Evolution of secretin family GPCR members in the metazoa

Abstract

Background: Comparative approaches using protostome and deuterostome data have greatly contributed to understanding gene function and organismal complexity. The family 2 G-protein coupled receptors (GPCRs) are one of the largest and best studied hormone and neuropeptide receptor families. They are suggested to have arisen from a single ancestral gene via duplication events. Despite the recent identification of receptor members in protostome and early deuterostome genomes, relatively little is known about their function or origin during metazoan divergence. In this study a comprehensive description of family 2 GPCR evolution is given based on in silico and expression analyses of the invertebrate receptor genes.

Results: Family 2 GPCR members were identified in the invertebrate genomes of the nematodes C. elegans and C. briggsae, the arthropods D. melanogaster and A. gambiae (mosquito) and in the tunicate C. intestinalis. This suggests that they are of ancient origin and have evolved through gene/genome duplication events. Sequence comparisons and phylogenetic analyses have demonstrated that the immediate gene environment, with regard to gene content, is conserved between the protostome and deuterostome receptor genomic regions. Also that the protostome genes are more like the deuterostome Corticotrophin Releasing Factor (CRF) and Calcitonin/Calcitonin Gene-Related Peptide (CAL/CGRP) receptors members than the other family 2 GPCR members. The evolution of family 2 GPCRs in deuterostomes is characterised by acquisition of new family members, with SCT (Secretin) receptors only present in tetrapods. Gene structure is characterised by an increase in intron number with organismal complexity with the exception of the vertebrate CAL/CGRP receptors.

Conclusion: The family 2 GPCR members provide a good example of gene duplication events occurring in tandem with increasing organismal complexity during metazoan evolution. The putative ancestral receptors are proposed to be more like the deuterostome CAL/CGRP and CRF receptors and this may be associated with their fundamental role in calcium regulation and the stress response, both of which are essential for survival.

Figure 1

Phylogenetic position of the protostome and deuterostome genomes analysed. Simplified phylogeny of the metazoan evolution based on molecular data indicating the positions of protostome (nematodes and arthropods) and deuterostome (tunicate and vertebrate) genomes analysed (adapted from Giribet., 2002 [79]; Gerhart et al., 2005 [80; Delsuc et al., 2006 [81).

Figure 2

Phylogenetic relationship of the metazoan family 2 GPCRs. Consensus phylogenetic tree (neighbour joining method, pairwise gap deletion, Poisson correction distance and 1000 bootstraps) produced with family 2 GPCR TM domains (TM2, TM4, TM5 and TM6). The protostome (nematodes and arthropods) and tunicate (Ciona) receptors are underlined and the bootstrap values for each fork is indicated. Bootstrap values less than 50 were removed. Annotation of the receptor subfamilies was carried out according to Donnelly (1997) [13]. The human (Hsa), Takifugu (Tru), nematodes, C. elegans (Cel) and C. briggsae (Cbr), arthropod D. melanogaster (Dme) and A. gambiae (Aga) and Ciona (Cin) receptor sequences were obtained as described in the methods section.

Figure 3

Comparison of the N-terminal end of metazoa family 2 GPCRs. Multiple sequence alignment of the N-terminal domain of the family 2 GPCR receptor genes identified and characterised in protostomes and deuterostomes. Conserved cysteine residues are indicated by ''•'' and the conserved amino acid motifs are boxed. The TM1 domain is annotated. Accession numbers of the human family 2 GPCRs: HsaGLP1R (P43220), HsaCRF1R (P34998) and HsaCALR (P30988). Accession numbers of the protostome family 2 receptor genes: House cricket (Acheta domesticus) Diuretic hormone receptor (AdoDHR, Q16983), Drosophila melanogaster (DmeCG13758, NP_570007; DmeCG8422, NP_610960; DmeCG32843, XP_396046), Anopheles gambiae (AgaP14164, EAA11768), Caenorhabditis. elegans (CelC18B12.2, NP_510496; CelC13B9.4, NP_498465) and Caenorhabditis briggsae (CbrCAE70126, CAE70126; CbrCAE63268, CAE63268). EST data was used to obtain the N-terminal region of the incomplete receptor sequences (Additional file 1). The N-terminal region of Ciona CinS752 was predicted by NIX and that of CinS5A by sequence comparison with CinS5B. Only the clones for which a putative N-terminal domain was identified were included in the analysis. For figure simplicity, an arrow indicates the region of the receptor CbrCAE70126 that was eliminated since it did not align with any other sequences present.

Figure 4

Gene environment comparison between C. elegans and Drosophila family 2 GPCRs genomic regions. Short-range linkage analysis of the region surrounding family 2 GPCRs on the C. elegans chromosome III and X with the Drosophila chromosome regions containing family 2 GPCR receptor genes. Genes are represented by horizontal bars and gene identification and chromosome position is given at the side. Family 2 GPCRs members are highlighted in bold. The lines represent the correspondence between the genes in each species. The dashed lines represent the common genes that were identified in both protostome and deuterostome genomes. For simplicity, only genes that are in common are represented.

Figure 5

Gene environment comparison between C. elegans and Takifugu and Human family 2 GPCRs genomic regions. Short-range linkage analysis of the region surrounding family 2 GPCRs on the C. elegans chromosome III and Takifugu and human chromosome regions containing family 2 GPCR receptor genes. Genes are represented by horizontal bars and gene identification and chromosome position is given at the side. Family 2 GPCRs members are highlighted in bold. The lines represent the correspondence between the genes in each species. The dashed lines represent the common genes that were identified in both protostome and deuterostome genomes. For simplicity, only genes that are in common are represented.

Figure 6

Evolution of family 2 GPCRs in the metazoa. Diagram illustrating the increase in gene number and gene structure complexity of the TM domains regions of family 2 GPCR receptor genes in protostomes (nematodes and arthropods) and deuterostomes (tunicate and vertebrate). The proposed species-specific gene duplications are indicated in the figure. Putative gene or genome duplication events of ancestral family 2 GPCRs during metazoan evolution are indicated by arrows (dashed arrow – represent several duplication events within the chordate lineage). The number of receptors identified in each species is within brackets. Exons are represented by blocks and introns by lines and the molecular evolutionary time for each species is indicated in MYA (million years ago) and was obtained from Hedges and Kumar, 2003 [82] and (*) from Dehal et al. 2002 [42]. The TM domain regions are represented by shaded regions and numbered. Intron/exon boundary phases are indicated below each exon. The tunicate and vertebrates family 2 GPCR members have been grouped according to their common gene organisation. The figure is not drawn to scale.