedures we purified eight of nine proteins from xenotropic murine leukemia virus-related virus (XMRV) on the first attempt at large scale.

Characterization of retroviral and lentiviral vectors pseudotyped with xenotropic murine leukemia virus-related virus envelope glycoprotein.

Retroviral and lentiviral vectors are effective gene delivery vehicles that are being evaluated in clinical trials. Variations in the viral envelope (Env) glycoproteins, which are used to pseudotype retroviral or lentiviral vectors, can alter vector performance, including stability, titers, host range, and tissue tropism. Xenotropic murine leukemia virus (MLV)-related virus (XMRV) is a novel human retrovirus identified in patients with prostate cancer. XMRV targets XPR1 cell surface receptor, which is expressed in a broad range of human tissues including hematopoietic stem cells. Pseudotyping with XMRV Env would allow targeting of XPR1-expressing tissues. Here, we characterized XMRV Env-pseudotyped retroviral and lentiviral vectors. Although HIV and MLV vectors were poorly pseudotyped with wild-type XMRV Env, replacement of the C-terminal 11 amino acid residues in the transmembrane domain of XMRV Env with the corresponding 6 amino acid residues of amphotropic MLV Env (XMRV/R(ampho)) significantly increased XMRV Env-pseudotyped HIV and MLV vector titers. The transduction efficiency in human CD34(+) cells when using the XMRV/R(ampho)-pseudotyped HIV vector (10-20%) was comparable to that achieved when using the same infectious units of vesicular stomatitis virus G glycoprotein-pseudotyped vector (25%); thus the modified XMRV Env offers an alternative pseudotyping strategy for XPR1-mediated gene delivery.