Myelin structure and composition in zebrafish

To establish a standard for genotype/phenotype studies on the myelin of zebrafish (Danio rerio), an organism increasingly popular as a model system for vertebrates, we have initiated a detailed characterization of the structure and biochemical composition of its myelinated central and peripheral nervous system (CNS; PNS) tissues. Myelin periods, determined by X-ray diffraction from whole, unfixed optic and lateral line nerves, were approximately 153 and approximately 162 Angstrom, respectively. In contrast with the lability of PNS myelin in higher vertebrates, zebrafish lateral line nerve myelin exhibited structural stability when exposed to substantial changes in pH and ionic strength. Neither optic nor lateral line nerves showed swelling at the cytoplasmic apposition in CaCl(2)-containing Ringer's solution, in contrast with nerves from other teleost and elasmobranch fishes. Zebrafish optic nerve showed greater stability against changes in NaCl and CaCl(2) than lateral line nerve. The nerves from zebrafish having mutations in the gene for myelin basic protein (mbpAla2Thr and mbpAsp25Val) showed similar myelin periods as the wildtype (WT), but gave approximately 20% less compact myelin. Analysis of proteins by SDS-PAGE and Western blotting identified in both CNS and PNS of WT zebrafish two orthologues of myelin P0 glycoprotein that have been characterized extensively in trout--intermediate protein 1 (24 kDa) and intermediate protein 2 (28 kDa). Treatment with endoglycosidase-F demonstrated a carbohydrate moiety of approximately 7 kDa, which is nearly threefold larger than for higher vertebrates. Thin-layer chromatography for lipids revealed a similar composition as for other teleosts. Taken together, these data will serve as a baseline for detecting changes in the structure and/or amount of myelin resulting from mutations in myelin-related genes or from exogenous, potentially cytotoxic compounds that could affect myelin formation or stability.