The nuclear receptors of Biomphalaria glabrata and Lottia gigantea: implications for developing new model organisms

Abstract

Nuclear receptors (NRs) are transcription regulators involved in an array of diverse physiological functions including key roles in endocrine and metabolic function. The aim of this study was to identify nuclear receptors in the fully sequenced genome of the gastropod snail, Biomphalaria glabrata, intermediate host for Schistosoma mansoni and compare these to known vertebrate NRs, with a view to assessing the snail's potential as a invertebrate model organism for endocrine function, both as a prospective new test organism and to elucidate the fundamental genetic and mechanistic causes of disease. For comparative purposes, the genome of a second gastropod, the owl limpet, Lottia gigantea was also investigated for nuclear receptors. Thirty-nine and thirty-three putative NRs were identified from the B. glabrata and L. gigantea genomes respectively, based on the presence of a conserved DNA-binding domain and/or ligand-binding domain. Nuclear receptor transcript expression was confirmed and sequences were subjected to a comparative phylogenetic analysis, which demonstrated that these molluscs have representatives of all the major NR subfamilies (1-6). Many of the identified NRs are conserved between vertebrates and invertebrates, however differences exist, most notably, the absence of receptors of Group 3C, which includes some of the vertebrate endocrine hormone targets. The mollusc genomes also contain NR homologues that are present in insects and nematodes but not in vertebrates, such as Group 1J (HR48/DAF12/HR96). The identification of many shared receptors between humans and molluscs indicates the potential for molluscs as model organisms; however the absence of several steroid hormone receptors indicates snail endocrine systems are fundamentally different.

Fig 1. Phylogenetic relationships of NRs in molluscs, humans, fly, nematode and trematode.

NRs from six species B. glabrata (Bg), L. gigantea (Lg), H. sapiens (Hs), D. melanogaster (Dm), C. elegans (Ce) and S. mansoni (Sm), were subjected to phylogenetic comparisons using Bayesian inference, maximum parsimony and maximum likelihood methods. The Bayesian tree (midpoint rooted) is shown with posterior probability values from Bayesian inference and bootstrap values from maximum parsimony and maximum likelihood trees. The value of 1 on each node represents 100% posterior probability/bootstrap support; an X indicates an area of disagreement from the Bayesian tree (S1 Fig). Scale bar, 0.1 expected changes/site.

Fig 2. Phylogenetic relationships of peroxisome proliferator-activated receptors (PPAR α, β and γ).

PPAR sequences from different species were subjected to phylogenetic analyses using Bayesian inference, maximum likelihood and maximum parsimony. Accession numbers of the PPARs of the different species in the phylogenetic tree: Heterocephalus glaber XP_004846774; Physeter catodon XP_007109986; Leptonychotes weddellii γ XP_006734113; L. weddellii α XP_006750071; L. weddellii δ XP_006737129; Saccoglossus kowalevskii XP_006819446; Strongylocentrotus purpuratus α XP_781750; S. purpuratus γ XP_784429; C. gigas EKC18691; H. sapiens α AB32649; H. sapiens γ AAH06811; H. sapiens β AAA36469; Mus musculus α NP_035274; M. musculus γ NP_035276; M. musculus β NP_035275; Oryzias latipus α XP_004069934; O. latipus β NP_001265836; O. latipus γ NP_001158348; B. glabrata 1 Contig2052; B. glabrata 2 Contig1275; L. gigantea 1 ProteinID174409; L. gigantea 2 ProteinID238472; H. sapiens ROR AAH0883. Scale bar, 0.1 expected changes/site.

Fig 3. Phylogenetic relationships of 2DBD nuclear receptors (alpha, beta and gamma).

2DBD NR sequences from different species were subjected to phylogenetic analyses using Bayesian inference, maximum likelihood and maximum parsimony. Accession numbers of the 2DBD NRs of the different species in the phylogenetic tree: D. pulex FE382753; Hymenolepis microstoma CDS31978; Echinococcus multilocularis CDI99377; Echinococcus granulosus CDS24085; S. mansoni α AAW88533; S. mansoni β AAW88534; S. mansoni γ AAW88550; Dugesia japonica BP186725; L. gigantea 2DBD1 ProteinID168696; L. gigantea 2DBD2 ProteinID173632; B. glabrata 2DBD1 Contig304; B. glabrata 2DBD2 Contig1296. Scale bar, 0.1 expected changes/site.