C-mannosylation is an evolutionarily conserved but poorly understood glycosylation process, particularly regarding its physiological roles and pathological relevance. Here, we propose that C-mannosylation plays a key role in maintaining body axis straightness during zebrafish embryogenesis. Using Drosophila S2 cells and mass spectrometry, we show that zebrafish Dpy19-like 1, like (Dpy19l1l) and Dpy19-like 3 (Dpy19l3) catalyze substrate-specific C-mannosylation. Knockout of dpy19l1l resulted in a scoliosis-like “curly tail down” phenotype, whereas dpy19l3 knockout showed no obvious abnormalities. Based on its consensus sequence and biological function, we identified the giant extracellular glycoprotein SCO-spondin as a candidate substrate of Dpy19l1l and confirmed its C-mannosylation by using S2 cells. Live imaging of transgenic zebrafish expressing GFP-tagged SCO-spondin revealed that in dpy19l1l mutants, SCO-spondin fails to be secreted into the cerebrospinal fluid and accumulates at the flexural organ and floor plate, resulting in the loss of the Reissner fiber. These findings uncover a novel in vivo function of C-mannosylation and provide new insights into the molecular pathogenesis of scoliosis.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.