[Characterization of Class F G protein - coupled receptors and functional analysis of Smoothened receptor in Schistosoma japonicum].

OBJECTIVE: To characterize Schistosoma japonicum Class F G protein-coupled receptors (GPCRs), and to decipher the biological function of S. japonicum Smoothened receptor (SjSMO), so as to provide insights into screening of novel anti-schistosomal drug targets.

METHODS: Six Class F GPCRs were identified from the S. japonicum GPCR library using a hidden Markov model (HMM) and genome annotation filese, including five frizzled (FZD) receptors (SjFZD4, SjFZD5, SjFZD7, SjFZD8, SjFZD10) and one Smoothened receptor (SjSMO). Protein domain analysis was performed at the SMART website, and a phylogenetic tree was built with the software MEGA 11.0 and refined with the Interactive Tree Of Life (iTOL) tool. The expression profile and tissue localization of S. japonicum class F GPCRs were analyzed using the real-time fluorescence-based quantitative PCR (RT-qPCR) assay and in situ hybridization, and the effects of SjSMO on parasite growth, development, reproduction, and oviposition were examined using in vivo and in vitro RNA interference (RNAi). S. japonicum adult worms were selected for double-stranded RNAi (dsRNA) in in vitro experiments, and a green fluorescent protein (GFP) negative control group and SjFZD4, SjFZD5, SjFZD7, SjFZD8, SjFZD10, and SjSMO dsRNAi groups were assigned to observe the parasite vitality and state and calculate the female and male rating rate. For in-vivo animal experiments, 6-week-old SPF female mice of the Kunming strain were randomly divided into a GFP dsRNA negative control group and SjFZD4, SjFZD5, SjFZD7, SjFZD8, SjFZD10, and SjSMO dsRNAi groups. Worms and liver tissues were collected for measurement of the worm length and estimation of the worm burdens, and changes in the parasite reproductive organs were observed using alum carmine staining. In addition, the number of S. japonicum eggs in mouse livers was counted and the pathological damages of mouse livers were recorded.

RESULTS: The five FZD receptors and one Smoothened receptor all contained a conserved Frizzled domain. Phylogenetic analysis revealed that SjSMO formed an independent evolutionary clade, which exhibited a far genetic distance from the FZD family. RT-qPCR assay quantified stable SjSMO expression in male worms but gradually decreased in female worms with worm development, and in situ hybridization revealed high systemic expression of SjSMO. In vitro RNAi results revealed a significant difference in pairing rates among all groups ( χ2 = 130.530, P < 0.000 1). Only the SjSMO interference group exhibited a markedly reduced worm pairing rate of 20.00%, which was significantly lower than that of the control group (94.74%) (P < 0.000 1), accompanied by decreased worm vitality. In contrast, no statistically significant differences in pairing rates were observed between each FZD family gene interference group and the control group (all P > 0.05). In vivo RNAi assay revealed that SjSMO dsRNAi 1 to 26 days post-infection resulted in a significant reduction in worm length, with reductions in the male worm length by 32.27% and female worm length by 36.77% as compared to controls [male worms: (6.51 ± 1.77) mm vs. (9.62 ± 1.44) mm, t = 7.196, P < 0.000 1; female worms: (5.49 ± 1.36) mm vs. (8.68 ± 2.60) mm, t = 5.953, P < 0.000 1], as well as arrest of testicular development in male worms and ovarian development in females. In addition, SjSMO dsRNAi 26 to 42 days post-infection was found to significantly reduce hepatic egg burdens in mice, with a 49.10% reduction in the count of liver eggs in the SjSMO dsRNAi group relative to the GFP dsRNA negative control group [(2 824 ± 844) eggs per gram vs. (5 548 ± 797) eggs per gram; t = 4.064, P < 0.05], and to alleviate granuloma formation and severity of hepatic fibrosis.

CONCLUSIONS: SjSMO is a key receptor that regulates the growth, development, reproduction, and oviposition of S. japonicum, which provides a new potential target for development of anti-schistosomal drugs.