Receptor-regulated smads (R-Smads) in the liver fluke Fasciola gigantica: Characterization, comparative sequence analysis, and life stage-specific expression

The transforming growth factor-beta (TGF-β) signaling pathway is a key conserved developmental pathway, demonstrating remarkable conservation in its components, structures, and functions across diverse animal species. This study presents a comprehensive characterization and comparative analysis of receptor-regulated Smads (R-Smads) in Fasciola gigantica, a major parasite affecting cattle, buffalo, and ruminants in tropical regions. Four R-Smads were identified: FgSmad1/5, FgSmad2, FgSmad3, and FgSmad8. Full-length cDNAs for these R-Smads, isolated using the rapid amplification of cDNA ends (RACE) approach, were 3348, 3010, 5172, and 1991 bp in length, encoding proteins of 464, 506, 678, and 404 amino acids, respectively. FgSmad1/5 and FgSmad8 were classified as bone morphogenetic protein (BMP)-specific R-Smads (BR-Smads) due to their conserved signature motifs and similarity to those found in parasitic platyhelminths and mammalian hosts. Conversely, FgSmad2 and FgSmad3 were identified as TGF-β/activin-specific R-Smads (AR-Smads), displaying a distinctive pattern of conserved motifs commonly observed among trematode parasites. Notably, FgSmad2 exhibited an atypical feature for R-Smads, as it lacked the MH1 domain. The expressions of F. gigantica R-Smad genes were observed across all developmental stages, with the highest levels of most R-Smads occurring in the unembryonated egg stage (primarily BR-Smads) and the metacercaria stage (predominantly AR-Smads), underscoring a significant shift in TGF-β and BMP signaling dynamics during development. These findings greatly advance our understanding of R-Smad proteins within the TGF-β signaling pathway of F. gigantica and provide a solid foundation for future research into their roles and regulatory mechanisms.