Glycosaminoglycans in FGF/FGFR Signaling

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DESCRIPTION (provided by applicant): Fibroblast growth factor (FGF) signaling is contingent on the formation of a ternary complex with the FGF receptor (FGFR) and with specific structures of glycosaminoglycan (GAG) chains. Genetic defects in FGFs, FGFRs or in GAG biosynthesis have been shown to be lethal or result in overgrowth, dwarfism, and other syndromes. Animals with defects in GAG biosynthesis show a complete loss of FGF/FGFR signaling pathways. We recently discovered that chondroitin sulfate could activate FGF/FGFR signaling in a fashion similar to heparan sulfate. Both heparan sulfate and chondroitin sulfate are highly sulfated linear GAG chains abundantly expressed on the cell surface and in the extracellular matrix in the forms of heparan sulfate, chondroitin sulfate, or heparan/chondroitin sulfate hybrid proteoglycans. The FGF/FGFR/GAG interactions depend to a large extent on the specific GAG sequences. GAG sequences are not directly encoded by genes, but are put together in the Golgi by enzymes encoded by over 40 genes. This generates a huge variety of GAG sequences. The overall goal of the current grant proposal is to understand how GAGs coordinately regulate FGF/FGFR signaling at the molecular level. In the first aim, we will dissect the roles of heparan and chondroitin sulfate in FGF/FGFR signaling in a defined cellular system, The central hypothesis of this aim is that specific heparan and chondroitin sulfate sequences regulate distinct aspects of FGF/FGFR signaling. Since most animal cells make both heparan and chondroitin sulfate, our goal is to prepare a comprehensive library of CHO cell lines that express defined heparan or chondroitin sulfate sequences. This library will be a unique tool for establishing GAG structure-function relationships in FGF/FGFR signaling. In the second aim, we will clone the long sought epimerase of chondroitin sulfate biosynthesis and use it as a tool to rebuild chondroitin sulfate structures that activate FGF/FGFR signaling. In the third aim, we will determine by mass spectrometry the fine structures of GAGs in growth plates that are involved in FGF/FGFR signaling. The central hypothesis of this aim is that specific heparan and chondroitin sulfate structures function to enhance or suppress FGF signals at different biological sites within the growth plate. The goal of this aim is to discover such GAG structures. The long-term outcome of this work will contribute to a better understanding of diseases caused by disordered GAG-dependant FGF/FGFR signaling. The work may suggest novel therapeutic applications of GAG biology.