The Structure and Function of Ionotropic Receptors in Drosophila

Abstract

Ionotropic receptors (IRs) are a highly divergent subfamily of ionotropic glutamate receptors (iGluR) and are conserved across Protostomia, a major branch of the animal kingdom that encompasses both Ecdysozoa and Lophothrochozoa. They are broadly expressed in peripheral sensory systems, concentrated in sensory dendrites, and function in chemosensation, thermosensation, and hygrosensation. As iGluRs, four IR subunits form a functional ion channel to detect environmental stimuli. Most IR receptors comprise individual stimulus-specific tuning receptors and one or two broadly expressed coreceptors. This review summarizes the discoveries of the structure of IR complexes and the expression and function of each IR, as well as discusses the future direction for IR studies.

Keywords: Drosophila; chemosensation; gustation; hygrosensation; ionotropic receptors; olfaction; thermosensation.

Figure 1

Schematic of the IR coreceptor and tuning receptor. ATD, the amino-terminal domain; LBD, the ligand-binding domain; S1 and S2, two half-domains of LBD that form a “Venus flytrap” structure; CREL, the coreceptor extra loop; TMD, the transmembrane domain; Pore, the ion channel pore region; CTD, the carboxy-terminal domain.

Figure 2

The adult fly head to show the olfactory organs (the third antennal segment and maxillary palp), gustatory organs [the labellar taste sensillum, labellar taste peg, labial sense organ (LSO), ventral cibarial sense organ (VCSO), and dorsal cibarial sense organ (DCSO)], thermosensory organ (the arista), and hygrosensory organ (the sacculus).

Figure 3

The larval anterior part to show the chemosensory organs, including the terminal organ ganglion (TOG), dorsal organ ganglion (DOG), ventral organ ganglion (VOG), dorsal pharyngeal sensilla (DPS), ventral pharyngeal sensilla (VPS), posterior pharyngeal sensilla (PPS), and dorsal pharyngeal organ (DPO). Larval thermosensitive cells are housed in the DOG. Upper panel: top view; lower panel: side view. The orientation is shown by double headed arrows. A, anterior; P, posterior; V, ventral; D, dorsal; L, lateral.