C-mannosyltransferase Dpy19l1l regulates body axis formation via secretion of SCO-spondin in zebrafish

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.