Axon myelin transfer of a non-enveloped virus

Abstract

We showed previously that Theiler's virus, a neurotropic non-enveloped picornavirus of mouse, traffics from the axon of infected neurons into the surrounding myelin. When this traffic is interrupted, as in the shiverer mouse which bears a mutation in the myelin basic protein gene, the virus is unable to persist in the central nervous system. In the present work, we used the Wld(s) mutant mouse, a strain in which axonal degeneration is considerably slowed down, to show that axon to myelin traffic takes place in the absence of axon degeneration. Our results suggest the existence of a mechanism of transfer of axonal cytoplasm into the myelin which Theiler's virus might exploit to ensure its persistence.

Figure 1. Amount of viral RNA in the retina, 2 days post-inoculation.

Total RNA was extracted from the eye. The number of TMEV negative-strand RNA copies was measured in each sample by real time RT-PCR. Negative-strand RNA is diagnostic of replication whereas positive-strand RNA can be residual inoculum. Each circle corresponds to a different mouse. There is no statistically significant difference of viral load between control and Wld^s mice (p = 0.40, Mann-Whitney test).

Figure 2. Number of infected cells in the LGN, 4 days post-inoculation.

Mice were inoculated intraocularly in one eye. Serial sections of the entire corresponding geniculate nucleus were stained with a fluorescent anti-viral capsid antibody. The total number of fluorescent neuron cell body was counted. Each circle corresponds to a different mouse. There is no statistically significant difference of viral load between control and Wld^s mice (p = 0.61, Mann-Whitney test).

Figure 3. Phenotype of TMEV-infected optic nerves of wild type and Wld^s mice.

A: Longitudinal sections of optic nerves reacted with the SMI32 antibody (green). Nuclei were stained with DAPI (blue). SMI32 staining was detected in wild type samples only. B: Electron microscopy. Swollen axons with disorganized cytoskeleton are present in wild type mice only (star); bar = 5 µm. Mice shown in A and B were sacrificed 3 days post inoculation. C: Frozen sections were stained for CNPase (oligodendrocytes) or GFAP (astrocytes) and for TMEV to determine the percentage of TMEV⁺ cells which were CNPase⁺. No difference between wild type and Wld^s mice was found (p = 0.31, Mann-Whitney test). D: The same sections were used to measure the density of CNPase⁺, TMEV⁺ cells (infected oligodendrocytes/mm²). No difference between wild type and Wld^s mice was found (p = 0.40, Mann-Whitney test). In C and D, each circle corresponds to a different mouse.