Differences in retinoid levels and metabolism among gastropod lineages: Imposex-susceptible gastropods lack the ability to store retinoids in the form of retinyl esters

• Until recently, the presence of an elaborated retinoid system was believed to be a chordate/vertebrate novelty.
• Patella depressa has the capacity to store retinoids in the form of inactive retinyl esters.
• The imposex-susceptible species Nucella lapillus and Nassarius reticulatus lacked detectable amounts of retinol or retinyl esters.
• An exposure test with N. lapillus confirmed that this species is not able to synthesize retinyl esters in vivo.
• The incapacity to inactivate retinoid precursors might negatively affect retinoid signaling homeostasis.

The presence of a complex retinoid system was long believed to be a chordate/vertebrate novelty. However, recent findings indicate otherwise since the gastropod mollusk Osilinus lineatus was found to have the capacity to store retinoids in the form of retinyl esters (REs), a key feature to maintain a homeostatic control of retinoid levels. Here, we investigated whether such a complex retinoid system is widely distributed among gastropod lineages. Additionally, since one of the most spectacular examples of endocrine disruption in the wild, the masculinization of female gastropods (imposex) by the retinoid X receptor (RXR) agonist, tributyltin (TBT), has been linked with perturbed retinoid signaling, we also investigated if retinoid storage mechanisms in the form of retinyl esters were present in imposex-susceptible gastropods. Initially, we determined the presence of both polar (active retinoic acid isomers) and nonpolar retinoids (retinol, REs) in selected gastropod species: the limpet Patella depressa and the imposex-susceptible whelks Nucella lapillus and Nassarius reticulatus. Although all species presented active retinoid forms, N. lapillus and N. reticulatus were shown to lack nonpolar retinoids. The absence of REs, which are the common retinoid storage form found in vertebrates and in O. lineatus suggest that those species are unable to use them to maintain a homeostatic control of their retinoid levels. In order to further clarify the retinoid metabolic pathways in imposex-susceptible gastropods, a retinoid exposure study was carried out with N. lapillus. The results demonstrate that although N. lapillus is able to metabolize several retinoid precursors, it lacks the capacity to store retinoids as REs. Whether the lack of retinoid storage mechanisms in the form of REs in imposex-susceptible gastropods plays an important role in the susceptibility to RXR agonists warrants additional studies.