Functional properties of insect olfactory receptors: ionotropic receptors and odorant receptors

Abstract

The majority of insect olfactory receptors belong to two distinct protein families, the ionotropic receptors (IRs), which are related to the ionotropic glutamate receptor family, and the odorant receptors (ORs), which evolved from the gustatory receptor family. Both receptor types assemble to heteromeric ligand-gated cation channels composed of odor-specific receptor proteins and co-receptor proteins. We here present in short the current view on evolution, function, and regulation of IRs and ORs. Special attention is given on how their functional properties can meet the environmental and ecological challenges an insect has to face.

Keywords: Adaptation; Insect olfaction; Ion channel; Ionotropic receptor; Odorant receptor; Olfactory sensory neuron; Sensitization; Signal transduction.

Fig. 1

Functional roles and coding strategies of insect IRs and ORs. a Sensory modalities of ionotropic receptors (IRs) and odorant receptors (ORs). IRs are a multi-modal gene family for the detection of multiple taste qualities, volatile acids and amines, and environmental stimuli as humidity and cooling temperatures (Rimal and Lee 2018). ORs instead are specialized for the detection of a wide variety of volatile and semi-volatile compounds. b Distribution pattern of IRs and ORs in the adult vinegar fly Drosophila melanogaster. Candidate IR taste and pheromone receptors are expressed in the anterior margin of the wing, on abdominal multidendritic neurons, on the tibiae and tarsi of the legs, and on the labellar, labral and cibarial sense organs (Koh et al. ; Sánchez-Alcañiz et al. 2018). The antennal funiculus and the maxillary palps are the two main olfactory organs and they are covered with porous sensilla. The coeloconic sensilla house olfactory neurons expressing mainly IRs, while the basiconic, intermediate, and trichoid sensilla house OR-expressing neurons. The antenna houses two other sensory structures, namely, the sacculus and the arista, that express IRs involved in humidity and cooling sensing (Frank et al. ; Knecht et al. 2017). c IRs and ORs allow insects to detect a wide range of ecologically relevant chemicals through a combinatorial code and a set of labeled lines. In a combinatorial code, each odor is detected by multiple broadly tuned receptors and elicits a unique activation pattern of the antennal lobe glomeruli. Such mechanism allows the fly to exploit a wide array of food sources through the detection of multiple acetate esters produced by yeasts (Mansourian and Stensmyr 2015). Odors detected through labeled lines activate only one tuning receptor and trigger specific innate behaviors (Grosjean et al. ; Min et al. ; Stensmyr et al. 2012)

Fig. 2

Evolution of arthropod ionotropic and odorant receptors. a Emergence of the three main classes of arthropod chemoreceptors: gustatory receptors (GRs), ionotropic receptors (IRs), and odorant receptors (ORs). GRs arose early in the evolution of Metazoa. While being lost in Deuterostomia (Chordata and Echinodermata), they represent the most ancient arthropod chemosensory receptor class between the three. IRs emerged with Protostomia (Croset et al. 2010), while ORs evolved from GRs and they represent the most recent chemosensory class (Robertson et al. 2003). b Conserved antennal IR subfamily gain dynamics. Based on their expression pattern, IRs can be divided in conserved “antennal” IRs and species-specific “divergent” IRs (Croset et al. 2010). Gain of IR gene subfamilies is shown as green triangles. The gain and loss dynamics for the six arthropod antennal IR subfamilies is highlighted in green. Recent results show that none of these families is unique to insects (Eyun et al. 2017). c Evolution of insect ORs. The emergence of ORs pre-dates the evolution of winged insects (Pterygota) and OR genes have been detected in basal insects. The conserved olfactory receptor-coreceptor (Orco) has not been detected in Archaeognatha and may be evolved later or being lost in this Order (Brand et al. ; Thoma et al. 2019). Winged insects show a huge expansion in their number of OR genes (blue triangle)

Fig. 3

Structure-function of insect IRs and ORs. a Schematic representation of insect IRs. The functional unit of IRs is considered to be a heterotetramer made of two tuning receptor (IRX) and two coreceptor subunits (IRcoY) (Abuin et al. ; Abuin et al. 2019). The transmembrane domain (TMD) of both tuning receptor and coreceptor consists of four helices (M1-4). In the closely related AMPA ionotropic glutamate receptor (Croset et al. 2010), the re-entrant portion of the M2 loops forms the ion selectivity filter (Twomey et al. 2017). Both the tuning and coreceptor subunits possess the extracellular ligand-binding domain (LBD), but only the two coreceptors (i.e., Ir25a and Ir8a) possess an amino-terminal domain (ATD) (Croset et al. 2010). b–c Ribbon representation of a tuning (Ir84a) and a coreceptor (Ir8a) IR and characterized function of selected amino acid residues. b A glutamine in position 401 (in brown) in the M2 region of Ir84a is responsible for the Ca²⁺-dependent conductance of the Ir84a/Ir8a channels (Abuin et al. 2011). The LBD of tuning subunits houses the amino acid residues that form the ligand binding pocket and define the response specificity (Prieto-Godino et al. ; Prieto-Godino et al. 2017). In particular, for Ir84a see (Abuin et al. ; Cicconardi et al. 2017). c The LBD of the Ir8a coreceptor instead houses residues involved in the trafficking and correct cellular localization of the IR heteromers such as the coreceptor extracellular loop (CREL, in blue) (Abuin et al. 2019) and residues affecting IR localization to the sensory cilia (in orange) (Abuin et al. 2011). The coreceptor ATD also plays a major role in protein folding, heteromeric protein assembly, and/or cilia targeting (Abuin et al. 2011). Ir8a and Ir84a homology models were created in SWISS-MODEL (Waterhouse et al. 2018) with the R. norvegicus GluA2 structure (PDB: 6DLZ) (Twomey et al. 2018) as template following (Abuin et al. 2019). d Schematic representation of the 7 transmembrane-domain insect ORs. The functional unit of ORs is formed by heteromers made of a tuning OR and a coreceptor named Orco. e Top view ribbon representation of the tetrameric cryo-EM structure of Orco from the parasitic wasp Apocrypta bakeri (Butterwick et al. 2018). Highlighted are the binding pocket (dashed cicles), the anchor domain regions (star symbols), and the lateral conducts stemming from of the channel pore (dotted lines). f Functionally relevant residues of insect ORs, identified through mutagenesis studies, mapped on a Drosophila melanogaster Orco monomer. Critical residues are involved in the ligand binding and selectivity of ORs (in violet) (Corcoran et al. ; Hopf et al. 2015), a putative calmodulin binding site with modulatory function (Mukunda et al. 2014), K⁺ selectivity filter (in brown) (Wicher et al. 2008), and ion channel function (in green) (Hopf et al. 2015). The ribbon representation of D. melanogaster Orco was modeled on the A. bakeri cryo-EM structure using the I-TASSER server (Yang and Zhang 2015) and optimized using FoldX (Schymkowitz et al. 2005)

Fig. 4

Function and regulation of IRs and ORs. a IRs are ligand-gated ion channels mainly permeable to monovalent cations (Na⁺, K⁺). Ca²⁺ conductance in, e.g., Ir84a depends on the Q401 residue in the channel M2 region described in Fig. 3. IR channels are activated by long-lasting stimuli, and their activity is not modulated by sensitization or adaptation mechanisms (Getahun et al. 2012). b–c ORs are nonselective cation channels modulated by intracellular signaling cascades. Activation of these cascades depends on the stimulus strength and may lead to receptor sensitization (b) or adaptation (c). b Activation of a tuning OrX can produce an increase in intracellular cAMP levels in a Ca²⁺-dependent and Ca²⁺-independent manner, leading to the activation of adenylyl cyclases (AC) (Miazzi et al. 2016). Upon phosphorylation of Orco via protein kinase C (PKC), cAMP can increase the activity of OR channels (bold arrow) when repeatedly exposed to sub-threshold stimuli (Getahun et al. ; Getahun et al. ; Sargsyan et al. ; Wicher et al. 2008). Receptor sensitization requires also calmodulin (CaM), as this process can be suppressed via CaM pharmacological inhibition (Mukunda et al. 2016). c A single Orco residue, S289, governs OR adaptation and desensitization. When Ca²⁺-dependent phosphatases (PA) dephosphorylate Orco at position S298, the sensitivity of ORs to their agonist is diminished (light arrow) (Guo et al. ; Guo and Smith 2017)

Fig. 5

Additional players in the modulation of insect olfactory receptors response. Several mechanisms add layers of complexity to the modulation of insect olfactory receptors. a Olfactory sensory neurons (OSNs) housed within the same sensillum can differ in their morphology and influence each other’s activity by means of ephaptic interactions (Zhang et al. 2019). b Modulation of olfactory receptors’ activity at the interface between the extracellular sensillum lymph and the cytoplasm. OR-ligand interactions are influenced by odor- and pheromone-binding proteins (OBPs and PBPs) and odor- or pheromone-degradating enzymes (ODEs, PDEs) (Larter et al. ; Leal ; Xiao et al. 2019). In addition, lipid-derived pheromones can require additional membrane proteins for efficient detection, such as SNMP1 (Gomez-Diaz et al. 2016). Activation of ORs and a subset of IRs, e.g., IR84a, increases the intracellular Ca²⁺ concentration ([Ca²⁺]_i). In Or47b- or Ir84a-expressing OSNs this [Ca²⁺]_i increase can sensitize the OSN in a CaM-dependent way through the DEG/ENaC channel PPK25 (Ng et al. 2019). Cytoplasmic Ca²⁺ can be sequestered in the mitochondria through the mitochondrial calcium uniporter (mCU), while Ca²⁺ can be release from this organelle through the mitochondrial permeability transition pore (mPTP) (Lucke et al. 2020). Moreover, cytoplasmic Ca²⁺ can be extruded through the Na⁺/Ca²⁺ exchanger (CALX) (Halty-deLeon et al. 2018). OR function and/or intracellular trafficking is also affected by other proteins, such as the ATP8B flippase, that is involved in maintaining the phospholipid asymmetry of the plasma membrane (Ha et al. ; Liu et al. 2014). c The endoplasmic reticulum (ER) instead plays a role in the adaptation after long-lasting stimuli. After an odor response, the opening of voltage-gated Ca²⁺ channels (VGCCs) at the presynaptic terminus can lead to the opening of ryanodine receptors (RyRs) and trigger a Ca²⁺-induced Ca²⁺ release (CICR). The resulting release of acetylcholine (ACh) stimulates the projection neuron's (PN) ACh receptors (AChRs) and may also activate—directly or indirectly—the release of GABA from associated local interneurons (LN). GABA release can in turn activate the OSN inositol 1,4,5-triphosphate receptors (IP₃Rs) via the phospholipase C (PLC) pathway activated by GABAB receptors. The resulting Ca²⁺ release from the ER can activate RyRs and lead to an additional amplification of the signal through CICR (Murmu et al. , 2011)