Glycosaminoglycan Biosynthesis and Function in Zebrafish Development : Sugars Shaping Skeletons

Abstract: Heparan sulfate (HS) and chondroitin/dermatan sulfate (CS/DS) proteoglycans are glycosylated proteins with important roles in animal development and homeostasis. HS and CS/DS are long, linear glycosaminoglycan (GAG) polysaccharides and attached to a core protein they form proteoglycans. GAGs on proteoglycans are often modified by sulfate groups and mainly found in the extracellular matrix or associated to the cell membrane. They interact with different proteins, for example signaling molecules, and influence developmental processes. Cells in cartilage produce a functionally specialized dense extracellular matrix, full of proteoglycans. Using the zebrafish as a model to study GAG biosynthesis we discovered that HS production is prioritized over CS/DS production, if the availability of link structures is restricted. We also found that the effects of removing HS and CS/DS biosynthetic enzymes in zebrafish larvae typically differ from what could be hypothesized solely from knowledge of the activity of each enzyme. These findings indicated a highly complex regulation of GAG biosynthesis and we thus proceeded to identify novel GAG biosynthetic enzymes in zebrafish and characterized their expression during early development. Notably, strong expression of CS/DS glycosyltransferases was found in cartilage structures, correlating with a drastic increase of CS/DS synthesis after two days of development, and high CS/DS deposition in cartilage. Finally, to understand how different GAG biosynthetic enzymes affect zebrafish development, we decided to use the CRISPR/Cas9 technology to generate new loss of function alleles for enzymes in HS and CS/DS biosynthesis. Some mutants show disturbed larval development or adult morphology, but we found many mutants to develop into adults without major morphological abnormalities, suggesting a high redundancy for GAG biosynthetic enzymes. Many GAG glycosyltransferases and modification enzymes have multiple isoforms, suggesting that a combination of mutations in one individual will become necessary to study the loss of specific modifications. To conclude, the zebrafish model gives new insights into the GAG machinery and the CRSIPR/Cas9 technology allows for swift production of new loss of function zebrafish lines with defective GAG biosynthesis.