Zebrafish model of demyelination and remyelination

Abstract

Myelin is a protective layer wrapped around axons which helps them conduct electrical signals rapidly, and provides them with metabolic support. In the central nervous system (CNS), myelin is produced by specialised glial cells called oligodendrocytes. Loss of myelin (demyelination) is associated with degeneration of axons and many neurodegenerative disorders, including multiple sclerosis (MS). The restoration of myelin sheaths by remyelination may protect axons and help functional recovery of patients, but achieving this requires better understanding of how the process unfolds at the cellular level. To investigate the processes of de- and remyelination in vivo, I have characterised a transgenic zebrafish line in which expression of the bacterial enzyme nitroreductase (NTR) is driven under the myelin basic protein promoter, thus in myelinating glia. I treat larvae with the NTR substrate metronidazole (Mtz). The reaction between NTR and Mtz results in a toxic metabolite which selectively kills NTR-expressing cells. The treatment with Mtz consistently ablates two-thirds of oligodendrocytes while not harming the animals otherwise. Myelin sheaths continue to deteriorate after the end of the treatment, such that seven days later, extensive demyelination is observed by electron microscopy. By 16 days after Mtz-treatment, robust recovery has occurred, with no discernible axon loss and myelin thickness restored to control levels. At this time point, oligodendrocyte numbers have also returned to control levels. During the demyelinated phase, I observe a striking increase in microglia and macrophages in the spinal cord. In order to study the role of the innate immune system in recovery, I used a mutant line, irf8-/- which lacks a transcription factor essential for development of microglia and macrophages. I am in the process of determining the ability of these mutants to regenerate their oligodendrocytes and myelin; preliminary results suggest that they are able to restore their myelin sheaths fully, but seem to have a delay in regenerating their oligodendrocytes compared to wild-types. The model I have established can be used in the future to better understand the consequences of demyelination to axon health, as well as chemical screening to identify compounds that could accelerate the remyelination process or enhance the thickness of myelin generated during remyelination. Insights arising from such studies will be useful in designing strategies to reduce axon loss and improve myelin regeneration in demyelinating diseases