Breaking the MF curse: the regulatory role of the Methyl farnesoate – MEKRE93 pathway in crustacean molting

Abstract

Ecdysis, or the active shedding of the exoskeleton, is critical for arthropod growth, development, and/or regeneration of lost or damaged appendages. The antagonistic interaction between the steroid hormone 20-hydroxyecdysone (20-E) and the sesquiterpenoid juvenile hormone (JH) control insect molting and development, respectively. On the other hand, crustacean molting is primarily regulated through the endocrine crosstalk between 20-E and the neuropeptide molt-inhibiting hormone (MIH). MIH secretion by the X-organ, sinus gland complex (XO) inhibits Y-organ (YO) synthesis of 20-E. Molting is initiated by the decrease in MIH titers thereby allowing increasing 20-E concentrations in the hemolymph. The molting process is divided into different stages where the YO exhibits different phenotypic states: intermolt (IM)– basal, early premolt (EP)– activated, mid premolt (MP)– committed, late premolt (LP)– committed/repressed, ecdysis (E)– repressed, and postmolt (PM)– repressed/basal. The mechanistic target of rapamycin (mTOR) and transforming growth factor beta (TGF-β) signaling leads to YO activation and commitment, respectively. However, the YO transition to- and from- the repressed state is unknown. Methyl farnesoate (MF), commonly referred to as the crustacean JH, is produced by the mandibular organ (MO) and is suppressed by the mandibular organ-inhibiting hormone (MOIH). MF regulates several physiological processes in crustaceans including metamorphosis, development, reproduction, morphogenesis, and molting; however, the underlying mechanism remains unknown and thereby is considered to be "cursed". The differential effects MF has on molting and ecdysteroidogenesis is hypothesized to be regulated through the Methoprene tolerant– Krüppel homolog 1– E93 (MEKRE93) transcriptional cascade. Using bioinformatic approaches, the components of the MF signaling pathway were identified in the European green shore crab (Carcinus maenas) and the blackback land crab (Gecarcinus lateralis) YO transcriptomes, including the MF/JH receptor Methoprene tolerant (Met), the zinc finger transcription factor Krüppel homolog 1 (Kr-h1), Ecdysone response gene 93 (E93), Steroid receptor coactivator (Src), and transcription comediators CREB–binding protein (CBP) and C-terminal–binding protein (CtBP). Additionally, genes encoding for the MF synthetic pathway enzymes were also identified in the YO transcriptomes including 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), farnesoic acid O-methyltransferase (FAMeT), and FAMeT2. Additionally, a FAMeT2 transcript was also identified in the YO and contains an unconventional domain organization compared to annotated FAMeT. Nonetheless, phylogenetic analysis of each gene was overall highly conserved across pancrustaceans (and occaisionally panarthropodans). Furthermore, in vitro assays showed that C. maenas and G. lateralis YOs were responsive to JH-mimics (e.g. pyriproxyfen, fenoxycarb, methoprene, and hydroprene), but not to MF. Taken altogether, these data suggest that the YO can respond to MF and may have its own MF-innate system serving as an autocrine factor to regulate the YO by acting through a MEKRE93 transcriptional network that can be mediated by coregulators.