INVESTIGATING THE ROLE OF CELL SURFACE PROTEINS IN CYTONEME-MEDIATED FGF SIGNALING IN DROSOPHILA

Abstract

Precise coordination of cell signaling in space and time is essential for proper tissue development, homeostasis, and regeneration. This spatiotemporal coordination relies on the regulated delivery of key secreted signaling ligands, such as FGF, Wnt, Hedgehog, BMP, and EGF, to their appropriate target cells. Increasing evidence suggests that spatially coordinated, target-specific signal delivery is mediated by a specialized cellular mechanism, in which cells extend thin, actin-rich, filopodia-like projections called cytonemes. These dynamic structures reach out to establish direct contact with signaling partners over long distances. By physically bridging signal-sending and signal-receiving cells, cytonemes serve as conduits for long-range, contact-dependent transport of signaling molecules and/or their receptors, thereby enabling precise, target-selective ligand-receptor interactions and signal exchange through their synapse-like signaling junctions. Initially identified in Drosophila for BMP signaling, cytonemes have since been implicated in many conserved signaling pathways, including FGF, Dpp/BMP, Wnt, and Hedgehog, across diverse organisms and developmental contexts. However, despite their broad biological relevance, the molecular mechanisms that regulate cytoneme formation and function remain poorly understood. This study investigates the roles of two cell surface proteins, Syndecan, a transmembrane heparan sulfate proteoglycan (HSPG), and the Matrix Metalloproteinase 2 (MMP2), in controlling cytoneme behavior using Drosophila trachea as a genetic model system. Through a combination of genetic, cell biological, and high- resolution imaging approaches, this research thoroughly dissected the roles of Sdc in cytonemes from FGF-receiving tracheal cells that carry FGF-Receptor (FGFR) and receive a GPI-anchored FGF from the surface of the FGF-producing cells. This research uncovered that Sdc is specifically recruited to the FGF-exchanging cytoneme synapse from the pre-synaptic, FGF-receiving cells and promotes cytoneme stability and FGF signaling. This work also provides evidence for the role of Sdc as an organizer of the cytoneme synapse, where it promotes the peri-synaptic recruitment of its interacting partner, Lar, in trans, from the post-synaptic FGF-producing cells. Through this mechanism, Sdc facilitates functional coupling between signal-sending and -receiving cells in an interdependent and spatially coordinated manner. While the above work provides insights into the roles of cell surface Sdc in cytoneme synapse organization, this work also explored the roles of MMP2, a GPI-anchored cell surface Metalloproteinase, in shedding several cytoneme surface proteins, and, thereby, modulating the target-specificity and functions of cytonemes. Altogether, this study provides new insights into how cell surface proteins can regulate cytoneme-mediated target-specific communication during development.