Analysis of cellular mutants resistant to Theiler's virus infection: differential infection of L929 cells by persistent and neurovirulent strains

Injection of the sciatic nerve with TMEV: a new model for peripheral nerve demyelination

Demyelination of the human peripheral nervous system (PNS) can be caused by diverse mechanisms including viral infection. Despite association of several viruses with the development of peripheral demyelination, animal models of the condition have been limited to disease that is either autoimmune or genetic in origin. We describe here a model of PNS demyelination based on direct injection of sciatic nerves of mice with the cardiovirus, Theiler's murine encephalomyelitis virus (TMEV). Sciatic nerves of FVB mice develop inflammatory cell infiltration following TMEV injection. Schwann cells and macrophages are infected with TMEV. Viral replication is observed initially in the sciatic nerves and subsequently the spinal cord. Sciatic nerves are demyelinated by day 5 post-inoculation (p.i.). Injecting sciatic nerves of scid mice resulted in increased levels of virus recovered from the sciatic nerve and spinal cord relative to FVB mice. Demyelination also occurred in scid mice and by 12 days p.i., hindlimbs were paralyzed. This new model of virus-induced peripheral demyelination may be used to dissect processes involved in protection of the PNS from viral insult and to study the early phases of lesion development.

Evolution of virulence in picornaviruses

The Picornaviridae encompass many positive-strand RNA viruses, all of which share a generally similar genome design and capsid structure, but which induce quite diverse diseases in humans and other animals. Picornavirus strains of the same serotype have been shown to express different virulence (or pathogenic) phenotypes when studied in animal models, demonstrating that key elements of pathogenesis reside in the viral genome. However, the genetics that determine the virulence phenotype of any picornavirus are poorly understood. Picornaviruses do not have virulence genes per se, but the design ofthe capsid andhow it interacts with the virus receptor expressed on the host cell surface, specific sequences within the nontranslated regions of the viral genome, as well as coding sequences that result in different protein sequences may all have a part in determining the virulence phenotype. Virulence may be better understood as a continuum from an apparent inability to induce disease to the ability to cause severe pathogenic changes. Ultimately, the ability of a picornavirus to induce disease depends upon viral genetics and how they are modulated by the host environment.

Replication of Theiler’s murine encephalomyelitis virus in Xenopus laevis oocytes

Xenopus laevis oocytes can be used as an alternative system to study replication of Theiler's murine encephalomyelitis virus (TMEV). We have shown that transcript RNA, containing full-length viral genome, can be directly used to programme the oocytes. In the programmed oocytes, there is correct viral translation, polyprotein processing and assembly of capsid proteins leading to the production of infectious TMEV. The vast majority of de novo synthesised virions were found in the medium in which the programmed oocytes were incubated and not in the oocytes.

Heparan sulfate-independent infection attenuates high-neurovirulence GDVII virus-induced encephalitis

The high-neurovirulence Theiler's murine encephalomyelitis virus (TMEV) strain GDVII uses heparan sulfate (HS) as a coreceptor to enter target cells. We report here that GDVII virus adapted to growth in HS-deficient cells exhibited two amino acid substitutions (R3126L and N1051S) in the capsid and no longer used HS as a coreceptor. Infectious-virus yields in CHO cells were 25-fold higher for the adapted virus than for the parental GDVII virus, and the neurovirulence of the adapted virus in intracerebrally inoculated mice was substantially attenuated. The adapted virus showed altered cell tropism in the central nervous systems of mice, shifting from cerebral and brainstem neurons to spinal cord anterior horn cells; thus, severe poliomyelitis, but not acute encephalitis, was observed in infected mice. These data indicate that the use of HS as a coreceptor by GDVII virus facilitates cell entry and plays an important role in cell tropism and neurovirulence in vivo.

Galactose is needed only for expression of co-receptors used by Theiler's murine encephalomyelitis virus as the virus does not directly bind galactose or use the UDP-galactose transporter as a receptor

Theiler's murine encephalomyelitis virus (TMEV) infects most mammalian cells, but a TMEV receptor has not been identified. Studies have demonstrated that the UDP-galactose transporter (UGT) is critical for TMEV attachment and entry into mammalian cells (Hertzler et al., Virology 286, 336-344, 2001). It was suggested that UGT might function as a TMEV receptor. We have demonstrated that polyclonal rabbit antibodies to human UGT that cross-react with hamster UGT do not block binding to or infection of mammalian cells by either high- or low-neurovirulence TMEV. In addition, incubation of virus with galactose, or blocking galactose on the cell surface with lectins, does not inhibit TMEV binding or infection. Thus, TMEV needs UGT for its transporter activity and galactose for assembly of its co-receptors (attachment factors) but does not bind directly to galactose. Excluding direct involvement of UGT and galactose in TMEV binding and entry provides further insight into how TMEV interacts with the host cell and should facilitate ongoing studies to identify a TMEV receptor.

Mutations that affect the tropism of DA and GDVII strains of Theiler's virus in vitro influence sialic acid binding and pathogenicity

Theiler's murine encephalomyelitis virus (TMEV) is a natural pathogen of the mouse. The different strains of TMEV are divided into two subgroups according to the pathology they provoke. The neurovirulent strains GDVII and FA induce an acute fatal encephalitis, while persistent strains, like DA and BeAn, cause a chronic demyelinating disease associated with viral persistence in the central nervous system. Different receptor usage was proposed to account for most of the phenotype difference between neurovirulent and persistent strains. Persistent but not neurovirulent strains were shown to bind sialic acid. We characterized DA and GDVII derivatives adapted to grow on CHO-K1 cells. Expression of glycosaminoglycans did not influence infection of CHO-K1 cells by parental and adapted viruses. Mutations resulting from adaptation of DA and GDVII to CHO-K1 cells notably mapped to the well-characterized VP1 CD and VP2 EF loops of the capsid. Adaptation of the DA virus to CHO-K1 cells correlated with decreased sialic acid usage for entry. In contrast, adaptation of the GDVII virus to CHO-K1 cells correlated with the appearance of a weak sialic acid usage for entry. The sialic acid binding capacity of the GDVII variant resulted from a single amino acid mutation (VP1-51, Asn-->Ser) located out of the sialic acid binding region defined for virus DA. Mutations affecting tropism in vitro and sialic acid binding dramatically affected the persistence and neurovirulence of the viruses.

Heparan sulfate mediates infection of high-neurovirulence Theiler's viruses

The mechanisms by which Theiler's murine encephalomyelitis virus (TMEV) binds and enters host cells and the molecules involved are not completely understood. In this study, we demonstrate that the high-neurovirulence TMEV GDVII virus uses the glycosaminoglycan heparan sulfate (HS) as an attachment factor that is required for efficient infection. Studies based on soluble HS-mediated inhibition of attachment and infection, removal of HS with specific enzymes, and blocking with anti-HS antibodies establish that HS mediates GDVII virus entry into mammalian cells. Data from defined proteoglycan-deficient Chinese hamster ovary mutant cells further support the role of HS in GDVII infection and indicate that the extent of sulfation is critical for infection. Neuraminidase treatment of proteoglycan-deficient cells restores permissiveness to GDVII virus, indicating that sialic acid hinders direct access of virus to the protein entry receptor. A model of the potential steps in GDVII virus entry into mammalian cells involving HS is proposed.

Altered cell growth and morphology in a BHK-21 cell mutant that lacks a receptor for Theiler's murine encephalomyelitis virus

The receptor for Theiler's murine encephalomyelitis virus (TMEV) remains unknown. In vitro, BHK-21 cells are permissive to infection by TMEV. Selecting mutants of BHK-21 cells produced a cell line (BHKR-) resistant to infection by TMEV. Viral persistence was ruled out by immunofluorescent staining for viral antigens. BHKR- cells were nonpermissive to infection even at high multiplicities of infection. In contrast, cells were able to support one round of virus replication when transfected with infectious TMEV RNA. Binding studies indicated that TMEV was unable to attach to these cells. These data are consistent with the BHKR- cells lacking a receptor for TMEV. Interestingly, BHKR- cells were larger in size and had a significant lag in growth after subculture versus BHK-21 cells. This suggests that the TMEV receptor on BHK-21 cells could play an important role in cell growth and morphology under physiologic conditions. BHKR- cells should facilitate the search for TMEV receptors.

UDP-galactose transporter is required for Theiler's virus entry into mammalian cells

Theiler's murine encephalomyelitis viruses (TMEV) are divided into two groups: high-neurovirulence strains, such as GDVII, cause fatal encephalitis, while low-neurovirulence strains, such as BeAn and DA, cause persistent infection and demyelination in mice. Cell surface sialic acid is bound by the low-neurovirulence DA and BeAn viruses, but not by the high-neurovirulence GDVII virus. We have identified a clone from a BHK-21 cell cDNA library that mediates TMEV entry and infection by viruses of both TMEV groups in a receptor-negative BHK-21 cell variant (R26). The sequence of this clone is 96.4% identical to the human UDP-galactose transporter (UGT), which belongs to a family of nucleotide-sugar transporter proteins involved in the biosynthesis of complex carbohydrate structures in the trans-Golgi network. UGT mRNA from R26 cells was found to have a 490-nucleotide deletion involving the C-terminal amino acids 255 to 392 and 81 nucleotides of the 3' noncoding region. These results suggest two possibilities by which UGT may mediate TMEV entry and infection. The most likely one relates to the transporter function of adding galactose to another receptor protein. This possibility suggests the requirement for a specific glycoprotein interaction for GDVII virus cell binding and entry, e.g., galactose for GDVII and sialic acid for BeAn. Alternatively, UGT might be a TMEV receptor itself, acting via UGT cycling to the cell surface.

Selection and characterization of a BHK-21 cell line resistant to infection by Theiler's murine encephalomyelitis virus due to a block in virus attachment and entry

A clonal population of BHK-21 cells resistant to infection with the low-neurovirulence BeAn strain of Theiler's murine encephalomyelitis virus (TMEV) was derived after four cycles of infection and characterized. These cells were resistant to both low- and high-neurovirulence TMEV strains due to a block in virus attachment and entry and not in virus replication, since transfection of these cells with TMEV RNA to bypass the entry step(s) induced virus replication and assembly. The resistance to infection was stable for more than a year, suggesting that it is a heritable property arising from a mutation in the susceptible parent BHK-21 population. This cell line is being used to identify a receptor for TMEV.