Capillary endothelial cell tropism of PVC-211 murine leukemia virus and its application for gene transduction

O-sulfate groups of heparin are critical for inhibition of ecotropic murine leukemia virus infection by heparin

There is increasing evidence that soluble glycosaminoglycans such as heparin can interfere with the infectivity of various viruses, including ecotropic murine leukemia viruses (MLVs). The ecotropic MLV, Friend MLV (F-MLV) and the neuropathogenic variants A8 MLV and PVC-211 MLV, were susceptible to heparin-mediated inhibition of infection of NIH 3T3 cells. To investigate the interaction between the envelope glycoprotein (Env) of MLV and heparin, we prepared vesicular stomatitis virus-based pseudotyped viruses carrying the Env of F-, A8, or PVC-211 MLVs. Surface plasmon resonance analyses indicated that the Env of A8 and PVC-211 MLVs had a higher binding activity to heparin than that of F-MLV. We examined the influence of N- or O-sulfation of heparin on binding activity to Env and on the inhibition of the infectivity of MLV and pseudotyped viruses carrying Env. This analysis indicated that the O-sulfate groups of heparin play a major role in determining Env-dependent inhibitory effects.

Spongiform degeneration induced by neuropathogenic murine coronavirus infection

Soluble receptor‐resistant mutant 7 (ssr7) is isolated from a highly neurovirulent mouse hepatitis virus (MHV) JHMV cl‐2 strain (cl‐2). srr7 exhibits lower virulence than its maternal strain in infected mice, which is typically manifested in a longer lifespan. In this study, during the course of infection with srr7, small spongiotic lesions became apparent at 2 days post‐inoculation (pi), they spread out to form spongiform encephalopathy by 8 to 10 days pi. We recently reported that the initial expressions of viral antigens in the brain are detected in the infiltrating monocyte lineage and in ependymal cells. Here, we demonstrate that the next viral spread was observed in glial fibrillary acidic protein‐positive cells or nestin‐positive progenitor cells which take up positions in the subventricular zone (SVZ). From this restricted site of infection in the SVZ, a large area of gliosis extended deep into the brain parenchyma where no viral antigens were detected but vacuolar degeneration started at 48 h pi of the virus. The extremely short incubation period compared with other experimental models of infectious spongiform degeneration in the brain would provide a superior experimental model to investigate the mechanism of spongiotic lesions formation.

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.

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.

Disparate regions of envelope protein regulate syncytium formation versus spongiform encephalopathy in neurological disease induced by murine leukemia virus TR

The murine leukemia virus (MLV) TR1.3 provides an excellent model to study the wide range of retrovirus-induced central nervous system (CNS) pathology and disease. TR1.3 rapidly induces thrombotic events in brain microvessels and causes cell-specific syncytium formation of brain capillary endothelial cells (BCEC). A single amino acid substitution, W102G, in the MLV envelope protein (Env) regulates the pathogenic effects. The role of Env in determining this disease phenotype compared to the induction of spongiform encephalomyelitis with a longer latency, as seen in several other MLV and in human retroviruses, was determined by studying in vitro-attenuated TR1.3. Virus cloned from this selection, termed TRM, induced progressive neurological disease characterized by ataxia and paralysis and the appearance of spongiform neurodegeneration throughout the brain stem and spinal cord. This disease was associated with virus replication in both BCEC and highly ramified glial cells. TRM did not induce syncytium formation, either in vivo or in vitro. Sequence and mutational analyses demonstrated that TRM contained a reversion of Env G102W but that neurological disease mapped to the single amino acid substitution Env S159P. The results demonstrate that single nucleotide changes within disparate regions of Env control dramatically different CNS disease patterns.

Host range and receptor binding properties of vectors bearing feline leukemia virus subgroup B envelopes can be modulated by envelope sequences outside of the receptor binding domain

To evaluate host range differences between two different strains of feline leukemia virus subgroup B (FeLV-B), we compared the binding and infectivity patterns of retrovirus vectors bearing either FeLV-B-90Z or FeLV-B-GA envelopes. We report here that the ability of these envelopes to utilize different Pit1 orthologs is mediated primarily by the receptor binding domain; however, in the case of FeLV-B-90Z, the C terminus also contributes to the recognition of certain Pit1 orthologs.

A unique heparin-binding domain in the envelope protein of the neuropathogenic PVC-211 murine leukemia virus may contribute to its brain capillary endothelial cell tropism

Previous studies from our laboratory demonstrated that PVC-211 murine leukemia virus (MuLV), a neuropathogenic variant of Friend MuLV (F-MuLV), had undergone genetic changes which allowed it to efficiently infect rat brain capillary endothelial cells (BCEC) in vivo and in vitro. Two amino acid changes from F-MuLV in the putative receptor binding domain (RBD) of the envelope surface protein of PVC-211 MuLV (Glu-116 to Gly and Glu-129 to Lys) were shown to be sufficient for conferring BCEC tropism on PVC-211 MuLV. Recent examination of the unique RBD of PVC-211 MuLV revealed that the substitution of Lys for Glu at position 129 created a new heparin-binding domain that overlapped a heparin-binding domain common to ecotropic MuLVs. In this study we used heparin-Sepharose columns to demonstrate that PVC-211 MuLV, but not F-MuLV, can bind efficiently to heparin and that one or both of the amino acids in the RBD of PVC-211 MuLV that are associated with BCEC tropism are responsible. We further showed that heparin can enhance or inhibit MuLV infection and that the mode of action is dependent on heparin concentration, sulfation of heparin, and the affinity of the virus for heparin. Our results suggest that the amino acid changes that occurred in the envelope surface protein of PVC-211 MuLV may allow the virus to bind strongly to the surface of BCEC via heparin-like molecules, increasing the probability that the virus will bind to its cell surface receptor and efficiently infect these cells.

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

Induction of syncytia by neuropathogenic murine leukemia viruses depends on receptor density, host cell determinants, and the intrinsic fusion potential of envelope protein

Infection by the neuropathogenic murine leukemia virus (MLV) TR1.3 results in hemorrhagic disease that correlates directly to in vivo syncytium formation of brain capillary endothelial cells (BCEC). This phenotype maps to amino acid 102 in the envelope (Env) protein of TR1.3. Substitution of glycine (G) for tryptophan (W) at this position (W102G Env) in the nonpathogenic MLV FB29 induces both syncytium formation and neurologic disease in vivo. Using an in vitro gene reporter cell fusion assay, we showed that fusion either with murine NIH 3T3 cells or with nonmurine target cells that expressed receptors at or below endogenous murine levels mirrored that seen in BCEC in vivo. In these instances only TR1.3 and W102G Env induced cell fusion. In contrast, when receptor levels on nonmurine cells were raised above endogenous murine levels, FB29 Env was as fusogenic as the neuropathogenic TR1.3 and W102G Env. These results indicate that TR1.3 Env and W102G Env are intrinsically more fusogenic than FB29 Env, that the induction of fusion requires a threshold number of receptors that is greater for FB29 Env than for TR1.3 or W102G Env, and that receptor density on murine NIH 3T3 cells and BCEC is below the threshold for FB29-dependent fusion. Surprisingly, receptor density on NIH 3T3 cells could not be increased by stable expression of exogenous receptors, and FB29-dependent fusion was not observed in NIH 3T3 cells that transiently expressed elevated receptor numbers. These results suggest that an additional undefined host cell factor(s) may limit both receptor expression and fusion potential in murine cells.

Analysis of receptor usage by ecotropic murine retroviruses, using green fluorescent protein-tagged cationic amino acid transporters

Entry of ecotropic murine leukemia virus (MuLV) into host cells is initiated by interaction between the receptor-binding domain of the viral SU protein and the third extracellular domain (TED) of the receptor, cationic amino acid transporter 1 (CAT1). To study the molecular basis for the retrovirus-receptor interaction, mouse CAT1 (mCAT1) was expressed in human 293 cells as a fusion protein with jellyfish green fluorescent protein (GFP). Easily detected by fluorescence microscopy and immunoblot analysis with anti-GFP antibodies, the mCAT1-GFP fusion protein was expressed in an N-glycosylated form on the cell surface and in the Golgi apparatus, retaining the ecotropic receptor function. The system was applied to compare Friend MuLV (F-MuLV) and its neuropathogenic variant, PVC-211 MuLV, which exhibits a unique cellular tropism and host range, for the ability to use various CAT family members as a receptor. The results indicated that F-MuLV and PVC-211 MuLV could infect the cells expressing wild-type mCAT1 at comparable efficiencies and that rat CAT3, but not mCAT2, conferred a low but detectable level of susceptibility to F-MuLV and PVC-211 MuLV. The data also suggested that CAT proteins might be expressed in an oligomeric form. Further application of the system developed in this study may provide useful insights into the entry mechanism of ecotropic MuLV.