Among putative PDEs, the carboxylesterases (CCEs; EC 3.1.1.1) have been the most studied, but few molecular and functional data are still available on antennal CCEs. Through molecular cloning, one antennal esterase gene has been identified in the noctuid moths Mamestra brassicae, Spodoptera littoralis and Sesamia nonagrioides, in the beetle Popilia japonica (PjapPDE) and in the bee Apis mellifera.^5-8 Three CCEs have been cloned in the wild silkmoth Antheraea polyphemus,^9,10 whereas five genes have been identified in the tortricid Epiphyas postvittana by transcriptomic analysis.¹¹ All these CCEs were predicted as extracellular and could thus potentially interact with the odorant molecules within the sensillum lymph, in the vicinity of the ORs. Functional data were only available for two male antennal specific CCEs, ApolPDE from A. polyphemus^9,12 and PjapPDE.⁷ Their rapid kinetics toward pheromones strongly suggests that enzymatic inactivation could play a role in the dynamic of signal termination.

In the moth Spodoptera littoralis, a worldwide crop pest, the sex pheromone blend is a mix of esters, suggesting the involvement of CCEs in pheromone degradation.¹³ To investigate the structural and functional diversity of S. littoralis antennal CCEs, we have developed a transcriptomic approach on male antennae.¹⁴ Phylogenetic analysis of the identified sequences coupled to the study of their tissue-related distribution has allowed us to precise their putative function. We have then selected two CCEs restricted to the antennae to test their catalytic properties toward pheromones but also toward plant components. Indeed, the ability of CCEs to hydrolyze other odorants than pheromones was surprisingly not studied, despite that esters are widespread among odorants emitted by plants.¹⁵

Our results show that the antennae of this species expressed a surprising diversity of CCEs for such a specialized tissue. Twenty CCEs^6,16 were first identified and ten additional sequences were isolated more recently (Maïbèche-Coisne et al.,⁵ pers. com., GenBank acc. numbers HQ122611 to HQ122620), leading to a final number of 30 genes. This is very high, especially when compared with the whole repertoires of CCEs identified in other species through genome analysis (from 24 in the bee to 69 in the silkworm B. mori).^17,18 Phylogenetic analysis revealed that S. littoralis antennal sequences are distributed among the three classes of CCEs already defined.¹⁷ Eight sequences clustered within the first class, which contains predominantly intracellular active enzymes. Most of them are supposed to have digestive or detoxification functions, based on their expression in insect midgut, or have been implicated in insecticide resistance.¹⁹ Two sequences clustered within the third class, which brought together proteins implicated in neuro/developmental functions,²⁰ such as acetylcholinesterases and other non-catalytic proteins. All the others sequences from S. littoralis antennae belong to the second class, which contains mostly secreted and generally catalytically active enzymes. For a few, functional data suggest a role in hormone and pheromone processing.¹⁹ This class includes noteworthy ApolPDE and PjapPDE, as well as Juvenile-Hormone (JH) esterases involved in JH catabolism.

We have functionally characterized two CCEs, SlCXE10 and SlCXE7, belonging to the first and second class, respectively.^21,22 In situ hybridization revealed that these two genes were associated with long and short olfactory sensilla, tuned to pheromones and plant odorants, respectively. We showed that SlCXE10 was able to efficiently hydrolyze a green leaf ester emitted by host plants, but not the pheromone components. The intracellular location of SlCXE10 suggests also that the CCE-based metabolism of odorant is not restricted to the sensillum lymph and could occur within the cells. SlCXE7 is predicted as extracellular and presented different substrate specificities as it was able to efficiently hydrolyze the two sex pheromone components, but also the plant odor. This result demonstrates that a same CCE could have a dual function depending of its sensillar expression: acting as a PDE and reducing plant’s odorant background noise within the pheromone-sensitive sensilla, or acting as an ODE within the sensilla tuned to this plant volatile. Interestingly, for both genes, the transcript levels were induced by male exposure to the odorants, showing that odorant molecules could modulate CCE expression, allowing the insect to adapt its antennal catabolism to the fluctuations of its odorant environment.

Durand N, Carot-Sans G, Bozzolan F, Rosell G, Siaussat D, Debernard S, et al. Degradation of pheromone and plant volatile components by a same Odorant-Degrading Enzyme in the cotton leafworm, Spodoptera littoralis PLoS One 2011 6 e29147 doi: 10.1371/journal.pone.0029147.