Remyelination describes the regeneration of myelin sheaths, and is considered one of the most promising strategies for improving the prognosis of demyelinating diseases such as multiple sclerosis. Data from animal models and human studies have shown that remyelination can occur extensively in the central nervous system (CNS), leading to functional recovery and axonal protection. However, remyelination does not always proceed to completion, and its failure is associated with progressive neurological disability. Thus, there is clinical need for interventions that can optimise the conditions for remyelination.

Recent advances in genomics and animal husbandry have kindled an interest in the microbiome as a means to influence processes throughout the body. Our commensal microbes communicate with host cells at epithelial barriers, stimulate neural and endocrine axes and directly produce a plethora of long-range signalling molecules. Critically, the development and maintenance of the immune system depend on signals from the microbiota, and we know that a well-coordinated immune response is a key determinant of the success of remyelination. This thesis explores how the microbiome can influence CNS remyelination.

To do so, I have studied remyelination in three murine models of microbiome alteration. Firstly, long-term oral administration of an antibiotic cocktail was used to deplete the microbiota of adult mice. Following focal demyelination, these mice had deficits in their inflammatory response, clearance of myelin debris and differentiation of new oligodendrocytes from oligodendrocyte progenitor cells (OPCs). Faecal microbial transplant was able to rescue aspects of the inflammatory response and phagocytosis, but not OPC differentiation.

Secondly, I looked at remyelination in germ-free (GF) mice following cuprizone-induced demyelina- tion. As with the antibiotics-treated mice, there were deficits in inflammation following demyelination, which tended to peak later than in control mice.

Finally, I investigated the potential of a therapeutic probiotic (VSL#3) to improve remyelination in aged mice. In contrast to antibiotic treatment, probiotic administration caused a slight enhancement in the onset of inflammation following focal demyelination. However, there was no significant improvement in OPC differentiation or toluidine blue rank analysis, suggesting these changes in inflammation were not sufficient to positively modulate remyelination.

The results from these three studies introduce a significant but previously unconsidered environmental influence on remyelination in the CNS. Whilst the effects are subtle relative to more direct interventions, the microbiome can be manipulated simply and non-invasively, which may provide a useful adjunct to other strategies for optimising remyelination.