The Calcitonin/Calcitonin Gene-Related Peptide Family in Invertebrate Deuterostomes

Abstract

Calcitonin (CT)/CT gene-related peptide (CGRP) family peptides (CT/CGRP family peptides) including CT, CGRP, adrenomedullin, amylin, and CT receptor-stimulating peptide have been identified from various vertebrates and perform a variety of important physiological functions. These peptides bind to two types of receptors including CT receptor (CTR) and CTR-like receptor (CLR). Receptor recognition of CT/CGRP family peptides is determined by the heterodimer between CTR/CLR and receptor activity-modifying protein (RAMP). Comparative studies of the CT/CGRP family have been exclusively performed in vertebrates from teleost fishes to mammals and strongly manifest that the CGRP family system containing peptides, their receptors, and RAMPs was derived from a common ancestor. In addition, CT/CGRP family peptides and their receptors are also identified and inferred from various invertebrate species. However, the evolutionary process of the CT/CGRP family from invertebrates to vertebrates remains enigmatic. In this review, I principally summarize the CT/CGRP family peptides and their receptors in invertebrate deuterostomes, highlighting the study of invertebrate chordates including ascidians and amphioxi. The CT/CGRP family peptide that shows similar molecular structure and function with that of vertebrate CT has been identified from ascidian, Ciona intestinalis. Amphioxus, Branchiostoma floridae also possessed three CT/CGRP family peptides, one CTR/CLR receptor, and three RAMP-like proteins. The molecular function of the receptor complex formed by amphioxus CTR/CLR and a RAMP-like protein was clarified. Moreover, CT/CGRP family peptides have been identified in the superphylum Ambulacraria, which is close to Chordata. Finally, this review provides potential hypotheses of the evolution of CGRP family peptides and their receptors from invertebrates to vertebrates.

Keywords: amphioxus; ascidian; calcitonin/calcitonin gene-related peptide family; evolution; invertebrate deuterostome.

Figure 1

Molecular features of vertebrate CT/CGRP family peptides. (A) Primary structure of CT/CGRP family peptides in mammals. All identical and more than half of similar amino acid residues are denoted as a red and yellow box, respectively. The GenBank accession numbers of sequences are as follows: human CT, NP_001029124.1; human CGRP, CAA34070.1; human AMY, NP_001316130.1; human AM, AAC60642.1; human AM2, BAE46395.1; pig AM5, BAF64272.1; pig CRSP, NP_998907.1. Pig AM5 and CRSP were selected owing to the lack of human AM and CRSP sequences. The mature form of pig AM5 was predicted by using the signalP4.1 web server (http://www.cbs.dtu.dk/services/SignalP/). (B) The secondary structure of CT/CGRP family peptides. The N-terminal circular region, the central α-helical region, and the C-terminal amidated residue are conserved in vertebrates. The schematic diagram is based on the structure of CT.

Figure 2

Multiple alignment of CT/CGRP family peptides in invertebrate deuterostomes. (A) Multiple alignment of CT/CGRP family peptides of protochordates and human. The GenBank accession numbers of sequences are as follows: Ci-CT, BAI63095.1; Bf-CTFP1, BAU51800.1; Bf-CTFP2, BAU51801.1; Bf-CTFP3, BAU51802.1; human CT, NP_001029124.1; human CGRP, CAA34070.1; human AMY, NP_001316130.1; human AM, AAC60642.1. (B) Multiple alignment of CT/CGRP family peptides of Ambulacraria and human. Accession numbers for the corresponding precursors are as follows: SpCTLP, XP_001177490.1 (GenBank); ArCTP, ALJ99957.1 (GenBank); OvCT1 and 2, MF15231 and MF15235 (GenBank). Amino acid sequences of SkCTLP1-2, AjCTLP1-2, and HscCTP1-2 were obtained from the following publications: SkCTLP1-2, Rowe et al. (25); AjCTLP1-2, Rowe et al. (25); HscCTP1-2, Suwansa-Ard et al. (36).

Figure 3

A schema predicting the diversification process of the CT/CGRP family peptide gene during chordate evolution. This schema is described as a modification of Takei's report (66). The dotted line and dotted box represent uncertain pathway and unidentified gene, respectively. Question mark on dotted line depicts the ambiguous pathway. Dotted boxes with a question mark exhibit that comprehensive identification of cartilaginous fish and cyclostome CT/CGRP family peptide is incomplete. 2R and 3R show second-round whole genome duplication and third-round whole genome duplication, respectively. CT, calcitonin; CGRP, CT gene-related peptide, AM, adrenomedullin; AMY, amylin; CRSP, CT receptor-stimulating peptide.

Figure 4

Three possible evolutionary scenarios of the CT/CGRP family receptors and RAMPs in chordates. (A) A common ancestor of chordates might have already possessed the RAMP-independent CTR-type gene, RAMP-dependent CLR-type gene, and RAMP gene. Gene loss of the CTR-type gene and CLR-type gene might have occurred in the amphioxus and ascidian lineage, respectively. (B) A primitive chordate might have had only a CLR-type gene and RAMP gene. Although an ascidian RAMP-like gene exists, this gene has not yet been identified. A CTR-type gene might have arisen in the vertebrate lineage. (C) A common ancestor of chordates might have possessed a CTR-type gene. CTR-type gene might have been lost in extant amphioxus. Furthermore, both amphioxus and vertebrates might have independently acquired the RAMP-like gene and CLR-type gene.