Hemocyte-secreted papilin bearing mucin-type O-glycans regulates peripodial stalk formation via epidermal JAK/STAT signaling in Drosophila

Protein glycosylation is an essential post-translational modification. In evolutionarily conserved mucin-type <i>O</i>-glycosylation, the most common <i>O</i>-glycan, T antigen, is synthesized by core 1 β1,3-galactosyltransferase 1 (C1GalT1). Loss of <i>C1GalT1</i> leads to developmental defects across organisms. We previously found that <i>Drosophila C1GalT1</i> mutants exhibit malformed legs, but the underlying mechanism was unclear. Here, we identify a glycan-mediated inter-tissue signaling mechanism wherein embryonic hemocytes regulate leg morphogenesis. We show that T antigen-modified Papilin (Ppn), an extracellular matrix (ECM) protein secreted by embryonic hemocytes, suppresses JAK/STAT signaling in the epidermis surrounding Keilin's organ. This repression is essential for proper tubulogenesis of the peripodial stalk anchoring the leg disc and ensuring its correct positioning during development. Disrupted mucin-type <i>O</i>-glycosylation impairs Ppn secretion and causes mislocalized leg discs and morphogenetic defects. These findings identify Ppn carrying mucin-type <i>O</i>-glycan as long-range modulators of epithelial signaling and underscore the role of immune-like cells in coordinating organogenesis via ECM.