William Talbot

1. Genetic dissection of myelin formation in zebrafish. The myelin sheath allows for rapid axonal conduction in vertebrates. Disruption of myelin underlies important human diseases, including Multiple Sclerosis and hereditary peripheral neuropathy. The formation of myelin, which involves reciprocal signaling between neurons and glial cells, a dramatic morphological transformation of the glial cells, and organization of the axon into different specialized domains, is fascinating but nonetheless poorly understood. We investigate myelination in the zebrafish, a vertebrate model system well suited for large-scale genetic studies and exquisite cellular analysis.

Our goal is to define new genes with essential functions in the development of myelinated nerves using genetic approaches in zebrafish. In genetic screens, we have identified more than 20 mutations that specifically disrupt the development of myelinated nerves. These mutations define at least 15 different genes, indicating that continued screening will identify a large number of new genes. Phenotypic characterization demonstrates that the mutations disrupt a wide array of processes in the developmental progression of myelinating glia, ranging from fate specification through terminal differentiation. Examples of genes defined in the screen include a transcription factor that regulates midline signals important for glial development in the CNS, a receptor tyrosine kinase that controls migration of glial cells along growing axons, and a motor protein that is essential for the morphological specialization of myelinating glial cells. In addition, we are working to positionally clone a mutated gene that is required for the localization of sodium channel proteins at the nodes of Ranvier in myelinated axons.

To elucidate the functions of the mutated genes, we are pursuing detailed phenotypic studies with marker genes, in vivo analysis of transgenes expressing GFP in glia, and electron microscopy to examine myelin ultrastructure. This project will discover new genes with essential functions in myelination, define new zebrafish models of important myelin disorders in humans, and provide new avenues toward therapies for myelin repair and prevention of axonal damage after demyelination.


2. Functional genomics in zebrafish. Recent progress in genome sequencing has demonstrated that duplicated genes and chromosomal segments are a general feature of vertebrate genomes. It has long been thought that duplication events provide the “raw material” for the acquisition of new gene functions, but it is unclear how genes acquire new activities that propel functional innovations in evolution. To answer important outstanding questions about the mechanisms by which duplicated genes adopt new functions, we are using high-throughput methods to determine expression patterns and loss-of-function phenotypes for a large number of these zebrafish duplicate gene pairs. In addition, we are working to develop technology and genomic infrastructure to accelerate the molecular analysis of zebrafish mutations.

3. Axis formation and cell fate specification in zebrafish. The vertebrate body plan emerges during gastrulation, when cell rearrangements form the three germ layers and signaling interactions establish organ rudiments in their proper positions. To understand patterning events in the early embryo, we are characterizing mutations that disrupt germ layer formation and dorsoventral patterning. We have shown that the squint gene encodes a Nodal-related TGF-beta family signal that induces organizer development in the early embryo. In addition, our genetic analysis indicates that the mutually repressive interactions among the bozozok, vox, and vent genes are essential to establish the organizer on the dorsal side of the embryo. We are working to understand the interactions among these genes and to identify new genes with important patterning functions in genetic screens.