The lipidic purge of pesticides: physiological mechanisms and evolutionary conservation. – LIPIDIC_PURGE

Les hypothèses élaborées sont testées de deux manières complémentaires. Il s’agit d’une part d’observer et de mesurer le paramètre d’intérêt et d’autre part de perturber génétiquement le système investigué et d’étudier alors les conséquences de ces manipulations sur le paramètre ainsi que sur la capacité des drosophiles de survivre à une exposition au fipronil.

Chez la drosophile, nos observations n’ont pas permis de valider notre modèle initial basé sur une perte des lipides prenant place au niveau de l’intestin. Nos résultats nous amènent plutôt à privilégier un effet prenant place directement au niveau du tissu adipeux, sous le contrôle du système nerveux où fonctionne le récepteur ciblé par le fipronil.
Au niveau de cellules intestinales humaines en culture, nos résultats confirment un relargage de lipides dans le milieu de culture suite à une exposition à des doses faibles de fipronil.

NA

NA

The diminution of biodiversity in our environment made the news; it is marked for birds and alarming for insects. Honeybee colony collapse disorder, which mirrors this trend, has multiple causes. Infections have been incriminated, yet, pesticides are the chief suspects. In a previous ANR-funded project, we have reproduced in a model genetic organism, the fruitfly Drosophila melanogaster, a finding made initially in honeybees infected by an intracellular fungal parasite, a microsporidium, also exposed to sublethal concentrations of a pesticide found in the fields as well as in our households, fipronil. Exposed insects succumb earlier to the infection, yet without any increase of the parasitic load. Our studies on microsporidian infections in Drosophila led to the discovery that a critical limiting metabolite for parasite proliferation is phosphatidate and that the parasite exhausts its host's lipidic reserves. Fat store depletion occurs faster when flies are also exposed to low doses of fipronil at the same time; strikingly, exposure to fipronil alone also leads to the exhaustion of lipidic reserves. Thus, the insect succumbs to the depletion of its energy stores caused by the unexpected competition between the parasite and the pesticide for host lipids. Lipids do not get oxidized after exposure to fipronil but are lost at the level of some enterocytes by the extrusion of large lipid globules in a process that evokes lactation in mammals. This purge results from a nervous reflex whereby fipronil indirectly activates neurons that innervate the proximal part of the midgut by inhibiting the ionotropic GABAA receptor. The proximal intestinal epithelium would respond to this stimulus by emitting a signal that would reach the region where the lipidic purge takes place, far away from any innervation. We hypothesize that this proximal signal triggers a transcriptional program in competent enterocytes that leads to the uptake of lipid particles from the hemolymph, the formation of numerous lipid droplets, their apical migration and fusion that precedes their eventual extrusion into the gut lumen. Upon chronic exposure to fipronil, the normal flux of lipids from the gut to the fat body gets reversed, progressively leading to the exhaustion of lipidic stores and the ultimate demise of the host.

Here, we propose to answer three questions:
i) How do neurons communicate with the proximal midgut and what is the ensuing signal that activates the lipid-rich regions of the midgut into purge-mode. One hypothesis is that neurons form an intestinal "synapse" akin to the neuromuscular plate in which neurons directly instruct a specific cell type of the underlying intestinal epithelium. What then is the transcriptional program that drives the purge in the lipid-rich enterocytes?

ii) How does the organism react to this sustained loss of lipids? Is it a passive or an active process driven by a signal emitted by the purging enterocytes? The answer to the last question in i) may very well provide interesting leads in this respect. We shall also extensively rely on a very rich knowledge base on lipid metabolism in Drosophila and use it for a candidate gene approach, which will be complemented by a more systematic approach.

iii) Is the lipidic purge evolutionarily conserved up to mammals? A dual approach will be conducted in cultured intestinal cells on the one hand, and in vivo in mice on the other.

This study will initiate a finely-grained dissection of a paradigm-changing detoxification mechanism that remains mysterious to this day and may also likely be involved in our own health while it threatens the fitness of insects chronically exposed to pesticides.