Identification of neuropeptide networks involved in the ecdysis program of a crustacean model: Carcinus maenas reveal similarities and differences to insects that reflect evolutionary divergence in structure and function

BACKGROUND: Arthropods require periodic molting (ecdysis) for growth. While the neuroendocrine orchestration of ecdysis is well characterized in insects, it remains comparatively poorly understood in crustaceans. In insects, ecdysis-triggering hormone (ETH) from epitracheal cells and eclosion hormone (EH) from the brain initiate and coordinate ecdysis. ETH triggers pre-ecdysis behaviors, while EH amplifies ETH release and promotes neuropeptide secretion of crustacean cardioactive peptide (CCAP), myoinhibitory peptide (MIP) for exuviation. Definitive evidence for functional ETH and EH in crustaceans is lacking. Here, we investigate ETH and EH in the crab Carcinus maenas by functionally characterizing the ETH receptor, comprehensively mapping transcript and peptide localization of ETH and EH, and quantifying ETH neurohormone and transcript throughout the molt cycle. RESULTS: We identified a single CamETH GPCR with high specificity for arthropod ETHs, signaling via calcium and cAMP. ETHR mRNA is expressed in multiple tissues, but ETH is restricted to the central nervous system (CNS) with a complex neuroarchitecture in pericardial organs, adjacent to the branchiocardiac veins. EH is limited to CNS, notably in the eyestalk ganglia. Of note, complex EH immunopositive fibers in the abdominal ganglion, adjacent to CCAP/allatostatin-C (AST-C)/bursicon cells, represent a novel putative neurohemal site. Measurements of ETH mRNA and peptide through the molt cycle showed that transcription peaks in late premolt, while peptide is released during active ecdysis, later than observed in insects. CONCLUSIONS: While crustacean and insect ecdysis share neuroendocrine components, reflecting common ancestry, there is clear divergence in terms of functionality. We propose a new crustacean ecdysis cascade: ETH released from pericardial organs may stimulate EH to initiate CCAP/AST-C and bursicon release from adjacent neurons.