Frontier Science Research Center, University of Miyazaki, 5200 Kihara, Kiyotake, Miyazaki, Miyazaki 889-1692, Japan.
Family B G protein-coupled receptors (GPCRs) consist of 15 members that are preferentially coupled to Gs proteins for cAMP signaling [1-2]. Notably, their ligands are all biologically active peptides. Among these, calcitonin (CT), two CT gene-related peptides (CGRP andCGRP), amylin (AMY or islet amyloid polypeptide: IAPP), adrenomedullin (AM) and AM2 (or intermedin) are members of CT family peptides . These six peptides are deeply involved in many diseases, including cardiovascular diseases, diabetes mellitus and Alzheimer’s disease. Interestingly, CGRPs, AM, AMY and AM2 bind to the heterodimeric receptors consisting of two Family B GPCRs and three receptor activity-modifying proteins (RAMP1, RAMP2 and RAMP3), which are single-transmembrane domain accessory proteins that were discovered in 1998 . This special issue focuses on the recent notable findings of the RAMP-interacting Family B GPCRs (calcitonin (CT) receptor (CTR)-like receptor (CLR) and CTR) and their specific peptide ligands, particularly two CGRPs, AM, AMY and AM2 (Table 1).
Since the discovery of the three RAMPs , the concept of GPCRs has dramatically changed. The individual RAMPs primarily interact with eight Family B GPCRs (Table 1): the CLR, the CTR, the vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating polypeptide (PACAP) 1 (VPAC1) receptor, the parathyroid hormone 1 (PTH1) receptor, the PTH2 receptor, the glucagon receptor and the secretin receptor . Among these GPCRs, the CLR, CTR and VPAC1 receptor are coupled to each of the three RAMPs. The three RAMPs are required for CLR transport to the cell surface, where they form a CGRP receptor (CLR/RAMP1 heterodimer) and two AM receptors (CLR/RAMP2 and CLR/RAMP3 heterodimers). No self-transport of CLR to the plasma membrane has been observed. In contrast, other RAMP-interacting GPCRs can be expressed at the cell surface in the absence of RAMPs. Notably, the individual RAMPs alter the phenotype of the CTR to form three functional AMY receptors (CTR/RAMP1, CTR/RAMP2 and CTR/RAMP3 heterodimers). Although the three RAMPs do not change the phenotype of the VPAC1 receptor, RAMP2 co-expression augments intracellular IP3, but not cAMP, signaling of the VPAC1 receptor. Similarly, RAMP2 overexpression enables greater elevation of intracellular calcium, but not cAMP production, through the enhancement of Gq coupling to the CRF1 receptor. The PTH1 and glucagon receptors selectively interact with RAMP2, while the PTH2 and secretin receptors selectively interact with RAMP3.
The first half of this special issue describes the three CT family peptides, AM, CGRPs, AM2 and AMY. Among these bioactive peptides, it is well known that AM, CGRPs and AM2 can powerfully protect against multi-organ damage . AM was first isolated from human pheochromocytoma tissue extracts by Kitamura et al. in 1993 . In this issue, Kitamura et al. review the strong anti-inflammatory effect of AM in the context of inflammatory bowel disease, discussing a wide range of studies from basic science to a clinical study performed by the authors . Nishikimi et al. recently stated that “mid-regional proAM (MR-proAM) is an important biomarker of mortality and morbidity in patients with cardiovascular diseases,” whereas Holzmann referred to the anti-inflammatory activities of CGRP, which modulate innate immune responses in inflammatory bowel disease and sepsis . Takatori et al. have demonstrated the possible mechanisms underlying insulin resistance-induced hypertension and a role of perivascular adrenergic and CGRPergic nerves in the development of hypertension. However, two mammalian AMs (AM2 and AM5) and five vertebrate animal AMs (AM1 through AM5) were discovered in a genome-wide screen by Takei et al. , although these peptides have been not isolated from tissues. The five AM genes form a group that is independent of CGRP and amylin in the molecular phylogenetic tree. In this issue, Takei et al. refer to potential unidentified receptors specific for AM2 and AM5 by comparing the biological activities of AM (or AM1). Holmes et al. have reviewed the protective effects of AM2 in cardiovascular, renal and pulmonary diseases by clearly comparing the effects of AM and CGRP . Additionally, Westermark et al. have reviewed the involvement of AMY in the development of type 2 diabetes, which results from its pathological aggregation into amyloid  and its effect on insulin secretion from pancreatic cells. Fu et al. focused on the recent progress in linking AMY and its receptor to neurodegenerative diseases of the brain .
The second half of this special issue discusses the heterodimeric receptors for AM, CGRPs, AM2 and AMY. Shindo et al. generated knockout (KO) mice for RAMP2 and AM . Using these KO mice, they have demonstrated that the AM-RAMP2 system has protective effects on organs and the vasculature at postnatal stages and that it plays an important role in morphogenesis during development. Woolley et al. reviewed the structure and functional analyses of the CGRP and AM receptors (CLR/RAMP1 and CLR/RAMP2, respectively). Notably, Kusano et al. [14-15] first reported the crystal structures of the ectodomains (ECDs) of human RAMP1 (for CGRP receptor) and RAMP2 (for AM receptor). In this issue, they review the distinct features of crystal ECDs of the human CGRP and AM receptors by comparing the numerous mutagenesis data. They also refer to the comparison with the ECDs of the six Family B GPCRs, including the CLR and PTH1 and CRF1 receptors, which can interact with at least one RAMP. Edvinsson et al. reviewed the effects of clinical CGRP receptor (CLR/RAMP1) antagonists on migraine . Chang et al. comprehensively reviewed the effects of CGRP, AM and AM2 on reproduction and development as well as their receptor functions. Dickerson primarily reviewed the CGRP-receptor component protein (RCP) in heterodimers consisting of CLR and three RAMPs. Interestingly, overexpression of the second intracellular loop (ICL2) of CLR can markedly inhibit CGRP receptor signaling by blocking the interaction between RCP and endogenous CLR ICL2 . We have primarily reviewed our current view of the functions of the third extracellular loop (ECL3) and the eighth helix (helix 8) in the activation of Family B GPCRs associated with RAMPs and the roles of GPCR kinases (GRKs) in trafficking, notably internalization, of this family of receptor complexes [18-19]. Finally, Larráyoz et al. have referred to the unique idea of proposing the cytoskeleton as a cellular receptor for AM and its related peptide.
There are still many issues to be solved before the clinical application of the RAMP-interacting Family B GPCRs and their specific peptide ligands. Currently, the largest hurdles are the clarification of the crystal structures of all heterodimers consisting of Family B GPCRs and their RAMPs and the development of the related non-peptide low-molecular weight compounds. I truly hope that this special issue will help to promote the development of research on the RAMP-interacting Family B GPCRs and their specific ligands.