Functional evolution of thyrotropin-releasing hormone neuropeptides: Insights from an echinoderm

Feeding behavior is regulated by a complex network of endogenous neuropeptides. In chordates, this role is suggested to be under the control of diverse factors including thyrotropin-releasing hormone (TRH). However, whether this regulatory activity of TRH is functionally conserved in non-chordate metazoans, and to what extent this process is underpinned by interactions of TRH with other neuropeptides such as cholecystokinin (CCK, known as a satiety signal), remain unclear. This study investigated the TRH signaling system in the echinoderm Apostichopus japonicus. Bioinformatic analyses and ligand-binding assays identified a functional TRH receptor (AjTRHR) that activated signaling via the MAPK/ERK1/2 pathways. Experimental administration of TRH significantly reduced feeding activity, while up-regulating CCK expression. RNA interference (RNAi) experiments confirmed that both CCK and TRH are essential components of satiety signaling, working synergistically to mediate feeding inhibition. Evolutionary analysis of TRH-type peptides revealed greater conservation of the short isoform of TRH compared to the long isoform, probably driven by strong selection acting on the functional redundancy. These findings provide compelling evidence of a TRH-mediated signaling system in non-chordate deuterostomes, expanding our understanding of neuropeptide-regulated feeding mechanisms in marine invertebrates.