Promoteur de thèse: Monsieur Igor Eeckhaut (UMONS)
Co-promoteur: Mme Magali Deleu (ULiège)

Résumé de la dissertation :
Triterpenoids are a group of organic chemicals that consist of over 100 distinct backbones
including sterols and saponins (both triterpenoidal-; and steroidal-glycosides). Saponins includes
a variety of specialized defensive, membranolytic metabolites found mainly in plants but also in
two classes of Echinoderms; holothuroids (sea cucumbers) and asteroids (sea stars). Saponins
are widely studied for their pharmacological properties, attributed to their ability to complex
plasma membrane cholesterol, and the formation of pores. However, paradoxically, how sea
cucumbers tolerate this membranolytic toxin, the potential biological roles (other than defensive),
the biosynthesis and evolution of these highly intriguing molecules in their host sea cucumbers
remains enigmatic. Here after we explore these fundamental questions focused around the
unique saponic system of sea cucumbers.
Sea cucumbers are thought to produce Δ7 sterols instead of Δ5 cholesterol, in order to have
saponin tolerant plasma membranes. The interactions between holothuroid-like Δ7 sterols and
cholesterol with the holothuroid saponin Frondoside A was explored through the use of
complementary biophysical experiments. The saponin caused significantly less permeabilization
in liposomes containing the holothuroid sterol than those containing cholesterol and resulted in
endothermic interactions versus exothermic interactions with cholesterol containing liposomes.
Lipid phase simulations revealed that Frondoside A has an agglomerating effect on cholesterol
domains, however, induced small irregular Δ7 sterol clusters. These findings suggest that sterols
and saponins have co-evolved in sea cucumbers.
This alternate adaptation of a fundamental biological building block (membrane sterols), suggests
that having saponins has given a significant evolutionary benefit to sea cucumbers, perhaps more
than simple defensive toxin. The high diversity of saponins found in the surrounding water around
sea cucumbers further supports the hypothesis that saponin mixtures of holothuroids may play
multiple biological roles for them. Statistically significant aggregation was observed in the spatial
distribution of the aqua cultivated sea cucumber, Holothuria scabra, both as adults in large sea
pens, and as juveniles in aquaria. The role of olfaction and the mechanisms of chemical
communication in this behavior was investigated using olfactory experimental assays. In
complementation with comparative mass spectrometry a distinct saponin profile/mixture was
found to function as a sea cucumber pheromone. Saponins are therefore not simple defensive
toxins, but a complex mixture of semiochemicals used to communicate with hetero- and
conspecifics.
These findings suggest that the diversification of saponins and the co-evolution of saponins and
sterols in sea cucumbers has been essential to the success of these macroinvertebrates in the
benthic marine ecosystems. The common biosynthetic trunk of sterols and saponins from 2,3-
oxidosqualene although well described in plants, remains poorly investigated in sea cucumbers.
The complex composition of free and conjugated sterols of holothuroid tissue, was concluded to
be exclusively due de novo biosynthesis (no dietary sequestration). This composition was
integrated with the identification of orthologue and paralogue genes of the biosynthetic pathway.
Missing enzymes, and highly divergent sequences suggest a highly distinct triterpenoid pathway
in sea cucumbers. The functional expression of the first enzyme in the biosynthesis of sea
cucumber triterpenoids, Oxidosqualene Cyclase, revealed the unique production of parkeol in sea
cucumbers, and not lanosterol, as found in all other animals. Further suggesting a unique
enzymatic adaptation for the biosynthesis of both sterols and saponins.
In summary, this PhD brings new light to the biological roles and benefits of the diversification of
saponins, and suggests a role of saponins well beyond that of a simple toxin. It also highlights for
the first time the complexity of the saponin-sterol system of sea cucumbers, and the enzymatic
machinery responsible for this unique co-evolution between toxin (saponins) and toxicity
mediators (sterols).

La défense publique de la thèse de Madame Emily Claereboudt aura lieu le 30 avril 2021 à 9h par vidéo-conférence, le lien actif sera publié la veille sur ce site.

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