Elsevier

Neuroscience

Volume 111, Issue 2, 10 May 2002, Pages 399-411
Neuroscience

Preservation of neurologic function during inflammatory demyelination correlates with axon sparing in a mouse model of multiple sclerosis

https://doi.org/10.1016/S0306-4522(02)00012-XGet rights and content

Abstract

Axonal injury has been proposed as the basis of permanent deficits in the inflammatory, demyelinating disease, multiple sclerosis. However, reports on the degree of injury are highly variable, and the responsible mechanisms are poorly understood. We examined the relationships among long-term demyelination, inflammation, axonal injury, and motor function in a model of multiple sclerosis, in which mice develop chronic, immune-mediated demyelination of the spinal cord resulting from persistent infection with Theiler’s virus. We studied two strains of mice, inbred SJL/J and C57BL/6×129 mice deficient in β2-microglobulin and therefore CD8 lymphocytes. After 8 months of disease, SJL mice had considerably worse motor function than β2-microglobulin-deficient mice. Motor dysfunction correlated linearly with the extent of demyelinated lesions in the spinal cord (lesion load) within each strain, but no difference in lesion load was present between strains. Also, the extent of remyelination did not differ between strains. Instead, the disparity in motor deficits reflected differences in the integrity of descending neurons. That is, retrograde labeling of reticulospinal, vestibulospinal, and rubrospinal neurons, although reduced in all chronically diseased mice, was two to seven times higher in β2-microglobulin-deficient mice. The labeling was superior in β2-microglobulin-deficient mice despite the fact that lesion expanse and therefore the number of axons traversing lesions were similar in both strains. Thus, by all criteria axons were equivalently demyelinated in SJL and β2-microglobulin-deficient mice, but the extent of axonal injury differed significantly.

These results indicate that mechanisms of demyelination and axonal injury are at least partly separable, and are consistent with the hypothesis that cytotoxic CD8 lymphocytes may selectively injure demyelinated axons. Additionally, the data suggest that axonal injury obligatorily results from chronic inflammatory demyelination and significantly contributes to neurological deficits.

Section snippets

Mice and viral infection

C57BL/6×129/J (H-2b, β2m−/−) mice were acquired from the Mayo Clinic Immunogenetic Mouse Colony, C. David Director. SJL/J mice were from Jackson Laboratories. Six- to eight-week-old mice were intracerebrally injected with 2.0×106 plaque forming units of Daniel’s strain of TMEV and maintained until experimental analysis at 8–9 months post-infection.

Motor function

Chronically infected mice (infected for 8–9 months) and uninfected littermates were tested on a Rotamex Rotarod (Columbus Instruments, Columbus, OH,

Relative preservation of motor function in β2m−/− mice

Chronically infected β2m−/− and SJL mice (8–9 months post-infection) both have inflammatory demyelination of the spinal cord but only SJL mice have obvious reductions in spontaneous activity (Rivera-Quinones et al., 1998). As a more sensitive and objective measure of motor function, we assayed mice by rotarod, a rotating cylinder upon which mice run under accelerating conditions (McGavern et al., 1999b). We found that most chronically infected mice from both strains performed more poorly than

Discussion

This study was undertaken principally to understand how neuronal integrity is affected by inflammatory demyelination and how changes in neuronal integrity relate to motor function. The Theiler’s virus model offers many advantages for addressing these issues. One advantage is that the duration of pathology is precisely known, in contrast to the unknown histories of post-mortem or biopsied MS plaques. Without knowing lesion age or its changing character, the rate at which neurons are injured or

Conclusion

Our results emphasize the need to consider neuronal injury as a major factor in neurological dysfunction in demyelinating disease. The data suggest that axonal destruction is an inevitable consequence of chronic disease, but could be ameliorated by therapies that target cytotoxic CD8 lymphocytes or activated immune or glial cells that specifically damage axons. Alternative therapies might involve neurotrophic factors or other molecules that bolster endogenous neuroprotective mechanisms or

Acknowledgements

This work was supported by NIH Grant NS32129 and the generous support of Ms. Kathryn Petersen and Mr. and Mrs. Eugene Applebaum. D.R.U. was supported by a fellowship from the Multiple Sclerosis Society of Canada and NIH Grants NS32129 and PO1-NS38468. The authors would like to thank Laurie Zoecklein for technical support.

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