The myelin sheath is a multilayered membrane generated by specialized glial cells that iteratively spiral their plasma membranes around axon segments in vertebrate nervous systems. Myelination serves to increase the conduction velocity of action potentials and provide trophic support that is vital for neuronal survival. In the peripheral nervous system (PNS), myelin is made by Schwann cells (SCs). Disruptions in SC development and myelination lead to devastating symptoms in several neurological disorders, including Charcot-Marie-Tooth disease. Additionally, mutations in genes that are important regulators of normal SC development can result in both benign and malignant peripheral nerve tumors. In order to develop effective therapies for these patients, we must first understand the molecular mechanisms that govern SC development and myelination. To this end, we recently conducted a large-scale forward genetic screen in zebrafish for mutations that affect myelinating glial development. From this effort, we recovered stl64 mutants, which display dramatic increases in the expression of myelin-related genes and hypermyelination in both the CNS and PNS. Using whole genome sequencing, we determined that stl64 disrupts fbxw7, which encodes the substrate recognition of E3 ubiquitin ligase complexes that are responsible for catalyzing the addition of ubiquitin moieties to proteins to mark them for degradation by the proteasome. Some of the most notable Fbxw7 targets are master regulators of transcription and cell cycle including: mTOR, c-Myc, c-Jun, Notch, and cyclin E. Thus, Fbxw7 function is required for critical cellular processes such as proliferation and differentiation. Our preliminary analyses of fbxw7stl64 zebrafish mutants provide the first evidence that Fbxw7 is an important modulator of Schwann cell development and myelination. In the first aim of this proposal, I will use zebrafish and mouse models to clarify the cell-autonomous roles of Fbxw7 throughout SC development and myelination.
The second aim of this proposal seeks to address the mechanisms by which Fbxw7 controls SC development and myelination. Using TALEN mediated genome editing in zebrafish, I will generate isoform-specific loss-of-function mutants to determine the critical Fbxw7 splice isoform(s) in the context of SCs. Additionally, I will test he hypothesis that loss of Fbxw7 regulation of mTOR is the primary cause of the SC defects observed in stl64 mutants. Together the proposed experiments will define the cellular autonomy of Fbxw7 and elucidate the mechanisms by which Fbxw7 controls proper SC development and myelination.
Disruptions in genes that regulate Schwann cell development and myelination result in debilitating symptoms in several neurological disorders, and can contribute to some types of peripheral nerve tumors. Unfortunately, therapies that reliably prevent demyelination and/or promote remyelination are ineffective, and we do not understand the molecular mechanisms that control the formation and progression of Schwann cell-derived tumors. Here, we propose to determine how a component of the protein degradation pathway, Fbxw7, regulates Schwann cell development and myelination, which is critical for the development of new therapeutic avenues for these patients.
