Identification and characterization of the kynurenine pathway in the pond snail Lymnaea stagnalis

Dysregulation of the kynurenine pathway (KP) is implicated in many human diseases and disorders, from immunological, metabolic, neurodegenerative, and neuropsychiatric conditions to cancer, and represents an appealing target for new therapeutic approaches. In this intricate scenario, invertebrates, like Lymnaea stagnalis (LS), provide a flexible tool to unravel the complexity of the KP. Starting from the available LS genome and transcriptome, we identified putative transcripts of all KP enzymes containing an ORF; each predicted protein possessed a high degree of sequence conservation to known orthologues of other invertebrate and vertebrate model organisms. Sequences were confirmed by qualitative PCR and sequencing. At the same time, the qRT-PCR analysis revealed that Lym IDO-like, Lym TDO-like, Lym AFMID-like, Lym KMO-like, Lym AADAT-like, Lym KYAT I/III-like, Lym KYNU-like, Lym HAAO-like, and Lym ACMSD-like showed widespread tissue expression. Then, tryptophan, kynurenine, kynurenic acid, anthranilic acid, 3-hydroxy-kynurenine, xanthurenic acid, picolinic acid, and quinolinic acid were identified in the hemolymph of LS by UHPLC-Q exactive mass spectrometer. Our study provides the most thorough characterization to date of the KP in an invertebrate model, supporting the value of LS for future functional studies of this pathway at the cellular, synaptic, and behavioral levels.