Spike Protein of Mouse Hepatitis Virus Mediates Axonal Injury and Demyelination

Biswas, Kaushiki (2014) Spike Protein of Mouse Hepatitis Virus Mediates Axonal Injury and Demyelination. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) of unknown etiology. In addition to autoimmune etiologies, viral-induced mechanisms of disease have been suggested. Mouse hepatitis virus (MHV) strain A59 infection in mice causes demyelination and axonal loss in the CNS, including optic nerve, and is used as a model of MS and experimental optic neuritis. The spike glycoprotein mediates neurotropic effects of MHV, as isogenic strains which differ only in their spike protein differ in their ability to cause demyelination, axonal loss, and loss of retinal ganglion cells (RGCs). RSA59, a demyelinating strain containing MHV-A59 recombined with its wild-type A59 spike gene, causes axonal loss concurrent with macrophage mediated demyelination. In contrast, optic nerve and spinal cord infected with RSMHV2, a non-demyelinating strain containing the MHV2 spike gene in the MHV-A59 genome, show only rare early axonal degeneration. After intracranial inoculation, both RSA59 and RSMHV2 spread to spinal cord, likely through axonal transport within infected neurons, but only RSA59 spreads from spinal cord gray matter into white matter, suggesting a role of spike in specifically mediating anterograde transport. These findings suggest that spike is responsible for differential axonal transport and spread in the CNS, but the mechanisms by which spike facilitates interaction with axonal transport machinery are unknown. Transport studies in spinal cord are complicated by the presence of multiple pathways, whereas RGCs that comprised the optic nerve represent a unidirectional pathway that is relatively homogeneous and readily accessible. Optic neuritis is one of the most common lesions of MS, and understanding mechanisms of RGC damage in this model may help direct design of future therapies for optic neuritis and MS. MHV-A59 spike is synthesized as a 180-kDa glycosylated precursor that is posttranslationally cleaved into two 90-kDa subunits, S1 and S2, with a receptor binding domain in the S1 subunit responsible for initial attachment of MHV to cell surface receptors. S1 binding and cleavage triggers a conformational change in spike that allows S2 to initiate fusion of virus and host membranes. A candidate fusion peptide domain has been identified within S2; however, the actual fusion peptide has not been definitively identified. MHV-A59 and MHV-2 spike proteins have 82% amino acid sequence identity and 94% similarity, with the S2 domain more conserved and S1 more variable. They also differ in the cleavage signal site whereby MHV-A59 spike is cleaved into S1 and S2 subunits, but MHV-2 spike protein is not cleaved and is unable to cause fusion in vivo and in vitro. Thus, variable regions of the S1 domain or differences in the cleavage signal site and or fusion domain between strains may explain the differential axonal transport/spread and successive demyelination/axonal loss observed in our studies.

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