Infectious DNA of the human spumaretrovirus

The DACH1 Gene Transcriptional Activation and Protein Degradation Mediated by Transactivator Tas of Prototype Foamy Virus

Foamy viruses are members of the Retroviridae family's Spumaretrovirinae subfamily. They induce cell vacuolation and exhibit a foamy pathogenic impact after infecting cells. DACH1 (dachshund family transcription factor 1) is a crucial cytokine linked to tumor development, and is associated with the growth of many different malignant tumor cells. Additionally, DACH1 suppresses pancreatic cell proliferation and is involved in diabetes insulin signaling. Prototype foamy viruses (PFVs) were used for the investigation of the regulatory mechanism of FVs on cellular DACH1 expression. The results show that DACH1 expression in PFV-infected cells was inconsistent at both the transcriptional and protein levels. At the transcriptional level, DACH1 was significantly activated by PFV transactivator Tas, and dual-luciferase reporter gene tests, EMSA, and ChIP assays found a Tas response element of 21 nucleotides in the DACH1 promoter. PFV and Tas did not boost the levels of DACH1 protein in a manner consistent with the high levels of DACH1 transcription expression. It was noted that Tas increased the expression of the Ser/Thr protein phosphatase PPM1E, causing PPM1E-mediated post-translational SUMOylation alterations of DACH1 to prompt DACH1 to degrade. The reason for DACH1 protein degradation is that DACH1 inhibits PFV replication. To sum up, these findings show that PFV upregulated the transcription of DACH1, while urging its protein into PPM1E-mediated SUMOylation, to eliminate the adverse effect of DACH1 overexpression of host cells on viral replication and promote virus survival.

Balance and Stealth: The Role of Noncoding RNAs in the Regulation of Virus Gene Expression

In the past two decades, our knowledge of gene regulation has been greatly expanded by the discovery of microRNAs (miRNAs). miRNAs are small (19-24 nt) noncoding RNAs (ncRNAs) found in metazoans, plants, and some viruses. They have been shown to regulate many cellular processes, including differentiation, maintenance of homeostasis, apoptosis, and the immune response. At present, there are over 300 known viral miRNAs encoded by diverse virus families. One well-characterized function of some viral miRNAs is the regulation of viral transcripts. Host miRNAs can also regulate viral gene expression. We propose that viruses take advantage of both host and viral ncRNA regulation to balance replication and infectious state (for example, latent versus lytic infection). As miRNA regulation can be reversed upon certain cellular stresses, we hypothesize that ncRNAs can serve viruses as barometers for cellular stress.

Heparan sulfate is an attachment factor for foamy virus entry

The cellular receptor of foamy viruses (FVs) is unknown. The broad spectrum of permissive cells suggests that the cellular receptor is a molecular structure with almost ubiquitous prevalence. Here, we investigated the ability of heparan sulfate (HS), a glycosaminoglycan (GAG) present on the extracellular matrix of many cells, to bind FV particles and to permit prototype FV (PFV) and feline FV (FFV) entry. Permissivity of different cell lines for FV entry correlated with the amount of heparan sulfate present on the cell surface. The resulting 50% cell culture infectious doses (CCID(50)s) were distributed over a range of 4 logs, which means that the most susceptible cell line tested (HT1080) was more than 10,000 times more susceptible for PFV infection than the least susceptible cell line (CRL-2242). HS surface expression varied over a range of 2 logs. HS expression and FV susceptibility were positively correlated (P < 0.001). Enzymatic digestion of heparan sulfate on HT1080 cells diminished permissivity for PFV entry by a factor of at least 500. Using fast protein liquid chromatography (FPLC), we demonstrated binding of FV vector particles to a gel filtration column packed with heparin, a molecule structurally related to heparan sulfate, allowing for the purification of infectious particles. Both PFV and FFV infection were inhibited by soluble heparin. Our results show that FVs bind to HS and that this interaction is a pivotal step for viral entry, suggesting that HS is a cellular attachment factor for FVs.

Molecular ecology and natural history of simian foamy virus infection in wild-living chimpanzees

Identifying microbial pathogens with zoonotic potential in wild-living primates can be important to human health, as evidenced by human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2) and Ebola virus. Simian foamy viruses (SFVs) are ancient retroviruses that infect Old and New World monkeys and apes. Although not known to cause disease, these viruses are of public health interest because they have the potential to infect humans and thus provide a more general indication of zoonotic exposure risks. Surprisingly, no information exists concerning the prevalence, geographic distribution, and genetic diversity of SFVs in wild-living monkeys and apes. Here, we report the first comprehensive survey of SFVcpz infection in free-ranging chimpanzees (Pan troglodytes) using newly developed, fecal-based assays. Chimpanzee fecal samples (n = 724) were collected at 25 field sites throughout equatorial Africa and tested for SFVcpz-specific antibodies (n = 706) or viral nucleic acids (n = 392). SFVcpz infection was documented at all field sites, with prevalence rates ranging from 44% to 100%. In two habituated communities, adult chimpanzees had significantly higher SFVcpz infection rates than infants and juveniles, indicating predominantly horizontal rather than vertical transmission routes. Some chimpanzees were co-infected with simian immunodeficiency virus (SIVcpz); however, there was no evidence that SFVcpz and SIVcpz were epidemiologically linked. SFVcpz nucleic acids were recovered from 177 fecal samples, all of which contained SFVcpz RNA and not DNA. Phylogenetic analysis of partial gag (616 bp), pol-RT (717 bp), and pol-IN (425 bp) sequences identified a diverse group of viruses, which could be subdivided into four distinct SFVcpz lineages according to their chimpanzee subspecies of origin. Within these lineages, there was evidence of frequent superinfection and viral recombination. One chimpanzee was infected by a foamy virus from a Cercopithecus monkey species, indicating cross-species transmission of SFVs in the wild. These data indicate that SFVcpz (i) is widely distributed among all chimpanzee subspecies; (ii) is shed in fecal samples as viral RNA; (iii) is transmitted predominantly by horizontal routes; (iv) is prone to superinfection and recombination; (v) has co-evolved with its natural host; and (vi) represents a sensitive marker of population structure that may be useful for chimpanzee taxonomy and conservation strategies.

A PCR-based protocol for generating West Nile virus replicons

A new protocol for the generation of West Nile virus (WNV) replicons was developed. Fragmented cDNAs that covered the entire WNV RNA sequence, except the sequence corresponding to nucleotides 190-2379, were amplified separately by polymerase chain reactions (PCRs) using primer set franking with overlapping sequences of 40-50 bp at the 5'- and the 3'-ends of each fragment. All amplified fragments were mixed together and annealed to each other at the overlapping sequences. The annealed-DNA fragments were elongated by DNA polymerase and amplified by short-cycle PCRs to generate full-sized WNV replicon cDNAs. The WNV replicons were transcribed in vitro using the replicon cDNAs as templates. When the in vitro-transcribed replicon was introduced into mammalian cells, the viral envelope protein and viral positive- and negative-strand RNAs were detected in the replicon-transfected cells. It is noteworthy that the synthesis of the replicon cDNAs and the replicons took just 1 week, and that the use of a high-fidelity DNA polymerase afforded stability to the sequence of the synthetic replicon.

Cell-cycle dependence of foamy virus vectors

Retroviruses differ in the extent to which they are dependent on host-cell proliferation for their replication, an aspect of their replication that impacts on their vector potential. Foamy viruses offer distinct advantages over other retroviruses for development as vectors for gene therapy. A vector derived from the prototypic foamy virus (PFV), formerly known as human foamy virus (HFV), transduced aphidicolin-arrested cells five- to tenfold more efficiently than one derived from murine leukemia virus (MLV), but several-fold less efficiently than a human immunodeficiency virus type 1 (HIV-1) vector. The same relative efficiency was found following transduction of cells that had been arrested by gamma-irradiation or with mitomycin C. Cells that were exposed to vector during aphidicolin arrest and were subsequently allowed to cycle were transduced significantly better by PFV than by MLV. Quiescent human CD34+ progenitor cells were transduced as efficiently by PFV as by HIV vectors (40-50 %) when transduction was assayed after the cells were allowed to cycle.

Purifying Selection Masks the Mutational Flexibility of HIV-1 Reverse Transcriptase

DNA and RNA polymerases share a core architecture composed of three structurally conserved motifs: A, B, and C. Although the amino acid sequences of these motifs are highly conserved between closely related organisms, variation across broader evolutionary distances suggests that only a few residues in each motif are indispensable for polymerase function. To test this, we constructed libraries of human immunodeficiency virus type-1 (HIV-1) containing random single amino acid replacements in motif B of reverse transcriptase (RT), and we used selection in culture to assess RT function. Despite the nearly absolute constancy of motif B in vivo, virus replicating in culture tolerated a range of conservative and nonconservative substitutions at 10 of the 11 amino acid positions examined. These included residues that are invariant across all retroviral subfamilies and highly conversed in diverse retroelements. Several mutants retained wild type infectivity, and serial passage experiments revealed replacements that were neutral or even beneficial to viral fitness. In addition, a number of the selected variants exhibited altered susceptibility to the nucleoside analog inhibitors AZT and 3TC. Taken together, these data indicate that HIV-1 tolerates a range of substitutions at conserved RT residues and that selection against slightly deleterious mutations (purifying selection) in vivo masks a large repertoire of viable phenotypic variants. This mutational flexibility likely contributes to HIV-1 evolution in response to changing selection pressures in infected individuals.

Determinants of foamy virus envelope glycoprotein mediated resistance to superinfection

Little is known about the nature of foamy virus (FV) receptor molecules on target cells and their interaction with the viral glycoproteins. Similar to other viruses, cellular expression of the FV Env protein is sufficient to induce resistance to exogenous FV, a phenomenon called superinfection resistance (SIR). In this study we define determinants of the FV Env protein essential for mediating SIR. FV Env requires the extracellular domains of the SU and the TM subunits as well as membrane anchorage, efficient cell surface transport, and most probably correct subunit processing. This is in contrast to murine leukemia virus where secreted proteins comprising the receptor-binding domain in SU are sufficient to induce SIR. Furthermore, we demonstrate that cellular expression of the prototype FV envelope proteins induces SIR against pseudotypes with glycoproteins of other FV species, including of simian, feline, bovine, and equine origin. This implies that all of them use the same receptor molecules for viral entry.

Foamy virus vectors

Gene therapy is a promising novel treatment for a variety of human diseases. Successful application of gene therapy requires the availability of vehicles with the ability to efficiently deliver and express genes. Viral vectors are efficient means of transferring a gene of interest into target cells. Current available vehicles for gene transfer are either inefficient or potentially unsafe for human gene therapy applications. Foamy viruses offer a fresh alternative vector system for gene transfer with the potential to overcome the concerns of the current vectors. Foamy viruses are nonpathogenic and have a broad host range with the ability to infect various types of cells from different species. Foamy virus replication is distinct and may provide an edge for foamy virus vector usage over other retroviral vectors. These features offer the foamy vectors unique opportunities to deliver several genes into a number of different cell types in vivo safely and efficiently. The principal problems for the design of foamy virus vectors have been solved, and several foamy virus vectors that efficiently transduce a variety of cell types are available. This chapter reviews specific features of foamy virus vector systems and recent advances in the development and use of these vectors.

Palindromic sequence plays a critical role in human foamy virus dimerization

The retroviral RNA genome is dimeric, consisting of two identical strands of RNA linked near their 5' ends by a dimer linkage structure. Previously it was shown that human foamy virus (HFV) RNA transcribed in vitro contained three sites, designated SI, SII, and SIII, which contributed to the dimerization process (O. Erlwein, D. Cain, N. Fischer, A. Rethwilm, and M. O. McClure, Virology 229:251-258, 1997). To characterize these sites further, a series of mutants were designed and tested for their ability to dimerize in vitro. The primer binding site and a G tetrad in SI were dispensable for dimerization. However, a mutant that changed the 3' end of SI migrated slower on nondenaturing gels than wild-type RNA dimers. The sequence composition of the SII palindrome, consisting of 10 nucleotides, proved to be critical for in vitro dimerization, since mutations within this sequence or replacement of the sequence with a different palindrome of equal length impaired in vitro dimerization. The length of the palindrome also seems to play an important role. A moderate extension to 12 nucleotides was tolerated, whereas an extension to 16 nucleotides or more impaired dimerization. When nucleotides flanking the palindrome were mutated in a random fashion, dimerization was unaffected. Changing the SIII sequence also led to decreased dimer formation, confirming its contribution to the dimerization process. Interesting mutants were cloned into the infectious molecular clone of HFV, HSRV-2, and were transfected into BHK-21 cells. Mutations in SII that reduced dimerization in vitro also abolished virus replication. In contrast, constructs containing mutations in SI and SIII replicated to some extent in cell culture after an initial drop in viral replication. Analysis of the SIM1 mutant revealed reversion to the wild type but with the insertion of an additional two nucleotides. Analysis of cell-free virions demonstrated that both replication-competent and replication-defective mutants packaged nucleic acid. Thus, efficient dimerization is a critical step for HFV to generate infectious virus, but HFV RNA dimerization is not a prerequisite for packaging.

Historical perspective of foamy virus epidemiology and infection

Foamy viruses (FV) are complex retroviruses which are widespread in many species. Despite being discovered over 40 years ago, FV are among the least well characterized retroviruses. The replication of these viruses is different in many interesting respects from that of all other retroviruses. Infection of natural hosts by FV leads to a lifelong persistent infection, without any evidence of pathology. A large number of studies have looked at the prevalence of primate foamy viruses in the human population. Many of these studies have suggested that FV infections are prevalent in some human populations and are associated with specific diseases. More recent data, using more rigorous criteria for the presence of viruses, have not confirmed these studies. Thus, while FV are ubiquitous in all nonhuman primates, they are only acquired as rare zoonotic infections in humans. In this communication, we briefly discuss the current status of FV research and review the history of FV epidemiology, as well as the lack of pathogenicity in natural, experimental, and zoonotic infections.

Characterization of the R572T point mutant of a putative cleavage site in human foamy virus Env

A putative cleavage site of the human foamy virus (HFV) envelope glycoprotein (Env) was altered. Transient env expression revealed that the R572T mutant Env was normally expressed and modified by asparagine-linked oligosaccharide chains. However, this single-amino-acid substitution was sufficient to abolish all detectable cleavage of the gp130 precursor polyprotein. Cell surface biotinylation demonstrated that the uncleaved mutant gp130 was transported to the plasma membrane. The uncleaved mutant protein was incapable of syncytium formation. Glycoprotein-driven virion budding, a unique aspect of HFV assembly, occurred despite the absence of Env cleavage. We then substituted the R572T mutant env into a replication-competent HFV molecular clone. Transfection of the mutant viral DNA into BHK-21 cells followed by viral titration with the FAB (foamy virus-activated beta-galactosidase expression) assay revealed that proteolysis of the HFV Env was essential for viral infectivity. Wild-type HFV Env partially complemented the defective virus phenotype. Taken together, these experimental results established the location of the HFV Env proteolytic site; the effects of cleavage on Env transport, processing, and function; and the importance of Env proteolysis for virus maturation and infectivity.

Properties of human foamy virus relevant to its development as a vector for gene therapy

The Spumaviridae (foamy viruses) are increasingly being considered as potential vectors for gene therapy, yet little has been documented of their basic cell biology. This study demonstrates that human foamy virus (HFV) has a broad tropism and that the receptor for HFV is expressed not only on many mammalian, but on avian and reptilian cells. Receptor interference assays using an envelope-expressing cell line and a vesicular stomatitis virus/HFV pseudotype virus demonstrate that the cellular receptor is common to all primate members of the genus. The majority of foamy virus particles assemble and remain sequestered intracellularly. A rapid and quantitative method of assaying foamy virus infectivity by reverse transcriptase activity facilitates the use of classical protocols to increase infectious virus titres in vitro to > or = 10(6) TCID/ml.

Specific binding of recombinant foamy virus envelope protein to host cells correlates with susceptibility to infection

The interaction of simian foamy viruses (FVs) with their putative cellular receptor(s) was studied with two types of recombinant envelope protein (Env). Transient expression of full-length Env in BHK-21 cells induced syncytia formation. However, selected stable transfectants fused with naive cells but not with each other. A soluble fusion protein of the Env surface domain with the Fc fragment of a human IgG1 heavy chain (EnvSU-Ig) was produced in the baculovirus expression system, purified to homogeneity, and used for binding and competition analyses. EnvSU-Ig but not unrelated Ig fusion proteins bound to cells specifically. Neutralizing serum blocked binding of EnvSU-Ig and, vice versa, serum-mediated neutralization was abrogated by the chimeric protein. Concomitant reduction of EnvSU-Ig binding and FV susceptibility was seen in Env-expressing target cells. Although EnvSU-Ig did not inhibit FV infection, very likely due to its displacement by multivalent virus-cell interactions, this divalent ligand should help to characterize functionally and to identify the ubiquitous FV receptor.

Sequences in pol are required for transfer of human foamy virus-based vectors

A series of vectors with heterologous genes was constructed from HSRV1, an infectious clone of human foamy virus (HFV), and transfected into baby hamster kidney cells to generate stably transfected vector cell lines. Two cis-acting sequences were required to achieve efficient rescue by helper virus. The first element was located at the 5' end upstream of position 1274 of the proviral DNA. Interestingly, a mutation in the leader sequence which decreased the ability to dimerize in vitro inhibited transfer by helper HFV. A second element that was important for vector transfer was located in the pol gene between positions 5638 and 6317. Constructs lacking this element were only poorly transferred by helper HFV, even though their RNA was produced in the vector cell lines. This finding rules out the possibility that the observed lack of transfer was due to RNA instability. A minimal vector containing only these two elements could be successfully delivered by helper HFV, confirming that all essential cis-acting sequences were present. The presence of a sequence described as a second polypurine tract in HFV was not necessary for transfer. Our data identified the minimal sequence requirements for HFV vector transfer for the development of useful vector systems.

Prevalence of human foamy virus-related sequences in the Korean population

The possible association of human foamy virus (HFV) with human thyroid disorders such as Graves' disease (GD) has been a topic of controversy due to the inconsistent results reported by several groups of investigators. Here we report the investigation of the presence of HFV-related sequences in the Korean population. DNA was obtained from peripheral blood lymphocytes from 24 GD patients and 23 healthy blood donors and subjected to PCR amplification using three sets of nested primers derived from gag, env, and LTR regions of the HFV genome. Contrary to previously reported studies, our analysis identified HFV-related sequences in the genomes of both healthy individuals and the GD patients. However, the nature of the HFV genome present in each group appeared to be different. We detected all 3 regions of HFV-related sequences in 29% of the HFV-positive GD patients, while no samples in the control group amplified all three regions. This suggests that the LTR may be used as a tool for screening for HFV in GD patients. Our data favor the hypothesis of a relationship between GD and the presence of HFV-related sequences, though in a complex way.

Characterization of provirus clones of simian foamy virus type 1

We have cloned proviral DNA of simian foamy virus type 1 (SFV-1) from linear unintegrated DNA (pSFV-1). Transfection of pSFV-1 induces cytopathology in several cell lines with supernatants from the transfected cell culture containing infectious viral particles. Electron microscopy of the transfected cells revealed foamy virus particles. Deletion analysis of pSFV-1 indicated that the transcriptional transactivator (tas) gene located between env and the long terminal repeat is critical for virus replication, whereas the second open reading frame (ORF-2) in this region is dispensable. Although the tas and ORF-2 regions of foamy viruses have significantly diverged, the results presented here suggested that the gene products have similar functions. Recombinant pSFV-1 containing the cat gene was able to transduce the heterologous gene, indicating the utility of SFV-1 as a vector. An infectious clone of SFV-1 which is distantly related to the human foamy virus will provide a means to understand the biology of this unique group of viruses.

Gene transfer using replication-defective human foamy virus vectors

Replication-defective vectors based on an infectious molecular clone of human foamy virus (HFV) were constructed by deletion and replacement of the accessory genes with expression cassettes for puromycin-resistance and beta-glucouronidase. Cell lines which produced in excess of 10(5) helper virus-free transducing units/ml were generated by trans-complementation of the replication defect using a BHK-21-derived cell line expressing the Bel-1 transactivator. Vectors based on the HFV genome may provide useful alternatives to existing retroviral vectors.

Efficient pseudotyping of murine leukemia virus particles with chimeric human foamy virus envelope proteins

Incorporation of human foamy virus (HFV) envelope proteins into murine leukemia virus (MuLV) particles was studied in a transient transfection packaging cell system. We report here that wild-type HFV envelope protein can pseudotype MuLV particles, albeit at low efficiency. Complete or partial removal of the HFV cytoplasmic tail resulted in an abolishment or reduction of HFV-mediated infectivity, implicating a role of the HFV envelope cytoplasmic tail in the pseudotyping of MuLV particles. Mutation of the endoplasmic reticulum retention signal present in the HFV envelope cytoplasmic tail did not result in a higher relative infectivity of pseudotyped retroviral vectors. However, a chimeric envelope protein, containing an unprocessed MuLV envelope cytoplasmic domain fused to a truncated HFV envelope protein, showed an enhanced HFV specific infectivity as a result of an increased incorporation of chimeric envelope proteins into MuLV particles.

Long terminal repeat U3 length polymorphism of human foamy virus

Size determination of the long terminal repeat (LTR) of an early (1985) and a more recent (1993) passage of wild-type human foamy virus (HFV) revealed that the virus has undergone substantial deletions in the U3 region upon replication in tissue culture. Two LTR deletion variants (HSRV1 and 2) have been characterized in the past and used to construct molecular clones which are replication competent in cell culture. We now report the molecular cloning, sequencing, and biological characterization of an HFV genome with full-length LTR (pHFV2). Sequence analysis revealed that the deletions in HSRV1 and 2 are nonrandom and probably occurred by misalignment during reverse transcription. The comparative analysis of HFV2 and the variant with the largest U3 deletion, HSRV2, revealed a differential ability to replicate in human cell cultures. While HSRV2 replicated faster in diploid human fibroblasts, cells which have been used extensively for amplification of HFV in the past, replication of HFV2 was faster in a lymphoblastoid cell line. Reporter gene assays indicated that the cell-type specific ability of the LTRs to respond to the viral transcriptional transactivator may be a likely, reason for the different growth properties of both viruses and for the occurrence of the HFV U3 deletions. In foamy virus-infected chimpanzees only the full-length type of LTR was observed; however, the HSRV1 deletion variant was detected as the dominating virus in an accidentally HFV-infected human.

Identification of sites that act together to direct dimerization of human foamy virus RNA in vitro

Retroviral particles contain two molecules of genomic RNA, which are noncovalently linked near their 5' ends in a region called the dimer linkage structure (DLS). By using complementary DNA oligonucleotides and deletion mutants to impair RNA dimerization of the human foamy virus (HFV), three sites, designated SI, SII, and SIII, were found within a 159-nucleotide RNA fragment of HFV that are involved in dimerization in vitro. SI overlaps the primer-binding site; and SII contains the palindromic sequence, UCCCUAGGGA, the disruption of which impairs dimer formation; and SIII extends into the gag gene. The first two sites are highly conserved in the other primate foamy viruses, SFV-1, SFV-3, and SFVcpz, whereas the third appears to be shared only by HFV and SFVcpz. RNA of HFV and SFV-3 could form heterodimers, indicating that both viruses dimerize by similar mechanisms. On testing thermal stability, dimers of the 159-nucleotide fragment dissociated between 40 and 70 degrees, with half of the dimers dissociating at 55 degrees. Since the splice donor site of HFV is located at position 51 of viral RNA, the DLS is part of the genomic RNA exclusively.

In vitro infection of primary and retrovirus-infected human leukocytes by human foamy virus

The infectivity of human foamy virus (HFV) was examined in primary and cultured human leukocytes. Cell-free infectious viral stocks of HFV were prepared from the human kidney cell line 293 transfected with an infectious molecular clone of HFV. HFV productively infects a variety of human myeloid and lymphoid cell lines. In addition, primary cell cultures enriched for human CD4+, monocytes and brain-derived microglial cells, were readily infected by HFV. Interestingly, while infected primary CD4+ lymphocytes and microglial cells showed marked cytopathology characteristic of foamy virus, HFV-infected monocyte-derived macrophages failed to show any cytopathology. In addition, marked cytotoxicity due to HFV infection was seen in both human T-cell leukemia virus type 1- and human immunodeficiency virus type 1-infected T-cell lines and in human immunodeficiency virus type 1-infected monocytoid cell lines. Thus, HFV infection produces differential cytopathology in a wide host range of primary human leukocytes and hematopoietic cell lines.

Transgenic and knockout mice in the study of neurodegenerative diseases

Accurate animal models are essential for detailed analysis of the mechanisms underlying human neurodegenerative diseases. In addition, they can offer useful paradigms for the development and evaluation of new therapeutic strategies. We review the most popular techniques for modification of the mammalian genome in vivo, and provide a critical evaluation of the available transgenic mouse models for several neurological conditions of humans, including prion diseases, human retroviral diseases, Alzheimer's disease, and motor neuron diseases.

Foamy virus vectors

Human foamy virus (HFV) is a retrovirus of the spumavirus family. We have constructed vectors based on HFV that encode neomycin phosphotransferase and alkaline phosphatase. These vectors are able to transduce a wide variety of vertebrate cells by integration of the vector genome. Unlike vectors based on murine leukemia virus, HFV vectors are not inactivated by human serum, and they transduce stationary-phase cultures more efficiently than murine leukemia virus vectors. These properties, as well as their large packaging capacity, make HFV vectors promising gene transfer vehicles.

Functional analysis of human spuma retrovirus genome

Human spuma retrovirus (HSRV) belongs to retroviruses that possess a complex genome organization. HSRV carries at least three extra genes in the region between env and the 3' long terminal repeat, which are not found in simple retroviruses. Via alternative splicing, these HSRV genes can encode several proteins. To genetically study the requirements of these viral proteins for viral replication in tissue cultures, a number of mutant viruses were constructed from an infectious molecular clone of HSRV. All mutants grew normally in the cell lines tested, except for those lacking transcriptional transactivator activity. By reporter-based transient assay systems, no Rev/Rex equivalent was detected in the HSRV proteins.

Phylogenesis and genetic complexity of the nonhuman primate retroviridae

The three known groups of nonhuman primate retroviruses (simian immunodeficiency virus, simian T cell lymphotropic/leukemic virus type I, and simian foamy virus) are thought to have equivalent human counterparts. This is clearly the case with human immunodeficiency virus types 1 and 2, the causative agents of acquired immunodeficiency syndrome, and with human T cell lymphotropic/leukemia virus type I (HTLV-I), which causes T cell leukemia and a progressive form of myelopathy (tropical spastic paraparesis/HTLV-I-associated myelopathy), and HTLV-II. However, the presence of spumaviruses (foamy viruses) in humans remains uncertain. Data accumulated in the last 5 years suggest the possibility that the human retroviruses are indeed the result of transmission of simian retroviruses to humans. In this article we attempt to parallel the genetic features of the simian retroviridae with their human counterparts and argue for the possibility of horizontal transmission of these viruses from monkeys to humans.

Structure and function of the long terminal repeat of the chimpanzee foamy virus isolates (SFV-6)

The complete long terminal repeat (LTR) nucleotide sequence of the chimpanzee foamy virus isolate SFV-6 was determined. Its 1761-bp size makes it the longest LTR reported to date among all retroviruses. Since the length of its LTR is similar to that of other simian isolates while its sequence homology is closer to that of HFV, SFV-6 genetic structure appears to be intermediate between simian and human foamy viruses. Transient expression assays demonstrate that SFV-6 encodes a transactivator of viral gene expression directed either by its own LTR or by heterologous promoters like HFV and HIV-1 LTRs. Our data also provide evidence for cross-transactivation between SFV-6 and HFV.

Nuclear localization of foamy virus Gag precursor protein

All foamy viruses give rise to a strong nuclear staining when infected cells are reacted with sera from infected hosts. This nuclear fluorescence distinguishes foamy viruses from all other retroviruses. The experiments reported here indicate that the foamy virus Gag precursor protein is transiently located in the nuclei of infected cells and this is the likely reason for the typical foamy virus nuclear fluorescence. By using the vaccinia virus expression system, a conserved basic sequence motif in the nucleocapsid domain of foamy virus Gag proteins was identified to be responsible for the nuclear transport of the gag precursor molecule. This motif was also found to be able to direct a heterologous protein, the Gag protein of human immunodeficiency virus, into the nucleus.

Expression of the human foamy virus bel-1 transactivator in insect cells

The human foamy virus (HFV) bel-1 transactivator protein was expressed in insect cells by a recombinant baculovirus. For the generation of the recombinant baculovirus, Acbel-1, the bel-1 gene of an HFV mutant was used, that bears truncations in the bel-1 overlapping bel-2 open reading frame. Acbel-1 infected Sf9 cells produced high amounts of recombinant protein of the same electrophoretic mobility (36 kD) as bel-1 expressed in mammalian cells. The baculovirus expressed bel-1 protein was readily identified by a polyclonal rabbit serum directed against bel-1 in immunoblot assay. As in mammalian cells, bel-1 was predominantly localized to the nucleus of Acbel-1 infected insect cells. The baculovirus expressed bel-1 protein will be of use to determine the action of this novel viral transactivator more precisely.

Functional analysis of human foamy virus accessory reading frames

Foamy viruses belong to the retroviruses which possess a complex genome structure. The human foamy virus (HFV) isolate bears three open reading frames (the so-called bel genes) in the 3' region of the genome which have been reported to give rise to possibly six different proteins via alternative splicing (W. Muranyi and R. M. Flügel, J. Virol. 65:727-735, 1991). In order to analyze the requirements of these proteins for HFV replication in vitro, we constructed a set of single and combinatory bel gene mutants of an infectious molecular clone of HFV. The mutant which lacked the transacting activator, bel-1, was found to be replication incompetent. All other mutants replicated equally well and gave rise to comparable titers of infectious cell-free virus. When HFV proviruses were put under the control of a heterologous promoter (simian virus 40), none of the accessory gene products was found to be required for expression of structural (gag) proteins. There was no evidence for a posttranscriptional regulatory protein that is present in other complex retroviruses.

Human foamy virus polypeptides: identification of env and bel gene products

Human foamy virus (HFV) proteins were identified in human cells cultured in vitro by immunoprecipitation and immunoblotting with specific antisera. Among several viral polypeptides, four glycoproteins of approximately 160, 130, 70, and 48 kDa were identified in HFV-infected cells. gp130 was shown to represent the intracellular env precursor, and gp70 and gp48 were shown to represent the external and transmembrane env proteins, respectively. The nature of gp160, which shares sequences with the env, bel1, and bel2 proteins, is not yet resolved. In addition, a p62 identified with bel1- and bel2-specific antisera likely corresponds to the bet gene product.

The foamy virus family: molecular biology, epidemiology and neuropathology

The family of foamy viruses designates a group of retroviruses which share a specific morphology and provoke characteristic cytopathic effects in cultured cells. Like HTLV and HIV, foamy viruses are complex viruses encoding a number of ancillary genes in addition to gag, pol and env, including a transcriptional transactivator. Foamy viruses are endemic in various primate species, and human foamy viruses (HFV) have been isolated from patients with various neoplastic and degenerative diseases. Despite a growing body of knowledge on the biology of foamy viruses, it has not yet been possible to identify a disease specifically caused by foamy virus infection. After reviewing the epidemiology and molecular biology of the various animal foamy viruses, this article focuses on the pathogenic properties of HFV in transgenic mouse systems. HFV transgenes exhibit a striking neurotropism and elicit a progressive degenerative disease of the central nervous system and striated muscle. Similarly to patients with HIV-associated encephalopathy, HFV transgenic mice develop accumulations of syncytial giant cells in their brains. The relevance of these findings for human neuropathology is discussed.

Human foamy virus proteins accumulate in neurons and induce multinucleated giant cells in the brain of transgenic mice

Human foamy virus (HFV) is a retrovirus encoding structural genes and, like human immunodeficiency virus and human T cell leukemia virus I, several ancillary reading frames collectively termed the be1 genes. We have previously shown that HFV transgenic mice develop an encephalopathy with neuronal loss in hippocampus and cerebral cortex. We have now raised and characterized rabbit antisera to various recombinant portions of gag, pol, env, and bel-1, the viral trans-activator. Immunoreactivity for gag and bel-1 was observed in nuclei and processes of hippocampal and cortical neurons before the onset of morphological lesions and correlated with the appearance of HFV mRNA. Astrocyte-derived multinucleated giant cells containing HFV proteins were present in the brain of transgenic mice coexpressing full-length HFV genes but not in mice expressing truncated gag and env, suggesting that these genes contain a fusogenic domain. Expression of full-length structural genes decreased the life expectancy of transgenic mice, implying an adjuvant role for these proteins in HFV-induced brain damage.

Functional dissection of the human spumaretrovirus transactivator identifies distinct classes of dominant-negative mutants

The bel1 gene of human spumaretrovirus (HSRV) codes for a 300-amino-acid nuclear protein, termed Bel1, that can strongly activate transcription from the cognate long terminal repeat (LTR) by at least 200-fold. Bel1 can also activate human immunodeficiency virus type 1 (HIV-1) LTR expression. By using site-directed mutagenesis, we have identified distinct regions of Bel1 essential for HSRV LTR activation. The amino-terminal 55 residues, which comprise a highly acidic region followed by a short basic stretch, were dispensable for activation. The distribution of functionally defective mutants indicates that two distinct regions between residues 56 and 300 cooperate to confer full activator function. The larger, more amino-terminal region between residues 56 and 227 is sufficient to minimally activate the HSRV LTR. It contains a region between residues 88 and 110 that is strongly conserved between the simian and human spumavirus transactivators but otherwise lacks obvious homology to known transcriptional activators except for an Arg-rich nuclear localization sequence (NLS) between residues 211 and 225 that can be functionally substituted for by the NLS of the simian virus 40 large T antigen. The carboxy-terminal 73 residues contain two functionally redundant regions that can independently augment the activity of the more N-terminal minimal activator domain by 30- to 90-fold. Comparative analysis of the effect of Bel1 mutations on HSRV and HIV-1 LTR expression revealed a similar requirement of Bel1 domains for activation of the two LTRs. Bel1 is phosphorylated in vivo, and a nuclear localization-defective mutant lacking residues 211 to 222 was severely defective for phosphorylation, whereas various deletion mutations in residues 228 to 300 resulted in a four- to eightfold reduction in phosphate incorporation. When functionally defective bel1 mutants were examined for a dominant-negative phenotype, only mutants lacking a proline-rich basic region between residues 194 and 200 or the NLS between residues 211 and 222 that were found to occupy predominantly nuclear and cytoplasmic locations, respectively, could suppress wild-type Bel1 function efficiently. In identifying two classes of dominant-negative mutants with distinct subcellular localization phenotypes, the mutational analysis of Bel1 has revealed a feature unusual for known transcriptional activators.

Identification of conserved amino acid residues critical for human immunodeficiency virus type 1 integrase function in vitro

We have probed the structural organization of the human immunodeficiency virus type 1 integrase protein by limited proteolysis and the functional organization by site-directed mutagenesis of selected amino acid residues. A central region of the protein was relatively resistant to proteolysis. Proteins with altered amino acids in this region, or in the N-terminal part of the protein that includes a putative zinc-binding motif, were purified and assayed for 3' processing, DNA strand transfer, and disintegration activities in vitro. In general, these mutations had parallel effects on 3' processing and DNA strand transfer, suggesting that integrase may utilize a single active site for both reactions. The only proteins that were completely inactive in all three assays contained mutations at conserved amino acids in the central region, suggesting that this part of the protein may be involved in catalysis. In contrast, none of the mutations in the N-terminal region resulted in a protein that was inactive in all three assays, suggesting that this part of integrase may not be essential for catalysis. The disintegration reaction was particularly insensitive to these amino acid substitutions, indicating that some function that is important for 3' processing and DNA strand transfer may be dispensable for disintegration.

Mechanism of action of regulatory proteins encoded by complex retroviruses

Complex retroviruses are distinguished by their ability to control the expression of their gene products through the action of virally encoded regulatory proteins. These viral gene products modulate both the quantity and the quality of viral gene expression through regulation at both the transcriptional and posttranscriptional levels. The most intensely studied retroviral regulatory proteins, termed Tat and Rev, are encoded by the prototypic complex retrovirus human immunodeficiency virus type 1. However, considerable information also exists on regulatory proteins encoded by human T-cell leukemia virus type I, as well as several other human and animal complex retroviruses. In general, these data demonstrate that retrovirally encoded transcriptional trans-activators can exert a similar effect by several very different mechanisms. In contrast, posttranscriptional regulation of retroviral gene expression appears to occur via a single pathway that is probably dependent on the recruitment of a highly conserved cellular cofactor. These two shared regulatory pathways are proposed to be critical to the ability of complex retroviruses to establish chronic infections in the face of an ongoing host immune response.

Transactivation of human immunodeficiency virus type 1 long terminal repeat-directed gene expression by the human foamy virus bel1 protein requires a specific DNA sequence

Human foamy virus (HFV) encodes the transcriptional transactivator bel1. The bel1 protein transactivates HFV long terminal repeat (LTR)-directed gene expression by recognizing a region in U3. It also transactivates human immunodeficiency virus type 1 (HIV-1) LTR-directed gene expression in transient transfection assays. To identify the specific region in HIV-1 LTR responsible for bel1 action, we examined the effect of bel1 on chloramphenicol acetyltransferase (CAT) gene expression in transfected cells with a series of mutant HIV-1 LTR/CAT plasmids. The region between -158 and -118 from the transcription initiation site, immediately upstream of the core enhancer element, was identified as responsible for the transactivation by bel1. In addition, bel1 transactivated a heterologous promoter when this region was positioned upstream of it in the sense and antisense orientations. Optimal transactivation of the HIV-1 LTR by bel1 did not require an intact TAR sequence, suggesting that the binding of tat to the TAR sequence is not a prerequisite for bel1 function in HIV-1 LTR-directed gene expression. In the region of the HIV-1 LTR that is necessary for the bel1-mediated transactivation, we have found a sequence which is conserved between HIV-1 and HFV. Our results suggest that the bel1 action on HIV-1 seems to be mediated by a specific DNA sequence which is shared by both the HIV-1 LTR and HFV LTR.

Construction of an infectious DNA clone of the full-length human spumaretrovirus genome and mutagenesis of the bel 1 gene

An infectious and full-length molecular clone of genomic human spumaretrovirus (HSRV) DNA was constructed. The infectivity of the pHSRV13 clone was demonstrated after transfection into susceptible cells by passage of HSRV-specific cytopathic effects as a cell-free culture supernatant, by electron microscopy of HSRV particles in pHSRV13 DNA-transfected cells, by detection of HSRV transcripts, and by identification of HSRV-encoded proteins with Env- and Bel-specific antisera in indirect immunofluorescence assays and in protein blotting. The predominant HSRV protein detected in immunoblots by both Bel 1- and Bel 2-specific antisera had an apparent molecular weight of 56 kDa and corresponds to Bet. The amino-terminus of Bet is encoded by part of a Bel 1-specific RNA and the larger Bet domain by an RNA species from the bel 2 gene (Muranyi, W., and Flügel, R. M. J. Virology 65, 727-735, 1991). HSRV-specific proteins of 36 and 43 kDa reacted with Bel 1 and Bel 2 antisera, consistent with the values calculated for the bel 1 and bel 2 gene products, respectively. Deletion mutagenesis of the transcriptional HSRV-specific trans-activator bel 1 and the bet genes completely abolished the infectivity of the pHSRV13 clone. The defect in RNA, protein, and virion synthesis was trans-complemented by cotransfection of an expression clone harboring the complete bel coding region. This result demonstrates that the bel 1 gene is required for viral replication. It remains to be determined whether other HSRV gene products, like bet that share a common region with bel 1, contributed to the defect observed.

Progressive encephalopathy and myopathy in transgenic mice expressing human foamy virus genes

Transgenic mice carrying the bel region of human foamy retrovirus (HFV) under transcriptional control of its own long terminal repeat expressed the transgene in their central nervous systems and in smooth and striated muscle tissues. The animals developed a progressive degenerative disease of the central nervous system and of the striated muscle. Because expression of the transgene was closely correlated with the appearance of structural damage and inflammatory reactions were scanty, the disease is likely to be caused directly by the HFV proteins. These unexpected findings call for a reevaluation of the pathogenic potential of HFV in humans.

Distinct cis-acting regions in U3 regulate trans-activation of the human spumaretrovirus long terminal repeat by the viral bel1 gene product

The human spumaretrovirus (HSRV) genome contains, in addition to coding information for the structural proteins, open reading frames (ORFs) for at least three additional genes termed bel1, bel2 and bel3. We report here the localization of the transcriptional activator of HSRV to the bel1 ORF. In reporter-based transient expression assays in COS cells utilizing the bacterial CAT gene linked to HSRV LTR sequences between -710 and +309 with respect to the transcriptional initiation site, co-expression of the bel1 gene product alone caused an over 100 to 300-fold increase in the level of LTR activity. High-level trans-activation by bel1 was specific for the HSRV LTR, as relatively minor positive and negative regulatory effects were observed on HIV-1 LTR and RSV LTR expression, respectively, whereas HTLV-1 LTR activity remained unaffected. Distinct regions of the HSRV LTR were found to be involved in bel1-induced trans-activation. Specifically, deletions between -500 and -389 and between -136 and -62 in the U3 region resulted in a 4- and 30 to 35-fold decline, respectively, in the response to bel1. Limited mutagenesis of the bel1 ORF indicated that most of the bel1 coding region, except for the carboxy-terminal 27 residues, is essential for the activation function.

Characterization of the transcriptional trans activator of human foamy retrovirus

The human foamy viruses, or spumaviruses, a distinct subfamily of complex human retroviruses, remain poorly understood both in terms of their pathogenic potential and in terms of the regulatory mechanisms that govern their replication. Here, we demonstrate that the human spumaretrovirus shares with other complex human retroviruses the property of encoding a transcriptional trans activator of the homologous viral long terminal repeat. This regulatory protein is encoded by the viral Bel-1 open reading frame and is localized to the nucleus of expressing cells. The Bel-1 trans activator is shown to function effectively in cell lines derived from human, simian, murine, and avian sources. The viral target sequence for Bel-1 has been mapped 5' to the start of viral transcription and is therefore likely to be recognized as a DNA sequence. Our results further suggest that the mechanism of action of the Bel-1 protein may be distinct from those reported for the transcriptional trans activators encoded by members of the other human retroviral subfamilies.

The transcriptional transactivator of human foamy virus maps to the bel 1 genomic region

The human foamy virus (HFV) genome possesses three open reading frames (bel 1, 2, and 3) located between env and the 3' long terminal repeat. By analogy to other human retroviruses this region was selected as the most likely candidate to encode the viral transactivator. Results presented here confirmed this and showed further that a deletion introduced only into the bel 1 open reading frame of a plasmid derived from an infectious molecular clone of HFV abolished transactivation. In contrast, deletions in bel 2 and bel 3 had only minor effects on the ability to transactivate. The role of the bel 1 genomic region as a transactivator was further investigated by eukaryotic expression of a genome fragment of HFV spanning the bel 1 open reading frame. A construct expressing bel 1 under control of a heterologous promoter was found to transactivate the HFV long terminal repeat in a dose-dependent fashion. Furthermore, it is shown that the U3 region of the HFV long terminal repeat is sufficient to respond to the HFV transactivator.