The role of Orco in hawkmoth pheromone transduction and in the control of circadian changes in olfactory sensitivity

FG Tierphysiolgie

Project members: Katrin Schröder, Assoc. Prof. Dr. Hülya Altuntaş, Prof. Dr. Monika Stengl

Insect odor transduction is still not resolved, despite the fact that its understanding is a crucial prerequisite to controlling the behaviour of pest, vector, or beneficial insects. Odours from the environment and pheromones (species-specific odours) can be detected via odour receptor neurons (ORNs) on insect antennae. It is well known that insect olfactory receptors (ORs) expressed in ORNs consist of a highly conserved olfactory receptor co-receptor (Orco) subunit and a highly variable ligand-binding OR subunit. Orco locates and maintains ORs in ciliary membranes as prerequisite to odor responses (termed chaperone function). Despite several excellent investigations, it remains unclear how Orco and OR proteins interact with each other during odor transduction.

In vivo tip recordings of pheromone-sensitive moth trichoid sensilla of the hawkmoth Manduca sexta (Sphingidae: Lepidoptera) showed that Orco did not affect the primary pheromone response, thus not taking part in an ionotropic pheromone transduction mechanism. However, it affected the late, long-lasting pheromone response that appears to maintain a memory of the odor stimulus over seconds to minutes (Gawelek and Stengl 2018). The mechanisms of this late activation of Orco are not known. Insect pheromones rapidly and transiently activate a PLC-β-dependent metabotropic transduction cascade, increasing intracellular Ca2+ (Breer et al., 1990; Stengl et al., 1992). In addition, delayed rises in cGMP are triggered (Ziegelberger et al., 1990; Boeckhoff et al., 1994). Thus, it was hypothesized that pheromone-dependently protein kinase C activation causes phosphorylation of Orco. Then, the phorsphorylated Orco might be able to be actiated by delayed rises of cGMP. Since Orco controls spontaneous activity in ORNs (termed pacemaker channel function) in hawkmoths, we are testing our hypothesis of late Orco activation in tip recordings of single pheromone-sensitive sensilla in vivo. Furthermore, ORNs are peripheral circadian pacemaker neurons, expressing the circadian clock gene period. Previous tip recordings showed that cAMP and cGMP can modulate the sensitivity and time course of ORNs during the course of the day (Flecke et al., 2006, 2010), connected to circadian rhythms of the biogenic amine and hormone octopamin (Flecke and Stengl, 2009; Schendzielorz et al., 2012). Thus, we examine whether Orco may be targeted by hormones to express circadian rhythms in spontaneous activity of ORNs. All of our experiments will be performed in the photophase of the adults at different Zeitgebertimes (ZTs) 1-3 and 9-11. The results of this study will provide important information about the signal transduction cascade in insect ORNs. Furthermore, it will provide insights to the role of Orco in odour transduction and in its functions in the control of circadian rhythms of ORNs that are peripheral circadian clocks. The project will be achieved in cooperation with Prof. Dr. Herberg (Department of Biochemistry), as well as with Prof. Dr. Garcia (Department of Theoretical Physics) and Prof. Dr. Stursberg (Department of Electrical Engineering and Systems Control). (Financial support: provided by DFG-grants STE531/20-1,2, SPP 1392; STE531/26-1, SPP 2041 to Monika Stengl, and by “clocks” funded by the University of Kassel).