Analysis of the Structure, Expression and Evolution of the Shark Myelin Proteins and Genes

Citation

Fors, Lance (1990) Analysis of the Structure, Expression and Evolution of the Shark Myelin Proteins and Genes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/840h-0p67. https://resolver.caltech.edu/CaltechETD:etd-06072007-082551

Abstract

Myelin is a compacted multilamellar membrane which encases axons to provide electrical insulation and facilitates the rapid transmission of nerve impulses. The major myelin structural proteins produced by oligodendrocyctes in the central nervous system (CNS) of mammals are proteolipid protein (PLP) and myelin basic protein (MBP). In contrast, the major myelin structural proteins produced by the Schwann cells in the peripheral nervous system (PNS) of mammals are protein zero (Po) and MBP. Sharks (class Chondrichthyes) are the oldest living vertebrates that have a concentric multilamellar "mammalian-like" myelin structure around axons. In addition to this structural similarity, the shark and mammalian myelin proteins appeared to be distantly related biochemically and immunologically even though they diverged from each other about 400 million years ago. Logically those regions in the shared proteins, genes and promoters which are most similar between sharks and mammals are likely to be functionally important to both. Therefore by analyzing these elements in shark myelin and comparing them to what is already known about mammalian myelin we could learn about shark myelin, its evolution and what regions are essential for the proper function and expression of mammalian myelin. This thesis contains an analysis of the structure, expression and evolution of the shark myelin proteins and genes.

The first chapter (Saavedra, R., Fors, L., Aebersold, R., Arden, B., Horvath, S., Sanders, J., and Hood, L. J. Mol. Evol. 29:149) describes the isolation and sequencing of the two major shark CNS proteins Po and MBP and their corresponding cDNAs. This study shows that the myelin proteins of the shark brain are similar to the myelin proteins of the mammalian peripheral nervous system in both primary and secondary structures.

The second chapter (Fors, L., Saavedra, R., and Hood, L. Nuc. Acids Res., Submitted) contains a novel genomic walking technique that was developed to clone the shark Po and MBP promoters. Using this technique it was possible to clone approximately 400 nucleotides immediately upstream of the shark Po and MBP transcription initiation sites. This genomic walking technique will be generally useful for cloning promoters or other sequences of interest without the need for constructing or screening genomic libraries.

The third chapter presents and discusses the similarity between these shark Po and MBP promoters, the JC virus enhancer (which directs tissue-specific expression in oligodendrocytes), and the mouse Po and MBP promoters. The implications of these findings on nervous system specific and CNS vs. PNS specific gene expression are discussed.

Lastly, the appendix describes the current status of Shiverer transgenic mouse experiments in which constructs bearing the shark MBP gene are injected into mouse eggs. These transgenic experiments are testing if the structural similarity between shark and mammalian MBPs translates into any measurable functional similarity in vivo.

Thesis Files