Figure 1.

Fbxw7 negatively regulates myelination. A–F, Transverse sections of 4 dpf larvae at the level of the trunk spinal cord (sc), with dorsal up, processed for in situ RNA hybridization to detect myelin gene expression. Wild-type larvae express plp1a, cldnk, and mbp in cells (arrows) adjacent to the pial surface of dorsal and ventral spinal cord (A–C). fbxw7 mutant larvae express plp1a and cldnk ectopically (arrowheads; D, E) and cldnk and mbp expression appears elevated (E, F). Scale bar, 10 μm. G, H, Graphs showing relative levels of myelin gene (G) and cholesterol pathway gene (H) transcripts in 4 dpf wild-type (wt) and fbxw7 mutant (m) larvae measured by qPCR. n = 3 biological replicates consisting of 20 larvae for each measurement. *p < 0.05, **p < 0.01, ***p < 0.005, two-tailed Mann–Whitney test. Error bars indicate SEM. I–L, Transverse sections of 7 dpf wild-type and fbxw7 mutant larvae processed to reveal Mbp by immunohistochemistry (I, K) and myelin using Fluoromyelin staining (J, L). Dashed circles outline the spinal cord. Processing was performed in parallel and images acquired using identical exposure settings. In wild-type, myelin proteins are mostly localized to dorsal and ventral longitudinal axon tracts (asterisks). Myelin protein labeling appears brighter in fbxw7 mutant sections than in corresponding wild-type sections. Scale bar, 10 μm. M, N, Electron micrographs of transverse ventral spinal cord sections from 8 dpf wild-type and mutant larvae. Arrows indicate myelinated axons. Scale bar, 1 μm. O, Graph showing average number of myelin membrane wraps ensheathing axons in 8 dpf wild-type and fbxw7 mutant larvae. n = 3 larvae for each genotype. ****p < 0.0001, unpaired two-tailed Student's t test. Error bars indicate SEM. Only axons with a cross-sectional area of 0.401–3.0 μm² were used for this analysis. P, Graph showing the size distribution of wild-type and mutant myelinated axons in ventral spinal cord of 8 dpf larvae.