Heparan sulfate proteoglycans in signaling and development

Project Summary The long-term goal of our research is to understand fundamental principles of cell communications mediated by heparan sulfate proteoglycans (HSPGs). HSPGs are a special type of carbohydrate-modified proteins that serve as co-receptors for various growth factors, including bone morphogenetic proteins, Wnt/Wingless, and Hedgehog. These HSPG co-receptors play critical roles in quantitative and robust control of signaling output. We study in vivo functions of HSPGs using the Drosophila model. Our earlier research has established critical roles of HSPGs in development, namely in morphogen signaling/gradient formation and stem cell control. Although proteoglycan biology has made significant progress, several major questions remain to be elucidated. For example, the molecular mechanisms of co-receptor activities of HSPGs are poorly understood. It is also unknown how distinct HS structures regulate specific signaling and patterning events. Furthermore, despite extensive studies of HS functions in paracrine signaling, the involvement of HS in inter-organ communications remains to be determined. Our previous studies suggested that HSPGs cooperate with other factors to exert co-receptor activity on the cell surface. Using proteomic and genetic approaches, we recently identified candidate molecules of the HSPG regulators that can be classified into three groups: (1) secreted glypican-binding proteins, (2) transmembrane proteins, and (3) a different class of proteoglycans—chondroitin sulfate proteoglycans (CSPGs). We will study their roles in morphogen signaling and stem cell control. To understand how HS structures affect signaling and morphogenesis, we have developed an "in vitro" model using Drosophila cells. Using these cell lines, we will establish a direct link between detailed structural information of Drosophila HS and a wealth of knowledge on biological phenotypic information obtained over the last two decades using this animal model. Several lines of evidence suggest that Drosophila activins are novel HS-dependent factors. Three activins, Actß, Dawdle, and Myoglianin, regulate body size, sugar homeostasis, metabolism, and pH balance through inter-organ signaling. We will study the functions of HS in trapping the source tissue, systemic transport, recruiting to the target tissue, release from latency, and signal reception of activins. Together, this application will advance our field by: (1) defining fundamental molecular mechanisms of co-receptor function, (2) developing a comprehensive understanding of the structure-function relationship of HS, and (3) identifying novel functions of HSPGs in development and physiology.