Evolution of Vertebrate Myelin-Associated Proteins: Insights from a Basal Vertebrate (Sea Lamprey - Petromyzon marinus)

Sea lampreys (Petromyzon marinus) are basal vertebrates that diverged from our own lineage approximately 500 million years ago. Sequencing of the lamprey genome revealed homologs of a large number of genes that are known to function in the formation and maintenance of compact myelin. In humans, disorders that cause instability or variations in expression levels of these genes cause deterioration of myelin resulting in an array of afflictions, including Charcot-Marie-Tooth and Dejerine-Sottas diseases. Although the lamprey genome possesses most genes necessary for myelination, lamprey lack both of the cell types that produce myelin [oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS)], raising fundamental questions as to the function of “myelination” genes in the lamprey and ancestral vertebrate lineages.

To shed additional light on the function of these proteins within lamprey, we performed rtPCR on RNAs that were derived multiple somatic tissues and in situ hybridization on tissues and developing embryos. Typically, expression of these proteins is localized to the CNS and PNS in jawed vertebrates. However, lamprey homologs do not exhibit the same patterns of localized expression. Some of these genes are expressed in both CNS and PNS, or across several tissues (including non-neuronal tissues: blood). Notably, expression of the CNP gene in lamprey appears to be localized to the PNS, contrasting the pattern of CNS-specific expression typically observed among jawed vertebrates. These analyses suggest that most CNS/PNS-associated expression patterns evolved uniquely within the jawed vertebrate lineage.