A 0.3-kb fragment containing the R-U5-5' leader sequence is essential for the induction of spongiform neurodegeneration by A8 murine leukemia virus

Splicing of Friend Murine Leukemia Virus env-mRNA Enhances Its Ability to Form Polysomes

Friend murine leukemia virus (MLV) belongs to the gamma retroviruses of the Retroviridae family. The positive-sense RNA of its genome contains a 5' long terminal repeat (LTR), 5' leader sequence, gag, pol, env, and 3' LTR. Transcription from proviral DNA begins from the R region of the 5' LTR and ends at the polyadenylation signal located at the R region of the other end of the 3' LTR. There is a 5' splice site in the 5' leader sequence and a 3' splice site at the 3' end of the pol region. Both full-length unspliced mRNAs and a singly spliced mRNA (env-mRNA) are produced in MLV-infected cells. The MLV Env protein plays important roles both in viral adsorption to host cells and in neuropathogenic disease in MLV-infected mice and rats. Understanding the regulatory mechanisms controlling Env expression is important for determining the functions of the Env protein. We have previously shown that splicing increases env-mRNA stability and translation efficiency. Generally, mRNA polysome formation correlates with translation efficiency. Therefore, here we investigated the effects of env-mRNA splicing on polysome formation to identify mechanisms for Env up-regulation due to splicing. We performed polysome profile analyses using Env-expression plasmids producing spliced or unspliced env-mRNA and showed that the former formed polysomes more efficiently than the latter. Thus, splicing of env-mRNA facilitated polysome formation, suggesting that this contributes to up-regulation of Env expression. We replaced the env region of the expression plasmids with a luciferase (luc) gene, and found that in this case both unspliced and spliced luc-mRNA formed polysomes to a similar extent. Thus, we conclude that whether mRNA polysome formation is affected by splicing depends on the structure of gene in question.

Polyadenylation of Friend murine leukemia virus env-mRNA is affected by its splicing

As splicing was previously found to be important for increasing Friend murine leukemia virus env-mRNA stability and translation, we investigated whether splicing of env-mRNA affected the poly(A) tail length using env expression vectors that yielded unspliced or spliced env-mRNA. Incomplete polyadenylation was detected in a fraction of the unspliced env-mRNA products in an env gene-dependent manner, showing that splicing of Friend murine leukemia virus plays an important role in the efficiency of complete polyadenylation of env-mRNA. These results suggested that the promotion of complete polyadenylation of env-mRNA by splicing might partially explain up-regulation of Env protein expression as a result of splicing.

Protease 3C of hepatitis A virus induces vacuolization of lysosomal/endosomal organelles and caspase-independent cell death

BACKGROUND

3C proteases, the main proteases of picornaviruses, play the key role in viral life cycle by processing polyproteins. In addition, 3C proteases digest certain host cell proteins to suppress antiviral defense, transcription, and translation. The activity of 3C proteases per se induces host cell death, which makes them critical factors of viral cytotoxicity. To date, cytotoxic effects have been studied for several 3C proteases, all of which induce apoptosis. This study for the first time describes the cytotoxic effect of 3C protease of human hepatitis A virus (3Cpro), the only proteolytic enzyme of the virus.

RESULTS

Individual expression of 3Cpro induced catalytic activity-dependent cell death, which was not abrogated by the pan-caspase inhibitor (z-VAD-fmk) and was not accompanied by phosphatidylserine externalization in contrast to other picornaviral 3C proteases. The cell survival was also not affected by the inhibitors of cysteine proteases (z-FA-fmk) and RIP1 kinase (necrostatin-1), critical enzymes involved in non-apoptotic cell death. A substantial fraction of dying cells demonstrated numerous non-acidic cytoplasmic vacuoles with not previously described features and originating from several types of endosomal/lysosomal organelles. The lysosomal protein Lamp1 and GTPases Rab5, Rab7, Rab9, and Rab11 were associated with the vacuolar membranes. The vacuolization was completely blocked by the vacuolar ATPase inhibitor (bafilomycin A1) and did not depend on the activity of the principal factors of endosomal transport, GTPases Rab5 and Rab7, as well as on autophagy and macropinocytosis.

CONCLUSIONS

3Cpro, apart from other picornaviral 3C proteases, induces caspase-independent cell death, accompanying by cytoplasmic vacuolization. 3Cpro-induced vacuoles have unique properties and are formed from several organelle types of the endosomal/lysosomal compartment. The data obtained demonstrate previously undocumented morphological characters of the 3Cpro-induced cell death, which can reflect unknown aspects of the human hepatitis A virus-host cell interaction.

A 38 nt region and its flanking sequences within gag of Friend murine leukemia virus are crucial for splicing at the correct 5' and 3' splice sites

The genome of the Friend murine leukemia virus (Fr-MLV) contains a 5' splice site (5'ss) located at 205 nt and a 3'ss located at 5489 nt. In our previous studies, it was shown that if the HindIII-BglII (879-1904 bp) fragment within gag is deleted from the proA8m1 vector, which carries the entire Fr-MLV sequence, then cryptic splicing of env-mRNA occurs. Here, attempts were made to identify the genomic segment(s) in this region that is/are essential to correct splicing. First, vectors with a serially truncated HindIII-BglII fragment were constructed. The vector, in which a 38 bp fragment (1612-1649 bp) is deleted or reversed in proA8m1, only produced splice variants. It was found that a 38 nt region within gag contains important elements that positively regulate splicing at the correct splice sites. Further analyses of a series of vectors carrying the 38 bp fragment and its flanking sequences showed that a region (1183-1611 nt) upstream of the 38 nt fragment also contains sequences that positively or negatively influence splicing at the correct splice sites. The SphI-NdeI (5140-5400 bp) fragment just upstream of the 3'ss was deleted from vectors that carried the 38 bp fragment and its flanking sequences, which yielded correctly spliced mRNA; interestingly, these deleted vectors showed cryptic splicing. These findings suggest that the 5140-5400 nt region located just upstream of the 3'ss is required for the splicing function of the 38 nt fragment and its flanking sequences.

Kinetic studies of the effect of a 17-nucleotide difference in the 0.3-kb region containing the R-U5-5' leader sequence of Friend murine leukemia virus on viral gene expression

In addition to the env gene, a 0.3-kb fragment containing the R-U5-5' leader sequence is essential for the induction of spongiform neurodegeneration by Friend murine leukemia virus (Fr-MLV) clone A8 and it also influences expression of the Env protein. Kinetic studies were carried out using two recombinant viruses, R7f, carrying the A8 0.3-kb fragment, and Rec5, carrying the 0.3-kb fragment of the non-neuropathogenic Fr-MLV clone 57. These analyses suggested that the 0.3-kb fragment influenced the expression level of the Env protein by regulating the amount of spliced env-mRNA rather than the amount of total viral mRNA or viral production.

The 0.3-kb fragment containing the R-U5-5'leader sequence of Friend murine leukemia virus influences the level of protein expression from spliced mRNA

Background

A neuropathogenic variant of Friend murine leukemia virus (Fr-MLV) clone A8 induces spongiform neurodegeneration when infected into neonatal rats. Studies with chimeras constructed from the A8 virus and the non-neuropathogenic Fr-MLV clone 57 identified a 0.3-kb KpnI-AatII fragment containing a R-U5-5’leader sequence as an important determinant for inducing spongiosis, in addition to the env gene of A8 as the primary determinant. This 0.3-kb fragment contains a 17-nucleotide difference between the A8 and 57 sequences. We previously showed that the 0.3-kb fragment influences expression levels of Env protein in both cultured cells and rat brain, but the corresponding molecular mechanisms are not well understood.

Results

Studies with expression vectors constructed from the full-length proviral genome of Fr-MLV that incorporated the luciferase (luc) gene instead of the env gene found that the vector containing the A8-0.3-kb fragment yielded a larger amount of spliced luc-mRNA and showed higher expression of luciferase when compared to the vector containing the 57-0.3-kb fragment. The amount of total transcripts from the vectors, the poly (A) tail length of their mRNAs, and the nuclear-cytoplasm distribution of luc-mRNA in transfected cells were also evaluated. The 0.3-kb fragment did not influence transcription efficiency, mRNA polyadenylation or nuclear export of luc-mRNA. Mutational analyses were carried out to determine the importance of nucleotides that differ between the A8 and 57 sequences within the 0.3-kb fragment. In particular, seven nucleotides upstream of the 5’splice site (5’ss) were found to be important in regulating the level of protein expression from spliced messages. Interestingly, these nucleotides reside within the stem-loop structure that has been speculated to limit the recognition of 5’ss.

Conclusions

The 0.3-kb fragment containing the R-U5-5’leader sequence of Fr-MLV influences the level of protein expression from the spliced-mRNA by regulating the splicing efficiency rather than transcription, nuclear export of spliced-mRNA, or poly (A) addition to mRNA. Seven nucleotides in the 0.3-kb fragment, which reside within the stem-loop structure that has been speculated to limit recognition of the 5’ss, could pinpoint the function of this region.

Narrowing down the critical region within env gene for determining neuropathogenicity of murine leukemia virus A8

Friend murine leukemia virus clone A8 causes spongiform neurodegeneration in the rat brain, and the env gene of A8 is a primary determinant of neuropathogenicity. In order to narrow down the critical region within the env gene that determines neuropathogenicity, we constructed chimeric viruses having chimeric env between A8 and non-neuropathogenic 57 on the background of A8 virus. After replacement of the BamHI (at nucleotide 5715)-AgeI (at nucleotide 6322) fragment of A8 virus with the corresponding fragment of 57, neuropathogenicity was lost. In contrast, the chimeric viruses that have the BamHI (5715)-AgeI (6322) fragment of A8 induced spongiosis in 100% of infected rats at the same or slightly lower intensity than A8 virus. These results indicate that the BamHI (5715)-AgeI (6322) fragment of A8, which contains the signal sequence and the N-terminal half of RBD, is crucial for the induction of spongiform neurodegeneration. In the BamHI (5715)-AgeI (6322) fragment, seven amino acids differed between A8 and 57, one in the signal sequence and six in RBD, which suggests that these amino acids significantly contribute to the neuropathogenicity of A8.

Analysis of cis-regulatory elements in the 5' untranslated region of murine leukemia virus controlling protein expression

It has previously been reported by us that high-level expression of the Env protein of Fr-MLV clone A8 in brains is crucial for induction of spongiform neurodegeneration, and that the 0.3-kb fragment containing the R, U5, and the 5' leader sequence of A8 is responsible for neuropathogenicity. In the present study, the role of the 5' untranslated region in protein expression was investigated. Luciferase expression vectors containing the LTR (R-U3-U5) and 5' leader sequence of A8 and non-neuropathogenic 57 Fr-MLV, designated gl-A8 and gl-57, and their chimeric vectors, were constructed, and transfected into rat glial cells F10. Replacement of the region containing the 3' half of R, U5, and 5' leader sequence of gl-A8 with that of 57 showed a reduction in luciferase activities, and replacement of this region of gl-57 with that of A8 showed increased luciferase activity. These results show that the region containing the 3' half of R, U5, and 5' leader sequence of A8 more efficiently up-regulates protein expression than 57. In particular, the 3' half of 5' leader of A8 was most responsible for the up-regulation of protein expression. Of interest, after replacement of the fragments between A8 and 57, changes in the activities of vectors containing A8-U3 paralleled the amount of mRNA, but the activities of vectors containing 57-U3 did not. Furthermore, it is suggested that the region containing R, U5, and the 5' leader sequence influences transcriptional or post-transcriptional steps, depending on the upstream sequence containing enhancer elements and promoter.

A viral non-coding region determining neuropathogenicity of murine leukemia virus A8 is responsible for envelope protein expression in the rat brain

Friend murine leukemia virus clone A8 causes spongiform neurodegeneration in the rat brain. A 0.3-kb fragment containing the R-U5-5' leader sequence of A8 is required in addition to the A8-env gene to induce spongiosis. The A8-env gene is a primary determinant of neuropathogenicity. Comparative studies of the neuropathogenic virus R7f, which carries the 0.3-kb fragment of A8 and A8-env on the background of the non-neuropathogenic clone 57, and the non-neuropathogenic virus Rec5, which carries A8-env on the background of 57, showed that the 0.3-kb fragment of A8 was responsible for increasing the ratio of Env/Gag expression in the brain, but not in the spleen.

Neuropathology induced by infection with Friend murine leukemia viral clone A8-V depends upon the level of viral antigen expression

A8-V is a neuropathogenic clone isolated from the Friend murine leukemia virus which causes spongiosis in the rat brain after infection at birth. Serial studies using chimeric viruses derived from the A8-V and the 57 virus (57-V), which is a non-neuropathogenic strain of Friend murine leukemia virus, proved that the long terminal repeat (LTR) and 5' leader (LTR-leader/A8) derived from A8-V, in addition to the env gene (env/A8) of A8-V, are necessary for the neuropathogenesis of A8-V. The enhancer element within the LTR of A8-V (LTR/A8) has been supposed to contribute to the severe manifestation of spongiosis by inducing high levels of viral production in the brain after A8-V infection. However, the recombinant viruses R7c and R7f, which lack the enhancer element of A8-V, induced spongiosis with high incidence rates, although the isolated viral titers of the infected brain display very low levels, which are even comparable to the 57-V infection. Immunohistochemical studies demonstrated that infection with neuropathogenic chimerae, R7c and R7f, induced increased expression of viral antigens than that produced by infection with non-neuropathogenic chimeric virus, Rec5, despite the fact that R7c, R7f and Rec5 all exhibited similar levels of viral proliferation in the brain postinfection. Thus, neuropathology induced by A8 infection is not dependent upon the viral proliferation rate but rather the level of viral antigen expression.

Cerebral metabolism in brains of rats infected with neuropathogenic murine leukemia viruses

Friend murine leukemia virus A8 and PVC211 cause spongiform neurodegeneration in rat brains. Glutamate is an important neurotransmitter synthesized from alpha-ketoglutaric acid, an intermediate product of the citric acid cycle, and glutamine is synthesized from glutamate. To examine the brain metabolism of rats infected with neuropathogenic viruses, the amount of glutamate and glutamine in the brains of rats infected with A8, PVC211, and non-neuropathogenic 57 was measured using high performance liquid chromatography, and the (13)C-label incorporation into the C4 position of glutamate and glutamine from [1-(13)C] glucose was measured with (13)C nuclear magnetic resonance. In the cerebral hemisphere and region containing the brain stem and basal ganglia of rats infected with A8 and PVC211 at 8-9 weeks post-infection (wpi), the amount of glutamine was decreased compared with the 57-infected rats. The amount of glutamate was decreased in the cerebral hemisphere of the A8-infected rats and the region containing the brain stem and basal ganglia of PVC211-infected rats at 8-9 wpi. The amount of [4-(13)C] glutamine and [4-(13)C] glutamate in the cerebral hemisphere and region containing the brain stem and basal ganglia of rats infected with A8 and PVC211 at 8-9 wpi was equivalent to that of the 57-infected rats. These results suggest that in the brains of rats infected with neuropathogenic viruses, de novo synthesis of glutamate and glutamine is not decreased, but the ability to maintain quantitative levels of glutamate and glutamine is decreased compared with the brains of rats infected with non-neuropathogenic virus.

Analysis of the distribution of neuropathogenic retroviral antigens following PVC211 or A8-V infection

A8-V and PVC211 are neuropathogenic strains of the Friend murine leukemia virus (Fr-MLV) that cause spongiosis in the rat brain after infection at birth. PVC211 exhibited stronger neuropathogenicity than A8-V, and induced more severe neurological symptoms such as hind-leg paralysis. These symptoms correlated with the neuropathological spread and intensity, which were more severe in the spinal cord of rats infected with PVC211 than in those infected with A8-V, without exhibiting neuropathological differences in other areas of the CNS. Interestingly, virus titers recovered from infected spinal cords were similar in PVC211 and A8-V infected animals. However, in the spinal cord infected with PVC211, glial cells attained higher immunohistochemical expression scores for the viral surface antigen, gp70 (Env) than in the A8-V infected spinal cord, although expression levels of viral antigens in blood vessel walls were similar in A8-V and PVC211 infections. Furthermore, many of those glial cells which carried viral antigens were found, by double immunostaining, to be microglia. The results suggested that the spread of viral antigen positive microglia plays an important role in forming the different neuro-pathogenicity observed in A8-V and PVC211 infections.