Astrocytes regulate myelin clearance through recruitment of microglia during cuprizone-induced demyelination.

Recent evidence suggests that astrocytes play an important role in regulating de- and remyelination in multiple sclerosis. The role of astrocytes is controversial, and both beneficial as well as detrimental effects are being discussed. We performed loss-of-function studies based on astrocyte depletion in a cuprizone-induced rodent model of demyelination. This led to strong astrogliosis accompanied by microgliosis and demyelination in C57BL/6 wild-type mice. Ablation of astrocytes in glial fibrillary acidic protein-thymidine kinase transgenic mice was associated with a failure of damaged myelin removal and a consecutive delay in remyelination. Despite oligodendrocyte death, myelin was still present, but ultrastructual investigations showed that the myelin structure was loosened and this damaged myelin did not protect axons. These alterations were associated with a decrease in microglial activation. Thus, our results show that astrocyte loss does not prevent myelin damage, but clearance of damaged myelin through recruitment of microglia is impaired. Further studies suggest that this process is regulated by the chemokine CXCL10. As a consequence of the delayed removal of myelin debris, remyelination and oligodendrocyte precursor cell proliferation were impaired. Experiments omitting the influence of myelin debris demonstrated an additional beneficial effect of astrocytes on oligodendrocyte regeneration during remyelination. In conclusion, these data demonstrate for the first time in vivo that astrocytes provide the signal environment that forms the basis for the recruitment of microglia to clear myelin debris, a process required for subsequent repair mechanisms. This is of great importance to understanding regenerative processes in demyelinating diseases such as multiple sclerosis.

De- and remyelination in the CNS white and grey matter induced by cuprizone: the old, the new, and the unexpected.

The copper chelator cuprizone (bis-cyclohexanone oxaldihydrazone) was established as a neurotoxin in rodents in 1966 by Carlton. During the following years the usefulness of cuprizone feeding in mice to induce oligodendrocyte death with secondary demyelination of the superior cerebellar peduncles was described by Blakemore. In 1998 the cuprizone model experienced a renaissance as the group of Matsushima described the effects of cuprizone on the white matter of the cerebrum and focussed on demyelination in the corpus callosum, where the extent of demyelination could be scored more easily and consistently. Since then the toxic cuprizone model has been widely used to study experimental de- and remyelination in the corpus callosum. Recently, we and others have extended these studies and have shown several new aspects characteristic for this model. Many lessons can be learned from these recent findings that have implications for the basic understanding of remyelination and the design of remyelinating and neuroprotective strategies in demyelinating diseases of the CNS. Although the model is often mentioned in the context of multiple sclerosis, it must always be kept in mind that this model has a fundamentally different induction of demyelination. We highlight the important findings delineated from this model and critically discuss both the advantages and the shortcomings of cuprizone induced demyelination.