Absence of serological evidence for foamy virus infection in patients with amyotrophic lateral sclerosis

Retroviral Elements in Pathophysiology and as Therapeutic Targets for Amyotrophic Lateral Sclerosis

The study of the role of retroviruses in amyotrophic lateral sclerosis (ALS) dates back to the 1960s shortly after transposable elements themselves were first discovered. It was quickly realized that in wild mice both horizontal and vertical transmissions of retroviral elements were key to the development of an ALS-like syndrome leading to the postulate that endogenous retroviruses (ERVs) contribute significantly to the pathogenicity of this disease. Subsequent studies identified retroviral reverse transcriptase activity in brains of individuals with ALS from Guam. However, except for a single study from the former Soviet Union, ALS could not be transmitted to rhesus macaques. The discovery of an ALS-like syndrome in human immunodeficiency virus (HIV) and human T cell leukemia virus infected individuals led to renewed interest in the field and reverse transcriptase activity was found in the blood and cerebrospinal fluid of individuals with sporadic ALS. However, exogenous retroviruses could not be found in individuals with ALS which further reinforced the possibility of involvement of a human ERV (HERV). The first demonstration of the involvement of a HERV was the discovery of the activation of human endogenous retrovirus-K subtype HML-2 in the brains of individuals with ALS. The envelope protein of HML-2 is neurotoxic and transgenic animals expressing the envelope protein develop an ALS-like syndrome. Activation of HML-2 occurs in the context of generalized transposable element activation and is not specific for ALS. Individuals with HIV-associated ALS show a remarkable response to antiretroviral therapy; however, antiretroviral trials in ALS down-regulate HML-2 without ameliorating the disease. This highlights the need for specific drugs to be developed against HML-2 as a novel therapeutic target for ALS. Other approaches might include antisense oligonucleotides, shRNA targeted against the envelope gene or antibodies that can target the extracellular envelope protein. Future clinical trials in ALS should consider combination therapies to control these ERVs.

A chemogenomic approach is required for effective treatment of amyotrophic lateral sclerosis

ALS is a fatal untreatable disease involving degeneration of motor neurons. Μultiple causative genes encoding proteins with versatile functions have been identified indicating that diverse biological pathways lead to ALS. Chemical entities still represent a promising choice to delay ALS progression, attenuate symptoms and/or increase life expectancy, but also gene-based and stem cell-based therapies are in the process of development, and some are tested in clinical trials. Various compounds proved effective in transgenic models overexpressing distinct ALS causative genes unfortunately though, they showed no efficacy in clinical trials. Notably, while animal models provide a uniform genetic background for preclinical testing, ALS patients are not stratified, and the distinct genetic forms of ALS are treated as one group, which could explain the observed discrepancies between treating genetically homogeneous mice and quite heterogeneous patient cohorts. We suggest that chemical entity-genotype correlation should be exploited to guide patient stratification for pharmacotherapy, that is administered drugs should be selected based on the ALS genetic background.

Identification and functional characterization of Bet protein as a negative regulator of BFV3026 replication

Foamy virus (FV) establishes persistent infection in the host without causing apparent disease. Besides the transactivator Tas protein, another auxiliary protein--Bet--has been reported in prototype foamy virus, equine foamy virus, and feline foamy virus. Here, we found the putative bbet gene in clone C74 from a cDNA library of bovine foamy virus strain 3026 (BFV3026) by comparison of gene localization, composition, and splicing features with other known bet genes. Subsequently, BBet protein was detected in BFV3026-infected cells by Western blot and immunofluorescence analyses. Analysis of the BBet mutant infectious clone (pBS-BFVdelBBet) revealed that BBet could inhibit BFV3026 replication. Consistent with this result, overexpression of BBet in Cf2Th cells reduced BFV replication by approximately threefold. Furthermore, virus replication levels similarly were reduced by approximately threefold in pBS-BFV-transfected and BFV3026-infected Cf2Th cells stably expressing BBet compared with control cells. After three passages, BFV3026 replicated more slowly in BBet-expressing cells. This study implicates BBet as a negative regulator of BFV replication and provides a resource for future studies on the function of this protein in the virus lifecycle.

Retroviruses and amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive, invariably fatal neurologic disorder resulting from upper and lower motor neuron degeneration, which typically develops during the sixth or seventh decade of life, and is diagnosed based on standard clinical criteria. Its underlying cause remains undetermined. The disease may occur with increased frequency within certain families, often in association with specific genomic mutations, while some sporadic cases have been linked to environmental toxins or trauma. Another possibility, first proposed in the 1970s, is that retroviruses play a role in pathogenesis. In this paper, we review the published literature for evidence that ALS is associated either with infection by an exogenous retrovirus or with the expression of human endogenous retroviral (HERV) sequences in cells of the central nervous system. A small percentage of persons infected with the human immunodeficiency virus-1 (HIV-1) or human T cell leukemia virus-1 (HTLV-1) develop ALS-like syndromes. While HTLV-1 associated ALS-like syndrome has several features that may distinguish it from classical ALS, HIV-infected patients may develop neurological manifestations that resemble classical ALS although it occurs at a younger age and they may show a dramatic improvement following the initiation of antiretroviral therapy. However, most patients with probable or definite ALS show no evidence of HIV-1 or HTLV-1 infection. In contrast, recent reports have shown a stronger association with HERV, as analysis of serum samples, and postmortem brain tissue from a number of patients with a classical ALS has revealed significantly increased expression of HERV-K, compared to controls. These findings suggest that endogenous retroviral elements are involved in the pathophysiology of ALS, but there is no evidence that they are the primary cause of the syndrome.

Motor neuron-immune interactions: the vicious circle of ALS

Because microglial cells, the resident macrophages of the CNS, react to any lesion of the nervous system, they have for long been regarded as potential players in the pathogenesis of several neurodegenerative disorders including amyotrophic lateral sclerosis, the most common motor neuron disease in the adult. In recent years, this microglial reaction to motor neuron injury, in particular, and the innate immune response, in general, has been implicated in the progression of the disease, in mouse models of ALS. The mechanisms by which microglial cells influence motor neuron death in ALS are still largely unknown. Microglial activation increases over the course of the disease and is associated with an alteration in the production of toxic factors and also neurotrophic factors. Adding to the microglial/macrophage response to motor neuron degeneration, the adaptive immune system can likewise influence the disease process. Exploring these motor neuron-immune interactions could lead to a better understanding in the physiopathology of ALS to find new pathways to slow down motor neuron degeneration.

Sporadic amyotrophic lateral sclerosis: a hypothesis of persistent (non-lytic) enteroviral infection

Because of recently reported reverse transcriptase polymerase chain reaction evidence of enterovirus in sporadic amyotrophic lateral sclerosis (SALS) and because of newly available anti-enteroviral drugs binding enteroviral capsids, it is reasonable to re-formulate an enteroviral hypothesis of SALS using recent advances in molecular virology. Viral persistence is non-lytic and non-cytopathic infection that evades host's immune surveillance. Enteroviruses are known to cause persistent as well as lytic infection both in vitro and in vivo. Both virion as well as host factors modulate between persistent and lytic infection. Apoptosis, or programmed cell death, is a process of active non-necrotic cell death. It has complex interplay with viruses and may be either promoted or opposed by them. Apoptosis is a major factor in motor neuron death in SALS. Viral tropism is the process by which viruses select and propagate to target cells. It is controlled by capsid conformation and surface receptors on host cells. Enteroviruses have a region on their capsids known as the canyon which docks on such receptors. Docking induces conformational changes of the capsid and genome release. Poliovirus, tropic for motor neurons, docks on the poliovirus receptor, about which much is known. The virus penetrates the motor system focally after crossing either the blood-muscle or the blood-brain barriers. It propagates bidirectionally along axons and synapses to contiguous motor neurons, upper as well as lower, which sequester infection and create avenues for spread over long distances. If chronic and persistent rather than acute and lytic, such viruses trafficking in a finite system of non-dividing cells and inducing apoptosis would cause cell death that summates linearly rather than exponentially. Taken together, these explain signature clinical features of SALS - focal onset weakness, contiguous or regional spread of weakness, confinement to upper and lower motor neurons, and linear rates of progression. The hypothesis predicts the following testable investigations: 1) viral detection may be possible by applying amplification technology to optimally acquired nervous tissue processed by laser microdissection; 2) genetic susceptibility factors such as cell surface receptor polymorphisms may combine with sporadic exposure and chance penetration of the motor system in SALS; 3) a transgenic animal model might be created by inserting such genetic factors into an animal host and inoculating intramuscularly rather than intracerebrally biochemical fractions of SALS motor neurons at vulnerable periods in the developmental life cycle of the transgenic host; and 4) continual long-term administration of anti-enteroviral agents called capsid-binding compounds which stabilize capsids and prevent genome release might be efficacious.

Replication of primate foamy viruses in natural and experimental hosts

Foamy viruses (FVs) are common apathogenic retroviruses readily spread by horizontal transmission in nonhuman primate and some other mammalian host populations. Primate FV infections have been known for half a century, i.e., 15 years before the definition of retroviruses and another 15 years before the detection of primate immune deficiency viruses. The emerging interest in human retroviruses included primate FV, and although the role of human hosts for FV was greatly overestimated temporarily, enthusiastic researchers compiled invaluable data on molecular biology and classic as well as molecular epidemiology of these viruses. It has been shown that lytic FV infection in a wide range of cell cultures is in great contrast to the silent state of the infection in animals. Once transmitted by saliva via biting, FVs reside in all tissues as DNA copies, but their replication is untraceable except in oral submucosal cells, which are thought to supply the virus for transmission. FVs have not definitely been associated with any disease, regardless of viral phylogenetic differences. Various primate and nonprimate species have been used for studies on the natural carrier state and primary infection. Experimental infections have mostly proven to be inefficient in primates as well as lower laboratory animals. However, investigation of the immune response in FV-infected animals has only partly explained the control of FV replication in the animal host. Thus, the biological role of FV remains an enigma to be resolved in the future.

Human infection with foamy viruses

Virtually all nonhuman primate species investigated thus far including prosimians, New World and Old World monkeys and apes all harbor distinct and species-specific clades of simian foamy virus (SFV). However, evidence supporting the existence of a human-specific foamy virus (FV) is not yet available. Early reports describing widespread infection of healthy and sick humans with FV could not be confirmed. In contrast, all FV infections documented in humans are of zoonotic origin and are identified in persons occupationally exposed to nonhuman primates. The introduction of SFV into humans raises several public health questions regarding disease outcomes and potential for human-to-human transmissibility. The available data from a very limited number of SFV-infected humans suggest that these infections are nonpathogenic and are not easily transmissible. Additional studies are needed to better define the prevalence and natural history of SFV in humans.

Inflammatory processes in amyotrophic lateral sclerosis

Neuroinflammation is a characteristic of pathologically affected tissue in several neurodegenerative disorders. These changes can be observed in the brainstem and spinal cord of amyotrophic lateral sclerosis (ALS) cases and in mouse models of the disease. They include an accumulation of large numbers of activated microglia and astrocytes, as well as small numbers of T cells, mostly adhering to postcapillary venules. Accompanying biochemical alterations include the appearance of numerous molecules characteristic of free-radical attack, the occurrence of proteins associated with activation of the complement cascade, and a sharp upregulation of the enzyme cyclooxygenase 2 (COX-2). Anti-inflammatory agents may have a role to play in treating ALS. COX-2 is a particularly attractive target because of its marked increase in ALS spinal cord.

Simian foamy virus infection in a blood donor

BACKGROUND

Infections with simian foamy virus (SFV) are widely prevalent in nonhuman primates. SFV infection was confirmed in a worker, occupationally exposed to nonhuman primates, who donated blood after the retrospectively documented date of infection. Human-to-human transmission of SFV through transfusion and its pathogenicity have not been studied.

STUDY DESIGN AND METHODS

Recipients of blood from this donor were identified and blood samples from such recipients were tested for SFV infection by Western blot and PCR assay.

RESULTS

One recipient of RBCs and another recipient of FFP had died; retroviral infections were not implicated. One platelet recipient could not be tested. Recipients of RBCs (two), a WBC-reduced RBC unit (one), and a platelet unit (one) tested SFV-negative 19 months to 7 years after transfusion. Tested recipients had transfusions 3 to 35 days after blood donation. Samples of one lot of albumin and three lots of plasma protein fraction (manufactured from recovered plasma from two donations) tested negative both for antibodies and for viral RNA.

CONCLUSION

SFV transmission through transfusion was not identified among four recipients of cellular blood components from one SFV-infected donor. Derivatives containing plasma from that donor tested negative for SFV.

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.

Detection of reverse transcriptase activity in the serum of patients with motor neurone disease

The recognition that both human and murine retroviruses can cause motor neurone disease-like syndromes has raised the possibility that a retrovirus may be involved in the aetiology of motor neurone disease. This possibility was explored by looking for evidence of reverse transcriptase in the serum of motor neurone disease patients. Sera from 56 patients with motor neurone disease and 58 controls were tested by the product-enhanced reverse transcriptase assay, a technique that is approximately a million fold more sensitive than conventional reverse transcriptase assays and capable of detecting very low numbers of retroviral particles. Cell-free reverse transcriptase activity was detected in the serum of 33 of the 56 motor neurone disease patients (59%) but in only 3 of the controls (P < 0.00001). The reverse transcriptase activity was detectable in the presence of a large excess of an effective inhibitor of human cellular DNA polymerases and was therefore tentatively considered to be compatible with a retroviral origin. The reverse transcriptase activity, however, was not found to be due to the presence of known human exogenous retroviruses including HIV-1, HIV-2, HTLV-I, HTLV-II, HRV-5 or human foamy virus, as assessed by PCR-based assays. Further investigations will be required to determine the source of the reverse transcriptase activity observed in these motor neurone disease patient sera.

Complex effects of deletions in the 5' untranslated region of primate foamy virus on viral gene expression and RNA packaging

Due to various advantageous features there is current interest in retroviral vectors derived from primate foamy viruses (PFVs). Two PFV cis-acting sequences have been mapped in the 5' region of the RNA (pre-)genome and in the 3' pol genomic region. In order to genetically separate PFV packaging constructs from vector constructs, we investigated the effect of deletions in the 5' untranslated region (UTR) of PFV packaging constructs and vectors on gene expression and RNA incorporation into viral particles. Our results indicate that the 5' UTR serves different previously unknown functions. First, the R region of the long terminal repeat was found to be required for PFV gag gene expression. This regulation of gene expression appeared to be mainly posttranscriptional. Second, constructs with sequence deletions between the R region and the gag gene start codon packaged as much viral mRNA into particles as the undeleted construct, and RNA from such a 5'-UTR-deleted packaging construct was copackaged into vector-virus particles, together with vector RNA which was preferentially packaged. Finally, in the U5 region a sequence was identified that was required to allow cleavage of the Gag precursor protein by the pol gene-encoded protease, suggesting a role of RNA in PFV particle formation. Taken together, the results indicate that complex interactions of the viral RNA, capsid, and polymerase proteins take place during PFV particle formation and that a clear separation of PFV vector and packaging construct sequences may be difficult to achieve.

Sites of simian foamy virus persistence in naturally infected African green monkeys: latent provirus is ubiquitous, whereas viral replication is restricted to the oral mucosa

Foamy viruses (FV), retroviruses of the genus Spumavirus, are able to infect a wide variety of animal species and replicate in nearly all types of cultured cells. To identify the cells targeted by FV in the natural host and define the sites of viral replication, multiple organs of four African green monkeys naturally infected with simian FV type 3 were investigated for the presence of FV proviral DNA and viral transcripts. All organs contained significant amounts of FV proviral DNA. In addition to proviruses containing the complete transactivator gene taf, proviral genomes carrying a specific 295-bp deletion in the taf gene were detected in all monkeys. As in the case of human foamy virus the deletion leads to the formation of the bet gene that is regarded to be instrumental in the regulation of viral persistence. FV RNA was detected by RT-PCR and in situ hybridization only in the oral mucosa of one monkey. No other samples contained detectable levels of viral transcripts. Histopathological changes were not observed in any of the tissue samples analyzed. Our results show that the natural history of FV is characterized by latent infection in all organs of the host and by minimal levels of harmless viral replication in the oral mucosa. The broad host cell range in vivo further encourages the development of FV-derived vectors for therapeutic gene delivery.

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.

Cells expressing the human foamy virus (HFV) accessory Bet protein are resistant to productive HFV superinfection

Bet is a foamy virus (FV) accessory protein not required for virus replication. The function of Bet is not understood. We report on the generation of cell lines stably expressing the HFV Bet protein. In Bet+ cells, HFV replication was reduced by approximately 3-4 orders of magnitude compared with control cells. The HFV Bet-expressing cells only partially resisted infection by the distantly related feline FV (FFV). Pseudotyping experiments, using murine retroviral vectors with an HFV envelope, revealed that the resistance was not due to downregulation of the unknown HFV receptor. In transfection experiments, using proviral reporter gene constructs and infectious proviruses, no significant differences were detected between Bet+ and control cells. In infection experiments, HFV vectors expressing an indicator gene under control of the HFV promoters showed no activity in Bet+ cells. The results are best compatible with the hypothesis that the main block to productive superinfection of Bet+ cells occurs at an early stage of replication between virus entry and provirus establishment. We suggest that inhibition of provirus integration by Bet protein may serve a distinct function in the unique foamy virus replication cycle.

Degeneration of the cerebellar granule cell layer in transgenic mice expressing genes of human foamy virus

Transgenic mice expressing various combinations of structural and regulatory genes of human foamy virus (HFV) develop a neurodegenerative syndrome. delta gpe transgenic mice (which express the auxiliary bel-1 and bet genes along with truncated forms of gag, pol, and env) develop a severe neurological syndrome consisting mainly of spastic tetraparesis and blindness, and show neuronal loss in the hippocampus and cerebral cortex. In addition, mice in two of eight delta gpe lines developed an ataxic gait. Here we studied the phenotype of these two lines, and show that these mice exhibit progressive degeneration of their cerebellar granule cells beginning at 4-8 weeks of age. Transgenic mRNA and HFV proteins accumulate in cerebellar granule cells immediately before the onset of degeneration. The Purkinje cell layer is largely unaffected by this pathological process. Probably due to the loss of granule cell processes, the cerebellar molecular layer is narrowed in the late stages of the disease. These findings indicate that HFV gene products can be neurotoxic for cerebellar granule cells.

Simian foamy virus isolated from an accidentally infected human individual

Evidence for natural foamy virus (FV) infections in humans is still lacking. However, accidental infections of humans with simian FV have been demonstrated by serology and PCR, but all previous attempts to recover infectious virus in such cases have failed. Here we describe the isolation of a simian FV from peripheral blood mononuclear cells (PBMC) of a healthy animal caretaker, who acquired the virus 20 years ago from an African green monkey (AGM) bite. Properties of the human isolate such as host range in cell cultures including human PBMC and ability to induce neutralizing antibodies in the primate host proved to be similar to those of FV obtained from AGM. The genomic sequence of the isolate was found to be virtually identical to the proviral sequence present in the host lymphocytes and related to AGM isolates but distinct from those of all FV isolates handled in the laboratory. For successful virus isolation, it was essential to stimulate the host lymphocytes by phytohemagglutinin and interleukin-2 for 2 weeks prior to cocultivation with permissive cells. In contrast to the situation found in FV-infected monkeys, virus isolation from the saliva of the animal caretaker was not possible, and no evidence for FV transmission to family contacts was obtained. We conclude that, in contrast to active infection in monkeys, FV persists in a state of latency following accidental infection of humans.

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.