GPI-mannosyltransferase 2 shapes the Hedgehog morphogen gradient. Yi-Nan Lee¹, Haiwei Pi², Cheng-Ting Chien¹. 1) Institute of Molecular Biology, Academia Sinica, Taipei 115, Taiwan; 2) Department of Biomedical Sciences, Chang-Gang University, Taoyuan 333, Taiwan.

   Hedgehog (Hh) signaling controls a wide spectrum of developmental processes such as tissue patterning and axon guidance. Hh is concentrated at cell surface by the interaction of lipid moieties with heparin sulfate proteoglypicans (HSPGs) residing in the extracellular matrix. Glypicans are HSPGs with a glycosylphosphatidylinositol (GPI) modification at the C-terminus of core proteins for membrane anchorage. In Drosophila, two functionally interchangeable glypicans Dally and Dally-like (Dlp) are required for Hh gradient formation in wing discs. Signaling-coupled internalization of Hh in a complex with cognate receptor Patched (Ptc) also depends on the function of the membrane attachment of glypicans. However, non-cell autonomous activities of Dally and Dlp have been observed in wing development. In this study, we investigate the functions of GPI anchor on Hh responses and morphogen gradient formation by studying mutants for GPI mannosyltransferase II (GPI-MT2), an enzyme for GPI anchor synthesis. We show that Dlp modification, processing and localization are affected in gpi-mt2 mutants. In genetic mosaic assay, gpi-mt2 is required cell autonomously for the expressions of short and medium ranges of Hh target genes. However, low levels of Hh signaling are unaffected in mutant clones for null gpi-mt2 . In contrast, the responses to low and intermediate levels of Hh signaling are enhanced in gpi-mt2 hypomorphic mutants. In correlation with the long-range effect on Hh signaling activity, both secreted and anchorless forms of Dlp are increased in the larval hemolymph, and Hh levels are increased in the anterior compartment of wing discs. Finally, we show that knockdown of gpi-mt2 in wind discs cause defect in wing development, which can be rescued by the coexpression of secreted and membrane-anchored forms of Dlp. We propose that secreted and anchored Dlp forms play distinct roles in Hh signaling with secreted Dlp in promoting Hh long-range diffusion.