Substitution of the U3 long terminal repeat region of the neurotropic Cas-Br-E retrovirus affects its disease-inducing potential

Elevated levels of transcripts encoding a human retroviral envelope protein (syncytin) in muscles from patients with motor neuron disease

Retroviral components of both exogenous and endogenous origins have been associated with nervous system diseases in both animals and humans. In the present study, the levels of transcripts from elements in the human endogenous retrovirus (HERV) W family were determined in muscle biopsies from patients with motor neuron disease (MND) and control subjects. Transcripts from the HERV-W element on chromosome 7q21.2 encoding syncytin and from the SOD1 gene were detected at elevated levels in biopsies from the most affected muscles from MND patients compared to biopsies from control individuals. According to a recent study, syncytin is expressed in microglia in normal brain and can be up-regulated in macrophages/microglia during inflammation. Although syncytin may have cytotoxic effects, it is therefore more likely that the present findings reflect a macrophage response in the muscles undergoing neurogenic atrophy than a primary pathogenetic event in MND.

Retroviral insertion mutagenesis in mice as a comparative oncogenomics tool to identify disease genes in human leukemia

Retroviral insertion mutagenesis has recently received much attention because of its adverse effects in the application of retroviral vector-based gene therapy, resulting in leukemia in certain patients. At the same time, retroviral mutagenesis in mice is being considered a powerful forward genetic strategy to identify disease genes involved in cancer. The publication of the mouse genome sequence and the development of high-throughput genomic approaches have given a further boost to this rapidly evolving field. The increasing numbers of new potential oncogenes identified in retroviral screens have given a valuable basis for a better understanding of cancer related pathways in mice. Important challenges that now lie ahead of us are (i) to determine the relevance and causal relationship of these genes with various types of human cancer (ii) to develop strategies to identify tumor suppressor genes on a large scale, (iii) to place the disease genes into regulatory networks to better understand their role in the complex pathogenesis of cancer, and (iv) to determine their value for diagnosis refinement and therapeutic target intervention in human disease. In this review, we will give a brief update of the current state-of-the-art and thoughts concerning these issues. We will specifically focus on the value of employing retroviral insertion mutagenesis in mice and gene expression profiling in man in the context of acute myeloid leukemia.

Sequence analysis of a neuropathogenic variant of Moloney murine leukemia virus ts1: evidence for recombination

The nucleotide sequence of the complete genome of ts1, a neuropathogenic variant of Moloney murine leukemia virus, has been determined. Although it is possible that the genome of ts1 accumulated point mutations during reverse transcription, our results now indicate that some of these mutations may have resulted from recombination.

At least four non-env factors that reside in the LTR, in the 5'-non-coding region, in gag and in part of pol affect neuropathogenicity of PVC-441 murine leukemia virus (MuLV)

PVC-441 murine leukemia virus (MuLV) is neuropathogenic in F344 rats. Recently, an infectious DNA clone was isolated and its nucleotide sequence was determined (J. Virol. 72: 3423-3426. 1998). To identify the viral determinants of neuropathogenicity of the molecularly cloned PVC-441 MuLV, chimeras were constructed between PVC-441 MuLV and F-MuLV clones at appropriate restriction enzyme sites that divide the viral genome approximately in LTR-non-coding, gag-, pol-, and env-gene regions. Results indicated that the LTR-non-coding and the gag-gene regions of PVC-441 MuLV affected independently the neuropathogenicity in combination with the env gene region as evidenced clinically and pathologically. Studies on the distribution of vacuolar degeneration suggested that the pons and cervical spinal cord areas were the primary targets and the large brain was the latest target of PVC-441 MuLV. Further studies with chimeric viruses that were formed in the LTR-non-coding and the gag gene regions revealed that at least four factors affected the neuropathogenicity of PVC-441 MuLV. Two factors were found in the U3, and R-U5-5'-non-coding regions, and at least two factors in the gag gene region that contained the N-terminal part of the pol gene. Among these factors, at least two factors seemed to be 'cis-acting' from each other

Nuclear factors that bind to the U3 region of two murine myeloid leukemia-inducing retroviruses, Cas-Br-E and Graffi

Cas-Br-E and Graffi are two myeloid leukemia-inducing murine viruses. Cas-Br-E induces, in NIH-Swiss mice, mostly non-T, non-B leukemia composed of very immature cells with no specific characteristics (Bergeron et al. (1993). Leukemia 7, 954-962). The Graffi murine leukemia virus causes exclusively myeloid leukemia, but the tumor cells are clearly of granulocytic nature (Ru et al. (1993). J. Virol. 67, 4722). We were interested to understand the role of the long terminal repeat (LTR) U3 region in the myeloid specificity of these two retroviruses. We used DNase I footprinting and gel mobility shift assays to identify a number of protein binding sites within Cas-Br-E and Graffi U3 regions. The pattern of protected regions is highly similar for the two viruses. Some factors identified in other murine leukemia viruses, like the core binding factor, also bind to Cas-Br-E and Graffi LTR; however, other binding sites seem specific for these two viruses. Only one difference between them was noted, at the 5' end of the U3 region. Transcriptional activity of both LTRs was also analyzed in various cell lines and compared with other murine leukemia viruses. The results show a slight myeloid specificity for the two LTRs, and indicate that the Graffi enhancer is quite strong in a broad range of cell types.

Studies of the susceptibility of nude, CD4 knockout, and SCID mutant mice to the disease induced by the murine AIDS defective virus

Murine AIDS (MAIDS) is induced by a defective retrovirus that infects lymphocyte cells of the B lineage. To determine whether functional T cells are required for the infection of B cells, T-cell-deficient mice (nude, CD4 knockout, and SCII)) were infected with helper-free stocks of the MAIDS defective virus. Infection of B cells was monitored by Northern blot analysis and in situ hybridization. The C57BL/6 nude mice contained clusters of infected B cells, but less so than did the euthymic mice. In contrast, the (C57BL/6 x BALB/c)F1 nude mice harbored more infected B cells than did their euthymic littermates when maintained in a pathogen-free environment. Clusters of infected B cells were also detected in the MAIDS virus-infected CD4-/- knockout mice despite the total absence of CD4+ T cells in these mice. However, infected cells were not detected in SCID mice (deficient in mature T and B cells) inoculated with the same virus, indicating that precursor B cells are not a target of the virus in the absence of mature CD4+ T cells. These data confirm that the primary event in the development of MAIDS is the infection of relatively mature peripheral B cells and that CD4+ T cells are required to promote the expansion of these infected B cells.

HTLV-I transgenic models: an overview

Human T cell leukemia virus type I (HTLV-I) is the agent of a wide spectrum of human diseases. The mechanisms by which a single virus can cause neurodegenerative disorders as well as leukemia is still a matter of debate. Transgenic mice have been used to assess the contribution of different viral elements in viral tropism as well as on cell transformation in vivo. In particular, transgenic models were generated to study the tissue specificity of expression directed by the viral long terminal repeat and the pathological effects induced by the Tax protein of HTLV-I. These models have led to a description of the cell types able to support the viral expression in vivo, and the use of Tax-transgenic mice has demonstrated that this protein is oncogenic and able to induce muscular atrophy and arthropathies. Finally, these models could provide a useful system to study therapeutic approaches for HTLV-I-associated diseases.

Expression of human immunodeficiency virus type 1 in the nervous system of transgenic mice leads to neurological disease

Patients infected with the human immunodeficiency virus type 1 (HIV-1) frequently develop central and peripheral nervous system complications, some of which may reflect the effect of the virus itself. In order to elucidate the pathogenic mechanisms of HIV in neurological disease in a small animal model, we generated transgenic mice expressing the entire HIV genome under control of the promoter for the human neurofilament NF-L gene. The transgene was predominantly expressed in anterior thalamic and spinal motor neurons. Animals developed a neurological syndrome characterized by hypoactivity and weakness and by axonal degeneration in peripheral nerves. These results provide evidence for a role of HIV in affecting both the central and peripheral nervous systems. This animal model may also facilitate the development of therapeutic agents against the human disease.

Myristylation of Pr60gag of the murine AIDS-defective virus is required to induce disease and notably for the expansion of its target cells

Murine AIDS (MAIDS) is characterized by severe lymphadenopathy and splenomegaly. The proliferation of the infected target B cells is also an important manifestation of the disease (M. Huang, C. Simard, D. G. Kay, and P. Jolicoeur, J. Virol. 65:6562-6571, 1991). The etiologic agent of MAIDS is a defective murine leukemia virus that is deleted of most of its pol and env genes and appears to encode a single protein, the Gag precursor Pr60gag protein. Pr60gag is myristylated and attached to the plasma membrane. To study the role myristylation on the function of Pr60gag, we have generated a myristylation-negative (Myr-) mutant of the MAIDS defective virus. We found that Myr- Pr60gag interacted less tightly with the plasma membrane. In addition, the Myr- MAIDS defective virus mutant was unable to induce expansion of infected cells and was nonpathogenic. These results emphasize the essential role of Pr60gag in the disease process. Our data also suggest that Pr60gag, once recruited to the cell membrane through its myristylation, interacts with other membrane-bound effectors to send signals to induce proliferation of the infected cells and to initiate immune dysfunctions.

Identification of a sequence in the unique 5' open reading frame of the gene encoding glycosylated Gag which influences the incubation period of neurodegenerative disease induced by a murine retrovirus

Neonatal inoculation of the wild-mouse ecotropic retrovirus CasBrE (clone 15-1) causes a noninflammatory spongiform neurodegenerative disease with an incubation period of > or = 6 months. Introduction of sequences from Friend murine leukemia virus (clone FB29) into the genome of CasBrE results in a marked shortening of the incubation period. The FB29 sequences which influence the incubation period were previously localized to the 5' leader sequence of the viral genome (M. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 66:3298-3305, 1992). In the current study, we constructed a series of chimeric viruses consisting of the genome of CasBrE containing various segments of the leader sequence from FB29. A 41-nucleotide element (positions 481 through 521) near the 3' end of the leader was found to have a strong influence on the incubation period. This element influenced the kinetics of virus replication and/or spread in nonneuronal tissues, a property which was shown previously to determine the extent of central nervous system infection (M. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 66:3298-3305, 1992). Curiously, this sequence had no demonstrable effect on virus replication in vitro in a fibroblastic cell line from Mus dunni. This segment encodes 14 of the unique 88-amino-acid N terminus of pr75gag, the precursor of a glycosylated form of the gag polyprotein which is expressed at the cell surface. Previous in vitro studies of mutants of Moloney murine leukemia virus lacking expression of glycosylated Gag failed to reveal a function for this protein in virus replication. We mutated the Kozak consensus sequence around the initiation codon for this protein in the chimeric virus CasFrKP, a virus which induces neurologic disease with a short (18- to 23-day) incubation period. M. dunni cells infected with the mutants lacked detectable cell surface Gag, but, compared with CasFrKP, no effect on replication kinetics in vitro was observed. In contrast, there was a marked slowing of the replication kinetics in vivo and a dramatic attenuation of neurovirulence. These studies indicate that glycosylated Gag has an important function in virus replication and/or spread in the mouse and further suggest that the sequence of its N terminus is a critical, though likely indirect, determinant of neurovirulence.

Murine AIDS is initiated in the lymph nodes draining the site of inoculation, and the infected B cells influence T cells located at distance, in noninfected organs

The infection of cells which belong to the B-cell lineage is thought to be the primary event leading to the phenotypic and functional alterations seen in the murine AIDS (M. Huang, C. Simard, D. Kay, and P. Jolicoeur, J. Virol. 65:6562-6571, 1991). Using in situ hybridization, we studied the time course of the anatomic distribution of the murine AIDS-infected B cells in C57BL/6 mice inoculated intraperitoneally or in the foot pad with helper-free stocks of the defective murine AIDS virus. The local lymph nodes draining the injection site (the mediastinal or popliteal lymph nodes) were the primary organs in which infected B cells could be detected. From this initial site, the proliferating infected B cells were found to migrate progressively to most of the other lymph nodes and to the spleen. The bone marrow cells (containing the precursor B cells) were not found to be infected by the virus. These results suggest that the defective murine AIDS virus infects mature Ly-1- B cells present in lymph nodes. We compared the concanavalin A response of the T cells at an early time postinoculation, before all lymphoid organs are infiltrated with infected B cells. In lymphoid organs free of infected B cells, T cells were found to be hyperresponsive. In lymphoid organs in which infected B cells were present, T cells were hyporesponsive. These data suggest that infected B cells influence distant T cells, maybe by the release of a circulating factor or through another uninfected cell population activated by the infected B cells.

Identification of the infected target cell type in spongiform myeloencephalopathy induced by the neurotropic Cas-Br-E murine leukemia virus

The Cas-Br-E murine leukemia virus (MuLV) induces a progressive hindlimb paralysis accompanied by a spongiform myeloencephalopathy in susceptible mice. In order to better understand the pathological process leading to these neurodegenerative lesions, we have investigated the nature of the cell type(s) infected by the virus during the course of the disease in CFW/D and SWR/J mice. For this purpose, we used in situ hybridization with virus-specific probes in combination with cell-type-specific histochemical (lectin) and immunological markers as well as morphological assessment. In the early stage of infection, endothelial cells represented the main cell type expressing viral RNA in the central nervous system (CNS). With disease progression and the appearance of lesions, microglial cells became the major cell type infected, accounting for up to 65% of the total infected cell population in diseased areas. Morphologically, these cells appeared activated and were frequently found in clusters. Infection and activation of microglial cells were almost exclusively restricted to diseased regions of the CNS. Neurons in diseased regions were not discernibly infected with virus at either early or late times of disease progression. Similarly, the proportion of infected astrocytes was typically < 1%. Although some endothelial cells and oligodendrocytes were infected by the virus, their infection was not limited to diseased CNS regions. These results are consistent with a model of indirect motor neuron degeneration, subsequent to the infection of nonneuronal CNS cells and especially of microglial cells. Infected microglial cells may play a role in the disease process by releasing not only virions or viral env-gene-encoded gp70 proteins but also other factors which may be directly or indirectly toxic to neurons. Parallels between microglial cell infection by MuLV and by lentiviruses, and specifically by human immunodeficiency virus, are discussed.

Intracerebral hemorrhages and syncytium formation induced by endothelial cell infection with a murine leukemia virus

The mechanisms of endothelial cell damage that lead to cerebral hemorrhage are not completely understood. In this study, a cloned murine retrovirus, TR1.3, that uniformly induced stroke in neonatal BALB/c mice is described. Restriction digest mapping suggests that TR1.3 is part of the Friend murine leukemia virus (FMuLV) family. However, unlike mice exposed to other FMuLVs, mice infected with TR1.3 virus developed tremors and seizures within 8 to 18 days postinoculation. This was uniformly followed by paralysis and death within 1 to 2 days. Postmortem examination of TR1.3-inoculated mice revealed edematous brain tissue with large areas of intracerebral hemorrhage. Histologic analysis revealed prominent small vessel pathology including syncytium formation of endothelial cells. Immunohistochemical analysis of frozen brain sections using double fluorescence staining demonstrated that TR1.3 virus specifically infected small vessel endothelial cells. Although infection of vessel endothelial cells was detected in several organs, only brain endothelial cells displayed viral infection associated with hemorrhage. The primary determinant of TR1.3-induced neuropathogenicity was found to reside within a 3.0-kb fragment containing the 3' end of the pol gene, the env gene, and the U3 region of the long terminal repeat. The restricted tropism and acute pathogenicity of this cloned murine retrovirus provide a model for studying virus-induced stroke and for elucidating the mechanisms involved in syncytium formation by retroviruses in vivo.

Viruses and behavioural changes: a review of clinical and experimental findings

This review focuses on behavioural neurovirology. Profound changes in behaviour are observed following infection of the central nervous system by some viruses. Irritability, insomnia, hyperactivity and learning disability are some of the behavioural disturbances that have been described in both humans and animals with central nervous system infection. The reticular core neurons which innervate the entire brain play an important role in regulating behaviour. Some of these neurons--locus coeruleus, raphe and diagonal bands--send projections to the olfactory bulbs and can be targets for exogenous agents attacking the olfactory epithelium. In infant rats, vesicular stomatitis virus is transported along the olfactory pathway by retrograde transport and reaches the reticular core neurons causing destruction of raphe, diagonal bands and, to a lesser extent, the locus coeruleus. As the neurons degenerate, the viral antigens disappear and the animals sustain severe deficits in neurotransmitter levels and behaviour. Such a "hit and run" effect of the virus suggests the possibility that a similar mechanism may be operating in some human disorders. Apart from their intrinsic interest as possible aetiological factors, viruses may provide valuable tools in experimental work seeking to correlate behaviour, morphology and neurotransmitter function.

Neurological disease induced in transgenic mice expressing the env gene of the Cas-Br-E murine retrovirus

The Cas-Br-E murine leukemia virus induces a spongiform myeloencephalopathy in susceptible mice. We constructed transgenic mice harboring either the viral genome (in a replication-defective form) or only its env gene. Low levels of expression of either transgene resulted in mild neuropathology and/or signs of neurological disease in more than half of these mice. These results indicate that the disease can occur in the absence of virus replication and strongly suggest that the env gp70/p15E complex is sufficient to induce disease.

Comparative analysis of HTLV-I promoter activities reveals no disease-linked pattern of expression

To investigate the possibility of an association between the type of pathology caused by HTLV-I and the activity of its promoter, we compared the levels of transcription obtained with six LTRs isolated from patients with two different HTLV-I-related diseases: ATL and TSP/HAM. The patients came from different geographical endemic areas. The LTR region was amplified by polymerase chain reaction (PCR) from the DNA of uncultured peripheral blood lymphocytes, and directly cloned upstream of the luciferase reporter gene. Constructs were tested by a transient transfection assay in a variety of cell lines. Although the activities of these LTRs were statistically different in some of the cell lines tested, no correlation could be demonstrated between the promoter activity and the nature of the disease. Thus, the data suggest that the LTR is not a major determinant of the nature of the disease associated with the infection by HTLV-I.

The pathogenesis of murine retroviral infection of the central nervous system

Several decades have lapsed since the original description of retroviral infection of the central nervous system (CNS) appeared. With the recent arrival of the autoimmune deficiency syndrome (AIDS) epidemic and the associated human retroviral encephalitis, interest in murine models has been rekindled. In most of the published studies, susceptible mouse strains are infected as neonates with molecularly cloned type-C retroviruses. In most models, a spongiform encephalopathy follows an early CNS endothelial cell infection. The subsequent pathogenesis of this encephalopathy is unknown. In some models neuronal and glial infection is seen, while in others only non-neuroglial elements are infected. This variation can be traced to differences in strains of mice and viruses in addition to differences in assays. The different models offer fertile experimental ground to decipher the role of direct versus indirect neuroglial damage. Reconciliation of these various models where the final neuropathology appears so similar, may be the key to understanding their pathogenesis.

Functional analysis of two long terminal repeats from the HTLV-I retrovirus

Human T-cell leukemia virus type I (HTLV-I) is associated with a large spectrum of clinical manifestations in man. Viral and host factors are probably involved in determining the consequences of infection. Although most of the genome of HTLV-I appears remarkably stable, considerable variation is observed in the long terminal repeat (LTR) which harbors the promoter region. So far, no correlation between specific mutations and pathogenesis has been found, and the current opinion is that sequence variations reflect the geographical origin of the isolate more than the associated pathology. To assess whether the mutations observed between two HTLV-I LTRs were functionally significant, two LTRs, which differ by ten mutations, were coupled to the highly sensitive eukaryotic luciferase-encoding reporter gene, luc, and tested by transfection in a variety of cell lines. Marked differences in promoter activity were observed in some of the cells tested, whereas in other both LTRs were equally active. This result demonstrates that the minor differences observed between two HTLV-I LTRs can affect the activity level of the promoter in some cellular environments, a result which could point to the LTR as one determinant of HTLV-I cell tropism in vivo.

Murine retrovirus-induced spongiform encephalomyelopathy: host and viral factors which determine the length of the incubation period

A molecular clone of wild mouse ecotropic retrovirus CasBrE (clone 15-1) causes a spongiform neurodegenerative disease with a long incubation period, greater than or equal to 6 months. This virus infects the central nervous system (CNS) at low levels. In contrast, a chimeric virus, FrCasE, containing env and 3' pol sequences of 15-1 in a Friend murine leukemia virus background, infects the CNS at high levels and causes a rapid neurodegenerative disease with an incubation period of only 16 days. With both viruses, the induction of neurologic disease is dependent on inoculation during the perinatal period. Since the length of the incubation period of this disease appears to be a function of the relative level of CNS infection, we have attempted to identify the viral and host factors which determine the relative level of virus infection of the CNS. It was previously shown that the CNS is susceptible to infection only during the perinatal period (M. Czub, S. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 65:2539-2544, 1991). Here we have found that the susceptibility of the CNS wanes progressively or gradually as a function of the age of the host, this age-dependent resistance being complete by 12 to 14 days of age. Utilizing a group of chimeric viruses, we found that the relative level of CNS infection achieved after inoculation of mice at 1 day of age was a function of the kinetics of virus replication and spread in peripheral organs. Viruses which reached peak viremia titers early (5 to 7 days of age) infected the CNS at high levels, and viruses which reached peak titers later infected the CNS at lower levels. Among the group of viruses examined in the current study, the kinetics of peripheral virus replication and spread appeared to be influenced primarily by sequences within the R-U5-5' leader region of the viral genome. These results suggested that the relative level of CNS infection was determined very early in life and appeared to be a function of a dynamic balance between the kinetics of virus replication in the periphery and a progressively developing restriction of virus replication in the CNS.

Tissue expression pattern directed in transgenic mice by the LTR of an HTLV-I provirus isolated from a case of tropical spastic paraparesis

Human T lymphotropic virus type I (HTLV-I) causes adult T cell leukemia/lymphoma and a chronic neurological disease called either tropical spastic paraparesis (TSP) or HTLV-I-associated myelopathy. The different outcomes of this infection could be due to both host and viral factors and it has been proposed that genetic differences could make some HTLV-I strains neurotropic. In this paper, we examined the pattern of tissue-specific expression determined by a long terminal repeat (LTR) obtained from a case of TSP. We constructed transgenic mice in which this LTR controlled the expression of the nlslacZ reporter gene. We observed that in three independent lines of transgenic mice, the reporter gene was expressed predominantly in the central nervous system (CNS), in choroid plexus, and in cells of the hippocampus and cerebellum. Our observations indicate the existence of CNS cells permissive for the expression of HTLV-I and which may be of importance in the pathogenesis of TSP.

Potential role of viruses in neurodegeneration

Viruses have the capacity to induce alterations and degenerations of neurons by different direct and indirect mechanisms. In the review, we have focused on some examples that may provide new avenues for treatment or altering the course of infections, i.e., antibodies to fusogenic virus membrane proteins, drugs that interfere with lipid metabolism, calcium channel blockers, immunoregulatory molecules, and, and inhibitors of excitotoxic amino acids. Owing to their selectivity in attack on regions of nervous tissue, governed by viral factors and by routes of invasion, viral receptors or metabolic machineries of infected cells, certain viral infections show similarities in distribution of their resulting lesions in the nervous system to that of the common human neurodegenerative diseases (namely, motor neurons disease, Parkinson's disease, and Alzheimer's disease). However, it should be emphasized that no infectious agent has as yet provided a complete animal model for any of these diseases, nor has any infectious agent been linked to them from observations on clinical or postmortem materials.

The majority of cells infected with the defective murine AIDS virus belong to the B-cell lineage

Murine AIDS (MAIDS) is caused by a defective retrovirus which encodes a gag fusion protein (Pr60gag). We previously reported that this virus induced an oligoclonal proliferation of infected cells and suggested that this cell expansion was an important event in the pathogenesis of MAIDS. To identify these target cells, we constructed novel defective viruses whose genomes could be detected with specific probes. Helper-free stocks of these viruses induced MAIDS. Using in situ hybridization and immunocytochemistry and Southern analysis, we found that most infected cells belong to the B-cell lineage. Transformation of these B cells appears to be the primary event responsible for the development of immunodeficiency. This animal model may be relevant to our understanding of AIDS, of the immunodeficiencies associated with B-cell lymphoproliferative disorders, and of the role of B-cell proliferation and transformation in the effects of superantigens, since Pr60gag appears to be a superantigen.

Alteration from T- to B-cell tropism reduces thymic atrophy and cytocidal effects in thymocytes but not neurovirulence induced by ts1, a mutant of Moloney murine leukemia virus TB

The ts1 mutant of Moloney murine leukemia virus TB causes degenerative neurologic and immunologic disease in mice, characterized by development of spongiform encephalomyelopathy resulting in hindlimb paralysis, marked thymic atrophy associated with immunodeficiency, and generalized body wasting. To investigate the pathogenesis of the thymic atrophy caused by ts1, we constructed a chimeric virus, ts1-Cas(NS), in which a major portion of the U3 region of the long terminal repeat of ts1, a T-lymphotropic and neurovirulent murine leukemia virus, was replaced by the corresponding U3 region of Cas-Br-E, a B-lymphotropic and neurovirulent murine leukemia virus. In FVB/N mice, ts1-Cas(NS) induced paralytic and wasting disease with incidence, severity, and latency similar to that induced by ts1, but it failed to cause thymic atrophy as severe as that observed in ts1-infected mice. Furthermore, thymocytes cultured from ts1-Cas(NS)-infected mice died at a much slower rate than those of ts1-infected mice. The U3 substitution in ts1-Cas(NS) specifically diminished the ability of the virus to replicate in the thymus, whereas viral replication in the spinal cord was not significantly affected; thus, neurovirulence was not changed. The correlation of reduced thymic atrophy with decreased thymic viral titers and the decreased ability of ts1-Cas(NS) to cause thymocyte death in mice suggest strongly that the marked thymic atrophy in ts1-infected mice is not an indirect effect occurring secondary to neurodegenerative and wasting disease but is a direct cytopathic effect of high-level viral replication in the thymus.

Neuronal loss in a lower motor neuron disease induced by a murine retrovirus

An animal model of neuronal loss induced by a murine retrovirus is described. The neurological disease is manifested by a lower limb paralysis and is characterized as a spongiform myeloencephalopathy. The primary determinant of neurovirulence has been mapped within the viral env sequences and a secondary determinant is present within the LTR region. A model of pathogenesis, suggesting that the disease is receptor-mediated, is proposed.

The R-U5-5' leader sequence of neurovirulent wild mouse retrovirus contains an element controlling the incubation period of neurodegenerative disease

The wild mouse ecotropic retrovirus CasBrE causes a spongiform neurodegenerative disease after neonatal inoculation, with an incubation period ranging from 2 to 12 months. We previously showed that introduction of long terminal repeat (LTR) and gag-pol sequences from a strain of Friend murine leukemia virus (FB29) resulted in a dramatic acceleration of the onset of the disease. The chimeric virus FrCasE, which consisted of the FB29 genome containing 3' pol and env sequences from the wild mouse virus, induced a highly predictable, lethal neurodegenerative disease with an incubation period of only 16 days. Here we report that the sequences which are primary determinants of the length of the incubation period are located in the 5' end of the viral genome between a KpnI site in the R region of the LTR and a PstI site immediately 5' of the start codon for pr65gag (R-U5-5' leader). This region contains the tRNA primer binding site, splice donor site for the subgenomic env mRNA, and the packaging sequence. Computer-assisted sequence analysis failed to find evidence of a consensus sequence for a DNA enhancer in this region. In addition, sequences within a region of the genome between a ClaI site at the 3' end of env to the KpnI site in the R region of the LTR (inclusive of U3) also influenced the incubation period of the disease, but the effect was distinctly weaker than that of the R-U5-5' leader sequence. This U3 effect, however, appeared to be independent of the number of direct repeats, since deletion of one of two duplicated 42-base repeats containing consensus sequences of nuclear-factor binding domains had no effect on the incubation period of the disease. On the basis of Southern blot analysis of total viral DNA in the tissues, the effect of these sequences on the incubation period appeared to be related to the level of virus replication in the central nervous system. All of the chimeric viruses analyzed, irrespective of neurovirulence, replicated to comparable levels in the spleen and induced comparable levels of viremia.

Retrovirus-induced spongiform myeloencephalopathy in mice: regional distribution of infected target cells and neuronal loss occurring in the absence of viral expression in neurons

The Cas-Br-E murine leukemia virus (MuLV) induces a spongiform myeloencephalopathy resulting in a progressive hindlimb paralysis. We have used in situ hybridization with a Cas-Br-E MuLV-specific probe to study viral expression in the central nervous system. Infected cells were concentrated in regions where spongiform lesions and gliosis are detected (lumbosacral spinal cord, brainstem, deep cerebellar regions), suggesting a causative link between the level of virus expression and the degree of pathological changes in this disease. However, viral expression was not in itself sufficient to cause disease, since significant viral expression was observed in regions that did not exhibit pathological changes (cerebellar cortex, hippocampus, corpus callosum, peripheral nervous system). In both diseased and nondiseased regions, endothelial and glial cells were identified as the main target cells. Neurons in diseased regions did not show viral expression. The regional distribution of the spongiform changes appears to be laid down very early following infection, since expression could be detected at 10 days postinfection in regions that become diseased. These results indicate that nonneuronal cells have distinct properties in various regions of the central nervous system and suggest an indirect mechanism of neuronal loss consequent to viral expression in nonneuronal cells.