In vivo analysis of Drosophila chondroitin sulfate biosynthetic genes

Chondroitin sulfate (CS) is an evolutionarily conserved class of glycosaminoglycans and is found in most animal species. Previous studies of CS-deficient Drosophila models, Chondroitin sulfate synthase (Chsy), and Chondroitin polymerizing factor (Chpf) mutants demonstrated the importance of CS in the structural integrity of the basement membrane and organ shape maintenance. However, biosynthetic mechanisms of Drosophila CS remain to be elucidated. To investigate the CS biosynthesis in Drosophila, we generated mutants for two additional biosynthetic enzyme genes, CS N-acetylgalactosaminyltransferase (Csgalnact) and CS 4-O sulfotransferase (C4st), using CRISPR-Cas9 mutagenesis. Csgalnact-null mutants show moderate changes in CS biosynthesis, including reduced CS in the larval brain and altered CS chain length. We found that this gene is dispensable for normal viability and morphogenesis. On the other hand, C4st mutants show more severe defects, including a high level of lethality and a folded wing phenotype. The C4st mutation not only eliminates CS sulfation but increases production of unsulfated chondroitin, suggesting the existence of a compensatory feedback mechanism. Both Csgalnact and C4st mutants show impaired adult negative geotaxis behavior, consistent with the role of CS proteoglycan in the neuromuscular systems. Our study revealed unique and poorly understood features of invertebrate CS biosynthesis and provides novel in vivo toolsets to investigate CSPG functions in development.