Fig. 4

The MSE is not active in Schwann cells of gpr126 mutants, but activity is restored during the initiation of myelination after elevating cAMP. (A-F) Lateral views (anterior to the left, dorsal to the top) of zebrafish larvae imaged live at the indicated stage carrying the MSE:EGFP transgene. Heterozygous fish are in the left column and mutants in the right column. White arrows in A, C and D indicate Schwann cells along the PLLn. Arrowheads in A and C indicate Schwann cells along motor nerves. (A) EGFP-positive Schwann cells are present in gpr126^st49/+ fish (n=10/10 transgenic fish). (B) Mutant larvae do not express EGFP in Schwann cells (n=0/6 transgenic fish). (C) Elevating cAMP with a pulse of FSK from 50 to 55 hpf has no effect on MSE:EGFP expression in gpr126^st49/+ fish (n=5/5 transgenic fish). (D) Elevating cAMP rescues the expression of MSE:EGFP in Schwann cells along the PLLn in mutant larvae treated with a FSK pulse from 50 to 55 hpf (n=5/5 transgenic fish). (E) Schwann cells along the PLLn continue to express EGFP at 11 dpf in gpr126^st49/+ control larvae (n=3/3 transgenic fish). (F) Schwann cells along the PLLn continue to express EGFP at 11 dpf in mutant larvae that were transiently treated with FSK from 50 to 55 hpf (n=7/7 transgenic fish). (G,H) Lateral view of the PLLn from a mutant larva that was treated with FSK from 50 to 55 hpf and then fixed and stained for MBP expression at 20 dpf. EGFP expression is nearly undetectable (G) but MBP expression is strong in rescued Schwann cells (H) (n=10). Scale bars: 50 μm.