Zebrafish myelination: a transparent model for remyelination?

Abstract

There is currently an unmet need for a therapy that promotes the regenerative process of remyelination in central nervous system diseases, notably multiple sclerosis (MS). A high-throughput model is, therefore, required to screen potential therapeutic drugs and to refine genomic and proteomic data from MS lesions. Here, we review the value of the zebrafish (Danio rerio) larva as a model of the developmental process of myelination, describing the powerful applications of zebrafish for genetic manipulation and genetic screens, as well as some of the exciting imaging capabilities of this model. Finally, we discuss how a model of zebrafish myelination can be used as a high-throughput screening model to predict the effect of compounds on remyelination. We conclude that zebrafish provide a highly versatile myelination model. As more complex transgenic zebrafish lines are developed, it might soon be possible to visualise myelination, or even remyelination, in real time. However, experimental outputs must be designed carefully for such visual and temporal techniques.

Fig. 1

Zebrafish myelination: high-throughput drug screening. (A) Anatomy of a transparent 3 d.p.f. zebrafish larva. (B) Several zebrafish larvae were placed in each well of a 96-well plate, with different drugs in each well. (C) Live sagittal image of a 3 d.p.f. olig2:EGPF zebrafish larva spinal cord. Dorsally migrated cells were counted in response to different drugs. Green dots in the middle of the magnified region indicate some of these cells. Examples of fainter, elongated migrating cells can be seen below the green star. The green arrow indicates a cell on the border of the pMN. (D) An automated system was used to count olig2-positive cells. Larvae (the arrow indicates one larva) were aspirated in methylcellulose into a capillary tube. The capillary was inserted through a water chamber (blue square) to reduce refraction of light, then anchored onto a movable stage (arrowhead) and remotely positioned until the larvae were orientated sagittally above an inverted microscope (outer circle). A z-stack of images was taken and combined into one collapsed image. The number of dorsally migrated olig2-positive cells was automatically counted. (E) Mbp immunohistochemistry of a transverse hindbrain section from a 5 d.p.f. zebrafish larva. Arrows indicate the medial longitudinal fascicle and arrowheads indicate the ventral commissure. (F) Electron micrograph of a transverse spinal cord section from a 10 d.p.f. zebrafish larva. Arrows indicate the large Mauthner axons. These are surrounded ventrally by axons with a smaller diameter.

Fig. 2

A hierarchy for identifying remyelination-enhancing therapies. Here, we indicate where zebrafish fit among other experimental models. In vitro myelinating co-cultures and in vivo zebrafish larval models of myelination should be used alongside each other to efficiently refine candidate drugs or genes of interest. These should then be investigated further in vivo in mammalian systems before moving to clinical trials. This hierarchy provides a resource-effective method of developing a therapy that targets remyelination.