1.A.69 The Heteromeric Odorant Receptor Channel (HORC) Family
In insects, each olfactory sensory neuron expresses between one and three ligand-binding members of the olfactory receptor (OR) gene family, along with the highly conserved and broadly expressed Or83b co-receptor. The functional insect OR consists of a heteromeric complex of unknown stoichiometry but comprising at least one variable odorant-binding subunit and one constant Or83b family subunit. Insect ORs lack homology to G-protein-coupled chemosensory receptors in vertebrates and possess a distinct seven-transmembrane topology with the amino terminus located intracellularly. Sato et al. (2008) and Touhara (2009) showed that heteromeric insect ORs comprise a new class of ligand-activated non-selective cation channels. Heterologous cells expressing silkmoth, fruitfly or mosquito heteromeric OR complexes show extracellular Ca2+ influx and cation-non-selective ion conductance on stimulation with odorant or pheromone. Odour-evoked OR currents are independent of known G-protein-coupled second messenger pathways. The fast response kinetics and OR-subunit-dependent K+ ion selectivity of the insect OR complex support the hypothesis that the complex between OR and Or83b itself confers channel activity. The ligand (odorant)-gated ion channels formed by an insect OR complex seem to be the basis for a unique strategy that insects have acquired to respond to the olfactory environment (Sato et al., 2008).
Insect odorant receptors are composed of conventional odorant receptors (for example, Or22a), dimerized with a ubiquitously expressed chaperone protein, such as Or83b in Drosophila. Or83b has a structure akin to GPCRs, but has an inverted orientation in the plasma membrane. However, G proteins are expressed in insect olfactory receptor neurons, and olfactory perception is modified by mutations affecting the cAMP transduction pathway. Application of odorants to mammalian cells co-expressing Or22a and Or83b results in non-selective cation currents activated by means of ionotropic and metabolotropic pathways, and a subsequent increase in the intracellular Ca2+ concentration (Wicher et al., 2008). Expression of Or83b alone leads to functional ion channels not directly responding to odorants, but being directly activated by intracellular cAMP or cGMP. Insect odorant receptors thus form ligand-gated channels as well as complexes of odorant-sensing units and cyclic-nucleotide-activated non-selective cation channels. They, thereby, provide rapid and transient as well as sensitive and prolonged odorant signalling (Wicher et al., 2008).
ORs have been identified from four insect orders (Coleoptera, Lepidoptera, Diptera, and Hymenoptera). Although all ORs share the same G-protein coupled receptor structure with seven transmembrane domains, they present poor sequence homologies within and between species. D. melanogaster is the only insect species where Ors have been extensively studied from expression pattern establishment to functional investigations (Jacquin-Joly and Merlin, 2004). One OR type is selectively expressed in a subtype of olfactory receptor neurons, and one olfactory neuron expresses only one type of OR. In addition, all olfactory neurons expressing one OR type converge to the same glomerulus in the antennal lobe. The olfactory mechanism, thus, appears to be conserved between insects and vertebrates (Jacquin-Joly and Merlin, 2004).
After the discovery of the complete repertoire of D. melanogaster Olfactory Receptors (ORs), candidate ORs have been identified from at least 12 insect species from four orders (Coleoptera, Lepidoptera, Diptera, and Hymenoptera). Although all ORs share the same G-protein coupled receptor structure with seven TMSs, they share poor sequence identity. One OR type is selectively expressed in a subtype of olfactory receptor neurons, and one olfactory neuron expresses only one type of OR. The olfactory mechanism, further, appears to be conserved between insects and vertebrates. The C-terminal region (TMSs4-7) of OR83b is involved in homodimer and heterodimer formation (with OR22a) which suggests why the C-termini of insect ORs are highly conserved. There may be two possible ion channel pathways, one formed by the TMS4-5 region with the intracellular pore-forming domain and the other formed by TM5-6 with the extracellular pore forming domain. Odorant receptors generally comprise the obligate co-receptor, Orco, and one of a family of highly divergent odorant 'tuning' receptors. The two subunits are thought to come together at some as-yet unknown stoichiometry to form a functional complex that is capable of both ionotropic and metabotropic signalling. Segments and residues involved in this interaction have been identified (Carraher et al. 2015).
The generalized reaction catalyzed by HORC is:
cations (in) cations (out)
Heteromeric odorant receptor, OR (Sato et al., 2008). OR22a senses fruit-derived esters. These olfactory receptors may have 3-d structures resembling animal rhodopsins, human citronellic terpenoid receptors, OR1A1 and OA1A2 and the mouse eugenol receptor, OR-EG (Ramdya and Benton, 2010). Molecular modelling of oligomeric states of DmOR83b has been reported (Harini and Sowdhamini, 2012). Recombinant receptor together with the co-receptor, Orco, has been overproduced, purified and reconstituted in a lipid bilayer (Carraher et al. 2013). Orco (Or83b) forms a dimer that is fully functional for Ca2+ transport, is regulated by calmodulin and interacts normally with Or22a. The native Orco is therefore probably a dimer (Mukunda et al. 2014).
Heterometic odorant receptor (OR) of Drosophila melanogaster:
Odorant receptor, OR2 (Carraher et al., 2012).
OR2 of Anopheles gambiae (Q8WTE6)
Odorant receptor 56a. Mediates aversive responses to harmful microbial (bacterial and fungal) products such as geosmin (trans-1,10-dimetnyl-trans-9-decalol). (Stensmyr et al. 2012).
OR56a of Drosophila melanogaster
Ordorant receptor 67b of 421 aas and 8 TMSs (Identical to Or67b of D. melanogaster)
Or67b of Drosophila simulans
Odorant receptor 10b of 406 aas and 7 TMSs
Or10b of Drosophila melanogaster
The insect heteromeric CO2 receptor: GR21a (Olfactory receptor 21a; 454 aas) GR63a (Olfactory receptor 63a; 512 aas) are coexpressed in antennal neurons of insects and together comprise the peripheral sensory receptor for CO2 (Ramdya and Benton, 2010). These proteins are members of the 7Tm-7 superfamily of putative 7TMS proteins.
The gustatory receptor for CO2, GR21a/GRG3a of Drosophila melanogaster
Fructose-regulated Ca2+/cation channel, Gustatory (fructose) receptor-9, Gr9 (Sato et al., 2011). Gr9 is widely expressed in the central nervous system (CNS), as well as oral sensory organs and is involved in the promotion of feeding behaviors (Mang et al. 2016).
GR-9 of Bombyx mori (B3GTD7)
Gustatory receptor 43a isoform A. Functions as a narrowly tuned fructose receptor in taste neurons (Miyamoto et al. 2012), being both necessary and sufficient to sense hemolymph fructose.
GR43a of Drosophila melanogaster (Q9V4K2)
Gustatory receptor 28b isoform D
GR28b of Drosophila melanogaster (Q9VM08)
Gustatory receptor 2a isoform B
GR2a of Drosophila melanogaster (Q9W594)
High energy light unresponsive protein 1, Lite1; chemoreceptor GUR-2 of 439 aas nad 8 TMSs.
GUR-2 of Caenorhabditis elegans
The pheromone receptor, Or-1 (Nakagawa et al., 2012)
Or-1 of Bombyx mori (Q5WA61)
Sex pheromone receptor of 416 aas and 7 TMSs (Miura et al. 2010).
pheromone receptor of Ostrinia nubilalis (European corn borer) (Pyralis nubilalis)
Odorant receptor 3, Or3 of 410 aas and 7 TMSs.
Or3 of Epiphyas postvittana (Light brown apple moth)
Odorant receptor 85b (or85b) of 302 aas and 5 putative TMSs. Binds the odorant, heptanone, for activation; 2-nananone is a competitive antagonist. The second half of TMS3 is involved in odorant binding and activation (Nichols and Luetje 2010).
animals (Invertebrates; insects)
Or85b of Drosophila melanogaster
Odorant receptor 22 of 312 aas and 6 TMSs
Or22 of Bombyx mori
Odorant receptor 17 of 401 aas and 8 TMSs
Or17 of Bombyx mori (Silk moth)
Odorant recpetor 278 if 385 aas and 8 TMSs
Or278 of Tribolium castaneum (Red flour beetle)
Odorant receptor 205 of 406 aas and 9 putative TMSs.
Or205 of Tribolium castaneum (Red flour beetle)